Pyruvate enhances M1 microglial polarization via NF-κB/MCT1/iNOS signaling axis under low glucose condition

Abstract

Background Proinflammatory microglia rely predominantly on glycolysis to maintain cytokine production during an inflammatory response. However, during ischemia, where glucose supply is low, inducible nitric oxide synthase (iNOS) production remains high accompanied by an increase in monocarboxylate transporter 1 (MCT1) expression. In this study, we explored whether there is a link between iNOS and MCT1 expressions, and whether pyruvate can act as an energy source to sustain the M1 phenotype. Methods Using a mouse model with laser-induced brain ischemia and cell culture with low-glucose treatment, we examined responses from microglia. Results The expressions of iNOS and MCT1, as well as arginase-1 (ARG1), were increased in the brain of the ischemic mouse model. In the BV2 microglial cell line and primary microglia treated under low glucose condition, iNOS and MCT1 also increased, while ARG1 decreased. The addition of pyruvate under low-glucose or lipopolysaccharide (LPS) treatment enhanced iNOS and MCT1 expressions compared with groups without pyruvate. MCT1 knockdown resulted in decreased iNOS while MCT1 overexpression increased iNOS. Furthermore, inhibitor of nuclear factor-kappaB (NF-κB) reduced both iNOS and MCT1. Conclusion Our data suggested that after proinflammatory microglial polarization, MCT1 is upregulated through the NF-κB signaling pathway, which leads to iNOS production. We speculate that microglia may continuously pick up monocarboxylates such as pyruvate through MCT1 to sustain the M1 phenotype.