S49 mouse lymphoma cells are deficient in hypoxanthine transport

Hypoxanthine uptake at the fetal side of human placenta proceeds through a nucleobase-preferring carrier and a non-saturable process

Uptake and metabolism of hypoxanthine by human placenta were studied using the single-circulation paired-tracer technique. In isolated cotyledons perfused through the fetal (basal) circulation, at mean pressures of 31.7 +/- 4.0 mmHg and mean flow rates maintained at 5.5 +/- 0.15 ml/min, the [3H]hypoxanthine uptake was 36 +/- 2.4 per cent (16.5 +/- 1.1 pmol/g wet weight). Hypoxanthine uptake was significantly inhibited by unlabelled (mM) hypoxanthine (0.5), adenine (0.5), guanine (0.5) and papaverine (15.0), but was unaffected by nitrobenzylthioinosine (0.01). Adenosine failed to inhibit hypoxanthine uptake. The kinetic analysis of hypoxanthine uptake showed it to be partially mediated by a saturable (apparent K(m) = 12.1 +/- 1.85 microns; Jmax = 7.1 +/- 0.52 nmol/min) and Na(+)-dependent mechanism. A greater fraction of hypoxanthine influx proceeded through a non-saturable process. Thin layer chromatographic analysis of venous perfusate after the intra-arterial injection of [3H]hypoxanthine showed a negligible degradation of nucleobase. These overall results show that hypoxanthine uptake at the fetal side of human placenta occurs by a saturable plus a non-saturable process. The carrier showed specificity for nucleobases and high affinity-low capacity for hypoxanthine. Since the fetal blood concentration of hypoxanthine is normally low, its uptake would be mediated by the high affinity transport system. Because the non-saturable mechanism can be operative at high concentrations of hypoxanthine, it may have primary importance to clear the nucleobase coming from the fetus during intrauterine hypoxia.

Na(+)-dependent, active nucleoside transport in S49 mouse lymphoma cells and loss in AE-1 mutant deficient in facilitated nucleoside transport

S49 murine lymphoma cells were examined for expression of various nucleoside transport systems using a non-metabolized nucleoside, formycin B, as substrate. Nitrobenzylthioinosine (NBTI)-sensitive, facilitated transport was the primary nucleoside transport system of the cells. The cells also expressed very low levels of NBTI-resistant, facilitated nucleoside transport as well as of Na(+)-dependent, concentrative formycin B transport. Concentrative transport was specific for uridine and purine nucleosides, just as the concentrative nucleoside transporters of other mouse and rat cells. A nucleoside transport mutant of S49 cells, AE-1, lacked both the NBTI-sensitive, facilitated and Na(+)-dependent, concentrative formycin B transport activity, but Na(+)-dependent, concentrative transport of alpha-aminoisobutyrate was not affected.

Defective drug uptake contributing to multidrug resistance in hepatoma cells can be evaluated in vitro

In clinical practice the acquired or de novo resistance of tumors to antitumor chemotherapy remains a big problem. However, in the past few years some progress has been made in understanding the two principal mechanisms: metabolic alterations leading to a reduced cytostatic or cytotoxic effect of drugs, and reduced accumulation of drugs within the tumor cells [15, 34, 35]. The second phenomenon is usually attributed to the ability of tumor cells to accelerate the efflux of various xenobiotics. This phenomenon is considered primarily responsible for the development of multidrug resistance (MDR). However, loss or impairment of drug uptake by the tumor cells may also contribute to resistance to antitumor drugs. This paper focuses on recent findings with hepatoma cells, which support this view.

Effects of Ca2+-channel antagonists on nucleoside and nucleobase transport in human erythrocytes and cultured mammalian cells

Lidoflazine strongly inhibited the equilibrium exchange of uridine in human erythrocytes (Ki approximately 16 nM). Uridine zero-trans influx was similarly inhibited by lidoflazine in cultured HeLa cells (IC50 approximately to 80 nM), whereas P388 mouse leukemia and Novikoff rat hepatoma cells were three orders of magnitude more resistant (IC50 greater than 50 microM). Uridine transport was also inhibited by nifedipine, verapamil, diltiazem, prenylamine and trifluoperazine, but only at similarly high concentrations in both human erythrocytes and the cell lines. IC50 values ranged from about 10 microM for nifedipine and about 20 microM for verapamil to more than 100 microM for diltiazem, prenylamine and trifluoperazine. The concentrations required for inhibition of nucleoside transport are several orders higher than those blocking Ca2+ channels. Lidoflazine competitively inhibited the binding of nitrobenzylthioinosine to high-affinity sites in human erythrocytes, but did not inhibit the dissociation of nitrobenzylthioinosine from these sites on the transporter as is observed with dipyridamole and dilazep.

Residual nitrobenzylthioinosine-resistant nucleoside transport in a transport mutant (AE1) of S49 murine T-lymphoma cells

The uptake of various nucleosides by S49 mouse T-lymphoma cells and that by a single-step nucleoside transport-defective mutant thereof (AE1) were compared. Residual nucleoside entry into AE1 cells occurred via two routes, nonmediated permeation and saturable, non-concentrative transport with broad substrate specificity and a Michaelis-Menten constant approximating that for thymidine transport in wild-type cells. However, in contrast to nucleoside transport in wild-type cells, residual nucleoside transport in AE1 cells was resistant to inhibition by nitrobenzylthioinosine. In its properties the latter resembled nitrobenzylthioinosine-resistant nucleoside transport observed in other types of mammalian cells. It amounted to less than 1% of the total nucleoside transport activity of wild-type S49 cells. The results indicate that nitrobenzylthioinosine-resistant and -sensitive nucleoside transports are genetically distinguishable. In wild-type cells, the salvage of thymidine, when present at concentrations higher than 1 to 10 microM, was limited by phosphorylation, because of the saturation of thymidine kinase. In AE1 cells, entry into the cells mainly limited thymidine salvage, but at high thymidine concentrations the combined entry via residual transport and nonmediated permeation was sufficiently rapid to support intracellular thymidine phosphorylation at rates comparable to those observed in wild-type cells.

Nucleoside and nucleobase transport and metabolism in wild type and nucleoside transport-deficient Aedes albopictus cells

Nucleoside and nucleobase transport and metabolism were measured in ATP-depleted and normal Aedes albopictus mosquito cells (line C-7-10) by rapid kinetic techniques. The cells possess a facilitated diffusion system for nucleosides, which in its broad substrate specificity and kinetic properties resembles that present in many types of mammalian cells. The Michaelis-Menten constant for uridine transport at 28 degrees C is about 180 microM. However, the nucleoside transporter of the mosquito cells is resistant to inhibition by nmolar concentrations of nitrobenzylthioinosine and the cells lack high affinity nitrobenzylthioinosine binding sites. The cells also possess an adenine transporter, which is distinct from the nucleoside transporter. They lack, however, a hypoxanthine transport system and are deficient in hypoxanthine phosphoribosyltransferase activity, which explains their failure to efficiently salvage hypoxanthine from the medium. The cells possess uridine and thymidine phosphorylase activities and, in contrast to cultured mammalian cells, efficiently convert uracil to nucleotides. An adenosine-resistant variant (CAE-3-6) of the C-7-10 cell line is devoid of significant nucleoside transport activity but transports adenine normally. Residual entry of various nucleosides into these cells and of hypoxanthine and cytosine into wild type and mutant cells is strictly non-mediated. The rate of permeation of various nucleosides and of hypoxanthine into the CAE-3-6 cells is related to their hydrophobicity. Uridine permeation into CAE-3-6 cells exhibits an activation energy of about 20 kcal/mol. At high uridine concentrations permeation is sufficiently rapid to partly overcome the limitation in nucleoside salvage imposed by the nucleoside transport defect in these cells.

Residual nitrobenzylthioinosine-resistant nucleoside transport in a transport mutant (AE1) of S49 murine T-lymphoma cells

The uptake of various nucleosides by S49 mouse T-lymphoma cells and that by a single-step nucleoside transport-defective mutant thereof (AE1) were compared. Residual nucleoside entry into AE1 cells occurred via two routes, nonmediated permeation and saturable, non-concentrative transport with broad substrate specificity and a Michaelis-Menten constant approximating that for thymidine transport in wild-type cells. However, in contrast to nucleoside transport in wild-type cells, residual nucleoside transport in AE1 cells was resistant to inhibition by nitrobenzylthioinosine. In its properties the latter resembled nitrobenzylthioinosine-resistant nucleoside transport observed in other types of mammalian cells. It amounted to less than 1% of the total nucleoside transport activity of wild-type S49 cells. The results indicate that nitrobenzylthioinosine-resistant and -sensitive nucleoside transports are genetically distinguishable. In wild-type cells, the salvage of thymidine, when present at concentrations higher than 1 to 10 microM, was limited by phosphorylation, because of the saturation of thymidine kinase. In AE1 cells, entry into the cells mainly limited thymidine salvage, but at high thymidine concentrations the combined entry via residual transport and nonmediated permeation was sufficiently rapid to support intracellular thymidine phosphorylation at rates comparable to those observed in wild-type cells.