Circulating irisin levels are associated with lipid and uric acid metabolism in a Chinese population

FNDC5 Attenuates Atherosclerotic Plaque Formation and Regulates PPARα/HO-1 in ApoE-/- Mice

INTRODUCTION

This study attempted to observe the role of fibronectin type III domain-containing protein 5 (FNDC5) in atherosclerosis development and the underlying mechanism.

METHODS

After being fed a high-fat diet (HFD), ApoE-/- mice were injected with saline, control adenovirus (Ad-vector), or FNDC5 overexpressing adenovirus (Ad-FNDC5). ApoE-/- mice fed with a chow diet were considered the control. After 12 weeks of treatment, the levels of serum high-density lipoprotein cholesterol (HDL-C), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), triglyceride (TG), and irisin were detected by commercial kits.

RESULTS

Compared with the control, the serum TG, TC, and LDL-C levels, aortic plaque area, and weight were significantly increased, while serum HDL-C and irisin levels were reduced in HFD mice. Treating with Ad-FNDC5 could alleviate these changes in HFD mice and cause the activation of PPARα/HO-1 signaling in aortic tissue. After co-treating with GW6471, a PPARα antagonist, the effects of Ad-FNDC5 on the weight, serum LDL-C, TC, TG, and HDL-C levels, and aortic plaque of HFD mice were partly blocked.

CONCLUSION

Elevated FNDC5 has a delaying effect on atherosclerotic plaque formation, which may be related to the upregulation of PPARα/HO-1 signaling.

Association between irisin and metabolic parameters in nondiabetic, nonobese adults: a meta-analysis

BACKGROUND

Irisin has been proposed to have a beneficial influence on the metabolic status of animals and humans. However, the relationship between circulating irisin levels and the risks of metabolic components in humans remains unclear. In the present meta-analysis, we aimed to evaluate the association between circulating irisin and metabolic parameters in nonobese, nondiabetic adults.

METHODS

We searched PubMed, Embase, the Cochrane Library, Web of Science and ClinicalTrial.gov using the main search terms and identified original articles published prior to March 7, 2022. Studies that met our inclusion criteria and reported the association between irisin and metabolic parameters were included in our meta-analysis. We used the Newcastle Ottawa scale to assess the quality of the included studies.

RESULTS

A total of 14 studies (711 subjects) in 11 articles were included for qualitative and quantitative synthesis. The pooled results showed that circulating irisin was positively and significantly correlated with fasting blood glucose (r = 0.159), HOMA-IR (r = 0.217) and waist-to-hip ratio (WHR) (r = 0.168). However, no significant association was detected between irisin levels and other metabolic parameters.

CONCLUSIONS

Thus, these findings indicated the possible link between irisin levels and part of the metabolic parameters in apparently metabolically normal individuals. However, the regulation of irisin in metabolism in humans remains to be fully elucidated, and well-designed prospective studies will be needed in the future. Trial registration The review protocol was registered in the International Prospective Register of Systematic Reviews (PROSPERO): CRD42022315269.

Irisin, an Effective Treatment for Cardiovascular Diseases?

Irisin, as one of the myokines induced by exercise, has attracted much attention due to its important physiological functions such as white fat browning, the improvement in metabolism, and the alleviation of inflammation. Despite the positive role that irisin has been proven to play in the prevention and treatment of cardiovascular diseases, whether it can become a biomarker and potential target for predicting and treating cardiovascular diseases remains controversial, given the unreliability of its detection methods, the uncertainty of its receptors, and the species differences between animals and humans. This paper was intended to review the role of irisin in the diagnosis and treatment of cardiovascular diseases, the potential molecular mechanism, and the urgent problems to be solved in hopes of advancing our understanding of irisin as well as providing data for the development of new and promising intervention strategies by discussing the causes of contradictory results.

Relationships between Uncoupling Protein Genes UCP1, UCP2 and UCP3 and Irisin Levels in Residents of the Coldest Region of Siberia

Currently, it is known that irisin can participate in the processes of thermoregulation and browning of adipose tissue, and, therefore, it is possible that it is involved in the microevolutionary mechanisms of adaptation to a cold. The aim of this study is to investigate the relationship between the uncoupling protein genes (UCP1, UCP2, UCP3) and the irisin levels in the residents of the coldest region of Siberia. The sample consisted of 279 Yakut people (185 females, 94 males, average age 19.8 ± 2.03 years). The females plasma irisin concentration was 8.33 ± 2.74 mcg/mL and the males was 7.76 ± 1.86 mcg/mL. Comparative analysis of irisin levels with the genotypes of six studied SNP-markers in females revealed a significant association of irisin with rs1800849-UCP3. The TT genotype of rs1800849 was associated with elevated levels of irisin (p = 0.01). It was also found that this TT genotype in females was associated with reduced weight and height (p = 0.03). We searched for natural selection signals for the T-allele rs1800849-UCP3; as a result of which, it was found that this allele has a significantly high frequency of distribution in northern (45%, CI: 0.42–0.484) compared with southern Asian populations (28%, CI: 0.244–0.316) (p = 0.01). The results obtained indicate the probable involvement of irisin and the UCP3 gene in thermoregulation, and the spread of its allelic variants is probably related to adaptation to a cold climate.

Irisin in atherosclerosis

Irisin, a novel exercise-induced myokine, has been shown to play important roles in increasing white adipose tissue browning, regulating energy metabolism and improving insulin resistance. Growing evidence suggests a direct role for irisin in preventing atherosclerosis (AS) by inhibiting oxidative stress, improving dyslipidemia, facilitating anti-inflammation, reducing cellular damage and recovering endothelial function. In addition, some studies have noted that serum irisin levels play an essential role in cardiovascular diseases (CVDs) risk prediction, highlighting that irisin has the potential to be a useful predictive marker and therapeutic target of AS, especially in monitoring therapeutic efficacy. This review summarizes the understanding of irisin-mediated regulation in essential biological pathways and functions in atherosclerosis and prompts further exploitation of the biological properties of irisin in the pathogenesis of atherosclerosis.

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.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

The Controversial Role of Irisin in Clinical Management of Coronary Heart Disease

Irisin, a PGC1α-dependent myokine, was once believed to have beneficial effects induced by exercise. Since its first discovery of adipose browning in 2012, multiple studies have been trying to explore the metabolic functions of irisin, such as glucose and lipid metabolism. However, recently many studies with irisin concentration measuring were doubt for methodological problems, which may account for the continuous inconsistencies. New tools like recombinant irisin and gene-knockout mice are required to reconfirm the questioned functions of irisin. In this paper, we make a critical introduction to the latest researches concerning the relationship between irisin and coronary heart disease, which includes atherosclerosis, stable angina pectoris and acute coronary syndromes. These studies provided various controversial evidence of short and long-term monitoring and therapeutic effect from molecular cellular mechanisms, in vivo experiments and epidemiological investigation. But with ambiguities, irisin still has a long way to go to identify its functions in the clinical management.

The Potential Role of Irisin in Vascular Function and Atherosclerosis: A Review

Exercise is an effective intervention for both the prevention and the treatment of obesity and insulin resistance because skeletal muscle secretes many bioactive proteins that contribute to the beneficial effect of exercise. It has been revealed that irisin plays an important role in metabolic homeostasis and both acute and chronic exercises increase circulating irisin in experimental animal models and in humans. Although previous studies have reported that the irisin-related signaling mechanism may play a beneficial role in the treatment of metabolic diseases including obesity, metabolic syndrome, insulin resistance, and diabetes mellitus, studies on whether irisin plays a key role in vascular function and vascular complications are still insufficient. Therefore, the current review aims to summarize the accumulating evidence showing the potential role of irisin, especially in vascular reactivity and vascular abnormalities such as atherosclerosis.

Irisin Is Controlled by Farnesoid X Receptor and Regulates Cholesterol Homeostasis

Objective

The aim of this study was to investigate whether the nuclear receptor farnesoid X receptor (FXR) could regulate FNDC5/Irisin expression and the role of Irisin in hyperlipidemia and atherosclerosis in ApoE-/- mice.

Methods and Results

We treated primary human hepatocytes, HepG2 cells, and Rhesus macaques with FXR agonist (CDCA, GW4064, and ivermectin). FNDC5 expression was highly induced by CDCA and GW4064 in hepatocytes, HepG2 cells, and the circulating level of Irisin increased in Rhesus macaques. Luciferase reporter and CHIP assays were used to determine whether FXR could regulate FNDC5 promoter activity. Irisin-ApoE-/- and ApoE-/- mice were used to study the metabolic function of Irisin in dyslipidemia and atherosclerosis. Irisin-ApoE-/- mice showed improved hyperlipidemia and alleviated atherosclerosis as compared with ApoE-/- mice. Irisin upregulated the expression of Abcg5/Abcg8 in liver and intestine, which increased the transport of biliary cholesterol and fecal cholesterol output.

Conclusion

Activation of FXR induces FNDC5 mRNA expression in human and increased the circulating level of Irisin in Rhesus macaques. FNDC5/Irisin is a direct transcriptional target of FXR. Irisin may be a novel therapeutic strategy for dyslipidemia and atherosclerosis.

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

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

A glance at the therapeutic potential of irisin against diseases involving inflammation, oxidative stress, and apoptosis: An introductory review

Irisin is a hormone-like molecule mainly released by skeletal muscles in response to exercise. Irisin induces browning of the white adipose tissue and has been shown to regulate glucose and lipid homeostasis. Keeping its energy expenditure and metabolic properties in view, numerous studies have focused on its therapeutic potential for the treatment of metabolic disorders like obesity and type 2 diabetes. Recently, the anti-inflammatory, anti-apoptotic and anti-oxidative properties of irisin have received a great deal of attention of the scientific society. These pathogenic processes are often associated with initiation, progression, and prognosis of numerous diseases like myocardial infarction, kidney diseases, cancer, lung injury, inflammatory bowel diseases, atherosclerosis, liver diseases, obesity and type 2 diabetes. In the current review, we present evidence regarding the anti-inflammatory, anti-apoptotic and anti-oxidative potential of irisin pertaining to various pathological conditions. Here, we explore multiple molecular pathways targeted by irisin therapy. Given the promising effects of irisin, many diseases with evident oxidative stress, inflammation and apoptosis can be targeted by irisin.

Irisin in metabolic diseases

INTRODUCTION

Irisin is a myokine/adipokine induced by the exercise in mice and humans, which is proposed to induce "browning" of white adipose tissue, its primary target, thus increasing thermogenesis and energy expenditure. Since its identification, irisin has been linked to favorable effects on metabolic diseases, including obesity, type 2 diabetes mellitus (T2DM), lipid metabolism and cardiovascular disease (CVD), nonalcoholic fatty liver disease (NAFLD), polycystic ovary syndrome (PCOS), and metabolic bone diseases. Generally, despite the promising profile of irisin in rodents, its effects on human are less recognized.

REVIEW

Most, but not all studies show a positive association between irisin and indices of adiposity. In T2DM, NAFLD, and CVD, most observational studies reported lower irisin levels in patients than controls. Regarding metabolic bone diseases, irisin is positively associated with bone mineral density and strength in athletes, and inversely associated with osteoporotic fractures in postmenopausal osteoporosis. In PCOS, data remain largely conflicting. Irisin does not seem to be further reduced when two metabolic diseases, e.g., T2DM and NAFLD, or obesity and NAFLD exist though more data are needed. Furthermore, it seems that diverse confounders may have affected the results of different clinical studies.

CONCLUSION

Irisin remains an appealing molecule from a pathophysiological point of view and an appealing therapeutic target for metabolic diseases, albeit much research is still needed.

Irisin Inhibits Hepatic Cholesterol Synthesis via AMPK-SREBP2 Signaling

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

Functional alpha 1 protease inhibitor produced by a human hepatoma cell line

Alpha 1 protease inhibitor antigen was identified in the culture medium of the human ascites hepatoma cell line SK-HEP-1. Trypsin inhibitory activity and alpha 1 Pl antigen accumulated in serum-free medium concomitantly over a period of several days. Radioactive alpha 1 Pl antigen was detected in conditioned medium from cultures supplemented with 35S-L-methionine, indicating a synthesis and release of the protein. Alpha 1 Pl antigen in conditioned medium appeared to be antigenically identical to that in human plasma, and the newly synthesized (radiolabeled) antigen co-migrated with plasma, alpha 1 Pl after immunoelectrophoresis or SDS-polyacrylamide gel electrophoresis. Moreover, evidence is presented that the synthesized inhibitor exhibits functional activity, since the 35S-labeled alpha 1 Pl in conditioned medium complexes with trypsin. We conclude that SK-HEP-1 cells in culture produce functionally active alpha 1 Pl which may be identical to that in plasma.