Endoglin (CD105): a Marker of Tumor Vasculature and Potential Target for Therapy Nikolaos A

Review

Endoglin (CD105): A Marker of Tumor Vasculature and Potential Target for Therapy Nikolaos A. Dallas,1Shaija Samuel,2 Ling Xia,2 Fan Fan,2 Michael J. Gray,1Sherry J. Lim,1and Lee M. Ellis1, 2

Abstract Endoglin (CD105) is an accessory protein of the transforming growth factor-h receptor system expressed on vascular endothelial cells. Mutation of the endoglin gene is associated with hereditary hemorrhagic telangiectasias, or Osler-Weber-Rendu syndrome, and has been studied extensively in the context of this disease.The expression of endoglin is elevated on the endothelial cells of healing wounds, developing embryos, inflammatory tissues, and solid tumors. Endoglin is a marker of activated endothelium, and its vascular expression is limited to proliferating cells. Recent studies identified endoglin expression in several solid tumor types, with the level of expression correlating with various clinicopathologic factors including decreased survival and presence of metastases. Attempts to target endoglin and the cells that express this protein in tumor-bearing mice have yielded promising results.

Endoglin (CD105) is a transmembrane glycoprotein expressed syndrome, which affects 1 in 8,000 individuals worldwide, is an on activated vascular endothelial cells (1). It is an accessory inherited autosomal-dominant and highly penetrant disorder protein of the transforming growth factor-h (TGF-h) receptor characterized by vascular dysplasias, frequent episodes of system and composed of two disulfide-linked subunits of 95 epistaxis, mucocutaneous telangiectases, and arteriovenous kDa each forming a 180-kDa homodimeric mature protein (2). malformations of the lung, brain, liver, and gastrointestinal In addition to its expression on endothelial cells, endoglin is tract (1, 2, 10–12). There are two genetic forms of the disease: found on the surface of several other cell types (Table 1; hereditary hemorrhagic telangiectasia 1, characterized by a reviewed in refs. 2–5). mutation of endoglin itself, and hereditary hemorrhagic The endoglin gene is 40 kb long and located on human telangiectasia 2, characterized by a mutation of the endoglin- chromosome 9q34 (6, 7). Gene transcription results in a mRNA associated protein activin-like kinase (ALK) 1 (10, 13). With product with a length of 3.4 kb and 14 exons; exons 1 to 12 both forms, patients have dilation of postcapillary venules and encode the extracellular domain, exon 13 encodes the direct arteriovenous connections that bypass capillary networks transmembrane domain, and exon 14 encodes the cytoplasmic (7). In a transgenic murine model of hereditary hemorrhagic domain. Endoglin is expressed as two isoforms, designated long telangiectasia with a heterozygous genotype for endoglin (L-CD105) and short (S-CD105), based on the length of the (eng+/-), mice exhibit large numbers of irregular, dilated, and cytoplasmic domain (5, 8). Endoglin was assigned the cluster of thinner-walled vessels with fewer associated vascular smooth differentiation number 105 at the Fifth International Workshop muscle cells than their wild-type counterparts. Whereas on Human Leukocyte Differentiation Antigens and thus is also endoglin haploinsufficient mice survive to adulthood, homo- known as CD105 (9). zygous mutation of endoglin (eng-/-) results in embryologic The importance of endoglin in vascular homeostasis is lethality by day E11.5 due to defective yolk sac vascularization, reflected by the fact that mutations in endoglin or its heart valve abnormalities, and irregular ventricular develop- downstream signaling mediator lead to hereditary hemorrhagic ment (11, 14, 15). telangiectasias (Osler-Weber-Rendu syndrome; ref. 10). This

Overview of theTGF-B System and Endoglin

1 2 Authors’ Affiliations: Departments of Surgical Oncology and Cancer Biology, As mentioned above, endoglin is an accessory protein of the The University ofTexas M. D. Anderson Cancer Center, Houston,Texas h h Received 10/17/07; revised 11/30/07; accepted 12/20/07. TGF- receptor family. TGF- ligands and receptors make up a Grant support: NIH grant T32 CA09599 (N.A. Dallas and S.J. Lim); R.E.‘‘Bob’’ complex signaling system involved in many developmental, Smith Fund for Cancer Research (S. Samuel);The John E. and Dorothy Harris Fund physiologic, and pathologic cellular processes (16). The TGF-h for Gastrointestinal Cancer Research (L.M. Ellis);TheWilliam C. Liedtke, Jr., Fund for family of ligands includes TGF-h1,TGF-h2,andTGF-h3 Cancer Research (L.M. Ellis); and NIH grant RO1CA112390 (L.M. Ellis). isoforms, activins, and bone morphogenic proteins (BMP; The costs of publication of this article were defrayed in part by the payment of page h h charges. This article must therefore be hereby marked advertisement in accordance ref. 2). There are seven type I (T R-I), five type II (T R-II), and with 18 U.S.C. Section 1734 solely to indicate this fact. two type III (ThR-III) receptors (11, 13, 17). ThR-I and ThR-II Requests for reprints: Lee M. Ellis, Department of Surgical Oncology, The are serine/threonine kinase receptors and their kinase activity is University of Texas M. D. Anderson Cancer Center, Unit 444, P. O. Box 301402, necessary for all responses to TGF-h (1). In contrast, the type III Houston, TX 77230-1402. Phone: 713-792-6926; Fax: 713-792-4689; E-mail: [email protected]. receptors, endoglin and betaglycan, lack kinase activity and are F 2008 American Association for Cancer Research. regulators of binding and signaling. As shown by coimmuno- doi:10.1158/1078-0432.CCR-07-4478 precipitation experiments, TGF-h does not bind endoglin alone

signaling by competing with receptor-mediated Smads for Table 1. Endoglin (CD105) expression in normal receptor binding. Inhibitory Smads can also dephosphorylate human tissues the activated type I receptor and recruit ubiquitin ligases to the cell membrane to promote the degradation of the receptor Location Cell type (10, 13, 21). Cardiovascular system Endothelial cells Vascular smooth muscle cells Endoglin and Endothelial Cell Signaling Hematologic system Bone marrow h stromal fibroblasts TGF- signaling in endothelial cells can mediate a variety of Progenitor B cells processes, including proliferation, migration, and adhesion (7). Erythroid precursors TGF-h is expressed in endothelial cells of healing wounds, + Circulating CD34 cells embryos, infected tissues, psoriatic skin, inflamed synovial Reticuloendothelial Interstitial macrophages h system in splenic red pulp arthritis, and solid tumors (2, 20). Whether TGF- stimulates or Activated monocytes inhibits these processes depends on experimental conditions. Differentiated macrophages For example, lowdoses of TGF- h in vitro (0.25-0.70 ng/mL) Follicular dendritic cells stimulate endothelial cell proliferation and migration, whereas in lymphoid organs Genitourinary system Kidney extraglomerular higher doses inhibit them, causing endothelial cell quiescence interstitial cells (22–24). Kidney glomerular The stimulatory phenotype in endothelial cells is mediated mesangial cells by ALK1-dominated signaling, and endoglin expression is Seminiferous tubule necessary for activation of this pathway. ALK1 activation causes basal cells Integumentary system Melanocytes an increase in endothelial cell proliferation and migration as Embryo Syncytiotrophoblasts well as transcription of the proangiogenic genes Id1, interleukin- ofplacenta 1 receptor-like 1, and endoglin itself (11, 25). In contrast, ALK5 Embryonic heart signaling induces endothelial cell quiescence by inhibiting mesenchymal cells proliferation and migration and increases expression of maturation-specific genes, such as fibronectin, connexin-37, a3/b1 integrin,andplasminogen activator inhibitor-1 (10, 11, 13). but rather requires ThR-II (2). Endoglin can, however, bind to ALK5 signaling on endothelial cells also promotes the recruit- ThR-II in the absence of ligand. In endothelial cells, the widely ment and differentiation of vascular smooth muscle cells expressed type I receptor ALK5 and the endothelial cell–specific during vessel formation (11). ALK5 is the predominant h type I receptor ALK1 mediate endoglin activity in the presence mediator of TGF- signaling in quiescent endothelial cells, of ThR-II (2, 13, 17). Endoglin can also bind other ligands of but during angiogenesis, ALK1 is preferentially activated the TGF-h system, including activin-A, BMP-2, BMP-7, and (11, 13). In addition, the complete absence of ALK5 in cells BMP-9, in other receptor complexes (18, 19). causes a decrease in ALK1 signaling, indicating that the pre- Endoglin signaling is initiated by binding of TGF-h and sence of ALK5 is also necessary for signaling via ALK1 (26). results in a series of activation steps leading to transcriptional Alteration of the patterns of expression of molecules in these activity. TGF-h is secreted in its latent form and activated by pathways has led to several interesting insights into their proteases or thrombospondin, converting it into its active form, necessary for binding to specific type I or type II TGF-h receptors (13). The sequential process of TGF-h binding and signaling in endothelial cells is illustrated in Fig. 1. TGF-h ligand binds to a ThR-II homodimer, which then recruits the type I receptor ALK1 or ALK5 to form a heterotetrameric receptor complex (2, 11–13). In endoglin-expressing cells, endoglin is included as a dimer in this receptor complex. ThR-II then phosphorylates either ALK1 or ALK5 in a highly conserved glycine/serine-rich cytoplasmic domain, causing a conforma- tional change in the receptor (13). Signaling is propagated to the nucleus via phosphorylation of Smad proteins, which act as transcriptional coactivators or corepressors (17). There are eight identified Smad proteins, which are catego- rized as receptor-mediated, common-mediator, or inhibitory Smads (13). Receptor-mediated Smads are phosphorylated in the cytoplasm by membrane-bound receptors and include Smad-1, Smad-2, Smad-3, Smad-5, and Smad-8. ALK1 associates with and activates Smad-1, Smad-5, and Smad-8, and ALK-5 associates with Smad-2 and Smad-3 (13). Smad-4, the Fig. 1. The role of endoglin in endothelial cell signaling. ALK1is phosphorylated in the common-mediator Smad, binds activated receptor-mediated presence of endoglin, which in turn phosphorylates Smad-1/Smad-5. Activated Smad-1/Smad-5 associates with Smad-4 and translocates to the nucleus, Smads and translocates to the nucleus to propagate signaling. propagating angiogenic signaling. ALK5 mediatesTGF-h signaling in quiescent The inhibitory Smads Smad-6 and Smad-7 interfere with Smad endothelial cells.

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Fig. 2. Vascular markers in colon cancer and normal mucosa. Antibodies to CD31, a pan-endothelial marker, stain vessels in both normal and neoplastic specimens. Endoglin (CD105), a marker of angiogenic endothelium, is expressed only in tumor-associated blood vessels (i.e., activated endothelium). Photomicrographs were obtained at Â10 magnification.

signaling. Overexpression of ALK1 in endothelial cells in vitro bind TGF-h in a receptor complex, but this finding has causes a decrease in cell attachment and a loss of proliferation unknown functional significance (9, 29, 30). and overall viability; additional forced overexpression of Experimental alteration of endoglin transcription and transla- endoglin opposes this phenotype, thus underscoring the tion has also led to other interesting findings that highlight the importance of endoglin in ALK1 signaling (17). In addition, importance of endoglin. When endoglin expression was reduced reduction of ALK5 signaling in endothelial cells rescues the by an antisense approach, the inhibitory effects of TGF-h on effect of small interfering RNA–mediated silencing of endoglin endothelial cell proliferation, migration, and tube formation expression (26). Furthermore, focused gene chip array studies were heightened (20). These effects were mediated by ALK5 showed that constitutively active ALK1 led to an up-regulation signaling, emphasizing that endoglin is a negative regulator of of the angiogenic mediators interleukin-8 (18-fold), jagged-1 ALK5 activity (13). Stable transfection of endothelial cells with (3-fold), and endothelin-1 (3-fold; ref. 27). In contrast to TGF- small interfering RNA to endoglin also enhanced the ability of h, BMP-9, another ALK1 and endoglin ligand, activates the TGF-h to suppress growth and migration (26, 31). Furthermore, ALK1/Smad-1/Smad-5/Smad-8 pathway but phenotypically when endoglin small interfering RNA–transfected cells were leads to a decrease in endothelial cell migration and prolifer- irradiated in vitro, they showed a decrease in cell viability, with ation (18). BMP-9 also signals via BMP receptor II in these cells, elevation of p53 (proapoptotic) and reduction of bcl-2 and the interaction of signaling via these two receptor (antiapoptotic) levels compared with mock-transfected cells complexes may explain the seemingly contradictory results. (31). Endoglin is thus important in endothelial cell survival In endothelial cells, endoglin expression is up-regulated by during radiation exposure in addition to growth and migration. hypoxia, TGF-h stimulation, and irradiation in vitro, whereas Although it is clear that endoglin expression in tissue sections endoglin is down-regulated by tumor necrosis factor-a (13, 19). is a marker of proliferating endothelial cells, there is no Endoglin is constitutively phosphorylated in vascular endothe- consensus as to the exact role of endoglin in developmental lial cells on two serine residues, Ser634 and Ser635 (8, 17, 20). angiogenesis. Endothelial cells from eng+/+ and eng-/- mouse TGF-h signaling in the presence of endoglin causes an embryos were isolated, immortalized, and cultured (32). eng-/- activation of ALK1-associated Smad proteins as well as negative mouse embryonic endothelial cells showed a higher prolifer- regulation of the ALK5-associated Smad-3 protein. Ectopic ation rate by [3H]thymidine incorporation and achieved higher endoglin expression, in fact, inhibits Smad-3 transcriptional cell densities relative to eng+/+ mouse embryonic endothelial activity via c-Jun activation and thus negatively regulates ALK5 cells. This finding is in contrast to previous work that showed pathway activation (11, 13, 28). Interestingly, endoglin an increase in proliferation of endoglin-overexpressing cells expression increases Smad-2 activity, leading to an increase in (26). Furthermore, eng-/- cells showed increased levels of endothelial nitric oxide synthase (eNOS) expression (discussed phosphorylated Smad-1/Smad-5/Smad-8 when compared with below; refs. 8, 12). This overlapping regulation highlights the eng+/+ mouse embryonic endothelial cells in the former study, complexity of ThR-I/endoglin interaction and signaling. Inter- whereas the latter study showed the opposite. It is hypothesized estingly, only 1% of endoglin molecules on an endothelial cell that the discordant results of these studies were due to the

Table 2. Endoglin expression and associated clinicopathologic variables in human solid malignancies

Tumor type Lymph node Distant Higher tumor Decreased Comments References metastasis metastasis grade survival Gastric cancer + + Ascites Ding et al. (38) Nikiteas et al. (39) Esophageal cancer + + Saad et al. (40) Colorectal cancer + + + + Duff et al. (8) Fonsatti and Maio (4) Saad et al. (41) Akagi et al. (42) Breast cancer + + + Response to neoadjuvant Duff et al. (8) chemotherapy Fonsatti and Maio (4) Gomez-Esquer et al. (43) Li et al. (44) Beresford et al. (45) Hepatocellular carcinoma + + Yang et al. (46) Non – small cell lung cancer + Fonsatti and Maio (4) Prostate cancer + + + + Preoperative PSA Duff et al. (8) El-Gohary et al. (47) Fonsatti and Maio (4) Head and neck cancers + + + VEGF levels Chien et al. (48) Chuang et al. (49) Marioni et al. (50) Primary brain malignancies + + VEGF levels Behrem et al. (51) Yao et al. (52) Endometrial carcinoma + VEGF levels Erdem et al. (53) Fonsatti and Maio (4) Ovarian cancer + Suboptimal cytoreduction Taskiran et al. (54)

Abbreviations: PSA, prostate-specific antigen; VEGF, vascular endothelial growth factor. differences between RNA interference technology and gene has a rapid turnover and has been termed ‘‘activated’’ (35). One ablation. Specifically, small interfering RNA silencing of potential marker of this activated endothelium is endoglin. In endoglin led to decreased ALK5 levels, whereas ALK5 levels in several malignancies, endoglin is found on peritumoral and eng-/- mouse embryonic endothelial cells were elevated (32). intratumoral vessels. Our laboratory has evaluated patterns of The above reinforces the fact that endoglin signaling is complex endoglin expression in primary colon adenocarcinomas and and requires further study. normal colonic mucosa, along with CD31 expression. Whereas Endoglin downstream signaling also interacts with the nitric anti-CD31 antibodies stained blood vessels in both normal and oxide pathway in endothelial cells. Transcription of eNOS is malignant colon equally, endoglin expression was observed regulated by endothelial sheer stress, hypoxia, hormones, and primarily in malignant lesions, with little to no expression in various growth factors and mediators including TGF-h1 (12). In the vessels of the nonmalignant mucosa (Fig. 2). Furthermore, endoglin haploinsufficient mice (eng+/-), levels of eNOS are many investigators have shown that endoglin expression serves constitutively reduced (33, 34). Modulation of endoglin as a better prognostic marker of patient outcomes than more expression has been shown to regulate nitric oxide–dependent traditional vascular markers, such as CD31 and von Willebrand vasodilation in addition to eNOS expression and activity in factor (3, 4, 36, 37). both in vitro and in vivo models (12). Using a doxycycline- Table 2 summarizes studies that have examined endoglin inducible endoglin system in endothelial cells, it has been (CD105) expression on tumor endothelium and clinicopath- shown that increases in endoglin correspond with increases in ologic factors associated with its expression (4, 8, 38–54). In all eNOS mRNA and protein levels; similarly, reduction in malignancies studied, endoglin expression as determined endoglin causes a decrease in both basal and TGF-h1 –induced by immunohistochemical staining was consistently associated eNOS expression (12). This regulation occurs through interac- with lower patient survival rates. This is not surprising because tion of endoglin with ALK5/Smad-2, causing an increase in increased tumor vasculature is an established marker of poor phosphorylation of the latter in a ThR-I–independent manner. prognosis. Furthermore, in gastrointestinal, breast, prostate, These findings suggest that endoglin expression and nitric oxide and head and neck malignancies, endoglin expression was regulation are intimately related and that eNOS plays a major associated with the presence of distant metastatic disease role in endoglin-dependent angiogenesis. (38, 40, 41, 44, 46–48). Endoglin expression thus seems to have prognostic value in a variety of solid cancers. Endoglin and Angiogenesis In preliminary studies, endoglin expression has also been shown to be predictive of response to chemotherapeutic Endothelial cells in normal quiescent endothelium are stable regimens. In patients with breast cancer, low endoglin and have a very lowturnover rate, witha doubling time of expression in pretreatment biopsies of women receiving more than 1,000 days. Angiogenic endothelium, in contrast, neoadjuvant chemotherapy (5-fluorouracil, epirubicin, and

Clin Cancer Res 2008;14(7) April 1, 2008 1934 www.aacrjournals.org Downloaded from clincancerres.aacrjournals.org on September 25, 2021. © 2008 American Association for Cancer Research. Endoglin as aTumor Vascular Marker cyclophosphamide) was associated with an increased clinical conjugates were injected into mice bearing orthotopic human response rate (45). The differences between pathologic res- pancreatic cancers, and tumors were visualized by ultrasonog- ponders and nonresponders did not reach statistical signifi- raphy. The excised tumors were examined using fluorescent cance, but this may be attributed to the elimination of complete immunohistochemistry after pancreatectomy and found to responders from the analysis because they had no postoperative correlate quantitatively with preoperative ultrasound findings. tumor to evaluate. If endoglin imaging can specifically identify tumor vasculature in vivo, then perhaps endoglin-expressing cells can be Endoglin and Hypoxia targeted therapeutically as well. Endoglin haploinsufficient (eng+/-) mice bearing s.c. lung carcinomas have been shown to Hypoxia is a potent stimulus for tumor-associated induction have decreased tumor growth rates and lower capillary densities of angiogenic factors. Given the role of endoglin in activated even in the absence of therapy compared with wild-type endothelium, it is no surprise that there is a correlation (eng+/+) mice (60). In an in vitro study, investigators incubated between hypoxia and endoglin expression. In fact, hypoxia is endoglin-expressing human umbilical vascular endothelial cells one of the fewknownstimuli of endoglin induction. In vitro, with anti-endoglin mAbs (61) and found that mAb treatment hypoxia (0.2% O2) causes an increase in endoglin mRNA levels caused significant growth inhibition and that the addition of as early as 1 h after the onset of hypoxia exposure, with TGF-h1 had a synergistic effect. mAbs to endoglin were also maximum RNA transcription at 3 h; endoglin protein levels are injected in vivo in mice bearing breast and colon cancer elevated to maximum levels at 16 h after exposure to hypoxia xenografts (62). The mAb-treated mice showed significantly (30). Regulation of endoglin expression by hypoxia occurs at smaller tumors and greater survival rates than controls. the transcriptional level as evidenced by standard reporter Recently, investigators developed a human/mouse chimeric assays (30). In vivo, hypoxia induced by occlusion of the antibody to endoglin and generated encouraging pharmacoki- middle cerebral artery in a mouse model caused up-regulation netic and toxicity data in nonhuman primates in anticipation of of endoglin on the endothelium of focally ischemic areas in the initiating phase I clinical trials in the near future (63). brain (55). Functionally, endoglin expression not only pro- Several groups have conjugated anti-endoglin mAbs to toxic motes angiogenesis by activating endothelial proliferation moieties in an attempt to increase the efficacy of treatment. pathways but also activates antiapoptotic signaling in hypoxic One group of investigators conjugated Auger electron emitters, endothelial cells (13). In cells treated with antisense to which act by damaging DNA after internalization, to anti- endoglin, TGF-h augments the apoptotic effect of hypoxia endoglin mAbs and treated mice with the conjugate beginning (29, 30). Induction of endoglin by hypoxia seems to be 3 days after they were inoculated with human breast cancers regulated by intermediates such as p38 and c-Jun NH2-terminal (64). The treated mice showed significant tumor growth kinase (55). These finding suggest that endoglin may be inhibition relative to control mice and had no significant involved in the stimulation of tumor angiogenesis via hypoxic toxicity, weight loss, or organ damage at autopsy 100 days after regulation. treatment. Other researchers conjugated anti-endoglin mAbs to deglycosylated ricin A and treated mice bearing human breast Endoglin in Diagnosis and Therapy cancer xenografts (65). More than half of the mice showed tumor regression, and no progression was seen even after 100 As discussed above, endoglin is preferentially expressed in days with no further therapy. Other investigators conjugated the angiogenic endothelium of tumors. The expression of packaging molecules, such as adenovirus or cationic liposomes, endoglin on tumor endothelium may have diagnostic value if to anti-endoglin mAbs to more specifically deliver their its expression can be imaged in vivo. Investigators attempted contents to activated endothelial cells, causing significant to showthis principle using technetium-99m–labeled anti- antitumor activity (66, 67). endoglin monoclonal antibodies (mAb) to perfuse fresh A unique therapeutic approach involves sensitizing host nephrectomy specimens from patients with renal cell carcino- immune cells to the endoglin protein prophylactically so that ma (56). Radiographic hotspots corresponded to preoperative future angiogenic vessels are seen as antigenic and targeted by magnetic resonance imaging–identified tumors in all seven the host’s own immune system. This approach was used by cases studied. In one case, technetium-99m imaging was able to investigators using a double-attenuated Salmonella-based oral identify a malignant mass that was missed on preoperative vaccine containing a murine endoglin-expressing plasmid (68). magnetic resonance imaging, suggesting that labeled anti- After vaccination, mice were injected with D2F2 mouse endoglin mAbs may be superior to magnetic resonance imaging mammary carcinoma, and tumor progression was evaluated. in identifying renal tumors. In another study, investigators Unvaccinated mice had significantly more lung metastases, and injected iodine-125–labeled anti-endoglin mAbs into two dogs their tumors had a more disseminated distribution. Tumors in with spontaneous mammary carcinoma and imaged them 8 h vaccinated mice were less angiogenic, and the vaccinated mice later (57). Tumors showed rapid and efficient uptake of had a longer overall survival. Given the above findings, radioactivity with high signal-to-noise ratios. A diagnosis endoglin seems to be a unique and optimal target for antitumor of ductal adenocarcinoma was verified by surgical excision therapies and needs to be explored further. 10 days later, and the dogs showed no systemic side effects during the 3-month follow-up period. Others used indium- Conclusion 111–labeled anti-endoglin mAbs to identify B16 melanoma- bearing tumors in mice by in vivo scintigraphy (58). Another Recent advances in antiangiogenic therapies for solid group of investigators conjugated microbubbles, which can be malignancies have improved patient survival and thus validated visualized by ultrasonography, to anti-endoglin mAbs (59). The the tumor vasculature as a target in anticancer therapy.

Targeting the angiogenic mediator vascular endothelial growth of endoglin in endothelial cell signaling is complex and factor has proven efficacious in several solid malignancies; remains to be completely elucidated, its presence in solid however, targeting the tumor-associated endothelial cell direct- tumor vasculature has prognostic value, suggesting its potential ly may also be a successful strategy (69). Endoglin has been as a therapeutic target. Targeting endoglin on the activated identified as a unique marker of proliferating (activated) endothelial cells that express this target has produced promis- endothelial cells in vitro and in vivo. Although the exact role ing preclinical results and thus warrants further investigation.

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