Mechanism of insulin resistance in A-ZIP/F-1 fatless mice

Understanding the cause of type 2 diabetes

Type 2 diabetes has long been thought to have heterogenous causes, even though epidemiological studies uniformly show a tight relationship with overnutrition. The twin cycle hypothesis postulated that interaction of self-reinforcing cycles of fat accumulation inside the liver and pancreas, driven by modest but chronic positive calorie balance, could explain the development of type 2 diabetes. This hypothesis predicted that substantial weight loss would bring about a return to the non-diabetic state, permitting observation of the pathophysiology determining the transition. These changes were postulated to reflect the basic mechanisms of causation in reverse. A series of studies over the past 15 years has elucidated these underlying mechanisms. Together with other research, the interaction of environmental and genetic factors has been clarified. This knowledge has led to successful implementation of a national programme for remission of type 2 diabetes. This Review discusses the paucity of evidence for heterogeneity in causes of type 2 diabetes and summarises the in vivo pathophysiological changes, which cause this disease of overnutrition. Type 2 diabetes has a homogenous cause expressed in genetically heterogenous individuals.

Leptin replacement therapy in the management of lipodystrophy syndromes

Lipodystrophy syndromes are rare diseases of genetic or acquired origin, characterized by quantitative and qualitative defects in adipose tissue. The metabolic consequences of lipodystrophy syndromes, such as insulin resistant diabetes, hypertriglyceridemia and hepatic steatosis, are frequently very difficult to treat, resulting in significant risks of acute and/or chronic complications and of decreased quality of life. The production of leptin by lipodystrophic adipose tissue is decreased, more severely in generalized forms of lipodystrophy, where adipose tissue is absent from almost all body fat depots, than in partial forms of the disease, where lipoatrophy affects only some parts of the body and can be associated with increased body fat in other anatomical regions. Several lines of evidence in preclinical and clinical models have shown that leptin replacement therapy could improve the metabolic complications of lipodystrophy syndromes. Metreleptin, a recombinant leptin analogue, was approved as an orphan drug to treat the metabolic complications of leptin deficiency in patients with generalized lipodystrophy in the USA or with either generalized or partial lipodystrophy in Japan and Europe. In this brief review, we will discuss the benefits and limitations of this therapy, and the new expectations arising from the recent development of a therapeutic monoclonal antibody able to activate the leptin receptor.

Overnutrition, Hyperinsulinemia and Ectopic Fat: It Is Time for A Paradigm Shift in the Management of Type 2 Diabetes

The worldwide incidence of prediabetes/type 2 has continued to rise the last 40 years. In the same period, the mean daily energy intake has increased, and the quality of food has significantly changed. The chronic exposure of pancreatic β-cells to calorie excess (excessive energy intake) and food additives may increase pancreatic insulin secretion, decrease insulin pulses and/or reduce hepatic insulin clearance, thereby causing chronic hyperinsulinemia and peripheral insulin resistance. Chronic calorie excess and hyperinsulinemia may promote lipogenesis, inhibit lipolysis and increase lipid storage in adipocytes. In addition, calorie excess and hyperinsulinemia can induce insulin resistance and contribute to progressive and excessive ectopic fat accumulation in the liver and pancreas by the conversion of excess calories into fat. The personal fat threshold hypothesis proposes that in susceptible individuals, excessive ectopic fat accumulation may eventually lead to hepatic insulin receptor resistance, the loss of pancreatic insulin secretion, hyperglycemia and the development of frank type 2 diabetes. Thus, type 2 diabetes seems (partly) to be caused by hyperinsulinemia-induced excess ectopic fat accumulation in the liver and pancreas. Increasing evidence further shows that interventions (hypocaloric diet and/or bariatric surgery), which remove ectopic fat in the liver and pancreas by introducing a negative energy balance, can normalize insulin secretion and glucose tolerance and induce the sustained biochemical remission of type 2 diabetes. This pathophysiological insight may have major implications and may cause a paradigm shift in the management of type 2 diabetes: avoiding/reducing ectopic fat accumulation in the liver and pancreas may both be essential to prevent and cure type 2 diabetes.

Adipose transplantation improves metabolism and atherosclerosis but not perivascular adipose tissue abnormality or vascular dysfunction in lipodystrophic Seipin/Apoe null mice

Adipose dysfunction in lipodystrophic SEIPIN deficiency is associated with multiple metabolic disorders and increased risks of developing cardiovascular diseases, such as atherosclerosis, cardiac hypertrophy, and heart failure. Recently, adipose transplantation has been found to correct adipose dysfunction and metabolic disorders in lipodystrophic Seipin knockout mice; however, whether adipose transplantation could improve lipodystrophy-associated cardiovascular consequences is still unclear. Here, we aimed to explore the effects of adipose transplantation on lipodystrophy-associated metabolic cardiovascular diseases in Seipin knockout mice crossed into atherosclerosis-prone apolipoprotein E (Apoe) knockout background. At 2 months of age, lipodystrophic Seipin/Apoe double knockout mice and nonlipodystrophic Apoe knockout controls were subjected to adipose transplantation or sham operation. Seven months later, mice were euthanized. Our data showed that although adipose transplantation had no significant impact on endogenous adipose atrophy or gene expression, it remarkably increased plasma leptin but not adiponectin concentration in Seipin/Apoe double knockout mice. This led to significantly reduced hyperlipidemia, hepatic steatosis, and insulin resistance in Seipin/Apoe double knockout mice. Consequently, atherosclerosis burden, intraplaque macrophage infiltration, and aortic inflammatory gene expression were all attenuated in Seipin/Apoe double knockout mice with adipose transplantation. However, adipocyte morphology, macrophage infiltration, or fibrosis of the perivascular adipose tissue was not altered in Seipin/Apoe double knockout mice with adipose transplantation, followed by no significant improvement of vasoconstriction or relaxation. In conclusion, we demonstrate that adipose transplantation could alleviate lipodystrophy-associated metabolic disorders and atherosclerosis but has an almost null impact on perivascular adipose abnormality or vascular dysfunction in lipodystrophic Seipin/Apoe double knockout mice.NEW & NOTEWORTHY Adipose transplantation (AT) reverses multiply metabolic derangements in lipodystrophy, but whether it could improve lipodystrophy-related cardiovascular consequences is unknown. Here, using Seipin/Apoe double knockout mice as a lipodystrophy disease model, we showed that AT partially restored adipose functionality, which translated into significantly reduced atherosclerosis. However, AT was incapable of reversing perivascular adipose abnormality or vascular dysfunction. The current study provides preliminary experimental evidence on the therapeutic potential of AT on lipodystrophy-related metabolic cardiovascular diseases.

Bioelectrical Impedance Analysis Detects Body Fat Changes After Surgical Subcutaneous Fat Removal

Background: The risk and metabolic effects of obesity are determined by the distribution of fat throughout the body. It has been proposed that the distribution of abdominal fat is more closely related to the metabolic risks of obesity. High prevalence of overweight and obesity has thereby contributed to an increased uptake of surgical subcutaneous fat removal (SSFR) procedures. The goal of this study was to determine whether bioelectrical impedance analysis (Tanita system) can be used to detect the removal of excess abdominal subcutaneous fat tissue during SSFR when studying the metabolic effects of such procedures. Methods: Study population comprised patients who received body contouring procedures at the Hamad General Hospital's plastic surgery department between November 2020 and December 2022. To evaluate the factors of interest, subjects were prospectively followed up at two time points: within 1 week before the surgery and within 1-2 weeks thereafter. The following factors were measured: body weight, body fat percentage, body fat mass, body mass index (BMI), fat-free mass, estimated muscle mass, total body water, visceral fat score, and basal metabolic rate. Results: In total, 22 patients were included in the study. The two visits' medians for height, weight, BMI, fat percent (fat%), fat mass, visceral fat rating, and Doi's weighted average glucose (dwAG) were compared. Only in the case of Tanita fat% and fat mass, were the preoperative and postoperative medians significantly different. Furthermore, there was no association between these Tanita measures and dwAG or homeostatic model assessment (HOMA; insulin resistance [IR]) changes (before and after surgery). Tanita measures overestimated fat loss, as seen by the mountain plot and Bland-Altman plot agreement methods. Conclusions: Our findings indicated that the only two Tanita measures exhibited meaningful early associations with the amount of tissue excised which were fat mass and fat% differences. Although dwAG and HOMA-IR are not impacted immediately postsurgery, a trend was seen that suggested improvements in those parameters, even though the changes are not clinically significant.

O-linked N-acetylglucosamine modification is essential for physiological adipose expansion induced by high-fat feeding

Adipose tissues accumulate excess energy as fat and heavily influence metabolic homeostasis. O-linked N-acetylglucosamine (O-GlcNAc) modification (O-GlcNAcylation), which involves the addition of N-acetylglucosamine to proteins by O-GlcNAc transferase (Ogt), modulates multiple cellular processes. However, little is known about the role of O-GlcNAcylation in adipose tissues during body weight gain due to overnutrition. Here, we report on O-GlcNAcylation in mice with high-fat diet (HFD)-induced obesity. Mice with knockout of Ogt in adipose tissue achieved using adiponectin promoter-driven Cre recombinase (Ogt-FKO) gained less body weight than control mice under HFD. Surprisingly, Ogt-FKO mice exhibited glucose intolerance and insulin resistance, despite their reduced body weight gain, as well as decreased expression of de novo lipogenesis genes and increased expression of inflammatory genes, resulting in fibrosis at 24 weeks of age. Primary cultured adipocytes derived from Ogt-FKO mice showed decreased lipid accumulation. Both primary cultured adipocytes and 3T3-L1 adipocytes treated with OGT inhibitor showed increased secretion of free fatty acids. Medium derived from these adipocytes stimulated inflammatory genes in RAW 264.7 macrophages, suggesting that cell-to-cell communication via free fatty acids might be a cause of adipose inflammation in Ogt-FKO mice. In conclusion, O-GlcNAcylation is important for healthy adipose expansion in mice. Glucose flux into adipose tissues may be a signal to store excess energy as fat.NEW & NOTEWORTHY We evaluated the role of O-GlcNAcylation in adipose tissue in diet-induced obesity using adipose tissue-specific Ogt knockout mice. We found that O-GlcNAcylation in adipose tissue is essential for healthy fat expansion and that Ogt-FKO mice exhibit severe fibrosis upon long-term overnutrition. O-GlcNAcylation in adipose tissue may regulate de novo lipogenesis and free fatty acid efflux to the degree of overnutrition. We believe that these results provide new insights into adipose tissue physiology and obesity research.

Achievements, prospects and challenges in precision care for monogenic insulin-deficient and insulin-resistant diabetes

Integration of genomic and other data has begun to stratify type 2 diabetes in prognostically meaningful ways, but this has yet to impact on mainstream diabetes practice. The subgroup of diabetes caused by single gene defects thus provides the best example to date of the vision of 'precision diabetes'. Monogenic diabetes may be divided into primary pancreatic beta cell failure, and primary insulin resistance. In both groups, clear examples of genotype-selective responses to therapy have been advanced. The benign trajectory of diabetes due to pathogenic GCK mutations, and the sulfonylurea-hyperresponsiveness conferred by activating KCNJ11 or ABCC8 mutations, or loss-of-function HNF1A or HNF4A mutations, often decisively guide clinical management. In monogenic insulin-resistant diabetes, subcutaneous leptin therapy is beneficial in some severe lipodystrophy. Increasing evidence also supports use of 'obesity therapies' in lipodystrophic people even without obesity. In beta cell diabetes the main challenge is now implementation of the precision diabetes vision at scale. In monogenic insulin-resistant diabetes genotype-specific benefits are proven in far fewer patients to date, although further genotype-targeted therapies are being evaluated. The conceptual paradigm established by the insulin-resistant subgroup with 'adipose failure' may have a wider influence on precision therapy for common type 2 diabetes, however. For all forms of monogenic diabetes, population-wide genome sequencing is currently forcing reappraisal of the importance assigned to pathogenic mutations when gene sequencing is uncoupled from prior suspicion of monogenic diabetes.

Reducing ether lipids improves Drosophila overnutrition-associated pathophysiology phenotypes via a switch from lipid storage to beta-oxidation

High-calorie diets increase the risk of developing obesity, cardiovascular disease, type-two diabetes (T2D), and other comorbidities. These "overnutrition" diets also promote the accumulation of a variety of harmful lipids in the heart and other peripheral organs, known as lipotoxicity. However, the mechanisms underlying lipotoxicity and its influence on pathophysiology remain unknown. Our study uses genetics to identify the role of ether lipids, a class of potential lipotoxins, in a Drosophila model of overnutrition. A high-sugar diet (HSD) increases ether lipids and produces T2D-like pathophysiology phenotypes, including obesity, insulin resistance, and cardiac failure. Therefore, we targeted ether lipid biosynthesis through the enzyme dihydroxyacetonephosphate acyltransferase (encoded by the gene DHAPAT). We found that reducing DHAPAT in the fat body improved TAG and glucose homeostasis, cardiac function, respiration, and insulin signaling in flies fed a HSD. The reduction of DHAPAT may cause a switch in molecular signaling from lipogenesis to fatty acid oxidation via activation of a PPARα-like receptor, as bezafibrate produced similar improvements in HS-fed flies. Taken together, our findings suggest that ether lipids may be lipotoxins that reduce fitness during overnutrition.

Uniting epidemiology and experimental models: pancreatic steatosis and pancreatic cancer

Research from epidemiologic studies and experimental animal models provide insights into the role of pancreatic steatosis in the development of pancreatic cancer. Epidemiologic data demonstrate that pancreatic steatosis is widely prevalent and significantly associated with both development and progression of pancreatic cancer. By focusing on current experimental models, this review elucidates potential cellular mechanisms underlying not only the pathophysiology of pancreatic steatosis itself, but also the pathogenesis behind pancreatic steatosis's role in changing the tumour microenvironment and accelerating the development of pancreatic cancer. This review further explores the impact of bariatric surgery on pancreatic steatosis and pancreatic cancer. Synthesizing knowledge from both epidemiologic studies and experimental animal models, this review identifies gaps in current knowledge regarding pancreatic steatosis and its role in carcinogenesis and proposes future research directions to elucidate the possible mechanisms underlying other obesity-associated cancers.

Adipocyte Gq signaling is a regulator of glucose and lipid homeostasis in mice

Obesity is the major driver of the global epidemic in type 2 diabetes (T2D). In individuals with obesity, impaired insulin action leads to increased lipolysis in adipocytes, resulting in elevated plasma free fatty acid (FFA) levels that promote peripheral insulin resistance, a hallmark of T2D. Here we show, by using a combined genetic/biochemical/pharmacologic approach, that increased adipocyte lipolysis can be prevented by selective activation of adipocyte Gq signaling in vitro and in vivo (in mice). Activation of this pathway by a Gq-coupled designer receptor or by an agonist acting on an endogenous adipocyte Gq-coupled receptor (CysLT2 receptor) greatly improved glucose and lipid homeostasis in obese mice or in mice with adipocyte insulin receptor deficiency. Our findings identify adipocyte Gq signaling as an essential regulator of whole-body glucose and lipid homeostasis and should inform the development of novel classes of GPCR-based antidiabetic drugs.

Systems-level analysis of insulin action in mouse strains provides insight into tissue- and pathway-specific interactions that drive insulin resistance

Skeletal muscle and adipose tissue insulin resistance are major drivers of metabolic disease. To uncover pathways involved in insulin resistance, specifically in these tissues, we leveraged the metabolic diversity of different dietary exposures and discrete inbred mouse strains. This revealed that muscle insulin resistance was driven by gene-by-environment interactions and was strongly correlated with hyperinsulinemia and decreased levels of ten key glycolytic enzymes. Remarkably, there was no relationship between muscle and adipose tissue insulin action. Adipocyte size profoundly varied across strains and diets, and this was strongly correlated with adipose tissue insulin resistance. The A/J strain, in particular, exhibited marked adipocyte insulin resistance and hypertrophy despite robust muscle insulin responsiveness, challenging the role of adipocyte hypertrophy per se in systemic insulin resistance. These data demonstrate that muscle and adipose tissue insulin resistance can occur independently and underscore the need for tissue-specific interrogation to understand metabolic disease.

The metabolic triad of non-alcoholic fatty liver disease, visceral adiposity and type 2 diabetes: Implications for treatment

Non-alcoholic fatty liver disease (NAFLD) is associated with visceral obesity, insulin resistance, type 2 diabetes (T2D) and has been often considered as the hepatic expression of the metabolic syndrome (MetS). Epidemiological studies highlight a bidirectional relationship of NAFLD with T2D in which NAFLD increases the risk of incident T2D and T2D increases the risk of severe non-alcoholic steatohepatitis (NASH) and liver fibrosis. Regarding the molecular determinants of NAFLD, we specifically focused in this review on adipocyte dysfunction as a key molecular link between visceral adipose tissue, MetS and NAFLD. Notably, the subcutaneous white adipose tissue expandability appears a critical adaptive buffering mechanism to prevent lipotoxicity and its related metabolic complications, such as NAFLD and T2D. There is a clinical challenge to consider therapeutic strategies targeting the metabolic dysfunction common to NASH and T2D pathogenesis. Strategies that promote significant and sustained weight loss (~10% of total body weight) such as metabolic and bariatric surgery or incretin-based therapies (GLP-1 receptor agonists or dual GLP-1/GIP or GLP-1/glucagon receptor co-agonists) are among the most efficient ones. In addition, insulin sensitizers such as PPARγ (pioglitazone) and pan-PPARs agonists (lanifibranor) have shown some beneficial effects on both NASH and liver fibrosis. Since NASH is a complex and multifactorial disease, it is conceivable that targeting different pathways, not only insulin resistance but also inflammation and fibrotic processes, is required to achieve NASH resolution.

Insulin Resistance: From Mechanisms to Therapeutic Strategies

Insulin resistance is the pivotal pathogenic component of many metabolic diseases, including type 2 diabetes mellitus, and is defined as a state of reduced responsiveness of insulin-targeting tissues to physiological levels of insulin. Although the underlying mechanism of insulin resistance is not fully understood, several credible theories have been proposed. In this review, we summarize the functions of insulin in glucose metabolism in typical metabolic tissues and describe the mechanisms proposed to underlie insulin resistance, that is, ectopic lipid accumulation in liver and skeletal muscle, endoplasmic reticulum stress, and inflammation. In addition, we suggest potential therapeutic strategies for addressing insulin resistance.

Adipocyte Biology from the Perspective of In Vivo Research: Review of Key Transcription Factors

Obesity and type 2 diabetes are both significant contributors to the contemporary pandemic of non-communicable diseases. Both disorders are interconnected and associated with the disruption of normal homeostasis in adipose tissue. Consequently, exploring adipose tissue differentiation and homeostasis is important for the treatment and prevention of metabolic disorders. The aim of this work is to review the consecutive steps in the postnatal development of adipocytes, with a special emphasis on in vivo studies. We gave particular attention to well-known transcription factors that had been thoroughly described in vitro, and showed that the in vivo research of adipogenic differentiation can lead to surprising findings.

Adipose Lipolysis Regulates Cardiac Glucose Uptake and Function in Mice under Cold Stress

The heart primarily uses fatty acids as energy substrates. Adipose lipolysis is a major source of fatty acids, particularly under stress conditions. In this study, we showed that mice with selective inactivation of the lipolytic coactivator comparative gene identification-58 (CGI-58) in adipose tissue (FAT-KO mice), relative to their littermate controls, had lower circulating FA levels in the fed and fasted states due to impaired adipose lipolysis. They preferentially utilized carbohydrates as energy fuels and were more insulin sensitive and glucose tolerant. Under cold stress, FAT-KO versus control mice had >10-fold increases in glucose uptake in the hearts but no increases in other tissues examined. Plasma concentrations of atrial natriuretic peptide and cardiac mRNAs for atrial and brain-type natriuretic peptides, two sensitive markers of cardiac remodeling, were also elevated. After one week of cold exposure, FAT-KO mice showed reduced cardiac expression of several mitochondrial oxidative phosphorylation proteins. After one month of cold exposure, hearts of these animals showed depressed functions, reduced SERCA2 protein, and increased proteins for MHC-β, collagen I proteins, Glut1, Glut4 and phospho-AMPK. Thus, CGI-58-dependent adipose lipolysis critically regulates cardiac metabolism and function, especially during cold adaptation. The adipose-heart axis may be targeted for the management of cardiac dysfunction.

Revisiting the contribution of mitochondrial biology to the pathophysiology of skeletal muscle insulin resistance

While the etiology of type 2 diabetes is multifaceted, the induction of insulin resistance in skeletal muscle is a key phenomenon, and impairments in insulin signaling in this tissue directly contribute to hyperglycemia. Despite the lack of clarity regarding the specific mechanisms whereby insulin signaling is impaired, the key role of a high lipid environment within skeletal muscle has been recognized for decades. Many of the proposed mechanisms leading to the attenuation of insulin signaling - namely the accumulation of reactive lipids and the pathological production of reactive oxygen species (ROS), appear to rely on this high lipid environment. Mitochondrial biology is a central component to these processes, as these organelles are almost exclusively responsible for the oxidation and metabolism of lipids within skeletal muscle and are a primary source of ROS production. Classic studies have suggested that reductions in skeletal muscle mitochondrial content and/or function contribute to lipid-induced insulin resistance; however, in recent years the role of mitochondria in the pathophysiology of insulin resistance has been gradually re-evaluated to consider the biological effects of alterations in mitochondrial content. In this respect, while reductions in mitochondrial content are not required for the induction of insulin resistance, mechanisms that increase mitochondrial content are thought to enhance mitochondrial substrate sensitivity and submaximal adenosine diphosphate (ADP) kinetics. Thus, this review will describe the central role of a high lipid environment in the pathophysiology of insulin resistance, and present both classic and contemporary views of how mitochondrial biology contributes to insulin resistance in skeletal muscle.

The Effect of High Fat Diet on Cerebrovascular Health and Pathology: A Species Comparative Review

In both humans and animal models, consumption of a high-saturated-fat diet has been linked to vascular dysfunction and cognitive impairments. Laboratory animals provide excellent models for more invasive high-fat-diet-related research. However, the physiological differences between humans and common animal models in terms of how they react metabolically to high-fat diets need to be considered. Here, we review the factors that may affect the translatability of mechanistic research in animal models, paying special attention to the effects of a high-fat diet on vascular outcomes. We draw attention to the dissociation between metabolic syndrome and dyslipidemia in rodents, unlike the state in humans, where the two commonly occur. We also discuss the differential vulnerability between species to the metabolic and vascular effects of macronutrients in the diet. Findings from animal studies are better interpreted as modeling specific aspects of dysfunction. We conclude that the differences between species provide an opportunity to explore why some species are protected from the detrimental aspects of high-fat-diet-induced dysfunction, and to translate these findings into benefits for human health.

Mechanisms and disease consequences of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the leading chronic liver disease worldwide. Its more advanced subtype, nonalcoholic steatohepatitis (NASH), connotes progressive liver injury that can lead to cirrhosis and hepatocellular carcinoma. Here we provide an in-depth discussion of the underlying pathogenetic mechanisms that lead to progressive liver injury, including the metabolic origins of NAFLD, the effect of NAFLD on hepatic glucose and lipid metabolism, bile acid toxicity, macrophage dysfunction, and hepatic stellate cell activation, and consider the role of genetic, epigenetic, and environmental factors that promote fibrosis progression and risk of hepatocellular carcinoma in NASH.

Mitophagy-mediated adipose inflammation contributes to type 2 diabetes with hepatic insulin resistance

White adipose tissues (WAT) play crucial roles in maintaining whole-body energy homeostasis, and their dysfunction can contribute to hepatic insulin resistance and type 2 diabetes mellitus (T2DM). However, the mechanisms underlying these alterations remain unknown. By analyzing the transcriptome landscape in human adipocytes based on available RNA-seq datasets from lean, obese, and T2DM patients, we reveal elevated mitochondrial reactive oxygen species (ROS) pathway and NF-κB signaling with altered fatty acid metabolism in T2DM adipocytes. Mice with adipose-specific deletion of mitochondrial redox Trx2 develop hyperglycemia, hepatic insulin resistance, and hepatic steatosis. Trx2-deficient WAT exhibited excessive mitophagy, increased inflammation, and lipolysis. Mechanistically, mitophagy was induced through increasing ROS generation and NF-κB-dependent accumulation of autophagy receptor p62/SQSTM1, which recruits damaged mitochondria with polyubiquitin chains. Importantly, administration of ROS scavenger or NF-κB inhibitor ameliorates glucose and lipid metabolic disorders and T2DM progression in mice. Taken together, this study reveals a previously unrecognized mechanism linking mitophagy-mediated adipose inflammation to T2DM with hepatic insulin resistance.

β-Cell Dysfunction, Hepatic Lipid Metabolism, and Cardiovascular Health in Type 2 Diabetes: New Directions of Research and Novel Therapeutic Strategies

Cardiovascular disease (CVD) remains a major problem for people with type 2 diabetes mellitus (T2DM), and dyslipidemia is one of the main drivers for both metabolic diseases. In this review, the major pathophysiological and molecular mechanisms of β-cell dysfunction and recovery in T2DM are discussed in the context of abnormal hepatic lipid metabolism and cardiovascular health. (i) In normal health, continuous exposure of the pancreas to nutrient stimulus increases the demand on β-cells. In the long term, this will not only stress β-cells and decrease their insulin secretory capacity, but also will blunt the cellular response to insulin. (ii) At the pre-diabetes stage, β-cells compensate for insulin resistance through hypersecretion of insulin. This increases the metabolic burden on the stressed β-cells and changes hepatic lipoprotein metabolism and adipose tissue function. (iii) If this lipotoxic hyperinsulinemic environment is not removed, β-cells start to lose function, and CVD risk rises due to lower lipoprotein clearance. (iv) Once developed, T2DM can be reversed by weight loss, a process described recently as remission. However, the precise mechanism(s) by which calorie restriction causes normalization of lipoprotein metabolism and restores β-cell function are not fully established. Understanding the pathophysiological and molecular basis of β-cell failure and recovery during remission is critical to reduce β-cell burden and loss of function. The aim of this review is to highlight the link between lipoprotein export and lipid-driven β-cell dysfunction in T2DM and how this is related to cardiovascular health. A second aim is to understand the mechanisms of β-cell recovery after weight loss, and to explore new areas of research for developing more targeted future therapies to prevent T2DM and the associated CVD events.

Domeless receptor loss in fat body tissue reverts insulin resistance induced by a high-sugar diet in Drosophila melanogaster

Insulin resistance is a hallmark of type 2 diabetes resulting from the confluence of several factors, including genetic susceptibility, inflammation, and diet. Under this pathophysiological condition, the dysfunction of the adipose tissue triggered by the excess caloric supply promotes the loss of sensitivity to insulin at the local and peripheral level, a process in which different signaling pathways are involved that are part of the metabolic response to the diet. Besides, the dysregulation of insulin signaling is strongly associated with inflammatory processes in which the JAK/STAT pathway plays a central role. To better understand the role of JAK/STAT signaling in the development of insulin resistance, we used a simple organism, Drosophila melanogaster, as a type 2 diabetes model generated by the consumption of a high-sugar diet. In this model, we studied the effects of inhibiting the expression of the JAK/STAT pathway receptor Domeless, in fat body, on adipose metabolism and glycemic control. Our results show that the Domeless receptor loss in fat body cells reverses both hyperglycemia and the increase in the expression of the insulin resistance marker Nlaz, observed in larvae fed a high sugar diet. This effect is consistent with a significant reduction in Dilp2 mRNA expression and an increase in body weight compared to wild-type flies fed high sugar diets. Additionally, the loss of Domeless reduced the accumulation of triglycerides in the fat body cells of larvae fed HSD and also significantly increased the lifespan of adult flies. Taken together, our results show that the loss of Domeless in the fat body reverses at least in part the dysmetabolism induced by a high sugar diet in a Drosophila type 2 diabetes model.

Molecular and Cellular Bases of Lipodystrophy Syndromes

Lipodystrophy syndromes are rare diseases originating from a generalized or partial loss of adipose tissue. Adipose tissue dysfunction results from heterogeneous genetic or acquired causes, but leads to similar metabolic complications with insulin resistance, diabetes, hypertriglyceridemia, nonalcoholic fatty liver disease, dysfunctions of the gonadotropic axis and endocrine defects of adipose tissue with leptin and adiponectin deficiency. Diagnosis, based on clinical and metabolic investigations, and on genetic analyses, is of major importance to adapt medical care and genetic counseling. Molecular and cellular bases of these syndromes involve, among others, altered adipocyte differentiation, structure and/or regulation of the adipocyte lipid droplet, and/or premature cellular senescence. Lipodystrophy syndromes frequently present as systemic diseases with multi-tissue involvement. After an update on the main molecular bases and clinical forms of lipodystrophy, we will focus on topics that have recently emerged in the field. We will discuss the links between lipodystrophy and premature ageing and/or immuno-inflammatory aggressions of adipose tissue, as well as the relationships between lipomatosis and lipodystrophy. Finally, the indications of substitutive therapy with metreleptin, an analog of leptin, which is approved in Europe and USA, will be discussed.

Ovariectomy Impaired Hepatic Glucose and Lipid Homeostasis and Altered the Gut Microbiota in Mice With Different Diets

The lower incidence of metabolic diseases of women than men and the increasing morbidity of metabolic disorders of menopausal women indicated that hormones produced by ovaries may affect homeostasis of glucose and lipid metabolism, but the underlying mechanisms remain unclear. To explore the functions of ovaries on regulating glucose and lipid metabolism in females, 8 weeks old C57BL/6 mice were preformed ovariectomy and administrated with normal food diet (NFD) or high fat diet (HFD). Six weeks after ovariectomy, blood biochemical indexes were tested and the morphology and histology of livers were checked. The expression levels of genes related to glucose and lipid metabolism in liver were detected through transcriptome analysis, qPCR and western blot assays. 16S rDNA sequence was conducted to analyze the gut microbiota of mice with ovariectomy and different diets. The serum total cholesterol (TC) was significantly increased in ovariectomized (OVX) mice fed with NFD (OVXN), and serum low density lipoprotein-cholesterol (LDL-C) was significantly increased in both OVXN mice and OVX mice fed with HFD (OVXH). The excessive glycogen storage was found in livers of 37.5% mice from OVXN group, and lipid accumulation was detected in livers of the other 62.5% OVXN mice. The OVXN group was further divided into OVXN-Gly and OVXN-TG subgroups depending on histological results of the liver. Lipid drops in livers of OVXH mice were more and larger than other groups. The expression level of genes related with lipogenesis was significantly increased and the expression level of genes related with β-oxidation was significantly downregulated in the liver of OVXN mice. Ovariectomy also caused the dysbiosis of intestinal flora of OVXN and OVXH mice. These results demonstrated that hormones generated by ovaries played important roles in regulating hepatic glucose and lipid metabolism and communicating with the gut microbiota in females.

Lack of adipocyte purinergic P2Y6 receptor greatly improves whole body glucose homeostasis

Uridine diphosphate (UDP)-activated purinergic receptor P2Y₆ (P2Y₆R) plays a crucial role in controlling energy balance through central mechanisms. However, P2Y₆R's roles in peripheral tissues regulating energy and glucose homeostasis remain unexplored. Here, we report the surprising finding that adipocyte-specific deletion of P2Y₆R protects mice from diet-induced obesity, improving glucose tolerance and insulin sensitivity with reduced systemic inflammation. These changes were associated with reduced JNK signaling and enhanced expression and activity of PPARα affecting downstream PGC1α levels leading to beiging of white fat. In contrast, P2Y₆R deletion in skeletal muscle reduced glucose uptake, resulting in impaired glucose homeostasis. Interestingly, whole body P2Y₆R knockout mice showed metabolic improvements similar to those observed with mice lacking P2Y₆R only in adipocytes. Our findings provide compelling evidence that P2Y₆R antagonists may prove useful for the treatment of obesity and type 2 diabetes.

The NLRP3 inflammasome regulates adipose tissue metabolism

Adipose tissue regulates metabolic homeostasis by participating in endocrine and immune responses in addition to storing and releasing lipids from adipocytes. Obesity skews adipose tissue adipokine responses and degrades the coordination of adipocyte lipogenesis and lipolysis. These defects in adipose tissue metabolism can promote ectopic lipid deposition and inflammation in insulin-sensitive tissues such as skeletal muscle and liver. Sustained caloric excess can expand white adipose tissue to a point of maladaptation exacerbating both local and systemic inflammation. Multiple sources, instigators and propagators of adipose tissue inflammation occur during obesity. Cross-talk between professional immune cells (i.e. macrophages) and metabolic cells (i.e. adipocytes) promote adipose tissue inflammation during metabolic stress (i.e. metaflammation). Metabolic stress and endogenous danger signals can engage pathogen recognition receptors (PRRs) of the innate immune system thereby activating pro-inflammatory and stress pathways in adipose tissue. The Nod-like receptor protein 3 (NLRP3) inflammasome can act as a metabolic danger sensor to a wide range of pathogen- and damage-associated molecular patterns (PAMPs and DAMPs). Activation of the NLRP3 inflammasome facilitates caspase-1 dependent production of the pro-inflammatory cytokines IL-1β and IL-18. Activation of the NLRP3 inflammasome can promote inflammation and pyroptotic cell death, but caspase-1 is also involved in adipogenesis. This review discusses the role of the NLRP3 inflammasome in adipose tissue immunometabolism responses relevant to metabolic disease. Understanding the potential sources of NLRP3 activation and consequences of NLRP3 effectors may reveal therapeutic opportunities to break or fine-tune the connection between metabolism and inflammation in adipose tissue during obesity.

Controlled-release mitochondrial protonophore (CRMP) reverses dyslipidemia and hepatic steatosis in dysmetabolic nonhuman primates

Nonalcoholic fatty liver disease (NAFLD) is estimated to affect up to one-third of the general population, and new therapies are urgently required. Our laboratory previously developed a controlled-release mitochondrial protonophore (CRMP) that is functionally liver-targeted and promotes oxidation of hepatic triglycerides. Although we previously demonstrated that CRMP safely reverses hypertriglyceridemia, fatty liver, hepatic inflammation, and fibrosis in diet-induced rodent models of obesity, there remains a critical need to assess its safety and efficacy in a model highly relevant to humans. Here, we evaluated the impact of longer-term CRMP treatment on hepatic mitochondrial oxidation and on the reversal of hypertriglyceridemia, NAFLD, and insulin resistance in high-fat, fructose-fed cynomolgus macaques (n = 6) and spontaneously obese dysmetabolic rhesus macaques (n = 12). Using positional isotopomer nuclear magnetic resonance tracer analysis (PINTA), we demonstrated that acute CRMP treatment (single dose, 5 mg/kg) increased rates of hepatic mitochondrial fat oxidation by 40%. Six weeks of CRMP treatment reduced hepatic triglycerides in both nonhuman primate models independently of changes in body weight, food intake, body temperature, or adverse reactions. CRMP treatment was also associated with a 20 to 30% reduction in fasting plasma triglycerides and low-density lipoprotein (LDL)-cholesterol in dysmetabolic nonhuman primates. Oral administration of CRMP reduced endogenous glucose production by 18%, attributable to a 20% reduction in hepatic acetyl-coenzyme A (CoA) content [as assessed by whole-body β-hydroxybutyrate (β-OHB) turnover] and pyruvate carboxylase flux. Collectively, these studies provide proof-of-concept data to support the development of liver-targeted mitochondrial uncouplers for the treatment of metabolic syndrome in humans.

Adipocyte Gi signaling is essential for maintaining whole-body glucose homeostasis and insulin sensitivity

Adipocyte dysfunction links obesity to insulin resistance and type 2 diabetes. Adipocyte function is regulated by receptor-mediated activation of heterotrimeric G proteins. Little is known about the potential in vivo metabolic roles of G_i-type G proteins expressed by adipocytes, primarily due to the lack of suitable animal models. To address this question, we generated mice lacking functional G_i proteins selectively in adipocytes. Here we report that these mutant mice displayed significantly impaired glucose tolerance and reduced insulin sensitivity when maintained on an obesogenic diet. In contrast, using a chemogenetic strategy, we demonstrated that activation of G_i signaling selectively in adipocytes greatly improved glucose homeostasis and insulin signaling. We also elucidated the cellular mechanisms underlying the observed metabolic phenotypes. Our data support the concept that adipocyte G_i signaling is essential for maintaining euglycemia. Drug-mediated activation of adipocyte G_i signaling may prove beneficial for restoring proper glucose homeostasis in type 2 diabetes.

Gs-coupled receptor signaling is well known to modulate adipocyte metabolism, but the role of Gi-coupled receptors in adipose tissue is less well understood. Here the authors show that signaling via Gi-type G proteins expressed by adipocytes is essential for maintaining proper blood glucose homeostasis.

β-arrestin-1 suppresses myogenic reprogramming of brown fat to maintain euglycemia

A better understanding of the signaling pathways regulating adipocyte function is required for the development of new classes of antidiabetic/obesity drugs. We here report that mice lacking β-arrestin-1 (barr1), a cytoplasmic and nuclear signaling protein, selectively in adipocytes showed greatly impaired glucose tolerance and insulin sensitivity when consuming an obesogenic diet. In contrast, transgenic mice overexpressing barr1 in adipocytes were protected against the metabolic deficits caused by a high-calorie diet. Barr1 deficiency led to a myogenic reprogramming of brown adipose tissue (BAT), causing elevated plasma myostatin (Mstn) levels, which in turn led to impaired insulin signaling in multiple peripheral tissues. Additional in vivo studies indicated that barr1-mediated suppression of Mstn expression by BAT is required for maintaining euglycemia. These findings convincingly identify barr1 as a critical regulator of BAT function. Strategies aimed at enhancing barr1 activity in BAT may prove beneficial for the treatment of type 2 diabetes.

Beneficial metabolic role of β-arrestin-1 expressed by AgRP neurons

β-Arrestin-1 and β-arrestin-2 have emerged as important signaling molecules that modulate glucose fluxes in several peripheral tissues. The potential roles of neuronally expressed β-arrestins in regulating glucose homeostasis remain unknown. We here report that mice lacking β-arrestin-1 (barr1) selectively in AgRP neurons displayed impaired glucose tolerance and insulin sensitivity when consuming an obesogenic diet, while mice overexpressing barr1 selectively in AgRP neurons were protected against obesity-associated metabolic impairments. Additional physiological, biochemical, and electrophysiological data indicated that the presence of barr1 is essential for insulin-mediated hyperpolarization of AgRP neurons. As a result, barr1 expressed by AgRP neurons regulates efferent neuronal pathways that suppress hepatic glucose production and promote lipolysis in adipose tissue. Mice lacking β-arrestin-2 (barr2) selectively in AgRP neurons showed no substantial metabolic phenotypes. Our data suggest that agents able to enhance the activity of barr1 in AgRP neurons may prove beneficial as antidiabetic drugs.

TMAVA, a Metabolite of Intestinal Microbes, Is Increased in Plasma From Patients With Liver Steatosis, Inhibits γ-Butyrobetaine Hydroxylase, and Exacerbates Fatty Liver in Mice

BACKGROUND & AIMS

Nonalcoholic fatty liver disease is characterized by excessive hepatic accumulation of triglycerides. We aimed to identify metabolites that differ in plasma of patients with liver steatosis vs healthy individuals (controls) and investigate the mechanisms by which these might contribute to fatty liver in mice.

METHODS

We obtained blood samples from 15 patients with liver steatosis and 15 controls from a single center in China (discovery cohort). We performed untargeted liquid chromatography with mass spectrometry analysis of plasma to identify analytes associated with liver steatosis. We then performed targeted metabolomic analysis of blood samples from 2 independent cohorts of individuals who underwent annual health examinations in China (1157 subjects with or without diabetes and 767 subjects with or without liver steatosis; replication cohorts). We performed mass spectrometry analysis of plasma from C57BL/6J mice, germ-free, and mice given antibiotics. C57BL/6J mice were given 0.325% (m/v) N,N,N-trimethyl-5-aminovaleric acid (TMAVA) in their drinking water and placed on a 45% high-fat diet (HFD) for 2 months. Plasma, liver tissues, and fecal samples were collected; fecal samples were analyzed by 16S ribosomal RNA gene sequencing. C57BL/6J mice with CRISPR-mediated disruption of the gene encoding γ-butyrobetaine hydroxylase (BBOX-knockout mice) were also placed on a 45% HFD for 2 months. Hepatic fatty acid oxidation (FAO) in liver tissues was determined by measuring liberation of ³H₂O from [³H] palmitic acid. Liver tissues were analyzed by electron microscopy, to view mitochondria, and proteomic analyses. We used surface plasmon resonance analysis to quantify the affinity of TMAVA for BBOX.

RESULTS

Levels of TMAVA, believed to be a metabolite of intestinal microbes, were increased in plasma from subjects with liver steatosis compared with controls, in the discovery and replication cohorts. In 1 replication cohort, the odds ratio for fatty liver in subjects with increased liver plasma levels of TMAVA was 1.82 (95% confidence interval [CI], 1.14-2.90; P = .012). Plasma from mice given antibiotics or germ-free mice had significant reductions in TMAVA compared with control mice. We found the intestinal bacteria Enterococcus faecalis and Pseudomonas aeruginosa to metabolize trimethyllysine to TMAVA; levels of trimethyllysine were significantly higher in plasma from patients with steatosis than controls. We found TMAVA to bind and inhibit BBOX, reducing synthesis of carnitine. Mice given TMAVA had alterations in their fecal microbiomes and reduced cold tolerance; their plasma and liver tissue had significant reductions in levels of carnitine and acyl-carnitine and their hepatocytes had reduced mitochondrial FAO compared with mice given only an HFD. Mice given TMAVA on an HFD developed liver steatosis, which was reduced by carnitine supplementation. BBOX-knockout mice had carnitine deficiency and decreased FAO, increasing uptake and liver accumulation of free fatty acids and exacerbating HFD-induced fatty liver.

CONCLUSIONS

Levels of TMAVA are increased in plasma from subjects with liver steatosis. In mice, intestinal microbes metabolize trimethyllysine to TMAVA, which reduces carnitine synthesis and FAO to promote steatosis.

Mitochondrial Dysfunction, Insulin Resistance, and Potential Genetic Implications

Insulin resistance (IR) is fundamental to the development of type 2 diabetes (T2D) and is present in most prediabetic (preDM) individuals. Insulin resistance has both heritable and environmental determinants centered on energy storage and metabolism. Recent insights from human genetic studies, coupled with comprehensive in vivo and ex vivo metabolic studies in humans and rodents, have highlighted the critical role of reduced mitochondrial function as a predisposing condition for ectopic lipid deposition and IR. These studies support the hypothesis that reduced mitochondrial function, particularly in insulin-responsive tissues such as skeletal muscle, white adipose tissue, and the liver, is inextricably linked to tissue and whole body IR through the effects on cellular energy balance. Here we discuss these findings as well as address potential mechanisms that serve as the nexus between mitochondrial malfunction and IR.

Hepatic Lipoprotein Export and Remission of Human Type 2 Diabetes after Weight Loss

The role of hepatic lipoprotein metabolism in diet-induced remission of type 2 diabetes is currently unclear. Here, we determined the contributions of hepatic VLDL1-triglyceride production rate and VLDL1-palmitic acid content to changes in intra-pancreatic fat and return of first phase insulin response in a subgroup of the Diabetes Remission Clinical Trial. Liver fat, VLDL1-triglyceride production, and intra-pancreatic fat decreased after weight loss and remained normalized after 24 months of remission. First-phase insulin response remained increased only in those maintaining diabetes remission. Compared with those in remission at 24 months, individuals who relapsed after initial remission had a greater rise in the content of VLDL1-triglyceride and VLDL1-palmitic acid, re-accumulated intra-pancreatic fat, and lost first-phase response by 24 months. Thus, we observed temporal relationships between VLDL1-triglyceride production, hepatic palmitic acid flux, intra-pancreatic fat, and β-cell function. Weight-related disordered fat metabolism appears to drive development and reversal of type 2 diabetes.

Pathogenesis of Insulin Resistance and Atherogenic Dyslipidemia in Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease in the developed world, with a global prevalence of around 25%. NAFLD is considered to be the hepatic manifestation of metabolic syndrome and is strongly associated with obesity, insulin resistance and dyslipidemia. Insulin resistance plays a pivotal role in the development of NAFLD-related dyslipidemia, which ultimately increases the risk of premature cardiovascular diseases, a leading cause of morbidity and mortality in patients with NAFLD. Insulin affects hepatic glucose and lipid metabolism by hepatic or extrahepatic pathways. Aside from insulin resistance, several other factors also contribute to the pathogenesis of atherogenic dyslipidemia in patients with NAFLD. These include diet composition, gut microbiota and genetic factors, to name a few. The identification of potentially modifiable risk factors of NAFLD is of importance, so as to target those who may benefit from lifestyle changes and to help develop targeted therapies that decrease the risk of cardiovascular diseases in patients with NAFLD.

Recent advances in understanding lipodystrophy: a focus on lipodystrophy-associated cardiovascular disease and potential effects of leptin therapy on cardiovascular function

Lipodystrophy is a disease characterized by a partial or total absence of adipose tissue leading to severe metabolic derangements including marked insulin resistance, type 2 diabetes, hypertriglyceridemia, and steatohepatitis. Lipodystrophy is also a source of major cardiovascular disorders which, in addition to hepatic failure and infection, contribute to a significant reduction in life expectancy. Metreleptin, the synthetic analog of the adipocyte-derived hormone leptin and current therapy of choice for patients with lipodystrophy, successfully improves metabolic function. However, while leptin has been associated with hypertension, vascular diseases, and inflammation in the context of obesity, it remains unknown whether its daily administration could further impair cardiovascular function in patients with lipodystrophy. The goal of this short review is to describe the cardiovascular phenotype of patients with lipodystrophy, speculate on the etiology of the disorders, and discuss how the use of murine models of lipodystrophy could be beneficial to address the question of the contribution of leptin to lipodystrophy-associated cardiovascular disease.

The Role of NADPH Oxidases in the Etiology of Obesity and Metabolic Syndrome: Contribution of Individual Isoforms and Cell Biology

Significance: Highly prevalent in Western cultures, obesity, metabolic syndrome, and diabetes increase the risk of cardiovascular morbidity and mortality and cost health care systems billions of dollars annually. At the cellular level, obesity, metabolic syndrome, and diabetes are associated with increased production of reactive oxygen species (ROS). Increased levels of ROS production in key organ systems such as adipose tissue, skeletal muscle, and the vasculature cause disruption of tissue homeostasis, leading to increased morbidity and risk of mortality. More specifically, growing evidence implicates the nicotinamide adenine dinucleotide phosphate oxidase (NOX) enzymes in these pathologies through impairment of insulin signaling, inflammation, and vascular dysfunction. The NOX family of enzymes is a major driver of redox signaling through its production of superoxide anion, hydrogen peroxide, and attendant downstream metabolites acting on redox-sensitive signaling molecules. Recent Advances: The primary goal of this review is to highlight recent advances and survey our present understanding of cell-specific NOX enzyme contributions to metabolic diseases. Critical Issues: However, due to the short half-lives of individual ROS and/or cellular defense systems, radii of ROS diffusion are commonly short, often restricting redox signaling and oxidant stress to localized events. Thus, special emphasis should be placed on cell type and subcellular location of NOX enzymes to better understand their role in the pathophysiology of metabolic diseases. Future Directions: We discuss the targeting of NOX enzymes as potential therapy and bring to light potential emerging areas of NOX research, microparticles and epigenetics, in the context of metabolic disease.

Impaired metabolic and hepatic functions following subcutaneous lipectomy in adult obese rats

NEW FINDINGS

What is the central question of this study? What is the impact and drawbacks of subcutaneous lipectomy on body metabolism? What is the main finding and its importance? Subcutaneous lipectomy resulted in deterioration of hepatic functions, atherosclerotic lipid profile and disturbed redox state. While the results support lipectomy as an effective treatment for obesity, lipectomy induces unfavourable changes in health.

ABSTRACT

The number of obese older adults is on the rise, but data about proper treatment of obesity in the elderly is controversial. The present study was designed to investigate the effectiveness and consequences of partial subcutaneous lipectomy, as a rapid medical intervention against increased accumulation of body fat, in adult obese rats. The study was conducted on adult (9-12 months) female rats, in which obesity was induced by bilateral surgical ovariectomy. They were randomized into two main groups: short term (5 weeks) and long term (10 weeks). Both groups were subdivided into control, ovariectomized (OVX) and ovariectomized lipectomized groups. Body weight (BW) was measured and body mass index (BMI) calculated. Fasting blood glucose, lipid profile and plasma levels of total proteins, albumin, liver enzymes, malondialdehyde (MDA), leptin and adiponectin were determined. The content of both blood and hepatic tissue of reduced glutathione was estimated. In addition, histological study of the liver, aorta and peri-renal fat was performed. Compared to controls, OVX rats showed significant increase in BW, BMI and plasma levels of liver enzymes, MDA and leptin. Histological study revealed vacuolated ballooned hepatocytes and enlarged irregular visceral adipocytes with atherosclerotic changes in the wall of aorta. Following subcutaneous lipectomy, rats exhibited significant fasting hyperglycaemia, dyslipidaemia, lowered plasma albumin and disturbed redox state with aggravation of the histological changes. The findings indicate that although subcutaneous lipectomy appears to be effective in combating obesity in older females, it has unfavourable effects on both metabolic and hepatic functions.

Kinin B1 Receptor Acts in Adipose Tissue to Control Fat Distribution in a Cell-Nonautonomous Manner

The kinin B₁ receptor (B₁R) plays a role in inflammatory and metabolic processes. B₁R deletion (B₁ ^-/-) protects mice from diet-induced obesity and improves insulin and leptin sensitivity. In contrast, genetic reconstitution of B₁R exclusively in adipose tissue reverses the lean phenotype of B₁ ^-/- mice. To study the cell-nonautonomous nature of these effects, we transplanted epididymal white adipose tissue (eWAT) from wild-type donors (B₁ ^+/+) into B₁ ^-/- mice (B₁ ^+/+→B₁ ^-/-) and compared them with autologous controls (B₁ ^+/+→B₁ ^+/+ or B₁ ^-/-→B₁ ^-/-). We then fed these mice a high-fat diet for 16 weeks and investigated their metabolic phenotypes. B₁ ^+/+→B₁ ^-/- mice became obese but not glucose intolerant or insulin resistant, unlike B₁ ^-/-→B₁ ^-/- mice. Moreover, the endogenous adipose tissue of B₁ ^+/+→B₁ ^-/- mice exhibited higher expression of adipocyte markers (e.g., Fabp4 and Adipoq) and changes in the immune cell pool. These mice also developed fatty liver. Wild-type eWAT transplanted into B₁ ^-/- mice normalized circulating insulin, leptin, and epidermal growth factor levels. In conclusion, we demonstrated that B₁R in adipose tissue controls the response to diet-induced obesity by promoting adipose tissue expansion and hepatic lipid accumulation in cell-nonautonomous manners.

Novel metabolic disorders in skeletal muscle of Lipodystrophic Bscl2/Seipin deficient mice

Bscl2^-/- mice recapitulate many of the major metabolic manifestations in Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) individuals, including lipodystrophy, hepatosteatosis, muscular hypertrophy, and insulin resistance. Metabolic defects in Bscl2^-/- mice with regard to glucose and lipid metabolism in skeletal muscle have never been investigated. Here, we identified Bscl2^-/- mice displayed reduced intramyocellular triglyceride (IMTG) content but increased glycogen storage predominantly in oxidative type I soleus muscle (SM). These changes were associated with increased incomplete fatty acid oxidation and glycogen synthesis. Interestingly, SM in Bscl2^-/- mice demonstrated a fasting duration induced insulin sensitivity which was further confirmed by hyperinsulinemic-euglycemic clamp in SM of overnight fasted Bscl2^-/- mice but reversed by raising circulating NEFA levels through intralipid infusion. Furthermore, mice with skeletal muscle-specific inactivation of BSCL2 manifested no changes in muscle deposition of lipids and glycogen, suggesting BSCL2 does not play a cell-autonomous role in muscle lipid and glucose homeostasis. Our study uncovers a novel link between muscle metabolic defects and insulin resistance, and underscores an important role of circulating NEFA in regulating oxidative muscle insulin signaling in BSCL2 lipodystrophy.

Effect of Olea europaea leaves extract on streptozotocin induced diabetes in male albino rats

The present study was aimed to evaluate the effect of olive (Olea europaea) leaves extract on streptozotocin (STZ)-induced diabetic male rats. The experimental rats were divided into six groups. Rats of the first group were served as normal controls. Rats of the second group were diabetic control. The third and fourth groups were diabetic rats, treated with olive leaves extract at low and high doses respectively. The fifth and sixth groups were non diabetic rats, subjected to olive leaves extract at the same doses given to the third and fourth groups respectively. The minimum of body weigh gain was noted in diabetic rats of the second group. the levels of serum glucose, insulin, total protein, albumin, triglycerides, cholesterol, low density lipoprotein cholesterol (LDL-C), very low density lipoprotein cholesterol (VLDL-C), creatine kinase (CK), lactate dehydrogenase (LDH) and malondialdehyde (MDA) were significantly increased, while the levels of high density lipoprotein cholesterol (HDL-C), superoxide dismutase, (SOD) glutathione (GSH) and catalase (CAT) were statistically decreased in diabetic rats of the second group. The levels of liver insulin receptor substrate 1 (IRS1) and insulin receptor A (IRA) were significantly declined in diabetic rats of the second group. The diabetic pancreatic sections from diabetic rats of the second group showed several histopathological changes. Administration of low and high doses of olive leaves extract improved the observed physiological, molecular and histopathological alterations. Collectively, the obtained results confirmed that the protective effects of olive leaves extract are attributed to the antioxidant activities of olive leaves extract and its active constituents.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Bmal1 in Perivascular Adipose Tissue Regulates Resting-Phase Blood Pressure Through Transcriptional Regulation of Angiotensinogen

BACKGROUND

The perivascular adipose tissue (PVAT) surrounding vessels constitutes a distinct functional integral layer of the vasculature required to preserve vascular tone under physiological conditions. However, there is little information on the relationship between PVAT and blood pressure regulation, including its potential contributions to circadian blood pressure variation.

METHODS

Using unique brown adipocyte-specific aryl hydrocarbon receptor nuclear translocator-like protein 1 (Bmal1) and angiotensinogen knockout mice, we determined the vasoactivity of homogenized PVAT in aortic rings and how brown adipocyte peripheral expression of Bmal1 and angiotensinogen in PVAT regulates the amplitude of diurnal change in blood pressure in mice.

RESULTS

We uncovered a peripheral clock in PVAT and demonstrated that loss of Bmal1 in PVAT reduces blood pressure in mice during the resting phase, leading to a superdipper phenotype. PVAT extracts from wild-type mice significantly induced contractility of isolated aortic rings in vitro in an endothelium-independent manner. This property was impaired in PVAT from brown adipocyte-selective Bmal1-deficient (BA-Bmal1-KO) mice. The PVAT contractile properties were mediated by local angiotensin II, operating through angiotensin II type 1 receptor-dependent signaling in the isolated vessels and linked to PVAT circadian regulation of angiotensinogen. Indeed, angiotensinogen mRNA and angiotensin II levels in PVAT of BA-Bmal1-KO mice were significantly reduced. Systemic infusion of angiotensin II, in turn, reduced Bmal1 expression in PVAT while eliminating the hypotensive phenotype during the resting phase in BA-Bmal1-KO mice. Angiotensinogen, highly expressed in PVAT, shows circadian expression in PVAT, and selective deletion of angiotensinogen in brown adipocytes recapitulates the phenotype of selective deletion of Bmal1 in brown adipocytes. Furthermore, angiotensinogen is a transcriptional target of Bmal1 in PVAT.

CONCLUSIONS

These data indicate that local Bmal1 in PVAT regulates angiotensinogen expression and the ensuing increase in angiotensin II, which acts on smooth muscle cells in the vessel walls to regulate vasoactivity and blood pressure in a circadian fashion during the resting phase. These findings will contribute to a better understanding of the cardiovascular complications of circadian disorders, alterations in the circadian dipping phenotype, and cross-talk between systemic and peripheral regulation of blood pressure.

Eosinophils support adipocyte maturation and promote glucose tolerance in obesity

Accumulating data have indicated a fundamental role of eosinophils in regulating adipose tissue homeostasis. Here, we performed whole-genome RNA sequencing of the small intestinal tract, which suggested the presence of impaired lipid metabolism in eosinophil-deficient ΔdblGATA mice. ΔdblGATA mice fed a high-fat diet (HFD) showed reduced body fat mass, impaired enlargement of adipocytes, decreased expression of adipogenic genes, and developed glucose intolerance. HFD induced accumulation of eosinophils in the perigonadal white adipose tissue. Concordantly, adipocyte-differentiated 3T3-L1 cells promoted the migration of eosinophils through the expression of CCL11 (eotaxin-1) and likely promoted their survival through the expression of interleukin (IL)-3, IL-5, and granulocyte-macrophage colony-stimulating factor. HFD-fed ΔdblGATA mice showed increased infiltration of macrophages, CD4⁺ T-cells, and B-cells, increased expression of interferon-γ, and decreased expression of IL-4 and IL-13 in white adipose tissue. Interferon-γ treatment significantly decreased lipid deposition in adipocyte-differentiated 3T3-L1 cells, while IL-4 treatment promoted lipid accumulation. Notably, HFD-fed ΔdblGATA mice showed increased lipid storage in the liver as compared with wild-type mice. We propose that obesity promotes the infiltration of eosinophils into adipose tissue that subsequently contribute to the metabolic homeostasis by promoting adipocyte maturation.

Connecting homocysteine and obesity through pyroptosis, gut microbiome, epigenetics, peroxisome proliferator-activated receptor γ, and zinc finger protein 407

Although homocysteine (Hcy), a part of the epigenome, contributes to cell death by pyroptosis and decreases peroxisome proliferator-activated receptor γ (PPARγ) levels, the mechanisms are unclear. Hcy is found in high concentrations in the sera of obese individuals, which can elicit an immune response as well by hypermethylating CpG islands of specific gene promoters, a marker of epigenetics. Hcy has also been established to chelate divalent metal ions like Cu²⁺ and Zn²⁺, but this role of Hcy has not been established in relationship with obesity. It has been known for a while that PPARγ dysregulation results in various metabolic disorders including glucose and lipid metabolism. Recently, zinc finger protein 407 (Zfp407) is reported to regulate PPARγ target gene expression without affecting PPARγ transcript and protein levels by synergistically working with PPARγ. However, the mechanism(s) of this synergy, as well as other factors contributing to or inhibiting this synergism, have not been proven. This review suggests that Hcy contributes to pyroptosis, changes gut microbiome, and alters PPARγ-dependent mechanism(s) via Zfp407-mediated upregulated adipogenesis and misbalanced fatty acid metabolism, which can predispose to obesity and, consequently, obesity-related metabolic disorders.

Insulin-like growth factor binding protein-3 links obesity and breast cancer progression

Obesity is associated epidemiologically with poor breast cancer prognosis, but the mechanisms remain unclear. Since IGF binding protein-3 (IGFBP-3) influences both breast cancer growth and adipocyte maturation, it may impact on how obesity promotes breast oncogenesis. This study investigated the role of endogenous IGFBP-3 on the development of obesity and subsequently on breast tumor growth. Wild-type (WT) C57BL/6 or IGFBP-3-null (BP3KO) mice were fed a high-fat diet (HFD) or control chow-diet for 15 weeks before orthotopic injection with syngeneic EO771 murine breast cancer cells. When the largest tumor reached 1000 mm3, tissues and tumors were excised for analysis. Compared to WT, BP3KO mice showed significantly reduced weight gain and mammary fat pad mass (contralateral to tumor) in response to HFD, despite similar food intake. EO771 tumor weight and volume were increased by HFD and decreased by BP3KO. Despite differences in tumor size, tumors in BP3KO mice showed no differences from WT in the number of mitotically active (Ki67+) and apoptotic (cleaved caspase-3+) cells, but had greater infiltration of CD3+ T-cells. These data suggest that endogenous (circulating and/or stromal) IGFBP-3 is stimulatory to adipose tissue expansion and enhances mammary tumor growth in immune-competent mice, potentially by suppressing T-cell infiltration into tumors.

Nonalcoholic Fatty Liver Disease as a Nexus of Metabolic and Hepatic Diseases

NAFLD is closely linked with hepatic insulin resistance. Accumulation of hepatic diacylglycerol activates PKC-ε, impairing insulin receptor activation and insulin-stimulated glycogen synthesis. Peripheral insulin resistance indirectly influences hepatic glucose and lipid metabolism by increasing flux of substrates that promote lipogenesis (glucose and fatty acids) and gluconeogenesis (glycerol and fatty acid-derived acetyl-CoA, an allosteric activator of pyruvate carboxylase). Weight loss with diet or bariatric surgery effectively treats NAFLD, but drugs specifically approved for NAFLD are not available. Some new pharmacological strategies act broadly to alter energy balance or influence pathways that contribute to NAFLD (e.g., agonists for PPAR γ, PPAR α/δ, FXR and analogs for FGF-21, and GLP-1). Others specifically inhibit key enzymes involved in lipid synthesis (e.g., mitochondrial pyruvate carrier, acetyl-CoA carboxylase, stearoyl-CoA desaturase, and monoacyl- and diacyl-glycerol transferases). Finally, a novel class of liver-targeted mitochondrial uncoupling agents increases hepatocellular energy expenditure, reversing the metabolic and hepatic complications of NAFLD.

Lipolysis in Brown Adipocytes Is Not Essential for Cold-Induced Thermogenesis in Mice

Lipid droplet (LD) lipolysis in brown adipose tissue (BAT) is generally considered to be required for cold-induced nonshivering thermogenesis. Here, we show that mice lacking BAT Comparative Gene Identification-58 (CGI-58), a lipolytic activator essential for the stimulated LD lipolysis, have normal thermogenic capacity and are not cold sensitive. Relative to littermate controls, these animals had higher body temperatures when they were provided food during cold exposure. The increase in body temperature in the fed, cold-exposed knockout mice was associated with increased energy expenditure and with increased sympathetic innervation and browning of white adipose tissue (WAT). Mice lacking CGI-58 in both BAT and WAT were cold sensitive, but only in the fasted state. Thus, LD lipolysis in BAT is not essential for cold-induced nonshivering thermogenesis in vivo. Rather, CGI-58-dependent LD lipolysis in BAT regulates WAT thermogenesis, and our data uncover an essential role of WAT lipolysis in fueling thermogenesis during fasting.