Abstract 3872: The role and regulation of C/EBPalpha in the DNA damage-induced G1 checkpoint
Authors: Hall Jonathan R. , Thompson Elizabeth Anderson , Smart Robert C. 
Published: February 2011

Abstract
The ability of cells to correctly respond to DNA damage is a vital step in maintaining genome stability and suppressing tumorigenesis. The C/EBP family of transcription factors is involved in the regulation of cell proliferation, growth arrest, differentiation, inflammation, senescence and energy metabolism in a range of cell types. Previously, we have shown that C/EBPα is highly inducible by DNA damage through a p53-dependent mechanism and has a novel role in the DNA damage G1 checkpoint. Furthermore, cells deficient in C/EBPα display an impaired G1 checkpoint and inappropriate entry into S-phase following DNA damage; suggesting loss of C/EBPα could result in genome/checkpoint instability. While it is known that C/EBPα has a critical role in the DNA damage-induced G1 checkpoint, the molecular mechanism through which it regulates the DNA damage G1 checkpoint is unknown. Moreover, it is not known whether C/EBPα itself is regulated by post-translational modifications (PTMs) downstream of DNA damage.
We report that C/EBPα undergoes PTMs involving phosphorylation in response to UVB-induced DNA damage. These modifications are in part dependent of the activity of the DNA damage checkpoint kinase Chk1 and occur in both cultured mouse keratinocytes and in vivo mouse epidermis that have been exposed to UVB. Co-immunoprecipitation studies revealed that C/EBPα forms a complex with E2F1 and this interaction is increased following UVB-induced DNA damage. We observed that ectopic expression of C/EBPα in keratinocytes inhibits E2F1 transcriptional activity. These findings indicate that C/EBPα can function downstream of DNA damage to regulate E2F1and induce a cell cycle arrest independent of C/EBPα transcriptional activity.
Additionally, we observe that C/EBPα functions in the canonical DNA damage-induced G1 checkpoint response through regulation of the CDK inhibitor p21. Ablation of C/EBPα in primary mouse keratinocytes or in vivo mouse epidermis results in severely impaired induction of p21 in response to DNA damage. Furthermore, we show that the diminished p21 protein levels in C/EBPα deficient mice following exposure to UVB occur without any changes in p21 mRNA levels. We propose that C/EBPα is required to stabilize p21 in response to DNA damage; enabling p21 to accumulate to levels necessary to elicit the DNA damage G1 checkpoint response.
In conclusion, we report that C/EBPα undergoes post-translational modifications downstream of DNA damage and has a multi-faceted role in the DNA damage-induced G1 checkpoint response. We propose C/EBPα inhibits cell cycle progression in response to DNA damage through suppression of E2F1 transcriptional activity and stabilization of the CDK inhibitor p21 protein. This study describes a novel mechanism for the function and regulation of C/EBPα in the DNA damage-induced G1 checkpoint.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3872.