Regular paper TNFα-induced activation of NFκB protects against UV-induced apoptosis specifically in p53-proficient cells

The signaling pathways that depend on p53 or NFκB transcription factors are essential components of cellular responses to stress. In general, p53 is involved in either activation of cell cycle arrest or induction of apoptosis, while NFκB exerts mostly anti-apoptotic functions; both regulatory pathways apparently interfere with each other. Here we aimed to analyze the effects of NFκB activation on DNA damage-induced apoptosis, either p53-dependent or p53-independent, in a set of human cell lines. Four cell lines, HCT116 and RKO colon carcinoma, NCI-H1299 lung carcinoma and HL60 myeloblastoma, each of them in two congenic variants either containing or lacking transcriptionally competent p53, were used. Cells were incubated with TNFα cytokine to activate NFκB and then treated with ultraviolet or ionizing radiation to induce apoptosis, which was assessed by measurement of the sub-G1 cell fraction. We observed that treatment with TNFα resulted in a significant reduction in the frequency of apoptotic cells in UV-irradiated p53-proficient lines (with exception of the UV-resistant NCI-H1299 cells). This anti-apoptotic effect was lost when cells were pretreated with parthenolide, an inhibitor of NFκB activation. In marked contrast, TNFα-pretreatment of p53-deficient lines resulted in an increased frequency of apoptotic cells after UV irradiation (with exception of HL60 cells). Such antiand pro-apoptotic influence of TNFα was less obvious in cells treated with ionizing radiation. The data clearly indicates functional interference of both signaling pathways upon the damage-induced apoptotic response, yet the observed effects are both cell typeand stimulus-specific.