Role of endogenous PGE2 in osteoblastic functions of a clonal osteoblast-like cell, MC3T3-E1

MC3T3-E1 cells actively synthesized and released prostaglandin E2 (PGE2) during culture. PGE2 release was minimal on day 9 and gradually increased with culture up to day 27. DNA content gradually increased until day 27. Alkaline phosphatase (ALP) activity increased up to day 15 and decreased thereafter. In contrast to the decrease in ALP activity, calcium accumulation increased rapidly after day 21, possibly due to mineralization by the cells. Indomethacin, a cyclooxygenase inhibitor, blocked PGE2 production completely at concentrations higher than 0.3 mumol/L. In the presence of indomethacin (3 mumol/L), DNA content was slightly decreased on day 27. Furthermore, ALP activity on day 15 was greater than that of the control and this high activity was maintained until day 27. However, calcium accumulation was not affected by the addition of indomethacin. These results suggest that endogenous PGE2 down-regulates ALP activity and slightly stimulates the proliferation of MC3T3-E1 cells as an autocrine mediator, although it does not directly influence the cells' mineralizing activity.