Yin Yang-1 Inhibits Vascular Smooth Muscle Cell Growth and Intimal Thickening by Repressing p21 WAF1/Cip1 Transcription and p21 WAF1/Cip1 -Cdk4-Cyclin D1 Assembly

Vascular injury initiates a cascade of phenotype-altering molecular events. Transcription factor function in this process, particularly that of negative regulators, is poorly understood. We demonstrate here that the forced expression of the injury-inducible GLI-Krüppel zinc finger protein Yin Yang-1 (YY1) inhibits neointima formation in human, rabbit and rat blood vessels. YY1 inhibits p21 WAF1/Cip1 transcription, prevents assembly of a p21 WAF1/Cip1 -cdk4-cyclin D1 complex, and blocks downstream pRb Ser249/Thr252 phosphorylation and expression of PCNA and TK-1. Conversely, suppression of endogenous YY1 elevates levels of p21 WAF1/Cip1 , PCNA, pRb Ser249/Thr252 and TK-1, and increases intimal thickening. YY1 binds Sp1 and prevents its occupancy of a distinct element in the p21 WAF1/Cip1 promoter without YY1 itself binding the promoter. Additionally, YY1 induces ubiquitination and proteasome-dependent degradation of p53, decreasing p53 immunoreactivity in the artery wall. These findings define a new role for YY1 as both an inducer of p53 instability in smooth muscle cells, and an indirect repressor of p21 WAF1/Cip1 transcription, p21 WAF1/Cip1 -cdk4-cyclin D1 assembly and intimal thickening.