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Multidrug-Resistant Tumor Cells Remain Sensitive to a Recombinant Interleukin-4-Pseudomonas Exotoxin, Except When Overexpressing the Multidrug Resistance Protein MRP1

Departments of Pathology [M. C. d. J., G. L. S., R. J. S.], Medical Oncology [H. J. B.], and Pediatric Hematology/Oncology [J. H. H.], VU University Medical Center, 1081 HV Amsterdam, the Netherlands; Department of Molecular Recognition, Institute for Animal Science and Health (ID-DLO), 8200 AB Lelystad, the Netherlands [J. W. S., R. H. M.]; Laboratory of Molecular Biology, Division of Basic Sciences, National Cancer Institute, NIH, Bethesda, Maryland 20892 [R. J. K.]; and Laboratory of Molecular Tumor Biology, Division of Cellular and Gene Therapies, Center for Biologics Evaluation and Research, Food and Drug Administration, Bethesda, Maryland 20892 [S. R. H., B. H. J., R. K. P.]

Tumor cells may become resistant to conventional anticancer drugs through the occurrence of transmembrane transporter proteins such as P-glycoprotein (ABCB1), breast cancer resistance protein (ABCG2), or members of the multidrug resistance-associated protein family (MRP1–MRP5; ABCC1–ABCC5). In this report, we studied whether tumor cells that are cytostatic drug resistant because of overexpression of one of the above mentioned proteins are sensitive to a new anticancer agent, interleukin-4 toxin (IL-4 toxin). IL-4 toxin is a fusion protein composed of circularly permuted IL-4 and a truncated form of Pseudomonas exotoxin (PE) [IL-4(38–37)-PE38KDEL]. Ninety-six-h cytotoxicity assays and 10-day clonogenic assays showed that drug-selected multidrug resistant (MDR) tumor cells that overexpress P-glycoprotein or breast cancer resistance proteins are still sensitive to IL-4 toxin. Also, tumor cells transfected with cDNA for MRP2–5 showed no resistance, or marginal resistance, only to the toxin as compared with the parent cells. In contrast, MRP1-overexpressing cells, both drug selected and MRP1 transfected, are clearly resistant to IL-4 toxin with resistance factors of 4.3 to 8.4. MRP1-overexpressing cells were not resistant to PE itself. IL-4 toxin resistance in MRP1-overexpressing cells could be reversed by the MRP1 inhibitors probenecid or MK571 and were not affected by glutathione depletion by DL-buthionine-S,R-sulfoximine. In a transport assay using plasma membrane vesicles prepared from MRP1-overexpressing cells, IL-4 toxin and IL-4, but not PE, inhibited the translocation of the known MRP1 substrate 17ß-estradiol 17-(ß-D-glucuronide) (E₂17ßG). These data suggest that MRP1-overexpressing cells are resistant to IL-4 toxin because of extrusion of this agent by MRP1. Still, the results of this study demonstrate that IL-4 toxin effectively kills most MDR tumor cells and, therefore, represents a promising anticancer drug.

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