CAMPTOTHECIN RESISTANCE INVOLVING STEPS SUBSEQUENT TO THE FORMATION OF PROTEIN-LINKED DNA BREAKS IN HUMAN CAMPTOTHECIN-RESISTANT KB CELL-LINES

To identify mechanisms of camptothecin (CPT) resistance/toxicity, subl ines from a human KB cell line were made resistant to CPT by continuou s selection in increasing concentrations of CPT. Two CPT-resistant lin es, 100 and 300, were 32- and 54-fold resistant to the growth-inhibito ry properties of CPT compared to the KB line. After CPT-free culturing , partial revertant lines were established from each resistant line, T hese partial revertant lines, 100(rev) and 300(rev), were 2.5- and 3.2 -fold resistant to CPT compared to KB. When growth inhibition and toxi city were compared, the resistant lines alone displayed an enhanced cy tostatic response to CPT. The resistant and partial revertant lines di splayed no cross-resistance to etoposide or cisplatin. Comparisons of topoisomerase I (TOP1) activity, content, and protein-linked DNA break production by CPT revealed that resistant and partial revertant lines had one-half the levels as KB, with TOP1 activity that was equally se nsitive to CPT in all cell lines tested. However, double-stranded DNA break induction by CPT was significantly reduced only in the resistant lines, Coincubation with 3-aminobenzamide, an inhibitor of poly(ADP-r ibosyl) polymerase, potentiated CPT toxicity in the resistant lines al one, without affecting CPT: TOP1 interactions, Therefore, CPT resistan ce in the 100 and 300 lines was characterized by factors independent o f TOP1, specific for CPT, and attenuated by poly(ADP-ribosyl) polymera se inhibition. This resistant phenotype produced fewer double-stranded DNA breaks and enhanced a cytostatic response to CPT.