Cadmium-induced calcium release and prostaglandin E2 production in neonatal mouse calvaria are dependent on cox-2 induction and protein kinase C activation.

Abstract

The mechanisms by which cadmium (Cd) causes skeletal impairment have not been fully clarified. Release of calcium from neonatal mouse calvaria in organ culture is stimulated by submicromolar concentrations of Cd, an effect that is associated with increased production of prostaglandin E2 (PGE2). The prostaglandin-synthesising enzyme cyclooxygenase (cox) exists in two forms, one constitutive (cox-1) and the other inducible (cox-2). Cox-2 can be induced by mitogenic stimuli and inflammatory cytokines, such as parathyroid hormone (PTH), interleukin-1alpha and tumour necrosis factor-alpha. Cd potently activates protein kinase C (PKC). which in turn induces cox-2 production in several cell types. Our aim was to determine whether Cd-induced Ca release and PGE2 production in neonatal mouse calvaria involve induction of cox-2 and, if so, to ascertain whether that effect is mediated by activation of PKC. Cd dose-dependently stimulated Ca release from cultured neonatal mouse calvaria, with a maximal effect at 0.4-0.8 microM. Different sensitivity was observed to Cd-induced Ca release between two breeds of mice suggesting that the susceptibility to Cd may be genetically determined. Dexamethasone (10 microM) added to the culture medium abolished the Ca releasing effect of Cd, an effect not overcome by addition of arachidonic acid (10 microM). The cox-2-selective inhibitors NS-398 and DFU and the less selective inhibitor meloxicam, potently impeded Cd-induced Ca release (IC50 of 1 nM, 41 nM and 7 nM, respectively) and calvarial production of PGE2. Cd-induced and phorbol 12-myristate 13-acetate (PMA; 20 nM)-induced Ca release was inhibited by the PKC inhibitor calphostin C (0.5 microM) and by NS-398. The effects of PMA and Cd on Ca release were not additive, suggesting that both operated via the PKC pathway. We suggest that Cd-induced Ca release from neonatal mouse calvaria in culture depends on induction of cox-2 that occurs via the PKC signalling pathway.