Impaired glucose tolerance is accompanied by decreased insulin sensitivity in tissues of mice implanted with cells that overexpress resistin

Fucoxanthin: A Promising Phytochemical on Diverse Pharmacological Targets

Fucoxanthin (FX) is a special carotenoid having an allenic bond in its structure. FX is extracted from a variety of algae and edible seaweeds. It has been proved to contain numerous health benefits and preventive effects against diseases like diabetes, obesity, liver cirrhosis, malignant cancer, etc. Thus, FX can be used as a potent source of both pharmacological and nutritional ingredient to prevent infectious diseases. In this review, we gathered the information regarding the current findings on antimicrobial, antioxidant, anti-inflammatory, skin protective, anti-obesity, antidiabetic, hepatoprotective, and other properties of FX including its bioavailability and stability characteristics. This review aims to assist further biochemical studies in order to develop further pharmaceutical assets and nutritional products in combination with FX and its various metabolites.

Molecular and expression characteristics of resistin (RETN) and its effects on the differentiation of intramuscular preadipocyte in goat

Resistin (RETN) is a hormone secreted by adipocytes, which plays an important role in glucose and lipid metabolism. The aim of this study is to clone and obtain the full length open reading frame (ORF) of goat RETN gene sequence, and to reveal its molecular and expression characteristics. Simultaneously, we explore its effect on the differentiation of intramuscular preadipocytes in goat. The full length ORF sequence of goat RETN gene was cloned by RT-PCR technique, and bioinformatics analysis was performed though relevant biological softwares. In this study, the expression of RETN mRNA in goat tissues and intramuscular preadipocytes during differentiation was detected by qPCR technique. Furthermore, RNA interference was used to explore the effects of RETN on intramuscular preadipocytes differentiation in goat. The results showed that the cloned goat RETN gene sequence was 428 bp in length, of which the ORF was 330 bp, encoding 109 amino acids. Sequence analysis revealed that it had 12 phosphorylation sites and an O-glycosylation site, and its protein contained a signal peptide sequence. Also, the RETN gene is expressed in goat various analyzed tissues, and the results showed that the expression of RETN gene in lung tissue was higher than that in other analyzed tissues of goat (p < .01). Moreover, the expression level of RETN gene in the goat's intramuscular preadipocytes decreased first and then increased, and reached the highest on the fifth day, which was significantly higher than that of undifferentiated intramuscular preadipocytes (p < .001). After transfecting intramuscular preadipocyte with siRNA, we found that the mRNA level of RETN was significantly down-regulated by 70% and 87% (p < .01). Oil red O staining results showed that the interference of RETN gene can promote the differentiation of intramuscular preadipocytes. After knockdown of RETN with siRNA, the PPARγ, AP2, C/EBPα, C/EBPβ and SREBP1 genes were significantly up-regulated (p < .01). Thus, it can be inferred that RETN inhibits the differentiation of goat intramuscular preadipocytes, probably through regulating the expression of C/EBPα, C/EBPβ, PPARγ, AP2 and SREBP-1 genes.

Adipocytes as a vehicle for ex vivo gene therapy: Novel replacement therapy for diabetes and other metabolic diseases

Because of its availability and recent advances in cell biology, adipose tissue is now considered an ideal target site for the preparation of recipient cells and for the transplantation of gene‐transduced cells for supplementation of therapeutic proteins. Inherited or acquired serum protein deficiencies are the ideal targets for gene therapy. However, to develop an effective ex vivo gene therapy‐based protein replacement treatment, the requirements for the recipient cells are different from those for standard gene therapy that is intended to correct the function of the recipient cells themselves. To meet the requirements for such a therapeutic strategy, recent in vitro and animal model studies have developed new methods for the preparation, culture, expansion and manipulation of adipose cells using advanced gene transduction methods and transplantation scaffolds. In this short review, we introduce the progress made in novel adipose tissue‐based therapeutic strategies for the treatment of protein deficiencies by our group and other investigators, and describe their future applications for diabetes and other metabolic diseases. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2011.00133.x, 2011)

Serum concentrations of resistin and adiponectin and their relationship to insulin resistance in subjects with impaired glucose tolerance

OBJECTIVES

This study measured the serum concentrations of resistin and adiponectin in Chinese subjects with impaired glucose tolerance (IGT) and investigated their association with insulin resistance, metabolic parameters and circulating inflammatory markers.

METHODS

A total of 124 subjects participated in the study (44 with IGT and 80 with normal glucose tolerance [NGT]). Fasting serum concentrations of lipids, glucose, insulin and adipocytokines (resistin, adiponectin, leptin, tumour necrosis factor-α [TNF-α], interleukin-6 and C-reactive protein) were measured.

RESULTS

Serum resistin concentrations were similar in the IGT and NGT groups but were significantly higher in overweight/ obese IGT subjects than in those of normal weight. Serum adiponectin concentrations were significantly lower in the IGT group than in the NGT group. In the IGT group, resistin was positively correlated with age, body mass index and TNF-α, and adiponectin was correlated positively with high-density lipoprotein cholesterol and negatively with TNF-α and waist/hip ratio.

CONCLUSIONS

Circulating resistin is unlikely to be a major mediator of glucose tolerance in humans but it may have an inflammatory role in IGT. The data support the theory that circulating adiponectin has an anti-inflammatory and anti-insulin resistance function.

Disturbed apolipoprotein A-I-containing lipoproteins in fish-eye disease are improved by the lecithin:cholesterol acyltransferase produced by gene-transduced adipocytes in vitro

We report the in vitro efficacy of recombinant LCAT produced by lcat gene-transduced proliferative adipocytes (ccdPA/lcat), which has been developed for enzyme replacement therapy. ApoA-I-specific immunodetection in combination with 1D and 2D gel electrophoreses showed that the disturbed high-density lipoprotein subpopulation profile was clearly ameliorated by the in vitro incubation with ccdPA/lcat-derived recombinant LCAT. Thus, these results using ccdPA/lcat strongly suggest the cell implantation could contribute the enzyme replacement for the patients with LCAT deficiency.

Failure of the Adipocytokine, Resistin, to Protect the Heart From Ischemia-Reperfusion Injury

Experimental studies have linked the adipocytokines with acute cardioprotection. Whether the adipocytokine, resistin, confers protection is, however, debatable. In the current study, the actions of resistin, administered at reperfusion, were investigated in in vivo and in vitro rodent and in vitro human models of myocardial ischemia-reperfusion (I/R) injury. Resistin did not reduce infarct size in Langendorff-perfused rat hearts or murine hearts perfused in vivo. Resistin also did not protect human atrial muscle subjected to hypoxia-reoxygenation. Although cyclosporin A delayed mitochondrial permeability transition pore (MPTP) opening in murine cardiomyocytes, resistin was ineffective. Western blot analysis revealed that resistin treatment was associated with enhanced phosphorylation of Akt, at both the serine-473 (+ 51.9%, P = .01) and threonine-308 (+107%, P < .01) phosphorylation sites, although not to the extent seen with ischemic preconditioning (+132.5%, P = .002 and +389.1%, P < .01, respectively). We conclude that resistin administered at reperfusion at concentrations/doses equivalent to normal (upper end) and pathological serum levels does not protect against I/R injury or inhibit MPTP opening.

Diabetes and apoptosis: liver

The liver is a central regulator of glucose homeostasis and stores or releases glucose according to metabolic demands. In insulin resistant states or diabetes the dysregulation of hepatic glucose release contributes significantly to the pathophysiology of these conditions. Acute or chronic liver disease can aggravate insulin resistance and the physiological effects of insulin on hepatocytes are disturbed. Insulin resistance has also been recognized as an independent risk factor for the development of liver injury. In the healthy liver tissue homeostasis is achieved through cell turnover by apoptosis and dysregulation of the physiological process resulting in too much or too little cell death can have potentially devastating effects on liver tissue. The delineation of the signaling pathways that mediate apoptosis changed the paradigms of understanding of many liver diseases. These signaling events include cell surface based receptor-ligand systems and intracellular signaling pathways that are regulated through kinases on multiple levels. The dissection of these signaling pathways has shown that the regulators of apoptosis signaling events in hepatocytes can also modulate insulin signaling pathways and that mediators of insulin resistance in turn influence liver cell apoptosis. This review will summarize the potential crosstalk between apoptosis and insulin resistance signaling events and discuss the involved mediators.

Effects of resistin expression on glucose metabolism and hepatic insulin resistance

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

Ang II-stimulated migration of vascular smooth muscle cells is dependent on LR11 in mice

Medial-to-intimal migration of SMCs is critical to atherosclerotic plaque formation and remodeling of injured arteries. Considerable amounts of the shed soluble form of the LDL receptor relative LR11 (sLR11) produced by intimal SMCs enhance SMC migration in vitro via upregulation of urokinase-type plasminogen activator receptor (uPAR) expression. Here, we show that circulating sLR11 is a novel marker of carotid intima-media thickness (IMT) and that targeted disruption of the LR11 gene greatly reduces intimal thickening of arteries through attenuation of Ang II-induced migration of SMCs. Serum concentrations of sLR11 were positively correlated with IMT in dyslipidemic subjects, and multivariable regression analysis suggested sLR11 levels as an index of IMT, independent of classical atherosclerosis risk factors. In Lr11-/- mice, femoral artery intimal thickness after cuff placement was decreased, and Ang II-stimulated migration and attachment of SMCs from these mice were largely abolished. In isolated murine SMCs, sLR11 caused membrane ruffle formation via activation of focal adhesion kinase/ERK/Rac1 accompanied by complex formation between uPAR and integrin alphavbeta3, a process accelerated by Ang II. Overproduction of sLR11 decreased the sensitivity of Ang II-induced activation pathways to inhibition by an Ang II type 1 receptor blocker in mice. Thus, we demonstrate a requirement for sLR11 in Ang II-induced SMC migration and propose what we believe is a novel role for sLR11 as a biomarker of carotid IMT.

Resistin and insulin resistance in hepatocytes: resistin disturbs glycogen metabolism at the protein level

Resistin has been considered to link obesity with type 2 diabetes. Liver glycogen metabolism plays an essential role in maintaining glucose homeostasis, we investigated the effect of resistin on liver glycogen metabolism and attempted to identify its role in initiating insulin resistance and type 2 diabetes. Primary culture of rat hepatocytes was treated by resistin and insulin. Glycogen content was determined by the anthrone-reagent method. Real-time PCR, Western blot and enzymatic activity assay were used to detect key enzymes and genes involved in glucose metabolism. Hepatocytes exposed to resistin, but only in the presence of insulin, show a decrease in insulin-stimulated glycogen content. Decreased insulin receptor expression and GS activity and elevated GP activity was observed after the treatment of hepatocytes with resistin. No significant changes in the expression of the genes for these proteins were observed. These results strongly suggest that resistin effects glycogen metabolism at the protein level, and resistin is highly associated with insulin resistance and type 2 diabetes and is a candidate for the prevention and treatment of type 2 diabetes. Our results should lead to the development of novel strategies for the treatment of type 2 diabetes.

Effects of over-expressing resistin on glucose and lipid metabolism in mice

Resistin, a newly discovered peptide hormone mainly secreted by adipose tissues, is present at high levels in serum of obese mice and may be a potential link between obesity and insulin resistance in rodents. However, some studies of rat and mouse models have associated insulin resistance and obesity with decreased resistin expression. In humans, no relationship between resistin level and insulin resistance or adiposity was observed. This suggests that additional studies are necessary to determine the specific role of resistin in the regulation of energy metabolism and adipogenesis. In the present study, we investigated the effect of resistin in vivo on glucose and lipid metabolism by over-expressing resistin in mice by intramuscular injection of a recombinant eukaryotic expression vector pcDNA3.1-Retn encoding porcine resistin gene. After injection, serum resistin and serum glucose (GLU) levels were significantly increased in the pcDNA3.1-Retn-treated mice; there was an obvious difference in total cholesterol (TC) level between the experiment and the control groups on Day 30. In pcDNA3.1-Retn-treated mice, both free fatty acid (FFA) and high density lipoprotein (HDL) cholesterol levels were markedly lower than those of control, whereas HDL cholesterol and triglyceride (TG) levels did not differ between the two groups. Furthermore, lipase activity was expressly lower on Day 20. Our data suggest that resistin over-expressed in mice might be responsible for insulin resistance and parameters related to glucose and lipid metabolism were changed accordingly.

Dietary combination of fucoxanthin and fish oil attenuates the weight gain of white adipose tissue and decreases blood glucose in obese/diabetic KK-Ay mice

Fucoxanthin is a marine carotenoid found in edible brown seaweeds. We previously reported that dietary fucoxanthin attenuates the weight gain of white adipose tissue (WAT) of diabetic/obese KK- A(y) mice. In this study, to evaluate the antiobesity and antidiabetic effects of fucoxanthin and fish oil, we investigated the effect on the WAT weight, blood glucose, and insulin levels of KK- A(y) mice. Furthermore, the expression level of uncoupling protein 1 (UCP1) and adipokine mRNA in WAT were measured. After 4 weeks of feeding, 0.2% fucoxanthin in the diet markedly attenuated the gain of WAT weight in KK- A(y) mice with increasing UCP1 expression compared with the control mice. The WAT weight of the mice fed 0.1% fucoxanthin and 6.9% fish oil was also significantly lower than that of the mice fed fucoxanthin alone. In addition, 0.2% fucoxanthin markedly decreased the blood glucose and plasma insulin concentrations in KK- A(y) mice. The mice fed with the combination diet of 0.1% fucoxanthin and fish oil also showed improvements similar to that of 0.2% fucoxanthin. Leptin and tumor necrosis factor (TNFalpha) mRNA expression in WAT were significantly down-regulated by 0.2% fucoxanthin. These results suggest that dietary fucoxanthin decreases the blood glucose and plasma insulin concentration of KK- A(y) along with down-regulating TNFalpha mRNA. In addition, the combination of fucoxanthin and fish oil is more effective for attenuating the weight gain of WAT than feeding with fucoxanthin alone.

Resistin, an adipocytokine, offers protection against acute myocardial infarction

Resistin, an adipocyte-derived hormone, is thought to represent a link between obesity and insulin-resistant diabetes. The potential role of resistin as a cardioprotective agent has not been explored. Our hypothesis is that resistin has a cardioprotective effect that is mediated by the resistin receptor-coupled activation of PI3K/Akt/PKC/K(ATP) dependent pathways. Our studies demonstrated that pretreatment of mouse hearts with 10 nM resistin for 5 min protected the heart against I/R injury in a mouse heart perfusion model. When mouse hearts were subjected to 60 min of LAD ligation followed by 4 h of reperfusion, resistin pretreatment (33 microg/kg) for 30 min or 24 h before ligation was able to significantly reduce the infarct size/risk area. The protective effect of resistin was abolished by wortmannin, as well as by an Akt inhibitor, triciribine. Resistin's protective effect was absent in Akt kinase-deficient mutant mice. The protective effect was also blocked by chelerythrine, a PKC inhibitor, and epsilonV1-2, a PKCepsilon inhibitor. Finally, the protective effect was blocked by 5-hydroxydecanoate, which blocks the opening of mitoK(ATP) channels. Resistin-induced Akt phosphorylation in HL-1 cells was inhibited by wortmannin and triciribine. Resistin also induced PKCepsilon phosphorylation, which was blocked by triciribine. These studies demonstrate that resistin's cardioprotective effect is mediated by PI3K/Akt/PKC dependent pathways. In addition to cardiomyocytes, resistin also induced Akt phosphorylation in endothelial cells and smooth muscle cells, suggesting that resistin receptors are present in these cells. The effect of resistin on apoptosis was assessed in hearts subjected to 30 min of ischemia and 3 h of reperfusion. There were significantly fewer in situ oligo ligation-positive myocyte nuclei in mice treated with resistin. Our results show that resistin can dramatically reduce apoptosis and infarct size, thus protecting the heart against I/R injury.

Serum resistin level is associated with insulin sensitivity in Japanese patients with type 2 diabetes mellitus

Impaired insulin secretion and decreased insulin sensitivity are the main pathophysiologic features responsible for development of hyperglycemia in type 2 diabetes mellitus. Insulin resistance is often associated with increased adipose tissue mass. To examine which variables influence insulin sensitivity, we compared metabolic parameters, serum resistin, leptin, and adiponectin concentrations to the insulin sensitivity, obtained by frequently sampled intravenous glucose tolerance test using the minimal model analysis, in 113 Japanese patients with type 2 diabetes mellitus. Duration of diabetes, fasting plasma glucose, fasting insulin, homeostasis model assessment of insulin resistance index, and serum resistin concentration were significantly higher in the insulin-resistant subgroup compared with the insulin-sensitive subgroup and correlated with insulin sensitivity. Stepwise regression analysis also identified these parameters as independent regulators of insulin sensitivity. The present study reconfirmed that fasting insulin level or homeostasis model assessment of insulin resistance would be a surrogate measure of insulin resistance and demonstrated that insulin resistance increases progressively after the onset of overt diabetes and that the serum resistin level is associated with insulin sensitivity, suggesting that resistin plays an important role in the development of insulin resistance in Japanese patients with type 2 diabetes mellitus.

High fat intake induces a population of adipocytes to co-express TLR2 and TNFα in mice with insulin resistance

Cytokine production in fat tissue plays a key role in insulin resistance. The aim of study is to know the phenotypic changes of adipocytes with high fat-induced insulin resistance. High fat intake induced the expression of tumor necrosis factor alpha (TNFalpha) in visceral fat tissue as well as development of insulin resistance. Analysis of the gene expression profiles in adipocytes showed that high fat intake induced the expression of toll-like receptor 2 (TLR2) in addition to TNFalpha. Flow cytometry analysis revealed the presence of adipocytes co-expressing TLR2 and TNFalpha (TLR2/TNFalpha-adipocytes), and the number of TLR2/TNFalpha-adipocytes in visceral fat tissues was increased by high fat intake compared to that in subcutaneous fat tissues. Free fatty acids increased TNFalpha expression in 3T3-L1 adipocytes through TLR2 signals. These results indicate that TLR2/TNFalpha-adipocytes possibly cause the induction of TNFalpha expression in visceral fat tissues, being associated with the development of high fat-induced insulin resistance.

Resistin gene polymorphisms and progression of glycaemia in southern Chinese: a 5-year prospective study

OBJECTIVE

Human resistin gene (RETN) polymorphisms have been found to be associated with type 2 diabetes (T2DM), insulin resistance and/or obesity. We evaluated, in a 5-year prospective study, whether RETN polymorphisms could predict the progression of glycaemia in southern Chinese.

DESIGN AND PATIENTS

We conducted a systematic search for variants in RETN in 70 southern Chinese subjects. This was followed by the genotyping in 624 unrelated nondiabetic subjects of two polymorphisms, -420C-->G and +62G-->A, previously reported in cross-sectional studies to be associated with T2DM in Asians, to examine their relationship with the progression of glycaemia in this cohort.

RESULTS

We identified 15 polymorphisms, including 2 novel but rare polymorphisms (-319G-->A and +63G-->C). Compared to subjects with the CC genotype, -420GG subjects had higher 2-h glucose (7.7 +/- 1.8 vs. 7.2 +/- 2.0 mmol/l, P = 0.011) and insulin (101.6 +/- 69.5 vs. 79.8 +/- 59.5 mU/l, P = 0.021) during an oral glucose tolerance test. Carriers of the +62A allele had higher body mass indices (25.3 +/- 4.0 vs. 24.5 +/- 3.6 kg/m(2) in GG, P = 0.02). The presence of the allele -420G (OR 2.15, 95% CI 1.28-3.60, P = 0.004) or +62A (OR1.86, 95% CI 1.08-3.21, P = 0.025) predicted the progression of glycaemia at Year 5, after adjustment for sex, age or body mass index. The haplotype G-A also conferred a higher risk of progression in glycaemia (P = 0.002).

CONCLUSION

Our study would support the role of the resistin gene in obesity, insulin resistance and progression of glycaemia in southern Chinese.

Effect of PPARalpha activation of macrophages on the secretion of inflammatory cytokines in cultured adipocytes

The relationship between adipocytes and infiltrated macrophages in fat tissue is important for the pathogenesis of insulin resistance through the activation of cytokines. Peroxisome proliferator-activated receptors (PPARs) play a role in the regulation of cytokine secretion in these cells. We studied the effect of the PPARalpha activation of macrophages on the modulation of the tumor necrosis factor alpha (TNFalpha) expression in adipocytes using a cell culture system. A conditioned medium of lipopolysaccharide (LPS)-stimulated RAW264.7 cells, a macrophage cell line, induced the level of TNFalpha mRNA in 3T3-L1 adipocytes. This effect was inhibited by the addition of neutralizing antibody against interleukin 6 (IL-6) in the conditioned medium or the preincubation of RAW264.7 cells with a specific PPARalpha agonist, K-111 (2,2-dichloro-12-(4-chlorophenyl)dodecanoic acid). K-111 reduced both the IL-6 production and mRNA expression in RAW264.7 cells, and its effect was stronger than that of rosiglitazone, a PPARgamma agonist. The activation of the stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) pathway and nuclear factor kappa B (NF-kappaB) subunits of p65 was significantly inhibited by K-111. The blocking of IL-6 production through the SAPK/JNK pathway or by transfection with siRNA specific for IL-6 abolished the inhibitory effect of K-111 on the TNFalpha expression in the 3T3-L1 adipocytes. As a result, the IL-6 produced by RAW264.7 cells is an inducer of TNFalpha expression in 3T3-L1 adipocytes, and the IL-6 secretion is inhibited by the activation of PPARalpha. The PPARalpha activators may suppress the pathogenetical secretion of TNFalpha in the adipocytes through the functional modulation of the infiltrated macrophages.

Resistin induces insulin resistance in pancreatic islets to impair glucose-induced insulin release

An adipokine resistin, a small cysteine-rich protein, is one of the major risk factors of insulin resistance. In the present study, transiently resistin-expressing mice using adenovirus method showed an impaired glucose tolerance due to insulin resistance. We found that resistin-expressing mice exhibited impaired insulin secretory response to glucose. In addition, in vitro treatment with resistin for 1 day induced insulin resistance in pancreatic islets and impaired glucose-stimulated insulin secretion by elevating insulin release at basal glucose (2.8 mM) and suppressing insulin release at stimulatory glucose (8.3 mM). In addition, resistin inhibited insulin-induced phosphorylation of Akt in islets as well as other insulin target organs. Furthermore, resistin induced SOCS-3 expression in beta-cells. In conclusion, resistin induces insulin resistance in islet beta-cells at least partly via induction of SOCS-3 expression and reduction of Akt phosphorylation and impairs glucose-induced insulin secretion.

Increased matrix metalloproteinase-3 mRNA expression in visceral fat in mice implanted with cultured preadipocytes

Using preadipocyte implantation methods, we recently demonstrated that adipocytes in the visceral area change their function, as the expression of tumor necrosis factor-alpha (TNF-alpha) increases, thereby causing insulin resistance. In order to clarify the mechanism for changes in the function of adipocytes in visceral area, we examined the mRNA expression profiles in visceral fat tissue specimens. Four weeks after cell implantation, we performed a microarray analysis using the RNA of fat tissue specimens implanted either with 3T3-L1 cells or PBS alone. Sixty-three genes were thus isolated and the expression of matrix metalloproteinase-3 (MMP-3) mRNA was found to dramatically increase in the fat tissue specimens. The neutralization of MMP-3 protein inhibited adipogenesis and the free fatty acid-induced TNF-alpha secretion in 3T3-L1 adipocytes. These results suggest a potential role of MMP-3, which promotes the TNF-alpha secretion, thus contributing to the disturbance of the functions in the adipocytes which accumulate in the visceral area.

Activation of diacylglycerol O-acyltransferase 1 gene results in increased tumor necrosis factor-alpha gene expression in 3T3-L1 adipocytes

The tumor necrosis factor-alpha (TNF-alpha) expression has been reported to be largely dependent on the size of adipocytes. We herein investigated the gene regulation of diacylglycerol O-acyltransferase (DGAT) in order to clarify the mechanism of TNF-alpha expression induced in large adipocytes. 3T3-L1 cells were cultured in the presence of 5 mM or 25 mM glucose to generate adipocytes from which the triglyceride content differs. The expression of TNF-alpha, DGAT1, and DGAT2 were upregulated in adipocytes cultured with 25 mM glucose. Furthermore, knockdown of DGAT1 gene significantly inhibited the TNF-alpha expression. Finally, the DGAT1 expression levels were closely related to the TNF-alpha level in 3T3-L1 adipocytes.

Up-regulation of PTEN (phosphatase and tensin homolog deleted on chromosome ten) mediates p38 MAPK stress signal-induced inhibition of insulin signaling. A cross-talk between stress signaling and insulin signaling in resistin-treated human endothelial cells

The key feature of metabolic syndrome, a cluster of metabolic and cardiovascular disorders, is systemic insulin resistance, which is associated with dysregulated endothelial nitric-oxide synthase (eNOS). Stress signaling induced by inflammation can inhibit insulin signaling. However, molecular mechanisms for the cross-talk between stress signaling and insulin resistance are only partially understood. Resistin, an adipokine/cytokine, is involved in inflammatory processes that could lead to insulin resistance status and vascular diseases. In the current study, we observed that resistin inhibited insulin signaling and eNOS activation in endothelial cells. Up-regulation of PTEN (phosphatase and tensin homolog deleted on chromosome ten) expression by resistin may mediate the inhibitory effects. Activated stress signaling p38 MAPK, but not JNK, is involved in PTEN up-regulation. We further found that p38 target transcriptional factor activating transcription factor-2 (ATF-2) bound to ATF sites in the PTEN promoter. The phosphorylation/activation of ATF-2 and its binding to PTEN promoter were increased by resistin treatment. In summary, up-regulation of PTEN is involved in the inhibitory effects of resistin on insulin signaling and eNOS activation in endothelial cells. Resistin induces PTEN expression by activating stress signaling p38 pathway, which may activate target transcription factor ATF-2, which in turn induces PTEN expression. Our findings suggest that resistin-mediated inhibition of insulin signaling and eNOS activation may contribute to cardiovascular diseases.

Resistin as a putative modulator of insulin action in the daily feeding/fasting rhythm

Resistin and adiponectin are adipokines with postulated opposite functions. Resistin has been related with insulin resistance in obesity, while adiponectin could be associated to higher insulin sensitivity. We have determined whether the production of these two adipokines during the day is related to the feeding rhythm in rats. Resistin mRNA levels in adipose tissue correlated positively with the gastric contents and serum insulin concentration, showing higher levels during the dark phase (period of the highest food intake), especially in the mesenteric depot, while levels decreased during the light phase. The diurnal pattern of resistin expression was not directly reflected in the circulating levels, but it showed a 6-h delay and correlated negatively with the gastric contents and serum insulin. Adiponectin expression followed an opposite pattern, not apparently related to feeding or insulin release, and not translated into changes in circulating levels. Moreover, considering that insulin stimulates resistin expression and that circulating resistin follows a contrary circadian pattern in comparison to insulin, resistin, apart from its role in the increased insulin resistance associated to obesity, could also act as a putative modulator of insulin in the daily feeding/fasting rhythm through a negative feedback regulation of its action.

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

AIMS/HYPOTHESIS

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

METHODS

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

RESULTS

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

CONCLUSIONS/INTERPRETATION

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

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

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

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

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