Comprehensive messenger ribonucleic acid profiling reveals that peroxisome proliferator-activated receptor gamma activation has coordinate effects on gene expression in multiple insulin-sensitive tissues

Isomer-specific effects of CLA on gene expression in human adipose tissue depending on PPARgamma2 P12A polymorphism: a double blind, randomized, controlled cross-over study

BACKGROUND

Peroxisome proliferator-activated receptor (PPAR)gamma is a key regulator in adipose tissue. The rare variant Pro12Ala of PPARgamma2 is associated with a decreased risk of insulin resistance. Being dietary PPARgamma ligands, conjugated linoleic acids (CLAs) received considerable attention because of their effects on body composition, cancer, atherosclerosis, diabetes, obesity and inflammation, although some effects were only demonstrated in animal trials and the results in human studies were not always consistent. In the present study effects of CLA supplementation on genome wide gene expression in adipose tissue biopsies from 11 Ala12Ala and 23 Pro12Pro men were investigated. Subjects underwent four intervention periods (4 wk) in a randomized double blind cross-over design receiving 4.25 g/d of either cis-9, trans-11 CLA, trans-10,cis-12 CLA, 1:1 mixture of both isomers or a reference linoleic acid oil preparation. After each intervention biopsies were taken, whole genome expression microarrays were applied, and genes of interest were verified by realtime PCR.

RESULTS

The following genes of lipid metabolism were regulated by CLA: LDLR, FASN, SCD, FADS1 and UCP2 were induced, while ABCA1, CD36 and CA3 were repressed. Transcription factors PPARgamma, NFAT5, CREB5 and EBF1, the adipokine NAMPT, members of the insulin signaling cascade SORBS1 and IGF1 and IL6ST were repressed, while the adipokine THBS1 and GLUT4 involved in insulin signaling were induced. Compared to trans-10,cis-12 CLA and the CLA mixture the cis-9, trans-11 CLA isomer exerted weaker effects. Only CD36 (-1.2 fold) and THBS1 (1.5 fold) were regulated. The CLA effect on expression of PPARgamma and leptin genes depends on the PPARgamma2 genotype.

CONCLUSION

The data suggest that the isomer specific influence of CLA on glucose and lipid metabolism is genotype dependent and at least in part mediated by PPARgamma.

TRIAL REGISTRATION

http://www.controlled-trials.com: ISRCTN91188075.

Fat poetry: a kingdom for PPAR gamma

Adipose tissue is not an inert cell mass contributing only to the storage of fat, but a sophisticated ensemble of cellular components with highly specialized and complex functions. In addition to managing the most important energy reserve of the body, it secretes a multitude of soluble proteins called adipokines, which have beneficial or, alternatively, deleterious effects on the homeostasis of the whole body. The expression of these adipokines is an integrated response to various signals received from many organs, which depends heavily on the integrity and physiological status of the adipose tissue. One of the main regulators of gene expression in fat is the transcription factor peroxisome proliferator-activated receptor gamma (PPARgamma), which is a fatty acid- and eicosanoid-dependent nuclear receptor that plays key roles in the development and maintenance of the adipose tissue. Furthermore, synthetic PPARgamma agonists are therapeutic agents used in the treatment of type 2 diabetes.This review discusses recent knowledge on the link between fat physiology and metabolic diseases, and the roles of PPARgamma in this interplay via the regulation of lipid and glucose metabolism. Finally, we assess the putative benefits of targeting this nuclear receptor with still-to-be-identified highly selective PPARgamma modulators.

Pyruvate dehydrogenase kinase 4: regulation by thiazolidinediones and implication in glyceroneogenesis in adipose tissue

OBJECTIVE

Pyruvate dehydrogenase complex (PDC) serves as the metabolic switch between glucose and fatty acid utilization. PDC activity is inhibited by PDC kinase (PDK). PDC shares the same substrate, i.e., pyruvate, as glyceroneogenesis, a pathway controlling fatty acid release from white adipose tissue (WAT). Thiazolidinediones activate glyceroneogenesis. We studied the regulation by rosiglitazone of PDK2 and PDK4 isoforms and tested the hypothesis that glyceroneogenesis could be controlled by PDK.

RESEARCH DESIGN AND METHODS

Rosiglitazone was administered to Zucker fa/fa rats, and then PDK4 and PDK2 mRNAs were examined in subcutaneous, periepididymal, and retroperitoneal WAT, liver, and muscle by real-time RT-PCR. Cultured WAT explants from humans and rats and 3T3-F442A adipocytes were rosiglitazone-treated before analyses of PDK2 and PDK4 mRNA and protein. Small interfering RNA (siRNA) was transfected by electroporation. Glyceroneogenesis was determined using [1-(14)C]pyruvate incorporation into lipids.

RESULTS

Rosiglitazone increased PDK4 mRNA in all WAT depots but not in liver and muscle. PDK2 transcript was not affected. This isoform selectivity was also found in ex vivo-treated explants. In 3T3-F442A adipocytes, Pdk4 expression was strongly and selectively induced by rosiglitazone in a direct and transcriptional manner, with a concentration required for half-maximal effect at 1 nmol/l. The use of dichloroacetic acid or leelamine, two PDK inhibitors, or a specific PDK4 siRNA demonstrated that PDK4 participated in glyceroneogenesis, therefore altering nonesterified fatty acid release in both basal and rosiglitazone-activated conditions.

CONCLUSIONS

These data show that PDK4 upregulation in adipocytes participates in the hypolipidemic effect of thiazolidinediones through modulation of glyceroneogenesis.

Pioglitazone decreases fasting and postprandial endogenous glucose production in proportion to decrease in hepatic triglyceride content

OBJECTIVE

Hepatic triglyceride is closely associated with hepatic insulin resistance and is known to be decreased by thiazolididinediones. We studied the effect of pioglitazone on hepatic triglyceride content and the consequent effect on postprandial endogenous glucose production (EGP) in type 2 diabetes.

RESEARCH DESIGN AND METHODS

Ten subjects with type 2 diabetes on sulfonylurea therapy were treated with pioglitazone (30 mg daily) for 16 weeks. EGP was measured using a dynamic isotopic methodology after a standard liquid test meal both before and after pioglitazone treatment. Liver and muscle triglyceride levels were measured by (1)H magnetic resonance spectroscopy, and intra-abdominal fat content was measured by magnetic resonance imaging.

RESULTS

Pioglitazone treatment reduced mean plasma fasting glucose and mean peak postprandial glucose levels. Fasting EGP decreased after pioglitazone treatment (16.6 +/- 1.0 vs. 12.2 +/- 0.7 micromol . kg(-1) . min(-1), P = 0.005). Between 80 and 260 min postprandially, EGP was twofold lower on pioglitazone (2.58 +/- 0.25 vs. 1.26 +/- 0.30 micromol . kg(-1) . min(-1), P < 0.001). Hepatic triglyceride content decreased by approximately 50% (P = 0.03), and muscle (anterior tibialis) triglyceride content decreased by approximately 55% (P = 0.02). Hepatic triglyceride content was directly correlated with fasting EGP (r = 0.64, P = 0.01) and inversely correlated to percentage suppression of EGP (time 150 min, r = -0.63, P = 0.02). Muscle triglyceride, subcutaneous fat, and visceral fat content were not related to EGP. CONCLUSIONS Reduction in hepatic triglyceride by pioglitazone is very closely related to improvement in fasting and postprandial EGP in type 2 diabetes.

Hepatic dysfunction and insulin insensitivity in type 2 diabetes mellitus: a critical target for insulin-sensitizing agents

The liver plays an essential role in maintaining glucose homeostasis, which includes insulin-mediated processes such as hepatic glucose output (HGO) and uptake, as well as in clearance of insulin itself. In type 2 diabetes, the onset of hyperglycaemia [itself a potent inhibitor of hepatic glucose output (HGO)], alongside hyperinsulinaemia, indicates the presence of hepatic insulin insensitivity. Increased HGO is central to the onset of hyperglycaemia and highlights the need to target hepatic insulin insensitivity as a central component of glucose-lowering therapy. The mechanisms underlying the development of hepatic insulin insensitivity are not well understood, but may be influenced by factors such as fatty acid oversupply and altered adipocytokine release from dysfunctional adipose tissue and increased liver fat content. Furthermore, although the impact of insulin insensitivity as a marker of cardiovascular disease is well known, the specific role of hepatic insulin insensitivity is less clear. The pharmacological tools available to improve insulin sensitivity include the biguanides (metformin) and thiazolidinediones (rosiglitazone and pioglitazone). Data from a number of sources indicate that thiazolidinediones, in particular, can improve multiple aspects of hepatic dysfunction, including reducing HGO, insulin insensitivity and liver fat content, as well as improving other markers of liver function and the levels of mediators with potential involvement in hepatic function, including fatty acids and adipocytokines. The current review addresses this topic from the perspective of the role of the liver in maintaining glucose homeostasis, its key involvement in the pathogenesis of type 2 diabetes and the tools currently available to reduce hepatic insulin insensitivity.

Adipose tissue inflammation and liver fat in patients with highly active antiretroviral therapy-associated lipodystrophy

In this cross-sectional study, we sought to determine whether gene expression of macrophage markers and inflammatory chemokines in lipoatrophic subcutaneous abdominal adipose tissue and liver fat content are increased and interrelated in human immunodeficiency virus (HIV)-1-positive, highly active antiretroviral therapy (HAART)-treated patients with lipodystrophy (HAART+LD+; n = 27) compared with those without (HAART+LD-; n = 13). The study groups were comparable with respect to age, gender, and body mass index. The HAART+LD+ group had twofold more intra-abdominal (P = 0.01) and 1.5-fold less subcutaneous (P = 0.091) fat than the HAART+LD- group. As we have reported previously, liver fat was 10-fold higher in the HAART+LD+ compared with the HAART+LD- group (P = 0.00003). Inflammatory gene expression was increased in HAART-lipodystrophy: CD68 4.5-fold (P = 0.000013), tumor necrosis factor (TNF)-alpha 2-fold (P = 0.0094), chemokine (C-C motif) ligand (CCL) 2 2.5-fold (P = 0.0024), CCL3 7-fold (P = 0.0000017), integrin alphaM (ITGAM) 3-fold (P = 0.00067), epidermal growth factor-like module containing, mucin-like, hormone receptor-like (EMR)1 2.5-fold (P = 0.0038), and a disintegrin and metalloproteinase domain (ADAM)8 3.5-fold (P = 0.00057) higher in the HAART+LD+ compared with the HAART+LD- group. mRNA concentration of CD68 (r = 0.37, P = 0.019), ITGAM (r = 0.35, P = 0.025), CCL2 (r = 0.39, P = 0.012), and CCL3 (r = 0.54, P = 0.0003) correlated with liver fat content. In conclusion, gene expression of markers of macrophage infiltration and adipose tissue inflammation is increased in lipoatrophic subcutaneous abdominal adipose tissue of patients with HAART-associated lipodystrophy compared with those without. CD68, ITGAM, CCL2, and CCL3 expression is significantly associated with accumulation of liver fat.

PPARgamma regulates adipose triglyceride lipase in adipocytes in vitro and in vivo

Peroxisome proliferator-activated receptor-gamma (PPARgamma) regulates adipocyte genes involved in adipogenesis and lipid metabolism and is the molecular target for thiazolidinedione (TZD) antidiabetic agents. Adipose triglyceride lipase (ATGL) is a recently described triglyceride-specific lipase that is induced during adipogenesis and remains highly expressed in mature adipocytes. This study evaluates the ability of PPARgamma to directly regulate ATGL expression in adipocytes in vitro and in vivo. In fully differentiated 3T3-L1 adipocytes, ATGL mRNA and protein are increased by TZD and non-TZD PPARgamma agonists in a dose- and time-dependent manner. Rosiglitazone-mediated induction of ATGL mRNA is rapid and is not inhibited by the protein synthesis inhibitor cycloheximide, indicating that intervening protein synthesis is not required for this effect. Rosiglitazone-mediated induction of ATGL mRNA and protein is inhibited by the PPARgamma-specific antagonist GW-9662 and is also significantly reduced following siRNA-mediated knockdown of PPARgamma, supporting the direct transcriptional regulation of ATGL by PPARgamma. In vivo, ATGL mRNA and protein are increased by rosiglitazone treatment in white and brown adipose tissue of mice with and without obesity due to high-fat diet or leptin deficiency. Thus, PPARgamma positively regulates ATGL mRNA and protein expression in mature adipocytes in vitro and in adipose tissue in vivo, suggesting a role for ATGL in mediating PPARgamma's effects on lipid metabolism.

Hepatic response to restoration of GLUT4 in skeletal muscle of GLUT4 null mice

Expression of GLUT4 in fast-twitch skeletal muscle fibers of GLUT4 null mice (G4-MO) normalized glucose uptake in muscle and restored peripheral insulin sensitivity. GLUT4 null mice exhibit altered carbohydrate and lipid metabolism in liver and skeletal muscle. To test the hypothesis that increased glucose utilization by G4-MO muscle would normalize the changes seen in the GLUT4 null liver, serum metabolites and hepatic metabolism were compared in control, GLUT4 null, and G4-MO mice. The fed serum glucose and triglyceride levels of G4-MO mice were similar to those of control mice. In addition, the alternations in liver metabolism seen in GLUT4 nulls including increased GLUT2 expression and fatty acid synthesis accompanied by an increase in the oxidative arm of the pentose phosphate pathway were absent in G4-MO mice. The transgene used for GLUT4 restoration in muscle was specific for fast-twitch muscle fibers. The mitochondria hypertrophy/hyperplasia in all GLUT4 null skeletal muscles was absent in transgene-positive extensor digitorum longus muscle but present in transgene-negative soleus muscle of G4-MO mice. Results of this study suggest that the level of muscle GLUT4 expression influences mitochondrial biogenesis. These studies also demonstrate that the type and amount of substrate that muscle takes up and metabolizes, determined in part by GLUT4 expression levels, play a major role in directing hepatic carbohydrate and lipid metabolism.

Selective modulation of promoter recruitment and transcriptional activity of PPARgamma

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor regulated by the insulin-sensitizing thiazolidinediones (TZDs). We studied selective modulation of endogenous genes by PPARgamma ligands using microarray, RNA expression kinetics, and chromatin immunoprecipitation (ChIP) in 3T3-L1 adipocytes. We found over 300 genes that were significantly regulated the TZDs pioglitazone, rosiglitazone, and troglitazone. TZD-mediated expression profiles were unique but overlapping. Ninety-one genes were commonly regulated by all three ligands. TZD time course and dose-response studies revealed gene- and TZD-specific expression kinetics. PEPCK expression was induced rapidly but PDK4 expression was induced gradually. Troglitazone EC50 values for PEPCK, PDK4, and RGS2 regulation were greater than those for pioglitazone and rosiglitazone. TZDs differentially induced histone acetylation of and PPARgamma recruitment to target gene promoters. Selective modulation of PPARgamma by TZDs resulted in distinct expression profiles and transcription kinetics which may be due to differential promoter activation and chromatin remodeling of target genes.

RAR and RXR modulation in cancer and metabolic disease

Retinoic acid receptors (RARs) are ligand-controlled transcription factors that function as heterodimers with retinoid X receptors (RXRs) to regulate cell growth and survival. The success of RAR modulation in the treatment of acute promyelocytic leukaemia (APL) has stimulated considerable interest in the development of RAR and RXR modulators. This has been aided by recent advances in the understanding of the biological role of RARs and RXRs and in the design of selective receptor modulators that might overcome the limitations of current drugs. Here, we discuss the challenges and opportunities for therapeutic strategies based on RXR and RAR modulators, with a focus on cancer and metabolic diseases such as diabetes and obesity.

Adipose tissue integrity as a prerequisite for systemic energy balance: a critical role for peroxisome proliferator-activated receptor gamma

Peroxisome proliferator-activated receptor gamma (PPARgamma) is an essential regulator of adipocyte differentiation, maintenance, and survival. Deregulations of its functions are associated with metabolic diseases. We show here that deletion of one PPARgamma allele not only affected lipid storage but, more surprisingly, also the expression of genes involved in glucose uptake and utilization, the pentose phosphate pathway, fatty acid synthesis, lipolysis, and glycerol export as well as in IR/IGF-1 signaling. These deregulations led to reduced circulating adiponectin levels and an energy crisis in the WAT, reflected in a decrease to nearly half of its intracellular ATP content. In addition, there was a decrease in the metabolic rate and physical activity of the PPARgamma(+/-) mice, which was abolished by thiazolidinedione treatment, thereby linking regulation of the metabolic rate and physical activity to PPARgamma. It is likely that the PPARgamma(+/-) phenotype was due to the observed WAT dysfunction, since the gene expression profiles associated with metabolic pathways were not affected either in the liver or the skeletal muscle. These findings highlight novel roles of PPARgamma in the adipose tissue and underscore the multifaceted action of this receptor in the functional fine tuning of a tissue that is crucial for maintaining the organism in good health.

Suppression of PPAR-gamma attenuates insulin-stimulated glucose uptake by affecting both GLUT1 and GLUT4 in 3T3-L1 adipocytes

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) plays a critical role in regulating insulin sensitivity and glucose homeostasis. In this study, we identified highly efficient small interfering RNA (siRNA) sequences and used lentiviral short hairpin RNA and electroporation of siRNAs to deplete PPAR-gamma from 3T3-L1 adipocytes to elucidate its role in adipogenesis and insulin signaling. We show that PPAR-gamma knockdown prevented adipocyte differentiation but was not required for maintenance of the adipocyte differentiation state after the cells had undergone adipogenesis. We further demonstrate that PPAR-gamma suppression reduced insulin-stimulated glucose uptake without affecting the early insulin signaling steps in the adipocytes. Using dual siRNA strategies, we show that this effect of PPAR-gamma deletion was mediated by both GLUT4 and GLUT1. Interestingly, PPAR-gamma-depleted cells displayed enhanced inflammatory responses to TNF-alpha stimulation, consistent with a chronic anti-inflammatory effect of endogenous PPAR-gamma. In summary, 1) PPAR-gamma is essential for the process of adipocyte differentiation but is less necessary for maintenance of the differentiated state, 2) PPAR-gamma supports normal insulin-stimulated glucose transport, and 3) endogenous PPAR-gamma may play a role in suppression of the inflammatory pathway in 3T3-L1 cells.

Transcriptional activation of SHP by PPAR-gamma in liver

The mechanism of how PPARgamma decrease gluconeogenic gene expressions in liver is still unclear. Since PPARgamma is a transcriptional activator, it requires a mediator to decrease the transcription of gluconeogenic genes. Recently, SHP has been shown to mediate the bile acid-dependent down regulation of gluconeogenic gene expression in liver. This led us to explore the possibility that SHP may mediate the antigluconeogenic effect of PPARgamma. In the present study, we have identified and characterized the presence of functional PPRE in human SHP promoter. We show the binding of PPARgamma/RXRalpha heterodimer to the PPRE and increased SHP expression by rosiglitazone in primary rat hepatocytes. Taken together with the previous reports about the function of SHP on gluconeogenesis, our results indicate that SHP can mediate the acute antigluconeogenic effect of PPARgamma.

The small molecule harmine is an antidiabetic cell-type-specific regulator of PPARgamma expression

PPARgamma is the master regulator of adipogenesis and the molecular target of the thiazolidinedione antidiabetic drugs. By screening for compounds that promote adipogenesis, we identified a small molecule that targets the PPARgamma pathway by a distinct mechanism. This molecule, harmine, is not a ligand for the receptor; rather, it acts as a cell-type-specific regulator of PPARgamma expression. Administration of harmine to diabetic mice mimics the effects of PPARgamma ligands on adipocyte gene expression and insulin sensitivity. Unlike thiazolidinediones, however, harmine does not cause significant weight gain or hepatic lipid accumulation. Molecular studies indicate that harmine controls PPARgamma expression through inhibition of the Wnt signaling pathway. This work validates phenotypic screening of adipocytes as a promising strategy for the identification of bioactive small molecules and suggests that regulators of PPARgamma expression may represent a complementary approach to PPARgamma ligands in the treatment of insulin resistance.

Molecular Genetics of Experimental Hypertension and the Metabolic Syndrome

Genetic studies of human and experimental hypertension provide a means to identify key pathways that predispose individuals to increased blood pressure and associated risk factors for cardiovascular and metabolic diseases. The pathways so identified can then serve as targets for therapeutic intervention. This article discusses genetic studies in animal models of hypertension in which specific genes have been identified that regulate blood pressure and biochemical features of the metabolic syndrome. Consistent with studies in humans with monogenic disorders of blood pressure regulation, studies in rat models have demonstrated that naturally occurring genetic variation in pathways regulating sodium chloride transport can contribute to inherited variation in blood pressure. Such studies have also indicated that naturally occurring variation in genes, such as Cd36, that regulate fatty acid metabolism and ectopic accumulation of fat and fat metabolites can influence both biochemical and hemodynamic features of the metabolic syndrome and mediate the antidiabetic effects of drugs that activate the peroxisome proliferator-activated receptor-gamma. Angiotensin II receptor blockers with the ability to selectively modulate activity of peroxisome proliferator-activated receptor-gamma and expression of genes in these fat metabolism pathways may represent useful prototypes for a new class of transcription modulating drugs aimed at treating patients with hypertension and the metabolic syndrome.

Response to pioglitazone treatment is associated with the lipoprotein lipase S447X variant in subjects with type 2 diabetes mellitus

To investigate the influence of the S447X variant in lipoprotein lipase (LPL) gene on the response rate to therapy with the thiazolidinedione pioglitazone. A total of 113 diabetic patients were treated with pioglitazone 30 mg for 10 weeks. Response to the pioglitazone treatment was defined by either a >10% relative reduction in fasting blood glucose (FBG) or a more than 1% decrease in glycosylated haemoglobin (HbA1c) values after 10 weeks of pioglitazone treatment. The genotypes were determined by polymerase chain reaction-restriction fragment length polymorphism method. Using the criteria >10% relative reduction in FBG after 10 weeks of pioglitaone treatment, responder frequency to pioglitazone treatment in S447S genotype group is significantly higher than S447X genotype group. Meanwhile, the S447X genotype conferred a statistically significant 0.538-fold reduction in response rate to pioglitazone treatment relative to the S447S genotype. Moreover, pioglitazone treatment has significantly beneficial effects on serum lipid profile and blood pressure in S447S genotype carriers. The S447X variant in LPL gene may be a cause for therapy modification by pioglitazone.

beta-cell failure in diabetes and preservation by clinical treatment

There is a progressive deterioration in beta-cell function and mass in type 2 diabetics. It was found that islet function was about 50% of normal at the time of diagnosis, and a reduction in beta-cell mass of about 60% was shown at necropsy. The reduction of beta-cell mass is attributable to accelerated apoptosis. The major factors for progressive loss of beta-cell function and mass are glucotoxicity, lipotoxicity, proinflammatory cytokines, leptin, and islet cell amyloid. Impaired beta-cell function and possibly beta-cell mass appear to be reversible, particularly at early stages of the disease where the limiting threshold for reversibility of decreased beta-cell mass has probably not been passed. Among the interventions to preserve or "rejuvenate" beta-cells, short-term intensive insulin therapy of newly diagnosed type 2 diabetes will improve beta-cell function, usually leading to a temporary remission time. Another intervention is the induction of beta-cell "rest" by selective activation of ATP-sensitive K+ (K(ATP)) channels, using drugs such as diazoxide. A third type of intervention is the use of antiapoptotic drugs, such as the thiazolidinediones (TZDs), and incretin mimetics and enhancers, which have demonstrated significant clinical evidence of effects on human beta-cell function. The TZDs improve insulin secretory capacity, decrease beta-cell apoptosis, and reduce islet cell amyloid with maintenance of neogenesis. The TZDs have indirect effects on beta-cells by being insulin sensitizers. The direct effects are via peroxisome proliferator-activated receptor gamma activation in pancreatic islets, with TZDs consistently improving basal beta-cell function. These beneficial effects are sustained in some individuals with time. There are several trials on prevention of diabetes with TZDs. Incretin hormones, which are released from the gastrointestinal tract in response to nutrient ingestion to enhance glucose-dependent insulin secretion from the pancreas, aid the overall maintenance of glucose homeostasis through slowing of gastric emptying, inhibition of glucagon secretion, and control of body weight. From the two major incretins, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), only the first one or its mimetics or enhancers can be used for treatment because the diabetic beta-cell is resistant to GIP action. Because of the rapid inactivation of GLP-1 by dipeptidyl peptidase (DPP)-IV, several incretin analogs were developed: GLP-1 receptor agonists (incretin mimetics) exenatide (synthetic exendin-4) and liraglutide, by conjugation of GLP-1 to circulating albumin. The acute effect of GLP-1 and GLP-1 receptor agonists on beta-cells is stimulation of glucose-dependent insulin release, followed by enhancement of insulin biosynthesis and stimulation of insulin gene transcription. The chronic action is stimulating beta-cell proliferation, induction of islet neogenesis, and inhibition of beta-cell apoptosis, thus promoting expansion of beta-cell mass, as observed in rodent diabetes and in cultured beta-cells. Exenatide and liraglutide enhanced postprandial beta-cell function. The inhibition of the activity of the DPP-IV enzyme enhances endogenous GLP-1 action in vivo, mediated not only by GLP-1 but also by other mediators. In preclinical studies, oral active DPP-IV inhibitors (sitagliptin and vildagliptin) also promoted beta-cell proliferation, neogenesis, and inhibition of apoptosis in rodents. Meal tolerance tests showed improvement in postprandial beta-cell function. Obviously, it is difficult to estimate the protective effects of incretin mimetics and enhancers on beta-cells in humans, and there is no clinical evidence that these drugs really have protective effects on beta-cells.

Acute and selective regulation of glyceroneogenesis and cytosolic phosphoenolpyruvate carboxykinase in adipose tissue by thiazolidinediones in type 2 diabetes

AIMS/HYPOTHESIS

Regulation of glyceroneogenesis and its key enzyme cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) plays a major role in the control of fatty acid release from adipose tissue. Here we investigate the effect of rosiglitazone on the expression of genes involved in fatty acid metabolism and the resulting metabolic consequences.

MATERIALS AND METHODS

Rosiglitazone was administered to Zucker fa/fa rats for 4 days and to 24 diabetic patients for 12 weeks, then mRNA expression for the genes encoding PEPCK-C, mitochondrial PEPCK, adipocyte lipid-binding protein, glycerol kinase, lipoprotein lipase and glycerol-3-phosphate dehydrogenase was examined in s.c. adipose tissue by real-time RT-PCR. Glyceroneogenesis was determined using [1-(14)C]pyruvate incorporation into lipids. Cultured adipose tissue explants from overweight women undergoing plastic surgery were incubated with rosiglitazone for various times before mRNA determination and analysis of PEPCK-C protein, activity and glyceroneogenesis.

RESULTS

Rosiglitazone administration to rats induced the expression of the gene encoding PEPCK-C mRNA (PCK1) and PEPCK-C activity in adipose tissue with a resulting 2.5-fold increase in glyceroneogenesis. This was accompanied by an improvement in dyslipidaemia as demonstrated by the decrease in plasma NEFAs and triacylglycerol. In rosiglitazone-treated diabetic patients, PCK1 mRNA was raised 2.5-fold in s.c. adipose tissue. Rosiglitazone treatment of adipose tissue explants from overweight women caused a selective augmentation in PCK1 mRNA which reached a maximum of 9-fold at 14 h, while mRNA for other genes remained unaffected. Experiments with inhibitors showed a direct and transcription-only effect, which was followed by an increase in PEPCK-C protein, enzyme activity and glyceroneogenesis.

CONCLUSIONS/INTERPRETATION

These results favour adipocyte glyceroneogenesis as the initial thiazolidinedione-responsive pathway leading to improvement in dyslipidaemia.

Britt C, G. Wilson J, O. Milliken N, G. Binz J, C. Lobe D, R. Oliver W, C. Lewis M, M. Ignar D. The Effect of PPARalpha, PPARdelta, PPARgamma, and PPARpan Agonists on Body Weight, Body Mass, and Serum Lipid Profiles in Diet-Induced Obese AKR/J Mice

Activation of peroxisome proliferator-activated receptor (PPAR) alpha, delta, and gamma subtypes increases expression of genes involved in fatty acid transport and oxidation and alters adiposity in animal models of obesity and type-2 diabetes. PPARpan agonists which activate all three receptor subtypes have antidiabetic activity in animal models without the weight gain associated with selective PPARgamma agonists. Herein we report the effects of selective PPAR agonists (GW9578, a PPARalpha agonist, GW0742, a PPARdelta agonist, GW7845, a PPARgamma agonist), combination of PPARalpha and delta agonists, and PPARpan (PPARalpha/gamma/delta) activators (GW4148 or GW9135) on body weight (BW), body composition, food consumption, fatty acid oxidation, and serum chemistry of diet-induced obese AKR/J mice. PPARalpha or PPARdelta agonist treatment induced a slight decrease in fat mass (FM) while a PPARgamma agonist increased BW and FM commensurate with increased food consumption. The reduction in BW and food intake after cotreatment with PPARalpha and delta agonists appeared to be synergistic. GW4148, a PPARpan agonist, induced a significant and sustained reduction in BW and FM similar to an efficacious dose of rimonabant, an antiobesity compound. GW9135, a PPARpan agonist with weak activity at PPARdelta, induced weight loss initially followed by rebound weight gain reaching vehicle control levels by the end of the experiment. We conclude that PPARalpha and PPARdelta activations are critical to effective weight loss induction. These results suggest that the PPARpan compounds may be expected to maintain the beneficial insulin sensitization effects of a PPARgamma agonist while either maintaining weight or producing weight loss.

Intrahepatic insulin resistance in a murine model of steatohepatitis: effect of PPARgamma agonist pioglitazone

Hepatic insulin resistance is associated with hepatic steatosis and is thought to play an important role in the pathogenesis of steatohepatitis. Using a murine model of steatohepatitis (mice fed a diet deficient in methionine and choline-MCD diet), we tested the effects of the insulin-sensitising, PPARgamma agonist drug pioglitazone (PGZ) on systemic and intrahepatic insulin sensitivity and on liver pathology. Compared to controls, mice fed the MCD diet develop a significant steatohepatitis, have enhanced glucose tolerance and enhanced systemic response to insulin. PGZ did not affect the systemic insulin sensitivity in control or MCD-fed mice as assessed in vivo by intraperitoneal glucose or insulin dynamic tests. However, PGZ prevented hepatic fat accumulation and steatohepatitis induced by the MCD diet. This effect was associated with an increased mass of adipose tissue and increased expression and release of adiponectin, while hepatic acyl co-enzyme A oxidase and acyl-co-enzyme A carboxylase, regulating hepatic beta-oxidation of fatty acid, remained unchanged. Steatohepatitis in MCD-diet-fed mice was associated with intrahepatic insulin resistance as shown by a reduced phosphorylation of hepatic insulin receptor, and Akt in response to an insulin stimulus. PGZ to MCD-fed mice restored the activation of the insulin receptor and of the Akt pathway in response to insulin. In conclusion, PGZ alleviates steatosis and steatohepatitis induced by the MCD diet, an effect associated with correction of intrahepatic insulin resistance.

Molecular Mechanisms of Cross-talk between Thyroid Hormone and Peroxisome Proliferator Activated Receptors: Focus on the Heart

Thyroid hormone receptors (TR) and peroxisome proliferator activated receptors (PPAR) regulate cardiac metabolism. Numerous studies have examined TR and PPAR function since PPAR was first discovered in the early 1990s, however few have evaluated TR and PPAR interactions. Although ligands for these members of the nuclear steroid receptor family are under evaluation for treatment of congestive heart failure and various metabolic diseases, their interactions have not been investigated in detail in heart. These interactions are remarkably complicated. Nevertheless, their identification and elucidation is extremely important for further development of specific drugs. We review here the fundamental ways TRs and PPARs are regulated and how their cross-talk patterns mediate transcription of their target genes.

Regulation of peroxisome proliferator-activated receptor-gamma in liver fibrosis

The peroxisome proliferator-activated receptors (PPARs) impart diverse cellular effects in biological systems. Because stellate cell activation during liver injury is associated with declining PPARgamma expression, we hypothesized that its expression is critical in stellate cell-mediated fibrogenesis. We therefore modulated its expression during liver injury in vivo. PPARgamma was depleted in rat livers by using an adenovirus-Cre recombinase system. PPARgamma was overexpressed by using an additional adenoviral vector (AdPPARgamma). Bile duct ligation was utilized to induce stellate cell activation and liver fibrosis in vivo; phenotypic effects (collagen I, smooth muscle alpha-actin, hydroxyproline content, etc.) were measured. PPARgamma mRNA levels decreased fivefold and PPARgamma protein was undetectable in stellate cells after culture-induced activation. During activation in vivo, collagen accumulation, assessed histomorphometrically and by hydroxyproline content, was significantly increased after PPARgamma depletion compared with controls (1.28 +/- 0.14 vs. 1.89 +/- 0.21 mg/g liver tissue, P < 0.03). In isolated stellate cells, AdPPARgamma overexpression resulted in significantly increased adiponectin mRNA expression and decreased collagen I and smooth muscle alpha-actin mRNA expression compared with controls. During in vivo fibrogenesis, rat livers exposed to AdPPARgamma had significantly less fibrosis than controls. Collagen I and smooth muscle alpha-actin mRNA expression were significantly reduced in AdPPARgamma-infected rats compared with controls (P < 0.05, n = 10). PPARgamma-deficient mice exhibited enhanced fibrogenesis after liver injury, whereas PPARgamma receptor overexpression in vivo attenuated stellate cell activation and fibrosis. The data highlight a critical role for PPARgamma during in vivo fibrogenesis and emphasize the importance of the PPARgamma pathway in stellate cells during liver injury.

Peroxisome proliferator-activated receptor gamma (PPARgamma) regulates trefoil factor family 2 (TFF2) expression in gastric epithelial cells

Although trefoil factor family 2 (TFF2) plays a critical role in the defense and repair of gastric mucosa, the regulatory mechanism of TFF2 expression is not fully understood. In this study, we investigated the regulation of TFF2 expression by peroxisome proliferator-activated receptor gamma (PPARgamma) in gastric epithelial cells. MKN45 gastric cells were used. TFF2 mRNA expression was analyzed by real-time quantitative RT-PCR. The promoter sequence of the human TFF2 gene was cloned into pGL3-basic vector for reporter gene assays. Ciglitazone was mainly used as a specific PPARgamma ligand. MKN45 cells expressed functional PPARgamma proteins. Endogenous TFF2 mRNA expression and TFF2 reporter gene transcription was significantly up-regulated by ciglitazone in a dose-dependent manner. Reporter gene assays showed that two distinct cis-elements are involved in the response to PAPRgamma activation. Within one of these element (nucleotides -558 to -507), we identified a functional peroxisome proliferator responsive element (PPRE) at -522 (5'-GGGACAAAGGGCA-3'). Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) assay confirmed the binding of PPARgamma to this sequence. Another element (nucleotides -407 to -358) appeared to be a composite enhancer element indirectly regulated by PPARgamma and a combination of these two cis-elements was required for the full response of the human TFF2 gene expression to PPARgamma. These data demonstrate that human TFF2 gene is a direct target of PPARgamma in gastric epithelial cells. Since TFF2 is a critical gastroprotective agent, PPARgamma may be involved in the gastric mucosal defense through regulating TFF2 expression in humans.

Overexpression of rat long chain acyl-coa synthetase 1 alters fatty acid metabolism in rat primary hepatocytes

Long chain acyl-CoA synthetases (ACSL) activate fatty acids (FA) and provide substrates for both anabolic and catabolic pathways. We have hypothesized that each of the five ACSL isoforms partitions FA toward specific downstream pathways. Acsl1 mRNA is increased in cells under both lipogenic and oxidative conditions. To elucidate the role of ACSL1 in hepatic lipid metabolism, we overexpressed an Acsl1 adenovirus construct (Ad-Acsl1) in rat primary hepatocytes. Ad-ACSL1, located on the endoplasmic reticulum but not on mitochondria or plasma membrane, increased ACS specific activity 3.7-fold. With 100 or 750 mum [1-(14)C]oleate, Ad-Acsl1 increased oleate incorporation into diacylglycerol and phospholipids, particularly phosphatidylethanolamine and phosphatidylinositol, and decreased incorporation into cholesterol esters and secreted triacylglycerol. Ad-Acsl1 did not alter oleate incorporation into triacylglycerol, beta-oxidation products, or total amount of FA metabolized. In pulse-chase experiments to examine the effects of Ad-Acsl1 on lipid turnover, more labeled triacylglycerol and phospholipid, but less labeled diacylglycerol, remained in Ad-Acsl1 cells, suggesting that ACSL1 increased reacylation of hydrolyzed oleate derived from triacylglycerol and diacylglycerol. In addition, less hydrolyzed oleate was used for cholesterol ester synthesis and beta-oxidation. The increase in [1,2,3-(3)H]glycerol incorporation into diacylglycerol and phospholipid was similar to the increase with [(14)C]oleate labeling suggesting that ACSL1 increased de novo synthesis. Labeling Ad-Acsl1 cells with [(14)C]acetate increased triacylglycerol synthesis but did not channel endogenous FA away from cholesterol ester synthesis. Thus, consistent with the hypothesis that individual ACSLs partition FA, Ad-Acsl1 increased FA reacylation and channeled FA toward diacylglycerol and phospholipid synthesis and away from cholesterol ester synthesis.

Type II nuclear hormone receptors, coactivator, and target gene repression in adipose tissue in the acute-phase response

The acute-phase response (APR) leads to alterations in lipid metabolism and type II nuclear hormone receptors, which regulate lipid metabolism, are suppressed, in liver, heart, and kidney. Here, we examine the effect of the APR in adipose tissue. In mice, lipopolysaccharide produces a rapid, marked decrease in mRNA levels of nuclear hormone receptors [peroxisome proliferator-activated receptor gamma (PPARgamma), liver X receptor alpha (LXRalpha) and LXRbeta, thyroid receptor alpha (TRalpha) and TRbeta, and retinoid X receptor alpha (RXRalpha) and RXRbeta] and receptor coactivators [cAMP response element binding protein, steroid receptor coactivator 1 (SRC1) and SRC2, thyroid hormone receptor-associated protein, and peroxisome proliferator-activated receptor gamma co-activator 1alpha (PGC1alpha) and PGC1beta] along with decreased expression of target genes (adipocyte P2, phosphoenolpyruvate carboxykinase, glycerol-3-phosphate acyltransferase, ABCA1, apolipoprotein E, sterol-regulatory element binding protein-1c, glucose transport protein 4 (GLUT4), malic enzyme, and Spot14) involved in triglyceride (TG) and carbohydrate metabolism. We show that key TG synthetic enzymes, 1-acyl-sn-glycerol-3-phosphate acyltransferase-2, monoacylglycerol acyltransferase 1, and diacylglycerol acyltransferase 1, are PPARgamma-regulated genes and that they also decrease in the APR. In 3T3-L1 adipocytes, tumor necrosis factor-alpha (TNF-alpha) significantly decreases PPARgamma, LXRalpha and LXRbeta, RXRalpha and RXRbeta, SRC1 and SRC2, and PGC1alpha and PGC1beta mRNA levels, which are associated with a marked reduction in receptor-regulated genes. Moreover, TNF-alpha significantly reduces PPAR and LXR response element-driven transcription. Thus, the APR suppresses the expression of many nuclear hormone receptors and their coactivators in adipose tissue, which could be a mechanism to coordinately downregulate TG biosynthesis and thereby redirect lipids to other critical organs during the APR.

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.

The effect of pioglitazone on the liver: role of adiponectin

OBJECTIVE

Diabetic hyperglycemia results from insulin resistance of peripheral tissues and glucose overproduction due to increased gluconeogenesis (GNG). Thiazolidinediones (TZDs) improve peripheral insulin sensitivity, but the effect on the liver is less clear. The goal of this study was to examine the effect of TZDs on GNG.

RESEARCH DESIGN AND METHODS

Twenty sulfonylurea-treated type 2 diabetic subjects were randomly assigned (double-blind study) to receive pioglitazone (PIO group; 45 mg/day) or placebo (Plc group) for 4 months to assess endogenous glucose production (EGP) (3-(3)H-glucose infusion), GNG (D2O technique), and insulin sensitivity by two-step hyperinsulinemic-euglycemic clamp (240 and 960 pmol/min per m2).

RESULTS

Fasting plasma glucose (FPG) (10.0 +/- 0.8 to 7.7 +/- 0.7 mmol/l) and HbA1c (9.0 +/- 0.4 to 7.3 +/- 0.6%) decreased in the PIO and increased in Plc group (P < 0.05 PIO vs. Plc). Insulin sensitivity increased approximately 40% during high insulin clamp after pioglitazone (P < 0.01) and remained unchanged in the Plc group (P < 0.05 PIO vs. Plc). EGP did not change, while GNG decreased in the PIO group (9.6 +/- 0.7 to 8.7 +/- 0.6 micromol x min(-1) x kg(ffm)(-1)) and increased in the Plc group (8.0 +/- 0.5 to 9.6 +/- 0.8) (P < 0.05 PIO vs. Plc). Change in FPG correlated with change in GNG flux (r = 0.63, P < 0.003) and in insulin sensitivity (r = 0.59, P < 0.01). Plasma adiponectin increased after pioglitazone (P < 0.001) and correlated with delta FPG (r = -0.54, P < 0.03), delta GNG flux (r = -0.47, P < 0.05), and delta insulin sensitivity (r = 0.65, P < 0.005). Plasma free fatty acids decreased after pioglitazone and correlated with delta GNG flux (r = 0.54, P < 0.02). From stepwise regression analysis, the strongest determinant of change in FPG was change in GNG flux.

CONCLUSIONS

Pioglitazone improves FPG, primarily by reducing GNG flux in type 2 diabetic subjects.

Decreased expression of adipogenic genes in obese subjects with type 2 diabetes

OBJECTIVE

Our objective was to delineate the potential role of adipogenesis in insulin resistance and type 2 diabetes. Obesity is characterized by an increase in adipose tissue mass resulting from enlargement of existing fat cells (hypertrophy) and/or from increased number of adipocytes (hyperplasia). The inability of the adipose tissue to recruit new fat cells may cause ectopic fat deposition and insulin resistance.

RESEARCH METHODS AND PROCEDURES

We examined the expression of candidate genes involved in adipocyte proliferation and/or differentiation [CCAAT/enhancer-binding protein (C/EBP) alpha, C/EBPdelta, GATA domain-binding protein 3 (GATA3), C/EBPbeta, peroxisome proliferator-activated receptor (PPAR) gamma2, signal transducer and activator of transcription 5A (STAT5A), Wnt-10b, tumor necrosis factor alpha, sterol regulatory element-binding protein 1c (SREBP1c), 11 beta-hydroxysteroid dehydrogenase, PPARG angiopoietin-related protein (PGAR), insulin-like growth factor 1, PPARgamma coactivator 1alpha, PPARgamma coactivator 1beta, and PPARdelta] in subcutaneous adipose tissue from 42 obese individuals with type 2 diabetes and 25 non-diabetic subjects matched for age and obesity.

RESULTS

Insulin sensitivity was measured by a 3-hour 80 mU/m2 per minute hyperinsulinemic glucose clamp (100 mg/dL). As expected, subjects with type 2 diabetes had lower glucose disposal (4.9 +/- 1.9 vs. 7.5 +/- 2.8 mg/min per kilogram fat-free mass; p < 0.001) and larger fat cells (0.90 +/- 0.26 vs. 0.78 +/- 0.17 microm; p = 0.04) as compared with obese control subjects. Three genes (SREBP1c, p < 0.01; STAT5A, p = 0.02; and PPARgamma2, p = 0.02) had significantly lower expression in obese type 2 diabetics, whereas C/EBPbeta only tended to be lower (p = 0.07).

DISCUSSION

This cross-sectional study supports the hypothesis that impaired expression of adipogenic genes may result in impaired adipogenesis, potentially leading to larger fat cells in subcutaneous adipose tissue and insulin resistance.

Halofenate is a selective peroxisome proliferator-activated receptor gamma modulator with antidiabetic activity

Halofenate has been shown previously to lower triglycerides in dyslipidemic subjects. In addition, significant decreases in fasting plasma glucose were observed but only in type 2 diabetic patients. We hypothesized that halofenate might be an insulin sensitizer, and we present data to suggest that halofenate is a selective peroxisome proliferator-activated receptor (PPAR)-gamma modulator (SPPARgammaM). We demonstrate that the circulating form of halofenate, halofenic acid (HA), binds to and selectively modulates PPAR-gamma. Reporter assays show that HA is a partial PPAR-gamma agonist, which can antagonize the activity of the full agonist rosiglitazone. The data suggest that the partial agonism of HA may be explained in part by effective displacement of corepressors (N-CoR and SMRT) coupled with inefficient recruitment of coactivators (p300, CBP, and TRAP 220). In human preadipocytes, HA displays weak adipogenic activity and antagonizes rosiglitazone-mediated adipogenic differentiation. Moreover, in 3T3-L1 adipocytes, HA selectively modulates the expression of multiple PPAR-gamma-responsive genes. Studies in the diabetic ob/ob mouse demonstrate halofenate's acute antidiabetic properties. Longer-term studies in the obese Zucker (fa/fa) rat demonstrate halofenate's comparable insulin sensitization to rosiglitazone in the absence of body weight increases. Our data establish halofenate as a novel SPPARgammaM with promising therapeutic utility with the potential for less weight gain.

Insulin regulation of skeletal muscle PDK4 mRNA expression is impaired in acute insulin-resistant states

We previously showed that insulin has a profound effect to suppress pyruvate dehydrogenase kinase (PDK) 4 expression in rat skeletal muscle. In the present study, we examined whether insulin's effect on PDK4 expression is impaired in acute insulin-resistant states and, if so, whether this change is accompanied by decreased insulin's effects to stimulate Akt and forkhead box class O (FOXO) 1 phosphorylation. To induce insulin resistance, conscious overnight-fasted rats received a constant infusion of Intralipid or lactate for 5 h, while a control group received saline infusion. Following the initial infusions, each group received saline or insulin infusion (n = 6 or 7 each) for an additional 5 h, while saline, Intralipid, or lactate infusion was continued. Plasma glucose was clamped at basal levels during the insulin infusion. Compared with the control group, Intralipid and lactate infusions decreased glucose infusion rates required to clamp plasma glucose by approximately 60% (P < 0.01), confirming the induction of insulin resistance. Insulin's ability to suppress PDK4 mRNA level was impaired in skeletal muscle with Intralipid and lactate infusions, resulting in two- to threefold higher PDK4 mRNA levels with insulin (P < 0.05). Insulin stimulation of Akt and FOXO1 phosphorylation was also significantly decreased with Intralipid and lactate infusions. These data suggest that insulin's effect to suppress PDK4 gene expression in skeletal muscle is impaired in insulin-resistant states, and this may be due to impaired insulin signaling for stimulation of Akt and FOXO1 phosphorylation. Impaired insulin's effect to suppress PDK4 expression may explain the association between PDK4 overexpression and insulin resistance in skeletal muscle.

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.

The Peroxisome Proliferator-Activated Receptor N-Terminal Domain Controls Isotype-Selective Gene Expression and Adipogenesis

Peroxisome proliferator-activated receptors (PPARγ, PPARα, and PPARδ) are important regulators of lipid metabolism. Although they share significant structural similarity, the biological effects associated with each PPAR isotype are distinct. For example, PPARα and PPARδ regulate fatty acid catabolism, whereas PPARγ controls lipid storage and adipogenesis. The different functions of PPARs in vivo can be explained at least in part by the different tissue distributions of the three receptors. The question of whether the receptors have different intrinsic activities and regulate distinct target genes, however, has not been adequately explored. We have engineered cell lines that express comparable amounts of each receptor. Transcriptional profiling of these cells in the presence of selective agonists reveals partially overlapping but distinct patterns of gene regulation by the three PPARs. Moreover, analysis of chimeric receptors points to the N terminus of each receptor as the key determinant of isotype-selective gene expression. For example, the N terminus of PPARγ confers the ability to promote adipocyte differentiation when fused to the PPARδ DNA binding domain and ligand binding domain, whereas the N terminus of PPARδ leads to the inappropriate expression of fatty acid oxidation genes in differentiated adipocytes when fused to PPARγ. Finally, we demonstrate that the N terminus of each receptor functions in part to limit receptor activity because deletion of the N terminus leads to nonselective activation of target genes. A more detailed understanding of the mechanisms by which the individual PPARs differentially regulate gene expression should aid in the design of more effective drugs, including tissue- and target gene-selective PPAR modulators.

Drug therapy of the metabolic syndrome: minimizing the emerging crisis in polypharmacy

The metabolic syndrome--a collection of factors associated with increased risk for cardiovascular disease and diabetes--is becoming increasingly common, largely as a result of the increase in the prevalence of obesity. Although it is generally agreed that first-line clinical intervention for the metabolic syndrome is lifestyle change, this is insufficient to normalize the risk factors in many patients, and so residual risk could be high enough to justify drug therapy. However, at present there are no approved drugs that can reliably reduce all of the metabolic risk factors over the long term, and so there is growing interest in therapeutic strategies that might target multiple risk factors more effectively, thereby minimizing problems with polypharmacy. This review summarizes current understanding of the nature of the metabolic syndrome, and discusses each of the risk factors of the metabolic syndrome as possible primary drug targets; potential secondary or tertiary targets are also considered.

Necdin and E2F4 are modulated by rosiglitazone therapy in diabetic human adipose and muscle tissue

To identify novel pathways mediating molecular mechanisms of thiazolidinediones (TZDs) in humans, we assessed gene expression in adipose and muscle tissue from six subjects with type 2 diabetes before and after 8 weeks of treatment with rosiglitazone. mRNA was analyzed using Total Gene Expression Analysis (TOGA), an automated restriction-based cDNA display method with quantitative analysis of PCR products. The expression of cell cycle regulatory transcription factors E2F4 and the MAGE protein necdin were similarly altered in all subjects after rosiglitazone treatment. E2F4 expression was decreased by 10-fold in muscle and 2.5-fold in adipose tissue; necdin was identified in adipose tissue only and increased 1.8-fold after TZD treatment. To determine whether changes were related to an effect of the drug or adipogenesis, we evaluated the impact of rosiglitazone and differentiation independently in 3T3-L1 adipocytes. While treatment of differentiated adipocytes with rosiglitazone did not alter E2F4 or necdin, expression of both genes was significantly altered during differentiation. Differentiation was associated with increased cytosolic localization of E2F4. Moreover, necdin overexpression potently inhibited adipocyte differentiation and cell cycle progression. These data suggest that changes in necdin and E2F4 expression after rosiglitazone exposure in humans are associated with altered adipocyte differentiation and may contribute to improved insulin sensitivity in humans treated with TZDs.

The G0/G1 switch gene 2 is a novel PPAR target gene

PPARs (peroxisome-proliferator-activated receptors) alpha, beta/delta and gamma are a group of transcription factors that are involved in numerous processes, including lipid metabolism and adipogenesis. By comparing liver mRNAs of wild-type and PPARalpha-null mice using microarrays, a novel putative target gene of PPARalpha, G0S2 (G0/G1 switch gene 2), was identified. Hepatic expression of G0S2 was up-regulated by fasting and by the PPARalpha agonist Wy14643 in a PPARalpha-dependent manner. Surprisingly, the G0S2 mRNA level was highest in brown and white adipose tissue and was greatly up-regulated during mouse 3T3-L1 and human SGBS (Simpson-Golabi-Behmel syndrome) adipogenesis. Transactivation, gel shift and chromatin immunoprecipitation assays indicated that G0S2 is a direct PPARgamma and probable PPARalpha target gene with a functional PPRE (PPAR-responsive element) in its promoter. Up-regulation of G0S2 mRNA seemed to be specific for adipogenesis, and was not observed during osteogenesis or myogenesis. In 3T3-L1 fibroblasts, expression of G0S2 was associated with growth arrest, which is required for 3T3-L1 adipogenesis. Together, these data indicate that G0S2 is a novel target gene of PPARs that may be involved in adipocyte differentiation.

Critical roles of the p160 transcriptional coactivators p/CIP and SRC-1 in energy balance

Several transcriptional coactivators have been implicated in modulating the transcriptional activities of nuclear hormone receptors in vitro. Potential roles of these cofactors in important physiological processes such as energy homeostasis remain unknown. We report here that a developmental arrest in interscapular brown fat and defective adaptive thermogenesis occur in mice lacking both the p160 family transcriptional coactivators SRC-1 and p/CIP due to a failure in induction of selective PPARgamma target genes involved in adipogenesis and mitochondrial uncoupling. In the absence of p/CIP and SRC-1, mice eat more food on both regular chow and a high-fat diet because of decreased blood leptin levels. However, the p/CIP(-/-)/SRC-1(-/-) mice are lean and resistant to high-fat-diet-induced obesity. They exhibit increased basal metabolic rates and heightened levels of physical activity. Therefore, p/CIP and SRC-1 play critical roles in energy balance by controlling both energy intake and energy expenditure.

Different residues mediate recognition of 1-O-oleyllysophosphatidic acid and rosiglitazone in the ligand binding domain of peroxisome proliferator-activated receptor gamma

Here we showed that a naturally occurring ether analog of lysophosphatidic acid, 1-O-octadecenyl-2-hydroxy-sn-glycero-3-phosphate (AGP), is a high affinity partial agonist of the peroxisome proliferator-activated receptor gamma (PPARgamma). Binding studies using the PPARgamma ligand binding domain showed that [32P]AGP and [3H]rosiglitazone (Rosi) both specifically bind to PPARgamma and compete with each other. [32P]AGP bound PPARgamma with an affinity (Kdapp 60 nm) similar to that of Rosi. However, AGP displaced approximately 40% of bound [3H]Rosi even when applied at a 2000-fold excess. Activation of PPARgamma reporter gene expression by AGP and Rosi showed similar potency, yet AGP-mediated activation was approximately 40% that of Rosi. A complex between AGP and PPARgamma was generated using molecular modeling based on a PPARgamma crystal structure. AGP-interacting residues were compared with Rosi-interacting residues identified within the Rosi-PPARgamma co-crystal complex. These comparisons showed that the two ligands occupy partially overlapping positions but make different hydrogen bonding and ion pairing interactions. Site-specific mutants of PPARgamma were prepared to examine individual ligand binding. H323A and H449A mutants showed reduced binding of Rosi but maintained binding of AGP. In contrast, the R288A showed reduced AGP binding but maintained Rosi binding. Finally, alanine replacement of Tyr-473 abolished binding and activation by Rosi and AGP. These observations indicate that the endogenous lipid mediator AGP is a high affinity ligand of PPARgamma but that it binds via interactions distinct from those involved in Rosi binding. These distinct interactions are likely responsible for the partial PPARgamma agonism of AGP.

Gene expression profiling of potential peroxisome proliferator-activated receptor (PPAR) target genes in human hepatoblastoma cell lines inducibly expressing different PPAR isoforms

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors and commonly play an important role in the regulation of lipid homeostasis. To identify human PPARs-responsive genes, we established tetracycline-regulated human hepatoblastoma cell lines that can be induced to express each human PPAR and investigated the gene expression profiles of these cells.

RESULTS

The expression of each introduced PPAR gene was investigated using the various concentrations of doxycycline in the culture media. We found that the expression of each PPAR subtype was tightly controlled by the concentration of doxycycline in these established cell lines. DNA microarray analyses using these cell lines were performed with or without adding each subtype ligand and provided much important information on the PPAR target genes involved in lipid metabolism, transport, storage and other activities. Interestingly, it was noted that while ligand-activated PPARdelta induced target gene expression, unliganded PPARdelta repressed these genes. The real-time RT-PCR was used to verify the altered expression of selected genes by PPARs and we found that these genes were induced to express in the same pattern as detected in the microarray analyses. Furthermore, we analysed the 5'-flanking region of the human adipose differentiation-related protein (adrp) gene that responded to all subtypes of PPARs. From the detailed analyses by reporter assays, the EMSAs, and ChIP assays, we determined the functional PPRE of the human adrp gene.

CONCLUSION

The results suggest that these cell lines are important tools used to identify the human PPARs-responsive genes.

Tungstate decreases weight gain and adiposity in obese rats through increased thermogenesis and lipid oxidation

The increasing worldwide incidence of obesity and the limitations of current treatments raise the need for finding novel therapeutic approaches to treat this disease. The purpose of the current study was first to investigate the effects of tungstate on body weight and insulin sensitivity in a rat model of diet-induced obesity. Second, we aimed to gain insight into the molecular mechanisms underlying its action. Oral administration of tungstate significantly decreased body weight gain and adiposity without modifying caloric intake, intestinal fat absorption, or growth rate in obese rats. Moreover, the treatment ameliorated dislipemia and insulin resistance of obese rats. These effects were mediated by an increase in whole-body energy dissipation and by changes in the expression of genes involved in the oxidation of fatty acids and mitochondrial uncoupling in adipose tissue. Furthermore, treatment increased the number of small adipocytes with a concomitant induction of apoptosis. Our results indicate that tungstate treatment may provide the basis for a promising novel therapy for obesity.

Elevated stearoyl-CoA desaturase-1 expression in skeletal muscle contributes to abnormal fatty acid partitioning in obese humans

Obesity and type 2 diabetes are strongly associated with abnormal lipid metabolism and accumulation of intramyocellular triacylglycerol, but the underlying cause of these perturbations are yet unknown. Herein, we show that the lipogenic gene, stearoyl-CoA desaturase 1 (SCD1), is robustly up-regulated in skeletal muscle from extremely obese humans. High expression and activity of SCD1, an enzyme that catalyzes the synthesis of monounsaturated fatty acids, corresponded with low rates of fatty acid oxidation, increased triacylglycerol synthesis and increased monounsaturation of muscle lipids. Elevated SCD1 expression and abnormal lipid partitioning were retained in primary skeletal myocytes derived from obese compared to lean donors, implying that these traits might be driven by epigenetic and/or heritable mechanisms. Overexpression of human SCD1 in myotubes from lean subjects was sufficient to mimic the obese phenotype. These results suggest that elevated expression of SCD1 in skeletal muscle contributes to abnormal lipid metabolism and progression of obesity.

Diverging regulation of pyruvate dehydrogenase kinase isoform gene expression in cultured human muscle cells

The pyruvate dehydrogenase complex occupies a central and strategic position in muscle intermediary metabolism and is primarily regulated by phosphorylation/dephosphorylation. The identification of multiple isoforms of pyruvate dehydrogenase kinase (PDK1-4) and pyruvate dehydrogenase phosphatase (PDP1-2) has raised intriguing new possibilities for chronic pyruvate dehydrogenase complex control. Experiments to date suggest that PDK4 is the major isoenzyme responsible for changes in pyruvate dehydrogenase complex activity in response to various different metabolic conditions. Using a cultured human skeletal muscle cell model system, we found that expression of both PDK2 and PDK4 mRNA is upregulated in response to glucose deprivation and fatty acid supplementation, the effects of which are reversed by insulin treatment. In addition, insulin directly downregulates PDK2 and PDK4 mRNA transcript abundance via a phosphatidylinositol 3-kinase-dependent pathway, which may involve glycogen synthase kinase-3 but does not utilize the mammalian target of rapamycin or mitogen-activated protein kinase signalling pathways. In order to further elucidate the regulation of PDK, the role of the peroxisome proliferators-activated receptors (PPAR) was investigated using highly potent subtype selective agonists. PPARalpha and PPARdelta agonists were found to specifically upregulate PDK4 mRNA expression, whereas PPARgamma activation selectively decreased PDK2 mRNA transcript abundance. PDP1 mRNA expression was unaffected by all conditions analysed. These results suggest that in human muscle, hormonal and nutritional conditions may control PDK2 and PDK4 mRNA expression via a common signalling mechanism. In addition, PPARs appear to independently regulate specific PDK isoform transcipt levels, which are likely to impart important metabolic mediation of fuel utilization by the muscle.

Therapeutic roles of peroxisome proliferator-activated receptor agonists

Peroxisome proliferator-activated receptors (PPARs) play key roles in the regulation of energy homeostasis and inflammation, and agonists of PPARalpha and -gamma are currently used therapeutically. Fibrates, first used in the 1970s for their lipid-modifying properties, were later shown to activate PPARalpha. These agents lower plasma triglycerides and VLDL particles and increase HDL cholesterol, effects that are associated with cardiovascular benefit. Thiazolidinediones, acting via PPARgamma, influence free fatty acid flux and thus reduce insulin resistance and blood glucose levels. PPARgamma agonists are therefore used to treat type 2 diabetes. PPARalpha and -gamma agonists also affect inflammation, vascular function, and vascular remodeling. As knowledge of the pleiotropic effects of these agents advances, further potential indications are being revealed, including roles in the management of cardiovascular disease (CVD) and the metabolic syndrome. Dual PPARalpha/gamma agonists (currently in development) look set to combine the properties of thiazolidinediones and fibrates, and they hold considerable promise for improving the management of type 2 diabetes and providing an effective therapeutic option for treating the multifactorial components of CVD and the metabolic syndrome. The functions of a third PPAR isoform, PPARdelta, and its potential as a therapeutic target are currently under investigation.

Activation of PPARgamma enhances myocardial glucose oxidation and improves contractile function in isolated working hearts of ZDF rats

It is suggested that insulin resistance and metabolic maladaptation of the heart are causes of contractile dysfunction. We tested the hypothesis whether systemic PPARgamma activation, by changing the metabolic profile in a model of insulin resistance and type 2 diabetes (the ZDF rat) in vivo, improves contractile function of the heart in vitro. Male Zucker diabetic fatty (ZDF) and Zucker lean (ZL) rats, at 53-56 days of age, were treated with either GI-262570 (a nonthiazolidinedione PPARgamma agonist; A) or vehicle (V) for 1 wk. Agonist treatment resulted in correction of hyperglycemia and dyslipidemia, as well as in reduced hyperinsulinemia. The accumulation of triacylglycerols in the myocardium, characteristic of the ZDF rat, disappeared with treatment. Cardiac power and rates of glucose oxidation in the isolated working heart were significantly reduced in ZDF-V rats, but both parameters increased to nondiabetic levels with agonist treatment. In ZDF-V hearts, transcript levels of PPARalpha-regulated genes and of myosin heavy chain-beta were upregulated, whereas GLUT4 was downregulated compared with ZL. Agonist treatment of ZDF rats reduced PPARalpha-regulated genes and increased transcripts of GLUT4 and GLUT1. In conclusion, by changing the metabolic profile, reducing myocardial lipid accumulation, and promoting the downregulation of PPARalpha-regulated genes, PPARgamma activation leads to an increased capacity of the myocardium to oxidize glucose and to a tighter coupling of oxidative metabolism and contraction in the setting of insulin resistance and type 2 diabetes.

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.

Catalase and PPARgamma2 genotype and risk of rheumatoid arthritis in Koreans

Catalase (CAT) and peroxisome proliferator activated receptor-gamma2 (PPARgamma2) are important regulators of oxidative stress and inflammation, and may contribute to the development of rheumatoid arthritis (RA). We investigated the association between CAT and PPARgamma2 genotypes and risk and severity of RA using 474 cases and 400 controls. Genotyping for the -262C-->T polymorphism of CAT and the Pro12Ala polymorphism of PPARgamma2 was performed by PCR-RFLP analysis. Severity of RA was assessed by the anatomical stage according to Steinbrocker, and a Korean language version of a Health Assessment Questionnaire (KHAQ). No association was observed between CAT and PPARgamma2 genotypes and risk of RA. Our results suggest that genetic polymorphisms of CAT and PPARgamma2 do not play a significant role in the susceptibility to RA among Koreans.

A Peroxisome Proliferator-Activated Receptor α/γ Dual Agonist with a Unique in Vitro Profile and Potent Glucose and Lipid Effects in Rodent Models of Type 2 Diabetes and Dyslipidemia

LSN862 is a novel peroxisome proliferator-activated receptor (PPAR)α/γ dual agonist with a unique in vitro profile that shows improvements on glucose and lipid levels in rodent models of type 2 diabetes and dyslipidemia. Data from in vitro binding, cotransfection, and cofactor recruitment assays characterize LSN862 as a high-affinity PPARγ partial agonist with relatively less but significant PPARα agonist activity. Using these same assays, rosiglitazone was characterized as a high-affinity PPARγ full agonist with no PPARα activity. When administered to Zucker diabetic fatty rats, LSN862 displayed significant glucose and triglyceride lowering and a significantly greater increase in adiponectin levels compared with rosiglitazone. Expression of genes involved in metabolic pathways in the liver and in two fat depots from compound-treated Zucker diabetic fatty rats was evaluated. Only LSN862 significantly elevated mRNA levels of pyruvate dehydrogenase kinase isozyme 4 and bifunctional enzyme in the liver and lipoprotein lipase in both fat depots. In contrast, both LSN862 and rosiglitazone decreased phosphoenol pyruvate carboxykinase in the liver and increased malic enzyme mRNA levels in the fat. In addition, LSN862 was examined in a second rodent model of type 2 diabetes, db/db mice. In this study, LSN862 demonstrated statistically better antidiabetic efficacy compared with rosiglitazone with an equivalent side effect profile. LSN862, rosiglitazone, and fenofibrate were each evaluated in the humanized apoA1 transgenic mouse. At the highest dose administered, LSN862 and fenofibrate reduced very low-density lipoprotein cholesterol, whereas, rosiglitazone increased very low-density lipoprotein cholesterol. LSN862, fenofibrate, and rosiglitazone produced maximal increases in high-density lipoprotein cholesterol of 65, 54, and 30%, respectively. These findings show that PPARγ full agonist activity is not necessary to achieve potent and efficacious insulin-sensitizing benefits and demonstrate the therapeutic advantages of a PPARα/γ dual agonist.

Increased Akt protein expression is associated with decreased ceramide content in skeletal muscle of troglitazone-treated mice

Although it is generally believed that thiazolidinediones ameliorate insulin resistance by lowering circulating free fatty acids, direct effects of these drugs in skeletal muscle may also contribute to their antidiabetic action. We report that troglitazone administration to mice for 1 day increased the protein expression of Akt (two-fold induction, P<0.001) in skeletal muscle without significant changes in the levels of free fatty acids in plasma. Increased Akt protein expression was associated with reduced phospho-AMP-activated protein kinase abundance and with a fall in the phosphorylation of acetyl-CoA carboxylase, which in turn resulted in an increase in the content of muscular malonyl-CoA (2.4-fold, P<0.05) and lactate (1.4-fold, P<0.05). Troglitazone treatment did not affect the mRNA levels of either Akt1 or Akt2, suggesting that a transcriptional mechanism was not involved, but caused a dramatic reduction in the content of muscular ceramides (76%, P<0.001), lipid-derived second messengers known to increase Akt degradation. Our data indicate that troglitazone treatment inhibited de novo ceramide synthesis, since the content of its precursor, palmitoyl-CoA, was reduced (55%, P=0.05). These results were confirmed in C2C12 myotubes, where troglitazone treatment increased Akt protein expression and prevented the reduction of this protein and the increase in ceramide levels caused by palmitate. These findings implicate ceramide as an important intermediate in the regulation of Akt after troglitazone treatment.

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.