Induction of G2 Retardation and Polyploidy by Epstein-Barr Virus EBNA2 Protein in Human Epithelial Cells

• Main Authors: Chia-Ching Tai, 戴嘉靚
• Other Authors: 王萬波
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/03587724283111198281

碩士 === 國立臺灣大學 === 微生物學研究所 === 93 === Epstein-Barr Virus (EBV) is a herpesvirus which exists commonly in the human population and has been shown to be associated with many human malignancies, notably Burkett’s lymphoma and nasopharyngeal carcinoma. Eight latent proteins of EBV have been shown to be critical for EBV transformation of B-cells. Among them, EBV nuclear antigen 2 (EBNA2), a transcriptional activator, has been shown to be able to induce tumor in transgenic mice. Although the transactivation function of EBNA2 has been proposed to contribute to its transformation function, the mechanism underlying EBNA2 tumor-inducing function remains unclear. Previously we reported that EBNA2 can retard epithelial cell growth possibly through stimulating p53 activity and inducing p21 expression. To further investigate the effects of EBNA2 on cell proliferation, we used flow cytometric analysis to determine which phase of cell cycle was affected by EBNA2. We found that EBNA2 could retard cell cycle progression at G2/M phase. To more specifically pinpoint whether G2 or M phase was affected by EBNA2, we measured the phosphorylation status of histone H3 after cells were released from G1/S block. Our data indicated that EBNA2 specifically retarded cell cycle progression at G2 phase. This conclusion was also confirmed by the data that measured the phosphorylation status of Cdk1 after cells were released from G1/S block. Previous studies indicated that agents that induce cell cycle G2 arrest often cause cells to become polyploid and contain multinuclei. Since EBNA2 could induce G2 arrest, we also test whether EBNA2 could induce polyploidy. Indeed, we found that EBNA2 could induce polyploidy by flow cytometric analysis. To investigate the mechanism underlying EBNA2 induction of polyploidy, we asked whether EBNA2 could disrupt mitotic spindle checkpoint. By analyzing the phosphorylation status of histone H3 and mitotic index, we found that EBNA2 indeed could disrupt the mitotic spindle checkpoint. Through disrupting spindle checkpoint, EBNA2 may cause chromosomes to be unevenly divided into daughter cells. Polyploidy cells may then arise after endoreduplication of these unevenly divided chromosomes. Multinucleation is frequently accompanied with disruption of spindle checkpoint and polyploidy. We also found that many EBNA2-expressing cells contained multinuclei. Together, the above data reveal a novel function of EBNA2, that is, its ability to induce genome instability. We believe that this function of EBNA2 may be more important than other known functions of EBNA2 in inducing tumor formation.