A possible link between skeletal muscle mitochondrial efficiency and age-induced insulin resistance

Organotypic cultures as aging associated disease models

Aging remains a primary risk factor for a host of diseases, including leading causes of death. Aging and associated diseases are inherently multifactorial, with numerous contributing factors and phenotypes at the molecular, cellular, tissue, and organismal scales. Despite the complexity of aging phenomena, models currently used in aging research possess limitations. Frequently used in vivo models often have important physiological differences, age at different rates, or are genetically engineered to match late disease phenotypes rather than early causes. Conversely, routinely used in vitro models lack the complex tissue-scale and systemic cues that are disrupted in aging. To fill in gaps between in vivo and traditional in vitro models, researchers have increasingly been turning to organotypic models, which provide increased physiological relevance with the accessibility and control of in vitro context. While powerful tools, the development of these models is a field of its own, and many aging researchers may be unaware of recent progress in organotypic models, or hesitant to include these models in their own work. In this review, we describe recent progress in tissue engineering applied to organotypic models, highlighting examples explicitly linked to aging and associated disease, as well as examples of models that are relevant to aging. We specifically highlight progress made in skin, gut, and skeletal muscle, and describe how recently demonstrated models have been used for aging studies or similar phenotypes. Throughout, this review emphasizes the accessibility of these models and aims to provide a resource for researchers seeking to leverage these powerful tools.

Skeletal muscle mitochondrial dysfunction and muscle and whole body functional deficits in cancer patients with weight loss

Reductions in skeletal muscle mass and function are often reported in patients with cancer-associated weight loss and are associated with reduced quality of life, impaired treatment tolerance, and increased mortality. Although cellular changes, including altered mitochondrial function, have been reported in animals, such changes have been incompletely characterized in humans with cancer. Whole body and skeletal muscle physical function, skeletal muscle mitochondrial function, and whole body protein turnover were assessed in eight patients with cancer-associated weight loss (10.1 ± 4.2% body weight over 6-12 mo) and 19 age-, sex-, and body mass index (BMI)-matched healthy controls to characterize skeletal muscle changes at the whole body, muscle, and cellular level. Potential pathways involved in cancer-induced alterations in metabolism and mitochondrial function were explored by interrogating skeletal muscle and plasma metabolomes. Despite similar lean mass compared with control participants, patients with cancer exhibited reduced habitual physical activity (57% fewer daily steps), cardiorespiratory fitness [22% lower V̇o_2peak (mL/kg/min)] and leg strength (35% lower isokinetic knee extensor strength), and greater leg neuromuscular fatigue (36% greater decline in knee extensor torque). Concomitant with these functional declines, patients with cancer had lower mitochondrial oxidative capacity [25% lower State 3 O₂ flux (pmol/s/mg tissue)] and ATP production [23% lower State 3 ATP production (pmol/s/mg tissue)] and alterations in phospholipid metabolite profiles indicative of mitochondrial abnormalities. Whole body protein turnover was unchanged. These findings demonstrate mitochondrial abnormalities concomitant with whole body and skeletal muscle functional derangements associated with human cancer, supporting future work studying the role of mitochondria in the muscle deficits associated with cancer.NEW & NOTEWORTHY To our knowledge, this is the first study to suggest that skeletal muscle mitochondrial deficits are associated with cancer-associated weight loss in humans. Mitochondrial deficits were concurrent with reductions in whole body and skeletal muscle functional capacity. Whether mitochondrial deficits are causal or secondary to cancer-associated weight loss and functional deficits remains to be determined, but this study supports further exploration of mitochondria as a driver of cancer-associated losses in muscle mass and function.

Codeine exerts cardiorenal injury via upregulation of adenine deaminase/xanthine oxidase and caspase 3 signaling

AIMS

Codeine treatment has been shown to be associated with glucolipid deregulation, though data reporting this are inconsistent and the mechanisms are not well understood. Perturbation of glutathione-dependent antioxidant defense and adenosine deaminase (ADA)/xanthine oxidase (XO) signaling has been implicated in the pathogenesis of cardiometabolic disorders. We thus, hypothesized that depletion of glutathione contents and upregulation of ADA/XO are involved in codeine-induced glucolipid deregulation. The present study also investigated whether or not codeine administration would induce genotoxicity and apoptosis in cardiac and renal tissues.

MATERIALS AND METHODS

Male New Zealand rabbits received per os distilled water or codeine, either in low dose (4 mg/kg) or high dose (10 mg/kg) for 6 weeks.

KEY FINDINGS

Codeine treatment led to reduced absolute and relative cardiac and renal mass independent of body weight change, increased blood glucose, total cholesterol (TC), triglycerides (TG), and low-density lipoprotein (LDL-C), as well as increased atherogenic indices and triglyceride-glucose index (TyG). Codeine administration significantly increased markers of cardiac and renal injury, as well as impaired cardiorenal functions. Codeine treatment also resulted in increased cardiac and renal malondialdehyde, Advanced Glycation Endproducts (AGE) and 8-hydroxydeoxyguanosine (8-OH-dG), and myeloperoxidase (MPO), ADA, XO, and caspase 3 activities. These observations were accompanied by impaired activities of cardiac and renal proton pumps.

SIGNIFICANCE

Findings of this study demonstrate that upregulation of ADA/XO and caspase 3 signaling are, at least partly, contributory to the glucolipid deregulation and cardiorenal injury induced by codeine.

Suppression of uric acid and lactate production by sodium acetate ameliorates hepatic triglyceride accumulation in fructose-insulin resistant pregnant rats

High fructose intake has been associated with perturbed lipid, uric acid and lactate homeostasis. However, consumption of fructose-sweetened beverages is not usually regulated during pregnancy. The effect of short-chain fatty acid (acetate) on the metabolic effects of high fructose intake during pregnancy is not known. We hypothesized that acetate prevents gestational fructose-induced hepatic triglyceride (TG) accumulation by suppressing uric acid and lactate production. Pregnant Wistar rats were randomly separated into three groups (n = 6/group) receiving drinking water (CON), 10 % (w/v) fructose drink (FRU) and 10 % (w/v) fructose with 200 mg/kg (w/w; p.o.) sodium acetate (FRU + ACE) daily for nineteen days. Fructose intake resulted in increased body weight gain, liver weight, fluid intake, visceral fat, insulin resistance, fasting blood glucose, insulin, plasma and hepatic TG, total cholesterol, free fatty acid, lipid peroxidation, adenosine deaminase, xanthine oxidase, uric acid, lactate, lactate dehydrogenase, and liver injury marker enzymes. However, gestational high fructose intake led to depressed plasma and hepatic glucose-6-phosphate dehydrogenase (G6PD)-dependent antioxidant barrier, adenosine and food intake. All these effects except water intake and food intake were abated by sodium acetate. These results demonstrate that maternal fructose-enriched drink would cause hepatic TG accumulation that is associated with perturbed glucose, uric acid, lactate homeostasis, and G6PD-dependent antioxidant barrier. These results also demonstrate that acetate protects the liver against gestational fructose-induced TG accumulation by inhibiting uric acid and lactate production. Thus, acetate may be useful in the treatment of hyperuricemia- and hyperlactatemia-related disorders.

l-glutamine supplementation exerts cardio-renal protection in estrogen-progestin oral contraceptive-treated female rats

Glycogen and lipid disruptions represent a spectrum of metabolic disorders that are crucial risk factors for cardiovascular disease in estrogen-progestin oral contraceptive (COC) users. l-glutamine (GLN) has been shown to exert a modulatory effect in metabolic disorders-related syndromes. We therefore hypothesized that GLN supplementation would protect against myocardial and renal glycogen-lipid mishandling in COC-treated animals by modulation of Glucose-6-phosphate dehydrogenase (G6PD) and xanthine oxidase (XO) activities. Adult female Wistar rats were randomly allotted into control, GLN, COC and COC + GLN groups (six rats per group). The groups received vehicle (distilled water, p.o.), GLN (1 g/kg), COC containing 1.0 μg ethinylestradiol plus 5.0 μg levonorgestrel and COC plus GLN respectively, daily for 8 weeks. Data showed that treatment with COC led to metabolically-induced obesity with correspondent increased visceral and epicardial fat mass. It also led to increased plasma, myocardial and renal triglyceride, free fatty acid, malondialdehyde (MDA), XO activity, uric acid content and decreased glutathione content and G6PD activity. In addition, COC increased myocardial but not renal glycogen content, and increased myocardial and renal glycogen synthase activity, increased plasma and renal lactate production and plasma aspartate transaminase/alanine aminotransferase (AST/ALT) ratio. However, these alterations were attenuated when supplemented with GLN except plasma AST/ALT ratio. Collectively, the present results indicate that estrogen-progestin oral contraceptive causes metabolically-induced obesity that is accompanied by differential myocardial and renal metabolic disturbances. The findings also suggest that irrespective of varying metabolic phenotypes, GLN exerts protection against cardio-renal dysmetabolism by modulation of XO and G6PD activities.

Oral ethinylestradiol–levonorgestrel normalizes fructose-induced hepatic lipid accumulation and glycogen depletion in female rats

The present study investigated the effects of oral ethinylestradiol–levonorgestrel (EEL) on hepatic lipid and glycogen contents during high fructose (HF) intake, and determined whether pyruvate dehydrogenase kinase-4 (PDK-4) and glucose-6-phosphate dehydrogenase (G6PD) activity were involved in HF and (or) EEL-induced hepatic dysmetabolism. Female Wistar rats weighing 140–160 g were divided into groups. The control, EEL, HF, and EEL+HF groups received water (vehicle, p.o.), 1.0 μg ethinylestradiol plus 5.0 μg levonorgestrel (p.o.), fructose (10% w/v), and EEL plus HF, respectively, on a daily basis for 8 weeks. Results revealed that treatment with EEL or HF led to insulin resistance, hyperinsulinemia, increased hepatic uric acid production and triglyceride content, reduced glycogen content, and reduced production of plasma or hepatic glutathione- and G6PD-dependent antioxidants. HF but not EEL also increased fasting glucose and hepatic PDK-4. Nonetheless, these alterations were attenuated by EEL in HF-treated rats. Our results demonstrate that hepatic lipid accumulation and glycogen depletion induced by HF is accompanied by increased PDK-4 and defective G6PD activity. The findings also suggest that EEL would attenuate hepatic lipid accumulation and glycogen depletion by suppression of PDK-4 and enhancement of a G6PD-dependent antioxidant barrier.

Dipeptidyl peptidase-4 inhibition protects the liver of insulin-resistant female rats against triglyceride accumulation by suppressing uric acid

Dipeptidyl peptidase-4 (DPP-4) inhibition has been shown to exert beneficial effects against insulin resistance (IR) and type 2 diabetes. Combined oral contraceptive (COC) treatment is associated with impaired glucose and lipid metabolism but the mechanisms are elusive. We therefore, hypothesized that DPP-4 inhibition ameliorates COC-induced glucose dysregulation and hepatic triglyceride (TG) accumulation through adenosine deaminase (ADA) /xanthine oxidase (XO) /uric acid-dependent pathway. Female Wistar rats received (po) vehicle and COC (1.0 μg ethinylestradiol plus 5.0 μg levonorgestrel; po) with or without DPP-4 inhibitor (sitagliptin; 100 mg/kg; po) for 8 weeks (n = 6/group). Glucose dysmetabolism was assessed by elevated fasting blood glucose, impaired oral glucose tolerance test and homeostatic model assessment of IR. Treatment with COC led to increased plasma fasting glucose, triglyceride-glucose index, 1-h postload glucose response, insulin, free fatty acid, IR and impaired glucose tolerance. COC treatment also resulted in increased plasma and hepatic TG, TG/HDL-cholesterol ratio, malondialdehyde, uric acid (plasma; 25.2 ± 0.6 mg/dl; hepatic 128.9 ± 8.0 mg/100 mg tissue), lactate dehydrogenase, DPP-4, ADA and XO (plasma;10.5 ± 1.1 U/L; hepatic 21.2 ± 1.4 U/g protein) activities. Likewise, COC led to reduction in nitric oxide level. However, DPP-4 inhibition significantly ameliorated these alterations induced by COC treatment through suppression of uric acid (plasma; 15.1 ± 1.0 mg/dl, hepatic; 75.6 ± 5.0 mg/100 mg tissue), XO (plasma; 4.1 ± 0.9 U/L, hepatic; 8.7 ± 0.4 U/g protein), ADA and DPP-4 activities suggesting their involvement in glucose dysregulation and hepatic TG accumulation induced by COC treatment. Therefore, DPP-4 inhibition would impact positively on cardiometabolic disorders, at least in part, through XO, ADA and uric acid suppression.

Oral ethinylestradiol-levonorgestrel attenuates cardiac glycogen and triglyceride accumulation in high fructose female rats by suppressing pyruvate dehydrogenase kinase-4

Fructose (FRU) intake has increased dramatically in recent decades with a corresponding increased incidence of insulin resistance (IR), particularly in young adults. The use of oral ethinylestradiol-levonorgestrel (EEL) formulation is also common among young women worldwide. The present study aimed at determining the effect of EEL on high fructose-induced cardiac triglyceride (TG) and glycogen accumulation. The study also investigated the possible involvement of pyruvate dehydrogenase kinase-4 (PDK-4) in EEL and/or high fructose metabolic effects on the heart. Ten-week-old female Wistar rats were allotted into four groups. The control, EEL, FRU, and EEL + FRU rats received distilled water (vehicle, p.o.), 1.0 μg ethinylestradiol plus 5.0 μg levonorgestrel (p.o.), 10% fructose (w/v), and 1.0 μg ethinylestradiol plus 5.0 μg levonorgestrel and 10% fructose, respectively, daily for 8 weeks. Data showed that EEL or high fructose caused IR' impaired glucose tolerance' hyperlipidemia' increased plasma lactate, lactate dehydrogenase, PDK-4, uric acid, xanthine oxidase (XO), adenosine deaminase (ADA), malondialdehyde (MDA), cardiac uric acid, TG, TG/HDL- cholesterol, glycogen synthesis, MDA, and visceral fat content and reduced glutathione. High fructose also resulted in impaired pancreatic β-cell function, hyperglycemia, and increased cardiac PDK-4, lactate synthesis, and mass. Nonetheless, these alterations were ameliorated in EEL plus high fructose rats. This study demonstrates that high fructose-induced myocardial TG and glycogen accumulation is attributable to increased PDK-4. Besides, EEL could be a useful pharmacological utility for protection against cardiac dysmetabolism by inhibiting PDK-4.

The saturation degree of fatty acids and their derived acylcarnitines determines the direct effect of metabolically active thyroid hormones on insulin sensitivity in skeletal muscle cells

Using differentiated rat L6 cells, we studied the direct effect of 3,5,3'-triiodo-l-thyronine (T3) and 3,5-diiodo-l-thyronine (T2) on the response to insulin in presence of fatty acids with a varying degree of saturation. We found that T3 and T2 both invert the response to insulin by modulating Akt Ser473 phosphorylation in the presence of palmitate and oleate. Both hormones prevented palmitate-induced insulin resistance, whereas increased insulin sensitivity in the presence of oleate was reduced, with normalization to (or, in the case of T3, even below) control levels. Both hormones effectively reduced intracellular acylcarnitine concentrations. Interestingly, insulin sensitization was lowered by incubation of the myotubes with relevant concentrations of palmitoylcarnitines (C16) and increased by oleylcarnitines and linoleylcarnitines (C18:1 and C18:2, respectively). The efficiency of mitochondrial respiration decreased in the order palmitate-oleate-linoleate; in the presence of palmitate, only T3 increased ATP synthesis-independent cellular respiration and mitochondrial respiratory complex activities. Both hormones modulated gene expression and enzyme activities related to insulin sensitivity, glucose metabolism, and lipid handling. Although T2 and T3 differentially regulated the expression of relevant genes involved in glucose metabolism, they equally stimulated related metabolic activities. T2 and T3 differentially modulated mitochondrial fatty acid uptake and oxidation in the presence of each fatty acid. The results show that T2 and T3 both invert the fatty acid-induced response to insulin but through different mechanisms, and that the outcome depends on the degree of saturation of the fatty acids and their derived acylcarnitines.-Giacco, A., delli Paoli, G., Senese, R., Cioffi, F., Silvestri, E., Moreno, M., Ruoppolo, M., Caterino, M., Costanzo, M., Lombardi, A., Goglia, F., Lanni, A., de Lange, P. The saturation degree of fatty acids and their derived acylcarnitines determines the direct effect of metabolically active thyroid hormones on insulin sensitivity in skeletal muscle cells.

Plasma Amino Acids Stimulate Uncoupled Respiration of Muscle Subsarcolemmal Mitochondria in Lean but Not Obese Humans

CONTEXT

Obesity is associated with mitochondrial dysfunction in skeletal muscle. Increasing the plasma amino acid (AA) concentrations stimulates mitochondrial adenosine triphosphate (ATP) production in lean individuals.

OBJECTIVE

To determine whether acute elevation in plasma AAs enhances muscle mitochondrial respiration and ATP production in subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria in obese adults.

DESIGN

Assessment of SS and IMF mitochondrial function during saline (i.e., control) and AA infusions.

PARTICIPANTS

Eligible participants were healthy lean (body mass index, <25 kg/m2; age, 37 ± 3 years; n = 10) and obese (body mass index >30 kg/m2; age 35 ± 3 years; n = 11) subjects.

INTERVENTION

Single trial of saline infusion followed by AA infusion. SS and IMF mitochondria were isolated from muscle biopsies collected at the end of the saline and AA infusions.

MAIN OUTCOMES

Mitochondrial respiration and ATP production.

RESULTS

AA infusion increased adenosine 5'-diphosphate (ADP)-stimulated respiration and ATP production rates of SS mitochondria in the lean (P < 0.05), but not obese, subjects. Furthermore, AA infusion increased the uncoupled (i.e., non-ADP-stimulated) respiration of SS mitochondria in the lean subjects only (P < 0.05). AA infusion had no effect on any of these parameters in IMF mitochondria in either lean or obese subjects (P > 0.05).

CONCLUSIONS

Increasing the plasma AA concentrations enhances the capacity for respiration and ATP production of muscle SS, but not IMF, mitochondria in lean individuals, in parallel with increases in uncoupled respiration. However, neither of these parameters increases in muscle SS or IMF mitochondria in obese individuals.

Muscle Mass Index and Animal Source of Dietary Protein Are Positively Associated with Insulin Resistance in Participants of the NuAge Study

OBJECTIVES

Contribute evidence towards the complex interrelationships of body composition, insulin sensitivity and protein intake independently from adiposity in an older population.

DESIGN

This is a cross-sectional analysis of an existing dataset in which a literature-supported model linking together the variables of interest is tested using path analysis.

SETTING

The loss of muscle mass has been implicated in the development of insulin resistance. We propose to test associations of muscle mass with insulin sensitivity and their respective associations with animal and vegetable sources of protein intake, independently from adiposity.

PARTICIPANTS

Non-diabetic participants aged 68-82 years from the NuAge study with all available measures (n=441) were included.

MEASUREMENTS

A model considering age, sex, chronic diseases, physical activity; smoking and sources of protein intake influencing body composition components and insulin sensitivity was created and tested with Path Analysis for their independent associations. Muscle mass index (MMI; kg/height in m2) and % body fat were derived from DXA and BIA. Insulin resistance was estimated by the Homeostatic Model Assessment for Insulin Resistance (HOMA-IR) score and physical activity by the Physical Activity Scale for the Elderly (PASE) questionnaire. Protein intakes were obtained from three non-consecutive 24h-diet recalls.

RESULTS

In the final model, direct positive associations were observed between HOMA-IR score and MMI (ß=0.42; 95%CI: 0.24; 0.6) and % body fat (ß=0.094; 95%CI: 0.07; 0.11). There were no direct associations between animal protein intake and MMI or with HOMA-IR. There was a significant direct negative association between plant protein intake and MMI (ß= -0.068; 95%CI: -0.13; -0.003) and significant indirect associations mediated through MMI and % body fat between HOMA-IR and animal protein intake (ß=0.0321; 95%CI: 0.01; 0.05), as well as plant protein intake (ß= -0.07; 95%CI: -0.1; 0.0).

CONCLUSIONS

Our final model indicated that MMI and HOMA score were significantly positively associated. Protein intake sources were related to HOMA-IR score differently through MMI and % body fat, respectively.

Higher blood hematocrit predicts hyperuricemia: a prospective study of 62,897 person-years of follow-up

This prospective study aimed to investigate the relationship between higher hematocrit (Hct) level and hyperuricemia (HU) incidence. A total of 27,540 subjects were included. Baseline Hct was classified into four categories based on the quartile distribution of the study population. A cox proportional hazards regression was used to evaluate the risk of HU incidence across the Hct quartiles after adjusting a number of potential confounding factors. Out of the 62,897 person-years of follow-up, 2745 new cases of HU were developed. In models adjusted for known risk factors of HU, higher Hct was used to predict HU incidence independently in a graded manner (p = 0.02): compared with subjects in the lowest quartile, subjects in the highest quartile of Hct (hazard ratio = 1.20; 95% confidence interval: 1.03-1.41) were n20% more likely to develop HU. Sensitivity analysis indicated that the hazard ratios increased with the extension of the minimum follow-up interval. When the minimum follow-up interval was restricted to 4 years, subjects in the highest quartile of Hct were 70% more likely to develop HU, compared with the lowest quartile. Higher Hct, a routinely measured inexpensive biomarker was independently associated with the incidence of HU even within the normal range.

Skeletal muscle mitochondrial H2 O2 emission increases with immobilization and decreases after aerobic training in young and older men

Currently, it is not known whether impaired mitochondrial function contributes to human ageing or whether potential impairments in mitochondrial function with age are secondary to physical inactivity. The present study investigated mitochondrial respiratory function and reactive oxygen species emission at a predefined membrane potential in young and older men subjected to 2 weeks of one-leg immobilization followed by 6 weeks of aerobic cycle training. Immobilization increased reactive oxygen species emission and decreased ATP generating respiration. Subsequent aerobic training reversed these effects. By contrast, age had no effect on the measured variables. The results of the present study support the notion that increased mitochondrial reactive oxygen species production mediates the detrimental effects seen after physical inactivity and that ageing per se does not cause mitochondrial dysfunction. Mitochondrial dysfunction, defined as increased oxidative stress and lower capacity for energy production, may be seen with ageing and may cause frailty, or it could be that it is secondary to physical inactivity. We studied the effect of 2 weeks of one-leg immobilization followed by 6 weeks of supervised cycle training on mitochondrial function in 17 young (mean ± SEM: 23 ± 1 years) and 15 older (68 ± 1 years) healthy men. Submaximal H2 O2 emission and respiration were measured simultaneously at a predefined membrane potential in isolated mitochondria from skeletal muscle using two protocols: pyruvate + malate (PM) and succinate + rotenone (SR). This allowed measurement of leak and ATP generating respiration from which the coupling efficiency can be calculated. The protein content of the anti-oxidants manganese superoxide dismuthase (MnSOD), CuZn superoxide dismuthase, catalase and gluthathione peroxidase 1 was measured by western blotting. Immobilization decreased ATP generating respiration using PM and increased H2 O2 emission using both PM and SR similarly in young and older men. Both were restored to baseline after the training period. Furthermore, MnSOD and catalase content increased with endurance training. The young men had a higher leak respiration at inclusion using PM and a higher membrane potential in State 3 using both substrate combinations. Collectively, the findings of the present study support the notion that increased mitochondrial reactive oxygen species mediates the detrimental effects seen after physical inactivity. Age, on the other hand, was not associated with impairments in anti-oxidant protein levels, mitochondrial respiration or H2 O2 emission using either protocol.

Skeletal muscle mitochondrial energetic efficiency and aging

Aging is associated with a progressive loss of maximal cell functionality, and mitochondria are considered a key factor in aging process, since they determine the ATP availability in the cells. Mitochondrial performance during aging in skeletal muscle is reported to be either decreased or unchanged. This heterogeneity of results could partly be due to the method used to assess mitochondrial performance. In addition, in skeletal muscle the mitochondrial population is heterogeneous, composed of subsarcolemmal and intermyofibrillar mitochondria. Therefore, the purpose of the present review is to summarize the results obtained on the functionality of the above mitochondrial populations during aging, taking into account that the mitochondrial performance depends on organelle number, organelle activity, and energetic efficiency of the mitochondrial machinery in synthesizing ATP from the oxidation of fuels.

Mitochondrial efficiency and insulin resistance

Insulin resistance, “a relative impairment in the ability of insulin to exert its effects on glucose, protein and lipid metabolism in target tissues,” has many detrimental effects on metabolism and is strongly correlated to deposition of lipids in non-adipose tissues. Mitochondria are the main cellular sites devoted to ATP production and fatty acid oxidation. Therefore, a role for mitochondrial dysfunction in the onset of skeletal muscle insulin resistance has been proposed and many studies have dealt with possible alteration in mitochondrial function in obesity and diabetes, both in humans and animal models. Data reporting evidence of mitochondrial dysfunction in type two diabetes mellitus are numerous, even though the issue that this reduced mitochondrial function is causal in the development of the disease is not yet solved, also because a variety of parameters have been used in the studies carried out on this subject. By assessing the alterations in mitochondrial efficiency as well as the impact of this parameter on metabolic homeostasis of skeletal muscle cells, we have obtained results that allow us to suggest that an increase in mitochondrial efficiency precedes and therefore can contribute to the development of high-fat-induced insulin resistance in skeletal muscle.

Subsarcolemmal and intermyofibrillar mitochondrial responses to short-term high-fat feeding in rat skeletal muscle

OBJECTIVES

We assessed the alterations in mitochondrial function in skeletal muscle that were elicited by short-term high-fat feeding in sedentary rats.

METHODS

Two groups of rats were pair-fed for 1 wk and received a low-fat or high-fat diet. Body composition, energy balance, and glucose homeostasis were measured. Mitochondrial mass, oxidative capacity, and energetic efficiency as well as parameters of oxidative stress and antioxidant defense were evaluated in subsarcolemmal and intermyofibrillar mitochondria from the skeletal muscle.

RESULTS

Body energy, lipid content, and metabolic efficiency were significantly higher and energy expenditure was significantly decreased among rats that were fed a high-fat diet, as compared with controls. Skeletal muscle mitochondrial energetic efficiency, oxidative capacity for lipid substrates, and antioxidant defense were significantly increased in rats that were fed a high-fat diet as compared with controls.

CONCLUSIONS

Acute isocaloric high-fat feeding is able to induce increased phosphorylation efficiency in skeletal muscle subsarcolemmal and intermyofibrillar mitochondria. This modification implies a reduced oxidation of energy substrates that may contribute to the early onset of obesity.

Alterations in proton leak, oxidative status and uncoupling protein 3 content in skeletal muscle subsarcolemmal and intermyofibrillar mitochondria in old rats

BACKGROUND

We considered of interest to evaluate how aging affects mitochondrial function in skeletal muscle.

METHODS

We measured mitochondrial oxidative capacity and proton leak, together with lipid oxidative damage, superoxide dismutase specific activity and uncoupling protein 3 content, in subsarcolemmal and intermyofibrillar mitochondria from adult (six months) and old (two years) rats. Body composition, resting metabolic rate and plasma non esterified fatty acid levels were also assessed.

RESULTS

Old rats displayed significantly higher body energy and lipids, while body proteins were significantly lower, compared to adult rats. In addition, plasma non esterified fatty acid levels were significantly higher, while resting metabolic rates were found to be significantly lower, in old rats compared to adult ones. Significantly lower oxidative capacities in whole tissue homogenates and in intermyofibrillar and subsarcolemmal mitochondria were found in old rats compared to adult ones. Subsarcolemmal and intermyofibrillar mitochondria from old rats exhibited a significantly lower proton leak rate, while oxidative damage was found to be significantly higher only in subsarcolemmal mitochondria. Mitochondrial superoxide dismutase specific activity was not significantly affected in old rats, while significantly higher content of uncoupling protein 3 was found in both mitochondrial populations from old rats compared to adult ones, although the magnitude of the increase was lower in subsarcolemmal than in intermyofibrillar mitochondria.

CONCLUSIONS

The decrease in oxidative capacity and proton leak in intermyofibrillar and subsarcolemmal mitochondria could induce a decline in energy expenditure and thus contribute to the reduced resting metabolic rate found in old rats, while oxidative damage is present only in subsarcolemmal mitochondria.

Mitochondrial reactive oxygen species production and respiratory complex activity in rats with pressure overload-induced heart failure

We investigated the impact of cardiac reactive oxygen species (ROS) during the development of pressure overload-induced heart failure. We used our previously described rat model where transverse aortic constriction (TAC) induces compensated hypertrophy after 2 weeks, heart failure with preserved ejection fraction at 6 and 10 weeks, and heart failure with systolic dysfunction after 20 weeks. We measured mitochondrial ROS production rates, ROS damage and assessed the therapeutic potential of in vivo antioxidant therapies. In compensated hypertrophy (2 weeks of TAC) ROS production rates were normal at both mitochondrial ROS production sites (complexes I and III). Complex I ROS production rates increased with the appearance of diastolic dysfunction (6 weeks of TAC) and remained high thereafter. Surprisingly, maximal ROS production at complex III peaked at 6 weeks of pressure overload. Mitochondrial respiratory capacity (state 3 respiration) was elevated 2 and 6 weeks after TAC, decreased after this point and was significantly impaired at 20 weeks, when contractile function was also impaired and ROS damage was found with increased hydroxynonenal. Treatment with the ROS scavenger α-phenyl-N-tert-butyl nitrone or the uncoupling agent dinitrophenol significantly reduced ROS production rates at 6 weeks. Despite the decline in ROS production capacity, no differences in contractile function between treated and untreated animals were observed. Increased ROS production occurs early in the development of heart failure with a peak at the onset of diastolic dysfunction. However, ROS production may not be related to the onset of contractile dysfunction.

Plasma lactate and diabetes risk in 8045 participants of the atherosclerosis risk in communities study

PURPOSE

Determinants of oxidative capacity, such as fitness and level of adiposity, are strongly associated with type 2 diabetes. Whether decreased oxidative capacity itself is a cause or consequence of insulin resistance and diabetes is unknown.

METHODS

We examined the association of plasma lactate, a marker of oxidative capacity, with incident diabetes in 8045 participants from the Atherosclerosis Risk in Communities (ARIC) Study with no history of subclinical or diagnosed diabetes at baseline (1996-1998). Incident diabetes was self-reported during annual telephone calls.

RESULTS

During a median follow-up of 12 years, there were 1513 new cases of diabetes. In Cox proportional hazards models, baseline plasma lactate (per 10 mg/dL) was significantly associated with diabetes (hazard ratio, 1.20; 95% confidence interval, 1.01-1.43), even after adjustment for diabetes risk factors, fasting glucose, and insulin. The upper quartile of baseline lactate (≥ 8.1 mg/dL) was also significantly associated with diabetes risk (hazard ratio, 1.20; 95% confidence interval, 1.02-1.41) compared with the lowest quartile (≤ 5.1 mg/dL). Significant associations persisted among persons without insulin resistance (homeostatic model assessment insulin resistance index < 2.6 U) (P-trend < .01).

CONCLUSIONS

These findings suggest that low oxidative capacity may precede diabetes. Future studies should evaluate the physiological origins of elevated lactate to better understand its possible role in the pathogenesis of diabetes.

Lactate and risk of incident diabetes in a case-cohort of the atherosclerosis risk in communities (ARIC) study

Background

Oxidative capacity is decreased in type 2 diabetes. Whether decreased oxidative capacity is a cause or consequence of diabetes is unknown. Our purpose is to evaluate whether lactate, a marker of oxidative capacity, is associated with incident diabetes.

Methods and Findings

We conducted a case-cohort study in the Atherosclerosis Risk in Communities (ARIC) study at year 9 of follow-up. We evaluated lactate’s association with diabetes risk factors at baseline and estimated the hazard ratio for incident diabetes by quartiles of plasma lactate in 544 incident diabetic cases and 533 non-cases. Plasma lactate showed a graded positive relationship with fasting glucose and insulin (P<0.001). The relative hazard for incident diabetes increased across lactate quartiles (P-trend ≤0.001). Following adjustment for demographic factors, medical history, physical activity, adiposity, and serum lipids, the hazard ratio in the highest quartile was 2.05 times the hazard in the lowest quartile (95% CI: 1.28, 3.28). After including fasting glucose and insulin the association became non-significant.

Conclusions

Lactate, an indicator of oxidative capacity, predicts incident diabetes independent of many other risk factors and is strongly related to markers of insulin resistance. Future studies should evaluate the temporal relationship between elevated lactate and impaired fasting glucose and insulin resistance.

Skeletal muscle mitochondria and aging: a review

Aging is characterized by a progressive loss of muscle mass and muscle strength. Declines in skeletal muscle mitochondria are thought to play a primary role in this process. Mitochondria are the major producers of reactive oxygen species, which damage DNA, proteins, and lipids if not rapidly quenched. Animal and human studies typically show that skeletal muscle mitochondria are altered with aging, including increased mutations in mitochondrial DNA, decreased activity of some mitochondrial enzymes, altered respiration with reduced maximal capacity at least in sedentary individuals, and reduced total mitochondrial content with increased morphological changes. However, there has been much controversy over measurements of mitochondrial energy production, which may largely be explained by differences in approach and by whether physical activity is controlled for. These changes may in turn alter mitochondrial dynamics, such as fusion and fission rates, and mitochondrially induced apoptosis, which may also lead to net muscle fiber loss and age-related sarcopenia. Fortunately, strategies such as exercise and caloric restriction that reduce oxidative damage also improve mitochondrial function. While these strategies may not completely prevent the primary effects of aging, they may help to attenuate the rate of decline.

Obesity augments the age-induced increase in mitochondrial capacity for H2O2 release in Zucker fatty rats

AIM

Mitochondrial dysfunction has been suggested to play a significant role in obesity and insulin resistance. The aim of the present study was to investigate if changes in obesity and insulin resistance were related to similar changes in mitochondrial capacity for hydrogen peroxide release in Zucker diabetic fatty rats and their lean littermates.

METHODS

Thirty-four rats were used in this study. Rats were either lean or obese Zucker rats killed at 5-6 (young) or 12-14 (adults) weeks of age. Mitochondria were isolated from soleus muscles; respiration and release of hydrogen peroxide were determined and related to citrate synthase activity to determine intrinsic mitochondrial function. Mitochondrial-specific super-oxide dismuthase (MnSOD) protein content was determined in isolated mitochondria and muscle homogenate. Catalase protein content was determined in muscle homogenate.

RESULTS

Young lean and obese rats had a higher mitochondrial respiration when using palmitoyl-l-carnitine as substrate compared with adult lean and obese rats. The obese strain had higher mitochondrial hydrogen peroxide release but only in the adult animals. In both lean and obese animals, increased age was associated with increased mitochondrial hydrogen peroxide release. MnSOD tended to be higher in the obese strain in the isolated mitochondria. Regardless of age, catalase protein content was significantly lower in the obese rats.

CONCLUSIONS

This study shows that the augmented increase in obesity and insulin resistance seen in Zucker diabetic fatty rats is associated with increased capacity for mitochondrial hydrogen peroxide release.

Biphasic response of skeletal muscle mitochondria to chronic cardiac pressure overload - role of respiratory chain complex activity

Pressure overload induced heart failure affects cardiac mitochondrial function and leads to decreased respiratory capacity during contractile dysfunction. A similar cardiac mitochondrial dysfunction has been demonstrated by studies which induce heart failure through myocardial infarction or pacing. These heart failure models differ in their loading conditions to the heart and show nevertheless the same cardiac mitochondrial changes. Based on these observations we speculated that a workload independent mechanism may be responsible for the impairment in mitochondrial function after pressure overload, which may then also affect the skeletal muscle. We aimed to characterize changes in mitochondrial function of skeletal muscle during the transition from pressure overload (PO) induced cardiac hypertrophy to chronic heart failure. PO by transverse aortic constriction caused compensated hypertrophy at 2 weeks, HF with normal ejection fraction (EF) at 6 and 10 weeks, and hypertrophy with reduced EF at 20 weeks. Cardiac output was normal at all investigated time points. PO did not cause skeletal muscle atrophy. Mitochondrial respiratory capacity in soleus and gastrocnemius muscles showed an early increase (up to 6 weeks) and a later decline (significant at 20 weeks). Respiratory chain complex activities responded to PO in a biphasic manner. At 2 weeks, activity of complexes I and II was increased. These changes pseudo-normalized within the 6-10 week interval. At 20 weeks, all complexes showed reduced activities which coincided with clinical heart failure symptoms. However, both protein expression and supercomplex assembly (Blue-Native gel) remained normal. There were also no relevant changes in mRNA expression of genes involved in mitochondrial biogenesis. This temporal analysis reveals that mitochondrial function of skeletal muscle is changed early in the development of pressure overload induced heart failure without being directly influenced by an increased loading condition. The observed early increase and the later decline in respiratory capacity can be explained by concomitant activity changes of complex I and complex II and is not due to differences in gene expression or supercomplex assembly.

Platelet mitochondrial dysfunction is evident in type 2 diabetes in association with modifications of mitochondrial anti-oxidant stress proteins

OBJECTIVE

Mitochondrial dysfunction and oxidative stress in insulin responsive tissues is implicated in the pathogenesis of type 2 diabetes. Whether these perturbations extend to other tissues and contribute to their pathophysiology is less well established. The objective of this study was to investigate platelet mitochondria to evaluate whether type 2 diabetes associated mitochondrial dysfunction is evident in circulating cells.

METHOD

A pilot study of mitochondrial respiratory function and proteomic changes comparing platelets extracted from insulin sensitive (n=8) and type 2 diabetic subjects (n=7).

RESULTS

In-situ platelet mitochondria show diminished oxygen consumption and lower oxygen-dependent ATP synthesis in diabetic vs. control subjects. Mass spectrometric identification and confirmatory immunoblot analysis identifies induction of the mitochondrial anti-oxidant enzymes superoxide dismutase 2 and thioredoxin-dependent peroxide reductase 3 in platelets of diabetic subjects. As oxidative stress upregulates anti-oxidant enzymes we assessed mitochondrial protein carbonylation as an index of oxidative-stress. Platelets of diabetic subjects exhibit significantly increased protein carbonylation compared to controls.

CONCLUSIONS

As platelets are anuclear fragments of megakaryocytes, our data suggest that the bone marrow compartment in type 2 diabetic subjects is exposed to increased mitochondrial oxidative stress with upregulation of nuclear-encoded antioxidant mitochondrial enzymes. This 'stress-signature' in platelets of diabetic subjects is associated with a diminution of their mitochondrial contribution to energy production and support that mitochondrial perturbations in type 2 diabetes extends beyond the classical insulin responsive tissues. Platelets, as "accessible human tissue", may be useful to measure the mitochondrial modulatory effects of emerging anti-diabetic therapeutics.

Increased mitochondrial substrate sensitivity in skeletal muscle of patients with type 2 diabetes

AIMS/HYPOTHESIS

Mitochondrial respiration has been linked to insulin resistance. We studied mitochondrial respiratory capacity and substrate sensitivity in patients with type 2 diabetes (patients), and obese and lean control participants.

METHODS

Mitochondrial respiration was measured in permeabilised muscle fibres by respirometry. Protocols for respirometry included titration of substrates for complex I (glutamate), complex II (succinate) and both (octanoyl-carnitine). Myosin heavy chain (MHC) composition, antioxidant capacity (manganese superoxide dismutase [MnSOD]), citrate synthase activity and maximal oxygen uptake (VO2) were also determined. Insulin sensitivity was determined with the isoglycaemic-hyperinsulinaemic clamp technique.

RESULTS

Insulin sensitivity was different (p < 0.05) between the groups (patients<obese controls<lean controls). MnSOD was lower in patients than in lean controls. MHC I content was lowest in patients (37 ± 11% [mean ± SE] vs 53 ± 6% and 56 ± 4%) vs obese controls and lean controls, respectively. VO2 was highest in lean controls (40 ± 3 ml min(-1) kg(-1) [mean ± SE]) compared with patients (25 ± 2) and obese controls (27 ± 2). Mitochondrial content (citrate synthase) was higher (p < 0.05) in lean controls than in patients and obese controls. When normalised for mitochondrial content by citrate synthase, mitochondrial respiratory capacity was similar in all groups. However, the half maximal substrate concentration (C(50)) for complex I was significantly lower (p = 0.03) in patients (1.1 ± 0.2 mmol/l [mean ± SE]) than in obese (2.0 ± 0.3) and lean (1.8 ± 0.3) controls. Likewise, C(50) for complex II was lower (p = 0.02) in patients (3.5 ± 0.2 mmol/l [mean ± SE]) than in obese controls (4.1 ± 0.2), but did not differ from that in lean controls (3.8 ± 0.4). Substrate sensitivity for octanoyl-carnitine did not differ between groups.

CONCLUSIONS/INTERPRETATION

Increased mitochondrial substrate sensitivity is seen in skeletal muscle from type 2 diabetic patients and is confined to non-lipid substrates. Respiratory capacity per mitochondrion is not decreased with insulin resistance.

Skeletal muscle mitochondrial uncoupling, adaptive thermogenesis and energy expenditure

PURPOSE OF REVIEW

The prevalence of obesity is still increasing, despite obesity treatment strategies that aim at reducing energy intake. In addition to this, exercise programmes designed to increase energy expenditure have only a low efficiency and have generated mixed results. Therefore, strategies based on increasing energy expenditure via nonexercise means are currently under investigation. One novel strategy is the modulation of adaptive thermogenesis.

RECENT FINDINGS

Among others, adaptive thermogenesis can be modulated by changing dietary composition, treatment with hormone mimetics as well as by cold exposure. In humans, a large part of the adaptive thermogenic response is, in addition to a putative role of brown adipose tissue, determined by the skeletal muscle mass via the process of mitochondrial uncoupling. Here, we describe the molecular processes involved in mitochondrial uncoupling, state-of-the-art techniques to measure mitochondrial uncoupling in vitro and in vivo, as well as the current strategies to mitochondrial uncoupling.

SUMMARY

Data generated in rodents and humans implicate that increasing adaptive thermogenesis by increasing skeletal muscle mitochondrial uncoupling indeed elevates total energy expenditure and thus may provide a promising target for the treatment of obesity.

Uncoupled respiration, ROS production, acute lipotoxicity and oxidative damage in isolated skeletal muscle mitochondria from UCP3-ablated mice

The function of uncoupling protein 3 (UCP3) is still not established. Mitochondrial uncoupling, control of ROS production, protection against lipotoxicity and protection against oxidative stress are functions classically discussed. To establish a role for UCP3 in these functions, we have here used UCP3 (-/-) mice, backcrossed for 10 generations on a C57Bl/6 background. In isolated skeletal muscle mitochondria, we examined uncoupled respiration, both unstimulated and in the presence of fatty acids. We did not observe any difference between mitochondria from wildtype and UCP3 (-/-) mice. We measured H(2)O(2) production rate and respiration rate under reactive oxygen species-generating conditions (succinate without rotenone) but found no effect of UCP3. We tested two models of acute lipotoxicity-fatty acid-induced oxidative inhibition and fatty acid-induced swelling-but did not observe any protective effect of UCP3. We examined oxidative stress by quantifying 4-hydroxynonenal protein adducts and protein carbonyls in the mitochondria-but did not observe any protective effect of UCP3. We conclude that under the experimental conditions tested here, we find no evidence for the function of UCP3 being basal or induced uncoupling, regulation of ROS production, protection against acute lipotoxicity or protection against oxidative damage.

Dietary inorganic nitrate improves mitochondrial efficiency in humans

Nitrate, an inorganic anion abundant in vegetables, is converted in vivo to bioactive nitrogen oxides including NO. We recently demonstrated that dietary nitrate reduces oxygen cost during physical exercise, but the mechanism remains unknown. In a double-blind crossover trial we studied the effects of a dietary intervention with inorganic nitrate on basal mitochondrial function and whole-body oxygen consumption in healthy volunteers. Skeletal muscle mitochondria harvested after nitrate supplementation displayed an improvement in oxidative phosphorylation efficiency (P/O ratio) and a decrease in state 4 respiration with and without atractyloside and respiration without adenylates. The improved mitochondrial P/O ratio correlated to the reduction in oxygen cost during exercise. Mechanistically, nitrate reduced the expression of ATP/ADP translocase, a protein involved in proton conductance. We conclude that dietary nitrate has profound effects on basal mitochondrial function. These findings may have implications for exercise physiology- and lifestyle-related disorders that involve dysfunctional mitochondria.

High levels of whole-body energy expenditure are associated with a lower coupling of skeletal muscle mitochondria in C57Bl/6 mice

Considerable variation in energy expenditure is observed in C57Bl/6 mice on a high-fat diet. Because muscle tissue is a major determinant of whole-body energy expenditure, we set out to determine the variation in energy expenditure and its possible association with skeletal muscle mitochondrial function upon high-fat diet intervention. Metabolic cages using indirect calorimetry were used to assess whole-body energy metabolism in C57Bl/6 male mice during the first 3 days of high-fat diet intervention. Mice were grouped in a negative or positive residual nocturnal energy expenditure group after correction of total nocturnal energy expenditure for body mass by residual analysis. The positive residual energy expenditure group was characterized by higher uncorrected total nocturnal energy expenditure and food intake. On day 7, mitochondria were isolated from the skeletal muscle of the hind limb. Mitochondrial density was determined by mitochondrial protein content and did not differ between the positive and negative residual energy expenditure groups. Using high-resolution respirometry, mitochondrial oxidative function was assessed using various substrates. Mitochondria from the positive residual energy expenditure group were characterized by a lower adenosine diphosphate-stimulated respiration and lower respiratory control rates using palmitoyl-coenzyme A as substrate. These results indicate that reduced mitochondrial coupling is associated with positive residual energy expenditure and high rates of total energy expenditure in vivo.

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

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

Obesity in the elderly: is faulty metabolism to blame?

The fastest growing segment of the US population, and that of other developed countries, is the oldest-old (aged >85 years). Many children born after the year 2000 in countries with the longest lived residents may live to see their 100th birthday. The combination of reduced mortality along with reduced fertility in developed countries is producing 'population aging', and the comorbidities associated with aging are becoming important public health issues. Age-associated obesity is one such important public health issue. Aging is associated with significant changes in body composition, including loss of skeletal muscle mass and increased visceral fat accumulation. The loss of muscle mass is accompanied by a disproportionate decline in muscle strength (up to three-times greater than the loss of mass), indicative of reduced muscle 'quality' or muscle dysfunctionality. Aging is characterized by markedly reduced physical activity and a drop in resting metabolic rate that is disproportionate to the loss of muscle mass, with a shift towards preferentially oxidizing carbohydrate at the expense of fat. A combination of these factors may act to increase muscular lipid infiltration and decrease insulin sensitivity; however, the cause and effect relationship remains undetermined. Changes in cellular energy (i.e., ATP) requirement owing to decreased ion channel activity, decreased protein synthesis or increased mitochondrial energy efficiency may underlie the decreased resting metabolic rate. Increasing energy demand through physical activity may alleviate some of the adverse metabolic changes that are associated with aging.

Cold tolerance of UCP1-ablated mice: a skeletal muscle mitochondria switch toward lipid oxidation with marked UCP3 up-regulation not associated with increased basal, fatty acid- or ROS-induced uncoupling or enhanced GDP effects

Mice lacking the thermogenic mitochondrial membrane protein UCP1 (uncoupling protein 1)--and thus all heat production from brown adipose tissue--can still adapt to a cold environment (4 degrees C) if successively transferred to the cold. The mechanism behind this adaptation has not been clarified. To examine possible adaptive processes in the skeletal muscle, we isolated mitochondria from the hind limb muscles of cold-acclimated wild-type and UCP1(-/-) mice and examined their bioenergetic chracteristics. We observed a switch in metabolism, from carbohydrate towards lipid catabolism, and an increased total mitochondrial complement, with an increased total ATP production capacity. The UCP1(-/-) muscle mitochondria did not display a changed state-4 respiration rate (no uncoupling) and were less sensitive to the uncoupling effect of fatty acids than the wild-type mitochondria. The content of UCP3 was increased 3-4 fold, but despite this, endogenous superoxide could not invoke a higher proton leak, and the small inhibitory effect of GDP was unaltered, indicating that it was not mediated by UCP3. Double mutant mice (UCP1(-/-) plus superoxide dismutase 2-overexpression) were not more cold sensitive than UCP1(-/-), bringing into question an involvement of reactive oxygen species (ROS) in activation of any alternative thermogenic mechanism. We conclude that there is no evidence for an involvement of UCP3 in basal, fatty-acid- or superoxide-stimulated oxygen consumption or in GDP sensitivity. The adaptations observed did not imply any direct alternative process for nonshivering thermogenesis but the adaptations observed would be congruent with adaptation to chronically enhanced muscle activity caused by incessant shivering in these mice.

The role of mitochondria in the pathophysiology of skeletal muscle insulin resistance

Multiple organs contribute to the development of peripheral insulin resistance, with the major contributors being skeletal muscle, liver, and adipose tissue. Because insulin resistance usually precedes the development of type 2 diabetes mellitus (T2DM) by many years, understanding the pathophysiology of insulin resistance should enable development of therapeutic strategies to prevent disease progression. Some subjects with mitochondrial genomic variants/defects and a subset of lean individuals with hereditary predisposition to T2DM exhibit skeletal muscle mitochondrial dysfunction early in the course of insulin resistance. In contrast, in the majority of subjects with T2DM the plurality of evidence implicates skeletal muscle mitochondrial dysfunction as a consequence of perturbations associated with T2DM, and these mitochondrial deficits then contribute to subsequent disease progression. We review the affirmative and contrarian data regarding skeletal muscle mitochondrial biology in the pathogenesis of insulin resistance and explore potential therapeutic options to intrinsically modulate mitochondria as a strategy to combat insulin resistance. Furthermore, an overview of restricted molecular manipulations of skeletal muscle metabolic and mitochondrial biology offers insight into the mitochondrial role in metabolic substrate partitioning and in promoting innate adaptive and maladaptive responses that collectively regulate peripheral insulin sensitivity. We conclude that skeletal muscle mitochondrial dysfunction is not generally a major initiator of the pathophysiology of insulin resistance, although its dysfunction is integral to this pathophysiology and it remains an intriguing target to reverse/delay the progressive perturbations synonymous with T2DM.

High-hydroxypropylated tapioca starch improves insulin resistance in genetically diabetic KKAy mice

The hypoglycemic and antidiabetic effect of hydroxypropyl tapioca starch (HPTS) with a varying degree of substitution (DS: 0.058, 0.091, and 0.180) was investigated in rats and KKAy mice, an animal model of type 2 diabetes. The positive incremental area under the curve (IAUC) for glucose significantly decreased as the DS of HPTS increased. The IAUC after intragastric intubation of the highest HPTS (HPTS-III, DS = 0.180) was 55% of the IAUC of tapioca starch (TS). After 28 d, fasting blood glucose and insulin concentrations were significantly lower in rats fed HPTS-III (50 g/kg diet) than in those fed TS (P < 0.05). In KKAy mice fed HPTS-III (50 or 100 g/kg diet) for 33 d, as compared with TS, there was a delay in the detection of glucose in urine and also a decreased incidence of finding glucose in urine on days 7, 21, and 28; in addition, the AUCs for glucose in the oral glucose tolerance test on days 14 and 28 were significantly lower (P < 0.05 and P < 0.05, respectively). The plasma adiponectin concentration and the quantitative insulin sensitivity check index (QUICKI) were significantly higher in mice fed HPTS-III than in those fed TS (P < 0.01), whereas the homeostasis model assessment of insulin resistance (HOMA-IR) was lower (P < 0.01). Energy intake was significantly lower in mice fed HPTS-III than in those fed TS. These findings show that HPTS with a high DS resists digestion by alpha-amylase and improves insulin resistance in KKAy mice by decreasing energy intake. However, the potential mechanism by which HPTS-III decreases energy intake is unclear at present.

PPARalpha activation and increased dietary lipid oppose thyroid hormone signaling and rescue impaired glucose-stimulated insulin secretion in hyperthyroidism

The aim of the study was to investigate the impact of hyperthyroidism on the characteristics of the islet insulin secretory response to glucose, particularly the consequences of competition between thyroid hormone and peroxisome proliferator-activated receptor (PPAR)alpha in the regulation of islet adaptations to starvation and dietary lipid-induced insulin resistance. Rats maintained on standard (low-fat/high-carbohydrate) diet or high-fat/low-carbohydrate diet were rendered hyperthyroid (HT) by triiodothyronine (T(3)) administration (1 mg.kg body wt(-1).day(-1) sc, 3 days). The PPARalpha agonist WY14643 (50 mg/kg body wt ip) was administered 24 h before sampling. Glucose-stimulated insulin secretion (GSIS) was assessed during hyperglycemic clamps or after acute glucose bolus injection in vivo and with step-up and step-down islet perifusions. Hyperthyroidism decreased the glucose responsiveness of GSIS, precluding sufficient enhancement of insulin secretion for the degree of insulin resistance, in rats fed either standard diet or high-fat diet. Hyperthyroidism partially opposed the starvation-induced increase in the glucose threshold for GSIS and decrease in glucose responsiveness. WY14643 administration restored glucose tolerance by enhancing GSIS in fed HT rats and relieved the impact of hyperthyroidism to partially oppose islet starvation adaptations. Competition between thyroid hormone receptor (TR) and PPARalpha influences the characteristics of GSIS, such that hyperthyroidism impairs GSIS while PPARalpha activation (and increased dietary lipid) opposes TR signaling and restores GSIS in the fed hyperthyroid state. Increased islet PPARalpha signaling and decreased TR signaling during starvation facilitates appropriate modification of islet function.

Blood viscosity and hematocrit as risk factors for type 2 diabetes mellitus: the atherosclerosis risk in communities (ARIC) study

Several lines of evidence support the notion that elevated blood viscosity may predispose to insulin resistance and type 2 diabetes mellitus by limiting delivery of glucose, insulin, and oxygen to metabolically active tissues. To test this hypothesis, the authors analyzed longitudinal data on 12,881 initially nondiabetic adults, aged 45–64 years, who were participants in the Atherosclerosis Risk in Communities (ARIC) Study (1987–1998). Whole blood viscosity was estimated by using a validated formula based on hematocrit and total plasma proteins at baseline. At baseline, estimated blood viscosity was independently associated with several features of the metabolic syndrome. In models adjusted simultaneously for known predictors of diabetes, estimated whole blood viscosity and hematocrit predicted incident type 2 diabetes mellitus in a graded fashion (P_{trend (linear)} < 0.001): Compared with their counterparts in the lowest quartiles, adults in the highest quartile of blood viscosity (hazard ratio = 1.68, 95% confidence interval: 1.53, 1.84) and hematocrit (hazard ratio = 1.63, 95% confidence interval: 1.49, 1.79) were over 60% more likely to develop diabetes. Therefore, elevated blood viscosity and hematocrit deserve attention as emerging risk factors for insulin resistance and type 2 diabetes mellitus.

Diet does not explain the high prevalence of dyslipidaemia in paediatric renal transplant recipients

Dyslipidaemia exists frequently after renal transplantation (RTx) and promotes atherosclerosis. In this study, we examined the association between daily intake of nutrients and serum lipids after paediatric RTx. We studied 45 children with acceptably functioning kidney grafts and adequately completed food records at a median age of 10.6 years (range 4.3-17.2 years), a median 5.2 years (range 1.0-11.0) after RTx, and 178 healthy controls at a median age of 9.0 years (range 3.2-18.7 years). Serum total cholesterol (TC), triglyceride, and apolipoprotein B concentrations were higher in the RTx patients than in the controls (P < 0.001), despite similar dietary intakes of saturated and polyunsaturated fats, and cholesterol. Both the RTx patients and controls ingested a low amount of polyunsaturated fats [mean (SD) percent of total calories (E%) 4.8 (1.3) and 4.6 (1.5), respectively] and an excessive amount of saturated fats [mean (SD) E% 14.4 (2.4) and 14.1 (2.8), respectively]. In multiple regression analyses, dietary fibre was negatively associated with serum TC concentration. The standard deviation score for body mass index was negatively associated with serum concentration of high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein diameter, and positively with serum triglyceride concentration. In addition, dietary total fat intake was positively associated with serum HDL-C. In conclusion, the higher prevalence of dyslipidaemia in our paediatric RTx patients than in the controls was not explained by the diet. However, the type of fat consumed implicates the counselling for a healthier dietary lifestyle, with an increase in the ingestion of polyunsaturated fats and a decrease in that of saturated fats.

Chronic estradiol treatment improves brain homeostasis during aging in female rats

Aging is associated with a reduction in metabolic function, insulin resistance, increased incidence of neurodegenerative diseases, and memory or cognitive dysfunction. In aging females, loss of gonadal function determines the beginning of the period of reduced metabolic function. Estrogens have neuroprotective effects, but the mechanisms by which they exert these effects remain unclear. The effects of estradiol treatment on the activation of the insulin receptor substrate (IRS)-1 signaling pathway, the interactions between estrogen receptor (ER)-alpha and IRS-1 and the p85alpha subunit of phosphatidylinositol-3 kinase, together with the possible effects of estradiol treatment on glucose transporter-3 and -4 levels, were investigated in female rats. The level of expression of each glucose transporter was greater in control and estradiol-treated groups than in the ovariectomized group. Interactions of ERalpha46-IRS-1, ERalpha46-p85alpha, and p85alpha-IRS-1, as well as IRS-1 phosphorylation, appeared to increase with estradiol treatment. The results indicate that estradiol treatment improves some aspects of neuronal homeostasis that are affected by aging; this may indicate that estradiol has neuroprotective effects in female rats. Additional animal studies are required to clarify the neuroprotective role of estradiol in relation to other important molecules involved in the IRS-1-phosphatidylinositol-3 kinase signaling pathway.

Skeletal muscle subsarcolemmal mitochondrial dysfunction in high-fat fed rats exhibiting impaired glucose homeostasis

OBJECTIVE

To investigate whether changes in body energy balance induced by long-term high-fat feeding in adult rats could be associated with modifications in energetic behaviour and oxidative stress of skeletal muscle subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondrial populations.

DESIGN

Adult rats were fed low-fat or high-fat diet for 7 weeks.

MEASUREMENTS

Body energy balance and composition analysis together with plasma insulin and glucose level determination in the whole animal. Oxidative capacity, basal and induced proton leaks as well as aconitase and superoxide dismutase activities in SS and IMF mitochondria from skeletal muscle.

RESULTS

High-fat fed rats exhibit increased body lipid content, as well as hyperinsulinemia, hyperglycaemia and higher plasma non-esterified fatty acids. In addition, SS mitochondria display lower respiratory capacity and a different behaviour of SS and IMF mitochondria is found in the prevention from oxidative damage.

CONCLUSIONS

A deleterious consequence of decreased oxidative capacity in SS mitochondria from rats fed high-fat diet would be a reduced utilization of energy substrates, especially fatty acids, which may lead to intracellular triglyceride accumulation, lipotoxicity and insulin resistance development. Our results thus reveal a possible role for SS mitochondria in the impairment of glucose homeostasis induced by high-fat diet.

Mitochondrial respiration is decreased in skeletal muscle of patients with type 2 diabetes

We tested the hypothesis of a lower respiratory capacity per mitochondrion in skeletal muscle of type 2 diabetic patients compared with obese subjects. Muscle biopsies obtained from 10 obese type 2 diabetic and 8 obese nondiabetic male subjects were used for assessment of 3-hydroxy-Acyl-CoA-dehydrogenase (HAD) and citrate synthase activity, uncoupling protein (UCP)3 content, oxidative stress measured as 4-hydroxy-2-nonenal (HNE), fiber type distribution, and respiration in isolated mitochondria. Respiration was normalized to citrate synthase activity (mitochondrial content) in isolated mitochondria. Maximal ADP-stimulated respiration (state 3) with pyruvate plus malate and respiration through the electron transport chain (ETC) were reduced in type 2 diabetic patients, and the proportion of type 2X fibers were higher in type 2 diabetic patients compared with obese subjects (all P < 0.05). There were no differences in respiration with palmitoyl-l-carnitine plus malate, citrate synthase activity, HAD activity, UCP3 content, or oxidative stress measured as HNE between the groups. In the whole group, state 3 respiration with pyruvate plus malate and respiration through ETC were negatively associated with A1C, and the proportion of type 2X fibers correlated with markers of insulin resistance (P < 0.05). In conclusion, we provide evidence for a functional impairment in mitochondrial respiration and increased amount of type 2X fibers in muscle of type 2 diabetic patients. These alterations may contribute to the development of type 2 diabetes in humans with obesity.

PPARalpha activation reverses adverse effects induced by high-saturated-fat feeding on pancreatic beta-cell function in late pregnancy

We examined whether the additional demand for insulin secretion imposed by dietary saturated fat-induced insulin resistance during pregnancy is accommodated at late pregnancy, already characterized by insulin resistance. We also assessed whether effects of dietary saturated fat are influenced by PPARalpha activation or substitution of 7% of dietary fatty acids (FAs) with long-chain omega-3 FA, manipulations that improve insulin action in the nonpregnant state. Glucose tolerance at day 19 of pregnancy in the rat was impaired by high-saturated-fat feeding throughout pregnancy. Despite modestly enhanced glucose-stimulated insulin secretion (GSIS) in vivo, islet perifusions revealed an increased glucose threshold and decreased glucose responsiveness of GSIS in the saturated-fat-fed pregnant group. Thus, insulin resistance evoked by dietary saturated fat is partially countered by augmented insulin secretion, but compensation is compromised by impaired islet function. Substitution of 7% of saturated FA with long-chain omega-3 FA suppressed GSIS in vivo but did not modify the effect of saturated-fat feeding to impair GSIS by perifused islets. PPARalpha activation (24 h) rescued impaired islet function that was identified using perifused islets, but GSIS in vivo was suppressed such that glucose tolerance was not improved, suggesting modification of the feedback loop between insulin action and secretion.

Obesity-related derangements in metabolic regulation

An epidemic surge in the incidence of obesity has occurred worldwide over the past two decades. This alarming trend has been triggered by lifestyle habits that encourage overconsumption of energy-rich foods while also discouraging regular physical activity. These environmental influences create a chronic energy imbalance that leads to persistent weight gain in the form of body fat and a host of other abnormalities in metabolic homeostasis. As adiposity increases, so does the risk of developing comorbidities such as diabetes, hypertension, and cardiovascular disease. The intimate association between obesity and systemic metabolic dysregulation has inspired a new area of biochemistry research in which scientists are seeking to understand the molecular mechanisms that link chronic lipid oversupply to tissue dysfunction and disease development. The purpose of this chapter is to review recent findings in this area, placing emphasis on lipid-induced functional impairments in the major peripheral organs that control energy flux: adipose tissue, the liver, skeletal muscle, and the pancreas.

Pathogenesis of non-alcoholic steatohepatitis: human data

Non-alcoholic fatty liver disease (NAFLD) has moved rapidly to the forefront of clinical disease, with a prevalence of 30% in the adult United States population and a definite but yet uncertain rate of progression to cirrhosis and end-stage liver disease. This disease has an impact on all areas of clinical medicine, with increasing prevalence and adversity. It is essential to understand the pathophysiologic mechanisms involved in NAFLD, so that therapeutic strategies can be developed. Although fatty liver may be caused by other factors, this review concentrates on fatty liver associated with insulin resistance, sometimes referred to as the primary form.

Altered skeletal muscle subsarcolemmal mitochondrial compartment during catch-up fat after caloric restriction

An accelerated rate of fat recovery (catch-up fat) and insulin resistance are characteristic features of weight recovery after caloric restriction, with implications for the pathophysiology of catch-up growth and weight fluctuations. Using a previously described rat model of weight recovery in which catch-up fat and skeletal muscle insulin resistance have been linked to suppressed thermogenesis per se, we investigated alterations in mitochondrial energetics and oxidative stress in subsarcolemmal (SS) and intermyofibrillar (IMF) skeletal muscle mitochondria. After 2 weeks of semistarvation followed by 1 week of refeeding, the refed rats show persistent and selective reductions in SS mitochondrial mass (assessed from citrate synthase activity in tissue homogenate and isolated mitochondria) and oxidative capacity. Furthermore, the refed rats show, in both SS and IMF muscle mitochondria, a lower aconitase activity (whose inactivation is an index of increased reactive oxygen species [ROS]), associated with higher superoxide dismutase activity and increased proton leak. Taken together, these studies suggest that diminished skeletal muscle mitochondrial mass and function, specifically in the SS mitochondrial compartment, contribute to the high metabolic efficiency for catch-up fat after caloric restriction and underscore a potential link between diminished skeletal muscle SS mitochondrial energetics, increased ROS concentration, and insulin resistance during catch-up fat.

Positive effects of 17beta-estradiol on insulin sensitivity in aged ovariectomized female rats

Aging is associated with insulin resistance, which represents a common factor in age-related diseases. We aimed to determine the role of 17beta-estradiol on insulin sensitivity and memory during aging using ovariectomized rats (2-26 months of age) treated with physiological doses of 17beta-estradiol. Our results indicate a lack of effect of 17beta-estradiol replacement on spatial memory assessed in a water maze. Conversely, estradiol treatment improved insulin sensitivity in aging rats. These data imply that relatively low doses of 17beta-estradiol may have beneficial effects on glucose homeostasis due to the protective effects of estrogen. However, estradiol treatment used in the present study did not prevent memory impairment associated with aging.

Metallothionein antagonizes aging-induced cardiac contractile dysfunction: role of PTP1B, insulin receptor tyrosine phosphorylation and Akt

Aging is often accompanied by reduced insulin sensitivity and cardiac dysfunction. However, the causal relationship between the two remains poorly understood. This study was designed to determine the impact of cardiac-specific overexpression of antioxidant metallothionein (MT) on aging-associated cardiac dysfunction and impaired insulin signaling. Contractile and intracellular Ca(2+) properties were evaluated in left ventricular myocytes including peak shortening (PS), maximal velocity of shortening/relengthening (+/- dL/dt), time-to-PS (TPS), time-to-90% relengthening (TR(90)), fura-2 fluorescence intensity change (DeltaFFI) and intracellular Ca(2+) decay rate. Expression of insulin receptor, protein-tyrosine phosphatase 1B (PTP1B), phosphorylation of insulin receptor (Tyr1146) and Akt were evaluated by Western blot analysis. Aged wild-type FVB and MT transgenic mice (26-28 months old) displayed glucose intolerance and hyperinsulinemia. Cardiomyocytes from aged FVB mice exhibited prolonged TR(90) and intracellular Ca(2+) decay associated with normal PS, +/- dL/dt, TPS and DeltaFFI compared with those from young (2-3 months old) mice. Western blot analysis revealed reduced Akt expression and insulin (5 mU g(-1))-stimulated Akt phosphorylation, elevated PTP1B expression and diminished basal insulin receptor tyrosine phosphorylation associated with comparable insulin receptor expression in aged FVB mouse hearts. All of these aging-related defects in cardiac contractile function and insulin signaling (although not hyperinsulinemia and glucose intolerance) were significantly attenuated or ablated by MT transgene. These data indicate that enhanced antioxidant defense is beneficial for aging-induced cardiac contractile dysfunction and alteration in insulin signaling.

Hepatic insulin gene therapy prevents deterioration of vascular function and improves adipocytokine profile in STZ-diabetic rats

Hepatic insulin gene therapy (HIGT) ameliorates hyperglycemia in diabetic rodents, suggesting that similar approaches may eventually provide a means to improve treatment of diabetes mellitus. However, whether the metabolic and hormonal changes produced by HIGT benefit vascular function remains unclear. The impact of HIGT on endothelium-dependent vasodilation, nitrosyl-hemoglobin content (NO-Hb), and insulin sensitivity were studied using aortic ring preparations, electron spin resonance spectroscopy (ESR), homeostasis assessment of insulin resistance (HOMA-IR) calculations, and insulin tolerance testing (ITT). Data were correlated with selected hormone and adipocytokine concentrations. Rats made diabetic with streptozotocin were treated with subcutaneous insulin pellets dosed to sustain body weights and hyperglycemia or with HIGT; nondiabetic rats served as controls. Hyperglycemic rats demonstrated impaired endothelium-dependent vasodilation, reduced levels of NO-Hb, and diminished insulin, leptin, and adiponectin concentrations compared with controls. In contrast, HIGT treatment significantly reduced blood sugars and sustained both endothelium-mediated vasodilation and NO-Hb at control levels. HOMA-IR calculations and ITT indicated enhanced insulin sensitivity among HIGT-treated rats. HIGT partially restored suppressed leptin levels in hyperglycemic rats and increased adiponectin concentrations to supranormal levels, consistent with indicators of insulin sensitivity. Our findings indicate that the metabolic milieu produced by HIGT is sufficient to preserve vascular function in diabetic rodents. These data suggest that improved glycemia, induction of a beneficial adipocytokine profile, and enhanced insulin sensitivity combine to preserve endothelium-dependent vascular function in HIGT-treated diabetic rats. Consequently, HIGT may represent a novel and efficacious approach to reduce diabetes-associated vascular dysfunction.