Regulation of glucose transport by pioglitazone in cultured muscle cells

A Fluorescence-Based Assay for Measuring Glucose Uptake in Living Melanoma Cells

Melanoma cells have high glycolytic capacity. Glucose uptake is a key rate-limiting step in glucose utilization. Here we describe a simple protocol for measuring direct glucose uptake in living melanoma cells by flow cytometry.

Telmisartan induces melanoma cell apoptosis and synergizes with vemurafenib in vitro by altering cell bioenergetics

Objective

Despite recent advancements in targeted therapy and immunotherapies, prognosis for metastatic melanoma patients remains extremely poor. Development of resistance to previously effective treatments presents a serious challenge and new approaches for melanoma treatment are urgently needed. The objective of this study was to examine the effects of telmisartan, an AGTR1 inhibitor and a partial agonist of PPARγ, on melanoma cells as a potential agent for repurposing in melanoma treatment.

Methods

Expression of AGTR1 and PPARγ mRNA in melanoma patient tumor samples was examined in publicly available datasets and confirmed in melanoma cell lines by qRT-PCR. A panel of melanoma cell lines was tested in viability, apoptosis and metabolic assays in presence of telmisartan by flow cytometry and immunocytochemistry. A cytotoxic effect of combinations of telmisartan and targeted therapy vemurafenib was examined using the Chou-Talalay combination index method.

Results

Both AGTR1 and PPARγ mRNA were expressed in melanoma patient tumor samples and decreased compared to the expression in the healthy skin. In vitro, we found that telmisartan decreased melanoma cell viability by inducing cell apoptosis. Increased glucose uptake, but not utilization, in the presence of telmisartan caused the fission of mitochondria and release of reactive oxygen species. Telmisartan altered the cell bioenergetics, thereby synergizing with vemurafenib in vitro, and even sensitized vemurafenib-resistant cells to the treatment.

Conclusions

Given that the effective doses of telmisartan examined in our study can be administered to patients and that telmisartan is a widely used and safe antihypertensive drug, our findings provide the scientific rationale for testing its efficacy in treatment of melanoma progression.

Thiazolidinedione-induced effects beyond glycaemic control

The thiazolidinediones exert their insulin sensitising effect by binding to the nuclear receptors (transcription factors) peroxisome proliferator activated receptor (PPAR) γ and, to varying degrees, to PPARα. Several different genes are activated by thiazolidinediones, many of which contribute to the increase in insulin sensitivity (eg. an increase in glucose uptake and utilisation, a decrease in gluconeogenesis and in insulin-antagonistic cytokines, such as tumour necrosis factor α). Activation of other genes indirectly reduces insulin resistance by, for example, increasing free fatty acid (FFA) uptake and oxidation resulting in lower circulating FFA levels. The action of thiazolidinediones at PPARγ is generally responsible for their insulin sensitising effects while action at PPARα contributes to their lipid lowering effects. Therefore, the relative affinities of the different thiazolidinediones for PPARγ and PPARα will also lead to a different spectrum of actions for each agent.

Sildenafil does not enhance but rather attenuates vasorelaxant effects of antidiabetic agents

Type 2 diabetic men commonly experience erectile dysfunction for which phosphodiesterase-5 (PDE5) inhibitors like sildenafil (Viagra) are often recommended. By preventing degradation of cyclic guanosine monophosphate (cGMP) in vascular smooth muscle, these inhibitors also enhance arterial vasorelaxant effects of nitric oxide donors (which stimulate cGMP synthesis). In the present work, we confirmed this enhancing effect after co-administration of sildenafil with nitroprusside to freshly-isolated rat tail arterial tissues. However, in the same tissues we also observed that sildenafil does not enhance but rather attenuates vasorelaxant effects of three commonly-used antidiabetic drugs, i.e. the biguanide metformin and the thiazolidinediones pioglitazone and rosiglitazone. Indeed, sildenafil completely blocked vasorelaxant effects of low concentrations of these drugs. In addition, we found that this same novel anti-vasorelaxant interaction of sildenafil with these agents was abolished by either 1) omitting extracellular glucose or 2) inhibiting specific smooth muscle glycolytic pathways; pathways known to preferentially utilize extracellular glucose to fuel certain adenosine triphosphate (ATP)-dependent ion transporters: e.g. ATP-sensitive K channels, sarcoplasmic reticulum Ca-ATPase, plasma membrane Ca-ATPase and Na/K-ATPase. Accordingly, we suspect that altered activity of one or more of these ion transporters mediates the observed attenuating (anti-vasorelaxant) interaction of sildenafil with the antidiabetic drugs. The present results are relevant because hypertension is so common and difficult to control in Type 2 diabetes. The present data suggest that sildenafil might interfere with the known antihypertensive potential of metformin and the thiazolidinediones. However, they do not suggest that it will interact with them to cause life-threatening episodes of severe hypotension, as can occur when it is co-administered with nitrates.

Pioglitazone treatment increases survival and prevents body weight loss in tumor-bearing animals: possible anti-cachectic effect

Cachexia is a multifactorial syndrome characterized by profound involuntary weight loss, fat depletion, skeletal muscle wasting, and asthenia; all symptoms are not entirely attributable to inadequate nutritional intake. Adipose tissue and skeletal muscle loss during cancer cachexia development has been described systematically. The former was proposed to precede and be more rapid than the latter, which presents a means for the early detection of cachexia in cancer patients. Recently, pioglitazone (PGZ) was proposed to exhibit anti-cancer properties, including a reduction in insulin resistance and adipose tissue loss; nevertheless, few studies have evaluated its effect on survival. For greater insight into a potential anti-cachectic effect due to PGZ, 8-week-old male Wistar rats were subcutaneously inoculated with 1 mL (2×10⁷) of Walker 256 tumor cells. The animals were randomly assigned to two experimental groups: TC (tumor + saline-control) and TP5 (tumor + PGZ/5 mg). Body weight, food ingestion and tumor growth were measured at baseline and after removal of tumor on days 7, 14 and 26. Samples from different visceral adipose tissue (AT) depots were collected on days 7 and 14 and stored at -80o C (5 to 7 animals per day/group). The PGZ treatment showed an increase in the survival average of 27.3% (P< 0.01) when compared to TC. It was also associated with enhanced body mass preservation (40.7 and 56.3%, p< 0.01) on day 14 and 26 compared with the TC group. The treatment also reduced the final tumor mass (53.4%, p<0.05) and anorexia compared with the TC group during late-stage cachexia. The retroperitoneal AT (RPAT) mass was preserved on day 7 compared with the TC group during the same experimental period. Such effect also demonstrates inverse relationship with tumor growth, on day 14. Gene expression of PPAR-γ, adiponectin, LPL and C/EBP-α from cachectic rats was upregulated after PGZ. Glucose uptake from adipocyte cells (RPAT) was entirely re-established due to PGZ treatment. Taken together, the results demonstrate beneficial effects of PGZ treatment at both the early and final stages of cachexia.

Expression and regulation of facilitative glucose transporters in equine insulin-sensitive tissue: from physiology to pathology

Glucose uptake is the rate-limiting step in glucose utilization in mammalians and is tightly regulated by a family of specialized proteins, called the facilitated glucose transporters (GLUTs/SLC2). GLUT4, the major isoform in insulin-responsive tissue, translocates from an intracellular pool to the cell surface and as such determines insulin-stimulated glucose uptake. However, despite intensive research over 50 years, the insulin-dependent and -independent pathways that mediate GLUT4 translocation are not fully elucidated in any species. Insulin resistance (IR) is one of the hallmarks of equine metabolic syndrome and is the most common metabolic predisposition for laminitis in horses. IR is characterized by the impaired ability of insulin to stimulate glucose disposal into insulin-sensitive tissues. Similar to other species, the functional capability of the insulin-responsive GLUTs is impaired in muscle and adipose tissue during IR in horses. However, the molecular mechanisms of altered glucose transport remain elusive in all species, and there is still much to learn about the physiological and pathophysiological functions of the GLUT family members, especially in regard to class III. Since GLUTs are key regulators of whole-body glucose homeostasis, they have received considerable attention as potential therapeutic targets to treat metabolic disorders in human and equine patients.

Imaging of a glucose analog, calcium and NADH in neurons and astrocytes: dynamic responses to depolarization and sensitivity to pioglitazone

Neuronal Ca(2+) dyshomeostasis associated with cognitive impairment and mediated by changes in several Ca(2+) sources has been seen in animal models of both aging and diabetes. In the periphery, dysregulation of intracellular Ca(2+) signals may contribute to the development of insulin resistance. In the brain, while it is well-established that type 2 diabetes mellitus is a risk factor for the development of dementia in the elderly, it is not clear whether Ca(2+) dysregulation might also affect insulin sensitivity and glucose utilization. Here we present a combination of imaging techniques testing the disappearance of the fluorescent glucose analog 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-2-deoxyglucose (2-NBDG) as an indication of glycolytic activity in neurons and astrocytes. Our work shows that glucose utilization at rest is greater in neurons compared to astrocytes, and ceases upon activation in neurons with little change in astrocytes. Pretreatment of hippocampal cultures with pioglitazone, a drug used in the treatment of type 2 diabetes, significantly reduced glycolytic activity in neurons and enhanced it in astrocytes. This series of experiments, including Fura-2 and NADH imaging, provides results that are consistent with the idea that Ca(2+) levels may rapidly alter glycolytic activity, and that downstream events beyond Ca(2+) dysregulation with aging, may alter cellular metabolism in the brain.

Pioglitazone does not increase cerebral glucose utilisation in a murine model of Alzheimer's disease and decreases it in wild-type mice

AIMS/HYPOTHESIS

Clinical trials are in progress to test thiazolidinediones in neurodegenerative diseases such as Alzheimer's disease that involve deficiencies in brain glucose metabolism. While thiazolidinediones enhance glucose uptake in non-cerebral tissues, their impact on brain energy metabolism has not been investigated in vivo. We thus determined whether the thiazolidinedione pioglitazone reverses the decrease in cerebral glucose utilisation (CGU) in a model of brain metabolic deficiency related to Alzheimer's disease. Results are relevant to diabetes because millions of diabetic patients take pioglitazone as an insulin-sensitising drug, and diabetes increases the risk of developing Alzheimer's disease.

MATERIALS AND METHODS

The regional pattern of CGU was measured with the 2-deoxy [(14)C] glucose autoradiographic technique in adult awake mice overexpressing transforming growth factor beta1 (TGFbeta1), and in wild-type littermates. Mice were treated with pioglitazone for 2 months.

RESULTS

Measurement of CGU in 27 brain regions confirmed that TGFbeta1 overexpression induced hypometabolism across the brain. Pioglitazone did not reverse the effect of TGFbeta1 overexpression and decreased regional CGU in control animals by up to 23%. The extent of the regional CGU decrease induced by pioglitazone, but not that induced by TGFbeta1, correlated strongly with basal CGU, suggesting that the higher the local metabolic rate the greater the reduction of CGU effected by pioglitazone.

CONCLUSIONS/INTERPRETATION

In contrast to its stimulatory effect in non-cerebral tissues, chronic treatment with pioglitazone decreases CGU in vivo. This evidence does not support the hypothesis that pioglitazone could act as a metabolic enhancer in Alzheimer's disease, and raises the question of how thiazolidinediones could be beneficial in neurodegenerative diseases.

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.

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.

Muscle-specific Pparg deletion causes insulin resistance

Thiazolidinediones (TZDs) are insulin-sensitizing drugs and are potent agonists of the nuclear peroxisome proliferator-activated receptor-gamma (PPAR-gamma). Although muscle is the major organ responsible for insulin-stimulated glucose disposal, PPAR-gamma is more highly expressed in adipose tissue than in muscle. To address this issue, we used the Cre-loxP system to knock out Pparg, the gene encoding PPAR-gamma, in mouse skeletal muscle. As early as 4 months of age, mice with targeted disruption of PPAR-gamma in muscle showed glucose intolerance and progressive insulin resistance. Using the hyperinsulinemic-euglycemic clamp technique, the in vivo insulin-stimulated glucose disposal rate (IS-GDR) was reduced by approximately 80% and was unchanged by 3 weeks of TZD treatment. These effects reveal a crucial role for muscle PPAR-gamma in the maintenance of skeletal muscle insulin action, the etiology of insulin resistance and the action of TZDs.

Effect of thiazolidinediones on glucose and fatty acid metabolism in patients with type 2 diabetes

The current study aimed to compare the effects of treatment (2 months) with thiazolidinediones (TZDs) and placebo on glucose and fat metabolism in patients with type 2 diabetes (T2DM) in a crossover design. Eight patients received placebo (2 months) followed by TZD (2 months). Two-stage (1.5 and 6.0 pmol/kg min) hyperinsulinemic-euglycemic clamps were performed in all 8 patients pre- and post-placebo and post-TZD (post-placebo = pre-TZD). We determined rates of glucose disappearance (G(Rd)), glycolysis (GLS), glycogen synthesis (GS) (all with 3-(3)H glucose), carbohydrate (CHO) oxidation (indirect calorimetry), endogenous glucose production (EGP), free fatty acid (FFA) release ((2)H(5) glycerol), and oxidation (indirect calorimetry) and re-esterification, as well as plasma adiponectin and leptin concentrations, and fat cell size and number (determined in upper thigh biopsy samples). Placebo treatment had no effects on any of the measured parameters. TZD treatment improved insulin-stimulated glucose uptake (ISGU) from 17.1 to 26.4 micromol/kg min (P <.01) and insulin-stimulated GS from 4.8 to 13.4 micromol/kg min (P < 0.03), potentiated insulin-induced suppression of lipolysis from 4.3 to 2.3 micromol/kg min (P <.03) and FFA re-esterification from 1.9 to 1.0 micromol/kg min (P <.02), increased plasma adiponectin levels from 2.7 to 7.2 microg/mL (P <.05), and decreased plasma leptin levels from 30.8 to 23.4 ng/mL (P <.02). In addition, TZD tended to increase the number of small adipocytes (<50 microm) in subcutaneous adipose tissue. We conclude that TZDs have multiple actions and that many, but perhaps not all, can be accounted for by their action on adipose tissue.

Differences in troglitazone action on glucose metabolism in freshly isolated vs long-term incubated rat skeletal muscle

1. Exposure of isolated skeletal muscle to troglitazone has resulted in inconsistent findings ranging from inhibition to stimulation of fuel oxidation and the glycogenic pathway. To better understand such variation in outcome, the present study used isolated rat soleus muscle strips to examine the interdependent influences of prolonged maintenance in vitro and of troglitazone exposure. 2. If freshly isolated muscle strips were exposed to troglitazone (1 micro mol l(-1)) for 24 h, glucose oxidation was markedly reduced (-26+/-1%, P<0.0001), whereas glycogen synthesis remained unaffected (+9+/-7%, n.s.). 3. In contrast, extended exposure to troglitazone for 72 h increased both glucose oxidation (+65+/-28%, P<0.05) and glycogen synthesis (+46+/-11%, P<0.005), and a similar stimulatory effect was also observed in muscles exposed to troglitazone only during the last 24 h of their 72 h preincubation period (glucose oxidation: +61+/-15%, P<0.001; glycogen synthesis: +43+/-15%, P<0.01). 4. Troglitazone thus stimulated glucose utilization in long-term incubated muscle independent of the duration of exposure (24 or 72 h), whereas it inhibited glucose utilization in freshly isolated muscle. 5. The observed differences in troglitazone action on freshly isolated vs long-term incubated muscle suggest that findings on muscle tissue subject to prolonged maintenance in vitro cannot be extrapolated to native muscle in vivo.

Protective action of the peroxisome proliferator-activated receptor-gamma agonist pioglitazone in a mouse model of Parkinson's disease

We examined the effect of pioglitazone, a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist of the thiazolidinedione class, on dopaminergic nerve cell death and glial activation in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. The acute intoxication of C57BL/6 mice with MPTP led to nigrostriatal injury, as determined by tyrosine hydroxylase (TH) immunocytochemistry, and HPLC detection of striatal dopamine and metabolites. Damage to the nigrostriatal dopamine system was accompanied by a transient activation of microglia, as determined by macrophage antigen-1 (Mac-1) and inducible nitric oxide synthase (iNOS) immunoreactivity, and a prolonged astrocytic response. Orally administered pioglitazone (approximately 20 mg/kg/day) attenuated the MPTP-induced glial activation and prevented the dopaminergic cell loss in the substantia nigra pars compacta (SNpc). In contrast, there was little reduction of MPTP-induced dopamine depletion, with no detectable effect on loss of TH immunoreactivity and glial response in the striatum of pioglitazone-treated animals. Low levels of PPARgamma expression were detected in the ventral mesencephalon and striatum, and were unaffected by MPTP or pioglitazone treatment. Since pioglitazone affects primarily the SNpc in our model, different PPARgamma-independent mechanisms may regulate glial activation in the dopaminergic terminals compared with the dopaminergic cell bodies after acute MPTP intoxication.

The Anti-diabetic drugs rosiglitazone and metformin stimulate AMP-activated protein kinase through distinct signaling pathways

AMP-activated protein kinase (AMPK) is activated within the cell in response to multiple stresses that increase the intracellular AMP:ATP ratio. Here we show that incubation of muscle cells with the thiazolidinedione, rosiglitazone, leads to a dramatic increase in this ratio with the concomitant activation of AMPK. This finding raises the possibility that a number of the beneficial effects of the thiazolidinediones could be mediated via activation of AMPK. Furthermore, we show that in addition to the classical activation pathway, AMPK can also be stimulated without changing the levels of adenine nucleotides. In muscle cells, both hyperosmotic stress and the anti-diabetic agent, metformin, activate AMPK in the absence of any increase in the AMP:ATP ratio. However, although activation is no longer dependent on this ratio, it still involves increased phosphorylation of threonine 172 within the catalytic (alpha) subunit. AMPK stimulation in response to hyperosmotic stress does not appear to involve phosphatidylinositol 3-phosphate kinase, protein kinase C, mitogen-activated protein (MAP) kinase kinase, or p38 MAP kinase alpha or beta. Our results demonstrate that AMPK can be activated by at least two distinct signaling mechanisms and suggest that it may play a wider role in the cellular stress response than was previously understood.

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

Thiazolidinediones (TZDs) are believed to induce insulin sensitization by modulating gene expression via agonistic stimulation of the nuclear peroxisome proliferator-activated receptor-gamma (PPAR-gamma). We have shown earlier that the TZD troglitazone inhibits mitochondrial fuel oxidation in isolated rat skeletal muscle. In the present study, rat soleus muscle strips were exposed to TZDs to examine whether the inhibition of fuel oxidation is mediated by PPAR-gamma activation. Our findings consistently indicated direct, acute, and PPAR-gamma-independent TZD action on skeletal muscle fuel metabolism. Rapid stimulation of lactate release by 20 micromol/l troglitazone within 30 min suggested that direct TZD action on skeletal muscle in vitro does not rely on changes in gene expression rates (12.6 +/- 0.6 [control] vs. 16.0 +/- 0.8 micromol. g(-1). h(-1) [troglitazone]; P < 0.01). This conclusion was supported by the failure of actinomycin D and cycloheximide to block the effects of troglitazone. Mitochondrial fuel oxidation was consistently inhibited by six different TZDs (percent inhibition of CO(2) production from palmitate after 25 h: troglitazone, -61 +/- 2%; pioglitazone, -43 +/- 7%; rosiglitazone, -22 +/- 6%; BM13.1258, -47 +/- 9%; BM15.2054, -51 +/- 4%; and T-174, -59 +/- 4% [P < 0.005 each]), but not by PPAR-gamma agonistic compounds not belonging to the TZD class (JTT-501, -5 +/- 7% [NS]; prostaglandin J(2), 17 +/- 7% [P < 0.05]), which further argues against dependence on PPAR-gamma activation. In summary, our findings provided good evidence that direct inhibition of mitochondrial fuel oxidation in isolated skeletal muscle is a group-specific effect of TZDs and is independent of PPAR-gamma-mediated gene expression.

Pioglitazone ameliorates tumor necrosis factor-alpha-induced insulin resistance by a mechanism independent of adipogenic activity of peroxisome proliferator--activated receptor-gamma

Tumor necrosis factor (TNF)-alpha is one of the candidate mediators of insulin resistance associated with obesity, a major risk factor for the development of type 2 diabetes. The insulin resistance induced by TNF-alpha is antagonized by thiazolidinediones (TZDs), a new class of insulin-sensitizing drugs. The aim of the current study was to dissect the mechanism whereby pioglitazone, one of the TZDs, ameliorates TNF-alpha-induced insulin resistance in 3T3-L1 adipocytes. Pioglitazone restored insulin-stimulated 2-deoxyglucose (DOG) uptake, which was reduced by TNF-alpha, with concomitant restorations in tyrosine phosphorylation and protein levels of insulin receptor (IR) and insulin receptor substrate (IRS)-1, as well as association of the p85 regulatory subunit of phosphatidylinositol (PI) 3-kinase with IRS-1 and PI 3-kinase activity. Adenovirus-mediated gene transfer of either wild-type human peroxisome proliferator-activated receptor (PPAR)-gamma2 or a mutant carrying a replacement at the consensus mitogen-activated protein kinase phosphorylation site (hPPAR-gamma2-S112A) promoted adipogenesis of 3T3-L1 fibroblasts and restored TNF-alpha-induced decrease of triglyceride in adipocytes as effectively as pioglitazone. Overexpression of the PPAR-gamma proteins in TNF-alpha-treated adipocytes restored protein levels of IR/IRS-1, but did not improve insulin-stimulated tyrosine phosphorylation of IR/IRS-1 or insulin-stimulated 2-DOG uptake. These results indicate that the ability of pioglitazone to restore insulin-stimulated tyrosine phosphorylation of IR/IRS-1, which is necessary for amelioration of TNF-alpha-induced insulin resistance, may be independent of the adipogenic activity of PPAR-gamma that regulates protein levels of IR/IRS-1.

Troglitazone induces GLUT4 translocation in L6 myotubes

A number of studies have demonstrated that insulin resistance in the skeletal muscle plays a pivotal role in the insulin resistance associated with obesity and type 2 diabetes. A decrease in GLUT4 translocation from the intracellular pool to the plasma membranes in skeletal muscles has been implicated as a possible cause of insulin resistance. Herein, we examined the effects of an insulin-sensitizing drug, troglitazone (TGZ), on glucose uptake and the translocation of GLUT4 in L6 myotubes. The prolonged exposure (24 h) of L6 myotubes to TGZ (10(-5) mol/l) caused a substantial increase in the 2-deoxy-[3H]D-glucose (2-DG) uptake without changing the total amount of the glucose transporters GLUT4, GLUT1, and GLUT3. The TGZ-induced 2-DG uptake was completely abolished by cytochalasin-B (10 micromol/l). The ability of TGZ to translocate GLUT4 from light microsomes to the crude plasma membranes was greater than that of insulin. Both cycloheximide treatment (3.5 x 10(-6) mol/l) and the removal of TGZ by washing reversed the 2-DG uptake to the basal level. Moreover, insulin did not enhance the TGZ-induced 2-DG uptake additively. The TGZ-induced 2-DG uptake was only partially reversed by wortmannin to 80%, and TGZ did not change the expression and the phosphorylation of protein kinase B; the expression of protein kinase C (PKC)-lambda, PKC-beta2, and PKC-zeta; or 5'AMP-activated protein kinase activity. a-Tocopherol, which has a molecular structure similar to that of TGZ, did not increase 2-DG uptake. We conclude that the glucose transport in L6 myotubes exposed to TGZ for 24 h is the result of an increased translocation of GLUT4. The present results imply that the effects of troglitazone on GLUT4 translocation may include a new mechanism for improving glucose transport in skeletal muscle.

Rosiglitazone, insulin treatment, and fasting correct defective activation of protein kinase C-zeta/lambda by insulin in vastus lateralis muscles and adipocytes of diabetic rats

Atypical protein kinases C (PKCs), zeta and lambda, and protein kinase B (PKB) are thought to function downstream of phosphatidylinositol 3-kinase (PI 3-kinase) and regulate glucose transport during insulin action in skeletal muscle and adipocytes. Insulin-stimulated glucose transport is defective in type II diabetes mellitus, and this defect is ameliorated by thiazolidinediones and lowering of blood glucose by chronic insulin therapy or short-term fasting. Presently, we evaluated the effects of these insulin-sensitizing modalities on the activation of insulin receptor substrate-1 (IRS-1)-dependent PI 3-kinase, PKC-zeta/lambda, and PKB in vastus lateralis skeletal muscles and adipocytes of nondiabetic and Goto-Kakizaki (GK) diabetic rats. Insulin provoked rapid increases in the activity of PI 3-kinase, PKC-zeta/lambda, and PKB in muscles and adipocytes of nondiabetic rats, but increases in IRS-1-dependent PI 3-kinase and PKC-zeta/lambda, but not PKB, activity were substantially diminished in GK muscles and adipocytes. Rosiglitazone treatment for 10-14 days, 10-day insulin treatment, and 60-h fasting reversed defects in PKC-zeta/lambda activation in GK muscles and adipocytes and increased glucose transport in GK adipocytes, without necessarily increasing IRS-1-dependent PI 3-kinase or PKB activation. Our findings suggest that insulin-sensitizing modalities, viz. thiazolidinediones, chronic insulin treatment, and short-term fasting, similarly improve defects in insulin-stimulated glucose transport at least partly by correcting defects in insulin-induced activation of PKC-zeta/lambda.

Arachidonic acid stimulates glucose uptake in 3T3-L1 adipocytes by increasing GLUT1 and GLUT4 levels at the plasma membrane. Evidence for involvement of lipoxygenase metabolites and peroxisome proliferator-activated receptor gamma

Exposure of insulin-sensitive tissues to free fatty acids can impair glucose disposal through inhibition of carbohydrate oxidation and glucose transport. However, certain fatty acids and their derivatives can also act as endogenous ligands for peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear receptor that positively modulates insulin sensitivity. To clarify the effects of externally delivered fatty acids on glucose uptake in an insulin-responsive cell type, we systematically examined the effects of a range of fatty acids on glucose uptake in 3T3-L1 adipocytes. Of the fatty acids examined, arachidonic acid (AA) had the greatest positive effects, significantly increasing basal and insulin-stimulated glucose uptake by 1.8- and 2-fold, respectively, with effects being maximal at 4 h at which time membrane phospholipid content of AA was markedly increased. The effects of AA were sensitive to the inhibition of protein synthesis but were unrelated to changes in membrane fluidity. AA had no effect on total cellular levels of glucose transporters, but significantly increased levels of GLUT1 and GLUT4 at the plasma membrane. While the effects of AA were insensitive to cyclooxygenase inhibition, the lipoxygenase inhibitor, nordihydroguaiaretic acid, substantially blocked the AA effect on basal glucose uptake. Furthermore, adenoviral expression of a dominant-negative PPARgamma mutant attenuated the AA potentiation of basal glucose uptake. Thus, AA potentiates basal and insulin-stimulated glucose uptake in 3T3-L1 adipocytes by a cyclooxygenase-independent mechanism that increases the levels of both GLUT1 and GLUT4 at the plasma membrane. These effects are at least partly dependent on de novo protein synthesis, an intact lipoxygenase pathway and the activation of PPARgamma with these pathways having a greater role in the absence than in the presence of insulin.

Role of skeletal muscle in thiazolidinedione insulin sensitizer (PPARgamma agonist) action

Thiazolidinedione (TZD) insulin sensitizers are specific agonists of peroxisome proliferator activated receptor (PPAR)gamma. However, their mechanism of action and the in vivo target tissue(s) that mediate insulin sensitization remain poorly defined. Although PPARgamma messenger RNA expression has been reported in skeletal muscle, the expression of PPARgamma within myocytes in intact muscle tissue has not been examined. An antipeptide PPARgamma antibody was generated; immunohistochemistry was then used to demonstrate that PPARgamma is present within nuclei of myocytes [in both skeletal (white and red fibers) and cardiac tissue (rodent and human)]. The effect of insulin sensitizer treatment on muscle insulin action was studied using ob/ob mice after 4 days dosing with a potent (6 nM PPARgamma Kd) TZD (10 mg/kg x day). 2-deoxyglucose (2-DOG) uptake was then assessed in freshly isolated soleus muscles from lean vs. ob/ob vs. TZD-treated ob/ob mice. In lean mouse muscles, 2-DOG uptake was stimulated by 82%, 95%, 165% (with 25, 100, 2000 microU/ml insulin); muscles from ob/ob were severely insulin resistant (<80% stimulation with 2000 microU/ml insulin). Muscles from TZD-treated ob/ob displayed a normal insulin response with 100 (71%) or 2000 (158%) microU/ml insulin. Additional studies were performed using ZDF rats treated with/without TZD for 7 days. In vivo 2-DOG glucose uptake into soleus, gastrocnemius, and diaphragm muscles was measured during euglycemic-hyperinsulinemic clamp. Compared with lean rats, muscle 2-DOG uptake in ZDF was reduced by 52% (soleus) or 71% (diaphragm). Partial (40-60%) normalization of the reduced 2-DOG uptake was evident in TZD-treated ZDF rats. In contrast to the effect of in vivo treatment on muscle insulin action, preincubation of isolated soleus muscles from naive lean or ob/ob mice for 5 h with 100 nM TZD did not affect insulin-stimulated 2-DOG uptake. We conclude: 1) PPARgamma is expressed in myocytes within skeletal and cardiac muscle. 2) In vivo activation of PPARgamma by treatment of insulin-resistant mice/rats with a potent TZD corrects impaired muscle insulin action. 3) The lack of a direct effect on muscle after 5 h in vitro TZD incubation suggests that changes in insulin action may require a longer duration of PPARgamma activation or that improved muscle insulin sensitivity may result from an indirect in vivo effect of PPARgamma activation (e.g. changes in systemic lipid metabolism).

The new antidiabetic drug MCC-555 acutely sensitizes insulin signaling in isolated cardiomyocytes

Freshly isolated adult rat ventricular cardiomyocytes have been used to characterize the action profile of the new thiazolidinedione antidiabetic drug MCC-555. Preincubation of cells with the compound (100 microM for 30 min or 10 microM for 2 h) did not modify basal 3-O-methylglucose transport, but produced a marked sensitizing effect (2- to 3-fold increase in insulin action at 3 x 10(-11) M insulin) and a further enhancement of maximum insulin action (1.8-fold). MCC-555 did not modulate autophosphorylation of the insulin receptor and tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1). However, insulin action (10(-10) and 10(-7) M) on IRS-1-associated phosphatidylinositol (PI) 3-kinase activity was enhanced 2-fold in the presence of MCC-555. Association of the p85 adapter subunit of PI 3-kinase to IRS-1 was not modified by the drug. Immunoblotting experiments demonstrated expression of the peroxisomal proliferator-activated receptor-gamma in cardiomyocytes reaching about 30% of the abundance observed in adipocytes. The insulin-sensitizing effect of MCC-555 was lost after inhibition of protein synthesis by preincubation of the cells with cycloheximide (1 mM; 30 min). Cardiomyocytes from obese Zucker rats exhibited a completely blunted response of glucose transport at 3 x 10(-11) M insulin. MCC-555 ameliorates this insulin resistance, producing a 2-fold stimulation of glucose transport, with maximum insulin action being 1.6-fold higher than that in control cells. This drug effect was paralleled by a significant dephosphorylation of IRS-1 on Ser/Thr. In conclusion, MCC-555 rapidly sensitizes insulin-stimulated cardiac glucose uptake by enhancing insulin signaling resulting from increased intrinsic activity of PI 3-kinase. Acute activation of protein expression leading to a modulation of the Ser/Thr phosphorylation state of signaling proteins such as IRS-1 may be underlying this process. It is suggested that MCC-555 may provide a causal therapy of insulin resistance by targeted action on the defective site in the insulin signaling cascade.

Actions of novel antidiabetic thiazolidinedione, T-174, in animal models of non-insulin-dependent diabetes mellitus (NIDDM) and in cultured muscle cells

1. The antihyperglycaemic effect and the possible mechanism of action of T-174, a novel thiazolidinedione derivative, was determined in vivo and in vitro. 2. Oral administration of T-174 markedly improved hyperglycaemia, hyperinsulinaemia, hyperlipidaemia, and glucose intolerance in genetically obese and diabetic yellow KK (KK-Ay) mice (0.2-15.5 mg kg(-1) day(-1), for 7 days) and Zucker fatty rats (1.4-11.4 mg kg(-1) day(-1), for 6 days). The ED50 values for the glucose lowering action of T-174 and pioglitazone, another thiazolidinedione antidiabetic, were 1.8 and 29 mg kg(-1) day(-1), respectively in KK-Ay mice; T-174 was about 16 times more potent than pioglitazone. 3. The hypoglycaemic effect of exogenous insulin in KK-Ay mice was enhanced after the administration of T-174. A hyperinsulinaemic euglycaemic clamp study in Zucker fatty rats showed an amelioration of whole-body insulin resistance by the T-174 treatment. 4. Insulin-stimulated glucose metabolism was enhanced in adipocytes from KK-Ay mice treated with T-174. The insulin receptor number of the adipocytes was increased without a change in the affinity of the receptor. 5. The hypomagnesaemia in KK-Ay mice was completely restored by T-174. 6. In cultured L6 myotubes, glucose consumption and [3H]-2-deoxy-glucose transport were enhanced by T-174 (EC50; 6 and 4 microM, respectively). Combination of insulin with T-174 was additive to stimulate glucose disposal. 7. These results suggest that the antihyperglycaemic effect of T-174 was mediated by enhanced insulin action. This was associated with amelioration of the hypomagnesaemia and T-174 directly increased basal and insulin-stimulated glucose utilization by cultured muscle cells.

Effects of troglitazone on hepatic and peripheral insulin resistance induced by growth hormone excess in rats

It is well known that short-term growth hormone (GH) administration in humans and animals induces insulin resistance and glucose intolerance. The purpose of the present study was to clarify whether troglitazone, a new insulin-sensitizing drug of the thiazolidinedione class, counteracts the insulin antagonistic effects of recombinant human (rh) GH on glucose metabolism in rats. Male Wistar rats weighing 184 to 226 g were treated either with rhGH (n = 8) or rhGH plus troglitazone (n = 8). rhGH (20 IU/kg body weight/d) was given by subcutaneous injection twice daily for 2 days. Troglitazone was given at 100 mg/kg/d orally for 5 days before and 2 days during rhGH. Saline was injected to the control rats (n = 7). Euglycemic clamp studies with an insulin infusion rate of 8 mU/kg/min were performed in these rats after an overnight fast. Hepatic glucose output (HGO), glucose infusion rate (GIR), and glucose disappearance rate (GDR) were measured. Fasting levels of plasma glucose (6.6 +/- 0.1, 6.1 +/- 0.3, 6.5 +/- 0.2 mmol/L), insulin (187.5 +/- 24.1, 206.4 +/- 24.1, 182.3 +/- 31.0 pmol/L), and serum free fatty acid (FFA) (1.58 +/- 0.18, 1.43 +/- 0.16, 1.61 +/- 0.25 mEq/L) were comparable among rats treated with rhGH, rhGH plus troglitazone, and controls, respectively. Basal HGO was also comparable among the three treatment groups. HGO was suppressed significantly during the hyperinsulinemic glucose clamp in control rats, but not in rhGH rats. When troglitazone was coadministered with rhGH, suppressibility of HGO during the glucose clamp was comparable to that of controls. GIR (13.5 +/- 4.5 v 24.1 +/- 4.1 mg/kg/min) and GDR (18.1 +/- 5.8 v 30.3 +/- 5.2 mg/kg/min) were decreased by rhGH treatment compared with control values. They returned to normal levels in rats treated with both rhGH and troglitazone (GIR, 22.4 +/- 5.9; GDR, 24.7 +/- 7.1). From these results, it is evident that rhGH treatment impaired insulin's ability to suppress HGO and stimulate peripheral glucose utilization. Troglitazone could block the insulin antagonistic effects of GH on hepatic glucose output and peripheral glucose utilization.

Effects of troglitazone and pioglitazone on cytokine-mediated endothelial cell proliferation in vitro

We examined whether troglitazone and pioglitazone, antidiabetic thiazolidinediones, would directly induce endothelial cell proliferation or influence cytokine-driven proliferation in vitro. Monolayers of Balb/c mouse aortic endothelial cells were treated with troglitazone or pioglitazone in the absence of fetal bovine serum. Endothelial cells also were exposed to varying concentrations of basic fibroblast growth factor (bFGF) or insulin with or without either thiazolidinedione. After 48 h, 3-[4,5-dimethylthiozol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) assays were performed to quantitate endothelial cell proliferation by using the various treatment regimens. The data demonstrate that the antidiabetic thiazolidinediones troglitazone and pioglitazone negligibly affect direct endothelial cell proliferation in vitro. Furthermore, troglitazone and pioglitazone significantly inhibit bFGF-induced endothelial cell mitogenesis, whereas only high concentrations of troglitazone affect insulin-mediated proliferation.

Troglitazone regulation of glucose metabolism in human skeletal muscle cultures from obese type II diabetic subjects

To determine the effects of troglitazone on abnormal skeletal muscle glucose metabolism, muscle cultures from type II diabetic patients were grown for 4-6 weeks and then fused for 4 days either without or with troglitazone (1-5 micrograms/mL; chronic studies) or had troglitazone added for 90 min (1-5 micrograms/mL) at completion of fusion (acute studies). Acute troglitazone treatment stimulated glucose uptake, but not glycogen synthase (GS) activity 2-fold (P < 0.05) in a dose-dependent fashion and to the same extent as the addition of maximal (33 nmol/L) insulin. Maximal chronic troglitazone (5 micrograms/mL for 4 days) increased both glucose uptake (from 9.0 +/- 1.5 to 40.9 +/- 8.1 pmol/mg protein.min; P < 0.05) and GS fractional velocity (from 5.4 +/- 0.7% to 20.6 +/- 6.3%; P < 0.05) by approximately 4-fold. At each concentration of chronic troglitazone, glucose uptake rates were similar in the absence and presence of maximal (33 nmol/L) insulin concentrations. In contrast, insulin-stimulated GS activity was greater (P < 0.05) when maximal chronic troglitazone and acute insulin were combined than when chronic troglitazone alone was used. After 4 days of troglitazone, GLUT1 messenger ribonucleic acid and protein increased about 2-fold (P < 0.05) without a change in GLUT4 or GS messenger ribonucleic acid and protein. We conclude that troglitazone has both acute and chronic effects to improve skeletal muscle glucose metabolism of obese type II diabetic subjects. These effects involve direct insulin mimetic stimulatory actions as well as indirect insulin-sensitizing properties.

Troglitazone increases system A amino acid transport in 3T3-L1 cells

System A is one of the most highly regulated transport systems for transport of neutral amino acids into mammalian cells. Stimulation of uptake of alpha-[3H]methylaminoisobutyric acid (MeAIB), a nonmetabolizable system A substrate, by a novel insulin-sensitizing agent, troglitazone, in 3T3-L1 adipocytes was investigated. Treating adipocytes with troglitazone alone resulted in a time- and dose-dependent increase in the uptake of MeAIB. The peak stimulation appeared about 24 h after troglitazone addition. Both troglitazone- and insulin-stimulated transport activities increased markedly after the induction of differentiation of preadipocytes into adipocytes, and declined to a steady state level in adipocytes. The stimulated MeAIB uptake exhibited substrate specificity typical of system A and was mediated by a single component as determined by Eadie-Hofstee plots. The stimulation by troglitazone and that by insulin were similarly sensitive to actinomycin D and cycloheximide, suggesting that both agents may induce de novo synthesis of the same type of system A transport. Apart from the insulin-independent effect, troglitazone also showed an insulin-dependent action characterized by enhanced sensitivity to insulin. The synergistic stimulation of MeAIB uptake by coadministration of insulin and troglitazone was most prominent at the early stages of adipocyte differentiation. Pretreating cells with troglitazone during the differentiation attenuated the sensitivity of insulin to inhibition by actinomycin D, suggesting that troglitazone may enhance the insulin action by stabilizing messenger RNA involved in system A function.

Effects of troglitazone on dexamethasone-induced insulin resistance in rats

Troglitazone, a thiazolidinedione derivative, has been shown to counteract insulin resistance in obesity and non-insulin-dependent diabetes mellitus (NIDDM). To test its effects on dexamethasone-induced insulin resistance, we measured hepatic glucose production (HGP) and the insulin-stimulated glucose disposal rate (Rd) by a euglycemic-hyperinsulinemic glucose clamp technique coupled with 3-3H-glucose infusion in male Wistar rats treated with low-dose dexamethasone ([LoDex] 0.05 mg/kg/d, n = 7), high-dose dexamethasone ([HiDex] 0.1 mg/kg/d, n = 7), or dexamethasone plus troglitazone (LoDex + T, n = 8; HiDex + T, n = 6). Dexamethasone was injected subcutaneously for 4 days. Troglitazone was administered orally at 20 mg/d for 3 days before and for 4 days along with the dexamethasone treatment. The glucose clamp study was performed after an overnight fast in chronically catheterized conscious rats with a continuous insulin infusion of 57.4 pmol/kg/min. Basal HGP was comparable among the control (45.8 +/- 2.1 micromol/kg/min, n = 7), LoDex (47.9 +/- 4.7 micromol/kg/min), LoDex + T (46.0 +/- 2.6 micromol/kg/min), and HiDex + T (54.7 +/- 3.4 micromol/kg/min) groups. It increased about twofold in the HiDex group (80.1 +/- 5.2 micromol/kg/min, P < .05 v control). Under hyperinsulinemia, HGP was suppressed to a similar level in the control (11.3 +/- 8.8 micromol/kg/min), LoDex (10.2 +/- 8.4 micromol/kg/min), and LoDex + T (7.8 +/- 7.9 micromol/kg/min) groups. The suppressive effect of insulin on steady-state HGP during the clamp was impaired in HiDex (63.7 +/- 9.7 micromol/kg/min, P < .05) and HiDex + T (64.0 +/- 6.5 micromol/kg/min, P < .05). Rd decreased 27% in LoDex (81.5 +/- 5.8 micromol/kg/min, P < .05) and 36% in HiDex (71.3 +/- 9.4 micromol/kg/min, P < .05) compared with the controls (111.4 +/- 7.4 micromol/kg/min). Troglitazone prevented the decrease in Rd in LoDex + T (102.6 +/- 5.7 micromol/kg/min), but not in HiDex + T (67.0 +/- 6.4 micromol/kg/min). These results indicate that the development of peripheral insulin resistance was prevented by troglitazone in LoDex rats. Troglitazone may be a useful drug to treat steroid-induced diabetes.

Troglitazone enhances glucose uptake and inhibits mitogen-activated protein kinase in human aortic smooth muscle cells

The thiazolidinedione analogue troglitazone is an antidiabetic agent that improves insulin resistance in rodents and humans. Although coronary artery disease is common in patients with the insulin resistance syndrome, the effects of troglitazone on smooth muscle cells (SMC) have not been fully elucidated. We therefore examined the effects of troglitazone on cell growth and glucose uptake in human aortic SMC. Mitogen-activated protein (MAP) kinase activity and glucose transporter (Glut) 1 mRNA levels were also studied. In the absence of troglitazone, insulin (10(-7) M) caused a 2-fold increase of DNA synthesis in SMC and troglitazone suppressed the increase of DNA synthesis in a dose-dependent manner. This growth suppression was accompanied by inhibition of MAP kinase activity. On the other hand, troglitazone significantly increased Glut 1 mRNA and enhanced glucose uptake in SMC. These results suggest that troglitazone affects the insulin signaling pathways in SMC and suppresses growth while promoting glucose uptake. Our findings support the application of troglitazone as an inhibitor of SMC proliferation in patients with insulin resistance.

Effects of troglitazone and metformin on glucose and lipid metabolism: alterations of two distinct molecular pathways

Troglitazone and metformin are antidiabetic agents that belong to the thiazolidinedione and biguanide classes of drugs, respectively. To evaluate how these drugs influence fuel utilization, we compared their effects on several pathways regulating carbohydrate and lipid metabolism in vitro. Both drugs stimulated glucose transport and utilization in C3H10T1/2 cells, a cell line capable of differentiating into adipocytes when treated with thiazolidinediones. However, we observed that these drugs had a number of different in vitro effects. Unlike metformin, troglitazone stimulated beta3-adrenergic receptor-mediated lipolysis, lipogenesis, and transcriptional activity of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma). Further, by using a mitochondrial-specific fluorescent dye, we found troglitazone to be more effective than metformin at increasing mitochondrial mass. In contrast to troglitazone, metformin was more effective at increasing mitochondrial fatty acid beta-oxidation, peroxisomal fatty acid beta-oxidation, and anaerobic respiration (i.e. lactate production). Additionally, metformin stimulated and troglitazone inhibited both aerobic respiration and basal lipolysis. Insulin enhanced the effects of troglitazone, but not those of metformin, on these cells. Taken together, the data show that troglitazone and metformin affect two distinct metabolic pathways: one that is anabolic (i.e. troglitazone) and the other that is catabolic (i.e. metformin). Further, these observations suggest that the metabolic activity of mitochondria may be lower in cells treated with troglitazone than with metformin.

Thiazolidinediones block tumor necrosis factor-alpha-induced inhibition of insulin signaling

TNF-alpha has been shown to be an important mediator of insulin resistance linked to obesity. This cytokine induces insulin resistance, at least in part, through inhibition of the tyrosine kinase activity of the insulin receptor. Recently, a new class of compounds, the antidiabetic thiazolidinediones (TZDs), has been shown to improve insulin resistance in obesity and non-insulin-dependent diabetes mellitus in both rodents and man. Here we show that TZDs have powerful effects on the ability of TNF-alpha to alter the most proximal steps of insulin signaling, including tyrosine phosphorylation of the insulin receptor and its major substrate, IRS-1, and activation of PI3-kinase. Troglitazone or pioglitazone essentially eliminate the reduction in tyrosine phosphorylation of IR and IRS-1 caused by TNF-alpha in fat cells, even at relatively high doses (25 ng/ml). That this effect of TZDs operates through activation of the nuclear receptor PPARgamma/ RXR complex is shown by the fact that similar effects are observed with other PPARgamma/RXR ligands such as 15 deoxy Delta12,14PGJ2 and LG268. The TZDs do not inhibit all TNF-alpha signaling in that the transcription factor NF-kB is still induced well. These data indicate that TZDs can specifically block certain actions of TNF-alpha related to insulin resistance, suggesting that this block may contribute to their antidiabetic actions.

KB-R7785, a novel matrix metalloproteinase inhibitor, exerts its antidiabetic effect by inhibiting tumor necrosis factor-alpha production

It has been suggested that tumor necrosis factor-alpha (TNF-alpha) is a key mediator of insulin resistance in non-insulin-dependent diabetes mellitus (NIDDM). TNF-alpha is synthesized as a membrane-bound precursor; this is proteolytically processed to an active form by a matrix metalloproteinase (MMP)-like enzyme. In this study, we have used KKAy mice which show insulin resistance like NIDDM to investigate the effects of KB-R7785, a novel MMP inhibitor, on blood glucose and insulin levels. Subcutaneous administration of KB-R7785 at 100 mg/kg twice daily (i.e., 200 mg/kg/day) for 4 weeks resulted in a significant decrease in plasma glucose levels which was observed after 3 weeks. Oral administration of pioglitazone (20 mg/kg twice daily or 40 mg/kg/day for 4 weeks), an agent known to ameliorate insulin sensitivity, significantly decreased plasma glucose levels during the treatment period. KB-R7785, but not pioglitazone, also significantly decreased plasma insulin levels. Lipopolysaccharide (LPS) increased plasma TNF-alpha levels to a significantly greater degree in KKAy mice than in normal C57BL mice; this was inhibitable in KKAy mice by KB-R7785. In contrast, pioglitazone did not affect the LPS-induced increase in plasma TNF-alpha levels in KKAy mice. These results suggest that KB-R7785 exerts its antidiabetic effect by ameliorating insulin sensitivity through the inhibition of TNF-alpha production.

Peroxisome proliferator-activated receptors: a nuclear receptor signaling pathway in lipid physiology

Peroxisome proliferator-activated receptors (PPARs) are lipid-activated transcription factors that belong to the steroid/thyroid/retinoic acid receptor superfamily. All their characterized target genes encode proteins that participate in lipid homeostasis. The recent finding that antidiabetic thiazolidinediones and adipogenic prostanoids are ligands of one of the PPARs reveals a novel signaling pathway that directly links these compounds to processes involved in glucose homeostasis and lipid metabolism including adipocyte differentiation. A detailed understanding of this pathway could designate PPARs as targets for the development of novel efficient treatments for several metabolic disorders.

Antidiabetic thiazolidinediones inhibit leptin (ob) gene expression in 3T3-L1 adipocytes

Lack of leptin (ob) protein causes obesity in mice. The leptin gene product is important for normal regulation of appetite and metabolic rate and is produced exclusively by adipocytes. Leptin mRNA was induced during the adipose conversion of 3T3-L1 cells, which are useful for studying adipocyte differentiation and function under controlled conditions. We studied leptin regulation by antidiabetic thiazolidinedione compounds, which are ligands for the adipocyte-specific nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) that regulates the transcription of other adipocyte-specific genes. Remarkably, leptin gene expression was dramatically repressed within a few hours after thiazolidinedione treatment. The ED50 for inhibition of leptin expression by the thiazolidinedione BRL49653 was between 5 and 50 nM, similar to its Kd for binding to PPARgamma. The relatively weak, nonthiazolidinedione PPAR activator WY 14,643 also inhibited leptin expression, but was approximately 1000 times less potent than BRL49653. These results indicate that antidiabetic thiazolidinediones down-regulate leptin gene expression with potencies that correlate with their abilities to bind and activate PPARgamma.

Regulation of glucose transport in cultured muscle cells by novel hypoglycemic agents

The antidiabetic agent troglitazone (CS-045) and a metabolite designated M3 have potent blood glucose-lowering actions. The mechanism of the hypoglycemic effects of troglitazone and M3 was investigated in cultured L6 muscle cells. Short-term (2-hour) exposure of fully differentiated myotubes to troglitazone had no effect on glucose transport activity; M3 exposure caused a modest (50% to 60%) increase in basal and insulin-stimulated transport. Long-term (72-hour) treatment of myotubes with troglitazone resulted in a doubling of glucose transport in the absence of insulin, whereas M3 treatment resulted in a fivefold increase in basal glucose transport. Transport activity in M3-treated myotubes was greater than that seen after short-term insulin treatment. Insulin did not stimulate transport further in long-term M3-treated cells. A similar effect of prolonged exposure to M3 was observed in nondifferentiated myocytes. The agent had no influence on cell growth or the extent of differentiation. Augmentation of basal glucose transport by M3 was slow in onset, requiring 18 to 24 hours before significant effects were observed and 72 hours for full stimulation. M3 action on glucose transport was also dose-dependent, with half-maximal stimulation at 5 micrograms/mL of the agent and full effects at 10 to 20 micrograms/mL. Total membranes were prepared from control and M3-treated L6 myocytes and myotubes, and glucose transporter (GLUT1 and GLUT4) protein levels were measured by Western blotting. GLUT1 content was increased 2.9- +/- 1.3- and 2.8- +/- .2-fold by M3 treatment in myocytes and myotubes, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)