Expression of constitutively active phosphatidylinositol 3-kinase inhibitsactivation of caspase 3 and apoptosis of cardiac muscle cells

Apoptosis of cardiac muscle cells contributes to the development of cardiom yopathy. Recent studies showed that insulin-like growth factor I (IGF-I) in hibits apoptosis of cardiac muscle cells and improves myocardial function i n experimental heart failure. This study was carried out to elucidate the r ole of phosphatidylinositol 3-kinase (PI 3-kinase) in the anti-apoptotic ac tions of IGF-I in cardiomyocytes and to explore whether expression of const itutively active PI 3-kinase can inhibit apoptosis in cardiomyocytes. Apopt osis of primary cardiomyocytes was induced by doxorubicin treatment and ser um withdrawal. Transduction of cardiomyocytes with constitutively active PI 3-kinase specifically lead to serine phosphorylation of Akt, whereas phosp horylation of IGF-I receptor, IRS1/2 and p44/42 mitogen-activated protein k inase were not increased. In the cardiomyocytes transduced with constitutiv ely active PI 3-kinase, activation of the pro-apoptotic caspase 3 was atten uated and fragmentation of DNA was reduced. Preincubating cells with PI 3-k inase inhibitor LY294002 was associated with loss of anti-apoptotic actions of IGF-I and PI S-kinase. Neither IGF-I nor constitutively active PI 3-kin ase lead to serine phosphorylation of Bad, suggesting that the anti-apoptot ic effects of PI 3-kinase are not mediated through Bad phosphorylation in c ardiac muscle cells. To determine whether activation of caspase 3 is suffic ient to induce apoptosis in cardiomyocytes, an engineered TAT-caspase 3 pro tein was introduced to cardiomyocytes. Significant reduction of cell viabil ity occurred in the cardiomyocytes transduced with active caspase 3, indica ting that activation of caspase 3 is sufficient to cause cardiomyocyte deat h. These findings indicate the existence of an IGF-I receptor-PI 3-kinase-c aspase 3 pathway in cardiomyocytes that plays an important role in the anti -apoptotic actions of IGF-I in heart. Moreover, these data suggest that mod ulation of PI 3-kinase activities may represent a potential therapeutic str ategy to counteract the occurrence of apoptosis in cardiomyopathy.