Free fatty acid receptor 4 in cardiac myocytes ameliorates ischemic cardiomyopathy

Aims

Free fatty acid receptor 4 (Ffar4) is a receptor for long-chain fatty acids that attenuates heart failure driven by increased afterload. Recent findings suggest that Ffar4 prevents ischemic injury in brain, liver, and kidney, and therefore, we hypothesized that Ffar4 would also attenuate cardiac ischemic injury.

Methods and Results

Using a mouse model of ischemia-reperfusion (I/R), we found that mice with systemic deletion of Ffar4 (Ffar4KO) demonstrated impaired recovery of left ventricular systolic function post-I/R with no effect on initial infarct size. To identify potential mechanistic explanations for the cardioprotective effects of Ffar4, we performed bulk RNAseq to compare the transcriptomes from wild-type (WT) and Ffar4KO infarcted myocardium 3-days post-I/R. In the Ffar4KO infarcted myocardium, gene ontology (GO) analyses revealed augmentation of glycosaminoglycan synthesis, neutrophil activation, cadherin binding, extracellular matrix, rho signaling, and oxylipin synthesis, but impaired glycolytic and fatty acid metabolism, cardiac repolarization, and phosphodiesterase activity. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated impaired AMPK signaling and augmented cellular senescence in the Ffar4KO infarcted myocardium. Interestingly, phosphodiesterase 6c (PDE6c), which degrades cGMP, was the most upregulated gene in the Ffar4KO heart. Further, the soluble guanylyl cyclase stimulator, vericiguat, failed to increase cGMP in Ffar4KO cardiac myocytes, suggesting increased phosphodiesterase activity. Finally, cardiac myocyte-specific overexpression of Ffar4 prevented systolic dysfunction post-I/R, defining a cardioprotective role of Ffa4 in cardiac myocytes.

Conclusions

Our results demonstrate that Ffar4 in cardiac myocytes attenuates systolic dysfunction post-I/R, potentially by attenuating oxidative stress, preserving mitochondrial function, and modulation of cGMP signaling.

Atrial Myopathy Quantified by Speckle-tracking Echocardiography in Mice

BACKGROUND

Emerging evidence suggests that atrial myopathy may be the underlying pathophysiology that explains adverse cardiovascular outcomes in heart failure (HF) and atrial fibrillation. Lower left atrial (LA) function (strain) is a key biomarker of atrial myopathy, but murine LA strain has not been described, thus limiting translational investigation. Therefore, the objective of this study was to characterize LA function by speckle-tracking echocardiography in mouse models of atrial myopathy.

METHODS

We used 3 models of atrial myopathy in wild-type male and female C57Bl6/J mice: (1) aged 16 to 17 months, (2) Ang II (angiotensin II) infusion, and (3) high-fat diet+Nω-nitro-L-arginine methyl ester (HF with preserved ejection fraction, HFpEF). LA reservoir, conduit, and contractile strain were measured using speckle-tracking echocardiography from a modified parasternal long-axis window. Left ventricular systolic and diastolic function, and global longitudinal strain were also measured. Transesophageal rapid atrial pacing was used to induce atrial fibrillation.

RESULTS

LA reservoir, conduit, and contractile strain were significantly reduced in aged, Ang II and HFpEF mice compared with young controls. There were no sex-based interactions. Left ventricular diastolic function and global longitudinal strain were lower in aged, Ang II and HFpEF, but left ventricular ejection fraction was unchanged. Atrial fibrillation inducibility was low in young mice (5%), moderately higher in aged mice (20%), and high in Ang II (75%) and HFpEF (83%) mice.

CONCLUSIONS

Using speckle-tracking echocardiography, we observed reduced LA function in established mouse models of atrial myopathy with concurrent atrial fibrillation inducibility, thus providing the field with a timely and clinically relevant platform for understanding the pathophysiology and discovery of novel treatment targets for atrial myopathy.

FFAR4 regulates cardiac oxylipin balance to promote inflammation resolution in HFpEF secondary to metabolic syndrome

Heart failure with preserved ejection fraction (HFpEF) is a complex clinical syndrome, but a predominant subset of HFpEF patients has metabolic syndrome (MetS). Mechanistically, systemic, nonresolving inflammation associated with MetS might drive HFpEF remodeling. Free fatty acid receptor 4 (Ffar4) is a GPCR for long-chain fatty acids that attenuates metabolic dysfunction and resolves inflammation. Therefore, we hypothesized that Ffar4 would attenuate remodeling in HFpEF secondary to MetS (HFpEF-MetS). To test this hypothesis, mice with systemic deletion of Ffar4 (Ffar4KO) were fed a high-fat/high-sucrose diet with L-NAME in their water to induce HFpEF-MetS. In male Ffar4KO mice, this HFpEF-MetS diet induced similar metabolic deficits but worsened diastolic function and microvascular rarefaction relative to WT mice. Conversely, in female Ffar4KO mice, the diet produced greater obesity but no worsened ventricular remodeling relative to WT mice. In Ffar4KO males, MetS altered the balance of inflammatory oxylipins systemically in HDL and in the heart, decreasing the eicosapentaenoic acid-derived, proresolving oxylipin 18-hydroxyeicosapentaenoic acid (18-HEPE), while increasing the arachidonic acid-derived, proinflammatory oxylipin 12-hydroxyeicosatetraenoic acid (12-HETE). This increased 12-HETE/18-HEPE ratio reflected a more proinflammatory state both systemically and in the heart in male Ffar4KO mice and was associated with increased macrophage numbers in the heart, which in turn correlated with worsened ventricular remodeling. In summary, our data suggest that Ffar4 controls the proinflammatory/proresolving oxylipin balance systemically and in the heart to resolve inflammation and attenuate HFpEF remodeling.

Loss of DNA repair mechanisms in cardiac myocytes induce dilated cardiomyopathy

Cardiomyopathy is a progressive disease of the myocardium leading to impaired contractility. Genotoxic cancer therapies are known to be potent drivers of cardiomyopathy, whereas causes of spontaneous disease remain unclear. To test the hypothesis that endogenous genotoxic stress contributes to cardiomyopathy, we deleted the DNA repair gene Ercc1 specifically in striated muscle using a floxed allele of Ercc1 and mice expressing Cre under control of the muscle-specific creatinine kinase (Ckmm) promoter or depleted systemically (Ercc1-/D mice). Ckmm-Cre+/- ;Ercc1-/fl mice expired suddenly of heart disease by 7 months of age. As young adults, the hearts of Ckmm-Cre+/- ;Ercc1-/fl mice were structurally and functionally normal, but by 6-months-of-age, there was significant ventricular dilation, wall thinning, interstitial fibrosis, and systolic dysfunction indicative of dilated cardiomyopathy. Cardiac tissue from the tissue-specific or systemic model showed increased apoptosis and cardiac myocytes from Ckmm-Cre+/- ;Ercc1-/fl mice were hypersensitive to genotoxins, resulting in apoptosis. p53 levels and target gene expression, including several antioxidants, were increased in cardiac tissue from Ckmm-Cre+/- ;Ercc1-/fl and Ercc1-/D mice. Despite this, cardiac tissue from older mutant mice showed evidence of increased oxidative stress. Genetic or pharmacologic inhibition of p53 attenuated apoptosis and improved disease markers. Similarly, overexpression of mitochondrial-targeted catalase improved disease markers. Together, these data support the conclusion that DNA damage produced endogenously can drive cardiac disease and does so mechanistically via chronic activation of p53 and increased oxidative stress, driving cardiac myocyte apoptosis, dilated cardiomyopathy, and sudden death.

Signaling through Free Fatty Acid Receptor 4 Attenuates Cardiometabolic Disease

A surge in the prevalence of obesity and metabolic syndrome, which promote systemic inflammation, underlies an increase in cardiometabolic disease. Free fatty acid receptor 4 is a nutrient sensor for long-chain fatty acids, like ω3-polyunsaturated fatty acids (ω3-PUFAs), that attenuates metabolic disease and resolves inflammation. Clinical trials indicate ω3-PUFAs are cardioprotective, and this review discusses the mechanistic links between ω3-PUFAs, free fatty acid receptor 4, and attenuation of cardiometabolic disease.

Epitope-tagged and phosphomimetic mouse models for investigating natriuretic peptide-stimulated receptor guanylyl cyclases

The natriuretic peptide receptors NPR1 and NPR2, also known as guanylyl cyclase A and guanylyl cyclase B, have critical functions in many signaling pathways, but much remains unknown about their localization and function in vivo. To facilitate studies of these proteins, we developed genetically modified mouse lines in which endogenous NPR1 and NPR2 were tagged with the HA epitope. To investigate the role of phosphorylation in regulating NPR1 and NPR2 guanylyl cyclase activity, we developed mouse lines in which regulatory serines and threonines were substituted with glutamates, to mimic the negative charge of the phosphorylated forms (NPR1-8E and NPR2-7E). Here we describe the generation and applications of these mice. We show that the HA-NPR1 and HA-NPR2 mice can be used to characterize the relative expression levels of these proteins in different tissues. We describe studies using the NPR2-7E mice that indicate that dephosphorylation of NPR2 transduces signaling pathways in ovary and bone, and studies using the NPR1-8E mice that indicate that the phosphorylation state of NPR1 is a regulator of heart, testis, and adrenal function.

Free fatty acid receptor 4 responds to endogenous fatty acids to protect the heart from pressure overload

AIMS

Free fatty acid receptor 4 (Ffar4) is a G-protein-coupled receptor for endogenous medium-/long-chain fatty acids that attenuates metabolic disease and inflammation. However, the function of Ffar4 in the heart is unclear. Given its putative beneficial role, we hypothesized that Ffar4 would protect the heart from pathologic stress.

METHODS AND RESULTS

In mice lacking Ffar4 (Ffar4KO), we found that Ffar4 is required for an adaptive response to pressure overload induced by transverse aortic constriction (TAC), identifying a novel cardioprotective function for Ffar4. Following TAC, remodelling was worsened in Ffar4KO hearts, with greater hypertrophy and contractile dysfunction. Transcriptome analysis 3-day post-TAC identified transcriptional deficits in genes associated with cytoplasmic phospholipase A2α signalling and oxylipin synthesis and the reduction of oxidative stress in Ffar4KO myocytes. In cultured adult cardiac myocytes, Ffar4 induced the production of the eicosapentaenoic acid (EPA)-derived, pro-resolving oxylipin 18-hydroxyeicosapentaenoic acid (18-HEPE). Furthermore, the activation of Ffar4 attenuated cardiac myocyte death from oxidative stress, while 18-HEPE rescued Ffar4KO myocytes. Systemically, Ffar4 maintained pro-resolving oxylipins and attenuated autoxidation basally, and increased pro-inflammatory and pro-resolving oxylipins, including 18-HEPE, in high-density lipoproteins post-TAC. In humans, Ffar4 expression decreased in heart failure, while the signalling-deficient Ffar4 R270H polymorphism correlated with eccentric remodelling in a large clinical cohort paralleling changes observed in Ffar4KO mice post-TAC.

CONCLUSION

Our data indicate that Ffar4 in cardiac myocytes responds to endogenous fatty acids, reducing oxidative injury, and protecting the heart from pathologic stress, with significant translational implications for targeting Ffar4 in cardiovascular disease.

Guanylyl cyclase-A phosphorylation decreases cardiac hypertrophy and improves systolic function in male, but not female, mice

Atrial natriuretic peptide (NP) and BNP increase cGMP, which reduces blood pressure and cardiac hypertrophy by activating guanylyl cyclase (GC)-A, also known as NPR-A or Npr1. Although GC-A is highly phosphorylated, and dephosphorylation inactivates the enzyme, the significance of GC-A phosphorylation to heart structure and function remains unknown. To identify in vivo processes that are regulated by GC-A phosphorylation, we substituted glutamates for known phosphorylation sites to make GC-A^8E/8E mice that express an enzyme that cannot be inactivated by dephosphorylation. GC-A activity, but not protein, was increased in heart and kidney membranes from GC-A^8E/8E mice. Activities were threefold higher in female compared to male cardiac ventricles. Plasma cGMP and testosterone were elevated in male and female GC-A^8E/8E mice, but aldosterone was only increased in mutant male mice. Plasma and urinary creatinine concentrations were decreased and increased, respectively, but blood pressure and heart rate were unchanged in male GC-A^8E/8E mice. Heart weight to body weight ratios for GC-A^8E/8E male, but not female, mice were 12% lower with a 14% reduction in cardiomyocyte cross-sectional area. Subcutaneous injection of fsANP, a long-lived ANP analog, increased plasma cGMP and decreased aldosterone in male GC-A^WT/WT and GC-A^8E/8E mice at 15 min, but only GC-A^8E/8E mice had elevated levels of plasma cGMP and aldosterone at 60 min. fsANP reduced ventricular ERK1/2 phosphorylation to a greater extent and for a longer time in the male mutant compared to WT mice. Finally, ejection fractions were increased in male but not female hearts from GC-A^8E/8E mice. We conclude that increased phosphorylation-dependent GC-A activity decreases cardiac ERK activity, which results in smaller male hearts with improved systolic function.

Abstract 255: Novel Role for Free Fatty Acid Receptor 4 in Response to Pathologic Pressure Overload-Induced Heart Failure in Mice

Free fatty acid receptor 4 (Ffar4, GPR120) is a GPCR for long chain fatty acids, including omega-3 polyunsaturated fatty acids (ω3-PUFAs). Previously, we demonstrated that eicosapentaenoic acid (EPA), an ω3-PUFA, prevents interstitial fibrosis and contractile dysfunction in pressure overload heart failure (transverse aortic constriction, TAC), but EPA was not incorporated into cardiac myocytes or fibroblasts, the traditional mechanism of action. Therefore, we hypothesized that Ffar4 is necessary for the anti-fibrotic and cardioprotective effects of EPA. In fact, we previously found that in primary cardiac fibroblasts, Ffar4 was sufficient and necessary to prevent TGFβ1-induced fibrosis. Here, we were surprised to find that Ffar4 also has important ω3-independent effects in cardiac myocytes in vivo . In mice with systemic deletion of Ffar4 (Ffar4KO mice) on a standard, corn oil diet (no EPA supplementation), TAC induced significantly more cardiac hypertrophy and more systolic (ejection fraction) and diastolic dysfunction (E/A ratio), but without excess fibrosis after 4 weeks. Mechanistically, transcriptome analysis in cardiac myocytes 3 days post-TAC indicated a deficiency of transcription in Ffar4KO myocytes. Sorting based on gene ontology indicated that genes functionally categorized as cell death and inflammation, including ilrn1 , hmox1 , Il33 , and ptgs2 (cox2) were not induced in the Ffar4KO myocytes. Ffar4 also activates cytoplasmic phospholipase A2 (cPLA2) inducing the production of oxylipins, and systemic deletion of cPLA2 induces exaggerated hypertrophy during development and following TAC. TAC increased cPLA2 expression in WT, but not Ffar4KO cardiac myocytes and oxylipin production was reduced in Ffar4KO mice. Finally, we confirmed that Ffar4 is expressed in human heart and down-regulated in heart failure. In summary, our data identify a novel, ω3-independent cardioprotective role for Ffar4 in cardiac myocytes.

Abstract 526: Free Fatty Acid Receptor 4 is Required for an Adaptive Response to Pathologic Pressure Overload-induced Heart Failure in Mice

Background: Our previous results indicate that eicosapentaenoic acid (EPA) prevents interstitial fibrosis and pathologic remodeling in the transverse aortic constriction (TAC) model of pressure overload induced heart failure in mice. However, EPA supplementation did not lead to incorporation into cardiac myocytes or fibroblasts in vivo , a traditional mechanism of action. Interestingly, we found that free fatty acid receptor 4 (Ffar4), a G-protein coupled receptor for long-chain fatty acids, was expressed in cardiac myocytes and fibroblasts. In primary cultures of cardiac fibroblasts, Ffar4 was sufficient and required to prevent TGFβ1-induced fibrosis, suggesting Ffar4 might mediate EPA cardioprotection.

Methods: To understand the function of Ffar4 in the heart in vivo , we examined the response to TAC in male and female Ffar4 knockout mice (Ffar4KO) under basal conditions without EPA supplementation.

Results: In male Ffar4KO mice, TAC induced a worsened dilated cardiomyopathy relative to wild-type (WT) mice. Specifically, in male Ffar4KO mice, TAC produced more hypertrophy (Heart weight: WT: 175±8 mg, n=14; Ffar4KO: 223±14 mg, n=10; P <0.05), and generated more severe systolic and diastolic dysfunction (Ejection Fraction: WT: 42±3%, n=14; Ffar4KO: 28±4%, n=10; P <0.05) (E/A ratio: WT: 2.0±0.2, n=12; Ffar4KO: 3.7±0.5, n=9; P <0.05). However, TAC induced similar levels of interstitial fibrosis in male Ffar4 KO and WT mice (WT: 9.6±1.8%, n=14; Ffar4KO: 7.4±1.1%, n=10; ns ). While TAC worsened outcomes in male Ffar4KO mice, TAC in Ffar4 KO female mice produced no significant differences in the degree of hypertrophy or contractile dysfunction relative to WT mice.

Discussion: Our results indicate that Ffar4 is required for an adaptive response to pathologic pressure overload in male mice. The lack of an exaggerated fibrotic response was surprising given our previous results indicating that in cardiac fibroblasts, Ffar4 prevents TGFβ1-induced fibrosis. This might indicate that Ffar4 signaling is minimal in fibroblasts under conditions of low omega-3 fatty acid levels. This result also suggests that the worsened contractile dysfunction in Ffar4KO mice is unrelated to fibrosis, but involves a direct effect in cardiac myocytes.

Subcellular compartmentalization of proximal Gαq-receptor signaling produces unique hypertrophic phenotypes in adult cardiac myocytes

G protein-coupled receptors that signal through Gα_q (G_q receptors), such as α₁-adrenergic receptors (α₁-ARs) or angiotensin receptors, share a common proximal signaling pathway that activates phospholipase Cβ1 (PLCβ1), which cleaves phosphatidylinositol 4,5-bisphosphate (PIP₂) to produce inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol. Despite these common proximal signaling mechanisms, G_q receptors produce distinct physiological responses, yet the mechanistic basis for this remains unclear. In the heart, G_q receptors are thought to induce myocyte hypertrophy through a mechanism termed excitation-transcription coupling, which provides a mechanistic basis for compartmentalization of calcium required for contraction versus IP₃-dependent intranuclear calcium required for hypertrophy. Here, we identified subcellular compartmentalization of G_q-receptor signaling as a mechanistic basis for unique G_q receptor-induced hypertrophic phenotypes in cardiac myocytes. We show that α₁-ARs co-localize with PLCβ1 and PIP₂ at the nuclear membrane. Further, nuclear α₁-ARs induced intranuclear PLCβ1 activity, leading to histone deacetylase 5 (HDAC5) export and a robust transcriptional response (i.e. significant up- or down-regulation of 806 genes). Conversely, we found that angiotensin receptors localize to the sarcolemma and induce sarcolemmal PLCβ1 activity, but fail to promote HDAC5 nuclear export, while producing a transcriptional response that is mostly a subset of α₁-AR-induced transcription. In summary, these results link G_q-receptor compartmentalization in cardiac myocytes to unique hypertrophic transcription. They suggest a new model of excitation-transcription coupling in adult cardiac myocytes that accounts for differential G_q-receptor localization and better explains distinct physiological functions of G_q receptors.

EPA, not DHA, prevents fibrosis in pressure overload-induced heart failure: potential role of free fatty acid receptor 4

Heart failure with preserved ejection fraction (HFpEF) is half of all HF, but standard HF therapies are ineffective. Diastolic dysfunction, often secondary to interstitial fibrosis, is common in HFpEF. Previously, we found that supra-physiologic levels of ω3-PUFAs produced by 12 weeks of ω3-dietary supplementation prevented fibrosis and contractile dysfunction following pressure overload [transverse aortic constriction (TAC)], a model that resembles aspects of remodeling in HFpEF. This raised several questions regarding ω3-concentration-dependent cardioprotection, the specific role of EPA and DHA, and the relationship between prevention of fibrosis and contractile dysfunction. To achieve more clinically relevant ω3-levels and test individual ω3-PUFAs, we shortened the ω3-diet regimen and used EPA- and DHA-specific diets to examine remodeling following TAC. The shorter diet regimen produced ω3-PUFA levels closer to Western clinics. Further, EPA, but not DHA, prevented fibrosis following TAC. However, neither ω3-PUFA prevented contractile dysfunction, perhaps due to reduced uptake of ω3-PUFA. Interestingly, EPA did not accumulate in cardiac fibroblasts. However, FFA receptor 4, a G protein-coupled receptor for ω3-PUFAs, was sufficient and required to block transforming growth factor β1-fibrotic signaling in cultured cardiac fibroblasts, suggesting a novel mechanism for EPA. In summary, EPA-mediated prevention of fibrosis could represent a novel therapy for HFpEF.

Nuclear localization of a1A-adrenergic receptors is required for signaling in cardiac myocytes: an “inside-out” a1-AR signaling pathway

Background

Recent studies indicate that α1‐adrenergic receptors (α1‐ARs) are cardioprotective by preventing cardiac myocyte death and augmenting contractility in heart failure. Although G‐protein‐coupled receptors are assumed to localize to and signal at the plasma membrane, we previously demonstrated that endogenous α1‐ARs localize to the nuclei in adult cardiac myocytes. However, the functional consequence of this nuclear localization remains unclear. Here, we attempted to reconcile nuclear localization of α1‐ARs with their physiologic function by examining α1‐AR‐induced contractility in adult cardiac myocytes.

Methods and Results

By measuring shortening in unloaded, cultured adult cardiac myocytes, we found that the α1A‐subtype regulated contractility through phosphorylation of cardiac troponin I (cTnI) at the protein kinase C (PKC) site, threonine 144. Reconstitution of an α1A‐subtype nuclear localization mutant in cardiac myocytes lacking α1‐ARs failed to rescue nuclear α1A‐mediated phosphorylation of cTnI and myocyte contractility. Leptomycin B, the nuclear export inhibitor, also blocked α1A‐mediated phosphorylation of cTnI. These data indicate that α1‐AR signaling originates in the nucleus. Consistent with these observations, we localized the α1A‐subtype to the inner nuclear membrane, identified PKCα, δ, and ε in the nucleus, and found that α1‐ARs activate PKCδ in nuclei isolated from adult cardiac myocytes. Finally, we found that a PKCδ nuclear localization mutant blunted α1‐induced phosphorylation of cTnI.

Conclusions

Together, our data identify a novel, “inside‐out” nuclear α1A‐subtype/PKCδ/cTnI‐signaling pathway that regulates contractile function in adult cardiac myocytes. Importantly, these data help resolve the discrepancy between nuclear localization of α1‐ARs and α1‐AR‐mediated physiologic function.

Abstract P193: Nuclear Localization of the α1B-Adrenergic Receptor Subtype Is Required for Hypertrophic Signaling in Cardiac Myocytes

We previously demonstrated that α1-adrenergic receptors (α1-ARs) in the heart are required for physiologic hypertrophy during development and prevent a maladaptive response to pathologic stress. We have also shown that the major subtypes, α1A and α1B, both localize to the nucleus in adult cardiac myocytes. Importantly, we have defined a nuclear α1A-ERK survival and an α1A-PKCδ-cTnI inotropic signaling pathway that both originate at the nucleus and are transduced to cytosolic targets, suggesting that the α1A is required for cardiac myocyte survival and contractility. However, less is known about the molecular function of the α1B. In the current study, we examined the role of the α1B-subtype in hypertrophic signaling. First, we identified a bi-partite nuclear localization sequence (NLS) in the carboxy-terminal tail of the receptor. Mutation of the NLS (α1B-NLSmut) disrupted its localization to the nucleus when expressed in adult cardiac myocytes. We then compared hypertrophic signaling of the wild-type α1B- to the mutated receptor by reconstitution in cardiac myocytes lacking endogenous α1B (α1BKO) receptors. Activation of the wild-type receptor by the α1-agonist phenylephrine in α1BKO myocytes restored hypertrophic signaling, as we observed increased phosphorylation of protein kinase C (PKC) isoforms δ and ε, and histone deacetylases (HDAC) 4 and 5. We also observed increased expression of the hypertrophic gene marker, atrial natriuretic factor (ANF). Expression of the α1B-NLSmut failed to activate hypertrophic signaling in α1BKO cardiac myocytes despite phenylephrine stimulation. Together, our data show that nuclear localization of the α1B-subtype is required for hypertrophic signaling and overall further suggest that α1-adrenergic receptors are functional only at the nucleus in cardiac myocytes.

Omega-3 fatty acids prevent pressure overload-induced cardiac fibrosis through activation of cyclic GMP/protein kinase G signaling in cardiac fibroblasts

BACKGROUND

Omega-3 polyunsaturated fatty acids (eicosapentaenoic acid and docosahexaenoic acid) from fish oil ameliorate cardiovascular diseases. However, little is known about the effects of ω-3 polyunsaturated fatty acids on cardiac fibrosis, a major cause of diastolic dysfunction and heart failure. The present study assessed the effects of ω-3 polyunsaturated fatty acids on cardiac fibrosis.

METHODS AND RESULTS

We assessed left ventricular fibrosis and pathology in mice subjected to transverse aortic constriction after the consumption of a fish oil or a control diet. In control mice, 4 weeks of transverse aortic constriction induced significant cardiac dysfunction, cardiac fibrosis, and cardiac fibroblast activation (proliferation and transformation into myofibroblasts). Dietary supplementation with fish oil prevented transverse aortic constriction-induced cardiac dysfunction and cardiac fibrosis and blocked cardiac fibroblast activation. In heart tissue, transverse aortic constriction increased active transforming growth factor-β1 levels and phosphorylation of Smad2. In isolated adult mouse cardiac fibroblasts, transforming growth factor-β1 induced cardiac fibroblast transformation, proliferation, and collagen synthesis. Eicosapentaenoic acid and docosahexaenoic acid increased cyclic GMP levels and blocked cardiac fibroblast transformation, proliferation, and collagen synthesis. Eicosapentaenoic acid and docosahexaenoic acid blocked phospho-Smad2/3 nuclear translocation. DT3, a protein kinase G inhibitor, blocked the antifibrotic effects of eicosapentaenoic acid and docosahexaenoic acid. Eicosapentaenoic acid and docosahexaenoic acid increased phosphorylated endothelial nitric oxide synthase and endothelial nitric oxide synthase protein levels and nitric oxide production.

CONCLUSION

Omega-3 fatty acids prevent cardiac fibrosis and cardiac dysfunction by blocking transforming growth factor-β1-induced phospho-Smad2/3 nuclear translocation through activation of the cyclic GMP/protein kinase G pathway in cardiac fibroblasts.

An association between gene expression and better survival in female mice following myocardial infarction

Following myocardial infarction, the prognosis for females is better than males. Estrogen is thought to be protective, but clinical trials with hormone replacement failed to show protection. Here, we sought to identify novel mechanisms that might explain this sex-based difference. By diverging from the traditional focus on sex hormones, we employed a conceptually novel approach to this question by using a non-biased approach to measure global changes in gene expression following infarction. We hypothesized that specific gene programs are initiated in the heart following infarction that might account for this sex-based difference. We induced small, medium, and large infarcts in male and female mice and measured changes in gene expression by microarray following infarction. Regardless of infarct size, survival was better in females, while mortality occurred 3-10 days following infarction in males. Two days following infarction, males developed significant ventricular dilation, the best predictor of mortality in humans. Three days following infarction, we measured gene expression by microarray, comparing male versus female and sham versus surgery/infarction. In general, our results indicate a higher relative level of gene induction in females versus males and identified programs for angiogenesis, extracellular matrix remodeling, and immune response. This pattern of gene expression was linked to less pathologic remodeling in female hearts, including increased capillary density and decreased fibrosis. In summary, our results suggest an association between improved survival and less pathologic remodeling and the relative induction of gene expression in females following myocardial infarction.

Nuclear alpha1-adrenergic receptors signal activated ERK localization to caveolae in adult cardiac myocytes

We previously identified an alpha1-AR-ERK (alpha1A-adrenergic receptor-extracellular signal-regulated kinase) survival signaling pathway in adult cardiac myocytes. Here, we investigated localization of alpha1-AR subtypes (alpha1A and alpha1B) and how their localization influences alpha1-AR signaling in cardiac myocytes. Using binding assays on myocyte subcellular fractions or a fluorescent alpha1-AR antagonist, we localized endogenous alpha1-ARs to the nucleus in wild-type adult cardiac myocytes. To clarify alpha1 subtype localization, we reconstituted alpha1 signaling in cultured alpha1A- and alpha1B-AR double knockout cardiac myocytes using alpha1-AR-green fluorescent protein (GFP) fusion proteins. Similar to endogenous alpha1-ARs and alpha1A- and alpha1B-GFP colocalized with LAP2 at the nuclear membrane. alpha1-AR nuclear localization was confirmed in vivo using alpha1-AR-GFP transgenic mice. The alpha1-signaling partners Galphaq and phospholipase Cbeta1 also colocalized with alpha1-ARs only at the nuclear membrane. Furthermore, we observed rapid catecholamine uptake mediated by norepinephrine-uptake-2 and found that alpha1-mediated activation of ERK was not inhibited by a membrane impermeant alpha1-blocker, suggesting alpha1 signaling is initiated at the nucleus. Contrary to prior studies, we did not observe alpha1-AR localization to caveolae, but we found that alpha1-AR signaling initiated at the nucleus led to activated ERK localized to caveolae. In summary, our results show that nuclear alpha1-ARs transduce signals to caveolae at the plasma membrane in cardiac myocytes.

GATA4 is a survival factor in adult cardiac myocytes but is not required for alpha1A-adrenergic receptor survival signaling

Recently, we defined an alpha1A-adrenergic receptor-ERK (alpha1A-AR-ERK) survival signaling pathway in adult cardiac myocytes. Previous studies in neonatal cardiac myocytes indicated that the cardiac-specific transcription factor GATA4 is a downstream mediator of alpha1-ERK signaling and that phosphorylation of GATA4 by ERK increases DNA binding and transcriptional activity. Therefore, we examined GATA4 as a potential downstream effector of alpha1A-ERK survival signaling in adult cardiac myocytes. We measured norepinephrine (NE)-induced cell death in cultured cardiac myocytes lacking alpha1-ARs (cultured from alpha1A/B-AR double-knockout mice, alpha1ABKO mice) that are susceptible to cell death induced by several proapoptotic stimuli, including NE. Our results show that overexpression of GATA4 is sufficient to protect alpha1ABKO cardiac myocytes from NE-induced cell death. However, we found that the alpha1A-subtype did not induce phosphorylation or increase the activity of GATA4 in adult mouse cardiac myocytes in culture or in vivo. Furthermore, we examined the effect of siRNA-mediated knockdown of GATA4 on alpha1A-survival signaling. In alpha1B-knockout cardiac myocytes, which express only the alpha1A-subtype and are protected from NE-induced cell death, GATA4 knockdown did not reverse alpha1A-survival signaling in response to NE. In summary, we found that GATA4 acted as a survival factor by preventing cell death in alpha1ABKO cardiac myocytes, but GATA4 was not activated by alpha1-AR stimulation and was not required for alpha1A-survival signaling in adult cardiac myocytes. This also identifies an important mechanistic difference in alpha1-signaling between adult and neonatal cardiac myocytes.