The mechanisms by which both heterozygous peroxisome proliferator-activated receptor gamma (PPARgamma) deficiency and PPARgamma agonist improve insulin resistance

Enterostatin decreases postprandial pancreatic UCP2 mRNA levels and increases plasma insulin and amylin

This study investigated the chronic effect of enterostatin on body weight and some of the associated changes in postprandial metabolism. Rats were adapted to 6 h of food access/day and a choice of low-fat and high-fat (HF) food and then given enterostatin or vehicle by an intraperitoneally implanted minipump delivering 160 nmol enterostatin/h continuously over a 5-day infusion period. Enterostatin resulted in a slight but significant reduction of HF intake and body weight. After the last 6-h food access period, enterostatin-treated animals had lower plasma triglyceride and free fatty acid but higher plasma glucose and lactate levels than control animals. Enterostatin infusion resulted in increased uncoupling protein-2 (UCP2) expression in various tissues, including epididymal fat and liver. UCP2 was reduced in the pancreas of enterostatin-treated animals, and this was associated with increased plasma levels of insulin and amylin. Whether these two hormones are involved in the observed decreased food intake due to enterostatin remains to be determined. As lipid metabolism appeared to be altered by enterostatin, we measured peroxisome proliferator-activated receptor (PPAR) expression in tissues and observed that PPARalpha, -beta, -gamma1, and -gamma2 expression were modified by enterostatin in epididymal fat, pancreas, and liver. This further links altered lipid metabolism with body weight loss. Our data suggest that alterations in UCP2 and PPARgamma2 play a role in the control of insulin and amylin release from the pancreas. This implies that enterostatin changes lipid and carbohydrate metabolic pathways in addition to its effects on food intake and energy expenditure.

PPAR gamma, 10 years later

It is now over 10 years since the discovery of peroxisome proliferator activated receptor gamma (PPAR gamma) and its unique role in adipogenesis. The subsequent identification of PPAR gamma as the target of insulin sensitizing drugs certified this ligand-regulated transcription factor as an exciting link between adipocyte biology and peripheral insulin resistance. Here, I summarize the great progress that has been made over the past decade in elucidating the biology of PPAR gamma and its role in adipogenesis and glucose metabolism. Prospects for future research leading to new therapies for obesity and diabetes are also discussed.

Transgenic and Knockout Mice in Diabetes Research: Novel Insights into Pathophysiology, Limitations, and Perspectives

Insulin resistance and type 2 diabetes are serious public health threats. Although enormous research efforts have been focused on the pathogenesis of these diseases, the underlying mechanisms remain only partly understood. Here we review mouse phenotypes resulting from inactivation of molecules responsible for the control of glucose metabolism that have led to novel insights into insulin action and the development of insulin resistance. In addition, more sophisticated strategies to manipulate genes in mice in the future are presented.

The link between nutritional status and insulin sensitivity is dependent on the adipocyte-specific peroxisome proliferator-activated receptor-gamma2 isoform

The nuclear receptor peroxisome proliferator-activated receptor-gamma (PPARgamma) is critically required for adipogenesis. PPARgamma exists as two isoforms, gamma1 and gamma2. PPARgamma2 is the more potent adipogenic isoform in vitro and is normally restricted to adipose tissues, where it is regulated more by nutritional state than PPARgamma1. To elucidate the relevance of the PPARgamma2 in vivo, we generated a mouse model in which the PPARgamma2 isoform was specifically disrupted. Despite similar weight, body composition, food intake, energy expenditure, and adipose tissue morphology, male mice lacking the gamma2 isoform were more insulin resistant than wild-type animals when fed a regular diet. These results indicate that insulin resistance associated with ablation of PPARgamma2 is not the result of lipodystrophy and suggests a specific role for PPARgamma2 in maintaining insulin sensitivity independently of its effects on adipogenesis. Furthermore, PPARgamma2 knockout mice fed a high-fat diet did not become more insulin resistant than those on a normal diet, despite a marked increase in their mean adipocyte cell size. These findings suggest that PPARgamma2 is required for the maintenance of normal insulin sensitivity in mice but also raises the intriguing notion that PPARgamma2 may be necessary for the adverse effects of a high-fat diet on carbohydrate metabolism.

Peroxisome proliferator-activated receptor-gamma2 P12A polymorphism and risk of coronary heart disease in US men and women

OBJECTIVE

Activation of the peroxisome proliferator-activated receptor-gamma (PPARgamma) improves insulin sensitivity and exerts antiatherogenic effects. A common alanine for proline substitution at codon 12 in the PPARG2 gene is related to lower receptor activity. Studies suggest that the A12 allele is associated with reduced risk of type 2 diabetes; however, data on the risk of coronary heart disease (CHD) are scarce and controversial.

METHODS AND RESULTS

We examined the relationship between PPARG2 P12A and CHD risk in women (Nurses' Health Study) and men (Health Professionals Follow-Up Study) in nested case control settings. Among participants free of cardiovascular disease at baseline, 249 women and 266 men developed nonfatal myocardial infarction (MI) or fatal CHD during 8 and 6 years of follow-up, respectively. Using risk-set sampling, controls were selected 2:1 matched on age, smoking, and date of blood draw. The relative risk (RR) of nonfatal MI or fatal CHD of carriers compared with noncarriers of the A12 allele was 1.17 (95% CI, 0.82 to 1.68) among women and 1.44 (95% CI, 1.00 to 2.07) among men (pooled RR, 1.30 [95% CI, 1.00 to 1.67]). We found a significantly increased risk associated with the A12 allele among individuals with a body mass index > or =25 kg/m2 (women: RR, 1.88; 95% CI, 1.01 to 3.50; men: RR, 1.55; 95% CI, 0.92 to 2.60; pooled: RR, 1.68; 95% CI, 1.13 to 2.50) but not among those <25 kg/m2 (pooled RR, 0.86; 95% CI, 0.37 to 1.97; P heterogeneity overweight versus nonoverweight 0.16).

CONCLUSIONS

These data do not support the hypothesis that the A12 allele is associated with a decreased risk of CHD. The potential interaction between PPARG2 P12A, overweight, and increased CHD risk needs further evaluation.

Failure of fat cell proliferation, mitochondrial function and fat oxidation results in ectopic fat storage, insulin resistance and type II diabetes mellitus

BACKGROUND

It is widely accepted that increasing adiposity is associated with insulin resistance and increased risk of type II diabetes. The predominant paradigm used to explain this link is the portal/visceral hypothesis. This hypothesis proposes that increased adiposity, particularly in the visceral depots, leads to increased free-fatty acid flux and inhibition of insulin-action via Randle's effect in insulin-sensitive tissues.

OBJECTIVES

In this review, limitations of this paradigm will be discussed and two other paradigms that may explain established links between adiposity and insulin resistance/diabetes will be presented. (A) Ectopic fat storage syndrome. Three lines of evidence support this concept. Firstly, failure to develop adequate adipose tissue mass (also known as 'lipodystrophy') results in severe insulin resistance and diabetes. This is thought to be the result of ectopic storage of lipid into liver, skeletal muscle and the pancreatic insulin-secreting beta cell. Secondly, most obese patients also shunt lipid into the skeletal muscle, the liver and probably the beta cell. The importance of this finding is exemplified by several studies demonstrating that the degree of lipid infiltration into skeletal muscle and liver highly correlates with insulin resistance. Thirdly, increased fat cell size is highly associated with insulin resistance and the development of diabetes. Increased fat cell size may represent the failure of the adipose tissue mass to expand and therefore to accommodate an increased energy influx. Taken together, these observations support the 'acquired lipodystrophy' hypothesis as a link between adiposity and insulin resistance. Ectopic fat deposition is therefore the result of additive or synergistic effects including increased dietary intake, decreased fat oxidation and impaired adipogenesis. (B) Endocrine paradigm. This concept was developed in parallel with the 'ectopic fat storage syndrome' hypothesis. Adipose tissue secretes a variety of endocrine hormones such as leptin, interleukin-6, angiotensin II, adiponectin and resistin. From this viewpoint, adipose tissue plays a critical role as an endocrine gland, secreting numerous factors with potent effects on the metabolism of distant tissues.

CONCLUSIONS

The novel paradigms of ectopic fat and fat cell as an endocrine organ probably will constitute a new framework for the study of the links between our obesigenic environment and the risk of developing diabetes.

Adiponectin and adiponectin receptors

Metabolic syndrome is thought to result from obesity and obesity-linked insulin resistance. Obesity in adulthood is characterized by adipocyte hypertrophy. Adipose tissue participates in the regulation of energy homeostasis as an important endocrine organ that secretes a number of biologically active "adipokines."Heterozygous peroxisome proliferator-activated receptor-gamma knockout mice were protected from high-fat diet induced obesity, adipocyte hypertrophy, and insulin resistance. Systematic gene profiling analysis of these mice revealed that adiponectin/Acrp30 was overexpressed. Functional analyses including generation of adiponectin transgenic or knockout mice have revealed that adiponectin serves as an insulin-sensitizing adipokine. In fact, obesity-linked down-regulation of adiponectin was a mechanism whereby obesity could cause insulin resistance and diabetes. Recently, we have cloned adiponectin receptors in the skeletal muscle (AdipoR1) and liver (AdipoR2), which appear to comprise a novel cell-surface receptor family. We showed that AdipoR1 and AdipoR2 serve as receptors for globular and full-length adiponectin and mediate increased AMP-activated protein kinase, peroxisome proliferator-activated receptor-alpha ligand activities, and glucose uptake and fatty-acid oxidation by adiponectin. Obesity decreased expression levels of AdipoR1/R2, thereby reducing adiponectin sensitivity, which finally leads to insulin resistance, the so-called "vicious cycle." Most recently, we showed that osmotin, which is a ligand for the yeast homolog of AdipoR (PHO36), activated AMPK via AdipoR in C2C12 myocytes. This may facilitate efficient development of adiponectin receptor agonists. Adiponectin receptor agonists and adiponectin sensitizers should serve as versatile treatment strategies for obesity-linked diseases such as diabetes and metabolic syndrome.

Deletion of PPARgamma in adipose tissues of mice protects against high fat diet-induced obesity and insulin resistance

Peroxisome proliferator-activated receptor gamma (PPARgamma) plays a crucial role in adipocyte differentiation, glucose metabolism, and other physiological processes. To further explore the role of PPARgamma in adipose tissues, we used a Cre/loxP strategy to generate adipose-specific PPARgamma knockout mice. These animals exhibited marked abnormalities in the formation and function of both brown and white adipose tissues. When fed a high-fat diet, adipose-specific PPARgamma knockout mice displayed diminished weight gain despite hyperphagia, had diminished serum concentrations of both leptin and adiponectin, and did not develop glucose intolerance or insulin resistance. Characterization of in vivo glucose dynamics pointed to improved hepatic glucose metabolism as the basis for preventing high-fat diet-induced insulin resistance. Our findings further illustrate the essential role for PPARgamma in the development of adipose tissues and suggest that a compensatory induction of hepatic PPARgamma may stimulate an increase in glucose disposal by the liver.

Regulation of adipocyte differentiation and function by polyunsaturated fatty acids

A diet enriched in PUFAs, in particular of the n-3 family, decreases adipose tissue mass and suppresses development of obesity in rodents. Although several nuclear hormone receptors are identified as PUFA targets, the precise molecular mechanisms underlying the effects of PUFAs still remain to be elucidated. Here we review research aimed at elucidating molecular mechanisms governing the effects of PUFAs on the differentiation and function of white fat cells. This review focuses on dietary PUFAs as signaling molecules, with special emphasis on agonistic and antagonistic effects on transcription factors currently implicated as key players in adipocyte differentiation and function, including peroxisome proliferator activated receptors (PPARs) (alpha, beta and gamma), sterol regulatory element binding proteins (SREBPs) and liver X receptors (LXRs). We review evidence that dietary n-3 PUFAs decrease adipose tissue mass and suppress the development of obesity in rodents by targeting a set of key regulatory transcription factors involved in both adipogensis and lipid homeostasis in mature adipocytes. The same set of factors are targeted by PUFAs of the n-6 family, but the cellular/physiological responses are dependent on the experimental setting as n-6 PUFAs may exert either an anti- or a proadipogenic effect. Feeding status and hormonal background may therefore be of particular importance in determining the physiological effects of PUFAs of the n-6 family.

Implication of the Pro12Ala polymorphism of the PPAR-gamma 2 gene in type 2 diabetes and obesity in the French population

Background

The Pro12Ala Single Nucleotide Polymorphism (SNP) of the Peroxisome Proliferator-Activated Receptor gamma 2 (PPAR-gamma 2) has been associated with insulin resistance and type 2 diabetes (T2D) and also inconsistently with obesity. The aim of this study was to evaluate the impact of this SNP with regards to T2D and childhood and adult obesity in the French Caucasian population.

Methods

We conducted three independent case/control studies encompassing 2126 cases and 1124 controls.

Results

We found a significant association between PPAR-gamma 2 Pro12Ala SNP and T2D (p = 0.04, OR = 1.37), which was stronger when the T2D cohort was stratified according to the obesity status (p = 0.03, OR = 1.81 in obese T2D subjects). In contrast, there was no association between the Pro12Ala SNP and childhood and adulthood obesity. In normal glucose tolerant obese adults (but not in lean subjects), the Pro12 allele was associated with a significant increase in fasting insulin levels (p = 0.01), and in insulin resistance estimated by the Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) (p = 0.003), after adjustment for age, gender and BMI. We didn't detect evidence for an interaction effect between the Pro12Ala SNP and the obesity status with respect to the HOMA-IR index in normal glucose tolerant children, but we found a borderline interaction (p = 0.06) in normal glucose tolerant adults.

Conclusion

Our results showed that the Pro12Ala polymorphism is not associated with childhood or adult obesity in the French Caucasian population. In contrast, we confirm a contribution of the PPAR-gamma 2 Pro12 allele in the genetic risk forT2D, especially in obese subjects, where this allele worsens insulin resistanceand increases fasting insulin levels.

Sustained rise in triacylglycerol synthesis and increased epididymal fat mass when rats cease voluntary wheel running

Four-week-old, Fischer-Brown Norway F1-generation male rats were given access to voluntary running wheels for 21 days, and then the wheels were locked for 5 (WL5), 10 (WL10), 29 (WL29), or 53 (WL53) hours. Two other groups (SED5 and SED10) had no access to voluntary running wheels and were killed at the same time as WL5 and WL10, respectively. Absolute and relative epididymal fat mass, mean cell volume, and amount of lipid per cell increased in WL53 relative to all other groups, with no change in cell number. C/EBPalpha protein levels in epididymal fat were 30% greater in SED5 than in WL5. The rate of triacylglycerol synthesis in epididymal fat was 4.2-fold greater in SED5 than in WL5, increased 14-fold between WLS and WL10, and was 79% lower in SED10 than in WL10. Triacylglycerol synthesis remained at this elevated level (at least 3.5-fold greater than SED5) through WL53. Thus, the rapid increase in epididymal fat mass with the cessation of voluntary wheel running is associated with a prolonged overshoot in epididymal fat triacylglycerol synthesis. Moreover, rats without running wheels had a 9.4% lower body mass after 21 days than those with running wheels. The individual mass of seven different muscles from the hindlimb, upper forelimb, and back were each lower in animals without running wheels, suggesting that physical activity in rapidly growing rats may be requisite for optimal muscle development.

Monogenic syndromes of abnormal glucose homeostasis: clinical review and relevance to the understanding of the pathology of insulin resistance and beta cell failure

Type 2 diabetes mellitus is caused by a combination of insulin resistance and beta cell failure. The polygenic nature of type 2 diabetes has made it difficult to study. Although many candidate genes for this condition have been suggested, in most cases association studies have been equivocal. Monogenic forms of diabetes have now been studied extensively, and the genetic basis of many of these syndromes has been elucidated, leading to greater understanding of the functions of the genes involved. Common variations in the genes causing monogenic disorders have been associated with susceptibility to type 2 diabetes in several populations and explain some of the linkage seen in genome-wide scans. Monogenic disorders are also helpful in understanding both normal and disordered glucose and insulin metabolism. Three main areas of defect contribute to diabetes: defects in insulin signalling leading to insulin resistance; defects of insulin secretion leading to hypoinsulinaemia; and apoptosis leading to decreased beta cell mass. These three pathological pathways are reviewed, focusing on rare genetic syndromes which have diabetes as a prominent feature. Apoptosis seems to be a final common pathway in both type 1 and type 2 diabetes. Study of rare forms of diabetes may help ion determining new therapeutic targets to preserve or increase beta cell mass and function.

Effect of the peroxisome proliferator-activated receptor-gamma C161T polymorphism on lipid profile in Brazilian patients with Type 2 diabetes mellitus

AIM

The aim of the present study was to examine the effects of the C161T polymorphism of the peroxisome proliferator-activated receptor gamma (PPARgamma) gene in Brazilian subjects with Type 2 diabetes mellitus (T2DM) and controls residing in Sao Paulo City, Brazil.

METHODS

Genomic DNA was obtained from 207 patients with T2DM and 170 unrelated normoglycemic individuals (CG). Anthropometric data included: body mass index, waist, hip, waist-to-hip ratio; biochemical parameters: fasting plasma glucose, total cholesterol, HDL- and LDL-cholesterol, triglycerides, glycated hemoglobin and insulin. Systolic and diastolic blood pressure was also measured. Screening for mutations in the entire coding region of the PPARgamma gene was carried out by PCR, single strand conformational polymorphism analysis (SSCP) and sequencing. C161T polymorphism was analyzed by PCR-RFLP.

RESULTS

The C161T polymorphism was the only variant found in exon 6 of the PPARgamma gene. The frequency of the 161T allele in T2DM (0.10) was similar to that found in CG (0.07, p=0.210). Serum triglycerides (p=0.040), VLDL-cholesterol (p=0.040) and Atherogenic Index of Plasma (AIP; p=0.003) were significantly lower in 161T allele carriers than non-carriers in women of the T2DM group.

CONCLUSIONS

Our results show that the C161T polymorphism in the PPARgamma gene is not associated with variables related to T2DM or insulin resistance in the Brazilian population. However, a reduction of serum triglycerides and AIP was observed in women with 161T allele of the C161T polymorphism of the PPARgamma gene.

Gene expression profile of isolated rat adipocytes treated with anthocyanins

Adipocyte dysfunction is strongly associated with the development of obesity and insulin resistance. It is accepted that the regulation of adipocytokine secretion or the adipocyte specific gene expression is one of the most important targets for the prevention of obesity and amelioration of insulin sensitivity. Recently, we demonstrated that anthocyanins, which are pigments widespread in the plant kingdom, have the potency of anti-obesity in mice and the enhancement adipocytokine secretion and adipocyte gene expression in adipocytes. In this study, we have shown for the first time the gene expression profile in isolated rat adipocytes treated with anthocyanins (cyanidin 3-glucoside; C3G or cyanidin; Cy). The rat adipocytes were treated with 100 muM C3G, Cy or vehicle for 24 h. The total RNA from the adipocytes was isolated and carried out GeneChip microarray analysis. A total of 633 or 427 genes was up-regulated (>1.5-fold) by the treatment of adipocytes with C3G or Cy, respectively. The up-regulated genes include lipid metabolism and signal transduction-related genes, however, the altered genes were partly different between the C3G- and Cy-treated groups. Based on the gene expression profile, we demonstrated the up-regulation of hormone sensitive lipase and enhancement of the lipolytic activity by the treatment of adipocytes with C3G or Cy. These data have provided an overview of the gene expression profiles in adipocytes treated with anthocyanins and identified new responsive genes with potentially important functions in adipocytes related with obesity and diabetes that merit further investigation.

Influence of Rosiglitazone on Flow-Mediated Dilation and Other Markers of Cardiovascular Risk in HIV-Infected Patients with Lipoatrophy

Background

Antiretroviral therapy for HIV infection is commonly complicated by lipoatrophy, insulin resistance and dyslipidaemia. In HIV-uninfected adults with insulin resistance or type 2 diabetes, thiazolidinediones can lower blood pressure and improve both insulin sensitivity and endothelial function. This study sought to investigate the effects of rosiglitazone on endothelial function and other markers of cardiovascular risk in patients with HIV-related lipoatrophy.

Methods

HIV-infected, lipoatrophic adults receiving anti-retroviral therapy were randomized to receive either rosiglitazone 4 mg or matched placebo, twice daily. Percentage flow-mediated forearm arterial dilation (FMD%) was measured at weeks 0, 12, 24 and 48, together with other markers of vascular risk (blood pressure, lipids, glycaemic parameters, adiponectin and leptin).

Results

Out of 64 enrolled adults, 44 (69%) attended all visits (23 rosiglitazone, 21 placebo). Relative to placebo, at week 48, rosiglitazone decreased systolic blood pressure (8 mmHg, P=0.03), insulin (3 μIU/ml, P=0.02), insulin resistance ( P=0.03) and leptin (0.6 ng/ml, P=0.02), whilst adiponectin was increased (3.3 μg/ml, P<0.0001). However, rosiglitazone increased total cholesterol (49.1 mg/dl, P=0.001), low-density lipoprotein cholesterol (23.5 mg/dl, P=0.01) and triglycerides (146 mg/dl, P=0.06). Mean baseline FMD% for the entire cohort was moderately impaired (4.5%). Compared with baseline, mean on-treatment FMD% increased by 0.8% with rosiglitazone and decreased by 0.3% with placebo (mean difference 1.1%, 95% CI -0.2 to 2.5, P=0.09).

Conclusions

Rosiglitazone has minimal effect on flow-mediated dilation in HIV-infected lipoatrophic adults. However, despite worsening of the lipid profile, the overall effect of rosiglitazone on the cardiovascular risk profile in these subjects was positive.

Nuclear Receptors as Targets for Drug Development: Molecular Mechanisms for Regulation of Obesity and Insulin Resistance by Peroxisome Proliferator-Activated Receptor γ, CREB-Binding Protein, and Adiponectin

Obesity is defined as increased mass of adipose tissue, conferring a higher risk of cardiovascular and metabolic disorders such as diabetes, hyperlipidemia, and coronary heart disease. To investigate the role of transcriptional factors, which are involved in adipocytes differentiation and adiposity, we have generated peroxisome proliferator-activated receptor (PPAR) gamma or CREB-binding protein (CBP)-deficient mice by gene targeting. Heterozygous PPARgamma-deficient mice were protected from the development of insulin resistance due to adipocyte hypertrophy under a high-fat diet. Heterozygous CBP-deficient mice showed increased insulin sensitivity and were completely protected from body weight gain induced by a high-fat diet. PPARgamma or CBP deficiency results in increased effects of hormones such as adiponectin and leptin. Adiponectin was decreased in obesity and lipoatrophy, and replenishment of adiponectin ameliorated insulin resistance. Moreover, adiponectin-deficient mice showed insulin resistance and atherogenic phenotype. Finally, cDNA encoding adiponectin receptors (AdipoR1/R2) have been identified by expression cloning. The expression of AdipoR1/R2 appears to be inversely regulated by insulin in physiological and pathophysiological states such as fasting/refeeding, insulin deficiency, and hyperinsulinemia models, and it is correlated with adiponectin sensitivity. These results facilitate the understanding of molecular mechanisms of adiponectin actions and obesity-linked diseases such as diabetes and atherosclerosis and propose the molecular targets for anti-diabetic and anti-atherogenic drugs.

Effect of pioglitazone on body composition and energy expenditure: a randomized controlled trial

BACKGROUND

Several clinical studies have demonstrated that body weight increases after treatment with thiazolidinediones (TZDs). Prior studies have demonstrated an increase in insulin-stimulated lipid storage in adipose tissue. Some, but not all, studies demonstrate reductions in visceral adipose tissue. Changes in body weight are the result of changes in energy intake, energy expenditure, or both.

OBJECTIVES

Based on these findings, the primary aim of this study was to evaluate the effect of TZDs on visceral, subcutaneous, and total body fat. Secondary aims were to determine the effects of pioglitazone on (a) energy expenditure, (b) hunger and satiety, (c) blood lipids, and (d) the role of insulinemia/sulfonylurea usage on weight gain in patients with type 2 diabetes.

SUBJECTS AND METHODS

We performed a randomized, double-blind, placebo-controlled trial in 48 men and women with type 2 diabetes who had not previously received treatment with TZDs. Patients were treated for 24 weeks with 45 mg/d of pioglitazone or a matching placebo. Body composition was measured by dual-energy x-ray absorptiometry. Visceral and subcutaneous fat were measured by computed tomography. Resting metabolic rate and thermogenic response to a test meal were measured by indirect calorimetry before and after a standardized meal. Hunger and satiety were measured with visual analog scales before and after the same test meal. Blood was collected for the measurement of fasting glucose and insulin levels, hemoglobin A 1c levels, and lipid content.

RESULTS

Pioglitazone treatment resulted in a decrease in hemoglobin A(1c) level by 0.96 +/- 1.1% vs 0.11 +/- 0.8% in the placebo group (P < .005). Body weight and fat increased steadily in the patients treated with pioglitazone during the 6 months of the study (+3.9 +/- 3.1 kg at 6 months in pioglitazone-treated patients vs -0.8 +/- 3.4 kg in the placebo-treated patients). Subcutaneous fat in the trunk, arms, and legs were all increased in the pioglitazone-treated group. Visceral fat did not change significantly in either group. Neither resting metabolic rate nor the thermogenic responses to a meal were altered by pioglitazone. Subjective measures of hunger (visual analog scale) did not change with pioglitazone treatment. Triglycerides fell in the pioglitazone-treated group (-58.5 +/- 124 mg/dL, P < .003). Neither the prior use of sulfonylureas nor the level of insulinemia before treatment was a predictor of weight or fat change.

CONCLUSION

Pioglitazone increased subcutaneous body fat, but not visceral fat. There was no measurable effect on energy expenditure or hunger/satiety. In contrast to the placebo-treated patient with diabetes, weight gain occurs in the face of falling hemoglobin A(1c) and triglyceride levels.

Metabolic syndrome and robustness tradeoffs

The metabolic syndrome is a highly complex breakdown of normal physiology characterized by obesity, insulin resistance, hyperlipidemia, and hypertension. Type 2 diabetes is a major manifestation of this syndrome, although increased risk for cardiovascular disease (CVD) often precedes the onset of frank clinical diabetes. Prevention and cure for this disease constellation is of major importance to world health. Because the metabolic syndrome affects multiple interacting organ systems (i.e., it is a systemic disease), a systems-level analysis of disease evolution is essential for both complete elucidation of its pathophysiology and improved approaches to therapy. The goal of this review is to provide a perspective on systems-level approaches to metabolic syndrome, with particular emphasis on type 2 diabetes. We consider that metabolic syndromes take over inherent dynamics of our body that ensure robustness against unstable food supply and pathogenic infections, and lead to chronic inflammation that ultimately results in CVD. This exemplifies how trade-offs between robustness against common perturbations (unstable food and infections) and fragility against unusual perturbations (high-energy content foods and low-energy utilization lifestyle) is exploited to form chronic diseases. Possible therapeutic approaches that target fragility of emergent robustness of the disease state have been discussed. A detailed molecular interaction map for adipocyte, hepatocyte, skeletal muscle cell, and pancreatic beta-cell cross-talk in the metabolic syndrome can be viewed at http://www.systems-biology.org/001/003.html.

Thiazolidinediones: a review of their mechanisms of insulin sensitization, therapeutic potential, clinical efficacy, and tolerability

The thiazolidinediones (TZDs) rosiglitazone and pioglitazone are newer additions to the antidiabetic armamentarium and are indicated for the treatment of type 2 diabetes mellitus (T2DM) in the United States. The TZDs are peroxisome proliferator-activated receptor-gamma (PPAR-gamma) agonists that provide clinically effective glycemic control and unique pharmacologic effects on multiple risk factors for T2DM-related morbidity, including improvement of insulin sensitivity and endothelial dysfunction, reduction of blood pressure, and amelioration of dyslipidemia. Weight gain and fluid retention occur with TZD therapy, especially when they are administered in higher doses and in combination with insulin. Although fluid retention associated with the use of TZDs is generally mild and reversible, these agents should not be used in patients with New York Heart Association Class III or IV heart failure symptoms. The findings of ongoing, long-term, prospective studies will clarify the role of the TZDs in the treatment of T2DM, particularly in terms of the durability of improvements in glycemic control, insulin sensitivity, pancreatic beta- cell function, and cardiovascular health.

Common polymorphisms of the PPAR-gamma2 (Pro12Ala) and PGC-1alpha (Gly482Ser) genes are associated with the conversion from impaired glucose tolerance to type 2 diabetes in the STOP-NIDDM trial

AIM/HYPOTHESIS

We investigated the effects of the common polymorphisms in the peroxisome proliferator-activated receptor gamma2 (PPAR-gamma2; Pro12Ala) and in PPAR-gamma coactivator 1alpha (PGC-1alpha; Gly482Ser) genes on the conversion from impaired glucose tolerance to type 2 diabetes in participants in the STOP-NIDDM trial. This trial aimed to study the effect of acarbose in the prevention of type 2 diabetes.

METHODS

Genotyping was performed in 770 study subjects whose DNA was available. The Gly482Ser variant in the PGC-1alpha gene was determined with the polymerase chain reaction amplification, Hpa II enzyme digestion, and gel electrophoresis. The Pro12Ala polymorphism of the PPAR-gamma2 gene was determined by the polymerase chain reaction-single-strand conformation polymorphism analysis.

RESULTS

The Pro12Pro genotype of the PPAR-gamma2 gene predicted the conversion to diabetes in women in the acarbose group (odds ratio 2.89, 95% CI 1.20 to 6.96; p=0.018). The 482Ser allele of the PGC-1alpha gene had a significant interaction with the mode of treatment (p=0.012), and in the placebo group the 482Ser allele was associated with a 1.6-fold higher risk for type 2 diabetes compared to the Gly482Gly genotype (95% CI 1.06 to 2.33; p=0.023). Acarbose prevented the development of diabetes independently of the genotype of the PPAR-gamma2 gene, but only the carriers of the 482Ser allele of the PGC-1alpha gene were responsive to acarbose treatment.

CONCLUSION/INTERPRETATION

We conclude that the Pro12Pro genotype of the PPAR-gamma2 gene and the 482Ser allele of the PGC-1alpha gene are associated with the conversion from impaired glucose tolerance to type 2 diabetes in the STOP-NIDDM trial.

A pilot study of vitamin E versus vitamin E and pioglitazone for the treatment of nonalcoholic steatohepatitis

BACKGROUND & AIMS

Insulin resistance and oxidative stress contribute to the pathogenesis of nonalcoholic steatohepatitis (NASH). We conducted a pilot study for the following reasons: (1) to test the hypothesis that a combination of an antioxidant (vitamin E) and an insulin sensitizer (pioglitazone) would be superior to vitamin E alone for the treatment of NASH, and (2) to define the effects of these interventions on insulin-sensitive metabolic functions and correlate the effects with changes in liver histology.

METHODS

A randomized prospective trial was performed to compare the efficacy and safety of vitamin E alone (400 IU/day) vs. vitamin E (400 IU/day) and pioglitazone (30 mg/day) in nondiabetic, noncirrhotic subjects with NASH. Metabolic functions were assessed by a 2-step, hyperinsulinemic (10 and 40 mU/m2/min) euglycemic clamp.

RESULTS

A total of 10 patients were randomized to each arm. Two patients on combination therapy discontinued treatment; one because of pregnancy and the other because of hepatotoxicity. Treatment with vitamin E only produced a significant decrease in steatosis (mean grade, 2.2 vs. 1.4; P < .02). Compared with baseline, combination therapy produced a significant decrease in steatosis (mean, 2.3 vs. 1; P < .002), cytologic ballooning (1.3 vs. 0.2; P < .01), Mallory's hyaline (0.7 vs. 0.2; P < .04), and pericellular fibrosis (1.2 vs. 0.6; P < .03). Although vitamin E had no significant effects, combination therapy produced a significant increase in metabolic clearance of glucose and a decrease in fasting free fatty acid (FFA) and insulin. The decrease in fasting FFA and insulin independently predicted improvement in hepatic steatosis and cytologic ballooning.

CONCLUSIONS

A combination of vitamin E and pioglitazone produces a greater improvement in NASH histology. The improvement in steatosis and cytologic ballooning are related to treatment-associated decreases in fasting FFA and insulin levels.

Weight Loss–Associated Induction of Peroxisome Proliferator–Activated Receptor-α and Peroxisome Proliferator–Activated Receptor-γ Correlate With Reduced Atherosclerosis and Improved Cardiovascular Function in Obese Insulin-Resistant Mice

Background— Weight loss in obese insulin-resistant but not in insulin-sensitive persons reduces coronary heart disease risk. To what extent changes in gene expression are related to atherosclerosis and cardiovascular function is unknown.

Methods and Results— We studied the effect of diet restriction–induced weight loss on gene expression in the adipose tissue, the heart, and the aortic arch and on atherosclerosis and cardiovascular function in mice with combined leptin and LDL-receptor deficiency. Obesity, hypertriglyceridemia, and insulin resistance are associated with hypertension, impaired left ventricular function, and accelerated atherosclerosis in those mice. Compared with lean mice, peroxisome proliferator–activated receptors (PPAR)-α and PPAR-γ expression was downregulated in obese double-knockout mice. Diet restriction caused a 45% weight loss, an upregulation of PPAR-α and PPAR-γ, and a change in the expression of genes regulating glucose transport and insulin sensitivity, lipid metabolism, oxidative stress, and inflammation, most of which are under the transcriptional control of these PPARs. Changes in gene expression were associated with increased insulin sensitivity, decreased hypertriglyceridemia, reduced mean 24-hour blood pressure and heart rate, restored circadian variations of blood pressure and heart rate, increased ejection fraction, and reduced atherosclerosis. PPAR-α and PPAR-γ expression was inversely related to plaque volume and to oxidized LDL content in the plaques.

Conclusions— Induction of PPAR-α and PPAR-γ in adipose tissue, heart, and aortic arch is a key mechanism for reducing atherosclerosis and improving cardiovascular function resulting from weight loss. Improved lipid metabolism and insulin signaling is associated with decreased tissue deposition of oxidized LDL that increases cardiovascular risk in persons with the metabolic syndrome.

Small-molecule insulin mimetic reduces hyperglycemia and obesity in a nongenetic mouse model of type 2 diabetes

Adiposity positively correlates with insulin resistance and is a major risk factor of type 2 diabetes. Administration of exogenous insulin, which acts as an anabolic factor, facilitates adipogenesis. Recently nonpeptidal insulin receptor (IR) activators have been discovered. Here we evaluate the effects of the orally bioavailable small-molecule IR activator (Compound-2) on metabolic abnormalities associated with type 2 diabetes using a nongenetic mouse model in comparison with the effects of a novel non-thiazolidinedione (nTZD) peroxisome proliferator-activated receptor-gamma agonist. Both Compound-2 and nTZD alleviated fasting and postprandial hyperglycemia; accelerated glucose clearance rate; and normalized plasma levels of nonesterified fatty acids, triglycerides, and leptin. Unlike nTZD, which increased body weight gain, and total fat mass, which is a common feature for PPARgamma agonists, Compound-2 prevented body weight gain and hypertrophy of brown, and white adipose tissue depots and the development of hepatic steatosis in the mouse model of type 2 diabetes. The effect of the two compounds on proximal steps in insulin signal transduction pathway was analyzed in tissues. Compound-2 enhanced insulin-stimulated phosphorylation of IR tyrosine and/or Akt in the liver, skeletal muscle, and white adipose tissue, whereas nTZD potentiated the phosphorylation of IR and Akt in the adipose tissue only. In conclusion, small-molecule IR activators have unique features as insulin sensitizers and hold potential utility in the treatment of type 2 diabetes and obesity.

Pioglitazone reduces islet triglyceride content and restores impaired glucose-stimulated insulin secretion in heterozygous peroxisome proliferator-activated receptor-gamma-deficient mice on a high-fat diet

Heterozygous peroxisome proliferator-activated receptor-gamma (PPAR-gamma)-deficient (PPARgamma(+/-)) mice were protected from high-fat diet-induced insulin resistance. To determine the impact of systemic reduction of PPAR-gamma activity on beta-cell function, we investigated insulin secretion in PPARgamma(+/-) mice on a high-fat diet. Glucose-induced insulin secretion in PPARgamma(+/-) mice was impaired in vitro. The tissue triglyceride (TG) content of the white adipose tissue, skeletal muscle, and liver was decreased in PPARgamma(+/-) mice, but it was unexpectedly increased in the islets, and the increased TG content in the islets was associated with decreased glucose oxidation. Administration of a PPAR-gamma agonist, pioglitazone, reduced the islet TG content in PPARgamma(+/-) mice on a high-fat diet and ameliorated the impaired insulin secretion in vitro. Our results demonstrate that PPAR-gamma protects islets from lipotoxicity by regulating TG partitioning among tissues and that a PPAR-gamma agonist can restore impaired insulin secretion under conditions of islet fat accumulation.

The role of sulphonylureas in the management of type 2 diabetes mellitus

The sulphonylureas act by triggering insulin release from the pancreatic beta cell. A specific site on the adenosine triphosphate (ATP)-sensitive potassium channels is occupied by sulphonylureas leading to closure of the potassium channels and subsequent opening of calcium channels. This results in exocytosis of insulin. The meglitinides are not sulphonylureas but also occupy the sulphonylurea receptor unit coupled to the ATP-sensitive potassium channel. Glibenclamide (glyburide), gliclazide, glipizide and glimepiride are the primary sulphonylureas in current clinical use for type 2 diabetes mellitus. Glibenclamide has a higher frequency of hypoglycaemia than the other agents. With long-term use, there is a progressive decrease in the effectiveness of sulphonylureas. This loss of effect is the result of a reduction in insulin-producing capacity by the pancreatic beta cell and is also seen with other antihyperglycaemic agents. The major adverse effect of sulphonylureas is hypoglycaemia. There is a theoretical concern that sulphonylureas may affect cardiac potassium channels resulting in a diminished response to ischaemia. There are now many choices for initial therapy of type 2 diabetes in addition to sulphonylureas. Metformin and thiazolidinediones affect insulin sensitivity by independent mechanisms. Disaccharidase inhibitors reduce rapid carbohydrate absorption. No single agent appears capable of achieving target glucose levels in the majority of patients with type 2 diabetes. Combinations of agents are successful in lowering glycosylated haemoglobin levels more than with a single agent. Sulphonylureas are particularly beneficial when combined with agents such as metformin that decrease insulin resistance. Sulphonylureas can also be given with a basal insulin injection to provide enhanced endogenous insulin secretion after meals. Sulphonylureas will continue to be used both primarily and as part of combined therapy for most patients with type 2 diabetes.

PPAR- and LXR-dependent pathways controlling lipid metabolism and the development of atherosclerosis

The nuclear receptor superfamily is composed of transcription factors that positively and negatively regulate gene expression in response to the binding of a diverse array of lipid-derived hormones and metabolites. Intense efforts are currently being directed at defining the biological roles and mechanisms of action of liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs). LXRs have been found to play essential roles in the regulation of whole body cholesterol absorption and excretion, in the efflux of cholesterol from peripheral cells, and in the biosynthesis and metabolism of very low density lipoproteins. PPARs have been found to regulate diverse aspects of lipid metabolism, including fatty acid oxidation, fat cell development, lipoprotein metabolism, and glucose homeostasis. Intervention studies indicate that activation of PPARalpha, PPARgamma, and LXRs by specific synthetic ligands can inhibit the development of atherosclerosis in animal models. Here, we review recent studies that provide new insights into the mechanisms by which these subclasses of nuclear receptors act to systemically influence lipid and glucose metabolism and regulate gene expression within the artery wall.

A novel IKKβ inhibitor stimulates adiponectin levels and ameliorates obesity-linked insulin resistance

Adiponectin is an anti-diabetic and anti-atherogenic hormone that is exclusively secreted from fat cells. Serum adiponectin levels are reduced in obese patients and obese model mice, despite increased adipose tissue mass. Elucidation of the mechanism(s) by which plasma adiponectin levels are decreased in obese and diabetic patients would provide insight into the cause of obesity-induced diabetes and the development of therapeutic advances. In the present study, the regulation of adiponectin secretion was investigated using 3T3-L1 adipocytes and a diabetic-/obese-mouse model. A novel insulin sensitizer, IkappaB kinase beta (IKKbeta) inhibitor, ameliorated insulin resistance and up-regulated plasma levels of adiponectin without producing a significant change in body weight in KKAy mice that were fed a high-fat diet. The IKKbeta inhibitor cancelled the TNFalpha-mediated down-regulation of adiponectin secretion and simultaneously up-regulated the phosphorylation of Akt in 3T3-L1 adipocytes. Using dominant-negative mutants of Akt or PKClambda (downstream effectors of phosphoinositide 3-kinase), insulin-stimulated Akt activity was found to be important in the regulation of adiponectin secretion by insulin in 3T3-L1 adipocytes. These observations suggest that "insulin-stimulated Akt activity in adipocytes" may play an important role in the regulation of adiponectin secretion.

PPARalpha governs glycerol metabolism

Glycerol, a product of adipose tissue lipolysis, is an important substrate for hepatic glucose synthesis. However, little is known about the regulation of hepatic glycerol metabolism. Here we show that several genes involved in the hepatic metabolism of glycerol, i.e., cytosolic and mitochondrial glycerol 3-phosphate dehydrogenase (GPDH), glycerol kinase, and glycerol transporters aquaporin 3 and 9, are upregulated by fasting in wild-type mice but not in mice lacking PPARalpha. Furthermore, expression of these genes was induced by the PPARalpha agonist Wy14643 in wild-type but not PPARalpha-null mice. In adipocytes, which express high levels of PPARgamma, expression of cytosolic GPDH was enhanced by PPARgamma and beta/delta agonists, while expression was decreased in PPARgamma(+/-) and PPARbeta/delta(-/-) mice. Transactivation, gel shift, and chromatin immunoprecipitation experiments demonstrated that cytosolic GPDH is a direct PPAR target gene. In line with a stimulating role of PPARalpha in hepatic glycerol utilization, administration of synthetic PPARalpha agonists in mice and humans decreased plasma glycerol. Finally, hepatic glucose production was decreased in PPARalpha-null mice simultaneously fasted and exposed to Wy14643, suggesting that the stimulatory effect of PPARalpha on gluconeogenic gene expression was translated at the functional level. Overall, these data indicate that PPARalpha directly governs glycerol metabolism in liver, whereas PPARgamma regulates glycerol metabolism in adipose tissue.

Insulin Regulation of Sterol Regulatory Element-binding Protein-1 Expression in L-6 Muscle Cells and 3T3 L1 Adipocytes

Sterol regulatory element-binding proteins (SREBPs) are transcription factors that regulate enzymes required for cholesterol and fatty acid synthesis. Expression of SREBP-1 is enhanced by insulin; however, the actual insulin-signaling cascades employed are yet unclear. We determined the roles of the phosphatidylinositol (PI) 3-kinase and mitogen-activated protein (MAP) kinase-dependent pathways in the effect of mediating insulin on SREBP-1 in L-6 skeletal muscle cells and 3T3 L1 adipocytes, using wortmannin or LY294002 to inhibit the PI 3-kinase pathway, and PD98059 to inhibit the MAP kinase-dependent pathway. In myocytes, insulin increased SREBP-1 protein in a dose-dependent manner. 1 and 10 nm insulin significantly increased expression of total cellular SREBP-1 protein at 24 and 48 h, nuclear SREBP-1 protein at 24 h, and SREBP-1a mRNA at 24 h. Although wortmannin and LY294002 had no effect on this aspect of insulin action, PD98059 completely blocked each of these responses. Transfection of a dominant negative mutant of Ras similarly blocked the insulin effect on SREBP-1. In contrast, in adipocytes, the insulin effect on SREBP-1 was mediated via the PI 3-kinase and not the MAP kinase pathway. In conclusion, although insulin increases skeletal muscle SREBP-1 expression in a dose-dependent fashion via the MAP kinase-dependent signaling pathway, insulin action on adipocyte SREBP-1 is mediated via the PI 3-kinase signaling pathway. In the state of insulin resistance, characterized by selective inhibition of the PI 3-kinase pathway, the usual stimulation of lipogenesis by insulin in adipocytes may be inhibited, whereas intramyocellular lipogenesis via the MAP kinase pathway of insulin may continue unabated.

Actions of PPARgamma agonism on adipose tissue remodeling, insulin sensitivity, and lipemia in absence of glucocorticoids

Peroxisome proliferator-activated receptor gamma (PPARgamma) agonists improve insulin sensitivity and lipemia partly through enhancing adipose tissue proliferation and capacity for lipid retention. The agonists also reduce local adipose glucocorticoid production, which may in turn contribute to their metabolic actions. This study assessed the effects of a PPARgamma agonist in the absence of glucocorticoids (adrenalectomy, ADX). Intact, ADX, and intact pair-fed (PF) rats were treated with the PPARgamma agonist rosiglitazone (RSG) for 2 wk. RSG increased inguinal (subcutaneous) white (50%) and brown adipose tissue (6-fold) weight but not that of retroperitoneal (visceral) white adipose tissue. ADX but not PF reduced fat accretion in both inguinal and retroperitoneal adipose depots but did not affect brown adipose mass. RSG no longer increased inguinal weight in ADX and PF rats but increased brown adipose mass, albeit less so than in intact rats. RSG increased cell proliferation in white (3-fold) and brown adipose tissue (6-fold), as assessed microscopically and by total DNA, an effect that was attenuated but not abrogated by ADX. RSG reduced the expression of the glucocorticoid-activating enzyme 11beta-hydroxysteroid dehydrogenase 1 (11beta-HSD1) in all adipose depots. RSG improved insulin sensitivity (reduction in fasting insulin and homeostasis model assessment of insulin resistance, both -50%) and triacylglycerolemia (-75%) regardless of the glucocorticoid status, these effects being fully additive to those of ADX and PF. In conclusion, RSG partially retained its ability to induce white and brown adipose cell proliferation and brown adipose fat accretion and further improved insulin sensitivity and lipemia in ADX rats, such effects being therefore independent from the PPARgamma-mediated modulation of glucocorticoids.

Intramyocellular lipids and insulin resistance

Lipids are stored not only in adipocytes but also 'ectopically' in tissues such as muscle, liver, beta cells and others. From a metabolic perspective, intramyocellular lipids (IMCLs) have recently become a focus of interest. This review summarizes history, measurement techniques and interpretation of muscle lipid data. Problems in biopsies with the separation of those metabolically active lipid droplets in the cytoplasm of myocytes from further lipids in adipocytes are discussed as well as considerations important for analysis of correlations between IMCL content and insulin sensitivity under various circumstances. The relatively new approach to non-invasive assessment of the IMCL content by magnetic resonance spectroscopy (MRS) is described in detail and exemplary spectra from different skeletal muscle types in humans are presented. The MRS technique allows human examinations of large cohorts for a detailed assessment of the interactions among metabolic parameters such as age, measures of adiposity, hormonal and ethnic factors and insulin resistance. IMCLs are generally positively correlated with measures of obesity and negatively with insulin sensitivity. Paradoxically, physical fitness (maximal aerobic capacity) increases both IMCL content and insulin sensitivity and therefore has to be taken into account as a confounding factor. Intervention studies with MRS further allowed to elucidate the regulation of IMCL. Molecular mechanisms and potential genetic factors on IMCL regulation are discussed as well as possible mechanisms of current treatment strategies for improving insulin sensitivity.

Effects of obesity on the relationship of leptin mRNA expression and adipocyte size in anatomically distinct fat depots in mice

In support of leptin's physiological role as humoral signal of fat mass, we have shown that adipocyte volume is a predominant determinant of leptin mRNA levels in anatomically distinct fat depots in lean young mice in the postabsorptive state. In this report, we investigated how obesity may affect the relationship between leptin mRNA levels and adipocyte volume in anatomically distinct fat depots in mice with genetic ( Lepob/ Lepoband Ay/+), diet-induced, and aging-related obesity. In all of the obese mice examined, tissue leptin mRNA levels relative to the average adipocyte volume were lower in the perigonadal and/or retroperitoneal than in the inguinal fat depots and were lower than those of the lean young mice in the perigonadal fat depot. A close, positive correlation between leptin mRNA level and adipocyte volume was present from small to hypertrophic adipocytes within each perigonadal and inguinal fat pad in the obese mice, but the slopes of the regression lines relating leptin mRNA level to adipocyte volume were significantly lower in the perigonadal than in the inguinal fat pads of the same mice. These results suggest that obesity per se is associated with a decreased leptin gene expression per unit of fat mass in mice and that the positive correlation between leptin mRNA level and adipocyte volume is an intrinsic property of adipocytes that is not disrupted by adipocyte hypertrophy in obese mice.

Role of adiponectin in insulin-resistant hypertension and atherosclerosis

Insulin resistance is one of the major risk factors associated with development of hypertension and atherosclerosis. Recent studies have shown that adiponectin, an adipocyte-derived hormone, may be involved in insulin resistance and development of atherosclerosis in diabetes patients. The aim of this study was to examine adiponectin levels in patients with essential hypertension to determine the relationships between adiponectin levels and insulin sensitivity and to examine the relationship of adiponectin with pulse wave velocity (PWV) in a general population based on the results of an epidemiological survey in Japan. In a clinical study, 20 normotensives (NT) and 30 non-treated essential hypertensives (EHT) were hospitalized, and euglycemic hyperinsulinemic glucose clamp (GC) was performed to evaluate insulin sensitivity defined as M value. EHT were divided into insulin-resistant EHT (EHT-R) and insulin-nonresistant EHT (EHT-N) according to the mean -1 SD of the M value of NT as a cut-off point. Fasting plasma glucose (FPG), immunoreactive insulin (IRI), and adiponectin concentrations were measured. There were no significant differences in body mass index (BMI) or FPG among the NT, EHT-N, and EHT-R groups. The M value and adiponectin concentration in EHT-R were significantly lower than those in the NT or EHT-N. The IRI level in the EHT-R was significantly higher than those in the other groups. A positive correlation between adiponectin concentration and M value was found in all subjects, and adiponectin concentration and M value were found to be significant determinants of each other in multiple regression analysis. In an epidemiological study, we studied 391 male inhabitants of rural communities in Hokkaido, Japan. Systolic blood pressure (SBP), BMI, FPG, IRI, and adiponectin were measured in all subjects early in the morning. Homeostasis model assessment (HOMA) values were calculated as an index of insulin sensitivity, and PWV was used as an index of atherosclerosis. A negative correlation between HOMA values and adiponectin concentration was found in all of the subjects. Multiple regression analysis revealed that adiponectin was a significant determinant for PWV in subjects less than 70 years of age. The results of the clinical study indicate that EHT-R had not only hyperinsulinemia but also a low concentration of adiponectin. The results of multiple regression analysis for determinants of degree of PWV using data obtained in the epidemiological study suggest that adiponectin plays a role in antiatherosclerosis, partly through improvement of insulin resistance.

PPARs and the complex journey to obesity

Obesity and the related disorders of dyslipidemia and diabetes (components of syndrome X) have become global health epidemics. Over the past decade, the elucidation of key regulators of energy balance and insulin signaling have revolutionized our understanding of fat and sugar metabolism and their intimate link. The three 'lipid-sensing' peroxisome proliferator-activated receptors (PPAR-alpha, PPAR-gamma and PPAR-delta) exemplify this connection, regulating diverse aspects of lipid and glucose homeostasis, and serving as bona fide therapeutic targets. With molecular underpinnings now in place, new pharmacologic approaches to metabolic disease and new questions are emerging.

Isohumulones, bitter acids derived from hops, activate both peroxisome proliferator-activated receptor alpha and gamma and reduce insulin resistance

The peroxisome proliferator-activated receptors (PPARs) are dietary lipid sensors that regulate fatty acid and carbohydrate metabolism. The hypolipidemic effects of fibrate drugs and the therapeutic benefits of the thiazolidinedione drugs are due to their activation of PPARalpha and -gamma, respectively. In this study, isohumulones, the bitter compounds derived from hops that are present in beer, were found to activate PPARalpha and -gamma in transient co-transfection studies. Among the three major isohumulone homologs, isohumulone and isocohumulone were found to activate PPARalpha and -gamma. Diabetic KK-Ay mice that were treated with isohumulones (isohumulone and isocohumulone) showed reduced plasma glucose, triglyceride, and free fatty acid levels (65.3, 62.6, and 73.1%, respectively, for isohumulone); similar reductions were found following treatment with the thiazolidinedione drug, pioglitazone. Isohumulone treatment did not result in significant body weight gain, although pioglitazone treatment did increase body weight (10.6% increase versus control group). C57BL/6N mice fed a high fat diet that were treated with isohumulones showed improved glucose tolerance and reduced insulin resistance. Furthermore, these animals showed increased liver fatty acid oxidation and a decrease in size and an increase in apoptosis of their hypertrophic adipocytes. A double-blind, placebo-controlled pilot study for studying the effect of isohumulones on diabetes suggested that isohumulones significantly decreased blood glucose and hemoglobin A1c levels after 8 weeks (by 10.1 and 6.4%, respectively, versus week 0). These results suggest that isohumulones can improve insulin sensitivity in high fat diet-fed mice with insulin resistance and in patients with type 2 diabetes.

Rosiglitazone improves intestinal lipoprotein overproduction in the fat-fed Syrian Golden hamster, an animal model of nutritionally-induced insulin resistance

We have recently shown that the fructose-fed Syrian Golden hamster, a non-diabetic animal model of nutritionally-induced insulin resistance and hyperlipidemia, is characterized by intestinal lipoprotein overproduction. In order to determine whether intestinal lipoprotein overproduction is specific to fructose feeding or applies generally to other models of insulin resistance, we studied intestinal lipoprotein production and the response to insulin sensitization in the high fat-fed Syrian Golden hamster. Syrian Golden Hamsters were fed either (1). chow (CHOW), (2). 60% fat (FAT) or (3). 60% fat with rosiglitazone, 20 micromol/kg per day (FAT + RSG) for 5 weeks. Euglycemic hyperinsulinemic clamp studies confirmed that FAT is a good model of insulin resistance and rosiglitazone treatment resulted in a significant improvement in insulin sensitivity. In addition, there was a significant approx. two- to four-fold increase in intestinal apoB48 particle production in FAT. Rosiglitazone treatment resulted in partial normalization of apoB48-containing intestinal lipoprotein secretion. In summary: (1). the fat-fed Syrian Golden Hamster is a good model of nutritionally-induced insulin resistance, (2). intestinal overproduction of lipoproteins appear to contribute to the hypertriglyceridemia of insulin resistance in this animal model and (3). insulin sensitization with rosiglitazone ameliorates intestinal apoB48 particle overproduction in the fat-fed Syrian Golden Hamster. These data further support the link between insulin resistance and intestinal lipoprotein overproduction.

Peroxisome proliferator-activated receptor γ in diabetes and metabolism

The peroxisome proliferator-activated receptor gamma (PPAR-gamma) has been the focus of intense research during the past decade because ligands for this receptor have emerged as potent insulin sensitizers used in the treatment of type 2 diabetes. Recent advances include the discovery of novel genes that are regulated by PPAR-gamma, which helps explain how activation of this adipocyte-predominant transcription factor regulates glucose and lipid homeostasis. Increased levels of circulating free fatty acids and lipid accumulation in non-adipose tissue have been implicated in the development of insulin resistance. This situation is improved by PPAR-gamma ligands, which promote fatty acid storage in fat depots and regulate the expression of adipocyte-secreted hormones that impact on glucose homeostasis. The net result of the pleiotropic effects of PPAR-gamma ligands is improvement of insulin sensitivity, although undesired side-effects limit the utility of this therapy. It might be possible to dissociate the anti-diabetic and adverse effects through selective modulation of PPAR-gamma activity.

PPARgamma insufficiency enhances osteogenesis through osteoblast formation from bone marrow progenitors

Based on the fact that aging is associated with a reciprocal decrease of osteogenesis and an increase of adipogenesis in bone marrow and that osteoblasts and adipocytes share a common progenitor, this study investigated the role of PPARgamma, a key regulator of adipocyte differentiation, in bone metabolism. Homozygous PPARgamma-deficient ES cells failed to differentiate into adipocytes, but spontaneously differentiated into osteoblasts, and these were restored by reintroduction of the PPARgamma gene. Heterozygous PPARgamma-deficient mice exhibited high bone mass with increased osteoblastogenesis, but normal osteoblast and osteoclast functions, and this effect was not mediated by insulin or leptin. The osteogenic effect of PPARgamma haploinsufficiency became prominent with aging but was not changed upon ovariectomy. The PPARgamma haploinsufficiency was confirmed to enhance osteoblastogenesis in the bone marrow cell culture but did not affect the cultures of differentiated osteoblasts or osteoclast-lineage cells. This study demonstrates a PPARgamma-dependent regulation of bone metabolism in vivo, in that PPARgamma insufficiency increases bone mass by stimulating osteoblastogenesis from bone marrow progenitors.

Anthocyanin enhances adipocytokine secretion and adipocyte-specific gene expression in isolated rat adipocytes

Adipocyte dysfunction is strongly associated with the development of obesity and insulin resistance. It is accepted that the regulation of adipocytokine secretion or the adipocyte-specific gene expression is one of the most important targets for the prevention of obesity and amelioration of insulin sensitivity. In this study, we demonstrated that anthocyanins (cyanidin or cyanidin 3-glucoside) have the potency of a unique pharmacological function in isolated rat adipocytes. Treated adipocytes with anthocyanins enhanced adipocytokine (adiponectin and leptin) secretion and up-regulated the adipocyte specific gene expression without activation of PPARgamma in isolated rat adipocytes. The gene expression of adiponectin was also up-regulated in white adipose tissue in mice fed an anthocyanin supplemented diet. As one of the possible mechanisms, AMP-activated protein kinase activation would be associated with these changes, nevertheless, the AMP:ATP ratio was significantly decreased by administration of the anthocyanins. These data suggest that anthocyanins have a potency of unique therapeutic advantage and also have important implications for preventing obesity and diabetes.

Peroxisome proliferator-activated receptor γ: Its role in metabolic syndrome

Here we review PPARgamma function in relation to human adipogenesis, insulin sensitization, lipid metabolism, blood pressure regulation and prothrombotic state to perhaps provide justification for this nuclear receptor remaining a key therapeutic target for the continuing development of agents to treat human metabolic syndrome.

Novel adipose tissue-mediated resistance to diet-induced visceral obesity in 11 beta-hydroxysteroid dehydrogenase type 1-deficient mice

The metabolic syndrome (visceral obesity, insulin resistance, type 2 diabetes, and dyslipidemia) resembles Cushing's Syndrome, but without elevated circulating glucocorticoid levels. An emerging concept suggests that the aberrantly elevated levels of the intracellular glucocorticoid reamplifying enzyme 11 beta-hydroxysteroid dehydrogenase type 1 (11 beta-HSD-1) found in adipose tissue of obese humans and rodents underlies the phenotypic similarities between idiopathic and "Cushingoid" obesity. Transgenic overexpression of 11 beta-HSD-1 in adipose tissue reproduces a metabolic syndrome in mice, whereas 11 beta-HSD-1 deficiency or inhibition has beneficial metabolic effects, at least on liver metabolism. Here we report novel protective effects of 11 beta-HSD-1 deficiency on adipose function, distribution, and gene expression in vivo in 11 beta-HSD-1 nullizygous (11 beta-HSD-1(-/-)) mice. 11 beta-HSD-1(-/-) mice expressed lower resistin and tumor necrosis factor-alpha, but higher peroxisome proliferator-activated receptor-gamma, adiponectin, and uncoupling protein-2 mRNA levels in adipose, indicating insulin sensitization. Isolated 11 beta-HSD-1(-/-) adipocytes exhibited higher basal and insulin-stimulated glucose uptake. 11 beta-HSD-1(-/-) mice also exhibited reduced visceral fat accumulation upon high-fat feeding. High-fat-fed 11 beta-HSD-1(-/-) mice rederived onto the C57BL/6J strain resisted diabetes and weight gain despite consuming more calories. These data provide the first in vivo evidence that adipose 11 beta-HSD-1 deficiency beneficially alters adipose tissue distribution and function, complementing the reported effects of hepatic 11 beta-HSD-1 deficiency or inhibition.

Cellular and molecular mechanisms of adipose tissue plasticity in muscle insulin receptor knockout mice

White adipose tissue (WAT) plays a critical role in the development of insulin resistance via secretion of free fatty acids (FFA) and adipocytokines. Muscle-specific insulin receptor knockout (MIRKO) mice do not develop insulin resistance or diabetes under physiological conditions despite a marked increase in adiposity and plasma FFA. On the contrary, WAT of MIRKO is sensitized to insulin action during a euglycemic clamp, and WAT glucose utilization is dramatically increased. To get insight into the potential antidiabetic role of MIRKO adiposity, we have studied insulin action in WAT during a euglycemic, hyperinsulinemic clamp, and we have characterized the morphology and biology of WAT. During the clamp, there is no alteration in the expression or activation in the insulin signaling molecules involved in glucose transport through the phosphoinositide 3-kinase/Akt and CAP/Cbl pathways in WAT from MIRKO. The 53% increase in WAT mass results from a 48% increase in adipocyte number (P < 0.05) without alteration in cell size and contemporary to a 300% increase in mRNA levels of the adipogenic transcription factor CCAAT enhancer binding protein-alpha (C/EBP-alpha) (P < 0.05). There is a 39.5% increase in serum adiponectin (P < 0.01) without modification in serum leptin, resistin, and TNF-alpha. In conclusion, the MIRKO mouse displays muscle insulin resistance, visceral obesity, and dyslipidemia but does not develop hyperinsulinemia or diabetes. There is an accelerated differentiation of small insulin sensitive adipocytes, an increased secretion of the insulin sensitizer adiponectin, and maintenance of leptin sensitivity. The MIRKO mouse confirms the importance of WAT plasticity in the maintenance of whole body insulin sensitivity and represents an interesting model to search for new secreted molecules that positively alter adipose tissue biology.

Thiazolidinediones, like metformin, inhibit respiratory complex I: a common mechanism contributing to their antidiabetic actions?

Metformin and thiazolidinediones (TZDs) are believed to exert their antidiabetic effects via different mechanisms. As evidence suggests that both impair cell respiration in vitro, this study compared their effects on mitochondrial functions. The activity of complex I of the respiratory chain, which is known to be affected by metformin, was measured in tissue homogenates that contained disrupted mitochondria. In homogenates of skeletal muscle, metformin and TZDs reduced the activity of complex I (30 mmol/l metformin, -15 +/- 2%; 100 micromol/l rosiglitazone, -54 +/- 7; and 100 micromol/l pioglitazone, -12 +/- 4; P < 0.05 each). Inhibition of complex I was confirmed by reduced state 3 respiration of isolated mitochondria consuming glutamate + malate as substrates for complex I (30 mmol/l metformin, -77 +/- 1%; 100 micromol/l rosiglitazone, -24 +/- 4; and 100 micromol/l pioglitazone, -18 +/- 5; P < 0.05 each), whereas respiration with succinate feeding into complex II was unaffected. In line with inhibition of complex I, 24-h exposure of isolated rat soleus muscle to metformin or TZDs reduced cell respiration and increased anaerobic glycolysis (glucose oxidation: 270 micromol/l metformin, -30 +/- 9%; 9 micromol/l rosiglitazone, -25 +/- 8; and 9 micromol/l pioglitazone, -45 +/- 3; lactate release: 270 micromol/l metformin, +84 +/- 12; 9 micromol/l rosiglitazone, +38 +/- 6; and 9 micromol/l pioglitazone, +64 +/- 11; P < 0.05 each). As both metformin and TZDs inhibit complex I activity and cell respiration in vitro, similar mitochondrial actions could contribute to their antidiabetic effects.

Update on adipocyte hormones: regulation of energy balance and carbohydrate/lipid metabolism

Hormones produced by adipose tissue play a critical role in the regulation of energy intake, energy expenditure, and lipid and carbohydrate metabolism. This review will address the biology, actions, and regulation of three adipocyte hormones-leptin, acylation stimulating protein (ASP), and adiponectin-with an emphasis on the most recent literature. The main biological role of leptin appears to be adaptation to reduced energy availability rather than prevention of obesity. In addition to the well-known consequences of absolute leptin deficiency, subjects with heterozygous leptin gene mutations have low circulating leptin levels and increased body adiposity. Leptin treatment dramatically improves metabolic abnormalities (insulin resistance and hyperlipidemia) in patients with relative leptin deficiency due to lipoatrophy. Leptin production is primarily regulated by insulin-induced changes of adipocyte metabolism. Dietary fat and fructose, which do not increase insulin secretion, lead to reduced leptin production, suggesting a mechanism for high-fat/high-sugar diets to increase energy intake and weight gain. ASP increases the efficiency of triacylglycerol synthesis in adipocytes leading to enhanced postprandial lipid clearance. In mice, ASP deficiency results in reduced body fat, obesity resistance, and improved insulin sensitivity. Adiponectin production is stimulated by thiazolidinedione agonists of peroxisome proliferator-activated receptor-gamma and may contribute to increased insulin sensitivity. Adiponectin and leptin cotreatment normalizes insulin action in lipoatrophic insulin-resistant animals. These effects may be mediated by AMP kinase-induced fat oxidation, leading to reduced intramyocellular and liver triglyceride content. The production of all three hormones is influenced by nutritional status. These hormones, the pathways controlling their production, and their receptors are promising targets for managing obesity, hyperlipidemia, and insulin resistance.

Lack of hypotriglyceridemic effect of gemfibrozil as a consequence of age-related changes in rat liver PPARalpha

We have investigated if changes in hepatic lipid metabolism produced by old age are related to changes in liver peroxisome proliferator-activated receptor alpha (PPARalpha). Our results indicate that 18-month-old rats showed a marked decrease in the expression and activity of liver PPARalpha, as shown by significant reductions in PPARalpha mRNA, protein and binding activity, resulting in a reduction in the relative mRNA levels of PPARalpha target genes, such as liver-carnitine-palmitoyl transferase-I (CPT-I) and mitochondrial medium-chain acyl-CoA dehydrogenase (MCAD). Further, in accordance with a liver PPARalpha deficiency in old rats, treatment of old animals with a therapeutic dose of gemfibrozil (GFB) (3mg/kg per day, 21 days) was ineffective in reducing plasma triglyceride concentrations (TG), despite attaining a 50% reduction in TG when GFB was administered to young animals at the same dose and length of treatment. We hypothesize that the decrease in hepatic PPARalpha can be related to a state of leptin resistance present in old animals.

Decreased plasma adiponectin concentrations are closely related to hepatic fat content and hepatic insulin resistance in pioglitazone-treated type 2 diabetic patients

The effect of pioglitazone (PIO) on plasma adiponectin concentration, endogenous glucose production (EGP), and hepatic fat content (HFC) was studied in 11 type 2 diabetic patients (age, 52 +/- 2 yr; body mass index, 29.6 +/- 1.1 kg/m(2); HbA(1c), 7.8 +/- 0.4%). HFC (magnetic resonance spectroscopy) and basal plasma adiponectin concentration were quantitated before and after PIO (45 mg/d) for 16 wk. Subjects received a 3-h euglycemic insulin (100 mU/m(2).min) clamp combined with 3-[(3)H] glucose infusion to determine rates of EGP and tissue glucose disappearance (Rd) before and after PIO. PIO reduced fasting plasma glucose (10.0 +/- 0.7 to 7.2 +/- 0.6 mmol/liter, P < 0.01) and HbA(1c) (7.8 +/- 0.4 to 6.5 +/- 0.3%, P < 0.01) despite increased body weight (83.0 +/- 3.0 to 86.4 +/- 3.0 kg, P < 0.01). PIO improved Rd (6.6 +/- 0.6 vs. 5.2 +/- 0.5 mg/kg.min, P < 0.005) and reduced EGP (0.23 +/- 0.04 to 0.05 +/- 0.02 mg/kg.min, P < 0.01) during the 3-h insulin clamp. After PIO treatment, HFC decreased from 21.3 +/- 4.2 to 11.0 +/- 2.4% (P < 0.01), and plasma adiponectin increased from 7 +/- 1 to 21 +/- 2 micro g/ml (P < 0.0001). Plasma adiponectin concentration correlated negatively with HFC (r = -0.60, P < 0.05) and EGP (r = -0.80, P < 0.004) and positively with Rd before (r = 0.68, P < 0.02) pioglitazone treatment; similar correlations were observed between plasma adiponectin levels and HFC (r = -0.65, P < 0.03) and Rd after (r = 0.70, P = 0.01) pioglitazone treatment. EGP was almost completely suppressed after pioglitazone treatment; taken collectively, plasma adiponectin concentration, before and after pioglitazone treatment, still correlated negatively with EGP during the insulin clamp (r = -0.65, P < 0.001). In conclusion, PIO treatment in type 2 diabetes causes a 3-fold increase in plasma adiponectin concentration. The increase in plasma adiponectin is strongly associated with a decrease in hepatic fat content and improvements in hepatic and peripheral insulin sensitivity. The increase in plasma adiponectin concentration after thiazolidinedione therapy may play an important role in reversing the abnormality in hepatic fat mobilization and the hepatic/muscle insulin resistance in patients with type 2 diabetes.

[The mechanisms by which PPARgamma and adiponectin regulate glucose and lipid metabolism]

Obesity, a state of increased adipose tissue mass, is a major cause for type 2 diabetes, hyperlipidemia, and hypertension, resulting in clustering of risk factors for atherosclerosis. Heterozygous PPARgamma knockout mice and KKA(y) mice administered with a PPARgamma antagonist were protected from high-fat diet-induced adipocyte hypertrophy and insulin resistance. Moderate reduction of PPARgamma activity prevented adipocyte hypertrophy, thereby diminution of TNFalpha, resistin, and FFA and upregulation of adiponectin and leptin. These alterations led to reduction of tissue TG content in muscle/liver, thereby ameliorating insulin resistance. Insulin resistance in the lipoatrophic mice and KKA(y) mice were ameliorated by replenishment of adiponectin. Moreover, adiponectin transgenic mice ameliorated insulin resistance and diabetes, but not the obesity of ob/ob mice. Furthermore, targeted disruption of the adiponectin gene caused moderate insulin resistance and glucose intolerance. In muscle, adiponectin activated AMP kinase and PPARgamma pathways, thereby increasing beta-oxidation of lipids, leading to decreased TG content, which ameliorated muscle insulin resistance. In the liver, adiponectin also activated AMPK, thereby downregulating PEPCK and G6Pase, leading to decreased glucose output from the liver. In conclusion, PPARgamma plays a central role in the regulation of adipocyte hypertrophy and insulin sensitivity. The upregulation of the adiponectin pathway by PPARgamma may play a role in the increased insulin sensitivity of heterozygous PPARgamma knockout mice, and activation of adiponectin pathway may provide novel therapeutic strategies for obesity-linked disorders such as type 2 diabetes and metabolic syndrome.