Reduced skeletal muscle mitochondrial respiration and improved glucose metabolism in nondiabetic obese women during a very low calorie dietary intervention leading to rapid weight loss

The Impact of Weight Loss on Inflammation, Oxidative Stress, and Mitochondrial Function in Subjects with Obesity

Inflammation, oxidative stress, and mitochondrial function are implicated in the development of obesity and its comorbidities. The purpose of this study was to assess the impact of weight loss through calorie restriction on the metabolic profile, inflammatory and oxidative stress parameters, and mitochondrial respiration in an obese population. A total of 109 subjects underwent two cycles of a very low-calorie diet alternated with a low-calorie diet (24 weeks). We analyzed biochemical and inflammatory parameters in serum, as well as oxidative stress markers, mRNA antioxidant gene expression, and mitochondrial respiration in peripheral blood mononuclear cells (PBMCs). After the intervention, there was an improvement in both insulin resistance and lipid profiles, including cholesterol subfractions. Weight loss produced a significant reduction in mitochondrial ROSs content and an increase in glutathione levels, coupled with an enhancement in the mRNA expression of antioxidant systems (SOD1, GSR, and CAT). In addition, a significant improvement in basal oxygen consumption, maximal respiration, and ATP production was observed. These findings demonstrate that moderate weight loss can improve insulin resistance, lipid profiles and subfractions, inflammatory and oxidative stress parameters, and mitochondrial respiration. Therefore, we can affirm that dietary intervention can simultaneously achieve significant weight loss and improve metabolic profile and mitochondrial function in obesity.

Exercise training modifies skeletal muscle clock gene expression but not 24-hour rhythmicity in substrate metabolism of men with insulin resistance

Twenty-four hour rhythmicity in whole-body substrate metabolism, skeletal muscle clock gene expression and mitochondrial respiration is compromised upon insulin resistance. With exercise training known to ameliorate insulin resistance, our objective was to test if exercise training can reinforce diurnal variation in whole-body and skeletal muscle metabolism in men with insulin resistance. In a single-arm longitudinal design, 10 overweight and obese men with insulin resistance performed 12 weeks of high-intensity interval training recurrently in the afternoon (between 14.00 and 18.00 h) and were tested pre- and post-exercise training, while staying in a metabolic research unit for 2 days under free-living conditions with regular meals. On the second days, indirect calorimetry was performed at 08.00, 13.00, 18.00, 23.00 and 04.00 h, muscle biopsies were taken from the vastus lateralis at 08.30, 13.30 and 23.30 h, and blood was drawn at least bi-hourly over 24 h. Participants did not lose body weight over 12 weeks, but improved body composition and exercise capacity. Exercise training resulted in reduced 24-h plasma glucose levels, but did not modify free fatty acid and triacylglycerol levels. Diurnal variation of muscle clock gene expression was modified by exercise training with period genes showing an interaction (time × exercise) effect and reduced mRNA levels at 13.00 h. Exercise training increased mitochondrial respiration without inducing diurnal variation. Twenty-four-hour substrate metabolism and energy expenditure remained unchanged. Future studies should investigate alternative exercise strategies or types of interventions (e.g. diet or drugs aiming at improving insulin sensitivity) for their capacity to reinforce diurnal variation in substrate metabolism and mitochondrial respiration. KEY POINTS: Insulin resistance is associated with blunted 24-h flexibility in whole-body substrate metabolism and skeletal muscle mitochondrial respiration, and disruptions in the skeletal muscle molecular circadian clock. We hypothesized that exercise training modifies 24-h rhythmicity in whole-body substrate metabolism and diurnal variation in skeletal muscle molecular clock and mitochondrial respiration in men with insulin resistance. We found that metabolic inflexibility over 24 h persisted after exercise training, whereas mitochondrial respiration increased independent of time of day. Gene expression of Per1-3 and Rorα in skeletal muscle changed particularly close to the time of day at which exercise training was performed. These results provide the rationale to further investigate the differential metabolic impact of differently timed exercise to treat metabolic defects of insulin resistance that manifest at a particular time of day.

Mitochondrial Function in Healthy Human White Adipose Tissue: A Narrative Review

As ¾ of the global population either have excess or insufficient fat, it has become increasingly critical to understand the functions and dysfunctions of adipose tissue (AT). AT serves as a key organ in energy metabolism, and recently, attention has been focused on white AT, particularly its mitochondria, as the literature evidence links their functions to adiposity. This narrative review provides an overview of mitochondrial functionality in human white AT. Firstly, it is noteworthy that the two primary AT depots, subcutaneous AT (scAT) and visceral AT (vAT), exhibit differences in mitochondrial density and activity. Notably, vAT tends to have a higher mitochondrial activity compared to scAT. Subsequently, studies have unveiled a negative correlation between mitochondrial activity and body mass index (BMI), indicating that obesity is associated with a lower mitochondrial function. While the impact of exercise on AT mitochondria remains uncertain, dietary interventions have demonstrated varying effects on AT mitochondria. This variability holds promise for the modulation of AT mitochondrial activity. In summary, AT mitochondria exert a significant influence on health outcomes and can be influenced by factors such as obesity and dietary interventions. Understanding the mechanisms underlying these responses can offer potential insights into managing conditions related to AT and overall health.

High-Intensity Interval Training Improves Glycemic Control, Cellular Apoptosis, and Oxidative Stress of Type 2 Diabetic Patients

Background and Objectives: Physical exercise is an important therapeutic modality for treating and managing diabetes. High-intensity interval training (HIIT) is considered one of the best non-drug strategies for preventing and treating type 2 diabetes mellitus (T2DM) by improving mitochondrial biogenesis and function. This study aimed to determine the effects of 12 weeks of HIIT training on the expression of tumor suppressor protein-p53, mitochondrial cytochrome c oxidase (COX), and oxidative stress in patients with T2DM. Methods: A total of thirty male sedentary patients aged (45-60 years) were diagnosed with established T2DM for more than five years. Twenty healthy volunteers, age- and sex-matched, were included in this study. Both patients and control subjects participated in the HIIT program for 12 weeks. Glycemic control variables including p53 (U/mL), COX (ng/mL), total antioxidant capacity (TAC, nmole/µL), 8-hydroxy-2'-deoxyguanosine (8-OHdG, ng/mL), as well as genomic and mitochondrial DNA content were measured in both the serum and muscle tissues of control and patient groups following exercise training. Results: There were significant improvements in fasting glucose levels. HbA1c (%), HOMA-IR (mUmmol/L2), fasting insulin (µU/mL), and C-peptide (ng/mL) were reported in T2DM and healthy controls. A significant decrease was also observed in p53 protein levels. COX, 8-OhdG, and an increase in the level of TAC were reported in T2DM following 12 weeks of HIIT exercise. Before and after exercise, p53; COX, mt-DNA content, TAC, and 8-OhdG showed an association with diabetic control parameters such as fasting glucose (FG), glycated hemoglobin (HbA1C, %), C-peptide, fasting insulin (FI), and homeostatic model assessment for insulin resistance (HOMA-IR) in patients with T2DM. These findings support the positive impact of HIIT exercise in improving regulation of mitochondrial biogenesis and subsequent control of diabetes through anti-apoptotic and anti-oxidative pathways. Conclusions: A 12-week HIIT program significantly improves diabetes by reducing insulin resistance; regulating mitochondrial biogenesis; and decreasing oxidative stress capacity among patients and healthy controls. Also; p53 protein expression; COX; 8-OhdG; and TAC and mt-DNA content were shown to be associated with T2DM before and after exercise training.

Weight loss increases skeletal muscle mitochondrial energy efficiency in obese mice

Weight loss from an overweight state is associated with a disproportionate decrease in whole-body energy expenditure that may contribute to the heightened risk for weight regain. Evidence suggests that this energetic mismatch originates from lean tissue. Although this phenomenon is well documented, the mechanisms have remained elusive. We hypothesized that increased mitochondrial energy efficiency in skeletal muscle is associated with reduced expenditure under weight loss. Wildtype (WT) male C57BL6/N mice were fed with high fat diet for 10 weeks, followed by a subset of mice that were maintained on the obesogenic diet (OB) or switched to standard chow to promote weight loss (WL) for additional 6 weeks. Mitochondrial energy efficiency was evaluated using high-resolution respirometry and fluorometry. Mass spectrometric analyses were employed to describe the mitochondrial proteome and lipidome. Weight loss promoted ~50% increase in the efficiency of oxidative phosphorylation (ATP produced per O2 consumed, or P/O) in skeletal muscle. However, weight loss did not appear to induce significant changes in mitochondrial proteome, nor any changes in respiratory supercomplex formation. Instead, it accelerated the remodeling of mitochondrial cardiolipin (CL) acyl-chains to increase tetralinoleoyl CL (TLCL) content, a species of lipids thought to be functionally critical for the respiratory enzymes. We further show that lowering TLCL by deleting the CL transacylase tafazzin was sufficient to reduce skeletal muscle P/O and protect mice from diet-induced weight gain. These findings implicate skeletal muscle mitochondrial efficiency as a novel mechanism by which weight loss reduces energy expenditure in obesity.

Mitochondrial adaptations to calorie restriction and bariatric surgery in human skeletal muscle: a systematic review with meta-analysis

OBJECTIVE

We performed a meta-analysis to determine the changes induced by calorie restriction (CR) and bariatric surgery on human skeletal muscle mitochondria.

METHODS

A systematic search of Medline and Web of Science was conducted. Controlled trials exploring CR (≥14 days) and mitochondrial function and/or content assessment were included. Moreover, studies analyzing weight loss following gastric surgery were included for comparison purposes. Human muscle data from 28 studies assessing CR (520 muscle samples) and from 10 studies assessing bariatric surgery (155 muscle samples) were analyzed in a random effect meta-analysis with three a priori chosen covariates.

MAIN RESULTS

We report a decrease (p < 0.05) (mean (95 % CI)) in maximal mitochondrial state 3 respiration in response to CR (-0.44 (-0.85, -0.03)) but not in response to surgery (-0.33 (-1.18, 0.52)). No changes in mitochondrial content were reported after CR (-0.05 (-0.12, 0.13)) or in response to surgery (0.23 (-0.05, 0.52)). Moreover, data from CR subjects showed a reduction in complex IV (CIV) activity (-0.29 (-0.56, -0.03)) but not in CIV content (-0.21 (-0.63, 0.22)). Similar results were obtained when the length of the protocol, the initial body mass index, and the estimated energy deficit were included in the model as covariates.

CONCLUSION

The observation of reduced maximal mitochondrial state 3, uncoupled respiration, and CIV activity without altering mitochondrial content suggests that, in human skeletal muscle, CR mainly modulates intrinsic mitochondrial function.

High intramuscular triglyceride turnover rates and the link to insulin sensitivity: influence of obesity, type 2 diabetes and physical activity

Large intramuscular triglyceride (IMTG) stores in sedentary, obese individuals have been linked to insulin resistance, yet well-trained athletes exhibit high IMTG levels whilst maintaining insulin sensitivity. Contrary to previous assumptions, it is now known that IMTG content per se does not result in insulin resistance. Rather, insulin resistance is caused, at least in part, by the presence of high concentrations of harmful lipid metabolites, such as diacylglycerols and ceramides in muscle. Several mechanistic differences between obese sedentary individuals and their highly trained counterparts have been identified, which determine the differential capacity for IMTG synthesis and breakdown in these populations. In this review, we first describe the most up-to-date mechanisms by which a low IMTG turnover rate (both breakdown and synthesis) leads to the accumulation of lipid metabolites and results in skeletal muscle insulin resistance. We then explore current and potential exercise and nutritional strategies that target IMTG turnover in sedentary obese individuals, to improve insulin sensitivity. Overall, improving IMTG turnover should be an important component of successful interventions that aim to prevent the development of insulin resistance in the ever-expanding sedentary, overweight and obese populations. Novelty: A description of the most up-to-date mechanisms regulating turnover of the IMTG pool. An exploration of current and potential exercise/nutritional strategies to target and enhance IMTG turnover in obese individuals. Overall, highlights the importance of improving IMTG turnover to prevent the development of insulin resistance.

Impact of physical exercise and caloric restriction in patients with type 2 diabetes: Skeletal muscle insulin resistance and mitochondrial dysfunction as ideal therapeutic targets

Skeletal muscle insulin resistance and mitochondrial dysfunction are some of the major pathological defects implicated in the development of type 2 diabetes (T2D). Therefore, it has become necessary to understand how common interventions such as physical exercise and caloric restriction affect metabolic function, including physiological processes that implicate skeletal muscle dysfunction within a state of T2D. This review critically discusses evidence on the impact of physical exercise and caloric restriction on markers of insulin resistance and mitochondrial dysfunction within the skeletal muscle of patients with T2D or related metabolic complications. Importantly, relevant information from clinical studies was acquired through a systematic approach targeting major electronic databases and search engines such as PubMed, Google Scholar, and Cochrane library. The reported evidence suggests that interventions like physical exercise and caloric restriction, within a duration of approximately 2 to 4 months, can improve insulin sensitivity, in part by targeting the phosphoinositide 3-kinases/protein kinase B pathway in patients with T2D. Furthermore, both physical exercise and caloric restriction can effectively modulate markers related to improved mitochondrial function and dynamics. This was consistent with an improved modulation of mitochondrial oxidative capacity and reduced production of reactive oxygen species in patients with T2D or related metabolic complications. However, such conclusions are based on limited evidence, additional clinical trials are required to better understand these interventions on pathological mechanisms of T2D and related abnormalities.

Reduced mitochondrial DNA and OXPHOS protein content in skeletal muscle of children with cerebral palsy

AIM

To provide a detailed gene and protein expression analysis related to mitochondrial biogenesis and assess mitochondrial content in skeletal muscle of children with cerebral palsy (CP).

METHOD

Biceps brachii muscle samples were collected from 19 children with CP (mean [SD] age 15y 4mo [2y 6mo], range 9-18y, 16 males, three females) and 10 typically developing comparison children (mean [SD] age 15y [4y], range 7-21y, eight males, two females). Gene expression (quantitative reverse transcription polymerase chain reaction [PCR]), mitochondrial DNA (mtDNA) to genomic DNA ratio (quantitative PCR), and protein abundance (western blotting) were analyzed. Microarray data sets (CP/aging/bed rest) were analyzed with a focused query investigating metabolism- and mitochondria-related gene networks.

RESULTS

The mtDNA to genomic DNA ratio was lower in the children with CP compared to the typically developing group (-23%, p=0.002). Out of five investigated complexes in the mitochondrial respiratory chain, we observed lower protein levels of all complexes (I, III, IV, V, -20% to -37%; p<0.05) except complex II. Total peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) messenger RNA (p<0.004), isoforms PGC1α1 (p=0.05), and PGC1α4 (p<0.001) were reduced in CP. Transcriptional similarities were observed between CP, aging, and 90 days' bed rest.

INTERPRETATION

Mitochondrial biogenesis, mtDNA, and oxidative phosphorylation protein content are reduced in CP muscle compared with typically developing muscle. Transcriptional pathways shared between aging and long-term unloading suggests metabolic dysregulation in CP, which may guide therapeutic strategies for combatting CP muscle pathology. What this paper adds Cerebral palsy (CP) muscle contains fewer energy-generating organelles than typically developing muscle. Gene expression in CP muscle is similar to aging and long-term bed rest.

Acute endurance exercise stimulates circulating levels of mitochondrial derived peptides in humans

Mitochondrial derived peptides (MDPs) humanin (HN) and mitochondrial open reading frame of the 12S rRNA-c (MOTS-c) are involved in cell survival, suppression of apoptosis and metabolism. Circulating levels of MDPs are altered in chronic diseases such as diabetes type 2 and chronic kidney disease. Whether acute resistance (RE) or endurance (EE) exercise modulates circulating levels of HN and MOTS-c in humans is unknown. Following familiarization, subjects were randomized to EE (n=10, 45 min cycling at 70% of estimated VO₂max), RE (n=10, 4 sets x 7RM, leg press and knee extension), or control (CON, n=10). Skeletal muscle biopsies and blood samples were collected before and at 30 minutes and 3 hours following exercise. Plasma concentration of HN and MOTS-c, skeletal muscle MOTS-c as well as gene expression of exercise related genes were analyzed. Acute EE and RE promoted changes in skeletal muscle gene expression typically seen in response to each exercise modality (c-Myc, 45S pre-rRNA, PGC-1α-total and PGC-1α-ex1b). At rest, circulating levels of HN were positively correlated to MOTS-c levels and age. Plasma levels of MDPs were not correlated to fitness outcomes (VO₂max, leg strength or muscle mitochondrial (mt) DNA copy number). Circulating levels of HN were significantly elevated by acute EE but not RE. MOTS-C levels showed a trend to increase after EE. These results indicate that plasma MDP levels are not related to fitness status but that acute EE increases circulating levels of MDPs, in particular HN.

Mitochondrial bioenergetic pathways in blood leukocyte transcriptome decrease after intensive weight loss but are rescued following weight regain in female physique athletes

Prolonged periods of energy deficit leading to weight loss induce metabolic adaptations resulting in reduced energy expenditure, but the mechanisms for energy conservation are incompletely understood. We examined 42 healthy athletic females (age 27.5 ± 4.0 years, body mass index 23.4 ± 1.7 kg/m² ) who volunteered into either a group dieting for physique competition (n = 25) or a control group (n = 17). The diet group substantially reduced their energy intake and moderately increased exercise levels to induce loss of fat mass that was regained during a voluntary weight regain period. The control group maintained their typical lifestyle habits and body mass as instructed. From the diet group, fasting blood samples were drawn at baseline (PRE), after 4- to 5-month weight loss (PRE-MID), and after 4- to 5-month weight regain (MID-POST) as well as from the control group at similar intervals. Blood was analyzed to determine leukocyte transcriptome by RNA-Sequencing and serum metabolome by nuclear magnetic resonance (NMR) platform. The intensive weight loss period induced several metabolic adaptations, including a prominent suppression of transcriptomic signature for mitochondrial OXPHOS and ribosome biogenesis. The upstream regulator analysis suggested that this reprogramming of cellular energy metabolism may be mediated via AMPK/PGC1-α signaling and mTOR/eIF2 signaling-dependent pathways. Our findings show for the first time that prolonged energy deprivation induced modulation of mitochondrial metabolism can be observed through minimally invasive measures of leukocyte transcriptome and serum metabolome at systemic level, suggesting that adaptation to energy deficit is broader in humans than previously thought.

Are Alterations in Skeletal Muscle Mitochondria a Cause or Consequence of Insulin Resistance?

As a major site of glucose uptake following a meal, skeletal muscle has an important role in whole-body glucose metabolism. Evidence in humans and animal models of insulin resistance and type 2 diabetes suggests that alterations in mitochondrial characteristics accompany the development of skeletal muscle insulin resistance. However, it is unclear whether changes in mitochondrial content, respiratory function, or substrate oxidation are central to the development of insulin resistance or occur in response to insulin resistance. Thus, this review will aim to evaluate the apparent conflicting information placing mitochondria as a key organelle in the development of insulin resistance in skeletal muscle.

Cell energy metabolism: An update

In living cells, growth is the result of coupling between substrate catabolism and multiple metabolic processes that take place during net biomass formation and maintenance processes. During growth, both ATP/ADP and NADH/NAD⁺ molecules play a key role. Cell energy metabolism hence refers to metabolic pathways involved in ATP synthesis linked to NADH turnover. Two main pathways are thus involved in cell energy metabolism: glycolysis/fermentation and oxidative phosphorylation. Glycolysis and mitochondrial oxidative phosphorylation are intertwined through thermodynamic and kinetic constraints that are reviewed herein. Further, our current knowledge of short-term and long term regulation of cell energy metabolism will be reviewed using examples such as the Crabtree and the Warburg effect.

"Labile" heme critically regulates mitochondrial biogenesis through the transcriptional co-activator Hap4p in Saccharomyces cerevisiae

Heme (iron protoporphyrin IX) is a well-known prosthetic group for enzymes involved in metabolic pathways such as oxygen transport and electron transfer through the mitochondrial respiratory chain. However, heme has also been shown to be an important regulatory molecule (as "labile" heme) for diverse processes such as translation, kinase activity, and transcription in mammals, yeast, and bacteria. Taking advantage of a yeast strain deficient for heme production that enabled controlled modulation and monitoring of labile heme levels, here we investigated the role of labile heme in the regulation of mitochondrial biogenesis. This process is regulated by the HAP complex in yeast. Using several biochemical assays along with EM and epifluorescence microscopy, to the best of our knowledge, we show for the first time that cellular labile heme is critical for the post-translational regulation of HAP complex activity, most likely through the stability of the transcriptional co-activator Hap4p. Consequently, we found that labile heme regulates mitochondrial biogenesis and cell growth. The findings of our work highlight a new mechanism in the regulation of mitochondrial biogenesis by cellular metabolites.

Dynamic changes of muscle insulin sensitivity after metabolic surgery

The mechanisms underlying improved insulin sensitivity after surgically-induced weight loss are still unclear. We monitored skeletal muscle metabolism in obese individuals before and over 52 weeks after metabolic surgery. Initial weight loss occurs in parallel with a decrease in muscle oxidative capacity and respiratory control ratio. Persistent elevation of intramyocellular lipid intermediates, likely resulting from unrestrained adipose tissue lipolysis, accompanies the lack of rapid changes in insulin sensitivity. Simultaneously, alterations in skeletal muscle expression of genes involved in calcium/lipid metabolism and mitochondrial function associate with subsequent distinct DNA methylation patterns at 52 weeks after surgery. Thus, initial unfavorable metabolic changes including insulin resistance of adipose tissue and skeletal muscle precede epigenetic modifications of genes involved in muscle energy metabolism and the long-term improvement of insulin sensitivity.

Surgical weight-loss interventions improve insulin sensitivity via incompletely understood mechanisms. Here the authors assess skeletal muscle epigenetic changes in individuals with obesity following metabolic surgery and compare them with data from individuals without obesity.

Reduced skeletal muscle expression of mitochondrial-derived peptides humanin and MOTS-C and Nrf2 in chronic kidney disease

Advanced chronic kidney disease (CKD) is characterized by a premature aging phenotype of multifactorial origin. Mitochondrial dysfunction is prevalent in CKD and has been proposed as a major contributor to poor muscle function. Although the mitochondria-derived peptides (MDPs) humanin and mitochondrial open reading frame of 12S rRNA-c (MOTS-c) are involved in cell survival, suppression of apoptosis, and glucose control, the implications of MDP in CKD are unknown. We investigated humanin and MOTS-c protein expression in skeletal muscle and serum levels in CKD at stage 5 (glomerular filtration rate: <15 ml/min) patients and age-matched controls with normal renal function. Whereas circulating levels of humanin were increased in CKD, local muscle expression was reduced. In contrast, MOTS-c levels were reduced in both skeletal muscle and serum in CKD. Humanin in serum correlated positively to circulating TNF levels. Reduced MDP levels in skeletal muscle were associated with lower mitochondrial density and evidence of oxidative stress. These results indicate a differential regulation of MDPs in CKD and suggest an alternative site for humanin production than skeletal muscle in the uremic milieu. MDP levels were linked to systemic inflammation and evidence of oxidative stress in the muscle, two hallmark features of premature aging and uremia.

Incomplete Healing as a Cause of Aging: The Role of Mitochondria and the Cell Danger Response

The rate of biological aging varies cyclically and episodically in response to changing environmental conditions and the developmentally-controlled biological systems that sense and respond to those changes. Mitochondria and metabolism are fundamental regulators, and the cell is the fundamental unit of aging. However, aging occurs at all anatomical levels. At levels above the cell, aging in different tissues is qualitatively, quantitatively, and chronologically distinct. For example, the heart can age faster and differently than the kidney and vice versa. Two multicellular features of aging that are universal are: (1) a decrease in physiologic reserve capacity, and (2) a decline in the functional communication between cells and organ systems, leading to death. Decreases in reserve capacity and communication impose kinetic limits on the rate of healing after new injuries, resulting in dyssynchronous and incomplete healing. Exercise mitigates against these losses, but recovery times continue to increase with age. Reinjury before complete healing results in the stacking of incomplete cycles of healing. Developmentally delayed and arrested cells accumulate in the three stages of the cell danger response (CDR1, 2, and 3) that make up the healing cycle. Cells stuck in the CDR create physical and metabolic separation—buffer zones of reduced communication—between previously adjoining, synergistic, and metabolically interdependent cells. Mis-repairs and senescent cells accumulate, and repeated iterations of incomplete cycles of healing lead to progressively dysfunctional cellular mosaics in aging tissues. Metabolic cross-talk between mitochondria and the nucleus, and between neighboring and distant cells via signaling molecules called metabokines regulates the completeness of healing. Purinergic signaling and sphingolipids play key roles in this process. When viewed against the backdrop of the molecular features of the healing cycle, the incomplete healing model provides a new framework for understanding the hallmarks of aging and generates a number of testable hypotheses for new treatments.

Mitochondrial Dysfunction and Diabetes: Is Mitochondrial Transfer a Friend or Foe?

Obesity, insulin resistance and type 2 diabetes are accompanied by a variety of systemic and tissue-specific metabolic defects, including inflammation, oxidative and endoplasmic reticulum stress, lipotoxicity, and mitochondrial dysfunction. Over the past 30 years, association studies and genetic manipulations, as well as lifestyle and pharmacological invention studies, have reported contrasting findings on the presence or physiological importance of mitochondrial dysfunction in the context of obesity and insulin resistance. It is still unclear if targeting mitochondrial function is a feasible therapeutic approach for the treatment of insulin resistance and glucose homeostasis. Interestingly, recent studies suggest that intact mitochondria, mitochondrial DNA, or other mitochondrial factors (proteins, lipids, miRNA) are found in the circulation, and that metabolic tissues secrete exosomes containing mitochondrial cargo. While this phenomenon has been investigated primarily in the context of cancer and a variety of inflammatory states, little is known about the importance of exosomal mitochondrial transfer in obesity and diabetes. We will discuss recent evidence suggesting that (1) tissues with mitochondrial dysfunction shed their mitochondria within exosomes, and that these exosomes impair the recipient's cell metabolic status, and that on the other hand, (2) physiologically healthy tissues can shed mitochondria to improve the metabolic status of recipient cells. In this context the determination of whether mitochondrial transfer in obesity and diabetes is a friend or foe requires further studies.

Mitochondrial Biogenesis and Mitochondrial Reactive Oxygen Species (ROS): A Complex Relationship Regulated by the cAMP/PKA Signaling Pathway

Mitochondrial biogenesis is a complex process. It requires the contribution of both the nuclear and the mitochondrial genomes and therefore cross talk between the nucleus and mitochondria. Cellular energy demand can vary by great length and it is now well known that one way to adjust adenosine triphosphate (ATP) synthesis to energy demand is through modulation of mitochondrial content in eukaryotes. The knowledge of actors and signals regulating mitochondrial biogenesis is thus of high importance. Here, we review the regulation of mitochondrial biogenesis both in yeast and in mammalian cells through mitochondrial reactive oxygen species.

Influence of exercise amount and intensity on long-term weight loss maintenance and skeletal muscle mitochondrial ROS production in humans

Sustaining a weight loss after a lifestyle intervention is challenging. The objective of the present study was to investigate if mitochondrial function is associated with the ability to maintain a weight loss. Sixty-eight former participants in an 11-12-week lifestyle intervention were recruited into 2 groups; weight loss maintenance (WLM; body mass index (BMI): 32 ± 1 kg/m²) and weight regain (WR; BMI: 43 ± 2 kg/m²) based on weight loss measured at a follow-up visit (WLM: 4.8 ± 0.4; WR: 7.6 ± 0.8 years after lifestyle intervention). Maximal oxygen consumption rate, physical activity level, and blood and muscle samples were obtained at the follow-up experiment. Mitochondrial respiratory capacity and reactive oxygen species (ROS) production were measured. Fasting blood samples were used to calculate glucose homeostasis index. WR had impaired glucose homeostasis and decreased maximal oxygen uptake and physical activity level compared with WLM. The decreased physical activity in WR was due to a lower activity level at vigorous and moderate intensities. Mitochondrial respiratory capacity and citrate synthase (CS) activity was higher in WLM, but intrinsic mitochondrial respiratory capacity (mitochondrial respiratory capacity corrected for mitochondrial content (CS activity)) was similar. ROS production was higher in WR compared with WLM, which was accompanied by a decreased content of antioxidant proteins in WR. Intrinsic mitochondrial respiratory capacity in skeletal muscle is not associated with the ability to maintain a long-term weight loss. WLM had a higher maximal oxygen uptake, physical activity level, mitochondrial respiratory capacity and CS activity compared with WR. The reduced glucose tolerance was concurrent with increased ROS production per mitochondria in WR, and could also be associated with the lower physical activity level in this group.

Mitochondrial respiratory capacity remains stable despite a comprehensive and sustained increase in insulin sensitivity in obese patients undergoing gastric bypass surgery

AIM

It has been proposed, but not yet demonstrated by convincing evidence in published articles, that insulin resistance and mitochondrial respiratory function are causally related physiological phenomena. Here, we tested the prediction that weight loss-induced increase in insulin sensitivity will correlate with a corresponding change in mitochondrial respiratory capacity over the same time period.

METHODS

Insulin sensitivity was evaluated using the hyperinsulinaemic-euglycaemic clamp technique, and skeletal muscle mitochondrial respiratory capacity was evaluated by high-resolution respirometry in 26 patients with obesity. Each experiment was performed ~2 months and 1-2 weeks before, and ~4 and ~19 months after Roux-en-Y gastric bypass (RYGB) surgery.

RESULTS

A substantial weight loss was observed in all patients, and insulin sensitivity increased in all patients over the 21-months time period of the study. In contrast, skeletal muscle mitochondrial respiratory capacity, intrinsic mitochondrial respiratory capacity and mitochondrial content remained unchanged over the same time period.

CONCLUSION

Among obese patients with and without type 2 diabetes undergoing RYGB surgery, intrinsic mitochondrial respiratory capacity in skeletal muscle is not correlated with insulin sensitivity before or after the surgical intervention. Mitochondrial respiratory function may not be germane to the pathophysiology and/or aetiology of obesity and/or type 2 diabetes.

Prior weight loss exacerbates the biological drive to gain weight after the loss of ovarian function

Both the history of obesity and weight loss may change how menopause affects metabolic health. The purpose was to determine whether obesity and/or weight loss status alters energy balance (EB) and subsequent weight gain after the loss of ovarian function. Female lean and obese Wistar rats were randomized to 15% weight loss (WL) or ad libitum fed controls (CON). After the weight loss period, WL rats were kept in EB at the reduced weight for 8 weeks prior to ovariectomy (OVX). After OVX, all rats were allowed to eat ad libitum until weight plateaued. Energy intake (EI), spontaneous physical activity, and total energy expenditure (TEE) were measured with indirect calorimetry before OVX, immediately after OVX, and after weight plateau. Changes in energy intake (EI), TEE, and weight gain immediately after OVX were similar between lean and obese rats. However, obese rats gained more total weight and fat mass than lean rats over the full regain period. Post-OVX, EI increased more (P ≤ 0.03) in WL rats (58.9 ± 3.5 kcal/d) than CON rats (8.5 ± 5.2 kcal/d), and EI partially normalized (change from preOVX: 20.5 ± 4.2 vs. 1.5 ± 4.9 kcal/day) by the end of the study. As a result, WL rats gained weight (week 1:44 ± 20 vs. 7 ± 25 g) more rapidly (mean = 44 ± 20 vs. 7 ± 25 g/week; P < 0.001) than CON Prior obesity did not affect changes in EB or weight regain following OVX, whereas a history of weight loss prior to OVX augmented disruptions in EB after OVX, resulting in more rapid weight regain.

Repeated Hypoglycemic Episodes with Postprandial Hyperinsulinemia after the Recovery from Acute Weight Loss Revealed by Continuous Glucose Monitoring and the Oral Glucose Tolerance Test

We herein report a case of a 20-year-old woman who experienced hypoglycemia in parallel with acute weight loss confirmed by continuous glucose motoring (CGM). When she recovered from the acute weight loss, CGM revealed nocturnal and postprandial hypoglycemia. Six months were required to resolve the hypoglycemia and hyperinsulinemia after the recovery of her weight. Our case suggests that the adaption of insulin secretion to the rapid loss of weight and to the recovery of weight may require a long period of time, leading to the excessive secretion of insulin relative to the glucose level and repeated hypoglycemic episodes with postprandial hyperinsulinemia.

Humanin skeletal muscle protein levels increase after resistance training in men with impaired glucose metabolism

Humanin (HN) is a mitochondrially encoded and secreted peptide linked to glucose metabolism and tissue protecting mechanisms. Whether skeletal muscle HN gene or protein expression is influenced by exercise remains unknown. In this intervention study we show, for the first time, that HN protein levels increase in human skeletal muscle following 12 weeks of resistance training in persons with prediabetes. Male subjects (n = 55) with impaired glucose regulation (IGR) were recruited and randomly assigned to resistance training, Nordic walking or a control group. The exercise interventions were performed three times per week for 12 weeks with progressively increased intensity during the intervention period. Biopsies from the vastus lateralis muscle and venous blood samples were taken before and after the intervention. Skeletal muscle and serum protein levels of HN were analyzed as well as skeletal muscle gene expression of the mitochondrially encoded gene MT‐RNR2, containing the open reading frame for HN. To elucidate mitochondrial training adaptation, mtDNA, and nuclear DNA as well as Citrate synthase were measured. Skeletal muscle HN protein levels increased by 35% after 12 weeks of resistance training. No change in humanin protein levels was seen in serum in any of the intervention groups. There was a significant correlation between humanin levels in serum and the improvements in the 2 h glucose loading test in the resistance training group. The increase in HN protein levels in skeletal muscle after regular resistance training in prediabetic males may suggest a role for HN in the regulation of glucose metabolism. Given the preventative effect of exercise on diabetes type 2, the role of HN as a mitochondrially derived peptide and an exercise‐responsive mitokine warrants further investigation.

Dietary supplementation of heat-treated Gracilaria and Ulva seaweeds enhanced acute hypoxia tolerance in gilthead sea bream (Sparus aurata)

Intensive aquaculture practices involve rearing fish at high densities. In these conditions, fish may be exposed to suboptimal dissolved O2 levels with an increased formation of reactive O2 species (ROS) in tissues. Seaweeds (SW) contain biologically active substances with efficient antioxidant capacities. This study evaluated the effects of dietary supplementation of heat-treated SW (5% Gracilaria vermiculophylla or 5% Ulva lactuca) on stress bioindicators in sea bream subjected to a hypoxic challenge. 168 fish (104.5 g average weight) were distributed in 24 tanks, in which eight tanks were fed one of three experimental diets for 34 days: (i) a control diet without SW supplementation, (ii) a control diet supplemented with Ulva, or (iii) a control diet with Gracilaria Thereafter, fish from 12 tanks (n=4 tanks/dietary treatment) were subjected to 24 h hypoxia (1.3 mg O2 l-1) and subsequent recovery normoxia (8.6 mg O2 l-1). Hypoxic fish showed an increase in hematocrit values regardless of dietary treatment. Dietary modulation of the O2-carrying capacity was conspicuous during recovery, as fish fed SW supplemented diets displayed significantly higher haemoglobin concentration than fish fed the control diet. After the challenge, survival rates in both groups of fish fed SW were higher, which was consistent with a decrease in hepatic lipid peroxidation in these groups. Furthermore, the hepatic antioxidant enzyme activities were modulated differently by changes in environmental O2 condition, particularly in sea bream fed the Gracilaria diet. After being subjected to hypoxia, the gene expression of antioxidant enzymes and molecular chaperones in liver and heart were down regulated in sea bream fed SW diets. This study suggests that the antioxidant properties of heat-treated SW may have a protective role against oxidative stress. The nature of these compounds and possible mechanisms implied are currently being investigated.

Supramolecular structure of dietary fat in early life modulates expression of markers for mitochondrial content and capacity in adipose tissue of adult mice

Background

Previous studies have shown that early life nutrition can modulate the development of white adipose tissue and thereby affect the risk on obesity and metabolic disease later in life. For instance, postnatal feeding with a concept infant milk formula with large, phospholipid coated lipid droplets (Concept, Nuturis®), resulted in reduced adiposity in adult mice. The present study investigated whether differences in cell energy metabolism, using markers of mitochondrial content and capacity, may contribute to the observed effects.

Methods

C57Bl/6j male mice were exposed to a rodent diet containing the Concept (Concept) or standard (CTRL) infant milk formula from postnatal day 16 until postnatal day 42, followed by a western style diet challenge until postnatal day 98. Markers for mitochondrial content and capacity were analyzed in retroperitoneal white adipose tissue and gene expression of metabolic markers was measured in both retroperitoneal white adipose tissue and muscle tibialis (M. tibialis) at postnatal day 98.

Results

In retroperitoneal white adipose tissue, the Concept group showed higher citrate synthase activity and mitochondrial DNA expression compared to the CTRL group (p < 0.05). In addition, protein expression of mitochondrial cytochrome c oxidase subunit I of the oxidative phosphorylation pathway/cascade was increased in the Concept group compared to CTRL (p < 0.05). In the M. tibialis, gene expression of uncoupling protein 3 was higher in the Concept compared to the CTRL group. Other gene and protein expression markers for mitochondrial oxidative capacity were not different between groups.

Conclusion

Postnatal feeding with large, phospholipid coated lipid droplets generating a different supramolecular structure of dietary lipids enhances adult gene and protein expression of specific mitochondrial oxidative capacity markers, indicative of increased substrate oxidation in white adipose tissue and skeletal muscle. Although functional mitochondrial capacity was not measured, these results may suggest that adaptations in mitochondrial function via early feeding with a more physiological structure of dietary lipids, could underlie the observed beneficial effects on later life adiposity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-017-0191-5) contains supplementary material, which is available to authorized users.

Mitochondrial function in heart failure: The impact of ischemic and non-ischemic etiology

BACKGROUND

Although cardiac mitochondrial dysfunction is associated with heart failure (HF), this is a complex syndrome with two predominant etiologies, ischemic HF (iHF) and non-ischemic HF (niHF), and the exact impact of mitochondrial dysfunction in these two distinct forms of HF is unknown.

METHODS AND RESULTS

To determine the impact of HF etiology on mitochondrial function, respiration was measured in permeabilized cardiac muscle fibers from patients with iHF (n=17), niHF (n=18), and healthy donor hearts (HdH). Oxidative phosphorylation capacity (OXPHOS), assessed as state 3 respiration, fell progressively from HdH to niHF, to iHF (Complex I+II: 54±1; 34±4; 27±3pmol·s(-1)·mg(-1)) as did citrate synthase activity (CSA: 206±18; 129±6; 82±6nmol·mg(-1)·min(-1)). Although still significantly lower than HdH, normalization of OXPHOS by CSA negated the difference in mass specific OXPHOS between iHF and niHF. Interestingly, Complex I state 2 respiration increased progressively from HdH, to niHF, to iHF, whether or not normalized for CSA (0.6±0.2; 1.1±0.3; 2.3±0.3; pmol·mg(-1)·CSA), such that the respiratory control ratio (RCR), fell in the same manner across groups. Finally, both the total free radical levels (60±6; 46±4AU) and level of mitochondrial derived superoxide (1.0±0.2; 0.7±0.1AU) were greater in iHF compared to niHF, respectively.

CONCLUSIONS

Thus, the HF-related attenuation in OXPHOS actually appears to be independent of etiology when the lower mitochondrial content of iHF is taken into account. However, these findings provide evidence of deleterious intrinsic mitochondrial changes in iHF, compared to niHF, including greater proton leak, attenuated OXPHOS efficiency, and augmented free radical levels.

Exercise and Weight Loss Improve Muscle Mitochondrial Respiration, Lipid Partitioning, and Insulin Sensitivity After Gastric Bypass Surgery

Both Roux-en-Y gastric bypass (RYGB) surgery and exercise can improve insulin sensitivity in individuals with severe obesity. However, the impact of RYGB with or without exercise on skeletal muscle mitochondria, intramyocellular lipids, and insulin sensitivity index (SI) is unknown. We conducted a randomized exercise trial in patients (n = 101) who underwent RYGB surgery and completed either a 6-month moderate exercise (EX) or a health education control (CON) intervention. SI was determined by intravenous glucose tolerance test. Mitochondrial respiration and intramyocellular triglyceride, sphingolipid, and diacylglycerol content were measured in vastus lateralis biopsy specimens. We found that EX provided additional improvements in SI and that only EX improved cardiorespiratory fitness, mitochondrial respiration and enzyme activities, and cardiolipin profile with no change in mitochondrial content. Muscle triglycerides were reduced in type I fibers in CON, and sphingolipids decreased in both groups, with EX showing a further reduction in a number of ceramide species. In conclusion, exercise superimposed on bariatric surgery-induced weight loss enhances mitochondrial respiration, induces cardiolipin remodeling, reduces specific sphingolipids, and provides additional improvements in insulin sensitivity.

Adipose tissue mitochondrial respiration and lipolysis before and after a weight loss by diet and RYGB

OBJECTIVE

To study adipose tissue mitochondrial respiration and lipolysis following a massive weight loss.

METHODS

High resolution respirometry of adipose tissue biopsies and tracer determined whole body lipolysis. Sixteen obese patients with type 2 diabetes (T2DM) and 27 without (OB) were studied following a massive weight loss by diet and Roux-en-Y gastric bypass (RYGB).

RESULTS

The mitochondrial respiratory rates were similar in OB and T2DM, and the mass-specific oxygen flux increased significantly 4 and 18 months post-surgery (P < 0.05). With normalization to mitochondrial content, no differences in oxidative capacity after RYGB were seen. The ratio between the oxidative phosphorylation system capacity (P) and the capacity of the electron transfer system (E) increased 18 months after RYGB in both groups (P < 0.05). Lipolysis per fat mass was similar in the two groups and was increased (P < 0.05) and lipid oxidation during hyperinsulinemia decreased 4 months post-surgery. In T2DM, visceral fat mass was always higher relative to the body fat mass (%) compared to OB.

CONCLUSIONS

Adipose tissue mitochondrial respiratory capacity increases with RYGB. Adipocytes adapt to massive weight loss by increasing the phosphorylation system ratio (P/E), suggesting an increased ability to oxidize substrates after RYGB. Lipolysis increases in the short term post-surgery, and insulin sensitivity for suppression of lipolysis increases with RYGB.

Effects of weight loss via high fat vs. low fat alternate day fasting diets on free fatty acid profiles

Cardiovascular disease risk is associated with excess body weight and elevated plasma free fatty acid (FFA) concentrations. This study examines how an alternate-day fasting (ADF) diet high (HF) or low (LF) in fat affects plasma FFA profiles in the context of weight loss, and changes in body composition and lipid profiles. After a 2-week weight maintenance period, 29 women (BMI 30-39.9 kg/m(2)) 25-65 years old were randomized to an 8-week ADF-HF (45% fat) diet or an ADF-LF (25% fat) diet with 25% energy intake on fast days and ad libitum intake on feed days. Body weight, BMI and waist circumference were assessed weekly and body composition was measured using dual x-ray absorptiometry (DXA). Total and individual FFA and plasma lipid concentrations were measured before and after weight loss. Body weight, BMI, fat mass, total cholesterol, LDL-C and triglyceride concentrations decreased (P < 0.05) in both groups. Total FFA concentrations also decreased (P < 0.001). In the ADF-LF group, decreases were found in several more FFAs than in the ADF-HF group. In the ADF-HF group, FFA concentrations were positively correlated with waist circumference. Depending on the macronutrient composition of a diet, weight loss with an ADF diet decreases FFA concentrations through potentially different mechanisms.

Subsarcolemmal lipid droplet responses to a combined endurance and strength exercise intervention

Muscle lipid stores and insulin sensitivity have a recognized association although the mechanism remains unclear. We investigated how a 12‐week supervised combined endurance and strength exercise intervention influenced muscle lipid stores in sedentary overweight dysglycemic subjects and normal weight control subjects (n = 18). Muscle lipid stores were measured by magnetic resonance spectroscopy (MRS), electron microscopy (EM) point counting, and direct EM lipid droplet measurements of subsarcolemmal (SS) and intramyofibrillar (IMF) regions, and indirectly, by deep sequencing and real‐time PCR of mRNA of lipid droplet‐associated proteins. Insulin sensitivity and VO₂max increased significantly in both groups after 12 weeks of training. Muscle lipid stores were reduced according to MRS at baseline before and after the intervention, whereas EM point counting showed no change in LD stores post exercise, indicating a reduction in muscle adipocytes. Large‐scale EM quantification of LD parameters of the subsarcolemmal LD population demonstrated reductions in LD density and LD diameters. Lipid droplet volume in the subsarcolemmal LD population was reduced by ~80%, in both groups, while IMF LD volume was unchanged. Interestingly, the lipid droplet diameter (n = 10 958) distribution was skewed, with a lack of small diameter lipid droplets (smaller than ~200 nm), both in the SS and IMF regions. Our results show that the SS LD lipid store was sensitive to training, whereas the dominant IMF LD lipid store was not. Thus, net muscle lipid stores can be an insufficient measure for the effects of training.

We have investigated the muscle storage lipids responses to exercise, finding that subsarcolemmal lipid droplets are reduced 80%. Interestingly, we find that the lipid droplet diameter distribution was skewed, with a marked lack of lipid droplets smaller than 200 nm.

Allyl isothiocyanate ameliorates insulin resistance through the regulation of mitochondrial function

Mitochondrial dysfunction is associated with the pathophysiology of insulin resistance. Allylisothiocyanate (AITC) is found in many cruciferous vegetables and has been reported to possess anticancer activity. However, the effect of AITC on insulin resistance and mitochondrial function has not yet been investigated. Here, we show that AITC increased glucose uptake in insulin-resistant C2C12 myotubes and augmented glucose transporter 4 (GLUT4) translocation in L6-GLUT4myc cells. AITC recovered the impaired insulin signaling evoked by free fatty acid exposure and increased mitochondrial membrane potential and mitochondrial DNA content. AITC also elevated the rate of oxygen consumption in C2C12 cells. Furthermore, mice that were fed a high-fat diet with AITC for 10 weeks had reduced diet-induced obesity and hepatic steatosis. AITC also inhibited the hyperglycemia and hyperinsulinemia induced by the consumption of a high-fat diet. Glucose and insulin tolerance tests indicated that AITC improved both glucose tolerance and insulin sensitivity. In addition, AITC inhibited hepatic gluconeogenesis and ameliorated high fat diet-induced mitochondrial dysfunction. Collectively, these data suggest that the protective effect of AITC on insulin resistance is partly mediated through the modulation of mitochondrial dysfunction.

Carotid intima-media thickness is predicted by combined eotaxin levels and severity of hepatic steatosis at ultrasonography in obese patients with Nonalcoholic Fatty Liver Disease

BACKGROUND

Non-Alcoholic Fatty Liver Disease (NAFLD) is a distinct coronary artery disease (CAD) risk factor. The atherosclerotic process predisposing to CAD includes altered lipid profile and inflammatory processes. The available evidence suggests that increased circulating levels of eotaxin, an eosinophil chemoattractant cytokine implicated in allergic responses, are detected in the serum of patients with CAD. Relationships were sought between serum eotaxin on the one hand, and intima-media thickness--an early predictor of the atherosclerotic process, hepatic steatosis, arterial blood pressure values, as well as inflammation/immune markers and angiogenetic factors--on the other.

METHODS

Eighty obese patients with NAFLD, diagnosed at ultrasonography, without evident cytolysis, formed our study population. Anthropometric measures, metabolic profile, serum concentrations of interleukin-1β, C-reactive protein, interleukin-6, fibrinogen, ferritin, TNF-α, spleen size, vascular endothelial growth factor, platelet-derived growth factor-BB and heat shock protein-70 were evaluated.

RESULTS

Serum eotaxin concentrations were distinctly associated with TNF α, IL-6, IL-1β, VEGF and PDGF-BB levels but not with CRP, fibrinogen, heat shock protein-70 or spleen size. Among the metabolic and anthropometric parameters, a significant predictive power emerged when comparing eotaxin to insulin resistance, expressed as HOMA. NAFLD was distinctly associated with HOMA (P = 0.0005). Intima-media thickness was well predicted by both eotaxin levels and severity of NAFLD at ultrasonography, although no relation was detected between these last two variables.

DISCUSSION AND CONCLUSION

A role for insulin resistance in mediating the interplay between eotaxin and other inflammation/immune parameters could be evidenced in the induction/maintenance of atherosclerosis of obese patients with NAFLD.

Relationships between mitochondrial function and metabolic flexibility in type 2 diabetes mellitus

Introduction

Mitochondrial dysfunction, lipid accumulation, insulin resistance and metabolic inflexibility have been implicated in the etiology of type 2 diabetes (T2D), yet their interrelationship remains speculative. We investigated these interrelationships in a group of T2D and obese normoglycemic control subjects.

Methods

49 non-insulin dependent male T2D patients and 54 male control subjects were enrolled, and a hyperinsulinemic-euglycemic clamp and indirect calorimetry were performed. A muscle biopsy was taken and intramyocellular lipid (IMCL) was measured. In vivo mitochondrial function was measured by PCr recovery in 30 T2D patients and 31 control subjects.

Results

Fasting NEFA levels were significantly elevated in T2D patients compared with controls, but IMCL was not different. Mitochondrial function in T2D patients was compromised by 12.5% (p<0.01). Whole body glucose disposal (WGD) was higher at baseline and lower after insulin stimulation. Metabolic flexibility (ΔRER) was lower in the type 2 diabetic patients (0.050±0.033 vs. 0.093±0.050, p<0.01). Mitochondrial function was the sole predictor of basal respiratory exchange ratio (RER) (R² = 0.18, p<0.05); whereas WGD predicted both insulin-stimulated RER (R² = 0.29, p<0.001) and metabolic flexibility (R² = 0.40, p<0.001).

Conclusions

These results indicate that defects in skeletal muscle in vivo mitochondrial function in type 2 diabetic patients are only reflected in basal substrate oxidation and highlight the importance of glucose disposal rate as a determinant of substrate utilization in response to insulin.

Insulin resistance and mitochondrial function in skeletal muscle

The mechanism(s) behind the decreased ability of insulin to facilitate glucose uptake in insulin sensitive tissues as seen in type 2 diabetes is not resolved. With the rapidly increasing prevalence of this disease world-wide, and the many complications that follow the disease, large resources are used in the attempt to resolve the mechanisms of insulin resistance. In this context, a dysfunction of mitochondria in the skeletal muscle has been suggested to play a pivotal role. It has been postulated that a decrease in the content of mitochondria in the skeletal muscle can explain the insulin resistance. Complementary to this also specific defects of components in the respiratory chain in the mitochondria have been suggested to play a role in insulin resistance. A key element in these mechanistic suggestions is inability to handle substrate fluxes and subsequently an accumulation of ectopic intramyocellular lipids, interfering with insulin signaling. In this review we will present the prevailing view-points and argue for the unlikelihood of this scenario being instrumental in human insulin resistance. This article is part of a Directed Issue entitled: Bioenergetic dysfunction.

Progression of type 2 diabetes in GK rats affects muscle and liver mitochondria differently: pronounced reduction of complex II flux is observed in liver only

Impaired mitochondrial function is implicated in the development of type 2 diabetes mellitus (T2DM). This was investigated in mitochondria from skeletal muscle and liver of the Goto-Kakizaki (GK) rat, which spontaneously develops T2DM with age. The early and the manifest stage of T2DM was studied in 6- and 16-wk-old GK rats, respectively. In GK16 compared with GK6 animals, a decrease in state 3 respiration with palmitoyl carnitine (PC) as substrate was observed in muscle. Yet an increase was seen in liver. To test the complex II contribution to the state 3 respiration, succinate was added together with PC. In liver mitochondria, this resulted in an ∼50% smaller respiratory increase in the GK6 group compared with control and no respiratory increase at all in the GK16 animals. Yet no difference between groups was seen in muscle mitochondria. RCR and P/O ratio was increased (P < 0.05) in liver but unchanged in muscle in both GK groups. We observed increased lipid peroxidation and decreased Akt phosphorylation in liver with the progression of T2DM but no change in muscle. We conclude that, during the progression of T2DM in GK rats, liver mitochondria are affected earlier and/or more severely than muscle mitochondria. Succinate dehydrogenase flux in the presence of fatty acids was reduced severely in liver but not in muscle mitochondria during manifest T2DM. The observations support the notion that T2DM pathogenesis is initiated in the liver and that only later are muscle mitochondria affected.

The influence of age and aerobic fitness: effects on mitochondrial respiration in skeletal muscle

AIM

Mitochondrial function has previously been studied in ageing, but never in humans matched for maximal oxygen uptake ((V)·O2max). Furthermore, the influence of ageing on mitochondrial substrate sensitivity is not known.

METHODS

Skeletal muscle mitochondrial respiratory capacity and mitochondrial substrate sensitivity were measured by respirometry in young (23 ± 3 years) and middle-aged (53 ± 3 years) male subjects with similar (V)·O2max. Protocols for respirometry included titration of substrates for complex I (glutamate), complex II (succinate) and both (octanoyl carnitine) for calculation of substrate sensitivity (C(50) ). Myosin heavy chain (MHC) isoforms, citrate synthase (CS) and β-hydroxy-acyl-CoA-dehydrogenase (HAD) activity, mitochondrial DNA (mtDNA) content, protein levels of complexes I-V and antioxidant defence system [manganese superoxide dismutase (MnSOD)] were measured.

RESULTS

No differences were found in maximal mitochondrial respiration or C(50) with glutamate (2.0 ± 0.3 and 1.8 ± 0.3 mm), succinate (3.7 ± 0.2 and 3.8 ± 0.4 mm) or octanoyl carnitine (47 ± 8 and 56 ± 7 μm) in young and middle-aged subjects respectively. Normalizing mitochondrial respiration to mtDNA young subjects had a higher (P < 0.05) respiratory capacity per mitochondrion compared to middle-aged subjects. HAD activity and mtDNA per mg tissue were higher in middle-aged compared to young subjects. Middle-aged had a higher MHC I isoform and a lower MHC IIX isoform content compared to young subjects.

CONCLUSION

Mitochondrial substrate sensitivity is not affected by ageing. When young and middle-aged men are carefully matched for (V)·O2max, mitochondrial respiratory capacity is also similar. However, per mitochondrion respiratory capacity was lower in middle-aged compared to young subjects. Thus, when matched for (V)·O2max, middle-aged seem to require a higher mitochondrial content than young subjects.

The effect of a very low calorie diet on insulin sensitivity, beta cell function, insulin clearance, incretin hormone secretion, androgen levels and body composition in obese young women

OBJECTIVE

Evaluation of the effect of an 8-week very low calorie diet (VLCD, 500-600 kcal daily) on weight, body fat distribution, glucose, insulin and lipid metabolism, androgen levels and incretin secretion in obese women.

METHODS

Seventeen overweight women (BMI > 28) were recruited to the study. Glucose, insulin and lipid metabolism were evaluated by euglycemic clamp technique, indirect calorimetry and an oral glucose tolerance test (OGTT). Insulin sensitivity was calculated as glucose disposal rate (GDR) and insulin sensitivity index (ISI), and also by HOMA-IR. Insulin secretion rate (ISR) was calculated from plasma C-peptide measurements. Secretion of glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) was measured during an oral glucose tolerance test. Abdominal fat distribution was assessed by dual x-ray absorptiometry scan and computed tomography.

RESULTS

Ten women completed the intervention. The subjects lost an average 11% of their baseline weight. There was a significant loss of subcutaneous abdominal fatty tissue (p < 0.01) and intra-abdominal fatty tissue (p =0.05). Whole body (HOMA-IR) (p < 0.05) insulin sensitivity increased significantly, but peripheral (ISI) insulin sensitivity was unaltered after weight loss. GIP increased (p < 0.05) and GLP-1 was unaltered after the dietary intervention. Insulin responses did not differ before and after dietary intervention, however, a significant increase in insulin clearance (p < 0.05) was observed. The weight loss resulted in a significant decrease in free testosterone.

CONCLUSION

A VLCD is an effective weight loss treatment, which results in an immediate improvement in several metabolic parameters.

Biomarkers of mitochondrial content in skeletal muscle of healthy young human subjects

Skeletal muscle mitochondrial content varies extensively between human subjects. Biochemical measures of mitochondrial proteins, enzyme activities and lipids are often used as markers of mitochondrial content and muscle oxidative capacity (OXPHOS). The purpose of this study was to determine how closely associated these commonly used biochemical measures are to muscle mitochondrial content and OXPHOS. Sixteen young healthy male subjects were recruited for this study. Subjects completed a graded exercise test to determine maximal oxygen uptake (VO2peak) and muscle biopsies were obtained from the vastus lateralis. Mitochondrial content was determined using transmission electron microscopy imaging and OXPHOS was determined as the maximal coupled respiration in permeabilized fibres. Biomarkers of interest were citrate synthase (CS) activity, cardiolipin content, mitochondrial DNA content (mtDNA), complex I–V protein content, and complex I–IV activity. Spearman correlation coefficient tests and Lin's concordance tests were applied to assess the absolute and relative association between the markers and mitochondrial content or OXPHOS. Subjects had a large range of VO2peak (range 29.9–71.6ml min−1 kg−1) and mitochondrial content (4–15% of cell volume).Cardiolipin content showed the strongest association with mitochondrial content followed by CS and complex I activities. mtDNA was not related to mitochondrial content. Complex IV activity showed the strongest association with muscle oxidative capacity followed by complex II activity.We conclude that cardiolipin content, and CS and complex I activities are the biomarkers that exhibit the strongest association with mitochondrial content, while complex IV activity is strongly associated with OXPHOS capacity in human skeletal muscle.

Lymphocytes mitochondrial physiology as biomarker of energy metabolism during fasted and fed conditions

Mitochondria are central coordinators of energy metabolism, and changes of their physiology have long been associated with metabolic disorders. Thus, observations of energy dynamics in different cell types are of utmost importance. Therefore, tools with quick and easy handling are needed for consistent evaluations of such interventions. In this paper, our main hypothesis is that during different nutritional situations lymphocytes mitochondrial physiology could be associated with the metabolism of other cell types, such as cardiomyocytes, and consequently be used as metabolic biomarker. Blood lymphocytes and heart muscle fibers were obtained from both fed and 24 h-fasted mice, and mitochondrial analysis was assessed by high-resolution respirometry and western blotting. Carbohydrate-linked oxidation and fatty acid oxidation were significantly higher after fasting. Carnitine palmitoil transferase 1 and uncouple protein 2 contents were increased in the fasted group, while the glucose transporters 1 and 4 and the ratio phosphorylated AMP-activated protein kinase/AMPK did not change between groups. In summary, under a nutritional status modification, mitochondria demonstrated earlier adaptive capacity than other metabolic sensors such as glucose transporters and AMPK, suggesting the accuracy of mitochondria physiology of lymphocytes as biomarker for metabolic changes.

5'-AMP Activated Protein Kinase is Involved in the Regulation of Myocardial β-Oxidative Capacity in Mice

5′-adenosine monophosphate-activated protein kinase (AMPK) is considered central in regulation of energy status and substrate utilization within cells. In heart failure the energetic state is compromised and substrate metabolism is altered. We hypothesized that this could be linked to changes in AMPK activity and we therefore investigated mitochondrial oxidative phosphorylation capacity from the oxidation of long- and medium-chain fatty acids (LCFA and MCFA) in cardiomyocytes from young and old mice expressing a dominant negative AMPKα2 (AMPKα2-KD) construct and their wildtype (WT) littermates. We found a 35–45% (P < 0.05) lower mitochondrial capacity for oxidizing MCFA in AMPKα2-KD of both age-groups, compared to WT. This coincided with marked decreases in protein expression (19/29%, P < 0.05) and activity (14/21%, P < 0.05) of 3-hydroxyacyl-CoA-dehydrogenase (HAD), in young and old AMPKα2-KD mice, respectively, compared to WT. Maximal LCFA oxidation capacity was similar in AMPKα2-KD and WT mice independently of age implying that LCFA-transport into the mitochondria was unaffected by loss of AMPK activity or progressing age. Expression of regulatory proteins of glycolysis and glycogen breakdown showed equivocal effects of age and genotype. These results illustrate that AMPK is necessary for normal mitochondrial function in the heart and that decreased AMPK activity may lead to an altered energetic state as a consequence of reduced capacity to oxidize MCFA. We did not identify any clear aging effects on mitochondrial function.

Calorie restriction: is AMPK a key sensor and effector?

Dietary restriction can extend life span in most organisms tested to date, suggesting that mechanisms sensing nutrient and energy availability might regulate longevity. The AMP-activated protein kinase (AMPK) has emerged as a key energy sensor with the ability to transcriptionally reprogram the cell and metabolically adapt to external cues. In this review, we will discuss the possible role of AMPK in the beneficial effects of calorie restriction on health and life span.

Dietary restriction increases skeletal muscle mitochondrial respiration but not mitochondrial content in C57BL/6 mice

Dietary restriction (DR) is suggested to induce mitochondrial biogenesis, although recently this has been challenged. Here we determined the impact of 1, 9 and 18 months of 30% DR in male C57BL/6 mice on key mitochondrial factors and on mitochondrial function in skeletal muscle, relative to age-matched ad libitum (AL) controls. We examined proteins and mRNAs associated with mitochondrial biogenesis and measured mitochondrial respiration in permeabilised myofibres using high resolution respirometry. 30% DR, irrespective of duration, had no effect on citrate synthase activity. In contrast, total and nuclear protein levels of PGC-1α, mRNA levels of several mitochondrial associated proteins (Pgc-1α, Nrf1, Core 1, Cox IV, Atps) and cytochrome c oxidase content were increased in skeletal muscle of DR mice. Furthermore, a range of mitochondrial respiration rates were increased significantly by DR, with DR partially attenuating the age-related decline in respiration observed in AL controls. Therefore, DR did not increase mitochondrial content, as determined by citrate synthase, in mouse skeletal muscle. However, it did induce a PGC-1α adaptive response and increased mitochondrial respiration. Thus, we suggest that a functionally 'efficient' mitochondrial electron transport chain may be a critical mechanism underlying DR, rather than any net increase in mitochondrial content per se.

Skeletal muscle mitochondrial function in polycystic ovarian syndrome

OBJECTIVE

Polycystic ovarian syndrome (PCOS) is associated with skeletal muscle insulin resistance (IR), which has been linked to decreased mitochondrial function. We measured mitochondrial respiration in lean and obese women with and without PCOS using high-resolution respirometry.

METHODS

Hyperinsulinemic-euglycemic clamps (40  mU/min per m(2)) and muscle biopsies were performed on 23 women with PCOS (nine lean (body mass index (BMI) <25 kg/m(2)) and 14 obese (BMI >25 kg/m(2))) and 17 age- and weight-matched controls (six lean and 11 obese). Western blotting and high-resolution respirometry was used to determine mitochondrial function.

RESULTS

Insulin sensitivity decreased with PCOS and increasing body weight. Mitochondrial respiration with substrates for complex I and complex I+II were similar in all groups, and PCOS was not associated with a decrease in mitochondrial content as measured by mitochondrial DNA/genomic DNA. We found no correlation between mitochondrial function and indices of insulin sensitivity.

CONCLUSIONS

In contrast to previous reports, we found no evidence that skeletal muscle mitochondrial respiration is reduced in skeletal muscle of women with PCOS compared with control subjects. Furthermore, mitochondrial content did not differ between our control and PCOS groups. These results question the causal relationship between reduced mitochondrial function and skeletal muscle IR in PCOS.

The role of mitochondria in insulin resistance and type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) has been related to alterations of oxidative metabolism in insulin-responsive tissues. Overt T2DM can present with acquired or inherited reductions of mitochondrial oxidative phosphorylation capacity, submaximal ADP-stimulated oxidative phosphorylation and plasticity of mitochondria and/or lower mitochondrial content in skeletal muscle cells and potentially also in hepatocytes. Acquired insulin resistance is associated with reduced insulin-stimulated mitochondrial activity as the result of blunted mitochondrial plasticity. Hereditary insulin resistance is frequently associated with reduced mitochondrial activity at rest, probably due to diminished mitochondrial content. Lifestyle and pharmacological interventions can enhance the capacity for oxidative phosphorylation and mitochondrial content and improve insulin resistance in some (pre)diabetic cases. Various mitochondrial features can be abnormal but are not necessarily responsible for all forms of insulin resistance. Nevertheless, mitochondrial abnormalities might accelerate progression of insulin resistance and subsequent organ dysfunction via increased production of reactive oxygen species. This Review discusses the association between mitochondrial function and insulin sensitivity in various tissues, such as skeletal muscle, liver and heart, with a main focus on studies in humans, and addresses the effects of therapeutic strategies that affect mitochondrial function and insulin sensitivity.