PKC‐alpha regulates expression of the hsp70 promoter via its two proximal AP‐1 binding elements

Abstract

Research has shown that PKC isozymes and heat shock proteins contribute to cardioprotection. Recent investigations into the interaction between these protein families revealed that PKC-alpha can confer protection against simulated ischemia by mediating the induction of Hsp70i. Interestingly, Hsp70i expression was found to be independent of the conventional heat shock transcription factor HSF-1. In an effort to elucidate a potentially novel mechanism, luciferase reporter assays were performed on chimeric rat Hsp70i promoter constructs transfected into myogenic H9c2 cells. Adenoviral overexpression of wild-type PKC-alpha demonstrates that inducibility of the rat Hsp70 promoter is preserved within 175 base pairs from the start of transcription. This segment contains two canonic AP-1 binding elements. Although both AP-1 sites retain modest induction in isolation, a more robust response occurs when present in tandem. Treatment with inhibitors targeting RhoA (C3 exoenzyme), ROCK (Y27693) and MEK (PD98059) suggest PKC-alpha transduces its signal through the raf-1/MEK/ERK pathway to upregulate c-Fos expression. These observations are supported by electrophoretic mobility shift assays confirming that c-Fos and c-Jun components bind to both the distal and proximal AP-1 elements. Potentially, these investigations may lead to new therapeutic approaches to harness the cardioprotective effects of hsp70.