THERMOTOLERANCE INHIBITS VARIOUS STRESS-INDUCED APOPTOSIS IN NIH3T3 CELLS

When NIH3T3 cells were exposed to mild heat and recovered at 37 degree s C for various time intervals, they were thermotolerant and resistant to subsequent stresses including heat, oxidative stresses, and antitu mor drug methotrexate which are apoptotic inducers. The induction kine tics of apoptosis by stresses were determined by DNA fragmentation and protein synthesis using [S-35]methionine pulse labeling. We investiga ted the hypothesis that thermotolerant cells were resistant to apoptot ic cell death compared to control cells when both cells were exposed t o various stresses inducing apoptosis. The cellular changes in thermot olerant cells were ex amined to determine which components are involve d in this resistance. At first, the degree of resistance correlates wi th the extent of heat shock protein synthesis which were varied depend ing on the heating times at 45 degrees C and recovery times at 37 degr ees C after heat shock. Secondly, membrane permeability change was obs erved in thermotolerant cells. When cells pre[abe[ed with [H-3]thymidi ne were exposed to various amounts of heat and recovered at 37 degrees C for 1/2 to 24 h, the permeability of cytosolic [H-3]thymidine in th ermotolerant cells was 4 fold higher than that in control cells. Third ly, the protein synthesis rates in thermotolerant and control cells we re measured after exposing the cells to the same extent of stress. It turned out that thermotolerant cells were less damaged to same amount of stress than control cells, although the recovery rates are very sim ilar to each other. These results demonstrate that an increase of heat shock proteins and membrane changes in thermotolerant cells may prote ct the cells from the stresses and increase the resistance to apoptoti c cell death, even though the exact mechanism should be further studie d.