title: Down-regulation of topoisomerase I in mammalian cells following ionizing radiation.
abstract: Modulators of the DNA unwinding enzyme, topoisomerase I (Topo I), inhibit DNA repair and augment the lethal effects of X-rays and other agents that create breaks in DNA. To investigate the role of Topo I in DNA repair, we examined Topo I activity before and after X-irradiation using confluence-arrested hamster and human cells. Topo I activities were higher in unirradiated neoplastic compared to normal hamster or human cells. Following ionizing radiation, however, enzyme activities were dramatically down-regulated to a greater extent in tumor than in normal cells. The extent of Topo I down-regulation correlated to some extent with survival enhancement in irradiated cells. Enzyme activities were down-regulated within 5 min in hamster and human cells. Recovery of Topo I activities in X-irradiated hamster cells required 12 h, whereas irradiated human cells recovered in only 70 min. Decreased Topo I activity was also noted after UV irradiation. In contrast, Topo I protein and mRNA levels remained unchanged following radiation. Administration of 5 mM 3-aminobenzamide or 0.5 mM PD 128763, inhibitors of poly(ADP-ribose) transferase, prevented Topo I down-regulation in X-irradiated or UV-irradiated human or hamster cells. Thus, decreases in Topo I activity following DNA damage are likely caused by ADP-ribosylation of the enzyme. Down-regulation of Topo I may prevent its binding to single-stranded DNA nicks created by X-irradiation, allowing the DNA repair complex (which is concomitantly activated by ADP-ribosylation) access to these lesions.
authors.affiliation: Department of Human Oncology, University of Wisconsin-Madison 53792.
authors.forename: D A
authors.initials: DA
authors.lastname: Boothman
authors.forename: N
authors.initials: N
authors.lastname: Fukunaga
authors.forename: M
authors.initials: M
authors.lastname: Wang
mesh_terms: D000246:Adenosine Diphosphate Ribose / Q000378:metabolism; D000818:Animals; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus / Q000196:embryology; D004264:DNA Topoisomerases, Type I / Q000737:chemistry / Q000378:metabolism* / Q000528:radiation effects*; D015536:Down-Regulation; D006801:Humans; D008545:Melanoma / Q000378:metabolism; D008970:Molecular Weight; D011499:Protein Processing, Post-Translational; D012333:RNA, Messenger / Q000378:metabolism / Q000528:radiation effects; D011829:Radiation Dosage; D014407:Tumor Cells, Cultured
chemical_list: D012333:RNA, Messenger; D000246:Adenosine Diphosphate Ribose; D004264:DNA Topoisomerases, Type I