Direct thiazolidinedione action on isolated rat skeletal muscle fuel handling is independent of peroxisome proliferator-activated receptor-gamma-mediated changes in gene expression

Insulin Sensitizing Pharmacology of Thiazolidinediones Correlates with Mitochondrial Gene Expression rather than Activation of PPARγ

Insulin sensitizing thiazolidinediones (TZDs) are generally considered to work as agonists for the nuclear receptor peroxisome proliferative activated receptor-gamma (PPARγ). However, TZDs also have acute, non-genomic metabolic effects and it is unclear which actions are responsible for the beneficial pharmacology of these compounds. We have taken advantage of an analog, based on the metabolism of pioglitazone, which has much reduced ability to activate PPARγ. This analog (PNU-91325) was compared to rosiglitazone, the most potent PPARγ activator approved for human use, in a variety of studies both in vitro and in vivo. The data demonstrate that PNU-91325 is indeed much less effective than rosiglitazone at activating PPARγ both in vitro and in vivo. In contrast, both compounds bound similarly to a mitochondrial binding site and acutely activated PI-3 kinase-directed phosphorylation of AKT, an action that was not affected by elimination of PPARγ activation. The two compounds were then compared in vivo in both normal C57 mice and diabetic KKAy mice to determine whether their pharmacology correlated with biomarkers of PPARγ activation or with the expression of other gene transcripts. As expected from previous studies, both compounds improved insulin sensitivity in the diabetic mice, and this occurred in spite of the fact that there was little increase in expression of the classic PPARγ target biomarker adipocyte binding protein-2 (aP2) with PNU-91325 under these conditions. An examination of transcriptional profiling of key target tissues from mice treated for one week with both compounds demonstrated that the relative pharmacology of the two thiazolidinediones correlated best with an increased expression of an array of mitochondrial proteins and with expression of PPARγ coactivator 1-alpha (PGC1α), the master regulator of mitochondrial biogenesis. Thus, important pharmacology of the insulin sensitizing TZDs may involve acute actions, perhaps on the mitochondria, that are independent of direct activation of the nuclear receptor PPARγ. These findings suggest a potential alternative route to the discovery of novel insulin sensitizing drugs.

Mitochondrial target of thiazolidinediones

Insulin-sensitizing thiazolidinediones exert a pleiotropic pharmacology with therapeutic potential in a number of disease states ranging from metabolic syndrome and diabetes to neurodegeneration and cancer. A growing understanding of their mechanism of action, working from the site of their binding in the mitochondrion, provides insight into the mechanism of action of the insulin sensitizers and the reasons for their pleiotropic pharmacology. This review helps to frame the direction of future work that should be helpful in setting a new direction for the discovery and development of new, more useful therapeutic agents for metabolic disease.

The effect of IGF2BP2 gene polymorphisms on pioglitazone response in Chinese type 2 diabetes patients

BACKGROUND

Genome-wide association studies identified that insulin-like growth factor 2 mRNA-binding protein 2 (IGF2BP2) genetic polymorphisms are related to type 2 diabetes mellitus (T2DM) in several populations. This study aimed to investigate whether the IGF2BP2 gene rs1470579 and rs4402960 polymorphisms were associated with T2DM and pioglitazone efficacy in Chinese T2DM patients.

METHODS

A total of 281 T2DM patients and 111 healthy volunteers were enrolled to identify the IGF2BP2 gene rs1470579 and rs4402960 polymorphisms; 86 patients were randomly selected and given a 12-week pioglitazone treatment (30 mg/day). Fasting plasma glucose, postprandial plasma glucose (PPG), glycated hemoglobin, serum triglycerides (TG), total cholesterol, low-density lipoprotein cholesterol and high-density lipoprotein cholesterol (HDL-C) were determined before and after pioglitazone treatment.

RESULTS

The results showed that the IGF2BP2 gene rs1470579 and rs4402960 polymorphisms were associated with T2DM in a Chinese population (OR = 2.002, 95% CI 1.170-3.426, p < 0.05; OR = 1.879, 95% CI 1.110-3.182, p < 0.05). The effect of pioglitazone on PPG (p < 0.05), TG (p < 0.01) and HDL-C (p < 0.05) was lower in patients with the rs1470579 AC+CC genotypes than in AA genotype carriers. Its effect on PPG level was also lower in patients with the GT+TT genotypes of rs4402960 than in patients with the GG genotype (p < 0.05).

CONCLUSIONS

The IGF2BP2 gene rs1470579 and rs4402960 polymorphisms were associated with T2DM and therapeutic efficacy of pioglitazone in this Chinese population.

Pioglitazone leads to an inactivation and disassembly of complex I of the mitochondrial respiratory chain

BACKGROUND

Thiazolidinediones are antidiabetic agents that increase insulin sensitivity but reduce glucose oxidation, state 3 respiration, and activity of complex I of the mitochondrial respiratory chain (MRC). The mechanisms of the latter effects are unclear. The aim of this study was to determine the mechanisms by which pioglitazone (PGZ), a member of the thiazolidinedione class of antidiabetic agents, decreases the activity of the MRC. In isolated mitochondria from mouse liver, we measured the effects of PGZ treatment on MRC complex activities, fully-assembled complex I and its subunits, gene expression of complex I and III subunits, and [3H]PGZ binding to mitochondrial complexes.

RESULTS

In vitro, PGZ decreased activity of complexes I and III of the MRC, but in vivo only complex I activity was decreased in mice treated for 12 weeks with 10 mg/kg/day of PGZ. In vitro treatment of isolated liver mitochondria with PGZ disassembled complex I, resulting in the formation of several subcomplexes. In mice treated with PGZ, fully assembled complex I was increased and two additional subcomplexes were found. Formation of supercomplexes CI+CIII2+CIVn and CI+CIII2 decreased in mouse liver mitochondria exposed to PGZ, while formation of these supercomplexes was increased in mice treated with PGZ. Two-dimensional analysis of complex I using blue native/sodium dodecyl sulfate polyacrylamide gel electrophoresis (BN/SDS-PAGE) showed that in vitro PGZ induced the formation of four subcomplexes of 600 (B), 400 (C), 350 (D), and 250 (E) kDa, respectively. Subcomplexes B and C had NADH:dehydrogenase activity, while subcomplexes C and D contained subunits of complex I membrane arm. Autoradiography and coimmunoprecipitation assays showed [3H]PGZ binding to subunits NDUFA9, NDUFB6, and NDUFA6. Treatment with PGZ increased mitochondrial gene transcription in mice liver and HepG2 cells. In these cells, PGZ decreased intracellular ATP content and enhanced gene expression of specific protein 1 and peroxisome-proliferator activated receptor (PPAR)γ coactivator 1α (PGC-1α).

CONCLUSIONS

PGZ binds complex I subunits, which induces disassembly of this complex, reduces its activity, depletes cellular ATP, and, in mice and HepG2 cells, upregulates nuclear DNA-encoded gene expression of complex I and III subunits.

PPAR-γ2 and PTPRD gene polymorphisms influence type 2 diabetes patients' response to pioglitazone in China

AIM

To investigate the influence of peroxisome proliferator-activated receptor γ2 (PPAR-γ2) gene polymorphism rs1801282 and protein tyrosine phosphatase receptor type D (PTPRD) gene polymorphism rs17584499 on the occurrence of type 2 diabetes and pioglitazone efficacy in a Chinese Han population.

METHODS

One hundred ninety seven type 2 diabetes patients and 212 healthy controls were enrolled. Among them, 67 type 2 diabetes patients were administered pioglitazone (30 mg/d, po) for 3 months. All the subjects were genotyped for genetic variants in PPAR-γ2 and PTPRD using MALDI-TOF mass spectrometry. Fasting plasma glucose, postprandial plasma glucose, glycated hemoglobin, serum triglyceride, total cholesterol, low-density and high-density lipoprotein-cholesterol were determined.

RESULTS

The PPAR-γ2 gene rs1801282 polymorphism was significantly associated with type 2 diabetes susceptibility (OR=0.515, 95% CI 0.268-0.990) and the PTPRD gene rs17584499 polymorphism was also significantly associated with type 2 diabetes (OR=1.984, 95% CI 1.135-3.469) in a dominant model adjusted for age, gender and BMI. After pioglitazone treatment for 3 months, the type 2 diabetes patients with PPAR-γ2 rs1801282 CG genotypes significantly showed higher differential values of postprandial plasma glucose and serum triglyceride compared with those with rs1801282 CC genotype. The patients with PTPRD rs17584499 CT+TT genotypes showed significantly lower differential value of postprandial plasma glucose compared to those with rs17584499 CC genotype.

CONCLUSION

Diabetes risk alleles in PPAR-γ2 (rs1801282) and PTPRD (rs17584499) are associated with pioglitazone therapeutic efficacy.

Peroxisome proliferator-activated receptor γ decouples fatty acid uptake from lipid inhibition of insulin signaling in skeletal muscle

Peroxisome proliferator-activated receptor γ (PPARγ) is expressed at low levels in skeletal muscle, where it protects against adiposity and insulin resistance via unclear mechanisms. To test the hypothesis that PPARγ directly modulates skeletal muscle metabolism, we created two models that isolate direct PPARγ actions on skeletal myocytes. PPARγ was overexpressed in murine myotubes by adenotransfection and in mouse skeletal muscle by plasmid electroporation. In cultured myotubes, PPARγ action increased fatty acid uptake and incorporation into myocellular lipids, dependent upon a 154 ± 20-fold up-regulation of CD36 expression. PPARγ overexpression more than doubled insulin-stimulated thymoma viral proto-oncogene (AKT) phosphorylation during low lipid availability. Furthermore, in myotubes exposed to palmitate levels that inhibit insulin signaling, PPARγ overexpression increased insulin-stimulated AKT phosphorylation and glycogen synthesis over 3-fold despite simultaneously increasing myocellular palmitate uptake. The insulin signaling enhancement was associated with an increase in activating phosphorylation of phosphoinositide-dependent protein kinase 1 and a normalized expression of palmitate-induced genes that antagonize AKT phosphorylation. In vivo, PPARγ overexpression more than doubled insulin-dependent AKT phosphorylation in lipid-treated mice but did not augment insulin-stimulated glucose uptake. We conclude that direct PPARγ action promotes myocellular storage of energy by increasing fatty acid uptake and esterification while simultaneously enhancing insulin signaling and glycogen formation. However, direct PPARγ action in skeletal muscle is not sufficient to account for the hypoglycemic actions of PPARγ agonists during lipotoxicity.

Role of PPARg2 transcription factor in thiazolidinedione-induced insulin sensitization

OBJECTIVES

Adipose tissue is the key regulator of energy balance, playing an active role in lipid storage and metabolism and may be a dynamic buffer to control fatty acid flux. Peroxisome proliferator-activated receptor gamma isoform-2 (PPARg2), an isoform of the nuclear hormone receptor superfamily, has been implicated in almost all aspects of human metabolic alterations such as obesity, insulin resistance, type-2 diabetes and dyslipidaemia. The PPARg2 isoform is highly present in adipose tissue where it functions as a thrifty phenotype, which promotes adipocyte differentiation and triglyceride storage. Thiazolidinediones, antidiabetic drugs, induce insulin sensitivity by controlling adipokines. The thiazolidinediones bind with PPARg2 in adipocytes and exert an agonist effect by enhancing adipogenesis and fatty acid uptake. Thiazolidinediones stimulate PPARg2, by which they down-regulate tumour necrosis factor-α, leptin, interleukin-6 and plasminogen and also enhance insulin sensitivity. The aim of this work is to define role of PPARg2 transcription factor in thiazolidinedione-induced insulin sensitization.

KEY FINDINGS

The PPARg2 alters the transcription of the target gene. This altered gene transcription results in the up-regulation of insulin-sensitizing factors and down-regulation of insulin-resistant factors. The variant Pro12Ala of the PPARg2 gene is an important modulator in metabolic control in the body. Thiazolidinediones stimulate PPARg2 transcription factor by which PPARg2 binds to responsive elements located in the promoter regions of many genes and modulates their transcriptive activity. There is a strong mutual relationship between receptor binding and agonism, which is evidence of the insulin-sensitizing target of thiazolidinediones in PPARg2. This evidently increases the biological potency of the glucose-lowering effect of thiazolidinediones in vivo as well as their antidiabetic activity.

CONCLUSIONS

PPARg2 transcription factor plays an important role in treatment of type-2 diabetes with thiazolidindiones. The variant Pro12Ala of the PPARg2 gene promotes the activity of thiazolidinediones in minimizing insulin resistance. Transcriptional activity of Pro12Ala variant improves the activity of insulin. Thus thiazolidinediones promote the phosphorylation of PPARg2 to induce insulin sensitivity.

Pioglitazone limits myocardial infarct size, activates Akt, and upregulates cPLA2 and COX-2 in a PPAR-γ-independent manner

Pioglitazone (PIO), a PPAR-γ agonist, limits myocardial infarct size by activating Akt and upregulating cytosolic phospholipase A(2) (cPLA(2)) and cyclooxygenase (COX)-2. However, PIO has several PPAR-γ-independent effects. We assessed whether PIO limits myocardial infarct size in PPAR-γ-knockout mice, attenuates hypoxia-reoxygenation injury and upregulates P-Akt, cPLA(2), and COX-2 expression in PPAR-γ-knockout cardiomyocytes. Cardiac-specific inducible PPAR-γ knockout mice were generated by crossing αMHC-Cre mice to PPAR-γ(loxp/loxp) mice. PPAR-γ deletion was achieved after 7 days of intraperitoneal tamoxifen (20 mg/kg/day) administration. Mice received PIO (10 mg/kg/day), or vehicle, for 3 days and underwent coronary occlusion (30 min) followed by reperfusion (4 h). We assessed the area at risk by blue dye and infarct size by TTC. Cultured adult cardiomyocytes of PPAR-γ(loxp/loxp/cre) mice without or with pretreatment with tamoxifen were incubated with or without PIO and subjected to 2 h hypoxia/2 h reoxygenation. Cardiac-specific PPAR-γ knockout significantly increased infarct size. PIO reduced infarct size by 51% in PPAR-γ knockout mice and by 55% in mice with intact PPAR-γ. Deleting the PPAR-γ gene increased cell death in vitro. PIO reduced cell death in cells with and without intact PPAR-γ. PIO similarly increased myocardial Ser-473 P-Akt, cPLA(2), and COX-2 levels after hypoxia/reoxygenation in cells with and without intact PPAR-γ. PIO limited infarct size in mice in a PPAR-γ-independent manner. PIO activated Akt, increased the expression of cPLA(2) and COX-2, and protected adult cardiomyocytes against the effects of hypoxia/reoxygenation independent of PPAR-γ activation.

Lipophilicity as a determinant of thiazolidinedione action in vitro: findings from BLX-1002, a novel compound without affinity to PPARs

The pharmacology of thiazolidinediones (TZDs) seems to be driven not only by activation of peroxisome proliferator-activated receptor-γ (PPARγ), but also by PPARγ-independent effects on mitochondrial function and cellular fuel handling. This study portrayed such actions of the novel hydrophilic TZD compound BLX-1002 and compared them to those of conventional TZDs. Mitochondrial function and fuel handling were examined in disrupted rat muscle mitochondria, intact rat liver mitochondria, and specimens of rat skeletal muscle. BLX-1002 was superior to most other TZDs as an inhibitor of respiratory complex 1 in disrupted mitochondria, but had less effect than any other TZD on oxygen consumption by intact mitochondria and on fuel metabolism by intact tissue. The latter finding was obviously related to the hydrophilic properties of BLX-1002, because high potentials of individual TZDs to shift muscle fuel metabolism from the aerobic into the anaerobic pathway were associated with high ClogP values indicative of high lipophilicity and low hydrophilicity (e.g., % increase in lactate release induced by 10 μmol/l of respective compound: BLX-1002, ClogP 0.39, +10 ± 8%, not significant; pioglitazone, ClogP 3.53, +68 ± 12%, P < 0.001; troglitazone, ClogP 5.58, +157 ± 14%, P < 0.001). The observed specific properties of BLX-1002 could result from relatively strong direct affinity to an unknown mitochondrial target, but limited access to this target. Results suggest 1) that impairment of mitochondrial function and increased anaerobic fuel metabolism are unlikely to account for PPARγ-independent glucose lowering by BLX-1002, and 2) that higher lipophilicity of an individual TZD is associated with stronger acceleration of anaerobic glycolysis.

Agonism of Peroxisome Proliferator Receptor-Gamma may have Therapeutic Potential for Neuroinflammation and Parkinson's Disease

Evidence suggests inflammation, mitochondria dysfunction, and oxidative stress play major roles in Parkinson's disease (PD), where the primary pathology is the significant loss of dopaminergic neurons in the substantia nigra (SN). Current methods used to treat PD focus mainly on replacing dopamine in the nigrostriatal system. However, with time these methods fail and worsen the symptoms of the disease. This implies there is more to the treatment of PD than just restoring dopamine or the dopaminergic neurons, and that a broader spectrum of factors must be changed in order to restore environmental homeostasis. Pharmacological agents that can protect against progressive neuronal degeneration, increase the level of dopamine in the nigrostriatal system, or restore the dopaminergic system offer various avenues for the treatment of PD. Drugs that reduce inflammation, restore mitochondrial function, or scavenge free radicals have also been shown to offer neuroprotection in various animal models of PD. The activation of peroxisome proliferator receptor- gamma (PPAR-gamma ) has been associated with altering insulin sensitivity, increasing dopamine, inhibiting inflammation, altering mitochondrial bioenergetics, and reducing oxidative stress - a variety of factors that are altered in PD. Therefore, PPAR-gamma activation may offer a new clinically relevant treatment approach to neuroinflammation and PD related neurodegeneration. This review will summarize the current understanding of the role of PPAR-gamma agonists in neuroinflammation and discuss their potential for the treatment of PD.

PPARgamma1 and LXRalpha face a new regulator of macrophage cholesterol homeostasis and inflammatory responsiveness, AEBP1

Peroxisome proliferator-activated receptor γ1 (PPARγ1) and liver X receptor α (LXRα) are nuclear receptors that play pivotal roles in macrophage cholesterol homeostasis and inflammation; key biological processes in atherogenesis. The activation of PPARγ1 and LXRα by natural or synthetic ligands results in the transactivation of ABCA1, ABCG1, and ApoE; integral players in cholesterol efflux and reverse cholesterol transport. In this review, we describe the structure, isoforms, expression pattern, and functional specificity of PPARs and LXRs. Control of PPARs and LXRs transcriptional activity by coactivators and corepressors is also highlighted. The specific roles that PPARγ1 and LXRα play in inducing macrophage cholesterol efflux mediators and antagonizing macrophage inflammatory responsiveness are summarized. Finally, this review focuses on the recently reported regulatory functions that adipocyte enhancer-binding protein 1 (AEBP1) exerts on PPARγ1 and LXRα transcriptional activity in the context of macrophage cholesterol homeostasis and inflammation.

Acute exposure to rosiglitazone does not affect glucose transport in intact human skeletal muscle

Thiazolidinediones (TZDs) such as rosiglitazone are widely used as antidiabetic drugs. Animal studies suggest that TZDs may have direct metabolic actions in skeletal muscle. Here, we examined if acute exposure to rosiglitazone stimulates glucose transport rate and affects proximal insulin signaling in isolated skeletal muscle strips from nondiabetic men. Open muscle biopsies were obtained from musculus vastus lateralis from 15 nondiabetic men (50 +/- 3 years old, 26.9 +/- 1.1 kg/m(2)). Skeletal muscle strips were isolated and exposed to rosiglitazone (1 or 10 micromol/L), 5-aminoimidazole-4-carboxamide 1-beta-D-ribonucleoside (1 mmol/L), insulin (120 nmol/L), or a combination of insulin (120 nmol/L) and rosiglitazone (10 micromol/L) in vitro for 1 hour. Glucose transport was analyzed by accumulation of intracellular 3-O-methyl [(3)H] glucose; phosphorylation of Akt-Ser(473) and Akt-Thr(308) and phosphorylation of acetyl coenzyme A carboxylase beta were determined using phosphospecific antibodies. 5-Aminoimidazole-4-carboxamide 1-beta-d-ribonucleoside and insulin increased glucose transport rate 1.5-fold (P < .05) and 1.7-fold (P < .01) in isolated muscle strips, respectively. Exposure to rosiglitazone transiently increased phosphorylation of acetyl coenzyme A carboxylase beta, with a maximum effect at 15 minutes and return to baseline at 60 minutes. However, rosiglitazone did not affect basal or insulin-stimulated glucose transport rate, or phosphorylation of Akt-Ser(473) or Akt-Thr(308) in isolated muscle strips. In conclusion, acute exposure to rosiglitazone does not affect glucose transport in human skeletal muscle.

The effects of amino acids on glucose metabolism of isolated rat skeletal muscle are independent of insulin and the mTOR/S6K pathway

Two mechanisms have been proposed for the modulation of skeletal muscle glucose metabolism by amino acids. Whereas studies on humans and cultured cells suggested acute insulin desensitization via mammalian target of rapamycin (mTOR) and its downstream target p70 S6 kinase (S6K), investigations using native specimens of rat muscle hinted at impairment of glucose oxidation by competition for mitochondrial oxidation. To better understand these seemingly contradictory findings, we explored the effects of high concentrations of mixed amino acids on fuel metabolism and S6K activity in freshly isolated specimens of rat skeletal muscle. In this setting, increasing concentrations of amino acids dose-dependently reduced the insulin-stimulated rates of CO(2) production from glucose and palmitate (decrease in glucose oxidation induced by addition of 5.5, 11, 22, and 44 mmol/l amino acids:--16 +/- 3, -25 +/- 7, -44 +/- 4, -62 +/- 4%; P < 0.02 each). This effect could not be attributed to insulin desensitization, because it was not accompanied by any reduction of insulin-stimulated glucose transport [+12 +/- 16, +17 +/- 22, +21 +/- 33, +13 +/- 12%; all nonsignificant (NS)] or glycogen synthesis (+1 +/- 6, -5 +/- 6, -9 +/- 8, +6 +/- 5%; all NS) and because it persisted without insulin stimulation. Abrogation of S6K activity by the mTOR blocker rapamycin failed to counteract amino acid-induced inhibition of glucose and palmitate oxidation, which therefore was obviously independent of mTOR/S6K signaling (decrease in glucose oxidation by addition of 44 mmol/l amino acids: without rapamycin, -60 +/- 4%; with rapamycin, -50 +/- 13%; NS). We conclude that amino acids can directly affect muscle glucose metabolism via two mechanisms, mTOR/S6K-mediated insulin desensitization and mitochondrial substrate competition, with the latter predominating in isolated rat muscle.

Peroxisome proliferator-activated receptor-gamma agonist improves skeletal muscle insulin signaling in the pregestational intrauterine growth-restricted rat offspring

The effect of early intervention with a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist on skeletal muscle GLUT4 translocation and insulin signaling was examined in intrauterine (IUGR) and postnatal (PNGR) growth-restricted pregestational female rat offspring. Rosiglitazone [11 mumol/day provided from postnatal day (PN)21 to PN60] improved skeletal muscle insulin sensitivity and GLUT4 translocation in prenatal nutrient restriction [50% calories from embryonic day (e)11 to e21; IUGR] with (IUGR+PNGR) and without (IUGR) postnatal nutrient restriction (50% calories from PN1 to PN21; PNGR) similar to that of control (ad libitum feeds throughout; Con) (n = 6 each). This was accomplished by diminished basal and improved insulin-responsive GLUT4 association with the plasma membrane in IUGR, IUGR+PNGR, and PNGR mimicking that in Con (P < 0.005). While no change in p85-phosphatidylinositol 3-kinase (PI3-K) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) was observed, a decrease in protein tyrosine phosphatase 1B (PTP1B; P < 0.0002) and SH2-containing protein tyrosine phosphatase 2 (SHP2; P < 0.05) contributing to the rosiglitazone-induced insulin sensitivity was seen only in IUGR+PNGR. In contrast, an increase in phosphorylated 5'-adenosine monophosphate kinase (pAMPK; P < 0.04) and insulin responsiveness of phosphorylated phosphoinositide-dependent protein kinase-1 (pPDK1; P < 0.05), pAkt (P < 0.01), and particularly pPKCzeta (P < 0.0001) and its corresponding enzyme activity (P < 0.005) were observed in all four experimental groups. We conclude that early introduction of PPARgamma agonist improved skeletal muscle activation of AMPK and insulin signaling, resulting in insulin-independent AMPK and insulin-responsive GLUT4 association with plasma membranes in IUGR, IUGR+PNGR, and PNGR adult offspring, similar to that of Con. These findings support a role for insulin sensitizers in preventing the subsequent development of gestational or type 2 diabetes mellitus in intrauterine and postnatal growth-restricted offspring.

PPAR-gamma-independent inhibitory effect of rosiglitazone on glucose synthesis in primary cultured rabbit kidney-cortex tubules

Therapeutic effect of rosiglitazone has been reported to result from an improvement of insulin sensitivity and inhibition of glucose synthesis. As the latter process occurs in both liver and kidney cortex the aim of this study was to elucidate the rosiglitazone action on glucose formation in both tissues. Primary cultured cells of both liver and kidney cortex grown in defined medium were use throughout. To identify the mechanism responsible for drug-induced changes, intracellular gluconeogenic intermediates and enzyme activities were determined. In contrast to hepatocytes, the administration of a 10 micromol/L concentration of rosiglitazone to renal tubules resulted in about a 70% decrease in the rate of gluconeogenesis, accompanied by an approximately 75% decrease in alanine utilization and a 35% increase in lactate synthesis. The effect of rosiglitazone was not abolished by GW9662, the PPAR-gamma irreversible antagonist, indicating that this action is not dependent on PPAR-gamma activation. In view of rosiglitazone-induced changes in gluconeogenic intermediates and a diminished incorporation of 14CO2 into pyruvate, it is likely that the drug causes a decline in flux through pyruvate carboxylase and (or) phosphoenolpyruvate carboxykinase. It is likely that the hypoglycemic action of rosiglitazone is PPAR-gamma independent and results mainly from its inhibitory effects on renal gluconeogenesis.

The high affinity peroxisome proliferator-activated receptor-gamma agonist RS5444 inhibits both initiation and progression of colon tumors in azoxymethane-treated mice

We evaluated RS5444, a thiazolidinedione high affinity PPARgamma agonist, for the ability to inhibit colon carcinogenesis in azoxymethane (AOM)-treated mice. In our initial experiment, mice were treated with RS5444 during AOM treatment, and the drug was withdrawn 12 weeks after the last injection of AOM. RS5444 significantly inhibited aberrant crypt focus formation under these circumstances. Furthermore, exposure to RS5444 during the course of AOM treatment effectively blocked colon tumor formation after withdrawal of the agonist. PPARgamma expression and nuclear localization were reduced in adenomas. RS5444 did not inhibit DNA synthesis in tumor cells, suggesting that PPARgamma activity was impaired in adenomas. To test this hypothesis, pre-existing adenomas were treated with RS5444 for 16 weeks. We observed a slight, albeit not statistically significant, reduction in tumor incidence in RS5444-treated mice. However, histological examination revealed that tumors from RS5444-treated mice were of significantly lower grade, as evaluated by the extent of dysplasia. Furthermore, carcinoma in situ was observed in about one-third of control tumors, but was never observed in RS5444-treated tumors. We conclude that RS5444 inhibits both initiation and progression of colon tumors in the AOM model of sporadic colon carcinogenesis.

Pioglitazone enhances mitochondrial biogenesis and ribosomal protein biosynthesis in skeletal muscle in polycystic ovary syndrome

Insulin resistance is a common metabolic abnormality in women with PCOS and leads to an elevated risk of type 2 diabetes. Studies have shown that thiazolidinediones (TZDs) improve metabolic disturbances in PCOS patients. We hypothesized that the effect of TZDs in PCOS is, in part, mediated by changes in the transcriptional profile of muscle favoring insulin sensitivity. Using Affymetrix microarrays, we examined the effect of pioglitazone (30 mg/day for 16 weeks) on gene expression in skeletal muscle of 10 obese women with PCOS metabolically characterized by a euglycemic-hyperinsulinemic clamp. Moreover, we explored gene expression changes between these PCOS patients before treatment and 13 healthy women. Treatment with pioglitazone improved insulin-stimulated glucose metabolism and plasma adiponectin, and reduced fasting serum insulin (all P<0.05). Global pathway analysis using Gene Map Annotator and Pathway Profiler (GenMAPP 2.1) and Gene Set Enrichment Analysis (GSEA 2.0.1) revealed a significant upregulation of genes representing mitochondrial oxidative phosphorylation (OXPHOS), ribosomal proteins, mRNA processing reactome, translation factors, and proteasome degradation in PCOS after pioglitazone therapy. Quantitative real-time PCR suggested that upregulation of OXPHOS genes was mediated by an increase in PGC-1α expression (P<0.05). Pretreatment expression of genes representing OXPHOS and ribosomal proteins was down-regulated in PCOS patients compared to healthy women. These data indicate that pioglitazone therapy restores insulin sensitivity, in part, by a coordinated upregulation of genes involved in mitochondrial OXPHOS and ribosomal protein biosynthesis in muscle in PCOS. These transcriptional effects of pioglitazone may contribute to prevent the onset of type 2 diabetes in these women.

Pioglitazone induces lipid accumulation in the rat heart despite concomitant reduction in plasma free fatty acid availability

Thiazolidinediones are insulin-sensitizing drugs which have been proved to be effective in the treatment of type 2 diabetes. However, the action of thiazolidinediones on myocardial metabolism is only poorly recognized. Therefore, the aim of our study was to investigate the effects of two-week pioglitazone treatment (3 mg/kg/d) on lipid and carbohydrate metabolism in the heart of rats fed on a standard chow or on a high-fat diet (HFD) for three weeks. High-fat feeding increased myocardial protein expression of all peroxisome proliferator-activated receptor (PPAR) isoforms. The greatest response was, however, noted in the case of PPARgamma. Surprisingly, administration of pioglitazone induced accumulation of free fatty acids (FFA) and diacylglycerol in the heart in both groups, despite concomitant reduction in plasma FFA concentration. The content of triacylglycerol was increased only in the HFD group. Pioglitazone treatment also shifted myocardial substrate utilization towards greater contribution of glucose in both groups, as evidenced by decreased rate of palmitate oxidation and higher 2-deoxyglucose uptake and elevated glycogen content. This could induce a mismatch between the rate of myocardial fatty acid uptake and oxidation leading to increased intracellular availability of fatty acids for non-oxidative metabolic pathways like synthesis of acylglycerols. Our data suggests that thiazolidinediones improve cardiac insulin sensitivity by mechanisms other than reduction in intramyocardial lipid content.

A new mechanism involving ERK contributes to rosiglitazone inhibition of tumor necrosis factor-alpha and interferon-gamma inflammatory effects in human endothelial cells

OBJECTIVE

Microvascular endothelium is one of the main targets of the inflammatory response. On specific activation, endothelial cells recruit Th1-lymphocytes at the inflammatory site. We investigated the intracellular signaling mediating tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma inflammatory response in human microvascular endothelial cells (HMEC-1) and the interfering effects of the peroxisome-proliferator-activated-receptor (PPARgamma) agonist, rosiglitazone (RGZ).

METHODS AND RESULTS

TNFalpha and IFNgamma, mainly when combined, stimulate IFNgamma-inducible protein of 10 kDa (IP10) and fractalkine production evaluated by ELISA and TaqMan analyses. This effect is not only mediated by activation of the NFkB and Stat1 classic pathways, but also involves a rapid increase in phosphorylation and activation of extracellular signal-regulated kinases (ERK1/2) as measured by Western blot. RGZ interferes with TNFalpha and IFNgamma stimulation of IP10, fractalkine, and adhesion molecule through a novel rapid mechanism which involves the blocking of ERK activation.

CONCLUSIONS

Our findings shed new light on the mechanisms underlying the inflammatory response of microvascular endothelium and on the possible therapeutic use of RGZ in vasculopathies involving Th1-responses.

Apolipoprotein AII is a regulator of very low density lipoprotein metabolism and insulin resistance

Apolipoprotein AII (apoAII) transgenic (apoAIItg) mice exhibit several traits associated with the insulin resistance (IR) syndrome, including IR, obesity, and a marked hypertriglyceridemia. Because treatment of the apoAIItg mice with rosiglitazone ameliorated the IR and hypertriglyceridemia, we hypothesized that the hypertriglyceridemia was due largely to overproduction of very low density lipoprotein (VLDL) by the liver, a normal response to chronically elevated insulin and glucose. We now report in vivo and in vitro studies that indicate that hepatic fatty acid oxidation was reduced and lipogenesis increased, resulting in a 25% increase in triglyceride secretion in the apoAIItg mice. In addition, we observed that hydrolysis of triglycerides from both chylomicrons and VLDL was significantly reduced in the apoAIItg mice, further contributing to the hypertriglyceridemia. This is a direct, acute effect, because when mouse apoAII was injected into mice, plasma triglyceride concentrations were significantly increased within 4 h. VLDL from both control and apoAIItg mice contained significant amounts of apoAII, with approximately 4 times more apoAII on apoAIItg VLDL. ApoAII was shown to transfer spontaneously from high density lipoprotein (HDL) to VLDL in vitro, resulting in VLDL that was a poorer substrate for hydrolysis by lipoprotein lipase. These results indicate that one function of apoAII is to regulate the metabolism of triglyceride-rich lipoproteins, with HDL serving as a plasma reservoir of apoAII that is transferred to the triglyceride-rich lipoproteins in much the same way as VLDL and chylomicrons acquire most of their apoCs from HDL.

Progesterone impairs cell respiration and suppresses a compensatory increase in glucose transport in isolated rat skeletal muscle: a non-genomic mechanism contributing to metabolic adaptation to late pregnancy?

AIMS/HYPOTHESIS

The aim of the study was to gain better insight into the mechanisms responsible for impaired glucose metabolism during late pregnancy. We explored the direct effects of progesterone on glucose metabolism of skeletal muscle.

METHODS

Specimens of skeletal muscle from untreated rats were incubated with progesterone and rates of substrate fluxes through the various pathways of glucose metabolism were analysed.

RESULTS

Progesterone dose-dependently reduced the rates of glucose and pyruvate oxidation (insulin-stimulated rates after 5 h of exposure to 1 and 10 mumol/l progesterone: glucose oxidation, -6 +/- 4%, NS, and -39 +/- 4%, p < 0.001; pyruvate oxidation, -28 +/- 2% and -55 +/- 4%, p < 0.001 each) and increased lactate release (+28 +/- 4% and +58 +/- 9%, p < 0.005 each), which indicated inhibition of mitochondrial respiratory function. Impairment of cell respiration, e.g. by the specific inhibitor rotenone, is known to trigger a compensatory increase in glucose transport, but this response was blunted in the case of progesterone (change of glucose transport in response to 10 mumol/l progesterone vs 60 nmol/l rotenone, both causing a reduction in glucose oxidation by -39%: progesterone, +14 +/- 8% vs rotenone, +84 +/- 23%, p < 0.03). Further experiments dealt with the underlying mechanisms and revealed a rapid mode of action (50 mumol/l progesterone, reduction in insulin-stimulated glucose oxidation after 30 min: -29 +/- 7%, p < 0.01) not affected by blockers of gene expression or the nuclear progesterone receptor.

CONCLUSIONS/INTERPRETATION

Progesterone inhibits cell respiration and at the same time suppresses a compensatory increase in glucose transport, causing cellular carbohydrate deficiency in isolated rat skeletal muscle. This effect is mediated by a direct, rapid and non-genomic mechanism and could contribute to pregnancy-associated changes in glucose homeostasis.

Insulin does not regulate glucose transport and metabolism in human endothelium

BACKGROUND

Although endothelial cells express insulin receptors, it is controversially discussed whether the endothelium represents an insulin-responsive tissue. Since available data are primarily restricted to animal endothelial cells, this study tested (i) whether insulin affects glucose metabolism in human endothelium; (ii) whether insulin sensitivity is different in micro- versus macrovascular endothelial cells; and (iii) whether glucose concentration in the incubation medium affects the cells' response to insulin.

MATERIALS AND METHODS

Human umbilical vein endothelial cells (HUVECs), human adult saphenous vein endothelial cells (HAVECs), human aortic endothelial cells (HAEC), and human retinal endothelial cells (HRECs) as well as human smooth muscle cells were incubated with/without insulin (0.3 nmol L(-1) or 1 micromol L(-1)). Glucose transport, glycogen synthesis, glycogen content, lactate release, and expression of phospho-Akt, Akt, and endothelial nitric oxide synthase (eNOS) were determined.

RESULTS

In HUVECs and HRECs, insulin (1 micromol L(-1)) increased (P < 0.05) eNOS expression by ~70% and doubled Akt phosphorylation, but the latter was by far more pronounced in human smooth muscle cells (+1093 +/- 500%, P < 0.05). In human smooth muscle cells, insulin (1 micromol L(-1)) stimulated glycogen synthesis by 67 +/- 11% (P < 0.01). In human micro- (HRECs) and macrovascular endothelial cells (HUVECs, HAVECs and HAECs), insulin, however, failed to stimulate glucose transport, glycogen synthesis, glycogen content, or lactate release under various conditions, i.e. after glucose deprivation or in medium with normal (5.5 mmol L(-1)) or high glucose (30 mmol L(-1)).

CONCLUSIONS

Insulin stimulated glycogen synthesis and Akt phosphorylation in human smooth muscle cells. In human micro- and macrovascular endothelial cells, insulin, however, failed to affect glucose uptake and metabolism under all experimental conditions applied, whereas it increased Akt phosphorylation and eNOS expression.

Direct rosiglitazone-induced modifications in insulin secretion, action and clearance: a single-dose hyperglycaemic clamp study

AIMS/HYPOTHESIS

In addition to the improvement in insulin sensitivity, it has been shown that thiazolidinediones modulate beta cell function and insulin clearance in type 2 diabetic subjects. However, interactions between all these actions, and confounding factors due to co-morbidities and co-treatments in diabetic individuals, complicate the identification of specific effects. The aim of this pilot study was to investigate the potential acute effects of rosiglitazone on beta cell function and insulin sensitivity by the hyperglycaemic clamp technique in healthy volunteers.

SUBJECTS AND METHODS

Twelve healthy men were included in a randomised, double-blind crossover study. Rosiglitazone (8 mg) or placebo was given orally 45 min before the hyperglycaemic clamp (10 mmol/l for 2 h).

RESULTS

The second phase of the insulin response was significantly decreased by rosiglitazone: 13,066 +/- 1,531 vs 16,316 +/- 2,813 pmol l(-1) 110 min in controls (p < 0.05), without change in the first phase. Serum C-peptide was not modified. Rosiglitazone treatment significantly increased insulin clearance (molar ratio of the C-peptide to insulin AUCs: 12.80 +/- 1.34 vs 11.38 +/- .33, p < 0.05) and the insulin sensitivity index (12.0 +/- 1.5 vs 8.5 +/- 1.1 micromol m(-2) min(-1) pmol(-1)l, p < 0.01).

CONCLUSIONS/INTERPRETATION

The present results show that a single dose of rosiglitazone rapidly increases insulin clearance and insulin sensitivity index in healthy volunteers, with no direct effect on insulin secretion. The precise mechanisms mediating these actions remain to be determined.

Pro-MMP-2 activation by the PPARgamma agonist, ciglitazone, induces cell invasion through the generation of ROS and the activation of ERK

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a nuclear receptor modulating a variety of biological functions including cancer cell proliferation and differentiation. However, the role of PPARgamma and its ligands in tumor invasion is unclear. To evaluate a possible role for PPARgamma ligands in tumor invasion, we examined whether PPARgamma agonists including pioglitazone, troglitazone, rosiglitazone, and ciglitazone could affect the activity of matrix metalloproteinases (MMPs) in the HT1080 cell line, a well-studied and well-characterized cell line for MMP research. The gelatin zymography assay showed that ciglitazone activated pro-MMP-2 significantly. In addition, ciglitazone increased the expression of MMP-2, which was accompanied by an increase of membrane type 1-MMP (MT1-MMP) expression. The PPARgamma antagonist, GW9662 attenuated the ciglitazone-induced PPARgamma activation but it did not affect the pro-MMP2 activation by ciglitazone, suggesting that the action of ciglitazone on the pro-MMP-2 activation bypassed the PPARgamma pathway. Antioxidants and various inhibitors of signal transduction were used to investigate the mechanism of ciglitazone-induced pro-MMP-2 activation. We found that the sustained production of reactive oxygen species (ROS) was required for pro-MMP-2 activation by ciglitazone. We also found that PB98059, an inhibitor of MEK-ERK, significantly blocked ciglitazone-induced pro-MMP-2 activation and that extracellular signal-regulated kinase (ERK) was hyperphosphorylated by ciglitazone. Moreover, cell invasion was significantly increased by ciglitazone in the HT1080 cell lines, whereas cell motility was not affected. This study suggests that ciglitazone-induced pro-MMP-2 activation increases PPARgamma-independent tumor cell invasion through ROS production and ERK activation in some types of cancer cells.

Effect of rosiglitazone on visfatin and retinol-binding protein-4 plasma concentrations in HIV-positive patients

Thiazolidinediones (TZD) may improve insulin resistance in patients with diabetes and HIV. The novel adipocytokines visfatin and retinol-binding protein-4 (RBP-4) have been proposed to influence the development of impaired glucose tolerance. The impact of TZD on these cytokines is yet unknown. In this randomized, double-blind, placebo-controlled parallel group study, 37 lean HIV-positive subjects aged 19-50 years were treated with 8 mg/day rosiglitazone (n=20) or placebo (n=17) for 6 months. Insulin sensitivity was estimated from the homeostasis model assessment (HOMA) index. Fasting visfatin, RBP-4, leptin, and adiponectin plasma concentrations were analyzed by immunoassays. Rosiglitazone had no effect on impaired insulin sensitivity, but increased median plasma visfatin from 6.2 ng/ml (95% CI: 5.9; 6.5) to 13.7 ng/ml (12.6; 19.1) (P<0.001) and adiponectin from 3.2 ng/ml (2.2; 4.0) to 4.0 ng/ml (3.3; 8.5; P<0.001). RBP-4 was lowered from 21.0 ng/ml (19.6; 23.1) to 16.3 ng/ml (15.2; 17.0; P<0.001), and leptin concentrations were unchanged. Adipocytokine concentrations were stable in subjects receiving placebo, where a deterioration in insulin sensitivity was detectable (P<0.05). Changes in visfatin and RBP-4 were correlated in subjects receiving rosiglitazone (r=-0.64, P<0.01) but not placebo (r=0.12, P=0.15). TZD treatment affects circulating adipocytokine concentrations in subjects with HIV. Reductions in RBP-4 and increases in visfatin may contribute to the pharmacodynamic action of TZD on glucose homeostasis. Quantification of adipocytokines might be useful to assess TZD treatment effectiveness in insulin-resistant subjects with HIV.

Pioglitazone acutely influences glucose-sensitive insulin secretion in normal and diabetic human islets

We have studied acute effects of the PPARgamma agonist pioglitazone in vitro on human islets from both non-diabetic and type 2 diabetic subjects. In 5 mM glucose, pioglitazone caused a transient increase in insulin secretion in non-diabetic, but not diabetic, islets. Continuous presence of the drug suppressed insulin release in both non-diabetic and diabetic islets. In islets from non-diabetic subjects, both high glucose and tolbutamide-stimulated insulin secretion was inhibited by pioglitazone. When islets were continuously perifused with 5 mM glucose, short-term pretreatment with pioglitazone caused approximately 2-fold increase in insulin secretion after drug withdrawal. Pioglitazone pretreatment of diabetic islets restored their glucose sensitivity. Examination of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) in non-diabetic islets revealed slight Ca(2+) transient by pioglitazone at 3 mM glucose with no significant changes at high glucose. Our data suggest that short-term pretreatment with pioglitazone primes both healthy and diabetic human islets for enhanced glucose-sensitive insulin secretion.

Activation of PPAR-delta in isolated rat skeletal muscle switches fuel preference from glucose to fatty acids

AIMS/HYPOTHESIS

GW501516, an agonist of peroxisome proliferator-activated receptor-delta (PPAR-delta), increases lipid combustion and exerts antidiabetic action in animals, effects which are attributed mainly to direct effects on skeletal muscle. We explored such actions further in isolated rat skeletal muscle.

MATERIALS AND METHODS

Specimens of rat skeletal muscle were pretreated with GW501516 (0.01-30 mumol/l) for 0.5, 4 or 24 h and rates of fuel metabolism were then measured. In addition, effects on mitochondrial function were determined in isolated rat liver mitochondria.

RESULTS

At concentrations between 0.01 and 1 mumol/l, GW501516 dose-dependently increased fatty acid oxidation but reduced glucose utilisation in isolated muscle. Thus after 24 h of preincubation with 1 mumol/l GW501516, palmitate oxidation increased by +46+/-10%, and the following decreased as specified: glucose oxidation -46+/-8%, glycogen synthesis -42+/-6%, lactate release -20+/-2%, glucose transport -15+/-6% (all p<0.05). Reduction of glucose utilisation persisted independently of insulin stimulation or muscle fibre type, but depended on fatty acid availability (the effect on glucose transport in the absence of fatty acids was an increase of 30+/-9%, p<0.01), suggesting a role for the glucose-fatty acid cycle. At higher concentrations, GW501516 uncoupled oxidative phosphorylation by direct action on isolated mitochondria.

CONCLUSIONS/INTERPRETATION

GW501516-induced activation of PPAR-delta reduces glucose utilisation by skeletal muscle through a switch in mitochondrial substrate preference from carbohydrate to lipid. High concentrations of GW501516 induce mitochondrial uncoupling independently of PPAR-delta.

Free fatty acids normalize a rosiglitazone-induced visfatin release

The detrimental effect of elevated free fatty acids (FFAs) on insulin sensitivity can be improved by thiazolidinediones (TZDs) in patients with type 2 diabetes mellitus. It is unknown whether this salutary action of TZD is associated with altered release of the insulin-mimetic adipocytokine visfatin. In this study, we investigated whether visfatin concentrations are altered by FFA and TZD treatment. In a randomized, double-blind, placebo-controlled, parallel-group study 16 healthy volunteers received an infusion of triglycerides/heparin to increase plasma FFA after 3 wk of treatment with rosiglitazone (8 mg/day, n = 8) or placebo (n = 8), and circulating plasma visfatin was measured. As a corollary, human adipocytes were incubated with synthetic fatty acids and rosiglitazone to assess visfatin release in vitro. The results were that rosiglitazone treatment increased systemic plasma visfatin concentrations from 0.6 +/- 0.1 to 1.7 +/- 0.2 ng/ml (P < 0.01). Lipid infusion caused a marked elevation of plasma FFA but had no effect on circulating visfatin in controls. In contrast, elevated visfatin concentrations in subjects receiving rosiglitazone were normalized by lipid infusion. In isolated adipocytes, visfatin was released into supernatant medium by acute addition and long-term treatment of rosiglitazone. This secretion was blocked by synthetic fatty acids and by inhibition of phosphatidylinositol 3-kinase or Akt. In conclusion, release of the insulin-mimetic visfatin may represent a major mechanism of metabolic TZD action. The presence of FFA antagonizes this action, which may have implications for visfatin bioactivity.

Dehydroepiandrosterone inhibits complex I of the mitochondrial respiratory chain and is neurotoxic in vitro and in vivo at high concentrations

Dehydroepiandrosterone (DHEA) is widely used as a food supplement and considered to be relatively safe. In animal studies, however, additions of high concentrations of DHEA to the diet have led to hepatotoxicity as well as liver mitochondrial dysfunction. This study was therefore designed to find out whether DHEA is able to inhibit the respiratory activity also in neuronal mitochondria and to reveal whether this leads to functional disturbance in the brain. Using different mitochondrial substrates, we show here that DHEA suppresses the mitochondrial respiration in permeabilized neurons (half maximal inhibitory concentration 13 microM) by inhibiting complex I of the mitochondrial electron transport chain. Treatment with DHEA was associated with increased glucose expenditure in intact cultures and led to neuronal death. The latter was most prominent in hypoglycemic conditions. Mice fed with pellet containing 0.6% DHEA for 3 months showed a significant neuronal loss in the cerebral cortex and hippocampus, a slightly decreased dopamine/dihydroxyphenylacetic acid ratio, as well as motor impairment. The main conclusion of the present study is that high concentrations of DHEA inhibit complex I of the mitochondrial respiratory chain and are neurotoxic in vitro and in vivo.

Insulin sensitizers may prevent metabolic inflammation

The relative decreased response of peripheral tissues to insulin (insulin resistance) is a key metabolic disturbance that predisposes a large percentage of individuals to the development of type 2 diabetes and to cardiovascular disease. As detailed in an extensive literature over the last two decades, insulin resistance co-exists in varying degrees with a variety of other key risk factors, including dyslipidemia, hypertension, and vascular inflammation, that contribute to poor cardiovascular outcomes of individuals with type 2 diabetes and metabolic syndrome. Whereas insulin resistance is generally thought of as pathology unto itself, this commentary suggests that insulin resistance is a physiological compensation to inappropriate oxidative metabolism that induces a metabolic inflammatory response. Via signaling of this inflammatory response, the protective compensation to excessive oxidative metabolism dampens metabolism by reducing insulin action, fatty acid oxidation, and eventually mitochondrial function and numbers. Such a scenario could explain the coexistence of these phenomena with obesity and reduced mitochondrial function. Recent evidence suggests that thiazolidinediones exert pharmacology through modifications of mitochondrial metabolism, preventing the metabolic inflammation and allowing the up regulation of mitochondrial biogenesis. A further understanding of these mechanisms, which are likely to involve key redox signaling events emanating from mitochondrial biochemistry, is needed to fuel new therapeutic advances for the treatment of metabolic syndrome.

Thiazolidinediones can rapidly activate AMP-activated protein kinase in mammalian tissues

Thiazolidinediones (TZDs) are insulin-sensitizing agents used in the treatment of type 2 diabetes. A widely held view is that their action is secondary to transcriptional events that occur when TZDs bind to the nuclear receptor PPARgamma in the adipocyte and stimulate adipogenesis. It has been proposed that this increases insulin sensitivity, at least in part, by increasing the expression and release of adiponectin, an adipokine that activates the fuel-sensing enzyme AMP-activated protein kinase (AMPK). In this study, we report that TZDs also acutely activate AMPK in skeletal muscle and other tissues by a mechanism that is likely independent of PPARgamma-regulated gene transcription. Thus incubation of isolated rat EDL muscles in medium containing 5 microM troglitazone for 15 min (too brief to be attributable to transcription) significantly increased pAMPK and pACC. At a concentration of 100 microM, troglitazone maximally increased these parameters and caused twofold increases in 2-deoxy-d-glucose uptake and the oxidation of exogenous [(14)C]palmitate. Time course studies revealed that troglitazone-induced increases in pAMPK and pACC abundance at 15 min were paralleled by an increase in the AMP-to-ATP ratio and that by 60 min all of these parameters had returned to baseline values. Increases in pAMPK and pACC were also observed in skeletal muscle, liver, and adipose tissue in intact rats 15 min after the administration of a single dose of troglitazone (10 mg/kg, ip). Likewise, troglitazone and another TZD, pioglitazone, caused rapid increases in pAMPK and pACC of equal magnitude in Swiss 3T3 fibroblasts with and without sufficient PPARgamma to mediate the expression of target genes. The results indicate that TZDs can act within minutes to activate AMPK in mammalian tissues. They suggest that this effect is associated with a change in cellular energy state and that it is not dependent on PPARgamma-mediated gene transcription.

Roles of skeletal muscle and peroxisome proliferator-activated receptors in the development and treatment of obesity

Metabolic disturbances associated with alterations in lipid metabolism, such as obesity, type 2 diabetes, and syndrome X, are becoming more and more prominent in Western societies. Despite extensive research in such pathologies and their molecular basis, we are still far from completely understanding how these metabolic perturbations are produced and interrelate and, consequently, how to treat them efficiently. The discovery that adipose tissue is, in fact, an endocrine tissue able to secrete active molecules related to lipid homeostasis--the adipokines--has dramatically changed our understanding of the molecular events that take place in such diseases. This knowledge has been further improved by the discovery of peroxisome proliferator-activated receptors and their ligands, at present commonly used for the clinical treatment of lipid disturbances. However, a key point remains to be solved, and that is the role of muscle lipid metabolism, notably because of the main role played by this tissue in the development of such pathologies. In addition, a reciprocal regulation between adipose tissue and skeletal muscle has been proposed. New discoveries on the role of peroxisome proliferator-activated receptor-delta in skeletal muscle functions as well as the secretory capabilities of muscle, now considered as an endocrine tissue, have changed the general point of view on lipid homeostasis, opening new and promising doors for the treatment of lipid disorders.

Effects of rosiglitazone and metformin on pancreatic beta cell gene expression

AIMS/HYPOTHESIS

Rosiglitazone and metformin are two oral antihyperglycaemic drugs used to treat type 2 diabetes. While both drugs have been shown to improve insulin-sensitive glucose uptake, the direct effects of these drugs on pancreatic beta cells is only now beginning to be clarified. The aim of the present study was to determine the direct effects of these agents on beta cell gene expression.

METHODS

We used reporter gene analysis to examine the effects of rosiglitazone and metformin on the activity of the proinsulin and insulin promoter factor 1 (IPF1) gene promoters in the glucose-responsive mouse beta cell line Min6. Western blot and gel retardation analyses were used to examine the effects of both drugs on the regulation of IPF1 protein production, nuclear accumulation and DNA binding activity in both Min6 cells and isolated rat islets of Langerhans.

RESULTS

Over 24 h, rosiglitazone promoted the nuclear accumulation of IPF1 and forkhead homeobox A2 (FOXA2), independently of glucose concentration, and stimulated a two-fold increase in the activity of the Ipf1 gene promoter (p<0.01). Stimulation of the Ipf1 promoter by rosiglitazone was unaffected by the presence of the peroxisome proliferator activated receptor gamma antagonist GW9662. No effect of either rosiglitazone or metformin was observed on proinsulin promoter activity. Metformin stimulated IPF1 nuclear accumulation and DNA binding activity in a time-dependent manner, with maximal effects observed after 2 h.

CONCLUSIONS/INTERPRETATION

Metformin and rosiglitazone have direct effects on beta cell gene expression, suggesting that these agents may play a previously unrecognised role in the direct regulation of pancreatic beta cell function.

What has prevented the expansion of insulin sensitisers?

The ability to improve insulin sensitivity with synthetic compounds was uncovered by empirical discoveries by Takeda in the late 1970s. The potential of a class of thiazolidinediones for the treatment of Type 2 diabetes, by decreasing glucose and triglycerides alongside lowering circulating insulin, was made public during the 1980s. As the first of the chemicals (pioglitazone, troglitazone and rosliglitazone) proceeded to clinical trials, these observations were soon extended to demonstrate a rich and complex pharmacology. The promise of this mode of action included prevention of diabetes as well as making a significant impact on the incidence and severity of the life-shortening consequences of the established disease. There are now two of these drugs on the market: pioglitazone and rosiglitazone, and they are being used to treat significant numbers of diabetic patients. However, the use of these drugs and development of future generations of successful candidates has not met the expectations that were held out in the early 1980s. This can be attributed to two major prevailing conditions. Troglitazone became the first thiazolidinedione to be approved as a result of delays in the development of pioglitazone. Unfortunately, troglitazone produced a unique idiosyncratic and sometimes fatal, hepatoxicity that necessitated its removal from the marketplace; second, there has been an incomplete understanding of the biochemical mechanism of action of these drugs that has slowed (and perhaps derailed) attempts to produce second-generation compounds. The latter issue is the subject of this editorial, which suggests that it is time to take a fresh look at the pharmacology of insulin sensitisers.

Chronic rosiglitazone treatment restores AMPKalpha2 activity in insulin-resistant rat skeletal muscle

Rosiglitazone (RSG) is an insulin-sensitizing thiazolidinedione (TZD) that exerts peroxisome proliferator-activated receptor-gamma (PPARgamma)-dependent and -independent effects. We tested the hypothesis that part of the insulin-sensitizing effect of RSG is mediated through the action of AMP-activated protein kinase (AMPK). First, we determined the effect of acute (30-60 min) incubation of L6 myotubes with RSG on AMPK regulation and palmitate oxidation. Compared with control (DMSO), 200 microM RSG increased (P < 0.05) AMPKalpha1 activity and phosphorylation of AMPK (Thr172). In addition, acetyl-CoA carboxylase (Ser218) phosphorylation and palmitate oxidation were increased (P < 0.05) in these cells. To investigate the effects of chronic RSG treatment on AMPK regulation in skeletal muscle in vivo, obese Zucker rats were randomly allocated into two experimental groups: control and RSG. Lean Zucker rats were treated with vehicle and acted as a control group for obese Zucker rats. Rats were dosed daily for 6 wk with either vehicle (0.5% carboxymethylcellulose, 100 microl/100 g body mass), or 3 mg/kg RSG. AMPKalpha1 activity was similar in muscle from lean and obese animals and was unaffected by RSG treatment. AMPKalpha2 activity was approximately 25% lower in obese vs. lean animals (P < 0.05) but was normalized to control values after RSG treatment. ACC phosphorylation was decreased with obesity (P < 0.05) but restored to the level of lean controls with RSG treatment. Our data demonstrate that RSG restores AMPK signaling in skeletal muscle of insulin-resistant obese Zucker rats.

Rosiglitazone ameliorates abnormal expression and activity of protein tyrosine phosphatase 1B in the skeletal muscle of fat-fed, streptozotocin-treated diabetic rats

Protein tyrosine phosphatase 1B (PTP1B) acts as a physiological negative regulator of insulin signaling by dephosphorylating the activated insulin receptor (IR). Here we examine the role of PTP1B in the insulin-sensitizing action of rosiglitazone (RSG) in skeletal muscle and liver. Fat-fed, streptozotocin-treated rats (10-week-old), an animal model of type II diabetes, and age-matched, nondiabetic controls were treated with RSG (10 micromol kg(-1) day(-1)) for 2 weeks. After RSG treatment, the diabetic rats showed a significant decrease in blood glucose and improved insulin sensitivity. Diabetic rats showed significantly increased levels and activities of PTP1B in the skeletal muscle (1.6- and 2-fold, respectively) and liver (1.7- and 1.8-fold, respectively), thus diminishing insulin signaling in the target tissues. We found that the decreases in insulin-stimulated glucose uptake (55%), tyrosine phosphorylation of IRbeta-subunits (48%), and IR substrate-1 (IRS-1) (39%) in muscles of diabetic rats were normalized after RSG treatment. These effects were associated with 34 and 30% decreases in increased PTP1B levels and activities, respectively, in skeletal muscles of diabetic rats. In contrast, RSG did not affect the increased PTP1B levels and activities or the already reduced insulin-stimulated glycogen synthesis and tyrosine phosphorylation of IRbeta-subunits and IRS-2 in livers of diabetic rats. RSG treatment in normal rats did not significantly change PTP1B activities and levels or protein levels of IRbeta, IRS-1, and -2 in diabetic rats. These data suggest that RSG enhances insulin activity in skeletal muscle of diabetic rats possibly by ameliorating abnormal levels and activities of PTP1B.

Receptor-independent actions of PPAR thiazolidinedione agonists: is mitochondrial function the key?

Agonists of the peroxisome proliferator activated receptor gamma (PPAR(gamma)) are currently used for treatment of type 2 diabetes due to their insulin sensitizing and glucose metabolism stabilizing effects. More recently some of these same agonists were shown to exert anti-inflammatory and anti-proliferative effects as well. Although PPAR(gamma) agonists can operate via receptor-mediated events occurring at the genomic level, thereby causing long lasting changes in gene expression patterns, recent studies demonstrate non-genomic as well as genomic actions, and receptor-dependent as well as receptor-independent effects of the thiazolidinedione (TZD) class of PPAR(gamma) agonists. In this review we will summarize data describing some of these novel, receptor independent actions of TZDs, review evidence that TZDs directly influence mitochondrial function, and attempt to reconcile how changes in mitochondrial function could contribute to other receptor-independent actions of these drugs.

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.

Therapeutic Actions of an Insulin Receptor Activator and a Novel Peroxisome Proliferator-Activated Receptor γ Agonist in the Spontaneously Hypertensive Obese Rat Model of Metabolic Syndrome X

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

Thiazolidinediones inhibit proliferation of microvascular and macrovascular cells by a PPARgamma-independent mechanism

AIMS/HYPOTHESIS

This study evaluated the hypothesis that peroxisome proliferator-activated receptor-gamma (PPARgamma) agonists, including thiazolidinediones (TZDs) and the rexinoid LG100268 (LG), directly affect human vascular cell function (proliferation, cell cycle, protein expression, lactate release) independently of (1) their PPARgamma-activating potential and (2) the cells' vascular origin.

METHODS

Human umbilical vein endothelial cells (HUVECs), human adult vein endothelial cells (HAVECs), human retinal endothelial cells (HRECs) and human retinal pericytes (HRPYCs) were incubated (48 h) with 2-50 micromol/l rosiglitazone (RSG), RWJ241947 (RWJ), pioglitazone (PIO), troglitazone (TRO), 15-deoxy-Delta(12,14)-prostaglandin J2 (PGJ2) and LG. Proliferation, cell cycle distribution, protein expression, peroxisome proliferator-activated receptor responsive element (PPRE) transcriptional activity and mitochondrial effects were determined by [3H]thymidine incorporation, FACS analyses, western blots, reporter assays and lactate release respectively.

RESULTS

In HUVECs, RSG, RWJ, PIO, TRO, PGJ2 and LG reduced (p<0.01) proliferation (due to a G0/G1 cell cycle arrest) by up to 23%, 36%, 38%, 86%, 99% and 93% respectively. The antiproliferative response was similar in HRPYCs and HAVECs, but was attenuated in HRECs. Whereas p21WAF-1/Cip1 and p27Kip were differently affected in HUVECs, all agents reduced (p<0.05) expression of cyclins (D3, A, E, B), cyclin-dependent kinase-2 and hyperphosphorylated retinoblastoma protein. The rank order of the antiproliferative effects of TZDs in HUVECs (RSG approximately PIO approximately RWJ<TRO) contrasted their PPRE transcriptional activities (TRO<PIO<RSG<RWJ), but correlated with cellular lactate release. Proliferation inhibition and lactate release were mimicked by rotenone (mitochondrial complex I inhibitor).

CONCLUSIONS/INTERPRETATION

In conclusion, this study suggests that the antiproliferative action of the TZDs in vascular cells is independent of their PPARgamma-activating and associated insulin-sensitising potential, but could relate to mitochondrial mechanisms.

Rosiglitazone upregulates caveolin-1 expression in THP-1 cells through a PPAR-dependent mechanism

Peroxisome proliferator-activated receptor gamma (PPARgamma) activation or overexpression induces caveolin-1 (cav-1) expression in several cell types. The objective of this study was to investigate if PPAR agonists could also regulate the cav-1 gene in macrophages and to explore the mechanisms involved. Our experiments demonstrated that rosiglitazone dose- and time-dependently increased cav-1 mRNA and protein in THP-1 macrophages. This induction was not observed in the presence of inhibitors of transcription or de novo protein synthesis. We also showed that the increase in cav-1 elicited by rosiglitazone was not related either to macrophage differentiation or to cellular apoptosis. The inductive effect seems to be dependent on PPAR activation, as the PPAR antagonist GW9662 abolished it. The activation of the liver X receptor with 22(R)-hydroxycholesterol also increased cav-1 mRNA, whereas the inactive (S) isomer did not. Finally, we identified a functional peroxisome proliferator response element in the cav-1 promoter that was activated upon rosiglitazone treatment in THP-1 macrophages.

Expression of uncoupling protein-3 mRNA in rat skeletal muscle is acutely stimulated by thiazolidinediones: an exercise-like effect?

AIMS/HYPOTHESIS

We examined whether thiazolidinediones (TZDs) acutely affect uncoupling protein-3 ( UCP-3) expression in skeletal muscle and plasma NEFA in Sprague-Dawley rats.

METHODS

Expression of UCP-3 mRNA in hindlimb muscles and plasma NEFA were measured after a single intraperitoneal injection of TZDs in healthy male rats.

RESULTS

Independent of which TZD was injected (50 micromol/kg), UCP-3 expression in gastrocnemius muscle was distinctly increased after 6 h (increase vs vehicle-injected control: pioglitazone, 10.3+/-3.2-fold, p=0.03; rosiglitazone, 8.7+/-1.2-fold, p=0.001; RWJ241947, 9.5+/-2.7-fold, p=0.03). This was accompanied by elevated plasma NEFA (control 158+/-13 micromol/l; pioglitazone, 281+/-40 micromol/l, p=0.03; rosiglitazone, 276+/-27 micromol/l, p=0.005; RWJ241947, 398+/-51 micromol/l, p=0.004). The increase in plasma NEFA could in part have mediated TZD-induced UCP-3 expression, but increased UCP-3 mRNA was also found in isolated muscle after 2 h of TZD exposure in vitro (25 micromol/l pioglitazone, 1.7+/-0.3-fold, p=0.046), suggesting that TZDs act directly and independently of NEFA on skeletal muscle.

CONCLUSIONS/INTERPRETATION

In healthy rats, a single dose of TZDs rapidly increases UCP-3 mRNA in skeletal muscle and plasma NEFA. This effect resembles the acute response to a bout of exercise.

Fenofibrate, Troglitazone, and 15-Deoxy-Δ12,14-prostaglandin J2Close KATPChannels and Induce Insulin Secretion

It is known that peroxisome proliferator-activated receptor-gamma (PPAR-gamma) ligands stimulate acute-phase insulin secretion with a rapid Ca2+ influx into pancreatic beta-cells, but the precise mechanisms are not clear. The effects of PPAR-alpha ligands on pancreatic beta-cells also remain unclear. We investigated the effects of PPAR-alpha ligands (fenofibrate and fenofibric acid), a PPAR-gamma ligand (troglitazone), and an endogenous ligand of PPAR-gamma [15-deoxy-Delta12,14-prostaglandin J2 (15-deoxy-Delta12,14-PGJ2)] on KATP channel activity in clonal hamster insulinoma cell line, HIT-T15 cells. As assessed by whole-cell patch clamp, fenofibrate, fenofibric acid, troglitazone, and 15-deoxy-Delta12,14-PGJ2 reduced the KATP channel currents, and inhibition continued after washout of these agents. The concentration-response curves of fenofibrate, fenofibric acid, troglitazone, and 15-deoxy-Delta12,14-PGJ2 showed half-maximal inhibition of KATP channel currents (IC50) at 3.26, 94, 2.1, and 7.3 micromol/l, respectively. Fenofibrate (> or = 10(-6) mol/l), 15-deoxy-Delta12,14-PGJ2 (> or = 5 x 10(-5) mol/l), and troglitazone (> or = 10(-6) mol/l) inhibited [3H]glibenclamide binding, but fenofibric acid did not. In addition, fenofibrate (> or = 10(-6) mol/l), fenofibric acid (10(-4) mol/l), troglitazone (10(-4) mol/l), and 15-deoxy-Delta12,14-PGJ2 (> or = 10(-5) mol/l) increased insulin secretion from HIT-T15 when applied for 10 min. Our data suggest that PPAR-alpha and -gamma ligands interact directly with the beta-cell membrane and stimulate insulin secretion.

Fenofibrate Impairs Rat Mitochondrial Function by Inhibition of Respiratory Complex I

Fibrates are used for the treatment of dyslipidemia and known to affect mitochondrial function in vitro. To better understand the mechanisms underlying their mitochondrial effects, fibrate actions on complex I of the respiratory chain and cell respiration were studied in vitro. In homogenates of rat skeletal muscle, fenofibrate, and to a lesser extent clofibrate, reduced the activity of complex I (10, 30, and 100 microM fenofibrate: -41 +/- 7%, -70 +/- 2%, and -78 +/- 4%; 100 microM clofibrate: -27 +/- 7%; p < 0.005 each). Inhibition of complex I by fenofibrate (100 microM) was confirmed by reduced state 3 respiration of isolated mitochondria consuming glutamate + malate as substrates for complex I (-33 +/- 4%; p < 0.0005), but not of such consuming succinate as substrate for complex II (-8 +/- 4%; NS). In isolated rat muscle, 24-h fenofibrate exposure (25, 50, and 100 microM) decreased CO(2) production from palmitate (-15 +/- 7%, -23 +/- 8%, and -22 +/- 7%; p < 0.05 each) and increased lactate release (+15 +/- 5%, +14 +/- 5%, and + 17 +/- 6%; p < 0.02 each) indicating impaired cell respiration. Ciprofibrate and gemfibrocil (but not bezafibrate) impaired cell respiration without any inhibition of complex I. Our findings support the notion that individual fibrates induce mitochondrial dysfunction via different molecular mechanisms and show that fenofibrate predominantly acts by inhibition of complex I of the respiratory chain.