Regulation of C/EBPbeta1 by Ras in mammary epithelial cells and the role of C/EBPbeta1 in oncogene-induced senescence.
Overexpression of Ras(V12) in MCF10A cells, an immortalized mammary epithelial cell line, leads to transformation of these cells. We demonstrate that this is accompanied by degradation of C/EBPbeta1. C/EBPbeta is a transcription factor in which three protein isoforms exist because of alternative translation at three in-frame methionines. When C/EBPbeta1 is expressed in MCF10A-Ras(V12) cells, immunoblot analysis reveals that C/EBPbeta1 is degraded in these cells. Treatment of MCF10A-Ras(V12)-C/EBPbeta1 cells with the cdk inhibitor roscovitine leads to stabilization of C/EBPbeta1. It has been previously shown that cdk2 phosphorylates C/EBPbeta on Thr235. We demonstrate that mutation of Thr235 to alanine in C/EBPbeta1 is sufficient to restore the stability of C/EBPbeta1 expression in MCF10A-Ras(V12) cells. Overexpression of Ras(V12) in primary cells induces senescence rather than transformation, thus suppressing tumorigenesis. C/EBPbeta is required for Ras(V12)-induced senescence in primary mouse embryonic fibroblasts. Upregulation of interleukin-6 (IL6) by C/EBPbeta has been shown to be necessary for oncogene-induced senescence, but the specific isoform of C/EBPbeta has not been investigated. We show that the C/EBPbeta1 isoform upregulates IL6 when introduced into normal fibroblasts. In addition, we show that C/EBPbeta1 induces senescence. Taken together, degradation of C/EBPbeta1 by Ras activation may represent a mechanism to bypass OIS.
Atwood
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AA
Department of Cancer Biology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
Sealy
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L
D001940:Breast; D022782:CCAAT-Enhancer-Binding Protein-beta; D022762:CCAAT-Enhancer-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D002460:Cell Line; D016922:Cellular Senescence / Q000187:drug effects*; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors; D004847:Epithelial Cells; D005347:Fibroblasts; D005786:Gene Expression Regulation; D006801:Humans; D007223:Infant; D015850:Interleukin-6 / Q000378:metabolism; D009154:Mutation; D015689:Oncogene Protein p21(ras) / Q000494:pharmacology*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D011687:Purines / Q000494:pharmacology; D000077546:Roscovitine; D015854:Up-Regulation; D018631:ras Proteins / Q000494:pharmacology*
D022782:CCAAT-Enhancer-Binding Protein-beta; D022762:CCAAT-Enhancer-Binding Proteins; C545012:CEBPB protein, human; D015850:Interleukin-6; D047428:Protein Kinase Inhibitors; D011687:Purines; D000077546:Roscovitine; D018844:Cyclin-Dependent Kinases; D015689:Oncogene Protein p21(ras); D018631:ras Proteins
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