Higher baseline irisin concentrations are associated with greater reductions in glycemia and insulinemia after weight loss in obese subjects

White-to-Beige and Back: Adipocyte Conversion and Transcriptional Reprogramming

Obesity has become a pandemic, as currently more than half a billion people worldwide are obese. The etiology of obesity is multifactorial, and combines a contribution of hereditary and behavioral factors, such as nutritional inadequacy, along with the influences of environment and reduced physical activity. Two types of adipose tissue widely known are white and brown. While white adipose tissue functions predominantly as a key energy storage, brown adipose tissue has a greater mass of mitochondria and expresses the uncoupling protein 1 (UCP1) gene, which allows thermogenesis and rapid catabolism. Even though white and brown adipocytes are of different origin, activation of the brown adipocyte differentiation program in white adipose tissue cells forces them to transdifferentiate into "beige" adipocytes, characterized by thermogenesis and intensive lipolysis. Nowadays, researchers in the field of small molecule medicinal chemistry and gene therapy are making efforts to develop new drugs that effectively overcome insulin resistance and counteract obesity. Here, we discuss various aspects of white-to-beige conversion, adipose tissue catabolic re-activation, and non-shivering thermogenesis.

Comprehensive Strategies for Metabolic Syndrome: How Nutrition, Dietary Polyphenols, Physical Activity, and Lifestyle Modifications Address Diabesity, Cardiovascular Diseases, and Neurodegenerative Conditions

Several hallmarks of metabolic syndrome, such as dysregulation in the glucose and lipid metabolism, endothelial dysfunction, insulin resistance, low-to-medium systemic inflammation, and intestinal microbiota dysbiosis, represent a pathological bridge between metabolic syndrome and diabesity, cardiovascular, and neurodegenerative disorders. This review aims to highlight some therapeutic strategies against metabolic syndrome involving integrative approaches to improve lifestyle and daily diet. The beneficial effects of foods containing antioxidant polyphenols, intestinal microbiota control, and physical activity were also considered. We comprehensively examined a large body of published articles involving basic, animal, and human studie, as well as recent guidelines. As a result, dietary polyphenols from natural plant-based antioxidants and adherence to the Mediterranean diet, along with physical exercise, are promising complementary therapies to delay or prevent the onset of metabolic syndrome and counteract diabesity and cardiovascular diseases, as well as to protect against neurodegenerative disorders and cognitive decline. Modulation of the intestinal microbiota reduces the risks associated with MS, improves diabetes and cardiovascular diseases (CVD), and exerts neuroprotective action. Despite several studies, the estimation of dietary polyphenol intake is inconclusive and requires further evidence. Lifestyle interventions involving physical activity and reduced calorie intake can improve metabolic outcomes.

Associations between blood manganese levels and sarcopenia in adults: insights from the National Health and Nutrition Examination Survey

Background

While increasing concerns arise about the health effects of environmental pollutants, the relationship between blood manganese (Mn) and sarcopenia has yet to be fully explored in the general population.

Objective

This study aims to investigate the association between blood manganese (Mn) levels and sarcopenia in adults.

Methods

In our study, we evaluated 8,135 individuals aged 18-59 years, utilizing data from the National Health and Nutrition Examination Survey (NHANES) spanning 2011 to 2018. We employed generalized additive model (GAM) to discern potential non-linear relationships and utilized the two-piecewise linear regression model to probe the association between blood Mn levels and sarcopenia.

Results

After adjusting for potential confounders, we identified non-linear association between blood Mn levels and sarcopenia, with an inflection point at 13.45 μg/L. The effect sizes and the confidence intervals on the left and right sides of the inflection point were 1.006 (0.996 to 1.048) and 1.082 (1.043 to 1.122), respectively. Subgroup analysis showed that the effect sizes of blood Mn on sarcopenia have significant differences in gender and different BMI groups.

Conclusion

Our results showed that a reverse U-shaped curve between blood Mn levels and sarcopenia, with an identified the inflection point at blood Mn level of 13.45 μg/L.

Irisin Regulates Cardiac Responses to Exercise in Health and Diseases: a Narrative Review

Exercise has been recognized as an important non-pharmacological approach for the prevention, treatment, and rehabilitation of cardiovascular diseases, but the mechanisms of exercise in promoting cardiovascular health remain unclear. Exercise generates cardiac benefits via stimulating muscle to secret hundreds of myokines that directly enter circulation and target heart tissue. Therefore, inter-organ communication between skeletal muscle and heart may be one important regulating pattern, and such communication can occur through secretion of molecules, frequently known as myokines. Irisin, a newly identified myokine, is cleaved from fibronectin type III domain-containing protein 5 (FNDC5) and secreted by the stimulation of exercise. Recently, accumulating evidence focusing on the interaction between irisin and cardiac function has been reported. This review highlights the molecular signaling by which irisin regulates the benefits of exercise on cardiac function both in physiological and pathological process, and discusses the clinical potential of irisin in treating heart diseases. Exercise generates various cardiovascular benefits through stimulating skeletal muscle to secrete irisin. The exercise "hormone" irisin, both produced by exercise or recombinant form, exerts therapeutic effects in a group of cardiovascular disorders including heart failure, myocardial infarction, atherosclerosis and hypertension. However, the molecular mechanisms involved remain ambiguous.This review highlights the most up-to-date findings to bridge the gap between exercise, irisin and cardiovascular diseases, and discusses the potential clinical prospect of irisin.

What Is the Relationship Between Body Mass Index, Sex Hormones, Leptin, and Irisin in Children and Adolescents? A Path Analysis

OBJECTIVE

The present research aimed to study the relationship between body mass index (BMI), sex hormones, leptin, and irisin in children and adolescents with different body types.

METHODS

In this study, a stratified cluster random sampling method was used to select students aged 8-15 years from two 9-year schools as the research subjects. Based on a case-control study, 183 overweight/obese students were selected. After using sex and age matching to create a matched sample of normal-weighted students, a total of 366 students, including 214 boys (58.5%) and 152 girls (41.5%) were included. We measured their height and weight and calculated their body mass index BMI. Afterward, their concentrations of leptin, irisin, oestradiol (E2), and testosterone (T) in the serum were detected.

RESULTS

There were significant differences in T, E2, leptin, and irisin between normal-weighted boys and girls (p < 0.05). There were statistically significant differences in T, E2, and irisin between overweight/obese boys and girls (p < 0.05). Overweight/obese students had higher concentrations of irisin and leptin than normal-weight students (p < 0.05). The direct effect of BMI on irisin was not statistically significant in either normal or overweight/obese students, but their indirect effects via leptin were statistically significant (for normal-weight boys and girls, standardized indirect effect coefficient: 0.29 and 0.38, respectively; for overweight/obese boys and girls, standardized indirect effect coefficient: 0.36 and 0.34, respectively). There was a negative pathway of E2 → leptin → irisin in normal-weight boys (standardized indirect effect coefficient: -0.24) and a negative pathway of T → leptin → irisin in overweight/obese boys (standardized indirect effect coefficient: -0.27).

CONCLUSION

The indirect effects of BMI on irisin via leptin exist in children and adolescents of different body types. E2 was negatively correlated with leptin in normal-weight boys, whereas T was negatively correlated with leptin in overweight/obese boys.

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.

Novel Roles of Follistatin/Myostatin in Transforming Growth Factor-β Signaling and Adipose Browning: Potential for Therapeutic Intervention in Obesity Related Metabolic Disorders

Obesity is a global health problem and a major risk factor for several metabolic conditions including dyslipidemia, diabetes, insulin resistance and cardiovascular diseases. Obesity develops from chronic imbalance between energy intake and energy expenditure. Stimulation of cellular energy burning process has the potential to dissipate excess calories in the form of heat via the activation of uncoupling protein-1 (UCP1) in white and brown adipose tissues. Recent studies have shown that activation of transforming growth factor-β (TGF-β) signaling pathway significantly contributes to the development of obesity, and blockade or inhibition is reported to protect from obesity by promoting white adipose browning and increasing mitochondrial biogenesis. Identification of novel compounds that activate beige/brown adipose characteristics to burn surplus calories and reduce excess storage of fat are actively sought in the fight against obesity. In this review, we present recent developments in our understanding of key modulators of TGF-β signaling pathways including follistatin (FST) and myostatin (MST) in regulating adipose browning and brown adipose mass and activity. While MST is a key ligand for TGF-β family, FST can bind and regulate biological activity of several TGF-β superfamily members including activins, bone morphogenic proteins (BMP) and inhibins. Here, we review the literature supporting the critical roles for FST, MST and other proteins in modulating TGF-β signaling to influence beige and brown adipose characteristics. We further review the potential therapeutic utility of FST for the treatment of obesity and related metabolic disorders.

The Usefulness of Diagnostic Panels Based on Circulating Adipocytokines/Regulatory Peptides, Renal Function Tests, Insulin Resistance Indicators and Lipid-Carbohydrate Metabolism Parameters in Diagnosis and Prognosis of Type 2 Diabetes Mellitus with Obesity

The quantitative analysis of selected regulatory molecules, i.e., adropin, irisin, and vaspin in the plasma of obese patients with newly diagnosed, untreated type 2 diabetes mellitus, and in the same patients after six months of using metformin, in relation to adropinemia, irisinemia and vaspinemia in obese individuals, was performed. The relationship between plasma concentration of the adipocytokines/regulatory peptides and parameters of renal function (albumin/creatinine ratio-ACR, estimated glomerular filtration rate-eGFR), values of insulin resistance indicators (Homeostatic Model Assessment of Insulin Resistance (HOMA-IR2), Homeostatic Model Assessment of Insulin Sensitivity (HOMA-S), Homeostatic Model Assessment of β-cell function (HOMA-B), quantitative insulin sensitivity check index (QUICKI), insulin), and parameters of carbohydrate-lipid metabolism (fasting plasma glucose-FPG, glycated hemoglobin-HbA_1C, estimated glucose disposal rate-eGDR, fasting lipid profile, TG/HDL ratio) in obese type 2 diabetic patients was also investigated. Circulating irisin and vaspin were found significantly different in subjects with metabolically healthy obesity and in type 2 diabetic patients. Significant increases in blood levels of both analyzed adipokines/regulatory peptides were observed in diabetic patients after six months of metformin treatment, as compared to pre-treatment levels. The change in plasma vaspin level in response to metformin therapy was parallel with the improving of insulin resistance/sensitivity parameters. An attempt was made to identify a set of biochemical tests that would vary greatly in obese non-diabetic subjects and obese patients with type 2 diabetes, as well as a set of parameters that are changing in patients with type 2 diabetes under the influence of six months metformin therapy, and thus differentiating patients' metabolic state before and after treatment. For these data analyses, both statistical measures of strength of the relationships of individual parameters, as well as multidimensional methods, including discriminant analysis and multifactorial analysis derived from machine learning methods, were used. Adropin, irisin, and vaspin were found as promising regulatory molecules, which may turn out to be useful indicators in the early detection of T2DM and differentiating the obesity phenotype with normal metabolic profile from T2DM obese patients. Multifactorial discriminant analysis revealed that irisin and vaspin plasma levels contribute clinically relevant information concerning the effectiveness of metformin treatment in T2D patients. Among the sets of variables differentiating with the highest accuracy the metabolic state of patients before and after six-month metformin treatment, were: (1) vaspin, HbA1c, HDL, LDL, TG, insulin, and HOMA-B (ACC = 88 [%]); (2) vaspin, irisin, QUICKI, and eGDR (ACC = 86 [%]); as well as, (3) vaspin, irisin, LDL, HOMA-S, ACR, and eGFR (ACC = 86 [%]).

Irisin induces trophoblast differentiation via AMPK activation in the human placenta

Irisin, an adipokine, regulates differentiation and phenotype in various cell types including myocytes, adipocytes, and osteoblasts. Circulating irisin concentration increases throughout human pregnancy. In pregnancy disorders such as preeclampsia and gestational diabetes mellitus, circulating irisin levels are reduced compared to healthy controls. To date, there are no data on the role and molecular function of irisin in the human placenta or its contribution to pathophysiology. Aberrant trophoblast differentiation is involved in the pathophysiology of preeclampsia. The current study aimed to assess the molecular effects of irisin on trophoblast differentiation and function. First-trimester placental explants were cultured and treated with low (10 nM) and high (50 nM) physiological doses of irisin. Treatment with irisin dose-dependently increased both in vitro placental outgrowth (on Matrigel™) and trophoblast cell-cell fusion. Adenosine monophosphate-activated protein kinase (AMPK) signaling, an important regulator of cellular energy homeostasis that is involved in trophoblast differentiation and pathology, was subsequently investigated. Here, irisin exposure induced placental AMPK activation. To determine the effects of irisin on trophoblast differentiation, two trophoblast-like cell lines, HTR-8/SVneo and BeWo, were treated with irisin and/or a specific AMPK inhibitor (Compound C). Irisin-induced AMPK phosphorylation in HTR-8/SVneo cells. Additionally, as part of the differentiation process, integrin switching from α6 to α1 occurred as well as increased invasiveness. Overall, irisin promoted differentiation in villous and extravillous cell-based models via AMPK pathway activation. These findings provide evidence that exposure to irisin promotes differentiation and improves trophoblast functions in the human placenta that are affected in abnormal placentation.

Circulating Irisin and Myostatin as Markers of Muscle Strength and Physical Condition in Elderly Subjects

Background and objective

Aging is a physiological process known to produce changes in body composition, affecting the musculature and leading to decreased muscle strength. Muscle in response to exercise acts as an endocrine organ, producing and releasing myokines such as irisin and myostatin that modulate muscular growth. Here, we aimed to evaluate the effects of low intensity resistance exercise, with or without protein supplementation, on body composition, anthropometric parameters and circulating irisin and myostatin in elderly subjects.

Methods

This is a prospective and controlled clinical trial in which subjects were randomized into 3 groups: (1) control group (n = 20), (2) low intensity resistance exercise group (RE) (n = 14), and (3) low intensity resistance exercise and nutritional support group (RENS) (n = 9). Participants, aged 60-75 years, were studied at baseline and 16 weeks thereafter. Body composition was evaluated through bioelectric impedance. Serum irisin and myostatin was measured using ELISA.

Results

At follow-up, RENS resulted in a significant increase in fat free mass (47.4 ± 7.4 vs. 46.5 ± 7.4, p = 0.046), the calf muscle circumference (36.4 ± 1.3 vs. 32.3 ± 4.3, p = 0.025), and circulating irisin (3 ± 1.1 vs. 2.6 ± 1.3, p = 0.030) compared to baseline. RE resulted in a significant increase in grip strength (17.2 ± 4.6 vs. 15.3 ± 4.6, p = 0.011) and irisin (3.1 ± 0.8 vs. 2.4 ± 0.3, p = 0.011) and decreased walking speed at different distance (p < 0.02). Opposite findings in these parameters were observed in control intervention. In line with these findings, the percent change of calf muscle circumference (p = 0.003) and fat free mass (p < 0.0001) were significantly increased in RENS compared to control, whereas fat mass (p = 0.033) was decreased. Interestingly, in this group, strength was positively correlated with fat free mass (r = 0.782, p = 0.008), and circulating irisin was significantly decreased in those participants with strength loss at the end of the study (p = 0.002). No significant correlation between circulating irisin and myostatin in any group was observed.

Conclusion

Circulating irisin, but not myostatin, constitutes a marker for improved muscular performance in elderly subjects.

Metformin-induced autophagy and irisin improves INS-1 cell function and survival in high-glucose environment via AMPK/SIRT1/PGC-1α signal pathway

In order to explore the protective function of metformin on pancreatic β cells to alleviate insulin resistance and underlying mechanisms, INS-1 cells were cultured into normal control (N), high glucose (H), high glucose and metformin (H + Met), high glucose and chloroquine (H + CQ), and high glucose and Ex527 (H + Ex527) groups, respectively. Upon 24-hr cultivation, the proliferation and glucose-stimulated insulin secretion (GSIS) of INS-1 cells were determined, and the expression of irisin and other proteins associated with AMPK/SIRT1/PGC-1α signal pathway, autophagy, and apoptosis was evaluated. Compared with the N group, the cells from the H group revealed lower proliferation, GSIS, and expression of irisin and proteins associated with AMPK/SIRT1/PGC-1α signal pathway and autophagy, but higher expression of proteins associated with apoptosis; in contrast, metformin could significantly rescue lower cell proliferation, GSIS, and expression of proteins associated with AMPK/SIRT1/PGC-1α signal pathway and autophagy, as well as irisin, and suppress apoptosis in high-glucose environment. Meanwhile, autophagy inhibitor CQ and SIRT1 inhibitor Ex527 can block above functions of metformin. Therefore, metformin can promote INS-1 cell proliferation, enhance GSIS, and suppress apoptosis by activating AMPK/SIRT1/PGC-1α signal pathway, up-regulating irisin expression, and inducing autophagy in INS-1 cells in high-glucose environment.

Higher Baseline Serum Irisin Decreases Risk for Body Mass Index Increment in Chinese Populations: A 3.2-Year Cohort Study

INTRODUCTION

Irisin, a newly discovered myokine, exerts beneficial effects on energy metabolism. However, published results from studies examining the relationship between irisin concentration and obesity have been conflicting. The aim of our study was to investigate the association between serum irisin level and obese individuals with different body mass index (BMI) values and to explore the question of whether serum irisin can predict the risk of increases in the BMI.

METHODS

This study based on the data collected in the Risk Evaluation of cAncers in Chinese diabeTic Individuals: a lONgitudinal (REACTION). The cross-sectional cohort study was carried out from May 2011 to August 2011, and a longitudinal cohort study was conducted from July 2014 to October 2014 to complete the first 3.2-year follow-up. We enrolled 93 low-weight subjects (BMI < 18.5 kg/m²), 94 normal-weight subjects (BMI 18.5-23.9 kg/m²), 98 overweight subjects (BMI 24.0-27.9 kg/m²) and 93 obese subjects (BMI ≥ 28 kg/m²). Subjects in the normal-weight, overweight and obese groups were selected to match low-weight subjects by age and sex. Serum samples were obtained from all subjects to determine the irisin level.

RESULTS

Subjects with a higher serum irisin level tended to have significantly lower changes in BMI and body fat percentage and higher baseline high-density lipoprotein cholesterol level (p < 0.05). No significant correlation was observed between serum irisin level and the baseline obesity index. Serum irisin level was positively correlated to an active lifestyle (i.e. physical activity; β = 1.138, p  = 0.032) and negatively correlated to fasting plasma glucose level (β = - 0.996, p  = 0.023), changes in BMI (β = - 0.533, p  = 0.002), waist circumference (β = - 0.102, p  = 0.018), body fat percentage (β = - 0.457, p  = 0.001) and Chinese visceral adiposity index (β = - 0.280, p  = 0.028). After adjustment for cofactors, higher baseline serum irisin was an independent factor for a decreased BMI increment (baseline serum irisin: odds ratio 0.747, 95% confidence interval 0.652-0.949, p  = 0.002).

CONCLUSIONS

Higher serum irisin at baseline independently predicted a lower BMI increment in Chinese populations.

Irisin interaction with adipose tissue secretions by exercise training and flaxseed oil supplement

BACKGROUND

Previous studies have shown that physical training and natural diet able to change the expression and concentration of peptides and proteins. Myokines and adipokines play an important role in metabolism and metabolic syndrome. Therefore, the purpose of the present study was to investigate the effect of high-intensity interval training (HIIT) and supplementation of flaxseed oil on plasma irisin, nesfatin-1 and resistin in male rats.

METHODS

Forty adult male rats were randomly divided into four groups (ten in each group) including Control-Saline (CS), Training-Saline (TS), Control-FlaxOil supplement (CO), and Training-FlaxOil supplement (TO). The training groups performed for 10 weeks and 5 sessions each week, interval training with 90-95% VO2max on rodent treadmill, and supplement groups received flaxseed oil (300 mg / kg). Five days after the last training session, rats were sacrificed. Blood samples were taken from the heart and plasma was evaluated.

RESULTS

Exercise Training significantly increased plasma levels of irisin (P = 0.019), nesfatin-1 (P = 0.01), and decreased resistin (P = 0.01). Flaxseed oil significantly reduced plasma resistin levels (P = 0.02). Plasma irisin levels in the supplementation group were higher than all groups (P = 0.041).

CONCLUSION

There was a significant positive correlation between plasma levels of irisin with nesfatin-1 and negative correlation with resistin. HIIT program with flaxseed oil as a modality can create a metabolic crosstalk between skeletal muscle and adipose tissues and have health benefits.

Irisin: A Hope in Understanding and Managing Obesity and Metabolic Syndrome

White adipose tissue (WAT) is an endocrine organ highly integrated in homeostasis and capable of establishing ways of communicating and influencing multiple metabolic processes. Brown adipose tissue promotes energy expenditure by incorporating the uncoupling protein 1 (UCP1), also known as thermogenin, which decouples cellular respiration and heat production, in the mitochondrial membranes. Recent data suggest the presence of a thermogenic cell formation from white adipocytes (beige or brite cells) with a potential role in preventing obesity and metabolic syndrome. The formation of these cells is influenced by physical exertion that induces expression of PPARγ coactivator-1 (PGC1) and downstream membrane protein, fibronectin type III domain-containing protein 5 (FNDC5) in skeletal muscle. Irisin, a thermogenic adipomyokine produced by FNDC5 cleavage is involved in the browning of adipose tissue. While animal studies are congruent with regard to the relationship between physical exertion and irisin release, the results from human studies are less than clear. Therefore, this review focuses on recent advances in our understanding of muscle and adipose tissue thermogenesis. Further, it describes the molecular mechanisms by which irisin impacts exercise, glucose homeostasis and obesity. Finally, the review discusses current gaps and controversies related to irisin release, its mode of action and its future potential as a therapeutic tool in managing obesity and metabolic syndrome.

The Role of Irisin, Insulin and Leptin in Maternal and Fetal Interaction

OBJECTIVE

Insulin is an important hormone for intrauterine growth. Irisin is an effective myokine in the regulation of physiological insulin resistance in pregnancy. Leptin and insulin are associated with fetal growth and fetal adiposity. In this study, we aimed to investigate the relationships between irisin, insulin and leptin levels and maternal weight gain, as well as anthropometric measurements in the newborn.

METHODS

Eighty-four mothers and newborns were included in the study. Irisin, leptin and insulin levels were measured in the mothers and in cord blood. Anthropometric measurements in the newborn, maternal weight at the beginning of the pregnancy and at delivery were recorded.

RESULTS

Birth weight were classified as small for gestational age (SGA), appropriate for gestational age (AGA) and large for gestational age (LGA). There was no difference in irisin levels among the groups. Leptin and insulin levels were found to change significantly according to birth weight (p=0.013, and p=0.012, respectively). There was a negative correlation between the anthropometric measurements of the AGA newborns and irisin levels. This correlation was not observed in SGA and LGA babies. Leptin levels were associated with fetal adiposity.

CONCLUSION

While irisin levels are not affected by weight gain during pregnancy nor by birth weight, they show a relationship with anthropometric measurements in AGA infants. These results may lead to the understanding of metabolic disorders that will occur in later life.

Abnormal irisin level in serum and endometrium is associated with metabolic dysfunction in polycystic ovary syndrome patients

INTRODUCTION

Polycystic ovary syndrome (PCOS) is the most common endocrinopathy in reproductive-age women. Irisin is considered to play a role in metabolic disorder and PCOS. However, correlation between irisin and metabolic disorder in PCOS is not clear.

MATERIAL AND METHODS

This is a prospective study. Forty patients with PCOS and thirty patients without PCOS were recruited for in vitro fertilization and embryo transfer (IVF-ET). All PCOS women fulfilled all three Rotterdam consensus criteria. In each group, patients were also divided into obese and nonobese patients, and patients with or without dyslipidaemia.

RESULTS

Serum irisin level in PCOS patients was significantly reduced. Irisin level in obese PCOS patients was significantly lower than in nonobese PCOS patients. Irisin level in PCOS patients with dyslipidaemia was significantly higher than in PCOS patients with normal blood lipid profile. In both PCOS and control patients, serum irisin level was negatively correlated with body weight and waist-hip ratio (WHR). Moreover, serum irisin level was positively correlated with body fat rate, BAI, fasting plasma glucose (FPG) and HOMA-IR in PCOS patients. In addition, serum irisin level was positively correlated with HOMA-IR in control patients. In PCOS patients, body weight and HOMA-IR could predict the level of irisin. In control patients, body mass index (BMI) could predict the level of irisin. Expression of irisin in PCOS patients was lower than that in control patients. However, there was no significant difference of irisin expression between the subdivided groups in PCOS and control patients.

CONCLUSIONS

Taken together, the present findings enriched the knowledge about the role of irisin in metabolic dysfunction in PCOS patients.

Plasma irisin is increased following 12 weeks of Nordic walking and associates with glucose homoeostasis in overweight/obese men with impaired glucose regulation

Irisin is a myokine that is thought to be secreted in response to exercise that may help to prevent obesity and maintain normal glucose metabolism. In this study we investigated the associations between irisin and glucose homeostasis in middle-aged, overweight and obese men (n =  144) with impaired glucose regulation, and the impact of exercise training on these relationships. The participants underwent 12 weeks of resistance or aerobic (Nordic walking) exercise training three times per week, 60 minutes per session. Venous blood (n = 105) and skeletal muscle samples (n = 45) were obtained at baseline and post-intervention. Compared to controls, Nordic walking, but not resistance training, increased irisin levels in plasma (9.6 ± 4.2%, P = 0.014; 8.7 ±  4.9%, P = 0.087; respectively) compared to controls. When considering all subjects, baseline irisin correlated positively with atherogenic index of plasma (r = 0.244, P = 0.013) and 2-hour insulin levels (r = 0.214, P = 0.028), and negatively with age (r = -0.262, P = 0.007), adiponectin (r = -0.240, P = 0.014) and McAuley index (r = -0.259, P = 0.008). Training-induced FNDC5 mRNA changes were negatively correlated with HbA1c (r = -0.527, P = 0.030) in the resistance training group and with chemerin in the Nordic walking group (r = -0.615, P = 0.033). In conclusion, 12-weeks of Nordic walking was more effective than resistance training in elevating plasma irisin, in middle-aged men with impaired glucose tolerance. Thus, the change in irisin in response to exercise training varied by the type of exercise but showed limited association with improvements in glucose homeostasis.

Exercise-induced irisin release as a determinant of the metabolic response to exercise training in obese youth: the EXIT trial

The mechanisms underlying the metabolic improvements following aerobic exercise training remain poorly understood. The primary aim of this study was to determine if an adipomyokine, irisin, responded to acute exercise was associated with the metabolic adaptations to chronic aerobic exercise in obese youth. The acute response to exercise was assessed in 11 obese youth following 45‐min acute bouts of aerobic (AE) and resistance exercise (RE). The irisin area under the curve (pre‐exercise, 15, 30, and 45 min) during these AE sessions were the main exposure variables. The primary outcome measure was the change in insulin sensitivity using the Matsuda index, following 6 weeks of RE training, delivered for 45 min, three times per week at 60–65% 1RM. Participants were also categorized as either responders (above) or nonresponders (below) based on the percentage change in the Matsuda index following the 6‐week intervention. Irisin increased significantly during the acute bout of AE from 29.23 ± 6.96 to 39.30 ± 7.05 ng/mL; P = 0.028, but not significantly during the RE session (P = 0.182). Absolute and relative change in irisin during the acute bout of AE was associated with absolute and relative change in Matsuda index (r = 0.68; P = 0.022 and r = 0.63; P = 0.037) following the 6‐week RE intervention. No such association was observed with the irisin response to acute RE (all P > 0.05). Responders to the 6‐week RE intervention displayed a fourfold greater irisin response to acute AE (90.0 ± 28.0% vs. 22.8 ± 18.7%; P = 0.024) compared to nonresponders. Irisin increases significantly following an acute bout of AE, but not RE, and this response is associated with a greater improvement in insulin sensitivity in response to chronic resistance training.

Fasting and glucose induced thermogenesis in response to three ambient temperatures: a randomized crossover trial in the metabolic syndrome

BACKGROUND/OBJECTIVES

Cold exposure increases thermogenesis and could improve insulin sensitivity. We hypothesized a blunted response in the metabolic syndrome (MetS).

SUBJECTS/METHODS

Twenty older adults 59 ± 10.4 years (with MetS, MetS+, n = 9; without MetS, MetS-, n = 11) completed a randomized crossover design of 3.5 h exposures to 20, 25 and 27 °C on three visits. After an hour's rest at the desired temperature, resting metabolic rate (RMR), respiratory quotient (RQ), forearm to fingertip gradients (FFG), and in the ear temperature (IET) were measured over 30 min. An oral glucose tolerance test followed, and serial measurements were continued for 2 h. Venous blood was sampled for clinical chemistry, irisin, and fibroblast growth factor 21(FGF21). A mixed model ANCOVA adjusted data for age, gender, fat mass, fat-free mass and seasonality.

RESULTS

There was a significant MetS×temperature interaction where adjusted RMR was significantly higher in MetS+ compared to MetS- by 12% at 20 °C and by 6% at 25 °C, but similar at 27 °C. FFG increased and IET decreased with decreasing temperature to the same extent in both groups. Fasting irisin and FGF21 did not vary with temperature but the former was significantly higher in MetS-. Adjusted postprandial RQ and insulin to glucose ratios were significantly higher at 20 °C relative to 25 °C. Partial correlation analysis of differences between 27 and 20 °C indicated significant positive relationships between fasting as well as postprandial RQ and the respective changes in irisin and FGF21.

CONCLUSIONS

There could be an upward shift of the TNZ in MetS+, but this needs reevaluation.

Obese with higher FNDC5/Irisin levels have a better metabolic profile, lower lipopolysaccharide levels and type 2 diabetes risk

OBJECTIVE

Thus, the aim of this study was to compare if higher or smaller fibronectin type 3 domain-containing protein 5 (FNDC5)/irisin levels are associated with inflammatory and metabolic markers, caloric/macronutrient intake, physical fitness and type 2 diabetes mellitus (T2DM) risk in obese middle-aged men, and also to correlate all variables analyzed with FNDC5/irisin.

SUBJECTS AND METHODS

On the basis of a cluster study, middle-aged obese men (IMC: 31.01 ± 1.64 kg/m2) were divided into groups of higher and smaller levels of FNDC5/irisin. The levels of leptin, resistin, adiponectin, tumor necrosis factor alpha (TNFα), interleukin 6 and 10 (IL6, IL10), lipopolysaccharide (LPS), glucose, insulin, glycated hemoglobin, insulin resistance and sensibility, lipid profile, risk of T2DM development, body composition, rest energy expenditure, caloric/macronutrient intake and physical fitness were measured.

RESULTS

The higher FNDC5/ irisin group presented improved insulin sensibility (homeostasis model assessment - sensibility (HOMA-S) (p = 0.01) and QUICKI index (p < 0.01)), insulin (p = 0.02) and triglyceride levels (p = 0.01), lower insulin resistance (homeostasis model assessment - insulin resistance (HOMA-IR) (p = 0.01), triglycerides/glucose (TYG index) (p = 0.02), neck circumference (p = 0.02), risk of T2DM development (p = 0.02), tendency to decrease serum resistin (p = 0.08) and significant lower LPS levels (p = 0.02). Inverse correlations between FNDC5/irisin and body weight (r -0.46, p = 0.04), neck circumference (r -0.51, p = 0.02), free fat mass (r -0.49, p = 0.02), triglycerides (r -0.43, p = 0.05) and risk of developing T2DM (r -0.61, p = 0.04) were observed.

CONCLUSIONS

These results suggest that higher FNDC5/irisin levels in obese middle-aged men are related to a better metabolic profile and lower risk of T2DM development and serum LPS, a potential inducer of insulin resistance.

Lack of Adipocyte-Fndc5/Irisin Expression and Secretion Reduces Thermogenesis and Enhances Adipogenesis

Irisin is a browning-stimulating molecule secreted from the fibronectin type III domain containing 5 precursor (FNDC5) by muscle tissue upon exercise stimulation. Despite its beneficial role, there is an unmet and clamorous need to discern many essential aspects of this protein and its mechanism of action not only as a myokine but also as an adipokine. Here we contribute to address this topic by revealing the nature and role of FNDC5/irisin in adipose tissue. First, we show that FNDC5/irisin expression and secretion are induced by adipocyte differentiation and confirm its over-secretion by human obese visceral (VAT) and subcutaneous (SAT) adipose tissues. Second, we show how secreted factors from human obese VAT and SAT decrease PGC1α, FNDC5 and UCP1 gene expression on differentiating adipocytes; this effect over UCP1 is blunted by blocking irisin in obese secretomes. Finally, by stable gene silencing FNDC5 we reveal that FNDC5-KO adipocytes show reduced UCP1 expression and enhanced adipogenesis.

Increased FNDC5 is associated with insulin resistance in high fat-fed mice

FNDC5/irisin, has recently been identified as a novel protein that stimulates the "browning" of white adipose by inducing thermogenesis via increased uncoupling protein 1 (UCP1). We tested the hypothesis that high fat diet-induced prediabetic mice would exhibit increased FNDC5 and this effect would be attenuated by chronic exercise. C57BL/6 mice were randomized into three groups for the 4 week intervention: Standard diet (Std, n = 12), High fat diet (HF, n = 14), or High fat diet and free access to a running wheel (HFEX, n = 14). Body weight, glucose, insulin, and the homeostatic model assessment of insulin resistance (HOMA-IR) were greater in HF compared to Std and HFEX after the 4 week intervention. In support of our hypothesis, FNDC5 was higher in HF in both skeletal muscle and adipose compared to Std and was lower in adipose only in HFEX compared to HF mice. Following the same pattern, PGC-1α was significantly higher in HF compared to Std in skeletal muscle and significantly lower in HFEX compared to HF in adipose. UCP1 was significantly lower in HFEX versus Std (in skeletal muscle) and versus HF (in adipose). HOMA-IR was significantly correlated with FNDC5 protein levels in adipose. Increased FNDC5 in adipose and skeletal muscle may be a compensatory mechanism to offset high fat diet-induced weight gain and insulin resistance by increasing energy expenditure.

Anti-Inflammatory Properties of Irisin, Mediator of Physical Activity, Are Connected with TLR4/MyD88 Signaling Pathway Activation

Irisin, an adipomiokine known as a mediator of physical activity, induces the browning of adipose tissue and it has potentially protective properties in the development of obesity-related states, such as insulin resistance, arteriosclerosis, and type 2 diabetes. Despite numerous studies conducted on this factor, still little is known about its impact on the functioning of immunocompetent cells, but its potential anti-inflammatory properties were previously suggested. In the current study we investigated the role of irisin (0-100 nM) in the downstream pathway activation of Toll-like receptor 4 (TLR4) in RAW 264.7 macrophages stimulated with lipopolysaccharide (LPS; 100 ng/mL). The results have shown that irisin in high concentrations (50, 100 nM) significantly decreased the TLR4 and MyD88 protein levels, as well as the phosphorylation of nuclear factor-κB (NF-κB), consequently leading to the reduction in the release of crucial pro-inflammatory cytokines. The above was confirmed for interleukin 1β (IL-1β), tumor necrosis factor α (TNFα), interleukin 6 (IL-6), keratinocyte chemoattractant (KC), monocyte chemotactic protein 1 (MCP-1), as well as for high mobility group box 1 (HMGB1). Moreover, our results indicate that this effect is connected with irisin's impact on the phosphorylation of mitogen-activated protein kinases (MAPKs), where a significant reduction in p-JNK and p-ERK but not p-p38 was observed. In conclusion, these data suggest that irisin has potentially anti-inflammatory properties connected with the downregulation of downstream pathways of TLR4/MyD88.

Irisin acts as a regulator of macrophages host defense

AIM

Irisin, the adipomyokine, released mainly by exercised muscles, participate in the browning of adipose tissue and contribute to the restriction of insulin resistance and diabetes mellitus 2 development. Because of the limited reports describing the effect of irisin on inflammation and immunocompetent cells activation, the present study attempted to assess the influence of various irisin concentrations on basic macrophage activity.

MAIN METHODS

Studies were carried out on murine RAW 264.7 macrophages cultured in medium enriched with irisin (0nM, 10nM, 50nM, or 100nM). General cell activity, viability, and proliferation were assessed along with phagocytosis process, and respiratory burst generation.

KEY FINDING

Irisin level positively correlates with general cell activity and cell cycle progression as well as with phagocytosis intensity, but negatively correlates with the intensity of respiratory burst generation. No influence of irisin on quiescent cell viability, including apoptosis or necrosis, was observed.

SIGNIFICANCE

This research is the first to show that irisin modulates macrophage activity by reducing reactive oxygen species (ROS) overproduction, which could suggest its potential anti-inflammatory properties. Therefore, further studies are needed for the evaluation of influence of irisin on immunocompetent cell function.

Effects of Dietary Macronutrient Composition on FNDC5 and Irisin in Mice Skeletal Muscle

BACKGROUND

Fibronectin type III domain containing 5 (FNDC5) and its protein product Irisin are therapeutic targets for obesity-associated disorders. Irisin plays an important role in energy regulation, inducing browning of white adipocytes, and improving obesity. We aimed to investigate the association between muscle Irisin expression and dietary quality.

METHODS

Twenty-eight female mice were divided into four groups and fed the following experimental diets for 60 days: standard diet (SD), high-carbohydrate diet (HCD), high-fat diet (HFD), and high-protein diet (HPD). We evaluated body weight, food intake, serum total cholesterol, triacylglycerol, and glucose. We also performed glucose tolerance and insulin sensitivity tests. Expression of FNDC5 was evaluated by quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR) of soleus muscle. Western blot was used to assess Irisin protein expression.

RESULTS

The major finding of the present study was that HFD and HCD were associated with a downregulation of FNDC5. In addition to these results, we noted a significant reduction in skeletal muscle Irisin level. HPD prevented reductions of both FNDC5 and Irisin levels, as well as increased brown adipose tissue, compared to the control group.

CONCLUSIONS

In conclusion, we observed that the HPD type of diet can change both FNDC5 expression and Irisin levels. Thus, the HPD might be the most appropriate diet to achieve high amounts of Irisin, a target molecule for the treatment of obesity and its co-morbidities.

Levels of metabolic markers in drug-naive prediabetic and type 2 diabetic patients

AIMS

Type 2 diabetes mellitus (T2DM) and prediabetes (pre-DM) are associated with changes in levels of metabolic markers. The main aim of this study is to compare the levels of omentin, irisin, endothelin-1, nesfatin, hepatocyte growth factor (HGF), fibroblast growth factor, and oxytocin (OXT) between normoglycemic and pre-DM/T2DM obese Jordanian patients.

METHODS

One hundred and ninety-eight adult Jordanian subjects were recruited. Demographic data and clinical parameters were collected. The serum levels of biomarkers were measured by enzymatic assay procedure.

RESULTS

Compared to normoglycemic (95 subjects), pre-DM/T2DM (103 subjects) displayed higher HGF (ng/ml) = 78.8 (71.4-104) versus 55.9 (45.3-66.6), p < 0.0001; and nesfatin (ng/ml) = 0.5 (0.4-0.7) versus 0.2 (0.1-0.4), p < 0.0001; betatrophin (ng/ml) = 1.2 (0.8-1.6) versus 0.22 (0.15-0.41), p < 0.0001. On the other hand, they had lower levels of omentin (ng/ml) = 2.1 (0.9-3.3) versus 3.6 (2.0-6.4), p < 0.0001, irisin (ng/ml) = 113.7 (88.9-142.9) versus 132.6 (110.7-147.8), p < 0.0001; and oxytocin (pg/ml) = 1077.9 (667.3-1506.0) versus 2180.1 (1464.5-2795.6), p < 0.0001; respectively. In comparison, FGF-21 (ng/ml) = 0.3 (0.2-0.5) versus 0.2 (0.1-0.4), and endothelin (pg/ml) = 2.7 (1.3-5.2) versus 2.8 (1.6-5.6) did not differ between the two groups (p > 0.05).

CONCLUSIONS

In the present study, patients with pre-DM and T2DM have higher serum levels of metabolic HGF, nesfatin, and betatrophin and lower levels of omentin, irisin, and OXT. Future longitudinal and interventional studies are required to confirm the utility of these markers as novel progression or therapeutic targets in the pharmacotherapy of diabetes.

Relationship between Irisin Concentration and Serum Cytokines in Mother and Newborn

Introduction

Irisin is considered to be a myokine and adipokine that may also participate in reproductive functions, as it increases significantly throughout pregnancy. However, the regulation of circulating irisin and its relationship with other cytokines has not been assessed thus far in pregnant women and their offspring.

Objective

The aim of this study was to evaluate differences in irisin and cytokine concentrations between women at the end of pregnancy and their offspring, as well as the relationship between maternal and newborn irisin and maternal and newborn biomarkers.

Methods

Twenty-eight mother/newborn pairs were included in this study. The following biomarkers were evaluated in maternal venous and arterial umbilical cord blood samples: irisin, 27 cytokine panel, total antioxidant capacity (TAC), total plasma protein, and free fatty acid concentration.

Results

The newborns had significantly lower irisin concentrations compared to their mothers (p = 0.03), but this difference was present only in babies born from mothers without labor prior to cesarean section delivery (p = 0.01). No significant differences in maternal and newborn irisin concentrations were found between diabetic and non-diabetic mothers or between overweight/obese and normal weight mothers. A significant positive correlation was found between TAC level and irisin concentration in newborns. Maternal and newborn interleukin (IL)-1β, IL-1RA, IL-5, IL-7, and interferon gamma-induced protein (IP)-10 levels were significantly positively correlated with irisin concentrations in both study groups. In addition, maternal IL1β, IL-5, IL-7, and IP-10 levels positively predicted maternal irisin concentrations. Furthermore, arterial cord blood TAC and IL-1β and IL1-RA levels positively predicted newborn irisin concentrations. Multiple regression analyses showed that maternal IL-13 negatively predicted offspring irisin levels (p = 0.03) and that maternal IL-1β positively predicted newborn irisin concentrations (p = 0.046).

Conclusion

No evidence was found that serum irisin concentrations in mothers at pregnancy termination or those of their newborns correlated with maternal body mass index, the presence of diabetes mellitus, or free fatty acid levels. However, the results of this study indicated that cytokines might predict irisin concentration in mothers and their offspring, although interactions between irisin levels during pregnancy and the newborn have not yet been fully elucidated.

Short-term safety, tolerability and efficacy of a very low-calorie-ketogenic diet interventional weight loss program versus hypocaloric diet in patients with type 2 diabetes mellitus

Brackground:The safety and tolerability of very low-calorie-ketogenic (VLCK) diets are a current concern in the treatment of obese type 2 diabetes mellitus (T2DM) patients.

OBJECTIVE

Evaluating the short-term safety and tolerability of a VLCK diet (<50 g of carbohydrate daily) in an interventional weight loss program including lifestyle and behavioral modification support (Diaprokal Method) in subjects with T2DM.

METHODS

Eighty-nine men and women, aged between 30 and 65 years, with T2DM and body mass index between 30 and 35 kg m(-)(2) participated in this prospective, open-label, multi-centric randomized clinical trial with a duration of 4 months. Forty-five subjects were randomly assigned to the interventional weight loss (VLCK diet), and 44 to the standard low-calorie diet.

RESULTS

No significant differences in the laboratory safety parameters were found between the two study groups. Changes in the urine albumin-to-creatinine ratio in VLCK diet were not significant and were comparable to control group. Creatinine and blood urea nitrogen did not change significantly relative to baseline nor between groups. Weight loss and reduction in waist circumference in the VLCK diet group were significantly larger than in control subjects (both P<0.001). The decline in HbA1c and glycemic control was larger in the VLCK diet group (P<0.05). No serious adverse events were reported and mild AE in the VLCK diet group declined at last follow-up.

CONCLUSIONS

The interventional weight loss program based on a VLCK diet is most effective in reducing body weight and improvement of glycemic control than a standard hypocaloric diet with safety and good tolerance for T2DM patients.

Novel Molecules Regulating Energy Homeostasis: Physiology and Regulation by Macronutrient Intake and Weight Loss

Excess energy intake, without a compensatory increase of energy expenditure, leads to obesity. Several molecules are involved in energy homeostasis regulation and new ones are being discovered constantly. Appetite regulating hormones such as ghrelin, peptide tyrosine-tyrosine and amylin or incretins such as the gastric inhibitory polypeptide have been studied extensively while other molecules such as fibroblast growth factor 21, chemerin, irisin, secreted frizzle-related protein-4, total bile acids, and heme oxygenase-1 have been linked to energy homeostasis regulation more recently and the specific role of each one of them has not been fully elucidated. This mini review focuses on the above mentioned molecules and discusses them in relation to their regulation by the macronutrient composition of the diet as well as diet-induced weight loss.

Circulating Levels of Irisin in Hypopituitary and Normal Subjects

Context

The recently identified myokine irisin conveys some of the benefits of exercise. Hypopituitarism with adult growth hormone deficiency (HP) is a situation characterized by decreased GH secretion and an altered body composition.

Objective

Our aim was to study the skeletal muscle hormone irisin in HP, and compare the results with a similar group of normal subjects.

Participants and Methods

Seventeen HP patients and fifty-one normal subjects of similar age and sex were studied. The diagnosis of GH deficiency was confirmed by the presence of pituitary disease and a peak GH secretion below 3 μg/L after an insulin tolerance test. The patients were adequately treated for all pituitary hormone deficits, except for GH. Fasting serum irisin was measured with an enzyme immunoassay, and HOMA-IR, QUICKI and HOMA-β were calculated.

Results

Fasting irisin levels (ng/ml) were similar in normal [208.42 (168.44–249.23)] and HP patients [195.13 (178.44–241.44)]. In the control group there were moderate significant positive correlations between irisin and BMI, waist circumference, leptin, fasting insulin, HOMA-IR, HOMA-β, triglycerides, and cholesterol. In the control group there were moderate significant negative correlations between irisin and IGF-I and QUICKI. In the hypopituitary group there were moderate significant positive correlations between irisin and body fat and HOMA-β.

Conclusions

We found similar irisin levels in GH deficiency hypopituitary patients when compared with normal subjects. The correlation between irisin and adiposity related factors suggests that that in the case of this clinical model, irisin is regulated by adiposity and not by GH.

Association of circulating irisin levels with normal weight obesity, glycemic and lipid profile

Background

Irisin, a recently identified myokine/adipokine, has potential role in type 2 diabetes and obesity. Normal weight obesity (NWO) is associated with a significantly higher risk of developing metabolic syndrome and cardiometabolic dysfunction. The aim of this study was to investigate association of irisin level with NWO, glycemicand lipid profile in women.

Methods

In this matchedcase-control study, 38 subjects with NWO (body mass index (BMI) <25 kg/m2 and BF % > 30) as case and 26 controls (BMI <25 kg/m2 and BF % < 30) were selected randomly from sport clubs in the East area of Tehran, Iran. In addition to anthropometric variables, including BMI and body composition, fasting blood sugar (FBS), fasting levels of irisin andinsulin, triglyceride (TG), total cholesterol (TC), high-density lipoprotein (HDL) and low-density lipoprotein (LDL) cholesterol were measured. All statistical analyses were performed with SPSS 18.0.

Results

In univariate analysis, levels of irisin were significantly higher in subjects with NWO compared to controls (0.81 ± 0.41vs. 0.58 ± 0.26 ng/ml, P = 0.009). This association remained significant after adjusting for confounders (adjusted for energy intake, physical activity, waist circumference and BMI) (P = 0.049). In NWO, irisin level was not significantly correlated with all glycemic and lipid profile. In control group, only correlation ofirisin with insulin was statistically significant (P = 0.03).

Conclusion

Serum irisin levels were higher in NWO subjects than controls. In control group, only the negative association between irisin and insulin levels was statistically significant. Further studies with larger sample size are clearly needed to evaluate the potential role of irisin in NWO subject and other disturbed metabolic conditions.

Association between circulating irisin and insulin resistance in non-diabetic adults: A meta-analysis

INTRODUCTION

Exogenous administration of recombinant irisin improves glucose metabolism. However, the association of endogenous circulating (plasma/serum) irisin with insulin resistance remains poorly delineated. This study was aimed to examine this association by meta-analyzing the current evidence without study design restriction in non-diabetic adults.

MATERIALS/METHODS

Peer-reviewed studies written in English from 3 databases were searched to December 2015. Studies that reported the association between circulating irisin and insulin resistance (or its reverse, insulin sensitivity) in non-diabetic non-pregnant adults (mean ages ≥18years) were included. The pooled correlation coefficient (r) and 95% confidence intervals (CIs) were calculated using a random-effects model. Subgroup analyses and meta-regression were performed to explore potential sources of heterogeneity.

RESULTS

Of the 195 identified publications, 17 studies from 15 articles enrolling 1912 participants reported the association between circulating irisin and insulin resistance. The pooled effect size was 0.15 (95% CI: 0.07 to 0.22) with a substantial heterogeneity (I(2)=55.5%). This association seemed to be modified by glycemic status (fasting blood glucose ≥6.1mmol/L versus <6.1mmol/L) and racial-ethnic difference (Asians versus Europeans versus Americans), but not by sex difference, sampling time-point, blood sample type, ELISA kits used, baseline age, or body mass index. Circulating irisin was inversely associated with insulin sensitivity (6 studies; r=-0.17, 95% CI: -0.25 to -0.09).

CONCLUSIONS

Circulating irisin is directly and positively associated with insulin resistance in non-diabetic adults. However, this association is rather small and requires further clarification, in particular by well-designed large epidemiological studies with overall, race-, and sex-specific analyses.

Circulating irisin levels reflect visceral adiposity in non-diabetic patients undergoing hemodialysis

BACKGROUND

Recent evidence suggests that increased visceral adiposity is a strong independent risk factor for cardiovascular death and all-cause mortality in hemodialysis (HD) patients. Irisin, which is a novel myokine, can play critical roles in diabetes and adiposity. The purpose of our study was to investigate whether serum irisin levels are associated with body mass index, waist circumference (WC), and total fat mass in non-diabetic patients undergoing maintenance HD.

METHODS

This cross-sectional study included 108 non-diabetic HD patients and 40 age- and sex-matched apparently healthy subjects. Serum irisin concentrations were determined using an enzyme-linked immunosorbent assay. Body fat composition (TBF-410 Tanita Body Composition Analyzer) was measured and calculated.

RESULTS

Serum irisin levels did not differ between HD patients and the healthy controls (523.50 ± 229.32 vs. 511.28 ± 259.74, p = 0.782). Serum irisin levels were associated with age (r = 0.314; p =0.006), HOMA-IR (r = 0.472; p = 0.003), WC (r = 0.862; p < 0.001), and total fat mass (r = 0.614; p < 0.001). In multivariate regression analysis, WC (β = 1.240, p < 0.001) and total fat mass (β = 0.792, p = 0.015) were the variables that were significantly associated with irisin concentrations (R(2 )=( )0.684, p < 0.001) after adjusting for confounding factors (age and HOMA-IR).

CONCLUSIONS

These results suggest that serum irisin levels are related to visceral adiposity in non-diabetic HD patients.

Irisin Inhibits Hepatic Cholesterol Synthesis via AMPK-SREBP2 Signaling

Irisin, a myokine released during exercise, promotes browning of subcutaneous adipose tissue and regulates energy homeostasis. Although exercise constantly reduces blood cholesterol, whether irisin is involved in the regulation of cholesterol remains largely unknown. In the present study, subcutaneous infusion of irisin for 2weeks induced a reduction in plasma and hepatic cholesterol in high fat diet-induced obese (DIO) mice. These alterations were associated with an activation of 5' AMP-activated protein kinase (AMPK) and inhibition of sterol regulatory element-binding transcription factor 2 (SREBP2) transcription and nuclear translocation. In primary hepatocytes from either lean or DIO mice, irisin significantly decreased cholesterol content via sequential activation of AMPK and inhibition of SREBP2. Suppression of AMPK by compound C or AMPKα1 siRNA blocked irisin-induced alterations in cholesterol contents and SREBP2. In conclusion, irisin could suppress hepatic cholesterol production via a mechanism dependent of AMPK and SREBP2 signaling. These findings suggest that irisin is a promising therapeutic target for treatment of hypercholesterolemia.

Central and peripheral irisin differentially regulate blood pressure

INTRODUCTION

Irisin is a newly identified 112 amino acid hormone, derived as a product of fibronectin type III domain containing 5 (FNDC5), which is highly related to metabolic activity in skeletal muscle and brown fat. The effects of irisin on cardiovascular functions are unknown.

PURPOSE

To explore the effects of central and peripheral irisin on cardiovascular functions.

METHODS

Irisin was either administrated into 3rd ventricle of rats or intravenously, and its effects on blood pressure and cardiac contractibility measured.

RESULTS

Administration of recombinant human irisin into the 3rd brain ventricle of rats activated neurons in the paraventricular nuclei of the hypothalamus. Central administration of irisin increased blood pressure and cardiac contractibility. Exogenous irisin reversed atenolol-induced inhibition of cardiac contractibility. In contrast, peripheral administration of irisin reduced blood pressure in both control and spontaneously hypertensive rats. Irisin dilated mesenteric artery rings through ATP-sensitive potassium channels.

CONCLUSION

Our studies indicate that central and peripheral irisin may differentially regulate cardiovascular activities.

Irisin, an exercise-induced myokine as a metabolic regulator: an updated narrative review

Irisin, as a new hormone-like myokine, is discovered in the presence of exercise-induced peroxisome proliferator-activated receptor gamma coactivator-1-alpha (PGC-1α). Which substance plays an important role in energy metabolism in each organ in the body and the regulation of metabolic diseases such as obesity and diabetes. The finding of irisin can contribute to the exploration of the novel and effective therapeutic targets or therapeutic strategies of these metabolic diseases or metabolism-associated health issues. To date, little is known regarding the functions and regulatory mechanisms of irisin with respect to metabolic diseases or metabolism-associated health issues. In this narrative review article, we systematically introduce its structural characteristics, production and distribution in tissues and organs, and the regulation and corresponding mechanisms for metabolic diseases or metabolism-associated health issues of irisin. Meanwhile, its future prospects and the development of irisin-related products for the promotion of human health have also been proposed, which will benefit future research and application of irisin. Copyright © 2015 John Wiley & Sons, Ltd.

Circulating Irisin Levels Are Not Regulated by Nutritional Status, Obesity, or Leptin Levels in Rodents

Irisin is a cleaved and secreted fragment of fibronectin type III domain containing 5 (FNDC5) that is mainly released by skeletal muscle and was proposed to mediate the beneficial effects of exercise on metabolism. In the present study we aim to investigate the regulation of the circulating levels of irisin in obese animal models (diet-induced obese (DIO) rats and leptin-deficient (ob/ob) mice), as well as the influence of nutritional status and leptin. Irisin levels were measured by Enzyme-Linked Immunosorbent Assay (ELISA) and Radioimmunoassay (RIA). Serum irisin levels remained unaltered in DIO rats and ob/ob mice. Moreover, its circulating levels were also unaffected by fasting, leptin deficiency, and exogenous leptin administration in rodents. In spite of these negative results we find a negative correlation between irisin and insulin in DIO animals and a positive correlation between irisin and glucose under short-term changes in nutritional status. Our findings indicate that serum irisin levels are not modulated by different physiological settings associated to alterations in energy homeostasis. These results suggest that in rodents circulating levels of irisin are not involved in the pathophysiology of obesity and could be unrelated to metabolic status; however, further studies should clarify its precise role in states of glucose homeostasis imbalance.

Irisin levels before and after physical activity among school-age children with different BMI: a direct relation with leptin

OBJECTIVE

Irisin is a novel myokine that seems to mediate the beneficial effects of exercise. Levels of circulating irisin before and after an 8-month physical activity program (PAP) in school-age children were evaluated.

METHODS

Irisin and leptin were measured at baseline and at follow-up among 85 children with different BMI.

RESULTS

Of the 85 children (mean age 8.9; 47% female), 25 children had normal weight, 23 were overweight, and 37 had obesity. We observed no significant difference in irisin serum levels between boys and girls. Irisin was positively associated with BMI before and after the PAP (r(before) = 0.42; r(after) = 0.37, P < 0.001), with the highest levels in children with obesity. There was a slight decrease of circulating irisin after PAP, but this decrease was not of statistical significance. We observed a high and positive association between irisin and leptin levels before and after the PAP (r(before) = 0.78; r(after) = 0.82, P < 0.001). Moreover, changes in leptin correlated with changes in irisin (r = 0.72, P < 0.001).

CONCLUSIONS

Circulating irisin is positively linked to BMI and leptin in school-age children, supporting the notion that that irisin is produced by adipose tissue. As in previous reports, this study failed to observe changes in irisin levels after exercise, likely because higher irisin levels are produced only during exercise.

Irisin: 'fat' or artefact

Soon after the discovery of the muscle-derived factor irisin, a great controversy arose in the literature regarding certain inconsistencies in the regulation of the fibronectin type III domain containing 5 protein (FNDC5/irisin) after exercise, as well as the unpredicted association of circulating irisin levels with parameters of adiposity in humans. Due to these questionable findings, doubts as to the identity of the soluble portion of FNDC5 as well as the real role of irisin and its possible therapeutic applications in the treatment of obesity and diabetes have proliferated. We recently postulated that FNDC5/irisin is an adipokine expressed and secreted by white adipose tissue in rats and humans. Its circulating concentration correlates with adiposity in humans among independent cohorts of patients. Further analysis, focused on obesity-related metabolic disorders, has shown that irisin could play a role in promoting insulin resistance or act as an adaptive response to counteract disturbances in glucose and lipid homoeostasis in obesity. Overall, this leads us to raise the question whether the new factor, increased in circulation of obese patients, is really irisin-reflecting fat mass or it is an artefact. Therefore, the current review is focused on the potential participation of adipose tissue in irisin circulating levels, and the role of irisin in metabolic pathologies associated with obesity in an attempt to clarify the controversy generated by these recently published reports.

Circulating irisin and glucose metabolism in overweight/obese women: effects of α-lipoic acid and eicosapentaenoic acid

Irisin is a myokine/adipokine with potential role in obesity and diabetes. The objectives of the present study were to analyse the relationship between irisin and glucose metabolism at baseline and during an oral glucose tolerance test (OGTT) and to determine the effects of eicosapentaenoic acid (EPA) and/or α-lipoic acid treatment on irisin production in cultured human adipocytes and in vivo in healthy overweight/obese women following a weight loss program. Seventy-three overweight/obese women followed a 30% energy-restricted diet supplemented without (control) or with EPA (1.3 g/day), α-lipoic acid (0.3 g/day) or both EPA + α-lipoic acid (1.3 + 0.3 g/day) during 10 weeks. An OGTT was performed at baseline. Moreover, human adipocytes were treated with EPA (100-200 μM) or α-lipoic acid (100-250 μM) during 24 h. At baseline plasma, irisin circulating levels were positively associated with glucose levels; however, serum irisin concentrations were not affected by the increment in blood glucose or insulin during the OGTT. Treatment with α-lipoic acid (250 μM) upregulated Fndc5 messenger RNA (mRNA) and irisin secretion in cultured adipocytes. In overweight/obese women, irisin circulating levels decreased significantly after weight loss in all groups, while no additional differences were induced by EPA or α-lipoic acid supplementation. Moreover, plasma irisin levels were positively associated with higher glucose concentrations at beginning and at endpoint of the study. The data from the OGTT suggest that glucose is not a direct contributing factor of irisin release. The higher irisin levels observed in overweight/obese conditions could be a protective response of organism to early glucose impairments.

Association of irisin concentrations with the presence of diabetic nephropathy and retinopathy

OBJECTIVE

Irisin, a recently identified myokine, is involved in the protection of mice against obesity and diabetes. This study aims to determine the serum and vitreous concentrations of irisin in patients with diabetic nephropathy and diabetic retinopathy.

METHODS

A total of 178 patients with type 2 diabetes mellitus, as well as 22 type 2 diabetes mellitus patients without diabetic retinopathy and 35 patients with proliferative diabetic retinopathy were enrolled in this study.

RESULTS

Serum irisin concentrations were significantly elevated in the control group compared with those in type 2 diabetes mellitus patients. Furthermore, type 2 diabetes mellitus patients with macroalbuminuria exhibited significantly lower serum irisin concentrations than the controls and type 2 diabetes mellitus patients with normoalbuminuria and microalbuminuria. Simple regression analysis showed that the serum irisin concentrations in type 2 diabetes mellitus patients were negatively correlated with age, fasting plasma glucose, homeostasis model assessment of insulin resistance, blood urea nitrogen, creatinine, and positively correlated with creatinine clearance and angiotensin-converting enzyme inhibitors/angiotensin receptor blockers treatment. Proliferative diabetic retinopathy patients showed significantly decreased serum and vitreous irisin concentrations compared with the control group and type 2 diabetes mellitus patients without diabetic retinopathy. Furthermore, decreased serum and vitreous irisin concentrations were found in type 2 diabetes mellitus patients without diabetic retinopathy than those in the controls.

CONCLUSION

Irisin concentrations are associated with the presence of diabetic nephropathy and diabetic retinopathy.

Increased 24-hour ad libitum food intake is associated with lower plasma irisin concentrations the following morning in adult humans

BACKGROUND

The relationship between food intake and irisin concentrations in humans is unclear.

OBJECTIVES

To determine whether the previous day's intake impacts fasting plasma irisin concentrations, or whether fasting irisin concentrations associate with subsequent ad libitum food intake.

METHODS

Sixty-six nondiabetic adults (42 men) were admitted for a study of the determinants of energy intake. After 6 days of a weight maintaining diet, ad libitum energy intake over 3 days was assessed using a vending machine paradigm. Fasting plasma irisin concentrations were measured on the morning of the second day of the vending period.

RESULTS

There were no correlations between irisin and demographic or anthropometric parameters. On day 1, subjects consumed 144 ± 52% of weight maintaining energy needs. Every additional 500 kcal consumed on day 1 associated with a 3.4% lower irisin concentration the following morning (95% CI -6.2, -0.4%, p = 0.01; adjusted for age, sex and race). If energy intake was expressed as a percentage of weight maintaining energy needs, every 10% increase associated with a 1.9% lower irisin concentration (95% CI -3.7, -0.1%; adjusted p = 0.02). A 100 kcal increase in carbohydrate or fat consumption associated with a 1.3% (95% CI -2.5, -0.1%, p = 0.01) and a 0.6% (95% CI -1.1, -0.0%, p = 0.02) lower irisin concentration, respectively. There was no association between fasting irisin concentrations and subsequent energy intake on day 2 (r = 0.19, p = 0.1).

CONCLUSIONS

Higher ad libitum 24 h energy intake was associated with lower fasting irisin concentrations the following morning, but fasting irisin concentrations did not predict subsequent energy intake. The decrease in irisin concentrations with increased energy intake is consistent with the detrimental metabolic effects of overeating.

FNDC5 (irisin) gene and exceptional longevity: a functional replication study with rs16835198 and rs726344 SNPs

Irisin might play an important role in reducing the risk of obesity, insulin resistance, or several related diseases, and high irisin levels may contribute to successful aging. Thus, the irisin precursor (FNDC5) gene is a candidate to influence exceptional longevity (EL), i.e., being a centenarian. It has been recently shown that two single-nucleotide polymorphisms (SNPs) in the FNDC5 gene, rs16835198 and rs726344, are associated with in vivo insulin sensitivity in adults. We determined luciferase gene reporter activity in the two above-mentioned SNPs and studied genotype distributions among centenarians (n = 175, 144 women) and healthy controls (n = 347, 142 women) from Spain. We also studied an Italian [79 healthy centenarians (40 women) and 316 healthy controls (156 women)] and a Japanese cohort [742 centenarians (623 women) and 499 healthy controls (356 women)]. The rs726344 SNP had functional significance, as shown by differences in luciferase activity between the constructs of this SNP (all P ≤ 0.05), with the variant A-allele having higher luciferase activity compared with the G-allele (P = 0.04). For the rs16835198 SNP, the variant T-allele tended to show higher luciferase activity compared with the G-allele (P = 0.07). However, we found no differences between genotype/allele frequencies of the two SNPs in centenarians versus controls in any cohort, and no significant association (using logistic regression adjusted by sex) between the two SNPs and EL. Further research is needed with different cohorts as well as with additional variants in the FNDC5 gene or in other genes involved in irisin signaling.