Topoisomerase II inhibitor-induced apoptosis in thymocytes and lymphoma cells.

DNA topoisomerase II is a nuclear enzyme that modulates DNA topology during several metabolic processes and is the target of several antitumor drugs. The primary effect of anticancer agents is to induce apoptosis. The present study showed that etoposide, a topoisomerase II inhibitor which forms cleavable complexes, induced apoptosis in nonproliferative thymocytes and proliferative RVC cells, whereas ICRF-154, a bis(2,6-dioxopiperazine) derivative which does not form a cleavable complex, induced apoptosis only in thymocytes. Both etoposide and ICRF-154 inhibited topoisomerase II activity in thymocytes and RVC cells to a similar extent. Etoposide had no effect on the cell cycle of RVC cells, but ICRF-154 induced cell cycle arrest at the G2/M stage followed by cell death without forming a DNA ladder on an agarose gel. Incubation with ICRF-154 reduced the expression of topoisomerase IIa in thymocytes and IIb in RVC cells. These findings suggest that the catalytic inhibitor, ICRF-154, has a mechanism of cytotoxicity which differs from that of etoposide. In RVC cells exposed to etoposide, we identified two clones that were suppressed early in the incubation. One was highly homologous to hnRNP A1 which modulates splicing of selected transcripts or stabilizes mRNAs. The other was a novel gene of which the function remains unknown. These genes were also altered in RVC cells exposed to camptothecin, which underwent apoptosis, but not in those incubated with ICRF-154, indicating that the suppression of these genes is related to inhibitor-induced DNA breaks resulting in apoptosis. In thymocytes, however, a cleavable complex by topoisomerase II inhibitors is not essential for the induction of apoptosis, since it was induced by ICRF-154. This suggests that tissue-specific nuclear matrix proteins other than topoisomerase II, including SATP-1 in the thymus, should also be considered. The present findings also suggest that bis(2,6-dioxopiperazine) derivatives are useful agents with which to study the role of topoisomerase II in the regulation of gene expression as well as the role of the nuclear matrix.