TRB3 augments IL1β-TLR4 signaling by engaging Flightless-homolog 1.

Signaling via IL1{beta} and TLR4 receptors (IL1R-TLR4) plays a crucial role in cytokine and fatty acid-induced beta cell inflammation, in type 1 and type 2 diabetes respectively. IL1R-TLR4 share signaling mechanisms via a common, cytoplasmic, toll-like-receptor domain to activate proinflammatory JNK and IKK kinases. We have previously reported that in response to IL1{beta}, pancreatic islets isolated from TRB3 knockout (TRB3KO) mice show attenuated kinetics of activation for MAP3K MLK3, and JNK stress kinases. Here we report that similar to MLK3 and JNK, TRB3KO islets also show a decrease in amplitude and duration of IL1{beta}/LPS-stimulated TAK1 and IKK phosphorylation. Thus, loss of TRB3 attenuates both pathways critically required for a full-blown, cytokine-inducible, proapoptotic response in beta cells. TRB3KO islets display a sharp decrease in cytokine-induced beta cell death, accompanied by a decrease in select downstream NFkB targets, most notably, inducible Nitric Oxide Synthase (iNOS/NOS2), a well-characterized mediator of beta cell dysfunction and death. In order to better understand the molecular basis of TRB3-enhanced IL1R-TLR4 signaling, we interrogated the TRB3 interactome and identified Flightless-homolog 1 (Fli1), an immunomodulatory, actin-binding, leucine-rich-repeat protein, as a novel TRB3-interaction factor. TRB3 binds and disrupts Fli1-dependent sequestration of MyD88, thereby increasing availability of this proximal adaptor to participate in IL1R-TLR4 signaling. Fli1 forms a multiprotein complex that can disconnect IL1R-TLR4 from MyD88, resulting in a brake on assembly of downstream signaling complexes. By interacting with Fli1, TRB3 lifts the brake on IL1R-TLR4 signaling to augment the proinflammatory response in beta cells.