Expression of ABCC-Type Nucleotide Exporters in Blasts of Adult Acute Myeloid Leukemia: Relation to Long-term Survival

Purpose: Successful treatment of acute myeloid leukemia (AML) remains a therapeutic challenge, with a high percentage of patients suffering from persistent or relapsed disease. Resistance to drug therapy can develop from increased drug export and/or altered intracellular signaling. Both mechanisms are mediated by the efflux transporters ABCC4 (MRP4), ABCC5 (MRP5), and ABCC11 (MRP8), which are involved in cellular efflux of endogenous signaling molecules (e.g., cyclic adenosine 3',5'-monophosphate and cyclic guanosine 3',5'-monophosphate) and nucleoside analogues. The nucleoside analogue cytosine arabinoside (AraC) is administered to all patients with AML. Experimental Design: Expression of ABCC transporters MRP4, MRP5, and MRP8 in blast samples from 50 AML patients was investigated by real-time reverse transcription-PCR analysis and correlated with clinical outcome measures. Accumulation of radiolabeled AraC, transport of AraC metabolites, and AraC cytotoxicity were analyzed in MRP8-transfected LLC-PK1 cells. Results: Regression analysis revealed that high expression of MRP8 is associated with a low probability of overall survival assessed over 4 years (P < 0.03). MRP8-transfected LLC-PK1 cells accumulated reduced intracellular levels of AraC (63% of the parental vector-transfected LLC-PK1 control cells) as well as AraC metabolites. Furthermore, AraC monophosphate was transported by MRP8-enriched membrane vesicles (116 +/- 6 versus 65 +/- 13 pmol/mg/10 minutes by control vesicles), and MRP8-transfected cells were resistant to AraC. Conclusion: These data suggest that MRP8 is differentially expressed in AML blasts, that expression of MRP8 serves as a predictive marker for treatment outcome in AML, and that efflux of AraC metabolites by MRP8 is a mechanism that contributes to resistance of AML blasts.