Effects of Troglitazone (CS-045) on insulin secretion in isolated rat pancreatic islets and HIT cells: an insulinotropic mechanism distinct from glibenclamide

Comparisons between dipeptidyl peptidase-4 inhibitors and other classes of hypoglycemic drugs using two distinct biomarkers of pancreatic beta-cell function: A meta-analysis

Background and objective

Dipeptidyl peptidase-4 (DPP-4) inhibitors have been suggested to have pancreatic beta-cell preserving effect according to studies using homeostatic model of assessment for beta-cell function (HOMA-β). However, whether HOMA-β is a suitable biomarker for comparisons between hypoglycemic drugs with different mechanisms of action remains unclear. Therefore, we conducted a meta-analysis to compare the effects of DPP-4 inhibitors and other classes of hypoglycemic drugs on HOMA-β and proinsulin-to-insulin ratio (PIR).

Methods

We searched MEDLINE, CENTRAL, and Ichushi-web for the period of 1966 to May 2020. We collected randomized, controlled clinical trials in patients with type 2 diabetes mellitus comparing DPP-4 inhibitors and other classes of hypoglycemic agents [α-glucosidase inhibitors (α-GIs), glucagon-like peptide-1 (GLP-1) analogues, metformin, sodium-glucose cotransporter 2 (SGLT2) inhibitors, sulfonylureas, or thiazolidinediones]. Weighted mean differences and 95% confidence intervals of changes in HOMA-β or PIR during study periods were calculated for pairwise comparisons.

Results

Thirty-seven and 21 relevant trials were retrieved for comparisons of HOMA-β and PIR, respectively. HOMA-β and PIR consistently showed superiority of DPP-4 inhibitors compared with α-GIs. Both biomarkers consistently supported inferiority of DPP-4 inhibitors compared with GLP-1 analogues. However, PIR showed inferiority of DPP-4 inhibitors compared with metformin, and superiority compared with SGLT2 inhibitors, whereas HOMA-β showed no significant differences between DPP-4 inhibitors and the two other agents.

Conclusion

DPP-4 inhibitors appear to be superior to α-GIs but inferior to GLP-1 analogues in preservation of beta-cell function assessed by either HOMA-β or PIR. DPP-4 inhibitors seem to be superior to SGLT2 inhibitors but inferior to metformin on islet function assessed only by PIR. Because HOMA-β and PIR may indicate different aspects of beta-cell function, results of beta-cell function preserving effects of hypoglycemic agents should be interpreted with caution.

GPR55-dependent stimulation of insulin secretion from isolated mouse and human islets of Langerhans

AIMS

The novel cannabinoid receptor GPR55 is expressed by rodent islets and it has been implicated in β-cell function in response to a range of ligands. This study evaluated the effects of GPR55 ligands on intracellular calcium ([Ca²⁺ ]_i ) levels and insulin secretion from islets isolated from GPR55 knockout (GPR55 ^-/- ) mice, age-matched wildtype (WT) mice and human pancreas.

MATERIALS AND METHODS

GPR55 expression was determined by Western blotting and fluorescent immunohistochemistry. Changes in [Ca²⁺ ]_i were measured by Fura-2 microfluorimetry. Dynamic insulin secretion was quantified by radioimmunoassay following perifusion of isolated islets. RhoA activity was monitored using a Rho binding domain pull down assay.

RESULTS

Western blotting indicated that MIN6 β-cells, mouse and human islets express GPR55 and its localization on human β-cells was demonstrated by fluorescent immunohistochemistry. The pharmacological GPR55 agonist O-1602 (10 μM) significantly stimulated [Ca²⁺ ]_i and insulin secretion from WT mouse islets and these stimulatory effects were abolished in islets isolated from GPR55 ^-/- mice. In contrast, while the putative endogenous GPR55 agonist lysophosphatidylinositol (LPI, 5 µM) and the GPR55 antagonist cannabidiol (CBD, 1 µM) also elevated [Ca²⁺ ]_i and insulin secretion, these effects were sustained in islets from GPR55 ^-/- mice. Stimulatory effects of O-1602 on [Ca²⁺ ]_i and insulin secretion were also observed in experiments using human islets, but O-1602 did not activate RhoA in MIN6 β-cells.

CONCLUSIONS

Our results therefore suggest that GPR55 plays an important role in the regulation of mouse and human islet physiology, but LPI and CBD exert stimulatory effects on islet function by a GPR55-independent pathway(s).

The effects of Leptin and Adiponectin on Pdx1, Foxm1, and PPARγ Transcription in Rat Islets of Langerhans

BACKGROUND

Leptin and adiponectin are two hormones, which are released from adipocytes in order to control energy expenditure. Both hormones are also involved in glucose homeostasis through control of insulin secretion from pancreatic islets. Since Pdx1, PPARγ, and foxm1 play important roles in islets function, it is essential to understand how these genes are regulated in the islets of Langerhans.

OBJECTIVES

We have designed an experiment to identify the effect of leptin and adiponectin treatment on Pdx1, PPARγ, and foxm1 transcription.

MATERIALS AND METHODS

Islets were isolated from adult male rats by collagenase and incubated with different concentrations of leptin and adiponectin for 24 hours. Next, by means of real time PCR, we evaluated the gene transcription related to a housekeeping gene. The effect of leptin and adiponectin on insulin secretion was evaluated by ELISA.

RESULTS

Leptin decreased PPARγ transcription and insulin secretion, while adiponectin significantly increased Pdx1 and PPARγ transcription and insulin secretion in rat islets. The transcription of foxm1 did not change in the islet cells treated with leptin or adiponectin.

CONCLUSIONS

These findings indicate the possibility that Pdx1 and PPARγ transcription is a mediator of leptin and adiponectin function in control of insulin secretion and glucose homeostasis in pancreatic islets.

Pleiotropic effects of thiazolidinediones: implications for the treatment of patients with type 2 diabetes mellitus

Thiazolidinediones (TZDs) are insulin-sensitizing antidiabetes agents that act through the peroxisome proliferator-activated receptor-γ to cause a durable improvement in glycemic control in patients with type 2 diabetes mellitus. Although less well recognized, TZDs also exert a protective effect on β-cell function. In addition to their beneficial effects on glucose homeostasis, TZDs-especially pioglitazone-exert a number of other pleiotropic effects that make them ideal agents as monotherapy or in combination with other oral agents, glucagon-like peptide-1 analogs, or insulin. Pioglitazone improves endothelial dysfunction, reduces blood pressure, corrects diabetic dyslipidemia, and reduces circulating levels of inflammatory cytokines and prothrombotic factors. Pioglitazone also redistributes fat and toxic lipid metabolites in muscle, liver, β cells, and arteries, and deposits the fat in subcutaneous adipocytes where it cannot exert its lipotoxic effects. Consistent with these antiatherogenic effects, pioglitazone reduced major adverse cardiac event endpoints (ie, mortality, myocardial infarction, and stroke) in the Prospective Pioglitazone Clinical Trial in Macrovascular Events and in a meta-analysis of all other published pioglitazone trials. Pioglitazone also mobilizes fat out of the liver, improving liver function and histologic abnormalities in patients with nonalcoholic fatty liver disease and nonalcoholic steatohepatitis. Pioglitazone also reduces proteinuria, all-cause mortality, and cardiovascular events in patients with type 2 diabetes mellitus with a reduced glomerular filtration rate. These benefits must be weighed against the side effects of the drug, including weight gain, fluid retention, atypical fractures, and, possibly, bladder cancer. When low doses of pioglitazone are used (eg, 7.5-30 mg/d) with gradual titration, and physician recognition of the potential side effects are applied, the risk-to-benefit ratio is very favorable. Despite having similar effects on glycemic control, pioglitazone and rosiglitazone appear to have different effects on cardiovascular outcomes. Rosiglitazone has been associated with an increased risk of myocardial infarction, and its use in the United States is restricted because of cardiovascular safety concerns.

PPARs: history and advances

Peroxisome proliferator-activated receptors (PPARs) are members of the steroid hormone receptor superfamily, discovered in 1990. To date, three PPAR subtypes have been identified; PPARα, PPAR β/δ, and PPARγ. These receptors share a high degree of homology but differ in tissue distribution and ligand specificity. PPARs have been implicated in the etiology as well as treatment of several important diseases and pathological conditions such as diabetes, inflammation, senescence-related diseases, regulation of fertility, and various types of cancer. Consequently, significant efforts to discover novel PPAR roles and delineate molecular mechanisms involved in their activation and repression as well as develop safer and more effective PPAR modulators, as therapeutic agents to treat a myriad of diseases and conditions, are underway. This volume of Methods in Molecular Biology contains details of experimental protocols used in researching these receptors.

Glitazones exert multiple effects on β-cell stimulus-secretion coupling

Earlier studies suggest that glitazones exert beneficial effects in patients with type 2 diabetes by directly affecting insulin secretion of β-cells, besides improving the effectiveness of insulin in peripheral tissues. The effects of glitazones on stimulus-secretion coupling (SSC) are poorly understood. We tested the influence of troglitazone and pioglitazone on different parameters of SSC, including insulin secretion (radioimmunoassay), cell membrane potential, various ion currents (patch-clamp), mitochondrial membrane potential (ΔΨ), and cytosolic Ca(2+) concentration (fluorescence). Troglitazone exerted stimulatory, inhibitory, or no effects on insulin secretion depending on the drug and glucose concentration. It depolarized the ΔΨ, thus lowering ATP production, which resulted in opening of ATP-dependent K(+) channels (K(ATP) channels) and reduced insulin secretion. However, it also exerted direct inhibitory effects on K(ATP) channels that can explain enhanced insulin secretion. Troglitazone also inhibited the currents through voltage-dependent Ca(2+) and K(+) channels. Pioglitazone was less effective than troglitazone on all parameters tested. The effects of both glitazones were markedly reduced in the presence of bovine serum albumin. Glitazones exert multiple actions on β-cell SSC that have to be considered as undesired side effects because the influence of these compounds on β-cells is not controllable. The final effect on insulin secretion depends on many parameters, including the actual glucose and drug concentration, protein binding of the drug, and the drug by itself. Troglitazone and pioglitazone differ in their influence on SSC. It can be assumed that the effects of pioglitazone on β-cells are negligible under in vivo conditions.

Suppression effects of AICAR on insulin secretion involved in peroxisome proliferator-activated receptor gamma changes in INS-1 cells

BACKGROUND

AMP-activated protein kinase (AMPK) activation is known to attenuate glucose-stimulated insulin secretion (GSIS) in pancreatic beta cells. However, the underlying mechanisms are poorly understood. The purpose of this study was to examine the effects of AMPK activation on insulin secretion and to determine whether peroxisome proliferator-activated receptors (PPAR) are involved in the effects on INS-1 cells.

METHODS

INS-1 cells, insulinoma cell lines, were treated with an activator (AICAR) or inhibitor (Compound C) of AMPK as well as inhibitors of PPAR [MK886 and biphenol A diglycidyl ether (BADGE)] for different treatment times.

RESULTS

AICAR-induced AMPK activation significantly attenuated GSIS as well as insulin content. Meanwhile, AMPK activation increased the mRNA levels of both PPARalpha and PPARgamma. However, with regard to DNA binding, AMPK activation upregulated PPARgamma only, and it was possible to reduce the increment with the AMPK inhibitor. Moreover, the AICAR-induced suppression of insulin secretion can be counteracted by the PPARgamma inhibitor, BADGE but not the PPARalpha inhibitor.

CONCLUSIONS

AICAR-induced glucose-stimulated insulin secretion reduction correlates mainly with PPARgamma changes.

Serine-385 phosphorylation of inwardly rectifying K+ channel subunit (Kir6.2) by AMP-dependent protein kinase plays a key role in rosiglitazone-induced closure of the K(ATP) channel and insulin secretion in rats

AIMS/HYPOTHESIS

Rosiglitazone, an insulin sensitiser, not only improves insulin sensitivity but also enhances insulin secretory capacity by ameliorating gluco- and lipotoxicity in beta cells. Rosiglitazone can stimulate insulin secretion at basal and high glucose levels via a phosphatidylinositol 3-kinase (PI3K)-dependent pathway. We hypothesised that regulation of phosphorylation of the ATP-sensitive potassium (K(ATP)) channel might serve as a key step in the regulation of insulin secretion.

METHODS

Insulin secretory responses were studied in an isolated pancreas perfusion system, cultured rat islets and MIN6 and RINm5F beta cells. Signal transduction pathways downstream of PI3K were explored to link rosiglitazone to K(ATP) channel conductance with patch clamp techniques and insulin secretion measured by ELISA.

RESULTS

Rosiglitazone stimulated AMP-activated protein kinase (AMPK) activity and induced inhibition of the K(ATP) channel conductance in islet beta cells; both effects were blocked by the PI3K inhibitor LY294002. Following stimulation of AMPK by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR), a pharmacological activator, both AICAR-stimulated insulin secretion and inhibition of K(ATP) channel conductance were unaffected by LY294002, indicating that AMPK activation occurs at a site downstream of PI3K activity. The serine residue at amino acid position 385 of Kir6.2 was found to be the substrate phosphorylation site of AMPK when activated by rosiglitazone or AICAR.

CONCLUSIONS/INTERPRETATION

Our data indicate that PI3K-dependent activation of AMPK is required for rosiglitazone-stimulated insulin secretion in pancreatic beta cells. Phosphorylation of the Ser(385) residue of the Kir6.2 subunit of the K(ATP) channel by AMPK may play a role in insulin secretion.

Insulin-dependent actions of pioglitazone in newly diagnosed, drug naïve patients with type 2 diabetes

The aim of this study was to study the effects of pioglitazone on several diabetic parameters with subjects possessing distinct levels of insulin. Treatment naive patients with type 2 diabetes received 15-30 mg/day pioglitazone monotherapy. At 3 months, levels of insulin, C-peptide, HbA1c, HOMA-R, HOMA-B and BMI were compared with those at baseline between the low (below 5.9 microU/ml, n = 48), medium (11.9-6 microU/ml n = 39) and high (above 12 microU/ml, n = 33) insulin groups. At baseline, differences existed in the levels of HbA1c, insulin, C-peptide, HOMA-R, HOMA-B, and BMI between these groups. In the high-insulin group significant reductions of insulin/C-peptide levels were observed, while in the low-insulin group significant increases of insulin/C-peptide were observed. In the medium-insulin group, no significant changes were observed. In contrast, the HbA1c levels significantly and similarly decreased in all the groups. Significant correlations between the changes of insulin/C-peptide levels with pioglitazone and the baseline insulin/C-peptide levels were observed. HOMA-R showed greater reductions in the high-insulin group, while HOMA-B showed greater increases in the low-insulin group in comparison to other groups. Multiple regression analysis revealed that the baseline insulin level is the predominant determinant of the changes of insulin levels with pioglitazone. These results suggest that pioglitazone appears to have two effects: to reduce insulin resistance (and lower insulin) and to improve beta-cell function (and increase insulin). The predominance of these effects appears to be determined by the insulin levels. Based on these data, a novel physiological model showing that pioglitazone may shift the natural history of diabetes toward an earlier stage (rejuvenation of beta-cell function) will be presented.

Beta-cell preservation with thiazolidinediones

Progressive beta-cell dysfunction and beta-cell failure are fundamental pathogenic features of type 2 diabetes. Ultimately, the development and continued progression of diabetes is a consequence of the failure of the beta-cell to overcome insulin resistance. Strategies that aim to prevent diabetes must, therefore, ultimately aim to stabilize the progressive decline of the beta-cell. Clinical study evidence from several sources now suggests that thiazolidinediones (TZDs) have profound effects on the beta-cell, such as improving insulin secretory capacity, preserving beta-cell mass and islet structure and protecting beta-cells from oxidative stress, as well as improving measures of beta-cell function, such as insulinogenic index and homeostasis model assessment of beta-cell function (HOMA-%B). Furthermore, intervention studies suggest that TZDs have the potential to delay, stabilize and possibly even prevent the onset on diabetes in high-risk individuals, and these effects appear to accompany improvements in beta-cell function. Here, we review the evidence, from in vitro studies to large intervention trials, for the effects of TZDs on beta-cell function and the consequences for glucose-lowering therapy.

Effect of pioglitazone on the metabolic and hormonal response to a mixed meal in type II diabetes

We explored the mechanisms by which a 4-month, placebo-controlled pioglitazone treatment (45 mg/day) improves glycemic control in type II diabetic patients (T2D, n=27) using physiological testing (6-h mixed meal) and a triple tracer technique ([6,6-(2)H(2)]glucose infusion, (2)H(2)O and [6-(3)H]glucose ingestion) to measure endogenous glucose production (EGP), gluconeogenesis (GNG), insulin-mediated glucose clearance and beta-cell glucose sensitivity (by c-peptide modeling). Compared to sex/age/weight-matched non-diabetic controls, T2D patients showed inappropriately (for prevailing insulinemia) raised glucose production (1.05[0.53] vs 0.71[0.36]mmol min(-1) kg(ffm)(-1) pM, P=0.03) because of enhanced GNG (73.1+/-2.4 vs 59.5+/-3.6%, P<0.01) persisting throughout the meal, reduced insulin-mediated glucose clearance (6[5] vs 12[13]ml min(-1) kg(ffm)(-1) nM(-1), P<0.005), and impaired beta-cell glucose-sensitivity (27[38] vs 71[37]pmol min(-1) m(-2) mM(-1), P=0.002). Compared to placebo, pioglitazone improved glucose overproduction (P=0.0001), GNG and glucose underutilization (P=0.05) despite lower insulinemia. GNG improvement was quantitatively related to raised adiponectin. beta-cell glucose sensitivity was unchanged. In mild-to-moderate T2D, pioglitazone monotherapy decreased fasting and post-prandial glycemia, principally via inhibition of gluconeogenesis, improved hepatic and peripheral insulin resistance.

Effect of PPAR-gamma activation and inhibition on glucose-stimulated insulin release in INS-1e cells

Peroxisome proliferator-activated receptor (PPAR)-gamma is expressed in human beta-cells and in the rat beta-cell line INS-1. Previous studies have suggested that PPAR-gamma agonism (e.g., thiazolidinediones) enhances glucose-stimulated insulin secretion (GSIS) from islets or INS-1 cells. We tested the direct effect on insulin release by INS-1e of a PPAR-gamma agonist (Ro4389679-000-001 at 0.2 and 0.4 micromol/l) and a PPAR-gamma antagonist (SR202 at 0.2 and 0.4 mmol/l). Cells were incubated in 11 mmol/l glucose for 96 h and then challenged with 3.3, 7.5, 11.0, and 20.0 mmol/l glucose for 1 h. Under these control conditions, insulin concentrations in the medium rose from 19 +/- 4 ng/ml (mean +/- SE) to 82 +/- 5, 107 +/- 11, and 103 +/- 10 ng/ml (P <0.0001 by ANOVA). Preincubation for 48 h with the PPAR-gamma agonist potentiated GSIS (to 154 +/- 14 and 156 +/- 12 ng/ml at 20 mmol/l glucose, P <0.01). Cell insulin content was not altered by either acute glucose challenge or PPAR-gamma agonist coincubation. Preincubation for 48 h with SR202 at the higher dose caused a 30% inhibition of GSIS, with no change in cell insulin contents. When cells were preincubated with 11 mmol/l glucose plus 1 mmol/l oleate, GSIS was significantly potentiated (by 30%, P <0.0001); adding Ro4389679-000-001 or SR202 to these preincubations reduced GSIS to the respective levels seen in the absence of oleate (P <0.0001 for both effects). In conclusion, INS-1e cells display a PPAR-gamma tone that is symmetrically modulated and competitively stimulated by oleate.

Pathways in beta-cell stimulus-secretion coupling as targets for therapeutic insulin secretagogues

Physiologically, insulin secretion is subject to a dual, hierarchal control by triggering and amplifying pathways. By closing ATP-sensitive K+ channels (KATP channels) in the plasma membrane, glucose and other metabolized nutrients depolarize beta-cells, stimulate Ca2+ influx, and increase the cytosolic concentration of free Ca2+ ([Ca2+]i), which constitutes the indispensable triggering signal to induce exocytosis of insulin granules. The increase in beta-cell metabolism also generates amplifying signals that augment the efficacy of Ca2+ on the exocytotic machinery. Stimulatory hormones and neurotransmitters modestly increase the triggering signal and strongly activate amplifying pathways biochemically distinct from that set into operation by nutrients. Many drugs can increase insulin secretion in vitro, but only few have a therapeutic potential. This review identifies six major pathways or sites of stimulus-secretion coupling that could be aimed by potential insulin-secreting drugs and describes several strategies to reach these targets. It also discusses whether these perspectives are realistic or theoretical only. These six possible beta-cell targets are 1) stimulation of metabolism, 2) increase of [Ca2+]i by closure of K+ ATP channels, 3) increase of [Ca2+]i by other means, 4) stimulation of amplifying pathways, 5) action on membrane receptors, and 6) action on nuclear receptors. The theoretical risk of inappropriate insulin secretion and, hence, of hypoglycemia linked to these different approaches is also envisaged.

Pleiotropic effects of thiazolidinediones: taking a look beyond antidiabetic activity

Thiazolidinediones (TZD) [Troglitazone (TRO), Pioglitazone (PGZ), Rosiglitazone, (RGZ)] are a novel class of antidiabetic drugs for patients with Type-2 diabetes mellitus (T2DM) able to decrease blood glucose, working through a reduction of insulin resistance. The family of TZD exerts its effect specifically bound to peroxisome proliferator-activated receptor y (PPARy). This is a member of the nuclear hormone receptor superfamily of ligand-dependent transcription factors, together with PPARalpha and deltabeta. Although PPARgamma is essentially expressed in adipose tissue, it has also been found in endothelial cells, macrophages, vascular smooth muscle cells, glomerular mesangial cells, hepatic stellate cells and in several cancer cell lines. In these cells, the PPARgamma activation by TZD determines modulatory effects on growth factor release, production of cytokine, cell proliferation and migration, extracellular matrix remodeling and control on cell cycle progression and differentiation. In addition, TZD have been shown to have a potent antioxidant effect. This review, taking a quick look beyond the antidiabetic activity of PPARgamma, shows the dramatic ranging of medical implications that the use of TZD could have modulating the PPARgamma activity in several diseases with a strong social impact, such as insulin resistance syndrome, chronic inflammation, atherosclerosis and cancer.

Impact of PPARgamma overexpression and activation on pancreatic islet gene expression profile analyzed with oligonucleotide microarrays

Peroxisome proliferator-activated receptor-gamma (PPARgamma) serves as a target for the thiazolidinedione class of antidiabetic drugs and is an important regulator of adipose tissue differentiation. By contrast, the principal target genes for PPARgamma in the pancreatic islet and the impact of their induction on insulin secretion are largely undefined. Here, we show that mRNAs encoding both isoforms of rodent PPARgamma, gamma1 and gamma2, are expressed in primary rat islets and are upregulated by overexpresssion of the lipogenic transcription factor sterol response element-binding protein 1c. Unexpectedly, however, oligonucleotide microarray analysis demonstrates that graded activation of PPARgamma achieved with 1) the thiazolidinedione GW-347845, 2) transduction with adenoviral PPARgamma1, or 3) a combination of both treatments progressively enhances the expression of genes involved in fatty acid oxidation and transport. Moreover, maximal activation of PPARgamma1 reduces islet triglyceride levels and enhances the oxidation of exogenous palmitate while decreasing glucose oxidation, cellular ATP content, and glucose-, but not depolarization-stimulated, insulin secretion. We conclude that, in the context of the pancreatic islet, the principal response to PPARgamma expression and activation is the activation of genes involved in the disposal, rather than the synthesis, of fatty acids. Although fatty acid oxidation may have beneficial effects on beta-cell function in the longer term by countering beta-cell "lipotoxicity," the acute response to this metabolic shift is a marked inhibition of insulin secretion.

Expression and functional activity of PPARgamma in pancreatic beta cells

Rosiglitazone is an agonist of peroxisome proliferator activated receptor-gamma (PPARgamma) and ameliorates insulin resistance in type II diabetes. In addition, it may also promote increased pancreatic beta-cell viability, although it is not known whether this effect is mediated by a direct action on the beta cell. We have investigated this possibility. Semiquantitative real-time reverse transcription-polymerase chain reaction analysis (Taqman) revealed that freshly isolated rat islets and the clonal beta-cell line, BRIN-BD11, express PPARgamma, as well as PPARalpha and PPARdelta. The levels of expression of PPARgamma were estimated by reference to adipose tissue and were found to represent approximately 60% (islets) and 30% (BRIN-BD11) of that found in freshly isolated visceral adipose tissue. Western blotting confirmed the presence of immunoreactive PPARgamma in rat (and human) islets and in BRIN-BD11 cells. Transfection of BRIN-BD11 cells with a PPARgamma-sensitive luciferase reporter construct was used to evaluate the functional competence of the endogenous PPARgamma. Luciferase activity was modestly increased by the putative endogenous ligand, 15-deoxy-Delta12,14 prostaglandin J2 (15dPGJ2). Rosiglitazone also caused activation of the luciferase reporter construct but this effect required concentrations of the drug (50-100 microm) that are beyond the expected therapeutic range. This suggests that PPARgamma is relatively insensitive to activation by rosiglitazone in BRIN-BD11 cells. Exposure of BRIN-BD11 cells to the lipotoxic effector, palmitate, caused a marked loss of viability. This was attenuated by treatment of the cells with either actinomycin D or cycloheximide suggesting that a pathway of programmed cell death was involved. Rosiglitazone failed to protect BRIN-BD11 cells from the toxic actions of palmitate at concentrations up to 50 microm. Similar results were obtained with a range of other PPARgamma agonists. Taken together, the present data suggest that, at least under in vitro conditions, thiazolidinediones do not exert direct protective effects against fatty acid-mediated cytotoxicity in pancreatic beta cells.

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

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

Role of peroxisome proliferator-activated receptor-gamma in the glucose-sensing apparatus of liver and beta-cells

Type 2 diabetes develops in the context of both insulin resistance and beta-cell failure. Thiazolidinediones are a class of antidiabetic agents that are known to improve insulin sensitivity in various animal models of diabetes. The improved insulin sensitivity may be achieved either by systemic insulin sensitization or by direct action of peroxisome proliferator-activated receptor (PPAR)-gamma on the transcription of genes involved in glucose disposal. Evidence supporting the direct action of PPAR-gamma on glucose metabolism is observed in the genes involved in insulin-stimulated glucose disposal. We already showed that GLUT2 and beta-glucokinase were directly activated by PPAR-gamma. Recently, we have identified and characterized the functional PPAR response element in the GLUT2 and liver type glucokinase (LGK) promoter of the liver. It is well known that adipose tissue plays a crucial role in antidiabetic action of PPAR-gamma. In addition, PPAR-gamma can directly affect liver and pancreatic beta-cells to improve glucose homeostasis.

Contrasting effects of nateglinide and rosiglitazone on insulin secretion and phospholipase C activation

When stimulated with 6 mmol/L glucose, a minimal, transient insulin secretory response was observed from perifused rat islets. The inclusion of 5 micromol/L nateglinide significantly amplified release. Elevating glucose to 8 or 10 mmol/L resulted in an increasing insulin secretory response that was again markedly potentiated by the further inclusion of nateglinide. The calcium channel antagonist, nitrendipine, abolished secretion to 8 mmol/L glucose plus nateglinide. Unlike nateglinide, rosiglitazone (5 micromol/L), troglitazone (1 to 10 micromol/L), or darglitazone (10 micromol/L), 3 peroxisome proliferator-activated receptor gamma (PPARgamma) agonists, were without any acute stimulatory effect on insulin release in the simultaneous presence of 6 to 10 mmol/L glucose. Glucose (8 to 10 mmol/L) significantly increased inositol phosphate accumulation. Nateglinide amplified this response. Nitrendipine reduced inositol phosphate (IP) accumulation in response to the combination of 8 mmol/L glucose plus 5 micromol/L nateglinide. Rosiglitazone had no effect on IP accumulation. These results confirm the efficacy of nateglinide as a potent glucose-dependent insulin secretagogue that exerts its stimulatory effect, at least in part, through the activation of phospholipase C (PLC). No acute potentiating effect of rosiglitazone on either insulin secretion or IP accumulation could be detected in isolated rat islets.

Combination therapy with PPARgamma and PPARalpha agonists increases glucose-stimulated insulin secretion in db/db mice

Although peroxisome proliferator-activated receptor (PPAR)gamma agonists ameliorate insulin resistance, they sometimes cause body weight gain, and the effect of PPAR agonists on insulin secretion is unclear. We evaluated the effects of combination therapy with a PPARgamma agonist, pioglitazone, and a PPARalpha agonist, bezafibrate, and a dual agonist, KRP-297, for 4 wk in male C57BL/6J mice and db/db mice, and we investigated glucose-stimulated insulin secretion (GSIS) by in situ pancreatic perfusion. Body weight gain in db/db mice was less with KRP-297 treatment than with pioglitazone or pioglitazone + bezafibrate treatment. Plasma glucose, insulin, triglyceride, and nonesterified fatty acid levels were elevated in untreated db/db mice compared with untreated C57BL/6J mice, and these parameters were significantly ameliorated in the PPARgamma agonist-treated groups. Also, PPARgamma agonists ameliorated the diminished GSIS and insulin content, and they preserved insulin and GLUT2 staining in db/db mice. GSIS was further increased by PPARgamma and -alpha agonists. We conclude that combination therapy with PPARgamma and PPARalpha agonists may be more useful with respect to body weight and pancreatic GSIS in type 2 diabetes with obesity.

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.

Insulin resistance, diabetes, and atherosclerosis: thiazolidinediones as therapeutic interventions

The insulin resistance syndrome, a cluster of metabolic abnormalities involving dyslipidemia, hypertension, diabetes, impaired glucose tolerance, and hypercoagulability, carries an increased risk of atherosclerosis. Although interventions targeting elements of this syndrome have dramatically reduced cardiovascular risk, the impact of glucose-lowering has been more disappointing. Thiazolidinediones (TZDs) are a new class of insulin-sensitizing agents that activate the nuclear receptor peroxisome proliferator-activated receptor-g. TZDs may improve not only glucose levels but also other metabolic parameters associated with insulin resistance. The TZD data are reviewed, with a focus on their potential cardiovascular effects.

Biology and toxicology of PPARgamma ligands

The peroxisome proliferator activated receptor-gamma (PPARgamma) is an attractive target for therapeutic intervention, as modulation of PPARgamma-regulated pathways is potentially beneficial in a number of disease areas. This review provides an overview of what is known about the biology of PPARgamma, and an indication of what progress has been made towards drug development in several therapy areas. As well as efficacy, the safety of drugs is of course an important issue, and a substantial volume of preclinical and clinical information has already accumulated for PPARgamma agonists. Here we discuss some of the major toxicology issues with PPARgamma agonists, and give a perspective on likely issues concerning the development of PPARgamma modulators in the future.

Troglitazone ameliorates lipotoxicity in the beta cell line INS-1 expressing PPAR gamma

To elucidate the mechanisms by which troglitazone, which is a direct ligand for peroxisome proliferator-activated receptor (PPAR) gamma, ameliorates insulin resistance, we have demonstrated that PPAR gamma is expressed in a pancreatic beta cell line, INS-1, using reverse transcription-polymerase chain reaction (RT-PCR). We incubated the cells with 5 micromol/l troglitazone and 1 mmol/l of each major free fatty acid (FFA; palmitic acid, oleic acid, and linoleic acid), alone or in combination, for 48 h. After that, we evaluated glucose-stimulated insulin secretion (GSIS) and 25 mmol/l KCl-induced insulin secretion in the presence of diazoxide, which clamps membrane potential. Our results showed: (1) treatment with troglitazone for 48 h caused enhancement of GSIS, although troglitazone significantly suppressed cell viability assessed by MTT assay. (2) In cells co-treated with troglitazone and FFA, troglitazone ameliorated lipotoxicity due to FFA. (3) In the presence of 300 micromol/l diazoxide and 25 mmol/l KCl, troglitazone did not affect the recovery of GSIS in INS-1 cells. These results suggest that insulin secretion from the rat insulinoma cell line, INS-1, is modulated by troglitazone, acting somewhere in the ATP-sensitive K(+) channel pathway, possibly through PPAR gamma.

Thiazolidinediones for the prevention of diabetes in the non-obese diabetic (NOD) mouse: implications for human type 1 diabetes

BACKGROUND

Thiazolidinediones (TZDs) are a recently introduced generation of drugs acting as receptor agonists to reduce insulin resistance and used currently in combination with other hypoglycaemic agents for the treatment of type 2 diabetes. In addition, TZDs possess anti-inflammatory properties that make them of interest for reducing the T-cell inflammation occurring in the islets in type 1 diabetes.

METHODS

The action of TZD treatment on diabetes incidence in the non-obese diabetic (NOD) mouse was studied by investigating the effect of rosiglitazone (RGL) (400 mg/kg body weight by gavage) from 3 weeks of age (soon after weaning) onwards and comparing its effect to that of troglitazone (TGL) given by the same route.

RESULTS

We found that RGL and TGL both significantly reduced diabetes incidence by >50% in the NOD mouse compared to litter-matched control NOD mice (p<0.05 and p<0.01, respectively). However, the withdrawal of TGL from the market due to hepatotoxicity led us to re-analyse our data for toxic liver effects. We found that TGL was more toxic to mice than RGL (causing ten deaths as compared with one death).

CONCLUSION

RGL reduces diabetes incidence in the NOD mouse model of type 1 diabetes. This may be an effect resulting from its action as on inhibitor of pro-inflammatory genes such as cytokines and metabolic proteases. Its use may be considered for trials designed to protect beta-cell function in humans, especially in patients with latent autoimmune diabetes of adults (LADA) and also for disease prevention.

Clinical evidence of thiazolidinedione-induced improvement of pancreatic beta-cell function in patients with type 2 diabetes mellitus

BACKGROUND

There is growing evidence in animal and in vitro studies showing that thiazolidinediones (TZDs) improve pancreatic beta cell (beta-cell) function. The aim of this study was to evaluate the effect of thiazolidinediones on the beta-cell function of patients with Type 2 diabetes mellitus (T2DM) in clinical practice.

PATIENTS AND METHODS

This is an observational, nested case-control study. We identified 28 patients (TZD group), with T2DM, who had had a meal-stimulated C-peptide level documented before the addition of troglitazone to a failing double-therapy regimen with metformin (MET) and sulphonylurea (SU). As a control group (CTRL), we identified 26 patients, with T2DM, who also had had a meal-stimulated C-peptide documented before adding MET to a failing SU monotherapy regimen. We then proceeded to prospectively remeasure their meal-stimulated C-peptide levels and compared the changes over time between the two groups.

RESULTS

Both groups were well matched for age, body mass index (BMI), and HgbA1c before and after treatment. The C-peptide in the TZD group increased significantly during therapy (from 3.2 +/- 0.5 to 4.2 +/- 0.5, p = 0.01), whereas it remained unchanged in the CTRL group (from 4.8 +/- 0.6 to 5.0 +/- 0.5, p = 0.74). The C-peptide/glucose ratio also increased significantly in the TZD group (from 1.9 +/- 0.3 to 3.1 +/- 0.3, p = 0.0003) whereas it remained unchanged in the CTRL group (from 3.4 +/- 0.7 to 3.4 +/- 0.3, p = 0.97). Furthermore, the C-peptide/glucose ratio of the TZD group, which was significantly lower at baseline compared with the CTRL group (1.9 +/- 0.3 vs. 3.4 +/- 0.7, p = 0.04), caught up to its level during treatment (3.1 +/- 0.3 vs. 3.4 +/- 0.3, p = 0.48).

CONCLUSION

Thiazolidinediones seem to induce recovery of pancreatic beta cell function, independently of the correction of glucose toxicity.

Rosiglitazone (BRL 49653) enhances insulin secretory response via phosphatidylinositol 3-kinase pathway

To elucidate the direct effect of rosiglitazone (RSG), a new thiazolidinedione antihyperglycemic agent, on pancreatic insulin secretion, an in situ investigation by rat pancreatic perfusion was performed. At a basal glucose concentration of 6 mmol/l, RSG (0.045-4.5 micromol/l) stimulated insulin release in a dose-dependent manner. In addition, 4.5 micromol/l RSG potentiated the glucose (10 mmol/l)-induced insulin secretion. Both the first and second phases of glucose-induced insulin secretion were significantly enhanced by RSG, by 80.7 and 52.4%, respectively. The effects of RSG on insulin secretion were inhibited by a phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002. In contrast, the glucose-stimulated insulin secretion was not affected by LY294002. The potentiation effect of RSG on glucose-stimulated insulin secretion, in both the first and second phases, was significantly blocked by LY294002. These results suggest that RSG has a direct potentiation effect on insulin secretion in the presence of 10 mmol/l glucose, mediated through PI3K activity. The inability of LY294002 to inhibit glucose-induced insulin secretion suggests that different pathways are responsible for glucose and RSG signaling.

Autoimmune diabetes not requiring insulin at diagnosis (latent autoimmune diabetes of the adult): definition, characterization, and potential prevention

Type 1 diabetes is caused by the immune-mediated destruction of islet insulin-secreting beta-cells. This chronic destructive process is associated with both cellular and humoral immune changes in the peripheral blood that can be detected months or even years before the onset of clinical diabetes. Throughout this prediabetic period, metabolic changes, including altered glucose tolerance and reduced insulin secretion, deteriorate at variable rates and eventually result in clinical diabetes. A fraction of individuals with humoral immunological changes have clinical diabetes that initially is not insulin-requiring. The onset of diabetes in these patients is usually in adult life, and because their diabetes is at least initially not insulin-requiring, they appear clinically to be affected by type 2 diabetes. Such patients probably have the same disease process as patients with type 1 diabetes in that they have similar HLA genetic susceptibility as well as autoantibodies to islet antigens, low insulin secretion, and a higher rate of progression to insulin dependency. These patients are defined as being affected by an autoimmune type of diabetes not requiring insulin at diagnosis, which is also named latent autoimmune diabetes of the adult (LADA). Special attention should be paid to diagnose such patients because therapy may influence the speed of progression toward insulin dependency, and in this respect, efforts should be made to protect residual C-peptide secretion. LADA can serve as a model for designing new strategies for prevention of type 1 diabetes but also as a target group for prevention in its own right.

Effects of high-dose troglitaz one on insulin sensitivity and beta-cell function in Watanabe heritable hyperlipidemic rabbits

To clarify the dose-response effects of troglitazone on insulin sensitivity and beta-cell function, we examined the effects of high-dose troglitazone (100 mg/day per animal, administered as a food admixture) on glucose and insulin metabolism in hyperinsulinemic Watanabe heritable hyperlipidemic (WHHL) rabbits, and compared the results with our previous results with low-dose troglitazone (10 mg /day per animal).

MATERIALS AND METHODS

Glucose and insulin metabolism were quantitatively characterized by a minimal model technique as reported previously.

RESULTS

When troglitazone was administrated at a high dose for 6 months, it reduced hyperinsulinemia as reflected by a reduced basal (steady-state) insulin concentration lb and the insulin response to a glucose load, improved beta-cell function as reflected by decreased second-phase post-hepatic insulin delivery to glucose phi2, and reduced insulin resistance as reflected by increased insulin sensitivity to glucose disposal Si, without affecting glucose tolerance as reflected by an unchanged rate of glucose utilization Kg or insulin-independent glucose disposal Sg. The reductions in Ib and phi2 and the increases in Si in WHHL rabbits treated with a high dose of troglitazone were greater (p<0.05) than those observed in WHHL rabbits treated with a low dose of troglitazone, as assessed by a two-way repeated measures analysis of variance and the Wilcoxon-Mann-Whitney test.

CONCLUSION

In WHHL rabbits, troglitazone dose-dependently reduced hyperinsulinemia, improved beta-cell function, and increased insulin sensitivity.

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.

Inhibitory action of troglitazone, an insulin-sensitizing agent, on the calcium current in cardiac ventricular cells of guinea pig

We investigated the effects of troglitazone, a new orally active hypoglycemic agent, on the voltage-dependent L-type Ca2+ current in single cardiac ventricular myocytes of guinea pigs by the whole-cell voltage clamp technique. Troglitazone blocked the Ca2+ currents in a concentration-dependent manner. The inhibitory effect was more potent at the holding potential (HP) of - 50 mV than at - 80 mV. The half-maximum inhibiting concentration (IC50) of troglitazone was 0.8 microM with the Hill coefficient of 0.84 at -50 mV HP. In contrast, the IC50 value was higher than 10 microM at -80 mV HP. These results suggest that troglitazone at therapeutic concentrations inhibit the Ca2+ channels and may exert cardioprotective effects in diabetic conditions.

Troglitazone but not pioglitazone affects ATP-sensitive K(+) channel activity

We compared the effects of the two thiazolidinedione derivatives, troglitazone and pioglitazone, on ATP-sensitive K(+) (K(ATP)) channel activities. Pancreatic beta-cell type and cardiac type K(ATP) channels were reconstituted in COS-1 cells (SV 40-transformed African green monkey kidney (AGMK) cells) by heterologously expressing sulfonylurea receptor 1 (SUR1) plus Kir6.2 and sulfonylurea receptor 2A (SUR2A) plus Kir6.2, respectively. Troglitazone inhibited [86Rb(+)] efflux in both K(ATP) channel types in the presence of metabolic inhibitors, which was confirmed by electrophysiological techniques. The [86Rb(+)] efflux increased by the channel openers diazoxide and pinacidil was abolished by troglitazone. In contrast, pioglitazone did not affect these channel activities in either type K(ATP) channel. These results suggest that troglitazone modulates the various cellular functions including insulin secretion by inhibiting the K(ATP) channels, while pioglitazone has no effect on K(ATP) channel activity.

Troglitazone inhibits expression of the phosphoenolpyruvate carboxykinase gene by an insulin-independent mechanism

Troglitazone is an oral insulin-sensitizing drug used to treat patients with type 2 diabetes. A major feature of this hyperglycemic state is the presence of increased rates of hepatic gluconeogenesis, which troglitazone is able to ameliorate. In this study, we examined the molecular basis for this property of troglitazone by exploring the effects of this compound on the expression of the two genes encoding the major regulatory enzymes of gluconeogenesis, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) in primary cultures of rat hepatocytes. Insulin is able to inhibit expression of both of these genes, which was verified in our model system. Troglitazone significantly reduced mRNA levels of PEPCK and G6Pase in rat hepatocytes isolated from normal and Zucker-diabetic rats, but to a lesser extent than that observed with insulin. Interestingly, troglitazone was unable to reduce cAMP-induced levels of PEPCK mRNA, suggesting that the molecular mechanism whereby troglitazone exerted its effects on gene expression differed from that of insulin. This was further supported by the observation that troglitazone was able to reduce PEPCK mRNA levels in the presence of the insulin signaling pathway inhibitors wortmannin, rapamycin, and PD98059. These results indicate that troglitazone can regulate the expression of specific genes in an insulin-independent manner, and that genes encoding gluconeogenic enzymes are targets for the inhibitory effects of this drug.

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.

Diazoxide- and leptin-activated K(ATP) currents exhibit differential sensitivity to englitazone and ciclazindol in the rat CRI-G1 insulin-secreting cell line

1. The effects of the antidiabetic agent englitazone and the anorectic drug ciclazindol on ATP-sensitive K+ (K(ATP)) channels activated by diazoxide and leptin were examined in the CRI-G1 insulin-secreting cell line using whole cell and single channel recording techniques. 2. In whole cell current clamp mode, the hyperglycaemic agent diazoxide (200 microM) and the ob gene product leptin (10 nM) hyperpolarised CRI-G1 cells by activation of K(ATP) currents. K(ATP) currents activated by either agent were inhibited by tolbutamide, with an IC50 for leptin-activated currents of 9.0 microM. 3. Application of englitazone produced a concentration-dependent inhibition of K(ATP) currents activated by diazoxide (200 microM) with an IC50 value of 7.7 microM and a Hill coefficient of 0.87. In inside-out patches englitazone (30 microM) also inhibited K(ATP) channel currents activated by diazoxide by 90.8+/-4.1%. 4. In contrast, englitazone (1-30 microM) failed to inhibit K(ATP) channels activated by leptin, although higher concentrations (> 30 microM) did inhibit leptin actions. The englitazone concentration inhibition curve in the presence of leptin resulted in an IC50 value and Hill coefficient of 52 microM and 3.2, respectively. Similarly, in inside-out patches englitazone (30 microM) failed to inhibit the activity of K(ATP) channels in the presence of leptin. 5. Ciclazindol also inhibited K(ATP) currents activated by diazoxide (200 microM) in a concentration-dependent manner, with an IC50 and Hill coefficient of 127 nM and 0.33, respectively. Furthermore, application of ciclazindol (1 microM) to the intracellular surface of inside-out patches inhibited K(ATP) channel currents activated by diazoxide (200 microM) by 86.6+/-8.1%. 6. However, ciclazindol was much less effective at inhibiting KATP currents activated by leptin (10 nM). Ciclazindol (0.1-10 microM) had no effect on K(ATP) currents activated by leptin, whereas higher concentrations (> 10 microM) did cause inhibition with an IC50 value of 40 microM and an associated Hill coefficient of 2.7. Similarly, ciclazindol (1 microM) had no significant effect on K(ATP) channel activity following leptin addition in excised inside-out patches. 7. In conclusion, K(ATP) currents activated by diazoxide and leptin show different sensitivity to englitazone and ciclazindol. This may be due to differences in the mechanism of activation of K(ATP) channels by diazoxide and leptin.

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.

Effect of englitazone on KATP and calcium-activated non-selective cation channels in CRI-G1 insulin-secreting cells

1. The effects of englitazone sodium, an antidiabetic agent, on ion channel activity in the CRI-G1 insulin secreting cell line was examined by use of the patch clamp technique. 2. Application of englitazone to the outside of CRI-G1 cells in the whole-cell recording configuration produced concentration-dependent inhibition of KATP currents with an IC50 value of 8 microM. The inhibition of the K+ current was not affected by the removal of Mg2+ ions from or the addition of trypsin to the solution bathing the intracellular surface of the cell membrane. 3. Englitazone also inhibited KATP channel activity in recordings from inside out excise membrane patches. The concentration-dependence of inhibition was identical to that observed in whole-cell recordings and was voltage-independent. Single channel recordings confirmed that neither the absence or presence of Mg2+ ions nor the addition of trypsin at the intracellular surface of the membrane influenced the inhibition of KATP channels by englitazone. 4. Englitazone also inhibited Ca(2+)-activated non-selective cation (NSCa) channels in inside-out patches in a concentration-dependent and voltage-independent manner with an IC50 value of 10 microM. In comparison, the non-sulphonylurea KATP channel blocker ciclazindol produced a slight voltage-dependent inhibition of the NSCa channel at a concentration of 20 microM. 5. In whole-cell recordings englitazone, at a relatively high concentration (50 microM) in comparison with that required to block KATP and NSCa channels, inhibited voltage-activated Ca2+ currents by 33% but did not inhibit voltage-activated K+ and Na+ currents. 6. It is concluded that englitazone is a novel blocker of NSCa and KATP channels. The inhibition of KATP channels occurs following procedures that dissociate sulphonylurea receptor coupling to the channel. The equipotent and voltage-independent inhibition of NSCa and KATP channels by englitazone may indicate a common mechanism of block.

Inhibition of KATP channel activity by troglitazone in CRI-G1 insulin-secreting cells

Patch-clamp recording techniques were used to examine the effect of troglitazone on KATP channel activity in Cambridge rat insulinoma-G1 (CRI-G1) insulin-secreting cells. In both inside-out and outside-out patch recordings, bath application of troglitazone reduced KATP channel activity. This inhibition was independent of the membrane voltage and was poorly reversible. In whole-cell studies, troglitazone inhibited KATP channel currents with an IC50 of 697 +/- 92 nM and an associated Hill coefficient of 1.2 +/- 0.2. In current clamp recordings 10 microM troglitazone depolarised the CRI-G1 cell membrane by 36.8 +/- 3.9 mV with a concomitant decrease in membrane conductance. However, in contrast to the rapid depolarisation produced by tolbutamide, the effects of troglitazone developed more slowly, usually taking 15-20 min to develop.

Acute and chronic effects of troglitazone (CS-045) on isolated rat ventricular cardiomyocytes

Freshly isolated and primary cultured adult rat cardiomyocytes were used to elucidate the mechanism of action of the new oral antidiabetic agent (+/-)-5-[4-(6-hydroxy-2, 5, 7, 8-tetramethyl-chroman-2-yl-methoxy)benzyl]-2,4-thiazolidinedione (troglitazone) on the heart. Interaction with protein kinase C (PKC) and regulation of glucose transport were evaluated as possible sites of drug action. Acute treatment (30 min) of cardiomyocytes with troglitazone did not affect the phorbolester-induced membrane association of PKC-delta and PKC-epsilon, which represent the major isoforms present in these cells. However, under these conditions the phorbolester-mediated increase in membrane associated PKC activity was inhibited by 43 +/- 4% (n = 4) without affecting the basal distribution of PKC activity. In contrast to these findings, troglitazone had no acute effect on basal or insulin-stimulated glucose transport in freshly isolated cardiomyocytes; even after 120 min treatment an unaltered release of lactate was determine in the presence of the drug. After 20 h in serum-free culture troglitazone induced a dose-dependent increase in 2-deoxyglucose uptake reaching a 40-fold stimulation at 5 mumol/l. This was paralleled by a dose-dependent increase of glucose transporter-1 (GLUT1) and GLUT4 protein expression to 320 +/- 80 and 156 +/- 15% of control, respectively. In addition, chronic exposure to troglitazone increased the GLUT4 abundance in a plasma membrane fraction about twofold. These data show that troglitazone exerts multiple effects on cardiomyocytes involving inhibition of PKC and regulation of glucose transporter expression and distribution. We suggest that an increased glucose supply may be beneficial for the diabetic heart and that modulation of PKC-activity could be relevant for improving insulin action in muscle tissue.