Substrate stiffness modulates cardiac fibroblast activation, senescence, and proinflammatory secretory phenotype

In vitro cultures of primary cardiac fibroblasts (CFs), the major extracellular matrix (ECM)-producing cells of the heart, are used to determine molecular mechanisms of cardiac fibrosis. However, the supraphysiologic stiffness of tissue culture polystyrene (TCPS) triggers the conversion of CFs into an activated myofibroblast-like state, and serial passage of the cells results in the induction of replicative senescence. These phenotypic switches confound the interpretation of experimental data obtained with cultured CFs. In an attempt to circumvent TCPS-induced activation and senescence of CFs, we used poly(ethylene glycol) (PEG) hydrogels as cell culture platforms with low and high stiffness formulations to mimic healthy and fibrotic hearts, respectively. Low hydrogel stiffness converted activated CFs into a quiescent state with a reduced abundance of α-smooth muscle actin (α-SMA)-containing stress fibers. Unexpectedly, lower substrate stiffness concomitantly augmented CF senescence, marked by elevated senescence-associated β-galactosidase (SA-β-Gal) activity and increased expression of p16 and p21, which are antiproliferative markers of senescence. Using dynamically stiffening hydrogels with phototunable cross-linking capabilities, we demonstrate that premature, substrate-induced CF senescence is partially reversible. RNA-sequencing analysis revealed widespread transcriptional reprogramming of CFs cultured on low-stiffness hydrogels, with a reduction in the expression of profibrotic genes encoding ECM proteins, and an attendant increase in expression of NF-κB-responsive inflammatory genes that typify the senescence-associated secretory phenotype (SASP). Our findings demonstrate that alterations in matrix stiffness profoundly impact CF cell state transitions, and suggest mechanisms by which CFs change phenotype in vivo depending on the stiffness of the myocardial microenvironment in which they reside.NEW & NOTEWORTHY Our findings highlight the advantages and pitfalls associated with culturing cardiac fibroblasts on hydrogels of varying stiffness. The findings also define stiffness-dependent signaling and transcriptional networks in cardiac fibroblasts.

Emerging epigenetic therapies of cardiac fibrosis and remodelling in heart failure: from basic mechanisms to early clinical development

Cardiovascular diseases and specifically heart failure (HF) impact global health and impose a significant economic burden on society. Despite current advances in standard of care, the risks for death and readmission of HF patients remain unacceptably high and new therapeutic strategies to limit HF progression are highly sought. In disease settings, persistent mechanical or neurohormonal stress to the myocardium triggers maladaptive cardiac remodelling, which alters cardiac function and structure at both the molecular and cellular levels. The progression and magnitude of maladaptive cardiac remodelling ultimately leads to the development of HF. Classical therapies for HF are largely protein-based and mostly are targeted to ameliorate the dysregulation of neuroendocrine pathways and halt adverse remodelling. More recently, investigation of novel molecular targets and the application of cellular therapies, epigenetic modifications, and regulatory RNAs has uncovered promising new avenues to address HF. In this review, we summarize the current knowledge on novel cellular and epigenetic therapies and focus on two non-coding RNA-based strategies that reached the phase of early clinical development to counteract cardiac remodelling and HF. The current status of the development of translating those novel therapies to clinical practice, limitations, and future perspectives are additionally discussed.

Chromatin Remodeling Drives Immune-Fibroblast Crosstalk in Heart Failure Pathogenesis

Chronic inflammation and tissue fibrosis are common stress responses that worsen organ function, yet the molecular mechanisms governing their crosstalk are poorly understood. In diseased organs, stress-induced changes in gene expression fuel maladaptive cell state transitions and pathological interaction between diverse cellular compartments. Although chronic fibroblast activation worsens dysfunction of lung, liver, kidney, and heart, and exacerbates many cancers, the stress-sensing mechanisms initiating the transcriptional activation of fibroblasts are not well understood. Here, we show that conditional deletion of the transcription co-activator Brd4 in Cx3cr1-positive myeloid cells ameliorates heart failure and is associated with a dramatic reduction in fibroblast activation. Analysis of single-cell chromatin accessibility and BRD4 occupancy in vivo in Cx3cr1-positive cells identified a large enhancer proximal to Interleukin-1 beta (Il1b), and a series of CRISPR deletions revealed the precise stress-dependent regulatory element that controlled expression of Il1b in disease. Secreted IL1B functioned non-cell autonomously to activate a p65/RELA-dependent enhancer near the transcription factor MEOX1, resulting in a profibrotic response in human cardiac fibroblasts. In vivo, antibody-mediated IL1B neutralization prevented stress-induced expression of MEOX1, inhibited fibroblast activation, and improved cardiac function in heart failure. The elucidation of BRD4-dependent crosstalk between a specific immune cell subset and fibroblasts through IL1B provides new therapeutic strategies for heart disease and other disorders of chronic inflammation and maladaptive tissue remodeling.

Inhibition of Eicosanoid Degradation Mitigates Fibrosis of the Heart

BACKGROUND

Organ fibrosis due to excessive production of extracellular matrix by resident fibroblasts is estimated to contribute to >45% of deaths in the Western world, including those due to cardiovascular diseases such as heart failure. Here, we screened for small molecule inhibitors with a common ability to suppress activation of fibroblasts across organ systems.

METHODS

High-content imaging of cultured cardiac, pulmonary, and renal fibroblasts was used to identify nontoxic compounds that blocked induction of markers of activation in response to the profibrotic stimulus, transforming growth factor-β1. SW033291, which inhibits the eicosanoid-degrading enzyme, 15-hydroxyprostaglandin dehydrogenase, was chosen for follow-up studies with cultured adult rat ventricular fibroblasts and human cardiac fibroblasts (CF), and for evaluation in mouse models of cardiac fibrosis and diastolic dysfunction. Additional mechanistic studies were performed with CFs treated with exogenous eicosanoids.

RESULTS

Nine compounds, including SW033291, shared a common ability to suppress transforming growth factor-β1-mediated activation of cardiac, pulmonary, and renal fibroblasts. SW033291 dose-dependently inhibited transforming growth factor-β1-induced expression of activation markers (eg, α-smooth muscle actin and periostin) in adult rat ventricular fibroblasts and normal human CFs, and reduced contractile capacity of the cells. Remarkably, the 15-hydroxyprostaglandin dehydrogenase inhibitor also reversed constitutive activation of fibroblasts obtained from explanted hearts from patients with heart failure. SW033291 blocked cardiac fibrosis induced by angiotensin II infusion and ameliorated diastolic dysfunction in an alternative model of systemic hypertension driven by combined uninephrectomy and deoxycorticosterone acetate administration. Mechanistically, SW033291-mediated stimulation of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase signaling was required for the compound to block CF activation. Of the 12 exogenous eicosanoids that were tested, only 12(S)-hydroxyeicosatetraenoic acid, which signals through the G protein-coupled receptor, GPR31, recapitulated the suppressive effects of SW033291 on CF activation.

CONCLUSIONS

Inhibition of degradation of eicosanoids, arachidonic acid-derived fatty acids that signal through G protein-coupled receptors, is a potential therapeutic strategy for suppression of pathological organ fibrosis. In the heart, we propose that 15-hydroxyprostaglandin dehydrogenase inhibition triggers CF-derived autocrine/paracrine signaling by eicosanoids, including 12(S)-hydroxyeicosatetraenoic acid, to stimulate extracellular signal-regulated kinase 1/2 and block conversion of fibroblasts into activated cells that secrete excessive amounts of extracellular matrix and contribute to heart failure pathogenesis.

Sex-specific responses to slow progressive pressure overload in a large animal model of HFpEF

Approximately 50% of all heart failure (HF) diagnoses can be classified as HF with preserved ejection fraction (HFpEF). HFpEF is more prevalent in females compared with males, but the underlying mechanisms are unknown. We previously showed that pressure overload (PO) in male felines induces a cardiopulmonary phenotype with essential features of human HFpEF. The goal of this study was to determine if slow progressive PO induces distinct cardiopulmonary phenotypes in females and males in the absence of other pathological stressors. Female and male felines underwent aortic constriction (banding) or sham surgery after baseline echocardiography, pulmonary function testing, and blood sampling. These assessments were repeated at 2 and 4 mo postsurgery to document the effects of slow progressive pressure overload. At 4 mo, invasive hemodynamic studies were also performed. Left ventricle (LV) tissue was collected for histology, myofibril mechanics, extracellular matrix (ECM) mass spectrometry, and single-nucleus RNA sequencing (snRNAseq). The induced pressure overload (PO) was not different between sexes. PO also induced comparable changes in LV wall thickness and myocyte cross-sectional area in both sexes. Both sexes had preserved ejection fraction, but males had a slightly more robust phenotype in hemodynamic and pulmonary parameters. There was no difference in LV fibrosis and ECM composition between banded male and female animals. LV snRNAseq revealed changes in gene programs of individual cell types unique to males and females after PO. Based on these results, both sexes develop cardiopulmonary dysfunction but the phenotype is somewhat less advanced in females.NEW & NOTEWORTHY We performed a comprehensive assessment to evaluate the effects of slow progressive pressure overload on cardiopulmonary function in a large animal model of heart failure with preserved ejection fraction (HFpEF) in males and females. Functional and structural assessments were performed at the organ, tissue, cellular, protein, and transcriptional levels. This is the first study to compare snRNAseq and ECM mass spectrometry of HFpEF myocardium from males and females. The results broaden our understanding of the pathophysiological response of both sexes to pressure overload. Both sexes developed a robust cardiopulmonary phenotype, but the phenotype was equal or a bit less robust in females.

HDAC6 modulates myofibril stiffness and diastolic function of the heart

Passive stiffness of the heart is determined largely by extracellular matrix and titin, which functions as a molecular spring within sarcomeres. Titin stiffening is associated with the development of diastolic dysfunction (DD), while augmented titin compliance appears to impair systolic performance in dilated cardiomyopathy. We found that myofibril stiffness was elevated in mice lacking histone deacetylase 6 (HDAC6). Cultured adult murine ventricular myocytes treated with a selective HDAC6 inhibitor also exhibited increased myofibril stiffness. Conversely, HDAC6 overexpression in cardiomyocytes led to decreased myofibril stiffness, as did ex vivo treatment of mouse, rat, and human myofibrils with recombinant HDAC6. Modulation of myofibril stiffness by HDAC6 was dependent on 282 amino acids encompassing a portion of the PEVK element of titin. HDAC6 colocalized with Z-disks, and proteomics analysis suggested that HDAC6 functions as a sarcomeric protein deacetylase. Finally, increased myofibril stiffness in HDAC6-deficient mice was associated with exacerbated DD in response to hypertension or aging. These findings define a role for a deacetylase in the control of myofibril function and myocardial passive stiffness, suggest that reversible acetylation alters titin compliance, and reveal the potential of targeting HDAC6 to manipulate the elastic properties of the heart to treat cardiac diseases.

Therapeutic targets for cardiac fibrosis: from old school to next-gen

Cardiovascular diseases remain the leading cause of death worldwide, with pathological fibrotic remodeling mediated by activated cardiac myofibroblasts representing a unifying theme across etiologies. Despite the profound contributions of myocardial fibrosis to cardiac dysfunction and heart failure, there currently exist limited clinical interventions that effectively target the cardiac fibroblast and its role in fibrotic tissue deposition. Exploration of novel strategies designed to mitigate or reverse myofibroblast activation and cardiac fibrosis will likely yield powerful therapeutic approaches for the treatment of multiple diseases of the heart, including heart failure with preserved or reduced ejection fraction, acute coronary syndrome, and cardiovascular disease linked to type 2 diabetes. In this Review, we provide an overview of classical regulators of cardiac fibrosis and highlight emerging, next-generation epigenetic regulatory targets that have the potential to revolutionize treatment of the expanding cardiovascular disease patient population.

Abstract P411: Sex-based Differences In A Feline Model Of Hfpef

Rationale: Heart Failure with preserved Ejection Fraction (HFpEF) accounts for approximately 50% of all HF diagnoses with no FDA approved therapies. HFpEF is more prevalent in females versus males, but the mechanisms driving the development of HFpEF as a sex-based disorder are not well understood. We have recently shown that slow progressive pressure overload (PO) in male felines induces a HFpEF phenotype but have not investigated the differences in response to the same physiological stress in females.

Hypothesis: Females will develop a phenotype that is distinct from males in response to PO.

Methods and Results: Male (m) and female (f) domestic short felines (age 2mo) underwent either a sham procedure (m: n=7; f: n=7) or aortic constriction (m: n=11; f: n=10) using a customized pre-shaped band. At baseline (prior to surgery), there was no difference in body weight between groups and echocardiography revealed no significant difference in the ratio of left atrium to aortic root (LA/Ao), LA ejection fraction (LA EF), left ventricle (LV) ejection fraction, LV wall-thickness, and E/A ratio. At 4mo post-surgery, both males and females developed cardiac dysfunction. Females gained significantly less weight than males throughout the study. Despite the size difference, both sexes developed comparable LV wall thickness and changes in E/A ratio vs. sham groups. There was no change in LV EF. Furthermore, there was a decrease in LA EF and increased LA/Ao, indicating LA dysfunction and enlargement. Invasive hemodynamics at 4mo post-surgery showed no differences between sexes for the systolic pressure gradient generated by the aortic banding. Banded males had a significantly higher LV end-diastolic pressure vs. banded females, but there was a trend towards prolongation of tau and lower dp/dt min in banded females, reflective of worse active relaxation. Both sexes had comparable dP/dt max . There were no differences between banded males and females in heart weight to body weight or cardiomyocyte cross-sectional area.

Conclusion: Despite similar pressure gradients as a result of PO and the development of similar cardiac hypertrophy between sexes and a higher LVEDP in males, females had a trend towards worse relaxation. Other causes of HFpEF may have sex-based differences.

A transcriptional switch governs fibroblast activation in heart disease

In diseased organs, stress-activated signalling cascades alter chromatin, thereby triggering maladaptive cell state transitions. Fibroblast activation is a common stress response in tissues that worsens lung, liver, kidney and heart disease, yet its mechanistic basis remains unclear1,2. Pharmacological inhibition of bromodomain and extra-terminal domain (BET) proteins alleviates cardiac dysfunction3-7, providing a tool to interrogate and modulate cardiac cell states as a potential therapeutic approach. Here we use single-cell epigenomic analyses of hearts dynamically exposed to BET inhibitors to reveal a reversible transcriptional switch that underlies the activation of fibroblasts. Resident cardiac fibroblasts demonstrated robust toggling between the quiescent and activated state in a manner directly correlating with BET inhibitor exposure and cardiac function. Single-cell chromatin accessibility revealed previously undescribed DNA elements, the accessibility of which dynamically correlated with cardiac performance. Among the most dynamic elements was an enhancer that regulated the transcription factor MEOX1, which was specifically expressed in activated fibroblasts, occupied putative regulatory elements of a broad fibrotic gene program and was required for TGFβ-induced fibroblast activation. Selective CRISPR inhibition of the single most dynamic cis-element within the enhancer blocked TGFβ-induced Meox1 activation. We identify MEOX1 as a central regulator of fibroblast activation associated with cardiac dysfunction and demonstrate its upregulation after activation of human lung, liver and kidney fibroblasts. The plasticity and specificity of BET-dependent regulation of MEOX1 in tissue fibroblasts provide previously unknown trans- and cis-targets for treating fibrotic disease.

HDAC Inhibition Reverses Preexisting Diastolic Dysfunction and Blocks Covert Extracellular Matrix Remodeling

BACKGROUND

Diastolic dysfunction (DD) is associated with the development of heart failure and contributes to the pathogenesis of other cardiac maladies, including atrial fibrillation. Inhibition of histone deacetylases (HDACs) has been shown to prevent DD by enhancing myofibril relaxation. We addressed the therapeutic potential of HDAC inhibition in a model of established DD with preserved ejection fraction.

METHODS

Four weeks after uninephrectomy and implantation with deoxycorticosterone acetate pellets, when DD was clearly evident, 1 cohort of mice was administered the clinical-stage HDAC inhibitor ITF2357/Givinostat. Echocardiography, blood pressure measurements, and end point invasive hemodynamic analyses were performed. Myofibril mechanics and intact cardiomyocyte relaxation were assessed ex vivo. Cardiac fibrosis was evaluated by picrosirius red staining and second harmonic generation microscopy of left ventricle (LV) sections, RNA sequencing of LV mRNA, mass spectrometry-based evaluation of decellularized LV biopsies, and atomic force microscopy determination of LV stiffness. Mechanistic studies were performed with primary rat and human cardiac fibroblasts.

RESULTS

HDAC inhibition normalized DD without lowering blood pressure in this model of systemic hypertension. In contrast to previous models, myofibril relaxation was unimpaired in uninephrectomy/deoxycorticosterone acetate mice. Furthermore, cardiac fibrosis was not evident in any mouse cohort on the basis of picrosirius red staining or second harmonic generation microscopy. However, mass spectrometry revealed induction in the expression of >100 extracellular matrix proteins in LVs of uninephrectomy/deoxycorticosterone acetate mice, which correlated with profound tissue stiffening based on atomic force microscopy. ITF2357/Givinostat treatment blocked extracellular matrix expansion and LV stiffening. The HDAC inhibitor was subsequently shown to suppress cardiac fibroblast activation, at least in part, by blunting recruitment of the profibrotic chromatin reader protein BRD4 (bromodomain-containing protein 4) to key gene regulatory elements.

CONCLUSIONS

These findings demonstrate the potential of HDAC inhibition as a therapeutic intervention to reverse existing DD and establish blockade of extracellular matrix remodeling as a second mechanism by which HDAC inhibitors improve ventricular filling. Our data reveal the existence of pathophysiologically relevant covert or hidden cardiac fibrosis that is below the limit of detection of histochemical stains such as picrosirius red, highlighting the need to evaluate fibrosis of the heart using diverse methodologies.

Cardiac Remodeling During Pregnancy With Metabolic Syndrome: Prologue of Pathological Remodeling

BACKGROUND

The heart undergoes physiological hypertrophy during pregnancy in healthy individuals. Metabolic syndrome (MetS) is now prevalent in women of child-bearing age and might add risks of adverse cardiovascular events during pregnancy. The present study asks if cardiac remodeling during pregnancy in obese individuals with MetS is abnormal and whether this predisposes them to a higher risk for cardiovascular disorders.

METHODS

The idea that MetS induces pathological cardiac remodeling during pregnancy was studied in a long-term (15 weeks) Western diet-feeding animal model that recapitulated features of human MetS. Pregnant female mice with Western diet (45% kcal fat)-induced MetS were compared with pregnant and nonpregnant females fed a control diet (10% kcal fat).

RESULTS

Pregnant mice fed a Western diet had increased heart mass and exhibited key features of pathological hypertrophy, including fibrosis and upregulation of fetal genes associated with pathological hypertrophy. Hearts from pregnant animals with WD-induced MetS had a distinct gene expression profile that could underlie their pathological remodeling. Concurrently, pregnant female mice with MetS showed more severe cardiac hypertrophy and exacerbated cardiac dysfunction when challenged with angiotensin II/phenylephrine infusion after delivery.

CONCLUSIONS

These results suggest that preexisting MetS could disrupt physiological hypertrophy during pregnancy to produce pathological cardiac remodeling that could predispose the heart to chronic disorders.

The black sheep of class IIa: HDAC7 SIKens the heart

Class IIa histone deacetylases (HDACs) repress cardiomyocyte hypertrophy through association with the prohypertrophic transcription factor (TF) myocyte enhancer factor-2 (MEF2). The four class IIa HDACs - HDAC4, -5, -7, and -9 - are subject to signal-dependent phosphorylation by members of the Ca2+/calmodulin-dependent protein kinase (CaMK) group. In response to stress, HDAC4, HDAC5, and HDAC9 undergo phosphorylation-induced nuclear export in cardiomyocytes, freeing MEF2 to stimulate progrowth genes; it was generally assumed that HDAC7 is also antihypertrophic. However, in this issue of the JCI, Hsu and colleagues demonstrate that, in sharp contrast to the other class IIa HDACs, HDAC7 is constitutively localized to the cardiomyocyte cytoplasm, where it promotes cardiac hypertrophy. Phosphorylation of HDAC7 by the CaMK group member salt-inducible kinase 1 (SIK1) stabilized the deacetylase, leading to increased expression of c-Myc, which in turn stimulated a pathological gene program. These unexpected findings highlight the SIK1/HDAC7 signaling axis as a promising target for the treatment of cardiac hypertrophy and heart failure.

Phosphorylation of Hsp20 Promotes Fibrotic Remodeling and Heart Failure

Cardiomyocyte-specific increases in phosphorylated Hsp20 (S16D-Hsp20) to levels similar to those observed in human failing hearts are associated with early fibrotic remodeling and depressed left ventricular function, symptoms which progress to heart failure and early death. The underlying mechanisms appear to involve translocation of phosphorylated Hsp20 to the nucleus and upregulation of interleukin (IL)-6, which subsequently activates cardiac fibroblasts in a paracrine fashion through transcription factor STAT3 signaling. Accordingly, treatment of S16D-Hsp20 mice with a rat anti-mouse IL-6 receptor monoclonal antibody (MR16-1) attenuated interstitial fibrosis and preserved cardiac function. These findings suggest that phosphorylated Hsp20 may be a potential therapeutic target in heart failure.

Pharmacological and Activated Fibroblast Targeting of Gβγ-GRK2 After Myocardial Ischemia Attenuates Heart Failure Progression

BACKGROUND

Cardiac fibroblasts are a critical cell population responsible for myocardial extracellular matrix homeostasis. Upon injury or pathological stimulation, these cells transform to an activated myofibroblast state and play a fundamental role in myocardial fibrosis and remodeling. Chronic sympathetic overstimulation, a hallmark of heart failure (HF), induces pathological signaling through G protein βγ (Gβγ) subunits and their interaction with G protein-coupled receptor kinase 2 (GRK2).

OBJECTIVES

This study investigated the hypothesis that Gβγ-GRK2 inhibition and/or ablation after myocardial injury would attenuate pathological myofibroblast activation and cardiac remodeling.

METHODS

The therapeutic potential of small molecule Gβγ-GRK2 inhibition, alone or in combination with activated fibroblast- or myocyte-specific GRK2 ablation-each initiated after myocardial ischemia-reperfusion (I/R) injury-was investigated to evaluate the possible salutary effects on post-I/R fibroblast activation, pathological remodeling, and cardiac dysfunction.

RESULTS

Small molecule Gβγ-GRK2 inhibition initiated 1 week post-injury was cardioprotective in the I/R model of chronic HF, including preservation of cardiac contractility and a reduction in cardiac fibrotic remodeling. Systemic small molecule Gβγ-GRK2 inhibition initiated 1 week post-I/R in cardiomyocyte-restricted GRK2 ablated mice (also post-I/R) still demonstrated significant cardioprotection, which suggested a potential protective role beyond the cardiomyocyte. Inducible ablation of GRK2 in activated fibroblasts (i.e., myofibroblasts) post-I/R injury demonstrated significant functional cardioprotection with reduced myofibroblast transformation and fibrosis. Systemic small molecule Gβγ-GRK2 inhibition initiated 1 week post-I/R provided little to no further protection in mice with ablation of GRK2 in activated fibroblasts alone. Finally, Gβγ-GRK2 inhibition significantly attenuated activation characteristics of failing human cardiac fibroblasts isolated from end-stage HF patients.

CONCLUSIONS

These findings suggested consideration of a paradigm shift in the understanding of the therapeutic role of Gβγ-GRK2 inhibition in treating HF and the potential therapeutic role for Gβγ-GRK2 inhibition in limiting pathological myofibroblast activation, interstitial fibrosis, and HF progression.

Abstract 422: Small Molecule and Activated Fibroblast Targeting of the Gβγ-GRK2 Interface After Myocardial Ischemia Attenuates Heart Failure Progression

Cardiac fibroblasts are a critical cell population responsible for myocardial extracellular matrix homeostasis. Upon injury or pathologic stimulation, these cells transform to an activated myofibroblast state and play a fundamental role in myocardial fibrosis and remodeling. Chronic sympathetic overstimulation, a hallmark of heart failure, induces pathologic signaling through G protein βγ subunits and their interaction with G protein-coupled receptor kinase 2 (GRK2). We hypothesized that Gβγ-GRK2 inhibition/ablation after myocardial injury would attenuate pathologic myofibroblast activation and cardiac remodeling. The therapeutic potential of small molecule Gβγ-GRK2 inhibition alone or in combination with activated fibroblast- or myocyte-specific GRK2 ablation, each initiated after myocardial ischemia/reperfusion (I/R) injury, was investigated to evaluate possible salutary effects on post-I/R fibroblast activation, pathologic remodeling and cardiac function. Small molecule Gβγ-GRK2 inhibition initiated one week post-injury was cardioprotective in the I/R model of chronic heart failure, including preservation of cardiac contractility and reduction in cardiac fibrotic remodeling. Systemic small molecule Gβγ-GRK2 inhibition initiated one week post-I/R in cardiomyocyte-restricted GRK2 ablated mice (also post-I/R) demonstrated additional cardioprotection, suggesting a potential protective role beyond the cardiomyocyte. Inducible ablation of GRK2 in activated fibroblasts (i.e. myofibroblasts) post-I/R injury demonstrated significant functional cardioprotection with reduced myofibroblast transformation and fibrosis. Systemic small molecule Gβγ-GRK2 inhibition initiated one week post-I/R provided little to no further protection in mice with ablation of GRK2 in activated fibroblasts alone. Finally, Gβγ-GRK2 inhibition significantly attenuated activation characteristics of failing human cardiac fibroblasts isolated from end stage heart failure patients. These findings suggest a potential therapeutic role for Gβγ-GRK2 inhibition in limiting pathologic myofibroblast activation, interstitial fibrosis and heart failure progression.

G Protein-Coupled Receptor-G-Protein βγ-Subunit Signaling Mediates Renal Dysfunction and Fibrosis in Heart Failure

Development of CKD secondary to chronic heart failure (CHF), known as cardiorenal syndrome type 2 (CRS2), clinically associates with organ failure and reduced survival. Heart and kidney damage in CRS2 results predominantly from chronic stimulation of G protein-coupled receptors (GPCRs), including adrenergic and endothelin (ET) receptors, after elevated neurohormonal signaling of the sympathetic nervous system and the downstream ET system, respectively. Although we and others have shown that chronic GPCR stimulation and the consequent upregulated interaction between the G-protein βγ-subunit (Gβγ), GPCR-kinase 2, and β-arrestin are central to various cardiovascular diseases, the role of such alterations in kidney diseases remains largely unknown. We investigated the possible salutary effect of renal GPCR-Gβγ inhibition in CKD developed in a clinically relevant murine model of nonischemic hypertrophic CHF, transverse aortic constriction (TAC). By 12 weeks after TAC, mice developed CKD secondary to CHF associated with elevated renal GPCR-Gβγ signaling and ET system expression. Notably, systemic pharmacologic Gβγ inhibition by gallein, which we previously showed alleviates CHF in this model, attenuated these pathologic renal changes. To investigate a direct effect of gallein on the kidney, we used a bilateral ischemia-reperfusion AKI mouse model, in which gallein attenuated renal dysfunction, tissue damage, fibrosis, inflammation, and ET system activation. Furthermore, in vitro studies showed a key role for ET receptor-Gβγ signaling in pathologic fibroblast activation. Overall, our data support a direct role for GPCR-Gβγ in AKI and suggest GPCR-Gβγ inhibition as a novel therapeutic approach for treating CRS2 and AKI.

Cardiac Fibrosis: The Fibroblast Awakens

Myocardial fibrosis is a significant global health problem associated with nearly all forms of heart disease. Cardiac fibroblasts comprise an essential cell type in the heart that is responsible for the homeostasis of the extracellular matrix; however, upon injury, these cells transform to a myofibroblast phenotype and contribute to cardiac fibrosis. This remodeling involves pathological changes that include chamber dilation, cardiomyocyte hypertrophy and apoptosis, and ultimately leads to the progression to heart failure. Despite the critical importance of fibrosis in cardiovascular disease, our limited understanding of the cardiac fibroblast impedes the development of potential therapies that effectively target this cell type and its pathological contribution to disease progression. This review summarizes current knowledge regarding the origins and roles of fibroblasts, mediators and signaling pathways known to influence fibroblast function after myocardial injury, as well as novel therapeutic strategies under investigation to attenuate cardiac fibrosis.

Abstract 319: Small Molecule Gβγ Inhibition Attenuates Cardiac Fibroblast Inflammatory and Pro-Fibrotic Signaling

Heart failure (HF) is a devastating disease characterized by chamber remodeling, interstitial fibrosis and reduced ventricular compliance. Prolonged sympathetic overstimulation promotes excess signaling through G-protein Gβγ subunits and ultimately results in pathologic GRK2-mediated β-adrenergic receptor (β-AR) downregulation. We have recently demonstrated the therapeutic potential of the small molecule Gβγ-GRK2 inhibitor Gallein in limiting HF progression. Pathologic activation of the cardiac fibroblast (CF) induces the transition to a myofibroblast phenotype, which plays a fundamental role in myocardial fibrosis and remodeling. We hypothesized that Gβγ-GRK2 inhibition plays an important functional role in the CF to attenuate pathologic CF activation, inflammation and interstitial fibrosis.

To explore the effect of Gβγ-GRK2 inhibition on inflammation and pro-fibrotic signaling, mice were subjected to 7 days of transverse aortic constriction, a pressure-overload model of HF. In addition to the attenuation in overall cardiac hypertrophy, animals treated with Gallein displayed reduced expression of pro-inflammatory cytokines, including macrophage inflammatory protein 1 alpha (MIP-1α) and MIP-1β, along with Interleukin-6, as assessed by qPCR. Gallein-treated animals also exhibited diminished pro-fibrotic signaling, as evidenced by a reduction in the expression of TGFβ, a major driver of myocardial fibrosis, and decreased expression of collagen. To recapitulate these findings in vitro, primary adult mouse ventricular fibroblasts were pathologically stimulated using Isoproterenol (ISO, β-AR agonist) or Angiotensin II and treated +/- Gallein for 24 hours. CFs treated with Gallein displayed an analogous reduction in the expression of these pro-inflammatory cytokines and collagen.

In summary, these data suggest a potential therapeutic role for small molecule Gβγ-GRK2 inhibition in limiting pathologic myofibroblast activation, inflammation and interstitial fibrosis. We believe this fibroblast-targeted approach will lead to the refinement of existing targets and compounds, and possibly the generation of novel therapeutics for the treatment of HF.

Abstract 293: Role Of G-protein Coupled Receptor Signaling In Cardio-renal Injury

The kidneys play an important role in cardiovascular disease (CVD), where renal co-morbidities accompany CVD in a large proportion of patients thus complicating their treatment regimen. Moreover, the incidence of acute renal injury after cardiac surgery plays an important role in disease progression. Emerging data suggest the importance of understanding the mechanisms of cardio-renal injury and the development of novel therapies that can be safely used with cardiovascular and renal co-existing pathologies. Although the role of G-protein coupled receptors (GPCRs) in CVD has been broadly recognized, their role in renal injury remains poorly understood. We have found, in a chronic mouse model of heart failure, attenuated renal fibrosis and attenuated pathologic RAAS activation by the small molecule GPCR-Gβγ inhibitor “gallein”. To investigate the direct effects of GPCR-Gβγ inhibition on renal injury, we utilized an acute renal ischemia-reperfusion (RIR) mouse model. Gβγ inhibition by gallein pretreatment attenuated the histopathological profile of RIR, including attenuation of tubular hypertrophy, apoptosis, cast formation, and tissue Lipocalin2 expression. This was accompanied by attenuated inflammation, reflected by reduced CCL2 and ICAM1 gene expression and cellular infiltration, in addition to reduced Collagen III gene expression. These preliminary results suggest a promising protective role for Gβγ inhibition in renal injury and remodeling. Future mechanistic investigation of this possible protective effect will provide better understanding of the role of GPCR-Gβγ signaling in cardio-renal injury and remodeling and possible novel therapeutic targets.

Simultaneous adrenal and cardiac g-protein-coupled receptor-gβγ inhibition halts heart failure progression

OBJECTIVES

The authors propose simultaneous inhibition of Gβγ signaling in the heart and the adrenal gland as a novel therapeutic approach for heart failure (HF).

BACKGROUND

Elevated sympathetic nervous system activity is a salient characteristic of HF progression. It causes pathologic desensitization of β-adrenergic receptors (β-AR), facilitated predominantly through Gβγ-mediated signaling. The adrenal glands are key contributors to the chronically elevated plasma catecholamine levels observed in HF, where adrenal α2-AR feedback inhibitory function is impaired also through Gβγ-mediated signaling.

METHODS

We investigated the efficacy of a small molecule Gβγ inhibitor, gallein, in a clinically relevant, pressure-overload model of HF.

RESULTS

Daily gallein treatment (10 mg/kg/day), initiated 4 weeks after transverse aortic constriction, improved survival and cardiac function and attenuated cardiac remodeling. Mechanistically, gallein restored β-AR membrane density in cardiomyocytes, attenuated Gβγ-mediated G-protein-coupled receptor kinase 2-phosphoinositide 3-kinase γ membrane recruitment, and reduced Akt (protein kinase B) and glycogen synthase kinase 3β phosphorylation. Gallein also reduced circulating plasma catecholamine levels and catecholamine production in isolated mouse adrenal glands by restoring adrenal α2-AR feedback inhibition. In human adrenal endocrine tumors (pheochromocytoma), gallein attenuated catecholamine secretion, as well as G-protein-coupled receptor kinase 2 expression and membrane translocation.

CONCLUSIONS

These data suggest small molecule Gβγ inhibition as a systemic pharmacologic therapy for HF by simultaneously normalizing pathologic adrenergic/Gβγ signaling in both the heart and the adrenal gland. Our data also suggest important endocrine/cardiovascular interactions and a possible role for small molecule Gβγ inhibition in treating endocrine tumors such as pheochromocytoma, in addition to HF.

Abstract 183: Small Molecule Gβ? Inhibition Reduces Pathologic Activation of Cardiac Fibroblasts

Heart failure (HF) is a devastating disease characterized by cardiac hypertrophy, fibrosis and inflammation. Excess signaling through Gβγ subunits leads to chronic β-adrenergic receptor (β-AR) downregulation, mediated predominantly by GRK2 in complex with PI3Kγ. Our recent work has demonstrated the therapeutic potential of the small molecule Gβγ-GRK2 inhibitor Gallein in limiting HF progression. Chronic activation of cardiac fibroblasts (CF), critical yet underappreciated myocardial cells, is a key contributor to pathologic cardiac remodeling. We hypothesized that Gβγ-GRK2 inhibition may limit pathologic CF activation.

CFs were stimulated with Isoproterenol (Iso, β-AR agonist), AngII, or vehicle (V), +/- Gβγ inhibition for 24hr. Gallein treatment attenuated the induction of αSMA expression, a marker of pathologic CF activation, and two inflammatory cytokines, IL-1β and IL-6 in response to these pathologic stimuli (Iso, AngII), as assessed by real time PCR. This data suggest that Gallein treatment may reduce pathologic CF activation. Iso stimulation also enhances the phosphorylation of Akt, a kinase downstream of PI3Kγ known to be involved in cellular proliferation. Gβγ inhibition mitigated this induction, decreasing Akt phosphorylation >60% in response to Iso. This phenomenon was also observed in failing human CFs, in which Gallein decreased Akt phosphorylation >70%.

We have recently demonstrated that the protease-activated receptor 1 (PAR1), a GPCR we have implicated in cardiac hypertrophy, is transactivated via chronic β-AR stimulation by induction of MMP-13, a protease we have found to be elevated in HF. Recent data from our lab and others have demonstrated that PAR1 is the most abundantly expressed GPCR in CFs, and that its stimulation in CFs may be pathologic. Interestingly, Gβγ inhibition treatment reduced PAR1 cleavage and activation in response to chronic Iso.

In summary, small molecule Gβγ inhibition appears to reduce pathologic CF activation. The reduction in β-AR-mediated PAR1 cleavage reveals an alternative role for Gβγ inhibition in preventing CF activation and proliferation. These data suggest a potential therapeutic role for small molecule Gβγ inhibition in limiting pathologic CF activation and cardiac hypertrophy.

G protein-coupled receptor kinases in cardiovascular disease: why "where" matters

Cardiac function is mainly controlled by β-adrenergic receptors (β-ARs), members of the G protein-coupled receptor (GPCR) family. GPCR signaling and expression are tightly controlled by G protein-coupled receptor kinases (GRKs), which induce GPCR internalization and signal termination through phosphorylation. Reduced β-AR density and activity associated with elevated cardiac GRK expression and activity have been described in various cardiovascular diseases. Moreover, alterations in extracardiac GRKs have been observed in blood vessels, adrenal glands, kidneys, and fat cells. The broad tissue distribution of GPCRs and GRKs suggests that a keen appreciation of integrative physiology may drive future therapeutic development. In this review, we provide a brief summary of GRK isoforms, subcellular localization, and interacting partners that impinge directly or indirectly on the cardiovascular system. We also discuss GRK/GPCR interactions and their implications in cardiovascular pathophysiology.