Regulation of p85alpha phosphatidylinositol-3-kinase expression by peroxisome proliferator-activated receptors (PPARs) in human muscle cells

Regulation of p85a phosphatidylinositol-3-kinase (p85alphaPI-3K) expression by peroxisome proliferator-activated receptor (PPAR) activators was studied in human skeletal muscle cells. Activation of PPARgamma or PPARbeta did not modify the expression of p85alphaPI-3K. In contrast, activation of PPARalpha increased p85alphaPI-3K mRNA. This effect was potentiated by 9-cis-retinoic acid, an activator of RXR. Up-regulation of p85alphaPI-3K gene expression resulted in a rise in p85alphaPI-3K protein level and in an increase in insulin-induced PI3-kinase activity. According to the role of p85alphaPI-3K in insulin action, these results suggest that drugs with dual action on both PPARgamma and PPARalpha can be of interest for the treatment of insulin resistance.     

Effects of rosiglitazone on gene expression in subcutaneous adipose tissue in highly active antiretroviral therapy-associated lipodystrophy

Highly active antiretroviral therapy (HAART) has improved the prognosis of human immunodeficiency virus (HIV)-infected patients but is associated with severe adverse events, such as lipodystrophy and insulin resistance. Rosiglitazone did not increase subcutaneous fat in patients with HAART-associated lipodystrophy (HAL) in a randomized, double-blind, placebo-controlled trial, although it attenuated insulin resistance and decreased liver fat content. The aim of this study was to examine effects of rosiglitazone on gene expression in subcutaneous adipose tissue in 30 patients with HAL. The mRNA concentrations in subcutaneous adipose tissue were measured using real-time PCR. Twenty-four-week treatment with rosiglitazone (8 mg/day) compared with placebo significantly increased the expression of adiponectin, peroxisome proliferator-activated receptor-gamma (PPARgamma), and PPARgamma coactivator 1 and decreased IL-6 expression. Expression of other genes involved in lipogenesis, fatty acid metabolism, or glucose transport, such as acyl-CoA synthase, adipocyte lipid-binding protein, CD45, fatty acid transport protein-1 and -4, GLUT1, GLUT4, keratinocyte lipid-binding protein, lipoprotein lipase, PPARdelta, and sterol regulatory element-binding protein-1c, remained unchanged. Rosiglitazone also significantly increased serum adiponectin concentration. The change in serum adiponectin concentration was inversely correlated with the change in fasting serum insulin concentration and liver fat content. In conclusion, rosiglitazone induced significant changes in gene expression in subcutaneous adipose tissue and ameliorated insulin resistance in patients with HAL. Increased expression of adiponectin might have mediated most of the favorable insulin-sensitizing effects of rosiglitazone in these patients.     

H-ras Induces Glucose Uptake in Brown Adipocytes in an Insulin- and Phosphatidylinositol 3-Kinase-Independent Manner

Fetal brown adipocytes (parental cells) expressed mainly Glut4 mRNA glucose transporter, the expression of Glut1 mRNA being much lower. At physiological doses, insulin stimulation for 15 min increased 3-fold glucose uptake and doubled the amount of Glut4 protein located at the plasma membrane. Moreover, phosphatidylinositol (PI) 3-kinase activity was induced by the presence of insulin in those cells, glucose uptake being precluded by PI 3-kinase inhibitors such as wortmannin or LY294002. H-ras^lys12-transformed brown adipocytes showed a 10-fold higher expression of Glut1 mRNA and protein than parental cells, Glut4 gene expression being completely down-regulated. Glucose uptake increased by 10-fold in transformed cells compared to parental cells; this uptake was unaltered in the presence of insulin and/or wortmannin. Transient transfection of parental cells with a dominant form of active Ras increased basal glucose uptake by 5-fold, no further effects being observed in the presence of insulin. However, PI 3-kinase activity (immunoprecipitated with anti-αp85 subunit of PI 3-kinase) remained unaltered in H-ras permanent and transient transfectants. Our results indicate that activated Ras induces brown adipocyte glucose transport in an insulin-independent manner, this induction not involving PI 3-kinase activation.     

A selective peroxisome proliferator-activated receptor-gamma (PPARgamma) modulator blocks adipocyte differentiation but stimulates glucose uptake in 3T3-L1 adipocytes

Peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists such as the thiazolidinediones are insulin sensitizers used in the treatment of type 2 diabetes. These compounds induce adipogenesis in cell culture models and increase weight gain in rodents and humans. We have identified a novel PPARgamma ligand, LG100641, that does not activate PPARgamma but selectively and competitively blocks thiazolidinedione-induced PPARgamma activation and adipocyte conversion. It also antagonizes target gene activation as well as repression in agonist-treated 3T3-L1 adipocytes. This novel PPARgamma antagonist does not block adipocyte differentiation induced by a ligand for the retinoid X receptor (RXR), the heterodimeric partner for PPARgamma, or by a differentiation cocktail containing insulin, dexamethasone, and 1-methyl-3-isobutylxanthine. Surprisingly, LG100641, like the PPARgamma agonist rosiglitazone, increases glucose uptake in 3T3-L1 adipocytes. Such selective PPARgamma antagonists may help determine whether insulin sensitization by thiazolidinediones is mediated solely through PPARgamma activation, and whether there are PPARgamma-ligand-independent pathways for adipocyte differentiation. If selective PPARgamma modulators block adipogenesis in vivo, they may prevent obesity, lower insulin resistance, and delay the onset of type 2 diabetes.     

Rosiglitazone prevents the impairment of human islet function induced by fatty acids: evidence for a role of PPARgamma2 in the modulation of insulin secretion

Peroxisome proliferator-activated receptors (PPARs) are a subgroup of the superfamily of nuclear receptors, with three distinct main types: alpha, beta and gamma (subdivided into gamma(1) and gamma(2)). Recently, the presence of PPARgamma has been reported in human islets. Whether other PPAR types can be found in human islets, how islet PPARgamma mRNA expression is regulated by the metabolic milieu, their role in insulin secretion, and the effects of a PPARgamma agonist are not known. In this study, human pancreatic islets were prepared by collagenase digestion and density gradient purification from nonobese adult donors. The presence of PPAR mRNAs was assessed by RT-PCR, and the effect was evaluated of exposure for up to 24 h to either 22.2 mmol/l glucose and/or 0.25, 0.5, or 1.0 mmol/l long-chain fatty acid mixture (oleate to palmitate, 2:1). PPARbeta and, to a greater extent, total PPARgamma and PPARgamma(2) mRNAs were expressed in human islets, whereas PPARalpha mRNA was not detected. Compared with human adipose tissue, PPARgamma mRNA was expressed at lower levels in the islets, and PPARbeta at similar levels. The expression of PPARgamma(2) mRNA was not affected by exposure to 22.2 mmol/l glucose, whereas it decreased markedly and time-dependently after exposure to progressively higher free fatty acids (FFA). This latter effect was not affected by the concomitant presence of high glucose. Exposure to FFA caused inhibition of insulin mRNA expression, glucose-stimulated insulin release, and reduction of islet insulin content. The PPARgamma agonists rosiglitazone and 15-deoxy-Delta-(12,14)prostaglandin J(2) prevented the cytostatic effect of FFA as well as the FFA-induced changes of PPAR and insulin mRNA expression. In conclusion, this study shows that PPARgamma mRNA is expressed in human pancreatic islets, with predominance of PPARgamma(2); exposure to FFA downregulates PPARgamma(2) and insulin mRNA expression and inhibits glucose-stimulated insulin secretion; exposure to PPARgamma agonists can prevent these effects.     

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.     

Estrogen augments glucose transporter and IGF1 expression in primate cerebral cortex.

Estrogen has many positive effects on neural tissue in experimental model systems, including stimulation of neurite growth and neurotransmitter synthesis and protection against diverse types of neural injury. In humans, estrogen treatment is reputed to protect against Alzheimer's disease. To investigate potential mediators of estrogen's action and determine whether selective estrogen receptor modulators (SERMs) such as tamoxifen have estrogen-like effects in the primate brain, we evaluated the expression of glucose transporters and insulin-like growth factor 1 (IGF1) and its receptor in the frontal cortex of ovariectomized rhesus monkeys. We treated one group for 3 days with vehicle, another with 17 beta estradiol (E2), and a third with tamoxifen. The expression of facilitative glucose transporters (Gluts) 1, 3, and 4 was investigated using in situ hybridization, immunohistochemistry, and immunoblot analysis. Gluts 3 and 4 were concentrated in cortical neurons and Glut1 in capillaries and glial cells. E2 treatment induced two- to fourfold increases in Glut3 and Glut4 mRNA levels and lesser but significant increases in Glut3 and 4 protein levels. E2 treatment induced an approximately 70% increase in parenchymal Glut1 mRNA levels, but did not appreciably affect vascular Glut1 gene expression. IGF1 and IGF1 receptor mRNAs were concentrated in cortical neurons in a distribution similar to Gluts 3 and 4. IGF1 mRNA levels were significantly increased in E2-treated animals but IGF1 receptor mRNA levels were not altered by hormone treatment. Tamoxifen increased cerebral cortical Glut3 and 4 mRNA levels, but did not affect Glut1, IGF1, or IGF1 receptor expression. This study provides novel data showing that Gluts 3 and 4 and IGF1 are coexpressed by primate cerebral cortical neurons, where their expression is enhanced by estrogen. These findings suggest that up-regulation of glucose transporter and IGF1 expression may contribute to estrogen's salutary effects on neural tissue. Tamoxifen, an antiestrogen at the breast, is shown to have estrogen-like effects on higher brain centers in the monkey, suggesting that some SERMs may share estrogen's neuroprotective potential for menopausal women.     

Peroxisome proliferator-activated receptor-gamma ligands inhibit adipocyte 11beta -hydroxysteroid dehydrogenase type 1 expression and activity

Peroxisome proliferator-activated receptor-gamma (PPARgamma) has been shown to play an important role in the regulation of expression of a subclass of adipocyte genes and to serve as the molecular target of the thiazolidinedione (TZD) and certain non-TZD antidiabetic agents. Hypercorticosteroidism leads to insulin resistance, a variety of metabolic dysfunctions typically seen in diabetes, and hypertrophy of visceral adipose tissue. In adipocytes, the enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD-1) converts inactive cortisone into the active glucocorticoid cortisol and thereby plays an important role in regulating the actions of corticosteroids in adipose tissue. Here, we show that both TZD and non-TZD PPARgamma agonists markedly reduced 11beta-HSD-1 gene expression in 3T3-L1 adipocytes. This diminution correlated with a significant decrease in the ability of the adipocytes to convert cortisone to cortisol. The half-maximal inhibition of 11beta-HSD-1 mRNA expression by the TZD, rosiglitazone, occurred at a concentration that was similar to its K(d) for binding PPARgamma and EC(50) for inducing adipocyte differentiation thereby indicating that this action was PPARgamma-dependent. The time required for the inhibitory action of the TZD was markedly greater for 11beta-HSD-1 gene expression than for leptin, suggesting that these genes may be down-regulated by different molecular mechanisms. Furthermore, whereas regulation of PPARgamma-inducible genes such as phosphoenolpyruvate carboxykinase was maintained when cellular protein synthesis was abrogated, PPARgamma agonist inhibition of 11beta-HSD-1 and leptin gene expression was ablated, thereby supporting the conclusion that PPARgamma affects the down-regulation of 11beta-HSD-1 indirectly. Finally, treatment of diabetic db/db mice with rosiglitazone inhibited expression of 11beta-HSD-1 in adipose tissue. This decrease in enzyme expression correlated with a significant decline in plasma corticosterone levels. In sum, these data indicate that some of the beneficial effects of PPARgamma antidiabetic agents may result, at least in part, from the down-regulation of 11beta-HSD-1 expression in adipose tissue.     

Downregulated IRS-1 and PPARgamma in obese women with gestational diabetes: relationship to FFA during pregnancy

Gestational diabetes mellitus (GDM) is associated with elevated postprandial free fatty acids (FFA) and insulin resistance; however, little is known about the cellular mechanisms underlying insulin resistance to suppress lipolysis during gestation. We evaluated the longitudinal changes in insulin suppression of FFA before pregnancy and in early (12-14 wk) and late (34-36 wk) gestation in obese subjects with normal glucose tolerance and in obese GDM subjects. Abdominal subcutaneous adipose tissue biopsies were also obtained during cesarean delivery from normal obese pregnant (Preg-Con), GDM, and nonpregnant obese control (Non-Preg-Con) subjects during gynecological surgery. GDM subjects had higher basal plasma FFA before pregnancy (P = 0.055). Insulin's ability to suppress FFA levels declined from early to late gestation in both GDM and Preg-Con subjects and was significantly less in GDM subjects compared with Preg-Con subjects over time (P = 0.025). Adipose tissue insulin receptor substrate (IRS)-1 protein levels were 43% lower (P = 0.02) and p85alpha subunit of phosphatidylinositol 3-kinase was twofold higher (P = 0.03) in GDM compared with Preg-Con subjects. The levels of peroxisome proliferator-activated receptor-gamma (PPARgamma) mRNA and protein were lower by 38% in Preg-Con (P = 0.006) and by 48% in GDM subjects (P = 0.005) compared with Non-Preg controls. Lipoprotein lipase and fatty acid-binding protein-2 mRNA levels were 73 and 52% lower in GDM compared with Preg-Con subjects (P < 0.002). Thus GDM women have decreased IRS-1, which may contribute to reduced insulin suppression of lipolysis with advancing gestation. Decreased PPARgamma and its target genes may be part of the molecular mechanism to accelerate fat catabolism to meet fetal nutrient demand in late gestation.     

The establishment of a novel non-alcoholic steatohepatitis model accompanied with obesity and insulin resistance in mice

Non-alcoholic steatohepatitis (NASH) is a hepatic manifestation of the metabolic syndrome that can progress to liver cirrhosis. The major aim of this study was to establish a novel NASH mouse model accompanied by obesity and insulin resistance, then explore the molecular mechanisms of NASH and evaluate the effects of both the peroxisome proliferator-activated receptor alpha (PPARalpha) agonist fenofibrate and the PPARgamma agonist rosiglitazone in this established NASH model. The novel model was induced in C57BL/6 mice by 23 weeks of ad libitum feeding of a modified high-fat diet (mHFD), with lower methinione and choline and higher fat content. In comparison to the controls, the model animals developed pronounced obesity, dyslipidemia and insulin resistance. Marked liver lesions characterized by severe steatosis, inflammation, fibrosis, increased hepatic triglyceride content, and elevated serum alanine aminotransferase (ALT) levels were observed in the models. In this novel model, treatment with fenofibrate or rosiglitazone significantly improved insulin sensitivity and corrected dyslipidemia; however, fenofibrate was more effective than rosiglitazone in improving hepatic morphology and ALT levels. Further study showed that long-term feeding of mHFD significantly increased expression of mRNA for hepatic PPARgamma, adipose fatty acid binding protein (ap2) and CD36 and suppressed expression of mRNA for hepatic PPARalpha and carnitine palmitoyl transferase-1a (CPT-1a). These results showed the successful establishment of the combined NASH and obese-insulin resistance mouse model. Additionally, aberrant expressions of hepatic PPARalpha and PPARgamma may play a major role in the pathogenesis of NASH by affecting hepatic lipogenesis and fatty acid oxidation in this novel model.