Targeting BCAA Catabolism to Treat Obesity-Associated Insulin Resistance

Abstract

Recent studies implicate a strong association between elevated plasma branched-chain amino acids (BCAAs) and insulin resistance (IR). However, a causal relationship and whether interrupted BCAA homeostasis can serve as a therapeutic target for diabetes remain to be established experimentally. In this study, unbiased integrative pathway analyses identified a unique genetic link between obesityassociated IR and BCAA catabolic gene expression at the pathway level in human and mouse populations. In genetically obese (ob/ob) mice, rate-limiting branched-chain a-keto acid (BCKA) dehydrogenase deficiency (i.e., BCAA and BCKA accumulation), a metabolic feature, accompanied the systemic suppression of BCAA catabolic genes. Restoring BCAA catabolic flux with a pharmacological inhibitor of BCKA dehydrogenase kinase (BCKDK) ( a suppressor of BCKA dehydrogenase) reduced the abundance of BCAA and BCKA and markedly attenuated IR in ob/ob mice. Similar outcomes were achieved by reducing protein (and thus BCAA) intake, whereas increasing BCAA intake did the opposite; this corroborates the pathogenic roles of BCAAs and BCKAs in IR in ob/ob mice. Like BCAAs, BCKAs also suppressed insulin signaling via activation of mammalian target of rapamycin complex 1. Finally, the small-molecule BCKDK inhibitor significantly attenuated IR in high-fat diet-induced obese mice. Collectively, these data demonstrate a pivotal causal role of a BCAA catabolic defect and elevated abundance of BCAAs and BCKAs in obesity-associated IR and provide proof-ofconcept evidence for the therapeutic validity of manipulating BCAA metabolism for treating diabetes.

Silencing branched-chain ketoacid dehydrogenase or treatment with branched-chain ketoacids ex vivo inhibits muscle insulin signaling

“…Ketoacids, in combination, significantly reduced AKT1 phosphorylation at Ser 473 in C2C12 cells ( Fig S2e). Our data are consistent with a recent report which demonstrated that BCKAs inhibited insulin signalling in 3T3-L1 adipocytes (26). Consistent with reduced AKT1 phosphorylation, insulin induced AKT1 activity was also decreased by ketoleucine and ketovaline (Fig 3d).…”

“…3,6-dichlorobenzothiophene-2-carboxylic acid (BT2) is a selective inhibitor of BCKDK (Fig 7a) which increases BCKDH activity (22,26) by inhibiting BCKDH phosphorylation. A 20-h exposure of C2C12 myotubes to BT2 reduced intracellular BCKAs (Fig 7b), signifying increased BCKDH activity, consistent with previous reports (22,26). Furthermore, BT2 markedly increased Klf15 and moderately increased Bcat2 mRNA levels (Fig 7c), indicating increased BCKAs flux towards oxidation.…”

“…Skeletal and cardiac muscle insulin signaling and sensitivity are critical for metabolic homeostasis and organ function (23,24). A strong association between plasma BCAAs, muscle BCAA catabolic gene expression and insulin resistance (IR) has been reported in several human and rodent models of obesity (25,26). Whether defects in muscle BCAA catabolizing enzyme expression and activity precede or follow obesity-induced IR remains unclear.…”

“…Whether defects in muscle BCAA catabolizing enzyme expression and activity precede or follow obesity-induced IR remains unclear. Strikingly, pharmacological inhibition of BCKDK in ob/ob and diet-induced obese (DIO) mice normalized BCAA catabolism and attenuated IR (26), signifying that restoring BCAA catabolism and clearing BCAA metabolites in endocrine disorders has beneficial functional outcomes. Plausibly a metabolic environment favouring the accumulation of BCAA catabolites reprograms substrate utilization and causes metabolic dysfunction.…”

Branched chain amino acids: A factor for zone 3 steatosis in non‐alcoholic fatty liver disease

“…A causal relationship between high BCAA level and NAFLD remains unclear; however, BCAAs are metabolized in extrahepatic tissues. The skeletal muscle is a major site of BCAA catabolism, and an impaired BCAA catabolism in the skeletal muscle might cause an elevation of BCAA levels, which leads to an increase in insulin resistance, a characteristic feature of NAFLD . Adipose tissue is also a site of BCAA catabolism, and alterations in mRNA levels of BCAA‐catabolizing genes in adipose tissue are associated with circulating BCAA levels .…”