Prostaglandin E 2 stimulates DNA synthesis by a cyclic AMP‐independent pathway in osteoblastic clone MC3T3‐E1 cells

The effect of prostaglandin E 2 (PGE 2 ) on osteoblastic cell proliferation was investigated using osteoblastic clone MC3T3‐E1 cells cultured in serum‐free medium. PGE 2 at 2 μg/ml increased the number of the cells by 2 days after its addition. PGE 2 raised the level of DNA synthesis in a dose‐related fashion after a constant lag time, the maximal effect being at 2–10 μg/ml and the level about fourfold over that of the control at 36 hr after its addition. However, at low doses (below 0.2 μg/ml), PGE 2 rather depressed DNA synthesis. Isobutyl methylxanthine counteracted the stimulation of DNA synthesis by PGE 2 , and forskolin depressed the synthesis, which was inversely correlated with increasing intracellular cAMP content. These results indicate that an increase in cAMP content inhibits DNA synthesis. In addition, 2′,5′‐dideoxyadenosine did not negate the stimulatory effect of PGE 2 on DNA synthesis, suggesting that PGE 2 increases DNA synthesis, probably via a pathway different from the adenylate cyclase/cAMP system. Moreover, at a high dose, PGE 2 stimulated both the production and degradation of cAMP; the elevationof cAMP content was rapidly depressed by the stimulated degradation system. Consequently, thestimulatory effect of PGE 2 on DNA synthesis would be released from the inhibition by cAMP, resulting in an increase in DNA synthesis. Taken together with data from our previous reports, these results indicate that PGE 2 enhances both the proliferation and differentiation of osteoblastic cells in vitro, which are probably mediated by two different second messengers dependent on the concentration of PGE 2 .