Isolated branched-chain amino acid intake and muscle protein synthesis in humans: a biochemical review / Consumo isolado de aminoácidos de cadeia ramificada e síntese de proteína muscular em humanos: uma revisão bioquímica

ABSTRACT Alongside a proper diet, ergogenic aids with potential direct and/or indirect physical performance enhancing effects are sought after for improved adaptation to physical training. Nutritional ergogenics include diet composition changes and/or dietary supplementation. Branched-chain amino acids valine, leucine and isoleucine are widely popular among products with ergogenic claims. Their major marketing appeal derives from allegations that branched-chain amino acids intake combined with resistance physical exercise stimulates muscle protein synthesis. Evidence supporting the efficacy of branched-chain amino acids alone for muscle hypertrophy in humans is somewhat equivocal. This brief review describes physiological and biochemical mechanisms underpinning the effects of complete protein source and branched-chain amino acid intake on skeletal muscle growth in the postabsorptive and post-exercise state. Evidence in favor of or against potential anabolic effects of isolated branched-chain amino acid intake on muscle protein synthesis in humans is also examined.

RESUMO No treinamento físico, buscam-se, além de uma dieta adequada, recursos ergogênicos que possam maximizar direta e/ou indiretamente o desempenho físico. Entre as categorias de recursos ergogênicos, o nutricional compreende a modulação da composição dietética e/ou uso de suplementação. A comercialização dos suplementos de aminoácidos de cadeia ramificada valina, leucina e isoleucina possui muita popularidade entre aqueles com alegação ergogênica. O principal marketing está na afirmação de que o consumo isolado de aminoácidos de cadeia ramificada associado ao exercício físico resistido estimula a síntese de proteína muscular. As evidências da eficácia da ingestão isolada de aminoácidos de cadeia ramificada para a hipertrofia muscular em humanos parecem equivocadas. Nesta breve revisão, apresentamos a compreensão fisiológica e bioquímica de como a ingestão de uma fonte completa de proteína e de aminoácidos de cadeia ramificada afeta o crescimento do músculo esquelético no estado pós-absortivo e pós-exercício. Mostramos também as evidências que suportam ou não a afirmação dos potenciais efeitos anabólicos na síntese de proteína muscular dos aminoácidos de cadeia ramificada quando consumidos isoladamente em humanos.

Branched chain amino acid suppresses hepatocellular cancer stem cells through the activation of mammalian target of rapamycin.

Differentiation of cancer stem cells (CSCs) into cancer cells causes increased sensitivity to chemotherapeutic agents. Although inhibition of mammalian target of rapamycin (mTOR) leads to CSC survival, the effect of branched chain amino acids (BCAAs), an mTOR complex 1 (mTORC1) activator remains unknown. In this study, we examined the effects of BCAA on hepatocellular carcinoma (HCC) cells expressing a hepatic CSC marker, EpCAM. We examined the effects of BCAA and/or 5-fluorouracil (FU) on expression of EpCAM and other CSC-related markers, as well as cell proliferation in HCC cells and in a xenograft mouse model. We also characterized CSC-related and mTOR signal-related molecule expression and tumorigenicity in HCC cells with knockdown of Rictor or Raptor, or overexpression of constitutively active rheb (caRheb). mTOR signal-related molecule expression was also examined in BCAA-treated HCC cells. In-vitro BCAA reduced the frequency of EpCAM-positive cells and improved sensitivity to the anti-proliferative effect of 5-FU. Combined 5-FU and BCAA provided better antitumor efficacy than 5-FU alone in the xenograft model. Stimulation with high doses of BCAA activated mTORC1. Knockdown and overexpression experiments revealed that inhibition of mTOR complex 2 (mTORC2) or activation of mTORC1 led to decreased EpCAM expression and little or no tumorigenicity. BCAA may enhance the sensitivity to chemotherapy by reducing the population of cscs via the mTOR pathway. This result suggests the utility of BCAA in liver cancer therapy.

Role of branched chain amino acids in cerebral ammonia homeostasis related to hepatic encephalopathy.

Hepatic encephalopathy (HE) is associated with increased ammonia levels in plasma and brain. Different treatment strategies have been developed to ameliorate the detrimental effects of the ammonia load. One such strategy is based on the finding of a low level of the branched chain amino acids (BCAAs) in plasma of patients suffering from HE and the assumption that in particular isoleucine could be beneficial to brain energy metabolism as it is metabolized to the tricarboxylic acid cycle intermediate and precursor succinyl-CoA and acetyl-CoA, respectively. This would enable de novo synthesis of glutamine via α-ketoglutarate and glutamate and at the same time stimulate oxidative metabolism. The present mini-review summarizes the metabolic basis for this hypothesis delineating studies in the brain in vivo as well as in cultured neural cells aimed at elucidating the metabolism of the BCAAs focusing on isoleucine. The conclusion is that isoleucine appears at least partially to act in this fashion albeit its metabolism is quantitatively relatively modest. In addition, a short section on the role of the BCAAs in synaptic ammonia homeostasis is included along with some thoughts on the role of the BCAAs in other pathologies such as cancer.

Mechanisms and treatment of anorexia in end-stage renal disease patients on hemodialysis.

Anorexia, defined as the loss of the desire to eat, is relatively common in hemodialysis patients, occurring in one third of such cases. The pathogenesis is essentially unknown. It has been proposed that uremic toxins such as middle molecules, inflammation, altered amino acid pattern, leptin, ghrelin, and neuropeptide Y are involved. Anorexia reduces oral energy and protein intakes, thus contributing to the development of malnutrition and cachexia. Unquestionably, it contributes to poor quality of life. The clinical relevance of anorexia as an independent prognostic factor in hemodialysis is debated. The treatment of this debilitating condition is based on a therapeutic strategy that may include daily dialysis sessions and nutritional counseling. Normalization of plasma branched chain amino acids through branched chain amino acid supplementation may decrease anorexia and improve energy and protein intake. The role of megestrol acetate as an appetite stimulant needs to be validated through adequate randomized trials. Subcutaneous ghrelin administration and melanocortin-receptor antagonists appear to be promising therapeutic interventions.

Hormonal and signaling role of branched-chain amino acids.

Amino acids (AAs), especially BCAAs, play pivotal roles in hormonal secretion and action as well as in intracellular signaling. There is emerging data showing that BCAAs regulate gene transcription and translation. Signaling proteins such as the mammalian target of rapamycin act as sensors of BCAAs, especially leucine, to modulate anabolic action. AAs stimulate protein synthesis and inhibit protein breakdown in skeletal muscle and liver. The specific role of BCAAs in regulating synthesis and breakdown of individual protein or proteins with common function or functions remains to be defined. Future studies should also focus on potential adverse effects of BCAAs on insulin sensitivity, renal function, and tumor growth. It also remains to be determined whether potential adverse effects of BCAA supplementation is similar in people of different age groups.

Branched-chain amino acids: a role in skeletal muscle proteolysis in catabolic states?

A 48-h starvation period resulted in a great increase in muscle proteolysis-as measured following the release of tyrosine into the medium-in incubated isolated rat extensor digitorum longus (EDL) muscles. We have quantified the contribution of the different proteolytic systems to the increased protein degradation and observed a considerable activation in the ATP-dependent proteolytic (60%) and in the calcium-dependent (125%) systems, while no increases were observed in lysosomal proteolysis. The addition of 10 mM leucine to the incubation medium did not result in any changes in either total proteolytic rate or the activity rates of any of the different systems studied. In addition, the presence of the amino acid did not influence the levels of mRNA for the different genes studied-ubiquitin, C8 proteasome subunit, E2 conjugating enzyme, m-calpain, and cathepsin B. In a similar way, as observed during starvation, tumor growth resulted in increased protein degradation in incubated isolated EDL muscles from animals bearing the Yoshida AH-130 ascites hepatoma. The increased rate of protein degradation affected all the proteolytic systems studied: ATP- and calcium-dependent and lysosomal. Finally, leucine addition (10 mM), although not able to revert the increased proteolytic rate, resulted in a decrease in the gene expression for ubiquitin, C8 proteasome subunit and cathepsin B.

Interactions among the branched-chain amino acids and their effects on methionine utilization in growing pigs: effects on nitrogen retention and amino acid utilization.

An experiment was conducted to investigate the effects of branched-chain amino acid (BCAA) interactions on their utilization by growing pigs and the effects of excessive amounts of BCAA (leucine, isoleucine, valine) on the utilization of methionine. A semipurified diet containing 100 g crude protein/kg with a balanced amino acid pattern was prepared using casein supplemented with free amino acids. Three further diets were made by reducing the concentration of methionine + cyst(e)ine, valine or isoleucine by 20%. Each of these four diets was then supplemented with leucine (50% excess) or a mixture of BCAA (50% excess of each but excluding the limiting amino acid). All diets were isoenergetic and were made isonitrogenous by replacement of glutamic and aspartic acids. The twelve diets were given to twenty-four growing pigs (30-40 kg) in three periods according to a randomized block design. Each period lasted 8 d and N retention was measured during the last 5 d of each period. Reducing dietary methionine, valine or isoleucine reduced the utilization of N (N retained/N digested) by approximately 20% (P < 0.05). Adding leucine to the isoleucine-limiting diet decreased the utilization of N by 9% (P < 0.05). This was reversed by simultaneous addition of valine. Excess leucine in a valine-deficient diet did not significantly reduce N utilization. In methionine-limiting diets an excess of either leucine alone or of all three BCAA increased the utilization of N by 8% (P < 0.05).

Muscle prostaglandin production in the rat. Effect of abdominal sepsis and different amino acid formulations.

Recent in vitro studies attribute regulatory functions to prostaglandins (PGs) in muscle protein metabolism, particularly enhancing proteolysis. In the present study, the amount of muscle PG production from endogenous precursors was determined in control and septic animals (cecal ligation and puncture) that were infused with 5% dextrose or dextrose with three amino acid formulations differing in their branched-chain amino acid (BCAA) content. We could not detect any differences in prostaglandin E, 6-keto-prostaglandin F1 alpha, and thromboxane B2 production between control and septic animals. Furthermore, the infusion of BCAAs, which have previously been shown to be nitrogen sparing following injury, did not influence the production of any of the PGs studied in either control or septic muscle. It is likely that the effects of the BCAAs on muscle synthesis and degradation are independent of the PGs.