2.A.41 The Concentrative Nucleoside Transporter (CNT) Family
Sequenced members of the CNT family are derived from Gram-negative and Gram-positive bacteria as well as yeast and animals. They are of about 400 residues (bacterial) or about 600-700 residues (eukaryotes) with 10-14 transmembrane α-helical spanners (TMSs). The rat CNT1 has been shown to have 13 TMSs with the hydrophilic N-terminus in the cytoplasm and the C-terminus on the extracellular side of the intestinal or renal brush border membranes of these polarized epithelial cells (Hamilton et al., 2001). The C. albicans homologue is probably of the same topology. The first 3 TMSs of the mammalian CNT1 are non-essential and are, in fact, absent from the bacterial systems (Hamilton et al., 2001). Nucleoside drug analogues and inhibitors used in cancer chemotherapy include docetaxel, uridine-furane and S-(4-nitrobenzyl)-6-thioinosine (Drápela et al. 2018).
In bacteria and yeast, CNT family members are energized by H+ symport, but in mammals they are energized by Na+ symport. The different transporters exhibit differing specificities for nucleosides. Thus, the E. coli NupC permease transports all nucleosides (both ribo- and deoxyribonucleosides) except hypoxanthine and guanine nucleosides. Another system in E. coli, NupG, a member of the MFS (TC #2.A.1.10.1), transports all ribo- and deoxyribonucleosides, while a NupG homologue, XapB (TC #2.A.1.10.2), apparently transports only xanthine. Similarly, in B. subtilis, there is evidence for three distinct nucleoside permeases, one specific for hypoxanthine and guanine nucleosides, a second specific for adenine nucleosides, and a third (B. subtilis NupC) specific for pyrimidine nucleosides (cytidine and uridine and the corresponding deoxyribonucleosides).
The mammalian permease members of the CNT family also exhibit differing specificities. Thus, rats possess at least two NupC homologues, one specific for both purine and pyrimidine nucleosides (gbU10279) and one specific for purine nucleosides (gbU25055). At least three paralogues have been characterized from humans. One human homologue (CNT1) transports pyrimidine nucleosides and adenosine, but deoxyadenosine and guanosine are poor substrates of this permease. Another (CNT2) is selective for purine nucleosides. Alteration of just a few amino acyl residues in TMSs 7 and 8 interconverts their specificities. A third homologue (CNT3) transports both purine and pyrimidine nucleosides with broad specificity (Ritzel et al., 2001). All of these transporters also accumulate various nucleoside analogue drugs such as cladribrine, 2CdA. The rat CNT2 transports this drug much better than the human orthologue due to residue substitutions in the C-terminal half of the proteins (Owen et al., 2006).
The phylogenetic tree for the CNT family shows three clusters. One includes the NupC proteins of E. coli and B. subtilis, the second includes all mammalian symporters, and the third includes functionally uncharacterized bacterial homologues (Saier et al., 1999).
The 7 known human nucleosides transporters (hNTs) exhibit varying permeant selectivities and are found into 2 protein families: the solute carrier (SLC) 29 (SLC29A1, SLC29A2, SLC29A3, SLC29A4) and SLC28 (SLC28A1, SLC28A2, SLC28A3) proteins, otherwise known, respectively, as the human equilibrative NTs (hENTs, hENT1, hENT2, hENT3, hENT4) and human concentrative NTs (hCNTs, hCNT1, hCNT2, hCNT3) (Elwi et al., 2006). The well characterized hENTs (hENT1 and hENT2) are bidirectional facilitative diffusion transporters in plasma membranes; hENT3 and hENT4 are much less well known, although hENT3, found in lysosomal membranes, transports nucleosides and is pH dependent. hENT4-PMAT is a H+/adenosine cotransporter as well as a monoamine-organic cation transporter. The 3 hCNTs are unidirectional secondary active Na+/nucleoside cotransporters. In renal epithelial cells, hCNT1, hCNT2, and hCNT3, at apical membranes, and hENT1 and hENT2 at basolateral membranes, apparently work in concert to mediate reabsorption of nucleosides from lumen to blood, driven by Na+ gradients. Secretion of some physiological nucleosides, therapeutic nucleoside analog drugs, and nucleotide metabolites of therapeutic nucleoside and nucleobase drugs likely occurs through various xenobiotic transporters in renal epithelia, including organic cation transporters, organic anion transporters, multidrug resistance related proteins, and multidrug resistance proteins. Mounting evidence suggests that hENT1 may have a presence at both apical and basolateral membranes of renal epithelia, and thus may participate in both selective secretory and reabsorptive fluxes of nucleosides (Elwi et al., 2006).
The generalized transport reaction for permeases of the CNT family is:
Nucleoside (out) + n[H+ or Na+] (out) → Nucleoside (in) + n[H+ or Na+] (in)