Abstract 3512: Regulation of hypoxia inducible factor alpha in clear cell renal cell carcinoma

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Abstract Clear cell renal cell carcinoma (CCRC) is the most common kidney cancer with high incidence of hypoxia-inducible factor 1 and 2 alpha (HIF-α) expression as a consequence of the nonfunctional Von Hippel-Lindau (VHL). The purpose of the study is: a) to investigate the expression levels and relationship between HIF-α and prolyl hydroxylases (PHDs), the negative regulators of HIF-α in tissue microarray (TMA) of human surgical specimens. b) to demonstrate whether methylselenenic acid (MSA) would degrade HIF-α, independent of VHL. c) to determine whether inhibition of HIF-α would enhance cytotoxicity of docetaxel.The results of immunohistochemical evaluation revealed that 92% (81 out of 88) combined incidence of both HIF-1α and HIF-2α with low levels of PHD2 (35%, 31 out of 88) and completely deficient with PHD3, the enzymes responsible for the hydroxylation of HIF-α and subsequent degradation in proteasome. In contrast, we found high incidence of PHD2 positive cases with low HIF-α in both head and neck (86% PHD2 with 38% HIF-α, n=210) and colorectal (90% PHD2 with 26% HIF-α, n=64) indicate high incidence of PHD2 is associated with low incidence of HIF-α. The expression of PHD3 was found in both head & neck (21%) and colorectal (50%) cancer specimens which have low HIF-α incidence compared to CCRC. These data indicate that there is an inverse relationship between HIF-α and PHD2 in CCRC clinical samples in addition to VHL deficiency. In agreement with the surgical specimens, CCRC cells RC2 and 786- O express PHD2 protein but not PHD3 with constitutive expression of HIF-1α in RC2 and HIF-2α in 786 O. We have demonstrated the degradation of both HIF-1α and HIF-2α by MSA that leads to the significant reduction of HIF-α transcriptionally regulated gene VEGF in RC2 and 786-O. MSA exhibits unique property of degrading both HIF-1α and HIF-2α that has not been reported earlier by any other agent. Proteasome inhibitor MG132 reversed the HIF-1α degradation by MSA indicates HIF-1α degradation is proteasome dependent and VHL independent since VHL is nonfunctional in CCRC. Based on these findings we hypothesize that the degradation of HIF-α by MSA is VHL independent. We found RC2 cells with constitutive HIF-1α are 5 fold resistant to docetaxel compared to RC2-VHL cells with no detectable HIF-1α due to overexpression of VHL. As we expected, the combination of MSA enhanced the docetaxel efficacy only in RC2 cells, but not in RC2-VHL shows that the effect of MSA is through HIF-1α. These findings demonstrated the therapeutic potential of selenium for the treatment of CCRC. Thus, the unique molecular properties of CCRC being VHL deficient, express high constitutive HIF-1α and HIF-2α protein with low PHD2 and deficient in PHD3 could provide the rational for the development and evaluation of selenium in combination with the clinically approved drugs for kidney cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3512. doi:10.1158/1538-7445.AM2011-3512