Impaired Bone Resorption to Prostaglandin E2 in Prostaglandin E Receptor EP4-knockout Mice

Abstract Prostaglandin E2(PGE2) acts as a potent stimulator of bone resorption. In this study, we first clarified in normal ddy mice the involvement of protein kinase A and induction of matrix metalloproteinases (MMPs) in PGE2-induced bone resorption, and then identified PGE receptor subtype(s) mediating this PGE2 action using mice lacking each subtype (EP1, EP2, EP3, and EP4) of PGE receptor. In calvarial culture obtained from normal ddy mice, both PGE2and dibutyryl cyclic AMP (Bt2cAMP) stimulated bone resorption and induced MMPs including MMP-2 and MMP-13. Addition of an inhibitor of protein kinase A, H89, or an inhibitor of MMPs, BB94, significantly suppressed bone-resorbing activity induced by PGE2. In calvarial culture from EP1-, EP2-, and EP3-knockout mice, PGE2 stimulated bone resorption to an extent similar to that found in calvaria from the wild-type mice. On the other hand, a marked reduction in bone resorption to PGE2 was found in the calvarial culture from EP4-knockout mice. The impaired bone resorption to PGE2 was also detected in long bone cultures from EP4-knockout mice. Bt2cAMP greatly stimulated bone resorption similarly in both wild-type and EP4-knockout mice. Induction of MMP-2 and MMP-13 by PGE2 was greatly impaired in calvarial culture from EP4-knockout mice, but Bt2cAMP stimulated MMPs induction similarly in the wild-type and EP4-knockout mice. These findings suggest that PGE2 stimulates bone resorption by a cAMP-dependent mechanism via the EP4 receptor.