EMERGENCE OF DIFFERENT MECHANISMS OF RESISTANCE IN THE EVOLUTION OF MULTIDRUG-RESISTANCE IN MURINE ERYTHROLEUKEMIA CELL-LINES

We examined the genetic and biochemical bases for drug resistance and the order of appearance of different mechanisms underlying the increas ingly more resistant murine erythroleukemia cell lines established in Adriamycin(R) (ADR). In the first-step low-level resistant cell line P C4-A5 (able to grow in 5 ng/mL ADR), there was a 2-fold reduction in t opoisomerase II alpha and topoisomerase II beta mRNA levels, as well a s topoisomerase II alpha protein and activity levels as compared with the parental cell line. The topoisomerase II alpha activity levels rem ained reduced as the cells became increasingly more resistant. In cont rast, the topoisomerase II mRNA and protein levels returned to approxi mately the parental levels in resistant cells growing in higher drug c oncentrations (40-160 ng/mL). Parental cells expressed the multidrug r esistance protein (MRP), but beginning with PC4-A5 MRP expression decr eased and remained reduced in increasingly resistant cell lines. At hi gh levels of ADR resistance, the cells expressed the mdr3 gene concomi tant with the appearance of vincristine resistance and energy-dependen t daunomycin and vincristine efflux. Glutathione levels, internal pH, and expression of the major vault protein (MVP) remained unchanged in all cell lines. Fluorescence microscopy revealed no alterations in dau nomycin distribution or vesicle numbers between the parental and resis tant cell Lines. Different resistance mechanisms emerge sequentially a s cells become more resistant to ADR; the mechanisms are retained duri ng the development of multidrug resistance (MDR). In intermediate leve l MDR cell lines (PC4-A10 and PC4-A20), resistance involves an as yet undetermined mechanism(s). (C) 1997 Elsevier Science Inc.