Failure of p53 to repress c-FLIP under hypoxic conditions contributes to TRAIL resistance of cancer cells    Free

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Cellular FLICE-like inhibitory protein (c-FLIP) is a well established antiapoptotic regulator of the Fas and TRAIL death-receptor signaling pathways. Overexpression of c-FLIP promotes resistance of cancer cells to TRAIL therapy. Here we show that c-FLIP is repressed in TRAIL-sensitive cancer cells cultured under hypoxic conditions similar to what is found in the tumor microenvironment. Further we show that repression of c-FLIP under hypoxic conditions is p53-dependent. Cells in the absence of p53 show higher basal levels of c-FLIP and fail to repress c-FLIP when exposed to hypoxia. Previously we have reported that HCT116 colon carcinoma cells null for p53 become more resistant to the effects of TRAIL under hypoxic conditions (0.2% O₂) while HCT116 cells wild-type for p53 do not (Mayes et. al. Cancer. Biol. Ther. 4:1068-1074, 2005). Here we show that p53-dependent repression of FLIP does not occur in HCT116 p53-null cells under hypoxic conditions, and that this may contribute to the hypoxia-induced resistance of these cells. Further we show that H460 lung carcinoma cells which are wild-type for p53 are dramatically sensitized to the effects of TRAIL under hypoxic conditions and that p53-dependent repression of FLIP correlates with TRAIL sensitization. This study is the first to show p53-dependent transcriptional repression of c-FLIP. Further, these findings show the importance of p53-status as well as p53-dependent repression of c-FLIP in determining cancer cell TRAIL-sensitivity under hypoxic conditions. This study has implications in guiding the use of TRAIL in the in vivo setting. In tumors with wild-type p53, hypoxic tumor cell fractions resistant to many therapies may in fact be sensitive to TRAIL therapy. Further, loss of wild-type p53 and p53-dependent repression of FLIP may be a mechanism by which cancer cells escape immune surveillance during tumorigenesis. This study demonstrates the importance of accounting for physiologic factors within the tumor microenvironment, such as hypoxia, when developing new agents such as TRAIL for cancer therapy.