Ca2+-ATPase inhibitors and Ca2+-ionophore induce osteoclast-like cell formation in the cocultures of mouse bone marrow cells and calvarial cells.

Osteoclasts which derive from hemopoietic cells are multinucleated cells responsible for bone resorption. We found that cyclopiazonic acid (CPA), thapsigargin (TG), and 2,5-di-(t-butyl)-1,4-hydroquinone (BHQ) induced osteoclast-like cell (OCL) formation in cocultures of mouse calvaria-derived stromal cells and hemopoietic cells such as bone marrow cells and spleen cells. OCLs induced by these compounds showed typical characteristics of osteoclasts such as tartrate-resistant acid phosphatase activity and pit forming activity. These compounds are known as endoplasmic reticulum (ER)/sarcoplasmic reticulum (SR) Ca2+-ATPase inhibitors that increase intracellular Ca2+ levels by inhibiting Ca2+-ATPase activity located in the membrane of ER/SR. Ca2+-ionophores such as ionomycin which increase intracellular Ca2+ levels also stimulated OCL formation in the cocultures. Differentiation of hemopoietic cells into OCLs induced by these compounds required the presence of calvarial cells. These results indicate that an increase of intracellular Ca2+ levels may be a part of signaling pathways to induce osteoclast differentiation in the presence of calvarial cells.