| Budget Amount *help |
¥3,700,000 (Direct Cost: ¥3,700,000)
Fiscal Year 2004: ¥1,800,000 (Direct Cost: ¥1,800,000)
Fiscal Year 2003: ¥1,900,000 (Direct Cost: ¥1,900,000)
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| Research Abstract |
Calreticulin is a Ca^<2+>-binding molecular chaperone of the lumen of the endoplasmic reticulum. Previously we reported that an overexpression of calreticulin promotes the differentiation-dependent apoptosis in rat myocardiac H9c2 cells by suppressing the Akt signaling pathway via the alteration of Ca^<2+> homeostasis. In the present study we examined how Akt pathway is influenced by a change in the cytoplasmic free calcium concentration ([Ca^<2+>]_i) via the regulation of a Ser/Thr phosphatase, protein phosphatase 2Ac (PP2Ac), in H9c2 cells. We found that Akt was down-regulated when [Ca^<2+>]_i was elevated by thapsigargin, an inhibitor of the endoplasmic reticulum Ca^<2+>-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 Ca^<2+> chelator. To investigate the mechanism of the Ca^<2+>-dependent regulation of Akt via the regulation of PP2A, we examined the transcriptional regulation of PP2Acα in H9c2 cells with Ca^<2+> modulators. Transcription of the PP2Acα 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 Ca^<2+>-dependent regulation of PP2Acα. Furthermore, phosphorylation of CRE-binding protein increased with thapsigargin but decreased with BAPTA-AM. Taken together, a long-term change of [Ca^<2+>]_i regulates PP2Acα gene transcription via CRE, resulting in a change in the activation status of Akt leading to an altered susceptibility to apoptosis. The results also suggest that calreticulin modulates Ca^<2+> homeostasis to regulate the Akt signaling pathway via the transcriptional regulation of PP2Ac gene expression.
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