Abrogation of etoposide-mediated cytotoxicity by cycloheximide.

The antitumor agent etoposide interacts with DNA topoisomerase II to produce a unique form of DNA-enzyme intermediate referred to as a "cleavable complex". These drug-induced DNA strand breaks initiate poorly defined cell processes which result in lethality. To explore the mechanism of etoposide cytotoxicity, we studied the effect of protein synthesis inhibitor on Balb/C 3T3 fibroblasts and CCRF-CEM and L1210 leukemia cells by exposing these cell lines to cycloheximide for various periods of time prior to etoposide challenge. Cycloheximide alone during these periods of exposure was not cytotoxic; however, it conferred increasing cytoprotection from etoposide in a time-dependent fashion when it preceded etoposide. Although cycloheximide did cause a decrease in enzyme content and in etoposide-induced DNA cleavage of Balb/C 3T3 and the CCRF-CEM cell lines, cytoprotection by cycloheximide could not be accounted for completely by these phenomena since, in L1210 cells, cytoprotection was observed without significant change in DNA cleavage or enzyme content. Cycloheximide diminished DNA synthesis as well as protein synthesis. However, DNA synthesis resumed within 6 hr after removal of cycloheximide, in spite of the fact that cytoprotection persisted. Cycloheximide did not alter cell cycle distribution as measured by flow cytometry. Our data, therefore, clearly demonstrate that cycloheximide can diminish the cytotoxic potential of etoposide-mediated topoisomerase-DNA complexes. Elucidation of the mechanism by which cytoprotection occurs should shed light on the basis for the cytotoxic effect of topoisomerase II-active drugs.