Prostaglandin Receptor EP₄ Mediates the Bone Anabolic Effects of PGE₂

Abstract

Prostaglandin (PG) E(2) is a potent inducer of cortical and trabecular bone formation in humans and animals. Although the bone anabolic action of PGE(2) is well documented, the cellular and molecular mechanisms that mediate this effect remain unclear. This study was undertaken to examine the effect of pharmacological inactivation of the prostanoid receptor EP(4), one of the PGE(2) receptors, on PGE(2)-induced bone formation in vivo. We first determined the ability of EP(4)A, an EP(4)-selective ligand, to act as an antagonist. PGE(2) increases intracellular cAMP and suppresses apoptosis in the RP-1 periosteal cell line. Both effects were reversed by EP(4)A, suggesting that EP(4)A acts as an EP(4) antagonist in the cells at concentrations consistent with its in vitro binding to EP(4). We then examined the effect of EP(4) on bone formation induced by PGE(2) in young rats. Five- to 6-week-old rats were treated with PGE(2) (6 mg/kg/day) in the presence or absence of EP(4)A (10 mg/kg/day) for 12 days. We found that treatment with EP(4)A suppresses the increase in trabecular bone volume induced by PGE(2). This effect is accompanied by a suppression of bone formation indices: serum osteocalcin, extent of labeled surface, and extent of trabecular number, suggesting that the reduction in bone volume is due most likely to decreased bone formation. The pharmacological evidence presented here provides strong support for the hypothesis that the bone anabolic effect of PGE(2) in rats is mediated by the EP(4) receptor.

Stimulation of bone formation and prevention of bone loss by prostaglandin E EP4 receptor activation

“…The role of this receptor in bone formation was suggested previously only indirectly by the use of a limited repertory of EP-acting compounds (18). We have found that activation of EP4 induced callus formation on the femur of mice and restored the volume of cancellous bone in OVX or immobilized rats.…”

The EP4 receptor antagonist, L-161,982, blocks prostaglandin E2-induced signal transduction and cell proliferation in HCA-7 colon cancer cells

“…Although recent studies suggest a role for the EP2 receptor in PGE 2 -induced cellular response, a caveat with these studies is the use of cell culture systems in which the EP2 receptor is ectopically expressed [40]. Our studies, which are in agreement with several other reports [36], indicate an important role for the EP4 receptor for the following reasons: 1) PGE1-OH (relatively selective EP4 agonist), but not 17-phenyltrinor PGE 2 (EP1/EP3 agonist) induced ERK phosphorylation; 2) pretreatment of HCA-7 cells with AH6809 (EP1/EP2 antagonist) did not have any effect on PGE 2 -induced ERK phosphorylation; 3) L-161,982 at concentrations that does not bind to either EP1 or EP2 receptors [18] abrogated PGE 2 -induced ERK phosphorylation. Nevertheless, the relative contribution of the EP2 and EP4 receptors to human colon cancer will require further study.…”

“…U0126 and H-89 were purchased from Cell signaling technology (Beverly, MA) and Calbiochem (La Jolla, CA), respectively. A selective EP4 receptor antagonist L-161,982 [18] was from Merck-Frosst laboratories (Merck-Frosst, Canada). Antibodies against phospho-ERK (Tyr 304), and Egr-1 (sc-110) were from Santa Cruz Biotechnology Inc. (Santa Cruz, CA).…”

Effects of selective prostaglandins E2 receptor agonists on cultured calvarial murine osteoblastic cells

“…Recent studies using animals with deletion of either the EP2 or EP4 receptors or using selective agonists for these receptors have suggested that both EP2 and EP4 can mediate anabolic responses to PGE 2 (4)(5)(6)(7)(8)(9)(10), although the EP1 receptor may also be involved (11). Activation of EP2 and EP4 receptors increases cAMP and can also induce COX-2, which can result in auto-amplification of PGE 2 signaling (12).…”