Exercise enhances branched-chain amino acid (BCAA) catabolism, and BCAA supplementation influences exercise metabolism. However, it remains controversial whether BCAA supplementation improves exercise endurance, and unknown whether the exercise endurance effect of BCAA supplementation requires catabolism of these amino acids. Therefore, we examined exercise capacity and intermediary metabolism in skeletal muscle of knockout (KO) mice of mitochondrial branched-chain aminotransferase (BCATm), which catalyzes the first step of BCAA catabolism. We found that BCATm KO mice were exercise intolerant with markedly decreased endurance to exhaustion. Their plasma lactate and lactate-to-pyruvate ratio in skeletal muscle during exercise and lactate release from hindlimb perfused with high concentrations of insulin and glucose were significantly higher in KO than wild-type (WT) mice. Plasma and muscle ammonia concentrations were also markedly higher in KO than WT mice during a brief bout of exercise. BCATm KO mice exhibited 43-79% declines in the muscle concentration of alanine, glutamine, aspartate, and glutamate at rest and during exercise. In response to exercise, the increments in muscle malate and alpha-ketoglutarate were greater in KO than WT mice. While muscle ATP concentration tended to be lower, muscle IMP concentration was sevenfold higher in KO compared with WT mice after a brief bout of exercise, suggesting elevated ammonia in KO is derived from the purine nucleotide cycle. These data suggest that disruption of BCAA transamination causes impaired malate/aspartate shuttle, thereby resulting in decreased alanine and glutamine formation, as well as increases in lactate-to-pyruvate ratio and ammonia in skeletal muscle. Thus BCAA metabolism may regulate exercise capacity in mice.

It remains unclear how ␣-ketoisocaproate (KIC) and leucine are metabolized to stimulate insulin secretion. Mitochondrial BCATm (branched-chain aminotransferase) catalyzes reversible transamination of leucine and ␣-ketoglutarate to KIC and glutamate, the first step of leucine catabolism. We investigated the biochemical mechanisms of KIC and leucine-stimulated insulin secretion (KICSIS and LSIS, respectively) using Although leucine oxidation and KIC transamination were blocked in BCATm ؊/؊ islets, KIC oxidation was unaltered.These data indicate that KICSIS requires transamination of KIC and glutamate to leucine and ␣-ketoglutarate, respectively. LSIS does not require leucine catabolism and may be through leucine activation of glutamate dehydrogenase. Thus, KICSIS and LSIS occur by enhancing the metabolism of glutamine/glutamate to ␣-ketoglutarate, which, in turn, is metabolized to produce the intracellular signals such as ATP and NADPH for insulin secretion.To maintain glucose homeostasis in response to a meal, insulin secretion is precisely stimulated by nutrients such as glucose, amino acids, and free fatty acids as well as incretin hormones such as glucagon-like peptide-1. Nutrients are thought to stimulate insulin secretion through metabolic secretion coupling to generate metabolic signals, i.e. second messengers or coupling factors. Extensive research has been conducted to determine how nutrients are metabolized to generate these coupling factors, e.g. ATP and NADPH. Although leucine and ␣-ketoisocaproate (KIC) 3 are potent insulin secretagogues (1), the underlying mechanisms of leucine and KIC-stimulated insulin secretion (LSIS and KICSIS, respectively) remain elusive. A key question is whether their oxidative decarboxylation is required for induction of insulin secretion.

a b s t r a c t TNF-a-induced insulin resistance is associated with generation of reactive oxygen species (ROS). This study aims at defining the link between ROS production and hepatic insulin resistance. Treatment with TNF-a increased ROS generation through activating NADPH oxidase 3 (NOX3) in HepG2 hepatocytes. Down-regulation of NOX3 using siRNA prevented TNF-a-induced decrease of cellular glycogen. In the cells treated with TNF-a, there were NOX3-dependent activation of JNK, inhibition of IRS1 and phosphorylation of AKT/PKB and GSK. In conclusion, the effects of TNF-a on hepatic insulin resistance appear to be, at least in part, mediated by NOX3-derived ROS through a JNK pathway.

Defects in insulin secretion by pancreatic cells and/or decreased sensitivity of target tissues to insulin action are the key features of type 2 diabetes. It has been shown that excessive generation of reactive oxygen species (ROS) is linked to glucose‐induced β‐cell dysfunction. However, cellular mechanisms involved in ROS generation in β‐cells and the link between ROS and glucose‐induced β‐cell dysfunction are poorly understood. Here, we demonstrate a key role of NADPH oxidase 2 (NOX2)‐derived ROS in the deterioration of β‐cell function induced by a high concentration of glucose. Sprague–Dawley rats were fed a high‐fat diet for 24 weeks to induce diabetes. Diabetic rats showed increased glucose levels and elevated ROS generation in blood, but decreased insulin content in pancreatic β‐cells. In vitro, increased ROS levels in pancreatic NIT‐1 cells exposed to high concentrations of glucose (33.3 mmol·L−1) were associated with elevated expression of NOX2. Importantly, decreased glucose‐induced insulin expression and secretion in NIT‐1 cells could be rescued via siRNA‐mediated NOX2 reduction. Furthermore, high glucose concentrations led to apoptosis of β‐cells by activation of p38MAPK and p53, and dysfunction of β‐cells through phosphatase and tensih homolog (PTEN)‐dependent Jun N‐terminal kinase (JNK) activation and protein kinase B (AKT/PKB) inhibition, which induced the translocation of forkhead box O1 and pancreatic duodenal homeobox‐1, followed by reduced insulin expression and secretion. In conclusion, NOX2‐derived ROS could play a critical role in high glucose‐induced β‐cell dysfunction through PTEN‐dependent JNK activation and AKT inhibition.