Up-regulation of hypoxia-inducible factors HIF-1alpha and HIF-2alpha under normoxic conditions in renal carcinoma cells by von Hippel-Lindau tumor suppressor gene loss of function

Abstract

Hypoxia induces transcription of a range of physiologically important genes including erythropoietin and vascular endothelial growth factor. The transcriptional activation is mediated by the hypoxia-inducible factor-1 (HIF-1), a heterodimeric member of the basic helix-loop-helix PAS family, composed of alpha and beta subunits. HIF-1alpha shares 48 per cent identity with the recently identified HIF-2alpha protein that is also stimulated by hypoxia. In a previous study of hemangioblastomas, the most frequent manifestation of hereditary von Hippel-Lindau disease (VHL), we found elevated levels of vascular endothelial growth factor and HIF-2alpha mRNA in stromal cells of the tumors. Mutations of the VHL tumor suppressor gene are associated with a variety of tumors such as renal clear cell carcinomas (RCC). In this study, we analysed the expression of the hypoxia-inducible factors HIF-1alpha and HIF-2alpha in a range of VHL wildtype and VHL deficient RCC cell lines. In the presence of functional VHL protein, HIF-1alpha mRNA levels are elevated, whereas HIF-2alpha mRNA expression is increased only in cells lacking a functional VHL gene product. On the protein levels, however, in VHL deficient cell lines, both HIF-alpha subunits are constitutively expressed, whereas re-introduction of a functional VHL gene restores the instability of HIF-1alpha and HIF-2alpha proteins under normoxic conditions. Moreover, immunohistochemical analyses of RCCs and hemangioblastomas demonstrate up-regulation of HIF-1alpha and HIF-2alpha in the tumor cells. The data presented here provide evidence for a role of the VHL protein in regulation of angiogenesis and erythropoiesis mediated by the HIF-1alpha and HIF-2alpha proteins.