Dissociation of 68,000 M(r) heat shock protein synthesis from thermotolerance expression in rat fibroblasts

Rat embryonic fibroblasts growing exponentially at either 35, 37, or 39°C were exposed to 42°C for times up to 6 hr. Cell survival was unaffected by this heat shock in cultures growing at 39°C but survival was decreased in a temperature dependent manner in cells growing at 37 or 35°C. Exposure to 42°C of cells previously adapted to 35 or 37°C resulted in the induction of heat shock proteins (hsps) with apparent molecular weights of 68,000 (hsp 68), 70,000 (hsp 70), and 89,000 (hsp 89); cells previously adapted to 39°C expressed all hsps except hsp 68. Inasmuch as the synthesis of certain hsps may function to protect cells from thermal damage, these data indicate that hsp 68 may not be required for this adaptation-related thermotolerant survival response. Hsp 68 may only be expressed in cells destined to die.