title: DNA topoisomerase III alpha regulates p53-mediated tumor suppression.
abstract: OBJECTIVE
  Human DNA topoisomerase III alpha (hTOP3α) is involved in DNA repair surveillance and cell-cycle checkpoints possibly through formatting complex with tumor suppressors. However, its role in cancer development remained unsolved.
  
  
  METHODS
  Coimmunoprecipitation, sucrose gradient, chromatin immunoprecipitation (ChIP), real time PCR, and immunoblotting analyses were performed to determine interactions of hTOP3α with p53. Paired cell lines with different hTOP3α levels were generated via ectopic expression and short hairpin RNA (shRNA)-mediated knockdown approaches. Cellular tumorigenic properties were analyzed using cell counting, colony formation, senescence, soft agar assays, and mouse xenograft models.
  
  
  RESULTS
  The hTOP3α isozyme binds to p53 and cofractionizes with p53 in gradients differing from fractions containing hTOP3α and BLM. Knockdown of hTOP3α expression (sh-hTOP3α) caused a higher anchorage-independent growth of nontumorigenic RHEK-1 cells. Similarly, sh-hTOP3α and ectopic expression of hTOP3α in cancer cell lines caused increased and reduced tumorigenic abilities, respectively. Genetic and mutation experiments revealed that functional hTOP3α, p53, and p21 are required for this tumor-suppressive activity. Mechanism-wise, ChIP data revealed that hTOP3α binds to the p53 and p21 promoters and positively regulates their expression. Two proteins affect promoter recruitments of each other and collaborate in p21 expression. Moreover, sh-hTOP3α and sh-p53 in AGS cells caused a similar reduction in senescence and hTOP3α mRNA levels were lower in gastric and renal tumor samples.
  
  
  CONCLUSIONS
  We concluded that hTOP3α interacts with p53, regulates p53 and p21 expression, and contributes to the p53-mediated tumor suppression.
authors: Authors' Affiliations: Department and Graduate Institute of Microbiology, College of Medicine, Department of Plan Pathology and Microbiology, College of Bioresources and Agriculture, and Center for Biotechnology, National Taiwan University, Taipei, Taiwan.
authors: Mei-Yi
authors: MY
authors: Hsieh
authors: Jia-Rong
authors: JR
authors: Fan
authors: Han-Wen
authors: HW
authors: Chang
authors: Hsiang-Chin
authors: HC
authors: Chen
authors: Tang-Long
authors: TL
authors: Shen
authors: Shu-Chun
authors: SC
authors: Teng
authors: Yen-Hsiu
authors: YH
authors: Yeh
authors: Tsai-Kun
authors: TK
authors: Li
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D047369:Chromatin Immunoprecipitation; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000378:metabolism*; D004264:DNA Topoisomerases, Type I / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic / Q000502:physiology*; D064593:Heterografts; D006801:Humans; D015151:Immunoblotting; D047468:Immunoprecipitation; D051379:Mice; D016688:Mice, Inbred NOD; D016513:Mice, SCID; D060888:Real-Time Polymerase Chain Reaction; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*
chemical_list: D050759:Cyclin-Dependent Kinase Inhibitor p21; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53; D004264:DNA Topoisomerases, Type I