A selective EP₄ receptor antagonist abrogates the stimulation of osteoblast recruitment from bone marrow stromal cells by prostaglandin E ₂ in vivo and in vitro

Recent evidence indicates that systemic administration of PGE₂ increases bone formation and bone mass via activation of the EP₄ receptor. Previously, we demonstrated that osteoblastic recruitment from rat bone marrow stromal cells (BMSC) is a major mechanism for the anabolic effect of PGE₂. In this study, we used a selective EP₄ antagonist to test if the stimulation of osteoblast differentiation from rat BMSC in vitro and in vivo involves the EP₄ receptor. In vitro, PGE₂ (100 nM) increased nodule formation and alkaline phosphatase (ALP) activity in cultures of rat BMSC 1.5- to 2-fold. These effects were abolished by the EP₄ antagonist at 10^-6 M but not 10 ^-9 M. Furthermore, PGE₂ increased the number of surviving adherent BMSC by ∼225% and the EP₄ antagonist prevented this effect as well. The antagonist had no effect on basal levels of nodule formation and adherent cell number. In vivo, daily systemic administration of PGE₂ at 6 mg/kg for 2 weeks increased cancellous bone area (by ∼50%) and increased nodule formation (measured as mineralized area) in ex vivo stromal cultures by ∼50%. Pre-administration of the EP₄ antagonist at 10 mg/kg abrogated both the increase in bone mass as well as the increase in nodule formation. These data indicate that PGE₂ stimulates osteoblastic commitment of BMSC via activation of the EP₄ receptor.