C/EBPβ cooperates with RB:E2F to implement RasV12-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 G(1) arrest that requires the p19(ARF)/p53 and p16(INK4a)/pRB pathways and may suppress tumorigenesis in vivo. Here we show that the transcription factor C/EBP beta is required for Ras(V12)-induced senescence. C/EBP beta(-/-) mouse embryo fibroblasts (MEFs) expressing Ras(V12) continued to proliferate despite unimpaired induction of p19(ARF) and p53, and lacked morphological features of senescent fibroblasts. Enforced C/EBP beta expression inhibited proliferation of wild-type MEFs and also slowed proliferation of p19(Arf-/-) and p53(-/-) cells, indicating that C/EBP beta acts downstream or independently of p19(ARF)/p53 to suppress growth. C/EBP beta 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/EBP beta decreased expression of several E2F target genes and was associated with their promoters in chromatin immunoprecipitation assays, suggesting that C/EBP beta functions by repressing genes required for cell cycle progression. C/EBP beta is therefore a novel component of the RB: E2F-dependent senescence program activated by oncogenic stress in primary cells.