New functions for old hormones: bone as an endocrine organ

Association of Serum Sclerostin Level, Coronary Artery Calcification, and Patient Outcomes in Maintenance Dialysis Patients

OBJECTIVE

The objective of this study is to investigate the association between the serum sclerostin, the coronary artery calcification (CAC), and patient outcomes in maintenance dialysis patients.

METHODS

We performed a prospective cohort study of 65 maintenance dialysis patients in 2014, including 39 patients on peritoneal dialysis and 26 on hemodialysis, and followed up for 5 years. Parameters of mineral metabolism including bone-specific alkaline phosphatase, fibroblast growth factor 23, sclerostin, and other biochemical factors were determined at the baseline. Meanwhile, the CAC score was analyzed by cardiac computed tomography.

RESULTS

Serum sclerostin in hemodialysis patients was significantly higher than that in peritoneal dialysis patients (632.35 ± 369.18 vs. 228.85 ± 188.92, p < 0.001). The patients with CAC were older, receiving hemodialysis, lower Kt/V, and had longer dialysis vintage, as well as higher levels of serum 25-(OH)-vit D and sclerostin. In multivariate logistic regression analysis, older age and lower Kt/V were risk factors for CAC. The area under the receiver operating characteristic curves for prediction of CAC by sclerostin was 0.74 (95% confidence interval 0.605-0.878, p = 0.03), and the cutoff value of sclerostin is 217.55 pg/mL with the sensitivity 0.829 and specificity 0.619. After 5 years of follow-up, 51 patients survived. The patients in the survival group had significantly lower age, sclerostin levels, and low CAC scores than the nonsurvival group. Old age (≥60 years, p < 0.001) and high CAC score (≥50 Agatston unit, p = 0.031) were significant risk factors for the patient survival.

CONCLUSIONS

Sclerostin is significantly elevated in dialysis patients with CAC. But sclerostin is not a risk factor for CAC. After 5 years of follow-up, patients in the survival group are younger and have lower sclerostin levels and CAC scores. But sclerostin levels are not independent risk factors for high mortality in dialysis patients.

Insulin and osteocalcin: further evidence for a mutual cross-talk

PURPOSE

In the last few years, bone has been recognized as an endocrine organ that modulates glucose metabolism by secretion of osteocalcin, an osteoblast-specific hormone, that influences fat deposition and blood sugar levels. To date, however, very few in vitro models have been developed to investigate, at the molecular levels, the relationship between glucose, insulin and osteocalcin. This study aims at covering this gap.

METHODS

We studied osteogenic differentiation, osteocalcin gene expression, and osteblast-mediated insulin secretion, using cultured MG-63 human osteoblast-like cells that underwent glucotoxicity and insulin resistance. In addition, we investigated whether a correlation existed between hyperglycemia and/or insulin resistance and total osteocalcin serum concentrations in patients.

RESULTS

While insulin and low glucose increased osteocalcin gene expression, disruption of insulin signaling in MG-63 osteoblasts and high glucose concentration in cell culture medium decreased osteocalcin gene transcription and reduced osteogenic differentiation. Concomitantly, insulin secretion was significantly impaired in rat INS-1 β-cells treated with conditioned medium from insulin resistant MG-63 cells or cells exposed to high glucose concentrations. Also, chronic hyperglycemia, but not insulin resistance, inversely correlated with circulating osteocalcin levels in patients.

CONCLUSION

Our results further support the existence of an endocrine axis between bone, where osteocalcin is produced, and pancreatic β-cells, and add new insights into the molecular details of this relationship. These findings may contribute to the understanding of osteocalcin regulation and its role in metabolism.