Epigenetic regulation of heart failure

Heart failure is a leading cause of death with multiple cell types involved, including cardiomyocytes and fibroblasts. Decreased contractile function of muscle cells, as well as increased extracellular matrix (ECM) production by fibroblasts, contribute to decreased cardiac output in the failing heart.¹ Programmatic changes in gene expression in cardiac fibroblasts and myocytes occur during the pathogenesis of heart failure and are affected by epigenetic mechanisms, including histone modifications.² These modifications include post-translational changes in histone acetylation and methylation associated with cardiac development and disease. While histone deacetylases (HDACs) in the heart have been studied for many years, the functions of histone demethylases in cardiac disease are just beginning to be reported.³ A new study by Huo et al reports cell type-specific functions for the histone lysine specific demethylase 1 (LSD1, also called KDM1A) in cardiac fibroblast activation and cardiomyocyte dysfunction in adult heart homeostasis and injury response.⁴

Epigenetic regulation of cell type-specific gene expression in development, homeostasis, and disease often involves changes in DNA accessibility due to post-translational modification of histone proteins. Histone methylation at arginines generally leads to gene activation, while methylation of lysines has more diverse effects on gene expression based on different combinatorial patterns of specific lysine residues of individual histones.³ Histone methylation is regulated by multiple methylases and demethylases with specificity for distinct histones and target amino acid residues. In the heart, individual histone demethylases have been implicated in gene expression changes in cardiac morphogenesis, as well as pathologic hypertrophy and fibrosis. LSD1 specifically demethylates Histone H3 at lysines K4 and K9, thus altering DNA accessibility of multiple transcription factors leading to coordinated changes in gene expression. Systemic loss of LSD1 leads to early embryonic lethality at E6.5 in mice and a homozygous LSD1 hypomorphic allele causes cardiac morphological defects and perinatal lethality.⁵ However, little is known about the cell type specificity of LSD1 function in cardiac development and disease.

The study by Huo et al. reports cell type-specific loss of LSD1 function in cardiomyocytes and fibroblasts during cardiac homeostasis and injury in adult mice (Figure).⁴ LSD1 expression is increased in human dilated cardiomyopathy and in mouse heart failure models, particularly in cardiac fibroblasts. Mice generated with tamoxifen-inducible loss of LSD1 specifically in periostin (Postn)-expressing activated fibroblasts or in cardiomyocytes were subjected to pressure-overload by transverse aortic constriction (TAC), which leads to cardiac fibrosis, hypertrophy, and heart failure over the course of several weeks. Loss of LSD1 in activated fibroblasts leads to improvement of cardiac function with reduced fibrosis and a blunted cardiomyocyte hypertrophic response. Changes in gene expression resulting from loss of LSD1 include reduced expression of multiple Transforming Growth Factor (TGF)β pathway and fibrotic ECM genes. Supernatant from cardiac fibroblasts with LSD1 loss-of-function inhibits cardiomyocyte hypertrophy, which is dependent on TGFβ signaling, demonstrating an indirect beneficial effect of fibroblast LSD1 loss-of-function on cardiomyocytes with normal levels of LSD1. In contrast, loss of LSD1 in cardiomyocytes leads to mild hypertrophy and fibrosis in the absence of injury and worsened cardiac function after TAC. Together, these studies demonstrate a role for LSD1 in induction of TGFβ pathway and fibrotic gene expression specifically in cardiac fibroblasts, as well as an indirect beneficial effect of inhibiting pathologic cardiac hypertrophy in cardiomyocytes. However, the direct consequences of loss of LSD1 in cardiomyocytes are pathological hypertrophy and reduced heart function.

Figure. Cell type-specific epigenetic mechanisms in fibroblasts and cardiomyocytes after cardiac injury.

Lysine-specific histone demethylase-1 (LSD1) inhibition has differential effects on fibroblasts and cardiomyocytes. The loss of LSD1 in Postn+ fibroblasts after transverse aortic constriction (TAC) reduces fibrosis, cardiomyocyte (CM) hypertrophy, extracellular matrix (ECM) deposition, and TGFβ activity, that also blunts CM hypertrophy. In contrast, the loss of LSD1 in cardiomyocytes leads to hypertrophy and fibrosis in the absence of injury, with increased cardiac pathology after TAC.

TGFβ signaling is a well-known regulator of fibrosis in multiple organ systems, including the heart, and includes multiple ligands, receptors, and mediators.⁶ Likewise tissue fibrosis results from excessive ECM production, notably fibrillar collagen. These fibrotic changes include dysregulation of numerous genes related to cell signaling, increased ECM production and tissue remodeling. In pulmonary fibrosis, inhibition of LSD1 reduces fibrosis by suppression of TGFβ/Smad3 signaling via repressive H3K9 methylation.⁷ In addition, histone methylation modifications H3K4 and H3K9 have been implicated in the progression of cardiac fibrosis. Together, these studies support a conserved role for histone demethylation in TGFβ regulation of fibroblast activation and fibrotic gene expression.

In some mouse models with targeted reduction of cardiac fibroblast activation, cardiomyocyte hypertrophy also is reduced and cardiac function is preserved, even in the presence of sustained pressure overload. Notably, this has been seen with fibroblast specific ablation of β-catenin or multiple mediators of TGFβ signaling.^{8, 9} While the mechanisms underlying these indirect effects were not identified, the study by Huo et al. supports a direct role for TGFβ1 secreted by fibroblasts in promoting pathologic hypertrophy of cardiomyocytes. Indeed, TGFβ has been demonstrated to promote cardiomyocyte hypertrophy through paracrine and autocrine mechanisms.¹⁰

While ablation of LSD1 in Postn+ cardiac fibroblasts directly inhibits cardiac fibrosis with indirect improvement in cardiomyocyte hypertrophy and function, loss of LSD1 in cardiomyocytes directly leads to pathologic changes in cardiomyocytes and indirect induction of cardiac fibrosis. The differences in downstream gene expression in cardiac fibroblasts and cardiomyocytes are attributed to cell type-specific interactions with other chromatin remodeling proteins. In cardiomyocytes, but not fibroblasts, loss of LSD1 results in decreased expression of transcriptional cofactor proteins including repressor element 1 silencing transcription factor (REST) and coREST, potentially through proteosomal degradation.⁴ REST is a component of a transcriptional complex that represses ANP and BNP fetal cardiac genes.¹¹ Similar to LSD1, cardiomyocyte-specific loss of REST leads to pathologic cardiac hypertrophy and induction of fetal gene expression profiles. These interacting transcriptional repressor proteins and downstream effects with loss of LSD1 are specific to cardiomyocytes and are sufficient to promote cardiac dysfunction and disease. Thus, inhibition of LSD1 function in the diseased heart has the potential to worsen cardiac function due to its role in cardiomyocyte contractile protein gene regulation.

Reversible histone demethylase inhibitors have been developed for treatment of a variety of conditions including Parkinson’s disease and cancer. LSD1 expression is increased in acute myeloid leukemia (AML) and specific LSD1 inhibitors, including ORY-1001, are currently in phase I/II clinical trials for treatment of AML.¹² In addition, treatment with an LSD1 inhibitor during development or at birth ameliorates cardiac developmental defects and adult cardiomyopathy in a mouse model of laminopathy.¹³ Interestingly, LSD1 inhibitors were not administered to adult mice, but Huo et al report detrimental effects of reduced LSD1 function in adult cardiomyocytes.⁴ At the same time, LSD1 inhibition is proposed as antifibrotic through repression of TGFβ pathway gene expression and signaling in cardiac fibroblasts. Together, these studies demonstrate heterogeneous effects of LSD1 inhibition, depending on age and cell-type in the developing and diseased heart. These pleiotropic effects must be taken into account in consideration of LSD1 inhibitors as therapeutics for cardiovascular disease.

Post-translational modifications of histones contribute to major transitions in cardiac gene expression in development and disease.² These epigenetic mechanisms represent attractive therapeutic targets in cardiac injury response and heart failure because they broadly affect gene expression regulatory networks. Histone deacetylase (HDAC) inhibitors have shown promising results in mouse models and are currently in preclinical testing for cardiac disease.¹⁴ Multiple studies in mice have demonstrated that HDAC loss-of-function prevents cardiomyocyte hypertrophy, inhibits oxidative damage, and improves cardiac output in heart failure models. Recently, Travers et al reported that treatment of mice with the HDAC inhibitor ITF2357/Givinostat reverses diastolic dysfunction and also prevents activation of fibrotic gene expression by inhibiting recruitment of the bromodomain protein BRD4 to ECM gene regulatory elements.¹⁵ Together, these studies show that HDAC inhibition can have beneficial effects on both cardiomyocytes and fibroblasts under disease conditions. In contrast, inhibition of histone demethylases, such as LSD1, may have opposing effects on cardiac function through specific cardiomyocyte and fibroblast-mediated mechanisms.

Preclinical studies for developing new cardiovascular therapeutics in mouse models benefit from increasingly sophisticated genetic tools for targeted manipulation of gene expression in specific cell types in normal and diseased hearts. These targeted studies have the potential to reveal opposing effects in cardiac fibroblasts and muscle cells during progression to heart failure. In addition, crosstalk between cardiac fibroblasts and muscle cells, such as via TGFβ signaling, can confound cell type-specific analyses. Effective therapies for human heart disease will need to have the benefits, while minimizing detrimental effects, that can occur at the same time with whole body exposure to medical therapies. Post-translational modification of histones, including methylation, have pleiotropic functions in diverse cell types leading to programmatic changes in gene expression. Thus, it may be more effective to target downstream pathways, such as TGFβ signaling in fibrosis, for optimal therapeutic benefit in cardiac fibrosis and heart failure treatment.