Transactivation of Herpes Simplex Virus 1 (HSV-1) Infected Cell Protein 4 Enhancer by Glucocorticoid Receptor and Stress-Induced Transcription Factors Requires Overlapping Krüppel-Like Transcription Factor 4/Sp1 Binding Sites.

Following acute infection, herpes simplex virus 1 (HSV-1) lytic cycle viral gene expression is silenced; consequently, lifelong latency in neurons is established. Certain external stimuli that trigger reactivation from latency also activate the glucocorticoid receptor (GR). The synthetic corticosteroid dexamethasone, but not a GR-specific antagonist, increases the frequency of explant-induced reactivation from latency and stimulates productive infection. Furthermore, dexamethasone increases expression of cellular transcription factors in trigeminal ganglionic neurons: for example, SLUG and three Krüppel-like transcription factor (KLF) family members, KLF4, KLF15, and promyelocytic leukemia zinc finger protein (PLZF). Consequently, we hypothesized that stress-induced transcription factors stimulate expression of ICP4, a viral transcriptional regulator required for productive infection. New studies demonstrated that GR and KLF4, PLZF, or SLUG cooperatively transactivate the ICP4 enhancer upstream of a minimal promoter in monkey kidney cells (Vero) and mouse neuroblastoma cells (Neuro-2A). Strikingly, mutagenesis of two KLF4/Sp1 binding sites reduced GR- plus KLF4-, PLZF-, or SLUG-mediated transactivation to basal levels. A consensus enhancer (E)-Box adjacent to a KLF4/Sp1 binding site was also required for GR- and SLUG-, but not KLF family member-, mediated transactivation of the ICP4 promoter. Chromatin immunoprecipitation studies (ChIP) revealed GR and stress-induced transcription factors occupy ICP4 enhancer sequences. Conversely, specific binding was generally reduced in the KLF4/Sp1 mutant. Furthermore, GR and SLUG occupancy of ICP4 enhancer sequences was reduced in the E-Box mutant. Based on these studies, we suggest stressful stimuli can trigger productive infection because GR and specific stress-induced transcription factors activate ICP4 expression.