Multiple splice variants of the human HIF-3α locus are targets of the von Hippel-Lindau E3 uhiquitin ligase complex

Functional inactivation of the von Hippel-Lindau (VHL) tumor suppressor protein is the cause of familial VHL disease and sporadic kidney cancer. The VHL gene product (pVHL) is a component of an E3 ubiquitin ligase complex that targets the hypoxia-inducible factor (HIF) 1 and 2 alpha subunits for polyubiquitylation. This process is dependent on the hydroxylation of conserved proline residues on the a subunits of HIF-1/2 in the presence of oxygen. In our effort to identify orphan HIF-like proteins in the data base that are potential targets of the pVHL complex, we report multiple splice variants of the human HIF-3alpha locus as follows: hHIF-3alpha1, hHIF-3alpha2 (also referred to as hIPAS; human inhibitory PAS domain protein), hHIF-3alpha3, hHIF-3alpha4, hHIF-3alpha5, and hHIF-3alpha6. We demonstrate that the common oxygen-dependent degradation domain of hHIF-3alpha1-3 splice variants is targeted for ubiquitylation by the pVHL complex an vitro and an vivo. This activity is enhanced in the presence of prolyl hydroxylase and is dependent on a proline residue at position 490. Furthermore, the ubiquitin conjugation occurs on lysine residues at position 465 and 568 within the oxygen-dependent degradation domain. These results demonstrate additional targets of the pVHL complex and suggest a growing complexity in the regulation of hypoxia-inducible genes by the HIF family of transcription factors.