CAMPTOTHECIN HYPER-RESISTANT P388 CELLS - DRUG-DEPENDENT REDUCTION INTOPOISOMERASE-I CONTENT

A subline of P388 leukemia made 10-fold resistant to camptothecin (CPT ) by serial passage in drug-treated mice was adapted to growth in tiss ue culture and made hyper-resistant to CPT by passage in the presence of increasing concentrations of the drug. Cells were obtained that wer e 1,000-fold resistant to CPT, compared to wild-type P388 cells. Neith er topoisomerase I mRNA nor 100 kDa topoisomerase I enzyme was detecta ble in these cells, and topoisomerase I activity extracted from nuclei was less than 4% of that extracted from nuclei of wild-type cells. An immunoreactive 130 kDa protein that could be an altered, inactive for m of topoisomerase I was evident in the hyper-resistant cells. In addi tion, the cells deficient in topoisomerase I contained enhanced topois omerase II activity. Maintenance of the hyper-resistant phenotype requ ired continued exposure to CPT; growth in its absence led to loss of h yper-resistance, increased topoisomerase I content and activity, and d ecreased topoisomerase II activity. The sensitivity of the cells to ki lling by a number of inhibitors of topoisomerases I and II was consist ent with these observations. Thus, P388 cells have the potential to be come highly resistant to CPT by severely curtailing topoisomerase I ex pression; in these circumstances, topoisomerase I and II activities ar e regulated coordinately.