Nucleoside transporter (NT) plays key roles in the physiology of
nucleosides and the pharmacology of its analogues in mammals. We previously cloned
Na+/nucleoside cotransporter CNT2 from mouse M5076 ovarian
sarcoma cells, the
peptide encoded by it differing from that by the previously reported mouse CNT2 in five substitutions, and observed that the transporter can take up
cytidine, like CNT1 and CNT3. In the present study, we examined which of the two aforementioned CNT2 is the normal one, and whether or not
cytidine is transported via the previously reported CNT2. The
peptide encoded by CNT2 derived from mouse intestine, liver, spleen, and ovary was identical to that previously reported. The uptake of [3H]
cytidine, but not [3H]
thymidine, by Cos-7 cells transfected with CNT2
cDNA obtained from mouse intestine was much greater than that by mock cells, as in the case of [3H]
uridine, a typical substrate of NT. [3H]
Cytidine and [3H]
uridine were taken up via CNT2, in temperature-, extracellular Na+-, and substrate concentration-dependent manners. The uptake of [3H]
cytidine and [3H]
uridine mediated by CNT2 was significantly inhibited by the variety of
nucleosides used in this study, except for
thymidine, and inhibition of the [3H]
uridine uptake by
cytidine was competitive. The [3H]
uridine uptake via CNT2 was significantly decreased by the addition of cytarabin or
gemcitabine,
antimetabolites of
cytidine analogue. These results indicated that the previously reported mouse CNT2 is the wild-type one, and
cytidine is transported mediated by the same recognition site on the CNT2 with
uridine, and furthermore,
cytidine analogues may be substrates for the transporter.