Human HIF-3 alpha 4 is a dominant-negative regulator of HIF-1 and is down-regulated in renal cell carcinoma

A universal response to changes in cellular oxygen tension is governed by a family of heterodimeric transcription factors called hypoxia-inducible factor (HIF). Tumor hypoxia, as well as various cancer-causing mutations, has been shown to elevate the level of HIF-1 alpha, signifying a critical role of the HIF pathway in cancer development. The recently identified third member of the human HIF-alpha family, HIF-3 alpha, produces multiple splice variants that contain extra DNA binding elements and protein-protein interaction motifs not found in HIF-1 alpha or HIF-2 alpha. Here we report the molecular cloning of the alternatively spliced human HIF-3 alpha variant HIF-3 alpha 4 and show that it attenuates the ability of HIF-1 to bind hypoxia-responsive elements located within the enhancer/promoter of HIF target genes. The overexpression of HIF-3 alpha 4 suppresses the transcriptional activity of HIF-1 and siRNA-mediated knockdown of the endogenous HIF-3 alpha 4 increases transcription by hypoxia-inducible genes. HIF3 alpha 4 itself is oxygen-regulated, suggesting a novel feedback mechanism of controlling HIF-1 activity. Furthermore, the expression of HIF-3 alpha 4 is dramatically down-regulated in the majority of primary renal carcinomas. These results demonstrate an important dominant-negative regulation of HIF-1-mediated gene transcription by HIF-3 alpha 4 in vivo and underscore its potential significance in renal epithelial oncogenesis.