p53, p21, and MDM2 Involvement in Proliferation and Apoptosis in an In Vitro Model of Conditionally Immortalized Human Vascular Smooth Muscle Cells

Using an in vitro model of a conditionally immortalized cell line, we investigated how human vascular smooth muscle cells (VSMCs) are affected by the expression of simian virus 40 (SV40) large T antigen (LT antigen), which binds to cell cycle regulators, such as the tumor suppressor protein p53. Cells were obtained after infection of saphenous vein–derived VSMCs with a nonreplicative retroviral vector containing a temperature-sensitive (ts) mutant of SV40 LT antigen and were shown to have maintained some characteristics and responses of VSMCs. Under permissive temperature conditions (36°C), the increased rate of cell proliferation was shown to be associated with expression of LT antigen and with LT-antigen binding to and inactivation of p53. p53 inactivation failed to block apoptosis induced by serum withdrawal or by UV irradiation. Downregulation of LT-antigen expression at the nonpermissive temperature (39°C) was shown to be associated with growth arrest, increased expression of the cell cycle inhibitor p21, increased MDM2-promoter activity, and differential expression of MDM2 gene products, suggesting that p53-induced transcription/transactivation may be involved in VSMC cell cycle control but not necessarily apoptosis. The established SMC line HVTs-SM1 may be a useful model for the study of processes involved in myointimal hyperplasia and cellular aging, as well as for the study of cell cycle control in general. (Arterioscler Thromb Vasc Biol. 2000;20:636-644.)