Exercise type and muscle fiber specific induction of caveolin-1 expression for insulin sensitivity of skeletal muscle

Modern aspects of therapy of metabolic associated liver disease in patients with type 2 diabetes mellitus

Metabolic associated fatty liver disease (MAFLD) is currently the most common liver disease. The main risk factors for its development are poor nutrition, sedentary lifestyle and obesity. MAFLD is associated with various cardiometabolic conditions – lipid metabolism disorders, cardiovascular pathology and carbohydrate metabolism disorders. The association of MAFLD and type 2 diabetes mellitus (DM) is common among patients, since these diseases have a common pathogenesis link – insulin resistance. The combination of these diseases has a mutual negative effect on each other and increases the risks of cardiovascular diseases, hospitalizations, as well as the risks of liver fibrosis progression. Therefore, the detection of MAFLD and its treatment in type 2 DM are extremely important to improve the patient’s prognosis. Diagnostics of MAFLD includes laboratory and instrumental research methods. The “gold standard” of diagnostics is considered to be liver biopsy, but due to the fact that this method is invasive, it is rarely used and only for differential diagnostics of MAFLD with other liver pathologies. The most accessible instrumental method for detecting liver steatosis is ultrasound. Treatment of MAFLD primarily involves lifestyle changes (rational nutrition with limitation of simple carbohydrates and animal fats, adequate physical activity) and weight loss. Also, hypoglycemic drugs used to treat type 2 diabetes (metformin, pioglitazone, glucagon-like peptide-1 agonists) can have a certain positive effect on MAFLD. Essential phospholipids, which have membrane-stabilizing, antioxidant and antifibrotic effects, are also an important component of MAFLD treatment. A number of domestic and foreign studies have shown the high efficiency of Essential Phospholipids both in relation to biochemical parameters in patients with a combination of type 2 diabetes and MAFLD, and in relation to ultrasound signs, improving the function and structure of the liver in MAFLD, as well as slowing the progression of liver fibrosis.

Diet and physical exercise as key players to tackle MASLD through improvement of insulin resistance and metabolic flexibility

Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) has emerged as a prevalent health concern, encompassing a wide spectrum of liver-related disorders. Insulin resistance, a key pathophysiological feature of MASLD, can be effectively ameliorated through dietary interventions. The Mediterranean diet, rich in whole grains, fruits, vegetables, legumes, and healthy fats, has shown promising results in improving insulin sensitivity. Several components of the Mediterranean diet, such as monounsaturated fats and polyphenols, exert anti-inflammatory and antioxidant effects, thereby reducing hepatic steatosis and inflammation. Furthermore, this dietary pattern has been associated with a higher likelihood of achieving MASLD remission. In addition to dietary modifications, physical exercise, particularly resistance exercise, plays a crucial role in enhancing metabolic flexibility. Resistance exercise training promotes the utilization of fatty acids as an energy source. It enhances muscle glucose uptake and glycogen storage, thus reducing the burden on the liver to uptake excess blood glucose. Furthermore, resistance exercise stimulates muscle protein synthesis, contributing to an improved muscle-to-fat ratio and overall metabolic health. When implemented synergistically, the Mediterranean diet and resistance exercise can elicit complementary effects in combating MASLD. Combined interventions have demonstrated additive benefits, including greater improvements in insulin resistance, increased metabolic flexibility, and enhanced potential for MASLD remission. This underscores the importance of adopting a multifaceted approach encompassing dietary modifications and regular physical exercise to effectively manage MASLD. This narrative review explores the biological mechanisms of diet and physical exercise in addressing MASLD by targeting insulin resistance and decreased metabolic flexibility.

Exploring protein relative relations in skeletal muscle proteomic analysis for insights into insulin resistance and type 2 diabetes

The escalating prevalence of insulin resistance (IR) and type 2 diabetes mellitus (T2D) underscores the urgent need for improved early detection techniques and effective treatment strategies. In this context, our study presents a proteomic analysis of post-exercise skeletal muscle biopsies from individuals across a spectrum of glucose metabolism states: normal, prediabetes, and T2D. This enabled the identification of significant protein relationships indicative of each specific glycemic condition. Our investigation primarily leveraged the machine learning approach, employing the white-box algorithm relative evolutionary hierarchical analysis (REHA), to explore the impact of regulated, mixed mode exercise on skeletal muscle proteome in subjects with diverse glycemic status. This method aimed to advance the diagnosis of IR and T2D and elucidate the molecular pathways involved in its development and the response to exercise. Additionally, we used proteomics-specific statistical analysis to provide a comparative perspective, highlighting the nuanced differences identified by REHA. Validation of the REHA model with a comparable external dataset further demonstrated its efficacy in distinguishing between diverse proteomic profiles. Key metrics such as accuracy and the area under the ROC curve confirmed REHA's capability to uncover novel molecular pathways and significant protein interactions, offering fresh insights into the effects of exercise on IR and T2D pathophysiology of skeletal muscle. The visualizations not only underscored significant proteins and their interactions but also showcased decision trees that effectively differentiate between various glycemic states, thereby enhancing our understanding of the biomolecular landscape of T2D.

Aerobic and Muscle-Strengthening Physical Activity, Television Viewing, and Nonalcoholic Fatty Liver Disease: The CARDIA Study

BACKGROUND

The prevalence of non-alcoholic fatty liver disease (NAFLD) in U.S. adults is over 30%, yet the role of lifestyle factors in the etiology of NAFLD remains understudied. We examined the associations of physical activity, by intensity and type, and television viewing with prevalent NAFLD.

METHODS

Cross-sectional analysis of a population-based sample of 2726 Black (49%) and White (51%) adults (Mean (SD) age, 50 (3.6) years; 57.3% female) from the CARDIA study. Exposures were aerobic activity by intensity (moderate, vigorous; hours/week); activity type (aerobic, muscle-strengthening; hours/week); and television viewing (hours/week), examined concurrently in all models and assessed by validated questionnaires. Our outcome was NAFLD (liver attenuation < 51 Hounsfield Units), measured by non-contrast computed tomography, after exclusions for other causes of liver fat. Covariates were sex, age, race, study center, education, diet quality, smoking status, alcohol consumption, and body mass index or waist circumference.

RESULTS

648 participants had NAFLD. In the fully adjusted modified Poisson regression model, the risk ratios per interquartile range of each exposure were moderate-intensity aerobic activity, 1.10 (95% CI, 0.97-1.26); vigorous-intensity aerobic activity, 0.72 (0.63-0.82); muscle-strengthening activity, 0.89 (0.80-1.01); and television viewing, 1.20 (1.10-1.32). Relative to less active participants with higher levels of television viewing, those who participated in ≥2 h/week of both vigorous-intensity aerobic and muscle-strengthening activity and <7 h/week of television viewing had 65% lower risk of NAFLD (risk ratio = 0.35, 95% CI = 0.23-0.51).

CONCLUSION

Adults who follow public health recommendations for vigorous-aerobic and muscle-strengthening activity, as well as minimize television viewing, are considerably less likely to have NAFLD than those who do not follow the recommendations and who have relatively high levels of television viewing.

Effects of different exercise modalities on lipid profile in the elderly population: A meta-analysis

BACKGROUND

Dyslipidemia, characterized by elevated total cholesterol (TC), triglycerides, and low-density lipoprotein cholesterol (LDL-C), as well as decreased high-density lipoprotein cholesterol (HDL-C), has long been recognized as a major risk factor for age-related diseases. Recent studies have shown that exercise can be effective in mitigating dyslipidemia in older adults. The aim of this study was to systematically evaluate the effects of different exercise modalities (aerobic exercise [AE], resistance exercise [RE], aerobic+resistance exercise [AE+RE], and high-intensity interval training) on lipid profile in the elderly population, in order to identify the most effective exercise modality for maintaining lipid profile health in older adults.

METHODS

A comprehensive database search of PubMed, EBSCO, Web of Science, China National Knowledge Infrastructure, and Wanfang database. Eligible studies were individually assessed according to the Cochrane Risk of Bias Tool version 2, and the data were analyzed and processed using RevMan 5.4.1 analysis software.

RESULTS

This study identified 20 randomized controlled trials with a total of 988 subjects, of which 541 were in the exercise group and 447 in the control group. Our analysis showed that AE significantly reduced TC (P < .05), triglycerides (P < .01), and LDL-C (P < .01), while increasing HDL-C (P < .01) in the elderly population. RE significantly reduced the elevation of TC (P < .01) and LDL-C (P < .05) indices in the elderly population, while AE+RE had a significant effect on decreasing TC (P < .01) and LDL-C (P < .01) indices.

CONCLUSION

Our analysis indicates that AE is the most effective exercise modality for reducing dyslipidemia in older adults, compared to RE, AE+RE, and high-intensity interval training. These findings suggest that AE should be promoted as an important lifestyle intervention to improve lipid profile health in the elderly population. However, further research is needed to investigate the optimal duration, frequency, and intensity of AE required to achieve the most beneficial effects on lipid profile health in older adults.

Association between muscle strength and advanced fibrosis in non-alcoholic fatty liver disease: a Korean nationwide survey

BACKGROUND

We investigated the association between muscle strength and the prevalence of advanced fibrosis among individuals with non-alcoholic fatty liver disease (NAFLD) using a nationwide cross-sectional survey.

METHODS

Individuals, 20 to 79 years of age, from the Korean National Health and Nutrition Examination Surveys (KNHANES) from 2014 to 2016 were selected (N = 14 861), with sample weights applied. Muscle strength was quantified as the handgrip strength divided by the body mass index (BMI); low muscle strength (LMS) was defined as the lowest quartile (Q1 ) of the handgrip strength/BMI for our sample population. NAFLD was defined as hepatic steatosis index >36. Advanced fibrosis was defined as a fibrosis-4 index score ≥1.30 (FibrosisFIB4 ).

RESULTS

The mean age of the study population was 45.6 ± 0.2 years, and 42.4% were male. As muscle strength increased, the mean BMI and age decreased accordingly, and the proportions of diabetes, dyslipidaemia, hypertension, and obesity decreased significantly (P < 0.001 for all). In a crude analysis, the LMS was associated with an increased prevalence of NAFLD (odds ratio [OR] 3.62, 95% confidence interval [CI] 3.25-4.03, P < 0.001), which remained significant even after adjustment for age, sex, obesity, insulin resistance, diabetes, hypertension, dyslipidaemia, and high-sensitivity C-reactive protein (OR 1.66, 95% CI 1.28-2.16, P < 0.001). In this logistic regression model, the prevalence of NAFLD decreased by 24% with each quartile increment in muscle strength (OR 0.76, 95% CI 0.68-0.85, P < 0.001). Among individuals with NAFLD (n = 2092), LMS was significantly associated with the presence of advanced fibrosis (FibrosisFIB4 ) independently of age, sex, obesity, diabetes, hypertension, dyslipidaemia, and high-sensitivity C-reactive protein (OR 1.66, 95% CI 1.01-2.49, P = 0.015), which lost its statistical significance after additional adjustment for insulin resistance.

CONCLUSIONS

Low muscle strength is independently associated with NAFLD. The significant association between LMS and advanced fibrosis in NAFLD may be mediated through insulin resistance.

GDM-Induced Macrosomia Is Reversed by Cav-1 via AMPK-Mediated Fatty Acid Transport and GLUT1-Mediated Glucose Transport in Placenta

Objective

To investigate if the role of Cav-1 in GDM-induced macrosomia is through regulating AMPK signaling pathway in placenta.

Methods

We used diagnostic criteria of gestational diabetes mellitus (GDM) and macrosomia to separate and compare placental protein and mRNA levels from GDM with macrosomia group (GDMM), GDM with normal birth weight group (GDMN) and normal glucose tolerance (NGT) with normal birth weight group (CON). Western blotting was performed to examine differentially expressed proteins of caveolin-1 (Cav-1) and Adenosine monophosphate-activated protein kinase (AMPK) signaling pathway related proteins, including phosphorylated-AMPKα(Thr172), AMPKα, phosphorylated-Acetyl-CoA carboxylase(Ser79) (p-ACC(Ser79)), ACC and glucose transporter 1 (GLUT1) in placenta between the three groups. The mRNA levels of Cav-1, AMPKα, ACC and GLUT1 in placenta were measured by real time-PCR.

Results

In the GDMM placenta group, both protein and mRNA levels of Cav-1 were down-regulated, while GLUT1 was up-regulated; the phosphorylation and mRNA levels of ACC and AMPKα were decreased, but total ACC protein levels were increased compared to both the GDMN (p<0.05) and CON groups (p<0.05). In GDMM placenta group, there was a significant negative correlation observed between neonatal birth weight (NBW) and protein expression levels of Cav-1, p-ACC(Ser79) and p-AMPKα(Thr172) (p<0.05), while positive relationship with ACC and GLUT1 protein levels. Besides, in GDMM group placental mRNA levels, NBW had a positive correlation with GLUT1 (p<0.05), while negative with Cav-1, AMPKα and ACC expression (p<0.05). Cav-1 protein expression was positively associated with p-AMPK and p-ACC (p<0.05), and negatively associated with GLUT1 (p<0.05). Interestingly, p-AMPK protein expression was closely related to p-ACC (p<0.05), but not with GLUT1.

Conclusion

GDM-induced macrosomias have more severe inhibition of Cav-1 expression in placenta. Cav-1 is associated with placental glucose and fatty acid transport via the induction of AMPK signaling pathway and the reduction of GLUT1 signaling pathway to reverse GDM-induced macrosomia.

Aerobic vs. resistance exercise in non-alcoholic fatty liver disease: A systematic review

BACKGROUND & AIMS

Exercise is a first-line therapy for patients with non-alcoholic fatty liver disease (NAFLD). We sought to: 1) summarize effective aerobic and resistance exercise protocols for NAFLD; and 2) compare the effects and energy consumption of aerobic and resistance exercises.

METHODS

A literature search was performed using PubMed, Web of Science, and Scopas to January 28, 2016. From a total of 95 articles, 23 studies including 24 aerobic and 7 resistance exercise protocols were selected for the summary of exercise protocols. Twelve articles including 13 aerobic and 4 resistance exercise protocols were selected for the comparative analysis.

RESULTS

For aerobic exercise, the median effective protocol was 4.8 metabolic equivalents (METs) for 40min/session, 3times/week for 12weeks. For resistance exercise, the median effective protocol was 3.5 METs for 45min/session, 3times/week for 12weeks. Aerobic and resistance exercise improved hepatic steatosis. No significant difference was seen in the duration, frequency, or period of exercise between the two exercise groups; however, %VO₂max and energy consumption were significantly lower in the resistance than in the aerobic group (50% [45-98] vs. 28% [28-28], p=0.0034; 11,064 [6394-21,087] vs. 6470 [4104-12,310] kcal/total period, p=0.0475).

CONCLUSIONS

Resistance exercise improves NAFLD with less energy consumption. Thus, resistance exercise may be more feasible than aerobic exercise for NAFLD patients with poor cardiorespiratory fitness or for those who cannot tolerate or participate in aerobic exercise. These data may indicate a possible link between resistance exercise and lipid metabolism in the liver.

LAY SUMMARY

Both aerobic and resistance exercise reduce hepatic steatosis in non-alcoholic fatty liver disease (NAFLD) with similar frequency, duration, and period of exercise (40-45min/session 3times/week for 12weeks); however, the two forms of exercise have different characteristics. Intensity and energy consumption were significantly lower for resistance than for aerobic exercise. Resistance exercise may be more feasible than aerobic exercise for NAFLD patients with poor cardiorespiratory fitness or for those who cannot tolerate or participate in aerobic exercise.

Exercise-Induced Changes in Caveolin-1, Depletion of Mitochondrial Cholesterol, and the Inhibition of Mitochondrial Swelling in Rat Skeletal Muscle but Not in the Liver

The reduction in cholesterol in mitochondria, observed after exercise, is related to the inhibition of mitochondrial swelling. Caveolin-1 (Cav-1) plays an essential role in the regulation of cellular cholesterol metabolism and is required by various signalling pathways. Therefore, the aim of this study was to investigate the effect of prolonged swimming on the mitochondrial Cav-1 concentration; additionally, we identified the results of these changes as they relate to the induction of changes in the mitochondrial swelling and cholesterol in rat skeletal muscle and liver. Male Wistar rats were divided into a sedentary control group and an exercise group. The exercised rats swam for 3 hours and were burdened with an additional 3% of their body weight. After the cessation of exercise, their quadriceps femoris muscles and livers were immediately removed for experimentation. The exercise protocol caused an increase in the Cav-1 concentration in crude muscle mitochondria; this was related to a reduction in the cholesterol level and an inhibition of mitochondrial swelling. There were no changes in rat livers, with the exception of increased markers of oxidative stress in mitochondria. These data indicate the possible role of Cav-1 in the adaptive change in the rat muscle mitochondria following exercise.

Fiber type-specific expression of low-density lipoprotein receptor-related protein 6 in human skeletal muscles

OBJECTIVE

Gene expression patterns differ in the two types of skeletal muscle fiber. The Wnt signaling pathway, which includes low-density lipoprotein receptor-related protein 6 (LRP6), has been associated with cell differentiation and glucose metabolism in skeletal muscles. We examined the relationships between muscle fiber types and LRP6 expression.

METHODS

Adenosine triphosphatase was assayed histochemically, and the levels of expression of LRP6 and myosin were analyzed immunohistochemically, in frozen sections of muscle fiber obtained from 16 muscle biopsy samples. The expression pattern of LRP6 in C2C12 cells was assayed by immunocytochemistry.

RESULTS

LRP6 was expressed only in type II fibers. Type IIc fibers showed variations in LRP6 expression. Expression of LRP6 was observed at the stage of myoblast differentiation.

CONCLUSION

Antibody to LRP6 may be useful for identifying type II skeletal muscle fibers. LRP6 may influence glucose metabolism in type II fibers of human skeletal muscles.

Differential expression of caveolins and myosin heavy chains in response to forced exercise in rats

Exercise training can improve strength and lead to adaptations in the skeletal muscle and nervous systems. Skeletal muscles can develop into two types: fast and slow, depending on the expression pattern of myosin heavy chain (MHC) isoforms. Previous studies reported that exercise altered the distribution of muscle fiber types. It is not currently known what changes in the expression of caveolins and types of muscle fiber occur in response to the intensity of exercise. This study determined the changes in expression of caveolins and MHC type after forced exercise in muscular and non-muscular tissues in rats. A control (Con) group to which forced exercise was not applied and an exercise (Ex) group to which forced exercise was applied. Forced exercise, using a treadmill, was introduced at a speed of 25 m/min for 30 min, 3 times/day (07:00, 15:00, 23:00). Homogenized tissues were applied to extract of total RNA for further gene analysis. The expression of caveolin-3 and MHC2a in the gastrocnemius muscle of female rats significantly increased in the Ex group compared with the Con group (P<0.05). Furthermore, in the gastrocnemius muscle of male rats, the expression of MHC2x was significantly different between the two groups (P<0.05). There was an increased expression in caveolin-3 and a slightly decreased expression in TGFβ-1 in muscular tissues implicating caveolin-3 influences the expression of MHC isoforms and TGFβ-1 expression. Eventually, it implicates that caveolin-3 has positive regulatory function in muscle atrophy induced by neural dysfunction with spinal cord injury or stroke.

Fucoxanthin promotes translocation and induction of glucose transporter 4 in skeletal muscles of diabetic/obese KK-A(y) mice

Fucoxanthin (Fx) isolated from Undaria pinnatifida suppresses the development of hyperglycemia and hyperinsulinemia of diabetic/obese KK-A(y) mice after 2 weeks of feeding 0.2% Fx-containing diet. In the soleus muscle of KK-A(y) mice that were fed Fx, glucose transporter 4 (GLUT4) translocation to plasma membranes from cytosol was promoted. On the other hand, Fx increased GLUT4 expression levels in the extensor digitorum longus (EDL) muscle, although GLUT4 translocation tended to increase. The expression levels of insulin receptor (IR) mRNA and phosphorylation of Akt, which are in upstream of the insulin signaling pathway regulating GLUT4 translocation, were also enhanced in the soleus and EDL muscles of the mice fed Fx. Furthermore, Fx induced peroxisome proliferator activated receptor γ coactivator-1α (PGC-1α), which has been reported to increase GLUT4 expression, in both soleus and EDL muscles. These results suggest that in diabetic/obese KK-A(y) mice, Fx improves hyperglycemia by activating the insulin signaling pathway, including GLUT4 translocation, and inducing GLUT4 expression in the soleus and EDL muscles, respectively, of diabetic/obese KK-A(y) mice.

MicroRNAs in hepatic pathophysiology in diabetes

MicroRNAs (miRNAs or miRs) are small approximately 22 nucleotide RNA species that are believed to regulate diverse metabolic and physiological processes. In the recent past, several reports have surfaced that demonstrate the role of miRNAs in various biological processes and numerous disease states. For a disease as complex as diabetes, the emergence of miRNAs as key regulators leading to the disease phenotype has added a novel dimension to the area of diabetes research. On the other hand, the liver, a metabolic hub, contributes in a major way towards maintaining normal glucose levels in the body as it can both stimulate and inhibit hepatic glucose output. This equilibrium is frequently disturbed in diabetes and hence, the liver assumes special significance considering the correlation between altered hepatic physiology and diabetes. While the understanding of the mechanisms behind this altered hepatic behavior is not yet completely understood, recent reports on the status and role of miRNAs in the diabetic liver have further added to the complexities of the knowledge of hepatic pathophysiology in diabetes. Here, we bring together the various miRNAs that play a role in the altered hepatic behavior during diabetes.

Divergent effects of GLP-1 analogs exendin-4 and exendin-9 on the expression of myosin heavy chain isoforms in C2C12 myotubes

Exendin 1-39 amide (Ex-4) and its truncated form exendin 9-39 amide (Ex-9) are peptides of non-mammalian nature, which act as an agonist and antagonist, respectively, of the glucagon-like peptide-1 (GLP-1) receptor in mammals. GLP-1 is an intestinal peptide that plays an important role in the regulation of glucose metabolism and glucose uptake in skeletal muscle; however, the effects of its two analogs (Ex-4 and Ex-9) on myofiber properties are still unclear. Here, we report the effects of Ex-4 and Ex-9 alone or in combination on the myosin heavy chain (MyHC) type composition and the glucose uptake capacity in differentiated C2C12 myotubes. Neither Ex-4 nor Ex-9 altered basal glucose uptake, whereas Ex-9 significantly increased insulin-stimulated glucose uptake, suggesting enhanced insulin sensitivity. The mRNA expression of MyHC I and 2A as well as the percentage of MyHC I protein was remarkably increased in Ex-9-treated myotubes. In contrast, Ex-4, alone or in combination with Ex-9, caused a significant reduction in MyHC 2A mRNA expression and the percentage of MyHC I protein. Consistent with the MyHC type switching peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α expression in myotubes was remarkably increased by Ex-9 yet was significantly inhibited by Ex-4. In addition, intracellular concentrations of free Ca(2+) were increased in all treatment groups, but only Ex-9-treated myotubes showed higher calcineurin A protein content. Taken together, our data suggest that Ex-9 promotes oxidative differentiation in myotubes to improve cell insulin sensitivity, probably through calcineurin and PGC-1α mediated pathways.

Modulation of insulin sensitivity and caveolin-1 expression by orchidectomy in a nonobese type 2 diabetes animal model

Previously, we found that male JYD mice developed type 2 diabetes but female mice did not, and that decreased expression levels of caveolin-1 were correlated with the development of a diabetic phenotype in these mice. Therefore, we hypothesized that sex hormones affect the expression of caveolin-1 and contribute to the development of insulin resistance and hyperglycemia in JYD mice. We used glucose and insulin tolerance tests to examine insulin sensitivity in male, female and orchidectomized male JYD mice. Glucose uptake was analyzed by using (18)F-fluorodeoxyglucose positron emission tomography. We also examined insulin-signaling molecules and caveolin proteins in various tissues in these mice by Western blotting. In addition, we examined changes of caveolin-1 expression in L6 skeletal muscle cells treated with 17-β estradiol or dihydroxytestosterone. We found that glucose and insulin tolerance were impaired and hyperglycemia developed in male, but not female, JYD mice. Expression of insulin-signaling molecules such as insulin receptor, protein kinase B, and glucose transporter-4 were decreased in male JYD mice compared with female mice. Orchidectomized JYD male mice showed improved glucose and insulin tolerance with a concomitant increase in the expression of insulin-signaling molecules and caveolin-1 in adipose tissue and skeletal muscle. Moreover, 17-β-estradiol treatment increased the expression of caveolin-1 in differentiated skeletal muscle cells. We conclude that sex hormones modulate the expression of caveolin-1 and insulin-signaling molecules, subsequently affecting insulin sensitivity and the development of type 2 diabetes in JYD mice.

Caveolin, GLUT4 and insulin receptor protein content in human arm and leg muscles

Recent evidence suggests that insulin sensitivity is relatively better preserved in arm muscle than in leg muscle in both healthy controls and type 2 DM, based on measurements of basal and insulin-mediated glucose clearance performed simultaneously in the two sets of muscles. It has also been reported that glucose uptake rates are higher in arm compared to leg muscles in the fasted state during normo-insulinaemia. However, the mechanism(s) for this are unknown. Currently, no information is available on the content of glucose transport proteins between arm and leg muscles. Therefore, we compared four proteins, Caveolin-1 (Cav-1), Caveolin-3 (Cav-3), GLUT4, and IR-beta, each of which plays an important role in regulating glucose transport between arm and leg muscles using muscle samples that were obtained from the deltoid (DEL) and vastus lateralis (VAS) of 14 male college pentathlon athletes before and after two swimming trials performed over 100 and 1,500 m. In the present study, we have shown the levels of Cav-1, -3, GLUT4, and IR-beta measured together for the first time in human arm and leg muscles. There was no difference in the levels of these proteins between arm and leg muscles. Cav-3, GLUT4, and IR-beta were unchanged from the resting levels after both exercise trials in DEL, while Cav-1 was increased (17%) at the end of the longer swim trial. In contrast, all measurements of Cav-1, -3, GLUT4, and IR-beta after the 1,500 m swim trial in VAS were increased, by 120, 46, 123, and 60%, respectively. These data imply that there was no functional difference in glucose transport capacity between arm and leg muscles in highly trained pentathlon athletes in the resting state. Although Cav-3, GLUT4, and IR-beta were unchanged from the resting levels at the end of both exercise trials in DEL, all measures, including Cav-1, increased after the 1,500 m swim trial in VAS.

A potential role for skeletal muscle caveolin-1 as an insulin sensitivity modulator in ageing-dependent non-obese type 2 diabetes: studies in a new mouse model

AIMS/HYPOTHESIS

Type 2 diabetes mellitus is a common age-dependent disease. We discovered that male offspring of non-diabetic C57BL/6 and DBA/2 mice, called JYD mice, develop type 2 diabetes when they grow old. JYD mice show characteristics of insulin resistance, hyperglycaemia and hyperinsulinaemia in old age without obesity. We postulated that the mechanism of age-dependent type 2 diabetes in this model relates to caveolin-1 status in skeletal muscle, which appears to regulate insulin sensitivity in the mice.

METHODS

We compared insulin sensitivity in aged C57BL/6 and JYD mice using glucose and insulin tolerance tests and (18)F-fluorodeoxyglucose positron emission tomography. We also determined insulin signalling molecules and caveolin proteins using western blotting, and altered caveolin-1 levels in skeletal muscle of C57BL/6 and JYD mice using viral vector systems, to examine the effect of this on insulin sensitivity.

RESULTS

In 30-week-old C57BL/6 and JYD mice, the basal levels of IRS-1, Akt and peroxisome proliferator-activated receptor-gamma decreased, as did insulin-stimulated phosphorylation of Akt and insulin receptor beta. However, caveolin-1 was only increased about twofold in 30-week-old JYD mice as compared with 3-week-old mice, whereas an eightfold increase was seen in C57BL/6 mice. Downregulation of caveolin-1 production in C57BL/6 mice caused severe impairment of glucose and insulin tolerance. Upregulation of caveolin-1 in aged diabetic JYD mice significantly improved insulin sensitivity with a concomitant increase of glucose uptake in the skeletal muscle.

CONCLUSIONS/INTERPRETATION

The level of skeletal muscle caveolin-1 is correlated with the progression of age-dependent type 2 diabetes in JYD mice.