Acute Myeloid Leukemia Cells Are Resistant to Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (Apo2L/TRAIL) by a Mechanism Independent of Multidrug Resistance.

Multidrug resistance (MDR) associated with overexpression of the ATP-binding cassette (ABC) proteins P-glycoprotein (Pgp), multidrug resistance protein (MRP-1) and breast cancer resistance protein (BCRP) is an obstacle to the successful treatment of AML. Apo2L/TRAIL binds to the extracellular death receptors DR4 and DR5 and induces tumor-specific apoptosis in diverse malignancies. Given that Apo2L/TRAIL induces apoptosis via signal transduction pathways, it has the potential to be effective in cells with MDR mediated by ABC proteins. Cell lines with MDR mediated by ABC proteins and acute myeloid leukemia (AML) cells from 10 patients (6 untreated, 4 relapsed/refractory) were tested for expression of the Apo2L/TRAIL receptors DR4 and DR5 and decoy receptors DcR1 and DcR2 by flow cytometry and for cytotoxic and pro-apoptotic effects of Apo2L/TRAIL (Genentech, Inc.). Parental HL60 and 8226 cells and resistant HL60/VCR (Pgp) and HL60/Adr (MRP-1) cells demonstrated expression of DR4 and DR5 and lack of expression of DcR1 and DcR2, while 8226/MR20 (BCRP) had only low-level expression of DR5. Parental HL60 and 8226 cells displayed dose-dependent cytotoxicity in response to Apo2L/TRAIL. HL60/VCR and HL60/Adr were 3- and 5-fold more resistant to Apo2L/TRAIL than parental HL60 cells, but were not sensitized by the Pgp- and MRP-1-specific modulators PSC-833 and MK571, demonstrating that resistance was not mediated by Pgp and MRP-1. 8226/MR20 (BCRP) cells were not resistant to Apo2L/TRAIL in relation to parental 8226 cells. Of note, cyclosporine A (CsA) sensitized all parental and multidrug resistant cells to Apo2L/TRAIL, lowering IC₅₀’s 2- to 5-fold, likely by a pro-apoptotic mechanism independent of MDR modulation. AML blasts did not express Apo2L/TRAIL receptors, with the exception of DR5 in two samples, both DR5 and DcR1 in one and DcR1 alone in a fourth. Moreover, AML blasts showed minimal or no apoptosis following 24- and 48-hour in vitro exposure to Apo2L/TRAIL in concentrations up to 1 ug/ml, 30-fold higher than the highest cytotoxic concentration in cell lines, and no enhancement of apoptosis by CsA. In conclusion, MDR mediated by ABC proteins does not appear to be associated with Apo2L/TRAIL resistance in cell line models, but AML cells are resistant to Apo2L/TRAIL, and this resistance is associated at least in part with lack of receptor expression. Strategies for upregulating Apo2L/TRAIL receptor expression on AML cells warrant further study.