TWEAK induces apoptosis through a death-signaling complex comprising receptor-interacting protein 1 (RIP1), Fas-associated death domain (FADD), and caspase-8

The tumor necrosis factor (TNF) superfamily member TNF-like weak inducer of Apoptosis (TNFSF12, CD255) (TWEAK) can stimulate Apoptosis in certain Cancer cells. Previous studies suggest that TWEAK activates cell death indirectly, by inducing TNFα-mediated autocrine signals. However, the underlying death-signaling mechanism has not been directly defined. Consistent with earlier work, TWEAK assembled a proximal signaling complex containing its cognate receptor FN14, the adaptor TRAF2, and cellular inhibitor of Apoptosis protein 1 (cIAP1). Neither the death domain adaptor Fas-associated death domain nor the apoptosis-initiating protease Caspase-8 associated with this primary complex. Rather, TWEAK induced TNFα secretion and TNF Receptor 1-dependent assembly of a death-signaling complex containing receptor-interacting protein 1 (RIP1), FADD, and Caspase-8. Knockdown of RIP1 by siRNA prevented TWEAK-induced association of FADD with Caspase-8 but not formation of the FN14-TRAF2-cIAP1 complex and inhibited Apoptosis activation. Depletion of the RIP1 E3 ubiquitin Ligase cIAP1 enhanced assembly of the RIP1-FADD-caspase-8 complex and augmented cell death. Conversely, knockdown of the RIP1 Deubiquitinase CYLD inhibited these functions. Depletion of FADD, Caspase-8, BID, or Bax and Bak but not RIP3 attenuated TWEAK-induced cell death. Pharmacologic inhibition of the NF-κB pathway or siRNA knockdown of RelA attenuated TWEAK induction of TNFα and association of RIP1 with FADD and Caspase-8. These results suggest that TWEAK triggers Apoptosis by promoting assembly of a RIP1-FADD-caspse-8 complex via autocrine TNFα-TNFR1 signaling. The proapoptotic activity of TWEAK is modulated by cIAP1 and CYLD and engages both the extrinsic and intrinsic signaling pathways.