ABROGATION OF ADRIAMYCIN TOXICITY INVIVO BY CYCLOHEXIMIDE

The processes involved in cell killing by Adriamycin (R) (ADR) and other agents that interact with topoisomerase II are unclear. To investigate the mode of ADR cytotoxicity in vivo, we have investigated the effects of the protein synthesis inhibitor, cycloheximide (CH), on cell killing by ADR in the murine intestinal tract. We have used morphological criteria to assay the cell death. ADR rapidly induces cell death in this tissue that has the morphology of apoptosis or programmed cell death. CH, when administered immediately after ADR, reduced the incidence of cell death by approximately 81% at 3 hr and approximately 51% at 6 hr after treatment. The inhibitor was only effective when administered within 0.5 hr of ADR suggesting that critical events leading to cell death may occur during this period. The inhibitor did not interfere with the ADR uptake or retention. Significant positive correlation was observed between protein and DNA synthesis inhibition (as measured by precursor uptake) and apoptosis inhibition. CH delayed progression of cells through all phases of the cell cycle except mitosis. However. ADR also had a similar effect, suggesting that progression through the cell cycle is not necessary for the expression of apoptosis. The effectiveness of CH in apoptosis inhibition, even when administered 0.5 hr after the ADR, coupled with the rapid uptake of ADR by the intestinal epithelium suggests that the mode of inhibition is unlikely to be modulation of cellular targets of ADR such as topoisomerase II or inhibition of formation of ADR-topoisomerase II complex. These data indicate that topoisomerase II-interacting agents such as ADR may induce apoptosis; the processes leading to cell death in this situation are thought to be gene dependent and require protein synthesis for their expression. Thus, the cytoprotective effect of CH may be due directly to the inhibition of protein synthesis.