Resistin/ADSF/FIZZ3 in obesity and diabetes

Crucial role of iron in epigenetic rewriting during adipocyte differentiation mediated by JMJD1A and TET2 activity

Iron metabolism is closely associated with the pathogenesis of obesity. However, the mechanism of the iron-dependent regulation of adipocyte differentiation remains unclear. Here, we show that iron is essential for rewriting of epigenetic marks during adipocyte differentiation. Iron supply through lysosome-mediated ferritinophagy was found to be crucial during the early stage of adipocyte differentiation, and iron deficiency during this period suppressed subsequent terminal differentiation. This was associated with demethylation of both repressive histone marks and DNA in the genomic regions of adipocyte differentiation-associated genes, 　including Pparg, which encodes PPARγ, the master regulator of adipocyte differentiation. In addition, we identified several epigenetic demethylases to be responsible for iron-dependent adipocyte differentiation, with the histone demethylase jumonji domain-containing 1A and the DNA demethylase ten-eleven translocation 2 as the major enzymes. The interrelationship between repressive histone marks and DNA methylation was indicated by an integrated genome-wide association analysis, and was also supported by the findings that both histone and DNA demethylation were suppressed by either the inhibition of lysosomal ferritin flux or the knockdown of iron chaperone poly(rC)-binding protein 2. In summary, epigenetic regulations through iron-dependent control of epigenetic enzyme activities play an important role in the organized gene expression mechanisms of adipogenesis.

Innate-Immunity Genes in Obesity

The main functions of adipose tissue are thought to be storage and mobilization of the body’s energy reserves, active and passive thermoregulation, participation in the spatial organization of internal organs, protection of the body from lipotoxicity, and ectopic lipid deposition. After the discovery of adipokines, the endocrine function was added to the above list, and after the identification of crosstalk between adipocytes and immune cells, an immune function was suggested. Nonetheless, it turned out that the mechanisms underlying mutual regulatory relations of adipocytes, preadipocytes, immune cells, and their microenvironment are complex and redundant at many levels. One possible way to elucidate the picture of adipose-tissue regulation is to determine genetic variants correlating with obesity. In this review, we examine various aspects of adipose-tissue involvement in innate immune responses as well as variants of immune-response genes associated with obesity.

Obesity in the pathogenesis of type 2 diabetes

Obesity, and especially visceral adiposity, escalates the development of insulin resistance and type 2 diabetes. Excess adipose tissue contributes to a chronic increase in circulating fatty acids reducing the usage of glucose as a source of cellular energy. Excess fatty acids also result in increased deposition of fat in muscle and liver, and increased metabolites such as diacylglycerol and ceramide which activate isoforms of protein kinase C that impede cellular insulin signalling. Chronically raised lipid levels also impair islet beta cell function, acting in conjuction with insulin resistance to aggravate hyperglycaemia. The detrimental effects of several adipokines such as TNFα, IL6 and RBP4, which are produced in excess by an increased adipose mass, and reduced production of adiponectin are further mechanisms through which obesity potentiates the development of type 2 diabetes.

White adipose tissue as endocrine organ and its role in obesity

Due to the public health problem represented by obesity, the study of adipose tissue, particularly of the adipocyte, is central to the understanding of metabolic abnormalities associated with the development of obesity. The concept of adipocyte as endocrine and functional cell is not totally understood and can be currently defined as the capacity of the adipocyte to sense, manage, and send signals to maintain energy equilibrium in the body. Adipocyte functionality is lost during obesity and has been related to adipocyte hypertrophy, disequilibrium between lipogenesis and lipolysis, impaired transcriptional regulation of the key factors that control adipogenesis, and lack of sensitivity to external signals, as well as a failure in the signal transduction process. Thus, dysfunctional adipocytes contribute to abnormal utilization of fatty acids causing lipotoxicity in non-adipose tissue such as liver, pancreas and heart, among others. To understand the metabolism of the adipocyte it is necessary to have an overview of the developmental process of new adipocytes, regulation of adipogenesis, lipogenesis and lipolysis, endocrine function of adipocytes and metabolic consequences of its dysfunction. Finally, the key role of adipose tissue is shown by studies in transgenic animals or in animal models of diet-induced obesity that indicate the contribution of adipose tissue during the development of metabolic syndrome. Thus, understanding of the molecular process that occurs in the adipocyte will provide new tools for the treatment of metabolic abnormalities during obesity.

Adiponectin and resistin response in the onset of obesity in male and female rats

OBJECTIVE

Studying the sex-dependent response of adiponectin and resistin adipose tissue expression and circulating levels in the onset of dietary obesity.

METHODS AND PROCEDURES

Male and female 4-week-old Wistar rats were fed a control or cafeteria (CAF) diet for 15 days. Body weight and energy intake were monitored. Gonadal (visceral), retroperitoneal (visceral) and inguinal (subcutaneous) white adipose tissue (WAT) depots were collected. Serum adiponectin and resistin and tissue mRNA levels were analyzed by western blot and reverse transcription-PCR, respectively. Serum levels of insulin, tumor necrosis factor-alpha (TNFalpha), and glucose were measured by enzyme-linked immunosorbent assay and by a glucose sensor. Insulin resistance was assessed by the homeostasis model assessment (HOMA).

RESULTS

Energy intake and adipose-tissue weight were significantly increased in the CAF rats, with higher increase in visceral than in subcutaneous fat, especially in females. The effective production of adiponectin and resistin (total levels adjusted per WAT weight) was decreased in the CAF groups, more markedly in females for adiponectin. This decrease was associated with the tendency to lower WAT mRNA levels for resistin, but not for adiponectin. Insulin levels were not significantly altered. Fasting glucose was slightly increased in CAF females. HOMA score was not significantly increased by CAF feeding, although it tended to be increased in a few CAF females.

DISCUSSION

Decrease of WAT adiponectin and resistin-effective production seems an early response to obesity development under a high-fat (CAF) diet, with sex-associated differences. This can probably be related to a physiological role of both adipokines modulating the insulin signaling system.

Peripheral tissue-brain interactions in the regulation of food intake

More than 70 years ago the glucostatic, lipostatic and aminostatic hypotheses proposed that the central nervous system sensed circulating levels of different metabolites, changing feeding behaviour in response to the levels of those molecules. In the last 20 years the rapid increase in obesity and associated pathologies in developed countries has involved a substantial increase in the knowledge of the physiological and molecular mechanism regulating body mass. This effort has resulted in the recent discovery of new peripheral signals, such as leptin and ghrelin, as well as new neuropeptides, such as orexins, involved in body-weight homeostasis. The present review summarises research into energy balance, starting from the original classical hypotheses proposing metabolite sensing, through peripheral tissue-brain interactions and coming full circle to the recently-discovered role of hypothalamic fatty acid synthase in feeding regulation. Understanding these molecular mechanisms will provide new pharmacological targets for the treatment of obesity and appetite disorders.

Treating insulin resistance: future prospects

Insulin resistance typically reflects multiple defects of insulin receptor and post-receptor signalling that impair a diverse range of metabolic and vascular actions. Many potential intervention targets and compounds with therapeutic activity have been described. Proof of principle for a non-peptide insulin mimetic has been demonstrated by specific activation of the intracellular B-subunit of the insulin receptor. Potentiation of insulin action has been achieved with agents that enhance phosphorylation and prolong the tyrosine kinase activity of the insulin receptor and its protein substrates after activation by insulin. These include inhibitors of phosphatases and serine kinases that normally prevent or terminate tyrosine kinase signalling. Additional approaches involve increasing the activity of phosphatidylinositol 3-kinase and other downstream components of the insulin signalling pathways. Experimental interventions to remove signalling defects caused by cytokines, certain adipocyte hormones, excess fatty acids, glucotoxicity and negative feedback by distal signalling steps have also indicated therapeutic possibilities. Several hormones, metabolic enzymes, minerals, co-factors and transcription co-activators have shown insulin-sensitising potential. Since insulin resistance affects many metabolic and cardiovascular diseases, it provides an opportunity for simultaneous therapeutic attack on a broad front.

Release of interleukins and other inflammatory cytokines by human adipose tissue is enhanced in obesity and primarily due to the nonfat cells

The white adipose tissue, especially of humans, is now recognized as the central player in the mild inflammatory state that is characteristic of obesity. The question is how the increased accumulation of lipid seen in obesity causes an inflammatory state and how this is linked to the hypertension and type 2 diabetes that accompanies obesity. Once it was thought that adipose tissue was primarily a reservoir for excess calories that were stored in the adipocytes as triacylglycerols. In times of caloric deprivation these stored lipids were mobilized as free fatty acids and the insulin resistance of obesity was attributed to free fatty acids. It is now clear that in humans the expansion of adipose tissue seen in obesity results in more blood vessels, more connective tissue fibroblasts, and especially more macrophages. There is an enhanced secretion of some interleukins and inflammatory cytokines in adipose tissue of the obese as well as increased circulating levels of many cytokines. The central theme of this chapter is that human adipose tissue is a potent source of inflammatory interleukins plus other cytokines and that the majority of this release is due to the nonfat cells in the adipose tissue except for leptin and adiponectin that are primarily secreted by adipocytes. Human adipocytes secrete at least as much plasminogen activator inhibitor-1 (PAI-1), MCP-1, interleukin-8 (IL-8), and IL-6 in vitro as they do leptin but the nonfat cells of adipose tissue secrete even more of these proteins. The secretion of leptin, on the other hand, by the nonfat cells is negligible. The amount of serum amyloid A proteins 1 & 2 (SAA 1 & 2), haptoglobin, nerve growth factor (NGF), macrophage migration inhibitory factor (MIF), and PAI-1 secreted by the adipocytes derived from a gram of adipose tissue is 144%, 75%, 72%, 37%, and 23%, respectively, of that by the nonfat cells derived from the same amount of human adipose tissue. However, the release of IL-8, MCP-1, vascular endothelial growth factor (VEGF), TGF-beta1, IL-6, PGE(2), TNF-alpha, cathepsin S, hepatocyte growth factor (HGF), IL-1beta, IL-10, resistin, C-reactive protein (CRP), and interleukin-1 receptor antagonist (IL-1Ra) by adipocytes is less than 12% of that by the nonfat cells present in human adipose tissue. Obesity markedly elevates the total release of TNF-alpha, IL-6, and IL-8 by adipose tissue but only that of TNF-alpha is enhanced in adipocytes. However, on a quantitative basis the vast majority of the TNF-alpha comes from the nonfat cells. Visceral adipose tissue also releases more VEGF, resistin, IL-6, PAI-1, TGF-beta1, IL-8, and IL-10 per gram of tissue than does abdominal subcutaneous adipose tissue. In conclusion, there is an increasing recognition that adipose tissue is an endocrine organ that secretes leptin and adiponectin along with a host of other paracrine and endocrine factors in addition to free fatty acids.

Resistin as a putative modulator of insulin action in the daily feeding/fasting rhythm

Resistin and adiponectin are adipokines with postulated opposite functions. Resistin has been related with insulin resistance in obesity, while adiponectin could be associated to higher insulin sensitivity. We have determined whether the production of these two adipokines during the day is related to the feeding rhythm in rats. Resistin mRNA levels in adipose tissue correlated positively with the gastric contents and serum insulin concentration, showing higher levels during the dark phase (period of the highest food intake), especially in the mesenteric depot, while levels decreased during the light phase. The diurnal pattern of resistin expression was not directly reflected in the circulating levels, but it showed a 6-h delay and correlated negatively with the gastric contents and serum insulin. Adiponectin expression followed an opposite pattern, not apparently related to feeding or insulin release, and not translated into changes in circulating levels. Moreover, considering that insulin stimulates resistin expression and that circulating resistin follows a contrary circadian pattern in comparison to insulin, resistin, apart from its role in the increased insulin resistance associated to obesity, could also act as a putative modulator of insulin in the daily feeding/fasting rhythm through a negative feedback regulation of its action.

Transgenic animal models for the study of adipose tissue biology

The traditional view of adipose tissue as a passive energy reservoir has changed. Adipose tissue is a complex, highly active metabolic and endocrine organ. With obesity as an increasingly important public health threat, a major development in the understanding of adipose tissue biology has come with observations in different biological spheres including whole-body physiology and application of transgenic animal models. Scientific progress has been made with the identification of several genes in spontaneous monogenic animal models of obesity, and in understanding the molecular mechanisms underlying phenotypes of altered body weight, adiposity and fat distribution by creating transgenic and knockout animal models. Mouse phenotypes resulting from inactivation or overexpression of molecules responsible for the regulation of adipose tissue metabolism have led to novel concepts in the understanding of adipocyte biology and development of obesity. This review presents an overview of transgenic animal models for the study of adipose tissue biology.

Hormonal regulation of food intake

Our knowledge of the physiological systems controlling energy homeostasis has increased dramatically over the last decade. The roles of peripheral signals from adipose tissue, pancreas, and the gastrointestinal tract reflecting short- and long-term nutritional status are now being described. Such signals influence central circuits in the hypothalamus, brain stem, and limbic system to modulate neuropeptide release and hence food intake and energy expenditure. This review discusses the peripheral hormones and central neuronal pathways that contribute to control of appetite.

Effects of adipocyte-derived cytokines on endothelial functions: implication of vascular disease

Adipose tissue has recently emerged as an active endocrine organ that secretes a variety of metabolically important substances, collectively called adipocytokines or adipokines. In this review we summarize the effects of the adipokines leptin, adiponectin, and resistin on the vasculature and their potential role for pathogenesis of vascular disease. Leptin is associated with arterial wall thickness, decreased vessel distensibility, and elevated C reactive protein (CRP) levels. Leptin possesses procoagulant and antifibrinolytic properties, and it promotes thrombus and atheroma formation, probably through the leptin receptors by promoting vascular inflammation, proliferation, and calcification, and by increasing oxidative stress. Research for development of pharmacologic antagonism for the leptin receptor is currently under way. Adiponectin inhibits the expression of the adhesion molecules ICAM-1, VCAM-1, and P selectin. Therefore, it interferes with monocyte adherence to endothelial cells and their subsequent migration to the subendothelial space, one of the initial events in the development of atherosclerosis. Adiponectin also inhibits the transformation of macrophages to foam cells in vitro and decreases their phagocytic activity. Resistin, discovered in 2001, represents the newest of the adipokines and was named for its ability to promote insulin resistance. Resistin increases the expression of the adhesion molecules VCAM-1 and ICAM-1, up-regulates the monocyte chemoattractant chemokine-1, and promotes endothelial cell activation via ET-1 release. Although many aspects of its function need further clarification, it appears that resistin will add significantly to our knowledge of the pathophysiology of vascular disease and the metabolic syndrome.

The effect of PPARgamma ligands on the adipose tissue in insulin resistance

Insulin resistance is frequently accompanied by obesity and both obesity and type 2 diabetes are associated with a mild chronic inflammation. Elevated levels of various cytokines, such as TNF-alpha and IL-6, are typically found in the adipose tissue in these conditions. It has been suggested that many cytokines produced in the adipose tissue are derived from infiltrated inflammatory cells. However, the adipose tissue itself has proven to be an important endocrine organ, secreting several hormones and cytokines, usually referred to as adipokines. Peroxisome proliferator-activated receptor (PPAR)gamma is essential for adipocyte proliferation and differentiation. In recent years, PPARgamma and its ligands, the thiazolidinediones (TZD), have achieved great attention due to their insulin sensitizing and anti-inflammatory properties. Treatment with TZDs result in improved insulin signaling and adipocyte differentiation, increased adipose tissue influx of free fatty acids and inhibition of cytokine expression and action. As a result, PPARgamma plays a central role in maintaining a functional and differentiated adipose tissue.

Two days of a very low calorie diet reduces endogenous glucose production in obese type 2 diabetic patients despite the withdrawal of blood glucose-lowering therapies including insulin

The mechanism of the blood glucose-lowering effect of a 2-day very low calorie diet (VLCD; 1890 kJ/d) in combination with the cessation of all blood glucose-lowering agents was studied in 12 (7 women, 5 men) obese (body mass index, 36.3 +/- 1.0 kg/m 2 [mean +/- SEM]) type 2 diabetic patients (age, 55 +/- 4 years; HbA 1c , 7.3% +/- 0.4%) undergoing insulin therapy. Endogenous glucose production (EGP) and whole body glucose disposal (6,6 2 H 2 -glucose), lipolysis ( 2 H 5 -glycerol), and substrate oxidation (indirect calorimetry) rates were measured before and after the intervention in basal and hyperinsulinemic conditions. After 2 days of a VLCD and discontinuation of all blood glucose-lowering therapies, fasting plasma glucose levels did not increase (11.3 +/- 1.3 vs 10.3 +/- 1.0 mmol/L). Basal EGP significantly declined (14.2 +/- 1.0 to 11.9 +/- 0.7 mu mol/kg per minute; P = .009). Basal metabolic clearance rate of glucose and rate of basal lipolysis did not change. During hyperinsulinemia, EGP (5.5 +/- 0.8 to 5.2 +/- 0.5 mu mol/kg per minute), whole body glucose disposal (12.1 +/- 0.7 to 11.3 +/- 1.0 mu mol/kg per minute), the metabolic clearance rate of glucose, and the rate of lipolysis did not change after the 2-day intervention. Cessation of blood glucose-lowering therapy in combination with a 2-day VLCD does not lead to hyperglycemia and is associated with a reduction in basal EGP. Insulin-stimulated whole body glucose disposal did not improve, nor did insulin suppressibility of EGP and lipolysis.