Transport of nucleosides

Deoxyribonucleoside toxicity in adenosine deaminase and purine nucleoside phosphorylase deficiency: implications for the development of new immunosuppressive agents

The immunodeficient state associated with adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiency may result from the selective phosphorylation by thymus-derived lymphocytes of the ADA substrate deoxyadenosine and the PNP substrate deoxyguanosine, leading to the intracellular trapping of toxic deoxyribonucleoside triphosphates. Agents such as deoxycytidine might be able to favourably modify the immunodeficient state by inhibiting deoxyribonucleoside phosphorylation. Deficiencies of other nucleotide catabolic enzymes, if selectively expressed by lymphocytes, might also lead to immunodeficiency via nucleoside trapping in lymphoid tissues. Purine deoxyribonucleoside analogues, either alone or in combination with ADA inhibitors, may have value as lymphospecific antimetabolites.

Adenosine and its receptor agonists regulate nitric oxide production and RAW 264.7 macrophages via both receptor binding and its downstream metabolites-inosine

Adenosine and its receptor agonists enhanced the production of nitric oxide (NO) in lipopolysaccharide (LPS)-treated RAW 264.7 cells. The enhancement of LPS-induced NO production by adenosine, as represented by the amount of its oxidation products, nitrite and nitrate, was inhibited by adenosine uptake inhibitors, such as dipyridamole, S(4-nitrobenzyl)-6-thioinosine (NBTI) and S(4-nitrobenzyl)-6-thioguanosine (NBTG). These indicate that the uptake of adenosine by macrophages is a prerequisite for the enhancement effects observed. A downstream metabolite of adenosine, inosine, also potentiated the LPS-induced NO production in a dose-dependent manner while its enhancement effect was also inhibited by dipyridamole. However, the degree of enhancement by inosine on NO production and nitric oxide synthase (NOS) activity in LPS-treated RAW 264.7 was weaker than the effect of adenosine. Furthermore, adenosine agonists also enhanced the NO production in a dose-dependent manner, but were not specific for A1, A2 nor A3 adenosine receptor. Adenosine uptake inhibitors had no effects on the enhancement activity of the adenosine receptor agonists. Thus, extracellular receptor/s may also play an important role in the observed enhancement responses. The results of this study indicate that the enhancement effects of adenosine on NO production in macrophages could be mediated by the extracellular adenosine receptors as well as the downstream metabolites of adenosine.

In vitro evaluation of nucleoside transport inhibition in vivo

A method is described which permits rapid evaluation of the effects of nucleoside transport inhibitors on [3H]thymidine uptake into mouse whole blood cells. The uptake is measured after 15-20 sec by withdrawing 100 microliters of blood from the orbital sinus into microcapillary pipets that have been prelabeled with [3H]thymidine. Blood cells are rapidly separated from plasma by centrifugation through a layer of dibutyl phthalate using an Eppendorf microcentrifuge. The acid-soluble radio-activity associated with the cell pellet is then determined. Treatment of mice with the nucleoside transport inhibitors dipyridamole or p-nitrobenzylthioinosine 5'-monophosphate reduces uptake of the radioisotope into the cells. The assay allows for estimation of the dose- and time-effect relationships for inhibition of nucleoside transport by these agents. Advantages of the method are its simplicity, its speed, and the fact that the biological sample and drug are not diluted before measurement of the drug effect.

Passive permeability of human red blood cells to calcium

Ca2+ influx was measured into human erythrocytes in which efflux was blocked by either introduction of an intracellular Ca2+ chelator, introduction of the Ca2+ chelator followed by ATP depletion, or depletion of the Ca2+ pump cofactors ATP and Mg2+. The Ca2+ influx under all three conditions was 14-20 mumol . 1 cells-1 . h-1, which is an order of magnitude higher than the influx previously reported for cells depleted of either ATP or Mg2+ separately. The difference between the two values was explained by the finding of substantial Ca2+ efflux from the Ca2+-loaded ATP-depleted cells, whereas this efflux was insignificant from cells loaded with quin 2 and then ATP depleted. Under these latter conditions Ca2+ influx estimates the unidirectional permeability to this cation. Studies using this technique showed that Ca2+ influx was the same in media of isotonic sodium, potassium, lithium, choline, or magnesium chlorides. Moreover the dependence of Ca2+ influx on external Ca2+ concentration was well described by the sum of saturable and nonsaturable (linear) components.

Dipyridamole decreases glomerular filtration in the sodium-depleted dog. Evidence for mediation by intrarenal adenosine

To determine the renal effects of inhibiting the uptake and subsequent metabolism of endogenous adenosine, dipyridamole, a nucleoside transport inhibitor, was infused intrarenally into anesthetized dogs. Dipyridamole (24 micrograms/kg per min) inhibited the cellular extraction of [14C]adenosine (72 +/- 3% vs. 9 +/- 3%) and elevated the excretion of endogenous adenosine (0.60 +/- 0.08 to 1.70 +/- 0.21 nmol/min, P less than 0.05). The action of exogenous adenosine to decrease glomerular filtration rate is known to be enhanced by sodium depletion, and is minimal or absent in sodium-loaded animals. To ascertain whether dietary sodium intake alters the renal effects of elevated endogenous adenosine, dipyridamole was infused into sodium-depleted and sodium-loaded dogs. In the sodium-depleted dogs (n = 9), dipyridamole infusion decreased the glomerular filtration rate by 59 +/- 7% (20 +/- 1 to 8 +/- 2 ml/min, P less than 0.05) which returned to control levels within 30 minutes after stopping infusion of dipyridamole. Renal vascular resistance was unchanged during dipyridamole infusion. In the sodium-loaded dogs (n = 5), dipyridamole had no effect on glomerular filtration rate (22 +/- 4 vs. 25 +/- 3 ml/min) or renal vascular resistance. In a separate series of sodium-depleted dogs (n = 8), the dipyridamole-induced decrease in glomerular filtration rate was completely reversed or inhibited by theophylline, an adenosine receptor antagonist. These experiments demonstrate that inhibition of cellular uptake of adenosine elevates adenosine levels, that dipyridamole decreases glomerular filtration rate in sodium-depleted but not sodium-loaded dogs, and that the decrease in glomerular filtration rate is inhibited by theophylline. We conclude that the decrease in glomerular filtration rate during dipyridamole administration is mediated by increased endogenous adenosine.

Adenosine release from stimulated mast cells

Adenosine release has been documented in lung tissue exposed to hypoxic conditions or antigen challenge. Exogenous adenosine potentiates mediator release from stimulated rat serosal and mouse bone marrow-derived mast cells. To investigate the production and release of adenosine from stimulated mast cells, rat serosal mast cells were purified on metrizamide gradients, sensitized with anti-dinitrophenol IgE for 30 min at 37 degrees C, and challenged in the presence of 1 microM deoxycoformycin with either dinitrophenol-conjugated bovine serum albumin antigen, the calcium ionophore A23187, or compound 48/80. Reactions were terminated by centrifugation, and the supernatants and pellets were assayed for adenosine and ATP content, respectively, by high performance liquid chromatography. The adenosine concentration of the supernatants increased from 0.036 +/- 0.003 nmol per 10(6) cells to 0.049, 0.056, and 0.129 nmol per 10(6) cells 60 sec after challenge with antigen, 48/80, or A23187, respectively. After ionophore stimulation, increased extracellular adenosine was evident by 15 sec, peaked by 60 sec, and remained constant for at least 5 min. A significant decline in stimulated ATP levels was observed within 30 sec, suggesting that the enhanced adenosine concentrations may result from the breakdown of ATP. Cultured mouse bone marrow-derived mast cells under similar conditions also displayed augmented extracellular adenosine levels with A23187 challenge. This endogenous source of adenosine may act locally through a positive feedback mechanism to potentiate immediate hypersensitivity reactions.

Effects of adenosine transport inhibitors on adenosine- and nerve-mediated relaxations of guinea pig taenia caeci

Dipyridamole and 6-thiobenzyl derivatives of guanosine and inosine were studied in isolated guinea pig taenia caeci with respect to their direct relaxing effects and their influence on relaxations caused by adenosine and nonadrenergic inhibitory nerve stimulation. Direct inhibitory effects were noted with all drugs at 10 microM and these were usually reversible. All inhibitors potentiated the relaxant effects of a submaximal concentration of adenosine in a concentration dependent manner and the potentiating effects persisted for at least 2-3 h after washout of inhibitors. Among the nucleoside-type inhibitors 6-[(4-nitrobenzyl-thio]-purine riboside (NBMPR) was slightly more potent and longer lasting than the others and showed less direct depressant actions. Adenosine concentration-response curves were shifted to the left by all inhibitors but relaxations caused by 2-chloroadenosine remained unaffected. Frequency response curves (0.2-5 s-1), obtained under conditions of selective stimulation of nonadrenergic inhibitory nerves, were not affected by any of the nucleoside-type inhibitors while dipyridamole caused small but significant enhancement of relaxations at low frequencies only. All inhibitors shifted concentration-response curves of adenosine, but not of 2-chloroadenosine, to the left.

On the functional symmetry of nucleoside transport in mammalian cells

The transport of uridine and thymidine has been examined in HeLa cells, in Novikoff rat hepatoma cells and in human erythrocytes, with the purpose of comparing influx, efflux and isotopic exchange at chemical equilibrium. The results support the following conclusions: (i) In all three cell types influx and efflux are comparable; (ii) HeLa and Novikoff cells show no trans-effect, while erythrocytes show a 5-fold trans-stimulation; (iii) a single kinetic entity accounts for nucleoside transport in HeLa and Novikoff cells - no parallel routes of permeation with Km less than 40 microM were detected. For the cultured cells, the flux data conform to the kinetic model of a single, carrier-mediated transport system symmetrical with respect to direction, and with equal mobilities of substrate-loaded and empty carrier.

Effect of arylazido aminopropionyl ATP (ANAPP3), on ATP responses of isolated guinea pig smooth muscle

Arylazido aminopropionyl ATP (ANAPP3), a photoaffinity analogue of adenosine 5'-triphosphate, photoactivated with visible light (+hv), specifically and irreversibly antagonized ATP contractions of the guinea pig vas deferens. ANAPP3 (30 microM) antagonized responses to exogenously added ATP in untreated, and in tissues pretreated with indomethacin (2.9 microM) and 6-(2-hydroxy-5-nitrobenzyl)-thio guanosine (10 microM). It was of interest to see if this pharmacological antagonist of ATP could be used to assess the validity of the purinergic nerve hypothesis by allowing a differentiation between, or proof of the identity of, responses to ATP and the non-adrenergic inhibitory transmitter in guinea pig stomach fundus. After photoactivation (+hv) in the organ bath and subsequent washout, ANAPP3 (30 and 100 microM) failed to antagonize relaxant responses to ATP (1.0 - 1000 microM) in fundic strips. In addition ANAPP3 failed to antagonize ATP-induced inhibition of the twitch response in electrically stimulated guinea pig ileum longitudinal muscle strips. We conclude that ANAPP3 does not antagonize all actions of ATP, which may limit its usefulness in assessing the above hypothesis. Results with this compound suggest that ATP excitatory receptors may differ from those mediating relaxation and other ATP actions.

Some biochemical properties of the rapid adenosine uptake system in rat brain synaptosomes

The rapid uptake of adenosine into rat brain cortical synaptosomes is mediated by a facilitated diffusion process. The carrier mediated uptake is sensitive to pH and temperature. The average Q10 value for the system is approximately 1.77 and the necessary activation energy (Ea) is estimated to be 8870 cal/mol. These values are essentially in agreement with values reported for adenosine uptake carriers of other tissues. Substrate specificity of the uptake system in the CNS demonstrates that nucleotides do not interact with the carrier until they have been hydrolyzed to nucleosides. Structural modification of the purine moiety at the "2" position did not have a profound effect on the ability of the molecule to serve as a substrate for the uptake system. Competitive inhibition by sulfhydryl reagents, p-chloromercuribenzoate, and N-ethylmaleimide on adenosine uptake suggests a direct involvement of sulfhydryl group(s) in the uptake mechanism. Other purines and pyrimidines also inhibit adenosine uptake, suggesting that a variety of nucleosides can interact with a common carrier system.

Toward a "bio-energy supplement" -- a prototype for functional orthomolecular supplementation

A broad-spectrum approach to the nutritional optimization of bioenergetics is discussed as a specific example of the principle of functional orthomolecular supplementation. Experimental and clinical studies with "metavitamins" such as lipoic acid, carnitine, coenzyme Q, and creatine, and with mitochondrial antioxidants, indicate that many nutritional agents involved in bioenergetics are often functionally sub-saturated. Numerous therapeutic applications for a well-designed "bio-energy supplement" can be postulated.

The rapid uptake and release of [3H]adenosine by rat cerebral cortical synaptosomes

Adenosine, a putative inhibitory transmitter or modulator in the brain, is rapidly transported by rat cerebral cortical synaptosomes. The uptake may represent a facilitated diffusion process, which is saturable and temperature-dependent. In this study, the uptake process was very rapid, reaching completion within 60 s of incubation at 37 degrees C, and had an apparent Km value of 0.9 microM and a Vmax value of 5.26 pmol/mg protein/30 s. Over 70% of the adenosine taken up remained unchanged, whereas 14% was metabolized to inosine. Twelve percent of the adenosine was converted to nucleotides. Rapid uptake of adenosine into rat cerebral cortical synaptosomes was partially inhibited by replacing Na+ with choline chloride in the medium. Ca2+ ion is important for the uptake process, as inhibition of adenosine uptake occurs in the presence of either Co2+ or EGTA. Rapid uptake of adenosine is apparently mediated by a nucleoside carrier, a conclusion based on its inhibition by a variety of purine and pyrimidine nucleosides. Uptake was inhibited by dipyridamole, hexobendine, papaverine, flurazepam, and morphine. Over 60% of the adenosine taken up by the rapid uptake system (30 s) was released by depolarizing agents. In contrast, only 30% of the adenosine taken up during a 15-min incubation period was released under the same conditions. [3H]Adenosine was the predominant purine released in the presence or absence of depolarizing agents. The basal and KCl-evoked release mechanisms were found to be at least partially Ca2+-dependent, however, the release of adenosine by veratridine was increased in the presence of EGTA. This finding is in agreement with the reported Ca2+-independent release of ATP from brain synaptosomes. The present findings suggest that there are at least two functional pools of adenosine in synaptosomes. Adenosine taken up by different uptake systems may be destined for different uses (metabolism or release) in the neuron.

Apamin, a nonspecific antagonist of smooth muscle relaxants

Apamin, a peptide of bee venom, was shown to inhibit the relaxant responses of guinea-pig taenia caeci to ATP, noradrenaline, adenosine and, less effectively, to stimulation of noradrenergic inhibitory nerves. Thus apamin acts nonspecifically and, contrary to the suggestion of Vladimirova and Shuba (1978), the fact that inhibitory responses due to nerve stimulation and ATP are blocked by the toxin does not allow conclusions as to the possible transmitter role of ATP in these nerves.

Relaxation of guinea-pig fundic strip by adenosine, adenosine triphosphate and electrical stimulation: lack of antagonism by theophylline or ATP treatment

1 Theophylline relaxed isolated strips of guinea-pig stomach fundus in a dose-dependent manner; above 50 to 100 microM responses showed no fade for up to 90 min. 2 Relaxant responses to adenosine, adenosine triphosphate (ATP), noradrenaline, and to electric field stimulation of non-adrenergic inhibitory nerves were not affected in a significant manner in the presence of 50 microM theophylline. 3 In tissues which showed complete fade of initial responses in the continued presence of 50 microM ATP, the effects of stimulation of non-adrenergic inhibitory nerves remained unaltered, suggesting that the ATP receptor has no function in non-adrenergic inhibitory transmission in this tissue. 4 These findings are opposite to those of Okwuasaba, Hamilton & Cook (1977), who claimed that 50 microM theophylline almost fully inhibited relaxation induced by adenosine ATP and nerve stimulation and that ATP-induced fade also abolished sensitivity to inhibitory nerve stimulation.

Alteration of human breast tumor cell membrane functions by chromosome-mediated gene transfer

BOT-2 cells (human breast tumor origin) have an impaired ability to utilize exogenous thymidine. Previous studies revealed this deficiency to be the permeation event rather than phosphorylation, since the cells have active thymidine kinase. Chromosome-mediated gene transfer was used to transfer genetic information in the form of metaphase chromosomes, from HeLa-65 cells to the BOT-2 cells, correcting the permease deficiency. Poly-L-ornithine or lipochromes were used for facilitation of chromosome uptake. After selection on HAT medium, transferant clones were isolated at a frequency of 4 x 10(-5) and 1 x 10(-5), respectively. Transferants MGP-1 and MGL-1 are stable after 18 months and have been characterized on the bases of purine and pyrimidine nucleoside uptake, relative thymidine kinase activities, alkaline phosphatase activities, and hydrocortisone-induced alkaline phosphatase activity. MGP-1 demonstrates positive thymidine uptake and incorporates radiolabeled thymidine into DNA. MGL-1 remains thymidine transport-deficient and surveys on HAT by increasing endogenous dihydrofolate reductase activity. Alkaline phosphatase activity in MGL-1 is similar to HeLa-65, 2% of that in BOT-2, and in addition, is inducible 25-30-fold by 3 micro M hydrocortisone. We have separated, genetically, a thymidine permease function from phosphorylation in cells of human origin and have transferred genetic information for the regulation of alkaline phosphatase.

Specificity of nucleoside transport in Neurospora crassa

The specificity of nucleoside uptake in germinating conidia of Neurospora crassa was investigated by examining the kinetics of [2-14C]uridine and [8-14C]-adenosine uptake in the wild-type, ad-8, and ud-1 pyr-1 strains. The results obtained strongly indicate that nucleoside transport in N. crassa is mediated solely by a general transport system which accepts both purine and pyrimidine nucleosides. Studies directed at characterizing the specificity of the transport system indicate that general structural features of the nucleoside which enhance its efficiency in binding to the transport system include: (i) a purine or pyrimidine as the heterocyclic ring, (ii) an unfunctionalized ribose or 2'-deoxyribose as the sugar unit, (iii) a beta-configuration about the anomeric carbon, (iv) the absence of substituents at C8 in the purine series and at C5 and C6 in the pyrimidine series, (v) the presence of a C5-C6 double bond in the pyrimidine series, and (vi) the absence of a charge on the heterocyclic ring.

The effect of chondrocyte growth factor on membrane transport by articular chondrocytes in monolayer culture

Chondrocyte growth factor (CGF), a contaminant of pituitary glycoprotein hormones, stimulates growth of cultured lapine articular chondrocytes while depressing SO4-proteoglycan synthesis. To study its effect on membrane transport, NIH-bTSH and two other preparations with comparable CGF activity were employed. In early log phase (36 hr) cultures CGF (64 microgram/ml) did not alter thymidine (dThd) uptake during the first 5 min. By 15 min however, TCA-precipitable dThd was 4-fold greater than in controls while the TCA-soluble fraction remained the same. CGF increased deoxy-glucose (DG) uptake in 36-hr old cultures. At 66 hr, CGF reduced DG transport. The transport of cycloleucine (CL) and aminoisobutyric acid (AIB) was reduced by CGF in 36 and 66-hr old cultures. There was a dose dependency between CGF concentration, the lowered uptake of DG, CL and AIB, and cell protein content. The effect of CGF on DG transport and dThd incorporation into DNA was not immediate but required prior exposure of the cells to CGF. CGF did not alter DG transport in rabbit or mouse fibrocytes or Chang liver cells. This and the reported finding that pituitary fibroblast growth factor (FGF), increases amino acid transport in other cells suggests that the biological specificity of CGF may not be identical to that of FGF.

Defective transport of thymidine by cultured cells resistant to 5-bromodeoxyuridine

A line of HeLa cells resistant to 5-bromo-2'-deoxyuridine (BUdR) was established by continuous culture in growth medium containing BUdR; during the selection period, BUdR concentrations, initially 15 micrometer, were gradually increased to 100 micrometer. Cells of a clone (HeLa/B5) established from this line were also resistant to 5-fluoro-2'-deoxyuridine (FUdR), but not to the free base, 5-fluorouracil. Although extracts of HeLa/B5 cells exhibited levels of thymidine kinase activity comparable to those of parental cells, rates of uptake of BUdR, FUdR, and thymidine into intact cells were much reduced. The kinetics of uptake of uridine and adenosine, nucleosides which appear to be transported independently of thymidine in HeLa cells, were similar for HeLa/B5 and the parental line (HeLa/O). Relative to thymidine uptake by HeLa/O cells, that by HeLa/B5 cells was distinctly less sensitive to nitrobenzylthioinosine (NBMPR), a specific inhibitor of nucleoside transport in various types of animal cells. Despite this difference in NBMPR sensitivity, both cell lines possessed the same number of high affinity NBMPR binding sites per mg cell protein. The altered kinetics of thymidine uptake and the NBMPR insensitivity of that function in HeLA/B5 cells suggest that resistance to BUdR is due to an altered thymidine transport mechanism.