C/EBPbeta cooperates with RB:E2F to implement Ras(V12)-induced cellular senescence.

In primary cells, overexpression of oncogenes such as Ras(V12) induces premature senescence rather than transformation. Senescence is an irreversible form of G1 arrest that requires the p19ARF/p53 and p16INK4a/pRB pathways and may suppress tumorigenesis in vivo. Here we show that the transcription factor C/EBPbeta is required for Ras(V12)-induced senescence. C/EBPbeta-/- mouse embryo fibroblasts (MEFs) expressing Ras(V12) continued to proliferate despite unimpaired induction of p19ARF and p53, and lacked morphological features of senescent fibroblasts. Enforced C/EBPbeta expression inhibited proliferation of wild-type MEFs and also slowed proliferation of p19Arf-/- and p53-/- cells, indicating that C/EBPbeta acts downstream or independently of p19ARF/p53 to suppress growth. C/EBPbeta was unable to inhibit proliferation of MEFs lacking all three RB family proteins or wild-type cells expressing dominant negative E2F-1 and, instead, stimulated their growth. C/EBPbeta decreased expression of several E2F target genes and was associated with their promoters in chromatin immunoprecipitation assays, suggesting that C/EBPbeta functions by repressing genes required for cell cycle progression. C/EBPbeta is therefore a novel component of the RB:E2F-dependent senescence program activated by oncogenic stress in primary cells.