Prostaglandin E2 (PGE2) increases the number of rat bone marrow osteogenic stromal cells (BMSC) via binding the EP4 receptor, activating sphingosine kinase and inhibiting caspase activity

Prostaglandin E2 (PGE2) is bone-anabolic, i.e. stimulates bone formation and increases bone mass. In this study, we explored possible intracellular mechanisms of its increase of osteogenic cells in rat bone marrow. Adherent rat bone marrow cells were counted after 12–48 h or cultured for 21 days and mineralized nodules were counted. Also, apoptosis of marrow cells was measured after in vivo PGE2 injection.PGE2 (100 nM) increased 2–3 fold the number of adherent BMSC, an effect which was mediated via binding the EP4 receptor since it was mimicked by forskolin and 11-deoxy-prostaglandin E1 (PGE1) and was blocked by DDA and L-161982 (EP4 antagonist). PGE2 stimulated sphingosine kinase (SPK) activity since its effects were blocked by DMS (SPK inhibitor) and mimicked by SPP (SPK product). PGE2 reduced the activity of caspase-3 and -8 in BMSC and their inhibitors increased BMSC number and nodule formation. In vivo, PGE2 prevented the increase in the apoptosis of bone marrow cells caused by indomethacin.We propose that PGE2 exerts an anti-apoptotic effect on BMSC, thereby increasing their number and subsequent osteoblastic differentiation. Such an effect could explain how PGE2 stimulates bone formation in vivo.