Hypoxia-Inducible Factor-2α Correlates to Distant Recurrence and Poor Outcome in Invasive Breast Cancer

Differential regulation as well as target gene specificity of the two hypoxia-inducible factor (HIF)-alpha subunits HIF-1 alpha and HIF-2 alpha in various tumors and cell lines have been suggested. In breast cancer, the prognostic significance of HIF-1 alpha is not clear-cut and that of HIF-2 alpha is largely unknown. Using IHC analyses of HIF-1 alpha, HIF-2 alpha, and vascular endothelial growth factor (VEGF) expression in a tissue microarray of invasive breast cancer specimens from 512 patients, we investigated the expression patterns of the 2 HIF-alpha subunits in relation to established clinicopathologic variables, VEGF expression, and survival. HIF-1 alpha and HIF-2 alpha protein levels and their effect on survival were additionally analyzed in a second cohort of 179 patients. To evaluate the individual role of each subunit in the hypoxic response and induction of VEGF, HIF-alpha protein and HIF-alpha and VEGF mRNA levels were further studied in cultured breast cancer cells after hypoxic induction and/or knockdown of HIF-alpha subunits by siRNA by Western blot and Quantitative Real-Time PCR techniques. We showed that although HIF-1 alpha and HIF-2 alpha protein levels in breast cancer specimens were not interrelated, high levels of both HIF-1 alpha and HIF-2 alpha associated to high VEGF expression. HIF-2 alpha expression was an independent prognostic factor associated to reduced recurrence-free and breast cancer-specific survival, whereas HIF-1 alpha did not exhibit these correlations. In cultured cells, acute hypoxia induced both HIF-proteins. At prolonged hypoxia, HIF-2 alpha remained accumulated, whereas HIF-1 alpha protein levels decreased, in agreement with the oxygen level and time-dependent induction of HIFs recently reported in neuroblastoma. [Cancer Res 2008;68(22):9212-20]