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Epstein-Barr virus (EBV) was the first human virus found to encode microRNAs (miRNAs), but the function of these miRNAs has been obscure. Nasopharyngeal carcinoma (NPC) is associated with EBV infection, and the EBV-encoded LMP1 is believed to be a key factor in NPC development. However, detection of LMP1 protein in NPC is variable. Here, we report that EBV-encoded BART miRNAs target the 3′ UTR of the LMP1 gene and negatively regulate LMP1 protein expression. These miRNAs also modulate LMP1-induced NF-κB signaling and alleviate the cisplatin sensitivity of LMP1-expressing NPC cells. Consistent with a previous study on the NPC C666-1 cell line and C15 xenograft, we found abundant expression of BART miRNAs in NPC tissues. Furthermore, DNA sequencing revealed that the 3′ UTR of LMP1 is highly conserved in NPC-derived EBV isolates. The data provide insight into the discrepancy between LMP1 transcript and protein detection in NPC and highlight the role of the EBV miRNAs in regulating LMP1 downstream signaling to promote cancer development. [ABSTRACT FROM AUTHOR]

Epstein-Barr virus infection in vitro immortalizes primary B cells. EBNA2 is an Epstein-Barr virus-encoded transcriptional transactivator that mimics the effects of activated Notch signaling and is essential for this proliferative response. An assay using Sindbis virus (SV) as a cell death inducer revealed that, like Notch, EBNA2 also has antiapoptotic activity. We show that Nur77 is a mediator of SV-induced cell death and that EBNA2 antiapoptotic activity results from interaction with Nur77. EBNA2 colocalized with Nur77 in transfected cells and coprecipitated with Nur77 in IB4 B cells. EBNA2 binds to Nut77 through sequences in the EBNA2 amino acid 123-147 conserved domain and an EBNA2 mutant unable to bind Nur77 also lost the ability to protect cells from SV-induced apoptosis. EBNA2 exerted its antideath function by retaining Nur77 in the nucleus and preventing Nur77 from targeting mitochondria in response to apoptotic stimuli. Thus, targeting of Nur77 can be added to the list of strategies used by viruses to counter apoptosis. [ABSTRACT FROM AUTHOR]

CBF1 is a member of the CSL family of DNA binding factors, which mediate either transcriptional repression or transcriptional activation. CSL proteins play a central role in Notch signaling and in Epstein-Barr virus-induced immortalization. Notch is a transmembrane protein involved in cell-fate decisions, and the cytoplasmic domain of Notch (NotchIC) targets CBF1. The Epstein-Barr virus-immortalizing protein EBNA2 activates both cellular and viral gene expression by targeting CBF1 and mimicking NotchIC. We have examined the mechanism of CBF1-mediated repression and show that CBF1 binds to a unique corepressor, CBF1 interacting corepressor (CIR). A CIR homolog is encoded by Caenorhabditis elegans, indicating that CIR is evolutionarily conserved. Two CBF1 mutants that were unable to bind CIR did not function as repressors, suggesting that targeting of CIR to CBF1 is an important component of repression. When expressed as a Gal4 fusion protein, CIR repressed reporter gene expression. CIR binds to histone deacetylase and to SAP30 and serves as a linker between CBF1 and the histone deacetylase complex.