Possible involvement of p38 MAP kinase in prostaglandin E1-induced ALP activity in osteoblast-like cells

Prostaglandins are now recognized to be important regulators for both bone formation and resorption. Among them, prostaglandin E₁ (PGE ₁) has been reported to stimulate cAMP accumulation and to induce alkaline phosphatase (ALP) activity, a marker of differentiation, in osteoblast-like cells. Recently, we have shown that p38 mitogen-activated protein (MAP) kinase pathway regulates ALP activity in response to activation of Gi protein-coupled receptors in mouse osteoblast-like MC3T3-E1 cells (Suzuki et al., Endocrinology 140 (1999) 3177). In the present study, we investigated whether p38 MAP kinase is involved in ALP activation by PGE₁ in MC3T3-E1 osteoblast-like cells. PGE₁ dose-dependently enhanced ALP activities in the concentration range between 1 nM and 1 μM in MC3T3-E1 cells. SB203580, a specific inhibitor of p38 MAP kinase, blocked the increase in ALP activity induced by PGE₁. Further analysis with western blotting suggested that PGE₁ induced an increase in tyrosine (Tyr) phosphorylation of p38 MAP kinase. Both Bt₂CAMP, a permeable analogue of cAMP, and forskolin, which directly activates adenylate cyclase, also induced an increase in Tyr phosphorylation of p38 MAP kinase. H-89, a potent inhibitor of protein kinase A (PKA), significantly suppressed PGE ₁-induced Tyr phosphorylation of p38 MAP kinase. The results of this study suggest that PGE₁ stimulates p38 MAP kinase through the activation of PKA, resulting in the enhancement of ALP activity.