Obesity-linked Insulin Resistance Research Articles

Adipose Tissue: A Novel Target of the Incretin Axis? A Paradigm Shift in Obesity-Linked Insulin Resistance.

Adipose tissue (AT) represents a plastic organ that can undergo significant remodeling in response to metabolic demands. With its numerous checkpoints, the incretin system seems to play a significant role in controlling glucose homeostasis and energy balance. The importance of the incretin hormones, namely the glucagon-like peptide-1 (GLP-1) and the glucose-dependent insulinotropic peptide (GIP), in controlling the function of adipose cells has been brought to light by recent studies. Notably, a "paradigm shift" in reevaluating the role of the incretin system in AT as a potential target to treat obesity-linked metabolic disorders resulted from the demonstration that a disruption of the GIP and GLP-1 signaling axis in fat is associated with adiposity-induced insulin-resistance (IR) and/or type 2 diabetes mellitus (T2D). We will briefly discuss the (patho)physiological functions of GLP-1 and GIP signaling in AT in this review, emphasizing their potential impacts on lipid storage, adipogenesis, glucose metabolism and inflammation. We will also address the conundrum with the perturbation of the incretin axis in white or brown fat tissue and the emergence of metabolic disorders. In order to reduce or avoid adiposity-related metabolic complications, we will finally go over a potential scientific rationale for suggesting AT as a novel target for GLP-1 and GIP receptor agonists and co-agonists.

Gender Differences in the Predictive Value of Obesity Indices for Insulin Resistance in Adult Mexican Individuals.

Obesity-linked insulin resistance (IR) is an important risk factor for metabolic diseases, and anthropometric indices are commonly used for risk assessment. The study aimed to assess possible differences between women and men in the predictive value and association of nine obesity indices with IR, as assessed by HOMA-IR, in a nondiabetic adult population. The cross-sectional study included individuals recruited from a hospital in Mexico City. Indices evaluated were waist circumference (WC), hip circumference (HC), body mass index (BMI), waist-to-hip ratio, waist-to-height ratio, visceral adiposity index, body adiposity index (BAI), relative fat mass (RFM), and conicity index (CI). Fasting plasma glucose and insulin were measured to calculate HOMA-IR. Correlation analysis was performed between obesity indices and HOMA-IR. Receiver operating characteristics curve analyses were performed to determine predictive accuracy and cut-off values of obesity indices for IR. A binary logistic regression (BLR) analysis with OR calculation was performed to determine the strength of association with HOMA-IR. We included 378 individuals (59% females, mean age 46.38 ±12.25 years). The highest Pearson coefficient value was observed for BMI among women, while among men, the highest values were found for BMI and BAI. WC among women, and BAI and RFM among men showed the highest sensitivity, while the highest specificity was observed for WHR among women and WC among men with respect to insulin prediction. In the adjusted BLR model, BMI, WC, and WHR among women and WC and RFM and BAI among men were independently associated with IR, showing the highest odds ratio (OR). In Mexican adults, WC, WHR, RFM and BAI could be complementary tools for BMI in screening for IR.

Adiponectin Resistance in Obesity: Adiponectin Leptin/Insulin Interaction.

The adiponectin (APN) levels in obesity are negatively correlated with chronic subclinical inflammation markers. The hypertrophic adipocytes cause obesity-linked insulin resistance and metabolic syndrome. Furthermore, macrophage polarization is a key determinant regulating adiponectin receptor (AdipoR1/R2) expression and differential adiponectin-mediated macrophage inflammatory responses in obese individuals. In addition to decrease in adiponectin concentrations, the decline in AdipoR1/R2 messenger ribonucleic acid (mRNA) expression leads to a decrement in adiponectin binding to cell membrane, and this turns into attenuation in the adiponectin effects. This is defined as APN resistance, and it is linked with insulin resistance in high-fat diet-fed subjects. The insulin-resistant group has a significantly higher leptin-to-APN ratio. The leptin-to-APN ratio is more than twofold higher in obese individuals. An increase in expression of AdipoRs restores insulin sensitivity and β-oxidation of fatty acids via triggering intracellular signal cascades. The ratio of high molecular weight to total APN is defined as the APN sensitivity index (ASI). This index is correlated to insulin sensitivity. Homeostasis model of assessment (HOMA)-APN and HOMA-estimated insulin resistance (HOMA-IR) are the most suitable methods to estimate the metabolic risk in metabolic syndrome. While morbidly obese patients display a significantly higher plasma leptin and soluble (s)E-selectin concentrations, leptin-to-APN ratio, there is a significant negative correlation between leptin-to-APN ratio and sP-selectin in obese patients. When comparing the metabolic dysregulated obese group with the metabolically healthy obese group, postprandial triglyceride clearance, insulin resistance, and leptin resistance are significantly delayed following the oral fat tolerance test in the first group. A neuropeptide, Spexin (SPX), is positively correlated with the quantitative insulin sensitivity check index (QUICKI) and APN. APN resistance together with insulin resistance forms a vicious cycle. Despite normal or high APN levels, an impaired post-receptor signaling due to adaptor protein-containing pleckstrin homology domain, phosphotyrosine-binding domain, and leucine zipper motif 1 (APPL1)/APPL2 may alter APN efficiency and activity. However, APPL2 blocks adiponectin signaling through AdipoR1 and AdipoR2 because of the competitive inhibition of APPL1. APPL1, the intracellular binding partner of AdipoRs, is also an important mediator of adiponectin-dependent insulin sensitization. The elevated adiponectin levels with adiponectin resistance are compensatory responses in the condition of an unusual discordance between insulin resistance and APN unresponsiveness. Hypothalamic recombinant adeno-associated virus (rAAV)-leptin (Lep) gene therapy reduces serum APN levels, and it is a more efficient strategy for long-term weight maintenance.

The Significance of Different Obesity Parameters in Obese Participants with Insulin Resistance

Background: Studies found a link between visceral obesity and insulin resistance (IR). Numerous attempts made to discover a link between fat lipotoxicity and IR by measuring waist circumference (WC) or body mass index (BMI). This study investigates the relationship between body adiposity index [BAI], a body shape index [ABSI], BMI, waist-to-hip ratio [WHR], TGI-WC = Triglyceride glucose index-× WC, and TGI-BMI = Triglyceride glucose index-BMI. Homeostatic model assessment of IR (HOMA-IR) is utilized to examine the significance of those indices in the target population with obesity-linked IR in Iraq. Objective: The aim is to identify the most relevant obesity index by examining the relationship between multiple obesity indices and IR in obese healthy individuals. Design and Methods: 100 obese and nonobese patients’ height, weight, and hip circumference were taken. Triglycerides (TGs), glycated hemoglobin (HbA1c), serum insulin, and fasting plasma glucose were obtained to calculate HOMA-IR, TGI-WC, and TGI-BMI. Results: Healthy adults 18–50 years were divided (obese and not-obese), clinical features were averaged, and standard errors of the mean calculated. Results for the first obese group were blood pressure [BP] = 114 ± 2/75 ± 9, HbA1c=5.76 ± 0.09, TG = 175.26 ± 3.66, fasting blood glucose [FBG] = 111.84 ± 1.57, WC = 106.62 ± 1.68, BMI = 32.02 ± 0.69, while those for the second group were BP = 111 ± 8/70 ± 6, HbA1c = 4.41 ± 0.06, TG = 96.36 ± 2.02, FBG = 82.40 ± 1.48, WC = 83.76 ± 0.85, and BMI = 22.84 ± 0.22. The relationships between obesity indices were analyzed with HOMA-IR being judged to be IR when it acquired a score value of ≥ 2. The Pearson’s correlation coefficient® used to represent was WHR: r = 0.58, P = 0.0001, ABSI: r = 0.44, P = 0.04, BAI: r = 0.50, P = 0.02, TGI-WC: r = 0.89, P = 0.0001, and TGI-BMI: r = 0.74, P = 0.74. Conclusion: All of the obesity and lipid-related indices investigated were positively correlated with IR. The use of TGI-WC is recommended since it is straightforward to calculate using accessible laboratory tests; it showed the highest (Pearson’s) coefficient (r = 0.89; P = 0.0001).

Non-invasive type 2 diabetes risk scores do not identify diabetes when the cause is β-cell failure: The Africans in America study.

BackgroundEmerging data suggests that in sub-Saharan Africa β-cell-failure in the absence of obesity is a frequent cause of type 2 diabetes (diabetes). Traditional diabetes risk scores assume that obesity-linked insulin resistance is the primary cause of diabetes. Hence, it is unknown whether diabetes risk scores detect undiagnosed diabetes when the cause is β-cell-failure.AimsIn 528 African-born Blacks living in the United States [age 38 ± 10 (Mean ± SE); 64% male; BMI 28 ± 5 kg/m2] we determined the: (1) prevalence of previously undiagnosed diabetes, (2) prevalence of diabetes due to β-cell-failure vs. insulin resistance; and (3) the ability of six diabetes risk scores [Cambridge, Finnish Diabetes Risk Score (FINDRISC), Kuwaiti, Omani, Rotterdam, and SUNSET] to detect previously undiagnosed diabetes due to either β-cell-failure or insulin resistance.MethodsDiabetes was diagnosed by glucose criteria of the OGTT and/or HbA1c ≥ 6.5%. Insulin resistance was defined by the lowest quartile of the Matsuda index (≤ 2.04). Diabetes due to β-cell-failure required diagnosis of diabetes in the absence of insulin resistance. Demographics, body mass index (BMI), waist circumference, visceral adipose tissue (VAT), family medical history, smoking status, blood pressure, antihypertensive medication, and blood lipid profiles were obtained. Area under the Receiver Operator Characteristics Curve (AROC) estimated sensitivity and specificity of each continuous score. AROC criteria were: Outstanding: >0.90; Excellent: 0.80–0.89; Acceptable: 0.70–0.79; Poor: 0.50–0.69; and No Discrimination: 0.50.ResultsPrevalence of diabetes was 9% (46/528). Of the diabetes cases, β-cell-failure occurred in 43% (20/46) and insulin resistance in 57% (26/46). The β-cell-failure group had lower BMI (27 ± 4 vs. 31 ± 5 kg/m2 P < 0.001), lower waist circumference (91 ± 10 vs. 101 ± 10cm P < 0.001) and lower VAT (119 ± 65 vs. 183 ± 63 cm3, P < 0.001). Scores had indiscriminate or poor detection of diabetes due to β-cell-failure (FINDRISC AROC = 0.49 to Cambridge AROC = 0.62). Scores showed poor to excellent detection of diabetes due to insulin resistance, (Cambridge AROC = 0.69, to Kuwaiti AROC = 0.81).ConclusionsAt a prevalence of 43%, β-cell-failure accounted for nearly half of the cases of diabetes. All six diabetes risk scores failed to detect previously undiagnosed diabetes due to β-cell-failure while effectively identifying diabetes when the etiology was insulin resistance. Diabetes risk scores which correctly classify diabetes due to β-cell-failure are urgently needed.

Adipocyte-specific Nos2 deletion improves insulin resistance and dyslipidemia through brown fat activation in diet-induced obese mice

ObjectiveInducible nitric oxide (NO) synthase (NOS2) is a well-documented inflammatory mediator of insulin resistance in obesity. NOS2 expression is induced in both adipocytes and macrophages within adipose tissue during high-fat (HF)-induced obesity. MethodsEight-week-old male mice with adipocyte selective deletion of the Nos2 gene (Nos2AD−KO) and their wildtype littermates (Nos2fl/fl) were subjected to chow or high-fat high-sucrose (HFHS) diet for 10 weeks followed by metabolic phenotyping and determination of brown adipose tissue (BAT) thermogenesis. The direct impact of NO on BAT mitochondrial respiration was also assessed in brown adipocytes. ResultsHFHS-fed Nos2AD−KO mice had improved insulin sensitivity as compared to Nos2fl/fl littermates. Nos2AD−KO mice were also protected from HF-induced dyslipidemia and exhibited increased energy expenditure compared with Nos2fl/fl mice. This was linked to the activation of BAT in HFHS-fed Nos2AD−KO mice as shown by increased Ucp1 and Ucp2 gene expression and augmented respiratory capacity of BAT mitochondria. Furthermore, mitochondrial respiration was inhibited by NO, or upon cytokine-induced NOS2 activation, but improved by NOS2 inhibition in brown adipocytes. ConclusionsThese results demonstrate the key role of adipocyte NOS2 in the development of obesity-linked insulin resistance and dyslipidemia, partly through NO-dependent inhibition of BAT mitochondrial bioenergetics.

Oxidative phenotype induced by aerobic physical training prevents the obesity-linked insulin resistance without changes in gastrocnemius muscle ACE2-Angiotensin(1-7)-Mas axis

BackgroundWe investigate the effect of aerobic physical training (APT) on muscle morphofunctional markers and Angiotensin Converting Enzyme 2/Angiotensin 1-7/Mas receptor (ACE2/Ang 1-7/Mas) axis in an obesity-linked insulin resistance (IR) animal model induced by cafeteria diet (CAF).MethodsMale C57BL/6J mice were assigned into groups CHOW-SED (chow diet, sedentary; n = 10), CHOW-TR (chow diet, trained; n = 10), CAF-SED (n = 10) and CAF-TR (n = 10). APT consisted in running sessions of 60 min at 60% of maximal speed, 5 days per week for 8 weeks.ResultsTrained groups had lower body weight and adiposity compared with sedentary groups. CAF-TR improved the glucose and insulin tolerance tests compared with CAF-SED group (AUC = 28.896 ± 1589 vs. 35.200 ± 1076 mg dL−1 120 min−1; kITT = 4.1 ± 0.27 vs. 2.5 ± 0.28% min−1, respectively). CHOW-TR and CAF-TR groups increased exercise tolerance, running intensity at which VO2 max was reached, the expression of p-AMPK, p-ACC and PGC1-α proteins compared with CHOW-SED and CAF-SED. Mithocondrial protein expression of Mfn1, Mfn2 and Drp1 did not change. Lipid deposition reduced in CAF-TR compared with CAF-SED group (3.71 vs. 5.53%/area), but fiber typing, glycogen content, ACE2 activity, Ang 1-7 concentration and Mas receptor expression did not change.ConclusionsThe APT prevents obesity-linked IR by modifying the skeletal muscle phenotype to one more oxidative independent of changes in the muscle ACE2/Ang 1-7/Mas axis.

Role of Grb10 in mTORC1-dependent regulation of insulin signaling and action in human skeletal muscle cells.

Growth factor receptor-bound protein 10 (Grb10) is an adaptor protein that binds to the insulin receptor, upon which insulin signaling and action are thought to be inhibited. Grb10 is also a substrate for the mechanistic target of rapamycin complex 1 (mTORC1) that mediates its feedback inhibition on phosphatidylinositide 3-kinase (PI3K)/Akt signaling. To characterize the function of Grb10 and its regulation by mTORC1 in human muscle, primary skeletal muscle cells were isolated from healthy lean young men and then induced to differentiate into myotubes. Knockdown of Grb10 enhanced insulin-induced PI3K/Akt signaling and glucose uptake in myotubes, reinforcing the notion underlying its function as a negative regulator of insulin action in human muscle. The increased insulin responsiveness in Grb10-silenced myotubes was associated with a higher abundance of the insulin receptor. Furthermore, insulin and amino acids independently and additively stimulated phosphorylation of Grb10 at Ser476. However, acute inhibition of mTORC1 with rapamycin blocked Grb10 Ser476 phosphorylation and repressed a negative-feedback loop on PI3K/Akt signaling that increased myotube responsiveness to insulin. Chronic rapamycin treatment reduced Grb10 protein abundance in conjunction with increased insulin receptor protein levels. Based on these findings, we propose that mTORC1 controls PI3K/Akt signaling through modulation of insulin receptor abundance by Grb10. These findings have potential implications for obesity-linked insulin resistance, as well as clinical use of mTORC1 inhibitors.

Metagenomic Analysis of Bacteria Phylum Firmicutes and Bacteroidetes in Women with Type 2 Diabetes

Diabetes mellitus (DM) is a serious health problem in Indonesia. Type 2 diabetes is a metabolic disease which primarily caused by obesity-linked insulin resistance. There is a link between insulin resistance and bacterial populations in the gut. Therefore, the aims of this study was to analyze composition of bacteria belong to the phylum Firmicutes and Bacteroidetes in women with type 2 diabetes and non-diabetic persons as control. The study included five female adults type 2 diabetes patients and five healthy controls. The bacteria composition was analyzed for abundance by denaturing gradient gel electrophoresis (DGGE). Metagenomic analysis based on 16S rRNA gene was represented by 12 DGGE bands. The twelve respective bands showed the similarity ranging from 77 up to 98%. Diversity of microbial composition in women type 2 diabetes was represented by five DGGE bands are Bacteroides fragilis, Bacteroides vulgatus, uncultured Bacteroides sp. clone Lb4eF4, uncultured bacterium clone 16sms90-5g05, and uncultured bacterium clone Malaga 1F14. Four DGGE bands in women healthy controls are closely related to uncultured bacterium isolate DGGE gel band Eub42, uncultured bacterium isolate DGGE gel band K115, uncultured bacterium clone HFV04255, and Enterococcus sp. the finding shows bacterial species belong to the phylum Bacteroidetes was mostly found in women diabetic group, while those belong to the phylum Firmicutes was mostly found in healthy controls.

Inhibition of mitochondrial complex 1 by the S6K1 inhibitor PF-4708671 partly contributes to its glucose metabolic effects in muscle and liver cells

mTOR complex 1 (mTORC1) and p70 S6 kinase (S6K1) are both involved in the development of obesity-linked insulin resistance. Recently, we showed that the S6K1 inhibitor PF-4708671 (PF) increases insulin sensitivity. However, we also reported that PF can increase glucose metabolism even in the absence of insulin in muscle and hepatic cells. Here we further explored the potential mechanisms by which PF increases glucose metabolism in muscle and liver cells independent of insulin. Time course experiments revealed that PF induces AMP-activated protein kinase (AMPK) activation before inhibiting S6K1. However, PF-induced glucose uptake was not prevented in primary muscle cells from AMPK α1/2 double KO (dKO) mice. Moreover, PF-mediated suppression of hepatic glucose production was maintained in hepatocytes derived from AMPK α1/2-dKO mice. Remarkably, PF could still reduce glucose production and activate AMPK in hepatocytes from S6K1/2 dKO mice. Mechanistically, bioenergetics experiments revealed that PF reduces mitochondrial complex I activity in both muscle and hepatic cells. The stimulatory effect of PF on glucose uptake was partially reduced by expression of the Saccharomyces cerevisiae NADH:ubiquinone oxidoreductase in L6 cells. These results indicate that PF-mediated S6K1 inhibition is not required for its effect on insulin-independent glucose metabolism and AMPK activation. We conclude that, although PF rapidly activates AMPK, its ability to acutely increase glucose uptake and suppress glucose production does not require AMPK activation. Unexpectedly, PF rapidly inhibits mitochondrial complex I activity, a mechanism that partially underlies PF's effect on glucose metabolism.

Adrenergic stimulation of adiponectin secretion in visceral mouse adipocytes is blunted in high-fat diet induced obesity

The hormone adiponectin is secreted by white adipocytes and has been put forward as a key mediator of obesity-linked insulin resistance and the metabolic syndrome. Although adiponectin was discovered two decades ago, the knowledge about the molecular and cellular regulation of its secretion is incomplete. Here we have investigated the adrenergic regulation of adiponectin secretion in primary visceral (gonadal) adipocytes isolated from lean or obese/diabetic mice. We show that visceral adipocyte adiponectin release is triggered by cAMP/catecholamines via signalling pathways involving adrenergic beta-3-receptors (β3ARs) and Exchange Protein directly Activated by cAMP, isoform 1 (Epac1). The adrenergically stimulated adiponectin secretion is blunted in visceral adipocytes isolated from obese and diabetic mice and our results suggest the existence of a secretory defect. We have previously shown that adiponectin secretion in subcutaneous adipocytes is abolished in the obese/diabetic state due to reduced abundance of β3ARs and Epac1. However, here we show that protein levels of β3ARs and Epac1 are maintained in visceral adipocytes from obese/diabetic mice proposing that other molecular defects underlie the blunted adiponectin release. Gene expression analysis indicate diabesity-associated disturbances of the signalling downstream of Epac1 and/or the exocytotic process itself. Our study proposes that visceral adipocytes partake in the regulated secretion of adiponectin and may thus influence circulating levels of the hormone, in health and in metabolic disease.

2090-P: Role of Thromboxane-Prostanoid Receptor in Obesity Related Insulin Resistance

Background: Thromboxane-Prostanoid Receptor (TP-R) is implicated in several chronic inflammatory disorders. Our preclinical animal studies showed that TP-R plays a critical role in obesity linked Insulin Resistance (IR). Aim: To determine the link between TP-R and obesity and/or obesity-associated inflammation/IR in human subjects. Methods: We collected PBMCs from lean normal and obese IR subjects and analyzed the expression of various inflammatory genes including TP-R, Thromboxane A2 Synthase (TXA2S), TNFα, and MCP-1. We obtained subcutaneous adipose tissue biopsies from obese subjects and analyzed the TP-R mRNA expression and its correlation with inflammatory cytokines mediating IR. Results: TP-R mRNA expression is significantly increased in PBMCs of obese IR subjects compared to lean normal subjects (P&amp;lt;0.04 by Wilcoxon rank-sum test). In addition, PBMC TP-R expression is positively correlated with the expression of TXA2S, a gene involved in thromboxane A2 synthesis (n=29, Spearman’s r=0.36, P=0.05). Moreover, TP-R expression is positively correlated with body weight, BMI and fat mass (Figure). In AT, TP-R expression is positively correlated with IL-8 expression (n=31, r=0.49, P&amp;lt;0.005). Conclusion: Our data demonstrate that TP-R expression is positively correlated with markers of obesity and/or IR and support our hypothesis that TP-R can be used as a biomarker or a therapeutic target for obesity related IR. Disclosure T. Thangavelu: None. S. Viswanathan: None. C. Desouza: Consultant; Self; Novo Nordisk A/S, Sanofi US. Funding National Institute of General Medical Sciences; Great Plains Institutional Developmental Award for Clinical and Translational Research

1765-P: Adipose Sirtuin 6 Drives Macrophage Polarization Toward M2 through IL-4 Production and Maintains Systemic Insulin Sensitivity in Mice and Human

Adipose tissue inflammation is a reproducible feature of obesity and obesity-linked insulin resistance. Here, using adipocyte-specific Sirt6 knockout mice, we explored whether adipocyte Sirt6 inhibits adipose tissue inflammation and its underlying mechanism. aS6KO mice fed normal chow gained more body weight and fat mass than wild type mice and exhibited glucose intolerance and systemic insulin resistance. Measurement of plasma and tissue cytokines and flow cytometric analysis of adipose stromal vascular cells indicated a decrease in alternatively activated M2 macrophages in adipose tissue of aS6KO mice. Mechanistically, Sirt6 regulated the expression of a canonical type 2 cytokine IL-4 by adipocytes in a cell autonomous manner, which in turn affects M2 macrophage polarization. Consistent with animal experiment data, the degree of obesity and insulin resistance demonstrated by body mass index, fasting blood glucose and HbA1c correlated negatively with the expression of Sirt6 in human visceral fat tissues. Collectively, these results suggest that adipocyte Sirt6 regulates body weight gain and insulin sensitivity independent of diet, and the increased IL-4 production by Sirt6 and resultant M2 polarization of adipose tissue macrophages may attenuate proinflammatory responses in adipose tissue. Disclosure M. Song: None.

Syntaxin 4 (STX4) Enrichment in Skeletal Muscle Remediates Insulin Resistance via and Improving Mitochondrial Function

Type 2 diabetes afflicts 10% of the US population and is the result of both peripheral insulin resistance (skeletal muscle and adipose tissue) as well as insulin secretion defects (pancreas). Insulin resistance, also commonly termed ‘pre‐diabetes’ is estimated to impact more than 30% of our population, and is caused by the inability of skeletal muscle to clear excess blood glucose from the circulation. Human skeletal muscle accounts for over 80% of glucose clearance in the body, a process important in maintaining glucose homeostasis. Currently the only therapeutics that directly improve muscle glucose uptake are diet and exercise. However, with only 3% of the population leading a “healthy lifestyle”, including exercise, directed therapeutics are required to restore skeletal muscle function in pre‐diabetics. Several studies have identified correlations between insulin resistance and reduced levels of a SNARE protein named Syntaxin 4 (STX4) in human skeletal muscle. STX4 interacts with other SNARE proteins to facilitate the fusion of glucose transporter 4 (GLUT4) vesicles with the plasma membrane, to then channel extracellular glucose to the interior of the muscle cell, ultimately clearing glucose from the circulation.To further define the function of STX4 in muscle, we developed a doxycycline Tet‐on inducible skeletal muscle specific STX4 overexpressing mouse model, under the muscle creatine kinase promoter (skm‐STX4 mice). Chow fed female mice overexpressing STX4 exhibited enhanced insulin sensitivity and improved glucose tolerance. Serum insulin levels from these mice were also reduced compared to littermate controls, concordant with their improved glucose tolerance. The data recapitulated the global overexpression model that we had previously published, suggesting that skeletal muscle is the primary driver in modulating glucose homeostasis. To establish that enriching STX4 can remediate insulin resistance, male skm‐STX4 mice were fed a 45% high fat diet (HF; to mimic a typical western diet), until they became insulin resistant. Upon establishing pre‐diabetes in the mice, STX4 enrichment was induced in the skeletal muscle (HFD+STX4 mice). Remarkably HFD+STX4 mice had the insulin sensitivity of a chow‐fed mouse, resolving the HFD‐linked insulin resistance in full. Moreover, the HFD+STX4 mice had significantly improved voluntary movement and increased respiratory exchange ratio, indicating increased reliance on carbohydrate rather than fat oxidation. The maximal oxygen consumption rate was also increased in primary myotubes from these HFD+ STX4 mice.Taken together, these results suggest that STX4 enrichment carries the potential as a therapeutic to remedy obesity‐linked insulin resistance, potentially via improving mitochondrial function. These results suggest that STX4 could be a viable therapeutic target for treatment of pre‐diabetes.Support or Funding InformationResearch was supported by the NIH (DK067912, DK102233, and DK112917) to D.C.T and by the H.N. &amp; Frances C. Berger Foundation, and Helen &amp; Payson Chu Fellowships to K.E.MThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Obesity-induced protein carbonylation in murine adipose tissue regulates the DNA-binding domain of nuclear zinc finger proteins

In obesity-linked insulin resistance, oxidative stress in adipocytes leads to lipid peroxidation and subsequent carbonylation of proteins by diffusible lipid electrophiles. Reduction in oxidative stress attenuates protein carbonylation and insulin resistance, suggesting that lipid modification of proteins may play a role in metabolic disease, but the mechanisms remain incompletely understood. Herein, we show that in vivo, diet-induced obesity in mice surprisingly results in preferential carbonylation of nuclear proteins by 4-hydroxy-trans-2,3-nonenal (4-HNE) or 4-hydroxy-trans-2,3-hexenal (4-HHE). Proteomic and structural analyses revealed that residues in or around the sites of zinc coordination of zinc finger proteins, such as those containing the C2H2 or MATRIN, RING, C3H1, or N4-type DNA-binding domains, are particularly susceptible to carbonylation by lipid aldehydes. These observations strongly suggest that carbonylation functionally disrupts protein secondary structure supported by metal coordination. Analysis of one such target, the nuclear protein estrogen-related receptor γ (ERR-γ), showed that ERR-γ is modified by 4-HHE in the obese state. In vitro carbonylation decreased the DNA-binding capacity of ERR-γ and correlated with the obesity-linked down-regulation of many key genes promoting mitochondrial bioenergetics. Taken together, these findings reveal a novel mechanistic connection between oxidative stress and metabolic dysfunction arising from carbonylation of nuclear zinc finger proteins, such as the transcriptional regulator ERR-γ.

Sphingolipid changes do not underlie fatty acid-evoked GLUT4 insulin resistance nor inflammation signals in muscle cells[S

Ceramides contribute to obesity-linked insulin resistance and inflammation in vivo, but whether this is a cell-autonomous phenomenon is debated, particularly in muscle, which dictates whole-body glucose uptake. We comprehensively analyzed lipid species produced in response to fatty acids and examined the consequence to insulin resistance and pro-inflammatory pathways. L6 myotubes were incubated with BSA-adsorbed palmitate or palmitoleate in the presence of myriocin, fenretinide, or fumonisin B1. Lipid species were determined by lipidomic analysis. Insulin sensitivity was scored by Akt phosphorylation and glucose transporter 4 (GLUT4) translocation, while pro-inflammatory indices were estimated by IκBα degradation and cytokine expression. Palmitate, but not palmitoleate, had mild effects on Akt phosphorylation but significantly inhibited insulin-stimulated GLUT4 translocation and increased expression of pro-inflammatory cytokines Il6 and Ccl2 Ceramides, hexosylceramides, and sphingosine-1-phosphate significantly heightened by palmitate correlated negatively with insulin sensitivity and positively with pro-inflammatory indices. Inhibition of sphingolipid pathways led to marked changes in cellular lipids, but did not prevent palmitate-induced impairment of insulin-stimulated GLUT4 translocation, suggesting that palmitate-induced accumulation of deleterious lipids and insulin resistance are correlated but independent events in myotubes. We propose that muscle cell-endogenous ceramide production does not evoke insulin resistance and that deleterious effects of ceramides in vivo may arise through ancillary cell communication.

Hyperlipidemia-induced hepassocin in the liver contributes to insulin resistance in skeletal muscle

Hepassocin (HPS) has recently been identified as a novel hepatokine that causes hepatic steatosis. However, the role of HPS in the development of insulin resistance in skeletal muscle under obesity remains unclear. The effect of hyperlipidemia on hepatic HPS expression was evaluated in primary hepatocytes and liver of mice. HPS-mediated signal pathways were explored using small interfering (si) RNAs of specific genes or inhibitors. We found that treatment of primary hepatocytes with palmitate could induce HPS expression through C/EBPβ-mediated transcriptional activation. Furthermore, increased HPS expression was observed in the liver of high fat diet (HFD)-fed or tunicamycin-treated mice. Pretreatment with 4-phenylbutyrate (4-BPA) (an endoplasmic reticulum (ER) stress inhibitor) and suppression of p38 by siRNA abrogated the effect of palmitate on HPS expression in primary hepatocytes. Treatment of differentiated C2C12cells with recombinant HPS caused c-Jun N-terminal kinase (JNK) phosphorylation and impairment of insulin sensitivity in a dose-dependent manner. siRNA-mediated suppression of JNK reduced the effect of HPS on insulin signaling. Furthermore, the suppression of epidermal growth factor receptor (EGFR) by siRNA mitigated both HPS-induced JNK phosphorylation and insulin resistance. In addition, HPS did not affect inflammation and ER stress in differentiated C2C12cells. In conclusion, we elucidated that ER stress induced by palmitate could increase the expression of HPS in hepatocytes and further contribute to the development of insulin resistance in skeletal muscle via EGFR/JNK-mediated pathway. Taken together, we suggest that HPS could be a therapeutic target for obesity-linked insulin resistance.

ER phospholipid composition modulates lipogenesis during feeding and in obesity.

Sterol regulatory element-binding protein 1c (SREBP-1c) is a central regulator of lipogenesis whose activity is controlled by proteolytic cleavage. The metabolic factors that affect its processing are incompletely understood. Here, we show that dynamic changes in the acyl chain composition of ER phospholipids affect SREBP-1c maturation in physiology and disease. The abundance of polyunsaturated phosphatidylcholine in liver ER is selectively increased in response to feeding and in the setting of obesity-linked insulin resistance. Exogenous delivery of polyunsaturated phosphatidylcholine to ER accelerated SREBP-1c processing through a mechanism that required an intact SREBP cleavage-activating protein (SCAP) pathway. Furthermore, induction of the phospholipid-remodeling enzyme LPCAT3 in response to liver X receptor (LXR) activation promoted SREBP-1c processing by driving the incorporation of polyunsaturated fatty acids into ER. Conversely, LPCAT3 deficiency increased membrane saturation, reduced nuclear SREBP-1c abundance, and blunted the lipogenic response to feeding, LXR agonist treatment, or obesity-linked insulin resistance. Desaturation of the ER membrane may serve as an auxiliary signal of the fed state that promotes lipid synthesis in response to nutrient availability.

Improvement of obesity-linked skeletal muscle insulin resistance by strength and endurance training

Obesity-linked insulin resistance is mainly due to fatty acid overload in non-adipose tissues, particularly skeletal muscle and liver, where it results in high production of reactive oxygen species and mitochondrial dysfunction. Accumulating evidence indicates that resistance and endurance training alone and in combination can counteract the harmful effects of obesity increasing insulin sensitivity, thus preventing diabetes. This review focuses the mechanisms underlying the exercise role in opposing skeletal muscle insulin resistance-linked metabolic dysfunction. It is apparent that exercise acts through two mechanisms: (1) it stimulates glucose transport by activating an insulin-independent pathway and (2) it protects against mitochondrial dysfunction-induced insulin resistance by increasing muscle antioxidant defenses and mitochondrial biogenesis. However, antioxidant supplementation combined with endurance training increases glucose transport in insulin-resistant skeletal muscle in an additive fashion only when antioxidants that are able to increase the expression of antioxidant enzymes and/or the activity of components of the insulin signaling pathway are used.

416 - Mitochondrial Oxidative Stress Induced by Downregulation of Antioxidant Enzymes Leads to Nuclear Protein Carbonylation by Retrograde Signaling in 3T3-L1 Adipocytes

Obesity-linked insulin resistance is mechanistically connected to local inflammation of adipose tissue, which produces a metabolic state characterized by oxidative stress and mitochondrial dysfunction. Antioxidants enzymes such as Glutathione S-transferase A4 (GSTA4), peroxiredoxin 3 (Prdx3) and glutathione peroxidase 4 (GPx4) expressions are selectively downregulated in adipose tissue of obese insulin-resistant mice and in human obesity-linked insulin resistance. Also, TNFα treatment of 3T3-L1 adipocytes resulted in decreased expression of GSTA4, GPx4, and Prdx3 and increased protein carbonylation. In addition, protein carbonylation is implicated as an initiating factor in mitochondrial dysfunction and ER-stress, providing a mechanistic connection between oxidative stress and metabolic disease. Histones are the primary components of chromatin and are notably susceptible to carbonylation because of their long lysine-rich tails. These modifications may have effects on histone code, leading to long lasting implications of human health, including insulin resistance. In this study GSTA4-Prdx3-GPx4-silenced 3T3-L1 adipocytes were evaluated for reactive oxygen species production (ROS), mitochondrial function and histones carbonylation. Downregulation of GSTA4, Prdx3 and GPx4 led to an significant increase in ROS, a significant increase in H3 and H4 histones carbonylation, and mitochondrial dysfunction. These results indicate that a downregulation of antioxidant enzymes in adipocytes leads to increased ROS production, mitochondrial dysfunction and nuclear protein carbonylation, and may contribute to the development of insulin resistance and type 2 diabetes.