Functional expression of human intestinal Na+-dependent and Na+-independent nucleoside transporters in Xenopus laevis oocytes

Inhibitor selectivity of CNTs and ENTs

The concentrative nucleoside transporters (CNT; solute carrier family 28 (SLC28)) and the equilibrative nucleoside transporters (ENT; solute carrier family 29 (SLC29)) are important therapeutic targets but may also mediate toxicity or adverse events. To explore the relative role of the base and the monosaccharide moiety in inhibitor selectivity we selected compounds that either harbor an arabinose moiety or a cytosine moiety, as these groups had several commercially available drug members. The screening data showed that more compounds harboring a cytosine moiety displayed potent interactions with the CNTs than compounds harboring the arabinose moiety. In contrast, ENTs showed a preference for compounds with an arabinose moiety. The correlation between CNT1 and CNT3 was good as five of six compounds displayed IC₅₀ values within the threefold threshold and one displayed a borderline 4-fold difference. For CNT1 and CNT2 as well as for CNT2 and CNT3 only two of six IC₅₀ values correlated and one displayed a borderline 4-fold difference. Interestingly, of the six compounds that potently interacted with both ENT1 and ENT2 only nelarabine displayed selectivity. Our data show differences between inhibitor selectivities of CNTs and ENTs as well as differences within the CNT family members.

Visualizing nucleic acid metabolism using non-natural nucleosides and nucleotide analogs

Nucleosides and their corresponding mono-, di-, and triphosphates play important roles in maintaining cellular homeostasis. In addition, perturbations in this homeostasis can result in dysfunctional cellular processes that cause pathological conditions such as cancer and autoimmune diseases. This review article discusses contemporary research areas applying nucleoside analogs to probe the mechanistic details underlying the complexities of nucleoside metabolism at the molecular and cellular levels. The first area describes classic and contemporary approaches used to quantify the activity of nucleoside transporters, an important class of membrane proteins that mediate the influx and efflux of nucleosides and nucleobases. A focal point of this section is describing how biophotonic nucleosides are replacing conventional assays employing radiolabeled substrates to study the mechanism of these proteins. The second section describes approaches to understand the utilization of nucleoside triphosphates by cellular DNA polymerases during DNA synthesis. Emphasis here is placed on describing how novel nucleoside analogs such as 5-ethynyl-2'-deoxyuridine are being used to quantify DNA synthesis during normal replication as well as during the replication of damaged DNA. In both sections, seminal research articles relevant to these areas are described to highlight how these novel probes are improving our understanding of these biological processes. This article is part of a Special Issue entitled: Physiological Enzymology and Protein Functions.

Role of the equilibrative and concentrative nucleoside transporters in the intestinal absorption of the nucleoside drug, ribavirin, in wild-type and Ent1(-/-) mice

Ribavirin is frontline treatment for hepatitis C virus infection. To determine the role of nucleoside transporters in the intestinal absorption of orally administered ribavirin, we perfused the intestines of Ent1(-/-) and wild-type mice, in situ, with [(3)H] ribavirin (20, 200, and 5000 μM) in the presence and absence of sodium. The decrease in luminal ribavirin concentration over 30 min was measured at 5 min intervals. Blood samples were collected approximately every 10 min. Ribavirin plus phosphorylated metabolite concentrations (hereafter referred to as ribavirin) were determined in tissue, blood, and plasma by HPLC fractionation and scintillation counting. There was no significant difference between wild-type and Ent1(-/-) mice in intestinal loss of ribavirin at any ribavirin concentration studied. Perfusions without sodium drastically reduced the intestinal loss of ribavirin in both wild-type and Ent1(-/-) mice. After 20 μM ribavirin perfusions, Ent1(-/-) intestinal tissue contained 8-fold greater ribavirin than wild-type mice (p < 0.01). Ribavirin concentrations in the wild-type intestinal tissue were 70-fold higher after 200 vs 20 μM perfusions (p < 0.001), indicating saturation of intestinal ribavirin efflux and possibly other processes as well. Ribavirin plasma concentrations were significantly higher in wild-type mice (2.7-fold) vs Ent1(-/-) mice at 30 min after the 20 μM perfusion (p < 0.01). These results suggest that, at lower intestinal concentrations of ribavirin, concentrative and equilibrative nucleoside transporters are important in the intestinal absorption of ribavirin. At higher intestinal concentrations, these transporters are saturated and other processes in the intestine (transport and/or metabolism) play an important role in the absorption of ribavirin.

The role of nucleoside transporters in the erythrocyte disposition and oral absorption of ribavirin in the wild-type and equilibrative nucleoside transporter 1-/- mice

Ribavirin [1-(beta-d-ribofuranosyl)-1H-1,2,4-triazole-3-carboxamide] is the treatment of choice for hepatitis C virus infection. Ribavirin is a substrate of several nucleoside transporters, including the equilibrative nucleoside transporter (Ent) and the concentrative nucleoside transporter 2. To determine the role of Ent1 in ribavirin absorption and erythrocyte distribution, we examined its pharmacokinetics in Ent1-null mice. After intravenous administration, we found that the erythrocyte area under the curve (AUC(0-12 h)) was reduced 3.05-fold along with 2.63-fold reduction of erythrocyte versus plasma AUC ratio in the Ent1(-/-) mice, whereas there was no significant difference in the plasma AUC(0-12 h) between Ent1(+/+) and Ent1(-/-) mice. After 48 h, we found a similar fraction of ribavirin or total radioactivity excreted in the urine between the Ent1(+/+) and Ent1(-/-) mice. After oral administration of three different doses, 0.024, 0.24, and 6.1 mg/kg, we found that the dose-normalized plasma AUC(0-12 h) of ribavirin was 69.7 +/- 12.0, 20.7 +/- 1.5, and 18.3 +/- 2.7 min/l, respectively, in the Ent1(+/+) mice and 18.9 +/- 2.8, 13.0 +/- 0.5, and 12.2 +/- 1.0 min/l, respectively, in the Ent1(-/-) mice. It is interesting that at the highest dose, the dose-normalized plasma AUC(0-30 min), AUC(0-12 h), and C(max) in the Ent1(+/+) mice were decreased 4.0-, 3.8-, and 3.4-fold, respectively, compared with the lowest dose, suggesting absorption was saturated at the highest dose we used. The dose-normalized plasma AUC(0-12 h) was 3.7- and 1.5-fold lower at the lowest and the highest dose, respectively, in the Ent1(-/-) mice compared with those of the Ent1(+/+) mice. Our findings indicate that Ent1 plays a significant role in the oral absorption and erythrocyte distribution of ribavirin.

New insights into the pharmacology and cytotoxicity of gemcitabine and 2',2'-difluorodeoxyuridine

In a clinical study with oral gemcitabine (2',2'-difluorodeoxycytidine, dFdC), 2',2'-difluorodeoxyuridine (dFdU) was extensively formed and accumulated after multiple oral dosing. Here, we have investigated the in vitro cytotoxicity, cellular uptake, efflux, biotransformation, and nucleic acid incorporation of dFdC and dFdU. Short-term and long-term cytotoxicity assays were used to assess the cytotoxicity of dFdC and dFdU in human hepatocellular carcinoma HepG2, human lung carcinoma A549, and Madin-Darby canine kidney cell lines transfected with the human concentrative or equilibrative nucleoside transporter 1 (hCNT1 or hENT1), or empty vector. Radiolabeled dFdC and dFdU were used to determine cellular uptake, efflux, biotransformation, and incorporation into DNA and RNA. The compounds dFdC, dFdU, and their phosphorylated metabolites were quantified by high-performance liquid chromatography with UV and radioisotope detection. dFdU monophosphate, diphosphate, and triphosphate (dFdU-TP) were formed from dFdC and dFdU. dFdU-TP was incorporated into DNA and RNA. The area under the intracellular concentration-time curve of dFdC-TP and dFdU-TP and their extent of incorporation into DNA and RNA inversely correlated with the IC(50) of dFdC and dFdU, respectively. The cellular uptake and cytotoxicity of dFdU were significantly enhanced by hCNT1. dFdU inhibited cell cycle progression and its cytotoxicity significantly increased with longer duration of exposure. dFdU is taken up into cells with high affinity by hCNT1 and phosphorylated to its dFdU-TP metabolite. dFdU-TP is incorporated into DNA and RNA, which correlated with dFdU cytotoxicity. These data provide strong evidence that dFdU can significantly contribute to the cytotoxicity of dFdC.

Expression and hepatobiliary transport characteristics of the concentrative and equilibrative nucleoside transporters in sandwich-cultured human hepatocytes

We previously reported that both the concentrative (hCNT) and equilibrative (hENT) nucleoside transporters are expressed in the human liver (21). Here we report a study that investigated the expression of these transporters (transcripts and proteins) and their role in the hepatobiliary transport of nucleosides/nucleoside drugs using sandwich-cultured human hepatocytes. In the hepatic tissue, the rank order of the mRNA expression of the transporters was hCNT1 approximately hENT1>hENT2 approximately hCNT2>hCNT3. In sandwich-cultured hepatocytes, the mRNA expression of hCNT2 and hENT2 was comparable to that in hepatic tissue, whereas the expression of corresponding transporters in the two-dimensional hepatocyte cultures was lower. Colocalization studies demonstrated predominant localization of these transporters at the sinusoidal membrane and of hENT1, hCNT1, and hCNT2 at the canalicular membrane. In the sandwich-cultured hepatocytes, ENTs were the major contributors to the transport of thymidine (hENT1, 63%; hENT2, 23%) or guanosine (hENT1, 53%; hENT2, 24%) into the hepatocytes followed by hCNT1 (10%) for thymidine or hCNT2 (23%) for guanosine. Although ribavirin was predominately transported (89%) into the hepatocytes by hENT1, fialuridine (FIAU) was transported by both hENT1 (30%) and hCNTs (61%). The extensively metabolized natural nucleosides were not effluxed into the bile, whereas significant biliary-efflux was observed of FIAU (19%), ribavirin (30%), and formycin B (35%). We conclude that the hepatic activity of hENT1 and hCNT1/2 transporters will determine the in vivo hepatic distribution and therefore the efficacy and/or toxicity of nucleoside drugs used to treat hepatic diseases.

Prolonged versus standard gemcitabine infusion: translation of molecular pharmacology to new treatment strategy

Gemcitabine is frequently used in the treatment of patients with solid tumors. Gemcitabine is taken up into the cell via human nucleoside transporters (hNTs) and is intracellularly phosphorylated by deoxycytidine kinase (dCK) to its monophosphate and subsequently into its main active triphosphate metabolite 2',2'-difluorodeoxycytidine triphosphate (dFdCTP), which is incorporated into DNA and inhibits DNA synthesis. In addition, gemcitabine is extensively deaminated to 2',2'-difluorodeoxyuridine, which is largely excreted into the urine. High expression levels of human equilibrative nucleoside transporter type 1 were associated with a significantly longer overall survival duration after gemcitabine treatment in patients with pancreatic cancer. Clinical studies in blood mononuclear and leukemic cells demonstrated that a lower infusion rate of gemcitabine was associated with higher intracellular dFdCTP levels. Prolonged infusion of gemcitabine at a fixed dose rate (FDR) of 10 mg/m2 per minute was associated with a higher intracellular accumulation of dFdCTP, greater toxicity, and a higher response rate than with the standard 30-minute infusion of gemcitabine in patients with pancreatic cancer. In the current review, we discuss the molecular pharmacology of nucleoside analogues and the influence of hNTs and dCK on the activity and toxicity of gemcitabine, which is the basis for clinical studies on FDR administration, and the results of FDR gemcitabine administration in patients. These findings might aid optimal clinical application of gemcitabine in the future.

In situ hybridization and immunolocalization of concentrative and equilibrative nucleoside transporters in the human intestine, liver, kidneys, and placenta

To better understand the role of human equilibrative (hENTs) and concentrative (hCNTs) nucleoside transporters in physiology and pharmacology, we investigated the regional, cellular, and spatial distribution of two hCNTs (hCNT1 and hCNT2) and two hENTs (hENT1 and hENT2) in four human tissues. Using in situ hybridization and immunohistochemical techniques, we found that the duodenum expressed hCNT1 and hCNT2 mRNAs in enterocytes and hENT1 and hENT2 mRNAs in crypt cells. In these cells, the hCNT and hENT proteins were predominantly localized in the apical and lateral membrane, respectively. Hepatocytes expressed higher levels of mRNAs of hENT1, hCNT1, and hENT2 than of hCNT2 and expressed all these proteins at hepatocyte cell borders and in the cytoplasm. While the kidney expressed hCNT1 and hCNT2 mRNAs in the proximal tubules, hENT1 and hENT2 mRNAs were present in the distal tubules, glomeruli, endothelial cells, and vascular smooth muscle cells. Proximal tubules adjacent to corticomedullary junctions expressed hENT1, hCNT1, and hCNT2 mRNA. Immunolocalization studies revealed predominant localization of hCNTs in the brush-border membrane of the proximal tubular epithelial cells and hENTs in the basolateral membrane of the distal tubular epithelial cells. Chorionic villi sections of human term placenta expressed mRNAs and proteins for hENT1 and hENT2 but only mRNA for hCNT2. Immunolocalization studies showed presence of hENT1 in the brush-border membrane of the syncytiotrophoblasts. These data are critical for a better understanding of the role of nucleoside transporters in the physiological and pharmacological effects of nucleosides and nucleoside drugs, respectively.

Localization of broadly selective equilibrative and concentrative nucleoside transporters, hENT1 and hCNT3, in human kidney

Nucleoside transporters in kidney mediate renal reabsorption and secretion of nucleosides. Using RT-PCR, we demonstrated mRNAs encoding hENT1, hENT2, hCNT1, hCNT2, and hCNT3 in both cortex and medulla. Immunoblotting with crude membrane preparations revealed abundant hENT1 and hCNT3 in both cortex and medulla, and little, if any, hENT2, hCNT1, or hCNT2, indicating that the latter were either absent or below limits of detection of immunoassays. hENT1 immunostaining was observed on apical surfaces of proximal tubules and on both apical and basal surfaces of thick ascending loops of Henle and collecting ducts. Prominent hCNT3 immunostaining was observed on apical surfaces of proximal tubules and thick ascending loops of Henle in addition to some cytoplasmic staining. Equilibrium binding of [3H]nitrobenzylmercaptopurine ribonucleoside (NBMPR), a high-affinity inhibitor of hENT1, to brush-border membrane vesicles from cortex confirmed the presence of hENT1 on apical surfaces of proximal tubules. Uptake of [3H]uridine by polarized renal proximal tubule cells exhibited a sodium-dependent component that was inhibited by thymidine and inosine as well as a sodium-independent component that was partially inhibited by NBMPR and completely inhibited by dilazep, indicating high levels of hENT1 and hCNT3 and low levels of hENT2 activities. The presence of 1) transcripts for hENT1/2 and hCNT1/2/3 and the hENT1 and hCNT3 proteins in human kidneys and 2) hENT1, hENT2, and hCNT3 activities in cultured proximal tubule cells suggest involvement of hENT1, hCNT3, and possibly also hENT2 in renal handling of nucleosides and nucleoside drugs.

Exogenous nucleic acids and nucleotides are efficiently hydrolysed and taken up as nucleosides by intestinal explants from suckling piglets

Human milk is a rich source of RNA, free nucleotides (NT) and nucleosides (NS). To determine the uptake of different NS sources by the intestinal epithelium, jejunal explants from suckling piglets were cultured in a medium supplemented with a mixture of NS (adenosine, cytidine, guanosine, inosine, uridine; 10 mg/l each), a mixture of five NT (AMP, CMP, GMP, IMP, UMP; 7 mg/l each) or RNA (60 mg/l), respectively. Aliquots from the media were taken at different times (0.5, 2, 5, 15, 30, 60, 180 min). NS and NT concentrations were analysed in the different supernatants at those periods using solid-phase extraction followed by HPLC. When explants were cultured in the presence of NS the concentration of these compounds, excepting cytidine, rapidly decreased, suggesting that they are efficiently taken up. When explants were incubated in the presence of NT, the total concentration of these compounds decreased while the total concentration of NS increased, suggesting that enterocytes efficiently hydrolyse NT into NS. Likewise, when explants were incubated in the presence of RNA, the total concentration of both NT and NS increased, indicating that intestinal explants are able to hydrolyse RNA to NT and then to NS in the absence of luminal enzymes. In conclusion, the jejunum of piglets at weaning is able to hydrolyse RNA and free NT to NS, and NS, excepting cytidine, are efficiently taken up by the small intestine. These results suggest that the current concentration of NT used to supplement infant formulas should be reconsidered.

Nucleoside transport in primary cultured rabbit tracheal epithelial cells

The present study aimed at elucidating the mechanisms of nucleoside transport in primary cultured rabbit tracheal epithelial cells (RTEC) grown on a permeable filter support. Uptake of (3)H-uridine, the model nucleoside substrate, from the apical fluid of primary cultured RTEC was examined with respect to its dependence on Na(+), substrate concentration, temperature and its sensitivity to inhibitors, other nucleosides and antiviral nucleoside analogs. Apical (3)H-uridine uptake in primary cultured RTEC was strongly dependent on an inward Na(+) gradient and temperature. Ten micromolar nitro-benzyl-mercapto-purine-ribose (NBMPR) (an inhibitor of es-type nucleoside transport in the nanomolar range) did not further inhibit this process. (3)H-uridine uptake from apical fluid was inhibited by basolateral ouabain (10 microM) and apical phloridzin (100 microM), indicating that uptake may involve a secondary active transport process. Uridine uptake was saturable with a K(m) of 3.4 +/- 1.8 microM and the V(max) of 24.3 +/- 5.2 pmoles/mg protein/30 s. Inhibition studies indicated that nucleoside analogs that have a substitution on the nucleobase competed with uridine uptake from apical fluid, but those with modifications on the ribose sugar including acyclic analogs were ineffective. The pattern of inhibition of apical (3)H-uridine, (3)H-inosine and (3)H-thymidine uptake into RTEC cells by physiological nucleosides was consistent with multiple systems: A pyrimidine-selective transport system (CNT1); a broad nucleoside substrate transport system that excludes inosine (CNT4) and an equilibrative NBMPR-insensitive nucleoside transport system (ei type). These results indicate that the presence of apically located nucleoside transporters in the epithelial cells lining the upper respiratory tract can lead to a high accumulation of nucleosides in the trachea. At least one Na(+)-dependent, secondary, active transport process may mediate the apical absorption of nucleosides or analogous molecules.

Functional and pharmacological mechanisms of nucleoside transport across the basolateral membrane of rabbit tracheal epithelial cells

The role of basolateral membrane nucleoside transport in primary cultured rabbit tracheal epithelial cells (RTEC) was studied. Primary cultured RTEC were grown on permeable support at an air-interface. Transport studies were conducted in the uptake, efflux, and transepithelial transport configurations using (3)H-uridine as a model substrate. Time, temperature and concentration dependency of (3)H-uridine transport were evaluated in parallel to the metabolism of this substrate using scintillation counting and thin layer chromatography. Inhibition of (3)H-uridine uptake from basolateral fluid was estimated in presence of all unlabeled natural nucleosides as well as analogs and nucleobases. Functional modulation pathways of (3)H-uridine uptake were studied after treatment of RTEC with pharmacological levels of A23187, forskolin, tamoxifen, H89 and colchicine. The basolateral aspect has a low-affinity and high-capacity transport system that exhibits characteristics of bi-directionality, temperature/concentration dependency, and broad specificity towards purines and pyrimidines without requiring Na(+). Basolateral equilibrative-sensitive/insensitive (es/ei) type transport machinery manifested as a biphasic dose response to nitro-benzyl-mercapto-purine-ribose (NBMPR) inhibition. In addition, a number of therapeutically relevant nucleoside analogs appeared to compete with the uptake of uridine from basolateral fluid. Short-term pre-incubation of primary cultured RTEC with the calcium ionophore A23187 inhibited basolateral uridine uptake without affecting the J(max) and K(m). The inhibitory effect was not reversible with a protein kinase C (PKC) antagonist, tamoxifen. In contrast, basolateral uridine uptake was increased by adenylyl cyclase activator forskolin (reversible with protein kinase A (PKA) inhibitor H89), resulting in a decreased K(m), but a lower J(max). Uridine exit across the basolateral membrane of primary cultured RTEC occurs via a facilitative diffusion carrier, which can be modulated by intracellular Ca(2+) levels and PKA. Information about these carriers will help improve the transportability of antitumor and antiviral nucleoside analogs in the pulmonary setting.

Long term endocrine regulation of nucleoside transporters in rat intestinal epithelial cells

We studied the regulation of nucleoside transporters in intestinal epithelial cells upon exposure to either differentiating or proliferative agents. Rat intestinal epithelial cells (line IEC-6) were incubated in the presence of differentiating (glucocorticoids) or proliferative (EGF and TGF-α) agents. Nucleoside uptake rates and nucleoside transporter protein and mRNA levels were assessed. The signal transduction pathways used by the proliferative stimuli were analyzed. We found that glucocorticoids induce an increase in sodium-dependent, concentrative nucleoside transport rates and in protein and mRNA levels of both rCNT2 and rCNT1, with negligible effects on the equilibrative transporters. EGF and TGF-α induce an increase in the equilibrative transport rate, mostly accounted for by an increase in rENT1 activity and mRNA levels, rENT2 mRNA levels remaining unaltered. This effect is mimicked by another proliferative stimulus that functions as an in vitro model of epithelial wounding. Here, rENT1 activity and mRNA levels are also increased, although the signal transduction pathways used by the two stimuli are different. We concluded that differentiation of rat intestinal epithelial cells is accompanied by increased mature enterocyte features, such as concentrative nucleoside transport (located at the brush border membrane of the enterocyte), thus preparing the cell for its ultimate absorptive function. A proliferative stimulus induces the equilibrative nucleoside activities (mostly through ENT1) known to be located at the basolateral membrane, allowing the uptake of nucleosides from the bloodstream for the increased demands of the proliferating cell.

Cell entry and export of nucleoside analogues

Some nucleoside analogues currently used as antiretroviral agents might promote mutagenesis besides their putative ability to interfere with endogenous nucleotide metabolism and/or inhibit viral transcription. The intracellular concentration of nucleosides and nucleobases is to some extent the result of the metabolic background of the specific cell line used for infection studies, its particular suit of enzymes and transporters. This review focuses on the transporter-mediated pathways implicated in either the uptake or the efflux of nucleoside- and nucleobase-derivatives. From a biochemical point of view, four different types of transport processes for nucleoside-related antiviral drugs have been described: (1) equilibrative uniport, (2) substrate exchange, (3) concentrative Na+- or H+-dependent uptake and finally, (4) substrate export through primary ATP-dependent active efflux pumps. These mechanisms are mainly related to the following set of transporter families: Concentrative Nucleoside Transporter (CNT), Equilibrative Nucleoside Transporter (ENT), Organic Anion Transporter (OAT) and Organic Cation Transporter (OCT), Peptide Transporter (PEPT) and Multidrug Resistance Protein (MRP). The basic properties of these carrier proteins and their respective role in the transport across the plasma membrane of nucleoside-derived antiviral drugs are reviewed.

Residues Met89 and Ser160 in the Human Equilibrative Nucleoside Transporter 1 Affect Its Affinity for Adenosine, Guanosine, S6-(4-Nitrobenzyl)-mercaptopurine Riboside, and Dipyridamole

The human equilibrative nucleoside transporter 1 (hENT1) is an important modulator of the physiological action of adenosine. We identified amino acid residues involved in adenosine transport using a yeast-based assay to rapidly screen and identify randomly generated hENT1 mutants that exhibited decreased sensitivity to inhibition of adenosine transport by various hENT1 competitive inhibitors. We identified Met89 and Ser160 as important in the affinity of hENT1 for various substrates and inhibitors. Mutation to Met89Cys or Ser160Cys significantly (p < 0.05) increased the S6-(4-nitrobenzyl)-mercaptopurine riboside (NBMPR) IC50 values by approximately 4- and 6-fold, respectively (42 +/- 13 and 65 +/- 1.6 nM) compared with the wild-type transporter (11 +/- 0.7 nM). The double mutant Met89Cys/Ser160Cys synergistically increased the NBMPR IC50 value to approximately 19-fold of that of the wild-type transporter. In contrast, compared with wild-type hENT1, the sensitivity to dipyridamole inhibition was significantly (p < 0.05) increased by only the Ser160Cys (approximately 2.6-fold) or the double mutant Met89Cys/Ser160Cys (approximately 4.7-fold) but not by the Met89Cys mutant. Mutation to Met89Cys or Ser160Cys increased the Km of adenosine (approximately 8- and 3-fold) and the Ki of guanosine (approximately 6- and 2-fold). The double mutant increased both the Km value of adenosine and the Ki value of guanosine by approximately 8-fold and seemed to confer no additional reduction in adenosine or guanosine affinity than that by mutation of Met89 alone. Together, these data indicate that transmembrane domains (TMDs) 2 (Met89) and 4 (Ser160) of hENT1 interact and are important in conferring sensitivity to NBMPR. In contrast, Ser160 and Met89 of hENT1, respectively, play a dominant role in conferring sensitivity to dipyridamole and adenosine/guanosine affinity.

Mutation of leucine-92 selectively reduces the apparent affinity of inosine, guanosine, NBMPR [S6-(4-nitrobenzyl)-mercaptopurine riboside] and dilazep for the human equilibrative nucleoside transporter, hENT1

We developed a yeast-based assay for selection of hENT1 (human equilibrative nucleoside transporter 1) mutants that have altered affinity for hENT1 inhibitors and substrates. In this assay, expression of hENT1 in a yeast strain deficient in adenine biosynthesis (ade2) permits yeast growth on a plate lacking adenine but containing adenosine, a hENT1 substrate. This growth was prevented when inhibitors of hENT1 [e.g. NBMPR [S6-(4-nitrobenzyl)-mercaptopurine riboside], dilazep or dipyridamole] were included in the media. To identify hENT1 mutants resistant to inhibition by these compounds, hENT1 was randomly mutagenized and introduced into this strain. Mutation(s) that allowed growth of yeast cells in the presence of these inhibitors were then identified and characterized. Mutants harbouring amino acid changes at Leu92 exhibited resistance to NBMPR and dilazep but not dipyridamole. The IC50 values of NBMPR and dilazep for [3H]adenosine transport by one of these mutants L92Q (Leu92-->Gln) were approx. 200- and 4-fold greater when compared with the value for the wild-type hENT1, whereas that for dipyridamole remained unchanged. Additionally, when compared with the wild-type transporter, [3H]adenosine transport by L92Q transporter was significantly resistant to inhibition by inosine and guanosine but not by adenosine or pyrimidines. The Km value for inosine transport was approx. 4-fold greater for the L92Q mutant (260+/-16 mM) when compared with the wild-type transporter (65+/-7.8 mM). We have identified for the first time an amino acid residue (Leu92) of hENT1 that, when mutated, selectively alters the affinity of hENT1 to transport the nucleosides inosine and guanosine and its sensitivity to the inhibitors NBMPR and dilazep.

Nucleoside transporters in chronic lymphocytic leukaemia

Nucleoside derivatives have important therapeutic activity in chronic lymphocytic leukaemia (CLL). Experimental evidence indicates that in CLL cells most of these drugs induce apoptosis ex vivo, suggesting that programmed cell death is the mechanism of their therapeutic action, relying upon previous uptake and metabolic activation. Although defective apoptosis and poor metabolism often cause resistance to treatment, differential uptake and/or export of nucleosides and nucleotides may significantly modulate intracellular drug bioavailability and, consequently, responsiveness to therapy. Two gene families, SLC28 and SLC29, encode transporter proteins responsible for concentrative and equilibrative nucleoside uptake (CNT and ENT, respectively). Furthermore, selected members of the expanding ATP-binding cassette (ABC) protein family have recently been identified as putative efflux pumps for the phosphorylated forms of these nucleoside-derived drugs, ABCC11 (MRP8) being a good candidate to modulate cell sensitivity to fluoropyrimidines. Sensitivity of CLL cells to fludarabine has also been recently correlated with ENT-type transport function, suggesting that, besides the integrity of apoptotic pathways and appropriate intracellular metabolism, transport across the plasma membrane is also a relevant event during CLL treatment. As long as nucleoside transporter expression in leukaemia cells is not constitutive, the possibility of regulating nucleoside transporter function by pharmacological means may also contribute to improve therapy.

Molecular requirements of the human nucleoside transporters hCNT1, hCNT2, and hENT1

Concentrative nucleoside transporters (CNTs) and equilibrative nucleoside transporters (ENTs) are important in physiological and pharmacological activity and disposition of nucleosides and nucleoside drugs. A better understanding of the structural requirements of inhibitors for these transporters will aid in designing therapeutic agents. To define the relative and unified structural requirements of nucleoside analogs for interaction with hCNT1, hCNT2, and hENT1, we applied an array of structure-activity techniques. Unique pharmacophore models for each respective nucleoside transporter were generated. These models reveal that hCNT2 affinity is dominated by hydrogen bonding features, whereas hCNT1 and hENT1 displayed mainly electrostatic and steric features. Hydrogen bond formation over 3'-OH is essential for all nucleoside transporters. Inhibition of nucleoside transporters by a series of uridine and adenosine analogs and a variety of drugs was analyzed by comparative molecular field analysis. Cross-validated r2 (q2) values were 0.65, 0.52, and 0.74 for hCNT1, hCNT2, and hENT1, respectively. The predictive quality of the models was further validated by successful prediction of the inhibition of a set of test compounds. Addition of a hydroxyl group around the 2-position of purine (or 3-position of pyrimidine) may increase inhibition to hCNT2 transporter; addition of hydroxyl group around the 2,7-position of purine (or the 3,5-position of pyrimidine) would increase the inhibition to hENT1 transporter. Utilization of these models should assist the design of high-affinity nucleoside transporter inhibitors and substrates for both anticancer and antiviral therapy.

Nucleoside anticancer drugs: the role of nucleoside transporters in resistance to cancer chemotherapy

The clinical efficacy of anticancer nucleoside drugs depends on a complex interplay of transporters mediating entry of nucleoside drugs into cells, efflux mechanisms that remove drugs from intracellular compartments and cellular metabolism to active metabolites. Nucleoside transporters (NTs) are important determinants for salvage of preformed nucleosides and mediated uptake of antimetabolite nucleoside drugs into target cells. The focus of this review is the two families of human nucleoside transporters (hENTs, hCNTs) and their role in transport of cytotoxic chemotherapeutic nucleoside drugs. Resistance to anticancer nucleoside drugs is a major clinical problem in which NTs have been implicated. Single nucleotide polymorphisms (SNPs) in drug transporters may contribute to interindividual variation in response to nucleoside drugs. In this review, we give an overview of the functional and molecular characteristics of human NTs and their potential role in resistance to nucleoside drugs and discuss the potential use of genetic polymorphism analyses for NTs to address drug resistance.

Genomic organization and functional characterization of the human concentrative nucleoside transporter-3 isoform (hCNT3) expressed in mammalian cells

Human CNT3 encodes the concentrative nucleoside transport N3 system. Previous expression studies in oocytes showed that the Km values for nucleosides of the cloned hCNT3 were 7- to 25-fold lower than the endogenous N3 transporter in HL60 cells. Therefore, in the present study we re-examined the kinetic properties of the cloned hCNT3 using mammalian cell expression systems by transient expression in Cos7L cells and stably expression in nucleoside transporter deficient PK15NTD cells. We demonstrated that hCNT3 is a Na-dependent, broadly-selective nucleoside transporter with affinities (<11 microM) for nucleosides closely resembling the endogenous N3 transporter. Pharmacological studies showed that phloridzin is a mixed-type inhibitor of hCNT3 (Ki=15 microM), and the dideoxyuridine analogs are poor substrates. By epitope-tagging, we further demonstrated that hCNT3 is N-glycosylated as PNGase F and Endo H deglycosylated hCNT3 from 67 kDa to 58 kDa. Searching the human genome database, we identified the genomic organization of hCNT3. This gene contains 19 exons and its exon-intron boundaries within the coding sequence exactly match with those of hCNT1 and hCNT2 with one additional exon in the N-terminus. Our data suggest that hCNT3 gene is evolutionarily conserved with hCNT1 and hCNT2. Physiologically, hCNT3 is a glycoprotein, which transports purine and pyrimidine nucleosides in a Na-dependent manner with high affinities.

Localization of human equilibrative nucleoside transporters, hENT1 and hENT2, in renal epithelial cells

Nucleoside transporters are important in the disposition of nucleosides and nucleoside analogs in the kidney. Two human equilibrative nucleoside transporters have been cloned and characterized, hENT1 and hENT2. The primary goal of this study was to localize these transporters in polarized renal epithelia. hENT1 and hENT2 were tagged with green fluorescence protein, stably expressed in renal epithelial cells, and localized by immunofluorescence and functional analysis. Our data demonstrated that both transporters are expressed on the basolateral membrane. hENT1 is also present on the apical membrane. Additionally, we examined the importance to basolateral targeting of two COOH-terminal targeting motifs: a RXXV motif for hENT1 and a dileucine repeat for hENT2. Neither motif appeared to affect targeting, but the dileucine repeat was implicated in surface expression of hENT2. In addition, a splice variant of hENT2 was identified that is predicted to result in a 156-residue COOH-terminal truncation. This variant had a tissue distribution similar to wild-type hENT2 but was retained intracellularly. These data suggest that hENT1 and hENT2 on the basolateral membrane function with concentrative nucleoside transporters on the apical membrane to mediate active reabsorption of nucleosides within the kidney.

Molecular biology and regulation of nucleoside and nucleobase transporter proteins in eukaryotes and prokaryotes

The molecular cloning of cDNAs encoding nucleoside transporter proteins has greatly advanced understanding of how nucleoside permeants are translocated across cell membranes. The nucleoside transporter proteins identified thus far have been categorized into five distinct superfamilies. Two of these superfamilies, the equilibrative and concentrative nucleoside transporters, have human members and these will be examined in depth in this review. The human equilibrative nucleoside transporters translocate nucleosides and nucleobases bidirectionally down their concentration gradients and are important in the uptake of anticancer and antiviral nucleoside drugs. The human concentrative nucleoside transporters cotranslocate nucleosides and sodium unidirectionally against the nucleoside concentration gradients and play a vital role in certain tissues. The regulation of nucleoside and nucleobase transporters is being studied more intensely now that more tools are available. This review provides an overview of recent advances in the molecular biology and regulation of the nucleoside and nucleobase transporters.

Ecto 5'-nucleotidase and nonspecific alkaline phosphatase. Two AMP-hydrolyzing ectoenzymes with distinct roles in human airways

In human airways, extracellular adenosine regulates epithelial functions supporting mucociliary clearance, an important airway defense mechanism against bacterial infection. Thus, defining the mechanisms of adenosine generation is critical for elucidating the role of this nucleoside in airway homeostasis. In this study, we identified the source of adenosine on the mucosal surface of human airway epithelia. Polarized primary cultures of human nasal or bronchial epithelial cells were assayed for transepithelial transport, cytosolic and cell surface adenosine production. Ussing chamber experiments indicated that serosal 1 microM [(3)H]adenosine was not transported to the mucosal compartment. Messenger RNA for the cytosolic AMP-specific 5'-nucleotidase (CN-I) was not detected in human bronchial epithelial cells, suggesting that mucosal adenosine did not originate from intracellular pools. In contrast, extracellular 0.1 mm ATP was rapidly dephosphorylated into adenosine on the mucosal epithelial surface. We identified two ectonucleotidases that mediated the conversion of AMP to adenosine: ecto 5'-nucleotidase (ecto 5'-NT, CD73) and alkaline phosphatase (AP). Both mucosal and serosal epithelial surfaces displayed ecto 5'-NT activity (K(m) = 14 microM, V(max) = 0.5 nmol x min(-1) x cm(-2)), whereas AP activity was restricted to the mucosal surface (K(m,)(high) = 36 microM, V(max) = 1.2 nmol x min(-1) x cm(-2); K(m,)(low) = 717 microM, V(max) = 2.8 nmol x min(-1) x cm(-2)). In bronchial cultures and tissues, ecto 5'-NT accounted for >80% of total activity toward 0.01 mm AMP, compared with <15% for 5 mm AMP. The proximal airway AP isoform was identified as nonspecific AP (NS AP) by levamisole sensitivity and mRNA expression. The two ectoenzymes presented opposite airway distributions, ecto 5'-NT and NS AP mRNA dominating in higher and lower airways, respectively. Collectively, these experiments support a major role for extracellular nucleotide catalysis and for ecto 5'-NT and NS AP in the regulation of adenosine concentrations on airway surfaces.

Nucleoside transporters in absorptive epithelia

There are two families of nucleoside transporters, concentrative (termed CNTs) and equilibrative (called ENTs). The members of both families mediate the transmembrane transport of natural nucleosides and some drugs whose structure is based on nucleosides. CNT transporters show a high affinity for their natural substrates (with Km values in the low micromolar range) and are substrate selective. In contrast, ENT transporters show lower affinity and are more permissive regarding the substrates they accept. Both types of transporters are tightly regulated in all cell types studied so far, both by endocrine and growth factors and by substrate availability. The degree of cell differentiation and the proliferation status of a cell also affect the pattern of expressed transporters. Although the presence of both types of transporters in the cells of absortive epithelia suggested the possibility of a transepithelial flux of nucleosides, their exact localization in the different plasma membrane domains of epithelial cells had not been demonstrated until recently. Concentrative transporters are found in the apical membrane while equlibrative transporters are located in the basolateral membrane, thus strengthening the hypothesis of a transepithelial flux of nucleosides.

Simultaneous expression of hCNT1-CFP and hENT1-YFP in Madin-Darby canine kidney cells. Localization and vectorial transport studies

To test the hypothesis that human concentrative and equilibrative nucleoside transporters (hCNT1 and hENT1) are present on the apical and basolateral membrane, respectively, we constructed a Madin-Darby canine kidney (MDCK) cell line that simultaneously and stably expresses recombinant hCNT1 and hENT1 gene products tagged with CFP and YFP fluorescent proteins, respectively. Using a confocal microscope, both hCNT1-CFP and hENT1-YFP were found to be distributed uniformly on the plasma membrane of undifferentiated MDCK cells. Upon differentiation of the MDCK cells on Transwell filter inserts, hCNT1-CFP was visualized exclusively on the apical membrane, whereas hENT1-YFP appeared predominantly on the basolateral membrane. As differentiation proceeded, there was an increase in alkaline phosphatase activity, and activity of hENT1 in the apical compartment decreased while hCNT1 activity remained constant. These results suggest that, on differentiation, hENT1 is sorted to the basolateral membrane. This was confirmed when the hCNT1-mediated uptake of [(3)H]uridine from the apical compartment of the differentiated cells was found to be approximately 20-fold higher and that for hENT1 was approximately 4-fold lower than the corresponding uptake from the basal compartment. As observed in vivo, the net transport of [(3)H]adenosine was from the apical to the basal compartment, whereas that for (14)C-deoxyadenosine was from the basal to the apical compartment. In summary, we have shown for the first time that hCNT1 and hENT1 are expressed in polarized MDCK cells on the apical and basolateral membrane, respectively, allowing vectorial transport in both directions depending on the relative activity (ratio of maximal transporter activity to affinity) of each transporter for their substrates.

Uptake of lamivudine by rat renal brush border membrane vesicles

Uptake of lamivudine, a nucleoside analogue antiviral agent, by brush border membrane vesicles (BBMV) prepared from rat renal cortex was investigated. Initial uptake of lamivudine by BBMV was stimulated in the presence of an outward pH gradient. Determination of the kinetic parameters of the initial uptake yielded apparent Km and Vmax values of 2.28 mm and 1.56 nmol (mg protein)(-1) (20 s)(-1), respectively. The pH-driven uptake of lamivudine was inhibited by organic cations such as trimethoprim and cimetidine. The inhibitory effect of trimethoprim on lamivudine uptake was competitive, with an apparent Ki of 27.6 microM. The uptake of lamivudine was also inhibited by nitrobenzylthioinosine, a representative inhibitor of nucleoside transport, and by other nucleoside analogues, such as azidothymidine and dideoxycytidine, that are excreted by renal tubular secretion. These findings suggest that efflux of lamivudine at the brush border membrane of renal tubular epithelium is mediated by an H+/lamivudine antiport system, which may correspond to the H+/organic cation antiport system, and that this system is also involved in the renal secretion of other nucleoside analogues.

Acquisition of human concentrative nucleoside transporter 2 (hcnt2) activity by gene transfer confers sensitivity to fluoropyrimidine nucleosides in drug-resistant leukemia cells

CEM-ARAC leukemia cells with resistance to cytarabine were shown to lack equilibrative transporter (hENT1) expression and activity. Stable transfer of hCNT2 cDNA into CEM-ARAC enabled Na(+)-dependent transport of purine and pyrimidine nucleoside analogs and provided a unique in vitro model for studying hCNT2. Analysis of [(3)H]uridine inhibitory activity by test substances in hCNT2 transfectant ARAC/D2 revealed structural requirements for interaction with hCNT2: 1) ribosyl and 2'-deoxyribosyl nucleosides were better inhibitors than 3'-deoxyribosyl, 2',3'-dideoxyribosyl or arabinosyl nucleosides; 2) uridine analogs with halogens at position 5 were better inhibitors than 5-methyluridine or thymidine; 3) 2-chloroadenosine was a better inhibitor than 2-chloro-2'-deoxyadenosine (cladribine); and 4) cytosine-containing nucleosides, 7-deazaadenosine and nucleobases were not inhibitors. Quantification of inhibitory capacity yielded K(i) values of 34-50 microM (5-halogenated uridine analogs, 2'-deoxyuridine), 82 microM (5-fluoro-2'-deoxyuridine), 197-246 microM (5-methyluridine < 5-bromo-2'-deoxyuridine < 5-iodo-2'-deoxyuridine), and 411 microM (5-fluoro-5'-deoxyuridine, capecitabine metabolite). Comparisons of hCNT2-mediated transport rates indicated halogenated uridine analogs were transported more rapidly than halogenated adenosine analogs, even though hCNT2 exhibited preference for physiologic purine nucleosides over uridine. Kinetics of hCNT2-mediated transport of 5-fluorouridine and uridine were similar (K(m) values, 43-46 microM). The impact of hCNT2-mediated transport on chemosensitivity was assessed by comparing antiproliferative activity of nucleoside analogs against hCNT2-containing cells with transport-defective, drug-resistant cells. Chemosensitivity was restored partially for cladribine, completely for 5-fluorouridine and 5-fluoro-2'-deoxyuridine, whereas there was little effect on chemosensitivity for fludarabine, 7-deazaadenosine, or cytarabine. These studies, which demonstrated hCNT2 uptake of halogenated uridine analogs, suggested that hCNT2 is an important determinant of cytotoxicity of this class of compounds in vivo.

Characterization of nucleoside transport systems in cultured rat epididymal epithelium

The nucleoside transport systems in cultured epididymal epithelium were characterized and found to be similar between the proximal (caput and corpus) and distal (cauda) regions of the epididymis. Functional studies revealed that 70% of the total nucleoside uptake was Na(+) dependent, while 30% was Na(+) independent. The Na(+)-independent nucleoside transport was mediated by both the equilibrative nitrobenzylthioinosine (NBMPR)-sensitive system (40%) and the NBMPR-insensitive system (60%), which was supported by a biphasic dose response to NBMPR inhibition. The Na(+)-dependent [(3)H]uridine uptake was selectively inhibited 80% by purine nucleosides, indicating that the purine nucleoside-selective N1 system is predominant. Since Na(+)-dependent [(3)H]guanosine uptake was inhibited by thymidine by 20% and Na(+)-dependent [(3)H]thymidine uptake was broadly inhibited by purine and pyrimidine nucleosides, this suggested the presence of the broadly selective N3 system accounting for 20% of Na(+)-dependent nucleoside uptake. Results of RT-PCR confirmed the presence of mRNA for equilibrative nucleoside transporter (ENT) 1, ENT2, and concentrative nucleoside transporter (CNT) 2 and the absence of CNT1. It is suggested that the nucleoside transporters in epididymis may be important for sperm maturation by regulating the extracellular concentration of adenosine in epididymal plasma.

Na+ reabsorption in cultured rat epididymal epithelium via the Na+/nucleoside cotransporter

The effect of nucleoside on Na+ reabsorption via Na+/nucleoside cotransporter in cultured rat epididymal epithelia was studied by short-circuit current (Isc) technique. Guanosine added apically stimulated Isc in a dose-dependent manner, with a median effective concentration (EC50) of 7 +/- 2 microM (mean +/- SEM). Removal of Na+ from the apical bathing solution or pretreatment with a nonspecific Na+/nucleoside cotransporter inhibitor, phloridzin, completely blocked the Isc response to guanosine. Moreover, the guanosine response was abolished by pretreatment of the tissue with ouabain, a Na+/K+-ATPase inhibitor, suggesting the involvement of Na+/nucleoside cotransporter on the apical side and Na+/K+-ATPase on the basolateral side in Na+ reabsorption. In contrast, the Isc response to guanosine was not affected after desensitization of purinoceptors by ATP. Addition of the Na+/K+/2Cl- symport inhibitor bumetanide to the basolateral side or the nonspecific Cl- channel blocker diphenylamine-2-carboxylate to the apical side showed no effect on the Isc response to guanosine, excluding stimulation of Cl- secretion by guanosine as the cause of the guanosine-induced Isc. The Isc response to purine nucleoside (guanosine and inosine) was much higher than that to pyrimidine nucleoside (thymidine and cytidine). Consistent with substrate specificity, results of reverse transcription-polymerase chain reaction revealed mRNA for concentrative nucleoside transporter (CNT2), which is a purine nucleoside-selective Na+/nucleoside cotransporter in the epididymis, but not for CNT1. It is suggested that the Na+/nucleoside cotransporter (i.e., CNT2) may be one of the elements involved in Na+ and fluid reabsorption in the epididymis, thereby providing an optimal microenvironment for the maturation and storage of spermatozoa.

Ontogenic and longitudinal activity of Na(+)-nucleoside transporters in the human intestine

The objectives of our study were to identify the types of nucleoside transporters present in the human fetal small intestine and to characterize their developmental activity, longitudinal distribution, and transport kinetics compared with those present in the adult intestine. Nucleoside uptake by intestinal brush-border membrane vesicles was measured by an inhibitor-stop rapid filtration technique. Only the purine-specific (N1; hCNT2) and the pyrimidine-specific (N2; hCNT1) Na(+)-dependent nucleoside transporters were found to be present on the brush-border membranes of the enterocytes along the entire length of the fetal and adult small intestines. The activity of these transporters was higher in the proximal than in the distal small intestine. Both the N1 and N2 transporters found in the fetal intestine shared similar kinetic properties (Michaelis-Menten constant and Na(+)-nucleoside stoichiometry) to those in the adult intestine. During the period of rapid morphogenesis (11-15 wk gestation), no temporal differences were apparent in the activity of the N1 and N2 transporters in the fetal small intestine. These findings have implications for the absorption of drugs from the amniotic fluid by the fetus after maternal drug administration of nucleoside drugs such as the antivirals zidovudine and didanosine.

Active intestinal absorption of nucleosides by Na+-dependent transport across the brush border membrane in cows

Transport of 3H-labeled nucleosides across the bovine intestinal brush border membrane (BBM) was characterized with BBM vesicles (BBMV) isolated from mid-jejunum of cows because large amounts of nucleic acids are digested in the small intestine of ruminants. Two Na+-dependent electrogenic nucleoside transporters with overlapping substrate specificity were shown to be present in the jejunal BBM, one for pyrimidine nucleosides and one for purine nucleosides. As indicated by inhibitory studies, thymidine seemed to be a specific substrate for the pyrimidine nucleoside transporter, while this applied to guanosine and deoxyguanosine for the purine nucleoside transporter. Uridine and adenosine appear to have an affinity to both transporters. This also applies to deoxyadenosine and deoxyuridine. Nucleobases (uracil, hypoxanthine) did not affect transport of nucleosides. The kinetic constants (Km and Vmax) for Na-dependent thymidine and guanosine transport were 29 and 24 micromol/L and 78 and 51 pmol (mg protein)(-1) s(-1), respectively. These values are much higher than those reported for monogastric species.

Drug transport and targeting. Intestinal transport

A wide variety of transporters are found in the intestine, and are involved in the membrane transport of daily nutrients as well as drugs. These intestinal transporters are located in the brush border membrane as well as basolateral membrane. Each transporter exhibits its own substrate specificity, and some have broader specificities than others. In addition, the distribution and characteristics of the intestinal transporters exhibit regional differences along the intestine, implying diverse physiologic functions and in some cases pathologic responses. Indeed several genetic disorders have been shown to result from deficient intestinal transporters. The development of prodrugs that target to intestinal transporters has been successful in improving oral absorption. For example, the intestinal peptide transporter is utilized in order to increase the bioavailability of several classes of peptidomimetic drugs, especially ACE inhibitors and beta-lactam antibiotics. The bioavailability of poorly absorbed drugs can be improved by utilization of the transporters responsible for the intestinal absorption of various solutes and/or by inhibiting the transporter involved in the efflux system. Recent advances in gene cloning and molecular biology techniques make it possible to study the characteristics and distribution of transporters at the molecular level. Based on molecular characterizations of membrane transporters and accumulated biochemical data on their specificities and kinetics, structural modification and targeting of a specific transporter is a promising strategy for the design of drugs that improve bioavailability and tissue distribution.

Gemcitabine transport in xenopus oocytes expressing recombinant plasma membrane mammalian nucleoside transporters

BACKGROUND

Gemcitabine, a pyrimidine analogue of deoxycytidine, is an anticancer nucleoside drug that requires functional plasma membrane nucleoside transporter proteins to reach its intracellular targets and cause cytotoxicity. Because of technical difficulties inherent in studying nucleoside transport in human cells, we rigorously defined gemcitabine membrane transportability by producing each of the available human (h) and rat (r) recombinant nucleoside transporters (NTs) individually in Xenopus laevis oocytes.

METHODS

Oocytes were microinjected with in vitro-transcribed RNAs derived from complementary DNAs encoding (C = concentrative) rCNT1, rCNT2, hCNT1, hCNT2, (E = equilibrative) rENT1, rENT2, hENT1, and hENT2. Uptake of [(3)H]gemcitabine and [(14)C] uridine was measured 3 days after microinjection to determine kinetic constants. We also used the two-electrode, voltage-clamp technique to investigate the electrophysiology of hCNT1-mediated gemcitabine transport.

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. 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. Gemcitabine had a limited ability to cross the lipid bilayer of oocyte membranes by simple diffusion. 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.

CONCLUSIONS

Mammalian nucleoside transporters vary widely in their affinity and capacity to transport gemcitabine. Variation in the tumor and tissue distribution of plasma membrane nucleoside transporter proteins may contribute to the solid tumor activities and schedule-dependent toxic effects of gemcitabine.

Involvement of multiple transporters in the oral absorption of nucleoside analogues

Many nucleoside analogues such as azt, ddI, ddC, d4T, 3TC, acv and vacv are currently being used in the treatment of patients infected with HIV, suffering from AIDS, or AIDS-related opportunistic infections. The transport of nucleoside analogues across the gastrointestinal tract is mediated by a number of transporters that fall into three broad categories, i.e., Na(+)-dependent concentrative transporters, Na(+)-independent equilibrative transporters and H(+)/peptide transporters. The first two transporter classes contain a large number of subtypes that are based on the substrate specificity. Recent studies have shown that most of the anti-HIV nucleoside analogues are transported by one or more of the nucleoside transporters. Furthermore, certain analogues, such as acv, appear to be absorbed by non-carrier-mediated diffusion, whereas vacv is apparently transported by non-nucleoside transporters (e.g., the oligopeptide transporter, PepT1 and possibly others). Thus, it is desirable to understand the precise nature of the absorption mechanism of these drugs to improve bioavailability and reduce the variability that is commonly observed in vivo in human patients. A complete understanding of the complex interactions of nucleoside analogues with the various transporters will help in designing better delivery systems and strategies to improve efficacy. In the current report, the mechanisms of nucleoside and nucleoside-analogue transport are reviewed. Also, methods of exploiting prodrugs to improve the bioavailability characteristics of drugs are highlighted.

Nucleoside transport in human colonic epithelial cell lines: evidence for two Na+-independent transport systems in T84 and Caco-2 cells

RT-PCR of RNA isolated from monolayers of the human colonic epithelial cell lines T84 and Caco-2 demonstrated the presence of mRNA for the two cloned Na+-independent equilibrative nucleoside transporters, ENT1 and ENT2, but not for the cloned Na+-dependent concentrative nucleoside transporters, CNT1 and CNT2. Uptake of [3H]uridine by cell monolayers in balanced Na+-containing and Na+-free media confirmed the presence of only Na+-independent nucleoside transport mechanisms. This uptake was decreased by 70-75% in the presence of 1 microM nitrobenzylthioinosine, a concentration that completely inhibits ENT1, and was completely blocked by the addition of 10 microM dipyridamole, a concentration that inhibits both ENT1 and ENT2. These findings indicate the presence in T84 and Caco-2 cells of two functional Na+-independent equilibrative nucleoside transporters, ENT1 and ENT2.

Molecular cloning, functional expression and chromosomal localization of a cDNA encoding a human Na+/nucleoside cotransporter (hCNT2) selective for purine nucleosides and uridine

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. 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). Here, the molecular cloning and functional characterization of a CNT2/SPNT-type transporter from human small intestine are described. The encoded 658-residue protein (hCNT2 in the nomenclature) had the same predicted amino acid sequence as human kidney hSPNT1, except for a polymorphism at residue 75 (Arg substituted by Ser), and was 83 and 72% identical to rCNT2 and hCNT1, respectively. Sequence differences between hCNT2 and rCNT2 were greatest at the N-terminus. In Xenopus oocytes, recombinant hCNT2 exhibited the functional characteristics of a Na(+)-dependent nucleoside transporter with selectivity for adenosine, other purine nucleosides and uridine (adenosine and uridine K(m) app values 8 and 40 microM, respectively). hCNT2 transcripts were found in kidney and small intestine but, unlike rCNT2, were not detected in liver. Deoxyadenosine, which undergoes net renal secretion in humans, was less readily transported than adenosine. hCNT2 also mediated small, but significant, fluxes of the antiviral purine nucleoside analogue 2',3'-dideoxyinosine. hCNT2 is, therefore potentially involved in both the intestinal absorption and renal handling of purine nucleosides (including adenosine), uridine and purine nucleoside drugs. The gene encoding hCNT2 was mapped to chromosome 15q15.