Reversal of resistance to methotrexate by hyperthermia in Chinese hamster ovary cells.

ABSTRACT

Our Chinese hamster ovary cells are extremely resistant to methotrexate (MTX) (100% survival after 500 microgram/ml for 13 hr). However, exposure to 43 degrees (but not 41 degrees or 42 degrees) for 1 hr sensitizes the cells to MTX so that a 50% cell kill in excess of that due to hyperthermia occurs. Treatment of cells at 43 degrees increases net MTX uptake by about 30% at 30 min but causes a substantial reduction after 1 hr. This negative effect is greater in cells continually heated at 43 degrees than in those exposed for only 1 hr. Treatment at 43 degrees for 1 hr also markedly increases efflux of MTX out of cells over the that 2 hr. Dihydrofolate reductase activity was found to decrease to about 50% of control values by 4 to 5 hr after exposure to 43 degrees. The biological half-life of dihydrofolate reductase in Chinese hamster ovary cells was determined to be about 4.5 hr, indicating that hyperthermia-induced cessation of protein synthesis may explain both the decrease in dihydrofolate reductase activity and the sensitization to MTX observed with heat exposure. In scheduling experiments, lethality due to exposure to 43 degrees for 1 hr in conjunction with MTX was maximum when 1-hr drug exposure began just at the end of heat treatment.