Exercise ameliorates adipose tissue insulin resistance by activating the lactate/GPR81 signaling pathway in DIO-IR mice.

Resulting from impaired collagen turnover, fibrosis is a hallmark of adipose tissue dysfunction and obesity-associated insulin resistance. Prolidase also known as Peptidase D (PEPD) plays a vital role in collagen turnover by degrading proline-containing dipeptides but its specific functional relevance in adipose tissue is unknown. Here we show that in human and murine obesity, PEPD expression and activity decrease in adipose tissue, and PEPD is released into the systemic circulation, which promotes fibrosis and adipose tissue insulin resistance. Loss of the enzymatic function of PEPD by genetic ablation or pharmacological inhibition causes adipose tissue fibrosis in mice. In addition to its intracellular enzymatic role, secreted extracellular PEPD protein enhances macrophage and adipocyte fibro-inflammatory responses via EGFR signaling, thereby promoting adipose tissue fibrosis and insulin resistance. We further show that decreased prolidase activity is coupled with increased systemic levels of PEPD that act as a pathogenic trigger of adipose tissue fibrosis and insulin resistance. Thus, PEPD produced by macrophages might serve as a biomarker of adipose tissue fibro-inflammation and could represent a therapeutic target for AT fibrosis and obesity-associated insulin resistance and type 2 diabetes.

Alteration of local adipose tissue trace element homeostasis as a possible mechanism of obesity-related insulin resistance

Apolipoprotein E (ApoE) deficiency is associated with reduced fat accumulation in white adipose tissue (WAT) and high liver triacylglycerol content. Elevated levels of endocannabinoids and cannabinoid receptor type 1 (CB(1)) receptors in the liver and in epididymal vs subcutaneous WAT are associated with fatty liver, visceral adipose tissue, inflammatory markers and insulin resistance. We investigated, in Apoe (-/-) and wild-type (WT) mice, the effect of a high-fat diet (HFD) on: (1) subcutaneous and epididymal WAT accumulation, liver triacylglycerols, phospholipid-esterified fatty acids, inflammatory markers in WAT and liver, and insulin resistance; and (2) endocannabinoid levels, and the gene expression levels of the Cb ( 1 ) receptor and endocannabinoid metabolic enzymes in liver and WAT. After a 16 week HFD, Apoe (-/-) mice exhibited lower body weight, WAT accumulation and fasting leptin, glucose and insulin levels, and higher hepatic steatosis, than WT mice. Glucose clearance and insulin-mediated glucose disposal following the HFD were slower in WT than Apoe (-/-) mice, which exhibited higher levels of mRNA encoding inflammatory markers (tumour necrosis factor-α [TNF-α], monocyte chemoattractant protein-1 [MCP-1], cluster of differentiation 68 [CD68] and EGF-like module-containing mucin-like hormone receptor-like 1 [EMR1]) in the liver, but lower levels in epididymal WAT. HFD-induced elevation of endocannabinoid levels in the liver or epididymal WAT was higher or lower, respectively, in Apoe (-/-) mice, whereas HFD-induced decrease of subcutaneous WAT endocannabinoid and CB(1) receptor levels was significantly less marked. Alterations in endocannabinoid levels reflected changes in endocannabinoid catabolic enzymes in WAT, or the availability of phospholipid precursors in the liver. Liver and adipose tissue endocannabinoid tone following an HFD is altered on Apoe deletion and strongly associated with inflammation, insulin resistance and hepatic steatosis, or lack thereof.

AbatractObesity is known to be associated with adipose tissue inflammation and insulin resistance. Importantly, in obesity, the accumulation of proinflammatory macrophages in adipose tissue correlates with insulin resistance. We hypothesized that the receptor for advanced glycation end products (RAGE) and associated ligands are involved in adipose tissue insulin resistance, and that the activation of the AGE–RAGE axis plays an important role in obesity-associated inflammation. C57BL/6J mice (WT) and RAGE deficient (RAGE−/−) mice were fed a high fat diet (HFD) and subjected to glucose and insulin tolerance tests. Epdidymal adipose tissue (eAT) was collected and adipose stromal vascular cells isolated using flow cytometry. Visceral adipose tissue macrophage polarization was assessed by quantitative real time PCR. Immunoblotting was performed to evaluate the insulin signaling in adipose tissues. In additional studies, cell trafficking was assessed by injecting labeled blood monocytes into recipient mice. RAGE−/− mice displayed improved insulin sensitivity and glucose tolerance, accompanied by decreased body weight and eAT mass. Exogenous methylglyoxal (MGO) impaired insulin-stimulated AKT signaling in adipose tissues from WT mice fed a normal chow diet, but not in RAGE−/− mice. In contrast, in obese mice, treatment with MGO did not reduce insulin-induced phosphorylation of AKT in WT-HFD mice. Moreover, insulin-induced AKT phosphorylation was found to be impaired in adipose tissue from RAGE−/−-HFD mice. RAGE−/− mice displayed improved inflammatory profiles and evidence for increased adipose tissue browning. This observation is consistent with the finding of reduced plasma levels of FFA, glycerol, IL-6, and leptin in RAGE−/− mice compared to WT mice. Collectively the data demonstrate that RAGE-mediated adipose tissue inflammation and insulin-signaling are potentially important mechanisms that contribute to the development of obesity-associated insulin resistance.

Luteolin reduces adipose tissue macrophage inflammation and insulin resistance in postmenopausal obese mice

Adiposity is commonly associated with adipose tissue dysfunction and many overnutrition-related metabolic diseases including type 2 diabetes. Much attention has been paid to reducing adiposity as a way to improve adipose tissue function and systemic insulin sensitivity. PFKFB3/iPFK2 is a master regulator of adipocyte nutrient metabolism. Using PFKFB3(+/-) mice, the present study investigated the role of PFKFB3/iPFK2 in regulating diet-induced adiposity and systemic insulin resistance. On a high-fat diet (HFD), PFKFB3(+/-) mice gained much less body weight than did wild-type littermates. This was attributed to a smaller increase in adiposity in PFKFB3(+/-) mice than in wild-type controls. However, HFD-induced systemic insulin resistance was more severe in PFKFB3(+/-) mice than in wild-type littermates. Compared with wild-type littermates, PFKFB3(+/-) mice exhibited increased severity of HFD-induced adipose tissue dysfunction, as evidenced by increased adipose tissue lipolysis, inappropriate adipokine expression, and decreased insulin signaling, as well as increased levels of proinflammatory cytokines in both isolated adipose tissue macrophages and adipocytes. In an in vitro system, knockdown of PFKFB3/iPFK2 in 3T3-L1 adipocytes caused a decrease in the rate of glucose incorporation into lipid but an increase in the production of reactive oxygen species. Furthermore, knockdown of PFKFB3/iPFK2 in 3T3-L1 adipocytes inappropriately altered the expression of adipokines, decreased insulin signaling, increased the phosphorylation states of JNK and NFkappaB p65, and enhanced the production of proinflammatory cytokines. Together, these data suggest that PFKFB3/iPFK2, although contributing to adiposity, protects against diet-induced insulin resistance and adipose tissue inflammatory response.

Induction of Yin Yang 1 (YY1) overexpression in mature adipocytes promotes dysfunctional adipose tissue and systemic insulin resistance in mice.

Macrophage HIF-2α ameliorates adipose tissue inflammation and insulin resistance in obesity.

Contribution of Abdominal Visceral Obesity and Insulin Resistance to the Cardiovascular Risk Profile of Postmenopausal Women

Semaphorin3E-Induced Inflammation Contributes to Insulin Resistance in Dietary Obesity

Luteolin Improves Insulin Resistance in Postmenopausal Obese Mice by Altering Macrophage Polarization (FS12-01-19)

Reverse causality has made it difficult to establish the causal directions between obesity and prediabetes and obesity and insulin resistance. To disentangle whether obesity causally drives prediabetes and insulin resistance already in non-diabetic individuals, we utilized the UK Biobank and METSIM cohort to perform a Mendelian randomization (MR) analyses in the non-diabetic individuals. Our results suggest that both prediabetes and systemic insulin resistance are caused by obesity (p = 1.2×10−3 and p = 3.1×10−24). As obesity reflects the amount of body fat, we next studied how adipose tissue affects insulin resistance. We performed both bulk RNA-sequencing and single nucleus RNA sequencing on frozen human subcutaneous adipose biopsies to assess adipose cell-type heterogeneity and mitochondrial (MT) gene expression in insulin resistance. We discovered that the adipose MT gene expression and body fat percent are both independently associated with insulin resistance (p≤0.05 for each) when adjusting for the decomposed adipose cell-type proportions. Next, we showed that these 3 factors, adipose MT gene expression, body fat percent, and adipose cell types, explain a substantial amount (44.39%) of variance in insulin resistance and can be used to predict it (p≤2.64×10−5 in 3 independent human cohorts). In summary, we demonstrated that obesity is a strong determinant of both prediabetes and insulin resistance, and discovered that individuals’ adipose cell-type composition, adipose MT gene expression, and body fat percent predict their insulin resistance, emphasizing the critical role of adipose tissue in systemic insulin resistance.

Reverse causality has made it difficult to establish the causal directions between obesity and prediabetes and obesity and insulin resistance. To disentangle whether obesity causally drives prediabetes and insulin resistance already in non-diabetic individuals, we utilized the UK Biobank and METSIM cohort to perform a Mendelian randomization (MR) analyses in the non-diabetic individuals. Our results suggest that both prediabetes and systemic insulin resistance are caused by obesity (p = 1.2×10-3 and p = 3.1×10-24). As obesity reflects the amount of body fat, we next studied how adipose tissue affects insulin resistance. We performed both bulk RNA-sequencing and single nucleus RNA sequencing on frozen human subcutaneous adipose biopsies to assess adipose cell-type heterogeneity and mitochondrial (MT) gene expression in insulin resistance. We discovered that the adipose MT gene expression and body fat percent are both independently associated with insulin resistance (p≤0.05 for each) when adjusting for the decomposed adipose cell-type proportions. Next, we showed that these 3 factors, adipose MT gene expression, body fat percent, and adipose cell types, explain a substantial amount (44.39%) of variance in insulin resistance and can be used to predict it (p≤2.64×10-5 in 3 independent human cohorts). In summary, we demonstrated that obesity is a strong determinant of both prediabetes and insulin resistance, and discovered that individuals' adipose cell-type composition, adipose MT gene expression, and body fat percent predict their insulin resistance, emphasizing the critical role of adipose tissue in systemic insulin resistance.

The rising number of obese individuals has become a major burden to the healthcare systems worldwide. Obesity includes not only the increase of adipose tissue mass but importantly also the altered cellular functions that collectively lead to a chronic state of adipose tissue inflammation, insulin resistance and impaired wound healing. Adipose tissue undergoing chronic inflammation shows altered cytokine expression and an accumulation of adipose tissue macrophages (ATM). The macrophage migration inhibitory factor (MIF) superfamily consists of MIF and the recently identified homolog D-dopachrome tautomerase (D-DT or MIF-2). MIF and D-DT, which both bind to the CD74/CD44 receptor complex, are differentially expressed in adipose tissue and have distinct roles in adipogenesis. MIF positively correlates with obesity as well as insulin resistance and contributes to adipose tissue inflammation by modulating ATM functions. D-DT, however, is negatively correlated with obesity and reverses glucose intolerance. In this review, their respective roles in adipose tissue homeostasis, adipose tissue inflammation, insulin resistance and impaired wound healing will be reviewed.

Acute-phase serum amyloid A (A-SAA) was shown recently to correlate with obesity and insulin resistance in humans. However, the mechanisms linking obesity-associated inflammation and elevated plasma A-SAA to insulin resistance are poorly understood. Using high-fat diet- (HFD-) fed mice, we found that plasma A-SAA was increased early upon HFD feeding and was tightly associated with systemic insulin resistance. Plasma A-SAA elevation was due to induction of Saa1 and Saa2 expression in liver but not in adipose tissue. In adipose tissue Saa3 was the predominant isoform and the earliest inflammatory marker induced, suggesting it is important for initiation of adipose tissue inflammation. To assess the potential impact of A-SAA on adipose tissue insulin resistance, we treated 3T3-L1 adipocytes with recombinant A-SAA. Intriguingly, physiological levels of A-SAA caused alterations in gene expression closely resembling those observed in HFD-fed mice. Proinflammatory genes (Ccl2, Saa3) were induced while genes critical for insulin sensitivity (Irs1, Adipoq, Glut4) were down-regulated. Our data identify HFD-fed mice as a suitable model to study A-SAA as a biomarker and a novel possible mediator of insulin resistance.