Discordance between intramuscular triglyceride and insulin sensitivity in skeletal muscle of Zucker diabetic rats after treatment with fenofibrate and rosiglitazone

Skeletal muscle fiber type-selective effects of acute exercise on insulin-stimulated glucose uptake in insulin-resistant, high-fat-fed rats

Insulin-stimulated glucose uptake (GU) by skeletal muscle is enhanced several hours after acute exercise in rats with normal or reduced insulin sensitivity. Skeletal muscle is composed of multiple fiber types, but exercise's effect on fiber type-specific insulin-stimulated GU in insulin-resistant muscle was previously unknown. Male rats were fed a high-fat diet (HFD; 2 wk) and were either sedentary (SED) or exercised (2-h exercise). Other, low-fat diet-fed (LFD) rats remained SED. Rats were studied immediately postexercise (IPEX) or 3 h postexercise (3hPEX). Epitrochlearis muscles from IPEX rats were incubated in 2-deoxy-[³H]glucose (2-[³H]DG) without insulin. Epitrochlearis muscles from 3hPEX rats were incubated with 2-[³H]DG ± 100 µU/ml insulin. After single fiber isolation, GU and fiber type were determined. Glycogen and lipid droplets (LDs) were assessed histochemically. GLUT4 abundance was determined by immunoblotting. In HFD-SED vs. LFD-SED rats, insulin-stimulated GU was decreased in type IIB, IIX, IIAX, and IIBX fibers. Insulin-independent GU IPEX was increased and glycogen content was decreased in all fiber types (types I, IIA, IIB, IIX, IIAX, and IIBX). Exercise by HFD-fed rats enhanced insulin-stimulated GU in all fiber types except type I. Single fiber analyses enabled discovery of striking fiber type-specific differences in HFD and exercise effects on insulin-stimulated GU. The fiber type-specific differences in insulin-stimulated GU postexercise in insulin-resistant muscle were not attributable to a lack of fiber recruitment, as indirectly evidenced by insulin-independent GU and glycogen IPEX, differences in multiple LD indexes, or altered GLUT4 abundance, implicating fiber type-selective differences in the cellular processes responsible for postexercise enhancement of insulin-mediated GLUT4 translocation.

Synthetic and natural Peroxisome Proliferator-Activated Receptor (PPAR) agonists as candidates for the therapy of the metabolic syndrome

INTRODUCTION

Peroxisome proliferator-activated receptors (PPARs) are the molecular targets of hypolipidemic and insulin-sensitizing drugs and implicated in a multitude of processes that fine-tune the functions of all organs in vertebrates. As transcription factors they sense endogenous and exogenous lipid signaling molecules and convert these signals into intricate gene responses that impact health and disease. The PPARs act as modulators of cellular, organ, and systemic processes, such as lipid and carbohydrate metabolism, making them valuable for understanding body homeostasis influenced by nutrition and exercise. Areas covered: This review concentrates on synthetic and natural PPAR ligands and how they have helped reveal many aspects of the transcriptional control of complex processes important in health. Expert opinion: The three PPARs have complementary roles in the fine-tuning of most fundamental body functions, especially energy metabolism. Understanding their inter-relatedness using ligands that simultaneously modulate the activity of more than one of these receptors is a major goal. This approach may provide essential knowledge for the development of dual or pan-PPAR agonists or antagonists as potential new health-promoting agents and for nutritional approaches to prevent metabolic diseases.

Identifying plausible adverse drug reactions using knowledge extracted from the literature

Pharmacovigilance involves continually monitoring drug safety after drugs are put to market. To aid this process; algorithms for the identification of strongly correlated drug/adverse drug reaction (ADR) pairs from data sources such as adverse event reporting systems or Electronic Health Records have been developed. These methods are generally statistical in nature, and do not draw upon the large volumes of knowledge embedded in the biomedical literature. In this paper, we investigate the ability of scalable Literature Based Discovery (LBD) methods to identify side effects of pharmaceutical agents. The advantage of LBD methods is that they can provide evidence from the literature to support the plausibility of a drug/ADR association, thereby assisting human review to validate the signal, which is an essential component of pharmacovigilance. To do so, we draw upon vast repositories of knowledge that has been extracted from the biomedical literature by two Natural Language Processing tools, MetaMap and SemRep. We evaluate two LBD methods that scale comfortably to the volume of knowledge available in these repositories. Specifically, we evaluate Reflective Random Indexing (RRI), a model based on concept-level co-occurrence, and Predication-based Semantic Indexing (PSI), a model that encodes the nature of the relationship between concepts to support reasoning analogically about drug-effect relationships. An evaluation set was constructed from the Side Effect Resource 2 (SIDER2), which contains known drug/ADR relations, and models were evaluated for their ability to "rediscover" these relations. In this paper, we demonstrate that both RRI and PSI can recover known drug-adverse event associations. However, PSI performed better overall, and has the additional advantage of being able to recover the literature underlying the reasoning pathways it used to make its predictions.

Rosiglitazone increases fatty acid Δ9-desaturation and decreases elongase activity index in human skeletal muscle in vivo

The ratio of unsaturated to saturated long-chain fatty acids (LC-FAs) in skeletal muscle has been associated with insulin resistance. Some animal data suggest a modulatory effect of peroxisome proliferator receptor γ (PPARγ) stimulation on stearoyl-CoA desaturase 1 (SCD1) and LC-FA composition in skeletal muscle, but human data are rare. We here investigate whether treatment with a PPARγ agonist affects myocellular SCD1 expression and modulates the intramyocellular fatty acid profile in individuals with impaired glucose tolerance. Muscle biopsies and hyperinsulinemic-euglycemic clamps were performed in 7 men before and after 8 weeks of rosiglitazone treatment. Intramyocellular saturated, monounsaturated, and polyunsaturated intramuscular fatty acid profiles were measured by gas chromatography. Effects on SCD1 messenger RNA expression were analyzed in C2C12 cells and in human biopsies before and after rosiglitazone treatment. As expected, treatment with the PPARγ activator rosiglitazone improved insulin sensitivity in humans. Myocellular SCD1 messenger RNA expression was increased in human biopsies and C2C12 cells. Although the total content of myocellular LC-FA was unchanged, a relative shift from saturated LC-FAs to unsaturated LC-FAs was observed in human biopsies. Particularly, the amount of stearate was reduced, whereas the amounts of palmitoleate as well as oleate and vaccenate were increased, after rosiglitazone therapy. These changes resulted in an increased fatty acid Δ9-desaturation index (16:1/16:0 and 18:1/18:0) in skeletal muscle and a decreased elongase activity index (18:0/16:0). The PPARγ associated phenotypes may be partially explained by an increased Δ9-desaturation and a decreased elongase activity of skeletal muscle.

AVE8134, a novel potent PPARα agonist, improves lipid profile and glucose metabolism in dyslipidemic mice and type 2 diabetic rats

AIM

AVE8134 is a structurally novel potent PPARα agonist. The aim of this study is to investigate the efficacy of AVE8134 on lipid profile and glucose metabolism in dyslipidemic mice and type 2 diabetic rats.

METHODS

A cell based PPAR Gal4 transactivation assay was constructed for testing the activities of AVE8134 at 3 different PPAR isoforms in vitro. Transgenic human Apo A1 (hApo A1) mice and insulin-resistant ZDF rats were used to evaluate the effects of AVE8134 in vivo.

RESULTS

AVE8134 was a full PPARα dominated PPAR agonist (the values of EC(50) for human and rodent PPARα receptor were 0.01 and 0.3 μmol/L, respectively). AVE8134 was not active at PPARδ receptor. In female hApo A1 mice, AVE8134 (1-30 mg·kg(-1)·d(-1), po for 12 d) dose-dependently lowered the plasma triglycerides, and increased the serum HDL-cholesterol, hApo A1 and mouse Apo E levels. In female ZDF rats, AVE8134 (3-30 mg·kg(-1)·d(-1) for 2 weeks) improved insulin-sensitivity index. In pre-diabetic male ZDF rats (at the age of 7 weeks), AVE8134 (10 mg·kg(-1)·d(-1) for 8 weeks) produced an anti-diabetic action comparable to rosiglitazone, without the PPARγ mediated adverse effects on body weight and heart weight. In male ZDF rats (at the age of 6 weeks), AVE8134 (20 mg·kg(-1)·d(-1) for 12 weeks) increased mRNA levels of the target genes LPL and PDK4 about 20 fold in the liver, and there was no relevant effect with rosiglitazone.

CONCLUSION

AVE8134 improves lipid profile and glucose metabolism in dyslipidemic mice and type 2 diabetic rats.

Fenofibrate administration does not affect muscle triglyceride concentration or insulin sensitivity in humans

Animal data suggest that males, in particular, rely on peroxisome proliferator activated receptor-α activity to maintain normal muscle triglyceride metabolism. We sought to examine whether this was also true in men vs women and its relationship to insulin sensitivity (Si). Normolipidemic obese men (n = 9) and women (n = 9) underwent an assessment of Si (intravenous glucose tolerance test) and intramuscular triglyceride (IMTG) metabolism (gas chromatography/mass spectrometry and gas chromatography-combustion isotope ratio mass spectrometry from plasma and muscle biopsies taken after infusion of [U-(13)C]palmitate) before and after 12 weeks of fenofibrate treatment. Women were more insulin sensitive (Si: 5.2 ± 0.7 vs 2.4 ± 0.4 ×10(-4)/ μU/mL, W vs M, P < .01) at baseline despite similar IMTG concentration (41.9 ± 15.5 vs 30.8 ± 5.1 μg/mg dry weight, W vs M, P = .43) and IMTG fractional synthesis rate (FSR) (0.27%/h ± 0.07%/h vs 0.35%/h ± 0.06%/h, W vs M, P = .41) as men. Fenofibrate enhanced FSR in men (0.35 ± 0.06 to 0.54 ± 0.06, P = .05), with no such change seen in women (0.27 ± 0.07 to 0.32 ± 0.13, P = .73) and no change in IMTG concentration in either group (23.0 ± 3.9 in M, P = .26 vs baseline; 36.3 ± 12.0 in W, P = .79 vs baseline). Insulin sensitivity was unaffected by fenofibrate (P ≥ .68). Lower percentage saturation of IMTG in women vs men before (29.1% ± 2.3% vs 35.2% ± 1.7%, P = .06) and after (27.3% ± 2.8% vs 35.1% ± 1.9%, P = .04) fenofibrate most closely related to their greater Si (R(2) = 0.34, P = .10) and was largely unchanged by the drug. Peroxisome proliferator activated receptor-α agonist therapy had little effect on IMTG metabolism in men or women. Intramuscular triglyceride saturation, rather than IMTG concentration or FSR, most closely (but not significantly) related to Si and was unchanged by fenofibrate administration.

Multi-tissue, selective PPARγ modulation of insulin sensitivity and metabolic pathways in obese rats

Peroxisome proliferator-activated receptor-γ (PPARγ) ligands, including the insulin-sensitizing thiazolidinedione drugs, transcriptionally regulate hundreds of genes. Little is known about the relationship between PPARγ ligand-specific modulation of cellular mechanisms and insulin sensitization. We characterized the insulin sensitivity and multitissue gene expression profiles of lean and insulin-resistant, obese Zucker rats untreated or treated with one of four PPARγ ligands (pioglitazone, rosiglitazone, troglitazone, and AG-035029). We analyzed the transcriptional profiles of adipose tissue, skeletal muscle, and liver from the rats and determined whether ligand treatment insulin-sensitizing potency was related to ligand treatment-induced alteration of functional pathways. Ligand treatments improved insulin sensitivity in obese rats to varying degrees. Adipose tissue profiles revealed ligand treatment-selective modulation of inflammatory and branched-chain amino acid (BCAA) metabolic pathways, which correlated with ligand treatment-specific insulin-sensitizing potency. Skeletal muscle profiles showed that obese rats exhibited elevated expression of adipocyte and slow-twitch fiber markers, which further increased after ligand treatment, but the magnitude of the treatment-induced changes was not correlated with insulin sensitization. Although PPARγ ligand treatments heterogeneously improved dysregulated expression of cholesterol and fatty acid biosynthetic pathways in obese rat liver, these alterations were not correlated with ligand insulin-sensitizing potency. PPARγ ligand treatment-specific insulin-sensitizing potency correlated with modulation of adipose tissue inflammatory and BCAA metabolic pathways, suggesting a functional relationship between these pathways and whole body insulin sensitivity. Other PPARγ ligand treatment-induced functional pathway changes were detected in adipose tissue, skeletal muscle, and liver profiles but were not related to degree of insulin sensitization.

Regression to normoglycaemia by fenofibrate in pre-diabetic subjects complicated with hypertriglyceridaemia: a prospective randomized controlled trial

AIMS

Lipotoxicity has recently been shown to be an important risk factor underlying the pathogenesis of pre-diabetes. However, clinical evidence supporting the treatment of pre-diabetes by improving lipotoxicity is lacking. Here, we conducted an open-label, randomized, controlled trial to investigate whether fenofibrate, the widely used hypolipidaemic agent, might benefit pre-diabetes, with metformin and diet control, the recommended intervention methods, as positive controls.

METHODS

Newly diagnosed pre-diabetes patients (n = 120) with hypertriglyceridaemia (plasma triglyceride levels between 1.8 and 4.5 mmol/l) were randomly assigned by computer-generated randomization sequence to either control group (no intervention), fenofibrate group (200 mg once a day), metformin group (500 mg three times a day) or diet-controlled group (diet recommendation). Plasma biochemistry examination was performed every 2 months. The primary endpoint was the outcome of the natural course of pre-diabetes, evaluated by oral glucose tolerance test after 6-month follow-up.

RESULTS

Twenty subjects in the fenofibrate group, 24 subjects in the metformin group and 25 subjects in both the diet-controlled group and the control group finished the trial. Fenofibrate, metformin and diet control had protective effects on hypertriglyceridaemic pre-diabetes, evidenced by 53.3, 70 and 30% participants regressed to normoglycaemia, respectively. The effects of fenofibrate and metformin were comparable (P > 0.05), while diet control was less effective (P < 0.05). Liver damage occurred in six subjects in the fenofibrate group and gastrointestinal symptoms occurred in four subjects in the metformin group. No serious adverse events occurred.

CONCLUSION

Controlling lipotoxicity by fenofibrate could effectively ameliorate the natural course of hypertriglyceridaemic pre-diabetes.

Early responses of insulin signaling to high-carbohydrate and high-fat overfeeding

Background

Early molecular changes of nutritionally-induced insulin resistance are still enigmatic. It is also unclear if acute overnutrition alone can alter insulin signaling in humans or if the macronutrient composition of the diet can modulate such effects.

Methods

To investigate the molecular correlates of metabolic adaptation to either high-carbohydrate (HC) or high-fat (HF) overfeeding, we conducted overfeeding studies in 21 healthy lean (BMI < 25) individuals (10 women, 11 men), age 20-45, with normal glucose metabolism and no family history of diabetes. Subjects were studied first following a 5-day eucaloric (EC) diet (30% fat, 50% CHO, 20% protein) and then in a counter balanced manner after 5 days of 40% overfeeding of both a HC (20% fat, 60% CHO) diet and a HF (50% fat, 30% CHO) diet. At the end of each diet phase, in vivo insulin sensitivity was assessed using the hyperinsulinemic-euglycemic clamp technique. Ex vivo insulin action was measured from skeletal muscle tissue samples obtained 15 minutes after insulin infusion was initiated.

Results

Overall there was no change in whole-body insulin sensitivity as measured by glucose disposal rate (GDR, EC: 12.1 ± 4.7; HC: 10.9 ± 2.7; HF: 10.8 ± 3.4). Assessment of skeletal muscle insulin signaling demonstrated increased tyrosine phosphorylation of IRS-1 (p < 0.001) and increased IRS-1-associated phosphatidylinositol 3 (PI 3)-kinase activity (p < 0.001) following HC overfeeding. In contrast, HF overfeeding increased skeletal muscle serine phosophorylation of IRS-1 (p < 0.001) and increased total expression of p85α (P < 0.001).

Conclusion

We conclude that acute bouts of overnutrition lead to changes at the cellular level before whole-body insulin sensitivity is altered. On a signaling level, HC overfeeding resulted in changes compatible with increased insulin sensitivity. In contrast, molecular changes in HF overfeeding were compatible with a reduced insulin sensitivity.

Obesity and diabetes: lipids, 'nowhere to run to'

Although specific pathogenic entities contributing to diabetic risk, such as central adiposity, ectopic fat accumulation, hyperlipidaemia and inflammation, are well-characterized, the response of cellular systems to such insults are less well understood. This short review highlights the effect of increasing fat mass on ectopic fat accumulation, the role of triacylglycerols (triglycerides) in Type 2 diabetes mellitus and cardiovascular disease pathogenesis, and selected current therapeutic strategies used to ameliorate these risk factors.

In vivo and in vitro effects of SREBP-1 on diabetic renal tubular lipid accumulation and RNAi-mediated gene silencing study

Lipid deposits can injury the kidney of diabetic patients and models. Sterol regulatory element binding protein-1 (SREBP-1) is transcription factor regulating the synthesis of fatty acid and triglyceride. At present whether the expression of SREBP-1 makes some effects on the lipid accumulation in diabetic kidney is not still clear completely. The purpose of our in vivo and in vitro study is to investigate the relationship between the expression of SREBP-1 and lipid abnormal metabolism in the type 1 diabetic rats and explore to inhibit SREBP-1 gene expression by RNA interfere in human renal proximal tubular epithelial cells line (HKC cells). The animal experiment showed that triglyceride and SREBP-1 were up-regulated in proximal tubule of diabetic rats' kidney, which may result in increase of transforming growth factor-beta1 (TGF-beta1) and accumulation of extracellular matrix (ECM). The further HKC cells experiment confirmed SREBP-1 increasing resulted into lipid droplet formation. The expression of fatty acid synthase (FAS) in HKC cells transfected with specific plasmid for SREBP-1 gene was significantly more than that of the cells transfected with the control plasmid pcDNA3.1 and that of the untransfected cells. Simultaneously, up-regulation of TGF-beta1 and fibronectin, an ECM glycoprotein, was evident in HKC cells transfected by specific SREBP-1 plasmid. Furthermore, we found that high glucose was a positive factor on the expression of SREBP-1 at protein and mRNA levels in HKC cells. High glucose makes effects on SREBP-1 in time-dependent manner, and the greatest effect was at 48 h. In addition, two effective eukaryotic expression plasmid vectors of shRNA aimed at SREBP-1 were designed and constructed successfully. Compared with the negative control plasmid group, the levels of the expression of SREBP-1 were inhibited by 24.11 and 36.15%, respectively, at mRNA level, 20.80 and 37.59%, respectively, at precursor segment of protein level, and 38.12 and 52.24%, respectively, at mature segment of protein level at 48 h after transfection. In vivo and in vitro study suggested that high glucose caused increasing SREBP-1 mRNA and protein in renal proximal tubule epithelial cells of type 1 diabetic rats. Increasing SREBP-1 plays an important role in the pathogenesis of renal lipid accumulation by up-regulation of FAS and ECM accumulation by inducing TGF-beta1 expression. The application of vector-mediated RNAi could markedly inhibit the expression of SREBP-1 in HKC cells, which is a promising tool for future research into the mechanisms of renal lipid accumulation in vivo.

Tissue-specific postprandial clearance is the major determinant of PPARgamma-induced triglyceride lowering in the rat

Peroxisome proliferator-activated receptor-gamma (PPARgamma) agonism potently reduces circulating triglycerides (TG) in rodents and more modestly so in humans. This study aimed to quantify in vivo the relative contribution of hepatic VLDL-TG secretion and tissue-specific TG clearance to such action. Rats were fed an obesogenic diet, treated with the PPARgamma full agonist COOH (30 mg.kg(-1).day(-1)) for 3 wk, and studied in both the fasted and refed (fat-free) states. Hepatic VLDL-TG secretion rate was not affected by chronic COOH in the fasted state and was only modestly decreased (-30%) in refed rats. In contrast, postprandial VLDL-TG clearance was increased 2.6-fold by COOH, which concomitantly stimulated adipose tissue TG-derived lipid uptake and one of its major determinants, lipoprotein lipase (LPL) activity, in a highly depot-specific manner. TG-derived lipid uptake and LPL were indeed strongly increased in subcutaneous inguinal white adipose tissue and in brown adipose tissue, independently of the nutritional state, whereas of the three visceral fat depots examined (epididymal, retroperitoneal, mesenteric) only the latter responded consistently to COOH. Robust correlations (0.5 < r < 0.9) were observed between TG-derived lipid uptake and LPL in adipose tissues. The agonist did not increase LPL in muscle, and its enhancing action on postprandial muscle lipid uptake appeared to be mediated by post-LPL processes involving increased expression of fatty acid binding/transport proteins (aP2, likely in infiltrated adipocytes, FAT/CD36, and FATP-1). The study establishes in a diet-induced obesity model the major contribution of lipid uptake by specific, metabolically safe adipose depots to the postprandial hypotriglyceridemic action of PPARgamma agonism, and suggests a key role for LPL therein.