Gene name: Solute carrier family 28 member 3 (SLC28A3)
Concentrative Nucleoside Transporter 3 (SLC28A3, CNT3) is a sodium-dependent transporter of naturally occurring nucleosides. CNT3 has a broad substrate specificity, accepting both purine and pyrimidine nucleosides, as well as anticancer and antiviral nucleoside derivative drugs. Current FDA and EMA recommendations do not include testing for CNT3 transporter liabilities.
Localization and expression
CNT3 protein is expressed on the apical membrane of cells and can be found in the pancreas, trachea, bone marrow, mammary gland, and intestine. CNT3 is also present, although with modest expression levels, in the liver, lung, placenta, prostate, testis, as well as in some regions of the brain and heart. Also expressed in nasal mucosa, CNT3 is thought to contribute to nasal absorption of substrate drugs . In HL 60 promyelocytic leukemia cells CNT3 expression increases with cell differentiation . Although Ritzel et al. were originally unable to detect CNT3 in the kidney, it was later confirmed by others in this location . Monocytes and monocyte-derived macrophages were also shown to express CNT3 . A truncated but functional splice variant was reported by Errasti-Murugarren et al. to be expressed in the endoplasmic reticulum (ER) in many cell types . Intracellular localization is a unique characteristic among CNTs.
Function, physiology and clinically significant polymorphisms
CNT3, cloned by Ritzel et al. in 2001 , is a 72-kDa protein with a putative structure of 13 transmembrane domains . Human CNT3 was shown to form a homotrimer . As CNT3 is widely expressed and accepts both purine and pyrimidine-based substrates, it has the theoretical capacity for functionally substituting CNT1 and CNT2 wherever it is co-expressed with either of these. CNT3-mediated transport is electrogenic, and a sigmoidal dependence of uridine influx on Na+ concentration indicated a Na+/uridine coupling ratio of at least 2:1 . A rare variant, Cys602Arg, was identified with a 1:1 ratio of cation/nucleoside transport . A distinguishing feature of CNT3 among CNTs is the acceptance of protons in its ion-binding pocket, making it the only H+-coupled nucleoside transporter in mammals [4, 8]. Proton-dependent transport is most relevant to the ER-related function of the protein.
The physiological role of this transporter has not been extensively investigated. Our general understanding that CNTs are mainly associated with cell differentiation and stress response derives from the studies of CNT1 and CNT2 (e. g. [18, 19, 20]). Notably, a recent study has shown that CNT3 transports deoxyadenosine monophosphate (dAMP), a nucleotide, albeit with about 500-fold less efficiently than it transports deoxyadenosine (dAdo) . As dAMP affects intestinal cell growth, this interaction may be of physiological significance. In addition, CNT3 transports adenosine, and thus contributes to purinergic regulation . Lactation has been shown to increase CNT3 mRNA levels 4-fold in lactating mammary epithelial cells, indicating a role for CNT3 in nucleoside accumulation in milk .
Important drug substrates of these proteins are listed comprehensively in , and include gemcitabine, fludarabine, cladribine, ddI, ddC, ribavirin, and entecavir .
CNT3 demonstrates low inter-individual diversity as only 10 nonsynonymous SNPs and 52 haplotypes were found in a study by Badagnani et al. Commonly occurring variants were tested for fludarabine and cladribine transport, but no significant changes were reported. Although the rare Gly367Arg SNP caused reduced transport of inosine and thymidine, interactions between the variant and clinically relevant drugs were not determined due to its low allele frequency. The glycine at position 367 is conserved not only in the SLC28A3 genes across mammalian species, but in SLC28A1 and SLC28A2 as well, indicating the significance of this residue .
A study from 2010 identified a haplotype with two separate nonsynonymous SNPs in the coding sequence, leading to decreased risk of hemolytic anemia in patients receiving ribavirin therapy, where the mutant haplotype is thought to take up less drug into the red blood cells, allowing them to survive .
Considering its wide substrate specificity covering both purine and pyrimidine analogue drugs, CNT3 is an obvious choice to be tested against a broad range of anticancer and antiviral agents. The analysis of thiopurine-resistant cell lines showed lower mRNA levels of CNT3 , while ribavirin was shown to be less effective after CNT3 overexpression in ribavirin-resistant cells . High expression of CNT3 in patients with chronic lymphocytic leukemia correlates with poor prognosis and, interestingly, immunostaining of the protein showed intracellular localization .
Currently, there is no recommendation by the FDA or EMA for the evaluation of CNT2. However, in vitro evaluations may be required on a case-by-case basis.
|Location||Endogenous substrates||In vitro substrates used experimentally||Substrate drugs||Inhibitors|
|pancreas, trachea, bone marrow, mammary gland, intestine, liver, lung, placenta, prostate, testis, brain, heart, immune cells||adenosine, uridine, cytidine, guanosine, inosine, thymidine,
|Adenosine Uridine||cladribine, fludarabine, 5-fluorouridine, 5-fluoro-29-deoxyuridine, zebularine, gemcitabine, AZT, ddC, ddI, araC, pirarubicine, ribavirin
|Partial inhibition by phloridzin
vidarabine, fludarabine-des-phosphate, decitabine, gemcitabine 
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4. Errasti-Murugarren, E., et al., A splice variant of the SLC28A3 gene encodes a novel human concentrative nucleoside transporter-3 (hCNT3) protein localized in the endoplasmic reticulum. FASEB J, 2009. 23(1): p. 172-82.
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19. Duflot, S., et al., ATP-sensitive K(+) channels regulate the concentrative adenosine transporter CNT2 following activation by A(1) adenosine receptors. Mol Cell Biol, 2004. 24(7): p. 2710-9.
20. Aymerich, I., et al., Extracellular adenosine activates AMP-dependent protein kinase (AMPK). J Cell Sci, 2006. 119(Pt 8): p. 1612-21.
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23. Ma, Z., et al., Multiple SLC and ABC Transporters Contribute to the Placental Transfer of Entecavir. Drug Metab Dispos, 2017. 45(3): p. 269-278.
24. Vasko, B., et al., Inhibitor selectivity of CNTs and ENTs. Xenobiotica, 2019. 49(7): p. 840-851.