siRNA-based gene silencing reveals specialized roles of IRS-1/Akt2 and IRS-2/Akt1 in glucose and lipid metabolism in human skeletal muscle

HIV protease inhibitors induce metabolic dysfunction in part via increased JNK1/2 pro-inflammatory signaling in L6 cells

Protease inhibitors (PIs), such as atazanavir sulfate and ritonavir, are used clinically to prevent the progression of HIV and are known to induce insulin resistance. To determine whether PI-mediated insulin resistance is induced by activation of pro-inflammatory cascades, L6 skeletal muscle cells were treated ±atazanavir sulfate, ritonavir, or atazanavir sulfate + ritonavir, and ±insulin. Treatment with atazanavir sulfate, ritonavir, or atazanavir sulfate + ritonavir for 24 or 48 h significantly increased basal glucose uptake (P<0.05) and atazanavir sulfate + ritonavir treatment increased basal glucose uptake significantly more than ritonavir or atazanavir sulfate treatment alone (P<0.05). Atazanavir sulfate + ritonavir treatment for 48 h completely prevented insulin stimulation of glucose uptake (P>0.05). When compared to untreated cells, basal palmitate uptake and oxidation was found to be significantly higher in cells treated with PIs alone or in combination (P<0.05). Prior PI treatment alone or in combination prevented (P>0.05) the insulin-mediated increase in palmitate uptake and the insulin-mediated decrease in palmitate oxidation observed in the control group. Atazanavir sulfate treatment alone or in combination with ritonavir significantly increased JNK1/2 phosphorylation when compared to the control or ritonavir group (P<0.05) and this was accompanied by a rise (P<0.05) in AKT(Ser473) phosphorylation in the basal state. Total JNK1/2 and p38 MAPK protein content and p38 MAPK phosphorylation state were not altered in any of the treatment groups (P>0.05). Our data indicate that, in muscle cells, PIs induce metabolic dysfunction that is not limited to insulin-sensitive metabolism and that is potentially mediated by a rise in JNK1/2 pro-inflammatory signaling.

Endothelin-1 reduces glucose uptake in human skeletal muscle in vivo and in vitro

OBJECTIVE

Endothelin (ET)-1 is a vasoconstrictor and proinflammatory peptide that may interfere with glucose uptake. Our objective was to investigate whether exogenous ET-1 affects glucose uptake in the forearm of individuals with insulin resistance and in cultured human skeletal muscle cells.

RESEARCH DESIGN AND METHODS

Nine male subjects (aged 61 ± 3 years) with insulin resistance (M value <5.5 mg/kg/min or a homeostasis model assessment of insulin resistance index >2.5) participated in a protocol using saline infusion followed by ET-1 infusion (20 pmol/min) for 2 h into the brachial artery. Forearm blood flow (FBF), endothelium-dependent vasodilatation, and endothelium-independent vasodilatation were assessed. Molecular signaling and glucose uptake were determined in cultured skeletal muscle cells.

RESULTS

ET-1 decreased forearm glucose uptake (FGU) by 39% (P < 0.05) after the 2-h infusion. ET-1 reduced basal FBF by 36% after the 2-h infusion (P < 0.05) and impaired both endothelium-dependent vasodilatation (P < 0.01) and endothelium-independent vasodilatation (P < 0.05). ET(A) and ET(B) receptor expression was detected on cultured skeletal muscle cells. One-hour ET-1 incubation increased glucose uptake in cells from healthy control subjects but not from type 2 diabetic patients. Incubation with ET-1 for 24 h reduced glucose uptake in cells from healthy subjects. ET-1 decreased insulin-stimulated Akt phosphorylation and increased phosphorylation of insulin receptor substrate-1 serine 636.

CONCLUSIONS

ET-1 not only induces vascular dysfunction but also acutely impairs FGU in individuals with insulin resistance and in skeletal muscle cells from type 2 diabetic subjects. These findings suggest that ET-1 may contribute to the development of insulin resistance in skeletal muscle in humans.

Testosterone or 17{beta}-estradiol exposure reveals sex-specific effects on glucose and lipid metabolism in human myotubes

Changes in sex hormone levels with aging or illness may lead to metabolic disorders. Moreover, the ratio changes in men versus women may have distinct pathological responses. Since little is known about sex hormone action on muscle metabolism, we examined the role of testosterone or 17β-estradiol (E(2)) in metabolism and investigated whether either hormone may mediate a sex-specific effect. Myotubes from postmenopausal women and age-matched male donors were treated with 10 nM testosterone or E(2) for 4 days, and assays were performed to measure metabolic readouts, signal transduction, and mRNA expression. Testosterone and E(2) treatment enhanced insulin-stimulated glucose incorporation into glycogen and AKT phosphorylation in myotubes from female donors, highlighting a sex-specific role of sex hormone in glucose metabolism. Testosterone treatment increased palmitate oxidation in myotubes from both female and male donors, while E(2) enhanced palmitate oxidation in myotubes from male donors only. Testosterone-mediated increase in palmitate oxidation was attenuated at the presence of androgen receptor antagonist, which may indicate a role of nuclear steroid receptor in muscle lipid oxidation. Testosterone treatment increased mRNA expression of the insulin receptor substrate 2 in myotubes from male and female donors, whereas it increased mRNA expression of glycogen synthase 1 only in myotubes from male donors. E(2) treatment increased pyruvate dehydrogenase kinase 4 mRNA expression in myotubes from female donors. Thus, our data suggest that testosterone or E(2) modulates muscle glucose and lipid metabolism and may play a role in metabolism in a sex-dependent manner.

Ankrd2/ARPP is a novel Akt2 specific substrate and regulates myogenic differentiation upon cellular exposure to H(2)O(2)

A proteomic-based search for novel substrates of Akt was undertaken in C₂C₁₂ murine muscle cells. Our data demonstrate that Akt isoform 2 phosphorylates Ankrd2 at Serine 99 in response to H₂O₂ stimuli, regulating muscle differentiation rate.

Activation of Akt-mediated signaling pathways is crucial for survival, differentiation, and regeneration of muscle cells. A proteomic-based search for novel substrates of Akt was therefore undertaken in C₂C₁₂ murine muscle cells exploiting protein characterization databases in combination with an anti–phospho-Akt substrate antibody. A Scansite database search predicted Ankrd2 (Ankyrin repeat domain protein 2, also known as ARPP) as a novel substrate of Akt. In vitro and in vivo studies confirmed that Akt phosphorylates Ankrd2 at Ser-99. Moreover, by kinase assay with recombinant Akt1 and Akt2, as well as by single-isoform silencing, we demonstrated that Ankrd2 is a specific substrate of Akt2. Ankrd2 is typically found in skeletal muscle cells, where it mediates the transcriptional response to stress conditions. In an attempt to investigate the physiological implications of Ankrd2 phosphorylation by Akt2, we found that oxidative stress induced by H₂O₂ triggers this phosphorylation. Moreover, the forced expression of a phosphorylation-defective mutant form of Ankrd2 in C₂C₁₂ myoblasts promoted a faster differentiation program, implicating Akt-dependent phosphorylation at Ser-99 in the negative regulation of myogenesis in response to stress conditions.

Increased levels of the Akt-specific phosphatase PH domain leucine-rich repeat protein phosphatase (PHLPP)-1 in obese participants are associated with insulin resistance

AIMS/HYPOTHESIS

We determined the contribution to insulin resistance of the PH domain leucine-rich repeat protein phosphatase (PHLPP), which dephosphorylates Akt at Ser473, inhibiting its activity. We measured the abundance of PHLPP in fat and skeletal muscle from obese participants. To study the effect of PHLPP on insulin signalling, PHLPP (also known as PHLPP1) was overexpressed in HepG2 and L6 cells.

METHODS

Subcutaneous fat samples were obtained from 82 morbidly obese and ten non-obese participants. Skeletal muscle samples were obtained from 12 obese and eight non-obese participants. Quantification of PHLPP-1 in human tissues was performed by immunoblotting. The functional consequences of recombinant PHLPP1 overexpression in hepatoma HepG2 cells and L6 myoblasts were investigated.

RESULTS

Of the 82 obese participants, 31 had normal fasting glucose, 33 impaired fasting glucose and 18 type 2 diabetes. PHLPP-1 abundance was twofold higher in the three obese groups than in non-obese participants (p = 0.004). No differences were observed between obese participants with normal fasting glucose, impaired fasting glucose or type 2 diabetes. PHLPP-1 abundance was correlated with basal Akt Ser473 phosphorylation (r = -0.48; p = 0.001), BMI (r = 0.44; p < 0.0001), insulin (r = 0.35; p < 0.0001) and HOMA (r = 0.38; p < 0.0001). PHLPP-1 abundance was twofold higher in the skeletal muscle of 12 obese participants than in that of eight non-obese participants (p < 0.0001). Insulin treatment of HepG2 cells resulted in a dose- and time-dependent upregulation of PHLPP-1. Overexpression of PHLPP1 in HepG2 cells and L6 myoblasts resulted in impaired insulin signalling involving Akt/glycogen synthase kinase 3, glycogen synthesis and glucose transport.

CONCLUSIONS/INTERPRETATION

Increased abundance of PHLPP-1, production of which is regulated by insulin, may represent a new molecular defect in insulin-resistant states such as obesity.

Serum starvation: caveat emptor

Serum starvation is one of the most frequently performed procedures in molecular biology and there are literally thousands of research papers reporting its use. In fact, this method has become so ingrained in certain areas of research that reports often simply state that cells were serum starved without providing any factual details as to how the procedure was carried out. Even so, we quite obviously lack unequivocal terminology, standard protocols, and perhaps most surprisingly, a common conceptual basis when performing serum starvation. Such inconsistencies not only hinder interstudy comparability but can lead to opposing and inconsistent experimental results. Although it is frequently assumed that serum starvation reduces basal activity of cells, available experimental data do not entirely support this notion. To address this important issue, we studied primary human myotubes, rat L6 myotubes and human embryonic kidney (HEK)293 cells under different serum starvation conditions and followed time-dependent changes in important signaling pathways such as the extracellular signal-regulated kinase 1/2, the AMP-activated protein kinase, and the mammalian target of rapamycin. Serum starvation induced a swift and dynamic response, which displayed obvious qualitative and quantitative differences across different cell types and experimental conditions despite certain unifying features. There was no uniform reduction in basal signaling activity. Serum starvation clearly represents a major event that triggers a plethora of divergent responses and has therefore great potential to interfere with the experimental results and affect subsequent conclusions.

Promiscuous affairs of PKB/AKT isoforms in metabolism

The protein kinase B (PKB) family encompasses three isoforms; PKBα (AKT1), PKBβ (AKT2) and PKBγ (AKT3). PKBα and PKBβ but not PKBγ, are prominently expressed in classical insulin-sensitive tissues like liver, muscle and fat. Transgenic mice deficient for PKBα, PKBβ or PKBγ have been analysed to study the roles of PKB isoforms in metabolic regulation. Until recently, only loss of PKBβ was reported to result in metabolic disorders, especially insulin resistance, in humans and mice. However, a new study has shown that PKBα-deficient mice can show enhanced glucose tolerance accompanied by improved β-cell function and higher insulin sensitivity in adipocytes. These findings prompted us to review the relevant literature on the regulation of glucose metabolism by PKB isoforms in liver, skeletal muscle, adipocytes and pancreas.

Insulin-induced serine phosphorylation of IRS-2 via ERK1/2 and mTOR: studies on the function of Ser675 and Ser907

The identity of specific serine phosphorylation residues of insulin receptor substrate (IRS)-2 and their impact on insulin signal transduction are largely unknown. Ser(675) and Ser(907) of mouse IRS-2 are adjacent to PI 3-kinase or Grb2 binding domains, respectively. Using monoclonal phosphosite-specific antibodies, we demonstrated the phosphorylation of both serines after stimulation of Fao hepatoma cells with insulin, anisomycin, or phorbol esters. Phosphorylation of both sites was a late and prolonged event during insulin treatment and was also detected in liver tissue of insulin-treated as well as refed mice. Inhibition and siRNA-mediated knockdown of ERK1/2 indicated that the insulin-induced phosphorylation of Ser(907) was ERK dependent. Phosphorylation of Ser(907) did not prevent the insulin-induced association of IRS-2 with Grb2, but phosphorylation of the adjacent Tyr(911) was proved to be crucial in HEK 293 cells expressing IRS-2 Ala mutants. The insulin-induced phosphorylation of Ser(675) was prevented by inhibition and siRNA-mediated knockdown of mTOR but not of p70(S6K1). Mutation of Ser(675) to Ala did not affect downstream insulin signaling but increased the half-life of the protein, suggesting an involvement of phospho-Ser(675) in an accelerated degradation of IRS-2. Moreover, the insulin-induced degradation of IRS-2 was blocked by inhibition of mTOR. We conclude that the two novel insulin-dependent serine phosphorylation sites of IRS-2 were not involved in the regulation of the adjacent PI 3-kinase and Grb2 binding domains but might be implicated in the ERK- and mTOR-mediated negative feedback control.

An amino acid mixture enhances insulin-stimulated glucose uptake in isolated rat epitrochlearis muscle

Protein and certain amino acids (AA) have been found to lower blood glucose. Although these glucose-lowering AA are important modulators of skeletal muscle metabolism, their impact on muscle glucose uptake remains unclear. We therefore examined how an AA mixture consisting of 2 mM isoleucine, 0.012 mM cysteine, 0.006 mM methionine, 0.0016 mM valine, and 0.014 mM leucine impacts skeletal muscle glucose uptake in the absence or presence of a submaximal (sINS) or maximal insulin (mINS) concentration. The AA mixture, sINS, and mINS significantly increased 2-[(3)H]deoxyglucose (2-DG) uptake by 63, 79, and 298% above basal, respectively. When the AA mixture was combined with sINS and mINS, 2-DG uptake was further increased significantly by 26% (P = 0.028) and 14% (P = 0.032), respectively. Western blotting analysis revealed that the AA mixture increased basal and sINS Akt substrate of 160 kDa (AS160) phosphorylation, while AA mixture did not change phosphorylation of Akt or mammalian target of rapamycin (mTOR) under these conditions. Interestingly, addition of the AA mixture to mINS increased phosphorylation of mTOR, Akt as well as AS160, compared with mINS alone. These data suggest that certain AA increase glucose uptake in the absence of insulin and augment insulin-stimulated glucose uptake in an additive manner. Furthermore, these effects appear to be mediated via a pathway that is independent from the canonical insulin cascade and therefore may prove effective as an alternative therapeutic treatment for insulin resistance.

Bimodal effect on pancreatic β-cells of secretory products from normal or insulin-resistant human skeletal muscle

OBJECTIVE

Type 2 diabetes is characterized by insulin resistance with a relative deficiency in insulin secretion. This study explored the potential communication between insulin-resistant human skeletal muscle and primary (human and rat) β-cells.

RESEARCH DESIGN AND METHODS

Human skeletal muscle cells were cultured for up to 24 h with tumor necrosis factor (TNF)-α to induce insulin resistance, and mRNA expression for cytokines was analyzed and compared with controls (without TNF-α). Conditioned media were collected and candidate cytokines were measured by antibody array. Human and rat primary β-cells were used to explore the impact of exposure to conditioned media for 24 h on apoptosis, proliferation, short-term insulin secretion, and key signaling protein phosphorylation and expression.

RESULTS

Human myotubes express and release a different panel of myokines depending on their insulin sensitivity, with each panel exerting differential effects on β-cells. Conditioned medium from control myotubes increased proliferation and glucose-stimulated insulin secretion (GSIS) from primary β-cells, whereas conditioned medium from TNF-α-treated insulin-resistant myotubes (TMs) exerted detrimental effects that were either independent (increased apoptosis and decreased proliferation) or dependent on the presence of TNF-α in TM (blunted GSIS). Knockdown of β-cell mitogen-activated protein 4 kinase 4 prevented these effects. Glucagon-like peptide 1 protected β-cells against decreased proliferation and apoptosis evoked by TMs, while interleukin-1 receptor antagonist only prevented the latter.

CONCLUSIONS

Taken together, these data suggest a possible new route of communication between skeletal muscle and β-cells that is modulated by insulin resistance and could contribute to normal β-cell functional mass in healthy subjects, as well as the decrease seen in type 2 diabetes.

Akt/protein kinase B in skeletal muscle physiology and pathology

The Akt/protein kinase B is critical regulator of cellular homeostasis with diminished Akt activity being associated with dysregulation of cellular metabolism and cell death while Akt over-activation has been linked to inappropriate cell growth and proliferation. Although the regulation of Akt function has been well characterized in vitro, much less is known regarding the function of Akt in vivo. Here we examine how skeletal muscle Akt expression and enzymatic activity are controlled, the role of Akt in the regulation of skeletal muscle contraction, stress response glucose utilization, and protein metabolism, and the potential participation of this important molecule in skeletal muscle atrophy, aging, and cancer.

Docking proteins

Docking proteins comprise a distinct category of intracellular, noncatalytic signalling protein, that function downstream of a variety of receptor and receptor-associated tyrosine kinases and regulate diverse physiological and pathological processes. The growth factor receptor bound 2-associated binder/Daughter of Sevenless, insulin receptor substrate, fibroblast growth factor receptor substrate 2 and downstream of tyrosine kinases protein families fall into this category. This minireview focuses on the structure, function and regulation of these proteins.

Functional characterization of a haplotype in the AKT1 gene associated with glucose homeostasis and metabolic syndrome

A small 12-kb haplotype upstream of the AKT1 gene has been found to be associated with insulin resistance phenotypes. We sought to define the functional consequences of the three component polymorphic loci (rs1130214, rs10141867, rs33925946) on AKT1 and the upstream ZBTB42 gene. 5' RACE analysis of AKT1 transcripts in human skeletal muscle biopsies showed the predominant promoter to be 2.5 kb upstream of exon 2, and distinct from those promoters previously reported in rat. We then studied the effect of each of the three haplotype polymorphisms in transcriptional reporter assays in muscle, bone, and fat cell culture models, and found that each modulated enhancer and repressor activity are in a cell-specific and differentiation-specific manner. Our results in promoter assays are consistent with the human phenotype data; we found an anabolic effect on muscle and bone with increased mRNA expression of AKT1, and catabolic effect on fat with decreased expression. To test the hypothesis that rs10141867 affects transcription levels of the novel zinc finger protein ZBTB42 in vivo, we developed the allele-specific expression assay using Taqman technology to test for allelic differences within heterozygotes. The allele containing the derived polymorphism (haplotype H2) showed a 1.75-fold increase in expression in human skeletal muscle. Our data show a particularly complex effect of the component polymorphisms of a single haplotype on cells and tissues, suggesting that the coordination of different tissue-specific effects may have driven selection for the H2 haplotype. In light of the recent abundance of SNP association studies, our approach can serve as a method for exploring the biological function of polymorphisms that show significant genotype/phenotype associations.

Differential aetiology and impact of phosphoinositide 3-kinase (PI3K) and Akt signalling in skeletal muscle on in vivo insulin action

AIMS/HYPOTHESIS

Insulin resistance in skeletal muscle is a key factor in the development of type 2 diabetes and although some studies indicate that this could be partly attributed to reduced content and activity of various proximal and distal insulin signalling molecules, consensus is lacking. We therefore aimed to investigate the regulation of proximal insulin signalling in skeletal muscle and its effect on glucose metabolism in a large non-diabetic population.

METHODS

We examined 184 non-diabetic twins with gold-standard techniques including the euglycaemic-hyperinsulinaemic clamp. Insulin signalling was evaluated at three key levels, i.e. the insulin receptor, IRS-1 and V-akt murine thymoma viral oncogene (Akt) levels, employing kinase assays and phospho-specific western blotting.

RESULTS

Proximal insulin signalling was not associated with obesity, age or sex. However, birthweight was positively associated with IRS-1-associated phosphoinositide 3-kinase (PI3K; IRS-1-PI3K) activity (p = 0.04); maximal aerobic capacity (VO2(max)), paradoxically, was negatively associated with IRS-1-PI3K (p = 0.02) and Akt2 activity (p = 0.01). Additionally, we found low heritability estimates for most measures of insulin signalling activity. Glucose disposal was positively associated with Akt-308 phosphorylation (p < 0.001) and Akt2 activity (p = 0.05), but not with insulin receptor tyrosine kinase or IRS-1-PI3K activity.

CONCLUSIONS/INTERPRETATION

With the exception of birthweight, 'classical' modifiers of insulin action, including genetics, age, sex, obesity and VO2(max) do not seem to mediate their most central effects on whole-body insulin sensitivity through modulation of proximal insulin signalling in skeletal muscle. We also demonstrated an association between Akt activity and in vivo insulin sensitivity, suggesting a role of Akt in control of in vivo insulin resistance and potentially in type 2 diabetes.

Molecular mechanisms controlling human adipose tissue development: insights from monogenic lipodystrophies

Appropriately functioning adipose tissue is essential for human health, a fact most clearly illustrated by individuals with lipodystrophy, who have impaired adipose development and often suffer severe metabolic disease as a result. Humans with obesity display a similar array of metabolic problems. This reflects failures in fat tissue function in obesity, which results in consequences similar to those seen when insufficient adipose tissue is present. Thus a better understanding of the molecules that regulate the development of fat tissue is likely to aid the generation of novel therapeutic strategies for the treatment of all disorders of altered fat mass. Single gene disruptions causing lipodystrophy can give unique insights into the importance of the proteins they encode in human adipose tissue development. Moreover, the mechanisms via which they cause lipodystrophy can reveal new molecules and pathways important for adipose tissue development and function as well as confirming the importance of molecules identified from studies of cellular and animal models.

The actin-bundling protein palladin is an Akt1-specific substrate that regulates breast cancer cell migration

The phosphatidylinositol 3-kinase (PI3K) signaling pathway is frequently deregulated in cancer. Downstream of PI3K, Akt1 and Akt2 have opposing roles in breast cancer invasive migration, leading to metastatic dissemination. Here, we identify palladin, an actin-associated protein, as an Akt1-specific substrate that modulates breast cancer cell invasive migration. Akt1, but not Akt2, phosphorylates palladin at Ser507 in a domain that is critical for F-actin bundling. Downregulation of palladin enhances migration and invasion of breast cancer cells and induces abnormal branching morphogenesis in 3D cultures. Palladin phosphorylation at Ser507 is required for Akt1-mediated inhibition of breast cancer cell migration and also for F-actin bundling, leading to the maintenance of an organized actin cytoskeleton. These findings identify palladin as an Akt1-specific substrate that regulates cell motility and provide a molecular mechanism that accounts for the functional distinction between Akt isoforms in breast cancer cell signaling to cell migration.

Prolonged activation of S6K1 does not suppress IRS or PI-3 kinase signaling during muscle cell differentiation

BACKGROUND

Myogenesis in C2C12 cells requires the activation of the PI3K/mTOR signaling pathways. Since mTOR signaling can feedback through S6K1 to inhibit the activation of PI3K, the aim of this work was to assess whether feedback from S6K1 played a role in myogenesis and determine whether siRNA mediated knockdown of S6K1 would lead to an increased rate of myotube formation.

RESULTS

S6K1 activity increased in a linear fashion following plating and was more than 3-fold higher after Day 3 of differentiation (subconfluent = 11.09 +/- 3.05, Day 3 = 29.34 +/- 3.58). IRS-1 levels tended to increase upon serum withdrawal but decreased approximately 2-fold (subconfluent = 0.88 +/- 0.10, Day 3 = 0.42 +/- 0.06) 3 days following differentiation whereas IRS-2 protein remained stable. IRS-1 associated p85 was significantly reduced upon serum withdrawal (subconfluent = 0.86 +/- 0.07, Day 0 = 0.31 +/- 0.05), remaining low through day 1. IRS-2 associated p85 decreased following serum withdrawal (subconfluent = 0.96 +/- 0.05, Day 1 = 0.56 +/- 0.08) and remained suppressed up to Day 3 following differentiation (0.56 +/- 0.05). Phospho-tyrosine associated p85 increased significantly from subconfluent to Day 0 and remained elevated throughout differentiation. siRNA directed against S6K1 and S6K2 did not result in changes in IRS-1 levels after either 48 or 96 hrs. Furthermore, neither 48 nor 96 hrs of S6K1 knockdown caused a change in myotube formation.

CONCLUSIONS

Even though S6K1 activity increases throughout muscle cell differentiation and IRS-1 levels decrease over this period, siRNA suggests that S6K1 is not mediating the decrease in IRS-1. The decrease in IRS-1/2 associated p85 together with the increase in phospho-tyrosine associated p85 suggests that PI3K associates primarily with scaffolds other than IRS-1/2 during muscle cell differentiation.

Smad5 regulates Akt2 expression and insulin-induced glucose uptake in L6 myotubes

Insulin-induced glucose uptake by skeletal muscle results from Akt2 activation and is severely impaired during insulin resistance. Recently, we and others have demonstrated that BMP9 improves glucose homeostasis in diabetic and non-diabetic rodents. However, the mechanism by which BMP9 modulates insulin action remains unknown. Here we demonstrate that Smad5, a transcription factor activated by BMP9, and Akt2, are upregulated in differentiated L6 myotubes. Smad5, rather than Smad1/8, is downregulated "in vivo" and "in vitro" by dexamethasone. Smad5 knockdown decreased Akt2 expression and serine phosphorylation and insulin-induced glucose uptake, and increased the expression of the lipid phosphatase Ship2. Additionally, binding of Smad5 to Akt2 gene is decreased in dexamethasone-treated rats and increased in L6 myotubes compared to myoblasts. The present study indicates that Smad5 regulates glucose uptake in skeletal muscle by controlling Akt2 expression and phosphorylation. These finding reveals Smad5 as a potential target for the therapeutic of type 2 diabetes.

Identification of G0S2 as a gene frequently methylated in squamous lung cancer by combination of in silico and experimental approaches

Epigenetic changes can lead to abnormal expression of genes in cancer, and several genes have been reported to have aberrant promoter DNA methylation in non-small-cell lung cancer (NSCLC). We identified aberrantly methylated genes in NSCLC by combination of in silico and experimental approaches. We first applied bioinformatics, and from microarray datasets, we selected genes with low expression and having functions related to cancer. Next, combined bisulfite restriction analysis was carried out in 10 pooled resected lung cancer tissues to screen for genes that were aberrantly methylated, and the methylation ratio (the fraction of methylated DNA in extracted DNA from a cancer tissue sample) was quantified using quantitative analysis of methylated alleles. We identified 8 methylated genes (ARPC1B, DNAH9, FLRT2, G0S2, IRS2, PKP1, SPOCK1 and UCHL1) previously unreported in NSCLC. Analyses of methylation profiles of 101 resected lung cancer tissue samples revealed quantitatively low methylation in whole, methylation ratios were almost less than 30% even in the methylated samples, and no significant correlation to prognosis after 2 years of follow-up using hierarchical clustering. DNA methylation of G0S2 gene was significantly more frequent in squamous lung cancer (n = 18, mean of methylation ratios: 15%) compared with nonsquamous lung cancer (n = 83, mean of methylation ratios: 2.6%) (Mann-Whitney U test, p < 0.001). DNA methylation of G0S2 can be an important biomarker for squamous lung cancer.

Activation of Wnt/beta-catenin signaling increases insulin sensitivity through a reciprocal regulation of Wnt10b and SREBP-1c in skeletal muscle cells

Background

Intramyocellular lipid accumulation is strongly related to insulin resistance in humans, and we have shown that high glucose concentration induced de novo lipogenesis and insulin resistance in murin muscle cells. Alterations in Wnt signaling impact the balance between myogenic and adipogenic programs in myoblasts, partly due to the decrease of Wnt10b protein. As recent studies point towards a role for Wnt signaling in the pathogenesis of type 2 diabetes, we hypothesized that activation of Wnt signaling could play a crucial role in muscle insulin sensitivity.

Methodology/Principal Findings

Here we demonstrate that SREBP-1c and Wnt10b display inverse expression patterns during muscle ontogenesis and regeneration, as well as during satellite cells differentiation. The Wnt/β-catenin pathway was reactivated in contracting myotubes using siRNA mediated SREBP-1 knockdown, Wnt10b over-expression or inhibition of GSK-3β, whereas Wnt signaling was inhibited in myoblasts through silencing of Wnt10b. SREBP-1 knockdown was sufficient to induce Wnt10b protein expression in contracting myotubes and to activate the Wnt/β-catenin pathway. Conversely, silencing Wnt10b in myoblasts induced SREBP-1c protein expression, suggesting a reciprocal regulation. Stimulation of the Wnt/β-catenin pathway i) drastically decreased SREBP-1c protein and intramyocellular lipid deposition in myotubes; ii) increased basal glucose transport in both insulin-sensitive and insulin-resistant myotubes through a differential activation of Akt and AMPK pathways; iii) restored insulin sensitivity in insulin-resistant myotubes.

Conclusions/Significance

We conclude that activation of Wnt/β-catenin signaling in skeletal muscle cells improved insulin sensitivity by i) decreasing intramyocellular lipid deposition through downregulation of SREBP-1c; ii) increasing insulin effects through a differential activation of the Akt/PKB and AMPK pathways; iii) inhibiting the MAPK pathway. A crosstalk between these pathways and Wnt/β-catenin signaling in skeletal muscle opens the exciting possibility that organ-selective modulation of Wnt signaling might become an attractive therapeutic target in regenerative medicine and to treat obese and diabetic populations.

Differential effects of protein kinase B/Akt isoforms on glucose homeostasis and islet mass

Protein kinase B (PKB)/Akt is considered to be a key target downstream of insulin receptor substrate 2 (IRS2) in the regulation of beta-cell mass. However, while deficiency of IRS2 in mice results in diabetes with insulin resistance and severe failure of beta-cell mass and function, only loss of the PKBbeta isoform leads to a mild metabolic phenotype with insulin resistance. Other isoforms were reported not to be required for metabolic regulation. To clarify the roles of the three PKB isoforms in the regulation of islet mass and glucose homeostasis, we assessed the metabolic and pancreatic phenotypes of Pkbalpha, Pkbbeta, and Pkbgamma-deficient mice. Our study uncovered a novel role for PKBalpha in the regulation of glucose homeostasis, whereas it confirmed that Pkbbeta(-/)(-) mice are insulin resistant with compensatory increase of islet mass. Pkbalpha(-/)(-) mice displayed an opposite phenotype with improved insulin sensitivity, lower blood glucose, and higher serum glucagon concentrations. Pkbgamma(-/)(-) mice did not show metabolic abnormalities. Additionally, our signaling analyses revealed that PKBalpha, but not PKBbeta or PKBgamma, is specifically activated by overexpression of IRS2 in beta-cells and is required for IRS2 action in the islets.

Evidence against a sexual dimorphism in glucose and fatty acid metabolism in skeletal muscle cultures from age-matched men and post-menopausal women

AIM

In vivo whole body differences in glucose/lipid metabolism exist between men and women. Thus, we tested the hypothesis that intrinsic sex differences exist in skeletal muscle gene expression and glucose/lipid metabolism using cultured myotubes.

METHODS

Myotube cultures were prepared for gene expression and metabolic studies from vastus lateralis skeletal muscle biopsies obtained from age-matched men (n = 11; 59 +/- 2 years) and post-menopausal women (n = 10; 60 +/- 1 years).

RESULTS

mRNA expression of several genes involved in glucose and lipid metabolism was higher in skeletal muscle biopsies from female vs. male donors, but unaltered between the sexes in cultured myotubes. Basal and insulin-stimulated glucose uptake, as well as glucose incorporation into glycogen, was similar in myotube cultures derived from male vs. female donors. In males vs. females, insulin increased glucose uptake (1.3 +/- 0.1 vs. 1.5 +/- 0.1-fold respectively) and incorporation into glycogen (2.3 +/- 0.3 vs. 2.0 +/- 0.3-fold respectively) to the same extent. Basal fatty acid oxidation and rate of uptake/accumulation was similar between sexes. In response to the 5'AMP-activated protein kinase activator AICAR, lipid oxidation was increased to the same extent in myotubes established from male vs. female donors (1.6 +/- 0.6 vs. 2.0 +/- 0.3-fold respectively). Moreover, the AICAR-induced rate of uptake/accumulation was similar between sexes.

CONCLUSION

Differences in metabolic parameters and gene expression profiles between age-matched men and post-menopausal women noted in vivo are not observed in cultured human skeletal muscle cells. Thus, the sexual dimorphism in glucose and lipid metabolism is likely a consequence of systemic whole body factors, rather than intrinsic differences in the skeletal muscle proper.

mTOR function in skeletal muscle: a focal point for overnutrition and exercise

The mammalian target of rapamycin (mTOR) is a highly conserved atypical serine–threonine kinase that controls numerous functions essential for cell homeostasis and adaptation in mammalian cells via 2 distinct protein complex formations. Moreover, mTOR is a key regulatory protein in the insulin signalling cascade and has also been characterized as an insulin-independent nutrient sensor that may represent a critical mediator in obesity-related impairments of insulin action in skeletal muscle. Exercise characterizes a remedial modality that enhances mTOR activity and subsequently promotes beneficial metabolic adaptation in skeletal muscle. Thus, the metabolic effects of nutrients and exercise have the capacity to converge at the mTOR protein complexes and subsequently modify mTOR function. Accordingly, the aim of the present review is to highlight the role of mTOR in the regulation of insulin action in response to overnutrition and the capacity for exercise to enhance mTOR activity in skeletal muscle.

The Akt kinases: isoform specificity in metabolism and cancer

The Akt (PKB) protein kinases are critical regulators of human physiology that control an impressive array of diverse cellular functions, including the modulation of growth, survival, proliferation and metabolism. The Akt kinase family is comprised of three highly homologous isoforms: Akt1 (PKBalpha), Akt2 (PKBbeta) and Akt3 (PKBgamma). Phenotypic analyses of Akt isoform knockout mice documented Akt isoform specific functions in the regulation of cellular growth, glucose homeostasis and neuronal development. Those studies establish that the functions of the different Akt kinases are not completely overlapping and that isoform-specific signaling contributes to the diversity of Akt activities. However, despite these important advances, a thorough understanding about the specific roles of Akt family members and the molecular mechanisms that determine Akt isoform functional specificity will be essential to elucidate the complexity of Akt regulated cellular processes and how Akt isoform-specific deregulation might contribute to disease states. Here, we summarize recent advances in understanding the roles of Akt isoforms in the regulation of metabolism and cancer, and possible mechanisms contributing to Akt isoform functional specificity.

Specificity of insulin signalling in human skeletal muscle as revealed by small interfering RNA

Insulin action on metabolically active tissues is a complex process involving positive and negative feedback regulation to control whole body glucose homeostasis. At the cellular level, glucose and lipid metabolism, as well as protein synthesis, are controlled through canonical insulin signalling cascades. The discovery of small interfering RNA (siRNA) allows for the molecular dissection of critical components of the regulation of metabolic and gene regulatory events in insulin-sensitive tissues. The application of siRNA to tissues of human origin allows for the molecular dissection of the mechanism(s) regulating glucose and lipid metabolism. Penetration of the pathways controlling insulin action in human tissue may aid in discovery efforts to develop diabetes prevention and treatment strategies. This review will focus on the use of siRNA to validate critical regulators controlling insulin action in human skeletal muscle, a key organ important for the control of whole body insulin-mediated glucose uptake and metabolism.

Altered skeletal muscle insulin signaling and mitochondrial complex II-III linked activity in adult offspring of obese mice

We recently reported insulin resistance in adult offspring of obese C57BL/6J mice. We have now evaluated whether parameters of skeletal muscle structure and function may play a role in insulin resistance in this model of developmental programming. Obesity was induced in female mice by feeding a highly palatable sugar and fat-rich diet for 6 wk prior to pregnancy, and during pregnancy and lactation. Offspring of obese dams were weaned onto standard laboratory chow. At 3 mo of age, skeletal muscle insulin signaling protein expression, mitochondrial electron transport chain activity (ETC), muscle fiber type, fiber density, and fiber cross-sectional area were compared with that of offspring of control dams weaned onto the chow diet. Female offspring of obese dams demonstrated decreased skeletal muscle expression of p110beta, the catalytic subunit of PI3K (P < 0.01), as well as reduced Akt phosphorylation at Serine residue 473 compared with control offspring. Male offspring of obese dams demonstrated increased skeletal muscle Akt2 and PKCzeta expression (P < 0.01; P < 0.001, respectively). A decrease in mitochondrial-linked complex II-III was observed in male offspring of obese dams (P < 0.01), which was unrelated to CoQ deficiency. This was not observed in females. There were no differences in muscle fiber density between offspring of obese dams and control offspring in either sex. Sex-related alterations in key insulin-signaling proteins and in mitochondrial ETC may contribute to a state of insulin resistance in offspring of obese mice.

Insulin receptor substrate-1/2 mediates IL-4-induced migration of human airway epithelial cells

Migration of airway epithelial cells (AEC) is an integral component of airway mucosal repair after injury. The inflammatory cytokine IL-4, abundant in chronic inflammatory airways diseases such as asthma, stimulates overproduction of mucins and secretion of chemokines from AEC; these actions enhance persistent airway inflammation. The effect of IL-4 on AEC migration and repair after injury, however, is not known. We examined migration in primary human AEC differentiated in air-liquid interface culture for 3 wk. Wounds were created by mechanical abrasion and followed to closure using digital microscopy. Concurrent treatment with IL-4 up to 10 ng/ml accelerated migration significantly in fully differentiated AEC. As expected, IL-4 treatment induced phosphorylation of the IL-4 receptor-associated protein STAT (signal transducer and activator of transcription)6, a transcription factor known to mediate several IL-4-induced AEC responses. Expressing a dominant negative STAT6 cDNA delivered by lentivirus infection, however, failed to block IL-4-stimulated migration. In contrast, decreasing expression of either insulin receptor substrate (IRS)-1 or IRS-2 using a silencing hairpin RNA blocked IL-4-stimulated AEC migration completely. These data demonstrate that IL-4 can accelerate migration of differentiated AEC after injury. This reparative response does not require STAT6 activation, but rather requires IRS-1 and/or IRS-2.

Inhibition of contraction-stimulated AMP-activated protein kinase inhibits contraction-stimulated increases in PAS-TBC1D1 and glucose transport without altering PAS-AS160 in rat skeletal muscle

OBJECTIVE

Phosphorylation of two members of the TBC1 domain family of proteins, Akt substrate of 160 kDa (AS160, also known as TBC1D4) and TBC1D1, has been implicated in the regulation of glucose transport in skeletal muscle. Insulin-stimulated phosphorylation (measured using the phospho-Akt substrate [PAS] antibody) of AS160 and TBC1D1 appears to occur in an Akt-dependent manner, but the kinases responsible for contraction-stimulated PAS-AS160 and PAS-TBC1D1 remain unclear. AMP-activated protein kinase (AMPK) and Akt, both activated by contraction, can each phosphorylate AS160 and TBC1D1 in cell-free assays.

RESEARCH DESIGN AND METHODS

To evaluate the roles of AMPK and Akt on insulin- or contraction-stimulated PAS-AS160, PAS-TBC1D1, and glucose transport, rat epitrochlearis was incubated with and without compound C (inhibitor of AMPK) or Wortmannin (inhibitor of phosphatidylinositol [PI] 3-kinase, which is upstream of Akt) before and during insulin stimulation or contraction.

RESULTS

Insulin-stimulated glucose transport and phosphorylation of both AS160 and TBC1D1 were completely inhibited by Wortmannin. Wortmannin eliminated contraction stimulation of phospho-Ser(21/9)glycogen synthase kinase 3alpha/beta (pGSK3; Akt substrate) and PAS-AS160 but did not significantly alter pAMPK, phospho-Ser79acetyl CoA carboxylase (pACC; AMPK substrate), PAS-TBC1D1, or glucose transport in contraction-stimulated muscle. Compound C completely inhibited contraction-stimulated pACC and PAS-TBC1D1 and partially blocked glucose transport, but it did not significantly alter pAkt, pGSK3, or PAS-AS160.

CONCLUSIONS

These data suggest that 1) insulin stimulates glucose transport and phosphorylation of AS160 and TBC1D1 in a PI 3-kinase/Akt-dependent manner, 2) contraction stimulates PAS-AS160 (but not PAS-TBC1D1 or glucose transport) in a PI 3-kinase/Akt-dependent manner, and 3) contraction stimulates PAS-TBC1D1 and glucose transport (but not PAS-AS160) in an AMPK-dependent manner.

Exercise-induced phospho-proteins in skeletal muscle

Efforts to identify exercise-induced signaling events in skeletal muscle have been influenced by ground-breaking discoveries in the insulin action field. Initial discoveries demonstrating that exercise enhances insulin sensitivity raised the possibility that contraction directly modulates insulin receptor signaling events. Although the acute effects of exercise on glucose metabolism are clearly insulin-independent, the canonical insulin signaling cascade has been used as a framework by investigators in an attempt to resolve the mechanisms by which muscle contraction governs glucose metabolism. This review focuses on recent advances in our understanding of exercise-induced signaling pathways governing glucose metabolism in skeletal muscle. Particular emphasis will be placed on the characterization of AS160, a novel Akt substrate that plays a role in the regulation of glucose transport.

Changes in insulin and IGF-I receptor expression during differentiation of human preadipocytes

Mature adipocytes originate from fibroblast-like precursor cells, preadipocytes, which differentiate to obtain the characteristics of adipocytes. Our aim was to investigate how differentiation of human preadipocytes affects the distribution of insulin receptors (IR) and IGF-I receptors (IGF-IR) and other cell characteristics. Preadipocytes were differentiated using indomethacine, dexamethasone, isobutyl-methylxantine (IBMX) and high concentration of insulin. Gene expression was quantified by real-time RT-PCR in preadipocytes (PA), differentiated preadipocytes (dPA) and mature adipocytes (mAD). The amount of expressed receptor protein was analyzed using receptor specific ELISAs and Western blot. We also studied DNA synthesis with radiolabeled thymidine incorporation and glucose accumulation with radiolabeled glucose. Differentiation of PA increased gene expression of IR but not IGF-IR. GLUT4, growth hormone receptor (GHR) and adiponectin appeared or increased. In PA and dPA only IR-A was expressed whereas also IR-B was detected in mAD. By Western blot and ELISA, IR and IGF-IR were detected in PA, dPA and mAD. During differentiation the ratio of IR to IGF-IR increased severalfold. In PA both the IR and the IGF-IR was phosphorylated by their own ligand at 1 nM and in dPA the activation of both receptors was stimulated by IGF-I, but not insulin, at 1 nM. Accumulation of glucose in PA was increased by insulin at 10nM and by IGF-I at 1 nM and 10nM. DNA synthesis was increased by insulin and IGF-I at 10nM. In conclusion, both IR and IGF-IR are present in human preadipocytes and adipocytes. Differentiation is characterized by an increased IR/IGF-IR ratio.

Reduced malonyl-CoA content in recovery from exercise correlates with improved insulin-stimulated glucose uptake in human skeletal muscle

This study evaluated whether improved insulin-stimulated glucose uptake in recovery from acute exercise coincides with reduced malonyl-CoA (MCoA) content in human muscle. Furthermore, we investigated whether a high-fat diet [65 energy-% (Fat)] would alter the content of MCoA and insulin action compared with a high-carbohydrate diet [65 energy-% (CHO)]. After 4 days of isocaloric diet on two occasions (Fat/CHO), 12 male subjects performed 1 h of one-legged knee extensor exercise (approximately 80% peak workload). Four hours after exercise, insulin-stimulated glucose uptake was determined in both legs during a euglycemic-hyperinsulinemic clamp. Muscle biopsies were obtained in both legs before and after the clamp. Four hours after exercise, insulin-stimulated glucose uptake was improved (approximately 70%, P<0.001) independent of diet composition and despite normal insulin-stimulated regulation of insulin receptor substrate-1-associated phosphatidylinositol 3-kinase, Akt, GSK-3, and glycogen synthase. Interestingly, exercise resulted in a sustained reduction (approximately 20%, P<0.05) in MCoA content 4 h after exercise that correlated (r=0.65, P<0.001) with improved insulin-stimulated glucose uptake. Four days of Fat diet resulted in an increased content of intramyocellular triacylglycerol (P<0.01) but did not influence muscle MCoA content or whole body insulin-stimulated glucose uptake. However, at the muscular level proximal insulin signaling and insulin-stimulated glucose uptake appeared to be compromised, although to a minor extent, by the Fat diet. Collectively, this study indicates that reduced muscle MCoA content in recovery from exercise may be part of the adaptive response leading to improved insulin action on glucose uptake after exercise in human muscle.

Distinct actions of Akt1 and Akt2 in skeletal muscle differentiation

Differentiation, maturation, and repair of skeletal muscle requires ongoing cooperation between signaling cascades activated by hormones and growth factors, and intrinsic regulatory programs controlled by myogenic transcription factors. The insulin-like growth factor--phosphatidylinositol-3 kinase--Akt pathway has been implicated in muscle growth and regeneration after injury, in counteracting sarcopenia during aging, and in maintaining muscle cell viability. Here we present evidence for distinct roles for Akt1 and Akt2 in different phases of muscle differentiation. Targeted knockdown of either Akt had no effect on C2 myoblast proliferation, even though Akt1 concentrations are markedly higher than Akt2 levels under growth-promoting conditions. Akt2 concentrations rose by nearly an order of magnitude during muscle differentiation, while Akt1 levels remained constant, yet loss of either protein did not increase myoblast death. Rather, knockdown or genetic knockout of Akt1 blocked differentiation at its earliest stages, preventing induction of muscle-specific proteins and inhibiting formation of multinucleated myofibers, while myoblasts lacking Akt2 differentiated normally, although resultant myofibers were thinner and incorporated fewer nuclei than controls. Forced expression of knockdown-resistant Akt1 partially reversed the deficit in differentiation seen in myoblasts lacking Akt1. Our results define isoform-specific Akt actions in muscle cells, and demonstrate that both Akts are necessary for full myoblast differentiation and maturation.

Glucocorticoid-mediated effects on metabolism are reversed by targeting 11 beta hydroxysteroid dehydrogenase type 1 in human skeletal muscle

BACKGROUND

Adipose tissue and liver play important roles in mediating the metabolic actions of glucocorticoids. However, the effects of glucocorticoids on glucose and lipid metabolism in skeletal muscle are not understood completely. Intracellular glucocorticoid action is dependent on 11 beta-hydroxysteroid dehydrogenase 1 (HSD1), an enzyme that converts cortisone to active cortisol.

METHODS

We investigated the direct role of HSD1 in cultured primary human skeletal muscle cells using siRNA and pharmacological inhibitors of the enzyme. Primary human skeletal muscle cells were cultured in the presence of 0.5 microM cortisone or 0.5 microM cortisol for eight days. siRNA was utilized to reduce expression of either HSD1 or pyruvate dehydrogenase kinase (PDK) 4. Effects of pharmacological inhibitors of HSD1 were also studied.

RESULTS

Exposure to cortisone or cortisol decreased basal glucose uptake and glucose incorporation into glycogen, but was without effect on the insulin-stimulated response. Glucocorticoid exposure increased palmitate oxidation, as well as the expression of PDK4. siRNA-mediated reduction or pharmacological inhibition of HSD1 prevented the effects of cortisone, but not cortisol, on metabolic responses. siRNA-mediated reduction of PDK4 prevented the effect of cortisol to attenuate glycogen synthesis.

CONCLUSION

Targeted reduction or pharmacological inhibition of HSD1 in primary human skeletal muscle cells prevents the effects of cortisone, but not cortisol, on glucose metabolism and palmitate oxidation. Furthermore, the glucocorticoid-mediated reductions in glucose metabolism are dependent on PDK4.

NDRG2, a novel regulator of myoblast proliferation, is regulated by anabolic and catabolic factors

Skeletal muscle tissue undergoes adaptive changes in response to stress and the genes that control these processes are incompletely characterised. NDRG2 (N-myc downstream-regulated gene 2), a stress- and growth-related gene, was investigated in skeletal muscle growth and adaption. While NDRG2 expression levels were found to be up-regulated in both differentiated human and mouse myotubes compared with undifferentiated myoblasts, the suppression of NDRG2 in C2C12 myoblasts resulted in slowed myoblast proliferation. The increased expression levels of the cell cycle inhibitors, p21 Waf1/Cip1 and p27 Kip1, and of various muscle differentiation markers in NDRG2-deficient myoblasts indicate that a lack of NDRG2 promoted cell cycle exiting and the onset of myogenesis. Furthermore, the analysis of NDRG2 regulation in C2C12 myotubes treated with catabolic and anabolic agents and in skeletal muscle from human subjects following resistance exercise training revealed NDRG2 gene expression to be down-regulated during hypertrophic conditions, and conversely, up-regulated during muscle atrophy. Together, these data demonstrate that NDRG2 expression is highly responsive to different stress conditions in skeletal muscle and suggest that the level of NDRG2 expression may be critical to myoblast growth and differentiation.

Novel D-xylose derivatives stimulate muscle glucose uptake by activating AMP-activated protein kinase alpha

Type 2 diabetes mellitus has reached epidemic proportions; therefore, the search for novel antihyperglycemic drugs is intense. We have discovered that D-xylose increases the rate of glucose transport in a non-insulin-dependent manner in rat and human myotubes in vitro. Due to the unfavorable pharmacokinetic properties of D-xylose we aimed at synthesizing active derivatives with improved parameters. Quantitative structure-activity relationship analysis identified critical hydroxyl groups in D-xylose. These data were used to synthesize various hydrophobic derivatives of D-xylose of which compound 19 the was most potent compound in stimulating the rate of hexose transport by increasing the abundance of glucose transporter-4 in the plasma membrane of myotubes. This effect resulted from the activation of AMP-activated protein kinase without recruiting the insulin transduction mechanism. These results show that lipophilic D-xylose derivatives may serve as prototype molecules for the development of novel antihyperglycemic drugs for the treatment of diabetes.

Altered PI3-kinase/Akt signalling in skeletal muscle of young men with low birth weight

BACKGROUND

Low birth weight (LBW) is associated with increased future risk of insulin resistance and type 2 diabetes mellitus. The underlying molecular mechanisms remain poorly understood. We have previously shown that young LBW men have reduced skeletal muscle expression of PI3K p85alpha regulatory subunit and p110beta catalytic subunit, PKCzeta and GLUT4 in the fasting state. The aim of this study was to determine whether insulin activation of the PI3K/Akt and MAPK signalling pathways is altered in skeletal muscle of young adult men with LBW.

METHODS

Vastus lateralis muscle biopsies were obtained from 20 healthy 19-yr old men with BW< or = 10th percentile for gestational age (LBW) and 20 normal birth weight controls (NBW), matched for physical fitness and whole-body glucose disposal, prior to (fasting state) and following a 4-hr hyperinsulinemic euglycemic clamp (insulin stimulated state). Expression and phosphorylation of selected proteins was determined by Western blotting.

PRINCIPAL FINDINGS

Insulin stimulated expression of aPKCzeta (p<0.001) and Akt1 (p<0.001) was decreased in muscle of LBW men when compared to insulin stimulated controls. LBW was associated with increased insulin stimulated levels of IRS1 (p<0.05), PI3K p85alpha (p<0.001) and p110beta (p<0.05) subunits, while there was no significant change in these proteins in insulin stimulated control muscle. In addition LBW had reduced insulin stimulated phospho-Akt (Ser 473) (p<0.01), indicative of reduced Akt signalling. Insulin stimulated expression/phosphorylation of all the MAPK proteins studied [p38 MAPK, phospho-p38 MAPK (Thr180/Tyr182), phospho-ERK (Thr 202/Tyr204), JNK1, JNK2 and phospho-JNK (Thr 183/Tyr185)] was not different between groups.

CONCLUSIONS

We conclude that altered insulin activation of the PI3K/Akt but not the MAPK pathway precedes and may contribute to development of whole-body insulin resistance and type 2 diabetes in men with LBW.

Globular adiponectin stimulates glucose transport in type 2 diabetic muscle

BACKGROUND

Adiponectin acts as an insulin sensitizer in rodent models. The direct effect of adiponectin in intact type 2 diabetic muscle is unknown. We examined whether adiponectin stimulates glucose transport in isolated skeletal muscle strips from type 2 diabetic men.

METHODS

We obtained open muscle biopsies from 12 men with type 2 diabetes (56 +/- 1 years, 30.5 +/- 1.1 kg/m(2)), and from 15 non-diabetic men (59 +/- 1 years, 28.0 +/- 1.0 kg/m(2)). Skeletal muscle strips were isolated and exposed to globular adiponectin (2.5 microg/mL), insulin (120 nM) and/or AICAR (1 mM) in vitro for 1 h. Glucose transport was analysed by accumulation of intracellular 3-O-methyl [(3)H] glucose, phosphorylation of Akt-Ser(473) and Akt-Thr(308) was determined using phosphospecific antibodies, and adiponectin receptor 1 and 2 content was measured using specific antibodies.

RESULTS

Globular adiponectin increased glucose transport rate by 1.3-fold (P < 0.01) in type 2 diabetic, but not in non-diabetic muscle. Insulin-stimulated glucose transport rate was unaltered by exposure to globular adiponectin in either group. AICAR increased glucose transport and enhanced insulin-stimulated glucose transport in type 2 diabetic and non-diabetic muscles. Insulin-stimulated phosphorylation of Akt-Ser(473) or Akt-Thr(308) was comparable in type 2 diabetic and non-diabetic muscles, and unaltered by the addition of globular adiponectin in either group. Adiponectin receptor expression was similar in skeletal muscle from type 2 diabetic and non-diabetic men.

CONCLUSIONS

Globular adiponectin directly increases glucose transport in skeletal muscle from type 2 diabetic patients. This may occur via Akt-independent signalling routes.

The protein phosphatase PP2A-B' subunit Widerborst is a negative regulator of cytoplasmic activated Akt and lipid metabolism in Drosophila

Inappropriate regulation of the PI3-kinase/PTEN/Akt kinase-signalling cassette, a key downstream target of insulin/insulin-like growth factor signalling (IIS), is associated with several major human diseases such as diabetes, obesity and cancer. In Drosophila, studies have recently revealed that different subcellular pools of activated, phosphorylated Akt can modulate different IIS-dependent processes. For example, a specific pool of activated Akt within the cytoplasm alters aspects of lipid metabolism, a process that is misregulated in both obesity and diabetes. However, it remains unclear how this pool is regulated. Here we show that the protein phosphatase PP2A-B' regulatory subunit Widerborst (Wdb), which coimmunoprecipitates with Akt in vivo, selectively modulates levels of activated Akt in the cytoplasm. It alters lipid droplet size and expression of the lipid storage perilipin-like protein LSD2 in the Drosophila ovary, but not in epithelial cells of the eye imaginal discs. We conclude that isoforms of PP2A-B' can act as subcellular-compartment-specific regulators of PI3-kinase/PTEN/Akt kinase signalling and IIS, potentially providing new targets for modulating individual subcellular pools of activated Akt in insulin-linked disease.

siRNA-mediated reduction of inhibitor of nuclear factor-kappaB kinase prevents tumor necrosis factor-alpha-induced insulin resistance in human skeletal muscle

OBJECTIVE

Proinflammatory cytokines contribute to systemic low-grade inflammation and insulin resistance. Tumor necrosis factor (TNF)-alpha impedes insulin signaling in insulin target tissues. We determined the role of inhibitor of nuclear factor-kappaB kinase (IKK)beta in TNF-alpha-induced impairments in insulin signaling and glucose metabolism in skeletal muscle.

RESEARCH DESIGN AND METHODS

Small interfering RNA (siRNA) was used to silence IKKbeta gene expression in primary human skeletal muscle myotubes from nondiabetic subjects. siRNA gene silencing reduced IKKbeta protein expression 73% (P < 0.05). Myotubes were incubated in the absence or presence of insulin and/or TNF-alpha, and effects of IKKbeta silencing on insulin signaling and glucose metabolism were determined.

RESULTS

Insulin increased glucose uptake 1.7-fold (P < 0.05) and glucose incorporation into glycogen 3.8-fold (P < 0.05) in myotubes from nondiabetic subjects. TNF-alpha exposure fully impaired insulin-mediated glucose uptake and metabolism. IKKbeta siRNA protected against TNF-alpha-induced impairments in glucose metabolism, since insulin-induced increases in glucose uptake (1.5-fold; P < 0.05) and glycogen synthesis (3.5-fold; P < 0.05) were restored. Conversely, TNF-alpha-induced increases in insulin receptor substrate-1 serine phosphorylation (Ser(312)), Jun NH(2)-terminal kinase phosphorylation, and extracellular signal-related kinase-1/2 mitogen-activated protein kinase (MAPK) phosphorylation were unaltered by siRNA-mediated IKKbeta reduction. siRNA-mediated IKKbeta reduction prevented TNF-alpha-induced insulin resistance on Akt Ser(473) and Thr(308) phosphorylation and phosphorylation of the 160-kDa Akt substrate AS160. IKKbeta silencing had no effect on cell differentiation. Finally, mRNA expression of GLUT1 or GLUT4 and protein expression of MAPK kinase kinase kinase isoform 4 (MAP4K4) was unaltered by IKKbeta siRNA.

CONCLUSIONS

IKKbeta silencing prevents TNF-alpha-induced impairments in insulin action on Akt phosphorylation and glucose uptake and metabolism in human skeletal muscle.

Insulin action on glucose transporters through molecular switches, tracks and tethers

Glucose entry into muscle cells is precisely regulated by insulin, through recruitment of GLUT4 (glucose transporter-4) to the membrane of muscle and fat cells. Work done over more than two decades has contributed to mapping the insulin signalling and GLUT4 vesicle trafficking events underpinning this response. In spite of this intensive scientific research, there are outstanding questions that continue to challenge us today. The present review summarizes the knowledge in the field, with emphasis on the latest breakthroughs in insulin signalling at the level of AS160 (Akt substrate of 160 kDa), TBC1D1 (tre-2/USP6, BUB2, cdc16 domain family member 1) and their target Rab proteins; in vesicle trafficking at the level of vesicle mobilization, tethering, docking and fusion with the membrane; and in the participation of the cytoskeleton to achieve optimal temporal and spatial location of insulin-derived signals and GLUT4 vesicles.

Malonyl CoenzymeA decarboxylase regulates lipid and glucose metabolism in human skeletal muscle

OBJECTIVE

Malonyl coenzyme A (CoA) decarboxylase (MCD) is a key enzyme responsible for malonyl-CoA turnover and functions in the control of the balance between lipid and glucose metabolism. We utilized RNA interference (siRNA)-based gene silencing to determine the direct role of MCD on metabolic responses in primary human skeletal muscle.

RESEARCH DESIGN AND METHODS

We used siRNA to silence MCD gene expression in cultured human myotubes from healthy volunteers (seven male and seven female) with no known metabolic disorders. Thereafter, we determined lipid and glucose metabolism and signal transduction under basal and insulin-stimulated conditions.

RESULTS

RNA interference-based silencing of MCD expression (75% reduction) increased malonyl-CoA levels twofold and shifted substrate utilization from lipid to glucose oxidation. RNA interference-based depletion of MCD reduced basal palmitate oxidation. In parallel with this reduction, palmitate uptake was decreased under basal (40%) and insulin-stimulated (49%) conditions compared with myotubes transfected with a scrambled sequence. MCD silencing increased basal and insulin-mediated glucose oxidation 1.4- and 2.6-fold, respectively, compared with myotubes transfected with a scrambled sequence. In addition, glucose transport and cell-surface GLUT4 content was increased. In contrast, insulin action on IRS-1 tyrosine phosphorylation, tyrosine-associated phosphatidylinositol (PI) 3-kinase activity, Akt, and glycogen synthase kinase (GSK) phosphorylation was unaltered between myotubes transfected with siRNA against MCD versus a scrambled sequence.

CONCLUSIONS

These results provide evidence that MCD silencing suppresses lipid uptake and enhances glucose uptake in primary human myotubes. In conclusion, MCD expression plays a key reciprocal role in the balance between lipid and glucose metabolism.

Satellite cell proliferation is reduced in muscles of obese Zucker rats but restored with loading

The obese Zucker rat (OZR) is a model of metabolic syndrome, which has lower skeletal muscle size than the lean Zucker rat (LZR). Because satellite cells are essential for postnatal muscle growth, this study was designed to determine whether reduced satellite cell proliferation contributes to reduced skeletal mass in OZR vs. LZR. Satellite cell proliferation was determined by a constant-release 5-bromo-2-deoxyuridine (BrdU) pellet that was placed subcutaneously in each animal. Satellite cell proliferation, as determined by BrdU incorporation, was significantly attenuated in control soleus and plantaris muscles of the OZR compared with that shown in the LZR. To determine whether this attenuation of satellite cell activity could be rescued in OZR muscles, soleus and gastrocnemius muscles were denervated, placing a compensatory load on the plantaris muscle. In the LZR and the OZR after 21 days of loading, increases of approximately 25% and approximately 30%, respectively, were shown in plantaris muscle wet weight compared with that shown in the contralateral control muscle. The number of BrdU-positive nuclei increased similarly in loaded plantaris muscles from LZR and OZR. Myogenin, MyoD, and Akt protein expressions were lower in control muscles of OZR than in those of the LZR, but they were all elevated to similar levels in the loaded plantaris muscles of OZR and LZR. These data indicate that metabolic syndrome may reduce satellite cell proliferation, and this may be a factor that contributes to the reduced mass in control muscles of OZR; however, satellite cell proliferation can be restored with compensatory loading in OZR.

High rates of muscle glycogen resynthesis after exhaustive exercise when carbohydrate is coingested with caffeine

We determined the effect of coingestion of caffeine (Caff) with carbohydrate (CHO) on rates of muscle glycogen resynthesis during recovery from exhaustive exercise in seven trained subjects who completed two experimental trials in a randomized, double-blind crossover design. The evening before an experiment subjects performed intermittent exhaustive cycling and then consumed a low-CHO meal. The next morning subjects rode until volitional fatigue. On completion of this ride subjects consumed either CHO [4 g/kg body mass (BM)] or the same amount of CHO + Caff (8 mg/kg BM) during 4 h of passive recovery. Muscle biopsies and blood samples were taken at regular intervals throughout recovery. Muscle glycogen levels were similar at exhaustion [ approximately 75 mmol/kg dry wt (dw)] and increased by a similar amount ( approximately 80%) after 1 h of recovery (133 +/- 37.8 vs. 149 +/- 48 mmol/kg dw for CHO and Caff, respectively). After 4 h of recovery Caff resulted in higher glycogen accumulation (313 +/- 69 vs. 234 +/- 50 mmol/kg dw, P < 0.001). Accordingly, the overall rate of resynthesis for the 4-h recovery period was 66% higher in Caff compared with CHO (57.7 +/- 18.5 vs. 38.0 +/- 7.7 mmol x kg dw(-1) x h(-1), P < 0.05). After 1 h of recovery plasma Caff levels had increased to 31 +/- 11 microM (P < 0.001) and at the end of the recovery reached 77 +/- 11 microM (P < 0.001) with Caff. Phosphorylation of CaMK(Thr286) was similar after exercise and after 1 h of recovery, but after 4 h CaMK(Thr286) phosphorylation was higher in Caff than CHO (P < 0.05). Phosphorylation of AMP-activated protein kinase (AMPK)(Thr172) and Akt(Ser473) was similar for both treatments at all time points. We provide the first evidence that in trained subjects coingestion of large amounts of Caff (8 mg/kg BM) with CHO has an additive effect on rates of postexercise muscle glycogen accumulation compared with consumption of CHO alone.

Rab GTPase-activating protein AS160 is a major downstream effector of protein kinase B/Akt signaling in pancreatic beta-cells

OBJECTIVE

Protein kinase B/Akt plays a central role in beta-cells, but little is known regarding downstream Akt substrates in these cells. Recently, Rab GTPase-activating protein AS160, a substrate of Akt, was shown to be involved in insulin modulation of GLUT4 trafficking in skeletal muscle and adipose tissue. The aim of this study was to investigate the expression and potential role of AS160 in beta-cells.

RESEARCH DESIGN AND METHODS

AS160 mRNA expression was measured in mouse and human islets and fluorescence-activated cell sorted beta-cells and compared in islets from control subjects versus individuals with type 2 diabetes. For knockdown experiments, transformed mouse insulin-secreting MIN6B1 cells were transfected with pSUPER-GFP plasmid encoding a small hairpin RNA against insulin receptor substrate (IRS)-2, AS160, or a negative control. Primary mouse islet cells were transfected with AS160 small interfering RNA.

RESULTS

AS160 was expressed in human and mouse pancreatic beta-cells and phosphorylated after glucose stimulation. AS160 mRNA expression was downregulated in pancreatic islets from individuals with type 2 diabetes. In MIN6B1 cells, glucose induced phosphorylation of Akt and AS160, and this was mediated by insulin receptor/IRS-2/phosphatidylinositol 3-kinase independently of changes in cytosolic Ca(2+). Knockdown of AS160 resulted in increased basal insulin secretion, whereas glucose-stimulated insulin release was abolished. Furthermore, beta-cells with decreased AS160 showed increased apoptosis and loss of glucose-induced proliferation.

CONCLUSIONS

This study shows for the first time that AS160, previously recognized as a key player in insulin signaling in skeletal muscle and adipose tissue, is also a major effector of protein kinase B/Akt signaling in the beta-cell.

Isoform-specific defects of insulin stimulation of Akt/protein kinase B (PKB) in skeletal muscle cells from type 2 diabetic patients

AIMS/HYPOTHESIS

The serine/threonine kinase Akt/protein kinase B (PKB) is required for the metabolic actions of insulin. Controversial data have been reported regarding Akt defective activation in the muscle of type 2 diabetic patients. Because three Akt isoforms exist, each having a distinct physiological role, we investigated the contribution of isoform-specific defects to insulin signalling in human muscle.

METHODS

The phosphorylation pattern and kinase activity of each Akt isoform were compared in primary myotubes from healthy control participants and type 2 diabetic patients. Phosphorylation of Ser(473) and of Thr(308) in each isoform was determined after immunoprecipitation in myotubes treated or not with insulin.

RESULTS

Muscle cells from diabetic patients displayed defective insulin action and a drastic reduction of insulin-stimulated activity of all Akt isoforms. This was associated with specific defects of their phosphorylation pattern in response to insulin, with impaired Akt2- (and to a lower extent Akt3-) Ser(473) phosphorylation, and with altered Akt1-Thr(308) phosphorylation. These defects were not due to faulty phosphoinositide-dependent protein kinase 1 (PDK1) production or activation. Rather, we found higher levels of the Akt2-Ser(473)-specific protein phosphatase PH domain leucine-rich repeat protein phosphatase 1 (PHLPP1) in muscle from diabetic patients, which may contribute to the alteration of Akt2-Ser(473) phosphorylation.

CONCLUSIONS/INTERPRETATION

These results suggest that several mechanisms affecting Akt isoforms, including deregulated production of PHLPP1, could underlie the alterations of skeletal muscle insulin signalling in type 2 diabetes. Taking into account the recently described isoform-specific metabolic functions of Akt, our results provide mechanistic insight that may contribute to the defective regulation of glucose and lipid metabolisms in the muscle of diabetic patients.

Phosphorylation of Ser357 of rat insulin receptor substrate-1 mediates adverse effects of protein kinase C-delta on insulin action in skeletal muscle cells

The activation of the protein kinase C (PKC) family of serine/threonine kinases contributes to the modulation of insulin signaling, and the PKC-dependent phosphorylation of insulin receptor substrate (IRS)-1 has been implicated in the development of insulin resistance. Here we demonstrate Ser(357) of rat IRS-1 as a novel PKC-delta-dependent phosphorylation site in skeletal muscle cells upon stimulation with insulin and phorbol ester using Ser(P)(357) antibodies and active and kinase dead mutants of PKC-delta. Phosphorylation of this site was simulated using IRS-1 Glu(357) and shown to reduce insulin-induced tyrosine phosphorylation of IRS-1, to decrease activation of Akt, and to subsequently diminish phosphorylation of glycogen synthase kinase-3. When the phosphorylation was prevented by mutation of Ser(357) to alanine, these effects of insulin were enhanced. When the adjacent Ser(358), present in mouse and rat IRS-1, was mutated to alanine, which is homologous to the human sequence, the insulin-induced phosphorylation of glycogen synthase kinase-3 or tyrosine phosphorylation of IRS-1 was not increased. Moreover, both active PKC-delta and phosphorylation of Ser(357) were shown to be necessary for the attenuation of insulin-stimulated Akt phosphorylation. The phosphorylation of Ser(357) could lead to increased association of PKC-delta to IRS-1 upon insulin stimulation, which was demonstrated with IRS-1 Glu(357). Together, these data suggest that phosphorylation of Ser(357) mediates at least in part the adverse effects of PKC-delta activation on insulin action.