Paradoxical upregulation of tumor suppressor protein p53 in serum-stimulated vascular smooth muscle cells: A novel negative-feedback regulatory mechanism

Background - The proliferative response of vascular smooth muscle cells (VSMCs) to various growth stimuli is critical for atherosclerosis and postangioplasty restenosis. Although tumor suppressor protein p53 plays a critical role in the elimination of cancerous cells, recent genetic studies have indicated that it also protects against atherosclerosis and restenosis. Methods and Results - We examined the levels of p53 protein in normal VSMCs before and after serum stimulation. The p53 protein levels increased robustly on stimulation. Upregulated p53 protein was capable of binding to the p53 consensus sequence, as shown by electrophoretic mobility shift assay. In addition, p53 upregulation was associated with increases in the transcript and protein levels of p21^WAF1/CIP1 and Bax, as shown by real-time reverse transcriptase-polymerase chain reaction and Western blot analysis, respectively. Furthermore, the upregulation of p21^WAF1/CIP1 and Bax was followed by cell-cycle arrest and apoptosis induction, as shown by 5-bromo-2′-dUTP incorporation assay and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling staining, respectively. Finally, double-staining analyses showed that the majority of p53-expressing cells also expressed p21^WAF1/CIP1 and Bax proteins. Conclusions - p53 protein expression in quiescent VSMCs is paradoxically increased by application of a growth stimulus. Through the mediation of p21^WAF1/CIP1 and Bax, the induced p53 protein negatively regulates the growth of dividing VSMCs, thereby minimizing the inappropriate accumulation of VSMCs. Therefore, p53 may be a negative regulator of VSMC growth.