Dexamethasone Treatment of Calves Latently Infected with Bovine Herpesvirus 1 Leads to Activation of the bICP0 Early Promoter, in Part by the Cellular Transcription Factor C/EBP-Alpha

Abstract

Sensory neurons within trigeminal ganglia (TG) are the primary site for bovine herpesvirus 1 (BHV-1) latency. During latency, viral gene expression is restricted to the latency-related (LR) gene and the open reading frame ORF-E. We previously constructed an LR mutant virus that expresses LR RNA but not any of the known LR proteins. In contrast to calves latently infected with wild-type (wt) BHV-1 or the LR rescued virus, the LR mutant virus does not reactivate from latency following dexamethasone (DEX) treatment. In this study, we demonstrated that bICP0, but not bICP4, transcripts were consistently detected in TG of calves infected with the LR mutant or LR rescued virus following DEX treatment. Calves latently infected with the LR rescued virus but not the LR mutant virus expressed late transcripts, which correlated with shedding of infectious virus following DEX treatment. The bICP4 and bICP0 genes share a common immediate-early promoter, suggesting that this promoter was not consistently activated during DEX-induced reactivation from latency. The bICP0 gene also contains a novel early promoter that was activated by DEX in mouse neuroblastoma cells. Expression of a cellular transcription factor, C/EBP-alpha, was stimulated by DEX, and C/EBPalpha expression was necessary for DEX induction of bICP0 early promoter activity. C/EBP-alpha directly interacted with bICP0 early promoter sequences that were necessary for trans activation by C/EBP-alpha. In summary, DEX treatment of latently infected calves induced cellular factors that stimulated bICP0 early promoter activity. Activation of bICP0 early promoter activity does not necessarily lead to late gene expression and virus shedding.

Productive Infection and bICP0 Early Promoter Activity of Bovine Herpesvirus 1 Are Stimulated by E2F1

“…In trigeminal ganglia of latently infected calves, bICP0 transcription is stimulated from the E promoter during DEXinduced reactivation from latency, regardless of whether infectious virus is detected (47). In addition, the bICP0 E promoter is stimulated by DEX, in part because of the cellular transcrip- Sixteen hours after infection, cells were fixed in 4% paraformaldehyde and stained for confocal microscopy as described in Materials and Methods.…”

“…bICP0 also contains an E promoter located near the 5Ј end of the bICP0 coding exon (e2). Recent evidence indicated that the bICP0 E promoter, but not the IEtu1 promoter, is stimulated by DEX treatment (47). Expression of the cellular transcription factor C/EBP-alpha is stimulated by DEX, thus stimulating bICP0 E promoter activity.…”

“…Expression of the bICP4 protein represses IEtu1 promoter activity, whereas bICP0 activates its own E promoter and all other viral promoters. A recent study demonstrated that during DEX-induced reactivation from latency, bICP0 mRNA, but not bICP4 mRNA, was consistently detected (47). In part, this was due to the fact that the bICP0 early promoter is activated by DEX induction of the cellular transcription factor CAAT-enhancer binding protein alpha (C/EBP-alpha) (47).…”

“…A recent study demonstrated that during DEX-induced reactivation from latency, bICP0 mRNA, but not bICP4 mRNA, was consistently detected (47). In part, this was due to the fact that the bICP0 early promoter is activated by DEX induction of the cellular transcription factor CAAT-enhancer binding protein alpha (C/EBP-alpha) (47). bICP0 transcription appears to be stimulated during reactivation from latency by cellular transcription factors that transactivate the bICP0 E promoter.…”

“…This oligonucleotide is a fluorescently conjugated control containing a scrambled sequence that does not reduce the level of any known mammalian gene. Forty-eight hours after transfection, whole-cell lysate was prepared as previously described (47). Protein concentrations were quantified by the Bradford assay.…”