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Ras^V12-Mediated Down-regulation of CCAAT/Enhancer Binding Protein β in Immortalized Fibroblasts Requires Loss of p19^Arf and Facilitates Bypass of Oncogene-Induced Senescence

Basic Research Laboratory, Center for Cancer Research, National Cancer Institute-Frederick, Frederick, Maryland

Requests for reprints: Peter F. Johnson, Basic Research Laboratory, Center for Cancer Research, National Cancer Institute-Frederick, Building 539, Room 122, Frederick, MD 21702-1201. Phone: 301-846-1627; Fax: 301-846-5991; E-mail: johnsopf{at}ncifcrf.gov.

Key Words: Ras • C/EBPβ • p19^Arf • oncogenesis • tumor suppression

The transcription factor CCAAT/enhancer binding protein β (C/EBPβ) is involved in cellular responses to oncogenic and physiologic Ras signals. C/EBPβ 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/EBPβ 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/EBPβ 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/EBPβ expression in Ink4a/Arf^–/– mouse embryo fibroblasts (MEF) but increased C/EBPβ levels in wild-type MEFs. C/EBPβ 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/EBPβ levels. Ectopic expression of p34 C/EBPβ (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/EBPβ is part of a p53-independent, p19^Arf-mediated network that enforces Ras-induced cell cycle arrest and tumor suppression in primary fibroblasts. [Cancer Res 2009;69(6):2588–98]