Reduced cellular transport and activation of fluoropyrimidine nucleosides and resistance in human lymphocytic cell lines selected for arabinosylcytosine resistance

Arabinofuranosylcytosine (araC) resistant H9-araC0.05 and H9-araC0.5 sublines were obtained following in vitro exposure of H9 cells to 0.05 and 0.5 muM araC, respectively. These cell lines were 83.3- and 266.7-fold, 21- and 80-fold, and 2.4- and 4.0-fold more resistant to 5-fluorouridine (FUR), 5-fluoro-2'-deoxyuridine (FdUR), and 5-fluorouracil (FU), respectively. Compared with H9 cells, the cellular accumulation of FUR was 2.2 and 0.2%, FdUR 15.6 and 0.9%, and FU 56.9 and 66.5% in H9-araC0.05 and H9-araC0.5 cells, respectively. An araC resistant HL60 cell line (promyelocytic cell line) was 5.0- and 1.7-fold resistant to FUR and FdUR, respectively, but displayed no resistance to FU. The lower FUR and FdUR nucleotide levels in the resistant cells were a result of reduced cellular transport and uridine kinase (UR kinase) and thymidine kinase (TK) activities. Compared with the parental cell line, the p-nitrobenzyl thioinosine (an inhibitor of nucleoside transport) binding sites also were lower in the araC resistant cells. There was no difference in the expression of multidrug-resistant protein and thymidylate synthase mRNA in the parental and the resistant cell lines. Data presented here suggest that araC exposure of H9 cells, in addition to araC resistance, induced/selected cells that were resistant to FUR and FdUR. These cells had altered cellular drug transport and lower TK and UR kinase activities. Further studies to understand molecular mechanisms of this phenomenon are warranted. (C) 2000 Elsevier Science Inc. All rights reserved.