Nucleoside transporter subtype expression: effects on potency of adenosinekinase inhibitors

1 Adenosine kinase (AK) inhibitors can enhance adenosine levels and potenti ate adenosine receptor activation. As the AK inhibitors 5' iodotubercidin ( ITU) and 5-amino-5'-deoxyadenosine (NH(2)dAdo) are nucleoside analogues, we hypothesized that nucleoside transporter subtype expression can affect the potency of these inhibitors in intact cells. 2 Three nucleoside transporter subtypes that mediate adenosine permeation o f rat cells have been characterized and cloned: equilibrative transporters rENT1 and rENT2 and concentrative transporter rCNT2. We stably transfected rat C6 glioma cells, which express rENT2 nucleoside transporters, with rENT 1 (rENT1-C6 cells) or rCNT2 (rCNT2-C6 cells) nucleoside transporters. 3 We tested the effects of ITU and NH(2)dAdo on [H-3]-adenosine uptake and conversion to [H-3]-adenine nucleotides in the three cell types. NH(2)dAdo did not show any cell type selectivity. In contrast, ITU showed significant inhibition of [H-3]-adenosine uptake and [H-3]-adenine nucleotide formatio n at concentrations less than or equal to 100 nm in rENT1-C6 cells, while c oncentrations greater than or equal to3 mum were required for C6 or rCNT2-C 6 cells. 4 Nitrobenzylthioinosine (NBMPR; 100 nm), a selective inhibitor of rENT1, a bolished the effects of nanomolar concentrations of ITU in rENT1-C6 cells. 5 This study demonstrates that the effects of ITU, but not NH(2)dAdo, in wh ole cell assays are dependent upon nucleoside transporter subtype expressio n. Thus, cellular and tissue differences in expression of nucleoside transp orter subtypes may affect the pharmacological actions of some AK inhibitors .