Signaling by transforming growth factor (TGF)-␤ family members is mediated by Smad proteins that regulate gene transcription through functional cooperativity and association with other DNA-binding proteins. The hypoxia-inducible factor (HIF)-1 is a transcriptional complex that plays a key role in oxygen-regulated gene expression. We demonstrate that hypoxia and TGF-␤ cooperate in the induction of the promoter activity of vascular endothelial growth factor (VEGF), which is a major stimulus in the promotion of angiogenesis. This cooperation has been mapped on the human VEGF promoter within a region at ؊1006 to ؊954 that contains functional DNA-binding sequences for HIF-1 and Smads. Optimal HIF-1␣-dependent induction of the VEGF promoter was obtained in the presence of Smad3, suggesting an interaction between these proteins. Consistent with this, co-immunoprecipitation experiments revealed that HIF-1␣ physically associates with Smad3. These results demonstrate that both TGF-␤ and hypoxia signaling pathways can synergize in the regulation of VEGF gene expression at the transcriptional level.

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Endoglin is a transforming growth factor-␤ (TGF-␤) co-receptor expressed mainly on endothelial cells and involved in cardiovascular development, angiogenesis, and vascular remodeling. This is illustrated by the fact that mutations in the endoglin gene give rise to hereditary hemorrhagic telangiectasia type 1, a dominant vascular disease with clinical manifestations that originate by a mechanism of haploinsufficiency. Thus, studies on the regulated expression of endoglin are crucial to devising therapeutic strategies for hereditary hemorrhagic telangiectasia type 1. Endoglin is highly expressed in the neovasculature associated with hypoxia such as ischemic tissues and tumors, but the molecular mechanism of this up-regulation is unknown. Here, we have investigated the possible regulation of endoglin expression by hypoxia. Surface protein, transcript, and promoter activity levels of endoglin were found to be up-regulated by hypoxia, indicating that the regulation takes place at the transcriptional level. A hypoxia-responsive element downstream of the main transcription start site of the endoglin gene was functionally characterized. Whereas hypoxia alone moderately stimulated endoglin transcription, addition of TGF-␤ under hypoxic conditions resulted in transcriptional cooperation between both signaling pathways, leading to marked stimulation of endoglin expression. Because basal endoglin transcription is sustained by Sp1, and TGF-␤ and hypoxia signaling pathways are mediated by Smad proteins and hypoxia-inducible factor-1 (HIF-1), respectively, the involvement of these transcription factors was analyzed. Functional and co-immunoprecipitation experiments demonstrated the existence of a multiprotein complex (Sp1⅐Smad3⅐HIF-1) on the endoglin promoter, mediating the cooperation between the hypoxia and TGF-␤ pathways. Within this multiprotein complex, Smad3 appears to function not only as a coactivator factor, but also as an adaptor between HIF-1 and Sp1. We propose that basal endoglin transcription (highly dependent on Sp1) may switch from a constitutive to an inducible state through Sp1 interaction with HIF-1 and Smad transcription factors, induced by hypoxia and TGF-␤, respectively.

Endoglin is a transmembrane glycoprotein that acts as an auxiliary receptor for transforming growth factor-b (TGF-b) and modulates cellular responses to this pleiotropic cytokine. Endoglin is strongly expressed in endothelial cells, where it appears to exert a crucial role in vascular development and angiogenesis. Two endoglin isoforms (L and S), differing in their cytoplasmic domains, have been previously characterized in human tissues. We now demonstrate the existence of similar L-and Sendoglin variants in murine tissues with 47 and 35 amino acids, respectively, in their cytoplasmic tail. RT-PCR analysis showed that L is the predominant endoglin isoform expressed in mouse tissues, although S-endoglin mRNA is significantly expressed in liver and lung, as well as in endothelial cell lines. Furthermore, a protein of size equivalent to recombinant S-endoglin expressed in mammalian cells was detected in mouse endothelial cells by Western blot analysis. L-and S-endoglin isoforms can form disulfide-linked heterodimers, as demonstrated by cotransfection of L-and S-endoglin constructs. To address the role of S-endoglin in vivo, an S-Eng þ transgenic mouse model that targets S-endoglin expression to the endothelium was generated. The lethal phenotype of endoglin-null (Eng À/À ) mice was not rescued by breeding S-Eng þ transgenic mice into the endoglin-null background. S-Eng þ mice exhibited reduced tumor growth and neovascularization after transplantation of Lewis lung carcinoma cells. In addition, S-Eng þ mice showed a drastic inhibition of benign papilloma formation when subjected to two-stage chemical skin carcinogenesis. These results point to S-endoglin as an antiangiogenic molecule, in contrast to L-endoglin which is proangiogenic.