NUCLEOSIDE TRANSPORTERS: CNTs AND ENTs - Drug Transporters: Molecular Characterization and Role in Drug Disposition - page 173

cells. Consequently, the expression and functional characteristics of these transporters
in target or nontarget cells, as well as in absorptive and excretory organs, will have
an important impact on the efficacy and toxicity of therapeutic nucleoside analogs.
Historically, nucleoside transport activities were first studied by functional and
kinetic analyses in mammalian cells and tissue preparations. 1 These early studies
established that there exist multiple nucleoside transporters, which can be classified
into two major systems: equilibrative and concentrative. 1 The equilibrative system
consists of some low-affinity facilitated carrier proteins that are further divided into
two subtypes, es and ei , based on their sensitivity to a highly specific inhibitor, ni-
trobenzylthioinosine [nitrobenzylmercaptopurine riboside (NBMPR)]. The es (equi-
librative inhibitor sensitive) subtype is potently inhibited by NBMPR ( K i , 0.1 to
10 nM), whereas the ei (equilibrative inhibitor insensitive) subtype is not affected
by NBMPR at concentrations below 1 M. 1 The es and ei transport both purine and

pyrimidine nucleosides and are widely distributed in different tissues and cell types.
The concentrative system consists of some high-affinity Na + -dependent transporters
that mediate uphill transport of nucleosides by coupling to the physiologic Na + gra-
dient. Several subtypes with distinct substrate specificity were recognized, with three
of them ( cit, cif, cib) well characterized. 1 The cit (N2) subtype is pyrimidine selective
but also transports adenosine. The cif (N1) subtype is mainly purine selective but
also transports uridine. The cib (N3) subtype is broadly selective, transporting both
purine and pyrimidine nucleosides. The Na + -dependent transporters appear to have
relatively limited tissue distributions and are typically found in epithelial tissues such
as the intestine, kidney, liver, and choroid plexus. 1
Molecular cloning studies in the past decade led to the isolation of genes en-
coding individual membrane proteins underlying the foregoing nucleoside transport
activities in human and other mammalian species. 3-5 They belong to two structurally
unrelated solute carrier gene families, SLC28 and SLC29. The SLC28 family consists
of three members, CNT1 (SLC28A1), CNT2 (SLC28A2), and CNT3 (SLC28A3),
which correspond to the previously characterized cit , cif , and cib subtypes respectively.
The SLC29 family consists of four members, ENT1 (SLC29A1), ENT2 (SLC29A2),
ENT3 (SLC29A3), and ENT4 (SLC29A4). ENT1 underlies the es activity, whereas
ENT2 is responsible for the ei activity. ENT3 and ENT4 are novel members identi-
fied from the human genome project. ENT3 is an intracellular low-affinity nucleo-
side transporter, 6 and ENT4 exhibits novel substrate specificity by functioning as a
polyspecific organic cation transporter. 7 , 8

μ

An overview of individual CNT and ENT

family members is presented in Table 8.1.

8.2. CONCENTRATIVE NUCLEOSIDE TRANSPORTERS (SLC28)

8.2.1. Family Members
CNT1 (SLC28A1) The first cloned Na + -dependent nucleoside transporter, rCNT1,
was isolated from a rat jejunal cDNA library in 1994 by expression cloning in Xenopus
laevis oocytes. 9 Its human homolog, hCNT1, was subsequently cloned from human
kidney. 10 The hCNT1 gene is localized to chromosome 15q25-26. hCNT1 (649 a.a.)
and rCNT1 (648 a.a.) are 85% identical in protein sequence, and are predicted to