Ras/ERK signaling pathway mediates activation of the p21WAF1 gene promoter in vascular smooth muscle cells by platelet-derived growth factor

We previously demonstrated that platelet-derived growth factor (PDGF) induces the cyclin-dependent kinase inhibitor p21/WAF1 promoter in vascular smooth muscle cells (VSMC) via activation of a Sp1 site in VSMC. In this report, the role and relevance of the signaling pathway in the transcriptional regulation of p21WAF1 in VSMC was examined. PDGF stimulated the expression of p21WAF1 in VSMC, as evidenced by Immunoblot and Northern blot analyses. Treatment with PD98059, a specific MEK inhibitor, and the transient expression of VSMC with DN-MEK1 plasmid effectively down-regulated PDGF-induced p21WAF1 expression and promoter activity, respectively. Furthermore, the transactivation of PDGF-stimulated Sp1 was inhibited by treatment with PD98059 and the transient expression of VSMC with the DN-MEK1 plasmid. Finally, the transient transfection of VSMC with a dominant negative Ras (RasN17) suppressed PDGF-induced ERK activity, p21WAF1 expression, and promoter activity. The overexpression of RasN17 also abolished PDGF-stimulated Sp1 activity. In conclusion, the findings herein presented indicate that the activation of the Ras/ERK pathway contributes to the induction of p21WAF1 expression in VSMC. In addition, the transcription factor Sp1 that is involved in the Ras/ERK-mediated control of p21WAF1 regulation in VSMC in response to PDGF has now been identified.