Expression of the cellular FLICE-inhibitory protein (c-FLIP) protects Hodgkin's lymphoma cells from autonomous Fas-mediated death

Hodgkin's lymphoma (HL) is characterized by the presence of malignant so-called Hodgkin's/Reed-Sternberg (HRS) cells, which display resistance to certain apoptotic stimuli, including a lack of sensitivity to Fas-mediated cell death. However, the mechanisms responsible for their resistance to apoptosis inducers have not been elucidated. Here we confirm that both HL-derived cell lines and the HRS cells of primary HL tissues express Fas ligand (FasL) along with the inhibitory c-FLIP protein. Down-regulation of cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (c-FLIP) through the use of specific small inhibitory RNAs (siRNAs) leads to reduced viability of the L428 and LS91 HL-derived cell lines. To determine whether endogenous FasL was responsible for the reduction in cell viability observed after down-regulation of c-FLIP, L428 and LS91 cells were treated with c-FLIP-specific siRNAs with and without siRNAs directed to FasL. Treatment of these cells with both c-FLIP- and FasL-specific siRNAs in combination restored cell viability to near control levels. Our results provide a mechanism whereby HRS cells are protected from autonomous FasL-mediated cell death while preserving their ability to evade immunosurveillance. Targeting c-FLIP could provide a novel approach to the treatment of HL.