Involvement of mitogen-activated protein kinases and protein kinase C in cadmium-induced prostaglandin E2 production in primary mouse osteoblastic cells

We previously reported that cadmium (Cd) induced prostaglandin E 2 (PGE 2 ) biosynthesis through the activation of cytosolic phospholipase A 2 (cPLA 2 ) and induction of cyclooxygenase 2 (COX-2) in primary mouse osteoblastic cells. In the present study, we further investigated the mechanism of PGE 2 production by Cd focusing on the main mitogen-activated protein kinase (MAPK) subfamilies that mediate prostaglandin synthesis, extracellular signal-regulated kinase (ERK1/2 MAPK), c-jun-amino-terminal kinase (JNK MAPK) and p38 MAPK, and protein kinase C (PKC) which is activated by Cd in several kinds of cells. Cd at 2 μM and above stimulated PGE 2 production in osteoblastic cells and its production was inhibited by the kinase-specific inhibitors PD98059, SB203580, curcumin, and calphostin C. Calphostin C also inhibited the production of PGE 2 by phorbol 12-myristate 13-acetate (PMA), which is a potent activator of PKC. PD98059 inhibited PGE 2 production stimulated by PMA as well as Cd, indicating that activation of PKC by ERK1/2 MAPK was necessary for Cd-stimulated PGE 2 production. Moreover, Cd stimulated the phosphorylation of these three MAPKs, and inhibition of the phosphorylation of ERK1/2 MAPK by calphostin C was also observed. On the other hand, Cd was found to phosphorylate cPLA 2 and the phosphorylation was inhibited by PD98059, indicating that cPLA 2 was activated by Cd through ERK1/2 MAPK and released arachidonic acid (AA), a substrate of COX-2, from membranous phospholipids. From these results, it was suggested that activation of each of the ERK1/2, p38, and JNK MAPK cascades in addition to that of PKC and cPLA 2 played an important role in the Cd-stimulated biosynthesis of PGE 2 in mouse osteoblastic cells.