title: Lactate boosts TLR4 signaling and NF-kappaB pathway-mediated gene transcription in macrophages via monocarboxylate transporters and MD-2 up-regulation.
abstract: It has been shown that lactate induces insulin resistance. However, the underlying mechanisms have not been well understood. Based on our observation that lactate augments LPS-stimulated inflammatory gene expression, we proposed that lactate may enhance TLR4 signaling in macrophages, which has been shown to play an important role in insulin resistance in adipocytes. In this study, we demonstrated that lactate stimulated MD-2, a coreceptor for TLR4 signaling activation, NF-kappaB transcriptional activity, and the expression of inflammatory genes in human U937 histiocytes (resident macrophages). Similar enhancement of the inflammatory gene expression by lactate was also observed in human monocyte-derived macrophages. The essential role of MD-2 in lactate-augmented TLR4 signaling was confirmed by observation that the suppression of MD-2 expression by small interfering RNA led to significant inhibition of inflammatory gene expression. To further elucidate how lactate treatment enhances TLR4 activation, we showed that the augmentation of inflammatory gene expression by lactate was abrogated by antioxidant treatment, suggesting a critical role of reactive oxygen species in the enhancement of TLR4 activation by lactate. Finally, we showed that alpha-cyano-4-hydroxycinnamic acid, a classic inhibitor for monocarboxylate transporters, blocked lactate-augmented inflammatory gene expression and nuclear NF-kappaB activity, indicating that lactate transport through monocarboxylate transporters is required for lactate-enhanced TLR4 activation. Collectively, this study documents that lactate boosts TLR4 activation and NF-kappaB-dependent inflammatory gene expression via monocarboxylate transporters and MD-2 up-regulation.
authors.affiliation: Division of Endocrinology, Diabetes and Medical Genetics, Department of Medicine, Medical University of South Carolina, Charleston, SC 29425, USA.
authors.forename: Devadoss J
authors.initials: DJ
authors.lastname: Samuvel
authors.forename: Kamala P
authors.initials: KP
authors.lastname: Sundararaj
authors.forename: Alena
authors.initials: A
authors.lastname: Nareika
authors.forename: Maria F
authors.initials: MF
authors.lastname: Lopes-Virella
authors.forename: Yan
authors.initials: Y
authors.lastname: Huang
mesh_terms: D000975:Antioxidants / Q000494:pharmacology; D004797:Enzyme-Linked Immunosorbent Assay; D005786:Gene Expression Regulation / Q000187:drug effects / Q000276:immunology; D006801:Humans; D019344:Lactic Acid / Q000276:immunology / Q000378:metabolism*; D008070:Lipopolysaccharides / Q000276:immunology; D051218:Lymphocyte Antigen 96 / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D008264:Macrophages / Q000276:immunology / Q000378:metabolism*; D027501:Monocarboxylic Acid Transporters / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D016328:NF-kappa B / Q000276:immunology / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000187:drug effects / Q000276:immunology; D051197:Toll-Like Receptor 4 / Q000276:immunology / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects / Q000276:immunology; D015854:Up-Regulation
chemical_list: D000975:Antioxidants; C495377:LY96 protein, human; D008070:Lipopolysaccharides; D051218:Lymphocyte Antigen 96; D027501:Monocarboxylic Acid Transporters; D016328:NF-kappa B; C495345:TLR4 protein, human; D051197:Toll-Like Receptor 4; D019344:Lactic Acid