Two Distinct Molecular Mechanisms Underlying Cytarabine Resistance in Human Leukemic Cells

To understand the mechanism of cellular resistance to the nucleoside analogue cytarabine (1-β-d-arabinofuranosylcytosine, AraC), two resistant derivatives of the human leukemic line CCRF-CEM were obtained by stepwise selection in different concentrations of AraC. CEM/4×AraC cells showed low AraC resistance, whereas CEM/20×AraC cells showed high resistance. Both cell lines showed similar patterns of cross-resistance to multiple cytotoxic nucleoside analogues, with the exception that CEM/20×AraC cells remained sensitive to 5-fluorouridine and 2-deoxy-5-fluorouridine. Both cell lines were sensitive to 5-fluorouracil and to a variety of natural product drugs. Although both CEM/4×AraC and CEM/20×AraC cells displayed reduced intracellular accumulation of [3H]AraC, only CEM/4×AraC cells showed reduced uptake of [3H]uridine, which was used to assess nucleoside transport activities. Genes encoding proteins known to be involved in nucleoside transport, efflux, and metabolism were analyzed for the presence of mutations in the two cell lines. In CEM/4×AraC cells, independent mutations were identified at each allele of human equilibrative nucleoside transporter 1 (hENT1; SLC29A1), one corresponding to a single-nucleotide change in exon 4, the other being a complex intronic mutation disrupting splicing of exon 13. In contrast to CEM/20×AraC cells, CEM/4×AraC cells did not bind the hENT1/SLC29A1 ligand nitrobenzylmercaptopurine ribonucleoside and lacked detectable hENT1/SLC29A1 protein. In CEM/20×AraC cells, independent intronic mutations impairing splicing of exons 2 and 3 were found at each allele of the deoxycytidine kinase gene. These studies point to at least two distinct mechanisms of AraC resistance in leukemic cells. [Cancer Res 2008;68(7):2349–57]