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 17beta-estradiol 17-(beta-D-glucuronide) (E(2)17betaG). 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.