Development of methotrexate-resistant human choriocarcinoma cells in culture.

A human choriocarcinoma cell line, HCCM-5, was fed with medium containing increasing concentrations of methotrexate (MTX). The initial MTX concentration, 10(-9) M which reduced the [3H] thymidine incorporation into DNA, was raised from 2- to 2.5-fold successively. After about 36 weeks of feeding, the cells became resistant to 5 X 10(-7) M which produced complete inhibition of the parent HCCM-5 cell growth. The parent line and its MTX-resistant subline (HCCM-5MTXr) had almost the same population doubling time. There were no apparent differences in morphology and human chorionic gonadotropin secretion between the two cell lines. The development of resistance was accompanied by a 10-fold decrease in the 3H-MTX uptake and a 5-fold elevation of the intracellular dihydrofolate reductase (DHFR) activity. The impairment of MTX transport in HCCM-5MTXr cells continued after transferring the HCCM-5MTXr cells into MTX-free medium, whereas the DHFR activity returned to the level found in the HCCM-5 cells. These results indicate that the MTX resistance acquired in choriocarcinoma cells chiefly involves the impaired transport of MTX and continues after the deprivation of the drug.