Measurement of transport versus metabolism in cultured cells

Purine uptake in Plasmodium: transport versus metabolism

In a recent paper, Quashie et al. have proposed that purine uptake into the intraerythrocytic malaria parasite involves four different plasma membrane transporters - two high affinity and two low affinity. They equate one of the two high-affinity transporters with PfNT1, a transporter reported previously to be a low-affinity system. Here, we offer an alternative interpretation of their data, suggesting that the conclusions drawn by Quashie et al. take insufficient account of metabolism.

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

9-(2-Phosphonylmethoxyethyl)-N6-cyclopropyl-2,6-diaminopurine (cpr-PMEDAP) as a prodrug of 9-(2-phosphonylmethoxyethyl)guanine (PMEG)

9-(2-Phosphonylmethoxyethyl)-N6-cyclopropyl-2,6-diaminopurine (cpr-PMEDAP) is an acyclic nucleotide analog of the [9-(2-phosphonylmethoxyethyl)-] (PME) series containing a cyclopropyl substituent on the N6 position of the 2,6-diaminopurine (DAP) base. Growth inhibition assays in a broad range of tumor cell lines demonstrated that this analog had potent antiproliferative activity with IC50 values similar to those of the structurally related guanine analog 9-(2-phosphonylmethoxyethyl)guanine (PMEG). A substantially lower growth inhibitory effect was observed for the 2,6-diaminopurine analog, PMEDAP. To dissect the basis for these varying potencies, the metabolism of the three analogs was examined in a human pancreatic carcinoma cell line, BxPC-3. HPLC analysis of the intracellular metabolites demonstrated that the cpr-PMEDAP was deaminated to PMEG and subsequently phosphorylated to PMEG mono- and diphosphates (PMEGp and PMEGpp). The level of PMEGpp generated from cpr-PMEDAP-treated cells was 50% greater than the level generated from cells incubated with PMEG. The presence of PMEG in the DNA of cells incubated with cpr-PMEDAP confirmed that the cpr-PMEDAP was converted to PMEG. In contrast, PMEDAP was not deaminated to PMEG, but directly phosphorylated to PMEDAPp and PMEDAPpp. The adenylate deaminase inhibitor 2'-deoxycoformycin (dCF) inhibited the conversion of cpr-PMEDAP in a rat liver cytosolic extract and increased the IC50 value for growth inhibition by 40-fold. The antiproliferative activities of PMEG and PMEDAP were unaffected by dCF. Thus, it appears that cpr-PMEDAP, but not PMEDAP, is converted by an adenylate deaminase-like enzyme and functions as a prodrug of PMEG.

Toxoplasma gondii tachyzoites possess an unusual plasma membrane adenosine transporter

Nucleoside transport may play a critical role in successful intracellular parasitism by Toxoplasma gondii. This protozoan is incapable of de novo purine synthesis, and must salvage purines from the host cell. We characterized purine transport by extracellular T. gondii tachyzoites, focusing on adenosine, the preferred salvage substrate. Although wild-type RH tachyzoites concentrated [3H]adenosine 1.8-fold within 30 s, approx. half of the [3H]adenosine was converted to nucleotide, consistent with the known high parasite adenosine kinase activity. Studies using an adenosine kinase deficient mutant confirmed that adenosine transport was non-concentrative. [14C]Inosine, [14C]hypoxanthine and [3H]adenine transport was also rapid and non-concentrative. Adenosine transport was inhibited by dipyridamole (IC50 approx. 0.7 microM), but not nitrobenzylthioinosine (15 microM). Transport of inosine, hypoxanthine and adenine was minimally inhibited by 10 microM dipyridamole, however. Competition experiments using unlabeled nucleosides and bases demonstrated distinct inhibitor profiles for [3H]adenosine and [14C]inosine transport. These results are most consistent with a single, dipyridamole-sensitive, adenosine transporter located in the T. gondii plasma membrane. Additional permeation pathways for inosine, hypoxanthine, adenine and other purines may also be present.

Transport of 9-(2-phosphonomethoxyethyl)adenine across plasma membrane of HeLa S3 cells is protein mediated

9-(2-Phosphonomethoxyethyl)adenine (PMEA) is an acyclic adenine nucleotide analog which exhibits potent and selective antiviral activity against herpesviruses and retroviruses. The study of [14C]PMEA uptake in HeLa S3 cells has shown that intracellular levels of the drug plateau after 1 h. Transport across the plasma membrane is saturable (concentration at half-maximal saturation [Kt], 0.39 microM; maximum rate of uptake [Vmax], 1.72 pmol/min.10(6) cells), and it can operate against the concentration gradient. Its significant dependence on temperature and on cellular density has been demonstrated. Following the treatment of cells with proteases, PMEA uptake strongly decreases. The transport process is considerably specific, since only a few phosphonate analogs act effectively as competitive inhibitors. Of these, 9-(2-phosphonomethoxyethyl)-2,6-diaminopurine (Ki = 0.24 microM) is the most efficient. Also, natural nucleotides competitively inhibit PMEA transport, depending on the nature of the nucleobase (thymine = adenine > guanine > cytosine < uracil) and on the position and number of phosphate groups. Nucleosides and nucleobases do not interfere with PMEA uptake. Cellular transport of adenosine and thymidine or uptake of AMP and ATP via conjugated activity of ectonucleotidases and nucleoside transporters is not affected by PMEA. By using vectorial labeling of plasma membrane proteins with Na125I combined with affinity chromatography, a 50-kDa protein which may mediate cellular transport of PMEA has been identified.

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.

Transport and metabolism of D-[3H]adenosine and L-[3H]adenosine in rat cerebral cortical synaptoneurosomes

The relationship between transport and metabolism in synaptoneurosomes was examined to determine the metabolic stability of rapidly accumulated D-[3H]adenosine and L-[3H]adenosine and the degree to which metabolism of the accumulated purines affected measurements of apparent KT and Vmax values for adenosine transport. For D-[3H]adenosine, high- and low-affinity accumulation processes were present. For the high-affinity system an inverse relationship was found between transport reaction times and KT and Vmax values. For incubations of 5, 15, and 600 s, which corresponded to 24, 32, and 76% phosphorylation of accumulated D-[3H]adenosine to nucleotides, apparent KT values were 9.4, 8.4, and 4.5 microM, respectively, and Vmax values were 850, 70, and 12 pmol/min/mg of protein, respectively. Pretreatment with 10 microM erythro-9-(2-hydroxy-3-nonyl)adenine, an adenosine deaminase inhibitor, and 5'-iodotubercidin, an adenosine kinase inhibitor, decreased the phosphorylation of accumulated D-[3H]adenosine to 6% with 5-s and 9% with 15-s incubations. This resulted in significantly higher KT values: 36 microM at 5 s and 44 microM at 15 s. At 10-min incubations in the presence of these inhibitors, metabolism of accumulated D-[3H]adenosine was 32%, and apparent KT and Vmax values at this time were not significantly different from those obtained without inhibitors. For L-[3H]adenosine, apparent KT and Vmax values for 20-s incubations were 38.7 microM and 330 pmol/min/mg of protein, respectively. Metabolism (mainly phosphorylation) of accumulated L-[3H]adenosine was observed only at incubations of greater than 30 s.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Mycoplasma contamination greatly enhances the apparent transport and concentrative accumulation of formycin B by mammalian cell culture

S49 mouse leukemia cells exhibit both equilibrative and Na(+)-dependent, concentrative formycin B transport. The latter represents only a minor nucleoside transport component and is detectable only when equilibrative nucleoside transport is inhibited by dipyridamole or another transport inhibitor. Thus in uncontaminated S49 cells formycin B accumulated only to slightly above the intracellular-extracellular equilibrium level. In contrast, in suspensions of S49 cells contaminated with mycoplasma, formycin B accumulated in the intracellular water space in unmodified form to 40-50-times the extracellular concentration in a dipyridamole-independent manner during 90 min of incubation at 37 degrees C. The mycoplasma active formycin B transport system was inhibited by all nucleosides tested, including thymidine and deoxycytidine, which are not substrates for the concentrative nucleoside transporter of S49 cells. Mycoplasma contamination was detected by the presence of cell-associated adenosine phosphorylase activity.

L-[3H]adenosine, a new metabolically stable enantiomeric probe for adenosine transport systems in rat brain synaptoneurosomes

The stereoenantimers D-[3H]adenosine and L-[3H]adenosine were used to study adenosine accumulation in rat cerebral cortical synaptoneurosomes. L-Adenosine very weakly inhibited rat brain adenosine deaminase (ADA) activity with a Ki value of 385 microM. It did not inhibit rat brain adenosine kinase (AK) activity, nor was it utilized as a substrate for either ADA or AK. The rate constants (fmol/mg of protein/s) for L-[3H]adenosine accumulation measured in assays where transport was stopped either with inhibitor-stop centrifugation or with rapid filtration methods were 82 +/- 14 and 75 +/- 10, respectively. Using the filtration method, the rates of L-[3H]adenosine accumulation were not significantly different from the value of 105 +/- 15 fmol/mg of protein/s measured for D-[3H]adenosine transport. Unlabeled D-adenosine and nitrobenzylthiolnosine, both at a concentration of 100 microM, reduced the levels and rates of L-[3H]adenosine accumulation by greater than 44%. These findings suggest that L-adenosine, a metabolically stable enantiomeric analog, and the naturally occurring D-adenosine are both taken up by rat brain synaptoneurosomes by similar processes, and as such L-adenosine may represent an important new probe with which adenosine uptake may be studied.

Characterization of Na(+)-dependent, active nucleoside transport in rat and mouse peritoneal macrophages, a mouse macrophage cell line and normal rat kidney cells

Peritoneal rat macrophages expressed solely an Na(+)-dependent, concentrative nucleoside transporter, which possesses a single Na(+)-binding site and transports purine nucleosides and uridine but not thymidine or deoxycytidine. The Michaelis-Menten constants for formycin B and Na+ were about 6 microns and 14 mM, respectively, and the estimated Na+:formycin B stoichiometry was 1:1. Rat macrophages accumulated 5 microM formycin B to a steady-state level exceeding that in the medium by about 500-fold during 60 min of incubation at 37 degrees C. Concentrative formycin B transport was resistant to inhibition by nitrobenzylthioinosine, lidoflazine, dilazep and nifedipine, but was slightly inhibited by high concentrations of dipyridamole (greater than 10 microM) and probenecid (greater than 100 microM). Mouse peritoneal macrophages and lines of mouse macrophages and normal rat kidney cells expressed Na(+)-dependent, active nucleoside transport but in addition significant Na(+)-independent, facilitated nucleoside transport. Facilitated nucleoside transport in these cells was sensitive to inhibition by nitrobenzylthioinosine, dilazep and dipyridamole. The presence of these inhibitors greatly enhanced the concentrative accumulation of formycin B by these cells by inhibiting the efflux via the facilitated transporter of the formycin B actively transported into the cells. Whereas rat macrophages lacked high-affinity nitrobenzylthioinosine-binding sites, mouse macrophages and normal rat kidney cells possessed about 10,000 such sites/cell. Rat and mouse erythrocytes, rat lymphocytes, and lines of Novikoff rat hepatoma cells, Chinese hamster ovary cells, Mus dunni cells and embryonic monkey kidney cells expressed only facilitated nucleoside transport.

Na(+)-dependent, active nucleoside transport in mouse spleen lymphocytes, leukemia cells, fibroblasts and macrophages, but not in equivalent human or pig cells; dipyridamole enhances nucleoside salvage by cells with both active and facilitated transport

Formycin B influx studies have shown that P388 and L1210 mouse leukemia cells, mouse L929 cells, mouse RAW 309 Cr.1 cells, LK35.2 mouse B-cell hybridoma cells and cultured mouse peritoneal macrophages express both Na(+)-dependent, active and nonconcentrative, facilitated nucleoside transport systems. In the mouse cell lines, active transport represented only a minor nucleoside transport component and was detected only by measuring formycin B uptake in the presence of dipyridamole or nitrobenzylthioinosine, strong inhibitors of facilitated, but not of active, nucleoside transport. Inhibition of facilitated transport resulted in the concentrative accumulation of formycin B in cells expressing active nucleoside transport. Concentrative formycin B accumulation was abolished by treatment of the cells with gramicidin or absence of Na+ in the extracellular medium and strongly inhibited by ATP depletion or ouabain treatment. Mouse macrophages accumulated formycin B to 70-times the extracellular concentration in the absence of dipyridamole during 90 min of incubation at 37 degrees C. Thus active transport represents a major nucleoside transport system of these cells, similarly as previously reported for mouse spleen lymphocytes. In contrast to the various types of mouse cells, active formycin B transport was not detected in human HeLa cells, human H9, Jurkat and CEM T lymphoidal cells and pig spleen lymphocytes. These cells expressed only facilitated nucleoside transport with kinetic properties similar to those of the facilitated transporters of other mammalian cells.