CCAAT/enhancer Binding Protein-alpha (C/EBP-alpha) is a DNA-damage Inducible p53-regulated Mediator of the G1 Checkpoint.

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Title: CCAAT/enhancer Binding Protein-alpha (C/EBP-alpha) is a DNA-damage Inducible p53-regulated Mediator of the G1 Checkpoint.
Author: Yoon, Kyungsil
Advisors: Robert C. Smart, Committee Chair
Jun Ninomiya-Tsuji, Committee Member
Yoshiaki Tsuji, Committee Member
William K. Kaufmann, Committee Member
Abstract: CCAAT/enhancer binding proteins (C/EBPs) are members of basic leucine zipper class of transcription factors, and C/EBPα is abundantly expressed in epidermal keratinocytes. C/EBPα has been shown to play an important role in cell cycle arrest and/or differentiation in various cell types, and forced expression of C/EBPα blocks keratinocyte proliferation suggesting a cell cycle regulatory function of C/EBPα in keratinocytes. Skin is constantly challenged by environmental stressors including sunlight, and there is a strong association between human skin cancers and exposure to UVB radiation. We report that UVB radiation is a potent inducer of C/EBPα in human and mouse keratinocytes as well as in mouse skin in vivo. UVB irradiation of keratinocytes resulted in the transcriptional upregulation of C/EBPα mRNA producing >70 fold increase in C/EBPα protein. Treatment of keratinocytes with other DNA damaging agents such as MNNG, etoposide and bleomycin also induced C/EBPα, suggesting that the induction of C/EBPα is a general DNA damage response in keratinocytes. The UVB-induction of C/EBPα was temporarily accompanied by UVB-induced p53 accumulation implying a possible relationship between these two proteins. Caffeine, an inhibitor of ATM and ATR kinases, inhibited UVB-induction of C/EBPα as well as p53 increase, and C/EBPα promoter was activated by p53 and UVB irradiation. Exposure of p53-deficient or mutant p53 containing keratinocytes to UVB failed to induce C/EBPα demonstrating that p53 is required for UVB-induction of C/EBPα. Enforced expression of C/EBPα in keratinocytes inhibited cell proliferation suggesting C/EBPα negatively regulates cell growth. UVB induced a more rapid growth arrest in keratinocytes overexpressing C/EBPα. Furthermore, UVB irradiation of C/EBPα knockdown keratinocytes displayed a greatly attenuated DNA damage G1 checkpoint and this was associated with increased sensitivity to UVB-induced apoptosis. Taken together, our results identify C/EBPα as a novel p53-regulated DNA damage-inducible gene that has a critical function in the DNA damage G1 checkpoint response in keratinocytes.
Date: 2005-05-10
Degree: PhD
Discipline: Toxicology

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