Cytochrome P-450 is an important bioactivation-detoxification system in vivo. Its expression is regulated by foreign chemicals and dietary factors, and lipids have been found to regulate its gene expression. We showed previously that prostaglandin E-2 (PGE(2)), a fatty acid metabolite, down-regulates cytochrome P-450 2B1 (CYP 2B1) expression induced by phenobarbital. The objective of the present study was to determine whether PGE(2) type 2 receptor (EP2)-which is coupled to Gs-protein when bound by PGE(2), leading to cAMP production-is involved in this down-regulation. We also determined the possible roles of EP2 downstream pathways in this down-regulation. We used a primary rat hepatocyte culture model in which EP, was shown to be present to study this question. The intracellular cAMP concentration in primary rat hepatocytes was significantly higher after treatment with 1 muM PGE(2) than after treatment with 0, 0.01, or 0.1 muM PGE(2). Butaprost, an EP2 agonist, down-regulated CYP 2B1 expression in a dose-dependent manner. SQ22536, an adenylate cyclase inhibitor, reversed the down-regulation by PGE(2) as did H-89, a protein kinase A inhibitor. These results suggest that EP, and the downstream pathways of cAMP and protein kinase A are involved in the down-regulation of CYP 2B1 expression by PGE(2) in the presence of phenobarbital. (C) 2004 Elsevier Inc. All rights reserved.