Cardiocyte apoptosis has been identified in many clinically important cardiac conditions including heart failure and ischemia-reperfusion injury. Insulin-like growth factor-1 (IGF-I) blocks apoptosis in many systems, and has beneficial effects on both cardiocyte function and survival. IGF-I administration reduces cardiocyte apoptosis after ischemia-reperfusion in animal models. However, the intracellular mechanisms controlling cardiocyte cell death and the benefits of IGF-I remain largely unexplored. The goal of this proposal is to understand the role of specific IGF-I receptor-mediated signaling pathways in blocking cardiocyte apoptosis. Binding of IGF-I to the IGF-I receptor (IGF-IR) initiates signaling through two distinct pathways: 1) phosphorylation of insulin receptor substrate (IRS)-I with activation of phosphatidylinositol (PI) 3-kinase and 2) the Ras/MAP (mitogen-activated protein) kinase pathway. In other systems, activation of PI 3-kinase is critical to the ability of IGF-I to block apoptosis, while MAP kinase activation has been associated with cardiac hypertrophy and malignant transformation. This proposal is based on three hypotheses: 1) that IGF-IR-induced activation of PI 3-kinase mediates the beneficial effects of IGF-I on cardiocyte survival, 2) that the benefit of IGF-I in ischemia-reperfusion is mediated by inhibition of caspase-dependent apoptosis, and 3) that apoptosis contributes significantly to the regional cardiac dysfunction seen after ischemia-reperfusion. To test these hypotheses, mutants of the IGF-IR (mIGF-IR) that constitutively and selectively activate PI 3-kinase will be utilized. Adenoviral gene transfer will overexpress mIGF-IR, the wild-type IGF-IR (wtIGF-IR), and crmA (a potent inhibitor of caspase-mediated apoptosis) in cardiomyocytes in in vivo and in vitro models.
Specific Aim 1 will develop and characterize wild-type and a series wild-IGF-receptor mutants in adenoviral vectors , as well as vectors encoding crmA, a specific caspase inhibitor.
Specific Aim 2 will examine the effects of wtIGF-IR, mIGF-IR, and crmA overexpression on cardiocyte apoptosis in vitro.
Specific Aim 3 will study the effects of mIGF-IR and crmA overexpression on cardiac apoptosis and function in a model of ischemia-reperfusion injury. Understanding the role of specific signaling pathways in cardiocyte apoptosis and developing approaches to local modulation of these pathways through somatic gene transfer, may provide novel therapeutic approaches for the management of many clinically important disorders.
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