Prostacylin Receptor Activation Inhibits Proliferation of Aortic Smooth Muscle Cells by Regulating cAMP Response Element-Binding Protein- and Pocket Protein-Dependent Cyclin A Gene Expression

The prostanoid prostacyclin (PGI 2 ) inhibits aortic smooth muscle cell proliferation by blocking cell cycle progression from G 1 -to S-phase. However, the mechanism of this inhibition is poorly understood. We report here that the PGI 2 mimetic, cicaprost, inhibits the induction of cyclin A and activation of the cyclin A promoter in primary and established rodent aortic smooth muscle cells. The inhibition of cyclin A gene expression is associated with a block in cyclin E-cdk2 activity and phosphorylation of both the retinoblastoma protein and p107. Inactivation of pocket proteins with human papilloma virus protein E7 partially, but not completely, restored cyclin A promoter activity in cicaprost-treated cells. Complementary studies showed that occupancy of the cAMP response element (CRE) is required for efficient activation of the cyclin A promoter in aortic smooth muscle cells, that the CRE is primarily occupied by the CRE-binding protein (CREB) and phospho-CREB, and that cicaprost blocks the binding of CREB and phospho-CREB to the cyclin A promoter CRE. Treatment with pertussis toxin reversed the inhibitory effects of cicaprost on CRE occupancy, cyclin E-cdk2 activity, and S phase entry, suggesting the involvement of G i signaling in cicaprost action. We conclude that PGI 2 inhibits proliferation of aortic smooth muscle cells by coordinately blocking CRE- and pocket protein-dependent cyclin A gene expression.