Reduction of intramyocellular lipid following short-term rosiglitazone treatment in Zucker fatty rats: an in vivo nuclear magnetic resonance study

Quantitative Analysis Concerning Atrophy and Fat Infiltration of the Multifidus Muscle with Magnetic Resonance Spectroscopy in Chronic Low Back Pain

INTRODUCTION

Magnetic resonance spectroscopy (MRS) enables detailed analysis of the composition of muscular fat tissues such as intramyocellular lipids (IMCLs) and extramyocellular lipids (EMCLs). The aim of this study was to analyze the EMCL and IMCL of the multifidus muscle (Mm) using MRS in chronic low-back pain (CLBP) patients and identify their possible correlations with age, body mass index (BMI), low-back pain (LBP) visual analog scale (VAS) score, cross-sectional area (CSA), and fat infiltration of the Mm.

METHODS

Eighty patients (32 men and 48 women; mean age, 64.7 ± 1.3 years; range, 22-83 years) with VAS scores >30 mm for CLBP were included. We analyzed the gender difference and the possible correlations of age, BMI, LBP VAS, CSA, and fat infiltration of the Mm with the IMCL and EMCL of the Mm. The subjects were divided into five groups as per their age range: < 40s, 50s, 60s, 70s, and 80s. We also analyzed the EMCL and IMCL of the Mm as per the fat infiltration classification.

RESULTS

CSA was larger in the male group, EMCL was higher in the female group, and there was no significant difference in IMCL between the female and male groups. There was a significant positive correlation of EMCL with age (r = 0.33, p < 0.01) and BMI (r = 0.42, p < 0.01) and a significant negative correlation of EMCL with CSA (r = -0.61, p < 0.01). There was a significant positive correlation between IMCL and VAS (r = 0.43, p < 0.01). The EMCL and CSA of the Mm decreased with age, whereas fat infiltration increased with age.

CONCLUSIONS

These results suggest that EMCL could indicate Mm degeneration associated with aging, and IMCL could be an effective objective indicator of CLBP. The EMCL and IMCL of the Mm may be useful prognostic markers in rehabilitation strategies.

Multifidus Muscles Lipid Content Is Associated with Intervertebral Disc Degeneration: A Quantitative Magnetic Resonance Imaging Study

Study Design

Cross-sectional study.

Purpose

To determine the association between fatty degeneration of the multifidus muscle (Mm) and intervertebral disc degeneration (IVDD) using quantitative magnetic resonance imaging (MRI).

Overview of Literature

Few studies have reported on quantitative MRI analysis of the relation between the Mm and IVDD.

Methods

The subjects with chronic low back pain comprised 45 patients (19 males, 26 females; mean age, 63.8±2.0 years; range, 41-79 years). We analyzed the intramyocellular lipids (IMCL) and extramyocellular lipids (EMCL) of the Mm using magnetic resonance spectroscopy. The T2 values of the anterior annulus fibrosus (AF), nucleus pulposus (NP), and posterior AF were evaluated using MRI T2 mapping. We compared the possible correlations of IMCL and EMCL of the Mm with the T2 values of anterior AF, NP, and posterior AF.

Results

There was a significant negative correlation between IMCL and T2 values of the anterior AF (r=-0.65, p<0.01). There were no significant correlations between the IMCL and T2 values of NP (r=-0.16, p=0.30) and posterior AF (r=0.07, p=0.62). There were no significant correlations between the EMCL and T2 values of the anterior AF (r=-0.11, p=0.46), NP (r=0.15, p=0.32), and posterior AF (r=0.07, p=0.66). After adjustment for age and sex using multiple linear regression analysis, there was a significant negative correlation between the IMCL and T2 values of anterior AF (standardized partial regression coefficient=-0.65, p<0.01).

Conclusions

The results indicated that IMCL of the Mm might be accompanied with anterior AF degeneration. Therapeutic exercises using IMCL of the Mm as evaluation index might have the potential to identify novel targets for the treatment and prevention of IVDD.

Magnetic resonance spectroscopic analysis of multifidus muscles lipid content and association with spinopelvic malalignment in chronic low back pain

OBJECTIVE

To analyze intramyocellular lipids (IMCLs) and extramyocellular lipids (EMCLs) of the multifidus muscle (Mm) using MR spectroscopy in chronic low back pain (CLBP) and control groups and to identify correlations with spinopelvic alignment.

METHODS

40 patients (16 males, 24 females; mean age, 62.9 ± 1.9 years) whose visual analogue scale scores were >30 mm for CLBP were included. Furthermore, 40 control participants matched with the CLBP group subjects by sample size, gender and age (17 males, 23 females; mean age, 65.0 ± 1.2 years) were included. We compared the body mass index, physical workload, leisure time physical activity level, spinopelvic parameters, and IMCLs and EMCLs of the Mm between the groups. We also evaluated possible correlations of spinopelvic parameters with IMCLs and EMCLs of the Mm in the groups.

RESULTS

There were no statistically significant differences in body mass index, physical workload, exercise intensity level, spinopelvic parameters and EMCLs between the groups. The IMCLs were significantly higher in the CLBP group than in the control group (p < 0.01). In the CLBP group, there was a significantly negative correlation between IMCLs and lumbar lordosis (r = -0.64, p < 0.01) and a significantly positive correlation between IMCLs and sagittal vertical axis (r = 0.43, p < 0.01).

CONCLUSION

The measurement of IMCLs might be a characteristic finding of CLBP as well as a precursor to spinal deformity. Advances in knowledge: IMCLs of the Mm may be a useful prognostic marker in rehabilitation strategies for patients with CLBP.

The effect of exercise training combined with PPARγ agonist on skeletal muscle glucose uptake and insulin sensitivity in induced diabetic obese Zucker rats

[Purpose]

Exercise training with PPARγ agonist is expected to increase glucose uptake and improve insulin sensitivity in skeletal muscle of patients with diabetes. However, its mechanisms to effect glucose uptake and insulin sensitivity in skeletal muscle are unclear.

[Methods]

The mechanism of action was determined by co-treatment with PPARγ agonist- rosiglitazone and exercise training in streptozotocin induced-diabetic obese Zucker rats. Exercise training was carried out for 6 weeks (swimming, 1 h/day, 5 times/week, 5% weight/g, 32±1℃) with rosiglitazone treatment (3mg/kg/day, 6weeks).

[Results]

Glucose uptake and insulin sensitivity was decreased in diabetic than normal animals. Exercise training and rosiglitazone treatment respectively increased the expression of PPAR(peroxisome proliferators-activated receptor)-α, -β/δ, -γ, PGC-1α(PPAR-γ coactivator-1α), adiponectin, GLUT-4(glucose transportor-4) and p-AMPK-α₂(phospho-AMP activated protein kinase-α₂) in EDL and SOL of diabetic, as compared to normal animals. Interestingly, training combined with rosiglitazone significantly increased glucose uptake and insulin sensitivity, which resulted in high expression of all molecules in diabetic than all other groups.

[Conclusion]

These results indicated that exercise training combined with rosiglitazone might mediate regulation of glucose uptake and insulin sensitivity in skeletal muscle. Therefore, exercise training combined with rosiglitazone may be recommended as complementary therapies for diabetes.

Evaluation of intramyocellular and extramyocellular lipids in the paraspinal muscle in patients with chronic low back pain using MR spectroscopy: preliminary results

OBJECTIVE:

To analyze the intramyocellular lipids (IMCL) and extramyocellular lipids (EMCL) in the psoas muscle (Pm) and multifidus muscle (Mm) using MR spectroscopy (MRS) in patients with chronic low back pain (CLBP) and asymptomatic volunteers.

METHODS:

This prospective study had institutional review board approval, and written informed consent was obtained from all individual participants. 20 patients with CLBP (mean age, 49.8 ± 12.5 years; age range, 28-66 years) with low back pain (LBP) duration of ≥3 months and asymptomatic volunteers (mean age, 44.0 ± 12.8 years; age range, 28-65 years) underwent MRS for the quantification of IMCL and EMCL of the right Pm and Mm in a volume of interest at the intervertebral level of L4 through L5. We compared patients with CLBP and asymptomatic volunteers for age and body mass index (BMI), IMCL and EMCL of Pm and Mm. The Mann-Whitney U-test was used for significant difference testing, with p-value <0.05 indicating statistical significance.

RESULTS:

No statistically significant differences were observed in the age and BMI of patients with CLBP and asymptomatic volunteers. There was no statistically significant difference in the EMCL and IMCL in Pm between the two groups. EMCL in Mm was not statistically significantly different; the IMCL of patients with CLBP was significantly higher than that of asymptomatic volunteers (p < 0.05).

CONCLUSION:

MRS demonstrates that IMCL in the Mm of patients with CLBP is significantly higher than that of asymptomatic volunteers. The increase of IMCL in the Mm may be a characteristic finding in CLBP.

ADVANCES IN KNOWLEDGE:

We investigated the detail of the fat content in the paraspinal muscles in patients CLBP and asymptomatic volunteers. The increase of fat content in the Mm of patients with LBP on previous studies was demonstrated to be the increase of IMCL.

Not only accumulation, but also saturation status of intramuscular lipids is significantly affected by PPARγ activation

AIM

Intramuscular lipid accumulation has been associated with insulin resistance, and after thiazolidinediones (TZD) treatment, it was shown to be reduced in some, but not all, studies. This work was undertaken to investigate the relationships between intramuscular lipids [free fatty acids (FFA), diacylglycerols (DAG), triacylglycerol (TAG) and phospholipids] and plasmalemmal expression of fatty acid (FA) transporter [FAT/CD36 and FABPpm] in the muscles of varying oxidative capacity, after peroxisome proliferator-activated receptors gamma (PPARγ) activation (rosiglitazone) in an animal model of high-fat-diet-induced insulin resistance. Endurance training was also included to further explore the differences in these relationships.

METHODS

We have used gas liquid chromatography to estimate FA content and composition in each lipid fraction. For sarcolemmal expression of FA transporters, subfractionation of skeletal muscles with subsequent western blot technique was applied.

RESULTS

High-fat diet induced intramuscular accumulation of FFA, DAG and TAG, irrespective of muscle's fibre composition. PPARγ activation (rosiglitazone) and, to a lesser extent, endurance training further increased TAG accumulation, while it reduced DAG in oxidative muscles (soleus and red gastrocnemius). Aforementioned interventions increased also sarcolemmal FAT/CD36 and FABPpm expressions in particular muscles. Irrespective of diet, rosiglitazone and exercise decreased significantly FA saturation status favouring proportionate enhancement in monounsaturated FA (rosiglitazone) or polyunsaturated FAs (endurance training).

CONCLUSION

These findings support the conclusion that not only the change in total lipid content (DAG and TAG), but also FA composition is affected by rosiglitazone in an animal model of high-fat-diet-induced insulin resistance.

The role of muscle insulin resistance in the pathogenesis of atherogenic dyslipidemia and nonalcoholic fatty liver disease associated with the metabolic syndrome

The metabolic syndrome is a clustering of cardiovascular risk factors, including insulin resistance, abdominal obesity, dyslipidemia, and hypertension, and is associated with other comorbidities such as a proinflammatory state and nonalcoholic fatty liver disease (NAFLD). Its prevalence is high, especially among developed countries, and mainly reflects overnutrition and sedentary lifestyle. Moreover, the developing countries are not spared, as obesity and its related problems such as the metabolic syndrome are increasing quickly. We review the potential primary role of skeletal muscle insulin resistance in the pathophysiology of the metabolic syndrome, showing that in lean, young, insulin-resistant individuals, impaired muscle glucose transport and glycogen synthesis redirect energy derived from carbohydrate into hepatic de novo lipogenesis, promoting the development of atherogenic dyslipidemia and NAFLD. The demonstration of a link between skeletal muscle insulin resistance and the metabolic syndrome offers opportunities in targeting early defects in muscle insulin action in order to counteract the development of the disease and its related complications.

Endogenous peroxisome proliferator-activated receptor-gamma augments fatty acid uptake in oxidative muscle

In the setting of insulin resistance, agonists of peroxisome proliferator-activated receptor (PPAR)-gamma restore insulin action in muscle and promote lipid redistribution. Mice with muscle-specific knockout of PPARgamma (MuPPARgammaKO) develop excess adiposity, despite reduced food intake and normal glucose disposal in muscle. To understand the relation between muscle PPARgamma and lipid accumulation, we studied the fuel energetics of MuPPARgammaKO mice. Compared with controls, MuPPARgammaKO mice exhibited significantly increased ambulatory activity, muscle mitochondrial uncoupling, and respiratory quotient. Fitting with this latter finding, MuPPARgammaKO animals compared with control siblings exhibited a 25% reduction in the uptake of the fatty acid tracer 2-bromo-palmitate (P < 0.05) and a 13% increase in serum nonesterified fatty acids (P = 0.05). These abnormalities were associated with no change in AMP kinase (AMPK) phosphorylation, AMPK activity, or phosphorylation of acetyl-CoA carboxylase in muscle and occurred despite increased expression of fatty acid transport protein 1. Palmitate oxidation was not significantly altered in MuPPARgammaKO mice despite the increased expression of several genes promoting lipid oxidation. These data demonstrate that PPARgamma, even in the absence of exogenous activators, is required for normal rates of fatty acid uptake in oxidative skeletal muscle via mechanisms independent of AMPK and fatty acid transport protein 1. Thus, when PPARgamma activity in muscle is absent or reduced, there will be decreased fatty acid disposal leading to diminished energy utilization and ultimately adiposity.

The influence of thiazolidinediones on adipogenesis in vitro and in vivo: Potential modifiers of intramuscular adipose tissue deposition in meat animals1,2

Thiazolidinediones (TZD) are insulin sensitizing agents currently used for the treatment of type 2 diabetes and are widely used as adipogenic agents because they are ligands of peroxisome proliferator-activated receptor gamma (PPARgamma), a key adipogenic transcription factor. In vivo and in vitro studies of TZD as potential modifiers of intramuscular or marbling adipogenesis are reviewed. Thiazolidinedione-induced adipogenesis has been reported in numerous cell culture systems, including rodent, human, bovine, and porcine adipose tissue stromal-vascular (S-V) cell cultures. Studies of porcine S-V cell cultures derived from semitendinosus muscle show that TZD can potentially modify intramuscular or marbling adipogenesis. Preadipocyte recruitment was TZD-dependent in muscle S-V cultures but TZD-independent in adipose S-V cultures. There appear to be differences between adipocytes in muscle and subcutaneous adipose tissue, reminiscent of differences observed in adipocytes from different adipose tissue depots. Troglitazone, a TZD, induces marbling adipogenesis without inhibiting myogenesis when cells are grown on laminin precoated culture dishes. Additionally, troglitazone treatment does not increase lipid content in porcine adipose tissue or muscle S-V cell cultures. Thiazolidinedione treatment increases lipid content of muscle in rodents and humans; however, rosiglitazone treatment for 49 d in pigs did not influence muscle lipid content and meat quality, but several significant changes in muscle fatty acid composition were observed. Although timing of treatment with TZD needs to be optimized, evidence suggests these compounds may enhance marbling deposition in swine.

Selective PPARdelta agonist treatment increases skeletal muscle lipid metabolism without altering mitochondrial energy coupling: an in vivo magnetic resonance spectroscopy study

Peroxisome proliferator-activated receptor-delta (PPARdelta) activation results in upregulation of genes associated with skeletal muscle fatty acid oxidation and mitochondrial uncoupling. However, direct, noninvasive assessment of lipid metabolism and mitochondrial energy coupling in skeletal muscle following PPARdelta stimulation has not been examined. Therefore, in this study we examined the response of a selective PPARdelta agonist (GW610742X at 5 or 100 mg.kg(-1).day(-1) for 8 days) on skeletal-muscle lipid metabolism and mitochondrial coupling efficiency in rats by using in vivo magnetic resonance spectroscopy (MRS). There was a decrease in the intramyocellular lipid-to-total creatine ratio as assessed by in vivo (1)H-MRS in soleus and tibialis anterior muscles by day 7 (reduced by 49 and 46%, respectively; P < 0.01) at the high dose. Following the (1)H-MRS experiment (day 8), [1-(13)C]glucose was administered to conscious rats to assess metabolism in the soleus muscle. The relative fat-vs.-carbohydrate oxidation rate increased in a dose-dependent manner (increased by 52 and 93% in the 5 and 100 mg.kg(-1).day(-1) groups, respectively; P < 0.05). In separate experiments where mitochondrial coupling was assessed in vivo (day 7), (31)P-MRS was used to measure hindlimb ATP synthesis and (13)C-MRS was used to measure the hindlimb tricarboxylic acid cycle flux (V(tca)). There was no alteration, at either dose, in mitochondrial coupling efficiency measured as the ratio of unidirectional ATP synthesis flux to V(tca). Soleus muscle GLUT4 expression was decreased by twofold, whereas pyruvate dehydrogenase kinase 4, carnitine palmitoyl transferase 1a, and uncoupling protein 2 and 3 expression was increased by two- to threefold at the high dose (P < 0.05). In summary, these are the first noninvasive measurements illustrating a selective PPARdelta-mediated decrease in muscle lipid content that was consistent with a shift in metabolic substrate utilization from carbohydrate to lipid. However, the mitochondrial-energy coupling efficiency was not altered in the presence of increased uncoupling protein expression.

Tissue-specific effects of rosiglitazone and exercise in the treatment of lipid-induced insulin resistance

Both pharmacological intervention (i.e., thiazolidinediones [TZDs]) and lifestyle modification (i.e., exercise training) are clinically effective treatments for improving whole-body insulin sensitivity. However, the mechanism(s) by which these therapies reverse lipid-induced insulin resistance in skeletal muscle is unclear. We determined the effects of 4 weeks of rosiglitazone treatment and exercise training and their combined actions (rosiglitazone treatment and exercise training) on lipid and glucose metabolism in high-fat-fed rats. High-fat feeding resulted in decreased muscle insulin sensitivity, which was associated with increased rates of palmitate uptake and the accumulation of the fatty acid metabolites ceramide and diacylglycerol. Impairments in lipid metabolism were accompanied by defects in the Akt/AS160 signaling pathway. Exercise training, but not rosiglitazone treatment, reversed these impairments, resulting in improved insulin-stimulated glucose transport and increased rates of fatty acid oxidation in skeletal muscle. The improvements to glucose and lipid metabolism observed with exercise training were associated with increased AMP-activated protein kinase alpha1 activity; increased expression of Akt1, peroxisome proliferator-activated receptor gamma coactivator 1, and GLUT4; and a decrease in AS160 expression. In contrast, rosiglitazone treatment exacerbated lipid accumulation and decreased insulin-stimulated glucose transport in skeletal muscle. However, rosiglitazone, but not exercise training, increased adipose tissue GLUT4 and acetyl CoA carboxylase expression. Both exercise training and rosiglitazone decreased liver triacylglycerol content. Although both interventions can improve whole-body insulin sensitivity, our results show that they produce divergent effects on protein expression and triglyceride storage in different tissues. Accordingly, exercise training and rosiglitazone may act as complementary therapies for the treatment of insulin resistance.

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.

Adipocyte differentiation-related protein in human skeletal muscle: relationship to insulin sensitivity

OBJECTIVE

To determine whether adipocyte differentiation-related protein (ADRP), a lipid droplet-associated protein that binds to and sequesters intracellular fatty acids, is 1) expressed in human skeletal muscle and 2) differentially regulated in human skeletal muscle obtained from obese non-diabetic (OND) and obese diabetic (OD) subjects.

RESEARCH METHODS AND PROCEDURES

Ten OND subjects and 15 OD subjects underwent a weight loss or pharmacological intervention program to improve insulin sensitivity. Anthropometric data, hemoglobin A(1C), fasting glucose, lipids, and glucose disposal rate were determined at baseline and at completion of studies. Biopsies of the vastus lateralis muscle (SkM) were obtained in the fasting state from OND and OD subjects. Protein expression was determined by Western blotting.

RESULTS

ADRP was highly expressed in SkM from OND (4.4 +/- 1.54 AU/10 microg, protein, n = 10) and OD (5.02 +/- 1.33 AU/10 microg, n = 12) subjects. OND subjects undergoing weight loss had decreased triglyceride levels and improved insulin action. SkM ADRP content increased with weight loss from 5.14 +/- 2.15 AU/10 microg to 9.92 +/- 1.57 AU/10 microg (p < 0.025). OD subjects were treated with either troglitazone or metformin, together with glyburide, for 3 to 4 months. Both treatments attained similar levels of glycemic control. OD subjects with lower baseline ADRP content (2.85 +/- 1.07 AU/10 microg, n = 6) displayed up-regulation of ADRP expression (to 9.27 +/- 2.76 AU/10 microg, p < 0.025).

DISCUSSION

ADRP is the predominant lipid droplet-associated protein in SkM, and low ADRP expression is up-regulated in circumstances of improved glucose tolerance. Up-regulation of ADRP may act to sequester fatty acids as triglycerides in discrete lipid droplets that could protect muscle from the detrimental effects of fatty acids on insulin action and glucose tolerance.

Correlation between insulin resistance and intramyocellular lipid levels in rats

Increased intramyocellular lipid (IMCL) content has been proposed as biomarker for insulin resistance (IR). An inverse correlation between IMCL and insulin sensitivity (IS) was found in nonathletic humans, whereas in animal models only a few validation studies have been performed. The aim of this study was to investigate the interrelation between IS indices determined by the glucose clamp technique (glucose disposal (GD), exogenous glucose infusion rates (GIR)) and IMCL content in the tibialis (TIB) and the soleus (SOL) muscle obtained by magnetic resonance spectroscopy in different rat models of IR. Diet-induced insulin-resistant Wistar rats as well as genetic disease models (ZDF rats) were used. In both muscles, elevated IMCL correlated with an impaired IS in all models of IR. The correlation of IMCL with both parameters for IS was comparable in TIB and SOL. The best fit between IMCL and IS was obtained using TIB and GIR data (r = -0.69, P < 0.001). Diabetic male ZDF rats exhibited comparatively low IMCL levels due to their catabolic state: exclusion of this group improved r. In summary, IMCL, especially in TIB, is a valid biomarker for IS in various rat models of IR with the advantage of a fast repeatable noninvasive measurement in individual animals.

Evaluation of adipose tissue distribution in obese fa/fa Zucker rats by in vivo MR imaging: effects of peroxisome proliferator-activated receptor agonists

High-resolution MRI of obese (fa/fa) Zucker rats was investigated to characterize and assess in vivo adipose tissue distribution. Thirty animals were gavaged with a placebo, a PPARgamma activator (pioglitazone), or a dual PPARalphagamma activator (LM 4156). At day 15, T1-weighted images were acquired in vivo using a 2TMRI system with a high in-plane spatial resolution (254 microm). Fat volumes of selected territories were measured by image segmentation, and the retroperitoneal fat was weighed post-mortem. Body-weight gain was significant with pioglitazone (101.8+/-5.9 g, p<0.01 vs. placebo). The good quality of MR images allowed the delimitation and quantification of different fat territories. In response to pioglitazone, the retroperitoneal fat was more important compared to placebo (+23%, p<0.01) while subcutaneous fat was not different. No significant effects were observed with LM 4156. In vivo measurements of fat volumes were strongly correlated with ex vivo tissue weights (r=0.91). High-resolution MRI provides an in vivo measurement of adipose tissue distribution in obese Zucker rats. Specific fat depots of regions that were particularly involved in drug response were determined in vivo. Fat remodeling was observed with pioglitazone but not with a dual PPARalphagamma activator (LM 4156).

Rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle

We hypothesized that improved glucose tolerance with rosiglitazone treatment would coincide with decreased levels of i.m. triacylglycerol (IMTG), diacylglycerol, and ceramide. Obese Zucker rats were randomly divided into two experimental groups: control (n = 9) and rosiglitazone (n = 9), with lean Zucker rats (n = 9) acting as a control group for obese controls. Rats received either vehicle or 3 mg/kg rosiglitazone for 6 wk. Glucose tolerance was impaired (P < 0.01) in obese compared with lean rats, but was normalized after rosiglitazone treatment. IMTG content was higher in obese compared with lean rats (70.5 +/- 5.1 vs. 27.5 +/- 2.0 micromol/g dry mass; P < 0.05) and increased an additional 30% (P < 0.05) with rosiglitazone treatment. Intramuscular fatty acid composition shifted toward a higher proportion of monounsaturates (P < 0.05) in obese rosiglitazone-treated rats due to an increase in palmitoleate (16:1; P < 0.05). Rosiglitazone treatment increased (P < 0.05) skeletal muscle diacylglycerol and ceramide levels by 65% and 100%, respectively, compared with obese rats, but elevated muscle diacylglycerol was not associated with changes in the total or membrane contents of the diacylglycerol-sensitive protein kinase C isoforms theta;, delta, alpha, and beta. In summary, we observed a disassociation among skeletal muscle IMTG, diacylglycerol and ceramide content, and glucose tolerance with rosiglitazone treatment in obese Zucker rats. Our data suggest, therefore, that rosiglitazone enhances glucose tolerance by mechanisms other than reduction of fatty acid accumulation within skeletal muscle.

Intramyocellular lipids and insulin resistance

Lipids are stored not only in adipocytes but also 'ectopically' in tissues such as muscle, liver, beta cells and others. From a metabolic perspective, intramyocellular lipids (IMCLs) have recently become a focus of interest. This review summarizes history, measurement techniques and interpretation of muscle lipid data. Problems in biopsies with the separation of those metabolically active lipid droplets in the cytoplasm of myocytes from further lipids in adipocytes are discussed as well as considerations important for analysis of correlations between IMCL content and insulin sensitivity under various circumstances. The relatively new approach to non-invasive assessment of the IMCL content by magnetic resonance spectroscopy (MRS) is described in detail and exemplary spectra from different skeletal muscle types in humans are presented. The MRS technique allows human examinations of large cohorts for a detailed assessment of the interactions among metabolic parameters such as age, measures of adiposity, hormonal and ethnic factors and insulin resistance. IMCLs are generally positively correlated with measures of obesity and negatively with insulin sensitivity. Paradoxically, physical fitness (maximal aerobic capacity) increases both IMCL content and insulin sensitivity and therefore has to be taken into account as a confounding factor. Intervention studies with MRS further allowed to elucidate the regulation of IMCL. Molecular mechanisms and potential genetic factors on IMCL regulation are discussed as well as possible mechanisms of current treatment strategies for improving insulin sensitivity.