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Physiologisches Institut, Justus-Liebig-Universität, D-35392 Giessen, Germany
1Correspondence: Physiologisches Institut, Justus-Liebig-Universität, Aulweg 129, D-35392 Giessen, Germany. E-Mail: Klaus-Dieter.Schlueter{at}physiologie.med.uni-giessen.de
| ABSTRACT |
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1-adrenoceptor stimulation
has been investigated in greatest detail and may therefore be taken as
a reference for other humoral stimuli. It involves the activation of
protein kinase C (PKC) and, downstream of PKC activation, of two
separate signaling pathways, one including the mitogen-activated
protein kinase and another including PI3-kinase and p70s6k
as key steps. Activation of the first pathway leads to re-expression of
fetal genes, activation of the second pathway to a general activation
of protein synthesis, and cellular growth. In neonatal cardiomyocytes,
mechanical stretch causes growth by an activation of an autocrine
mechanism including angiotensin II and endothelin. This mechanism does
not operate, however, in adult cardiomyocytes. A mechanism of
mechanotransduction has not yet been identified on adult cardiomyocytes
but integrins may play a part. In microgravity, the scenario of
myocardial growth stimulation is altered. On the systemic level, there
are changes in hemodynamic and neuroendocrine regulation that exert
indirect effects on the myocardium. Microgravity may also exert a
direct cellular effect by the absence of a constant gravitational load
component.Schlüter, K.-D., Piper, H. M. Regulation of
growth in the adult cardiomyocytes.
Key Words: adrenoceptors mechanotransduction microgravity MAP kinase PI3-kinase
| INTRODUCTION |
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Apart from mechanical stimuli, heart muscle cells in vivo
are under continuous neuroendocrine growth control. Myocardial
- and
ß-adrenoceptors play an important role in rapid functional
adjustments of the cardiac pump activity (5)
. They also
play a key role in structural adaptations of the heart. It has been
demonstrated in vitro and in vivo that
1-adrenoceptor stimulation promotes cardiac hypertrophy
(6
, 7
). This growth-promoting effect of
1-adrenoceptor stimulation represents a direct effect on
the cardiomyocyte because it has also been observed on isolated
cardiomyocytes (8-10)
.
Mechanical and neuroendocrine factors usually act together. Discrimination of one or the other of these influences in causal analysis in vivo cannot normally be achieved. The basic mechanisms of cardiac growth control have therefore been analyzed in experimental models using isolated cardiomyocytes. In these in vitro models selected growth-regulating stimuli can be studied specifically and influences of cell types other than cardiomyocytes can be excluded. Cell cultures may be prepared from fetal, neonatal, or adult animals. The third type of culture is more difficult to establish and is therefore used less frequently. The resulting cell culture models are different in several respects: immature and mature cardiomyocytes differ considerably in terms of cell size and structure, metabolism, gene expression, and receptor composition (11-13) . Cardiomyocytes from the adult myocardium represent the most appropriate model for studies interested in cardiac growth regulation in adults.
This brief review provides an overview of the signal transduction
mechanisms of growth regulation in the adult ventricular cardiomyocyte.
The growth response to
1-adrenoceptor stimulation is
chosen as reference because it has been investigated in greatest
detail. This review also mentions the current concepts of how
mechanical stress triggers cardiac growth. Finally, we discuss the
question of how microgravity may affect the growth control of the
cardiomyocyte.
| GROWTH RESPONSE TO CATECHOLAMINES |
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1-,
ß1-, and ß2-adrenoceptors. On newly
isolated quiescent adult cardiomyocytes, ß-adrenoceptor stimulation
does not accelerate protein synthesis. This finding was confirmed on
cardiomyocytes from various species (10
1-adrenoceptor
stimulation. First, it activates the protein synthesis. Second, it
causes an increase in the number of ribosomes that in total make up the
machinery of protein synthesis. Third, it induces changes in the
pattern of gene expression. Examples for the last are the re-expression
of the ß-isoform of myosin heavy chain
(ß-MHC)2, of the B-type isoform of creatine kinase (CK-B), or of the
atrionatriuretic factor (ANF).
What are the intracellular signals involved in the growth
response of adult cardiomyocytes to
1-adrenoceptor
stimulation?
1-Adrenoceptors belong to a classical
receptor family. These consist of seven-transmembrane-spanning domains
and are linked to G-proteins. Under agonist stimulation,
1-adrenoceptors activate Gq proteins and
subsequently the phospholipase Cß/protein kinase C (PLCß/PKC)
pathway. These components of signal transduction are also involved in
the hypertrophic response to
1-adrenoceptor stimulation
in cardiomyocytes. First, overexpression of constitutively active
1B-adrenoceptors causes myocardial hypertrophy
(15)
. Second, direct stimulation of PKC by phorbol esters
increases protein and RNA synthesis and induces re-expression of fetal
type proteins (10
, 16
). Third, inhibition of
PKC by pharmacologically different PKC inhibitors antagonizes the
hypertrophic response to
1-adrenoceptor stimulation
(10
, 17
). These results indicate a pivotal
role for the PLCß/PKC pathway in the signaling of
1-adrenoceptors to the hypertrophic growth response of
cardiomyocytes.
Intracellular signals, which follow PKC activation under
1-adrenoceptor stimulation, are only partially
characterized. Studies investigating potential downstream targets of
PKC focused on the mitogen-activated protein kinase (MAPK) pathway.
1-Adrenoceptor stimulation activates in a PKC-dependent
way classical isoforms of MAPK (p42MAPK and
p44MAPK, also known as Erk-2 and Erk-1). On neonatal
cardiomyocytes, MAPK activation seems to be required for the activation
of protein synthesis (18)
. In contrast, on adult
cardiomyocytes the inhibition of the MAPK pathway does not prevent the
activation of protein and RNA synthesis in response to
1-adrenoceptor stimulation (19)
. Induction
of fetal type proteins, however, depends on MAPK activation in either
cell type. Under pharmacological inhibition of the MAPK kinase (also
known as MEK),
1-adrenoceptor stimulation no longer
induces fetal-type proteins (19)
. Downstream targets of
the MAPK involved in this transcriptional activation have not yet been
characterized on adult cardiomyocytes. One may speculate that
MAPK-dependent transcription factors such as Elk-1 are also activated
in this cell type. It is important to note that there is no linkage
between the hypertrophic growth and the re-expression of fetal-type
proteins in cardiomyocytes. This not only holds for cardiomyocytes
in vitro, but also for the myocardium in vivo.
For example, triiodo-L-thyrodine provokes cardiac
hypertrophy in vivo without induction of fetal-type proteins
(20)
.
Apart from activation of MEK and MAPK, activation of PI3-kinase under
1-adrenoceptor stimulation represents another
intracellular signaling pathway (21)
. PI3-kinase is a key
enzyme for growth regulation in adult cardiomyocytes. Inhibition of
this kinase abolishes the hypertrophic response to
1-adrenoceptor stimulation (21)
. Its
activation occurs secondarily to the activation of PKC but
independently of the MAPK pathway. Activation of PI3-kinase seems to
represent a converging point in intracellular signaling for various
growth factors in adult cardiomyocytes. Classical growth
factors activate PI3-kinase via receptor tyrosine kinases; neuropeptide
Y activates PI3-kinase in a pertussis toxin-sensitive but
PKC-independent way (22)
.
Downstream of PI3-kinase, activation of p70s6k has been
identified as another key step in stimulation of protein synthesis
under
1-adrenoceptor stimulation (23)
. This
kinase phosphorylates the S6 protein of the 40-S subunit of the
ribosomes. This may increase their translational activity. Inhibition
of the activation of p70s6k attenuates the growth effect of
1-adrenoceptor agonists or neuropeptide Y in adult
cardiomyocytes. Another factor that contributes to the increase in
translational activity is the activation of the peptide chain
initiation factor elF-4E. The phosphorylation and therefore activation
of this factor depends also on the activation of PKC (24)
.
In summary, activation of PKC, PI3-kinase, and p70s6k
represent key steps of the intracellular signaling that control protein
synthesis in adult cardiomyocytes.
Ribosomes represent the cellular machinery for protein synthesis. It
seems that in resting cardiomyocytes activation of
p70s6k can recruit part of the ribosomes to participate in
protein synthesis. In mechanically active cardiomyocytes, however,
virtually all ribosomes seem functionally active (25)
. In
the latter case, presumably corresponding to the normal situation
in vivo, de novo synthesis of ribosomal RNA is
required for a substantial acceleration of protein synthesis. Under
1-adrenoceptor stimulation or direct stimulation of PKC,
RNA polymerase I is found activated and synthesis of rRNA increased
(26)
. The stimulation in RNA synthesis is also mediated
through PI3-kinase and p70s6k. This indicates that, in
addition to the S6 protein of ribosomes, p70s6k has other
targets. A candidate has been identified on non-cardiac cells, namely
the transcription factor CREM (27)
. p70s6k
thus seems to directly interfere with transcriptional regulation.
Figure 1
summarizes the intracellular signaling pathways of
1-adrenoceptor stimulation identified on adult
cardiomyocytes.
|
Under certain experimental conditions, ß2-adrenoceptor
stimulation stimulates cardiac protein and RNA synthesis
(28
, 29
). Such a response is observed in
isolated cardiomyocytes after exposure to active transforming growth
factor ß1 (TGF-ß1) (30)
. It
has been hypothesized that this observation has relevance for hearts at
the turning point between hypertrophy and heart failure because under
these conditions the intramyocardial expression of TGF-ß is
up-regulated (31)
. On the cellular level,
ß2-adrenoceptor stimulation evokes an increase in protein
synthesis by a mechanism depending on activation of adenylate cyclase
and, subsequently, PI3-kinase and p70s6k
(28-30)
. Activation of PKC and MAPK is not involved. Even
though the upstream signaling steps differ for
1- and
ß2-adrenoceptor stimulation, downstream signaling toward
protein synthesis seems the same.
The mechanism causing an increase in cellular RNA mass under
ß2-adrenoceptor stimulation differs entirely, however,
from that under
1-adrenoceptor stimulation. Under
ß2-adrenoceptor stimulation, RNA mass increases in the
absence of accelerated RNA synthesis (28)
. The degradation
of ribosomal RNA is probably prolonged under these conditions. The
induction of ornithine decarboxylase (ODC) seems to play a key role.
ODC is the rate-limiting enzyme of the polyamine metabolism. Polyamines
are polycationic molecules that stabilize nucleic acids and thereby
prolong the half-life of rRNA. ODC itself has an extremely short
half-life. Its induction requires transcriptional activation. A causal
involvement of ODC in the hypertrophic growth achieved by
ß-adrenoceptor stimulation is found in vivo and in
isolated cardiomyocytes (32
, 33
). In spite of
the differences in mechanisms, both
1- or
ß2-adrenoceptor stimulation supports the growth response
of adult cardiomyocytes by an enlargement of the ribosomal capacity of
protein synthesis. This supports the direct activating effect on
preexisting ribosomes in favor of an increase in protein synthesis.
| MECHANICAL STIMULATION OF CARDIAC PROTEIN SYNTHESIS |
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In summary, the mechanism of mechanical growth stimulation in the adult cardiomyocyte is still not known. Neonatal cardiomyocytes seem not to provide an adequate model.
| POSSIBLE INFLUENCE OF MICROGRAVITY ON CARDIOMYOCYTE GROWTH CONTROL |
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In the intact body, systemic effects of microgravity can also
indirectly influence growth control of the cardiomyocytes. Systemic
shifts of blood volume alter cardiac preload, namely the diastolic
distension of the myocardium. The altered passive stretch of the
cardiomyocytes may influence their trophic state. The systemic
neuroendocrine regulation also grossly alters in microgravity. This may
have numerous effects on the heart, independent of mechanical load
(Fig. 3
). Alterations in plasma catecholamine levels during
spaceflights have been documented. Investigations on astronauts of
SpaceHab flights indicate an activation of the sympathetic nerve system
caused by flight stress. As outlined above, catecholamines exert a
direct growth-promoting effect on the myocardium. One may speculate
that sympathetic activation partially counteracts the induction of
myocardial atrophy under hypogravity. In vivo, the indirect
effects of microgravity on cardiomyocyte growth control cannot be
distinguished from direct effects. Experimental investigations using
isolated cardiomyocytes in micro-, normo-, and hypergravity are needed
to make that distinction. Adult cardiomyocytes are preferable because
they represent more clearly the heart cell of the adults.
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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| REFERENCES |
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1-Adrenergic stimulation is coupled to cardiac myocyte hypertrophy. Am. J. Physiol. 260,H953-H956
-adrenoceptor mediated hypertrophy. Pfluegers Arch 433(Suppl. 33),R50
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