The identification of intracellular signal transduction pathways that regulate pathologic cardiac growth has been an area of intense interest. The identification of these pathways would establish novel targets, modulation of which could alter the progression of hypertrophy to heart failure. This has been a difficult goal to achieve, however, since it is apparent that multiple interacting and/or parallel pathways regulate growth of the heart. Our interests have focused on the glycogen synthase kinase-3 pathway. We have found that GSK-3 is a negative regulator of cardiac growth. We have created a transgenic mouse expressing GSK-3beta in the heart, and this mouse is characterized by markedly impaired cardiac growth. We have also followed the GSK-3 pathway downstream to identify targets of GSK-3 that may play a role in growth. We have now identified one target, beta-catenin, that is negatively regulated by GSK-3. Beta-catenin is a transcription factor that has been known for some time to regulate dorsoventral patterning during embryoganesis as part of the Wnt pathway. In addition, mutations in beta-catenin have been found in a number of cancers, attesting to its role in tumorigenesis. We propose three Specific Aims to further clarify the role of GSK-3beta and beta-catenin in normal and pathologic cardiomyocyte growth.
Specific Aim 1. Determine the role of beta-catenin in the development of cardiac hypertrophy. It is our hypothesis that beta-catenin is a critical regulator of physiologic and pathologic hypertrophy in vivo. We will determine whether beta-catenin is necessary, sufficient, or both, for the hypertrophic response in vivo using a variety of approaches including viral mediated gene transfer, transgenic models, and a cardiac-specific conditional knockout of beta-catenin.
Specific Aim 2. Determine the mechanisms by which expression of GSK-3beta leads to a phenotype of impaired growth and depressed systolic function of the heart. GSK-3beta has a number of targets, any of which could play a role in impaired growth. It is our hypothesis that GSK-3beta blocks physiologic hypertrophy primarily via its inhibitory effects on the beta-catenin/c-Myc pathway. We will evaluate this hypothesis, again employing viralmediated gene transfer, transgenics, and knockouts.
Specific Aim 3. Determine the mechanisms regulating beta-catenin stabilization in response to hypertrophic stress. Our preliminary studies have identified a novel mechanism by which beta-catenin is stabilized by hypertrophic stress, and this involves recruitment of the GSK-3beta inhibitor, PKB/Akt to the beta-catenin degradation complex. It is our hypothesis that the mechanism involves recruitment of downstream components of the Wnt pathway. We will attempt to identify the specific signaling factors involved. We believe that the studies outlined herein, which are a logical extension of the work performed under this grant over the past three years, will allow us to define a novel linear signaling pathway, including cytosolic and nuclear components, regulating physiologic and pathologic hypertrophy of the heart.
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