Indomethacin-induced radiosensitization and inhibition of ionizing radiation-induced NF-kappa B activation in HeLa cells occur via a mechanism involving p38 MAP kinase

Although ionizing radiation (IR) activates multiple cellular factors that v ary depending on dose and tissue specificity, the activation of NF-kappaB a ppears to be a well-conserved response in tumor cells exposed to IR. Recent ly, it also has been demonstrated that nonsteroidal anti-inflammatory agent s inhibit tumor necrosis factor and interleukin-1-induced NF-kappaB activat ion and act as radiosensitizing agents. These observations reinforce the gr owing notion that NF-kappaB may be a protective cellular factor responding to the cytotoxicity of IR and other damaging stimuli. As such, we addressed tile idea and mechanism that NF-kappaB is a downstream target of the nonst eroidal anti-inflaminatory agent indomethacin and is involved in the proces s of radiosensitization. In this study, we report that indomethacin inhibit ed IR-induced activation of NF-kappaB and sensitized HeLa cells to IR-induc ed cytotoxicity at similar concentrations. Pretreatment of HeLa cells with SB 203580, a pyridinyl imidazole compound that specifically inhibits p38 mi togen-activated protein kinase (MAPK), abrogated the ability of indomethaci n to inhibit IR-induced activation of NF-kappaB and diminished the indometh acin radiosensitizing effect. In addition, the transient genetic activation of p38(MAPK) inhibited IR induction of NF-kappaB gene expression in the ab sence of indomethacin. Finally, permanently transfected cell lines genetica lly unable to activate NF-kappaB, because of expression of a dominant negat ive I-kappaB alpha gene, demonstrated increased sensitivity to IR-induced c ytotoxicity. Taken together, these results suggest that p38 MAPK is a targe t involved in indomethacin-induced radiosensitization and that NF-kappaB ma y be one downstream target in this process.