Mobility of nucleoside transporter of human erythrocytes differs greatly when loaded with different nucleosides

Alternating carrier models of asymmetric glucose transport violate the energy conservation laws

Alternating access transporters with high-affinity externally facing sites and low-affinity internal sites relate substrate transit directly to the unliganded asymmetric "carrier" (Ci) distribution. When both bathing solutions contain equimolar concentrations of ligand, zero net flow of the substrate-carrier complex requires a higher proportion of unliganded low-affinity inside sites (proportional, variant 1/KD(in)) and slower unliganded "free" carrier transit from inside to outside than in the reverse direction. However, asymmetric rates of unliganded carrier movement, kij, imply that an energy source, DeltaGcarrier = RT ln (koi/kio) = RT ln (Cin/Cout) = RT ln (KD(in)/KD(out)), where R is the universal gas constant (8.314 Joules/M/K degrees), and T is the temperature, assumed here to be 300 K degrees , sustains the asymmetry. Without this invalid assumption, the constraints of carrier path cyclicity, combined with asymmetric ligand affinities and equimolarity at equilibrium, are irreconcilable, and any passive asymmetric uniporter or cotransporter model system, e.g., Na-glucose cotransporters, espousing this fundamental error is untenable. With glucose transport via GLUT1, the higher maximal rate and Km of net ligand exit compared to net ligand entry is only properly simulated if ligand transit occurs by serial dissociation-association reactions between external high-affinity and internal low-affinity immobile sites. Faster intersite transit rates occur from lower-affinity sites than from higher-affinity sites and require no other energy source to maintain equilibrium. Similar constraints must apply to cotransport.

Extracellular adenosine-induced apoptosis in mouse neuroblastoma cells: studies on involvement of adenosine receptors and adenosine uptake

The induction of apoptosis by adenosine was studied in the mouse neuroblastoma cell line N1E-115. Apoptosis was characterized by fluorescence and electron microscopy, fluorescence-activated cell sorter (FACS) analysis, and caspase activity assays. A sixteen-hour exposure to 100 microM of adenosine led to chromatin condensation and caspase activation. However, selective agonists for all four adenosine receptors were ineffective. Caspase activation could be blocked partially by an inhibitor of the nucleoside transporter, dipyridamole, and completely by uridine, a competing substrate for adenosine transport. 2'-Deoxycoformycin, an inhibitor of adenosine deaminase, enhanced caspase activation by adenosine but had no effect by itself. Caspase activation could be blocked by 5'-amino-5'-deoxyadenosine, which inhibits the phosphorylation of adenosine by adenosine kinase. These results indicate that adenosine receptors are not involved in adenosine-induced apoptosis in N1E-115 cells, but that uptake of adenosine and its subsequent phosphorylation is required.

Selective transport of adenosine into porcine coronary smooth muscle

Adenosine (ADO), an endogenous regulator of coronary vascular tone, enhances vasorelaxation in the presence of nucleoside transport inhibitors such as dipyridamole. We tested the hypothesis that coronary smooth muscle (CSM) contains a high-affinity transporter for ADO. ADO-mediated relaxation of isolated large and small porcine coronary artery rings was enhanced 12-fold and 3.4-fold, respectively, by the transport inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI). Enhanced relaxation was independent of endothelium and was selective for ADO over synthetic analogs. Uptake of [(3)H]ADO into freshly dissociated CSM cells or endothelium-denuded rings was linear and concentration dependent. Kinetic analysis yielded a maximum uptake (V(max)) of 67 +/- 7.0 pmol. mg protein(-1). min(-1) and a Michaelis constant (K(m)) of 10. 5 +/- 5.8 microM in isolated cells and a V(max) of 5.1 +/- 0.5 pmol. min(-1). mg wet wt(-1) and a K(m) of 17.6 +/- 2.6 microM in intact rings. NBTI inhibited transport into small arteries (IC(50) = 42 nM) and cells. Analyses of extracellular space and diffusion kinetics using [(3)H]sucrose indicate the V(max) and K(m) for ADO transport are sufficient to clear a significant amount of extracellular adenosine. These data indicate CSM possess a high-affinity nucleoside transporter and that the activity of this transporter is sufficient to modulate ADO sensitivity of large and small coronary arteries.

Endogenous ADP-ribosylation of the G protein beta subunit prevents the inhibition of type 1 adenylyl cyclase

Mono-ADP-ribosylation is a post-translational modification of cellular proteins that has been implicated in the regulation of signal transduction, muscle cell differentiation, protein trafficking, and secretion. In several cell systems we have observed that the major substrate of endogenous mono-ADP-ribosylation is a 36-kDa protein. This ADP-ribosylated protein was both recognized in Western blotting experiments and selectively immunoprecipitated by a G protein beta subunit-specific polyclonal antibody, indicating that this protein is the G protein beta subunit. The ADP-ribosylation of the beta subunit was due to a plasma membrane-associated enzyme, was sensitive to treatment with hydroxylamine, and was inhibited by meta-iodobenzylguanidine, indicating that the involved enzyme is an arginine-specific mono-ADP-ribosyltransferase. By mutational analysis, the target arginine was located in position 129. The ADP-ribosylated beta subunit was also deribosylated by a cytosolic hydrolase. This ADP-ribosylation/deribosylation cycle might be an in vivo modulator of the interaction of betagamma with specific effectors. Indeed, we found that the ADP-ribosylated betagamma subunit is unable to inhibit calmodulin-stimulated type 1 adenylyl cyclase in cell membranes and that the endogenous ADP-ribosylation of the beta subunit occurs in intact Chinese hamster ovary cells, where the NAD(+) pool was labeled with [(3)H]adenine. These results show that the ADP-ribosylation of the betagamma subunit could represent a novel cellular mechanism in the regulation of G protein-mediated signal transduction.

Age-related changes in A(1)-adenosine receptor-mediated bradycardia

The impact of age on functional sensitivity to A(1)-adenosine receptor activation was studied in Langendorff-perfused hearts from young (1-2 mo) and old (12-18 mo) male Wistar rats. Adenosine mediated bradycardia in young and old hearts, with sensitivity enhanced approximately 10-fold in old [negative logarithm of EC(50) (pEC(50)) = 4.56 +/- 0.11] versus young hearts (pEC(50) = 3.70 +/- 0. 09). Alternatively, the nonmetabolized A(1) agonists N(6)-cyclohexyladenosine and (R)-N(6)-phenylisopropyladenosine were equipotent in young (pEC(50) = 7.43 +/- 0.12 and 6.61 +/- 0.19, respectively) and old hearts (pEC(50) = 7.07 +/- 0.10 and 6.80 +/- 0. 11, respectively), suggesting a role for uptake and/or catabolism in age-related changes in adenosine sensitivity. In support of this suggestion, [(3)H]-adenosine uptake was approximately twofold greater in young than in old hearts (from 3-100 microM adenosine). However, although inhibition of adenosine deaminase and adenosine transport with 10 microM erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride and 10 microM S-(4-nitrobenzyl)-6-thioinosine increased adenosine sensitivity three- to fourfold, it failed to abolish the sensitivity difference in old (pEC(50) = 4.95 +/- 0.08) versus young (pEC(50) = 4.29 +/- 0.13) hearts. Data indicate that 1) age increases functional A(1) receptor sensitivity to adenosine without altering the sensitivity of the A(1) receptor itself, and 2) age impairs adenosine transport and/or catabolism, but this does not explain differing functional sensitivity to adenosine. This increased functional sensitivity to adenosine may have physiological significance in the older heart.

Enhancement of the functional stability of solubilized nucleoside transporters by substrates and inhibitors

Purification of functional nucleoside transporters has been hampered by the instability of detergent-solubilized proteins. The present study was undertaken to determine if the presence of specific transporter ligands in the solubilization medium could enhance the functional stability of the isolated proteins. Ehrlich cell plasma membranes were solubilized with 1% (w/v) octylglucoside (+/- transporter ligands) and reconstituted into liposomal membranes either immediately after solubilization or after storage for 48 hr at 6 degrees. Storage resulted in a parallel loss (approximately 60%) of [3H]nitrobenzylthioinosine (NBMPR) binding and reconstituted [3H]uridine uptake activities. furthermore, upon storage, the relative amount of NBMPR-resistant [3H]uridine uptake by the reconstituted system dropped from 19 +/- 2 to 8 +/- 1% of the total mediated influx. The inclusion of high concentrations (> 10 mM) of adenosine in the solubilization medium completely prevented the storage-induced loss of both [3H]NBMPR binding and [3H]uridine influx activity, and prevented the shift in NBMPR sensitivity. In addition, inclusion of adenosine in the solubilization procedure increased the relative amount of NBMPR-resistant [3H]uridine uptake to 33 +/- 2% of the total influx in proteoliposomes prepared immediately after the proteins were extracted from the plasma membrane (i.e. no storage). A partial protection of [3H]NBMPR binding activity was also obtained using 2'-deoxyadenosine, 2-chloroadenosine, uridine, and non-radiolabelled NBMPR, but not with cytidine, inosine, diazepam, dipyridamole, or dilazep. These results suggest that both NBMPR sensitivity and transporter stability are dependent upon the conformational state of the protein. The protective effects of adenosine analogues and other nucleosides are likely due to their binding to the substrate translocation site, thereby effectively "locking" the transporter in a stable conformation.

Adenosine transporters

1. In mammals, nucleoside transport is an important determinant of the pharmacokinetics, plasma and tissue concentration, disposition and in vivo biological activity of adenosine as well as nucleoside analogues used in antiviral and anticancer therapies. 2. Two broad types of adenosine transporter exist, facilitated-diffusion carriers and active processes driven by the transmembrane sodium gradient. 3. Facilitated-diffusion adenosine carriers may be sensitive (es) or insensitive (ei) to nanomolar concentrations of the transport inhibitor nitrobenzylthioinosine (NBMPR). Dipyridamole, dilazep and lidoflazine analogues are also more potent inhibitors of the es carrier than the ei transporter in cells other than those derived from rat tissues. 4. The es transporter has a broad substrate specificity (apparent Km for adenosine approximately 25 microM in many cells at 25 degrees C), is a glycoprotein with an average apparent Mr of 57,000 in human erythrocytes that has been purified to near homogeneity and may exist in situ as a dimer. However, there is increasing evidence to suggest the presence of isoforms of the es transporter in different cells and species, based on kinetic and molecular properties. 5. The ei transporter also has a broad substrate specificity with a lower affinity for some nucleoside permeants than the es carrier, is genetically distinct from es but little information exists as to the molecular properties of the protein. 6. Sodium-dependent adenosine transport is present in many cell types and catalysed by four distinct systems, N1-N4, distinguished by substrate specificity, sodium coupling and tissue distribution. 7. Two genes have been identified which encode sodium-dependent adenosine transport proteins, SNST1 from the sodium/glucose cotransporter (SGLT1) gene family and the rat intestinal N2 transporter (cNT1) from a novel gene family including a bacterial nucleoside carrier (NupC). Transcripts of cNT1, which encodes a 648-residue protein, are found in intestine and kidney only. 8. Success in cloning the remaining adenosine transporter genes will improve our understanding of the diversity of nucleoside transport processes, with a view to better targeting of therapeutic nucleoside analogues and protective use of transport inhibitors.

Membrane transport of nucleobases: interaction with inhibitors

1. The kinetic properties and the mechanism of nucleobase transport and transport inhibition are briefly reviewed. 2. Many purine derivatives even when bearing large substituents on N9 and C6 are inhibitors of nucleobase transport, some are also substrates. 3. Papaverine and other benzyl-isoquinolines are efficient inhibitors of facilitated transport of nucleobases. 4. Papaverine is a noncompetitive inhibitor of nucleobase transport in human erythrocytes. 5. Reduction of the aromatic isoquinoline to the tetrahydro form causes loss of inhibitory activity whereas replacement of methoxy groups by ethoxy groups leads to increased activity. 6. Papaverine also inhibits sodium dependent active nucleobase transport in pig kidney cells. 7. The nucleoside transport inhibitors dipyridamole and dilazep have no effect on facilitated diffusion transport of nucleobases, but inhibit in micromolar concentrations active sodium dependent nucleobase transport in pig kidney cells.

Deoxynucleoside induces neuronal apoptosis independent of neurotrophic factors

Postmitotic sympathetic neurons are known to undergo a programmed cell death (apoptosis) when they are deprived of nerve growth factor (NGF) or treated with arabinofuranosyl nucleoside antimetabolites. Here we report the existence of a biochemical mechanism for the induction of neuronal death by an endogenous nucleoside in the presence of NGF. In support of such a mechanism we show that 2-deoxyadenosine (dAdo) induces apoptosis in chick embryonic sympathetic neurons supported in culture by NGF, excess K+, phorbol 12,13-dibutyrate, or forskolin. Neuronal death was related to a dramatic increase in the dATP content of sympathetic neurons exposed to dAdo (34.96 +/- 5.98 versus 0.75 +/- 0.16 pmol/micrograms protein in untreated controls, n = 9), implicating dATP in the toxicity. Supportive evidence for a central role of dATP was gained by inhibition of kinases necessary for phosphorylation of dAdo. 5'-Iodotubercidin in nanomolar concentrations completely and dose-dependently inhibited formation of dATP and also protected against toxicity of submillimolar concentrations of dAdo in sympathetic neurons. Although some of these actions of dAdo were remarkably similar to those reported for human lymphoid cells, several were uniquely different. For example, [3H]dAdo was not transported into neurons by the nucleoside transporter, and therefore inhibition of the transporter (dilazep, nitrobenzylthioinosine) did not prevent neurotoxicity by dAdo. Precursors of pyrimidine synthesis (2'-deoxycytidine, uridine) or NAD+ synthesis (nicotinamide) were ineffective in protecting sympathetic neurons against dAdo toxicity. Finally, inhibition of adenosine deaminase by deoxycoformycin or erythro-9-(2-hydroxy-3-nonyl) adenine did not potentiate the toxic effects of dAdo. Our results provide evidence for the first time that neuronal cells are as susceptible to nucleoside lethality as human lymphocytes are, and provide a new model to study the salvage pathway of deoxyribonucleosides in controlling neuronal populations through programmed cell death.

Effects of chemical modification of nitrobenzylthioinosine on its binding to high-affinity membrane binding sites and inhibition of nucleoside transport

Nitrobenzylthioinosine (NBTI) was systematically modified by attachment of substituents at the 2-, 5'-, 3'- and 2'-positions in order to assess the importance of these positions in the binding of NBTI to high-affinity membrane binding sites (Kd < or = 1 nM) and the inhibition of NBTI-sensitive, equilibrative nucleoside transport by mammalian cells. We determined the effect of the derivatives on the equilibrium binding of 1 nM [3H]NBTI to human erythrocytes and mouse P388 leukemia cells and on the inhibition of zero-trans influx of formycin B in P388 cells and equilibrium exchange of uridine in human erythrocytes. Placement of substituent groups at the 5'-position of NBTI had relatively little effect on its binding to high-affinity binding sites or its inhibition of nucleoside transport, regardless of the size of the substituent group (up to about 1000 kDa). All substituents at the 2-position considerably reduced the affinity of NBTI to membrane binding sites and its potency as an inhibitor of nucleoside transport, but some substituent groups reduced the affinity of binding more than the inhibition of nucleoside transport. The effect of the 2-substituents was not directly related to their size. Attachment of a succinate at the 3'- or 5'-position also reduced to a greater extent the binding of NBTI than its inhibition of nucleoside transport, which was relatively little affected. Attachment of succinates at both the 3' and 5'-positions almost completely abolished both binding to high-affinity sites and inhibition of nucleoside transport. Both functions of NBTI were abolished completely by the simultaneous blockage of the 2'- and 3'-positions. None of the NBTI derivatives significantly inhibited NBTI-resistant equilibrative formycin B transport in P388 and Novikoff rat hepatoma cells at concentrations of < or = 1 microM.

High-affinity, equilibrative nucleoside transporter of pig kidney cell line (PK-15)

Nucleoside transport was determined in the cloned porcine kidney cell line PK-15 which exhibits properties of tubular cells. The cells did not express any Na(+)-dependent, concentrative nucleoside transport. They exhibited only nitrobenzylthioinosine-sensitive equilibrative nucleoside transport. Their transport activity, however, was only about 10% of that observed in many other mammalian cell lines. This low transport activity correlated with a relatively low number of high-affinity nitrobenzylthioinosine binding sites (5 x 10(3)/cell). Furthermore, although the equilibrative transporter of PK-15 cells exhibited a similar broad substrate specificity as the equilibrative nucleoside transporters of other mammalian cells, it exhibited a much higher affinity for certain nucleosides, especially cytidine and uridine, than the latter. The Michaelis-Menten constants for zero-trans transport and equilibrium exchange of uridine in ATP-depleted cells were about the same (about 40 microM), indicating equal mobility of the nucleoside-loaded and empty carrier. Concentrative nucleoside transport was detected in one set of PK-15 cultures, but was found to be due to mycoplasma contamination.

Nucleoside transport in cultured LLC-PK1 epithelia

The transport of nucleosides by LLC-PK1 cells, a continuous epithelial cell line derived from pig kidney, was characterised. Uridine influx was saturable (apparent Km approximately 34 microM at 22 degrees C) and inhibited by greater than 95% by nitrobenzylthioinosine (NBMPR), dilazep and a variety of purine and pyrimidine nucleosides. In contrast to other cultured animal cells, the NBMPR-sensitive nucleoside transporter in LLC-PK1 cells exhibited both a high affinity for cytidine (apparent Ki approximately 65 microM for influx) and differential 'mobility' of the carrier (the kinetic parameters of equilibrium exchange of formycin B are greater than those for formycin B influx). An additional minor component of sodium-dependent uridine influx in LLC-PK1 cells became detectable when the NBMPR-sensitive nucleoside transporter was blocked by the presence of 10 microM NBMPR. This active transport system was inhibited by adenosine, inosine and guanosine but thymidine and cytidine were without effect, inhibition properties identical to the N1 sodium-dependent nucleoside carrier in bovine renal outer cortical brush-border membrane vesicles (Williams and Jarvis (1991) Biochem. J. 274, 27-33). Late proximal tubule brush-border membrane vesicles of porcine kidney were shown to have a much reduced Na(+)-dependent uridine uptake activity compared to early proximal tubule porcine brush-border membrane vesicles. These results, together with the recent suggestion of the late proximal tubular origin of LLC-PK1 cells, suggest that in vivo nucleoside transport across the late proximal tubule cell may proceed mainly via a facilitated-diffusion process.

Adenine and hypoxanthine transport in human erythrocytes: distinct substrate effects on carrier mobility

Transport of adenine and hypoxanthine in human erythrocytes proceeds via two mechanisms: (1) a common carrier for both nucleobases and (2) unsaturable permeation 4-5-fold faster for adenine for hypoxanthine. The latter process was resistant to inactivation by diazotized sulfanilic acid. Carrier mediated transport of both substrates was investigated using zero-trans and equilibrium exchange protocols. Adenine displayed a much higher affinity for the carrier (Km approximately 5-8 microM) than hypoxanthine (Km approximately 90-120 microM) but maximum fluxes at 25 degrees C were generally 5-10-fold lower for adenine (Vmax approximately 0.6-1.4 pmol/microliters per s) than for hypoxanthine (Vmax approximately 9-11 pmol/microliters per s). The carrier behaved symmetrically with respect to influx and efflux for both substrates. Adenine, but not hypoxanthine reduced carrier mobility more than 10-fold. The mobility of the unloaded carrier, calculated from the kinetic data of either hypoxanthine or adenine transport, was the same thus providing further evidence that these substrates share a common transporter and that their membrane transport is adequately described by the alternating conformation model of carrier-mediated transport.

Na(+)-dependent, concentrative nucleoside transport in rat macrophages. Specificity for natural nucleosides and nucleoside analogs, including dideoxynucleosides, and comparison of nucleoside transport in rat, mouse and human macrophages

The effects of natural nucleosides and various analogs thereof on Na(+)-dependent, concentrative transport of formycin B by cultured rat macrophages were investigated. Concentrative transport is the sole nucleoside transport system of these cells. The results indicated that uridine, 5'-fluorouridine, all natural purine nucleosides, 2-chloroadenosine and 5'-deoxyadenosine are efficient substrates for the transporter. None of nine other pyrimidine nucleosides was transported. 3'-Deoxy-adenosine, 2',3'-dideoxyadenosine, 8-azidoadenosine, tubercidin, 5'-methylthioadenosine 6-mercaptopurine riboside and adenosine arabinoside were either poor substrates or not transported significantly. The substrate activity of some of the natural nucleosides and the lack of substrate activity of 3'-deoxyadenosine, 2',3'-dideoxyadenosine, 8-azidoadenosine and 2',3'-dideoxycytidine were confirmed by direct uptake measurements. No significant concentrative nucleoside transport was detected in cultured human monocytes/macrophages, whereas mouse macrophages possessed both concentrative and equilibrative nucleoside transporters.