Many aspects of cardiac contractile function are determined by the action of thyroid hormones, specifically triiodothyronine (T3), on myocyte-specific gene transcription that is mediated byT3 binding to one or more nuclear thyroid hormone receptors (TR). The precise role of the individual TRs (TR alpha I,TR beta 1) and their post-transcriptional modification in determining transcription of specific cardiomyocyte genes, including co-myosin heavy chain (alpha-MHC), remains unknown. The myocardium in conditions of pathologic and physiologic hypertrophy exhibits altered expression of many T3-responsive genes, which can be partially normalized by T3 treatment. The present application will test the hypothesis that impaired T3 responsiveness in the hypertrophied heart is the result of altered expression and phosphorylation of TR isoforms which affect all aspects of nuclear receptor function including DNA binding, dimerization, interaction with co-regulators, ligand affinity and transcriptional activity. In the first two specific aims, we propose to test the hypothesis that the hypertrophic cardiac phenotype is a result of changes in nuclear, content of TR isoforms and that the individual TR alpha I and beta 1 isoforms are differentially phosphorylated as assessed by isolation of nuclear proteins from purified adult rat ventricular myocytes. Our preliminary data suggest that the non-T3 binding isoform, TR alpha 2,has the unique ability to repress T3-inducible transcription of cardiac alpha-MHC and SERCA2 genes. A recombinant adenovirus strategy will be used in specific aim 3 to overexpress TR alpha 2 protein in cultured cardiomyocytes,and the resulting changes in transcription of the endogenous alpha-MHC gene will be measured by a novel assay to quantify alpha-MHC heteronuclear RNA.
Specific aim 4 will test will the hypothesis that protein kinase C signaling pathways that are activated in the hypertrophic myocardium result in phosphorylation of specific TR isoforms, which in turn mediate changes in cardiac phenotype remarkably similar to that of the hypothyroid heart. We will study the effects of adenoviral-mediated overexpression of individual PKC delta, epsilon, and zeta isoenzymes on specific TR isoform phosphorylation and alpha-MHC gene transcription. Completion of these studies will provide novel mechanistic information regarding the role of TRs in mediating the hypertrophic phenotype, and provide the rationale for the potential therapeutic utility of T3 in the setting of congestive heart failure.
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