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In this Perspective, Michael Czech presents evidence for whether hyperinsulinemia occurs before insulin resistance upon overfeeding or high-fat diet feeding, or whether insulin resistance causes hyperinsulinemia, thus attempting to delineate the relationship between hyperinsulinemia, obesity and insulin resistance. Nutritional excess is a major forerunner of type 2 diabetes. It enhances the secretion of insulin, but attenuates insulin's metabolic actions in the liver, skeletal muscle and adipose tissue. However, conflicting evidence indicates a lack of knowledge of the timing of these events during the development of obesity and diabetes, pointing to a key gap in our understanding of metabolic disease. This Perspective reviews alternate viewpoints and recent results on the temporal and mechanistic connections between hyperinsulinemia, obesity and insulin resistance. Although much attention has addressed early steps in the insulin signaling cascade, insulin resistance in obesity seems to be largely elicited downstream of these steps. New findings also connect insulin resistance to extensive metabolic cross-talk between the liver, adipose tissue, pancreas and skeletal muscle. These and other advances over the past 5 years offer exciting opportunities and daunting challenges for the development of new therapeutic strategies for the treatment of type 2 diabetes.

Adipose tissue comprises adipocytes and many other cell types that engage in dynamic crosstalk in a highly innervated and vascularized tissue matrix. Although adipose tissue has been studied for decades, it has been appreciated only in the past 5 years that extensive arborization of nerve fibres has a dominant role in regulating the function of adipose tissue. This Review summarizes the latest literature, which suggests that adipocytes signal to local sensory nerve fibres in response to perturbations in lipolysis and lipogenesis. Such adipocyte signalling to the central nervous system causes sympathetic output to distant adipose depots and potentially other metabolic tissues to regulate systemic glucose homeostasis. Paracrine factors identified in the past few years that mediate such adipocyte–neuron crosstalk are also reviewed. Similarly, immune cells and endothelial cells within adipose tissue communicate with local nerve fibres to modulate neurotransmitter tone, blood flow, adipocyte differentiation and energy expenditure, including adipose browning to produce heat. This understudied field of neurometabolism related to adipose tissue biology has great potential to reveal new mechanistic insights and potential therapeutic strategies for obesity and type 2 diabetes mellitus.This Review examines the signalling between cells resident in adipose tissue and local sensory and sympathetic nerve fibres. The potential of targeting these processes in new therapeutics for metabolic diseases is also discussed.

Hepatocytes secrete DPP4, which promotes adipose tissue inflammation and insulin resistance in obese mice, suggesting a new specific target for treatment of metabolic disorders. Liver enzyme DPP4 inflames fatty tissue in obese mice Previous studies have shown that liver secretory factors cause insulin resistance in muscle and impair the ability of pancreatic beta cells to secrete insulin. However, the role of hepatokines in promoting adipose pathobiology in obesity is not well understood. In this paper, Ira Tabas and colleagues show that obesity promotes the synthesis and secretion of hepatocyte dipeptidyl peptidase 4 (DPP4), which acts together with plasma factor Xa to inflame visceral adipose tissue macrophages. Silencing hepatocyte DPP4 improved metabolism in obese mice, suggesting that DPP4 may contribute to insulin resistance and systemic metabolic disease associated with obesity. Obesity-induced metabolic disease involves functional integration among several organs via circulating factors, but little is known about crosstalk between liver and visceral adipose tissue (VAT) 1 . In obesity, VAT becomes populated with inflammatory adipose tissue macrophages (ATMs) 2 , 3 . In obese humans, there is a close correlation between adipose tissue inflammation and insulin resistance 4 , 5 , and in obese mice, blocking systemic or ATM inflammation improves insulin sensitivity 6 , 7 , 8 . However, processes that promote pathological adipose tissue inflammation in obesity are incompletely understood. Here we show that obesity in mice stimulates hepatocytes to synthesize and secrete dipeptidyl peptidase 4 (DPP4), which acts with plasma factor Xa to inflame ATMs. Silencing expression of DPP4 in hepatocytes suppresses inflammation of VAT and insulin resistance; however, a similar effect is not seen with the orally administered DPP4 inhibitor sitagliptin. Inflammation and insulin resistance are also suppressed by silencing expression of caveolin-1 or PAR2 in ATMs; these proteins mediate the actions of DPP4 and factor Xa, respectively. Thus, hepatocyte DPP4 promotes VAT inflammation and insulin resistance in obesity, and targeting this pathway may have metabolic benefits that are distinct from those observed with oral DPP4 inhibitors.

Key Points Insulin resistance due to obesity is a primary event in the development of type 2 diabetes. Recent work indicates a central role of adipose tissue dysfunction in linking obesity to insulin resistance. Fatty acids and their derivatives are mediators of insulin resistance in skeletal muscle, probably through deleterious effects on the insulin signalling pathway. Increased exposure of skeletal muscle tissue to elevated fatty acids in rodents and humans impairs insulin-stimulated glucose uptake. The impaired ability of adipose tissue to sequester fatty acids in triglyceride stores results in increasing fatty acid concentrations in the circulation and the exposure of skeletal muscle to these high fatty acid levels. This impairment of adipose function can be caused by a chronic inflammatory state that arises within adipose tissue in obese animals and humans. Inflammatory cytokines, including tumour-necrosis factor-α (TNFα), have profound effects on adipocyte metabolism by impairing triglyceride synthesis and storage, and promoting the hydrolysis and release of triglycerides as free fatty acids. These effects are mediated in part through downregulation of the key adipocyte transcription factor PPARγ (peroxisome proliferator-activated receptor-γ). Recent data implicate lipid-droplet proteins, including newly described CIDE family proteins, in the promotion of triglyceride storage and as significant targets of PPARγ regulation. Through effects on these proteins, downregulation of PPARγ can mediate diminished lipid storage ability of inflamed adipose tissue. Expression of these proteins may be determinants of the differential ability to sequester fat away from the circulation and the propensity of obese humans to develop insulin resistance. Adipose tissue controls whole-body lipid flux, thereby modulating both glucose and lipid homeostasis in humans. Discovery of new targets that regulate fatty acids in adipocytes might lead to therapeutic modalities that can prevent insulin resistance and type 2 diabetes. Acquired resistance to the action of insulin to stimulate glucose transport in skeletal muscle is associated with obesity and promotes the development of type 2 diabetes. In skeletal muscle, insulin resistance can result from high levels of circulating fatty acids that disrupt insulin signalling pathways. However, the severity of insulin resistance varies greatly among obese people. Here we postulate that this variability might reflect differences in levels of lipid-droplet proteins that promote the sequestration of fatty acids within adipocytes in the form of triglycerides, thereby lowering exposure of skeletal muscle to the inhibitory effects of fatty acids.

George Kunos and his colleagues show in a rat model that endocannabinoid activation of the Nlrp3 inflammasome in macrophages results in death of pancreatic beta cells, and thus development of type 2 diabetes mellitus. These results suggest that preventing this process might be a therapeutic option for diabetes in the clinic. Type 2 diabetes mellitus (T2DM) progresses from compensated insulin resistance to beta cell failure resulting in uncompensated hyperglycemia, a process replicated in the Zucker diabetic fatty (ZDF) rat. The Nlrp3 inflammasome has been implicated in obesity-induced insulin resistance and beta cell failure. Endocannabinoids contribute to insulin resistance through activation of peripheral CB 1 receptors (CB 1 Rs) and also promote beta cell failure. Here we show that beta cell failure in adult ZDF rats is not associated with CB 1 R signaling in beta cells, but rather in M1 macrophages infiltrating into pancreatic islets, and that this leads to activation of the Nlrp3-ASC inflammasome in the macrophages. These effects are replicated in vitro by incubating wild-type human or rodent macrophages, but not macrophages from CB 1 R-deficient ( Cnr1 −/− ) or Nlrp3 −/− mice, with the endocannabinoid anandamide. Peripheral CB 1 R blockade, in vivo depletion of macrophages or macrophage-specific knockdown of CB 1 R reverses or prevents these changes and restores normoglycemia and glucose-induced insulin secretion. These findings implicate endocannabinoids and inflammasome activation in beta cell failure and identify macrophage-expressed CB 1 R as a therapeutic target in T2DM.

Adipocytes robustly synthesize fatty acids (FA) from carbohydrate through the de novo lipogenesis (DNL) pathway, yet surprisingly DNL contributes little to their abundant triglyceride stored in lipid droplets. This conundrum raises the hypothesis that adipocyte DNL instead enables membrane expansions to occur in processes like autophagy, which requires an abundant supply of phospholipids. We report here that adipocyte Fasn deficiency in vitro and in vivo markedly impairs autophagy, evident by autophagosome accumulation and severely compromised degradation of the autophagic substrate p62. Our data indicate the impairment occurs at the level of autophagosome-lysosome fusion, and indeed, loss of Fasn decreases certain membrane phosphoinositides necessary for autophagosome and lysosome maturation and fusion. Autophagy dependence on FA produced by Fasn is not fully alleviated by exogenous FA in cultured adipocytes, and interestingly, imaging studies reveal that Fasn colocalizes with nascent autophagosomes. Together, our studies identify DNL as a critical source of FAs to fuel autophagosome and lysosome maturation and fusion in adipocytes. The function of de novo lipogenesis (DNL) in adipocytes has been a mystery as it contributes little to fat storage in these cells. Here, the authors show that DNL is a critical source of fatty acids for membrane-expanding processes like autophagy.

Obesity and type 2 diabetes are associated with disturbances in insulin-regulated glucose and lipid fluxes and severe comorbidities including cardiovascular disease and steatohepatitis. Whole body metabolism is regulated by lipid-storing white adipocytes as well as “brown” and “brite/beige” adipocytes that express thermogenic uncoupling protein 1 (UCP1) and secrete factors favorable to metabolic health. Implantation of brown fat into obese mice improves glucose tolerance, but translation to humans has been stymied by low abundance of primary human beige adipocytes. Here we apply methods to greatly expand human adipocyte progenitors from small samples of human subcutaneous adipose tissue and then disrupt the thermogenic suppressor gene NRIP1 by CRISPR. Ribonucleoprotein consisting of Cas9 and sgRNA delivered ex vivo are fully degraded by the human cells following high efficiency NRIP1 depletion without detectable off-target editing. Implantation of such CRISPR-enhanced human or mouse brown-like adipocytes into high fat diet fed mice decreases adiposity and liver triglycerides while enhancing glucose tolerance compared to implantation with unmodified adipocytes. These findings advance a therapeutic strategy to improve metabolic homeostasis through CRISPR-based genetic enhancement of human adipocytes without exposing the recipient to immunogenic Cas9 or delivery vectors. Worldwide pandemics of obesity and diabetes prompt an urgent need for new approaches to their prevention and cure. Here the authors present a CRISPR-based strategy that enhances the therapeutic potential of human adipocytes when implanted in obese mice.

Non-alcoholic fatty liver disease is prevalent in human obesity and type 2 diabetes, and is characterized by increases in both hepatic triglyceride accumulation (denoted as steatosis) and expression of pro-inflammatory cytokines such as IL-1β. We report here that the development of hepatic steatosis requires IL-1 signaling, which upregulates Fatty acid synthase to promote hepatic lipogenesis. Using clodronate liposomes to selectively deplete liver Kupffer cells in ob/ob mice, we observed remarkable amelioration of obesity-induced hepatic steatosis and reductions in liver weight, triglyceride content and lipogenic enzyme expressions. Similar results were obtained with diet-induced obese mice, although visceral adipose tissue macrophage depletion also occurred in response to clodronate liposomes in this model. There were no differences in the food intake, whole body metabolic parameters, serum β-hydroxybutyrate levels or lipid profiles due to clodronate-treatment, but hepatic cytokine gene expressions including IL-1β were decreased. Conversely, treatment of primary mouse hepatocytes with IL-1β significantly increased triglyceride accumulation and Fatty acid synthase expression. Furthermore, the administration of IL-1 receptor antagonist to obese mice markedly reduced obesity-induced steatosis and hepatic lipogenic gene expression. Collectively, our findings suggest that IL-1β signaling upregulates hepatic lipogenesis in obesity, and is essential for the induction of pathogenic hepatic steatosis in obese mice.

Adipose tissue (AT) inflammation and infiltration by macrophages is associated with insulin resistance and type 2 diabetes in obese humans, offering a potential target for therapeutics. However, whether AT macrophages (ATMs) directly contribute to systemic glucose intolerance has not been determined. The reason is the lack of methods to ablate inflammatory genes expressed in macrophages specifically localized within AT depots, leaving macrophages in other tissues unaffected. Here we report that i.p. administration of siRNA encapsulated by glucan shells in obese mice selectively silences genes in epididymal ATMs, whereas macrophages within lung, spleen, kidney, heart, skeletal muscle, subcutaneous (SubQ) adipose, and liver are not targeted. Such administration of GeRPs to silence the inflammatory cytokines TNF-α or osteopontin in epididymal ATMs of obese mice caused significant improvement in glucose tolerance. These data are consistent with the hypothesis that cytokines produced by ATMs can exacerbate whole-body glucose intolerance.

Signalling pathways that control endothelial cell (EC) permeability, leukocyte adhesion and inflammation are pivotal for atherosclerosis initiation and progression. Here we demonstrate that the Sterile-20-like mitogen-activated protein kinase kinase kinase kinase 4 (MAP4K4), which has been implicated in inflammation, is abundantly expressed in ECs and in atherosclerotic plaques from mice and humans. On the basis of endothelial-specific MAP4K4 gene silencing and gene ablation experiments in Apoe −/− mice, we show that MAP4K4 in ECs markedly promotes Western diet-induced aortic macrophage accumulation and atherosclerotic plaque development. Treatment of Apoe −/− and Ldlr −/− mice with a selective small-molecule MAP4K4 inhibitor also markedly reduces atherosclerotic lesion area. MAP4K4 silencing in cultured ECs attenuates cell surface adhesion molecule expression while reducing nuclear localization and activity of NFκB, which is critical for promoting EC activation and atherosclerosis. Taken together, these results reveal that MAP4K4 is a key signalling node that promotes immune cell recruitment in atherosclerosis. Atherosclerosis is an inflammatory disease with limited therapeutic options. Here, the authors show that protein kinase MAP4K4 regulates vascular inflammation underlying atherosclerotic plaque development and that its inhibition prevents the disease and promotes lesion regression in mice, proposing a new atherosclerosis treatment.