Stimulative Effect of Cadmium on Prostaglandin E2Production in Primary Mouse Osteoblastic Cells

We have reported that cadmium (Cd) stimulates bone resorption via prostaglandin E2 (PGE2), which is mainly produced in osteoblasts. Prostaglandin (PGs) is regulated by arachidonic acid (AA) release by phospholi- pase A2 (PLA2) and its conversion to PGs by cyclooxygen- ase (COX). In the present study, we investigated the possi- bility that Cd-induced PGE2 synthesis was mediated through PLA2 or COX or both using primary mouse osteo- blastic cells in serum-free medium. Cd at 1 mM and above stimulated 14 C-AA release from 14 C-AA-prelabeled osteo- blastic cells. PLA2 activity of cytosolic fraction in Cd- treated cells preferentially hydrolyzed AA at the Sn2 posi- tion of phospholipids and was inhibited by arachidonyltri- fluoromethyl ketone (AACOCF3), an inhibitor of cytosolic PLA2 (cPLA2). Cd at 1 mM and above increased cPLA2 activity and the level of constitutive cPLA 2 mRNA. Secre- tory PLA 2 mRNA was not detected. On the other hand, Cd at 1 mM and above stimulated PGE 2 production and its production was inhibited by an inhibitor of COX-2 (NS- 398). Cd at 1 mM and above markedly stimulated COX-2 mRNA expression and slightly increased the level of COX-1 mRNA. An inhibitor of COX-1 (varelylsalicylic acid) did not affect Cd-induced PGE 2 production. In addi- tion, Cd-induced PGE 2 synthesis was inhibited by AA- COCF 3 , On the other hand, IL-1a, an inducer of COX-2, did not stimulated PGE 2 production in present culture sys- tem. When IL-1a- or Cd-treated cells were incubated with AA for 10 minutes, IL-1a-treated cells as well as Cd-treated ones caused an increase in PGE2 production. This suggests that the mechanism of Cd-induced PGE2 production is dif- ferent from that of IL-1a, which may require an activation of cPLA2. From these results, it was found that Cd by itself stimulated PGE2 production by two successive steps that Cd increased cPLA2 activity and then COX-2 induction.