HIF (hypoxia-inducible factor)-3α is the third member of the HIF transcription factor family. Whereas HIF-1α and -2α play critical roles in the cellular and systemic adaptation to hypoxia, little is known about the regulation and function of HIF-3α. At least five different splice variants may be expressed from the human HIF-3α locus that are suggested to exert primarily negative regulatory effects on hypoxic gene induction. In the present paper, we report that hypoxia induces the human HIF-3α gene at the transcriptional level in a HIF-1-dependent manner. HIF-3α2 and HIF-3α4 transcripts, the HIF-3α splice variants expressed in Caki-1 renal carcinoma cells, rapidly increased after exposure to hypoxia or chemical hypoxia mimetics. siRNA (small interfering RNA)-mediated HIF-α knockdown demonstrated that HIF-3α is a specific target gene of HIF-1α, but is not affected by HIF-2α knockdown. In contrast with HIF-1α and HIF-2α, HIF-3α is not regulated at the level of protein stability. HIF-3α protein could be detected under normoxia in the cytoplasm and nuclei, but increased under hypoxic conditions. Promoter analyses and chromatin immunoprecipitation experiments localized a functional hypoxia-responsive element 5′ to the transcriptional start of HIF-3α2. siRNA-mediated knockdown of HIF-3α increased transactivation of a HIF-driven reporter construct and mRNA expression of lysyl oxidase. Immunohistochemistry revealed an overlap of HIF-1α-positive and HIF-3α-positive areas in human renal cell carcinomas. These findings shed light on a novel aspect of HIF-3α as a HIF-1 target gene and point to a possible role as a modulator of hypoxic gene induction.

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