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Ludwig Institute for Cancer Research, Box 595, S-75124 Uppsala, Sweden
1Correspondence and current address: Division of Cellular Biochemistry, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands. E-mail: ptdijke{at}nki.nl
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ABSTRACT |
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 TOP ABSTRACT THE TGF-{beta} SUPERFAMILY AND...SIGNALING VIA TGF-{beta}...INTRACELLULAR SIGNALING BY TGF...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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Key Words: activin • bone morphogenetic protein • signal transduction • Smad • transforming growth factor-ß
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THE TGF-ß SUPERFAMILY AND ITS BINDING PROTEINS |
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TOPABSTRACTTHE TGF-{beta} SUPERFAMILY AND...SIGNALING VIA TGF-{beta}...INTRACELLULAR SIGNALING BY TGF...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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, 2
). BMPs are potent
inducers of bone and cartilage formation and play important
developmental roles in the induction of ventral mesoderm,
differentiation of neural tissue, and organogenesis (reviewed in refs 3
, 4
). Activins, named after their initial identification as activators
of follicle-stimulating hormone (FSH) secretion from pituitary glands,
are also known to promote erythropoiesis, mediate dorsal mesoderm
induction, and contribute to survival of nerve cells (reviewed in ref 5
). Several growth factors belonging to the TGF-ß superfamily play
important roles in embryonic patterning and tissue homeostasis. Their
inappropriate functioning has been implicated in several pathological
situations like fibrosis, rheumatoid arthritis, and carcinogenesis.
Distinct in vivo expression patterns of TGF-ß
isoforms
TGF-ß1, TGF-ß2, and TGF-ß3 are highly similar in their
biological activities in vitro. However, they differ in
their in vivo expression patterns (reviewed in ref 6
), which
largely explains the unique isoform-specific phenotypes displayed by
the TGF-ß knockout mice. TGF-ß1-deficient mice that are born alive
undergo early postnatal death due to excessive infiltration of
inflammatory lymphocytes and macrophages into several organs
(7
8
9)
. Half of the mice lacking TGF-ß1 die in
utero due to defects in vasculogenesis and hematopoiesis
(10)
. It is thought that maternally supplied TGF-ß1, in
combination with redundant expression of TGF-ß isoforms, contributes
to normal development of the embryos. TGF-ß1 null mice born from
TGF-ß1-deficient mothers display abnormal cardiac development
(11)
. TGF-ß2 knockout mice show multiple developmental
malformations of tissues and organs, leading to perinatal death
(12)
. TGF-ß3 null mice die shortly after birth,
displaying delayed pulmonary development and defective palate
development (13
, 14)
.
Analysis of the genes encoding the three TGF-ßs revealed that each
isoform is controlled by differentially regulated promoters (reviewed
in ref 15
). The existence of different genes encoding functionally
similar proteins, yet controlled by differentially regulated promoters,
might provide an important mechanism to ensure tissue-specific and
spatio-temporal expression patterns of the different TGF-ß isoforms,
thereby resulting in proper embryonic development and tissue
homeostasis.
In addition, posttranscriptional control mechanisms contribute to
regulation of the production of TGF-ß. TGF-ß1, TGF-ß2, and
TGF-ß3 that have long, GC-rich 5'-untranslated regions and
intramolecular duplex loops located in close proximity to the
transcriptional start site negatively modulate TGF-ß expression,
possibly by binding cell type-specific cytoplasmic molecules
(16)
.
Control of TGF-ß bioactivity
Members of the TGF-ß superfamily are synthesized as large
precursor molecules that are proteolytically processed in the Golgi
apparatus by the convertase family of endoproteases, of which furin is
a member. Furin cleaves the precursor into a mature TGF-ß and
amino-terminal precursor remnant, also termed latency-associated
protein, or LAP (reviewed in ref 17
and references therein; 18). LAP
remains noncovalently linked to TGF-ß and prevents binding of mature
TGF-ß to its receptors. These so-called small latent TGF-ß
complexes are significantly more stable than bioactive TGF-ß. Within
the Golgi, LAP covalently interacts with latent TGF-ß binding
proteins (LTBPs) to form large, latent complexes (reviewed in ref 19
and references therein). Four different LTBP homologs, LTBPs 1–4, have
been characterized, including several alternative splice variants
(reviewed in refs 19
, 20
). LTBPs function to enhance secretion and
stability of the TGF-ß-LAP complex, ensure correct folding of
TGF-ß, and target the latent TGF-ß complex to the extracellular
matrix of certain cells and tissues for storage or to the cell surface
where activation takes place (reviewed in refs 17
, 19
and references
therein). LTBP-1, -2, and -4 contain RGD sequences that are integrin
binding sites (20
, 21)
. It has been shown that large
latent TGF-ß complexes directly interact with integrin
v/ß1 at the cell
surface (21)
, which may enable TGF-ß to activate
integrin signaling. Activation of latent TGF-ß into biologically
active mature TGF-ß is controlled by proteases like plasmin or
cathepsin, which cleave LAP (Fig. 1
), and by binding of LAP to mannose-6-phosphate receptors (17, 19 and
references therein). Lipoprotein Lp(a) inhibits activation of
plasminogen to plasmin and thereby negatively affects activation of
TGF-ß (reviewed in ref 22
and references therein). Other mechanisms
that have been implicated in activation of latent TGF-ß are
deglycosylation of LAP (23)
, exposure to reactive oxygen
species (24)
, or acidic cellular microenvironments
(25)
. In addition, an important activator of TGF-ß
in vivo appears to be thrombospondin-1, which induces a
conformational change of LAP and thereby activates TGF-ß
(26)
.
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Signaling diversity by generation of heterodimeric ligands
After proteolytic cleavage of the mature carboxyl-terminal parts,
biologically active TGF-ß1, -ß2, or -ß3 homodimers are generated.
Homodimers are most abundant, but TGF-ß1.2 (27
, 28)
and
TGF-ß2.3 (28)
heterodimers have been identified in
vivo. For activin, four different ß chains have been
identified—ßA, ßB, ßC, and ßE (reviewed in ref 29
and
references therein)—of which ßA and ßB are known to form homo- as
well as heterodimers. Whether heterodimers can also be formed with the
ßC and ßE chains and whether these different isoforms exert
different biological activities is not clear. Inhibins, which are
heterodimers of inhibin chains and activin ß chains, are
functional antagonists of activin signaling (reviewed in ref 5
). In the
case of BMPs, BMP2/7 and BMP4/7 heterodimers are much more potent in
the induction of ventral mesoderm (30)
, as well as in bone
induction (31)
than their respective homodimers. The
coexpression of individual BMPs in several tissues suggests that
heterodimer formation might occur in vivo, and heterodimers
of BMP2 and BMP-7 have been isolated from bovine bone
(32)
. The mechanism underlying potentiated signaling by
heterodimers compared to homodimers has not been investigated, but
might be due to formation of receptor complexes consisting of, for
example, two different type II and two different type I receptors,
which may activate downstream signaling in a more potent manner.
TGF-ß superfamily binding proteins
After production of bioactive TGF-ß superfamily members, several
extracellular proteins can bind and modify their activity (Table 1
). The extracellular matrix proteoglycans decorin and biglycan have been
described to inhibit TGF-ß activity (33)
. Transgenic
mice lacking biglycan expression show dramatically decreased bone mass,
suggesting an important role for biglycan in the constitution of bone,
possibly by its function as a source for storage of TGF-ß superfamily
members (34)
.
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The serum protein 2-macroglobulin associates with circulating
TGF-ß superfamily members; via its interaction with the
2-macroglobulin receptor, it takes care of clearance of the growth
factors from serum (35
, 36)
. Fibronectin can bind TGF-ß
and promote its bioactivity, possibly by changing it from a latent to a
bioactive conformation (37)
, and collagen IV may store
bioactive TGF-ß in the extracellular matrix (38)
.
Factors known to play important roles during embryonic development are
follistatin, noggin, chordin, and Short gastrulation (Sog), the
Drosophila homologue of chordin (39)
. These
proteins are secreted by the Spemann organizer and compete with BMPs to
exert opposing effects on developmental fates of mesoderm and ectoderm
(reviewed in ref 40
and references therein). Mice deficient in noggin
expression display excessive cartilage formation and failure of joint
specification during skeletogenesis (41)
. The TGF-ß
superfamily member Xnr3 has been reported to interfere with BMP
signaling in the Xenopus Spemann organizer via an
intracellular antagonistic mechanism that prevents BMP synthesis
(reviewed in ref 40
).
Cerberus, DAN, and gremlin have recently been identified as antagonists
of BMP signaling during early embryogenesis (42)
. Cerberus
can also counteract signaling by activin and nodal. These proteins,
like noggin and chordin, have been shown to interact directly with the
growth factors, thereby preventing interaction of the ligands with
their signaling receptors (reviewed in refs 40
, 42
). Follistatin
counteracts activin signaling by a similar mechanism (reviewed in ref 36
), whereas the mechanism by which follistatin interferes with BMP
signaling is unclear; BMP–follistatin complexes were found to interact
normally with BMP receptors (43)
.
Interaction of chordin or Sog with BMPs is controlled by
metalloproteases of the astacin family, to which tolloid, the
Drosophila homologue of BMP1, Xenopus xolloid,
and zebrafish Ztld, belongs, and they are able to proteolytically
cleave chordin and Sog, thereby liberating bioactive BMPs (44; reviewed
in ref 45
).
An extensive number of membrane-bound or transmembrane TGF-ß
binding proteins are known to interfere with TGF-ß action. The type I
and type II serine/threonine kinase receptors, which will be discussed
in detail below, are directly involved in TGF-ß signal transduction.
TGF-ß receptor III (TßR-III; also known as betaglycan) and endoglin
are transmembrane proteins that share high sequence homology in their
short cytoplasmic tails rich in serine residues (46
, 47)
.
Betaglycan has several high-affinity binding sites for TGF-ß1,
TGF-ß2, and TGF-ß3 (48)
and facilitates binding of
TGF-ß to their type II receptors, a property that is especially
important for TGF-ß2, which has low intrinsic affinity for TßR-II
(Fig. 2
) (49
, 50)
. In contrast, soluble betaglycan acts as an
antagonist of TGF-ß bioactivity by preventing the interaction of
TGF-ß with the signaling receptors (46
, 51)
.
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The role of endoglin in modulation of TGF-ß signaling appears to be
different from betaglycan. In contrast to betaglycan, endoglin can
interact with TGF-ß1, TGF-ß3, activin A, BMP-2, and BMP-7 and
requires appropriate type I or type II receptors for efficient ligand
binding (52)
. Ectopic expression of endoglin in monocytes
results in a selective abrogation of TGF-ß1-induced growth inhibition
and fibronectin synthesis (53)
. Furthermore, mutations in
the genes encoding endoglin or ALK-1, a putative TGF-ß type I
receptor in endothelial cells, form the basis for the vascular disorder
hereditary hemorrhagic telangiectasia type 1 and type 2, respectively
(54
, 55)
, suggesting that endoglin and ALK-1 act in a
common signal transduction pathway.
In many vascular endothelial cells and hematopoietic progenitor
cells, TGF-ß1 and TGF-ß3 are equipotent in mediating signaling, but
TGF-ß2 has weaker biological effects (47
, 56)
. This
difference in potency between different TGF-ß isoforms can be
accounted for by the lack of betaglycan in these cells. Differential
activities of TGF-ß isoforms in other cell types have been observed,
but their modes of action have not been correlated to receptor binding
properties (57
, 58)
. Several cell membrane-associated
TGF-ß binding proteins, including certain
glycosyl-phosphatidylinositol (GPI) -anchored proteins (59
, 60)
, have been identified that are endowed with specific
affinity for certain TGF-ß isoforms, but their role in TGF-ß
signaling is not clear. Furthermore, a 60 kDa TGF-ß binding protein
has been characterized (61
, 62)
that exhibits TGF-ß1
binding specificity and interferes with binding of TGF-ß to the
signaling receptors (62)
.
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SIGNALING VIA TGF-ß SUPERFAMILY RECEPTORS |
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TOPABSTRACTTHE TGF-{beta} SUPERFAMILY AND...SIGNALING VIA TGF-{beta}...INTRACELLULAR SIGNALING BY TGF...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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.
TGF-ß1, TGF-ß3, and activins initially bind to their corresponding
type II receptors, after which the type I receptors are recruited into
the signaling complex. Type I and type II receptors have an intrinsic
affinity for each other, which contributes to stability of the
heteromeric complex (63)
. TGF-ß2, which does not bind to
TßR-II or TßR-I alone, can interact with and stabilize transiently
formed complexes of TßR-I and TßR-II (64)
. BMPs have
low affinity for type II or type I receptors individually, and
high-affinity binding requires formation of a heteromeric type I/type
II receptor complex (65
, 66)
. At present, only a few type
I and type II receptors for the TGF-ß superfamily are known. However,
depending on the ligand, multiple heterotetrameric complexes consisting
of two type I and two type II receptor homodimers are possible
(67)
. In view of the existence of heterodimeric ligands,
it is conceivable that two different type I and two different type II
receptors constitute the receptor complex, thereby creating
combinatorial signaling.
TGF-ßs form heteromeric complexes between TßR-II and TßR-I in
most cell types (68
, 69)
. In endothelial cells TGF-ß can
bind and signal through activin receptor-like kinase 1 (ALK-1); ALK-1
has therefore been implicated as an endothelial-specific TGF-ß type I
receptor (T. Imamura, P. ten Dijke, and K. Miyazono, personal
communication). Activins signal via combinations of ActR-II or ActR-IIB
and ActR-IB (68
, 70)
. Activins have also been shown to
interact with ActR-I (68
, 71)
, although this receptor
appears to play a minor role in activin signaling. BMPs interact with
ActR-II, ActR-IIB, or BMPR-II, in combination with ActR-I, BMPR-IA, or
BMPR-IB (65
, 66
, 72)
. For AMH, only a type II receptor has
been identified (73)
. A potential AMH type I receptor is
ActR-I, which is coexpressed with AMHR-II in the Müllerian duct
and can mediate AMH-specific repression of FSH-induced aromatase
activity (A. Themmen, personal communication). ALK-7 is an orphan
receptor (74)
. A novel type I receptor, designated zALK-8,
was recently cloned and shown to be widely expressed throughout early
zebrafish development (75)
. Mammalian homologues of zALK-8
as well as of zebrafish TARAM-A, which is a type I receptor involved in
induction of anterior dorsal mesoderm (76)
, have not yet
been identified. The ligands for these receptors have not been
elucidated.
Activation and regulation of serine/threonine kinase type I and
type II receptors
Type I receptor kinases become activated upon phosphorylation by
the constitutively active type II receptor kinase (Fig. 2)
. Thus, type
I receptors act downstream of type II receptors (77)
.
Consistent with this finding, type I receptors were found to determine
the specificity of the intracellular signals induced by different
TGF-ß superfamily members, whereas the L45 region in type I receptors
was found to be important in determining the specificity of type I
receptors (78
79
80)
.
The phosphorylation status of TßR-II has been shown to influence
TGF-ß signaling. TßR-II is a constitutively active kinase that
requires phosphorylation of Ser213 (located outside the kinase domain)
to mediate its autocatalytic effect. Ligand-dependent
autophosphorylation of TßR-II on either Ser409 or Ser416
differentially contributes to regulation of its activity, leading to
stimulation or inhibition of TGF-ß signaling, respectively
(81)
.
Differential kinetics in biosynthesis, ligand-induced internalization,
and down-regulation of type I vs. type II receptors have been
attributed to differential modulation of TGF-ß signaling
(82
83
84
85)
. The half-life and processing of TßR-II in the
endoplasmic reticulum are considerably shorter than that of TßR-I
(82
83
84
85)
. Furthermore, homo- and heteromeric receptor
complexes have distinct endocytotic fates (84)
. Rapid
modulation of TßR-II expression levels as well as ligand-bound
TßR-I/TßR-II complexes may be important for fine-tuning of
signaling by TGF-ß.
Diversity in receptor availability might be provided by the
existence of alternative receptor transcripts, as identified for
ActR-IIA (86)
, ActR-IB (87)
, ActR-IIB
(70)
, AMHR-II (73)
, and TßR-II
(88)
. In the case of ActR-IIB, differential affinity of
activin for the alternatively spliced gene products has been observed.
In addition, differences in the cytoplasmic domains offer possibilities
for differential modes of signaling (70)
.
Several growth factors are known to bind to the same type II
receptor—for example, activins and BMPs, which share ActR-II and
ActR-IIB (72
, 89)
. This enables signaling by several
growth factors via a limited scala of available receptor types, whereas
it can lead to competition of different ligands for the same type II
receptors, thereby fine-tuning signaling via type I receptors. This is
the case for activin and inhibin, since inhibin is known to counteract
many of activin functions by competing for binding to the activin type
II receptor (90)
. A similar mechanism has been suggested
to underlie the inhibitory action of zebrafish antivin, most closely
related to mouse lefty, upon activin signaling (91)
, and
appears to contribute to the functional antagonism observed between
activin and OP-1 in Tera-2 embryonal carcinoma cells (89)
.
Whether different ligands might also compete for binding to common type
I receptors is not clear at present.
Cytoplasmic TGF-ß receptor interacting proteins distinct from
Smads
A number of cytoplasmic proteins are now known to interact
with the kinase domain of type I and type II receptors, thereby
manipulating or mediating their signaling capacities (Table 2
). TRIP-1, a protein that contains five WD domains, is known to bind and
become phosphorylated by TßR-II. The interaction requires a
functional TßR-II kinase (92)
. Overexpression of TRIP-1
attenuates TGF-ß-induced PAI-1 transcriptional response, but not
TGF-ß-induced cyclin A response (93)
. The WD
domain-containing protein STRAP can associate with TßR-I and
TßR-II. STRAP and the inhibitory Smad7 synergistically block
TGF-ß-mediated transcriptional activation (94)
. Another
WD-40 repeat protein B, a subunit of protein phosphatase 2A, has
been reported to interact with and become phosphorylated by TßR-I.
B potentiates the antiproliferative effect mediated by TGF-ß
(95)
. TßR-I-associated protein-1 (TRAP-1) interacts
specifically with the activated TßR-I. However, its functional role
in TGF-ß signaling is unclear (96)
.
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The immunophilin FKBP12 interacts with type I serine/threonine kinase
receptors (97)
through an FK506-like Leu-Pro motif
preceding the kinase domain (98)
. FKBP-12 inhibits
phosphorylation of the type I receptors by the type II receptors,
possibly by sterical hindrance, and may function to prevent leaky
signaling in the basal cell state (99)
. FKBP12 dissociates
from TßR-I after ligand-induced phosphorylation of TßR-I by
TßR-II (97
, 98)
.
Farnesyltransferase (FT) has also been shown to interact with
and become phosphorylated by activin and TGF-ß type I receptors
(99)
. However, farnesyl transferase activity was shown to
be dispensable for TGF-ß signaling (100)
.
Type I receptors as determinants of downstream signal propagation
Activation of type I receptors by type II receptors occurs by
phosphorylation of serine and threonine residues within a juxtamembrane
region preceding the kinase domain of type I receptors that is
characterized by repeats of glycine and serine residues (also termed
the GS domain) (77)
. Specificity of signal propagation to
Smad molecules, the main downstream components in serine/threonine
kinase receptor signaling, is determined by type I receptors (reviewed
in refs 29
, 101
). More specifically, a region flanking ß strands 4
and 5 in the kinase domain, designated L45 loop, confers a high degree
of specificity in Smad interaction with the type I receptors (78
, 80)
. The L45 loop is highly conserved among type I receptors
with comparable signaling specificity, but differs significantly
between BMP and TGF-ß or activin type I receptors (79
, 80)
. Swapping the L45 loop between TßR-I and BMPR-IB, followed
by ligand-stimulation, was shown to result in exchange of Smad1 vs.
Smad2 recognition specificity, accompanied by a switch in
transcriptional responses (79
, 80)
. However, differential
effects between the two BMP type I receptors have been described
(102
103
104)
. CDMP-1 and CDMP-2, which both interact with
BMPR-IB and BMPR-II, differentially regulate osteogenesis (105
, 106)
. Thus, in addition to a role for the L45 loop, other
mechanisms must contribute to specify downstream signaling after
activation of type I receptors.
It has been shown that the cytoplasmic juxtamembrane region of TßR-I
is involved in specifying signal transduction, since mutations of
certain serine or threonine residues within this region selectively
impair TGF-ß-mediated growth inhibition, but do not affect
TGF-ß-induced PAI-1 or fibronectin synthesis (107)
.
However, in vivo phosphorylation of these sites by TßR-II
has not yet been reported. Mutation of Ser165, located amino-terminal
of the GS domain, results in potentiation of TGF-ß-mediated growth
inhibition and extracellular matrix induction but diminishes
TGF-ß-induced apoptosis (108)
. Thus, additional residues
in TßR-I appear to modulate distinct pathways triggered by TGF-ß.
In certain cell systems it has been observed that the expression level
of type I receptors correlates with extracellular matrix production and
gene transcription, whereas the expression level of type II receptors
correlates to the growth inhibitory response, suggesting participation
of type I and type II receptors in different signaling pathways.
However, the observations made are all compatible with an alternative
interpretation, which is that all signals are initiated by activation
of the type I receptor and that different activation thresholds of the
signaling pathways leading to growth inhibition vs. gene transcription
underlie the apparent differential modes of signaling by type I and
type II receptors (109)
.
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INTRACELLULAR SIGNALING BY TGF-ß SUPERFAMILY MEMBERS |
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TOPABSTRACTTHE TGF-{beta} SUPERFAMILY AND...SIGNALING VIA TGF-{beta}...INTRACELLULAR SIGNALING BY TGF...CONCLUSIONS AND PERSPECTIVESREFERENCES |
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, 111)
, was isolated from a genetic screen for
enhancers of a weak Dpp maternal phenotype. Subsequently, three
Mad homologues, called Sma-2, Sma-3,
and Sma-4, were picked up in C. elegans by
searching for genes whose mutations caused the same small body-size
phenotype as observed for mutant Daf4, a type II
serine/threonine kinase receptor (112)
. At present, eight
different Sma- and Mad-related proteins have been identified in
mammals, and are termed Smads (Fig. 3A
). They can be subdivided into three distinct subclasses:
receptor-activated Smads (R-Smads), common-partner Smads (Co-Smads),
and inhibitory Smads (anti-Smads). R-Smads and Co-Smads are homologous
in their amino- and carboxyl-terminals, called the MH1 and MH2 domains,
respectively. These domains are connected by a proline-rich linker
region. R-Smads contain SSXS phosphorylation motifs in their
own carboxy termini.
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Smad1, Smad5, and Smad8 are involved in BMP signaling and become
phosphorylated by ActR-I, BMPR-IA, or BMPR-IB (113
114
115
116
117)
.
Smad2 and Smad3 are mediators of TGF-ß and activin signaling and
interact with and become phosphorylated by TßR-I or ActR-IB
(118
119
120
121)
. Consistent with these findings, injection of
BMP-Smads into Xenopus animal caps results in
ventralization of mesoderm (115
, 116
, 122
, 123)
, and
ectopic expression of Smad2 in Xenopus embryos induces
dorsal mesoderm and secondary axis formation analogous to
activin/Vg-1-like responses (122
, 123)
. Smad2 null mice
lack anterior-posterior specification and fail to develop mesoderm
(124
, 125)
. The phenotype of Smad2 knockouts is strikingly
similar to Nodal knockouts and suggests that they cooperate in
regulation of gastrulation (124)
. Mutant mice lacking
Smad3 expression have been reported to be viable and fertile; however,
Smad3 -/- animals were found to exhibit limb malformations
(126)
, have a defect in immune function
(127)
, and develop colon carcinomas 4–6 months after
birth (128)
. Smad3 null cell lines showed strongly
impaired TGF-ß responsiveness and indicated an important role for
Smad3 in TGF-ß-mediated inhibition of cellular proliferation
(126)
. The Smad3 knockout phenotype observed by Zhu and
co-workers (128)
classifies Smad3 as a tumor suppressor
gene, although Smad3 mutations have not yet been detected in
tumorigenesis (129)
.
TGF-ß-mediated phosphorylation of Smad1 has been observed in breast
tumor cells, but the receptor by which this phosphorylation is mediated
has not been determined (130)
. Smad5 phosphorylation in
response to TGF-ß was detected in CD34+ cells (131)
.
ALK-1 has been shown to bind TGF-ß and to phosphorylate and activate
BMP-receptor-regulated Smads (113, 114; T. Imamura, P. ten Dijke, and
K. Miyazono, personal communication), and thus could possibly mediate
TGF-ß-induced Smad phosphorylation in these cells.
Three Co-Smads have been characterized: mammalian Smad4/DPC4 (deleted
in colon carcinoma; ref 132
), C. elegans Sma-4
(112)
, and Drosophila Medea
(133)
. Co-Smads play a critical role in signaling by
TGF-ß superfamily members, illustrated by the absence of TGF-ß
signaling and R-Smad functioning in Smad4-deficient tumor cell lines,
as well as by the synergistic action of Smad4 with R-Smads in
transcriptional activation of target genes and induction of mesoderm
(121
, 134
135
136
137)
. The importance of Smad4 functioning is
also shown by its tumor suppressor activity; mutation or deletion of
Smad4 is often associated with tumorigenesis (132
, 138
139
140
141)
. Cell lines expressing Smad4-mutants found in human
cancers failed to transcriptionally activate transfected luciferase
reporters containing Smad binding elements (140)
. Mice
lacking expression of Smad4 show a BMP knockout phenotype,
characterized by growth retardation and gastrulation defects
(142)
. However, the abnormalities in mesoderm development
of Smad4 null mice could be overcome by the presence of wild-type
extraembryonic tissue, indicating that Smad4 is not critical for early
gastrulation (142)
. Although several explanations are
possible, one reasonable argument is that a yet unidentified Co-Smad is
involved in mediating early embryonic differentiation induced by
members of the TGF-ß superfamily. At later stages of development,
Smad4 is important for anterior specifications (142)
.
Additional indications for the possible existence of novel Co-Smads
comes from studies in Drosophila where Medea, in
contrast to Mad, is dispensable for oogenesis
(143)
. An explanation for the fact that Mad knockouts have
a more severe phenotype than Medea knockouts might be that Mad is able
to signal certain responses independent of Medea. Furthermore,
mutations in Medea efficiently block signaling by Saxophone (Sax) but
fail to fully suppress signaling via Thick Veins (Tkv), both of which
are Dpp type I receptors (133). Although Medea might be abundantly
present to allow Tkv signaling even when hypomorphic alleles are
expressed, the inability to fully abrogate Tkv signaling might also
reside in the fact that another, yet unidentified, Co-Smad is involved
in Tkv signaling or that Tkv can signal independent of Medea. Besides
the XSmad4 ortholog, another Smad4/Medea-like protein was recently
identified in Xenopus (144, 145; C. Hill, personal
communication)
Mechanisms controlling Smad activation
R-Smads occur as monomers in the cytoplasm (146)
and,
after ligand-induced type I receptor activation, specifically interact
with the L45 loop in the kinase domain of type I receptors (79
, 80)
. Recruitment of R-Smads to the TGF-ß receptors is promoted
by the action of SARA, a FYVE zinc finger domain-containing protein
that interacts with both TGF-ß receptors and Smad2 or Smad3 (Fig. 2)
(147
, 148)
. The highly conserved L3 loop of R-Smads, which
protrudes from the MH2 core domain as observed in the Smad4-MH2 crystal
structure (139)
, is involved in specifying type I
interactions with TßR-I and BMPR-I (150)
. This loop
varies between Smad1 and Smad2 in only two amino acids, and exchanging
these residues between Smad1 and Smad2 enables their binding to TßR-I
and BMPR-I, respectively (79
, 149)
. The -helix 1 of
Smad1, located near the L3 loop in the 3 dimensional structure, has
been shown to be important for the interaction of Smad1 with ALK-1
(114)
.
After type I receptor interaction, R-Smads become activated by
phosphorylation of their carboxyl-terminal SS(M/V)S motifs (Fig. 3B
) (119
, 150
151
152)
, which triggers their
homodimerization and heteromerization with Co-Smads (134
, 153)
or other R-Smads (121
, 146)
. This leads to the
speculation that homomeric R-Smads, heteromeric R-Smads, and
R-Smad/Co-Smad complexes might control different biological responses.
In contrast to R-Smads, Co-Smads do not contain the carboxyl-terminal
phosphorylation motif SSXS and are not phosphorylated by the activated
type I receptor (119
, 120)
. Although the L3 loop of
Co-Smads shares high amino acid similarity with the L3 loop of R-Smads,
Co-Smads do not interact with type I receptors (120)
;
instead, their L3 loops are important for heteromerization with
phosphorylated R-Smads (139)
. Resolution of the crystal
structure of the Smad4-MH2 domain and analyses of Smad-deletion mutants
(139)
have indicated that the MH2 domains of R-Smads and
Co-Smads are involved in formation of homo-and heteromeric trimers.
After association of R-Smads with Co-Smads, the heteromeric complexes
are translocated into the nucleus (see Fig. 5
).
|
Domains controlling nuclear translocation
Whereas ligand-induced nuclear translocation of R-Smads is not
dependent on expression of Co-Smads (136)
, Co-Smads
require association with activated R-Smads in order to enter the
nucleus (136
, 151)
. A carboxyl-terminal truncation of
Smad4, corresponding to a mutation of Smad4 found in pancreatic tumors
(132)
and identified in an inactive form of Mad in
Drosophila (111)
, dominant-negatively
interferes with nuclear transport of all R- and Co-Smads
(137)
. In contrast, a comparable mutation in R-Smads does
not affect their nuclear localization (137)
. This
indicates that the carboxyl-terminal part of Co-Smads is required for
nuclear trafficking (137)
, whereas phosphorylation of
R-Smads, though required for their functionality, is not obligatory for
nuclear transport. How these mutant Co-Smads interfere with the nuclear
transport function of R-Smads needs to be further studied. Removal of
the NH2 terminus and linker region of R-Smads
results in strong, ligand-independent nuclear localization (122
, 137)
, whereas an additional carboxyl-terminal truncation of the
remaining MH2 domain results in cytoplasmic accumulation at the nuclear
membrane (137)
. Presumably, ligand-induced
heteromerization results in a conformational change of the Smad
molecules, thereby exposing nuclear translocation signals in R- and
Co-Smads. Whether the cytoplasmic residence of R-Smads and Co-Smads in
the absence of ligand is due merely to autoinhibition of the MH2
effector domain by association with the MH1 domain (154)
,
or whether a cytoplasmic tether exists that anchors R- and Co-Smads in
the cytoplasm in the absence of ligand, is not clear
(137)
.
R-Smads and Co-Smads as sequence-specific transcriptional
activators
Once in the nucleus, R-Smads and Co-Smads are involved in
transcriptional regulation of target genes (Fig. 3C
)
(reviewed in ref 155
), and the MH1 and MH2 domains
differentially contribute to this property. Direct binding of Smads to
DNA was first shown by the interaction of the Drosophila
Mad-MH1 domain with specific sequences in the promoter of the Dpp
target gene vestigial (156)
. A prerequisite
for the interaction of Mad, Smad3, but not Smad4, with DNA is the
dissociation of the MH2 domain from the MH1 domain, which is achieved
by ligand-induced phosphorylation of the SSXS motif in R-Smads
(154
, 156)
. Thus, the MH2 domain can exert an
autoinhibitory action on the MH1 DNA binding domain.
In several promoters of genes induced by TGF-ß signaling like
type VII Collagen, JunB, and
PAI-1 (157
, 158
, 160)
, unique elements have
been identified with which Smad3 and Smad4 physically interact. The
sequences of the Smad binding elements (SBEs) in these promoters are
highly related and indicate an AGAC-containing motif as binding site
for Smad3 and Smad4. This sequence was also picked up in a random
screening for Smad interacting DNA sequences (161)
, and
mutation of these SBEs in the PAI-1 promoter impairs TGF-ß
responsiveness (160)
.
Crystalization of the Smad3 MH1 domain bound to an optimal Smad binding
sequence revealed that the so-called ß-hairpin loop, protruding from
the MH1 core domain, interacts with DNA (162)
. The
ß-hairpin loop is highly conserved among R-Smads and Co-Smads.
However, Smad2 contains an extra exon preceding this region, encoding
30 amino acids, that might interfere with the correct conformation of
the ß-hairpin loop; in contrast to Smad3 and Smad4, Smad2 has not yet
been shown to interact with DNA. Removal of exon 3 in Smad2 enables its
efficient binding to DNA (163
, 164)
.
In several TGF-ß target genes, multiple copies of SBEs can be
identified, often located in close proximity to sites for other
transcription factors (158
, 160
, 165
166
167)
. Smads have
been shown to mediate transcriptional activation by interaction with
other regulatory promoter units, like the TPA-responsive elements
(TREs)/AP-1 sites in the collagenase promoter, which actually overlap
in sequence with SBEs (158
, 166)
. Smads cooperate with the
transcription factor TFE3 in the regulation of the PAI-1
gene (168)
. The p21/WAF1/Cip1 and p15/INK4B gene promoters
lack SBEs, but TGF-ß transcriptionally regulates these genes via the
Sp1 site (169
170
171)
. It has been suggested that the
interaction of Smads with additional transcription factors such as AP-1
(Fig. 4
; 166
, 172
, 173
), forkhead activin signal transducers
(FAST; 165, 167, 174), or Sp1 (169
, 171)
confers
additional DNA binding specificity to the Smad-containing
transcriptional complex (see below).
|
R-Smads and Co-Smads as transcriptional (co-)activators
Studies with Xenopus explants, in which injection of
the Smad2-MH2 domain fully induced dorsalization of mesoderm and
secondary axis formation analogous to activin signaling
(122)
, suggested an effector function for MH2 domains.
Furthermore, transcriptional activation properties of R- and Co-Smads
were shown by fusion constructs of Smad1-MH2 or Smad4-MH2 domains with
Gal4-DNA binding domains (175)
. Heteromerization of
Co-Smads with R-Smads is obligatory for their transcriptional
activation properties (136
, 153
, 176)
.
Several domains in Smad4 have now been identified to contribute to its
important role in signaling by members of the TGF-ß superfamily. As
mentioned above, the MH1 domain enables the interaction of
R-Smad/Co-Smad complexes with DNA, which is of particular importance
for Smad2-Smad4 heteromeric complexes (136)
since Smad2 is
not endowed with DNA binding capacities. The MH1 domain autosuppresses
the MH2 effector domain (154)
, and heteromerization of
Co-Smads with phosphorylated R-Smads has been suggested to release the
inhibitory association (177)
. Studies in which chimeras
between Smad1 and Smad4 were tested revealed that the proline-rich
linker region of Smad4 contains a domain that is important for its
transcriptional activity mediated by the MH2 domain (178)
.
This Smad activation domain (SAD) cooperates functionally with the
transcription coactivator MSG1 (178
, 179)
. MSG1 strongly
enhances Smad4 transcriptional activity, provided that Smad4 is present
in a heteromeric complex with R-Smads (179)
. Smad
heteromerization possibly introduces a configuration that allows
trans-activators to associate and endow Smad4 with
transcriptional potencies (179)
.
Smads interact with transcription factors
The first evidence that Smads interact and cooperate with
transcription factors comes from the interaction observed between
Smad2, Smad4, and Xenopus FAST-1 (XFAST-1), making a complex
that interacts with the activin responsive element (ARE) in the
Xenopus Mix2 promoter (see Figs. 4
, 5
) (136
, 180
, 181)
. Human FAST-1 and mouse FAST-2 (mFAST-2) have been
identified as well, but share little sequence homology with XFAST-1
(165
, 167
, 174)
. FAST-1 efficiently interacts with DNA,
and Smad2 and Smad4 contribute in providing additional DNA interactions
as well as essential transcriptional activation properties (136
, 167
, 174
, 180
, 181)
. Phosphorylation of the Smad2 SSMS motif is
required for interaction of -helix 2, exposed from the MH2-trimer
structure, with FAST-1 (136
, 167
, 180
, 181)
.
Whereas Smad2/Smad4/mFAST-2 complexes drive transcriptional activation
of the goosecoid reporter, Smad3/Smad4/mFAST-2 complexes were found to
suppress promoter activation, presumably because Smad3 and Smad4 share
the same DNA binding site (165)
. This suggests that the
relative expression levels of Smad2 and Smad3 in a cell might determine
the outcome of certain biological responses induced by TGF-ß.
Additional evidence for differential roles of Smad2 and Smad3 comes
from observations in human keratinocytes, where TGF-ß stimulates
Smad2 and Smad3 to comparable extents while activin predominantly
activates Smad3 (182)
. In human lung epithelial cells,
Smad3 more potently induces apoptosis compared to Smad2
(183)
.
Cooperation of Smads with the AP-1 complex
Characterization of Smad binding sites in a collagenase
promoter revealed that Smads can mediate transcription via
12-O-tetradecanoyl-13-acetate (TPA) -responsive elements (TREs; 120,
121, 158), to which the transcription factor AP-1, a dimer of c-Jun and
c-Fos, binds. These AP-1 binding sites overlap in sequence with
consensus Smad binding sites (158)
. Smad3 directly
interacts with TRE, and Smad3 and Smad4 activate the TGF-ß-inducible
transcription of TRE-Luc reporter in the absence of c-Jun and c-Fos
(166)
. However, TGF-ß-induced reporter activity was
further enhanced in the presence of c-Jun-c-Fos heterodimers
(166)
. After TGF-ß-induced Smad activation, Smad3 and
Smad4 can interact with JunB, c-Jun, and JunD as well as with c-Fos
(166
, 172)
. By footprint analysis and electromobility
shift assays, it was shown that c-Jun and Smad3 can bind simultaneously
to overlapping sequences in the TRE (166)
.
Crystallographic analysis of the Smad3 MH1 domain bound to DNA
indicated that a conformational change of the NH2
terminus of c-Fos would allow the multimeric Smad-AP-1 complex to
interact with a single AP-1 site (162)
. This cross talk
might be an important level of coordination between Smads and MAPK/JNK
signaling.
Interaction with the coactivators p300/CBP
Insight into how Smads mediate their transcriptional activating
function was recently unraveled by the interaction of R-Smads with the
coactivators p300 and CREB binding protein (Fig. 4)
(CBP; 176,
184–187). These coactivators contribute to transcriptional activation
by loosening the chromatin structure or by increasing the affinity of
certain transcription factors for DNA through their intrinsic (or
associated) acetyltransferase activity. In addition, they act as a
bridging factor between transcription factors and the basal
transcription machinery (184 and references therein). In view of the
importance of Smad4 for transcriptional activation of the Smad-p300/CBP
complex, Smad4 might function as a coactivator as well
(176)
. The interaction between the MH2 domain of R-Smads
and CBP/p300 is triggered by ligand-induced phosphorylation of the
carboxyl-terminal SSXS motif (176
, 184
, 186)
.
Phosphorylation most likely induces a conformational change in the
R-Smad molecule, which then results in exposure of the CBP/p300 binding
site; deletion of the NH2 terminus and part of
the carboxy terminus, leaving the CBP binding site intact, enhances the
affinity of Smad3 for CBP/p300 (184)
. The
carboxyl-terminal region of CBP is required for Smad interaction
(176
, 184
, 186)
and can associate with other transcription
factors like the adenoviral oncoprotein E1A. E1A is known to interfere
with TGF-ß-mediated signaling (188
, 189)
and is shown to
abrogate Smad-mediated transcriptional activation (176
, 184
, 186)
, most likely by direct competition with Smads for binding
to CBP/p300 (188
, 189)
.
Negative regulation of Smad function
Activin and BMPs have been shown to compete which each other in
Xenopus mesoderm development due to limiting amounts of
Smad4 (190). In view of the limited amounts of CBP/p300 in the cell
(184)
and the observed competition between different
signal transduction pathways at the level of coactivator availability
(191), competition between members of the TGF-ß superfamily as
observed in embryogenesis can also be expected to occur at the level of
CBP/p300.
Interaction of the first zinc finger domain of the nuclear protein
Evi-1 with the MH2 domain of Smad3 interferes with binding of Smad3 to
DNA (Fig. 4)
, thereby repressing transcriptional activity of Smad3 as
well as growth inhibition (192). Evi-1 shows a spatial and temporal
expression pattern during mouse embryogenesis, suggesting it could play
a regulatory role during development (193).
Inhibitory Smads
Mammalian Smad6, Smad7, and Drosophila Dad have been
characterized as inhibitors of TGF-ß signal transduction (Fig. 5
) (194
195
196
197
198)
. Whereas their amino-terminal domains are
highly diverse and share only weak similarity to other Smads, they are
homologous to the R- and Co-Smads in their MH2 domains. However,
inhibitory Smads lack the carboxyl-terminal SSXS phosphorylation motif,
which may enable them to stably associate with type I receptors and to
interfere with receptor binding and activation of R-Smads.
Phosphorylation of the type I receptors by the type II receptors is
essential for inhibitory Smad association (194
, 196
, 199)
and the MH2 domain of anti-Smads suffices in exhibiting the inhibitory
effect (194
, 196
, 199
, 200)
, as manifested by inhibition
of R-Smad phosphorylation and abolition of their heteromerization with
Co-Smad and nuclear translocation, as well as abrogation of growth
factor-induced transcriptional responses and growth inhibition
(194
195
196
, 198
199
200
201
202)
. It has been reported that Smad6 can
exert its inhibitory effect via an alternative mode that involves
BMP-induced association of Smad6 with phosphorylated Smad1, thereby
competing with activated Smad1 for heteromerization with Smad4
(200)
(Fig. 5)
.
The expression of the inhibitory Smads is quickly induced upon
stimulation by TGF-ß superfamily members and thereby provides an
autoinhibitory mechanism in TGF-ß signaling (196
, 197
, 201
, 203)
. The human Smad7 promoter contains an SBE consensus site
that, when fused to a luciferase gene, induces luciferase activity
after TGF-ß stimulation (G. Brodin and R. Heuchel, personal
communication). Thus, after receptor activation, R- and Co-Smads might
be directly involved in transcriptional regulation of the Smad7 gene.
The MH1 domain of inhibitory Smads might play a role in specification
of their inhibitory actions: Xenopus and mouse Smad7, which
share 96% identity in their carboxyl-terminal domain but only 51% in
the amino-terminal region, exert differential effects in inhibition of
TGF-ß and activin signaling (199). Mouse Smad7 inhibits
TßR-I-induced phosphorylation of Smad2 and Smad3 (194
, 196
, 199)
, BMPR-IA and BMPR-IB-mediated phosphorylation of Smad1 and
Smad5 (199), and ActR-I mediated phosphorylation of Smad1 (199); it
therefore appears to be a common inhibitor of R-Smad activation.
Whereas Smad6 has been implicated in inhibition of TGF-ß signaling
(194
, 204)
, it appears to play a more pronounced role in
inhibition of BMP signaling (Fig. 5)
(195
, 200
, 201
, 203
, 205
, 206)
. The mechanism of action for inhibitory Smads in TGF-ß
superfamily signaling is evolutionarily well conserved; it has recently
been shown that Dad, which is a Dpp-inducible inhibitory Smad in
Drosophila (197), interacts with Tkv and thereby prevents
binding and phosphorylation of Mad by Tkv (198).
The in vivo relevance of Smad6 and Smad7 is shown in several
systems. Smad6 and Smad7 were initially identified as genes induced by
shear stress in vascular endothelial cells (204)
. Smad7
inhibits activin-induced dorsal mesoderm formation and interferes with
inhibition of activin-mediated growth arrest and apoptosis in B-cells
(206)
. Smad6 and Smad7 antagonize BMP-induced mesoderm
development, have neural-inducing potencies in Xenopus, and
inhibit BMP-induced growth arrest and apoptosis in B cells (200
, 205
206
207)
.
Alternative functions for Smad6 and Smad7 may exist as well. In
Xenopus, XSmad6 was found to be partially or completely
restricted to the nuclei in most cells (207)
. In mammalian
cells, Smad7 has been located in the nucleus in the absence of ligand,
but rapidly accumulated in the cytoplasm after TGF-ß stimulation
(202)
. In accordance with its importance for inhibition of
R-Smad signaling, the inhibitory Smad carboxyl-terminal tail also
harbors domains required for transport across the nuclear membrane
(202)
. A nuclear localization of inhibitory Smads might be
required to allow phosphorylation of R-Smads after receptor activation;
the mechanism whereby Smad7 is translocated to the plasma membrane
after TGF-ß receptor activation is unknown. An interesting
possibility, which remains to be elucidated, is that inhibitory Smads
in addition function in transcriptional regulation. Differential
compartmentalization of the inhibitory Smads, in combination with
induction of their expression after signaling by TGF-ß superfamily
members, and the fact that inhibitory Smads can selectively eliminate
particular R-Smad pathways provide a tightly controlled cell-autonomous
regulatory mechanism.
Modulation of Smad signaling by MAP kinase pathways
The mitogen-activated protein kinase kinase kinase TAK1 (TGF-ß
activated kinase 1) has been shown to be phosphorylated and activated
upon TGF-ß or BMP-4 stimulation (208)
, and can mediate
transcriptional activation of a luciferase reporter driven by a
TGF-ß-inducible element of the PAI-1 promoter containing three AP-1
sites. This effect is strongly enhanced by the TAK1-activator TAB1
(209)
. The in vivo relevance of the cooperative
functioning of TAK1 and TAB1 was demonstrated in Xenopus
mesoderm development in which TAK1 and TAB1 promoted ventral mesoderm
induction and perturbed neural differentiation, thereby substituting
BMP signaling (210)
. The human X-chromosome-linked
inhibitor of apoptosis protein (XIAP) can interact with both BMP
receptors as well as with TAB1 and enhances ventralization of
Xenopus embryos in a TAB1-TAK1-dependent manner
(211)
.
Using the transmembrane and cytoplasmic domain of BMPR-IA as bait in
yeast two-hybrid screens, BRAM1 (BMP receptor-associated molecule) was
identified. The carboxyl-terminal region of BRAM1 is responsible for
specific interaction with BMPR-IA and does not associate with the
kinase domain of TßR-I (212)
. Furthermore, this region
in BRAM1 is also responsible for interaction with TAB1
(212)
. Expression of GST-coupled BMPR-IA, HA-tagged BRAM1,
and myc-tagged TAB1 in COS7 cells showed that the three proteins form a
ternary complex. BRAM1 might function as an adaptor protein for
positioning TAB1 in close proximity of BMPR-IA kinase domain
(212)
. The role of BRAM1 in activation of the TAB1/TAK1
signaling cascade, as well as its possible effect on Smad-receptor
interaction and Smad-activation is not clear at present.
A downstream phosphorylation target in the MAPK cascade triggered by
TAK1 is mitogen-activated protein kinase kinase 4
(MKK4)/stress-activated protein kinase/extracellular signal-regulated
kinase SEK1 (208
, 213)
, which is involved in the
stress-activated protein kinase (SAPK)/c-Jun
NH2-terminal kinase (JNK) pathway, finally
leading to activation of c-Jun. In that JNK activity induced by
hematopoietic progenitor kinase-1 (HPK-1) can be perturbed by
expression of a kinase inactive form of TAK1, there are indications
that TGF-ß might exert JNK activation via the HPK1-TAK1-SAPK/JNK
route (214)
. The small Rho-like GTPases Rho, Rac, and
cdc42, which signal via the SAPK/JNK pathway, have also been shown to
be important for certain aspects of TGF-ß signaling (215
, 216)
.
The SAPK/JNK pathway has been shown to affect TGF-ß signaling via the
Smad-pathway, since overexpression of dominant negative members of the
SAPK/JNK pathway prevent both TGF-ß- and Smad4-mediated
transcriptional response (217)
. The interface of SAPK/JNK-
and Smad-signaling might be at the level of interaction between c-Jun
and Smad3, since these transcription factors have been shown to
associate after TGF-ß stimulation and synergize in activating a
transcriptional reporter containing AP-1 binding sites (163
, 166)
.
Other levels of interaction between MAPK pathways and Smad signaling
have been elucidated. In the linker regions of R-Smads that connect the
MH1 DNA binding domain with the MH2 transcriptional domain of
receptor-activated Smads, several ERK recognition sites (PXSP, or PXTP
motifs) (Fig. 3B
) and JNK motifs (XXSP) are present in the
linker region of R-Smads. Phosphorylation of these sites in Smad1 by
EGF or HGF has been shown to circumvent nuclear translocation of Smad1
and indicates a role for ERK signaling in the modulation of TGF-ß
signaling (218)
. On the other hand, EGF and HGF have been
shown to mediate signaling via Smad2, activated by kinases downstream
of MEK1 (219)
. Furthermore, deletion of the
carboxyl-terminal phosphorylation motif SS(V/M)S abrogates HGF-induced
Smad1 or Smad2 phosphorylation, their nuclear translocation, and
HGF-induced activation of a Smad responsive reporter construct,
indicating that these residues are important for Smad functioning in
both TGF-ß and HGF or EGF signaling (219)
. It is not
known under which conditions the ERK signaling inhibits TGF-ß
family/Smad-induced responses or mediates Smad-dependent pathways.
Analogously, differences in cellular context might underlie the
apparently contradicting reports on the role of Ras/MAPK in TGF-ß
signaling in different tumor cell systems. Phosphorylation of ERK sites
in the linker region of Smad3 by activated Ras, MEK1, or
v-HA-Ras-transformed mouse mammary epithelial EpH4 cells inhibits
TGF-ß/Smad-mediated transcriptional responses, Smad nuclear
translocation, and growth inhibition (220)
. In contrast,
TGF-ß rapidly induces Ras/MEK signaling in intestinal epithelial
IEC1.4 cells; this signal transduction pathway cooperates in TGF-ß
mediated Smad-signaling, presumably by phosphorylation of Smad1 on the
four ERK-sites in the linker region (221)
.
|
CONCLUSIONS AND PERSPECTIVES |
|---|
|
TOPABSTRACTTHE TGF-{beta} SUPERFAMILY AND...SIGNALING VIA TGF-{beta}...INTRACELLULAR SIGNALING BY TGF...CONCLUSIONS AND PERSPECTIVESREFERENCES |
|---|
, 101
). On ligand-induced heteromeric complex formation of
distinct type I and type II receptors, particular R-Smads transiently
interact with and become phosphorylated by the activated type I
receptor kinase. Subsequently, R-Smads assemble with Co-Smads into
heteromeric complexes that translocate into the nucleus, where (in
combination with other transcription factors) they regulate the
transcription of target genes.
One TGF-ß superfamily member can couple with multiple type I
receptors and multiple Smads, each mediating a distinct set of
responses (reviewed in refs 29
, 101
). Thus, the receptor and Smad
expression profile in the target cell is an important factor that
decides which particular cellular responses are induced by a TGF-ß
superfamily member. In addition, the repertoire of transcription
factors that are present in the cell with which the Smads can interact
is a critical determinant. Although multimerized Smad binding element
is sufficient to drive TGF-ß-induced transcription (reviewed in ref 155
), emerging promoter analyses of TGF-ß superfamily target genes
indicate that transcriptional regulation is achieved by cooperation of
Smads with other transcription factors. Complex formation of Smads with
these transcription factors may occur independent of DNA binding or
require DNA binding for additional selectivity (reviewed in ref 155
).
The TGF-ß/Smad pathway has been implicated in many responses,
including growth inhibition, differentiation and many transcriptional
responses. An important issue that needs to be addressed is the
requirement of particular Smads in these responses. In addition, it
will be important to explore the possibility that Smad-independent
signaling also occurs. TGF-ß-induced transcription of the fibronectin
gene can occur independent of Smad4, but requires the JNK pathway and
activation of cAMP-responsive element by cJun/ATF2 heterodimer
(222)
.
An important task for the future is to determine the genetic programs that are triggered on challenging different cell types with different TGF-ß family members. This will become feasible by applying the newly developed functional genomics and proteomics technology and thereby determine the time- and dose-dependent effects of TGF-ß family members on the expression of thousands of genes/proteins simultaneously. In addition, changes in the expression patterns on ectopic expression of constitutively active or dominant negative versions of signaling components or on ligand stimulation in cells deficient in such components can be measured. The target genes/proteins identified will need experimental follow-up to evaluate their importance in various biological responses. Moreover, the involvement of different type I receptors and Smad molecules in various responses induced by TGF-ß family member can be elucidated using this methodology.
The multifunctional character of TGF-ß superfamily members implicates a need for tight control of their activities. Indeed, both positive and negative (feedback) regulatory mechanisms have been observed at nearly every step in the TGF-ß superfamily signaling cascade, from the release of biologically active ligand to the Smad-mediated transcriptional effects. Cross talk of the TGF-ß/Smad pathway with other pathways will be a theme of many future studies, e.g., how other signaling inputs affect the activity, expression, stability, or subcellular localization of TGF-ß superfamily receptors and Smads. In addition, the pleiotropic action of TGF-ß superfamily members has driven the evolution of multiple closely related TGF-ß superfamily members with different expression and activation patterns. The mammalian TGF-ß superfamily will likely continue to grow until the complete human sequence has been elucidated at the beginning of the next century.
Much of our insight into signaling mechanisms has come from the
genetically accessible model organisms Drosophila and
C. elegans (reviewed in refs 29
, 101
). In
Drosophila, dpp signals through two type I
receptors—saxophone and thick veins—and their downstream effector,
Mad. Recently, an activin/TGF-ß signal transduction pathway was
identified in Drosophila (223
, 224)
. Activation
of the activin type I receptor Atr-I stimulates dSmad2-dependent
pathways. The type II receptor punt and Medea are shared components
between the Dpp and activin/TGF-ß pathway. C. elegans
research obtained a boost by the recent completion of its entire genome
sequence (reviewed in ref 225
). This vertebrate has four TGF-ß-like
ligands (including Daf-7 and Daf-1), one type II receptor (Daf-4), and
two type I receptors (Daf-1 and Sma-6). Thus, Daf-4 is required for
both pathways and type I receptors to determine signaling specificity
(226); Daf-1 is important in dauer larva formation and Sma-6 for body
size determination and male tail development. Six Smad proteins are
present in the C. elegans genome: Sma-2, Sma-3, and Sma-4
have been implicated in the Daf-7/Daf-1 pathway, and the Smad mediators
Daf-8, Daf-14, and Daf-3 in Dbl-1/Sma-6-induced signaling responses.
Drosophila and C. elegans will continue to be
extremely important in elucidating the molecular mechanisms that
underlie TGF-ß superfamily signaling.
In mammals, the physiological significance of interactions between ligands and receptors, between receptors and Smads, and between Smads and Smad-interacting proteins, as well as the importance of the signaling pathways in which they act, need to be validated through comparison of the phenotypes of mice deficient in a particular TGF-ß superfamily member, receptor, Smad, or target gene. In those cases where a null mutation leads to an early embryonal lethal phenotype, conditional knock-out approaches will be required to study the role of this component in late development or adult tissues. In addition, transgenic approaches in mice with dominant negative or constitutively active forms of TGF-ß superfamily signal transducers under inducible or tissue-specific promoters will provide important information regarding issues of signaling specificity and diversity in different cellular contexts.
The improved understanding by which TGF-ß superfamily members elicit and regulate different responses will be essential in the design of new therapeutic approaches for various diseases caused by deregulated activity of TGF-ß family members. For example, antagonists of TGF-ß could be applied in various types of fibrosis that are due to TGF-ß overactivity and agonists of TGF-ß in diseases in which enhanced activity is beneficial, such as wound healing or immunosuppression. In most cases it will be advantageous not to inhibit or activate all activities of TGF-ß superfamily members. By understanding the molecular mechanisms that underlie specificity, diversity, and regulation in TGF-ß superfamily signaling, it will be possible to screen for pharmacological compounds that inhibit or mimic only defined activities of TGF-ß superfamily members.
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REFERENCES |
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TOPABSTRACTTHE TGF-{beta} SUPERFAMILY AND...SIGNALING VIA TGF-{beta}...INTRACELLULAR SIGNALING BY TGF...CONCLUSIONS AND PERSPECTIVESREFERENCES |
|---|
ß1. Mol. Biol. Cell 9,2627-26382-macroglobulin receptor-dependent mechanism for the plasma clearance of transforming growth factor-ß1 in mice. J. Clin. Invest. 87,39-44
2-macroglobulin are soluble binding proteins for inhibin and activin. Horm. Res. 45,207-210[Medline]
, a WD-40 repeat subunit of phosphatase 2A. Mol. Cell. Biol. 18,6595-6604 subunit in TGF-ß and activin signaling. Science 271,1120-1122[Abstract]
in yeast and mammalian cells. J. Biol. Chem. 271,13931-13934This article has been cited by other articles:
|
|
 |
|
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|
|
|
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 |
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|
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|
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 B R Klass, A O Grobbelaar, and K J Rolfe Transforming growth factor {beta}1 signalling, wound healing and repair: a multifunctional cytokine with clinical implications for wound repair, a delicate balance Postgrad. Med. J., January 1, 2009; 85(999): 9 - 14. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. Wang, Y. Shen, X. Yuan, K. Chen, X. Guo, Y. Chen, Y. Niu, J. Li, R.-H. Xu, X. Yan, et al. Dissecting Signaling Pathways That Govern Self-renewal of Rabbit Embryonic Stem Cells J. Biol. Chem., December 19, 2008; 283(51): 35929 - 35940. [Abstract] [Full Text] [PDF]
|
|
|
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|
|
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|
|
|
|
 |
|
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|
|
|
|
|
|
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|
|
|
|
|
|
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|
|
|
|
 |
|
 S. Velasco, P. Alvarez-Munoz, M. Pericacho, P. t. Dijke, C. Bernabeu, J. M. Lopez-Novoa, and A. Rodriguez-Barbero L- and S-endoglin differentially modulate TGF{beta}1 signaling mediated by ALK1 and ALK5 in L6E9 myoblasts J. Cell Sci., March 15, 2008; 121(6): 913 - 919. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. P. Gaide Chevronnay, P. B. Cornet, D. Delvaux, P. Lemoine, P. J. Courtoy, P. Henriet, and E. Marbaix Opposite Regulation of Transforming Growth Factors-{beta}2 and -{beta}3 Expression in the Human Endometrium Endocrinology, March 1, 2008; 149(3): 1015 - 1025. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. Hirose, K. Nakazato, H. Song, and N. Ishii TGF- 1 and TNF-{alpha} are involved in the transcription of type I collagen {alpha}2 gene in soleus muscle atrophied by mechanical unloading J Appl Physiol, January 1, 2008; 104(1): 170 - 177. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. J. You, M. W. Bruinsma, T. How, J. H. Ostrander, and G. C. Blobe The type III TGF- receptor signals through both Smad3 and the p38 MAP kinase pathways to contribute to inhibition of cell proliferation Carcinogenesis, December 1, 2007; 28(12): 2491 - 2500. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 L. J. McReynolds, S. Gupta, M. E. Figueroa, M. C. Mullins, and T. Evans Smad1 and Smad5 differentially regulate embryonic hematopoiesis Blood, December 1, 2007; 110(12): 3881 - 3890. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 N. Renlund, F. H O'Neill, L. Zhang, Y. Sidis, and J. Teixeira Activin receptor-like kinase-2 inhibits activin signaling by blocking the binding of activin to its type II receptor J. Endocrinol., October 1, 2007; 195(1): 95 - 103. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Cabello-Verrugio and E. Brandan A Novel Modulatory Mechanism of Transforming Growth Factor-beta Signaling through Decorin and LRP-1 J. Biol. Chem., June 29, 2007; 282(26): 18842 - 18850. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A.-C. Poncelet, H. W. Schnaper, R. Tan, Y. Liu, and C. E. Runyan Cell Phenotype-specific Down-regulation of Smad3 Involves Decreased Gene Activation as Well as Protein Degradation J. Biol. Chem., May 25, 2007; 282(21): 15534 - 15540. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Bujak and N. G. Frangogiannis The role of TGF-{beta} signaling in myocardial infarction and cardiac remodeling Cardiovasc Res, May 1, 2007; 74(2): 184 - 195. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H.-A Seong, H. Jung, K.-T. Kim, and H. Ha 3-Phosphoinositide-dependent PDK1 Negatively Regulates Transforming Growth Factor-beta-induced Signaling in a Kinase-dependent Manner through Physical Interaction with Smad Proteins J. Biol. Chem., April 20, 2007; 282(16): 12272 - 12289. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. D. Andl, B. B. Fargnoli, T. Okawa, M. Bowser, M. Takaoka, H. Nakagawa, A. Klein-Szanto, X. Hua, M. Herlyn, and A. K. Rustgi Coordinated Functions of E-Cadherin and Transforming Growth Factor {beta} Receptor II In vitro and In vivo. Cancer Res., October 15, 2006; 66(20): 9878 - 9885. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B.-Y. Bao, J. Yao, and Y.-F. Lee 1{alpha}, 25-dihydroxyvitamin D3 suppresses interleukin-8-mediated prostate cancer cell angiogenesis Carcinogenesis, September 1, 2006; 27(9): 1883 - 1893. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Bangi and K. Wharton Dual function of the Drosophila Alk1/Alk2 ortholog Saxophone shapes the Bmp activity gradient in the wing imaginal disc Development, September 1, 2006; 133(17): 3295 - 3303. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. Mikami, J. H. Lim, H. Ishinaga, U.-H. Ha, H. Gu, T. Koga, H. Jono, H. Kai, and J.-D. Li The Transforming Growth Factor-beta-Smad3/4 Signaling Pathway Acts as a Positive Regulator for TLR2 Induction by Bacteria via a Dual Mechanism Involving Functional Cooperation with NF-{kappa}B and MAPK Phosphatase 1-dependent Negative Cross-talk with p38 MAPK J. Biol. Chem., August 4, 2006; 281(31): 22397 - 22408. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 Q.-a. Yuan, H. H. Simmons, M. K. Robinson, M. Russeva, W. A. Marasco, and G. P. Adams Development of engineered antibodies specific for the Mullerian inhibiting substance type II receptor: a promising candidate for targeted therapy of ovarian cancer. Mol. Cancer Ther., August 1, 2006; 5(8): 2096 - 2105. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. Camoretti-Mercado, D. J. Fernandes, S. Dewundara, J. Churchill, L. Ma, P. C. Kogut, J. F. McConville, M. S. Parmacek, and J. Solway Inhibition of Transforming Growth Factor beta-enhanced Serum Response Factor-dependent Transcription by SMAD7 J. Biol. Chem., July 21, 2006; 281(29): 20383 - 20392. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Omori, K. Matsumoto, H. Sanjo, S. Sato, S. Akira, R. C. Smart, and J. Ninomiya-Tsuji TAK1 Is a Master Regulator of Epidermal Homeostasis Involving Skin Inflammation and Apoptosis J. Biol. Chem., July 14, 2006; 281(28): 19610 - 19617. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 Q. Luo, E. Nieves, J. Kzhyshkowska, and R. H. Angeletti Endogenous Transforming Growth Factor-{beta} Receptor-mediated Smad Signaling Complexes Analyzed by Mass Spectrometry Mol. Cell. Proteomics, July 1, 2006; 5(7): 1245 - 1260. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. G. Farnworth, P. G. Stanton, Y. Wang, R. Escalona, J. K. Findlay, and G. T. Ooi Inhibins Differentially Antagonize Activin and Bone Morphogenetic Protein Action in a Mouse Adrenocortical Cell Line Endocrinology, July 1, 2006; 147(7): 3462 - 3471. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 H. H. Kim, S. Cho, S. Lee, K. H. Kim, K. H. Cho, H. C. Eun, and J. H. Chung Photoprotective and anti-skin-aging effects of eicosapentaenoic acid in human skin in vivo J. Lipid Res., May 1, 2006; 47(5): 921 - 930. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. Ishida, T. Kondo, A. Kimura, K. Matsushima, and N. Mukaida Absence of IL-1 Receptor Antagonist Impaired Wound Healing along with Aberrant NF-{kappa}B Activation and a Reciprocal Suppression of TGF-beta Signal Pathway J. Immunol., May 1, 2006; 176(9): 5598 - 5606. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Tedgui and Z. Mallat Cytokines in Atherosclerosis: Pathogenic and Regulatory Pathways Physiol Rev, April 1, 2006; 86(2): 515 - 581. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. Bandyopadhyay, J. Fan, S. Guan, Y. Li, M. Chen, D. T. Woodley, and W. Li A "traffic control" role for TGF{beta}3: orchestrating dermal and epidermal cell motility during wound healing J. Cell Biol., March 27, 2006; 172(7): 1093 - 1105. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. G Farnworth, Y. Wang, P. Leembruggen, G. T Ooi, C. Harrison, D. M Robertson, and J. K Findlay Rodent adrenocortical cells display high affinity binding sites and proteins for inhibin A, and express components required for autocrine signalling by activins and bone morphogenetic proteins. J. Endocrinol., March 1, 2006; 188(3): 451 - 465. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K Bobacz, R Ullrich, L Amoyo, L Erlacher, J S Smolen, and W B Graninger Stimulatory effects of distinct members of the bone morphogenetic protein family on ligament fibroblasts Ann Rheum Dis, February 1, 2006; 65(2): 169 - 177. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Chen, M. Crawford, R. M. Day, V. R. Briones, J. E. Leader, P. A. Jose, and R. J. Lechleider RhoA Modulates Smad Signaling during Transforming Growth Factor-beta-induced Smooth Muscle Differentiation J. Biol. Chem., January 20, 2006; 281(3): 1765 - 1770. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H.-A. Seong, H. Jung, H.-S. Choi, K.-T. Kim, and H. Ha Regulation of Transforming Growth Factor-beta Signaling and PDK1 Kinase Activity by Physical Interaction between PDK1 and Serine-Threonine Kinase Receptor-associated Protein J. Biol. Chem., December 30, 2005; 280(52): 42897 - 42908. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C.L. Tower, S.L. Chappell, K. Morgan, N. Kalsheker, P.N. Baker, and L.J. Morgan Transforming growth factor {beta}1 regulates angiotensin II type I receptor gene expression in the extravillous trophoblast cell line SGHPL-4 Mol. Hum. Reprod., December 1, 2005; 11(12): 847 - 852. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E.-S. Akool, A. Doller, R. Muller, P. Gutwein, C. Xin, A. Huwiler, J. Pfeilschifter, and W. Eberhardt Nitric Oxide Induces TIMP-1 Expression by Activating the Transforming Growth Factor {beta}-Smad Signaling Pathway J. Biol. Chem., November 25, 2005; 280(47): 39403 - 39416. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Dimitriadis, C.A. White, R.L. Jones, and L.A. Salamonsen Cytokines, chemokines and growth factors in endometrium related to implantation Hum. Reprod. Update, November 1, 2005; 11(6): 613 - 630. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Fernandez-L, F. Sanz-Rodriguez, R. Zarrabeitia, A. Perez-Molino, R. P. Hebbel, J. Nguyen, C. Bernabeu, and L.-M. Botella Blood outgrowth endothelial cells from Hereditary Haemorrhagic Telangiectasia patients reveal abnormalities compatible with vascular lesions Cardiovasc Res, November 1, 2005; 68(2): 235 - 248. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. Vallier, M. Alexander, and R. A. Pedersen Activin/Nodal and FGF pathways cooperate to maintain pluripotency of human embryonic stem cells J. Cell Sci., October 1, 2005; 118(19): 4495 - 4509. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Moustakas and C.-H. Heldin Non-Smad TGF-{beta} signals J. Cell Sci., August 15, 2005; 118(16): 3573 - 3584. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
Y. Tang, V. Katuri, R. Srinivasan, F. Fogt, R. Redman, G. Anand, A. Said, T. Fishbein, M. Zasloff, E. P. Reddy, et al. Transforming Growth Factor-{beta} Suppresses Nonmetastatic Colon Cancer through Smad4 and Adaptor Protein ELF at an Early Stage of Tumorigenesis Cancer Res., May 15, 2005; 65(10): 4228 - 4237. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 U. Valcourt, M. Kowanetz, H. Niimi, C.-H. Heldin, and A. Moustakas TGF-{beta} and the Smad Signaling Pathway Support Transcriptomic Reprogramming during Epithelial-Mesenchymal Cell Transition Mol. Biol. Cell, April 1, 2005; 16(4): 1987 - 2002. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. L. Elliott and G. C. Blobe Role of Transforming Growth Factor Beta in Human Cancer J. Clin. Oncol., March 20, 2005; 23(9): 2078 - 2093. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. E. Runyan, H. W. Schnaper, and A.-C. Poncelet The Role of Internalization in Transforming Growth Factor {beta}1-induced Smad2 Association with Smad Anchor for Receptor Activation (SARA) and Smad2-dependent Signaling in Human Mesangial Cells J. Biol. Chem., March 4, 2005; 280(9): 8300 - 8308. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Gaedeke, T. Boehler, K. Budde, H.-H. Neumayer, and H. Peters Glomerular activin A overexpression is linked to fibrosis in anti-Thy1 glomerulonephritis Nephrol. Dial. Transplant., February 1, 2005; 20(2): 319 - 328. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. K. Kamaraju and A. B. Roberts Role of Rho/ROCK and p38 MAP Kinase Pathways in Transforming Growth Factor-{beta}-mediated Smad-dependent Growth Inhibition of Human Breast Carcinoma Cells in Vivo J. Biol. Chem., January 14, 2005; 280(2): 1024 - 1036. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 N. Kaivo-Oja, D. G. Mottershead, S. Mazerbourg, S. Myllymaa, S. Duprat, R. B. Gilchrist, N. P. Groome, A. J. Hsueh, and O. Ritvos Adenoviral Gene Transfer Allows Smad-Responsive Gene Promoter Analyses and Delineation of Type I Receptor Usage of Transforming Growth Factor-{beta} Family Ligands in Cultured Human Granulosa Luteal Cells J. Clin. Endocrinol. Metab., January 1, 2005; 90(1): 271 - 278. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Yang, C. Dai, and Y. Liu A Novel Mechanism by which Hepatocyte Growth Factor Blocks Tubular Epithelial to Mesenchymal Transition J. Am. Soc. Nephrol., January 1, 2005; 16(1): 68 - 78. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 B. F. Schrijvers, A. S. De Vriese, and A. Flyvbjerg From Hyperglycemia to Diabetic Kidney Disease: The Role of Metabolic, Hemodynamic, Intracellular Factors and Growth Factors/Cytokines Endocr. Rev., December 1, 2004; 25(6): 971 - 1010. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. G. Vicencio, C. G. Lee, S. J. Cho, O. Eickelberg, Y. Chuu, G. G. Haddad, and J. A. Elias Conditional Overexpression of Bioactive Transforming Growth Factor-{beta}1 in Neonatal Mouse Lung: A New Model for Bronchopulmonary Dysplasia? Am. J. Respir. Cell Mol. Biol., December 1, 2004; 31(6): 650 - 656. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. G.M. Molin, R. E. Poelmann, M. C. DeRuiter, M. Azhar, T. Doetschman, and A. C. Gittenberger-de Groot Transforming Growth Factor {beta}-SMAD2 Signaling Regulates Aortic Arch Innervation and Development Circ. Res., November 26, 2004; 95(11): 1109 - 1117. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. H. Lane, J. Sun, K. Devish, and W. J. Langer Dissociation of renal TGF-{beta} and hypertrophy in female rats with diabetes mellitus Am J Physiol Renal Physiol, November 1, 2004; 287(5): F1011 - F1020. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. V. Bakin, A. Safina, C. Rinehart, C. Daroqui, H. Darbary, and D. M. Helfman A Critical Role of Tropomyosins in TGF-{beta} Regulation of the Actin Cytoskeleton and Cell Motility in Epithelial Cells Mol. Biol. Cell, October 1, 2004; 15(10): 4682 - 4694. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Takeda, F. Otsuka, K. Nakamura, K. Inagaki, J. Suzuki, D. Miura, H. Fujio, H. Matsubara, H. Date, T. Ohe, et al. Characterization of the Bone Morphogenetic Protein (BMP) System in Human Pulmonary Arterial Smooth Muscle Cells Isolated from a Sporadic Case of Primary Pulmonary Hypertension: Roles of BMP Type IB Receptor (Activin Receptor-Like Kinase-6) in the Mitotic Action Endocrinology, September 1, 2004; 145(9): 4344 - 4354. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Henckaerts, J. C. Langer, J. Orenstein, and H.-W. Snoeck The Positive Regulatory Effect of TGF-{beta}2 on Primitive Murine Hemopoietic Stem and Progenitor Cells Is Dependent on Age, Genetic Background, and Serum Factors J. Immunol., August 15, 2004; 173(4): 2486 - 2493. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. A. Walker, K. S. Masters, D. N. Shah, K. S. Anseth, and L. A. Leinwand Valvular Myofibroblast Activation by Transforming Growth Factor-{beta}: Implications for Pathological Extracellular Matrix Remodeling in Heart Valve Disease Circ. Res., August 6, 2004; 95(3): 253 - 260. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. A. Pioli, K. E. Goonan, K. Wardwell, and P. M. Guyre TGF-{beta} regulation of human macrophage scavenger receptor CD163 is Smad3-dependent J. Leukoc. Biol., August 1, 2004; 76(2): 500 - 508. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. J. Sadlon, I. D. Lewis, and R. J. D'Andrea BMP4: Its Role in Development of the Hematopoietic System and Potential as a Hematopoietic Growth Factor Stem Cells, July 1, 2004; 22(4): 457 - 474. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. Selvamurugan, S. Kwok, and N. C. Partridge Smad3 Interacts with JunB and Cbfa1/Runx2 for Transforming Growth Factor-{beta}1-stimulated Collagenase-3 Expression in Human Breast Cancer Cells J. Biol. Chem., June 25, 2004; 279(26): 27764 - 27773. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C. Dai and Y. Liu Hepatocyte Growth Factor Antagonizes the Profibrotic Action of TGF-{beta}1 in Mesangial Cells by Stabilizing Smad Transcriptional Corepressor TGIF J. Am. Soc. Nephrol., June 1, 2004; 15(6): 1402 - 1412. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. Rios, S. Fernandez-Nocelos, I. Carneiro, V. M. Arce, and J. Devesa Differential Response to Exogenous and Endogenous Myostatin in Myoblasts Suggests that Myostatin Acts as an Autocrine Factor in Vivo Endocrinology, June 1, 2004; 145(6): 2795 - 2803. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. del Re, J. L. Babitt, A. Pirani, A. L. Schneyer, and H. Y. Lin In the Absence of Type III Receptor, the Transforming Growth Factor (TGF)-{beta} Type II-B Receptor Requires the Type I Receptor to Bind TGF-{beta}2 J. Biol. Chem., May 21, 2004; 279(21): 22765 - 22772. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. S. Siddiqui, Z. K. Siddiqui, and A. B. Malik Albumin endocytosis in endothelial cells induces TGF-{beta} receptor II signaling Am J Physiol Lung Cell Mol Physiol, May 1, 2004; 286(5): L1016 - L1026. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. W. Feinberg, M. Watanabe, M. A. Lebedeva, A. S. Depina, J.-i. Hanai, T. Mammoto, J. P. Frederick, X.-F. Wang, V. P. Sukhatme, and M. K. Jain Transforming Growth Factor-{beta}1 Inhibition of Vascular Smooth Muscle Cell Activation Is Mediated via Smad3 J. Biol. Chem., April 16, 2004; 279(16): 16388 - 16393. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Monteleone, J. Mann, I. Monteleone, P. Vavassori, R. Bremner, M. Fantini, G. Del Vecchio Blanco, R. Tersigni, L. Alessandroni, D. Mann, et al. A Failure of Transforming Growth Factor-{beta}1 Negative Regulation Maintains Sustained NF-{kappa}B Activation in Gut Inflammation J. Biol. Chem., February 6, 2004; 279(6): 3925 - 3932. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Shimasaki, R. K. Moore, F. Otsuka, and G. F. Erickson The Bone Morphogenetic Protein System In Mammalian Reproduction Endocr. Rev., February 1, 2004; 25(1): 72 - 101. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 U. Bartram and C. P. Speer The Role of Transforming Growth Factor {beta} in Lung Development and Disease Chest, February 1, 2004; 125(2): 754 - 765. [Abstract] [Full Text] [PDF]
|
|
|
|
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C. E. Runyan, H. W. Schnaper, and A.-C. Poncelet The Phosphatidylinositol 3-Kinase/Akt Pathway Enhances Smad3-stimulated Mesangial Cell Collagen I Expression in Response to Transforming Growth Factor-{beta}1 J. Biol. Chem., January 23, 2004; 279(4): 2632 - 2639. [Abstract] [Full Text] [PDF]
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 J. C. Langer, E. Henckaerts, J. Orenstein, and H.-W. Snoeck Quantitative Trait Analysis Reveals Transforming Growth Factor-{beta}2 as a Positive Regulator of Early Hematopoietic Progenitor and Stem Cell Function J. Exp. Med., January 5, 2004; 199(1): 5 - 14. [Abstract] [Full Text] [PDF]
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S. Saika, Y. Okada, T. Miyamoto, O. Yamanaka, Y. Ohnishi, A. Ooshima, C.-Y. Liu, D. Weng, and W. W.-Y. Kao Role of p38 MAP Kinase in Regulation of Cell Migration and Proliferation in Healing Corneal Epithelium Invest. Ophthalmol. Vis. Sci., January 1, 2004; 45(1): 100 - 109. [Abstract] [Full Text] [PDF]
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W. Li, W. Qiao, L. Chen, X. Xu, X. Yang, D. Li, C. Li, S. G. Brodie, M. M. Meguid, L. Hennighausen, et al. Squamous cell carcinoma and mammary abscess formation through squamous metaplasia in Smad4/Dpc4 conditional knockout mice Development, December 15, 2003; 130(24): 6143 - 6153. [Abstract] [Full Text] [PDF]
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T. G. Hofmann, N. Stollberg, M. L. Schmitz, and H. Will HIPK2 Regulates Transforming Growth Factor-{beta}-Induced c-Jun NH2-Terminal Kinase Activation and Apoptosis in Human Hepatoma Cells Cancer Res., December 1, 2003; 63(23): 8271 - 8277. [Abstract] [Full Text] [PDF]
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 A. Moustakas and C.-H. Heldin Ecsit-ement on the crossroads of Toll and BMP signal transduction Genes & Dev., December 1, 2003; 17(23): 2855 - 2859. [Full Text] [PDF]
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J. Yang, X. Zhang, Y. Li, and Y. Liu Downregulation of Smad Transcriptional Corepressors SnoN and Ski in the Fibrotic Kidney: An Amplification Mechanism for TGF-{beta}1 Signaling J. Am. Soc. Nephrol., December 1, 2003; 14(12): 3167 - 3177. [Abstract] [Full Text] [PDF]
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H. Takahashi, T. Shintani, H. Sakuta, and M. Noda CBF1 controls the retinotectal topographical map along the anteroposterior axis through multiple mechanisms Development, November 1, 2003; 130(21): 5203 - 5215. [Abstract] [Full Text] [PDF]
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N. Wright, T. L. de Lera, C. Garcia-Moruja, R. Lillo, F. Garcia-Sanchez, A. Caruz, and J. Teixido Transforming growth factor-{beta}1 down-regulates expression of chemokine stromal cell-derived factor-1: functional consequences in cell migration and adhesion Blood, September 15, 2003; 102(6): 1978 - 1984. [Abstract] [Full Text] [PDF]
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C. E. Runyan, H. W. Schnaper, and A.-C. Poncelet Smad3 and PKC{delta} mediate TGF-{beta}1-induced collagen I expression in human mesangial cells Am J Physiol Renal Physiol, September 1, 2003; 285(3): F413 - F422. [Abstract] [Full Text] [PDF]
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S. C. Hubchak, C. E. Runyan, J. I. Kreisberg, and H. W. Schnaper Cytoskeletal Rearrangement and Signal Transduction in TGF-{beta}1-Stimulated Mesangial Cell Collagen Accumulation J. Am. Soc. Nephrol., August 1, 2003; 14(8): 1969 - 1980. [Abstract] [Full Text] [PDF]
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 C. Merrihew, S. Soeder, D. C. Rueger, K. E. Kuettner, and S. Chubinskaya Modulation of Endogenous Osteogenic Protein-1 (OP-1) by Interleukin-1 in Adult Human Articular Cartilage J. Bone Joint Surg. Am., August 1, 2003; 85(90003): 67 - 74. [Abstract] [Full Text] [PDF]
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 L. Tadlock, Y. Yamagiwa, J. Hawker, C. Marienfeld, and T. Patel Transforming growth factor-{beta} inhibition of proteasomal activity: a potential mechanism of growth arrest Am J Physiol Cell Physiol, August 1, 2003; 285(2): C277 - C285. [Abstract] [Full Text] [PDF]
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H. Jono, H. Xu, H. Kai, D. J. Lim, Y. S. Kim, X.-H. Feng, and J.-D. Li Transforming Growth Factor-{beta}-Smad Signaling Pathway Negatively Regulates Nontypeable Haemophilus influenzae-induced MUC5AC Mucin Transcription via Mitogen-activated Protein Kinase (MAPK) Phosphatase-1-dependent Inhibition of p38 MAPK J. Biol. Chem., July 18, 2003; 278(30): 27811 - 27819. [Abstract] [Full Text] [PDF]
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D. Haller, L. Holt, S. C. Kim, R. F. Schwabe, R. B. Sartor, and C. Jobin Transforming Growth Factor-{beta}1 Inhibits Non-pathogenic Gramnegative Bacteria-induced NF-{kappa}B Recruitment to the Interleukin-6 Gene Promoter in Intestinal Epithelial Cells through Modulation of Histone Acetylation J. Biol. Chem., June 20, 2003; 278(26): 23851 - 23860. [Abstract] [Full Text] [PDF]
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 S. Luckhart, A. L. Crampton, R. Zamora, M. J. Lieber, P. C. Dos Santos, T. M. L. Peterson, N. Emmith, J. Lim, D. A. Wink, and Y. Vodovotz Mammalian Transforming Growth Factor {beta}1 Activated after Ingestion by Anopheles stephensi Modulates Mosquito Immunity Infect. Immun., June 1, 2003; 71(6): 3000 - 3009. [Abstract] [Full Text] [PDF]
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G. M. Wildey, S. Patil, and P. H. Howe Smad3 Potentiates Transforming Growth Factor beta (TGFbeta )-induced Apoptosis and Expression of the BH3-only Protein Bim in WEHI 231 B Lymphocytes J. Biol. Chem., May 9, 2003; 278(20): 18069 - 18077. [Abstract] [Full Text] [PDF]
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 T. Thyagarajan, S. Totey, M. J. S. Danton, and A. B. Kulkarni GENETICALLY ALTERED MOUSE MODELS: THE GOOD, THE BAD, AND THE UGLY Critical Reviews in Oral Biology & Medicine, May 1, 2003; 14(3): 154 - 174. [Abstract] [Full Text] [PDF]
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W.-y. Lui, W. M. Lee, and C. Y. Cheng Transforming Growth Factor {beta}3 Regulates the Dynamics of Sertoli Cell Tight Junctions Via the p38 Mitogen-Activated Protein Kinase Pathway Biol Reprod, May 1, 2003; 68(5): 1597 - 1612. [Abstract] [Full Text] [PDF]
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 P. C. Gray, C. A. Harrison, and W. Vale Cripto forms a complex with activin and type II activin receptors and can block activin signaling PNAS, April 29, 2003; 100(9): 5193 - 5198. [Abstract] [Full Text] [PDF]
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C. Mukasa, M. Nomura, T. Tanaka, K. Tanaka, Y. Nishi, T. Okabe, K. Goto, T. Yanase, and H. Nawata Activin Signaling through Type IB Activin Receptor Stimulates Aromatase Activity in the Ovarian Granulosa Cell-Like Human Granulosa (KGN) Cells Endocrinology, April 1, 2003; 144(4): 1603 - 1611. [Abstract] [Full Text] [PDF]
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C. Liu, M. D. A. Gaca, E. S. Swenson, V. F. Vellucci, M. Reiss, and R. G. Wells Smads 2 and 3 Are Differentially Activated by Transforming Growth Factor-beta (TGF-beta ) in Quiescent and Activated Hepatic Stellate Cells. CONSTITUTIVE NUCLEAR LOCALIZATION OF Smads IN ACTIVATED CELLS IS TGF-beta -INDEPENDENT J. Biol. Chem., March 21, 2003; 278(13): 11721 - 11728. [Abstract] [Full Text] [PDF]
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 J. Xu, A. R. Beyer, W. H. Walker, and E. A. McGee Developmental and Stage-Specific Expression of Smad2 and Smad3 in Rat Testis J Androl, March 1, 2003; 24(2): 192 - 200. [Abstract] [Full Text] [PDF]
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 S. Chen, M. Kulik, and R. J. Lechleider Smad proteins regulate transcriptional induction of the SM22{alpha} gene by TGF-{beta} Nucleic Acids Res., February 15, 2003; 31(4): 1302 - 1310. [Abstract] [Full Text] [PDF]
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H. W. Schnaper, T. Hayashida, S. C. Hubchak, and A.-C. Poncelet TGF-beta signal transduction and mesangial cell fibrogenesis Am J Physiol Renal Physiol, February 1, 2003; 284(2): F243 - F252. [Abstract] [Full Text] [PDF]
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B. Schmierer, M. K. Schuster, A. Shkumatava, and K. Kuchler Activin and Follicle-Stimulating Hormone Signaling Are Required for Long-Term Culture of Functionally Differentiated Primary Granulosa Cells from the Chicken Ovary Biol Reprod, February 1, 2003; 68(2): 620 - 627. [Abstract] [Full Text] [PDF]
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R. K. Moore, F. Otsuka, and S. Shimasaki Molecular Basis of Bone Morphogenetic Protein-15 Signaling in Granulosa Cells J. Biol. Chem., January 3, 2003; 278(1): 304 - 310. [Abstract] [Full Text] [PDF]
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H. Kim, X. Liu, T. Kobayashi, T. Kohyama, F.-Q. Wen, D. J. Romberger, H. Conner, P. S. Gilmour, K. Donaldson, W. MacNee, et al. Ultrafine Carbon Black Particles Inhibit Human Lung Fibroblast-Mediated Collagen Gel Contraction Am. J. Respir. Cell Mol. Biol., January 1, 2003; 28(1): 111 - 121. [Abstract] [Full Text] [PDF]
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A. V. Biankin, A. L. Morey, C.-S. Lee, J. G. Kench, S. A. Biankin, H. C. Hook, D. R. Head, T. B. Hugh, R. L. Sutherland, and S. M. Henshall DPC4/Smad4 Expression and Outcome in Pancreatic Ductal Adenocarcinoma J. Clin. Oncol., December 1, 2002; 20(23): 4531 - 4542. [Abstract] [Full Text] [PDF]
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 C. Huang, Y. Kim, M. L. A. Caramori, A. J. Fish, S. S. Rich, M. E. Miller, G. B. Russell, and M. Mauer Cellular Basis of Diabetic Nephropathy: II. The Transforming Growth Factor-{beta} System and Diabetic Nephropathy Lesions in Type 1 Diabetes Diabetes, December 1, 2002; 51(12): 3577 - 3581. [Abstract] [Full Text] [PDF]
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 S Saika, T Miyamoto, I Ishida, K Shirai, Y Ohnishi, A Ooshima, and J W McAvoy TGF{beta}-Smad signalling in postoperative human lens epithelial cells Br. J. Ophthalmol., December 1, 2002; 86(12): 1428 - 1433. [Abstract] [Full Text] [PDF]
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