Compensatory effects of the human nucleoside transporters on the response to nucleoside-derived drugs in breast cancer MCF7 cells

Nucleoside transporters (NTs) are involved in the cytotoxicity and transcriptomic response induced by nucleoside analogues. A relationship between the expression of nucleoside transporters and response to therapy has been demonstrated in solid tumours, although the pattern of such expression is highly variable. Thus, a question is whether the transporter expression pattern rather than specific NT proteins might better explain the ability of tumour cells to respond to nucleoside-derived drug therapy. In this study we used the breast cancer cell lines MCF7 and MCF7–hCNT1 (stably transfected with hCNT1) to determine whether hCNT1 expression can complement hENT1 functional loss in the cytotoxicity and transcriptomic response triggered by nucleoside analogues. Expression of hCNT1 slightly increased cell sensitivity to 5′-deoxy-5-fluorouridine (5′-DFUR). Inhibition of the endogenous equilibrative activity blocked 5′-DFUR cytotoxicity in MCF7 cells, but not in MCF7–hCNT1 cells. Moreover, under equilibrative transport inhibition conditions, induction of some transcriptional targets of 5′-DFUR was blocked in MCF7 cells, whereas ENT-inhibition had no effect on the transcriptional response to 5′-DFUR in MCF7–hCNT1 cells. To confirm the role of hCNT1 in 5′-DFUR treatment, a panel of nucleoside derivatives suitable for hCNT1-inhibition was obtained. The molecule T-Ala inhibited hCNT1-mediated transport. Furthermore, the cytotoxic action of 5′-DFUR and the transcriptional changes produced by this nucleoside analogue were partially inhibited by T-Ala in MCF7–hCNT1 cells. These results show a link between NT function and the pharmacogenomic response to nucleoside analogues and further support the hypothesis that the expression pattern rather than specific transporters determines the cytotoxic effect of nucleoside derivatives.