Adipokines in the treatment of diabetes mellitus and obesity

Background: The physiology of adipose tissue plays a main role in the pathogenesis of type 2 diabetes mellitus. The secretion of adipocyte-derived hormones, in either an autocrine or a paracrine manner, has been proposed as a relevant mechanism in this process. In this sense, the administration and regulation of hormones derived from adipose tissue arises as an attractive option for treating metabolic disorders. Objective: To review the current understanding of the implication of adipokines in the development of obesity and insulin resistance, as well as their potential use as therapeutic agents. Methodology: Review of scientific literature. Conclusions: This review describes the role of adipokines in generating insulin resistance and the chronic low-grade inflammatory profile accompanying visceral obesity.     

Perivascular fat,AMP-activated protein kinase and vascular diseases

Perivascular adipose tissue (PVAT) is an active endocrine and paracrine organ that modulates vascular function, with implications for the pathophysiology of cardiovascular disease (CVD). Adipocytes and stromal cells contained within PVAT produce mediators (adipokines, cytokines, reactive oxygen species and gaseous compounds) with a range of paracrine effects modulating vascular smooth muscle cell contraction, proliferation and migration. However, the modulatory effect of PVAT on the vascular system in diseases, such as obesity, hypertension and atherosclerosis, remains poorly characterized. AMP-activated protein kinase (AMPK) regulates adipocyte metabolism, adipose biology and vascular function, and hence may be a potential therapeutic target for metabolic disorders such as type 2 diabetes mellitus (T2DM) and the vascular complications associated with obesity and T2DM. The role of AMPK in PVAT or the actions of PVAT have yet to be established, however. Activation of AMPK by pharmacological agents, such as metformin and thiazolidinediones, may modulate the activity of PVAT surrounding blood vessels and thereby contribute to their beneficial effect in cardiometabolic diseases. This review will provide a current perspective on how PVAT may influence vascular function via AMPK. We will also attempt to demonstrate how modulating AMPK activity using pharmacological agents could be exploited therapeutically to treat cardiometabolic diseases.     

Adipose tissue: the link between obesity and cardiovascular disease

The ever-increasing prevalence of cardiovascular disease (CVD) associated with obesity is linked through signaling pathways within adipose tissue. Adipose tissue functions as an endocrine organ, producing and secreting a variety of bioactive molecules. In obesity, the adipose tissue itself undergoes changes in cell size which alters its normal physiological function. Altered adipocyte function changes production and secretion of adipokines, such as leptin, adiponectin, angiotensinogen, plasminogen activator inhibitor-1, resistin, and several inflammatory molecules. Adipokines interact with other tissues and cells in the body, including many pathways linked to CVD. Future research in the area of obesity-related CVD requires further investigation into a combination of lifestyle and pharmacological therapies that alter adipokine production by reducing adipocyte size.     

Adiponectin and lipid metabolism in skeletal muscle

White adipose tissue (WAT) is a key energy depot in humans and most animals. Traditionally, it is believed that WAT passively accumulates triglycerides or releases fatty acids to accommodate systemic energy metabolism. However, recent studies have demonstrated that WAT also actively participates in energy metabolism mainly through its secretion of cytokines and hormones. Therefore, at this time, WAT is recognized as an endocrine organ. Adiponectin is one of the key adipocyte-derived hormones that regulate systemic or tissue lipid and glucose metabolism. In contrast to most other adipocyte-derived hormones, adiponectin increases insulin sensitivity and improves lipid and glucose metabolism. Although the insulin-sensitizing function of adiponectin has been well established, recent studies have demonstrated that adiponectin also regulates metabolism through pathways independent of insulin signaling. Due to the massive tissue mass of skeletal muscle, lipid uptake and subsequent fatty acid oxidation in skeletal muscle have a big impact on maintaining systemic energy homeostasis. Furthermore, adiponectin gene expression is regulated by energy intake. Therefore, adiponectin serves as a coordinator of energy balance amongst WAT, skeletal muscle and other tissues. We summarize the regulatory effects of adiponectin on lipid and glucose metabolism in skeletal muscle. Future research directions have also been proposed.     

Autophagy induction: a critical event for the modulation of cell death/survival and inflammatory responses by adipokines

Adipose tissue acts as a dynamic endocrine organ playing critical roles in many metabolic and immune responses. Endocrine functions by adipose tissue are achieved by secretion of diverse cytokines and hormones, collectively called adipokines. Adiponectin and leptin the most abundantly expressed adipokines within adipose tissue have an impact on various physiological responses. While both adiponectin and leptin are secreted from the same location, their physiological functions are not identical. Adiponectin possesses potent anti-inflammatory properties and anti-tumor activities, whereas leptin acts as a pro-inflammatory hormone and generates tumor-promoting effects. Autophagy, a highly conserved intracellular self-digestive process, is implicated in the maintenance of diverse physiological responses. In particular, autophagy plays dual roles in the regulation of cell death/survival (e.g., inducing cell death and cytoprotection) and is associated with anti-inflammatory actions. Increasing recent evidence has indicated that autophagy is implicated in various biological responses by adipokines. Therefore, autophagy would be a promising target for the management of inflammation and tumor growth by adipokines. In this review, we summarize the effects of adiponectin and leptin on autophagy induction and highlight their implications in modulating inflammatory responses and tumor growth.     

Aldosterone, mineralocorticoid receptor and the metabolic syndrome: role of the mineralocorticoid receptor antagonists

Several lines of evidence suggest a detrimental effect of aldosterone excess on the development of metabolic alterations. Glucose metabolism derangements due to aldosterone action are frequently observed not only in patients with primary aldosteronism but also in patients with obesity. A contribution to the hyperaldosteronism observed in obese subjects can be attributed, at least in part, to the action of still unidentified adipocyte-derived factor. Aldosterone, through genomic and non-genomic actions contributes to induce several abnormalities: pancreatic fibrosis, impaired beta cell function, as well as reduced skeletal muscle and adipose tissue insulin sensitivity. Oxidative stress, systemic inflammation, together with these metabolic alterations may explain the appearance of the cardiometabolic syndrome and the progression of cardiovascular and renal diseases, in the presence of inappropriate aldosterone levels. The biological actions of aldosterone are mediated by mineralocorticoid receptor (MR), although MR can be activated through an aldosterone independent fashion. Besides salt-water homeostasis, MR activation promotes inflammation, endothelial dysfunction, cardiovascular remodelling and affects adipose tissue differentiation and function. Clinical and experimental studies have shown that MR blockade is able to suppress inflammation, to improve endothelium- dependent vasorelaxation, but most interestingly, to improve pancreatic insulin release as well as insulin-mediated glucose utilization. These actions indicate MR antagonists as a useful therapeutic tool able not only to reduce cardiovascular risk and renal damage, but also to improve metabolic sequaelae.     

Adipose tissue as a regulator of energy balance

Adipose tissue plays an active role in energy balance because it is not only a lipid storing and mobilizing tissue but consists of functionally specialized tissues able to produce heat (in brown adipose tissue) and to produce or release a vast number of so called adipokines or adipocytokines. These consist of polypeptides but also non-protein factors and are metabolically active molecules belonging to different functional categories like immunity (complement factors, haptoglobin), endocrine function (leptin, sex steroids, various growth factors), metabolic function (fatty acids, adiponectin, resistin), and cardiovascular function (angiotensinogen, PAI-1). Recent advances using genomic and proteomic approaches have identified numerous new adipocyte secreted factors whose function remain to be established. Too little as well as too much adipose tissue leads to metabolic disturbances like insulin resistance. Visceral obesity is especially strongly correlated with the development of diabetes, hypertension and cardio-vascular disease. Thermogenesis in brown adipose tissue is a means to dissipate excess energy, but in adult humans brown fat is very scarce and probably not functional. However, human white adipose tissue contains mesenchymal stem cells, and if these could be stimulated to differentiate into brown adipocytes, increased energy expenditure in white fat could help to shift energy balance towards a more negative state.     

The role of adipocytokines in adipocyte-related pathological processes

It is becoming clear that adipose tissue is not merely a storage for excess energy but that it secretes a number of biologically active soluble factors collectively termed adipocytokines that control glucose and fatty acid metabolism. Of these adipocytokines, adiponectin and resistin have been the objects of intensive research, as they are implicated in obesity and diabetes-related diseases. In this review, we summarize recent advances in understanding the roles of adiponectin and resistin in the causation of metabolic diseases and consider the prospects for treating metabolic disorders by targeting these two adipocytokines.     

Dietary phytochemicals and their potential effects on obesity: A review

The incidence of obesity is rising at an alarming rate and is becoming a major public health concern with incalculable social costs. Indeed, obesity facilitates the development of metabolic disorders such as diabetes, hypertension, and cardiovascular diseases in addition to chronic diseases such as stroke, osteoarthritis, sleep apnea, some cancers, and inflammation-bases pathologies. In this review we summarize the progresses made in our understanding of obesity, including the role of inflammation process, the recently understood endocrine function of adipose tissue, as well as passive roles of processes of energy storage and adipogenesis related to fat cell lifecycle: differentiation, maturation, and apoptosis. In addition, the article discusses the anti-obesity potential of dietary phytochemicals and analyzes their mechanisms of action, e.g. induction of apoptosis and lipolysis and inhibition of inflammation.     

Adipokines in inflammation, insulin resistance and cardiovascular disease

1. Obesity is a major determinant of cardiovascular disease (CVD). Studies in the past two decades have shown that adipose tissue is not merely an inert energy reserve of triglycerides, but also an active endocrine organ. 2. Adipose tissue can produce and secrete numerous bioactive peptides and/or proteins termed adipokines. These secretory factors are involved in the regulation of local and systemic inflammation and insulin sensitivity in a paracrine and/or endocrine manner. Inflammation and insulin resistance (IR) play critical roles in the obesity-linked development of CVD, such as atherosclerosis, hypertension and restenosis. 3. In the present minireview, we summarize the relationship between inflammation and IR, as well as their contribution to the development of CVD during adipose tissue dysfunction. In particular, we focus on the effects of various adipokines in pathological processes, which may provide an insight into obesity-linked CVD and facilitate the development of new therapeutic strategies.     

Aminoacyl-tRNA synthetase-interacting multi-functional protein 1/p43: an emerging therapeutic protein working at systems level

Introduction: Over the past 20 years, the idea that white adipose tissue (WAT) is simply an energy depot organ has been radically changed. Indeed, present understanding suggests WAT to be an endocrine organ capable of producing and secreting a wide variety of proteins termed adipokines. These adipokines appear to be relevant factors involved in a number of different functions, including metabolism, immune response, inflammation and bone metabolism.

Areas covered: In this review, the authors focus on the effects of several adipose tissue-derived factors in bone pathophysiology. They also consider how the modification of the adipokine network could potentially lead to promising treatment options for bone diseases.

Expert opinion: There are currently substantial developments being made in the understanding of the interplay between bone metabolism and the metabolic system. These insights could potentially lead to the development of new treatment strategies and interventions with the aim of successful outcomes in many people affected by bone disorders. Specifically, future research should look into the intimate mechanisms regulating peripheral and central activity of adipokines as it has potential for novel drug discovery.     

Estrogen regulation of adipose tissue functions: involvement of estrogen receptor isoforms

Adipose tissue has recently been described as one of the major endocrine gland that plays a role in energy homeostasis, lipid metabolism, immune response, and reproduction. An excess of white adipose tissue, caused by a complex interaction between genetic, hormonal, behavioral, and environmental factors, results in obesity: a heterogeneous disorder that predisposes humans to a variety of diseases. Among several hormones, estrogens promote, maintain, and control the typical distribution of body fat and adipose tissue metabolism through still unknown mechanisms. These steroids are known to regulate fat mass, adipose deposition and differentiation, and adipocyte metabolism. Moreover, estrogen deficiency results in increases in adipose tissue, preferentially in visceral fat, which would link obesity to the susceptibility of related disorders. In this review the role of estrogens in adipose tissue differentiation and in the protection against the onset of obesity will be discussed with particular attention being drawn to the underlying molecular mechanisms mediated by estrogen receptor isoforms ERalpha and ERbeta.     

Visceral and subcutaneous adiposity: are both potential therapeutic targets for tackling the metabolic syndrome?

The metabolic syndrome represents a constellation of co-morbidities that include central adiposity, insulin resistance, dyslipidemia and hypertension, which results from an elevated prevalence of obesity. An increased abdominal adiposity is observed in upper-body obesity with preferential accumulation of fat in the visceral depot, which renders these individuals more prone to metabolic and cardiovascular problems. The pathophysiology of the metabolic syndrome seems to be closely associated to an elevated efflux of free fatty acids from the visceral fat compartment and a dysregulation of the expression of adipose tissue-derived factors (also termed "adipokines"). Weight reduction and increased physical activity represent the main approach to tackle the "diabesity" epidemic. Nonetheless, taking advantage of the different biochemical and molecular characteristics of visceral and subcutaneous adipose tissue may open up novel pharmacological strategies to combat the metabolic and cardiovascular derangements accompanying the metabolic syndrome.     

The Role of Leptin in the Association between Obesity and Psoriasis

Adipose tissue secretes many adipokines which contribute to various metabolic processes, such as blood pressure, glucose homeostasis, inflammation and angiogenesis. The biology of adipose tissue in an obese individual is abnormally altered in a manner that increases the body’s vulnerability to immune diseases, such as psoriasis. Psoriasis is considered a chronic inflammatory skin disease which is closely associated with being overweight and obese. Additionally, secretion of leptin, a type of adipokine, increases dependently on adipose cell size and adipose accumulation. Likewise, high leptin levels also aggravate obesity via development of leptin resistance, suggesting that leptin and obesity are closely related. Leptin induction in psoriatic patients is mainly driven by the interleukin (IL)-23/helper T (Th) 17 axis pathway. Furthermore, leptin can have an effect on various types of immune cells such as T cells and dendritic cells. Here, we discuss the relationship between obesity and leptin expression as well as the linkage between effect of leptin on immune cells and psoriasis progression.     

Adipokines: therapeutic targets for metabolic syndrome

For a long time it has been known that obesity (adiposity) is linked to insulin resistance. Recently, many investigators have reported that adipocytes secrete a variety of bioactive molecules, termed adipokines (adipocytokines), including TNFalpha, IL-6, leptin, adiponectin, resistin and so on. These adipokines play pivotal roles in energy homeostasis by affecting insulin sensitivity, glucose and lipid metabolisms, food intake, the coagulation system and inflammation. This review provides a summary of these adipose tissue-secreting biomolecules and discusses their feasibilities as drug targets for the treatment of metabolic syndrome.     

AMP activated protein kinase: a next generation target for total metabolic control

Metabolic syndrome is characterized by a cluster of metabolic disorders, such as reduced glucose tolerance, hyperinsulinemia, hypertension, visceral obesity and lipid disorders. The benefit of exercise in maintaining total metabolic control is well known and recent research indicates that AMP-activated protein kinase (AMPK) may play an important role in exercise-related effects. AMPK is considered as a master switch in regulating glucose and lipid metabolism. AMPK is an enzyme that works as a fuel gauge, being activated in conditions of high phosphate depletion. In the liver, activation of AMPK results in decreased production of plasma glucose, cholesterol, triglyceride and enhanced fatty acid oxidation. AMPK is also robustly activated by skeletal muscle contraction and myocardial ischemia, and is involved in the stimulation of glucose transport and fatty acid oxidation by these stimuli. In adipose tissue, activated AMPK inhibits deposition of fat, but enhances breakdown and burning of stored fat, resulting in reduction of body weight. The two leading diabetic drugs, namely metformin and rosiglitazone, and adipokines, such as adiponectin and leptin, show their metabolic effects partially through AMPK. These data suggest that AMPK may be a key player in the development of new treatments for obesity, Type 2 diabetes and the metabolic syndrome. In this review, the author provide insight into the role of AMPK as a probable target for treatment of metabolic syndrome.     

Markedly reduced white adipose tissue and increased insulin sensitivity in adcyap1-deficient mice

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide implicated in several metabolic functions, including insulin secretion and sympathoadrenal activation. To clarify the roles of PACAP in maintenance of whole-body glucose and lipid homeostasis, the impact of the deletion of PACAP on glucose homeostasis, body weight, and adipose tissue mass was examined by comparing mice lacking the Adcyap1 gene encoding PACAP (Adcyap1(-/-)) with wild-type littermate controls. Adcyap1(-/-) mice showed significant hypoinsulinemia, although being normoglycemic, and lower body weight as well as reduced food intake. They also showed greatly reduced white adipose tissue mass, in which the mRNA expression of adipocyte fatty acid-binding protein (aP2), a marker of adipocyte differentiation, was decreased. Glucose and insulin tolerance tests revealed increased insulin sensitivity in Adcyap1(-/-) mice. In accordance with these observations, plasma levels of resistin, an adipocytokine implicated in insulin resistance, were decreased in Adcyap1(-/-) mice. After a high-fat dietary challenge for six weeks, Adcyap1(-/-) mice still showed lower body weights and increased insulin sensitivity. These results indicate the crucial roles of PACAP in energy metabolism, including lipid metabolism, and in the regulation of body weight, raising the possibility that the PACAP-signaling pathway that favors energy storage could be a therapeutic target for obesity.     

Vascular AMPK as an attractive target in the treatment of vascular complications of obesity

The key for the survival of all organisms is the regulation and control of energy metabolism. Thus, several strategies have evolved in each tissue in order to balance nutrient supply with energy demand. Adenosine monophosphate-activated protein kinase (AMPK) is now recognized as a key participant in energy metabolism. It ensures an appropriate energetic supply by promoting energy conserving pathways in detriment of anabolic processes not essential for cell survival. Vascular AMPK plays a critical role in the regulation of blood flow and vascular tone through several mechanisms, including vasodilation by stimulating nitric oxide release in endothelial cells. Since obesity leads to endothelial damage and AMPK dysregulation, AMPK activation might be an important strategy to restore vascular function in cardiometabolic alterations. In the present review we focus on the role of vascular AMPK in both endothelial and smooth muscle cells, paying special attention to its dysregulation in obesity- and high-fat diet-related complications, as well as to the mechanisms and benefits of vascular AMPK activation.