Retinoid Ligands Inhibit Cell Proliferation by Modulating Cell Cycle Regulators in Human Coronary Smooth Muscle Cells
Retinoids inhibit vascular smooth muscle cell proliferation in vitro and intimal hyperplasia in vivo in rats. We examined the underlying mechanism of the antiproliferative effect of retinoids on human coronary smooth muscle cells (CASMC). The retinoids tested included: RAR ligands, all-trans retinoic acid (atRA) and Ethyl-p-[(E)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl)-l-propenyl]-benzoic acid (TTNPB); a RXR and RAR ligand 9-cis retinoic acid (9cRA); and the potent RXRα- ligand AGN4204. All retinoids inhibited DNA synthesis in CASMC stimulated with platelet derived growth factor and insulin (IC50: TTNPB 0.063μM, atRA 0.123μM, AGN4204 0.461μM, 9cRA 1.529μM, all p<0.01, n=6). All retinoids substantially attenuated retinoblastoma protein phosphorylation which is required for CASMC proliferation (maximal inhibition: TTNPB and atRA, 2x10-7M; AGN4204 and 9cRA, 2x10-6M all p<0.01, n=3). None of the retinoids affected the expression of CDKs 2, 4, or 6 or cyclin E. TTNPB, atRA and AGN4204 inhibited the mitogenic induction of cyclin D1 (maximal inhibition: TTNPB, 2x10-9M; atRA, 2x10-7 M; AGN4204, 2x10-6M, all p<0.01, n=3), whereas 9cRA had no effect (n=3). All retinoids significantly attenuated mitogen-induced downregulation of CDKI p27Kip1, a major negative regulator of G1 cyclin/CDK activity (maximal effect: TTNPB and atRA, 2x10-7M; AGN4204 and 9cRA, 2x10-6M, all p<0.01, n=3). The present study demonstrate that retinoids have antiproliferative activity by modulating the G1→S cell cycle regulators in human CASMC, which may provide a novel therapeutic target for proliferative vascular diseases.