Diet-induced Insulin Resistance In Rats Research Articles

The specific mitochondrial unfolded protein response in fast- and slow-twitch muscles of high-fat diet-induced insulin-resistant rats.

Skeletal muscle insulin resistance (IR) plays an important role in the pathogenesis of type 2 diabetes mellitus. Skeletal muscle is a heterogeneous tissue composed of different muscle fiber types that contribute distinctly to IR development. Glucose transport shows more protection in slow-twitch muscles than in fast-twitch muscles during IR development, while the mechanisms involved remain unclear. Therefore, we investigated the role of the mitochondrial unfolded protein response (UPRmt) in the distinct resistance of two types of muscle in IR. Male Wistar rats were divided into high-fat diet (HFD) feeding and control groups. We measured glucose transport, mitochondrial respiration, UPRmt and histone methylation modification of UPRmt-related proteins to examine the UPRmt in the slow fiber-enriched soleus (Sol) and fast fiber-enriched tibialis anterior (TA) under HFD conditions. Our results indicate that 18 weeks of HFD can cause systemic IR, while the disturbance of Glut4-dependent glucose transport only occurred in fast-twitch muscle. The expression levels of UPRmt markers, including ATF5, HSP60 and ClpP, and the UPRmt-related mitokine MOTS-c were significantly higher in slow-twitch muscle than in fast-twitch muscle under HFD conditions. Mitochondrial respiratory function is maintained only in slow-twitch muscle. Additionally, in the Sol, histone methylation at the ATF5 promoter region was significantly higher than that in the TA after HFD feeding. The expression of proteins involved in glucose transport in slow-twitch muscle remains almost unaltered after HFD intervention, whereas a significant decline of these proteins was observed in fast-twitch muscle. Specific activation of the UPRmt in slow-twitch muscle, accompanied by higher mitochondrial respiratory function and MOTS-c expression, may contribute to the higher resistance to HFD in slow-twitch muscle. Notably, the different histone modifications of UPRmt regulators may underlie the specific activation of the UPRmt in different muscle types. However, future work applying genetic or pharmacological approaches should further uncover the relationship between the UPRmt and insulin resistance.

Lactobacillus rhamnosus GG administration partially prevents diet-induced insulin resistance in rats: a comparison with its heat-inactivated parabiotic.

Insulin resistance and type 2 diabetes are obesity-related health alterations, featuring an ever-increasing prevalence. Besides inadequate feeding patterns, gut microbiota alterations stand out as potential contributors to these metabolic disturbances. The aim of this study was to investigate whether the administration of a probiotic (Lactobacillus rhamnosus GG) effectively prevents diet-induced insulin resistance in rats and to compare these potential effects with those exerted by its heat-inactivated parabiotic. For this purpose, 34 male Wistar rats were fed a standard or a high-fat high-fructose diet, alone or supplemented with viable or heat-inactivated Lactobacillus rhamnosus GG. The body and white adipose tissue weight increases, induced by the obesogenic diet, were prevented by probiotic and parabiotic administration. The trend towards higher basal glucose levels and significantly higher serum insulin concentration observed in the non-treated animals fed with the obesogenic diet were effectively reverted by both treatments. Similar results were also found for serum adiponectin and leptin, whose levels were brought back by the probiotic and parabiotic administration to values similar to those of the control animals. Noteworthily, parabiotic administration significantly reduced skeletal muscle triglyceride content and activated CPT-1b compared to the non-treated animals. Finally, both treatments enhanced Akt and AS160 phosphorylation in the skeletal muscle compared to the non-treated animals; however, only parabiotic administration increased GLUT-4 protein expression in this tissue. These results suggest that heat-inactivated Lactobacillus rhamnosus GG seem to be more effective than its probiotic of origin in preventing high-fat high-fructose diet-induced insulin resistance in rats.

Novel dual PPARα/γ agonists protect against liver steatosis and improve insulin sensitivity while avoiding side effects

Insulin resistance is a feature of type 2 diabetes mellitus (T2D), and is strongly interconnected with non-alcoholic fatty liver disease (NAFLD). Peroxisome-proliferator activated receptor gamma (PPARγ) and peroxisome-proliferator activated receptor alpha (PPARα) are master regulators of insulin sensitivity and lipid metabolism, respectively. Thiazolidinediones (TZDs) such as pioglitazone, which target PPARα/γ, are highly effective at treating insulin resistance and NAFLD, but their clinical utility has been restricted by side effects such as weight gain, adipocyte hypertrophy and fluid retention. Therefore, there is urgent need for new safer and effective drugs. Thus, we aimed to develop novel dual PPARα/γ agonists to avoid their known side effects while preserving their overall therapeutic effects. Here, we show that our novel agonists G4 and G5 strongly stimulate glucose transporter 4 (GLUT4) translocation to the cell membrane in skeletal muscle cells, and manifest weaker lipogenic effect in adipocytes. Moreover, G4 and G5 improve systemic glucose metabolism, hyperinsulinemia, hyperlipidemia, and markers of liver injury in high fructose diet-induced insulin resistant rats. Mechanistic studies revealed that G4 and G5 enhance GLUT4, and AMPK in skeletal muscle and protect against liver steatosis by upregulating PPARα and improve whole-body insulin sensitivity by increasing PPARγ. Despite this increase in PPARγ activity, G4 and G5 inhibit the unwanted side effects such as weight gain due to adiposity, hypertrophy of adipocytes, and fluid retention unlike TZDs. These findings identify G4 and G5 as promising dual PPARα/γ agonists for the treatment of NAFLD and insulin resistance with improved safety.

Renoprotective effect of empagliflozin in cafeteria diet-induced insulin resistance in rats: Modulation of HMGB-1/TLR-4/NF-κB axis

AimCafeteria diet (CAF) is a well-established model used to mimic what occurs in human upon eating junk and ultra-processed food. This study aimed to investigate the possible protective impact of empagliflozin (EMPA) against CAF-induced insulin resistance (IR) in rats and the possible underlying mechanisms. Main methodsRats were fed on CAF diet for 12 weeks while treatment with EMPA (10 & 30 mg/kg/day, orally) and/or metformin (MET) (100 mg/kg/day, orally) started at day 29. Key findingsOral administration of EMPA and/or MET significantly and dose-dependently succeeded to attenuate CAF-induced obesity which was evidenced by decreased oral glucose tolerance test (AUCOGTT), insulin tolerance test (AUCITT) and decreased fasting serum insulin level besides improving the histopathological alterations induced by CAF. Moreover, EMPA significantly mitigated CAF-induced elevation in serum levels of creatinine urea, transaminases (ALT and AST), and increased albumin level as well as improving dyslipidemia and oxidative stress. Furthermore, EMPA markedly reduced renal levels of high mobility group box 1 (HMGB-1), toll like receptor4 (TLR-4) and nuclear factor κB (NF-κB) as well as decreasing the expression of tumor necrosis factor α (TNF-α) and Caspase 3. Combining EMPA30 with MET synergistically improved dyslipidemia, oxidative stress and enhanced kidney function. SignificanceEMPA administration could confer protection against CAF-induced IR and its complications through its hypoglycemic, insulin-sensitizing, hypolipidemic, hepatoprotective, renoprotective, anti-inflammatory, anti-oxidant and anti-apoptotic properties. Also, our findings highlighted the synergistic effect of combining EMPA30 with MET so this combination might be promising in treatment of IR.

Sirt1 and Sirt3 Activation Improved Cardiac Function of Diabetic Rats via Modulation of Mitochondrial Function.

In the present study, we aimed to evaluate the effect of Sirt1, Sirt3 and combined activation in high fructose diet-induced insulin resistance rat heart and assessed the cardiac function focusing on mitochondrial health and function. We administered the Sirt1 activator; SRT1720 (5 mg/kg, i.p.), Sirt3 activator; Oroxylin-A (10 mg/kg i.p.) and the combination; SRT1720 + Oroxylin-A (5 mg/kg and 10 mg/kg i.p.) daily from 12th week to 20th weeks of study. We observed significant perturbations of most of the cardiac structural and functional parameters in high fructose diet-fed animals. Administration of SRT1720 and Oroxylin-A improved perturbed cardiac structural and functional parameters by decreasing insulin resistance, oxidative stress, and improving mitochondrial function by enhancing mitochondrial biogenesis, OXPHOS expression and activity in high fructose diet-induced insulin-resistant rats. However, we could not observe the synergistic effect of SRT1720 and Oroxylin-A combination. Similar to in-vivo study, perturbed mitochondrial function and oxidative stress observed in insulin-resistant H9c2 cells were improved after activation of Sirt1 and Sirt3. We observed that Sirt1 activation enhances Sirt3 expression and mitochondrial biogenesis, and the opposite effects were observed after Sirt1 inhibition in cardiomyoblast cells. Taken together our results conclude that activation of Sirt1 alone could be a potential therapeutic target for diabetes-associated cardiovascular complications.

Potentiation of incretin hormones and modulation of metabolic enzymes as possible mechanisms behind the insulin sensitizing effects of cabbage-metformin treatment

In our study, we treated high fructose diet-induced insulin resistance in rats with any of metformin, cabbage (80%w/w) or combined metformin and cabbage (MetCabb), and observed the activities of glycolysis and gluconeogenesis regulatory enzymes, incretin hormones and other hormones affecting glucose homeostasis. Comparisons were made with normoglycemic noninsulin resistance rats (control) and insulin-resistant untreated rats (INres). Baseline analysis showing elevated fasting blood sugar (>250 mg/dl), insulin (>25 µIU/ml) and HOMA-IR (>10) satisfied the criteria for recruitment into the insulin-resistant groups. Treatment lasted for 12 weeks. HOMA-IR values significantly (P < 0.05) decreased from 24.7 to 5.5 and 10.6 respectively with MetCabb treatment. MetCabb normalized HOMA-IR values and mean β-cell responsiveness of the INres. Cabbage and metCabb normalized the leptin levels relative to control. The mean fasting blood sugar, insulin, and c-peptide levels with MetCabb treatment reverted to control levels. We found a strong positive linear correlation between the glucagon levels (r = 0.9145) and increasing HOMA-IR values while both incretin hormones; GLP-1 and GIP negatively regressed (r = -0.8084 and -0.8488). MetCab treatment produced comparable values of GLP-1 and GIP to the control. A strong positive correlation was found between the HOMA-IR values and the PEPCK (r = 0.9065), F-1,6-BPase (r = 0.7951), and G-6-Pase (r = 0.7893). The hexokinase (r = -0.807), PFK (r = -0.9151), and PK (r = -0.7448) levels regressed as HOMA-IR values increased. The glycolytic and gluconeogenic enzymes except PEPCK reverted to control levels with MetCabb treatment. Combination of metformin and cabbage was more effective than individual treatments.

Effects of dapagliflozin and statins attenuate renal injury and liver steatosis in high-fat/high-fructose diet-induced insulin resistant rats

High-fat high-fructose diet (HFF) in obesity can induce dyslipidemia and lipid accumulation both in kidney and liver which related to insulin resistance and lipotoxicity-induced cellular damage. We investigated whether dapagliflozin with or without atorvastatin could improve lipid accumulation-induced kidney and liver injury in HFF-induced insulin resistant rats. Male Wistar rats were fed with HFF for 16 weeks and then received drug treatments for 4 weeks; vehicle, dapagliflozin, atorvastatin and dapagliflozin plus atorvastatin treatment groups. HFF rats demonstrated insulin resistance, dyslipidemia, liver injury and renal dysfunction associated with impaired renal lipid metabolism and lipid accumulation. Dapagliflozin and combination treatment could improve HFF-induced insulin resistance, lipogenesis and lipotoxicity-related renal oxidative stress, inflammation, fibrosis and apoptosis leading to kidney dysfunction recovery. Liver injury-associated inflammation was also improved by these two regimens. Notably, the reduced lipid accumulation in liver and kidney that linked to an improvement of lipid oxidation was prominent in the combination treatment. Therefore, dapagliflozin combined with atorvastatin treatment exert the beneficial effects on lipid metabolism and lipotoxicity in liver and kidney injury via the attenuation of oxidative stress, fibrosis and apoptosis in insulin resistant model.

The Influence of Taurine Supplementation on Serum and Tissular Fe, Zn and Cu Levels in Normal and Diet-Induced Insulin-Resistant Rats

Taurine (Tau) is a β-sulphonated amino acid postulated to improve glucose homeostasis in insulin resistance and diabetes. Changes in carbohydrate metabolism are accompanied by oxidative stress, which may disturb the mineral balance. Therefore, the aim of this study was to assess the effect of Tau supplementation on the levels of trace elements in rats fed either a standard (AIN-93M, 4% fat) diet or a modified high-fat diet (30% fat). For 8 weeks, male Wistar rats were fed these diets supplemented with 3% Tau. Taurine supplementation normalized increased serum insulin concentration and insulin resistance index; however, it did not improve serum CRP concentration in high-fat diet fed rats. The high-fat diet supplemented with Tau decreased the renal and splenic Zn levels, but the tissular Fe content did not change. The effect of Tau supplementation on the mineral balance to some extent depended on the fat content in the rats’ diet. The high-fat diet supplemented with Tau decreased the rats’ splenic Zn levels but increased their femur levels. In the group fed the standard diet, Tau reduced the rats’ femur Zn level, whereas their splenic Zn level was comparable. Tau supplementation decreased the renal Cu level and serum ceruloplasmin concentration in the rats fed the standard diet, but this effect was not observed in the rats fed the high-fat diet. In conclusion, supplementary taurine failed to ameliorate disturbances in mineral homeostasis caused by high-fat diet feeding and led to tissular redistribution of Zn and Cu in the rat.

122 - Egg White Hydrolysate Effects in Adipose Tissue of High Fat Diet-Induced Insulin Resistant Rats

White adipose tissue (WAT) is no longer thought to be only an energy reservoir and, currently, its role in metabolic diseases, such as type 2 diabetes (T2D) and metabolic syndrome, is under extensive investigation. Egg white hydrolysate (EWH) has potential as an alternative method to manage metabolic conditions, improving both glucose tolerance and hypertension. Previously, we showed that feeding EWH for 6 weeks to insulin resistant rats increased insulin signalling (p-Akt) and PPARγ abundance, associated with reduced adipocyte size in WAT.

Egg white hydrolysate enhances insulin sensitivity in high-fat diet-induced insulin-resistant rats via Akt activation.

Agents that block the renin-angiotensin system (RAS) improve glucoregulation in the metabolic syndrome disorder. We evaluated the effects of egg white hydrolysate (EWH), previously shown to modulate the protein abundance of RAS component in vivo, on glucose homeostasis in diet-induced insulin-resistant rats. Sprague-Dawley rats were fed a high-fat diet (HFD) for 6 weeks to induce insulin resistance. They were then randomly divided into four groups receiving HFD or HFD supplemented with different concentrations of EWH (1, 2 and 4 %) for another 6 weeks in the first trial. In the second trial, insulin-resistant rats were divided into two groups receiving only HFD or HFD+4 % EWH for 6 weeks. Glucose homeostasis was assessed by oral glucose tolerance and insulin tolerance tests. Insulin signalling and protein abundance of RAS components, gluconeogenesis enzymes and PPARγ were evaluated in muscle, fat and liver. Adipocyte morphology and inflammatory markers were evaluated. In vivo administration of EWH increased insulin sensitivity, improved oral glucose tolerance (P < 0·0001) and reduced systemic inflammation (P < 0·05). EWH potentiated insulin-induced Akt phosphorylation in muscle (P = 0·0341) and adipose tissue (P = 0·0276), but minimal differences in the protein abundance of tissue RAS components between the EWH and control groups were observed. EWH treatment also reduced adipocyte size (P = 0·0383) and increased PPARγ2 protein abundance (P = 0·0237). EWH treatment yielded positive effects on the inflammatory profile, glucose tolerance, insulin sensitivity and adipocyte differentiation in HFD-induced insulin resistance rats. The involvement of local RAS activity requires further investigation.

Glucose mediates insulin sensitivity via a hepatoportal mechanism in high-fat-fed rats.

Poor nutrition plays a fundamental role in the development of insulin resistance, an underlying characteristic of type 2 diabetes. We have previously shown that high-fat diet-induced insulin resistance in rats can be ameliorated by a single glucose meal, but the mechanisms for this observation remain unresolved. To determine if this phenomenon is mediated by gut or hepatoportal factors, male Wistar rats were fed a high-fat diet for 3 weeks before receiving one of five interventions: high-fat meal, glucose gavage, high-glucose meal, systemic glucose infusion or portal glucose infusion. Insulin sensitivity was assessed the following day in conscious animals by a hyperinsulinaemic-euglycaemic clamp. An oral glucose load consistently improved insulin sensitivity in high-fat-fed rats, establishing the reproducibility of this model. A systemic infusion of a glucose load did not affect insulin sensitivity, indicating that the physiological response to oral glucose was not due solely to increased glucose turnover or withdrawal of dietary lipid. A portal infusion of glucose produced the largest improvement in insulin sensitivity, implicating a role for the hepatoportal region rather than the gastrointestinal tract in mediating the effect of glucose to improve lipid-induced insulin resistance. These results further deepen our understanding of the mechanism of glucose-mediated regulation of insulin sensitivity and provide new insight into the role of nutrition in whole body metabolism.

Metformin treatment affects adipocytokine secretion and lipid composition in adipose tissues of diet-induced insulin-resistant rats.

Adipose tissue plays a central role in the pathogenesis of insulin resistance (IR) and type 2 diabetes. However, the molecular changes that promote these diseases are not completely understood. Several studies demonstrated that ceramide (Cer) and diacylglycerol (DAG) accumulation in muscle is associated with IR. The aim of this study was to explain whether a high-fat diet (HFD) leads to bioactive lipid accumulation in adipose tissue and how metformin affects the lipid content in adipocytes and the concentration of plasma adipocytokines. The experiments were conducted on male Wistar rats divided into three groups: control, HFD-fed, and HFD-fed and treated with metformin. Cer and DAGs were analyzed by liquid chromatography tandem mass spectrometry. Phosphorylation of hormone-sensitive lipase (HSL) was analyzed by Western blot. Oral glucose tolerance and insulin tolerance tests were also performed. Plasma adiponectin and tumor necrosis factor (TNF)-α concentration were measured by enzyme-linked immunosorbent assay. HFD induced IR and elevated DAGs and Cer content in subcutaneous and visceral adipose tissues, which was accompanied by an increased phosphorylation of HSL. Metformin improved insulin sensitivity, decreased Cer and DAG levels, and attenuated the phosphorylation of HSL in both fat depots. Furthermore, we observed a strong correlation between adiponectin (negative) and TNF-α (positive) and bioactive lipids in both fat tissues. These results indicated that bioactive lipids accumulation in adipose tissue influences the induction of IR and, at least in part, answered the question of what the insulin-sensitizing effect of metformin at the level of adipose tissue is.

Dietary krill oil (Euphausia superba) alleviates oxidative stress and DNA damages bio-markers in an experimental model of cafeteria diet-induced insulin resistance in rats

Dietary krill oil (Euphausia superba) alleviates oxidative stress and DNA damages bio-markers in an experimental model of cafeteria diet-induced insulin resistance in rats

Abstract # P-20: 11b-Hydroxysteroid Dehydrogenase 1 Regulation in High Fat Diet Induced Insulin Resistant Rats and Rats Treated with Sutherlandia Frutescens

Abstract # P-20: 11b-Hydroxysteroid Dehydrogenase 1 Regulation in High Fat Diet Induced Insulin Resistant Rats and Rats Treated with Sutherlandia Frutescens

Trigonelline attenuates hepatic complications and molecular alterations in high-fat high-fructose diet-induced insulin resistance in rats.

The present study aimed to evaluate the effect of trigonelline (TRG) on the hepatic complications associated with high-fat high-fructose (HFHF) diet-induced insulin resistance (IR) in rats. IR was induced by giving a saturated fat diet and 10% fructose in drinking water to rats for 8 weeks. Insulin-resistant rats were orally treated with TRG (50 and 100 mg/kg), sitagliptin (SIT; 5 mg/kg), or a combination of TRG (50 mg/kg) and SIT (5 mg/kg) for 14 days. Liver homogenates were used for assessment of hepatic lipids, oxidative stress biomarkers, and inflammatory cytokines. Histopathological and DNA cytometry examinations were carried out for hepatic and pancreatic tissues. Hepatic tissues were examined using Fourier-transform infrared spectroscopy for assessment of any molecular changes. Results of the present study revealed that oral treatment of insulin-resistant rats with TRG or TRG in combination with SIT significantly decreased homeostatic model assessment of IR, hepatic lipids, oxidative stress biomarkers, and the inflammatory cytokines. TRG or TRG in combination with SIT ameliorated the histopathological, DNA cytometry, and molecular alterations induced by a HFHF diet. Finally, it can be concluded that TRG has beneficial effects on the hepatic complications associated with IR due to its hypoglycemic effect and antioxidant potential.

High-fructose diet is as detrimental as high-fat diet in the induction of insulin resistance and diabetes mediated by hepatic/pancreatic endoplasmic reticulum (ER) stress.

In the context of high human consumption of fructose diets, there is an imperative need to understand how dietary fructose intake influence cellular and molecular mechanisms and thereby affect β-cell dysfunction and insulin resistance. While evidence exists for a relationship between high-fat-induced insulin resistance and metabolic disorders, there is lack of studies in relation to high-fructose diet. Therefore, we attempted to study the effect of different diets viz., high-fat diet (HFD), high-fructose diet (HFS), and a combination (HFS+HFD) diet on glucose homeostasis and insulin sensitivity in male Wistar rats compared to control animals fed with normal pellet diet. Investigations include oral glucose tolerance test, insulin tolerance test, histopathology by H&E and Masson's trichrome staining, mRNA expression by real-time PCR, protein expression by Western blot, and caspase-3 activity by colorimetry. Rats subjected to high-fat/fructose diets became glucose intolerant, insulin-resistant, and dyslipidemic. Compared to control animals, rats subjected to different combination of fat/fructose diets showed increased mRNA and protein expression of a battery of ER stress markers both in pancreas and liver. Transcription factors of β-cell function (INSIG1, SREBP1c and PDX1) as well as hepatic gluconeogenesis (FOXO1 and PEPCK) were adversely affected in diet-induced insulin-resistant rats. The convergence of chronic ER stress towards apoptosis in pancreas/liver was also indicated by increased levels of CHOP mRNA & increased activity of both JNK and Caspase-3 in rats subjected to high-fat/fructose diets. Our study exposes the experimental support in that high-fructose diet is equally detrimental in causing metabolic disorders.

Moderate Exercise Prevents Functional Remodeling of the Anterior Pituitary Gland in Diet-Induced Insulin Resistance in Rats: Role of Oxidative Stress and Autophagy.

A sustained elevation of glucocorticoid production, associated with the establishment of insulin resistance (IR) could add to the deleterious effects of the IR state. The aim of this study is to analyze the consequences of long-term feeding with a sucrose-rich diet (SRD) on Pomc/ACTH production, define the underlying cellular processes, and determine the effects of moderate exercise (ME) on these parameters. Animals fed a standard chow with or without 30% sucrose in the drinking water were subjected to ME. Circulating hormone levels were determined, and pituitary tissues were processed and analyzed by immunobloting and quantitative real-time PCR. Parameters of oxidative stress (OxS), endoplasmic reticulum stress, and autophagy were also determined. Rats fed SRD developed a decrease in pituitary Pomc/ACTH expression levels, increased expression of antioxidant enzymes, and induction of endoplasmic reticulum stress and autophagy. ME prevented pituitary dysfunction as well as induction of antioxidant enzymes and autophagy. Reporter assays were performed in AtT-20 corticotroph cells incubated in the presence of palmitic acid. Pomc transcription was inhibited by palmitic acid-dependent induction of OxS and autophagy, as judged by the effect of activators and inhibitors of both processes. Long-term feeding with SRD triggers the generation of OxS and autophagy in the pituitary gland, which could lead to a decline in Pomc/ACTH/glucocorticoid production. These effects could be attributed to an increase in fatty acids availability to the pituitary gland. ME was able to prevent these alterations, suggesting additional beneficial effects of ME as a therapeutic strategy in the management of IR.

MicroRNA-16 modulates macrophage polarization leading to improved insulin sensitivity in myoblasts.

Uncontrolled inflammation leads to several diseases such as insulin resistance, T2D and several types of cancers. The functional role of microRNAs in inflammation induced insulin resistance is poorly studied. MicroRNAs are post-transcriptional regulatory molecules which mediate diverse biological processes. We here show that miR-16 expression levels are down-regulated in different inflammatory conditions such as LPS/IFNγ or palmitate treated macrophages, palmitate exposed myoblasts and insulin responsive tissues of high sucrose diet induced insulin resistant rats. Importantly, forced expression of miR-16 in macrophages impaired the production of TNF-α, IL-6 and IFN-β leading to enhanced insulin stimulated glucose uptake in co-cultured skeletal myoblasts. Further, ectopic expression of miR-16 enhanced insulin stimulated glucose uptake in skeletal myoblasts via the up-regulation of GLUT4 and MEF2A, two key players involved in insulin stimulated glucose uptake. Collectively, our data highlight the important role of miR-16 in ameliorating inflammation induced insulin resistance.

N-Acetylneuraminic Acid Supplementation Prevents High Fat Diet-Induced Insulin Resistance in Rats through Transcriptional and Nontranscriptional Mechanisms.

N-Acetylneuraminic acid (Neu5Ac) is a biomarker of cardiometabolic diseases. In the present study, we tested the hypothesis that dietary Neu5Ac may improve cardiometabolic indices. A high fat diet (HFD) + Neu5Ac (50 or 400 mg/kg BW/day) was fed to rats and compared with HFD + simvastatin (10 mg/kg BW/day) or HFD alone for 12 weeks. Weights and serum biochemicals (lipid profile, oral glucose tolerance test, leptin, adiponectin, and insulin) were measured, and mRNA levels of insulin signaling genes were determined. The results indicated that low and high doses of sialic acid (SA) improved metabolic indices, although only the oral glucose tolerance test, serum triglycerides, leptin, and adiponectin were significantly better than those in the HFD and HFD + simvastatin groups (P < 0.05). Furthermore, the results showed that only high-dose SA significantly affected the transcription of hepatic and adipose tissue insulin signaling genes. The data suggested that SA prevented HFD-induced insulin resistance in rats after 12 weeks of administration through nontranscriptionally mediated biochemical changes that may have differentially sialylated glycoprotein structures at a low dose. At higher doses, SA induced transcriptional regulation of insulin signaling genes. These effects suggest that low and high doses of SA may produce similar metabolic outcomes in relation to insulin sensitivity through multiple mechanisms. These findings are worth studying further.

Hypermethylation of Cox5a promoter is associated with mitochondrial dysfunction in skeletal muscle of high fat diet-induced insulin resistant rats.

High-fat diet (HFD) is an environmental factor that contributes to the pathogenesis of obesity and type 2 diabetes. A number of genes influencing oxidative phosphorylation (OXPHOS) were found to be downregulated in skeletal muscle of humans and rats treated with HFD and have been implicated in mitochondrial dysfunction, insulin resistance, and consequent type 2 diabetes. In this study, we hypothesized that DNA methylation plays a crucial role in the regulation of OXPHOS genes in skeletal muscle of rats exposed to HFD. Using whole genome promoter methylation analysis of skeletal muscle followed by qPCR and bisulfite sequencing analysis, we identified hypermethylation of Cox5a in HFD rats. Furthermore, we found that Cox5a hypermethylation was associated with downregulation of Cox5a expression at the mRNA and protein level, and a reduction in mitochondrial complex IV activity and ATP content in HFD-induced insulin resistant rats compared to controls. Moreover, we found that while exposure to palmitate resulted in hypermethylation of the Cox5a promoter in rat myotubes, demethylation with 5-aza-2′-deoxycytidine was associated with preserved Cox5a expression, as well as restoration of complex IV activity and cellular ATP content. These novel observations indicate that Cox5a hypermethylation is associated with mitochondrial dysfunction in skeletal muscle of HFD-induced insulin resistant rats.