Regulation of Progressive Liver Disease by Mitochondrial Carboxylate Transport

Abstract

Mitochondrial transporters link TCA cycle and cytosolic metabolism. Mitochondrial pyruvate import and citrate export are two especially important steps for metabolic regulation. Pyruvate is imported into mitochondria by the mitochondrial pyruvate carrier (MPC), whereas citrate is exported by the mitochondrial citrate carrier (CIC). During fatty liver disease and T2D, liver MPC expression increases and facilitates excessive gluconeogenesis driving chronic hyperglycemia. Liver MPC disruption attenuates hyperglycemia and decreases liver inflammation. Sustained MPC disruption provides long-term protection from hyperglycemia without apparent liver damage. Furthermore, MPC disruption markedly decreases oxidative stress-induced hepatocellular carcinoma, a highly lethal cancer for which fatty liver disease and fibrosis are a major risk factors. The mechanisms by which MPC disruption provides metabolic projection are not completely understood. One potential mechanism is that MPC disruption in-turn decreases mitochondrial citrate export. Cytosolic citrate stimulates increased gluconeogenesis and de novo lipogenesis, two features of fatty liver disease and T2D. Thus, CIC export of citrate derived from MPC-imported pyruvate could mediate these processes. To test the role of the CIC in fatty liver disease, we generated liver-specific CIC knockout mice. Liver CIC disruption markedly decreased de novo lipogenesis measured by deuterated water into lipids. To examine how altered transcriptional programming might contribute to decreased CIC LivKO lipogenesis, we analyzed WT and CIC LivKO livers by RNAseq and qPCR. Numerous genes involved in lipoprotein and fatty acid uptake were upregulated in CIC LivKO livers, including Acly and Fasn, suggesting transcriptional counter regulation for impaired lipogenesis. CIC disruption protected from glucose intolerance after Western diet feeding but did not change insulin tolerance. To understand this selective effect on glucose tolerance, we traced liver 13C-glucose metabolism in vivo. Mass spectrometry analysis of liver extracts showed that CIC disruption increased 13C enrichment into pyruvate and TCA cycle intermediates, consistent with disinhibited glycolysis and decreased mitochondrial citrate export. Together, these data provide evidence for an MPC-CIC axis as key mediator of fatty liver disease.