Irisin in humans: recent advances and questions for future research

Negative findings but positive contributions in cardiovascular research

Researchers have always concluded that results that do not support the hypothesis as unimportant, unworthy, or simply not good enough for publication. However, negative findings are essential for the progress of science and its self-correcting nature. We also believe in the importance and indispensability of negative results. Therefore, in this review, we discussed the factors contributing to the publication bias of negative results and the problems to assess the factuality and validity of negative results. Moreover, we emphasized the importance of reporting negative results in cardiovascular research, including treatments, and suggest that the negative results could clarify previously controversial topics in the treatment of cardiovascular diseases and prompt the translation of research on precision cardiovascular disease prevention and treatment.

Irisin and Bone in Sickness and in Health: A Narrative Review of the Literature

Irisin is a hormone-like myokine produced by the skeletal muscle in response to exercise. Upon its release into the circulation, it is involved in the browning process and thermogenesis, but recent evidence indicates that this myokine could also regulate the functions of osteoblasts, osteoclasts, and osteocytes. Most human studies have reported that serum irisin levels decrease with age and in conditions involving bone diseases, including both primary and secondary osteoporosis. However, it should be emphasized that recent findings have called into question the importance of circulating irisin, as well as the validity and reproducibility of current methods of irisin measurement. In this review, we summarize data pertaining to the role of irisin in the bone homeostasis of healthy children and adults, as well as in the context of primary and secondary osteoporosis. Additional research is required to address methodological issues, and functional studies are required to clarify whether muscle and bone damage per se affect circulating levels of irisin or whether the modulation of this myokine is caused by the inherent mechanisms of underlying diseases, such as genetic or inflammatory causes. These investigations would shed further light on the effects of irisin on bone homeostasis and bone disease.

Association of Irisin/FNDC5 with ERRα and PGC-1α Expression in NSCLC

The rapid growth and division of cancer cells are associated with mitochondrial biogenesis or switching to glycolysis. ERRα, PGC-1α and irisin/FNDC5 are some of the proteins that can influence these processes. The aim of this study was to determine the correlation of these proteins in non-small cell lung cancer (NSCLC) and to investigate their association with clinicopathological parameters. Immunohistochemistry reactions were performed on tissue microarrays (860 NSCLC, 140 non-malignant lung tissue). The normal fibroblast cell line (IMR-90) and lung cancer cell lines (NCI-H1703 and NCI-H522) were used as co-cultures. The mRNA levels of FNDC5 and ESRRA (encoding ERRα) were assessed in IMR-90 cells after co-culture with lung cancer cells. We observed a decreased level of ERRα with an increase in tumor size (T), stages of the disease, and lymph node metastases (N). In the adenocarcinoma (AC) subtype, patients with a higher ERRα expression had significantly longer overall survival. A moderate positive correlation was observed between FNDC5 mRNA and ESRRA mRNA in NSCLCs. The expression of FNDC5 mRNA in IMR-90 cells increased after 24 h, and ESRRA gene expression increased after 48 h of co-culture. The ERRα receptor with PGC-1α participates in the control of FNDC5/irisin expression. Normal fibroblasts revealed an upregulation of the FNDC5 and ESRRA genes under the influence of lung cancer cells.

Association of irisin levels with cardiac magnetic resonance, inflammatory, and biochemical parameters in patients with chronic heart failure versus controls

BACKGROUND AND AIMS

Chronic heart failure (CHF) represents a significant cause of morbidity and mortality globally. Metabolic maladaptation has proven to be critical in the progression of this condition. Preclinical studies have shown that irisin, an adipomyokine involved in metabolic regulations, can induce positive cardioprotective effects by improving cardiac remodeling, cardiomyocyte viability, calcium delivery, and reducing inflammatory mediators. However, data on clinical studies identifying the associations between irisin levels and functional imaging parameters are scarce in CHF patients. The objective of this study was to determine the association of irisin levels with cardiac imaging measurements through cardiac magnetic resonance, inflammatory markers, and biochemical parameters in patients with CHF compared with control subjects.

METHODS AND RESULTS

Thirty-two subjects diagnosed with CHF and thirty-two healthy controls were evaluated in a cross-sectional study. Serum irisin levels were significantly lower in patients with CHF than in controls. This is the first study to report a significant positive correlation between irisin levels and cardiac magnetic resonance parameters such as left ventricular ejection fraction, fraction shortening, and global radial strain. A negative correlation was demonstrated between irisin levels and brain natriuretic peptide, insulin levels, and Homeostatic model assessment for insulin resistance index. We did not observe significant correlations between irisin levels and inflammatory cytokines.

CONCLUSIONS

Given the importance of fraction shortening and global radial strain as accurate markers of ventricular wall motion, these results support the hypothesis that irisin may play an essential role in maintaining an adequate myocardial wall architecture, deformation, and thickness.

The Emerging Role of Irisin in Cardiovascular Diseases

Irisin, a novel hormone like polypeptide, is cleaved and secreted by an unknown protease from a membrane‐spanning protein, FNDC5 (fibronectin type III domain‐containing protein 5). The current knowledge on the biological functions of irisin includes browning white adipose tissue, regulating insulin use, and anti‐inflammatory and antioxidative properties. Dysfunction of irisin has shown to be involved in cardiovascular diseases such as hypertension, coronary artery disease, myocardial infarction, and myocardial ischemia–reperfusion injury. Moreover, irisin gene variants are also associated with cardiovascular diseases. In this review, we discuss the current knowledge on irisin‐mediated regulatory mechanisms and their roles in the pathogenesis of cardiovascular diseases.

Combined Physical Training Increases Plasma Brain-Derived Neurotropic Factor Levels, But Not Irisin in People Living with HIV/AIDS

This study evaluated plasma levels of brain-derived neurotropic factor (BDNF), irisin, and lactate in people living with HIV/AIDS who completed a combined physical training program. Nineteen HIV+ participants (age: 39.60 ± 10.96 years; carrier time: 7.75 ± 7.88 years; time of ART: 6.41 ± 5.93 years) performed strength/aerobic training (combined physical training) in the same session for 8 weeks and levels of BDNF, irisin, and lactate were assessed. BDNF (pg/mL) was higher post-CPT (Pre: 1258.73 ± 372.30; Post: 1504.17 ± 322.30; p < 0.001). Irisin (ng/mL) showed no change (Pre: 115.61 ± 72.41; Post: 125.87 ± 81.14; p = 0.973). There was positive correlation between irisin and lactate (mmol/L) pre (r = 0.55, p = 0.04), and lactate values were higher in the group with the highest value of irisin (3.65 ± 0.69 × 2.82 ± 0.59, p = 0.02). Combined physical training results in increased basal BDNF in people living with HIV/AIDS, this finding suggests that increased concentration of BDNF may be associated with decreased chances of developing cognitive disorders or HIV-associated dementia. Further studies involving molecular mechanisms on this subject are necessary.

Legend of Weight Loss: a Crosstalk Between the Bariatric Surgery and the Brain

In recent years, plenty of researches have reported in obese individuals with abnormal brain processes implicated in homeostatic regulation, reward, emotion, memory, attention, and executive function in eating behaviors. Thus, treating obesity cannot remain "brainless." Behavioral and psychological interventions activate the food reward, attention, and motivation system, leading to minimal weight loss and high relapse rates. Pharmacotherapy is an effective weight loss method and regulate brain activity but with concerns about its brain function safety problems. Obesity surgery, the most effective therapy currently available for obesity, shows pronounced effects on brain activity, such as deactivation of reward and attention system, and activation of inhibition control toward food cues. In this review, we present an overview of alterations in the brain after the three common weight loss methods.

Metabolic cross-talk between skeletal muscle and adipose tissue in high-intensity interval training vs. moderate-intensity continuous training by regulation of PGC-1α

PURPOSE

An imbalance in the production of adipokines and myokines impairs the energy expenditure, increases adipocyte and develops metabolic pathologies. Physical exercise is able to regulate the secretion of myokines and adipokines. The present study considers the metabolic cross talk between skeletal muscle and adipose tissue in high-intensity interval training vs. moderate-intensity continuous training by regulation of PGC-1α.

METHODS

A sample of 32 male Wistar rats (8 weeks old with mean weight 250 ± 55 g) were divided into four groups randomly: control of base (CO), control of 8 weeks (CO8w), moderate-intensity continuous training (MICT), and high-intensity interval training (HIIT). The rats were fed with standard chow diet. The CO group was killed at the start of the study and the CO8w group was kept alive for the same time as the experimental groups, but did not participate in any exercise. MICT and HIIT groups for 8 weeks were placed under the moderate-intensity continuous training (15-60 min, with speed of 15-30 m/min) and high-intensity interval training (8-4 intense period for 1 min, with speed of 28-55 m/min, with 3-7 slow-intensity period for 1 min, with a speed of 12-30 m/min) for 8 weeks, respectively. To measure the levels of serum irisin, nesfatin, and resistin the ELISA method was used and real-time PCR method was used to evaluate the relative expression of soleus PGC-1α gene mRNA.

RESULTS

The levels of irisin and nesfatin significantly increased in the HIIT compared with control groups (p = 0.001). Resistin values in both training groups showed a significant decrease compared to the control groups (p = 0.005). The level of PGC-1α gene expression in both HIIT and MICT groups was significantly increased in comparison with the control groups (p = 0.001).

DISCUSSION

The results showed that HIIT and MICT increase the transcription of the PGC-1α gene and possibly the increased expression of this gene after HIIT and MICT plays a central role in the secretion of skeletal muscle myokines and adipokines of adipose tissue.

LEVEL OF EVIDENCE

No Level of evidence: Animal study.

The diagnostic and prognostic roles of serum irisin in bladder cancer

BACKGROUND

Egypt is among the countries with the highest incidence of bladder cancer (BC). Adipokines involved in BC development. This study aimed to examine the diagnostic and prognostic roles of irisin in BC through its function as an adipokine.

PATIENTS AND METHODS

This study included 150 subjects; 75 patients newly diagnosed as BC and 75 apparently healthy subjects. Serum irisin levels were quantified by enzyme-linked immunosorbent assay.

RESULTS

Serum irisin levels exhibited significantly lower levels in BC patients compared to controls (1.07 [0.51-1.96] and 1.8 [0.5-2.44] µg/mL), respectively (P < 0.001). Serum irisin was positively correlated with BMI (r = 0.386, P = 0.001) and negatively correlated with serum cholesterol (r = -0.58, P < 0.0001). Irisin had 74.7% sensitivity and 90.7% specificity at a cutoff point of ≤1.2 µg/mL. Serum irisin levels reduction can predict the BC stages, when adjusted for BMI and serum cholesterol level, serum irisin had an adjusted odds ratio of 14 (P = 0.001). Low serum irisin patients had a higher mortality rate when compared to those with high levels (38.2% vs 5%).

CONCLUSION

BC patients had significantly lower levels of serum irisin. Serum irisin showed acceptable performance criteria in BC diagnosis. It had a limited role in BC grading but showed a decreasing trend in different BC stages. Serum irisin seems to be an excellent diagnostic and prognostic marker for BC.

Expression of Irisin/FNDC5 in Cancer Cells and Stromal Fibroblasts of Non-small Cell Lung Cancer

Background: Recent in vitro studies have indicated that irisin inhibits proliferation, migration and epithelial-mesenchymal transition. Irisin expression has not been studied in tumour tissues of non-small cell lung cancer (NSCLC) patients yet. The aim of the study was to determine the irisin expression in NSCLCs in comparison to the clinicopathological factors and expression of TTF-1, p63 and Ki-67. Material and methods: Tissue microarrays with 729 NSCLC and 140 non-malignant lung tissue (NMLT) were used to perform immunohistochemical reactions. Laser Capture Microdissection (LCM) was used to collect cancer and stromal cells from NSCLCs. FNDC5 expression was tested for LCM samples, 75 NSCLCs and 25 NMLTs with the RT-PCR technique. Western-blot, immunofluorescence reaction and RT-PCR assays were performed on lung cancer cell lines. Results: Irisin expression was observed in NSCLC cancer cells and stromal fibroblasts. In cancer cells, irisin expression was decreased in higher grades (G) of malignancy, tumour size (T) and according to lymph node metastasis. In stromal cells, irisin expression was increased in higher G and advanced T. A shorter overall survival was observed in patients with higher irisin expression in NSCLC stromal cells. Conclusions: Irisin expression in stromal fibroblasts may influence cancer cell proliferation and may be a prognostic factor for survival in NSCLC.

Comprehensive evaluation of irisin levels in fetomaternal circulation of pregnant women with obesity or gestational diabetes mellitus

AIM

To evaluate maternal and cord blood irisin levels in pregnant women with gestational diabetes mellitus (GDM) and in obese pregnant women without GDM.

METHODS

The study included 109 patients, with 34 patients in the GDM group, 40 in the obese non-GDM group, and 35 in the control group. Maternal serum irisin levels at the time of delivery were measured by an enzyme-linked immunosorbent assay kit. The correlation of serum irisin levels with metabolic parameters and anthropometric measurements was analyzed.

RESULTS

There were significant differences between the study groups in terms of cord arterial, cord venous, and maternal serum irisin levels (P < 0.001, P < 0.01, P < 0.001, respectively). Cord arterial, cord venous, and maternal serum irisin levels were higher in the obese group compared to the control (P < 0.01, P < 0.01, P < 0.01, respectively) and the GDM group (P < 0.001, P < 0.001, P < 0.001, respectively).

CONCLUSION

Elevation in irisin levels of women who have pregnancies complicated with obesity may be explained as part of the compensation mechanism against disturbed metabolic functions. Pregnant individuals with GDM have lower serum irisin levels in comparison to healthy pregnant women. In this regard, it is possible that the measurement of serum irisin levels may be utilized in the future for prediction, prevention, and treatment of GDM.

Beneficial Effect of Exercise on Cognitive Function during Peripheral Arterial Disease: Potential Involvement of Myokines and Microglial Anti-Inflammatory Phenotype Enhancement

Peripheral arterial disease (PAD), leading to intermittent claudication, critical ischemia with rest pain, and/or tissue damage, is a public health issue associated with significant morbidity and mortality. Little is known about the link between PAD, cognitive function, and whether exercise might reduce cognitive dysfunction in PAD patients, as previously observed concerning both quality of life and prognosis. This review highlights the fact that patients suffering from PAD often demonstrate cognitive dysfunction characterized by reduced performance in nonverbal reasoning, reduced verbal fluency, and decreased information processing speed and a greater risk for progression toward dementia. Further, the data presented support that physical exercise, likely through myokine secretion and microglial anti-inflammatory phenotype enhancement, might participate in the cognition protection in common clinical settings.

Association of Circulating Irisin Concentrations with Weight Loss after Roux-en-Y Gastric Bypass Surgery

Irisin is a myokine with potential anti-obesity properties that has been suggested to increase energy expenditure in obese patients. However, there is limited clinical information on the biology of irisin in humans, especially in morbidly obese patients undergoing bariatric surgery. We aimed to assess the association of circulating irisin concentrations with weight loss in obese patients undergoing bariatric surgery. This was a pilot, single-centre, longitudinal observational study. We recruited 25 morbidly obese subjects who underwent Roux-en-Y gastric bypass surgery (RYGBP), and blood samples from 12 patients were taken to measure serum irisin concentrations before, and one and nine months after surgery. Their clinical characteristics were measured for one year. The preoperative serum irisin concentration (mean 1.01 ± 0.23 μg/mL, range 0.73⁻1.49) changed bidirectionally one month after RYGBP. The mean concentration at nine months was 1.11 ± 0.15 μg/mL (range 0.92⁻1.35). Eight patients had elevated irisin levels compared with their preoperative values, but four did not. Elevations of irisin levels nine months, but not one month, after surgery, were associated with lower preoperative levels (p = 0.016) and worse weight reduction rates (p = 0.006 for the percentage excess weight loss and p = 0.032 for changes in body mass index). The preoperative serum irisin concentrations were significantly correlated with the percentage of excess weight loss for one year (R² = 0.612; p = 0.04) in our study. Our results suggest that preoperative circulating irisin concentrations may be at least in part associated with a weight loss effect of bariatric surgery in morbidly obese patients. Further large-scale clinical studies are needed to ratify these findings.

The Diagnostic Value of Irisin in Pediatric Patients with Acute Abdominal Pain

Objectives

Diagnosis of pediatric patients presenting to the Emergency Department with acute abdominal pain is not always easy. The purpose of this study was to investigate the effectiveness of irisin, a peptide hormone with reactivity shown in the appendix and neutrophils, in the differential diagnosis of pediatric patients with acute abdominal pain.

Methods

162 subjects consenting to participate, including 112 patients presenting to the Pediatric Emergency and Pediatric Surgery clinics with acute abdominal pain and 50 controls, were enrolled in the study. Blood was collected from all patients following initial examination for irisin, WBC, and CRP investigation.

Results

Mean irisin levels in cases of acute appendicitis (AA) and perforated appendicitis (PA) were statistically significantly higher compared to nonspecific abdominal pains and the control group. No statistically significant difference was observed in irisin levels between AA and PA cases. WBC and CRP levels were also significantly higher in cases of AA and PA compared to nonspecific abdominal pains.

Conclusions

Differential diagnosis of acute abdominal pains in children and deciding on surgery are a difficult and complex process. Our study shows that irisin can be a useful biomarker in differentiating AA and PA from other acute abdominal pains in children.

Th17 and Treg lymphocytes in obesity and Type 2 diabetic patients

Assumption that the pathogenesis of obesity-associated type 2 diabetes (T2DM) encompasses inflammation and autoimmune aspects is increasingly recognized. In the state of obesity and T2DM, the imbalance of T helper 17 (Th17) cells and regulatory T (Treg) cells are observed. These alterations reflect a loss of T cell homeostasis, which may contribute to tissue and systemic inflammation and immunity in T2DM. In this review we will discuss the accumulating data supporting the concept that Th17/Treg mediated immune responses are present in obesity-related T2DM pathogenesis, and provide evidences that restoration of Th17/Treg imbalance may be a possible therapeutic avenue for the prevention and treatment of T2DM and its complications.

Irisin Maternal Plasma and Cord Blood Levels in Mothers with Spontaneous Preterm and Term Delivery

Irisin, an adipomyokine identified in 2012, has been investigated in association with common pregnancy complications, including gestational diabetes mellitus, preeclampsia, and intrauterine growth restriction. The objective of this study is to examine the potential role of irisin in preterm birth (PTB) by comparing its level between mothers with term and preterm labor. Maternal peripheral blood and cord blood samples were collected from 30 mothers who delivered prematurely and from 35 mothers who delivered at term. Irisin concentrations were measured in all samples using ELISA, and four common single nucleotide polymorphisms in the irisin gene were determined (rs16835198, rs726344, rs3480, and rs1746661). Univariable and multivariable regression modeling was applied to evaluate maternal and cord blood irisin concentrations in relation to preterm/term labor. Irisin concentration in umbilical cord blood was found to be associated with PTB in the univariable model (p = 0.046). On the other hand, no differences in maternal blood irisin levels between mothers with preterm and term deliveries were established. To the best of our knowledge, this is the first study determining irisin levels in term and preterm deliveries in maternal peripheral blood and umbilical cord blood. Our study shows a possible association between cord blood irisin concentration and PTB occurrence.

Colostrum and mature breast milk analysis of serum irisin and sterol regulatory element-binding proteins-1c in gestational diabetes mellitus

Background: We aimed to evaluate irisin and SREBP-1c levels in serum, colostrum and mature breast milk in women with and without gestational diabetes (GDM); and to relate them with maternal glucose, lipid profile and weight status of babies. Methods: GDM positive women (n = 33) and normal glucose tolerant women (NGT) (n = 33) were recruited. Maternal blood samples were collected at 28th week of gestation and later at 6-week post-partum while breast milk samples of the lactating mothers were collected within 72 hours of birth (colostrum) and at 6 weeks post-partum (mature milk). Irisin and SREBP-1c levels were analyzed by commercially available ELISA kits for all maternal samples. Results: Lower levels of irisin were seen in serum, colostrum and mature breast milk of GDM females (p < .01). SREBP-1c profile showed a similar trend of low serum levels in GDM, however, they were undetectable in colostrum and mature breast milk. Weak to moderate correlations of serum irisin with BMI (r = 0.439; p < .001), GTT 0 hours (r = 0.403; p = .01), HbA1c (r = -0.312; p = .011), Fasting blood glucose (r = 0.992; p = .008), and baby weight at birth (r = 0.486; p < .001). Colostrum and mature breast milk irisin showed positive associations with baby weight at 6 weeks (r = 0.325; p = .017; r = 0.296; p = .022, respectively). Serum SREBP-1c at 6 weeks correlated with random blood glucose (r = 0.318; p = .009), and HbA1c (r= -0.292; p = .011). All correlations were lost once we adjusted for maternal BMI. Conclusions: Low irisin and SREBP1-c levels may favor development of GDM in pregnant subjects. Further, low mature breast milk levels may act as a continued stressor from fetal to infant life as long as breast-feeding is continued. Further studies are required to identify the mechanistic relationship between these biomarkers and GDM.

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.

Physiology and role of irisin in glucose homeostasis

Irisin is a myokine that leads to increased energy expenditure by stimulating the 'browning' of white adipose tissue. In the first description of this hormone, increased levels of circulating irisin, which is cleaved from its precursor fibronectin type III domain-containing protein 5, were associated with improved glucose homeostasis by reducing insulin resistance. Consequently, several studies attempted to characterize the role of irisin in glucose regulation, but contradictory results have been reported, and even the existence of this hormone has been questioned. In this Review, we present the current knowledge on the physiology of irisin and its role in glucose homeostasis. We describe the mechanisms involved in the synthesis, secretion, circulation and regulation of irisin, and the controversies regarding the measurement of irisin. We also discuss the direct effects of irisin on glucose regulatory mechanisms in different organs, the indirect effects and interactions with other hormones, and the important open questions with regard to irisin in those organs. Finally, we present the results from animal interventional studies and from human clinical studies investigating the association of irisin with obesity, insulin resistance, type 2 diabetes mellitus and the metabolic syndrome.

Crosstalk between adipokines and myokines in fat browning

Skeletal muscle is the largest organ determining whole-body insulin sensitivity and metabolic homoeostasis. Adaptive changes of skeletal muscle in response to physical activity include adjustments in the production and secretion of muscle-derived bioactive factors, known as myokines, such as myostatin, IL-4, IL-6, IL-7 and IL-15, myonectin, follistatin-like 1 or leukaemia inhibitory factor. These myokines not only act locally in the muscle in an autocrine/paracrine manner, but also are released to the bloodstream as endocrine factors to regulate physiological processes in other tissues. Irisin, derived from the cleavage of FNDC5 protein, constitutes a myokine that induces myogenesis and fat browning (switch of white adipocytes to brown fat-like cells) together with a concomitant increase in energy expenditure. Besides being a target for irisin actions, the adipose tissue also constitutes a production site of FNDC5. Interestingly, irisin secretion from subcutaneous and visceral fat depots is decreased by long-term exercise training and fasting, suggesting a discordant regulation of FNDC5/irisin in skeletal muscle and adipose tissue. Accordingly, our group has recently reported that the adipokine leptin differentially regulates FNDC5/irisin expression in skeletal muscle and fat, confirming the crosstalk between both tissues. Moreover, irisin secretion and function are regulated by other myokines, such as follistatin or myostatin, as well as by other adipokines, including fibroblast growth factor 21 and leptin. Taken together, myokines have emerged as novel molecular mediators of fat browning and their activity can be modulated by adipokines, confirming the crosstalk between skeletal muscle and adipose tissue to regulate thermogenesis and energy expenditure.

TRPM8 downregulation by angiotensin II in vascular smooth muscle cells is involved in hypertension

Angiotensin II (Ang II)-induced injury of vascular smooth muscle cells (VSMCs) serves an important role in hypertension and other cardiovascular disorders. Transient receptor potential melastatin 8 (TRPM8) is a thermally‑regulated Ca2+‑permeable channel that is activated by reduced body temperature. Although several recent studies have revealed the regulatory effect of TRPM8 in vascular tone and hypertension, the precise role of TRPM8 in dysfunction of vascular smooth muscle cells (VSMCs) induced by Ang II remains elusive. In the present study, the possible function of TRPM8 in Ang II‑induced VSMCs malfunction in vivo and in vitro was investigated. In the aortae from rats that had undergone a two‑kidney one‑clip operation, which is a widely‑used renovascular hypertension model, the mRNA and protein levels of TRPM8 were reduced. In addition, exogenous Ang II treatment decreased TRPM8 mRNA and protein expression levels in primary cultures of rat VSMCs. TRPM8 activation by menthol, a pharmacological agonist, in VSMCs, significantly attenuated the Ang II‑induced increase in reactive oxygen species and H2O2 production. In addition, TRPM8 activation reduced the Ang II‑induced upregulation of NADPH oxidase (NOX) 1 and NOX4 in VSMCs. Furthermore, TRPM8 activation relieved the Ang II‑induced activation of ras homolog gene family, member A‑rho associated protein kinase 2 and janus kinase 2 signaling pathways in VSMCs. In conclusion, the results presented in the current study indicated that TRPM8 downregulation by Ang II in VSMCs may be involved in hypertension.

Metabolic Adaptation in Obesity and Type II Diabetes: Myokines, Adipokines and Hepatokines

Obesity and type II diabetes are characterized by insulin resistance in peripheral tissues. A high caloric intake combined with a sedentary lifestyle is the leading cause of these conditions. Whole-body insulin resistance and its improvement are the result of the combined actions of each insulin-sensitive organ. Among the fundamental molecular mechanisms by which each organ is able to communicate and engage in cross-talk are cytokines or peptides which stem from secretory organs. Recently, it was reported that several cytokines or peptides are secreted from muscle (myokines), adipose tissue (adipokines) and liver (hepatokines) in response to certain nutrition and/or physical activity conditions. Cytokines exert autocrine, paracrine or endocrine effects for the maintenance of energy homeostasis. The present review is focused on the relationship and cross-talk amongst muscle, adipose tissue and the liver as secretory organs in metabolic diseases.

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.

Non-alcoholic fatty liver disease and dyslipidemia: An update

Non-alcoholic fatty liver (NAFLD) is the most common liver disease worldwide, progressing from simple steatosis to necroinflammation and fibrosis (leading to non-alcoholic steatohepatitis, NASH), and in some cases to cirrhosis and hepatocellular carcinoma. Inflammation, oxidative stress and insulin resistance are involved in NAFLD development and progression. NAFLD has been associated with several cardiovascular (CV) risk factors including obesity, dyslipidemia, hyperglycemia, hypertension and smoking. NAFLD is also characterized by atherogenic dyslipidemia, postprandial lipemia and high-density lipoprotein (HDL) dysfunction. Most importantly, NAFLD patients have an increased risk for both liver and CV disease (CVD) morbidity and mortality. In this narrative review, the associations between NAFLD, dyslipidemia and vascular disease in NAFLD patients are discussed. NAFLD treatment is also reviewed with a focus on lipid-lowering drugs. Finally, future perspectives in terms of both NAFLD diagnostic biomarkers and therapeutic targets are considered.

Irisin modulates the association of interleukin-17A with the presence of non-proliferative diabetic retinopathy in patients with type 2 diabetes

The role of inflammation in pathogenesis of diabetic retinopathy (DR) is getting increasingly recognized. However, it is unclear whether and how non-proliferative diabetic retinopathy (NPDR) is affected by Interleukin-17A (IL-17A) and Interleukin-22 (IL-22), two well-known inflammatory factors, and irisin, a novel potential anti-inflammatory factor. Here we recruited 40 type 2 diabetes mellitus (T2DM) patients with NPDR, 60 T2DM patients without DR (no-DR), and 20 normal glucose tolerance (NGT) controls. Serum levels of IL-17A, IL-22, and irisin were examined. Compared with NGT and no-DR subjects, NPDR group had significantly higher IL-17A levels. Irisin levels were significantly lower in T2DM patients, while IL-22 levels were not significantly different across all three groups. Multiple logistic regression analysis revealed that IL-17A significantly increased the risk of NPDR (OR = 1.22, P < 0.05) before adjusting for irisin. When irisin was included in the model, neither irisin nor IL-17A was associated with NPDR. Further partial correlation analysis showed that irisin was intrinsically correlated with IL-17A even after multiple adjustment (r = -0.252; P = 0.018). These findings suggest that IL-17A is an independent risk factor of NPDR, and irisin could protect against DR through potential anti-IL-17A effects.

Exercise-stimulated FGF23 promotes exercise performance via controlling the excess reactive oxygen species production and enhancing mitochondrial function in skeletal muscle

OBJECTIVE

Physical exercise induces many adaptive changes in skeletal muscle and the whole body and improves metabolic characteristics. Fibroblast growth-factor 23 (FGF23) is a unique member of the FGF family that acts as a hormone regulating phosphate metabolism, calcitriol concentration, and kidney functions. The role of FGF23 in exercise and skeletal muscle is largely unknown yet.

MATERIALS AND METHODS

C57BL/6J mice were exercised on a motor treadmill. Mice serum FGF23 levels; FGF23 mRNA expression in various organs including the liver, heart, skeletal muscle tissue, and thyroid; and FGF23 receptor Klotho mRNA expression were examined using enzyme-linked immunosorbent assay, real-time polymerase chain reaction, and immunoblotting, respectively, after a single bout of acute exercise (60min), exhaustive exercise, and chronic prolonged exercise (60min every day for one week). C57BL/6J mice were injected with recombinant FGF23 (100mg/kg, twice per day, i.p.) or vehicle control (saline) for 3days, and then the exercise performance, reactive oxygen species (ROS), H2O2 production, and mitochondrial functional biomarkers in muscle (gene expression of sirtuin 1, PPAR-δ, PGC-1α and mitochondrial transcription factor A [TFAM], and citrate synthase activity) were assayed.

RESULTS

Three forms of exercise, acute exercise, exhaustive exercise, and chronic exercise, increased serum FGF23 levels. However, only chronic exercise upregulated FGF23 mRNA and protein expression in skeletal muscle. FGF23 mRNA expression in the heart, liver, and thyroid was not affected. FGF23 protein was mainly located in the cytoplasm in skeletal muscle tissue and the localization of FGF23 was not altered by exercise. Exogenous FGF23 treatment significantly extended the time to exhaustion and reduced the exercise-induced ROS and H2O2 production. FGF23 treatment increased the mRNA level of PPAR-δ and citrate synthase activity, but did not influence the mRNA expression of sirtuin 1, PGC-1α, and TFAM in skeletal muscle.

CONCLUSION

These results demonstrate that exercise-stimulated FGF23 promotes exercise performance via controlling the excess ROS production and enhancing mitochondrial function in skeletal muscle, which reveals an entirely novel role of FGF23 in skeletal muscle.

Central nervous system regulation of eating: Insights from human brain imaging

Appetite and body weight regulation are controlled by the central nervous system (CNS) in a rather complicated manner. The human brain plays a central role in integrating internal and external inputs to modulate energy homeostasis. Although homeostatic control by the hypothalamus is currently considered to be primarily responsible for controlling appetite, most of the available evidence derives from experiments in rodents, and the role of this system in regulating appetite in states of hunger/starvation and in the pathogenesis of overeating/obesity remains to be fully elucidated in humans. Further, cognitive and affective processes have been implicated in the dysregulation of eating behavior in humans, but their exact relative contributions as well as the respective underlying mechanisms remain unclear. We briefly review each of these systems here and present the current state of research in an attempt to update clinicians and clinical researchers alike on the status and future directions of obesity research.

Skeletal muscle as an endocrine organ: Role of [Na+]i/[K+]i-mediated excitation-transcription coupling

During the last two decades numerous research teams demonstrated that skeletal muscles function as an exercise-dependent endocrine organ secreting dozens of myokines. Variety of physiological and pathophysiological implications of skeletal muscle myokines secretion has been described; however, upstream signals and sensing mechanisms underlying this phenomenon remain poorly understood. It is well documented that in skeletal muscles intensive exercise triggers dissipation of transmembrane gradient of monovalent cations caused by permanent activation of voltage-gated Na+ and K+ channels. Recently, we demonstrated that sustained elevation of the [Na+]i/[K+]i ratio triggers expression of dozens ubiquitous genes including several canonical myokines, such as interleukin 6 and cyclooxygenase 2, in the presence of intra- and extracellular Ca2+ chelators. These data allowed us to suggest a novel [Na+]i/[K+]i-sensitive, Ca2+i-independent mechanism of excitation-transcription coupling which triggers myokine production. This pathway exists in parallel with canonical signaling mediated by Ca2+i, AMP-activated protein kinase and hypoxia-inducible factor 1α (HIF-1α). In our mini-review we briefly summarize data supporting this hypothesis as well as unresolved issues aiming to forthcoming studies.

Cord blood irisin levels are positively correlated with birth weight in newborn infants

BACKGROUND

Irisin is a novel myokine, secreted from skeletal muscle after exercise. Irisin mediates exercise-related energy expenditure by turning white adipose tissue (WAT) into brown adipose tissue (BAT). Thus, irisin is considered as a potential biomarker for obesity and metabolic syndrome. Infants born small for gestational age (SGA) have increased risk for metabolic syndrome. However, the physiologic role of irisin in neonates remains to be studied.

OBJECTIVE

To evaluate the association of umbilical cord blood irisin levels with gestational age and birth weight categories in neonates.

METHODS

A cross-sectional study of 341 newborns, from 26 to 41weeks' gestation. We collected umbilical cord blood and analyzed plasma for irisin by ELISA.

RESULTS

Plasma irisin levels were positively correlated with gestational age (r=0.21, p<0.001), and birth weight Z-score (r=0.18, p<0.001). SGA infants had significantly lower irisin (median [interquartile range] 55.38 [46.56-65.72]ng/mL) compared to appropriate for gestational age infants (64.41 [53.87-76.76]ng/mL) and large for gestational age infants (68.70 [54.78-79.09]ng/mL, p<0.01). The association between SGA and lower irisin remained significant in multivariate analysis independent of gestational age, maternal age, maternal BMI, and gestational diabetes (p=0.03). In singleton infants, irisin was also significantly negatively associated with maternal preeclampsia (p=0.01).

CONCLUSIONS

Our results support the notion that irisin may have a physiologic role in neonates. We speculate that decreased levels of irisin in SGA infants may contribute to the development of catch-up growth and metabolic syndrome later in life.

Irisin-encoding gene (FNDC5) variant is associated with changes in blood pressure and lipid profile in type 2 diabetic women but not in men

INTRODUCTION

Irisin has recently been described as a novel myokine, which reduces visceral obesity and improves glucose metabolism in mice. Thus, polymorphisms in the gene encoding irisin, fibronectin type III domain containing 5 (FNDC5), may be associated with type 2 diabetes mellitus (T2DM) and related disorders. However, to date, no study has investigated the association between FNDC5 polymorphisms and susceptibility to T2DM.

OBJECTIVE

To investigate the association of FNDC5 rs3480 (A/G) and rs1746661 (G/T) polymorphisms, alone or in combination, with T2DM and its clinical features.

METHODS

We analyzed 1006 T2DM patients and 434 nondiabetic subjects. Polymorphisms were genotyped by real-time PCR using TaqMan MGB probes. Haplotypes constructed from the combination of rs1746661 and rs3480 polymorphisms were inferred using the Phase 2.1 program.

RESULTS

Genotype, allele and haplotype frequencies of rs1746661 and rs3480 polymorphisms did not differ significantly between nondiabetic subjects and T2DM patients. Women with T2DM carrying the G allele of rs3480 showed increased HbA1c levels compared with A/A carriers, adjusted for age. The T allele of rs1746661 was associated with increased systolic blood pressure, total cholesterol and LDL-cholesterol and decreased HDL-cholesterol in women with T2DM, adjusted for covariates. Moreover, prevalence of hypercholesterolemia was higher in women carrying the T allele of rs1746661 than in G/G carriers (72.4% vs. 58.7%, OR=2.010, 95% CI=1.210-3.390), but it was not significantly different in men.

CONCLUSIONS

These results indicate that, although not associated with T2DM, the G allele of rs3480 appears to be associated with increased HbA1c, while the T allele of rs1746661 appears to be associated with higher systolic blood pressure and dyslipidemia in women with T2DM.

Habitual physical activity is associated with circulating irisin in healthy controls but not in subjects with diabetes mellitus type 2

BACKGROUND

Irisin, a novel myokine, has been shown to increase following vigorous exercise, with studies suggesting that it mediates some of the beneficial effects of exercise. Irisin might play a role in 'browning' of white adipocytes, thus increasing energy expenditure. The role of irisin in exercise and energy expenditure in subjects with diabetes mellitus type 2 (DMT2) remains largely unknown. We aimed to investigate the association between circulating irisin and habitual physical activity in subjects with and without DMT2.

MATERIAL AND METHODS

In this cross-sectional study, 164 Saudi adults: 81 non-DMT2 controls [age: (mean ± SD) 51.6 ± 10.9; BMI: 29.6 ± 4.3 kg/m(2) ] and 83 DMT2 subjects [age: 54.3 ± 10.3 year; BMI: 29.4 ± 4.7 kg/m(2) ] were studied. Anthropometric and fasting serum biochemical data were collected. Circulating irisin was measured using an enzyme-linked immunosorbent assay (ELISA). Frequency intensity time (FIT) index was used to assess the level of habitual physical activity.

RESULTS

We observed significantly higher levels of irisin in DMT2 subjects than in controls (P < 0.001). FIT index was positively associated (r = 0.20, P = 0.03) with circulating irisin in controls only. Additionally, irisin levels were significantly higher in tertile 3 (0.75 ± 0.07 μg/mL) than tertile 1 (0.49 ± 0.06 μg/mL) of the FIT index in healthy controls, whilst no such relation with physical activity was observed in DMT2 subjects.

CONCLUSION

This cross-sectional study has shown a weak association of irisin with physical activity levels in healthy controls but not in DMT2 subjects, suggesting the possibility of discordant regulation in the condition of DMT2.

The 148 M allele of the PNPLA3 is associated with plasma irisin levels in a population sample of Caucasian children: The PANIC Study

BACKGROUND

There are no previous data on the association of PNPLA3 I148M polymorphism (rs738409) with circulating adipokines and myokines in children.

SUBJECTS/METHODS

Subjects were a population sample of 481 Caucasian children aged 6-8 years. We assessed circulating levels of irisin together with IL-6, TNF-α, leptin, high molecular weight (HMW)-adiponectin, alanine aminotransferase (ALT) and gamma-glutamyl transferase (GGT) while the subjects were stratified according to PNPLA3 I148M variants.

RESULTS

The PNPLA3 rs738409 polymorphism had a linear relationship with plasma levels of irisin after adjustment for age, sex and body height (p=0.007) but it was not associated with circulating levels of interleukin- 6 (IL-6), tumor-necrosis factor α (TNF-α), leptin or HMW-adiponectin.

CONCLUSION

PNPLA3 148M allele carriers had higher plasma levels of irisin than the non-carriers. This might be due to compensatory mechanism to limit early steatotic and inflammatory changes in the liver.

Circulating irisin concentrations in rat models of thyroid dysfunction -- effect of exercise

OBJECTIVES

The speculation that the myokine irisin could regulate whole body energy expenditure led to the anticipation that irisin may have therapeutic potential in metabolic diseases. Regulation of irisin under conditions of metabolic derangements in altered thyroid status, and the changes in irisin response to exercise remain to be investigated.

METHODS

Serum irisin concentration was measured in sixty male Wistar rats subjected to either sedentary life or 8-week chronic swimming exercise after induction of hyper- or hypothyroidism (10 rats/group). The effect of acute exercise on serum irisin was assessed in 10 additional rats subjected once to forced swimming against a load (5% of body weight) and compared to sedentary rats.

RESULTS

Serum irisin was significantly higher in both sedentary hyper- and hypothyroid rats (by 45%, p<0.001, and 30%, p<0.001, respectively) versus euthyroid controls. Serum irisin also increased after acute exercise (p<0.001 versus sedentary control). Chronic training episodes failed to significantly alter serum irisin in all thyroid hormone profiles. Serum irisin correlated positively with serum creatine kinase (r=0.45, p<0.001) and with muscle and liver concentrations of malondialdehyde (r=0.50 and r=0.47 respectively, p<0.001 for both), and negatively with muscle and liver content of reduced glutathione (r=-0.34, p=0.003 and r=-0.28, p=0.018 respectively) in pooled groups. However, significance of these associations was waived when analyzing each group separately. Serum irisin was not associated with skeletal muscle mass, insulin resistance, blood glucose, lipids or TSH.

CONCLUSIONS

Both hyper- and hypothyroidism are associated with up-regulation of serum irisin in male rats, possibly as a response to oxidative damage and/or myopathy observed in both conditions. Acute exercise, which is also associated with oxidative stress, increases serum irisin. No obvious association was detected linking serum irisin to metabolic abnormalities in thyroid dysfunction.

Sarcopenia--The search for emerging biomarkers

Sarcopenia, an age-related decline in skeletal muscle mass and function, dramatically affects the life quality of elder people. In view of increasing life expectancy, sarcopenia renders a heavy burden on the health care system. However, although there is a consensus that sarcopenia is a multifactorial syndrome, its etiology, underlying mechanisms, and even definition remain poorly delineated, thus, preventing development of a precise treatment strategy. The main aim of our review is to critically analyze potential sarcopenia biomarkers in light of the molecular mechanisms of their involvement in sarcopenia pathogenesis. Normal muscle mass and function maintenance are proposed to be dependent on the dynamic balance between the positive regulators of muscle growth such as bone morphogenetic proteins (BMPs), brain-derived neurotrophic factor (BDNF), follistatin (FST) and irisin, and negative regulators including TGFβ, myostatin, activins A and B, and growth and differentiation factor-15 (GDF-15). We hypothesize that the shift in this balance to muscle growth inhibitors, along with increased expression of the C- terminal agrin fragment (CAF) associated with age-dependent neuromuscular junction (NMJ) dysfunction, as well as skeletal muscle-specific troponin T (sTnT), a key component of contractile machinery, is a main mechanism underlying sarcopenia pathogenesis. Thus, this review proposes and emphasizes that these molecules are the emerging sarcopenia biomarkers.

Irisin has no effect on lipolysis in 3T3-L1 adipocytes or fatty acid metabolism in HepG2 hepatocytes

Irisin, a newly identified myokine responsible for browning of white or beige adipocytes, has been reported to be present at reduced levels in diabetic patients and associated with obesity, serum triglyceride (TG) levels, and intrahepatic TG levels. We wondered whether irisin could directly affect fatty acid and TG metabolism in adipocytes and hepatocytes. We examined the effects of various concentrations of irisin on lipolysis (according to Oil Red O staining, free fatty acid release, and glycerol release), protein expression of HSL and ATGL, and mRNA expression of other lipid-related genes (UCP-1, PPARγ, FABP-4, HSL, ATGL, PPARα, and CPT-1) in mature 3T3-L1 adipocytes, as well as mRNA levels of genes involved in the synthesis (SREBP-1C and FAS) and β-oxidation (PPARα and CPT-1) of fatty acids in HepG2 hepatocytes under physiological or hyperglycemic conditions. Our results revealed that although irisin significantly increased the mRNA levels of UCP-1 and PPARα, it failed to show detectable effects on lipolysis, HSL or ATGL protein levels, or the mRNA expression of other lipid-related genes in mature 3T3-L1 adipocytes. In HepG2 hepatocytes, high glucose induced the upregulation of SREBP-1C and FAS and the downregulation of PPARα; however, no significant effect of irisin on gene expression was observed under either physiological or hyperglycemic conditions. We therefore conclude that irisin has no significant direct effect on lipolysis in 3T3-L1 adipocytes or on fatty acid metabolism in HepG2 hepatocytes.

Association between serum irisin levels and non-alcoholic fatty liver disease in health screen examinees

Irisin is a recently found myokine that aids obesity control and improves glucose homeostasis by acting on white adipose tissue cells and increases total energy consumption. The aim of this study was to evaluate serum irisin levels in patients with non-alcoholic fatty liver disease (NAFLD) and to compare these levels with those of normal controls. Among 595 health screen examinees who had visited our institute between January 2013 to March 2013, 355 patients (84 NAFLD patients and 271 normal controls) were enrolled depending on whether they gave written informed consents and their history of alcohol intake, blood tests, and abdominal ultrasonographic findings. Age; sex; laboratory test parameters; homeostasis model assessment-insulin resistance; and levels of leptin, adiponectin, and irisin were assessed. Serum irisin levels (ng/ml) were significantly higher in the NAFLD group than in normal controls (63.4±32.6 vs. 43.0±29.7, p<0.001) and higher in the mild fatty liver group than in the moderate-to-severe fatty liver group (68.3±38.2 vs. 56.6±21.2, p<0.001). Additionally, serum irisin levels were not different between the non-obese and obese groups (48.4±34.2 vs. 45.8±22.9, p = 0.492); however, the levels were significantly lowest in normal controls and highest in the mild fatty liver group in the non-obese (44.9±31.7 vs. 73.1±48.5 vs 59.7±18.0, p<0.001) and obese groups (35.0±17.0 vs. 62.9±21.2 vs. 54.6±23.3, p<0.001). Serum irisin levels were significantly higher in NAFLD patients, which is not consistent with the results of previously published studies. Therefore, more studies are needed to confirm the role of irisin in NAFLD.

Circulating irisin, omentin-1, and lipoprotein subparticles in adults at higher cardiovascular risk

OBJECTIVE

Muscle and fat are now recognized as metabolism-regulating endocrine organs. However, muscle and adipocyte-derived novel cytokines such as irisin and omentin-1 remain understudied in relation to metabolic biomarkers that are associated with cardiovascular risk.

SUBJECTS AND METHODS

Thirty-nine subjects with mean (± SD) BMI of 29.2 ± 5.4 kg/m(2) and either diabetes or two other cardiovascular risk factors were enrolled in a 6-month randomized trial of low-dose ethanol. We examined cross-sectional data at baseline, 3-month, and 6-month visits to assess (1) within-person stability of novel cytokines (irisin, omentin-1, visfatin, resistin, and soluble tumor necrosis factor receptor II) and (2) their associations with metabolic parameters, particularly lipoprotein subparticle profile.

RESULTS

Repeated measures of irisin and omentin-1 were highly correlated, with intra-class correlations of 0.84 (95% CI: 0.74, 0.91; P < 0.001) and 0.81 (0.70, 0.89; P < 0.001), respectively. Irisin was negatively correlated with omentin-1 (7.4% irisin decrease per a 1-SD increment in omentin-1; 95% CI: 0.5%, 13.9%; P = 0.04). In models adjusted for age, sex, and race, irisin was negatively associated with HDL cholesterol (7.3% decrease per a 10mg/dL increment; 1.0%, 13.3%; P = 0.02) and large HDL particles (15.5% decrease per a 1-SD or 3.5-μmol/L increment; 5.2%, 24.7%; P=0.005). Omentin-1 was positively associated with mean VLDL size (3.8% increase per a 1-SD increment; 0.06%, 7.8%; P = 0.05). Adjustment for alcohol intervention, BMI, and other cytokines did not materially affect these associations.

CONCLUSIONS

Irisin and omentin-1 are stable within-person, inversely associated with each other, and closely related to lipoprotein profile. These molecules may be promising markers for cardiovascular risk.

Fighting obesity: When muscle meets fat

The prevalence of obesity has risen to an unprecedented level. According to World Health Organization, over 500 million adults, equivalent to 10%-14% of the world population, were obese with a body mass index (BMI) of 30 kg/m(2) or greater in 2008.(1) This rising prevalence and earlier onset of obesity is believed to be resulted from an interplay of genetic factors, over-nutrition and physical inactivity in modern lifestyles. Obesity also increases the susceptibility to metabolic syndromes, hypertension, cardiovascular diseases, Type 2 diabetes mellitus (T2DM) and cancer.(2-4) The global obesity epidemic has sparked substantial interests in the biology of adipose tissue (fat). In addition, the skeletal muscle and its secretive factors (myokines) have also been shown to play a critical role in controlling body energy balance, adipose homeostasis and inflammation status.(5) Interestingly, skeletal muscle cells share a common developmental origin with brown adipocytes,(6,7) which breaks down lipids to generate heat - thus reducing obesity. Here, we provide a brief overview of the basics and recent progress in muscle-fat crosstalk in the context of body energy metabolism, obesity, and diabetes. We summarize the different types of adipocytes, their developmental origins and implications in body composition. We highlight the role of several novel myokines in regulating fat mass and systemic energy balance, and evaluate the potential of skeletal muscles as a therapeutic target to treat obesity.

Circulating irisin in healthy, young individuals: day-night rhythm, effects of food intake and exercise, and associations with gender, physical activity, diet, and body composition

CONTEXT

The myokine irisin may increase energy expenditure and affect metabolism.

OBJECTIVE

The objective of the study was to elucidate predictors of irisin and study whether circulating irisin may have day-night rhythm in humans.

DESIGN

This was an observational, cross-sectional study with an additional 24-hour prospective observational arm (day-night rhythm substudy) and two prospective interventional arms (mixed meal substudy and exercise substudy).

SETTING

The study was conducted at the Hellenic Military School of Medicine (Thessaloniki, Greece).

PATIENTS AND INTERVENTIONS

One hundred twenty-two healthy, young individuals were subjected to anthropometric and body composition measurements, and their eating and exercise behavior profiles were assessed with validated questionnaires. Subgroups were subjected to day-night rhythm, standardized meal ingestion, and 30-minute aerobic exercise studies.

MAIN OUTCOME MEASURES

Circulating irisin levels were measured.

RESULTS

Ιrisin levels were lower in males than females (P = .02) after adjustment for lean body mass, which was its major determinant. Irisin levels followed a day-night rhythm (P < .001) with peak at 9:00 pm. Irisin levels were increased at the end of exercise (84.1 ± 10.0 vs 105.8 ± 14.3 ng/mL; P < .001). Irisin levels were not affected by intake of a standardized meal and were not associated with caloric intake or diet quality.

CONCLUSIONS

In healthy, young individuals, circulating irisin displays a day-night rhythm, is correlated with lean body mass, and increases acutely after exercise.