Selective activation of PPARgamma in skeletal muscle induces endogenous production of adiponectin and protects mice from diet-induced insulin resistance

Isolation and purification of high molecular weight adiponectin from human plasma fraction

Adiponectin, a crucial protein hormone originating from adipose tissue, regulates key metabolic processes, including lipid metabolism, mitochondrial activity, and insulin sensitivity. These pleiotropic roles of adiponectin, along with its inverse correlation with metabolic disorders such as obesity, type II diabetes, and atherosclerosis, establish this protein as a potential therapeutic target. However, due to this complexity, challenges have arisen in its production with a natural conformation in bacterial or mammalian expression systems, hindering clinical translation. Furthermore, while inducers for adiponectin secretion or chemical agonists targeting adiponectin receptors have shown promise in laboratory settings, clinical studies with these agents have not yet been conducted. This study proposes a method for isolating and purifying natural high molecular weight (HMW) adiponectin from discarded plasma fractions during the conventional pharmaceutical protein manufacturing process. The process involved Cohn-Oncley fractionation, initial chromatography using reduced cellufine formyl, and subsequent purification via DEAE Sepharose chromatography. Characterization involved gel electrophoresis and biological assays on a hepatocyte cell-line. The purification process effectively captured adiponectin from the I + III paste, demonstrating that this fraction contained a significant portion of total plasma adiponectin. The two-step chromatography led to highly purified HMW adiponectin, confirmed by native-PAGE showing a 780 kDa multimeric complex. Biological assessments demonstrated normal downstream signaling, with HMW adiponectin inducing AMPK phosphorylation. This study demonstrates the feasibility of obtaining purified HMW adiponectin by repurposing plasma fractionation processes. It offers a promising avenue for the HMW adiponectin production, tapping into HMW adiponectin's therapeutic potential against metabolic disorders while optimizing plasma resource utilization in healthcare.

Mitochondrial-derived microprotein MOTS-c attenuates immobilization-induced skeletal muscle atrophy by suppressing lipid infiltration

Mitochondrial open reading frame of the 12S ribosomal RNA type-c (MOTS-c), a mitochondrial microprotein, has been described as a novel regulator of glucose and lipid metabolism. In addition to its role as a metabolic regulator, MOTS-c prevents skeletal muscle atrophy in high fat-fed mice. Here, we examined the preventive effect of MOTS-c on skeletal muscle mass, using an immobilization-induced muscle atrophy model, and explored its underlying mechanisms. Male C57BL/6J mice (10 wk old) were randomly assigned to one of the three experimental groups: nonimmobilization control group (sterilized water injection), immobilization control group (sterilized water injection), and immobilization and MOTS-c-treated group (15 mg/kg/day MOTS-c injection). We used casting tape for the immobilization experiment. After 8 days of the experimental period, skeletal muscle samples were collected and used for Western blotting, RNA sequencing, and lipid and collagen assays. Immobilization reduced ∼15% of muscle mass, whereas MOTS-c treatment attenuated muscle loss, with only a 5% reduction. MOTS-c treatment also normalized phospho-AKT, phospho-FOXO1, and phospho-FOXO3a expression levels and reduced circulating inflammatory cytokines, such as interleukin-1b (IL-1β), interleukin-6 (IL-6), chemokine C-X-C motif ligand 1 (CXCL1), and monocyte chemoattractant protein 1 (MCP-1), in immobilized mice. Unbiased RNA sequencing and its downstream analyses demonstrated that MOTS-c modified adipogenesis-modulating gene expression within the peroxisome proliferator-activated receptor (PPAR) pathway. Supporting this observation, muscle fatty acid levels were lower in the MOTS-c-treated group than in the casted control mice. These results suggest that MOTS-c treatment inhibits skeletal muscle lipid infiltration by regulating adipogenesis-related genes and prevents immobilization-induced muscle atrophy.NEW & NOTEWORTHY MOTS-c, a mitochondrial microprotein, attenuates immobilization-induced skeletal muscle atrophy. MOTS-c treatment improves systemic inflammation and skeletal muscle AKT/FOXOs signaling pathways. Furthermore, unbiased RNA sequencing and subsequent assays revealed that MOTS-c prevents lipid infiltration in skeletal muscle. Since lipid accumulation is one of the common pathologies among other skeletal muscle atrophies induced by aging, obesity, cancer cachexia, and denervation, MOTS-c treatment could be effective in other muscle atrophy models as well.

The global perspective on peroxisome proliferator-activated receptor γ (PPARγ) in ectopic fat deposition: A review

Excessive expansion of adipocytes can have unhealthy consequences as excess free fatty acids enter other tissues and cause ectopic fat deposition by resynthesizing triglycerides. This lipid accumulation in various tissues is harmful and can increase the risk of related metabolic diseases such as type II diabetes, cardiovascular disease, and insulin resistance. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily that play a key role in energy metabolism as fatty acid metabolism sensors, and peroxisome proliferator-activated receptor γ (PPARγ) is the main subtype responsible for fat cell differentiation and adipogenesis. In this paper, we introduce the main structure and function of PPARγ and its regulatory role in the process of lipogenesis in the liver, kidney, skeletal muscle, and pancreas. This information can serve as a reference for further understanding the regulatory mechanisms and measures of the PPAR family in the process of ectopic fat deposition.

Muscle Injuries Induce a Prostacyclin-PPARγ/PGC1a-FAO Spike That Boosts Regeneration

It is well-known that muscle regeneration declines with aging, and aged muscles undergo degenerative atrophy or sarcopenia. While exercise and acute injury are both known to induce muscle regeneration, the molecular signals that help trigger muscle regeneration have remained unclear. Here, mass spectrometry imaging (MSI) is used to show that injured muscles induce a specific subset of prostanoids during regeneration, including PGG1, PGD2, and the prostacyclin PGI2. The spike in prostacyclin promotes skeletal muscle regeneration via myoblasts, and declines with aging. Mechanistically, the prostacyclin spike promotes a spike in PPARγ/PGC1a signaling, which induces a spike in fatty acid oxidation (FAO) to control myogenesis. LC-MS/MS and MSI further confirm that an early FAO spike is associated with normal regeneration, but muscle FAO became dysregulated during aging. Functional experiments demonstrate that the prostacyclin-PPARγ/PGC1a-FAO spike is necessary and sufficient to promote both young and aged muscle regeneration, and that prostacyclin can synergize with PPARγ/PGC1a-FAO signaling to restore aged muscles' regeneration and physical function. Given that the post-injury prostacyclin-PPARγ-FAO spike can be modulated pharmacologically and via post-exercise nutrition, this work has implications for how prostacyclin-PPARγ-FAO might be fine-tuned to promote regeneration and treat muscle diseases of aging.

Skeletal muscle fiber composition may modify the effect of nutrition on body composition in young females

BACKGROUND AND AIM

The aim of this study was to investigate the hypothesis that healthy, normal-weight females with greater proportions and sizes of the oxidative muscle fibers would also be characterized by a healthier body composition compared with individuals with increased glycolytic fibers, even if both follow similar nutritional plans.

METHODS AND RESULTS

Vastus lateralis muscle fiber-type composition, body composition through dual-energy X-ray absorptiometry, and dietary intakes through questionnaire were evaluated in twenty-two young, healthy, non-obese females (age: 21.3±1.8yrs, body mass: 67.5±6.2 kg, body height: 1.66±0.05m, body mass index (BMI): 24.2±2.6  kg m^-2). The participants were allocated into two groups according to their type I muscle fibers percentage [high (HI) and low (LI)]. The participants of the LI group were characterized by significantly higher body mass, fat mass, BMI, and cross-sectional and percentage cross-sectional area (%CSA) of type IIx muscle fibers compared with participants of the HI group (p < 0.021). In contrast, the HI group was characterized by higher cross-sectional and %CSA of type I muscle fibers compared with the LI group (p < 0.038). Significant correlations were observed between body fat mass, lean body mass, total energy intake, fat energy intake, and %CSAs of type I and IIx muscle fibers (r: -0.505 to 0.685; p < 0.05).

CONCLUSION

In conclusion, this study suggests that muscle fiber composition is an important factor that at least partly could explain the observed differential inter-individual responses of the body composition to nutrition in female individuals. Increased %CSAs of type I muscle fibers seem to act as a protective mechanism against obesity and favor a healthier body composition, neutralizing the negative effect of increased caloric fats intake on body composition, probably because of their greater oxidative metabolic properties and fat utilization capacities. In contrast, female individuals with low type I and high type IIx %CSAs of type I seem to be more metabolically inflexible and dietinduced obesity prone, even if they consume fewer total daily calories and fats.

Exercise attenuates angiotensinⅡ-induced muscle atrophy by targeting PPARγ/miR-29b

BACKGROUND

Exercise is beneficial for muscle atrophy. Peroxisome proliferator-activated receptor gamma (PPARγ) and microRNA-29b (miR-29b) have been reported to be responsible for angiotensinⅡ (AngⅡ)-induced muscle atrophy. However, it is unclear whether exercise can protect AngⅡ-induced muscle atrophy by targeting PPARγ/miR-29b.

METHODS

Skeletal muscle atrophy in both the control group and the run group was established by AngⅡ infusion; after 1 week of exercise training, the mice were sacrificed, and muscle weight was determined. Myofiber size was measured by hematoxylin-eosin and wheat-germ agglutinin staining. Apoptosis was evaluated by terminal deoxynucleotidyl transferase dUTP nick end labeling staining. The expression level of muscle atrogenes, including F-box only protein 32 (FBXO32, also called Atrogin-1) and muscle-specific RING-finger 1 (MuRF-1), the phosphorylation level of protein kinase B (PKB, also called AKT)/forkhead box O3A (FOXO3A)/mammalian target of rapamycin (mTOR) pathway proteins, the expression level of PPARγ and apoptosis-related proteins, including B-cell lymphoma-2 (Bcl-2), Bcl-2-associated X (Bax), cysteine-aspartic acid protease 3 (caspase-3), and cleaved-caspase-3, were determined by western blot. The expression level of miR-29b was checked by reverse-transcription quantitative polymerase chain reaction. A PPARγ inhibitor (T0070907) or adeno-associated virus serotype-8 (AAV8)-mediated miR-29b overexpression was used to demonstrate whether PPARγ activation or miR-29b inhibition mediates the beneficial effects of exercise in AngⅡ-induced muscle atrophy.

RESULTS

Exercise can significantly attenuate AngⅡ-induced muscle atrophy, which is demonstrated by increased skeletal muscle weight, cross-sectional area of myofiber, and activation of AKT/mTOR signaling and by decreased atrogenes expressions and apoptosis. In AngⅡ-induced muscle atrophy mice models, PPARγ was elevated whereas miR-29b was decreased by exercise. The protective effects of exercise in AngⅡ-induced muscle atrophy were inhibited by a PPARγ inhibitor (T0070907) or adeno-associated virus serotype-8 (AAV8)-mediated miR-29b overexpression.

CONCLUSION

Exercise attenuates AngⅡ-induced muscle atrophy by activation of PPARγ and suppression of miR-29b.

Slc12a8 in the lateral hypothalamus maintains energy metabolism and skeletal muscle functions during aging

Sarcopenia and frailty are urgent socio-economic problems worldwide. Here we demonstrate a functional connection between the lateral hypothalamus (LH) and skeletal muscle through Slc12a8, a recently identified nicotinamide mononucleotide transporter, and its relationship to sarcopenia and frailty. Slc12a8-expressing cells are mainly localized in the LH. LH-specific knockdown of Slc12a8 in young mice decreases activity-dependent energy and carbohydrate expenditure and skeletal muscle functions, including muscle mass, muscle force, intramuscular glycolysis, and protein synthesis. LH-specific Slc12a8 knockdown also decreases sympathetic nerve signals at neuromuscular junctions and β2-adrenergic receptors in skeletal muscle, indicating the importance of the LH-sympathetic nerve-β2-adrenergic receptor axis. LH-specific overexpression of Slc12a8 in aged mice significantly ameliorates age-associated decreases in energy expenditure and skeletal muscle functions. Our results highlight an important role of Slc12a8 in the LH for regulation of whole-body metabolism and skeletal muscle functions and provide insights into the pathogenesis of sarcopenia and frailty during aging.

Contribution of HIF-P4H isoenzyme inhibition to metabolism indicates major beneficial effects being conveyed by HIF-P4H-2 antagonism

Hypoxia-inducible factor (HIF) prolyl 4-hydroxylases (HIF-P4Hs 1-3) are druggable targets in renal anemia, where pan-HIF-P4H inhibitors induce an HIF-mediated erythropoietic response. HIF is also a potent regulator of energy metabolism. Preclinical data suggest that HIF-P4Hs could also be treatment targets for metabolic dysfunction, although the contributions of the isoenzymes and various tissues to the metabolic phenotype are inadequately understood. We used mouse lines that were gene-deficient for HIF-P4Hs 1-3 and two preclinical pan-HIF-P4H inhibitors to study the contributions of the isoenzymes to the anthropometric and metabolic outcome and HIF response. Both inhibitors induced the HIF response in wild-type white adipose tissue (WAT), liver and skeletal muscle and alleviated metabolic dysfunction during a six-week treatment period, but they did not alter healthy metabolism. Our data show that HIF-P4H-1 contributed especially to skeletal muscle and WAT metabolism and that its loss lowered body weight and serum cholesterol levels upon aging. HIF-P4H-3-mediated effects on the liver and WAT and its loss increased body weight, adiposity, liver weight and triglyceride levels, WAT inflammation and cholesterol levels and resulted in hyperglycemia and insulin resistance, especially upon aging. HIF-P4H-2 contributed to all the tissues studied and its inhibition lowered body and liver weight and serum cholesterol levels and improved glucose tolerance. There was specificity in the regulation of metabolic HIF target mRNAs in tissues, very few being regulated by the inhibition of all isoenzymes, thus suggesting a potential for selective therapeutic tractability. Altogether, these data provide specifications for the development of HIF-P4H inhibitors for metabolic diseases.

Cancer-Related Cachexia: The Vicious Circle between Inflammatory Cytokines, Skeletal Muscle, Lipid Metabolism and the Possible Role of Physical Training

Cachexia is a multifactorial and multi-organ syndrome that is a major cause of morbidity and mortality in late-stage chronic diseases. The main clinical features of cancer-related cachexia are chronic inflammation, wasting of skeletal muscle and adipose tissue, insulin resistance, anorexia, and impaired myogenesis. A multimodal treatment has been suggested to approach the multifactorial genesis of cachexia. In this context, physical exercise has been found to have a general effect on maintaining homeostasis in a healthy life, involving multiple organs and their metabolism. The purpose of this review is to present the evidence for the relationship between inflammatory cytokines, skeletal muscle, and fat metabolism and the potential role of exercise training in breaking the vicious circle of this impaired tissue cross-talk. Due to the wide-ranging effects of exercise training, from the body to the behavior and cognition of the individual, it seems to be able to improve the quality of life in this syndrome. Therefore, studying the molecular effects of physical exercise could provide important information about the interactions between organs and the systemic mediators involved in the overall homeostasis of the body.

Lower citrate synthase activity, mitochondrial complex expression, and fewer oxidative myofibers characterize skeletal muscle from growth-restricted fetal sheep

Skeletal muscle from the late gestation sheep fetus with intrauterine growth restriction (IUGR) has evidence of reduced oxidative metabolism. Using a sheep model of placental insufficiency and IUGR, we tested the hypothesis that by late gestation, IUGR fetal skeletal muscle has reduced capacity for oxidative phosphorylation because of intrinsic deficits in mitochondrial respiration. We measured mitochondrial respiration in permeabilized muscle fibers from biceps femoris (BF) and soleus (SOL) from control and IUGR fetal sheep. Using muscles including BF, SOL, tibialis anterior (TA), and flexor digitorum superficialis (FDS), we measured citrate synthase (CS) activity, mitochondrial complex subunit abundance, fiber type distribution, and gene expression of regulators of mitochondrial biosynthesis. Ex vivo mitochondrial respiration was similar in control and IUGR muscle. However, CS activity was lower in IUGR BF and TA, indicating lower mitochondrial content, and protein expression of individual mitochondrial complex subunits was lower in IUGR TA and BF in a muscle-specific pattern. IUGR TA, BF, and FDS also had lower expression of type I oxidative fibers. Fiber-type shifts that support glycolytic instead of oxidative metabolism may be advantageous for the IUGR fetus in a hypoxic and nutrient-deficient environment, whereas these adaptions may be maladaptive in postnatal life.

Role of Plant Derived Products Through Exhilarating Peroxisome Proliferator Activated Receptor-γ (ppar-γ) in the Amelioration of Obesity Induced Insulin Resistance

Abstract:

Insulin resistance is an elemental facet of the etiology of diabetes mellitus and the principal relating factor between obesity and diabetes. Oxidative stress, lipotoxicity, inflammation and receptor dysfunction are the underlying determinants of insulin resistance commencement in metabolic illnesses. ppar-γ is a nuclear transcription factor whose activation or inhibition directly influences insulin resistance and controls glucose and lipid homeostasis by modulating gene expression. Synthetic ligands of ppar-γ are therapeutically employed to counter the hyper-glycaemia associated with obesity and type 2 diabetes, but they possess severe side effects. In the modern era, bioactive phytochemicals have been employed in the drug development process and a considerable investigation has recently been initiated to analyze the ppar-γ activating ability of diverse phytochemicals. In this review, we outlined the role of phytochemicals in insulin resistance treatment through ppar-γ activation.

Skeletal muscle bioenergetic health and function in people living with HIV: association with glucose tolerance and alcohol use

At-risk alcohol use is prevalent and increases dysglycemia among people living with human immunodeficiency virus (PLWH). Skeletal muscle (SKM) bioenergetic dysregulation is implicated in dysglycemia and type 2 diabetes. The objective of this study was to determine the relationship between at-risk alcohol, glucose tolerance, and SKM bioenergetic function in PLWH. Thirty-five PLWH (11 females, 24 males, age: 53 ± 9 yr, body mass index: 29.0 ± 6.6 kg/m2) with elevated fasting glucose enrolled in the ALIVE-Ex study provided medical history and alcohol use information [Alcohol Use Disorders Identification Test (AUDIT)], then underwent an oral glucose tolerance test (OGTT) and SKM biopsy. Bioenergetic health and function and mitochondrial volume were measured in isolated myoblasts. Mitochondrial gene expression was measured in SKM. Linear regression adjusting for age, sex, and smoking was performed to examine the relationship between glucose tolerance (2-h glucose post-OGTT), AUDIT, and their interaction with each outcome measure. Negative indicators of bioenergetic health were significantly (P < 0.05) greater with higher 2-h glucose (proton leak) and AUDIT (proton leak, nonmitochondrial oxygen consumption, and bioenergetic health index). Mitochondrial volume was increased with the interaction of higher 2-h glucose and AUDIT. Mitochondrial gene expression decreased with higher 2-h glucose (TFAM, PGC1B, PPARG, MFN1), AUDIT (MFN1, DRP1, MFF), and their interaction (PPARG, PPARD, MFF). Decreased expression of mitochondrial genes were coupled with increased mitochondrial volume and decreased bioenergetic health in SKM of PLWH with higher AUDIT and 2-h glucose. We hypothesize these mechanisms reflect poorer mitochondrial health and may precede overt SKM bioenergetic dysregulation observed in type 2 diabetes.

Perinatal Nutritional and Metabolic Pathways: Early Origins of Chronic Lung Diseases

Lung development is not completed at birth, but expands beyond infancy, rendering the lung highly susceptible to injury. Exposure to various influences during a critical window of organ growth can interfere with the finely-tuned process of development and induce pathological processes with aberrant alveolarization and long-term structural and functional sequelae. This concept of developmental origins of chronic disease has been coined as perinatal programming. Some adverse perinatal factors, including prematurity along with respiratory support, are well-recognized to induce bronchopulmonary dysplasia (BPD), a neonatal chronic lung disease that is characterized by arrest of alveolar and microvascular formation as well as lung matrix remodeling. While the pathogenesis of various experimental models focus on oxygen toxicity, mechanical ventilation and inflammation, the role of nutrition before and after birth remain poorly investigated. There is accumulating clinical and experimental evidence that intrauterine growth restriction (IUGR) as a consequence of limited nutritive supply due to placental insufficiency or maternal malnutrition is a major risk factor for BPD and impaired lung function later in life. In contrast, a surplus of nutrition with perinatal maternal obesity, accelerated postnatal weight gain and early childhood obesity is associated with wheezing and adverse clinical course of chronic lung diseases, such as asthma. While the link between perinatal nutrition and lung health has been described, the underlying mechanisms remain poorly understood. There are initial data showing that inflammatory and nutrient sensing processes are involved in programming of alveolarization, pulmonary angiogenesis, and composition of extracellular matrix. Here, we provide a comprehensive overview of the current knowledge regarding the impact of perinatal metabolism and nutrition on the lung and beyond the cardiopulmonary system as well as possible mechanisms determining the individual susceptibility to CLD early in life. We aim to emphasize the importance of unraveling the mechanisms of perinatal metabolic programming to develop novel preventive and therapeutic avenues.

Interstitial Cell Remodeling Promotes Aberrant Adipogenesis in Dystrophic Muscles

Fibrosis and fat replacement in skeletal muscle are major complications that lead to a loss of mobility in chronic muscle disorders, such as muscular dystrophy. However, the in vivo properties of adipogenic stem and precursor cells remain unclear, mainly due to the high cell heterogeneity in skeletal muscles. Here, we use single-cell RNA sequencing to decomplexify interstitial cell populations in healthy and dystrophic skeletal muscles. We identify an interstitial CD142-positive cell population in mice and humans that is responsible for the inhibition of adipogenesis through GDF10 secretion. Furthermore, we show that the interstitial cell composition is completely altered in muscular dystrophy, with a near absence of CD142-positive cells. The identification of these adipo-regulatory cells in the skeletal muscle aids our understanding of the aberrant fat deposition in muscular dystrophy, paving the way for treatments that could counteract degeneration in patients with muscular dystrophy.

Evaluation of vitamin A status on myogenic gene expression and muscle fiber characteristics

A randomized complete block design experiment with 30 yearling crossbred steers (average BW = 436.3 ± 39.8 kg) fed a steam-flaked corn-based diet was used to evaluate the effects dietary vitamin A (Rovimix A 1000; DSM Nutritional Products Ltd., Sisseln, SUI) supplementation on myogenic gene expression and skeletal muscle fiber characteristics during the finishing phase. Steers were blocked by BW (n = 5 blocks; 6 steers/block), randomly assigned to pens (n = 2 steers/pen), and one of the following treatments: no added vitamin A (0 IU; 0.0 IU/kg of dietary dry matter intake of additional vitamin A), vitamin A supplemented at the estimated requirement (2,200 IU; 2,200 IU/kg of dietary dry matter (DM) of additional vitamin A), and vitamin A supplemented at 5× the estimated requirement (11,000 IU; 11,000 IU/kg of dietary DM of additional vitamin A). After all treatments underwent a 91-d vitamin A depletion period, additional vitamin A was top-dressed at feeding via a ground corn carrier. Blood, longissimus muscle, and liver biopsy samples were obtained on days 0, 28, 56, 84, and 112. Biopsy samples were used for immunohistochemical and mRNA analysis. Sera and liver samples were used to monitor circulating vitamin A and true vitamin A status of the cattle. Expression for myosin heavy chain (MHC)-I diminished and rebounded (P = 0.04) over time. The intermediate fiber type, MHC-IIA, had a similar pattern of expression (P = 0.01) to that of MHC-I. On day 84, C/EBPβ expression was also the greatest (P = 0.03). The pattern of PPARγ (P < 0.01) and PPARδ (P < 0.01) expression seemed to mimic that of MHC-I expression, increasing from days 84 to 112. Distribution of MHC-IIA demonstrated a change over time (P = 0.02). Muscle fiber cross-sectional area increased by day (P < 0.01) for each MHC with the notable increase between days 0 and 56. Total nuclei density decreased (P = 0.02) over time. Cells positive for only Myf5 increased (P < 0.01) in density early in the feeding period, then declined, indicating that satellite cells were fusing into fibers. The dual-positive (PAX7+Myf5) nuclei also peaked (P < 0.01) around day 56 then declined. These data indicated that gene expression associated with oxidative proteins may be independent of vitamin A status in yearling cattle.

Adiponectin: Structure, Physiological Functions, Role in Diseases, and Effects of Nutrition

Adiponectin (a protein consisting of 244 amino acids and characterized by a molecular weight of 28 kDa) is a cytokine that is secreted from adipose tissues (adipokine). Available evidence suggests that adiponectin is involved in a variety of physiological functions, molecular and cellular events, including lipid metabolism, energy regulation, immune response and inflammation, and insulin sensitivity. It has a protective effect on neurons and neural stem cells. Adiponectin levels have been reported to be negatively correlated with cancer, cardiovascular disease, and diabetes, and shown to be affected (i.e., significantly increased) by proper healthy nutrition. The present review comprehensively overviews the role of adiponectin in a range of diseases, showing that it can be used as a biomarker for diagnosing these disorders as well as a target for monitoring the effectiveness of preventive and treatment interventions.

The Effects of Muscle Cell Aging on Myogenesis

The process of myogenesis gradually deteriorates as the skeletal muscle ages, contributing to muscle mass loss. The aim of this study is to investigate the effect of senescence/aging on skeletal myogenesis, in vitro. A model of multiple cell divisions of C2C12 myoblasts was used to replicate cell senescence. Control and aged myoblasts were investigated during myogenesis, i.e., at days 0, 2, and 6of differentiation. SA-β-gal activity and comet assay were used as markers of aging and DNA damage. Flow cytometry was performed to characterize potential differences in cell cycle between control and aged cells. Alterations in the mRNA and/or protein expression of myogenic regulatory factors (MRFs), IGF-1 isoforms, apoptotic, atrophy, inflammatory, metabolic and aging-related factors were evaluated. Compared with the control cells, aged myoblasts exhibited G0/G1 cell cycle arrest, DNA damage, increased SA-β-gal activity, and increased expression of aging-related factors p16 and p21 during differentiation. Moreover, aged myoblasts showed a reduction in the expression of MRFs and metabolic/anabolic factors, along with an increased expression of apoptotic, atrophy and inflammatory factors. A diminished differentiation capacity characterized the aged myoblasts which, in combination with the induction of apoptotic and atrophy factors, indicated a disrupted myogenic lineage in the senescent muscle cells.

The Endocannabinoid System and PPARs: Focus on Their Signalling Crosstalk, Action and Transcriptional Regulation

Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear receptors including PPARα, PPARγ, and PPARβ/δ, acting as transcription factors to regulate the expression of a plethora of target genes involved in metabolism, immune reaction, cell differentiation, and a variety of other cellular changes and adaptive responses. PPARs are activated by a large number of both endogenous and exogenous lipid molecules, including phyto- and endo-cannabinoids, as well as endocannabinoid-like compounds. In this view, they can be considered an extension of the endocannabinoid system. Besides being directly activated by cannabinoids, PPARs are also indirectly modulated by receptors and enzymes regulating the activity and metabolism of endocannabinoids, and, vice versa, the expression of these receptors and enzymes may be regulated by PPARs. In this review, we provide an overview of the crosstalk between cannabinoids and PPARs, and the importance of their reciprocal regulation and modulation by common ligands, including those belonging to the extended endocannabinoid system (or "endocannabinoidome") in the control of major physiological and pathophysiological functions.

Molecular and Biochemical Pathways of Catalpol in Alleviating Diabetes Mellitus and Its Complications

Catalpol isolated from Rehmannia glutinosa is a potent antioxidant and investigated against many disorders. This review appraises the key molecular pathways of catalpol against diabetes mellitus and its complications. Multiple search engines including Google Scholar, PubMed, and Science Direct were used to retrieve publications containing the keywords “Catalpol”, “Type 1 diabetes mellitus”, “Type 2 diabetes mellitus”, and “diabetic complications”. Catalpol promotes IRS-1/PI3K/AKT/GLUT2 activity and suppresses Phosphoenolpyruvate carboxykinase (PEPCK) and Glucose 6-phosphatase (G6Pase) expression in the liver. Catalpol induces myogenesis by increasing MyoD/MyoG/MHC expression and improves mitochondria function through the AMPK/PGC-1α/PPAR-γ and TFAM signaling in skeletal muscles. Catalpol downregulates the pro-inflammatory markers and upregulates the anti-inflammatory markers in adipose tissues. Catalpol exerts antioxidant properties through increasing superoxide dismutase (sod), catalase (cat), and glutathione peroxidase (gsh-px) activity in the pancreas and liver. Catalpol has been shown to have anti-oxidative, anti-inflammatory, anti-apoptosis, and anti-fibrosis properties that in turn bring beneficial effects in diabetic complications. Its nephroprotective effect is related to the modulation of the AGE/RAGE/NF-κB and TGF-β/smad2/3 pathways. Catalpol produces a neuroprotective effect by increasing the expression of protein Kinase-C (PKC) and Cav-1. Furthermore, catalpol exhibits a cardioprotective effect through the apelin/APJ and ROS/NF-κB/Neat1 pathway. Catalpol stimulates proliferation and differentiation of osteoblast cells in high glucose condition. Lastly, catalpol shows its potential in preventing neurodegeneration in the retina with NF-κB downregulation. Overall, catalpol exhibits numerous beneficial effects on diabetes mellitus and diabetic complications.

Physiological Genomics of Adaptation to High-Altitude Hypoxia

Population genomic studies of humans and other animals at high altitude have generated many hypotheses about the genes and pathways that may have contributed to hypoxia adaptation. Future advances require experimental tests of such hypotheses to identify causal mechanisms. Studies to date illustrate the challenge of moving from lists of candidate genes to the identification of phenotypic targets of selection, as it can be difficult to determine whether observed genotype-phenotype associations reflect causal effects or secondary consequences of changes in other traits that are linked via homeostatic regulation. Recent work on high-altitude models such as deer mice has revealed both plastic and evolved changes in respiratory, cardiovascular, and metabolic traits that contribute to aerobic performance capacity in hypoxia, and analyses of tissue-specific transcriptomes have identified changes in regulatory networks that mediate adaptive changes in physiological phenotype. Here we synthesize recent results and discuss lessons learned from studies of high-altitude adaptation that lie at the intersection of genomics and physiology.

The effects of physical activity on adipokines in individuals with overweight/obesity across the lifespan: A narrative review

This narrative review summarizes current knowledge on the effects of physical activity (PA) on adipokine levels in individuals with overweight and obesity. Approximately 90 investigations including randomized control, cross-sectional and longitudinal studies that reported on the effects of a single session of PA (acute) or long-term PA (chronic) on adipokine levels in individuals with overweight/obesity were reviewed. The findings support the notion that there is consensus on the benefits of chronic exercise training-regardless of the mode (resistance vs. aerobic), intensity and cohort (healthy vs. diabetes)-on adipokine levels (such as tumour necrosis factor-alpha, interleukin-6, adiponectin, visfatin, omentin-1 and leptin). However, several confounding factors (frequency, intensity, time and type of exercise) can alter the magnitude of the effects of an acute exercise session. Available evidence suggests that PA, as a part of routine lifestyle behaviour, improves obesity complications by modulating adipokine levels. However, additional research is needed to help identify the most effective interventions to elicit the most beneficial changes in adipokine levels in individuals with overweight/obesity.

Cell autonomous requirement of neurofibromin (Nf1) for postnatal muscle hypertrophic growth and metabolic homeostasis

BACKGROUND

Neurofibromatosis type 1 (NF1) is a multi-organ disease caused by mutations in neurofibromin 1 (NF1). Amongst other features, NF1 patients frequently show reduced muscle mass and strength, impairing patients' mobility and increasing the risk of fall. The role of Nf1 in muscle and the cause for the NF1-associated myopathy are mostly unknown.

METHODS

To dissect the function of Nf1 in muscle, we created muscle-specific knockout mouse models for NF1, inactivating Nf1 in the prenatal myogenic lineage either under the Lbx1 promoter or under the Myf5 promoter. Mice were analysed during prenatal and postnatal myogenesis and muscle growth.

RESULTS

Nf1^Lbx1 and Nf1^Myf5 animals showed only mild defects in prenatal myogenesis. Nf1^Lbx1 animals were perinatally lethal, while Nf1^Myf5 animals survived only up to approximately 25 weeks. A comprehensive phenotypic characterization of Nf1^Myf5 animals showed decreased postnatal growth, reduced muscle size, and fast fibre atrophy. Proteome and transcriptome analyses of muscle tissue indicated decreased protein synthesis and increased proteasomal degradation, and decreased glycolytic and increased oxidative activity in muscle tissue. High-resolution respirometry confirmed enhanced oxidative metabolism in Nf1^Myf5 muscles, which was concomitant to a fibre type shift from type 2B to type 2A and type 1. Moreover, Nf1^Myf5 muscles showed hallmarks of decreased activation of mTORC1 and increased expression of atrogenes. Remarkably, loss of Nf1 promoted a robust activation of AMPK with a gene expression profile indicative of increased fatty acid catabolism. Additionally, we observed a strong induction of genes encoding catabolic cytokines in muscle Nf1^Myf5 animals, in line with a drastic reduction of white, but not brown adipose tissue.

CONCLUSIONS

Our results demonstrate a cell autonomous role for Nf1 in myogenic cells during postnatal muscle growth required for metabolic and proteostatic homeostasis. Furthermore, Nf1 deficiency in muscle drives cross-tissue communication and mobilization of lipid reserves.

PPARs and Microbiota in Skeletal Muscle Health and Wasting

Skeletal muscle is a major metabolic organ that uses mostly glucose and lipids for energy production and has the capacity to remodel itself in response to exercise and fasting. Skeletal muscle wasting occurs in many diseases and during aging. Muscle wasting is often accompanied by chronic low-grade inflammation associated to inter- and intra-muscular fat deposition. During aging, muscle wasting is advanced due to increased movement disorders, as a result of restricted physical exercise, frailty, and the pain associated with arthritis. Muscle atrophy is characterized by increased protein degradation, where the ubiquitin-proteasomal and autophagy-lysosomal pathways, atrogenes, and growth factor signaling all play an important role. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor family of transcription factors, which are activated by fatty acids and their derivatives. PPARs regulate genes that are involved in development, metabolism, inflammation, and many cellular processes in different organs. PPARs are also expressed in muscle and exert pleiotropic specialized responses upon activation by their ligands. There are three PPAR isotypes, viz., PPARα, -β/δ, and -γ. The expression of PPARα is high in tissues with effective fatty acid catabolism, including skeletal muscle. PPARβ/δ is expressed more ubiquitously and is the predominant isotype in skeletal muscle. It is involved in energy metabolism, mitochondrial biogenesis, and fiber-type switching. The expression of PPARγ is high in adipocytes, but it is also implicated in lipid deposition in muscle and other organs. Collectively, all three PPAR isotypes have a major impact on muscle homeostasis either directly or indirectly. Furthermore, reciprocal interactions have been found between PPARs and the gut microbiota along the gut–muscle axis in both health and disease. Herein, we review functions of PPARs in skeletal muscle and their interaction with the gut microbiota in the context of muscle wasting.

Short-chain fatty acids as potential regulators of skeletal muscle metabolism and function

A key metabolic activity of the gut microbiota is the fermentation of non-digestible carbohydrate, which generates short-chain fatty acids (SCFAs) as the principal end products. SCFAs are absorbed from the gut lumen and modulate host metabolic responses at different organ sites. Evidence suggests that these organ sites include skeletal muscle, the largest organ in humans, which plays a pivotal role in whole-body energy metabolism. In this Review, we evaluate the evidence indicating that SCFAs mediate metabolic cross-talk between the gut microbiota and skeletal muscle. We discuss the effects of three primary SCFAs (acetate, propionate and butyrate) on lipid, carbohydrate and protein metabolism in skeletal muscle, and we consider the potential mechanisms involved. Furthermore, we highlight the emerging roles of these gut-derived metabolites in skeletal muscle function and exercise capacity, present limitations in current knowledge and provide suggestions for future work.

Skeletal muscle adiponectin induction in obesity and exercise

Recent scientific efforts have focused on the detrimental effects that obesity has on the metabolic function of skeletal muscles and whether exercise can improve this dysfunction. In this regard, adiponectin, with important metabolic functions (e.g. insulin-sensitizer and anti-inflammatory), has been recently described as a myokine that acts in an autocrine/paracrine manner. Earlier studies reported that muscle adiponectin could be induced by pro-inflammatory mediators (e.g. lipopolysaccharide), cytokines, and high-fat diets, providing a protective mechanism of this tissue against metabolic insults. However, when metabolic insults such as high-fat diets are sustained this protective response becomes dysregulated, making the skeletal muscle susceptible to metabolic impairments. Recent studies have suggested that exercise could prevent or even reverse this process. Considering that most scientific knowledge on adiponectin dysregulation in obesity is from the study of adipose tissue, the present review summarizes and discusses the literature available to date regarding the effects of obesity on skeletal muscle adiponectin induction, along with the potential effects of different exercise prescriptions on this response in an obesity context.

The effects of vitamin A supplementation during late-stage pregnancy on longissimus dorsi muscle tissue development, birth traits, and growth performance in postnatal Korean native calves

OBJECTIVE

This study investigated the effects of vitamin A (VA) supplementation during late-stage pregnancy on longissimus dorsi muscle tissue development, birth traits, and growth performance of postnatal Korean native calves.

METHODS

In the preliminary experiment, twenty-six pregnant cattle (initial body weight [BW] = 319 kg (standard deviation [SD] = 30.1; 1st parity) were randomly assigned to the control and treatment groups. The treatment group received VA supplementation at 24,000 IU/d from gestational day 225 until delivery. In the main experiment, twelve pregnant cattle (initial BW = 317 kg [SD = 31.3]; 1st parity) were treated with VA supplementation at 24,000 IU/d (gestational days 150 to 225) and at 78,000 IU/d (gestational day 225 until delivery). Serum VA levels were analyzed in pregnant cattle, and the growth performance, gene expression, and serum VA levels were analyzed in the offspring.

RESULTS

Serum VA levels in pregnant cattle decreased the late gestation in both experiments (p<0.001). In the main experiment, pregnant cattle at parturition and offspring at birth in the treatment group had higher serum VA levels than those in the control group (p<0.05). In the treatment groups, an increased birth weight was observed in the main experimental group (p = 0.022), and a tendency (p = 0.088) toward an increased birth weight was observed in the preliminary experimental group. However, no differences were observed in the feed intake, average daily gain, gain-to-feed ratio, or BW of 31-day-old calves. Gene expression was analyzed in longissimus dorsi muscles of 31-day-old calves. VA supplementation in pregnant cattle stimulated postnatal muscle development in offspring by elevating myogenic factor 5 (MYF5), MYF6, and myoblast determination levels (p<0.05). Moreover, preadipocyte-related marker genes such as extracellular signal-regulated kinase 2 and krüppel-like factor 2 were higher in the treatment group than in the control group (p<0.05).

CONCLUSION

VA supplementation (78,000 IU/d) in late-stage pregnant cattle maintained serum VA levels. In addition, 78,000 IU/d VA supplementation increased the birth weight and expression of genes related to muscle and preadipocyte development in offspring. Overall, 78,000 IU/d VA supplementation in pregnant cattle is beneficial to newborn calves.

Delayed access to feed alters expression of genes associated with carbohydrate and amino acid utilization in newly hatched broiler chicks

Newly hatched chicks must transition from lipid-rich yolk to carbohydrate-rich feed as their primary nutrient source, and posthatch delays in access to feed can have long-term negative consequences on growth and metabolism. In this study, impacts of delayed access to feed at hatch on expression of genes related to nutrient uptake and utilization in two metabolically important tissues, liver and muscle, were determined in broiler (meat-type) chickens. Hatched chicks were given access to feed within 3 h (fed) or delayed access to feed for 48 h (delayed fed), and liver and breast muscle were collected from males at hatch and 4 h, 1 day, 2 days, 4 days, and 8 days posthatch for analysis of gene expression. Differential expression of carbohydrate response element-binding protein and peroxisome proliferator-activated receptor-γ in muscle and liver was observed, with results indicating a transitional delay from lipid to carbohydrate metabolism when hatched chicks were not given immediate access to feed. Extended upregulation of insulin receptor mRNA was observed in both tissues in delayed fed birds, suggesting increased sensitivity to circulating levels of the hormone. Developmental delays in expression patterns of cationic amino acid transporters 1 and 2 in both tissues and large neutral amino acid transporter 1 in muscle were also apparent when immediate feed access was prevented. These data suggest that delayed transition to carbohydrate use and altered nutrient transport and utilization within liver and breast muscle are key factors negatively affecting growth and metabolism following delayed feed access in broiler chickens.

PPARγ and Cognitive Performance

Recent findings have led to the discovery of many signaling pathways that link nuclear receptors with human conditions, including mental decline and neurodegenerative diseases. PPARγ agonists have been indicated as neuroprotective agents, supporting synaptic plasticity and neurite outgrowth. For these reasons, many PPARγ ligands have been proposed for the improvement of cognitive performance in different pathological conditions. In this review, the research on this issue is extensively discussed.

Adiponectin in Myopathies

In skeletal muscle, adiponectin has varied and pleiotropic functions, ranging from metabolic, anti-inflammatory, insulin-sensitizing to regenerative roles. Despite the important functions exerted by adiponectin, the study of the hormone in myopathies is still marginal. Myopathies include inherited and non-inherited/acquired neuromuscular pathologies characterized by muscular degeneration and weakness. This review reports current knowledge about adiponectin in myopathies, regarding in particular the role of adiponectin in some hereditary myopathies (as Duchenne muscular dystrophy) and non-inherited/acquired myopathies (such as idiopathic inflammatory myopathies and fibromyalgia). These studies show that some myopathies are characterized by decreased concentration of plasma adiponectin and that hormone replenishment induces beneficial effects in the diseased muscles. Overall, these findings suggest that adiponectin could constitute a future new therapeutic approach for the improvement of the abnormalities caused by myopathies.

Exercise-induced AMPK activation and IL-6 muscle production are disturbed in adiponectin knockout mice

BACKGROUND

Adiponectin exhibits anti-inflammatory actions and is mainly expressed in adipose tissue. However, recent studies have shown that adiponectin can also be secreted by skeletal muscle fibers with autocrine and paracrine effects.

OBJECTIVES

To analyze the role of adiponectin in the metabolic and inflammatory response of skeletal muscle after acute exhaustive aerobic exercise.

METHODS

C57BL/6 (WT) and adiponectin knockout (AdKO) mice underwent four days of treadmill running adaptation and at the fifth day, they performed an incremental maximum test to determine the maximum speed (Vmax). Acute exercise consisted of one hour at 60% Vmax. Mice were euthanatized 2 and 24 h after acute exercise session.

RESULTS

Serum and gastrocnemius adiponectin increased after 2-hours of acute exercise. NEFA concentrations were lower in non-exercise AdKO, and decreased 2-hours after exercise only in WT. No differences were found in muscle triacylglycerol content; however, glycogen content was higher in AdKO in non-exercise (p-value = 0.005). WT showed an increase in AMP-activated protein kinase (AMPK) phosphorylation 2-hours after exercise and its level went back to normal after 24-hours. Otherwise, exercise was not able to modify AMPK in the same way as in AdKO. WT showed an increase in the phosphorylation of ACC (Ser79) 2-hours after exercise and return to normal after 24-hours of exercise (p-value < 0.05), kinects that was not observed in AdKO mice. IL-10 and IL-6 concentration was completely different among genotypes. In WT, these cytokines were increased at 2 (p-value < 0.01) and 24 h (p-value < 0.001) after exercise when compared with AdKO. NF-κBp65 protein and gene expression were not different between genotypes.

CONCLUSION

Adiponectin influences muscle metabolism, mainly by the decrease in exercise-induced AMPK phosphorylation, inflammatory profile and IL-6 in the muscle.

Phenolic Compounds Inhibit 3T3-L1 Adipogenesis Depending on the Stage of Differentiation and Their Binding Affinity to PPARγ

Phenolic compounds might modulate adiposity. Here, we report our observation that polyphenols and phenolic acids inhibit adipogenesis in 3T3-L1 with different intensity depending on the family and the stage of differentiation. While quercetin and resveratrol inhibited lipid accumulation along the whole process of differentiation, apigenin and myricetin were active during the early and latest stages, but not intermediate, contrary to hesperidin. The activity of phenolic acids was limited to the early stages of the differentiation process, except p-coumaric and ellagic acids. This anti-adipogenic effect was accompanied by down-regulation of Scd1 and Lpl. Molecular docking analysis revealed that the inhibitory activity of these phenolic compounds over the early stages of adipogenesis exhibits a significant correlation (r = 0.7034; p = 0.005) with their binding affinity to the ligand-binding domain of PPARγ. Results show that polyphenols and phenolic acids would interact with specific residues of the receptor, which could determine their potential anti-adipogenic activity during the early stages of the differentiation. Residues Phe264, His266, Ile281, Cys285 and Met348 are the most frequently involved in these interactions, which might suggest a crucial role for these amino acids modulating the activity of the receptor. These data contribute to elucidate the possible mechanisms of phenolic compounds in the control of adipogenesis.

The Mitochondrial Basis for Adaptive Variation in Aerobic Performance in High-Altitude Deer Mice

Mitochondria play a central role in aerobic performance. Studies aimed at elucidating how evolved variation in mitochondrial physiology contributes to adaptive variation in aerobic performance can therefore provide a unique and powerful lens to understanding the evolution of complex physiological traits. Here, we review our ongoing work on the importance of changes in mitochondrial quantity and quality to adaptive variation in aerobic performance in high-altitude deer mice. Whole-organism aerobic capacity in hypoxia (VO2max) increases in response to hypoxia acclimation in this species, but high-altitude populations have evolved consistently greater VO2max than populations from low altitude. The evolved increase in VO2max in highlanders is associated with an evolved increase in the respiratory capacity of the gastrocnemius muscle. This appears to result from highlanders having more mitochondria in this tissue, attributed to a higher proportional abundance of oxidative fiber-types and a greater mitochondrial volume density within oxidative fibers. The latter is primarily caused by an over-abundance of subsarcolemmal mitochondria in high-altitude mice, which is likely advantageous for mitochondrial O2 supply because more mitochondria are situated adjacent to the cell membrane and close to capillaries. Evolved changes in gastrocnemius phenotype appear to be underpinned by population differences in the expression of genes involved in energy metabolism, muscle development, and vascular development. Hypoxia acclimation has relatively little effect on respiratory capacity of the gastrocnemius, but it increases respiratory capacity of the diaphragm. However, the mechanisms responsible for this increase differ between populations: lowlanders appear to adjust mitochondrial quantity and quality (i.e., increases in citrate synthase [CS] activity, and mitochondrial respiration relative to CS activity) and they exhibit higher rates of mitochondrial release of reactive oxygen species, whereas highlanders only increase mitochondrial quantity in response to hypoxia acclimation. In contrast to the variation in skeletal muscles, the respiratory capacity of cardiac muscle does not appear to be affected by hypoxia acclimation and varies little between populations. Therefore, evolved changes in mitochondrial quantity and quality make important tissue-specific contributions to adaptive variation in aerobic performance in high-altitude deer mice.

Dietary polyunsaturated fatty acids influence flight muscle oxidative capacity but not endurance flight performance in a migratory songbird

The migratory flights of birds are primarily fueled by fat; however, certain fatty acids may also enhance flight performance and the capacity to oxidize fat. The natural doping hypothesis posits that n-3 long-chain polyunsaturated fatty acids (PUFA) increase membrane fluidity and aerobic and fatty acid oxidative enzymes in the flight muscles, which enables prolonged endurance flight. Support for this hypothesis is mixed, and there is no empirical evidence for increased flight performance. We fed yellow-rumped warblers ( Setophaga coronata coronata) diets enriched in either n-3 or n-6 long-chain PUFA or low in long-chain PUFA and evaluated flight muscle metabolism and endurance performance in a wind tunnel flights lasting up to 6 h. Fatty acid profiles of muscle phospholipids confirmed enrichment of the targeted dietary fatty acids, whereas less substantial differences were observed in adipose triacylglycerol. Contrary to the predictions, feeding n-3 PUFA decreased peroxisome proliferator-activated receptors-β mRNA abundance and muscle oxidative enzyme activities. However, changes in muscle metabolism were not reflected in whole animal performance. No differences were observed in flight performance among diet treatments in terms of endurance capacity, energy costs, or fuel composition. These measures of flight performance were more strongly influenced by body mass and flight duration. Overall, we found no support for the natural doping hypothesis in a songbird. Furthermore, we caution against extending changes in flight muscle metabolic enzymes or fatty acid composition to changes to migratory performance without empirical evidence.

Examining the Potential of Developing and Implementing Use of Adiponectin-Targeted Therapeutics for Metabolic and Cardiovascular Diseases

Cardiometabolic diseases encompass those affecting the heart and vasculature as well as other metabolic problems, such as insulin resistance, diabetes, and non-alcoholic fatty liver disease. These diseases tend to have common risk factors, one of which is impaired adiponectin action. This may be due to reduced bioavailability of the hormone or resistance to its effects on target tissues. A strong negative correlation between adiponectin levels and cardiometabolic diseases has been well-documented and research shown that adiponectin has cardioprotective, insulin sensitizing and direct beneficial metabolic effects. Thus, therapeutic approaches to enhance adiponectin action are widely considered to be desirable. The complexity of adiponectin structure and function has so far made progress in this area less than ideal. In this article we will review the effects and mechanism of action of adiponectin on cardiometabolic tissues, identify scenarios where enhancing adiponectin action would be of clinical value and finally discuss approaches via which this can be achieved.

All-trans retinoic acid increases the expression of oxidative myosin heavy chain through the PPARδ pathway in bovine muscle cells derived from satellite cells

All-trans retinoic acid (ATRA) has been associated with various physiological phenomenon in mammalian adipose tissue and skeletal muscle. We hypothesized that ATRA may affect skeletal muscle fiber type in bovine satellite cell culture through various transcriptional processes. Bovine primary satellite cell (BSC) culture experiments were conducted to determine dose effects of ATRA on expression of genes and protein levels related to skeletal muscle fiber type and metabolism. The semimembranosus from crossbred steers (n = 2 steers), aged approximately 24 mo, were used to isolate BSC for 3 separate assays. Myogenic differentiation was induced using 3% horse serum upon cultured BSC with increasing doses (0, 1, 10, 100, and 1,000 nM) of ATRA. After 96 h of incubation, cells were harvested and used to measure the gene expression of protein kinase B (Akt), AMP-activated protein kinase alpha (AMPK), glucose transporter 4 (GLUT4), myogenin, lipoprotein lipase (LPL), myosin heavy chain (MHC) I, MHC IIA, MHC IIX, insulin like growth factor-1 (IGF-1), Peroxisome proliferator activated receptor gamma (PPARγ), PPARδ, and Smad transcription factor 3 (SMAD3) mRNA relative to ribosomal protein subunit 9 (RPS9). The mRNA expression of LPL was increased (P < 0.05) with 100 and 1,000 nM of ATRA. Expression of GLUT4 was altered (P < 0.05) by ATRA. The treatment of ATRA (1,000 nM) also increased (P < 0.05) mRNA gene expression of SMAD3. The gene expression of both PPARδ and PPARγ were increased (P < 0.05) with 1,000 nM of ATRA. Protein level of PPARδ was also affected (P < 0.05) by 1,000 nM of ATRA and resulted in a greater (P < 0.05) protein level of PPARδ compared to CON. All-trans retinoic acid (10 nM) increased gene expression of MHC I (P < 0.05) compared to CON. Expression of MHC IIA was also influenced (P < 0.05) by ATRA. The mRNA expression of MHC IIX was decreased (P < 0.05) with 100 and 1,000 nM of ATRA. In muscle cells, ATRA may cause muscle fibers to transition towards the MHC isoform that prefers oxidative metabolism, as evidenced by increased expression of genes associated with the MHC I isoform. These changes in MHC isoforms appeared to be brought about by changing PPARδ gene expression and protein levels.

Role of Adiponectin in Central Nervous System Disorders

Adiponectin, the most abundant plasma adipokine, plays an important role in the regulation of glucose and lipid metabolism. Adiponectin also possesses insulin-sensitizing, anti-inflammatory, angiogenic, and vasodilatory properties which may influence central nervous system (CNS) disorders. Although initially not thought to cross the blood-brain barrier, adiponectin enters the brain through peripheral circulation. In the brain, adiponectin signaling through its receptors, AdipoR1 and AdipoR2, directly influences important brain functions such as energy homeostasis, hippocampal neurogenesis, and synaptic plasticity. Overall, based on its central and peripheral actions, recent evidence indicates that adiponectin has neuroprotective, antiatherogenic, and antidepressant effects. However, these findings are not without controversy as human observational studies report differing correlations between plasma adiponectin levels and incidence of CNS disorders. Despite these controversies, adiponectin is gaining attention as a potential therapeutic target for diverse CNS disorders, such as stroke, Alzheimer's disease, anxiety, and depression. Evidence regarding the emerging role for adiponectin in these disorders is discussed in the current review.

Mitochondrial dynamics in cancer-induced cachexia

Cancer-induced cachexia has a negative impact on quality of life and adversely affects therapeutic outcomes and survival rates. It is characterized by, often severe, loss of muscle, with or without loss of fat mass. Insight in the pathophysiology of this complex metabolic syndrome and direct treatment options are still limited, which creates a research demand. Results from recent studies point towards a significant involvement of muscle mitochondrial networks. However, data are scattered and a comprehensive overview is lacking. This paper aims to fill existing knowledge gaps by integrating published data sets on muscle protein or gene expression from cancer-induced cachexia animal models. To this end, a database was compiled from 94 research papers, comprising 11 different rodent models. This was combined with four genome-wide transcriptome datasets of cancer-induced cachexia rodent models. Analysis showed that the expression of genes involved in mitochondrial fusion, fission, ATP production and mitochondrial density is decreased, while that of genes involved ROS detoxification and mitophagy is increased. Our results underline the relevance of including post-translational modifications of key proteins involved in mitochondrial functioning in future studies on cancer-induced cachexia.

Re-highlighting the action of PPARγ in treating metabolic diseases

Peroxisome proliferator-activated receptor γ (PPARγ) is a member of the nuclear receptor family and plays an important role in adipocyte differentiation, glucose homeostasis, and insulin sensitivity. Thiazolidinediones (TZDs), synthetic ligands of PPARγ, have been used for the treatment of diabetes mellitus for two decades. TZDs were expected to be amazing drugs not only for type 2 diabetes but also for metabolic syndrome and atherosclerotic vascular disease because they can reduce both insulin resistance and inflammation in experimental studies. However, serious unwanted effects pushed TZDs back to an optional second-tier drug for type 2 diabetes. Nevertheless, PPARγ is still one of the most important targets for the treatment of insulin resistance and diabetes mellitus, and novel strategies to modulate PPARγ activity to enhance its beneficial effects and reduce unwanted adverse effects are anticipated. Recent studies showed that post-translational modification (PTM) of PPARγ regulates PPARγ activity or stability and may be a novel way to optimize PPARγ activity with reduced adverse effects. In this review, we will focus on recent advances in PTM of PPARγ and the mechanisms regulating PPARγ function as well as in the development of PPARγ modulators or agonists.

An aPPARent Functional Consequence in Skeletal Muscle Physiology via Peroxisome Proliferator-Activated Receptors

Skeletal muscle comprises 30–40% of the total body mass and plays a central role in energy homeostasis in the body. The deregulation of energy homeostasis is a common underlying characteristic of metabolic syndrome. Over the past decades, peroxisome proliferator-activated receptors (PPARs) have been shown to play critical regulatory roles in skeletal muscle. The three family members of PPAR have overlapping roles that contribute to the myriad of processes in skeletal muscle. This review aims to provide an overview of the functions of different PPAR members in energy homeostasis as well as during skeletal muscle metabolic disorders, with a particular focus on human and relevant mouse model studies.

Adiponectin and Its Isoforms in Pathophysiology

Adiponectin circulates in blood in multiple isoforms. High molecular weight (HMW) adiponectin is thought to be most biologically active and promotes glucose uptake, insulin sensitivity, and fatty acid oxidation. In obesity, adiponectin isoform formation is disrupted, leading to an inverse association between metabolic disease and HMW and total adiponectin. Adiponectin isoforms also function as acute-phase reactants influencing inflammation in acute and chronic disease. Interestingly, adiponectin and mortality have a U-shaped association. Unfortunately, data concerning adiponectin and its pathophysiologic function conflict. This is predominantly due to difficulties in adequate measurement of adiponectin isoforms and lack of a gold standard. In this review we provide a general overview of the formation and function of adiponectin and its isoforms under physiologic conditions. We highlight the ways adiponectin isoform formation is disrupted in obesity and its ensuing pathologic conditions. Furthermore, we will elaborate on the role of adiponectin isoforms as inflammatory proteins with respect to cardiac and kidney disease and discuss the association of adiponectin with mortality. Finally, we will provide a historical perspective on the measurement of adiponectin isoforms, current limitations, and future challenges.

Small heterodimer partner deletion prevents hepatic steatosis and when combined with farnesoid X receptor loss protects against type 2 diabetes in mice

Nuclear receptors farnesoid X receptor (FXR) and small heterodimer partner (SHP) are important regulators of bile acid, lipid, and glucose homeostasis. Here, we show that global Fxr ^-/- Shp^-/- double knockout (DKO) mice are refractory to weight gain, glucose intolerance, and hepatic steatosis when challenged with high-fat diet. DKO mice display an inherently increased capacity to burn fat and suppress de novo hepatic lipid synthesis. Moreover, DKO mice were also very active and that correlated well with the observed increase in phosphoenolpyruvate carboxykinase expression, type IA fibers, and mitochondrial function in skeletal muscle. Mechanistically, we demonstrate that liver-specific Shp deletion protects against fatty liver development by suppressing expression of peroxisome proliferator-activated receptor gamma 2 and lipid-droplet protein fat-specific protein 27 beta.

CONCLUSION

These data suggest that Fxr and Shp inactivation may be beneficial to combat diet-induced obesity and uncover that hepatic SHP is necessary to promote fatty liver disease. (Hepatology 2017;66:1854-1865).

Activation of Adiponectin Receptor Regulates Proprotein Convertase Subtilisin/Kexin Type 9 Expression and Inhibits Lesions in ApoE-Deficient Mice

Objective—

The reduced adiponectin levels are associated with atherosclerosis. Adiponectin exerts its functions by activating adiponectin receptor (AdipoR). Proprotein convertase subtilisin kexin type 9 (PCSK9) degrades LDLR protein (low-density lipoprotein receptor) to increase serum LDL-cholesterol levels. PCSK9 expression can be regulated by PPARγ (peroxisome proliferator–activated receptor γ) or SREBP2 (sterol regulatory element-binding protein 2). The effects of AdipoR agonists on PCSK9 and LDLR expression, serum lipid profiles, and atherosclerosis remain unknown.

Approach and Results—

At cellular levels, AdipoR agonists (ADP355 and AdipoRon) induced PCSK9 transcription/expression that solely depended on activation of PPAR-responsive element in the PCSK9 promoter. AdipoR agonists induced PPARγ expression; thus, the AdipoR agonist-activated PCSK9 expression/production was impaired in PPARγ deficient hepatocytes. Meanwhile, AdipoR agonists transcriptionally activated LDLR expression by activating SRE in the LDLR promoter. Moreover, AMP-activated protein kinase α (AMPKα) was involved in AdipoR agonist-activated PCSK9 expression. In wild-type mice, ADP355 increased PCSK9 and LDLR expression and serum PCSK9 levels, which was associated with activation of PPARγ, AMPKα and SREBP2 and reduction of LDL-cholesterol levels. In contrast, ADP355 reduced PCSK9 expression/secretion in apoE-deficient (apoE −/− ) mice, but it still activated hepatic LDLR, PPARγ, AMPKα, and SREBP2. More importantly, ADP355 inhibited lesions in en face aortas and sinus lesions in aortic root in apoE −/− mice with amelioration of lipid profiles.

Conclusions—

Our study demonstrates that AdipoR activation by agonists regulated PCSK9 expression differently in wild-type and apoE −/− mice. However, ADP355 activated hepatic LDLR expression and ameliorated lipid metabolism in both types of mice and inhibited atherosclerosis in apoE −/− mice.

Role of Nuclear Receptors in Exercise-Induced Muscle Adaptations

Skeletal muscle is not only one of the largest, but also one of the most dynamic organs. For example, plasticity elicited by endurance or resistance exercise entails complex transcriptional programs that are still poorly understood. Various signaling pathways are engaged in the contracting muscle fiber and collectively culminate in the modulation of the activity of numerous transcription factors (TFs) and coregulators. Because exercise confers many benefits for the prevention and treatment of a wide variety of pathologies, pharmacological activation of signaling pathways and TFs is an attractive avenue to elicit therapeutic effects. Members of the nuclear receptor (NR) superfamily are of particular interest owing to the presence of well-defined DNA- and ligand-binding domains. In this review, we summarize the current understanding of the involvement of NRs in muscle biology and exercise adaptation.

Skeletal Muscle Nucleo-Mitochondrial Crosstalk in Obesity and Type 2 Diabetes

Skeletal muscle mitochondrial dysfunction, evidenced by incomplete beta oxidation and accumulation of fatty acid intermediates in the form of long and medium chain acylcarnitines, may contribute to ectopic lipid deposition and insulin resistance during high fat diet (HFD)-induced obesity. The present review discusses the roles of anterograde and retrograde communication in nucleo-mitochondrial crosstalk that determines skeletal muscle mitochondrial adaptations, specifically alterations in mitochondrial number and function in relation to obesity and insulin resistance. Special emphasis is placed on the effects of high fat diet (HFD) feeding on expression of nuclear-encoded mitochondrial genes (NEMGs) nuclear receptor factor 1 (NRF-1) and 2 (NRF-2) and peroxisome proliferator receptor gamma coactivator 1 alpha (PGC-1α) in the onset and progression of insulin resistance during obesity and how HFD-induced alterations in NEMG expression affect skeletal muscle mitochondrial adaptations in relation to beta oxidation of fatty acids. Finally, the potential ability of acylcarnitines or fatty acid intermediates resulting from mitochondrial beta oxidation to act as retrograde signals in nucleo-mitochondrial crosstalk is reviewed and discussed.

Fasting increases the phosphorylation of AMPK and expression of sirtuin1 in muscle of adult male northern elephant seals (Mirounga angustirostris)

Fasting typically suppresses thyroid hormone (TH)‐mediated cellular events and increases sirtuin 1 (SIRT1) activity. THs may regulate metabolism through nongenomic pathways and directly through activation of adenosine monophosphate‐activated protein kinase (AMPK). Adult male elephant seals (Mirounga angustirostris) are active, hypermetabolic, and normothermic during their annual breeding fast, which is characterized by stable TH levels. However, the contribution of TH to maintenance of their fasting metabolism is unknown. To investigate the fasting effects on cellular TH‐mediated events and its potential association with SIRT1 and AMPK, we quantified plasma TH levels, mRNA expressions of muscle SIRT1 and TH‐associated genes as well as the phosphorylation of AMPK in adult, male northern elephant seals (n = 10/fasting period) over 8 weeks of fasting (early vs. late). Deiodinase type I (DI1) expression increased twofold with fasting duration suggesting that the potential for TH‐mediated cellular signaling is increased. AMPK phosphorylation increased 61 ± 21% with fasting suggesting that cellular metabolism is increased. The mRNA expression of the TH transporter, monocarboxylate transporter 10 (MCT10), increased 2.4‐fold and the TH receptor (THrβ‐1) decreased 30‐fold suggesting that cellular uptake of T₄ is increased, but its subsequent cellular effects such as activation of AMPK are likely nongenomic. The up‐regulation of SIRT1 mRNA expression (2.6‐fold) likely contributes to the nongenomic activation of AMPK by TH, which may be necessary to maintain the expression of PGC‐1α. These coordinated changes likely contribute to the up‐regulation of mitochondrial metabolism to support the energetic demands associated with prolonged fasting in adult seals.

New targets to alleviate skeletal muscle inflammation: role of microRNAs regulated by adiponectin

Muscle inflammation worsens metabolic disorders as well as devastating myopathies. The hormone adiponectin (ApN) has emerged has a master regulator of inflammation/immunity in several tissues including the skeletal muscle. In this work, we explore whether microRNAs regulated by ApN may represent novel mechanisms for controlling muscle inflammation. By screening arrays, we found miR-711 as a strong candidate for mediating ApN action. Thus, ApN-knockout mice showed decreased muscular expression of miR-711 together with enhanced inflammation/oxidative stress markers, while mice overexpressing ApN showed increased miR-711 levels. Likewise, electrotransfer of the ApN gene in muscle of ApN-knockout mice upregulated miR-711 while reducing inflammation and oxidative stress. Similar data were obtained in murine C2C12 cells or in human primary myotubes treated with ApN. MiR-711 overexpression downregulated several components of the Toll-like receptor-4 (TLR4) pathway, which led to repression of NF-κB activity and downstream pro-inflammatory cytokines. MiR-711 blockade had opposite effects. Moreover, muscle electrotransfer of pre-miR-711 recapitulated in vivo the anti-inflammatory effects observed in vitro. Thus, miR-711, which is upregulated by ApN represses TLR4 signaling, acting therefore as a major mediator of the anti-inflammatory action of ApN. This novel miRNA and its related target genes may open new therapeutic perspectives for controlling muscle inflammation.

Acclimation to hypoxia increases carbohydrate use during exercise in high-altitude deer mice

The low O₂ experienced at high altitude is a significant challenge to effective aerobic locomotion, as it requires sustained tissue O₂ delivery in addition to the appropriate allocation of metabolic substrates. Here, we tested whether high- and low-altitude deer mice (Peromyscus maniculatus) have evolved different acclimation responses to hypoxia with respect to muscle metabolism and fuel use during submaximal exercise. Using F₁ generation high- and low-altitude deer mice that were born and raised in common conditions, we assessed 1) fuel use during exercise, 2) metabolic enzyme activities, and 3) gene expression for key transporters and enzymes in the gastrocnemius. After hypoxia acclimation, highland mice showed a significant increase in carbohydrate oxidation and higher relative reliance on this fuel during exercise at 75% maximal O₂ consumption. Compared with lowland mice, highland mice had consistently higher activities of oxidative and fatty acid oxidation enzymes in the gastrocnemius. In contrast, only after hypoxia acclimation did activities of hexokinase increase significantly in the muscle of highland mice to levels greater than lowland mice. Highland mice also responded to acclimation with increases in muscle gene expression for hexokinase 1 and 2 genes, whereas both populations increased mRNA expression for glucose transporters. Changes in skeletal muscle with acclimation suggest that highland mice had an increased capacity for the uptake and oxidation of circulatory glucose. Our results demonstrate that highland mice have evolved a distinct mode of hypoxia acclimation that involves an increase in carbohydrate use during exercise.

Zinc finger protein 407 overexpression upregulates PPAR target gene expression and improves glucose homeostasis in mice

The peroxisome proliferator-activated receptor (PPAR) family of nuclear receptors is central to the pathophysiology and treatment of metabolic disease through the receptors' ability to regulate the expression of genes involved in glucose homeostasis, adipogenesis, and lipid metabolism. However, the mechanism by which PPAR is regulated remains incompletely understood. We generated a transgenic mouse strain (ZFP-TG) that overexpressed Zfp407 primarily in muscle and heart. Transcriptome analysis by RNA-Seq identified 1,300 differentially expressed genes in the muscle of ZFP-TG mice, among which PPAR target genes were significantly enriched. Among the physiologically important PPARγ target genes, Glucose transporter (Glut)-4 mRNA and protein levels were increased in heart and muscle. The increase in Glut4 and other transcriptional effects of Zfp407 overexpression together decreased body weight and lowered plasma glucose, insulin, and HOMA-IR scores relative to control littermates. When placed on high-fat diet, ZFP-TG mice remained more glucose tolerant than their wild-type counterparts. Cell-based assays demonstrated that Zfp407 synergistically increased the transcriptional activity of all PPAR subtypes, PPARα, PPARγ, and PPARδ. The increased PPAR activity was not associated with increased PPAR mRNA or protein levels, suggesting that Zfp407 posttranslationally regulates PPAR activity. Collectively, these results demonstrate that Zfp407 overexpression improved glucose homeostasis. Thus, Zfp407 represents a new drug target for treating metabolic disease.