title: RasV12-mediated down-regulation of CCAAT/enhancer binding protein beta in immortalized fibroblasts requires loss of p19Arf and facilitates bypass of oncogene-induced senescence.
abstract: The transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) is involved in cellular responses to oncogenic and physiologic Ras signals. C/EBPbeta is required for premature senescence of primary mouse fibroblasts induced by expression of H-Ras(V12), demonstrating its role in oncogene-induced senescence. Here, we have investigated the mechanisms by which Ras inhibits proliferation of normal cells but transforms immortalized cells. We show that oncogenic Ras down-regulates C/EBPbeta expression in NIH 3T3 cells, which are immortalized by a deletion of the CDKN2A locus and, therefore, lack the p16(Ink4a) and p19(Arf) tumor suppressors. Ras(V12)-induced silencing of C/EBPbeta occurred at the mRNA level and involved both the Raf-mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase-ERK and phosphatidylinositol 3-kinase signaling pathways. Oncogenic Ras decreased C/EBPbeta expression in Ink4a/Arf(-/-) mouse embryo fibroblasts (MEF) but increased C/EBPbeta levels in wild-type MEFs. C/EBPbeta down-regulation in NIH 3T3 cells was reversed by expression of p19(Arf), but not of p53 or p16(Ink4a), highlighting a critical role for p19(Arf) in sustaining C/EBPbeta levels. Ectopic expression of p34 C/EBPbeta (LAP) inhibited Ras(V12)-mediated transformation of NIH 3T3 cells, suppressed their tumorigenicity in nude mice, and reactivated expression of the proapoptotic Fas receptor, which is also down-regulated by Ras. Our findings indicate that Cebpb gene silencing eliminates a growth inhibitory transcription factor that would otherwise restrain oncogenesis. We propose that C/EBPbeta is part of a p53-independent, p19(Arf)-mediated network that enforces Ras-induced cell cycle arrest and tumor suppression in primary fibroblasts.
authors.affiliation: Basic Research Laboratory, Center for Cancer Research, National Cancer Institute-Frederick, Frederick, Maryland, USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: Sebastian
authors.forename: Peter F
authors.initials: PF
authors.lastname: Johnson
mesh_terms: D000818:Animals; D022782:CCAAT-Enhancer-Binding Protein-beta / Q000096:biosynthesis / Q000235:genetics*; D002471:Cell Transformation, Neoplastic / Q000235:genetics / Q000378:metabolism; D016922:Cellular Senescence / Q000235:genetics; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000235:genetics* / Q000378:metabolism; D015536:Down-Regulation; D005347:Fibroblasts / Q000378:metabolism / Q000502:physiology; D011905:Genes, ras; D006801:Humans; D051379:Mice; D041681:NIH 3T3 Cells; D016159:Tumor Suppressor Protein p53 / Q000235:genetics; D019014:fas Receptor / Q000235:genetics; D018631:ras Proteins / Q000235:genetics* / Q000378:metabolism
chemical_list: D022782:CCAAT-Enhancer-Binding Protein-beta; C417743:Cdkn2a protein, mouse; D019941:Cyclin-Dependent Kinase Inhibitor p16; D016159:Tumor Suppressor Protein p53; D019014:fas Receptor; D018631:ras Proteins