Phorbol myristate acetate stimulates osteoclast formation in 1α, 25-dihydroxyvitamin D3-primed mouse embryonic calvarial cells by a Prostaglandin-dependent mechanism

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Abstract
Our previous study provided a novel assay system utilizing devitalized bone slices for study of the differentiation of osteoclast progenitors into preosteoclasts and mature osteoclasts among calvarial cells of mouse embryos. Using this assay system, we examined the effect of phorbol myristate acetate (PMA) on osteoclast formation as assessed by the appearance of tartrate-resistant acid phosphatase (TRAP)-positive cells and bone resorption lacunae. PMA alone was directly unable to induce the appearance of TRAP-positive cells and bone resorption lacunae of calvarial bone cells of mouse embryos. However, PMA markedly stimulated increases in the number of TRAP-positive cells and area of the resorption lacunae of the calvarial cells when the bone cells were primed by 1α,25-(OH)2D3. This stimulatory effect of PMA was dose dependent. H-7, having relatively high affinity for protein kinase C, strongly inhibited in a dose-dependent fashion the stimulatory effect of PMA on the bone resorption of the hormone-primed calvarial cells. We also examined the involvement of prostaglandin in this stimulatory effect of PMA. Indomethacin, a cyclooxygenase inhibitor, markedly abolished the stimulatory effect of PMA on the bone resorption of the calvarial cells. PMA stimulated prostaglandin E2 (PGE2) production by the calvarial cells primed with 1α, 25-(OH)2D3 in a dose-dependent fashion. However, the PMA stimulation of the PGE2 production was significantly inhibited by H-7 and also by indomethacin. Furthermore, we observed that the addition of PGE2 to the calvarial cells primed with 1α, 25-(OH)2D3 for 1 or 3 days resulted in an increased number of TRAP-positive cells and increased bone resorption. This stimulatory effect of the exogenous PGE2 was also observed in the presence of indomethacin. These results strongly suggest that PMA stimulates osteoclast formation in 1α, 25-(OH)2D3-primed calvarial cells by a prostaglandin-dependent mechanism.
Funding Information
  • The Chugai Pharmaceutical Co.