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

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-1 alpha and HIF-2 alpha. To investigate the HIF-1 alpha and HIF-2 alpha-dependent effect on VEGF gene induction in RCC, a panel of human RCC cell lines was analyzed. We found that a loss of HIF-1 alpha protein expression was a common event in RCC cell lines, which was associated not only with truncated HIF-1 alpha mRNA transcripts but also with transcriptional silencing. Since the CpG rich promoter region of the HIF-1 alpha 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-1 alpha defective cell lines, the knockdown of the HIF-2 alpha gene demonstrated that HIF-2 alpha regulated the VEGF production, irrespective of the VHL gene mutation status. In contrast, HIF-1 alpha played a predominant role in VEGF secretion in the cells expressing both wild-type HIF-1 alpha and HIF-2 alpha proteins. HIF-1 alpha may therefore represent an important target molecule for RCC therapy; however, HIF-2 alpha should be targeted in HIF-1 alpha defective renal cancer cells.