Regulation Of Insulin Action Research Articles

Exploring the Interplay between Vitamin D, Insulin Resistance, Obesity and Skeletal Health.

Vitamin D (ViD), plays an important role in calcium absorption and bone mineralization, is associated with bone mineral density. Severe deficiency in ViD has long been linked to conditions such as rickets in children and osteomalacia in adults, revealing its substantial role in skeletal health. Additionally, investigations show an existing interconnection between ViD and insulin resistance (Ins-R), especially in patients with type 2 diabetes mellitus (T2DM). Obesity, in conjunction with Ins-R, may augment the risk of osteoporosis and deterioration of skeletal health. This review aims to examine recent studies on the interplay between ViD, Ins-R, obesity, and their impact on skeletal health, to offer insights into potential therapeutic strategies. Cochrane Library, Google Scholar, and Pubmed were searched to investigate relevant studies until December 2023. Current research demonstrates ViD's impact on pancreatic β-cell function, systemic inflammation, and insulin action regulation. Our findings highlight an intricate association between ViD, Ins-R, obesity, and skeletal health, providing a perspective for the prevention and/or treatment of skeletal disorders in patients with obesity, Ins-R, and T2DM.

FRI032 Cellular Insights Into Metabolically Healthy And Unhealthy Obesity

Abstract Disclosure: M.C. Petersen: None. G.I. Smith: None. J. Yu: None. R.A. Barve: Other; Self; Royalties; PercayAI. J. Yoshino: None. G.I. Shulman: None. S. Klein: Advisory Board Member; Self; Merck, Altimmune. Some people with obesity are resistant to the adverse metabolic effects of excess adiposity and are considered to have “metabolically healthy obesity” (MHO). However, the mechanisms responsible for MHO are not clear. We evaluated whole-body insulin sensitivity (glucose infusion rate/plasma insulin during a hyperinsulinemic-euglycemic clamp) and several putative factors that regulate insulin action, including skeletal muscle diacylglycerol (DAG) and ceramide content and transcriptomic profiles in abdominal subcutaneous adipose tissue (ASAT) and muscle, in three groups separated by adiposity and metabolic function: i) 20 adults with MHO (BMI = 38.6 ± 1.2 kg/m2, normal fasting glucose, oral glucose tolerance, hemoglobin A1c, and intrahepatic triglyceride content); ii) 20 adults with MUO (BMI= 39.5 ± 1.1 kg/m2, prediabetes and hepatic steatosis); and iii) 15 adults who were metabolically-healthy and lean (MHL, BMI= 22.7 ± 0.4 kg/m2). Insulin sensitivity progressively decreased from MHL to MHO to MUO [109 ± 7 to 72 ± 7 to 30 ± 2 (μg glucose/kg FFM/min)/(μU/mL), respectively]. Muscle plasma membrane and total cellular sn-1,2-DAG contents were greater in the MHO and MUO groups compared with the MHL group, with no difference between MHO and MUO groups. In contrast, muscle ceramide content in multiple compartments progressively increased from MHL to MHO to MUO and were inversely correlated with insulin sensitivity among all subjects (plasma membrane, r = -0.61; mitochondria, r = -0.56; endoplasmic reticulum, r = -0.51, all P < 0.001). In ASAT, the expression of genes involved in extracellular matrix (ECM) remodeling and inflammation were greater in MUO than MHO, whereas the expression of genes involved in lipogenesis was greater in MHO than MUO. Sparse partial least-squares discriminant analysis, a classification algorithm, was used to identify the features that best discriminated the MHO and MUO groups. Of 125 variables, fasting plasma C-peptide concentration and skeletal muscle mitochondrial ceramide content were the best discriminants of MHO and MUO. Muscle mitochondrial ceramide content was negatively correlated with muscle transcripts involved in mitochondrial structure/function (e.g., ATP5MG [r = -0.69, P < 0.0001] and MPC1 [r = -0.67, P < 0.0001]) and PAQR4 (r = -0.71, P < 0.0001), which encodes a ceramidase in the adiponectin receptor family. In addition, the muscle transcripts that positively correlated with muscle mitochondrial ceramide content were strongly enriched for ECM and TGFβ-related pathways. Conclusions: Greater whole-body insulin sensitivity in people with MHO than MUO is associated with alterations in adipose tissue and skeletal muscle biology, namely decreased markers of ASAT ECM remodeling and inflammation, decreased skeletal muscle ceramide content, and increased skeletal muscle expression of PAQR4 and genes associated with mitochondrial content/function. Presentation: Friday, June 16, 2023

Subcutaneous adipose tissue circadian gene expression: Relationship with insulin sensitivity, obesity, and the effect of weight-reducing dietary intervention

ObjectiveThe circadian rhythms are controlled by the central clock in the hypothalamic suprachiasmatic nuclei and by the peripheral clocks in tissues, including adipose tissue. The adipose tissue circadian clock may be associated with the regulation of insulin action; however, human data are limited. The aim of this study was to analyze the expression of subcutaneous adipose tissue circadian genes as they relate to obesity and insulin sensitivity before and after diet-induced weight loss. MethodsThe study group comprised 38 individuals who were overweight or obese. The individuals completed a 12-wk dietary intervention program. Hyperinsulinemic-euglycemic clamp and subcutaneous adipose tissue biopsy were performed before and after the program. Sixteen normal weight individuals were examined at baseline and served as a control group. ResultsAt baseline, individuals who were overweight/obese had lower adipose tissue expression of NR1D1, NR1D2, DBP, PER1, and PER2 than normal weight individuals. The expression of ARNTL, CLOCK, and CRY did not differ between the groups. A weight-reducing dietary intervention resulted in an increase in the expression of adipose tissue NR1D2 and DBP, which was positively related to insulin sensitivity both before (in the entire study group and in the subgroup of overweight/obese individuals) and after the dietary intervention. ConclusionsAdipose tissue circadian gene expression is decreased in obesity and this decrease may be partially reversed by dietary intervention. Among circadian genes, NR1D2 and DBP seem to be specifically associated with insulin action.

Bisphenol A instigates diminished sirtuin-1 mediated vascular complications in type 2 diabetes mellitus

Endocrine society has evidence that plastics can disrupt the endocrine system to cause severe diseases in humans and animals. However, plastics are still used in a variety of models for their applications. The overproduction of plastics simultaneously increases health hazards in human and animal living society. The pancreas is an endocrine system to produces insulin to maintain the glucose level in the blood when that insulin level resists or does not synthesize causing diabetes mellitus. It is one of the highest prevalence diseases occurred in worldwide. Sirtuin-1 is a protein involved in glucose homeostasis and it regulates insulin action. In the human body, hampered the level of insulin, sirtuin-1, and glucose cause diabetes and its vascular complications. Furthermore, many researchers evidenced that the endocrine disruptor of bisphenol A (BPA) causes diabetes mellitus and its vascular complications. Nevertheless, the biological pathway is unclear. This review discusses the linkage between the endocrine disruptor of BPA and diabetes and explores Sirtuin-1 mediated BPA-induced diabetic vascular complications.

Insulin Receptor Substrate 1 Gly972Arg (rs1801278) Polymorphism Is Associated with Obesity and Insulin Resistance in Kashmiri Women with Polycystic Ovary Syndrome.

Background: Polycystic ovary syndrome (PCOS) is commonly associated with metabolic abnormalities such as hyperinsulinemia, insulin resistance and obesity. The genetic variants of genes regulating insulin action, expression and regulation are suggested as possible factors involved in development and severity of clinical manifestations in PCOS. Aim: We investigated whether IRS-1Gly972Arg (rs1801278) polymorphism is associated with increased risk of PCOS in Kashmiri women. The correlation of various clinical, metabolic and hormonal markers with rs1801278 single nucleotide polymorphism was analyzed. The genotypic–phenotypic association of clinical manifestations of PCOS with the tested genetic variant was also assessed. Results: There were no significant differences in allele frequency (OR = 0.87, CI = 0.59–1.29, χ2 = 0.456, p = 0.499) or genotypic distribution (χ2 = 3.73, p = 0.15) between PCOS women and controls. No significant association was also found in the dominant (OR = 1.63, χ2 = 0.377, p = 0.53), recessive (OR = 0.79, χ2 = 1.01, p = 0.31) or heterozygote vs. homozygote (OR = 1.34, χ2 = 1.53, p = 0.22) genotype model analysis. The genotype–phenotype correlation analysis showed that the Arg allele was significantly associated with increased central adiposity markers hip circumference (p = 0.012), and body adiposity index BAI (p = 0.002) in the recessive model in PCOS women. The two-hour glucose (p = 0.04) and insulin resistance marker HOMA (p = 0.44) were significantly higher in Arg allele carriers. The androgen excess markers dehydroepiandrosterone sulfate DHEAS (p = 0.02), Ferriman–Gallwey score (p = 0.012), prevalence of acne, alopecia and hirsutism (all p < 0.01) were significantly elevated in the wild-type GG genotype. Conclusions:IRS-1Gly972Arg genetic variant does not increase the risk of PCOS in Kashmiri women. However, this polymorphism is associated with clinical manifestations of insulin resistance, obesity and hyperandrogenism, suggesting its possible role in variable phenotypic manifestations of PCOS.

The Loss of Drp1 Improves Skeletal Muscle Insulin Action in Primary Myotubes Derived from Humans with Severe Obesity

Dynamin‐related protein 1 (Drp1) is a key regulator of mitochondrial fission. Excessive Drp1‐mediated mitochondrial fission in skeletal muscle from humans with severe obesity is associated with impaired insulin action. However, it remains unclear whether specific inhibition of Drp1 in skeletal muscle cells alleviates insulin resistance in obesity. Therefore, this study aims to determine the direct role of Drp1 on regulating insulin action in human skeletal muscle cells derived from humans with severe obesity. Human skeletal muscle cells from six lean, insulin‐sensitive (LN, BMI = 22.7 ± 1.2 kg/m2,HOMA‐IR = 1.9 ± 0.4) and six severely obese, insulin‐resistant (OB, BMI = 47.3 ± 2.8 kg/m2, HOMA‐IR = 3.4 ± 0.4) subjects were pooled together, respectively. At 90% confluency, myoblasts were transfected using polyethyleneimine with a Drp1 shRNA (shDrp1) or scramble shRNA constructs (shCtrl). After 48 h, the medium was replaced with differentiation media with puromycin. On day 7 of differentiation, the mitochondrial network, reactive oxygen species (ROS), insulin signaling, glucose uptake, and protein markers of mitochondrial dynamics and mitochondrial content were assessed. RNA sequencing was also performed on OB‐shCtrl and OB‐shDrp1 myotubes. Differentially regulated genes were identified, and a gene set enrichment was used to determine pathway modulations. Drp1 protein expression was reduced in OB‐shDrp1 myotubes compared to LN‐shCtrl and OB‐shCtrl (72% and 78%, respectively, P&lt;0.05). OB‐shCtrl myotubes exhibited fragmented mitochondrial networks with an increase in the number of non‐networked individual mitochondria compared to the LN‐shCtrl (P&lt;0.05). The loss of Drp1 in OB‐shDrp1 myotubes restored the mitochondrial network structure with the reduced number of non‐networked mitochondria compared to OB‐shCtrl (P&lt;0.05), and is not different from LN‐shCtrl. There were no differences in protein expression of markers of mitochondrial dynamics and content. Regarding insulin action, insulin‐stimulated Akt Ser473 phosphorylation and glucose uptake (over basal condition) were both reduced in OB‐shCtrl myotubes compared to LN‐shCtrl (P&lt;0.05). Importantly, OB‐shDrp1 myotubes significantly increased insulin‐stimulated Akt Ser473 phosphorylation and glucose uptake compared to OB‐shCtrl (P&lt;0.05). In addition, ROS was elevated in OB‐shCtrl myotubes when compared to LN‐shCtrl (P&lt;0.05) but was reduced in OB‐shDrp1 myotubes (P&lt;0.05). Lastly, the loss of Drp1 revealed an upregulation in genes responsible for fatty acid oxidation and downregulation in genes for glycolysis in OB‐shDrp1 myotubes compared to OB‐shCtrl. These results demonstrate that the loss of Drp1 improves mitochondrial morphology and enhances insulin action, which may, at least partially, be due to reduced ROS production and improved fat oxidation in skeletal muscle cells from humans with severe obesity. Our data suggest that Drp1 may serve as an important regulator of skeletal muscle insulin action.

T1R3 homomeric sweet taste receptor negatively regulates insulin-induced glucose transport through Gαs-mediated microtubules disassembly in 3T3-L1 adipocytes.

T1R3 is a class C G protein-coupled receptor family member that forms heterodimeric umami and sweet taste receptors with T1R1 and T1R2, respectively, in the taste cells of taste buds. T1R3 is expressed in 3T3-L1 cells in homomeric form and negatively regulates adipogenesis in a Gαs-dependent but cAMP-independent manner. Although T1R3 expression is markedly upregulated during adipogenesis, its physiological role in mature adipocytes remains obscure. Here, we show that stimulation of T1R3 with sucralose or saccharin induces microtubule disassembly in differentiated 3T3-L1 adipocytes. The effect was reproduced by treatment with cholera toxin or isoproterenol but not with forskolin. Treatment with sucralose or saccharin for 3 h inhibited insulin-stimulated glucose uptake by 32% and 45% in differentiated adipocytes, respectively, similar to the inhibitory effect of nocodazole (by 33%). Isoproterenol treatment inhibited insulin-stimulated glucose transport by 26%, whereas sucralose did not affect the intrinsic activity of the glucose transporter, indicating that it inhibited insulin-induced GLUT4 translocation to the plasma membrane. Immunostaining analysis showed that insulin-stimulated GLUT4 accumulation on the plasma membrane was abrogated in sucralose-treated cells, in association with depolymerization of microtubules. Sucralose-mediated inhibition of GLUT4 translocation was reversed by the overexpression of dominant-negative Gαs (Gαs-G226A) or knockdown of Gαs. Additionally, membrane fractionation analysis showed that sucralose treatment reduced GLUT4 levels in the plasma membrane fraction from insulin-stimulated adipocytes. We have identified a novel non-gustatory role for homomeric T1R3 in adipocytes, and activation of the T1R3 receptor negatively regulates insulin action of glucose transport via Gαs-dependent microtubule disassembly.

Histone modifications influence the insulin-signaling genes and are related to insulin resistance in human adipocytes.

Insulin resistance (IR) is a state when the physiological amount of insulin is not sufficient to evoke proper action, that is, glucose uptake. Numerous conditions lead to IR, including epigenetic components. Epigenetic modifications, associated with obesity and IR are one of the main mechanisms leading to IR pathogenesis. The adipose tissue samples (subcutaneous (SAT) and visceral (VAT)) were collected during abdominal surgery from 40 patients of a wide range of BMI, age, and insulin resistance ratios (F = 9, M = 31). IR was induced in 3T3-L1 adipocytes and human adipocytes collected from SAT and VAT of healthy subjects. Global and site-specific histone modifications (H3K4me3 and H3K9/14ac) were determined. We found lower histone modifications in adipose tissue of IR patients. Furthermore, numerous genes regulating insulin action (PPARG, SLC2A4, ADIPOQ) were differently marked by histone methylation and acetylation. Moreover, we noticed that epigenetic changes appear as soon as 72 h following IR induction. The epigenetic changes appeared to be mediated through the SIRT family. Based on obtained results, the histone marks related to insulin resistance mostly concerned PPARG and SLC2A4 genes. Furthermore, our results proved a vital role of the SIRT family in insulin action and IR pathogenesis.

PPARG Hypermethylation as the First Epigenetic Modification in Newly Onset Insulin Resistance in Human Adipocytes.

Insulin acts by binding with a specific receptor called an insulin receptor (INSR), ending up with glucose transporter activation and glucose uptake. Insulin resistance (IR) is a state when the physiological amount of insulin is not sufficient to evoke proper action, i.e., glucose uptake. Epigenetic modifications associated with obesity and IR are some of the main mechanisms leading to IR pathogenesis. The mesenchymal stem cells of adipose tissue (subcutaneous (SAT) and visceral (VAT)) were collected during abdominal surgery. IR was induced ex vivo by palmitic acid. DNA methylation was determined at a global and site-specific level. We found higher global DNA methylation in IR adipocytes after 72 h following IR induction. Furthermore, numerous genes regulating insulin action (PPARG, SLC2A4, ADIPOQ) were hypermethylated in IR adipocytes; the earliest changes in site-specific DNA methylation have been detected for PPARG. Epigenetic changes appear to be mediated through DNMT1. DNA methylation is an important component of IR pathogenesis; the PPARG and its epigenetic modification appear to be the very first epigenetic modification in newly onset IR and are probably of the greatest importance.

The association of laminin levels with insulin resistance and non-alcoholic hepatosteatosis

BackgroundLaminin, one of the largest glycoproteins of the basement membrane, is an important component of the extracellular matrix. Functions of the basement membrane include regulation of cell signaling behaviors and structural support. Laminin plays a critical role in the regulation of insulin action in muscle, liver, and adipose tissue. The study mainly investigates an association between the change in serum laminin levels and insulin resistance and non-alcoholic hepatosteatosis.MethodsThis prospective study included a total of 90 participants; 60 patients diagnosed with Grade 2–3 non-alcoholic hepatosteatosis and 30 age- and sex-matched healthy controls between December 2019 and December 2020. Routine laboratory tests including glucose, insulin, homeostatic model of assessment-insulin resistance (HOMA-IR), alanine aminotransferase (ALT), aspartate aminotransferase (AST), triglyceride, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and C-reactive protein and laminin levels were measured in the serum of the patient and control groups. Enzyme-linked immunosorbent assay was used for the measurement of laminin levels.ResultsThe median serum laminin levels were lower in patients with hepatic steatosis, compared to the control group (72 ng/L vs. 82 ng/L, respectively; p = 0.003). In the patients with insulin resistance, median laminin levels were lower, regardless of the presence of non-alcoholic hepatosteatosis (67 ng/L vs. 85 ng/L, respectively; p = 0.007). There was a weak, negative correlation between the laminin levels and HOMA-IR.ConclusionsOur study results suggest that, although there is no exact link between laminin and non-alcoholic hepatosteatosis, serum laminin levels are lower in patients with insulin resistance by regulating the insulin effect through integrins.

A step towards glucose control with a novel nanomagnetic-insulin for diabetes care

Massive efforts have been devoted to insulin delivery for diabetes care. Achieving a long-term tight-regulated blood glucose level with a low risk of hypoglycemia remains a great challenge. In this study we propose a novel strategy to efficiently regulate insulin action after insulin is injected or released into patient body aiming to achieve better glycemic control, which is achieved by the administration of insulin-conjugated magnetic nanoparticles (MNPs-Ins). We show that the locomotion of MNPs-Ins can be controlled to reach a target site on an in vitro microfluidic platform, which may open a way to modulate the physiological effect of insulin in a remote-control manner. Most importantly, the in vivo blood glucose regulation of the MNPs-Ins was performed on diabetic mice to understand the glycemic control performance. The results showed that the MNPs-Ins can achieve a better glycemic control with longer effective drug duration while not causing hypoglycemia and a magnetic-modulated hypoglycemic dynamics. Moreover, the in vivo histochemistry experiments confirmed the good biocompatibility of MNPs-Ins. Along with our on-going research on the possibility of the recycle and reuse of the MNPs-Ins, the finding presented in this paper may manifest a fascinating potential in insulin delivery in the near future.

Irisin counteracts high glucose and fatty acid-induced cytotoxicity by preserving the AMPK-insulin receptor signaling axis in C2C12 myoblasts.

Irisin, a newly identified myokine, is critical to modulating body metabolism and biological homeostasis. However, whether irisin protects the skeletal muscles against metabolic stresses remains unknown. In this study, we determine the effect of irisin on high glucose and fatty acid-induced damages using irisin-overexpressed mouse C2C12 (irisin-C2C12) myoblasts and skeletal muscle from irisin-injected mice. Compared with empty vector-transfected control C2C12 cells, irisin overexpression resulted in a marked increase in cell viability and decrease in apoptosis under high-glucose stress. Progression of the cell cycle into the G2/M phase in the proliferative condition was also observed with irisin overexpression. Furthermore, glucose uptake, glycogen accumulation, and phosphorylation of AMPKα/insulin receptor (IR) β-subunit/Erk1/2 in response to insulin stimulation were enhanced by irisin overexpression. In irisin-C2C12 myoblasts, these responses of phosphorylation were preserved under palmitate treatment, which induced insulin resistance in the control cells. These effects of irisin were reversed by inhibiting AMPK with compound C. In addition, high glucose-induced suppression of the mitochondrial membrane potential was also prevented by irisin. Moreover, suppression of IR in irisin-C2C12 myoblasts by cotransfection of shRNA against IR also mitigated the effects of irisin while not affecting AMPKα phosphorylation. As an in vivo study, soleus muscles from irisin-injected mice showed elevated phosphorylation of AMPKα and Erk1/2 and glycogen contents. Our results indicate that irisin counteracts the stresses generated by high glucose and fatty acid levels and irisin overexpression serves as a novel approach to elicit cellular protection. Furthermore, AMPK activation is a crucial factor that regulates insulin action as a downstream target.

Metabolic Role of PTEN in Insulin Signaling and Resistance.

Phosphatase and tensin homolog (PTEN) is most prominently known for its function in tumorigenesis. However, a metabolic role of PTEN is emerging as a result of its altered expression in type 2 diabetes (T2D), which results in impaired insulin signaling and promotion of insulin resistance during the pathogenesis of T2D. PTEN functions in regulating insulin signaling across different organs have been identified. Through the use of a variety of models, such as tissue-specific knockout (KO) mice and in vitro cell cultures, PTEN's role in regulating insulin action has been elucidated across many cell types. Herein, we will review the recent advancements in the understanding of PTEN's metabolic functions in each of the tissues and cell types that contribute to regulating systemic insulin sensitivity and discuss how PTEN may represent a promising therapeutic strategy for treatment or prevention of T2D.

Insulin-induced membrane permeability to glucose in human muscles at rest and following exercise.

Increased insulin action is an important component of the health benefits of exercise, but its regulation is complex and not fully elucidated. Previous studies of insulin-stimulated GLUT4 translocation to the skeletal muscle membrane found insufficient increases to explain the increases in glucose uptake. By determination of leg glucose uptake and interstitial muscle glucose concentration, insulin-induced muscle membrane permeability to glucose was calculated 4h after one-legged knee-extensor exercise during a submaximal euglycaemic-hyperinsulinaemic clamp. It was found that during submaximal insulin stimulation, muscle membrane permeability to glucose in humans increases twice as much in previously exercised vs. rested muscle and outstrips the supply of glucose, which then becomes limiting for glucose uptake. This methodology can now be employed to determine muscle membrane permeability to glucose in people with diabetes, who have reduced insulin action, and in principle can also be used to determine membrane permeability to other substrates or metabolites. Increased insulin action is an important component of the health benefits of exercise, but the regulation of insulin action in vivo is complex and not fully elucidated. Previously determined increases in skeletal muscle insulin-stimulated GLUT4 translocation are inconsistent and mostly cannot explain the increases in insulin action in humans. Here we used leg glucose uptake (LGU) and interstitial muscle glucose concentration to calculate insulin-induced muscle membrane permeability to glucose, a variable not previously possible to quantify in humans. Muscle membrane permeability to glucose, measured 4h after one-legged knee-extensor exercise, increased ∼17-fold during a submaximal euglycaemic-hyperinsulinaemic clamp in rested muscle (R) and ∼36-fold in exercised muscle (EX). Femoral arterial infusion of NG -monomethyl l-arginine acetate or ATP decreased and increased, respectively, leg blood flow (LBF) in both legs but did not affect membrane glucose permeability. Decreasing LBF reduced interstitial glucose concentrations to ∼2mM in the exercised but only to ∼3.5mM in non-exercised muscle and abrogated the augmented effect of insulin on LGU in the EX leg. Increasing LBF by ATP infusion increased LGU in both legs with uptake higher in the EX leg. We conclude that it is possible to measure functional muscle membrane permeability to glucose in humans and it increases twice as much in exercised vs. rested muscle during submaximal insulin stimulation. We also show that muscle perfusion is an important regulator of muscle glucose uptake when membrane permeability to glucose is high and we show that the capillary wall can be a significant barrier for glucose transport.

Mechanisms involved in follistatin-induced hypertrophy and increased insulin action in skeletal muscle.

BackgroundSkeletal muscle wasting is often associated with insulin resistance. A major regulator of muscle mass is the transforming growth factor β (TGF‐β) superfamily, including activin A, which causes atrophy. TGF‐β superfamily ligands also negatively regulate insulin‐sensitive proteins, but whether this pathway contributes to insulin action remains to be determined.MethodsTo elucidate if TGF‐β superfamily ligands regulate insulin action, we used an adeno‐associated virus gene editing approach to overexpress an activin A inhibitor, follistatin (Fst288), in mouse muscle of lean and diet‐induced obese mice. We determined basal and insulin‐stimulated 2‐deoxy‐glucose uptake using isotopic tracers in vivo. Furthermore, to evaluate whether circulating Fst and activin A concentrations are associated with obesity, insulin resistance, and weight loss in humans, we analysed serum from morbidly obese subjects before, 1 week, and 1 year after Roux‐en‐Y gastric bypass (RYGB).ResultsFst288 muscle overexpression markedly increased in vivo insulin‐stimulated (but not basal) glucose uptake (+75%, P < 0.05) and increased protein expression and intracellular insulin signalling of AKT, TBC1D4, PAK1, pyruvate dehydrogenase‐E1α, and p70S6K, while decreasing TBC1D1 signaling (P < 0.05). Fst288 increased both basal and insulin‐stimulated protein synthesis, but no correlation was observed between the Fst288‐driven hypertrophy and the increase in insulin‐stimulated glucose uptake. Importantly, Fst288 completely normalized muscle glucose uptake in insulin‐resistant diet‐induced obese mice. RYGB surgery doubled circulating Fst and reduced activin A (−24%, P < 0.05) concentration 1 week after surgery before any significant weight loss in morbidly obese normoglycemic patients, while major weight loss after 1 year did not further change the concentrations.ConclusionsWe here present evidence that Fst is a potent regulator of insulin action in muscle, and in addition to AKT and p70S6K, we identify TBC1D1, TBC1D4, pyruvate dehydrogenase‐E1α, and PAK1 as Fst targets. Circulating Fst more than doubled post‐RYGB surgery, a treatment that markedly improved insulin sensitivity, suggesting a role for Fst in regulating glycaemic control. These findings demonstrate the therapeutic potential of inhibiting TGF‐β superfamily ligands to improve insulin action and Fst's relevance to muscle wasting‐associated insulin‐resistant conditions in mice and humans.

Phosphorylation status of fetuin-A is critical for inhibition of insulin action and is correlated with obesity and insulin resistance.

Fetuin-A (Fet-A), a hepatokine associated with insulin resistance, obesity, and incident type 2 diabetes, is shown to exist in both phosphorylated and dephosphorylated forms in circulation. However, studies on fetuin-A phosphorylation status in insulin-resistant conditions and its functional significance are limited. We demonstrate that serum phosphofetuin-A (Ser312) levels were significantly elevated in high-fat diet-induced obese mice, insulin-resistant Zucker diabetic fatty rats, and in individuals with obesity who are insulin resistant. Unlike serum total fetuin-A, serum phosphofetuin-A was associated with body weight, insulin, and markers of insulin resistance. To characterize potential mechanisms, fetuin-A was purified from Hep3B human hepatoma cells. Hep3B Fet-A was phosphorylated (Ser312) and inhibited insulin-stimulated glucose uptake and glycogen synthesis in L6GLUT4 myoblasts. Furthermore, single (Ser312Ala) and double (Ser312Ala + Ser120Ala) phosphorylation-defective Fet-A mutants were without effect on glucose uptake and glycogen synthesis in L6GLUT4 myoblasts. Together, our studies demonstrate that phosphorylation status of Fet-A (Ser312) is associated with obesity and insulin resistance and raise the possibility that Fet-A phosphorylation may play a role in regulation of insulin action.

Muscle Insulin Resistance and the Inflamed Microvasculature: Fire from Within

Insulin is a vascular hormone and regulates vascular tone and reactivity. Muscle is a major insulin target that is responsible for the majority of insulin-stimulated glucose use. Evidence confirms that muscle microvasculature is an important insulin action site and critically regulates insulin delivery to muscle and action on myocytes, thereby affecting insulin-mediated glucose disposal. Insulin via activation of its signaling cascade in the endothelial cells increases muscle microvascular perfusion, which leads to an expansion of the endothelial exchange surface area. Insulin’s microvascular actions closely couple with its metabolic actions in muscle and blockade of insulin-mediated microvascular perfusion reduces insulin-stimulated muscle glucose disposal. Type 2 diabetes is associated with chronic low-grade inflammation, which engenders both metabolic and microvascular insulin resistance through endocrine, autocrine and paracrine actions of multiple pro-inflammatory factors. Here, we review the crucial role of muscle microvasculature in the regulation of insulin action in muscle and how inflammation in the muscle microvasculature affects insulin’s microvascular actions as well as metabolic actions. We propose that microvascular insulin resistance induced by inflammation is an early event in the development of metabolic insulin resistance and eventually type 2 diabetes and its related cardiovascular complications, and thus is a potential therapeutic target for the prevention or treatment of obesity and diabetes.

The Cell Junction Protein Afadin Negatively Regulates Insulin Action and Modulates Adipose Tissue Function

Adipose tissue dysfunction is one of the main culprits triggering metabolic imbalance associated with obesity. We identified the cell junction protein Afadin as one of the most highly phosphorylated proteins in response to insulin/IGF-1 stimulation using quantitative phospho-proteomic analysis in pre-adipocytes, and investigated the role of Afadin in mediating insulin action and in adipocyte biology. We found that phosphorylation of Afadin occurred in response to insulin in both pre- and mature adipocytes obtained from mice and human subjects. In addition, Afadin phosphorylation was observed in mouse adipose tissues following insulin injection, and after re-feeding, indicating physiological relevance. In obese mice, however, brown fat insulin-induced phosphorylation of Afadin was abolished. Surprisingly, glucose and insulin tolerance were improved in Fat-specific Afadin knock-out (KO) mice on a high fat diet, despite similar body weight gain and adipose mass as wild type controls. Furthermore, ablation of Afadin in vitro did not affect adipogenesis. Yet, prolonged insulin-induced phosphorylation of Akt was observed in KO adipocytes, and was associated with increased insulin-mediated fatty-acid uptake. Finally, phosphorylation of Afadin regulates the docking of several proteins, which may be involved in the regulation of insulin action and adipocyte metabolism. Thus, Afadin is a novel bona fide substrate of the insulin signaling pathway, and is involved in the regulation of insulin action and metabolism. Disclosure M. Lundh: None. M.S. Isidor: None. K. Plucinska: None. F. Shamsi: None. P. Petersen: None. Y. Tseng: Other Relationship; Self; Chugai Pharmaceutical Co., Ltd.. Research Support; Self; MedImmune. B. Emanuelli: None.

Proteomic Profile of Adipose Tissue-Derived Exosomes and Their Potential Role on Placental Glucose Metabolism in Gestational Diabetes Mellitus

Emerging studies suggest that adipose tissue derived exosomes involves in regulating insulin action and glucose homeostasis. In this study, we hypothesised the protein content of adipose tissue derived exosomes (exo-AT) differs in normal glucose tolerant (NGT) and gestational diabetes mellitus (GDM) pregnancies which lead to changes in placental glucose metabolism. Human omental adipose tissue was obtained from GDM (n=60) and BMI-matched NGT (n=48) pregnancies at the time of term Caesarean section. Exosomes were isolated from tissue-conditioned media by differential centrifugation and characterised using nanoparticle tracking analysis and SWATH mass spectrometry. The effect of exosomes on human placental cells was evaluated using a Human Glucose Metabolism RT² Profiler PCR Array. The total number of exosomes (number of vesicles/mg adipose tissue/24 hour) were significantly higher in GDM compared to NGT. The number of exosomes positively correlated with maternal BMI and fetal weight. We identified 122 proteins upregulated in the exo-AT from GDM compared to NGT. Ingenuity pathway analysis revealed the upregulated proteins are associated with mitochondrial dysfunction and oxidative phosphorylation (OXPHOS). When compared to NGT exo-AT, GDM exo-AT increased ALDOB, PGK2 and GCK in placental cells. The data obtained in this study suggest that exosomes secreted from adipose tissue regulates placental glucose metabolism in GDM. Improving the communication of exo-AT to placental tissues may serve as an effective intervention strategy to prevent the consequences of GDM such as fetal overgrowth. Disclosure N. Jayabalan: None. V. Ormazabal: None. A. Lai: None. M. Lappas: None.

Insulin-XTEN® Exhibits a Size-Dependent Alteration in Tissue Action in Rats

To optimize the action of exogenously administered insulin, we employed XTEN® technology to create insulins with variably sized XTEN amino acid polymers. Recombinant fusions of XTEN polymers linked to insulin lispro with an A21G mutation were prepared in various amino acid lengths. Insulin-XTEN molecules demonstrated 15-fold lower potency in binding and receptor phosphorylation than insulin lispro but did not differ from each other. These insulin-XTEN molecules were equally effective in lowering blood glucose at a 100nmol/kg dose in diabetic Sprague-Dawley rats. Furthermore, the larger insulin-XTEN molecules had a longer duration of glucose lowering. Insulin-XTENs were compared to insulin lispro in rat euglycemic clamp studies, using insulin doses that would elicit steady plasma insulin concentrations and equivalent increases in glucose infusion rate. Insulin-mediated suppression of endogenous glucose production was not significantly different among any of the administered insulins. However, plasma free fatty acids and soleus muscle glucose uptake were significantly decreased in an XTEN size-dependent manner when compared to insulin lispro. Additional studies demonstrated equal hepatic pAkt accumulation in rats treated with insulin lispro or any of the insulin-XTENs, but revealed a significant XTEN size-dependent reduction in skeletal muscle pAkt in rats administered insulin-XTENs compared to insulin lispro. These data suggest a possible XTEN size-dependent regulation of insulin action and that the differing sizes of the XTEN polymer may convey preferential tissue action. In conclusion, XTEN technology may permit “tuning” of the glucodynamic effects of the insulin, leading to an enhanced time extension and improved hepatic and peripheral pharmacodynamic action that could more closely mimic the action of endogenously secreted insulin into the portal circulation. Disclosure M.E. Christe: Employee; Self; Eli Lilly and Company. D. Konkol: None. J. Friedrich: None. J. Jacobs: None. E. Hawkins: Employee; Self; Eli Lilly and Company. J. Moyers: Employee; Self; Eli Lilly and Company. Stock/Shareholder; Self; Eli Lilly and Company. C. Zhang: Employee; Self; Eli Lilly and Company. S.D. Kahl: Employee; Self; Eli Lilly and Company. H.E. Baker: None. A.L. Cox: None. R.J. Hansen: Employee; Self; Eli Lilly and Company. Stock/Shareholder; Self; Eli Lilly and Company. A. Sperry: Employee; Self; Eli Lilly and Company. Stock/Shareholder; Self; Eli Lilly and Company. M. Michael: Employee; Self; Eli Lilly and Company. Stock/Shareholder; Self; Eli Lilly and Company. Employee; Spouse/Partner; Eli Lilly and Company. Stock/Shareholder; Spouse/Partner; Eli Lilly and Company. V. Schellenberger: None. D. Baldwin: None. J.M. Beals: Employee; Self; Eli Lilly and Company. A. Korytko: None.