Thiazolidinediones (AD-4833 and CS-045) improve hepatic insulin resistance in streptozotocin-induced diabetic rats

Thiazolidine-2,4-diones: progress towards multifarious applications

The promising activity shown by compounds containing thiazolidine-2,4-dione nucleus in numerous categories such as anti-hyperglycaemics, aldose reductase inhibitors, anti-cancer, anti-inflammatory, anti-arthritics, anti-microbials, etc. has made it an indispensable anchor for development of new therapeutic agents. Varied substituents on the thiazolidine-2,4-dione nucleus have provided a wide spectrum of biological activities. Importance of this nucleus in some activities like, peroxisome proliferator activated receptor γ (PPARγ) agonism and PPARγ-dependent and -independent anti-cancer activities are reviewed separately in literature. Short reviews on biological importance of this nucleus are also known in literature. However, owing to fast development of new drugs possessing thiazolidine-2,4-dione nucleus many research reports are generated in short span of time. So, there is a need to couple the latest information with the earlier information to understand the current status of thiazolidine-2,4-dione nucleus in medicinal chemistry research. In the present review, various derivatives of thiazolidine-2,4-diones with different pharmacological activities are described on the basis of substitution pattern around the nucleus combined with the docking studies performed in the active site of the corresponding receptors with an aim to help medicinal chemists for developing an SAR on thiazolidine-2,4-dione derived compounds for each activity. This discussion will further help in the development of novel thiazolidine-2,4-dione compounds.

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

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

FK-614, a selective peroxisome proliferator-activated receptor gamma agonist, improves peripheral glucose utilization while decreasing hepatic insulin extraction in alloxan-induced diabetic dogs

This is the first report of the effects of a nonthiazolidinedione activator of peroxisome proliferator-activated receptor (PPAR) gamma, that is, FK-614 (a benzimidazole derivative), on glucose metabolism in vivo. To investigate the effect of FK-614 on peripheral and hepatic insulin action, we performed hyperinsulinemic-hyperglycemic clamp studies combined with the triple-catheter technique and a double-tracer approach in alloxan-diabetic dogs with (n=5) or without (n=6) treatment with FK-614 (0.32 mg/kg per day orally for 10 days). Throughout the experiment, insulin was infused intraportally at 18 pmol/kg per minute and hyperglycemia (approximately 11 mmol/L) was maintained by a peripheral glucose infusion. After a 45-minute basal period (period I), a portal infusion of glucose labeled with [U-14C]-glucose, was administered for 120 minutes (period II) to measure hepatic glucose uptake. This was followed by 90-minute recovery (period III). FK-614 marginally improved peripheral insulin sensitivity, did not affect hepatic glucose uptake, and surprisingly increased tracer-determined hepatic glucose production (19.0+/-5.0 vs 10.6+/-1.7 mumol/kg per minute, P<.001). Hepatic insulin extraction was decreased by FK-614 (47.8%+/-1.6% vs 55.9%+/-3.4%, P<.01), which led to greater peripheral insulin levels and glucose utilization. FK-614 treatment also decreased the daily insulin requirements (regular insulin, 0.18+/-0.01 vs 0.32+/-0.01 U/kg per day; and NPH insulin, 0.53+/-0.02 vs 0.89+/-0.04 U/kg per day; P<.001) to maintain fasting plasma glucose at approximately 10 mmol/L for 7 days before the experiments. We conclude that FK-614 treatment, at the dose used, improves peripheral glucose utilization because of an improvement in peripheral insulin sensitivity and a decrease in insulin clearance, but impairs hepatic insulin action in alloxan-induced diabetic dogs. The reason for the effects of FK-614 on hepatic glucose and insulin metabolism is unclear but they are both consistent with reports of hepatic steatosis by PPARgamma activation when unopposed by concomitant activation of PPARalpha.

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.

Hepatic over-expression of peroxisome proliferator activated receptor gamma2 in the ob/ob mouse model of non-insulin dependent diabetes mellitus

Studies of the molecular basis of insulin resistance have focused on the peroxisome proliferator activated receptor gamma (PPARgamma, gamma1 and gamma2). The aim of this study was to determine whether the insulin resistance in liver of diabetic animals is associated with abnormal expression of these receptors. PPARgamma mRNA and protein expression levels were quantified in liver of 9-week-old male ob/ob mice as a model of diabetes and compared to age- and gender-matched wild type control animals of the same genetic background. Semi-quantitative reverse transcription-polymerase chain reaction, using 18S rRNA as an internal standard, indicated that PPARgamma2 mRNA was significantly upregulated in ob/ob liver vs. that in wild type mice. Western blotting revealed greater immunoreactivity of PPARgamma2 in liver from ob/ob mice relative to that in wild type mice. An index of insulin resistance (product of serum glucose and insulin concentration) was correlated with liver PPARgamma2 mRNA expression (r = 0.776; p < 0.001). The findings that liver PPARgamma2 expression is (1) significantly elevated in the ob/ob model of diabetes and (2) positively associated with an index of insulin resistance, suggests a possible compensatory response through which type II diabetic and obese organisms strive to maintain insulin sensitivity of the liver.

A novel potent antagonist of peroxisome proliferator-activated receptor gamma blocks adipocyte differentiation but does not revert the phenotype of terminally differentiated adipocytes

The antidiabetic thiazolidinediones, which include troglitazone and rosiglitazone, are ligands for the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma). Their antihyperglycemic effects seem to be linked to the regulation of PPARgamma-responsive genes. Here, we report the characterization of a specific PPARgamma antagonist that blocks several of the biological activities of the PPARgamma agonist rosiglitazone. PD068235 inhibited rosiglitazone-dependent PPARgamma transcriptional activity with an IC(50) of 0.8 microM and rosiglitazone-stimulated in vitro coactivator association. The role of PPARgamma in the initiation of differentiation is well documented. In this study, we used PD068235 as a tool to evaluate the functional role of PPARgamma in the maintenance of the terminally differentiated state. Treatment of confluent, growth-arrested 3T3-L1 preadipocytes with PD068235 blocked adipocyte differentiation induced by the standard adipogenic hormonal mixture (insulin/dexamethasone/isobutylmethylxanthin) and fully antagonized rosiglitazone-induced adipogenesis. In contrast, long-term treatment of terminally differentiated 3T3-L1 adipocytes with PD068235 did not induce any obvious morphological changes and had no effect on basal lipolysis rates. In addition, in fully differentiated adipocytes PD068235 did not alter the basal expression of PPARgamma target genes aP2 and CAP, but it effectively blocked rosiglitazone-induced expression of both genes. These results suggest that in terminally differentiated adipocytes, the PPARgamma activity is minimal and may not be required for the maintenance of PPARgamma target gene expression.

Enhancing insulin action: from chemical elements to thiazolidinediones

Type 2 diabetes is characterized by two fundamental biological defects: a reduced glucose-dependent insulin secretion and an increased resistance to the action of insulin at the level of various target tissues. While the use of agents to improve the insulin secretory activity of the islets of Langerhans has witnessed the flourishing of several new drugs over the years, a much greater difficulty has been experienced in the search for insulin-sensitizing drugs. The aim of this article is to critically review this topic, and to emphasize the importance of providing alternative strategies for the management of Type 2 diabetes.

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.

Differential regulation by troglitazone of plasminogen activator inhibitor type 1 in human hepatic and vascular cells

Troglitazone, a novel oral insulin sensitizer, normalizes increased plasma activity of plasminogen activator inhibitor type 1 (PAI-1) in hyperinsulinemic patients such as women with polycystic ovary syndrome and patients with type 2 diabetes mellitus. However, underlying mechanisms have not yet been fully elucidated. Human hepatic and vascular cells, the main sources of circulating PAI-1, were studied in cell culture. In human hepatic cells, PAI-1 accumulated in conditioned medium by 23% within 24 h after exposure to 3 microg/mL troglitazone (P = 0.001). The accumulation depended on the concentration of troglitazone, but not that of insulin (known to stimulate PAI-1 synthesis). By contrast, in human aortic smooth muscle cells, 3 microg/mL troglitazone decreased basal PAI-1 expression by 23% (P = 0.037) and decreased transforming growth factor-beta-induced expression by 34% (P = 0.026). Concomitant insulin had no effect. Tissue-type plasminogen activator was decreased by 38% (P = 0.002). In human endothelial cells, PAI-1 was diminished by 32% (P < 0.001), whereas tissue-type plasminogen activator was unaffected. The results suggest that the reduction in plasma activity of PAI-1 induced by troglitazone in patients may reflect both directly mediated diminution of its elaboration from vessel walls and indirectly mediated reduction of its hepatic synthesis secondary to attenuation of hyperinsulinemia (known to increase the hepatic synthesis of PAI-1).

Differential effects of troglitazone and D-chiroinositol on glucosamine-induced insulin resistance in vivo in rats

Troglitazone and D-chiroinositol have been shown to exert antidiabetic effects by either potentiating or mimicking insulin action. We studied whether pretreatment with these compounds can prevent the deleterious effects of glucosamine on insulin action that may play an important role in hyperglycemia-induced insulin resistance. Normal Wistar rats were pretreated with troglitazone (100 mg/kg/d), D-chiroinositol (100 mg/kg/d), or placebo (saline) for 7 days. Glucosamine (50 micromol/kg/min) was then infused for 210 minutes, and a euglycemic glucose clamp was performed during the last 120 minutes. Pretreatment with troglitazone or D-chiroinositol had no effect on fasting plasma glucose or insulin or basal hepatic glucose output (HGO). Under the euglycemic-hyperinsulinemic (956+/-93 pmol/L) clamp condition, HGO in glucosamine-infused placebo-treated rats was not suppressed, but instead was increased over the basal level, indicative of hepatic insulin resistance. In contrast, HGO failed to increase during glucosamine infusion in rats pretreated with troglitazone but was not normally suppressed. This may indicate a partial improvement in the hepatic insulin resistance. D-Chiroinositol pretreatment had no effect on the glucosamine-induced increase in HGO. The glucose disposal rate (GDR) was 25% lower in rats infused with glucosamine versus saline-infused rats (25.5+/-2.5 v 34.1+/-2.0 mg/kg/min), indicative of peripheral insulin resistance. Pretreatment with D-chiroinositol (34.5+/-2.3 mg/kg/min) prevented the glucosamine-induced decrease in the GDR, indicating an improvement in peripheral insulin resistance. Troglitazone (25.2+/-3.3 mg/kg/min) was without effect. In conclusion, (1) in normal control rats, glucosamine infusion induced hepatic and peripheral insulin resistance; (2) D-chiroinositol, but not troglitazone, pretreatment prevented glucosamine-induced peripheral insulin resistance; and (3) troglitazone, but not D-chiroinositol, partially blocked the glucosamine-induced hepatic insulin resistance. D-Chiroinositol may provide a novel pharmacological approach to hexosamine-induced peripheral insulin resistance.

Thiazolidinediones--the new insulin enhancers

Insulin resistance, characterized by reduced responsiveness to normal circulating levels of insulin, leads to hyperglycemia and hyperinsulinemia resulting in a deadly quartet of non-insulin dependent diabetes mellitus, obesity, hypertension and dyslipidemia These complications, also referred to as 'Syndrome X' have been associated with an increased risk of coronary heart disease. A number of non-pharmacological and pharmacological interventions are available for prevention and treatment of insulin resistance. However, introduction of thiazolidinediones, the new orally active class of drug, has proved to be a major breakthrough in this field. These agents have been shown to reduce insulin resistance by a novel mechanism of action. By interacting with a family of nuclear receptors known as peroxisome proliferator activated receptors thiazolidinediones are thought to enhance the actions of insulin, thereby increasing insulin dependent glucose disposal and reducing hepatic glucose output. A series of animal and clinical studies in patients with impaired Glucose Tolerance and NIDDM have demonstrated the safety and effect of various thiazolidinediones including ciglitazone, pioglitazone and troglitazone. Thus, thiazolidinediones by unlocking insulin resistance act as a key to glycemic control and hence are likely to prove a useful and rational therapy in NIDDM and possibly other disorders resulting from insulin resistance.

Troglitazone: Current Therapeutic Role in Type 2 Diabetes Mellitus

OBJECTIVE

To describe the role of troglitazone in the treatment of non-insulin-dependent diabetes mellitus.

METHODS

The potential mechanisms of action of the thiazolidinediones are outlined, and studies that have been conducted in animals and in humans are reviewed.

RESULTS

Although the precise mode of action of troglitazone, a thiazolidinedione, is unknown, this agent is an insulin sensitizer that has been shown to decrease fasting insulin, fasting plasma glucose, and blood pressure levels in humans. The effect of troglitazone is progressively greater over time; in several studies, the maximal action occurred as long as 12 weeks after initiation of treatment. The usual daily dose is 200 to 600 mg, and no dosage adjustment is necessary in patients with renal insufficiency. Adverse events, including fluid retention and hepatic dysfunction, may limit the utility of troglitazone in some clinical situations.

CONCLUSION

Both in monotherapy and in combination with sulfonylureas, insulin, or metformin, troglitazone has proved to be an effective agent for the treatment of type 2 diabetes mellitus.

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.

Effect of troglitazone in insulin-treated patients with type II diabetes mellitus. Troglitazone and Exogenous Insulin Study Group

BACKGROUND

Troglitazone is a new oral antidiabetic drug that increases the sensitivity of peripheral tissues to insulin. It may therefore increase the efficacy of exogenous insulin in patients with insulin-resistant diabetes mellitus.

METHODS

We studied the effect of troglitazone or placebo in 350 patients with poorly controlled non-insulin-dependent (type 2) diabetes mellitus (glycosylated hemoglobin values, 8 to 12 percent; normal, 4.3 to 6.1 percent) despite therapy with at least 30 U of insulin daily. The patients were randomly assigned to receive 200 mg of troglitazone (116 patients), 600 mg of troglitazone (116 patients), or placebo (118 patients) daily for 26 weeks. Insulin doses were not increased and were reduced only to prevent hypoglycemia. Glycosylated hemoglobin, serum glucose while fasting, serum total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and triglycerides were measured 5 times during an 8-week base-line period and 10 times during the 26-week treatment period. Daily insulin doses were recorded during both periods.

RESULTS

Ninety percent of the patients completed the study. The adjusted mean glycosylated hemoglobin values decreased by 0.8 and 1.4 percentage points, respectively, in the group given 200 mg of troglitazone and the group given 600 mg of troglitazone, and fasting serum glucose concentrations decreased by 35 and 49 mg per deciliter (1.9 and 2.7 mmol per liter), respectively, despite decreases in the insulin dose of 11 percent and 29 percent (P<0.001 for all comparisons with the placebo group). Serum total cholesterol, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol concentrations increased slightly and serum triglyceride concentrations decreased slightly in the troglitazone-treated patients.

CONCLUSIONS

When given in conjunction with insulin, troglitazone improves glycemic control in patients with type 2 diabetes mellitus.

Effect of troglitazone on blood insulin levels after pancreas transplantation with systemic venous drainage in rats

BACKGROUND

Troglitazone is a new oral antidiabetic agent and has been reported to reduce insulin resistance and improve peripheral hyperinsulinemia in patients with noninsulin-dependent diabetes mellitus. To examine the effect of troglitazone on insulin regulation after pancreas transplantation with systemic venous drainage, we measured peripheral glucose and insulin levels and performed an intravenous glucose tolerance test.

METHODS

We divided the rats into four groups: diabetic rats with a pancreas graft and administration of troglitazone at 40 mg/day orally (group P+T, n=4), rats with a pancreas graft only (group P, n=4), age-matched normal rats (group N, n=5), and diabetic rats (group DM, n=4).

RESULTS

Fasting insulin levels in group P were relatively higher than those in group N, whereas the values in group P+T were normalized. In the intravenous glucose tolerance test, troglitazone clearly regulates sigma immunoreactive insulin levels of pancreas transplanted rats (P vs. P+T: 244+/-23 vs. 145+/-14 microU/ml, P<0.05).

CONCLUSION

Hyperinsulinemia induced by systemic venous drainage, which may progress atherosclerosis, can be controlled with troglitazone treatment.

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.

Thiazolidinediones

The thiazolidinediones are a unique class of compounds that exert direct effects on the mechanisms of insulin resistance and result in improved insulin action and reduced hyperinsulinemia. Troglitazone is the first of these compounds to be approved for use in humans and has the potential not only to reduce glycemia and insulin requirements in type II diabetes but to improve other components of the insulin resistance syndrome including dyslipidemia, hypertension, and accelerated cardiovascular disease. Such compounds also hold promise for the prevention of type II diabetes and for the treatment of other insulin-resistant states including polycystic ovary disease. In addition to the novel mechanism of action through binding and activation of PPARs, troglitazone has other unique advantages, including once-a-day administration, a low incidence of minor side effects, no known drug interactions, hepatic metabolism and secretion, and potent antioxidant properties. Thiazolidinedione compounds such as troglitazone provide an important additional resource for the health care provider in the management of type II diabetes and other components of the insulin resistance syndrome.

Peroxisome proliferator-activated receptor gene expression in human tissues. Effects of obesity, weight loss, and regulation by insulin and glucocorticoids

The peroxisome proliferator activated receptor (PPAR gamma) plays a key role in adipogenesis and adipocyte gene expression and is the receptor for the thiazolidinedione class of insulin-sensitizing drugs. The tissue expression and potential for regulation of human PPAR gamma gene expression in vivo are unknown. We have cloned a partial human PPAR gamma cDNA, and established an RNase protection assay that permits simultaneous measurements of both PPAR gamma1 and PPAR gamma2 splice variants. Both gamma1 and gamma2 mRNAs were abundantly expressed in adipose tissue. PPAR gamma1 was detected at lower levels in liver and heart, whereas both gamma1 and gamma2 mRNAs were expressed at low levels in skeletal muscle. To examine the hypothesis that obesity is associated with abnormal adipose tissue expression of PPAR gamma, we quantitated PPARgamma mRNA splice variants in subcutaneous adipose tissue of 14 lean and 24 obese subjects. Adipose expression of PPARgamma 2 mRNA was increased in human obesity (14.25 attomol PPAR gamma2/18S in obese females vs 9.9 in lean, P = 0.003). This increase was observed in both male and females. In contrast, no differences were observed in PPAR gamma1/18S mRNA expression. There was a strong positive correlation (r = 0.70, P < 0.001) between the ratio of PPAR gamma2/gamma1 and the body mass index of these patients. We also observed sexually dimorphic expression with increased expression of both PPAR gamma1 and PPAR gamma2 mRNAs in the subcutaneous adipose tissue of women compared with men. To determine the effect of weight loss on PPAR gamma mRNA expression, seven additional obese subjects were fed a low calorie diet (800 Kcal) until 10% weight loss was achieved. Mean expression of adipose PPAR gamma2 mRNA fell 25% (P = 0.0250 after a 10% reduction in body weight), but then increased to pretreatment levels after 4 wk of weight maintenance. Nutritional regulation of PPAR gamma1 was not seen. In vitro experiments revealed a synergistic effect of insulin and corticosteroids to induce PPAR gamma expression in isolated human adipocytes in culture. We conclude that: (a) human PPAR gamma mRNA expression is most abundant in adipose tissue, but lower level expression of both splice variants is seen in skeletal muscle; to an extent that is unlikely to be due to adipose contamination. (b) RNA derived from adipose tissue of obese humans has increased expression of PPAR gamma 2 mRNA, as well as an increased ratio of PPAR gamma2/gamma1 splice variants that is proportional to the BMI; (c) a low calorie diet specifically down-regulates the expression of PPAR gamma2 mRNA in adipose tissue of obese humans; (d) insulin and corticosteroids synergistically induce PPAR gamma mRNA after in vitro exposure to isolated human adipocytes; and (e) the in vivo modulation of PPAR gamma2 mRNA levels is an additional level of regulation for the control of adipocyte development and function, and could provide a molecular mechanism for alterations in adipocyte number and function in obesity.

Slight but significant improvement of insulin resistance of Wistar fatty rats by treatment with a disaccharidase inhibitor, AO-128

To know whether the insulin resistance is improved by delaying carbohydrate absorption from the small intestine, we studied the effect of a disaccharidase inhibitor, AO-128, on insulin resistance of Wistar fatty rats. Rats were kept on standard laboratory chow with and without 10 ppm of AO-128 for 4 weeks, and then subjected to the glucose clamp. At the end of the 4-week treatment, plasma glucose level at 14:00 to 16:00 of AO-128 treated rats was 121 +/- 14 mg/100 ml (mean +/- S.D.), significantly lower than 226 +/- 72 mg/100 ml of the rats without AO-128. During clamp steady state under 20 mU.kg (-1).min (-1) continuous insulin infusion, glucose uptake of AO-128 treated rats was only 7.62 +/- 0.70 mg.kg (-1). min (-1), not different from 6.64 +/- 0.91 mg.kg (-1).min (-1) of rats without AO-128, but much lower than the lean littermates (20.81 +/- 3.11 mg.kg (-1).min (-1)). However, the percent suppression of hepatic glucose output was 55.2 +/- 23.8%, which, though incomplete, was significantly higher than 17.4 +/- 11.2% of rats without AO-128. The present study suggested that there were at least two, components of insulin resistance, a genetically determined and a poor-glycemic control-related, and that the latter insulin resistance was ameliorated by AO-128.

Regulation of PPAR gamma gene expression by nutrition and obesity in rodents

The orphan nuclear receptor, peroxisome proliferator-activated receptor (PPAR) gamma, is implicated in mediating expression of fat-specific genes and in activating the program of adipocyte differentiation. The potential for regulation of PPAR gamma gene expression in vivo is unknown. We cloned a partial mouse PPAR gamma cDNA and developed an RNase protection assay that permits simultaneous quantitation of mRNAs for both gamma l and gamma 2 isoforms encoded by the PPAR gamma gene. Probes for detection of adipocyte P2, the obese gene product, leptin, and 18S mRNAs were also employed. Both gamma l and gamma 2 mRNAs were abundantly expressed in adipose tissue. PPAR gamma 1 expression was also detected at lower levels in liver, spleen, and heart; whereas, gamma l and gamma 2 mRNA were expressed at low levels in skeletal muscle. Adipose tissue levels of gamma l and gamma 2 were not altered in two murine models of obesity (gold thioglucose and ob/ob), but were modestly increased in mice with toxigene-induced brown fat ablation uncoupling protein diphtheria toxin A mice. Fasting (12-48 h) was associated with an 80% fall in PPAR gamma 2 and a 50% fall in PPAR gamma mRNA levels in adipose tissue. Western blot analysis demonstrated a marked effect of fasting to reduce PPAR gamma protein levels in adipose tissue. Similar effects of fasting on PPAR gamma mRNAs were noted in all three models of obesity. Insulin-deficient (streptozotocin) diabetes suppressed adipose tissue gamma l and gamma 2 expression by 75% in normal mice with partial restoration during insulin treatment. Levels of adipose tissue PPAR gamma 2 mRNA were increased by 50% in normal mice exposed to a high fat diet. In obese uncoupling protein diphtheria toxin A mice, high fat feeding resulted in de novo induction of PPAR gamma 2 expression in liver. We conclude (a) PPAR gamma 2 mRNA expression is most abundant in adipocytes in normal mice, but lower level expression is seen in skeletal muscle; (b) expression of adipose tissue gamma1 or gamma2 mRNAs is increased in only one of the three models of obesity; (c) PPAR gamma 1 and gamma 2 expression is downregulated by fasting and insulin-deficient diabetes; and (d) exposure of mice to a high fat diet increases adipose tissue expression of PPAR gamma (in normal mice) and induces PPAR gamma 2 mRNA expression in liver (in obese mice). These findings demonstrate in vivo modulation of PPAR gamma mRNA levels over a fourfold range and provide an additional level of regulation for the control of adipocyte development and function.

Effects of seishin-renshi-in and Gymnema sylvestre on insulin resistance in streptozotocin-induced diabetic rats

Although there is no concept of insulin resistance in traditional Kampo (Chinese) medicine and Indian medicine, we had the hypothesis that some drug in a mixture of crude drugs which was believed to ameliorate diabetes mellitus may have had the effect of improving insulin resistance. To test this hypothesis, the effects of Seishin-renshi-in (Chinese medicine) and Gymnema sylvestre (Indian medicine) on the insulin resistance of streptozotocin-induced diabetic rats was studied by the glucose clamp technique. Oral administration of Seishin-renshi-in (800 mg/kg/day) with injections of a minimum dose of Ultralente insulin decreased urine volume and urinary glucose excretion during a 7-day treatment period and improved the insulin stimulated glucose uptake in peripheral tissues, as well as improving the insulin suppressed hepatic glucose output during glucose clamp. However, G. sylvestre (120 mg/kg/day) treatment did not improve insulin resistance. We conclude that Seishin-renshi-in, with a small dose of insulin, improved insulin resistance in streptozotocin-induced diabetic rats, but Gymnema sylvestre did not.

Insulin sensitization in diabetic rat liver by an antihyperglycemic agent

This study aimed to demonstrate directly that the thiazolidinedione pioglitazone acts as an insulin sensitizer. We tested the hypothesis that pioglitazone treatment of diabetic rats alters liver function such that responsiveness of selected genes to subsequent insulin regulation is enhanced. Although flux through gluconeogenic/glycolytic pathways involves regulation of many enzymes, we presently report the effects of insulin on expression of two key enzymes in these metabolic pathways, ie, phosphoenolpyruvate carboxykinase (PEPCK) and glucokinase (GK). Rats were either studied as nondiabetic controls or injected with streptozotocin as a model for insulin-deficient diabetes. Diabetic animals were treated without or with pioglitazone and subsequently examined for acute responses to insulin. Pioglitazone treatment of diabetic animals significantly enhanced the effects of insulin to reverse elevated blood glucose. Although the mean level of liver mRNA transcripts encoding PEPCK was increased to nearly 300% in diabetic animals as compared with nondiabetic controls (100%), it was significantly lower in pioglitazone-treated diabetic rats (119% of control) than in diabetic rats without pioglitazone (223% of control) after insulin treatment. By contrast, mRNA transcripts encoding GK were not detectable in diabetic animals, but were increased markedly by insulin treatment in all animal groups. Insulin-enhanced expression of GK was significantly greater in liver from animals that were treated earlier with pioglitazone (291% of control) than in liver from those that were untreated (214% of control). An amplified acute response of liver to insulin thus established pioglitazone as an insulin sensitizer. Our findings further showed that such sensitization can be developed even in the insulin-deficient state.(ABSTRACT TRUNCATED AT 250 WORDS)