Gender difference in regulation of branched-chain amino acid catabolism

Sportomics Analyses of the Exercise-Induced Impact on Amino Acid Metabolism and Acute-Phase Protein Kinetics in Female Olympic Athletes

BACKGROUND

Exercise can be used as a model to understand immunometabolism. Biological data on elite athletes are limited, especially for female athletes, including relevant data on acute-phase proteins and amino acid metabolism.

METHODS

We analyzed acute-phase proteins and amino acids collected at South American, Pan-American, and Olympic Games for 16 Olympic sports. We compared female and male elite athletes (447 vs. 990 samples) across four states (fasting, pre-exercise, post-exercise, and resting) to understand sex-specific immunometabolic responses in elite athletes.

RESULTS

Considering all states and sports, we found that elite female athletes exhibited higher concentrations of C-reactive protein, lipopolysaccharide-binding protein, myeloperoxidase, haptoglobin, and IGF1, with ratios ranging from 1.2 to 2.0 (p < 0.001). Women exhibited lower concentrations of most amino acids, except for glutamate and alanine. Although almost 30% lower in women, branched-chain amino acids (BCAAs) showed a similar pattern in all states (ρ ≥ 0.9; p < 0.001), while aromatic amino acids (AAAs) showed higher consumption during exercise in women.

CONCLUSION

We established sex dimorphism in elite athletes' metabolic and inflammatory responses during training and competition. Our data suggest that female athletes present a lower amino acid response towards central fatigue development than male athletes. Understanding these differences can lead to insights into sex-related immuno-metabolic responses in sports or other inflammatory conditions.

BCAAs acutely drive glucose dysregulation and insulin resistance: role of AgRP neurons

BACKGROUND

High-protein diets are often enriched with branched-chain amino acids (BCAAs) known to enhance protein synthesis and provide numerous physiological benefits, but recent studies reveal their association with obesity and diabetes. In support of this, protein or BCAA supplementation is shown to disrupt glucose metabolism while restriction improves it. However, it is not clear if these are primary, direct effects of BCAAs or secondary to other physiological changes during chronic manipulation of dietary BCAAs.

METHODS

Three-month-old C57Bl/6 mice were acutely treated with either vehicle/BCAAs or BT2, a BCAA-lowering compound, and detailed in vivo metabolic phenotyping, including frequent sampling and pancreatic clamps, were conducted.

RESULTS

Using a catheter-guided frequent sampling method in mice, here we show that a single infusion of BCAAs was sufficient to acutely elevate blood glucose and plasma insulin. While pre-treatment with BCAAs did not affect glucose tolerance, a constant infusion of BCAAs during hyperinsulinemic-euglycemic clamps impaired whole-body insulin sensitivity. Similarly, a single injection of BT2 was sufficient to prevent BCAA rise during fasting and markedly improve glucose tolerance in high-fat-fed mice, suggesting that abnormal glycemic control in obesity may be causally linked to high circulating BCAAs. We further show that chemogenetic over-activation of AgRP neurons in the hypothalamus, as present in obesity, significantly impairs glucose tolerance that is completely normalized by acute BCAA reduction. Interestingly, most of these effects were demonstrated only in male, but not in female mice.

CONCLUSION

These findings suggest that BCAAs per se can acutely impair glucose homeostasis and insulin sensitivity, thus offering an explanation for how they may disrupt glucose metabolism in the long-term as observed in obesity and diabetes. Our findings also reveal that AgRP neuronal regulation of blood glucose is mediated through BCAAs, further elucidating a novel mechanism by which brain controls glucose homeostasis.

Heterogeneously elevated branched-chain/aromatic amino acids among new-onset type-2 diabetes mellitus patients are potentially skewed diabetes predictors

BACKGROUND

The lack of specific predictors for type-2 diabetes mellitus (T2DM) severely impacts early intervention/prevention efforts. Elevated branched-chain amino acids (BCAAs: Isoleucine, leucine, valine) and aromatic amino acids (AAAs: Tyrosine, tryptophan, phenylalanine)) show high sensitivity and specificity in predicting diabetes in animals and predict T2DM 10-19 years before T2DM onset in clinical studies. However, improvement is needed to support its clinical utility.

AIM

To evaluate the effects of body mass index (BMI) and sex on BCAAs/AAAs in new-onset T2DM individuals with varying body weight.

METHODS

Ninety-seven new-onset T2DM patients (< 12 mo) differing in BMI [normal weight (NW), n = 33, BMI = 22.23 ± 1.60; overweight, n = 42, BMI = 25.9 ± 1.07; obesity (OB), n = 22, BMI = 31.23 ± 2.31] from the First People's Hospital of Yunnan Province, Kunming, China, were studied. One-way and 2-way ANOVAs were conducted to determine the effects of BMI and sex on BCAAs/AAAs.

RESULTS

Fasting serum AAAs, BCAAs, glutamate, and alanine were greater and high-density lipoprotein (HDL) was lower (P < 0.05, each) in OB-T2DM patients than in NW-T2DM patients, especially in male OB-T2DM patients. Arginine, histidine, leucine, methionine, and lysine were greater in male patients than in female patients. Moreover, histidine, alanine, glutamate, lysine, valine, methionine, leucine, isoleucine, tyrosine, phenylalanine, and tryptophan were significantly correlated with abdominal adiposity, body weight and BMI, whereas isoleucine, leucine and phenylalanine were negatively correlated with HDL.

CONCLUSION

Heterogeneously elevated amino acids, especially BCAAs/AAAs, across new-onset T2DM patients in differing BMI categories revealed a potentially skewed prediction of T2DM development. The higher BCAA/AAA levels in obese T2DM patients would support T2DM prediction in obese individuals, whereas the lower levels of BCAAs/AAAs in NW-T2DM individuals may underestimate T2DM risk in NW individuals. This potentially skewed T2DM prediction should be considered when BCAAs/AAAs are to be used as the T2DM predictor.

Untargeted metabolomics identifies the potential role of monocarboxylate transporter 6 (MCT6/SLC16A5) in lipid and amino acid metabolism pathways

Monocarboxylate transporter 6 (MCT6; SLC16A5) is an orphan transporter protein with expression in multiple tissues. The endogenous function of MCT6 related to human health and disease remains unknown. Our previous transcriptomic and proteomic analyses in Mct6 knockout (KO) mice suggested that MCT6 may play a role in lipid and glucose homeostasis, but additional evidence is required. Thus, the objective of this study was to further explore the impact of MCT6 on metabolic function using untargeted metabolomic analysis in Mct6 KO mice. The plasma from male and female mice and livers from male mice were submitted for global metabolomics analysis to assess the relative changes in endogenous small molecules across the liver and systemic circulation associated with absence of Mct6. More than 782 compounds were detected with 101 and 51 metabolites significantly changed in plasma of male and female mice, respectively, and 100 metabolites significantly changed in the livers of male mice (p < .05). Significant perturbations in lipid metabolism were annotated in the plasma and liver metabolome, with additional alterations in the amino acid metabolism pathway in plasma samples from male and female mice. Elevated lipid diacylglycerol and altered fatty acid metabolite concentrations were found in liver and plasma samples of male Mct6 KO mice. Significant reduction of N-terminal acetylated amino acids was found in plasma samples of male and female Mct6 KO mice. In summary, the present study confirmed the significant role of MCT6 in lipid and amino acid homeostasis, suggesting its contribution in metabolic diseases.

Serum Metabolite Profile Associated with Sex-Dependent Visceral Adiposity Index and Low Bone Mineral Density in a Mexican Population

Recent evidence shows that obesity correlates negatively with bone mass. However, traditional anthropometric measures such as body mass index could not discriminate visceral adipose tissue from subcutaneous adipose tissue. The visceral adiposity index (VAI) is a reliable sex-specified indicator of visceral adipose distribution and function. Thus, we aimed to identify metabolomic profiles associated with VAI and low bone mineral density (BMD). A total of 602 individuals from the Health Workers Cohort Study were included. Forty serum metabolites were measured using the targeted metabolomics approach, and multivariate regression models were used to test associations of metabolomic profiles with anthropometric, clinical, and biochemical parameters. The analysis showed a serum amino acid signature composed of glycine, leucine, arginine, valine, and acylcarnitines associated with high VAI and low BMD. In addition, we found a sex-dependent VAI in pathways related to primary bile acid biosynthesis, branched-chain amino acids, and the biosynthesis of pantothenate and coenzyme A (CoA). In conclusion, a metabolic profile differs by VAI and BMD status, and these changes are gender-dependent.

Branched-chain Amino Acids: Catabolism in Skeletal Muscle and Implications for Muscle and Whole-body Metabolism

Branched-chain amino acids (BCAAs) are critical for skeletal muscle and whole-body anabolism and energy homeostasis. They also serve as signaling molecules, for example, being able to activate mammalian/mechanistic target of rapamycin complex 1 (mTORC1). This has implication for macronutrient metabolism. However, elevated circulating levels of BCAAs and of their ketoacids as well as impaired catabolism of these amino acids (AAs) are implicated in the development of insulin resistance and its sequelae, including type 2 diabetes, cardiovascular disease, and of some cancers, although other studies indicate supplements of these AAs may help in the management of some chronic diseases. Here, we first reviewed the catabolism of these AAs especially in skeletal muscle as this tissue contributes the most to whole body disposal of the BCAA. We then reviewed emerging mechanisms of control of enzymes involved in regulating BCAA catabolism. Such mechanisms include regulation of their abundance by microRNA and by post translational modifications such as phosphorylation, acetylation, and ubiquitination. We also reviewed implications of impaired metabolism of BCAA for muscle and whole-body metabolism. We comment on outstanding questions in the regulation of catabolism of these AAs, including regulation of the abundance and post-transcriptional/post-translational modification of enzymes that regulate BCAA catabolism, as well the impact of circadian rhythm, age and mTORC1 on these enzymes. Answers to such questions may facilitate emergence of treatment/management options that can help patients suffering from chronic diseases linked to impaired metabolism of the BCAAs.

Antihypertensive drug valsartan as a novel BDK inhibitor

Catabolism of branched-chain amino acids (BCAAs) is affected by various physiological conditions and its abnormality is associated with glucose metabolism, heart disease, and neurological dysfunction. The first two steps of the BCAA metabolic pathway are common to the three BCAAs (leucine, isoleucine, and valine). The second step is an irreversible rate-limited reaction catalyzed by branched-chain α-keto acid dehydrogenase (BCKDH), which is bound to a specific kinase, BCKDH kinase (BDK), and inactivated by phosphorylation. Here, we investigated potential new BDK inhibitors and discovered valsartan, an angiotensin II type 1 receptor (AT1R) blocker, as a new BDK inhibitor. BCKDH phosphorylation and the BCKDH-BDK interaction were inhibited by valsartan in vitro. Valsartan administration in rats resulted in increased BCKDH activity by decreasing the dephosphorylated level of BCKDH complex, bound forms of BDK from BCKDH complex as well as decreased plasma BCAA concentrations. Valsartan is a novel BDK inhibitor that competes with ATP, via a different mechanism from allosteric inhibitors. The BDK inhibitor has been shown to preserve cardiac function in pressure overload-induced heart failure mice and to attenuate insulin resistance in obese mice. Our findings suggest that valsartan is a potent seed compound for developing a powerful BDK inhibitor and useful medication for treating heart failure and metabolic diseases with suppressed BCAA catabolism.

Association Between Plasma Metabolites and Psychometric Scores Among Children With Developmental Disabilities: Investigating Sex-Differences

Background: Developmental disabilities are defined by delays in learning, language, and behavior, yet growing evidence has revealed disturbances in metabolic systems that may also be present. Little is known about whether these metabolic issues contribute to the symptoms or severity of these disabilities, or whether sex plays a role in these associations, given that boys are disproportionately affected by some developmental disabilities. Here we sought to investigate the correlation between psychometric scores, sex, and the plasma metabolome. Methods: The plasma metabolomes of children with autism spectrum disorder (ASD; n = 167), idiopathic developmental delay (i-DD; n = 51), Down syndrome (DS; n = 31), and typically developing controls (TD; n = 193) were investigated using NMR spectroscopy. Spearman rank correlations and multiple linear regression models (adjusted for child's neurodevelopmental diagnosis, child's sex, child's age, child's race/ethnicity, maternal age at child's birth, and parental homeownership) were used to examine the association between plasma metabolites and sex in relation to psychometric measures of cognitive skills, adaptive behavior, and maladaptive behavior in our study population. Results: Higher levels of metabolites involved in cellular energy and mitochondrial function among children with ASD (fumarate and cis-aconitate), DS (lactate), and TD (pyruvate) are associated with poorer cognitive and adaptive subscales. Similarly, higher o-acetylcarnitine associated with deficits in cognitive subscales among all DS cases and TD boys, and carnitine correlated with increased maladaptive behavior among girls with ASD and girls with DS. Among children with DS, elevated myo-inositol, ornithine, and creatine correlated with poorer scores across several subscales. Even among TD cases, elevated 3-hydroxybutyrate correlated with decreased receptive language. In contrast, higher levels of glutamate were associated with better socialization skills among ASD cases. Even after adjusting for the child's neurodevelopmental diagnosis, sex, and other possible confounders, key metabolites including glycolysis metabolites (lactate and pyruvate), ketone bodies (3-hydroxybutyrate and acetoacetate), TCA cycle metabolites (cis-aconitate and fumarate), as well as ornithine were associated with deficits in multiple domains of cognitive function, adaptive skills, and aberrant behaviors. Conclusions: Our results highlight that some plasma metabolites may relate to specific functional subdomains within cognitive, adaptive, and behavioral development with some variation by diagnosis and sex.

Differential Network Analysis Reveals Molecular Determinants Associated with Blood Pressure and Heart Rate in Healthy Subjects

There is mounting evidence that subclinical nonpathological high blood pressure and heart rate during youth and adulthood steadily increase the risk of developing a cardiovascular disease at a later stage. For this reason, it is important to understand the mechanisms underlying the subclinical elevation of blood pressure and heart rate in healthy, relatively young individuals. In the present study, we present a network-based metabolomic study of blood plasma metabolites and lipids measured using nuclear magnetic resonance spectroscopy on 841 adult healthy blood donor volunteers, which were stratified for subclinical low and high blood pressure (systolic and diastolic) and heart rate. Our results indicate a rewiring of metabolic pathways active in high and low groups, indicating that the subjects with subclinical high blood pressure and heart rate could present latent cardiometabolic dysregulations.

Branched-chain and aromatic amino acids and cardiometabolic risk in Black African and Asian Indian populations

INTRODUCTION

Studies have shown that systemic levels of branched-chain amino acids (BCAAs) and aromatic amino acids (AAAs) are elevated in cardiometabolic diseases (CMDs) in populations resident in high income countries. However, little is known about the association of BCAAs and AAAs with metabolic syndrome and its components in Asian Indian (AI) and Black African (BA) populations.

OBJECTIVE

The aim of this study was to describe the association of BCAAs and AAAs with the metabolic syndrome, its individual components and insulin resistance in AI and BA populations.

METHODS

Serum samples collected from AI (n = 349) and BA (n = 369) subjects were used to measure levels of BCAAs and AAAs by ultra-pressure liquid chromatography tandem mass spectrometry (UPLC-MS/MS). Anthropometric, demographic and cardiometabolic variables were measured in all subjects.

RESULTS

The sum of BCAAs and AAAs was higher in AIs compared to BAs. The BCAAs and AAAs were positively associated with insulin resistance, metabolic syndrome and its individual components. This was particularly the case for AI subjects, in unadjusted regression models. However, these associations were non-significant after adjusting for co-variates, particularly visceral adipose tissue (VAT). Triglyceride levels were significantly associated with valine and leucine levels in BAs even after adjustment for co-variates. Lastly, we found that fasting circulatory BCAA and AAA levels are strongly correlated with VAT in both populations.

CONCLUSION

This study identified specific associations of serum valine and leucine levels with triglycerides in BAs. The association of amino acids with CMDs was observed in AIs, but was found to be the result of confounding by VAT. Further studies are required to determine whether BCAAs and AAAs are aetiological factors in CMDs and how VAT modulates their serum levels.

Effects of sex and chronic cigarette smoke exposure on the mouse cecal microbiome

RATIONALE

Chronic smoke exposure is associated with weight loss in patients with Chronic Obstructive Pulmonary Disease (COPD). However, the biological contribution of chronic smoking and sex on the cecal microbiome has not been previously investigated.

METHODS

Adult male, female and ovariectomized mice were exposed to air (control group) or smoke for six months using a standard nose-only smoke exposure system. DNA was extracted from the cecal content using the QIAGEN QIAamp® DNA Mini Kit. Droplet digital PCR was used to generate total 16S bacterial counts, followed by Illumina MiSeq® analysis to determine microbial community composition. The sequencing data were resolved into Amplicon Sequence Variants and analyzed with the use of QIIME2®. Alpha diversity measures (Richness, Shannon Index, Evenness and Faith's Phylogenetic Diversity) and beta diversity (based on Bray-Curtis distances) were assessed and compared according to smoke exposure and sex.

RESULTS

The microbial community was different between male and female mice, while ovariectomy made the cecal microbiome similar to that of male mice. Chronic smoke exposure led to significant changes in the cecal microbial community in both male and female mice. The organism, Alistipes, was the most consistent bacteria identified at the genus level in the cecal content that was reduced with chronic cigarette exposure and its expression was positively related to the whole-body weight of these mice.

CONCLUSION

Chronic smoke exposure is associated with changes in the cecal content microbiome; these changes may play a role in the weight changes that are observed in cigarette smokers.

Sex, gut microbiome, and cardiovascular disease risk

Key differences exist between men and women in the determinants and manifestations of cardiovascular and cardiometabolic diseases. Recently, gut microbiome-host relations have been implicated in cardiovascular disease and associated metabolic conditions; therefore, gut microbiota may be key mediators or modulators driving the observed sexual dimorphism in disease onset and progression. While current evidence regarding pure physiological sex differences in gut microbiome composition is modest, robust research suggests that gut microbiome-dependent metabolites may interact with important biological pathways under sex hormone control, including toll-like receptor and flavin monooxygenase signaling. Here, we review key sex differences in gut microbiome interactions with four primary determinants of cardiovascular disease, impaired glucose regulation, dyslipidemia, hypertension, and obesity. Through this process, we propose important sex differences in downstream metabolic pathways that may be at the interface of the gut microbiome and cardiovascular disease.

Change of gene expression on protein uptake composition and hindlimb-suspension in rat skeletal muscle

[Purpose]

This study was to investigate changes in BCAT and BCKDH genes by Hindlimb-Suspension (HS) and protein intake composition (casein, Whey protein) in rats.

[Methods]

Following 5-day preliminary feeding, forty-eight male 5 weeks old Sprague Dawley albino rats (110g) divided into 17% protein intake group (24 rats) and 30% protein intake group (24 rats), and each group divided further into Hindlimb-Suspension group (HS; 12 rats) and control group(CON; 12 rats). Eventually, this study was performed with Whey protein intake group (HS; 6 rats, CON; 6 rats) and casein intake group (HS; 6 rats, CON; 6 rats). For analysis purposes, total RNA was extracted from isolated skeletal muscles, and mRNA expression was analyzed using Real Time PCR. Two-way ANOVA was performed to examine the difference in BCATm and BCKDH mRNA expression on protein uptake and myoatrophy. post-hoc test was perform on interaction if any, and significance level was set at p<0.05.

[Results]

In this study, BCATm and BCKDH gene analysis in rat skeletal muscles by hindlimb-suspension and protein intake composition resulted in significant higher BCATm expression in 30% dietary protein group and hindlimb-suspension group that control group. In addition, regarding BCKDH, BCKDH was significantly higher in hindlimb-suspended 30% protein intake group than control group.

[Conclusion]

Overall, protein intake and myoatrophy demonstrated close relationship in skeletal muscles. Therefore, it is likely to affect effectively in prevention or recovery of exercise induced muscle disorder. This effect is considered to be applied to maintain and improve health of not only athletes but also the general public. Additionally it would be applied in convalescent rehabilitation due to skeletal muscle atrophy.

Gender-specific pathway differences in the human serum metabolome

The susceptibility for various diseases as well as the response to treatments differ considerably between men and women. As a basis for a gender-specific personalized healthcare, an extensive characterization of the molecular differences between the two genders is required. In the present study, we conducted a large-scale metabolomics analysis of 507 metabolic markers measured in serum of 1756 participants from the German KORA F4 study (903 females and 853 males). One-third of the metabolites show significant differences between males and females. A pathway analysis revealed strong differences in steroid metabolism, fatty acids and further lipids, a large fraction of amino acids, oxidative phosphorylation, purine metabolism and gamma-glutamyl dipeptides. We then extended this analysis by a network-based clustering approach. Metabolite interactions were estimated using Gaussian graphical models to get an unbiased, fully data-driven metabolic network representation. This approach is not limited to possibly arbitrary pathway boundaries and can even include poorly or uncharacterized metabolites. The network analysis revealed several strongly gender-regulated submodules across different pathways. Finally, a gender-stratified genome-wide association study was performed to determine whether the observed gender differences are caused by dimorphisms in the effects of genetic polymorphisms on the metabolome. With only a single genome-wide significant hit, our results suggest that this scenario is not the case. In summary, we report an extensive characterization and interpretation of gender-specific differences of the human serum metabolome, providing a broad basis for future analyses.

Brain insulin lowers circulating BCAA levels by inducing hepatic BCAA catabolism

Circulating branched-chain amino acid (BCAA) levels are elevated in obesity/diabetes and are a sensitive predictor for type 2 diabetes. Here we show in rats that insulin dose-dependently lowers plasma BCAA levels through induction of hepatic protein expression and activity of branched-chain α-keto acid dehydrogenase (BCKDH), the rate-limiting enzyme in the BCAA degradation pathway. Selective induction of hypothalamic insulin signaling in rats and genetic modulation of brain insulin receptors in mice demonstrate that brain insulin signaling is a major regulator of BCAA metabolism by inducing hepatic BCKDH. Short-term overfeeding impairs the ability of brain insulin to lower BCAAs in rats. High-fat feeding in nonhuman primates and obesity and/or diabetes in humans is associated with reduced BCKDH protein in liver. These findings support the concept that decreased hepatic BCKDH is a major cause of increased plasma BCAAs and that hypothalamic insulin resistance may account for impaired BCAA metabolism in obesity and diabetes.

The metabolite profiles of the obese population are gender-dependent

Studies have identified that several amino acids, in particular, branched-chain amino acids (BCAAs), have increased significantly in obese individuals when compared to lean individuals. Additionally, these metabolites were strongly associated with future diabetes, which rendered them prognostic markers suitable for obese populations. Here we report a metabonomic study that reveals new findings on the role of these amino acid markers, particularly BCAAs, in a Chinese cohort including 106 healthy obese and 105 healthy lean participants. We found that the BCAAs were correlated with insulin resistance and differentially expressed in obese men, but not in obese women. The results were verified with two independent groups of participants (Chinese, n = 105 and American, n = 72) and demonstrate that the serum metabolite profiles of the obese population are gender-dependent. The study supports the previous findings of a panel of several key metabolites as prognostic markers of the obese population and highlights the need to take into account gender differences when using these markers for risk assessment.

Decreased enzyme activity and contents of hepatic branched-chain alpha-keto acid dehydrogenase complex subunits in a rat model for type 2 diabetes mellitus

The mitochondrial branched-chain alpha-keto acid dehydrogenase complex (BCKDC) is responsible for the committed step in branched-chain amino acid catabolism. In the present study, we examined BCKDC regulation in Otsuka Long-Evans Tokushima Fatty (OLETF) rats both before (8 weeks of age) and after (25 weeks of age) the onset of type 2 diabetes mellitus. Long-Evans Tokushima Otsuka (LETO) rats were used as controls. Plasma branched-chain amino acid and branched-chain alpha-keto acid concentrations were significantly increased in young and middle-aged OLETF rats. Although the hepatic complex was nearly 100% active in all animals, total BCKDC activity and protein abundance of E1alpha, E1beta, and E2 subunits were markedly lower in OLETF than in LETO rats at 8 and 25 weeks of age. In addition, hepatic BCKDC activity and protein amounts were significantly decreased in LETO rats aged 25 weeks than in LETO rats aged 8 weeks. In skeletal muscle, E1beta and E2 proteins were significantly reduced, whereas E1alpha tended to increase in OLETF rats. Taken together, these results suggest that (1) whole-body branched-chain alpha-keto acid oxidation capacity is extremely reduced in OLETF rats independently of diabetes development, (2) the aging process decreases BCKDC activity and protein abundance in the liver of normal rats, and (3) differential posttranscriptional regulation for the subunits of BCKDC may exist in skeletal muscle.

Branched-chain amino acid catabolism in exercise and liver disease

Branched-chain alpha-keto acid dehydrogenase (BCKDH) complex, the enzyme catalyst for the second step of the BCAA catabolic pathway, plays a central role in the regulation of BCAA catabolism. The activity of the complex is regulated by a covalent modification cycle in which phosphorylation by BCKDH kinase inactivates and dephosphorylation by BCKDH phosphatase activates the complex. Many studies suggest that control of the activity of the kinase is a primary determinant of the activity of the complex. The kinase exists at all times in the mitochondrial matrix space in two forms, with a large amount being free and a smaller amount bound rather tightly to the BCKDH complex. Only the bound form of the kinase appears to be catalytically active and, therefore, responsible for phosphorylation and inactivation of the complex. alpha-Ketoisocaproate, the transamination product of leucine and the most important known physiological inhibitor of BCKDH kinase, promotes release of the kinase from the complex. alpha-Chloroisocaproate, the analogue of leucine and the most potent known inhibitor of the kinase, is more effective than alpha-ketoisocaproate in promoting release of BCKDH kinase from the complex. Exercise and chronic liver disease (liver cirrhosis) likewise decrease the amount of the kinase bound to the complex in rat liver. The resulting activation of the BCKDH complex appears responsible for the increase in BCAA catabolism caused by exercise and liver cirrhosis. Our findings support the use of BCAA supplements for patients with liver cirrhosis.

Activation of hepatic branched-chain alpha-keto acid dehydrogenase complex by tumor necrosis factor-alpha in rats

Tumor necrosis factor-alpha (TNFalpha) promotes oxidation of branched-chain amino acids (BCAA). BCAA catabolism is regulated by branched-chain alpha-keto acid dehydrogenase (BCKDH) complex, which is regulated by phosphorylation-dephosphorylation of the E1alpha subunit at Ser293. BCKDH kinase is responsible for inactivation of the complex by phosphorylation. In the present study, we examined the effects of TNFalpha administration on hepatic BCKDH complex and kinase in rats. Rats were intravenously administered with 25 or 50 microg TNFalpha/kg body weight 4 h prior to sacrifice. The TNFalpha treatment at both doses elevated the activity state (percentage of the active form) of BCKDH complex from 22% to 69% and 86%, respectively, and the amount of phospho-Ser293 on the E1alpha subunit in each group of rats corresponded inversely to the activity state of BCKDH complex. The TNFalpha treatment of rats significantly decreased the activity as well as the bound form of BCKDH kinase. These results suggest that the decrease in the bound form of kinase is involved in the mechanism responsible for TNFalpha-induced activation of the BCKDH complex.

The reduction of protein intake observed in old rats depends on the type of protein

Previous experiments have shown in Lou/c/jall rats growing old a deleterious reduction of protein intake, which occurs earlier in males than in females. We previously showed that this decrease could not be attributed to a loss of regulation of protein intake with age. Present studies were designed to investigate if the age-related decrease of protein intake was dependent on the type of protein used. In a first sectional study, adult, middle and old-aged Lou/c/jall rats were submitted to a self-selection procedure. They were fed successively with casein, whey protein and fish flour as protein. In a second longitudinal study, self-selected males and females were tested each 4 months (at 3, 7, 11, 15, 19, 23 and 27 months of age) with only casein and whey protein as protein. In the two experiments, the type of dietary protein had an influence on the protein intake: when casein is offered, the well-established decrease in protein consumption was seen after 15 months of age in male groups. The introduction of whey protein induced maintenance of protein intake in old male groups at the level of female's protein intake. Moreover, young females showed an obvious preference for casein versus whey protein. This preference disappeared in old ages. These data showed that casein, even if it was an appropriate protein for young animals, could become inadequate for old animals and could result in a protein aversion in old rats. On the contrary, whey protein seemed to be a more appropriate protein than casein for old rats.

Estrogen controls branched-chain amino acid catabolism in female rats

A diurnal rhythm occurs in the activity state of branched-chain alpha-keto acid dehydrogenase complex (BCKDC) in female but not male rats. We attempted to determine the role played by ovarian hormones in this difference in enzyme regulation. A series of experiments examined the effects of the 4-d estrous cycle, ovariectomy, and replacement of female sex steroids on the catabolism of BCAAs. A proestrous decrease in the activity state of the complex corresponded to an increase in the plasma 17beta-estradiol level. Withdrawal of gonadal steroids by ovariectomy resulted in an increase in the activity state of BCKDC and a decrease in the activity of the branched-chain alpha-keto acid dehydrogenase kinase (BDK). However, 17beta-estradiol reversed these effects, resulting in an increase in the BDK activity, thereby decreasing the activity of the complex. Progesterone administration was ineffective. The changes in the percentage of active BCKDC caused by 17beta-estradiol withdrawal and replacement resulted from changes in the amount of BDK protein associated with the complex and therefore its activity. Thus, the marked diurnal variation in the activity state of BCKDC exhibited by female rats involves estrogenic control of BDK activity. We hypothesize that the 17beta-estradiol-controlled feeding pattern produces these variations in BCKDC activity. This may function in female rats to conserve essential amino acids for protein synthesis.

Effects of liver failure on branched-chain alpha-keto acid dehydrogenase complex in rat liver and muscle: comparison between acute and chronic liver failure

BACKGROUND/AIMS

Branched-chain alpha-keto acid dehydrogenase (BCKDH) complex catalyses the committed step in the branched-chain amino acid (BCAA) catabolic pathway. In many cases of liver failure, the serum BCAAs/aromatic amino acids ratio (Fisher's ratio) decreases, and BCAAs have been administered to patients with liver failure to correct this ratio. We conducted an animal study to examine whether the effects on hepatic BCKDH complex differ between acute liver failure (ALF) and chronic liver failure (CLF).

METHODS

ALF and CLF was induced in rats by a single high-dose injection and 21 weeks of repeated low-dose injections of carbon tetrachloride, respectively. Plasma BCAA and branched-chain alpha-keto acid (BCKA) levels, and activities and protein amounts of hepatic BCKDH complex and kinase were measured.

RESULTS

ALF was characterized by elevated plasma BCAA and BCKA levels and decreased hepatic BCKDH activity. CLF was characterized by decreased plasma BCAA and BCKA levels and increased hepatic BCKDH activity. This increase in BCKDH activity in CLF was associated with the decreased BCKDH kinase, which is responsible for the BCKDH inactivation.

CONCLUSIONS

The results obtained in the present study suggest that BCAA catabolism is suppressed in ALF and increased in CLF.

Effects of liver failure on the enzymes in the branched-chain amino acid catabolic pathway

Branched-chain alpha-keto acid dehydrogenase (BCKDH) complex catalyzes the committed step of the catabolism of branched-chain amino acids (BCAA). The liver cirrhosis chemically induced in rats raised the activity of hepatic BCKDH complex and decreased plasma BCAA and branched-chain alpha-keto acid concentrations, suggesting that the BCAA requirement is increased in liver cirrhosis. Since the effects of liver cirrhosis on the BCKDH complex in human liver are different from those in rat liver, further studies are needed to clarify the differences between rats and humans. In the valine catabolic pathway, crotonase and beta-hydroxyisobutyryl-CoA hydrolase are very important to regulate the toxic concentration of mitochondrial methacrylyl-CoA, which occurs in the middle part of valine pathway and highly reacts with free thiol compounds. Both enzyme activities in human and rat livers are very high compared to that of BCKDH complex. It has been found that both enzyme activities in human livers were significantly reduced by liver cirrhosis and hepatocellular carcinoma, suggesting a decrease in the capability to dispose methacrylyl-CoA. The findings described here suggest that alterations in hepatic enzyme activities in the BCAA catabolism are associated with liver failure.

Clofibric acid stimulates branched-chain amino acid catabolism by three mechanisms

Clofibrate promotes catabolism of branched-chain amino acids by increasing the activity of the branched-chain alpha-keto acid dehydrogenase [BCKDH] complex. Depending upon the sex of the rats, nutritional state, and tissue being studied, clofibrate can affect BCKDH complex activity by three different mechanisms. First, by directly inhibiting BCKDH kinase activity, clofibrate can increase the proportion of the BCKDH complex in the active, dephosphorylated state. This occurs in situations in which the BCKDH complex is largely inactive due to phosphorylation, e.g., in the skeletal muscle of chow-fed rats or in the liver of female rats late in the light cycle. Second, by increasing the levels at which the enzyme components of the BCKDH complex are expressed, clofibrate can increase the total enzymatic activity of the BCKDH complex. This is readily demonstrated in livers of rats fed a low-protein diet, a nutritional condition that induces a decrease in the level of expression of the BCKDH complex. Third, by decreasing the amount of BCKDH kinase expressed and therefore its activity, clofibrate induces an increase in the percentage of the BCKDH complex in the active, dephosphorylated state. This occurs in the livers of rats fed a low-protein diet, a nutritional condition that causes inactivation of the BCKDH complex due to upregulation of the amount of BCKDH kinase. WY-14,643, which, like clofibric acid, is a ligand for the peroxisome-proliferator-activated receptor alpha [PPARalpha], does not directly inhibit BCKDH kinase but produces the same long-term effects as clofibrate on expression of the BCKDH complex and its kinase. Thus, clofibrate is unique in its capacity to stimulate BCAA oxidation through inhibition of BCKDH kinase activity, whereas PPARalpha activators in general promote BCAA oxidation by increasing expression of components of the BCKDH complex and decreasing expression of the BCKDH kinase.

Variations in mammary metabolism during the natural filling of the udder with milk over a 12-h period between two milkings

Two groups of four Holstein cows, one in their second and the other in their third or fourth lactation, were used to study temporal variations of mammary metabolism over a 12-h period between two milkings. Blood samples were collected every 30 min from an artery and a mammary vein during a 12-h interval between two milkings. Isoleucine, leucine, lysine, methionine, and phenylalanine mammary net fluxes varied or tended to change over time after milking with a similar pattern between whole blood and plasma. For these amino acids, whole blood and plasma net fluxes reached their maximum over the first 8 h after milking. Simultaneously, respiratory quotients decreased linearly and varied from 2.31 to 2.01 during the first 8 h of the period, suggesting active mammary lipogenesis. From 8 to 12 h after milking, mammary amino acid net fluxes decreased, while mammary oxygen uptake tended to increase with a concomitant decrease in the respiratory quotient reaching 1.84 to 1.40. These findings suggest that, beginning 8 h after milking, mammary uptake of amino acids starts to decrease and catabolic processes appear promoted; this phenomenon could help to explain the increase in milk production reported in the literature with increased milking frequency.

Uteroplacental insufficiency alters liver and skeletal muscle branched-chain amino acid metabolism in intrauterine growth-restricted fetal rats

Uteroplacental insufficiency causes intrauterine growth restriction (IUGR) and decreases plasma levels of the branched-chain amino acids in both humans and rats. Increased fetal oxidation of these amino acids may contribute to their decline in the IUGR fetus. The rate-limiting step of branched-chain amino acid oxidation is performed by the mitochondrial enzyme branched-chain alpha-keto acid dehydrogenase (BCKAD), which is regulated by a deactivating kinase. We therefore hypothesized that uteroplacental insufficiency increases BCKAD activity through altered mRNA and protein levels of BCKAD and/or the BCKAD kinase. In IUGR fetal liver, BCKAD activity was increased 3-fold, though no difference in hepatic BCKAD protein or mRNA levels were noted. Hepatic BCKAD kinase mRNA and protein levels were significantly decreased in association with the increase in BCKAD activity. In IUGR fetal skeletal muscle, BCKAD mRNA levels were significantly increased. IUGR skeletal muscle BCKAD protein levels as well as BCKAD kinase mRNA and protein levels were unchanged. We also quantified mRNA levels of two amino acid transporters: LAT1 (system L) and rBAT (cysteine and dibasic amino acids). Both hepatic and muscle LAT1 mRNA levels were significantly increased in the IUGR fetus. We conclude that uteroplacental insufficiency significantly increases hepatic BCKAD activity in association with significantly decreased mRNA and protein levels of the deactivating kinase. We speculate that these changes contribute to the decreased serum levels of branched-chain amino acids seen in the IUGR fetus and may be an adaptation to the deprived milieu associated with uteroplacental insufficiency.

Mechanism of activation of branched-chain alpha-keto acid dehydrogenase complex by exercise

Branched-chain alpha-keto acid dehydrogenase (BCKDH) complex catalyzes the committed step of branched-chain amino acid catabolism, and its activity is regulated by the phosphorylation-dephosphorylation cycle. BCKDH kinase is responsible for inactivation of the complex by phosphorylation. In the present study, we examined acute exercise on the activity state of the complex as well as the amounts of bound and free forms of the kinase in rat liver and skeletal muscle. Acute exercise activated the complex in association with a decrease in the bound form of kinase in both liver and muscle. The free form of kinase in both tissues was slightly increased but the total amount of the kinase was not affected by acute exercise. The protein amount ratio of bound kinase to E1beta component of the complex was much higher in muscle than in the liver of rats, reflecting the low activity state of the complex in muscle. These results suggest that the amount of the bound kinase plays an important role in regulation of the activity state of the complex. We propose that the alteration in the amount of bound BCKDH kinase is a short-term regulatory mechanism for determining the activity of BCKDH complex.

Regulation of branched-chain amino acid catabolism: nutritional and hormonal regulation of activity and expression of the branched-chain alpha-keto acid dehydrogenase kinase

Branched-chain alpha-keto acid dehydrogenase kinase is responsible for the inactivation and phosphorylation of the branched-chain alpha-keto acid dehydrogenase complex, the enzyme that catalyses the committed step of branched-chain amino acid catabolism. The activity of the branched-chain alpha-keto acid dehydrogenase complex is inversely correlated with kinase activity, suggesting that the relative activity of the kinase is the primary regulator of the activity of the complex. It has been shown that kinase activity and expression are affected by nutritional states imposed by low-protein diet feeding, starvation, diabetes, and exercise. Evidence has also been presented that certain hormones, particularly insulin, glucocorticoid, thyroid hormone and female sex hormones, affect the activity and expression of the kinase. The findings indicate that nutritional and hormonal control of the activity and expression of branched-chain alpha-keto acid dehydrogenase kinase provides an important means of control of the activity of the branched-chain alpha-keto acid dehydrogenase complex, with inactivation serving to conserve branched-chain amino acids for protein synthesis in some situations and activation serving to provide carbon for gluconeogenesis in others.

Regulation of branched-chain alpha-keto acid dehydrogenase kinase expression in rat liver

Branched-chain amino acids are toxic in excess but have to be conserved for protein synthesis. This is accomplished in large part by control of the activity of the branched-chain alpha-keto acid dehydrogenase complex by phosphorylation/dephosphorylation. Regulation of the activity of the hepatic enzyme appears particularly important, at least in rats, since an exceptional high activity of the complex in this tissue makes the liver the primary clearing house for excess branched-chain alpha-keto acids released by other tissues. The degree to which the branched-chain alpha-keto acid dehydrogenase complex is inactivated by phosphorylation is determined by the activity of the branched-chain alpha-keto acid dehydrogenase kinase, which is itself regulated by allosteric effectors as well as factors that affect its level of expression. Well established among these are the alpha-keto acid produced by leucine transamination, which is a potent inhibitor of the kinase, and starvation for dietary protein, which causes increased expression of the branched-chain alpha-keto acid dehydrogenase kinase. The latter finding resulted in the working hypothesis that nutrients and hormones regulate expression of the branched-chain alpha-keto acid dehydrogenase kinase. Evidence has been obtained for the involvement of thyroid hormone, glucocorticoids and ligands for peroxisome proliferator-activated receptor alpha. Thyroid hormone induces, whereas glucocorticoids and peroxisome proliferator-activated receptor alpha ligands repress, expression of the kinase. Increased blood levels of thyroid hormone are proposed to be responsible for increased expression of branched-chain alpha-keto acid dehydrogenase kinase in animals starved for protein.

Regulation of the activity of branched-chain 2-oxo acid dehydrogenase (BCODH) complex by binding BCODH kinase

Branched-chain 2-oxo acid dehydrogenase (BCODH) kinase is responsible for inactivation of BCODH complex by phosphorylation of the complex. Activity of the kinase towards its substrate, the E1 component of the BCODH complex, is known dependent upon binding of the kinase to the E2 component. The possible existence as well as importance of unbound mitochondrial BCODH kinase has been largely ignored in previous studies. Evidence is presented here for the existence of free and bound BCODH kinase in the matrix space of rat liver mitochondria. Furthermore, in female rats, in which diurnal variations in liver BCODH complex and kinase activities occur, the amount of the kinase bound to the complex changes between morning and evening without a change in total kinase protein. Activity of the kinase correlates with the amount of bound rather than total kinase protein, suggesting only the bound form is active. Changes in amount of kinase bound and therefore active appear responsible for diurnal variation in BCODH complex activity in the female rat. We propose that change in the amount of bound BCODH kinase is a key feature of a novel regulatory mechanism for determining the activity state of the BCODH complex.

The alpha-ketoisocaproate catabolism in human and rat livers

Catabolism of alpha-ketoisocaproate in liver is mediated by cytosolic alpha-ketoisocaproate dioxygenase (KICD) and mitochondrial branched-chain alpha-keto acid dehydrogenase complex (BCKDC). The latter is believed to be involved in the main pathway of the KIC catabolism. In the present study, we measured the activities of KICD and BCKDC in human and rat livers. The KICD activity in human liver was 0.9 mU/g tissue, which was 14.2% of the total activity of BCKDC, and that in rat liver was 4.2 mU/g tissue, which was only 1.0% of the total activity, suggesting that KICD in human liver plays a relatively important role in the alpha-ketoisocaproate catabolism. The KICD activity in human liver was significantly increased by cirrhosis. In rat liver, the enzyme activity was markedly increased by physical training and streptozotocin-induced diabetes, but not by feeding of a diet rich in branched-chain amino acids, although BCKDC activity was increased by feeding of the diet.

Experimental hyperthyroidism causes inactivation of the branched-chain alpha-ketoacid dehydrogenase complex in rat liver

Hyperthyroidism induced by 3-day treatment of rats with thyroid hormone (T(3); 3,5,3'-triiodothyronine) at 0.1 or 1 mg/kg body wt/day resulted in a reduced activity state (% of enzyme in its active, dephosphorylated state) of the hepatic branched-chain alpha-ketoacid dehydrogenase (BCKDH) complex. One treatment with 0.1 mg T(3)/kg body wt caused a significant effect on the activity state of BCKDH complex after 24 h, indicating that the reduction of the activity state was triggered by the first administration of T(3). Hyperthyroidism also caused a stable increase in BCKDH kinase activity, the enzyme responsible for phosphorylation and inactivation of the BCKDH complex, suggesting that T(3) caused inactivation of the BCKDH complex by induction of its kinase. Western blot analysis also revealed increased amounts of BCKDH kinase protein in response to hyperthyroidism. No change in the plasma levels of branched-chain alpha-keto acids was observed in T(3)-treated rats, arguing against an involvement of these known regulators of BCKDH kinase activity. Inactivation of the hepatic BCKDH complex as a consequence of overexpression of its kinase may save the essential branched-chain amino acids for protein synthesis during hyperthyroidism.

Regulation of branched-chain amino acid metabolism in the lactating rat

There is evidence that during lactation, uptake of the essential branched-chain amino acids (BCAA) by mammary glands exceeds their output in milk protein. In this study, we have measured the potential of lactating rats to catabolize BCAA. The activity, relative protein and specific mRNA levels of the first two enzymes in the BCAA catabolic pathway, branched-chain aminotransferase (BCAT) and branched-chain alpha-keto acid dehydrogenase (BCKD), were measured in mammary gland, liver and skeletal muscle obtained from rat dams at peak lactation (12 d), from rat dams 24 h after weaning at peak lactation and from age-matched virgin controls. Western analysis showed that the mitochondrial BCATm isoenzyme was found in mammary gland. Comparison of lactating and control rats revealed that tissue BCATm activity, protein and mRNA were at least 10-fold higher in mammary tissue during lactation. Values were 1.3- to 1. 9-fold higher after 24 h of weaning. In mammary gland of lactating rats, the BCKD complex was fully active. In virgin controls and weaning dams, only about 20% of the complex was in the active state. Hypertrophy of the liver and mammary gland during lactation resulted in a 73% increase in total oxidative capacity in lactating rats. The results are consistent with increased expression of the BCATm gene in the mammary gland during lactation, whereas oxidation appears to be regulated primarily by changes in activity state (phosphorylation state) of BCKD.

Murine branched chain alpha-ketoacid dehydrogenase kinase; cDNA cloning, tissue distribution, and temporal expression during embryonic development

These studies were designed to demonstrate the structural and functional similarity of murine branched chain alpha-ketoacid dehydrogenase and its regulation by the complex-specific kinase. Nucleotide sequence and deduced amino acid sequence for the kinase cDNA demonstrate a highly conserved coding sequence between mouse and human. Tissue-specific expression in adult mice parallels that reported in other mammals. Kinase expression in female liver is influenced by circadian rhythm. Of special interest is the fluctuating expression of this kinase during embryonic development against the continuing increase in the catalytic subunits of this mitochondrial complex during development. The need for regulation of the branched chain alpha-ketoacid dehydrogenase complex by kinase expression during embryogenesis is not understood. However, the similarity of murine branched chain alpha-ketoacid dehydrogenase and its kinase to the human enzyme supports the use of this animal as a model for the human system.