Abstract 736: Inducible cAMP Early Repressor Inhibits Growth of Vascular Smooth Muscle Cell
Intracellular cAMP regulates a broad spectrum of cellular functions. An increase in intracellular cAMP activates protein kinase A (PKA) resulting in an activation of many cellular proteins including transcription factors designated cAMP response element binding protein (CREB) family. Although inducible cAMP early repressor (ICER) belongs to the CREB family, ICER serves as a transcription repressor due to lack of transactivation domain. The activity of ICER depends on its expression level rather than phosphorylation. Because the role of ICER in the vascular remodeling process has not been reported, we examined whether ICER affects growth of vascular smooth muscle cells (VSMCs) and neointmal formation after balloon injury. Semi-quantitative RT-PCR and Western blot analysis showed that the expression of ICER was increased in beraprost (a prostaglandin I2 analogue)-stimulated VSMC in a time- and dose-dependent manner. The induction of ICER was inhibited by pretreatment with H89, a PKA inhibitor, suggesting that PKA mediates the induction of ICER expression. Beraprost suppressed platelet-derived growth factor-induced thymidine incorporation into VSMC, which was reversed by transfection of short interfering (si) RNA for ICER, not by scramble RNA. Introduction of siRNA for ICER further increased beraprost-induced cAMP response element (CRE)-dependent gene promoter activity, suggesting that ICER negatively regulates CRE-dependent gene transcription induced by beraprost. Overexpression of ICER by an adenovirus vector (AdICER) but not beraprost induced apoptosis of VSMC in vitro. Infection of AdICER after balloon injury of rat carotid artery attenuated neointimal formation (intima/media ratio) by 50% compared with overexpression of LacZ. The number of TUNEL positive cells was increased, and the number of Ki-67 positive cells, which indicates cell proliferation, was decreased in the neointima of the ICER-transduced artery. These results show that ICER negatively regulates survival and proliferation of VSMC, and plays a critical role in beraprost-mediated suppression of VSMC growth. ICER may be an important endogenous inhibitor of vascular proliferation and remodeling, and could be a target molecule to treat restenosis and atherosclerosis.