Lactate-Induced Lipid Accumulation in Hepatocytes through GPR81 Activation.

BACKGRUOUND: Lactate, traditionally considered a metabolic byproduct, is increasingly recognized as a signaling molecule involved in metabolic regulation. Its role in hepatic steatosis, particularly through G-protein-coupled receptor 81 (GPR81)-mediated pathways, remains underexplored. METHODS: We investigated the effects of lactate on hepatic lipid metabolism using in vitro alpha mouse liver 12 (AML12) cells, zebrafish, and two diet-induced nonalcoholic fatty liver disease (NAFLD) mouse models. Lipid accumulation, gene/protein expression, and 5' adenosine monophosphate-activated protein kinase (AMPK) signaling were assessed under lactate exposure, GPR81 knockdown, monocarboxylate transporter 1 (MCT1) inhibition, and AMPK activation conditions. RESULTS: Lactate treatment in hepatocytes increased de novo lipogenesis and fatty acid uptake while suppressing fatty acid oxidation and AMPK phosphorylation. These effects were reversed by GPR81 knockdown but not by MCT1 inhibition, suggesting a GPR81-dependent mechanism. AMPK activation with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) reduced lactate-induced lipid accumulation. In zebrafish, 10 mM lactate treatment for 24 hours significantly increased hepatic lipid content. In mice fed high-fat diet (HFD) or high-fat high-cholesterol (HFHC) diets for 12 weeks, hepatic lactate levels and GPR81 expression were elevated. Interestingly, p-AMPK expression decreased in HFD livers but increased in the HFHC group, indicating dietspecific regulation. CONCLUSION: Our findings demonstrate that lactate promotes hepatic steatosis primarily via the GPR81-AMPK signaling axis. GPR81 activation enhances lipogenesis and lipid uptake, independent of MCT1-mediated transport. These results position GPR81 as a promising therapeutic target for NAFLD.