A cysteine-rich adipose tissue-specific secretory factor inhibits adipocyte differentiation

Adenovirus-mediated high expression of resistin causes dyslipidemia in mice

The adipocyte-derived hormone resistin has been proposed as a possible link between obesity and insulin resistance in murine models. Many recent studies have reported physiological roles for resistin in glucose homeostasis, one of which is enhancement of glucose production from the liver by up-regulating gluconeogenic enzymes such as glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. However, its in vivo roles in lipid metabolism still remain to be clarified. In this study, we investigated the effects of resistin overexpression on insulin action and lipid metabolism in C57BL/6 mice using an adenoviral gene transfer technique. Elevated plasma resistin levels in mice treated with the resistin adenovirus (AdmRes) were confirmed by Western blotting analysis and RIAs. Fasting plasma glucose levels did not differ between AdmRes-treated mice and controls, but the basal insulin concentration was significantly elevated in AdmRes-treated mice. In AdmRes-treated mice, the glucose-lowering effect of insulin was impaired, as evaluated by insulin tolerance tests. Furthermore, total cholesterol and triglyceride concentrations were significantly higher, whereas the high-density lipoprotein cholesterol level was significantly lower. Lipoprotein analysis revealed that low-density lipoprotein was markedly increased in AdmRes-treated mice, compared with controls. In addition, in vivo Triton WR-1339 studies showed evidence of enhanced very low-density lipoprotein production in AdmRes-treated mice. The expressions of genes involved in lipoprotein metabolism, such as low-density lipoprotein receptor and apolipoprotein AI in the liver, were decreased. These results suggest that resistin overexpression induces dyslipidemia in mice, which is commonly seen in the insulin-resistant state, partially through enhanced secretion of lipoproteins.

Resistin mRNA levels are downregulated by estrogen in vivo and in vitro

Resistin, a hormone secreted by adipocytes, is suggested to be an important link between obesity and diabetes. The aim of this study was to evaluate the regulatory effect of estrogen on adipocyte resistin gene expression in ovariectomized (OVX) rats and in isolated rat adipocytes in vitro. Subcutaneous injection of estradiol benzoate reduced resistin mRNA levels in adipocytes isolated from the inguinal, parametrial, perirenal, retroperitoneal, or periovarian fat deposits of OVX rats, while an in vitro study showed that estradiol treatment decreased resistin mRNA levels in cultured rat periovarian fat adipocytes. Results of Western blotting analysis also showed that estrogen decreased adipose resistin contents in vivo and in vitro. These data suggest that estrogen is a pivotal negative regulator of resistin gene expression.

Role of glucocorticoids in the physiopathology of excessive fat deposition and insulin resistance

Glucocorticoids are important hormones in the regulation of metabolic homeostasis. We infused normal rats with dexamethasone given intracerebroventricularly (i.c.v.) for 3 days. This resulted in hyperphagia, hyperinsulinemia, and marked insulin resistance. Similar metabolic defects were observed following i.c.v. infusion of neuropeptide Y (NPY) in normal rats. As central dexamethasone infusion enhanced NPY content in the arcuate nucleus, it suggested that its metabolic effects are mediated by NPY. Moreover, due to the lack of effects observed in vagotomized animals, activation of the parasympathetic nervous system by central dexamethasone infusion is proposed. Glucocorticoid action is known to involve prereceptor metabolism by enzymes such as 11beta-HSD-1 that converts inactive into active glucocorticoids. Mice overexpressing 11beta-HSD-1 in adipose tissue were shown to be obese and insulin resistant. We recently observed that adipose tissue 11beta-HSD-1 mRNA expression is increased at the onset of high-fat diet-induced obesity and positively correlated with the degree of hyperglycemia. In human obesity, increased adipose tissue 11beta-HSD-1 expression and activity were also reported. Resistin is a new adipose tissue-secreted hormone shown to play a role in glucose homeostasis by increasing hepatic glucose production and inhibiting muscle and adipose tissue glucose utilization. We observed increased adipose tissue resistin expression in the early phase of high-fat diet-induced obesity as well as decreased resistin expression in response to leptin. A positive correlation between glycemia and adipose tissue resistin expression further suggested a role of this hormone in the development of insulin resistance. The melanocortin system is another important player in the regulation of energy balance. Peripheral administration of a melanocortin agonist decreased food intake and body weight and favored lipid oxidation, effects that were more marked in obese than in lean rats. It is proposed that both resistin and melanocortin agonists may influence adipose tissue 11beta-HSD-1, thereby decreasing or enhancing glucose metabolism.

An inflammatory cascade leading to hyperresistinemia in humans

BACKGROUND

Obesity, the most common cause of insulin resistance, is increasingly recognized as a low-grade inflammatory state. Adipocyte-derived resistin is a circulating protein implicated in insulin resistance in rodents, but the role of human resistin is uncertain because it is produced largely by macrophages.

METHODS AND FINDINGS

The effect of endotoxin and cytokines on resistin gene and protein expression was studied in human primary blood monocytes differentiated into macrophages and in healthy human participants. Inflammatory endotoxin induced resistin in primary human macrophages via a cascade involving the secretion of inflammatory cytokines that circulate at increased levels in individuals with obesity. Induction of resistin was attenuated by drugs with dual insulin-sensitizing and anti-inflammatory properties that converge on NF-kappaB. In human study participants, experimental endotoxemia, which produces an insulin-resistant state, causes a dramatic rise in circulating resistin levels. Moreover, in patients with type 2 diabetes, serum resistin levels are correlated with levels of soluble tumor necrosis factor alpha receptor, an inflammatory marker linked to obesity, insulin resistance, and atherosclerosis.

CONCLUSIONS

Inflammation is a hyperresistinemic state in humans, and cytokine induction of resistin may contribute to insulin resistance in endotoxemia, obesity, and other inflammatory states.

Variation in the resistin gene is associated with obesity and insulin-related phenotypes in Finnish subjects

AIMS/HYPOTHESIS

Resistin is a peptide hormone produced by adipocytes that is present at high levels in sera of obese mice and may be involved in glucose homeostasis through regulation of insulin sensitivity. Several studies in humans have found associations between polymorphisms in the resistin gene and obesity, insulin sensitivity and blood pressure. An association between variation in the resistin gene and type 2 diabetes has been reported in some, but not all studies. The aim of this study was to analyse variants of the resistin gene for association with type 2 diabetes and related traits in a Finnish sample.

METHODS

In 781 cases with type 2 diabetes, 187 spouse controls and 222 elderly controls of Finnish origin, we genotyped four previously identified non-coding single-nucleotide polymorphisms (SNPs): -420C>G from the promoter region, +156C>T and +298G>A from intron 2, and +1084G>A from the 3' untranslated region. We then tested whether these SNPs were associated with type 2 diabetes and related traits.

RESULTS

The SNPs were not significantly associated with type 2 diabetes. However, SNPs -420C>G, +156C>T and +298G>A and the common haplotype for these three markers were associated with increased values of weight-related traits and diastolic blood pressure in cases, lower weight in elderly control subjects, and lower insulin sensitivity and greater acute insulin response in spouses. Furthermore, the +1084G allele was associated with lower HDL cholesterol in both cases and controls, higher systolic blood pressure and waist circumference in cases, and greater acute insulin response in spouse controls.

CONCLUSIONS/INTERPRETATION

Our results add to growing evidence that resistin is associated with variation in weight, fat distribution and insulin resistance.

Insulin Resistance and Obesity: Resistin, A Hormone Secreted by Adipose Tissue

A newly discovered hormone named adipocyte-secreted factor, or resistin, is secreted by adipocytes in mice. Expression of resistin is low during food deprivation and in diabetes, and increased greatly during refeeding and insulin treatment. It is found in serum in mice and humans, and is greatly increased in obesity. Resistin inhibits adipocyte differentiation and may function as a feedback regulator of adipogenesis. Administration of resistin to mice resulted in increased glucose production and blood glucose levels. Therefore, resistin also functions as a regulator of glucose homeostasis and a physiologic antagonist to hepatic insulin action.

Plasma resistin concentration, hepatic fat content, and hepatic and peripheral insulin resistance in pioglitazone-treated type II diabetic patients

OBJECTIVES

To study the effect of pioglitazone (PIO) on plasma resistin concentration, endogenous glucose production (EGP), and hepatic fat content (HFC) in patients with type II diabetes (T2DM).

SUBJECTS

A total of 13 T2DM patients (age=51+/-2 y, BMI=29.7+/-1.1 kg/m(2), HbA(1c)=8.0+/-0.5%).

METHODS

HFC (magnetic resonance spectroscopy) and basal plasma resistin concentration were quantitated before and after PIO treatment (45 mg/day) for 16 weeks. Subjects received a 3 h euglycemic insulin (100 mU/m(2)/min) clamp with 3-[(3)H] glucose to determine rates of EGP and tissue glucose disappearance (Rd) before and after PIO.

RESULTS

PIO reduced fasting plasma glucose (10.3+/-0.7 to 7.6+/-0.6 mmol/l, P<0.001) and HbA(1c) (8.0+/-0.4 to 6.8+/-0.3%, P<0.001) despite increased body weight (83.2+/-3.4 to 86.3+/-3.4 kg, P<0.001). PIO improved Rd (4.9+/-0.4 to 6.6+/-0.5 mg/kg/min, P<0.005) and reduced EGP (0.22+/-0.04 to 0.06+/-0.02 mg/kg/min, P<0.01) during the insulin clamp. Following PIO, HFC decreased from 21.1+/-3.5 to 11.2+/-2.1% (P<0.005), and plasma resistin decreased from 5.3+/-0.6 to 3.5+/-0.3 ng/ml (P<0.01). Plasma resistin concentration correlated positively with HFC before (r=0.58, P<0.05) and after (r=0.55, P<0.05) PIO treatment. Taken collectively, plasma resistin concentration, before and after PIO treatment, correlated positively with hepatic fat content (r=0.66, P<0.001) and EGP during the insulin clamp (r=0.41, P<0.05). However, the plasma resistin concentration did not correlate with whole body glucose disposal (Rd) during the insulin clamp either before (r=-0.18, P=NS) or after (r=-0.13, P=NS) PIO treatment.

CONCLUSIONS

PIO treatment in T2DM causes a significant decrease in plasma resistin concentration. The decrease in plasma resistin is positively correlated with the decrease in hepatic fat content and improvement in hepatic insulin sensitivity.

Effect of dieting on plasma leptin, soluble leptin receptor, adiponectin and resistin levels in healthy volunteers

OBJECTIVE

Recent findings demonstrating important effects of the adipokines on metabolism, energy homeostasis and body weight regulation have prompted research on the possible role of negative energy balance in altering adipocytokine regulation. The goal of this study was to evaluate the effects of a hypocaloric diet in healthy normal-weight volunteers. An additional goal was to help clarify the contribution of restricted caloric intake to altered plasma adipokine levels in the eating disorders anorexia nervosa and bulimia nervosa.

DESIGN

Participants were studied before and after a 4-week reduced-calorie diet (1000-12000 kcal/day). patients Subjects included 15 healthy, normal-weight women (age 22 +/- 3 years).

MEASUREMENTS

Plasma concentrations of leptin, soluble leptin receptor protein (sOB-R), adiponectin, resistin, thyroid hormones and beta-hydroxybutyrate were determined following overnight fast before and after the 4-week reduced-calorie diet.

RESULTS

Subjects lost a mean of 3.4 +/- 2.1 kg in response to the reduced-calorie diet. The weight loss phase was associated with a 60.3% decrease in plasma leptin levels (P < 0.001), a 43.5% increase in sOB-R levels (P < 0.002) and a 16.2% decrease in plasma adiponectin levels (P < 0.04). There was no significant change in plasma resistin levels.

CONCLUSIONS

These results demonstrate that a modest decrease in energy intake sustained over several weeks may play an important role in altering levels of plasma leptin and sOB-R. The findings also provide preliminary evidence that, in contrast to previous results in obese subjects, caloric restriction with accompanying weight loss in healthy, normal-weight volunteers may lead to decreased circulating adiponectin levels. Additional studies will be needed to clarify the contribution of altered energy intake to abnormalities in cytokine levels in the eating disorders.

Leptin treatment markedly increased plasma adiponectin but barely decreased plasma resistin of ob/ob mice

Adiponectin (ApN) and leptin are two adipocytokines that control fuel homeostasis, body weight, and insulin sensitivity. Their interplay is still poorly studied. These hormones are either undetectable or decreased in obese, diabetic ob/ob mice. We examined the effects of leptin treatment on ApN gene expression, protein production, secretion, and circulating levels of ob/ob mice. We also briefly tackled the influence of this treatment on resistin, another adipocytokine involved in obesity-related insulin resistance. Leptin-treated (T) obese mice (continuous sc infusion for 6 days) were compared with untreated lean (L), untreated obese (O), and untreated pair-fed obese (PF) mice. Blood was collected throughout the study. At day 3 or day 6, fat pads were either directly analyzed (mRNA, ApN content) or cultured for up to 24 h (ApN secretion). The direct effect of leptin was also studied in 3T3-F442A adipocytes. Compared with L mice, ApN content of visceral or subcutaneous fat and ApN secretion by adipose explants were blunted in obese mice. Accordingly, plasma ApN levels of O mice were decreased by 50%. Leptin treatment of ob/ob mice increased ApN mRNAs, ApN content, and secretion from the visceral depot by 50-80%. Leptin also directly stimulated ApN mRNAs and secretion from 3T3-F442A adipocytes. After 6 days of treatment, plasma ApN of ob/ob mice increased 2.5-fold, a rise that did not occur in PF mice. Plasma resistin of T mice was barely decreased. Leptin treatment, but not mere calorie restriction, corrects plasma ApN in obese mice by restoring adipose tissue ApN concentrations and secretion, at least in part, via a direct stimulation of ApN gene expression. Such a treatment only minimally affects circulating resistin. ApN restoration could, in concert with leptin, contribute to the metabolic effects classically observed during leptin administration.

Desnutrin, an adipocyte gene encoding a novel patatin domain-containing protein, is induced by fasting and glucocorticoids: ectopic expression of desnutrin increases triglyceride hydrolysis

We have used rat cDNA microarrays to identify adipocyte-specific genes that could play an important role in adipocyte differentiation or function. Here, we report the cloning and identification of a 2.0-kb mRNA coding for a putative protein that we have designated as desnutrin. The novel gene is expressed predominantly in adipose tissue, and its expression is induced early during 3T3-L1 adipocyte differentiation. Desnutrin mRNA levels were regulated by the nutritional status of animals, being transiently induced during fasting. In vitro desnutrin gene expression was up-regulated by dexamethasone in a dose-dependent manner but not by cAMP, suggesting that glucocorticoids could mediate the increase in desnutrin mRNA levels observed during fasting. Desnutrin mRNA codes for a 486-amino acid putative protein containing a patatin-like domain, characteristic of many plant acyl hydrolases belonging to the patatin family. Confocal microscopy of enhanced green fluorescent protein-tagged desnutrin protein-transfected cells showed that the fusion protein localized in the cytoplasm. Moreover, cells overexpressing desnutrin by transfection showed an increase in triglyceride hydrolysis. Interestingly, we also found that the desnutrin gene expression level was lower in ob/ob and db/db obese mouse models. Overall, our data suggest that the newly identified desnutrin gene codes for an adipocyte protein that may function as a lipase and play a role in the adaptive response to a low energy state, such as fasting, by providing fatty acids to other tissues for oxidation. In addition, decreased expression of desnutrin in obesity models suggests its possible contribution to the pathophysiology of obesity.

Differences in mRNA expression of adipocyte-derived factors in response to fasting, refeeding and leptin

The present study examines whether and to what extent the profiles of adipose-derived factors are altered in epididymal and subcutaneous adipose tissues of long-term fasted/refed and of fasted rats treated by recombinant leptin. Fasting was characterized by three successive metabolic phases. Minor differences in the time-course and magnitude of response were detected between the two adipose sites. Leptin, adiponectin, resistin, adiponutrin, and insulin-like growth factor-1 (IGF-1) gene expressions differentially decreased according to the fasting duration. mRNA levels reached a minimum in late fasting for these secreted factors, being decreased by 60-90% for adiponectin, resistin, and IGF-1, 95-98% for leptin and by 100% for adiponutrin. Refeeding partially or totally restored their mRNA expression in epididymal adipose. Expression levels of apolipoprotein E (ApoE), angiotensinogen (AGT), adipsin and macrophage migration inhibitory factor (MIF) were either unchanged or slightly affected. In leptin-treated rats, leptin mRNA concentrations were significantly decreased in phase 2 of fasting (by 85%) from levels in control phosphate-buffered saline (PBS)-treated rats in both tissues. Leptin treatment also decreased resistin mRNA levels (by 78% in P2L and 63% in P3L relative to control groups) in subcutaneous adipose. These data suggest that adiponectin, resistin, adiponutrin, and IGF-1 could be involved in overall energy homeostasis during prolonged fasting, as leptin is. The mechanisms that underlie the expressions of these adipose-secreted factors remain to be determined.

Abnormal glucose homeostasis due to chronic hyperresistinemia

Resistin is an adipocyte-secreted protein that circulates at increased levels in obesity. Acute administration of resistin impairs glucose tolerance, but the effects of chronic hyperresistinemia have not been established. Here we describe the generation and characterization of transgenic mice that have high circulating levels of resistin in the setting of normal weight. Fasted blood glucose was higher in resistin-transgenic mice than in their nontransgenic littermates, and glucose tolerance was impaired in the hyperresistinemic mice. Metabolic studies in the setting of a hyperinsulinemic-euglycemic clamp protocol revealed that chronically hyperresistinemic mice have elevated glucose production. This increase in glucose production may be partly explained by increased expression of hepatic phosphoenolpyruvate carboxykinase. Thus, chronic hyperresistinemia impairs normal glucose metabolism.

Two variants in the resistin gene and the response to long-term overfeeding

OBJECTIVE AND SUBJECTS

To investigate the role of resistin gene variants on the adiposity and metabolic changes observed in response to a 100-day overfeeding protocol conducted with 12 pairs of monozygotic twins.

MEASUREMENTS

Body-fat measurements included hydrodensitometry and abdominal fat from computed tomography. Plasma glucose and insulin during fasting and in response to an oral glucose tolerance test (OGTT) were assayed. A 4.2 MJ test meal was consumed, after which calorimetric measurements were performed for 240 min.

RESULTS

Respiratory quotient (RQ) decreased (P=0.001) more in AA/AG than in GG subjects of the IVS2+181G>A polymorphism after the caloric surplus and the significance persisted when correction for multiple testing was performed. Total abdominal (P=0.027) and visceral (P=0.004) fat increased more in TC than in TT subjects of the IVS2+39C>T polymorphism. In response to overfeeding, glucose area under the curve during the OGTT showed a slight decrease (P=0.031) in the TC while it increased in TT subjects. OGTT insulin area tended to increase less (P=0.055) in TC than in TT subjects. After overfeeding, fasting insulin was lower in TC than in TT subjects (P=0.010). In addition, TC subjects experienced more decrease in RQ than TT subjects (P=0.034).

CONCLUSION

The IVS2+181G>A variant was associated with the changes in RQ in response to overfeeding. The IVS2+39C>T polymorphism was associated with overfeeding-induced changes in abdominal visceral fat, OGTT glucose area and RQ. The results suggest that sequence variation in the resistin gene is involved in the adaptation to chronic positive energy balance.

Adipose tissue and adipokines: for better or worse

It is now recognized that the white adipose tIssue (WAT) produces a variety of bioactive peptIdes, collectively termed "adipokines". Alteration of WAT mass in obesity or lipoatrophy, affects the production of most adipose secreted factors. Since both conditions are associated with multiple metabolic disorders and increased risk of cardiovascular diseases, the Idea has emerged that WAT could be instrumental in these complications, by virtue of its secreted factors. Several adipokines are increased in the obese state and have been implicated in hypertension (angiotensinogen), impaired fibrinolysis (PAI-1) and insulin resistance (ASP, TNFalpha, IL-6, resistin). Conversely, leptin and adiponectin both exert an insulin-sensitizing effect, at least in part, by favoring tIssue fatty-acId oxIdation through activation of AMP-activated kinase. In obesity, insulin resistance has been linked to leptin resistance and decreased plasma adiponectin. In lipoatrophic mice, where leptin and adiponectin circulating levels are low, administration of the two adipokines synergistically reverses insulin resistance. Leptin and adiponectin also have distinct properties: leptin, as a long-term integrative signal of energy store and adiponectin, as a potent anti-atherogenic agent. The thiazolIdinedione anti-diabetic drugs increase endogenous adiponectin production in rodents and humans, supporting the Idea that the development of new drugs targeting adipokines might represent a promising therapeutic approach to protect obese patients from insulin resistance and atherosclerosis.

TNFalpha release by the nonfat cells of human adipose tissue

OBJECTIVE

The primary aim was to investigate the relative importance of the adipocytes vs the nonfat cells present in human adipose tissue with respect to release of immunoreactive tumor necrosis factor-alpha (TNFalpha). The second aim was to examine the correlation between body mass index (BMI) and the subsequent release of adiponectin and TNFalpha by explants of human subcutaneous and visceral adipose tissue incubated in primary culture for 48 h.

RESULTS

We found that the maximal release of TNFalpha was seen during the first 4 h of a 48-h incubation by explants of human adipose tissue in primary culture. Over 95% of the TNFalpha released to the medium by human adipose tissue explants over a 4-h incubation came from the nonfat cells present in the adipose tissue. The release of TNFalpha by the nonfat cells released during collagenase digestion was slightly higher than that by the cells present in the adipose tissue matrix after collagenase digestion. TNFalpha release by the combined matrix and isolated nonfat cells was greater than that by explants of tissue indicating some upregulation induced by collagenase digestion. Immunoreactive TNFalpha disappeared from the medium with a half-time of approximately 10 h. There was a positive correlation coefficient of 0.79 between TNFalpha release by tissue explants and the BMI of the fat donors as well as a correlation of 0.52 between BMI and release by adipocytes. TNFalpha release negatively correlated [-0.60] with adiponectin release by adipose tissue. The release of TNFalpha was far less than that of adiponectin or IL-6, and less than that of plasminogen activator inhibitor-1, hepatocyte growth factor, or leptin over a 4-h incubation of human adipose tissue explants. TNFalpha release over 4 h was enhanced by lipopolysaccharide and inhibited by a cyclooxygenase-2 inhibitor.

CONCLUSION

The release of TNFalpha by adipose tissue of obese humans is primarily due to the nonfat cells present in adipose tissue. TNFalpha is a short-lived adipokine whose release by human adipose tissue in primary culture correlates with the BMI of the fat donors.

Novel expression of resistin in rat testis: functional role and regulation by nutritional status and hormonal factors

Resistin, a recently cloned adipose-secreted factor, is primarily involved in the modulation of insulin sensitivity and adipocyte differentiation. However, additional metabolic or endocrine functions of this molecule remain largely unexplored. In this study, a series of experiments were undertaken to explore the potential expression, regulation and functional role of this novel adipocytokine in rat testis. Resistin gene expression was demonstrated in rat testis throughout postnatal development, with maximum mRNA levels in adult specimens. At this age, resistin peptide was immunodetected in interstitial Leydig cells and Sertoli cells within seminiferous tubules. Testicular expression of resistin was under hormonal regulation of pituitary gonadotropins and showed stage-specificity, with peak expression values at stages II-VI of the seminiferous epithelial cycle. In addition, testicular resistin mRNA was down-regulated by the selective agonist of PPARγ, rosiglitazone, in vivo and in vitro. Similarly, fasting and central administration of the adipocyte-derived factor, leptin, evoked a significant reduction in testicular resistin mRNA levels, whereas they remained unaltered in a model of diet-induced obesity. From a functional standpoint, resistin, in a dose-dependent manner, significantly increased both basal and choriogonadotropin-stimulated testosterone secretion in vitro. Overall, our present results provide the first evidence for the expression, regulation and functional role of resistin in rat testis. These data underscore a reproductive facet of this recently cloned molecule, which may operate as a novel endocrine integrator linking energy homeostasis and reproduction.

Increased expression of resistin and tumour necrosis factor-alpha in pig adipose tissue as well as effect of feeding treatment on resistin and cAMP pathway

AIM

Resistin, a novel hormone, is expressed in mouse, rat and human adipose tissue. Its resistance to insulin is controversial. We hypothesized that resistin has the similar roles like those of tumour necrosis factor-alpha (TNF-alpha), a putative mediator of insulin resistance, and they are in common involved in regulating lipid metabolism in vivo.

METHODS

We demonstrated TNF-alpha and resistin expression in different growth-stage adipose tissue of genetically lean and obese pigs using semiquantitative reverse transcriptase-polymerase chain reaction and Western blotting analysis. Moreover, we analysed the variations of resistin and some parameters associated with lipolysis pathway after body-weight reduction in feeding-restricted obese pigs. In addition, we localized resistin expression in the cell types from pig adipose tissue.

RESULTS

Overall, the expression of TNF-alpha and resistin in obese pigs was statistically higher than their lean counterparts, although this difference in resistin mRNA expression tailed off with age. In individual pigs, their levels increased progressively with age and degree of obesity. After feeding restriction, resistin expression was gradually decreased. Simultaneously, feeding restriction also resulted in a significant decrease of serum triglyceride and plasma cAMP, free fatty acids and insulin as well as fat adenyl cyclase and cAMP. In addition, in isolated pig adipocytes, our data showed that resistin was localized not only to adipocytes but also to other unidentified stromal vascular cells.

CONCLUSIONS

TNF-alpha and resistin are increased in obesity. Weight loss results in the decrease of resistin and lipolysis-associated parameters, suggesting the possible association between resistin and lipolysis pathway. Cell localization of resistin expression implies the possibility of other biological actions of resistin.

Regulation of resistin expression and circulating levels in obesity, diabetes, and fasting

Resistin was originally reported as an adipose tissue-specific hormone that provided a link between obesity and diabetes. Resistin protein level was elevated in obese mice and decreased by insulin-sensitizing thiazolidinediones. Immunoneutralization of resistin improved insulin sensitivity in diet-induced obese mice, while the administration of exogenous resistin induced insulin resistance. More recently, we have shown that ablation of the resistin gene in mice decreased fasting glucose through impairment of gluconeogenesis, while resistin treatment in these knockout mice increased hepatic glucose production. However, the link between resistin and glucose homeostasis has been questioned by studies demonstrating reduced, rather than increased, resistin mRNA expression in obese and diabetic mice. To better understand the regulation of resistin, we developed a sensitive and specific RIA resistin that could accurately measure serum resistin levels in several mouse models. We show that while resistin mRNA is indeed suppressed in obese mice, the circulating resistin level is significantly elevated and positively correlated with insulin, glucose, and lipids. Both resistin mRNA expression and protein levels in Lep(ob/ob) mice are suppressed by leptin treatment in parallel with reductions in glucose and insulin. In wild-type mice, serum resistin increases after nocturnal feeding, concordant with rising levels of insulin. Resistin mRNA and protein levels decline in parallel with glucose and insulin during fasting and are restored after refeeding. We performed clamp studies to determine whether resistin is causally related to insulin and glucose. Adipose resistin expression and serum resistin increased in response to hyperinsulinemia and further in response to hyperglycemia. Taken together, these findings suggest that the nutritional regulation of resistin and changes in resistin gene expression and circulating levels in obesity are mediated, at least in part, through insulin and glucose.

A resistin gene polymorphism is associated with body mass index in women

The potential association of resistin (RETN) gene variability with obesity-related phenotypes was investigated in 585 non-diabetic individuals of European descent. The polymorphism studied (-420 C>G) is located in the RETN gene 5'-flanking region. A significant association between the polymorphism and body mass index and waist circumference was observed in the women subsample (n = 356), where the G allele was somewhat less frequent in the overweight/obese group than in normal-weight individuals (0.25 vs. 0.32; p = 0.040; OR=0.70 [0.50-0.98]). Female carriers of the G-allele presented a lower mean BMI than C/C homozygotes (25.5 vs. 26.8 kg/m(2); p = 0.010). Furthermore, when women were stratified by menopausal status, the association was restricted to premenopausal women (C/C homozygotes, mean BMI = 26.3 kg/m2; G-carriers, 24.4 kg/m2; p = 0.014). Our findings suggest that RETN gene variation has gender-specific effects on BMI and warrants further investigation of its implications for the development of obesity.

Disulfide-dependent multimeric assembly of resistin family hormones

Resistin, founding member of the resistin-like molecule (RELM) hormone family, is secreted selectively from adipocytes and induces liver-specific antagonism of insulin action, thus providing a potential molecular link between obesity and diabetes. Crystal structures of resistin and RELMbeta reveal an unusual multimeric structure. Each protomer comprises a carboxy-terminal disulfide-rich beta-sandwich "head" domain and an amino-terminal alpha-helical "tail" segment. The alpha-helical segments associate to form three-stranded coiled coils, and surface-exposed interchain disulfide linkages mediate the formation of tail-to-tail hexamers. Analysis of serum samples shows that resistin circulates in two distinct assembly states, likely corresponding to hexamers and trimers. Infusion of a resistin mutant, lacking the intertrimer disulfide bonds, in pancreatic-insulin clamp studies reveals substantially more potent effects on hepatic insulin sensitivity than those observed with wild-type resistin. This result suggests that processing of the intertrimer disulfide bonds may reflect an obligatory step toward activation.

The direct peroxisome proliferator-activated receptor target fasting-induced adipose factor (FIAF/PGAR/ANGPTL4) is present in blood plasma as a truncated protein that is increased by fenofibrate treatment

The fasting-induced adipose factor (FIAF, ANGPTL4, PGAR, HFARP) was previously identified as a novel adipocytokine that was up-regulated by fasting, by peroxisome proliferator-activated receptor agonists, and by hypoxia. To further characterize FIAF, we studied regulation of FIAF mRNA and protein in liver and adipose cell lines as well as in human and mouse plasma. Expression of FIAF mRNA was up-regulated by peroxisome proliferator-activated receptor alpha (PPARalpha) and PPARbeta/delta agonists in rat and human hepatoma cell lines and by PPARgamma and PPARbeta/delta agonists in mouse and human adipocytes. Transactivation, chromatin immunoprecipitation, and gel shift experiments identified a functional PPAR response element within intron 3 of the FIAF gene. At the protein level, in human and mouse blood plasma, FIAF was found to be present both as the native protein and in a truncated form. Differentiation of mouse 3T3-L1 adipocytes was associated with the production of truncated FIAF, whereas in human white adipose tissue and SGBS adipocytes, only native FIAF could be detected. Interestingly, truncated FIAF was produced by human liver. Treatment with fenofibrate, a potent PPARalpha agonist, markedly increased plasma levels of truncated FIAF, but not native FIAF, in humans. Levels of both truncated and native FIAF showed marked interindividual variation but were not associated with body mass index and were not influenced by prolonged semistarvation. Together, these data suggest that FIAF, similar to other adipocytokines such as adiponectin, may partially exert its function via a truncated form.

Resistin expression in different adipose tissue depots during rat development

Resistin is a hormonal factor synthesised by adipocytes that was first thought to be related with the resistance to insulin in obesity, but whose function is not yet completely established. Here we have studied the ontogenic pattern of resistin mRNA expression in different white adipose tissue depots (WAT)--epididymal, inguinal, mesenteric and retroperitoneal--and in brown adipose tissue (BAT), as well as the circulating resistin levels, in rats of different ages (from the suckling period to one year of age). Resistin mRNA was determined by Northern blotting, and serum levels by enzyme immunoassay. In WAT, resistin expression remains almost constant with age, except in early development, where there is a peak of expression in the epididymal and retroperitoneal depots, and a decrease in the inguinal one, while the expression remains constant for the mesenteric depot. Moreover, there is a site-specific difference regarding resistin expression: all the depots express characteristic levels of mRNA, especially at the age of 2 months, the moment when resistin mRNA levels are significantly higher in the epididymal and the retroperitoneal than in the inguinal and mesenteric WAT and than in the BAT. The transient increased resistin expression in the epididymal and the retroperitoneal WAT at a period of time in which there is a change in diet (from milk to chow) suggests a common nutritional regulation of the resistin gene. Circulating resistin levels increase with age probably reflecting the increase in the body fat content.

Changes in glycemia by leptin administration or high- fat feeding in rodent models of obesity/type 2 diabetes suggest a link between resistin expression and control of glucose homeostasis

Resistin is an adipose-derived hormone that has been proposed as a link among obesity, insulin resistance, and diabetes. In agreement with a role of resistin in insulin resistance, the administration of recombinant resistin led to glucose intolerance in mice and impaired insulin action in rat liver. However, the regulation of resistin expression by physiological conditions, hormones, or agents known to modulate insulin sensitivity does not always support the association between resistin and obesity-induced insulin resistance. In the present study we investigated the effects of leptin administration on adipose resistin expression in insulin-resistant and obese ob/ob mice. We show that the expression of resistin mRNA and protein in adipose tissue is lower in ob/ob than in wild-type control mice, in agreement with the reduced adipocyte resistin mRNA level reported in several models of obesity. Leptin administration in ob/ob mice resulted in improvement of insulin sensitivity concomitant with a decrease in resistin gene expression. The lack of effect of leptin on resistin in db/db mice indicated that the leptin inhibitory action on resistin expression requires the long leptin receptor isoform. In addition, we demonstrated that the effect of leptin on resistin expression was centrally mediated. High-fat feeding in C57BL/6J wild-type mice, which is known to induce the development of obesity and insulin resistance, produced an increase in resistin expression. Interestingly, in both ob/ob and high fat-fed mice we obtained a striking positive correlation between glycemia and resistin gene expression. In conclusion, our results demonstrate that leptin decreases resistin expression and suggest that resistin may influence glucose homeostasis.

High serum resistin is associated with an increase in adiposity but not a worsening of insulin resistance in Pima Indians

Resistin is an adipokine with putative prodiabetogenic properties. Like other hormones secreted by adipose tissue, resistin is being investigated as a possible etiologic link between excessive adiposity and insulin resistance. Although there is growing evidence that circulating levels of this adipokine are proportional to the degree of adiposity, an effect on insulin resistance in humans remains unproven. To evaluate the relations among resistin, obesity, and insulin resistance, we measured fasting serum resistin levels in 113 nondiabetic (75-g oral glucose tolerance test) Pima Indians (ages 29 +/- 7 years, body fat 31 +/- 8%, resistin 3.7 +/- 1.1 ng/ml [means +/- SD]), who were characterized for body composition (assessed by hydrodensitometry or dual-energy X-ray absorptiometry), whole-body insulin sensitivity (M; assessed by hyperinsulinemic clamp), basal hepatic glucose output (BHGO) and hepatic glucose output during low-dosage insulin infusion of a hyperinsulinemic clamp (HGO; a measure of hepatic insulin resistance), and acute insulin secretory response (AIR; assessed by 25-g intravenous glucose tolerance test). Follow-up measurements of M, BHGO, HGO, and AIR were available for 34 subjects who had normal glucose tolerance at baseline and remained nondiabetic at follow-up. The average time to follow-up was 4.5 +/- 2.7 years. In cross-sectional analyses, serum resistin levels were positively associated with percent body fat (r = 0.37, P = 0.0001) and 2-h glucose (r = 0.19, P = 0.04), respectively. Serum resistin levels were not associated with fasting glucose and insulin levels, M, BHGO, HGO, or AIR (r = 0.17, 0.12, -0.13, -0.06, -0.03, and -0.04, respectively; all P > 0.05). After adjusting for percent body fat, there was no association between serum resistin levels and 2-h glucose (r = 0.06, P = 0.6). In prospective analyses, high serum resistin levels at baseline were not associated with a decline in M (r = -0.1, P > 0.5). Resistin levels were, however, associated with increases in percent body fat, fasting plasma insulin, and HGO (r = 0.34, 0.36, and 0.37; all P < 0.05) after adjusting for sex, age, and time to follow-up. After additional adjustment for the change in percent body fat, there was no association between baseline serum resistin levels and changes in plasma insulin or HGO (r = 0.26 and 0.23; both P > 0.1). We conclude that in Pima Indians, like other human populations, circulating resistin levels are proportional to the degree of adiposity, but not the degree of insulin resistance. We unexpectedly found that high serum resistin levels do predict future increases in percent body fat. Our data suggest that resistin promotes obesity but not obesity-associated insulin resistance in humans.

Dominant inhibitory adipocyte-specific secretory factor (ADSF)/resistin enhances adipogenesis and improves insulin sensitivity

Adipocyte-specific secretory factor (ADSF)/resistin is a small cysteine-rich protein secreted from adipose tissue that belongs to a gene family found in inflammatory zone (FIZZ) or found in resistin-like molecule (RELM). ADSF has been implicated in modulating adipogenesis and insulin resistance. To examine the long-term function of ADSF in adipogenesis and glucose homeostasis, we constructed an expression vector for a dominant inhibitory form of ADSF by fusing it to the human IgGgamma constant region (hFc). ADSF-hFc not only homodimerizes but heterooligomerizes with ADSF/resistin and prevents ADSF/resistin inhibition of adipocyte differentiation of 3T3-L1 cells in a dominant negative manner. Transgenic mice overexpressing ADSF-hFc in adipose tissue show increased adiposity with elevated expression of adipocyte markers as well as enlarged adipocyte size. This finding clearly demonstrates in vivo the inhibitory role of ADSF in adipogenesis. ADSF-hFc transgenic mice with impaired ADSF function exhibit improved glucose tolerance and insulin sensitivity either on chow or high-fat diets. Because of the enhanced adipocyte differentiation, the ADSF-hFc transgenic mice show increased expression of leptin and adiponectin in adipose tissue. The elevated circulating levels for these adipocyte-derived hormones with decreased plasma triglyceride and free fatty acid levels may account for the improved glucose and insulin tolerance in these transgenic mice.

Dehydroepiandrosterone up-regulates resistin gene expression in white adipose tissue

Dehydroepiandrosterone (DHEA), the most abundant steroid hormone in human blood, is considered to be one of fat-reducing hormones. However, the molecular mechanisms underlying DHEA mode of action in obesity has not been fully clarified. The pivotal role in the maintenance of cellular lipid and energy balance is played by peroxisome proliferator-activated receptor alpha (PPARalpha) which acts as transcriptional activator of numerous genes encoding enzymes involved in fatty acid catabolism. Lately published papers suggest that resistin, a low molecular-weight protein produced by adipose tissue, may act as an inhibitor of adipocyte differentiation and could regulate adipose tissue mass. Recent studies have established that the promoter region of the resistin gene contains several putative PPAR response elements. Since DHEA has been characterized as a peroxisome proliferator able to induce hepatic genes through PPARalpha, we hypothesised that DHEA might affect PPARalpha and, subsequently, resistin gene expression in adipose tissue. In order to test this hypothesis, an experiment was performed comparing PPARalpha and resistin gene expression in white adipose tissue (WAT) of male Wistar rats fed standard or DHEA-supplemented (0.6% (w/w)) diet for 2 weeks. DHEA administration to the rats induced PPARalpha and resistin gene expression in WAT (3- and 2.25-fold, respectively; as determined by real-time reverse transcription-polymerase chain reaction (RT-PCR)); reduced body weight, epididymal adipose tissue mass and decreased serum leptin levels. We propose that DHEA may impact on the transcription of resistin gene through a mechanism involving PPARalpha and that an elevated resistin level may lead to an inhibition of adipogenesis and a decrease in adipose tissue mass.

Modulation of resistin expression by retinoic acid and vitamin A status

This work identifies retinoic acid (RA), the acid form of vitamin A, as a signal that inhibits the expression of resistin, an adipocyte-secreted protein previously proposed to act as an inhibitor of adipocyte differentiation and as a systemic insulin resistance factor. Both 9-cis and all-trans RA reduced resistin mRNA levels in white and brown adipocyte cell model systems; the effect was time- and dose-dependent, was followed by a reduced secretion of resistin, and was reproduced by selective agonists of both RA receptors and rexinoid receptors. Association of CCAAT/enhancer-binding protein alpha (a positive regulator of the resistin gene) and its coactivators p300, cAMP response element-binding protein binding protein, and retinoblastoma protein with the resistin gene promoter was reduced in RA-treated adipocytes. RA administration to normal mice resulted in reduced resistin mRNA levels in brown and white adipose tissues, reduced circulating resistin levels, reduced body weight, and improved glucose tolerance. Resistin expression was also downregulated after dietary vitamin A supplementation in mice. The results raise the possibility that vitamin A status may contribute to modulate systemic functions through effects on the production of adipocyte-derived protein signals.

Relationship between serum resistin concentrations and insulin resistance in nonobese, obese, and obese diabetic subjects

Early reports suggested that resistin is associated with obesity and insulin resistance in rodents. However, subsequent studies have not supported these findings. To our knowledge, the present study is the first assessment in human subjects of serum resistin and insulin sensitivity by the insulin clamp technique. Thirty-eight nonobese subjects [age, 23 +/- 4 yr; body mass index (BMI), 25.4 +/- 4.3 kg/m(2)], 12 obese subjects (age, 54 +/- 8 yr; BMI, 33.0 +/- 2.5 kg/m(2)), and 22 obese subjects with type 2 diabetes (age, 59 +/- 7 yr; BMI, 34.0 +/- 2.4 kg/m(2)) were studied. Serum resistin concentrations were not different among nonobese (4.1 +/- 1.7 ng/ml), obese (4.2 +/- 1.6 ng/ml), and obese diabetic subjects (3.7 +/- 1.2 ng/ml), and were not significantly correlated to glucose disposal rate during a hyperinsulinemic glucose clamp across groups. Serum resistin was, however, inversely related to insulin sensitivity in nonobese subjects only (r = -0.35; P = 0.05), although this association was lost after adjusting for percent body fat. Serum resistin was not related to percent fat, BMI, or fat cell size. A strong correlation was observed between serum resistin and resistin mRNA expression from abdominal sc adipose tissue in a separate group of obese subjects (r = 0.62; P < 0.01; n = 56). Although the exact function of resistin is unknown, we demonstrated only a weak relationship between resistin and insulin sensitivity in nonobese subjects, indicating that resistin is unlikely to be a major link between obesity and insulin resistance in humans.

Resistin serum levels in type 1 diabetes pre- and post-islet transplantation

Resistin is a recently described secretory protein produced in adipocytes that is thought to be involved in insulin resistance, diabetes, and inflammation. While resistin can be detected in mouse and human serum, very little is known about the regulation of serum resistin levels, especially in humans. To test whether resistin levels are affected by type 1 diabetes mellitus (T1DM), we measured serum resistin levels in samples from 5 healthy volunteers and 6 patients with T1DM pre- and 3 months post-islet transplantation using a human resistin enzyme immunoassay (EIA). Interestingly, serum resistin levels were significantly higher in T1DM patients before transplantation compared to normal controls, but decreased to normal levels after islet transplantation. Thus, our results suggest that human resistin may be involved in the pathophysiology of T1DM and thereby reveal a heretofore unappreciated aspect of human resistin biology.

The current biology of resistin

Obesity and noninsulin-dependent diabetes mellitus are globally epidemic. Insulin resistance is a major contributor to the pathogenesis of type II diabetes and plays a role in numerous other metabolic disorders including hypertension, dyslipidaemia and atherosclerosis. Obesity, in particular visceral adiposity, is positively correlated with insulin resistance. Although this correlation between adiposity and insulin resistance is well established in human beings as well as in rodent models, the mechanisms involved in obesity-related insulin resistance are not fully defined. One mechanism is that factors secreted from adipocytes can affect peripheral insulin resistance. One candidate for such a factor is resistin, an adipocyte-secreted hormone that impairs glucose homeostasis and insulin action in the mouse. This review will summarize our current understanding of resistin and will attempt to provide a framework for future study of its role in rodent and human physiology.

Modest weight loss and reduction in waist circumference after medical treatment are associated with favorable changes in serum adipocytokines

Modest weight loss if maintained is associated with significant metabolic benefits and reduction in cardiovascular risk. Adipose tissue secretes cytokines believed to contribute to the pathogenesis of insulin resistance and cardiovascular risk. We therefore observed the effect of modest weight loss on serum adipocytokines and their relationship with changes in anthropometric and metabolic parameters within a period of 6 months in the setting of a routine obesity hospital clinic after various medical treatments. In this prospective, nonrandomized, nonblinded observational study, patients were first given treatment (sibutramine or orlistat) as decided by the treating clinician and then allocated into 1 of 2 groups according to the treatment prescribed. The first group included 21 Caucasian nondiabetic female subjects, with a mean (+/-SD) age of 43 +/- 11 years and a mean body mass index (BMI) of 46 +/- 8.6 kg/m(2); subjects were treated with sibutramine 10 or 15 mg/d for weight loss. The second group included 20 Caucasian nondiabetic female subjects, mean age 42 +/- 9 years and mean BMI 45.2 +/- 5.2 kg/m(2); orlistat was introduced after 1 month on a low-fat (</=30%) diet in this group. Blood pressure and anthropometric measurements were performed before and after weight loss by a single observer. Serum glucose, insulin, lipid profile, C-reactive protein (CRP), resistin, leptin, and adiponectin were measured before and after weight loss on a fasting sample. After 6 months, the sibutramine group had a modest mean weight loss of 5.4% (P =.0001), and waist circumference was reduced by 4.5 +/- 1.4 cm. There was a decrease in serum resistin, leptin, and CRP levels, and a rise in serum adiponectin (P <.05). Change (%) (Delta) in BMI (DeltaBMI%) was associated with Deltainsulin(%) (P =.02, r = 0.53) and Deltaleptin(%) (P =.01, r = 0.58). Change in waist was associated with Deltainsulin(%) (P =.005, r = 0.75) and Deltaresistin(%) (P =.03, r = -0.55). The orlistat-treated group had a mean weight loss of 2.5%. Although this group did not show significant change in metabolic parameters, surprisingly there was a greater decrease of resistin (P =.02) associated with comparable (%) increase in adiponectin and (%) reduction of waist circumference and CRP. We conclude that modest weight loss (>5%) after medical treatment in a routine obesity hospital clinic is associated with improvements in insulin sensitivity and lipid profile. Modest weight loss is also associated with potentially favourably changes in serum adipocytokines, particularly in a rise of serum adiponectin. Reduction of waist circumference is associated with a change in serum resistin.

Regulation of fasted blood glucose by resistin

The association between obesity and diabetes supports an endocrine role for the adipocyte in maintaining glucose homeostasis. Here we report that mice lacking the adipocyte hormone resistin exhibit low blood glucose levels after fasting, due to reduced hepatic glucose production. This is partly mediated by activation of adenosine monophosphate-activated protein kinase and decreased expression of gluconeogenic enzymes in the liver. The data thus support a physiological function for resistin in the maintenance of blood glucose during fasting. Remarkably, lack of resistin diminishes the increase in post-fast blood glucose normally associated with increased weight, suggesting a role for resistin in mediating hyperglycemia associated with obesity.

Cloning and functional characterization of resistin-like molecule gamma

Resistin, a recently discovered hormone that may play a crucial role in obesity-associated diabetes, is the founding member of a novel family of cysteine-rich proteins that are secreted by specific cell types. Three other members of this family have been described to date and were termed resistin-like molecules (RELMs). Here we describe the cloning and functional characterization of RELMgamma. The mouse RELMgamma-cDNA encodes a protein of 117 amino acids that contains a signal peptide leading to secretion of the protein. By Northern blotting the RELMgamma-mRNA is detectable in bone marrow, spleen, and lung as well as in peripheral blood granulocytes. Promyelocytic HL60 cells transfected with a RELMgamma expression plasmid have an increased proliferation rate compared to mock-transfected cells and display an altered response to retinoic acid-induced granulocytic differentiation. Taken together, these data provide the first experimental evidence that RELMgamma is a secreted molecule with a restricted expression pattern that may play a role in promyelocytic differentiation.

Pituitary resistin gene expression: effects of age, gender and obesity

Resistin is a new adipocytokine which is expressed in rat, mouse and possibly human adipose tissue. Its putative role as a mediator of insulin resistance is controversial. We hypothesized that resistin, like leptin, would have multiple roles in non-adipose tissues and we reported that resistin is expressed in mouse brain and pituitary. Moreover, resistin expression in female mouse pituitary is developmentally regulated and maximal expression occurs peripubertally. Although the role of endogenous resistin in mouse brain and pituitary has not been determined, our data suggest that resistin could be important in the postnatal maturation of the hypothalamic-pituitary system. In the present study we compared the ontogeny of resistin gene expression in the pituitary of male and female mice using semi-quantitative RT-PCR analysis. We show that resistin expression is developmentally regulated in the pituitary of male and female CD1 mice. However, significant gender differences were evident (male > female at postnatal day 28 and 42) and this was not modified by neonatal treatment of female pups with testosterone. Since resistin expression in adipose tissue is also influenced by obesity, we evaluated resistin expression in fat, brain and pituitary of the obese ob/ob mouse. Resistin mRNA was significantly increased in both visceral and subcutaneous adipose depots in postnatal day 28 ob/ob mice compared to controls, but pituitary resistin expression was significantly reduced. In contrast to the prepubertal levels, and in agreement with other reports, adipose resistin expression was reduced in adult ob/ob mice. In a third set of experiments we examined the influence of food deprivation on pituitary and fat resistin mRNA. Resistin gene expression was severely down-regulated by a 24-hour fast in adipose and pituitary tissue but not in hypothalamus. In conclusion, pituitary resistin expression is age- and gender-dependent. In ob/ob mice, and in fasted mice, resistin is regulated in a tissue-specific manner. Thus in visceral fat obesity increases but starvation decreases resistin mRNA. In contrast, pituitary levels are decreased in the presence of both high (ob/ob) and low (fasting) adipose stores. Further studies are required to define the unexpected hormonal regulation of resistin gene expression in the pituitary.

The adipocyte as an endocrine cell1

Communication between adipose and other tissues has been hypothesized since at least the 1940s to be bidirectional. Despite this expectation, early progress was largely limited to adipose tissue's role in metabolism and storage of fatty acids, its development, and its response to endocrine and neural cues. However, efforts of the last decade have identified several molecules that are secreted from adipocytes, apparently for the purpose of signaling to other tissues. Cloning of the mouse obesity gene in 1994 is perhaps the most famous impetus for recognition that adipocytes are active in the regulation of multiple body functions. The product of this gene, leptin, has since been found to inhibit feeding, enhance energy expenditure, and stimulate gonadotropes. Evidence for the roles of other adipocyte-derived signals is being generated. Resistin is a protein that can cause whole-body insulin resistance. Its expression is correlated with body fatness and is inhibited by thiazolidinediones, perhaps mediating the association of type 2 diabetes with obesity, and the effectiveness of these drugs. Resistin and a related molecule, RELM alpha, can also inhibit differentiation of preadipocytes. Adiponectin/Acrp30 secretion from adipocytes is diminished in obese states. This protein can enhance use of fatty acids in lean tissues, inhibit glucose production by liver, and consequently decrease both blood glucose and BW. Adiponectin may also be responsible for the effectiveness of thiazolidinediones, given that these drugs promote adiponectin secretion. Secretion of complement proteins has been observed in adipocytes, and these interact to generate a signal called acylation-stimulating protein, which can promote triacylglycerol synthesis. These signals seem to be largely unique to adipocytes. Other signals are derived from adipose tissue, and it is unlikely that all the adipocyte's endocrine signals have been identified. Certainly, there is much to learn about how these signals function; however, it is clear that these biomedical research discoveries comprise a useful model for our study of growth and development in livestock.

Common genetic polymorphisms in the promoter of resistin gene are major determinants of plasma resistin concentrations in humans

AIMS/HYPOTHESIS

Resistin is thought to be an important link between obesity and insulin resistance. It has been suggested that genetic polymorphism in the promoter of resistin gene is a determinant of resistin mRNA expression and possibly associated with obesity and insulin resistance. In this study, we investigated the association between the genotype of resistin promoter and its plasma concentrations.

METHODS

We examined g.-537A>C and g.-420C>G polymorphisms in the resistin promoter and measured plasma resistin concentrations in Korean subjects with or without Type 2 diabetes. We also did haplotype-based promoter activity assays and the gel electrophoretic mobility shift assay.

RESULTS

The -420G and the -537A alleles, which were in linkage disequilibrium, were associated with higher plasma resistin concentrations. Individuals with haplotype A-G (-537A and -420G) had significantly higher plasma resistin concentrations than the others. Haplotype A-G had modestly increased promoter activity compared to the other haplotypes. Electrophoretic mobility shift assay showed that the -420G allele is specific for binding of nuclear proteins from adipocytes and monocytes. However, none of the two polymorphisms were associated with Type 2 diabetes or obesity in our study subjects.

CONCLUSIONS/INTERPRETATION

Polymorphisms in the promoter of resistin gene are major determinants of plasma resistin concentrations in humans.

Zinc-alpha2-glycoprotein, a lipid mobilizing factor, is expressed in adipocytes and is up-regulated in mice with cancer cachexia

Zinc-alpha2-glycoprotein (ZAG), a 43-kDa protein, is overexpressed in certain human malignant tumors and acts as a lipid-mobilizing factor to stimulate lipolysis in adipocytes leading to cachexia in mice implanted with ZAG-producing tumors. Because white adipose tissue (WAT) is an endocrine organ secreting a wide range of protein factors, including those involved in lipid metabolism, we have investigated whether ZAG is produced locally by adipocytes. ZAG mRNA was detected by RT-PCR in the mouse WAT depots examined (epididymal, perirenal, s.c., and mammary gland) and in interscapular brown fat. In WAT, ZAG gene expression was evident in mature adipocytes and in stromal-vascular cells. Using a ZAG Ab, ZAG protein was located in WAT by Western blotting and immunohistochemistry. Mice bearing the MAC16-tumor displayed substantial losses of body weight and fat mass, which was accompanied by major increases in ZAG mRNA and protein levels in WAT and brown fat. ZAG mRNA was detected in 3T3-L1 cells, before and after the induction of differentiation, with the level increasing progressively after differentiation with a peak at days 8-10. Both dexamethasone and a beta3 agonist, BRL 37344, increased ZAG mRNA levels in 3T3-L1 adipocytes. ZAG gene expression and protein were also detected in human adipose tissue (visceral and s.c.). It is suggested that ZAG is a new adipose tissue protein factor, which may be involved in the modulation of lipolysis in adipocytes. Overexpression in WAT of tumor-bearing mice suggests a local role for adipocyte-derived ZAG in the substantial reduction of adiposity of cancer cachexia.

Serum resistin levels in women with polycystic ovary syndrome

OBJECTIVE

To measure serum resistin levels in women with polycystic ovary syndrome (PCOS) and assess possible correlations of resistin to the hormonal and metabolic parameters of the syndrome.

DESIGN

Clinical study.

SETTING

University hospital.

PATIENT(S)

Ninety selected women were classified as follows: group I: 35 anovulatory women with PCOS (body mass index [BMI] >25 kg/m(2)); group II: 35 anovulatory women with PCOS (BMI <25 kg/m(2)); group III: 20 ovulating women (controls) without hyperandrogenemia (BMI <25 kg/m(2)); women of group III were volunteers.

INTERVENTION(S)

Blood samples were collected between the 3rd and the 6th day of the menstrual cycle of the ovulating women, and between the 3rd and the 6th day of a spontaneous bleeding of the anovulatory women, at 9 a.m., after an overnight fast.

MAIN OUTCOME MEASURE(S)

Serum levels of FSH, LH, PRL, 17alpha-hydroxyprogesterone, sex hormone-binding globulin, androgens, insulin, resistin, and glucose.

RESULT(S)

Resistin levels were found to be significantly increased in group I compared with those of group II and those of group III. No significant difference in resistin levels was found between groups II and III, despite significant differences in insulin levels and the glucose-to-insulin ratio. Multiple regression analysis showed that resistin levels do not correlate with any parameter independent of BMI.

CONCLUSION(S)

Based on the above findings, we presume that resistin is unlikely to be a major determining factor of PCOS-associated insulin resistance and is not actively involved in the pathogenesis of the syndrome.

Energy homeostasis, obesity and eating disorders: recent advances in endocrinology

Health problems resulting from obesity could offset many of the recent health gains achieved by modern medicine, and obesity may replace tobacco as the number one health risk for developed societies. An estimated 300,000 deaths per year and significant morbidity are directly attributable to obesity, mainly due to heart disease, diabetes, cancer, asthma, sleep apnea, arthritis, reproductive complications and psychological disturbances. In parallel with the increasing prevalence of obesity, there has been a dramatic increase in the number of scientific and clinical studies on the control of energy homeostasis and the pathogenesis of obesity to further our understanding of energy balance. It is now recognized that there are many central and peripheral factors involved in energy homeostasis, and it is expected that the understanding of these mechanisms should lead to effective treatments for the control of obesity. This brief review discusses the potential role of several recently discovered molecular pathways involved in the control of energy homeostasis, obesity and eating disorders.

Regulation of adipocytokines and insulin resistance

It has long been known that obesity and insulin resistance are linked. Recently, it has been shown that adipocytes secrete several proteins including tumour necrosis factor-alpha, interleukin-6, resistin, and adiponectin. Since several of these so-called adipocytokines influence insulin sensitivity and glucose metabolism profoundly, they might provide a molecular link between increased adiposity and impaired insulin sensitivity. Thiazolidinediones which decrease insulin resistance and are used in the treatment of Type 2 diabetes seem to mediate part of their insulin-sensitising effects via modulation of adipocytokine expression. Furthermore, hormones such as beta-adrenergic agonists, insulin, glucocorticoids, and growth hormone might impair insulin sensitivity at least in part via up-regulation or down-regulation of adipocytokine synthesis. We summarise the current knowledge on how major adipocyte-secreted proteins are regulated by hormones and drugs influencing insulin sensitivity and discuss its implications for insulin resistance and obesity.

Resistin and type 2 diabetes: regulation of resistin expression by insulin and rosiglitazone and the effects of recombinant resistin on lipid and glucose metabolism in human differentiated adipocytes

Resistin, an adipocyte secreted factor, has been suggested to link obesity with type 2 diabetes in rodent models, but its relevance to human diabetes remains uncertain. Although previous studies have suggested a role for this adipocytokine as a pathogenic factor, its functional effects, regulation by insulin, and alteration of serum resistin concentration by diabetes status remain to be elucidated. Therefore, the aims of this study were to analyze serum resistin concentrations in type 2 diabetic subjects; to determine the in vitro effects of insulin and rosiglitazone (RSG) on the regulation of resistin, and to examine the functional effects of recombinant human resistin on glucose and lipid metabolism in vitro. Serum concentrations of resistin were analyzed in 45 type 2 diabetic subjects and 34 nondiabetic subjects. Subcutaneous human adipocytes were incubated in vitro with insulin, RSG, and insulin in combination with RSG to examine effects on resistin secretion. Serum resistin was increased by approximately 20% in type 2 diabetic subjects compared with nondiabetic subjects (P = 0.004) correlating with C-reactive protein. No other parameters, including adiposity and fasting insulin levels, correlated with serum resistin in this cohort. However, in vitro, insulin stimulated resistin protein secretion in a concentration-dependent manner in adipocytes [control, 1215 +/- 87 pg/ml (mean +/- SEM); 1 nM insulin, 1414.0 +/- 89 pg/ml; 1 microM insulin, 1797 +/- 107 pg/ml (P < 0.001)]. RSG (10 nM) reduced the insulin-mediated rise in resistin protein secretion (1 nM insulin plus RSG, 971 +/- 35 pg/ml; insulin, 1 microM insulin plus RSG, 1019 +/- 28 pg/ml; P < 0.01 vs. insulin alone). Glucose uptake was reduced after treatment with 10 ng/ml recombinant resistin and higher concentrations (P < 0.05). Our in vitro studies demonstrated a small, but significant, reduction in glucose uptake with human recombinant resistin in differentiated preadipocytes. In human abdominal sc adipocytes, RSG blocks the insulin-mediated release of resistin secretion in vitro. In conclusion, elevated serum resistin in human diabetes reflects the subclinical inflammation prevalent in type 2 diabetes. Our in vitro studies suggest a modest effect of resistin in reducing glucose uptake, and suppression of resistin expression may contribute to the insulin-sensitizing and glucose-lowering actions of the thiazolidinediones.

Resistin is expressed in pancreatic islets

Resistin, a recently described adipocyte factor, is regulated by peroxisome proliferator-activated receptor gamma (PPARgamma) agonists. While resistin has been proposed to mediate insulin resistance in rodents, little is known about human resistin and its expression in pancreatic islets has not been tested. The goal of the present study was therefore to analyze whether resistin, like PPARgamma, is expressed in islets. Human islets from seven donors were analyzed by quantitative RT-PCR revealing resistin expression in all samples. Immunohistochemistry using a resistin-specific antibody on human pancreatic sections localized resistin protein to the islets. Mouse resistin was also detected in the Min6 beta cell line. Interestingly, we found a 4-fold increase in islet resistin expression in insulin resistant A-ZIP transgenic compared to wild-type mice. Our results demonstrate that resistin is expressed in islets and up-regulated in insulin resistance and thereby shed new light on the role of resistin in mice and humans.

Transgenic and recombinant resistin impair skeletal muscle glucose metabolism in the spontaneously hypertensive rat

Increased serum levels of resistin, a molecule secreted by fat cells, have been proposed as a possible mechanistic link between obesity and insulin resistance. To further investigate the effects of resistin on glucose metabolism, we derived a novel transgenic strain of spontaneously hypertensive rats expressing the mouse resistin gene under the control of the fat-specific aP2 promoter and also performed in vitro studies of the effects of recombinant resistin on glucose metabolism in isolated skeletal muscle. Expression of the resistin transgene was detected by Northern blot analysis in adipose tissue and by real-time PCR in skeletal muscle and was associated with increased serum fatty acids and muscle triglycerides, impaired skeletal muscle glucose metabolism, and glucose intolerance in the absence of any changes in serum resistin concentrations. In skeletal muscle isolated from non-transgenic spontaneously hypertensive rats, in vitro incubation with recombinant resistin significantly inhibited insulin-stimulated glycogenesis and reduced glucose oxidation. These findings raise the possibility that autocrine effects of resistin in adipocytes, leading to release of other prodiabetic effector molecules from fat and/or paracrine actions of resistin secreted by adipocytes embedded within skeletal muscle, may contribute to the pathogenesis of disordered skeletal muscle glucose metabolism and impaired glucose tolerance.

Alternative splicing generates a novel non-secretable resistin isoform in Wistar rats1

Resistin is a secreted adipose tissue hormone that belongs to the resistin-like molecule family. We report here a new alternatively spliced isoform of the rat resistin gene, named S-resistin (short resistin), detected in adipose tissue by reverse transcription-polymerase chain reaction (RT-PCR). A comparison of this cDNA variant and genomic sequences indicates the lack of the second exon containing the secretory consensus signal. Both cDNAs, resistin and S-resistin, were carboxy-tagged with FLAG epitope and transiently expressed in cultured cell lines. While the resistin-FLAG construct gives the expected pattern for a secretion protein, the S-resistin-FLAG construct yielded a predominant nuclear staining. These results indicate that this splicing event regulates the fate and probably the function of the mature protein.

Serum resistin (FIZZ3) protein is increased in obese humans

The role of resistin in obesity and insulin resistance in humans is controversial. Therefore, resistin protein was quantitated by ELISA in serum of 27 lean [13 women/14 men, body mass index (BMI) 21.7 +/- 0.4 kg/m(2), age 33 +/- 2 yr] and 50 obese (37 women/13 men, BMI 49.8 +/- 1.5 kg/m(2), age 47 +/- 1 yr) subjects. There was more serum resistin protein in the obese (mean +/- SEM: 5.3 +/- 0.4 ng/ml; range 1.8-17.9) than lean subjects (3.6 +/- 0.4 ng/ml; range 1.5-9.9; P = 0.001). The elevation of serum resistin in obese humans was confirmed by Western blot as was expression of resistin protein in human adipose tissue and isolated adipocytes. There was a significant positive correlation between resistin and BMI (r = 0.37; P = 0.002). Multiple regression analysis with predictors BMI and resistin explained 25% of the variance in homeostasis model assessment of insulin resistance score. BMI was a significant predictor of insulin resistance (P = 0.0002), but resistin adjusted for BMI was not (P = 0.11). The data demonstrate that resistin protein is present in human adipose tissue and blood, and that there is significantly more resistin in the serum of obese subjects. Serum resistin is not a significant predictor of insulin resistance in humans.

Bacterial colonization leads to the colonic secretion of RELMbeta/FIZZ2, a novel goblet cell-specific protein

BACKGROUND & AIMS

Goblet cells are highly polarized exocrine cells found throughout the small and large intestine that have a characteristic morphology due to the accumulation of apical secretory granules. These granules contain proteins that play important physiologic roles in cellular protection, barrier function, and proliferation. A limited number of intestinal goblet cell-specific proteins have been identified. In this study, we investigate the expression and regulation of RELMbeta, a novel colon-specific gene.

METHODS

The regulation of RELMbeta messenger RNA expression was determined in LS174T, Caco-2, and HT-29 cell lines in response to stimulation with interleukin 13 and lipopolysaccharide. Quantitative reverse-transcription polymerase chain reaction, immunoblots, and immunohistochemistry were used to examine the expression of RELMbeta in BALB/c and C.B17.SCID mice housed in conventional, germ-free, and gnotobiotic environments.

RESULTS

Messenger RNA for RELMbeta is restricted to the undifferentiated, proliferating colonic epithelium. Immunohistochemistry shows that this protein is expressed in goblet cells located primarily in the distal half of the colon and cecum with lower levels detectable in the proximal colon. High levels of RELMbeta can be detected in the stool of mice and humans, where it exists as a homodimer under nonreducing conditions. Interestingly, the secretion of RELMbeta is dramatically reduced in germ-free mice. Furthermore, introduction of germ-free mice into a conventional environment results in enhanced expression and robust secretion of RELMbeta within 48 hours.

CONCLUSIONS

These studies define a new goblet cell-specific protein and provide the first evidence that colon-specific gene expression can be regulated by colonization with normal enteric bacteria.

Myostatin signals through a transforming growth factor beta-like signaling pathway to block adipogenesis

Myostatin, a transforming growth factor beta (TGF-beta) family member, is a potent negative regulator of skeletal muscle growth. In this study we characterized the myostatin signal transduction pathway and examined its effect on bone morphogenetic protein (BMP)-induced adipogenesis. While both BMP7 and BMP2 activated transcription from the BMP-responsive I-BRE-Lux reporter and induced adipogenic differentiation, myostatin inhibited BMP7- but not BMP2-mediated responses. To dissect the molecular mechanism of this antagonism, we characterized the myostatin signal transduction pathway. We showed that myostatin binds the type II Ser/Thr kinase receptor. ActRIIB, and then partners with a type I receptor, either activin receptor-like kinase 4 (ALK4 or ActRIB) or ALK5 (TbetaRI), to induce phosphorylation of Smad2/Smad3 and activate a TGF-beta-like signaling pathway. We demonstrated that myostatin prevents BMP7 but not BMP2 binding to its receptors and that BMP7-induced heteromeric receptor complex formation is blocked by competition for the common type II receptor, ActRIIB. Thus, our results reveal a strikingly specific antagonism of BMP7-mediated processes by myostatin and suggest that myostatin is an important regulator of adipogenesis.

Low resistin levels in adipose tissues and serum in high-fat fed mice and genetically obese mice: development of an ELISA system for quantification of resistin

Obesity is a major risk factor for insulin resistance. Resistin, an adipocyte-derived hormone-like molecule, is considered to serve as an important link between obesity and insulin resistance. However, the physiological role of resistin and the mechanism by which it neutralizes insulin action are still unclear. There are also conflicting reports that cast doubt on the cause of insulin resistance. In this study, we developed an enzyme-linked immunosorbent assay (ELISA) system for quantification of mouse resistin levels, analyzed in relation to insulin resistance. C57BL/6J mice fed high-fat diet compared with normal diet had low resistin levels (by 70%, P<0.01) in epididymal adipose tissues. Genetically obese mice, db/db and KK-A(y), had hyperinsulinemia and hyperglycemia but low resistin levels (decreases by 83 and 90%, both P<0.01) compared with C57/BL6J mice in epididymal adipose tissues. Serum resistin levels determined by Western blotting showed a similar pattern to those in adipose tissues. Resistin levels in adipose tissues correlated with serum adiponectin concentrations positively (r=0.49). Our results indicate that the novel ELISA system is suitable for measurement of resistin levels in adipose tissues. The results do not support a role for resistin in insulin resistance.