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Vol. 60, Issue 1, 36-41, July 2001
Department of Bone Biology and Osteoporosis Research, Merck
Research Laboratories, West Point, Pennsylvania (M.M., S.H., C.T.L.,
G.S., S.B.R., G.A.R.); and Merck Frosst, Kirkland, Quebec, Canada
(M.L., M.G., S.H., N.L., N.S., D.S., K.M.M., R.Y.)
Prostaglandin (PG) E2 is a potent inducer of cortical
and trabecular bone formation in humans and animals. Although the bone anabolic action of PGE2 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 EP4, one of the
PGE2 receptors, on PGE2-induced bone formation
in vivo. We first determined the ability of EP4A, an
EP4-selective ligand, to act as an antagonist.
PGE2 increases intracellular cAMP and suppresses apoptosis
in the RP-1 periosteal cell line. Both effects were reversed by
EP4A, suggesting that EP4A acts as an
EP4 antagonist in the cells at concentrations consistent
with its in vitro binding to EP4. We then examined the
effect of EP4 on bone formation induced by PGE2
in young rats. Five- to 6-week-old rats were treated with
PGE2 (6 mg/kg/day) in the presence or absence of
EP4A (10 mg/kg/day) for 12 days. We found that treatment
with EP4A suppresses the increase in trabecular bone volume
induced by PGE2. 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
PGE2 in rats is mediated by the EP4 receptor.
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