Conventional and novel PKC isoenzymes modify the heat-induced stress response but are not activated by heat shock

In mammalian cells, the heat-induced stress response is mediated by the con stitutively expressed heat shock transcription factor 1 (HSF1). Upon exposu re to elevated temperatures, HSF1 undergoes several post-translational modi fications, including inducible phosphorylation or hyperphosphorylation. To date, neither the role of HSF1 hyperphosphorylation in regulation of the tr anscriptional activity of HSF1 nor the signaling pathways involved have bee n characterized. We have previously shown that the protein kinase C (PKC) a ctivator, 12-O-tetradecanoylphorbol 13-acetate (TPA), markedly enhances the heat-induced stress response, and in the present study we elucidate the me chanism by which PKC activation affects the heat shock response in human ce lls. Our results show that several conventional and novel PKC isoenzymes ar e activated during the TPA-mediated enhancement of the heat shock response and that the enhancement can be inhibited by the specific PKC inhibitor bis indolylmaleimide I. Furthermore, the potentiating effect of TPA on the heat -induced stress response requires an intact heat shock element in the hsp70 promoter, indicating that PKC-responsive pathways are able to modulate the activity of HSF1. We also demonstrate that PKC is not activated by heat st ress per se, These results reveal that PKC exhibits a significant modulator y role of the heat-induced stress response, but is not directly involved in regulation of the heat shock response.