Prostaglandin E1and F2αStimulate Differentiation and Proliferation, Respectively, of Clonal Osteoblastic MC3T3-E1 Cells by Different Second Messengersin Vitro*

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Abstract
The effect of several prostaglandins (PGs) on osteoblastic cells was investigated using clone MC3T3-E1 under serum-free conditions. PGA1, A2, B1, and B2 had little effect on intracellular cAMP, alkaline phosphatase (ALP) activity, and DNA synthesis in the cells. At 4–2000 ng/ml, PGE1 among PG analogs tested had a dose-dependent stimulatory effect on ALP activity in the cells, and this effect was amplified by isobutyl methylxanthine. Also, PGE1 strongly augmented the amount of intracellular cAMP over the same concentration range. However, PGEj had little effect on ornithine decarboxylase activity and DNA synthesis, and at high doses it rather depressed DNA synthesis. Furthermore, PGEi did not affect the intracellular cGMP level. The effect of PGE, on the cells closely mimics that of forskolin, suggesting that the PG stimulates the differentiation of the osteoblastic cells predominantly via the stimulation of adenylate cyclase. In contrast with PGE1, PGF2α strongly increased ornithine decarboxylase activity and DNA synthesis in the cells in a dose-related fashion at low concentrations (4–100 ng/ml), at which concentrations it had little effect on the intracellular cAMP or cGMP level and depressed ALP activity. Moreover, PGF2α depressed the stimulatory effect of PGE1 on ALP activity but did not affect the elevation of cAMP level by PGE1. The accumulation of inositol phosphates was greatly increased by PGF2α in the concentration range effective in stimulating DNA synthesis, but was increased little by PGE1, suggesting that PGF2α is a potent stimulator of phosphatidyl inositol turnover in the cells. In addition, A23187, a Ca ionophore, alone did not influence the DNA synthesis, but the effects of tetradecanoyl phorbol acetate, a direct activator of protein kinase C, were very similar to those of PGF2α. Moreover, the stimulation of DNA synthesis or the inhibition of ALP activity by PGF2α was partially counteracted by H-7, a strong inhibitor of protein kinase C. These results suggest that PGF2α stimulates the proliferation of osteoblastic cells predominantly through the phosphatidyl inositol turnover system following in part the activation of protein kinase C. Our data presented here indicate that PGEi and PGF2α are closely involved in the differentiation and proliferation, respectively, of osteoblasts in vitro and that their action may be mediated by second messengers which differ from each other. (Endocrinology121: 1966–1974, 1987)

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