Extracellular ligands stimulate cardiac growth and differentiation by activating a network of protein kinases that phosphorylate transcription factors and alter gene expression. Many of these mechanisms are resurrected in the damaged or failing heart in an attempt to compensate for contractile dysfunction. Our previous studies focused on the cellular actions of thrombin, a serine protease that is generated at sites of cardiac injury and proteolytically activates protease-activated receptor-1 (PAR-1), 3 a G protein-coupled receptor that activates a spectrum of effectors that contribute to cardiac fibroblast proliferation and cardiomyocyte hypertrophy (1). Certain aspects of PAR-1 signaling are cell-specific; PAR-1 activates ERK primarily via an epidermal growth factor receptor (EGFR) transactivation pathway in cardiac fibroblasts and a distinct pathway that does not require EGFR kinase activity in cardiomyocytes. Of note, the PAR-1 signaling pathway in cardiomyocytes triggers a form of cellular remodeling that resembles the changes observed in dilated cardiomyopathies (with pronounced cell elongation and relatively little increased cell width). This hypertrophic phenotype is morphologically distinct from the concentric hypertrophy induced by ␣ 1 -AR agonists such as norepinephrine (NE) or P. multocida toxin (PMT, a direct G␣ q agonist); NE and PMT induce very pronounced increases in overall cell size in association with enhanced sarcomeric organization and atrial natriuretic factor expression (2). cAMP response element-binding protein (CREB) is a bZip transcription factor that forms homo-or heterodimers with itself or with other CREB/ATF family members and binds to specific DNA elements (termed cAMP response elements or CREs) within the regulatory regions of CREB target genes. CREB has been implicated in the maintenance of normal ventricular structure and function; cardiac-specific overexpression of dominant-negative CREB leads to dilated cardiomyopathy and interstitial fibrosis (3). CREB also has been implicated in the electrophysiological remodeling that accompanies pacinginduced cardiac memory in dogs (4). CREB is regulated via phosphorylation at Ser 133 , which activates CREB-dependent gene transcription by recruiting a coactivator (CREB-binding protein, or CBP) to the promoters of CREB target genes.* This work was supported, in whole or in part, by National Institutes of Health Grants HL77860, HL-67101, and HL-28958. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Our data suggest that a lack of I(Ks) results in a greater dependence on I(Kr) for repolarization in neonates and is associated with exaggerated effects of I(Kr)-blockade on APD. However, APD prolongation alone is insufficient for expression of proarrhythmia, which also requires transmural dispersion of repolarization and EADs. The extent to which APD prolongation, transmural dispersion and EADs are manifested at various ages in the absence and presence of I(Kr) blocking drugs appears to be the ultimate determinant of proarrhythmia.

A transcriptionally induced change in epicardial I(Kr) contributes to the altered ventricular repolarization that characterizes CM.

Thrombin activates protease-activated receptor-1 (PAR-1) and engages signaling pathways that influence the growth and survival of cardiomyocytes as well as extracellular matrix remodeling by cardiac fibroblasts. This study examines the role of Shc proteins in PAR-1-dependent signaling pathways that influence ventricular remodeling. We show that thrombin increases p46Shc/p52Shc phosphorylation at and/or Tyr 317 phosphorylation). Importantly, p66Shc protein expression is detected in neonatal, but not adult, cardiomyocytes; p66Shc expression is induced (via a mechanism that requires protein kinase C and MEK activity) by Pasteurella multocida toxin, a G␣ q agonist that promotes cardiomyocyte hypertrophy. These results identify novel regulation of individual Shc isoforms in receptor-dependent pathways leading to cardiac hypertrophy and the transition to heart failure. The observations that p66Shc expression is induced by a G␣ q agonist and that PAR-1 activation leads to p66Shc-Ser 36 phosphorylation identifies p66Shc as a novel candidate hypertrophy-induced mediator of cardiomyocyte apoptosis and heart failure.Cardiac hypertrophy and the progression to heart failure is the result of a complex array of signaling events that lead to structural remodeling of the ventricle. The literature has focused primarily on signaling mechanisms that regulate cardiomyocyte growth and survival. However, cardiac fibroblasts (activated by fibrogenic growth factors and pro-inflammatory cytokines) also play an important role in influencing the architecture and mechanical behavior of the heart by regulating collagen matrix remodeling and by elaborating growth factors that promote cardiomyocyte hypertrophy. Extensive cardiac fibroblast-induced structural remodeling disrupts the normal transmission of electrical impulses, impairs contractile performance, and critically influences clinical heart failure syndromes.Protease-activated receptor-1 (PAR-1) 2 is a G protein-coupled receptor that is proteolytically activated by thrombin and other pro-inflammatory proteases. We previously demonstrated that chronic/persistent PAR-1 activation leads to cardiomyocyte hypertrophy and cardiac fibroblast proliferation (1). PAR-1-dependent growth responses are mediated by distinct signaling mechanisms in cardiomyocytes and cardiac fibroblasts. PAR-1 evokes a robust increase in ERK, p38-MAPK, and AKT and increases DNA synthesis via an epidermal growth factor receptor (EGFR) transactivation pathway in cardiac fibroblasts, whereas PAR-1 activates ERK (and induces only a relatively minor increase in AKT) via a mechanism that does not require EGFR kinase activity in cardiomyocytes. The contextual nature of PAR-1 signaling emphasizes the need to directly examine PAR-1 actions in physiologically relevant cardiac preparations.Shc proteins are molecular adapters that link surface receptors to mitogenic responses. Shc is expressed as three isoforms that are encoded by a single gene locus (2, 3). p46Shc and p52Shc are ubiquitous isoforms that arise through the use of ...

These results strengthen the hypothesis that down-regulation of CREB-mediated transcription underlies the attenuation of epicardial I(to) in LTCM. They also emphasize that ventricular pacing exerts effects at a subcellular level contributing to memory and conceivably to other forms of cardiac remodeling.