Interferon-independent and -induced regulation of Epstein-Barr virus EBNA-1 gene transcription in Burkitt lymphoma

Abstract

Replication of the Epstein-Barr virus (EBV) genome within latently infected cells is dependent on the EBVEBNA-1 protein. The objective of this study was to identify transcriptional regulatory proteins that mediate EBNA-1 expression via the viral promoter Qp, which is active in EBV-associated tumors such as Burkitt lymphoma and nasopharyngeal carcinoma. Results of a yeast one-hybrid screen suggested that a subset of the interferon regulatory factor (IRF) family may regulate EBNA-1 transcription by targeting an essential cisregulatory element of Qp, QRE-2. Further investigation indicated that the transcriptional activator IRF-1 and the closely related IRF-2, a repressor of interferon-induced gene expression, are both capable of activating Qp. However, the major QRE-2-specific binding activity detected within extracts of Burkitt lymphoma cells was attributed to IRF-2, suggesting that interferon-independent activation of Qp is largely mediated by IRF-2 in these cells. We observed no effect of gamma interferon on Qp activity in transfection assays, whereas we observed a moderate but significant repression of Qp activity in response to alpha interferon, possibly mediated by either the interferon consensus sequence binding protein or IRF-7, a novel alpha interferoninducible factor identified in this study. Since expression of IRF-1 and IRF-2 is increased in response to interferons, the Qp activity observed in the presence of interferon likely represented an equilibrium between IRF factors that activate and those that repress gene expression in response to interferon. Thus, by usurping both IRF-1 and its transcriptional antagonist IRF-2 to activate Qp, EBV has evolved not only a mechanism to constitutively express EBNA-1 but also one which may sustain EBNA-1 expression in the face of the antiviral effects of interferon.

Characterization and Cell Cycle Regulation of the Major Kaposi’s Sarcoma-Associated Herpesvirus (Human Herpesvirus 8) Latent Genes and Their Promoter

“…Oct-1 is a ubiquitous transcription factor that recognizes an octamer sequence also recognized by Oct-2, a lymphoid-specific transcription factor. Two conserved interferon regulatory factor (IRF1/2) binding sites and a semiconserved interferon-stimulated response element (ISGF3) are present, which suggests the promoter may be regulated by interferon signal transduction pathways as has been shown for the EBV EBNA-1 Qp promoter (31,46). Conserved c-Jun, One AP-1 conserved binding site is present, although LT1-2 transcription has not been found to be responsive to TPA (13,14,40).…”

Expression of MUM1/IRF4 selectively clusters with primary effusion lymphoma among lymphomatous effusions: implications for disease histogenesis and pathogenesis

“…Also, the interplay of cellular and viral IRFs may be crucial for the modulation of HHV‐8/KSHV latency genes, cell‐cell interactions or homing of virus‐infected cells to specific organs or tissues. Indeed, cellular IRFs regulate the expression of viral latency genes in cells infected by EBV, a herpesvirus closely related to HHV‐8/KSHV that is also frequently present in PEL (Nonkwelo et al , 1997; Zhang & Pagano, 1997,1999; Fassone et al , 2000). Overall, these observations prompt future studies aimed at clarifying the possible interactions between MUM1/IRF4 and vIRFs and addressing the precise biological relevance of these molecules in PEL pathogenesis.…”

Differential viral induction of distinct interferon-alpha genes by positive feedback through interferon regulatory factor-7

“…A human homolog of IRF7 previously was suggested to act as a repressor of Epstein-Barr virus (EBV) gene expression (Nonkwelo et al, 1997;Zhang and Pagano, 1997). To test whether IRF7 is responsible for induction of IFNα genes, its ability to induce IFNα4 and IFNα6 promoter-reporter constructs was tested by co-transfection.…”

“…Thus, if the initial burst of IFNα4 and IFNβ production successfully eliminated infection, no further IFN would be produced since the newly synthesized IRF7 would be inactive without further modification. In fact, human IRF7 has been suggested as a possible repressor involved in EBV latency (Nonkwelo et al, 1997;Zhang and Pagano, 1997). Perhaps in the absence of a specific activation event, the presence of non-phosphorylated IRF7 could compete for activator proteins.…”