The effect of CP 68,722, a thiozolidinedione derivative, on insulin sensitivity in lean and obese Zucker rats

The Role of Oxidized Cholesterol in Diabetes-Induced Lysosomal Dysfunction in the Brain

Abnormalities in lysosomal function have been reported in diabetes, aging, and age-related degenerative diseases. These lysosomal abnormalities are an early manifestation of neurodegenerative diseases and often precede the onset of clinical symptoms such as learning and memory deficits; however, the mechanism underlying lysosomal dysfunction is not known. In the current study, we investigated the mechanism underlying lysosomal dysfunction in the cortex and hippocampi, key structures involved in learning and memory, of a type 2 diabetes (T2D) mouse model, the leptin receptor deficient db/db mouse. We demonstrate for the first time that diabetes leads to destabilization of lysosomes as well as alterations in the protein expression, activity, and/or trafficking of two lysosomal enzymes, hexosaminidase A and cathepsin D, in the hippocampus of db/db mice. Pioglitazone, a thiazolidinedione (TZD) commonly used in the treatment of diabetes due to its ability to improve insulin sensitivity and reverse hyperglycemia, was ineffective in reversing the diabetes-induced changes on lysosomal enzymes. Our previous work revealed that pioglitazone does not reverse hypercholesterolemia; thus, we investigated whether cholesterol plays a role in diabetes-induced lysosomal changes. In vitro, cholesterol promoted the destabilization of lysosomes, suggesting that lysosomal-related changes associated with diabetes are due to elevated levels of cholesterol. Since lysosome dysfunction precedes neurodegeneration, cognitive deficits, and Alzheimer’s disease neuropathology, our results may provide a potential mechanism that links diabetes with complications of the central nervous system.

The formation of brown adipose tissue induced by transgenic over-expression of PPARγ2

Brown adipose tissue (BAT) is specialized to dissipate energy as heat, therefore reducing fat deposition and counteracting obesity. Brown adipocytes arise from myoblastic progenitors during embryonic development by the action of transcription regulator PRDM16 binding to PPARγ, which promotes BAT-like phenotype in white adipose tissue. To investigate the capability of converting white adipose tissue to BAT or browning by PPARγ in vivo, we generated transgenic mice with over-expressed PPARγ2. The transgenic mice showed strong brown fat features in subcutaneous fat in morphology and histology. To provide molecular evidences on browning characteristics of the adipose tissue, we employed quantitative real-time PCR to determine BAT-specific gene expressions. The transgenic mice had remarkably elevated mRNA level of UCP1, Elovl3, PGC1α and Cebpα in subcutaneous fat. Compared with wild-type mice, UCP1 protein levels were increased significantly in transgenic mice. ATP concentration was slightly decreased in the subcutaneous fat of transgenic mice. Western blotting analysis also confirmed that phosphorylated AMPK and ACC proteins were significantly (P<0.01) increased in the transgenic mice. Therefore, this study demonstrated that over-expression of PPARγ2 in skeletal muscle can promote conversion of subcutaneous fat to brown fat formation, which can have beneficial effects on increasing energy metabolisms and combating obesity.

The PPAR α / γ Agonist, Tesaglitazar, Improves Insulin Mediated Switching of Tissue Glucose and Free Fatty Acid Utilization In Vivo in the Obese Zucker Rat

Metabolic flexibility was assessed in male Zucker rats: lean controls, obese controls, and obese rats treated with the dual peroxisome proliferator activated receptor (PPAR) α/γ agonist, tesaglitazar, 3 μmol/kg/day for 3 weeks. Whole body glucose disposal rate (R_d) and hepatic glucose output (HGO) were assessed under basal fasting and hyperinsulinemic isoglycemic clamp conditions using [3,³H]glucose. Indices of tissue specific glucose utilization (R_g′) were measured at basal, physiological, and supraphysiological levels of insulinemia using 2-deoxy-D-[2,6-³H]glucose. Finally, whole body and tissue specific FFA and glucose utilization and metabolic fate were evaluated under basal and hyperinsulinemic conditions using a combination of [U-¹³C]glucose, 2-deoxy-D-[U-¹⁴C]glucose, [U-¹⁴C]palmitate, and [9,10-³H]-(R)-bromopalmitate. Tesaglitazar improved whole body insulin action by greater suppression of HGO and stimulation of R_d compared to obese controls. This involved increased insulin stimulation of R_g′ in fat and skeletal muscle as well as increased glycogen synthesis. Tesaglitazar dramatically improved insulin mediated suppression of plasma FFA level, whole body turnover (R_fa), and muscle, liver, and fat utilization. At basal insulin levels, tesaglitazar failed to lower HGO or R_fa compared to obese controls. In conclusion, the results demonstrate that tesaglitazar has a remarkable ability to improve insulin mediated control of glucose and FFA fluxes in obese Zucker rats.

Pioglitazone improves superoxide dismutase mediated vascular reactivity in the obese Zucker rat

OBJECTIVE

To test the hypothesis that the thiazolidinedione agent, pioglitazone, mediates its chronic BP lowering action via improving vascular reactivity.

METHODS AND RESULTS

Lean (Fa/fa) and obese (fa/fa) Zucker rats were treated with or without pioglitazone (20 mg/ kg/day) for 4 weeks (n=8 animals per group). Pioglitazone treatment was associated with a significant improvement in oral glucose tolerance in the obese animals (p<0.05 compared with untreated obese). Pioglitazone prevented the development of hypertension seen in obese untreated rats (SBP 126+/-1 versus 138+/-1 mmHg; p<0.0001). Aortic ring preparations from pioglitazone-treated obese rats showed improved relaxation responsiveness (ED(50) 0.28 versus 1.15 U/ ml, p<0.001) to SOD, a NO potentiator, compared with untreated obese animals.

CONCLUSIONS

SOD-mediated vasorelaxation may contribute to the chronic antihypertensive effect and/or the improvement in insulin sensitivity following pioglitazone treatment.

Pioglitazone Increases Gallbladder Volume in Insulin-Resistant Obese Mice

BACKGROUND

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Adiponectin and adiponectin receptors in insulin resistance, diabetes, and the metabolic syndrome

Adiponectin is an adipokine that is specifically and abundantly expressed in adipose tissue and directly sensitizes the body to insulin. Hypoadiponectinemia, caused by interactions of genetic factors such as SNPs in the Adiponectin gene and environmental factors causing obesity, appears to play an important causal role in insulin resistance, type 2 diabetes, and the metabolic syndrome, which are linked to obesity. The adiponectin receptors, AdipoR1 and AdipoR2, which mediate the antidiabetic metabolic actions of adiponectin, have been cloned and are downregulated in obesity-linked insulin resistance. Upregulation of adiponectin is a partial cause of the insulin-sensitizing and antidiabetic actions of thiazolidinediones. Therefore, adiponectin and adiponectin receptors represent potential versatile therapeutic targets to combat obesity-linked diseases characterized by insulin resistance. This Review describes the pathophysiology of adiponectin and adiponectin receptors in insulin resistance, diabetes, and the metabolic syndrome.

The novel hypoglycemic agent YM440 improves hepatic insulin resistance in obese Zucker fatty rats

The novel hypoglycemic agent YM440 ((Z)-1,4-bis{4-[(3,5-dioxo-1,2,4-oxadiazolidin-2-yl)methyl] phenoxy}but-2-ene) is a ligand of the peroxisome proliferator-activated receptor (PPAR) gamma. YM440 has unique pharmacological profiles both in vitro and in vivo, but, it is not clear whether the compound has a significant effect on hepatic or peripheral insulin response throughout the body. The aim of this study is to examine the effects of YM440 on hepatic and peripheral insulin resistance in Zucker fatty (ZF) rats using the euglycemic-hyperinsulinaemic clamp technique. Treatment of ZF rats with YM440 (300 mg/kg per day) for 2 weeks significantly decreased plasma concentrations of glucose and insulin without inducing obesity. YM440 caused a 2-fold increase in the glucose infusion rate during euglycemic clamping compared with the vehicle control. YM440 also decreased the percent change in hepatic glucose production rate caused by intravenous insulin infusion in ZF rats. YM440 had no significant effect on the glucose disposal rate. These results indicate that YM440 ameliorates hepatic, but not peripheral insulin resistance in ZF rats. These findings strongly suggest that the main target organ of YM440 is the liver, unlike other PPARgamma agonist.

Pioglitazone Ameliorates Insulin Resistance and Diabetes by Both Adiponectin-dependent and -independent Pathways

Thiazolidinediones have been shown to up-regulate adiponectin expression in white adipose tissue and plasma adiponectin levels, and these up-regulations have been proposed to be a major mechanism of the thiazolidinedione-induced amelioration of insulin resistance linked to obesity. To test this hypothesis, we generated adiponectin knock-out (adipo-/-) ob/ob mice with a C57B/6 background. After 14 days of 10 mg/kg pioglitazone, the insulin resistance and diabetes of ob/ob mice were significantly improved in association with significant up-regulation of serum adiponectin levels. Amelioration of insulin resistance in ob/ob mice was attributed to decreased glucose production and increased AMP-activated protein kinase in the liver but not to increased glucose uptake in skeletal muscle. In contrast, insulin resistance and diabetes were not improved in adipo-/-ob/ob mice. After 14 days of 30 mg/kg pioglitazone, insulin resistance and diabetes of ob/ob mice were again significantly ameliorated, which was attributed not only to decreased glucose production in the liver but also to increased glucose uptake in skeletal muscle. Interestingly, adipo-/-ob/ob mice also displayed significant amelioration of insulin resistance and diabetes, which was attributed to increased glucose uptake in skeletal muscle but not to decreased glucose production in the liver. The serum-free fatty acid and triglyceride levels as well as adipocyte sizes in ob/ob and adipo-/-ob/ob mice were unchanged after 10 mg/kg pioglitazone but were significantly reduced to a similar degree after 30 mg/kg pioglitazone. Moreover, the expressions of TNFalpha and resistin in adipose tissues of ob/ob and adipo-/-ob/ob mice were unchanged after 10 mg/kg pioglitazone but were decreased after 30 mg/kg pioglitazone. Thus, pioglitazone-induced amelioration of insulin resistance and diabetes may occur adiponectin dependently in the liver and adiponectin independently in skeletal muscle.

Insulin receptor exon 11+/− is expressed in Zucker (fa/fa) rats, and chlorogenic acid modifies their plasma insulin and liver protein and DNA

In vivo studies confirmed that chlorogenic acid (CGA) improved glucose tolerance and mineral pool distribution in obese Zucker (fa/fa) rats. We found a significant decrease (P<.05) in postprandial blood glucose concentrations, which may have been due to an improved sensitivity to insulin. Impaired glucose tolerance and insulin resistance have been associated with differences in the hepatic mRNA expression of the spliced variants of the insulin receptor at exon 11. Spliced variants of the insulin receptor have not been studied in obese Zucker (fa/fa) rats, and no information exists about the effects of CGA in vivo as a possible insulin sensitizer. Thus, we studied the in vivo effect of CGA on plasma insulin concentrations during a glucose tolerance test, liver protein and DNA concentrations, the hepatic activity of glucose-6-phosphatase (G-6-PASE) and the mRNA expression of the two variants of the insulin receptor at exon 11. Zucker (fa/fa) rats were implanted with jugular vein catheters. Chlorogenic acid was administered (5 mg/kg body weight per day) for 3 weeks via intravenous infusion. In the CGA-treated group, areas under the curve (AUC) for blood glucose and plasma insulin improved (P<.005), and the protein and DNA concentrations in the liver increased (P<.05). No significant differences (P>.05) were found between groups for the hepatic G-6-PASE activity. The insulin receptor exon 11(+) and the exon 11(-) variants were expressed in the liver of Zucker (fa/fa) rats without significant changes (P>.05). Chlorogenic acid improved some cellular mechanisms that are stimulated by insulin.

Englitazone administration to late pregnant rats produces delayed body growth and insulin resistance in their fetuses and neonates

The level of maternal circulating triacylglycerols during late pregnancy has been correlated with the mass of newborns. PPARgamma (peroxisome-proliferator-activated receptor gamma) ligands, such as TZDs (thiazolidinediones), have been shown to reduce triacylglycerolaemia and have also been implicated in the inhibition of tissue growth and the promotion of cell differentiation. Therefore TZDs might control cell proliferation during late fetal development and, by extension, body mass of pups. To investigate the response to EZ (englitazone), a TZD, on perinatal development, 0 or 50 mg of englitazone/kg of body mass was given as an oral dose to pregnant rats daily from day 16 of gestation until either day 20 for the study of their fetuses, or until day 21 of gestation for the study of neonates. EZ decreased maternal triacylglycerol levels at day 20 of gestation and neonatal mass, but not fetal mass. Fetuses and neonates from EZ-treated mothers exhibited high levels of insulin and were found to be hyperglycaemic. The apparent insulin-resistant state in neonates from EZ-treated pregnant rats was corroborated, since they showed higher plasma NEFA [non-esterified ('free') fatty acid] levels, ketonaemia and liver LPL (lipoprotein lipase) activity and lower plasma IGF-I (type 1 insulin-like growth factor) levels, in comparison with those from control mothers. Moreover, at the molecular level, an increase in Akt phosphorylation was found in the liver of neonates from EZ-treated mothers, which confirms that the insulin pathway was negatively affected. Thus the response of fetuses and neonates to maternal antidiabetic drug treatment is the opposite of what would be expected, and can be justified by the scarce amount of adipose tissue impeding a normal response to PPARgamma ligands and by hyperinsulinaemia as being responsible for a major insulin-resistant condition.

Mechanisms and clinical effects of thiazolidinediones

Studies of pioglitazone, troglitazone, BRL 49653 and other thiazolidinediones in preclinical animal models of non-insulin dependent diabetes mellitus (NIDDM) and obesity led to the observation that these compounds were effective in reducing hyperglycaemia and hyperlipidaemia. In these models, animals treated with thiazolidinediones had notable improvements in blood glucose levels, hepatic glucose output, peripheral insulin resistance, and serum lipid levels. Mechanistic studies indicate that thiazolidinediones act at many intracellular sites and can influence several processes to increase cell sensitivity to insulin. These include influence on insulin receptor kinase activity, control of insulin receptor phosphorylation, change in number of insulin receptors, quantity and activity of GLUT-4, modulation of tumour necrosis factor (TNF) activity, activation of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and alteration of hepatic glucose metabolism. Available data on pioglitazone and troglitazone from clinical studies support the efficacy and safety of this class of compounds in reducing hyperglycaemia, hypertriglyceridaemia and insulin resistance associated with NIDDM. Currently, only troglitazone is approved for use in the United States and only in combination with insulin. This new pharmacological class of drugs has great promise for the treatment of NIDDM and also as a valuable research tool to further the understanding of the mechanisms that underlie NIDDM and insulin resistance syndrome.

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.

Adipose tissue sensitization to insulin induced by troglitazone and MEDICA 16 in obese Zucker rats in vivo

The putative role played by insulin sensitizers in modulating adipose tissue lipolysis in the fasting state was evaluated in obese conscious Zucker rats treated with troglitazone or beta,beta'-tetramethylhexadecanedioic acid (MEDICA 16) and compared with nontreated lean and obese animals. The rates of appearance (R(a)) of glycerol and free fatty acid (FFA), primary intra-adipose reesterification, and secondary reuptake of plasma FFA in adipose fat were measured using constant infusion of stable isotope-labeled [(2)H(5)]glycerol, [2,2-(2)H(2)]palmitate, and radioactive [(3)H]palmitate. The overall lipolytic flux (R(a) glycerol) was increased 1.7- and 1.4-fold in obese animals treated with troglitazone or MEDICA 16, respectively, resulting in increased FFA export (R(a) FFA) in the troglitazone-treated rats. Primary intra-adipose reesterification of lipolysis-derived fatty acids was enhanced twofold by insulin sensitizers, whereas reesterification of plasma fatty acids was unaffected by either treatment. Despite the unchanged R(a) FFA in MEDICA 16 or the increased R(a) FFA induced by troglitazone, very low density lipoprotein production rates were robustly curtailed. Total adipose tissue reesterification, used as an estimate of glucose conversion to glyceride-glycerol, was increased 1.9-fold by treatment with the insulin sensitizers. Our results indicate that, in the fasting state, insulin sensitizers induce, in vivo, a significant activation rather than suppression of adipose tissue lipolysis together with stimulation of glucose conversion to glyceride-glycerol.

Troglitazone: the discovery and development of a novel therapy for the treatment of Type 2 diabetes mellitus

Prior to the introduction of troglitazone, it had been more than 30 years since the last significant improvement in antidiabetic therapy. In view of the pressing need for more effective oral agents for the treatment of Type 2 diabetes mellitus, troglitazone was granted priority review by the FDA and was launched in the USA in 1997. The first of the thiazolidinedione insulin sensitizing agents, troglitazone was quickly followed by rosiglitazone and pioglitazone. The glitazones proved to be effective not only in lowering blood glucose, but also to have beneficial effects on cardiovascular risk. Troglitazone was subsequently withdrawn because of concerns about hepatotoxicity, which appears to be less of a problem with rosiglitazone and pioglitazone. Recent insights into the molecular mechanism of action of the glitazones, which are ligands for the peroxisome proliferator-activated receptors, open the prospect of designing more effective, selective and safer antidiabetic agents. This document will review the history of troglitazone from discovery through clinical development.

Troglitazone antagonizes metabolic effects of glucocorticoids in humans: effects on glucose tolerance, insulin sensitivity, suppression of free fatty acids, and leptin

Glucocorticoids induce insulin resistance in humans, whereas thiazolidinediones enhance insulin sensitivity. Although the effects of glucocorticoids and thiazolidinediones have been assessed in isolation, interaction between these drugs, which both act as ligands for nuclear receptors, has been less well studied. Therefore, we examined the metabolic effects of dexamethasone and troglitazone, alone and in combination, for the first time in humans. A total of 10 healthy individuals with normal glucose tolerance (age 40 +/- 11 years, BMI 31 +/- 6.1 kg/m(2)) were sequentially studied at baseline, after 4 days of dexamethasone (4 mg/day), after 4-6 weeks on troglitazone alone (400 mg/day), and again after 4 days of dexamethasone added to troglitazone. Key metabolic variables included glucose tolerance assessed by blood glucose and insulin responses to an oral glucose tolerance test (OGTT), insulin sensitivity evaluated via hyperinsulinemic-euglycemic clamp, free fatty acids (FFAs) and FFA suppressibility by insulin during the clamp study, and fasting serum leptin. Dexamethasone drastically impaired glucose tolerance, with fasting and 2-h OGTT insulin values increasing by 2.3-fold (P < 0.001) and 4.4-fold (P < 0.001) over baseline values, respectively. The glucocorticoid also induced a profound state of insulin resistance, with a 34% reduction in maximal glucose disposal rates (GDRs; P < 0.001). Troglitazone alone increased GDRs by 20% over baseline (P = 0.007) and completely prevented the deleterious effects of dexamethasone on glucose tolerance and insulin sensitivity, as illustrated by a return of OGTT glucose and insulin values and maximal GDR to near-baseline levels. Insulin-mediated FFA suppressibility (FFA decline at 30 min during clamp/FFA at time 0) was also markedly reduced by dexamethasone (P = 0.002). Troglitazone had no effect per se, but it was able to normalize FFA suppressibility in subjects coadministered dexamethasone. Futhermore, the magnitudes of response of FFA suppressibility and GDR to dexamethasone were proportionate. The same was true for the reversal of dexamethasone-induced insulin resistance by troglitazone, but not in response to troglitazone alone. Leptin levels were increased 2.2-fold above baseline by dexamethasone. Again, troglitazone had no effect per se but blocked the dexamethasone-induced increase in leptin. Subjects experienced a 1.7-kg weight gain while taking troglitazone but no other untoward effects. We conclude that in healthy humans, thiazolidinediones antagonize the action of dexamethasone with respect to multiple metabolic effects. Specifically, troglitazone reverses both glucocorticoid-induced insulin resistance and impairment of glucose tolerance, prevents dexamethasone from impairing the antilipolytic action of insulin, and blocks the increase in leptin levels induced by dexamethasone. Even though changes in FFA suppressibility were correlated with dexamethasone-induced insulin resistance and its reversal by troglitazone, a cause-and-effect relationship cannot be established. However, the data suggest that glucocorticoids and thiazolidinediones exert fundamentally antagonistic effects on human metabolism in both adipose and muscle tissues. By preventing or reversing insulin resistance, troglitazone may prove to be a valuable therapeutic agent in the difficult clinical task of controlling diabetes in patients receiving glucocorticoids.

Long-term treatment with dipeptidyl peptidase IV inhibitor improves hepatic and peripheral insulin sensitivity in the VDF Zucker rat: a euglycemic-hyperinsulinemic clamp study

Upon release into circulation, the potent insulin secretagogues glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are rapidly cleaved and inactivated by the enzyme dipeptidyl peptidase IV (DP IV). Long-term administration of specific DP IV inhibitors, so as to enhance circulating active GIP and GLP-1 levels, has been shown to improve glucose tolerance and beta-cell glucose responsiveness and to reduce hyperinsulinemia in the Vancouver diabetic fatty (VDF) rat model of type 2 diabetes. Using the VDF model, the current study was undertaken to examine the effects of long-term DP IV inhibitor treatment on insulin sensitivity. Euglycemic-hyperinsulinemic clamps were performed on two sets of conscious VDF rats treated with or without the DP IV inhibitor P32/98 (20 mg. kg(-1). day(-1) for 12 weeks). The protocol consisted of three sequential 90-min periods with insulin infusion rates of 0, 5, and 15 mU. kg(-1). min(-1) and included a constant infusion of [ (3)H]glucose for measure of hepatic and peripheral insulin sensitivity. Relative to untreated littermates, the treated animals showed a left shift in the sensitivity of hepatic glucose output to insulin (average reduction approximately 6 micro mol. kg(-1). min(-1)) and a marked gain in peripheral responsiveness to insulin, with glucose disposal rates increasing 105 and 216% in response to the two insulin steps (versus 2 and 46% in controls). These results provide the first demonstration of improved hepatic and peripheral insulin sensitivity after DP IV inhibitor therapy, and coupled with apparent improvements in beta-cell function, they offer strong support for the utility of these compounds in the treatment of diabetes.

Induction of adipocyte complement-related protein of 30 kilodaltons by PPARgamma agonists: a potential mechanism of insulin sensitization

Adipocyte complement-related protein of 30 kDa (Acrp30, adiponectin, or AdipoQ) is a fat-derived secreted protein that circulates in plasma. Adipose tissue expression of Acrp30 is lower in insulin-resistant states and it is implicated in the regulation of in vivo insulin sensitivity. Here we have characterized the ability of PPARgamma agonists to modulate Acrp30 expression. After chronic treatment of obese-diabetic (db/db) mice with PPARgamma agonists (11 d), mean plasma Acrp30 protein levels increased (>3x). Similar effects were noted in a nongenetic type 2 diabetes model (fat-fed and low-dose streptozotocin-treated mice). In contrast, treatment of mice (db/db or fat-fed) with metformin or a PPARalpha agonist did not affect plasma Acrp30 protein levels. In a cohort of normal human subjects, 14-d treatment with rosiglitazone also produced a 130% increase in circulating Acrp30 levels vs. placebo. In addition, circulating Acrp30 levels were suppressed 5-fold in patients with severe insulin resistance in association with dominant-negative PPARgamma mutations. Thus, induction of adipose tissue Acrp30 expression and consequent increases in circulating Acrp30 levels represents a novel potential mechanism for PPARgamma-mediated enhancement of whole-body insulin sensitivity. Furthermore, Acrp30 is likely to be a biomarker of in vivo PPARgamma activation.

Chronic toxicity in monkeys with the thiazolidinedione antidiabetic agent troglitazone

The antidiabetic agent troglitazone was given to groups of 4 cynomolgus monkeys per sex at 300, 600, or 1200 mg/kg daily by gavage for 52 weeks. A group of 4 monkeys per sex received vehicle alone and served as controls. Emesis and soft stool or diarrhea occurred sporadically in all troglitazone-treated groups, but did not compromise animal health. There were no effects on body weight or food consumption, or ophthalmologic, electrocardiographic, or echocardiographic parameters. Erythrocyte count, hemoglobin, and hematocrit decreased 8% to 16% in males at all doses and serum cholesterol decreased 30% to 46% in both sexes at all doses. Urinary ketones were increased in several animals at 600 and 1200 mg/kg. Absolute and relative liver weights increased at all doses in both sexes by 40% to 71%. The only microscopic change attributable to troglitazone treatment was minimal to mild bile duct hyperplasia in males at all doses and in females at 600 and 1200 mg/kg. No differences in systemic exposure were apparent between sexes. Over the dose range tested, AUC(0-24) values were 27 to 102 micrograms.hr/ml of troglitazone, 401 to 2060 micrograms.hr/ml of its sulfate conjugate, and 34 to 312 micrograms.hr/ml of its quinone metabolite. Therefore, oral administration of troglitazone to monkeys at 300, 600, and 1200 mg/kg for 52 weeks resulted in significant systemic exposure, with only minimal gastrointestinal, hematologic, and hepatic effects.

Dehydroepiandrosterone alters phospholipid profiles in Zucker rat muscle tissue

Insulin-resistant muscle tissue contains low proportions of arachidonic acid (AA), and increased proportions of muscle AA correlate with improved insulin sensitivity. Dehydroepiandrosterone (DHEA) and AA, like the thiazolidinedione drugs that decrease insulin resistance (IR), are peroxisome proliferators. Long-chain fatty acids (FA) have been named the "one true" endogenous ligand for activating the peroxisome proliferator-activator receptor (PPAR), and DHEA has been named a "good candidate" as a naturally occurring indirect activator of PPAR. This study was conducted to determine DHEA's effects on lipid profiles of skeletal and cardiac muscle in lean and obese Zucker rats (ZR), a model of IR, type 2 diabetes mellitus, and obesity. We hypothesize that DHEA may alter long-chain FA profiles in muscle tissue of obese rats such that they more closely resemble that of the lean. In our experiments, we employed a DHEA and a pair-fed (PF) group (n = 6) for 12 lean and 12 obese ZR. For 30 d, the diet of the two DHEA groups was supplemented with 0.6% DHEA; PF groups were given the average daily calories consumed by their corresponding treatment group. Hearts and gastrocnemius muscles were assayed for phospholipid (PL), free FA, and triglyceride (TG) FA profiles. The proportion of PL AA was significantly greater in both muscle types of lean compared to obese rats. Hearts from both DHEA groups had greater PL proportions of AA and less oleic (18:1) acid than their PF controls. Likewise, 18:1 proportions were significantly lower in the gastrocnemius; however, AA proportions were not significantly different. Similar phenotypic profile differences were observed in the TG fraction of both muscle types. There were no DHEA-related TG FA profile alterations.

Inhibition of phosphoenolpyruvate carboxykinase (PEPCK) gene expression by troglitazone: a peroxisome proliferator-activated receptor-gamma (PPARgamma)-independent, antioxidant-related mechanism

Phosphoenolpyruvate carboxykinase (PEPCK) is the rate-limiting enzyme of gluconeogenesis. Enhanced expression of the PEPCK gene in liver is present in most models of diabetes, and is thought to contribute to the increased hepatic glucose output seen in this disease. Recently, we showed that troglitazone, the first thiazolidinedione (TZD) used clinically, inhibits expression of the PEPCK gene in isolated hepatocytes. We have pursued the molecular mechanism whereby troglitazone exerts this inhibition. TZDs are known to bind and activate peroxisome proliferator-activated receptor-gamma (PPARgamma), a nuclear receptor, which regulates expression of target genes. Initially, we examined the abilities of three other TZDs (rosiglitazone, englitazone, and ciglitazone) to inhibit expression of the PEPCK gene. Despite the fact that these agents are ligands for PPARgamma, they displayed little if any inhibitory activity on the expression of this gene. GW1929 [N-(2-benzoyl phenyl)-l-tyrosine], another potent PPARgamma ligand that is unrelated structurally to TZDs, had no inhibitory effect on PEPCK gene expression, while a natural PPARgamma ligand, the prostaglandin metabolite 15-PGJ2 (15-deoxy-Delta(12,14)-prostaglandin J2), displayed only modest inhibitory activity. Treatment of hepatocytes with ligands for other isoforms of PPAR also had no significant effect on PEPCK gene expression. Troglitazone has an alpha-tocopherol (vitamin E) moiety that is not present in other TZDs, and treatment of hepatocytes with vitamin E led to an inhibition of PEPCK gene expression. These observations support the conclusion that troglitazone inhibits the expression of the PEPCK gene by a PPARgamma-independent, antioxidant-related mechanism.

Thiazolidinediones (PPARgamma agonists) but not PPARalpha agonists increase IRS-2 gene expression in 3T3-L1 and human adipocytes

Thiazolidinediones (TZD) improve insulin sensitivity in human as well as in different animal models of insulin resistance and Type 2 diabetes. However, no clear link to the insulin signaling events has been identified. Using differentiated 3T3-L1 adipocytes, we found that TZD rapidly and markedly increased IRS-2 gene expression. This effect was specific for PPARgamma agonists and was not seen with PPARalpha agonists. It was rapidly induced (within 4 h) and maintained throughout the observation period of 48 h. It was also concentration dependent (EC50 approximately 50 nM) and not inhibited by cycloheximide, suggesting a direct effect on the IRS-2 promoter. There was no evidence that TZD altered IRS-2 mRNA stability, supporting that the increased mRNA levels were due to an increased gene transcription. IRS-2 protein expression was increased approximately 30% after 48 h and approximately 50% after 96 h. No effects of TZD were seen on IRS-1, PKB/Akt, or GLUT4 gene expression. TZD also increased IRS-2 mRNA levels in cultured human adipose tissue. These data show the first direct link between TZD and a critical molecule in insulin's signaling cascade in both 3T3-L1 and human adipocytes, and indicate a novel mode of action of these compounds.

Pathophysiology and pharmacological treatment of insulin resistance

Diabetes mellitus type 2 is a world-wide growing health problem affecting more than 150 million people at the beginning of the new millennium. It is believed that this number will double in the next 25 yr. The pathophysiological hallmarks of type 2 diabetes mellitus consist of insulin resistance, pancreatic beta-cell dysfunction, and increased endogenous glucose production. To reduce the marked increase of cardiovascular mortality of type 2 diabetic subjects, optimal treatment aims at normalization of body weight, glycemia, blood pressure, and lipidemia. This review focuses on the pathophysiology and molecular pathogenesis of insulin resistance and on the capability of antihyperglycemic pharmacological agents to treat insulin resistance, i.e., a-glucosidase inhibitors, biguanides, thiazolidinediones, sulfonylureas, and insulin. Finally, a rational treatment approach is proposed based on the dynamic pathophysiological abnormalities of this highly heterogeneous and progressive disease.

Effect of dietary Platycodon grandiflorum on the improvement of insulin resistance in obese Zucker rats

The effect of dietary Platycodon grandiflorum on the improvement of insulin resistance and lipid profile was investigated in lean (Fa/-) and obese (fa/fa) Zucker rats, a model for noninsulin dependent diabetes mellitus. Dietary Platycodon grandiflorum feeding for 4 weeks resulted in a significant decrease in the concentration of plasma triglyceride in both lean and obese Zucker rats. Furthermore, dietary Platycodon grandiflorum markedly decreased both plasma cholesterol and fasting plasma insulin levels, and significantly decreased the postprandial glucose level at 30 min during oral glucose tolerance test in obese Zucker rats. Although there was no statistical significance, the crude glucose transporter 4 protein level of obese rats fed Platycodon grandiflorum tended to increase when compared with that of obese control rats. Therefore, the present results suggested that dietary Platycodon grandiflorum may be useful in prevention and improvement of metabolic disorders characterized by hyperinsulinemia states such as noninsulin dependent diabetes mellitus, syndrome X, and coronary artery disease.

Retinoid X receptor agonists have anti-obesity effects and improve insulin sensitivity in Zucker fa/fa rats

OBJECTIVE

To investigate whether retinoid X receptor agonists act as insulin sensitizers and compare their effects with that of thiazolidinedione BRL 49653 in obese Zucker rats.

DESIGN

In two independent studies, obese Zucker rats were dosed orally once daily for 14 days with one of the following treatments: LG 100268 (20 mg/kg), LG 100324 (20 mg/kg), BRL 49653 (3 mg/kg) or vehicle.

MEASUREMENTS

Daily food intake and body weight gain, blood glucose, plasma and pancreatic insulin, whole body glucose disposal (by euglycaemic-hyperinsulinaemic clamp) and tissue glucose utilization.

RESULTS

The retinoid X receptor agonists (rexinoids) LG 100268 and LG 100324 caused a reduction in the food intake of obese Zucker rats relative to controls and to rats receiving BRL 49653. The two rexinoids also produced a marked decrease in the body weight gain, whereas the growth rate of rats treated with BRL 49653 tended to increase. Both rexinoids and BRL 49653 reduced the plasma insulin concentration of fed rats. LG 100268 and LG 100324 also significantly lowered blood glucose concentrations after 1 week of treatment. The 5 h fasted plasma insulin concentration was significantly lower in the rexinoid-treated groups and the terminal insulin level (at the end of the clamp) tended to be lower in all treated groups compared with animals given the dosing vehicle. However, pancreatic insulin content was not affected by any of the treatments. Under euglycaemic-hyperinsulinaemic clamp conditions, there were no significant differences in the rate of hepatic glucose output and whole body glucose disposal, except that, in experiment 1, BRL 49653 caused significant increase in the glucose infusion rate and muscle glucose utilization. In experiment 2, a similar glucose infusion rate to the controls was achieved in all treatment groups but the steady-state insulin concentration in the treated animals was only about 50% of that in the control animals, despite the fact that all rats received a similar insulin infusion concentration. This suggests that both the rexinoids and BRL 49653 increased insulin clearance.

CONCLUSIONS

Chronic administration of retinoid X receptor agonists LG 100268 and LG 100324 to Zucker fa/fa rats reduces food intake and body weight gain, lowers plasma insulin concentrations while maintaining normoglycaemia, indicating an improvement of insulin sensitivity.

Lipid lowering explains the insulin sensitivity enhancing effects of a thiazolidinedione, 5-(4-(2-(2-phenyl-4-oxazolyl)ethoxy)benzyl)-2,4 thiazolidinedione

AIM

Insulin resistance is a characteristic feature of type 2 diabetes and obesity. The present study examined the effects of TZD300512, a thiazolidinedione, on glucose and lipid metabolism in the fatty Zucker rat (fa/fa), a rat model of insulin resistance.

METHODS

TZD300512 was administered (2.0 mg/kg/day) in the diet for 1 week to chronically catheterized Zucker fa/fa rats. We measured triglyceride clearance rate and hepatic triglyceride output. We assessed baseline glucose metabolism, and insulin-mediated glucose uptake. We also determined whether the insulin sensitivity enhancing effects of TZD300512 could be reversed by infusion of Intralipid.

RESULTS

TZD300512 treatment markedly reduced fasting plasma triglyceride by 72% and nonesterified free fatty acids by 46%. Moreover, treatment significantly enhanced plasma triglyceride clearance (AUC; 60.36+/-11.50 v 131.44+/-18.45 mM/min), but hepatic triglyceride output was not altered. Drug treatment significantly reduced fasting plasma glucose by 25%, plasma insulin by 73%, and had no effect on glucagon levels. Glucose infusion rate (GIR) needed to maintain euglycemia during hyperinsulinemic clamp was significantly increased from 34.96+/-3.94 micromol/kg/min to 123.80+/-4.80 micromol/kg/min, while whole body glucose uptake was more than doubled (58.49+/-2.86 control vs. 126.97+/-3.8 treated micromol/kg/min). Insulin-induced suppression of hepatic glucose production was nearly complete with treatment. Intralipid infusion reversed drug-induced improvement in insulin sensitivity.

CONCLUSIONS

These results suggest that TZD300512-favourable alterations in lipid metabolism have a significant impact on its effectiveness in enhancing insulin sensitivity in a severely insulin resistant rodent model of type 2 diabetes.

Chronic and acute effects of thiazolidinediones BM13.1258 and BM15.2054 on rat skeletal muscle glucose metabolism

1 New thiazolidinediones BM13.1258 and BM15.2054 were studied with regard to their PPARgamma-agonistic activities and to their acute and chronic effects on glucose metabolism in soleus muscle strips from lean and genetically obese rats. 2 Both BM13.1258 and BM15.2054 revealed to be potent PPARgamma-activators in transient transfection assays in vitro. 3 In insulin-resistant obese rats, but not in lean rats, 10 days of oral treatment with either compound increased the stimulatory effect of insulin on muscle glycogen synthesis to a similar extent (insulin-induced increment in micromol glucose incorporated into glycogen g-1 h-1: control, +1.19+/-0.28; BM13.1258, +2.50+/-0.20; BM15.2054, +2.55+/-0.46; P<0.05 vs control each). 4 In parallel to insulin sensitization, mean glucose oxidation increased insulin-independently in response to BM13.1258 (to 191 and 183% of control in the absence and presence of insulin, respectively; P<0.01 each), which was hardly seen in response to BM15.2054 (to 137 and 124% of control, respectively; ns). 5 Comparable effects on PPARgamma activation and on amelioration of insulin resistance by BM13.1258 and BM15.2054 were therefore opposed by different effects on glucose oxidation. 6 In contrast to chronic oral treatment, acute exposure of muscles to BM13.1258 or BM15.2054 in vitro elicited a distinct catabolic response of glucose metabolism in specimens from both lean and obese rats. 7 The results provide evidence that BM13.1258 and BM15.2054 can affect muscle glucose metabolism via more than one mechanism of action. 8 Further efforts are required to clarify, to what extent other mechanisms besides insulin sensitization via the activation of PPARgamma are involved in the antidiabetic actions of thiazolidinediones.

Amelioration by KRP-297, a new thiazolidinedione, of impaired glucose uptake in skeletal muscle from obese insulin-resistant animals

We examined the effect of KRP-297, a new thiazolidinedione derivative, on glucose uptake in the soleus muscle of two animal models of insulin resistance that show moderate (ob/ob mice) and severe (db/db mice) hyperglycemia. Insulin-stimulated 2-deoxyglucose (2DG) uptake in soleus muscle was 53.8% lower in ob/ob mice versus lean mice (P < .05). When administered to ob/ob mice, KRP-297 (0.3 to 10 mg/kg) decreased plasma glucose and insulin levels and improved the impaired insulin-stimulated 2DG uptake in soleus muscle in a dose-dependent manner. Soleus muscle from db/db mice exhibited defects in both basal (35.0% decrease, P < .01) and insulin-stimulated (50.5% decrease, P < .01) 2DG uptake. These defects were improved by treatment with KRP-297 (0.3 to 10 mg/kg). Moreover, KRP-297 prevented severe hyperglycemia and the marked decrease in pancreatic insulin content in db/db mice. These results suggest that KRP-297 treatment is useful to prevent the development of diabetic syndromes in addition to ameliorating the impaired glucose transport in skeletal muscle.

[Current status of the treatment of type 2 diabetes mellitus. The revival of insulin-resistance drugs]

The relief of insulin resistance is one of the two therapeutic targets of the treatment of type 2 diabetes. Insulin-sensitizers are therefore complemental with other oral diabetic drugs. The treatment of insulin resistance was for a long time limited to dietary and exercise programmes, a biguanide, metformine, and benfluorex, a phenylethylamine derivative; the mechanisms of action of both drugs are now better understood and their indications more precisely targeted. A new therapeutic class, the thiazolidinediones (troglitazone, rosiglitazone, pioglitazone) has recently completed the family of insulin-sensitizing agents. These drugs, which should be soon available in France, act by a different way than metformin, which has been recently identified as the peroxisomes proliferator-activated receptor. The role of antilipolytic agents, which might increase glucose uptake by reducing free fatty acid production and oxidation is under evaluation, as well as the potential benefit of orlistat, an inhibitor of lipid digestion which has been proved effective, in addition to hypocaloric diet, in the management of obese patients.

Thiazolidinediones: a new class of antidiabetic drugs

Thiazolidinediones (TZDs) are a new class of oral antidiabetic agents. They selectively enhance or partially mimic certain actions of insulin, causing a slowly generated antihyperglycaemic effect in Type 2 (noninsulin dependent) diabetic patients. This is often accompanied by a reduction in circulating concentrations of insulin, triglycerides and nonesterified fatty acids. TZDs act additively with other types of oral antidiabetic agents (suphonylureas, metformin and acarbose) and reduce the insulin dosage required in insulin-treated patients. The glucose-lowering effect of TZDs is attributed to increased peripheral glucose disposal and decreased hepatic glucose output. This is achieved substantively by the activation of a specific nuclear receptor - the peroxisome proliferator-activated receptor-gamma (PPARgamma), which increases transcription of certain insulin-sensitive genes. To date one TZD, troglitazone, has been introduced into clinical use (in Japan, USA and UK in 1997). This was suspended after 2 months in the UK pending further investigation of adverse effects on liver function. TZDs have been shown to improve insulin sensitivity in a range of insulin-resistant states including obesity, impaired glucose tolerance (IGT) and polycystic ovary syndrome (PCOS). In Type 2 diabetes, the TZDs offer a new type of oral therapy to reduce insulin resistance and assist glycaemic control.

The emerging role of thiazolidinediones in the treatment of diabetes-mellitus and related disorders

Type II diabetes is a polygenic disorder, characterized in most cases by early onset of resistance to the action of insulin. Insulin sensitizers belonging to the thiazolidinedione class offer the first therapeutic option specifically targeting the underlying insulin resistance. Troglitazone is the prototype drug of this class and has been approved for marketing in several countries. Troglitazone offers several benefits over traditional oral hypoglycemic agents such as sulfonylureas and the biguanide metformin. Most of these advantages are related to better control of glycemic parameters with troglitazone alone or when added to existing treatment. In addition, it has interesting lipid lowering activity that may be of potential benefit in reducing morbidity from cardiovascular disease among diabetics. However, troglitazone may not be the ideal insulin sensitizer since 20-30% of diabetics do not respond to it. Also, it produces liver toxicity in 2% of patients, necessitating withdrawal of the drug. A number of second generation insulin sensitizers, belonging to the same chemical class as troglitazone, are in clinical development. The role of insulin sensitizers in the management of diabetes and other diseases in which insulin resistance is an underlying feature, is likely to undergo evolution as more information is obtained from clinical studies.

Improved metabolic status and insulin sensitivity in obese fatty (fa/fa) Zucker rats and Zucker Diabetic Fatty (ZDF) rats treated with the thiazolidinedione, MCC-555

1. We examined the effect of chronic (21 days) oral treatment with the thiazolidinedione, MCC-555 ((+)-5-[[6-(2-fluorbenzyl)-oxy-2-naphy]methyl]-2,4-thiazo lid inedione) on metabolic status and insulin sensitivity in obese (fa/fa) Zucker rats and Zucker Diabetic Fatty (ZDF) rats which display an impaired glucose tolerance (IGT) or overt diabetic symptoms, respectively. 2. MCC-555 treatment to obese Zucker rats (10 and 30 mg kg(-1)) and diabetic ZDF rats (10 mg kg(-1)) reduced non-esterified fatty acid concentrations in both rat strains and reduced plasma glucose and triglyceride concentrations in the obese Zucker rats. Liver glycogen concentrations were significantly increased by chronic MCC-555 treatment in both obese Zucker rats (30 mg kg(-1) day(-1)) and diabetic ZDF rats (10 mg kg(-1) day(-1)), as compared with vehicle-treated lean and obese rats and there was a significant increase in hepatic glycogen synthase activity in MCC-555-treated diabetic ZDF rats as compared to vehicle-treated controls. 3. During a euglycaemic hyperinsulinaemic clamp, MCC-555-treated obese Zucker rats and diabetic ZDF rats required significantly higher glucose infusion rates to maintain stable glucose concentrations (2.01+/-0.19 mg min(-1) and 6.42+/-1.03 mg min(-1), respectively) than vehicle-treated obese controls (0.71+/-0.17 mg min(-1) and 2.09+/-0.71 mg min(-1); P<0.05), demonstrating improved insulin sensitivity in both Zucker and ZDF rats. MCC-555 treatment also enhanced insulin-induced suppression of hepatic glucose production in ZDF rats as measured using infusions of [6-3H]-glucose under clamp conditions. 4. In conclusion, we have demonstrated that MCC-555 improves metabolic status and insulin sensitivity in obese Zucker and diabetic ZDF rats. MCC-555 may prove a useful compound for alleviating the metabolic disturbances and IGT associated with insulin resistance in man.

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).

Perturbation of fuel homeostasis caused by overexpression of the glucose-6-phosphatase catalytic subunit in liver of normal rats

The terminal step in hepatic gluconeogenesis is catalyzed by glucose-6-phosphatase, an enzyme activity residing in the endoplasmic reticulum and consisting of a catalytic subunit (glucose-6-phosphatase (G6Pase)) and putative accessory transport proteins. We show that Zucker diabetic fatty rats (fa/fa), which are known to exhibit impaired suppression of hepatic glucose output, have 2.4-fold more glucose-6-phosphatase activity in liver than lean controls. To define the potential contribution of increased hepatic G6Pase to development of diabetes, we infused recombinant adenoviruses containing the G6Pase cDNA (AdCMV-G6Pase) or the beta-galactosidase gene into normal rats. Animals were studied by one of three protocols as follows: protocol 1, fed ad libitum for 7 days; protocol 2, fed ad libitum for 5 days, fasted overnight, and subjected to an oral glucose tolerance test; protocol 3, fed ad libitum for 4 days, fasted for 48 h, subjected to oral glucose tolerance test, and then allowed to refeed overnight. Hepatic glucose-6-phosphatase enzymatic activity was increased by 1.6-3-fold in microsomes isolated from AdCMV-G6Pase-treated animals in all three protocols, and the resultant metabolic profile was similar in each case. AdCMV-G6Pase-treated animals exhibited several of the abnormalities associated with early stage non-insulin-dependent diabetes mellitus, including glucose intolerance, hyperinsulinemia, decreased hepatic glycogen content, and increased peripheral (muscle) triglyceride stores. These animals also exhibited significant decreases in circulating free fatty acids and triglycerides, changes not normally associated with the disease. Our studies show that overexpression of G6Pase in liver is sufficient to perturb whole animal glucose and lipid homeostasis, possibly contributing to the development of metabolic abnormalities associated with diabetes.

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.

Tissue-specific effects of in vivo adenosine receptor blockade on glucose uptake in Zucker rats

Previous studies have shown that treatment of obese Zucker rats with the adenosine receptor antagonist 1,3-dipropyl-8-(p-acrylic) phenyl xanthine (BWA1433) improves intraperitoneal glucose tolerance. In this study, a euglycemic hyperinsulinemic clamp was performed on obese (fa/fa) and lean (Fa/fa) Zucker rats that had been treated orally with BWA1433 or vehicle for 1 wk. A constant infusion of [3H]glucose was initiated in fasted animals to measure basal whole body glucose kinetics. No differences in glucose concentration or rates of glucose production/disappearance were observed between lean or obese animals with or without BWA1433. During the euglycemic hyperinsulinemic clamp, whole body glucose disposal in obese Zucker rats was only 22% of that observed in lean animals. BWA1433 treatment increased glucose disposal by 88% in obese Zucker rats. At the end of the clamp, [14C]-2-deoxyglucose was injected to determine tissue-specific differences in glucose uptake. Gastrocnemius, soleus, heart, and liver of untreated obese animals had significantly lower glucose uptake than lean controls under hyperinsulinemic conditions. BWA1433 treatment of obese animals increased glucose uptake in gastrocnemius and soleus muscles by 44 and 47%, respectively. Conversely, BWA1433 treatment decreased glucose uptake in adipose tissue by 54 and 49% in obese and lean Zucker rats, respectively. In summary, BWA1433 improves glucose tolerance by increasing glucose uptake in skeletal muscle while decreasing glucose uptake by adipose tissue. This study suggests that insulin resistance in obese Zucker rats is tissue specific and that signaling from adenosine receptors may be a factor contributing to tissue-specific insulin resistance.

Comparative effects of englitazone and glyburide on gluconeogenesis and glycolysis in the isolated perfused rat liver

Englitazone (CP 68,722, Pfizer) is a member of a family of drugs known as thiazolidinediones. One member of this family, troglitazone (Rezulin), is currently utilized in the treatment of Type 2 diabetes. Previous studies have focused on the ability of englitazone to increase insulin sensitivity in various tissues. However, little information is available regarding the direct effect of englitazone on hepatic glucose metabolism in the absence of insulin. Therefore, the following studies were conducted to comparatively evaluate the effect of englitazone and glyburide (a representative sulfonylurea) on gluconeogenesis and glycolysis from various substrates in the isolated perfused rat liver (IPRL). In isolated perfused rat livers of 24-hr fasted rats infused with lactate (2 mM), englitazone (6.25 to 50 microM) produced a concentration-dependent decrease (32-93%) in hepatic gluconeogenesis. When dihydroxyacetone (1 mM) and fructose (1 mM) were used as metabolic substrates, englitazone inhibited gluconeogenesis by 31 and 15%, respectively, while increasing glycolysis by 42 and 50%. Similar effects on gluconeogenesis and glycolysis were observed with glyburide, even though the effects with glyburide were more acutely evident, reversible, and of a greater magnitude. Such data suggest alterations in hepatic glucose production may contribute to the decrease in plasma glucose concentrations observed in individuals treated with englitazone and glyburide. These alterations may include effects on several regulatory enzymes (e.g. fructose-1,6-bisphosphatase, pyruvate kinase, and phosphoenolpyruvate carboxykinase), which warrant further investigation.

Metabolic effects of troglitazone in the Goto-Kakizaki rat, a non-obese and normolipidemic rodent model of non-insulin-dependent diabetes mellitus

Troglitazone (TRG) is an orally active antidiabetic agent that increases insulin sensitivity in models of non-insulin-dependent diabetes mellitus (NIDDM), subsequently reducing hyperinsulinemia and hyperglycemia. We examined the effects of TRG on the development and severity of diabetes in the Goto-Kakizaki (GK) rat, a spontaneous, non-obese model of NIDDM. TRG was administered at a dose of 30 mg/kg/d beginning at 4 weeks of age. TRG-treated GK rats were evaluated against Wistar and untreated GK rats at 8, 12, and 16 weeks of age. Untreated GK rats were nonketotic, normolipidemic, hyperglycemic, and had normal fasting insulin levels compared with Wistar rats. TRG treatment decreased glycosylated hemoglobin levels in the GK rat independently of its effects on plasma insulin. In untreated GK rats, intravenous glucose tolerance tests (IVGTTs) showed a hyperglycemic response to glucose loading with severely impaired glucose disposal relative to Wistar controls. TRG treatment was successful in decreasing the glucose area under the curve (AUC) (P < .03) but did not improve glucose disposal, suggesting a direct hepatic effect. Ex vivo evaluation of hepatic glucose output (HGO) further supported a direct hepatic action, with 50% reduction in HGO in TRG-treated GK rats (P < .004). A euglycemic-hyperinsulinemic clamp performed at 16 weeks of age showed severe insulin resistance in the untreated GK rat, with a glucose infusion rate (GIR) 33% lower than in Wistar rats (P < .004). TRG treatment had no effect on this insulin resistance. These results indicate that TRG selectively decreases hepatic glucose production in this unique model of NIDDM independently of its action on peripheral insulin sensitivity or hyperlipidemia.

Drugs on the horizon for treatment of type 2 diabetes

The only new pharmaceutical therapy for Type 2 (non-insulin-dependent) diabetes that has become available for clinical use in the last 40 years is the alpha-glucosidase inhibitor, acarbose, which reduces postprandial glucose levels by retarding digestion of complex carbohydrates in the gut. It has proved difficult to find other new metabolically active drugs that lack toxicity. Agents that reduce insulin resistance include the thiazolidinediones, which are very effective in animals. Of these, the only one that has been maintained in clinical evaluation appears from preliminary data to have an effect that although still useful, is not greater than that reported for current oral agents. Agents that reduce non-esterified fatty acid levels by inhibiting lipolysis, thereby allowing increased peripheral uptake of glucose, have so far given minimal reduction in glycaemia. The development of fatty acid oxidation inhibitors to reduce gluconeogenesis in the liver has been hampered by toxicity, but additional new agents are being studied. The most promising new approach for enhancing insulin secretion has been suggested by the demonstration that pharmacological doses of GLP-1 (7-36 amide), a natural enteric incretin hormone, improves pancreatic beta-cell and alpha-cell sensitivity to glucose and can induce normal basal glucose levels in diabetic man. The future development of GLP-1 agonists will be of great interest. This is timely as other insulin secretogogues, such as alpha 2 adrenergic blockers have proved relatively ineffective. Anti-obesity agents would in theory be beneficial, but have either had limited efficacy or have been avoided because of concern about long-term safety. Until new pharmaceutical agents become available, if near-normal glycaemia is to be achieved, many more Type 2 diabetic patients will need insulin therapy. When full insulin replacement therapy is not feasible, reducing the fasting blood glucose level towards normal with a single daily basal insulin supplement, either alone or in combination with oral agents, could become a more widely used therapy.

Effect of the new oral antidiabetic agent (-)-BM 13.0913.Na on insulin resistance in lean and obese Zucker rats

The new antidiabetic agent (-)-BM 13.0913.Na (BM) was administered to 12-week-old lean and obese Zucker rats, an animal model of insulin resistance, at a daily dose of 50 mg/kg for 14 days. Hyperinsulinemic-euglycemic clamps were performed on treated and untreated lean and obese Zucker rats. Basal hepatic glucose production (HGP) rates were similar in lean and obese untreated animals. Insulin-induced suppression of HGP was significantly less effective in obese animals. In addition, these animals exhibited the characteristic impaired glucose utilization. In obese animals, drug treatment improved insulin suppression of HGP and total glucose utilization (GU) during clamp studies. Furthermore, drug treatment decreased insulin levels during clamp studies, suggesting an acceleration of insulin clearance. Drug treatment also decreased basal plasma insulin levels and serum and liver concentrations of cholesterol in both fasted lean and obese rats. Additionally, blood glucose, plasma nonesterified fatty acids (NEFA), and serum triglyceride levels were reduced in fasted obese rats, but only minor changes in liver triglycerides were observed in lean and obese rats. On the basis of these results, we suggest that BM is an effective antidiabetic agent that may reduce abnormalities of glucose and lipid metabolism.

Actions of the novel antidiabetic agent englitazone in rat hepatocytes

We examined effects of a novel antidiabetic agent, racemic englitazone (CP 68,722, Pfizer), on normal rat hepatocytes in vitro. For optimal effects, CP 68,722 must be preincubated for approximately 20 minutes. CP 68,722 inhibited the actions of glucagon on glycogenolysis (measured by monitoring cyclic adenosine monophosphate [cAMP] levels, phosphorylase activation, and glucose output) and gluconeogenesis (from 14C-lactate). Since CP 68,722 was able to attenuate the ability of glucagon to increase cAMP levels, this may account for part of its inhibitory actions on glycogenolysis and gluconeogenesis. The observation that CP 68,722 also inhibits the ability of the cAMP analog, 8-(4-chlorophenylthio)-adenosine 3':5'-cyclic monophosphate (8 CPT cAMP), to stimulate phosphorylase a is consistent with an effect of CP 68,722 to activate cAMP-dependent phosphodiesterase. The ability of vasopressin (an agonist known to stimulate glycogenolysis via a Ca(2+)-dependent mechanism) to stimulate phosphorylase a was slightly inhibited by CP 68,722. Another site of action of CP 68,722 was to inhibit hormonal-mediated Ca2+ influx, an effect that would decrease intracellular free calcium ([Ca2+]i), thereby inhibiting the actions of the Ca(2+)-dependent hormones such as alpha 1-adrenergic agonists and vasopressin, agents known to promote glucose output from the liver. In summary, CP 68,722 inhibits glucagon-stimulated glycogenolysis and gluconeogenesis in hepatocytes by a mechanism that may include activation of cAMP phosphodiesterase and inhibition of Ca2+ influx.