Insulin resistance and impaired mitochondrial function in obese adolescent girls

The Role of Mitochondrial Adaptation and Metabolic Flexibility in the Pathophysiology of Obesity and Insulin Resistance: an Updated Overview

PURPOSE OF REVIEW

The term "metabolic flexibility" denotes the dynamic responses of the cellular oxidative machinery in order to adapt to changes in energy substrate availability. A progressive loss of this adaptive capacity has been implicated in the development of obesity-related comorbidities. Mitochondria are dynamic intracellular organelles which play a fundamental role in energy metabolism, and the mitochondrial adaptation to environmental challenges may be viewed as the functional component of metabolic flexibility. Herein, we attempt to comprehensively review the available evidence regarding the role of mitochondrial adaptation and metabolic flexibility in the pathogenesis of obesity and related morbidities, namely insulin resistance states and non-alcoholic fatty liver disease (NAFLD).

RECENT FINDINGS

Overall, there is a concrete body of evidence to support the presence of impaired mitochondrial adaptation as a principal component of systemic metabolic inflexibility in conditions related to obesity. There are still many unresolved questions regarding the relationship between the gradual loss of mitochondrial adaptability and the progression of obesity-related complications, such as causality issues, the timely appearance and reversibility of the described disturbances, and the generalizability of the findings to the mitochondrial content of every affected tissue or organ. The evidence regarding the causality between the observed associations remains inconclusive, although most of the available data points towards a bidirectional, potentially mutually amplifying relationship. The spectrum of NAFLD is of particular interest, since functional and pathological changes in the course of its development closely mirror the progression of dysmetabolism, if not constituting a dynamic component of the latter.

Association of dietary insulinaemic potential and odds of non-alcoholic fatty liver disease among adults: A case-control study

BACKGROUND

Hyperinsulinaemia is considered as a major risk factor for the development of a myriad of chronic diseases. We examined the association between the dietary insulinaemic potential and the odds of non-alcoholic fatty liver disease (NAFLD) among Iranian adults.

METHODS

After being subjected to a liver ultrasound, 166 patients with NAFLD and 200 controls were included in the study. The dietary intakes and the physical activity levels of the participants were evaluated using a validated semi-quantitative food frequency questionnaire and the International Physical Activity Questionnaire (short IPAQ), respectively. The insulinaemic potential of the diet was assessed by computing the scores of the Empirical Dietary Index for Hyperinsulinemia (EDIH) and the Empirical Dietary Index for Insulin Resistance (EDIR).

RESULTS

Compared with the control subjects, patients with NAFLD were significantly older; had higher values for body mass index, fasting blood sugar, triglycerides, low-density lipoprotein cholesterol, total cholesterol and alanine transaminase; and were more likely to smoke. Moreover, NAFLD patients had significant lower levels of high-density lipoprotein cholesterol and were less likely to perform physical activity. The risk of NAFLD was higher in the individuals in the highest tertile of the EDIH (odds ratio [OR] = 2.79; 95% confidence interval [CI] = 1.32-5.90; p value for trend < 0.05) and EDIR (OR = 2.42; 95% CI = 1.22-4.79; p value for trend < 0.05) compared to those in the lowest tertile of these scores.

CONCLUSIONS

Our study indicates that a higher dietary insulinaemic potential is associated with an increased risk of NAFLD.

Quantitative Imaging of Body Composition

Body composition refers to the amount and distribution of lean tissue, adipose tissue, and bone in the human body. Lean tissue primarily consists of skeletal muscle; adipose tissue comprises mostly abdominal visceral adipose tissue and abdominal and nonabdominal subcutaneous adipose tissue. Hepatocellular and myocellular lipids are also fat pools with important metabolic implications. Importantly, body composition reflects generalized processes such as increased adiposity in obesity and age-related loss of muscle mass known as sarcopenia.In recent years, body composition has been extensively studied quantitatively to predict overall health. Multiple imaging methods have allowed precise estimates of tissue types and provided insights showing the relationship of body composition to varied pathologic conditions. In this review article, we discuss different imaging methods used to quantify body composition and describe important anatomical locations where target tissues can be measured.

High-fat diet affects skeletal muscle mitochondria comparable to pressure overload-induced heart failure

In heart failure, high-fat diet (HFD) may exert beneficial effects on cardiac mitochondria and contractility. Skeletal muscle mitochondrial dysfunction in heart failure is associated with myopathy. However, it is not clear if HFD affects skeletal muscle mitochondria in heart failure as well. To induce heart failure, we used pressure overload (PO) in rats fed normal chow or HFD. Interfibrillar mitochondria (IFM) and subsarcolemmal mitochondria (SSM) from gastrocnemius were isolated and functionally characterized. With PO heart failure, maximal respiratory capacity was impaired in IFM but increased in SSM of gastrocnemius. Unexpectedly, HFD affected mitochondria comparably to PO. In combination, PO and HFD showed additive effects on mitochondrial subpopulations which were reflected by isolated complex activities. While PO impaired diastolic as well as systolic cardiac function and increased glucose tolerance, HFD did not affect cardiac function but decreased glucose tolerance. We conclude that HFD and PO heart failure have comparable effects leading to more severe impairment of IFM. Glucose tolerance seems not causally related to skeletal muscle mitochondrial dysfunction. The additive effects of HFD and PO may suggest accelerated skeletal muscle mitochondrial dysfunction when heart failure is accompanied with a diet containing high fat.

Sex-specific metabolic functions of adipose Lipocalin-2

OBJECTIVE

Lipocalin-2 (LCN2) is a secreted protein involved in innate immunity and has also been associated with several cardiometabolic traits in both mouse and human studies. However, the causal relationship of LCN2 to these traits is unclear, and most studies have examined only males.

METHODS

Using adeno-associated viral vectors we expressed LCN2 in either adipose or liver in a tissue specific manner on the background of a whole-body Lcn2 knockout or wildtype mice. Metabolic phenotypes including body weight, body composition, plasma and liver lipids, glucose homeostasis, insulin resistance, mitochondrial phenotyping, and metabolic cage studies were monitored.

RESULTS

We studied the genetics of LCN2 expression and associated clinical traits in both males and females in a panel of 100 inbred strains of mice (HMDP). The natural variation in Lcn2 expression across the HMDP exhibits high heritability, and genetic mapping suggests that it is regulated in part by Lipin1 gene variation. The correlation analyses revealed striking tissue dependent sex differences in obesity, insulin resistance, hepatic steatosis, and dyslipidemia. To understand the causal relationships, we examined the effects of expression of LCN2 selectively in liver or adipose. On a Lcn2-null background, LCN2 expression in white adipose promoted metabolic disturbances in females but not males. It acted in an autocrine/paracrine manner, resulting in mitochondrial dysfunction and an upregulation of inflammatory and fibrotic genes. On the other hand, on a null background, expression of LCN2 in liver had no discernible impact on the traits examined despite increasing the levels of circulating LCN2 more than adipose LCN2 expression. The mechanisms underlying the sex-specific action of LCN2 are unclear, but our results indicate that adipose LCN2 negatively regulates its receptor, LRP2 (or megalin), and its repressor, ERα, in a female-specific manner and that the effects of LCN2 on metabolic traits are mediated in part by LRP2.

CONCLUSIONS

Following up on our population-based studies, we demonstrate that LCN2 acts in a highly sex- and tissue-specific manner in mice. Our results have important implications for human studies, emphasizing the importance of sex and the tissue source of LCN2.

Identifying Metabolomic Profiles of Insulinemic Dietary Patterns

The food-based empirical dietary index for hyperinsulinemia (EDIH) score assesses the insulinemic potential of diet. This cross-sectional study evaluated associations between EDIH scores from food frequency questionnaires with c-peptide concentrations and with 448 metabolites, from fasting plasma samples, in multivariable linear regression analyses. Metabolites were measured with liquid chromatography tandem mass spectroscopy. Using a robust two-stage study design, discovery of metabolite associations was conducted among 1109 Women's Health Initiative (WHI) Hormone Therapy (HT) trial participants and results replicated in an independent dataset of 810 WHI Observational Study (OS) participants. In both discovery and replication datasets, statistical significance was based on the false-discovery rate adjusted P < 0.05. In the multivariable-adjusted analyses, EDIH was significantly associated with c-peptide concentrations among 919 women (HT & OS) with c-peptide data. On average, c-peptide concentrations were 18% higher (95% CI, 6%, 32%; P-trend < 0.0001) in EDIH quintile 5 compared to quintile 1. Twenty-six metabolites were significantly associated with EDIH in the discovery dataset, and 19 of these were replicated in the validation dataset. Nine metabolites were found to decrease in abundance with increasing EDIH scores and included: C14:0 CE, C16:1 CE, C18:1 CE, C18:3 CE, C20:3 CE, C20:5 CE, C36:1 PS plasmalogen, trigonelline, and eicosapentanoate, whereas the 10 metabolites observed to increase with increasing EDIH scores were: C18:2 SM, C36:3 DAG, C36:4 DAG-A, C51:3 TAG, C52:3 TAG, C52:4, TAG, C54:3 TAG, C54:4 TAG, C54:6 TAG, and C10:2 carnitine. Cholesteryl esters, phospholipids, acylglycerols, and acylcarnitines may constitute circulating metabolites that are associated with insulinemic dietary patterns.

The impact of a short-term high-fat diet on mitochondrial respiration, reactive oxygen species production, and dynamics in oxidative and glycolytic skeletal muscles of young rats

Multiple aspects of mitochondrial function and dynamics remain poorly studied in the skeletal muscle of pediatric models in response to a short-term high-fat diet (HFD). This study investigated the impact of a short-term HFD on mitochondrial function and dynamics in the oxidative soleus (SOL) and glycolytic extensor digitorum longus (EDL) muscles in young rats. Young male Wistar rats were submitted to either HFD or normal chow (NCD) diets for 14 days. Permeabilized myofibers from SOL and EDL were prepared to assess mitochondrial respiration and reactive oxygen species (ROS) production. The expression and content of protein involved in mitochondrial metabolism and dynamics (fusion/fission) were also quantified. While no effects of HFD was observed on mitochondrial respiration when classical complex I and II substrates were used, both SOL and EDL of rats submitted to a HFD displayed higher basal and ADP-stimulated respiration rates when Malate + Palmitoyl-L-carnitine were used as substrates. HFD did not alter ROS production and markers of mitochondrial content. The expression of CPT1b was significantly increased in SOL and EDL of HFD rats. Although the expression of UCP3 was increased in SOL and EDL muscles from HFD rats, mitochondrial coupling efficiency was not altered. In SOL of HFD rats, the transcript levels of Mfn2 and Fis1 were significantly upregulated. The expression and content of proteins regulating mitochondrial dynamics was not modulated by HFD in the EDL. Finally, DRP1 protein content was increased by over fourfold in the SOL of HFD rats. Taken altogether, our findings show that exposing young animals to short-term HFD results in an increased capacity of skeletal muscle mitochondria to oxidize fatty acids, without altering ROS production, coupling efficiency, and mitochondrial content. Our results also highlight that the impact of HFD on mitochondrial dynamics appears to be muscle specific.

Metabolomics allows the discrimination of the pathophysiological relevance of hyperinsulinism in obese prepubertal children

BACKGROUND/OBJECTIVES

Insulin resistance (IR) is the cornerstone of the obesity-associated metabolic derangements observed in obese children. Targeted metabolomics was employed to explore the pathophysiological relevance of hyperinsulinemia in childhood obesity in order to identify biomarkers of IR with potential clinical application.

SUBJECTS/METHODS

One hundred prepubertal obese children (50 girls/50 boys, 50% IR and 50% non-IR in each group), underwent an oral glucose tolerance test for usual carbohydrate and lipid metabolism determinations. Fasting serum leptin, total and high molecular weight-adiponectin and high-sensitivity C-reactive protein (CRP) levels were measured and the metabolites showing significant differences between IR and non-IR groups in a previous metabolomics study were quantified. Enrichment of metabolic pathways (quantitative enrichment analysis) and the correlations between lipid and carbohydrate metabolism parameters, adipokines and serum metabolites were investigated, with their discriminatory capacity being evaluated by receiver operating characteristic (ROC) analysis.

RESULTS

Twenty-three metabolite sets were enriched in the serum metabolome of IR obese children (P<0.05, false discovery rate (FDR)<5%). The urea cycle, alanine metabolism and glucose-alanine cycle were the most significantly enriched pathways (P_FDR<0.00005). The high correlation between metabolites related to fatty acid oxidation and amino acids (mainly branched chain and aromatic amino acids) pointed to the possible contribution of mitochondrial dysfunction in IR. The degree of body mass index-standard deviation score (BMI-SDS) excess did not correlate with any of the metabolomic components studied. In the ROC analysis, the combination of leptin and alanine showed a high IR discrimination value in the whole cohort (area under curve, AUC_ALL=0.87), as well as in boys (AUC_M=0.84) and girls (AUC_F=0.91) when considered separately. However, the specific metabolite/adipokine combinations with highest sensitivity were different between the sexes.

CONCLUSIONS

Combined sets of metabolic, adipokine and metabolomic parameters can identify pathophysiological relevant IR in a single fasting sample, suggesting a potential application of metabolomic analysis in clinical practice to better identify children at risk without using invasive protocols.

Insulin Resistance in Youth Without Diabetes Is Not Related to Muscle Mitochondrial Dysfunction

Context

Obesity, insulin resistance (IR), and diabetes are increasing in youth, especially in girls. IR is associated with muscle mitochondrial dysfunction in youth and adults with diabetes. However, it is unknown whether this relationship is present in youth prior to development of diabetes.

Objective

Assess IR and mitochondrial function, including sex differences, in nondiabetic youth.

Design

Cross-sectional study of youth in the Exploring Perinatal Outcomes among Children, Resistance to InSulin in Type 1 And Type 2 diabetes, and Androgens and Insulin Resistance Study cohorts.

Setting

Academic medical university.

Participants

Two hundred seventy-five youth, 13 to 19 years old [43% males: 17.1 (16.52, 17.63) years, body mass index z-score (BMI-Z) 0.36, 64.7% Tanner 5; 57% females: 17.2 (16.43, 17.67) years, BMI-Z 0.72, 78.9% Tanner 5].

Interventions

Fasting laboratories, oral glucose tolerance test, and 31P magnetic resonance spectroscopy.

Main Outcome Measures

IR [triglyceride:high-density lipoprotein (HDL) ratio, Matsuda index, and homeostasis model for insulin resistance (HOMA-IR)] and muscle mitochondrial function (adenosine 5'-diphosphate time constant and oxidative phosphorylation rate).

Results

Compared with males, females were more insulin resistant, with higher triglyceride:HDL ratio [1.95 (1.30, 2.79) vs 1.69 (1.21, 2.23), P = 0.042], HOMA-IR [3.18 (2.42, 4.39) vs 2.76 (2.02, 4.08), P = 0.035], and fasting free fatty acids (FFAs) and lower Matsuda score [3.98 (2.71, 5.96) vs 5.39 (3.43, 7.57), P < 0.001]. After adjustment for the higher BMI and Tanner stage and lower physical activity levels seen in females, there were no sex differences in mitochondrial function nor in any IR measure except FFAs. We did not find an association between measures of IR and mitochondrial function.

Conclusions

The greater IR seen in adolescent girls vs boys is mostly explained by differences in BMI and physical activity. Mitochondrial function does not appear to be related to IR in a large cohort of nondiabetic youth.

Muscle Strength and Fitness in Pediatric Obesity: a Systematic Review from the European Childhood Obesity Group

The increasing prevalence of paediatric obesity and related metabolic complications has been mainly associated with lower aerobic fitness while less is known regarding potential musculoskeletal impairments. The purpose of the present systematic review was to report the evidence regarding muscular fitness in children and adolescents with obesity. A systematic article search was conducted between November 2014 and June 2015 using MEDLINE, EMBASE, CINAHL psycINFO, SPORTDiscus and SocINDEX. Articles published in English and reporting results on muscle strength and muscular fitness in children and adolescents aged 6 to 18 years were eligible. Of 548 identified titles, 36 studies were included for analyses. While laboratory-based studies described higher absolute muscular fitness in youth with obesity compared with their lean peers, these differences are negated when corrected for body weight and lean mass, then supporting field-based investigations. All interventional studies reviewed led to improved muscular fitness in youth with obesity. Children and adolescents with obesity display impaired muscular fitness compared to healthy-weight peers, which seems mainly due to factors such as excessive body weight and increased inertia of the body. Our analysis also points out the lack of information regarding the role of age, maturation or sex in the current literature and reveals that routinely used field tests analysing overall daily muscular fitness in children with obesity provide satisfactory results when compared to laboratory-based data.

Obesity-related differences in neuromuscular fatigue in adolescent girls

PURPOSE

The aim of this study was to investigate the effect of obesity on neuromuscular fatigue in adolescent girls.

METHODS

Twelve lean (13.6 ± 0.8 years) and 12 obese (13.9 ± 0.9 years) girls repeated 5-s maximal voluntary contractions (MVC) of the knee extensors until the generated torque reached 55 % of its initial value. Magnetic stimulations were delivered to the femoral nerve every five MVCs to follow the course of voluntary activation (VA) and potentiated twitch torque (Qtwpot).

RESULTS

Torque reached 55 % of its initial value after 52.6 ± 20.4 and 74.9 ± 22.8 repetitions in obese and lean girls, respectively (p < 0.01). Furthermore, the decline of VA was smaller in obese girls (p < 0.001). In contrast, Qtwpot decreased to a greater extent in obese girls (p < 0.05).

CONCLUSIONS

Obese girls fatigue faster than their lean counterparts. The peripheral factors mainly account for fatigue in obese girls, whereas central factors are mainly involved in lean girls.

Mitochondria: diversity in the regulation of the NLRP3 inflammasome

Recent studies have identified new roles for mitochondria in the regulation of autoinflammatory processes. Emerging data suggests that the release of danger signals from mitochondria in response to stress and infection promotes the formation of the inflammatory signaling platform known as inflammasomes. Activation of inflammasomes by damaged mitochondria results in caspase-1-dependent secretion of the inflammatory cytokines interleukin-1β (IL-1β) and IL-18, and an inflammatory form of cell death referred to as pyroptosis. Here, we review recently described mechanisms that have been proposed to be involved in mitochondria-mediated regulation of inflammasome activation and inflammation. In addition, we highlight how aberrant regulation of mitochondria-induced inflammasome activation centrally contributes to the inflammatory process that is responsible for obesity and associated metabolic diseases.

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Effects of recombinant human growth hormone (rhGH) administration on body composition and cardiovascular risk factors in obese adolescent girls

Background

Obesity is associated with a relative deficiency of growth hormone, which is predictive of greater visceral fat and markers of cardiovascular risk. The study’s purpose was to use recombinant human growth hormone (rhGH) as a physiologic probe to assess the effects of reversing obesity-related GH deficiency on body composition, cardiovascular risk markers, and insulin resistance.

Methods

22 obese girls 13–21 years old were followed for a randomized 6-month trial of rhGH vs. placebo/no treatment. At baseline and 6-months, DXA was performed for body composition, MRI to measure visceral, subcutaneous and total adipose tissue (VAT, SAT and TAT), and fasting blood drawn for IGF-1, inflammatory cardiovascular risk markers [soluble intercellular adhesion molecule (sICAM), high sensitivity CRP], lipids and HbA1C. An oral glucose tolerance test (OGTT) was performed. Twelve girls completed the 6-month visit. Baseline and mean 6-month change were compared between the groups using the Student t-test and the relationship between variables was determined through multiple regression analysis.

Results

After 6-months, the rhGH group maintained IGF-1 levels, and had decreases in total cholesterol (p = 0.03), sICAM-1 (p = 0.04) and HbA1C (p = 0.03) compared to placebo/no treatment. The rhGH group trended towards greater decreases in LDL and 2-hour OGTT glucose. Glucose tolerance did not worsen with rhGH administration.

Conclusions

Administering rhGH in small doses is able to stabilize IGF-1 levels in obesity. We have also shown that rhGH administration leads to an improvement in some markers of cardiovacular risk with without adversely affecting glucose tolerance.

Trial registration

Clinical Trial Registration Number: NCT01169103.

Early oxidative shifts in mouse skeletal muscle morphology with high-fat diet consumption do not lead to functional improvements

Short‐term consumption of a high‐fat diet (HFD) can result in an oxidative shift in adult skeletal muscle. However, the impact of HFD on young, growing muscle is largely unknown. Thus, 4‐week‐old mice were randomly divided into sedentary HFD (60% kcal from fat), sedentary standard chow (control), or exercise‐trained standard chow. Tibialis anterior (TA) and soleus muscles were examined for morphological and functional changes after 3 weeks. HFD consumption increased body and epididymal fat mass and induced whole body glucose intolerance versus control mice. Compared to controls, both HFD and exercise‐trained TA muscles displayed a greater proportion of oxidative fibers and a trend for an increased succinate dehydrogenase (SDH) content. The soleus also displayed an oxidative shift with increased SDH content in HFD mice. Despite the aforementioned changes, palmitate oxidation rates were not different between groups. To determine if the adaptive changes with HFD manifest as a functional improvement, all groups performed pre‐ and postexperiment aerobic exercise tests. As expected, exercise‐trained mice improved significantly compared to controls, however, no improvement was observed in HFD mice. Interestingly, capillary density was lower in HFD muscles; a finding which may contribute to the lack of functional differences seen with HFD despite the oxidative shift in skeletal muscle morphology. Taken together, our data demonstrate that young, growing muscle exhibits early oxidative shifts in response to a HFD, but these changes do not translate to functional benefits in palmitate oxidation, muscle fatigue resistance, or whole body exercise capacity.

Young, growing animals consuming a short‐term high‐fat diet (HFD) exhibit morphological and metabolic changes akin to that of exercise‐trained mice. We hypothesized that these changes may manifest in a functional outcome that was similar to the exercise‐trained mice. Despite these muscle adaptations with HFD consumption, no improvement in exercise or in situ muscle performance was observed, suggesting other limiting factors (e.g., decreased capillary density) in the HFD mice.

Crosstalk between circulating peroxisome proliferator-activated receptor gamma, adipokines and metabolic syndrome in obese subjects

BACKGROUND

Peroxisome proliferator-activated receptor gamma (PPARγ) has direct and indirect function in adipokines production process. We aimed to assess the possible influence of circulating PPARγ on relative risk of metabolic syndrome and also examine the association between circulating PPARγ and adipokines levels among obese subjects.

METHODS

A total of 96 obese subjects (body mass index (BMI) ≥30) were included in the current cross-sectional study. We assessed the body composition with the use of Body Composition Analyzer BC-418MA - Tanita. The MetS (metabolic syndrome) was defined based on the National Cholesterol Education Program Adult Treatment Panel III. All baseline blood samples were obtained following an overnight fasting. Serum concentrations of adipokines including Retinol binding protein 4 (RBP4), omentin-1, vaspin, progranulin, nesfatin-1 and circulating PPARγ was measured with the use of an enzyme-linked immunosorbent assay method. Statistical analyses were performed using software package used for statistical analysis (SPSS).

RESULTS

We found main association between circulating PPARγ and body composition in obese population. The risk of metabolic syndrome in subjects with higher concentration of PPARγ was 1.9 fold in compared with lower concentration of PPARγ after adjustment for age, sex and BMI. There was significant association between PPARγ and adipokines, specially nesfatin-1 and progranulin. Defined adipokines pattern among participants demonstrated the markedly higher concentration of vaspin, RBP4 and nesfatin-1 in participants with MetS compared to non-MetS subjects.

CONCLUSIONS

It appears all of studied adipokines might have association with PPARγ level and might simultaneously be involve in some common pathway to make susceptible obese subjects for MetS.