Irisin Exerts Inhibitory Effect on Adipogenesis Through Regulation of Wnt Signaling

Differential mitochondrial adaptation and FNDC5 production in brown and white adipose tissue in response to cold and obesity

OBJECTIVE

Fibronectin type III domain-containing protein 5 (FNDC5) modulates adipocyte metabolism by increasing white and brown adipose tissue (WAT and BAT) browning and activity, respectively. We investigated whether FNDC5 can regulate visceral WAT and BAT adaptive thermogenesis by improving mitochondrial homeostasis in response to cold and obesity.

METHODS

Adipose tissue expression of FNDC5 and factors involved in mitochondrial homeostasis were determined in patients with normal weight and obesity (n = 159) and in rats with diet-induced obesity after 1 week of cold exposure (n = 61). The effect of different FNDC5 concentrations on mitochondrial biogenesis, dynamics, and mitophagy was evaluated in vitro in human adipocytes.

RESULTS

In human visceral adipocytes, FNDC5/irisin triggered mitochondrial biogenesis (TFAM) and fusion (MFN1, MFN2, and OPA1) while inhibiting peripheral fission (DNM1L and FIS1) and mitophagy (PINK1 and PRKN). Circulating and visceral WAT expression of FNDC5 was decreased in patients and experimental animals with obesity, whereas its receptor, integrin αV, was upregulated. Obesity increased mitochondrial fusion while decreasing mitophagy in visceral WAT from patients and rats. By contrast, in rat BAT, an upregulation of Fndc5 and genes involved in mitochondrial biogenesis and fission was observed. Cold exposure promoted mitochondrial biogenesis and healthy peripheral fission while repressing Fndc5 expression and mitophagy in BAT from rats.

CONCLUSIONS

Depot differences in FNDC5 production and mitochondrial adaptations in response to obesity and cold might indicate a self-regulatory mechanism to control thermogenesis in response to energy needs.

The therapeutic potential of irisin to mitigate the risk of metabolic syndrome in postmenopausal women

Oestradiol withdrawal at menopause predisposes women to metabolic syndrome, a cluster of interrelated conditions including obesity, insulin resistance, dyslipidaemia and hypertension that together confer an increased risk of developing type 2 diabetes mellitus and cardiovascular disease. Hormone replacement therapies are commonly used to treat acute symptoms of the perimenopausal period, and whilst they have been associated with metabolic improvements in many studies, long-term use is considered unviable. Novel approaches are required to mitigate the risk of postmenopausal metabolic syndrome. In 2012, the exercise-inducible myokine irisin was isolated from the skeletal muscle of mice and identified to have anti-obesity and antidiabetic effects in vivo. Irisin is now recognised to exert pleiotropic action on cognitive, bone and metabolic health. There is accumulating evidence from in vitro and in vivo rodent studies that irisin can mitigate each component condition of metabolic syndrome. In postmenopausal women, independent associations have been observed between (a) exercise and plasma irisin concentration and (b) plasma irisin concentration and reduced incidence of metabolic syndrome. To date, however, no study has considered the mechanistic basis by which irisin, whether exercise-induced or exogenously administered, could reduce the incidence or severity of metabolic syndrome in postmenopausal women. This review aims to analyse the literature concerning the metabolic actions of irisin, with a focus on its therapeutic potential for metabolic syndrome driven by a state of oestradiol depletion. It evaluates the practicality of exercise as a therapy and discusses other irisin-based therapeutic strategies that may alleviate postmenopausal metabolic syndrome. Finally, it highlights areas where future research is required to advance knowledge of irisin's biological action such that it could be considered a viable candidate for clinical application.

Liraglutide promotes UCP1 expression and lipolysis of adipocytes by promoting the secretion of irisin from skeletal muscle cells

Although Liraglutide (Lira) increases serum irisin levels in type 2 diabetes mellitus (T2DM), it is unclear whether it induces expression of uncoupling protein 1 (UCP1) of adipocytes via promoting irisin secretion from skeletal muscle. Male T2DM rats were treated with 0.4 mg/kg/d Lira twice a day for 8 weeks, and the protein expression of phosphorylated AMP kinase (p-AMPK), phosphorylated acetyl-CoA carboxylase 1 (p-ACC1) and UCP1 in white adipose tissues were detected. Differentiated C2C12 cells were treated with palmitic acid (PA) and Lira to detect the secretion of irisin. Differentiated 3T3-L1 cells were treated with irisin, supernatant from Lira-treated C2C12 cells, Compound C or siAMPKα1, the triglyceride (TG) content and the related gene expression were measured. The transcriptome in irisin-treated differentiated 3T3-L1 cells was analyzed. Lira elevated serum irisin levels, decreased the adipocyte size and increased the protein expression of UCP1, p-AMPK and p-ACC1 in WAT. Moreover, it promoted the expression of PGC1α and FNDC5, the secretion of irisin in PA-treated differentiated C2C12 cells. The irisin and supernatant decreased TG synthesis and promoted the expression of browning- and lipolysis-related genes in differentiated 3T3-L1 cells. While Compound C and siAMPKα1 blocked AMPK activities and expression, irisin partly reversed the pathway. Finally, the transcriptome analysis indicated that differently expressed genes are mainly involved in browning and lipid metabolism. Overall, our findings showed that Lira modulated muscle-to-adipose signaling pathways in diabetes via irisin-mediated AMPKα/ACC1/UCP1/PPARα pathway. Our results suggest a new mechanism for the treatment of T2DM by Lira.

Irisin alleviates obesity-induced bone loss by inhibiting interleukin 6 expression via TLR4/MyD88/NF-κB axis in adipocytes

INTRODUCTION

Obesity-induced bone loss affects the life quality of patients all over the world. Irisin, one of the myokines, plays an essential role in bone and fat metabolism.

OBJECTIVE

Investigate the effects of irisin on bone metabolism via adipocytes in the bone marrow microenvironment.

METHODS

In this study, we fed fibronectin type III domain-containing protein 5 (FNDC5, the precursor protein of irisin) knockout mice (FNDC5-/-) with a high-fat diet (HFD) for 10 weeks. The quality of bone mass was assessed by micro-CT analysis, histological staining, and dynamic bone formation. In vitro, the lipogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) was assayed by Oil Red O staining, and the osteogenic differentiation was assayed by alkaline phosphatase staining. Meanwhile, the gene expression in the BMSC-differentiated adipocytes by RNA sequence and the involved pathway of irisin were determined by western blot and qRT-PCR were performed.

RESULTS

The FNDC5-/- mice fed with a HFD showed an increased body weight, fat content of the bone marrow and bone, and a decreased bone formation compared with those with a standard diet (SD). In vitro, irisin inhibited the differentiation of BMSCs into adipocytes and alleviated the inhibition of osteogenesis derived from BMSCs by the adipocyte supernatant. RNA sequence and blocking experiment showed that irisin reduced the production of interleukin 6 (IL-6) in adipocytes through downregulating the TLR4/MyD88/NF-κB pathway. Immunofluorescence staining of bone marrow further confirmed an increased IL-6 expression in the FNDC5-/- mice fed with HFD compared with those fed with SD, which suffered serious bone loss.

CONCLUSION

Irisin downregulates activation of the TLR4/MyD88/NF-κB pathway, thereby reducing IL-6 production in adipocytes to enhance the osteogenesis of BMSCs. Thus, the rescue of osteogenesis of BMSCs, initially inhibited by IL-6, is a potential therapeutic target to mitigate obesity-induced osteoporosis.

The Role of the Myokine Irisin in the Protection and Carcinogenesis of the Gastrointestinal Tract

Recently discovered irisin, a member of the myokines family, is a potential mediator of exercise-induced energy metabolism and a factor promoting browning of the white adipose tissue. Recent evidence indicates that this myokine, released from contracting muscles, can mediate the beneficial effects of exercise on health. Irisin may be a potential therapeutic agent against obesity and has been shown to play an important role in the protection of various cells, tissues, and organs due to its anti-inflammatory, antioxidative, and anti-cancer properties. Our aim was to review the recent experimental and clinical studies on irisin and its expression, release into the bloodstream, tissue targets, and potential contribution to the protective effects of exercise in the gastrointestinal tract. Particular emphasis was placed on inflammatory bowel disease, intestinal ischemia/reperfusion injury, periodontitis, and other digestive tract disorders, including carcinogenesis. Overall, irisin holds significant potential as a novel target molecule, offering a safe and therapeutic approach to treating various gastrointestinal diseases.

Cytokines and exosomal miRNAs in skeletal muscle-adipose crosstalk

Skeletal muscle and adipose tissues (ATs) are secretory organs that release secretory factors including cytokines and exosomes. These factors mediate muscle-adipose crosstalk to regulate systemic metabolism via paracrine and endocrine pathways. Myokines and adipokines are cytokines secreted by skeletal muscle and ATs, respectively. Exosomes loaded with nucleic acids, proteins, lipid droplets, and organelles can fuse with the cytoplasm of target cells to perform regulatory functions. A major regulatory component of exosomes is miRNA. In addition, numerous novel myokines and adipokines have been identified through technological innovations. These discoveries have identified new biomarkers and sparked new insights into the molecular regulation of skeletal muscle growth and adipose deposition. The knowledge may contribute to potential diagnostic and therapeutic targets in metabolic disease.

Exercise-regulated white adipocyte differentitation: An insight into its role and mechanism

White adipocytes play a key role in the regulation of fat mass amount and energy balance. An appropriate level of white adipocyte differentiation is important for maintaining metabolic homeostasis. Exercise, an important way to improve metabolic health, can regulate white adipocyte differentiation. In this review, the effect of exercise on the differentiation of white adipocytes is summarized. Exercise could regulate adipocyte differentiation in multiple ways, such as exerkines, metabolites, microRNAs, and so on. The potential mechanism underlying the role of exercise in adipocyte differentiation is also reviewed and discussed. In-depth investigation of the role and mechanism of exercise in white adipocyte differentiation would provide new insights into exercise-mediated improvement of metabolism and facilitate the application of exercise-based strategy against obesity.

Irisin: A Possible Marker of Adipose Tissue Dysfunction in Obesity

Adipose tissue (AT) secretes pro- and anti-inflammatory cytokines involved in AT homeostasis, including tumor necrosis factor-α (TNFα) and irisin. The functionality of AT is based on a regulated equilibrium between adipogenesis and extracellular matrix (ECM) remodeling. We investigated the contributions of adipose progenitors (ASCs) and adipocytes (AMCs) to TNFα-induced ECM remodeling and a possible implication of irisin in AT impairment in obesity. ASCs and AMCs were exposed to TNFα treatment and nuclear factor-kappa (NF-kB) pathway was investigated: Tissue Inhibitor of Metalloproteinase (TIMP-1), Twist Family Transcription Factor 1 (TWIST-1), and peroxisome proliferator-activated receptor-γ (PPARγ) expression levels were analyzed. The proteolytic activity of matrix metalloproteinases (MMPs) -2 and -9 was analyzed by zymography, and the irisin protein content was measured by ELISA. In inflamed AMCs, a TIMP-1/TWIST-1 imbalance leads to a drop in PPARγ. Adipogenesis and lipid storage ability impairment come with local tissue remodeling due to MMP-9 overactivation. In vitro and ex vivo measurements confirm positive correlations among inflammation, adipose secreting irisin levels, and circulating irisin levels in patients with visceral obesity. Our findings identify the NF-kB downstream effectors as molecular initiators of AT dysfunction and suggest irisin as a possible AT damage and obesity predictive factor.

A 21-Day Individual Rehabilitation Exercise Training Program Changes Irisin, Chemerin, and BDNF Levels in Patients after Hip or Knee Replacement Surgery

Osteoarthritis (OA) is the most frequent worldwide cause of adult population disabilities. The study evaluated the effects of a 21-day individual rehabilitation exercise training program focused on improving patients' functional capacity. The study analyzed the changes in irisin, chemerin, and BDNF serum levels in 36 OA patients subjected to an individually-adjusted rehabilitation program 90 days after surgical hip or knee replacement. The changes in irisin, chemerin, and BDNF serum levels were measured using enzyme-linked immunosorbent assay (ELISA) kits. A 21-day individual rehabilitation exercise training program significantly increased irisin and BDNF, and decreased chemerin serum levels. The presented study indicates that individually-adjusted exercise training is an important modulator influencing serum levels of anti- and pro-inflammatory factors, leading to positive clinical outcomes in osteoarthritis therapy. Selected factors are considered potential markers of various pathophysiological conditions. The presented study brings new details to the discussion.

Mechanisms of sarcopenia in liver cirrhosis and the role of myokines

Sarcopenia is a syndrome characterized by a decline in skeletal muscle quantity and/or quality, strength and performance, leading to unfortunate events, such as injurious falls or even death. It is not identical to frailty and malnutrition, even though there is a significant overlap among these syndromes. In patients with liver cirrhosis (LC), sarcopenia is classified as secondary and has been associated with increased morbidity and mortality during the pre- and post-transplantation period. It can be a result of malnutrition, hyperammonemia, low physical activity, endocrine abnormalities, accelerated starvation, metabolic disturbances, altered gut function leading to chronic inflammation, and alcohol abuse. Myokines are peptides mainly synthesized by contracting muscle and adipose tissue cells and may play a key role in the pathophysiology of sarcopenia. More than a hundred myokines have been recognized, but only a few have been investigated. They can be classified as negative regulators, such as myostatin, tumor growth factor-β, activins, growth differentiation factor-11, and positive regulators of muscle growth including follistatin, bone morphogenic proteins, and irisin. So far, only myostatin, follistatin, irisin and decorin have been studied in LC-associated sarcopenia. In this review, we focused on the mechanisms of cirrhosis-related sarcopenia and the role of myokines that have already been studied in the literature, either as markers helping in the diagnostic evaluation of sarcopenia, or as prognostic factors of survival. Standard therapeutic options to prevent or treat sarcopenia in LC are also being reported, as well as the possible therapeutic implication of myokines.

Alcohol, White Adipose Tissue, and Brown Adipose Tissue: Mechanistic Links to Lipogenesis and Lipolysis

According to data from the World Health Organization, there were about 3 million deaths caused by alcohol consumption worldwide in 2016, of which about 50% were related to liver disease. Alcohol consumption interfering with the normal function of adipocytes has an important impact on the pathogenesis of alcoholic liver disease. There has been increasing recognition of the crucial role of adipose tissue in regulating systemic metabolism, far beyond that of an inert energy storage organ in recent years. The endocrine function of adipose tissue is widely recognized, and the significance of the proteins it produces and releases is still being investigated. Alcohol consumption may affect white adipose tissue (WAT) and brown adipose tissue (BAT), which interact with surrounding tissues such as the liver and intestines. This review briefly introduces the basic concept and classification of adipose tissue and summarizes the mechanism of alcohol affecting lipolysis and lipogenesis in WAT and BAT. The adipose tissue-liver axis is crucial in maintaining lipid homeostasis within the body. Therefore, this review also demonstrates the effects of alcohol consumption on the adipose tissue-liver axis to explore the role of alcohol consumption in the crosstalk between adipose tissue and the liver.

Adipose-Muscle crosstalk in age-related metabolic disorders: The emerging roles of adipo-myokines

Obesity and type 2 diabetes account for a considerable proportion of the global burden of age-related metabolic diseases. In age-related metabolic diseases, tissue crosstalk and metabolic regulation have been primarily linked to endocrine processes. Skeletal muscle and adipose tissue are endocrine organs that release myokines and adipokines into the bloodstream, respectively. These cytokines regulate metabolic responses in a variety of tissues, including skeletal muscle and adipose tissue. However, the intricate mechanisms underlying adipose-muscle crosstalk in age-related metabolic diseases are not fully understood. Recent exciting evidence suggests that myokines act to control adipose tissue functions, including lipolysis, browning, and inflammation, whereas adipokines mediate the beneficial actions of adipose tissue in the muscle, such as glucose uptake and metabolism. In this review, we assess the mechanisms of adipose-muscle crosstalk in age-related disorders and propose that the adipokines adiponectin and spexin, as well as the myokines irisin and interleukin-6 (IL-6), are crucial for maintaining the body's metabolic balance in age-related metabolic disorders. In addition, these changes of adipose-muscle crosstalk in response to exercise or dietary flavonoid consumption are part of the mechanisms of both functions in the remission of age-related metabolic disorders. A better understanding of the intricate relationships between adipose tissue and skeletal muscle could lead to more potent therapeutic approaches to prolong life and prevent age-related metabolic diseases.

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.

FNDC5 Promotes Adipogenic Differentiation of Primary Preadipocytes in Mashen Pigs

Fibronectin type III domain-containing protein 5 (FNDC5) plays an important role in fat deposition, which can be cut to form Irisin to promote fat thermogenesis, resulting in a decrease in fat content. However, the mechanism of FNDC5 related to fat deposition in pigs is still unclear. In this research, we studied the expression of FNDC5 on different adiposes and its function in the adipogenic differentiation of primary preadipocytes in Mashen pigs. The expression pattern of FNDC5 was detected by qRT-PCR and Western blotting in Mashen pigs. FNDC5 overexpression and interference vectors were constructed and transfected into porcine primary preadipocytes by lentivirus. Then, the expression of key adipogenic genes was detected by qRT-PCR and the content of lipid droplets was detected by Oil Red O staining. The results showed that the expression of FNDC5 in abdominal fat was higher than that in back subcutaneous fat in Mashen pigs, whereas the expression in back subcutaneous fat of Mashen pigs was significantly higher than that of Large White pigs. In vitro, FNDC5 promoted the adipogenic differentiation of primary preadipocytes of Mashen pigs and upregulated the expression of genes related to adipogenesis, but did not activate the extracellular signal-regulated kinase (ERK) signaling pathway. This study can provide a theoretical basis for FNDC5 in adipogenic differentiation in pigs.

Irisin enhances longevity by boosting SIRT1, AMPK, autophagy and telomerase

Ageing is characterised by the accumulation of molecular and cellular damage through time, leading to a decline in physical and mental abilities. Currently, society has experienced a rapid increase in life expectancy, which has led to an increase in age-associated diseases. Therefore, it is crucial to study the process of ageing to guarantee the best conditions in the final stages of life. In recent years, interest has increased in a myokine known as irisin, which is secreted during physical exercise. This polypeptide hormone is produced by various organs, mainly muscle, and once it is released into the blood, it performs a wide variety of functions that are involved in metabolic control and may be relevant during some of the diseases associated with ageing. The aim of this review is to highlight the recent studies of irisin, such as its mechanism of expression, blood release, distribution, tissue target and participation in various cellular metabolic reactions and the relationship with key anti-ageing pathways such as adenosine monophosphate-activated protein kinase, silent information regulator T 1, autophagy and telomerase. In conclusion, irisin is a key player during the ageing process and it could be a novel target molecule for the therapeutic approach to boost longevity pathways. However, more research will be necessary to use this promising hormone for this gain.

Gasdermin D Deficiency Does Not Protect Mice from High-Fat Diet-Induced Glucose Intolerance and Adipose Tissue Inflammation

The adipose tissue NLRP3 inflammasome has recently emerged as a contributor to obesity-related metabolic inflammation. Recent studies have demonstrated that the activation of the NLRP3 inflammasome cleaves gasdermin D (GSDMD) and induces pyroptosis, a proinflammatory programmed cell death. However, whether GSDMD is involved in the regulation of adipose tissue function and the development of obesity-induced metabolic disease remains unknown. The aim of the present study was to investigate the role of GSDMD in adipose tissue inflammation as well as whole-body metabolism using GSDMD-deficient mice fed a high-fat diet (HFD) for 30 weeks. The effects of GSDMD deficiency on adipose tissue, liver, and isolated macrophages from wild-type (WT) and GSDMD knockout (KO) mice were examined. In addition, 3T3-L1 cells were used to examine the expression of GSDMD during adipogenesis. The results demonstrate that although HFD-induced inflammation was partly ameliorated in isolated macrophages and liver, adipose tissue remained unaffected by GSDMD deficiency. Compared with the WT HFD mice, GSDMD KO HFD mice exhibited a mild increase in HFD-induced glucose intolerance with increased systemic and adipose tissue IL-1β levels. Interestingly, GSDMD deficiency caused accumulation of fat mass when challenged with HFD, partly by suppressing the expression of peroxisome proliferator-activated receptor gamma (PPARγ). The expression of GSDMD mRNA and protein was dramatically suppressed during adipocyte differentiation and was inversely correlated with PPARγ expression. Together, these findings indicate that GSDMD is not a prerequisite for HFD-induced adipose tissue inflammation and suggest a noncanonical function of GSDMD in regulation of fat mass through PPARγ.

Metabolic properties of irisin in health and in diabetes mellitus

Irisin is a polypeptide hormone of muscle tissue (myokine), the synthesis and secretion of which increase against the background of physical exertion, which plays a significant role in the metabolism of fat, muscle and bone tissues. It is known that irisin promotes the transformation of white adipose tissue into brown adipose tissue. It has also been experimentally proven that the introduction of irisin contributed to an increase in bone mass and the prevention of osteoporosis and muscular atrophy. There are works indicating a positive effect of irisin in the functioning of bone, fat and muscle tissues in humans. Diabetes mellitus (DM) is an independent risk factor for osteoporotic fractures and the development of specific diabetic myopathy, at the cellular level similar to the aging of muscle tissue, and type 2 diabetes is also associated with the presence of obesity. Thus, it is of particular interest to study the effect of irisin on the state of bone, muscle and adipose tissues and glucose homeostasis in patients with diabetes. This literature review highlights the biological functions of irisin in healthy people and patients with DM.

6-Bromoindirubin-3′-Oxime Regulates Colony Formation, Apoptosis, and Odonto/Osteogenic Differentiation in Human Dental Pulp Stem Cells

6-bromoindirubin-3′-oxime (BIO) is a candidate small molecule that effectively modulates Wnt signalling owing to its stable property. The present study investigated the influence of BIO on the odonto/osteogenic differentiation of human dental pulp stem cells (hDPSCs). hDPSCs were treated with 200, 400, or 800 nM BIO, and the effects on hDPSC responses and osteogenic differentiation were assessed. BIO-mediated Wnt activation was confirmed by β-catenin nuclear translocation detected by immunofluorescence staining. BIO attenuated colony formation and cell migration determined by in vitro wound-healing assay. BIO increased early apoptotic cell population evaluated using flow cytometry. For osteogenic induction, BIO promoted alkaline phosphatase (ALP) activity and mineralisation in a dose-dependent manner. ALP, RUNX2, OCN, OSX, ANKH, DMP1, and DSPP mRNA expression were significantly upregulated. The OPG/RANKL expression ratio was also increased. Further, BIO attenuated adipogenic differentiation as demonstrated by decreased lipid accumulation and adipogenic-related gene expression. Bioinformatic analysis of RNA sequencing data from the BIO-treated hDPSCs revealed that BIO modulated pathways related to autophagy and actin cytoskeleton regulation. These findings demonstrated that BIO treatment promoted hDPSC osteogenic differentiation. Therefore, this small molecule is a strong candidate as a bioactive molecule to enhance dentin repair.

Impact of fibronectin type III domain-containing family in the changes in metabolic and hormonal profiles during peripartum period in dairy cows

The peripartum period in dairy cows is frequently associated with excessive lipolysis due to Negative Energy Balance (NEB). These metabolic disorders are the cause of various pathologies. Some metabolites such as β-hydroxybutyrate (BHBA) and Non-Esterified Fatty Acids (NEFA) are known to be biomarkers of NEB in dairy cows. The involvement of adipokines, including adiponectin and leptin, during fat mobilization in the peripartum period is well described, but little is known about the impact of myokines at this time. Fibronectin type III domain-containing proteins (FNDC) are myokines and adipokines recently discovered to play a role in metabolic dysfunctions. This study aimed to evaluate some FNDCs (FNDC5, 4, 3A and B) as potential plasma and adipose tissue indicators of NEB in cattle. We measured plasma FNDC concentrations and adipose tissue FNDC gene expression during the peripartum period, 4 weeks before the estimated calving day (4WAP), one (1WPP) and 16 (16WPP) weeks postpartum in two groups of dairy cows with low NEB (LNEB, n = 8) and high NEB (HNEB, n = 13) at 1WPP. Using specific bovine ELISAs, only plasma FNDC5 concentrations varied during the peripartum period in both LNEB and HNEB animals; concentrations were higher at 1WPP as compared to 4WAP and 16 WPP. FNDC5 plasma concentrations was negatively correlated with dry matter intake, live body weight, variation of empty body weight and glucose concentrations, and positively correlated with plasma non-esterified fatty acids and BHBA concentrations. Subcutaneous adipose tissue contained abundant FNDC5 mRNA and protein, as measured by RT-qPCR and immunoblotting, respectively. We also observed that FNDC5 mRNA abundance in subcutaneous adipose tissue was higher at 1 WPP as compared to 4WAP and 16WPP in HNEB cows and higher at 1 WPP as compared to 4 WAP in LNEB cows, and was higher in HNEB than in LNEB animals during early lactation. Finally, we showed that recombinant human irisin (a fragmented product of FNDC5) increased the release of glycerol and abundance of mRNA encoding adipose triglyceride lipase and hormone-sensitive-lipase in bovine and human adipose tissue explants. In conclusion, FNDC5 is expressed in bovine adipose tissue and may be involved in lipid mobilization and regulation of NEB in cattle.

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.

The role of irisin in metabolic flexibility: beyond adipose tissue browning

Metabolic flexibility is the ability to adapt to physiological and environmental changes in metabolic demand. Irisin was originally discovered as an exercise-induced myokine involved in fat browning. In this review, we summarize emerging evidence for the role of irisin in regulating glucose metabolism and insulin sensitivity in skeletal muscle, neuroplasticity and satiety in central nervous system, β cell function and insulin secretion in the pancreas, bone remodeling, and adipose tissue function, which together orchestrate whole-body metabolic flexibility. Irisin is a key communicating mediator between skeletal muscle and other organs, and its manipulation could be a promising therapeutic strategy for treating obesity and related metabolic disorders. Teaser: This review summarizes recent progress in manipulating metabolic flexibility with irisin, and discusses its potential application as a drug target to treat obesity and related metabolic disorders.

FNDC5/Irisin: Physiology and Pathophysiology

A sedentary lifestyle or lack of physical activity increases the risk of different diseases, including obesity, diabetes, heart diseases, certain types of cancers, and some neurological diseases. Physical exercise helps improve quality of life and reduces the risk of many diseases. Irisin, a hormone induced by exercise, is a fragmented product of FNDC5 (a cell membrane protein) and acts as a linkage between muscles and other tissues. Over the past decade, it has become clear that irisin is a molecular mimic of exercise and shows various beneficial effects, such as browning of adipocytes, modulation of metabolic processes, regulation of bone metabolism, and functioning of the nervous system. Irisin has a role in carcinogenesis; numerous studies have shown its impact on migration, invasion, and proliferation of cancer cells. The receptor of irisin is not completely known; however, in some tissues it probably acts via a specific class of integrin receptors. Here, we review research from the past decade that has identified irisin as a potential therapeutic agent in the prevention or treatment of various metabolic-related and other diseases. This article delineates structural and biochemical aspects of irisin and provides an insight into the role of irisin in different pathological conditions.

Scientific Challenges on Theory of Fat Burning by Exercise

Exercise training decreases abdominal fat in an intensity-dependent manner. The fat loss effect of exercise has been intuitively thought to result from increased fat burning during and after exercise, defined by conversion of fatty acid into carbon dioxide in consumption of oxygen. Nevertheless, increasing exercise intensity decreases oxidation of fatty acids derived from adipose tissue despite elevated lipolysis. The unchanged 24-h fatty acid oxidation during and after exercise does not provide support to the causality between fat burning and fat loss. In this review, alternative perspectives to explain the fat loss outcome are discussed. In brief, carbon and nitrogen redistribution to challenged tissues (muscle and lungs) for fuel replenishment and cell regeneration against abdominal adipose tissue seems to be the fundamental mechanism underlying the intensity-dependent fat loss effect of exercise. The magnitude of lipolysis (fatty acid release from adipocytes) and the amount of post-meal carbon and nitrogen returning to abdominal adipose tissue determines the final fat tissue mass. Therefore, meal arrangement at the time when muscle has the greatest reconstruction demand for carbon and nitrogen could decrease abdominal fat accumulation while increasing muscle mass and tissue repair.

The Role of Peptide Hormones Discovered in the 21st Century in the Regulation of Adipose Tissue Functions

Peptide hormones play a prominent role in controlling energy homeostasis and metabolism. They have been implicated in controlling appetite, the function of the gastrointestinal and cardiovascular systems, energy expenditure, and reproduction. Furthermore, there is growing evidence indicating that peptide hormones and their receptors contribute to energy homeostasis regulation by interacting with white and brown adipose tissue. In this article, we review and discuss the literature addressing the role of selected peptide hormones discovered in the 21st century (adropin, apelin, elabela, irisin, kisspeptin, MOTS-c, phoenixin, spexin, and neuropeptides B and W) in controlling white and brown adipogenesis. Furthermore, we elaborate how these hormones control adipose tissue functions in vitro and in vivo.

The Role of Irisin in Exercise-Mediated Bone Health

Exercise training promotes physical and bone health, and is the first choice of non-drug strategies that help to improve the prognosis and complications of many chronic diseases. Irisin is a newly discovered peptide hormone that modulates energy metabolism and skeletal muscle mass. Here, we discuss the role of irisin in bone metabolism via exercise-induced mechanical forces regulation. In addition, the role of irisin in pathological bone loss and other chronic diseases is also reviewed. Notably, irisin appears to be a key determinant of bone mineral status and thus may serve as a novel biomarker for bone metabolism. Interestingly, the secretion of irisin appears to be mediated by different forms of exercise and pathological conditions such as diabetes, obesity, and inflammation. Understanding the mechanism by which irisin is regulated and how it regulates skeletal metabolism via osteoclast and osteoblast activities will be an important step toward applying new knowledge of irisin to the treatment and prevention of bone diseases such as osteolysis and other chronic disorders.

Irisin treatment lowers levels of phosphorylated tau in the hippocampus of pre-symptomatic female but not male htau mice

AIMS

Irisin is a hormone cleaved from fibronectin type-III domain-containing protein 5 in response to exercise and may be therapeutic in Alzheimer's disease (AD). Irisin is shown to repair damage caused by midlife cardiometabolic risk factors for AD (i.e., diabetes mellitus; hypertension), prevent neural amyloid beta aggregation and reduce neuroinflammation. However, there are no investigations of irisin's effect on AD-associated tauopathy in the brain. This study begins to address this gap in knowledge.

METHODS

Transgenic htau mice that selectively develop age-related tauopathy were treated with recombinant irisin (100 µg/kg weekly i.p.) beginning at a pre-symptomatic age (4 months) to determine if irisin could prevent emergence of early neuropathology. One month later, mice were sacrificed to collect brain tissue and serum. Protein levels of ptau (serine 202), inflammatory cytokine tumour necrosis factor alpha (TNFα) and FNDC5 were quantified using capillary-based western blotting (Wes).

RESULTS

Our data show that irisin treatment significantly reduced ptau and TNFα in the hippocampus and serum of female htau mice compared to vehicle-treated controls. Irisin treatment did not alter ptau levels in male htau hippocampus and appeared to enhance both neural and systemic TNFα levels.

CONCLUSIONS

This study provides the first evidence that enhancing the endogenous hormone irisin may be therapeutic against emerging neuropathology in a tauopathy-selective AD model. This is important because there are currently no disease-modifying therapeutics available for AD, and few agents in development address the multiple disease targets irisin appears to-making irisin an intriguing therapeutic candidate for further investigation.

Capmatinib attenuates lipogenesis in 3T3-L1 adipocytes through an adenosine monophosphate-activated protein kinase-dependent pathway

Recently, there is a rapid increase in the incidence of obesity, a condition for which there are no effective therapeutic agents. Capmatinib (CAP), a novel mesenchymal-to-epithelial transition inhibitor, is reported to attenuate pro-inflammatory mediators and oxidative stress. In this study, the effects of CAP on lipogenesis in the adipocytes were examined. Treatment with CAP dose-dependently suppressed lipid accumulation in, and differentiation of, and increased lipolysis in, 3T3-L1 adipocytes. Additionally, CAP treatment augmented adenosine monophosphate-activated protein kinase (AMPK) phosphorylation and FNDC5 expression in the adipocytes. Transfection with si-AMPK or si-FNDC5 mitigated the CAP-induced suppression of lipogenesis and enhanced lipolysis. Furthermore, transfection with si-FNDC5 mitigated the CAP-induced phosphorylation of AMPK. These results suggest that the anti-obesity effect of CAP is mediated through the irisin/AMPK pathway and that CAP is a novel therapeutic agent for obesity.

Associations of Circulating Irisin with FNDC5 Expression in Fat and Muscle in Type 1 and Type 2 Diabetic Mice

Irisin is an exercise-induced myokine, suggested to exert beneficial effects on metabolism. However, the studies on the regulation of irisin secretion and the expression of its precursor FNDC5 have shown conflicting data. The discrepancies among previous correlation studies in humans are related to the heterogeneity of the study population. The fact that irisin is not only a myokine but also an adipokine leads to the further complexity of the role of irisin in metabolic regulation. In this study, we examined the regulation of FNDC5 expression and irisin in circulation in both type 1 and type 2 diabetic mice, and their potential relationships with metabolic parameters. In streptozotocin (STZ)-induced type 1 diabetic mice, high-fat diet (HFD)-induced obese mice and db/db mice, the circulating irisin as well as FNDC5 gene expression in subcutaneous fat was downregulated. Muscle FNDC5 expression was only significantly lower in STZ mice, and epididymal fat FNDC5 expression was unaltered. It is interesting to note that plasma irisin levels correlated positively with subcutaneous fat FNDC5 expression, but not epididymal fat or muscle. Moreover, both irisin levels and subcutaneous fat FNDC5 correlated negatively with markers of insulin resistance. These results suggest a regulatory role for subcutaneous fat-derived FNDC5/irisin in metabolic disease.

The effect of Irisin on bone cells in vivo and in vitro

The myokine Irisin, produced during physical exercise, has an anabolic effect on bone, both in vitro and in vivo. Very recently, using a controlled in vitro 3D cell model to mimic the bone microenvironment aboard the International Space Station, it has been shown that Irisin treatment in microgravity prevents the down-regulation of the transcription factors Atf4, Runx2 and Osterix, as well as Collagen I and Osteoprotegerin proteins, crucial for osteoblast differentiation in physiologic conditions. Irisin action has also been investigated in human subjects, in which it correlates with bone health status, supporting its physiological importance also in human bone, both in healthy subjects and in patients suffering from diseases related to bone metabolism, such as hyperparathyroidism and type 1 diabetes. Low levels of circulating Irisin have been found in post-menopausal women affected by hyperparathyroidism. Furthermore, Irisin is positively correlated with bone strength in athletes and bone mineral density in football players. Moreover, in healthy children, Irisin is positively associated with bone mineral status and in children with type 1 diabetes, Irisin is positively correlated with improved glycemic control and skeletal health. In this review, we will focus on recent findings about Irisin action on microgravity induced bone loss and on osteocyte activity and survival through its αV/β5 integrin receptor.

Exercise-Induced Myokines can Explain the Importance of Physical Activity in the Elderly: An Overview

Physical activity has been found to aid the maintenance of health in the elderly. Exercise-induced skeletal muscle contractions lead to the production and secretion of many small proteins and proteoglycan peptides called myokines. Thus, studies on myokines are necessary for ensuring the maintenance of skeletal muscle health in the elderly. This review summarizes 13 myokines regulated by physical activity that are affected by aging and aims to understand their potential roles in metabolic diseases. We categorized myokines into two groups based on regulation by aerobic and anaerobic exercise. With aging, the secretion of apelin, β-aminoisobutyric acid (BAIBA), bone morphogenetic protein 7 (BMP-7), decorin, insulin-like growth factor 1 (IGF-1), interleukin-15 (IL-15), irisin, stromal cell-derived factor 1 (SDF-1), sestrin, secreted protein acidic rich in cysteine (SPARC), and vascular endothelial growth factor A (VEGF-A) decreased, while that of IL-6 and myostatin increased. Aerobic exercise upregulates apelin, BAIBA, IL-15, IL-6, irisin, SDF-1, sestrin, SPARC, and VEGF-A expression, while anaerobic exercise upregulates BMP-7, decorin, IGF-1, IL-15, IL-6, irisin, and VEGF-A expression. Myostatin is downregulated by both aerobic and anaerobic exercise. This review provides a rationale for developing exercise programs or interventions that maintain a balance between aerobic and anaerobic exercise in the elderly.

Irisin in Liver Cirrhosis

Background: Sarcopenia is a prevalent muscle abnormality characterized by progressive and generalized loss of skeletal muscle mass and strength, common among patients with decompensated advanced chronic liver disease (dACLD). Irisin is a recently identified myokine, which is mainly expressed and secreted by skeletal muscle. Pointing to the essential role of irisin in metabolic regulation and energy expenditure we hypothesize that it plays an important role in cirrhosis development and progression. Aim: To assess irisin serum levels in patients with dACLD, with different cirrhosis stage and etiology. To analyze relationship between sarcopenia and irisin serum levels. Methods: Serum irisin concentrations were measured with commercially available ELISA kits in 88 cirrhotic patients. Recorded parameters of muscle mass were hand-grip strength (HGS), mid-arm muscle circumference (MAC), and transversal psoas muscle index (TPMI). Results: There was no difference in serum irisin levels between cirrhotic patients with different Child-Pugh (CTP) and model of end-stage liver disease (MELD) score, and those with and without ascites. The Liver Frailty Index (LFI) was significantly higher in patients with more advanced liver disease according to CTP and MELD. There was no association between serum irisin level with MAC (r = 0.04, p = 0.74) nor with TPMI (r = 0.20, p = 0.06). We observed significant negative correlation between serum irisin level and age (r = −0.35, p < 0.001). Conclusions: Serum irisin levels did not correlate with sarcopenia. There was no difference in serum irisin levels between cirrhotic patients with and without diabetes. There was no difference in serum irisin levels among patients with more severe dACLD, although we observed significant LFI increase among patients with more advanced liver disease.

FNDC4, a novel adipokine that reduces lipogenesis and promotes fat browning in human visceral adipocytes

BACKGROUND

Fibronectin type IIIdomain-containing protein 4 (FNDC4) constitutes a secreted factor showing a high homology in the fibronectin type III and transmembrane domains with the exercise-associated myokine irisin (FNDC5). We sought to evaluate whether FNDC4 mimics the anti-obesity effects of FNDC5/irisin in human adipose tissue.

METHODS

Plasma and adipose tissue samples of 78 patients with morbid obesity undergoing bariatric surgery and 26 normal-weight individuals were used in the present study.

RESULTS

Plasma FNDC4 was decreased in patients with morbid obesity, related to obesity-associated systemic inflammation and remained unchanged six months after bariatric surgery. Visceral adipose tissue from patients with morbid obesity showed higher expression of FNDC4 and its putative receptor GPR116 regardless of the degree of insulin resistance. FNDC4 content was regulated by lipogenic, lipolytic and proinflammatory stimuli in human visceral adipocytes. FNDC4 reduced intracytosolic lipid accumulation and stimulated a brown-like pattern in human adipocytes, as evidenced by an upregulated expression of UCP-1 and the brown/beige adipocyte markers PRDM16, TMEM26 and CD137. Moreover, FNDC4 treatment upregulated mitochondrial DNA content and factors involved in mitochondrial biogenesis (TFAM, NRF1 and NRF2). Human FNDC4-knockdown adipocytes exhibited an increase in lipogenesis and a reduction of brown/beige-specific fat markers as well as factors involved in mitochondrial biogenesis.

CONCLUSIONS

Taken together, the novel adipokine FNDC4 reduces lipogenesis and increases fat browning in human visceral adipocytes. The upregulation of FNDC4 in human visceral fat might constitute an attempt to attenuate the adipocyte hypertrophy, inflammation and impaired beige adipogenesis in the obese state.

New insights into the cellular activities of Fndc5/Irisin and its signaling pathways

Fndc5, a well-defined myokine and also identified as an adipokine, has a critical role in modulation of metabolism and protection against obesity. These important functions are mediated by irisin, a secretory peptide produced from proteolytic processing of Fndc5. The other beneficial physiological effects of irisin are alleviation of oxidative stress, neuroprotective effects, and anti-inflammatory properties and associated anti-metastatic effects. Fndc5/irisin exerts its biological effects through several intracellular signaling pathways. The major signaling pathway is thought to be MAPK signaling pathways which are involved in neural differentiation, browning of white adipocytes, as well as osteoblast proliferation and differentiation. Other essential functions of Fndc5/irisin are mediated through additional pathways including AMPK pathway, PI3K/AKT, and STAT3/Snail. Thorough understanding of the mechanisms of irisin actions are essential in order to develop Fndc5/irisin for therapeutic purposes. In the present review, we focus on the current knowledge of the signaling pathways that elicit irisin actions.

Recombinant Irisin Prevents the Reduction of Osteoblast Differentiation Induced by Stimulated Microgravity through Increasing β-Catenin Expression

Background: Irisin, a novel exercise-induced myokine, was shown to mediate beneficial effects of exercise in osteoporosis. Microgravity is a major threat to bone homeostasis of astronauts during long-term spaceflight, which results in decreased bone formation. Methods: The hind-limb unloading mice model and a random position machine are respectively used to simulate microgravity in vivo and in vitro. Results: We demonstrate that not only are bone formation and osteoblast differentiation decreased, but the expression of fibronectin type III domain-containing 5 (Fdnc5; irisin precursor) is also downregulated under simulated microgravity. Moreover, a lower dose of recombinant irisin (r-irisin) (1 nM) promotes osteogenic marker gene (alkaline phosphatase (Alp), collagen type 1 alpha-1(ColIα1)) expressions, ALP activity, and calcium deposition in primary osteoblasts, with no significant effect on osteoblast proliferation. Furthermore, r-irisin could recover the decrease in osteoblast differentiation induced by simulated microgravity. We also find that r-irisin increases β-catenin expression and partly neutralizes the decrease in β-catenin expression induced by simulated microgravity. In addition, β-catenin overexpression could also in part attenuate osteoblast differentiation reduction induced by simulated microgravity. Conclusions: The present study is the first to show that r-irisin positively regulates osteoblast differentiation under simulated microgravity through increasing β-catenin expression, which may reveal a novel mechanism, and it provides a prevention strategy for bone loss and muscle atrophy induced by microgravity.