Adipose tissue insulin sensitivity and macrophage recruitment: Does PI3K pick the pathway?

The Complement Component 4 Binding Protein α Gene: A Versatile Immune Gene That Influences Lipid Metabolism in Bovine Mammary Epithelial Cell Lines

Complement component 4 binding protein α (C4BPA) is an immune gene which is responsible for the complement regulation function of C4BP by binding and inactivating the Complement component C4b (C4b) component of the classical Complement 3 (C3) invertase pathway. Our previous findings revealed that C4BPA was differentially expressed by comparing the transcriptome in high-fat and low-fat bovine mammary epithelial cell lines (BMECs) from Chinese Holstein dairy cows. In this study, a C4BPA gene knockout BMECs line model was constructed via using a CRISPR/Cas9 system to investigate the function of C4BPA in lipid metabolism. The results showed that levels of triglyceride (TG) were increased, while levels of cholesterol (CHOL) and free fatty acid (FFA) were decreased (p < 0.05) after knocking out C4BPA in BMECs. Additionally, most kinds of fatty acids were found to be mainly enriched in the pathway of the biosynthesis of unsaturated fatty acids, linoleic acid metabolism, fatty acid biosynthesis, and regulation of lipolysis in adipocyte. Meanwhile, the RNA-seq showed that most of the differentially expressed genes (DEGs) are related to PI3K-Akt signaling pathway. The expressions of 3-Hydroxy-3-Methylglutaryl-CoA Synthase 1 (HMGCS1), Carnitine Palmitoyltransferase 1A (CPT1A), Fatty Acid Desaturase 1 (FADS1), and Stearoyl-Coenzyme A desaturase 1 (SCD1) significantly changed when the C4BPA gene was knocked out. Collectively, C4BPA gene, which is an immune gene, played an important role in lipid metabolism in BMECs. These findings provide a new avenue for animal breeders: this gene, with multiple functions, should be reasonably utilized.

Targeting PI3K/AKT signaling pathway in obesity

Obesity is a disorder with an increasing prevalence, which impairs the life quality of patients and intensifies societal health care costs. The development of safe and innovative prevention strategies and therapeutic approaches is thus of great importance. The complex pathophysiology of obesity involves multiple signaling pathways that influence energy metabolism in different tissues. The phosphatidylinositol 3-kinases (PI3K)/protein kinase B (AKT) pathway is critical for the metabolic homeostasis and its function in insulin-sensitive tissues is described in the context of health, obesity and obesity-related complications. The PI3K family participates in the regulation of diverse physiological processes including but not limited to cell growth, survival, differentiation, autophagy, chemotaxis, and metabolism depending on the cellular context. AKT is downstream of PI3K in the insulin signaling pathway, and promotes multiple cellular processes by targeting a plethora of regulatory proteins that control glucose and lipid metabolism. Natural products are essential for prevention and treatment of many human diseases, including obesity. Anti-obesity natural compounds effect multiple pathophysiological mechanisms involved in obesity development. Numerous recent preclinical studies reveal the advances in using plant secondary metabolites to target the PI3K/AKT signaling pathway for obesity management. In this paper the druggability of PI3K as a target for compounds with anti-obesity potential is evaluated. Perspectives on the strategies and limitations for clinical implementation of obesity management using natural compounds modulating the PI3K/AKT pathway are suggested.

Maternal and Placental DNA Methylation Changes Associated with the Pathogenesis of Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) is an important metabolic complication of pregnancy, which affects the future health of both the mother and the newborn. The pathogenesis of GDM is not completely clear, but what is clear is that with the development and growth of the placenta, GDM onset and blood glucose is difficult to control, while gestational diabetes patients' blood glucose drops and reaches normal after placenta delivery. This may be associated with placental secretion of insulin-like growth factor, adipokines, tumor necrosis factor-α, cytokines and insulin resistance. Therefore, endocrine secretion of placenta plays a key role in the pathogenesis of GDM. The influence of DNA methylation of these molecules and pathway-related genes on gene expression is also closely related to the pathogenesis of GDM. Here, this review attempts to clarify the pathogenesis of GDM and the related maternal and placental DNA methylation changes and how they affect metabolic pathways.

The PI3K/Akt Pathway in Meta-Inflammation

Obesity is a global epidemic representing a serious public health burden as it is a major risk factor for the development of cardiovascular disease, stroke and all-cause mortality. Chronic low-grade systemic inflammation, also known as meta-inflammation, is thought to underly obesity's negative health consequences, which include insulin resistance and the development of type 2 diabetes. Meta-inflammation is characterized by the accumulation of immune cells in adipose tissue, a deregulation in the synthesis and release of adipokines and a pronounced increase in the production of proinflammatory factors. In this state, the infiltration of macrophages and their metabolic activation contributes to complex paracrine and autocrine signaling, which sustains a proinflammatory microenvironment. A key signaling pathway mediating the response of macrophages and adipocytes to a microenvironment of excessive nutrients is the phosphoinositide 3-kinase (PI3K)/Akt pathway. This multifaceted network not only transduces metabolic information but also regulates macrophages' intracellular changes, which are responsible for their phenotypic switch towards a more proinflammatory state. In the present review, we discuss how the crosstalk between macrophages and adipocytes contributes to meta-inflammation and provide an overview on the involvement of the PI3K/Akt signaling pathway, and how its impairment contributes to the development of insulin resistance.

KRAS Affects Adipogenic Differentiation by Regulating Autophagy and MAPK Activation in 3T3-L1 and C2C12 Cells

Kirsten rat sarcoma 2 viral oncogene homolog (Kras) is a proto-oncogene that encodes the small GTPase transductor protein KRAS, which has previously been found to promote cytokine secretion, cell survival, and chemotaxis. However, its effects on preadipocyte differentiation and lipid accumulation are unclear. In this study, the effects of KRAS inhibition on proliferation, autophagy, and adipogenic differentiation as well as its potential mechanisms were analyzed in the 3T3-L1 and C2C12 cell lines. The results showed that KRAS was localized mainly in the nuclei of 3T3-L1 and C2C12 cells. Inhibition of KRAS altered mammalian target of rapamycin (Mtor), proliferating cell nuclear antigen (Pcna), Myc, peroxisome proliferator-activated receptor γ (PPARγ), CCAAT/enhancer binding protein beta (C/ebp-β), diacylglycerol O-acyltransferase 1 (Dgat1), and stearoyl-coenzyme A desaturase 1 (Scd1) expression, thereby reducing cell proliferation capacity while inducing autophagy, enhancing differentiation of 3T3-L1 and C2C12 cells into mature adipocytes, and increasing adipogenesis and the capacity to store lipids. Moreover, during differentiation, KRAS inhibition reduced the levels of extracellular regulated protein kinases (ERK), c-Jun N-terminal kinase (JNK), p38, and phosphatidylinositol 3 kinase (PI3K) activation. These results show that KRAS has unique regulatory effects on cell proliferation, autophagy, adipogenic differentiation, and lipid accumulation.

Intermittent fasting benefits on alpha- and beta-cell arrangement in diet-induced obese mice pancreatic islet

AIMS

There is a pancreatic islet adaptation in obese subjects, resulting in insulin resistance and diabetes type 2. We studied the effect of intermittent fasting (IntF) on the islet structure of diet-induced obese (DIO) mice.

METHODS

Three-month-old male mice fed a control diet (C, 10% Kcal fat) or a high-fat diet (HF, 50% Kcal fat) for two months (n = 20 each group). Then, half of each group did IntF (alternating 24 h fed/24 h fast), continuing in their diets four more weeks: C, C-IntF, HF, HF-IntF. Islets were prepared to microscopy or isolated for molecular analysis.

RESULTS

HF group (vs. C group) showed hyperglycemia, hyperinsulinemia, hyperleptinemia, hypoadiponectinemia, glucose intolerance, insulin resistance, and islet hypertrophy with a consequent higher both the alpha-cell and beta-cell masses. In the HF group (vs. C), there was low PDX1 (pancreatic and duodenal homeobox 1), and IntF did not alter PDX1. There was a low p-AKT/AKT ratio (protein kinase B), and IntF enhanced it. Also, tumor suppressor p53 was increased, and IntF decreased it. IL (interleukin) -6 was higher in the HF group (vs. C), and HF-IntF (vs. C-IntF). Any significant change in NFkB was seen among groups.

CONCLUSIONS

IntF improves pancreatic islet structure in DIO mice, even with continued HF diet intake, primarily considering on the alpha- and beta-cell masses regulation, then improving insulin signaling and decreasing cell apoptosis. Future research should explore whether the shortening of the IntF extend could maintain the benefits observed in the long term.

Visceral adipose tissue alteration of PI3KR1 expression is associated with gestational diabetes but not promoter DNA methylation

Obesity and diabetes are at an epidemic rate, as well as growing incidences of gestational diabetes mellitus (GDM) which causes pregnancy risks, and harm in both maternal and child health. It remains unclear which molecular mechanisms are driving the functional differences between visceral and subcutaneous fat and how these types directly affect an individual’s health outcome. Paired abdominal subcutaneous and omental visceral adipose tissue were collected from women with GDM (n = 20) and with normal glucose tolerance (NGT, n = 22) during planned caesarian section. Both groups had similar maternal age (average 32.5 years) and BMI at delivery (average 33.3 kg/m²). Adipose tissue mRNA expression analyses of insulin signalling genes: PI3KCA, PI3KR1, IRS1 and IRS2 showed significantly decreased PI3KR1 expression (−23%) in visceral fat in GDM with no association to promoter DNA methylation. Reduced visceral fat PI3KR1 expression appears to be a pathogenic factor in GDM but not through altered promoter methylation.

mTORC2/Akt activation in adipocytes is required for adipose tissue inflammation in tuberculosis

Background

Mycobacterium tuberculosis has co-evolved with the human host, adapting to exploit the immune system for persistence and transmission. While immunity to tuberculosis (TB) has been intensively studied in the lung and lymphoid system, little is known about the participation of adipose tissues and non-immune cells in the host-pathogen interaction during this systemic disease.

Methods

C57BL/6J mice were aerosol infected with M. tuberculosis Erdman and presence of the bacteria and the fitness of the white and brown adipose tissues, liver and skeletal muscle were studied compared to uninfected mice.

Findings

M. tuberculosis infection in mice stimulated immune cell infiltration in visceral, and brown adipose tissue. Despite the absence of detectable bacterial dissemination to fat tissues, adipocytes produced localized pro-inflammatory signals that disrupted adipocyte lipid metabolism, resulting in adipocyte hypertrophy. Paradoxically, this resulted in increased insulin sensitivity and systemic glucose tolerance. Adipose tissue inflammation and enhanced glucose tolerance also developed in obese mice after aerosol M. tuberculosis infection. We found that infection induced adipose tissue Akt signaling, while inhibition of the Akt activator mTORC2 in adipocytes reversed TB-associated adipose tissue inflammation and cell hypertrophy.

Interpretation

Our study reveals a systemic response to aerosol M. tuberculosis infection that regulates adipose tissue lipid homeostasis through mTORC2/Akt signaling in adipocytes. Adipose tissue inflammation in TB is not simply a passive infiltration with leukocytes but requires the mechanistic participation of adipocyte signals.

Cellular signaling pathways regulating β-cell proliferation as a promising therapeutic target in the treatment of diabetes

It is established that a decrease in β-cell number and deficiency in the function of existing β-cells contribute to type 1 and type 2 diabetes mellitus. Therefore, a major focus of current research is to identify novel methods of improving the number and function of β-cells, so as to prevent and/or postpone the development of diabetes mellitus and potentially reverse diabetes mellitus. Based on prior knowledge of the above-mentioned causes, promising therapeutic approaches may include direct transplantation of islets, implantation and subsequent induced differentiation of progenitors/stem cells to β-cells, replication of pre-existing β-cells, or activation of endogenous β-cell progenitors. More recently, with regards to cell replacement and regenerative treatment for diabetes patients, the identification of cellular signaling pathways with related genes or corresponding proteins involved in diabetes has become a topic of interest. However, the majority of pathways and molecules associated with β-cells remain unresolved, and the specialized functions of known pathways remain unclear, particularly in humans. The current article has evaluated the progress of research on pivotal cellular signaling pathways involved with β-cell proliferation and survival, and their validity for therapeutic adult β-cell regeneration in diabetes. More efforts are required to elucidate the cellular events involved in human β-cell proliferation in terms of the underlying mechanisms and functions.

Dose-response effects of exercise on insulin among colon cancer survivors

Physical activity is associated with a lower risk of disease recurrence among colon cancer survivors. The pathways through which physical activity may alter disease outcomes are unknown, but may include changes in metabolic growth factors, such as insulin. Between January 2015 and August 2015, 39 stage I-III colon cancer survivors were randomized to one of the three groups: usual care control, 150 min/week of aerobic exercise (low-dose) and 300 min/week of aerobic exercise (high-dose) for six months. The pre-specified key metabolic growth factor outcome was fasting insulin. Insulin resistance was quantified using the homeostatic model assessment. Mean age was 56.5 ± 10.0 years, 51% had stage III disease, 72% were treated with chemotherapy and the mean time since finishing treatment was 10.9 ± 6.1 months. Over six months, the low-dose group completed 141.5 ± 9.9 min/week of aerobic exercise, and the high-dose group completed 247.2 ± 10.7 min/week of aerobic exercise. Fasting insulin concentrations decreased 7.4 ± 9.4 pmol/L in the control group, 28.0 ± 8.3 pmol/L in the low-dose group and 20.7 ± 9.3 pmol/L in the high-dose group (nonlinear Ptrend = 0.042). Insulin resistance decreased 0.11 ± 0.20 in the control group, 0.63 ± 0.17 in the low-dose group and 0.43 ± 0.19 in the high-dose group (nonlinear Ptrend = 0.012). Aerobic exercise reduces insulin concentrations and insulin resistance among patients with stage I-III colon cancer. Prescribing 150 min/week of aerobic exercise may be sufficient for reducing insulin concentrations and insulin resistance, which may partially mediate the relationship between physical activity and colon cancer prognosis.

Role of Perivascular Adipose Tissue in Health and Disease

Perivascular adipose tissue (PVAT) is cushion of fat tissue surrounding blood vessels, which is phenotypically different from other adipose tissue depots. PVAT is composed of adipocytes and stromal vascular fraction, constituted by different populations of immune cells, endothelial cells, and adipose-derived stromal cells. It expresses and releases an important number of vasoactive factors with paracrine effects on vascular structure and function. In healthy individuals, these factors elicit a net anticontractile and anti-inflammatory paracrine effect aimed at meeting hemodynamic and metabolic demands of specific organs and regions of the body. Pathophysiological situations, such as obesity, diabetes or hypertension, induce changes in its amount and in the expression pattern of vasoactive factors leading to a PVAT dysfunction in which the beneficial paracrine influence of PVAT is shifted to a pro-oxidant, proinflammatory, contractile, and trophic environment leading to functional and structural cardiovascular alterations and cardiovascular disease. Many different PVATs surrounding a variety of blood vessels have been described and exhibit regional differences. Both protective and deleterious influence of PVAT differs regionally depending on the specific vascular bed contributing to variations in the susceptibility of arteries and veins to vascular disease. PVAT therefore, might represent a novel target for pharmacological intervention in cardiovascular disease. © 2018 American Physiological Society. Compr Physiol 8:23-59, 2018.

Dose-response effects of aerobic exercise on body composition among colon cancer survivors: a randomised controlled trial

Background:

Physical activity is associated with a lower risk of disease recurrence among colon cancer survivors. Excess visceral adipose tissue is associated with a higher risk of disease recurrence among colon cancer survivors. The pathways through which physical activity may alter disease outcomes are unknown, but may be mediated by changes in visceral adipose tissue.

Methods:

Thirty-nine stage I–III colon cancer survivors were randomised to one of three groups: usual-care control, 150 min wk⁻¹ of aerobic exercise (low dose) and 300 min wk⁻¹ of aerobic exercise (high dose) for 6 months. The prespecified key body composition outcome was visceral adipose tissue quantified using dual energy X-ray absorptiometry.

Results:

Exercise reduced visceral adipose tissue in dose–response fashion (P_trend=0.008). Compared with the control group, the low- and high-dose exercise groups lost 9.5 cm² (95% CI: –22.4, 3.5) and 13.6 cm² (95% CI: –27.0, –0.1) in visceral adipose tissue, respectively. Each 60 min wk⁻¹ increase in exercise predicted a 2.7 cm² (95% CI: –5.4, –0.1) reduction in visceral adipose tissue.

Conclusions:

Aerobic exercise reduces visceral adipose tissue in dose–response fashion among patients with stage I–III colon cancer. Visceral adipose tissue may be a mechanism through which exercise reduces the risk of disease recurrence among colon cancer survivors.

Citral, a Monoterpene Inhibits Adipogenesis Through Modulation of Adipogenic Transcription Factors in 3T3-L1 Cells

Obesity is considered as a major global human health problem which significantly increases the risk for development of type 2 diabetes. Citral, a bioactive compound widely found in a variety of foods that are consumed daily. In this study, we investigated the inhibitory effect of citral against adipogenic genes in 3T3-L1 cells. The mouse fibroblast 3T3-L1 pre-adipocytes were differentiated into adipocytes using adipogenic cocktail (5 g/ml insulin, 0.5 mM isobutylmethylxanthine and 10 M dexamethasone). Differentiation of adipocytes was evaluated by assessing triglyceride accumulation assay and cell viability by MTT assay. The PI3K/AKT signaling, adipogenic specific transcription factors (PPARγ, SREBP-1c, FAS and CPD) and inflammatory genes (TNF-α, IL-6 and MCP-1) were analyzed by western blotting and reverse transcriptase PCR in differentiated 3T3-L1 cell lines. In this study, triglyceride accumulation was increased in adipogenic cocktail induced 3T3-L1 cells, whereas treatment of citral significantly decreased levels of triglyceride accumulation in concentration dependent manner. Further, MTT assay shows that there was no reduction of cell viability during the differentiation of 3T3-L1 cells. The differentiated 3T3-L1 cell significantly increases the expression of PI3K/AKT, adipogenic transcription factors (PPARγ, SREBP-1c, FAS and CPD) and inflammatory biomarkers (TNF-α, IL-6 and MCP-1). Conversely, cells were treated with citral significantly suppress the expression of PI3K/AKT, PPARγ, SREBP-1c, FAS, CPD, TNF-α, IL-6 and MCP-1 in dose dependent manner. Thus, citral exhibits beneficial effects to inhibit adipogenesis in 3T3-L1 adipocytes through the modulation of adipogenic transcription factors and inflammatory markers.

Marine Peptides as Potential Agents for the Management of Type 2 Diabetes Mellitus-A Prospect

An increasing prevalence of diabetes is known as a main risk for human health in the last future worldwide. There is limited evidence on the potential management of type 2 diabetes mellitus using bioactive peptides from marine organisms, besides from milk and beans. We summarized here recent advances in our understanding of the regulation of glucose metabolism using bioactive peptides from natural proteins, including regulation of insulin-regulated glucose metabolism, such as protection and reparation of pancreatic β-cells, enhancing glucose-stimulated insulin secretion and influencing the sensitivity of insulin and the signaling pathways, and inhibition of bioactive peptides to dipeptidyl peptidase IV, α-amylase and α-glucosidase activities. The present paper tried to understand the underlying mechanism involved and the structure characteristics of bioactive peptides responsible for its antidiabetic activities to prospect the utilization of rich marine organism proteins.

The insulin-signaling pathway of the pancreatic islet is impaired in adult mice offspring of mothers fed a high-fat diet

OBJECTIVE

Mothers fed a high-fat (HF) diet can cause different adverse alterations in their offspring. The study aimed to verify the pancreatic islet structure and insulin-signaling pathway in adulthood of offspring of mothers fed a HF diet during the pregnancy.

METHODS

Female mice (mothers) were randomly assigned to receive either standard chow (Mo-SC) or a HF diet (Mo-HF) ad libitum. After 2 mo on the experimental diets, 3-mo-old female mice were mated with male C57 BL/6 mice that were fed a SC diet. The male offspring was evaluated at 6 mo old.

RESULTS

At 6 mo of age, Mo-HF offspring had an increment in body mass and adiposity, hypercholesterolemia, and hypertriacylglycerolemia, higher levels of insulin, and leptin with a concomitant decrease in adiponectin levels. In the islet, we observed an alteration in the structure characterized by the migration of some alpha cells from the edge to the core of the islet in association with an increase in the masses of the islet, beta cell, and alpha cell, featuring a pancreatic islet remodeling. Additionally, the Mo-HF offspring demonstrated a decrease in IRS1, PI3 k p-Akt, Pd-1, and Glut2 protein expressions compared to Mo-SC offspring. However, an increase was observed in FOXO1 and insulin protein expressions in Mo-HF offspring compared to Mo-SC offspring.

CONCLUSION

The present study demonstrated that a maternal HF diet is responsible for remodeling the islet structure coupled with an adverse carbohydrate metabolism and impairment of the insulin-signaling pathway in adult male mice offspring.

Transcriptional profiling of subcutaneous adipose tissue in Italian Large White pigs divergent for backfat thickness

Fat deposition is a widely studied trait in pigs because of its implications with animal growth efficiency, technological and nutritional characteristics of meat products, but the global framework of the biological and molecular processes regulating fat deposition in pigs is still incomplete. This study describes the backfat tissue transcription profile in Italian Large White pigs and reports genes differentially expressed between fat and lean animals according to RNA-seq data. The backfat transcription profile was characterised by the expression of 23 483 genes, of which 54.1% were represented by known genes. Of 63 418 expressed transcripts, about 80% were non-previously annotated isoforms. By comparing the expression level of fat vs. lean pigs, we detected 86 robust differentially expressed transcripts, 72 more highly expressed (e.g. ACP5, BCL2A1, CCR1, CD163, CD1A, EGR2, ENPP1, GPNMB, INHBB, LYZ, MSR1, OLR1, PIK3AP1, PLIN2, SPP1, SLC11A1, STC1) and 14 lower expressed (e.g. ADSSL1, CDO1, DNAJB1, HSPA1A, HSPA1B, HSPA2, HSPB8, IGFBP5, OLFML3) in fat pigs. The main functional categories enriched in differentially expressed genes were immune system process, response to stimulus, cell activation and skeletal system development, for the overexpressed genes, and unfolded protein binding and stress response, for the underexpressed genes, which included five heat shock proteins. Adipose tissue alterations and impaired stress response are linked to inflammation and, in turn, to adipose tissue secretory activity, similar to what is observed in human obesity. Our results provide the opportunity to identify biomarkers of carcass fat traits to improve the pig production chain and to identify genetic factors that regulate the observed differential expression.

PI3K signaling in the pathogenesis of obesity: The cause and the cure

With the steady rise in the incidence of obesity and its associated comorbidities, in the last decades research aimed at understanding molecular mechanisms that control body weight has gained new interest. Fat gain is frequently associated with chronic adipose tissue inflammation and with peripheral as well as central metabolic derangements, resulting in an impaired hypothalamic regulation of energy homeostasis. Recent attention has focused on the role of phosphatidylinositol 3-kinase (PI3K) in both immune and metabolic response pathways, being involved in the pathophysiology of obesity and its associated metabolic diseases. In this review, we focus on distinct PI3K isoforms, especially class I PI3Ks, mediating inflammatory cells recruitment to the enlarged fat as well as intracellular responses to key hormonal regulators of fat storage, both in adipocytes and in the central nervous system. This integrated view of PI3K functions may ultimately help to develop new therapeutic interventions for the treatment of obesity.

Insulin resistance and muscle insulin receptor substrate-1 serine hyperphosphorylation

Insulin resistance in metabolic syndrome subjects is profound in spite of muscle insulin receptor and insulin-responsive glucose transporter (GLUT4) expression being nearly normal. Insulin receptor tyrosine kinase phosphorylation of insulin receptor substrate-1 (IRS-1) at Tyr896 is a necessary step in insulin stimulation of translocation of GLUT4 to the cell surface. Serine phosphorylation of IRS-1 by some kinases diminishes insulin action in mice. We evaluated the phosphorylation status of muscle IRS-1 in 33 subjects with the metabolic syndrome and seventeen lean controls. Each underwent euglycemic insulin clamps and a thigh muscle biopsy before and after 8 weeks of either strength or endurance training. Muscle IRS-1 phosphorylation at six sites was quantified by immunoblots. Metabolic syndrome muscle IRS-1 had excess phosphorylation at Ser337 and Ser636 but not at Ser307, Ser789, or Ser1101. Ser337 is a target for phosphorylation by glycogen synthase kinase 3 (GSK3) and Ser636 is phosphorylated by c-Jun N-terminal kinase 1 (JNK1). Exercise training without weight loss did not change the IRS-1 serine phosphorylation. These data suggest that baseline hyperphosphorylation of at least two key serines within muscle IRS-1 diminishes the transmission of the insulin signal and thereby decreases the insulin-stimulated translocation of GLUT4. Excess fasting phosphorylation of muscle IRS-1 at Ser636 may be a major cause of the insulin resistance seen in obesity and might prevent improvement in insulin responsiveness when exercise training is not accompanied by weight loss.