MODULATION OF THE EFFECT OF CAMPTOTHECIN IN X-IRRADIATED L5178Y-R ANDL5178Y-S CELLS BY BENZAMIDE

The L5178Y (LY) murine lymphoma subline, LY-R, is more radioresistant and more sensitive to camptothecin (CPT, inhibitor of topisomerase I) than the second subline used in our investigation, LY-S. Post-irradiat ion treatment with 3 mu M CPT enhanced the radiosensitivity of LY-S ce lls (D-0 decrease from 0.52 to 0.34 Gy), but did not change it in LY-R cells. Treatment with 2 mM benzamide [BZ, inhibitor of poly(ADP-ribos ylation)] before x-rays and CPT increased the radiosensitivity of LY-R cells (D-0 decrease from 1.15 to 0.52) without further modification o f radiosensitivity of LY-S cells. Activity of topoisomerase I was dimi nished 10 min after x-irradiation (5 Gy) in LY-S, but not in LY-R cell s. The data on DNA damage (fluorescent halo or comet assays) showed th at the ultimate fate of the cells did not depend on the DNA damage pat tern estimated immediately after treatment (e. g. the damage was great er in x-rays plus CPT than in BZ plus x-rays plus CPT treated LY-R cel ls, although the radiosensitivity was less). Aphidicolin (inhibitor of DNA polymerases alpha and delta) applied concomitantly with CPT in ce lls not pretreated with BZ prevented the increase in DNA damage in LY- R cells, but was without effect in LY-S cells. Taking into account the differential inhibition by x-rays of DNA synthesis in LY sublines and its reversion by BZ in LY-S but not in LY-R cells, we conclude that t he pattern of DNA damage observed by the methods applied depended on t he status of DNA replication.