The renin-angiotensin system (RAS) is critical for the maintenance of volume and electrolyte homeostatis in all mammalian and many lower animal species. Angiotensin II, the most biologically active peptide of the RAS affects several aspects of cardiac function including contractility, cellular growth, differentiation, apoptosis and gene expression. Clinical and experimental evidence indicates that a local RAS is involved in mediating ventricular growth in association with cardiac hypertrophy. We and others have demonstrated that mechanical stress upregulates RAS components in cardiac tissue. However, the sensors and signal transduction mechanisms responsible for linking mechanical stimuli to expression of RAS genes remain to be elucidated. Both integrin- dependent and independent mechanotransduction processess have been shown to be operational in cardiac tissue. Integrins have been demonstrated to activate focal-adhesion proteins and stimulate hypertrophic growth in cardiac myocytes. Stretch-activated channels (SACs) are non-integrin mechanosensors that couple to cytosolic calcium mobilization, but can also activate focal-adhesion complex proteins and the mitogen-activated protein (MAP) kinase pathways. The overall goal of this proposal is to identify the mechanotransducers (Aim 1) and associated effectors (Aim 2) responsible for mediating stretch-dependent angiotensinogen (Ao) gene expression in neonatal rat cardiac myocytes and fibroblasts. The goal of the first aim will be to test the hypothesis that integrin-dependent and -independent signaling mechanisms are responsible for stretch-dependent Ao gene expression in these cardiac cells. If integrins contribute to Ao gene expression, then a combination of pharmacologic agents, blocking antibodies and overexpression of chimeric integrins will be used to identify the specific integrin receptors involved. If SACs are involved, the role of calcium and possible interactions with integrin mechanosignaling mechanisms will be examined. Aligned myocyte cultures will be used to test the hypothesis that identified mechanosensors are differentially regulated by stretch applied across the longand short-axis of the cell. The goal of Aim 2 will be to identify the key signal transduction pathways responsible stretch-dependent Ao gene expression in cardiac myocytes and fibroblasts. These studies will focus on components of the focal-adhesion complex, small GTP-binding proteins and the MAP kinase family members.
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