Upregulation of iHsp70 by mild heat shock protects rabbit myogenic stem cells: involvement of JNK signalling and c-Jun.

iHsp70 [inducible Hsp70 (heat-shock protein 70)] family members (iHsp70, Hsp72 and Hsp70) are highly conserved proteins that act as molecular chaperones and promote cell survival during various forms of stress. Our data indicate that cultured adult rabbit myoblasts do not express iHsp70 under normal growth conditions, although increased expression was detectable 0.5-72 h following a 42°C heat shock for 15-60 min. The intracellular iHsp70 level reached a maximum 8 h after onset of the heat shock, which correlated with its increased accumulation in nuclei. Inhibition of iHsp70 expression by quercetin showed that sustained activation of JNK (c-Jun N-terminal kinase) 2 and suppression of c-Jun phosphorylation were responsible for myoblast death after heat shock. The data also demonstrate that activation of transcription factor c-Jun depends mostly on JNK1, whereas JNK2 had higher affinity and was translocated to nuclei together with c-Jun. We have also shown that the JNK signalling pathway is an upstream effect of iHsp70 expression. These findings provide further in-depth understanding of the implication of the pro-survival signalling kinases JNK1 and JNK2 and their target, c-Jun, in expression of iHsp70 and regulation of myogenic stem cell survival and death mechanisms after heat shock. Mild heat shock before transplantation might be a way of improving myogenic stem cell survival.