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

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Abstract
The prostanoid prostacyclin (PGI2) inhibits aortic smooth muscle cell proliferation by blocking cell cycle progression from G1-to S-phase. However, the mechanism of this inhibition is poorly understood. We report here that the PGI2 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 Gi signaling in cicaprost action. We conclude that PGI2 inhibits proliferation of aortic smooth muscle cells by coordinately blocking CRE- and pocket protein-dependent cyclin A gene expression.

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