Ionizing radiation up-regulates cyclooxygenase-2 in I407 cells through p38 mitogen-activated protein kinase

The epithelial cell line I407 up-regulates cyclooxygenase-2 (COX-2) mRNA and protein expression following ionizing radiation exposure. Prostaglandin E₂ (PGE₂) production is concomitantly up-regulated. Irradiation of I407 cells also results in phosphorylation of the p38 mitogen-activated protein kinase and the p38 inhibitor SB203580 abrogates radiation-induced PGE₂ synthesis. Wild-type p38α (p38αWT) and dominant-negative p38α (p38αDN) stable-transfectant clones of I407 cells were used to examine the role of the p38 mitogen-activated protein kinase pathway in the events controlling PGE₂ synthesis after ionizing radiation. Treatment of p38αWT clones with γ-radiation resulted in increased COX-2 protein levels and PGE₂ synthesis similar to treated control-transfected cells. In contrast, the p38αDN clones failed to up-regulate COX-2 protein or increase PGE₂ synthesis when irradiated. Exogenous arachidonate did not restore PGE₂ synthesis by p38αDN cells. Radiation increased COX-2 mRNA stability and the p38 inhibitor SB203580 attenuated COX-2 mRNA stability in irradiated I407 cells. In contrast, irradiation had no effect on transcription from a COX-2 promoter/luciferase reporter plasmid in the presence or absence of SB203580. The data demonstrate a crucial role for p38α in COX-2 expression and PGE₂ synthesis in an irradiated transformed epithelial cell line. Furthermore, they indicate that p38 activity is required at a step distal to arachidonate release, most probably COX-2 up-regulation, since exogenous arachidonate did not restore PGE₂ synthesis.