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Volume 272, Number 29, Issue of July 18, 1997 pp. 18026-18032
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

Characterization of a Novel Na⁺-dependent, Guanosine-specific, Nitrobenzylthioinosine-sensitive Transporter in Acute Promyelocytic Leukemia Cells

(Received for publication, March 20, 1997, and in revised form, May 13, 1997)

From the Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada

NB4 cells are the only bona fide in vitro model of human acute promyelocytic leukemia. We have examined cytidine and guanosine transport in this cell line and characterized a novel guanosine-specific transporter. Cytidine transport occurred predominately by equilibrative nitrobenzylthioinosine (NBMPR)-sensitive (es) transport. In the presence of Na⁺, guanosine at various concentrations accumulated at least 6-fold above equilibrium. The initial rate of guanosine transport in Na⁺ buffer decreased by 75% with the addition of 1 µM NBMPR and the IC₅₀ for NBMPR inhibition was 0.7 ± 0.1 nM. Replacement of Na⁺ with choline also resulted in a 75% decrease in total guanosine transport. The potent inhibition of guanosine transport by NBMPR and the loss of transport in choline suggested that a Na⁺-dependent NBMPR-sensitive transporter was responsible for the majority of guanosine uptake. This concentrative, sensitive transporter is Na⁺ dependent with a stoichiometric coupling ratio of 1:1. This novel transporter, referred to as csg, is guanosine-specific with total guanosine transport inhibited by only 50% in the presence of 1 mM competing nucleosides. HL-60, acute myelocytic leukemia cells, do not exhibit csg activity while L1210, murine acute lymphocytic leukemia cells, exhibit csg transport. The presence of the csg transporter suggests an important role for guanosine in particular forms of leukemia and may provide a new target for cytotoxic therapy.

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