Hypoxia upregulates von Hippel-Lindau tumor-suppressor protein through RhoA-dependent activity in renal cell carcinoma

Abstract

A key task for the multifunctional von Hippel-Lindau protein (pVHL) is regulation of the activity of hypoxia-inducible factor-1alpha (HIF-1alpha) by targeting it to the proteasome for degradation under normoxia. pVHL binding to HIF-1alpha is lost under low O2 tension, leading to transcription of several genes involved in the hypoxia response. However, regulation of pVHL by hypoxia remains to be investigated. We evaluated the effects of hypoxia on pVHL expression in carcinoma and endothelial cells. We showed that hypoxia stimulates pVHL levels (2.5-fold) in renal Caki-1 cells expressing wild-type VHL (VHL+/+). This upregulation was independent of VHL status, because hypoxia also increased pVHL expression in renal 786-O cells carrying mutated VHL (VHL-/-). Hypoxia did not affect pVHL expression in endothelial cells. Hypoxia-induced pVHL in Caki-1 cells was RhoA dependent, because inhibition by exotoxin C3 prevented pVHL stimulation. Furthermore, inhibition of Rho kinase by Y-27632 blocked pVHL induction by hypoxia. During normoxia, pVHL expression was also induced in cells transfected with dominant-active RhoA. Furthermore, disruption of actin organization by chemical agents or by hypoxia stimulated pVHL expression in kidney cells. On the other hand, inhibition of MAP kinases p38 and JNK, but not MAP kinase kinase (MEK1/2), reduced pVHL upregulation by 30 and 72%, respectively, during hypoxia, supporting a significant role for these signaling pathways. Expression and phosphorylation of c-Jun were stimulated in cells transfected with dominant-active RhoA. Together, these findings demonstrate that hypoxia induces pVHL expression in renal cancer cells, and this induction is mediated by RhoA-dependent pathways.