Mechanism of regulation of the hypoxia-inducible factor-l alpha by the vonHippel-Lindau tumor suppressor protein

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