In primary cells, overexpression of oncogenes such as RasV12 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/EBPβ is required for RasV12‐induced senescence. C/EBPβ−/− mouse embryo fibroblasts (MEFs) expressing RasV12 continued to proliferate despite unimpaired induction of p19ARF and p53, and lacked morphological features of senescent fibroblasts. Enforced C/EBPβ expression inhibited proliferation of wild‐type MEFs and also slowed proliferation of p19Arf−/− and p53−/− cells, indicating that C/EBPβ acts downstream or independently of p19ARF/p53 to suppress growth. C/EBPβ 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/EBPβ decreased expression of several E2F target genes and was associated with their promoters in chromatin immunoprecipitation assays, suggesting that C/EBPβ functions by repressing genes required for cell cycle progression. C/EBPβ is therefore a novel component of the RB:E2F‐dependent senescence program activated by oncogenic stress in primary cells.

The ancient, highly conserved, Wnt signaling pathway regulates cell fate in all metazoans. We have previously shown that combined null mutations of the specificity protein (Sp) 1/Klf-like zinc-finger transcription factors Sp5 and Sp8 (i.e., Sp5/8) result in an embryonic phenotype identical to that observed when core components of the Wnt/β-catenin pathway are mutated; however, their role in Wnt signal transduction is unknown. Here, we show in mouse embryos and differentiating embryonic stem cells that Sp5/8 are gene-specific transcriptional coactivators in the Wnt/β-catenin pathway. Sp5/8 bind directly to GC boxes in Wnt target gene enhancers and to adjacent, or distally positioned, chromatin-bound T-cell factor (Tcf) 1/lymphoid enhancer factor (Lef) 1 to facilitate recruitment of β-catenin to target gene enhancers. Because Sp5 is itself directly activated by Wnt signals, we propose that Sp5 is a Wnt/β-catenin pathway-specific transcripton factor that functions in a feed-forward loop to robustly activate select Wnt target genes.S ignaling pathways in multicellular organisms have evolved over millions of years to accommodate complex programs of tissue-specific gene expression. One such pathway, the Wnt/ β-catenin pathway, regulates gene expression by elevating the cytosolic levels of the transcription coactivator β-catenin (1). Stabilized β-catenin translocates to the nucleus, where it interacts with the DNA-bending, DNA-binding Tcf1 and Lef1 transcription factors (TFs), which subsequently replace Groucho/ Tcf3 repressor complexes on Wnt target gene enhancers (2). β-Catenin interacts with cell context-dependent cofactors (web. stanford.edu/group/nusselab/cgi-bin/wnt/) to associate with RNA polymerase II and the general transcription apparatus to activate transcription. However, the nature of the β-cateninTcf/Lef enhancer-binding protein complex and the mechanisms that facilitate its association with regulatory elements at Wnt target genes remain poorly understood.The formation of a Wnt signaling center during gastrulation is essential for animal development (3). Secreted Wnts emanating from the primitive streak regulate the fate of posterior progenitors, including the neuromesodermal progenitor (NMP), an embryonic cell that depends upon Wnt3a for self-renewal and mesodermal differentiation and that gives rise to the spinal cord, dermis, and musculoskeletal system of the trunk and tail (4-6). Embryos lacking Wnt3a, Ctnnb1 (β-catenin), T-cell factor (Tcf) 1 and lymphoid enhancer factor (Lef) 1, or specificity protein (Sp) 5 and Sp8 display similar severe posterior truncations caused by the loss of NMPs (7-10). These genes define a syn-phenotype group that, together with the genetic interactions observed between Sp5 and Wnt3a, suggests that Sp5/8 could be effectors of Wnt signaling. Sp5/8 are closely related to Sp1, which is one of the first identified eukaryotic TFs (11) and is frequently associated with the regulation of housekeeping genes. In contrast to the ubiquitously expressed Sp1, Sp5 expression is restricte...

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