Energy metabolism profile of the effects of amino acid treatment on skeletal muscle cells: Leucine inhibits glycolysis of myotubes

Excess Branched-Chain Amino Acids Suppress Mitochondrial Function and Biogenic Signaling but Not Mitochondrial Dynamics in a Myotube Model of Skeletal Muscle Insulin Resistance

Branched-chain amino acids (BCAA) are correlated with severity of insulin resistance, which may partially result from mitochondrial dysfunction. Mitochondrial dysfunction is also common during insulin resistance and is regulated in part by altered mitochondrial fusion and fission (mitochondrial dynamics). To assess the effect of BCAA on mitochondrial dynamics during insulin resistance, the present study examined the effect of BCAA on mitochondrial function and indicators of mitochondrial dynamics in a myotube model of insulin resistance. C2C12 myotubes were treated with stock DMEM or DMEM with additional BCAA at 0.2 mM, 2 mM, or 20 mM to achieve a continuum of concentrations ranging from physiologically attainable to supraphysiological (BCAA overload) both with and without hyperinsulinemia-mediated insulin resistance. qRT-PCR and Western blot were used to measure gene and protein expression of targets associated with mitochondrial dynamics. Mitochondrial function was assessed by oxygen consumption, and mitochondrial content was measured using mitochondrial-specific staining. Insulin resistance reduced mitochondrial function, peroxisome proliferator-activated receptor gamma coactivator 1-alpha mRNA, and citrate synthase expression mRNA, but not protein expression. Excess BCAA at 20 mM also independently reduced mitochondrial function in insulin-sensitive cells. BCAA did not alter indicators of mitochondrial dynamics at the mRNA or protein level, while insulin resistance reduced mitochondrial fission protein 1 mRNA, but not protein expression. Collectively, BCAA at excessively high levels or coupled with insulin resistances reduce mitochondrial function and content but do not appear to alter mitochondrial dynamics under the tested conditions.

Effects of Low-Salinity Stress on Histology and Metabolomics in the Intestine of Fenneropenaeus chinensis

Metabolomics has been used extensively to identify crucial molecules and biochemical effects induced by environmental factors. To understand the effects of acute low-salinity stress on Fenneropenaeus chinensis, intestinal histological examination and untargeted metabonomic analysis of F. chinensis were performed after exposure to a salinity of 15 ppt for 3, 7, and 14 d. The histological examination revealed that acute stress resulted in most epithelial cells rupturing, leading to the dispersion of nuclei in the intestinal lumen after 14 days. Metabolomics analysis identified numerous differentially expressed metabolites (DEMs) at different time points after exposure to low-salinity stress, in which some DEMs were steadily downregulated at the early stage of stress and then gradually upregulated. We further screened 14 overlapping DEMs, in which other DEMs decreased significantly during low-salinity stress, apart from L-palmitoylcarnitine and vitamin A, with enrichments in phenylalanine, tyrosine and tryptophan biosynthesis, fatty acid and retinol metabolism, and ABC transporters. ABC transporters exhibit significant abnormalities and play a vital role in low-salinity stress. This study provides valuable insights into the molecular mechanisms underlying the responses of F. chinensis to acute salinity stress.

The BCKDH kinase inhibitor BT2 promotes BCAA disposal and mitochondrial proton leak in both insulin-sensitive and insulin-resistant C2C12 myotubes

Elevated circulating branched-chain amino acids (BCAAs) have been correlated with the severity of insulin resistance, leading to recent investigations that stimulate BCAA metabolism for the potential benefit of metabolic diseases. BT2 (3,6-dichlorobenzo[b]thiophene-2-carboxylic acid), an inhibitor of branched-chain ketoacid dehydrogenase kinase, promotes BCAA metabolism by enhancing BCKDH complex activity. The purpose of this report was to investigate the effects of BT2 on mitochondrial and glycolytic metabolism, insulin sensitivity, and de novo lipogenesis both with and without insulin resistance. C2C12 myotubes were treated with or without low or moderate levels of BT2 with or without insulin resistance. Western blot and quantitative real-time polymerase chain reaction were used to assess protein and gene expression, respectively. Mitochondrial, nuclei, and lipid content were measured using fluorescent staining and microscopy. Cell metabolism was assessed via oxygen consumption and extracellular acidification rate. Liquid chromatography-mass spectrometry was used to quantify BCAA media content. BT2 treatment consistently promoted mitochondrial uncoupling following 24-h treatment, which occurred largely independent of changes in expressional profiles associated with mitochondrial biogenesis, mitochondrial dynamics, BCAA catabolism, insulin sensitivity, or lipogenesis. Acute metabolic studies revealed a significant and dose-dependent effect of BT2 on mitochondrial proton leak, suggesting BT2 functions as a small-molecule uncoupler. Additionally, BT2 treatment consistently and dose-dependently reduced extracellular BCAA levels without altering expression of BCAA catabolic enzymes or pBCKDHa activation. BT2 appears to act as a small-molecule mitochondrial uncoupler that promotes BCAA utilization, though the interplay between these two observations requires further investigation.

Citrus p-Synephrine Improves Energy Homeostasis by Regulating Amino Acid Metabolism in HFD-Induced Mice

p-Synephrine is a common alkaloid widely distributed in citrus fruits. However, the effects of p-synephrine on the metabolic profiles of individuals with energy abnormalities are still unclear. In the study, we investigated the effect of p-synephrine on energy homeostasis and metabolic profiles using a high fat diet (HFD)-induced mouse model. We found that p-synephrine inhibited the gain in body weight, liver weight and white adipose tissues weight induced by HFD. p-Synephrine supplementation also reduced levels of serum total cholesterol (TC), triglyceride (TG) and low-density lipoprotein cholesterol (LDL-C) but not to a statistically significant degree. Histological analysis showed that HFD induced excessive lipid accumulation and glycogen loss in the liver and adipocyte enlargement in perirenal fat tissue, while p-synephrine supplementation reversed the changes induced by HFD. Moreover, HFD feeding significantly increased mRNA expression levels of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) and reduced the mRNA expression level of interleukin-10 (IL-10) compared to the control group, while p-synephrine supplementation significantly reversed these HFD-induced changes. Liver and serum metabolomic analysis showed that p-synephrine supplementation significantly altered small molecule metabolites in liver and serum in HFD mice and that the changes were closely associated with improvement of energy homeostasis. Notably, amino acid metabolism pathways, both in liver and serum samples, were significantly enriched. Our study suggests that p-synephrine improves energy homeostasis probably by regulating amino acid metabolism in HFD mice, which provides a novel insight into the action mechanism of p-synephrine modulating energy homeostasis.

Loss of 4E-BPs prevents the hindlimb immobilization-induced decrease in protein synthesis in skeletal muscle

The present study was designed to test the hypothesis that upregulating protein synthesis attenuates the loss of muscle mass in a model of disuse atrophy. The studies compared the effect of unilateral hindlimb immobilization in wild-type (WT) mice and double-knockout (DKO) mice lacking the translational regulators 4E-BP1 and 4E-BP2. Immobilization-induced downregulation of protein synthesis occurred in both groups of mice, but protein synthesis was higher in gastrocnemius muscle from the immobilized hindlimb of fasted DKO compared with WT mice. Surprisingly, although protein synthesis was partially elevated in DKO compared with WT mice, atrophy occurred to the same extent in both groups of animals. This may be partially due to impaired leucine-induced stimulation of protein synthesis in DKO compared with WT mice due to downregulated eukaryotic initiation factor eIF4E expression in muscle of DKO compared with WT mice. Expression of the E3 ubiquitin ligases MAFbx and MuRF-1 mRNAs and total protein ubiquitylation was upregulated in the immobilized compared with the nonimmobilized hindlimb of both WT and DKO mice, with little difference in the magnitude of the upregulation between genotypes. Analysis of newly synthesized proteins revealed downregulation of several glycolytic enzymes in the gastrocnemius of DKO mice compared with WT mice, as well as in the immobilized compared with the nonimmobilized hindlimb. Overall, the results suggest that the elevated rate of protein synthesis during hindlimb immobilization in fasted DKO mice is insufficient to prevent disuse-induced muscle atrophy, probably due to induction of compensatory mechanisms including downregulation of eIF4E expression.NEW & NOTEWORTHY Basal rates of protein synthesis are elevated in skeletal muscle in the immobilized leg of mice lacking the translational repressors, 4E-BP1 and 4E-BP2 (knockout mice), compared with wild-type mice. However, disuse-induced muscle atrophy occurs to the same extent in both wild-type and knockout mice suggesting that compensatory mechanisms are induced that overcome the upregulation of muscle protein synthesis. Proteomic analysis revealed that mRNAs encoding several glycolytic enzymes are differentially translated in wild-type and knockout mice.

Branched-Chain Amino Acids and Mitochondrial Biogenesis: An Overview and Mechanistic Summary

Branched-chain amino acids (BCAA) are essential in the diet and promote several vital cell responses which may have benefits for health and athletic performance, as well as disease prevention. While BCAA are well-known for their ability to stimulate muscle protein synthesis, their effects on cell energetics are also becoming well-documented, but these receive less attention. In this review, much of the current evidence demonstrating BCAA ability (as individual amino acids or as part of dietary mixtures) to alter regulators of cellular energetics with an emphasis on mitochondrial biogenesis and related signaling is highlighted. Several studies have shown, both in vitro and in vivo, that BCAA (either individual or as a mixture) may promote signaling associated with increased mitochondrial biogenesis including the upregulation of master regulator of mitochondrial biogenesis peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), as well as numerous downstream targets and related function. However, sparse data in humans and the difficulty of controlling variables associated with feeding studies leave the physiological relevance of these findings unclear. Future well-controlled diet studies will be needed to assess if BCAA consumption is associated with increased mitochondrial biogenesis and improved metabolic outcomes in healthy and/or diseased human populations.

The Effect of Dietary Leucine Supplementation on Antioxidant Capacity and Meat Quality of Finishing Pigs under Heat Stress

This study examined the effects of dietary leucine supplements on antioxidant capacity and meat quality in growing-finishing pigs. A total of 24 crossbred (Duroc × Landrace × Yorkshire) pigs with an average initial weight of 68.33 ± 0.97 kg were randomly allotted to three treatment groups. All pigs were exposed to constant heat stress. Each group of pigs was fed a basal diet, or a diet supplemented with increasing levels of leucine (0.25% or 0.50%). The results showed that leucine intake could improve average daily gain and reduce feed/gain of finishing pigs under heat stress (p < 0.05). The supplementation of leucine could improve the carcass slant length (p = 0.09), and dramatically increased loin-eye area of the finishing pigs (p < 0.05) but had no significant effect on other carcass traits. Compared with the control group, 0.50% leucine markedly reduced drip loss and shear force of longissimus dorsi muscle, and increased pH value at 24 h after slaughter (p < 0.05). Dietary supplementation of 0.25% leucine increased the contents of inosine monophosphate and intramuscular fat in biceps femoris muscle (p < 0.05). Supplementation of 0.25% or 0.50% leucine significantly stimulated the activities of antioxidant enzymes while reduced the level of MDA in serum, liver and longissimus dorsi muscle (p < 0.05). Compared with the control group, 0.50% leucine supplementation markedly modulated the relative mRNA expression levels of genes related to muscle fiber type and mitochondrial function in longissimus dorsi muscle and the gene relative antioxidant in the liver (p < 0.05). In conclusion, dietary leucine supplementation could improve the growth performance and meat quality of the finishing pigs under heat stress, and the pathway of Keap1-NRF2 and PGC-1α-TFAM might be involved.

Leucine induces cardioprotection in vitro by promoting mitochondrial function via mTOR and Opa-1 signaling

BACKGROUND AND AIMS

Coronary heart disease is a major global health concern. Further, severity of this condition is greatly influenced by myocardial ischemia/reperfusion (I/R) injury. Branched-chain amino acids (BCAAs) have cardioprotective effects against I/R via mammalian target of rapamycin (mTOR) activity, wherein Leu is considered to particularly regulate mTOR activation. However, the mechanism underlying cardioprotective effects of Leu via mTOR activity is not fully elucidated. Here, we aimed to study the signaling pathway of cardioprotection and mitochondrial function induced by Leu treatment.

METHODS AND RESULTS

Cardiac myocytes isolated from adult male Wistar rats were incubated and exposed to simulated I/R (SI/R) injury by replacing the air content. Cardiac myocytes were treated with Leu and subsequently, their survival rate was calculated. To elucidate the signaling pathway and mitochondrial function, immunoblots and mitochondrial permeability transition pore were examined. Cell survival rate was decreased with SI/R but improved by 160 μM Leu (38.5 ± 3.6% vs. 64.5 ± 4.2%, respectively, p < 0.001). Although rapamycin (mTOR inhibitor) prevented this cardioprotective effect induced by Leu, wortmannin (PI3K inhibitor) did not interfere with this effect. In addition, we indicated that overexpression of Opa-1 and mitochondrial function are ameliorated via Leu-induced mitochondrial biogenesis. In contrast, knockdown of Opa-1 suppressed Leu-induced cardioprotection.

CONCLUSION

Leu treatment is critical in rendering a cardioprotective effect exhibited by BCAAs via mTOR signaling. Furthermore, Leu improved mitochondrial function.

Energy metabolism profile of the effects of amino acid treatment on hepatocytes: Phenylalanine and phenylpyruvate inhibit glycolysis of hepatocytes

OBJECTIVES

Amino acids are not only the building blocks of proteins, but also can be metabolized to energy substances or function as signaling molecules. The aim of this study was to profile whether amino acid treatment (essential amino acids and alanine) affects the energy metabolism (glycolysis, mitochondrial respiration) of cultured hepatocytes.

METHODS

AML12 hepatocytes were treated with 5 mM of each amino acid for 1 h and the energy metabolism was then measured by using an extracellular flux analyzer.

RESULTS

The results showed that phenylalanine and lysine decreased the extracellular acidification rate (ECAR), an indirect indicator of glycolysis, whereas isoleucine and histidine increased the ECAR. Amino acids did not affect the oxygen consumption rate, an indirect indicator of mitochondrial respiration. The glycolysis stress test revealed that treatment of the hepatocytes with phenylalanine inhibited glycolysis when the concentration of the substrate for glycolysis was sufficient in cultured media. We also investigated the effect of metabolites derived from conversion of phenylalanine on glycolysis in hepatocytes and found that phenylpyruvate inhibited glycolysis, whereas tyrosine and phenylethylamine did not affect glycolysis.

CONCLUSIONS

The findings from the present study complement basic knowledge of the effects of amino acid treatment on energy metabolism in cultured hepatocytes and indicate that phenylalanine and phenylpyruvate inhibit glycolysis.

Data on the proliferation and differentiation of C2C12 myoblast treated with branched-chain ketoacid dehydrogenase kinase inhibitor

The catabolism of branched chain amino acids (BCAAs) is mainly carried out in skeletal muscle myofibers. It is mediated by branched chain aminotransferase 2 and branched chain alpha ketoacid dehydrogenase (BCKDH) in mitochondria for energy supply, especially during exercise. BCKDH kinase (BCKDK) is a negative regulator of BCAAs catabolism by its inhibitory phosphorylation of the BCKDH E1a subunit. The data presented in this article are related to the research article that we previously have reported entitled “Energy metabolism profile of the effects of amino acid treatment on skeletal muscle cells: Leucine inhibits glycolysis of myotubes” (Suzuki et al., 2020)[1]. In this report, we have demonstrated that 1hour treatment of BT2, an inhibitor of BCKDK, decreased the glycolysis of C2C12 differentiated myotubes compared to the control. Although BCAAs metabolism is basically assumed to be carried out in differentiated myofibers, BCKDK is expressed in both undifferentiated myoblasts and differentiated myotubes, and the biological and physiological significance of BCAAs metabolism in myoblasts is still unclear. Present data demonstrate an in vitro assessment of BT2 on C2C12 myoblasts proliferation and differentiation. The data suggest that activation of BCAAs catabolism by the BCKDK inhibitor BT2 impairs C2C12 myoblasts proliferation and differentiation.