C/EBPβ Deficiency Enhances Keratinocyte Apoptosis after UVB-Induced DNA Damage via Regulation of the Type I IFN and TNF Responses

The epidermis is routinely subjected to DNA damage induced by solar (UVB) radiation. In addition to activating canonical DNA damage responses such as cycle cell checkpoints and DNA repair, UVB-induced DNA damage can also activate additional signaling pathways including inflammatory responses. The pathways activated downstream of UVB-induced DNA damage have a critical role in determining cellular survival to UVB radiation. Here we report that loss of CCAAT-enhancer/binding protein β (C/EBPβ) in mouse keratinocytes results in enhanced UVB-induced apoptosis through activation of extrinsic apoptosis genes cleaved caspase-8 and tBid. RNAseq and Ingenuity Pathway Analysis of UVB-treated C/EBPβ-/- primary keratinocytes revealed an enrichment of inflammatory signaling pathways, including the type I interferon (IFN-I) pathway as the most enriched pathway. Numerous IFN-I stimulated genes were up-regulated in UVB-treated C/EBPβ-/- keratinocytes, including genes that regulate extrinsic apoptosis. Inhibition of the interferon / receptor or the associated kinase Tyk2 greatly reduced cell death in UVB-exposed C/EBPβ deficient keratinocytes, demonstrating the dependence of IFN signaling in C/EBPβ regulated apoptosis. The apoptosis inducing cytokine, tumor necrosis factor alpha (TNF-α) was identified as one of the most significant upstream regulators activated in UVB exposed C/EBPβ-/- keratinocytes compared to UVB exposed wild type control. UVB exposed C/EBPβ-/- keratinocytes displayed increased expression of TNF-α and the enhanced apoptosis in C/EBPβ-/- keratinocytes was suppressed by a TNF-α neutralizing antibody. Our results indicate that loss of C/EBPβ enhances activation of a non-canonical UVB DNA damage response pathway involving interferon and TNF signaling to induce keratinocyte cell death.