Degradation of topoisomerase I induced by topoisomerase I inhibitors is dependent on inhibitor structure but independent of cell death

DNA topoisomerase I (top I) is the target of the antitumor drug camptothecin (CPT) and its analogs. CPT induces dose- and time-dependent degradation of top I. Degradation of top I also occurs in a CPT-resistant cell line and, therefore, is not a consequence of cell death. Top I degradation is preceded by the appearance of a high molecular weight ladder of top I immunoreactivity and can be blocked by specific inhibitors of the proteasome. We compared the effects of five top I poisons [CPT, topotecan, 6-N-formylamino-12,13-dihydro-1, 11-dihydroxy-13-(beta-D-glucopyranosyl)-5H-indolo[2,3-a]pyrrolo[3, 4-c]carbazole-5,7(6H)-dione (NB506), camptothecin-(para)-4beta-amino-4'-O-demethyl Epipodophyllotoxin (W1), and camptothecin-(ortho)-4beta-amino-4'-O-demethyl Epipodophyllotoxin (W2)] on cleavable complex formation and top I degradation. Although all five drugs induced cleavable complex formation, two of the drugs, NB506 and W1 did not induce top I degradation.