Reciprocal Regulation of Hypoxia-Inducible Factor 2α and GLI1 Expression Associated With the Radioresistance of Renal Cell Carcinoma

Purpose: Renal cell carcinoma (RCC) is often considered a radioresistant tumor, but the molecular mechanism underlying its radioresistance is poorly understood. This study explored the roles of hypoxia-inducible factor 2 alpha (HIF2 alpha) and sonic hedgehog (SHH)-GLI1 signaling in mediating the radioresistance of RCC cells and to unveil the interaction between these 2 signaling pathways. Methods and Materials: The activities of SHH-GLI1 signaling pathway under normoxia and hypoxia in RCC cells were examined by real-time polymerase chain reaction, Western blot, and luciferase reporter assay. The expression of HIF2 alpha and GLI1 in RCC patients was examined by immunohistochemistry, and their correlation was analyzed. Furthermore, RCC cells were treated with HIF2 alpha-specific shRNA(sh-HIF2 alpha), GLI1 inhibitor GANT61, or a combination to determine the effect of ionizing radiation (IR) on RCC cells based on clonogenic assay and double-strand break repair assay. Results: RCC cells exhibited elevated SHH-GLI1 activities under hypoxia, which was mediated by HIF2 alpha. Hypoxia induced GLI1 activation through SMO-independent pathways that could be ablated by PI3K inhibitor or MEK inhibitor. Remarkably, the SHH-GLI1 pathway also upregulated HIF2 alpha expression in normoxia. Apparently, there was a positive correlation between HIF2 alpha and GLI1 expression in RCC patients. The combination of sh-HIF2 alpha and GLI1 inhibitor significantly sensitized RCC cells to IR. Conclusions: Cross-talk between the HIF2 alpha and SHH-GLI1 pathways was demonstrated in RCC. Cotargeting these 2 pathways, significantly sensitizing RCC cells to IR, provides a novel strategy for RCC treatment. (C) 2014 Elsevier Inc.