Abstract

AimsThe local concentration of extracellular Ca2+ ([Ca2+]o) in bone microenvironment is accumulated during bone remodeling. In the present study we investigated whether elevating [Ca2+]o induced store-operated calcium entry (SOCE) in primary rat calvarial osteoblasts and further examined the contribution of elevating [Ca2+]o to osteoblastic proliferation.MethodsCytosolic Ca2+ concentration ([Ca2+]c) of primary cultured rat osteoblasts was detected by fluorescence imaging using calcium-sensitive probe fura-2/AM. Osteoblastic proliferation was estimated by cell counting, MTS assay and ATP assay. Agonists and antagonists of calcium-sensing receptors (CaSR) as well as inhibitors of phospholipase C (PLC), SOCE and voltage-gated calcium (Cav) channels were applied to study the mechanism in detail.ResultsOur data showed that elevating [Ca2+]o evoked a sustained increase of [Ca2+]c in a dose-dependent manner. This [Ca2+]c increase was blocked by TMB-8 (Ca2+ release inhibitor), 2-APB and BTP-2 (both SOCE blockers), respectively, whereas not affected by Cav channels blockers nifedipine and verapamil. Furthermore, NPS2143 (a CaSR antagonist) or U73122 (a PLC inhibitor) strongly reduced the [Ca2+]o-induced [Ca2+]c increase. The similar responses were observed when cells were stimulated with CaSR agonist spermine. These data indicated that elevating [Ca2+]o resulted in SOCE depending on the activation of CaSR and PLC in osteoblasts. In addition, high [Ca2+]o significantly promoted osteoblastic proliferation, which was notably reversed by BAPTA-AM (an intracellular calcium chelator), 2-APB, BTP-2, TMB-8, NPS2143 and U73122, respectively, but not affected by Cav channels antagonists.ConclusionsElevating [Ca2+]o induced SOCE by triggering the activation of CaSR and PLC. This process was involved in osteoblastic proliferation induced by high level of extracellular Ca2+ concentration.

Highlights

  • Bone is constantly remodeling and maintaining homeostasis between formation and resorption

  • 1z(x=EC50 )n zYmin, where is the response, Ymax is the asymptotic maximum, Ymin is the asymptotic minimum, x is the extracellular calcium concentration and n is the Thapsigargin induced store-operated calcium entry (SOCE) in rat calvarial osteoblasts Firstly, we checked the ability of generating SOCE in rat calvarial osteoblasts with endoplasmic reticulum (ER) Ca2+-pump blocker thapsigargin (TG), a drug widely used to test SOCE

  • This Ca2+ entry was strongly inhibited by the application of potent SOCE blockers 2-APB (25 mM) [35,36] or BTP-2 (YM-58483, 20 mM) [37] during the high [Ca2+]c plateau evoked by adding 2 mM CaCl2 (Figure 1C–F)

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Summary

Introduction

Bone is constantly remodeling and maintaining homeostasis between formation and resorption. Osteoblasts play a pivotal role in bone formation and mineralization by secreting bone matrix components and providing factors essential for osteoclast differentiation [4,5,6]. The resorptive action of osteoclasts results in a local increase of extracellular calcium concentration ([Ca2+]o) which can reach levels as high as 40 mM [7]. This high level of [Ca2+]o has been suggested to regulate bone formation by stimulating osteoblastic proliferation, chemotaxis, differentiation and mineralization [8,9,10]. In vitro studies showed that high [Ca2+]o promoted proliferation in a number of osteoblast cell lines including rat calvarial osteoblasts [10]

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