Abstract
Zinc is an essential trace element required for bone remodelling process, but its role in such process remains to be elucidated. In particular, inconsistent results have been reported on the effect of Zn on osteoclastic responses, and supplement of receptor activator of nuclear factor kappa-B ligand (RANKL) factors has been commonly adopted. Co-culture is a suitable approach to elucidating the role of Zn in bone remodelling process, by better imitating the cellular environment as the presence of osteoblasts plays critical role in modulating osteoclastic functions. In this study, zinc-substituted HA coatings have been deposited using a liquid precursor plasma spraying process at two different concentrations (1, 2 wt.%). The effect of zinc substitution on osteoblastic and osteoclastic differentiation has been studied in vitro. In particular, a cultivation regime was designed to first induce osteoblastic differentiation of rat bone marrow stromal cells (BMSCs) for 14 days, and then induce osteoclastic differentiation of osteoclast-like precursor RAW 264.7 cells through the aid of the osteoblasts formed for additional 14 days, in the absence of the external addition of RANKL. The results showed that Zn substitution moderately promoted the BMSC differentiation into the osteoblasts and reduced the osteoclastic activity in early time (1 day co-culture). However, promotion of the osteoclastic activity were observed at later stages, as indicated by the significantly enhanced expressions of trap5b and IL-1 (8- and 15-day co-culture) and moderate stimulation of the nucleus integration and formation of the multinucleated cells (14-day co-culture). Such stimulating effect of the osteoclastic activity was absent under mono-culture of RAW 264.7 cell, with simple RANKL supplementation. The results suggest that both the zinc and the presence of MSC/osteoblast play profound and highly interacted roles on osteoclast differentiation and activity, which is critical in modulating the bone remodelling process.
Highlights
Bone mass is maintained by repeating cycle of destruction and rebuilding to keep the balance between bone resorption and formation [1]
Shifting of the (2 2 1), (1 1 2) and (3 0 0) diffraction peaks were clearly observed for the zinc-substituted HA coatings especially in the blowup spectra (Fig. 2b), indicating the successful incorporation of Zn in the HA lattice and the formation of the solid solution
Zn has a stimulating effect on bone formation through stimulating cell proliferation, alkaline phosphatase (ALP) activity, collagen synthesis [23] and mineralization in vitro and in vivo
Summary
Bone mass is maintained by repeating cycle of destruction and rebuilding to keep the balance between bone resorption and formation [1]. The regulation of hormones and other molecules secreted by osteoblast (OB) and osteoclast (OC) control the dynamic and complex process of bone formation and reconstruction [2, 3]. Zn is an essential trace element to metabolism function. Zn deficiency reduces bone weight, delays bone metabolism and results in retardation of bone growth [4,5,6]. Zinc has a stimulatory effect on bone formation and mineralization in vitro and in vivo [7, 8]. Zn stimulated cell proliferation and differentiation, as well as protein synthesis in osteoblastic MC3T3-E1 cells [9,10,11]. The cellular mechanism of zinc function in OCs has not been fully understood. Zn has been generally reported to inhibit osteoclastic differentiation [12, 13].
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