Chapter 4 The Pathophysiology of Cardiac Hypertrophy and Heart Failure © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4.1 Increases in heart rate, cardiac output and force of contraction occur early during the course of heart failure and aid in maintaining tissue perfusion close to physiologic levels. Green, normal excitatory stimulus; Red, normal inhibitory stimulus; up-arrow, increase in heart failure; down-arrow, decrease in heart failure. © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4.2 Activation of the renin–angiotensin–aldosterone system in heart failure. The physiologic response to decreased cardiac output and mean arterial pressure is mediated by a number of neuro-endocrine intermediates, including angiotensinogen, angiotensin, aldosterone, and renin. In the context of heart failure, activation of the renin–angiotensin–aldosterone axis results in exacerbation of heart failure. Low cardiac output results in decreased renal perfusion, triggering activation of the system. Angiotensin causes vasoconstriction, increasing peripheral vascular resistance and mean arterial pressure, while aldosterone results in volume retention. In a pressure- and volume-overloaded heart, these mechanisms exacerbate pressure and volume overload, resulting in further declines in cardiac output and further decreases in renal perfusion, reactivating the system, and taking the patient downward. ACE, angiotensin-converting enzyme, AT1R, angiotensin II receptor type 1, CO, cardiac output, MAP, mean arterial pressure, green arrow, increase in heart failure, red arrow, decrease in heart function.22 © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4. 3 Intracellular signaling pathways FIGURE 4.3 Intracellular signaling pathways. Intracellular signaling pathways involved in pathological and physiologic hypertrophy. Activation of a Gαq/11 G-protein coupled receptor (Gα q/11) leads to activation of the small GTP-binding proteins, Ras and Rho, which promote pathological hypertrophy through activation of the mitogen-activated protein kinase (MAPK) signaling cascade. Rho also activates Rho kinase (ROCK), another activator of pathologic hypertrophy. Activation of a Gαq/11 coupled receptor additionally activates phospholipase-Cβ (PLCβ), resulting in inosital-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) production. IP3 binds to an IP3 receptor on the sarcoplasmic reticulum stimulating calcium release. Calcium and DAG activate protein kinase Cα (PKCα), which promotes pathological hypertrophy. Many forms of hypertrophic stimuli increase the amount of intracellular calcium, leading to the activation of the protein phosphatase, calcineurin. Activated calcineurin dephosphorylates the nuclear factor of activated T-cells (NFAT), allowing NFAT to enter the nucleus, interact with GATA4 and myocyte enhancer factor-2 (MEF2) leading to increased protein synthesis and pathological hypertrophy. Glycogen synthase kinase-3β (GSK-3β) can phosphorylate and thereby inhibit NFAT nuclear translocation. Stimulation of the insulin-like growth factor 1 receptor activates phosphatidylinositide 3-kinase (PI3K), which phosphorylates and activates Akt to promote physiologic hypertrophy. Akt further activates the mammalian target of rapamycin (mTOR) and inhibits GSK-3β. © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4.4 Cardiomyocyte G-protein-coupled receptor regulation of hypertrophy. The catecholamines epinephrine and norepinephrine can stimulate the cardiomyocyte β1 and α1 adrenergic receptors (β1- and α1-AR), which are coupled to a Gαs and Gα q/11 G-protein, respectively. Activation of the β1-AR stimulates adenylate cyclase (AC) to produce cyclic adenosine monophosphate (cAMP), which activates protein kinase A (PKA). PKA promotes increased chronotropy, inotropy, and hypertrophy. Stimulation of the α1-AR activates phospholipase-Cβ (PLCβ) to produce inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates calcium release from sarcoplasmic reticulum by binding to an IP3 receptor. Intracellular calcium and DAG activate protein kinase Cα (PKCα). Calcium can similarly activate calmodulin leading to the stimulation of calcium–calmodulin protein kinase II (CaMK). CaMK and PKCα promote pathological hypertrophy. The angiotensin 1 receptor (AT1R) is coupled to a Gαq/11 G-protein, with similar effects on IP3 and DAG. Additionally, stimulation of the AT1R activates the MAPK signaling cascade to promote hypertrophy: mitogen-activated protein kinase kinase kinase (MAPKKK) phosphorylates MAPKK, which phosphorylates and activates p38 MAPK and c-Jun N terminal kinase (JNK). © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4. 5 Calpains modulate hypertrophic signaling pathways FIGURE 4.5 Calpains modulate hypertrophic signaling pathways. Calpains are activated by intracellular calcium, including calcium released from the sarcoplasmic reticulum. Activated calpains are known to interact with and activate calcineurin, which dephosphorylates NFAT, and lead to pathological hypertrophy. Nuclear factor kappa B (NF-κB) is known to induce pathological hypertrophy but must first be released from its inhibitor, inhibitor of NF-κB (IκB). IκB is degraded by calpains, thus allowing NF-κB to enter the nucleus and promote pathological hypertrophy. Physiologic hypertrophy is repressed by calpains through inhibition of Akt. © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4. 6 The anti-hypertrophic mechanisms of PKG FIGURE 4.6 The anti-hypertrophic mechanisms of PKG. Natriuretic peptide binding to the natriuretic peptide receptor and nitric oxide (NO) binding to nitric oxide synthase (NOS) stimulate particulate or soluble guanylate cyclase respectively to produce cyclic guanosine monophosphate (cGMP). Phosphodiesterase-5 (PDE5) is responsible for the breakdown of cGMP. PDE5 inhibitors, such as sildenafil, inhibit cGMP breakdown thereby increasing intracellular cGMP. Once produced, cGMP can bind to and activate cGMP-dependent protein kinase (PKG). Activated PKG has many anti-hypertrophic effects including the activation of G-protein signaling 2/4 (RGS 2/4), and the inhibition of calcineurin/NFAT, Rho A, transient receptor potential cation channel 6 (TRPC6), PKCα, and MAPKs (ERK1/2). © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4. 7 Endothelial cell regulation of cardiomyocyte hypertrophy FIGURE 4.7 Endothelial cell regulation of cardiomyocyte hypertrophy. Expression of related transcription enhancer factor-1 (RTEF1) in endothelial cells is associated with increased production of vascular endothelial growth factor-B, which can bind to a vascular endothelial growth factor receptor on the cardiomyocytes leading to pathological hypertrophy via ERK1/2 activation. Endothelial cell overexpression of endothlin-1 (ET1) is sufficient to cause pathological hypertrophy in adjacent cardiomyocytes. Alternatively, nitric oxide synthase (NOS) in endothelial cells can produce nitric oxide (NO), which can diffuse into neighboring cardiomyocytes to stimulate the anti-hypertrophic PKG. © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4. 8 HDAC modulation of cardiac hypertrophy FIGURE 4.8 HDAC modulation of cardiac hypertrophy. Histone deacetylases (HDACs) participate in both forms of hypertrophy. Class I HDACs (HDACs 1, 2, 3, and 8) are thought to be pro-hypertrophic. Class II HDACs (HDACs 4, 5, 7, and 9) exert anti-hypertrophic effects by suppressing the pro-hypertrophic MEF2. Increased intracellular calcium activates PKCα and the calmodulin/calcium–calmodulin protein kinase II pathways. This phosphorylates class II HDACs, causing their nuclear export, and thus removing MEF2 inhibition. Sirtuin3 (Sirt3) activates FOXO3A (among other targets) to produce anti-hypertrophic effects. © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4.9 Mechanotransduction signaling pathways in cardiac remodeling. Mechanical stretch directly activates mechanosensors, like integrins, which up-regulate the expression of signal molecules such as thrombospondins. Thrombospondins activate signaling transduction cascades involving PI3K, Rho GTPase, NO, Smads, and transcription factors that regulate cardiac remolding. (A) Activation of FAK and Rho GTPase cascade mediates cardiac remodeling. (B) Integrin and FAK activate PI3K and AKT and lead to cardiac hypertrophy. (C) Integrin-mediated calpain and NF-κB cascades control cardiomyocyte survival. (D) Thrombospondins regulate integrin, MMPs and growth factor activation and subsequent cascades to control cardiac hypertrophy and remodeling. (E) BAMBI negatively regulates TGF-β receptor activation to modulate cardiac hypertrophy. ECM, extracellular matrix; MMPs, matrix metalloproteinases; FAK, focal adhesion kinase; NF-κB, nuclear factor kappa B; PKG, protein kinase G; eNOS, endothelial nitric oxide synthase; NO, nitric oxide; BAMBI, BMP and activin membrane-bound inhibitor. © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4.10 The ubiquitin proteasome system mitigates hypertrophy through targeted degradation. Protein degradation by the ubiquitin proteasome system (UPS) involves a series of ATP-dependent enzymatic reactions involving an ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2), and ubiquitin ligase (E3). This process attaches an ubiquitin moiety (Ub) to a substrate protein, thereby targeting the protein for degradation by the 26S proteasome. The proteasome removes and recycles ubiquitin. Atrogin-1 is an E3 that is capable of ubiquitinating calcineurin for proteasome-mediated degradation, which inhibits pathological hypertrophy. Atrogin-1 can also suppress NF-κB and Akt to inhibit physiologic hypertrophy. Muscle ring finger-1 (MuRF1) is known to target a key sarcomeric protein, troponin I for degradation. Additionally, MuRF1 can associate with and inhibit PKCε and serum response factor (SRF) to suppress hypertrophic growth. © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4. 11 Cardiac atrophy vs. hypertrophy: an unbalancing act FIGURE 4.11 Cardiac atrophy vs. hypertrophy: an unbalancing act. The mammalian target of rapamycin (mTOR), activated by Akt, can increase protein synthesis by targeting p70/85 S6 kinase-1 (S6K1) and p54/56 (S6K2). Protein translation is enhanced by mTOR by stimulating the dissociation of 4E-binding protein-1 from eIF4E, allowing eIF4E to bind to eIF4G. The heart is constantly being remodeled. However, a healthy heart maintains balance of protein synthesis and degradation. During cardiac atrophy an increase in protein degradation predominates over a relatively unchanged protein synthesis, indicating the primary role of the ubiquitin proteasome system (UPS) in mediating protein (including sarcomere) degradation. Acute cardiac hypertrophy is characterized by increased protein synthesis, while chronic hypertrophy shows impaired protein degradation. Both conditions tip the balance in favor of protein synthesis. © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease

FIGURE 4.12 Effects of inflammatory mediators on cardiomyocyte function. Pro-inflammatory mediators are associated with cardiomyocyte dysfunction (red). TNF-α, IL-1, and IL-6 are produced by multiple cell types including endothelial cells, monocytes, macrophages, and adipocytes. The primary signaling mediator utilized by inflammatory molecules is nitric oxide (NO) produced by inducible nitric oxide synthase (iNOS). High levels or chronic exposure of pro-inflammatory molecules have detrimental effects on cardiomyocytes (both through NO-dependent and NO-independent pathways) including contractile dysfunction, remodeling of the extracellular matrix (ECM), apoptosis, and hypertrophy. Pro-inflammatory molecules such as bradykinin (BK) and adrenomedullin (AM), as well as anti-inflammatory molecules such as adiponectin, have apparent cardioprotective effects (green). These include modulation of vascular tone and endothelial cell function (AM), inhibition of ECM remodeling and cardiomyocyte hypertrophy (BK), and activation of critical signaling enzymes such as AMP-activated kinase (AMPK).297 © 2014, Elsevier Inc., Willis, et.al., Cellular and Molecular Pathobiology of Cardiovascular Disease