C/EBPβ suppresses keratinocyte autonomous type 1 IFN response and p53 to increase cell survival and susceptibility to UVB-induced skin cancer

Abstract

p53 is activated by DNA damage and oncogenic stimuli to regulate senescence, apoptosis and cell-cycle arrest, which are essential to prevent cancer. Here, we utilized UVB radiation, a potent inducer of DNA damage, p53, apoptosis and skin cancer to investigate the mechanism of CCAAT/enhancer binding protein-β (C/EBPβ) in regulating p53-mediated apoptosis in keratinocytes and to test whether the deletion of C/EBPβ in epidermis can protect mice from UVB-induced skin cancer. UVB-treatment of C/EBPβ skin conditional knockout (CKOβ) mice increased p53 protein levels in epidermis and enhanced p53-dependent apoptotic activity 3-fold compared with UVB-treated control mice. UVB increased C/EBPβ levels through a p53-dependent pathway and stimulated the formation of a C/EBPβ-p53 protein complex; knockdown of C/EBPβ increased p53 protein stability in keratinocytes. These results suggest a p53-C/EBPβ feedback loop, whereby C/EBPβ, a transcriptional target of a p53 pathway, functions as a survival factor by negatively regulating p53 apoptotic activity in response to DNA damage. RNAseq analysis of UVB-treated CKOβ epidermis unexpectedly revealed that type 1 interferon (IFN) pathway was the most highly enriched pathway. Numerous pro-apoptotic interferon stimulated genes were upregulated including some known to enhance p53 apoptosis. Our results indicate that p53 and IFN pathways function together in response to DNA damage to result in the activation of extrinsic apoptosis pathways and caspase 8 cleavage. Last, we observed CKOβ mice were resistant to UVB-induced skin cancer. Our results suggest that C/EBPβ represses apoptosis through keratinocyte autonomous suppression of the type 1 IFN response and p53 to increase cell survival and susceptibility to UVB-induced skin cancer.

C/EBPβ deficiency enhances the keratinocyte innate immune response to direct activators of cytosolic pattern recognition receptors

“…Together, these results uncover a previously unknown function for C/EBPβ in regulating basal keratinocyte expression of Ifnβ and a subset of ISGs. Earlier studies in UVB-treated mouse epidermis revealed an increase in ISGs 22 ; the current results indicate that the increase in most ISGs is independent of UVB treatment and represents an intrinsic change in the innate immune system in keratinocytes.…”

“…mouse epidermis and regressing skin tumors. 22,24 However, these previous studies relied on DNA damage and; the current results indicate that the increase in most ISG is independent of UVB treatment and represents an intrinsic change in the innate immune system in keratinocytes.…”

“…C/EBPβ has been shown to regulate cell survival in response to DNA damage, toxicants or oncogenic stress. [22][23][24][25][26][27] Inducing cell death and eliminating infected Caspase-8 is critical in numerous cell death pathways including extrinsic apoptosis and is activated by cleavage. [28][29][30] Two active caspase-8 pools can be generated: a membrane associated pool of caspase-8 known as p43 and a cytosolic pool of casapase-8 known as p18.…”

“…C/EBPβ (also known as nuclear factor induced by IL-6 (NF-IL-6) has been shown to positively regulate numerous cell stress response pathways mediated by IL-6, TNFα, and IFNs. 22,24,32,33 IFNα, β, and-γ have also been shown to induce C/EBPβ expression and enhance C/EBPβ's transcriptional activity in macrophages. [34][35][36] Previous research indicating that C/EBPβ is a pro-inflammatory gene is in contrast with our reported findings that indicate that C/EBPβ is a suppressor of the keratinocyte type I IFN response.…”

Bellidifolin Inhibits Proliferation of A549 Cells by Regulating STAT3/COX-2 Expression and Protein Activity

“…STAT3 can also indirectly activate COX-2 by activating the transcription factor C/EBP- β . Studies have demonstrated that C/EBP- β has an important function in the tumorigenesis and development of a number of malignancies, such as prostate [ 44 ], gastric [ 45 ], and skin [ 46 ] cancers. In this regard, saikosaponin D, an active triterpene saponin, appears to inhibit COX-2 expression by downregulating phospho-STAT3 and C/EBP- β .…”