Endothelin-1 inhibits resistin secretion in 3T3-L1 adipocytes

Endothelin-1 induces lipolysis through activation of the GC/cGMP/Ca2+/ERK/CaMKIII pathway in 3T3-L1 adipocytes

Endothelin-1 (ET-1) is a potent vasoconstrictive peptide produced and secreted mainly by endothelial cells. Recent studies indicate that ET-1 can regulate lipid metabolism, which may increase the risk of insulin resistance. Our previous studies revealed that ET-1 induced lipolysis in adipocytes, but the underlying mechanisms were unclear. 3T3-L1 adipocytes were used to investigate the effect of ET-1 on lipolysis and the underlying mechanisms. Glycerol levels in the incubation medium and hormone-sensitive lipase (HSL) phosphorylation were used as indices for lipolysis. ET-1 significantly increased HSL phosphorylation and lipolysis, which were completely inhibited by ERK inhibitor (PD98059) and guanylyl cyclase (GC) inhibitor (LY83583). LY83583 reduced ET-1-induced ERK phosphorylation. A Ca2+-free medium and PLC inhibitor caused significant decreases in ET-1-induced lipolysis as well as ERK and HSL phosphorylation, and IP3 receptor activator (D-IP3) increased lipolysis. ET-1 increased cGMP production, which was not affected by depletion of extracellular Ca2+. On the other hand, LY83583 diminished the ET-1-induced Ca2+ influx. Transient receptor potential vanilloid-1 (TRPV-1) antagonist and shRNA partially inhibited ET-1-induced lipolysis. ET-1-induced lipolysis was completely suppressed by CaMKIII inhibitor (NH-125). These results indicate that ET-1 stimulates extracellular Ca2+ entry and activates the intracellular PLC/IP3/Ca2+ pathway through a cGMP-dependent pathway. The increased cytosolic Ca2+ that results from ET-1 treatment stimulates ERK and HSL phosphorylation, which subsequently induces lipolysis. ET-1 induces HSL phosphorylation and lipolysis via the GC/cGMP/Ca2+/ERK/CaMKIII signaling pathway in 3T3-L1 adipocytes.

Resistin is co-secreted with adiponectin in white mouse adipocytes

In the current work we have investigated the cellular and molecular regulation of resistin secretion in cultured and primary mouse adipocytes. Resistin is an adipose tissue hormone proposed to contribute to metabolic disease. In rodents, resistin is secreted from white adipocytes whereas it is in humans synthesised and released from other cell types within white adipose tissue. The metabolic importance of resistin has been studied in both mouse and man, but the regulation of its release remains poorly investigated. Here we define that, in mouse adipocytes, resistin secretion is triggered by an intracellular elevation of cAMP and/or Ca²⁺. Resistin release is stimulated via activation of beta 3 adrenergic receptors (β₃ARs) and the downstream signalling protein exchange protein activated by cAMP (Epac). The secretion of resistin is markedly abrogated in adipocytes isolated from obese and diabetic mice. Immunocytochemical staining demonstrates a significant overlap between signals for resistin and the adipocyte hormone adiponectin. Our data propose that resistin and adiponectin are contained within the same vesicles in mouse adipocytes and that the two hormones are co-secreted in response to the same exocytosis-triggering signals.

Repressive effects of red bean, Phaseolus angularis, extracts on obesity of mouse induced with high-fat diet via downregulation of adipocyte differentiation and modulating lipid metabolism

Obesity is generally caused by quantitative changes in adipocyte differentiation and fat metabolism. Only a few studies have been determined the effect of red beans extract on obesity and plasma cholesterol concentration. We have been studied the functional activities of red-bean extracts including anti-oxidative effect against DNA and cell damages. Histological study including micro CT analysis showed that the accumulation of fat in hepatocytes and intestines was significantly decreased in red bean extract treated group. In addition, plasma cholesterol and triglyceride levels were decreased in blood samples. In addition, it was confirmed that the red bean extract inhibited the expression of PPARγ, Fabp4 and RETN genes, which regulate total adipocyte differentiation and lipid metabolism. Red bean extract inhibits the expressions of transcription factors associated with adipocyte differentiation in a dose-dependent manner, thereby inhibiting fat accumulation and decreasing blood lipid levels in obese mice induced by high fat diet.

Endothelin-1 differentially directs lineage specification of adipose- and bone marrow-derived mesenchymal stem cells

Blood vessels composed of endothelial cells (ECs) contact with mesenchymal stem cells (MSCs) in different tissues, suggesting possible interaction between these 2 types of cells. We hypothesized that endothelin-1 (ET1), a secreted paracrine factor of ECs, can differentially direct the lineages of adipose-derived stem cells (ASCs) and bone marrow-derived MSCs (BMSCs). Predifferentiated ASCs and BMSCs were treated with ET1 for 2 cell passages and then induced for multilineage differentiation. Our results showed that adipogenesis of ET1-pretreated ASCs and osteogenesis of ET1-pretreated BMSCs were increased compared to those of control cells. The effect of ET1 on enhancing adipogenesis of ASCs and osteogenesis of BMSCs was attenuated by blocking endothelin receptor type A (ETAR) and/or endothelin receptor type B (ETBR). Western blot analysis indicated that regulation by ET1 was mediated through activation of the protein kinase B and ERK1/2 signaling pathways. We analyzed subpopulations of ASCs and BMSCs with or without ETAR and/or ETBR, and we found that ETAR⁺/ETBR^- and ETAR^-/ETBR⁺ subpopulations of ASCs and those of BMSCs pretreated with ET1 were prone to turning into adipocytes and osteoblasts, respectively, after differentiation induction. Our findings provide insight into the differential regulation of MSC specification by ET1, which may help develop viable approaches for tissue regeneration.-Lee, M.-S., Wang, J., Yuan, H., Jiao, H., Tsai, T.-L., Squire, M. W., Li, W.-J. Endothelin-1 differentially directs lineage specification of adipose- and bone marrow-derived mesenchymal stem cells.

PKA-independent cAMP stimulation of white adipocyte exocytosis and adipokine secretion: modulations by Ca2+ and ATP

We examined the effects of cAMP, Ca(2+) and ATP on exocytosis and adipokine release in white adipocytes by a combination of membrane capacitance patch-clamp recordings and biochemical measurements of adipokine secretion. 3T3-L1 adipocyte exocytosis proceeded even in the complete absence of intracellular Ca(2+) ([Ca(2+)]i; buffered with BAPTA) provided cAMP (0.1 mm) was included in the intracellular (pipette-filling) solution. Exocytosis typically plateaued within ∼10 min, probably signifying depletion of a releasable vesicle pool. Inclusion of 3 mm ATP in combination with elevation of [Ca(2+)]i to ≥700 nm augmented the rate of cAMP-evoked exocytosis ∼2-fold and exocytosis proceeded for longer periods (≥20 min) than with cAMP alone. Exocytosis was stimulated to a similar extent upon substitution of cAMP by the Epac (exchange proteins activated by cAMP) agonist 8-Br-2'-O-Me-cAMP (1 mm included in the pipette solution). Inhibition of protein kinase A (PKA) by addition of Rp-cAMPS (0.5 mm) to the cAMP-containing pipette solution was without effect. A combination of the adenylate cyclase activator forskolin (10 μm) and the phosphodiesterase inhibitor IBMX (200 μm; forsk-IBMX) augmented adiponectin secretion measured over 30 min 3-fold and 2-fold in 3T3-L1 and human subcutaneous adipocytes, respectively. This effect was unaltered by pre-loading of cells with the Ca(2+) chelator BAPTA-AM and 2-fold amplified upon inclusion of the Ca(2+) ionophore ionomycin (1 μm) in the extracellular solution. Adiponectin release was also stimulated by the membrane-permeable Epac agonist 8-Br-2'-O-Me-cAMP-AM but unaffected by inclusion of the membrane-permeable PKA inhibitor Rp-8-Br-cAMPS (200 μm). The adipokines leptin, resistin and apelin were present in low amounts in the incubation medium (1-6% of measured adiponectin). Adipsin was secreted in substantial quantities (50% of adiponectin concentration) but release of this adipokine was unaffected by forsk-IBMX. We propose that white adipocyte exocytosis is stimulated by cAMP/Epac-dependent but Ca(2+)- and PKA-independent release of vesicles residing in a readily releasable pool and that the release is augmented by a combination of Ca(2+) and ATP. We further suggest that secreted vesicles chiefly contain adiponectin.

Resistin in rodents and humans

Obesity is characterized by excess accumulation of lipids in adipose tissue and other organs, and chronic inflammation associated with insulin resistance and an increased risk of type 2 diabetes. Obesity, type 2 diabetes, and cardiovascular diseases are major health concerns. Resistin was first discovered as an adipose-secreted hormone (adipokine) linked to obesity and insulin resistance in rodents. Adipocyte-derived resistin is increased in obese rodents and strongly related to insulin resistance. However, in contrast to rodents, resistin is expressed and secreted from macrophages in humans and is increased in inflammatory conditions. Some studies have also suggested an association between increased resistin levels and insulin resistance, diabetes and cardiovascular disease. Genetic studies have provided additional evidence for a role of resistin in insulin resistance and inflammation. Resistin appears to mediate the pathogenesis of atherosclerosis by promoting endothelial dysfunction, vascular smooth muscle cell proliferation, arterial inflammation, and formation of foam cells. Indeed, resistin is predictive of atherosclerosis and poor clinical outcomes in patients with coronary artery disease and ischemic stroke. There is also growing evidence that elevated resistin is associated with the development of heart failure. This review will focus on the biology of resistin in rodents and humans, and evidence linking resistin with type 2 diabetes, atherosclerosis, and cardiovascular disease.

Serum resistin and inflammatory and endothelial activation markers in obese adolescents

OBJECTIVES

To assess the level of serum resistin in obese and lean children and to establish a relationship with circulating inflammatory and vascular markers.

STUDY DESIGN

This is a cross-sectional study including 67 obese and 62 lean children (mean age 10.9 ± 2.8 years, age range 5.4-16.6 years). We assessed circulating hormones (insulin, leptin, insulin-like growth factor 1), markers of inflammation (resistin, high sensitivity C-reactive protein, interleukin-6, chemokine ligand 2), and endothelial cell activation (vascular and intercellular adhesion molecules: vascular cell adhesion molecule 1 and intercellular adhesion molecule; E-selectin; P-selectin; endothelin 1).

RESULTS

Body weight, body mass index (BMI), insulin, leptin, high-sensitivity C-reactive protein, vascular adhesion molecule 1, and E-selectin levels were significantly higher in obese than in lean subjects. Resistin was similar among the groups in the prepubertal period, but increased significantly in the obese adolescents (18.6 ± 24.9) compared with lean subjects (7.9 ± 10.7 ng/mL; P = .038). Resistin was not associated with BMI z score (P > .05). Subjects with resistin levels above 9 (ng/mL) had higher concentration of interleukin-6, chemokine ligand 2, endothelin-1, and insulin-like growth factor 1 but not of leptin, insulin, or BMI.

CONCLUSION

Resistin was increased in obese adolescents independently of the quantity of the adipose tissue. In this population, increased resistin levels were related to inflammation and endothelial activation. We may hypothesize that interventions aiming to diminish resistin expression may slow down atherogenesis in adolescents.

Endothelin-1 stimulates suppressor of cytokine signaling-3 gene expression in adipocytes

Endothelin (ET)-1 and suppressor of cytokine signaling (SOCS)-3 were respectively found to regulate energy metabolism and hormone signaling in fat cells. Although ET-1 can also regulate the expression of SOCS-3-stimulating hormones, it is still unknown whether ET-1 regulates SOCS-3 gene expression. This study investigated the pathways involved in ET-1's modulation of SOCS-3 gene expression in 3T3-L1 adipocytes. ET-1 upregulated SOCS-3 mRNA and protein expression in dose- and time-dependent manners. The concentration of ET-1 that increased SOCS-3 mRNA levels by 250-400% was ∼100nM with 2-4h of treatment. Treatment with actinomycin D prevented ET-1-stimulated SOCS-3 mRNA expression, suggesting that the effect of ET-1 requires new mRNA synthesis. Pretreatment with the ET type A receptor (ET(A)R) antagonist, BQ-610, but not the ET type B receptor (ET(B)R) antagonist, BQ-788, prevented the stimulatory effect of ET-1 on SOCS-3 gene expression. The specific inhibitors of either MEK1 (U-0126 and PD-98059), JAK (AG-490), JNK (SP-600125), or PI3K (LY-294002 and wortmannin) reduced ET-1-increased levels of SOCS-3 mRNA and respectively inhibited ET-1-stimulated activities of MEK1, JAK, JNK, and PI3K. These results imply that the ET(A)R, ERK, JAK, JNK, and PI3K are functionally necessary for ET-1's stimulation of SOCS-3 gene expression. Moreover, ET-1 was observed to upregulate expressions of SOCS-1, -2, -3, -4, -5, and -6 mRNAs, but not SOCS-7 or cytokine-inducible SH2-containing protein-1 mRNAs. This suggests that ET-1 selectively affects particular types of SOCS family members. Changes in SOCS gene expressions induced by ET-1 may help explain the mechanism by which ET-1 modulates hormone signaling of adipocytes.

A novel cross-talk between endothelin-1 and cyclic AMP signaling pathways in the regulation of GLUT1 transcription in 3T3-L1 adipocytes

We showed previously that chronic exposure to both endothelin-1 (ET-1) and cAMP resulted in a synergistic increase in Glut1 transcription in 3T3-L1 adipocytes via a protein kinase C (PKC)-dependent mechanism. In the present study, we further examined the molecular mechanism involved. Employing transient transfections with Glut1 promoter/enhancer -luciferase reporter and several dominant negative or constitutively active PKC mutants, we identified PKCε as the responsible PKC. Investigation with deletion and mutation mutants of the promoter/enhancer reporter suggested that Sp1, CREB and AP-1 responsive elements on enhancer 2 were involved. Furthermore, chromatin immunoprecipitation and co-immunoprecipitation analysis were applied to characterize the interactions between these transcription factors and their bindings to enhancer 2 in vivo. The results indicate that there are both negative and positive interactions between ET-1 and cAMP signaling pathways. On the one hand, cAMP inhibits ET-1 induced NF-κB activation required for ET-1-stimulated Glut1 transcription; on the other hand, cAMP, via sustained CREB phosphorylation, may activate AP-1 and cooperate with ET-1-activated PKCε to enhance Sp1 expression and consequently to generate a stable enhancer 2-bound Sp1/pCREB/AP-1 complex, which can strongly facilitate Glut1 transcription more than the additive effect of ET-1 and cAMP alone.

Insulin regulates leptin secretion from 3T3-L1 adipocytes by a PI 3 kinase independent mechanism

To better define the molecular mechanisms underlying leptin release from adipocytes, we developed a novel protocol that maximizes leptin production from 3T3-L1 adipocytes. The addition of a PPARgamma agonist to the Isobutylmethylxanthine/Dexamethasone/Insulin differentiation cocktail increased leptin mRNA levels by 5-fold, maintained insulin sensitivity, and yielded mature phenotype in cultured adipocytes. Under these conditions, acute insulin stimulation for 2 h induced a two-fold increase in leptin secretion, which was independent of new protein synthesis, and was not due to alterations in glucose metabolism. Stimulation with insulin for 15 min induced the same level of leptin release and was blocked by Brefeldin A. Inhibiting PI 3-kinase with wortmannin had no effect on insulin stimulation of leptin secretion. These studies show that insulin can stimulate leptin release via a PI3K independent mechanism and provide a cellular system for studying the effect of insulin and potentially other mediators on leptin secretion.

Resistin Is a Key Mediator of Glucose-dependent Insulinotropic Polypeptide (GIP) Stimulation of Lipoprotein Lipase (LPL) Activity in Adipocytes

Studies on the physiological roles of the incretin hormone, glucose-dependent insulinotropic polypeptide (GIP) have largely focused on its insulinotropic action and ability to regulate beta-cell mass. In previous studies on the stimulatory effect of GIP on adipocyte lipoprotein lipase (LPL), a pathway was identified involving increased phosphorylation of protein kinase B (PKB) and reduced phosphorylation of LKB1 and AMP-activated protein kinase (AMPK). The slow time of onset of the responses suggested that GIP may have induced release of an intermediary molecule, and the current studies focused on the possible contribution of the adipokine resistin. In differentiated 3T3-L1 adipocytes, GIP, in the presence of insulin, increased resistin secretion through a pathway involving p38 mitogen-activated protein kinase (p38 MAPK) and the stress-activated protein kinase/Jun amino-terminal kinase (SAPK/JNK). The other major incretin hormone, glucagon-like peptide-1 (GLP-1), exhibited no significant effects. Chronic elevation of circulating GIP levels in the Vancouver Diabetic Fatty (VDF) Zucker rat resulted in increases in circulating resistin levels and activation of p38 MAPK or SAPK/JNK in epididymal fat tissue, suggesting the existence of identical pathways in vivo as well as in vitro. Administration of resistin to 3T3-L1 adipocytes mimicked the effects of GIP on the PKB/LKB1/AMPK/LPL pathway: increasing phosphorylation of PKB, reducing levels of phosphorylated LKB1 and AMPK, and increasing LPL activity. Knockdown of resistin using RNA interference attenuated the effect of GIP on the PKB/LKB1/AMPK/LPL pathway in 3T3-L1 adipocytes, supporting a role for resistin as a mediator.

Differential effects of peroxisome proliferator-activated receptor ligands and sulfonylurea plus statin treatment on plasma concentrations of adipokines in type 2 diabetes with dyslipidemia

OBJECTIVES

Peroxisome proliferator-activated receptor gamma is the master regulator of adipocyte differentiation and controls many adipocyte genes in response to anti-diabetic thiazolidinediones (TZDs) and lipid-lowering fibrates. We hypothesized that the combination of TZD+fibrate may be better than the sulfonylurea + statin approach regarding modifying the adipokine profile in diabetic patients with dyslipidemia.

METHODS

We measured the lipid profiles and circulating levels of adiponectin, resistin, and inflammatory markers before and after treatment in 24 type 2 diabetic patients with dyslipidemia (aged 64+/-9 years; M/F=5/19). The study patients were randomly assigned to receive an 8-week treatment of either rosiglitazone 4 mg daily and fenofibrate 160 mg daily (PPAR group) or glibenclamide 5 mg daily and atorvastatin 10 mg daily (non-PPAR group).

RESULTS

Even though the administration of sulfonylurea+statin can achieve a greater reduction of total cholesterol and LDL-cholesterol levels and a comparable glucose control compared to PPAR treatment, their administration did not change the plasma adipokine levels significantly. In contrast, a significant greater increase of the plasma concentrations of adiponectin (P<0.0001), a trend to a greater decrease of the plasma resistin levels (P=0.061), a significantly greater increase of HDL-cholesterol (P=0.002), and a significantly greater reduction of triglyceride levels (P=0.018) were seen in the PPAR group.

CONCLUSIONS

Considering the clinical significance of the adipokine-endothelial interaction in the progression and long-term prognosis of atherosclerosis, the differential effects of PPAR ligands and sulfonylurea+statin on plasma adipokine concentrations demonstrated in this study are interesting foci of investigation in the future.

Endothelin-1 inhibits adiponectin secretion through a phosphatidylinositol 4,5-bisphosphate/actin-dependent mechanism

Adiponectin is an adipokine with profound insulin-sensitizing, anti-inflammatory, and anti-atherogenic properties. Plasma levels of adiponectin are reduced in insulin resistant states such as obesity, type 2 diabetes and cardiovascular disease. However, the mechanism(s) by which adiponectin concentrations are decreased during disease development is unclear. Studies have shown that endothelin-1 (ET-1), a vasoconstrictor peptide, affects adipocyte glucose metabolism and secretion of adipokines such as leptin, resistin, and adiponectin. The goal of our study was to determine the mechanism by which ET-1 decreases adiponectin secretion. 3T3-L1 adipocytes were treated for 24h with ET-1 (10nM) and then stimulated with vehicle or insulin (100 nM) for a period of 1-2h. Chronic ET-1 (24h) treatment significantly decreased basal and insulin-stimulated adiponectin secretion by 66% and 47%, respectively. Inhibition of phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis by the PLCbeta inhibitor, U73122, or exogenous addition of PIP(2):histone carrier complex (1.25:0.625 microM) ameliorated the decrease in basal and insulin-stimulated adiponectin secretion observed with ET-1. However, treatment with exogenous PIP(2):histone carrier complex and the actin depolymerizing agent latrunculin B (20 microM) did not reverse the ET-1-mediated decrease in adiponectin secretion. In conclusion, we demonstrate that ET-1 inhibits basal and insulin-stimulated adiponectin secretion through PIP(2) modulation of the actin cytoskeleton.

Effect of endothelin-1 on lipolysis in rat adipocytes

OBJECTIVE

To explore the role of endothelin-1 (ET-1) on lipid metabolism, we examined the effect of ET-1 on lipolysis in rat adipocytes.

RESEARCH METHODS AND PROCEDURE

Adipocytes isolated from male Sprague-Dawley rats, weighing 400 to 450 grams, were incubated in Krebs-Ringer buffer with or without 10(-7) M ET-1 for various times or with various concentrations of ET-1 for 4 hours; then glycerol release into the incubation medium was measured. In addition, selective ET(A)R and ET(B)R blockers were used to identify the ET receptor subtype involved. We also explored the involvement of cyclic adenosine monophosphate (cAMP) in ET-1-stimulated lipolysis using an adenylyl cyclase inhibitor and by measuring changes in intracellular cAMP levels in response to ET-1 treatment. To further explore the underlying mechanism of ET-1 action, we examined the involvement of the extracellular signal-regulated kinase (ERK)-mediated pathways.

RESULTS

Our results showed that ET-1 caused lipolysis in rat adipocytes in a time- and dose-dependent manner. BQ610, a selective ET(A)R blocker, blocked this effect. The adenylyl cyclase inhibitor, 2',5'-dideoxyadenosine, had no effect on ET-1-stimulated lipolysis. ET-1 did not induce an increase in intracellular cAMP levels. In addition, ET-1-induced lipolysis was blocked by inhibition of ERK activation using PD98059. Coincubation of cells with ET-1 and insulin suppressed ET-1-stimulated lipolysis.

DISCUSSION

These findings show that ET-1 stimulates lipolysis in rat adipocytes through the ET(A)R and activation of the ERK pathway. The underlying mechanism is cAMP-independent. However, this non-conventional lipolytic effect of ET-1 is inhibited by the anti-lipolytic effect of insulin.

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

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

Effect of atorvastatin on in vitro expression of resistin in adipocytes and monocytes/macrophages and effect of atorvastatin treatment on serum resistin levels in patients with type 2 diabetes

Resistin is a novel cysteine-rich protein that plays a role in the development of insulin resistance and atherosclerosis. HMG-CoA reductase inhibitors (statins) possess anti-inflammatory properties that are independent of their lipid-lowering action. The aims of this study were to investigate the effect of atorvastatin on expression of resistin in vitro and to determine the effect of 6 months of treatment with atorvastatin on serum levels of resistin in patients with type 2 diabetes. 3T3-L1 adipocytes and human monocytes/macrophages and preadipocytes were incubated with 1 and 10 micromol/l atorvastatin for 24 and 48 h, followed by measurement of resistin mRNA by the quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR). Serum resistin concentration in the patients with type 2 diabetes was measured at baseline and after 6 months of atorvastatin treatment (10 mg/day). qRT-PCR analysis revealed that atorvastatin decreased resistin mRNA expression in a dose- and time-dependent manner. Serum resistin concentration tended to decrease after 6 months of atorvastatin treatment, although this decrease did not reach statistical significance. In conclusion, the findings of our in vitro study contribute to the growing volume of evidence on the anti-inflammatory and anti-atherosclerotic effects of statins, and led us to suggest that statins may control inflammatory responses by inhibiting expression of resistin mRNA. It is necessary to confirm the findings of our in vitro study by an appropriately designed large-scale clinical study.

Effects of free fatty acids on plasma resistin and insulin resistance in awake rats

Resistin has been postulated to play a role in obesity-related insulin resistance. To explore this possibility, we have investigated effects of acute euglycemic (5.2+/-0.1 mmol/L) hyperinsulinemia (96+/-8 microU/mL) with and without concurrent infusion of lipid plus heparin (to raise or lower plasma free fatty acid [FFA] levels) on glucose turnover and plasma resistin levels in alert rats. Plasma FFA concentrations increased during lipid/heparin (L/H) infusion (from 0.82 to 2.86 mmol/L, P<.001) and decreased (from 0.83 to 0.21 mmol/L, P<.001) in controls who were infused with insulin but not with L/H. L/H infusion reduced insulin suppression of endogenous glucose production by approximately 90% (from 28.9 to 3.1 mg. kg-1 . min-1, P<.001) and insulin-stimulated glucose uptake (glucose rate of disappearance) by 78% (from 30.8% to 6.9%, P<.001). Plasma resistin levels increased by 46% (from 39.9 to 58.4 microg/L, P<.05) during L/H infusion and did not change in controls (39.7 vs 39.3 microg/L). Plasma ghrelin levels decreased by 41% (from 892 to 584 ng/L, P<.05) in response to hyperinsulinemia, whereas concurrent L/H infusion had no additional effect on ghrelin levels (584+/-67 vs 548+/-82 ng/L). In summary, we found that FFA induced hepatic insulin resistance, and to a lesser extent, peripheral insulin resistance was associated with elevated plasma resistin levels. We conclude that FFA-induced release of resistin may contribute to the development of FFA-induced insulin resistance in rats.

IGF-I downregulates resistin gene expression and protein secretion

Resistin (Rstn) is known as an adipocyte-specific secretory factor that can cause insulin resistance and decrease adipocyte differentiation. Conversely, based on various studies, insulin-like growth factors (IGFs) can improve insulin resistance and stimulate adipocyte adipogenesis. Whether IGFs exert their effects through the control of Rstn's production or modulation of Rstn's action is unknown. This study was designed to examine the influence and the signaling of IGF-I on Rstn gene expression and protein secretion by 3T3-L1 adipocytes. We found that IGF-I suppressed Rstn mRNA expression and protein release in dose- and time-dependent manners. The IC50 of IGF-I was approximately 1 nM for a range of 6-10 h of treatment. Treatment with cycloheximide, but not with actinomycin D, prevented IGF-I-suppressed Rstn mRNA expression, suggesting that IGF-I destabilizes Rstn mRNA and that IGF-I's effect requires new protein, but not mRNA, synthesis. Pretreatment with IGF-I receptor (IGF-IR) antibody blocked IGF-I-altered IGF-IR activity and Rstn mRNA levels. Neither PD-98059, SB-203580, nor LY-294002 changed the IGF-I-decreased levels of Rstn mRNA, but they inhibited IGF-I-stimulated activities of MEK1, p38 MAPK, and phosphoinositide 3-kinase, respectively. However, SB-203580 antagonized the IGF-I-decreased Rstn protein release. These data demonstrate that IGF-I downregulates Rstn gene expression via IGF-IR-dependent and MEK1-, p38 MAPK-, and phosphoinositide 3-kinase-independent pathways and likely modifies the distribution of Rstn protein between the intracellular and extracellular compartments via a p38 MAPK-dependent pathway. Decreases in Rstn production and secretion induced by IGF-I may be related to the mechanism by which IGF-I modulates body weight and diabetes in animals.

Stimulated resistin expression in white adipose of rats with bile duct ligation-induced liver cirrhosis: relationship to cirrhotic hyperinsulinemia and increased tumor necrosis factor-alpha

Resistin, an adipose-derived polypeptide hormone, is proposed as a candidate of insulin resistance, although its roles in inhibiting adipogenesis and in inflammation have also been suggested. Liver cirrhosis is characterized by elevated circulating proinflammatory cytokines, including tumor necrosis factor-alpha (TNF-alpha), hyperinsulinemia and insulin resistance. The study aimed to examine resistin expression and its association with insulin and TNF-alpha in a cirrhotic rat model using bile duct ligation (BDL). The BDL-induced cirrhotic rats showed significantly lower fat mass, insulin sensitivity and elevated plasma insulin and TNF-alpha compared to sham animals. In addition, epididymal TNF-alpha and resistin mRNA and protein levels were higher in cirrhotic rats. In normal control rats, in vivo insulin infusion and ex vivo administration of TNF-alpha to cultured fat pads increased resistin gene expression significantly. These results implied that hyperinsulinemia and increased TNF-alpha levels might upregulate adipose resistin gene in BDL-induced liver cirrhosis. Further study is necessary to document the role of resistin in metabolic abnormalities of liver cirrhosis.

Maturation of the expression of adrenomedullin, endothelin-1 and nitric oxide synthases in adipose tissues from childhood to adulthood

OBJECTIVE

To investigate if the vasoactive systems adrenomedullin (ADM) and endothelin-1 (ET-1) are expressed in human adipose tissues in children and in adults and to determine the distribution pattern of nitric oxide synthases (NOS).

DESIGN AND SUBJECTS

Subcutaneous, mesenterial and omental adipose tissue specimens taken from 15 children (age 0.5-16 y, median 6 y) and 13 adults (age 43-79 y, median 60 y) were analyzed. The body mass indices (BMI) were within the normal range. All patients were normotensive, and were free of infectious disease, and metabolic or endocrine disorders. The specimens were taken during elective laparotomies after informed consent was obtained.

MEASUREMENTS

ADM, ET-1, the endothelial (eNOS) and inducible (iNOS) NOS as well as two housekeeping genes were measured using quantitative real-time PCR.

RESULTS

ADM gene expression was found at all locations, and was significantly higher in adults than in children (P<0.01 for subcutaneous and omental adipose tissue). ET-1 mRNA was distributed in a similar way, showing significantly higher levels in the subcutaneous and mesenterial adipose tissue sections of adults than of children. For eNOS, the adult patients exhibited a higher expression in subcutaneous and mesenterial specimens than the children (P<0.01 and P<0.05). The iNOS mRNA was increased in subcutaneous, mesenterial and omental adipose tissues in the adult cohort compared to the children's levels (P<0.05 to P<0.01).

CONCLUSION

Human adipose tissue expresses many vasoactive substances including ADM and ET-1. In adults, the amounts of ET-1 and ADM as well as eNOS and iNOS mRNA are higher, possibly due to a physiological upregulation with increasing age. Although there are differences depending on the locations of the tissues, the expression patterns of the antagonists ADM and ET-1 are quite similar, indicative of a well-balanced pattern of local gene expression in normotensive individuals with normal body weight.

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.

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.

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.

Regulation of adiponectin secretion by endothelin-1

Adiponectin is an adipocyte-derived hormone best known for its insulin-sensitizing ability. The expression and circulating concentration of adiponectin are decreased in type 2 diabetics and increase following treatment with thiazolidinediones. Endothelin-1 (ET-1) is a potent vasoconstrictor peptide whose levels are elevated in numerous disease states, including obesity and diabetes. ET-1 has profound effects on adipose tissue metabolism and alters the release of adipose-derived factors such as leptin and resistin, therefore we investigated the role of ET-1 in adiponectin secretion. 3T3-L1 adipocytes were treated with insulin (100 nM), ET-1 (100 nM), or the appropriate vehicle and adiponectin secretion into the media was determined by immunoblotting and densitometric analysis. Adiponectin secretion significantly increased 1h following insulin or ET-1 treatment, respectively. Pretreatment with ET-1 for 24h significantly inhibited the ability of insulin or ET-1 to acutely stimulate adiponectin secretion. The specific ET(A) receptor antagonist, BQ-610 (1 microM), significantly inhibited ET-1-stimulated adiponectin secretion. In summary, ET-1 acutely stimulates adiponectin secretion through the ET(A) receptor. Chronic exposure to ET-1 dramatically decreases the stimulatory effect of insulin and ET-1 on adiponectin secretion. Our findings suggest vascular factors such as ET-1 may play a role in the regulation of adiponectin secretion and whole body energy metabolism.

Resistin is expressed in different rat tissues and is regulated in a tissue- and gender-specific manner

Resistin is a polypeptide hormone first reported from human and rodent adipocytes. In order to better define the potential biological role of resistin we undertook a detailed analysis of its expression in different rat tissues. We demonstrate by real-time reverse transcription polymerase chain reaction, Southern blotting and immunohistochemistry that resistin is expressed not only in brown and white adipose tissue, but also in the stomach, small and large intestines, adrenal gland, and skeletal muscle. Food deprivation led to a decrease in resistin mRNA expression only in adipose tissue, not in any of the other tissues studied. Furthermore, resistin mRNA expression is higher in males than in females in adipose tissue, not in any of the other tissues. Thus, our data suggest that resistin is not exclusively localized in adipocytes, and indicate that its expression is regulated in a tissue- and sex-specific manner.

Obesity-associated activation of angiotensin and endothelin in the cardiovascular system

The renin-angiotensin system (RAS) and the endothelin system have been implicated in the pathogenesis of human cardiovascular and renal diseases, and inhibition of the RAS markedly improves morbidity and survival. Obesity in humans is associated with an increased risk for the development of hypertension, atherosclerosis and focal-segmental glomerulosclerosis, however the exact mechanisms underlying these pathologies in obese individuals are not known. This article discusses the clinical importance of obesity and the current evidence for local activation of the renin-angiotensin system and its interactions with the endothelin system in obesity and the cardiovascular pathologies associated with it.