Abstract
Apoptosis of cardiac muscle cells contributes to the development of cardiomyopathy. Recent studies showed that insulin-like growth factor I (IGF-I) inhibits apoptosis of cardiac muscle cells and improves myocardial function in experimental heart failure. This study was carried out to elucidate the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the anti-apoptotic actions of IGF-I in cardiomyocytes and to explore whether expression of constitutively active PI 3-kinase can inhibit apoptosis in cardiomyocytes. Apoptosis of primary cardiomyocytes was induced by doxorubicin treatment and serum withdrawal. Transduction of cardiomyocytes with constitutively active PI 3-kinase specifically lead to serine phosphorylation of Akt, whereas phosphorylation of IGF-I receptor, IRS1/2 and p44/42 mitogen-activated protein kinase were not increased. In the cardiomyocytes transduced with constitutively active PI 3-kinase, activation of the pro-apoptotic caspase 3 was attenuated and fragmentation of DNA was reduced. Preincubating cells with PI 3-kinase inhibitor LY294002 was associated with loss of anti-apoptotic actions of IGF-I and PI 3-kinase. Neither IGF-I nor constitutively active PI 3-kinase lead to serine phosphorylation of Bad, suggesting that the anti-apoptotic effects of PI 3-kinase are not mediated through Bad phosphorylation in cardiac muscle cells. To determine whether activation of caspase 3 is sufficient to induce apoptosis in cardiomyocytes, an engineered TAT-caspase 3 protein was introduced to cardiomyocytes. Significant reduction of cell viability occurred in the cardiomyocytes transduced with active caspase 3, indicating that activation of caspase 3 is sufficient to cause cardiomyocyte death. These findings indicate the existence of an IGF-I receptor-PI 3-kinase-caspase 3 pathway in cardiomyocytes that plays an important role in the anti-apoptotic actions of IGF-I in heart. Moreover, these data suggest that modulation of PI 3-kinase activities may represent a potential therapeutic strategy to counteract the occurrence of apoptosis in cardiomyopathy.
Highlights
Apoptosis of cardiac muscle cells have been observed during fetal development of cardiac tissue, in various models of myocardial injuries, and during the development of heart failure [1,2,3,4,5,6,7,8,9]
insulin-like growth factor I (IGF-I) Suppression of Cardiomyocytes Apoptosis Is Inhibited by LY294002—To test whether IGF-I suppression of cardiomyocytes apoptosis requires activation of PI 3-kinase, we used an in vitro model of cardiomyocytes apoptosis induced with doxorubicin [13, 14]
LY294002 aggravated doxorubicin-induced fragmentation of DNA. These results strongly suggest that the survival effects of IGF-I in cardiomyocytes require activation of PI 3-kinase pathway, and raised the possibility that activation of PI 3-kinase plays an important role in maintaining the viability of cardiomyocytes
Summary
Apoptosis of cardiac muscle cells have been observed during fetal development of cardiac tissue, in various models of myocardial injuries, and during the development of heart failure [1,2,3,4,5,6,7,8,9]. The aim of this study was to investigate whether activation of PI 3-kinase can lead to suppression of caspase 3 activation and in turn inhibit the occurrence of apoptosis in cardiac muscle cells. Since activation of PI 3-kinase was not associated with serine phosphorylation of Bad, we concluded that in cardiac muscle cells, PI 3-kinase/Akt signaling inhibited caspase cascades through a mechanism independent of Bad phosphorylation. These results provide a novel link between PI 3-kinase pathway and suppression of caspase 3 activation in a tissue where apoptosis plays a significant pathophysiological role
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