Elevated resistin levels in chronic kidney disease are associated with decreased glomerular filtration rate and inflammation, but not with insulin resistance

In the present study, we explore the role of decreased renal function and a genetic polymorphism on the recently discovered protein resistin, apparently able to inhibit hepatic insulin action in mice. We also investigate possible links with inflammation and the insulin resistance present in patients with chronic kidney disease (CKD). This is a post hoc, cross-sectional study comparing 239 prevalent CKD patients with varying degrees of renal function impairment with an age- and gender-matched randomly selected control group of 25 individuals. Glomerular filtration rate (GFR) was estimated by the mean of urea and creatinine clearance (24-h urine samples) (n=204) or by iohexol clearance (n=60). Plasma analysis of blood lipids, insulin, glucose, inflammatory markers (high-sensitivity C-reactive protein, interleukin-6, tumor necrosis factor-alpha, vascular cellular adhesion molecule, intercellular adhesion molecule) and resistin (kit from LINCO Research, St Charles, MS) was performed using commercially available assays or routine methods. Insulin resistance was estimated by quantitative insulin-sensitivity check index (QUICKI) and homeostasis model assessment for insulin resistance (HOMA-IR) and body composition by dual-energy X-ray absorptiometry. Genotyping of a C/G promoter single nucleotide polymorphism (n=168) at position -180 of the resistin gene was performed by PyroSequencing. Serum levels of resistin were markedly elevated in the CKD patients with both advanced (39.9+/-1.3 ng/ml) and mild to moderate (23.2+/-1.0 ng/ml) renal function impairment, as compared to controls (8.5+/-0.7 ng/ml; P<0.001). In a multiple linear regression model in patients (adjusted r(2)=0.60), only GFR (beta=3.4; P<0.0001), lean body mass (beta=2.2; P<0.001) and the inflammatory markers were independently associated with circulating resistin levels. There was a weak but significant impact of -180 C/G genotype on plasma levels of resistin (median 43.0+/-2.4 ng/ml in CC, 37.5+/-2.0 ng/ml in CG, and 41.1+/-4.9 ng/ml in GG; P<0.05). Univariate analysis of non-diabetic patients and controls showed that serum resistin was associated with markers of glucose metabolism. However, in a multiple regression model, resistin, as well as all the measured markers of inflammation, was only associated with insulin resistance if GFR was not taken into account. Circulating resistin levels are strongly associated with both GFR and inflammatory biomarkers in CKD. As the significant relationship between plasma resistin levels and insulin resistance was lost following the correction for GFR, resistin is not a likely mediator of insulin resistance in patients with CKD. Renal function is an important factor to take into account in clinical studies relating insulin sensitivity to inflammatory biomarkers in CKD as well as in patients with diabetes mellitus, who often have an impaired renal function.

Expanding role of AMPK in endocrinology

Adenosine 5' monophosphate-activated protein kinase (AMPK) is a regulator of cellular and systemic energy homeostasis. It mediates some of the effects of peripheral hormones such as leptin, ghrelin and adiponectin, and it is involved in the insulin-sensitizing role of the antidiabetic drug metformin. There is increasing evidence that AMPK has a central role in mediating the appetite-modulating and metabolic effects of many other hormones and substances, including the cannabinoids. Recent studies have illustrated the interaction between hormones and AMPK, and highlighted AMPK as a potential target for the development of tissue-specific AMPK modulators in the treatment of obesity and the metabolic syndrome.

The role of resistin in obesity-induced insulin resistance

Resistin is a 12.5-kDa polypeptide hormone produced by adipocytes and immunocompetent cells. It was originally proposed as a link between obesity and insulin resistance/diabetes. Later, studies revealed that substantial inter-species differences exist between the major sites of resistin production in rodents (adipocytes) and humans (immunocompetent cells). While in rodents resistin appears to have an important role in the development of liver insulin resistance, its role in humans is less clear, and it is probably involved in the regulation of inflammatory processes rather than in insulin sensitivity. Current experimental and clinical data concerning resistin physiology and pathophysiology, and its possible role in the development of insulin resistance and atherosclerosis are detailed in this review.

[To know is to relieve; "lifestyle-related diseases"; as they are now and how to prevent them]

Hypertension, diabetes mellitus, hyperlipidemia and obesity have recently defined as lifestyle-related diseases. A common background of these lifestyle-related disease is nutritional excess and its consequence, obesity. Recent advances in the biology of adipose tissue have revealed that adipose is not simply an energy storage organ but it also secretes a variety of molecules which affect the metabolism of the whole body. Dysregulation in the secretion of these adipose-specific secretory proteins may have important roles in the lifestyle-related diseases. It may be a great concern for many people to know about lifestyle-related diseases and how to manage and prevent them. The purpose of this citizen joint symposium is to broaden citizen's knowledge on the mechanism and appropriate management of lifestyle-related disease. We hope that this symposium provide useful informations to the participants about the current information in the management of lifestyle-related diseases.

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.

Adipocyte-derived hormones, cytokines, and mediators

Adipose tissue is responsive to both central and peripheral metabolic signals and is itself capable of secreting a number of proteins. These adipocyte-specific or enriched proteins, termed adipokines, have been shown to have a variety of local, peripheral, and central effects. These secreted proteins, which include tumor necrosis factor (TNF)-alpha, resistin, IL-6, IL-8, acylation-stimulating protein (ASP), angiotensinogen, plasminogen activator inhibitor-1 (PAI-1) ("bad" adipokines) and leptin, adiponectin ("good" adipokines) seem to play important regulatory roles in a variety of complex processes, including fat metabolism, feeding behavior, hemostasis, vascular tone, energy balance, and insulin sensitivity, but none is without controversy regarding its respective mechanism and scope of action. The present review is focused on the effects of free fatty acids and a restricted number of adipokines, which have been implicated in vascular (angiotensinogen, PAI-1) and energy and glucose homeostasis (ASP, TNFalpha, IL-6, resistin, leptin, adiponectin).

The new adipose tissue and adipocytokines

Obesity is a well-known risk factor for the development of insulin resistance, type 2 diabetes, dyslipidemia, hypertension, and cardiovascular disease. Rather than the total amount of fat, central distribution of adipose tissue is very important in the pathophysiology of this constellation of abnormalities termed metabolic syndrome. Adipose tissue, regarded only as an energy storage organ until the last decade, is now known as the biggest endocrine organ of the human body. This tissue secretes a number of substances--adipocytokines--with multiple functions in metabolic profile and immunological process. Therefore, excessive fat mass may trigger metabolic and hemostatic disturbances as well as CVD. Adipocytokines may act locally or distally as inflammatory, immune or hormonal signalers. In this review we discuss visceral obesity, the potential mechanisms by which it would be related to insulin resistance, methods for its assessment and focus on the main adipocytokines expressed and secreted by the adipose tissue. Particularly, we review the role of adiponectin, leptin, resistin, angiotensinogen, TNF-alpha, and PAI-1, describing their impact on insulin resistance and cardiovascular risk, based on more recent findings in this area.

Fat-specific transgenic expression of resistin in the spontaneously hypertensive rat impairs fatty acid re-esterification

OBJECTIVE

To investigate the mechanism by which fat-specific transgenic expression of resistin affects fatty acid metabolism in the spontaneously hypertensive rat (SHR).

DESIGN

Basal- and adrenaline-stimulated lipolysis, basal- and insulin-stimulated lipogenesis as well as the site (glycerol versus acyl moiety) of glucose incorporated into triglycerides were determined in adipose tissue isolated from SHR-Resistin transgenic and SHR control rats.

RESULTS

A moderate expression of transgenic resistin in adipose tissue was associated with significant increase in the FFA/glycerol ratio during adrenaline-stimulated lipolysis in the SHR-Resistin transgenic rats (3.27+/-0.26) compared to SHR controls (2.11+/-0.10, P=0.0005). Transgenic SHR also exhibited a significant decrease in FFA re-esterification in adipose tissue (approximately by 23%).

CONCLUSION

These findings raise the possibility that the prodiabetic effects of transgenic resistin may be partly mediated by increased FFA release from adipose tissue due to impaired FFA re-esterification in adipocytes.

Expression of Endothelin-1 and Adrenomedullin Was Not Altered by Leptin or Resistin in Bovine Brain Microvascular Endothelial Cells

Hypertension frequently occurs in obese subjects. It has been reported that leptin and resistin induce endothelin-1 expression in vascular endothelial cells. Altered function of brain microvascular endothelial cells may be related to increased occurrences of stroke in hypertensive patients. In the present study, we therefore studied the effects of leptin and resistin on the expression of endothelin-1 and adrenomedullin in bovine brain microvascular endothelial cells. Northern blot analysis showed that leptin (10(-10)-10(-8) mol/l), resistin (10(-10)-10(-8) mol/l) or a combination of leptin and resistin (10(-8) mol/l for each) had no significant effects on the expression of endothelin-1 mRNA or adrenomedullin mRNA in cultured bovine brain microvascular endothelial cells. On the other hand, hypoxia induced, and tumor necrosis factor-alpha (10 ng/ml) decreased, the expression levels of endothelin-1 and adrenomedullin mRNAs, indicating that the bovine brain microvascular endothelial cells were able to respond to hypoxia and tumor necrosis factor-alpha. Consistent with the results of Northern blot analysis, immunoreactive endothelin and immunoreactive adrenomedullin concentrations in the medium were not significantly changed by the treatment with leptin, resistin, or a combination of leptin and resistin. The present study thus showed that neither leptin nor resistin affects the expression of endothelin-1 or adrenomedullin in bovine brain microvascular endothelial cells.

Metabolic syndrome: the danger signal in atherosclerosis

Atherosclerosis is a chronic inflammatory disease characterized by infiltration of blood vessels by lipids and leukocytes. There is a growing body of evidence that among risk factors that promote atherosclerosis, the metabolic syndrome is a powerful and prevalent predictor of cardiovascular events. The systemic inflammatory process associated with the metabolic syndrome has numerous deleterious effects that promote plaque activation, which is responsible for clinical events. Interactions between the innate immune system with lipid-derived products seem to play a major role in the pathophysiology of atherosclerosis in relation with the metabolic syndrome. The multiple links among adipose tissue, the vascular wall, and the immune system are the topics of this review, which examines the roles of oxidized low-density lipoprotein, inflammatory cytokines, and adipokines in triggering and perpetuating a danger signal response that promotes the development of atherosclerosis. Furthermore, therapeutic options that specifically target the metabolic syndrome components are reviewed in light of recent developments.

Adipokine signaling in the peritoneal dialysis patient

Patients on peritoneal dialysis suffer from an increased incidence of cardiovascular disease, obesity, insulin resistance, as well as a markedly increased mortality rate. In both the general population and in renal patients current research implicates fat tissue as an important factor in modulating many of these complications through the secretion of various signaling peptides, mainly cytokines and adipokines. Whereas hemodialysis patients tend to lose fat mass after initiation of renal replacement therapy, peritoneal dialysis-patients instead gain fat, probably as a result of continuous glucose absorption. In the present overview we summarize some recent findings indicating that fat mass actively contributes to systemic inflammation and metabolic disturbances in chronic kidney disease. We conclude that adipokines are likely to influence several key survival factors in peritoneal dialysis patients, including systemic inflammation, endothelial health and appetite.

Adipocytokines: leptin--the classical, resistin--the controversical, adiponectin--the promising, and more to come

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 are increasingly shown to affect several aspects in the pathogenesis of obesity-related diseases. The cloning of the ob gene is consistent with this concept and suggests that body fat content in adult rodents is regulated by a negative feedback loop centred in the hypothalamus. In recent years, a number of additional signalling molecules secreted by adipose tissue have been discovered, commonly referred to as 'adipocytokines'. Among these, adiponectin is perhaps the most interesting and promising compound for the clinician since it has profound protective actions in the pathogenesis of diabetes and cardiovascular disease. Adiponectin is low in obese subjects and, in particular, insulin-resistant patients. In contrast, resistin seems to be of greater relevance in relation to the immune stress response than in the regulation of glucose homeostasis. However, inflammatory processes have recently been connected with the development of atherosclerosis. Finally, little is known regarding the clinical relevance of visfatin. Recent research has revealed many functions of adipocytokines extending far beyond metabolism, such as immunity, cancer and bone formation. This report aims to review some of the recent topics of adipocytokine research that may be of particular importance.

[Adipocytokins, obesity and development of type 2 diabetes]

Normal metabolic balance is maintained by a complex homeostatic system involving multiple tissues and organs. Acquired or inherited defects associated to environmental factors in any part of this system can lead to metabolic disorders such as the syndrome X which is presently a frequent syndrome in industrialized countries. It is characterized by a cluster of risk factors of atherosclerosis including insulin resistance, hyperinsulinemia, impaired glucose tolerance or type 2 diabetes, hypertension, dyslipidemia, and coagulation abnormalities. Its pathophysiology is likely to involve insulin resistance at the level of both skeletal muscle and visceral adipose tissue and altered fluxes of metabolic substrates between these tissues that in turn impair liver metabolism. Therapeutic intervention favours at present diet and exercise prescriptions. In addition, if necessary, specific treatment of the metabolic disorders is required. In the treatment of insulin resistance, new promising drugs are likely to be used in the next future. In this regard, adipose tissue, once thought to function primarily as a passive depot for the storage of excess lipid, is now understood to play a much more active role in metabolic regulation, secreting a variety of metabolic hormones and actively functioning to prevent deleterious lipid accumulation in other tissues and to modulate the insulin resistance. Here, we review new advances in our understanding of mechanisms leading to insulin resistance and type 2 diabetes from the perspective of the role and interactions of recently identified adipocyte-specific chemical messengers, the adipocytokines, such as adiponectin, tumor necrosis factor-alpha, interleukin 6, and resistin.

Adipose tissue: a regulator of inflammation

Adipose tissue is a highly active organ. In addition to storing calories as triglycerides, it also secretes a large variety of proteins, including cytokines, chemokines and hormone-like factors, such as leptin, adiponectin and resistin. Intriguingly, many, if not most, of these adipose-derived proteins have dual actions; cytokines have both immunomodulatory functions and act as systemic or auto-/paracrine regulators of metabolism, while proteins such as leptin and adiponectin are regulators of both metabolism and inflammation. The production of pro-atherogenic chemokines by adipose tissue is of particular interest since their local secretion, e.g. by perivascular adipose depots, may provide a novel mechanistic link between obesity and the associated vascular complications.

Resistin-like Molecule β Activates MAPKs, Suppresses Insulin Signaling in Hepatocytes, and Induces Diabetes, Hyperlipidemia, and Fatty Liver in Transgenic Mice on a High Fat Diet

Resistin and resistin-like molecules (RELMs) are a family of proteins reportedly related to insulin resistance and inflammation. Because the serum concentration and intestinal expression level of RELMbeta were elevated in insulin-resistant rodent models, in this study we investigated the effect of RELMbeta on insulin signaling and metabolism using transgenic mice and primary cultured hepatocytes. First, transgenic mice with hepatic RELMbeta overexpression were shown to exhibit significant hyperglycemia, hyperlipidemia, fatty liver, and pancreatic islet enlargement when fed a high fat diet. Hyperinsulinemic glucose clamp showed a decreased glucose infusion rate due to increased hepatic glucose production. In addition, the expression levels of IRS-1 and IRS-2 proteins as well as the degrees of insulin-induced phosphatidylinositol 3-kinase and Akt activations were attenuated in RELMbeta transgenic mice. Similar down-regulations of IRS-1 and IRS-2 proteins were observed in primary cultured hepatocytes chronically treated (for 24 h) with RELMbeta, suggesting the insulin resistance-inducing effect of RELMbeta to be direct. Furthermore, it was shown that RELMbeta acutely and markedly activates ERK and p38, while weakly activating JNK, in primary cultured hepatocytes. This increased basal p38 phosphorylation level was also observed in the livers of RELMbeta transgenic mice. In conclusion, RELMbeta, a gut-derived hormone, impairs insulin signaling probably via the activations of classic MAPKs, and increased expression of RELMbeta may be involved in the pathogenesis of glucose intolerance and hyperlipidemia in some insulin-resistant models. Thus, RELMbeta is a potentially useful marker for assessing insulin resistance and may also be a target for future novel anti-diabetic agents.

Resistin is not associated with insulin sensitivity or the metabolic syndrome in humans

AIMS/HYPOTHESIS

The aim of this study was to further elucidate the relationship between resistin and insulin sensitivity, body fat distribution and the metabolic syndrome in humans.

METHODS

We measured plasma resistin levels in 177 non-diabetic subjects (75 male, 102 female; age 32-75 years). BMI, waist circumference, blood pressure, lipids, glucose, plasminogen-activator inhibitor 1 (PAI-1), adiponectin and leptin levels were also measured. The insulin sensitivity index (S(I)) was quantified using Bergman's minimal model. Intra-abdominal fat (IAF) and subcutaneous fat (SQF) areas were quantified by CT scan. Presence of metabolic syndrome criteria was determined using the National Cholesterol Education Program Adult Treatment Panel III guidelines.

RESULTS

When subjects were divided into categories based on BMI (< or > or =27.5 kg/m(2)) and S(I) (< or > or = 7 x 10(-5) min(-1) [pmol/l](-1)), resistin levels did not differ between the lean, insulin-sensitive (n=53, 5.36+/-0.3 ng/ml), lean, insulin-resistant (n=67, 5.70+/-0.4 ng/ml) and obese, insulin-resistant groups (n=48, 5.94+/-0.4 ng/ml; ANOVA p=0.65). Resistin correlated with age (r=-0.22, p<0.01), BMI (r=0.16, p=0.03) and SQF (r=0.19, p=0.01) but not with S(I) (p=0.31) or IAF (p=0.52). Resistin did not correlate with the number of metabolic syndrome criteria or any of the individual metabolic syndrome criteria. In contrast, adiponectin, PAI-1 and leptin each correlated with IAF, SQF and S(I). Additionally, the number of metabolic syndrome criteria correlated with adiponectin (r=-0.32, p<0.001), leptin (r=0.31, p<0.001) and PAI-1 (r=0.26, p=0.001).

CONCLUSIONS/INTERPRETATION

In contrast to other adipokines, resistin is only weakly associated with body fat and is unlikely to be a major mediator of insulin resistance or the metabolic syndrome in humans.

Resistin in preterm and term newborns: relation to anthropometry, leptin, and insulin

This study aimed to investigate 1) the plasma resistin concentration at birth, 2) the relationship of resistin with leptin and insulin, and 3) the association of resistin with anthropometric indexes in newborn infants. Blood samples for hormonal assay were obtained from preterm and term newborns within the first 2 h of life and before milk feeding or energy intake. Although these infants required blood sampling for clinical reasons, all were proved to be noninfected. Plasma resistin was significantly higher in term than in preterm infants. It was also significantly correlated with serum leptin, and both hormones were significantly associated with gestational age and anthropometric indexes. Infants who were born vaginally were found to have significantly higher plasma resistin levels compared with those who were born by cesarean section. In the multivariate forward stepwise regression models, resistin was found to be significantly associated with the mode of delivery and gestational age or birth weight. The association among resistin, leptin, and anthropometric indexes suggested that both hormones might be gestation related. A high circulating resistin level at term gestation could be advantageous to the infant by promoting hepatic glucose production and preventing hypoglycemia after birth. Infants who were born vaginally had significantly higher plasma resistin levels, suggesting that this hormone might also be associated with stress or inflammation induced by the birth process.

The emerging role of adipocytokines as inflammatory mediators in inflammatory bowel disease

Anorexia, malnutrition, altered body composition and development of mesenteric obesity are well known features of inflammatory bowel disease (IBD). Recent data suggest that dysregulation of protein secretion by white adipose tissue is involved in these manifestations of patients with IBD. Adipocytes are recently recognized as endocrine cells that secrete a variety of bioactive substances known as adipocytokines. There is evidence that adipocytokines are involved in inflammatory and metabolic pathways in human beings. Overexpression of adipocytokines such as leptin, adiponectin and resistin in mesenteric adipose tissue of operated patients with Crohn's disease has recently been reported, suggesting that mesenteric adipocytes in IBD may act as immunoregulating cells. Therefore, it could be suggested that adipocytokines play an important role in the disease pathogenesis. Moreover, modulators of mesenteric adipose function have been suggested as potential therapeutic drugs in IBD. In this review, the importance of white adipose tissue function and adipocytokines, is discussed with respect to IBD.

Evidence for the involvement of resistin in inflammation and cardiovascular disease

The prevalence of obesity continues to increase throughout the world in an analogous way to that of type 2 diabetes mellitus (T2DM). Excess adiposity and accompanying insulin resistance is frequently associated to the development of cardiovascular disease. The circulating hormone resistin, which is produced mainly by adipocytes and appears to be increased in obesity and inflammation, seems to play a role in this association. Some studies indicate that T2DM patients have increased circulating concentrations of resistin, although these results need further confirmation. Increased resistin concentrations have been described in patients with severe inflammatory disease. However, the precise physiological role of resistin in the pathogenesis and perpetuation of inflammation remains unclear. Resistin exerts direct effects to promote the activation of endothelial cells inducing the release of endothelin-1, increasing the expression of adhesion molecules and chemokines, and potentiating the effect of the CD40 ligand. The present review summarizes recent advances in understanding the physiology of resistin and analyzes the involvement of this hormone in inflammation and cardiovascular disease.

Adipose tissue and its relation to inflammation: The role of adipokines

An activated inflammatory response is a common feature of end-stage renal disease (ESRD) patients and predicts outcome. Although various factors related to the dialysis procedure may contribute to inflammation in ESRD, a number of nondialysis-related factors also are of importance. Adipose tissue is a complex organ with functions far beyond the mere storage of energy and secretes a number of proinflammatory adipokines, such as leptin, resistin, tumor necrosis factor-alpha and interleukin-6, as well as one anti-inflammatory adipokine, adiponectin. It has been proposed that adipose tissue may be a significant contributor to increased systemic inflammation in nonrenal patients. In this review, we put forward the hypothesis that a reduction of renal mass will contribute to retention of proinflammatory adipokines, thus generating adipokine imbalance. Such an imbalance may, via effects on the central nervous system and the vasculature, contribute to wasting, atherosclerosis, and insulin resistance--all common features of ESRD.