The human HIF (hypoxia-inducible factor)-3α gene is a HIF-1 target gene and may modulate hypoxic gene induction

HIF (hypoxia-inducible factor)-3 alpha is the third member of the HIF transcription factor family. Whereas HIF-1 alpha and -2 alpha play critical roles in the cellular and systemic adaptation to hypoxia, little is known about the regulation and function of HIF-3 alpha. At least five different splice variants may be expressed from the human HIF-3 alpha locus that tire suggested to exert primarily negative regulatory effects on hypoxic gene induction. In the present paper, we report that hypoxia induces the human HIF-3 alpha gene at the transcriptional level in a HIF-1-dependent manner. HIF-3 alpha 2 and HIF-3 alpha 4 transcripts, the HIF-3 alpha 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-alpha knockdown demonstrated that HIF-3 alpha is a specific target gene of HIF-1 alpha, but is not affected by HIF-2 alpha knockdown. In contrast with HIF-1 alpha and HIF-2 alpha, HIF-3 alpha is not regulated at the level of protein stability. HIF-3 alpha 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 HTF-3 alpha 2. siRNA-mediated knockdown of HIF-3 alpha increased transactivation of a HIF-driven reporter construct and mRNA expression of lysyl oxidase. Immunohistochemistry revealed an overlap of HIF-1 alpha-positive and HIF-3 alpha-positive areas in human renal cell carcinomas. These findings shed light on a novel aspect of HIF-3 alpha as a HIF-1 target gene and point to a possible role as a modulator of hypoxic gene induction.