Functional characteristics of equilibrative nucleoside transporter 2 (ENT2/SLC29A2) for the transport of urate as a newly identified substrate

Equilibrative Nucleoside Transporters (ENTs) are known to be involved in the membrane transport of nucleosides and nucleobases. We here report our finding that ENT2/SLC29A2 can also transport urate, which is the final metabolite of purine nucleobases. In transiently transfected human embryonic kidney 293 (HEK293) cells, urate uptake by human ENT2 was quite efficient at an acidic pH of 5.5, being saturable with the K_m of 1.64 mM and inhibited by its nucleoside substrates and specific inhibitors. Although the uptake activity decreased with increasing pH, it was still detected at around near neutral pH. In the Caco-2 cell model, urate uptake was reduced by ENT2 inhibitors and by ENT2 knockdown, indicating the involvement of ENT2 in urate transport. In HEK293 cells transiently transfected with sodium-dependent nucleobase transporter 1, which can accumulate urate, urate uptake was reduced by cotransfection of ENT2, suggesting its operation for urate efflux. Interestingly, ENT2 with N68K mutation was found to have little urate uptake activity, whereas the urate efflux activity was maintained. This finding suggests that ENT2 may operate in different modes for the uptake and efflux of urate. Overall, this study provides novel insight into the function of ENT2 and its potential role in urate disposition.