Human SLC2A9a and SLC2A9b isoforms mediate electrogenic transport of urate with different characteristics in the presence of hexoses.
Human SLC2A9 (GLUT9) is a novel high-capacity urate transporter belonging to the facilitated glucose transporter family. In the present study, heterologous expression in Xenopus oocytes has allowed us to undertake an in-depth radiotracer flux and electrophysiological study of urate transport mediated by both isoforms of SLC2A9 (a and b). Addition of urate to SLC2A9-producing oocytes generated outward currents, indicating electrogenic transport. Urate transport by SLC2A9 was voltage dependent and independent of the Na(+) transmembrane gradient. Urate-induced outward currents were affected by the extracellular concentration of Cl(-), but there was no evidence for exchange of the two anions. [(14)C]urate flux studies under non-voltage-clamped conditions demonstrated symmetry of influx and efflux, suggesting that SLC2A9 functions in urate efflux driven primarily by the electrochemical gradient of the cell. Urate uptake in the presence of intracellular hexoses showed marked differences between the two isoforms, suggesting functional differences between the two splice variants. Finally, the permeant selectivity of SLC2A9 was examined by testing the ability to transport a panel of radiolabeled purine and pyrimidine nucleobases. SLC2A9 mediated the uptake of adenine in addition to urate, but did not function as a generalized nucleobase transporter. The differential expression pattern of the two isoforms of SLC2A9 in the human kidney's proximal convoluted tubule and its electrogenic transport of urate suggest that these transporters play key roles in the regulation of plasma urate levels and are therefore potentially important participants in hyperuricemia and hypouricemia.
Membrane Protein Disease Research Group, Department of Physiology, University of Alberta, Edmonton, Alberta, Canada.
Kate
K
Witkowska
Kyla M
KM
Smith
Sylvia Y M
SY
Yao
Amy M L
AM
Ng
Debbie
D
O'Neill
Edward
E
Karpinski
James D
JD
Young
Christopher I
CI
Cheeseman
D000818:Animals; D001692:Biological Transport; D055724:Electrophysiological Phenomena; D005786:Gene Expression Regulation / Q000502:physiology; D051246:Glucose Transport Proteins, Facilitative / Q000235:genetics / Q000378:metabolism*; D006601:Hexoses / Q000378:metabolism*; D006801:Humans; D015640:Ion Channel Gating; D009865:Oocytes; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D014527:Uric Acid / Q000378:metabolism*; D014982:Xenopus laevis
D051246:Glucose Transport Proteins, Facilitative; D006601:Hexoses; D020033:Protein Isoforms; C411811:SLC2A9 protein, human; D014527:Uric Acid