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Prostaglandin E 2 (PGE 2 ) synthesis modulates the response to radiation injury in the mouse intestinal epithelium through effects on crypt survival and apoptosis; however, the downstream signaling events have not been elucidated. WT mice receiving 16,16-dimethyl PGE 2 (dmPGE 2 ) had fewer apoptotic cells per crypt than untreated mice. Apoptosis in Bax -/-mice receiving 12 Gy was approximately 50% less than in WT mice, and the ability of dmPGE 2 to attenuate apoptosis was lost in Bax -/-mice. Positional analysis revealed that apoptosis in the Bax -/-mice was diminished only in the bax-expressing cells of the lower crypts and that in WT mice, dmPGE 2 decreased apoptosis only in the bax-expressing cells. The HCT-116 intestinal cell line and Bax -/-HCT-116 recapitulated the apoptotic response of the mouse small intestine with regard to irradiation and dmPGE 2 . Irradiation of HCT-116 cells resulted in phosphorylation of AKT that was enhanced by dmPGE 2 through transactivation of the EGFR. Inhibition of AKT phosphorylation prevented the reduction of apoptosis by dmPGE 2 following radiation. Transfection of HCT-116 cells with a constitutively active AKT reduced apoptosis in irradiated cells to the same extent as in nontransfected cells treated with dmPGE 2 . Treatment with dmPGE 2 did not alter bax or bcl-x expression but suppressed bax translocation to the mitochondrial membrane. Our in vivo studies indicate that there are bax-dependent and bax-independent radiation-induced apoptosis in the intestine but that only the bax-dependent apoptosis is reduced by dmPGE 2 . The in vitro studies indicate that dmPGE 2 , most likely by signaling through the E prostaglandin receptor EP 2 , reduces radiation-induced apoptosis through transactivation of the EGFR and enhanced activation of AKT and that this results in reduced bax translocation to the mitochondria.

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