LAUSR

The Krebs cycle metabolite succinate contributes to inflammatory conditions like arthritis and colitis by activating its receptor SUCNR1. We aimed to analyze whether the succinate-SUCNR1 pathway contributes to the leukocyte-endothelial cell interactions that initiate the inflammatory response. We evaluated leukocyte rolling and adhesion by intravital microscopy in cremaster venules of wild-type (WT) and Sucrn1−/− mice treated, 4h before, with succinate (1 Mm, intraescrotally), combined or not with the common pro-inflammatory cytokine TNFα (500 ng/mice, i.p.), or with their vehicles. We analyzed the activity of the NF-κB signaling pathway in endothelial cells (HUVEC) treated (4h) with succinate (0.5–1 mM) ± TNFα (20ng/ml), by detecting this transcription factor, its inhibitor iκBα, and their phosphorylated forms, by western blot. Results are expressed as mean±SEM (N≥6) and analyzed by ANOVA + Newman-Keuls test. In WT mice, succinate tended to induce leukocyte rolling and adhesion, and amplified the effects of TNFα on rolling. SUCNR1-deficient mice failed to increase leukocyte rolling in response to these treatments while showed an attenuated response to the effect of TNFα on leukocyte adhesion (Figure 1). Succinate treatment intensified the activation of the NF-κB pathway in HUVEC treated with TNFα, as shown by the increased IκBα phosphorylation, which allows its degradation, the reduced total IκBα, and the increased NF-κB/IκBα ratio (Figure 2). Our results suggest that the endogenous release of succinate and the consequent stimulation of the SUCNR1 mediates the leukocyte-endothelial cell interactions induced by TNFα, and this pro-inflammatory activity may be related with an increased activation of the NF-κB signaling pathway in endothelial cells. Thus, the accumulation of succinate observed in various inflammatory diseases probably contributes to the initiation of the inflammatory focus by promoting the accumulation of leukocytes.