Human equilibrative nucleoside transporter (ENT) family of nucleoside and nucleobase transporter proteins

Predicting gemcitabine transport and toxicity in human pancreatic cancer cell lines with the positron emission tomography tracer 3'-deoxy-3'-fluorothymidine

The abundance of human equilibrative nucleoside transporter 1 (hENT1) has recently been shown to be a predictive marker of benefit from gemcitabine therapy in patients with pancreatic cancer. Since hENT1 is also important for the uptake of positron emission tomography (PET) tracer 3'-deoxy-3'-fluorothymidine (FLT) in various cultured human cell lines, this study was undertaken to determine if FLT uptake predicts gemcitabine uptake and/or toxicity in a panel of human pancreatic cancer cell lines (Capan-2, AsPC-1, BxPC-3, PL45, MIA PaCa-2, and PANC-1). Capan-2 cells displayed the lowest levels of (1) extracellular nitrobenzylmercaptopurine ribonucleoside (NBMPR) binding, which represents cell-surface hENT1, (2) FLT and gemcitabine uptake during short (1-45s) and prolonged (1h) periods, and (3) gemcitabine sensitivity. Exposure to NBMPR (inhibits only hENT1) or dilazep (inhibits hENT1 and hENT2) reduced FLT and gemcitabine uptake and gemcitabine sensitivity, with dilazep having greater effects than NBMPR. Gemcitabine permeation was almost completely mediated, primarily by hENT1 and to a lesser extent by hENT2, whereas FLT permeation included a substantial component of passive diffusion. In five of six cell lines, correlations were observed between (1) FLT and gemcitabine initial rates of uptake, (2) gemcitabine uptake and gemcitabine toxicity, (3) FLT uptake and gemcitabine toxicity, and (4) ribonucleotide reductase subunit M1 expression and gemcitabine toxicity. FLT and gemcitabine uptake were comparable for predicting gemcitabine toxicity in the tested pancreatic cancer cell lines suggesting that FLT PET may provide clinically useful information about tumor gemcitabine transport capacity and sensitivity.

Mechanisms of multidrug resistance in cancer

The development of multidrug resistance (MDR) to chemotherapy remains a major challenge in the treatment of cancer. Resistance exists against every effective anticancer drug and can develop by numerous mechanisms including decreased drug uptake, increased drug efflux, activation of detoxifying systems, activation of DNA repair mechanisms, evasion of drug-induced apoptosis, etc. In the first part of this chapter, we briefly summarize the current knowledge on individual cellular mechanisms responsible for MDR, with a special emphasis on ATP-binding cassette transporters, perhaps the main theme of this textbook. Although extensive work has been done to characterize MDR mechanisms in vitro, the translation of this knowledge to the clinic has not been crowned with success. Therefore, identifying genes and mechanisms critical to the development of MDR in vivo and establishing a reliable method for analyzing clinical samples could help to predict the development of resistance and lead to treatments designed to circumvent it. Our thoughts about translational research needed to achieve significant progress in the understanding of this complex phenomenon are therefore discussed in a third section. The pleotropic response of cancer cells to chemotherapy is summarized in a concluding diagram.

Role of transmembrane domain 4 in ligand permeation by Crithidia fasciculata equilibrative nucleoside transporter 2 (CfNT2)

Equilibrative nucleoside transporters play essential roles in nutrient uptake, cardiovascular and renal function, and purine analog drug chemotherapies. Limited structural information is available for this family of transporters; however, residues in transmembrane domains 1, 2, 4, and 5 appear to be important for ligand and inhibitor binding. In order to identify regions of the transporter that are important for ligand specificity, a genetic selection for mutants of the inosine-guanosine-specific Crithidia fasciculata nucleoside transporter 2 (CfNT2) that had gained the ability to transport adenosine was carried out in the yeast Saccharomyces cerevisiae. Nearly all positive clones from the genetic selection carried mutations at lysine 155 in transmembrane domain 4, highlighting lysine 155 as a pivotal residue governing the ligand specificity of CfNT2. Mutation of lysine 155 to asparagine conferred affinity for adenosine on the mutant transporter at the expense of inosine and guanosine affinity due to weakened contacts to the purine ring of the ligand. Following systematic cysteine-scanning mutagenesis, thiol-specific modification of several positions within transmembrane domain 4 was found to interfere with inosine transport capability, indicating that this helix lines the water-filled ligand translocation channel. Additionally, the pattern of modification of transmembrane domain 4 suggested that it may deviate from helicity in the vicinity of residue 155. Position 155 was also protected from modification in the presence of ligand, suggesting that lysine 155 is in or near the ligand binding site. Transmembrane domain 4 and particularly lysine 155 appear to play key roles in ligand discrimination and translocation by CfNT2.

Equilibrative nucleoside transporter 1 plays an essential role in cardioprotection

To better understand the role of equilibrative nucleoside transporters (ENT) in purine nucleoside-dependent physiology of the cardiovascular system, we investigated whether the ENT1-null mouse heart was cardioprotected in response to ischemia (coronary occlusion for 30 min followed by reperfusion for 2 h). We observed that ENT1-null mouse hearts showed significantly less myocardial infarction compared with wild-type littermates. We confirmed that isolated wild-type adult mouse cardiomyocytes express predominantly ENT1, which is primarily responsible for purine nucleoside uptake in these cells. However, ENT1-null cardiomyocytes exhibit severely impaired nucleoside transport and lack ENT1 transcript and protein expression. Adenosine receptor expression profiles and expression levels of ENT2, ENT3, and ENT4 were similar in cardiomyocytes isolated from ENT1-null adult mice compared with cardiomyocytes isolated from wild-type littermates. Moreover, small interfering RNA knockdown of ENT1 in the cardiomyocyte cell line, HL-1, mimics findings in ENT1-null cardiomyocytes. Taken together, our data demonstrate that ENT1 plays an essential role in cardioprotection, most likely due to its effects in modulating purine nucleoside-dependent signaling and that the ENT1-null mouse is a powerful model system for the study of the role of ENTs in the physiology of the cardiomyocyte.

8-Cl-adenosine inhibits proliferation and causes apoptosis in B-lymphocytes via protein kinase A-dependent and independent effects: implications for treatment of Carney complex-associated tumors

CONTEXT

Carney complex, a multiple neoplasia syndrome, characterized primarily by spotty skin pigmentation and a variety of endocrine and other tumors, is caused by mutations in PRKAR1A, the gene that codes for the RIalpha subunit of protein kinase A (PKA). PKA controls cell proliferation in many cell types. The cAMP analogue 8-Cl-adenosine (8-Cl-ADO) is thought to inhibit cancer cell proliferation.

OBJECTIVE

The objective of the study was to study the antiproliferative effects of 8-Cl-ADO on growth and proliferation in B-lymphocytes of Carney complex patients that have PKA defects and to determine whether 8-CL-ADO could be used as a therapeutic agent in the treatment of Carney complex-associated tumors.

DESIGN

We used a multiparametric approach (i.e. growth and proliferation assays, PKA, and PKA subunit assays, cAMP and (3)H-cAMP binding assays, and apoptosis assays) to understand the growth and proliferative effects of 8-Cl-ADO on human B-lymphocytes.

RESULTS

8-Cl-ADO inhibited proliferation, mainly through its intracellular transport and metabolism, which induced apoptosis. PKA activity, cAMP levels, and (3)H-cAMP binding were increased or decreased, respectively, by 8-Cl-ADO, whereas PKA subunit levels were differentially affected. 8-Cl-ADO also inhibited proliferation induced by G protein-coupled receptors for isoproterenol and adenosine, as well as proliferation induced by tyrosine kinase receptors.

CONCLUSIONS

8-Cl-ADO in addition to unambiguously inhibiting proliferation and inducing apoptosis in a PKA-independent manner also has PKA-dependent effects that are unmasked by a mutant PRKAR1A. Thus, 8-Cl-ADO could serve as a therapeutic agent in patients with Carney complex-related tumors.

Towards to hENT1-nucleoside transporter selective imaging agents. Synthesis and in vitro evaluation of the radiolabeled SAENTA analogues

Three new potential hENT(1) inhibitors suitable for labeling with PET/SPECT radioisotopes were prepared from an advanced intermediate 4. They were tested for their capability to inhibit binding of SAENTA-fluorescein to HL60 leukemia cells in flow cytometry assay and SAENTA-I (5) was determined to be the most active compound. (131)I-5 showed high hENT(1)-specific binding (up to 54% ID) to 6 from 7 tested tumor cell lines and was chosen for further in vivo study.

Substituted cysteine accessibility method analysis of human concentrative nucleoside transporter hCNT3 reveals a novel discontinuous region of functional importance within the CNT family motif (G/A)XKX3NEFVA(Y/M/F)

The human SLC28 family of integral membrane CNT (concentrative nucleoside transporter) proteins has three members, hCNT1, hCNT2, and hCNT3. Na(+)-coupled hCNT1 and hCNT2 transport pyrimidine and purine nucleosides, respectively, whereas hCNT3 mediates transport of both pyrimidine and purine nucleosides utilizing Na(+) and/or H(+) electrochemical gradients. These and other eukaryote CNTs are currently defined by a putative 13-transmembrane helix (TM) topology model with an intracellular N terminus and a glycosylated extracellular C terminus. Recent mutagenesis studies, however, have provided evidence supporting an alternative 15-TM membrane architecture. In the absence of CNT crystal structures, valuable information can be gained about residue localization and function using substituted cysteine accessibility method analysis with thiol-reactive reagents, such as p-chloromercuribenzene sulfonate. Using heterologous expression in Xenopus oocytes and the cysteineless hCNT3 protein hCNT3C-, substituted cysteine accessibility method analysis with p-chloromercuribenzene sulfonate was performed on the TM 11-13 region, including bridging extramembranous loops. The results identified residues of functional importance and, consistent with a new revised 15-TM CNT membrane architecture, suggest a novel membrane-associated topology for a region of the protein (TM 11A) that includes the highly conserved CNT family motif (G/A)XKX(3)NEFVA(Y/M/F).

Conserved glutamate residues Glu-343 and Glu-519 provide mechanistic insights into cation/nucleoside cotransport by human concentrative nucleoside transporter hCNT3

Human concentrative nucleoside transporter 3 (hCNT3) utilizes electrochemical gradients of both Na(+) and H(+) to accumulate pyrimidine and purine nucleosides within cells. We have employed radioisotope flux and electrophysiological techniques in combination with site-directed mutagenesis and heterologous expression in Xenopus oocytes to identify two conserved pore-lining glutamate residues (Glu-343 and Glu-519) with essential roles in hCNT3 Na(+)/nucleoside and H(+)/nucleoside cotransport. Mutation of Glu-343 and Glu-519 to aspartate, glutamine, and cysteine severely compromised hCNT3 transport function, and changes included altered nucleoside and cation activation kinetics (all mutants), loss or impairment of H(+) dependence (all mutants), shift in Na(+):nucleoside stoichiometry from 2:1 to 1:1 (E519C), complete loss of catalytic activity (E519Q) and, similar to the corresponding mutant in Na(+)-specific hCNT1, uncoupled Na(+) currents (E343Q). Consistent with close-proximity integration of cation/solute-binding sites within a common cation/permeant translocation pore, mutation of Glu-343 and Glu-519 also altered hCNT3 nucleoside transport selectivity. Both residues were accessible to the external medium and inhibited by p-chloromercuribenzene sulfonate when converted to cysteine.

Reduced ribavirin antiviral efficacy via nucleoside transporter-mediated drug resistance

Treatment for hepatitis C virus infection currently consists of pegylated interferon and ribavirin (RBV), a nucleoside analog. Although RBV clearly plays a role in aiding the treatment response, its antiviral mechanism is unclear. Regardless of the specific mechanism of RBV, we hypothesize that differences in levels of cellular uptake of RBV may affect antiviral efficacy and treatment success and that cells may become RBV resistant through reduced uptake. We monitored RBV uptake in various cell lines and determined the effect of uptake capacity on viral replication. RBV-resistant cells demonstrated reduced RBV uptake and increased growth of a model RNA virus, poliovirus, in the presence of RBV. Overexpression of equilibrative nucleoside transporter 1 (ENT1) or concentrative nucleoside transporter 3 (CNT3) increased RBV uptake in RBV-sensitive cell lines and restored the uptake defect in most RBV-resistant cell lines. However, CNT3 is not expressed in Huh-7 liver cells, and inhibition of concentrative transport did not affect RBV uptake. Blocking equilibrative transport using the inhibitor nitrobenzylmercaptopurine riboside recapitulated the RBV-resistant phenotype in RBV-sensitive cell lines, with a reduction in RBV uptake and increased poliovirus growth. Taken together, these results indicate that RBV uptake is restricted primarily to ENT1 in the cell lines examined. Interestingly, some RBV-resistant cell lines may compensate for reduced ENT1-mediated nucleoside uptake by increasing the activity of an alternative nucleoside transporter, ENT2. It is possible that RBV uptake affects the antiviral treatment response, either through natural differences in patients or through acquired resistance.