Sex differences in insulin regulation of skeletal muscle glycogen synthase and changes during weight loss and exercise in adults

OBJECTIVE

The authors sought to understand sex differences in muscle metabolism in 73 older men and women.

METHODS

Body composition, VO2max, and insulin sensitivity (M) by 3-hour hyperinsulinemic-euglycemic clamp with vastus lateralis muscle biopsies were measured.

RESULTS

Women had lower body weight, VO2max, and fat-free mass than men. Men had lower M, lower change (insulin minus basal) in muscle glycogen synthase (GS) activity, and lower change in AKT protein expression than women. M was associated with the change (insulin-basal) in GS activity and the change in AKT protein expression. Sex differences (n = 60) were tested with 6-month weight loss or 3×/week aerobic exercise training. The postintervention minus preintervention change (insulin-basal) (∆∆) in GS activity (fractional, independent, total) was higher in men than women in the weight loss group and ∆∆ in GS fractional activity was higher in women than men in the aerobic exercise group. In all participants, ∆∆ in GS fractional and independent activities was related to ∆∆ in AKT expression and glycogen content.

CONCLUSIONS

Sex differences in insulin sensitivity may be explained at the cellular muscle level, and to improve skeletal muscle insulin action in older adults, it may be necessary to recommend different behavioral strategies depending on the individual's sex.

Toxoplasma gondii IgG Serointensity Is Positively Associated With Frailty

BACKGROUND

Persistent inflammation related to aging ("inflammaging") is exacerbated by chronic infections and contributes to frailty in older adults. We hypothesized associations between Toxoplasma gondii (T. gondii), a common parasite causing an oligosymptomatic unremitting infection, and frailty, and secondarily between T. gondii and previously reported markers of immune activation in frailty.

METHODS

We analyzed available demographic, social, and clinical data in Spanish and Portuguese older adults [N = 601; age: mean (SD) 77.3 (8.0); 61% women]. Plasma T. gondii immunoglobulin G (IgG) serointensity was measured with an enzyme-linked immunosorbent assay. The Fried criteria were used to define frailty status. Validated translations of Mini-Mental State Examination, Geriatric Depression Scale, and the Charlson Comorbidity Index were used to evaluate confounders. Previously analyzed biomarkers that were significantly associated with frailty in both prior reports and the current study, and also related to T. gondii serointensity, were further accounted for in multivariable logistic models with frailty as outcome.

RESULTS

In T. gondii-seropositives, there was a significant positive association between T. gondii IgG serointensity and frailty, accounting for age (p = .0002), and resisting adjustment for multiple successive confounders. Among biomarkers linked with frailty, kynurenine/tryptophan and soluble tumor necrosis factor receptor II were positively associated with T. gondii serointensity in seropositives (p < .05). Associations with other biomarkers were not significant.

CONCLUSIONS

This first reported association between T. gondii and frailty is limited by a cross-sectional design and warrants replication. While certain biomarkers of inflammaging were associated with both T. gondii IgG serointensity and frailty, they did not fully mediate the T. gondii-frailty association.

Promotion of Successful Weight Management in Overweight and Obese Veterans (POWER-VET): Trial Design and Methods

BACKGROUND

Almost 75% of US adults are overweight or obese. Though intentional weight loss of as little as 3% improves physical functioning and reduces cardiometabolic risk, most adults are unsuccessful at long-term weight maintenance. Our hypothesis is that intermittent fasting (IF: short periods of intense energy restriction) will reduce weight regain. IF may combat obesity due to its effects on nutrient-sensing signaling pathways and circadian rhythm. The objective of this randomized clinical trial is to test the effectiveness of an intensive body weight management program with and without IF.

METHODS

In the Promotion of Successful Weight Management in Overweight and Obese Veterans (POWER-VET) trial (NCT04131647), 154 middle-aged and older adults (50-75 years) who are overweight and obese (BMI: 25-40 kg/m2) and seen at either a Baltimore, MD or San Antonio, TX Veterans Affairs Medical Center will be enrolled. Participants will undergo 12 weeks of weight loss (including a low-calorie heart healthy (HH) diet and exercise). Following weight loss, participants will be randomly assigned to one of two 24-week weight maintenance (WM) interventions: WM alone (continuation of HH diet and exercise) or WM + IF. The primary aim is to determine the effect of WM + IF compared to WM alone on body weight maintenance after intentional weight loss.

DISCUSSION

Determining effective, translatable strategies that minimize weight regain following successful weight loss holds public health relevance. This POWER-VET trial introduces an innovative practice of IF to prevent weight regain after clinically significant weight reduction and could provide evidence-based recommendations to promote this type of intervention in middle aged and older adults.

The Role of Companion Dogs in the VA Maryland Health Care System Whole Health(y) GeroFit Program

GeroFit is a gym-based exercise program that promotes health and wellness among older sedentary veterans. The aims of the current study were to determine whether providing a companion dog as an alternative to gym-based exercise would similarly affect whole health outcomes. A total of 15 (n = 15) veterans (62 ± 11 years of age; 13 of 15 >54 years of age) underwent physical function testing, completed global and whole health questionnaires, and wore an accelerometer for 7 days before (baseline) and 3 months after a dog came into their home. The participants completed the Pet Attachment Scale (PAS), Dog Owner-Specific Quality of Life (DOQOL), and Canine Behavioral Assessment and Research questionnaires at 3 months. Cardiorespiratory endurance, lower body strength, daily steps, and time spent engaging in moderate physical activity all increased compared to the baseline levels. Body weight decreased among veterans whose body mass index was ≥30 (n = 11). The PAS and DOQOL scores indicated high attachment and positive effects on quality of life after having a dog in the home, with all veterans agreeing that having a dog improved the number of social activities they performed. We conclude that providing a companion dog to veterans not inclined to participate in gym-based exercise is an effective alternative method of promoting health and wellness in this population.

Insulin Resistance in Skeletal Muscle of Chronic Stroke

A stroke can lead to reduced mobility affecting skeletal muscle mass and fatty infiltration which could lead to systemic insulin resistance, but this has not been examined and the mechanisms are currently unknown. The objective was to compare the effects of in vivo insulin on skeletal muscle glycogen synthase (GS) activity in paretic (P) and nonparetic (NP) skeletal muscle in chronic stroke, and to compare to nonstroke controls. Participants were mild to moderately disabled adults with chronic stroke (n = 30, 60 ± 8 years) and sedentary controls (n = 35, 62 ± 8 years). Insulin sensitivity (M) and bilateral GS activity were determined after an overnight fast and during a hyperinsulinemic-euglycemic clamp. Stroke subjects had lower aerobic capacity than controls, but M was not significantly different. Insulin-stimulated activities of GS (independent, total, fractional), as well as absolute differences (insulin minus basal) and the percent change (insulin minus basal, relative to basal) in GS activities, were all significantly lower in P versus NP muscle. Basal GS fractional activity was 3-fold higher, and the increase in GS fractional activity during the clamp was 2-fold higher in control versus P and NP muscle. Visceral fat and intermuscular fat were associated with lower M. The effect of in vivo insulin to increase GS fractional activity was associated with M in control and P muscle. A reduction in insulin action on GS in paretic muscle likely contributes to skeletal muscle-specific insulin resistance in chronic stroke.

Insulin suppression of fatty acid skeletal muscle enzyme activity in postmenopausal women, and improvements in metabolic flexibility and lipoprotein lipase with aerobic exercise and weight loss

BACKGROUND

Obesity and insulin resistance are characterized by metabolic inflexibility, a condition described as an inability to switch from fat oxidation during fasting to carbohydrate oxidation during hyperinsulinemia. The purpose of this study was to examine predictors of metabolic flexibility in 103 obese (37-59% fat), sedentary (VO₂max: 19.4 ± 0.5 ml/kg/min), postmenopausal (45-76 years) women, and changes in metabolic flexibility with exercise and weight loss interventions.

METHODS

Insulin sensitivity (M) and metabolic flexibility via an 80 mU/m²/min hyperinsulinemic-euglycemic clamp, VO₂max, and body composition were measured. Metabolic flexibility was measured after 6-months aerobic training + weight loss (AEX + WL: n = 43) or weight loss (WL: n = 31). Basal and insulin-stimulated vastus lateralis skeletal muscle samples were available from a subset of these women (n = 45).

RESULTS

Metabolic flexibility correlated inversely with glucose₁₂₀ min of OGTT, fasting insulin, and the percent change (insulin-basal) in lipoprotein lipase (LPL) activity and positively with M, but not with VO₂max, total body fat, visceral fat, or subcutaneous abdominal fat. Skeletal muscle acyl-CoA synthase and citrate synthase activities decreased during hyperinsulinemia. Metabolic flexibility increased after AEX + WL but not WL, and the percent change in metabolic flexibility was inversely related to the percent change in insulin's effect on LPL activity.

CONCLUSION

Metabolic flexibility is related to insulin sensitivity and insulin's action on LPL. Furthermore, metabolic flexibility and insulin suppression of skeletal muscle LPL activity increase with AEX + WL in overweight and obese, sedentary older women.

Exercise with weight loss improves adipose tissue and skeletal muscle markers of fatty acid metabolism in postmenopausal women

OBJECTIVE

The effects of 6-month weight loss (WL) versus aerobic exercise training (AEX)+WL on fat and skeletal muscle markers of fatty acid metabolism were determined in normal (NGT) and impaired (IGT) glucose tolerant African-American and Caucasian postmenopausal women with overweight/obesity.

METHODS

Fat (gluteal and abdominal) lipoprotein lipase (LPL), skeletal muscle LPL, acyl-CoA synthase (ACS), ß-hydroxacyl-CoA dehydrogenase, carnitine palmitoyltransferase (CPT-1), and citrate synthase (CS) activities were measured at baseline (n = 104) and before and after WL (n = 34) and AEX+WL (n = 37).

RESULTS

After controlling for age and race, muscle LPL and CPT-1 were lower in IGT, and the ratios of fat/muscle LPL activity were higher in IGT compared to NGT. Muscle LPL was related to insulin sensitivity (M value) and inversely related to G₁₂₀ , fasting insulin, and homeostatic model assessment of insulin resistance. AEX+WL decreased abdominal fat LPL and increased muscle LPL, ACS, and CS. The ratios of fat/muscle LPL decreased after AEX+WL. The change in VO₂ max was related to the changes in LPL, ACS, and CS and inversely related to the changes in fat/muscle LPL activity ratios.

CONCLUSIONS

Six-month AEX+WL, and not WL alone, is capable of enhancing skeletal muscle fatty acid metabolism in postmenopausal African-American and Caucasian women with NGT, IGT, and overweight/obesity.

Skeletal muscle cellular metabolism in older HIV-infected men

Skeletal muscle mitochondrial dysfunction may contribute to low aerobic capacity. We previously reported 40% lower aerobic capacity in HIV-infected men compared to noninfected age-matched men. The objective of this study was to compare skeletal muscle mitochondrial enzyme activities in HIV-infected men on antiretroviral therapy (55 ± 1 years of age, n = 10 African American men) with age-matched controls (55 ± 1 years of age, n = 8 Caucasian men), and determine their relationship with aerobic capacity. Activity assays for mitochondrial function including enzymes involved in fatty acid activation and oxidation, and oxidative phosphorylation, were performed in homogenates prepared from vastus lateralis muscle. Hydrogen peroxide (H2O2), cardiolipin, and oxidized cardiolipin were also measured. β-hydroxy acyl-CoA dehydrogenase (β-HAD) (38%) and citrate synthase (77%) activities were significantly lower, and H2O2 (1.4-fold) and oxidized cardiolipin (1.8-fold) were significantly higher in HIV-infected men. VO2peak (mL/kg FFM/min) was 33% lower in HIV-infected men and was directly related to β-HAD and citrate synthase activity and inversely related to H2O2 and oxidized cardiolipin. Older HIV-infected men have reduced oxidative enzyme activity and increased oxidative stress compared to age-matched controls. Further research is crucial to determine whether an increase in aerobic capacity by exercise training will be sufficient to restore mitochondrial function in older HIV-infected individuals.

Increased Skeletal Muscle Capillarization Independently Enhances Insulin Sensitivity in Older Adults After Exercise Training and Detraining

Intramuscular signaling and glucose transport mechanisms contribute to improvements in insulin sensitivity after aerobic exercise training. This study tested the hypothesis that increases in skeletal muscle capillary density (CD) also contribute to exercise-induced improvements in whole-body insulin sensitivity (insulin-stimulated glucose uptake per unit plasma insulin [M/I]) independent of other mechanisms. The study design included a 6-month aerobic exercise training period followed by a 2-week detraining period to eliminate short-term effects of exercise on intramuscular signaling and glucose transport. Before and after exercise training and detraining, 12 previously sedentary older (65 ± 3 years) men and women underwent research tests, including hyperinsulinemic-euglycemic clamps and vastus lateralis biopsies. Exercise training increased Vo2max (2.2 ± 0.2 vs. 2.5 ± 0.2 L/min), CD (313 ± 13 vs. 349 ± 18 capillaries/mm(2)), and M/I (0.041 ± 0.005 vs. 0.051 ± 0.007 μmol/kg fat-free mass/min) (P < 0.05 for all). Exercise training also increased the insulin activation of glycogen synthase by 60%, GLUT4 expression by 16%, and 5' AMPK-α1 expression by 21%, but these reverted to baseline levels after detraining. Conversely, CD and M/I remained 15% and 18% higher after detraining, respectively (P < 0.05), and the changes in M/I (detraining minus baseline) correlated directly with changes in CD in regression analysis (partial r = 0.70; P = 0.02). These results suggest that an increase in CD is one mechanism contributing to sustained improvements in glucose metabolism after aerobic exercise training.

Aerobic exercise plus weight loss improves insulin sensitivity and increases skeletal muscle glycogen synthase activity in older men

The purpose of this study was to determine the effects of 6-month aerobic exercise training + weight loss (AEX + WL) on basal and insulin activation of glycogen synthase, basal citrate synthase activity, and Akt and AS160 phosphorylation in older, overweight/obese insulin-resistant men (n = 14; 63 ± 2 years; body mass index, 32 ± kg/m(2)). Muscle samples of the vastus lateralis were collected before and during a 3-hour 80 mU/m(2)/min hyperinsulinemic-euglycemic clamp. AEX + WL increased VO2max by 11% (p < .05) and decreased body weight (-9%, p < .001). AEX + WL increased basal citrate synthase activity by 46% (p < .01) and insulin activation of independent (2.9-fold) and fractional (2.3-fold) activities (both p < .001) of glycogen synthase. AEX + WL had no effect on phosphorylation of Akt or AS160. Glucose utilization (M) improved 25% (p < .01), and the change tended to be related to the increase in insulin activation of glycogen synthase fractional activity (r = .50, p = .08) following AEX + WL. In summary, AEX + WL has a robust effect on insulin activation of skeletal muscle glycogen synthase activity that likely contributes to improved glucose utilization in older insulin-resistant men.

Aerobic exercise + weight loss decreases skeletal muscle myostatin expression and improves insulin sensitivity in older adults

OBJECTIVE

To determine whether aerobic exercise training + weight loss (AEX + WL) would affect the expression of myostatin and its relationship with insulin sensitivity in a longitudinal, clinical intervention study.

DESIGN AND METHODS

Thirty-three obese sedentary postmenopausal women and men (n = 17 and 16, age: 61 ± 1 years, body mass index: 31 ± 1 kg/m(2) , VO2 max: 21.9 ± 1.0 mL/kg/min, X ± Standard error of the mean (SEM)) completed 6 months of 3 days/week AEX + WL. During an 80 mU m(-2) min(-1) hyperinsulinemic-euglycemic clamp, we measured glucose utilization (M), myostatin, myogenin, and MyoD gene expression by real-time RT-PCR in vastus lateralis muscle at baseline and 2 h.

RESULTS

Body weight (-8%) and fat mass (-17%) decreased after AEX + WL (P < 0.001). Fat-free mass (FFM) and mid-thigh muscle area by computed tomography did not change but muscle attenuation increased (P < 0.05). VO2 max increased 14% (P < 0.001). AEX + WL increased M by 18% (P < 0.01). Myostatin gene expression decreased 19% after AEX + WL (P < 0.05). Basal mRNA myostatin levels were negatively associated with M before the intervention (r = -0.43, P < 0.05). Insulin infusion increased myoD and myogenin expression before and after AEX + WL (both P < 0.001) but basal levels did not change. The insulin effect on myostatin expression was associated with the change in M after AEX + WL (r = 0.56, P < 0.005).

CONCLUSIONS

Exercise and weight loss results in a downregulation of myostatin mRNA and an improvement in insulin sensitivity in obese older men and women.

Diversity of Insulin Resistance in Monkeys with Normal Glucose Tolerance

Insulin resistance has been proposed as a critical factor in the development of Type II diabetes, hypertension, dyslipidemia, and coronary artery disease. However, even in normal healthy individuals, a wide range of in vivo insulin action has been found. In the present study we sought to examine this heterogeneity in insulin action in both normal and spontaneously obese nonhuman primates. Maximal insulin responsiveness as measured by a hyperinsulinemic euglycemic clamp, fasting plasma glucose, and insulin levels, beta-cell insulin response to glucose, glucose tolerance, and adiposity were measured in 22 male rhesus monkeys. Results showed that lean animals (body fat < or = 22%) had higher insulin-stimulated glucose uptake (M rate: 14.42+/-1.8 mg/kg FFM/min) compared to obese (8.08+/-0.8). The obese monkeys, with 23-49% body fat, had a wide range of M values (5.32-14.29 mg/kg FFM/min) which showed no relationship to degree of adiposity. In all monkeys, M values had a strong inverse correlation with fasting plasma insulin levels (r=-0.76; p<0.001), but not with fasting glucose or glucose disappearance rate. We conclude that neither degree of obesity above a critical threshold nor range of glucose tolerance is related to insulin resistance; however, in individuals with normal glucose tolerance an early reliable indicator of defective insulin action appears to be fasting insulin concentration. Longitudinal determination of basal insulin levels obtained under standardized conditions so as to minimize extraneous variability is likely to strengthen the ability to predict insulin resistance and possible later development of overt Type II diabetes.

Exercise with calorie restriction improves insulin sensitivity and glycogen synthase activity in obese postmenopausal women with impaired glucose tolerance

Our objective was to compare the effects of in vivo insulin on skeletal muscle glycogen synthase (GS) activity in normal (NGT) vs. impaired glucose-tolerant (IGT) obese postmenopausal women and to determine whether an increase in insulin activation of GS is associated with an improvement in insulin sensitivity (M) following calorie restriction (CR) and/or aerobic exercise plus calorie restriction (AEX + CR) in women with NGT and IGT. We did a longitudinal, clinical intervention study of CR compared with AEX + CR. Overweight and obese women, 49-76 yr old, completed 6 mo of CR (n = 46) or AEX + CR (n = 50) with Vo(2 max), body composition, and glucose tolerance testing. Hyperinsulinemic euglycemic (80 mU·m(-2)·min(-1)) clamps (n = 73) and skeletal muscle biopsies (before and during clamp) (n = 58) were performed before and after the interventions (n = 50). After 120 min of hyperinsulinemia during the clamp, GS fractional activity and insulin's effect to increase GS fractional activity (insulin - basal) were significantly lower in IGT vs. NGT (P < 0.01) at baseline. GS total activity increased during the clamp in NGT (P < 0.05), but not IGT, at baseline. CR and AEX + CR resulted in a significant 8% weight loss with reductions in total fat mass, visceral fat, subcutaneous fat, and intramuscular fat. Overall, M increased (P < 0.01), and the change in M (postintervention - preintervention) was associated with the change in insulin-stimulated GS fractional activity (partial r = 0.44, P < 0.005). In IGT, the change (postintervention - preintervention) in insulin-stimulated GS total activity was greater following AEX + CR than CR alone (P < 0.05). In IGT, insulin-stimulated GS-independent (P < 0.005) and fractional activity (P = 0.06) increased following AEX + CR. We conclude that the greatest benefits at the whole body and cellular level (insulin activation of GS) in older women at highest risk for diabetes are derived from a lifestyle intervention that includes exercise and diet.

Insulin signaling and insulin sensitizing in muscle and liver of obese monkeys: peroxisome proliferator-activated receptor gamma agonist improves defective activation of atypical protein kinase C

Obesity, the metabolic syndrome, and aging share several pathogenic features in both humans and non-human primates, including insulin resistance and inflammation. Since muscle and liver are considered key integrators of metabolism, we sought to determine in biopsies from lean and obese aging rhesus monkeys the nature of defects in insulin activation and, further, the potential for mitigation of such defects by an in vivo insulin sensitizer, rosiglitazone, and a thiazolidinedione activator of the peroxisome proliferator-activated receptor gamma. The peroxisome proliferator-activated receptor gamma agonist reduced hyperinsulinemia, improved insulin sensitivity, lowered plasma triglycerides and free fatty acids, and increased plasma adiponectin. In muscle of obese monkeys, previously shown to exhibit defective insulin signaling, the insulin sensitizer improved insulin activation of atypical protein kinase C (aPKC), the defective direct activation of aPKC by phosphatidylinositol (PI)-3,4,5-(PO₄)₃, and 5'-AMP-activated protein kinase and increased carnitine palmitoyltransferase-1 mRNA expression, but it did not improve insulin activation of insulin receptor substrate (IRS)-1-dependent PI 3-kinase (IRS-1/PI3K), protein kinase B, or glycogen synthase. We found that, although insulin signaling was impaired in muscle, insulin activation of IRS-1/PI3K, IRS-2/PI3K, protein kinase B, and aPKC was largely intact in liver and that rosiglitazone improved insulin signaling to aPKC in muscle by improving responsiveness to PI-3,4,5-(PO₄)₃.

Leptin augments the acute suppressive effects of insulin on hepatic very low-density lipoprotein production in rats

It is well established that leptin increases the sensitivity of carbohydrate metabolism to the effects of insulin. Leptin and insulin also have potent effects on lipid metabolism. However, the effects of leptin on the regulation of liver lipid metabolism by insulin have not been investigated. The current study addressed the effects of leptin on insulin-regulated hepatic very low-density lipoprotein (VLDL) metabolism in vivo in rats. A 90-min hyperinsulinemic/euglycemic clamp (4 mU/kg x min(-1)) reduced plasma VLDL triglyceride (TG) by about 50% (P < 0.001 vs. saline control). Importantly, a leptin infusion (0.2 microg/kg x min(-1)) in combination with insulin reduced plasma VLDL-TG by about 80% (P < 0.001 vs. insulin alone). These effects did not require altered skeletal muscle lipoprotein lipase activity but did include differential effects of insulin and leptin on liver apolipoprotein (apo) B and TG metabolism. Thus, insulin decreased liver and plasma apoB100/B48 levels (approximately 50%, P < 0.01), increased liver TGs (approximately 20%, P < 0.05), and had no effect on fatty acid oxidation. Conversely, leptin decreased liver TGs (approximately 50%, P < 0.01) and increased fatty acid oxidation (approximately 50%, P < 0.01) but had no effects on liver or plasma apoB levels. Importantly, the TG-depleting and prooxidative effects of leptin were maintained in the presence of insulin. We conclude that leptin additively increases the suppressive effects of insulin on hepatic and systemic VLDL metabolism by stimulating depletion of liver TGs and increasing oxidative metabolism. The net effect of the combined actions of insulin and leptin is to decrease the production and TG content of VLDL particles.

Exercise training and calorie restriction increase SREBP-1 expression and intramuscular triglyceride in skeletal muscle

Intramuscular triglyceride (IMTG) deposition in skeletal muscle is associated with obesity and type 2 diabetes (T2DM) and is thought to be related to insulin resistance (IR). Curiously, despite enhanced skeletal muscle insulin sensitivity, highly trained athletes and calorie-restricted (CR) monkeys also have increased IMTG. Sterol regulatory element-binding proteins (SREBPs) are transcription factors that regulate the biosynthesis of cholesterol and fatty acids. SREBP-1 is increased by insulin in skeletal muscle in vitro and in skeletal muscle of IR subjects, but SREBP-1 expression has not been examined in exercise training or calorie restriction. We examined the relationship between IMTG and SREBP-1 expression in animal models of exercise and calorie restriction. Gastrocnemius and soleus muscle biopsies were obtained from 38 Sprague-Dawley rats (18 control and 20 exercise trained). Triglyceride content was higher in the gastrocnemius and soleus muscles of the trained rats. SREBP-1c mRNA, SREBP-1 precursor and mature proteins, and fatty acid synthase (FAS) protein were increased with exercise training. Monkeys (Macaca mulatta) were CR for a mean of 10.4 years, preventing weight gain and IR. Vastus lateralis muscle was obtained from 12 monkeys (6 CR and 6 controls). SREBP-1 precursor and mature proteins and FAS protein were higher in the CR monkeys. In addition, phosphorylation of ERK1/ERK2 was increased in skeletal muscle of CR animals. In summary, SREBP-1 protein and SREBP-1c mRNA are increased in interventions that increase IMTG despite enhanced insulin sensitivity. CR and exercise-induced augmentation of SREBP-1 expression may be responsible for the increased IMTG seen in skeletal muscle of highly conditioned athletes.

Effects of Aerobic and Resistive Exercise Training on Glucose Disposal and Skeletal Muscle Metabolism in Older Men

BACKGROUND

Aging is associated with insulin resistance, primarily as a result of physical inactivity and increased abdominal obesity. We hypothesized that aerobic (AEX) or resistive (RT) exercise training would result in comparable improvements in glucose disposal in older men, but that there would be different metabolic adaptations in skeletal muscle.

METHODS

Thirty-nine older (63+/-1 years, mean+/-standard error of the mean), overweight and obese (body mass index=30.3+/-0.4 kg/m2) men were assigned to AEX (treadmill walking and/or jogging, n=19) or RT (upper and lower body, n=20) programs 3 d/wk for 6 months, with 9 completing AEX and 13 completing RT. Testing before and after the exercise programs included body composition, euglycemic-hyperinsulinemic clamps, and vastus lateralis muscle biopsies.

RESULTS

Maximal oxygen consumption (VO2max) increased by 16% after AEX (p<.01), while leg and arm muscle strength increased by 45+/-5% and 27+/-5% after RT (p<.0001). Although participants were monitored to maintain their body weight during the exercise program, body weight decreased by 2% after AEX (p<.05), and increased by 2% after RT (p<.05). Whole-body glucose disposal, determined during the last 30 minutes of a 2-hour 480 pmol/m2/min euglycemic-hyperinsulinemic clamp, increased comparably by 20%-25% after AEX (51+/-5 to 61+/-5 microM/kgfat-free mass/min, p<.05) and RT (49+/-3 to 58+/-3 microM/kgfat-free mass/min, p<.05). The increase in vastus lateralis muscle glycogen synthase fractional activity in response to insulin stimulation was significantly higher after AEX compared to after RT (279+/-59% compared to 100+/-28% change, p<.05). Neither AEX nor RT altered muscle glycogen synthase total activity, glycogen content, or levels of phosphotidylinositol 3-kinase.

CONCLUSION

These results suggest that AEX and RT result in comparable improvements in glucose metabolism in older men, whereas an increase in insulin activation of glycogen synthase occurred only with AEX. These improvements in insulin sensitivity could reduce the risk of metabolic syndrome and type 2 diabetes and attenuate the development of cardiovascular disease.

Chronic calorie restriction alters glycogen metabolism in rhesus monkeys

Chronic caloric restriction (CR) prevents the development of obesity and maintains health, slows aging processes, and prevents or substantially delays the development of non-insulin-dependent diabetes. Because changes in energy metabolism could be involved in all of these positive effects of CR, we examined glycogen synthase (GS) and glycogen phosphorylase (GP) activities and glucose 6-phosphate (G6P) and glycogen concentrations in skeletal muscle samples before and during a euglycemic hyperinsulinemic clamp in 6 older aged monkeys in which CR had been continued for 10.4 +/- 2.1 years. Basal GS activity (fractional velocity and independent) was significantly higher in the CR monkeys than has been previously shown in normal, hyperinsulinemic and diabetic monkeys. The normal effect of insulin to activate GS was absent in the CR group due to the paradoxical finding in some of these monkeys of a reduction in GS activity by insulin. Insulin also had the unexpected effect of increasing the independent activity of GP above basal activity (p<0.05). There was an inverse relationship between the change (insulin-stimulated minus basal) in GS fractional velocity and GP activity ratio (r=-0.91, p<0.005). The basal independent activities of GS and GP were also inversely correlated (r=-0.79, p<0.05). The insulin-stimulated concentration of G6P tended to be higher than the basal concentration (p<0.06) and was significantly higher than that previously shown in normal monkeys (p<0.05). We suggest that long-term calorie restriction (1) results in alterations in glycogen metabolism that may be important to the anti-diabetogenic and antiaging effects of CR and (2) unmasks early defects which may indicate the likelihood of ultimately developing obesity and diabetes.

Skeletal muscle glycogen synthase subcellular localization: effects of insulin and PPAR-α agonist (K-111) administration in rhesus monkeys

Insulin covalently and allosterically regulates glycogen synthase (GS) and may also cause the translocation of GS from glycogen-poor to glycogen-rich locations. We examined the possible role of subcellular localization of GS and glycogen in insulin activation of GS in skeletal muscle of six obese monkeys and determined whether 1) insulin stimulation during a hyperinsulinemic euglycemic clamp and/or peroxisome proliferator-activated receptor (PPAR)-α agonist treatment (K-111, 3 mg·kg−1·day−1; Kowa) induced translocation of GS and 2) translocation of GS was associated with insulin activation of GS. GS and glycogen were present in all fractions obtained by differential centrifugation, except for the cytosolic fraction, under both basal and insulin-stimulated conditions. We found no evidence for translocation of GS by insulin. GS total (GST) activity was strongly associated with glycogen content ( r = 0.70, P < 0.001). Six weeks of treatment with K-111 increased GST activity in all fractions, except the cytosolic fraction, and mean GST activity, GS independent activity, and glycogen content were significantly higher in the insulin-stimulated samples compared with basal samples, effects not seen with vehicle. The increase in GST activity was strongly related to the increase in glycogen content during the hyperinsulinemic euglycemic clamp after K-111 administration ( r = 0.74, P < 0.001). Neither GS protein expression nor GS gene expression was affected by insulin or by K-111 treatment. We conclude that 1) in vivo insulin does not cause translocation of GS from a glycogen-poor to a glycogen-rich location in primate skeletal muscle and 2) the mechanism of action of K-111 to improve insulin sensitivity includes an increase in GST activity without an increase in GS gene or protein expression.

Mortality and morbidity in laboratory-maintained Rhesus monkeys and effects of long-term dietary restriction

Mortality and morbidity were examined in 117 laboratory-maintained rhesus monkeys studied over approximately 25 years (8 dietary-restricted [DR] and 109 ad libitum-fed [AL] monkeys). During the study, 49 AL monkeys and 3 DR monkeys died. Compared with the DR monkeys, the AL monkeys had a 2.6-fold increased risk of death. Hyperinsulinemia led to a 3.7-fold increased risk of death (p <.05); concordantly, the risk of death decreased by 7%, per unit increase in insulin sensitivity (M). There was significant organ pathology in the AL at death. The age at median survival in the AL was approximately 25 years compared with 32 years in the DR. The oldest monkey was a diabetic female (AL) that lived to be 40 years of age. These results suggest that dietary restriction leads to an increased average age of death in primates, associated with the prevention of hyperinsulinemia and the mitigation of age-related disease.

Skeletal muscle insulin resistance in obesity-associated type 2 diabetes in monkeys is linked to a defect in insulin activation of protein kinase C-zeta/lambda/iota

Rhesus monkeys frequently develop obesity and insulin resistance followed by type 2 diabetes when allowed free access to chow. This insulin resistance is partly due to defective glucose transport into skeletal muscle. In this study, we examined signaling factors required for insulin-stimulated glucose transport in muscle biopsies taken during euglycemic-hyperinsulinemic clamps in nondiabetic, obese prediabetic, and diabetic monkeys. Insulin increased activities of insulin receptor substrate (IRS)-1-dependent phosphatidylinositol (PI) 3-kinase and its downstream effectors, atypical protein kinase Cs (aPKCs) (zeta/lambda/iota) and protein kinase B (PKB) in muscles of nondiabetic monkeys. Insulin-induced increases in glucose disposal and aPKC activity diminished progressively in prediabetic and diabetic monkeys. Decreases in aPKC activation appeared to be at least partly due to diminished activation of IRS-1-dependent PI 3-kinase, but direct activation of aPKCs by the PI 3-kinase lipid product PI-3,4,5-(PO(4))(3) was also diminished. In conjunction with aPKCs, PKB activation was diminished in prediabetic muscle but, differently from aPKCs, seemed to partially improve in diabetic muscle. Interestingly, calorie restriction and avoidance of obesity largely prevented development of defects in glucose disposal and aPKC activation. Our findings suggest that defective activation of aPKCs contributes importantly to obesity-dependent development of skeletal muscle insulin resistance in prediabetic and type 2 diabetic monkeys.

Elevated plasma cell membrane glycoprotein levels and diminished insulin receptor autophosphorylation in obese, insulin-resistant rhesus monkeys

In obese humans, insulin resistance is accompanied by elevated levels of plasma cell membrane glycoprotein (PC-1) and decreased insulin receptor (IR) tyrosine kinase activity in skeletal muscle. PC-1 overexpression inhibits IR tyrosine kinase and possibly other downstream signaling events. The rhesus monkey in captivity is susceptible to obesity with concomitant insulin resistance. In the present study we analyzed obese (n = 10, 29.4% +/- 1.2% body fat) and non-obese (n = 12, 19.4% +/- 1.9% body fat) rhesus monkeys. Glucose clearance during an euglycemic hyperinsulinemic (400 mU/m(2) body surface area/min) clamp was lower for the obese group (non-obese, 9.7 +/- 0.9; obese, 3.2 +/- 0.7 mg/kg fat-free mass [FFM]/min; P <.01). We performed vastus lateralis muscle biopsies prior to and during the clamp. We measured PC-1 levels in these muscle samples to determine whether PC-1 content is elevated in this primate model of insulin resistance. PC-1 levels were determined by assay of phosphodiesterase activity and specific PC-1 enzyme-linked immunosorbent assay (ELISA). In the obese group, both PC-1 content and activity were 2-fold higher than in the non-obese group (P <.05). In order to investigate the ability of insulin to stimulate IR signaling in vivo in these 2 groups of monkeys, we then measured tyrosine autophosphorylation of the IR by specific ELISA. The increase in IR autophosphorylation in the non-obese group was twice that of the obese group (fold increase over basal: non-obese, 3.7 +/- 0.3; obese, 1.9 +/- 0.6; P <.05). We conclude that insulin resistance secondary to obesity in rhesus monkeys is associated with increased levels of PC-1 and decreased IR signaling capacity in skeletal muscle.

Prostaglandylinositol cyclic phosphate synthase activity in the liver of insulin-resistant rhesus monkeys before and after a euglycemic hyperinsulinemic clamp

Prostaglandylinositol cyclic phosphate (cPIP), functionally a cAMP antagonist, is a novel, low-molecular weight mediator of insulin action. Both essential hypertension and type 2 diabetes may be associated with a reduction of cPIP synthesis. In intact cells and in plasma membranes, cPIP synthesis is stimulated by insulin, which activates cPIP synthase by tyrosine phosphorylation. We measured the activities of cPIP synthase in the homogenates of freeze-clamped and then lyophilized liver samples from five insulin-resistant, adult rhesus monkeys, obtained under basal fasting conditions and again under maximal insulin stimulation during a euglycemic hyperinsulinemic clamp. The mean cPIP synthase activity in basal samples (0.33 +/- 0.09 pmol/min/mg protein) was not significantly different at the end of the clamp (0.24 +/- 0.11 pmol/min/mg protein). Basal cPIP synthase activityVoL 12, No. 1, 2001 was directly related to both basal cAMP content and basal fractional activity of cAMP-dependent protein kinase (PKA): r=0.85, p<0.05 and r=0.86, p<0.05, respectively. In turn, insulin-stimulated cPIP synthase activity was inversely related to both the insulin-stimulated fractional activity of PKA (r=0.89, p<0.02) and the insulin-stimulated total PKA activity: r=0.94, p<0.005. The findings suggest that in the liver of insulin-resistant rhesus monkeys, cPIP synthase activity, which leads to the synthesis of the low-molecular weight mediator cPIP, may oppose cAMP synthesis and PKA activity.

Prostaglandylinositol cyclic phosphate (cPIP): a novel second messenger of insulin action. Comparative analysis of two kinds of "insulin mediators"

Insulin induces a broad spectrum of effects over a wide time interval. It also stimulates the phosphorylation of some cellular proteins, while decreasing the state of phosphorylation of others. These observations indicate the presence of different, but not necessarily mutually exclusive, pathways of insulin action. One well-known pathway represents a phosphorylation cascade initiated by the tyrosine kinase activity of the insulin receptor followed by involvement of different MAP-kinases. Another pathway suggests the existence of low molecular weight insulin mediators whose synthesis and/or release is initiated by insulin. Comparable analysis of two kinds of insulin mediators, namely inositolphosphoglycans and prostaglandylinositol cyclic phosphate (cPIP), has been carried out. It has been shown that the expression of a number of enzymes, such as phospholipase A(2), phospholipase C, cyclo-oxygenase and IRS-1-like enzyme, could regulate the biosynthesis of cPIP in both normal and diabetes-related conditions. Data on the activity of a key enzyme of cPIP biosynthesis termed cPIP synthase (IRS-1-like enzyme) in various monkey tissues before and twice during an euglycemic hyperinsulinemic clamp have been presented. It has been concluded that in vivo insulin increases cPIP synthase activity in both liver and subcutaneous adipose tissue of lean normal monkeys. It has been also suggested that abnormal production of cPIP could be related to several pathologies including glucocorticoid-induced insulin resistance and diabetic embryopathy. Further studies on cPIP and other types of insulin mediators are necessary to aid our understanding of insulin action.

Circulating concentrations of the adipocyte protein adiponectin are decreased in parallel with reduced insulin sensitivity during the progression to type 2 diabetes in rhesus monkeys

Adiponectin is an adipose-specific plasma protein whose plasma concentrations are decreased in obese subjects and type 2 diabetic patients. This protein possesses putative antiatherogenic and anti-inflammatory properties. In the current study, we have analyzed the relationship between adiponectin and insulin resistance in rhesus monkeys (Macaca mulatta), which spontaneously develop obesity and which subsequently frequently progress to overt type 2 diabetes. The plasma levels of adiponectin were decreased in obese and diabetic monkeys as in humans. Prospective longitudinal studies revealed that the plasma levels of adiponectin declined at an early phase of obesity and remained decreased after the development of type 2 diabetes. Hyperinsulinemic-euglycemic clamp studies revealed that the obese monkeys with lower plasma adiponectin showed significantly lower insulin-stimulated peripheral glucose uptake (M rate). The plasma levels of adiponectin were significantly correlated to M rate (r = 0.66, P < 0.001). Longitudinally, the plasma adiponectin decreased in parallel to the progression of insulin resistance. No clear association was found between the plasma levels of adiponectin and its mRNA levels in adipose tissue. These results suggest that reduction in circulating adiponectin may be related to the development of insulin resistance.

In vivo insulin regulation of skeletal muscle glycogen synthase in calorie-restricted and in ad libitum-fed rhesus monkeys

Chronic calorie restriction in primates has been shown to have profound and unexpected effects on basal and on in vivo insulin action on skeletal muscle glycogen synthase (GS) activity. The decreased ability of insulin to activate skeletal muscle GS is a hallmark of insulin resistance and type 2 diabetes. The mechanism and role of in vivo insulin regulation of skeletal muscle GS are not fully understood. Two pathways for the activation of GS by insulin have been described by Larner and others: 1) insulin activates glucose transport that results in an increase in glucose-6-phosphate (G6P), thereby activating protein phosphatase-1, which in turn dephosphorylates and activates GS, therefore, pushing substrate into glycogen; and 2) insulin activates GS (perhaps by forming low-molecular-weight mediators which may activate protein phosphatase-1 and 2C) and activated GS subsequently pulls intermediates (e.g., G6P and uridine 5'-diphosphoglucose) into glycogen. To determine whether in vivo insulin regulates glycogen synthesis primarily via a push or pull mechanism and how this mechanism might be affected by long-term calorie restriction, skeletal muscle samples were obtained before and during a euglycemic hyperinsulinemic clamp from 41 rhesus monkeys. The monkeys varied widely in their degree of insulin sensitivity and age and included chronically calorie-restricted (CR) monkeys and ad libitum-fed monkeys. The ad libitum-fed monkeys included spontaneously type 2 diabetic, prediabetic and clinically normal animals. The apparent affinity of GS for the allosteric activator G6P (G6P Ka of GS) was measured and compared with G6P content in the muscle samples. Basal G6P Ka of GS was lower in the CR monkeys compared with the 3 ad libitum-fed groups (P: < or = 0.05). Only the normal ad libitum-fed monkeys had a decrease in the G6P Ka of GS with insulin (P: < 0.005). The insulin effect (insulin-stimulated minus basal) on the G6P Ka of GS was strongly positively related to the insulin effect on G6P content (r = 0.80, P: < 0.0001) across the entire group of monkeys. This finding supports the hypothesis that activation/dephosphorylation of GS by insulin is related to a decrease in G6P content and that paradoxical inactivation/phosphorylation of GS by insulin is related to an increase in G6P content (as demonstrated in 4 of 6 CR monkeys). Therefore, during a euglycemic hyperinsulinemic clamp, insulin regulates skeletal muscle glycogen synthesis primarily via a pull mechanism in both CR and in ad libitum-fed rhesus monkeys.

A thiazolidinedione improves in vivo insulin action on skeletal muscle glycogen synthase in insulin-resistant monkeys

Thiazolidinediones (TZD) have been shown to have anti-diabetic effects including the ability to decrease fasting hyperglycemia and hyperinsulinemia, increase insulin-mediated glucose disposal rate (M) and decrease hepatic glucose production, but the mechanisms of action are not well established. To determine whether a TZD (R-102380, Sankyo Company Ltd., Tokyo, Japan) could improve insulin action on skeletal muscle glycogen synthase (GS), the rate-limiting enzyme in glycogen synthesis, 4 insulin-resistant obese monkeys were given 1 mg/kg/day R-102380 p.o. for a 6-week period. Skeletal muscle GS activity and glucose 6-phosphate (G6P) content were compared between pre-dosing and dosing periods before and during the maximal insulin-stimulation of a euglycemic hyperinsulinemic clamp. Compared to pre-dosing, insulin-stimulated GS activity and G6P content were increased by this TZD: GS independent activity (p = 0.02), GS total activity (p = 0.005), GS fractional activity (p = 0.06) and G6P content (p = 0.02). The change in GS activity induced by in vivo insulin (insulin-stimulated minus basal) was also increased by this TZD: GS independent activity (p = 0.03) and GS fractional activity (p = 0.04). We conclude that the TZD R-102380 improves insulin action at the skeletal muscle in part by increasing the activity of glycogen synthase. This improvement in insulin sensitivity may be a key factor in the anti-diabetic effect of the thiazolidinedione class of agents.

Calorie restriction in nonhuman primates: mechanisms of reduced morbidity and mortality

Long term chronic calorie restriction (CR) of adult nonhuman primates significantly reduces morbidity and increases median age of death. The present review is focused upon an ongoing study of sustained adult-onset calorie restriction, which has been underway for 15 years. Monkeys, initially calorie restricted at about 10 years of age, are now approximately 25 years old. The median life span of these restricted monkeys is increasing, now exceeding that of ad libitum (AL)-fed monkeys. In our laboratory, maximum life span for AL-fed monkeys appears to be about 40 years. Thus, whether CR can also increase maximal life span, as it does in rodents, cannot be determined for at least another 15 years. The earliest detectable positive benefit on morbidity in these monkeys was previously reported as the prevention of obesity. Current evidence, as reviewed here, suggests that much obesity-associated morbidity is also mitigated by sustained calorie restraint in nonhuman primates. Furthermore, probably because of the prevention of obesity, diabetes has also been prevented. Recent findings include the identification of extraordinary changes in the glycogen synthesis pathway, and on the phosphorylation of glycogen synthase in response to insulin. This calorie restriction-induced prevention of morbidity does not require excessive leanness, but is clearly present when body fat is within the normal range of 10 to 22%, and this is likely to be true in humans as well.

Paradoxical phosphorylation of skeletal muscle glycogen synthase by in vivo insulin in very lean young adult rhesus monkeys

Calorie restriction (CR) has previously been shown to unexpectedly induce a reversal of in vivo insulin action (phosphorylation instead of dephosphorylation) on skeletal muscle glycogen synthase (GS) in four out of six long-term calorie-restricted (CR) monkeys. The purpose of the present study was to determine whether this increase in Ka (concentration of glucose 6-phosphate [G6P] at which GS activity is half-maximal) during insulin is also present in very lean (VL) young adult monkeys maintained on a controlled feeding regimen. Muscle samples from 10 VL monkeys (10 +/- 2% body fat; 7 years old) were obtained before and during a euglycemic hyperinsulinemic clamp and the Ka was determined and compared to the Ka of two other groups of monkeys, one matched in age but fully ad libitum (AL)-fed (n = 9.8 +/- 1 years old, 20 +/- 3% body fat, p = 0.01 vs. VL monkeys), and the other our previously described weight-clamped long-term CR monkeys (n = 6.20 +/- 1 years old, 21 +/- 2% body fat, p = 0.01 vs. VL monkeys). All of the AL monkeys had the expected decrease in Ka with insulin; however, similar to the 4 out of 6 CR monkeys, 7 out of 10 VL monkeys had an increase in Ka with insulin. The 11 monkeys with an increase in Ka (+Ka) (7 VL + 4 CR) were compared to the 14 monkeys with a decrease in Ka with insulin (-Ka) (3 VL + 2 CR + 9 AL). The +Ka monkeys had lower basal Ka (p = 0.0001), higher basal GS fractional activity (p = 0.0003), lower basal G6P content (p = 0.002), lower glycogen phosphorylase fractional activity (p = 0.01), and lower whole-body insulin-mediated glucose disposal rate (p < 0.05) than the -Ka monkeys. We conclude that the condition of steady-state restrained calorie intake (as in the CR monkeys and in the controlled feeding VL monkeys) produces the paradoxical action of in vivo insulin to phosphorylate muscle GS, and raises the possibility that the presence of the unusual response to insulin may serve as a marker in calorie-restrained individuals for the genotype of obesity, insulin resistance and/or Type 2 diabetes.

Age-related adipose tissue mRNA expression of ADD1/SREBP1, PPARgamma, lipoprotein lipase, and GLUT4 glucose transporter in rhesus monkeys

Aging has been shown to have an effect on the capacity to differentiate preadipocytes and on the expression of some genes expressed in adipose tissue. The mRNA levels of adipocyte differentiation-related genes were examined in rhesus monkeys (Macaca Mulatta) ranging in age from 7 to 30 years. The effect of aging on the expression of peroxisome proliferator activated receptor gamma (PPARgamma), adipocyte determination- and differentiation-dependent factor 1/sterol regulatory element binding protein 1 (ADD1/SREBP1), CCAAT/enhancer binding protein alpha (C/EBPalpha), lipoprotein lipase (LPL), GLUT4 glucose transporter, and adipsin were examined by slot blot analysis. Significant inverse correlations were observed between age and the mRNA levels of PPARgamma, ADD1/SREBP1, LPL, and GLUT4. The coordinate downregulation of these genes may be linked to the declining fat mass of senescent animals. There was no correlation between age and the mRNA levels of adipsin. The mRNA levels of these genes were not correlated to body weight orfasting plasma insulin. These findings indicate that aging may have an effect on the adipocyte differentiation program and this effect appears to be gene specific.

Insulin unexpectedly increases the glucose 6-phosphate Ka of skeletal muscle glycogen synthase in calorie-restricted monkeys

In skeletal muscle of normal subjects, the concentration of glucose 6-phosphate (G6P) at which the activity of glycogen synthase (GS) is half maximal (Ka) is decreased by in vivo insulin, and the fractional activity is increased without a change in GS maximal activity (Vmax). We have shown that moderate chronic calorie restriction, previously shown in rodents to be effective in slowing aging, resulted in the prevention of obesity and type 2 diabetes in primates (rhesus monkeys, Macaca mulatta). However, unexpectedly, in a subgroup of calorie-restricted monkeys, insulin during a euglycemic hyperinsulinemic clamp caused an unanticipated decrease in skeletal muscle GS fractional activity. These same monkeys had the lowest whole-body glucose disposal rate (M), the greatest increase in skeletal muscle G6P content and the greatest increase in skeletal muscle glycogen phosphorylase activity during the euglycemic hyperinsulinemic clamp compared to the remaining calorie-restricted monkeys with normal insulin action. To determine whether this highly unusual insulin-mediated decrease in GS fractional activity was due to increased phosphorylation (increased Ka), we measured the activity of skeletal muscle GS at 9 different G6P concentrations before and during the euglycemic hyperinsulinemic clamp in 6 calorie-restricted monkeys. G6P Ka increased (n = 4) and Vmax decreased (n = 5) during the clamp. Basal G6P Ka was inversely related to basal GSfv (r = -0.94, p < 0.002). G6P Ka and skeletal muscle G6P content were positively related under insulin-stimulated conditions (r = 0.93, p < 0.005). The change in G6P Ka (insulin-stimulated minus basal) was inversely related to M (r = -0.94, p < 0.002) and positively related to the change in skeletal muscle G6P content (r = 0.93, p < 0.005). We conclude that moderate calorie restriction results in a reversal of normal insulin action at the skeletal muscle with inactivation of glycogen synthase which is likely to be due to an increase in phosphorylation of GS together with a decrease in Vmax of GS during a euglycemic hyperinsulinemic clamp in most of the calorie-restricted monkeys. These alterations are likely to be involved in the anti-diabetogenic effects of calorie restriction.

Insulin increases liver protein phosphatase-1 and protein phosphatase-2C activities in lean, young adult rhesus monkeys

Liver glycogen synthase activity is increased, and glycogen phosphorylase activity and glucose 6-phosphate content reduced by in vivo insulin during a euglycemic hyperinsulinemic clamp in lean young adult rhesus monkeys. To examine the mechanism of dephosphorylation of liver glycogen synthase and glycogen phosphorylase, the enzyme activities of protein phosphatase-1, protein phosphatase-2C, cAMP-dependent protein kinase, glycogen synthase kinase-3, protein kinase C and protein tyrosine kinase were determined before and after three hours of in vivo insulin in these same monkeys. The bioactivity of an inositol phosphoglycan insulin mediator (pH 2.0) and cAMP concentrations were also measured in the liver before and after insulin administration. Insulin caused significant increases in protein phosphatase-1 (p = 0.005) and in protein phosphatase-2C activities (p = 0.001). Insulin-stimulated minus basal bioactivity of the pH 2.0 insulin mediator was strongly inversely related to the insulin-stimulated minus basal glucose 6-phosphate content (r = -0.93, p < 0.0001). These findings suggest that protein phosphatase-1 and protein phosphatase-2C may be involved in the mechanism of in vivo insulin activation of liver glycogen synthase and inactivation of liver glycogen phosphorylase.

Relationships of PPARgamma and PPARgamma2 mRNA levels to obesity, diabetes and hyperinsulinaemia in rhesus monkeys

OBJECTIVE

To examine the expression of peroxisome proliferator-activated receptor gamma (PPARgamma) together with CCAAT/enhancer binding protein alpha (C/EBPalpha), lipoprotein lipase (LPL) and glucose transporter (GLUT4) mRNA in adipose tissue of rhesus monkeys in relation to obesity.

DESIGN

Cloning of the PPARgamma1 and gamma2 cDNAs and analysis of PPARgamma, C/EBPalpha, LPL and GLUT4 mRNA levels in the adipose tissue of lean and obese monkeys.

SUBJECTS

28 rhesus monkeys (Macaca mulatta) with a wide range of body weights (9.2-22.6 kg) and with or without type 2 diabetes.

MEASUREMENTS

Sequence of PPARgamma1 and gamma2. Tissue distribution of PPARgamma1 and gamma2. The mRNA levels of PPARgamma, C/EBPalpha, LPL and GLUT4 in adipose tissue. The ratio of PPARgamma2 mRNA to total PPARgamma mRNA.

RESULTS

The monkey PPARgamma2 protein showed 99% identity with the human protein. PPARgamma1 mRNA was shown to be expressed in various tissues and most abundantly in adipose tissue. PPARgamma2 existed mainly in adipose tissue. A significant correlation between the ratio of PPARgamma2 mRNA to total PPARgamma mRNA and obesity was observed, whereas total PPARgamma mRNA levels showed no significant relationships to obesity. There was also a significant relationship between the ratio of PPARgamma2 mRNA to total PPARgamma mRNA and fasting plasma insulin concentration. The mRNA levels of C/EBPalpha, LPL and GLUT4 were highly correlated to that of total PPARgamma mRNA. They were also significantly correlated to the mRNA levels of PPARgamma1 and PPARgamma2.

CONCLUSIONS

The ratio of PPARgamma2 mRNA to total PPARgamma mRNA is related to obesity in the rhesus monkey and mRNA expression of PPARgamma1, PPARgamma2, C/EBPalpha, LPL and GLUT4 appear to be coordinated in vivo.

Monkey leptin receptor mRNA: sequence, tissue distribution, and mRNA expression in the adipose tissue of normal, hyperinsulinemic, and type 2 diabetic rhesus monkeys

OBJECTIVE

We have cloned the rhesus monkey leptin receptor and examined its mRNA expression levels in the adipose tissue of monkeys to investigate the regulation of gene expression of the leptin receptor.

RESEARCH METHODS AND PROCEDURES

Monkey leptin receptor cDNA was cloned by reverse transcriptase-polymerase chain reaction (RT-PCR). Tissue distribution of monkey leptin receptor was examined by Northern blot analysis and RT-PCR. The mRNA levels of monkey leptin receptor in adipose tissue of normal (n=10), hyperinsulinemic obese (n=8), and type 2 diabetic monkeys (n=8) were measured by quantitative RT-PCR.

RESULTS

Monkey leptin receptor cDNA had at least two alternatively spliced isoforms (long and short forms). The long form of the leptin receptor mRNA was expressed relatively highly in liver, adipose tissue, hypothalamus, and choroid plexus, whereas the total leptin receptors were expressed in every tissue examined. The mRNA levels of the long form of the leptin receptor in adipose tissue were not correlated to body weight, fasting plasma insulin, plasma glucose, or plasma leptin levels. The mRNA levels of the long form of the leptin receptor were highly correlated to that of the total leptin receptor (long and short form).

DISCUSSION

The long form of leptin receptor mRNA existed in adipose tissue as well as in liver and hypothalamus, suggesting that the leptin receptor in adipose tissue may be functional in adipose tissue. The expression of the leptin receptor mRNA in adipose tissue is not affected by obesity, hyperinsulinemia, or diabetes.

Insulin decreases skeletal muscle cAMP-dependent protein kinase (PKA) activity in normal monkeys and increases PKA activity in insulin-resistant rhesus monkeys

Insulin activation of skeletal muscle glycogen synthase and glucose disposal is defective in both prediabetic and diabetic primates. Reduction in the activation of glycogen synthase by insulin could be the cause of lower glucose disposal rates, and could be the result, at least in part, of the failure of insulin to inhibit cAMP-dependent protein kinase activity (protein kinase A, PKA). To examine this proposed mechanism, PKA activity was measured in skeletal muscle (vastus lateralis) samples freeze-clamped in situ under basal fasting conditions before, and again during a euglycemic hyperinsulinemic clamp in 27 rhesus monkeys. Nine of the monkeys were normal (normal fasting glucose and insulin), eight were prediabetic (normal fasting glucose and hyperinsulinemia) and ten had spontaneous non-insulin-dependent diabetes (hyperglycemia). Insulin lowered PKA activity ratio in normal monkeys (basal vs insulin-stimulated, 14.4 +/- 3.2 vs 8.1 +/- 1.8%, p < 0.05), but raised PKA activity ratio in prediabetic monkeys (5.4 +/- 1.4 vs 10.5 +/- 2.6%, p < 0.05). PKA activity ratio was unaffected by insulin in the diabetic monkeys (6.7 +/- 1.8 vs 7.5 +/- 1.4%). Basal PKA activity ratio was higher in normal monkeys compared to prediabetic (p < 0.05) and diabetic monkeys (p < 0.05). Basal PKA activity ratio was inversely related to the insulin-stimulated change in PKA activity ratio (r = -0.72, p < 0.001). We conclude that in vivo insulin during euglycemic hyperinsulinemic clamp decreases skeletal muscle PKA activity ratio in normal monkeys but fails to decrease the activity ratio of PKA in insulin resistant (prediabetic and diabetic) monkeys. The insulin resistant state is characterized by low basal fasting skeletal muscle PKA activity ratio.

Lack of defect in insulin action on hepatic glycogen synthase and phosphorylase in insulin-resistant monkeys

It is well known that an alteration in insulin activation of skeletal muscle glycogen synthase is associated with insulin resistance. To determine whether this defect in insulin action is specific to skeletal muscle, or also present in liver, simultaneous biopsies of these tissues were obtained before and during a euglycemic hyperinsulinemic clamp in spontaneously obese insulin-resistant male rhesus monkeys. The activities of glycogen synthase and glycogen phosphorylase and the concentrations of glucose 6-phosphate and glycogen were measured. There were no differences between basal and insulin-stimulated glycogen synthase and glycogen phosphorylase activities or in glucose 6-phosphate and glycogen contents in muscle. Insulin increased the activities of liver glycogen synthase (P < 0.05) and decreased the activities of liver glycogen phosphorylase (P 0.001). Insulin also caused a reduction in liver glucose 6-phosphate (P = 0.05). We conclude that insulin-resistant monkeys do not have a defect in insulin action on liver glycogen synthase, although a defect in insulin action on muscle glycogen synthase is present. Therefore, tissue-specific alterations in insulin action on glycogen synthase are present in the development of insulin resistance in rhesus monkeys.

Relationship of glycogen synthase and glycogen phosphorylase to protein phosphatase 2C and cAMP-dependent protein kinase in liver of obese rhesus monkeys

The regulation of glycogen synthase (GS) and glycogen phosphorylase (GP) activity by phosphorylation/ dephosphorylation has been proposed to be via changes in activities of several different protein (serine/threonine) phosphatases and kinases, including protein phosphatase (PP) 1/2A, PP2C, and cAMP-dependent protein kinase (PKA). In order to determine whether PP1/2A, PP2C, and/or PKA activities are related to GS and/or GP activities, these enzymes were measured in freeze-clamped liver biopsies obtained under basal fasting conditions from 16 obese monkeys. Four monkeys were normoglycemic and normoinsulinemic, five were hyperinsulinemic, and seven had type 2 diabetes (NIDDM). Liver glycogen and glucose 6-phosphate (G6P) contents were also determine. Basal enzyme activities and basal substrate concentrations were not significantly different between the three group of obese monkeys; however, there were several significant linear relationships observed when the monkeys were treated as one group. Therefore, multiple regression was used to determine the correlation between key variables. GS fractional activity was correlated to GP fractional activity (p < 0.05) and to PP2C activity (p = 0.005) (adjusted R2, 53%). GP independent activity was correlated to GS independent activity (p < 0.07) and to PKA fractional activity (p = 0.005) (adjusted R2, 64%). PP2C activity was correlated to GS fractional activity (p < 0.0005) and to PP1/2A activity (p < 0.0001) (adjusted R2, 83%). PKA fractional activity was correlated to GP total activity (p < 0.0005) and to age (p = 0.001) (adjusted R2, 82%). G6P content was correlated to glycogen content (p < 0.05) and to PP2C activity (p = 0.0005) (adjusted R2, 73%). In conclusion, PP2C and PKA are involved in the regulation of GS and GP activity in the basal state in liver of obese monkeys with a wide range of glucose tolerance.

Insulin regulates liver glycogen synthase and glycogen phosphorylase activity reciprocally in rhesus monkeys

In skeletal muscle of both humans and monkeys, the effects of in vivo insulin during a euglycemic hyperinsulinemic clamp on the enzymes and substrates of glycogen metabolism have been well established. In liver, such effects of insulin during a clamp have not been previously studied in primates. To examine insulin action at the liver, euglycemic hyperinsulinemic clamps were performed in 10 lean young adult male rhesus monkeys. Liver biopsies were obtained at three time points: basal (fasting), that is, immediately before the onset of the clamp, and during insulin infusion at 130 and 195 min. Glycogen synthase (GS), glycogen phosphorylase (GP), glucose 6-phosphate (G-6-P), and glycogen were determined at each time point, with the greatest effects observed most frequently at 195 min. Whole body insulin-mediated glucose disposal rate was related to the change in the independent activity of GS (r = 0.63, P < 0.05). Insulin increased the GS fractional activity (P < 0.005) and decreased the activity ratio of GP (P < 0.001) compared with basal. The changes in fractional activity of GS and in activity ratio of GP were inversely related (r = - 0.68, P < 0.05), G-6-P concentration was decreased during insulin stimulation compared with basal (P = 0.01). Glycogen concentration was not significantly different between the basal and insulin-stimulated time points. We conclude that insulin during a euglycemic clamp activates liver GS while inhibiting liver GP and that insulin action on liver GS is positively related to whole body insulin-mediated glucose disposal rates in lean young adult rhesus monkeys.

Hyperleptinemia: relationship to adiposity and insulin resistance in the spontaneously obese rhesus monkey

Plasma leptin levels in normal-weight and spontaneously obese male rhesus monkeys, and the relationships of circulating leptin to beta-cell basal secretion, glucose-stimulated responsiveness and peripheral insulin sensitivity, were determined. Basal leptin in normal lean adult monkeys averaged 6.0 +/- 1.3 ng/ml and in the obese monkeys averaged 22.6 +/- 2.9 ng/ml. In all monkeys, plasma leptin concentration was significantly related to body weight, body fat, fasting plasma insulin, acute insulin response to intravenous glucose, and peripheral insulin sensitivity but not to fasting glucose or glucose tolerance. Body fat and plasma insulin concentration were the best predictors of circulating leptin levels (R2 = 62.6%) independent of peripheral insulin sensitivity. Four of 17 obese monkeys had plasma leptin concentrations in the normal range, a finding that may be related to the heterogeneity of obesity. The close association of plasma leptin to body fat and plasma insulin (both basal and glucose-stimulated) support the possibility of a role of leptin in the link between obesity and beta-cell hypersecretion. However, the potential role of leptin in the development of peripheral insulin resistance, hyperglycemia and type 2 diabetes will require further study.

Dietary myoinositol results in lower urine glucose and in lower postprandial plasma glucose in obese insulin resistant rhesus monkeys

In a previous study, D-chiroinositol added to a meal (0.5 g/kg) resulted in significantly lower postprandial plasma glucose concentrations without an increase in insulin concentrations in obese insulin-resistant monkeys. The present report describes the effects of another isomer of inositol, myoinositol, on postprandial plasma glucose and insulin concentrations and on urine glucose concentrations in 6 similarly insulin-resistant monkeys. The three 5 day study periods included a control period (liquid diet ad libitum) and 2 experimental periods (liquid diet ad libitum with either 1.5 g/kg/day myoinositol or D-chiroinositol added). Twenty-four hour urine samples were collected during each 5 day period. On the sixth day of each period the monkeys were anesthetized 110 min after completing either the control meal (15 ml/kg) or the experimental meals (1.5 g/kg myoinositol or D-chiroinositol) and plasma samples were obtained at 120, 150, 180, 210, 240, 270 and 300 min. The plasma glucose concentration was lower after the meal with myoinositol compared to the control meal at 120, 150 and 180 min (p's < 0.05). The plasma insulin concentration was lower after the meal with myoinositol compared to the control meal at 150 and 180 min (p's < 0.05). In addition, 24 hour urine glucose concentrations were lower during the myoinositol diet compared to the control diet (p < 0.001). The plasma glucose concentration was lower after the meal with D-chiroinositol compared to the control meal at 150, 240, 270 and 300 min (p's < or = 0.05). In obese insulin-resistant monkeys, myoinositol added to the diet lowers urine glucose concentrations and both myoinositol and D-chiroinositol added to a meal lower postprandial plasma glucose concentrations without increasing postprandial insulin concentrations. Therefore, myoinositol, like D-chiroinositol, may be a useful agent for reducing meal-induced hyperglycemia without inducing hyperinsulinemia in insulin-resistant subjects.

Regulation of obese (ob) mRNA and plasma leptin levels in rhesus monkeys. Effects of insulin, body weight, and non-insulin-dependent diabetes mellitus

We have cloned the rhesus monkey obese cDNA and have analyzed its expression in monkeys with a wide range of body weights (lean to very obese) and with or without non-insulin-dependent diabetes mellitus to examine the relationship of ob gene expression to obesity and non-insulin-dependent diabetes mellitus. The sequence of monkey ob protein, excluding the signal peptide, showed 91% identity with the human protein. We observed a significant correlation between the level of ob mRNA and body weight. We also found a significant relationship between ob mRNA and fasting plasma insulin concentration; however, insulin stimulation during a 100-140-min euglycemic/hyperinsulinemic clamp did not result in any changes in ob mRNA levels. Circulating levels of the ob gene product leptin were also significantly correlated with body weight. These results show that ob gene expression is related to body weight and is not acutely regulated by insulin.

Altered insulin receptor messenger ribonucleic acid splicing in liver is associated with deterioration of glucose tolerance in the spontaneously obese and diabetic rhesus monkey: analysis of controversy between monkey and human studies

There are two insulin receptor (IR) isoforms (designated type A and type B), derived from alternative splicing of exon 11 of the IR gene. Recently, we reported (Huang Z., Bodkin N.L., Ortmeyer H.K., Hansen B.C., Shuldiner A. R., 1994, J Clin Invest, 94:1289-1296) that an increase in the exon 11- (i.e. lacking exon 11) (type A) IR messenger RNA (mRNA) variant in muscle is associated with hyperinsulinemia, an early risk factor for noninsulin-dependent diabetes mellitus (NIDDM), in the spontaneously obese, diabetic rhesus monkey. To explore further the role of IR mRNA splicing in insulin resistance of NIDDM, we studied liver, another target organ that is resistant to insulin action in NIDDM. The relative amounts of the two IR mRNA-splicing variants in liver were quantitated by RT-PCR in normal, prediabetic, and diabetic (NIDDM) monkeys. The percentage of the exon 11- mRNA variant in liver (n = 24) was significantly correlated with fasting plasma glucose (r = 0.55, P < 0.01) and intravenous glucose disappearance rate (r = -0.45, P < 0.05). The exon 11- mRNA variant was increased significantly from 29.8 +/- 1.6% in monkeys with normal fasting glucose to 39.2 +/- 2.9% in monkeys with elevated fasting glucose (P < 0.01). These studies provide the first direct evidence in vivo that the relative expression of the two IR mRNA-splicing variants is altered in liver and suggest that increased expression of the exon 11- IR isoform may contribute to hepatic insulin resistance and NIDDM or may compensate for some yet unidentified defect.

Glycogen phosphorylase activity and glycogen concentration in muscle of normal to overtly diabetic rhesus monkeys

The effect of insulin to increase the activity of glycogen synthase (GS) in muscle has been well documented, however, the effect of in vivo insulin to inactivate glycogen phosphorylase (GP) has not been previously shown. To determine the effects of insulin on glycogenolysis in rhesus monkeys, GP and glycogen were determined in muscle samples obtained under basal fasting and insulin-stimulated conditions during a euglycemic hyperinsulinemic clamp in a group of 27 monkeys ranging from normal to overtly diabetic (NIDDM) and compared to GS activity previously examined. The diabetic monkeys had lower basal and insulin-stimulated glycogen concentrations compared to the normal and hyperinsulinemic monkeys (p < 0.05). The response of GP activity ratio (AR) to insulin (delta) was inversely correlated to delta GS fractional velocity (fv) (r = -0.57, p < 0.002) in all of the monkeys. The AR of GP was inversely correlated to the fv of GS measured under insulin-stimulated conditions (r = -0.60, p < 0.05) in the 11 normal monkeys. In the normal group, the range in response of GS to insulin (delta GSfv) was previously shown to be 3-22%, with n = 6 < 11% ('low normals') and n = 5 > 11% ('high normals'). In the present study, the low normals were shown to have (1) higher delta GP independent activity and delta GP total activity compared to the high normals and hyperinsulinemic monkeys (p less than or equal to 0.05), (2) higher insulin-stimulated GP independent activity and GP total activity compared to the other three groups (p < 0.05), (3) higher insulin-stimulated GP activity ratio compared to the high normals and hyperinsulinemic monkeys (p < 0.05), (4) and lower whole-body insulin-mediated glucose disposal rates compared to the high normals (p < 0.05). We conclude that NIDDM is accompanied by low glycogen content in the muscle, and that some clinically normal monkeys have an alteration in insulin action on muscle GS, GP, and whole-body glucose disposal rates that may precede the development of hyperinsulinemia.

Effects of D-chiroinositol added to a meal on plasma glucose and insulin in hyperinsulinemic rhesus monkeys

We have previously demonstrated that D-chiroinositol, administered intravenously to insulin-resistant monkeys, increases the rate of disappearance of plasma glucose and insulin. The purpose of the present study was to determine whether orally administered D-chiroinositol might also similarly improve the postprandial plasma glucose profile of hyperinsulinemic insulin-resistant monkeys. A complete liquid diet meal (15 ml/kg body weight) was ingested by each of six monkeys on two occasions separated by 10 days, with conditions identical except D-chiroinositol (500 mg/kg body weight) was added to the second meal. At 110 minutes following each meal, the monkeys were anesthetized and blood samples obtained at 120, 150, 180, 210, 240, 270 and 300 minutes. Plasma glucose and insulin concentrations were determined. The mean plasma glucose concentration (120-300 minutes) was significantly lower after the meal containing D-chiroinositol compared to the control meal (7.1 +/- 1.2 vs. 7.8 +/- 1.2 mM) (p < 0.05). Plasma insulin concentrations tended to be lower after the meal containing D-chiroinositol compared to the control meal (3930 +/- 1068 vs. 4518 +/- 1200 pM) (p < 0.15, ns). We conclude that in hyperinsulinemic monkeys, D-chiroinositol added to a meal lowers postprandial plasma glucose without an increase in plasma insulin, and therefore may be a useful agent for reducing meal-induced hyperglycemia without inducing hyperinsulinemia.