Antibiotic C3368-A, a fungus-derived nucleoside transport inhibitor, potentiates the activity of antitumor drugs

Maintenance of differential methotrexate toxicity between cells expressing drug-resistant and wild-type dihydrofolate reductase activities in the presence of nucleosides through nucleoside transport inhibition

Methotrexate (MTX), a potent inhibitor of dihydrofolate reductase (DHFR), has been used widely as a chemotherapeutic agent and as a selective agent for cells expressing drug-resistant DHFR activity. MTX deprives rapidly dividing cells of reduced folates that are necessary for thymidylate synthesis and de novo purine nucleotide synthesis. However, MTX toxicity can be circumvented by salvaging thymidine (TdR) and purine nucleosides. Here we have investigated conditions under which nucleoside transport inhibition can be used to maintain differential MTX toxicity between unmodified cells and cells expressing drug-resistant DHFR activity in the presence of exogenous nucleosides. PA317 cells (a 3T3 derivative cell line) were rescued from the toxicity of 0.1 microM MTX by 1.0 microM TdR in the presence of 100 microM inosine. The nucleoside transport inhibitor dipyridamole (DP) resensitized these cells to MTX, even in the presence of exogenous nucleosides. Furthermore, PA317 cells transduced with any of three retroviruses encoding drug-resistant DHFRs remained resistant to MTX over all concentrations tested (up to 10.0 microM) in the presence of DP. Similar results were obtained in transduced HuH7 and K562 cell lines, a human hepatoma and a human leukemia cell line, respectively. We conclude that nucleoside transport inhibition increases the toxicity and selectivity of MTX in cultured cells, and therefore is an effective way to maintain differential MTX toxicity between unmodified and DHFR-modified cells. Our results support the use of nucleoside transport inhibition in in vivo selection protocols involving the liver and hematopoietic systems.

Antitumor components from an actinomycete strain 6011W

Three bioactive compounds that inhibited nucleoside transport were isolated from the cultured broth of Streptoverticillium sp. 6011W. The structures of those compounds were characterized as cinnamamide, N-(tetrahydro-2-oxo-3-thienyl)-acetamide and benzamide, respectively. They all inhibited radiolabeled thymidine transport into Ehrlich carcinoma cells, with IC50 values of 30.4, 97.2 and 85.4 microM, respectively. When administered i.p., cinnamamide not only inhibited the growth of transplanted tumors but also reduced the number of lung metastases in mice bearing Lewis lung carcinoma. The results suggest that nucleoside transport inhibition assay is a valuable model to search for antitumor agents of natural origin.

Possibility of contribution of nucleoside transport systems to pirarubicin uptake by HL60 cells but not mononuclear cells

Previously, we reported that pirarubicin (THP), an anthracycline, was taken up, at least in part, by both human leukemic HL60 cells and mononuclear cells (MNCs) via a carrier-mediated system. In this study, the possibility of a contribution of nucleoside transport systems to the uptake of THP by HL60 cells and MNCs was investigated. The experiments were performed after both types of cells had been pretreated with a metabolic inhibitor, 2,4-dinitrophenol, to deplete cellular ATP. In HL60 cells, THP uptake was increased and decreased significantly by treatment with equilibrative nucleoside transport inhibitors, nitrobenzylthioinosine (NBMPR), nitrobenzylthioguanosine and dilazep, in the presence and absence, respectively, of an inwardly directed Na+-gradient. THP uptake by HL60 cells showed an overshoot in the presence of the gradient, and was decreased by treatment of the cells with monensin, indicating that the uptake partially depended on the Na+-gradient. In HL60 cells in which equilibrative nucleoside transport was inhibited by NBMPR, THP uptake in the presence of the gradient was inhibited by Na+-dependent concentrative nucleoside transport inhibitors, but no inhibition was observed in the absence of the gradient. In MNCs, conversely, there was no effect of any equilibrative nucleoside transport inhibitor or the Na+-gradient on THP uptake. These results suggested that THP was taken up, at least in part, via both equilibrative and concentrative nucleoside transport systems in HL60 cells, but not in MNCs.