(The FASEB Journal. 2003;17:984-992.)
© 2003 FASEB
CD105 is important for angiogenesis: evidence and potential applications
SARAH E. DUFF1,
CHENGGANG LI1,
JOHN M. GARLAND2 and
SHANT KUMAR3
Department of Pathology, Medical School, University of Manchester, Manchester, UK
3Correspondence: Department of Pathology, Medical School, University of Manchester, M13 9PT, UK. E-mail: MDDPSSK2{at}FS1.SCG.MAN.AC.UK
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ABSTRACT
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Angiogenesis is the propelling force for tumor growth and metastasis, and antiangiogenic therapy represents one of the most promising modalities for cancer treatment. CD105 (endoglin) is a proliferation-associated and hypoxia-inducible protein abundantly expressed in angiogenic endothelial cells (EC). It is a receptor for transforming growth factor (TGF) -ß1 and -ß3 and modulates TGF-ß signaling by interacting with TGF-ß receptors I and/or II. Immunohistochemistry studies have revealed that CD105 is strongly expressed in blood vessels of tumor tissues. Intratumoral microvessel density (MVD) determined using antibodies to CD105 has been found to be an independent prognostic indicator, wherein increased MVD correlates with shorter survival. CD105 is able to be shed into the circulation, with elevated levels detected in patients with various types of cancer and positively correlated with tumor metastasis. Tangible evidence of its proangiogenic role comes from knockout studies in which CD105 null mice die in utero as a result of impaired angiogenesis in the yolk sac and heart defects. The potential usefulness of CD105 for tumor imaging has been evaluated in tumor-bearing mice and dogs that have shown the rapid accumulation of radiolabeled anti-CD105 monoclonal antibody in the tumors with a high tumor-to-background ratio. The anti-CD105 antibody conjugated with immunotoxins and immunoradioisotopes efficiently suppressed/abrogated tumor growth in murine models bearing breast and colon carcinoma without any significant systemic side effects. Immunoscintigraphy in patients with renal cell carcinomas has shown specific localization of 99Tcm-labeled CD105 mab in tumor endothelial cells. Thus, CD105 is a promising vascular target that can be used for tumor imaging, prognosis, and bears therapeutic potential in patients with solid tumors and other angiogenic diseases.—Duff, S. E., Li, C., Garland, J. M., Kumar, S. CD105 is important for angiogenesis: evidence and potential applications.
Key Words: HHT1 • tumor prognosis • cancer
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INTRODUCTION
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ANGIOGENESIS, the formation of new blood vessels from preexisting ones, is essential for tumor growth and metastasis. The original concept of antiangiogenic therapy proposed by Folkman as an alternative adjuvant to traditional anti-cancer therapies has attracted enormous attention for the past three decades (1
, 2)
. The goal of the antiangiogenic approach is to deliver an effector or cytotoxic agent specifically to the vasculature of a solid tumor to eliminate blood supply to the tumor while causing minimal perturbation in blood flow to normal tissues. Over the years great effort has been made to find specific markers for the angiogenic endothelial cells that can be exploited by vascular targeting agents. Although a marker that is strictly specific for tumor vasculature probably does not exist, a variety of potential therapeutically viable candidates are still under investigation (2)
. These can be categorized as markers of endothelial cell activation and proliferation, markers of hypoxic stress, growth factor receptors, undefined endothelial cell antigens, and proteins that are expressed in newly formed or remodeled basement membrane. Equally important to antiangiogenic therapy is the search for markers that can be used for tumor imaging, monitoring the efficacy of antiangiogenic therapy and predicting tumor progression. CD105 (endoglin) is predominantly expressed in angiogenic endothelial cells (EC) and up-regulated by hypoxia (2
, 3
and our unpublished results). Such properties have made it a prime target for tumor imaging, prognosis, and antiangiogenic therapy. The importance of angiogenesis in several physiological and pathological conditions such as embryogenesis, wound healing, low back pain, myocardial infarction, stroke, rheumatic arthritis, psoriasis, scleroderma, etc., is well recognized (4
5
6
7)
. This review will focus on CD105 and angiogenesis and its potential for vascular targeting in malignant disease. It may be appropriate first to give a short account of the structure, function, and cellular signaling of CD105, then its role in tumor biology.
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MOLECULAR CHARACTERISTICS OF CD105
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Protein structure
CD105 is expressed on the cell surface as a 180 kDa homodimeric transmembrane protein (8
, 9)
. The molecule isolated by immunoaffinity using mab 44G4 from HOON cells is a glycoprotein since it binds to ricinus communis agglutinin, wheat germ agglutinin, and peanut agglutinin lectins (10)
. The external domain binds TGF-ß1 and -3 isoforms with high affinity (50 nM) (11)
. The transmembrane and the intracellular domains of CD105 share a 71% sequence similarity with betaglycan (11)
. The human CD105 gene is located on chromosome 9q34, identified using fluorescence in situ hybridization, and the coding region contains 14 exons (12
, 13)
. Two different isoforms (L and S) of CD105 with capacity to bind TGF-ß have been characterized (14)
which differ in the amino acid composition of their cytoplasmic tails. The L-CD105 consists of 633 amino acid residues with 47 amino acid residues in the cytoplasmic tail as opposed to the S-CD105, which consists of 600 amino acid residues with a 14 amino acid cytoplasmic tail. Both isoforms have been detected using RT-PCR in EC, promonocytic cell lines HL-60 and U-937, and placenta (14)
, but only the L-CD105 can be detected in EC using immunoblotting, suggesting that the L-CD105 is the predominant form (ref 15
and our unpublished results). Although the in vivo significance of the S-CD105 remains unclear, it has been useful to study the contribution to function of the cytoplasmic domain in transfection studies (16
, 17)
. CD105 is constitutively phosphorylated in endothelial cells, mainly on serine and threonine residues. It has recently been shown that the constitutive phosphorylation of CD105 is due to the constitutively active TGF-ßRII within the cell (18)
. The extent of L-CD105 phosphorylation was eightfold higher than that of S-CD105 (19)
. TGF-ß binding to CD105 results in down-regulation of phosphorylation, similar to effects seen with protein kinase C inhibitors (19)
. The CD105 amino acid sequence contains the tripeptide arginine-glycine-aspartic acid (RGD) located in an exposed region of the extracellular domain (11)
. The RGD peptide is a key recognition structure found on ECM proteins such as fibronectin, vitronectin, von Willebrand factor (vWF), type I collagen, and fibrinogen and is recognized by cell surface integrins (20
, 21)
. Integrin adhesion has been implicated in hemostasis, thrombosis, and inflammation, processes in which the endothelium plays a critical role (20
, 21)
. The identity of integrin-like molecules that may interact with the RGD sequence of CD105 and the role of this protein in the specialized function of EC are unknown at present. In a comparison between human and murine CD105, the latter, though it has an overall identify of 72% with its human counterpart, lacks the RGD sequence (22)
. The most conserved regions of CD105 are the transmembrane and cytoplasmic domains, with 95% identity between human and murine forms, which strongly suggests that the cytoplasmic domain of CD105 interacts with other intracellular proteins and is likely to be involved in the transduction of signals associated with the TGF-ß receptor system (22)
. Epitopes recognized by a panel of mabs have been mapped by recombinant CD105 cDNA techniques (13)
. Epitopes recognized by mabs P3D1 and TEC4 are between amino acids 26 and 230 encoded by exons 1 to 5; mabs 44G4, E9, P4A4, and MAEND 3 react with epitopes between amino acids 277 and 331 encoded essentially by exon 7; mabs CLE4, RMAC8, and CLB-HEC-19 react with sequences between amino acids 332 and 586 corresponding to exons 8 to 12. That different domains of CD105 are always accessible to antibodies, particularly when CD105 is associated with other molecules, may not be the case and may explain differences in its recorded distributions (23)
.
Molecular function
Following the discovery of CD105 by Letarte’s group (8
, 9)
, a variety of important functions of CD105 have been uncovered; most of its functions are likely to be associated with TGF-ß signaling. TGF-ß signals through heterodimeric receptors consisting of serine kinases, receptor I (RI), and receptor II (RII). Binding of TGF-ß to the external domains of the receptor unmasks the cytoplasmic RII kinase activity that phosphorylates the TGF-ßRI, which can then interact with downstream signalers such as the Smad proteins. CD105 forms part of the TGF-ß receptor complex but it can exist independently on the cell surface. While binding TGF-ß1 and -ß3, the binding domains on CD105 are currently unknown. In many cells in vitro, CD105 suppresses TGF-ß signaling (15
, 17)
. A number of perplexing aspects arise concerning its function. First, only a small percentage of surface-expressed CD105 actually binds TGF-ß (11)
; presumably these are associated with TGF-ß receptors. Second, despite the lack of signaling domains, CD105 overexpression affects cell morphology and adhesion in the absence of TGF-ß (24)
, suggesting a function independent of its modulation of TGF-ß signaling. CD105 binds other growth factors such as activin A and bone morphogenic proteins (BMP) -7 and -2 (25)
.
Mechanism of action of CD105
Little is known about the mechanism of action of CD105 (26
, 27)
. The picture is complicated by the fact that the different receptors for the growth factors binding to CD105 belong to a family of serine/threonine kinases. Binding of TGF-ß or other growth factor ligands to CD105 requires the presence of at least the receptor RII (25)
, and it cannot bind ligands by itself. In the case of the BMP-7 receptors, CD105 interaction also needs the RI (25)
. However, CD105 association with receptors does not alter their affinity for the ligand itself. One postulate is that CD105 acts in a ligand-presenting capacity that would not accord with its known suppression of TGF-ß signaling unless it presented it in a nonfunctional way, effectively blocking TGF-ß receptors. Recent progress has shed light on the interaction between CD105 and the signaling receptors; a model illustrating the role of CD105 in the TGF-ß receptor complex has been proposed by Bernabeu’s group (Fig. 1
) (18)
. Full-length CD105 can bind to TGF-ßRI and TGF-ßRII in the absence of TGF-ß, but the CD105/TGF-ßRI interaction is different from that of CD105/TGF-ßRII. TGF-ßRII interacts with the amino acids 437–558 of the extracellular domain of CD105, whereas TGF-ßRI interacts with the region 437–558 and the protein region between amino acid 437 and the amino terminus. As mentioned earlier, the pattern of CD105/TGF-ßRI interaction is distinct from that of CD105/TGF-ßRII interaction. TGF-ßRI only interacts when its kinase domain is inactive, whereas both active and inactive TGF-ßRII remain associated with CD105. Upon association, the cytoplasmic domain of CD105 is phosphorylated by TGF-ßRI and TGF-ßRII; then TGF-ßRI, but not TGF-ßRII, kinase dissociates from the receptor complex. Furthermore, CD105 expression inhibits phosphorylation levels of TGF-ßRII but increases that of TGF-ßRI, resulting in increased phosphorylation of Smad 2 but not Smad 3. Since Smad 2 can interact with a variety of transcription factors, coactivators, and suppressors, phosphorylated Smad 2 may act as an integrator of multiple signals to modulate gene transcription. Thus, CD105 modulates TGF-ß functions via interaction with TGF-ßRI and TGF-ßRII and modifies the phosphorylation of downstream Smad proteins.
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Regulation of CD105 expression
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The CD105 promoter is 2.6 kb in length but does not contain TATA or CAAT transcription initiation boxes. However, it has two GC-rich regions, consensus motifs for Sp1, ets, GATA, AP-2, NF
B, and Mad, as well as TGF-ß response elements (28)
. Nonetheless, CD105 has a relatively restricted cellular distribution. The basal level of transcription appears to require an ets site at position -68 and the Sp1 sites, but the relative restriction of expression, for example, to endothelial cells, appears to involve multiple regulatory regions—in particular, one at -1294 to -932 and another very close to the transcription initiation site (29)
. CD105 is up-regulated by TGF-ß, and this has been shown to require a critical Sp1 site at -37 to -29, also involving one or more juxtaposed upstream SBE sites binding Smads 3 and/or 4 (which are activated by TGF-ß signaling) (30)
. In addition, vascular defects similar to hereditary hemorrhagic telangiectasia type 1 (HHT1) seen in adult heterozygous mice are strain dependent (31
, 32)
, suggesting that other modifier genes, such as those involved in TGF-ß regulation (33)
, may exert paracrine and autocrine regulatory effects on CD105 expression. Hypoxia is a common feature of ischemic tissues and tumors. We and others have shown that hypoxia is a potent stimulator for CD105 gene expression in vascular EC and such an effect is potentiated in combination with TGF-ß1 (ref 3
and our unpublished results). The up-regulated CD105 appears to exert a self-protective role in EC under hypoxic stress (our unpublished results).
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CD105 AND CELLULAR SIGNAL TRANSDUCTION
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Vascular EC are the major source of CD105. Other cell types including vascular smooth muscle cells (34)
, fibroblasts (22)
, macrophages (35)
, leukemic cells of pre-B and myelomonocytic origin (8
, 36)
, and erythroid precursors (37)
express CD105 to a lesser extent. Syncytiotrophoblasts of term placenta contain a high level of CD105 (38)
. As mentioned, CD105 binds TGF-ß1 and TGF-ß3 and forms heterocomplexes with TGF-ßRI and/or RII in EC, indicating that CD105 may be involved in TGF-ß signal transduction. Human U937 myelomonocytic cells transfected with CD105 cDNA overexpress CD105. Cellular responses of these transfectants to TGF-ß1, including inhibition of cellular proliferation, down-regulation of c-myc mRNA, stimulation of fibronectin synthesis and cellular adhesion, phosphorylation of platelet/endothelial cell adhesion molecule-1 (PECAM-1/CD31), were all inhibited (17)
. Similar results have been observed in CD105 transfected rat myoblasts transfected with human CD105 (16)
. In human umbilical vein EC, which strongly expresses CD105, an antisense approach was used to suppress CD105 protein translation. In these cells in which CD105 mRNA and protein expression were considerably reduced, the inhibitory effects of TGF-ß1 on cell proliferation, migration, and microvessel formation were significantly heightened, implying that the abundant presence of CD105 on these cells may insulate EC from the negative control effects of TGF-ß1 and thus contribute to angiogenesis (15)
. These data demonstrate that CD105 is an important component in the TGF-ß receptor complex that modulates the effects of TGF-ß.
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CD105 AND HEREDITARY HEMORRHAGIC TELANGIECTASIA TYPE 1
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HHT is an autosomal dominant disorder characterized by multisystemic vascular dysplasia and recurrent hemorrhage (39)
. Telangiectases, arteriovenous malformations of the skin, mucosa, and viscera are characteristics of this disease. Whereas ALK1 mutations lead to HHT2 (40)
, HHT1 is the result of CD105 gene mutations (41)
. Mutations identified so far are scattered widely in the external domain of CD105 from HHT1 patients (42
, 43
, 44)
, and these mutations apparently yield truncated forms of CD105. These truncated mutants are either undetectable, present at low levels as transient intracellular forms, or expressed as partially glycosylated premature forms. Most important, such mutants do not form heterodimers with normal CD105 and nor do they interfere with its normal trafficking to the cell surface (45)
. These data suggest that HHT1 is caused by the expression of a single allele rather than by interference from a mutated copy. Mice expressing a single CD105 allele develop external signs of disease similar to human HHT1, supporting the haploinsufficiency model for HHT1 (31
, 33)
.
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CD105 AND ANGIOGENESIS
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The proposition that CD105 may be associated with tumor angiogenesis developed from observations that CD105 was strongly up-regulated in the endothelium of various tumor tissues compared with that in normal tissues (46
47
48)
. Studies performed in different laboratories using various antibodies to CD105 have revealed CD105 up-regulation in a wide range of tumor endothelia including that within colon, breast, brain, lung, prostate, and cervical cancer (Table 1
),suggestive of the possible involvement of CD105 in tumor angiogenesis (47
48
49
50
51)
. The discrepancies of intensity of tissue staining between the different mabs could be explained in several ways, such as the criteria for grading, the negative and positive controls used, the distinctive epitopes recognized by the mabs, and the definition of "normal" tissue. Nevertheless, these mabs have all revealed a stronger expression of CD105 in tumor endothelium than corresponding normal tissues. A study using an antisense approach has demonstrated that suppression of CD105 expression in HUVEC resulted in marked inhibition of in vitro angiogenesis in combination with TGF-ß1, suggesting that CD105 is a proangiogenic component in the endothelial cells (15)
. Further evidence of the important role of CD105 in angiogenesis comes from CD105 knockout mice. The CD105 null mice exhibit multiple vascular and cardiac defects leading to death at an early embryonic stage (32
, 52
, 53)
. From embryonic day 9.0, the primitive vascular plexus of the yolk sac failed to remodel into mature vessels, causing vascular channel dilation, rupture, and hemorrhage. Vessel fragility resulted in internal bleeding. These severe vascular impairments observed in CD105 null mice suggest that CD105 is required for the formation of mature blood vessels in the extraembryonic vasculature. Failed endocardial cushion formation, essential for valve development and heart septation, and pericardial edema were also noticed in the CD105 null mice, indicating another crucial role of CD105 in cardiac development.
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Table 1. Summary table showing immunohistochemical staining intensity of a panel of mabs to CD105 with endothelial cells in tumors and normal tissuesa
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CD105 AND TUMOR PROGNOSIS
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The use of diagnostic modalities based on CD105 may have an important role in clinical management. Areas with clinical potential include diagnosis, follow-up, prediction of response to treatment, and prognostic determination. Shed CD105 levels may be useful as an indicator for disease progression and to identify patients at risk of recurrence and metastasis. In patients with breast cancer, shed CD105 levels were markedly elevated in plasma samples compared with healthy controls. CD105 levels were significantly increased in those patients who subsequently developed distant metastasis (54)
. Takahashi et al. (55)
have confirmed these findings in serum samples from patients with colorectal, breast and other types of cancer. Patients with metastatic disease had significantly elevated CD105 levels in comparison with metastasis-negative individuals and healthy controls. Chemotherapy significantly reduced the levels of serum CD105. The results of these studies suggest that circulating levels of CD105 can detect patients with more advanced disease, predict those at risk of metastasis, and alter in response to chemotherapy. CD105 levels might also help in measuring the response of an individual to treatment, especially to antiangiogenic modalities and in follow-up to monitor for disease recurrence.
Microvessel density (MVD) has been reported to be an independent prognostic indicator of outcome in a variety of human malignancies, with increased MVD correlating with shorter overall and relapse-free survival rates (56
57
58
59
60)
. However, a few investigators have failed to confirm these findings (61
, 62)
. Discrepancies between these studies may be due to the various methods of staining tissues using different pan-endothelial marker antibodies and different methods of counting microvessels. Pan-endothelial markers such as antibodies against vWF stain endothelial cells well in large blood vessels but fail to localize some microvessels. The use of such markers would therefore underestimate the MVD in tumors. Attempts have been made to standardize procedures for staining and counting of microvessels; adherence to such a consensus methodology may ensure that MVD values are comparable between studies (63)
. CD105 antibodies have shown a greater specificity for tumor vasculature in comparison to pan-endothelial markers (23
, 46
, 47
, 51
, 64
65
66
67)
. Similarly, MVD measured using antibodies directed against CD105 has been compared with MVD measured with pan-endothelial markers (51
, 66
, 67)
. Kumar et al. (51)
used anti-CD105 mab E9 and anti-CD34 to quantify MVD in human breast carcinoma. The MVD counts using anti-CD105 antibody significantly correlated with overall and disease-free survival, whereas no such correlation was seen using the pan-endothelial marker (CD34). Multivariate analysis confirmed that CD105-determined MVD was an independent prognostic factor. Similar findings have been reported by others in non-small cell lung cancer patients (67)
. Tanaka et al. (67)
found that MVD obtained using anti-CD105 antibody correlated with the expression of vascular endothelial growth factor, and patients with lower MVD counts survived longer. Higher MVD identified with mab to CD105 was observed in colorectal mucosa with high-grade vs. low-grade dysplasia and in carcinoma vs. high-grade dysplasia, i.e., MVD predicted the risk of development from dysplasia to carcinoma (68)
. In prostate cancer, MVD score using CD105 mab was associated with Gleason score, metastasis, tumor stage, tumor cell proliferation index, and survival, but not when an antibody to vWF was used (69)
. We would like to add a cautionary note that all mabs against CD105 do not show the same specificity (Fig. 2
and Table 1
). Indeed, it has already been the cause of some debate about the usefulness of CD105 as a target for antiangiogenesis therapy (23
, 70)
. To clarify which react more strongly with tumor endothelium, a panel of mabs to CD105 should be evaluated using the same tumor specimens in future studies.
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Figure 2. Immunohistochemical staining of consecutive cryosections of breast cancer using two mabs to CD105. Mab CLE4 (A) stained both large and microvessels whereas mab E9 (B) decorated mainly microvessels of the breast cancer tissue.
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CD105 AND TUMOR IMAGING
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Promising early studies suggest that the use of radiolabeled antibodies against CD105 is a feasible approach for the immunoscintigraphy of tumors. The overexpression of CD105 in proliferating endothelium and its location on the luminal surface make it an ideal target for tumor detection by an antibody (Fig. 3
). Localization of CD105 to angiogenic tissue has potential for selecting patients who would benefit from antiangiogenic therapies and measuring the response to such a therapy. Optimization of imaging an endothelial target requires that background blood pool activity and ligand extravasation is minimized. This can be achieved by using small doses of ligand so that the high-affinity receptors on the endothelial cells are not saturated and imaging relatively soon after injection of labeled ligand before extravasation into the extracellular space has occurred.
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Figure 3. Tumor imaging using mab to CD105. After perfusion of a kidney from a patient with renal cell carcinoma with 99Tcm-labeled Mab E9, immunoscintigraphy clearly shows two hot spots of radioactivity. Although a presurgery MRI scan found only one tumor mass (a), the histopathology report identified two tumors—one corresponding to "a" and another, a well-vascularized 0.75 cm diameter tumor (b). These findings strongly indicate that our 99Tcm-labeled Mab E9 should be useful for detection of occult metastatic disease that conventional imaging modalities are unable to locate.
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Intravenous injection of 111In-labeled CD105 specific rat anti-mouse mab into tumor-bearing C57BL/6 mice resulted in accumulation of radioactivity in the tumors (71)
. Pharmacokinetics demonstrated that 97% of the injected dose of anti-CD105 antibody was removed from circulation within 15 min and the blood half-life was less than 1 min, binding to tumor vasculature occurred early. Binding to vascular endothelial cells in liver, kidney, and heart was also seen. Autoradiography and immunohistology showed the anti-CD105 antibody accumulated at the tumor edge, where the highest density of blood vessels was found, whereas the tumor centers were stained heterogeneously. Fonsatti et al. (72)
used a spontaneous canine mammary adenocarcinoma model and 125I-labeled mab MAEND3 (antihuman CD105 monoclonal antibody that cross reacts with rat CD105). Immunoscintigraphy 8 h after injection demonstrated rapid and intense uptake into the tumor areas with a high tumor:background ratio without systemic effects.
These studies suggest that CD105 is a good target for tumor imaging and may be useful in other angiogenic diseases. It may be particularly valuable in monitoring response to novel antiangiogenic therapies and in situations where conventional imaging techniques perform poorly, e.g., discrimination between tumor recurrence and postoperative/radiotherapeutic changes in tissues.
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TARGETING CD105 FOR ANTIANGIOGENIC THERAPY
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Antiangiogenic therapies theoretically take two approaches: vascular targeting is therapy directed at preexisting blood vessels whereas antiangiogenic therapy aims to prevent the development of neovasculature. Such approaches have several potential advantages over conventional chemotherapeutic agents (73
74
75)
: 1) ease of accessibility of vascular endothelium via the bloodstream; 2) the genetic stability of endothelial cells, unlike the tumor cells that depend on them, so development of drug resistance is unlikely; 3) limitation of toxicity and side effects in normal tissues by specific targeting of antiangiogenic treatments to the proliferating endothelial cells; 4) a single blood vessel supports the survival of many thousands of tumor cells so blood vessel destruction leads to an inbuilt avalanche of tumor cell death; 5) avoidance of the problems of tumor antigen heterogeneity as angiogenic endothelial cells will homogeneously up-regulate antigen expression; 6) applicable to many solid tumors, all of which depend on a vascular supply. Potential disadvantages of antiangiogenic strategies are 1) the possibility of interfering with physiological angiogenesis, e.g., female reproductive tract, developing growth plate in bone, wound healing; 2) need for long-term treatment, as tumor cells are not targeted themselves but maintained in a dormant state by deprivation of blood supply; 3) danger of immunogenicity against antibodies; humanization of mabs will overcome this problem.
CD105 potentially represents an ideal target for antiangiogenic therapies; however, it is difficult to test these using in vivo animal tumor models, because most anti-CD105 antibodies do not react with the vasculature in xenografts of human tumors, the reason being that endothelial cells in the transplanted tumors are derived from the host’s blood vessels. Seon’s group has partially overcome this problem (76
77
78
79)
by developing a series of anti-CD105 antibodies that cross-react weakly with murine CD105. Their studies have demonstrated long-lasting complete abrogation of human breast tumors in SCID mice using CD105 mab that has been conjugated to immunotoxins (76
, 77)
and growth suppression of human solid tumors using radiolabeled mab to CD105 (79)
. They have observed synergism between naked anti-CD105 antibodies and conventional chemotherapeutic schedules in a human skin/SCID mouse chimera model (78)
. In our study using 99Tcm-labeled mab E9, we have shown specific localization of the labeled antibody in the tumor vasculature of the kidneys (Fig. 3
, unpublished results). These studies illustrate the potential of CD105 as a target in antiangiogenic therapies.
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CONCLUSIONS AND FUTURE PROSPECTS
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Evidence supports the important role of CD105 in angiogenesis and in tumor progression. CD105 represents an ideal target for antiangiogenic therapy and a good marker for tumor prognosis. However, the mechanisms underlying the proangiogenic action of CD105 have not been fully elucidated. TGF-ß inhibits EC proliferation, migration, and formation of microvessels, whereas CD105 counteracts these actions, thereby promoting angiogenesis. In endothelial cells, only a small proportion of CD105 interacts with TGF-ß; the function of the vast majority of CD105 is not known. This is a critical question that may have an important bearing on the pharmacokinetics of receptor:ligand interaction and thus on their functional consequences. Regarding the interaction between CD105 and TGF-ß receptors, CD105 is recognized as a modulator of TGF-ß signaling, but the intracellular mediators have not been identified. Further studies are required to search for the intracellular partners interacting with CD105. Targeted gene delivery is a promising approach for therapies directed to angiogenic diseases, including cancer; it aims to reduce the unintended gene expression in normal tissues. Tissue-specific and/or hypoxia-inducible promoters are potential candidates for such strategies. The CD105 promoter is predominantly active in proliferating EC and its activity is induced by hypoxia, which could be important for targeted gene delivery to manage human malignant disease.
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FOOTNOTES
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1 Both authors made an equal contribution to this paper. 
2 Present address: Cardiovascular Central Clinical Academic Group, Manchester University Medical School, Stopford Building, Manchester M13 9PT, UK.
Received for publication September 12, 2002.
Accepted for publication February 14, 2003.
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ABSTRACT
INTRODUCTION
