Inactivation of nuclear factor kappaB by soy isoflavone genistein contributes to increased apoptosis induced by chemotherapeutic agents in human cancer cells.

Cancer chemotherapeutic strategies commonly require multiple agents. However, use of multiple agents contributes to added toxicity resulting in poor treatment outcome. Thus, combination chemotherapy must be optimized to increase tumor response and at the same time lower its toxicity. Chemotherapeutic agents are known to induce nuclear factor kappaB (NF-kappaB) activity in tumor cells, resulting in lower cell killing and drug resistance. In contrast, genistein has been shown to inhibit the activity of NF-kappaB and the growth of various cancer cells without causing systemic toxicity. We therefore investigated whether the inactivation of NF-kappaB by genistein before treatment of various cancer cells with chemotherapeutic agents could lead to better tumor cell killing as tested by in vitro studies using gene transfections and also by animal studies. PC-3 (prostate), MDA-MB-231 (breast), H460 (lung), and BxPC-3 (pancreas) cancer cells were pretreated with 15 to 30 micromol/L genistein for 24 hours and then exposed to low doses of chemotherapeutic agents for an additional 48 to 72 hours. We found that 15 to 30 micromol/L genistein combined with 100 to 500 nmol/L cisplatin, 0.5 to 2 nmol/L docetaxel, or 50 ng/mL doxorubicin resulted in significantly greater inhibition of cell growth and induction of apoptosis compared with either agent alone. Moreover, we found that the NF-kappaB activity was significantly increased within 2 hours of cisplatin and docetaxel treatment and that the NF-kappaB inducing activity of these agents was completely abrogated in cells pretreated with genistein. These results were also supported, for the first time, by animal experiments, p65 cDNA transfection and p65 small interfering RNA studies, which clearly showed that a specific target (NF-kappaB) was affected in vivo. Collectively, our results clearly suggest that genistein pretreatment inactivates NF-kappaB and may contribute to increased growth inhibition and apoptosis induced by cisplatin, docetaxel, and doxorubicin in prostate, breast, lung, and pancreatic cancer cells. Theses results warrant carefully designed clinical studies investigating the combination of soy isoflavones and commonly used chemotherapeutic agents for the treatment of human cancers.