Cellular and viral microRNAs (miRNAs) are involved in many different processes of key importance and more than 10,000 miRNAs have been identified so far. In general, relatively little is known about their biological functions in mammalian cells because their phenotypic effects are often mild and many of their targets still await identification. The recent discovery that Epstein-Barr virus (EBV) and other herpesviruses produce their own, barely conserved sets of miRNAs suggests that these viruses usurp the host RNA silencing machinery to their advantage in contrast to the antiviral roles of RNA silencing in plants and insects. We have systematically introduced mutations in EBV's precursor miRNA transcripts to prevent their subsequent processing into mature viral miRNAs. Phenotypic analyses of these mutant derivatives of EBV revealed that the viral miRNAs of the BHRF1 locus inhibit apoptosis and favor cell cycle progression and proliferation during the early phase of infected human primary B cells. Our findings also indicate that EBV's miRNAs are not needed to control the exit from latency. The phenotypes of viral miRNAs uncovered by this genetic analysis indicate that they contribute to EBV-associated cellular transformation rather than regulate viral genes of EBV's lytic phase.

Epstein-Barr virus (EBV) has evolved exquisite controls over its host cells, human B lymphocytes, not only directing these cells during latency to proliferate and thereby expand the pool of infected cells, but also to survive and thereby persist for the lifetime of the infected individual. Although these activities ensure the virus is successful, they also make the virus oncogenic, particularly when infected people are immunosuppressed. Here we show, strikingly, that one set of EBV’s miRNAs both sustain BL (Burkitt’s lymphoma) cells in the absence of other viral oncogenes and promote the transformation of primary B lymphocytes. Burkitt’s Lymphoma cells were engineered to lose EBV and found to die by apoptosis and could be rescued by constitutively expressing viral miRNAs in them. Two of these EBV miRNAs were found to target Caspase 3 to inhibit apoptosis at physiological concentrations.

The BARF1 gene is located in the BamHI-A fragment of the Epstein-Barr virus (EBV) genome, encodes 221 amino acids, and has activity as an oncogene. Several reports have demonstrated that BARF1 is expressed in the tissues of various EBV-associated epithelioid malignancies. However,BARF1 is thought to be a lytic gene, since its expression is induced upon induction of the lytic cycle in Burkitt's lymphoma cell lines. Therefore, the possibility cannot be excluded that BARF1 expression in EBV-associated epithelioid malignancies reflects spontaneous induction of the lytic cycle in carcinoma cells. The present study aimed to clarify whether BARF1 was expressed as a latent gene or a lytic gene in epithelioid malignancies. Quantitative real-time RT-PCR assay revealed that BARF1 was highly expressed in nasopharyngeal carcinoma (NPC) and EBV-positive gastric carcinoma tissues in the absence of expression of lytic genes. On the other hand, BARF1 protein was detectable only in two of seven NPC tissue samples by immunoblot analysis. Analysis of BARF1-transfected CNE1 cells revealed that BARF1 was quickly secreted into culture medium and was hardly detectable in the cell lysate, which would account for why some NPC tissues were negative for BARF1 protein expression even though they were strongly positive forBARF1 expression at the transcriptional level. The present findings indicate that BARF1 is expressed in NPC and EBV-positive gastric carcinoma tissues as a latent gene and suggest that BARF1 plays a role in the pathogenesis of these malignancies.

Epstein-Barr virus (EBV) infection has been postulated to be an early event involved in the pathogenesis of nasopharyngeal carcinomas (NPC). The lack of representative premalignant nasopharyngeal epithelial cell system for EBV infection has hampered research investigation into the regulation and involvement of EBV infection in NPC pathogenesis. We have compared the efficiency of EBV infection in nasopharyngeal epithelial cells with different biological properties including immortalized, primary and cancerous nasopharyngeal epithelial cells. EBV infection could be achieved in all the nasopharyngeal epithelial cells examined with variable infection rate. TGF-b effectively enhanced EBV infection into nasopharyngeal epithelial cells both in the immortalized and primary nasopharyngeal epithelial cells. Stable infection of EBV was achieved in a telomerase-immortalized nasopharyngeal epithelial cell line, NP460hTert. The expression pattern of EBVencoded genes and biological properties of this EBV infected cell line on long-term propagation were monitored. The EBVinfected nasopharyngeal epithelial cells acquired anchorage-independent growth and exhibited invasive growth properties on prolonged propagation. A distinguished feature of this EBV-infected nasopharyngeal epithelial cell model was its enhanced ability to survive under growth factor and nutrient starvation. This was evidenced by the suppressed activation of apoptotic markers and sustained activation of pAkt of EBV-infected cells compared to control cells under nutrient starvation. Examination of cytokine profiles of EBV-infected NP460hTert cells to nutrient and growth factor deprivation revealed upregulation of expression of MCP-1 and GRO-a. The establishment of a stable EBV infection model of premalignant nasopharyngeal epithelial cells will facilitate research investigation into the pathogenic role of EBV in NPC development. Infection of Epstein-Barr virus (EBV) is ubiquitous and more than 95% of the adult population world-wide is infected with this virus.1 EBV infection is life-long and largely asymptomatic. EBV is implicated as an etiological agent in several human malignancies of either lymphoid or epithelial origin. These include Burkitt's lymphoma, Hodgkin's lymphoma, nasal T/NK lymphomas, undifferentiated nasopharyngeal carcinomas (NPC), gastric adenocarcinomas, smooth muscle tumors and various B-cells lymphomas in AIDS patients or transplant recipients with compromised immune functions. 2-4EBV infection can be detected in most, if not all, undifferentiated NPC, regardless of geographical and ethnical origin of NPC patients.2 Expression of latent EBV genes including EBER, EBNA1, LMP1 and LMP2A as well as lytic EBV genes, such as BZLF1, could be detected in cancer cells inside the nasopharyngeal carcinoma specimens. Clonal infection of EBV has been detected in precancerous lesions of NPC, implicating its involvement at the early stage of NPC pathogenesis.5 EBV infection has long been postulated to play an important role in transformation of premalignant ...