Potential role of peroxisome proliferator-activated receptor-alpha in the modulation of glucose-stimulated insulin secretion

Indirect comparison of efficacy and safety of chiglitazar and thiazolidinedione in patients with type 2 diabetes: A meta-analysis

BACKGROUND

Chiglitazar is an emerging pan-agonist of all peroxisome proliferator activated receptors (PPAR)-α, δ and γ, and has therapeutic potential for type 2 diabetes (T2D). However, to date, no clinical studies or meta-analyses have compared the efficacy and safety of chiglitazar and traditional PPAR-γ agonist thiazolidinediones (TZDs). A meta-analysis concerning this topic is therefore required.

AIM

To compare the efficacy and safety of chiglitazar and TZD in patients with T2D.

METHODS

PubMed, Medline, Embase, the Cochrane Central Register of Controlled Trials, Reference Citation Analysis and Clinicaltrial.gov websites were searched from August 1994 to March 2022. Randomized controlled trials (RCTs) of chiglitazar or TZD vs placebo in patients with T2D were included. Indirect comparisons and sensitivity analyses were implemented to evaluate multiple efficacy and safety endpoints of interest.

RESULTS

We included 93 RCTs that compared TZD with placebo and one that compared chiglitazar with placebo. For efficacy endpoints, the augmented dose of chig-litazar resulted in greater reductions in hemoglobin (Hb)A1c [weighted mean difference (WMD) = -0.15%, 95% confidence interval (CI): -0.27 to -0.04%], triglycerides (WMD = -0.17 mmol/L, 95%CI: -0.24 to -0.11 mmol/L) and alanine aminotransferase (WMD = -5.25 U/L, 95%CI: -8.50 to -1.99 U/L), and a greater increase in homeostasis model assessment-β (HOMA-β) (WMD = 17.75, 95%CI: 10.73-24.77) when compared with TZD treatment. For safety endpoints, the risks of hypoglycemia, edema, bone fractures, upper respiratory tract infection, urinary tract infection, and weight gain were all comparable between the augmented dose of chiglitazar and TZD. In patients with baseline HbA1c ≥ 8.5%, body mass index ≥ 30 kg/m2 or diabetes duration < 10 years, the HbA1c reduction and HOMA-β increase were more conspicuous for the augmented dose of chiglitazar compared with TZD.

CONCLUSION

Augmented dose of chiglitazar, a pan-activator of PPARs, may serve as an antidiabetic agent with preferable glycemic and lipid control, better β-cell function preserving capacity, and does not increase the risk of safety concerns when compared with TZD.

Modulation of PPAR signaling disrupts pancreas development in the zebrafish, Danio rerio

Peroxisome Proliferator Activated Receptors (PPARs) are transcription factors that regulate processes such as lipid and glucose metabolism. Synthetic PPAR ligands, designed as therapeutics for metabolic disease, provide a tool to assess the relationship between PPAR activity and pancreas development in vivo, an area that remains poorly characterized. Here, we aim to assess the effects of PPAR agonists and antagonists on gene expression, embryonic morphology and pancreas development in transgenic zebrafish embryos. To evaluate developmental perturbations, we assessed gross body and pancreas morphology at 4 days post fertilization (dpf) in response to developmental exposures with PPARα, PPARγ, and PPARβ/δ agonists and antagonists at 0, 0.01, 0.1, 1, and 10 μM concentrations. All ligand exposures, with the exception of the PPARα agonist, resulted in significantly altered fish length and yolk sac area. PPARγ agonist and antagonist had higher incidence of darkened yolk sac and craniofacial deformities, whereas PPARα antagonist had higher incidence of pericardial edema and death. Significantly reduced endocrine pancreas area was observed in both PPARγ ligands and PPARα agonist exposed embryos, some of which also exhibited aberrant endocrine pancreas morphology. Both PPARβ/δ ligands caused reduced exocrine pancreas length and novel aberrant phenotype, and disrupted gene expression of pancreatic targets pdx1, gcga, and try. Lipid staining was performed at 8 dpf and revealed altered lipid accumulation consistent with isoform function. These data indicate chronic exposure to synthetic ligands may induce morphological and pancreatic defects in zebrafish embryos.

Pathways Enrichment Analysis of Gene Expression Data in Type 2 Diabetes

Profiling genome-wide transcriptional changes with advanced high-throughput transcriptional profiling techniques has led to a revolution in biomedical science. It has been challenging to handle the massive data generated by these techniques and draw meaningful conclusions from it. Therefore, computational biologists have developed a number of innovative methods of varying complexity and effectiveness to analyze such complex data. Over the past decade, rich information in pathway repositories has attracted and motivated researchers to incorporate such existing biological knowledge into computational analysis tools to develop what is known as pathway enrichment analysis tools. This chapter describes a new sophisticated pathway enrichment tool that exploits topology of pathway as well as expression of significantly changed genes to identify biologically significant pathways for high-dimensional gene expression datasets. Also, we demonstrate the use of this tool to analyze gene expression data from a type 2 diabetes dataset to identify a list of significantly enriched metabolic pathways.

Association between the membrane transporter proteins and type 2 diabetes mellitus

Introduction: The prevalence rate of diabetes is increasing day by day and the current scenario of the available agents for its treatment has given rise to stimulation in the search for new therapeutic targets and agents. Therefore the present review will examine the role of membrane composition in the pathophysiology of Type 2 Diabetes and the possible therapeutic approaches for this.Areas covered: Glucose transporter proteins (GLUTs) are integral membrane proteins which are responsible for facilitated glucose transport over the plasma membrane into cells. Thus, this chapter is an attempt to interpret the co-relation between membrane transporter proteins and lipid molecules of cell membrane and their implications in type 2 diabetes mellitus. The relationship between the composition controlled flexibility of the membrane in the insertion of GLUTs into cell membrane as well as its fusion with the membrane is the focus of this chapter.Expert opinion: There is increasing data on the central role of phospholipid composition toward T2DM. Plasma membrane lipid composition plays a key role in maintaining the machinery for insulin-independent GLUT insertion into the membrane as well as insulin-dependent GLUT4 containing vesicles. As a therapeutic option, the designing of new chemical entities should be aimed to decrease saturated fatty acids of lipid bilayer phospholipids to target type 2 diabetes mellitus.

Diagnosis and treatment of hyperinsulinaemic hypoglycaemia and its implications for paediatric endocrinology

Glucose homeostasis requires appropriate and synchronous coordination of metabolic events and hormonal activities to keep plasma glucose concentrations in a narrow range of 3.5–5.5 mmol/L. Insulin, the only glucose lowering hormone secreted from pancreatic β-cells, plays the key role in glucose homeostasis. Insulin release from pancreatic β-cells is mainly regulated by intracellular ATP-generating metabolic pathways. Hyperinsulinaemic hypoglycaemia (HH), the most common cause of severe and persistent hypoglycaemia in neonates and children, is the inappropriate secretion of insulin which occurs despite low plasma glucose levels leading to severe and persistent hypoketotic hypoglycaemia. Mutations in 12 different key genes (ABCC8, KCNJ11, GLUD1, GCK, HADH, SLC16A1, UCP2, HNF4A, HNF1A, HK1, PGM1 and PMM2) constitute the underlying molecular mechanisms of congenital HH. Since insulin supressess ketogenesis, the alternative energy source to the brain, a prompt diagnosis and immediate management of HH is essential to avoid irreversible hypoglycaemic brain damage in children. Advances in molecular genetics, imaging methods (¹⁸F–DOPA PET-CT), medical therapy and surgical approach (laparoscopic and open pancreatectomy) have changed the management and improved the outcome of patients with HH. This up to date review article provides a background to the diagnosis, molecular genetics, recent advances and therapeutic options in the field of HH in children.

Prolonged episodes of hypoglycaemia in HNF4A-MODY mutation carriers with IGT. Evidence of persistent hyperinsulinism into early adulthood

AIMS

HNF4A is an established cause of maturity onset diabetes of the young (MODY). Congenital hyperinsulinism can also be associated with mutations in the HNF4A gene. A dual phenotype is observed in HNF4A-MODY with hyperinsulinaemic hypoglycaemia in the neonatal period progressing to diabetes in adulthood. The nature and timing of the transition remain poorly defined. We performed an observational study to establish changes in glycaemia and insulin secretion over a 6-year period. We investigated glycaemic variability and hypoglycaemia in HNF4A-MODY using a continuous glucose monitoring system (CGMS).

METHODS

An OGTT with measurement of glucose, insulin and C-peptide was performed in HNF4A participants with diabetes mellitus (DM) (n = 14), HNF4A-IGT (n = 7) and age- and BMI-matched MODY negative family members (n = 10). Serial assessment was performed in the HNF4A-IGT cohort. In a subset of HNF4A-MODY mutation carriers (n = 10), CGMS was applied over a 72-h period.

RESULTS

There was no deterioration in glycaemic control in the HNF4A-IGT cohort. The fasting glucose-to-insulin ratio was significantly lower in the HNF4A-IGT cohort when compared to the normal control group (0.13 vs. 0.24, p = 0.03). CGMS profiling demonstrated prolonged periods of hypoglycaemia in the HNF4A-IGT group when compared to the HNF4A-DM group (432 vs. 138 min p = 0.04).

CONCLUSIONS

In a young adult HNF4A-IGT cohort, we demonstrate preserved glucose, insulin and C-peptide secretory responses to oral glucose. Utilising CGMS, prolonged periods of hypoglycaemia are evident despite a median age of 21 years. We propose a prolonged hyperinsulinaemic phase into adulthood is responsible for the notable hypoglycaemic episodes.

Molecular mechanisms of congenital hyperinsulinism

Congenital hyperinsulinism (CHI) is a complex heterogeneous condition in which insulin secretion from pancreatic β-cells is unregulated and inappropriate for the level of blood glucose. The inappropriate insulin secretion drives glucose into the insulin-sensitive tissues, such as the muscle, liver and adipose tissue, leading to severe hyperinsulinaemic hypoglycaemia (HH). At a molecular level, genetic abnormalities in nine different genes (ABCC8, KCNJ11, GLUD1, GCK, HNF4A, HNF1A, SLC16A1, UCP2 and HADH) have been identified which cause CHI. Autosomal recessive and dominant mutations in ABCC8/KCNJ11 are the commonest cause of medically unresponsive CHI. Mutations in GLUD1 and HADH lead to leucine-induced HH, and these two genes encode the key enzymes glutamate dehydrogenase and short chain 3-hydroxyacyl-CoA dehydrogenase which play a key role in amino acid and fatty acid regulation of insulin secretion respectively. Genetic abnormalities in HNF4A and HNF1A lead to a dual phenotype of HH in the newborn period and maturity onset-diabetes later in life. This state of the art review provides an update on the molecular basis of CHI.

A MATLAB tool for pathway enrichment using a topology-based pathway regulation score

BACKGROUND

Handling the vast amount of gene expression data generated by genome-wide transcriptional profiling techniques is a challenging task, demanding an informed combination of pre-processing, filtering and analysis methods if meaningful biological conclusions are to be drawn. For example, a range of traditional statistical and computational pathway analysis approaches have been used to identify over-represented processes in microarray data derived from various disease states. However, most of these approaches tend not to exploit the full spectrum of gene expression data, or the various relationships and dependencies. Previously, we described a pathway enrichment analysis tool created in MATLAB that yields a Pathway Regulation Score (PRS) by considering signalling pathway topology, and the overrepresentation and magnitude of differentially-expressed genes (J Comput Biol 19:563-573, 2012). Herein, we extended this approach to include metabolic pathways, and described the use of a graphical user interface (GUI).

RESULTS

Using input from a variety of microarray platforms and species, users are able to calculate PRS scores, along with a corresponding z-score for comparison. Further pathway significance assessment may be performed to increase confidence in the pathways obtained, and users can view Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway diagrams marked-up to highlight impacted genes.

CONCLUSIONS

The PRS tool provides a filter in the isolation of biologically-relevant insights from complex transcriptomic data.

Effects of d-α-tocopherol supplements on lipid metabolism in a high-fat diet-fed animal model

High-fat diet up-regulates either insulin resistance or triglycerides, which is assumed to be related to the expression of peroxisome proliferator-activated receptor (PPAR)-α and PPAR-γ. The beneficial effects of vitamin E on insulin resistance are well known; however, it is not clear if vitamin E with a high-fat diet alters the expression of PPAR-α and PPAR-γ. We investigated the effects of d-α-tocopherol supplementation on insulin sensitivity, blood lipid profiles, lipid peroxidation, and the expression of PPAR-α and PPAR-γ in a high-fat (HF) diet-fed male C57BL/6J model of insulin resistance. The animals were given a regular diet (CON; 10% fat), a HF diet containing 45% fat, or a HF diet plus d-α-tocopherol (HF-E) for a period of 20 weeks. The results showed that the HF diet induced insulin resistance and altered the lipid profile, specifically the triglyceride (TG) and total cholesterol (TC) levels (P < 0.05). In this animal model, supplementation with d-α-tocopherol improved insulin resistance as well as the serum levels of TG and very-low-density lipoprotein-cholesterol (VLDL-C) (P < 0.05). Moreover, the treatment decreased the levels of malondialdehyde (MDA) in the serum and liver while increasing hepatic PPAR-α expression and decreasing PPAR-γ expression. In conclusion, the oral administration of d-α-tocopherol with a high-fat diet had positive effects on insulin resistance, lipid profiles, and oxidative stress through the expression of PPAR-α and PPAR-γ in a high-fat diet-fed male mice.

Human adipose microRNA-221 is upregulated in obesity and affects fat metabolism downstream of leptin and TNF-α

AIMS/HYPOTHESIS

MicroRNAs (miRNAs) are short endogenous RNAs that regulate multiple biological processes including adipogenesis and fat metabolism. We sought to identify miRNAs that correlate with BMI and to elucidate their upstream regulation and downstream targets.

METHODS

Microarray-based expression profiling of 233 miRNAs was performed on subcutaneous abdominal adipose tissue biopsies from 29 non-diabetic Pima Indian participants. Correlation of the expression levels of eight miRNAs with BMI was assessed by quantitative reverse transcription (QRT) PCR in adipose samples from 80 non-diabetic Pima Indians with a BMI of 21.6-54.0 kg/m(2). The upstream regulation of one of these miRNAs, miR-221, was tested by treating cultured human pre-adipocytes with leptin, TNF-α and insulin. Predicted targets of miR-221 were validated using QRT-PCR, immunoblots and luciferase assays. The downstream effects of miR-221 overexpression were assayed by proteomic analysis.

RESULTS

Expression levels of miR-221 were positively correlated with BMI (particularly in women) and fasting insulin concentrations, while the levels of miR-193a-3p and miR-193b-5p were negatively correlated with BMI; other miRNAs did not show significant associations in the 80 samples. miR-221 was downregulated by leptin and TNF-α treatment in cultured human pre-adipocytes. Conversely, miR-221 overexpression upregulated several proteins involved in fat metabolism, mimicking peroxisome proliferator-activated receptor (PPAR) activation. Furthermore, miR-221 directly downregulated the adiponectin receptor 1 (ADIPOR1) and the transcription factor v-ets erythroblastosis virus E26 oncogene homolog 1 (ETS1). Adiponectin signalling is known to promote insulin sensitivity, and ETS1 is crucial for angiogenesis.

CONCLUSIONS/INTERPRETATION

Our data suggest that miR-221 may contribute to the development of the insulin resistance that typically accompanies obesity, by affecting PPAR signalling pathways and by directly downregulating ADIPOR1 and ETS1.

Dietary fish oil normalized glucose-stimulated insulin secretion in isolated pancreatic islets of dyslipemic rats through mechanisms involving glucose phosphorylation, peroxisome proliferator-activated receptor γ and uncoupling protein 2

This study evaluates some possible mechanisms behind the beneficial effects of dietary fish oil (FO) on β cell dysfunction in rats fed a sucrose-rich diet (SRD). Rats were fed a SRD for 6 months. Thereafter, half the rats received a SRD in which corn oil was partially replaced by FO up to 8 months. The other half continued consuming the SRD up to 8 months. A control group was fed a control diet throughout the experimental period. In isolated islets of SRD-fed rats dietary FO normalized the reduced glucose phosphorylation, the altered glucose oxidation, the triglyceride content, the increased protein mass levels of peroxisome proliferator-activated receptor γ (PPARγ) and uncoupling protein 2 without changes in GLUT2 and PPARα. These finding suggest that the changes mentioned above could be involved in the normalization of the altered glucose-stimulated insulin secretion pattern in this nutritional model of dyslipidemia and insulin resistance.

Treatment with CNX-011-67, a novel GPR40 agonist, delays onset and progression of diabetes and improves beta cell preservation and function in male ZDF rats

BACKGROUND

The role of G protein-coupled receptor (GPR40), which is highly expressed in pancreatic beta cells, has been studied extensively in the amelioration of beta cell dysfunction in T2D using rat and mouse islets, beta cell lines and in animal models of diabetes. But its potential as a therapeutic target has not been fully explored. This aim of the study is to evaluate the therapeutic potential of CNX-011-67, a highly selective, potent and orally bioavailable GPR40 agonist, in controlling diabetes and other metabolic parameters.

METHODS

Seven week old male ZDF rats were treated with either vehicle or CNX-011-67, 5 mg/kg twice daily, for seven weeks. The animals were subjected to oral glucose tolerance and insulin tolerance tests. Plasma glucose, insulin, triglyceride, HbA1c, fructosamine and free fatty acids were measured at selected time points. Pancreas from control and treated animals were subjected to insulin and pancreatic and duodenal homeobox 1 (PDX1) immunohistochemistry and were also evaluated by electron microscopy. Also the potential impact of CNX-011-67 on islet insulin secretion, content, ATP levels and markers of both glucose oxidation, beta cell health in rat islets under chronic glucolipotoxic conditions was evaluated.

RESULTS

Treatment of male ZDF rats with CNX-011-67 for 7 weeks significantly enhanced insulin secretion in response to oral glucose load, delayed the onset of fasting hyperglycemia by 3 weeks, reduced nonfasting glucose excursions, fasting free fatty acids and triglyceride levels. A significant increase in PDX1 expression and insulin content and reduction in plasma fructosamine, HOMA-IR, and beta cell apoptosis were observed. CNX-011-67 improves glucose mediated insulin secretion, insulin gene transcription and islet insulin content in cultured rat islets under chronic glucolipotoxic condition. Also enhanced glucose oxidation in the form of increased islet ATP content and overall improvement in beta cell health in the form of reduced expression of stress markers (TXNIP and CHOP mRNA) were observed.

CONCLUSIONS

These findings, suggest that long-term oral therapy with CNX-011-67 could be of clinical value to provide good glycemic control and improve islet beta cell function.

Balanced pan-PPAR activator bezafibrate in combination with statin: comprehensive lipids control and diabetes prevention?

All fibrates are peroxisome proliferators-activated receptors (PPARs)-alpha agonists with ability to decrease triglyceride and increase high density lipoprotein- cholesterol (HDL-C). However, bezafibrate has a unique characteristic profile of action since it activates all three PPAR subtypes (alpha, gamma and delta) at comparable doses. Therefore, bezafibrate operates as a pan-agonist for all three PPAR isoforms. Selective PPAR gamma agonists (thiazolidinediones) are used to treat type 2 diabetes mellitus (T2DM). They improve insulin sensitivity by up-regulating adipogenesis, decreasing free fatty acid levels, and reversing insulin resistance. However, selective PPAR gamma agonists also cause water retention, weight gain, peripheral edema, and congestive heart failure. The expression of PPAR beta/ delta in essentially all cell types and tissues (ubiquitous presence) suggests its potential fundamental role in cellular biology. PPAR beta/ delta effects correlated with enhancement of fatty acid oxidation, energy consumption and adaptive thermogenesis. Together, these data implicate PPAR beta/delta in fuel combustion and suggest that pan-PPAR agonists that include a component of PPAR beta/delta activation might offset some of the weight gain issues seen with selective PPAR gamma agonists, as was demonstrated by bezafibrate studies. Suggestively, on the whole body level all PPARs acting as one orchestra and balanced pan-PPAR activation seems as an especially attractive pharmacological goal. Conceptually, combined PPAR gamma and alpha action can target simultaneously insulin resistance and atherogenic dyslipidemia, whereas PPAR beta/delta properties may prevent the development of overweight. Bezafibrate, as all fibrates, significantly reduced plasma triglycerides and increased HDL-C level (but considerably stronger than other major fibrates). Bezafibrate significantly decreased prevalence of small, dense low density lipoproteins particles, remnants, induced atherosclerotic plaque regression in thoracic and abdominal aorta and improved endothelial function. In addition, bezafibrate has important fibrinogen-related properties and anti-inflammatory effects. In clinical trials bezafibrate was highly effective for cardiovascular risk reduction in patients with metabolic syndrome and atherogenic dyslipidemia. The principal differences between bezafibrate and other fibrates are related to effects on glucose level and insulin resistance. Bezafibrate decreases blood glucose level, HbA1C, insulin resistance and reduces the incidence of T2DM compared to placebo or other fibrates. Currently statins are the cornerstone of the treatment and prevention of cardiovascular diseases related to atherosclerosis. However, despite the increasing use of statins as monotherapy for low density lipoprotein- cholesterol (LDL-C) reduction, a significant residual cardiovascular risk is still presented in patients with atherogenic dyslipidemia and insulin resistance, which is typical for T2DM and metabolic syndrome. Recently, concerns were raised regarding the development of diabetes in statin-treated patients. Combined bezafibrate/statin therapy is more effective in achieving a comprehensive lipid control and residual cardiovascular risk reduction. Based on the beneficial effects of pan-PPAR agonist bezafibrate on glucose metabolism and prevention of new-onset diabetes, one could expect a neutralization of the adverse pro-diabetic effect of statins using the strategy of a combined statin/fibrate therapy.

Balanced pan-PPAR activator bezafibrate in combination with statin: comprehensive lipids control and diabetes prevention?

All fibrates are peroxisome proliferators-activated receptors (PPARs)-alpha agonists with ability to decrease triglyceride and increase high density lipoprotein- cholesterol (HDL-C). However, bezafibrate has a unique characteristic profile of action since it activates all three PPAR subtypes (alpha, gamma and delta) at comparable doses. Therefore, bezafibrate operates as a pan-agonist for all three PPAR isoforms. Selective PPAR gamma agonists (thiazolidinediones) are used to treat type 2 diabetes mellitus (T2DM). They improve insulin sensitivity by up-regulating adipogenesis, decreasing free fatty acid levels, and reversing insulin resistance. However, selective PPAR gamma agonists also cause water retention, weight gain, peripheral edema, and congestive heart failure. The expression of PPAR beta/ delta in essentially all cell types and tissues (ubiquitous presence) suggests its potential fundamental role in cellular biology. PPAR beta/ delta effects correlated with enhancement of fatty acid oxidation, energy consumption and adaptive thermogenesis. Together, these data implicate PPAR beta/delta in fuel combustion and suggest that pan-PPAR agonists that include a component of PPAR beta/delta activation might offset some of the weight gain issues seen with selective PPAR gamma agonists, as was demonstrated by bezafibrate studies. Suggestively, on the whole body level all PPARs acting as one orchestra and balanced pan-PPAR activation seems as an especially attractive pharmacological goal. Conceptually, combined PPAR gamma and alpha action can target simultaneously insulin resistance and atherogenic dyslipidemia, whereas PPAR beta/delta properties may prevent the development of overweight. Bezafibrate, as all fibrates, significantly reduced plasma triglycerides and increased HDL-C level (but considerably stronger than other major fibrates). Bezafibrate significantly decreased prevalence of small, dense low density lipoproteins particles, remnants, induced atherosclerotic plaque regression in thoracic and abdominal aorta and improved endothelial function. In addition, bezafibrate has important fibrinogen-related properties and anti-inflammatory effects. In clinical trials bezafibrate was highly effective for cardiovascular risk reduction in patients with metabolic syndrome and atherogenic dyslipidemia. The principal differences between bezafibrate and other fibrates are related to effects on glucose level and insulin resistance. Bezafibrate decreases blood glucose level, HbA1C, insulin resistance and reduces the incidence of T2DM compared to placebo or other fibrates. Currently statins are the cornerstone of the treatment and prevention of cardiovascular diseases related to atherosclerosis. However, despite the increasing use of statins as monotherapy for low density lipoprotein- cholesterol (LDL-C) reduction, a significant residual cardiovascular risk is still presented in patients with atherogenic dyslipidemia and insulin resistance, which is typical for T2DM and metabolic syndrome. Recently, concerns were raised regarding the development of diabetes in statin-treated patients. Combined bezafibrate/statin therapy is more effective in achieving a comprehensive lipid control and residual cardiovascular risk reduction. Based on the beneficial effects of pan-PPAR agonist bezafibrate on glucose metabolism and prevention of new-onset diabetes, one could expect a neutralization of the adverse pro-diabetic effect of statins using the strategy of a combined statin/fibrate therapy.

Congenital hyperinsulinism: current trends in diagnosis and therapy

Congenital hyperinsulinism (HI) is an inappropriate insulin secretion by the pancreatic β-cells secondary to various genetic disorders. The incidence is estimated at 1/50, 000 live births, but it may be as high as 1/2, 500 in countries with substantial consanguinity. Recurrent episodes of hyperinsulinemic hypoglycemia may expose to high risk of brain damage. Hypoglycemias are diagnosed because of seizures, a faint, or any other neurological symptom, in the neonatal period or later, usually within the first two years of life. After the neonatal period, the patient can present the typical clinical features of a hypoglycemia: pallor, sweat and tachycardia. HI is a heterogeneous disorder with two main clinically indistinguishable histopathological lesions: diffuse and focal. Atypical lesions are under characterization. Recessive ABCC8 mutations (encoding SUR1, subunit of a potassium channel) and, more rarely, recessive KCNJ11 (encoding Kir6.2, subunit of the same potassium channel) mutations, are responsible for most severe diazoxide-unresponsive HI. Focal HI, also diazoxide-unresponsive, is due to the combination of a paternally-inherited ABCC8 or KCNJ11 mutation and a paternal isodisomy of the 11p15 region, which is specific to the islets cells within the focal lesion. Genetics and ¹⁸F-fluoro-L-DOPA positron emission tomography (PET) help to diagnose diffuse or focal forms of HI. Hypoglycemias must be rapidly and intensively treated to prevent severe and irreversible brain damage. This includes a glucose load and/or a glucagon injection, at the time of hypoglycemia, to correct it. Then a treatment to prevent the recurrence of hypoglycemia must be set, which may include frequent and glucose-enriched feeding, diazoxide and octreotide. When medical and dietary therapies are ineffective, or when a focal HI is suspected, surgical treatment is required. Focal HI may be definitively cured when the partial pancreatectomy removes the whole lesion. By contrast, the long-term outcome of diffuse HI after subtotal pancreatectomy is characterized by a high risk of diabetes, but the time of its onset is hardly predictable.

Peroxisome proliferator-activated receptor {alpha} is responsible for the up-regulation of hepatic glucose-6-phosphatase gene expression in fasting and db/db Mice

Glucose-6-phosphatase (G6Pase) is a key enzyme that is responsible for the production of glucose in the liver during fasting or in type 2 diabetes mellitus (T2DM). During fasting or in T2DM, peroxisome proliferator-activated receptor α (PPARα) is activated, which may contribute to increased hepatic glucose output. However, the mechanism by which PPARα up-regulates hepatic G6Pase gene expression in these states is not well understood. We evaluated the mechanism by which PPARα up-regulates hepatic G6Pase gene expression in fasting and T2DM states. In PPARα-null mice, both hepatic G6Pase and phosphoenolpyruvate carboxykinase levels were not increased in the fasting state. Moreover, treatment of primary cultured hepatocytes with Wy14,643 or fenofibrate increased the G6Pase mRNA level. In addition, we have localized and characterized a PPAR-responsive element in the promoter region of the G6Pase gene. Chromatin immunoprecipitation (ChIP) assay revealed that PPARα binding to the putative PPAR-responsive element of the G6Pase promoter was increased in fasted wild-type mice and db/db mice. These results indicate that PPARα is responsible for glucose production through the up-regulation of hepatic G6Pase gene expression during fasting or T2DM animal models.

Antihyperglycemic and antihyperlipidemic action of Cinnamomi Cassiae (Cinnamon bark) extract in C57BL/Ks db/db mice

In previous study, the anti-diabetic effect of Cinnamomi Cassiae extract (Cinnamon bark: Lauraceae) in a type II diabetic animal model (C57BIKsj db/db) has been reported. To explore their mechanism of action, in present study, the effect of cinnamon extract on anti-hyperglycemia and anti-hyperlipidemia was evaluated by measuring the blood glucose levels, serum insulin, and adiponectin levels, serum and hepatic lipids, PPARalpha mRNA expression in liver and PPARgamma mRNA expression in adipose tissue, respectively. Male C57BIKs db/db mice were divided into a diabetic group and cinnamon extract treated group and examined for a period of 12 weeks (200 mg/kg, p.o). The fasting blood glucose and postprandial 2 h blood glucose levels in the cinnamon treated group were significantly lower than those in the control group (p < 0.01), whereas the serum insulin and adiponectin levels were significantly higher in the cinnamon treated group than in the control group (p < 0.05). The serum lipids and hepatic lipids were improved in the cinnamon administered group. Also the PPARalpha mRNA (liver) and PPARgamma mRNA (adipose tissue) expression levels were increased significantly in the cinnamon treated group (p < 0.05). Our results suggest that cinnamon extract significantly increases insulin sensitivity, reduces serum, and hepatic lipids, and improves hyperglycemia and hyperlipidemia possibly by regulating the PPAR-medicated glucose and lipid metabolism.

Nutrient modulation of insulin secretion

The presence of different nutrients regulates the beta-cell response to secrete insulin to maintain glucose in the physiological range and appropriate levels of fuels in different organs and tissues. Glucose is the only nutrient secretagogue capable of promoting alone the release of insulin release. The mechanisms of Insulin secretion are dependent or independent of the closure of ATP-sensitive K(+) channels. In addition, insulin secretion in response to glucose and other nutrients is modulated by several hormones as incretins, glucagon, and leptin. Fatty acids (FAs), amino acids, and keto acids influence secretion as well. The exact mechanism for which nutrients induce insulin secretion is complicated because nutrient signaling shows one of the most complex transduction systems, which exists for the reason that nutrient have to be metabolized. FAs in the absence of glucose induce FA oxidation and insulin secretion in a lesser extent. However, FAs in the presence of glucose produce high concentration of malonyl-CoA that repress FA oxidation and increase the formation of LC-CoA amplifying the insulin release. Long-term exposure to fatty acids and glucose results in glucolipotoxicity and decreases in insulin release. The amino acid pattern produced after the consumption of a dietary protein regulates insulin secretion by generating anaplerotic substrates that stimulates ATP synthesis or by activating specific signal transduction mediated by mTOR, AMPK, and SIRT4 or modulating the expression of genes involved in insulin secretion. Finally, dietary bioactive compounds such as isoflavones play an important role in the regulation of insulin secretion.

PPARalpha activation and increased dietary lipid oppose thyroid hormone signaling and rescue impaired glucose-stimulated insulin secretion in hyperthyroidism

The aim of the study was to investigate the impact of hyperthyroidism on the characteristics of the islet insulin secretory response to glucose, particularly the consequences of competition between thyroid hormone and peroxisome proliferator-activated receptor (PPAR)alpha in the regulation of islet adaptations to starvation and dietary lipid-induced insulin resistance. Rats maintained on standard (low-fat/high-carbohydrate) diet or high-fat/low-carbohydrate diet were rendered hyperthyroid (HT) by triiodothyronine (T(3)) administration (1 mg.kg body wt(-1).day(-1) sc, 3 days). The PPARalpha agonist WY14643 (50 mg/kg body wt ip) was administered 24 h before sampling. Glucose-stimulated insulin secretion (GSIS) was assessed during hyperglycemic clamps or after acute glucose bolus injection in vivo and with step-up and step-down islet perifusions. Hyperthyroidism decreased the glucose responsiveness of GSIS, precluding sufficient enhancement of insulin secretion for the degree of insulin resistance, in rats fed either standard diet or high-fat diet. Hyperthyroidism partially opposed the starvation-induced increase in the glucose threshold for GSIS and decrease in glucose responsiveness. WY14643 administration restored glucose tolerance by enhancing GSIS in fed HT rats and relieved the impact of hyperthyroidism to partially oppose islet starvation adaptations. Competition between thyroid hormone receptor (TR) and PPARalpha influences the characteristics of GSIS, such that hyperthyroidism impairs GSIS while PPARalpha activation (and increased dietary lipid) opposes TR signaling and restores GSIS in the fed hyperthyroid state. Increased islet PPARalpha signaling and decreased TR signaling during starvation facilitates appropriate modification of islet function.

Effects of Momordica charantia on insulin resistance and visceral obesity in mice on high-fat diet

We examined the preventive effect of Momordica charantia L. fruit (bitter melon) on hyperglycemia and insulin resistance in C57BL/6J mice fed with a high-fat (HF) diet. Firstly, mice were divided randomly into two groups: the control group was fed low-fat (LF) diet, whereas the experimental group was fed with a 45% HF diet last for 12 weeks. After 8 week of induction, the HF group was subdivided into six groups and was given orally with or without M. charantia or rosiglitazone 4 weeks afterward. We demonstrated that bitter melon was effective in ameliorating the HF diet-induced hyperglycemia, hyperleptinemia, and decreased the levels of blood glycated hemoglobin (HbA1c) and free fatty acid (FFA) (P<0.01, P<0.05, P<0.05, respectively), whereas increased the adipose PPARgamma and liver PPARalpha mRNA levels. Additionally, bitter melon significantly decreased the weights of epididymal white adipose tissue and visceral fat, and decreased the adipose leptin and resistin mRNA levels. It is tempting to speculate that at least a portion of bitter melon effects is due to be through PPARgamma-mediated pathways, resulting in lowering glucose levels and improving insulin resistance, and partly be through PPARalpha-mediated pathways to improve plasma lipid profiles. This is the first report demonstrating that bitter melon, is a food factor, but not a medicine, itself could influence dual PPARalpha/PPARgamma expression and the mediated gene expression, is effective in ameliorating insulin resistance and visceral obesity.

Persistent hyperinsulinemic hypoglycemia and maturity-onset diabetes of the young due to heterozygous HNF4A mutations

OBJECTIVE

Mutations in the human HNF4A gene encoding the hepatocyte nuclear factor (HNF)-4alpha are known to cause maturity-onset diabetes of the young (MODY), which is characterized by autosomal-dominant inheritance and impaired glucose-stimulated insulin secretion from pancreatic beta-cells. HNF-4alpha has a key role in regulating the multiple transcriptional factor networks in the islet. Recently, heterozygous mutations in the HNF4A gene were reported to cause transient hyperinsulinemic hypoglycemia associated with macrosomia.

RESEARCH DESIGN AND METHODS

Three infants presented with macrosomia and severe hypoglycemia with a positive family history of MODY. The hypoglycemia was confirmed to be due to hyperinsulinism, and all three patients required diazoxide therapy to maintain normoglycemia. Two of the three infants are still requiring diazoxide therapy at 8 and 18 months, whereas one of them had resolution of hyperinsulinemic hypoglycemia at 32 months of age.

RESULTS

Sequencing of the HNF4A gene identified heterozygous mutations in all three families. In family 1, a frameshift mutation L330fsdel17ins9 (c.987 1003del17ins9; p.Leu330fs) was present in the proband; a mutation affecting the conserved A nucleotide of the intron 2 branch site (c.264-21A>G) was identified in the proband of family 2; and finally a nonsense mutation, Y16X (c.48C>G, p.Tyr16X), was found in the proband of family 3.

CONCLUSIONS

Heterozygous HNF4A mutations can therefore cause both transient and persistent hyperinsulinemic hypoglycemia associated with macrosomia. We recommend that macrosomic infants with transient or persistent hyperinsulinemic hypoglycemia should be screened for HNF4A mutations if there is a family history of youth-onset diabetes.

The malonyl CoA axis as a potential target for treating ischaemic heart disease

Cardiovascular disease is the leading cause of death and disability for people living in western societies, with ischaemic heart disease accounting for the majority of this health burden. The primary treatment for ischaemic heart disease consists of either improving blood and oxygen supply to the heart or reducing the heart's oxygen demand. Unfortunately, despite recent advances with these approaches, ischaemic heart disease still remains a major health problem. Therefore, the development of new treatment strategies is still required. One exciting new approach is to optimize cardiac energy metabolism, particularly by decreasing the use of fatty acids as a fuel and by increasing the use of glucose as a fuel. This approach is beneficial in the setting of ischaemic heart disease, as it allows the heart to produce energy more efficiently and it reduces the degree of acidosis associated with ischaemia/reperfusion. Malonyl CoA is a potent endogenous inhibitor of cardiac fatty acid oxidation, secondary to inhibiting carnitine palmitoyl transferase-I, the rate-limiting enzyme in the mitochondrial uptake of fatty acids. Malonyl CoA is synthesized in the heart by acetyl CoA carboxylase, which in turn is phosphorylated and inhibited by 5'AMP-activated protein kinase. The degradation of myocardial malonyl CoA occurs via malonyl CoA decarboxylase (MCD). Previous studies have shown that inhibiting MCD will significantly increase cardiac malonyl CoA levels. This is associated with an increase in glucose oxidation, a decrease in acidosis, and an improvement in cardiac function and efficiency during and following ischaemia. Hence, the malonyl CoA axis represents an exciting new target for the treatment of ischaemic heart disease.

Peroxisome proliferator-activated receptor-alpha regulates the expression of pancreatic/duodenal homeobox-1 in rat insulinoma (INS-1) cells and ameliorates glucose-induced insulin secretion impaired by palmitate

Both peroxisome proliferator-activated receptor-alpha (PPARalpha) and pancreatic/duodenal homeobox-1 (PDX-1) have been reported to be associated with glucose-stimulated insulin secretion (GSIS), but the relationship between PPARalpha and PDX-1 is not yet fully understood. In the present study, we tested the hypothesis that PPARalpha regulates the expression of PDX-1 in beta-cells. Isolated pancreatic islets from Wistar rats and rat pancreatic insulinoma (INS-1) beta-cells were cultured in media supplemented with and without 0.2 or 0.4 mm palmitate, and treated with and without a PPARalpha agonist (fenofibrate) or PPARalpha antagonist (MK886). Results indicated that treatment with fenofibrate significantly enhanced PPARalpha mRNA and protein expression in cells cultured with elevated palmitate concentrations compared with cells that did not receive fenofibrate treatment. In turn, this enhanced expression led to an increase in PDX-1 mRNA and nuclear protein, as well as DNA binding activity of PDX-1 with the insulin promoter. Accordingly, the expression of the PDX-1 downstream targets, insulin and glucose transporter-2, increased, resulting in increased intracellular insulin content and GSIS. Treatment with MK886 inhibited expression of PPARalpha, blocking PPARalpha-regulated PDX-1 expression, and the downstream transcription events of PDX-1. EMSA revealed that nuclear protein might bind with the peroxisome proliferator response element sequence located in the PDX-1 promoter. Collectively, these results demonstrate a regulatory relationship between PPARalpha and PDX-1 in INS-1 cells. Furthermore, PPARalpha activation potentiates GSIS under elevated palmitate conditions possibly via up-regulation of PDX-1. Our findings have potential clinical implications for the use of PPARalpha agonists in the treatment of type 2 diabetes.

Inability to fully suppress sterol synthesis rates with exogenous sterol in embryonic and extraembyronic fetal tissues

The requirement for cholesterol is greater in developing tissues (fetus, placenta, and yolk sac) as compared to adult tissues. Here, we compared cholesterol-induced suppression of sterol synthesis rates in the adult liver to the fetal liver, fetal body, placenta, and yolk sac of the Golden Syrian hamster. Sterol synthesis rates were suppressed maximally in non-pregnant adult livers when cholesterol concentrations were increased. In contrast, sterol synthesis rates were suppressed only marginally in fetal livers, fetal bodies, placentas, and yolk sacs when cholesterol concentrations were increased. To begin to elucidate the mechanism responsible for the blunted response of sterol synthesis rates in fetal tissues to exogenous cholesterol, the ratio of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) to Insig-1 was measured in these same tissues since the ratio of SCAP to the Insigs can impact SREBP processing. The fetal tissues had anywhere from a 2- to 6-fold greater ratio of SCAP to Insig-1 than did the adult liver, suggesting constitutive processing of the SREBPs. As expected, the level of mature, nuclear SREBP-2 was not different in the fetal tissues with different levels of cholesterol whereas it was different in adult livers. These findings indicate that the suppression of sterol synthesis to exogenous sterol is blunted in developing tissues and the lack of response appears to be mediated at least partly through relative levels of Insigs and SCAP.

Pancreatic Insulin Secretion in Rats Fed a Soy Protein High Fat Diet Depends on the Interaction between the Amino Acid Pattern and Isoflavones

Obesity is frequently associated with the consumption of high carbohydrate/fat diets leading to hyperinsulinemia. We have demonstrated that soy protein (SP) reduces hyperinsulinemia, but it is unclear by which mechanism. Thus, the purpose of the present work was to establish whether SP stimulates insulin secretion to a lower extent and/or reduces insulin resistance, and to understand its molecular mechanism of action in pancreatic islets of rats with diet-induced obesity. Long-term consumption of SP in a high fat (HF) diet significantly decreased serum glucose, free fatty acids, leptin, and the insulin:glucagon ratio compared with animals fed a casein HF diet. Hyperglycemic clamps indicated that SP stimulated insulin secretion to a lower extent despite HF consumption. Furthermore, there was lower pancreatic islet area and insulin, SREBP-1, PPARgamma, and GLUT-2 mRNA abundance in comparison with rats fed the casein HF diet. Euglycemic-hyperinsulinemic clamps showed that the SP diet prevented insulin resistance despite consumption of a HF diet. Incubation of pancreatic islets with isoflavones reduced insulin secretion and expression of PPARgamma. Addition of amino acids resembling the plasma concentration of rats fed casein stimulated insulin secretion; a response that was reduced by the presence of isoflavones, whereas the amino acid pattern resembling the plasma concentration of rats fed SP barely stimulated insulin release. Infusion of isoflavones during the hyperglycemic clamps did not stimulate insulin secretion. Therefore, isoflavones as well as the amino acid pattern seen after SP consumption stimulated insulin secretion to a lower extent, decreasing PPARgamma, GLUT-2, and SREBP-1 expression, and ameliorating hyperinsulinemia observed during obesity.

In appreciation of Sir Philip Randle: The glucose-fatty acid cycle

The coordinated regulation of metabolic fuel selection is crucial to energy homeostasis. Philip Randle and his colleagues developed the fundamental concept of interplay between carbohydrate and lipid fuels in relation to the requirement for energy utilisation and storage. Their insight has fashioned current understanding of the regulation of metabolism in health and disease, as well as providing a springboard for research into the roles of lipid derivatives in insulin resistance and, at the transcriptional level, lipid-regulated nuclear hormone receptors.

Regulation of insulin secretion by uncoupling protein

UCPs (uncoupling proteins) can regulate cellular ATP production by uncoupling oxidative phosphorylation. UCP2 is expressed in islet beta-cells and its induction reduces glucose-stimulated insulin secretion. Under physiological conditions, superoxide, formed as a by-product of respiration, activates UCP2. This leads to reduced ATP production, which impairs closure of the ATP-dependent K+ channels to prevent insulin secretion. It is suggested that the physiological role of UCP2 is to prevent excessive superoxide generation through a feedback loop. UCP2 induction may also alter fatty acid metabolism by altering NAD/NADH or by facilitating cycling of fatty acid anions. Recently, UCP2 has been proposed to keep insulin secretion low during starvation, a function under the control of the transcription co-repressor, surtuin-1, which has been shown to bind to the UCP2 promoter. Pathological UCP2 expression or activation may suppress glucose-stimulated insulin secretion to the extent that diabetes onset is hastened. In ob/ob mice, induction of UCP2 at age 5 weeks precedes development of insulin secretion defects and hyperglycaemia. Activating protein kinase A-dependent pathways can normalize insulin secretion in UCP2-overexpressing islets. Conversely, lowering UCP2 expression may promote increased insulin secretion. UCP2 knockout mice were protected from the diabetogenic effects of a high-fat diet and their islets exhibited increased sensitivity to glucose and elevated ATP/ADP. These results support a role for UCP2 as a gene contributing to the pathogenesis of Type 2 diabetes.

Activation of Mouse and Human Peroxisome Proliferator–Activated Receptors (α, β/δ, γ) by Perfluorooctanoic Acid and Perfluorooctane Sulfonate

This study evaluates the potential for perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) to activate peroxisome proliferator-activated receptors (PPARs), using a transient transfection cell assay. Cos-1 cells were cultured in Dulbecco's Minimal Essential Medium (DMEM) with fetal bovine serum in 96-well plates and transfected with mouse or human PPARalpha, beta/delta, or gamma reporter plasmids. Transfected cells were exposed to PFOA (0.5-100 microM), PFOS (1-250 microM), positive controls (i.e., known agonists and antagonists), and negative controls (i.e., DMEM, 0.1% water, and 0.1% dimethyl sulfoxide). Following treatment for 24 h, activity was measured using the Luciferase reporter assay. In this assay, PFOA had more transactivity than PFOS with both the mouse and human PPAR isoforms. PFOA significantly increased mouse and human PPARalpha and mouse PPARbeta/delta activity relative to vehicle. PFOS significantly increased activation of mouse PPARalpha and PPARbeta/delta isoforms. No significant activation of mouse or human PPARgamma was observed with PFOA or PFOS. The PPARalpha antagonist, MK-886, significantly suppressed PFOA and PFOS activity of mouse and human PPARalpha. The PPARgamma antagonist, GW9662, significantly suppressed PFOA activity on the human isoform. In conclusion, this study characterized the dose response and differential activation of mouse and human PPARalpha, beta/delta, gamma by PFOA and PFOS. While this model allows opportunities to compare potential activation by perfluoroalkyl acids, it only evaluates the interaction and activation of the PPAR reporter constructs and is not necessarily predictive of a toxicological response in vivo.

Long-term effect of bezafibrate on pancreatic beta-cell function and insulin resistance in patients with diabetes

OBJECTIVE

Development of insulin resistance (IR) and the progressive failure of the pancreatic beta-cell function (BCF) may be important in the pathogenesis of type 2 diabetes. Influence of peroxisome proliferator-activated receptors ligand bezafibrate on BCF and IR in patients with diabetes is unknown. The present study was aimed to investigate the long-term effect of bezafibrate on these parameters in diabetic patients enrolled in the Bezafibrate Infarction Prevention (BIP) Study.

METHODS

Metabolic and inflammatory parameters were analyzed from stored frozen plasma samples obtained from 351 diabetic patients (168 treated by bezafibrate and 183 by placebo) who completed a 2-year of randomized, double-blind, placebo-controlled study period. The homeostatic indexes of BCF (HOMA-BCF) and IR (HOMA-IR) were calculated according to the homeostasis model of assessment.

RESULTS

Both groups displayed similar baseline characteristics. During follow-up, in the placebo group there was 28% rise of HOMA-IR (p<0.001). In contrast, HOMA-IR in patients in the bezafibrate group did not change (p=0.99). The intergroup differences in HOMA-IR percentage changes were in favor of bezafibrate (p=0.01). HOMA-BCF values have significantly decreased by 13.9% (p=0.04) in patients of placebo group, whereas in patients of bezafibrate group HOMA-BCF was stable during follow-up and its alterations (-2.9%) were non-significant (p=0.59).

CONCLUSIONS

Diabetic patients from the placebo group demonstrated a progressive declining of BCF and an increasing of IR over 2 years of follow-up. These longitudinal changes were attenuated when patients used bezafibrate.

Investigational PPAR-gamma agonists for the treatment of Type 2 diabetes

The tremendous increase in the global prevalence of Type 2 diabetes (T2D) and its conglomeration of metabolic disorders has dramatically intensified the search for innovative therapies to fight this emerging epidemic. Over the last decade, the family of nuclear receptors, especially the peroxisome proliferator-activated receptors (PPARs), has emerged as one of the most important drug targets aimed at combating the metabolic syndrome. Consequently, compounds that activate the PPARs have served as potential therapeutics for the treatment of T2D and the metabolic anomalies associated with this disorder. This review focuses on the currently marketed compounds and also describes the discovery and development of the next generation of PPAR ligands that are under investigation for the potential treatment of T2D and the metabolic syndrome.

The clinical applications of a systems approach

In the second of a two part series, Ahn and colleagues provide a practical discussion of how a systems approach will affect clinical medicine, using diabetes as an example.

Effects of fenofibrate and insulin on the biosynthesis of unsaturated fatty acids in streptozotocin diabetic rats

Both insulin and PPAR-alpha up-modulate hepatic Delta9, Delta6 and Delta5 desaturating enzymes involved in the biosynthesis of mono- and polyunsaturated fatty acids. Currently, we have examined for 9 days the independent and simultaneous effects of daily glargine insulin and fenofibrate administration on the insulinemia, glycemia, hepatic acyl-CoA oxidase activity and mRNAs and enzymatic activities of stearoyl-CoA desaturase-1 (SCD-1) and Delta5 desaturase in streptozotocin diabetic rats. Glargine insulin depressed the hyperglycemia of diabetic rats at 4h, but not after 24h of injection. Fenofibrate increased the radioimmunoreactive insulinemia in non-diabetic rats without changing the glycemia. Insulin increased the mRNAs and activities of SCD-1 and Delta5 desaturase depressed in diabetic rats. Fenofibrate increased acyl-CoA oxidase activity, and the mRNAs and activities of both desaturating enzymes in non-diabetic, diabetic and insulin-treated diabetic rats, but was less effective in the mRNAs modification of diabetic animals. Therefore, insulin, and fenofibrate through PPAR-alpha activation, enhance liver mRNAs and activities of SCD-1 and Delta5 desaturases independently and synergistically through different mechanisms. Insulin and fenofibrate independently increased the 18:1/18:0 ratio in liver lipids, increasing the fluidity of the membranes. The 20:4/18:2 ratio was maintained. Fenofibrate increased palmitic acid, but decreased stearic acid percentage in liver lipids.

Interactive influences of peroxisome proliferator-activated receptor alpha activation and glucocorticoids on pancreatic beta cell compensation in insulin resistance induced by dietary saturated fat in the rat

AIMS/HYPOTHESIS

We sought to elucidate whether excess glucocorticoids and increased dietary lipids act synergistically to impair glucose tolerance and, if so, whether activation of peroxisome proliferator-activated receptor alpha (PPARalpha) has an adverse or beneficial effect on glucose tolerance.

METHODS

Dexamethasone (100 microg kg(-1) body weight day(-1); 5 days) was administered to insulin-resistant rats fed a high-saturated-fat (HF) diet for 4 weeks. The PPARalpha agonist WY14643 was administered (50 mg kg(-1) body weight intraperitoneally) 24 h before sampling. Glucose-stimulated insulin secretion (GSIS) was assessed in vivo after an acute glucose bolus injection, and in vitro using step-up and step-down islet perifusions.

RESULTS

Although neither PPARalpha activation nor dexamethasone alone affected fasting glycaemia in the HF group, dexamethasone in combination with PPARalpha activation elicited marked postabsorptive hyperglycaemia. Dexamethasone treatment of HF rats had little effect on GSIS after an acute glucose challenge in vivo, but induced glucose intolerance. PPARalpha activation augmented GSIS in dexamethasone-treated HF rats in vivo, restoring glucose tolerance. Contrasting with data obtained in vivo, greatly enhanced peak rates of GSIS were observed ex vivo in perifusions of islets from dexamethasone-treated HF rats compared with those from untreated HF rats, an effect attenuated by antecedent PPARalpha activation.

CONCLUSIONS/INTERPRETATION

The study demonstrates that glucocorticoid excess precipitates the development of glucose intolerance in rats maintained on a high-saturated-fat diet. It does this by interrupting the negative feedback loop between insulin sensitivity and secretion in vivo, such that further enhancement of compensatory insulin secretion is not possible. PPARalpha activation restores the coupling between insulin secretion and action.

Gene-nutrient interactions during fetal development

Eukaryotic cells have evolved a complex series of nutrient sensors that protect them from damage caused by acute deficiencies and also mediate adaptive responses to prolonged excess or deficiency of particular nutrients. In adults gene expression is regulated by nutrients interacting with pathways involving mammalian target of rapamycin (mTOR), CCAAT/ enhancer-binding proteins (C/EBPs) and peroxisome proliferator activator proteins (PPARs). These systems are also present in key cells of the developing oocyte, embryo and fetus. In this review we will consider the role of interactions between genes and nutrients during reproduction with a particular emphasis on their possible involvement in the prenatal programming of glucose metabolism in the adult.

Peroxisome proliferator-activated receptor alpha (PPARalpha) potentiates, whereas PPARgamma attenuates, glucose-stimulated insulin secretion in pancreatic beta-cells

Fatty acids (FAs) are known to be important regulators of insulin secretion from pancreatic beta-cells. FA-coenzyme A esters have been shown to directly stimulate the secretion process, whereas long-term exposure of beta-cells to FAs compromises glucose-stimulated insulin secretion (GSIS) by mechanisms unknown to date. It has been speculated that some of these long-term effects are mediated by members of the peroxisome proliferator-activated receptor (PPAR) family via an induction of uncoupling protein-2 (UCP2). In this study we show that adenoviral coexpression of PPARalpha and retinoid X receptor alpha (RXRalpha) in INS-1E beta-cells synergistically and in a dose- and ligand-dependent manner increases the expression of known PPARalpha target genes and enhances FA uptake and beta-oxidation. In contrast, ectopic expression of PPARgamma/RXRalpha increases FA uptake and deposition as triacylglycerides. Although the expression of PPARalpha/RXRalpha leads to the induction of UCP2 mRNA and protein, this is not accompanied by reduced hyperpolarization of the mitochondrial membrane, indicating that under these conditions, increased UCP2 expression is insufficient for dissipation of the mitochondrial proton gradient. Importantly, whereas expression of PPARgamma/RXRalpha attenuates GSIS, the expression of PPARalpha/RXRalpha potentiates GSIS in rat islets and INS-1E cells without affecting the mitochondrial membrane potential. These results show a strong subtype specificity of the two PPAR subtypes alpha and gamma on lipid partitioning and insulin secretion when systematically compared in a beta-cell context.

Effects of dietary mulberry, Korean red ginseng, and banaba on glucose homeostasis in relation to PPAR-alpha, PPAR-gamma, and LPL mRNA expressions

Despite lack of scientific evidences to support its therapeutic efficacy, the use of herbal supplements has significantly increased. The purpose of this study was to evaluate the effects of traditional anti-diabetic herbs on the progress of diabetes in db/db mice, a typical non-insulin-dependent model. Five different experimental diets were as follows: control diet, 0.5% mulberry leaf water extract diet, 0.5% Korean red ginseng diet, 0.5% banaba leaf water extract diet, and 0.5% combination diet (mulberry leaf water extract/Korean red ginseng/banaba leaf water extract, 1:1:1). Blood levels of glucose, insulin, HbA1c, and triglyceride were measured every 2 weeks. At 12 weeks of age, animals were sacrificed, and tissue mRNA levels of PPAR-alpha, PPAR-gamma, and LPL were determined. Results indicated that mulberry leaf water extract, Korean red ginseng, banaba leaf water extract, and the combination of above herbs effectively reduced blood glucose, insulin, TG, and percent HbA1c in study animals (p<0.05). We also observed that the increased expressions of liver PPAR-alpha mRNA and adipose tissue PPAR-gamma mRNA in animals fed diets supplemented with test herbs. The expression of liver LPL mRNA was also increased with experimental diets containing herbs. The efficacy was highest in animals fed the combination diet for all of the markers used. These results suggest that mulberry leaf water extract, Korean red ginseng, banaba leaf water extract, and the combination of these herbs fed at the level of 0.5% of the diet significantly increase insulin sensitivity, and improve hyperglycemia possibly through regulating PPAR-mediated lipid metabolism.

PDH kinase inhibitors: a novel therapy for Type II diabetes?

The pyruvate dehydrogenase multienzyme complex catalyses the oxidative decarboxylation of pyruvate, which is an important regulatory step in oxidative metabolism. Phosphorylation of the E1 (pyruvate decarboxylase) subunit on one of three specific serine residues results in loss of enzyme activity. Four dedicated PDHK (pyruvate dehydrogenase kinase) isoenzymes have been identified, each of which display a distinct tissue-specific expression profile, and have differential regulatory properties. Thus PDHK play a key role in controlling the balance between glucose and lipid oxidation according to substrate supply. Increasing glucose oxidation by inhibiting PDHK may be an effective mechanism to increase glucose utilization; additionally, increasing pyruvate oxidation may further contribute to lowering of glucose level by decreasing the supply of gluconeogenic substrates. A number of PDHK inhibitors are now available to enable this mechanism to be evaluated as a therapy for diabetes. The isoenzyme selectivity profile of AZD7545 and related compounds will be described and evidence for their non-ATP-competitive mode of action presented. These compounds increase PDH activity in vivo, and when dosed chronically, improve glycaemic control in Zucker rats. Furthermore, glucose lowering has been demonstrated in the hyperglycaemic Zucker diabetic fatty rat. This result supports the hypothesis that inhibition of PDHK may be an effective therapy for Type II diabetes.

The MODY1 gene HNF-4alpha regulates selected genes involved in insulin secretion

Mutations in the gene encoding hepatocyte nuclear factor-4alpha (HNF-4alpha) result in maturity-onset diabetes of the young (MODY). To determine the contribution of HNF-4alpha to the maintenance of glucose homeostasis by the beta cell in vivo, we derived a conditional knockout of HNF-4alpha using the Cre-loxP system. Surprisingly, deletion of HNF-4alpha in beta cells resulted in hyperinsulinemia in fasted and fed mice but paradoxically also in impaired glucose tolerance. Islet perifusion and calcium-imaging studies showed abnormal responses of the mutant beta cells to stimulation by glucose and sulfonylureas. These phenotypes can be explained in part by a 60% reduction in expression of the potassium channel subunit Kir6.2. We demonstrate using cotransfection assays that the Kir6.2 gene is a transcriptional target of HNF-4alpha. Our data provide genetic evidence that HNF-4alpha is required in the pancreatic beta cell for regulation of the pathway of insulin secretion dependent on the ATP-dependent potassium channel.

The MODY1 gene HNF-4alpha regulates selected genes involved in insulin secretion

Mutations in the gene encoding hepatocyte nuclear factor-4alpha (HNF-4alpha) result in maturity-onset diabetes of the young (MODY). To determine the contribution of HNF-4alpha to the maintenance of glucose homeostasis by the beta cell in vivo, we derived a conditional knockout of HNF-4alpha using the Cre-loxP system. Surprisingly, deletion of HNF-4alpha in beta cells resulted in hyperinsulinemia in fasted and fed mice but paradoxically also in impaired glucose tolerance. Islet perifusion and calcium-imaging studies showed abnormal responses of the mutant beta cells to stimulation by glucose and sulfonylureas. These phenotypes can be explained in part by a 60% reduction in expression of the potassium channel subunit Kir6.2. We demonstrate using cotransfection assays that the Kir6.2 gene is a transcriptional target of HNF-4alpha. Our data provide genetic evidence that HNF-4alpha is required in the pancreatic beta cell for regulation of the pathway of insulin secretion dependent on the ATP-dependent potassium channel.

Regulation of human hepatic hydroxysteroid sulfotransferase gene expression by the peroxisome proliferator-activated receptor alpha transcription factor

Human hydroxysteroid sulfotransferase or (HUMAN)SULT2A1 catalyzes the sulfonation of procarcinogen xenobiotics, hydroxysteroids, and bile acids and plays a dynamic role in hepatic cholesterol homeostasis. The treatment of primary cultured human hepatocytes with a peroxisome proliferator-activated receptor alpha (PPARalpha)-activating concentration of ciprofibrate (10(-) (4) M) increased (HUMAN)SULT2A1 mRNA, immunoreactive protein, and enzymatic activity levels by approximately 2-fold. By contrast, expression of (RAT)SULT2A3, the rat counterpart to (HUMAN)SULT2A1, was induced by treatment of primary hepatocyte cultures with an activator of the pregnane X receptor, but not PPARalpha. In HepG2 cells, transient transfection analyses of luciferase reporter constructs containing upstream regions of the (HUMAN)SULT2A1 gene implicated a candidate peroxisome proliferator response element (PPRE) at nucleotides (nt) -5949 to -5929 relative to the transcription start site. Site-directed mutagenesis and electrophoretic mobility shift assay studies confirmed that this distal PPRE (dPPRE), a direct repeat nuclear receptor motif containing one intervening nt, represented a functional PPRE. Chromatin immunoprecipitation analysis indicated that the (HUMAN)SULT2A1 dPPRE was also a functional element in the context of the human genome. These data support a major role for the PPARalpha transcription factor in the regulation of hepatic (HUMAN)SULT2A1. Results also indicate that important species differences govern the transactivation of SULT2A gene transcription by nuclear receptors.

Transcriptional regulation of lipid metabolism by fatty acids: a key determinant of pancreatic beta-cell function

BACKGROUND: Optimal pancreatic beta-cell function is essential for the regulation of glucose homeostasis in both humans and animals and its impairment leads to the development of diabetes. Type 2 diabetes is a polygenic disease aggravated by environmental factors such as low physical activity or a hypercaloric high-fat diet. RESULTS: Free fatty acids represent an important factor linking excess fat mass to type 2 diabetes. Several studies have shown that chronically elevated free fatty acids have a negative effect on beta-cell function leading to elevated insulin secretion basally but with an impaired response to glucose. The transcription factors PPARalpha, PPARgamma and SREBP-1c respond to changing fat concentrations in tissues, thereby coordinating the genomic response to altered metabolic conditions to promote either fat storage or catabolism. These transcription factors have been identified in beta-cells and it appears that each may exert influence on beta-cell function in health and disease. CONCLUSION: The role of the PPARs and SREBP-1c as potential mediators of lipotoxicity is an emerging area of interest.

Nuclear Hormone Receptors, Metabolism, and Aging: What Goes Around Comes Around

Previous studies have linked the mysterious and inevitable process of aging to essential processes such as metabolism, maturation, and fecundity. Each of these processes is controlled to a large extent by nuclear hormone receptors (NHRs). NHRs also play important roles in the control of periodical processes, the most recently implicated being circadian rhythm. This Review stresses the mounting evidence for tight relationships between each of these NHR-regulated processes and the processes of aging.