FADD Negatively Regulates Lipopolysaccharide Signaling by Impairing Interleukin-1 Receptor-Associated Kinase 1-MyD88 Interaction

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Abstract
Lipopolysaccharide (LPS) engages Toll-like receptor 4 (TLR4) on various cells to initiate inflammatory and angiogenic pathways. FADD is an adaptor protein involved in death receptor-mediated apoptosis. Here we report a role for FADD in regulation of TLR4 signals in endothelial cells. FADD specifically attenuates LPS-induced activation of c-Jun NH(2)-terminal kinase and phosphatidylinositol 3'-kinase in a death domain-dependent manner. In contrast, FADD-null cells show hyperactivation of these kinases. Examining physical associations of endogenous proteins, we show that FADD interacts with interleukin-1 receptor-associated kinase 1 (IRAK1) and MyD88. LPS stimulation increases IRAK1-FADD interaction and recruitment of the IRAK1-FADD complex to activated MyD88. IRAK1 is required for FADD-MyD88 interaction, as FADD does not associate with MyD88 in IRAK1-null cells. By shuttling FADD to MyD88, IRAK1 provides a mechanism for controlled and limited activation of the TLR4 signaling pathway. Functionally, enforced FADD expression inhibited LPS- but not vascular endothelial growth factor-induced endothelial cell sprouting, while FADD deficiency led to enhanced production of proinflammatory cytokines induced by stimulation of TLR4 and TLR2, but not TLR3. Reconstitution of FADD reversed the enhanced production of proinflammatory cytokines. Thus, FADD is a physiological negative regulator of IRAK1/MyD88-dependent responses in innate immune signaling.