Abstract 15063: The Atf6-inducible Selenoprotein, Vimp, Regulates Proteostasis and Pressure Overload-induced Heart Failure

Introduction: Cardiac hypertrophy is associated with an increase in protein synthesis, which must coordinate with protein folding and degradation to allow for homeostatic growth without affecting the functional integrity of cardiac myocytes (i.e. proteostasis). There is a gap in our understanding of how proteostasis becomes imbalanced during chronic pathological hypertrophy. Hypothesis: The objective of this study to examine the mechanisms regulating the protein degradation aspect of proteostasis, ER associated degradation (ERAD), by focusing on the role in cardiac hypertrophy of the unique selenoprotein and functional initiator of the ERAD molecular machinery, VIMP, which is induced in cardiac pathology by the nodal proteostasis transcription factor, ATF6. Methods and Results: VIMP was increased in failing human and in mouse hearts subjected to TAC-induced heart failure in an ATF6-dependent manner. AAV9-shRNA-mediated VIMP knockdown in the heart dramatically suppressed cardiac hypertrophy and staved off decompensation and heart failure, while AAV9-mediated VIMP overexpression exacerbated cardiac decompensation in response to TAC, an effect not observed with overexpression of an ERAD-null mutant of VIMP. Unexpectedly, VIMP knockdown enhanced ERAD-mediated degradation of the cytosolic pro-hypertrophic kinase, serum/glucocorticoid-regulated kinase 1, SGK1. Furthermore, AAV9-mediated overexpression of SGK1 negated the beneficial effects of VIMP knockdown; knockdown of the SGK1-binding protein, GILZ enhanced ERAD-mediated degradation of SGK1 and blunted cardiac hypertrophy. A peptide inhibiting the SGK1-GILZ interaction enhanced the ERAD-mediated degradation of SGK1 and ameliorated agonist-induced cardiac myocyte hypertrophy. Conclusions: Here we studied ERAD for the first time in any organ, in vivo , finding that an ATF6-inducible selenoprotein, VIMP, regulates the ERAD-dependent degradation of SGK1, thus accelerating TAC-induced cardiac decompensation and heart failure.

Impact of hyperthyroidism on cardiac hypertrophy

The cardiac growth process (hypertrophy) is a crucial phenomenon conserved across a wide array of species and is critically involved in the maintenance of cardiac homeostasis. This process enables an organism to adapt to changes in systemic demand and occurs due to a plethora of responses, depending on the type of signal or stimuli received. The growth of cardiac muscle cells in response to environmental conditions depends on the type, strength and duration of stimuli, and results in adaptive physiological responses or non-adaptive pathological responses. Thyroid hormones (TH) have a direct effect on the heart and induce a cardiac hypertrophy phenotype, which may evolve to heart failure. In this review, we summarize the literature on TH function in the heart by presenting results from experimental studies. We discuss the mechanistic aspects of TH associated with cardiac myocyte hypertrophy, increased cardiac myocyte contractility and electrical remodeling, as well as the associated signaling pathways. In addition to classical crosstalk with the sympathetic nervous system (SNS), emerging work pointing to the new endocrine interaction between TH and the renin-angiotensin system (RAS) is also explored. Given the inflammatory potential of the angiotensin II peptide, this new interaction may open the door for new therapeutic approaches which target the key mechanisms responsible for TH-induced cardiac hypertrophy.

Golgi localized β1-adrenergic receptors stimulate Golgi PI4P hydrolysis by PLCε to regulate cardiac hypertrophy

Increased adrenergic tone resulting from cardiovascular stress leads to development of heart failure, in part, through chronic stimulation of β1 adrenergic receptors (βARs) on cardiac myocytes. Blocking these receptors is part of the basis for β-blocker therapy for heart failure. Recent data demonstrate that G protein-coupled receptors (GPCRs), including βARs, are activated intracellularly, although the biological significance is unclear. Here we investigated the functional role of Golgi βARs in rat cardiac myocytes and found they activate Golgi localized, prohypertrophic, phosphoinositide hydrolysis, that is not accessed by cell surface βAR stimulation. This pathway is accessed by the physiological neurotransmitter norepinephrine (NE) via an Oct3 organic cation transporter. Blockade of Oct3 or specific blockade of Golgi resident β1ARs prevents NE dependent cardiac myocyte hypertrophy. This clearly defines a pathway activated by internal GPCRs in a biologically relevant cell type and has implications for development of more efficacious β-blocker therapies.

Golgi localized β1-adrenergic receptors stimulate Golgi PI4P hydrolysis by PLCε to regulate cardiac hypertrophy

Increased adrenergic tone resulting from cardiovascular stress leads to development of heart failure, in part, through chronic stimulation of β1 adrenergic receptors (βARs) on cardiac myocytes. Blocking these receptors is part of the basis for β-blocker therapy for heart failure. Recent data demonstrate that G protein-coupled receptors (GPCRs), including βARs, are activated intracellularly, although the biological significance is unclear. Here we investigated the functional role of Golgi βARs in cardiac myocytes and found they activate Golgi localized, prohypertrophic, phosphoinositide hydrolysis, that is not accessed by cell surface βAR stimulation. This pathway is accessed by the physiological neurotransmitter norepinephrine (NE) via an Oct3 organic cation transporter. Blockade of Oct3 or specific blockade of Golgi resident β1ARs prevents NE dependent cardiac myocyte hypertrophy. This clearly defines a physiological pathway activated by internal GPCRs in a biologically relevant cell type and has implications for development of more efficacious β-blocker therapies.

Variation in stiffness regulates cardiac myocyte hypertrophy via signaling pathways

Much diseased human myocardial tissue is fibrotic and stiff, which increases the work that the ventricular myocytes must perform to maintain cardiac output. The hypothesis tested is that the increased load due to greater stiffness of the substrata drives sarcomere assembly of cells, thus strengthening them. Neonatal rat ventricular myocytes (NRVM) were cultured on polyacrylamide or polydimethylsiloxane substrates with stiffness of 10 kPa, 100 kPa, or 400 kPa, or glass with stiffness of 61.9 GPa. Cell size increased with stiffness. Two signaling pathways were explored, phosphorylation of focal adhesion kinase (p-FAK) and lipids by phosphatidylinositol 4,5-bisphosphate (PIP2). Subcellular distributions of both were determined in the sarcomeric fraction by antibody localization, and total amounts were measured by Western or dot blotting, respectively. More p-FAK and PIP2 distributed to the sarcomeres of NRVM grown on stiffer substrates. Actin assembly involves the actin capping protein Z (CapZ). Both actin and CapZ dynamic exchange were significantly increased on stiffer substrates when assessed by fluorescence recovery after photobleaching (FRAP) of green fluorescent protein tags. Blunting of actin FRAP by FAK inhibition implicates linkage from mechano-signalling pathways to cell growth. Thus, increased stiffness of cardiac disease can be modeled with polymeric materials to understand how the microenvironment regulates cardiac hypertrophy.