Stimulative effect of cadmium on prostaglandin E-2 production in primary mouse osteoblastic cells

We have reported that cadmium (Cd) stimulates bone resorption via prostagla ndin E-2 (PGE(2)). which is mainly produced in osteoblasts. Prostaglandin ( PGs) is regulated by arachidonic acid (AA) release by phospholipase A(2) (P LA(2)) and its conversion to PGs by cyclooxygenase (COX). In the present st udy, we investigated the possibility that Cd-induced PGE(2) synthesis was m ediated through PLA(2) or COX or both using primary mouse osteoblastic cell s in serum-free medium. Cd at 1 muM and above stimulated C-14-AA release fr om C-14-AA-prelabeled osteoblastic cells. PLA(2) activity of cytosolic frac tion in Cd-treated cells preferentially hydrolyzed AA at the Sn2 position o f phospholipids and was inhibited by arachidonyltri-fluoromethyl ketone (AA COCF3), an inhibitor of cytosolic PLA(2) (cPLA(2)). Cd at 1 muM and above i ncreased cPLA(2) activity and the level of constitutive cPLA(2) mRNA. Secre tory PLA(2) mRNA was not detected. On the other hand, Cd at 1 muM and above stimulated PGE(2) production and its production was inhibited by an inhibi tor of COX-2 (NS-398). Cd at 1 muM and above markedly stimulated COX-2 mRNA expression and slightly increased the level of COX-I mRNA. An inhibitor of COX-I (varelylsalicylic acid) did not affect Cd-induced PGE(2) production. In addition, Cd-induced PGE(2) synthesis was inhibited by AACOCF(3), On th e other hand, IL-1 alpha. an inducer of COX-2, did not stimulated PGE(2) pr oduction in present culture system. When IL-1 alpha- or Cd-treated cells we re incubated with AA for 10 minutes, IL-1 alpha -treated cells as well as C d-treated ones caused an increase in PGE(2) production. This suggests that the mechanism of Cd-induced PGE(2) production is different from that of IL- 1 alpha, which may require an activation of cPLA(2). From these results, it was found that Cd by itself stimulated PGE(2) production by two successive steps that Cd increased cPLA2 activity and then COX-2 induction.