Hsp72-mediated suppression of c-Jun N-terminal kinase is implicated in development of tolerance to caspase-independent cell death

Pretreatment with mild heat shock is known to protect cells from severe str ess (acquired thermotolerance). Here we addressed the mechanism of this phe nomenon by using primary human fibroblasts. Severe heat shock (45 degrees C , 75 min) of the fibroblasts caused cell death displaying morphological cha racteristics of apoptosis; however, it was caspase independent. This cell d eath process was accompanied by strong activation of Akt, extracellular sig nal-regulated kinase 1 (ERK1) and ERK2, p38, and c-Jun N-terminal (JNK) kin ases. Suppression of Akt or ERK1 and -2 kinases increased cell thermosensit ivity. In contrast, suppression of stress kinase JNK rendered cells thermor esistant. Development of thermotolerance was not associated with Akt or ERK 1 and -2 regulation, and inhibition of these kinases did not reduce acquire d thermotolerance. On the other hand, acquired tolerance to severe heat sho ck was associated with downregulation of JNK. Using an antisense-RNA approa ch, we found that accumulation of the heat shock protein Hsp72 is necessary for JNK downregulation and is critical for thermotolerance. The capability of naive cells to withstand moderate heat treatment also appears to be dep endent on the accumulation of Hsp72 induced by this stress. Indeed, exposur e to 45 degrees C for 45 min caused only transient JNK activation and was n onlethal, while prevention of Hsp72 accumulation prolonged JNK activation a nd led to massive cell death. We also found that JNK activation by UV irrad iation, interleukin-1, or tumor necrosis factor was suppressed in thermotol erant cells and that Hsp72 accumulation was responsible for this effect. Hs p72-mediated suppression of JNK is therefore critical for acquired thermoto lerance and may play a role in tolerance to other stresses.