Lipolysis in skeletal muscle is decreased in high-fat-fed rats

Dietary enrichment with fish oil prevents high fat-induced metabolic dysfunction in skeletal muscle in mice

High saturated fat (HF-S) diets increase intramyocellular lipid, an effect ameliorated by omega-3 fatty acids in vitro and in vivo, though little is known about sex- and muscle fiber type-specific effects. We compared effects of standard chow, HF-S, and 7.5% HF-S replaced with fish oil (HF-FO) diets on the metabolic profile and lipid metabolism gene and protein content in red (soleus) and white (extensor digitorum longus) muscles of male and female C57BL/6 mice (n = 9-12/group). Weight gain was similar in HF-S- and HF-FO-fed groups. HF-S feeding increased mesenteric fat mass and lipid marker, Oil Red O, in red and mixed muscle; HF-FO increased interscapular brown fat mass. Compared to chow, HF-S and HF-FO increased expression of genes regulating triacylglycerol synthesis and fatty acid transport, HF-S suppressed genes and proteins regulating fatty acid oxidation, whereas HF-FO increased oxidative genes, proteins and enzymes and lipolytic gene content, whilst suppressing lipogenic genes. In comparison to HF-S, HF-FO further increased fat transporters, markers of fatty acid oxidation and mitochondrial content, and reduced lipogenic genes. No diet-by-sex interactions were observed. Neither diet influenced fiber type composition. However, some interactions between muscle type and diet were observed. HF-S induced changes in triacylglycerol synthesis and lipogenic genes in red, but not white, muscle, and mitochondrial biogenesis and oxidative genes were suppressed by HF-S and increased by HF-FO in red muscle only. In conclusion, HF-S feeding promotes lipid storage in red muscle, an effect abrogated by the fish oil, which increases mediators of lipolysis, oxidation and thermogenesis while inhibiting lipogenic genes. Greater storage and synthesis, and lower oxidative genes in red, but not white, muscle likely contribute to lipid accretion encountered in red muscle. Despite several gender-dimorphic genes, both sexes exhibited a similar HF-S-induced metabolic and gene expression profile; likewise fish oil was similarly protective in both sexes.

Mitochondrial metabolism and diabetes

The oversupply of calories and sedentary lifestyle has resulted in a rapid increase of diabetes prevalence worldwide. During the past two decades, lines of evidence suggest that mitochondrial dysfunction plays a key role in the pathophysiology of diabetes. Mitochondria are vital to most of the eukaryotic cells as they provide energy in the form of adenosine triphosphate by oxidative phosphorylation. In addition, mitochondrial function is an integral part of glucose-stimulated insulin secretion in pancreatic β-cells. In the present article, we will briefly review the major functions of mitochondria in regard to energy metabolism, and discuss the genetic and environmental factors causing mitochondrial dysfunction in diabetes. In addition, the pathophysiological role of mitochondrial dysfunction in insulin resistance and β-cell dysfunction are discussed. We argue that mitochondrial dysfunction could be the central defect causing the abnormal glucose metabolism in the diabetic state. A deeper understanding of the role of mitochondria in diabetes will provide us with novel insights in the pathophysiology of diabetes. (J Diabetes Invest, doi: 10.1111/j.2040-1124.2010.00047.x, 2010).

Prenatal and early postnatal exposure to high-saturated-fat diet represses Wnt signaling and myogenic genes in offspring rats

The prenatal and early postnatal period is a key developmental window for nutrition status, and high-fat exposure in this period has been shown to be associated with type 2 diabetes, obesity and other features of metabolic disorders later in life. The present study was designed to investigate the underlying molecular mechanisms and role of relative genes involved in this process. We investigated the impact of prenatal and early postnatal exposure to a high-saturated-fat diet on the regulation of the Wnt signaling pathway and myogenic genes in skeletal muscle of rat offspring as well as the serum and muscle physiological outcomes. Timed-pregnant Sprague-Dawley rats were fed either a control (C, 16% kcal fat) or high-saturated-fat diet (HF, 45% kcal fat) throughout gestation and lactation. After weaning, female offspring were fed a control diet to generate two offspring groups: control diet-fed offspring of control diet-fed dams (C/C) and control diet-fed offspring of HF diet-fed dams (HF/C). The serum glucose of the HF/C offspring (5.58 ± 0.26 mmol/L) was significantly higher than that of C/C offspring (4.97 ± 0.28 mmol/L), and the Homeostasis Model Assessment-Insulin Resistance of HF/C offspring (2.00 ± 0.11) was also significantly higher when compared with C/C (1.84 ± 0.09). Furthermore, HF/C offspring presented excessive intramuscular fat accumulation (1.8-fold, P < 0.05) and decreased muscle glycogen (1.3-fold, P < 0.05), as well as impairment of muscle development at the age of 12 weeks. Meanwhile, we observed the repression of Wnt/β-catenin signaling and myogenic genes in HF/C offspring. The present study indicates that prenatal and early postnatal exposure to a high-saturated-fat diet suppresses the development of skeletal muscle and myogenic genes via Wnt/β-catenin signaling, and the inappropriate muscle development could potentially contribute to the predisposition of offspring to develop metabolic-syndrome-like phenotype in adulthood.

Lipid overaccumulation and drastic insulin resistance in adult catch-up growth rats induced by nutrition promotion after undernutrition

This study was designed to explore the metabolic changes resulting from catch-up growth in adult (CUGA) induced by varying degrees of nutrition promotion after undernutrition and to confirm whether these changes are transient or not. The CUGA models were developed on rats refed on intakes of normal chow or high-fat diet after a period of caloric restriction. The growth of the rats measured by body weight and length stagnated during caloric restriction and then rapidly accelerated following refeeding. Catch-up growth in adult resulted in an increase in intramuscular and intrahepatic lipid content, visceral fat deposition, and insulin resistance, which is consistent with a transient rise in food efficiency during the early stage of refeeding. In addition, ectopic lipid deposition, visceral fat accumulation, and insulin resistance were more severe in rats refed the high-fat diet than rats refed the normal chow. These findings suggest that CUGA induced by rapid nutrition promotion could result in persistent lipid overaccumulation (increased visceral fat and ectopic lipid deposition) and drastic systemic insulin resistance. The effects of CUGA on metabolic characteristics are dependent on the type of diet that is used for refeeding, especially on the amount of fat intake.

Adenine nucleotide translocator as a regulator of mitochondrial function: implication in the pathogenesis of metabolic syndrome

Mitochondria play key roles in energy production and intracellular reactive oxygen species (ROS) generation. Lines of evidence have shown that mitochondrial dysfunction contributes to the development of metabolic syndrome. The causes of mitochondrial dysfunction are complex, but overnutrition and sedentary living are among the best known causes of mitochondrial dysfunction. ATP synthesized in the mitochondria is exchanged for cytosolic ADP by adenine nucleotide translocator (ANT) to provide a continuous supply of ADP to mitochondria. We recently found that ANT function is essential for peroxisome proliferator-activated receptor-γ coactivator 1-α (PGC-1α)'s action on endothelial cells. PGC-1α is a transcriptional coactivator of nuclear receptors, playing an important role in fatty acid oxidation and mitochondrial biogenesis. Recent studies have shown that PGC-1α decreases intracellular ROS generation by increasing the expression of antioxidant genes. In our study, PGC-1α reduced cell apoptosis and ROS generation in endothelial cells by increasing ATP/ADP translocase activity of ANT and ANT1 expression. Here we review the role of ANT in maintaining proper mitochondrial function, and possible role of ANT dysfunction in the pathogenesis of metabolic syndrome.

Efeitos do exercício físico na expressão e atividade da AMPKα em ratos obesos induzidos por dieta rica em gordura

INTRODUÇÃO: A ingestão de dieta hiperlipídica é um fator de risco singular no desenvolvimento de resistência à insulina e diabetes do tipo 2. OBJETIVO: O estudo investigou os efeitos do exercício físico na expressão e atividade da AMPKα em ratos obesos. MÉTODOS: Foram utilizados ratos Wistar, aleatoriamente divididos em quatro grupos, que receberam dieta padrão de manutenção (grupo controle) ou dieta hiperlipídica (DHL) (grupos sedentários e exercitados), por período de quatro meses. Dois diferentes protocolos de exercícios foram utilizados: exercício agudo ou crônico de natação. O teste de tolerância à insulina foi realizado para estimar a sensibilidade à insulina. Os níveis protéicos da AMPKα e do GLUT4 e também de p-AMPKα e pACC no músculo esquelético dos ratos foram determinados através da técnica de Western blot. RESULTADOS: O teste de tolerância à insulina revelou significativo prejuízo na ação da insulina após a alimentação com a DHL, indicando insulino-resistência quando comparado com grupo controle (p < 0,05). O tratamento por quatro meses com a DHL resultou em significativa redução no conteúdo protéico de AMPKα (2,2 vezes) e do GLUT4 (2,5 vezes) e nos níveis de p-AMPKα (2,4 vezes) e p-ACC (2,5 vezes) no músculo esquelético dos ratos sedentários quando comparado aos ratos controles. Ambos os protocolos de exercícios resultaram em aumento na fosforilação da AMPKα e ACC e aumento da sensibilidade à insulina, enquanto apenas o programa de exercício crônico promoveu o aumento da expressão dessas proteínas (p < 0,05). CONCLUSÃO: A alimentação com uma DHL causa redução na expressão e na atividade da AMPKα, enquanto a ativação da AMPKα pelo exercício físico melhora a sensibilidade à insulina, indicando que ratos obesos mantêm preservada a funcionalidade da via AMPKα.

Chronic insulin treatment suppresses PTP1B function, induces increased PDGF signaling, and amplifies neointima formation in the balloon-injured rat artery

We tested the hypothesis that hyperinsulinemia induces the suppression of protein tyrosine phosphatase 1B (PTP1B) function, leading to enhanced PDGF receptor (PDGFR) signaling and neointimal hyperplasia. Rats were implanted with insulin-releasing pellets or sham pellets. Blood glucose levels, insulin levels, food and water intake, body weights, and blood pressures were measured. Neointimal hyperplasia was assessed by computerized morphometry 14 days after carotid balloon injury. PTP1B protein expression in injured arteries was determined via Western blot analysis, whereas PTP1B activity was determined via an immunophosphatase assay. Serum insulin levels were two- to threefold greater in hyperinsulinemic rats, whereas systolic blood pressures, food and water intake, serum triglyceride levels, plasma cortisol levels, and urinary catecholamine levels were not affected. Fourteen days after injury, neointima-to-media area ratios were 0.89 +/- 0.23 and 1.35 +/- 0.22 in control and hyperinsulinemic rats, respectively (P < 0.01). PTP1B protein levels and total PTP1B activity in injured carotid arteries from the insulin-treated group were significantly decreased 7 or 14 days after injury, whereas PTP1B specific activity was decreased only 14 days after injury. These findings were associated with decreased PTP1B mRNA levels and increased PDGFR tyrosyl phosphorylation in insulin-treated rats. These observations support the hypothesis that hyperinsulinemia induces the suppression of PTP1B function, leading to enhanced PDGFR signaling and neointimal hyperplasia.

High-fat- and lipid-induced insulin resistance in rats: the comparison of glucose metabolism, plasma resistin and adiponectin levels

AIMS

In animal models, both an acute elevation in plasma free fatty acid (FFA) via intravenous infusion of a lipid emulsion and a chronic elevation in plasma FFA via high-fat feeding have been shown to induce skeletal muscle and liver insulin resistance. However, there have been very few studies comparing the effects of high-fat- and lipid-induced insulin resistance on glucose metabolism and adipocytokines.

METHODS

In the current study, we used lipid infusion and a high-fat feed in combination with the hyperinsulinemic-euglycemic clamp technique to assess the impact of acute and chronically elevated FFA levels on overall glucose metabolism and insulin action; two adipocytokines, resistin and adiponectin, were used.

RESULTS

At baseline, plasma FFA levels were significantly increased in the high-fat diet (HF) group compared to the control group (p < 0.05). During clamp steady-state, the FFA levels were reduced by approximately 25% in the control and approximately 48% in the HF groups. In contrast, there was a significant increase in the plasma FFA level in the lipid-infused group (from 0.82 +/- 0.03 to 2.87 +/- 0.18 mmol/l). The glucose infusion rates (GIRs) in the HF and lipid groups were obviously lower than in the control group (p < 0.01). Moreover, GIR was lower in the lipid group compared with the HF group (p < 0.05). The rate of glucose disappearance (G(Rd)) was significantly lower in the lipid group compared with the control group. Hepatic glucose production in the control group was suppressed by approximately 15% compared with the HF and lipid groups where it was suppressed by only approximately 72 and approximately 91%, respectively. The resistin level of muscle tissues in the lipid group was significantly higher compared with the control and HF groups (both p < 0.05). After the insulin clamp, the circulating adiponectin level was significantly decreased in the lipid group compared with the control and HF groups (p < 0.05).

CONCLUSIONS

Lipid infusion, which was more effective than a high-fat diet, can induce peripheral and hepatic insulin resistance in rats. Insulin-induced resistance might be associated with elevated resistin and decreased adiponectin.

Cellular and biochemical features of skeletal muscle in obese Yucatan minipigs

OBJECTIVE

To examine cellular and biochemical features of skeletal muscle in response to dietary-induced obesity in a novel Yucatan minipig model of childhood obesity.

RESEARCH METHODS AND PROCEDURES

From 4 to 16 months of age, minipigs were fed either a recommended human-type diet (NF; n = 4) or were overfed a western-type diet with saturated fat and high-glycemic index carbohydrates (OF, n = 4). Muscle samples (biceps femoris) were histochemically stained for the identification of intramuscular adipocytes, intramyocellular lipid aggregates (oil red O), and myofiber types (myosin ATPase, succinate dehydrogenase). Gene expressions and/or activities of factors involved in lipogenesis, lipolysis, or energetic metabolism were quantified in muscle.

RESULTS

Cross-sectional areas of myofibers paralleled pig body weight (r = 0.86, p < 0.01). The size of intramuscular adipocytes, the relative proportion of oil red O-stained fibers, and total muscle lipid content tended (p < or = 0.10) to increase in response to OF diet. Hormone-sensitive lipase, carnitine palmityl transferase-I, and uncoupling protein 2 mRNA levels were lower (p < 0.05) in OF pigs than in NF pigs. Activities of beta-hydroxyacyl-coenzyme A dehydrogenase and citrate synthase assessing post-carnitine palmityl transferase I events and the proportion of oxidative myofibers were not altered by OF diet. Activity and gene expression of fatty acid synthase were lower (p < 0.02) in OF pigs than in NF pigs.

DISCUSSION

Overfeeding in Yucatan minipigs reduced the expression levels of three catabolic steps in skeletal muscle that are involved also in the etiology of human obesity.

High-fat diet feeding impairs both the expression and activity of AMPKa in rats' skeletal muscle

OBJECTIVE

To investigate the effects of high-fat feeding on the expression and activity of AMPK in rats' skeletal muscle.

METHODS

Total 40 male Wistar rats were randomly divided into three groups and received either a rat maintenance diet (Control group) or an isocaloric rich-fat diet (HF group and MET group) for five months. Metformin was administered orally with the daily dose of 300mg in MET group during the last month of high-fat feeding. Hyperinsulinemic-euglycemic clamp study was performed to estimate whole-body insulin sensitivity. The ability of insulin-stimulated glucose uptake in isolated skeletal muscle was detected just before execution. mRNA levels of AMPKa1, AMPKa2, and Glut4 of rats' skeletal muscle were determined using real-time PCR. Protein contents of AMPKa, P-AMPKa, P-ACC, and Glut4 in rats' skeletal muscle were measured using Western blot.

RESULTS

(1) Hyperinsulinemic-euglycemic clamp study revealed a significantly impaired insulin action at the whole-body level after high-fat feeding (p<0.01). Also, both basal and insulin-stimulated glucose uptake in isolated skeletal muscle decreased after high-fat feeding (p<0.05), indicating onset of high-fat induced insulin resistance. (2) Five months of high-fat treatment induced a significant decrease of AMPKa protein contents and AMPKa2 mRNA levels in rats' skeletal muscles (p<0.05), while it did not alter AMPKa1 mRNA levels. Protein levels of P-AMPKa also decreased after high-fat feeding (p<0.01). These data suggest that high-fat exposure might impair AMPKa expression and activities. (3) P-ACC protein contents, mRNA and protein levels of Glut4 in rats' skeletal muscles also decreased after high-fat treatment (p<0.05). (4) Compared with HF group, although no significant alternations of AMPKa expression in rats' skeletal muscles were detected, P-AMPKa levels revealed a 162% increase after metformin treatment (p<0.05), demonstrating the AMPK-activating effect of metformin. Accompanied with activation of AMPKa, rats in MET group exhibited significantly elevated P-ACC contents, Glut4 mRNA and protein levels, and an obviously enhanced insulin sensitivity at both whole-body and skeletal muscle levels (p<0.05).

CONCLUSIONS

High-fat feeding impaired both the expression and activities of AMPKa, while activating AMPKa by metformin obviously ameliorated high-fat induced insulin resistance, thus indicating a possible role of AMPKa in lipotoxicity.