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Topoisomerase II inhibitor-induced apoptosis in thymocytes and lymphoma cells
Abstract:
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.
Insights
Etoposide and ICRF-154 inhibit DNA topoisomerase II, but ICRF-154 induces apoptosis differently, suggesting varied cytotoxic mechanisms and roles for nuclear proteins in drug response.
Area of Science:
- Molecular Biology
- Pharmacology
- Cell Biology
Background:
- DNA topoisomerase II is a crucial nuclear enzyme regulating DNA topology and a target for antitumor drugs.
- Anticancer agents targeting topoisomerase II primarily induce apoptosis, programmed cell death.
Purpose of the Study:
- To investigate the distinct mechanisms of cytotoxicity induced by etoposide and ICRF-154, topoisomerase II inhibitors.
- To explore the role of DNA breaks and nuclear matrix proteins in drug-induced apoptosis.
Main Methods:
- Comparison of etoposide (cleavable complex-forming) and ICRF-154 (catalytic inhibitor) effects on thymocytes and RVC cells.
- Assessment of topoisomerase II activity, cell cycle progression, gene expression changes, and apoptosis induction.
- Analysis of specific gene alterations in response to different topoisomerase II inhibitors.
Main Results:
- Both etoposide and ICRF-154 inhibited topoisomerase II activity, but ICRF-154 induced apoptosis in thymocytes without forming cleavable complexes.
- ICRF-154 caused G2/M cell cycle arrest and cell death in RVC cells, distinct from etoposide's effects.
- Etoposide-induced apoptosis in RVC cells involved suppression of hnRNP A1 and a novel gene, linked to DNA breaks, which ICRF-154 did not affect.
Conclusions:
- ICRF-154 exhibits a unique cytotoxic mechanism compared to etoposide, highlighting differences in topoisomerase II inhibition.
- Apoptosis induction in thymocytes by ICRF-154 suggests that cleavable complexes are not essential, implicating tissue-specific nuclear matrix proteins.
- Bis(2,6-dioxopiperazine) derivatives like ICRF-154 are valuable tools for studying topoisomerase II function and nuclear matrix roles in gene regulation.