Caspase inhibition prolongs inflammation by promoting a signaling complex with activated RIPK1

Activation of inflammation by lipopolysaccharide (LPS) is an important innate immune response. Here we investigated the
contribution of caspases to the LPS-mediated inflammatory response and discovered distinctive temporal roles of RIPK1 in
mediating proinflammatory cytokine production when caspases are inhibited. We propose a biphasic model that
differentiates the role of RIPK1 in early versus late phase. The early production of proinflammation cytokines stimulated by
LPS with caspase inhibition is mediated by the NF-κB pathway that requires the scaffold function of RIPK1 but is kinase
independent. Autocrine production of TNFα in the late phase promotes the formation of a novel TNFR1-associated complex
with activated RIPK1, FADD, caspase-8, and key mediators of NF-κB signaling. The production of proinflammatory cytokines in
the late phase can be blocked by RIPK1 kinase inhibitor Nec-1s. Our study demonstrates a mechanism by which the activation of
RIPK1 promotes its own scaffold function to regulate the NF-κB–mediated proinflammatory cytokine production that is
negatively regulated by caspases to restrain inflammatory signaling.