Mitochondrial pyruvate carrier inhibition initiates metabolic crosstalk to stimulate branched chain amino acid catabolism.

The mitochondrial pyruvate carrier (MPC) has emerged as a therapeutic target for treating insulin resistance, type 2 diabetes, and nonalcoholic steatohepatitis (NASH). We evaluated whether MPC inhibitors (MPCi) might correct impairments in branched chain amino acid (BCAA) catabolism, which are predictive of developing diabetes and NASH. Circulating BCAA concentrations were measured in people with NASH and type 2 diabetes, who participated in a recent randomized, placebo-controlled Phase IIB clinical trial to test the efficacy and safety of the MPCi MSDC-0602K (EMMINENCE; NCT02784444). In this 52-week trial, patients were randomly assigned to placebo (n = 94) or 250 mg MSDC-0602K (n = 101). Human hepatoma cell lines and mouse primary hepatocytes were used to test the direct effects of various MPCi on BCAA catabolism in vitro. Lastly, we investigated how hepatocyte-specific deletion of MPC2 affects BCAA metabolism in the liver of obese mice and MSDC-0602K treatment of Zucker diabetic fatty (ZDF) rats. In patients with NASH, MSDC-0602K treatment, which led to marked improvements in insulin sensitivity and diabetes, had decreased plasma concentrations of BCAAs compared to baseline while placebo had no effect. The rate-limiting enzyme in BCAA catabolism is the mitochondrial branched chain ketoacid dehydrogenase (BCKDH), which is deactivated by phosphorylation. In multiple human hepatoma cell lines, MPCi markedly reduced BCKDH phosphorylation and stimulated branched chain keto acid catabolism; an effect that required the BCKDH phosphatase PPM1K. Mechanistically, the effects of MPCi were linked to activation of the energy sensing AMP-dependent protein kinase (AMPK) and mechanistic target of rapamycin (mTOR) kinase signaling cascades in vitro. BCKDH phosphorylation was reduced in liver of obese, hepatocyte-specific MPC2 knockout (LS-Mpc2-/-) mice compared to wild-type controls concomitant with activation of mTOR signaling in vivo. Finally, while MSDC-0602K treatment improved glucose homeostasis and increased the concentrations of some BCAA metabolites in ZDF rats, it did not lower plasma BCAA concentrations. These data demonstrate novel cross talk between mitochondrial pyruvate and BCAA metabolism and suggest that MPC inhibition leads to lower plasma BCAA concentrations and BCKDH phosphorylation by activating the mTOR axis. However, the effects of MPCi on glucose homeostasis may be separable from its effects on BCAA concentrations.

Ferguson D ，Eichler SJ ，Yiew NKH ，Colca JR ，Cho K ，Patti GJ ，Shew TM ，Lutkewitte AJ ，Mukherjee S ，McCommis KS ，Niemi NM ，Finck BN ... -  《Molecular Metabolism》

Diabetes and branched-chain amino acids: What is the link?

Branched-chain amino acids (BCAA) have increasingly been studied as playing a role in diabetes, with the PubMed search string "diabetes" AND "branched chain amino acids" showing particular growth in studies of the topic over the past decade (Fig. ). In the Young Finn's Study, BCAA and, to a lesser extent, the aromatic amino acids phenylalanine and tyrosine were associated with insulin resistance (IR) in men but not in women, whereas the gluconeogenic amino acids alanine, glutamine, or glycine, and several other amino acids (i.e. histidine, arginine, and tryptophan) did not show an association with IR. Obesity may track more strongly than metabolic syndrome and diabetes with elevated BCAA. In a study of 1302 people aged 40-79; higher levels of BCAA tracked with older age, male sex, and metabolic syndrome, as well as with obesity, cardiovascular risk, dyslipidemia, hypertension, and uric acid. Medium- and long-chain acylcarnitines, by-products of mitochondrial catabolism of BCAAs, as well as branched-chain keto acids and the BCAA themselves distinguished obese people having versus not having features of IR, and in a study of 898 patients with essential hypertension, the BCAA and tyrosine and phenylalanine were associated with metabolic syndrome and impaired fasting glucose. In a meta-analysis of three genome-wide association studies, elevations in BCAA and, to a lesser extent, in alanine tracked with IR, whereas higher levels of glutamine and glycine were associated with lesser likelihood of IR. Given these associations with IR, it is not surprising that a number of studies have shown higher BCAA levels in people with and prior to development of type 2 diabetes (T2D), although this has particularly been shown in Caucasian and Asian ethnic groups while not appearing to occur in African Americans. Similarly, higher BCAA levels track with cardiovascular disease. [Figure: see text] The metabolism of BCAA involves two processes: (i) a reversible process catalysed by a branched-chain aminotransferase (BCAT), either cytosolic or mitochondrial, requiring pyridoxal to function as an amino group carrier, by which the BCAA with 2-ketoglutarate produce a branched-chain keto acid plus glutamate; and (ii) the irreversible mitochondrial process catalysed by branched-chain keto acid dehydrogenase (BCKDH) leading to formation of acetyl-coenzyme A (CoA), propionyl-CoA, and 2-methylbutyryl-CoA from leucine, valine, and isoleucine, respectively, which enter the tricarboxylic acid (Krebs) cycle as acetyl-CoA, propionyl-CoA, and 2-methylbutyryl-CoA, respectively, leading to ATP formation. The BCAA stimulate secretion of both insulin and glucagon and, when given orally, of both glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), with oral administration leading to greater and more prolonged insulin and glucagon secretion. Insulin may particularly reduce BCAA turnover to a greater extent than that of other amino acids, and decreases the appearance and increases the uptake of amino acids. However, older studies of the effect of glucose or insulin on BCAA concentrations and rates of leucine appearance and oxidation showed no reduction in T2D, although the higher baseline levels of BCAA in obesity have long been recognized. Impaired function of BCAT and BCKDH has been posited, either as a primary genetic abnormality or due to effects of elevated fatty acids, proinflammatory cytokines, or insulin levels with consequent accumulation of branched-chain keto acids and metabolites such as diacylglycerol and ceramide, potentially contributing to the development of further insulin resistance, and decreased skeletal muscle BCAT and BCKDH expression has been shown in people with diabetes, supporting this concept. A Mendelian randomization study used measured variation in genes involved in BCAA metabolism to test the hypothesis of a causal effect of modifiable exposure on IR, showing that variants in protein phosphatase, Mg2+ /Mn2+ dependent 1K (PPM1K), a gene encoding the mitochondrial phosphatase activating the BCKDH complex, are associated with T2D, but another such study suggested that genetic variations associated with IR are causally related to higher BCAA levels. Another hypothesis involves the mammalian target of rapamycin complex 1 (mTORC1), which is activated by BCAA, as well as by insulin and glucose via cellular ATP availability. If this is the relevant pathway, BCAA overload may cause insulin resistance by activation of mammalian target of rapamycin (mTOR), as well as by leading to increases in acylcarnitines, with mTOR seen in this scenario as a central signal of cross-talk between the BCAA and insulin. At this point, whether whole-body or tissue-specific BCAA metabolism is increased or decreased in states of insulin-resistant obesity and T2D is uncertain. Insulin action in the hypothalamus induces but overfeeding decreases hepatic BCKDH, leading to the concept that hypothalamic insulin resistance impairs BCAA metabolism in obesity and diabetes, so that plasma BCAAs may be markers of hypothalamic insulin action rather than direct mediators of changes in IR. A way to address this may be to understand the effects of changes in diet and other interventions on BCAA, as well as on IR and T2D. In an animal model, lowering dietary BCAA increased energy expenditure and improved insulin sensitivity. Two large human population studies showed an association of estimated dietary BCAA intake with T2D risk, although another population study showed higher dietary BCAA to be associated with lower T2D risk. Ethnic differences, reflecting underlying differences in genetic variants, may be responsible for such differences. In the study of Asghari et al. in the current issue of the Journal of Diabetes, BCAA intake was associated with the development of subsequent IR. Studies of bariatric surgery suggest lower basal and post-insulin infusion BCAA levels are associated with greater insulin sensitivity, with reductions in BCAA not seen with weight loss per se with gastric band procedures, but occurring after Roux-en-Y gastric bypass, an intervention that may have metabolic benefits over and above those from reduction in body weight. The gut microbiota may be important for the supply of the BCAA to mammalian hosts, either by de novo biosynthesis or by modifying nutrient absorption. A final fascinating preliminary set of observations is that of the effects of empagliflozin on metabolomics; evidence of increased Krebs cycle activation and of higher levels of BCAA metabolites, such as acylcarnitines, suggests that sodium-glucose cotransporter 2 (SGLT2) inhibition may, to some extent, involve BCAA metabolism. Certainly, we do not yet have a full understanding of these complex associations. However, the suggestion of multiple roles of BCAA in the development of IR promises to be important and to lead to the development of novel effective T2D therapies.

Bloomgarden Z 《-》

Insulin sensitizer MSDC-0602K in non-alcoholic steatohepatitis: A randomized, double-blind, placebo-controlled phase IIb study.

MSDC-0602K is a novel insulin sensitizer designed to preferentially target the mitochondrial pyruvate carrier while minimizing direct binding to the transcriptional factor PPARγ. Herein, we aimed to assess the efficacy and safety of MSDC-0602K in patients with non-alcoholic steatohepatitis. Patients with biopsy-confirmed NASH and fibrosis (F1-F3) were randomized to daily oral placebo, or 1 of 3 MSDC-0602K doses in a 52-week double-blind study. The primary efficacy endpoint was hepatic histological improvement of ≥2 points in non-alcoholic fatty liver disease activity score (NAS) with a ≥1-point reduction in either ballooning or lobular inflammation and no increase in fibrosis stage at 12 months. Secondary endpoints included NAS improvement without worsening fibrosis, NASH resolution, and fibrosis reduction. Exploratory endpoints included changes in insulin sensitivity, liver injury and liver fibrosis markers. Patients were randomly assigned to placebo (n = 94), or 62.5 mg (n = 99), 125 mg (n = 98), or 250 mg (n = 101) of MSDC-0602K. At baseline, glycated hemoglobin was 6.4 ± 1.0%, 61.5% of patients had fibrosis F2/F3 and the average NAS was 5.3. The primary endpoint was reached in 29.7%, 29.8%, 32.9% and 39.5% of patients in the placebo, 62.5 mg, 125 mg and 250 mg dose arms, respectively, with adjusted odds ratios relative to placebo of 0.89 (95% CI 0.44-1.81), 1.22 (95% CI 0.60-2.48), and 1.64 (95% CI 0.83-3.27). The 2 highest doses of MSDC-0602K led to significant reductions in glucose, glycated hemoglobin, insulin, liver enzymes and NAS compared to placebo. The incidence of hypoglycemia and PPARγ-agonist-associated events such as edema and fractures were similar in the placebo and MSDC-0602K groups. MSDC-0602K did not demonstrate statistically significant effects on primary and secondary liver histology endpoints. However, effects on non-invasive measures of liver cell injury and glucose metabolism support further exploration of MSDC-0602K's safety and potential efficacy in patients with type 2 diabetes and liver injury. [ClinicalTrials.gov Identifier: NCT02784444]. First-generation insulin sensitizers are used to treat type 2 diabetes, but are associated with side effects including edema, bone fractures, and hypoglycemia. MSDC-0602K is a second-generation insulin sensitizer designed to reduce these side effects. We hypothesized that insulin sensitization could improve non-alcoholic steatohepatitis. In the current study of patients with non-alcoholic steatohepatitis, MSDC-0602K did not demonstrate significant effects on liver histology with the biopsy techniques used. However, useful information was gained for the design of future studies and MSDC-0602K significantly decreased fasting glucose, insulin, glycated hemoglobin, and markers of liver injury without dose-limiting side effects.

Harrison SA ，Alkhouri N ，Davison BA ，Sanyal A ，Edwards C ，Colca JR ，Lee BH ，Loomba R ，Cusi K ，Kolterman O ，Cotter G ，Dittrich HC ... -  《-》

Insulin action, type 2 diabetes, and branched-chain amino acids: A two-way street.

A strong association of obesity and insulin resistance with increased circulating levels of branched-chain and aromatic amino acids and decreased glycine levels has been recognized in human subjects for decades. More recently, human metabolomics and genetic studies have confirmed and expanded upon these observations, accompanied by a surge in preclinical studies that have identified mechanisms involved in the perturbation of amino acid homeostasis- how these events are connected to dysregulated glucose and lipid metabolism, and how elevations in branched-chain amino acids (BCAA) may participate in the development of insulin resistance, type 2 diabetes (T2D), and other cardiometabolic diseases and conditions. In human cohorts, BCAA and related metabolites are now well established as among the strongest biomarkers of obesity, insulin resistance, T2D, and cardiovascular diseases. Lowering of BCAA and branched-chain ketoacid (BCKA) levels by feeding BCAA-restricted diet or by the activation of the rate-limiting enzyme in BCAA catabolism, branched-chain ketoacid dehydrogenase (BCKDH), in rodent models of obesity have clear salutary effects on glucose and lipid homeostasis, but BCAA restriction has more modest effects in short-term studies in human T2D subjects. Feeding of rats with diets enriched in sucrose or fructose result in the induction of the ChREBP transcription factor in the liver to increase expression of the BCKDH kinase (BDK) and suppress the expression of its phosphatase (PPM1K) resulting in the inactivation of BCKDH and activation of the key lipogenic enzyme ATP-citrate lyase (ACLY). These and other emergent links between BCAA, glucose, and lipid metabolism motivate ongoing studies of possible causal actions of BCAA and related metabolites in the development of cardiometabolic diseases.

White PJ ，McGarrah RW ，Herman MA ，Bain JR ，Shah SH ，Newgard CB ... -  《Molecular Metabolism》

Regulation of adipose branched-chain amino acid catabolism enzyme expression and cross-adipose amino acid flux in human obesity.

Lackey DE ，Lynch CJ ，Olson KC ，Mostaedi R ，Ali M ，Smith WH ，Karpe F ，Humphreys S ，Bedinger DH ，Dunn TN ，Thomas AP ，Oort PJ ，Kieffer DA ，Amin R ，Bettaieb A ，Haj FG ，Permana P ，Anthony TG ，Adams SH ... -  《-》