c-FLIP: A Key Regulator of Colorectal Cancer Cell Death

c-FLIP is an inhibitor of apoptosis mediated by the death receptors Fas, DR4, and DR5 and is expressed as long (c-FLIP L ) and short (c-FLIP S ) splice forms. We found that small interfering RNA (siRNA)-mediated silencing of c-FLIP induced spontaneous apoptosis in a panel of p53 wild-type, mutant, and null colorectal cancer cell lines and that this apoptosis was mediated by caspase-8 and Fas-associated death domain. Further analyses indicated the involvement of DR5 and/or Fas (but not DR4) in regulating apoptosis induced by c-FLIP siRNA. Interestingly, these effects were not dependent on activation of DR5 or Fas by their ligands tumor necrosis factor–related apoptosis-inducing ligand and FasL. Overexpression of c-FLIP L , but not c-FLIP S , significantly decreased spontaneous and chemotherapy-induced apoptosis in HCT116 cells. Further analyses with splice form–specific siRNAs indicated that c-FLIP L was the more important splice form in regulating apoptosis in HCT116, H630, and LoVo cells, although specific knockdown of c-FLIP S induced more apoptosis in the HT29 cell line. Importantly, intratumoral delivery of c-FLIP–targeted siRNA duplexes induced apoptosis and inhibited the growth of HCT116 xenografts in BALB/c severe combined immunodeficient mice. In addition, the growth of c-FLIP L –overexpressing colorectal cancer xenografts was more rapid than control xenografts, an effect that was significantly enhanced in the presence of chemotherapy. These results indicate that c-FLIP inhibits spontaneous death ligand–independent, death receptor–mediated apoptosis in colorectal cancer cells and that targeting c-FLIP may have therapeutic potential for the treatment of colorectal cancer. [Cancer Res 2007;67(12):5754–62]