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 phosphatase type 1 and 2A, on prostaglandin E₁ (PGE₁)-induced alkaline phosphatase (ALP) activity in osteoblast-like MC3T3-E1 cells. PGE₁ increased ALP activity dose dependently in the range between 10 nM and 0.3 μM in these cells. The pretreatment with okadaic acid enhanced the PGE₁-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 PGE₁-induced ALP activity. Tautomycin, an another inhibitor of protein phosphatase type 1 and 2A, also enhanced the PGE₁-induced ALP activity. PGE₁ stimulated cAMP accumulation dose dependently in the range between 10 nM and 0.3 μM. However, PGE₁ had no effect on the formation of inositol phosphates. Okadaic acid did not affect the PGE₁-induced cAMP accumulation. Okadaic acid dose dependently enhanced the dibutyryl cAMP-induced ALP activity. These results strongly suggest that protein phosphatase type 1 and/or 2A act as a regulator of ALP activity at a point downstream from protein kinase A in osteoblast-like cells.