Pathological cardiac remodeling results from myocardial stresses including pressure and volume overload, neurohumoral activation, myocardial infarction, and hypothyroidism. These pathological conditions converge on altered cell signaling and subsequent transcriptional cardiac remodeling that is thought to initially be an adaptive response but ultimately drives progression to heart failure (HF). Mediator complex is a transcriptional co-activator that coordinates signal dependent transcription factors with basal transcriptional machinery. This thesis specifically addresses the role of two Mediator kinase submodule proteins, cyclin-dependent kinase 8 (Cdk8) and Mediator subunit 13 (Med13), in regulating pathological transcriptional remodeling in response to cardiac hypertrophy and hypothyroidism, respectively.
Cdk8 kinase activity regulates transcriptional responses to pathological cardiac hypertrophy
Pathological cardiac hypertrophy represents a major risk factor for HF. The hypertrophic response is orchestrated in part through transcriptional alterations that ultimately modify cardiac function. Mediator is a multiprotein complex that bridges signal dependent transcription factors with basal transcriptional machinery. Cdk8 is a Mediator kinase demonstrated to have a complex role in transcriptional regulation through mechanisms involving both transcriptional activation and inhibition. We have previously demonstrated that cardiac-specific overexpression of Cdk8 results in eccentric cardiac hypertrophy resulting in HF. The studies presented herein demonstrate that Cdk8 activity is induced in numerous cardiac hypertrophy models. To specifically assess the role of Cdk8 kinase activity in regulating hypertrophic transcriptional programs, Cdk8 kinase inhibitors, Senexin A and CCT251545, were utilized in an in vitro neonatal rat cardiomyocyte (NRCM) hypertrophy model. Inhibition of Cdk8 activity resulted in blunted cardiomyocyte hypertrophy and altered regulation of hypertrophic transcriptional programs. These studies demonstrate a role for Cdk8 activity in transcriptional regulation of gene programs associated with pathological cardiac remodeling that ultimately drives HF.
Regulation of cardiac transcription by thyroid hormone and Med13
Thyroid hormone (TH) is a key regulator of transcriptional homeostasis in the heart. While hypothyroidism is known to result in adverse cardiac effects, the molecular mechanisms that modulate TH signaling are not completely understood. Mediator complex protein, Med13, was previously demonstrated to repress numerous thyroid receptor (TR) response genes in the heart. Further, cardiac-specific overexpression of Med13 in mice that were treated with propylthiouracil (PTU), an inhibitor of the biosynthesis of the active TH, triiodothyronine (T3), resulted in resistance to PTU-dependent decreases in cardiac contractility. Therefore, these studies aimed to determine if Med13 is necessary for the cardiac response to hypothyroidism. Here we demonstrate that Med13 expression is induced in the hearts of mice with hypothyroidism. To elucidate the role of Med13 in regulating gene transcription in response to TH signaling in cardiac tissue, we utilized an unbiased RNA sequencing approach to define the TH-dependent alterations in gene expression in wild-type mice or those with a cardiac-specific deletion in Med13 (Med13cKO). Mice were fed a diet of PTU to induce a hypothyroid state or normal chow for either 4 or 16 weeks, and an additional group of mice on a PTU diet were treated acutely with T3 to re-establish a euthyroid state. Echocardiography revealed that wild-type mice had a decreased heart rate in response to PTU with a trend toward a reduced cardiac ejection fraction. Furthermore, loss of cardiac Med13 resulted in upregulation of genes associated with cardiac fibrosis and inflammation following PTU treatment. Notably, cardiomyocyte-specific deletion of Med13 exacerbated cardiac dysfunction. Collectively, these studies reveal cardiac transcriptional pathways regulated in response to hypothyroidism and re-establishment of a euthyroid state and define molecular pathways that are regulated by Med13 in response to TH signaling.