1-1. Cardiac remodeling
1-1-1. Concepts of cardiac remodeling
Cardiovascular disease will be the greatest health care burden of the twenty-first century [Crackower et al., 2002]. The term “remodeling” implies changes that result in the
rearrangement of normally existing structures [Swynghedauw, 1999]. Cardiac remodeling (CR) is defined as genome expression resulting in molecular, cellular and interstitial changes and manifested clinically as changes in size, shape and function of the heart resulting from cardiac load or injury, cardiac remodeling is influenced by hemodynamic load,
neurohormonal activation and other factors still under investigation [Cohn et al., 2000]. The concept of myocardial remodeling excludes concomitant changes in the cardiac atria, valves, blood vessels, and pericardium [Swynghedauw, 1999].
Cardiac remodeling is generally accepted as a determinant of the clinical course of heart failure (HF). Heart failure is an all-too-frequent outcome of hypertension and arterial vascular disease, making it a major concern in public heath and preventive medicine. It is a common cause of morbidity and mortality, and the incidence is increasing [Tyagi et al., 1995;
Kannel, 2000; Rodeheffer, 2003; Izzo and Gradman, 2004; Mathew et al., 2004; Franklin and Aurigemma, 2005; Hunt et al., 2005; Weir et al., 2006]. Following a specific cardiovascular stress, a cascade of compensatory structural events occurs within the myocardium and
contributes to eventual left ventricular (LV) dysfunction and the manifestation of the heart failure syndrome.
The time course of events is influenced, however, by the severity of the underlying disease, secondary events (such as recurrent MI), other factors (such as ischemia or
neuroendocrine activation), genotype and treatment [Hutchins and Bulkley, 1978; Weisman et al., 1985]. Animal studies also show that infarct expansion, regional dilation and thinning of the infarct zone can occur within one day of an MI [Weisman et al., 1985]. Severe
impairment of global ventricular function, a functional and clinical phenomenon that can be differentiated clearly from LV remodeling, can be observed within two days of an insult [Anversa et al., 1991]. The changes that occur after an insult are summarized in Table 1-1.
There is little doubt that remodeling and its role in disease progression are
multi-mechanistic and complex. Few clinical trials have specifically addressed the role of remodeling in disease progression. The key next steps will be the determination of how the information generated from cellular and molecular models can be used, together with data from clinical trials, to ensure that patients receive optimal therapy at an appropriate time to slow disease progression [Cohn et al., 2000].
Table 1-1. Processes occurring in ventricular remodeling
Processes occurring Description References
Cardiomyocyte lengthening Cardiomyocyte lengthening due to series addition of new sarcomeres and consequent fall in the short/long axis ratio.
Weisman et al., 1985; Anversa et al., 1991
Ventricular wall thins Ventricular compliance depends upon the thickness of the ventricular wall and
on factors, such as fibrosis, that alter the stiffness of the ventricle. Weisman et al., 1985; McKay et al., 1986; Anversa et al., 1991
Infarct expansion rather than extension occurs
Infarct expansion and infarct extension are events early in the course of myocardial infarction with serious short- and long-term consequences.
Expansion has an adverse effect on infarct structure and functional infarct size is increased because of infarct segment lengthening, and expansion results in over-all ventricular dilatation. Infarct extension is defined clinically as early in-hospital reinfarction after a myocardial infarction.
Hutchins and Bulkley, 1978;
Weisman et al., 1985
Inflammation and resorption of necrotic tissue
If early thinning and dilatation did not occur after myocardial infarction, the process of remodeling with resorption of necrotic tissue, laying down of granulation tissue and scar formation would probably result in a healed area that was somewhat thinned but generally preserved normal LV contour.
Hochman and Bulkley, 1982;
Weisman et al., 1985
Scar formation Scar formation is a natural part of the healing process. A scar forms from excess amounts of collagen in the wound as the body attempts a repair.
Zdrojewski et al., 2002
Continued expansion of infarct zone
Infarct expansion is a progressive thinning and dilation of the infarcted zone.
A progressive increase in infarct expansion is associated with increased left ventricular volume and predisposes to remodeling of the non infarcted segment.
Hutchins and Bulkley, 1978
Table 1-1. Continued
Processes occurring Description References
Dilation and reshaping of the left ventricle
Surgically reshaping the adversely remodeled dilated left ventricle is a concept that holds promise in the management of patients with dilated cardiomyopathy.
Weisman et al., 1985; McKay et al., 1986; Olivetti et al., 1990; Gaudron et al., 1993 Myocyte hypertrophy Hypertrophy of the surviving myocytes is an important adaptive response to
loss of contractile fuction. A decrease in cardiac fuction leads to increased levels of norepinephrine and activation of the renin-angiotensin system, leading to release of angiotensn II. Angiotensin II and mechanical stress induce a number of cellular signaling pathways important in the development of cellular hypertrophy.
Olivetti et al., 1992; Kajstura et al., 1994
Ongoing myocyte loss Recent studies in experimental animals have shown that cardiac myocyte loss through apoptosis, or programmed cell death, occurs following myocardial infarction, in the presence of cardiac hypertrophy, in the aging heart, and in the setting of chronic heart failure.
Weisman et al., 1985; McKay et al., 1986; Olivetti et al., 1990; Anversa et al., 1991
Excessive accumulation of collagen in the cardiac interstitium
The accumulation of excess collagen is believed tobe an important
pathophysiological process that contributesto diastolic heart failure. Diastolic heart failure accountsfor 30% to 50% of heart failure in clinical practice, and hypertensivedisease is the major cause of this type of heart failure.
Weber and Brilla, 1991;
Dostal, 2001
1-1-2. The effect of cardiac remodeling
Cardiac remodeling can be described as a physiologic and pathologic condition that may occur after myocardial infarction (MI), pressure overload (aortic stenosis, hypertension), inflammatory heart muscle disease (myocarditis), idiopathic dilated cardiomyopathy or volume overload (valvular regurgitation) [Fedak et al., 2005].
With an increased workload during hypertension, the heart eventually undergoes hypertrophic (enlargement) and fibrotic responses. Myocyte hypertrophy, when
accompanied by fibrosis can lead to a decrease in cardiac function. This cardiac hypertrophy and inappropriate interstitial collagen formation can contribute to increased wall stiffness and diastolic dysfunction. Thus the remodeling process, which could accompany hypertension, would consist of changes in the architecture of the heart, including myocardial fibrosis, and medial thickening of intramyocardial coronary arteries, in addition to the myocyte
hypertrophy. Therefore, ventricular remodeling after myocardial infarction is a risk factor for development of heart failure and sudden cardiac death [Cohn et al., 2000; Fedak et al., 2005; Grobe et al., 2007]. The prevention of ventricular remodeling after myocardial infarction is an important strategy in reducing mortality from myocardial infarction.
1-1-3. Critical factors involved in cardiac remodeling
The renin-angiotensin system (RAS) has previously been established to play an
important role in the progression of cardiac remodeling, and inhibition of a hyperactive RAS provides a protection from cardiac remodeling and subsequent heart failure [Dzau, 1993;
Cockcroft et al., 1995; Parmley, 1998; Bader et al., 2001; Ruiz-Ortega et al., 2001a; Grobe et al., 2007].
1-2. Renin-angiotensin system (RAS)
1-2-1. Physiological and patho-physiological roles of local RAS
RAS is a coordinated hormonal cascade in the control of cardiovascular, renal, and adrenal function that governs body fluid and electrolyte balance, as well as arterial pressure [Peach, 1977]. RAS is well known for its effects on the cardiovascular system and fluid homeostasis. Classically, these effects were thought to result primarily from the systemic production of angiotensin II (Ang II) [Paul et al., 2006]. Circulating Ang II stimulates Ang II type 1 receptors (AT1R) present in the kidney and the vasculature to produce
vasoconstriction but also water and salt reabsorption [Davisson, 2003; Lavoie and Sigmund, 2003].
It has become clear that a local RAS is present in several tissues, for example, the heart, adipose, vasculature, and bone marrow, with similar effects to the endocrine RAS but also more specific functions depending on the individual system [Paul et al., 2006]. One of these local systems, the brain RAS, has long been considered pivotal in cardiovascular regulation and important in the pathogenesis of hypertension and heart failure [Davisson, 2003]. Yet the brain RAS remains poorly understood, because of the difficulty in experimentally dissecting the brain RAS at the cellular, regional, and whole organism levels.
1-2-2. RAS in regulation of cardiovascular homeostasis
Numerous clinical and laboratory data are now available supporting the hypothesis that the renin-angiotensin system (RAS) is relevant in the pathogenesis of cardiovascular diseases [Ruiz-Ortega et al., 2001a; Boos and Lip, 2004; Levy, 2004; Healey et al., 2005]. RAS plays a major role in regulating the cardiovascular system, and disorders of the RAS contribute largely to the pathophysiology of hypertension, renal diseases, myocardial
infarction [Hanatani et al., 1995; Sutton and Sharpe, 2000], atrial fibrillation [Freestone et al., 2004; Savelieva and John Camm, 2004], and chronic heart failure [Weber et al., 1993; Shi et al., 2002]. This is to say that the emergence of cardiovascular diseases is largely related to the regulation of RAS.
1-2-3. Heart failure and RAS
Most cardiovascular diseases are multifactorial quantitative traits controlled by both genetic and environmental factors [Jacob, 1999]. One major factor for cardiovascular disease is the RAS. Due to this continuing morbidity and mortality, significant efforts have
been made to identify new drug targets in the RAS. We note that in human patients, inhibition of angiotensin-converting enzyme (ACE) or Ang II receptors can improve the outcome of heart failure [Garg and Yusuf, 1995; Boos and Lip, 2004; Madrid et al., 2004;
Healey et al., 2005]. To solve the severe issue in the increasingly cardiovascular diseases such as heart failure, the endless stage of heart diseases, a novel efficient approach may be considered.
1-2-4. The components of peptide converting involved in RAS
The protease renin is synthesized and released from the kidney and acts on a circulating inactive peptide, angiotensinogen [angiotensinogen (Asp-Arg-Val-Tyr-Ile-His-Pro-Phe-His- Leu-Val-Ile)], produced by the liver, giving rise to angiotensin I [angiotensin I; Ang I (Asp-Arg-Val-Tyr- Ile-His-Pro-Phe-His-Leu)]. Ang I is then transformed into the
biologically active octapeptide, Ang II [Ang II; Asp-Arg-Val-Tyr-Ile-His-Pro-Phe], through enzymatic cleavage by ACE [Skeggs et al., 1980; Carey and Siragy, 2003a; Lambert et al., 2008], which is a critical regulator of the RAS and the target of a number of highly effective therapeutic agents used to treat cardiovascular and renal diseases. ACE is also a
metalloproteinase which converts the inactive decapeptide Ang I into the potent
vasoconstrictor and mitogen Ang II [Skeggs et al., 1980; Lambert et al., 2008], which can contribute to hypertension by promoting vascular smooth muscle vasoconstriction and renal tubule sodium reabsorption [Skeggs et al., 1980].
In common with ACE, ACE2 is a type I transmembrane metallopeptidase; however, unlike ACE, ACE2 functions as a carboxypeptidase, cleaving a single C-terminal residue from a distinct range of substrates. One such substrate is angiotensin II, which is hydrolysed by ACE2 to the vasodilatory/anti-hypertrophic peptide angiotensin 1-7 [Ang 1-7;
Asp-Arg-Val-Tyr-Ile-His-Pro] [Lambert et al., 2008]. The schematic conversion of angiotensin peptides and balance between ACE/ACE2 was shown in Figure 1-1.
Figure 1-1. Schematic represents conversion of angiotensin peptides and balance between ACE/ACE2 in RAS.
1-3. Angiotensin peptides and core enzyme
1-3-1. Angiotensin II
Ang II is the main effector peptide of the RAS, acting in an endocrine,
autocrine/paracrine, and intracrine hormone involved in the regulation of blood pressure, vascular tone, waster as well as electrolyte balance [Skeggs et al., 1980; Parfrey, 2008].
Historically, Ang II was only seen as a regulatory hormone that regulates blood pressure, aldosterone release, and sodium reabsorption. Now it is generally accepted that locally formed Ang II could activate the cells regulating the expression of many substances,
including growth factors, cytokines, chemokines, and adhesion molecules, which are involved in cell growth/apoptosis, procoagulation, fibrosis, and inflammation (Figure 1-2) [Matsubara, 1998; Sadoshima, 2000; Bader et al., 2001; Ruiz-Ortega et al., 2001b].
Figure 1-2. Abnormal Ang II generation results in cardiac and renal damage. Ang II
activates the transcription factors nuclear factor κB (NF-κB) and activator protein 1 (AP-1) as well as proinflammatory and profibrotic genes. Ang II signaling also leads to impaired balance of cell growth and apoptosis and pro- and anti- coagulative systems. bFGF Basic fibroblast growth factor; TGF-β transforming growth factor β; PDGF platelet-derived growth factor; VEGF vascular endothelial growth factor [Bader et al., 2001].
Ang II binds and activates G protein–coupled receptors, the AT1R and angiotensin II type 2 receptor (AT2R), to mediate its actions [Carey et al., 2000; Shi et al., 2002].
Activation of AT1R mediates most of the cardiovascular responses attributed to Ang II (ie, vasoconstriction, mitogenic and hypertrophic effects, fibrosis, inflammation, and fluid retention) [Booz and Baker, 1995; Booz and Baker, 1996; Unger, 2002]. In contrast, AT2R activation may cause opposing physiological responses that are increased in several disease processes [Ohkubo et al., 1997; van Kesteren et al., 1997]. Multiple lines of evidence indicate that stimulation of Ang II plays a key role in the development of pathological ventricular remodeling [Pfeffer and Braunwald, 1990]. Role of Ang II in the inflammatory response and cardiac damage was shown in Figure 1-3.
Figure 1-3. Role of Ang II in the inflammatory response in vascular injury. Ang II activates mononuclear cells, causing direct chemotaxis and proliferation. In both resident and
infiltrating cells, Ang II, by NF-κB pathway and redox mechanisms, upregulates proinflammatory mediators, such as adhesion molecules, chemokines, and cytokines.
1-3-2. Angiotensin 1-7
Ang 1-7 is a biologically active peptide of the RAS that is known to potentiate the vasodilatory effects of bradykinin [Greco et al., 2006], stimulate NO and prostaglandin release [Rajendran et al., 2005], and antagonize the actions of Ang II [Grobe et al., 2007].
Ang 1-7 has been reported to act as an antagonist to the AT1R and may also work by
antagonizing ACE, which is involved in both the production of Ang II and the degradation of Ang 1-7 [Ferrario, 1998; Castro et al., 2005; Igase et al., 2005].
In the current study, Ang 1-7 infusion prevented cardiac hypertrophy and fibrosis
without having any effect on the elevated blood pressure induced by chronic Ang II treatment.
It has been well documented that Ang 1-7 levels are elevated during pharmacological ACE inhibition and blockade of AT1R [Ferrario et al., 2005a; Ferrario et al., 2005c; Igase et al., 2005], and it has been proposed that these cardioprotective inhibitors may actually work through the actions of increased Ang 1-7 [Ferrario, 1998]. Correlative studies have shown that ACE2 and Ang 1-7 levels are increased by cardiac myocytes in hearts following
myocardial infarction in both rats and human [Averill et al., 2003; Burrell et al., 2005].
Iwata et al. [2005] recently demonstrated that Ang 1-7 attenuates profibrotic signaling within the myocardium, through direct actions on cardiac fibroblasts. In the current study, chronic in vivo administration of Ang 1-7 also appears to have effects on hypertrophic actions on the cardiomyocytes that are induced by Ang II. This observation has also been observed in vitro, as Tallant et al. [2005a] showed that Ang 1-7 acts on cultured cardiac myocytes to inhibit hypertrophic responses through the Mas receptor.
Collectively, these findings suggest that elevated Ang 1-7 may also protect against cardiac hypertrophy in some forms of hypertension. Ang 1-7 delivery has been shown to delay development of cardiac hypertrophy [Santos et al., 2004], inhibit vascular growth [Tallant and Clark, 2003], attenuate development of heart failure [Loot et al., 2002], reduce cardiac Ang II levels [Mendes et al., 2005], and reduce Ang II receptor populations [Clark et al., 2003]. Evidence presented here would support the hypothesis that Ang 1-7 is a
cardioprotective peptide.
1-3-3. Angiotensin-converting enzyme II (ACE2)
At the turn of the millennium, a homologous enzyme, termed ACE2, was identified which increasingly shares the limelight with its better-known homologue, ACE [Donoghue et
al., 2000b; Tipnis et al., 2000; Lazartigues et al., 2007]. In vivo, ACE2 is predominantly expressed in the heart, kidneys and testes. In human study, confirming that ACE2 is expressed in human heart, kidney and testis, consistent with a possible role in cardio-renal function [Harmer et al., 2002]. The quantitative expression map for ACE 2 across 72 human tissues is shown in Figure 1-4.
Figure 1-4. Stellar plot illustrating the mRNA copy number in logarithmic form in 72 human tissues. For ACE (black), ACE 2 (red) and ACE testicular (blue). Each point represents the geometric mean copy number from determinations in three donors. Gene copy number increases logarithmically moving from the centre to the periphery of the circle. The tissues used are: 1. heart: left atrium; 2. heart: left ventricle; 3. blood vessel: coronary artery; 9.
duodenum; 11. ileum; 14. caecum; 15. colon; 49. kidney: cortex; 50. kidney: medulla; 51.
kidney: pelvis [Harmer et al., 2002].
In the heart, ACE2 is essentially confined to the endothelia [Donoghue et al., 2000b;
Tipnis et al., 2000; Guy et al., 2008; Pan et al., 2008].
ACE2 is able to cleave both Ang I and Ang II, to Ang 1-9 and Ang 1-7, respectively.
The high level of expression of ACE2 in the heart together with its ability to hydrolyse angiotensin peptides have suggested a role for ACE2 in maintaining cardiovascular physiology [Lambert et al., 2008]. The potential role of Ang 1-7 as a cardioprotective peptide having vasodilator, anti-growth and antiproliferative actions has been recognized [Ferrario, 1992b; Ferrario, 1992a; Clark et al., 2001; Carey and Siragy, 2003b; Burrell et al., 2004; Lambert et al., 2008]. It was shown that ACE2 provides a counter-regulatory system to Ang II [Carey and Siragy, 2003b; Burrell et al., 2004]. Crackower et al. [2002] showed that deletion of ACE2 in mice resulted in elevated cardiac and plasma Ang II together with impaired cardiac contractility which increased with age. These changes were associated with an upregulation of hypoxia-induced genes, consistent with a role for Ang II in the ACE2 null phenotype.
In humans, single nucleotide polymorphisms associated with increased risk of
cardiovascular disease have been identified within the ACE2 gene locus [Yang et al., 2006].
Disturbance of the balance of expression of ACE2 and its homologue ACE could alter the levels of Ang II and contribute to the development of a range of pathologies.
1-3-4. The counterbalance between Ang II and Ang 1-7
Further evidence for a role of ACE2 in maintaining cardiovascular homeostasis via Ang II regulation is provided by studies conducted by Zisman et al. [2003] which detected
increased ACE2 and Ang 1-7 forming activity in failing human hearts. Hence, the ability of ACE2 to degrade Ang II and simultaneously increase Ang 1-7 would effectively oppose the actions of ACE, suggesting the balance of the levels of the two enzymes would be critical in pathologies in the aetiologies of which Ang II is implicated. It is likely that ACE2 may play a protective role in the early stages of heart failure by elevating Ang 1-7 levels. In another study, Grobe et al. [2007] suggested that infusion of Ang II into adult Sprague-Dawley rats resulted in significantly increased blood pressure, myocyte hypertrophy, and midmyocardial interstitial fibrosis. Coinfusion of Ang 1-7 resulted in significant attenuations of myocyte hypertrophy and interstitial fibrosis, without significant effects on blood pressure. Another findings demonstrate that, in human endothelial cells, Ang 1-7 negatively modulates Ang II/AT1R–activated c-Src and its downstream targets ERK1/2 and NADPH oxidase. These
phenomena may represent a protective mechanism in the endothelium whereby potentially deleterious effects of Ang II are counterregulated by Ang1-7 [Sampaio et al., 2007]. The cascade of the processing of angiotensin peptides and their interaction with AT1R and Ang 1-7 receptor systems was shown in Figure 1-5.
Figure 1-5. Cascade of the processing of angiotensin peptides and their interaction with AT1R and Ang 1-7 receptor systems. ACE cleaves Ang I, releasing the dipeptide His-Leu to form Ang II, and ACE2 subsequently hydrolyzes Ang II to Ang1-7. ACE also metabolizes Ang 1-7 to Ang 1-5 and the dipeptide His-Pro. Ang 1-12 may be cleaved from angiotensinogen (Aogen) and potentially processed (Æ) directly to Ang II or Ang 1-7. Ang1-7 may attenuate the inflammatory and fibrotic actions of the Ang II-AT1R pathway through inhibition (-) of the MAP kinase kinase (MAPKK) pathway, the potential stimulation (+) of cellular
phosphatases, the inhibition of cyclooxygenase-2 (COX2) and other proinflammatory agents, as well as the stimulation of NO. Although not shown, the AT2R and bradykinin receptor systems may interact with these pathways as well [Chappell, 2007].
1-4. Angiotensin associated receptors
1-4-1. Angiotensin receptors and blockades
Angiotensin II has two major receptor subtypes, the angiotensin II type 1 receptor (AT1R) and angiotensin II type 2 receptor (AT2R). Two subtypes of angiotensin II (Ang II) receptors have been defined on the basis of their differential pharmacological and biochemical properties. AT1R, which are involved in most of the well-known physiological effects of Ang II, and AT2R, which have a less well-defined role but appear capable of
counterbalancing some of the effects of AT1R stimulation [Levy, 2004]. Importantly, AT1R antagonists are associated with a rise in plasma Ang II concentration due to the inhibition of
counterbalancing some of the effects of AT1R stimulation [Levy, 2004]. Importantly, AT1R antagonists are associated with a rise in plasma Ang II concentration due to the inhibition of