Relationship between plasma levels of sclerostin, calcium–phosphate disturbances, established markers of bone turnover, and inflammation in hemodialysis patients.
Also see- 25-OH-Vitamin D, DRG:HYBRiD-XL

Abstract

Introduction

Sclerostin is a protein secreted by osteocytes that prevents excessive bone formation. It activates Wnt pathway that inhibits the differentiation of the mesodermal stem cells towards preosteoblasts and osteoblasts proliferation. The sclerostin signal in osteoblasts is further modulated by several other factors, e.g. calcitriol, parathyroid hormone (PTH), glucocorticoids, and tumour necrosis factor-α (TNF-α). Calcitriol induces expression of low-density lipoprotein receptor-related protein 5 (LRP5) and inhibits bone marrow stromal cells, expression of Dickkopf-related protein 1 (DKK1) and secreted frizzled-related protein 2 (SFRP2)—antagonists of Wnt signalling pathway, and transcriptional regulation of key proteins osteoblasts, including non-collagenous proteins e.g. Osteocalcin (OC).

Simultaneously, calcitriol stimulates osteoclastogenesis by increasing the expression of receptor activator of nuclear factor kappa-B ligand (RANKL) and inhibiting the expression of Osteoprotegerin (OPG) in preosteoblasts. Previously, elevated sclerostin levels have been observed in subjects with a higher bone mineral mass. It suggested that circulating sclerostin level is an important marker of the pool of mature osteocytes. As a consequence of greater bone mineral mass, circulating sclerostin levels are higher in men than in women and due to decreased clearance its level increases with the decline of renal function and age. The sclerostin level is also considered as a potential biomarker of decreased bone formation.

In hemodialysis (HD) patients, bone biopsy remains a gold standard in the assessment of renal osteodystrophy (ROD). However, in daily clinical practice, several traditional biochemical surrogates of bone turnover are used. The assessment of iPTH, a bone isoenzyme of alkaline phosphatase (b-ALP), and 25-OH-D have an established position in the diagnosis of mineral and bone disorders in patients with chronic kidney disease (CKD-MBD). During the last decade, some novel potential markers of bone turnover, used in the diagnosis and management of osteoporosis, including osteocalcin, N-terminal propeptide of type I procollagen (P1NP)—markers of bone formation; C-terminal cross-linked alpha-chain telopeptide of type I collagen (CTx), N-terminal cross-linked telopeptide of the alpha chain of type I collagen (NTx), and pyridinoline and deoxypirydoline (PYD and DPD) were extensively investigated, but to date, they are not widely applied in CKD patients.

Of note, in histomorphometric cross-sectional study performed in HD patients by Cejka et al., serum sclerostin concentration negatively correlated with parameters of bone turnover, as well as with osteoblastic number and function determined in bone biopsy [10]. Furthermore, an excessive accumulation of sclerostin was shown to inhibit PTH secretion in patients with chronic kidney disease, which may potentially slow an excessive bone turnover, induced by secondary hyperparathyroidism. On the other hand, sclerostin secretion by osteocytes seems to be also stimulated by the inflammation in HD patients.

The aim of this study was to evaluate the associations between plasma levels of sclerostin and calcium–phosphate metabolism disturbances, traditional and novel markers of bone turnover as well as inflammation in HD patients.

Purpose

Data concerning the relation between increased levels of circulating sclerostin (a physiological inhibitor of bone formation) and bone turnover in patients with chronic renal failure (CRF) are limited. Therefore, the aim of this study was to evaluate associations between plasma sclerostin levels and calcium–phosphate disturbances, markers of bone turnover as well as inflammation in hemodialysis (HD) patients.

Methods

In plasma samples obtained in 150 stable HD patients (92 men) aged 40–70 years, levels of sclerostin, fibroblast growth factor (cFGF23), osteocalcin, the N-terminal propeptide of type I procollagen, C-terminal telopeptide of the alpha chain of type I collagen (β-CTx), and inflammatory markers (IL-6 and TNF-α) in addition to routine parameters (calcium, phosphorus, parathyroid hormone—iPTH, 25-OH-D, alkaline phosphatase) were measured.

Laboratory measurements

In frozen plasma samples, sclerostin, cFGF23, 25-OH-D, osteocalcin, N-terminal propeptide of type I procollagen (total P1NP), and terminal C-terminal telopeptide of the alpha chain of type I collagen (β-CTx) were assessed. Commercially available ELISA kits were used for measurements of plasma levels of sclerostin (TECOmedical AG, Sissach, Switzerland; the mean intra- and inter-assay coefficients < 4.0% and the < 4.8%, respectively), cFGF23 (Immutopics. San Clemente, CA, U.S.; the mean intra- and inter-assay coefficients < 2.4% and < 4.7%, respectively), and 25-OH-Vitamin D (DRG Instruments GmbH for Hybrid XL, Marburg, Germany; the inter-assay precision < 14.2%). Osteocalcin, β-CTx, total P1NP, iPTH were assessed by ECLIA (Roche Diagnostics GmBH, Mannheim, Germany for Cobas e 411 analyser) with precision < 3.3%, < 4.2%, < 4.1%, and < 6.5%, respectively. The other parameters used in this analysis (calcium—Ca, phosphate—P, and alkaline phosphatase—ALP) were retrieved from medical records. Besides, some previously assessed inflammatory markers (IL-6 and TNF-α—R&D Systems, Minnesota, MN, U.S., hs-CRP—DRG GmbH, Marburg, Germany) were also included in the present analysis.

Results

Plasma sclerostin concentrations were significantly higher in HD men than women (2.61 vs. 1.88 ng/mL, p < 0.01). Patients with sclerostin levels above median were characterized by lower iPTH and IL-6, but higher cFGF23 and TNF-α (significantly only in men) concentrations. Plasma sclerostin concentration positively correlated with serum 25-OH-D (τ = 0.204), phosphorus (τ = 0.1482), and TNF-α (τ = 0.183) and inversely with iPTH (τ = − 0.255), alkaline phosphatase (τ = − 0.203), IL-6 (τ =− 0.201), and β-CTx (τ = − 0.099) levels. In multivariate regression analysis, variability of sclerostin levels was explained by sex and 25-OH-D and phosphorus levels.

Conclusion

Increased circulating sclerostin levels seem to reflect slower bone turnover in HD patients. Low levels of sclerostin are associated with vitamin D deficiency and good phosphates alignment.