New role of irisin in hepatocytes: The protective effect of hepatic steatosis in vitro

Exercise enhances hepatic mitochondrial structure and function while preventing endoplasmic reticulum stress and metabolic dysfunction-associated steatotic liver disease in mice fed a high-fat diet

Metabolic dysfunction-associated steatotic liver disease (MASLD) has attracted increasing attention from the scientific community because of its severe but silent progression and the lack of specific treatment. Glucolipotoxicity triggers endoplasmic reticulum (ER) stress with decreased beta-oxidation and enhanced lipogenesis, promoting the onset of MASLD, whereas regular physical exercise can prevent MASLD by preserving ER and mitochondrial function. Thus, the hypothesis of this study was that high-intensity interval training (HIIT) could prevent the development of MASLD in high-fat (HF)-fed C57BL/6J mice by maintaining insulin sensitivity, preventing ER stress, and promoting beta-oxidation. Forty male C57BL/6J mice (3 months old) comprised 4 experimental groups: the control (C) diet group, the C diet + HIIT (C-HIIT) group, the HF diet group, and the HF diet + HIIT (HF-HIIT) group. HIIT sessions lasted 12 minutes and were performed 3 times weekly by trained mice. The diet and exercise protocols lasted for 10 weeks. The HIIT protocol prevented weight gain and maintained insulin sensitivity in the HF-HIIT group. A chronic HF diet increased ER stress-related gene and protein expression, but HIIT helped to maintain ER homeostasis, preserve mitochondrial ultrastructure, and maximize beta-oxidation. The increased sirtuin-1/peroxisome proliferator-activated receptor-gamma coactivator 1-alpha expression implies that HIIT enhanced mitochondrial biogenesis and yielded adequate mitochondrial dynamics. High hepatic fibronectin type III domain containing 5/irisin agreed with the antilipogenic and anti-inflammatory effects observed in the HF-HIIT group, reinforcing the antisteatotic effects of HIIT. Thus, we confirmed that practicing HIIT 3 times per week maintained insulin sensitivity, prevented ER stress, and enhanced hepatic beta-oxidation, impeding MASLD development in this mouse model even when consuming high energy intake from saturated fatty acids.

Free radical scavenging polyphenols isolated from Phyllanthus niruri L. ameliorates hyperglycemia via SIRT1 induction and GLUT4 translocation in in vitro and in vivo models

Type 2 diabetes milletus (T2DM) is a complex multifaceted disorder characterized by insulin resistance in skeletal muscle. Phyllanthus niruri L. is well reported sub-tropical therapeutically beneficial ayurvedic medicinal plant from Euphorbiaceae family used in various body ailments such as metabolic disorder including diabetes. The present study emphasizes on the therapeutic potential of Phyllanthus niruri L. and its phytochemical(s) against insulin resistance conditions and impaired antioxidant activity thereby aiding as an anti-hyperglycemic agent in targeting T2DM. Three compounds were isolated from the most active ethyl acetate fraction namely compound 1 as 1-O-galloyl-6-O-luteoyl-β-D-glucoside, compound 2 as brevifolincarboxylic acid and compound 3 as ricinoleic acid. Compounds 1 and 2, the two polyphenols enhanced the uptake of glucose and inhibited ROS levels in palmitate induced C2C12 myotubes. PNEAF showed the potent enhancement of glucose uptake in palmitate-induced insulin resistance condition in C2C12 myotubes and significant ROS inhibition was observed in skeletal muscle cell line. PNEAF treated IR C2C12 myotubes and STZ induced Wistar rats elevated SIRT1, PGC1-α signaling cascade through phosphorylation of AMPK and GLUT4 translocation resulting in insulin sensitization. Our study revealed an insight into the efficacy of marker compounds isolated from P. niruri and its enriched ethyl acetate fraction as ROS scavenging agent and helps in attenuating insulin resistance condition in C2C12 myotubes as well as in STZ induced Wistar rat by restoring glucose metabolism. Overall, this study can provide prospects for the marker-assisted development of P. niruri as a phytopharmaceutical drug for the insulin resistance related diabetic complications.

The Role of Irisin throughout Women's Life Span

Since its discovery, much attention has been drawn to irisin's potential role in metabolic and reproductive diseases. This narrative review summarizes and updates the possible role played by this fascinating molecule in different physiological (puberty and menopause) and pathological (polycystic ovary syndrome (PCOS), functional hypothalamic amenorrhea (FHA), endometriosis, and gestational diabetes) conditions that can affect women throughout their entire lives. Irisin appears to be an important factor for the hypothalamic-pituitary-gonadal axis activation, and appears to play a role in the timing of puberty onset. Serum irisin levels have been proposed as a biomarker for predicting the future development of gestational diabetes (GDM). Its role in PCOS is still controversial, although an "irisin resistance" mechanism has been hypothesized. In addition to its impact on metabolism, irisin also appears to influence bone health. Irisin levels are inversely correlated with the prevalence of fractures in postmenopausal women. Similar mechanisms have also been postulated in young women with FHA. In clinical settings, further controlled, prospective and randomized clinical trials are needed to investigate the casual relationship between irisin levels and the conditions described and, in turn, to establish the role of irisin as a prognostic/diagnostic biomarker or a therapeutic target.

Therapeutic role of FNDC5/irisin in attenuating liver fibrosis via inhibiting release of hepatic stellate cell-derived exosomes

OBJECTIVE

Cleavage of fibronectin type III domain-containing protein 5 (FNDC5), a membrane-bound precursor protein, would cleave into a myokine, irisin, which is also expressed in the liver. FNDC5/Irisin has been reported to play a critical role in maintaining glucose and lipid homeostasis in the liver and in combating liver fibrosis. Recently, several studies have shown that extracellular vesicles (EVs) derived from hepatic stellate cells (HSCs) could modulate liver fibrosis; however, there is a large gap in understanding whether inhibition of fibrogenic EVs derived from HSCs could alleviate the progression of liver fibrosis. Here, we investigated the role of FNDC5/irisin in liver fibrosis and the mechanism of its inhibitory role in the release of HSC-derived fibrogenic EVs.

METHODS

Experiments were performed in wild-type and FNDC5-/- mice, primary mouse HSCs, and human hepatic stellate cell line (LX2). Mice were treated with carbon tetrachloride (CCl4) or bile duct ligation (BDL) to induce liver fibrosis. EVs derived from HSCs were purified and injected intraperitoneally into mice.

RESULTS

Our results showed that FNDC5 deficiency exacerbated CCl4-induced liver fibrosis and activation of HSCs in mice. Moreover, fibrogenic EVs derived from PDGF-BB-treated HSCs promoted HSC migration in vitro and liver fibrosis in vivo. However, administration of irisin, a cleavage of FNDC5, inhibited the release of fibrogenic EVs and activation of HSCs by promoting ubiquitylation degradation of Rab27b. In vivo, the promoting role of HSC-derived fibrogenic EVs in liver fibrosis was also reversed by irisin.

CONCLUSION

All these results demonstrate that FNDC5/irisin is a novel therapeutic agent for chronic liver fibrosis.

AAV-mediated skeletal muscle specific irisin expression does not contribute to weight loss in mice

Obesity, a chronic disease, significantly increases the risk of various diseases, including diabetes, cardiovascular diseases, and cancers. Exercise is crucial for weight management not only through energy expenditure by muscle activity but also through stimulating the secretion of myokines, which affect various tissues. Irisin, derived from the proteolytic processing of fibronectin type III domain-containing protein 5 (Fndc5), is a well-studied myokine with beneficial effects on metabolism. This study explored the feasibility of adeno-associated virus (AAV)-mediated Fndc5 gene therapy to treat obesity in a mouse model using the AAV-DIO system to express Fndc5 specifically in skeletal muscle, and investigated its anti-obesity effect. Although Fndc5 was specifically expressed in the muscle, no significant impact on body weight under normal chow or high-fat diets was observed, and no change in thermogenic gene expression in inguinal white adipose tissue was detected. Notably, Fndc5 transduction did affect bone metabolism, consistent with previous reports. These findings suggest that AAV-mediated Fndc5 gene therapy may not be an efficient strategy for obesity, contrary to our expectations. Further research is needed to elucidate the complex mechanisms involved in irisin's role in obesity and related disorders.

Potential role of irisin in digestive system diseases

Digestive system diseases (DSD) are very complex conditions that severely threaten human health. Therefore, there is an urgent need to develop new pharmacological treatment strategies. Irisin, a myokine discovered in 2012, is produced by fibronectin type III domain-containing protein 5 (FNDC5), which is a transmembrane protein. Irisin is involved in promoting the browning of white adipose tissue, the regulation of energy metabolism, and the improvement of insulin resistance. Irisin is also an essential mediator of the inflammatory response, oxidative stress, and cell apoptosis. Recent studies have proved that irisin concentration is altered in DSD and exerts pivotal effects on the initiation, progression, and prognosis of these diseases through various mechanisms. Therefore, studying the expression and function of irisin may have great significance for the diagnosis and treatment of DSD. Here, we focus on irisin and explore the multiple molecular pathways targeted by irisin therapy. This review indicates that irisin can serve as a diagnostic marker or potential therapeutic agent for DSD. DATA AVAILABILITY: Not applicable.

Fish oil and chicoric acid combination protects better against palmitate-induced lipid accumulation via regulating AMPK-mediated SREBP-1/FAS and PPARα/UCP2 pathways

Non-alcoholic fatty liver disease (NAFLD) is associated with lipid accumulation and lipotoxicity. The main aim of this study is to evaluate the synergistic treatment effect of fish oils (FOs) and chicoric acid (CA) in palmitate (PA)-induced NAFLD HepG2 model. HepG2 cells were pre-treated with palmitate (0.75 mM) for 24 h, and then were exposed to CA, FOs and combination of these chemicals for another 24 h. Gene expression and protein levels were determined using qRT-PCR and western blotting or ELISA analysing, respectively. The combination index (CI) values of FOs and CA in HepG2 cells were calculated according to the Chou-Talalay equation using the CompuSyn software. FOs and CA acid together synergistically reduced lipid accumulation as indicated by decreased oil red O staining (vehicle-treated control: 1 ± 0.1; PA-treated control: 4.7 ± 0.4; PA + CA100: 3.9 ± 0.4; PA + CA200: 2.4 ± 0.3; PA + FOs: 2.7 ± 0.1; PA + CA200 + FOs: 1.5 ± 0.1) and triglyceride (vehicle-treatedcontrol:10 ± 1.2; PA-treated control: 25.8 ± 2.7; PA + CA100: 18.9 ± 2.5; PA + CA200: 14.4 ± 1.8; PA + FOs: 15.2 ± 2.4; PA + CA200 + FOs: 11.9 ± 1.5) levels in PA-treated HepG2 cells. Gene expression and Immunoblotting analysis confirmed the combination effect of FOs and CA in up-regulation of AMPK-mediated PPARα/UCP2 and down-regulation of AMPK-mediated SREBP-1/FAS signalling pathways. Collectively, these results suggest that combining FOs with CA can serve as a potential combination therapy for NAFLD.

Protein arginine methyltransferase 3: A crucial regulator in metabolic reprogramming and gene expression in cancers

Post-translational modification (PTM) of proteins increases proteome diversity, which is critical for maintaining cellular homeostasis. The importance of protein methylation in the regulation of diverse biological processes has been highlighted in the past decades. Methylation of the arginine residue on proteins is catalyzed by members of the protein arginine methyltransferase (PRMT) family. PRMTs play indispensable roles in various pathways that regulate cancer development, progression, and drug response. In this review, we discuss the role of PRMT3, a member of the PRMT family, in controlling oncogenic processes. Additionally, the effects of PRMT3 on the methylation of regulatory proteins involved in transcription, post-transcriptional control, ribosomal maturation, translation, biological synthesis, and metabolic signaling are summarized. Moreover, recent progresses in the development of PRMT3 inhibitors are introduced. Overall, this review highlights the importance of PRMT3 in tumorigenesis and discusses the underlying mechanisms by which PRMT3 modulates cellular metabolism and gene expression. These results also provide a molecular basis for therapeutic modalities by targeting PRMT3.

Neuroplasticity to autophagy cross-talk in a therapeutic effect of physical exercises and irisin in ADHD

Adaptive neuroplasticity is a pivotal mechanism for healthy brain development and maintenance, as well as its restoration in disease- and age-associated decline. Management of mental disorders such as attention deficit hyperactivity disorder (ADHD) needs interventions stimulating adaptive neuroplasticity, beyond conventional psychopharmacological treatments. Physical exercises are proposed for the management of ADHD, and also depression and aging because of evoked brain neuroplasticity. Recent progress in understanding the mechanisms of muscle-brain cross-talk pinpoints the role of the myokine irisin in the mediation of pro-cognitive and antidepressant activity of physical exercises. In this review, we discuss how irisin, which is released in the periphery as well as derived from brain cells, may interact with the mechanisms of cellular autophagy to provide protein recycling and regulation of brain-derived neurotrophic factor (BDNF) signaling via glia-mediated control of BDNF maturation, and, therefore, support neuroplasticity. We propose that the neuroplasticity associated with physical exercises is mediated in part by irisin-triggered autophagy. Since the recent findings give objectives to consider autophagy-stimulating intervention as a prerequisite for successful therapy of psychiatric disorders, irisin appears as a prototypic molecule that can activate autophagy with therapeutic goals.

Chemical composite of indigenous whole grain scented joha rice varietal prevents type 2 diabetes in rats through ameliorating insulin sensitization by the IRS-1/AKT/PI3K signalling cascade

Preventive measures to lower the prevalence of type-2 diabetes development using dietary phytochemicals are most realistic. A phytochemical composite derived from whole grain scented joha rice (PCKJ), which is indigenous to the North-eastern Region, India, was investigated to understand its preventive efficacy in rats in which type 2 diabetes was induced using a high-fat high-fructose (HFHF) diet and a low dose of streptozotocin, and the findings were correlated with those in L6-myotubes. Studies on cultured L6 myotubes revealed that treatment with PCKJ facilitated glucose uptake and GLUT-4 translocation to the plasma membrane, as evidenced by confocal microscopy and/or cell fractionation studies. Furthermore, the FFA-induced L6 myotubes were identified as having elevated levels of PI3K, p-AKT (Ser473) and GLUT-4, which returned to the basal level upon exposure to PCKJ. The administration of PCKJ (100 mg per kg body weight, oral gavage, 24 weeks) to rats significantly reduced their blood glucose levels along with common lipid and liver biomarkers (LDL, triglycerides, cholesterol, ALT, and AST) compared to the control group. Moreover, immunoblotting analysis showed that upon PCKJ treatment, PI3K, p-AKT and GLUT-4 levels are upregulated in the skeletal tissue of HFHF-fed rats, similar to the in vitro model. The alteration in the levels of inflammatory cytokines IL-6, IL-10 and IFN-γ in diabetic rats returned to normal levels upon exposure to PCKJ. Histological analysis of vital tissues further strengthens the findings of the preventive value of PCKJ against the development of insulin resistance. In conclusion, this study showed the prophylactic effect of PCKJ as a potent chemical composite, which can be used to develop functional foods (nutraceuticals) for ameliorating type-2 diabetes by improving insulin sensitization and thereby glucose metabolism.

The potential role of FNDC5/irisin in various liver diseases: awakening the sleeping beauties

Fibronectin type III domain-containing protein 5 (FNDC5) is a transmembrane protein and the precursor of irisin, which serves as a systemic exerkine/myokine with multiple origins. Since its discovery in 2012, this hormone-like polypeptide has rapidly evolved to a component significantly involved in a gamut of metabolic dysregulations and various liver diseases. After a decade of extensive investigation on FNDC5/irisin, we are still surrounded by lots of open questions regarding its diagnostic and therapeutic values. In this review, we first concentrated on the structure-function relationship of FNDC5/irisin. Next, we comprehensively summarised the current knowledge and research findings regarding pathogenic roles/therapeutic applications of FNDC5/irisin in the context of non-alcoholic fatty liver disease, fibrosis, liver injury due to multiple detrimental insults, hepatic malignancy and intrahepatic cholestasis of pregnancy. Moreover, the prominent molecules involved in the underlying mechanisms and signalling pathways were highlighted. As a result, emerging evidence reveals FNDC5/irisin may act as a proxy for diagnosing liver disease pathology, a sensitive biomarker for assessing damage severity, a predisposing factor for surveilling illness progression and a treatment option with protective/preventive impact, all of which are highly dependent on disease grading and contextually pathological features.

NADPH Oxidases Connecting Fatty Liver Disease, Insulin Resistance and Type 2 Diabetes: Current Knowledge and Therapeutic Outlook

Nonalcoholic fatty liver disease (NAFLD), characterized by ectopic fat accumulation in hepatocytes, is closely linked to insulin resistance and is the most frequent complication of type 2 diabetes mellitus (T2DM). One of the features connecting NAFLD, insulin resistance and T2DM is cellular oxidative stress. Oxidative stress refers to a redox imbalance due to an inequity between the capacity of production and the elimination of reactive oxygen species (ROS). One of the major cellular ROS sources is NADPH oxidase enzymes (NOX-es). In physiological conditions, NOX-es produce ROS purposefully in a timely and spatially regulated manner and are crucial regulators of various cellular events linked to metabolism, receptor signal transmission, proliferation and apoptosis. In contrast, dysregulated NOX-derived ROS production is related to the onset of diverse pathologies. This review provides a synopsis of current knowledge concerning NOX enzymes as connective elements between NAFLD, insulin resistance and T2DM and weighs their potential relevance as pharmacological targets to alleviate fatty liver disease.

Molecular Basis of Irisin Regulating the Effects of Exercise on Insulin Resistance

Insulin resistance is recognized as one major feature of metabolic syndrome, and frequently emerges as a difficult problem encountered during long-term pharmacological treatment of diabetes. Insulin resistance often causes organs or tissues, such as skeletal muscle, adipose, and liver, to become less responsive or resistant to insulin. Exercise can promote the physiological function of those organs and tissues and benefits insulin action via increasing insulin receptor sensitivity, glucose uptake, and mitochondrial function. This is done by decreasing adipose tissue deposition, inflammatory cytokines, and oxidative stress. However, understanding the mechanism that regulates the interaction between exercise and insulin function becomes a challenging task. As a novel myokine, irisin is activated by exercise, released from the muscle, and affects multi-organ functions. Recent evidence indicates that it can promote glucose uptake, improve mitochondrial function, alleviate obesity, and decrease inflammation, as a result leading to the improvement of insulin action. We here will review the current evidence concerning the signaling pathways by which irisin regulates the effect of exercise on the up-regulation of insulin action in humans and animals.

Irisin protects cardiomyocytes against hypoxia/reoxygenation injury via attenuating AMPK mediated endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress plays a central role in myocardial ischemia/reperfusion (I/R) injury. Irisin has been reported to have protective properties in ischemia disease. In this study, we aimed at investigating whether irisin could alleviate myocardial I/R injury by ER stress attenuation. The in vitro model of hypoxia/reoxygenation (H/R) was established, which resembles I/R in vivo. Cell viability and apoptosis were estimated. Expressions of cleaved caspase-3, cytochrome c, GRP78, pAMPK, CHOP, and eIF2α were assessed by western blot. Our results revealed that pre-treatment with irisin significantly decreased cytochrome c release from mitochondria and caspase-3 activation caused by H/R. Irsin also reduced apoptosis and increased cell viability. These effects were abolished by AMPK inhibitor compound C pre-treatment. Also, GRP78 and CHOP expressions were up-regulated in the H/R group compared to the control group; however, irisin attenuated their expression. The pAMPK level was significantly decreased compared to the control, and this effect could be partly reversed by metformin pre-treatment. These results suggest that ER stress is associated with cell viability decreasing and cardiomyocytes apoptosis induced by H/R. Irisin could efficiently protect cardiomyocytes from H/R-injury via attenuating ER stress and ER stress-induced apoptosis.

Antioxidant Effects of Irisin in Liver Diseases: Mechanistic Insights

Oxidative stress is a crucial factor in the development of various liver diseases. Irisin, a metabolic hormone discovered in 2012, is mainly produced by proteolytic cleavage of fibronectin type III domain containing 5 (FNDC5) in skeletal muscles. Irisin is induced by physical exercise, and a rapidly growing body of literature suggests that irisin is, at least partially, responsible for the beneficial effects of regular exercise. The major biological function of irisin is believed to be involved in the maintenance of metabolic homeostasis. However, recent studies have suggested the therapeutic potential of irisin against a variety of liver diseases involving its antioxidative function. In this review, we aim to summarize the accumulating evidence demonstrating the antioxidative effects of irisin in liver diseases, with an emphasis on the current understanding of the potential molecular mechanisms.

Irisin: circulating levels in serum and its relation to gonadal axis

Irisin is an exercise-induced myokine/adipokine in mice and humans that plays an important role in 'browning' of white adipose tissue and has shown great potential as a treatment for some metabolic diseases, such as obesity, insulin resistance, and inflammation. The circulating irisin level is reported to be associated with exercise, obesity, diet, diseases, and exposure to different pharmacological agents. Several studies have attempted to characterize the role of irisin in PCOS and other reproductive diseases, but contradictory results have been reported. Our previous study showed that irisin may serve further functions in folliculogenesis and fertility. In this review, we present the current knowledge on the physiology of irisin and its role in gonadal axis. Firstly, we describe irisin circulating levels and speculate on the potential mechanisms involved in irisin secretion and regulation. Then, we focus on the irisin levels in PCOS, and explore the relationships between, BMI, insulin resistance, and hyperandrogenism. Finally, we present the results from animal interventional studies and in vitro experiments to investigate the relationship between irisin and gonadal axis, indicating its novel effects on reproduction and fertility.

Irisin alleviates FFA induced β-cell insulin resistance and inflammatory response through activating PI3K/AKT/FOXO1 signaling pathway

PURPOSE

Type 2 diabetes mellitus is characterized by insulin resistance and β-cell dysfunction. Elevated free fatty acids-induced lipotoxicity may play a vital role in the pathogenesis of β-cell insulin resistance. Exercise-stimulated myokine irisin has been reported to be closely related to T2DM. However, its function on β-cell insulin signaling and the underlying mechanisms are only partially elucidated as yet.

METHODS

High-fat diet-fed C57BL/6J mice and palmitic acid-treated MIN6 cell models were utilized as lipotoxic models. Factors associated with β-cell insulin signaling transduction and inflammatory responses were assessed in these models. Furthermore, the role of irisin in β-cells and the underlying mechanisms were also explored.

RESULTS

Irisin effectively decreased lipid levels in HFD mice, enhanced glucose-stimulated insulin secretion and nullified the expressions of inflammatory cytokines in vivo and in vitro experiments. Moreover, irisin improved PI3K/AKT insulin signaling pathway and inhibited TLR4/NF-κB inflammatory signaling pathway in both islets of HFD mice and PA-treated MIN6 cells. Mechanistic analysis indicated that FOXO1 might serve as a bridge between the two pathways.

CONCLUSION

Irisin alleviates lipotoxicity-induced β-cell insulin resistance and inflammatory response through the activation of PI3K/AKT/FOXO1 signaling pathways and the inhibition of TLR4/NF-κB signaling pathways. Irisin might provide a novel therapeutic strategy for T2DM.

Irisin at the crossroads of inter-organ communications: Challenge and implications

The physiological functions of organs are intercommunicated occurring through secreted molecules. That exercise can improve the physiological function of organs or tissues is believed by secreting myokines from muscle to target remote organs. However, the underlying mechanism how exercise regulates the inter-organ communications remains incompletely understood yet. A recently identified myokine-irisin, primarily found in muscle and adipose and subsequently extending to bone, heart, liver and brain, provides a new molecular evidence for the inter-organ communications. It is secreted under the regulation of exercise and mediates the intercommunications between exercise and organs. To best our understanding of the regulatory mechanism, this review discusses the recent evidence involving the potential molecular pathways of the inter-organ communications, and the interactions between signalings and irisin in regulating the impact of exercise on organ functions are also discussed.

Exercise-Induced Irisin Decreases Inflammation and Improves NAFLD by Competitive Binding with MD2

Non-alcoholic fatty liver disease (NAFLD) is a global clinical problem. The MD2-TLR4 pathway exacerbates NAFLD progression by promoting inflammation. Long-term exercise is considered to improve NAFLD but the underlying mechanism is still unclear. In this study, we examined the protective effect and molecular mechanism of exercise on high-fat diet (HFD)-induced liver injury. In an HFD-induced NAFLD mouse model, exercise training significantly decreased hepatic steatosis and fibrosis. Interestingly, exercise training blocked the binding of MD2-TLR4 and decreased the downstream inflammatory response. Irisin is a myokine that is highly expressed in response to exercise and exerts anti-inflammatory effects. We found that circulating irisin levels and muscle irisin expression were significantly increased in exercised mice, suggesting that irisin could mediate the effect of exercise on NAFLD. In vitro studies showed that irisin improved lipid metabolism, fibrosis, and inflammation in palmitic acid (PA)-stimulated AML12 cells. Moreover, binding assay results showed that irisin disturbed MD2-TLR4 complex formation by directly binding with MD2 but not TLR4, and interfered with the recognition of stimuli such as PA and lipopolysaccharide with MD2. Our study provides novel evidence that exercise-induced irisin inhibits inflammation via competitive binding with MD2 to improve NAFLD. Thus, irisin could be considered a potential therapy for NAFLD.

Novel Biomolecules in the Pathogenesis of Gestational Diabetes Mellitus

Gestational diabetes mellitus (GDM) is one of the most common metabolic diseases in pregnant women. Its early diagnosis seems to have a significant impact on the developing fetus, the course of delivery, and the neonatal period. It may also affect the later stages of child development and subsequent complications in the mother. Therefore, the crux of the matter is to find a biopredictor capable of singling out women at risk of developing GDM as early as the very start of pregnancy. Apart from the well-known molecules with a proven and clear-cut role in the pathogenesis of GDM, e.g., adiponectin and leptin, a potential role of newer biomolecules is also emphasized. Less popular and less known factors with different mechanisms of action include: galectins, growth differentiation factor-15, chemerin, omentin-1, osteocalcin, resistin, visfatin, vaspin, irisin, apelin, fatty acid-binding protein 4 (FABP4), fibroblast growth factor 21, and lipocalin-2. The aim of this review is to present the potential and significance of these 13 less known biomolecules in the pathogenesis of GDM. It seems that high levels of FABP4, low levels of irisin, and high levels of under-carboxylated osteocalcin in the serum of pregnant women can be used as predictive markers in the diagnosis of GDM. Hopefully, future clinical trials will be able to determine which biomolecules have the most potential to predict GDM.

Deletion of PRAK Mitigates the Mitochondria Function and Suppresses Insulin Signaling in C2C12 Myoblasts Exposed to High Glucose

Background: p38 regulated/activated protein kinase (PRAK) plays a crucial role in modulating cell death and survival. However, the role of PRAK in the regulation of metabolic stress remains unclear. We examined the effects of PRAK on cell survival and mitochondrial function in C2C12 myoblasts in response to high glucose stresses.

Methods: PRAK of C2C12 myoblasts was knocked out by using CRISPR/Cas-9 genome editing technology. Both wild type and PRAK^−/− C2C12 cells were exposed to high glucose at the concentration of 30 mmol/L to induce metabolic stress. The effect of irisin, an adipomyokine, on both wild type and PRAK^−/− cells was determined to explore its relationship with RPAK. Cell viability, ATP product, glucose uptake, mitochondrial damage, and insulin signaling were assessed.

Results: PRAK knockout decreased C2C12 viability in response to high glucose stress as evident by MTT assay in association with the reduction of ATP and glucose uptake. PRAK knockout enhanced apoptosis of C2C12 myoblasts in response to high glucose, consistent with an impairment in mitochondrial function, by decreasing mitochondrial membrane potential. PRAK knockout induced impairment of mitochondrial and cell damage were rescued by irisin. PRAK knockout caused decrease in phosphorylated PI3 kinase at Tyr 485, IRS-1 and AMPKα and but did not affect non-phosphorylated PI3 kinase, IRS-1 and AMPKα signaling. High glucose caused the further reduction of phosphorylated PI3 kinase, IRS-1 and AMPKα. Irisin treatment preserved phosphorylated PI3 kinase, IRS-1by rescuing PRAK in high glucose treatment.

Conclusion: Our finding indicates a pivotal role of PRAK in preserving cellular survival, mitochondrial function, and high glucose stress.

Non-Alcoholic Fatty Liver Disease in Lean and Non-Obese Individuals: Current and Future Challenges

Non-alcoholic fatty liver disease (NAFLD), which approximately affects a quarter of the world’s population, has become a major public health concern. Although usually associated with excess body weight, it may also affect normal-weight individuals, a condition termed as lean/non-obese NAFLD. The prevalence of lean/non-obese NAFLD is around 20% within the NAFLD population, and 5% within the general population. Recent data suggest that individuals with lean NAFLD, despite the absence of obesity, exhibit similar cardiovascular- and cancer-related mortality compared to obese NAFLD individuals and increased all-cause mortality risk. Lean and obese NAFLD individuals share several metabolic abnormalities, but present dissimilarities in genetic predisposition, body composition, gut microbiota, and susceptibility to environmental factors. Current treatment of lean NAFLD is aimed at improving overall fitness and decreasing visceral adiposity, with weight loss strategies being the cornerstone of treatment. Moreover, several drugs including PPAR agonists, SGLT2 inhibitors, or GLP-1 receptor agonists could also be useful in the management of lean NAFLD. Although there has been an increase in research regarding lean NAFLD, there are still more questions than answers. There are several potential drugs for NAFLD therapy, but clinical trials are needed to evaluate their efficacy in lean individuals.

Positive Effects of Exercise Intervention without Weight Loss and Dietary Changes in NAFLD-Related Clinical Parameters: A Systematic Review and Meta-Analysis

One of the focuses of non-alcoholic fatty liver disease (NAFLD) treatment is exercise. Randomized controlled trials investigating the effects of exercise without dietary changes on NAFLD-related clinical parameters (liver parameters, lipid metabolism, glucose metabolism, gut microbiota, and metabolites) were screened using the PubMed, Scopus, Web of Science, and Cochrane databases on 13 February 2020. Meta-analyses were performed on 10 studies with 316 individuals who had NAFLD across three exercise regimens: aerobic exercise, resistance training, and a combination of both. No studies investigating the role of gut microbiota and exercise in NAFLD were found. A quality assessment via the (RoB)2 tool was conducted and potential publication bias, statistical outliers, and influential cases were identified. Overall, exercise without significant weight loss significantly reduced the intrahepatic lipid (IHL) content (SMD: −0.76, 95% CI: −1.04, −0.48) and concentrations of alanine aminotransaminase (ALT) (SMD: −0.52, 95% CI: −0.90, −0.14), aspartate aminotransaminase (AST) (SMD: −0.68, 95% CI: −1.21, −0.15), low-density lipoprotein cholesterol (SMD: −0.34, 95% CI: −0.66, −0.02), and triglycerides (TG) (SMD: −0.59, 95% CI: −1.16, −0.02). The concentrations of high-density lipoprotein cholesterol, total cholesterol (TC), fasting glucose, fasting insulin, and glycated hemoglobin were non-significantly altered. Aerobic exercise alone significantly reduced IHL, ALT, and AST; resistance training alone significantly reduced TC and TG; a combination of both exercise types significantly reduced IHL. To conclude, exercise overall likely had a beneficial effect on alleviating NAFLD without significant weight loss. The study was registered at PROSPERO: CRD42020221168 and funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement no. 813781.

The Physiological Role of Irisin in the Regulation of Muscle Glucose Homeostasis

Irisin is a myokine that primarily targets adipose tissue, where it increases energy expenditure and contributes to the beneficial effects of exercise through the browning of white adipose tissue. As our knowledge has deepened in recent years, muscle has been found to be a major target organ for irisin as well. Several studies have attempted to characterize the role of irisin in muscle to improve glucose metabolism through mechanisms such as reducing insulin resistance. Although they are very intriguing reports, some contradictory results make it difficult to grasp the whole picture of the action of irisin on muscle. In this review, we attempted to organize the current knowledge of the role of irisin in muscle glucose metabolism. We discussed the direct effects of irisin on glucose metabolism in three types of muscle, that is, skeletal muscle, smooth muscle, and the myocardium. We also describe irisin’s effects on mitochondria and its interactions with other hormones. Furthermore, to consider the relationship between the irisin-induced improvement of glucose metabolism in muscle and systemic disorders of glucose metabolism, we reviewed the results from animal interventional studies and human clinical studies.

[Myokines and adipomyokines: inflammatory mediators or unique molecules of targeted therapy for obesity?]

Skeletal muscles make up about 25% of the total mass in children and more than 40% in adults. Studies of the last twenty years have shown that along with the main functions, muscle tissue has hormonal activity. It was found that myocytes are able to release signaling molecules-myokines. They act auto-and paracrine within the muscle, and at a high level-through the systemic circulation, carrying out interactions between skeletal muscles and various organs and tissues, such as the liver, bone and adipose tissue, the brain. It is proved that the key factor in the expression of myokines is physical activity, and their level largely depends on physical fitness, the amount of skeletal muscle mass and its composition (the ratio of fast and slow fibers), on the intensity and duration of physical activity. Myokines have a wide range of physiological effects: myostatin suppresses the growth and differentiation of muscle tissue, and decorin, acting as its antagonist, promotes muscle hypertrophy. Interleukin 6 provides an energy substrate for contracting muscle fibers, fibroblast growth factor 21 activates the mechanisms of energy production during fasting and improves tissue sensitivity to insulin; irisin stimulates thermogenesis, glucose uptake by myocytes, and also contributes to an increase in bone mineral density. The study of myokines is one of the key links in understanding the mechanisms underlying obesity and metabolic complications, the consequences of a sedentary lifestyle, as well as the implementation of the action of physical activity. Taking into account the physiological effects of myokines in the body, in the future they can become therapeutic targets for the treatment of these conditions.

The Essential Role of PRAK in Preserving Cardiac Function and Insulin Resistance in High-Fat Diet-Induced Diabetes

Regulated/activated protein kinase (PRAK) plays a crucial role in modulating biological function. However, the role of PRAK in mediating cardiac dysfunction and metabolic disorders remains unclear. We examined the effects of deletion of PRAK on modulating cardiac function and insulin resistance in mice exposed to a high-fat diet (HFD). Wild-type and PRAK^-/- mice at 8 weeks old were exposed to either chow food or HFD for a consecutive 16 weeks. Glucose tolerance tests and insulin tolerance tests were employed to assess insulin resistance. Echocardiography was employed to assess myocardial function. Western blot was used to determine the molecular signaling involved in phosphorylation of IRS-1, AMPKα, ERK-44/42, and irisin. Real time-PCR was used to assess the hypertrophic genes of the myocardium. Histological analysis was employed to assess the hypertrophic response, interstitial myocardial fibrosis, and apoptosis in the heart. Western blot was employed to determine cellular signaling pathway. HFD-induced metabolic stress is indicated by glucose intolerance and insulin intolerance. PRAK knockout aggravated insulin resistance, as indicated by glucose intolerance and insulin intolerance testing as compared with wild-type littermates. As compared with wild-type mice, hyperglycemia and hypercholesterolemia were manifested in PRAK-knockout mice following high-fat diet intervention. High-fat diet intervention displayed a decline in fractional shortening and ejection fraction. However, deletion of PRAK exacerbated the decline in cardiac function as compared with wild-type mice following HFD treatment. In addition, PRAK knockout mice enhanced the expression of myocardial hypertrophic genes including ANP, BNP, and βMHC in HFD treatment, which was also associated with an increase in cardiomyocyte size and interstitial fibrosis. Western blot indicated that deletion of PRAK induces decreases in phosphorylation of IRS-1, AMPKα, and ERK44/42 as compared with wild-type controls. Our finding indicates that deletion of PRAK promoted myocardial dysfunction, cardiac remodeling, and metabolic disorders in response to HFD.

Effect of Heat Stimulation on Circulating Irisin in Humans

High temperatures lead to oxidative stress. The aim of the study was to determine whether heat stimulation-induced hyperthermia can increase the level of circulating irisin. Twenty-one healthy female subjects (age, 26.3 ± 2.71 years; height, 162.1 ± 3.15 cm; weight, 54.2 ± 3.86 kg; and body surface area, 1.57 ± 0.11 m2) not taking contraceptives participated in this study. All experiments were performed individually for each participant when they were in the early proliferative menstrual phase. In an automated climate chamber (25 ± 0.5°C), the heat load was applied via half-body immersion into a hot water bath (42 ± 0.5°C). Five-minutes break was provided every after 5 min of immersion and the total passive heating time was 30 min. Tympanic temperature (Tt y) and skin temperature (Ts) were measured. Mean body temperature (mTb) was calculated. Blood samples were collected before and immediately after immersion. Levels of irisin, cortisol, creatine kinase (CK), and lactate dehydrogenase (LDH) were analyzed. Tty, mTb and serum irisin levels increased after hot water immersion. The blood levels of cortisol, CK, and LDH were also elevated after hot water immersion. Heat stimulation might increase the levels of circulating irisin in humans in response to oxidative stress.

Increased FNDC5/IRISIN protein expression in breast cancer tissue is associated with obesity in postmenopausal women

AIM

To analyse the fibronectin type III domain containing 5 (FNDC5)/irisin expression in tumour tissue of postmenopausal women presenting breast cancer and different body mass indexes (BMIs), proposing that obesity deregulates the expression of FNDC5/irisin at the breast tumour level. In addition, we investigated if different breast cancer cell lines are capable to synthesise this protein.

METHODS

A total of 150 postmenopausal women (50 with a normal BMI, 50 presenting overweight and 50 having obesity) diagnosed with operable breast cancer were included. FNDC5/irisin expression was determined by immunohistochemistry or by immunocytochemistry. Qualitative analysis of protein expression was performed by the H-Score method, through ImageJ's IHC Profiler software. Statistical analyses were carried out using STATA V.14.0 (Texas, USA); p value<0.05 was accepted as statistically significant. Statistical power of the study was >80% with a p<0.05.

RESULTS

FNDC5/irisin expression in breast cancer tissue of postmenopausal women with obesity was significantly increased when compared with FNDC5/irisin expression in women with a normal BMI (p=0.001). Furthermore, three breast cancer cell lines studied were capable to synthesise and express FNDC5/irisin, being the BT-474 cell line the one that exhibited the highest intensity of expression.

CONCLUSIONS

Our results confirm that women with breast cancer and obesity exhibit an increased irisin expression in their tumorous tissue compared with women with breast cancer and normal BMI. Likewise, in vitro breast cancer cell lines have the capacity to synthesise and express FNDC5/irisin, without any extracellular stimuli, however the microenvironment surrounding these cells in vivo participates in its regulation.

Lower levels of irisin in patients with type 2 diabetes mellitus: A meta-analysis

AIMS

This study aimed to provide evidence on the levels of circulating irisin in patients with type 2 diabetes mellitus (T2DM).

METHODS

A meta-analysis was conducted to compare the irisin levels in patients with T2DM with the levels in control patients. PubMed, Embase, CENTRAL databases, and other sources were searched from inception through September 2020. Review manager software version 5.4 was used to calculate the pooled outcomes. Heterogeneity was measured using I² statistics.

RESULTS

Twenty-six studies involving 3667 participants met the inclusion criteria and were analyzed in this study. We found that irisin levels were significantly lower in patients with T2DM [Standard (Std.) Mean Difference, -1.02; 95% confidence interval (95% CI), -1.37 to -0.67; p < 0.00001]. Sensitivity analysis confirmed the robustness of this result (Std. Mean Difference, -0.56; 95% CI, -0.73 to -0.39; p < 0.00001).

CONCLUSIONS

As compared to the control group and irrespective of differences in ethnicities, age groups, study designs, blood samples, sample sizes, language used for the study, or ELISA kits, lower levels of irisin were observed in patients with T2DM.

Gamma-glutamyl carboxylated Gas6 facilitates the prophylactic effect of vitamin K in inhibiting hyperlipidemia-associated inflammatory pathophysiology via arresting MCP-1/ICAM-1 mediated monocyte-hepatocyte adhesion

Role of growth arrest-specific 6 (Gas6), member of vitamin K (VK)-dependent protein family in hyperlipidemia-associated inflammation remains unresolved. To address this, blood samples were collected from hyperlipidemic subjects and age-matched healthy controls and observed that gamma-glutamyl carboxylated Gas6 (Gla-Gas6) but not total Gas6 were significantly lower while pro-inflammatory markers, MCP-1 and ICAM-1 were remarkably higher in hyperlipidemic subjects compared to control. Correlation analyses demonstrated that Gla-Gas6 levels were inversely correlated with MCP-1 and ICAM-1 but positively with plasma VK in hyperlipidemic subjects but not in control. This suggests that boosting VK level might ameliorate the hyperlipidemia-associated inflammatory pathophysiology via augmenting Gla-Gas6. Further studies with high fat diet (HFD)-fed mice demonstrated that VK supplementation (1, 3, and 5 µg/kg BW, 8 weeks) dose-dependently reduced both hepatic and plasma levels of MCP-1 and ICAM-1 while elevating that of Gla-Gas6 but not total Gas6 in HFD-fed mice. Cell culture studies with gamma-glutamyl carboxylase (enzyme causes VK-dependent carboxylation of Gas6) knockdown hepatocytes and monocytes dissected the direct role of Gla-Gas6 in inhibiting high palmitic acid (0.75 mM)-induced inflammation via arresting MCP-1/ICAM-1 mediated hepatocyte-monocyte adhesion. The present study demonstrated an important role of Gla-Gas6 in facilitating the prophylactic effect of VK against hyperlipidemia associated inflammation.

Effect of Chronic Resistance Training on Circulating Irisin: Systematic Review and Meta-Analysis of Randomized Controlled Trials

Irisin seems to play an important role in several chronic diseases, however, the interactions between chronic training and irisin are still unclear. The purpose of this systematic review and meta-analysis was to examine the effect of chronic resistance training on circulating irisin in adults. Literature search was conducted in PubMed, Web of Science and EBSCOhost (Academic Search Complete) until December 2020. Randomized controlled trials researching irisin levels after a resistance training program for at least 8 weeks among an adult population were eligible. Other inclusion criteria comprised recruiting a control group and reporting circulating irisin through ELISA kits. Cohen's d effect size and subgroup analyses (95% confidence level) were calculated using a random effects analysis model. Data of the seven included studies comprising 282 individuals showed an increasing and non-significant tendency after a resistance training program (d = 0.58, 95% CI: -0.25 to 1.40, p = 0.17). Subgroup analyses showed significant increases for the older adults group (p < 0.001) and when training is demanding and progressive in terms of intensity (p = 0.03). Data suggest that resistance training programs seem to increase circulating irisin, especially in older adults and in demanding and progressive training programs. However, more studies should be conducted using robust measurement methods, such as mass spectrometry, to better understand the interaction between chronic resistance exercise and irisin.

Association of Circulating Irisin Levels and the Characteristics and Prognosis of Coronary Artery Disease

BACKGROUND

Irisin is a new muscle factor discovered in recent years that shows a strong association with metabolic diseases. However, its role in coronary artery disease (CAD) is still controversial. We performed this study to determine the relationship of serum irisin with the characteristics and prognosis of CAD.

MATERIALS AND METHODS

Patients with acute coronary syndrome (ACS) (n = 355), stable coronary artery disease (SCAD) (n = 162), nonobstructive coronary artery disease (NO-CAD) (n = 126) and normal coronary arteries (n = 109) were enrolled. An enzyme-linked immunosorbent assay kit was used to measure serum irisin concentrations. Major adverse cardiovascular events (MACEs) of patients with SCAD (n = 132) and ACS (n = 331) after percutaneous coronary intervention (PCI) were recorded during a 12-month follow-up. Receiver-operator characteristic (ROC) curve analysis was used to explore predictors of CAD. Kaplan-Meier survival analysis and the Cox proportional hazards regression model were used to explore the association between serum irisin levels and MACEs.

RESULTS

Serum irisin levels in patients with ACS, SCAD, NO-CAD and normal coronary arteries were 196.62±72.05 ng/ml, 216.81±79.69 ng/ml, 245.26±77.92 ng/ml and 300.17±76.74 ng/ml, respectively (p<0.001). ROC curve analysis indicated that serum irisin concentrations were a valuable biomarker of coronary lesions (AUC=0.799), CAD (AUC=0.734) and ACS (AUC=0.681). Survival analysis demonstrated that patients with high irisin levels exhibited a higher event-free survival rate in both the SCAD and ACS groups after successful PCI.

CONCLUSIONS

Serum irisin levels were significantly decreased in patients with CAD. Patients with ACS exhibited the lowest serum irisin levels. Furthermore, serum irisin levels were interrelated with prognosis in patients with CAD after PCI.

Irisin and Incretin Hormones: Similarities, Differences, and Implications in Type 2 Diabetes and Obesity

Incretins are gut hormones that potentiate glucose-stimulated insulin secretion (GSIS) after meals. Glucagon-like peptide-1 (GLP-1) is the most investigated incretin hormone, synthesized mainly by L cells in the lower gut tract. GLP-1 promotes β-cell function and survival and exerts beneficial effects in different organs and tissues. Irisin, a myokine released in response to a high-fat diet and exercise, enhances GSIS. Similar to GLP-1, irisin augments insulin biosynthesis and promotes accrual of β-cell functional mass. In addition, irisin and GLP-1 share comparable pleiotropic effects and activate similar intracellular pathways. The insulinotropic and extra-pancreatic effects of GLP-1 are reduced in type 2 diabetes (T2D) patients but preserved at pharmacological doses. GLP-1 receptor agonists (GLP-1RAs) are therefore among the most widely used antidiabetes drugs, also considered for their cardiovascular benefits and ability to promote weight loss. Irisin levels are lower in T2D patients, and in diabetic and/or obese animal models irisin administration improves glycemic control and promotes weight loss. Interestingly, recent evidence suggests that both GLP-1 and irisin are also synthesized within the pancreatic islets, in α- and β-cells, respectively. This review aims to describe the similarities between GLP-1 and irisin and to propose a new potential axis-involving the gut, muscle, and endocrine pancreas that controls energy homeostasis.

Irisin Protects Against Hind Limb Ischemia Reperfusion Injury

Aim

The aim of this study was to evaluate the effects of irisin in a murine model of hind limb ischemia reperfusion (I/R).

Methods

The mice were divided into four groups (n = 6 in each group): control, irisin, ischemia reperfusion (I/R), and irisin-ischemia reperfusion (I-I/R). Irisin (0.5 µg.g^-1, intraperitoneally [i.p.]) was administered 30 min before the I/R procedure. After 2 h of ischemia and 2.5 h of reperfusion, blood and tissue samples were taken for biochemical and histopathological analysis. The results were analyzed by Kruskal-Wallis and Mann-Whitney U-tests.

Results

There was a statistically significant difference in the total antioxidant status (TAS) and total oxidant status (TOS) levels in all the groups. The TAS level in the I/R group was significantly lower than that in the control, irisin, and I-I/R groups, whereas the TOS level was significantly higher in the I/R group as compared with that in the other groups. Caspase-3 activity and caspase-8 activity, indicators of inflammation, were significantly higher in the I/R and I-I/R groups as compared with those in the control and irisin groups.

Conclusion

Irisin may have protective effects in skeletal muscle ischemia reperfusion injury.

PRMT3 interacts with ALDH1A1 and regulates gene-expression by inhibiting retinoic acid signaling

Protein arginine methyltransferase 3 (PRMT3) regulates protein functions by introducing asymmetric dimethylation marks at the arginine residues in proteins. However, very little is known about the interaction partners of PRMT3 and their functional outcomes. Using yeast-two hybrid screening, we identified Retinal dehydrogenase 1 (ALDH1A1) as a potential interaction partner of PRMT3 and confirmed this interaction using different methods. ALDH1A1 regulates variety of cellular processes by catalyzing the conversion of retinaldehyde to retinoic acid. By molecular docking and site-directed mutagenesis, we identified the specific residues in the catalytic domain of PRMT3 that facilitate interaction with the C-terminal region of ALDH1A1. PRMT3 inhibits the enzymatic activity of ALDH1A1 and negatively regulates the expression of retinoic acid responsive genes in a methyltransferase activity independent manner. Our findings show that in addition to regulating protein functions by introducing methylation modifications, PRMT3 could also regulate global gene expression through protein-protein interactions.

Here, the authors demonstrate that protein arginine methyltransferase 3 (PRMT3) interacts with and inhibits the retinal dehydrogenase ALDH1A1, negatively regulating the expression of retinoic acid responsive genes. This study shows that PRMT3 affects diverse biological processes not only by globally regulating protein function through methylation but also by regulating gene expression.

The role of Irisin in multiorgan protection

Physical exercise is an effective strategy for improving human health. Various organs, including the heart, lung and kidney, can benefit from exercise. However, the underlying molecular mechanisms by which exercise protects organs remain unknown. Irisin, a myokine secreted from muscle in response to exercise, has attracted increased attention from researchers. The role of irisin in multiorgan protection has been gradually revealed, and this muscle-derived circulating factor is regarded as an essential bridge linking exercise and organ health. The mechanisms by which irisin protects diverse organs are different. Here, we review the research progress on the multiorgan protective effects of irisin and discuss the underlying molecular mechanisms.

The Effect of Irisin as a Metabolic Regulator and Its Therapeutic Potential for Obesity

Obesity is a worldwide health problem due to the imbalance of energy intake and energy expenditure. Irisin, a newly identified exercise-responsive myokine, which is produced by the proteolytic cleavage of fibronectin type III domain-containing protein 5 (FNDC5), has emerged as a promising therapeutic strategy to combat obesity and obesity-related complications. Various studies in mice have shown that irisin could respond to systematic exercise training and promote white-to-brown fat transdifferentiation, but the role and function of irisin in humans are controversial. In this review, we systematically introduced and analyzed the factors that may contribute to these inconsistent results. Furthermore, we also described the potential anti-inflammatory properties of irisin under a variety of inflammatory conditions. Finally, the review discussed the existing unresolved issues and controversies about irisin, including the transcription of the irisin precursor FNDC5 gene in humans, the cleavage site of the yet unknown proteolytic enzyme that cleaves irisin from FNDC5, and the reliability of irisin levels measured with available detection methods.

Resistance Exercise Regulates Hepatic Lipolytic Factors as Effective as Aerobic Exercise in Obese Mice

Non-alcoholic fatty liver disease (NAFLD) is associated with obesity. The effect of resistance exercise without dietary restriction on the regulation of hepatic lipolytic factors is unclear. This study aimed to analyze the effects of aerobic and resistance exercise on hepatic lipolytic factors of obese mice. High-fat diet (HFD)-induced obese mice were divided into HFD + sedentary (HF), HFD + aerobic exercise, and HFD + resistance exercise groups. Exercise group mice were subjected to treadmill or ladder climbing exercise for 8 weeks. Fat mass and liver triglycerides were significantly decreased in both aerobic and resistance training groups. In the results of protein levels related to hepatic steatosis, HFD significantly increased liver cannabinoid receptor 1 and sterol-regulatory element binding protein 1 (SREBP-1). Both aerobic and resistance training significantly (p < 0.05) increased liver carnitine palmitoyltransferase-1, phosphor-AMP-activated protein kinase (p-AMPK), and p-AMPK/AMPK and decreased liver SREBP-1. However, the type of exercise did not exert any significant effects on these protein levels. Thus, resistance exercise, similarly to aerobic exercise, effectively regulated hepatic lipolytic factors of obese mice. Therefore, a sustainable type of exercise selected based on the fitness level, disease type, musculoskeletal disorder status, and preference of the patients is the best exercise intervention for alleviating NAFLD.

Association between muscle strength and advanced fibrosis in non-alcoholic fatty liver disease: a Korean nationwide survey

BACKGROUND

We investigated the association between muscle strength and the prevalence of advanced fibrosis among individuals with non-alcoholic fatty liver disease (NAFLD) using a nationwide cross-sectional survey.

METHODS

Individuals, 20 to 79 years of age, from the Korean National Health and Nutrition Examination Surveys (KNHANES) from 2014 to 2016 were selected (N = 14 861), with sample weights applied. Muscle strength was quantified as the handgrip strength divided by the body mass index (BMI); low muscle strength (LMS) was defined as the lowest quartile (Q₁ ) of the handgrip strength/BMI for our sample population. NAFLD was defined as hepatic steatosis index >36. Advanced fibrosis was defined as a fibrosis-4 index score ≥1.30 (Fibrosis_FIB4 ).

RESULTS

The mean age of the study population was 45.6 ± 0.2 years, and 42.4% were male. As muscle strength increased, the mean BMI and age decreased accordingly, and the proportions of diabetes, dyslipidaemia, hypertension, and obesity decreased significantly (P < 0.001 for all). In a crude analysis, the LMS was associated with an increased prevalence of NAFLD (odds ratio [OR] 3.62, 95% confidence interval [CI] 3.25-4.03, P < 0.001), which remained significant even after adjustment for age, sex, obesity, insulin resistance, diabetes, hypertension, dyslipidaemia, and high-sensitivity C-reactive protein (OR 1.66, 95% CI 1.28-2.16, P < 0.001). In this logistic regression model, the prevalence of NAFLD decreased by 24% with each quartile increment in muscle strength (OR 0.76, 95% CI 0.68-0.85, P < 0.001). Among individuals with NAFLD (n = 2092), LMS was significantly associated with the presence of advanced fibrosis (Fibrosis_FIB4 ) independently of age, sex, obesity, diabetes, hypertension, dyslipidaemia, and high-sensitivity C-reactive protein (OR 1.66, 95% CI 1.01-2.49, P = 0.015), which lost its statistical significance after additional adjustment for insulin resistance.

CONCLUSIONS

Low muscle strength is independently associated with NAFLD. The significant association between LMS and advanced fibrosis in NAFLD may be mediated through insulin resistance.

TLR4/AP-1-Targeted Anti-Inflammatory Intervention Attenuates Insulin Sensitivity and Liver Steatosis

Insulin resistance has been shown to be the common pathogenesis of many metabolic diseases. Metainflammation is one of the important characteristics of insulin resistance. Macrophage polarization mediates the production and development of metainflammation. Toll-like receptor 4 (TLR4) mediates macrophage activity and is probably the intersection of immunity and metabolism, but the detailed mechanism is probably not fully understood. Activated protein 1 (AP1) signaling pathway is very important in macrophage activation-mediated inflammation. However, it is unclear whether AP1 signaling pathway mediates metabolic inflammation in the liver. We aimed to investigate the effects of macrophage TLR4-AP1 signaling pathway on hepatocyte metabolic inflammation, insulin sensitivity, and lipid deposition, as well as to explore the potential of TLR4-AP1 as new intervention targets of insulin resistance and liver steatosis. TLR4 and AP1 were silenced in the RAW264.7 cells by lentiviral siRNA transfection. In vivo transduction of lentivirus was administered in mice fed with high-fat diet. Insulin sensitivity and inflammation were evaluated in the treated cells or animals. Our results indicated that TLR4/AP-1 siRNA transfection alleviated high-fat diet-induced systemic and hepatic inflammation, obesity, and insulin resistance in mice. Additionally, TLR4/AP-1 siRNA transfection mitigated palmitic acid- (PA-) induced inflammation in RAW264.7 cells and metabolic abnormalities in cocultured AML hepatocytes. Herein, we propose that TLR4-AP1 signaling pathway activation plays a crucial role in high fat- or PA-induced metabolic inflammation and insulin resistance in hepatocytes. Intervention of the TLR4 expression regulates macrophage polarization and metabolic inflammation and further alleviates insulin resistance and lipid deposition in hepatocytes.

Irisin Attenuates Myocardial Ischemia/Reperfusion Injury and Improves Mitochondrial Function Through AMPK Pathway in Diabetic Mice

Aims

Several recent reports have shown irisin protects the heart against ischemia/reperfusion injury. However, the effect of irisin on I/R injury in diabetic mice has not been described. The present study was designed to investigate the role of irisin in myocardial ischemia-reperfusion (MI/R) injury in diabetic mice.

Methods

A mouse model of diabetes was established by feeding wild type or gene-manipulated adult male mice with a high-fat diet. All the mice received intraperitoneal injection of irisin or PBS. Thirty minutes after injection, mice were subjected to 30 min of myocardial ischemia followed by 3h (for cell apoptosis and protein determination), 24 h (for infarct size and cardiac function).

Results

Knock-out of gene FNDC5 augmented MI/R injury in diabetic mice, while irisin treatment attenuated MI/R injury, improved cardiac function, cellular ATP biogenetics, mitochondria potential, and impaired mitochondrion-related cell death. More severely impaired AMPK pathway was observed in diabetic FNDC5^-/- mice received MI/R. Knock-out of gene AMPK blocks the beneficial effects of irisin on MI/R injury, cardiac function, cellular ATP biogenetics, mitochondria potential, and mitochondrion-related cell death.

Conclusions

Our present study demonstrated that irisin improves the mitochondria function and attenuates MI/R injury in diabetic mice through AMPK pathway.

Serum Irisin Levels, Endothelial Dysfunction, and Inflammation in Pediatric Patients with Type 2 Diabetes Mellitus and Metabolic Syndrome

The prevalence of type 2 diabetes mellitus (T2DM) and metabolic syndrome (MetS) has increased in the pediatric population. Irisin, an adipomyokine, is involved in white adipose tissue browning, energy expenditure, insulin sensitivity, and anti-inflammatory pathways. Data on the associations among circulating irisin levels, soluble cell adhesion molecules (sCAMs), and inflammatory cytokines is scarce in children and adolescents with MetS and T2DM. Subjects aged 6-16 years were grouped into T2DM, MetS, and healthy controls. Serum irisin levels were significantly lower in the MetS (6.6 [2.8-18.0] ng/mL) and T2DM (6.8 [2.2-23.2] ng/mL) groups compared with controls (30.3 [24.6-57.1] ng/mL). Negative correlations between irisin and the BMI percentile (R = -0.358), WC percentile (R = -0.308), and triglycerides (R = -0.284) were identified, while positive associations with TC (R = 0.287), HDL-c (R = 0.488), and LDL-c (R = 0.414) were observed. Significant negative correlations were found between irisin and sNCAM (R = -0.382), sICAM-2 (R = -0.300), sVCAM-1 (R = -0.292), MCP-1 (R = -0.308), and IFN-α2 (R = -0.406). Of note, lower concentrations of most sCAMs (sICAM-1, sPSGL-1, sP-selectin, sEpCAM, sICAM-2, sALCAM, sPECAM-1, sCD44, sVCAM-1, sICAM-3, sL-selectin, and sNCAM) were shown in T2DM subjects compared with MetS patients. Lower irisin levels induce a lack of inhibition of oxidative stress and inflammation. In T2DM, higher ROS, AGEs, glucotoxicity, and inflammation trigger endothelial cell apoptosis, which downregulates the sCAM expression as a compensatory mechanism to prevent further vascular damage. In opposition, in subjects with MetS that have not yet developed T2DM and its accompanying stressors, the upregulation of the sCAM expression is ensued.

Myokines mediate the cross talk between skeletal muscle and other organs

Myokines are muscle-derived cytokines and chemokines that act extensively on organs and exert beneficial metabolic functions in the whole-body through specific signal networks. Myokines as mediators provide the conceptual basis for a whole new paradigm useful for understanding how skeletal muscle communicates with other organs. In this review, we summarize and discuss classes of myokines and their physiological functions in mediating the regulatory roles of skeletal muscle on other organs and the regulation of the whole-body energy metabolism. We review the mechanisms involved in the interaction between skeletal muscle and nonmuscle organs through myokines. Moreover, we clarify the connection between exercise, myokines and disease development, which may contribute to the understanding of a potential mechanism by which physical inactivity affects the process of metabolic diseases via myokines. Based on the current findings, myokines are important factors that mediate the effect of skeletal muscle on other organ functions and whole-body metabolism.

Irisin Regulates the Functions of Hepatic Stellate Cells

BACKGROUND

Hepatic stellate cells (HSCs) are known to play a fundamental role in the progression of liver fibrosis. Once HSCs are activated, they are involved in proliferation, migration, and contractility which are characteristics of liver fibrogenesis. Recent studies have shown that irisin, a myokine secreted during physical exercise, has a protective effect in various metabolic diseases, especially in renal fibrosis. However, whether irisin is involved in HSC activation and other processes associated with liver fibrosis has not yet been investigated. In this study, we reveal the role of irisin in HSC activation as well as in proliferation, migration, and contractile properties of HSCs in vitro.

METHODS

LX-2 cells, immortalized human HSCs, were treated with transforming growth factor beta 1 (TGF-β1), a core regulator of HSC fibrosis, with or without irisin, and markers of the aforementioned processes were analyzed. Further, an inflammatory response was stimulated with TGF-β1 and lipopolysaccharide (LPS) in combination with irisin and the expression of cytokines was measured.

RESULTS

Recombinant irisin significantly suppressed the expression of TGF-β1-stimulated fibrosis markers including alpha-smooth muscle actin and collagen type 1 alpha 1 and prevented the TGF-β1-induced proliferation, migration, and contractility of LX-2 cells. Additionally, irisin ameliorated the production of interleukin-6 (IL-6) and IL-1β induced by TGF-β1 and LPS treatments.

CONCLUSION

These findings suggested that irisin potently improved the progression of hepatic fibrosis by regulating HSC activation, proliferation, migration, contractility, and HSC-mediated production of inflammatory cytokine.

Serum irisin levels are decreased in patients with sepsis, and exogenous irisin suppresses ferroptosis in the liver of septic mice

BACKGROUND

Sepsis remains a major health issue without an effective therapy. Ferroptosis, an iron-dependent programmed cell death, has been proposed to be related to the pathogenesis of sepsis. Irisin, a myokine released during exercise, improves mitochondrial function under various conditions. Ferroptosis is closely related to mitochondrial function. However, the role of irisin in sepsis-induced ferroptosis and mitochondrial dysfunction in the liver remained unknown. Thus, we hypothesize that irisin treatment suppresses ferroptosis and improves mitochondrial function in sepsis.

METHODS

To study this, we first explored the role of serum irisin levels in patients with sepsis, and then determined the effect of irisin administration on ferroptosis and mitochondrial function in the liver of septic mice.

RESULTS

Serum irisin levels were decreased and negatively correlated with the APACHE II scores in patients with sepsis. In mice subjected to cecal ligation and puncture (CLP), exogenous irisin administration suppressed ferroptosis, inhibited inflammatory response, decreased reactive oxygen species (ROS) production, restored abnormal mitochondrial morphology, and increased mtDNA copy number and adenosine triphosphate (ATP) content. The effect of irisin on ferroptosis was confirmed in LPS-treated hepatocytes and CLP-induced septic mice. Inhibition of glutathione peroxidase 4 (GPX4), a central regulator of ferroptosis, reduced irisin's protective effects in LPS-treated hepatocytes and CLP-induced septic mice, while blocking the irisin receptor with RGD peptide or Echistain decreased irisin-induced GPX4 expression.

CONCLUSIONS

Serum irisin levels are decreased and negatively correlated with disease severity in patients with sepsis, and irisin treatment suppresses ferroptosis and restores mitochondrial function in experimental sepsis. Irisin may offer therapeutic potential in the management of sepsis.