Abstract
Branched-chain α-keto acids (BCKAs) are catabolites of branched-chain amino acids (BCAAs). Intracellular BCKAs are cleared by branched-chain ketoacid dehydrogenase (BCKDH), which is sensitive to inhibitory phosphorylation by BCKD kinase (BCKDK). Accumulation of BCKAs is an indicator of defective BCAA catabolism and has been correlated with glucose intolerance and cardiac dysfunction. However, it is unclear whether BCKAs directly alter insulin signaling and function in the skeletal and cardiac muscle cell. Furthermore, the role of excess fatty acids (FAs) in perturbing BCAA catabolism and BCKA availability merits investigation. By using immunoblotting and ultra-performance liquid chromatography MS/MS to analyze the hearts of fasted mice, we observed decreased BCAA-catabolizing enzyme expression and increased circulating BCKAs, but not BCAAs. In mice subjected to diet-induced obesity (DIO), we observed similar increases in circulating BCKAs with concomitant changes in BCAA-catabolizing enzyme expression only in the skeletal muscle. Effects of DIO were recapitulated by simulating lipotoxicity in skeletal muscle cells treated with saturated FA, palmitate. Exposure of muscle cells to high concentrations of BCKAs resulted in inhibition of insulin-induced AKT phosphorylation, decreased glucose uptake, and mitochondrial oxygen consumption. Altering intracellular clearance of BCKAs by genetic modulation of BCKDK and BCKDHA expression showed similar effects on AKT phosphorylation. BCKAs increased protein translation and mTORC1 activation. Pretreating cells with mTORC1 inhibitor rapamycin restored BCKA's effect on insulin-induced AKT phosphorylation. This study provides evidence for FA-mediated regulation of BCAA-catabolizing enzymes and BCKA content and highlights the biological role of BCKAs in regulating muscle insulin signaling and function.
Highlights
Branched-chain amino acids (BCAAs) and their catabolites are bioactive molecules with a broad repertoire of metabolic actions in cellular health and disease [1,2,3]
To confirm whether changes in circulating branched-chain keto acid (BCKA) were an outcome of altered BCAA catabolism, we examined protein markers of the BCAA catabolic pathway in the gastrocnemius muscle and heart
Because nutritional changes are accompanied by changes in circulating insulin, we examined whether protein expression of BCAA-catabolizing enzymes corresponded with changes in AKT phosphorylation, an effector of insulin signaling
Summary
Branched-chain amino acids (BCAAs) and their catabolites are bioactive molecules with a broad repertoire of metabolic actions in cellular health and disease [1,2,3]. In the skeletal muscle but not the heart of mice subjected to DIO, circulating BCKA levels increased with a concomitant decrease in BCAA-catabolizing enzyme expression. Phosphorylated BCKDE1a Ser293 levels in the heart (Fig. S1A) but not the gastrocnemius muscles (Fig. S1B) correlated with circulating BCKAs. Because nutritional changes are accompanied by changes in circulating insulin (decreased in fasting and increased in refeeding), we examined whether protein expression of BCAA-catabolizing enzymes corresponded with changes in AKT phosphorylation, an effector of insulin signaling.
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