Glutamine Regulates Skeletal Muscle Immunometabolism in Type 2 Diabetes.

Dysregulation of skeletal muscle metabolism influences whole-body insulin sensitivity and glucose homeostasis. We hypothesized that type 2 diabetes-associated alterations in the plasma metabolome directly contribute to skeletal muscle immunometabolism and the subsequent development of insulin resistance. To this end, we analyzed the plasma and skeletal muscle metabolite profile and identified glutamine as a key amino acid that correlates inversely with BMI and insulin resistance index (HOMA-IR) in men with normal glucose tolerance or type 2 diabetes. Using an in vitro model of human myotubes and an in vivo model of diet-induced obesity and insulin resistance in male mice, we provide evidence that glutamine levels directly influence the inflammatory response of skeletal muscle and regulate the expression of the adaptor protein GRB10, an inhibitor of insulin signaling. Moreover, we demonstrate that a systemic increase in glutamine levels in a mouse model of obesity improves insulin sensitivity and restores glucose homeostasis. We conclude that glutamine supplementation may represent a potential therapeutic strategy to prevent or delay the onset of insulin resistance in obesity by reducing inflammatory markers and promoting skeletal muscle insulin sensitivity.

Discovery of thymosin ß4 as a human exerkine and growth factor.

Skeletal muscle is an endocrine organ secreting exercise-induced factors (exerkines), which play a pivotal role in interorgan cross talk. Using mass spectrometry (MS)-based proteomics, we characterized the secretome and identified thymosin ß4 (TMSB4X) as the most upregulated secreted protein in the media of contracting C2C12 myotubes. TMSB4X was also acutely increased in the plasma of exercising humans irrespective of the insulin resistance condition or exercise mode. Treatment of mice with TMSB4X did not ameliorate the metabolic disruptions associated with diet induced-obesity, nor did it enhance muscle regeneration in vivo. However, TMSB4X increased osteoblast proliferation and neurite outgrowth, consistent with its WADA classification as a prohibited growth factor. Therefore, we report TMSB4X as a human exerkine with a potential role in cellular cross talk.

Modified UCN2 peptide treatment improves skeletal muscle mass and function in mouse models of obesity-induced insulin resistance.

BACKGROUND: Type 2 diabetes and obesity are often seen concurrently with skeletal muscle wasting, leading to further derangements in function and metabolism. Muscle wasting remains an unmet need for metabolic disease, and new approaches are warranted. The neuropeptide urocortin 2 (UCN2) and its receptor corticotropin releasing factor receptor 2 (CRHR2) are highly expressed in skeletal muscle and play a role in regulating energy balance, glucose metabolism, and muscle mass. The aim of this study was to investigate the effects of modified UCN2 peptides as a pharmaceutical therapy to counteract the loss of skeletal muscle mass associated with obesity and casting immobilization. METHODS: High-fat-fed mice (C57Bl/6J; 26 weeks old) and ob/ob mice (11 weeks old) were injected daily with a PEGylated (Compound A) and non-PEGylated (Compound B) modified human UCN2 at 0.3 mg/kg subcutaneously for 14 days. A separate group of chow-fed C57Bl/6J mice (12 weeks old) was subjected to hindlimb cast immobilization and, after 1 week, received daily injections with Compound A. In vivo functional tests were performed to measure protein synthesis rates and skeletal muscle function. Ex vivo functional and molecular tests were performed to measure contractile force and signal transduction of catabolic and anabolic pathways in skeletal muscle. RESULTS: Skeletal muscles (extensor digitorum longus, soleus, and tibialis anterior) from high-fat-fed mice treated with Compound A were ~14% heavier than muscles from vehicle-treated mice. Chronic treatment with modified UCN2 peptides altered the expression of structural genes and transcription factors in skeletal muscle in high-fat diet-induced obesity including down-regulation of Trim63 and up-regulation of Nr4a2 and Igf1 (P < 0.05 vs. vehicle). Signal transduction via both catabolic and anabolic pathways was increased in tibialis anterior muscle, with increased phosphorylation of ribosomal protein S6 at Ser235/236 , FOXO1 at Ser256 , and ULK1 at Ser317 , suggesting that UCN2 treatment modulates protein synthesis and degradation pathways (P < 0.05 vs. vehicle). Acutely, a single injection of Compound A in drug-naïve mice had no effect on the rate of protein synthesis in skeletal muscle, as measured via the surface sensing of translation method, while the expression of Nr4a3 and Ppargc1a4 was increased (P < 0.05 vs. vehicle). Compound A treatment prevented the loss of force production from disuse due to casting. Compound B treatment increased time to fatigue during ex vivo contractions of fast-twitch extensor digitorum longus muscle. Compound A and B treatment increased lean mass and rates of skeletal muscle protein synthesis in ob/ob mice. CONCLUSIONS: Modified human UCN2 is a pharmacological candidate for the prevention of the loss of skeletal muscle mass associated with obesity and immobilization.

Branched-chain amino acid metabolism is regulated by ERRα in primary human myotubes and is further impaired by glucose loading in type 2 diabetes.

AIMS/HYPOTHESIS: Increased levels of branched-chain amino acids (BCAAs) are associated with type 2 diabetes pathogenesis. However, most metabolomic studies are limited to an analysis of plasma metabolites under fasting conditions, rather than the dynamic shift in response to a metabolic challenge. Moreover, metabolomic profiles of peripheral tissues involved in glucose homeostasis are scarce and the transcriptomic regulation of genes involved in BCAA catabolism is partially unknown. This study aimed to identify differences in circulating and skeletal muscle BCAA levels in response to an OGTT in individuals with normal glucose tolerance (NGT) or type 2 diabetes. Additionally, transcription factors involved in the regulation of the BCAA gene set were identified. METHODS: Plasma and vastus lateralis muscle biopsies were obtained from individuals with NGT or type 2 diabetes before and after an OGTT. Plasma and quadriceps muscles were harvested from skeletal muscle-specific Ppargc1a knockout and transgenic mice. BCAA-related metabolites and genes were assessed by LC-MS/MS and quantitative RT-PCR, respectively. Small interfering RNA and adenovirus-mediated overexpression techniques were used in primary human skeletal muscle cells to study the role of PPARGC1A and ESRRA in the expression of the BCAA gene set. Radiolabelled leucine was used to analyse the impact of oestrogen-related receptor α (ERRα) knockdown on leucine oxidation. RESULTS: Impairments in BCAA catabolism in people with type 2 diabetes under fasting conditions were exacerbated after a glucose load. Branched-chain keto acids were reduced 37-56% after an OGTT in the NGT group, whereas no changes were detected in individuals with type 2 diabetes. These changes were concomitant with a stronger correlation with glucose homeostasis biomarkers and downregulated expression of branched-chain amino acid transaminase 2, branched-chain keto acid dehydrogenase complex subunits and 69% of downstream BCAA-related genes in skeletal muscle. In primary human myotubes overexpressing peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α, encoded by PPARGC1A), 61% of the analysed BCAA genes were upregulated, while 67% were downregulated in the quadriceps of skeletal muscle-specific Ppargc1a knockout mice. ESRRA (encoding ERRα) silencing completely abrogated the PGC-1α-induced upregulation of BCAA-related genes in primary human myotubes. CONCLUSIONS/INTERPRETATION: Metabolic inflexibility in type 2 diabetes impacts BCAA homeostasis and attenuates the decrease in circulating and skeletal muscle BCAA-related metabolites after a glucose challenge. Transcriptional regulation of BCAA genes in primary human myotubes via PGC-1α is ERRα-dependent.

Quantitative phosphoproteomic analysis of IRS1 in skeletal muscle from men with normal glucose tolerance or type 2 diabetes: A case-control study.

BACKGROUND & AIMS: The physiological regulation and contribution of the multiple phosphorylation sites of insulin receptor substrate 1 (IRS1) to the pathogenesis of insulin resistance is unknown. Our aims were to map the phosphorylated motifs of IRS1 in skeletal muscle from people with normal glucose tolerance (NGT; n = 11) or type 2 diabetes mellitus (T2DM; n = 11). METHODS: Skeletal muscle biopsies were obtained under fasted conditions or during a euglycemic clamp and IRS1 phosphorylation sites were identified by mass spectrometry. RESULTS: We identified 33 phosphorylation sites in biopsies from fasted individuals, including 2 previously unreported sites ([Ser393] and [Thr1017]). In men with NGT and T2DM, insulin increased phosphorylation of 5 peptides covering 10 serine or threonine sites and decreased phosphorylation of 6 peptides covering 9 serine, threonine or tyrosine sites. Insulin-stimulation increased phosphorylation of 2 peptides, and decreased phosphorylation of 2 peptides only in men with NGT. Insulin increased phosphorylation of 2 peptides only in men with T2DM. CONCLUSIONS: Despite severe skeletal muscle insulin resistance, the pattern of IRS1 phosphorylation was not uniformly altered in T2DM. Our results contribute to the evolving understanding of the physiological regulation of insulin signaling and complement the comprehensive map of IRS1 phosphorylation in T2DM.

Modified UCN2 Peptide Acts as an Insulin Sensitizer in Skeletal Muscle of Obese Mice.

The neuropeptide urocortin 2 (UCN2) and its receptor corticotropin-releasing hormone receptor 2 (CRHR2) are highly expressed in skeletal muscle and play a role in regulating energy balance and glucose metabolism. We investigated a modified UCN2 peptide as a potential therapeutic agent for the treatment of obesity and insulin resistance, with a specific focus on skeletal muscle. High-fat-fed mice (C57BL/6J) were injected daily with a PEGylated UCN2 peptide (compound A) at 0.3 mg/kg subcutaneously for 14 days. Compound A reduced body weight, food intake, whole-body fat mass, and intramuscular triglycerides compared with vehicle-treated controls. Furthermore, whole-body glucose tolerance was improved by compound A treatment, with increased insulin-stimulated Akt phosphorylation at Ser473 and Thr308 in skeletal muscle, concomitant with increased glucose transport into extensor digitorum longus and gastrocnemius muscle. Mechanistically, this is linked to a direct effect on skeletal muscle because ex vivo exposure of soleus muscle from chow-fed lean mice to compound A increased glucose transport and insulin signaling. Moreover, exposure of GLUT4-Myc-labeled L6 myoblasts to compound A increased GLUT4 trafficking. Our results demonstrate that modified UCN2 peptides may be efficacious in the treatment of type 2 diabetes by acting as an insulin sensitizer in skeletal muscle.

Suppression of 5'-nucleotidase enzymes promotes AMP-activated protein kinase (AMPK) phosphorylation and metabolism in human and mouse skeletal muscle.

The 5'-nucleotidase (NT5) family of enzyme dephosphorylates non-cyclic nucleoside monophosphates to produce nucleosides and inorganic phosphates. We hypothesized that gene silencing of NT5 enzymes to increase the intracellular availability of AMP would increase AMP-activated protein kinase (AMPK) activity and metabolism. We determined the role of cytosolic NT5 in metabolic responses linked to the development of insulin resistance in obesity and type 2 diabetes. Using siRNA to silence NT5C2 expression in cultured human myotubes, we observed a 2-fold increase in the AMP/ATP ratio, a 2.4-fold increase in AMPK phosphorylation (Thr(172)), and a 2.8-fold increase in acetyl-CoA carboxylase phosphorylation (Ser(79)) (p < 0.05). siRNA silencing of NT5C2 expression increased palmitate oxidation by 2-fold in the absence and by 8-fold in the presence of 5-aminoimidazole-4-carboxamide 1-ß-d-ribofuranoside. This was paralleled by an increase in glucose transport and a decrease in glucose oxidation, incorporation into glycogen, and lactate release from NT5C2-depleted myotubes. Gene silencing of NT5C1A by shRNA injection and electroporation in mouse tibialis anterior muscle reduced protein content (60%; p < 0.05) and increased phosphorylation of AMPK (60%; p < 0.05) and acetyl-CoA carboxylase (50%; p < 0.05) and glucose uptake (20%; p < 0.05). Endogenous expression of NT5C enzymes inhibited basal lipid oxidation and glucose transport in skeletal muscle. Reduction of 5'-nucleotidase expression or activity may promote metabolic flexibility in type 2 diabetes.

Insulin-stimulated phosphorylation of the Rab GTPase-activating protein TBC1D1 regulates GLUT4 translocation.

Insulin stimulates the translocation of the glucose transporter GLUT4 from intracellular locations to the plasma membrane in adipose and muscle cells. Prior studies have shown that Akt phosphorylation of the Rab GTPase-activating protein, AS160 (160-kDa Akt substrate; also known as TBC1D4), triggers GLUT4 translocation, most likely by suppressing its Rab GTPase-activating protein activity. However, the regulation of a very similar protein, TBC1D1 (TBC domain family, member 1), which is mainly found in muscle, in insulin-stimulated GLUT4 translocation has been unclear. In the present study, we have identified likely Akt sites of insulin-stimulated phosphorylation of TBC1D1 in C2C12 myotubes. We show that a mutant of TBC1D1, in which several Akt sites have been converted to alanine, is considerably more inhibitory to insulin-stimulated GLUT4 translocation than wild-type TBC1D1. This result thus indicates that similar to AS160, Akt phosphorylation of TBC1D1 enables GLUT4 translocation. We also show that in addition to Akt activation, activation of the AMP-dependent protein kinase partially relieves the inhibition of GLUT4 translocation by TBC1D1. Finally, we show that the R125W variant of TBC1D1, which has been genetically associated with obesity, is equally inhibitory to insulin-stimulated GLUT4 translocation, as is wild-type TBC1D1, and that healthy and type 2 diabetic individuals express approximately the same level of TBC1D1 in biopsies of vastus lateralis muscle. In conclusion, phosphorylation of TBC1D1 is required for GLUT4 translocation. Thus, the regulation of TBC1D1 resembles that of its paralog, AS160.

Kinetics of GLUT4 trafficking in rat and human skeletal muscle.

OBJECTIVE: In skeletal muscle, insulin stimulates glucose transport activity three- to fourfold, and a large part of this stimulation is associated with a net translocation of GLUT4 from an intracellular compartment to the cell surface. We examined the extent to which insulin or the AMP-activated protein kinase activator AICAR can lead to a stimulation of the exocytosis limb of the GLUT4 translocation pathway and thereby account for the net increase in glucose transport activity. RESEARCH DESIGN AND METHODS: Using a biotinylated photoaffinity label, we tagged endogenous GLUT4 and studied the kinetics of exocytosis of the tagged protein in rat and human skeletal muscle in response to insulin or AICAR. Isolated epitrochlearis muscles were obtained from male Wistar rats. Vastus lateralis skeletal muscle strips were prepared from open muscle biopsies obtained from six healthy men (age 39 +/- 11 years and BMI 25.8 +/- 0.8 kg/m2). RESULTS: In rat epitrochlearis muscle, insulin exposure leads to a sixfold stimulation of the GLUT4 exocytosis rate (with basal and insulin-stimulated rate constants of 0.010 and 0.067 min(-1), respectively). In human vastus lateralis muscle, insulin stimulates GLUT4 translocation by a similar sixfold increase in the exocytosis rate constant (with basal and insulin-stimulated rate constants of 0.011 and 0.075 min(-1), respectively). In contrast, AICAR treatment does not markedly increase exocytosis in either rat or human muscle. CONCLUSIONS: Insulin stimulation of the GLUT4 exocytosis rate constant is sufficient to account for most of the observed increase in glucose transport activity in rat and human muscle.

Activation of the thiazide-sensitive Na+-Cl- cotransporter by the WNK-regulated kinases SPAK and OSR1.

Mutations increasing WNK1 kinase expression in humans cause the pseudohypoaldosteronism type II hypertension syndrome. This condition is treated effectively by thiazide diuretics, which exert their effects by inhibiting the Na+-Cl(-) cotransporter (NCC), suggesting a link between WNK1 and NCC. Here, we demonstrate that the SPAK and OSR1 kinases that are activated by WNK1 phosphorylate human NCC at three conserved residues (Thr46, Thr55 and Thr60). Activation of the WNK1-SPAK/OSR1 signalling pathway by treatment of HEK293 or mpkDCT kidney distal-convoluted-tubule-derived cells with hypotonic low-chloride conditions induced phosphorylation of NCC at residues phosphorylated by SPAK/OSR1. Efficient phosphorylation of NCC was dependent upon a docking interaction between an RFXI motif in NCC and SPAK/OSR1. Mutation of Thr60 to Ala in NCC markedly inhibited phosphorylation of Thr46 and Thr55 as well as NCC activation induced by hypotonic low-chloride treatment of HEK293 cells. Our results establish that the WNK1-SPAK/OSR1 signalling pathway plays a key role in controlling the phosphorylation and activity of NCC. They also suggest a mechanism by which increased WNK1 overexpression could lead to hypertension and that inhibitors of SPAK/OSR1 might be of use in reducing blood pressure by suppressing phosphorylation and hence activity of NCC.

Insulin signaling and glucose transport in insulin resistant human skeletal muscle.

Insulin increases glucose uptake and metabolism in skeletal muscle by signal transduction via protein phosphorylation cascades. Insulin action on signal transduction is impaired in skeletal muscle from Type 2 diabetic subjects, underscoring the contribution of molecular defects to the insulin resistant phenotype. This review summarizes recent work to identify downstream intermediates in the insulin signaling pathways governing glucose homeostasis, in an attempt to characterize the molecular mechanism accounting for skeletal muscle insulin resistance in Type 2 diabetes. Furthermore, the effects of pharmaceutical treatment of Type 2 diabetic patients on insulin signaling and glucose uptake are discussed. The identification and characterization of pathways governing insulin action on glucose metabolism will facilitate the development of strategies to improve insulin sensitivity in an effort to prevent and treat Type 2 diabetes mellitus.

Low-intensity exercise increases skeletal muscle protein expression of PPARdelta and UCP3 in type 2 diabetic patients.

BACKGROUND: Physical exercise provides health benefits for people with type 2 diabetes mellitus, partly by enhancing skeletal muscle insulin action. We tested the hypothesis that changes in expression of key genes in skeletal muscles relate to exercise-induced improvements in type 2 diabetic patients. METHODS: We determined mRNA expression of 20 selected genes following a self-supervised program of walking (> 150 min per week) over a 4-month period. RESULTS: This level of physical activity improved clinical parameters in approximately half the participants, as determined by reduced hypertension and enhanced insulin sensitivity (defined by reduced plasma-insulin levels and improved homeostasis model assessment (HOMA)). Skeletal muscle mRNA expression of Cbl-associated protein (CAP), diacylglycerol kinase (DGK)delta, uncoupling protein (UCP) 3, nuclear respiratory factor (NRF)-1, and peroxisome proliferator-activated receptor (PPAR)delta tended to increase in type 2 diabetic patients with an improved clinical profile. Skeletal muscle protein expression of PPARdelta and UCP3 was increased significantly after physical exercise in patients with an improved clinical profile, but were unchanged in patients who did not show exercise-mediated improvements in clinical parameters. CONCLUSIONS: This study provides clinical evidence that improvements in insulin sensitivity can be achieved in type 2 diabetic patients after individually executed low-intensity exercise training. Moreover, the positive clinical response to exercise is correlated with changes in skeletal muscle proteins involved in the regulation of mitochondrial biogenesis and metabolism. These changes in skeletal muscle gene expression offer a possible molecular explanation for the improvements in clinical outcomes.

Functional interactions of the SPAK/OSR1 kinases with their upstream activator WNK1 and downstream substrate NKCC1.

The SPAK (STE20/SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase-1) kinases interact and phosphorylate NKCC1 (Na+-K+-2Cl- co-transporter-1), leading to its activation. Recent studies indicated that SPAK and OSR1 are phosphorylated and activated by the WNK1 [with no K (lysine) protein kinase-1] and WNK4, genes mutated in humans affected by Gordon's hypertension syndrome. In the present study, we have identified three residues in NKCC1 (Thr175/Thr179/Thr184 in shark or Thr203/Thr207/Thr212 in human) that are phosphorylated by SPAK and OSR1, and have developed a peptide substrate, CATCHtide (cation chloride co-transporter peptide substrate), to assess SPAK and OSR1 activity. Exposure of HEK-293 (human embryonic kidney) cells to osmotic stress, which leads to phosphorylation and activation of NKCC1, increased phosphorylation of NKCC1 at the sites targeted by SPAK/OSR1. The residues on NKCC1, phosphorylated by SPAK/OSR1, are conserved in other cation co-transporters, such as the Na+-Cl- co-transporter, the target of thiazide drugs that lower blood pressure in humans with Gordon's syndrome. Furthermore, we characterize the properties of a 92-residue CCT (conserved C-terminal) domain on SPAK and OSR1 that interacts with an RFXV (Arg-Phe-Xaa-Val) motif present in the substrate NKCC1 and its activators WNK1/WNK4. A peptide containing the RFXV motif interacts with nanomolar affinity with the CCT domains of SPAK/OSR1 and can be utilized to affinity-purify SPAK and OSR1 from cell extracts. Mutation of the arginine, phenylalanine or valine residue within this peptide abolishes binding to SPAK/OSR1. We have identified specific residues within the CCT domain that are required for interaction with the RFXV motif and have demonstrated that mutation of these in OSR1 inhibited phosphorylation of NKCC1, but not of CATCHtide which does not possess an RFXV motif. We establish that an intact CCT domain is required for WNK1 to efficiently phosphorylate and activate OSR1. These data establish that the CCT domain functions as a multipurpose docking site, enabling SPAK/OSR1 to interact with substrates (NKCC1) and activators (WNK1/WNK4).

Insulin signaling and glucose transport in skeletal muscle from first-degree relatives of type 2 diabetic patients.

Aberrant insulin signaling and glucose metabolism in skeletal muscle from type 2 diabetic patients may arise from genetic defects and an altered metabolic milieu. We determined insulin action on signal transduction and glucose transport in isolated vastus lateralis skeletal muscle from normal glucose-tolerant first-degree relatives of type 2 diabetic patients (n = 8, 41 +/- 3 years, BMI 25.1 +/- 0.8 kg/m(2)) and healthy control subjects (n = 9, 40 +/- 2 years, BMI 23.4 +/- 0.7 kg/m(2)) with no family history of diabetes. Basal and submaximal insulin-stimulated (0.6 and 1.2 nmol/l) glucose transport was comparable between groups, whereas the maximal response (120 nmol/l) was 38% lower (P < 0.05) in the relatives. Insulin increased phosphorylation of Akt and Akt substrate of 160 kDa (AS160) in a dose-dependent manner, with comparable responses between groups. AS160 phosphorylation and glucose transport were positively correlated in control subjects (R(2) = 0.97, P = 0.01) but not relatives (R(2) = 0.46, P = 0.32). mRNA of key transcriptional factors and coregulators of mitochondrial biogenesis were also determined. Skeletal muscle mRNA expression of peroxisome proliferator-activated receptor (PPAR) gamma coactivator (PGC)-1alpha, PGC-1beta, PPARdelta, nuclear respiratory factor-1, and uncoupling protein-3 was comparable between first-degree relatives and control subjects. In conclusion, the uncoupling of insulin action on Akt/AS160 signaling and glucose transport implicates defective GLUT4 trafficking as an early event in the pathogenesis of type 2 diabetes.

IRS-1 serine phosphorylation and insulin resistance in skeletal muscle from pancreas transplant recipients.

Insulin-dependent diabetic recipients of successful pancreas allografts achieve self-regulatory insulin secretion and discontinue exogenous insulin therapy; however, chronic hyperinsulinemia and impaired insulin sensitivity generally develop. To determine whether insulin resistance is accompanied by altered signal transduction, skeletal muscle biopsies were obtained from pancreas-kidney transplant recipients (n = 4), nondiabetic kidney transplant recipients (receiving the same immunosuppressive drugs; n = 5), and healthy subjects (n = 6) before and during a euglycemic-hyperinsulinemic clamp. Basal insulin receptor substrate (IRS)-1 Ser (312) and Ser (616) phosphorylation, IRS-1-associated phosphatidylinositol 3-kinase activity, and extracellular signal-regulated kinase (ERK)-1/2 phosphorylation were elevated in pancreas-kidney transplant recipients, coincident with fasting hyperinsulinemia. Basal IRS-1 Ser (312) and Ser (616) phosphorylation was also increased in nondiabetic kidney transplant recipients. Insulin increased phosphorylation of IRS-1 at Ser (312) but not Ser (616) in healthy subjects, with impairments noted in nondiabetic kidney and pancreas-kidney transplant recipients. Insulin action on ERK-1/2 and Akt phosphorylation was impaired in pancreas-kidney transplant recipients and was preserved in nondiabetic kidney transplant recipients. Importantly, insulin stimulation of the Akt substrate AS160 was impaired in nondiabetic kidney and pancreas-kidney transplant recipients. In conclusion, peripheral insulin resistance in pancreas-kidney transplant recipients may arise from a negative feedback regulation of the canonical insulin-signaling cascade from excessive serine phosphorylation of IRS-1, possibly as a consequence of immunosuppressive therapy and hyperinsulinemia.

Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise.

BCAAs (leucine, isoleucine, and valine), particularly leucine, have anabolic effects on protein metabolism by increasing the rate of protein synthesis and decreasing the rate of protein degradation in resting human muscle. Also, during recovery from endurance exercise, BCAAs were found to have anabolic effects in human muscle. These effects are likely to be mediated through changes in signaling pathways controlling protein synthesis. This involves phosphorylation of the mammalian target of rapamycin (mTOR) and sequential activation of 70-kD S6 protein kinase (p70 S6 kinase) and the eukaryotic initiation factor 4E-binding protein 1. Activation of p70 S6 kinase, and subsequent phopsphorylation of the ribosomal protein S6, is associated with enhanced translation of specific mRNAs. When BCAAs were supplied to subjects during and after one session of quadriceps muscle resistance exercise, an increase in mTOR, p70 S6 kinase, and S6 phosphorylation was found in the recovery period after the exercise with no effect of BCAAs on Akt or glycogen synthase kinase 3 (GSK-3) phosphorylation. Exercise without BCAA intake led to a partial phosphorylation of p70 S6 kinase without activating the enzyme, a decrease in Akt phosphorylation, and no change in GSK-3. It has previously been shown that leucine infusion increases p70 S6 kinase phosphorylation in an Akt-independent manner in resting subjects; however, a relation between mTOR and p70 S6 kinase has not been reported previously. The results suggest that BCAAs activate mTOR and p70 S6 kinase in human muscle in the recovery period after exercise and that GSK-3 is not involved in the anabolic action of BCAAs on human muscle.

Insulin-stimulated phosphorylation of the Akt substrate AS160 is impaired in skeletal muscle of type 2 diabetic subjects.

AS160 is a newly described substrate for the protein kinase Akt that links insulin signaling and GLUT4 trafficking. In this study, we determined the expression of and in vivo insulin action on AS160 in human skeletal muscle. In addition, we compared the effect of physiological hyperinsulinemia on AS160 phosphorylation in 10 lean-to-moderately obese type 2 diabetic and 9 healthy subjects. Insulin infusion increased the phosphorylation of several proteins reacting with a phospho-Akt substrate antibody. We focused on AS160, as this Akt substrate has been linked to glucose transport. A 160-kDa phosphorylated protein was identified as AS160 by immunoblot analysis with an AS160-specific antibody. Physiological hyperinsulinemia increased AS160 phosphorylation 2.9-fold in skeletal muscle of control subjects (P < 0.001). Insulin-stimulated AS160 phosphorylation was reduced 39% (P < 0.05) in type 2 diabetic patients. AS160 protein expression was similar in type 2 diabetic and control subjects. Impaired AS160 phosphorylation was related to aberrant Akt signaling; insulin action on Akt Ser(473) phosphorylation was not significantly reduced in type 2 diabetic compared with control subjects, whereas Thr(308) phosphorylation was impaired 51% (P < 0.05). In conclusion, physiological hyperinsulinemia increases AS160 phosphorylation in human skeletal muscle. Moreover, defects in insulin action on AS160 may impair GLUT4 trafficking in type 2 diabetes.

Effects of metformin and rosiglitazone treatment on insulin signaling and glucose uptake in patients with newly diagnosed type 2 diabetes: a randomized controlled study.

The effect of metformin or rosiglitazone monotherapy versus placebo on insulin signaling and gene expression in skeletal muscle of patients with newly diagnosed type 2 diabetes was determined. A euglycemic-hyperinsulinemic clamp, combined with skeletal muscle biopsies and glucose uptake measurements over rested and exercised muscle, was performed before and after 26 weeks of metformin (n = 9), rosiglitazone (n = 10), or placebo (n = 11) treatment. Insulin-mediated whole-body and leg muscle glucose uptake was enhanced 36 and 32%, respectively, after rosiglitazone (P < 0.01) but not after metformin or placebo treatment. Insulin increased insulin receptor substrate 1 (IRS-1) tyrosine phosphorylation, IRS-1-associated phosphatidylinositol (PI) 3-kinase activity, and phosphorylation of Akt Ser473 and AS160, a newly described Akt substrate that plays a role in GLUT4 exocytosis, approximately 2.3 fold before treatment. These insulin signaling parameters were unaltered after metformin, rosiglitazone, or placebo treatment. Expression of selected genes involved in glucose and fatty acid metabolism in skeletal muscle was unchanged between the treatment groups. Low-intensity acute exercise increased insulin-mediated glucose uptake but was without effect on insulin signaling. In conclusion, the insulin-sensitizing effects of rosiglitazone are independent of enhanced signaling of IRS-1/PI 3-kinase/Akt/AS160 in patients with newly diagnosed type 2 diabetes.

Relationship between serum amyloid A level and Tanis/SelS mRNA expression in skeletal muscle and adipose tissue from healthy and type 2 diabetic subjects.

Tanis is a recently described protein reported to be a putative receptor for serum amyloid A and found to be dysregulated with diabetes in the Israeli sand rat Psamommys obesus. Tanis has also been identified as a selenoprotein, one of the first two identified membrane selenoproteins. We determined mRNA expression of the human homologue of Tanis, SelS/AD-015, in skeletal muscle and adipose tissue biopsies obtained from 10 type 2 diabetic patients and 11 age- and weight-matched healthy subjects. Expression of Tanis/SelS mRNA in skeletal muscle and adipose tissue biopsies was similar between diabetic and control subjects. A subset of subjects underwent a euglycemic-hyperinsulinemic clamp, and adipose tissue expression of Tanis/SelS was determined after in vivo insulin stimulation. Adipose tissue Tanis/SelS mRNA expression was unchanged after insulin infusion in control subjects, whereas Tanis/SelS mRNA increased in seven of eight subjects following insulin stimulation in diabetic subjects. Skeletal muscle and adipose tissue Tanis/SelS mRNA expression were positively correlated with plasma serum amyloid A. In conclusion, there is a strong trend toward upregulation of Tanis/SelS following insulin infusion in adipose tissue from type 2 diabetic subjects. Moreover, the positive relationship between Tanis mRNA and the acute-phase protein serum amyloid A suggests an interaction between innate immune system responses and Tanis expression in muscle and adipose tissue.