Down-regulation of topoisomerase I in mammalian cells following ionizing radiation.

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.