Circulating resistin in lean, obese, and insulin-resistant mouse models: lack of association with insulinemia and glycemia

An increase in inflammation and islet dysfunction is a feature of prediabetes

AIMS

Type 2 diabetes (T2D) is a global health problem that will be diagnosed in almost 300 million people by 2025 according to the World Health Organization. Before being diagnosed with T2D, individuals may have glucose levels above normal but below the diabetic range. This condition is known as prediabetes. Studies showed that people with prediabetes had an increase in several pro-inflammatory cytokines in their serum and in their fasting glucose levels. The answer remains unclear when inflammation begins in the pancreas and islets, and what is the extent of this inflammation.

METHODS

Subjects with haemoglobin A1c levels from 5.7% to 6.4% were classified as pre-diabetic. Sections of pancreas and isolated islets from normal donors and donors with prediabetes were tested for markers of inflammation and glucose-stimulated insulin secretion (GSIS).

RESULTS

Gene and protein expression of the inflammatory markers resistin, interleukin-1 beta, tumour necrosis factor-alpha, interleukin-6, and monocyte chemoattractant protein-1 increased in donors with prediabetes compared to normal donors. GSIS response was significantly decreased in pre-diabetic islets compared to normal islets. Donors with prediabetes also had decreased expression of CD163+ cells but not CD68+ cells.

CONCLUSIONS

Based on our findings, inflammation and islet dysfunction may be more significant than originally thought in people with prediabetes. Rather than being in a normal state before diabetes occurs, it appears that subjects are already in an early diabetic condition resembling more closely T2D.

Cardiovascular and Metabolic Crosstalk in the Brain: Leptin and Resistin

Leptin and resistin are cytokines whose plasma levels correlate with adiposity. Leptin is a hormone synthesised and released from adipocytes and can be transported into the brain. Resistin is produced in adipocytes in rodents and in macrophages in humans, particularly macrophages that have infiltrated adipose tissue. Both hormones can act within the brain to influence sympathetic nerve activity. Leptin appears to have a generalised sympatho-excitatory actions whilst resistin appears to increase sympathetic nerve activity affecting the cardiovascular system but inhibits sympathetic nerve activity to brown adipose tissue, which contrasts with leptin. Since both hormones can be elevated in conditions of metabolic dysfunction, interactions/crosstalk between these two hormones in the brain is a real possibility. This review describes the current knowledge regarding such crosstalk within the central nervous system. The evidence suggests that with respect to sympathetic nerve activity, crosstalk between leptin and resistin can elicit enhanced sympatho-excitatory responses to the kidneys. In contrast, with respect to food intake, resistin has weaker effects, but in regard to insulin secretion and thermogenesis, leptin and resistin have opposing actions. Thus, in conditions in which there is increased resistin and leptin levels, the result of crosstalk in the central nervous system could contribute to worse cardiovascular and metabolic complications.

Comparative Intracerebroventricular and Intrathecal Administration of a Nanomolar Macrocyclic Melanocortin Receptor Agonist MDE6-5-2c (c[Pro-His-DPhe-Arg-Trp-Dap-Ala-DPro]) Decreases Food Intake in Mice

There is a critical need to find safe therapeutics to treat an increasingly obese population and diseases associated with an imbalance in energy homeostasis. The melanocortin-3 receptor (MC3R) and melanocortin-4 receptor (MC4R) ligands have long been the focus to help scientists understand energy homeostasis and the regulation of feeding behavior. Herein, we use a nanomolar macrocyclic melanocortin receptor agonist ligand MDE6-5-2c (c[Pro-His-DPhe-Arg-Trp-Dap-Ala-DPro) to examine metabolic and energy hemostasis profiles upon intrathecal (IT) administration directly into the spinal cord as compared to intracerebroventricular (ICV) administration directly into the brain. Overall, central ICV administration of MDE6-5-2c resulted in decreased food intake, in a dose-dependent manner, and decreased respiratory exchange ratio (RER). Comparison of IT versus ICV routes of MDE6-5-2c administration resulted in MDE6-5-2c possessing a longer duration of action on both feeding behavior and RER via IT. The C-peptide, ghrelin, GIP, leptin, IL-6, and resistin plasma hormones and biomarkers were compared using IT versus ICV MDE6-5-2c routes of administration. Plasma resistin levels were decreased upon ICV treatment of MDE6-5-2c, as compared to ICV vehicle control treatment. Intrathecal treatment resulted in significantly decreased inflammatory cytokine interleukin-6 (IL-6) levels compared to ICV administration. Investigation of the nonselective MC3R and MC4R macrocyclic agonist MDE6-5-2c molecule revealed differences in food intake, RER, and plasma biomarker profiles based upon ICV or IT routes of administration and characterize this novel molecular chemotype as a molecular probe to study the melanocortin system in vivo.

The Effect of Vitamin D Administration on Leptin, Adiponectin and mRNA MCP-1 Levels in Adipose Tissue of Obese Female Wistar Rats

In obesity, there is an accumulation of adipocytes which produces adipokine that are pro-inflammatory substance, such as leptin and MCP-1 and anti-inflammatory substance, such as adiponectin, while the bioavailability of vitamin D is decreased. This research aimed to study the effect of vitamin D administration on leptin, MCP-1, and adiponectin levels in adipose tissue rats with obesity. Vitamin D was administered to the obese model of 6-9 months old female Wistar rats. This experiment was a randomized control group design with a post-test group design only. Twenty-seven (27) female obese Wistar rats were included in this study. The animals were divided randomly into 3 groups: 9 rats were given 2400 IU vitamin D (group A), 9 rats were given 800 IU vitamin D (group B) and 9 rats were given a placebo as control (group C). The administration of Vitamin D was given once daily for 8 weeks. The visceral adipose tissue was taken to measure the level of leptin, adiponectin and mRNA MCP-1. Data among groups was analyzed by using one-way ANOVA and followed by LSD test, at a significance level of p <0.05. The lowest level of leptin (1059.15+135.20 pg/ml) and mRNA MCP-1 (2.36 + 0.75 fg/ml) and the highest adiponectin level (3.43 + 0.47 ng/ml) were found in group A. In conclusion, oral administration of vitamin D (2400 IU) decreased pro-inflammatory substances, such as leptin and mRNA MCP-1 and increased anti-inflammatory substances, such as adiponectin, in visceral adipose tissue of obese female Wistar rats.

Effects of Aerobic Training With No Caloric Restriction on Serum Resistin and Lipid Profile in Inactive Overweight Women

Introduction: Obesity is a major risk factor for inflammation and cardiovascular diseases. We tried to assess whether 12 weeks of aerobic exercises affect serum resistin level and lipid profile in overweight females or not. Methods: For this purpose, 32 non-trained adult females with the mean age of 38 ± 7 years old and body mass index (BMI) of 32 ± 3 kg/m2 were randomly assigned to aerobic exercise (n=16) and control (n=16) groups. Twelve weeks of aerobic exercise program including 3 sessions weekly at 60-75% of maximum heart rate was considered. Fasting serum resistin level, lipid profile (total cholesterol [TC], low-density lipoprotein [LDL] and high-density lipoprotein [HDL] cholesterol, and triglyceride [TG]), and anthropometric indexes were measured and compared between the two groups before and after training. Results: Serum resistin, lipid profile markers, and anthropometrical indexes were not significantly different at baseline between the two groups (P>0.05). TG, TC, LDL, and serum resistin did not change significantly after the intervention in the exercise group (P>0.05). However, a significant increase in HDL (exercise: 46.3 ± 9.6 vs. 51.4 ± 8.5, P=0.011; control: 45.8 ± 7.9 vs. 46.8 ± 6.9 P=0.326) and a significant decrease in LDL/HDL ratio (exercise: 2.94 ± 0.12 vs. 2.47 ± 0.19, P=0.019; control: 3.14 ± 0.23 vs. 3.18 ± 0.63, P=0.265) were observed after the intervention. Other variables in the control group remained unchanged. Conclusion: Aerobic exercises had no impacts on the inflammatory profile but could improve lipid profile with an emphasis on HDL in adult obese women.

Resistin induces breast cancer cells epithelial to mesenchymal transition (EMT) and stemness through both adenylyl cyclase-associated protein 1 (CAP1)-dependent and CAP1-independent mechanisms

Breast cancer incidence and metastasis in postmenopausal women are known to associate with obesity, but the molecular mechanisms behind this association are largely unknown. We investigated the effect of adipokine resistin on epithelial to mesenchymal transition (EMT) and stemness in breast cancer cells in vitro. Previous reports demonstrated that the inflammatory actions of resistin are mediated by the adenylyl cyclase-associated protein 1 (CAP1), which serves as its receptor. As a model for our study, we used MCF-7 and MDA-MB-231 breast cancer and MCF-10A breast epithelial cells. We showed that in MCF-7 cells resistin increases the migration of MCF-7 and MDA-MB-231 cells and induces the formation of cellular protrusions through reorganization of F-actin filaments. Resistin upregulated the expression of mesenchymal markers involved in EMT (SNAIL, SLUG, ZEB1, TWIST1, fibronectin, and vimentin), and downregulated those of epithelial markers (E-cadherin and claudin-1). Resistin also potentiated the nuclear translocation of SNAIL protein, indicating initiation of EMT reprogramming. We further induced EMT in non-carcinogenic breast epithelial MCF-10A cells demonstrating that the effects of resistin on EMT were not breast cancer cell specific. In order to assess whether resistin-induced EMT depends on CAP1, we used siRNA approach to silence CAP1 gene in MCF-7 cells. Results demonstrated that when CAP1 was silenced, the induction of SNAIL, ZEB1 and vimentin expression by resistin as well as SNAIL and ZEB1 nuclear translocation, were abolished. Additionally, CAP1 silencing resulted in a suppression of MCF-7 cells migration. We performed quantitative PCR array profiling the expression of 84 genes related to cancer stem cells (CSC), pluripotency and metastasis and selected a set of genes (ALDH1A1, ITGA4, LIN28B, SMO, KLF17, PTPRC, PROM1, SIRT1, and PECAM1) that were modulated by resistin. Further experiments demonstrated that the effect of resistin on the expression of some of these genes (PROM1, PTPRC, KLF17, SIRT1, and PECAM1) was also dependent on CAP1. Our results demonstrate that resistin promotes the metastatic potential of breast cancer cells by inducing EMT and stemness and some of these effects are mediated by CAP1.

A Direct in Vivo Comparison of the Melanocortin Monovalent Agonist Ac-His-DPhe-Arg-Trp-NH2 versus the Bivalent Agonist Ac-His-DPhe-Arg-Trp-PEDG20-His-DPhe-Arg-Trp-NH2: A Bivalent Advantage

Bivalent ligands targeting putative melanocortin receptor dimers have been developed and characterized in vitro; however, studies of their functional in vivo effects have been limited. The current report compares the effects of homobivalent ligand CJL-1-87, Ac-His-DPhe-Arg-Trp-PEDG20-His-DPhe-Arg-Trp-NH₂, to monovalent ligand CJL-1-14, Ac-His-DPhe-Arg-Trp-NH₂, on energy homeostasis in mice after central intracerebroventricular (ICV) administration into the lateral ventricle of the brain. Bivalent ligand CJL-1-87 had noteworthy advantages as an antiobesity probe over CJL-1-14 in a fasting-refeeding in vivo paradigm. Treatment with CJL-1-87 significantly decreased food intake compared to CJL-1-14 or saline (50% less intake 2-8 h after treatment). Furthermore, CJL-1-87 treatment decreased the respiratory exchange ratio (RER) without changing the energy expenditure indicating that fats were being burned as the primary fuel source. Additionally, CJL-1-87 treatment significantly lowered body fat mass percentage 6 h after administration (p < 0.05) without changing the lean mass percentage. The bivalent ligand significantly decreased insulin, C-peptide, leptin, GIP, and resistin plasma levels compared to levels after CJL-1-14 or saline treatments. Alternatively, ghrelin plasma levels were significantly increased. Serum stability of CJL-1-87 and CJL-1-14 (T_1/2 = 6.0 and 16.8 h, respectively) was sufficient to permit physiological effects. The differences in binding affinity of CJL-1-14 compared to CJL-1-87 are speculated as a possible mechanism for the bivalent ligand's unique effects. We also provide in vitro evidence for the formation of a MC3R-MC4R heterodimer complex, for the first time to our knowledge, that may be an unexploited neuronal molecular target. Regardless of the exact mechanism, the advantageous ability of CJL-1-87 compared to CJL-1-14 to increase in vitro binding affinity, increase the duration of action in spite of decreased serum stability, decrease in vivo food intake, decrease mice's body fat percent, and differentially affect mouse hormone levels demonstrates the distinct characteristics achieved from the current melanocortin agonist bivalent design strategy.

Adiponectin induced AMP-activated protein kinase impairment mediates insulin resistance in Bama mini-pig fed high-fat and high-sucrose diet

OBJECTIVE

Adipose tissue is no longer considered as an inert storage organ for lipid, but instead is thought to play an active role in regulating insulin effects via secretion adipokines. However, conflicting reports have emerged regarding the effects of adipokines. In this study, we investigated the role of adipokines in glucose metabolism and insulin sensitivity in obese Bama mini-pigs.

METHODS

An obesity model was established in Bama mini-pigs, by feeding with high-fat and high-sucrose diet for 30 weeks. Plasma glucose and blood biochemistry levels were measured, and intravenous glucose tolerance test was performed. Adipokines, including adiponectin, interleukin-6 (IL-6), resistin and tumor necrosis factor alpha (TNF-α), and glucose-induced insulin secretion were also examined by radioimmunoassay. AMP-activated protein kinase (AMPK) phosphorylation in skeletal muscle, which is a useful insulin resistance marker, was examined by immunoblotting. Additionally, associations of AMPK phosphorylation with plasma adipokines and homeostasis model assessment of insulin resistance (HOMA-IR) index were assessed by Pearce's correlation analysis.

RESULTS

Obese pigs showed hyperglycemia, high triglycerides, and insulin resistance. Adiponectin levels were significantly decreased (p<0.05) and IL-6 amounts dramatically increased (p<0.05) in obese pigs both in serum and adipose tissue, corroborating data from obese mice and humans. However, circulating resistin and TNF-α showed no difference, while the values of TNF-α in adipose tissue were significantly higher in obese pigs, also in agreement with data from obese humans but not rodent models. Moreover, strong associations of skeletal muscle AMPK phosphorylation with plasma adiponectin and HOMA-IR index were obtained.

CONCLUSION

AMPK impairment induced by adiponectin decrease mediates insulin resistance in high-fat and high-sucrose diet induction. In addition, Bama mini-pig has the possibility of a conformable model for human metabolic diseases.

Adipose tissue in control of metabolism

Adipose tissue plays a central role in regulating whole-body energy and glucose homeostasis through its subtle functions at both organ and systemic levels. On one hand, adipose tissue stores energy in the form of lipid and controls the lipid mobilization and distribution in the body. On the other hand, adipose tissue acts as an endocrine organ and produces numerous bioactive factors such as adipokines that communicate with other organs and modulate a range of metabolic pathways. Moreover, brown and beige adipose tissue burn lipid by dissipating energy in the form of heat to maintain euthermia, and have been considered as a new way to counteract obesity. Therefore, adipose tissue dysfunction plays a prominent role in the development of obesity and its related disorders such as insulin resistance, cardiovascular disease, diabetes, depression and cancer. In this review, we will summarize the recent findings of adipose tissue in the control of metabolism, focusing on its endocrine and thermogenic function.

Resistin's, obesity and insulin resistance: the continuing disconnect between rodents and humans

PURPOSE

This review aimed to discuss the conflicting findings from resistin research in rodents and humans as well as recent advances in our understanding of resistin's role in obesity and insulin resistance.

METHODS

A comprehensive review and synthesis of resistin's role in obesity and insulin resistance as well as conflicting findings from resistin research in rodents and humans.

RESULTS

In rodents, resistin is increased in high-fat/high-carbohydrate-fed, obese states characterized by impaired glucose uptake and insulin sensitivity. Resistin plays a causative role in the development of insulin resistance in rodents via 5' AMP-activated protein kinase (AMPK)-dependent and AMPK-independent suppressor of cytokine signaling-3 (SOCS-3) signaling. In contrast to rodents, human resistin is primarily secreted by peripheral-blood mononuclear cells (PBMCs) as opposed to white adipocytes. Circulating resistin levels have been positively associated with central/visceral obesity (but not BMI) as well as insulin resistance, while other studies show no such association. Human resistin has a role in pro-inflammatory processes that have been conclusively associated with obesity and insulin resistance. PBMCs, as well as vascular cells, have been identified as the primary targets of resistin's pro-inflammatory activity via nuclear factor-κB (NF-κB, p50/p65) and other signaling pathways.

CONCLUSION

Mounting evidence reveals a continuing disconnect between resistin's role in rodents and humans due to significant differences between these two species with respect to resistin's gene and protein structure, differential gene regulation, tissue-specific distribution, and insulin resistance induction as well as a paucity of evidence regarding the resistin receptor and downstream signaling mechanisms of action.

Secret talk between adipose tissue and central nervous system via secreted factors-an emerging frontier in the neurodegenerative research

First seen as a storage organ, the white adipose tissue (WAT) is now considered as an endocrine organ. WAT can produce an array of bioactive factors known as adipokines acting at physiological level and playing a vital role in energy metabolism as well as in immune response. The global effect of adipokines in metabolic activities is well established, but their impact on the physiology and the pathophysiology of the central nervous system (CNS) remains poorly defined. Adipokines are not only produced by the WAT but can also be expressed in the CNS where receptors for these factors are present. When produced in periphery and to affect the CNS, these factors may either cross the blood brain barrier (BBB) or modify the BBB physiology by acting on cells forming the BBB. Adipokines could regulate neuroinflammation and oxidative stress which are two major physiological processes involved in neurodegeneration and are associated with many chronic neurodegenerative diseases. In this review, we focus on four important adipokines (leptin, resistin, adiponectin, and TNFα) and one lipokine (lysophosphatidic acid-LPA) associated with autotaxin, its producing enzyme. Their potential effects on neurodegeneration and brain repair (neurogenesis) will be discussed. Understanding and regulating these adipokines could be an interesting lead to novel therapeutic strategy in order to counteract neurodegenerative disorders and/or promote brain repair.

Role of resistin in insulin resistance and obesity

Resistin is an adipose-derived hormone postulated to link adiposity to insulin resistance. Rodent animal experiments and in vitro experimental studies showed that resistin can induce insulin resistance, glucose and lipid metabolism disorders, and be closely related to metabolic syndrome. However, the specific mechanisms of action of resistin in humans are not clear. There is still controversy over the relationship between resistin and obesity. This review aims to elucidate the role of resistin in insulin resistance and discuss the relationship between resistin and obesity.

Metformin decreases plasma resistin concentrations in pediatric patients with impaired glucose tolerance: a placebo-controlled randomized clinical trial

The objective was to determine the effect of metformin on the concentrations of resistin and other markers of insulin resistance or inflammation (C-reactive protein, cytokines, body weight, HbA1c, among others) in minors with glucose intolerance. Patients aged 4 to 17 years with glucose intolerance were studied. They were randomized to receive 850 mg of either metformin or placebo twice daily for 12 weeks, during which all followed an iso-caloric diet and an exercise program. High sensitivity C-reactive protein, TNF-alpha, IL-6, IL1-beta, resistin, leptin, adiponectin, glucose, insulin, HbA1c, lipid profile and transaminases were measured at the beginning and at the end of the period. Fifty-two patients were included, 11.9±2.6 years old; 28 (12 males/16 females) received metformin and 24 placebo (11 males/13 females). Baseline characteristics were similar between groups (except for body mass index, which in the metformin group was slightly higher). Percentage weight loss was greater in the metformin group (-5.86% vs 2.75%, P<.05). At study end, there were statistically significant differences in resistin concentrations, even after adjusting for confounding variables (F=7.714; P<.006). Also, metformin was associated with a significant decrease in HOMA-IR index (P=.032) and HbA1c levels (P=.001), but no change was observed in the concentration of other markers of inflammation. Metformin resulted in significant reductions of plasma resistin levels in minors with glucose intolerance. This change is independent of its effects on body weight. In contrast, metformin did not alter the concentration of inflammatory markers.

Resistin: functional roles and therapeutic considerations for cardiovascular disease

Resistin, originally described as an adipocyte-specific hormone, has been suggested to be an important link between obesity, insulin resistance and diabetes. Although its expression was initially defined in adipocytes, significant levels of resistin expression in humans are mainly found in mononuclear leukocytes, macrophages, spleen and bone marrow cells. Increasing evidence indicates that resistin plays important regulatory roles apart from its role in insulin resistance and diabetes in a variety of biological processes: atherosclerosis and cardiovascular disease (CVD), non-alcoholic fatty liver disease, autoimmune disease, malignancy, asthma, inflammatory bowel disease and chronic kidney disease. As CVD accounts for a significant amount of morbidity and mortality in patients with diabetes and without diabetes, it is important to understand the role that adipokines such as resistin play in the cardiovascular system. Evidence suggests that resistin is involved in pathological processes leading to CVD including inflammation, endothelial dysfunction, thrombosis, angiogenesis and smooth muscle cell dysfunction. The modes of action and signalling pathways whereby resistin interacts with its target cells are beginning to be understood. In this review, the current knowledge about the functions and pathophysiological implications of resistin in CVD development is summarized; clinical translations, therapeutic considerations and future directions in the field of resistin research are discussed.

LINKED ARTICLES

This article is part of a themed section on Fat and Vascular Responsiveness. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-3.

Resistin: functional roles and therapeutic considerations for cardiovascular disease

Resistin, originally described as an adipocyte-specific hormone, has been suggested to be an important link between obesity, insulin resistance and diabetes. Although its expression was initially defined in adipocytes, significant levels of resistin expression in humans are mainly found in mononuclear leukocytes, macrophages, spleen and bone marrow cells. Increasing evidence indicates that resistin plays important regulatory roles apart from its role in insulin resistance and diabetes in a variety of biological processes: atherosclerosis and cardiovascular disease (CVD), non-alcoholic fatty liver disease, autoimmune disease, malignancy, asthma, inflammatory bowel disease and chronic kidney disease. As CVD accounts for a significant amount of morbidity and mortality in patients with diabetes and without diabetes, it is important to understand the role that adipokines such as resistin play in the cardiovascular system. Evidence suggests that resistin is involved in pathological processes leading to CVD including inflammation, endothelial dysfunction, thrombosis, angiogenesis and smooth muscle cell dysfunction. The modes of action and signalling pathways whereby resistin interacts with its target cells are beginning to be understood. In this review, the current knowledge about the functions and pathophysiological implications of resistin in CVD development is summarized; clinical translations, therapeutic considerations and future directions in the field of resistin research are discussed.

LINKED ARTICLES

This article is part of a themed section on Fat and Vascular Responsiveness. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-3.

Growth hormone and IGF-1 deficiency exacerbate high-fat diet-induced endothelial impairment in obese Lewis dwarf rats: implications for vascular aging

Previous studies suggest that the age-related decline in circulating growth hormone (GH) and insulin-like growth factor-1 (IGF-1) levels significantly contribute to vascular dysfunction in aging by impairing cellular oxidative stress resistance pathways. Obesity in elderly individuals is increasing at alarming rates, and there is evidence suggesting that elderly individuals are more vulnerable to the deleterious cardiovascular effects of obesity than younger individuals. However, the specific mechanisms through which aging, GH/IGF-1 deficiency, and obesity interact to promote the development of cardiovascular disease remain unclear. To test the hypothesis that low circulating GH/IGF-1 levels exacerbate the pro-oxidant and proinflammatory vascular effects of obesity, GH/IGF-1-deficient Lewis dwarf rats and heterozygous control rats were fed either a standard diet or a high-fat diet (HFD) for 7 months. Feeding an HFD resulted in similar relative weight gains and increases in body fat content in Lewis dwarf rats and control rats. HFD-fed Lewis dwarf rats exhibited a relative increase in blood glucose levels, lower insulin, and impaired glucose tolerance as compared with HFD-fed control rats. Analysis of serum cytokine expression signatures indicated that chronic GH/IGF-1 deficiency exacerbates HFD-induced inflammation. GH/IGF-1 deficiency also exacerbated HFD-induced endothelial dysfunction, oxidative stress, and expression of inflammatory markers (tumor necrosis factor-α, ICAM-1) in aortas of Lewis dwarf rats. Overall, our results are consistent with the available clinical and experimental evidence suggesting that GH/IGF-1 deficiency renders the cardiovascular system more vulnerable to the deleterious effects of obesity.

Inflammation, a link between obesity and cardiovascular disease

Obesity, the most common nutritional disorder in industrialized countries, is associated with an increased mortality and morbidity of cardiovascular disease (CVD). Obesity is primarily considered to be a disorder of energy balance, and it has recently been suggested that some forms of obesity are associated with chronic low-grade inflammation. The present paper focuses on the current status of our knowledge regarding chronic inflammation, a link between obesity and CVDs, including heart diseases, vascular disease and atherosclerosis. The paper discusses the methods of body fat evaluation in humans, the endocrinology and distribution of adipose tissue in the genders, the pathophysiology of obesity, the relationship among obesity, inflammation, and CVD, and the adipose tissue-derived cytokines known to affect inflammation. Due to space limitations, this paper focuses on C-reactive protein, serum amyloid A, leptin, adiponectin, resistin, visfatin, chemerin, omentin, vaspin, apelin, and retinol binding protein 4 as adipokines.

Diet-genotype interactions in the early development of obesity and insulin resistance in mice with a genetic deficiency in tumor necrosis factor-alpha

The onset of insulin resistance, the sites of action, and the mechanisms through which tumor necrosis factor-alpha (TNF-alpha) exacerbates the increase in adiposity and the development of insulin resistance in mice fed high-fat (HF) diet remain unclear. Here we investigated the effect of TNF-alpha deficiency on adiposity and insulin resistance during the initial 1 to 4 weeks of HF feeding. We examined body weight; the distribution of white adipose tissue (WAT); homeostasis model assessment; and levels of leptin, resistin, and adiponectin in the initial 4 weeks of HF feeding in TNF-alpha knockout (KO) mice and wild-type (WT) controls. Through 4 weeks of HF feeding, KO mice, unlike WT mice, maintained normal insulin sensitivity. Although WT-HF and KO-HF mice had similar levels of WAT at this time, KO-HF mice had more subcutaneous and less epididymal fat than WT-HF mice. The KO-HF mice also had less liver fat than the WT-HF mice. Finally, KO-HF mice had lower plasma levels of resistin than WT-HF mice. These data demonstrate that genetic lack of TNF-alpha protects insulin sensitivity during the early phase of HF feeding in the absence of altered total WAT. The data also suggest that the mechanism maintaining insulin sensitivity in the absence of TNF-alpha may involve redirection of the fat deposition to the metabolically more inert subcutaneous depot or decreases in circulating resistin and resultant decrease in liver fat deposition. The efficacy of therapeutic measures designed to counteract the effects of TNF-alpha may be increased during the early stages of obesity and insulin resistance.

Reviews: adipocytokines in normal and complicated pregnancies

Human pregnancy is characterized by insulin resistance, traditionally attributed to the effects of placental hormones. Normal pregnancy-induced insulin resistance is further enhanced in pregnancy complications, associated with disturbed placental function, such as gestational diabetes mellitus, preeclampsia, and intrauterine growth restriction. Compelling evidence suggests that these pregnancy disorders are associated with future development of maternal metabolic syndrome. However, the pathogenetic mechanisms underlying the association between abnormal placental development, insulin resistance, and maternal metabolic syndrome are not fully understood. A large body of evidence has recently supported the role of adipose tissue in the regulation of insulin resistance in both nonpregnant and pregnant participants. In this respect, adipocytokines, which are adipocyte-derived hormones, have been implicated in the regulation of maternal metabolism and gestational insulin resistance. Adipocytokines, including leptin, adiponectin, tumor necrosis factor alpha, interleukin 6, as well as the newly discovered resistin, visfatin, and apelin, are also known to be produced within the intrauterine environment. However, data concerning the pattern of adipocytokines secretion in normal and complicated pregnancies are still limited and partially contradictory. Given the importance of adipose tissue and its hormones in terms of adequate metabolic control and energy homeostasis, we present a review of published data related to the role of adipocytokines in pregnancy, especially in relation to pregnancy complications. Focus will be placed on the functions and other potential roles of the novel adipocytokines resistin, visfatin, and apelin.

Effects of resistin expression on glucose metabolism and hepatic insulin resistance

In order to observe the effect of increased serum resistin on glucose metabolism, insulin sensitivity, and hepatic insulin resistance (IR), mice were intravenously injected with recombinant adenovirus carrying the resistin gene (Adv-resistin-EGFP). Changes in hepatic glucose metabolism were observed using the Periodic Acid-Schiff method. Hepatic AMP-activated protein kinase (AMPK) activation was assessed by Western blot analysis, and glucose-6-phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression was determined using real-time RT-PCR. Although no effect on fasting blood glucose was detected, increased fasting insulin levels, decreased glucose tolerance and insulin sensitivity, and reduced hepatic glycogen levels and AMPK activation were seen in the Adv-resistin-EGFP mice. Finally, elevated G6Pase and PEPCK mRNA expression levels were detected upon overexpression of resistin. Resistin may inhibit hepatic AMPK activity, which results in elevated expression of gluconeogenic enzymes thereby affecting glucose metabolism and leading to decreased glycogen storage that contributes to the development of hepatic IR.

Adipose tissue-mediated inflammation: the missing link between obesity and cardiovascular disease?

Until relatively recently, the role of adipose tissue in the development of obesity and its consequences was considered to be a passive one. Mounting evidence highlights the role of adipose tissue in the development of a systemic inflammatory state that contributes to obesity-associated vasculopathy and cardiovascular risk. It is now clear that, in addition to storing calories as triglycerides, adipocytes secrete a large variety of cytokines, chemokines and hormone-like factors, such as leptin, resistin, and acute-phase proteins. In addition, insulin resistance, both in nondiabetic and diabetic subjects, is frequently associated with obesity, particularly with an excess of intraabdominal fat. This production of pro-atherogenic substances is of particular interest since an increase in the plasma levels of these mediators may provide a novel mechanistic link between obesity and its vascular complications.

Effects of a high-fat, low- versus high-glycemic index diet: retardation of insulin resistance involves adipose tissue modulation

Beneficial effects of low glycemic index (GI) diets in rodents have been studied using healthy low-fat diets, while the effects might be different on high-fat diets inducing progression of insulin resistance. We fed C57BL/6J male mice high-fat low/high-GI (LGI/HGI) diets for 13 wk. Glucose and insulin tolerance and serum substrates, including adipokines, were measured longitudinally. The LGI group showed a significantly higher glucose tolerance from wk 2 onwards, which was supported by lower serum insulin and free fatty acids levels at 8 wk, and a tendency for lower leptin levels, while resistin levels remained similar. At 11 wk, when differences in serum resistin started to increase, differences in serum insulin were diminished. Although food intake was similar throughout the study, body weights and epididymal adipose tissue mass became significantly lower in the LGI group at necropsy. Several serum substrates and adipose tissue leptin mRNA levels, as analyzed by Q-PCR, were, again, significantly lower, whereas adiponectin mRNA levels were higher. Taken together, an LGI high-fat diet maintains higher glucose tolerance and insulin sensitivity via adipose tissue modulation solely because of a difference in the type of carbohydrate, supporting a nutritional approach in the fight against insulin resistance.

Adherence to healthy eating patterns is associated with higher circulating total and high-molecular-weight adiponectin and lower resistin concentrations in women from the Nurses' Health Study

BACKGROUND

Adherence to a healthy dietary pattern, such as the Alternate Healthy Eating Index (AHEI), is associated with a lower risk of diabetes and atherosclerosis.

OBJECTIVE

We aimed to determine whether adherence to the AHEI is associated with higher plasma total and high-molecular-weight (HMW) adiponectin concentrations and lower concentrations of resistin, as well as biomarkers of inflammation, endothelial dysfunction, and insulin resistance.

DESIGN

The study evaluated 1922 women from the Nurses' Health Study (62% of whom were overweight) who had no history of diabetes or cardiovascular disease. Their plasma biomarker concentrations were measured in 1990, and data on dietary intake from semiquantitative food-frequency questionnaires administered in 1984, 1986, and 1990 were averaged to account for long-term dietary exposure and to reduce within-subject variability.

RESULTS

After adjustment for age and energy intake, women with the highest adherence to the AHEI had 24% higher median total adiponectin and 32% higher median HMW adiponectin concentrations, as well as 16% lower resistin, 41% lower CRP, 19% lower sE-selectin, and 24% lower ferritin concentrations (P < 0.01 for all) than did women with the lowest adherence to the AHEI. These associations remained significant after adjustment for potential confounders. Inverse associations between the AHEI and soluble tumor necrosis factor-alpha receptor II, interleukin-6, soluble intercellular adhesion molecule 1, soluble vascular cell adhesion molecule 1, C-peptide, insulin, and glycated hemoglobin were evident, but they were not significant after adjustment for body mass index.

CONCLUSION

The preventive effects of healthier dietary patterns on risk for diabetes and atherosclerosis may be mediated by improvements in plasma concentrations of adipokines or other biomarkers of risk for diabetes and cardiovascular disease.

Mechanisms of antidiabetogenic and body weight-lowering effects of estrogen in high-fat diet-fed mice

The high-fat diet (HFD)-fed mouse is a model of obesity, impaired glucose tolerance, and insulin resistance. The main objective of this study was to elucidate the molecular mechanisms underlying the antidiabetogenic and weight-lowering effects of 17beta-estradiol (E(2)) in this mouse model. C57BL/6 female mice (8 wk old) were fed on a HFD for 10 mo. E(2), given daily (50 microg/kg s.c.) during the last month of feeding, decreased body weight and markedly improved glucose tolerance and insulin sensitivity. Plasma levels of insulin, leptin, resistin, and adiponectin were decreased. We demonstrated that E(2) treatment decreased the expression of genes encoding resistin and leptin in white adipose tissue (WAT), whereas adiponectin expression was unchanged. Furthermore, in WAT we demonstrated decreased expression levels of sterol regulatory element-binding protein 1c (SREBP1c) and its lipogenic target genes, such as fatty acid synthase and stearoyl-CoA desaturase 1 (SCD1). In the liver, the expression levels of transcription factors such as liver X receptor alpha and SREBP1c were not changed by E(2) treatment, but the expression of the key lipogenic gene SCD1 was reduced. This was accompanied by decreased hepatic triglyceride content. Importantly, E(2) decreased the hepatic expression of glucose-6-phosphatase (G-6-Pase). We conclude that E(2) treatment exerts antidiabetic and antiobesity effects in HFD mice and suggest that this is related to decreased expression of lipogenic genes in WAT and liver and suppression of hepatic expression of G-6-Pase. Decreased plasma levels of resistin probably also play an important role in this context.

The role of adiposity as a determinant of an inflammatory milieu

With the growing prevalence of obesity, scientific interest in the biology of adipose tissue has been extended to the secretory products of adipocytes, since they have been shown increasingly to affect several aspects of the pathogenesis of obesity-related diseases. Until relatively recently, the role of adipose tissue itself in the development of obesity and its consequences was considered to be a passive one. It is now clear that, in addition to storing energy in the form of triglycerides, adipocytes also secrete a large variety of proteins, including cytokines, chemokines and hormone-like factors. This production of proatherogenic chemokines by adipose tissue is of particular interest, since their local secretion, for example by perivascular adipose depots, may provide a novel mechanistic link between obesity and associated vascular complications.

Viral and host correlates of serum resistin in simian AIDS

Resistin is an adipocytokine with a proposed dual role in metabolism and inflammation. In light of the ability to promote inflammatory responses, adipocytokines may prove key factors in modulating the host response to HIV. This study utilizes the simian immunodeficiency virus (SIV) model of HIV/AIDS to investigate changes in serum resistin levels following dietary intervention and SIV infection and determine associations with measures of body composition and disease severity. Resistin levels, body composition (n = 34), and insulin resistance (n = 16) were determined in healthy rhesus macaques. A subset of animals (n = 8) was placed on an atherogenic diet (AD) and subsequently inoculated with SIVmac239. Longitudinal measures of serum resistin, cytokines, viral load, lymphocyte subsets, and body composition were obtained. In healthy macaques consuming a standard diet, resistin levels correlated positively with total fat mass (r = 0.49; p < 0.01) and tissue fat percent (r = 0.53; p < 0.01) but failed to associate with measures of insulin resistance. In contrast, a negative correlation was noted between these measures of adiposity and resistin following SIV inoculation (r = -0.27; p < 0.05 and r = -0.24; p < 0.05, respectively). Viral load correlated positively with serum resistin (r = 0.32; p < 0.01). Serum levels of MCP-1 and sTNF RII demonstrated no correlation with resistin in normal animals on a standard diet, while a significant positive correlation was observed following SIV infection (r = 0.52; p < 0.0001 and r = 0.59; p < 0.0001, respectively). Findings indicate a fundamental difference in the relationship between resistin and body composition following SIV infection and suggest that elevations in resistin parallel measures of disease severity including loss of body fat and viral replication.

Effect of Jiangzhi Yigan Chongji on resistin mRNA in the liver of rats with non-alcoholic steatohepatitis

AIM: To explore the effects of resistin in the pathogenesis of a rat model of non-alcoholic steatohepatitis (NASH), and to observe the effect of Jiangzhi Yigan Chongji on resistin mRNA in the liver of that model, to explore its mechanism of action.

METHODS: 24 male Wistar rats were randomly divided into three groups (eight rats in each): normal group fed with standard diet, model group, and treated group fed with a high-fat diet. The treated group was given Jiangzhi Yigan Chongji (1 mL/100 g, its concentration is 0.8 g/mL, twice daily) by intragastric administration after 12 weeks. Meanwhile, the normal and model groups were separately given drinking water by the same means. After 17 weeks, they were killed. Resistin mRNA in the liver of the rat model of NASH induced by high-fat diet was detected with RT-PCR. Level of alanine aminotransferase (ALT) and asparate aminotransferase (AST) was detected by an automatic biochemistry analyzer; fasting blood glucose (FBG) was detected by glucose oxidase method; and tumor necrosis factor α (TNF-α) and fasting insulin (FINS) in serum were detected by radioimmunoassay. Degrees of steatosis and steatohepatitis were observed.

RESULTS: A positive correlation was found between mRNA expression of resistin and TNF-α, ALT and AST, and the degree of steatohepatitis (r = 0.873, 0.772, 0.716 and 0.892, P < 0.05) in the model group, but had no correlation with ISI. The relative level of resistin mRNA in the liver tissue of the model group (2.14 ± 0.11) was significantly higher than that in the normal group (0.42 ± 0.08), and that in the treated group (0.90 ± 0.06) was significantly lower than that in the model group (P < 0.01).

CONCLUSION: Resistin may play an important role in the damage of NASH. Jiangzhi Yigan Chongji can significantly decrease the expression of resistin mRNA in liver tissue of NASH rats, and resistin may be a pharmacological target.

Resistin down-regulates insulin receptor expression, and modulates cell viability in rodent pancreatic beta-cells

The adipokine resistin is known to induce insulin resistance in rodent tissues. Increases in adipose tissue mass are known to have a negative effect on pancreatic beta-cell function, although the mechanisms are poorly understood. This study investigated the effects of resistin on insulin secretion, insulin receptor expression and cell viability in pancreatic beta-cells. BTC-6 or BRIN-BD11 cells were treated for 24h with resistin, and insulin receptor expression, insulin secretion and cell viability were measured. Incubation with 40ng/ml resistin caused significant decreases in insulin receptor mRNA and protein expression, but did not affect insulin secretion. At low concentrations, resistin caused significant increases in cell viability. These data implicate resistin as a factor that may regulate beta-cell function/viability, and suggests a potential mechanism by which increased adiposity causes beta-cell dysfunction.

Biomarkers of obesity and subsequent cardiovascular events

Obesity is a major risk factor for cardiovascular diseases, but the mechanisms for increased cardiovascular risk in obesity are still unclear. Inflammation and increased oxidative stress are two potential mechanisms proposed to play a major role in the morbidity associated with obesity. Studies that investigate these mechanisms rely on biomarkers, but validated biomarkers for obesity-related cardiovascular outcomes are lacking. By finding optimal biomarkers, diagnostic criteria for cardiovascular diseases can be refined in the obese beyond "traditional" risk factors to identify early pathologic processes. The objective of this review is to identify potential early biomarkers resulting from obesity and associated with cardiovascular disease. Studies were initially identified through the search engine PubMed by using the keywords "obesity" and "biomarker." Subsequently, combinations of the keywords "obesity," "biomarker," "cardiovascular risk," "adipose tissue," "adipokine," "adipocytokine," and "oxidative stress" were used. The SOURCE database and Online Mendelian Inheritance in Man (OMIM) were used to obtain more information on the biomarkers. Results of the searches yielded a large number of potential biomarkers that occur in obesity and which either correlate with traditional cardiovascular risk factors or predict subsequent cardiovascular events. Several biomarkers are promising regarding their biologic properties, but they require further validation in humans.

High-fat diets: modeling the metabolic disorders of human obesity in rodents

RESEARCH METHODS AND PROCEDURES

High-fat (HF) diet feeding can induce obesity and metabolic disorders in rodents that resemble the human metabolic syndrome. However, this dietary intervention is not standardized, and the HF-induced phenotype varies distinctly among different studies. The question which HF diet type is best to model the metabolic deterioration seen in human obesity remains unclear. Therefore, in this review, metabolic data obtained with different HF diet approaches are compiled. Both whole-body and organ-specific diet effects are analyzed.

RESULTS

On the basis of these results, we conclude that animal fats and omega-6/omega-9-containing plant oils can be used to generate an obese and insulin-resistant phenotype in rodents, whereas fish oil-fed animals do not develop these disorders.

DISCUSSION

Looking at the present data, it does not seem possible to define an ideal HF diet, and an exact definition of diet composition and a thorough metabolic characterization of the HF diet effects in a researcher's specific laboratory setting remains essential for metabolic studies with this model.

Maternal and foetal resistin and adiponectin concentrations in normal and complicated pregnancies

OBJECTIVE

The aim of this study was to evaluate how resistin and adiponectin (ApN) are involved in maternal energy metabolism and foetal growth.

DESIGN

A cross-sectional study.

PATIENTS AND MEASUREMENTS

Resistin and ApN were measured in 30 healthy nonpregnant women, 73 pregnant women [10-41 weeks of gestation; 18 with gestational diabetes mellitus (GDM), five with pregnancy-induce hypertension (PIH), nine with pre-eclampsia (PE), eight with chronic hypertension (CH) and 33 normal] and 40 foetal samples (20-41 weeks of gestation; 18 from GDM mothers and 22 from normal mothers).

RESULTS

Resistin levels were significantly higher in normal pregnant women than in nonpregnant controls (13.7 +/- 2.1 vs. 6.3 +/- 1.6 ng/ml; P < 0.005) and showed a negative correlation with gestational age (P < 0.0001, r = -0.7). Only women with PE presented resistin levels significantly lower than normotensive women of the same gestational age (8.2 +/- 1.2 vs. 17.9 +/- 4.3 ng/ml; P < 0.005). ApN levels, although similar in normal pregnant women to those in nonpregnant controls, were significantly lower in women with GDM (37-41 weeks; 5.2 +/- 0.5 vs. 8.2 +/- 0.8 mg/l; P < 0.0001) and PE (20-37 weeks; 5.0 +/- 0.7 vs. 9.5 +/- 0.7 mg/l; P = 0.008) than those found in normal women matched for gestational age. Resistin was detected in the umbilical venous blood in foetuses from 20 to 41 weeks of gestation. In all newborns, both resistin and ApN levels were significantly higher than those recorded in adult life and did not correlate with maternal levels (P = ns, r = 0.03 for resistin and P = ns, r = -0.3 for ApN). Foetuses from diabetic mothers had ApN significantly lower than normal foetuses (26.8 +/- 2.6 vs. 37.5 +/- 3.5 mg/l; P = 0.02), while resistin levels were similar (17.3 +/- 3.7 vs. 18.2 +/- 1.5 ng/ml; P = ns).

CONCLUSION

The secretion pattern of ApN in normal and complicated pregnancies strongly suggests an involvement of ApN in insulin resistance during gestation, while resistin seems to have a minor role. Moreover, the detection of high levels of resistin and ApN in cord blood during gestation is consistent with a regulatory action of these adipokines on tissue differentiation and foetal growth.

Resistin worsens cardiac ischaemia-reperfusion injury

We provide the first report of direct effects of resistin upon haemodynamic and neurohumoral parameters in isolated perfused rat heart preparations. Pre-conditioning with 1 nmol L-1 recombinant human resistin prior to ischaemia significantly impaired contractile recovery during reperfusion, compared with vehicle-infused hearts (P<0.05, n=12). This was accompanied by a significant increase in both A-type and B-type natriuretic peptides (P<0.05, n=12 both ANP and BNP vs vehicle), creatine kinase, and tumour necrosis factor-alpha (TNF-alpha) release in resistin-infused hearts. Resistin had no significant effect on myocardial glucose uptake. Co-infusion of resistin with Bay 11 7082 (an NF-kappaB inhibitor) improved contractile recovery following ischaemia and reduced both natriuretic peptide and creatine kinase release. This is the first evidence indicating resistin impairs cardiac recovery following ischaemia, stimulates cardiac TNF-alpha secretion, and modulates reperfusion release of natriuretic peptides and biochemical markers of myocardial damage. A TNF-alpha signalling related mechanism is suggested as one component underlying these effects.

Plasma adipokines and body composition in response to modest dietary manipulations in the mouse

OBJECTIVE

The relationship between adipokine levels and body composition has not been carefully examined. Most studies in humans are cross-sectional, and the few studies in mice have been restricted to a comparison of control animals with markedly obese, insulin-resistant mice. Our objective was to study changes in adipokine levels and body composition in response to modest dietary intervention.

RESEARCH METHODS AND PROCEDURES

Plasma resistin, adiponectin, and leptin levels were examined in mice fed ad libitum, a 75% restricted diet, or a diet supplemented with 10% sucrose. Body composition was determined by whole-body DXA.

RESULTS

The percentage body fat was reduced in mice subjected to the restricted diet and increased in mice supplemented with 10% dextrose. Adipokine levels were not different in either of these groups compared with the control mice. A significant inverse correlation was observed between resistin levels and total body fat, whereas there was no significant correlation between body fat and adiponectin levels. Positive correlations were observed between leptin levels and percentage body fat, total body fat, and abdominal fat. Leptin levels correlated with plasma glucose, but multivariate analysis revealed that this correlation was the result of a strong positive correlation between leptin and insulin levels. There were no correlations between glycemia and resistin or glycemia and adiponectin levels, and no correlation was observed between any of the adipokine levels and bone mineral content or density.

DISCUSSION

These data suggest that in the mouse, modest dietary perturbations have little effect on resistin and adiponectin levels despite significant effects on glycemia, insulin levels, and bone parameters.

Evidence that oestrogen receptor-alpha plays an important role in the regulation of glucose homeostasis in mice: insulin sensitivity in the liver

AIMS/HYPOTHESIS

We used oestrogen receptor-alpha (ERalpha) knockout (ERKO) and receptor-beta (ERbeta) knockout (BERKO) mice to investigate the mechanism(s) behind the effects of oestrogens on glucose homeostasis.

METHODS

Endogenous glucose production (EGP) was measured in ERKO mice using a euglycaemic-hyperinsulinaemic clamp. Insulin secretion was determined from isolated islets. In isolated muscles, glucose uptake was assayed by using radiolabelled isotopes. Genome-wide expression profiles were analysed by high-density oligonucleotide microarray assay, and the expression of the genes encoding steroyl-CoA desaturase and the Leptin receptor (Scd1 and Lepr, respectively) was confirmed by RT-PCR.

RESULTS

ERKO mice had higher fasting blood glucose, plasma insulin levels and IGT. The plasma leptin level was increased, while the adiponectin concentration was decreased in ERKO mice. Levels of both glucose- and arginine-induced insulin secretion from isolated islets were similar in ERKO and wild-type mice. The euglycaemic-hyperinsulinaemic clamp revealed that suppression of EGP by increased insulin levels was blunted in ERKO mice, which suggests a pronounced hepatic insulin resistance. Microarray analysis revealed that in ERKO mice, the genes involved in hepatic lipid biosynthesis were upregulated, while genes involved in lipid transport were downregulated. Notably, hepatic Lepr expression was decreased in ERKO mice. In vitro studies showed a modest decrease in insulin-mediated glucose uptake in soleus and extensor digitorum longus (EDL) muscles of ERKO mice. BERKO mice demonstrated normal glucose tolerance and insulin release.

CONCLUSIONS/INTERPRETATION

We conclude that oestrogens, acting via ERalpha, regulate glucose homeostasis mainly by modulating hepatic insulin sensitivity, which can be due to the upregulation of lipogenic genes via the suppression of Lepr expression.

Adipokines: implications for female fertility and obesity

Obesity is associated with a diverse set of metabolic disorders, and has reproductive consequences that are complex and not well understood. The adipose tissue-produced leptin has dominated the literature with regards to female fertility complications, but it is pertinent to explore the likely role of other adipokines--adiponectin and resistin--as our understanding of their biological functions emerge. Leptin influences the developing embryo, the functioning of the ovary and the endometrium, interacts with the release and activity of gonadotrophins and the hormones that control their synthesis. In this review such biological actions and potential roles of the adipokines leptin, adiponectin and resistin are explored in relation to female fertility and the complexity of the obese metabolic state.

Regulation of insulin signalling, glucose uptake and metabolism in rat skeletal muscle cells upon prolonged exposure to resistin

AIMS/HYPOTHESIS

Debate exists regarding the role of resistin in the pathophysiology of insulin resistance. The aim of this study was to directly assess the effects of resistin (0-24 h) on basal and insulin-stimulated glucose uptake and metabolism in skeletal muscle cells and to investigate the mechanisms responsible for the effects of resistin.

METHODS

We used L6 rat skeletal muscle cells and examined [(3)H]2-deoxyglucose uptake, GLUT4 translocation and GLUT protein content. We assessed glucose metabolism by measuring the incorporation of D-[U-(14)C]glucose into glycogen, (14)CO(2) and lactate production, as well as the phosphorylation level and total protein content of insulin signalling proteins, including insulin receptor beta-subunit (IRbeta), insulin receptor substrate (IRS), Akt and glycogen synthase kinase-3beta (GSK-3beta).

RESULTS

Treatment of L6 rat skeletal muscle cells with recombinant resistin (50 nmol/l, 0-24 h) reduced levels of basal and insulin-stimulated 2-deoxyglucose uptake and decreased insulin-stimulated GLUT4myc content at the cell surface, with no alteration in the production of GLUT4 or GLUT1. Resistin also decreased glycogen synthesis and GSK-3beta phosphorylation. Insulin-stimulated oxidation of glucose via the Krebs cycle was reduced by resistin, whereas lactate production was unaltered. Although insulin receptor protein level and phosphorylation were unaltered by resistin, production of IRS-1, but not IRS-2, was downregulated and a decreased tyrosine phosphorylation of IRS-1 was detected. Reduced phosphorylation of Akt on T308 and S473 was observed, while total Akt and Akt1, but not Akt2 or Akt3, production was decreased.

CONCLUSIONS/INTERPRETATION

Our data show that resistin regulates the function of IRS-1 and Akt1 and decreases GLUT4 translocation and glucose uptake in response to insulin. Selective decreases in insulin-stimulated glucose metabolism via oxidation and conversion to glycogen were also induced by resistin. These observations highlight the potential role of resistin in the pathophysiology of type 2 diabetes in obesity.

Role of resistin in obesity, insulin resistance and Type II diabetes

Resistin is a member of a class of cysteine-rich proteins collectively termed resistin-like molecules. Resistin has been implicated in the pathogenesis of obesity-mediated insulin resistance and T2DM (Type II diabetes mellitus), at least in rodent models. In addition, resistin also appears to be a pro-inflammatory cytokine. Taken together, resistin, like many other adipocytokines, may possess a dual role in contributing to disease risk. However, to date there has been considerable controversy surrounding this 12.5 kDa polypeptide in understanding its physiological relevance in both human and rodent systems. Furthermore, this has led some to question whether resistin represents an important pathogenic factor in the aetiology of T2DM and cardiovascular disease. Although researchers still remain divided as to the role of resistin, this review will place available data on resistin in the context of our current knowledge of the pathogenesis of obesity-mediated diabetes, and discuss key controversies and developments.

Circulating resistin concentrations in children depend on renal function

BACKGROUND

Resistin is a newly discovered peptide hormone that inhibits adipogenesis. Furthermore, it may be involved in regulative processes taking place in insulin resistance and inflammation. In human beings as well as in rodents, the exact physiological role of resistin is unknown. The objective of this study was to examine whether resistin serum concentrations in childhood are regulated by renal function.

METHODS

Fifteen patients with end stage renal disease treated by haemodialysis (HD) (6m, 9f; age (median (25-75% percentile)) 16.1 (14.2-16.9) years; body mass index (BMI) 17.8 (17.6-20.6) kg/m2; glomerular filtration rate (GFR)<10 ml/min/1.73 m2), 11 patients with chronic renal failure (CRF) (4m, 7f; age 11.3 (9.2-16.9) years; BMI 17.9 (15.4-22.1) kg/m2; GFR 21.2 (19.8-39.4) ml/min/1.73 m2) and 23 healthy children (13m, 10f; age 7.8 (4.7-14.5) years; BMI 16.5 (14.9-18.1) kg/m2; GFR 168.5 (154.0-197.2) ml/min/1.73 m2) were included. Resistin concentration in serum was measured by ELISA, leptin by RIA.

RESULTS

Resistin concentrations were significantly elevated in HD (6.6 (5.7-8.7) ng/ml) and CRF (4.8 (4.1-6.2) ng/ml) compared to healthy controls [2.7 (2.0-3.6) ng/ml; P<0.0001 by multiple regression analysis]. Furthermore, resistin concentrations showed a tendency to be higher in children below the age of 5.5 years (youngest tertile) and above 12.5 years of age (oldest tertile), compared to children aged between 5.5 and 12.5 years (P=0.053 and P=0.043, respectively). Gender (P=0.686) and BMI (P =0.663) did not have a significant influence on resistin concentrations. Resistin and leptin serum concentrations correlated in HD patients only (r=0.62, P=0.013 by Spearman correlation). Haemodialysis did not eliminate resistin.

CONCLUSIONS

With decreasing renal function, resistin concentration in serum increases in our small paediatric cohort. Age possibly influences circulating resistin concentration. The hypothesis that elevated serum resistin in children with chronic renal failure or end stage renal disease may add to malnutrition and reduced BMI needs further investigation and is not supported by our data.

Regulation of fatty acid uptake and metabolism in L6 skeletal muscle cells by resistin

Resistin has been proposed as a potential link between obesity and insulin resistance. It is also well established that altered metabolism of fatty acids by skeletal muscle can lead to insulin resistance and lipotoxicity. However, little is known about the effect of resistin on long chain fatty acid uptake and metabolism in skeletal muscle. Here we show that treating rat skeletal muscle cells with recombinant resistin (50 nM, 24 h) decreased uptake of palmitate. This correlated with reduced cell surface CD36 content and lower expression of FATP1, but no change in FATP4 or CD36 expression. We also found that resistin decreased fatty acid oxidation by measuring 14CO2 production from [1-14C] oleate and an increase in intracellular lipid accumulation was detected in response to resistin. Decreased AMPK and ACC phosphorylation were observed in response to resistin while expression of ACC and AMPK isoforms was unaltered. Resistin mediated these effects without altering cell viability. In summary, our results demonstrate that chronic incubation of skeletal muscle cells with resistin decreased fatty acid uptake and metabolism via a mechanism involving decreased cell surface CD36 content, FATP1 expression and a decrease in phosphorylation of AMPK and ACC.