Implication of a small GTPase Rac1 in the activation of c-Jun N-terminal kinase and heat shock factor in response to heat shock

Heat shock induces c-Jun N-terminal kinase (JNK) activation as well as heat shock protein (HSP) expression through activation of the heat shock factor (HSP), but its signal pathway is not clearly understood. Since a small GTPase Rad has been suggested to participate in the cellular response to stresses, we examined whether Rad is involved in the heat shock response. Here we show that moderate heat shock (39-41 degreesC) induces membrane translocation of Rad and membrane ruffling in a Rad-dependent manner. In addition, Rac1N17, a dominant negative mutant of Rad, significantly inhibited JNK activation by heat shock. Since Rac1V12 was able to activate JNK, it is suggested that heat shock may activate JNK via Rad, Similar inhibition by Rac1N17 of HSP activation in response to heat shock was observed. However, inhibitory effects of Rac1N17 on heat shock-induced JNK and HSF activation were reduced as the heat shock temperature increased. Rac1N17 also inhibited HSF activation by L-azetidine-2-carboxylic acid, a proline analog, and heavy metals (CdCl2), suggesting that Rad may be linked to HSF activation by denaturation of polypeptides in response to various proteotoxic stresses. However, Rac1N17 did not prevent phosphorylation of HSF1 in response to these proteotoxic stresses. Interestingly, a constitutively active mutant Rac1V12 did not activate the HSF. Therefore, Rad activation may be necessary, but not sufficient, for heat shock-inducible HSF activation and HSP expression, or otherwise a signal pathway(s) involving Rad may be indirectly involved in the HSF activation. In sum, we suggest that Rad may play a critical role(s) in several aspects of the heat shock response.