C/EBPα regulates CRL4Cdt2-mediated degradation of p21 in response to UVB-induced DNA damage to control the G1/S checkpoint

The bZIP transcription factor, C/EBP is highly inducible by UVB and other DNA damaging agents in keratinocytes. C/EBP-deficient keratinocytes fail to undergo cell cycle arrest in G(1) in response to UVB-induced DNA damage and mice lacking epidermal C/EBP are highly susceptible to UVB-induced skin cancer. The mechanism through which C/EBP regulates the cell cycle checkpoint in response to DNA damage is unknown. Here we report untreated C/EBP-deficient keratinocytes have normal levels of the cyclin-dependent kinase inhibitor, p21, however, UVB-treated C/EBP-deficient keratinocytes fail to up-regulate nuclear p21 protein levels despite normal up-regulation of Cdkn1a mRNA levels. UVB-treated C/EBP-deficient keratinocytes displayed a 4-fold decrease in nuclear p21 protein half-life due to the increased proteasomal degradation of p21 via the E3 ubiquitin ligase CRL4(Cdt2). Cdt2 is the substrate recognition subunit of CRL4(Cdt2) and Cdt2 mRNA and protein levels were up-regulated in UVB-treated C/EBP-deficient keratinocytes. Knockdown of Cdt2 restored p21 protein levels in UVB-treated C/EBP-deficient keratinocytes. Lastly, the failure to accumulate p21 in response to UVB in C/EBP-deficient keratinocytes resulted in decreased p21 interactions with critical cell cycle regulatory proteins, increased CDK2 activity, and inappropriate entry into S-phase. These findings reveal C/EBP regulates G(1)/S cell cycle arrest in response to DNA damage via the control of CRL4(Cdt2) mediated degradation of p21.