| Summary: | 碩士 === 國立臺灣大學 === 微生物學研究所 === 93 === Epstein-Barr virus (EBV) contains two phases in its life cycle: latent and lytic stages. According to previous studies, the progression of lytic cycle may affect many cellular events. To address this issue, we performed a microarray analysis on two groups of EBV-infected 293 cells that were distinguished into latent and lytic clones. We found that the expression of early growth response-1 (Egr-1), a zinc finger transcription factor, was increased in lytic clones, but not in latent clones. Immunoblot analysis confirmed that Egr-1 protein expression was associated with EBV lytic cycle. Zta is an immediate-early protein encoded by BZLF1 gene and plays a significant role to initiate viral lytic cascades. By using BZLF1-targetd siRNA to block EBV lytic progression, the expression of Egr-1 protein was reduced. Therefore, we suggested that the expression of Egr-1 protein was the downstream event of EBV lytic cycle. Furthermore, by using single gene transfection assay, we identified that Zta could induce Egr-1 protein expression in two EBV-negative cell lines, NPC-TW01 and NPC-TW04. In addition, in a Zta-inducible cell line (HONE-1-52), Egr-1 protein was also induced during Zta expression. As the regulator of EBV lytic cycle, Zta also affects the cellular gene expression at transcriptional level either by directly binding to a consensus DNA sequence or indirectly activating cellular signaling pathways. According to the reporter assays, Zta could activate Egr-1 promoter. By analysis of the deletion mutants of Egr-1 promoter, two regions relative to the transcriptional start site, -503bp~-438bp and –438bp~-395bp, were proven to critical for Zta-mediated activation. These two promoter regions contained putative ZREs (Zta response elements) and Elk-1/SRF binding site, respectively. The Elk-1/SRF binding site was crucial for Egr-1 gene expression upon Ras/Raf/Mek/Erk/Elk signaling pathway triggered by serum and many growth factors. Interestingly, Zta could dramatically increase the phosphorylattion status of Erk-1 protein in our study. In the presence of Erk phosphorylation inhibitos, PD98059 and U0126, Zta-induced Egr-1 protein expression was decreased significantly. Taken together, we demonstrate that the EBV lytic transactivator Zta can induce the expression of cellular Egr-1 gene, and the mechanism is dependent on direct DNA binding and indirect Erk signal activation. The biological meanings of Zta-induced Egr-1 protein remain be investigated in future study.
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