Hyperaminoacidaemia at postprandial levels does not modulate glucose metabolism in type 2 diabetes mellitus

Does high dietary protein intake contribute to the increased risk of developing prediabetes and type 2 diabetes?

Insulin resistance is a complex metabolic disorder implicated in the development of many chronic diseases. While it is generally accepted that body mass loss should be the primary approach for the management of insulin resistance-related disorders in overweight and obese individuals, there is no consensus among researchers regarding optimal protein intake during dietary restriction. Recently, it has been suggested that increased plasma branched-chain amino acids concentrations are associated with the development of insulin resistance and type 2 diabetes. The exact mechanism by which excessive amino acid availability may contribute to insulin resistance has not been fully investigated. However, it has been hypothesised that mammalian target of rapamycin (mTOR) complex 1 hyperactivation in the presence of amino acid overload contributes to reduced insulin-stimulated glucose uptake because of insulin receptor substrate (IRS) degradation and reduced Akt-AS160 activity. In addition, the long-term effects of high-protein diets on insulin sensitivity during both weight-stable and weight-loss conditions require more research. This review focusses on the effects of high-protein diets on insulin sensitivity and discusses the potential mechanisms by which dietary amino acids can affect insulin signalling. Novelty: Excess amino acids may over-activate mTOR, resulting in desensitisation of IRS-1 and reduced insulin-mediated glucose uptake.

Interorgan Metabolic Crosstalk in Human Insulin Resistance

Excessive energy intake and reduced energy expenditure drive the development of insulin resistance and metabolic diseases such as obesity and type 2 diabetes mellitus. Metabolic signals derived from dietary intake or secreted from adipose tissue, gut, and liver contribute to energy homeostasis. Recent metabolomic studies identified novel metabolites and enlarged our knowledge on classic metabolites. This review summarizes the evidence of their roles as mediators of interorgan crosstalk and regulators of insulin sensitivity and energy metabolism. Circulating lipids such as free fatty acids, acetate, and palmitoleate from adipose tissue and short-chain fatty acids from the gut effectively act on liver and skeletal muscle. Intracellular lipids such as diacylglycerols and sphingolipids can serve as lipotoxins by directly inhibiting insulin action in muscle and liver. In contrast, fatty acid esters of hydroxy fatty acids have been recently shown to exert a series of beneficial effects. Also, ketoacids are gaining interest as potent modulators of insulin action and mitochondrial function. Finally, branched-chain amino acids not only predict metabolic diseases, but also inhibit insulin signaling. Here, we focus on the metabolic crosstalk in humans, which regulates insulin sensitivity and energy homeostasis in the main insulin-sensitive tissues, skeletal muscle, liver, and adipose tissue.

Plasma Amino Acids vs Conventional Predictors of Insulin Resistance Measured by the Hyperinsulinemic Clamp

Context:

Specific plasma amino acid (AA) profiles including elevated postabsorptive branched-chain amino acids (BCAAs) have been associated with insulin resistance (IR), mostly estimated by homeostatic model assessment. This study assessed the associations of postabsorptive AAs with IR directly measured by insulin-mediated glucose disposal and determined the quantitative value of AAs and conventional IR predictors.

Design:

Fifty-one healthy, 31 overweight or obese (Ow/Ob), and 52 men and women with type 2 diabetes (T2D) were studied retrospectively. The main outcome measures were the glucose disposal (M/I) index (using 3-[³H]-glucose) during a hyperinsulinemic–euglycemic clamp and whole-body protein turnover (using 1-[¹³C]-leucine).

Results:

Compared with healthy participants, M/I was lower in Ow/Ob participants and lowest in those with T2D. BCAAs, glutamate, and lysine were higher in the Ow/Ob and T2D groups than in healthy participants; glycine and threonine were lower. Most AAs were higher in men. Principal component analysis identified component 1 (C1: BCAAs, methionine) and C3 (glycine, threonine, serine). Glutamate, C1, ornithine, lysine, methionine, and tyrosine correlated negatively with M/I; C3 and glycine correlated positively. Waist circumference and sex strongly influenced AA–IR relationships; only glutamate correlated after these factors were controlled for. From regression analysis, waist circumference, fasting glucose, insulin, and free fatty acids (FFAs) negatively predicted 64% of the M/I variance; glutamate added 2% more. In nondiabetic participants, IR was predicted by waist circumference, insulin, and FFAs, without contribution from AAs.

Conclusion:

Several postabsorptive AAs correlated with IR but added limited predictive value to conventional markers because levels were determined largely by abdominal adiposity. Data suggest a sex-specific regulation of AA metabolism by excess adiposity, particularly the BCAAs, warranting investigation.

Leucine and Mammalian Target of Rapamycin-Dependent Activation of Muscle Protein Synthesis in Aging

The preservation or restoration of muscle mass is of prime importance for healthy aging. However, aging has been repeatedly shown to be associated with resistance of muscle to the anabolic effects of feeding. Leucine supplementation has been proposed as a possible strategy because of its regulatory role on protein homeostasis. Indeed, it acts independently of growth factors and leads to enhanced cap-dependent mRNA translation initiation and increased protein synthesis. Leucine acts as a signaling molecule directly at the muscle level via the activation of mammalian/mechanistic target of rapamycin complex 1 (mTORC1). However, in aged muscle, mTORC1 activation seems to be impaired, with decreased sensitivity and responsiveness of muscle protein synthesis to amino acids, whereas the phosphorylation state of several components of this signaling pathway appears to be higher in the basal state. This may stem from specific age-related impairment of muscle signaling and from decreased nutrient and growth factor delivery to the muscle. Whether aging per se affects mTORC1 signaling remains to be established, because aging is frequently associated with inadequate protein intake, decreased insulin sensitivity, inactivity, inflammatory processes, etc. Whatever its origin, this anabolic resistance to feeding can be mitigated by quantitative and qualitative manipulation of protein supply, such as leucine supplementation; however, there remains the question of possible adverse effects of long-term, high-dose leucine supplementation in terms of insulin resistance and tumorigenesis.

Differential control of muscle mass in type 1 and type 2 diabetes mellitus

Diabetes mellitus--whether driven by insulin deficiency or insulin resistance--causes major alterations in muscle metabolism. These alterations have an impact on nutrient handling, including the metabolism of glucose, lipids, and amino acids, and also on muscle mass and strength. However, the ways in which the distinct forms of diabetes affect muscle mass differ greatly. The most common forms of diabetes mellitus are type 1 and type 2. Thus, whereas type 1 diabetic subjects without insulin treatment display a dramatic loss of muscle, most type 2 diabetic subjects show no changes or even an increase in muscle mass. However, the most commonly used rodent models of type 2 diabetes are characterized by muscle atrophy and do not mimic the features of the disease in humans in terms of muscle mass. In this review, we analyze the processes that are differentially regulated under these forms of diabetes and propose regulatory mechanisms to explain them.

Protein and glucose metabolic responses to hyperinsulinemia, hyperglycemia, and hyperaminoacidemia in obese men

OBJECTIVE

In insulin-resistant states, resistance of protein anabolism occurs concurrently with that of glucose, but can be compensated for by abundant amino acid (AA) provision. This effect and its mechanism were sought in obesity.

METHODS

Pancreatic clamps were performed in 8 lean and 11 obese men, following 5-h postabsorptive, 3-h infusions of octreotide, basal glucagon, and growth hormone, with clamped postprandial-level insulin, glucose, and AA. Whole-body [1-(13) C]-leucine and [3-(3) H]-glucose kinetics, skeletal muscle protein ((2) H5 -phenylalanine) fractional synthesis rates, and insulin signaling were determined.

RESULTS

Clamp Δ insulin and Δ branched-chain AA did not differ; fasting glucagon and growth hormone were maintained. Glucose uptake was 20% less in obese concurrent with less Akt(Ser473) , but also less IRS-1(Ser636/639) phosphorylation. Stimulation of whole-body, myofibrillar, and sarcoplasmic protein synthesis was similar. Whole-body protein catabolism suppression tended to be less (P=0.06), resulting in lesser net balance (1.09 ± 0.07 vs. 1.31 ± 0.08 μmol [kg FFM(-1) ] min(-1) , P=0.048). Increments in muscle S6K1(Thr389) phosphorylation were less in the obese, but 4E-BP1(Ser65) did not differ.

CONCLUSIONS

Hyperaminoacidemia with hyperinsulinemia stimulated protein synthesis (possibly via nutrient signaling) normally in obesity, but suppression of proteolysis may be compromised. Whether long-term high protein intakes could compensate for the insulin resistance of protein anabolism remains to be determined.

Muscle p70S6K phosphorylation in response to soy and dairy rich meals in middle aged men with metabolic syndrome: a randomised crossover trial

Background

The mammalian target of rapamycin (mTOR) pathway is the primary regulator of muscle protein synthesis. Metabolic syndrome (MetS) is characterized by central obesity and insulin resistance; little is known about how MetS affects the sensitivity of the mTOR pathway to feeding.

Methods

The responsiveness of mTOR pathway targets such as p706Sk to a high protein meal containing either dairy or soy foods was investigated in healthy insulin sensitive middle-aged men and those presenting with metabolic syndrome (MetS). Twenty male subjects (10 healthy controls, 10 MetS) participated in a single-blinded randomized cross-over study. In a random sequence, subjects ingested energy-matched breakfasts composed predominately of either dairy-protein or soy-protein foods. Skeletal muscle biopsies were collected in the fasted state and at 2 and 4 h post-meal ingestion for the analysis of mTOR- and insulin-signalling kinase activation.

Results

Phosphorylated Akt and Insulin receptor substrate 1 (IRS1) increased during the postabsorptive period with no difference between groups. mTOR (Ser448) and ribosomal protein S6 phosphorylation increased 2 h following dairy meal consumption only. p70S6K (Thr389) phosphorylation was increased after feeding only in the control subjects and not in the MetS group.

Conclusions

These data demonstrate that the consumption of a dairy-protein rich but not a soy-protein rich breakfast activates the phosphorylation of mTOR and ribosomal protein S6, required for protein synthesis in human skeletal muscle. Unlike healthy controls, subjects with MetS did not increase muscle p70S6K(Thr389) phosphorylation in response to a mixed meal.

Trial registration

This trial was registered with the Australian New Zealand Clinical Trials Registry (ANZCTR) as ACTRN12610000562077.

Exercise in ZDF rats does not attenuate weight gain, but prevents hyperglycemia concurrent with modulation of amino acid metabolism and AKT/mTOR activation in skeletal muscle

PURPOSE

Protein metabolism is altered in obesity, accompanied by elevated plasma amino acids (AA). Previously, we showed that exercise delayed progression to type 2 diabetes in obese ZDF rats with maintenance of β cell function and reduction in hyperglucocorticoidemia. We hypothesized that exercise would correct the abnormalities we found in circulating AA and other indices of skeletal muscle protein metabolism.

METHODS

Male obese prediabetic ZDF rats (7-10/group) were exercised (swimming) 1 h/day, 5 days/week from ages 6-19 weeks, and compared with age-matched obese sedentary and lean ZDF rats.

RESULTS

Food intake and weight gain were unaffected. Protein metabolism was altered in obese rats as evidenced by increased plasma concentrations of essential AA, and increased muscle phosphorylation (ph) of Akt(ser473) (187%), mTOR(ser2448) (140%), eIF4E-binding protein 1 (4E-BP1) (111%), and decreased formation of 4E-BP1*eIF4E complex (75%, 0.01 ≤ p ≤ 0.05 for all measures) in obese relative to lean rats. Exercise attenuated the increase in plasma essential AA concentrations and muscle Akt and mTOR phosphorylation. Exercise did not modify phosphorylation of S6K1, S6, and 4E-BP1, nor the formation of 4E-BP1*eIF4E complex, mRNA levels of ubiquitin or the ubiquitin ligase MAFbx. Positive correlations were observed between ph-Akt and fed circulating branched-chain AA (r = 0.56, p = 0.008), postprandial glucose (r = 0.42, p = 0.04) and glucose AUC during an IPGTT (r = 0.44, p = 0.03).

CONCLUSION

Swimming exercise-induced attenuation of hyperglycemia in ZDF rats is independent of changes in body weight and could result in part from modulation of muscle AKT activation acting via alterations of systemic AA metabolism.

Amino acid-induced impairment of insulin sensitivity in healthy and obese rats is reversible

High‐protein diets (HPDs) promote weight loss but other studies implicate these diets and their constituent amino acids (AAs) in insulin resistance. We hypothesized that AA‐induced insulin resistance is a temporal and reversible metabolic event. L6 myotubes were serum deprived for 4 h and then incubated in AA and/or insulin (100 nmol/L). Another group of cells was incubated overnight in AA + insulin, starved again, and then reincubated with AA and insulin. Mammalian (mechanistic) target of rapamycin complex 1 (mTORC1) signaling and glucose uptake were then measured. Healthy or insulin‐resistant rats were gavaged with leucine (0.48 g/kg) and insulin sensitivity was examined. In myotubes, incubation with AA and insulin significantly (P <0.05) increased the phosphorylation of the mTORC1 substrate ribosomal protein S6 kinase 1 (S6K1, T389) and of insulin receptor substrate 1 (IRS‐1, serine residues), but suppressed insulin‐stimulated glucose uptake by 40% (P <0.01). These modifications were mTORC1‐dependent and were reversible. In vivo, leucine gavage reversibly increased S6K1 phosphorylation and IRS‐1 serine phosphorylation 5‐ to 12‐fold in skeletal muscle and impaired insulin tolerance of glucose (P <0.05) in lean rats. In insulin‐resistant rats, the impairment of whole blood glucose and AA metabolism induced by leucine gavage (0.001 < P <0.05) was more severe than that observed in lean rats; however, the impairment was reversible within 24 h of treatment. If these data are confirmed in long‐term studies, it would imply that the use of leucine/HPD in treating metabolic diseases is unlikely to have lasting negative effects on insulin sensitivity.

Using in vitro and in vivo systems, we show that amino acid‐induced modulation of mTORC1/S6K1 signaling and serine phosphorylation of IRS‐1 in muscle cells/skeletal muscle are temporal and reversible. In addition, in both lean and obese rats, leucine‐induced impairment of whole body insulin sensitivity of glucose and amino acid is reversible within 24 h of treatment. Our data suggest that the use of leucine/amino acid in body weight management is unlikely to carry any lasting negative effect on insulin sensitivity.

Insulin resistance of protein metabolism in type 2 diabetes and impact on dietary needs: a review

Evidence shows that the metabolism of protein is altered in type 2 diabetes mellitus and insulin resistance not only applies to glucose and lipid but protein metabolism as well. Population surveys report greater susceptibility to loss of lean tissue and muscle strength with aging in diabetes. Prevention of sarcopenia requires that protein receives more attention in dietary prescriptions. Protein intake of 1-1.2 g/kg of body weight (with weight at a body mass index of 25 kg/m(2))/day may be distributed equally among 3 meals a day, including breakfast, to optimize anabolism. Adopting a dietary pattern that provides a high plant-to-animal ratio and greater food volume favouring consumption of vegetables, legumes, fruits, complemented with fish, low fat dairy and meat (preferably cooked slowly in moisture), soy and nuts may assist with metabolic and weight control. Depending on the magnitude of energy restriction, usual protein intake should be maintained or increased, and the caloric deficit taken from fat and carbohydrate foods. Exercise before protein-rich meals improves skeletal muscle protein anabolism. Because high levels of amino acids lower glucose uptake in individuals without diabetes, the challenge remains to define the optimal protein intake and exercise regimen to protect from losses of muscle mass and strength while maintaining adequate glucose control in type 2 diabetes.

Muscle protein anabolism in type 2 diabetes

PURPOSE OF REVIEW

Protein anabolism is abnormal in human type 2 diabetes (T2DM). We review studies of anabolic stimuli that identify potential causes. If uncorrected, and combined with aging effects, they will compromise muscle function and mass. Knowing causes can guide studies of preventive and treatment measures.

RECENT FINDINGS

T2DM accelerates age-related decreases in muscle mass. This could be related to insulin resistance of whole-body protein anabolism demonstrated in hyperglycemic obese men. In contrast, their protein anabolic response to hyperaminoacidemia suggested that ample amino acid administration, especially branched chain amino acids might overcome such insulin resistance. One study of chronic leucine supplementation in elderly T2DM patients did not increase muscle mass. However, they lacked sarcopenia and had adequate dietary protein intake, so may be atypical. Exercise induced similar increases in muscle protein synthesis, mass and strength in healthy and T2DM patients suggesting that physical activity might also overcome insulin resistance of protein anabolism.

SUMMARY

Muscle protein anabolism in T2DM is resistant to the action of insulin but perhaps not to amino acid supply or exercise. Whether leucine supplementation improves muscle mass and function in persons with T2DM (especially elderly) with reduced protein intake or muscle mass needs to be determined.

Amino acids potentiate insulin signaling in CHO-K1 at high glucose conditions

BACKGROUND AND AIMS

Amino acids reportedly increase the glucose uptake under high glucose conditions. However, there are controversies in the role of amino acids in diabetes mellitus. The present study explores the insulin signaling pathway involved in glucose uptake mediated by amino acids in CHO-K1 cells.

METHODS

CHO-K1 cells were exposed to normal (7 mM) and high glucose (17 and 27 mM) with 100 nM insulin in the presence and absence of amino acid mixtures (AAM) in varying concentration (5 and 20 mM) followed by the assays, insulin receptor tyrosine kinase (IRTK) and phosphatidylinositol 3 kinase (PI3K) by autoradiography, protein kinase B (Akt) and glucose transporter (GLUT4) by Western blot and glycogen synthase (GS) by HPLC.

RESULTS

The addition of 5 and 20 mM AAM significantly increased IRTK and PI3K activity (ANOVA p = 0.025, p = 0.003, respectively) with increasing glucose concentration. Addition of 5 mM AAM in the presence of normal glucose significantly increased the levels of phosphorylated Akt Ser473 (p = 0.02) with no significant change at high glucose. At 20 mM AAM there was a significant decrease in Akt phosphorylation (p = 0.035) that was increased by high glucose concentration. GLUT4 protein levels were increased with AAM (5 mM) along with increase in glycogen synthase activity at all glucose concentrations (p <0.05).

CONCLUSIONS

Amino acids as a mixture is beneficial in augmenting insulin signaling pathway via IRTK/PI3K/GLUT4 pathway along with activation of GS in CHO-K1 cells, thereby ensuring increased intracellular glucose availability.