TWEAK induces TNFα and drives TNFR1-dependent apoptosis signaling. A, cells were transfected with TNFR1 siRNA for 48 h, stimulated with TWEAK (3 μg/ml, 6 h), and association of RIP1 and caspase-8 was analyzed by caspase-8 IP followed by immunoblot analysis as indicated. B, cells were transfected with TNFR1 siRNA and treated with TWEAK as in A, and assembly of the FN14-TRAF2-cIAP1 complex was examined by FN14 IP followed by immunoblot analysis as indicated. C–E, cells were transfected with TNFR1 siRNA, treated with TWEAK as in A, and analyzed for viability (C), caspase-3/7 activity (D), and MOMP (E). In B, IB analysis shows siRNA knockdown levels. For the IP experiments, zVAD-FMK (20 μm) was added 1 h prior to incubation with TWEAK. F, HSC3 cells were treated with TWEAK (3 μg/ml) for 6 h in the presence of zVAD-FMK (20 μm) and TNFR1:FC (100 μg/ml). Cell lysates were immunoprecipitated with caspase-8 followed by IB analysis of RIP1, FADD, and caspase-8.