Disease relevance of SLC28A1

• CONCLUSION: In summary, human pancreatic adenocarcinoma cells overexpress hENT1, although they retain the ability to express a functional hCNT1 transporter, an isoform that confers sensitivity to gemcitabine [1].
• Nucleoside transporter profiles in human pancreatic cancer cells: role of hCNT1 in 2',2'-difluorodeoxycytidine- induced cytotoxicity [1].
• Both proteins belong to a gene family that includes the NupC proton/nucleoside symporter of E. coli. cDNAs encoding members of the CNT family have been isolated from rat tissues (jejunum, brain, liver; rCNT1 and rCNT2/SPNT) and, most recently, human kidney (hCNT1 and hSPNT1) [2].
• These 2 antisera, along with a previously characterized antibody that specifically recognizes the high-affinity Na-dependent concentrative NT, hCNT1, have been used to analyze, using a tissue array approach, NT expression in gynecologic cancers (90 ovarian, 80 endometrial and 118 uterine cervix carcinomas) [3].
• Only a few endometrial carcinomas (15%) were found to be negative for hCNT1, but they all retained hENT1 and hENT2 expression [3].

High impact information on SLC28A1

• RESULTS: Gemcitabine was transported by most of the tested proteins (the exceptions being the purine-selective rCNT2 and hCNT2), with the greatest uptake occurring in oocytes producing recombinant rCNT1 and hCNT1 [4].
• Influxes of gemcitabine mediated by hCNT1, hENT1, and hENT2 were saturable and conformed to Michaelis-Menten kinetics with apparent K(m) values of 24, 160, and 740 microM, respectively [4].
• External application of gemcitabine to oocytes producing recombinant hCNT1 induced an inward current, which demonstrated that hCNT1 functions as a Na(+)/nucleoside co-transport protein and confirmed the transporter's ability to transport gemcitabine [4].
• We also used the two-electrode, voltage-clamp technique to investigate the electrophysiology of hCNT1-mediated gemcitabine transport [4].
• Thus, hCNT1 cannot be considered a broad-selectivity pyrimidine nucleoside carrier; in fact, very slight changes in substrate structure provoke a dramatic shift in selectivity [5].

Biological context of SLC28A1

• HNF4alpha is a major determinant of SLC28A1 expression, whereas C/EBPalpha and HNF3gamma modulate SLC28A2 gene expression [6].
• Cells expressing hCNT1 in a more constitutive manner were cloned after stable transfection of hCNT1 [1].
• NP cells transiently express hCNT1-type Na(+)-dependent nucleoside transport activity at low/medium cell density but not in confluent cultures [1].
• Two Na(+)-dependent nucleoside transporters implicated in adenosine and uridine transport in mammalian cells are distinguished functionally on the basis of substrate specificity: CNT1 is selective for pyrimidine nucleosides but also transports adenosine; CNT2 (also termed SPNT) is selective for purine nucleosides but also transports uridine [2].
• Animal and cell models genetically modified to alter nucleoside transporter expression levels may help to elucidate the particular roles of CNT proteins in cell physiology [7].

Anatomical context of SLC28A1

• Reverse transcription-PCR analysis of the other four cloned plasma membrane transporter mRNAs revealed very different expression patterns among NP cell lines, with apparent selective loss or decrease of hCNT mRNAs [1].
• In HeLa cells with recombinant human concentrative nucleoside transporter (hCNT) 1 or hCNT3 and pharmacologically blocked hENT1 and hENT2, transport of 10 microM[3H]TaraC and [3H]araC was not detected [8].
• The CNT1 and CNT2 transporters are co-expressed in liver parenchymal cells and macrophages, two suitable models in which to study cell cycle progression [7].
• When produced in Xenopus oocytes, recombinant hCNT1 is selective for pyrimidine nucleosides (system cit), whereas hCNT2 is selective for purine nucleosides (system cif) [9].
• In uterine cervix tumors, the loss of expression of hCNT1 was significantly associated with the adenocarcinoma subtype [3].

Associations of SLC28A1 with chemical compounds

• The apparent affinities of recombinant transporters (produced in yeast) for a panel of cytosine-containing nucleosides yielded results that were consistent with the observed low-permeant activities of TaraC and araC for hENT1/2 and negligible permeant activities for hCNT1/2/3 [8].
• These subtypes differ in their substrate specificities: CNT1 is pyrimidine-nucleoside preferring, CNT2 is purine-nucleoside preferring, and CNT3 transports both pyrimidine and purine nucleosides [10].
• On its own, the S353T mutation converted hCNT1 into a transporter with novel uridine-selective transport properties [9].
• Additional mutation of Leu(354) to Val (which had no effect on its own) increased the adenosine transport capability of hCNT1/S319G/Q320M/S353T, producing a full cif-type transporter phenotype [9].
• Mutation of Ser(319) in TM 7 of hCNT1 to Gly enabled transport of purine nucleosides, whereas concurrent mutation of Gln(320) to Met (which had no effect on its own) augmented this transport [9].
• Glu-322, the residue having the greatest influence on hCNT1 transport function, exhibited uridine-protected inhibition by p-chloromercuriphenyl sulfonate and 2-aminoethyl methanethiosulfonate when converted to cysteine [11].
• Although this cysteine residue is also present in hCNT1 and hCNT2, neither transporter was affected by PCMBS [12].

Other interactions of SLC28A1

• The introduction by gene transfer of hENT2 and hCNT1 activities, respectively, into nucleoside transport-defective CEM cells increased sensitivity to both drugs moderately and slightly [8].
• The human concentrative nucleoside transporter, CNT3 (SLC28A3), plays an important role in mediating the cellular entry of a broad array of physiological nucleosides and synthetic anticancer nucleoside analog drugs [13].
• Simultaneous expression of hCNT1-CFP and hENT1-YFP in Madin-Darby canine kidney cells. Localization and vectorial transport studies [14].
• By contrast, CNT1, which is a target of multifunctional cytokines involved in liver cell proliferation, does not respond to TGF-beta1 treatment [15].
• The percentage of hCNT1-positive cells correlated positively with the expression of thymidine phosphorylase and dihydropyrimidine dehydrogenase [16].

Analytical, diagnostic and therapeutic context of SLC28A1

• Chimeric studies between hCNT1 and hCNT3 located hCNT3-specific cation interactions to the C-terminal half of hCNT3, setting the stage for site-directed mutagenesis experiments to identify the residues involved [17].
• Immunohistochemistry for human Concentrative Nucleoside Transporter 3 revealed moderate to high-intensity staining in all adenocarcinoma tissue samples [18].
• Molecular cloning and functional expression of cDNAs encoding a human Na+-nucleoside cotransporter (hCNT1) [19].
• The increment of purine specific sodium nucleoside cotransporter mRNA in experimental fibrotic liver induced by bile duct ligation and scission [20].
• The production of recombinant hCNT1 and hCNT3 in a nucleoside-transporter deficient strain of yeast was confirmed by immunoblotting, and uridine transport parameters (Km, Vmax) were determined by defining the concentration dependence of initial rates of uptake of [3H]uridine by intact yeast [21].

References