Radiation-induced HIF-1α cell survival pathway is inhibited by soy isoflavones in prostate cancer cells

We previously showed that treatment of prostate cancer cells with soy isoflavones and radiation resulted in greater cell killing in vitro, and caused downregulation of NF-kappa B and APE1/Ref-1. APE1/Ref-1 functions as a redox activator of transcription factors, including NF-kappa B and HIF-1 alpha. These molecules are upregulated by radiation and implicated in radioresistance of cancer cells. We extended our studies to investigate the role of HIF-1a survival pathway and its upstream Sire and STAT3 molecules in isoflavones and radiation interaction. Radiation induced phosphorylation of Src and STAT3 leading to induction of HIF-1 alpha. Genistein, daidzein or a mixture of soy isoflavones did not activate this pathway. These data were observed both in PC-3 (AR-) and C4-2B (AR+) androgen-independent cell lines. Pretreatment with isoflavones inhibited Src/STAT3/HIF-1 alpha activation by radiation and nuclear translocation of HIF-1 alpha. These findings correlated with decreased expression of APE1/Ref-1 and DNA binding activity of HIF-1 alpha and NF-kappa B. In APEI/Ref-I cDNA transfected cells, radiation caused a greater increase in HIF-1 alpha and NF-kappa B activities but this effect was inhibited by pretreatment with soy prior to radiation. Transfection experiments indicate that APE1/Ref-1 inhibition by isoflavones impairs the radiation-induced transcription activity of NF-kappa B and HIF-1 alpha. This mechanism could result in the inhibition of genes essential for tumor growth and angiogenesis, as demonstrated by inhibition of VEGF production and HUVECs tube formation. Our novel findings suggest that the increased responsiveness to radiation mediated by soy isoflavones could be due to pleiotropic effects of isoflavones blocking cell survival pathways induced by radiation including Src/STAT3/HIF-1 alpha, APEI/Ref-I and NF-kappa B. (C) 2008 Wiley-Liss, Inc.