Adenovirus E1A-Regulated Transcription Factor p120 E4F Inhibits Cell Growth and Induces the Stabilization of the cdk Inhibitor p21 WAF1

Abstract

ABSTRACT Adenovirus E1A proteins influence cell growth and phenotype through physical interactions with cellular proteins that regulate basic processes such as cell cycle progression, DNA synthesis, and differentiation. p120 E4F is a low-abundance cellular transcription factor that represses the adenovirus E4 promoter and is regulated by E1A, through a phosphorylation-induced reduction of its DNA binding activity, to permit activation of the E4 promoter during early infection. To determine the normal biological role of p120 E4F , we assessed its ability to influence fibroblast cell growth and transformation. p120 E4F suppressed NIH 3T3 fibroblast colony formation but had little effect when coexpressed with E1A and/or activated ras . Cells that overexpressed p120 E4F were inhibited in their ability to enter S phase, had elevated levels of the cdk inhibitor p21 WAF1 , and reduced cyclin D-cdk4/6 kinase activity. The increase of p21 WAF1 levels occurred through a p53-independent posttranscriptional mechanism that included a three- to fourfold increase in the half-life of p21 WAF1 protein. Coexpression of activated ras with p120 E4F stimulated cyclin D1 expression, elevated cyclin D-cdk4/6 kinase activity, and accelerated cell growth. These data suggest an important role for p120 E4F in normal cell division and demonstrate that p21 WAF1 can be regulated by protein turnover.