c-FLIP confers resistance to FAS-mediated apoptosis in anaplastic large-cell lymphoma

We hypothesized that inhibition of the FAS-mediated apoptosis pathway by FLICE-like inhibitory protein (c-FLIP) may contribute to oncogenesis in ALK ⁺ anaplastic large-cell lymphoma (ALCL). Treatment with increasing concentrations of CH-11 (CD95/FAS agonistic antibody) had no effect on cell viability of 2 ALK⁺ ALCL cell lines, Karpas 299 and SU-DHL1, each expressing high levels of c-FLIP. However, inhibition of endogenous c-FLIP expression by specific c-FLIP siRNA in Karpas 299 and SU-DHL1 cells treated with CH-11 resulted in FAS-mediated cell death associated with increased annexin V binding, apoptotic morphology, and cleavage of caspase-8. In 26 ALK⁺ ALCL tumors, assessed for expression of DISC-associated proteins, CD95/FAS and c-FLIP were commonly expressed, in 23 (92%) of 25 and 21 (91%) of 23 tumors, respectively. By contrast, CD95L/FASL was expressed in only 3 (12%) of 26 ALCL tumors, although it was strongly expressed by surrounding small reactive lymphocytes. Our findings suggest that overexpression of c-FLIP protects ALK⁺ ALCL cells from death-receptor-induced apoptosis and may contribute to ALCL pathogenesis.