Calcific Aortic Valve Disease: Part 2—Morphomechanical Abnormalities, Gene Reexpression, and Gender Effects on Ventricular Hypertrophy and Its Reversibility

Abstract

In part 1, we considered cytomolecular mechanisms underlying calcific aortic valve disease (CAVD), hemodynamics, and adaptive feedbacks controlling pathological left ventricular hypertrophy provoked by ensuing aortic valvular stenosis (AVS). In part 2, we survey diverse signal transduction pathways that precede cellular/molecular mechanisms controlling hypertrophic gene expression by activation of specific transcription factors that induce sarcomere replication in-parallel. Such signaling pathways represent potential targets for therapeutic intervention and prevention of decompensation/failure. Hypertrophy provoking signals, in the form of dynamic stresses and ligand/effector molecules that bind to specific receptors to initiate the hypertrophy, are transcribed across the sarcolemma by several second messengers. They comprise intricate feedback mechanisms involving gene network cascades, specific signaling molecules encompassing G protein-coupled receptors and mechanotransducers, and myocardial stresses. Future multidisciplinary studies will characterize the adaptive/maladaptive nature of the AVS-induced hypertrophy, its gender- and individual patient-dependent peculiarities, and its response to surgical/medical interventions. They will herald more effective, precision medicine treatments.

Introduction

We have a tendency to attach a kind of quasi-religious significance to our DNA, to be more deterministic than we should.—Francis Collins, NIH.

Man is an instrument over which a series of external and internal impressions are driven, like the alternations of an ever-changing wind over an Æolian lyre; which move it, by their motion, to ever-changing melody.—A Defence of Poetry (1821), Percy Bysshe Shelley.

Aortic valvular stenosis (AVS), produced by progressive calcific aortic valve disease (CAVD) leading to cusp rigidity and incomplete cusp opening, afflicts mostly older persons and imposes a gradual left ventricular (LV) pressure overload (PO) accompanied by compensatory LV hypertrophy (LVH). If untreated, it leads to diastolic and systolic LV dysfunction and ultimately heart failure with life-threatening complications; accordingly, its effective management currently entails valve replacement. As noted in part 1 [1], CAVD is a complex disease in which multiple cellular and molecular mechanisms have been identified; moreover, the presence of additional comorbidities and clinical risk factors points toward a multifactorial pathogenesis. CAVD had been considered “degenerative,” but newer studies have shown that it is actively mediated, encompassing genetic mechanisms and processes similar to atherogenesis: genetic predisposition and signaling pathways, endothelial dysfunction, lipoprotein deposits, chronic inflammation, and oxidative stress, which lead to valve remodeling with calcification and osteogenesis. Consequently, CAVD may eventually be controlled/reversed by lifestyle and pharmacogenomic interventions, which are likely to retard these processes and could potentially delay the disease progression and postpone the need for valve replacement.

As the Epigraphs suggest, by ascertaining genetic attributes first, we can then go on to understand important environmental factors that contribute to organ/organismal phenotype; i.e., if we can ascertain the nature, we can exploit this knowledge to identify the consequences of nurture. It is likely that CAVD-induced cardiac hypertrophy in each patient possesses a distinct phenotype, depending on precisely how it is stimulated by distinct pathological AVS dynamic characteristics and mediated by the interacting biochemical pathways involved in its pathogenesis. Ideally, its management should be comprehensive, embracing not only the valve but also the left ventricle and the arterial system with their interdependent morphomechanics and hemodynamics [2], which modulate the ensuing diastolic and systolic LV dysfunction. And, when we treat patients with cardiac hypertrophy, we should strive to preserve its beneficial/adaptive aspects and target only the detrimental components.

In part 2, the compensatory hypertrophic process and its reversal after aortic valve replacement are examined, including gender effects having a bearing on ventricular function and prognosis. The hypertrophy is not consistent in patients with comparable degrees of AVS and its regression after valve replacement is variable, implying that factors unrelated to the PO burden have to account for these differences. Indeed, mechanisms other than compensatory hypertrophy appear capable of counteracting the adverse effects of the afterload excess in AVS. Shortcomings in myocardial pumping effectiveness are primarily expressed through changes in intracardiac plus great vessel root blood flow dynamics and circulatory insufficiency. They can be transduced into cellular and global cardiac morphomechanical changes (phenotypic plasticity) through three cooperating pathways: by direct mechanical action on myocardial cell structure, intermediated by a spatial reorganization of the cytoskeleton and the extracellular matrix (ECM); by autonomic neurohumoral agents; and by autocrine and paracrine intracardiac chemical signaling [3].

Hypertrophy provoking signals come in the form of dynamic stresses and ligand/effector molecules that bind to specific sarcolemmal receptors to initiate the hypertrophy and are communicated across the sarcolemma via an assortment of second messengers. Signal transduction pathways precede cellular/molecular mechanisms and control hypertrophic gene expression by activation of specific gene regulatory proteins or transcription factors. They entail intricate feedback mechanisms involving PO-intensified systolic and diastolic myocardial stresses, gene network cascades, specific signaling molecules encompassing G protein-coupled receptors (GPCR) and mechanotransducers, and posttranscriptional modulators (microRNAs) of gene expression and phenotype (Fig. 1).

Fig. 1.

Pathways involved in physiological and in the pathological cardiac hypertrophy of calcific aortic valve disease (CAVD). Signal transduction pathways pave the way for cellular mechanisms controlling gene expression; a cascade of molecules leading to the activation of one or more specific transcription factors is actuated. The extracellular signals in the form of ligands/effector molecules, which bind to specific receptors to initiate the hypertrophy-producing pathways, are transcribed across the sarcolemma via an assortment of second messengers. In physiological forms of cardiomyocyte/myocardial growth, only direct mechanotransduction routes and the PI3K(p110α) lipid kinase-Akt serine/threonine kinase pathway are activated, downstream from receptor tyrosine kinases (RTK), leading to left ventricular (LV) remodeling involving replication of cardiomyocyte sarcomeres both in-parallel and in-series and to an eccentric LV hypertrophy pattern with amplified systolic myocardial performance. On the other hand, in the pathological form of hypertrophy that is induced in conjunction with the LV pressure overload of CAVD (or hypertension), there ensues activation of a diverse, wider variety of signaling pathways, involving G protein-coupled receptors (GPCR). GPCR mediate pathological cardiac hypertrophy through downstream mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinases 1 and 2 (ERK1/2) and calcineurin, a Ca²⁺-dependent phosphatase that controls hypertrophic gene transcription by dephosphorylating transcription factors such as nuclear factor of activated T cells (NFAT). This ultimately leads to the identifiable expression of a maladaptive genetic program, with activation of protein translation concluding with replication of cardiomyocyte sarcomeres in-parallel and a typically concentric LV hypertrophy; these effects are complicated by cellular apoptosis and ECM fibrosis, by (subendocardial) ischemia with diastolic and systolic dysfunction, and by transition to heart failure with subsequent LV chamber dilatation (see particulars in text). Modified from Pasipoularides [1] with permission from the Journal of Cardiovascular Translational Research

Excessively high LV mass levels in AVS-induced PO may well have a detrimental impact on ventricular function, patient outcome, and prognosis and should no longer be considered as a purely advantageous adaptive mechanism in AVS; this has implications for the timing of valve replacement, as AVS patients with depressed ventricular function exhibit high operative mortality and poor long-term outcome. The negative effect of excessive hypertrophy may be due to the inception of cardiomyocyte degeneration with apoptosis and myocardial fibrosis in the (disproportionately) hypertrophied ventricle, which usher in what Sir William Osler described as a “broken compensation” [4]. Future pluridisciplinary studies will be necessary to develop carefully chosen plasma biomarkers that, in conjunction with genetic testing and gene-environment interaction analyses, can give the information needed to optimize forthcoming therapeutic interventions and to better and more completely characterize the molecular dynamics of ventricular remodeling. Such studies will point to more effective, personalized medicine treatments of CAVD and AVS.

Cardiac Hypertrophic and Hyperplastic Growth

For about the past 50 years, the heart has been viewed as a terminally differentiated, postmitotic organ incapable of cytokinesis-entailing, hyperplastic growth in response to an overload. In fact, postpartum cardiomyocytes stop proliferating and myocardial growth is achieved by increasing cardiomyocyte volume by sarcomerogenesis and the formation of binucleated syncytia; binucleation is advantageous because it allows cardiomyocytes to generate at roughly twice the mononucleate DNA transcription rate the quantity of assorted interacting RNA molecules needed to produce contractile and structural proteins during hypertrophic stimulation. During the adaptive hypertrophic response to AVS, individual myocytes grow in size without dividing, by assembling supplementary sarcomeres to maximize systolic force generation. While this response may provide initial compensatory advantages, such as the normalization of systolic wall stresses, prolonged hypertrophy in response to pathological PO is associated with increased morbidity and mortality due to both systolic and diastolic dysfunction, as is discussed later on.

The coexistence in cardiomyocytes during the embryonic stage of both mitotic cell division and terminal differentiated traits such as contractile myofibrillar cytoarchitecture represents a biological enigma worthy of exacting note. The sarcomere is enormously complex and may well be one of the most intricate macromolecular assemblies in biology [5, 6]; understanding the interactions between the proteins of this heterogeneous complex and its assembling will surely represent a formidable challenge to decipher through forthcoming research. The inability of postnatal/adult cardiomyocytes to divide and respond with hyperplasia to the overload of AVS is problematical. This inability is attributable to the complex processes of myofibrillar sarcomeric disassembly and reassembly, involving the participation of transient cytoskeletal scaffolds and adaptors, notably the microtubule network [6], that mitosis would entail—furthermore, too many cytoskeletal microtubular elements might physically impede cell division, as well.

Remarkably, during postnatal development, it appears that heart size can expand by two- to threefold solely as a result of non-proliferative protein synthesis and sarcomerogenesis producing cellular hypertrophy (see Fig. 1). Nevertheless, emerging evidence supports a dynamic view of the myocardium in which cardiomyocyte death and renewal, mediated by exogenous and endogenous stem/progenitor cells, are vital components of cardiac remodeling processes that lead to homeostatic dynamic outcomes [7–9]. Thus, it now appears likely that, since there are cardiac progenitor cells that may systematically replace damaged cardiomyocytes, then it will probably be possible over the next few years to augment cardiac regeneration via their stimulation, culture, expansion, and/or transplantation.

Resident Endogenous Stem/Progenitor Cells and Myocardial Growth

It has been demonstrated that in AVS-induced PO, the growth in cardiac mass is the product not only of hypertrophy of existing cardiomyocytes but also of hyperplasia caused by the differentiation of stem-like cells committed to the myocyte lineage, which transition to cardiogenic and myocyte precursors, as well as very primitive myocytes that turn into terminally differentiated myocytes, providing a link between cardiac stem cells and cardiomyocyte differentiation [10, 11]. Adult stem cells appear to be the only cells in the body that are both permanent residents—in protected niches in their specific tissues—and can also replicate extensively; but they endure quietly, not replicating, until their tissue is injured or otherwise endangered. Then they are actuated to slowly replicate, while concurrently turning out slightly more differentiated and rapidly proliferating, progenitor cells. Those stem-like progenitors sense, as they replicate, how many new cells the tissue needs, and they differentiate into more limited progenitors that can barely replicate at all. The latter differentiate into the final product: the mature cardiomyocytes that the myocardium needs to replace; as we just saw, these differentiated mature cells generally do not replicate but can undergo hypertrophy.

Indications are that the Notch signaling pathway could be important for postnatal cardiac tissue renewal and/or growth. Notch proteins are cell surface transmembrane-spanning receptors whose ligands initiate a signaling cascade that governs cell fate decisions such as differentiation, proliferation, and apoptosis [12, 13]. Considering that adult cardiomyocytes are resistant to cytokinesis with cell cycle reentry, many of the same intracellular signaling pathways that in other cell types regulate cell proliferation most likely regulate cardiomyocyte hypertrophy, as a substitute. The Notch signaling pathway is primarily a communication system between two adjacent cells: a signal-sending cell that expresses the ligand and a signal-receiving cell that expresses the transmembrane receptor. Within a population of cells with similar developmental potential, each cell is committed to a particular cell fate owing to extrinsic or intrinsic events and inhibits surrounding cells to adopt its same fate; Notch controls binary cell fate decision, i.e., lateral inhibition and/or induction of cell fates between adjacent cells [14]. To wit, as we saw in the companion part 1 of this survey [1], the expression of a stable and narrow distribution of cardiomyocyte diameter/size in the (hypertrophic) myocardium brings in focus the obvious need for an inhibitory (negative) feedback linking the growth rate of sarcomeres—being replicated in-parallel (Fig. 1)—to the cardiomyocyte diameter; Notch signaling could be subserving this need.

Notch signaling originates in the myocardial cell surface at a single-pass transmembrane protein receptor, passes through the cytoplasm following activation, and operates as a transcription factor upon gaining access into the nucleus. This spread allows for potential interactions and crosstalk with other signaling components and pathways; e.g., crosstalk between Notch and Akt can occur through receptor tyrosine kinase-mediated activation of PI3K or through activation of the PI3K/Akt pathway [15].

Notch signaling regulates stem/progenitor cell differentiation into cardiomyocytes and interacts with other signaling pathways to improve myocardial dysfunction, in part by inducing angiogenesis to enhance myocardial perfusion with rising myocardial energy demands; inhibition of Notch signaling might underlie decreased angiogenesis [16]. By controlling the preservation and commitment of a cardiac stem cell compartment, Notch may also protect the heart from an excessive and detrimental hypertrophic response and could thus foster cardiomyocyte survival; the biological outcome of Notch action is dosage-sensitive [17]. In this age of regenerative medicine, the potential of the Notch signaling pathway as a therapeutic target to bring about myocardial regeneration should render it a subject of concerted investigation. Then, as it frequently happens, the reward for the answers to our questions will be the challenge of new questions; the scientific unknown generally expands far more rapidly than it can be explored.

Mitochondrial Function Quality Control and Turnover

Prevention or palliation of maladaptive LV hypertrophy remains a major clinical challenge of translational cardiological research. Disrupted calcium homeostasis (Fig. 1) is a conspicuous feature of the transition from adaptive/compensatory hypertrophy to maladaptive, pathological hypertrophy exhibiting contractile dysfunction, rhythm disturbances, and heart failure [18]. Physiological cardiac hypertrophy is associated with an induction of signaling and transcriptional pathways, such as the peroxisome proliferator-activated receptor gamma coactivator-1α (PGC-1α), which appears to be the mediator of the myocardial and skeletal muscle health benefits accruing from regular exercise [19]. PGC-1α promotes mitochondrial biogenesis, defined as the growth and division of pre-existing mitochondria at times of stress, e.g., in response to “environmental” stimuli.

Mitochondria are direct descendants of an alpha proteobacterial (named after Proteus, a Greek god of the sea capable of assuming many different shapes) endosymbiont [20], which some 1.5 billion years ago became established within a host cell and can autoreplicate [21]. This engulfing of one cell by another, and the merging/fusing of their properties, is designated as endosymbiosis (Gk: living together within), an ability effectively allowing for step changes in living cells through the all-inclusive merger of capabilities that evolved separately in different organisms. A clue as to mitochondrial evolutionary origin is contained in mitochondrial DNA (mtDNA), which is stored in loops and kept separate from the genetic material in the cell nucleus. Bacteria also store their DNA in loops, and this is not a coincidence, since the mitochondria were once free-living bacteria.

Mitochondria generate ATP for the sarcomeric contractile units through an electron transport chain (ETC) system. A transcriptional regulatory network enables the heart to coordinate mitochondrial ATP-producing capacity with energy needs under diverse physiological or pathological circumstances, and the mitochondrial biogenic and maintenance circuitry is closely orchestrated with mitochondrial function quality control and turnover [22]. Human mitochondria contain about 1500 different proteins [23]. Of these, about 1 % (mostly membrane components of the ETC) are encoded by the mitochondrial (mt) genome and are synthesized on ribosomes in the mitochondrial matrix. However, the vast majority are encoded by nuclear genes, are synthesized outside the mitochondria on sarcoplasmic polyribosomes, and must be imported.

Mitochondrial function quality control and turnover are essential because of the generation of reactive oxygen species (ROS)—including superoxides, hydroxyl anions, hydrogen peroxide, and singlet oxygen—as by-products of the oxidative mitochondrial phosphorylation process. ROS activate a broad variety of hypertrophy signaling kinases and transcription factors, like MAP kinase, and subserve both protective and injurious functions but, in large quantities, they can damage irreversibly proteins, lipids, and mitochondrial DNAs [24]. Mitochondrial dysfunction affects cardiomyocyte viability through a wide range of effects, which comprise loss of ATP synthesis and increased ATP hydrolysis, formation of ROS, and release of proapoptotic proteins [25].

Augmented chronic mitochondrial ROS production may perhaps contribute to CAVD/AVS-induced myocardial dysfunction and heart failure. Mitochondrial dynamics, specifically mitochondrial fusion and fission, are important processes for mitochondrial homeostasis and for safeguarding normal mitochondrial function [26]. Myocardial mitochondria are constantly undergoing fission and fusion to adapt to alterations in the loading conditions. Fusion raises mitochondrial oxidative capacity and limits the accumulation of mtDNA mutations. Fusion of reversibly damaged mitochondria with healthy ones can favor their functional repair. Fission allows the conservation of functional mitochondria by separating out degraded portions; specifically, mitochondrial fission accomplishes the selective removal/recycling of damaged mitochondria by autophagy [27].

Autophagy (Gk: a process involving self-eating) is an evolutionarily preserved activity that targets impaired/aging proteins and cellular organelles for lysosomal degradation and recycling of components and can be either indiscriminate or item-specific; mitochondrial-specific autophagy is designated as mitophagy. Autophagy and mitophagy are involved integrally in the regulation of cardiac homeostasis and the myocardial response to stress [28]. Biosynthesis of mitochondrial proteins entails contributions from both mitochondria and the nucleus, but most of them are encoded by nuclear genes and are synthesized outside the mitochondria. Mitochondrial biogenesis results in added mitochondrial tissues, increasing metabolic enzymes available for oxidative phosphorylation and, consequently, providing a greater mitochondrial metabolic capacity in response to the initiating myocardial stress signals.

Mitochondria (heart muscle “sarcosomes”) are especially important as storage reservoirs for Ca²⁺. [Ca²⁺] is an important second messenger in cells; it must be precisely regulated, or cellular function can be compromised. Mitochondria may act as “sinks,” accessory to the sarcoplasmic reticulum, to buffer the effects of cytoplasmic Ca²⁺ overload [29]. It now appears that in a process characterized as excitation–energetics coupling, Ca²⁺ signals exert beat-to-beat regulation of mitochondrial ATP production that can closely couple energy production with demand; in addition, in a process labeled as excitation–transcription coupling, Ca²⁺ acting primarily through signal transduction pathways also regulates gene transcription processes [30].

Mitochondrial membranes feature numerous transport systems for the import of metabolites and high-energy intermediates of oxidative phosphorylation as well as inorganic phosphate returned to the mitochondrial matrix and for the export of mitochondrial synthesized ATP to be utilized in the sarcomeres and the sarcoplasm of cardiomyocytes. Direct measurements of substrate utilization and mitochondrial function in physiological hypertrophy have demonstrated increased rates of fatty acid and glucose oxidation, augmented mitochondrial capability for fatty acid utilization, increased ATP generation, and diminished oxidative stress with maintenance of “redox homeostasis” [31, 32]. Oxidative stress can lead to lipid peroxidation resulting in injury to cell membranes and tissue damage by ROS. Mitochondrial electron transport chains hold the potential to “leak” electrons to oxygen, resulting in superoxide and ROS formation. ROS are involved in signal transduction from membrane receptors in various physiological processes and in intercellular and intracellular signaling; e.g., addition of superoxide or hydrogen peroxide to cultured cells leads to an increased rate of DNA replication, as occurs during mitochondrial biogenesis [33].

Contrasting Physiological and Pathological Cardiac Hypertrophy

Mechanical loading is one of the most decisive determinants of cardiac muscle mass because, on the basis of Laplace’s law, increased wall thickness of a ventricular chamber moderates the systolic wall stress of pressure overload [3, 91]. Consequently, cardiac hypertrophy is a compensatory adaptation, and its shortfall can lead to transition to LV dysfunction; however, chronic cardiac hypertrophy is also a prominent risk factor for morbidity and mortality in an array of clinical and experimental settings [34, 35]. Clearly then, hypertrophy is a Janus-faced (double-sided) myocardial response (see Table 1): in normal development, exercise, or pregnancy, it is termed physiological hypertrophy and is beneficial and associated with preservation or improvement of cardiac pumping function; on the other hand, in the setting of the chronic PO of aortic stenosis and hypertensive heart disease, it is termed pathological hypertrophy and is potentially maladaptive and predisposing to myocardial dysfunction and heart failure.

Table 1.

Important features of pathological vs. physiological myocardial hypertrophy

	Physiological cardiac hypertrophy	Pathological cardiac hypertrophy in CAVD
Stimuli	Exercise training, pregnancy, normal development	Pressure overload caused by AVS
Signaling pathways	PI3K-Akt-mTOR pathway	Calcineurin-NFAT and MAPK signaling cascades
Cardiomyocyte morphology	Increased myocyte volume: width and length Replication of sarcomeres both in-series and in-parallel	Increased myocyte volume: width Replication of sarcomeres in-parallel
Remodeling/myocardial morphology	Typified by eccentric LV hypertrophy with proportionate increase in wall thickness	Typified by concentric LV hypertrophy with disproportionate increase in wall thickness; elderly female patients with AVS exhibit greater LV hypertrophy than males Interstitial fibrosis Cardiomyocyte apoptosis Associated with heart failure/mortality
Fetal gene expression	Normal	Upregulated
Mitochondrial biogenesis	Proportionate to cellular hypertrophy	Unchanged or diminished in heart failure
ATP production	Appropriate	Compromised
LV myocardial ischemia/hypoxia	No	Yes: subendocardial–regional–global
Myocardial function	Normal or improved	Progressively impaired over time; higher mortality in men Elderly female patients with AVS exhibit better systolic function than males
Reversibility-regression	Readily reversible (training–detraining)	Considerably less reversible; may not regress completely More rapid/marked regression after aortic valve replacement (TAVR/SAVR) in females than males
Progression to decompensation/heart failure	No	Yes

The heart can transition from a normal morphomechanical state to one of physiological hypertrophy and back, because morphological alterations that occur during training are reduced upon detraining [36], but pathological hypertrophy that engenders myocardial dysfunction with eventual heart failure and chamber dilatation may be considerably less reversible. Nonetheless, profound diastolic functional abnormalities arising from markedly altered loading conditions but unassociated with structural or biochemical changes in interstitial collagen, significant myocardial ischemia or ventricular hypertrophy, have been shown to be entirely reversible by reducing the elevated end-systolic and diastolic stresses even in early stages of a resultant heart failure [37]. Such conditional reversibility should be born in mind when evaluating or managing the effects of pathologic conditions on cardiac function in experimental and clinical settings, when loading conditions cannot be controlled.

Exercise-Induced, Readily Reversible, Physiological Cardiac Growth

Exercise-induced cardiac growth is governed in large part by growth factors/hormones, such as insulin-like growth factor-1 (IGF-1), through signaling by way of the phosphoinositide 3 kinase (PI3K)/Akt pathway (see Fig. 1), which plays a central role in the development of exercise-induced physiological hypertrophy [38] that does not lead to heart failure; transcription factors upregulated in exercised myocardium include GATA4—discussed in a subsequent section. In contrast, pathological or compensatory cardiac trophic response is elicited by autocrine and paracrine neurohormonal mechanisms mobilized by the biomechanical stress of PO; these signal by way of the Gαq/phospholipase pathway, provoking an upsurge in cytosolic Ca²⁺ and activation of the calcineurin-NFAT and MAPK signaling cascades [39, 40]. These cascade mechanisms are interdependent and, acting together and with other collateral pathways, affect nuclear transcription factors and the regulation of gene expression, ultimately drawing together the pathological hypertrophic myocardial response (see Fig. 1).

Physiological hypertrophy is characterized by absence of fibrosis and by lack of fetal gene reexpression [41], as well as by an LV wall thickness growth that is matched to chamber enlargement [42]. Physiological hypertrophy stimuli, such as PI3K–Akt–mTOR (see Fig. 1), promote protein synthesis by coordinating the regulation of ribosome biogenesis, translation initiation, and translation elongation [43], and they tend to inhibit pathological hypertrophy pathways, as is indicated in Fig. 1.

Chronic Pressure Overload Hypertrophy, with Emergent Maladaptive Behavior

Deviating from physiological cardiac growth, pressure overload typically results in disproportionate to chamber size concentric hypertrophy that is accompanied by fetal gene expression and by cardiomyocyte programmed cell death or apoptosis (Gk: falling-off, as leaves from a tree; programmed cell necrosis as signaled by the nuclei) triggered by a blending of the actions of angiotensin II (Ang II) [44], extraordinary direct mechanical compressive forces, and myocardial ischemia (Fig. 1) that ensues, in part, because of the increased myocardial component (due to myocardial compressive stresses) of coronary vascular resistance and also because of increased capillary-to-myocyte distances in pathological hypertrophy. The enzyme caspase, acting like molecular scissors that cut up selected cellular proteins, commences the chain reaction of changes that lead to a cell’s apoptosis; one of the sensors that activate caspase is a well-studied protein called p53 [44]. The foregoing combination of events triggers myocardial fibrosis that is exacerbated by profibrotic mediators, such as Ang II and transforming growth factor (TGF)-β [45]. There is a coordinated recapitulation of gene expression programs found in the developing heart that recurs during maladaptive myocardial hypertrophy and is tied in with impaired systolic function.

Clearly, while adaptive mechanisms are generally beneficial, they are not perfect; the preceding statement is both comforting and alarming. In complex adaptive systems, the component parts have the freedom and ability to respond to (patho) physiological stimuli in many different and fundamentally unpredictable ways. For this reason, emergent surprising behavior that is not explicitly described by the behavior of the components of the system but emerges due to interactions among them (likewise, flocking of birds cannot be described by the behavior of individual birds) is a real possibility. Such emergent behavior can be for better or for worse; that is, it can manifest itself as either compensatory adaptation or dangerous maladaptation. Additionally, such unforeseen behavior can occur at both the microsystem and macrosystem levels of the organism. While it is comforting to know that, given sufficient time, adaptive systems tend to concentrate on the healthier outcomes, occasionally they will end up in bad outcomes. There is always a chance that the compensatory system will move from a desirable outcome to a life-threatening one, as can happen in pathologic (mal) adaptive myocardial hypertrophy.

Ang II, in particular, is a potent multifunctional agent; it engenders profibrotic synthesis of ECM collagenous proteins but it also can promote autophagy in macrophages and in cardiomyocytes recapitulating gene expression programs found in the developing/neonatal heart. Autophagy, which was introduced earlier, is a cellular process involving a complex orchestration of regulatory gene products [46]. It normally aids survival when cells are suffering from nutrient deprivation, as it involves engulfing, breaking-up, and recycling cellular organelles and macromolecules to make available nutrients and energy; besides, it performs an essential function in sustaining homeostasis. However, excessive autophagy can eliminate essential cellular components and it plausibly may provoke apoptosis. Intriguingly, many studies have shown that baseline and upregulated autophagy is beneficial in myocardial hypertrophy, while others observed that upregulated autophagy can lead to cardiomyopathies [47]. Thus, autophagy and Ang II can have Janus-faced protective as well as harmful effects in myocardial hypertrophy.

Ang II induces mitochondrial autophagy and biogenesis in mouse hearts through mitochondrial ROS; the biogenesis is interpreted as an attempt to replenish the damaged mitochondria and restore production of energy supply molecules [48]. The apparent dichotomy of Ang II action, as an inducer both of ECM synthesis and of autophagy that promotes ECM degradation thus limiting the net buildup of ECM, concurrently supports the development of myocardial fibrosis and opposes it. This allows Ang II to play an essential, finely tuned role in maintaining ECM homeostasis: the induction of autophagy may serve as a homeostatic mechanism suppressing an excessive accumulation of ECM, possibly induced by Ang II [49].

Northern and Western blot analyses have identified several candidate genes that are differentially expressed during the fetal (reduplicative with cytokinesis) and postnatal (binucleation with karyokinesis but without cytokinesis) phases of cardiomyocyte growth [50]. Important among genes that normally are active only during fetal development are genes encoding atrial natriuretic peptide (ANP), myosin light chain 2, α-skeletal actin, and the β-isoform of myosin heavy chain [51]; this specific isoform exhibits a lower ATPase activity and a slower rate of contraction [52]. On the contrary, in physiological hypertrophy, it is the expression of the α-myosin heavy chain that increases, which leads to high ATPase activity and augmented contractility.

Both physiological and pathological hypertrophy varieties are characterized by varying degrees of cardiomyocyte enlargement and cardiac remodeling, entailing de novo synthesis of contractile and structural proteins; this is achieved by means of a complex series of epigenetic activities that bring about gene expression reprogramming and regulate gene expression without altering genomic DNA base pairs [3, 5, 6, 53–55]. However, only the pathological form is complicated by regional (subendocardial) ischemia, cardiomyocyte apoptosis that weakens the heart muscle, and increased fibrosis, as well as by diminished sarcomere contractile force and relaxation velocity and by diminished chamber and myocardial compliance [56–63]. Diastolic dysfunction and ensuing diastolic heart failure (HF) are risk factors for the subsequent development of systolic/contractile dysfunction with depressed ejection fraction and cardiac decompensation. The accumulation of collagen intrinsically promotes HF by impeding the contraction and relaxation of cardiomyocytes and by establishing electrophysiological remodeling with rhythm abnormalities [63–65]. There is also ion channel remodeling and altered cellular Ca²⁺ homeostasis involving SERCA2a downregulation and dysfunction, with increased levels of cytosolic Ca²⁺ [66] contributing to the relaxation abnormalities, tachyarrhythmias, and HF (see Fig. 1).

Pathological Hypertrophy in CAVD

The hypertrophic reaction to CAVD-related AVS and PO develops in response to the chronic augmentation of the hemodynamic load and is characterized by structural alterations at the cardiomyocyte level, which are represented by the global geometric adaptations that are collectively designated as LV concentric remodeling. Functional benefits of the latter consist of an increase in the number of in-parallel arranged contractile elements and a lowering of the average mural contractile stress levels with the increased wall thickness (h). However, the widened h has the detrimental side effect of an exacerbated nonuniformity in mural regional stress distributions [3], subjecting preferentially the subendocardial layers to higher systolic and diastolic stresses [67], which raise their systolic energy demands while also compromising their blood supply through stronger intramural vessel compression (the “mural component” of coronary resistance) [3, 58, 68, 69]. The intramural compressive forces reflect the nonlinear radial stress distribution, which decreases from the endocardium (intraluminal P) to the epicardium (pericardial P), and especially in LVH play a critical role in determining the distribution of coronary blood flow, which also decreases from endocardium to epicardium.

Subendocardial ischemia and impaired subendocardial coronary reserve in CAVD/AVS-induced LVH are referable mainly to the distribution of compressive myocardial mural stresses across the wall thickness. Because perfusion of the subendocardial layers is related to the diastolic coronary pressure minus the back pressure, which is exerted by the mural compressive forces, it follows that perfusion of the subendocardial layers would be especially impaired with concentric wall hypertrophy, which accentuates the nonuniformity in the transmural distribution of compressive forces and myocardial blood flow. In addition, and in sharp contrast to physiological hypertrophy, which is characterized by normal or increased capillary density, the density of the coronary microvascular network does not grow sufficiently to withstand the increasing CAVD-related AVS burden, so that coronary flow reserve is progressively compromised [3, 70].

Thus, in AVS, myocardial oxygen demand is raised substantially by the combination of enlarged myocardial mass and augmented afterload, while supply is compromised, rendering the LV myocardium vulnerable to (subendocardial) ischemia [67], even in absence of demonstrable coronary vascular disease. Notably, in conscious, chronically instrumented dog models of severe pressure overload LV hypertrophy, major alterations in diastolic function, including impaired relaxation, were not apparent at rest and were only noted in response to stress, which also induced subendocardial ischemia [67, 71].

Such a stress-induced impairment of LV relaxation is embodied in a prolongation of tau (τ) [72] and renders the left ventricle less tolerant to tachycardia than normal; it responds to tachycardia with marked increases in LV diastolic pressure levels, which in turn accentuate wall stress and pulmonary venous pressures causing pulmonary congestion [56, 58, 59]. By reducing the heart rate, myocardial oxygen consumption decreases and coronary flow increases, resulting in a better balance between O₂ supply and demand. This is true regardless of whether there coexists epicardial coronary disease or there is just myocardial hypertrophy and subendocardial ischemia. Matters may be exacerbated in sympathoadrenally mediated tachycardia, which accompanies heart failure and causes diastolic time to wane [73]. Tachycardia increases the relative time spent in systole, thus increasing the net extravascular compressive forces acting on the intramural coronary microvasculature, in particular in the subendocardial layers. In severe AVS, the potential for progressive coronary insufficiency is an important issue, given that there ensues diminished myocardial perfusion even with normal epicardial coronary arteries. This results from the reduced capillary density with increased capillary-to-myocyte distances and the augmented mural component of the coronary vascular resistance, both of which were already discussed earlier in this section, and it is correlated with stepped up cardiomyocyte apoptosis [74].

Fluid Dynamic Factors Triggering Functional Coronary Insufficiency in AVS

Severe aortic valvular stenosis in advanced CAVD prevents vortex formation in the sinuses of Valsalva: the flow streamline along each incompletely opening cusp no longer strikes the sinus ridge, so that no part of it curls into the corresponding sinus [3]. The turbulent systolic jet emerging from the stenotic orifice formed by the free margins of the conical/tapering aortic cusp conformation induces, by the Venturi effect, a rather uniformly distributed low pressure in the region between the sinus walls and the cusps. By this Venturi action, pressure in this annular region surrounding the jet gets depressed the most during the peak of systolic ejection, when ρv²/2 is highest [3]. Consequently, the pressure at the coronary ostia within the sinuses of Valsalva may be lowered quite substantially during ejection, whose duration is prolonged in AVS.

The preceding fluid dynamic factors may cause the effective systolic values of the pressure difference between the coronary ostia and the subepicardial coronary arteries to attain high negative magnitudes, especially when the cardiac output and the jet velocities are augmented, intensifying the Venturi effect. This is quite a drastic change from the normal/physiological hypertrophy states in which the mean systolic pressure difference between the coronary ostia and the subepicardial coronary arteries is positive and tends to rise with the cardiac output. An early systolic coronary inflow surge into the subepicardial coronary arteries is known to occur [68, 69], especially during exercise and high cardiac output states, which then transitions into the normally much larger early diastolic upsurge, as is demonstrated in the coronary hemodynamics recordings, which—to avoid confounding effects of anesthesia and recent surgery [75]—were obtained in the chronically instrumented, conscious dog, and are illustrated in Fig. 2.

Fig. 2.

Multichannel oscillograph recordings of coronary hemodynamics obtained in the chronically instrumented, conscious dog. Phasic waveforms for aortic root pressure, left circumflex coronary artery diameter, blood flow, and left ventricular pressure are shown at rapid paper speed. Before construction of this figure, the pens on the oscillograph were aligned to illustrate accurately the ≈10-ms delay between the rise in aortic pressure and the rise in coronary artery diameter; this delay most likely reflects the fact that the coronary dimension measurements are derived just distal to the measurement of aortic pressure. Note the similar features of the aortic root pressure and coronary diameter waveforms. Coronary blood flow was measured just distal to the coronary artery diameter. The waveform for coronary diameter was similar to that of coronary flow in that both showed downward deflections with atrial systole and isovolumic ventricular contraction. The marked sinus arrhythmia is characteristic of the conscious dog. Adapted, slightly modified, from Vatner et al. [68] with permission from the Journal of Clinical Investigation

Aortic valvular stenosis may therefore impede systolic coronary inflow during exertion and may even induce abnormal blood suction out of the subepicardial coronaries during the ejection phase [11, 13]. This may well be a fluid dynamic reason for the syndrome of angina on exertion, which is also accompanied by tachycardia that extends/increases the extravascular compressive forces acting on the intramural coronary microvasculature (see discussions above), and is a frequent complication of severe aortic stenosis with a strong outflow jet formation [3].

Apoptosis and Its Consequences in Pathological Hypertrophy

Apoptosis with loss of cardiomyocytes is actuated in pathological hypertrophy. The mitochondria, which once were thought solely to generate metabolic energy, actually manage apoptosis by releasing death-promoting factors residing in the mitochondrial intermembrane space into the cytosol/sarcoplasm; one of these is cytochrome c, a freely diffusible protein molecule ordinarily shuttling electrons between protein complexes in the inner mitochondrial membrane. Once released, cytochrome c helps activate the caspases (cysteine-aspartic proteases), a group of killer proteases in the process of programmed cell death. Release of cytochrome c from the mitochondria is controlled by proteins of the Bcl-2 family: those that prevent death (Bcl-2 and Bcl-xL) inhibit its release, whereas those that provoke death (Bax and Bak) prompt this release [76, 77], by inducing or preventing permeabilization of the outer mitochondrial membrane.

Apoptosis is counteracted with excessive collagen, viz., fibrosis, through the activation of fibroblast proliferation and activity by growth factors, such as TGF, cytokines, and chemokines and the replacement of myocardial muscle fibers with fibrous connective tissue [78]. Disproportionate accumulation of collagen stiffens the myocardium, impairs systolic and diastolic function, downgrades the electrical coupling of cardiomyocytes, and reduces myocardial capillary density and metabolic blood-tissue exchanges. The main fibrillar collagen scaffold in cardiac fibrosis is type I collagen; fibrosis with diminished capillary density decreases the effective diffusion coefficient of oxygen for transport through the tissue and increases diffusion distances, compounding myocardial ischemic effects and contributing to the transition from adaptive to maladaptive hypertrophy and heart failure.

Enhancing myocardial angiogenesis during pathological hypertrophy has been shown to improve outcome in heart failure models using mice subjected to transverse aortic constriction [79]. Pathological hypertrophy is associated with a switch from fatty acid to glucose utilization—i.e., to a more glycolytic metabolism characteristic of the fetal stage (low pO₂ environment) and accompanied by the activation of fetal cardiac genes [80]—however, glucose metabolism also decreases with the progression to heart failure. In pathological cardiac hypertrophy, although mitochondrial respiratory capacity remains relatively intact, fatty acid oxidation rates are decreased, and there is not increased ATP generation despite the need. In notable contrast to physiological hypertrophy where there is an increase in mitochondrial biogenesis, there is actually a repression of PGC-1α, the transcriptional coactivator that regulates several metabolic processes including mitochondrial biogenesis and respiration. Mitochondrial dysfunction promotes oxidative stress, leading to a vicious cycle of progressive mitochondrial damage and to decompensation and heart failure [31, 81].

One reason for the decompensation and failure is that pathologic myocardial hypertrophy itself can actually destabilize local mechanics becoming maladaptive, as hypertrophic myocardium has altered systolic contractile and diastolic passive filling and relaxation properties [56–60, 82]. In investigations involving conditions of impaired (delayed) myocardial relaxation, it may be particularly advantageous to apply a comprehensive model [56, 58, 83] allowing assessment of passive diastolic properties after subtracting decaying contractile components from measured total diastolic pressures and wall stresses. The complicated derangements accruing in the chronic PO of AVS usher in operational disparities between myocardial load and performance [3, 60]. Furthermore, it appears that molecular responses to the (subendocardial) ischemia/hypoxia that is induced by these derangements might initiate the reactivation of genes normally expressed in embryonic myocardium and the downregulation of various adult isoforms, as discussed later; thus, they modify the cellular phenotype and result in altered calcium kinetics, cardiomyocyte apoptosis with myocardial structural disarray, and the reactive interstitial collagen deposition.

Myocardial Maladaptive ECM Remodeling

Every myocardial cell is normally surrounded by an extensive ECM collagenous meshwork enwrapping it as a thick fibrous lattice, which is essential for development and transmission of diastolic filling forces and for providing anchoring against which cardiomyocyte contractile stresses can be generated in systole [3, 5, 6, 54–57]. The ECM also supports and preserves the 3D arrangement of cardiomyocytes and myocardial blood and lymphatic vessels (see Fig. 3) [3, 5, 6]. Processes of ECM assembly and disassembly by proteases (viz., metalloproteinases that after activation can digest all the protein components of the ECM, which encompass glycoproteins, collagen, laminins, and basement membrane proteins) [6] are central to vital cardiac functions, most notably ventricular morphomechanical remodeling in response to environmental cues, including changes in myocardial stresses brought about by PO and its reversal [84, 85].

Fig. 3.

Myocardial microstructure. Clockwise, Top panel: Cardiomyocytes represent the major myocardial constituent by volume, although myocardial fibroblasts are the most numerous cell type. Fibroblasts deposit and remodel the extracellular matrix (ECM) in which all myocardial cells reside and function. Fibrillar ECM proteins like collagen form the scaffold within which these cells organize, while proteins such as laminin and fibronectin are present in smaller amounts with multiple cell binding sites. Bottom panel: Cardiomyocytes in the myocardium are arrayed into a complex crisscrossing pattern of layered sheets; within each sheet, they are mechanically connected to one another by extracellular adherens junctions and desmosomes, which connect intracellularly to actin and desmin filaments of the cytoskeleton, respectively; cadherin/catenin complexes at adherens junctions link the ends of myofibrils from neighboring cells to transmit intercellular active/passive stresses. Left panel: The sarcomeric Z-disks define the lateral borders of individual sarcomeres and are important for intracellular signaling, stretch mechanosensation, and mechanotransduction mechanisms of cardiomyocytes; at costameres, integrins (green) link the ECM to the Z-disc and transmit stresses bidirectionally. Within the sarcomere, titin (the largest known protein in the human body; Gk mythology: the Titans were giants, immortal and powerful) extends between the Z-disc and the M-line (not shown) in the middle of the A-band and acts as an intracellular strain sensor; it encompasses a huge spring-like structure that can stretch or compress as needed building up force as the shape changes; when the cardiomyocytes relax/recoil, it is titin that allows restitution of the original size/configuration

Maladaptive remodeling exemplifies an overall imbalance in ECM turnover that can bring about excess accumulation or breakdown of the ECM structural proteins, predominantly collagens. The turnover rate for collagen is about tenfold slower than non-collagen proteins (>10 times slower synthesis, ~10 times longer half-life) [86]. Thus, the myocardium is very vulnerable to aberrant remodeling under conditions of stepped up ECM degradation. Conversely, even if there is significant increase in collagen production, the replacement collagen can be poorly crosslinked and this may compromise its supportive scaffolding role [86]. Depending on the resultant morphomechanical modifications, maladaptive remodeling can give rise to altered spatial layouts of myocardial cells and coronary microvasculature components, impaired systolic performance, and/or diminished LV compliance with diastolic dysfunction [56, 58–60].

Notably, in the context of CAVD/AVS-induced LVH and subendocardial ischemia, aging by itself can result in progressive cardiomyocyte hypertrophy and interstitial fibrosis. In the normal myocardium, thin layers of perimysium and endomysium surround myocardial bundles and individual myocytes, respectively; adventitial fibroblasts in the walls of blood vessels contribute to the endomysial collagen network. In the senescent heart, there is hypertrophy of cardiomyocytes, transition of cardiac fibroblasts into myofibroblasts, that is, a fibroblast-smooth muscle cell hybrid that is highly responsive to growth factors and inflammatory mediators, and is not normally present in the adult heart except for within the valve leaflets [87]. Cardiac myofibroblasts more effectively secrete and remodel ECM fibrous proteins and proteoglycans and provoke a gradual accumulation of ECM proteins in the interstitium; these adjustments lead to perivascular, endomysial, and perimysial fibrosis. [88, 89]. Thus, in many ways, myofibroblasts emerge as the effectors of transition to disease through overcompensation which leads to the establishment of a fibrogenic milieu.

Fibroblast biology will play a decisive role in ongoing efforts to regenerate cardiac tissue using stem cell methodologies, since cardiac fibroblasts must construct the ECM in which the regenerated myocytes dwell; furthermore, because of gap junctions [6], cardiac fibroblasts are electrically coupled with cardiomyocytes and thereby can significantly modulate electrophysiological properties of the cardiac fibers. Excessive collagen deposition, fibrosis, and diastolic dysfunction are truly a hallmark of aging; caution is therefore necessary in extrapolating to elderly humans findings from young animal studies investigating fibrotic pathophysiology and disorders.

Cardiomyocyte Hypertrophy and Augmented LV Mass May Elicit Systolic Dysfunction

In response to the elevation of peak developed systolic wall stresses in AVS PO, sarcomere replication in-parallel [90] with thickening of cardiomyocyte width or cross-sectional area and LVH develop (see Fig. 1). They are considered as an essentially compensatory homeostatic response reducing myocardial wall stress (force/cross-sectional area) and upholding an effective LV systolic function in the face of the outflow obstruction [56, 58, 60, 91–93]. Then again, if not corrected, chronic PO-induced LVH can set off chains of molecular events culminating in extensive myocardial fibrosis and ensuing diminution of contractility, myocardial fiber slippage and creep (time-dependent myocardial stretch) with diminished wall thickness and LV chamber dilatation, and myocardial dysfunction with decompensation, heart failure, and death [3, 56, 58–60, 73, 94–101].

Augmented LV mass has been shown to predict the presence of systolic dysfunction and HF independently of the severity of valvular obstruction, evoking the radical interpretation that the myocardial hypertrophy may be maladaptive rather than beneficial in AVS in man [102–105]. This provocative possibility brings to mind the incisive dictum of the sixteenth century irascible iconoclast, the Swiss physician Paracelsus (Theophrastus Bombastus von Hohenheim), who averred: Dosis sola facit venenum—only the dose determines the harm. In the current context, the implication of Paracelsus’s proclamation is that “dose and effect” progress together in a predictably monotonic fashion and that lower degrees and durations of hypertrophy will therefore regularly generate lower risk of transition to maladaptation. Still, the possibility of non-monotonic “dose–response” curves, deviating from the familiar ski-slope shape and forming inverted U and U shapes, or even undulating profiles at times [1, 60] should be born in mind by researcher and clinician alike.

Reprogramming and Gene Reexpression in CAVD with Therapeutic Corollaries

The global morphomechanical interconnectedness of the myocardial ECM, cytoskeleton (CSK), and nucleoskeleton (NSK) has pivotal repercussions. As I have detailed recently [3, 5, 6, 54–56], mechanical signaling and force transmission are important factors in the myocardial mechanotransduction process. Neurohumoral stresses and autocrine and paracrine factors are sensed by sarcolemma-bound receptors (sarco [from sarx] Gk: flesh, lemma Gk: sheath), while mechanical stresses are detected by both membrane and sarcomeric stretch-activated mechanisms (Fig. 3).

The CSK is a key constituent in the structural link between the cell membrane and putative intracellular stress-sensing components, which respond to mechanical signals transmitted both directly by cytoskeletal “solid-state” conduction, involving stimuli-sensitive supramolecular polymer networks dynamically interconnected by noncovalent bonds, and through multiple biochemical pathways, each involving various cascades of internal molecular interactions [3, 6, 106]. The network of transcriptional regulatory proteins in a cardiomyocyte is an indispensable determinant of its gene expression profile, dictating the form and function of the cell during the hypertrophic response to evolving AVS. Regulation of transcription factors has therefore been a predominant theme in understanding the molecular control of the development, physiology, and pathology of myocardial hypertrophy. Through the intermediation of the ECM, the CSK, and the NSK, operative extracellular and intracellular forces alter nuclear shape and structure, promote nucleosome disruption and chromatin relaxation, and alter gene transcription [3–5, 54]. Thus, the nucleus can be readily implicated in the processes of mechanotransduction that lead to dynamic changes in genomic and proteomic activities and in the myocardial phenotype in CAVD.

The nucleus, which contains almost all of the human genome, is the central site of transcriptional regulation. Forces applied onto myocardial cell nuclei induce modifications in nuclear shape, which bring about changes in chromatin state and organization [3, 6, 54, 55]; these can, in turn, affect transcription and regulate gene expression. Therefore, force generation and transmission can be important mechanisms of cardiac morphomechanical phenotypic abnormalities and pathology appearing in CAVD. It is noteworthy that already back in 1985 Dr. George Cooper et al. [107] demonstrated in aorta-constricted cats that papillary muscle whose tendon had been cut to release the tension failed to exhibit hypertrophy, whereas neighboring uncut papillary muscles displayed marked hypertrophy; and that myocardial hypertrophy is induced by PO even with denervation of ventricular adrenoceptors.

The pathological stress of the AVS-associated PO induces multiple epigenetic and transcriptional changes/adjustments. These are conducive to a myocardial genetic reprogramming with reexpression of multiple arrays of fetal genes, reflecting a return to an earlier state of myocyte differentiation, as well as a modulation of the activity of an assortment of adult genes. The reexpression of fetal genes encoding contractile proteins that are normally present in embryonic cardiomyocytes is a hallmark of pathological hypertrophy and exemplifies the direct effect of epigenetic internal (cellular) and external (extracellular) conditions that, in combination with genotype, regulate genetic expression and myocardial phenotype [3, 5, 6, 54, 55, 108]. Such considerations raise the possibility of managing pathological hypertrophy with epigenetics modifying agents; many such pharmaceuticals are already approved for clinical use, including inhibitors of DNA methyltransferases (DNMTs) [109].

Additionally, histones—the structural components of nucleosomes—are proteins involved in the nuclear packaging of DNA into chromatin, and chromatin modifications and remodeling are key regulatory elements of gene expression [3, 5, 6, 55]. DNA–histone interactions give a high degree of stability to nucleosomes, while allowing them to undergo considerable structural modifications without becoming unstable. The dynamic interplay between stability and plasticity is a critical quality of histones, considering that it permits them to fulfill both DNA-sequestering structural and regulatory roles. The number and specific functions of histone posttranslational modifications allude to a complex network of regulatory factors that, in combination, set the potential for transcription in a given region of chromatin. Histones could be modified by therapeutic posttranslational modifications (e.g., acetylation/deacetylation) targeted to modulate to advantage DNA-specific transcription processes subserving various features of AVS-associated pathological hypertrophy.

Cardiomyocytes are vulnerable to hypoxia in the adult but adapted to the relative hypoxia of the womb. At birth, when oxygen levels become abundant, myocardial metabolism switches from the hypoxic intrauterine mode of ATP generation predominantly from glucose via glycolysis to the aerobic mitochondrial β-oxidation of lipids and the tricarboxylic acid cycle as the primary source of energy, and a mature set of enzyme proteins is expressed. Relatively little is known about the mechanisms regulating these changes in cardiac energy metabolism. Specifically, the molecular link between oxygen availability and molecular control of energy metabolism remains ill-defined. With pathologic hypertrophy, accompanied by (subendocardial) ischemia and hypoxia, the fetal metabolic program can become reactivated [108–111] to varying degrees, resulting in upregulation of genes encoding embryonic isoforms of proteins that govern contractility, calcium handling, and energetics, with a concomitant downregulation of adult isoforms. Indeed, the striking reexpression of fetal genes including those for β-myosin heavy chain (β-MyHC), atrial and brain natriuretic peptides (ANP and BNP), and skeletal α-actin (SKA), as well as enhanced expression levels of the adenylyl cyclase type 5 protein isoform, characterize PO hypertrophy [112–115]. Fairly recent work [116] indicates that two qualitatively distinct myocyte populations exist within the same hypertrophic heart: one that is hypertrophic and expresses predominantly the α-MyHC fetal isomorph and a second that is not hypertrophic and expresses both β- and α-MyHC. This implies that β-MyHC reexpression is a biomarker of cellular normotrophy, viz., normal development with neither hypertrophy nor hypotrophy, under circumstances that overall induce cardiomyocyte hypertrophy [117]. Perhaps the increase in total myosin heavy chain that results in the β-MyHC cardiomyocytes makes them resistant to increases in cell size; consequently, the induction of β-MyHC effectively retains normotrophy.

Knowledge of the genetic basis of pathological morphomechanical remodeling in AVS is of substantial potential value to its medical management, as suggested by the Epigraphs. Beyond discerning biomarkers useful in diagnosis and treatment, it can also lead to identification of candidate genes for therapeutic interventions and to useful genetic tests for disease susceptibility in the aging population. If genes that are displaying intensified or weakened expression in hypertrophying cells act in an identified pathway, protein complex, or regulatory circuit, then that process may be modified/corrected by developing appropriate therapeutic agents and interventions; this expectation is strengthened if two or more genes are implicated in the same pathway or complex. In due course, molecular and genetic variation within the nonhomogeneous CAVD cohort may suggest personalized medicine treatments. It is becoming evident that transcription factor activity and subsequently the orchestration of gene expression bring about epigenetic regulation and adaptations that are customized to the cellular context and to signals emanating from the myocardial biomechanical Bernardian environment [3, 6, 55]. A number of transcriptional regulator systems have been identified as playing important roles in cardiac gene expression during the development of (mal) adaptive cardiac hypertrophy, as we shall see next.

Transmembrane G Protein-Coupled and Tyrosine Kinase-Associated Receptor Signaling Regulating Myocardial Hypertrophy

The myocardium senses many disease-inducing stimuli, such as the PO-prompted chronic demand for higher systolic force production in AVS, either directly through biomechanical stretch-sensitive receptors [5, 6, 118–120] or through an array of ligand-responsive membrane receptors. The latter include G protein (heterotrimeric guanine nucleotide-binding protein)-coupled transmembrane receptors (GPCRs) and tyrosine kinase-associated transmembrane receptors (RTKs) that have a cytosolic intracellular domain with protein-tyrosine kinase activity (see Fig. 1). Other protein kinases are operative, too, in a similar fashion—e.g., serine/threonine kinases. Achieving optimal molecular recognition rests on the employment of receptor-substrate pairings presenting complementarity in molecular geometry and interactions. GPCR and RTK receptors can recognize and bind signal proteins/ligands in blood and myocardial extracellular tissue fluid, which induce strong cardiomyocyte hypertrophy, such as peptide growth factors including insulin-like growth factor 1 [121], which induce strong anabolic/hypertrophic effects, hormones including catecholamines, endothelin-1 [122], cytokines including cardiotrophin 1 [123], chemokines, and Ang II.

Recently, synthetic gene networks have been established that record the activation of plasma membrane receptors, such as GPCRs and RTKs [124]. The initiating stimuli then converge on a finite array of intracellular signal transduction pathways (see Fig. 1) involving protein cascades that convey the information to the nucleus to mediate the cardiac adaptive/maladaptive hypertrophic transcriptional response [3, 5, 6, 54, 55, 125]; this response includes the re-induction of the fetal gene program and new sarcomeric growth. Receptor-mediated activation of the guanine nucleotide-binding protein Gαq, one of the three subunits (α, β, and γ) of heterotrimeric G proteins of the Gq family, stimulates myocardial hypertrophy. Although hypertrophy is the initial result of Gαq activation, with time and sufficient strength, this signal/stimulus mediates progression to pathology with fetal gene expression and apoptosis (Fig. 1); thus, compensated LVH and HF may be two different physiologic states representing different stages of one and the same evolving process, initiated by PO [126, 127].

G protein-coupled receptor kinases (GRKs) together with β-arrestins, which are versatile adapter molecules that sterically hinder the G protein coupling of activated GPCRs, normally desensitize receptor signal transduction, thus preventing hyperactivation of GPCR second messenger cascades, released in response to the exposure to extracellular signaling molecules, and ultimately resulting in receptor desensitization [128, 129]. Similarly, the binding of ligands (e.g., growth factors) to the extracellular domains of the transmembrane receptors activates their cytosolic kinase domains, resulting in phosphorylation of both the receptor proteins themselves and intracellular target proteins, and setting off a signaling cascade that can then propagate the signal initiated by growth factor/ligand binding (see Fig. 1). Transmission of the signal to effector molecules occurs down this signaling cascade where every protein in succession changes the conformation of the next down the path, most commonly by phosphorylation (via kinases) or dephosphorylation (via phosphatases). The final effect is to trigger myocardial adaptive/maladaptive cell responses, by the activation of intranuclear gene transcription processes.

Because they entail quite a few intracellular signaling steps that eventually lead to alterations in cardiomyocyte gene expression that give rise to hypertrophic growth, the responses to the signal proteins/ligands usually evolve slowly. Transmembrane receptor (e.g., G protein-coupled and tyrosine kinase-coupled) signaling pathways are not mutually exclusive of one another and often function as partners, with interactions occurring at multiple levels between various molecules downstream of the receptors [130–132]. The involvement of common molecules brings about an integration of distinct stimuli through complex cross-communication/crosstalk and affords intricate control over regulatory mechanisms that affect cardiac cell function, growth, and survival and ECM morphomechanical traits.

By way of the transmembrane signaling systems and in response to cardiac stress resulting from CAVD-associated hemodynamic overload, several neurohumoral adaptations occur, including the activation of the renin–angiotensin and the sympathetic nervous systems, bringing about heightened and continuous GPCR stimulation in the heart [133]. GPCR stimulation promotes G protein signaling and, to limit unrestrained stimulation in the presence of continuous agonist stimulation, activation of G protein-coupled receptor kinases leads to agonist-dependent receptor phosphorylation, which promotes receptor “desensitization” as well as β-arrestin binding to the receptors [134]; this prevents further G protein signaling. Although such neurohumoral activation contributes to compensation and the maintenance of perfusion of vital organs, it can instigate impaired GPCR signaling and several maladaptive intracellular consequences, leading to inappropriate cardiac remodeling and eventually to failure [135]. Indeed, adrenergic receptor blockers, angiotensin receptor blockers, and angiotensin-converting enzyme inhibitors can reverse cardiac remodeling, reduce cardiac hypertrophy, slow the progression of heart failure, and improve patient survival [135–137].

Renin-Angiotensin System

Locally expressed or intra-organ renin-angiotensin systems (RAS) have been described in a number of organs including the heart and have been shown to be responsive to many stimuli of (patho) physiological importance. They comprise locally generated biologically active angiotensin peptide fragments (e.g., angiotensin I, II, III, IV and angiotensin 1–7), which act through distinct cellular receptors and have a plethora of effects implicated in hormonal secretion, cell growth, anti-proliferation, reactive oxygen species generation, apoptosis, pro-inflammatory, and pro-fibrogenic processes [138].

The principal cardiac receptors for Ang II have been classified into two main subtypes: the Ang II type 1 receptor (AT1-R) and the Ang II type 2 receptor (AT2-R). AT1-R mediates most of the cardiovascular effects of Ang II. AT2-R is expressed strongly in the developing fetus; its expression is normally very subdued in the adult cardiovascular system but it can be upregulated in pathological states associated with tissue remodeling, as in cardiac hypertrophy and fibrosis [139]. Ang II plays a critical role in cardiac remodeling and promotes cardiac myocyte hypertrophy. The AT1-R on cardiac myocytes is a seven-transmembrane heterotrimeric G protein-coupled receptor of the G_αq/α11 subclass; it can be directly activated by mechanical stress and is involved in PO-induced cardiac hypertrophy [133]. The activated AT1-R initiates complicated intracellular signaling cascades by way of G protein-dependent and G protein-independent processes. G_αq/α11 coupling is a necessary occurrence in the induction of pathological myocardial hypertrophy, whereas IGF-1 signaling controls physiological hypertrophy, as we saw in an earlier section.

AT1-R blockers (ARBs) antagonize the activation of AT1-R and consequently can regress cardiac remodeling. Some ARBs exhibit properties of inverse agonism and arrestin-biased agonism at the AT1-R [140], which perhaps render them potential therapeutic targets for the treatment of PO-induced maladaptive cardiac hypertrophy. Inverse agonists bind to the same receptor as an agonist but induce an opposite response. “Biased” agonists selectively stabilize only a subset of receptor conformations induced by the natural “unbiased” ligand, thus preferentially activating only certain signaling mechanisms; such biased agonists thus open up the intriguing prospect of directed cellular signaling, ensuring unprecedented precision and specificity. Accordingly, there arises the alluring concept that biased agonists may in the future pin point innovative classes of effective therapeutic agents that could give rise to fewer side effects.

Sympathetic Nervous System

The sympathetic transmitters, norepinephrine (NE) and epinephrine (EPI), bind to specific adrenergic receptors (ARs), which are specialized macromolecules embedded in the cell membrane and signal primarily through interaction with heterotrimeric G proteins. The ARs form the interface between the sympathetic fibers and the cardiovascular system. The human heart contains beta1, beta2, and beta3 receptors [141]; β1- and β2-AR subtype stimulation exerts positive inotropic, chronotropic, and lusitropic effects, as well as positive dromotropic effect for impulse conduction through the atrioventricular node. Beta3-ARs are predominantly inactive during normal physiologic conditions but their stimulation elicits a negative inotropic effect, opposed to that induced by β1- and β2-ARs and acting as a relief valve during stress-induced intense adrenergic stimulation [142].

Cardiac pressure overload hypertrophy is regulated by a coordination of β1- and β2-AR signaling, acting in concert to develop an effective hypertrophic response; β2-AR signaling, linked to both similar and divergent downstream effectors compared with the β1-AR, may limit deleterious remodeling by putting a brake on the hypertrophic process. The heterogeneity of the G protein alpha subunit, of which there are many subtypes, is at the bottom of G protein-coupled receptor signaling, allowing both “receptor-accelerator” and “receptor-brake” actions [143].

GATA4 and GATA6 as Key Transcriptional Regulators of Hypertrophic Gene Expression

Following the activation of membrane-bound receptors, several transcriptional regulators that are actuated by the intracellular signaling cascades operate on molecular pathways that control cardiac hypertrophic gene expression [144]. Notable among them is the GATA4, GATA5, and GATA6 transcription factor family, which mediates hypertrophic gene expression in the myocardium. GATA5 is restricted to the endocardium, whereas GATA4 and GATA6 are expressed in the developing and postnatal myocardium [145]. There they regulate differentiation-specific gene expression both in the course of fetal cardiac development and during (patho) physiological stress and mediate the induction of genes that are involved in physiological and in pathological cardiac hypertrophy [146–148], as occurs in AVS. Indeed, the hypertrophic response of the adult heart involves reexpression of many fetal genes, suggesting that the developmental and disease gene programs share common regulatory events, potentially through GATA4/5/6 [149–151]. The overexpression of GATA4 in the heart was found to produce cardiac hypertrophy and GATA4 to be important for the preservation of cardiac function in response to PO. Moreover, the tissue-specific deletion of Gata4 from the heart attenuated cardiac hypertrophy following both PO, as occurs in CAVD/AVS, and exercise stimulation, as occurs in the athlete’s heart [152].

Expressed in the adult heart, GATA4 and GATA6 function as key transcriptional regulators of numerous cardiac genes, including ANF for atrial natriuretic factor, BNP for b-type natriuretic peptide, α-MHC and β-MHC for α- and β-myosin heavy chain, CARP for cardiac-restricted ankyrin repeat protein, and many others [148, 153]. Besides GATA transcription factors, other transcriptional regulators thought to mediate the reexpression of the “fetal program” and genes implicated in regulating the hypertrophic adaptive response process in adult hearts could become interesting therapeutic targets, too. They include the nuclear factor of activated T cells (NFAT, see Fig. 1), myocyte enhancer factor 2 (MEF2) transcription factors, and the homeobox cardiac transcription factor Csx/Nkx2–5 [151]. Elucidation of the mechanisms of action of these transcription factors should offer a molecular framework that will aid our comprehension of the molecular basis of physiological and pathological myocardial growth/hypertrophy.

MicroRNAS as Posttranscriptional Modulators of Gene Expression and Cardiac Phenotype

“Non-protein-coding RNAs” (ncRNAs), including microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), impact epigenetic biologic responses through the regulation of mRNA transcription and, no less importantly, translation. LncRNAs are transcribed RNA molecules >200 nucleotides in length that have no significant protein-coding potential and can be divided into cytoplasmic lncRNAs and nuclear lncRNAs. The latter guide chromatin modifiers such as DNA or histone methyltransferases to specific genomic loci; the effect is to engender a repressive heterochromatin state and, therefore, a downregulation of transcription [154].

Substantive evidence now implies that the expression of a transcriptional regulatory factor, in itself, is not necessarily a dependable indicator of the levels of active molecular/cellular product(s). Protein–protein interactions and posttranslational protein macromolecule-modifying electrostatic and steric mechanisms can serve to modulate activity dynamically. Consequently, differences in translation efficiency and posttranslational protein modifications or interactions may also be co-determinants in regulating active molecular/cellular product(s) levels, myocardial structure, and function [3, 6, 55]. Cytoplasmic lncRNAs regulate gene expression either constructively or obstructively at the translational level by binding to targeted mRNAs or by sequestering miRNAs and preventing them from accomplishing translational repression of their targets. LncRNAs form a novel, poorly characterized type of ncRNAs, and it is still a major question to understand their precise roles in relation to normal physiology and disease. Therefore, literature on the role(s) of lncRNAs is scarce [155].

Considering that our understanding of the (patho) physiological relevance of lncRNAs is currently confined within the context of heart regeneration, it is envisioned that they will initially form an essential component of the emerging field of myocardial regenerative medicine [7–9, 156–160]. It is not yet established whether lncRNAs are significantly involved in the regulation of cardiac hypertrophy, but one lncRNA (cardiac hypertrophy related factor, CHRF) serves as an endogenous sponge of miR-489 and allows the expression of the myeloid differentiation primary response gene 88 (Myd88), which activates hypertrophic adaptive response processes [161]. To date, most studies of the role of noncoding RNAs have focused on miRNAs.

The understanding of the functionality of RNAs as both elegant messengers for protein translation and self-contained versatile regulatory elements has greatly expanded in recent years. MicroRNAs are noncoding single-stranded RNA molecules, about 18–25 nucleotides long that regulate protein expression of target mRNAs having complementary sequences; a single mRNA may be simultaneously targeted by one or more different miRNAs that cooperate synergistically to repress the mRNA, and a single miRNA is liable to have multiple mRNA targets [162–164]. The first miRNA was discovered in 1993, when a gene identified in the transparent-bodied worm Caenorhabditis elegans as critical for development did not prove to encode a protein, but its RNA was responsible for silencing another gene [165]. MiRNAs regulate gene activity [166], by switching gene expression on-or-off, by tweaking genes controlling important adaptive/maladaptive processes, such as cardiomyocyte hypertrophy, and by translational suppression or degradation of their target mRNAs [167]. Remarkably, regulatory miRNAs can exert posttranscriptional simultaneous repression of hundreds of genes, by inhibiting mRNA translation into protein [6].

Since miRNAs are themselves transcribed from DNA, rendering their own expression subject to transcriptional control, miRNA effects on mRNA translation encompass means of fine-tuning cardiomyocyte structural and functional protein expression through complex feedback, feedforward, and crosstalk regulatory pathways [1]. This renders them attractive therapeutic targets for managing/reversing cardiac hypertrophy and heart failure [168–171]. Advancements in genomic techniques involving linear amplification of nanogram amounts of mRNA nowadays permit transcriptional profiling of small tissue regions and samples that are reflective of any applying operating steady-state of interest in vivo [172, 173]. The ensuing enhanced sensitivity of detection augments the discovery potential of high-throughput screening by microarrays [6, 55].

Attendant to pathological cardiac growth/remodeling and heart failure, a reexpression of fetal genes, mediated by epigenetic processes involving miR-208, a cardiac-specific microRNA transcribed from the a-myosin heavy chain (a-MHC) gene locus [174], was found to bring forth temporary stabilizing effects—in a standard miRNA nomenclature system, the prefix “miR” is followed by a dash and a number usually indicating order of naming. In subsequent studies, miRNAs have proved to be key regulators of the responses to disease of the adult heart [175–177]; e.g., upregulation of miR-21 in response to biomechanical myocardial stress/injury protects fibroblasts against apoptosis and leads to LV interstitial fibrosis with ensuing hemodynamic dysfunction, as discussed earlier.

The regulatory role of miR-21 in cardiomyocyte hypertrophy remains controversial, however [175]; whereas some studies find that miR-21 promotes the hypertrophic growth of cultured neonatal cardiomyocytes, others report that miR-21 represses the hypertrophic growth of myocytes. The circulating levels of miR-21 were shown to be higher in AVS patients when compared to cohorts of healthy individuals [175]. Moreover, the plasma levels of miR-21 directly correlated with the myocardial expression levels of miR-21, collagen I, and fibronectin, and they maintained a direct linear relationship with the mean aortic transvalvular pressure gradients [175]. Similarly, circulating levels of miR-378 have been shown to gauge LV hypertrophy in patients with AVS [178]. Overexpression of miR-133 or miR-1 has been shown to inhibit cardiac hypertrophy; in contrast, suppression of miR-133 induced hypertrophy [179].

Such intriguing findings suggest that, during development of CAVD-induced PO cardiac hypertrophy, progressive changes in the plasma levels of carefully chosen microRNAs could constitute a clinically relevant measurable characteristic and might serve as biomarkers conveying information on the extent of myocardial fibrosis and its modifications that should ensue with effective pharmacological/surgical treatments. Interestingly, as key regulators of cardiac hypertrophy, selected miRNAs represent potential therapeutic targets in maladaptive LVH accompanying PO in CAVD and in systemic hypertension; optimally, methodologies will need to be developed to affect miRNAs selectively in cardiomyocytes and not in other tissues or organs.

LVH Reversal After Aortic Valve Replacement

In the recent past, balloon aortic valvuloplasty (BAV) [93, 180] showed great promise for AVS relief and transaortic pressure gradient reduction (Fig. 4), but it has subsequently proved to have only disappointing longer-term hemodynamic usefulness, which is outweighed by the significant risk of procedural complications and the high odds of recurrent stenosis within 6 months [181]. It is now restricted to occasional patients with hemodynamic compromise, as a bridge to more definitive procedures [182]. Surgical aortic valve replacement (SAVR) and the minimally invasive transcatheter aortic valve implantation/replacement (TAVI/TAVR) are generally salutary therapeutic interventions [183–187]. Indeed, a large, multi-center Japanese registry has recently suggested that, for severe (defined by a peak aortic jet velocity ≥4.0 m/s, mean aortic pressure gradient ≥40 mmHg, or aortic valve area ≤1.0 cm²) but asymptomatic AVS, AVR was associated with better long-term outcomes than “watchful waiting” for symptoms to develop [188]. Without interventional treatment, symptomatic patients with severe AVS have a bleak prognosis with the 1-year mortality exceeding 50 % [189].

Fig. 4.

Immediately after balloon aortic valvuloplasty (BAV) on a patient with AVS, there is a spectacular reduction of the ventricular ejection pressure gradient, which is offset by a complementary increase of the aortic root systolic pressure. LV systolic pressure (total systolic load) remains unchanged; it does not decrease acutely. This is a manifestation of complementarity and competitiveness—see discussion in part 1 [1]—between the competing intrinsic component (ventricular ejection pressure gradient, hatched area) and extrinsic component (aortic root systolic pressure) of the total ventricular systolic load (LV systolic pressure). AoP: aortic root pressure; LVP: left ventricular pressure; these pressure waveforms are ensemble averages of many steady-state beats obtained by a double-tip multisensor Millar left-heart catheter. Redrawn from Shim et al. [93] by permission of Circulation Research

TAVR was developed for the treatment of patients with severe symptomatic AVS who have an unacceptably high estimated surgical risk [189] or in whom TAVR is indicated due to technical problems with surgery, e.g., a porcelain aorta (extensively calcified ascending aorta and/or aortic arch) complicating CAVD. Relief of the mechanical LV outflow obstruction generally leads to spectacular immediate hemodynamic improvement, akin to that of successful balloon aortic valvuloplasty (see Fig. 4), and induces a variable reversal of the adaptive hypertrophy—reverse remodeling. LVH is dissimilar in patients with a comparable grades of stenosis (see also the next section), and its extent of regression following surgical correction is also inconsistent [190].

Regression of experimental PO-induced (aortic arch banding/debanding) LVH is not merely the reverse of LVH induction; it actually proceeds as a distinct process with a specialized gene expression profile [191–193]. Stepped-up examination of such genomic response profiles should improve management of pathological LVH and could lead to an unconventional new therapeutic paradigm focused on stimulating regression of myocardial hypertrophy. Genetic predisposition of a patient, resulting from often inherited specific genetic variations, probably interacts with the effective hemodynamic loading pattern [3, 5, 6, 54, 55, 60, 194] (the Bernardian “environment,” see Epigraphs) to determine the time-related evolution and morphomechanical characteristics of LVH, its regression, and the overall clinical progress after valvular surgery. It follows that LVH regression cannot be viewed as a simple “degrowth,” or passive reversal of growth triggered by reduction in afterload, but also requires the induction of specialized gene expression program(s) [191–193].

Regression of LVH starts straightaway after AVR but may go on for years [195]. Among patients with severe AVS who are at increased risk for surgery, earlier recovery of LV systolic function [196] and significantly higher 1- and 2-year survival rates have been achieved in those treated with TAVR than in the SAVR cohort [197, 198]. A higher prevalence of prosthesis–patient mismatch in SAVR is partially offset by a higher occurrence of paravalvular regurgitant leaks in TAVR [199, 200]. The number of TAVRs has grown considerably, and this will likely continue as this technology becomes progressively more available [201]. Clinical status and long-term survival after AVR depend on a number of clinical variables, including the pattern of pre-operative LVH, especially the severity of myocardial fibrosis, and the presence and extent of myocardial dysfunction with LV dilatation and congestive failure [202].

Gender Effects on LVH and Its Post-op Reversibility

Female AVS patients and experimental animals acquire a more concentric LVH pattern with smaller effective chamber dimensions, higher relative wall thicknesses, and better systolic but worse diastolic function than males with comparable levels of PO; additionally, supervening diastolic heart failure with normal ejection fraction (HFNEF) is much more common in women than men [203, 204]. In comparison, males are usually characterized by a more eccentric hypertrophy with greater chamber capacity, systolic dysfunction with decreased ejection fraction culminating in HF, and increased interstitial collagen deposition with myocardial fibrosis [3, 205–211]. Higher levels of transforming growth factor beta 1 (TGFβ-1) that helps control the growth and apoptosis of cardiomyocytes, SMAD2 phosphorylation that activates TGF-β superfamily signaling, and the protein periostin that is a potent osteogenic molecule involved in osteogenesis in inflamed valves with dysregulated mineral metabolism are found in men than in women [212]. A greater capacity of female hearts to develop physiological hypertrophy could be a factor in the more favorable morphomechanical remodeling in women than men under pathological PO [205, 206]. The sexually dimorphic cardiac response of the heart to exercise is modulated by estrogen receptor beta (ERβ) [213], a nuclear receptor which is activated by estrogen (17β-estradiol) and is encoded by the ESR2 gene [214, 215].

In the current context, a few additional observations are noteworthy. LVH is the most important initiating risk factor for the eventual development of HF [216]. Measurements in AVS patients at the time of AVR have revealed higher mRNA expression of fibrosis-associated genes in men as compared to women [217]; specifically, they concern genes for type I and type III fibrillar collagen [218], which form hybrid fibrillar bundles with the help of glycoproteins and proteoglycans and are responsible for elevated myocardial and LV chamber stiffness [56, 58–60]. Higher too in men as compared to women are gene expression levels for the matrix metalloproteinases MMP-2 and MMP-9, which are cardiac fibroblast-derived proteases that are important for ECM degradation [219]. Combined with the higher mRNA expression of fibrosis-associated genes in men, this allows fine-tuned control of ECM degradation and remodeling within the hypertrophying LV myocardium. Furthermore, MMPs themselves can be inhibited by a class of soluble proteins known as tissue inhibitors of metalloproteinases (TIMPs) [219]. Thus, myocardial cells secrete ECM proteins as well as the proteases that degrade them and their inhibitors, thereby establishing ordinarily an abundantly sensitive balance between matrix protein assembly and degradation.

In hypertrophying myocardium, there is accumulation of type I and type III fibrillar collagenous connective tissue, as a result of the relative preponderance of MMP inhibitors. In supervening LV failure with structural dilatation, there is no TIMP preponderance and there is heightened collagen degradation characterized by an increased MMP activity. This can lead to disruption of crosslinking of collagen fibrils with the corollary of reduced LV mural strength and stiffness and ensuing myocardial creep [3, 6, 73, 96]. In mice subjected to transverse aortic constriction-induced LV hypertrophy and failure, estrogens moderate the progress of fibrosis and apoptosis [220]. Moreover, it is well appreciated that premenopausal women with AVS have a more favorable prognosis than men [221].

In evaluating the above findings regarding gender and myocardial PO-associated fibrosis, it should be born in mind that estrogens appear to have antiproliferative effects on cardiac fibroblasts, while androgens have contrary effects. Consequently, estrogens in women may restrain the upregulation of fibrosis-associated genes and collagen, and this may contribute to the more beneficial morphomechanical remodeling in female hearts vs. male hearts under stress conditions of PO accompanying AVS [222, 223]. Female sex was shown to offer protection against LV chamber dilatation in a mouse model of AVS-induced PO and both female sex and estrogen receptor-β to attenuate the development of fibrosis and apoptosis, thus slowing the progression to HF [224]. However, aging enters into the picture too: given that older patients have relative hypogonadal hormone levels, it is likely that this protective effect in women relative to men is lost progressively with aging. There are only few studies looking into the effect of gender on post-operative survival and reversibility of LVH after AVR; nonetheless, there is persuasive evidence that after TAVR/SAVR women reverse the hypertrophy more rapidly and that early LVH regression is more marked in women than men [217, 225–227].

Patient-specific genetic predisposition undoubtedly interacts with hemodynamic load to regulate the pattern of LVH, its regression, and the clinical course post-operatively. Correlating gender-related differences in pre-operative LVH patterns to regression and survival after AVR may expand our appreciation of their consequences and contribute fresh, gender-specific parameters and strategies for a most advantageous scheduling of interventions.

Recap and Prospects

Proper management of CAVD/AVS patients should entail a comprehensive approach that encompasses not only the aortic valve but also the hypertrophic left ventricle and the systemic arterial system, with their inexorably interdependent morphomechanics and hemodynamics. Physiological hypertrophy, due to training, is a beneficial process. The pathological hypertrophy caused by the progressively increasing wall stresses of AVS in CAVD may initially be a compensatory response to the systolic PO that only later on becomes maladaptive, or it may represent a myocardial growth process that is detrimental from its inception. In the latter case, it follows that incongruent molecular pathways are implicated in pathological as opposed to physiological hypertrophy. Moreover, it is gradually becoming apparent that the initiation/inhibition of cardiac hypertrophy encompasses multiple signaling pathways and that these diverging types of hypertrophy differ on both the morphological and the molecular levels (see Fig. 1).

How signaling pathways can be targeted therapeutically in the setting of CAVD/AVS is a captivating prospect and a formidable challenge. Inherent in this challenge are two concepts: (a) that certain of the differential gene expression patterns across hypertrophic pathophysiologies may be beneficial and some others can be detrimental to patient survival and (b) that changes in gene expression, and hence cellular and organ cardiac phenotype, may not follow a unidirectional pathway, as is strikingly demonstrated by the ability of appropriate somatic cell genetic reprogramming to turn out pluripotent stem cells from terminally differentiated somatic cells. This perspective offers hope that undesirable gene expression in pathological hypertrophy can be corrected, either by in situ myocardial gene expression modulation or through the generation of myocardial cells that exhibit a more favorable gene expression profile. Considering our rising knowledge of the regulatory functions of ncRNAs in cardiac homeostasis and disease, it may be envisioned that ncRNAs will form an essential component of the emerging field of myocardial regenerative medicine for years to come. Human musings on regeneration are ancient, as is epitomized by the tale of the luckless Greek Titan, Prometheus, who by day was tormented by an eagle ripping out his liver, only for the organ to regenerate overnight, ready to be torn out again the following day.

CAVD-associated, PO-induced LVH is generally accompanied by cardiomyocyte apoptosis, myocardial ECM fibrosis, large increases in cardiomyocyte diameters and ventricular wall thickness, regional (subendocardial) ischemia, and fetal gene expression. Conversely, physiological remodeling is usually not accompanied by ischemia, fetal gene expression, apoptosis, or an accumulation of collagen in the myocardium and does not go above a modest increase (say, ≤10 %) in LV wall thickness. The isoform expression of a-/β-MHCs is regulated in opposite directions in exercise as against AVS-induced cardiac hypertrophy. However, some hypertrophic pathways seem to be activated in both pathological and physiological LVH. These differences and commonalities suggest that physiological and pathological PO-induced hypertrophy differ at the molecular level but that this does not preclude that a number of common mechanisms, pathways, and cascades may be implicated in both of these varieties/phenotypes of myocardial hypertrophy.

Signal transduction pathways precede cellular/molecular mechanisms controlling myocardial gene expression; molecular cascades leading to activation of one or more specific transcription factors are actuated. The extracellular signals, in the form of dynamic stresses and ligand/effector molecules that bind to specific receptors to initiate the hypertrophy-producing pathways, are transcribed across the sarcolemma via an assortment of second messengers. It thus seems proper to view hypertrophic signaling as an energetic network, integrating and modulating a large number of input signals. This implies that even if some treatments bring about a similar extent of inhibition of LVH, the functional repercussions could vary markedly. Along these lines, various interventions restraining to a comparable extent the hypertrophic response to PO may preserve a normal LV systolic function or could precipitate a depressed systolic function with decompensation, implying that the hypertrophic response could be dissociated from myocardial contractile (dys) function.

A comprehensive itemization of hypertrophic signaling may in due course make it practicable to obtain therapeutic benefit from selected aspects of hypertrophy—e.g., improved sarcomere organization and mitochondrial function—while avoiding others—e.g., unsafe excessive increase in myocardial mass and ventricular wall thickness or myosin isoform switching. An intriguing target for prevention and treating is offered by the transcriptional pathways in pathological myocardial growth, because they are controlled by a rather small number of molecules, such as the transcriptional regulators GATA4 and GATA6, NFAT, MEF2, and the homeobox cardiac transcription factor Csx/Nkx2–5. Epigenetic agents bringing about gene expression reprogramming, such as histone deacetylases, negatively regulate these mediators of cardiac hypertrophy and thus could represent a particularly attractive potential focus for an inhibitory line of attack. Although inhibition of LV hypertrophy might be beneficial, even if the original stimulus (viz., raised mural stresses associated with surgically uncorrected AVS) persists, it is possible that long-term therapeutic inhibition of cardiac hypertrophy with elevated wall stresses might in due course still phase-in heart failure. Whatever the therapeutic antihypertrophic intervention may ultimately be, its effect should be maintained indefinitely and it should not adversely affect contractile function. Furthermore, antihypertrophic treatments will likely have to be combined with complementing strategies, such as the modulation of Ca²⁺ cycling, to enhance contractile function and prevent HF.

Upcoming studies will be needed to better and more completely characterize the overall dynamics, encompassing intricate feedback mechanisms involving gene network cascades, specific signaling molecules, including mechanoreceptors and mechanotransducers, and hemodynamic/myocardial stresses. In closely investigating the pertinent genomics and transcriptomics, post-genomic technologies such as high-throughput genotyping will certainly be of extraordinary importance for collecting the large-scale data on the genetic variations in individual CAVD patients and in population samples. Such meticulous and comprehensive characterization should pave the way to ascertaining and understanding the gender- and individual patient-dependent specific processes that underlie PO-induced LVH and should provide clinically valuable biomarkers. It should also clarify conclusively the mechanisms by which adaptive LVH in CAVD transitions into maladaptive, presaging congestive HF.

In performing such big data studies, following Aristotle’s paradigm, one should always remember that statistical correlation/regression cannot by itself yield causal explanations (understanding) and prediction, although it commonly provides a useful conceptual aid and it can be a solid source of insights. In absence of a cogent, validated theoretical framework, however, mere correlation does not imply causation, and one cannot predict what will result from changing either the structure of the population sample or the biochemistry of the individual(s) investigated. The comprehensive, detailed molecular mechanisms underlying myocardial cell reprogramming in PO-induced LVH still remain elusive. Bearing these facts in mind, our gradually/rapidly (?) developing future knowledge will all along be more or less directly harvestable, allowing us to treat CAVD/AVS in the aging population more rationally and strategically.

Conclusion

I have addressed the present survey to a multidisciplinary audience interested in the topic of CAVD and AVS, each individual reader approaching it from different perspectives and vantage points. Accordingly, a balance had to be struck allowing optimal breadth and depth of coverage, relying in part on an extensive list of cited references, pieced together to fill in plausibly desirable additional pertinent information. Analogous considerations pertain to readability and balance, in view of the intended pluridisciplinary audience of JCTR. What to one particular reader is necessary and to another not in accomplishing my goal is, in large measure, uneven and subjective by necessity. It cannot apply uniformly for a clinical cardiologist, a biochemist, a molecular biologist, an interventional catheterization expert, a geneticist, a bioinformatics specialist, a physiologist, a histomorphologist, a fluid dynamicist, a cardiac surgeon, and so on. And yet, these readers are linked by their shared interest in cardiovascular translational research as applied to CAVD and to multifaceted as well as multiscale (mal) adaptive myocardial responses to this disease and its management. Cardiovascular science has proceeded by developing ever more detailed maps of decreasingly small phenomena and this tradition has indeed led to prodigious advances in invaluable translational knowledge. Still, systems are embedded within multifaceted and multiscale organismal assemblies, and this “embeddedness” matters. Detailed attention to a piece of glass does not help one see, and appreciate, the image that emerges from a stained-glass window. And, while complex organs/organisms can be broken down into elemental parts and processes which are interesting and illuminating in and of themselves, every compensatory/adaptive change within an organ or the organism as a whole changes the context for and impacts other morphomechanical aspects in ways that we could not expect/predict. It is for this reason that, e.g., despite reams of meteorological data and enormously powerful supercomputers, accurate, detailed long-range weather forecasting is effectively unachievable.

Real understanding lies in the complex way that the multiscale organ/organismal parts and processes come together and are interconnected to fulfill the cardiac vital functions in health and disease. The strategic view of my treatment in this two-part survey is a “helicopter excursion view between 500 and 1000 feet,” enabling my nonhomogeneous audience to clearly recognize the multifaceted essence of what they are looking at, but with the benefit of seeing it from a different, higher and more comprehensive perspective, beyond the confines of their distinct specialty. At this higher level, they can expand their own discipline’s perspective to understand how the core-foundational cross-disciplinary aspects of CAVD fit together, interact, and evolve and should thus derive better comprehensive understanding and stronger foundations of translational knowledge. Of course, any such excursion must, by necessity, be a selective swath through a large field of existing ideas and data. Along with its companion paper (part 1) [1], this survey (part 2) aims at a pressing need as a contribution to the cross-disciplinary topic of CAVD: to transcend the limits of reductionism and expose new connections between translational cardiology facets of CAVD.