Contribution of an unidentified sodium-dependent nucleoside transport system to the uptake and cytotoxicity of anthracycline in mouse M5076 ovarian sarcoma cells

In the present study, we investigated whether an unidentified system for Na+-dependent nucleoside transport is expressed by mouse M5076 ovarian sarcoma cells, besides concentrative nucleoside transporter 2 (CNT2M), and is involved in the uptake and cytotoxicity of anthracyclines. In a transport assay involving CNT2M-transfectants, CNT2M was found to transport [3H]cytidine in a Na+-dependent manner, and 500 μM cytidine completely inhibited the Na+-dependent uptake of [3H]uridine via the transporter. In contrast, the Na+-dependent [3H]uridine uptake by M5076 cells decreased with 500 μM cytidine only to 70% of the control level. Furthermore, transfection of CNT2M-specific siRNAs into M5076 cells resulted in a reduction in the Na+-dependent uptake of [3H]uridine by only 23%, although the expression of CNT2M mRNA and Na+-dependent uptake of [3H]cytidine disappeared in the cells. The uptake of pirarubicin (THP), an anthracycline, by M5076 cells requiring extracellular Na+ was significantly inhibited by 500 μM uridine, but not 500 μM cytidine. The Na+-dependent and cytidine-insensitive uptake of [3H]uridine and the that of THP by M5076 cells significantly increased on cotreatment with both cholate and taurocholate, and the enhancement of THP uptake by the bile acids was reversed by cotreatment with 500 μM uridine. Furthermore, the cytotoxicity of THP and doxorubicin, which were previously reported to be taken up via the same transporter, toward M5076 cells was enhanced by cotreatment with both the bile acids. Therefore, it was indicated that an unidentified Na+-dependent transport system for nucleosides is expressed by M5076 cells, and contributes to the uptake and cytotoxicity of the anthracyclines.