Previous results showed that intermittently administered irisin improves bone mass in normal mice and prevents the development of disuse‐induced osteoporosis and muscular atrophy in hindlimb‐suspended mice, a murine model able to mimic the absence of mechanical loading. A recent study showed that irisin increases survival of osteocytes acting through integrin αV/β5 receptors. To better understand the action of irisin on these cells, we investigated the downstream signaling cascades in osteocyte‐like cells (MLO‐Y4) treated with recombinant irisin (rec‐irisin) in vitro and we analyzed survival of osteocytes and caspase activation in cortical bone of osteoporotic mice treated with rec‐irisin in vivo. Our results revealed that rec‐irisin activated the MAP kinases Erk1 and Erk2 and increased the expression of the transcription factor Atf4 (2.5‐fold, p < .05) through an Erk‐dependent pathway in osteocytes. Some key genes expressed by MLO‐Y4 cells were modulated by long‐term irisin treatment, either continuously administered or given with intermittent short pulses. Interestingly, Sost mRNA was severely downregulated only upon intermittent irisin administration (10‐fold, p < .001). Furthermore, rec‐irisin upregulated Tfam mRNA (fourfold, p < .05) and Bcl2/Bax ratio (twofold, p < .05) in MLO‐Y4 cells. By detecting caspase‐9 and caspase‐3, we also found that rec‐irisin inhibited apoptosis induced by hydrogen peroxide and dexamethasone, respectively. In cortical bone of unloading C57BL6 mice treated with vehicle (unload‐veh), irisin prevented disuse‐induced reduction of viable osteocytes (+30% versus unload‐veh, p < .05) and increase of empty lacunae (+110% versus unload‐veh, p < .05), as well as caspase‐9 (threefold, p < .05) and caspase‐3 (twofold, p < .05) activations. Our findings revealed underlying mechanisms of irisin action on osteocytes, which increases their functions and exerts anti‐apoptotic effects, confirming that mechanosensor cells of bone are sensitive to the exercise‐mimetic myokine irisin. © 2019 American Society for Bone and Mineral Research.

Irisin is a myokine produced by skeletal muscle during exercise in both mice and humans. We previously showed that irisin treatment ameliorates immobility‐induced osteoporosis and muscular atrophy in mice. Data in humans showed a positive association between irisin and bone mineral density (BMD) in athletes and a population of healthy children. However, the role of this myokine regarding the state of muscle and bone in the same population remained to be determined. For this purpose, 62 patients (age 68.71 ± 12.31 years) undergoing total hip or knee replacement were recruited. Our results showed that irisin serum levels negatively correlated with age (R = −0.515; p = .000018) and positively correlated with femoral BMD (R = 0.619; p = .001) and vertebral BMD (R = 0.201; p = .0001). Irisin was also positively associated with Fndc5 mRNA in muscle biopsies (R = 0.248; p = .016), as well as with Osteocalcin (Ocn) mRNA in bone biopsies (R = 0.708; p = .006). In skeletal muscle, FNDC5 positive fibers positively correlate with BMD of total femur (R = 0.765; p = .0014) and BMD of femoral neck (R = 0.575; p = .031), Interestingly, by analyzing patients divided by their T‐score, we found lower irisin levels (p = .0011) in patients with osteopenia/osteoporosis (OP) compared to healthy controls matched for age and sex. By analyzing the senescence marker p21, we found a significant increase of its mRNA expression in the bone biopsies of OP patients compared to control ones. Therefore, we investigated in vitro whether rec‐irisin had a direct effect on this senescence marker, showing that p21 mRNA expression was significantly downregulated in osteoblasts by the treatment with irisin. Overall, these results indicate that higher irisin levels are associated with a lower rate of age‐related osteoporosis and that irisin could be effective in delaying the osteoblast aging process, suggesting a potential senolytic action of this myokine. © 2020 American Society for Bone and Mineral Research (ASBMR).

BACKGROUND: Irisin is a myokine secreted by skeletal muscle during physical activity. Irisin treatment increased cortical bone mineral density (BMD) in young healthy mice and restored bone and muscle mass loss in a mouse model of disuse-induced osteoporosis and muscular atrophy. In humans, Irisin was positively correlated with BMD in young athletes. Considering that the bone mass reached during childhood is one of the most important determinants of lifelong skeletal health, we sought to determine if Irisin levels were correlated with bone mineral status in children. METHODS: Irisin and bone metabolic markers were quantified in sera and bone mineral status was evaluated by quantitative ultrasound in a population of 34 healthy children (9.82 ± 3.2 years). RESULTS: We found that Irisin levels were positively correlated with the amplitude-dependent speed of sound Z-score (r = 0.305; p < 0.001), bone transmission time Z-score (r = 0.375; p < 0.001) and osteocalcin (r = 0.370; p < 0.001), and negatively with Dickkopf WNT Signaling Pathway Inhibitor 1 (r = −0.274; p < 0.001). CONCLUSION: In a regression analysis model, Irisin was one of the determinants of bone mineral status to a greater extent than bone alkaline phosphatase and parathyroid hormone, indicating that Irisin might be considered as one of the bone formation markers during childhood.

Almost four years after the discovery of the anabolic action of irisin on bone in mice, ample clinical evidence is emerging in support of its additional physiological relevance in human bone. Irisin inversely correlates with sclerostin levels in adults with prediabetes and with vertebral fragility fractures in post-menopausal women. Furthermore, in athletes we observed a positive correlation between irisin and bone mineral density at different anatomical sites. Our group also described a positive association between serum irisin and bone status in healthy children and multivariate regression analysis showed that irisin is a stronger determinant of bone mineral status than bone alkaline phosphatase. In children with type 1 diabetes mellitus, serum irisin concentrations are positively associated with bone quality and with glycemic control following continuous subcutaneous insulin infusion. Additionally, our in vitro studies suggest the existence of a negative interplay between PTH and irisin biology and these results were also supported by the observation that post-menopausal women with primary hyperparathyroidism have lower levels of irisin compared to matched controls. In this review, we will focus on recent findings about circulating level of irisin in different populations of human subjects and its correlation with their bone status.

Multiple cytokines produced by immune cells induce remodeling and aid in maintaining bone homeostasis through differentiation of bone-forming osteoblasts and bone-resorbing osteoclasts. Here, we investigate bone remodeling controlled by the tumor necrosis factor (TNF) superfamily cytokine LIGHT. LIGHT-deficient mice (Tnfsf14 ) exhibit spine deformity and reduced femoral cancellous bone mass associated with an increase in the osteoclast number and a slight decrease of osteoblasts compared with WT mice. The effect of LIGHT in bone cells can be direct or indirect, mediated by both the low expression of the anti-osteoclastogenic osteoprotegerin (OPG) in B and T cells and reduced levels of the pro-osteoblastogenic Wnt10b in CD8 T cells in Tnfsf14 mice. LIGHT stimulation increases OPG levels in B, CD8 T, and osteoblastic cells, as well as Wnt10b expression in CD8 T cells. The high bone mass in Light and T- and B-cell-deficient mice (Rag /Tnfsf14 ) supports the cooperative role of the immune system in bone homeostasis. These results implicate LIGHT as a potential target in bone disease. © 2017 American Society for Bone and Mineral Research.

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Tumor necrosis factor superfamily member 14 (TNFSF14), LIGHT, is a component of the cytokine network that regulates innate and adaptive immune responses, which promote homeostasis of lymphoid organs, liver, and bone. Metastatic tumors often disrupt the tissue microenvironment, thus altering the homeostasis of the invaded organ; however, the underlying mechanisms required further studies. We investigated the role of LIGHT in osteolytic bone disease induced by metastatic non‐small cell lung cancer (NSCLC). Patients diagnosed with NSCLC bone metastasis show significantly higher levels of LIGHT expressed in monocytes compared with non‐bone metastatic tumors and healthy controls. Serum LIGHT levels were also higher in patients with bone metastases than in controls, suggesting a role for LIGHT in stimulating osteoclast precursors. In bone metastatic patients, we also detected increased RNA expression and serum RANKL levels, thus by adding anti‐LIGHT or RANK–fragment crystallizable region (RANK‐Fc) in PBMC cultures, a significant inhibition of osteoclastogenesis was observed. To model this observation in mice, we used the mouse lung cancer cell line LLC‐1. After intratibial implantation, wild‐type mice showed an increased number of osteoclasts but reduced numbers of osteoblasts and decreased osteoid formation. In contrast, Tnfsf14−/− mice showed no significant bone loss or other changes in bone homeostasis associated with this model. These data indicate LIGHT is a key control mechanism for regulating bone homeostasis during metastatic invasion. Thus, LIGHT may be a novel therapeutic target in osteolytic bone metastases. © 2019 American Society for Bone and Mineral Research.

Depression is a psychiatric disorder increasingly diffused worldwide. Evidence suggests that irisin, a myokine secreted by contracting muscle, mediates beneficial effects on several targets, including the brain. Here, the potential antidepressant properties of long-term intermittent systemic irisin administration (100 µg/kg/weekly for 1 month) were evaluated in mice by the Tail Suspension Test (TST), Forced Swim Test (FST), and Open Field Test (OFT). Furthermore, to deepen the molecular pathways underlying irisin treatment, the expression of irisin precursor, neurotrophic/growth factors, and cytokines was analyzed. Irisin treatment significantly decreased the immobility time in the TST and FST, suggesting an antidepressant effect. Additionally, irisin seemed to display an anxiolytic-like effect increasing the time spent in the OFT arena center. These findings were probably due to the modulation of endogenous brain factors as the gene expression of some neurotrophins, such as brain-derived neurotrophic factor (BDNF) and insulin-like growth factor (IGF-1), was upregulated only in irisin-treated mouse brain. Moreover, irisin modulated the expression of some cytokines (IL-1β, IL-4, IL-6, and IL-10). To the best of our knowledge, this is the first study demonstrating that the irisin antidepressant effect may be observed even with a systemic administration in mice. This could pave the way toward intriguing preclinical research in humans.