Cellular Uptake of Decitabine by Equilibrative Nucleoside Transporters in HCT116 Cells

DNA hypermethylation, an epigenetic change that silences gene expression without altering nucleotide sequences, plays a critical role in the formation and progression of colorectal cancers as well as in the acquisition of drug resistance. Decitabine (DAC), a DNA methyltransferase 1 inhibitor of nucleoside analogues, has been shown to restore gene expression silenced by hypermethylation. In the present study, the mechanisms underlying both uridine and DAC uptake were examined in the human colon cancer cell line HCT116. Real-time polymerase chain reaction analysis revealed that ENT1 mRNA was the most abundant among the nucleoside transporters examined in HCT116 cells. The ENT1 protein was detected in the membrane fraction, as determined by Western blotting. The uptake of uridine or DAC was time- and concentration-dependent, but also Na(+)-independent. The uptake of these agents was inhibited by S-(4-nitrobenzyl)-6-thioinosine (NBMPR), an inhibitor of equilibrative nucleoside transporters (ENTs), and was also decreased in cells treated with ENT1 small interfering RNA. The uptake of both uridine and DAC was inhibited by uridine, cytidine, adenosine, or inosine, while that of DAC was also inhibited by thymidine. The expression of MAGEA1 mRNA, the DNA of which was methylated in HCT116 cells, was increased by DAC treatment, and this increment was attenuated by concomitant treatment with NBMPR. The IC50 value of DAC was also increased in the presence of NBMPR. These results suggest that DAC is mainly taken up by ENT1 and that this uptake is one of the key determinants of the activity of DAC in HCT116 cells.