Serum Irisin Level Is Positively Associated with Bone Mineral Density in Patients on Maintenance Hemodialysis

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.

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.

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.

Serum Sclerostin Level Is Negatively Associated with Bone Mineral Density in Hemodialysis Patients

Background and Objectives: Sclerostin and Dickkopf-1 (DKK1) modulate osteoblastogenesis, but their role in bone loss in hemodialysis (HD) patients is inconclusive. This study investigated relationships among lumbar bone mineral density (BMD), serum sclerostin, and DKK1 in HD patients. Materials and Methods: Blood samples were obtained from 75 HD patients. Dual-energy X-ray absorptiometry measured lumbar BMD of the lumbar vertebrae (L2−L4). Enzyme-linked immunosorbent assay revealed serum sclerostin and DKK1 concentrations. Results: There were 10 (13.3%), 20 (26.7%), and 45 (60%) patients defined as presenting with osteoporosis, osteopenia, or normal BMD, respectively. Age, alkaline phosphatase, urea reduction rate, fractional clearance index for urea, sclerostin level, and percentage of female patients are significantly negatively associated with the lumbar BMD and T-score, while the body mass index and waist circumference significantly positively associated with the lumbar BMD and T-score. Multivariate forward stepwise linear regression analysis indicated that serum sclerostin (β = −0.546, adjusted R2 change = 0.454; p < 0.001), age (β = −0.216, adjusted R2 change = 0.041; p = 0.007), and percentage of female HD patients (β = −0.288, adjusted R2 change = 0.072; p = 0.0018) were significantly negatively associated with lumbar BMD in HD patients. Conclusions: Advanced age, female gender, and serum sclerostin level, but not DKK1, were negatively associated with BMD in HD patients.

The Association of Serum Irisin with Bone Mineral Density and Turnover Markers in New-Onset Type 2 Diabetic Patients

BACKGROUND

Irisin, an exercise-induced myokine and adipocytokine, has been reported to decrease in type 2 diabetic patients. Recently, several research studies indicated that circulating levels were correlated with bone mineral density (BMD). To evaluate bone metabolism, bone turnover markers (BTMs) should be included. However, with respect to newly diagnosed T2DM patients, the relevance of their irisin levels to their BTMs and BMD remains unclear. The investigation of serum irisin levels in patients who have been newly diagnosed with type 2 diabetes and illumination of the relationship between serum irisin levels and those two indices of BMD and BTMs mentioned above are the intention of this cross-sectional study.

METHODS

66 new-onset type 2 diabetic patients (T2DM group), together with 82 control subjects (NGT group), were recruited in this study. Serum irisin concentrations and BTMs (including osteocalcin (OC), procollagen type 1 N-terminal propeptide (P1NP), and β-C-terminal telopeptides of type I collagen (β-CTX)) were determined by the enzyme-linked immunosorbent assay (ELISA). Glucose, lipid profile, and insulin were considered as measuring indicators as well. Dual-energy X-ray absorptiometry (DXA) was utilized to evaluate the indicator of BMD. Serum irisin, BTMs, and BMD were compared between diabetic patients and healthy individuals. Pearson and Spearman correlation analyses were applied as well to assess correlations between irisin and BTMs and BMD. Multiple stepwise regression analysis was conducted to identify the independent factors of irisin. ROC curve analyses were carried out for serum irisin prediction for osteoporosis/osteopenia (OP).

RESULTS

The serum levels of irisin, procollagen type 1, intact N-terminal propeptide (P1NP), and osteocalcin (OC) were evidently lower in T2DM subjects than in NGT subjects (10.90 ± 1.88 vs .11.69 ± 2.06 ng/mL, P < 0.05; 36.42(25.68,51.70) vs. 44.52(35.73,58.05)ng/ml, P < 0.05; 16.15(12.40,21.66) vs. 18.70(15.56, 23.22)ng/ml, P < 0.05). Among patients with T2DM, the circulating irisin level of those with OP was lower than that of normal BMD (9.98 ± 2.09 vs. 11.39 ± 1.57 ng/ml, P < 0.01); irisin had a negative correlation with β-C-terminal telopeptides of type I collagen (β-CTX) (r = -0.496, P < 0.001) and came back unrelated to Lumbar BMD; Lumbar BMD was negatively relevant to OC (r = -0.274, P < 0.05) and β-CTX (r = -0.410, P < 0.01). Multiple linear regression analyses of stepwise models implied that TG, LDL-C, and β-CTX were independently associated with serum irisin concentrations (P < 0.01 or P < 0.05).

CONCLUSION

Serum irisin level was declined in patients with type 2 diabetes diagnosed in the near term and had a certain association with bone turnover markers. It is suggested to consider irisin as a potential biomarker of bone metabolic disorder in T2DM patients with the initial diagnosis.

Evaluation of the Effects of Systemic Irisin Hormone Application on Osseointegration of Titanium Implants: An Experimental Study

ABSTRACT

This study aimed to conduct a biomechanical investigation of the effects of systemic irisin hormone application on the osseointegration of titanium implants in rat tibias. After surgical implementation of titanium implants in the metaphyseal part of the tibiae of rats, the rats were randomly divided into 2 equal groups: control group (n = 10) and irisin group (n = 10). After surgery in the control group, the rats received no further treatment during the 4-week experimental period. The rats in the irisin group were given 100 ng/kg irisin every day for the 4-week experimental period after surgery. At the end of the experimental period, the rats were euthanized. Implants and surrounding bone tissues were collected for biomechanical (Newton) bone implant connection analysis. The Student t test was used for statistical analysis. There were no significant differences in the biomechanical osseointeration values (Newton) of the groups (P > 0.05, P = 0.59). Also, in the irisin group, there was numerically but not statistically more bone implant connection than in the controls. Within the limitations of this study, irisin did not affect the osseointegration of titanium implants.

Role of the Myokine Irisin on Bone Homeostasis: Review of the Current Evidence

Bone is a highly dynamic tissue that is constantly adapting to micro-changes to facilitate movement. When the balance between bone building and resorption shifts more towards bone resorption, the result is reduced bone density and mineralization, as seen in osteoporosis or osteopenia. Current treatment strategies aimed to improve bone homeostasis and turnover are lacking in efficacy, resulting in the search for new preventative and nutraceutical treatment options. The myokine irisin, since its discovery in 2012, has been shown to play an important role in many tissues including muscle, adipose, and bone. Evidence indicate that irisin is associated with increased bone formation and decreased bone resorption, leading to reduced risk of osteoporosis in post-menopausal women. In addition, low serum irisin levels have been found in individuals with osteoporosis and osteopenia. Irisin targets key signaling proteins, promoting osteoblastogenesis and reducing osteoclastogenesis. The present review summarizes the existing evidence regarding the effects of irisin on bone homeostasis.

Serum Osteoprotegerin Level Is Negatively Associated with Bone Mineral Density in Patients Undergoing Maintenance Hemodialysis

BACKGROUND AND OBJECTIVES

Osteoprotegerin (OPG), a potent osteoclast activation inhibitor, decreases bone resorption and plays a role in mediating bone mineral density (BMD). Our aim was to evaluate the relationship between BMD and serum OPG in maintenance hemodialysis (MHD) patients.

MATERIALS AND METHODS

Fasting blood samples were obtained from 75 MHD patients. BMD was measured by dual-energy X-ray absorptiometry in lumbar vertebrae (L2-L4). The WHO classification criteria were applied to define osteopenia and osteoporosis. A commercial enzyme-linked immunosorbent assay was used to measure serum OPG values.

RESULTS

Among all MHD patients, seven (9.3%) and 20 patients (26.7%) were defined as osteoporosis and osteopenia, respectively. Female patients had lower lumbar BMD than males (p = 0.002). Older age (p = 0.023), increased serum OPG (p < 0.001) urea reduction rate (p = 0.021), Kt/V (p = 0.027), and decreased body mass index (p = 0.006) and triglycerides (p = 0.020) were significantly different between the normal, osteopenia, and osteoporosis groups. Lumbar spine BMD was positively correlated with body mass index (BMI) (p < 0.001) but negatively correlated with OPG (p < 0.001) and age (p = 0.003). After grouping patients into T scores < -1 and < -2.5, female sex and OPG (adjusted odds ratio [aOR] 1.022, 95% confidence interval [C.I.] 1.011-1.034, p < 0.001) were predictors of T scores < -1, whereas only OPG was predictive of T scores < -2.5 (aOR 1.015, 95% C.I. 1.005-1.026, p = 0.004) by multivariate stepwise logistic regression analysis. The areas under the curve for predicting T scores < -1 or < -2.5 were 0.920 (95% C.I. 0.834-0.970, p < 0.001) and 0.958 (95% C.I. 0.885-0.991, p < 0.001), respectively.

CONCLUSIONS

Increased serum OPG negatively correlated with lumbar BMD and could be a potential biomarker predictive of osteoporosis in MHD patients.