(1/189) [3H]gemcitabine uptake by nucleoside transporters in a human head and neck squamous carcinoma cell line.

Cellular uptake of many chemotherapeutic nucleoside analogs is dependent on the activity of a family of nucleoside transport proteins located in the cell plasma membrane. In the present study, we examined the role of these transporters in the accumulation of gemcitabine by a human head and neck squamous carcinoma cell line. The uptake of [3H]gemcitibine was compared with that of [3H]uridine and [3H]formycin B in the parent cell line (HN-5a) and in a gemcitabine-resistant variant (GEM-8e). The HN-5a and GEM-8e cells were similar in their transport characteristics and expressed predominantly the es (equilibrative, inhibitor-sensitive) transporter subtype; less than 10% of the influx of [3H]formycin B or [3H]uridine was mediated by the ei (equilibrative inhibitor-resistant) system, and there was no evidence for Na+-dependent nucleoside transporters. [3H]Gemcitabine (10 microM) entered these cells via both the es and ei transporters with an initial rate of uptake similar to that seen with the use of [3H]formycin B or [3H]uridine. In addition, ATP-replete cells accumulated significantly less [3H]gemcitabine than did ATP-depleted cells, which is indicative of an active efflux mechanism for gemcitabine. These results show that gemcitabine is a substrate for both the es and ei nucleoside transporters of HN-5a and GEM-8e cells and that gemcitabine resistance of the GEM-8e cells cannot be attributed to changes in transporter activity. Further studies to define the characteristics of the putative efflux mechanism are clearly warranted because this system has the potential to significantly affect the clinical efficacy of gemcitabine.  (+info)

(2/189) Nucleoside transport in human colonic epithelial cell lines: evidence for two Na+-independent transport systems in T84 and Caco-2 cells.

RT-PCR of RNA isolated from monolayers of the human colonic epithelial cell lines T84 and Caco-2 demonstrated the presence of mRNA for the two cloned Na+-independent equilibrative nucleoside transporters, ENT1 and ENT2, but not for the cloned Na+-dependent concentrative nucleoside transporters, CNT1 and CNT2. Uptake of [3H]uridine by cell monolayers in balanced Na+-containing and Na+-free media confirmed the presence of only Na+-independent nucleoside transport mechanisms. This uptake was decreased by 70-75% in the presence of 1 microM nitrobenzylthioinosine, a concentration that completely inhibits ENT1, and was completely blocked by the addition of 10 microM dipyridamole, a concentration that inhibits both ENT1 and ENT2. These findings indicate the presence in T84 and Caco-2 cells of two functional Na+-independent equilibrative nucleoside transporters, ENT1 and ENT2.  (+info)

(3/189) Antitumor activity of P-4055 (elaidic acid-cytarabine) compared to cytarabine in metastatic and s.c. human tumor xenograft models.

The antineoplastic efficacy of P-4055, a 5'-elaidic acid (C18:1, unsaturated fatty acid) ester of cytarabine, a nucleoside antimetabolite frequently used in the treatment of hematological malignancies, was examined in several in vivo models for human cancer. In initial dose-finding studies in nude mice, the efficacy of P-4055 was highest when using schedules with repeated daily doses. In a Raji Burkitt's lymphoma leptomeningeal carcinomatosis model in nude rats, the control cytarabine- and saline-treated animals (five in each group) had a mean survival time of 13.2 days, whereas treatment with P-4055 resulted in three of five long-time survivors (>70 days). In a systemic Raji leukemia model in nude mice, 8 of 10 of the P-4055-treated animals survived (>80 days), compared with none of the cytarabine-treated animals (mean survival time, 34.2 days). In s.c. xenograft models, the effects of maximum tolerated doses of P-4055 and cytarabine, given in four weekly cycles of daily bolus i.v. injections for 5 subsequent days, against seven tumors (three melanomas, one lung adenocarcinoma, one breast cancer, and two osteogenic sarcomas) were investigated. P-4055 induced partial or complete tumor regression of the lung carcinoma, as well as of all three malignant melanomas. In two of the melanomas the activity was highly superior to that of cytarabine, and both P-4055 and cytarabine were, in general, more effective than several clinically established drugs previously tested in the same tumor models. In in vitro studies, inhibitors of nucleoside carrier-dependent transport, nitrobenzylmercaptopurine riboside and dipyridamol, reduced strongly the cellular sensitivity to cytarabine, but not to P-4055, indicating that P-4055 uses an alternative/additional mechanism of internalization into the cell compared with cytarabine. The results explain, at least in part, the observed differences between the two compounds in in vivo efficacy, and together the data strongly support the evaluation of P-4055 in clinical studies.  (+info)

(4/189) A nucleoside transporter from Trypanosoma brucei involved in drug resistance.

Drug resistance of pathogens is an increasing problem whose underlying mechanisms are not fully understood. Cellular uptake of the major drugs against Trypanosoma brucei spp., the causative agents of sleeping sickness, is thought to occur through an unusual, so far unidentified adenosine transporter. Saccharomyces cerevisiae was used in a functional screen to clone a gene (TbAT1) from Trypanosoma brucei brucei that encodes a nucleoside transporter. When expressed in yeast, TbAT1 enabled adenosine uptake and conferred susceptibility to melaminophenyl arsenicals. Drug-resistant trypanosomes harbor a defective TbAT1 variant. The molecular identification of the entry route of trypanocides opens the way to approaches for diagnosis and treatment of drug-resistant sleeping sickness.  (+info)

(5/189) Uptake of NO-releasing drugs by the P2 nucleoside transporter in trypanosomes.

Nitric oxide (NO.) has been identified as a principal regulatory molecule of the immune system and the major cytotoxic mediator of activated immune cells. NO. can also react rapidly with a variety of biological species, particularly with the superoxide radical anion O2.- at almost diffusion-limited rates to form peroxynitrite anion (ONOO-). ONOO- and its proton-catalyzed decomposition products are capable of oxidizing a great diversity of biomolecules and can act as a source of toxic hydroxyl radicals. As a consequence, a strategy for the development of molecules with potential trypanocidal activities could be developed to increase the concentration of nitric oxide in the parasites through NO.-releasing compounds. In this way, the rate of formation of peroxynitrite from NO. and O2.- would be faster than the rate of dismutation of superoxide radicals by superoxide dismutases which constitute the primary antioxidant enzymatic defense system in trypanosomes. The adenosine transport systems of parasitic protozoa, which are also in certain cases implicated in the selective uptake of active drugs such as melarsoprol or pentamidine, could be exploited to specifically target these NO.-releasing compounds inside the parasites. In this work, we present the synthesis, characterization and biological evaluation of a series of molecules that contain both a group which would specifically target these drugs inside the parasites via the purine transporter, and an NO.-donor group that would exert a specific pharmacological effect by increasing NO level, and thus the peroxynitrite concentration inside the parasite.  (+info)

(6/189) A Na(+)-dependent nucleoside transporter in microglia.

In the central nervous system, HIV-1 has a defined tropism for brain macrophages and microglia. Nucleoside analog drugs such as zidovudine improve the clinical and neuropsychological functions in HIV-demented patients. Multiple carrier-mediated transport systems can play an important role in the membrane permeation of nucleosides and nucleoside analog drugs in a number of cells. The purpose of this project was to characterize the uptake properties of the pyrimidine nucleoside probe thymidine by a continuous rat microglia cell line (MLS-9) grown as a monolayer on an impermeable substratum. Approximately 50% of thymidine (10 microM) uptake by the monolayer cells was found to be Na(+) dependent. Kinetics of specific thymidine uptake showed a single saturation system (K(m) = 44 microM at 37 degrees C) and a Na(+)/thymidine stoichiometry of 2:1. Pyrimidine and purine nucleoside probes (50 microM) exerted a competitive inhibitory effect on specific thymidine uptake with K(i) values of 40, 38, 45, and 39 microM for adenosine, uridine, guanosine, and cytidine, respectively. In addition, nucleoside analog drugs significantly decreased specific thymidine uptake, with IC(50) values of 135.1 microM for abacavir and 0.6 microM for zidovudine, which inhibited in a noncompetitive manner. These results suggest that a Na(+)-dependent nucleoside transport system is present in rat microglia and that long-range interactions between antiretroviral nucleoside analog drugs and the nucleoside substrates may occur at the transporter sites.  (+info)

(7/189) Nitric oxide regulates nucleoside transport in activated B lymphocytes.

Activation of human B lymphocytes by lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA) results in the differential regulation of nucleoside uptake [Soler, C., Felipe, A., Mata, J. F., Casado, F. J., Celada, A., Pastor-Anglada, M. (1998) J. Biol. Chem. 273, 26939-26945]. Because nitric oxide (NO) is involved in the modulation of the apoptotic response of B cells, the effects of NO on the regulatory responses of these transport systems to phorbol esters has been studied in Raji cells by a combination of approaches that involve arginine depletion, inhibition of nitric oxide synthase, and non-enzymatic production of NO using a donor. Human B lymphocytes express three transport systems involved in nucleoside uptake: N1 and N5, which are concentrative and Na+-dependent, and the nitrobenzylthioinosine-sensitive equilibrative system es. Raji cells do not express significant amounts of iNOS mRNA or protein; thus, NO production is presumably constitutive. The data are consistent with a role of NO in maintaining the basal transport activities of the three systems: N1, N5, and es. However, the up-regulatory effect of PMA on N1 and N5 does not require NO, whereas the inhibition of es transport activity does. In summary, NO differentially modulates nucleoside transport systems in activated human B lymphocytes and thus, NO may also be involved in the regulation of nucleoside (i.e., adenosine) disposal by activated B cells.  (+info)

(8/189) Cloning of a novel inosine-guanosine transporter gene from Leishmania donovani by functional rescue of a transport-deficient mutant.

Purine transport is an indispensable nutritional function for protozoan parasites, since they are incapable of purine biosynthesis and must, therefore, acquire purines from the host milieu. Exploiting a mutant cell line (FBD5) of Leishmania donovani deficient in inosine and guanosine transport activity, the gene encoding this transporter (LdNT2) has been cloned by functional rescue of the mutant phenotype. LdNT2 encodes a polypeptide of 499 amino acids that shows substantial homology to other members of the equilibrative nucleoside transporter family. Molecular analysis revealed that LdNT2 is present as a single gene copy within the leishmanial genome and encodes a single transcript of 3 kilobase pairs. Transfection of FBD5 parasites with LdNT2 re-established their ability to take up inosine and guanosine with a concurrent restoration of sensitivity to the inosine analog formycin B. Kinetic analyses reveal that LdNT2 is highly specific for inosine (K(m) = 0.3 micrometer) and guanosine (K(m) = 1.7 micrometer) and does not recognize other naturally occurring nucleosides. Expression of LdNT2 cRNA in Xenopus oocytes significantly augmented their ability to take up inosine and guanosine, establishing that LdNT2 by itself suffices to mediate nucleoside transport. These results authenticate genetically and biochemically that LdNT2 is a novel nucleoside transporter with an unusual and strict specificity for inosine and guanosine.  (+info)