Cardioprotective Mechanisms against Reperfusion Injury in AMI: Comparison
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Please note this is a comparison between Version 2 by Jason Zhu and Version 1 by Gabriel Méndez-Valdés.

Ischemia/reperfusion injury is a process associated with cardiologic interventions, such as percutaneous coronary angioplasty after an acute myocardial infarction. Blood flow restoration causes a quick burst of reactive oxygen species (ROS), which generates multiple organelle damage, leading to the activation of cell death pathways. Therefore, the intervention contributes to a greater necrotic zone, thus increasing the risk of cardiovascular complications. A major cardiovascular ROS source in this setting is the activation of multiple NADPH oxidases, which could result via the occupancy of type 1 angiotensin II receptors (AT1R); hence, the renin angiotensin system (RAS) is associated with the generation of ROS during reperfusion.

  • reperfusion injury
  • oxidative stress
  • ARBs
  • AT1R
  • AT2R

1. Introduction

Coronary heart disease is the leading cause of death worldwide [1]. When a patient presents with an acute myocardial infarction (AMI), the most effective therapeutic strategy to improve clinical outcomes is blood flow restoration through percutaneous coronary intervention (PCI), with or without prior thrombolysis [2]. However, this treatment has also been associated with a type of myocardial injury known as reperfusion injury [3], a process associated with an increase in the infarct size [2], where viable cardiomyocytes die due to the activation of specific signaling pathways. Studies in animal models suggest that reperfusion injury could account for up to 50% of the final infarct size [2]. Likewise, this event is associated with complications such as myocardial stunning, reperfusion arrhythmias, the non-reflux phenomenon and diastolic dysfunction [4].
Reperfusion damage is a consequence of blood flow restoration in the tissue affected by ischemia. After PCI, the rapid and massive production of reactive oxygen species (ROS) occurs, affecting the protein conformation, enzymatic activity, ligand binding and protein–protein interactions, inducing inflammation and damaging organelles along with biomolecules crucial for cell viability [4]. This ROS burst triggers different cell death pathways, such as apoptosis, necrosis or ferroptosis [2,4][2][4].
Reactive oxygen species are mainly generated in the mitochondria, where the most important dysfunction is that of the electron transport chain, followed by nicotinamide adenine dinucleotide (NADPH) oxidases [5]. One of the main agents related to the increase in ROS in cardiomyocytes is the RAS [6]. It has a significant role in the regulation of blood pressure and hydro-electrolyte homeostasis. The renin angiotensin system can be found in circulating form (circulating RAS −25%) or synthesized from local tissues (paracrine RAS −75%). The latter has its main effect in different locations, including the cardiovascular tissue [7], and it can exert an intracrine effect (intracellular RAS) [8].
Angiotensin II (Ang II) is a central multifunctional hormone of RAS that exerts its action mainly through two G protein-coupled receptors: type 1 (AT1R) and type 2 (AT2R) [8]. The angiotensin II type 1 receptor leads to prooxidant and proinflammatory activity associated with increased ROS production, vasoconstriction and cardiac remodeling, ending in cardiac hypertrophy [9]. In contrast, AT2R exerts antioxidant and anti-inflammatory activity [8]. The latter receptor modulates and counterbalances AT1R’s action by promoting vasodilation via nitric oxide (NO), reducing ROS and inflammation [8]. Interestingly, some experimental studies have shown that the application of type 1 Ang II receptor antagonists (ARBs) can reduce the size of the infarcted area in a dose-dependent way [10].
It has been suggested that Ang II, through the induction of moderate amounts of ROS and protein kinase C epsilon (PKCε) activation, can be involved in ischemic preconditioning (IPC) in Langendorff models. This mechanism of Ang II occurs through the activation of both AT1R and AT2R. Since IPC and Ang II share common signaling pathways at a mitochondrial level, such as the mitogen-activated protein kinase (MAPK) and NADPH oxidase pathway, it is expected that the use of IPC together with Ang II-induced preconditioning (APC) would have synergistic effects in reducing the infarct zone size [11].

2. Angiotensin Axis

The use of ARBs as a pharmacological therapy has a substantial clinical benefit because of their action on the RAS system. Through the inhibition of the angiotensin-converting enzyme (ACE) and selective antagonism of Ang II receptors, its main use is the reduction of blood pressure in patients with arterial hypertension. The selective antagonism of the angiotensin II receptor not only allows the control of blood pressure, but it also has a complex effect on the heart and kidneys. Indeed, ARBs guarantee protection against the local effects of high blood pressure, thus avoiding the remodeling of vital structures such as the blood vessels, heart, kidneys and brain.
In accordance with this information, several studies have demonstrated that the consequences of ARB usage lead to protection against several disease states, including systolic dysfunction, systolic dysfunction after myocardial infarction (MI) and diabetic nephropathy [12].
Typically, ARBs are administered as pills per os, and four to six weeks of therapy are required to achieve the full therapeutic effects. Angiotensin receptor blockers are generally well tolerated and have a low incidence of side effects. Since ARBs do not increase bradykinin levels, the incidence of angioedema and cough is lower with respect to that of angiotensin-converting enzyme inhibitors (ACEI). There are few cases where the use of ARBs is contraindicated, specifically in patients with bilateral renal artery stenosis or patients with heart failure who have hypotension, since, sometimes, ARBs can cause hypotension [13].

2.1. Ang II/AT1R Axis

The physiological and pathophysiological effects of the activation of the Ang II/AT1R axis are well known. Angiotensin II exerts its effect through binding to a G-protein-coupled receptor and its main role is the maintenance of renal and cardiovascular homeostasis. Other functions are the release of proinflammatory cytokines, an increase in oxidative stress, the suppression of NO synthesis and the activation of the nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB). This latter function has been reported to have an important role in angiogenesis [14]. There are two types of RAS: one on a local level and the other systemic. They are independently regulated, and their alterations trigger different pathological mechanisms [15]. The activation of this receptor is regulated by Ang II levels. Acutely, increased levels of Ang II increase the activation of this receptor [16].

2.2. Ang II/AT2R Axis

Type 2 Ang II receptor activation is generally described as a counter-modulatory effect of AT1R activation. It increases NO synthesis both by direct stimulation and by type 2 bradykinin receptor through endogenous bradykinin output [17,18][17][18]. AT2R also enhances dephosphorylation and participates in the tyrosine kinase growth pathway and in the activation of phosphatases. This latter action leads to the decreased expression of NF-κB and cyclooxygenase 2, and the inhibition of the JAK-STAT pathway. Due to this, AT2R activation also has an anti-inflammatory effect, which results in lower levels of both proinflammatory cytokines and prostaglandins [19,20,21][19][20][21]. It has been described that in some pathological processes, AT2R mimics the action of AT1R, promoting effects such as vasoconstriction and hypertrophy [22]. Its expression also increases cardiac cell numbers [23,24,25,26][23][24][25][26].

2.3. Angiotensin 1–7/Mas Receptor Axis

Angiotensin 1–7 (Ang 1–7) has been described to have a wide range of effects at a cardiovascular level through the activation of the Mas receptor (MasR). In cardiomyocytes, acute exposure to Ang 1–7 stimulates NO release by activating eNOS and neuronal nitric oxide synthase (nNOS) [27,28][27][28]. In addition, it has been seen that in animal models, it has different cardioprotective effects; among them are a reduction in the generation of ischemia–reperfusion-induced arrhythmias and the improvement of post-ischemic cardiac function [29,30][29][30].

3. Intracellular Renin Angiotensin System

The existence of an intracellular RAS (iRAS) has been studied for several years. Fibroblasts, endothelial cells, kidney cells and cardiac cells have been shown to synthesize intracellular Ang II, which has physiological effects on nuclear expression, extracellular matrix conformation, cell proliferation and vascular contraction [15]. Intracellular Ang II also stimulates nuclear AT1R and AT2R in cardiac fibroblasts, promoting the generation of NO and an increase in the calcium-dependent inositol triphosphate receptor (IP3R), modulating cell proliferation, mRNA and collagen production [31]. Extracellular ARBs do not interact with intracellular Ang II receptors [31,32][31][32]. Overstimulation of extracellular RAS in pathological processes increases intracellular RAS activity, leading to the stimulation of cardiac hypertrophy, apoptosis, oxidative stress and the increased expression of NF-κB and transforming growth factor β, thus leading to organ damage [33,34][33][34].

3.1. Mitochondrial iRAS

Among the organelles in which various RAS components have been described, one of the most relevant and studied is the presence of the different G-protein-coupled Ang II receptors in the mitochondrial membrane. These receptors are not encoded directly by mitochondrial DNA, but they come from the plasma membrane or can be found in the cytosol. Angiotensin II intracellular receptors have been identified primarily in mitochondria isolated from adrenal cortex cells [35] but they have also been observed in other cells [36], such as cardiomyocytes [11,36][11][36]. Protein kinase C epsilon is a protein responsible for the translocation of AT1R and AT2R from the plasma membrane to the mitochondrial membrane, as well as the activation of cardioprotective pathways such as p38, ERK 1/2, JNK and Akt [8]. This has been proven by the use of chelerythrine, a selective PKC blocker, which inhibits the activation of PKCε and blocks the translocation of the components mentioned above [37].

3.1.1. Mitochondrial AT1R

It has been demonstrated that the activation of the mitochondrial AT1R stimulates NADPH oxidase 4 (NOX4) [36], which increases the superoxide concentration within the mitochondria, accelerating mitochondrial respiration. By creating high concentrations of Ang II and by using an AT2R antagonist (PD123319), researchers have obtained evidence of the activation of oxidative phosphorylation, the maximum respiratory rate and ROS production [36].

3.1.2. Mitochondrial AT2R

Angiotensin II type 2 receptor is abundant in the mitochondria of non-ischemic cells, where the AT2R/AT1R ratio is higher [11]. During ischemia, this ratio is reduced, but it rises again if an APC before global ischemia is performed. This happens because Ang II promotes oxidative stress, through the induction of a compensatory increase in AT2R in the mitochondria. This mechanism has been observed in neuronal cells where oxidative stress was induced [38]. This receptor in the mitochondria activates mitochondrial NOS via the G-protein receptor pathway, which leads to an increase in the levels of NO within the mitochondria. This NO increase counteracts the effects of the superoxide produced by NOX4 and therefore reduces mitochondrial respiration [38]. This mechanism suggests that whenever a reperfusion is performed, the reduction in mitochondrial respiration would reduce the energy and oxygen demands of the heart, also reducing the production of ROS by the electron transport chain (ETC) and, thus, activating cardioprotection pathways such as hypoxia-inducible factor (HIF) and Nrf2 [5], dependent on PKCε, or via the NO/cGMP pathway [8].

3.2. Nuclear iRAS

Several studies have demonstrated that Ang II receptors can be present in the nuclei of multiple cells, including cardiomyocytes [38]. Following the binding of Ang II with AT1R, the receptor is translocated to the nucleus. The transport of AT2R has less evidence, but it is believed that it could be transported to the nucleus via active transport [39]. The main function of these receptors in the nucleus is believed to be related to an amplification of the response generated by the activation of Ang II receptors in the plasma membrane. The activation of nuclear AT1R would generate an increase in intranuclear superoxide via NOX4 activation [8]. If this increase in superoxide occurs in low amounts, it can stimulate the gene transcription of antioxidant substances in the same way as occurs with the moderate activation of the AT1R of the plasma membrane when APC is performed [38]. An increase in the superoxide concentration also promotes an increase in nuclear Ca2+ [40]. This increase is due to the activation of several transcription factors, such as DREAM, which increases the transcription of the mRNA of proteins that counter-modulate AT1R activity, such as IGF-1, AT2R and PGC-1α, leading to a reduction in mitochondrial respiration and therefore a reduction in ROS production [38]. For example, IGF-1 and PGC-1α interact with SIRT-1, thus increasing mitochondrial protection and reducing ROS. However, superoxide also increases the expression of prorenin, renin and angiotensinogen mRNA, which leads to increased intracellular Ang II levels [38]. High levels of intracellular Ang II are also associated with nuclear damage. Therefore, this apparently cardioprotective action of nuclear AT1R would only be generated in low to moderate amounts, which has not yet been determined.
Meanwhile, the activation of nuclear AT2R and MasR leads to an increase in NO levels, which would regulate AT1R [38]. Interestingly, the activation of the nuclear MasR, without activating AT2R, leads to a significant decrease in the expression of AT2R mRNA, but not of MasR, suggesting a type of regulation between AT2R and this receptor [38]. Given this evidence, blocking AT1R, which would be overexpressed in an ischemia event, could be expected to contribute to the increased generation of ROS and inflammation in affected cells. This could be a pharmacological target that would not only reduce the direct effect of the Ang II/AT1R axis but would also promote cardioprotective axes such as the Ang II/AT2R axis. In this way, this latter axis could exert a synergistic action with other drugs that have different pharmacological targets but share common signaling pathways, such as akt, MAPK and PKCε.

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