Expression of the cyclin-dependent kinase inhibitor, p21Waf-1/Cip-1/Sdi-1, in human vascular smooth muscle cells in the proliferating state.

Excess proliferation of vascular smooth muscle cells (SMC) is an important aspect of atherogenesis. Cell-cycle regulatory proteins such as cyclin and cyclin-dependent kinases are vital for cell-cycle progression. To understand the role of the cyclin-dependent kinase inhibitor, p21Waf-1/Cip-1/Sdi-1 (p21Waf-1), on human atherogenesis, we tested p21Waf-1 expression in human atherosclerotic lesions and cultured SMC. Immunohistochemical staining revealed that SMC in neointimal lesions expressed p21Waf-1. No evidence of the p53 protein could be detected. By Western blotting, cultured SMC obtained from a neonate revealed that a higher level of p21Waf-1 expression correlated with a higher expression of proliferating cell nuclear antigen and a lower expression of the contractile protein than that observed in cells obtained from aged donors. When the phenotypes of SMC were changed by modification of serum concentration and cell densities, p21Waf- expression was maximal in serum-stimulated SMC at low cell densities despite the low expression of p53. Furthermore, serum stimulation transiently increased the p21Waf-1 expression of quiescent SMC, which was synchronized with the transition from the G0/G1 to the S phase as well as with cyclin D1 expression. These results may suggest that the negative regulator of cell-cycle progression also plays a role in regulating the appropriate cell-cycle progression in SMC. Growth stimuli may induce both growth-promoting and growth-inhibitory factors in SMC. The balance between these two opposing factors may play an important role in the determination of cell-cycle progression.