Contribution of C/EBP proteins to Epstein-Barr virus lytic gene expression and replication in epithelial cells

The contribution of C/EBP proteins to Epstein-Barr virus (EBV) lytic gene expression and replication in epithelial cells was examined. Nasopharyngeal carcinoma cell lines constitutively expressed C/EBP beta and had limited C/EBP alpha expression, while the AGS gastric cancer cell line expressed significant levels of both C/EBP alpha and C/EBP beta. Induction of the lytic cycle in EBV-positive AGS/BX1 cells with phorbol ester and sodium butyrate treatment led to a transient stimulation of C/EBP beta expression and a prolonged increase in C/EBP alpha expression. In AGS/BX1 cells, endogenous C/EBP alpha and C/EBP beta proteins were detected associated with the ZTA and oriLyt promoters but not the RTA promoter. Electrophoretic mobility shift assays confirmed binding of C/EBP proteins to multiple sites in the ZTA and oriLyt promoters. The response of these promoters in reporter assays to transfected C/EBP alpha and C/EBP beta proteins was consistent with the promoter binding assays and emphasized the relative importance of C/EB beta s for activation of the ZTA promoter. Mutation of the oriLyt promoter proximal C/EB beta site had little effect on ZTA activation of the promoter in a reporter assay. However, this mutation impaired oriLyt DNA replication, suggesting a separate replication-specific contribution for C/EB beta proteins. Finally, the overall importance of C/EBP proteins for lytic gene expression was demonstrated using CHOP10 to antagonize C/EBP DNA binding activity. Introduction of CHOP10 significantly impaired induction of the ZTA, RTA, and BMRF1 proteins in chemically treated AGS/BX1 cells. Thus, C/EBP beta and C/EBP alpha expression are associated with lytic induction in AGS cells, and expression of C/EBP proteins in epithelial cells may contribute to the tendency of these cells to exhibit constitutive low-level ZTA promoter activity.