The Fas-associated death domain protein suppresses activation of NF-κB by LPS and IL-1β

Activation of NF-kappaB by bacterial LPS promotes the upregulation of proinflammatory cytokines that contribute to the pathogenesis of Gram-negative septic shock. LPS activation of NF-kappaB is dependent upon the interaction of two death domain-containing (DD-containing) proteins, MyD88 and IL-1 receptor-associated kinase IRAK. Another DD-containing protein, Fas-associated death domain (FADD), also binds MyD88 through respective D-DD interactions. Although FADD has been classically described as a proapoptotic signaling molecule, several reports have implicated a role for FADD in mediating NF-kappaB activation. In the present report, we investigated whether FADD could mediate LPS activation of NF-kappaB. Overexpression of FADD blocked LPS-induced NF-kappaB activation, whereas absence of FADD enhanced activation of NF-kappaB by LPS. Further, LPS-induced expression of two NF-kappaB-dependent gene products, IL-6 and KC, was enhanced in FADD(-/-) mouse embryo fibroblasts (MEFs) compared with wild-type. This increase in NF-kappaB activity correlated with enhanced IkappaB degradation. FADD(-/-) MEFs were also resistant to NF-kappa,B activation induced by IL-1 P. Finally, reconstitution of full-length FADD in the FADD(-/-) MEFs completely reversed the enhanced activation of NF-kappaB elicited by either LPS or IL-1beta. Together, these data indicate that FADD negatively regulates LPS- and IL-1beta-induced NF-kappaB activation and that this regulation occurs upstream of IkappaB degradation.