Insulin sensitization in diabetic rat liver by an antihyperglycemic agent

Thiazolidinedione-induced effects beyond glycaemic control

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

Gene Expression Changes Induced by PPAR Gamma Agonists in Animal and Human Liver

Thiazolidinediones are a class of Peroxisome Proliferator Activated Receptor γ (PPARγ) agonists that reduce insulin resistance in type 2 diabetic patients. Although no detectable hepatic toxicity has been evidenced in animal studies during preclinical trials, these molecules have nevertheless induced hepatic adverse effects in some treated patients. The mechanism(s) of hepatotoxicity remains equivocal. Several studies have been conducted using PCR analysis and microarray technology to identify possible target genes and here we review the data obtained from various in vivo and in vitro experimental models. Although PPARγ is expressed at a much lower level in liver than in adipose tissue, PPARγ agonists exert various PPARγ-dependent effects in liver in addition to PPARγ-independent effects. Differences in effects are dependent on the choice of agonist and experimental conditions in rodent animal studies and in rodent and human liver cell cultures. These effects are much more pronounced in obese and diabetic liver. Moreover, our own recent studies have shown major interindividual variability in the response of primary human hepatocyte populations to troglitazone treatment, supporting the occurrence of hepatotoxicity in only some individuals.

Functional characterisation of the regulation of CAAT enhancer binding protein alpha by GSK-3 phosphorylation of Threonines 222/226

BACKGROUND

Glycogen Synthase Kinase-3 (GSK3) activity is repressed following insulin treatment of cells. Pharmacological inhibition of GSK3 mimics the effect of insulin on Phosphoenolpyruvate Carboxykinase (PEPCK), Glucose-6 Phosphatase (G6Pase) and IGF binding protein-1 (IGFBP1) gene expression. CAAT/enhancer binding protein alpha (C/EBPalpha) regulates these gene promoters in liver and is phosphorylated on two residues (T222/T226) by GSK3, although the functional outcome of the phosphorylation has not been established. We aimed to establish whether CEBPalpha is a link between GSK3 and these gene promoters.

RESULTS

C/EBPalpha represses the IGFBP1 thymine-rich insulin response element (TIRE), but mutation of T222 or T226 of C/EBPalpha to non-phosphorylatable alanines has no effect on C/EBPalpha activity in liver cells (towards the TIRE or a consensus C/EBP binding sequence). Phosphorylation of T222/T226 is decreased by GSK3 inhibition, suggesting GSK3 does phosphorylate T222/226 in intact cells. However, phosphorylation was not altered by treatment of liver cells with insulin. Meanwhile C/EBPalpha activity in 3T3 L1 preadipocytes was enhanced by mutation of T222/T226 and/or S230 to alanine residues. Finally, we demonstrate that C/EBPalpha is a very poor substrate for GSK3 in vitro and in cells.

CONCLUSION

The work demonstrates an important role for this domain in the regulation of C/EBPalpha activity in adipocytes but not hepatocytes, however GSK3 phosphorylation of these residues does not mediate regulation of this C/EBP activity. In short, we find no evidence that C/EBPalpha activity is regulated by direct phosphorylation by GSK3.

A Peroxisome Proliferator-Activated Receptor α/γ Dual Agonist with a Unique in Vitro Profile and Potent Glucose and Lipid Effects in Rodent Models of Type 2 Diabetes and Dyslipidemia

LSN862 is a novel peroxisome proliferator-activated receptor (PPAR)α/γ dual agonist with a unique in vitro profile that shows improvements on glucose and lipid levels in rodent models of type 2 diabetes and dyslipidemia. Data from in vitro binding, cotransfection, and cofactor recruitment assays characterize LSN862 as a high-affinity PPARγ partial agonist with relatively less but significant PPARα agonist activity. Using these same assays, rosiglitazone was characterized as a high-affinity PPARγ full agonist with no PPARα activity. When administered to Zucker diabetic fatty rats, LSN862 displayed significant glucose and triglyceride lowering and a significantly greater increase in adiponectin levels compared with rosiglitazone. Expression of genes involved in metabolic pathways in the liver and in two fat depots from compound-treated Zucker diabetic fatty rats was evaluated. Only LSN862 significantly elevated mRNA levels of pyruvate dehydrogenase kinase isozyme 4 and bifunctional enzyme in the liver and lipoprotein lipase in both fat depots. In contrast, both LSN862 and rosiglitazone decreased phosphoenol pyruvate carboxykinase in the liver and increased malic enzyme mRNA levels in the fat. In addition, LSN862 was examined in a second rodent model of type 2 diabetes, db/db mice. In this study, LSN862 demonstrated statistically better antidiabetic efficacy compared with rosiglitazone with an equivalent side effect profile. LSN862, rosiglitazone, and fenofibrate were each evaluated in the humanized apoA1 transgenic mouse. At the highest dose administered, LSN862 and fenofibrate reduced very low-density lipoprotein cholesterol, whereas, rosiglitazone increased very low-density lipoprotein cholesterol. LSN862, fenofibrate, and rosiglitazone produced maximal increases in high-density lipoprotein cholesterol of 65, 54, and 30%, respectively. These findings show that PPARγ full agonist activity is not necessary to achieve potent and efficacious insulin-sensitizing benefits and demonstrate the therapeutic advantages of a PPARα/γ dual agonist.

Different mechanisms are used by insulin to repress three genes that contain a homologous thymine-rich insulin response element

Insulin rapidly and completely inhibits expression of the hepatic insulin-like growth factor binding protein-1 (IGFBP-1), phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) genes. This inhibition is mediated through a phosphatidyl inositol 3-kinase-dependent regulation of a DNA element, termed the thymine-rich insulin response element, found within the promoters of each of these genes. This has led to the conclusion that these three promoters are regulated by insulin using the same molecular mechanism. However, we recently found that the regulation of the IGFBP1 but not the PEPCK or G6Pase genes by insulin was sensitive to rapamycin, an inhibitor of mTOR. Here, we present further evidence that different regulatory pathways mediate the insulin regulation of these promoters. Importantly, we identify a protein phosphatase activity in the pathway connecting mTOR to the IGFBP-1 promoter. These data have major implications for the development of molecular therapeutics for the treatment of insulin-resistant states such as diabetes and hypertension.

Peroxisome proliferator-activated receptor gamma and metabolic disease

The nuclear peroxisome proliferator-activated receptor gamma (PPAR gamma) is a transcription factor that is activated by polyunsaturated fatty acids and their metabolites and is essential for fat cell formation. Although obesity is a strong risk factor for type 2 diabetes mellitus and other metabolic diseases, potent PPAR gamma activators such as the glitazone drugs lower glucose and lipid levels in patients with type 2 diabetes and also have antiatherosclerotic and antihypertensive effects. We review recent studies providing insight into the paradoxical relationship between PPAR gamma and metabolic disease. We also review recent advances in understanding the structural basis for PPAR gamma activation by ligands. The unusual ligand-binding properties of PPAR gamma suggest that it will be possible to discover new chemical classes of receptor "modulators" with distinct pharmacological activities for the treatment of type 2 diabetes and other metabolic diseases.

Peroxisome proliferator-activated receptor (PPAR) modulators: diabetes and beyond

Peroxisome proliferator-activated receptors (PPARs) are a class of ligand modulated transcription factors with a prominent role in the regulation of metabolic processes. This report is intended to provide a limited introduction to the PPAR field, sketched with reference to one early series of PPAR ligands.

Comprehensive messenger ribonucleic acid profiling reveals that peroxisome proliferator-activated receptor gamma activation has coordinate effects on gene expression in multiple insulin-sensitive tissues

Peroxisome proliferator-activated receptor gamma (PPAR gamma) agonists, including the glitazone class of drugs, are insulin sensitizers that reduce glucose and lipid levels in patients with type 2 diabetes mellitus. To more fully understand the molecular mechanisms underlying their therapeutic actions, we have characterized the effects of the potent, tyrosine-based PPAR gamma ligand GW1929 on serum glucose and lipid parameters and gene expression in Zucker diabetic fatty rats. In time-course studies, GW1929 treatment decreased circulating FFA levels before reducing glucose and triglyceride levels. We used a comprehensive and unbiased messenger RNA profiling technique to identify genes regulated either directly or indirectly by PPAR gamma in epididymal white adipose tissue, interscapular brown adipose tissue, liver, and soleus skeletal muscle. PPAR gamma activation stimulated the expression of a large number of genes involved in lipogenesis and fatty acid metabolism in both white adipose tissue and brown adipose tissue. In muscle, PPAR gamma agonist treatment decreased the expression of pyruvate dehydrogenase kinase 4, which represses oxidative glucose metabolism, and also decreased the expression of genes involved in fatty acid transport and oxidation. These changes suggest a molecular basis for PPAR gamma-mediated increases in glucose utilization in muscle. In liver, PPAR gamma activation coordinately decreased the expression of genes involved in gluconeogenesis. We conclude from these studies that the antidiabetic actions of PPAR gamma agonists are probably the consequence of 1) their effects on FFA levels, and 2), their coordinate effects on gene expression in multiple insulin-sensitive tissues.

Role of glucose and insulin in thiazolidinedione-induced alterations in hepatic gluconeogenesis

Previous studies from our laboratory as well as from others have suggested that the thiazolidinediones have the capacity to act as insulinomimetic agents, especially in the liver. In order to further characterize this insulinomimetic action, we evaluated the effect of troglitazone, a representative thiazolidinedione, on lactate- and glucagon-stimulated gluconeogenesis, in the presence or absence of insulin (10 nM) in isolated rat hepatocytes. The antigluconeogenic effect of troglitazone under basal (lactate-stimulated) conditions was found to be due to an elevation in the fructose 2,6-bisphosphate content, which was, however, not mediated by an activation of 6-phosphofructo 2-kinase. Troglitazone (125 and 250 microM) in the absence of insulin, produced a dose-dependent reduction in glucagon-stimulated gluconeogenesis, thereby suggesting an insulinomimetic effect. In addition, troglitazone (125 and 250 microM), in combination with insulin, produced an additive inhibition of gluconeogenesis during glucagon-stimulated conditions. However, unlike insulin, the metabolic mechanism responsible for these effects (in the presence or absence of insulin) does not involve fructose 2,6-bisphosphate.

Alteration in expression profiles of a series of diabetes-related genes in db/db mice following treatment with thiazolidinediones

We studied the effect of pioglitazone on the transcription of 42 genes associated with diabetes to examine the relationship between the antidiabetic action of thiazolidinediones (TZDs) and their ability to modulate transcription through their peroxisome proliferater-activated receptor (PPAR)-agonistic activity. Diabetic (db/db) mice were orally administered with pioglitazone for two weeks. Total RNA was prepared from liver, muscle and adipocytes and the quantity of mRNA was determined by comparative RT-PCR. The expression of diabetes-related genes was compared between lean and untreated db/db mice and between untreated and drug-treated db/db mice. The onset of diabetes was associated with a considerable alteration in the expression of a large number of diabetes-related genes. Treatment of db/db mice with pioglitazone modulated the expression of genes involved in the metabolism of glucose, lipids and lipoproteins. This included genes for phosphoenolpyruvate carboxykinase, beta-oxidation enzymes, lipoprotein lipase, apolipoprotein AI and uncoupling proteins. Most of the genes responsible for insulin signaling were unaffected. Administration of pioglitazone was also shown to induce PPARgamma expression in liver and muscle. It is therefore possible to hypothesize that TZDs may ameliorate diabetes through a mechanism of action involving a direct decrease in plasma glucose and triglyceride levels and improvements in free fatty acid-induced insulin resistance.

Pioglitazone

Pioglitazone is an orally administered insulin sensitising thiazolidinedione agent that has been developed for the treatment of type 2 diabetes mellitus. Pioglitazone activates the nuclear peroxisome proliferator activated receptor-gamma (PPAR-gamma), which leads to the increased transcription of various proteins regulating glucose and lipid metabolism. These proteins amplify the post-receptor actions of insulin in the liver and peripheral tissues, which leads to improved glycaemic control with no increase in the endogenous secretion of insulin. In placebo-controlled clinical trials, monotherapy with pioglitazone 15 to 45 mg/day has been shown to decrease blood glycosylated haemoglobin (HbA1c) levels in patients with type 2 diabetes mellitus. The addition of pioglitazone 30 mg/day to preexisting therapy with metformin, or of pioglitazone 15 or 30 mg/day to sulphonylurea, insulin or voglibose therapy, has been shown to decrease HbA1c and fasting blood glucose levels significantly in patients with poorly controlled type 2 diabetes mellitus. Pioglitazone has also been associated with improvements in serum lipid profiles in randomised placebo-controlled clinical studies. The drug has been well tolerated by adult patients of all ages in clinical studies. Oedema has been reported with monotherapy, and pooled data have shown hypoglycaemia in 2 to 15% of patients after the addition of pioglitazone to sulphonylurea or insulin treatment. There have been no reports of hepatotoxicity.

Dominant negative forms of Akt (protein kinase B) and atypical protein kinase Clambda do not prevent insulin inhibition of phosphoenolpyruvate carboxykinase gene transcription

Transcriptional regulation of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting enzyme in hepatic gluconeogenesis, by insulin was investigated with the use of adenovirus vectors encoding various mutant signaling proteins. Insulin inhibited transcription induced by dexamethasone and cAMP of a chloramphenicol acetyltransferase (CAT) reporter gene fused with the PEPCK promoter sequence in HL1C cells stably transfected with this construct. A dominant negative mutant of phosphoinositide (PI) 3-kinase blocked insulin inhibition of transcription of the PEPCK-CAT fusion gene, whereas a constitutively active mutant of PI 3-kinase mimicked the effect of insulin. Although a constitutively active mutant of Akt (protein kinase B) inhibited PEPCK-CAT gene transcription induced by dexamethasone and cAMP, a mutant Akt (Akt-AA) in which the phosphorylation sites targeted by insulin are replaced by alanine did not affect the ability of insulin to inhibit transcription of the fusion gene. Akt-AA almost completely inhibited insulin-induced activation of both endogenous and recombinant Akt in HL1C cells. Furthermore, neither a kinase-defective mutant protein kinase Clambda (PKClambda), which blocked insulin-induced activation of endogenous PKClambda, nor a dominant negative mutant of the small GTPase Rac prevented inhibition of PEPCK-CAT gene transcription by insulin. These data suggest that phosphoinositide 3-kinase is important for insulin-induced inhibition of PEPCK gene transcription and that a downstream effector of phosphoinositide 3-kinase distinct from Akt, PKClambda, and Rac may exist for mediating the effect of insulin.

Current views on the mechanism of action of thiazolidinedione insulin sensitizers

Resistance to the action of insulin in its target tissues in a major predisposing factor for the development of type 2 diabetes and is also tightly associated with a common pattern of cardiovascular risk factors that characterize the "insulin resistance syndrome." The thiazolidinediones are a new class of drugs that act as insulin sensitizers with well-documented-efficacy in the control of hyperglycemia in patients with overt diabetes. A growing body of evidence also suggests that thiazolidinediones may preserve beta-cell function and protect cardiovascular and renal function in patients with type 2 diabetes. This review will summarize our current notions of the mechanism of action of thiazolidinediones, which appears to involve a fascinating interplay between the partitioning of triglyceride stores, circulating free fatty acids and insulin signaling pathways. A detailed understanding of the action of thiazolidinediones will provide new insights into the pathogenesis of insulin resistance, diabetes and some of the causes of increased cardiovascular mortality in these conditions.

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.

PPARgamma induces the insulin-dependent glucose transporter GLUT4 in the absence of C/EBPalpha during the conversion of 3T3 fibroblasts into adipocytes

To define the molecular mechanisms that control GLUT4 expression during adipogenesis, NIH-3T3 fibroblasts ectopically expressing different adipogenic transcription factors (C/EBPbeta, C/EBPdelta, C/EBPalpha, and PPARgamma) under the control of a tetracycline-responsive inducible (C/EBPs) or a constitutive retroviral (PPARgamma) expression system were used. Enhanced production of C/EBPbeta (beta2 cell line), C/EBPbeta together with C/EBPdelta (beta/delta39 cell line), C/EBPalpha (alpha1 cell line), or PPARgamma (Pgamma2 cell line) in cells exposed to dexamethasone and the PPARgamma ligand ciglitazone (a thiazolidinedione) resulted in expression of GLUT4 mRNA as well as other members of the adipogenic gene program, including aP2 and adipsin. Focusing our studies on the beta/delta39 cells, we have demonstrated that C/EBPbeta along with C/EBPdelta in the presence of dexamethasone induces PPARgamma, adipsin, and aP2 mRNA production; however, GLUT4 mRNA is only expressed in cells exposed to ciglitazone. In addition, enhanced expression of a ligand-activated form of PPARgamma in the beta/delta39 fibroblasts stimulates synthesis of GLUT4 protein and gives rise to a population of adipocytic cells that take up glucose in direct response to insulin. C/EBPalpha is not expressed in the beta/delta39 cells under conditions that stimulate the adipogenic program. This observation suggests that PPARgamma alone or in combination with C/EBPbeta and C/EBPdelta is capable of activating GLUT4 gene expression.