Mechanism of regulation of the hypoxia-inducible factor-1α by the von Hippel-Lindau tumor suppressor protein

In normoxic cells the hypoxia-inducible factor-1α (HIF-1α) is rapidly degraded by the ubiquitin-proteasome pathway, and activation of HIF-1α to a functional form requires protein stabilization. Here we show that the product of the von Hippel-Lindau (VHL) tumor suppressor gene mediated ubiquitylation and proteasomal degradation of HIF-1α under normoxic conditions via interaction with the core of the oxygen-dependent degradation domain of HIF-1α. The region of VHL mediating interaction with HIF-1α overlapped with a putative macromolecular binding site observed within the crystal structure of VHL. This motif of VHL also represents a mutational hotspot in tumors, and one of these mutations impaired interaction with HIF-1α and subsequent degradation. Interestingly, the VHL binding site within HIF-1α overlapped with one of the minimal transactivation domains. Protection of HIF-1α against degradation by VHL was a multistep mechanism, including hypoxia-induced nuclear translocation of HIF-1α and an intranuclear hypoxia-dependent signal. VHL was not released from HIF-1α during this process. Finally, stabilization of HIF-1α protein levels per se did not totally bypass the need of the hypoxic signal for generating the transactivation response.