The Repression Functions of Simian Virus 40 T/t-common polypeptide

• Main Authors: YaLing Huang, 黃雅玲
• Other Authors: Won-Bo Wang
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/21634331242811142047

碩士 === 國立臺灣大學 === 微生物學研究所 === 89 === Overexpression of HER-2/neu proto-oncogene frequently occurs in many types of human cancers, including ovarian carcinoma and breast cancers, and is known to enhance tumor metastasis and chemoresistance. Previously our laboratory has found that the N-terminal common domain of simian virus 40 (SV40) large T and small t antigen (called T/t-common polypeptide here) can repress HER-2/neu expression and consequently suppress the tumorigenic potential of the HER-2/neu-overexpressing ovarian cancer cells. To investigate the probability of using the T/t-common gene in cancer gene therapy, I have constructed a recombinant adenovirus, rAdNT/t, carrying the T/t-common gene. Infection of the HER-2/neu-overexpressing cancer cells, including ovarian carcinoma, SK-OV-3 and SK-OV-3ip1, and breast carcinoma AU565, by rAdNT/t could lead to dramatical suppression of the soft-agarose-colony formation ability of these cancer cells. When these rAdNT/t-infected cancer cells were incubated over 60 days, the number of the soft-agarose colonies observed was as low as that observed at 20 days postseeding. Since T/t-common was expressed only transiently in these rAdNT/t-infected cells, the above observation suggest that T/t-common may induce cell death, rather than simply suppressing the growth of these infected cells. Indeed, when we cotransfected the T/t-common-expressing plasmid with the neomycin-resistance gene into the HER-2/neu-overexpressing SK-OV-3 and SK-OV-3ip1 cells, we found that the number of G418-resistant colonies obtained was significantly lower than that obtained by cotransfection of the vector plasmid and the neomycin-resistance gene. These data demonstrate that T/t-common can induce death in these HER-2/neu-overexpressing cancer cells. Similar conclusion was reached in transient cotransfection assays using the T/t-common-expressing plasmid and the β-galactosidase- expressing plasmid. In contrast, when HER-2/neu-nonoverexpressing cancer cells, NPC-TW04 and HEp-2, was used in the similar experiment, the cell-killing effect of T/t-common was not observed. Taken together, we proposed that T/t-common may specifically kill HER-2/neu- overexpressing cancer cells, but not HER-2/neu-nonoverexpressing cancer cells, and this killing may reflect the ability of T/t-common to repress HER-2/neu expression. Previously our laboratory has found that SV40 small t could interact with the transcriptional coactivator p300 and inhibit the coactivating activity of p300. In my experiments, I further demonstrated that small t could directly interact with p300 by GST pull-down assays. Small t could interact with the N-terminal, middle, and C-terminal portions of p300, and its interaction with the N-terminal and C-terminal regions of p300 was most likely mediated through CH1 and CH3, respectively, motifs of p300. To investigate whether small t’s repression function is related to its ability to interact with p300, we tested the ability of small t mutants, tsb40-45 and tΔ65-70, to interact with p300 by GST pull-down assays. We found that tsb40-45, which is defective in repression function, lost its ability to interact with p300, while tΔ65-70, which is partially defective in repression function, retained its ability to interact with p300 though at lower capacity. These results suggest that small t’s ability to repress transcription is related to its ability to interact and inhibit p300. We also found that T/t-common could interact with p300 by GST pull-down assays. Since p300 plays an important role in activation of the HER-2/neu promoter, our above finding may elucidate the mechanism by which T/t-common represses HER-2/neu expression.