Recent studies have described that macrophages undergo mitochondrial changes after stimulation of Toll-like receptors (TLR) such as TLR4 following stimulation with the lipopolysaccharide (LPS). Mitochondrial changes include switching from oxidative phosphorylation to glycolysis which give the Macrophages the energy and the metabolic by-products to initiate the inflammatory state, akin to that observed with the Warburg effect in cancer. However, the molecular mechanisms as to how TLRs signal transduction to the mitochondria are unknown.
Upon TLR stimulation, canonical signalling leads to expression of inflammatory cytokines such as IL-1b, TNFα, IL6, and IFNβ. Typically, IL-6 and IFNs subsequently induce autocrine activation of the JAK-STAT signalling pathway which induces Tyr705 phosphorylation of STAT3. We have found that multiple TLRs can induce a rapid Ser727, but not Tyr705 STAT3 phosphorylation which subsequently translocates to the mitochondria. Importantly, STAT3 directly interacts with TRAF6 and TBK-1 with TBK-1 identified as the kinase responsible for Ser727 STAT3 phosphorylation. Using a genetically engineered mouse model incapable of undergoing STAT3 Ser727 phosphorylation, we determined that non-canonical STAT3 signalling is critical for LPS-induced glycolytic reprogramming and the subsequent inflammatory response. This study identifies non-canonical STAT3 activation as the crucial signalling intermediary for TLR4-induced glycolysis, macrophage metabolic reprogramming and inflammation.