ΔNp63α promotes Epstein-Barr virus latency in undifferentiated epithelial cells.

Epstein-Barr virus (EBV) is a human herpesvirus that causes infectious mononucleosis and contributes to both B-cell and epithelial-cell malignancies. EBV-infected epithelial cell tumors, including nasopharyngeal carcinoma (NPC), are largely composed of latently infected cells, but the mechanism(s) maintaining viral latency are poorly understood. Expression of the EBV BZLF1 (Z) and BRLF1 (R) encoded immediate-early (IE) proteins induces lytic infection, and these IE proteins activate each other's promoters. ΔNp63α (a p53 family member) is required for proliferation and survival of basal epithelial cells and is over-expressed in NPC tumors. Here we show that ΔNp63α promotes EBV latency by inhibiting activation of the BZLF1 IE promoter (Zp). Furthermore, we find that another p63 gene splice variant, TAp63α, which is expressed in some Burkitt and diffuse large B cell lymphomas, also represses EBV lytic reactivation. We demonstrate that ΔNp63α inhibits the Z promoter indirectly by preventing the ability of other transcription factors, including the viral IE R protein and the cellular KLF4 protein, to activate Zp. Mechanistically, we show that ΔNp63α promotes viral latency in undifferentiated epithelial cells both by enhancing expression of a known Zp repressor protein, c-myc, and by decreasing cellular p38 kinase activity. Furthermore, we find that the ability of cis-platinum chemotherapy to degrade ΔNp63α contributes to the lytic-inducing effect of this agent in EBV-infected epithelial cells. Together these findings demonstrate that the loss of ΔNp63α expression, in conjunction with enhanced expression of differentiation-dependent transcription factors such as BLIMP1 and KLF4, induces lytic EBV reactivation during normal epithelial cell differentiation. Conversely, expression of ΔNp63α in undifferentiated nasopharyngeal carcinoma cells and TAp63α in Burkitt lymphoma promotes EBV latency in these malignancies

Differentiation-Dependent KLF4 Expression Promotes Lytic Epstein-Barr Virus Infection in Epithelial Cells.

Epstein-Barr virus (EBV) is a human herpesvirus associated with B-cell and epithelial cell malignancies. EBV lytically infects normal differentiated oral epithelial cells, where it causes a tongue lesion known as oral hairy leukoplakia (OHL) in immunosuppressed patients. However, the cellular mechanism(s) that enable EBV to establish exclusively lytic infection in normal differentiated oral epithelial cells are not currently understood. Here we show that a cellular transcription factor known to promote epithelial cell differentiation, KLF4, induces differentiation-dependent lytic EBV infection by binding to and activating the two EBV immediate-early gene (BZLF1 and BRLF1) promoters. We demonstrate that latently EBV-infected, telomerase-immortalized normal oral keratinocyte (NOKs) cells undergo lytic viral reactivation confined to the more differentiated cell layers in organotypic raft culture. Furthermore, we show that endogenous KLF4 expression is required for efficient lytic viral reactivation in response to phorbol ester and sodium butyrate treatment in several different EBV-infected epithelial cell lines, and that the combination of KLF4 and another differentiation-dependent cellular transcription factor, BLIMP1, is highly synergistic for inducing lytic EBV infection. We confirm that both KLF4 and BLIMP1 are expressed in differentiated, but not undifferentiated, epithelial cells in normal tongue tissue, and show that KLF4 and BLIMP1 are both expressed in a patient-derived OHL lesion. In contrast, KLF4 protein is not detectably expressed in B cells, where EBV normally enters latent infection, although KLF4 over-expression is sufficient to induce lytic EBV reactivation in Burkitt lymphoma cells. Thus, KLF4, together with BLIMP1, plays a critical role in mediating lytic EBV reactivation in epithelial cells

Hypoxia-inducible factor-1α plays roles in Epstein-Barr virus’s natural life cycle and tumorigenesis by inducing lytic infection through direct binding to the immediate-early <i>BZLF1</i> gene promoter

<div><p>When confronted with poor oxygenation, cells adapt by activating survival signaling pathways, including the oxygen-sensitive transcriptional regulators called hypoxia-inducible factor alphas (HIF-αs). We report here that HIF-1α also regulates the life cycle of Epstein-Barr virus (EBV). Incubation of EBV-positive gastric carcinoma AGS-Akata and SNU-719 and Burkitt lymphoma Sal and KemIII cell lines with a prolyl hydroxylase inhibitor, L-mimosine or deferoxamine, or the NEDDylation inhibitor MLN4924 promoted rapid and sustained accumulation of both HIF-1α and lytic EBV antigens. ShRNA knockdown of HIF-1α significantly reduced deferoxamine-mediated lytic reactivation. HIF-1α directly bound the promoter of the EBV primary latent-lytic switch <i>BZLF1</i> gene, Zp, activating transcription via a consensus hypoxia-response element (HRE) located at nt -83 through -76 relative to the transcription initiation site. HIF-1α did not activate transcription from the other EBV immediate-early gene, <i>BRLF1</i>. Importantly, expression of HIF-1α induced EBV lytic-gene expression in cells harboring wild-type EBV, but not in cells infected with variants containing base-pair substitution mutations within this HRE. Human oral keratinocyte (NOK) and gingival epithelial (hGET) cells induced to differentiate by incubation with either methyl cellulose or growth in organotypic culture accumulated both HIF-1α and Blimp-1α, another cellular factor implicated in lytic reactivation. HIF-1α activity also accumulated along with Blimp-1α during B-cell differentiation into plasma cells. Furthermore, most <i>BZLF1</i>-expressing cells observed in lymphomas induced by EBV in NSG mice with a humanized immune system were located distal to blood vessels in hypoxic regions of the tumors. Thus, we conclude that HIF-1α plays central roles in both EBV’s natural life cycle and EBV-associated tumorigenesis. We propose that drugs that induce HIF-1α protein accumulation are good candidates for development of a lytic-induction therapy for treating some EBV-associated malignancies.</p></div

KLF4 binds to both the Zp and Rp IE EBV promoters, and enhances their association with activated RNA polymerase II.

<p>A) Various 5’ Rp deletion luciferase constructs were co-transfected into EBV-negative NOKs cells with either control vector or a KLF4 expression vector, and luciferase assay was performed 2 days after transfection. The KLF4-induced fold-change in luciferase activity for each construct is shown relative to the activity of the promoter in presence of control vector (set to 1). Values shown are the average +/- the standard deviation of results from two replicates. B) The EBV Rp sequence located between -551 and -441 relative to the transcriptional start site is shown. KLF4 consensus binding sites are highlighted in red. C) The Rp -551 construct (with or without KLF4 consensus site mutants) were co-transfected into EBV negative NOKs cells with control vector or KLF4 expression vector. Luciferase activity for each of the conditions is shown. Values are given as average +/- the standard deviations of results from two replicates. KLF4 mutant 1 alters the CACCC motif and mutant 2 alters the GGGTG motif. D) HONE-Akata cells were transfected with either control vector or a KLF4 expression vector, and ChIP assay was performed 48 hours after transfection. Cross-linked DNA-protein complexes were immunoprecipitated using anti-KLF4 antibody (top panel), or anti-phospho-RNA polymerase II antibody (bottom panel) and control IgG antibody in each case. Quantitative PCR was performed to quantitate the amount of DNA pulled down for the IE Rp (left panel), Zp (middle panel) and negative control Cp (right panel) EBV promoters.</p

Lytic EBV protein expression in NOKs-Akata cells is restricted to the more differentiated cell layers.

<p>Two different independently generated NOKs-Akata cell lines (panels A and B) were grown in organotypic air-interface raft culture, and <i>in situ</i> hybridization or immunohistochemistry was performed to detect expression of the EBV EBERs or lytic EBV proteins (Z and BMRF1) as indicated. Examples of Z and BMRF1 stained cells are indicated by red arrows. C) NOKs-Akata cells grown in organotypic air-interface raft culture were examined by immunofluorescence using both anti-K10 (red) and anti-Z (green) antibodies. An example of a Z and K10 co-staining cell is shown in the left panel, and a Z-positive/K10 negative cell is shown in the right panel.</p

EBV transcript expression OHL lesions.

<p>EBV transcript expression OHL lesions.</p