Modulation of radiation-induced strand break repair by cisplatin in mammalian cells

PURPOSE: To investigate the repair of ionizing radiation-induced DNA lesions in human skin fibroblasts in the presence of cisplatin-DNA adducts and to determine the persistence of DNA repair inhibition by cisplatin. MATERIALS AND METHODS: Normal human fibroblasts (AG 1522) treated with cisplatin were exposed to 4 Gy 60Co gamma-radiation and assayed for repair of radiation-induced damage under growth-permissive conditions. DNA damage was measured by the fluorescence analysis of DNA unwinding (FADU) and cytokinesis-blocked micronucleus assays. RESULTS: Rejoining of strand breaks caused by 4 Gy radiation in cells without cisplatin pre-treatment appeared to be biphasic with an initial fast component (up to 15 min of repair time) followed by a slower component, and was completed by 90 min. Cisplatin treatment (10 microg/ml, 30 min) immediately before irradiation had no effect on the fast rejoining component, but inhibited the slow component (p<0.01). The same cisplatin treatment 24 h prior to irradiation inhibited both slow and fast components (p<0.01). In contrast, decreasing the cisplatin exposure to 1.0 microg/ml for 30 min, 24h prior to irradiation, resulted in an increased amount of strand break repair at each time point measured compared with irradiated control cells. This mild cisplatin treatment (95% survival) also resulted in a reduction of radiation-generated micronuclei indicating an adaptive response. CONCLUSIONS: Cisplatin used in combination with ionizing radiation can produce differential cellular responses depending upon the severity of the cisplatin treatment and the time interval between cisplatin and radiation exposures.