OKADAIC ACID REVERSES THE INHIBITORY EFFECT OF PROTEIN-KINASE-C ON ALKALINE-PHOSPHATASE ACTIVITY IN OSTEOBLAST-LIKE CELLS

We previously reported that fetal calf serum-induced alkaline phosphat ase activity is suppressed due to the activation of protein kinase C i n osteoblast-like MC3T3-E1 cells (Miwa et al. (1991) Bone Miner, 14, 1 5-25; Kotoyori et al. (1994) Horm. Metab. Res. 26, 116-118). In the pr esent study, we examined the effect of okadaic acid, a potent and spec ific inhibitor of protein phosphatase type 1 and 2A, on fetal call ser um-induced alkaline phosphatase activity in MC3T3-E1 cells. The pretre atment with okadaic acid enhanced the fetal calf serum-induced alkalin e phosphatase activity in a dose-dependent manner in the range between 0.1 and 5 nM. 1-Norokadaone, a less potent analogue of okadaic acid, had little effect on the fetal calf serum-induced alkaline phosphatase activity. Okadaic acid partially reversed the suppression of fetal ca lf serum-induced alkaline phosphatase activity by 12-O-tetradecanoylph orbol-13-acetate, a protein kinase C activator. The effect of okadaic acid was dose-dependent in the range between 0.1 and 5 nM. The pattern s of the dose-dependency of both okadaic acid effects on fetal calf se rum-induced alkaline phosphatase activity and on the suppression by 12 -O-tetradecanoylphorbol-13-acetate were similar. These results strongl y suggest that protein phosphatase type 1 and/or 2A act as a regulator of alkaline phosphatase activity at a point downstream from protein k inase C in osteoblast-like cells.