Nitric oxide and ionizing radiation synergistically promote apoptosis and growth inhibition of cancer by activating p53

Nitric oxide (NO) is a potent tumor radiosensitizer; however, its clinical use is limited by systemic side effects. We have demonstrated previously that gene transfer of the human inducible NO synthase (iNOS) gene into tumor cells and tumors induces high-output NO production that significantly enhances tumor radioresponsiveness, with no observed side effects. Notably, iNOS gene transfer enhances tumor radioresponsiveness via apoptotic cell death. Because NO and ionizing radiation are both known to promote p53-dependent apoptosis, we hypothesized that p53 activation might be a primary mechanism for the synergy of these two genotoxic stresses. We report that NO and ionizing radiation synergistically activate p53 in colorectal cancers grown in athymic mice by augmenting phosphorylation of p53 at serine 15. The effect of NO and ionizing radiation on tumor cell apoptosis and tumor radioresponsiveness is significantly reduced in p53 knockout isogenic cancer cell lines. Furthermore, the transfer of both p53 and iNOS genes into tumor cells lacking functional p53 enhanced their radioresponsiveness more than transfer of either gene alone.