Effects of pioglitazone on adipose tissue remodeling within the setting of obesity and insulin resistance

Fixed-dose combination of pioglitazone and glimepiride in the treatment of Type 2 diabetes mellitus

Type 2 diabetes is characterized by impaired insulin sensitivity and disturbances in β-cell function. While glimepiride stimulates β-cell secretion and leads to a reduction of blood glucose levels, pioglitazone activates peroxisome proliferator-activated receptor-γ and improves insulin resistance. Combining these two modes of action has been shown to improve glucose and lipid metabolism, and to improve the overall cardiovascular risk in patients with Type 2 diabetes. The combination of glimepiride and pioglitazone is generally well tolerated and a fixed combination may lead to an improved compliance in patients. The purpose of this review is to evaluate the clinical data that has been published on this combination, appearing to represent a convenient way to obtain therapeutic targets in patients with Type 2 diabetes mellitus.

Tesaglitazar, a PPARα/γ Agonist, Induces Interstitial Mesenchymal Cell DNA Synthesis and Fibrosarcomas in Subcutaneous Tissues in Rats

The development of the dual peroxisome proliferator-activated receptor (PPAR) alpha/gamma agonist tesaglitazar as an oral antidiabetic was recently discontinued. Here we present tumor data from a 2-year carcinogenicity study in rats given 0.3, 1, 3, and 10 micromol/kg tesaglitazar is presented with focus on the findings of subcutaneous fibrosarcomas. To investigate the mechanism for induction of fibrosarcomas, replicative DNA synthesis (immunohistochemical detection of BrdU-labeled cells) and expression of PPARgamma (immunohistochemistry and reverse transcription-polymerase chain reaction) in subcutaneous adipose tissues was assessed in rats administered 1 or 10 micromol/kg for 2 weeks or 3 months. Poorly differentiated subcutaneous mesenchymal sarcomas with a predominant spindle cell appearance occurred at the highest dose level of 10 micromol/kg in both sexes, and these tumors were diagnosed as fibrosarcomas. The 10-micromol/kg dose was at or above the maximum tolerated dose and caused considerable cardiovascular mortality. Tesaglitazar stimulated DNA synthesis mainly in subcutaneous interstitial mesenchymal cells. The percentage of BrdU-labeled interstitial cells was increased at 1 and 10 micromol/kg after 2 weeks. The increase in DNA synthesis was still significant at the end of the 12-week treatment at 10 mumol/kg, the dose producing fibrosarcoma. However, at 1 micromol/kg, a dose below the no-observed-effect level for fibrosarcoma, the level of DNA synthesis was similar to control levels at 12 weeks. Immunohistochemical analyses showed no detectable PPARgamma protein in the majority of BrdU-labeled interstitial mesenchymal cells in white and brown fat. This indicates that stimulation of DNA synthesis is not mediated via direct activation of PPARgamma in these cells. The results suggest that the induction of rat fibrosarcoma by tesaglitazar, at exposures 100-fold above the human therapeutic exposure, may involve proliferation of undifferentiated mesenchymal cells in subcutaneous tissues.

Improved glucose tolerance in mice receiving intraperitoneal transplantation of normal fat tissue

AIMS/HYPOTHESIS

The association between increased (visceral) fat mass, insulin resistance and type 2 diabetes mellitus is well known. Yet, it is unclear whether the mere increase in intra-abdominal fat mass, or rather functional alterations in fat tissue in obesity contribute to the development of insulin resistance in obese patients. Here we attempted to isolate the metabolic effect of increased fat mass by fat tissue transplantation.

METHODS

Epididymal fat pads were removed from male C57Bl6/J mice and transplanted intraperitoneally into male littermates (recipients), increasing the combined perigonadal fat mass by 50% (p < 0.005). At 4 and 8 weeks post-transplantation, glucose and insulin tolerance tests were performed, and insulin, NEFA and adipokines measured.

RESULTS

Circulating levels of NEFA, adiponectin and leptin were not significantly different between transplanted and sham-operated control mice, while results of the postprandial insulin tolerance test were similar between the two groups. In contrast, under fasting conditions, the mere increase in intra-abdominal fat mass resulted in decreased plasma glucose levels (6.9 +/- 0.4 vs 8.1 +/- 0.3 mmol/l, p = 0.03) and a approximately 20% lower AUC in the glucose tolerance test (p = 0.02) in transplanted mice. Homeostasis model assessment of insulin resistance (HOMA-IR) was 4.1 +/- 0.4 in transplanted mice (vs 6.2 +/- 0.7 in sham-operated controls) (p = 0.02), suggesting improved insulin sensitivity. Linear regression modelling revealed that while total body weight positively correlated, as expected, with HOMA-IR (beta: 0.728, p = 0.006), higher transplanted fat mass correlated with lower HOMA-IR (beta: -0.505, p = 0.031).

CONCLUSIONS/INTERPRETATION

Increasing intra-abdominal fat mass by transplantation of fat from normal mice improved, rather than impaired, fasting glucose tolerance and insulin sensitivity, achieving an effect opposite to the expected metabolic consequence of increased visceral fat in obesity.

Subcutaneous fat in normal and diseased states

The quest for effective strategies to treat obesity has propelled fat research into an exploration of the molecular processes that drive adipocyte formation, and hence body fat mass. The development of obesity is dependent on the coordinated interplay of adipocyte hypertrophy (increased fat cell size), adipocyte hyperplasia (increased fat cell number), and angiogenesis. Evidence suggests that adipocyte hyperplasia, or adipogenesis, occurs throughout life, both in response to normal cell turnover as well as in response to the need for additional fat mass stores that arises when caloric intake exceeds nutritional requirements. Adipogenesis involves two major events-the recruitment and proliferation of adipocyte precursor cells, called preadipocytes, followed by the subsequent conversion of preadipocytes, or differentiation, into mature fat cells. In vitro studies using experimental and primary preadipocyte cell lines have uncovered the mechanisms that drive the adipogenic process, a tightly controlled sequence of events guided by the strict temporal regulation of multiple inhibitory and stimulatory signaling events involving regulators of cell-cycle functions and differentiation factors. This article reviews the current understanding of adipogenesis with emphasis on the various stages of adipocyte development; on key hormonal, nutritional, paracrine, and neuronal control signals; as well as on the components involved in cell-cell or cell-matrix interactions that are pivotal in regulating fat cell formation. Special consideration is given to clinical applications derived from adipogenesis research with impact on medical, surgical and cosmetic fields.

Absence of an adipogenic effect of rosiglitazone on mature 3T3-L1 adipocytes: increase of lipid catabolism and reduction of adipokine expression

AIMS/HYPOTHESIS

The thiazolidinedione (TZD) rosiglitazone is a peroxisome proliferator-activated receptor-gamma agonist that induces adipocyte differentiation and, hence, lipid accumulation. This is in apparent contrast to the long-term glucose-lowering, insulin-sensitising effect of rosiglitazone. We tested whether the action of rosiglitazone involves specific effects on mature adipocytes, which are different from those on preadipocytes.

MATERIALS AND METHODS

Differentiated mature 3T3-L1 adipocytes were used as an in vitro model. Transcriptomics, proteomics and assays of metabolism were applied to assess the effect of rosiglitazone in different insulin and glucose conditions.

RESULTS

Rosiglitazone does not induce an increase, but rather a decrease in the lipid content of mature adipocytes. Analysis of transcriptome data, confirmed by quantitative RT-PCR and measurements of lipolysis, indicates that an altered energy metabolism may underlie this change. The pathway analysis shows a consistent picture dominated by lipid catabolism. In addition, we confirmed at both mRNA level and protein level that rosiglitazone represses adipokine expression and production, except for genes encoding adiponectin and apolipoprotein E. Moreover, transcriptome changes indicate that a general repression of genes encoding secreted proteins occurs.

CONCLUSIONS/INTERPRETATION

Our findings suggest that the change of adiposity as seen in vivo reflects a shift in balance between the different effects of TZDs on preadipocytes and on mature adipocytes, while the changes in circulating adipokine levels primarily result from an effect on mature adipocytes.

PPARalpha and PPARgamma regulation of liver and adipose proteins in obese and dyslipidemic rodents

Zucker fatty rats and ob/ob mice are both frequently used hyperlipidemic and insulin-resistant spontaneous genetic models of obesity. We used them to study the effect of PPAR agonists on the protein-expression level in liver and white adipose tissue. PPARalpha-agonist treatments of the rats resulted in that 27% of the quantified hepatic proteins were altered; implicating pronounced peroxisome proliferation and increase in capacity for beta-oxidation of fatty acids although no correction of plasma triglycerides were obtained. On treatment with PPARgamma agonists, adipose proteins were regulated to a much larger extent in the rats compared to mice, 18% and 2%, respectively.

Dietary abscisic acid ameliorates glucose tolerance and obesity-related inflammation in db/db mice fed high-fat diets

BACKGROUND & AIMS

Despite their efficacy in improving insulin sensitivity, thiazolidinediones (TZDs) are associated with a number of side effects (i.e. weight gain, hepatotoxicity, congestive heart failure) that have limited their use by millions of diabetic patients. We have investigated whether abscisic acid (ABA), a naturally occurring phytochemical with structural similarities to TZDs, could be used as an alternative to TZDs to improve glucose homeostasis.

METHODS

We first examined whether ABA, similar to TZDs, activates PPARgamma in vitro. We next determined the lowest effective dose of dietary ABA (100 mg/kg) and assessed its effect on glucose tolerance, obesity-related inflammation, and mRNA expression of PPARgamma and its responsive genes in white adipose tissue (WAT) of db/db mice fed high-fat diets.

RESULTS

We found that ABA induced transactivation of PPARgamma in 3T3-L1 pre-adipocytes in vitro. Dietary ABA-supplementation for 36 days decreased fasting blood glucose concentrations, ameliorated glucose tolerance, and increased mRNA expression of PPARgamma and its responsive genes (i.e., adiponectin, aP2, and CD36) in WAT. We also found that adipocyte hypertrophy, tumor necrosis factor-alpha (TNF-alpha) expression, and macrophage infiltration in WAT were significantly attenuated in ABA-fed mice.

CONCLUSIONS

These findings suggest that ABA could be used as a nutritional intervention against type II diabetes and obesity-related inflammation.

Review: Peroxisome proliferator-activated receptor gamma and adipose tissue--understanding obesity-related changes in regulation of lipid and glucose metabolism

CONTEXT

Adipose tissue is a metabolically dynamic organ, serving as a buffer to control fatty acid flux and a regulator of endocrine function. In obese subjects, and those with type 2 diabetes or the metabolic syndrome, adipose tissue function is altered (i.e. adipocytes display morphological differences alongside aberrant endocrine and metabolic function and low-grade inflammation).

EVIDENCE ACQUISITION

Articles on the role of peroxisome proliferator-activated receptor gamma (PPARgamma) in adipose tissue of healthy individuals and those with obesity, metabolic syndrome, or type 2 diabetes were sourced using MEDLINE (1990-2006).

EVIDENCE SYNTHESIS

Articles were assessed to provide a comprehensive overview of how PPARgamma-activating ligands improve adipose tissue function, and how this links to improvements in insulin resistance and the progression to type 2 diabetes and atherosclerosis.

CONCLUSIONS

PPARgamma is highly expressed in adipose tissue, where its activation with thiazolidinediones alters fat topography and adipocyte phenotype and up-regulates genes involved in fatty acid metabolism and triglyceride storage. Furthermore, PPARgamma activation is associated with potentially beneficial effects on the expression and secretion of a range of factors, including adiponectin, resistin, IL-6, TNFalpha, plasminogen activator inhibitor-1, monocyte chemoattractant protein-1, and angiotensinogen, as well as a reduction in plasma nonesterified fatty acid supply. The effects of PPARgamma also extend to macrophages, where they suppress production of inflammatory mediators. As such, PPARgamma activation appears to have a beneficial effect on the relationship between the macrophage and adipocyte that is distorted in obesity. Thus, PPARgamma-activating ligands improve adipose tissue function and may have a role in preventing progression of insulin resistance to diabetes and endothelial dysfunction to atherosclerosis.

Search for the preadipocyte progenitor cell

An increase in adipocyte number is a major contributor to the increase in adipose tissue mass that is characteristic of obesity. The identity and regulation of the adipocyte precursor cell (or preadipocyte) and the preadipocyte precursor cell (or progenitor cell) have been intensely studied for many years. In this issue of the JCI, Crossno et al. report that progenitor cells originating from outside the adipose tissue, in particular the bone marrow, can contribute to an increase in adipocyte number (see the related article beginning on page 3220). Their study in mice reveals that treatment with the thiazolidinedione rosiglitazone or exposure to a high-fat diet promotes the trafficking of circulating bone marrow-derived progenitor cells into adipose tissue, where they become multilocular adipocytes. This adds a new and unexpected dimension to this research arena.

Effects of Pioglitazone Hydrochloride on Japanese Patients with Type 2 Diabetes Mellitus

AIM

The effects of pioglitazone hydrochloride monotherapy on abnormal lipid control were evaluated in Japanese patients with type 2 diabetes mellitus, comparing with glibenclamide monotherapy.

METHODS

Patients were randomly assigned to receive, once daily, pioglitazone hydrochloride, at 15 mg or 30 mg (n=46), or glibenclamide, at 1.25 mg or 2.5 mg (n=46). The 24-week study included patients with type 2 diabetes having high levels of triglyceride (TG).

RESULTS

Pioglitazone hydrochloride produced beneficial effects on dyslipidemia in patients with type 2 diabetes, compared with the baseline and the glibenclamide group, as demonstrated by increases in high-density lipoprotein cholesterol (HDL-C) levels and low-density lipoprotein cholesterol (LDL) particle size, a fall in TG levels, and an increased ratio of visceral to subcutaneous fat volumes (V/S). Pioglitazone hydrochloride reduced fasting serum insulin levels, with low fasting plasma glucose (FPG) and glycohemoglobin levels, compared to the baseline, suggesting an improvement of insulin resistance.

CONCLUSION

As expected, glibenclamide reduced FPG levels through increased insulin secretion. Pioglitazone hydrochloride and glibenclamide were well tolerated. Pioglitazone hydrochloride improved dyslipidemia related to insulin resistance, whereas glibenclamide enhanced insulin secretion, with only a minor effect on lipid control, in Japanese patients with type 2 diabetes.

Dietary conjugated linoleic acid preserves pancreatic function and reduces inflammatory markers in obese, insulin-resistant rats

Pancreatic preservation is an important part of diabetes management that may occur with improved peripheral insulin sensitivity and attenuated low-grade adipose tissue inflammation. The objective of the current study was to determine the response of obese, insulin-resistant fa/fa Zucker rats vs lean controls to dietary conjugated linoleic acid (CLA) supplementation with respect to pancreatic islet size, insulin resistance, and markers of inflammation and adipose glucose uptake. Six-week-old fa/fa and lean Zucker rats (n = 20 per genotype) were fed either a 1.5% CLA mixture or control diet for 8 weeks. Oral glucose tolerance testing was conducted at 7.5 weeks. Fasting serum haptoglobin, insulin, and C-peptide were assayed, and select messenger RNA (mRNA) and protein markers of inflammation and glucose metabolism were measured in adipose and liver tissues. CLA-fed fa/fa Zucker rats had smaller islet cell size, improved oral glucose tolerance and insulinemia, and attenuated serum haptoglobin levels compared with control-fed fa/fa Zucker rats, despite no differences in body weight and a slightly higher visceral adipose mass. CLA did not alter insulin sensitivity or islet size in lean Zucker rats. The CLA-fed fa/fa rats also had greater adipose glucose transporter-4 mRNA and less adipose tumor necrosis factor alpha mRNA and protein compared with control-fed fa/fa rats. In contrast, other markers of inflammation and glucose metabolism including adipose macrophage inflammatory factor, macrophage inflammatory protein-2, and liver pyruvate carboxylase and pyruvate dehydrogenase kinase 4 were not significantly changed. These results suggest that CLA supplementation preserved pancreatic function in conjunction with improved peripheral glucose use and reduced inflammation in fa/fa Zucker rats.

Involvement of adipose tissues in the early hypolipidemic action of PPARgamma agonism in the rat

Agonists of the peroxisome proliferator-activated receptor gamma (PPARgamma) are insulin sensitizers that potently improve lipemia in rodents. This study aimed to determine the contribution of lipid secretion vs. clearance and the involvement of white adipose tissue (WAT) and brown adipose tissue (BAT) in the rapid hypolipidemic action of PPARgamma agonism. Male rats were treated with rosiglitazone (RSG; 15 mg x kg(-1) x day(-1)) for 1 to 4 days, and determinants of lipid metabolism were assessed postprandially. Serum triglycerides (TG) were lowered (-54%) after 3 days of RSG treatment, due to accelerated clearance from blood without contribution of changes in secretion rates. Both BAT and WAT were the major sites of RSG action on TG clearance, the increase in TG-derived fatty acid (FA) uptake reaching threefold in BAT and 60-90% in WAT depots. Accelerated TG clearance was associated with increased lipoprotein lipase (LPL) activity mostly in BAT. Serum nonesterified FA were lowered (-20%) by a single dose of RSG, an effect associated with increased expression levels of FA binding/transport (fatty acid binding protein-4), esterification (diacylglycerol acyltransferase-1), and recycling glycerol kinase and phosphoenolpyruvate carboxykinase enzymes in BAT and WAT, suggesting FA trapping. After 4 days of RSG treatment, nonesterified fatty acid (NEFA) uptake was also stimulated in both BAT (2.5-fold) and WAT (40%). These findings demonstrate the causal involvement of increased efficiency of LPL-mediated TG clearance and reveal the important contribution of TG-derived and albumin-bound FA uptake by BAT in the rapid hypolipidemic action of PPARgamma agonism in the rat.

Effects of rosiglitazone and high fat diet on lipase/esterase expression in adipose tissue

A number of intracellular lipase/esterase have been reported in adipose tissue either by functional assays of activity or through proteomic analysis. In the current work, we have studied the relative expression level of 12 members of the lipase/esterase family that are found in white adipose tissue. We found that the relative mRNA levels of ATGL and HSL are the most abundant, being 2-3 fold greater than TGH or ADPN; whereas other intracellular neutral lipase/esterases were expressed at substantially lower levels. High fat feeding did not alter the mRNA expression levels of most lipase/esterases, but did reduce CGI-58 and WBSCR21. Likewise, rosiglitazone treatment did not alter the mRNA expression levels of most lipase/esterases, but did increase ATGL, TGH, CGI-58 and WBSCR21, while reducing ADPN. WAT from HSL-/- mice showed no compensatory increase in any lipase/esterases, rather mRNA levels of most lipase/esterases were reduced. In contrast, BAT from HSL-/- mice showed an increase in ATGL expression, as well as a decrease in ES-1, APEH and WBSCR21. Analysis of the immunoreactive protein levels of some of the lipases confirmed the results seen with mRNA. In conclusion, these data highlight the complexity of the regulation of the expression of intracellular neutral lipase/esterases involved in lipolysis.

Pioglitazone Increases Gallbladder Volume in Insulin-Resistant Obese Mice

BACKGROUND

Both obesity and diabetes are associated with an increased incidence of gallstones. Recent animal and human data from our laboratory suggest that insulin resistance is associated with increased gallbladder volume and/or impaired gallbladder emptying. Pioglitazone is a thiazolidinedione that has been shown to improve insulin resistance. Therefore, we tested the hypothesis that pioglitazone would improve insulin resistance, decrease resting gallbladder volume and improve gallbladder response to neurotransmitters in insulin-resistant obese mice fed a 25% carbohydrate diet.

MATERIALS AND METHODS

Twenty eight-week-old insulin-resistant obese (Lep(ob)) mice fed a 25% carbohydrate diet for 4 weeks. Half of the animals had 0.3 g/kg pioglitazone added to their diet. At 12 weeks all animals were fasted and underwent cholecystectomy. Gallbladder volume and weight were measured, and fresh gallbladders were placed in a muscle bath to assess response to acetylcholine (ACh 10(-5)M), neuropeptide Y (NPY 10(-8,-7,-6)M) and cholecystokinin (CCK 10(-10,-9,-8,-7)M). Serum glucose and insulin were measured, and HOMA Index, a measure of insulin resistance, was calculated.

RESULTS

Fasting serum insulin and HOMA Index were significantly decreased (P < 0.03), but gallbladder volume was significantly increased (P < 0.03) in the pioglitazone treated group. Pioglitazone did not alter gallbladder weight or response to ACh, NPY, or CCK.

CONCLUSION

These data suggest that in insulin-resistant obese mice pioglitazone 1) lowers insulin-resistance, 2) increases resting gallbladder volume, and 3) does not alter gallbladder response to neurotransmitters. Therefore, we conclude that pioglitazone, while improving insulin resistance, paradoxically increases gallbladder volume and, thereby, may increase the propensity for gallstone formation by enhancing gallbladder stasis.

Review: The role of insulin resistance in nonalcoholic fatty liver disease

CONTEXT

Insulin resistance is an almost universal finding in nonalcoholic fatty liver disease (NAFLD). This review outlines the evidence linking insulin resistance and NAFLD, explores whether liver fat is a cause or consequence of insulin resistance, and reviews the current evidence for treatment of NAFLD.

EVIDENCE ACQUISITION

Evidence from epidemiological, experimental, and clinical research studies investigating NAFLD and insulin resistance was reviewed.

EVIDENCE SYNTHESIS

Insulin resistance in NAFLD is characterized by reductions in whole-body, hepatic, and adipose tissue insulin sensitivity. The mechanisms underlying the accumulation of fat in the liver may include excess dietary fat, increased delivery of free fatty acids to the liver, inadequate fatty acid oxidation, and increased de novo lipogenesis. Insulin resistance may enhance hepatic fat accumulation by increasing free fatty acid delivery and by the effect of hyperinsulinemia to stimulate anabolic processes. The impact of weight loss, metformin, and thiazolidinediones, all treatments aimed at improving insulin sensitivity, as well as other agents such as vitamin E, have been evaluated in patients with NAFLD and have shown some benefit. However, most intervention studies have been small and uncontrolled.

CONCLUSION

Insulin resistance is a major feature of NAFLD that, in some patients, can progress to steatohepatitis. Treatments aimed at reducing insulin resistance have had some success, but larger placebo-controlled studies are needed to fully establish the efficacy of these interventions and possibly others in reducing the deleterious effects of fat accumulation in the liver.

Mechanisms of the depot specificity of peroxisome proliferator-activated receptor gamma action on adipose tissue metabolism

In this study, we aimed to establish the mechanisms whereby peroxisome proliferator-activated receptor gamma (PPARgamma) agonism brings about redistribution of fat toward subcutaneous depots and away from visceral fat. In rats treated with the full PPARgamma agonist COOH (30 mg x kg(-1) x day(-1)) for 3 weeks, subcutaneous fat mass was doubled and that of visceral fat was reduced by 30% relative to untreated rats. Uptake of triglyceride-derived nonesterified fatty acids was greatly increased in subcutaneous fat (14-fold) and less so in visceral fat (4-fold), with a concomitant increase, restricted to subcutaneous fat only, in mRNA levels of the uptake-, retention-, and esterification-promoting enzymes lipoprotein lipase, aP2, and diacylglycerol acyltransferase 1. Basal lipolysis and fatty acid recycling were stimulated by COOH in both subcutaneous fat and visceral fat, with no frank quantitative depot specificity. The agonist increased mRNA levels of enzymes of fatty acid oxidation and thermogenesis much more strongly in visceral fat than in subcutaneous fat, concomitantly with a stronger elevation in O2 consumption in the former than in the latter. Mitochondrial biogenesis was stimulated equally in both depots. These findings demonstrate that PPARgamma agonism redistributes fat by stimulating the lipid uptake and esterification potential in subcutaneous fat, which more than compensates for increased O2 consumption; conversely, lipid uptake is minimally altered and energy expenditure is greatly increased in visceral fat, with consequent reduction in fat accumulation.

Acquired obesity increases CD68 and tumor necrosis factor-alpha and decreases adiponectin gene expression in adipose tissue: a study in monozygotic twins

CONTEXT

Both acquired and genetic factors regulate adipose tissue function.

OBJECTIVE

We determined whether adipose tissue mRNA expression is regulated by obesity, independently of genetic effects, by studying monozygotic (MZ) twins.

DESIGN

Seventeen healthy pairs of MZ twins aged 24-27 yr (body mass index 20.0-33.9 kg/m(2), intrapair differences in body weight 0.1-24.7 kg), were identified from the population-based FinnTwin16 cohort. Body fat percent was determined by dual-energy x-ray absorptiometry, sc and intraabdominal fat by magnetic resonance imaging, liver fat by proton spectroscopy, and insulin sensitivity by using the euglycemic insulin clamp technique. Adipocyte cell size and expression of 10 genes (real-time PCR) were determined in sc adipose tissue biopsies. Serum levels of some of the genes were measured using ELISA.

RESULTS

Within MZ twin pairs, acquired obesity was significantly related to increased adipocyte size and increased adipose tissue mRNA expressions of leptin, TNFalpha and the macrophage marker CD68, and decreased mRNA expressions of adiponectin and peroxisome proliferator-activated receptor-gamma. Intrapair differences in liver fat correlated directly with those in leptin and CD68 expression. CD68 expression and serum TNFalpha concentrations were correlated with insulin resistance.

CONCLUSIONS

Acquired obesity independent of genetic influences is able to increase expression of macrophage and inflammatory markers and decrease adiponectin expression in adipose tissue.

Telmisartan Prevents Obesity and Increases the Expression of Uncoupling Protein 1 in Diet-Induced Obese Mice

The aim of the present study was to clarify the effect of telmisartan, an angiotensin II receptor blocker, on the development of obesity and related metabolic disorders in diet-induced obese mice. Treatment with telmisartan dissolved in drinking water at a dosage of 5 mg/kg per day for 14 days attenuated the diet-induced weight gain without affecting food intake in diet-induced obese mice compared with controls using nontreated water. Telmisartan treatment decreased the weight of visceral adipose tissue and the triglyceride content in the liver and skeletal muscle. In addition, hyperglycemia, hyperinsulinemia, and hypertriglyceridemia in diet-induced obese mice all improved with telmisartan treatment. Furthermore, telmisartan treatment increased adiponectin mRNA in visceral white adipose tissue and was associated with a concomitant change in the serum adiponectin level. In contrast, the treatment reduced the serum level of resistin. Finally, telmisartan treatment increased the mRNA expression of uncoupling protein 1 in brown adipose tissue and was accompanied by an increase in oxygen consumption. In conclusion, telmisartan treatment might prevent the development of obesity and related metabolic disorders by altering the levels of adiponectin, resistin, and uncoupling protein 1 in diet-induced obese mice. Our results indicate that telmisartan can be used as a therapeutic tool for metabolic syndrome, including visceral obesity.

Endocrine-disrupting organotin compounds are potent inducers of adipogenesis in vertebrates

Dietary and xenobiotic compounds can disrupt endocrine signaling, particularly of steroid receptors and sexual differentiation. Evidence is also mounting that implicates environmental agents in the growing epidemic of obesity. Despite a long-standing interest in such compounds, their identity has remained elusive. Here we show that the persistent and ubiquitous environmental contaminant, tributyltin chloride (TBT), induces the differentiation of adipocytes in vitro and increases adipose mass in vivo. TBT is a dual, nanomolar affinity ligand for both the retinoid X receptor (RXR) and the peroxisome proliferator-activated receptor gamma (PPARgamma). TBT promotes adipogenesis in the murine 3T3-L1 cell model and perturbs key regulators of adipogenesis and lipogenic pathways in vivo. Moreover, in utero exposure to TBT leads to strikingly elevated lipid accumulation in adipose depots, liver, and testis of neonate mice and results in increased epididymal adipose mass in adults. In the amphibian Xenopus laevis, ectopic adipocytes form in and around gonadal tissues after organotin, RXR, or PPARgamma ligand exposure. TBT represents, to our knowledge, the first example of an environmental endocrine disrupter that promotes adipogenesis through RXR and PPARgamma activation. Developmental or chronic lifetime exposure to organotins may therefore act as a chemical stressor for obesity and related disorders.

Pioglitazone: a review of its use in type 2 diabetes mellitus

Pioglitazone is an antihyperglycaemic agent that, in the presence of insulin resistance, increases hepatic and peripheral insulin sensitivity, thereby inhibiting hepatic gluconeogenesis and increasing peripheral and splanchnic glucose uptake. Pioglitazone is generally well tolerated, weight gain and oedema are the most common emergent adverse events, and there are no known drug interactions between pioglitazone and other drugs. In clinical trials in patients with type 2 diabetes mellitus, pioglitazone as monotherapy, or in combination with metformin, repaglinide, insulin or a sulphonylurea, induced both long- and short-term improvements in glycaemic control and serum lipid profiles. Pioglitazone was also effective in reducing some measures of cardiovascular risk and arteriosclerosis. Pioglitazone thus offers an effective treatment option for the management of patients with type 2 diabetes.

Recent findings concerning thiazolidinediones in the treatment of diabetes

Thiazolidinediones (TZDs) are peroxisomal proliferator-activated receptor (PPAR)-gamma agonists. They increase insulin action through several mechanisms including: stimulation of the expression of genes that increase fat oxidation and lower plasma free fatty acid levels; increased expression, synthesis and release of adiponectin; and stimulation of adipocyte differentiation resulting in more and smaller fat cells. TZDs lower blood sugar comparably to sulfonylureas and metformin. The clinical use of TZDs is limited due to the long duration of time required before they reach their full blood sugar-lowering action (3-4 months) and adverse effects such as fluid retention, resulting in excessive weight gain and occasionally in peripheral and/or pulmonary oedema and congestive heart failure. Troglitazone, a TZD that has since been removed from the market because of hepatoxicity, has been demonstrated to decrease the progression from normal or impaired glucose tolerance to overt Type 2 diabetes mellitus. Pioglitazone, another TZD, marginally decreased the incidence of cardiovascular complications in patients with Type 2 diabetes mellitus (PROactive trial). Other, as yet, unapproved uses of TZDs include: non-alcoholic fatty liver disease, in which TZDs reduced hepatic fat accumulation and improved liver function tests; polycystic ovary syndrome, where TZDs improved ovulation, hirsutism and endothelial dysfunction; and lipodystrophies, where TZDs increased body fat (marginally) and decrease liver size. Lastly, because PPAR-alpha and -gamma agonists improve atherosclerotic vascular disease and insulin sensitivity, respectively, dual PPAR-alpha/gamma agonists, which are currently undergoing clinical trials, may be useful in treating patients with the metabolic syndrome.

Anti-hyperglycemic effects of plum in a rat model of obesity and type 2 diabetes, Wistar fatty rat

Dried plums, considered a healthy food in the West and used as medicine in India, contain phenolic compounds with protective actions against age-related diseases. Effects of oral plum ekisu (concentrated juice) on lipid and glucose tolerance were assessed in insulin-resistant obese Wistar fatty rats. Plum ingestion decreased blood glucose (P < 0.05) and plasma triglyceride concentrations (P < 0.01) compared with controls. Plum treatment for 2 weeks reduced areas under the curve (AUCs) for glucose and insulin during a glucose tolerance test. In db/db mice, plum decreased these AUCs, and also blood glucose during an insulin tolerance test. Plum treatment significantly increased plasma adiponectin concentrations and PPARgamma mRNA expression in adipose tissue from Wistar fatty rats. Plum thus may increase insulin sensitivity in these rats via adiponectin-related mechanisms.

Thiazolidinediones for the Therapeutic Management of Polycystic Ovary Syndrome

Polycystic ovary syndrome (PCOS) is a diagnosis made between late adolescence and the menopause in 5-10% of women. PCOS is a heterogeneous disorder of unknown etiology characterized by hyperandrogenic chronic anovulation. This syndrome consists of a diverse constellation of signs and symptoms, such as hirsutism, acne, acanthosis nigricans, obesity, menstrual irregularities, anovulation, and/or infertility. Features of the metabolic syndrome, including obesity, insulin resistance, and dyslipidemia, are common in this patient population. Recent insights into the pathophysiology of PCOS have shown insulin resistance and hyperinsulinemia to play a substantial role. Insulin resistance is increasingly recognized as a chronic, low-level, inflammatory state. Recent studies show that serum levels of inflammatory mediators, such as tumor necrosis factor-alpha and interleukin-6, are increased in the insulin-resistant conditions of obesity and PCOS. The optimal modality for long-term treatment should have positive effects on androgen synthesis, sex hormone-binding globulin production, the lipid profile, insulin sensitivity, inflammatory mediators, and clinical symptoms including acne, hirsutism, and irregular menstrual cycles. Treatment with insulin-sensitizing agents is a relatively new therapeutic strategy in women with PCOS. Current research has shown that the use of diabetes mellitus management practices aimed at reducing insulin resistance and hyperinsulinemia (such as weight reduction and the administration of oral antidiabetic drugs) can not only reverse testosterone and luteinizing hormone abnormalities and restore menstrual cycles, but can also improve glucose, insulin, proinflammatory cytokine, and lipid profiles.Clinical treatment with troglitazone, a member of the thiazolidinedione family, for the management of PCOS complications such as insulin resistance, hyperandrogenism, and anovulation was found to have beneficial effects; however, it was taken off the market over concerns of hepatotoxicity. Although troglitazone is no longer available in the US, numerous clinical trials have established the role of thiazolidinediones in the treatment of women with PCOS. Clinical data emerging regarding the utility of two of the newer, safer thiazolidinediones, pioglitazone and rosiglitazone, for this patient population, consistently demonstrate effective improvements of endocrine and ovulatory performance in women with PCOS. The benefit and importance of lifestyle modification and weight reduction, when it can be achieved, is still an important component in the long-term treatment of PCOS. Pharmacologic reduction in insulin levels using thiazolidinediones appears to offer another therapeutic modality for PCOS, which may ameliorate the progress of both hyperinsulinemia and hyperandrogenism. However, additional studies of patients so treated are necessary before these agents can be considered first-line treatment for PCOS. Convincing data from randomized controlled trials with sufficient power to detect both the benefits and risks of long-term treatment with thiazolidinediones in women with PCOS remain to be obtained.

Metformin and pioglitazone: Effectively treating insulin resistance

BACKGROUND

Insulin resistance has a complex etiology, with multiple manifestations across the organ systems involved in glucose homeostasis. Glucose-lowering drug therapies that target insulin resistance can therefore utilize different mechanistic approaches. Two key classes of insulin-sensitizing agents--the biguanides (principally metformin) and thiazolidinediones (pioglitazone and rosiglitazone)--have distinct molecular mechanisms of action and differing effects on metabolic dysfunction. This provides an opportunity for complementary beneficial effects in the treatment of type 2 diabetes and on the potential consequences of insulin resistance, such as dyslipidemia and atherosclerosis.

SCOPE

This review (based upon EMBASE and MEDLINE searches from January 1990 to April 2006) highlights the mechanistic distinctions and clinical data that support the rationale for thiazolidinedione/metformin combination therapy in patients with type 2 diabetes.

FINDINGS

The different insulin-sensitizing mechanisms of metformin and the thiazolidinediones are manifest in partially distinct effects on hepatic and peripheral glucose homeostasis, and clinical studies show improved glucose control with combination therapy. Both metformin and thiazolidinediones may also have pancreatic beta-cell preserving properties. Furthermore, they have different beneficial effects on several other metabolic risk markers and risk factors for cardiovascular disease. Whereas the thiazolidinediones (particularly pioglitazone) have greater effects on multiple aspects of dyslipidemia, metformin has anorexigenic properties. They also have distinct effects on levels of mediators involved in inflammation and endothelial dysfunction, and outcome studies suggest that either pioglitazone or metformin may reduce the risk of macrovascular events.

CONCLUSION

The distinct, but complementary, mechanisms of action of the thiazolidinediones and metformin provide the opportunity for effective combination therapy with two insulin-sensitizing agents. Such an approach has consequences, not only for improved glucose control, but also for reducing metabolic risk and potentially improving major cardiovascular disease outcomes.

Muraglitazar, a dual (α/γ) PPAR activator: A randomized, double-blind, placebo-controlled, 24-week monotherapy trial in adult patients with type 2 diabetes

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) present a therapeutic target, and simultaneous activation of PPAR-alpha and PPAR-gamma may provide improvements in glycemic control and dyslipidemia in patients with type 2 diabetes.

OBJECTIVE

The goal of this study was to evaluate the efficacy and safety of muraglitazar, a dual (alpha/gamma) PPAR activator, in adult patients with type 2 diabetes whose disease was inadequately controlled by diet and exercise.

METHODS

This was a randomized, double-blind, placebo-controlled, parallel-group, multicenter, 24-week monotherapy study in drug-naive, type 2 diabetes patients with inadequate glycemic control. Men and women aged 18 to 70 years with a body mass index < or =41 kg/m(2) and serum triglyceride levels < or =600 mg/dL were eligible for study participation. The study included double-blind and open-label treatment phases. Patients with glycosylated hemoglobin (HbA(1c)) levels > or =7.0% and < or =10.0% at screening were enrolled in the double-blind treatment phase. These patients received treatment with muraglitazar 2.5 mg, muraglitazar 5 mg, or placebo. Patients with HbA(1c) levels >10.0% and < or =12.0% who met all other study criteria were eligible for enrollment in a 24-week, open-label evaluation of muraglitazar 5 mg. The primary end point was the mean change from baseline in HbA(1c) levels after 24 weeks of treatment.

RESULTS

A total of 340 patients (179 men, 161 women) participated in the double-blind treatment phase of the study. Patients had mean baseline HbA(1c) levels of 7.9% to 8.0%. Monotherapy with muraglitazar 2.5 and 5 mg significantly reduced HbA(1c) levels (-1.05% and -1.23%, respectively) compared with placebo (-0.32%; P < 0.001). At week 24, 58%, 72%, and 30% of the patients receiving muraglitazar 2.5 mg, muraglitazar 5 mg, and placebo, respectively, achieved the American Diabetes Association-recommended HbA(1c) goal of <7.0%. Fasting plasma glucose, free fatty acids, and fasting plasma insulin levels significantly decreased during muraglitazar treatment (P < 0.001), suggesting an increase in insulin sensitivity. Muraglitazar 2.5 and 5 mg provided improvements from baseline in triglyceride (-18% and -27%), high-density lipoprotein (HDL) cholesterol (10% and 16%), apolipoprotein B (-7% and -12%), and non-HDL cholesterol levels (-3% and -5%) (P < 0.05 vs placebo for each). In a parallel, open-label cohort of 109 drug-naive patients (56 men, 53 women; mean baseline HbA(1c) level, 10.6%), muraglitazar 5 mg decreased the overall mean HbA(1c) level from baseline by 2.62% (last observation carried forward) and by 3.49% in the 62 patients completing 24 weeks of study. Changes in lipid parameters during open-label treatment were similar to those observed during double-blind treatment. Muraglitazar was generally well tolerated. Edema-related adverse events of mild to moderate severity occurred in 8% to 11% of patients in all groups. Mean changes from baseline weight in the double-blind treatment groups were 1.1 kg for muraglitazar 2.5 mg, 2.1 kg for muraglitazar 5 mg, and -0.8 kg for placebo (P < 0.001); there was a mean 2.9-kg increase in the open-label muraglitazar 5-mg group.

CONCLUSION

In this study, 24 weeks of treatment with muraglitazar 2.5 or 5 mg was an effective treatment option for these patients with type 2 diabetes whose disease was inadequately controlled with diet and exercise.

PPARgamma regulates adipocyte cholesterol metabolism via oxidized LDL receptor 1

In addition to its role in energy storage, adipose tissue also accumulates cholesterol. Concentrations of cholesterol and triglycerides are strongly correlated in the adipocyte, but little is known about mechanisms regulating cholesterol metabolism in fat cells. Here we report that antidiabetic thiazolidinediones (TZDs) and other ligands for the nuclear receptor PPARgamma dramatically upregulate oxidized LDL receptor 1 (OLR1) in adipocytes by facilitating the exchange of coactivators for corepressors on the OLR1 gene in cultured mouse adipocytes. TZDs markedly stimulate the uptake of oxidized LDL (oxLDL) into adipocytes, and this requires OLR1. Increased OLR1 expression, resulting either from TZD treatment or adenoviral gene delivery, significantly augments adipocyte cholesterol content and enhances fatty acid uptake. OLR1 expression in white adipose tissue is increased in obesity and is further induced by PPARgamma ligand treatment in vivo. Serum oxLDL levels are decreased in both lean and obese diabetic animals treated with TZDs. These data identify OLR1 as a novel PPARgamma target gene in adipocytes. While the physiological role of adipose tissue in cholesterol and oxLDL metabolism remains to be established, the induction of OLR1 is a potential means by which PPARgamma ligands regulate lipid metabolism and insulin sensitivity in adipocytes.

Combined thiazolidinedione-insulin therapy: should we be concerned about safety?

Thiazolidinediones, also called glitazones, are insulin sensitisers that act as agonists of the peroxisome proliferator-activated receptors-gamma (PPARgamma). After the withdrawal of troglitazone due to hepatotoxicity, only pioglitazone and rosiglitazone can be used for treating patients with type 2 diabetes mellitus, either as monotherapy or in combination with metformin or with sulphonylureas (or glinides). The combination of glitazones with insulin is also appealing, as it allows improvement of glycaemic control while decreasing the daily insulin requirement. Insulin dosage has to be adjusted regularly to avoid hypoglycaemic episodes. However, some concerns have been raised about such combined glitazone-insulin therapy because it may favour weight gain due to both enhanced adipogenesis and fluid retention. Such adverse effects are commonly observed in all diabetic individuals receiving glitazones, whatever the mode of use, but they appear to be exacerbated in insulin-treated patients. Body fat gain is a major drawback of treatment with adipogenic compounds such as glitazones. However, some evidence suggests that the fat is redistributed in a favourable direction, that is, from visceral to subcutaneous depots, although no long-term follow-up is yet available. An estimated 2-5% of patients receiving glitazone monotherapy and 5-15% receiving concomitant insulin therapy experience peripheral oedema. Some anecdotal cases of pulmonary oedema have also been reported, especially in insulin-treated patients, although the actual incidence of this complication is unknown. All glitazones increase the intravascular volume by approximately 6-7% in a dose-dependent manner. Rather than a direct effect on cardiac or renal function, fluid retention and tissue oedema seem to be part of a vascular 'leak' syndrome. Such a phenomenon may have greater consequences in patients with type 2 diabetes treated with insulin because such patients are usually older, have had the disease long-term and have worse cardiac or renal function. Additionally, glitazones may potentiate the renal effects of insulin on sodium and water retention. Regardless of the mechanism, it is conceivable that additional fluid retention caused by glitazones may alter the already precarious volume status in patients with underlying cardiac or renal dysfunction, thus leading to oedema and congestive heart failure. Thus, it is prudent to either avoid glitazones or use them cautiously in individuals with impaired cardiac function. Further studies are clearly needed to define the mechanisms of fluid retention associated with glitazone use and to determine the safety of cautious use of these new insulin sensitisers in insulin-treated patients with type 2 diabetes.

Therapeutic actions of an insulin receptor activator and a novel peroxisome proliferator-activated receptor gamma agonist in the spontaneously hypertensive obese rat model of metabolic syndrome X

Insulin resistance clusters with hyperlipidemia, impaired glucose tolerance, and hypertension as metabolic syndrome X. We tested a low molecular weight insulin receptor activator, demethylasterriquinone B-1 (DMAQ-B1), and a novel indole peroxisome proliferator-activated receptor gamma agonist, 2-(2-(4-phenoxy-2-propylphenoxy)ethyl)indole-5-acetic acid (PPEIA), in spontaneously hypertensive obese rats (SHROB), a genetic model of syndrome X. Agents were given orally for 19 days. SHROB showed fasting normoglycemia but impaired glucose tolerance after an oral load, as shown by increased glucose area under the curve (AUC) [20,700 mg x min/ml versus 8100 in lean spontaneously hypertensive rats (SHR)]. Insulin resistance was indicated by 20-fold excess fasting insulin and increased insulin AUC (6300 ng x min/ml versus 990 in SHR). DMAQ-B1 did not affect glucose tolerance (glucose AUC = 21,300) but reduced fasting insulin 2-fold and insulin AUC (insulin AUC = 4300). PPEIA normalized glucose tolerance (glucose AUC = 9100) and reduced insulin AUC (to 3180) without affecting fasting insulin. PPEIA also increased food intake, fat mass, and body weight gain (81 +/- 12 versus 45 +/- 8 g in untreated controls), whereas DMAQ-B1 had no effect on body weight but reduced subscapular fat mass. PPEIA but not DMAQ-B1 reduced blood pressure. In skeletal muscle, insulin-stimulated phosphorylation of the insulin receptor and insulin receptor substrate protein 1-associated phosphatidylinositol 3-kinase activity were decreased by 40 to 55% in SHROB relative to lean SHR. PPEIA, but not DMAQ-B1, enhanced both insulin actions. SHROB also showed severe hypertriglyceridemia (355 +/- 42 mg/dl versus 65 +/- 3 in SHR) attenuated by both agents (DMAQ-B1, 228 +/- 18; PPEIA, 79 +/- 3). Both these novel antidiabetic agents attenuate insulin resistance and hypertriglyceridemia associated with metabolic syndrome but via distinct mechanisms.

The effect of PPARgamma ligands on the adipose tissue in insulin resistance

Insulin resistance is frequently accompanied by obesity and both obesity and type 2 diabetes are associated with a mild chronic inflammation. Elevated levels of various cytokines, such as TNF-alpha and IL-6, are typically found in the adipose tissue in these conditions. It has been suggested that many cytokines produced in the adipose tissue are derived from infiltrated inflammatory cells. However, the adipose tissue itself has proven to be an important endocrine organ, secreting several hormones and cytokines, usually referred to as adipokines. Peroxisome proliferator-activated receptor (PPAR)gamma is essential for adipocyte proliferation and differentiation. In recent years, PPARgamma and its ligands, the thiazolidinediones (TZD), have achieved great attention due to their insulin sensitizing and anti-inflammatory properties. Treatment with TZDs result in improved insulin signaling and adipocyte differentiation, increased adipose tissue influx of free fatty acids and inhibition of cytokine expression and action. As a result, PPARgamma plays a central role in maintaining a functional and differentiated adipose tissue.

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.

Pioglitazone induces mitochondrial biogenesis in human subcutaneous adipose tissue in vivo

Thiazolidenediones such as pioglitazone improve insulin sensitivity in diabetic patients by several mechanisms, including increased uptake and metabolism of free fatty acids in adipose tissue. The purpose of the present study was to determine the effect of pioglitazone on mitochondrial biogenesis and expression of genes involved in fatty acid oxidation in subcutaneous fat. Patients with type 2 diabetes were randomly divided into two groups and treated with placebo or pioglitazone (45 mg/day) for 12 weeks. Mitochondrial DNA copy number and expression of genes involved in mitochondrial biogenesis were quantified by real-time PCR. Pioglitazone treatment significantly increased mitochondrial copy number and expression of factors involved in mitochondrial biogenesis, including peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1alpha and mitochondrial transcription factor A. Treatment with pioglitazone stimulated the expression of genes in the fatty acid oxidation pathway, including carnitine palmitoyltransferase-1, malonyl-CoA decarboxylase, and medium-chain acyl-CoA dehydrogenase. The expression of PPAR-alpha, a transcriptional regulator of genes encoding mitochondrial enzymes involved in fatty acid oxidation, was higher after pioglitazone treatment. Finally, the increased mitochondrial copy number and the higher expression of genes involved in fatty acid oxidation in human adipocytes may contribute to the hypolipidemic effects of pioglitazone.

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.

Divergent effects of rosiglitazone on protein-mediated fatty acid uptake in adipose and in muscle tissues of Zucker rats

Thiazolidinediones (TZDs) increase tissue insulin sensitivity in diabetes. Here, we hypothesize that, in adipose tissue, skeletal muscle, and heart, alterations in protein-mediated FA uptake are involved in the effect of TZDs. As a model, we used obese Zucker rats, orally treated for 16 days with 5 mg rosiglitazone (Rgz)/kg body mass/day. In adipose tissue from Rgz-treated rats, FA uptake capacity increased by 2.0-fold, coinciding with increased total contents of fatty acid translocase (FAT/CD36; 2.3-fold) and fatty acid transport protein 1 (1.7-fold) but not of plasmalemmal fatty acid binding protein, whereas only the plasmalemmal content of FAT/CD36 was changed (increase of 1.7-fold). The increase in FA uptake capacity of adipose tissue was associated with a decline in plasma FA and triacylglycerols (TAGs), suggesting that Rgz treatment enhanced plasma FA extraction by adipocytes. In obese hearts, Rgz treatment had no effect on the FA transport system, yet the total TAG content decreased, suggesting enhanced insulin sensitivity. Also, in skeletal muscle, the FA transport system was not changed. However, the TAG content remained unaltered in skeletal muscle, which coincided with increased cytoplasmic adipose-type FABP content, suggesting that increased extramyocellular TAGs mask the decline of intracellular TAG in muscle. In conclusion, our study implicates FAT/CD36 in the mechanism by which Rgz increases tissue insulin sensitivity.

Thiazolidinediones upregulate fatty acid uptake and oxidation in adipose tissue of diabetic patients

Thiazolidinediones (TZDs) are a new class of insulin-sensitizing drugs. To explore how and in which tissues they improve insulin action, we obtained fat and muscle biopsies from eight patients with type 2 diabetes before and 2 months after treatment with rosiglitazone (n = 5) or troglitazone (n = 3). TZD treatment was associated with a coordinated upregulation in the expression of genes and synthesis of proteins involved in fatty acid uptake, binding, beta-oxidation and electron transport, and oxidative phosphorylation in subcutaneous fat but not in skeletal muscle. These changes were accompanied by a 13% increase in total body fat oxidation, a 20% decrease in plasma free fatty acid levels, and a 46% increase in insulin-stimulated glucose uptake. We conclude that TZDs induced a coordinated stimulation of fatty acid uptake, oxidation, and oxidative phosphorylation in fat of diabetic patients and thus may have corrected, at least partially, a recently recognized defect in patients with type 2 diabetes consisting of reduced expression of genes related to oxidative metabolism and mitochondrial function.

Improved insulin sensitivity and adipose tissue dysregulation after short-term treatment with pioglitazone in non-diabetic, insulin-resistant subjects

AIMS/HYPOTHESIS

We examined whether short-term treatment with a thiazolidinedione improves insulin sensitivity in non-obese but insulin-resistant subjects and whether this is associated with an improvement in dysregulated adipose tissue (reduced expression of IRS-1, GLUT4, PPARgamma co-activator 1 and markers of terminal differentiation) that we have previously documented to be associated with insulin resistance.

METHODS

Ten non-diabetic subjects, identified as having low IRS-1 and GLUT-4 protein in adipose cells as markers of insulin resistance, underwent 3 weeks of treatment with pioglitazone. The euglycaemic-hyperinsulinaemic clamp technique was used to measure insulin sensitivity before and after treatment. Serum samples were analysed for glucose, insulin, lipids, total and high-molecular-weight (HMW) adiponectin levels. Biopsies from abdominal subcutaneous adipose tissue were taken, cell size measured, mRNA and protein extracted and quantified using real-time RT-PCR and Western blot.

RESULTS

Insulin sensitivity was improved after 3 weeks treatment and circulating total as well as HMW adiponectin increased in all subjects, while no effect was seen on serum lipids. In the adipose cells, gene and protein expression of IRS-1 and PPARgamma co-activator 1 remained unchanged, while adiponectin, adipocyte P 2, uncoupling protein 2, GLUT4 and liver X receptor-alpha increased. Insulin-stimulated tyrosine phosphorylation and p-ser-PKB/Akt increased, while no significant effect of thiazolidinedione treatment was seen on the inflammatory status of the adipose tissue in these non-obese subjects.

CONCLUSIONS/INTERPRETATION

Short-term treatment with pioglitazone improved insulin sensitivity in the absence of any changes in circulating NEFA or lipid levels. Several markers of adipose cell differentiation, previously shown to be reduced in insulin resistance, were augmented, supporting the concept that insulin resistance in these individuals is associated with impaired terminal differentiation of the adipose cells.

Evaluation of adipose tissue distribution in obese fa/fa Zucker rats by in vivo MR imaging: effects of peroxisome proliferator-activated receptor agonists

High-resolution MRI of obese (fa/fa) Zucker rats was investigated to characterize and assess in vivo adipose tissue distribution. Thirty animals were gavaged with a placebo, a PPARgamma activator (pioglitazone), or a dual PPARalphagamma activator (LM 4156). At day 15, T1-weighted images were acquired in vivo using a 2TMRI system with a high in-plane spatial resolution (254 microm). Fat volumes of selected territories were measured by image segmentation, and the retroperitoneal fat was weighed post-mortem. Body-weight gain was significant with pioglitazone (101.8+/-5.9 g, p<0.01 vs. placebo). The good quality of MR images allowed the delimitation and quantification of different fat territories. In response to pioglitazone, the retroperitoneal fat was more important compared to placebo (+23%, p<0.01) while subcutaneous fat was not different. No significant effects were observed with LM 4156. In vivo measurements of fat volumes were strongly correlated with ex vivo tissue weights (r=0.91). High-resolution MRI provides an in vivo measurement of adipose tissue distribution in obese Zucker rats. Specific fat depots of regions that were particularly involved in drug response were determined in vivo. Fat remodeling was observed with pioglitazone but not with a dual PPARalphagamma activator (LM 4156).

Soluble fibroin enhances insulin sensitivity and glucose metabolism in 3T3-L1 adipocytes

Type 2 diabetes is characterized by hyperglycemia and hyperinsulinemia, features of insulin resistance. In vivo treatment of ob/ob mice with hydrolyzed fibroin reverses these pathological attributes. To explore the mechanism underlying this effect, we used the murine, 3T3-L1 adipocyte cell line, which has been used extensively to model adipocyte function. Chronic exposure of 3T3-L1 adipocytes to insulin leads to a 50% loss of insulin-stimulated glucose uptake. Chronic exposure to different preparations of fibroin partially blocked the response to insulin but also increased the sensitivity of control cells to the acute action of insulin. The latter effect was most robust at physiologic concentrations of insulin. Fibroin did not prevent the insulin-induced downregulation of the insulin receptor or the tyrosine kinase activity associated with the receptor. Further, fibroin had no effect on the activity of the insulin-sensitive downstream kinase, Akt. Interestingly, fibroin accelerated glucose metabolism and glycogen turnover independent of insulin action. In addition, fibroin upregulated glucose transporter (GLUT)1, which increased its expression at the cell surface and enhanced GLUT4 translocation. Together, these phenomena may underlie the improvement in diabetic hyperglycemia noted in vivo in response to fibroin.

Antiobesity effects of A-331440, a novel non-imidazole histamine H3 receptor antagonist

Histamine affects homeostatic mechanisms, including food and water consumption, by acting on central nervous system (CNS) receptors. Presynaptic histamine H(3) receptors regulate release of histamine and other neurotransmitters, and histamine H(3) receptor antagonists enhance neurotransmitter release. A-331440 [4'-[3-(3(R)-(dimethylamino)-pyrrolidin-1-yl)-propoxy]-biphenyl-4-carbonitrile] is a histamine H(3) receptor antagonist which binds potently and selectively to both human and rat histamine H(3) receptors (K(i)<==25 nM). Mice were stabilized on a high-fat diet (45 kcal % lard) prior to 28-day oral b.i.d. dosing for measurement of obesity-related parameters. A-331440 administered at 0.5 mg/kg had no significant effect on weight, whereas 5 mg/kg decreased weight comparably to dexfenfluramine (10 mg/kg). A-331440 administered at 15 mg/kg reduced weight to a level comparable to mice on the low-fat diet. The two higher doses reduced body fat and the highest dose also normalized an insulin tolerance test. These data show that the histamine H(3) receptor antagonist, A-331440, has potential as an antiobesity agent.

Role of a critical visceral adipose tissue threshold (CVATT) in metabolic syndrome: implications for controlling dietary carbohydrates: a review

There are likely many scenarios and pathways that can lead to metabolic syndrome. This paper reviews mechanisms by which the accumulation of visceral adipose tissue (VAT) may contribute to the metabolic syndrome, and explores the paradigm of a critical VAT threshold (CVATT). Exceeding the CVATT may result in a number of metabolic disturbances such as insulin resistance to glucose uptake by cells. Metabolic profiles of patients with visceral obesity may substantially improve after only modest weight loss. This could reflect a significant reduction in the amount of VAT relative to peripheral or subcutaneous fat depots, thereby maintaining VAT below the CVATT. The CVATT may be unique for each individual. This may help explain the phenomena of apparently lean individuals with metabolic syndrome, the so-called metabolically normal weight (MONW), as well as the obese with normal metabolic profiles, i.e., metabolically normal obese (MNO), and those who are "fit and fat." The concept of CVATT may have implications for prevention and treatment of metabolic syndrome, which may include controlling dietary carbohydrates. The identification of the CVATT is admittedly difficult and its anatomical boundaries are not well-defined. Thus, the CVATT will continue to be a work in progress.

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.

The effect of pioglitazone on peroxisome proliferator-activated receptor-gamma target genes related to lipid storage in vivo

OBJECTIVE

Pioglitazone is a member of the thiazolidinediones (TZDs), insulin-sensitizing agents used to treat type 2 diabetes. The aim of this study was to define the effect of pioglitazone on the expression of genes related to carbohydrate and lipid metabolism in subcutaneous fat obtained from type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

Forty-eight volunteers with type 2 diabetes were divided into two groups treated for 12 weeks with placebo or pioglitazone (30 mg/day). The expression of several genes was quantified by real-time RT-PCR.

RESULTS

Pioglitazone treatment increased the expression of genes involved in glycerol-3-phosphate synthesis. The mRNA expression of PEPCK-C and glycerol-3-phosphate dehydrogenase (GPDH) increased (P < 0.01) in patients treated with pioglitazone. There was no difference in glycerol kinase (GyK) mRNA levels. The expression of genes that regulate fatty acid availability in adipocytes, including lipoprotein lipase (LPL) and acetyl-CoA synthetase (ACS), was higher (P < 0.01) in pioglitazone-treated patients. Pioglitazone stimulated (P < 0.0001) expression of c-Cbl-associated protein (CAP), whereas tumor necrosis factor-alpha, leptin, resistin, angiopoietin like-4, and 11-beta-hydroxysteroid dehydrogenase type 1 (11beta HSD 1) were not affected by pioglitazone. The baseline peroxisome proliferator-activated receptor (PPAR)-gamma1 mRNA was significantly correlated with mRNA for LPL, CAP, ACS, 11beta HSD 1, GyK, fatty acid synthase, leptin, and GPDH, whereas PPAR-gamma2 mRNA was correlated with CAP, PEPCK-C, leptin, and GPDH.

CONCLUSIONS

Treatment with pioglitazone increased body weight, and this is associated with upregulation of some, but not all, genes previously demonstrated as "TZD responsive" in subcutaneous fat. The results suggest that TZDs might increase body weight through the upregulation of genes facilitating adipocyte lipid storage in vivo.

A comparison of the effects of thiazolidinediones and metformin on metabolic control in patients with type 2 diabetes mellitus

BACKGROUND

Type 2 diabetes mellitus is a condition characterized by impaired insulin secretion and resistance to insulin-mediated glucose uptake and utilization. A number of oral antidiabetic medication are available for its treatment, including metformin and the thiazolidinediones (TZDs). The TZDs have been shown to improve insulin resistance, and it has been suggested that metformin has similar effects. Although both types of agents improve glycemic control, their mechanisms of action and effects on metabolic processes differ.

OBJECTIVE

The goal of this review was to compare the effects of TZDs and metformin on metabolic control in patients with type 2 diabetes.

METHODS

A search of MEDLINE to March 2004 using the terms metformin and biguanides, and thiazolidinediones and glitazones was conducted to identify preclinical and clinical studies focusing on the mechanisms of action and comparative effects of TZDs and metformin. Also searched were published abstracts from recent major diabetes and endocrinology conferences.

RESULTS

In the studies reviewed, both TZDs and metformin demonstrated the ability to improve glycemic control, although long-term monotherapy with TZDs appeared to be more effective than metformin. There continues to be debate about whether metformin is more effective than TZDs in terms of inhibition of hepatic glucose production. However, various studies have found TZDs to be more effective in promoting an increase in whole-body insulin sensitivity. With respect to lipid metabolism, patients who received TZDs had a greater reduction in concentrations of both plasma triglycerides and free fatty acids. Metformin was more effective in promoting weight loss in patients with type 2 diabetes, although TZDs may decrease visceral fat levels. Treatment with either metformin or TZDs was associated with a reduction in the risk of cardiovascular disease, although the mechanisms by which they accomplished this seem to differ.

CONCLUSIONS

The evidence suggests that the predominant effect of metformin is inhibition of hepatic glucose production, whereas the primary effects of TZDs is reduction of insulin resistance and promotion of peripheral glucose uptake. TZDs appear to have more positive effects on other metabolic processes and to be associated with greater improvements in cardiovascular risk factors compared with metformin.

Insulin resistance: from predisposing factor to therapeutic target in type 2 diabetes

BACKGROUND

Insulin resistance contributes to the pathogenesis of type 2 diabetes and is closely linked with cardiovascular risk factors and premature cardiovascular disease.

OBJECTIVE

The purpose of this paper was to review the importance of insulin resistance as a core defect in type 2 diabetes, a potential contributor to accelerated atherosclerosis, and a potential target for insulin-sensitizing agents.

METHODS

Articles considered for inclusion in this review were identified through a search of MEDLINE/PubMed for reports published from 1966 to April 2003. Search terms used were insulin resistance, diabetes, insulin sensitivity, obesity, cardiovascular disease, metformin, thiazolidinediones, pioglitazone, rosiglitazone, and troglitazone.

RESULTS

An overview of the epidemiology, natural history, and pathophysiology of type 2 diabetes is provided, with a focus on insulin resistance and a related discussion of the impact of current therapies used to treat insulin-resistant patients. In particular, information on insulin-sensitizing agents-metformin and the currently available thiazolidinediones (TZDs), pioglitazone and rosiglitazone-is presented. Although metformin has been shown to indirectly reduce insulin resistance, TZDs are the only available agents that have been shown to directly lower insulin resistance.

CONCLUSIONS

Recent evidence indicates that metformin, pioglitazone, and rosiglitazone may improve the dyslipidemic profile, reduce vascular inflammation, and improve endothelial dysfunction, all of which may be particularly important to physicians seeking treatment options to prevent or reduce cardiovascular complications in patients with type 2 diabetes.

Effects of pioglitazone versus diet and exercise on metabolic health and fat distribution in upper body obesity

OBJECTIVE

Insulin resistance is associated with visceral adiposity, and interventions that reduce this depot, e.g., diet and exercise, improve insulin resistance. Thiazolidinediones (TZDs) also improve insulin action but paradoxically increase total fat mass, perhaps through remodeling (recruitment of smaller fat cells) and redistribution of adipose tissue. We assessed the effects of pioglitazone versus diet and exercise on fat distribution and the relationship between fat distribution and insulin sensitivity in upper body obesity.

RESEARCH DESIGN AND METHODS

Thirty-nine upper body obese, insulin-resistant, nondiabetic men and premenopausal women were randomly assigned to receive either 30 mg/day pioglitazone or a diet and exercise program for 20 weeks. Before and after the intervention, insulin sensitivity, body composition, body fat distribution (waist-to-hip ratio [WHR], computed tomography abdomen, and dual-energy X-ray absorptiometry), and abdominal and femoral fat cell size were assessed.

RESULTS

Diet and exercise resulted in an 11.8 +/- 1.1 kg weight loss. Both diet and exercise and pioglitazone improved insulin sensitivity, but only the former was associated with loss of intra-abdominal fat. Pioglitazone increased total body fat, which preferentially accumulated in the lower body depot in both men and women. WHRs decreased in both groups. Abdominal fat cell size decreased (P = 0.06) after diet and exercise. No statistically significant changes in fat cell size were observed in pioglitazone-treated volunteers.

CONCLUSIONS

In nondiabetic upper body obese subjects, increasing insulin sensitivity via diet and exercise accompanies reductions in visceral fat. Pioglitazone treatment also improves insulin sensitivity and lowers WHR, but this is due to a selective increase in lower body fat. This confirms a site-specific responsiveness of adipose tissue to TZD and suggests that improvements in insulin sensitivity by pioglitazone are achieved independent of changes in intra-abdominal fat.

Differential influences of peroxisome proliferator-activated receptors gamma and -alpha on food intake and energy homeostasis

Chronic treatment with compounds activating peroxisome proliferator-activated receptor (PPAR)gamma and -alpha influences body energy stores, but the underlying mechanisms are only partially known. In a chronic-dosing study, equiefficacious antihyperglycemic doses of the PPAR gamma agonist pioglitazone and PPAR alpha/gamma dual activator ragaglitazar were administered to obesity-prone male rats. The PPAR alpha agonist fenofibrate had no effect on insulin sensitivity. Pioglitazone transiently increased and fenofibrate transiently decreased food intake, whereas ragaglitazar had no impact on feeding. As a result, body adiposity increased in pioglitazone-treated rats and decreased in fenofibrate-treated rats. PPAR gamma compounds markedly increased feed efficiency, whereas PPAR alpha agonist treatment decreased feed efficiency. In fenofibrate-treated rats, plasma acetoacetate was significantly elevated. Plasma levels of this potentially anorectic ketone body were unaffected in pioglitazone- and ragaglitazar-treated rats. High-fat feeding markedly increased visceral fat pads, and this was prevented by pioglitazone and ragaglitazar treatment. Pioglitazone treatment enlarged subcutaneous adiposity in high-fat-fed rats. In conclusion, PPAR gamma activation increases both food intake and feed efficiency, resulting in net accumulation of subcutaneous body fat. The impact of PPAR gamma activation on feeding and feed efficiency appears to be partially independent because the PPAR alpha component of ragaglitazar completely counteracts the orexigenic actions of PPAR gamma activation without marked impact on feed efficiency.

Variation in type 2 diabetes--related traits in mouse strains susceptible to diet-induced obesity

C57BL/6J (B6) and AKR/J (AKR) inbred strains of mice develop a comparable degree of obesity when fed a high-fat diet. However, although obese B6 mice are more glucose intolerant, obese AKR mice are more insulin resistant. To understand the basis for these strain differences, we characterized features of adiposity and glucose homeostasis in mice fed a high-fat diet for 8 weeks. The results indicated that despite hyperglycemia and impaired glucose tolerance, B6 mice have lower plasma insulin and are more insulin sensitive than AKR mice. Compared with adipose tissue of AKR mice, adipose tissue of B6 mice contained about threefold higher levels of total membrane-bound GLUT4 protein, whereas in skeletal muscle the levels were similar. Uptake of 2-[(14)C]deoxyglucose in vivo was reduced by a high-fat diet in adipose tissue, but not in skeletal muscle. Surprisingly, no significant differences in uptake occurred between the strains, despite the differences in GLUT4; however, glucose flux was calculated to be slightly higher in B6 mice. Higher expression of PEPCK in the liver of B6 mice, under both standard-diet and high-fat-diet conditions, suggests a plausible mechanism for elevated glycemia in these mice. In conclusion, phenotypic variation in insulin resistance and glucose production in the B6 and AKR strains could provide a genetic system for the identification of genes controlling glucose homeostasis.