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

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.