Endoglin is a transforming growth factor b (TGFb) superfamily auxiliary receptor. We had previously shown that it suppressed prostate cancer (PCa) cell motility, and that its expression was lost during PCa progression. The mechanism by which endoglin inhibits PCa cell motility is unknown. Here we demonstrate that endoglin abrogates TGFb-mediated cell motility, but does not alter cell surface binding of TGFb. By measuring Smad-specific phosphorylation and Smad-responsive promoter activity, endoglin was shown to constitutively activate Smad1, with little-to-no effect upon Smad3. Knockdown of Smad1 increased motility and abrogated endoglin's effects. As type I activin receptor-like kinases (ALKs) are necessary for Smad activation, we went on to show that knockdown of ALK2, but not TGFbRI (ALK5), abrogated endoglinmediated decreases in cell motility and constitutively active ALK2 was sufficient to restore a low-motility phenotype in endoglin deficient cells. These findings provide the first evidence that endoglin decreases PCa cell motility through activation of the ALK2-Smad1 pathway.

BMP9 signaling has been implicated in hereditary hemorrhagic telangiectasia (HHT) and vascular remodeling, acting via the HHT target genes, endoglin and ALK1. This study sought to identify endothelial BMP9-regulated proteins that could affect the HHT phenotype. Gene ontology analysis of cDNA microarray data obtained after BMP9 treatment of primary human endothelial cells indicated regulation of chemokine, adhesion, and inflammation pathways. These responses included the up-regulation of the chemokine CXCL12/SDF1 and down-regulation of its receptor CXCR4. Quantitative mass spectrometry identified additional secreted proteins, including the chemokine CXCL10/IP10. RNA knockdown of endoglin and ALK1 impaired SDF1/CXCR4 regulation by BMP9. Because of the association of SDF1 with ischemia, we analyzed its expression under hypoxia in response to BMP9 in vitro, and during the response to hindlimb ischemia, in endoglin-deficient mice. BMP9 and hypoxia were additive inducers of SDF1 expression. Moreover, the data suggest that endoglin deficiency impaired SDF1 expression in endothelial cells in vivo. Our data implicate BMP9 in regulation of the SDF1/CXCR4 chemokine axis in endothelial cells and point to a role for BMP9 signaling via endoglin in a switch from an SDF1-responsive autocrine phenotype to an SDF1 nonresponsive paracrine state that represses endothelial cell migration and may promote vessel maturation. IntroductionEndoglin directly interacts with the TGF-␤ receptors, 1 including ALK1, 2 and modulates TGF-␤ and bone morphogenetic protein (BMP) signaling. 3 Mutations in either endoglin 4 or ALK1 5 increase the risk of hereditary hemorrhagic telangiectasia (HHT1 and HHT2, respectively), whose symptoms include arteriovenous malformation, tissue ischemia, and reperfusion defects. 6 The ALK1-endoglin signaling complex in endothelial cells is activated by BMP9, 7 a circulating cytokine produced in the liver reticuloendothelium 8 and endothelial cells, including those lining the mouse aorta. 9 BMP9 interacts with endoglin and ALK1 to activate signaling pathways 7 that promote endothelial cell quiescence 10 and vessel maturation. 11 Several endothelial cell-derived factors, including BMP9, are known to regulate vessel maturation via paracrine recruitment of other cell types. 12 Moreover, our recent work using nonendothelial cells implicates endoglin in the regulation of tumor neoangiogenesis via the secreted insulin-like growth factor binding protein 4. 13 Therefore, elucidation of the role of BMP9 signaling, specifically in terms of its effects on the expression of endothelial cell-secreted factors, is needed to better understand the mechanisms by which BMP9 affects vessel maturation, integrity, the vascular response to injury, and how deficiency in either endoglin or ALK1 impacts vessel integrity and cause HHT.Stromal-derived factor 1 (SDF1, CXCL12) is a chemokine that signals via the chemokine receptor, CXCR4, to modulate hypoxiainduced angiogenesis. 14 SDF1 regulates both endothelial cellmediated paracrine signaling ...

Endoglin is an integral membrane glycoprotein primarily expressed in the vascular endothelium, but also found on macrophages and stromal cells. It binds several members of the transforming growth factor (TGF)-b family of growth factors and modulates TGF-b 1 -dependent cellular responses. However, it lacks cytoplasmic signaling motifs and is considered as an auxiliary receptor for TGF-b. We show here that endoglin is expressed in mouse and human epidermis and in skin appendages, such as hair follicles and sweat glands, as determined by immunohistochemistry. In normal interfollicular epidermis, endoglin was restricted to basal keratinocytes and absent in differentiating cells of suprabasal layers. Follicular expression of endoglin was high in hair bulb keratinocytes, but decreased in parts distal from the bulb. To address the role of endoglin in skin carcinogenesis in vivo, Endoglin heterozygous mice were subjected to long-term chemical carcinogenesis treatment. Reduction in endoglin had a dual effect during multistage carcinogenesis, by inhibiting the early appearance of benign papillomas, but increasing malignant progression to highly undifferentiated carcinomas. Our results are strikingly similar to those previously reported for transgenic mice overexpressing TGF-b 1 in the epidermis. These data suggest that endoglin might attenuate TGF-b 1 signaling in normal epidermis and interfere with progression of skin carcinogenesis.