Prostaglandin mediates resistance to γ-irradiation induced apoptosis in colon cancer cells

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Background: Although it is recognized that prostaglandin production promotes tumorigenesis in many tumors, including colorectal cancers, the exact mechanisms by which prostaglandins promote tumorigenesis is not well understood. Aim: We designed this study to determine if a prostaglandin E2 analog, dmPGE2, protects the human colon cancer cell line HCT-116 from irradiation-induced apoptosis. Furthermore we attempted to define the transduction signal pathways mediating the pro-survival effects of PGE2. Methods: HCT-116 cells, Bax+/− or Bax −/− were irradiated or not with a dose of 6Gy in the presence and absence of dmPGE2. Twenty-four hours later, floating and adherent cells were harvested, processed for (i) flow cytometry analysis of apoptosis after annexin V-propidium iodide staining, or (ii) for cytosol/mitochondria fractionation before western blotting with Bax antibody. The contribution of MEK, p38 MAPK and PI3K kinase-pathways to the ability of PGE2 to promote apoptosis-resistance from γ-irradiation was studied by using pharmacological inhibitors and cells transiently transfected with an AKT-siRNA or stably transfected with a plasmid containing constitutively active AKT. Results: dmPGE2 inhibited completely apoptosis induced by γ-irradiation. Western blot analysis showed that dmPGE2 inhibited Bax translocation from the cytosol to mitochondria. Furthermore activation of the PI3K/AKT pathway, but not the MEK or p38 MAPK pathways was indispensable to the protective effects of dmPGE2 on radiation-induced apoptosis in HCT-116. Conclusion: Prostaglandin E2 protects HCT-116 cells from radiation-induced apoptosis by inducing AKT phosphorylation which in turn inhibits Bax translocation from cytosol to mitochondria. Therefore, we speculate that prostaglandin E2 overproduction by tumor cells, allows them to resist to programmed cell death in part by inhibiting Bax mediated apoptosis.