Unbalanced Redox With Autophagy in Cardiovascular Disease

Abstract

Precise redox balance is essential for the optimum health and physiological function of the human body. Furthermore, an unbalanced redox state is widely believed to be part of numerous diseases, ultimately resulting in death. In this review, we discuss the relationship between redox balance and cardiovascular disease (CVD). In various animal models, excessive oxidative stress has been associated with increased atherosclerotic plaque formation, which is linked to the inflammation status of several cell types. However, various antioxidants can defend against reactive oxidative stress, which is associated with an increased risk of CVD and mortality. The different cardiovascular effects of these antioxidants are presumably due to alterations in the multiple pathways that have been mechanistically linked to accelerated atherosclerotic plaque formation, macrophage activation, and endothelial dysfunction in animal models of CVD, as well as in in vitro cell culture systems. Autophagy is a regulated cell survival mechanism that removes dysfunctional or damaged cellular organelles and recycles the nutrients for the generation of energy. Furthermore, in response to atherogenic stress, such as the generation of reactive oxygen species, oxidized lipids, and inflammatory signaling between cells, autophagy protects against plaque formation. In this review, we characterize the broad spectrum of oxidative stress that influences CVD, summarize the role of autophagy in the content of redox balance-associated pathways in atherosclerosis, and discuss potential therapeutic approaches to target CVD by stimulating autophagy.

INTRODUCTION

Reactive oxygen species (ROS) are undesirable byproducts of the cellular respiration system.¹,²,³,⁴ However, cellular respiration-based energy metabolism is an essential process to produce adenosine triphosphate (ATP) for the continuous maintenance of essential cellular functions, such as cellular signaling and protein synthesis. Electron transport through the mitochondrial respiratory chain leaks approximately 1%–2% of electrons, generating superoxide (O²⁻), hydrogen peroxide (H₂O₂), and hydroxyl radical (OH·).⁵,⁶ To protect the cell against ROS, cells have an equally ubiquitous antioxidant defense system, which is thought to be important for the balance of the intracellular redox environment.⁷ Superoxide dismutase (SOD) converts O²⁻ into H₂O₂, which is then reduced to water by several cellular enzymes. Peroxiredoxins (Prdx), glutathione peroxidase (Gpx), catalase, and thioredoxin reductase (Trx) are important cellular reductases in eukaryotic cells.⁸,⁹,¹⁰

The results from numerous studies have demonstrated that a relationship exists between ROS and the incidence and mortality of various diseases, presumably because high oxidative stress results in increased inflammation and contributes to the increased occurrence of apoptosis.¹¹,¹²,¹³,¹⁴ Inflammation and apoptosis are major risk factors for cardiovascular disease (CVD), whereby activated endothelial cells initiate the recruitment of macrophages to the arterial walls and are associated with several abnormalities involved in the pathogenesis of CVD, including dyslipidemia and hypertension.¹⁵ Thus, the removal of oxidative stress and management of the redox balance form the cornerstone of the prevention and treatment of CVD.

Autophagy is a catabolic pathway for the degradation or recycling of cytoplasmic organelles and aggregated proteins using the lysosomal apparatus.¹⁶,¹⁷ Autophagy is highly inducible and triggered by environmental stresses such as oxidative stress by ROS.¹⁸,¹⁹,²⁰ Oxidative stress not only involves the induction of autophagy, but can also serve as an intracellular signaling regulator by reversibly oxidizing the essential signaling components. Recent reports have revealed that acquired defects in autophagy exacerbate atherosclerosis, suggesting that autophagy exerts an anti-atherogenic function.

The oxidative stress associated with antioxidant dysfunction has adverse consequences on cardiovascular health because it is associated with an increased risk of atherogenesis. Numerous studies involving in vivo models and in vitro cell culture models have demonstrated a direct mechanistic link between the dysfunction of antioxidants and their related signaling pathways and atherosclerosis. In the following sections, we review the current evidence relating to oxidative stress and atherosclerotic risk and explore potential cellular and molecular mechanisms connecting autophagy with atherosclerosis.

ATHEROSCLEROSIS

Atherosclerosis is a gradually progressive disease characterized by the formation of lipid- and inflammatory component-rich plaques, resulting in the thickening of the arterial walls and narrowing of the arterial lumen,²¹,²² and it is considered the major cause of CVD. Research on atherosclerosis has provided insights into the relationship between plasma lipid abnormalities and inflammation on the arterial wall, including both vascular wall resident and immune cells. One of the most transformative paradigms in atherosclerosis research has been the discovery of the role of inflammation.²³,²⁴ For most of the 1900s, atherosclerosis was viewed as a hyperlipidemic disease, but in the late 1990s, the role of inflammation in atherosclerosis was identified. These initial findings gave rise to the field of immunometabolism, which is the study of how immune cells respond to and regulate hyperlipidemic states.

Atherosclerosis is initiated by endothelial dysfunction from lipids within the blood.²⁵ Endothelial cells form a monolayer on the arterial luminal surface because activated endothelial cells release several surface adhesion molecules, inducing the recruitment and adhesion of immune cells, especially monocytes.²⁶ One of the unique characteristics of endothelial cells is that they can sense vascular flow dynamics. In straight and normal laminar flow regions, endothelial cells produce few adhesion molecules, whereas in the curvatured and shear flow regions, the amount of surface adhesion molecules is greatly increased. Monocytes are the first immune cells that respond to the adhesion molecules on the endothelial cells and they continuously infiltrate into arterial intima lesions, which are a major area of plaque formation.²⁷,²⁸ After infiltration, monocytes differentiate into macrophages and respond to various signaling pathways, including the formation of foam cells by the uptake of modified lipoproteins and the secretion of many kinds of chemokines, cytokines, and proteases. Smooth muscle cells are another important mediator in atherogenesis.²⁹,³⁰ Although most smooth muscle cells in the arterial wall are located in the medial layer, in the late stage of atherogenesis, smooth muscle cells proliferate from the media, migrate into the intimal area, and secrete extracellular matrix proteins to create a fibrous cap that stabilizes the plaque. However, the migrated smooth muscle cells also take up modified lipoproteins, contributing to foam cell formation and further plaque complexity. The pathological process of atherosclerosis is summarized in Fig. 1.

Fig. 1. Schematic overview of the role of oxidative stress in atherosclerosis.

Atherosclerosis is the progressive narrowing of the arteries due to plaque formation and is considered the major cause of cardiovascular disease. Oxidative stress is a major stressor that affects the development of atherosclerosis, including lipid oxidation, endothelial activation, macrophage foam cell formation, and inflammation.

ROS, reactive oxygen species; LDL, low-density lipoprotein; EC, endothelial cell; VSMC, vascular smooth muscle cell; oxLDL, oxidized low-density lipoprotein; mtDNA, mitochondrial DNA; SMC, smooth muscle cell.

Several studies have evaluated the relationship between each cell type and the risk of atherosclerosis. The key cellular features and results from some of the most comprehensive studies are reviewed in the following sections.

1. Oxidized lipids and atherosclerosis

Oxidative stress during atherogenesis can generate various oxidized lipids, including oxidized cholesterol.³¹,³² Macrophages are the cells that contribute the most during early atherogenesis through the uptake of oxidized low-density lipoprotein (oxLDL) via scavenger receptors, including scavenger receptor A (SR-A), scavenger receptor BI (SR-BI), CD36, and CD68, and this uptake promotes cellular cholesterol accumulation in the arterial wall. The increase in oxLDL-related macrophage scavenger receptors has also been implicated in the progression of atherosclerosis. Peritoneal macrophages from mice supplemented with oxLDL revealed elevated expression of the scavenger receptors CD36 and SR-A1, which are key mediators of cholesterol metabolism.³³,³⁴ Numerous in vivo studies have implicated both CD36 and SR-A1 in early foam cell formation, altered low-density lipoprotein (LDL) uptake, and atherosclerotic plaque progression, complexity, and necrosis.³⁴,³⁵,³⁶,³⁷,³⁸,³⁹

Recent studies have shown that oxidized lipids not only induced the formation of lipid-enriched foam cells and their accumulation in plaques, but also enhanced vascular inflammation and gave rise to autoimmune reactions in the vascular walls.⁴⁰,⁴¹,⁴² In vitro studies have revealed that oxidative stress is a major stressor of the vasculature, which is associated with chronic inflammatory conditions and increases with aging.⁴³,⁴⁴ Another important oxLDL-mediated effect on atherosclerosis involves smooth muscle cell proliferation within the vasculature.⁴⁵,⁴⁶

2. Mitochondrial dysfunction in atherosclerosis

Recent studies have demonstrated that mitochondrial damage or dysfunction mediated the activation of oxidative stress, which is pro-atherogenic, by inhibiting mitochondria-dependent metabolism.⁴⁷,⁴⁸,⁴⁹,⁵⁰ Mitochondria are essential double membrane-bound subcellular organelles that play a central role in various metabolic process, including the synthesis of ATP through oxidative phosphorylation, and prevents various metabolic diseases by controlling apoptosis. Thus, defects due to mitochondrial dysfunction or impairment disrupt metabolic homeostasis and result in excessive ROS production, which leads to serious metabolic diseases.⁵¹,⁵²,⁵³ Moreover, damaged mitochondria correlate with a rapid rise in mitochondrial DNA (mtDNA) damage and, consequently, elevated apoptosis.⁵⁴,⁵⁵ Increased mtDNA damage results in manganese SOD2 deficiency, which inhibits mitochondrial function, leading to accelerated atherosclerosis. This ultimately induces mitochondria-dependent apoptosis and secondary necrotic core expansion in the endothelial cells.⁵⁶,⁵⁷ Endothelial and vascular smooth muscle cell (VSMC) accumulation, along with oxidative stress, elevates mtDNA damage, alters gene expression, and induces mitochondrial dysfunction; these changes are associated with atherosclerotic plaque formation, further supporting an oxidative stress-dependent mechanism for atherogenesis.⁴⁸,⁵⁸,⁵⁹

3. Inflammation in atherogenesis

In light of evidence reported in the literature, the involvement of inflammation has become increasingly accepted as a requirement for atherosclerotic plaque progression.²²,⁶⁰,⁶¹,⁶²,⁶³ As mentioned above, oxidative stress-induced lipid modification is an important factor in the initiation of atherogenesis, and oxidative stress also leads to inflammation during atherogenesis.⁴⁵,⁶⁴,⁶⁵ Multiple observational studies have suggested that a lack of antioxidant protection is associated with inflammatory cytokines and chemokine secretion in both animal and human disease models.⁶⁶,⁶⁷,⁶⁸ Inflammation within the atherosclerotic plaque induces ROS production, which further activates the oxidative stress-inflammation feedback loop.⁶⁹

Inflammasomes are induced by lipoprotein-derived lipids in macrophages and vascular wall cells and have recently been highlighted as an important mediator of atherosclerosis.⁷⁰,⁷¹,⁷² The most well-studied inflammasome is NLR family pyrin domain containing 3 (NLRP3). The NLRP3 inflammasome is potentially activated through 2 steps. The first step is priming, which is induced by the recognition of pathogen-associated molecular patterns, such as bacterial lipopolysaccharide or an endogenous danger-associated molecular pattern. The activation of the NLRP3 inflammasome results in the proteolytic activation and secretion of 2 proinflammatory cytokines, interleukin (IL)-1β and IL-18.⁷³ Because atherosclerotic plaques are filled with various potential danger signals for the activation of the NLRP3 inflammasome, including oxLDL, cholesterol crystals, and double-strand DNA, it subsequently binds the macrophage CD36 with the Toll-like receptor complex, leading to the priming and activation of the NLRP3 inflammasome.⁷⁴,⁷⁵,⁷⁶ The cholesterol crystals in the atherosclerotic plaque can activate the NLRP3 inflammasome, and an NLRP3 component deficiency, such as in an LDL receptor-deficient or apolipoprotein E (ApoE)-deficient mouse model, reduces atherosclerosis.⁷¹,⁷⁷,⁷⁸,⁷⁹ Recent studies have shown that human atherosclerotic lesions have increased expression of the NLRP3 inflammasome.⁸⁰,⁸¹,⁸²

OXIDATIVE STRESS

Oxidative stress is defined as an imbalance between the production of ROS and the antioxidant capacity of the cell. Several studies on the atherogenic effect of oxidative stress have relied on various animal models, including rodents (rats and mice) and human participants. In almost all studies, high oxidative stress induced by ROS has been found to be associated with advanced atherosclerotic plaque formation. Among the several types of ROS, nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) and mitochondrial electron chain reactions are closely associated with atherosclerosis (Fig. 2). Recent studies conducted both in vivo and in vitro, in cell culture systems, have provided insights into the molecular/signaling pathways involved in the adverse effect of oxidative stress on arterial disease.

Fig. 2. NOX and mitochondria proton transport are major sources of ROS.

NOX and mitochondrial proton transport are major sources of ROS. Because ROS control is crucial for survival, organelles have various antioxidant enzymes to scavenge ROS.

NOX, nicotinamide adenine dinucleotide phosphate oxidase; ROS, reactive oxygen species; SOD, superoxide dismutase; Gpx, glutathione peroxidase; Prdx, peroxiredoxins; NADPH, nicotinamide adenine dinucleotide phosphate; NADH, nicotinamide adenine dinucleotide (NAD⁺) hydrogen (H); NAD⁺, nicotinamide adenine dinucleotide; FADH₂, dihydroflavine-adenine dinucleotide; FAD, flavin adenine dinucleotide; ADP, adenosine diphosphate; ATP, adenosine triphosphate.

In the following sections, we review the critical mechanisms causally linking oxidative stress to cardiovascular risk. These data provide insights into the potential mechanisms that help explain the risks associated with different types of oxidative stress on the various aspects of atherosclerosis.

1. NOX

Several primary ROS-producing systems have been elucidated depending on the type of oxidative stress. Among the several ROS-producing systems, including xanthine oxidase, uncoupled endothelial nitric oxide synthase, enzymes of the mitochondrial respiratory chain, and NOX,⁸³,⁸⁴,⁸⁵ there is increasing evidence regarding membrane-bound NOX as the major source of superoxide generation. In cultured vascular wall cells, NOX with O²⁻ and H₂O₂ production are increased by several agonists that are associated with pathogenesis, such as angiotensin II (Ang II), thrombin, platelet-derived growth factor, and tumor necrosis factor-α in a time-dependent manner.⁸⁶,⁸⁷,⁸⁸,⁸⁹ The pro-atherogenic effect of NOX has been demonstrated in both animals and humans, with a deficiency of NOX correlating with mitigated atherosclerotic plaque progression. Knockdown of the p47phox gene, which is a cytosolic component of the NOX complex, resulted in lower levels of aortic O₂ ⁻ production and reduced the lesion size in ApoE-deficient mice. NOX2, a specific NOX isoform, is critical for atherogenesis. Higher expression of NOX2 was observed in aortic endothelial cells and macrophages of ApoE-deficient mice, with increased production of aortic O₂ ⁻.⁹⁰ In addition, NOX2 deficiency was associated with decreased vascular ROS production, increased NO bioavailability, and substantially mitigated early lesion development.

Further support for NOX-induced oxidative stress in atherosclerosis comes from human studies, which showed a significant positive association of NOX-derived oxidative stress with human atherosclerotic plaques. Similar results were obtained in a mice study where O₂ ⁻ production increased with p22phox, a critical component of NOX, and oxidative modification of LDL increased, which was related to atherosclerotic coronary artery disease (CAD).⁹¹ The conclusion of these studies was that O₂ generation by NOX resulted in foam cell formation and apoptosis, which may be a risk factor for the development of plaque stability and atherogenesis. Similar results were observed in experiments with human intimal smooth muscle and endothelial cells.⁹²,⁹³

2. Mitochondrial ROS

As mentioned above, mitochondria are essential cell organelles that provide energy via oxidative phosphorylation and are a potential cellular source of ROS production. Results from the majority of the reviewed studies suggest that the increased generation of mitochondrial ROS and mitochondrial dysfunction increase the risk for CAD.⁵¹,⁹⁴,⁹⁵,⁹⁶,⁹⁷ Among several targets that are damaged by the increased production of ROS in the mitochondria, mtDNA is the most sensitive to ROS generation as it lacks the protection of a histone-like structure, has an impaired DNA damage-repair system, and shows reduced mtRNA transcription.

Recent studies have directly addressed the relationship between mtDNA damage and the risk of atherosclerosis, as many other studies have focused on exploring atherogenesis with regard to mtDNA damage. The outcomes of human studies have shown mtDNA damage in the aortas of patients with atherosclerosis. Moreover, animal studies have found that mtDNA was increased in macrophages in atherosclerotic plaques of atherosclerosis model mice, such as ApoE- or LDL receptor-deficient mice, and mitochondrial oxidative stress was associated with atherosclerosis.⁹⁸,⁹⁹ Interestingly, mitochondrial dysfunction resulting from manganese SOD2 deficiency was associated with increased mtDNA damage even in the early stages and accelerated atherosclerosis formation in ApoE-deficient mice. The overexpression of catalase, an important antioxidant enzyme, effectively suppressed mitochondrial oxidative stress, which was associated with several downstream effects, including the reduction of atherosclerosis and macrophage inflammation by decreasing the activation of the nuclear factor-κB (NF-κB) pathway.

Mitochondrial ROS have been shown to induce NLRP3 inflammasome-dependent lysosomal damage and inflammasome activation.⁷⁴,⁸²,¹⁰⁰,¹⁰¹,¹⁰² Damaged mitochondria subsequently bind to NLRP3 to form an inflammasome complex, which triggers NLRP3 inflammasome activation, and this process inhibits autophagy/mitophagy with the release of IL-1β from macrophages. Research on autophagy gene-deficient mice demonstrated that mtDNA release induced by the accumulation of ROS-damaged mitochondria resulted in the activation of the NLRP3 inflammasome.¹⁰³,¹⁰⁴ Thus, multiple mechanisms may be involved in the mitochondrial ROS-mediated initiation and progression of atherosclerosis.

3. Antioxidants

As mentioned above, to maintain the balance of proper ROS levels, cells have several antioxidant enzymes, including SOD, Prdx, Gpx, and Trx. Many studies over the past few decades have provided substantial evidence that intracellular dysfunction or deficiency of these enzymes induces numerous diseases and mortality due to the ROS imbalance.¹⁰⁵,¹⁰⁶,¹⁰⁷ Because increased oxidative stress has been considered as a major cause of CVD, the delicate regulation of ROS by antioxidant enzymes is essential for maintaining cardiovascular health.

AUTOPHAGY AND OXIDATIVE STRESS IN CVD

Autophagy is a highly conserved cellular degradation system for the elimination of unwanted, damaged organelles or aggregated proteins by fusion with lysosomes to protect the cell from environmental stress, such as nutrient depletion or oxidative stress. Autophagy activation has been considered as a survival defense mechanism against cellular damage, and its potential to provide essential amino acids for protein synthesis has been highlighted. Autophagy has recently been considered as a possible connecter between oxidative stress and cardiovascular pathology. Not only is the activation of autophagy a defense against extracellular or intracellular stress, but it also inversely induces autophagic cell death by destroying important cellular organelles, such as mitochondria and the endoplasmic reticulum (ER), when excessively activated.

Here we explore the growing evidence for the role of autophagy in the etiology of CVD, particularly its correlation with the previously described factors that are associated with CVD and enhance its pathogenesis.

1. Evidence that autophagy regulates atherosclerosis

The observed evidence indicates the essential role of autophagy in the development of CVD; in particular, both autophagy and atherosclerosis are closely related to cholesterol metabolism and vascular inflammation. Interestingly, the current understanding of autophagy is that it plays an essential role in the regulation of lipid metabolism. Autophagy delivers lipids from lipid droplets to lysosomes, where they can be hydrolyzed by the lysosomal acid lipase for cholesterol efflux from macrophage foam cells, thereby inhibiting atherogenesis.¹⁰⁸,¹⁰⁹,¹¹⁰,¹¹¹ Indeed, wild-type p53-induced phosphatase 1 (Wip1) deficiency induced the suppression of foam cell formation by controlling the autophagy-dependent cholesterol efflux from macrophages.¹¹²,¹¹³ In addition, the suppression of autophagy via the silencing of autophagy protein 5 (ATG5), a major component of autophagy initiation, or treatment with autophagy inhibitor 3-methyladenine significantly suppressed cholesterol efflux from macrophages by attenuating autophagy activation.¹¹⁴ Moreover, observations of ATG5-deficient macrophages demonstrated that autophagy became dysfunctional in atherogenesis, and its deficiency promoted atherosclerosis through macrophage foam cell formation and inflammasome hyperactivation through oxidative stress.¹⁰⁸,¹¹⁵ Additionally, ATG16L1, an essential protein for the early stage of autophagy, has been found to be abundantly expressed in phagocytic cells associated with foam cell formation and could contribute to atherogenesis and the development of plaque vulnerability.¹¹⁶ Existing studies have focused on oxidative stress and lipid metabolism, both of which cause and increase the incidence of cardiovascular-related atherosclerosis.¹¹⁷ Prdx is an antioxidant enzyme that is highly dependent on H₂O₂ which is a major ROS in atherosclerosis. Excessive oxidative stress from Prdx1-deficient macrophages enhanced the autophagic dysfunction of lipid metabolism in atherosclerosis.¹¹⁸

However, results from studies on the cells of the vascular wall suggest that excessive autophagy increases the risk of cell death.¹¹⁹ Macrophage-specific ATG5 deficiency promotes oxidative stress, which activates NF-κB in macrophages and inflammation hyperactivation, thus enhancing plaque necrosis.¹²⁰ Macrophage apoptosis is considered a promising therapeutic approach for plaque stabilization, and these results suggest that intact autophagy stabilizes atherosclerotic plaques by suppressing macrophage inflammation. Recent evidence has suggested that the phagocytosis of macrophages that are dying via autophagy results in inflammasome activation and inflammatory factor release. Similar to Prdx1 deficiency, atherosclerosis was accelerated in Prdx2-deficient mice by enhancing inflammation through the NF-κB signaling pathway, including p65, c-Jun, c-Jun N-terminal kinases, and p38 mitogen-activated protein kinase.¹²¹ Although this paper did not elucidate the autophagy function in context, the possible autophagy function in oxidative stress-mediated atherogenesis could not be excluded.

Further evidence of the relationship between autophagy and oxidative stress has been found in endothelial cells. It has been shown that oxidative stress initiates autophagy activation in human endothelial cells.¹²² Recent findings indicate that autophagy in endothelial cells is a key regulator of the maintenance of redox and inflammation balance in endothelial cell responses to shear stress.¹²³,¹²⁴ However, similar to macrophages, excessive autophagy can induce the autophagic death of endothelial cells, and inefficient autophagy contributes to the development of atherosclerosis with inflammation, apoptosis, and senescent phenotypes.⁴⁴,¹²⁵,¹²⁶,¹²⁷ These findings reveal that autophagy not only plays a protective role in maintaining endothelial cell function, but may also be a key inducer of atherogenesis when excessively activated or dysfunctional.

Recent evidence has elucidated that smooth muscle cells are crucial cells during atherogenesis and could be regulated by autophagy.¹²⁸,¹²⁹,¹³⁰ As mentioned above, autophagy is activated by cellular stress, especially oxidative stress or oxLDL, in atherogenesis. Mitophagy of smooth muscle cells plays a protective role in VSMCs against apoptosis induced by oxLDL through the removal of damaged mitochondria and promotes cell survival during atherogenesis.¹³¹ However, high levels of oxLDL augment autophagy, and excessive autophagy activation in smooth muscle cells causes autophagic cell death of the smooth muscle cells on the fibrous cap, which may lead to plaque destabilization. VSMCs of ATG7 knockout mice showed accelerated atherosclerotic plaque formation with enhanced plaque cell death, an increased number of infiltrating macrophages, fibrous cap thickening, and increased collagen content.¹³² To summarize, autophagy can protect against cell damage and cellular death or it can activate atherogenesis in various atherosclerosis-related cell types, depending on the extent of autophagy activation through oxidative stress.

2. Evidence that autophagy regulates aortic aneurysm

Aortic aneurysm is the second deadliest aortic disease, and it can be induced by a chronic inflammatory disease, such as atherosclerosis. Although the development of aortic aneurysm has been thoroughly elucidated, the precise mechanism by which cellular and molecular factors induce the pathogenesis of aortic aneurysm remains unclear.

A few studies have explored the possible relationship between the role of autophagy and the pathogenesis of abdominal aortic aneurysm. Similar to atherosclerosis, NADPH is upregulated in human abdominal aortic aneurysm samples. However, NOX deficiency enhanced macrophage inflammation through the secretion of IL-1β and matrix metalloproteinase-9, thereby disrupting the tissue-remodeling function in a mouse abdominal aortic aneurysm model.¹³³ Additionally, Prdx2-deficient mice also showed an increase in oxidative stress and the inflammatory response, as well as accelerated abdominal aortic aneurysm progression.¹³⁴ In both studies, a possible association was observed between autophagy activation in the context of oxidative stress in conjunction with macrophage inflammation activation in mouse models of abdominal aortic aneurysm.

Direct evidence has been adduced from a VSMC autophagy-associated aortic aneurysm model.¹³⁵,¹³⁶ The VSMC phenotype changed, showing functional differentiation towards phagocytic-like phenotypes during aortic aneurysm formation. Moreover, the role of autophagy in aortic aneurysm was identified as critical for the preservation of vessel integrity through the limitation of VSMC death and endoplasmic reticulum stress-dependent inflammation in ATG5-deficient mice. Similar to mice, human aortic aneurysm samples showed increased expression of autophagy and ER stress markers in VSMCs. Deficiency in another important autophagy initiation marker, ATG7, also led to significantly increased aortic aneurysm formation compared to the control. This supports the critical role of autophagy in VSMCs, because ATG7 deficiency increased the frequency of aortic aneurysm formation and rupture.¹³⁷,¹³⁸,¹³⁹ Although few studies have investigated the link between oxidative stress-induced autophagy and thoracic aortic aneurysm,¹⁴⁰,¹⁴¹ a recent study in mice and humans provided potential mechanisms that explain the risk of aortic aneurysm induced by oxidative stress with autophagy.

3. Evidence that autophagy regulates myocardial infarction

The role of autophagy has recently been investigated in myocardial infarction. The final stage of chronic CVD is heart failure, which is induced by myocardial infarction under ischemic stress or ischemia/reperfusion injury and is associated with increased mortality.¹⁴²

Clinical studies involving patients with CAD or acute myocardial infarction showing upregulated autophagy have suggested the relevance of autophagy because of hypoxia/ischemia in myocardial infarction.⁸⁰,¹⁴³,¹⁴⁴ Research has clarified that the autophagy function is required not only to maintain cardiac function, but also to remove damaged mitochondria to protect the cell against hypoxic/ischemic stress.¹⁴⁵ Studies have shown that mitophagy is readily detected in ischemic/reperfusion models to protect cells from mitochondrial abnormalities.¹²⁵,¹⁴⁶ Moreover, results from mice deficient in autophagy-related genes, including Ulk1, Atg7, Atg13, beclin-1, and damage-regulated autophagy modulator 2, have shown significant increases in the pathology of cardiomyocytes.¹⁴⁷,¹⁴⁸,¹⁴⁹,¹⁵⁰,¹⁵¹,¹⁵²

Interestingly, the excessive activation of autophagy induces severe cardiac damage in response to reoxygenation following a hypoxic/ischemic injury.¹⁵³ Reperfusion after ischemia triggers an increase in autophagosome abundance compared to hypoxic/ischemic injuries, suggesting severe induced autophagy dysfunction or impaired autophagic flux.¹⁵⁴ Moreover, ischemia/reperfusion injuries impair autophagosome clearance through a ROS-induced reduction in lysosomal-associated membrane protein-2 and beclin-1 induction, resulting in cardiomyocyte death.¹⁵⁴,¹⁵⁵,¹⁵⁶ In addition, autophagy-targeting drugs have been developed, supporting the relationship between autophagy and myocardial infarction.¹⁵⁷,¹⁵⁸,¹⁵⁹,¹⁶⁰,¹⁶¹ Taken together, these previous studies demonstrated that autophagy activation is precisely regulated at each stage of ischemia/reperfusion and is important for enhancing cell survival in ischemic injuries to protect against cardiomyocyte death from reperfusion injuries.

4. Evidence that autophagy regulates hypertension

Lastly, an increasing amount of evidence indicates that autophagy plays a role in the pathophysiological process of ocular hypertension.¹⁶²,¹⁶³ Hypertension is a systemic disease characterized by persistently elevated blood pressure during the systolic phase (over 130 mmHg) and/or diastolic phase (over 80 mmHg) in systemic circulation and is associated with mortality.¹⁶⁴ Since 1990, the number of people with hypertension worldwide has doubled.¹⁶⁵ Although there is a strong correlation between CVD mortality and hypertension, hypertension provokes few notable symptoms; therefore, the recommendation for initial hypertension treatment to prevent CVD is easily overlooked.

Several studies have demonstrated that excessive and dysfunctional autophagy can cause cell death and senesce in an autophagy machinery depletion model.¹⁶⁶,¹⁶⁷ In addition, the inhibition of autophagy or silencing of the gene coding for LC3-II in sympathetic premotor neurons resulted in antihypertension in spontaneously hypertensive rats.¹⁶⁸ Moreover, an autophagy inhibitor, 3-methyladenine, significantly reduced blood pressure and arterial wall thickness, leading to improved vascular relaxation in Ang II-treated mice.¹⁶⁹ Although several studies have suggested that autophagy is associated with the pathogenesis of hypertension, a deeper understanding of the pathological mechanism of autophagy involved in hypertension is required for hypertension prevention and treatment.

CONCLUSIONS AND FUTURE PERSPECTIVES

Oxidative stress occurs abundantly throughout all organisms, and antioxidants play a crucial role in maintaining the redox balance and cellular homeostasis. As discussed in this review, autophagy is positively associated with high oxidative stress, and its dysfunction can induce atherogenesis or abdominal aortic aneurysm, which is one of the most frequently occurring pathologies of the arterial wall (Fig. 3). Although little is known regarding the expression and function of autophagy in CVD risk, recent studies have suggested that autophagy is a strong therapeutic candidate to prevent and treat numerous diseases, including CVD. Further mechanistically-oriented clinical trials are required to definitively assess the effects of autophagy on CVD risk and various roles of autophagy in the prevention and treatment of CVD by maintaining redox balance.

Fig. 3. Effects of impaired autophagy and potential mechanisms affecting CVD.

Excessive oxidative stress impairs autophagy and could affect CVD through these pathogenetic mechanisms.

CVD, cardiovascular disease; VSMC, vascular smooth muscle cell; ECM, extracellular matrix.