Discrepancy between the initial DNA damage and cell survival after camptothecin treatment in two murine lymphoma L5178Y sublines

Two L5178Y (LY) murine lymphoma cell sublines, LY-R, resistant, and LY-S, sensitive, to X-irradiation display inverse cross-sensitivity to camptothecin (CPT): LY-R cells were more susceptible to this specific topoisomerase I inhibitor than LY-S cells. After 1 h incubation with CPT, the doses that inhibited growth by 50 per cent (ID50) after 48 h of incubation were 0.54 microM for LY-R cells and 1.25 microM for LY-S cells. Initial numbers of DNA-protein crosslinks (DPCs) measured at this level of growth inhibition were two-fold higher in LY-R (5.6 Gray-equivalents) than in LY-S cells (3.1 Gray-equivalents), which corresponds well with the greater in vitro sensitivity of Topo I from LY-R cells to CPT. Conversely, the initial levels of single-strand DNA breaks (SSBs) and double-strand DNA breaks (DSBs) were lower in LY-R cells (4.2 Gray-equivalent SSBs and 5.8 Gray equivalent DSBs) than in LY-S cells (8.0 Gray-equivalent SSBs and 12.0 Gray-equivalent DSBs). Dissimilarity in the replication-dependent DNA damage observed after 1 h of treatment with CPT was not due to a difference in the rate of DNA synthesis between the two cell lines, but may have arisen from a substantially slower repair of DNA breaks in LY-S cells. Release from G2 block by caffeine co-treatment significantly increased cell killing in the LY-S subline, and only slightly inhibited growth of LY-R cells. These results show that after CPT treatment cells arrest in G2, allowing them time to repair the long-lived DSBs. As LY-S cells are slower in repairing the DSBs, they were more susceptible to CPT in the presence of caffeine.