Metabolic role of the hepatic valine/3-hydroxyisobutyrate (3-HIB) pathway in fatty liver disease
Background The valine (branched-chain amino acid) metabolite 3-hydroxyisobutyrate (3-HIB), produced by 3-Hydroxyisobutyryl-CoA Hydrolase (HIBCH), is strongly associated with insulin resistance and type-2 diabetes, but the implicated tissues and cellular mechanisms are poorly understood. We here hypothesized that HIBCH and 3-HIB regulate hepatic lipid accumulation.   Methods HIBCH mRNA in human liver biopsies and plasma 3-HIB were correlated with fatty liver and metabolic markers. Human Huh7 hepatocytes were supplemented with fatty acids (FAs) to induce lipid accumulation. Following HIBCH overexpression, siRNA knockdown, inhibition of PDK4 (a marker of FA ß-oxidation) or 3-HIB supplementation, we performed RNA- seq, targeted metabolite analyses and functional assays.   Findings We identify a regulatory feedback loop between the valine/3-HIB pathway and PDK4 that shapes FA metabolism and liver health and can be controlled via 3-HIB treatment of hepatocytes. HIBCH overexpression causes increased 3-HIB efflux and FA uptake, while knockdown increases cellular respiration and decreases reactive oxygen species (ROS) generation, associated with metabolic shifts via upregulation of PDK4. Treatment with PDK4 inhibitor lowered 3-HIB efflux and increased FA uptake, while increasing HIBCH mRNA. Consistent with a role for this regulatory loop in fatty liver, human cohorts show positive correlations of liver fat with hepatic HIBCH and PDK4 expression as well as with plasma 3-HIB. Hepatic 3-HIB supplementation lowered HIBCH expression and FA uptake and increased cellular respiration and ROS generation.  Interpretation These data implicate the valine/3-HIB pathway in key metabolic processes that contribute to fatty liver, reflected in increased plasma 3-HIB concentrations, and present possible targets for therapeutic intervention.