Serum levels of irisin in postmenopausal women with osteoporotic hip fractures

Irisin in degenerative musculoskeletal diseases: Functions in system and potential in therapy

Degenerative musculoskeletal diseases are a class of diseases related to the gradual structural and functional deterioration of muscles, joints, and bones, including osteoarthritis (OA), osteoporosis (OP), sarcopenia (SP), and intervertebral disc degeneration (IDD). As the proportion of aging people around the world increases, degenerative musculoskeletal diseases not only have a multifaceted impact on patients, but also impose a huge burden on the medical industry in various countries. Therefore, it is crucial to find key regulatory factors and potential therapeutic targets. Recent studies have shown that irisin plays an important role in degenerative musculoskeletal diseases, suggesting that it may become a key molecule in the prevention and treatment of degenerative diseases of the musculoskeletal system. Therefore, this review provides a comprehensive description of the release and basic functions of irisin, and summarizes the role of irisin in OA, OP, SP, and IDD from a cellular and tissue perspective, providing comprehensive basis for clinical application. In addition, we summarized the many roles of irisin as a key information molecule in bone-muscle-adipose crosstalk and a regulatory molecule involved in inflammation, senescence, and cell death, and proposed the interesting possibility of irisin in degenerative musculoskeletal diseases.

Irisin Protects Musculoskeletal Homeostasis via a Mitochondrial Quality Control Mechanism

Irisin, a myokine derived from fibronectin type III domain-containing 5 (FNDC5), is increasingly recognized for its protective role in musculoskeletal health through the modulation of mitochondrial quality control. This review synthesizes the current understanding of irisin's impact on mitochondrial biogenesis, dynamics, and autophagy in skeletal muscle, elucidating its capacity to bolster muscle strength, endurance, and resilience against oxidative-stress-induced muscle atrophy. The multifunctional nature of irisin extends to bone metabolism, where it promotes osteoblast proliferation and differentiation, offering a potential intervention for osteoporosis and other musculoskeletal disorders. Mitochondrial quality control is vital for cellular metabolism, particularly in energy-demanding tissues. Irisin's influence on this process is highlighted, suggesting its integral role in maintaining cellular homeostasis. The review also touches upon the regulatory mechanisms of irisin secretion, predominantly induced by exercise, and its systemic effects as an endocrine factor. While the therapeutic potential of irisin is promising, the need for standardized measurement techniques and further elucidation of its mechanisms in humans is acknowledged. The collective findings underscore the burgeoning interest in irisin as a keystone in musculoskeletal health and a candidate for future therapeutic strategies.

The association between circulating irisin levels and osteoporosis in women: a systematic review and meta-analysis of observational studies

Objective

This systematic review and meta-analysis aimed to investigate the association between circulating irisin levels and osteoporosis in women, exploring irisin's potential role in the pathophysiology and management of osteoporosis.

Method

We searched PubMed, Embase, Web of Science, Cochrane Library, CNKI, WanFang, and VIP databases up to January 2023. The inclusion criteria were observational studies reporting on circulating irisin levels in women. The standardized mean difference (SMD) and correlation coefficients with a 95% confidence interval (CI) were used as the main effect measures under a random-effects model. Heterogeneity was evaluated using the Cochrane Q statistic and the I2 statistics. Subgroup analysis and univariate meta-regression analysis were performed to identify the sources of heterogeneity. The quality of the included study was assessed by the Newcastle-Ottawa Score. The quality of evidence was evaluated using the GRADE system. Publication bias was assessed using Begg's and Egger's test, and the trim-and-fill method. Sensitivity analysis was performed to assess the stability of the results.

Results

Fifteen studies with a total of 2856 participants met the criteria. The analysis showed significantly lower irisin levels in postmenopausal osteoporotic women compared to non-osteoporotic controls (SMD = -1.66, 95% CI: -2.43 to -0.89, P < 0.0001; I2 = 98%, P < 0.00001) and in postmenopausal individuals with osteoporotic fractures than in non-fractures controls (SMD = -1.25, 95% CI: -2.15 to -0.34, P = 0.007; I2 = 97%, P < 0.00001). Correlation analysis revealed that irisin levels positively correlated with lumbar spine BMD (r = 0.37, 95% CI: 0.18 to 0.54), femoral BMD (r = 0.30, 95% CI: 0.18 to 0.42), and femoral neck BMD (r = 0.31, 95% CI: 0.14 to 0.47) in women. Despite significant heterogeneity, the robustness of the results was supported by using the random effects model and sensitivity analysis.

Conclusion

The current evidence suggests that lower irisin levels are significantly associated with osteoporosis and fracture in postmenopausal women, suggesting its utility as a potential biomarker for early detection of osteoporosis and therapeutic target. However, further high-quality prospective research controlling for confounding factors is needed to clarify the relationship between irisin levels and osteoporotic outcomes.

Systematic review registration

https://www.crd.york.ac.uk/PROSPERO, identifier CRD42023410264.

Irisin as an agent for protecting against osteoporosis: A review of the current mechanisms and pathways

BACKGROUND

Osteoporosis is recognized as a skeletal disorder characterized by diminished bone tissue quality and density. Regular physical exercise is widely acknowledged to preserve and enhance bone health, but the detailed molecular mechanisms involved remain unclear. Irisin, a factor derived from muscle during exercise, influences bone and muscle. Since its discovery in 2012, irisin has been found to promote bone growth and reduce bone resorption, establishing a tangible link between muscle exertion and bone health. Consequently, the mechanism by which irisin prevents osteoporosis have attracted significant scientific interest.

AIM OF THE REVIEW

This study aims to elucidate the multifaceted relationship between exercise, irisin, and bone health. Focusing on irisin, a muscle-derived factor released during exercise, we seek to understand its role in promoting bone growth and inhibiting resorption. Through a review of current research article on irisin in osteoporosis, Our review provides a deep dive into existing research on influence of irisin in osteoporosis, exploring its interaction with pivotal signaling pathways and its impact on various cell death mechanisms and inflammation. We aim to uncover the molecular underpinnings of how irisin, secreted during exercise, can serve as a therapeutic strategy for osteoporosis.

KEY SCIENTIFIC CONCEPTS OF THE REVIEW

Irisin, secreted during exercise, plays a vital role in bridging muscle function to bone health. It not only promotes bone growth but also inhibits bone resorption. Specifically, Irisin fosters osteoblast proliferation, differentiation, and mineralization predominantly through the ERK, p38, and AMPK signaling pathways. Concurrently, it regulates osteoclast differentiation and maturation via the JNK, Wnt/β-catenin and RANKL/RANK/OPG signaling pathways. This review further delves into the profound significance of irisin in osteoporosis and its involvement in diverse cellular death mechanisms, including apoptosis, autophagy, ferroptosis, and pyroptosis.

Bone-muscle crosstalk under physiological and pathological conditions

Anatomically connected bones and muscles determine movement of the body. Forces exerted on muscles are then turned to bones to promote osteogenesis. The crosstalk between muscle and bone has been identified as mechanotransduction previously. In addition to the mechanical features, bones and muscles are also secretory organs which interact closely with one another through producing myokines and osteokines. Moreover, besides the mechanical features, other factors, such as nutrition metabolism, physiological rhythm, age, etc., also affect bone-muscle crosstalk. What's more, osteogenesis and myogenesis within motor system occur almost in parallel. Pathologically, defective muscles are always detected in bone associated diseases and induce the osteopenia, inflammation and abnormal bone metabolism, etc., through biomechanical or biochemical coupling. Hence, we summarize the study findings of bone-muscle crosstalk and propose potential strategies to improve the skeletal or muscular symptoms of certain diseases. Altogether, functional improvement of bones or muscles is beneficial to each other within motor system.

Irisin and Its Role in Postmenopausal Osteoporosis and Sarcopenia

Menopause, an extremely delicate phase in a woman's life, is characterized by a drop in estrogen levels. This decrease has been associated with the onset of several diseases, including postmenopausal osteoporosis and sarcopenia, which often coexist in the same person, leading to an increased risk of fractures, morbidity, and mortality. To date, there are no approved pharmacological treatments for sarcopenia, while not all of those approved for postmenopausal osteoporosis are beneficial to muscles. In recent years, research has focused on the field of myokines, cytokines, or peptides secreted by skeletal muscle fibers following exercise. Among these, irisin has attracted great interest as it possesses myogenic properties but at the same time exerts anabolic effects on bone and could therefore represent the link between muscle and bone. Therefore, irisin could represent a new therapeutic strategy for the treatment of osteoporosis and also serve as a new biomarker of sarcopenia, thus facilitating diagnosis and pharmacological intervention. The purpose of this review is to provide an updated summary of what we know about the role of irisin in postmenopausal osteoporosis and sarcopenia.

FNDC5/irisin ameliorates bone loss of type 1 diabetes by suppressing endoplasmic reticulum stress‑mediated ferroptosis

BACKGROUND

Ferroptosis is known to play a crucial role in diabetic osteopathy. However, key genes and molecular mechanisms remain largely unclear. This study aimed to identify a crucial ferroptosis-related differentially expressed gene (FR-DEG) in diabetic osteopathy and investigate its potential mechanism.

METHODS

We identified fibronectin type III domain-containing protein 5 (FNDC5)/irisin as an essential FR-DEG in diabetic osteopathy using the Ferroptosis Database (FerrDb) and GSE189112 dataset. Initially, a diabetic mouse model was induced by intraperitoneal injection of streptozotocin (STZ), followed by intraperitoneal injection of irisin. MC3T3-E1 cells treated with high glucose (HG) were used as an in vitro model. FNDC5 overexpression plasmid was used to explore underlying mechanisms in vitro experiments. Femurs were collected for micro-CT scan, histomorphometry, and immunohistochemical analysis. Peripheral serum was collected for ELISA analysis. Cell viability was assessed using a CCK-8 kit. The levels of glutathione (GSH), malondialdehyde (MDA), iron, reactive oxygen species (ROS), and lipid ROS were detected by the corresponding kits. Mitochondria ultrastructure was observed through transmission electron microscopy (TEM). Finally, mRNA and protein expressions were examined by quantitative real-time PCR (qRT-PCR) and western blot analysis.

RESULTS

The expression of FNDC5 was found to be significantly decreased in both in vivo and in vitro models. Treatment with irisin significantly suppressed ferroptosis and improved bone loss. This was demonstrated by reduced lipid peroxidation and iron overload, increased antioxidant capability, as well as the inhibition of the ferroptosis pathway in bone tissues. Furthermore, in vitro studies demonstrated that FNDC5 overexpression significantly improved HG-induced ferroptosis and promoted osteogenesis. Mechanistic investigations revealed that FNDC5 overexpression mitigated ferroptosis in osteoblasts by inhibiting the eukaryotic initiation factor 2 alpha (eIF2α)/activated transcription factor 4 (ATF4)/C/EBP-homologous protein (CHOP) pathway.

CONCLUSIONS

Collectively, our study uncovered the important role of FNDC5/irisin in regulating ferroptosis of diabetic osteopathy, which might be a potential therapeutic target.

The Role of Irisin throughout Women's Life Span

Since its discovery, much attention has been drawn to irisin's potential role in metabolic and reproductive diseases. This narrative review summarizes and updates the possible role played by this fascinating molecule in different physiological (puberty and menopause) and pathological (polycystic ovary syndrome (PCOS), functional hypothalamic amenorrhea (FHA), endometriosis, and gestational diabetes) conditions that can affect women throughout their entire lives. Irisin appears to be an important factor for the hypothalamic-pituitary-gonadal axis activation, and appears to play a role in the timing of puberty onset. Serum irisin levels have been proposed as a biomarker for predicting the future development of gestational diabetes (GDM). Its role in PCOS is still controversial, although an "irisin resistance" mechanism has been hypothesized. In addition to its impact on metabolism, irisin also appears to influence bone health. Irisin levels are inversely correlated with the prevalence of fractures in postmenopausal women. Similar mechanisms have also been postulated in young women with FHA. In clinical settings, further controlled, prospective and randomized clinical trials are needed to investigate the casual relationship between irisin levels and the conditions described and, in turn, to establish the role of irisin as a prognostic/diagnostic biomarker or a therapeutic target.

Muscle-bone crosstalk via endocrine signals and potential targets for osteosarcopenia-related fracture

Background

Osteosarcopenia is a syndrome coexisting sarcopenia and osteopenia/osteoporosis, with a high fracture risk. Recently, skeletal muscle and bone have been recognized as endocrine organs capable of communication through secreting myokines and osteokines, respectively. With a deeper understanding of the muscle-bone crosstalk, these endocrine signals exhibit an important role in osteosarcopenia development and fracture healing.

Methods

This review summarizes the role of myokines and osteokines in the development and treatment of osteosarcopenia and fracture, and discusses their potential for osteosarcopenia-related fracture treatment.

Results

Several well-defined myokines (myostatin and irisin) and osteokines (RANKL and SOST) are found to not only regulate skeletal muscle and bone metabolism but also influence fracture healing processes. Systemic interventions targeting these biochemical signals has shown promising results in improving the mass and functions of skeletal muscle and bone, as well as accelerating fracture healing processes.

Conclusion

The regulation of muscle-bone crosstalk via biochemical signals presents a novel and promising strategy for treating osteosarcopenia and fracture by simultaneously enhancing bone and muscle anabolism. We propose that myostatin, irisin, RANKL, and SOST may serve as potential targets to treat fracture patients with osteosarcopenia.

The translational potential of this article

Osteosarcopenia is an emerging geriatric syndrome where sarcopenia and osteoporosis coexist, with high fracture risk, delayed fracture healing, and increased mortality. However, no pharmacological agent is available to treat fracture patients with osteosarcopenia. This review summarizes the role of several myokines and osteokines in the development and treatment of osteosacropenia and fracture, as well as discusses their potential as intervention targets for osteosarcopenia-related fracture, which provides a novel and promising strategy for future osteosarcopenia-related fracture treatment.

Irisin Protects against Loss of Trabecular Bone Mass and Strength in Adult Ovariectomized Mice by Stimulating Osteoblast Activity

Irisin is a peptide secreted by skeletal muscle that plays a major role in bone metabolism. Experiments in mouse models have shown that administration of recombinant irisin prevents disuse-induced bone loss. In this study, we aimed to evaluate the effects of irisin treatment for the prevention of bone loss in the ovariectomized (Ovx) mouse, the animal model commonly used to investigate osteoporosis caused by estrogen deficiency. Micro-Ct analysis conducted on Sham mice (Sham-veh) and Ovx mice treated with vehicle (Ovx-veh) or recombinant irisin (Ovx-irisn) showed bone volume fraction (BV/TV) decreases in femurs (Ovx-veh 1.39± 0.71 vs. Sham-veh 2.84 ± 1.23; p = 0.02) and tibia at both proximal condyles (Ovx-veh 1.97 ± 0.68 vs. Sham-veh 3.48 ± 1.26; p = 0.03) and the subchondral plate (Ovx-veh 6.33 ± 0.36 vs. Sham-veh 8.18 ± 0.41; p = 0.01), which were prevented by treatment with a weekly dose of irisin for 4 weeks. Moreover, histological analysis of trabecular bone showed that irisin increased the number of active osteoblasts per bone perimeter (Ovx-irisin 32.3 ± 3.9 vs. Ovx-veh 23.5 ± 3.6; p = 0.01), while decreasing osteoclasts (Ovx-irisin 7.6 ± 2.4 vs. Ovx-veh 12.9 ± 3.04; p = 0.05). The possible mechanism by which irisin enhances osteoblast activity in Ovx mice is upregulation of the transcription factor Atf4, one of the key markers of osteoblast differentiation, and osteoprotegerin, thereby inhibiting osteoclast formation.

Serum irisin levels and osteoporosis in patients with advanced chronic kidney disease and renal transplant recipients

AIM

To elucidate the association of serum irisin levels with bone mineral density (BMD) and calcium-phosphorus metabolism parameters in chronic kidney disease (CKD) patients and renal transplant recipients (RTRs).

METHODS

This is a cross-sectional study involving CKD patients and RTRs. Healthy volunteers served as controls. Age, gender, and dialysis vintage were recorded. Serum irisin, creatinine, glucose, calcium, albumin, 25(OH) vitamin D, ferritin, C-reactive protein, A1C, and lipid profile were studied in all participants. Estimated glomerular filtration rate (eGFR), corrected calcium, and body mass index (BMI) were calculated.

RESULTS

Overall, 49 patients (23 hemodialysis, 26 RTRs) and 25 control subjects were included. In hemodialysis (HD) group, 8 patients (34.8%) had osteoporosis, and 12 patients (52.2%) had osteopenia. In RTR group, 3 patients (11.5%) had osteoporosis, while 15 patients (57.7%) had osteopenia. Among controls, one had osteoporosis, and 7 had osteopenia. There was no significant difference between HD and RTRs; however, osteoporosis rate was significantly lower in control subjects. BMD measurements (femur and lumbar T- and Z-scores) were comparable between HD and RTR groups. Control group DEXA values were similar to RTRs; however, they were significantly higher compared to HD group. 25(OH) vitamin D levels were comparable between the HD and RTR groups, and these were significantly lower compared to values of the control group. Mean serum irisin level was 426.6 ± 191.2 pg/mL in hemodialysis group, 342.6 ± 174.8 in the RTR group, and 208.0 ± 186.1 in controls. Serum irisin levels were similar in RTR and HD groups, but their values were significantly higher compared to controls. When we compared serum irisin levels between patients with and without osteoporosis in the whole cohort and hemodialysis and RTR groups, there was no difference. Serum irisin was positively correlated with lumbar T-score both in hemodialysis and RTR groups.

CONCLUSION

Our study is the first in the literature revealing the positive correlation of serum irisin level with femur T-score in RTRs. Serum irisin level was also positively correlated with femur T-scores in hemodialysis patients.

Serum Irisin level is associated with fall risk, muscle strength, and cortical porosity in postmenopausal women

BACKGROUND

Irisin plays a role in bone-muscle crosstalk, but the relationship between the serum irisin level and bone microarchitecture remains unknown.

OBJECTIVE

This study aimed to investigate the relationships between serum irisin level and fall risk, muscle strength, bone mineral density (BMD), and bone microarchitecture among Chinese postmenopausal women.

METHODS

In all 138 postmenopausal women, handgrip strength, short physical performance battery (SPPB), and the timed up-and-go test were performed to evaluate muscle strength, physical performance, and fall risk, respectively. The serum irisin was measured. High-resolution peripheral quantitative computed tomography (HR-pQCT) was performed to acquire volumetric BMD and bone microarchitecture. Bivariate analysis was used to explore relationships between serum irisin level and muscle strength and HR-pQCT parameters. Univariate and multivariate linear regression analyses were performed to determine associations between serum irisin level and vBMD and cortical porosity (Ct.Po).

RESULTS

All participants had a median serum irisin level of 3.91 μg/ml. Participants with high fall risk had significantly lower serum irisin levels than those with low fall risk (2.22 μg/ml vs. 4.16 μg/ml, p=0.024). Irisin level was positively related to handgrip strength (rs=0.185, p=0.030) and SPPB performance. In univariate linear regression, serum irisin level was positively associated with cortical volumetric BMD (Ct.vBMD, radius: standardized β=0.184, p=0.031; tibia: standardized β=0.242, p=0.004), but it had no significant associations with Ct.vBMD after multivariate adjustment. After adjusting by age, height, serum sclerostin level, and body fat ratio, only Ct.Po at the distal radius had a significantly negative association with serum irisin level (standardized β=-0.276, p=0.003).

CONCLUSION

Postmenopausal women with lower serum irisin levels have a higher fall risk, weaker muscle strength, and higher cortical porosity. Moreover, serum irisin level has a positive association with Ct.vBMD, but it is affected by factors such as age.

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

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

Role of irisin in physiology and pathology

Irisin, out-membrane part of fibronectin type III domain-containing 5 protein (FNDC5), was activated by Peroxisome proliferator-activated receptor γ (PPARγ) coactivator-1α (PGC-1α) during physical exercise in skeletal muscle tissues. Most studies have reported that the concentration of irisin is highly associated with health status. For instance, the level of irisin is significantly lower in patients with obesity, osteoporosis/fractures, muscle atrophy, Alzheimer's disease, and cardiovascular diseases (CVDs) but higher in patients with cancer. Irisin can bind to its receptor integrin αV/β5 to induce browning of white fat, maintain glucose stability, keep bone homeostasis, and alleviate cardiac injury. However, it is unclear whether it works by directly binding to its receptors to regulate muscle regeneration, promote neurogenesis, keep liver glucose homeostasis, and inhibit cancer development. Supplementation of recombinant irisin or exercise-activated irisin might be a successful strategy to fight obesity, osteoporosis, muscle atrophy, liver injury, and CVDs in one go. Here, we summarize the publications of FNDC5/irisin from PubMed/Medline, Scopus, and Web of Science until March 2022, and we review the role of FNDC5/irisin in physiology and pathology.

FNDC5/Irisin polymorphisms are associated with osteopenia in postmenopausal Mayan-Mestizo women

OBJECTIVE

This study aimed to analyze the association between rs3480 and rs16835198 of FNDC5/Irisin and their haplotypes with variations in bone mineral density (BMD) and osteopenia/osteoporosis in postmenopausal Mayan-Mestizo women.

METHODS

We studied 547 postmenopausal women of Maya-Mestizo origin. BMD was measured in the lumbar spine and total hip by dual-energy X-ray absorptiometry. DNA was obtained from blood leukocytes. rs3480 and rs16835198 of FNDC5/Irisin were studied using real-time PCR allelic discrimination. Differences between the means of BMD according to genotype were analyzed with covariance. Allele frequency differences were assessed by χ² and logistic regression was used to test for associations. Pairwise linkage disequilibrium between polymorphisms was calculated by direct correlation r², and haplotype analysis was conducted.

RESULTS

Under a recessive model, we observed a significant association of rs3480 with the presence of osteopenia at the total hip and femoral neck (p = 0.008 and p = 0.003, respectively). For rs16835198, we found an association with osteopenia at the total hip and femoral neck in a dominant model (p = 0.043 and p = 0.009, respectively).

CONCLUSIONS

We found an association of rs3480 with risk to present osteopenia at the total hip and femoral neck, while rs16835198 was associated as a protector for presence of osteopenia only at the femoral neck.