The prostacyclin receptor induces human vascular smooth muscle cell difRecent studies of COX-2 inhibitors suggest that the balance between thromboxane and prostacyclin is a critical factor in cardiovascular homeostasis. Disruption of prostacyclin signaling by genetic deletion of the receptor or by pharmacological inhibition of COX-2 is associated with increased atherosclerosis and restenosis after injury in animal models, and adverse cardiovascular events in clinical trials (Vioxx(R)). Human vascular smooth muscle cells (VSMC) in culture exhibit a dedifferentiated, migratory, proliferative phenotype, similar to what occurs following arterial injury. We report that the prostacyclin analog iloprost induces differentiation of VSMC from this synthetic, proliferative phenotype, to a quiescent, contractile phenotype. Iloprost induced expression of smooth muscle-specific differentiation markers, including smooth muscle myosin heavy chain (SM-MHC), calponin, h-caldesmon and smooth muscle alpha-actin, as determined by western blotting and RT-PCR analysis. Iloprost activated cAMP/PKA signaling in human VSMC, and the cell permeable cAMP analog 8-Br-cAMP mimicked the iloprost-induced differentiation. Both myristoylated Protein Kinase A Inhibitor Amide 14-22 (myrPKI) (specific PKA inhibitor) as well as ablation of the catalytic subunits of PKA by small interfering RNA, opposed the upregulation of contractile markers induced by iloprost. These data suggest that iloprost modulates VSMC phenotype via Gs activation of the cAMP/PKA pathway. These studies reveal regulation of VSMC differentiation as a potential mechanism for the cardiovascular protective effects of prostacyclin. This provides important mechanistic insights into the induction of cardiovascular events with the use of selective COX-2 inhibitors.