Reduced intracellular content of methotrexate in an isolated MTX-resistant cell line of Nicotiana plumbaginifolia.

In plant cells methotrexate (MTX) may exert its toxic effect through several mechanisms, including inhibition of its target protein dihydrofolate reductase. Resistance based on a mechanism operating before MTX binds to proteins should confer protection to plant cells. A methotrexate-resistant cell line of Nicotiana plumbaginifolia was isolated by a stepwise selection procedure. This cell line survived in the presence of 10 μM MTX which is 50-100 fold higher than the lethal dose for the wild type cells. Neither alteration in kinetic characteristics of dihydrofolate reductase, nor elevated binding capacity of (3)H-MTX to target protein(s), were observed. However, in comparison with wild type cells, markedly lower amounts of intracellular (3)H-MTX were found after the selected cell line was incubated with (3)H-MTX, indicating that either reduced uptake or enhanced efflux of MTX is the major reason for MTX-resistance in this cell line.