Arterial remodeling in response to pathological insult is a complex process that depends in part on the balance between vascular cell apoptosis and proliferation. Studies in experimental models suggest that HO-1 mediates neointimal formation while limiting lumen stenosing, indicating a differential effect on vascular endothelial (EC) and smooth muscle cells (SMC). We investigated the effect of HO-1 expression on cell cycle progression in EC and SMC. The addition of SnMP (10 muM), an inhibitor of HO activity, to EC or SMC for 24 h, resulted in significant abnormalities in DNA distribution and cell cycle progression compared to cells treated with the HO-1 inducers, heme (10 muM) or SnCl2 (10 muM). SnMP increased G(1) phase and decreased S and G(2)/M phases in EC while heme or SnCl2 decreased G(1) phase, but increased S and G(2)/M phases (p<0.05). Opposite effects were obtained in SMC. SnMP decreased G(1) phase and increased S and G(2)/M phases while heme or SnCl2 increased G(1) phase but decreased S and G(2)/M phases (p<0.05). Our data demonstrate that HO-1 regulates the cell cycle in a cell-specific manner; it increases EC but decreases SMC cycle progression. The mechanisms underlying the HO-1 cell-specific effect on cell cycle progression within the vascular wall are yet to be explored. Nevertheless, these findings suggest that cell-specific targeting of HO-1 expression may provide a novel therapeutic strategy for the treatment of cardiovascular diseases. (C) 2002 Elsevier Science (USA). All rights reserved.