C/EBPα expression is partially regulated by C/EBPβ in response to DNA damage and C/EBPα-deficient fibroblasts display an impaired G1 checkpoint

We observed that CCAAT/enhancer-binding protein (C/EBP)α is highly inducible in primary fibroblasts by DNA-damaging agents that induce strand breaks, alkylate and crosslink DNA as well as those that produce bulky DNA lesions. Fibroblasts deficient in C/EBPα (C/EBPα−/−) display an impaired G1 checkpoint as evidenced by an inappropriate entry into the S-phase in response to DNA damage, and these cells also display an enhanced G1/S transition in response to mitogens. The induction of C/EBPα by DNA damage in fibroblasts does not require p53. Electrophoretic mobility shift assay (EMSA) analysis of nuclear extracts prepared from ultraviolet B (UVB)- and N-methyl-N′-nitro-N-nitrosoguanidine (MNNG)-treated fibroblasts showed increased binding of C/EBPβ to a C/EBP consensus sequence and chromatin immunoprecipitation (ChIP) analysis also showed increased C/EBPβ binding to the C/EBPα promoter. To determine whether C/EBPβ has a function in the regulation of C/EBPα, we treated C/EBPβ−/− fibroblasts with UVB or MNNG. We observed that C/EBPα induction was impaired in both UVB- and MNNG-treated C/EBPβ−/− fibroblasts. Our study shows a novel function for C/EBPβ in the regulation of C/EBPα in response to DNA damage and provides definitive genetic evidence that C/EBPα has a critical role in the DNA damage G1 checkpoint.