| Summary: | 博士 === 國立臺灣大學 === 微生物學研究所 === 102 === The human DNA topoisomerase III alpha (hTOP3a) isozyme is involved in DNA-repair surveillance and cell cycle checkpoints through the formation of a complex with tumor suppressors. However, the role of hTOP3a in tumorigenesis remains unexplored. Here, we report novel interactions between the epigenetic chromatin modulator hTOP3a and the transcription factor p53 that are important for tumorigenesis. We have determined that (i) hTOP3a knockdown (sh-hTOP3a) in non-tumorigenic RHEK-1 cells caused a higher anchorage-independent growth; (ii) modulation of hTOP3a levels by shRNA knockdown or ectopic expression approaches in different cancer cell lines also cause an increase or a reduction in tumorigenic abilities, respectively; (iii) hTOP3a binds to p53 and interacts with p53 functionally to suppress tumor growth; and (iv) the tumor-suppressive activity of hTOP3a requires functional hTOP3a, p53 and p21. To study the underlying mechanisms of hTOP3a in cancer, chromatin immunoprecipitation data revealed that hTOP3a binds to both the p53 and p21 promoters, and hTOP3a and p53 affect the recruitments of each other to corresponding promoters. Moreover, hTOP3a not only positively regulates p53 expression but also cooperates with p53 to promote p21 transcription. We defined the molecular basis of this collaboration with p21 expression and identified that hTOP3a is recruited to the p21 promoter via p53. Functionally, sh-hTOP3a and sh-p53 in AGS cells caused a similar amount of reduced senescence. Above interactions require functional hTOP3a and p53, as the active-site mutant of hTOP3a and the DNA-binding mutant of p53 were unable to promote p21 expression and to suppress anchorage-independent growth. Therefore, we suggest that hTOP3a contributes significantly to tumorigenesis in part through interactions with p53. In support of this hypothesis, hTOP3a mRNA levels were lower in gastric and renal tumor samples. Together, our results suggest that hTOP3a regulates p53 and p21 expression and might contributes to the p53-mediated senescence and tumor suppression.
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