Downregulation of integrins by von Hippel-Lindau (VHL) tumor suppressor protein is independent of VHL-directed hypoxia-inducible factor alpha degradation

Inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene occurs in the majority of clear-cell renal cell carcinomas (RCCs). It was previously shown that VHL decreased the abundance of integrins α2, α5, and β1, which is consistent with VHL-associated changes in cell–cell and cell – extracellular matrix adhesions. We investigated the mechanism by which VHL downregulates integrins. Although VHL can target hypoxia-inducible factor alpha (HIFα) subunits for degradation, VHL-dependent reduction of integrins was independent of O2 concentration and HIFα levels. VHL reduced the half-lives of integrins, and this activity was blocked by proteasomal inhibition. Although ectopically expressed FLAG-VHL retained HIFα degradation activity, it neither downregulated integrins nor promoted adherens and tight intercellular junctions, in contrast to expressed wild-type VHL. Moreover, integrins co-immunoprecipitated with wild-type VHL, but not FLAG-VHL. These data indicate that the downregulation of integrins by VHL is distinct from the regulation of HIFα subunits by VHL, and suggests that the loss of this activity contributes to VHL-associated RCC development through disruption of adherens and tight junctions.

Cancer-Causing Mutations in a Novel Transcription-Dependent Nuclear Export Motif of VHL Abrogate Oxygen-Dependent Degradation of Hypoxia-Inducible Factor

ABSTRACT It is thought that degradation of nuclear proteins by the ubiquitylation system requires nuclear-cytoplasmic trafficking of E3 ubiquitin ligases. The von Hippel-Lindau (VHL) tumor suppressor protein is the substrate recognition component of a Cullin-2-containing E3 ubiquitin ligase that recruits hypoxia-inducible factor (HIF) for oxygen-dependent degradation. We demonstrated that VHL engages in nuclear-cytoplasmic trafficking that requires ongoing transcription to promote efficient HIF degradation. Here, we report the identification of a discreet motif, DXGX2DX2L, that directs transcription-dependent nuclear export of VHL and which is targeted by naturally occurring mutations associated with renal carcinoma and polycythemia in humans. The DXGX2DX2L motif is also found in other proteins, including poly(A)-binding protein 1, to direct its transcription-dependent nuclear export. We define DXGX2DX2L as TD-NEM (transcription-dependent nuclear export motif), since inhibition of transcription by actinomycin D or 5,6-dichlorobenzimidazole abrogates its nuclear export activity. Disease-causing mutations of key residues of TD-NEM restrain the ability of VHL to efficiently mediate oxygen-dependent degradation of HIF by altering its nuclear export dynamics without affecting interaction with its substrate. These results identify a novel nuclear export motif, further highlight the role of nuclear-cytoplasmic shuttling of E3 ligases in degradation of nuclear substrates, and provide evidence that disease-causing mutations can target subcellular trafficking.