A selective EP 4 receptor antagonist abrogates the stimulation of osteoblast recruitment from bone marrow stromal cells by prostaglandin E 2 in vivo and in vitro

Abstract

Recent evidence indicates that systemic administration of PGE 2 increases bone formation and bone mass via activation of the EP 4 receptor. Previously, we demonstrated that osteoblastic recruitment from rat bone marrow stromal cells (BMSC) is a major mechanism for the anabolic effect of PGE 2. In this study, we used a selective EP 4 antagonist to test if the stimulation of osteoblast differentiation from rat BMSC in vitro and in vivo involves the EP 4 receptor. In vitro, PGE 2 (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 EP 4 antagonist at 10 −6 M but not 10 −9 M. Furthermore, PGE 2 increased the number of surviving adherent BMSC by ∼225% and the EP 4 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 PGE 2 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 EP 4 antagonist at 10 mg/kg abrogated both the increase in bone mass as well as the increase in nodule formation. These data indicate that PGE 2 stimulates osteoblastic commitment of BMSC via activation of the EP 4 receptor.