OKADAIC ACID ENHANCES PROSTAGLANDIN E1-INDUCED ALKALINE-PHOSPHATASE ACTIVITY IN OSTEOBLAST-LIKE CELLS - REGULATION AT A POINT DOWNSTREAM FROM PROTEIN-KINASE-A

We examined the effect of okadaic acid, an inhibitor of protein phosph atase type 1 and 2A, on prostaglandin E(1) (PGE(1))-induced alkaline p hosphatase (ALP) activity in osteoblast-like MC3T3-E1 cells. PGE(1) in creased ALP activity dose dependently in the range between 10 nM and 0 .3 mu M in these cells. The pretreatment with okadaic acid enhanced th e PGE(1)-induced ALP activity in a dose-dependent manner in the range between 0.1 and 5 nM. On the other hand, 1-norokadaone, a less potent analogue of okadaic acid, had no effect on the PGE(1)-induced ALP acti vity. Tautomycin, an another inhibitor of protein phosphatase type 1 a nd 2A, also enhanced the PGE(1)-induced ALP activity. PGE(1) stimulate d cAMP accumulation dose dependently in the range between 10 nM and 0. 3 mu M. However, PGE(1) had no effect on the formation of inositol pho sphates. Okadaic acid did not affect the PGE,-induced cAMP accumulatio n. Okadaic acid dose dependently enhanced the dibutyryl cAMP-induced A LP activity. These results strongly suggest that protein phosphatase t ype 1 and/or 2A act as a regulator of ALP activity at a point downstre am from protein kinase A in osteoblast-like cells.