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

Maternal diethylhexyl phthalate exposure affects adiposity and insulin tolerance in offspring in a PCNA-dependent manner

The ubiquitous plasticizer, diethylhexyl phthalate (DEHP), is a known endocrine disruptor. However, DEHP exposure effects are not well understood. Changes in industrial and agricultural practices have resulted in increased prevalence of DEHP exposure and has coincided with the heightened occurrence of metabolic syndrome and obesity. DEHP and its metabolites are detected in the umbilical cord blood of newborns; however, the prenatal and perinatal effects of DEHP exposure have not been intensively studied. Previously, we discovered that phosphorylation (p) of proliferating cell nuclear antigen (PCNA) at tyrosine 114 (Y114) is required for adipogenesis and diet-induced obesity in mice. Here, we show the unique ability of DEHP to induce p-Y114 in PCNA in vitro. We also show that while DEHP promotes adipogenesis of wild type (WT) murine embryonic fibroblasts, mutation of Y114 to phenylalanine (Y114F) in PCNA blocked adipocyte differentiation. Given the induction of p-Y114 in PCNA by DEHP and the relationship to obesity, WT and Y114F PCNA mice were exposed to DEHP during gestation or lactation, followed by high fat diet feeding. Paradoxically, in utero exposure of Y114F PCNA females to DEHP led to a significant increase in body mass and was associated with augmented expression of PPARγ, a critical regulator of obesity, compared to WT controls. In utero exposure of WT mice to DEHP led to insulin sensitivity while Y114F mutation ablated this phenotype, indicating that PCNA is an important regulator of early DEHP exposure and ensuing metabolic phenotypes.

Regulation of hepatic glucose metabolism in health and disease

The liver is crucial for the maintenance of normal glucose homeostasis - it produces glucose during fasting and stores glucose postprandially. However, these hepatic processes are dysregulated in type 1 and type 2 diabetes mellitus, and this imbalance contributes to hyperglycaemia in the fasted and postprandial states. Net hepatic glucose production is the summation of glucose fluxes from gluconeogenesis, glycogenolysis, glycogen synthesis, glycolysis and other pathways. In this Review, we discuss the in vivo regulation of these hepatic glucose fluxes. In particular, we highlight the importance of indirect (extrahepatic) control of hepatic gluconeogenesis and direct (hepatic) control of hepatic glycogen metabolism. We also propose a mechanism for the progression of subclinical hepatic insulin resistance to overt fasting hyperglycaemia in type 2 diabetes mellitus. Insights into the control of hepatic gluconeogenesis by metformin and insulin and into the role of lipid-induced hepatic insulin resistance in modifying gluconeogenic and net hepatic glycogen synthetic flux are also discussed. Finally, we consider the therapeutic potential of strategies that target hepatosteatosis, hyperglucagonaemia and adipose lipolysis.

Maternal Exercise Improves Glucose Tolerance in Female Offspring

Poor maternal diet can lead to metabolic disease in offspring, whereas maternal exercise may have beneficial effects on offspring health. In this study, we determined ifmaternal exercise could reverse the detrimental effects of maternal high-fat feeding on offspring metabolism of female mice. C57BL/6 female mice were fed a chow (21%) or high-fat (60%) diet and further divided by housing in static cages or cages with running wheels for 2 weeks prior to breeding and throughout gestation. Females were bred with chow-fed sedentary C57BL/6 males. High fat-fed sedentary dams produced female offspring with impaired glucose tolerance compared with offspring of chow-fed dams throughout their first year of life, an effect not present in the offspring from high fat-fed dams that had trained. Offspring from high fat-fed trained dams had normalized glucose tolerance, decreased fasting insulin, and decreased adiposity. Liver metabolic function, measured by hepatic glucose production in isolated hepatocytes, hyperinsulinemic-euglycemic clamps, liver triglyceride content, and liver enzyme expression, was enhanced in offspring from trained dams. In conclusion, maternal exercise negates the detrimental effects of a maternal high-fat diet on glucose tolerance and hepatocyte glucose metabolism in female offspring. The ability of maternal exercise to improve the metabolic health of female offspring is important, as this intervention could combat the transmission of obesity and diabetes to subsequent generations.

Adipose tissue: between the extremes

Adipose tissue represents a critical component in healthy energy homeostasis. It fulfills important roles in whole-body lipid handling, serves as the body's major energy storage compartment and insulation barrier, and secretes numerous endocrine mediators such as adipokines or lipokines. As a consequence, dysfunction of these processes in adipose tissue compartments is tightly linked to severe metabolic disorders, including obesity, metabolic syndrome, lipodystrophy, and cachexia. While numerous studies have addressed causes and consequences of obesity-related adipose tissue hypertrophy and hyperplasia for health, critical pathways and mechanisms in (involuntary) adipose tissue loss as well as its systemic metabolic consequences are far less understood. In this review, we discuss the current understanding of conditions of adipose tissue wasting and review microenvironmental determinants of adipocyte (dys)function in related pathophysiologies.

Roles of Diacylglycerols and Ceramides in Hepatic Insulin Resistance

Although ample evidence links hepatic lipid accumulation with hepatic insulin resistance, the mechanistic basis of this association is incompletely understood and controversial. Diacylglycerols (DAGs) and ceramides have emerged as the two best-studied putative mediators of lipid-induced hepatic insulin resistance. Both lipids were first associated with insulin resistance in skeletal muscle and were subsequently hypothesized to mediate insulin resistance in the liver. However, the putative roles for DAGs and ceramides in hepatic insulin resistance have proved more complex than originally imagined, with various genetic and pharmacologic manipulations yielding a vast and occasionally contradictory trove of data to sort. In this review we examine the state of this field, turning a critical eye toward both DAGs and ceramides as putative mediators of lipid-induced hepatic insulin resistance.

The glucoregulatory actions of leptin

Background

The hormone leptin is an important regulator of metabolic homeostasis, able to inhibit food intake and increase energy expenditure. Leptin can also independently lower blood glucose levels, particularly in hyperglycemic models of leptin or insulin deficiency. Despite significant efforts and relevance to diabetes, the mechanisms by which leptin acts to regulate blood glucose levels are not fully understood.

Scope of review

Here we assess literature relevant to the glucose lowering effects of leptin. Leptin receptors are widely expressed in multiple cell types, and we describe both peripheral and central effects of leptin that may be involved in lowering blood glucose. In addition, we summarize the potential clinical application of leptin in regulating glucose homeostasis.

Major conclusions

Leptin exerts a plethora of metabolic effects on various tissues including suppressing production of glucagon and corticosterone, increasing glucose uptake, and inhibiting hepatic glucose output. A more in-depth understanding of the mechanisms of the glucose-lowering actions of leptin may reveal new strategies to treat metabolic disorders.

The impact of maternal obesity on inflammatory processes and consequences for later offspring health outcomes

Obesity is a global epidemic, affecting both developed and developing countries. The related metabolic consequences that arise from being overweight or obese are a paramount global health concern, and represent a significant burden on healthcare systems. Furthermore, being overweight or obese during pregnancy increases the risk of offspring developing obesity and other related metabolic complications in later life, which can therefore perpetuate a transgenerational cycle of obesity. Obesity is associated with a chronic state of low-grade metabolic inflammation. However, the role of maternal obesity-mediated alterations in inflammatory processes as a mechanism underpinning developmental programming in offspring is less understood. Further, the use of anti-inflammatory agents as an intervention strategy to ameliorate or reverse the impact of adverse developmental programming in the setting of maternal obesity has not been well studied. This review will discuss the impact of maternal obesity on key inflammatory pathways, impact on pregnancy and offspring outcomes, potential mechanisms and avenues for intervention.

A controlled-release mitochondrial protonophore reverses hypertriglyceridemia, nonalcoholic steatohepatitis, and diabetes in lipodystrophic mice

Lipodystrophy is a rare disorder characterized by complete or partial loss of adipose tissue. Patients with lipodystrophy exhibit hypertriglyceridemia, severe insulin resistance, type 2 diabetes, and nonalcoholic steatohepatitis (NASH). Efforts to ameliorate NASH in lipodystrophies with pharmacologic agents have met with limited success. We examined whether a controlled-release mitochondrial protonophore (CRMP) that produces mild liver-targeted mitochondrial uncoupling could decrease hypertriglyceridemia and reverse NASH and diabetes in a mouse model (fatless AZIP/F-1 mice) of severe lipodystrophy and diabetes. After 4 wk of oral CRMP (2 mg/kg body weight per day) or vehicle treatment, mice underwent hyperinsulinemic-euglycemic clamps combined with radiolabeled glucose to assess liver and muscle insulin responsiveness and tissue lipid measurements. CRMP treatment reversed hypertriglyceridemia and insulin resistance in liver and skeletal muscle. Reversal of insulin resistance could be attributed to reductions in diacylglycerol content and reduced PKC-ε and PKC-θ activity in liver and muscle respectively. CRMP treatment also reversed NASH as reflected by reductions in plasma aspartate aminotransferase and alanine aminotransferase concentrations; hepatic steatosis; and hepatic expression of IL-1α, -β, -2, -4, -6, -10, -12, CD69, and caspase 3 and attenuated activation of the IRE-1α branch of the unfolded protein response. Taken together, these results provide proof of concept for the development of liver-targeted mitochondrial uncoupling agents as a potential novel therapy for lipodystrophy-associated hypertriglyceridemia, NASH and diabetes.-Abulizi, A., Perry, R. J., Camporez, J. P. G., Jurczak, M. J., Petersen, K. F., Aspichueta, P., Shulman, G. I. A controlled-release mitochondrial protonophore reverses hypertriglyceridemia, nonalcoholic steatohepatitis, and diabetes in lipodystrophic mice.

Bone marrow adipose tissue as an endocrine organ: close to the bone?

White adipose tissue (WAT) is a major endocrine organ, secreting a diverse range of hormones, lipid species, cytokines and other factors to exert diverse local and systemic effects. These secreted products, known as 'adipokines', contribute extensively to WAT's impact on physiology and disease. Adipocytes also exist in the bone marrow (BM), but unlike WAT, study of this bone marrow adipose tissue (MAT) has been relatively limited. We recently discovered that MAT contributes to circulating adiponectin, an adipokine that mediates cardiometabolic benefits. Moreover, we found that MAT expansion exerts systemic effects. Together, these observations identify MAT as an endocrine organ. Additional studies are revealing further secretory functions of MAT, including production of other adipokines, cytokines and lipids that exert local effects within bone. These observations suggest that, like WAT, MAT has secretory functions with diverse potential effects, both locally and systemically. A major limitation is that these findings are often based on in vitro approaches that may not faithfully recapitulate the characteristics and functions of BM adipocytes in vivo. This underscores the need to develop improved methods for in vivo analysis of MAT function, including more robust transgenic models for MAT targeting, and continued development of techniques for non-invasive analysis of MAT quantity and quality in humans. Although many aspects of MAT formation and function remain poorly understood, MAT is now attracting increasing research focus; hence, there is much promise for further advances in our understanding of MAT as an endocrine organ, and how MAT impacts human health and disease.

The Metabolic Phenotype in Obesity: Fat Mass, Body Fat Distribution, and Adipose Tissue Function

The current obesity epidemic poses a major public health issue since obesity predisposes towards several chronic diseases. BMI and total adiposity are positively correlated with cardiometabolic disease risk at the population level. However, body fat distribution and an impaired adipose tissue function, rather than total fat mass, better predict insulin resistance and related complications at the individual level. Adipose tissue dysfunction is determined by an impaired adipose tissue expandability, adipocyte hypertrophy, altered lipid metabolism, and local inflammation. Recent human studies suggest that adipose tissue oxygenation may be a key factor herein. A subgroup of obese individuals - the 'metabolically healthy obese' (MHO) - have a better adipose tissue function, less ectopic fat storage, and are more insulin sensitive than obese metabolically unhealthy persons, emphasizing the central role of adipose tissue function in metabolic health. However, controversy has surrounded the idea that metabolically healthy obesity may be considered really healthy since MHO individuals are at increased (cardio)metabolic disease risk and may have a lower quality of life than normal weight subjects due to other comorbidities. Detailed metabolic phenotyping of obese persons will be invaluable in understanding the pathophysiology of metabolic disturbances, and is needed to identify high-risk individuals or subgroups, thereby paving the way for optimization of prevention and treatment strategies to combat cardiometabolic diseases.

Effect of 3-keto-1,5-bisphosphonates on obese-liver's rats

Obesity is associated with an oxidative stress status, which is defined by an excess of reactive oxygen species (ROS) vs. the antioxidant defense system. We report in this present work, the link between fat deposition and oxidative stress markers using a High Fat Diet-(HFD) induced rat obesity and liver-oxidative stress. We further determined the impact of chronic administration of 3-keto-1, 5-BPs 1 (a & b) (40μg/kg/8 weeks/i.p.) on liver's level. In fact, exposure of rats to HFD during 16 weeks induced body and liver weight gain and metabolic disruption with an increase on liver Alanine amino transférase (ALAT) and Aspartate aminotransférase (ASAT) concentration. HFD increased liver calcium level as well as free iron, whereas, it provoked a decrease on liver lipase activity. HFD also induced liver-oxidative stress status vocalized by an increase in reactive oxygen species (ROS) as superoxide radical (O₂), hydroxyl radical (OH) and Hydrogen peroxide (H₂O₂). Consequently, different deleterious damages as an increase on Malon Dialdehyde MDA, Carbonyl protein PC levels with a decrease in non-protein sulfhydryls NPSH concentrations, have been detected. Interestingly, our results demonstrate a decrease in antioxidant enzymes activities such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidases (GPx) and peroxidases (POD). Importantly, 3-keto-1,5-bisphosphonates treatment corrected the majority of the deleterious effects caused by HFD, but it failed to correct some liver's disruptions as mineral profile, oxidative damages (PC and NPSH levels) as well as SOD and lipase activities. Our investigation point that 3-keto-1,5-bisphosphonates could be considered as safe antioxidant agents on the hepatic level that should also find other potential biological applications.

Animal models of insulin resistance: A review

Insulin resistance can be seen as a molecular and genetic mystery, with a role in the pathophysiology of type 2 diabetes mellitus. It is a basis for a number of chronic diseases like hypertension, dyslipidemia, glucose intolerance, coronary heart disease, cerebral vascular disease along with T2DM, thus the key is to cure and prevent insulin resistance. Critical perspicacity into the etiology of insulin resistance have been gained by the use of animal models where insulin action has been modulated by various transgenic and non-transgenic models which is not possible in human studies. The following review comprises the pathophysiology involved in insulin resistance, various factors causing insulin resistance, their screening and various genetic and non-genetic animal models highlighting the pathological and metabolic characteristics of each.

Pleotropic effects of leptin to reverse insulin resistance and diabetic ketoacidosis

In this review we discuss the mechanisms for the pleotropic effects of leptin replacement therapy to reverse liver and muscle insulin resistance in lipodystrophic individuals, as well as insulin-independent effects of leptin replacement therapy to suppress white adipose tissue lipolysis, hepatic gluconeogenesis and fasting hyperglycaemia in rodent models of poorly controlled diabetes. On the basis of these studies we conclude with a view of the potential therapeutic applications of leptin replacement therapy in humans. This review summarises a presentation given at the 'Is leptin coming back?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Thomas Meek and Gregory Morton, DOI: 10.1007/s00125-016-3898-3 , and by Christoffer Clemmensen and colleagues, DOI: 10.1007/s00125-016-3906-7 ) and an overview by the Session Chair, Ulf Smith (DOI: 10.1007/s00125-016-3894-7 ).

The pathogenesis of insulin resistance: integrating signaling pathways and substrate flux

Insulin resistance arises when the nutrient storage pathways evolved to maximize efficient energy utilization are exposed to chronic energy surplus. Ectopic lipid accumulation in liver and skeletal muscle triggers pathways that impair insulin signaling, leading to reduced muscle glucose uptake and decreased hepatic glycogen synthesis. Muscle insulin resistance, due to ectopic lipid, precedes liver insulin resistance and diverts ingested glucose to the liver, resulting in increased hepatic de novo lipogenesis and hyperlipidemia. Subsequent macrophage infiltration into white adipose tissue (WAT) leads to increased lipolysis, which further increases hepatic triglyceride synthesis and hyperlipidemia due to increased fatty acid esterification. Macrophage-induced WAT lipolysis also stimulates hepatic gluconeogenesis, promoting fasting and postprandial hyperglycemia through increased fatty acid delivery to the liver, which results in increased hepatic acetyl-CoA content, a potent activator of pyruvate carboxylase, and increased glycerol conversion to glucose. These substrate-regulated processes are mostly independent of insulin signaling in the liver but are dependent on insulin signaling in WAT, which becomes defective with inflammation. Therapies that decrease ectopic lipid storage and diminish macrophage-induced WAT lipolysis will reverse the root causes of type 2 diabetes.

Gsα deficiency in adipose tissue improves glucose metabolism and insulin sensitivity without an effect on body weight

Gsα, the G protein that transduces receptor-stimulated cAMP generation, mediates sympathetic nervous system stimulation of brown adipose tissue (BAT) thermogenesis and browning of white adipose tissue (WAT), which are both potential targets for treating obesity, as well as lipolysis. We generated a mouse line with Gsα deficiency in mature BAT and WAT adipocytes (Ad-GsKO). Ad-GsKO mice had impaired BAT function, absent browning of WAT, and reduced lipolysis, and were therefore cold-intolerant. Despite the presence of these abnormalities, Ad-GsKO mice maintained normal energy balance on both standard and high-fat diets, associated with decreases in both lipolysis and lipid synthesis. In addition, Ad-GsKO mice maintained at thermoneutrality on a standard diet also had normal energy balance. Ad-GsKO mice had improved insulin sensitivity and glucose metabolism, possibly secondary to the effects of reduced lipolysis and lower circulating fatty acid binding protein 4 levels. Gsα signaling in adipose tissues may therefore affect whole-body glucose metabolism in the absence of an effect on body weight.

Downregulation of de Novo Fatty Acid Synthesis in Subcutaneous Adipose Tissue of Moderately Obese Women

The purpose of this work was to evaluate the expression of fatty acid metabolism-related genes in human adipose tissue from moderately obese women. We used qRT-PCR and Western Blot to analyze visceral (VAT) and subcutaneous (SAT) adipose tissue mRNA expression involved in de novo fatty acid synthesis (ACC1, FAS), fatty acid oxidation (PPARα, PPARδ) and inflammation (IL6, TNFα), in normal weight control women (BMI < 25 kg/m², n = 35) and moderately obese women (BMI 30–38 kg/m², n = 55). In SAT, ACC1, FAS and PPARα mRNA expression were significantly decreased in moderately obese women compared to controls. The downregulation reported in SAT was more pronounced when BMI increased. In VAT, lipogenic-related genes and PPARα were similar in both groups. Only PPARδ gene expression was significantly increased in moderately obese women. As far as inflammation is concerned, TNFα and IL6 were significantly increased in moderate obesity in both tissues. Our results indicate that there is a progressive downregulation in lipogenesis in SAT as BMI increases, which suggests that SAT decreases the synthesis of fatty acid de novo during the development of obesity, whereas in VAT lipogenesis remains active regardless of the degree of obesity.

Metabolic syndrome and nonalcoholic fatty liver disease: Is insulin resistance the link?

Metabolic syndrome (MetS) is a disease composed of different risk factors such as obesity, type 2 diabetes or dyslipidemia. The prevalence of this syndrome is increasing worldwide in parallel with the rise in obesity. Nonalcoholic fatty liver disease (NAFLD) is now the most frequent chronic liver disease in western countries, affecting more than 30% of the general population. NAFLD encompasses a spectrum of liver manifestations ranging from simple steatosis to nonalcoholic steatohepatitis (NASH), fibrosis and cirrhosis, which may ultimately progress to hepatocellular carcinoma. There is accumulating evidence supporting an association between NAFLD and MetS. Indeed, NAFLD is recognized as the liver manifestation of MetS. Insulin resistance is increasingly recognized as a key factor linking MetS and NAFLD. Insulin resistance is associated with excessive fat accumulation in ectopic tissues, such as the liver, and increased circulating free fatty acids, which can further promote inflammation and endoplasmic reticulum stress. This in turn aggravates and maintains the insulin resistant state, constituting a vicious cycle. Importantly, evidence shows that most of the patients developing NAFLD present at least one of the MetS traits. This review will define MetS and NAFLD, provide an overview of the common pathophysiological mechanisms linking MetS and NAFLD, and give a perspective regarding treatment of these ever growing metabolic diseases.

Adipocyte Metrnl Antagonizes Insulin Resistance Through PPARγ Signaling

Adipokines play important roles in metabolic homeostasis and disease. We have recently identified a novel adipokine Metrnl, also known as Subfatin, for its high expression in subcutaneous fat. Here, we demonstrate a prodifferentiation action of Metrnl in white adipocytes. Adipocyte-specific knockout of Metrnl exacerbates insulin resistance induced by high-fat diet (HFD), whereas adipocyte-specific transgenic overexpression of Metrnl prevents insulin resistance induced by HFD or leptin deletion. Body weight and adipose content are not changed by adipocyte Metrnl. Consistently, no correlation is found between serum Metrnl level and BMI in humans. Metrnl promotes white adipocyte differentiation, expandability, and lipid metabolism and inhibits adipose inflammation to form functional fat, which contributes to its activity against insulin resistance. The insulin sensitization of Metrnl is blocked by PPARγ inhibitors or knockdown. However, Metrnl does not drive white adipose browning. Acute intravenous injection of recombinant Metrnl has no hypoglycemic effect, and 1-week intravenous administration of Metrnl is unable to rescue insulin resistance exacerbated by adipocyte Metrnl deficiency. Our results suggest adipocyte Metrnl controls insulin sensitivity at least via its local autocrine/paracrine action through the PPARγ pathway. Adipocyte Metrnl is an inherent insulin sensitizer and may become a therapeutic target for insulin resistance.

[Role of metabolic lipases and lipotoxicity in the development of non-alcoholic steatosis and non-alcoholic steatohepatitis]

Non-alcoholic fatty liver disease (NAFLD) has become the most common liver disease in developed countries, covering a spectrum of pathological conditions ranging from single steatosis to non-alcoholic steatohepatitis, cirrhosis and hepatocellular carcinoma. Its pathogenesis has been often interpreted by the "double-hit" hypothesis, where the lipid accumulation in the liver is followed by proinflammatory mediators inducing inflammation, hepatocellular injury and fibrosis. Nowadays, a more complex model suggests that free fatty acids and their metabolites could be the true lipotoxic agents that contribute to the development of NAFLD and hepatic insulin resistance, suggesting a central role for metabolic lipases in that process.

Increasing adipocyte lipoprotein lipase improves glucose metabolism in high fat diet-induced obesity

Lipid accumulation in liver and skeletal muscle contributes to co-morbidities associated with diabetes and obesity. We made a transgenic mouse in which the adiponectin (Adipoq) promoter drives expression of lipoprotein lipase (LPL) in adipocytes to potentially increase adipose tissue lipid storage. These mice (Adipoq-LPL) have improved glucose and insulin tolerance as well as increased energy expenditure when challenged with a high fat diet (HFD). To identify the mechanism(s) involved, we determined whether the Adipoq-LPL mice diverted dietary lipid to adipose tissue to reduce peripheral lipotoxicity, but we found no evidence for this. Instead, characterization of the adipose tissue of the male mice after HFD challenge revealed that the mRNA levels of peroxisome proliferator-activated receptor-γ (PPARγ) and a number of PPARγ-regulated genes were higher in the epididymal fat pads of Adipoq-LPL mice than control mice. This included adiponectin, whose mRNA levels were increased, leading to increased adiponectin serum levels in the Adipoq-LPL mice. In many respects, the adipose phenotype of these animals resembles thiazolidinedione treatment except for one important difference, the Adipoq-LPL mice did not gain more fat mass on HFD than control mice and did not have increased expression of genes in adipose such as glycerol kinase, which are induced by high affinity PPAR agonists. Rather, there was selective induction of PPARγ-regulated genes such as adiponectin in the adipose of the Adipoq-LPL mice, suggesting that increasing adipose tissue LPL improves glucose metabolism in diet-induced obesity by improving the adipose tissue phenotype. Adipoq-LPL mice also have increased energy expenditure.

Lipodystrophic diabetes mellitus: a lesson for other forms of diabetes?

Lipodystrophies are a genetically heterogeneous group of disorders characterized by loss of subcutaneous adipose tissue and metabolic dysfunction, including insulin resistance, increased levels of free fatty acids, abnormal adipocytokine secretion, and ectopic fat deposition, which are also observed in patients with visceral obesity and/or type 2 diabetes mellitus. Pathophysiological, biochemical, and genetic studies suggest that impairment in multiple adipose tissue functions, including adipocyte maturation, lipid storage, formation and/or maintenance of the lipid droplet, membrane composition, DNA repair efficiency, and insulin signaling, results in severe metabolic and endocrine consequences, ultimately leading to specific lipodystrophic phenotypes. In this review, recent evidences on the causes and metabolic processes of lipodystrophies will be presented, proposing a disease model that could be potentially informative for better understanding of common metabolic diseases in humans, including obesity, metabolic syndrome, and type 2 diabetes.

Fatty acid synthase cooperates with glyoxalase 1 to protect against sugar toxicity

Fatty acid (FA) metabolism is deregulated in several human diseases including metabolic syndrome, type 2 diabetes and cancers. Therefore, FA-metabolic enzymes are potential targets for drug therapy, although the consequence of these treatments must be precisely evaluated at the organismal and cellular levels. In healthy organism, synthesis of triacylglycerols (TAGs)—composed of three FA units esterified to a glycerol backbone—is increased in response to dietary sugar. Saturation in the storage and synthesis capacity of TAGs is associated with type 2 diabetes progression. Sugar toxicity likely depends on advanced-glycation-end-products (AGEs) that form through covalent bounding between amine groups and carbonyl groups of sugar or their derivatives α-oxoaldehydes. Methylglyoxal (MG) is a highly reactive α-oxoaldehyde that is derived from glycolysis through a non-enzymatic reaction. Glyoxalase 1 (Glo1) works to neutralize MG, reducing its deleterious effects. Here, we have used the power of Drosophila genetics to generate Fatty acid synthase (FASN) mutants, allowing us to investigate the consequence of this deficiency upon sugar-supplemented diets. We found that FASN mutants are lethal but can be rescued by an appropriate lipid diet. Rescued animals do not exhibit insulin resistance, are dramatically sensitive to dietary sugar and accumulate AGEs. We show that FASN and Glo1 cooperate at systemic and cell-autonomous levels to protect against sugar toxicity. We observed that the size of FASN mutant cells decreases as dietary sucrose increases. Genetic interactions at the cell-autonomous level, where glycolytic enzymes or Glo1 were manipulated in FASN mutant cells, revealed that this sugar-dependent size reduction is a direct consequence of MG-derived-AGE accumulation. In summary, our findings indicate that FASN is dispensable for cell growth if extracellular lipids are available. In contrast, FA-synthesis appears to be required to limit a cell-autonomous accumulation of MG-derived-AGEs, supporting the notion that MG is the most deleterious α-oxoaldehyde at the intracellular level.

Consumption of sugar and lipid (fat) enriched food increases the risk of developing metabolic diseases and cancers. However, lipids are essential molecules for life, as they are the major components of cell membranes. Metabolism refers to biochemical reactions that transform nutrients into molecules required by an organism, although toxic by-products can also formed. Sugars or their derivatives are likely to induce toxic effects by forming stable conjugates with proteins. To neutralize their toxic potential, sugars are metabolized and stored as fat. Here, we have used the fruitfly model to investigate the consequences of lipogenesis deficiency upon ingestion of sugar-enriched diets. We show that lipogenesis deficient animals are dramatically sensitive to dietary sugar. Further, we have identified the sugar by-product responsible for intracellular toxicity, in the context of lipogenesis inhibition. Our study reveals that inhibiting lipogenesis does not disrupt cellular growth if extracellular lipids are available. In contrast lipogenesis inhibition may have deleterious consequences due to accumulation of toxic by-products. The efficacy of lipogenic inhibitors in fighting cancers and metabolic diseases is currently under investigation. Therefore, to evaluate the clinical benefit of these inhibitors, accumulation of the toxic molecules should be monitored in both sick and healthy cells.

Leptin in congenital and HIV-associated lipodystrophy

Leptin is a hormone secreted by adipocytes that regulates energy metabolism via peripheral action on glucose synthesis and utilization as well as through central regulation of food intake. Patients with decreased amounts of fat in their adipose tissue (lipoatrophy) will have low leptin levels, and hypoleptinemic states have been associated with a variety of metabolic dysfunctions. Pronounced complications of insulin resistance, dyslipidemia and fatty liver are observed in patients suffering from congenital or acquired generalized lipodystrophy while somewhat less pronounced abnormalities are associated with human immunodeficiency virus (HIV) and the use of highly active antiretroviral therapy, the so-called HIV-associated lipodystrophy. Previous uncontrolled open-label studies have demonstrated that physiological doses of leptin repletion have corrected many of the metabolic derangements observed in subjects with rare fat maldistribution syndromes such as generalized lipodystrophy. In the much more commonly encountered HIV-associated lipodystrophy, leptin replacement has been shown to decrease central fat mass and to improve insulin sensitivity, dyslipidemia, and glucose levels. The United States Food and Drug Administration has recently granted approval for recombinant leptin therapy for congenital and acquired generalized lipodystrophy, however large, well-designed, placebo-controlled studies are needed to assess long-term efficacy, safety and adverse effects of leptin replacement. In this review, we present the role of leptin in the metabolic complications of congenital and acquired lipodystrophy and discuss current and emerging clinical therapeutic uses of leptin in humans with lipodystrophy.

Ectopic visceral fat: a clinical and molecular perspective on the cardiometabolic risk

Worldwide, cardiovascular diseases (CVDs) are a leading cause of mortality. While in many westernized societies there has been a decrease prevalence of smoking and that a special emphasis has been put on the urgency to control the, so called, classical risk factors, it is more and more recognized that there remains a residual risk, which contributes to the development of CVDs. Imaging studies conducted over two decades have highlighted that the accumulation of ectopic visceral fat is associated with a plethora of metabolic dysfunctions, which have complex and intertwined interactions and participate to the development/progression/events of many cardiovascular disorders. The contribution of visceral ectopic fat to the development of coronary artery disease (CAD) is now well established, while in the last several years emerging evidence has pointed out that accumulation of harmful ectopic fat is associated with other cardiovascular disorders such as calcific aortic valve disease (CAVD), atrial fibrillation and left ventricular dysfunction. We review herein the key molecular processes linking the accumulation of ectopic fat to the development of CVDs. We have attempted, whenever possible, to use a translational approach whereby the pathobiology processes are linked to clinical observations.

In vivo 2H2O administration reveals impaired triglyceride storage in adipose tissue of insulin-resistant humans

Indirect evidence suggests that impaired triglyceride storage in the subcutaneous fat depot contributes to the development of insulin resistance via lipotoxicity. We directly tested this hypothesis by measuring, in vivo, TG synthesis, de novo lipogenesis (DNL), adipocyte proliferation, and insulin suppression of lipolysis in subcutaneous adipose tissue of BMI-matched individuals classified as insulin resistant (IR) or insulin sensitive (IS). Nondiabetic, moderately obese subjects with BMI 25-35 kg/m(2), classified as IR or IS by the modified insulin suppression test, consumed deuterated water ((2)H2O) for 4 weeks. Deuterium incorporation into glycerol, palmitate, and DNA indicated TG synthesis, DNL, and adipocyte proliferation, respectively. Net TG synthesis and DNL in adipose cells were significantly lower in IR as compared with IS subjects, whereas adipocyte proliferation did not differ significantly. Plasma FFAs measured during an insulin suppression test were 2.5-fold higher in IR subjects, indicating resistance to insulin suppression of lipolysis. Adipose TG synthesis correlated directly with DNL but not with proliferation. These results provide direct in vivo evidence for impaired TG storage in subcutaneous adipose tissue of IR as compared with IS. Relative inability to store TG in the subcutaneous depot may represent a mechanism contributing to the development of insulin resistance in the setting of obesity.

Molecular mechanisms underlying fasting modulated liver insulin sensitivity and metabolism in male 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, hepatomegly, hepatic steatosis, and insulin resistance. The mechanisms that underlie hepatic steatosis and insulin resistance in Bscl2(-/-) mice are poorly understood. To address this issue, we performed hyperinsulinemic-euglycemic clamp on Bscl2(-/-) and wild-type mice after an overnight (16-h) fast, and found that Bscl2(-/-) actually displayed increased hepatic insulin sensitivity. Interestingly, liver in Bscl2(-/-) mice after a short term (4-h) fast had impaired acute insulin signaling, a defect that disappeared after a 16-hour fast. Notably, fasting-dependent hepatic insulin signaling in Bscl2(-/-) mice was not associated with liver diacylglyceride and ceramide contents, but could be attributable in part to the expression of hepatic insulin signaling receptor and substrates. Meanwhile, increased de novo lipogenesis and decreased β-oxidation led to severe hepatic steatosis in fed or short-fasted Bscl2(-/-) mice whereas liver lipid accumulation and metabolism in Bscl2(-/-) mice was markedly affected by prolonged fasting. Furthermore, mice with liver-specific inactivation of Bscl2 manifested no hepatic steatosis even under high-fat diet, suggesting Bscl2 does not play a cell autonomous role in regulating liver lipid homeostasis. Overall, our results offered new insights into the metabolic adaptations of liver in response to fasting and uncovered a novel fasting-dependent regulation of hepatic insulin signaling in a mouse model of human BSCL2.

Downregulation of lipogenesis and fatty acid oxidation in the subcutaneous adipose tissue of morbidly obese women

OBJECTIVE

The aim of this study was to analyse the expression of crucial genes in fatty acid metabolism in visceral (VAT) and subcutaneous (SAT) adipose tissue samples from morbidly obese women.

METHODS

The VAT and SAT expression of key genes in 145 morbidly obese women (MO, BMI > 40 Kg/m(2) ) and 18 normal weight control women by RT-PCR and Western Blot was analyzed.

RESULTS

In SAT, the expression levels of the genes related to lipogenesis and fatty acid oxidation were significantly lower in MO than in controls. In VAT, most of the lipogenic genes studied had similar expression levels in MO and control cohort. Regarding inflammation, IL6 was significantly higher in MO in both tissues whereas TNFα mRNA expression was significantly higher only in VAT.

CONCLUSIONS

Our results indicate that in morbidly obese patients, lipogenesis and fatty acid oxidation are downregulated in SAT, whereas in VAT these pathways are almost unchanged. By contrast, inflammation is induced in both adipose tissues. It is hypothesized that, in this type of extreme obesity, SAT works to limit any further development of fat mass, decreasing the expression of lipogenic and FA oxidative genes whereas VAT depot might have lost this capability.

Type 2 diabetes as a disease of ectopic fat?

BACKGROUND

Although obesity and diabetes commonly co-exist, the evidence base to support obesity as the major driver of type 2 diabetes mellitus (T2DM), and the mechanisms by which this occurs, are now better appreciated.

DISCUSSION

This review briefly examines several sources of evidence - epidemiological, genetic, molecular, and clinical trial - to support obesity being a causal risk factor for T2DM. It also summarises the ectopic fat hypothesis for this condition, and lists several pieces of evidence to support this concept, extending from rare conditions and drug effects to sex- and ethnicity-related differences in T2DM prevalence. Ectopic liver fat is the best-studied example of ectopic fat, but more research on pancreatic fat as a potential cause of β-cell dysfunction seems warranted. This ectopic fat concept, in turn, broadly fits with the observation that individuals of similar ages can develop diabetes at markedly different body mass indexes (BMIs). Those with risk factors leading to more rapid ectopic fat gain - for example, men (compared with women), certain ethnicities, and potentially those with a family history of diabetes, as well as others with genes linked to a reduced subcutaneous adiposity - are more likely to develop diabetes at a younger age and/or lower BMI than those without.

SUMMARY

Obesity is the major risk factor for T2DM and appears to drive tissue insulin resistance in part via gain of ectopic fat, with the best-studied organ being the liver. However, ectopic fat in the pancreas may contribute to β-cell dysfunction. In line with this observation, rapid resolution of diabetes linked to a preferential and rapid reduction in liver fat has been noted with significant caloric reduction. Whether these observations can help develop better cost-effective and sustainable lifestyle /medical interventions in patients with T2DM requires further study.

Carotid body denervation prevents fasting hyperglycemia during chronic intermittent hypoxia

Obstructive sleep apnea causes chronic intermittent hypoxia (IH) and is associated with impaired glucose metabolism, but mechanisms are unknown. Carotid bodies orchestrate physiological responses to hypoxemia by activating the sympathetic nervous system. Therefore, we hypothesized that carotid body denervation would abolish glucose intolerance and insulin resistance induced by chronic IH. Male C57BL/6J mice underwent carotid sinus nerve dissection (CSND) or sham surgery and then were exposed to IH or intermittent air (IA) for 4 or 6 wk. Hypoxia was administered by decreasing a fraction of inspired oxygen from 20.9% to 6.5% once per minute, during the 12-h light phase (9 a.m.-9 p.m.). As expected, denervated mice exhibited blunted hypoxic ventilatory responses. In sham-operated mice, IH increased fasting blood glucose, baseline hepatic glucose output (HGO), and expression of a rate-liming hepatic enzyme of gluconeogenesis phosphoenolpyruvate carboxykinase (PEPCK), whereas the whole body glucose flux during hyperinsulinemic euglycemic clamp was not changed. IH did not affect glucose tolerance after adjustment for fasting hyperglycemia in the intraperitoneal glucose tolerance test. CSND prevented IH-induced fasting hyperglycemia and increases in baseline HGO and liver PEPCK expression. CSND trended to augment the insulin-stimulated glucose flux and enhanced liver Akt phosphorylation at both hypoxic and normoxic conditions. IH increased serum epinephrine levels and liver sympathetic innervation, and both increases were abolished by CSND. We conclude that chronic IH induces fasting hyperglycemia increasing baseline HGO via the CSN sympathetic output from carotid body chemoreceptors, but does not significantly impair whole body insulin sensitivity.

Chlorinated persistent organic pollutants, obesity, and type 2 diabetes

Persistent organic pollutants (POPs) are lipophilic compounds that travel with lipids and accumulate mainly in adipose tissue. Recent human evidence links low-dose POPs to an increased risk of type 2 diabetes (T2D). Because humans are contaminated by POP mixtures and POPs possibly have nonmonotonic dose-response relations with T2D, critical methodological issues arise in evaluating human findings. This review summarizes epidemiological results on chlorinated POPs and T2D, and relevant experimental evidence. It also discusses how features of POPs can affect inferences in humans. The evidence as a whole suggests that, rather than a few individual POPs, background exposure to POP mixtures-including organochlorine pesticides and polychlorinated biphenyls-can increase T2D risk in humans. Inconsistent statistical significance for individual POPs may arise due to distributional differences in POP mixtures among populations. Differences in the observed shape of the dose-response curves among human studies may reflect an inverted U-shaped association secondary to mitochondrial dysfunction or endocrine disruption. Finally, we examine the relationship between POPs and obesity. There is evidence in animal studies that low-dose POP mixtures are obesogenic. However, relationships between POPs and obesity in humans have been inconsistent. Adipose tissue plays a dual role of promoting T2D and providing a relatively safe place to store POPs. Large prospective studies with serial measurements of a broad range of POPs, adiposity, and clinically relevant biomarkers are needed to disentangle the interrelationships among POPs, obesity, and the development of T2D. Also needed are laboratory experiments that more closely mimic real-world POP doses, mixtures, and exposure duration in humans.

The role of hepatic lipids in hepatic insulin resistance and type 2 diabetes

Non-alcoholic fatty liver disease and its downstream sequelae, hepatic insulin resistance and type 2 diabetes, are rapidly growing epidemics, which lead to increased morbidity and mortality rates, and soaring health-care costs. Developing interventions requires a comprehensive understanding of the mechanisms by which excess hepatic lipid develops and causes hepatic insulin resistance and type 2 diabetes. Proposed mechanisms implicate various lipid species, inflammatory signalling and other cellular modifications. Studies in mice and humans have elucidated a key role for hepatic diacylglycerol activation of protein kinase Cε in triggering hepatic insulin resistance. Therapeutic approaches based on this mechanism could alleviate the related epidemics of non-alcoholic fatty liver disease and type 2 diabetes.

Changes in FGF21 serum concentrations and liver mRNA expression in an experimental model of complete lipodystrophy and insulin-resistant diabetes

Patients with obesity and type 2 diabetes often display high levels of the anti-diabetic factor fibroblast growth factor-21 (FGF21), suggesting that the overproduction of FGF21 may result from increased adiposity in an attempt by white adipose tissue (WAT) to counteract insulin resistance. However, the production of FGF21 diabetes in the absence of WAT has not been examined. In this study, we investigated the effects of lipodystrophy in A-ZIP F-1 mice on FGF21 production in relation to diabetes. A-ZIP F-1 mice displayed high FGF21 plasma levels resulting from enhanced FGF21 mRNA expression in the liver. Concomitant enhancement of FGF21 receptor (FGFR1) and glucose transporter 1 (GLUT-1) mRNA expression was observed in the muscles of A-ZIP F-1 mice. Furthermore, the activation of hypothalamic NPY and AgRP mRNA expression positively correlated with plasma levels of FGF21 but not active ghrelin. Our study demonstrates that an increased FGF21 plasma level in lipodystrophic A-ZIP F-1 mice results mainly from up-regulated liver production but does not suffice to overcome the lipodystrophy-induced severe type 2-diabetes and insulin resistance in the liver linked to the augmented liver fat deposition.

Biphasic response of mitochondrial biogenesis to oxidative stress in visceral fat of diet-induced obesity mice

AIMS

Studies in skeletal muscle demonstrate a strong association of mitochondrial dysfunction with insulin resistance (IR). However, there is still a paucity of knowledge regarding the alteration of mitochondria in adipose tissue (AT) in the pathogenesis of IR in obesity. We investigated the mitochondrial biogenesis in visceral fat (VF) and subcutaneous fat (SF) in C57BL/6J mice fed a high-fat high-sucrose diet for 12 months.

RESULTS

Impairment of glucose tolerance and insulin sensitivity developed after 1 month of the diet and was associated with a prompt increase of VF. The VF adipocytes were larger than those in the SF and had increased expressions of HIF-1α and p-NFκB p65. However, the alteration of mitochondrial biogenesis did not occur in the early stage when increased intracellular reactive oxygen species (ROS), mitochondrial oxygen consumption rate, and mitochondrial ROS emerged at the 1st, 2nd and 2nd month, respectively. Until the 6th month, the VF had markedly increased mitochondrial DNA content and expression of PGC-1α, Tfam, ATP5A, and MnSOD. This increase of mitochondrial biogenesis was followed by a generalized decrease at the 12th month and the mitochondrial morphology altered markedly. In the late stage, although mitochondrial ROS decreased, the increased expression of 8-OHdG in VF continued.

INNOVATION AND CONCLUSION

These data suggest that IR and ROS production occur before the biphasic changes of mitochondrial biogenesis in AT, and the VF plays a more crucial role.

Saponin as regulator of biofuel: implication for ethnobotanical management of diabetes

There has been a sharp rise in the global prevalence of diabetes, obesity, and their comorbid conditions within the last decade prompting significant research into possible causes and cure via therapeutic intervention and lifestyle adjustments. Here, the molecular bases of antidiabetic plants used in the prehistorical treatment of diabetes and obesity are reviewed with particular focus on saponin as the phytotherapeutic principle. Until recently, the phytotherapeutic potentials of saponins have been masked in the heterogeneity of phytochemicals co-extractable during traditional preparations. With improved technique of purification and cutting edge biological assay methods, saponins have emerged as a regulator of primary biofuel availability through direct interaction with energy metabolism, cell signaling, and gene expression. Specific cases of lipoprotein lipase/peroxisome proliferator-activated receptor (PPAR)-gamma/phosphatidylinositide 3-kinase (PI-3-K)/protein kinase B (Akt) activation, adiponectin gene upregulation, fatty acid binding protein 4 repression (FABP4), and glucose transporter type 4 (Glut4) membrane exocytosis have been documented which provide molecular basis for hypocholesterolemic, hypoglycemic, and anti-obesity manifestations observed in experimental animals following saponin treatment. Although intensified research is required to characterize the pharmacophoric features in saponins exhibiting these interactions, however, this preliminary lead is valuable if the world will be free of diabetes, obesity, hypertension, hyperlipidemia, and atherosclerosis in no distant future.

Nonalcoholic fatty liver disease, hepatic insulin resistance, and type 2 diabetes

Nonalcoholic fatty liver disease (NAFLD), hepatic insulin resistance, and type 2 diabetes are all strongly associated and are all reaching epidemic proportions. Whether there is a causal link between NAFLD and hepatic insulin resistance is controversial. This review will discuss recent studies in both humans and animal models of NAFLD that have implicated increases in hepatic diacylglycerol (DAG) content leading to activation of novel protein kinase Cϵ (PKCϵ) resulting in decreased insulin signaling in the pathogenesis of NAFLD-associated hepatic insulin resistance and type 2 diabetes. The DAG-PKCϵ hypothesis can explain the occurrence of hepatic insulin resistance observed in most cases of NAFLD associated with obesity, lipodystrophy, and type 2 diabetes.

Macrophage-mediated glucolipotoxicity via myeloid-related protein 8/toll-like receptor 4 signaling in diabetic nephropathy

Dyslipidemia is an independent risk factor for the development and progression of diabetic nephropathy (DN). In this review, we summarize mouse models with both diabetes and dyslipidemia, and their associated complications. We then discuss molecules potentially involved in deterioration of DN by dyslipidemia. We focus especially upon toll-like receptor 4 (TLR4) and one of its endogenous ligands, myeloid-related protein 8 (MRP8 or S100A8), since we have found that their mRNA levels are commonly increased in glomeruli of type 1 (streptozotocin [STZ]-induced) and type 2 (A-ZIP/F-1 lipoatrophic) diabetic mice. Gene expression of MRP8 and Tlr4 is further upregulated during worsening of STZ-induced DN by a high fat diet (HFD). Moreover, these HFD-induced changes are accompanied by enhanced gene expression of CCAAT element binding protein β and phosphorylation of c-Jun N-terminal kinase in the kidney, which have also been reported in pancreatic β cells under diabetic-hyperlipidemic conditions. Effects of a HFD upon DN are cancelled in Tlr4 knockout mice. Macrophages are the predominant source of MRP8 in glomeruli. In cultured macrophages, combinatorial treatment with high glucose and palmitate amplifies MRP8 expression in a Tlr4-dependent manner, and recombinant MRP8 protein markedly increases gene expression of the inflammatory cytokines interleukin-1β and tumor necrosis factor α. Here, we propose ‘macrophage-mediated glucolipotoxicity’ via activation of MRP8/TLR4 signaling as a novel mechanism of pathophysiology for DN.

Citrange fruit extracts alleviate obesity-associated metabolic disorder in high-fat diet-induced obese C57BL/6 mouse

Obesity is becoming one of the global epidemics of the 21st century. In this study, the effects of citrange (Citrus sinensis × Poncirus trifoliata) fruit extracts in high-fat (HF) diet-induced obesity mice were studied. Female C57BL/6 mice were fed respectively a chow diet (control), an HF diet, HF diet supplemented with 1% w/w citrange peel extract (CPE) or 1% w/w citrange flesh and seed extract (CFSE) for 8 weeks. Our results showed that both CPE and CFSE regulated the glucose metabolic disorders of obese mice. In CPE and CFSE-treated groups, the body weight gain, blood glucose, serum total cholesterol (TC) and low density lipoprotein cholesterol (LDL-c) levels were significantly (p < 0.05) reduced relative to those in the HF group. To explore the mechanisms of action of CPE and CFSE on the metabolism of glucose and lipid, related genes’ expressions in liver were assayed. In liver tissue, the expression level of peroxisome proliferator-activated receptor γ (PPARγ) and its target genes were down-regulated by CPE and CFSE supplementation as revealed by qPCR tests. In addition, both CPE and CFSE decreased the expression level of liver X receptor (LXR) α and β, which are involved in lipid and glucose metabolism. Taken together, these results suggest that CPE and CFSE administration could ameliorate obesity and related metabolic disorders in HF diet-induced obesity mice probably through the inhibition of PPARγ and LXRs gene expressions.

Leptin ameliorates insulin resistance and hepatic steatosis in Agpat2-/- lipodystrophic mice independent of hepatocyte leptin receptors

Leptin is essential for energy homeostasis and regulation of food intake. Patients with congenital generalized lipodystrophy (CGL) due to mutations in 1-acylglycerol-3-phosphate-O-acyltransferase 2 (AGPAT2) and the CGL murine model (Agpat2(-/-) mice) both have severe insulin resistance, diabetes mellitus, hepatic steatosis, and low plasma leptin levels. In this study, we show that continuous leptin treatment of Agpat2(-/-) mice for 28 days reduced plasma insulin and glucose levels and normalized hepatic steatosis and hypertriglyceridemia. Leptin also partially, but significantly, reversed the low plasma thyroxine and high corticosterone levels found in Agpat2(-/-) mice. Levels of carbohydrate response element binding protein (ChREBP) were reduced, whereas lipogenic gene expression were increased in the livers of Agpat2(-/-) mice, suggesting that deregulated ChREBP contributed to the development of fatty livers in these mice and that this transcription factor is a target of leptin's beneficial metabolic action. Leptin administration did not change hepatic fatty acid oxidation enzymes mRNA levels in Agpat2(-/-) mice. The selective deletion of leptin receptors only in hepatocytes did not prevent the positive metabolic actions of leptin in Agpat2(-/-) mice, supporting the notion that the majority of metabolic actions of leptin are dependent on its action in nonhepatocyte cells and/or the central nervous system.

Immune cells and metabolic dysfunction

Throughout evolution, effective nutrient sensing and control of systemic energy homeostasis have relied on a close physical and functional interaction between immune and metabolically active cells. However, in today's obesogenic environment, this fine-tuned immunometabolic interface is perturbed. As a consequence, chronic inflammatory conditions and aberrant activation of immune cells have emerged as key features of obesity-related metabolic disorders, including insulin resistance, cardiovascular complications, and type 2 diabetes, whereas a major research focus has been placed on the adipocyte-macrophage interaction in the context of metabolic dysfunction; recent studies have not only expanded the scope of relevant immune cells in this setting but also highlight the impact of distinct metabolic organs, including the liver, on immunometabolic control, metabolic disease development, and potential anti-inflammatory therapeutic options in obesity-driven pathologies. This review will thus summarize recent progress in this emerging area of metabolic research.

Adipokines in the HIV/HAART-associated lipodystrophy syndrome

The use of highly active antiretroviral therapy (HAART) in the treatment of human immunodeficiency virus has dramatically altered both the landscape of this disease and the prognosis for those affected. With more patients now receiving HAART, adverse effects such as lipodystrophy and metabolic syndrome have emerged. In HIV/HAART-associated lipodystrophy syndrome (HALS), patients demonstrate fat maldistribution with dyslipidemia, insulin resistance, and other metabolic complications. Recent studies have contributed to the elucidation of the pathophysiological abnormalities seen in this syndrome and have provided guidance for the study and use of potential treatments for these patients, but widely accepted guidelines have not yet been established. Two adipokines, leptin and adiponectin, are decreased in patients with HALS and lipoatrophy or lipodystrophy. Further, recent proof-of-concept clinical trials have proven the efficacy of leptin replacement and medications that increase circulating adiponectin levels in improving the metabolic profile of HALS patients. This review article highlights recent evidence on leptin replacement and compares leptin's efficacy to that of other treatments, including metformin and thiazolidinediones, on metabolic abnormalities such as impaired insulin-glucose homeostasis associated with lipodystrophy in patients receiving HAART. It is hoped that forthcoming large phase III clinical trials will allow the addition of leptin to our therapeutic armamentarium for use in patients suffering from this disease state.

The liver and the kidney: two critical organs influencing the atherothrombotic risk in metabolic syndrome

The increased atherothrombotic risk in patients with metabolic syndrome (MetS) has been classically explained by the multiplicative effect of systemic concomitant pro-atherosclerotic factors. In particular, centripetal obesity, dyslipidaemia, glucose intolerance, hypertension (differently combined in the diagnosis of the disease) would be expected to act as classical cardiovascular risk conditions underlying accelerated atherogenesis. In order to better understand specific atherosclerotic pathophysiology in MetS, emerging evidence focused on the alterations in different organs that could serve as both pathophysiological targets and active players in the disease. Abnormalities in adipose tissue, heart and arteries have been widely investigated in a variety of basic research and clinical studies in MetS. In this narrative review, we focus on pathophysiological activities of the liver and kidney. Considering its key role in metabolism and production of soluble inflammatory mediators (such as C-reactive protein [CRP]), the liver in MetS has been shown to be altered both in its structure and function. In particular, a relevant amount of the fat accumulated within this organ has been shown to be associated with different degrees of inflammation and potential insulin resistance. In humans, non-alcoholic fatty liver disease (NAFLD) has been described as the hepatic manifestation of MetS. In an analogous manner, epidemiological evidence strongly suggested a "guilty" association between MetS and chronic kidney disease (CKD). Some biomarkers of hepatic (such as C-reactive protein, TNF-alpha or other cytokines) and renal diseases (such as uric acid) associated with MetS might be particularly useful to better manage and prevent the atherothrombotic risk.

Type 2 diabetes with partial lipodystrophy of the limbs: a new lipodystrophy phenotype

OBJECTIVE

Lipodystrophies are categorized by the extent of fat loss (generalized vs. partial) and by inheritance (congenital vs. acquired). We examined whether a group of patients with partial lipodystrophy of the limbs (PLL), type 2 diabetes mellitus (T2DM), and an absence of a family history of lipodystrophy constitute a new clinical subtype.

RESEARCH DESIGN AND METHODS

Ten women with T2DM and PLL were identified in academic diabetes clinics and were matched by age, sex, BMI, ethnicity, and diabetes status with 10 women with control T2DM without lipodystrophy. All patients were characterized by clinical evaluation and hyperinsulinemic clamp.

RESULTS

Patients with T2DM and PLL exhibited symmetrical loss of subcutaneous fat in forearms, or forearms plus calves, and acanthosis nigricans. Maximally stimulated glucose disposal rates were markedly reduced by 56% in the T2DM with PLL group compared with the control T2DM patients, whether normalized by body weight or surface area. Most PLL patients exhibited little or no insulin-mediated glucose uptake after subtraction of non-insulin-mediated glucose uptake. The T2DM with PLL group also had greater elevations in hepatic transaminases and triglycerides and earlier onset of diabetes compared with control T2DM.

CONCLUSIONS

T2DM with PLL represents a previously unrecognized phenotype of lipodystrophy and of T2DM. These T2DM patients exhibit symmetrical lipodystrophy of the distal limbs, acanthosis nigricans, marked insulin resistance with little insulin-mediated glucose uptake, hypertriglyceridemia, and hepatic transaminase elevations, which are greater in severity than observed in patients with common T2DM.

Adiposity and insulin resistance in humans: the role of the different tissue and cellular lipid depots

Human adiposity has long been associated with insulin resistance and increased cardiovascular risk, and abdominal adiposity is considered particularly adverse. Intra-abdominal fat is associated with insulin resistance, possibly mediated by greater lipolytic activity, lower adiponectin levels, resistance to leptin, and increased inflammatory cytokines, although the latter contribution is less clear. Liver lipid is also closely associated with, and likely to be an important contributor to, insulin resistance, but it may also be in part the consequence of the lipogenic pathway of insulin action being up-regulated by hyperinsulinemia and unimpaired signaling. Again, intramyocellular triglyceride is associated with muscle insulin resistance, but anomalies include higher intramyocellular triglyceride in insulin-sensitive athletes and women (vs men). Such issues could be explained if the "culprits" were active lipid moieties such as diacylglycerol and ceramide species, dependent more on lipid metabolism and partitioning than triglyceride amount. Subcutaneous fat, especially gluteofemoral, appears metabolically protective, illustrated by insulin resistance and dyslipidemia in patients with lipodystrophy. However, some studies suggest that deep sc abdominal fat may have adverse properties. Pericardial and perivascular fat relate to atheromatous disease, but not clearly to insulin resistance. There has been recent interest in recognizable brown adipose tissue in adult humans and its possible augmentation by a hormone, irisin, from exercising muscle. Brown adipose tissue is metabolically active, oxidizes fatty acids, and generates heat but, because of its small and variable quantities, its metabolic importance in humans under usual living conditions is still unclear. Further understanding of specific roles of different lipid depots may help new approaches to control obesity and its metabolic sequelae.