Correlation between insulin resistance and intramyocellular lipid levels in rats

Gender Differences in the Impact of a High-Fat, High-Sugar Diet in Skeletal Muscles of Young Female and Male Mice

In the context of the increasing number of obese individuals, a major problem is represented by obesity and malnutrition in children. This condition is mainly ascribable to unbalanced diets characterized by high intakes of fat and sugar. Childhood obesity and malnutrition are not only associated with concurrent pathologies but potentially compromise adult life. Considering the strict correlation among systemic metabolism, obesity, and skeletal muscle health, we wanted to study the impact of juvenile malnutrition on the adult skeletal muscle. To this aim, 3-week-old C56BL/6 female and male mice were fed for 20 weeks on a high-fat. high-sugar diet, and their muscles were subjected to a histological evaluation. MyHCs expression, glycogen content, intramyocellular lipids, mitochondrial activity, and capillary density were analyzed on serial sections to obtain the metabolic profile. Our observations indicate that a high-fat, high-sugar diet alters the metabolic profile of skeletal muscles in a sex-dependent way and induces the increase in type II fibers, mitochondrial activity, and lipid content in males, while reducing the capillary density in females. These data highlight the sex-dependent response to nutrition, calling for the development of specific strategies and for a systematic inclusion of female subjects in basic and applied research in this field.

Modeling insulin resistance in rodents by alterations in diet: what have high-fat and high-calorie diets revealed?

For over half a century, researchers have been feeding different diets to rodents to examine the effects of macronutrients on whole body and tissue insulin action. During this period, the number of different diets and the source of macronutrients employed have grown dramatically. Because of the large heterogeneity in both the source and percentage of different macronutrients used for studies, it is not surprising that different high-calorie diets do not produce the same changes in insulin action. Despite this, diverse high-calorie diets continue to be employed in an attempt to generate a "generic" insulin resistance. The high-fat diet in particular varies greatly between studies with regard to the source, complexity, and ratio of dietary fat, carbohydrate, and protein. This review examines the range of rodent dietary models and methods for assessing insulin action. In almost all studies reviewed, rodents fed diets that had more than 45% of dietary energy as fat or simple carbohydrates had reduced whole body insulin action compared with chow. However, different high-calorie diets produced significantly different effects in liver, muscle, and whole body insulin action when insulin action was measured by the hyperinsulinemic-euglycemic clamp method. Rodent dietary models remain an important tool for exploring potential mechanisms of insulin resistance, but more attention needs to be given to the total macronutrient content and composition when interpreting dietary effects on insulin action.

Treatment with Rhodiola crenulata root extract ameliorates insulin resistance in fructose-fed rats by modulating sarcolemmal and intracellular fatty acid translocase/CD36 redistribution in skeletal muscle

Background

Rhodiola species have been used for asthenia, depression, fatigue, poor work performance and cardiovascular diseases, all of which may be associated with insulin resistance. To disclose the underlying mechanisms of action, the effect of Rhodiola crenulata root (RCR) on insulin resistance was investigated.

Methods

Male Sprague-Dawley rats were treated with liquid fructose in their drinking water over 18 weeks. The extract of RCR was co-administered (once daily by oral gavage) during the last 5 weeks. The indexes of lipid and glucose homeostasis were determined enzymatically and/or by ELISA. Gene expression was analyzed by Real-time PCR, Western blot and/or confocal immunofluorescence.

Results

RCR extract (50 mg/kg) suppressed fructose-induced hyperinsulinemia and the increases in the homeostasis model assessment of insulin resistance index and the adipose tissue insulin resistance index in rats. Additionally, this treatment had a trend to restore the ratios of glucose to insulin and non-esterified fatty acids (NEFA) to insulin. Mechanistically, RCR suppressed fructose-induced acceleration of the clearance of plasma NEFA during oral glucose tolerance test (OGTT), and decreased triglyceride content and Oil Red O staining area in the gastrocnemius. Furthermore, RCR restored fructose-induced sarcolemmal overexpression and intracellular less distribution of fatty acid translocase/CD36 that contributes to etiology of insulin resistance by facilitating fatty acid uptake.

Conclusion

These results suggest that RCR ameliorates insulin resistance in fructose-fed rats by modulating sarcolemmal and intracellular CD36 redistribution in the skeletal muscle. Our findings may provide a better understanding of the traditional use of Rhodila species.

Investigation of Fat Metabolism during Antiobesity Interventions by Magnetic Resonance Imaging and Spectroscopy

The focus of current treatments for obesity is to reduce the body weight or visceral fat, which requires longer duration to show effect. In this study, we investigated the short-term changes in fat metabolism in liver, abdomen, and skeletal muscle during antiobesity interventions including Sibutra mine treatment and diet restriction in obese rats using magnetic resonance imaging, magnetic resonance spectroscopy, and blood chemistry. Sibutramine is an antiobesity drug that results in weight loss by increasing satiety and energy expenditure. The Sibutramine-treated rats showed reduction of liver fat and intramyocellular lipids on day 3. The triglycerides (TG) decreased on day 1 and 3 compared to baseline (day 0). The early response/nonresponse in different fat depots will permit optimization of treatment for better clinical outcome rather than staying with a drug for longer periods.

Effect of Hypericum perforatum L. extract on insulin resistance and lipid metabolic disorder in high-fat-diet induced obese mice

Natural product Hypericum perforatum L. has been used in folk medicine to improve mental performance. However, the effect of H. perforatum L. on metabolism is still unknown. In order to test whether H. perforatum L. extract (EHP) has an effect on metabolic syndrome, we treated diet induced obese (DIO) C57BL/6J mice with the extract. The chemical characters of EHP were investigated with thin-layer chromatography, ultraviolet, high-performance liquid chromatography (HPLC), and HPLC-mass spectrometry fingerprint analysis. Oral glucose tolerance test (OGTT), insulin tolerance test (ITT), and the glucose infusion rate (GIR) in hyperinsulinemic–euglycemic clamp test were performed to evaluate the glucose metabolism and insulin sensitivity. Skeletal muscle was examined for lipid metabolism. The results suggest that EHP can significantly improve the glucose and lipid metabolism in DIO mice. In vitro, EHP inhibited the catalytic activity of recombinant human protein tyrosine phosphatase 1B (PTP1B) and reduced the protein and mRNA levels of PTP1B in the skeletal muscle. Moreover, expressions of genes related to fatty acid uptake and oxidation were changed by EHP in the skeletal muscle. These results suggest that EHP may improve insulin resistance and lipid metabolism in DIO mice. © 2014 The Authors. Phytotherapy Research published by John Wiley & Sons Ltd.

A small volatile bacterial molecule triggers mitochondrial dysfunction in murine skeletal muscle

Mitochondria integrate distinct signals that reflect specific threats to the host, including infection, tissue damage, and metabolic dysfunction; and play a key role in insulin resistance. We have found that the Pseudomonas aeruginosa quorum sensing infochemical, 2-amino acetophenone (2-AA), produced during acute and chronic infection in human tissues, including in the lungs of cystic fibrosis (CF) patients, acts as an interkingdom immunomodulatory signal that facilitates pathogen persistence, and host tolerance to infection. Transcriptome results have led to the hypothesis that 2-AA causes further harm to the host by triggering mitochondrial dysfunction in skeletal muscle. As normal skeletal muscle function is essential to survival, and is compromised in many chronic illnesses, including infections and CF-associated muscle wasting, we here determine the global effects of 2-AA on skeletal muscle using high-resolution magic-angle-spinning (HRMAS), proton (¹H) nuclear magnetic resonance (NMR) metabolomics, in vivo³¹P NMR, whole-genome expression analysis and functional studies. Our results show that 2-AA when injected into mice, induced a biological signature of insulin resistance as determined by ¹H NMR analysis-, and dramatically altered insulin signaling, glucose transport, and mitochondrial function. Genes including Glut4, IRS1, PPAR-γ, PGC1 and Sirt1 were downregulated, whereas uncoupling protein UCP3 was up-regulated, in accordance with mitochondrial dysfunction. Although 2-AA did not alter high-energy phosphates or pH by in vivo³¹P NMR analysis, it significantly reduced the rate of ATP synthesis. This affect was corroborated by results demonstrating down-regulation of the expression of genes involved in energy production and muscle function, and was further validated by muscle function studies. Together, these results further demonstrate that 2-AA, acts as a mediator of interkingdom modulation, and likely effects insulin resistance associated with a molecular signature of mitochondrial dysfunction in skeletal muscle. Reduced energy production and mitochondrial dysfunctional may further favor infection, and be an important step in the establishment of chronic and persistent infections.

Improvement on Lipid Metabolic Disorder by 3′-Deoxyadenosine in High-Fat-Diet-Induced Fatty Mice

This study explores the effects of 3′-deoxyadenosine, a compound from Cordyceps militaris, on lipid metabolic disorder induced by a high-fat-diet in C57BL/6 mice. These mice had an obese body, lipid metabolic disorder and insulin resistance and were treated orally with 100 mg/kg/day 3′-deoxyadenosine (DA), 15 mg/kg/day rosiglitazone and 150 mg/kg/day fenofibrate, respectively. Compared to the model mice, the body weight gain in DA-treated mice were decreased by 66.5%, serum triglyceride and total cholesterol levels were decreased by 20.7% and 16.7%, respectively, and the triglyceride content in the skeletal muscle was reduced by 41.2%. This treatment also had a significant effect on insulin resistance. In DA-treated mice, the serum insulin levels and homeostasis model assessment of the insulin resistance index were decreased by 30% and 46%, respectively, and the areas under the glucose-time curve were depressed by 18% in the insulin tolerance test and by 21.5% in the oral glucose tolerance test. Finally, the value of glucose infusion rates and insulin induced-glucose uptake into the skeletal muscle in the hyperinsulinemic-euglycemic clamp test were increased by 18% and 41%, respectively, compared to those in the model mice. This data suggests that the effects of DA on lipid metabolic disorder induced by a high-fat-diet may be linked to its improvement on insulin resistance, especially concerning the increase of insulin sensitivity in the skeletal muscle.

Multitissue assessment of in vivo postprandial intracellular lipid partitioning in rats using localized 1H-[13C] magnetic resonance spectroscopy

Excess accumulation of lipids in nonadipose tissues such as skeletal muscle and liver has been implicated in the development of obesity-related disorders, but the cause of this ectopic lipid overload remains unknown. The aim of this study was to determine in vivo postprandial lipid partitioning in rat skeletal muscle and liver, using localized 1H-[13C] magnetic resonance spectroscopy in combination with the oral administration of 13C-labeled lipids. Six rats were measured at baseline and 5 and 24 h after administration of 400 mg [U-13C]-labeled algal lipids. Five hours after administration, fractional 13C enrichments of the lipid pools in muscle and liver were increased 3.9-fold and 4.6-fold (P<0.05), respectively, indicating that part of the ingested lipids had been taken up by muscle and liver tissue. At 24 h, fractional 13C enrichments of muscle and liver lipids were decreased 1.6-fold and 2.2-fold (P<0.05), respectively, compared with the 5 h values. This can be interpreted as a depletion of 13C-labeled lipids from the intracellular lipid pools as a consequence of lipid turnover. In conclusion, the novel application of 1H-[13C] magnetic resonance spectroscopy in combination with the oral administration of 13C-labeled lipids is applicable for the longitudinal assessment of in vivo lipid partitioning between multiple tissues.

Longitudinal evaluation of intramyocellular lipids (IMCLs) in tibialis anterior muscle of ob/ob and ob/+ control mice using a cryogenic surface coil at 9.4 T

Insulin resistance is a central feature of type II diabetes and is associated with alterations in skeletal muscle lipid metabolism, which manifest themselves, in part, in increased intramyocellular lipid (IMCL) accumulation. The objective of this study was to assess noninvasively the levels of IMCL longitudinally in the tibialis anterior muscle of Lep(ob) /Lep(ob) (ob/ob) mice, a genetic model of obesity and mild diabetes, and Lep(ob) /+ (ob/+) heterozygous control animals, using (1) H MRS at 9.4 T. The use of a cryogenic surface coil transceiver leads to significant increases in sensitivity. Method implementation included the assessment of the reproducibility and spatial heterogeneity of the IMCL signal and the determination of T(2) relaxation times, as IMCL levels were expressed relative to the total creatine signal, and therefore the signal ratios had to be corrected for differences in T(2) relaxation. IMCL levels were found to be significantly higher in ob/ob mice relative to ob/+ heterozygous control mice that do not develop disease. An increase in IMCL levels was observed for ob/ob mice until weeks 16/17; after this time point, IMCL levels decreased again, reaching final levels that were slightly higher than the initial values. These noninvasively detected alterations in skeletal muscle lipid metabolism in ob/ob mice were accompanied by a transient increase in plasma insulin concentrations. This study indicates that IMCL may be reliably assessed in mouse tibialis anterior muscle using a cryogenic surface coil, implying that (1) H MRS at 9.4 T represents a useful technology for the noninvasive measurement of changes in lipid metabolism in the skeletal muscle that accompany obesity.

Effects of encapsulated propolis on blood glycemic control, lipid metabolism, and insulin resistance in type 2 diabetes mellitus rats

The present study investigates the encapsulated propolis on blood glycemic control, lipid metabolism, and insulin resistance in type 2 diabetes mellitus (T2DM) rats. The animal characteristics and biological assays of body weight, fasting blood glucose (FBG), fasting serum insulin (FINS), insulin act index (IAI), triglycerides (TG), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), and low-density lipoprotein cholesterol (LDL-C) were measured and euglycemic hyperinsulinemic glucose clamp technique were used to determine these effects. Our findings show that oral administration of encapsulated propolis can significantly inhibit the increasing of FBG and TG in T2DM rats and can improve IAI and M value in euglycemic hyperinsulinemic clamp experiment. There was no significant effects on body weight, TC, HDL-C, and LDL-C in T2DM rats treated with encapsulated propolis. In conclusion, the results indicate that encapsulated propolis can control blood glucose, modulate lipid metabolism, and improve the insulin sensitivity in T2DM rats.

Effects of castration-induced visceral obesity and antioxidant treatment on lipid profile and insulin sensitivity in New Zealand white rabbits

Molecular mechanisms, responsible for the impaired insulin-sensitivity state due to the obesity are not fully understood in both humans and animals. The purpose of this study was to investigate the effects of castration-induced visceral obesity and the influence of two antioxidants on constituents of blood lipid profile and insulin sensitivity in New Zealand white rabbits. Twenty-six clinically healthy male New Zealand white rabbits were used in the experiment and were divided into 3 groups: first group (CI, n=7) - castrated-obese and treated with antioxidants "Immunoprotect" for 2months; second group (CO, n=7) - castrated-obese; third group (NC, n=12) - control group (non-castrated, non-obese). At the end of the follow-up period of 2months after castration an intravenous glucose tolerance test (IVGTT) was performed after a 12-h fasting period as the blood samples for determination of glucose and insulin and their kinetic parameters were obtained at 5 and 0min before and at 5, 10, 30, 60 and 120min after the infusion of the glucose. The constituents of lipid profile, triglycerides (TG), total cholesterol (TC) and HDL-cholesterol (HDL-C) were also assessed in the overnight fasting blood samples. The body weight (BW), body mass index (BMI), amount of the visceral fat (VF) and VF/BW ratio were both measured and calculated before the IVGTT and at the end of the experimental period. All measured markers of obesity (BW, BMI, VF, VF/BW) were significantly higher in both groups of castrated rabbits than in the control group. Apart HDL-C, the plasma concentrations of all constituents of lipid profile (TG, TC, HDL-C) were the highest in CO group. There were generally no differences between CI and NC groups for the same traits. After glucose injection blood glucose concentrations and glucose and insulin kinetic parameters were considerably higher (except of glucose elimination rate) in CO rabbits than in NC ones. Castrated rabbits treated with "Immunoprotect" showed lower fasting plasma insulin and improved glucose kinetics dynamics than CO rabbits, but commensurable values of glucose and insulin kinetics parameters than NC group. The results of the current study clearly indicated that castration-induced visceral obesity affected negatively the lipid profile and insulin sensitivity and/or responsiveness. Treatment with antioxidant supplementation, consisted of d-limonene and vitamin E, improved blood lipid profile, fatty liver, glucose homeostasis and insulin sensitivity in obese rabbits. In addition, based on our results we may suggest that castrated male New Zealand white rabbits might be considered as an appropriate animal model to study various metabolic abnormalities related to visceral obesity, such as dyslipidemia and impaired insulin sensitivity.

Inductively coupled Helmholtz coil on a dedicated imaging platform for the in vivo 1H-MRS measurement of intramyocellular lipids in the hind leg of rats

Skeletal muscle triglycerides are markers for insulin resistance in type 2 diabetes. Recently, MR spectroscopy was adapted for in vivo measurement of triglycerides in animal models and for the characterization of new therapeutic approaches. Because of small MR spectroscopy voxel sizes used in skeletal muscles, surface coils are used for signal reception. Furthermore, to obtain well-resolved and undistorted lipid spectra, muscle fibers must be aligned parallel to the magnetic field. Consequently, to achieve a high signal-to-noise ratio and spectral quality, a coil setup must combine high sensitivity with a reliable and reproducible positioning of muscle and voxel. These demands are difficult to match using surface coils. Here, a coil platform is described, which uses inductively coupled Helmholtz coil setup combined with a leg retainer system for rats. The new system allows for measurement of intramyocellular lipids with high signal-to-noise ratio and for significantly improved animal handling, positioning, and throughput.

Adipocyte differentiation-related protein and OXPAT in rat and human skeletal muscle: involvement in lipid accumulation and type 2 diabetes mellitus

SETTING

A disordered lipid metabolism is implicated in the development of skeletal muscle insulin resistance. Lipid droplet proteins of the PAT [perilipin, adipocyte differentiation-related protein (ADRP), and TIP47] family have been shown to regulate lipid accumulation and intracellular metabolism in other tissues.

OBJECTIVE

This study aimed to explore the role of the PAT proteins OXPAT and ADRP in skeletal muscle lipid metabolism and their putative role in modulating insulin sensitivity.

DESIGN

Muscle OXPAT and ADRP protein content was examined during the development of insulin resistance in Zucker diabetic fatty (ZDF) rats and in type 2 diabetes patients and BMI-matched control subjects. Furthermore, we examined the effect of 8 wk of insulin sensitizing by rosiglitazone on muscle OXPAT and ADRP content.

RESULTS

OXPAT and ADRP protein expression is muscle fiber type specific in humans and rats, with highest protein content in fibers containing most intramyocellular lipids (IMCL). Muscle OXPAT and ADRP protein content was 2- to 3-fold higher in ZDF rats during the progression of type 2 diabetes than in lean normoglycemic control rats, which was paralleled by high IMCL levels. Muscle OXPAT and ADRP content, as well as IMCL level, was not different between type 2 diabetes patients and control subjects. ADRP content was negatively associated with insulin-stimulated glucose uptake (r = -0.50; P = 0.017). Interestingly, rosiglitazone treatment decreased muscle OXPAT (-29%) and ADRP (-28%) content in diabetes patients, without affecting IMCL.

CONCLUSIONS

These results indicate involvement of OXPAT and ADRP in muscular lipid accumulation and type 2 diabetes.

In vivo assessment of mouse hindleg intramyocellular lipids by 1H-MR spectroscopy

RATIONALE AND OBJECTIVES

(1)H-magnetic resonance spectroscopy ((1)H-MRS) has proved to be the sole in vivo technique able to measure intramyocellular lipids (IMCL) in both humans and animals. Mouse models are now widely used for physiologic studies and drug discovery. However, IMCL assessment using (1)H-MRS is hindered in this animal model by the small muscle size and strong contamination from the extramyocellular lipid (EMCL) signal. The objective of this study was to the use of (1)H-MRS for IMCL quantification in mice at different ages.

MATERIALS AND METHODS

Noninvasive IMCL quantification was performed at 7 T in tibialis anterior (TA) muscles of healthy male C57/BL6 mice (n = 9; age, 13.6 +/- 1 months), db/db mice (n = 4), and their C57BL/KSJ control littermates (n = 4) at 7 and 17 weeks of age.

RESULTS

The IMCL content of diabetic mice TA was significantly higher than their littermates (2.41 +/- 0.5 vs. 1.21 +/- 0.35, P < .01). An age effect was observed, with TA IMCL levels being lower in older than younger control mice, but increasing between 7 and 17 weeks in the db/db mice.

CONCLUSIONS

The feasibility of (1)H-MRS spectroscopy was demonstrated in mice muscle, despite its small size, and used to assess IMCL content in db/db mice.

Dietary fructose during the suckling period increases body weight and fatty acid uptake into skeletal muscle in adult rats

OBJECTIVE

The suckling period is one potentially "critical" period during which nutritional intake may permanently "program" metabolism to promote increased adult body weight and insulin resistance in later life. This study determined whether fructose introduced during the suckling period altered body weight and induced changes in fatty acid transport leading to insulin resistance in adulthood in rats.

METHODS AND PROCEDURES

Pups were randomly assigned to one of four diets: suckle controls (SCs), rat milk substitute formula (Rat Milk Substitute), fructose-containing formula (Fructose), or galactose-containing formula (Galactose). Starting at weaning, all pups received the same diet; at 8 weeks of age, half of the SC rats began ingesting a diet containing 65% kcal fructose (SC-Fructose). This continued until animals were 12 weeks old and the study ended.

RESULTS

At weeks 8, 10, and 11, the Fructose group weighed more than SC and SC-Fructose groups (P < 0.05). At weeks 8 and 10 of age, the Fructose group had significantly higher insulin concentrations vs. rats in the SC-Fructose group. (3)H-Palmitate transport into vesicles from hind limb skeletal muscle was higher in Fructose vs. SC rats (P < 0.05). CD36 expression was increased in the sarcolemma but not in whole tissue homogenates from skeletal muscle from Fructose rats (P < 0.05) suggesting a redistribution of this protein associated with fatty acid uptake across the plasma membrane. This change in subcellular localization of CD36 is associated with insulin resistance in muscle.

DISCUSSION

Consuming fructose during suckling may result in lifelong changes in body weight, insulin secretion, and fatty acid transport involving CD36 in muscle and ultimately promote insulin resistance.

Increased intramyocellular lipid content but normal skeletal muscle mitochondrial oxidative capacity throughout the pathogenesis of type 2 diabetes

Currently inherited or acquired skeletal muscle mitochondrial dysfunction is linked to dysregulated fatty acid metabolism, resulting in increased levels of intramyocellular lipids (IMCLs) and lipid intermediates, inducing insulin resistance. The present study aimed to clarify the order of changes in IMCL levels and skeletal muscle mitochondrial function during the development of type 2 diabetes in Zucker diabetic fatty (ZDF) rats. IMCL levels and skeletal muscle oxidative capacity were determined in vivo, using localized (1)H magnetic resonance spectroscopy (MRS) and dynamic (31)P MRS, respectively. In parallel, in vitro activities were measured from enzymes involved in fatty acid oxidation, the tricarboxylic acid cycle and the electron transport chain. Fa/fa ZDF rats were studied at 3 different ages corresponding to different stages of type 2 diabetes, whereas fa/+ rats served as controls. Fa/fa ZDF rats had higher IMCL contents than controls throughout the duration of the study. In vivo muscle oxidative capacity was not different in fa/fa animals compared to controls, and in vitro enzyme activity data suggested improved functionality of enzymes involved in fat oxidation in type 2 diabetic animals. Accordingly, we can conclude that in the ZDF rat model, type 2 diabetes develops in the absence of skeletal muscle mitochondrial dysfunction.

Insulin Resistance, Hepatic Lipid and Adipose Tissue Distribution in HIV-Infected Men

Background

A large proportion of HIV-infected patients on antiretroviral medication develop insulin resistance, especially in the context of fat redistribution. This study investigates the interrelationships among fat distribution, hepatic lipid content, and insulin resistance in HIV-infected men.

Methods

We performed a cross-sectional analysis of baseline data from 23 HIV-infected participants in three prospective clinical studies. Magnetic resonance spectroscopy was used to quantify hepatic lipid concentrations. Magnetic resonance imaging was used to quantify whole-body adipose tissue compartments: that is, subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) volumes, as well as the intermuscular adipose tissue (IMAT) subcompartment and the omental-mesenteric adipose tissue (OMAT) and retroperitoneal adipose tissue (RPAT) subcompartments of VAT. The homeostasis model for assessment of insulin resistance (HOMA-IR) was calculated from fasting glucose and insulin concentrations.

Results

Hepatic lipid content correlated significantly with total VAT (r=0.62, P=0.0014), but not with SAT (r=0.053, P=0.81). In univariate analysis, hepatic lipid content was associated with the OMAT (r=0.67, P=0.0004) and RPAT (r=0.53, P=0.009) subcompartments; HOMA-IR correlated with both VAT and hepatic lipid contents (r=0.61, P=0.057 and r=0.68, P=0.0012, respectively). In stepwise linear regression models, hepatic lipid had the strongest associations with OMAT and with HOMA-IR.

Conclusion

Hepatic lipid content is associated with VAT volume, especially the OMAT subcompartment, in HIV-infected men. Hepatic lipid content is associated with insulin resistance in HIV-infected men. Hepatic lipid content might mediate the relationship between VAT and insulin resistance among treated, HIV-infected men.

Effect of dexamethasone on glucose tolerance and fat metabolism in a diet-induced obesity mouse model

Prolonged exposure to elevated glucocorticoid levels is known to produce insulin resistance (IR), a hallmark of diabetes mellitus. Although not fully elucidated, the underlying molecular mechanisms by which glucocorticoids induce IR may provide potential targets for pharmacological interventions. Here we characterized muscle lipid metabolism in a dexamethasone-aggravated diet-induced obesity murine model of IR. Male C57BL/6 mice on a high-fat diet for 2 months when challenged with dexamethasone showed elevated food consumption and weight gain relative to age and diet-matched animals dosed with saline only. Dexamethasone treatment impaired glucose tolerance and significantly increased the intramyocellular lipid content in the tibialis anterior muscle (TA). A good correlation (r = 0.76, P < 0.01) was found between accumulation in intramyocellular lipid content in the TA and visceral adiposity. The linoleic acid (18:2) to polyunsaturated acid ratio was increased in the dexamethasone-treated animals (+29%; P < 0.01), suggesting a possible increase in stearoyl-CoA desaturase 2 activity, as reported in Sertoli cells. The treatment was also accompanied by a reduction in the percent fraction of omega-3 and long-chain polyunsaturated fatty acids in the TA. Analysis of the low-molecular-weight metabolites from muscle extracts showed that there was no dysregulation of muscle amino acids, as has been associated with dexamethasone-induced muscle proteolysis. In conclusion, dexamethasone-induced insulin resistance in diet-induced obese mice is associated with a profound perturbation of lipid metabolism. This is particularly true in the muscle, in which an increased uptake of circulating lipids along with a conversion into diabetogenic lipids can be observed.

Dysregulated pyruvate dehydrogenase complex in Zucker diabetic fatty rats

The mitochondrial pyruvate dehydrogenase complex (PDC) is inactivated in many tissues during starvation and diabetes. We investigated carbohydrate oxidation (CHO) and the regulation of the PDC in lean and obese Zucker diabetic fatty (ZDF) rats during fed and starved conditions as well as during an oral glucose load without and with pharmacologically reduced levels of free fatty acids (FFA) to estimate the relative contribution of FFA on glucose tolerance, CHO, and PDC activity. The increase in total PDC activity (20-45%) was paralleled by increased protein levels ( approximately 2-fold) of PDC subunits in liver and muscle of obese ZDF rats. Pyruvate dehydrogenase kinase-4 (PDK4) protein levels were higher in obese rats, and consequently PDC activity was reduced. Although PDK4 protein levels were rapidly downregulated (57-62%) in both lean and obese animals within 2 h after glucose challenge, CHO over 3 h as well as the peak of PDC activity (1 h after glucose load) in liver and muscle were significantly lower in obese rats compared with lean rats. Similar differences were obtained with pharmacologically suppressed FFA by nicotinic acid, but with significantly improved glucose tolerance in obese rats, as well as increased CHO and delta increases in PDC activity (0-60 min) both in muscle and liver. These results demonstrated the suppressive role of FFA acids on the measured parameters. Furthermore, the results clearly demonstrate a rapid reactivation of PDC in liver and muscle of lean and obese rats after a glucose load and show that PDC activity is significantly lower in obese ZDF rats.

Intramyocellular lipid content is lower with a low-fat diet than with high-fat diets, but that may not be relevant for health

BACKGROUND

Fat deposition in muscle has been found to be related to metabolic risk.

OBJECTIVE

This study compared soleus intramyocellular lipid (IMCL) concentrations after consumption of weight-maintaining, controlled diets differing in total fat and fat type.

DESIGN

This study consisted of 3 phases of 25 d each in a crossover, controlled feeding design. The low-fat (LF) diet provided 30.8% and 5.2% of energy from fat and polyunsaturated fat (PUFA), respectively. Two higher-fat diets were tested: the high-fat (HF) diet provided 37.9% and 5.8% of energy from fat and PUFA, respectively, and the high-PUFA (HPUFA) diet provided 36.3% and 9.7% of energy from fat and PUFA, respectively. Twenty-four men and women [age range: 19-65 y; body mass index (in kg/m(2)): 20-35] whose LDL and glucose concentrations were between 130 and 180 mg/dL and <126 mg/dL, respectively, completed all study phases.

RESULTS

IMCL content was 1.88 times as high after the HF diet (P = 0.005) and 1.71 times as high after the HPUFA diet (P = 0.002) as after the LF diet. There was no significant correlation between percentage fat mass or waist circumference and IMCL content. With pooled data from all diets, there was no significant correlation between IMCL content and insulin or glucose concentration. There was no significant difference in IMCL content in subjects with or without the metabolic syndrome or in subjects with LDL particle pattern A or B.

CONCLUSIONS

Our results suggest that IMCL content is not modulated by dietary fat type but by total fat intake and that reducing fat intake effectively lowers IMCL. However, the metabolic implications of having lower IMCL concentrations are not clear.

Regional variations in intramyocellular lipid concentration correlate with muscle fiber type distribution in rat tibialis anterior muscle

1H MR spectroscopy (MRS) has proved to be a valuable noninvasive tool to measure intramyocellular lipids (IMCL) in research focused on insulin resistance and type II diabetes in both humans and rodents. An important determinant of IMCL is the muscle fiber type, since oxidative type I fibers can contain up to three times more IMCL than glycolytic type II muscle fibers. Because these different muscle fiber types are inhomogeneously distributed in rodent muscle, in the present study we investigated the distribution of IMCL within the rat tibialis anterior muscle (TA) in vivo using single-voxel 1H MRS along with the muscle fiber distribution in the TA ex vivo determined from immunohistological assays. IMCL levels in the TA differed by up to a factor of 3 depending on the position of the voxel. The distribution of IMCL over the TA cross section was not random, but emerged in a pattern similar to the distribution of the predominantly oxidative muscle fiber types. Dietary interventions, such as high-fat feeding and 15 hr of fasting, did not significantly change this typical fiber type-dependent pattern of IMCL content. These results stress the importance of voxel positioning when single-voxel 1H MRS is used to study IMCL in rodent muscle.

Role of proton MR for the study of muscle lipid metabolism

1H-MR spectroscopy (MRS) of intramyocellular lipids (IMCL) became particularly important when it was recognized that IMCL levels are related to insulin sensitivity. While this relation is rather complex and depends on the training status of the subjects, various other influences such as exercise and diet also influence IMCL concentrations. This may open insight into many metabolic interactions; however, it also requires careful planning of studies in order to control all these confounding influences. This review summarizes various historical, methodological, and practical aspects of 1H-MR spectroscopy (MRS) of muscular lipids. That includes a differentiation of bulk magnetic susceptibility effects and residual dipolar coupling that can both be observed in MRS of skeletal muscle, yet affecting different metabolites in a specific way. Fitting of the intra- (IMCL) and extramyocellular (EMCL) signals with complex line shapes and the transformation into absolute concentrations is discussed. Since the determination of IMCL in muscle groups with oblique fiber orientation or in obese subjects is still difficult, potential improvement with high-resolution spectroscopic imaging or at higher field strength is considered. Fat selective imaging is presented as a possible alternative to MRS and the potential of multinuclear MRS is discussed. 1H-MRS of muscle lipids allows non-invasive and repeated studies of muscle metabolism that lead to highly relevant findings in clinics and patho-physiology.

Effect of nutritional counselling on hepatic, muscle and adipose tissue fat content and distribution in non-alcoholic fatty liver disease

AIM: To assess the effectiveness of the current UK clinical practice in reducing hepatic fat (IHCL).

METHODS: Whole body MRI and ¹H MRS were obtained, before and after 6 mo nutritional counselling, from liver, soleus and tibialis muscles in 10 subjects with non-alcoholic fatty liver disease (NAFLD).

RESULTS: A 500 Kcal-restricted diet resulted in an average weight loss of 4% (-3.4 kg,) accompanied by significant reductions in most adipose tissue (AT) depots, including subcutaneous (-9.9%), abdominal subcutaneous (-10.2%) and intra-abdominal-AT (-11.4%). Intramyocellular lipids (IMCL) were significantly reduced in the tibialis muscle (-28.2%). Decreases in both IHCL (-39.9%) and soleus IMCL (-12.2%) content were also observed, although these were not significant. Several individuals showed dramatic decreases in IHCL, while others paradoxically showed increases in IHCL content. Changes in body composition were accompanied by improvements in certain liver function tests: serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT). Significant correlations were found between decreases in IHCL and reductions in both intra-abdominal and abdominal subcutaneous AT. Improvements in liver function tests were associated with reductions in intra-abdominal AT, but not with changes in IHCL.

CONCLUSION: This study shows that even a very modest reduction in body weight achieved through lifestyle modification can result in changes in body fat depots and improvements in LFTs.

Muscle type-specific fatty acid metabolism in insulin resistance: an integrated in vivo study in Zucker diabetic fatty rats

Intramyocellular lipid content (IMCL) serves as a good biomarker of skeletal muscle insulin resistance (IR). However, intracellular fatty acid metabolites [malonyl-CoA, long-chain acyl-CoA (LCACoA)] rather than IMCL are considered to be responsible for IR. This study aimed to investigate dynamics of IMCL and fatty acid metabolites during fed-to-starved-to-refed transition in lean and obese (IR) Zucker diabetic fatty rats in the following different muscle types: soleus (oxidative), extensor digitorum longus (EDL, intermediary), and white tibialis anterior (wTA, glycolytic). In the fed state, IMCL was significantly elevated in obese compared with lean rats in all three muscle types (soleus: 304%, EDL: 333%, wTA: 394%) in the presence of elevated serum triglycerides but similar levels of free fatty acids (FFA), malonyl-CoA, and total LCACoAs. During starvation, IMCL in soleus remained relatively constant, whereas in both rat groups IMCL increased significantly in wTA and EDL after comparable dynamics of starvation-induced FFA availability. The decreases of malonyl-CoA in wTA and EDL during starvation were more pronounced in lean than in obese rats, although there were no changes in soleus muscles for both groups. The concomitant increase in IMCL with the fall of malonyl-CoA support the concept that, as a reaction to starvation-induced FFA availability, muscle will activate lipid oxidation more the lower its oxidative capacity and then store the rest as IMCL.