Collateral Sensitivity to Methotrexate in Cells Resistant to Adriamycin

Chinese hamster ovary (CHO) cells, maintained and treated in log-phase growth, are extremely sensitive to Adriamycin but are very resistant to methotrexate-induced cell killing. The survival response to Adriamycin is biphasic, with the sensitive population showing a D₀ of 0.08 µzg/ml for 1 hr, while essentially no cell killing is produced by methotrexate treatments of up to 500 µg/ml for either 1 or 13 hr. Cells surviving Adriamycin treatment (5 µg/ml for 1 hr) and isolated in colony form were more resistant (D₀= 0.2 µg/ml) to subsequent treatments with Adriamycin than were the parental population, although the survival response was still biphasic. These Adriamycin-resist-ant cells (CHO-R/ADR), however, had become sensitive to methotrexate, with a nearly 50% cell killing achieved by treatment with 5 µg/ml for 1 hr. Uptake studies indicate that the acquired Adriamycin resistance is not due to a decrease in Adriamycin uptake, whereas the increase in methotrexate sensitivity is, in part, due to an increased uptake and a lower efflux of methotrexate. Levels of dihydrofolate reductase activity in the Adriamycin-sensitive methotrexate-resistant CHO cells are substantially higher (34%) than levels in Adriamycin-resistant (CHO-R/ADR) cultures. Titration of CHO cells with methotrexate doses of up to 500 µg/ml (for only 1 hr) reduced the dihydrofolate reductase activity to the level found in CHO-R/ADR cells treated with doses up to 5 µg/ml for 1 hr. Survival studies confirmed that Adriamycin resistance could be conferred to the CHO cells in a dose-dependent fashion by pre-treating normal cultures with methotrexate. These and other data suggest that the collateral methotrexate sensitivity in the CHO-R/ADR cells is on the basis of both decreased levels of dihydrofolate reductase and increased net uptake of methotrexate. Dihydrofolate reductase levels seem to be a measure of Adriamycin sensitivity. These results may have important implications in the use of these two drugs, either alone or in combination, in cancer chemotherapy.