Renal cancer cells lacking hypoxia inducible factor (HIF)-1alpha expression maintain vascular endothelial growth factor expression through HIF-2alpha.

Recent efforts have been aimed at targeting the hypoxia inducible factor (HIF)-mediated hypoxia-induced gene pathway for renal cell carcinomas (RCC) therapy. Among the various genes induced by HIF, vascular endothelial growth factor (VEGF) is one of the critical mediators in angiogenesis, tumor growth and metastasis. To date, however, limited information is available on the functional differences regarding VEGF transcription between the HIF subunits, namely HIF-1alpha and HIF-2alpha. To investigate the HIF-1alpha and HIF-2alpha-dependent effect on VEGF gene induction in RCC, a panel of human RCC cell lines was analyzed. We found that a loss of HIF-1alpha protein expression was a common event in RCC cell lines, which was associated not only with truncated HIF-1alpha mRNA transcripts but also with transcriptional silencing. Since the CpG rich promoter region of the HIF-1alpha gene contained a similar frequency of methylated CpG dinucleotides in RCC cell lines, a complex and non-uniform mechanism may be involved in this phenomenon. In these HIF-1alpha defective cell lines, the knockdown of the HIF-2alpha gene demonstrated that HIF-2alpha regulated the VEGF production, irrespective of the VHL gene mutation status. In contrast, HIF-1alpha played a predominant role in VEGF secretion in the cells expressing both wild-type HIF-1alpha and HIF-2alpha proteins. HIF-1alpha may therefore represent an important target molecule for RCC therapy; however, HIF-2alpha should be targeted in HIF-1alpha defective renal cancer cells.