Evaluation of signal transduction mechanisms for the mitogenic effects of prostaglandin E2 in normal human bone cells in vitro.

Prostaglandin E2 (PGE2) is one of the most potent stimulators of bone formation in vivo. In these studies, we investigated the mechanism(s) underlying PGE2 effects on human bone formation by evaluating the effects of PGE2 on normal human bone cell (HBC) proliferation in vitro. Cell proliferation of normal HBCs was increased by PGE2 as measured by increased [3H]thymidine incorporation after 18 h and increased cell number after 48 h of treatment. The effect of PGE2 to stimulate cell proliferation was biphasic, with a maximum stimulation between 0.01 and 1.0 nM PGE2 in different experiments. At higher concentrations of PGE2 (0.1 microM), HBC proliferation was inhibited. Signal transduction for PGE2 has been reported to include both protein kinase A (PKA) and protein kinase C (PKC) pathways. In these studies, concentrations of PGE2 which stimulated cell proliferation did not increase cyclic adenosine monophosphate (cAMP) production. However, higher concentrations of PGE2 increased cAMP production (7- to 12-fold at 1-10 microM) and inhibited cell proliferation. Because stimulators of PKC, such as phorbol esters, have been reported to stimulate cell proliferation, the action of PKC inhibitors were tested. Both staurosporine and sangivamysin (PKC inhibitors) totally abrogated the effect of PGE2 to stimulate cell proliferation. Additional studies revealed that PGE2 increased 45Ca uptake in a dose-dependent manner with a peak response occurring between 1 and 10 nM PGE2 concentrations in different experiments. Furthermore, when the calcium channel blocker, verapamil, was added to HBC cultures treated with PGE2, the stimulation of 45Ca uptake and cell proliferation by PGE2 was completely blocked. These data suggest that PGE2 increases cell proliferation through activation of a verapamil-sensitive calcium channel. In conclusion, these data are consistent with a model in which stimulation of HBC proliferation by low doses of PGE2 is mediated by an enhancement of phospholipase C, which results in both an increase in PKC activity and an increase in intracellular calcium influx.