Immunotoxin resistance in multidrug resistant cells.

Multidrug resistance (MDR) can be mediated, in part, by overexpression of P-glycoprotein (P-gp) and is characterized by broad resistance to several structurally, chemically, and pharmacologically distinct chemotherapeutic compounds. It has been hypothesized that immunological approaches to cytolysis may be used to overcome drug resistance. RV+ is a P-gp-expressing variant of the human myeloid leukemic cell line HL60 that displays a typical MDR phenotype. MDR RV+ cells displayed relative resistance to the immunotoxin (IT) HuM195-gelonin and to free rGelonin. K562 leukemia cells retrovirally infected to overexpress P-gp are also resistant to HuM195-gelonin. In addition, a monoclonal antibody capable of inhibiting the function of P-gp was able to partially reverse resistance to the IT. These data indicated that the expression of P-gp may contribute to IT resistance in RV+. Resistance to the IT was not mediated through decreased binding to cells, nor reduced internalization into the cell because the IT displayed similar kinetics of binding and internalization for both the parental HL60 and MDR RV+ cell lines. Comparison of the cytotoxicity of other ribosome-inactivating toxins indicated that RV+ cells were not universally resistant to toxins: RV+ cells were sensitive to the actions of ricin A chain, which acts on precisely the same RNase target as gelonin. Sensitivity of the MDR RV+ cells to the protein synthesis inhibitor cycloheximide, saponin, and Pseudomonas exotoxin A additionally confirmed that the resistance was not mediated through the ribosome and that pathways downstream from the inactivation of protein synthesis leading to cell death were not substantially perturbed in the MDR cells. Resistance could be partially abrogated by bafilomycin A, which inhibits lysosomal function. Moreover, direct visualization by confocal microscopy of the intracellular trafficking route of the IT showed that the IT accumulated preferentially in the lysosome in MDR RV+ cells but not in sensitive cells. These observations implicated the process of increased lysosomal degradation as the most likely basis for resistance. Such pathways of resistance may be important in the therapeutic applications of ITs, now becoming available for human use.