Genistein inhibits radiation-induced activation of NF-κB in prostate cancer cells promoting apoptosis and G(2)/M cell cycle arrest

BACKGROUND: New cancer therapeutic strategies must be investigated that enhance prostate cancer treatment while minimizing associated toxicities. We have previously shown that genistein, the major isoflavone found in soy, enhanced prostate cancer radiotherapy in vitro and in vivo. In this study, we investigated the cellular and molecular interaction between genistein and radiation using PC-3 human prostate cancer cells. METHODS: Tumor cell survival and progression was determined by clonogenic analysis, flow cytometry, EMSA analysis of NF-κB, and western blot analysis of cyclin B1, p21(WAF1/Cip1), and cleaved PARP protein. RESULTS: Genistein combined with radiation caused greater inhibition in PC-3 colony formation compared to genistein or radiation alone. Treatment sequence of genistein followed by radiation and continuous exposure to genistein showed optimal effect. Cell cycle analysis demonstrated a significant dose- and time-dependent G(2)/M arrest induced by genistein and radiation that correlated with increased p21(WAF1/Cip1 )and decreased cyclin B1 expression. NF-κB activity was significantly decreased by genistein, yet increased by radiation. Radiation-induced activation of NF-κB activity was strongly inhibited by genistein pre-treatment. A significant and striking increase in cleaved PARP protein was measured following combined genistein and radiation treatment, indicating increased apoptosis. CONCLUSION: A mechanism of increased cell death by genistein and radiation is proposed to occur via inhibition of NF-κB, leading to altered expression of regulatory cell cycle proteins such as cyclin B and/or p21(WAF1/Cip1), thus promoting G(2)/M arrest and increased radiosensitivity. These findings support the important and novel strategy of combining genistein with radiation for the treatment of prostate cancer.