ATP-sensitive K(+) channels regulate the concentrative adenosine transporter CNT2 following activation by A(1) adenosine receptors

Nucleoside transporters and immunosuppressive adenosine signaling in the tumor microenvironment: Potential therapeutic opportunities

Adenosine compartmentalization has a profound impact on immune cell function by regulating adenosine localization and, therefore, extracellular signaling capabilities, which suppresses immune cell function in the tumor microenvironment. Nucleoside transporters, responsible for the translocation and cellular compartmentalization of hydrophilic adenosine, represent an understudied yet crucial component of adenosine disposition in the tumor microenvironment. In this review article, we will summarize what is known regarding nucleoside transporter's function within the purinome in relation to currently devised points of intervention (i.e., ectonucleotidases, adenosine receptors) for cancer immunotherapy, alterations in nucleoside transporter expression reported in cancer, and potential avenues for targeting of nucleoside transporters for the desired modulation of adenosine compartmentalization and action. Further, we put forward that nucleoside transporters are an unexplored therapeutic opportunity, and modulation of nucleoside transport processes could attenuate the pathogenic buildup of immunosuppressive adenosine in solid tumors, particularly those enriched with nucleoside transport proteins.

Metabolic Aspects of Adenosine Functions in the Brain

Adenosine, acting both through G-protein coupled adenosine receptors and intracellularly, plays a complex role in multiple physiological and pathophysiological processes by modulating neuronal plasticity, astrocytic activity, learning and memory, motor function, feeding, control of sleep and aging. Adenosine is involved in stroke, epilepsy and neurodegenerative pathologies. Extracellular concentration of adenosine in the brain is tightly regulated. Adenosine may be generated intracellularly in the central nervous system from degradation of AMP or from the hydrolysis of S-adenosyl homocysteine, and then exit via bi-directional nucleoside transporters, or extracellularly by the metabolism of released nucleotides. Inactivation of extracellular adenosine occurs by transport into neurons or neighboring cells, followed by either phosphorylation to AMP by adenosine kinase or deamination to inosine by adenosine deaminase. Modulation of the nucleoside transporters or of the enzymatic activities involved in the metabolism of adenosine, by affecting the levels of this nucleoside and the activity of adenosine receptors, could have a role in the onset or the development of central nervous system disorders, and can also be target of drugs for their treatment. In this review, we focus on the contribution of 5'-nucleotidases, adenosine kinase, adenosine deaminase, AMP deaminase, AMP-activated protein kinase and nucleoside transporters in epilepsy, cognition, and neurodegenerative diseases with a particular attention on amyotrophic lateral sclerosis and Huntington's disease. We include several examples of the involvement of components of the adenosine metabolism in learning and of the possible use of modulators of enzymes involved in adenosine metabolism or nucleoside transporters in the amelioration of cognition deficits.

Adenosine and adenosine-5'-monophosphate ingestion ameliorates abnormal glucose metabolism in mice fed a high-fat diet

Background

We have previously reported that ingestion of adenosine (ADN) and adenosine-5′-monophosphate (AMP) improves abnormal glucose metabolism in the stroke-prone spontaneously hypertensive rat model of non-obesity-associated insulin resistance. In this study, we investigated the effect of ADN and AMP ingestion on glucose metabolism in mice with high-fat diet-induced obesity.

Methods

Seven-week-old C57BL/6 J mice were administered distilled water (as a control), 10 mg/L ADN, or 13 mg/L AMP via their drinking water for 14 or 25 weeks, during which they were fed a high-fat diet. Oral glucose tolerance test (OGTT) was conducted on 21-week-old mice fasted for 16 h. Insulin tolerance test (ITT) was performed on 22-week-old mice fasted for 3 h. Blood and muscle were collected for further analysis of serum parameters, gene and protein expression levels, respectively.

Results

Glucose metabolism in the ADN and AMP groups was significantly improved compared with the control. OGTT and ITT showed that ADN and AMP groups lower than control group. Furthermore, phosphorylation of AMP-activated protein kinase (AMPK) and mRNA levels of genes involved in lipid oxidation were enhanced in the skeletal muscle of ADN- and AMP-treated mice.

Conclusion

These results indicate that ingestion of ADN or AMP induces activation of AMPK in skeletal muscle and mitigates insulin resistance in mice with high-fat diet-induced diabetes.

Electronic supplementary material

The online version of this article (10.1186/s12906-018-2367-6) contains supplementary material, which is available to authorized users.

Inhibition of histone deacetylase 7 reverses concentrative nucleoside transporter 2 repression in colorectal cancer by up-regulating histone acetylation state

BACKGROUND AND PURPOSE

The concentrative nucleoside transporter 2 (CNT2) mediates the uptake of both natural nucleosides and nucleoside-derived drugs. Therefore, it is important both physiologically and pharmacologically. However, CNT2 expression is significantly repressed in colorectal cancer (CRC). Here, we have elucidated the mechanism(s) underlying CNT2 repression in CRC.

EXPERIMENTAL APPROACH

Repression of CNT2 in tumour samples from patients with CRC was identified using Western blot and RT-qPCR. The histone acetylation state at the CNT2 promoter region was then evaluated with chromatin immunoprecipitation and trichostatin A (TSA) treatment. To find the key enzyme responsible for hypoacetylation at the CNT2 promoter region, siRNA knockdown and RT-qPCR were used. Effects of combining HDAC inhibitors and cladribine were studied in HCT15 and HT29 cells.

KEY RESULTS

Histone deacetylase 7 was significantly up-regulated in CRC, leading to histone hypoacetylation at the CNT2 promoter region, especially at sites H3K9Ac, H3K18Ac and H4Ac. This hypoacetylation condensed the chromatin structure and reduced CNT2 expression. All these effects were reversed by treatment with TSA, a histone deacetylase inhibitor. In HCT15 and HT29 cells, inhibition of histone deacetylase increased cell uptake and decreased IC₅₀ for cladribine.

CONCLUSIONS AND IMPLICATIONS

Histone hypoacetylation due to increased levels of histone deacetylase 7 results in CNT2 repression in CRC tumour tissue and could lead to decreased uptake of and consequent resistance to nucleoside anti-cancer agents. Such resistance could be overcome by combining inhibitors of histone deacetylase with the nucleoside anti-cancer agent.

Intestinal Nucleoside Transporters: Function, Expression, and Regulation

The gastrointestinal tract is the absorptive organ for nutrients found in foods after digestion. Nucleosides and, to a lesser extent nucleobases, are the late products of nucleoprotein digestion. These metabolites are absorbed by nucleoside (and nucleobase) transporter (NT) proteins. NTs are differentially distributed along the gastrointestinal tract showing also polarized expression in epithelial cells. Concentrative nucleoside transporters (CNTs) are mainly located at the apical side of enterocytes, whereas equilibrative nucleoside transporters (ENTs) facilitate the basolateral efflux of nucleosides and nucleobases to the bloodstream. Moreover, selected nucleotides and the bioactive nucleoside adenosine act directly on intestinal cells modulating purinergic signaling. NT-polarized insertion is tightly regulated. However, not much is known about the modulation of intestinal NT function in humans, probably due to the lack of appropriate cell models retaining CNT functional expression. Thus, the possibility of nutritional regulation of intestinal NTs has been addressed using animal models. Besides the nutrition-related role of NT proteins, orally administered drugs also need to cross the intestinal barrier, this event being a major determinant of drug bioavailability. In this regard, NT proteins might also play a role in pharmacology, thereby allowing the absorption of nucleoside- and nucleobase-derived drugs. The relative broad selectivity of these membrane transporters also suggests clinically relevant drug-drug interactions when using combined therapies. This review focuses on all these physiological and pharmacological aspects of NT protein biology. © 2017 American Physiological Society. Compr Physiol 8:1003-1017, 2018.

Who Is Who in Adenosine Transport

Extracellular adenosine concentrations are regulated by a panel of membrane transporters which, in most cases, mediate its uptake into cells. Adenosine transporters belong to two gene families encoding Equilibrative and Concentrative Nucleoside Transporter proteins (ENTs and CNTs, respectively). The lack of appropriate pharmacological tools targeting every transporter subtype has introduced some bias on the current knowledge of the role of these transporters in modulating adenosine levels. In this regard, ENT1, for which pharmacology is relatively well-developed, has often been identified as a major player in purinergic signaling. Nevertheless, other transporters such as CNT2 and CNT3 can also contribute to purinergic modulation based on their high affinity for adenosine and concentrative capacity. Moreover, both transporter proteins have also been shown to be under purinergic regulation via P1 receptors in different cell types, which further supports its relevance in purinergic signaling. Thus, several transporter proteins regulate extracellular adenosine levels. Moreover, CNT and ENT proteins are differentially expressed in tissues but also in particular cell types. Accordingly, transporter-mediated fine tuning of adenosine levels is cell and tissue specific. Future developments focusing on CNT pharmacology are needed to unveil transporter subtype-specific events.

Emerging Roles of Nucleoside Transporters

Since human Nucleoside Transporters (hNTs) were identified by their activity as transport systems, extensive work has been done to fully characterize them at the molecular and physiological level. Many efforts have been addressed to the identification of their selectivity for natural substrates and nucleoside analogs used to treat several diseases. hNTs belong to two different gene families, SLC28 and SLC29, encoding human Concentrative Nucleoside Transporters (hCNTs) and human Equilibrative Nucleoside Transporters (hENTs), respectively. hCNTs and hENTs are integral membrane proteins, albeit structurally unrelated. Both families share common features as substrate selectivity and often tissue localization. This apparent biological redundancy may anticipate some different roles for hCNTs and hENTs in cell physiology. Thus, hENTs may have a major role in maintaining nucleoside homeostasis, whereas hCNTs could contribute to nucleoside sensing and signal transduction. In this sense, the ascription of hCNT1 to a transceptor reinforces this hypothesis. Moreover, some evidences could suggest a putative role of hCNT2 and hCNT3 as transceptors. The interacting proteins identified for hCNT2 suggest a link to energy metabolism. Moreover, the ability of hCNT2 and hCNT3 to transport adenosine links both proteins to purinergic signaling. On the other hand, the broad selectivity transporters hENTs have a crucial role in salvage pathways and purinergic signaling by means of nucleoside pools regulation. In particular, the two new hENT2 isoforms recently described together with hENT2 seem to be key elements controlling nucleoside and nucleotide pools for DNA synthesis. This review focuses on all these NTs functions beyond their mere translocation ability.

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.

Transportome Profiling Identifies Profound Alterations in Crohn's Disease Partially Restored by Commensal Bacteria

BACKGROUND AND AIMS

Several transport alterations have been described in intestinal inflammatory diseases. This is relevant because the primary function of the intestine is nutrient and mineral absorption. However, analysis of the transportome as a whole and the effect of commensal bacteria on it have not been addressed so far.

METHODS

Five healthy and 6 Crohn's disease (CD) samples were hybridized to human HT-12 V4 Illumina GeneChip. Results were validated by reverse transcription-polymerase chain reaction (RT-PCR) analysis and with additional array data. Organ culture assays were performed from mucosa ileal wall specimens collected at surgery. Samples were incubated with or without commensal bacteria for 4 hours. Finally, RNA was isolated for microarray processing.

RESULTS

The analysis of CD versus healthy ileal mucosa demonstrated upregulation of previously described genes involved in immunity and the inflammatory response in this disease. Interestingly, whole transcriptional analysis revealed profound alterations in the transportome profile. Sixty-two solute carrier (SLC) transporters displayed different expression patterns, most of them being downregulated. Changes were confirmed by RT-PCR in a randomly chosen subset of SLCs. A large number of amino acid transporters and most members of the enteric purinome were found to be altered. Most of these proteins were found at the apical membrane of the enterocyte, which could impair both amino acid absorption and purinergic signalling. Treatment of ileum specimen explants with commensal bacteria restored almost all CD transportome alterations.

CONCLUSIONS

These results describe the altered transportome profile in CD and open the possibility of restoring transportome complications with commensal bacteria.

Fluorescent nucleoside derivatives as a tool for the detection of concentrative nucleoside transporter activity using confocal microscopy and flow cytometry

The abundance and function of transporter proteins at the plasma membrane are likely to be crucial in drug responsiveness. Functional detection of human concentrative nucleoside transporters (hCNTs) is of interest for predicting drug sensitivity because of their ability to transport most nucleoside-derived drugs. In the present study, two fluorescent nucleoside analogues, uridine-furan and etheno-cytidine, were evaluated as tools to study in vivo nucleoside transporter-related functions. These two molecules showed high affinity interactions with hCNT1 and hCNT3 and were shown to be substrates of both transporters. Both fluorescence microscopy and flow cytometry experiments showed that uridine-furan uptake was better suited for distinguishing cells that express hCNT1 or hCNT3. These data highlight the usefulness of fluorescent nucleoside derivatives, as long as they fulfill the requirements of confocal microscopy and flow cytometry, for in vivo analysis of hCNT-related function.

Concentrative nucleoside transporter 3 as a prognostic indicator for favorable outcome of t(8;21)-positive acute myeloid leukemia patients after cytarabine-based chemotherapy

Although acute myeloid leukemia (AML) exhibits diverse responses to chemotherapy, patients harboring the t(8;21) translocation are part of a favorable risk group. However, the reason why this subgroup is more responsive to cytarabine-based therapy has not been elucidated. In the present study, we analyzed expression levels of cytarabine metabolism-related genes in patients diagnosed with AML with or without t(8;21) and investigated their correlation with clinical outcomes after cytarabine-based therapy. Among the 8 genes studied, expression of the concentrative nucleoside transporter 3 (CNT3) gene was significantly higher in t(8;21)-positive patients compared to the others in the test population and the validation cohort (P<0.001 in Mann-Whitney U test; P<0.002 in Pearson's correlation analysis). Additionally, in both multivariate and univariate analyses, t(8;21)-positive patients categorized in a higher CNT3 expression tertile had longer disease-free survival [hazard ratio (HR), 0.117; 95% confidence interval (CI), 0.025-0.557; P=0.008] and overall survival (HR, 0.062; 95% CI, 0.007-0.521; P=0.010) compared to t(8;21)-positive patients in a lower CNT3 expression tertile. Notably, these trends did not occur in t(8;21)-negative patients. Our results demonstrate that CNT3 expression is associated with overall favorable outcomes and is predictive of clinical outcomes in AML patients with t(8;21). This suggests that CNT3 expression can be used to optimize treatment strategies for AML patients.

Nucleoside transporter proteins as biomarkers of drug responsiveness and drug targets

Nucleoside and nucleobase analogs are currently used in the treatment of solid tumors, lymphoproliferative diseases, viral infections such as hepatitis and AIDS, and some inflammatory diseases such as Crohn. Two gene families are implicated in the uptake of nucleosides and nucleoside analogs into cells, SCL28 and SLC29. The former encodes hCNT1, hCNT2, and hCNT3 proteins. They translocate nucleosides in a Na⁺ coupled manner with high affinity and some substrate selectivity, being hCNT1 and hCNT2 pyrimidine- and purine-preferring, respectively, and hCNT3 a broad selectivity transporter. SLC29 genes encode four members, being hENT1 and hENT2 the only two which are unequivocally implicated in the translocation of nucleosides and nucleobases (the latter mostly via hENT2) at the cell plasma membrane. Some nucleoside-derived drugs can also interact with and be translocated by members of the SLC22 gene family, particularly hOCT and hOAT proteins. Inter-individual differences in transporter function and perhaps, more importantly, altered expression associated with the disease itself might modulate the transporter profile of target cells, thereby determining drug bioavailability and action. Drug transporter pharmacology has been periodically reviewed. Thus, with this contribution we aim at providing a state-of-the-art overview of the clinical evidence generated so far supporting the concept that these membrane proteins can indeed be biomarkers suitable for diagnosis and/or prognosis. Last but not least, some of these transporter proteins can also be envisaged as drug targets, as long as they can show “transceptor” functions, in some cases related to their role as modulators of extracellular adenosine levels, thereby providing a functional link between P1 receptors and transporters.

Functional crosstalk between the adenosine transporter CNT3 and purinergic receptors in the biliary epithelia

BACKGROUND & AIMS

Both hepatocytes and cholangiocytes release ATP into the bile, where it acts as a potent autocrine/paracrine stimulus that activates biliary secretory mechanisms. ATP is known to be metabolized into multiple breakdown products, ultimately yielding adenosine. However, the elements implicated in the adenosine-dependent purinergic regulation of cholangiocytes are not known.

METHODS

Normal rat cholangiocytes (NRCs) were used to study the expression of adenosine receptors and transporters and their functional interactions at the apical and basolateral membrane domains of polarized cholangiocytes.

RESULTS

We found that: (1) cholangiocytes exclusively express two concentrative nucleoside transporters (CNT) known to be efficient adenosine carriers: CNT3, located at the apical membrane, and CNT2, located at apical and basolateral membrane domains; (2) in both domains, NRCs also express the high affinity adenosine receptor A2A, which modulated the activity of apical CNT3 in a domain-specific manner; (3) the regulation exerted by A2A on CNT3 was dependent upon the cAMP/PKA/ERK/CREB axis, intracellular trafficking mechanisms and AMPK phosphorylation; (4) secretin increased the activity of the apically-located CNT3, and promoted additional basolateral CNT3-related activity; and (5) extracellular ATP (a precursor of adenosine) was able to exert an inhibitory effect on the apical activity of both CNT3 and CNT2.

CONCLUSIONS

This study uncovered the functional expression of nucleoside transporters in cholangiocytes and provides evidence for direct crosstalks between adenosine transporters and receptors for adenosine and its natural extracellular precursor, ATP. Our data anticipate the possibility of adenosine playing a major role in the physiopathology of the biliary epithelia.

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.

Nucleoside transporters in the purinome

The purinome is a rich complex of proteins and cofactors that are involved in fundamental aspects of cellular homeostasis and cellular responses. The purinome is evolutionarily ancient and is made up of thousands of members. Our understanding of the mechanisms linking some parts of this complex network and the physiological relevance of the various connections is well advanced. However, our understanding of other parts of the purinome is less well developed. Our research focuses on the adenosine or nucleoside transporters (NTs), which are members of the membrane purinome. Nucleoside transporters are integral membrane proteins that are responsible for the flux of nucleosides, such as adenosine, and nucleoside analog drugs, used in a variety of anti-cancer, anti-viral and anti-parasite therapies, across cell membranes. Nucleoside transporters form the SLC28 and SLC29 families of solute carriers and the protein members of these families are widely distributed in human tissues including the central nervous system (CNS). NTs modulate purinergic signaling in the CNS primarily through their effects on modulating prevailing adenosine levels inside and outside the cell. By clearing the extracellular milieu of adenosine, NTs can terminate adenosine receptor-dependent signaling and this raises the possibility of regulatory feedback loops that tie together receptor signaling with transporter function. Despite the important role of NTs as modulators of purinergic signaling in the human body, very little is known about the nature or underlying mechanisms of regulation of either the SLC28 or SLC29 families, particularly within the context of the CNS purinome. Here we provide a brief overview of our current understanding of the regulation of members of the SLC29 family and highlight some interesting avenues for future research.

Hypoxia and P1 receptor activation regulate the high-affinity concentrative adenosine transporter CNT2 in differentiated neuronal PC12 cells

Under several adverse conditions, such as hypoxia or ischaemia, extracellular levels of adenosine are elevated because of increased energy demands and ATP metabolism. Because extracellular adenosine affects metabolism through G-protein-coupled receptors, its regulation is of high adaptive importance. CNT2 (concentrative nucleoside transporter 2) may play physiological roles beyond nucleoside salvage in brain as it does in other tissues. Even though nucleoside transport in brain has mostly been seen as being of equilibrative-type, in the present study, we prove that the rat phaeochromocytoma cell line PC12 shows a concentrative adenosine transport of CNT2-type when cells are differentiated to a neuronal phenotype by treatment with NGF (nerve growth factor). Differentiation of PC12 cells was also associated with the up-regulation of adenosine A1 receptors. Addition of adenosine receptor agonists to cell cultures increased CNT2-related activity by a mechanism consistent with A₁ and A2A receptor activation. The addition of adenosine to the culture medium also induced the phosphorylation of the intracellular regulatory kinase AMPK (AMP-activated protein kinase), with this effect being dependent upon adenosine transport. CNT2-related activity of differentiated PC12 cells was also dramatically down-regulated under hypoxic conditions. Interestingly, the analysis of nucleoside transporter expression after experimental focal ischaemia in rat brain showed that CNT2 expression was down-regulated in the infarcted tissue, with this effect somehow being restricted to other adenosine transporter proteins such as CNT3 and ENT1 (equilibrative nucleoside transporter 1). In summary, CNT2 is likely to modulate extracellular adenosine and cell energy balance in neuronal tissue.

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.

mTOR signaling regulates the processing of pre-rRNA in human cells

Signaling through the mammalian target of rapamycin, complex 1 (mTORC1), positively regulates the transcription of ribosomal RNA (rRNA) and the synthesis of ribosomal proteins, thereby promoting the complex process of ribosome biogenesis. The major rRNAs are transcribed as a single precursor, which must be processed to create the 5.8S, 18S and 28S rRNAs. We used a new non-radioactive labeling approach to study the effects of rapamycin, an inhibitor of mTORC1, on rRNA synthesis. Rapamycin not only impaired synthesis of new 18S, 28S or 5S rRNA but also induced their decay. This prompted us to examine the effects of rapamycin on rRNA processing. We show that rapamycin also interferes with the processing events that generate 18S and 28S rRNA. rRNA transcription and processing occur in regions of the nucleus known as nucleoli. We find that the mTORC1 components raptor and mTOR are both present in nucleoli, where they may regulate rRNA maturation events. While rapamycin has no effect on overall nucleolar morphology or its proteome, it does induce loss of mTOR and raptor from them. These data show that mTORC1 is located in nucleoli where it acts to regulate events involved in ribosome biogenesis including the maturation of rRNA molecules.

Impaired glucose tolerance in the absence of adenosine A1 receptor signaling

OBJECTIVE

The role of adenosine (ADO) in the regulation of glucose homeostasis is not clear. In the current study, we used A1-ADO receptor (A1AR)-deficient mice to investigate the role of ADO/A1AR signaling for glucose homeostasis.

RESEARCH DESIGN AND METHODS

After weaning, A1AR(-/-) and wild-type mice received either a standard diet (12 kcal% fat) or high-fat diet (HFD; 45 kcal% fat). Body weight, fasting plasma glucose, plasma insulin, and intraperitoneal glucose tolerance tests were performed in 8-week-old mice and again after 12-20 weeks of subsequent observation. Body composition was quantified by magnetic resonance imaging and epididymal fat-pad weights. Glucose metabolism was investigated by hyperinsulinemic-euglycemic clamp studies. To describe pathophysiological mechanisms, adipokines and Akt phosphorylation were measured.

RESULTS

A1AR(-/-) mice were significantly heavier than wild-type mice because of an increased fat mass. Fasting plasma glucose and insulin were significantly higher in A1AR(-/-) mice after weaning and remained higher in adulthood. An intraperitoneal glucose challenge disclosed a significantly slower glucose clearance in A1AR(-/-) mice. An HFD enhanced this phenotype in A1AR(-/-) mice and unmasked a dysfunctional insulin secretory mechanism. Insulin sensitivity was significantly impaired in A1AR(-/-) mice on the standard diet shortly after weaning. Clamp studies detected a significant decrease of net glucose uptake in A1AR(-/-) mice and a reduced glucose uptake in muscle and white adipose tissue. Effects were not triggered by leptin deficiency but involved a decreased Akt phosphorylation.

CONCLUSIONS

ADO/A1AR signaling contributes importantly to insulin-controlled glucose homeostasis and insulin sensitivity in C57BL/6 mice and is involved in the metabolic regulation of adipose tissue.

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.

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.

SLC28 genes and concentrative nucleoside transporter (CNT) proteins

The human concentrative nucleoside transporter (hCNT) protein family has three members, hCNT1, 2, and 3, encoded by SLC28A1, A2, and A3 genes, respectively. hCNT1 and hCNT2 translocate pyrimidine- and purine-nucleosides, respectively, by a sodium-dependent mechanism, whereas hCNT3 shows broad substrate selectivity and the unique ability of translocating nucleosides both in a sodium- and a proton-coupled manner. hCNT proteins are also responsible for the uptake of most nucleoside-derived antiviral and anticancer drugs. Thus, hCNTs are key pharmacological targets. This review focuses on several crucial aspects of hCNT biology and pharmacology: protein structure-function, structural determinants for transportability, pharmacogenetics of hCNT-encoding genes, role of hCNT proteins in nucleoside-based therapeutics, and finally hCNT physiology.

Physiological and pharmacological roles of nucleoside transporter proteins

Nucleoside transporter proteins, CNT and ENT, encoded by gene families SLC28 and SLC29, respectively, mediate the uptake of natural nucleosides (among them adenosine) and are major routes of entry for a variety of nucleoside analogs used in anticancer and antiviral therapies. Expression of NT proteins is apparently redundant in most cell types, and the elucidation of their particular physiological roles still remains elusive. Moreover, transporter-mediated uptake of nucleoside-derived anticancer drugs is crucial for the pharmacogenomic response triggered by these molecules in tumor cells. This review focuses on recent data demonstrating that nucleoside transporters, particularly CNTs, can play physiological roles other than salvage, whereas particular NT isoforms can significantly contribute to the transcriptomic response triggered by nucleoside analogs in cancer cells.

Is human hibernation possible?

The induction of hypometabolism in cells and organs to reduce ischemia damage holds enormous clinical promise in diverse fields, including treatment of stroke and heart attack. However, the thought that humans can undergo a severe hypometabolic state analogous to hibernation borders on science fiction. Some mammals can enter a severe hypothermic state during hibernation in which metabolic activity is extremely low, and yet full viability is restored when the animal arouses from such a state. To date, the underlying mechanism for hibernation or similar behaviors remains an enigma. The beneficial effect of hypothermia, which reduces cellular metabolic demands, has many well-established clinical applications. However, severe hypothermia induced by clinical drugs is extremely difficult and is associated with dramatically increased rates of cardiac arrest for nonhibernators. The recent discovery of a biomolecule, 5'-AMP, which allows nonhibernating mammals to rapidly and safely enter severe hypothermia could remove this impediment and enable the wide adoption of hypothermia as a routine clinical tool.

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.

Altered Expression of Nucleoside Transporter Genes (SLC28 and SLC29) in Adipose Tissue from HIV-1–Infected Patients

Background

Nucleoside transporter proteins (NTs) encoded by members of the SLC28 and SLC29 gene families contribute to nucleoside and nucleobase recycling but also modulate extracellular adenosine levels and thus adenosine-regulated metabolic targets.

Methods

We have examined the expression pattern of NT-encoding genes in human adipose tissue and we have further analysed whether the mRNA related to these genes show changes in their amounts associated with either HIV-1 infection, highly active antiretroviral therapy (HAART) or development of HIV-1-associated lipodystrophy syndrome (HALS).

Results

Human adipocytes express SLC28A1, SLC28A2 and SLC28A3 (encoding hCNT1, hCNT2 and hCNT3, respectively) and SLC29A1 and SLC29A2 (encoding hENT1 and hENT2, respectively). HIV-1 infection, prior to HAART and HALS development, is associated with the upregulation of the mRNA levels of the genes encoding hCNT1, hCNT3 and hENT2. The increase in the mRNA amounts for the former two genes may be due to the action of tumour necrosis factor-α (TNF-α), a cytokine with enhanced expression in adipose tissue following HIV-1 infection, as the effect is also observed in human adipocytes in culture after treatment with TNF-α. HAART and HALS development are associated with the upregulation of the mRNA levels encoding hCNT2 and hENT1, and further enhancement of hCNT1, hCNT3 and hENT2 gene expression.

Conclusions

These data suggest that selected genes of the SLC28 and SLC29 families are not only targets of HIV-1 infection, but might also contribute to the development of adipose tissue alterations leading to lipodystrophy.

Role of CNT3 in the transepithelial flux of nucleosides and nucleoside-derived drugs

We examined the role of the concentrative nucleoside transporter CNT3 in the establishment of a transepithelial flux of natural nucleosides and their pharmacologically active derivatives in renal epithelial cell lines. Murine PCT cells grown on a transwell dish showed endogenous CNT3 activity at their apical membrane that was responsible for the sodium-dependent transepithelial flux of both purine and pyrimidine nucleosides. hCNT3 was also identified in human kidney and its role in the transport of nucleosides was tested. To this end, MDCK cells, lacking endogenous CNT3 activity, were genetically engineered to express the human orthologue of CNT3 (hCNT3-MDCK cells). In these cells, hCNT3 was inserted into the apical membrane, thus generating, as for PCT cells, a transepithelial flux of both nucleosides and nucleoside-derived drugs. Apical-to-basolateral transepithelial flux was present in all cells expressing a functional CNT3 transporter and was significantly higher than that found either in PCT cells in absence of sodium or in mock-transfected MDCK cells. Nevertheless in all cases a significant amount of the transported nucleoside was retained and transformed inside cells. However release to the opposite compartment was CNT3 dependent, not only in terms of absolute flux (much higher when an apical CNT3 transporter was active) but also regarding metabolic transformations of the apically absorbed nucleosides. These results underline a critical role of CNT3 in the renal reabsorption of nucleosides and their derivatives as well as in their intracellular metabolism.

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.

Transcription factors involved in the expression of SLC28 genes in human liver parenchymal cells

Human nucleoside transporters are encoded by SLC28 (hCNTs) and SLC29 (hENTs) genes. These proteins mediate the uptake of anticancer and some antiviral drugs and are also suitable candidates to facilitate nucleoside-derived drug uptake into hepatocytes for detoxification. Despite the putative relevance of these genes in liver physiology, the human SLC28 and SLC29 expression pattern is not known and suitable cell models are not available. These issues have been addressed by examining NT expression in human liver and primary cultures of human hepatocytes. Moreover, the effect of specific liver enriched transcription factors (LETFs) in hCNTs expression has been analyzed. Human hepatocytes express hCNT1, hCNT2, hENT1, and hENT2. Loss of the hepatic phenotype in primary culture is associated with a decrease in hCNT1 and hCNT2 mRNA levels. Selected LETFs are involved in the regulation of SLC28 genes in an isoform-specific manner. HNF4alpha is a major determinant of SLC28A1 expression, whereas C/EBPalpha and HNF3gamma modulate SLC28A2 gene expression.

Opioid receptor-independent protection of ischemic rat hepatocytes by morphine

We studied the role of morphine in anoxia/reoxygenation injury to hepatocytes. Overnight cultured rat hepatocytes were incubated in anoxic buffer at pH 6.2 for 4h and reoxygenated at pH 7.4 for 2h to simulate anoxia/reoxygenation. Some hepatocytes were preincubated with 50 microM morphine for 10 min prior to onset of anoxia/reoxygenation. To study the effect of morphine on nitric oxide (NO), hepatocytes were loaded with 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM). Changes in NO concentration were assessed with a multi-well fluorescence reader and confocal microscopy. Morphine substantially improved cell viability after reoxygenation and increased NO generation, which was blocked by ATP-sensitive potassium channel blockers. Confocal images revealed that the increase in NO occurred mainly at the cytosol. However, treatment with opioid receptor antagonists did not reverse cytoprotection by morphine. These results indicate that morphine prevents anoxia/reoxygenation injury to hepatocytes. Protective mechanisms are associated with the potassium channels and NO, but are independent of opioid receptor-mediated signaling.

Potassium channels: new targets in cancer therapy

BACKGROUND

Potassium channels (KCh) are the most diverse and ubiquitous class of ion channels. KCh control membrane potential and contribute to nerve and cardiac action potentials and neurotransmitter release. KCh are also involved in insulin release, differentiation, activation, proliferation, apoptosis, and several other physiological functions. The aim of this review is to provide an updated overview of the KCh role during the cell growth. Their potential use as pharmacological targets in cancer therapies is also discussed.

METHODS

We searched PubMed (up to 2005) and identified relevant articles. Reprints were mainly obtained by on line subscription. Additional sources were identified through cross-referencing and obtained from Library services.

RESULTS

KCh are responsible for some neurological and cardiovascular diseases and for a new medical discipline, channelopathies. Their role in congenital deafness, multiple sclerosis, episodic ataxia, LQT syndrome and diabetes has been proven. Furthermore, a large body of information suggests that KCh play a role in the cell cycle progression, and it is now accepted that cells require KCh to proliferate. Thus, KCh expression has been studied in a number of tumours and cancer cells.

CONCLUSIONS

Cancer is far from being considered a channelopathy. However, it seems appropriate to take into account the involvement of KCh in cancer progression and pathology when developing new strategies for cancer therapy.

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

CNT2 (concentrative nucleoside cotransporter) is a plasma membrane high-affinity Na+-coupled adenosine transporter, also localized in intracellular structures. This transporter protein may play additional roles other than nucleoside salvage, since it has recently been shown to be under purinergic control via K(ATP) channels, by a mechanism that does not seem to involve changes in its subcellular localization. In an attempt to identify the agents that promote CNT2 trafficking, bile acids were found to increase CNT2-related transport activity in a K(ATP) channel-independent manner in both Fao hepatoma and rat liver parenchymal cells. A maximum effect was recorded after treatment with hydrophilic anions such as TCA (taurocholate). However, this effect did not involve changes in the amount of CNT2 protein, it was instead associated with a subcellular redistribution of CNT2, resulting in an accumulation of the transporter at the plasma membrane. This was deduced from subcellular fractionation studies, biotinylation of plasma membrane proteins and subsequent CNT2 detection in streptavidin precipitates and in vivo confocal microscopic analysis of the distribution of a YFP (yellow fluorescent protein)-CNT2 construct. The induction of CNT2 translocation, triggered by TCA, was inhibited by wortmannin, dibutyryl-AMPc, PD98059 and colchicine, thus suggesting the involvement of the PI3K/ERK (phosphoinositide 3-kinase/extracellular-signal related kinase) pathway in microtubule-dependent activation of recombinant CNT2. These are novel effects of bile-acid physiology and provide the first evidence for short-term regulation of CNT2 translocation into and from the plasma membrane.

Extracellular adenosine activates AMP-dependent protein kinase (AMPK)

Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a heterotrimeric complex that senses intracellular energy status and exerts rapid regulation on energy-demanding and -consuming metabolic pathways. Although alterations in the intracellular adenosine nucleotide pool are traditionally assumed to be the consequence of changes in energy metabolism, in this study we have addressed the question of whether extracellular adenosine contributes to AMPK regulation. In the intestinal rat epithelial cell line IEC-6, addition of adenosine rapidly increases AMP intracellular concentrations and upregulates alpha1AMPK, thus promoting phosphorylation of its downstream target acetyl-CoA carboxylase (ACC). The effect of adenosine on AMPK signaling is completely blocked by transducing IEC-6 cells with an adenoviral vector expressing a mutated alpha1 subunit, resulting in a dominant-negative effect on endogenous AMPK activity. These effects are blocked by 5'-iodotubercidine (5'-ITU), an inhibitor of adenosine kinase. Moreover, inhibition of adenosine transport through the concentrative adenosine plasma membrane transporter CNT2 with formycin B results in the blockade of adenosine-mediated AMPK signaling. Extracellular adenosine is equally able to activate AMPK and promote ACC phosphorylation in liver parenchymal cell models in a manner that is also inhibited by 5'-ITU. In summary, this study shows that adenosine, when added at physiological concentrations, activates AMPK and promotes ACC phosphorylation. Adenosine must be transported and phosphorylated to exert its action. Thus, nucleoside transporters might be novel players in the complex regulation of AMPK and energy metabolism.

The effect of normothermic recirculation is mediated by ischemic preconditioning in NHBD liver transplantation

We have evaluated the involvement of hepatic preconditioning mediators (adenosine, adenosine A1 and A2 receptors) during normothermic recirculation (NR) in a model of liver transplantation from non-heart-beating donor (NHBD) pigs. Application of NR after 20 min of warm ischemia (WI) reversed the lethal injury associated with transplantation of NHBD livers (achieving 5-day survival and diminishing glutathione S-transferase (GST), aspartate aminotransferase (AST) and hyaluronic acid (HA)). Adenosine administration prior to WI simulated the effect of NR. Measuring adenosine, we found that during NR, hepatic adenosine levels increased and xanthine levels decreased. Then when we blocked A2 receptors the effect of NR was abolished, whereas the blocking of A1 receptors further protected the liver. Furthermore, A2 blocking improved hepatic perfusion during NR whereas A1 blocking reduced it. The study suggests that NR has a preconditioning effect by maintaining adequate adenosine and xanthine levels. During NR, adenosine protects the liver through A2 activation and damages it through A1 activation although simultaneous stimulation of both receptors exerts a clear beneficial effect. The possible relation of NR mechanism with other preconditioning mediators such as cAMP and nitric oxide synthesis are discussed.

Expression of concentrative nucleoside transporters SLC28 (CNT1, CNT2, and CNT3) along the rat nephron: Effect of diabetes

BACKGROUND

The renal reabsorption of natural nucleosides and a variety of nucleoside-derived drugs relies on the function of the apically located, Na(+)-dependent, concentrative nucleoside transporters CNT1, CNT2, and CNT3 (SLC28A1, SLC28A2, and SLC28A3). The aims of this study were to determine the segmental localization of the three SLC28 family members and to establish whether streptozotocin-induced diabetes alters their expression.

METHODS

SLC28 expression was measured by real-time polymerase chain reaction (PCR) on microdissected sections of rat nephrons. Diabetes was induced by streptozotocin treatment and the biochemical profiles of control, diabetic, and insulin-treated rats were established. The effect of diabetes on SLC28 expression was assessed in those segments that significantly express SLC28 genes.

RESULTS

CNT1-3 mRNAs were expressed in the proximal tubule and glomerulus. In addition, CNT2 and CNT3 mRNAs were expressed in the outer medullary and cortical collecting duct, respectively. Diabetes reduced expression of the three CNTs in almost all nephron segments, and this effect was not prevented by an insulin treatment that normalized all blood and urine parameters. Diabetes increased CNT1 and CNT3 expression in the glomerulus and insulin treatment decreased it.

CONCLUSION

The relative distribution of SLC28 gene expression suggests a role for the proximal tubule in renal nucleoside clearance and an accessory role for CNT2 and CNT3, in adenosine-mediated regulation of collecting duct functions. Diabetes probably may impair nucleoside clearance independently of insulin.

Adenosine A 2A receptors control the extracellular levels of adenosine through modulation of nucleoside transporters activity in the rat hippocampus

Adenosine, a neuromodulator of the CNS, activates inhibitory-A1 receptors and facilitatory-A2A receptors; its synaptic levels are controlled by the activity of bi-directional equilibrative nucleoside transporters. To study the relationship between the extracellular formation/inactivation of adenosine and the activation of adenosine receptors, we investigated how A1 and A2A receptor activation modifies adenosine transport in hippocampal synaptosomes. The A2A receptor agonist, CGS 21680 (30 nm), facilitated adenosine uptake through a PKC-dependent mechanism, but A1 receptor activation had no effect. CGS 21680 (30 nm) also increased depolarization-induced release of adenosine. Both effects were prevented by A2A receptor blockade. A2A receptor-mediated enhancement of adenosine transport system is important for formatting adenosine neuromodulation according to the stimulation frequency, as: (1) A1 receptor antagonist, DPCPX (250 nm), facilitated the evoked release of [(3)H]acetylcholine under low-frequency stimulation (2 Hz) from CA3 hippocampal slices, but had no effect under high-frequency stimulation (50 Hz); (2) either nucleoside transporter or A2A receptor blockade revealed the facilitatory effect of DPCPX (250 nm) on [3H]acetylcholine evoked-release triggered by high-frequency stimulation. These results indicate that A2A receptor activation facilitates the activity of nucleoside transporters, which have a preponderant role in modulating the extracellular adenosine levels available to activate A1 receptors.

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.

The concentrative nucleoside transporter family (SLC28): new roles beyond salvage?

The concentrative nucleoside transporter (CNT) family (SLC28) has three members: SLC28A1 (CNT1), SLC28A2 (CNT2) and SLC28A3 (CNT3). The CNT1 and CNT2 transporters are co-expressed in liver parenchymal cells and macrophages, two suitable models in which to study cell cycle progression. Despite initial observations suggesting that these transporter proteins might contribute to nucleoside salvage during proliferation, their subcellular localization and regulatory properties suggest alternative roles in cell physiology. In particular, CNT2 is a suitable candidate for modulation of purinergic responses, since it is under the control of the adenosine 1 receptor. Increasing evidence also suggests a role for CNT2 in energy metabolism, since its activation relies on the opening of ATP-sensitive K+ channels. 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.