Thermosensitization, heat shock protein synthesis and development of thermotolerance in M-14 human tumor cells subjected to step-down heating.

M-14 human tumor cells have been subjected to two regimens of step-down heating (SDH) consisting of a conditioning treatment at 42 degrees C for 1 h or at 44.5 degrees C for 20 min, immediately followed by heating at 40 degrees C. Both conditioning treatments thermosensitize the cells towards the subsequent heating at 40 degrees C; the thermosensitization ratio is 6.4 for cells conditioned at 42 degrees C for 1 h and 32.3 for cells conditioned at 44.5 degrees C for 20 min. The overall protein synthetic activity is reduced to 32.7% or 18.4% of control values following 1 h at 42 degrees C and 20 min at 44.5 degrees C, respectively; this inhibition is followed by a full recovery of the synthetic activity during the subsequent exposure at 40 degrees C. SDH-treated cells synthetize four heat shock proteins, with approximate molecular weights of 28, 64, 70 and 90 kDa. The pattern of HSPs induction observed in SDH-treated cells is similar to that found in cells subjected to single hyperthermic exposures. Cells subjected to the SDH sequence 42 degrees C/1 h-->40 degrees C/4 h develop thermotolerance, as indicated by a reduced sensitivity to further hyperthermic challenges.