Branched-chain amino acids at supraphysiological but not physiological levels reduce myotube insulin sensitivity

Chronic triclosan exposure induce impaired glucose tolerance by altering the gut microbiota

Triclosan (TCS) is an antimicrobial compound incorporated into more than 2000 consumer products. This compound is frequently detected in the human body and causes ubiquitous contamination in the environment, thereby raising concerns about its impact on human health and environmental pollution. Here, we demonstrated that 20 weeks' exposure of TCS drove the development of glucose intolerance by inducing compositional and functional alterations in intestinal microbiota in rats. Fecal-transplantation experiments corroborated the involvement of gut microbiota in TCS-induced glucose-tolerance impairment. 16S rRNA gene-sequencing analysis of cecal contents showed that TCS disrupted the gut microbiota composition in rats and increased the ratio of Firmicutes to Bacteroidetes. Cecal metabolomic analyses detected that TCS altered host metabolic pathways that are linked to host glucose and amino acid metabolism, particularly branched-chain amino acid (BCAA) biosynthesis. BCAA measurement confirmed the increase in serum BCAAs in rats exposed to TCS. Western blot and immunostaining results further confirmed that elevated BCAAs stimulated mTOR, a nutrient-sensing complex, and following IRS-1 serine phosphorylation, resulted in insulin resistance and glucose intolerance. These results suggested that TCS may induce glucose metabolism imbalance by regulating BCAA concentration by remodeling the gut microbiota.

5-Aza-2'-deoxycytidine-mediated DNA hypomethylation with and without concurrent insulin resistance suppresses myotube mitochondrial capacity

Type 2 diabetes is characterized by elevated blood glucose and reduced insulin sensitivity in target tissues. Moreover, reduced mitochondrial metabolism and expressional profile of genes governing mitochondrial metabolism (such as peroxisome proliferator-activated receptor gamma coactivator 1-alpha [PGC-1α]) are also reduced during insulin resistance. Epigenetic regulation via DNA methylation of genes including PGC-1α may contribute to diminished mitochondrial capacity, while hypomethylation of PGC-1α (such as that invoked by exercise) has been associated with increased PGC-1α expression and favorable metabolic outcomes. The purpose of the present report is to characterize the effects of DNA hypomethylation on myotube metabolism and expression of several related metabolic targets. C2C12 myotubes were treated with 5-Aza-2'-deoxycytidine (5-Aza) for either 24 or 72 h both with and without hyperinsulinemic-induced insulin resistance. Mitochondrial and glycolytic metabolism were measured via oxygen consumption and extracellular acidification rate, respectively. Metabolic gene and protein expression were assessed via quantitative real time polymerase chain reaction and western blot analysis, respectively. Though expression of PGC-1α and other related targets remained unaltered, insulin resistance and 5-Aza treatment significantly reduced mitochondrial metabolism. Similarly, peak glycolytic metabolism was diminished by 5-Aza-treated cells, while basal glycolytic metabolism was unaltered. 5-Aza also reduced the expression of branched-chain amino acid (BCAA) catabolic components, however BCAA utilization was enhanced during insulin resistance with 5-Aza treatment. Together the present work provides proof-of-concept evidence of the potential role of DNA methylation in the regulation of mitochondrial metabolism and the potential interactions with insulin resistance in a model of skeletal muscle.

Effect of AMPK activation and glucose availability on myotube LAT1 expression and BCAA utilization

Those with insulin resistance often display increased circulating branched-chain amino acids (BCAA), which has been largely attributable to reduced BCAA catabolic capacity. Metabolic stimuli such as exercise activates AMP-activated kinase (AMPK), which promotes the metabolism of BCAA and induction/activation of BCAA catabolic enzymes. Though much attention has been paid to BCAA catabolic machinery, few studies have assessed the effect of AMPK activation on the predominant BCAA transporter, L-type amino acid transporter 1 (LAT1). This study assessed the effect of AMPK activation on LAT1 expression via common chemical AMPK activators in a cell model of skeletal muscle. C2C12 myotubes were treated with either 1 mM AICAR, 1 mM Metformin, or filter-sterilized water (control) for 24 h with either low- (5 mM) or high-glucose (25 mM) media. LAT1 and pAMPK protein content were measured via western blot. BCAA media content was measured using liquid chromatography-mass spectrometry. AICAR treatment significantly increased pAMPK and reduced LAT1 expression. Collectively, pAMPK and LAT1 displayed a significant inverse relationship independent of glucose levels. During low-glucose experiments, AICAR-treated cells had higher BCAA media content compared to other groups, and an inverse relationship between LAT1 and BCAA media content was observed, however, these effects were not consistently observed during high-glucose conditions. Further investigation with AICAR with and without concurrent LAT1 inhibition (via JPH203) also revealed reduced BCAA utilization in AICAR-treated cells regardless of LAT1 inhibition (which also independently reduced BCAA utilization). pAMPK activation via AICAR (but not Metformin) may reduce LAT1 expression and BCAA uptake in a glucose-dependent manner.

Insulin resistance promotes extracellular BCAA accumulation without altering LAT1 content, independent of prior BCAA treatment in a myotube model of skeletal muscle

PURPOSE

Type 2 diabetes is characterized by reduced insulin sensitivity which correlates with increased circulating BCAA. These experiments investigated the effects of insulin resistance with and without excess BCAA on myotube insulin sensitivity and L-type amino acid transporter-1 (LAT1).

METHODS

C2C12 myotubes were treated with or without excess BCAA for 1 or 6 days, both with and without insulin resistance. Western blot was used to assess insulin sensitivity and LAT1 content. Liquid chromatography-mass spectrometry was used to evaluate BCAA media content.

RESULTS

Insulin resistance was associated with significantly increased extracellular BCAA accumulation independent of LAT1 content. Conversely, prior BCAA treatment was not associated with extracellular BCAA accumulation regardless of level of insulin sensitivity.

CONCLUSION

These data suggest insulin resistance, but not BCAA treatment, promotes extracellular BCAA accumulation independent of changes in LAT1 content, implicating insulin resistance as a causal agent of extracellular BCAA accumulation.

Role of branched-chain amino acid metabolism in the pathogenesis of obesity and type 2 diabetes-related metabolic disturbances BCAA metabolism in type 2 diabetes

Branched-chain amino acid (BCAA) catabolism has been considered to have an emerging role in the pathogenesis of metabolic disturbances in obesity and type 2 diabetes (T2D). Several studies showed elevated plasma BCAA levels in humans with insulin resistance and patients with T2D, although the underlying reason is unknown. Dysfunctional BCAA catabolism could theoretically be an underlying factor. In vitro and animal work collectively show that modulation of the BCAA catabolic pathway alters key metabolic processes affecting glucose homeostasis, although an integrated understanding of tissue-specific BCAA catabolism remains largely unknown, especially in humans. Proof-of-concept studies in rodents -and to a lesser extent in humans – strongly suggest that enhancing BCAA catabolism improves glucose homeostasis in metabolic disorders, such as obesity and T2D. In this review, we discuss several hypothesized mechanistic links between BCAA catabolism and insulin resistance and overview current available tools to modulate BCAA catabolism in vivo. Furthermore, this review considers whether enhancing BCAA catabolism forms a potential future treatment strategy to promote metabolic health in insulin resistance and T2D.

Excess branched-chain amino acids alter myotube metabolism and substrate preference which is worsened by concurrent insulin resistance

PURPOSE

Branched-chain amino acids (BCAA) have been shown to enhance several cellular signaling pathways including protein synthesis and mitochondrial biogenesis, yet population data demonstrate a correlation between circulating BCAA and severity of insulin resistance which has been hypothesized to be, in part, a byproduct of BCAA inhibition of mitochondrial function. The purpose of this study is to examine the effect of a BCAA mixture on muscle metabolism and related gene expression in vitro.

METHODS

C2C12 myotubes were treated with a BCAA mixture containing leucine:isoleucine:valine at a ratio of 2:1:1 at 0.2, 2, or 20 mM (based on leucine content) for 6 days. qRT-PCR was used to measure metabolic gene expression. Oxygen consumption and extracellular acidification were used to assess mitochondrial and glycolytic metabolism, respectively. Mitochondrial content was determined via mitochondrial-specific staining.

RESULTS

Despite significantly elevated mitochondrial staining, 6-day BCAA treatment reduced basal mitochondrial metabolism at a supraphysiological concentration (20 mM) in both insulin sensitive and resistant cells. Peak mitochondrial capacity was also reduced in insulin-resistant (but not insulin sensitive) cells. Conversely, basal glycolytic metabolism was elevated following 20 mM BCAA treatment, regardless of insulin resistance. In addition, insulin-resistant cells treated with 20 mM BCAA exhibited reduced gene expression of Ppargc1a, Cytc, Atp5b, Glut4, and several glycolytic enzymes versus insulin sensitive cells treated with 20 mM BCAA.

CONCLUSIONS

Collectively, these findings suggest BCAA at supraphysiologically high levels may negatively alter mitochondrial metabolism, and concurrent insulin resistance may also diminish peak mitochondrial capacity, as well as impede molecular adaptations that support a transition to a glycolytic preference/compensation.