Hypoxia-induced expression of RTEF-1 (related transcriptional enhancer factor-1) in endothelial cells is independent of HIF-1 (hypoxia-inducible factor-1)

Related transcriptional enhancer factor-1 (RTEF-1) plays an important role in transcriptional regulation of angiogenic genes in hypoxic endothelial cells. The mechanisms involved in the induction of RTEF-1 expression in hypoxia are poorly understood. In bovine aortic endothelial cells (BAEC) subjected to hypoxia, Western blot and quantitative PCR analysis revealed that RTEF-1 protein and mRNA levels were significantly increased by hypoxia. To address the potential role of the hypoxia-inducible factor-1 (HIF-1) in RTEF-1 induction, a hepatoma cell line deficient in HIF-1 (c4) and a control HIF-1 positive cell line (vT{l_brace}2{r_brace}) were exposed to hypoxia. We report that RTEF-1 protein expression assessed by either Western blotting or immunofluorescence was increased in both cell lines. This demonstrates that HIF-1 is not required for RTEF-1 upregulation by hypoxia. Conversely, RTEF-1 appeared to regulate the expression of HIF-1: HIF-1{alpha} promoter activity was increased (3.6-fold) by RTEF-1 overexpression in BAEC. Furthermore, RTEF-1 enhanced BAEC proliferation and tubule formation; these were inhibited by RTEF-1 knockdown with siRNA. We propose that RTEF-1, acting via HIF-1, is a key regulator of angiogenesis in response to hypoxia.