Bile acids alter the subcellular localization of CNT2 (concentrative nucleoside cotransporter) and increase CNT2-related transport activity in liver parenchymal cells

A guide to plasma membrane solute carrier proteins

This review aims to serve as an introduction to the solute carrier proteins (SLC) superfamily of transporter proteins and their roles in human cells. The SLC superfamily currently includes 458 transport proteins in 65 families that carry a wide variety of substances across cellular membranes. While members of this superfamily are found throughout cellular organelles, this review focuses on transporters expressed at the plasma membrane. At the cell surface, SLC proteins may be viewed as gatekeepers of the cellular milieu, dynamically responding to different metabolic states. With altered metabolism being one of the hallmarks of cancer, we also briefly review the roles that surface SLC proteins play in the development and progression of cancer through their influence on regulating metabolism and environmental conditions.

From Inflammation to the Onset of Fibrosis through A2A Receptors in Kidneys from Deceased Donors

Pretransplant graft inflammation could be involved in the worse prognosis of deceased donor (DD) kidney transplants. A2A adenosine receptor (A_2AR) can stimulate anti-inflammatory M2 macrophages, leading to fibrosis if injury and inflammation persist. Pre-implantation biopsies of kidney donors (47 DD and 21 living donors (LD)) were used to analyze expression levels and activated intracellular pathways related to inflammatory and pro-fibrotic processes. A_2AR expression and PKA pathway were enhanced in DD kidneys. A_2AR gene expression correlated with TGF-β1 and other profibrotic markers, as well as CD163, C/EBPβ, and Col1A1, which are highly expressed in DD kidneys. TNF-α mRNA levels correlated with profibrotic and anti-inflammatory factors such as TGF-β1 and A_2AR. Experiments with THP-1 cells point to the involvement of the TNF-α/NF-κB pathway in the up-regulation of A_2AR, which induces the M2 phenotype increasing CD163 and TGF-β1 expression. In DD kidneys, the TNF-α/NF-κB pathway could be involved in the increase of A_2AR expression, which would activate the PKA–CREB axis, inducing the macrophage M2 phenotype, TGF-β1 production, and ultimately, fibrosis. Thus, in inflamed DD kidneys, an increase in A_2AR expression is associated with the onset of fibrosis, which may contribute to graft dysfunction and prognostic differences between DD and LD transplants.

The effect of genetic variations on ribavirin pharmacokinetics and treatment response in HCV-4 Egyptian patients receiving sofosbuvir/daclatasvir and ribavirin

PURPOSE

This study aimed to investigate the effect of single nucleotide polymorphisms (SNPs) of genes involved in ribavirin (RBV) transport (SLC28A2 gene, ABCB1 gene and ABCB11 gene) on the clinical outcome and pharmacokinetics of ribavirin in HCV- 4 Egyptian patients.

METHOD

100 patients treated with sofosbuvir/daclatasvir and ribavirin for 12 weeks. The SNP genotyping was performed by real-time PCR using high resolution melting analysis. Ribavirin plasma trough concentrations were determined at week 4 of therapy using a liquid chromatography/tandem mass spectrometry (LC-MS/MS). For clinical outcomes, sustained virological response (SVR), liver function tests (ALT and AST), total bilirubin, albumin, serum creatinine, hemoglobin, leukocyte count, and platelet count were measured.

RESULTS

Concerning RBV pharmacokinetics, ABCB1 2677 G > T SNP and ABCB11 1331 T > C SNP were statistically associated with RBV C_trough levels after 4 weeks of therapy. ABCB11 1331 T > C SNP revealed significant association with clinical outcomes (SVR). SLC28A2-146 A > T SNP has not showed any statistically significant association with RBV plasma levels or response.

CONCLUSION

SNP genotyping for ABCB1 and ABCB11 genes can help in better personalized medicine for maximizing response for ribavirin as explored by the significant association between polymorphism in ABCB1 and ABCB11 genes and ribavirin pharmacokinetics and the significant association of ABCB11 1331 T > C SNP with clinical response.

Expression of Concentrative Nucleoside Transporters ( SLC28A) in the Human Placenta: Effects of Gestation Age and Prototype Differentiation-Affecting Agents

Equilibrative ( SLC29A) and concentrative ( SLC28A) nucleoside transporters contribute to proper placental development and mediate uptake of nucleosides/nucleoside-derived drugs. We analyzed placental expression of SLC28A mRNA during gestation. Moreover, we studied in choriocarcinoma-derived BeWo cells whether SLC29A and SLC28A mRNA levels can be modulated by activity of adenylyl cyclase, retinoic acid receptor activation, CpG islands methylation, or histone acetylation, using forskolin, all- trans-retinoic acid, 5-azacytidine, and sodium butyrate/sodium valproate, respectively. We found that expression of SLC28A1, SLC28A2, and SLC28A3 increases during gestation and reveals considerable interindividual variability. SLC28A2 was shown to be a dominant subtype in the first-trimester and term human placenta, while SLC28A1 exhibited negligible expression in the term placenta only. In BeWo cells, we detected mRNA of SLC28A2 and SLC28A3. Levels of the latter were affected by 5-azacytidine and all- trans-retinoic acid, while the former was modulated by sodium valproate (but not sodium butyrate), all- trans-retinoic acid, 5-azacytidine, and forskolin that caused 25-fold increase in SLC28A2 mRNA; we documented by analysis of syncytin-1 that the observed changes in SLC28A expression do not correlate with the morphological differentiation state of BeWo cells. Upregulated SLC28A2 mRNA was reflected in elevated uptake of [³H]-adenosine, high-affinity substrate of concentrative nucleoside transporter 2. Using KT-5720 and inhibitors of phosphodiesterases, we subsequently confirmed importance of cAMP/protein kinase A pathway in SLC28A2 regulation. On the other hand, SLC29A genes exhibited constitutive expression and none of the tested compounds increased SLC28A1 expression to detectable levels. In conclusion, we provide the first evidence that methylation status and activation of retinoic acid receptor affect placental SLC28A2 and SLC28A3 transcription and substrates of concentrative nucleoside transporter 2 might be taken up in higher extent in placentas with overactivated cAMP/protein kinase A pathway and likely in the term placenta.

Ribavirin uptake into human hepatocyte HHL5 cells is enhanced by interferon-α via up-regulation of the human concentrative nucleoside transporter (hCNT2)

Ribavirin is a broad spectrum antiviral that increases the response rate in chronic hepatitis C patients when administered in combination with IFNα. Ribavirin is a purine nucleoside derivative, transported into hepatocytes by nucleoside transporters. hCNT2 is the best candidate to mediate ribavirin uptake into hepatocytes due to its high-affinity for purines and its capacity to concentrate its substrates intracellularly. The aim of this study was to determine whether hCNT2 function is under IFNα modulation. IFNα treatment of the nontransformed human hepatocyte-derived cell line HHL5 induced a rapid and transient increase in hCNT2 activity after cytokine addition. hCNT2 activity up-regulation was associated with increased ribavirin accumulation into cells. This increase was consistent with the translocation of hCNT2-containing vesicles to the plasma membrane via a mechanism requiring ERK 1/2 and ROCK activation and cytoskeleton integrity. Longer treatments with IFNα induced transcriptional activation of the hCNT2-encoding gene (SLC28A2), resulting in a sustained increase in hCNT2-related activity. These observations are proof of concept for at least one of the putative mechanisms underlying the synergistic responses induced by combination therapy with IFNα and ribavirin.

Translocation of nucleoside analogs across the plasma membrane in hematologic malignancies

Nucleoside analogs are currently used in the treatment of various hematologic malignancies due to their ability to induce apoptosis of lymphoid cells. For nucleoside-derived drugs to exert their action, they must enter cells via nucleoside transporters from two gene families, SLC28 and SLC29 (CNT and ENT, respectively). Once inside the cell, these drugs must be phosphorylated to their active forms. In contrast, some members of the ATP-binding cassette (ABC) protein family have been identified as responsible for the efflux of the phosphorylated forms of these nucleoside-derived drugs. Here, we review the main nucleoside analogs used in hematologic malignancies and focus especially on those that are currently used in chronic lymphocytic leukemia (CLL). Moreover, we discuss the pharmacological profile of the nucleoside transporters, which determines the bioavailability of and cell sensitivity to these nucleoside-derived drugs. We also discuss the expression of nucleoside transporters and their activities in CLL as well as the possibility of modulating these transporter activities as a means of modulating intracellular drug availability and, consequently, responsiveness to therapy.

Structural determinants for rCNT2 sorting to the plasma membrane of polarized and non-polarized cells

rCNT2 (rat concentrative nucleoside transporter 2) (Slc28a2) is a purine-preferring concentrative nucleoside transporter. It is expressed in both non-polarized and polarized cells, where it is localized in the brush border membrane. Since no information about the domains implicated in the plasma membrane sorting of rCNT2 is available, the present study aimed to identify structural and functional requirements for rCNT2 trafficking. The comprehensive topological mapping of the intracellular N-terminal tail revealed two main features: (i) a glutamate-enriched region (NPGLELME) between residues 21 and 28 that seems to be implicated in the stabilization of rCNT2 in the cell surface, since mutagenesis of these conserved glutamates resulted in enhanced endocytosis; and (ii) mutation of a potential protein kinase CK2 domain that led to a loss of brush border-specific sorting. Although the shortest proteins assayed (rCNT2-74AA, -48AA and -37AA) accumulated intracellularly and lost their brush border membrane preference, they were still functional. A deeper analysis of CK2 implication in CNT2 trafficking, using a CK2-specific inhibitor [DMAT (2-dimethylamino-4,5,6,7-tetrabromo-1H-benzimidazole)] and other complementary mutations mimicking the negative charge provided by phosphorylation (S46D and S46E), demonstrated an effect of this kinase on rCNT2 activity. In summary, the N-terminal tail of rCNT2 contains dual sorting signals. An acidic region is responsible for its proper stabilization at the plasma membrane, whereas the putative CK2 domain (Ser46) is implicated in the apical sorting of the transporter.

Link between high-affinity adenosine concentrative nucleoside transporter-2 (CNT2) and energy metabolism in intestinal and liver parenchymal cells

Concentrative nucleoside transporter 2 (CNT2) is a high-affinity adenosine transporter that may play physiological roles beyond nucleoside salvage. Previous reports relate CNT2 function to modulation of purinergic signaling and energy metabolism in intestinal and liver parenchymal cells (Duflot et al., 2004, Mol Cell Biol 24:2710-2719; Aymerich et al., 2006, J Cell Sci 119:1612-1621). In the present study, to further examine the link between CNT2 and energy metabolism, CNT2 protein partners were identified using the bacterial two-hybrid and GST pull-down approaches. The N-terminal segment of CNT2 was used as bait, since proteins lacking this domain display impaired plasma membrane insertion and intracellular retention. Glucose-regulated protein 58 (GRP58) was identified as a potential rCNT2 partner in pull-down experiments. Two-hybrid screening performed against a liver human cDNA library led to the identification of aldolase B as another hCNT2 partner. Aldolase B-RFP and endogenous GRP58 separately co-localized with CNT2 in HeLa cells transfected with YFPrCNT2. CNT2 interaction with GRP58 was validated using co-immunoprecipitation experiments. In HeLa cells, fluorescence resonance energy transfer (FRET) efficiency increased upon fructose addition, consistent with a transient interaction between aldolase B and the transporter. The physiological basis for in vivo interactions was derived from experiments in which GRP58 was inhibited or overexpressed and aldolase B activity stimulated towards glycolysis. GRP58 appeared to be a negative effector of CNT2 function, whereas aldolase B flux modulated CNT2 activity via a mechanism involving acquisition of higher affinity for its substrates. These findings support the theory that CNT2 plays roles other than salvage and establishes links with energy metabolism.

Different N-terminal motifs determine plasma membrane targeting of the human concentrative nucleoside transporter 3 in polarized and nonpolarized cells

Human concentrative nucleoside transporter 3 (hCNT3) is a broad-selectivity, high-affinity protein implicated in the uptake of most nucleoside-derived anticancer and antiviral drugs. Regulated trafficking of hCNT3 has been recently postulated as a suitable way to improve nucleoside-based therapies. Moreover, the recent identification of a putative novel hCNT3-type transporter lacking the first 69 amino acids and retained at the endoplasmic reticulum anticipated that the N terminus of hCNT3 contains critical motifs implicated in trafficking. In the current study, we have addressed this issue by using deletions and site-directed mutagenesis and plasma membrane expression and nucleoside uptake kinetic analysis. Data reveal that 1) a segment between amino acids 50 and 62 contains plasma membrane-sorting determinants in nonpolarized cells; 2) in particular, the Val(57)-Thr(58)-Val(59) tripeptide seems to be the core of the export signal, whereas acidic motifs upstream and downstream of it seem to be important for the kinetics of the process; and 3) in polarized epithelia, the β-turn-forming motif (17)VGFQ(20) is necessary for proper apical expression of the protein.

All-trans-retinoic acid promotes trafficking of human concentrative nucleoside transporter-3 (hCNT3) to the plasma membrane by a TGF-beta1-mediated mechanism

Human concentrative nucleoside transporter-3 (hCNT3) is a sodium-coupled nucleoside transporter that exhibits high affinity and broad substrate selectivity, making it the most suitable candidate for mediating the uptake and cytotoxic action of most nucleoside-derived drugs. The drug of this class most commonly used in the treatment of chronic lymphocytic leukemia (CLL) is the pro-apoptotic nucleoside analog fludarabine (Flu), which enters CLL cells primarily through human equilibrative nucleoside transporters (hENTs). Although CLL cells lack hCNT3 activity, they do express this transporter protein, which is located mostly in the cytosol. The aim of our study was to identify agents and mechanisms capable of promoting hCNT3 trafficking to the plasma membrane. Here, we report that all-trans-retinoic acid (ATRA), currently used in the treatment of acute promyelocytic leukemia (APL), increases hCNT3-related activity through a mechanism that involves trafficking of pre-existing hCNT3 proteins to the plasma membrane. This effect is mediated by the autocrine action of transforming growth factor (TGF)-beta1, which is transcriptionally activated by ATRA in a p38-dependent manner. TGF-beta1 acts through activation of ERK1/2 and the small GTPase RhoA to promote plasma membrane trafficking of the hCNT3 protein.

Adenosine: An immune modulator of inflammatory bowel diseases

Inflammatory bowel disease (IBD) is a common and lifelong disabling gastrointestinal disease. Emerging treatments are being developed to target inflammatory cytokines which initiate and perpetuate the immune response. Adenosine is an important modulator of inflammation and its anti-inflammatory effects have been well established in humans as well as in animal models. High extracellular adenosine suppresses and resolves chronic inflammation in IBD models. High extracellular adenosine levels could be achieved by enhanced adenosine absorption and increased de novo synthesis. Increased adenosine concentration leads to activation of the A2a receptor on the cell surface of immune and epithelial cells that would be a potential therapeutic target for chronic intestinal inflammation. Adenosine is transported via concentrative nucleoside transporter and equilibrative nucleoside transporter transporters that are localized in apical and basolateral membranes of intestinal epithelial cells, respectively. Increased extracellular adenosine levels activate the A2a receptor, which would reduce cytokines responsible for chronic inflammation.

Identification and characterization of proximal promoter polymorphisms in the human concentrative nucleoside transporter 2 (SLC28A2)

The human concentrative nucleoside transporter 2 (CNT2) plays an important role in the absorption, disposition, and biological effects of endogenous nucleosides and nucleoside analog drugs. We identified genetic variation in the basal promoter region of CNT2 and characterized the function of the variants. We screened DNA from an ethnically diverse population and identified five basal promoter variants in CNT2. Three major haplotypes in the CNT2 basal promoter region were identified and were found at different allele frequencies in various ethnic groups. The common promoter variants and haplotypes were constructed and characterized for their promoter activity using luciferase reporter assays. One polymorphic variant, rs2413775 (-146T>A), with an allele frequency >20% in all populations, showed a gain of function in luciferase activity. Furthermore, in vivo mouse promoter assays of these nucleotide variants using the hydrodynamic tail vein injection, leading to their expression in the liver, demonstrated similar results. Transcription factor binding site (TFBS) analysis indicated this variant alters a hepatic nuclear factor (HNF) 1 TFBS. Electrophoretic mobility shift assay demonstrated stronger binding of HNF1alpha and weaker binding of HNF1beta to the -146T and -146A regions, whereas the single nucleotide polymorphism (SNP), -146A, exhibited enhanced binding to both HNF1alpha and HNF1beta, consistent with its greater activity in reporter assays. The data collectively suggest that the common variant, -146T>A, in the proximal promoter of CNT2 may result in an enhanced transcription rate of the gene and, thus, expression levels of CNT2. This SNP may play a role in variation in the pharmacokinetics and pharmacological effects of nucleoside analogs.

Physiology of Nucleoside Transporters: Back to the Future

Nucleoside transporters (NTs) are integral membrane proteins responsible for mediating and facilitating the flux of nucleosides and nucleobases across cellular membranes. NTs are also responsible for the uptake of nucleoside analog drugs used in the treatment of cancer and viral infections, and they are the target of certain compounds used in the treatment of some types of cardiovascular disease. The important role of NTs as drug transporters and therapeutic targets has necessarily led to intense interest into their structure and function and the relationship between these proteins and drug efficacy. In contrast, we still know relatively little about the fundamental physiology of NTs. In this review, we discuss various aspects of the physiology of NTs in mammalian systems, particularly noting tissues and cells where there has been little recent research. Our central thesis is reference back to some of the older literature, combined with current findings, will provide direction for future research into NT physiology that will lead to a fuller understanding of the role of these intriguing proteins in the everyday lives of cells, tissues, organs, and whole animals.

Characterization of guanine and guanosine transport in primary cultured rat cortical astrocytes and neurons

In this study, we examined the transport mechanisms for guanine and guanosine in rat neurons and astrocytes, and compared their characteristics. In the both types of cell, the uptake of [(3)H]guanine and [(3)H]guanosine was time-, temperature-, and concentration-dependent, and Na(+)-independent. Their uptake decreased on the addition of purine and pyrimidine nucleobases or nucleosides, and the inhibitory effect of the purine analogues was greater than that of the pyrimidine ones. In both cell types, equilibrative nucleoside transporter (ENT) 1 and ENT2 expression was confirmed at the mRNA level, and nitrobenzylmercaptopurine riboside, a representative inhibitor for ENT, decreased their uptake at concentrations of over 10 microM. Comparing uptake characteristics between the substrates, [(3)H]guanine uptake exhibited higher affinity and clearance than [(3)H]guanosine uptake in each type of cell. Although between neurons and astrocytes, there was no difference in the apparent uptake clearance for [(3)H]guanine and [(3)H]guanosine, which was calculated based upon the cellular protein content, the cellular uptake clearance was significantly greater in astrocytes than in neurons. These findings indicate that guanine and guanosine, of which the former is a preferable substrate, are taken up into both neurons and astrocytes via ENT2, and that the extracellular concentrations of guanine and guanosine are mainly regulated by astrocytes to maintain brain physiology.

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.

Concentrative nucleoside transporters (CNTs) in epithelia: from absorption to cell signaling

Concentrative and Equilibrative Nucleoside Transporter proteins (CNT and ENT, respectively) are encoded by gene families SLC28 and SLC29. They mediate the uptake of natural nucleosides and a variety of nucleoside-derived drugs, mostly used in anticancer therapy. CNT and ENT proteins are mostly localized in the apical and basolateral sides, respectively, in (re)absorptive epithelia. This anatomic distribution determines nucleoside and nucleoside-derived vectorial flux. CNT expression (particularly CNT2) is associated with differentiation and is also nutritionally regulated in intestinal epithelia, whereas ENT protein amounts (mostly ENT1) are increased when cells are exposed to proliferative stimuli such as EGF, TGF-alpha or wounding. Although all these features suggest a role for NT proteins in nucleoside salvage and (re)absorption, recent data demonstrate that CNT2 might be under purinergic control, in a manner that is dependent on energy metabolism. A physiological link between CNT2 function and intracellular metabolism is also supported by the evidence that extracellular adenosine can activate the AMP-dependent kinase (AMPK), by a mechanism which relies upon adenosine transport and phosphorylation. Thus the complex pattern of NT isoform expression in mammalian cells can fulfill physiological roles other than salvage.