Antiapoptotic activity of Akt is down-regulated by Ca2+ in myocardiac H9c2 cells -: Evidence of Ca2+-dependent regulation of protein phosphatase 2Ac

Cell survival signaling of the Akt/protein kinase B pathway was influenced by a change in the cytoplasmic free calcium concentration ([Ca2+](i)) for over 2 h via the regulation of a Ser/Thr phosphatase, protein phosphatase 2Ac (PP2Ac), in rat myocardiac H9c2 cells. Akt was down-regulated when [Ca2+](i) was elevated by thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, but was up-regulated when it was suppressed by 1,2-bis(o-aminophenoxy) ethane-N,N,N', N'-tetraacetic acid tetra( acetoxymethyl) ester (BAPTA-AM), a cell permeable Ca2+ chelator. The inactivation of Akt was well correlated with the susceptibility to oxidant-induced apoptosis in H9c2 cells. To investigate the mechanism of the Ca2+-dependent regulation of Akt via the regulation of PP2A, we examined the transcriptional regulation of PP2Acalpha in H9c2 cells with Ca2+ modulators. Transcription of the PP2Acalpha gene was increased by thapsigargin but decreased by BAPTA-AM. The promoter activity was examined and the cAMP response element (CRE) was found responsible for the Ca2+-dependent regulation of PP2Acalpha. Furthermore, phosphorylation of CRE-binding protein increased with thapsigargin but decreased with BAPTA-AM. A long term change of [Ca2+](i) regulates PP2Acalpha gene transcription via CRE, resulting in a change in the activation status of Akt leading to an altered susceptibility to apoptosis.