The Myokine Irisin Is Released in Response to Saturated Fatty Acids and Promotes Pancreatic β-Cell Survival and Insulin Secretion

A case-control study to evaluate irisin levels in patients with type 1 diabetes mellitus

BACKGROUND

Irisin is a precious hormone-like myokine that plays a key role in glucose/energy expenditure and metabolic regulation This paper aimed to determine the irisin levels in patients with type 1 diabetes mellitus and their correlation with insulin therapy and glycaemic control.

METHODS

Ninety type 1 diabetes mellitus patients were collected. The patients were subdivided into two groups: group I (37) newly diagnosed type 1 diabetes mellitus and group II (53) T1DM (on insulin injection); for comparison, 30 healthy individuals were included as control. The serum levels of irisin were estimated using ELISA. FSG and lipid profile were measured through spectrophotometrically. Glycated hemoglobin was determined using High-performance liquid chromatography.

RESULT

Serum levels of irisin were significantly lower (P = 0.01), as compared to the control group. Also irisin level was significantly lower in group I compared to group II. Fasting serum glucose, glycated hemoglobin, and lipid profile were significantly elevated in patient groups compared to the control group. Serum irisin was negatively correlated to fasting serum glucose, and glycated hemoglobin, whereas it positively correlated to serum lipid profile. In multiple stepwise regression, only glycated hemoglobin (β =  - 0.600, P = 0.040) was determined as an independent predictor for predicting the irisin levels. The AUC was excellent (AUC = 0.996, P = 0.0001), with high diagnostic accuracy (88.2) in differentiating newly diagnosed type 1 diabetes mellitus from the healthy subject group.

CONCLUSION

We demonstrated low irisin levels in type 1 diabetes mellitus and the association of the highest irisin amounts to an insulin therapy and a better glycaemic control. Furthermore, the measurement of irisin levels could be useful as laboratory markers to monitor type 1 diabetes mellitus severity and therapy response.

Impact of L-Arginine on diabetes-induced neuropathy and myopathy: Roles of PAI-1, Irisin, oxidative stress, NF-κβ, autophagy and microRNA-29a

BACKGROUND

T2DM is a chronic disorder with progressive neuromuscular alterations. L-arginine (ARG) is the most common semi-essential amino acid having several metabolic functions.

AIM

to investigate the impact of L-arginine in combating diabetic-induced neuromyopathy and its possible mechanisms.

MATERIALS & METHODS

24 rats were divided into CON, CON+ARG, DC, DC+ARG. Behavioral tests, Body weight (BW), fasting blood glucose (FBG), insulin, total antioxidant capacity (TAC), malondialdehyde (MDA), plasminogen activator inhibitor-1 (PAI-1), and irisin were done. Creatine kinase-MM (CK-MM), interleukin 4 (IL-4), interleukin 6 (IL-6), TAC, MDA, expression of microRNA-29a mRNA & light chain 3 protein were determined in muscle. Histological and NF-κβ immunohistochemical expression in muscle and nerve were assessed.

RESULTS

ARG supplementation to diabetic rats improved altered behavior, significantly increased BW, insulin, TAC, irisin and Il-4, decreased levels of glucose, microRNA-29a, NF-κβ and LC3 expression, PAI-1, CK-MM and restored the normal histological appearance.

CONCLUSIONS

ARG supplementation potently alleviated diabetic-induced neuromuscular alterations.

From Beats to Metabolism: the Heart at the Core of Interorgan Metabolic Cross Talk

The heart, once considered a mere blood pump, is now recognized as a multifunctional metabolic and endocrine organ. Its function is tightly regulated by various metabolic processes, at the same time it serves as an endocrine organ, secreting bioactive molecules that impact systemic metabolism. In recent years, research has shed light on the intricate interplay between the heart and other metabolic organs, such as adipose tissue, liver, and skeletal muscle. The metabolic flexibility of the heart and its ability to switch between different energy substrates play a crucial role in maintaining cardiac function and overall metabolic homeostasis. Gaining a comprehensive understanding of how metabolic disorders disrupt cardiac metabolism is crucial, as it plays a pivotal role in the development and progression of cardiac diseases. The emerging understanding of the heart as a metabolic and endocrine organ highlights its essential contribution to whole body metabolic regulation and offers new insights into the pathogenesis of metabolic diseases, such as obesity, diabetes, and cardiovascular disorders. In this review, we provide an in-depth exploration of the heart's metabolic and endocrine functions, emphasizing its role in systemic metabolism and the interplay between the heart and other metabolic organs. Furthermore, emerging evidence suggests a correlation between heart disease and other conditions such as aging and cancer, indicating that the metabolic dysfunction observed in these conditions may share common underlying mechanisms. By unraveling the complex mechanisms underlying cardiac metabolism, we aim to contribute to the development of novel therapeutic strategies for metabolic diseases and improve overall cardiovascular health.

The emerging roles of irisin in vascular calcification

Vascular calcification is a common accompanying pathological change in many chronic diseases, which is caused by calcium deposition in the blood vessel wall and leads to abnormal blood vessel function. With the progress of medical technology, the diagnosis rate of vascular calcification has explosively increased. However, due to its mechanism's complexity, no effective drug can relieve or even reverse vascular calcification. Irisin is a myogenic cytokine regulating adipose tissue browning, energy metabolism, glucose metabolism, and other physiological processes. Previous studies have shown that irisin could serve as a predictor for vascular calcification, and protect against hypertension, diabetes, chronic kidney disease, and other risk factors for vascular calcification. In terms of mechanism, it improves vascular endothelial dysfunction and phenotypic transformation of vascular smooth muscle cells. All the above evidence suggests that irisin plays a predictive and protective role in vascular calcification. In this review, we summarize the association of irisin to the related risk factors for vascular calcification and mainly explore the role of irisin in vascular calcification.

Mitochondrial Dysfunction, Oxidative Stress, and Inter-Organ Miscommunications in T2D Progression

Type 2 diabetes (T2D) is a heterogenous disease, and conventionally, peripheral insulin resistance (IR) was thought to precede islet β-cell dysfunction, promoting progression from prediabetes to T2D. New evidence suggests that T2D-lean individuals experience early β-cell dysfunction without significant IR. Regardless of the primary event (i.e., IR vs. β-cell dysfunction) that contributes to dysglycemia, significant early-onset oxidative damage and mitochondrial dysfunction in multiple metabolic tissues may be a driver of T2D onset and progression. Oxidative stress, defined as the generation of reactive oxygen species (ROS), is mediated by hyperglycemia alone or in combination with lipids. Physiological oxidative stress promotes inter-tissue communication, while pathological oxidative stress promotes inter-tissue mis-communication, and new evidence suggests that this is mediated via extracellular vesicles (EVs), including mitochondria containing EVs. Under metabolic-related stress conditions, EV-mediated cross-talk between β-cells and skeletal muscle likely trigger mitochondrial anomalies leading to prediabetes and T2D. This article reviews the underlying molecular mechanisms in ROS-related pathogenesis of prediabetes, including mitophagy and mitochondrial dynamics due to oxidative stress. Further, this review will describe the potential of various therapeutic avenues for attenuating oxidative damage, reversing prediabetes and preventing progression to T2D.

Nutritional Risk of Candidates for Simultaneous Pancreatic-Kidney Transplantation-A Narrative Review

INTRODUCTION

Not much is known about the significance of nutritional status and support in transplant surgery, least of all in simultaneous pancreatic and kidney transplantation. Malnutrition in the context of simultaneous pancreatic-kidney transplantation seems to be complex and a still poorly investigated problem. Since SPKTX is highly qualified and also has a small volume procedure, it is difficult to obtain data from large cohorts of patients. The aim of this article is to gather existing evidence and information about the subject, as well as to elicit some questions and goals for the future.

METHODS

We searched through the Pub-Med database using the keywords "pancreas and kidney transplantation" combined with "nutritional risk", "nutritional status", "malnutrition", "nutritional intervention", and "frailty", finding a total of 4103 matching results. We then narrowed it down to articles written in English with the full text available. We also researched through the references of articles most accurately matching our researched terms.

RESULTS

There are numerous tools that have been investigated for the screening of malnutrition, such as the NRI index, PNI index, NLR, SGA scale, and NRS-2002 scale, each of which proved to be of some use in predicting patient outcomes in different surgical settings. Since all of them differed in components and assessed parameters and, in the absence of more sensitive or infallible indicators, the most reasonable approach seems to evaluate them jointly.

CONCLUSION

It is important to underline the necessity of nutritional screening and the subsequent introduction of adequate therapy while awaiting transplantation in an attempt to improve results. Considering the complexity of surgical procedures and the severity of underlying diseases with their intense metabolic components, the patient's nutritional status seems to significantly influence results. Consequently, nutritional risk assessments should be a part of the routine care of patients qualified for transplantation.

Identification of myokines susceptible to improve glucose homeostasis after bariatric surgery

IMPORTANCE AND OBJECTIVE

The identification of myokines susceptible to improve glucose homeostasis following bariatric surgery could lead to new therapeutic approaches for type 2 diabetes.

METHODS

Changes in the homeostasis model assessment (HOMA) test were assessed in patients before and 3 months after bariatric surgery. Changes in myokines expression and circulating levels were assessed using real-time quantitative polymerase chain reaction (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA). Myokines known to regulate glucose homeostasis were identified using literature (targeted study) and putative myokines using RNA-sequencing (untargeted study). A linear regression analysis adjusted for age and sex was used to search for associations between changes in the HOMA test and changes in myokines.

RESULTS

In the targeted study, brain-derived neurotrophic factor (BDNF) expression was upregulated (+30%, P = .006) while BDNF circulating levels were decreased (-12%, P = .001). Upregulated BDNF expression was associated with decreased HOMA of insulin resistance (HOMA-IR) (adjusted estimate [95% confidence interval {CI}]: -0.51 [-0.88 to -0.13], P = .010). Decreased BDNF serum levels were associated with decreased HOMA of beta-cell function (HOMA-B) (adjusted estimate [95% CI] = 0.002 [0.00002-0.0031], P = .046). In the untargeted study, upregulated putative myokines included XYLT1 (+64%, P < .001), LGR5 (+57, P< .001), and SPINK5 (+46%, P < .001). Upregulated LGR5 was associated with decreased HOMA-IR (adjusted estimate [95% CI] = -0.50 [-0.86 to -0.13], P = .009). Upregulated XYLT1 and SPINK5 were associated with increased HOMA of insulin sensitivity (HOMA-S) (respectively, adjusted estimate [95% CI] = 109.1 [28.5-189.8], P = .009 and 16.5 [0.87-32.19], P = .039).

CONCLUSIONS

Improved glucose homeostasis following bariatric surgery is associated with changes in myokines expression and circulating levels. In particular, upregulation of BDNF, XYLT1, SPINK5, and LGR5 is associated with improved insulin sensitivity. These results suggest that these myokines could contribute to improved glucose homeostasis following bariatric surgery.

STUDY REGISTRATION

NCT03341793 on ClinicalTrials.gov (https://clinicaltrials.gov/).

The Muscle-Conditioned Medium Containing Protocatechuic Acid Improves Insulin Resistance by Modulating Muscle Communication with Liver and Adipose Tissue

Diabetes mellitus is a public health concern, affecting 10.5% of the population. Protocatechuic acid (PCA), a polyphenol, exerts beneficial effects on insulin resistance and diabetes. This study investigated the role of PCA in improving insulin resistance and the crosstalk between muscle with liver and adipose tissue. C2C12 myotubes received four treatments: Control, PCA, insulin resistance (IR), and IR-PCA. Conditioned media from C2C12 was used to incubate HepG2 and 3T3-L1 adipocytes. The impact of PCA was analyzed on glucose uptake and signaling pathways. PCA (80 µM) significantly enhanced glucose uptake in C2C12, HepG2, and 3T3-L1 adipocytes (p < 0.05). In C2C12, PCA significantly elevated GLUT-4, IRS-1, IRS-2, PPAR-γ, P-AMPK, and P-Akt vs. Control (p ≤ 0.05), and modulated pathways in IR-PCA. In HepG2, PPAR-γ and P-Akt increased significantly in Control (CM) vs. No CM, and PCA dose upregulated PPAR-γ, P-AMPK, and P-AKT (p < 0.05). In the 3T3-L1 adipocytes, PI3K and GLUT-4 expression was elevated in PCA (CM) vs. No CM. A significant elevation of IRS-1, GLUT-4, and P-AMPK was observed in IR-PCA vs. IR (p ≤ 0.001). Herein, PCA strengthens insulin signaling by activating key proteins of that pathway and regulating glucose uptake. Further, conditioned media modulated crosstalk between muscle with liver and adipose tissue, thus regulating glucose metabolism.

Exercise and inactivity as modifiers of β cell function and type 2 diabetes risk

Exercise and regular physical activity are beneficial for the prevention and management of metabolic diseases such as obesity and type 2 diabetes, whereas exercise cessation, defined as deconditioning from regular exercise or physical activity that has lasted for a period of months to years, can lead to metabolic derangements that drive disease. Adaptations to the insulin-secreting pancreatic β-cells are an important benefit of exercise, whereas less is known about how exercise cessation affects these cells. Our aim is to review the impact that exercise and exercise cessation have on β-cell function, with a focus on the evidence from studies examining glucose-stimulated insulin secretion (GSIS) using gold-standard techniques. Potential mechanisms by which the β-cell adapts to exercise, including exerkine and incretin signaling, autonomic nervous system signaling, and changes in insulin clearance, will also be explored. We will highlight areas for future research.

The impact of mild hypoxia exposure on myokine secretion in human obesity

BACKGROUND/OBJECTIVE

Compelling evidence indicates that myokines act in an autocrine, paracrine and endocrine manner to alter metabolic homeostasis. The mechanisms underlying exercise-induced changes in myokine secretion remain to be elucidated. Since exercise acutely decreases oxygen partial pressure (pO₂) in skeletal muscle (SM), the present study was designed to test the hypothesis that (1) hypoxia exposure impacts myokine secretion in primary human myotubes and (2) exposure to mild hypoxia in vivo alters fasting and postprandial plasma myokine concentrations in humans.

METHODS

Differentiated primary human myotubes were exposed to different physiological pO₂ levels for 24 h, and cell culture medium was harvested to determine myokine secretion. Furthermore, we performed a randomized single-blind crossover trial to investigate the impact of mild intermittent hypoxia exposure (MIH: 7-day exposure to 15% O₂, 3x2h/day vs. normoxia: 21% O₂) on in vivo SM pO₂ and plasma myokine concentrations in 12 individuals with overweight and obesity (body-mass index ≥ 28 kg/m²).

RESULTS

Hypoxia exposure (1% O₂) increased secreted protein acidic and rich in cysteine (SPARC, p = 0.043) and follistatin like 1 (FSTL1, p = 0.021), and reduced leukemia inhibitory factor (LIF) secretion (p = 0.009) compared to 3% O₂ in primary human myotubes. In addition, 1% O₂ exposure increased interleukin-6 (IL-6, p = 0.004) and SPARC secretion (p = 0.021), whilst reducing fatty acid binding protein 3 (FABP3) secretion (p = 0.021) compared to 21% O₂. MIH exposure in vivo markedly decreased SM pO₂ (≈40%, p = 0.002) but did not alter plasma myokine concentrations.

CONCLUSIONS

Hypoxia exposure altered the secretion of several myokines in primary human myotubes, revealing hypoxia as a novel modulator of myokine secretion. However, both acute and 7-day MIH exposure did not induce alterations in plasma myokine concentrations in individuals with overweight and obesity.

CLINICAL TRIALS IDENTIFIER

This study is registered at the Netherlands Trial Register (NL7120/NTR7325).

The Association of Serum Irisin with Impaired Glucose Before and After Laparoscopic Sleeve Gastrectomy in Obesity

BACKGROUND

To demonstrate the association of irisin levels with impaired glucose before and after laparoscopic sleeve gastrectomy (LSG) in patients with obesity.

METHODS

Thirty-six patients with obesity undergoing LSG were included. We tested the irisin levels before and after LSG and conducted an evaluation of baseline irisin levels with elevated glucose as well as irisin changes with weight loss and its association with glucose control after LSG.

RESULTS

Anthropometric measurements, body fat index, and metabolic parameters were significantly improved in 3 months following LSG (all p < 0.05). Baseline irisin levels were significantly higher in obesity with elevated fasting glucose than that with normal glucose (2.98 [2.37, 3.63] vs. 3.72 [3.06, 5.32], p = 0.031). After adjustment for sex, gender, and body mass index (BMI), obesity with higher irisin levels was prone to have impaired fasting glucose (OR = 2.499, 95% CI = 1.047-5.964). According to receiver operating characteristic curve analysis, the diagnostic accuracy and sensitivity of baseline irisin levels on impaired fasting glucose were 75% and 77.8%. Irisin levels decreased from 3.29 (2.67, 4.43) to 2.82 (2.41, 3.25) ng/mL (p = 0.009) after LSG. The decreases of weight, BMI, and FFA were more in irisin changes group (△irisin ≥ 0.5) than in no irisin changes group (△irisin < 0.5). And △irisin was negatively associated with postprandial glucose (PG) at 3 months after LSG (0.5 h-PG, r =  - 0.478, p = 0.029; 2 h-PG, r =  - 0.406, p = 0.017).

CONCLUSIONS

Elevated baseline irisin levels indicated the impaired glucose in obesity. The decrease of irisin with weight loss provided more evidence for the contribution of serum irisin secretion by fat mass in obesity.

Irisin expression and FNDC5 (rs3480) gene polymorphism in type 2 diabetic patients with and without CAD

BACKGROUND

Irisin was found to correlate with coronary artery disease (CAD) in diabetic patients. This study investigated the association of irisin and FNDC5 (SNP rs3480) with the presence and severity of CAD in T2DM.

METHODS

This cross-sectional study included 100 patients with T2DM divided into two groups, DM group (n = 50), including patients without CAD and CAD group (n = 50), including those confirmed to have CAD by coronary angiography. Irisin was measured. SNP rs3480 genotyping of FNDC5 was done.

RESULTS

Irisin levels were significantly lower in the CAD group (p < 0.001). The CAD group had significantly higher HbA1c and lower HDL (p < 0.001). Patients with controlled DM had significantly higher irisin levels (p < 0.001). single nucleotide polymorphism (SNP) rs3480 was not associated with irisin levels, and the FNDC5 rs3480 AA reference allele was significantly associated with significant CAD.

CONCLUSION

Irisin appears to be protective against developing CAD in diabetic patients. Irisin level was an independent predictor of significant CAD in diabetic patients combined with the FNDC5 rs3480 genotype.

CLINICAL TRIAL REGISTRATION NUMBER

NCT04957823.

Type 2 Diabetes and Alzheimer's Disease: The Emerging Role of Cellular Lipotoxicity

Type 2 diabetes (T2D) and Alzheimer's diseases (AD) represent major health issues that have reached alarming levels in the last decades. Although growing evidence demonstrates that AD is a significant comorbidity of T2D, and there is a ~1.4-2-fold increase in the risk of developing AD among T2D patients, the involvement of possible common triggers in the pathogenesis of these two diseases remains largely unknown. Of note, recent mechanistic insights suggest that lipotoxicity could represent the missing ring in the pathogenetic mechanisms linking T2D to AD. Indeed, obesity, which represents the main cause of lipotoxicity, has been recognized as a major risk factor for both pathological conditions. Lipotoxicity can lead to inflammation, insulin resistance, oxidative stress, ceramide and amyloid accumulation, endoplasmic reticulum stress, ferroptosis, and autophagy, which are shared biological events in the pathogenesis of T2D and AD. In the current review, we try to provide a critical and comprehensive view of the common molecular pathways activated by lipotoxicity in T2D and AD, attempting to summarize how these mechanisms can drive future research and open the way to new therapeutic perspectives.

Post-pancreatitis diabetes mellitus: insight on optimal management with nutrition and lifestyle approaches

Pancreatitis is the leading gastrointestinal cause of hospitalizations. There are multiple short- and long-term complications associated with pancreatitis. Post-pancreatitis diabetes mellitus (PPDM) is one of the less explored complications of pancreatitis. Nonetheless, it has attracted considerable attention during the last decade. PPDM is now the second most common cause of new-onset diabetes mellitus (DM) in adults after type II DM surpassing type 1 DM. However, there exists a knowledge gap amongst practitioners regarding diagnosis, complications, and management of PPDM. In this narrative, we aim to provide a brief review regarding risks, diagnosis and management of PPDM with a special focus on dietary and lifestyle management strategies.KEY MESSAGESPost-pancreatitis diabetes mellitus (PPDM) is now the second most common cause of new-onset diabetes mellitus (DM) in adults after type II DM surpassing type 1 DM.New-onset diabetes in patients with pancreatitis could also be an early marker of occult pancreatic malignancy.Management of PPDM is complex and requires a team-based approach including gastroenterologists, endocrinologists, primary care physicians, nutritionists, and behavioural health specialists.

The serum irisin response to prolonged physical activity in temperate and hot environments in older men with hypertension or type 2 diabetes

Current labor demographics are changing, with the number of older adults increasingly engaged in physically demanding occupations expected to continually rise, which are often performed in the heat. Given an age-related decline in whole-body heat loss, older adults are at an elevated risk of developing heat injuries that may be exacerbated by hypertension (HTN) and type 2 diabetes (T2D). Elevated irisin production may play a role in mitigating the excess oxidative stress and acute inflammation associated with physically demanding work in the heat. However, the effects of HTN and T2D on this response remain unclear. Therefore, we evaluated serum irisin before and after 3-h of moderate intensity exercise (metabolic rate: 200 W/m²) and at the end of 60-min of post-exercise recovery in a temperate (wet-bulb globe temperature (WBGT) 16 °C) and high-heat stress (WBGT 32 °C) environment in 12 healthy older men (mean ± SD; 59 ± 4 years), 10 men with HTN (60 ± 4 years), and 9 men with T2D (60 ± 5 years). Core temperature (T_co) was measured continuously. In the heat, total exercise duration was significantly lower in older men with HTN and T2D (both, p ≤ 0.049). Despite T_co not being different between groups, T_co was higher in the hot compared to the temperate condition for all groups (p < 0.001). Similarly, serum irisin concentrations did not differ between groups under either condition but were elevated relative to the temperate condition during post-exercise and end-recovery in the heat (+93.9 pg/mL SEM 26 and + 70.5 pg/mL SEM 38 respectively; both p ≤ 0.014). Thus, our findings indicate similar irisin responses in HTN and T2D compared to healthy, age-matched controls, despite reduced exercise tolerance during prolonged exercise in the heat. Therefore, older workers with HTN and T2D may exhibit greater cellular stress during prolonged exercise in the heat, underlying greater vulnerability to heat-induced cellular injury.

Systematic Review and Meta-Analysis of Circulating Irisin Levels Following Endurance Training: Results of Continuous and Interval Training

Background

Irisin has been suggested as a helpful hormone for adverse metabolic conditions. However, the interaction between acute endurance exercises and irisin is still unclear. The purpose of this systematic review and meta-analysis was to determine the acute effect of endurance training, either continuous or interval training, on circulating irisin in healthy adults.

Methods

Literature search was conducted in Web of Science, PubMed, Scopus and CINAHL until September 2022. Clinical trials measuring irisin levels following a single session of interval or continuous endurance training in healthy adults were eligible. Cohen’s d effect size (95% confidence level), subgroup analyses and univariate meta-regression were calculated using a random-effects model. The procedures described by PRISMA were followed and the protocol was prospectively registered with PROSPERO (CRD 42021240971).

Results

Data of the 16 included studies comprising 412 individuals showed a significant increase following one session of continuous endurance training (d = 0.33, 95% CI: 0.20 to 0.46 , p < 0.001), while interval training did not change circulating irisin (d = 0.16, 95% CI: −0.12 to 0.44 , p = 0.202). Both subgroup and univariate meta-regression analyses showed non-significant differences in the change of circulating irisin comparing blood measurement, exercise mode or previous level of physical activity of the participants and circulating irisin at baseline, duration, or intensity of the exercise, respectively.

Conclusion

Continuous method for endurance training increases circulating irisin in healthy adults, while studies measuring circulating irisin following interval training in healthy adults are still limited to be conclusive.

“Ferrocrinology”—Iron Is an Important Factor Involved in Gluco- and Lipocrinology

“Ferrocrinology” is the term used to describe the study of iron effects on the functioning of adipose tissue, which together with muscle tissue makes the largest endocrine organ in the human body. By impairing exercise capacity, reducing AMP-activated kinase activity, and enhancing insulin resistance, iron deficiency can lead to the development of obesity and type 2 diabetes mellitus. Due to impaired browning of white adipose tissue and reduced mitochondrial iron content in adipocytes, iron deficiency (ID) can cause dysfunction of brown adipose tissue. By reducing ketogenesis, aconitase activity, and total mitochondrial capacity, ID impairs muscle performance. Another important aspect is the effect of ID on the impairment of thermogenesis due to reduced binding of thyroid hormones to their nuclear receptors, with subsequently impaired utilization of norepinephrine in tissues, and impaired synthesis and distribution of cortisol, which all make the body’s reactivity to stress in ID more pronounced. Iron deficiency can lead to the development of the most common endocrinopathy, autoimmune thyroid disease. In this paper, we have discussed the role of iron in the cross-talk between glucocrinology, lipocrinology and myocrinology, with thyroid hormones acting as an active bystander.

Irisin improves adiposity and exercise tolerance in a rat model of postmenopausal obesity through enhancing adipo-myocyte thermogenesis

The prevalence of obesity and its associated metabolic disorders, along with their healthcare costs, is rising exponentially. Irisin, an adipomyokine, may serve as a critical cross-organ messenger, linking skeletal muscle with adipose tissue and the liver to integrate the energy homeostasis under diet-induced obesity. We aimed to explore the putative role of irisin in the protection against obesity in a postmenopausal rat model by modulating energy expenditure (EE). Bilateral ovariectomy (OVX) was performed. After 3 weeks of recovery, the OVX rats were classified according to their dietary protocol into rats maintained on normal diets (ND) (OVX) or high-fat diet (HFD) groups. The HFD-fed animals were equally divided into OVX/HFD, or irisin-treated OVX/HFD groups. Sham rats, maintained on ND, were selected as the control group. We evaluated anthropometric, EE, and molecular biomarkers of browning and thermogenesis in inguinal white adipose tissue and skeletal muscle, and the activity of the proteins related to mitochondrial long chain fatty acid transport, oxidation, and glycolysis. HFD of OVX further deteriorated the disturbed glucose homeostasis, lipid profile, and the reduced irisin, thermogenic parameters in adipose tissue and skeletal muscle, and EE. Irisin treatment improved the lipid profile and insulin resistance. That was associated with reduced hepatic gluconeogenic enzyme activities and restored hepatic glycogen content. Irisin reduced ectopic lipid infiltration. Irisin augmented EE by activating non-shivering thermogenesis in muscle and adipose tissues and decreasing metabolic efficiency. Our experimental evidence suggests irisin's use as a potential thermogenic agent, therapeutically targeting obesity in postmenopausal patients. Irisin modulates the non-shivering thermogenesis in skeletal muscle and adipose tissue in postmenopausal model.

The adipocyte-enriched secretory protein tetranectin exacerbates type 2 diabetes by inhibiting insulin secretion from β cells

Pancreatic β cell failure is a hallmark of diabetes. However, the causes of β cell failure remain incomplete. Here, we report the identification of tetranectin (TN), an adipose tissue-enriched secretory molecule, as a negative regulator of insulin secretion in β cells in diabetes. TN expression is stimulated by high glucose in adipocytes via the p38 MAPK/TXNIP/thioredoxin/OCT4 signaling pathway, and elevated serum TN levels are associated with diabetes. TN treatment greatly exacerbates hyperglycemia in mice and suppresses glucose-stimulated insulin secretion in islets. Conversely, knockout of TN or neutralization of TN function notably improves insulin secretion and glucose tolerance in high-fat diet-fed mice. Mechanistically, TN binds with high selectivity to β cells and inhibits insulin secretion by blocking L-type Ca²⁺ channels. Our study uncovers an adipocyte-β cell cross-talk that contributes to β cell dysfunction in diabetes and suggests that neutralization of TN levels may provide a new treatment strategy for type 2 diabetes.

Irisin Ameliorates Oxidative Stress-Induced Injury in Pancreatic Beta-Cells by Inhibiting Txnip and Inducing Stat3-Trx2 Pathway Activation

Lipotoxicity can lead to beta-cell dysfunction and apoptosis because it induces oxidative stress. Recent studies have found that Irisin prevents pancreatic beta-cell dysfunction induced by palmitic acid (PA). However, an association between the protection against oxidative stress conferred by Irisin and beta-cell dysfunction has not been fully elucidated. In this study, we observed that Irisin treatment prevented INS-1 cell apoptosis induced by PA treatment and preserved the insulin-secreting function of INS-1 cells in vitro. These effects probably resulted from the Irisin-induced decrease in intracellular ROS levels triggered by PA treatment. In addition, PA treatment induced oxidative stress partially by inhibiting the activation of thioredoxin 2 (Trx2) through its increase of thioredoxin-interacting protein (Txnip) expression. However, Irisin administration blocked the increase in Txnip expression, which reversed the PA-induced inactivation of Trx2. Irisin also increased the nuclear translocation of Stat3, and the inhibition of Stat3 by siRNAs blocked Irisin-induced Trx2 expression, indicating that both Txnip and Stat3 are involved in Irisin-induced activation of Trx2. Furthermore, blockade of Stat3 by siRNAs led to the decreased gene expression of MafA and Ins and to cessation of glucose-induced insulin secretion that had been enhanced by Irisin. In vivo, HFD treatment led to reduced glucose tolerance and an increase in the level of the oxidative marker malondialdehyde (MDA) compared to that in the control group. However, these effects were ameliorated by Irisin injection due to the inhibition of beta-cell apoptosis and the activation of Trx2, probably through Txnip inhibition and Stat3 activation. In conclusion, our results reveal a possible mechanism for Irisin-induced beta-cell protection, which is mediated through Txnip inhibition and activation of the Stat3-Trx2 pathway.

Resistance Training Improves Beta Cell Glucose Sensing and Survival in Diabetic Models

Resistance training increases insulin secretion and beta cell function in healthy mice. Here, we explored the effects of resistance training on beta cell glucose sensing and survival by using in vitro and in vivo diabetic models. A pancreatic beta cell line (INS-1E), incubated with serum from trained mice, displayed increased insulin secretion, which could be linked with increased expression of glucose transporter 2 (GLUT2) and glucokinase (GCK). When cells were exposed to pro-inflammatory cytokines (in vitro type 1 diabetes), trained serum preserved both insulin secretion and GCK expression, reduced expression of proteins related to apoptotic pathways, and also protected cells from cytokine-induced apoptosis. Using 8-week-old C57BL/6 mice, turned diabetic by multiple low doses of streptozotocin, we observed that resistance training increased muscle mass and fat deposition, reduced fasting and fed glycemia, and improved glucose tolerance. These findings may be explained by the increased fasting and fed insulinemia, along with increased beta cell mass and beta cell number per islet, observed in diabetic-trained mice compared to diabetic sedentary mice. In conclusion, we believe that resistance training stimulates the release of humoral factors which can turn beta cells more resistant to harmful conditions and improve their response to a glucose stimulus.

β-cell dynamics in type 2 diabetes and in dietary and exercise interventions

Pancreatic β-cell dysfunction and insulin resistance are two of the major causes of type 2 diabetes (T2D). Recent clinical and experimental studies have suggested that the functional capacity of β-cells, particularly in the first phase of insulin secretion, is a primary contributor to the progression of T2D and its associated complications. Pancreatic β-cells undergo dynamic compensation and decompensation processes during the development of T2D, in which metabolic stresses such as endoplasmic reticulum stress, oxidative stress, and inflammatory signals are key regulators of β-cell dynamics. Dietary and exercise interventions have been shown to be effective approaches for the treatment of obesity and T2D, especially in the early stages. Whilst the targeted tissues and underlying mechanisms of dietary and exercise interventions remain somewhat vague, accumulating evidence has implicated the improvement of β-cell functional capacity. In this review, we summarize recent advances in the understanding of the dynamic adaptations of β-cell function in T2D progression and clarify the effects and mechanisms of dietary and exercise interventions on β-cell dysfunction in T2D. This review provides molecular insights into the therapeutic effects of dietary and exercise interventions on T2D, and more importantly, it paves the way for future research on the related underlying mechanisms for developing precision prevention and treatment of T2D.

Adipose Tissue Secretion Pattern Influences β-Cell Wellness in the Transition from Obesity to Type 2 Diabetes

The dysregulation of the β-cell functional mass, which is a reduction in the number of β-cells and their ability to secure adequate insulin secretion, represents a key mechanistic factor leading to the onset of type 2 diabetes (T2D). Obesity is recognised as a leading cause of β-cell loss and dysfunction and a risk factor for T2D. The natural history of β-cell failure in obesity-induced T2D can be divided into three steps: (1) β-cell compensatory hyperplasia and insulin hypersecretion, (2) insulin secretory dysfunction, and (3) loss of β-cell mass. Adipose tissue (AT) secretes many hormones/cytokines (adipokines) and fatty acids that can directly influence β-cell function and viability. As this secretory pattern is altered in obese and diabetic patients, it is expected that the cross-talk between AT and pancreatic β-cells could drive the maintenance of the β-cell integrity under physiological conditions and contribute to the reduction in the β-cell functional mass in a dysmetabolic state. In the current review, we summarise the evidence of the ability of the AT secretome to influence each step of β-cell failure, and attempt to draw a timeline of the alterations in the adipokine secretion pattern in the transition from obesity to T2D that reflects the progressive deterioration of the β-cell functional mass.

Cyanidin-3-O-Glucoside Ameliorates Palmitic-Acid-Induced Pancreatic Beta Cell Dysfunction by Modulating CHOP-Mediated Endoplasmic Reticulum Stress Pathways

Cyanidin-3-O-glucoside (C3G) is a natural colorant with anti-diabetic properties, while its underlying mechanisms remain far from clear. Here, we investigated the protective role of C3G on palmitic acid (PA)-induced pancreatic beta cell dysfunction and further decipher its possible molecular mechanisms. Both primary isolated mouse islets and the INS-1E cell were used, and treated with a mixture of PA (0.5 mM) and C3G (12.5 µM, 25 µM, 50 µM) for different durations (12, 24, 48 h). We found that C3G could dose-dependently ameliorate beta cell secretory function and further alleviate cell apoptosis. Mechanistically, the primary role of the PKR-like ER kinase (PERK) endoplasmic reticulum (ER) stress pathway was detected by RNA sequencing, and the PERK-pathway-related protein expression, especially the pro-apoptotic marker C/EBP homologous protein (CHOP) expression, was significantly downregulated by C3G treatment. The critical role of CHOP in mediating the protective effect of C3G was further validated by small interfering RNA. Conclusively, C3G could ameliorate PA-induced pancreatic beta cell dysfunction targeting the CHOP-related ER stress pathway, which might be used as a nutritional intervention for the preservation of beta cell dysfunction in type 2 diabetes mellitus.

Mechanisms by Which Skeletal Muscle Myokines Ameliorate Insulin Resistance

The skeletal muscle is the largest organ in the body and secretes circulating factors, including myokines, which are involved in various cellular signaling processes. Skeletal muscle is vital for metabolism and physiology and plays a crucial role in insulin-mediated glucose disposal. Myokines have autocrine, paracrine, and endocrine functions, serving as critical regulators of myogenic differentiation, fiber-type switching, and maintaining muscle mass. Myokines have profound effects on energy metabolism and inflammation, contributing to the pathophysiology of type 2 diabetes (T2D) and other metabolic diseases. Myokines have been shown to increase insulin sensitivity, thereby improving glucose disposal and regulating glucose and lipid metabolism. Many myokines have now been identified, and research on myokine signaling mechanisms and functions is rapidly emerging. This review summarizes the current state of the field regarding the role of myokines in tissue cross-talk, including their molecular mechanisms, and their potential as therapeutic targets for T2D.

Chicoric acid does not restore palmitate-induced decrease in irisin levels in PBMCs of newly diagnosed patients with T2D and healthy subjects

Targeting irisin as a myokine/adipokine is a new therapeutic approach in the improvement of insulin-resistance (IR) during type 2 diabetes (T2D). In present study we evaluated the effects of palmitate and chicoric acid (CA) on irisin production in peripheral blood mononuclear cells (PBMCs) of patients with T2D. This study performed on 20 newly diagnosed patients with T2D and 20 healthy subjects. PBMCs treated with palmitate and CA. PPARGC1A and FNDC5 genes expression assessed using qRT-PCR. Irisin levels in cell culture medium measured by ELISA. Palmitate decreased PPARGC1A and FNDC5 genes expression, as well as irisin levels in PBMCs from T2D and healthy volunteers. CA significantly restored palmitate-induced decrease in PPARGC1A gene expression in PBMCs of healthy subjects. Although, FNDC5 gene expression and irisin levels were not induced significantly by CA. In conclusion, palmitate decreases irisin production through down-regulation of PPARGC1A and FNDC5 expressions. However, CA does not effect on irisin pathway.Key pointsPalmitate reduced PPARGC1A and FNDC5 genes expression, as well as irisin secretion in PBMCs.Palmitate-induced decrease in PPARGC1A gene expression significantly has been reversed by CA in PBMCs of healthy subjects.CA did not return palmitate-decreased in FNDC5 gene expression and irisin levels in PBMCs.

Effects of the FNDC5/Irisin on Elderly Dementia and Cognitive Impairment

Population aging is an inevitable problem nowadays, and the elderly are going through a lot of geriatric symptoms, especially cognitive impairment. Irisin, an exercise-stimulating cleaved product from transmembrane fibronectin type III domain-containing protein 5 (FNDC5), has been linked with favorable effects on many metabolic diseases. Recently, mounting studies also highlighted the neuroprotective effects of irisin on dementia. The current evidence remains uncertain, and few clinical trials have been undertaken to limit its clinical practice. Therefore, we provided an overview of current scientific knowledge focusing on the preventive mechanisms of irisin on senile cognitive decline and dementia, in terms of the possible connections between irisin and neurogenesis, neuroinflammation, oxidative stress, and dementia-related diseases. This study summarized the recent advances and ongoing studies, aiming to provide a better scope into the effectiveness of irisin on dementia progression, as well as a mediator of muscle brain cross talk to provide theoretical support for exercise therapy for patients with dementia. Whether irisin is a diagnostic or prognostic factor for dementia needs more researches.

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

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

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Data-independent acquisition-based proteomics analysis correlating type 2 diabetes mellitus with osteoarthritis in total knee arthroplasty patients

BACKGROUND

To explore the effects of type 2 diabetes mellitus (T2DM) on osteoarthritis (OA), 12 bone tissue samples were obtained surgically from the human total knee arthroplasty patients and analyzed by quantitative proteomics.

METHODS

Based on patient clinical histories, patient samples were assigned to diabetes mellitus osteoarthritis (DMOA) and OA groups. A data-independent acquisition method for data collection was used with proteomic data analysis to assess intergroup proteomic differences. Gene Ontology (GO) functional analysis and Kyoto Encyclopedia of Genes and Genome pathway enrichment analysis were used to further find the correlation between T2DM and OA.

RESULTS

GO functional analysis found 153 differentially expressed proteins between DMOA and OA groups, of which 92 differentially expressed proteins were significantly up-regulated and 61 were significantly down-regulated. Kyoto Encyclopedia of Genes and Genome pathway analysis found 180 pathways, including 9 pathways significantly enriched. Further data analysis revealed that 6 signaling pathways were closely associated with T2DM and OA.

CONCLUSION

OA and DMOA onset and progression were closely related to synthesis and metabolism of extracellular matrix components (e.g., fibronectin, decorin, etc.). The effects of T2DM on OA occur though 2 major ways of oxidative stress and low-grade chronic inflammation, involving in 2 inhibited signaling pathways and 4 activated signaling pathways.

Irisin-A Pancreatic Islet Hormone

Irisin is a myokine involved in glucose homeostasis. It is primarily expressed in skeletal muscle, but also in the pancreas. This study aimed to elucidate its presence and role in the islets of Langerhans-i.e., its effect on insulin and glucagon secretion as well as on blood flow in the pancreas. The precursor of irisin, fibronectin type III domain-containing protein 5 (FNDC5), was identified in rat and human islets by both qPCR and immunohistochemistry. Both α- and β-cells stained positive for FNDC5. In human islets, we found that irisin was secreted in a glucose-dependent manner. Neither irisin nor an irisin-neutralizing antibody affected insulin or glucagon secretion from human or rat islets in vitro. The insulin and glucagon content in islets was not altered by irisin. The intravenous infusion of irisin in Sprague Dawley rats resulted in nearly 50% reduction in islet blood flow compared to the control. We conclude that irisin is an islet hormone that has a novel role in pancreatic islet physiology, exerting local vascular effects by diminishing islet blood flow without affecting insulin secretion per se.

GLP-1 Induces the Expression of FNDC5 Derivatives That Execute Lipolytic Actions

Multiple GLP-1-derived therapeutics are clinically used to treat type 2 diabetes and obesity. However, the underlying mechanism of how these drugs regulate the body weight of obese patients remains incompletely understood. Here, we report that the lipolysis effects of GLP-1 on β cells can depend on its induced expression of fibronectin type III domain containing 5 (FNDC5). The transmembrane FNDC5 is a precursor of the recently identified hormone irisin that possesses a range of bioactivities, including anti-obesity and anti-diabetes. We revealed that GLP-1 upregulates the expression and secretion of FNDC5 in β cells, while GLP-1 itself fails to activate the lipolysis genes in FNDC5-knockout β cells. In addition, liraglutide, a clinically used GLP-1 receptor agonist, induced the expression of FNDC5 in mouse pancreas and brain tissues and increased the serum level of secreted FNDC5. Furthermore, we observed the expression of the well-known membrane-associated FNDC5 and a novel, secretable FNDC5 (sFNDC5) isoform in β cells and multiple rat tissues. Recombinant sFNDC5 stimulated lipolysis of wild type and FNDC5-knockout β cells. This new isoform further induced lipolysis and browning of adipocytes, and similar to irisin, executed potent anti-obesity activities in an obese mouse model. Overall, our studies provided new mechanistic insights into GLP-1’s anti-obesity actions in which GLP-1 induces the secretion of FNDC5 derivatives from its responsive organs that then mediate its anti-obesity activities.

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.

Irisin alleviates obesity-related spermatogenesis dysfunction via the regulation of the AMPKα signalling pathway

BACKGROUND

Infertility is a common complication in obese men. Oxidative stress and testicular apoptosis play critical roles in obesity-induced spermatogenesis dysfunction. It has been reported that irisin, an exercise-induced myokine, may attenuate oxidative damage and testicular apoptosis in several diseases; however, its role in obesity-induced spermatogenesis dysfunction remains unclear. The purpose of this study was to investigate the role and underlying mechanism of irisin in obesity-induced dysfunction of spermatogenesis.

METHODS

Male mice were fed a high-fat diet (HFD) for 24 weeks to establish a model of obesity-induced spermatogenesis dysfunction. To explore the effects of irisin, mice were subcutaneously infused with recombinant irisin for 8 weeks beginning at 16 weeks after starting a HFD. To confirm the role of AMP-activated protein kinase α (AMPKα), AMPKα-deficient mice were used.

RESULTS

The data showed decreased serum irisin levels in obese patients, which was negatively correlated with sperm count and progressive motility. Irisin was downregulated in the plasma and testes of obese mice. Supplementation with irisin protected against HFD-induced spermatogenesis dysfunction and increased testosterone levels in mice. HFD-induced oxidative stress, endoplasmic reticulum (ER) stress and testicular apoptosis were largely attenuated by irisin treatment. Mechanistically, we identified that irisin activated the AMPKα signalling pathway. With AMPKα depletion, we found that the protective effects of irisin on spermatogenesis dysfunction were abolished in vivo and in vitro.

CONCLUSIONS

In conclusion, we found that irisin alleviated obesity-related spermatogenesis dysfunction via activation of the AMPKα signalling pathway. Based on these findings, we hypothesized that irisin is a potential therapeutic agent against obesity-related spermatogenesis dysfunction.

Association between plasma irisin in pregnancy and postpartum glucose levels among Chinese women: A cohort study

AIMS/INTRODUCTION

The association between plasma irisin and glucose levels in the general population is controversial, and few studies have longitudinally detected this correlation. We aimed to examine whether irisin in pregnancy was associated with postpartum glucose levels among Chinese women and explore the modifiable factors.

MATERIALS AND METHODS

We carried out a prospective cohort study in Guangzhou, China, during 2017 and 2018, and 453 pregnant women (20-28 weeks) were enrolled. Plasma irisin levels in pregnancy were tested. At 6-8 weeks after delivery, 93 women with gestational diabetes mellitus (GDM) underwent a 75-g oral glucose tolerance test, and the other 360 women had a fasting blood glucose (FBG) test. Multivariable linear, quantile and logistic regressions were carried out.

RESULTS

The mean plasma irisin in mid-pregnancy was 13.73 ng/mL. We observed a significantly negative association between mid-pregnancy irisin and postpartum FBG (β: -0.056 ± 0.024). However, quantile regression showed the association was only significant in high percentiles of FBG levels (P50 to P95 ), and the magnitude showed an increasing trend. Higher baseline irisin was also associated with a lower risk of postpartum impaired fasting glucose (relative risk 0.563, 95% confidence interval 0.384-0.825). Furthermore, we found significant interactions between irisin and predominant breast-feeding on FBG and impaired fasting glucose (both Pinteraction  < 0.05). In women with GDM, baseline irisin was non-significantly associated with postpartum postprandial 2-h glucose levels (β: -0.305 ± 0.160, P = 0.061).

CONCLUSIONS

Plasma irisin levels in mid-pregnancy were negatively associated with FBG levels and impaired fasting glucose at 6-8 weeks postpartum among Chinese women, and stronger associations were observed in women with higher FBG values. Furthermore, breast-feeding might modify this relationship.

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

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

From Exercise to Cognitive Performance: Role of Irisin

The beneficial effects of exercise on the brain are well known. In general, exercise offers an effective way to improve cognitive function in all ages, particularly in the elderly, who are considered the most vulnerable to neurodegenerative disorders. In this regard, myokines, hormones secreted by muscle in response to exercise, have recently gained attention as beneficial mediators. Irisin is a novel exercise-induced myokine, that modulates several bodily processes, such as glucose homeostasis, and reduces systemic inflammation. Irisin is cleaved from fibronectin type III domain containing 5 (FNDC5), a transmembrane precursor protein expressed in muscle under the control of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α). The FNDC5/irisin system is also expressed in the hippocampus, where it stimulates the expression of the neurotrophin brain-derived neurotrophic factor in this area that is associated with learning and memory. In this review, we aimed to discuss the role of irisin as a key mediator of the beneficial effects of exercise on synaptic plasticity and memory in the elderly, suggesting its roles within the main promoters of the beneficial effects of exercise on the brain.

Progress and Challenges in the Biology of FNDC5 and Irisin

In 2002, a transmembrane protein-now known as FNDC5-was discovered and shown to be expressed in skeletal muscle, heart, and brain. It was virtually ignored for 10 years, until a study in 2012 proposed that, in response to exercise, the ectodomain of skeletal muscle FNDC5 was cleaved, traveled to white adipose tissue, and induced browning. The wasted energy of this browning raised the possibility that this myokine, named irisin, might mediate some beneficial effects of exercise. Since then, more than 1000 papers have been published exploring the roles of irisin. A major interest has been on adipose tissue and metabolism, following up the major proposal from 2012. Many studies correlating plasma irisin levels with physiological conditions have been questioned for using flawed assays for irisin concentration. However, experiments altering irisin levels by injecting recombinant irisin or by gene knockout are more promising. Recent discoveries have suggested potential roles of irisin in bone remodeling and in the brain, with effects potentially related to Alzheimer's disease. We discuss some discrepancies between research groups and the mechanisms that are yet to be determined. Some important questions raised in the initial discovery of irisin, such as the role of the mutant start codon of human FNDC5 and the mechanism of ectodomain cleavage, remain to be answered. Apart from these specific questions, a promising new tool has been developed-mice with a global or tissue-specific knockout of FNDC5. In this review, we critically examine the current knowledge and delineate potential solutions to resolve existing ambiguities.

Mechanisms of altered bone remodeling in children with type 1 diabetes

Bone loss associated with type 1 diabetes mellitus (T1DM) begins at the onset of the disease, already in childhood, determining a lower bone mass peak and hence a greater risk of osteoporosis and fractures later in life. The mechanisms underlying diabetic bone fragility are not yet completely understood. Hyperglycemia and insulin deficiency can affect the bone cells functions, as well as the bone marrow fat, thus impairing the bone strength, geometry, and microarchitecture. Several factors, like insulin and growth hormone/insulin-like growth factor 1, can control bone marrow mesenchymal stem cell commitment, and the receptor activator of nuclear factor-κB ligand/osteoprotegerin and Wnt-b catenin pathways can impair bone turnover. Some myokines may have a key role in regulating metabolic control and improving bone mass in T1DM subjects. The aim of this review is to provide an overview of the current knowledge of the mechanisms underlying altered bone remodeling in children affected by T1DM.

Cardiometabolic health, visceral fat and circulating irisin levels: results from a real-world weight loss study

BACKGROUND

The aim of this pragmatic intervention study was to investigate changes in cardiometabolic outcomes, irisin plasma concentration, and body composition during a 4-month intervention in unselected obese individuals.

MATERIALS AND METHODS

In 111 obese women aged 36.73 ± 7.2 years, we measured changes in weight, lipid profiles, glucose, insulin, Homeostatic Model Assessment-Insulin Resistance Index (HOMA-IR), uric acid, aminotransferases, and irisin. Body composition including lean mass (LM) and total (TF), gynoid (GF), android (AF), and visceral fat (VF) was assessed using densitometry. Physical activity was assessed using the International Physical Activity Questionnaire (IPAQ). The participants received tailored written advice targeting lifestyle according to current guidelines. At follow-up, patients rated their adherence in the self-administered questionnaire.

RESULTS

Mean weight loss in the whole group was 3.12 kg (- 3.3%); 26% of the women achieved the desired target of weight loss (> 5% of the initial weight), whereas weight decreased moderately in 50% and increased in 14%. In 86 women with weight loss, there were significant changes in HOMA-IR (- 13.8%), insulin (- 11.2%), alanine aminotransferase (- 8.0%), VF (- 7.0%), AF (- 5.4%), TF (- 4.7%), GF (- 2.8%) and LM (- 1.5%), whereas irisin and HDL-C levels and the mean IPAQ score did not change.

CONCLUSIONS

In this real-world evidence study, a successful weight loss achieved only 26% of patients, with overall much better adherence to diet restriction than to exercise. However, even mild to moderate weight loss resulted in significant improvements in cardiometabolic health. Weight loss was associated with a modest LM decrease but did not influence plasma irisin.

Irisin plays an important role in the outcomes of newly diagnosed prediabetes in adults in Guiyang, China

AIMS/INTRODUCTION

To explore the potential role of irisin in the outcomes of newly diagnosed prediabetes.

MATERIALS AND METHODS

Data were obtained from the Guiyang subcenter of the Risk Evaluation of cAncers in Chinese diabeTic Individuals: a lONgitudinal (REACTION) study. A total of 2,530 participants had newly diagnosed prediabetes at baseline and completed follow up. The nested 1:1 case-control study included 161 participants who developed diabetes mellitus at follow up, and 161 age- and sex-matched controls. The follow-up study included 86 matched case-control pairs. Fasting serum irisin levels were measured using enzyme-linked immunosorbent assay.

RESULTS

Baseline serum irisin levels were higher in the cases than in the controls (P = 0.002); high baseline serum irisin levels were an independent risk factor for the development of diabetes (odds ratio 1.235, 95% confidence interval 1.025-1.488). After adjustment for age, sex, body mass index, glycated hemoglobin (HbA1c), smoking, exercise, and family history of diabetes, subjects in the highest quartile of irisin levels had a higher risk of diabetes than those in the lowest quartile (odds ratio 3.065, 95% confidence interval 1.511-6.218). The extent of decrease in irisin levels during follow-up was greater in the cases than in the controls (P < 0.001). Baseline serum irisin levels were positively correlated with the extent of decrease in irisin during follow-up (r = 0.773, P < 0.001). After adjustment for confounding factors, subjects with a decrease of irisin above the median had much higher risk for diabetes (odds ratio 5.077, 95% confidence interval 2.112-12.206).

CONCLUSIONS

Irisin might play an important role in the outcomes of newly diagnosed prediabetes in adults in Guiyang, and can predict the risk for developing diabetes in these individuals.

Resistance exercise training improves glucose homeostasis by enhancing insulin secretion in C57BL/6 mice

Resistance exercise exerts beneficial effects on glycemic control, which could be mediated by exercise-induced humoral factors released in the bloodstream. Here, we used C57Bl/6 healthy mice, submitted to resistance exercise training for 10 weeks. Trained mice presented higher muscle weight and maximum voluntary carrying capacity, combined with reduced body weight gain and fat deposition. Resistance training improved glucose tolerance and reduced glycemia, with no alterations in insulin sensitivity. In addition, trained mice displayed higher insulinemia in fed state, associated with increased glucose-stimulated insulin secretion. Islets from trained mice showed reduced expression of genes related to endoplasmic reticulum (ER) stress, associated with increased expression of Ins2. INS-1E beta-cells incubated with serum from trained mice displayed similar pattern of insulin secretion and gene expression than isolated islets from trained mice. When exposed to CPA (an ER stress inducer), the serum from trained mice partially preserved the secretory function of INS-1E cells, and prevented CPA-induced apoptosis. These data suggest that resistance training, in healthy mice, improves glucose homeostasis by enhancing insulin secretion, which could be driven, at least in part, by humoral factors.

Molecular Mechanisms of Apoptosis Induction and Its Regulation by Fatty Acids in Pancreatic β-Cells

Pancreatic β-cell failure and death contribute significantly to the pathogenesis of type 2 diabetes. One of the main factors responsible for β-cell dysfunction and subsequent cell death is chronic exposure to increased concentrations of FAs (fatty acids). The effect of FAs seems to depend particularly on the degree of their saturation. Saturated FAs induce apoptosis in pancreatic β-cells, whereas unsaturated FAs are well tolerated and are even capable of inhibiting the pro-apoptotic effect of saturated FAs. Molecular mechanisms of apoptosis induction by saturated FAs in β-cells are not completely elucidated. Saturated FAs induce ER stress, which in turn leads to activation of all ER stress pathways. When ER stress is severe or prolonged, apoptosis is induced. The main mediator seems to be the CHOP transcription factor. Via regulation of expression/activity of pro- and anti-apoptotic Bcl-2 family members, and potentially also through the increase in ROS production, CHOP switches on the mitochondrial pathway of apoptosis induction. ER stress signalling also possibly leads to autophagy signalling, which may activate caspase-8. Saturated FAs activate or inhibit various signalling pathways, i.e., p38 MAPK signalling, ERK signalling, ceramide signalling, Akt signalling and PKCδ signalling. This may lead to the activation of the mitochondrial pathway of apoptosis, as well. Particularly, the inhibition of the pro-survival Akt signalling seems to play an important role. This inhibition may be mediated by multiple pathways (e.g., ER stress signalling, PKCδ and ceramide) and could also consequence in autophagy signalling. Experimental evidence indicates the involvement of certain miRNAs in mechanisms of FA-induced β-cell apoptosis, as well. In the rather rare situations when unsaturated FAs are also shown to be pro-apoptotic, the mechanisms mediating this effect in β-cells seem to be the same as for saturated FAs. To conclude, FA-induced apoptosis rather appears to be preceded by complex cross talks of multiple signalling pathways. Some of these pathways may be regulated by decreased membrane fluidity due to saturated FA incorporation. Few data are available concerning molecular mechanisms mediating the protective effect of unsaturated FAs on the effect of saturated FAs. It seems that the main possible mechanism represents a rather inhibitory intervention into saturated FA-induced pro-apoptotic signalling than activation of some pro-survival signalling pathway(s) or metabolic interference in β-cells. This inhibitory intervention may be due to an increase of membrane fluidity.

Insulin resistance and bone health in adolescents

Insulin resistance may be linked to bone health in young people. This study is the first on adolescents that jointly examined the association of bone health with insulin resistance and body composition. Our results revealed significant negative association between bone parameters and insulin resistance, even after adjustment for confounding factors.

PURPOSE

Previous studies are suggestive of the protective role of insulin on bone in adults. Whether this association exists in younger individuals is not clear, yet. This investigation aimed to evaluate the association between insulin resistance, bone parameters, and body composition amongst Iranian adolescents᾽ population.

METHODS

A cross-sectional study was conducted on 423 participants (224 girls and 199 boys) aged 9-19 years old. Insulin resistance was assessed, using a homeostatic model assessment of insulin resistance (HOMA-IR) and quantitative insulin sensitivity check index (QUICKI). Bone mineral density (BMD), bone mineral content (BMC), total body fat mass (TBFM), and total body lean mass (TBLM) were measured, using dual energy X-ray absorptiometry (DXA), and bone mineral apparent density (BMAD) was calculated.

RESULTS

In multiple regression analyses adjusted for potential confounders, the HOMA-IR showed statistically significant negative association with most of the bone parameters (β = - 1.1 to - 0.002, P = 0.004 to 0.036). On the subject of QUICKI index, this relationship was detected only for lumbar spine (LS) parameters (β = 0.062 to 37.21, P = 0.0001 to 0.026) and femoral neck bone mineral content (FNBMC) (β = 1.297, P = 0.013).

CONCLUSION

Our results suggest that insulin resistance may be inversely and independently associated with the bone indices in younger individuals. Whether high insulin levels have detrimental effects on growing bone is still unclear and has to be answered.

Meteorin-Like Ameliorates β Cell Function by Inhibiting β Cell Apoptosis of and Promoting β Cell Proliferation via Activating the WNT/β-Catenin Pathway

Meteorin-like (Metrnl) is a newly discovered myokine. Plasma Metrnl is decreased in subjects with newly diagnosed type 2 diabetes (T2D) and correlated with insulin resistance. This study aims to determine the effects of Metrnl on the apoptosis and proliferation of β cell. Mouse insulinoma MIN6 cells were divided into six groups: normal control, low glucose, high glucose, Vehicle, Metrnl, and Dickkopf 1 (DKK1) groups. MIN6 cells in Metrnl group were transfected with recombinant pCDH-Metrnl vector. WNT/β-catenin pathway was inhibited using DKK1. Then the apoptosis of MIN6 cells was detected using flow cytometry and TUNEL labeling. Immunofluorescence of Ki67 or Edu-594 was used to determine the β cell proliferation. db/db mice were confirmed as T2D group. Lentivirus-Metrnl was injected from the caudal vein of db/db mice once every two weeks for two times. High glucose induced the apoptosis of MIN6 cells and elevated expression of caspase 3. In addition, high glucose resulted in reduced β cell proliferation, cell viability, insulin secretion as well as decreased expression of β-catenin and TCF4. Metrnl ameliorated the above effects of high glucose. And the protecting role of Metrnl was inhibited by DKK1. T2D mice showed higher body weight and blood glucose compared with the controls. The β cell apoptosis was increased while the β cell proliferation and WNT/β-catenin pathway were inhibited in T2D mice. Metrnl treatment partly reversed the above changes in T2D mice. Metrnl ameliorates β cell function by inhibiting β cell apoptosis of and promoting β cell proliferation via activating the WNT/β-catenin pathway.

The Acute Effects of Swimming Exercise on PGC-1α-FNDC5/Irisin-UCP1 Expression in Male C57BL/6J Mice

Irisin is a myokine primarily secreted by skeletal muscles and is known as an exercise-induced hormone. The purpose of this study was to determine whether the PGC-1α -FNDC5 /Irisin-UCP1 expression which is an irisin-related signaling pathway, is activated by an acute swimming exercise. Fourteen to sixteen weeks old male C57BL/6J mice (n = 20) were divided into control (CON, n = 10) and swimming exercise groups (SEG, n = 10). The SEG mice performed 90 min of acute swimming exercise, while control (non-exercised) mice were exposed to shallow water (2 cm of depth) for 90 min. The mRNA and protein expression of PGC-1α, FNDC5 and browning markers including UCP1 were evaluated by quantitative real-time PCR and western blotting. Serum irisin concentration was measured by enzyme-linked immunosorbent assay. An acute swimming exercise did not lead to alterations in the mRNA and protein expression of PGC-1α in both soleus and gastrocnemius muscles, the mRNA and protein expression of UCP1 in brown adipose tissue, mRNA browning markers in visceral adipose tissue and circulating irisin when compared with the control group. On the other hand, an acute swimming exercise led to increases in the mRNA and protein expressions of FNDC5 in the soleus muscle, the protein expression of FNDC5 in the gastrocnemius muscles and the protein expression of UCP1 in subcutaneous adipose tissue.

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

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

Irisin Contributes to Neuroprotection by Promoting Mitochondrial Biogenesis After Experimental Subarachnoid Hemorrhage

Subarachnoid hemorrhage (SAH) is a devastating form of stroke, which poses a series of intractable challenges to clinical practice. Imbalance of mitochondrial homeostasis has been thought to be the crucial pathomechanism in early brain injury (EBI) cascade after SAH. Irisin, a protein related to metabolism and mitochondrial homeostasis, has been reported to play pivotal roles in post-stroke neuroprotection. However, whether this myokine can exert neuroprotection effects after SAH remains unknown. In the present study, we explored the protective effects of irisin and the underlying mechanisms related to mitochondrial biogenesis in a SAH animal model. Endovascular perforation was used to induce SAH, and recombinant irisin was administered intracerebroventricularly. Neurobehavioral assessments, TdT-UTP nick end labeling (TUNEL) staining, dihydroethidium (DHE) staining, immunofluorescence, western blot, and transmission electron microscopy (TEM) were performed for post-SAH assessments. We demonstrated that irisin treatment improved neurobehavioral scores, reduced neuronal apoptosis, and alleviated oxidative stress in EBI after SAH. More importantly, the administration of exogenous irisin conserved the mitochondrial morphology and promoted mitochondrial biogenesis. The protective effects of irisin were partially reversed by the mitochondrial uncoupling protein-2 (UCP-2) inhibitor. Taken together, irisin may have neuroprotective effects against SAH via improving the mitochondrial biogenesis, at least in part, through UCP-2 related targets.