Apoptosis-Related Diseases and Peroxisomes

Abstract

Apoptosis is a highly regulated cell death program that can be mediated by death receptors in the plasma membrane, as well as the mitochondria and the endoplasmic reticulum. Apoptosis plays a key role in the pathogenesis of a variety of human diseases. Peroxisomes are membrane-bound organelles occurring in the cytoplasm of eukaryotic cells. Peroxisomes engage in a functional interplay with mitochondria. They cooperate with each other to maintain the balance of reactive oxygen species homeostasis in cells. Given the key role of mitochondria in the regulation of apoptosis, there could also be an important relationship between peroxisomes and the apoptotic process. Peroxisome dysfunction severely affects mitochondrial metabolism, cellular morphological stability, and biosynthesis, and thus contributes directly or indirectly to a number of apoptosis-related diseases. This chapter provides an overview of the concept, characteristics, inducing factors, and molecular mechanisms of apoptosis, as well as evidence for apoptosis in cancer, cardiovascular diseases, and neurodegenerative disorders, and discusses the important role of the peroxisome in the apoptosis-associated diseases.

1. Introduction

Death is the final fate of cells and organisms and is a normal biological phenomenon in the living world. Cell death plays a crucial role in the development of plants and animals in nature and in maintaining ecological balance [1]. For example, in the developing vertebrate nervous system, as many as half or more of the nerve cells usually die soon after they are formed. In a healthy adult human, billions of cells die every hour in the bone marrow and intestines. So much cell death seems very wasteful, especially when the vast majority of cells are perfectly healthy at the time of suicide.

In general, cell death can be divided into two types: programmed cell death (PCD) and accidental cell death (necrosis) [2]. The former is a controlled process of intracellular death program, also vividly referred to as cellular suicide. The latter is caused by external factors (i.e., injury, infection, etc.). The study of PCD (especially apoptosis) processes has led to a better understanding of the pathogenesis of certain diseases. The 2002 Nobel Prize in Physiology and Medicine was awarded to Britons Sydney Brenner, Jone E. Sulston, and H Robert Horvitz for their discovery of how genes regulate organ growth and programmed cell suicide processes, using the nematode Caenorhabditis elegans (C. elegans) as an animal model.

A coordinated balance between cell proliferation and apoptosis is crucial for normal development and tissue homeostasis. Once this balance is permanently disrupted, normal cells may be transformed into mutant cells whose clonal survival and uncontrolled proliferation may lead to the development of tumors and various other diseases.

2. Apoptosis

2.1 The concept, characteristics, and inducing factors

2.1.1 Key concepts

Apoptosis is the process of cellular suicide by activating an intracellular death program or by the orderly breakdown of cells from within. The term was first introduced by Kerr J. F. R. in the 1970s and was not accepted by the general public until the 1990s.

Although apoptosis is only one form of Programmed cell death (PCD), it is by far the most common and well-understood form, and, confusingly, biologists often use the terms PCD and apoptosis interchangeably [3].

For a multicellular organism, a highly organized community, cell numbers are tightly regulated not only by controlling the rate of cell division but also by controlling the rate of cell death. Thus, apoptosis is important not only for tissue remodeling and elimination of transitional organs during the development of an organism, but also for the clearance of cellular senescence inactive metabolic organs, such as blood cells and epithelial cells in the digestive system, and cells with damaged or mutated DNA [4, 5, 6]. In a nutshell, apoptosis is an essential mechanism complementary to proliferation to ensure homeostasis in all tissues.

Unlike apoptosis, necrosis is a form of cell injury that leads to the premature death of cells in living tissues due to autolysis, usually caused by stronger external factors such as infection, toxins, or trauma, ultimately resulting in the unregulated of cellular components, always harmful and potentially fatal to the organism [7, 8]. Necrosis usually causes a local inflammatory response. The reason for this is that when nearby macrophages engulf these necrotic cells, they may release microorganisms that destroy the surrounding tissue causing collateral damage and inhibiting the healing process.

Typically, cell death due to necrosis does not follow the apoptotic signaling transduction pathway, but rather various receptors are activated, leading to loss of cell membrane integrity and uncontrolled release of cell death products into the extracellular space. In contrast, apoptosis is a naturally occurring programmed and targeted cause of cell death and usually provides beneficial effects to the organism. A brief comparison of them can be summarized as follows (Figure 1).

Figure 1.

Structural change of cells undergoing necrosis and apoptosis.

2.1.2 Characteristics

As mentioned above, necrosis is a form of traumatic cell death caused by acute cellular injury. In contrast, apoptosis is a process of active cellular suicide. Multicellular organisms eliminate mutated, damaged, or unwanted cells by this type of active suicide. Apoptosis plays an important role in tissue sculpting during embryonic development and in the maintenance of tissue homeostasis throughout life [6].

The process has distinct morphological features, including cell rounding and contraction, blebbing and PS externalization of the plasma membrane, cytoplasmic vacuolization including endoplasmic reticulum expansion and cisternae swelling to form vesicles and vacuoles, nuclear condensation, border aggregation or fragmentation, chromatin compaction, pyknosis, and ultimately fragmentation between nucleosomes by endonucleases, resulting in regular DNA degradation and inhibition of protein translation, and ultimately to the eventual rupture of the cell into small spheres surrounded by membranes called apoptotic bodies, which contain “packed” cell contents with an electron cloud density similar to chromatin; and a sub-G1 curve preceding the G1 phase peak is observed in cytometric histogram [9]. Apoptotic bodies can be recognized and digested by phagocytosis of neighboring macrophages through the presence of phosphatidylserine (PS) on their surface [10]. In this way, the apoptotic cells can be rapidly removed by tissue phagocytes through phagocytosis, without releasing harmful substances that can initiate inflammation, which can cause a significant amount of tissue damage. Because apoptotic cells are always rapidly eaten and digested, dead cells are usually rarely seen, even when large numbers of cells die from apoptosis. This may be the reason why biologists once ignored the phenomenon of apoptosis and may still underestimate its extent.

Abnormal apoptosis contributes to many important diseases, including cancer, autoimmune diseases, diabetes, and neurodegenerative diseases. Various types of cellular stress, such as DNA damage or growth factor deprivation, can trigger apoptosis through intrinsic or extrinsic pathways.

2.1.3 Inducing factors

Apoptosis can be triggered by both internal stimuli, such as abnormalities in DNA, and external stimuli, such as certain cytokines from different pathways, respectively [11]; or it can be induced by physiological or pathological factors.

Specifically, physiological triggers can include the following two aspects [12]: (1) Direct action of certain hormones and cytokines: for example, glucocorticoids are typical signals of apoptosis in lymphocytes; thyroxine plays an important role in the apoptotic degeneration of tadpoles’ tails; TNF can induce apoptosis in a variety of cells. (2) Indirect effects of certain hormones and cytokines: for example, testosterone deficiency caused by testicular dysplasia can lead to apoptosis of prostate epithelial cells. Inadequate secretion of adrenocorticotropic hormone by the pituitary gland can promote apoptosis of adrenocortical cells, etc.

While pathological triggers usually include the following two aspects: (1) It is generally believed that apoptosis can be induced by many factors that can cause damage to cells, such as stress, radiation, chemical toxins, viral infections, and chemotherapeutic drugs, and even malnutrition and excessive functional complexes can induce apoptosis. (2) Some factors such as various chemical carcinogens and certain viruses (e.g., EBV) inhibit apoptosis. Therefore, it is thought that the ability to induce cells may be related to the type, intensity, and duration of the harmful factors.

2.2 Molecular mechanisms of apoptosis (Signaling pathways and related enzymes)

2.2.1 Apoptotic Signaling pathways

The initiation of apoptosis is tightly regulated by different signaling pathways. The best-understood two are the intrinsic pathway (also known as the mitochondrial pathway) and the extrinsic pathway (also known as the death receptor pathway). The mitochondrial pathway is generally activated by intracellular signals and depends on proteins released from the intermembrane space between the mitochondrial bilayers. The death receptor pathway is activated by extracellular ligands, and the activated extracellular ligands bind to their specific death receptors on the cell surface, inducing the formation of death-inducing signaling complexes (DISC) [13, 14]. Here, we will discuss the extrinsic and intrinsic pathways separately. However, it should be noted that there is crosstalk between these pathways and that extracellular apoptotic signaling can also lead to activation of the intrinsic pathway.

2.2.1.1 The extrinsic pathway of apoptosis

The extrinsic death pathway triggers receptor-mediated apoptosis. Its major components include pro-apoptotic ligands, receptors that recognize/bind ligands, and adaptor proteins that bind to the cytoplasmic face of the receptor. In addition, the pathway recruits other molecules, including cysteine-specific proteases (caspases), the initiator of the death process, and the executors, to execute the apoptotic process [15]. For example, TNF is a common pro-apoptotic ligand and TNFR1 on the cell membrane is the receptor. When TNF binds to TNFR1, the activated receptor binds to two different cytoplasmic adaptor proteins (tumor necrosis factor-related death domain protein, TRADD, and fas-associating protein with death domain, FADD) and procaspase-8, forming a multi-protein complex on the inner surface of the plasma membrane, containing an 80 amino acid death structure domain through which a death-inducing signaling complex (DISC). The cytoplasmic structural domains of the TNF receptor, FADD, and TRADD interact through homologous regions called death structural domains present in each protein [16]. Procaspase-8 and FADD interact through homologous regions called death effector domains. Procaspase 8 in DISC is activated and active caspase 8 is released into the cytoplasm, where it cleaves and activates effector caspases (e.g., procaspase 3), triggering a caspase cascade that further cleaves a number of death substrates, including BID and cytoskeletal proteins, if glued, leading to apoptosis (Figure 2). Notably, inhibitors of apoptosis (IAPs) can inactivate caspases by specifically binding to their active sites. Caspase activator (SMAC)/Diablo and its functional homologs in flies, including Grim, Reaper, and Hid, can in turn target binding and degrade IAPs [17].

Figure 2.

Schematic diagram of apoptotic signaling.

In addition, it should be noted that the interaction between TNF and TNFR1 may also activate other signaling pathways and allow cell survival rather than self-destruction.

2.2.1.2 The intrinsic pathway of apoptosis

In general, internal stimuli such as irreparable genetic damage, hypoxia (lack of oxygen), very high concentrations of cytosolic Ca²⁺, viral infection, or severe oxidative stress (i.e., production of large amounts of damaging free radicals) and cytotoxic drug treatment trigger apoptosis via the intrinsic pathway.

The intrinsic death pathway, i.e., the mitochondrial-received apoptotic pathway, is a death receptor non-dependent apoptotic pathway [18]. This pathway is activated by the release of cytochrome C (Cyto C) from mitochondria in response to various stresses and developmental death cues. The process specifically involves multiple steps as follows: apoptotic signals (various types of cellular stress), lead to the insertion of pro-apoptotic members of the Bcl-2 family of proteins (e.g., Bax), into the outer mitochondrial membrane, forming pores that mediate the release of Cyto C from the mitochondrial intermembrane space into the cell membrane. Once in the cell membrane, Cyto C molecules bind to Apaf-1 (a homolog of mammalian CED4) and further recruit procaspase-9 to form a complex of multiple subunits called the apoptosome. Then procaspase-9 is activated to become active caspase-9. Then the caspase-9 molecule cleaves and activates the downstream executor caspase (Caspase-3, 6,7) to carry out the apoptotic process (Figure 2) [19].

Bcl-2, the mammalian homolog of Ced-9, prevents apoptosis by inhibiting the release of CytoC from mitochondria [20]. IAPs, second mitochondrial activators of caspases (Smac), endonuclease G (Endo G), and AIF also have important roles in the apoptotic process [21]. Notably, Endo G and AIF are specifically activated by apoptotic stimuli and are able to induce ribosomal breakage of DNA independently of caspases. Endo G is a mitochondria-specific nuclease that translocates to the nucleus and cleaves chromatin DNA during apoptosis. AIF is a flavin adenine dinucleotide-containing, NADH-dependent oxidoreductase that resides in the mitochondrial intermembrane space, and its specific enzymatic activity remains unknown. In the presence of apoptosis, AIF undergoes proteolysis and translocates to the nucleus, where it triggers chromatin condensation and massive DNA degradation in a caspase-independent manner.

2.2.1.3 ER-dependent apoptotic pathways

Previously, it was thought that the only apoptotic pathways were the mitochondrial pathway and the death receptor signaling pathway. Now, an increasing number of studies have shown that the endoplasmic reticulum (ER) also senses and transmits apoptotic signals [22, 23]. The sustained action of various apoptosis-inducing factors may induce a complex unfolded protein response (UPR) by interfering with the correct protein folding process. The UPR response causes endoplasmic reticulum stress, leading to cellular apoptosis due to the accumulation of intracellular misfolded proteins. ER, in addition to being the site of protein folding, it is also the main intracellular Ca²⁺ reservoir. Disturbing intracellular Ca²⁺ homeostasis can also induce the typical ER stress response. Interestingly, the localization of Bcl-2 family proteins (including Bcl-1, Bax, Bak, et al.) in the ER affects the cellular Ca²⁺ homeostasis. Overexpression of Bcl-2 or deficiency of Bax and Bak decreases the Ca²⁺ concentration in the ER and protects cells from apoptotic stimuli that trigger cell death by inducing Ca²⁺ influx from the ER to the cell membrane (Figure 2).

It has been suggested that procaspase-12 is a proximal effector of apoptosis associated with the ER. Recent studies have found that although caspase-12 is processed and activated in ER stress-induced apoptosis in mouse cells, the enzyme is not absolutely necessary for this process. On the other hand, cells lacking caspase-8 or caspase-9 were highly resistant to ER stress-induced apoptosis. One of the mechanisms that could explain caspase-8 activation in the ER involves the recent discovery of an ER-resident potential apoptosis initiator, named neurotrophic receptor-like death domain protein (NRADD). This protein has a transmembrane and cytoplasmic region that is highly homologous to the death receptor. Induction of apoptosis by NRADD is dependent on caspase-8 activation but does not require the mitochondrial component of the death program.

In addition to propagating death-inducing stress signals, ER contributes to apoptosis initiated by cell surface death receptors and to pathways resulting from DNA damage. Modulation of ER calcium stores can sensitize mitochondria to direct pro-apoptotic stimuli and promote activation of cytoplasmic death pathways.

In short, the extrinsic (receptor-mediated), intrinsic (mitochondria-mediated), and endoplasmic reticulum stress-mediated apoptotic pathways ultimately converge by activating the same caspases, which cleave the same cellular targets. Apoptosis-inducing factors can be involved in diseases by activating apoptotic pathways that affect the rate of apoptosis, and may predominantly involve the first two pathways or all three of these pathways.

2.2.2 Apoptosis-related enzymes

The mechanism by which apoptosis occurs is highly conserved in all animal cells. It is dependent on a family of proteins called caspases (c for cysteine and asp for aspartic acid). This family of proteins has many members and generally exists as inactive precursors (procaspases). Procaspases are generally activated by the catalytic cleavage of other (already active) caspases, forming an amplified network of protein cascades. The activation process of procaspases involves the formation of a heterodimer by cleavage and the combination of two dimers to form an active tetramer. During apoptosis, those responsible for initiation are known as initiator caspases; those responsible for cleavage of specific target proteins (e.g. nuclear lamina proteins, DNA degradation enzymes, cytoskeletal proteins, and cell–cell adhesion proteins) are the executor caspases.

Apoptotic mechanisms are present throughout the initial to final stages of animal development. Only the process requires a trigger to be activated for its occurrence. So, how is the first member of the caspase cascade reaction described above initiated? Initiator procaspases usually contain a caspase recruitment domain (CARD). This structural domain can assemble into an activation complex with an adaptor protein when the cell receives an apoptotic signal. The formation of this complex means that the promoter caspase will be activated by cleavage.

As mentioned above, there are numerous members of the caspases family, most of which are involved in apoptosis, but not all of them mediate apoptosis [24]. For example, the first discovered caspase, human interleukin-l-converting enzyme (ICE), was not associated with apoptosis but was responsible for mediating the inflammatory response. After the discovery of ICE, similar proteins to ICE were identified in C. elegans and were confirmed to be involved in the apoptotic process.

2.3 Apoptosis and peroxisomes

Peroxisomes, similar to the mitochondria, are a membranous subcellular organelle within eukaryotic cells. The peroxisome contains enzymes related to fatty acid and amino acid oxidation processes that produce hydrogen peroxide and also degrade hydrogen peroxide [25]. This gives the peroxisome its name and it plays an important role in maintaining intracellular oxidative metabolic homeostasis.

Because of the crucial role of the peroxisome, its dysfunction is associated with various pathological conditions, organ dysfunction, and aging [26, 27, 28]. For example, deficiency of Pex3, a peroxisomal membrane protein essential for membrane assembly, a member of the peroxisome (Pex) family, leads to complete loss of peroxisome function, while deficiency of Pex5, a peroxisome transporter, leads to Pex5 (a peroxisomal transporter) leads to the loss of peroxisomal matrix proteins. Mutations in this class of Pex genes may lead to human developmental abnormalities, such as human autosomal recessive disorders [29].

Peroxisomes play important roles in biosynthesis and signal transduction, which cannot be achieved without interaction with other organelles in the cell. In particular, peroxisomes interact functionally with mitochondria [30]. They cooperate with each other to perform biological functions such as production, fission, proliferation and degradation through vesicular transport, signaling, and membrane contact [31]. On the other hand, they can act synergistically to clear excess intracellular ROS, resist extracellular stresses through immune responses, and play an important role in the maintenance of lipid homeostasis through fatty acid β-oxidation [32, 33, 34]. In one word, peroxisomes are essential for the maintenance of normal mitochondrial and even whole cell function. Some chemotherapeutic drugs have been found to trigger mitochondrial dysfunction, leading to apoptosis by overwhelming cells with ROS. For example, Vorinostat (Vor), an FDA-approved histone deacetylase inhibitor (HDACi) for lymphoma treatment, has been well documented to trigger mitochondrial-mediated apoptosis through ROS accumulation. Acute Vor treatment has been shown to induce the expression of peroxisome proteins, thereby increasing peroxisome proliferation in a lymphoma model system. In addition, the knockdown of peroxisomes by gene silencing of Pex3 enhances Vor-induced ROS-mediated apoptosis [35].

In short, peroxisome dysfunction severely affects mitochondrial metabolism, cellular morphological stability, and biosynthesis, directly or indirectly contributing to a number of apoptosis-related diseases such as cancer [36, 37], cardiovascular disease [38, 39, 40], and neurodegenerative disorders [41].

2.4 Apoptosis-related diseases and peroxisomes

Apoptosis is an important way for the organism to maintain the numerical homeostasis of the cell population. Excessive or insufficient apoptosis can lead to disease.

2.4.1 Cancer

Crosstalk between mitochondria and other organelles is important in tumorigenesis. Mitochondria and peroxisomes are important organelles for ROS production and scavenging. Under normal conditions, both maintain intracellular ROS homeostasis. Impaired peroxisome function inevitably leads to increased levels of ROS in mitochondria, which impairs mitochondria, exacerbates impaired ROS clearance, leads to low levels of apoptosis, and thus promotes tumorigenesis and progression [42, 43, 44].

ROS act as signaling molecules to regulate various physiological and pathological processes [45]. H2O2 is a member of the ROS family and plays an important role in the signaling of epidermal growth factor (EGF) and platelet-derived growth factor (PDGF). H2O2 prevents protein tyrosine phosphatase 1B (PTP1B) from dephosphorylating EGF, thereby facilitating EGF stimulation. In addition, activation of PDGF requires H2O2 to promote oxidation and inactivation of PDGF-receptor-associated phosphatases and SHP-2, thereby facilitating the signaling pathway [46, 47]. Excessive ROS production can lead to cellular genomic instability (including mutations in the mitochondrial genome) on the one hand. Notably, ROS can promote tumor cell proliferation under hypoxic conditions. The reason for this is that the transcription factors hypoxia-inducible factors (HIFs) are upregulated under hypoxic conditions, thus promoting the expression of oncogenes. Although some proteases such as prolyl hydroxylases (PHDs) can degrade HIFs, the increased release of ROS induced by hypoxia can prevent the action of PHDs on HIFs. In this case, HIFs can then promote tumor progression under hypoxic conditions.

Briefly, because disruption of the functional balance between mitochondria and peroxidases may lead to increased ROS production, the increased ROS may inhibit apoptosis-inducing genes (bcl2 and p53, etc.), resulting in non-apoptosis of cells that should be apoptotic. Alternatively, the apoptotic process may be inhibited due to a decrease in the activity of apoptosis-related enzymes (caspases, etc.), leading to malignant cell transformation and tissue malignant proliferation. Both of these aspects are considered to be one of the important mechanisms leading to tumorigenesis and infiltrative metastasis.

2.4.2 Cardiovascular diseases (CVDs)

Apoptosis is a form of death of terminally differentiated cardiomyocytes. Clinical data suggest that ROS generation, DNA damage, and other factors activate apoptosis, resulting in the loss of large numbers of cardiomyocytes in patients with advanced congestive heart failure, patients with myocardial infarction, and patients with diabetic cardiomyopathy. The evidence suggests that apoptosis may be an important pathogenetic mechanism in cardiovascular disease [38]. Apoptosis, in concert with necrosis, may also lead to foam cell death and thus to the formation of a necrotic core, which contributes to lesion instability and increases the risk of lesion rupture and thrombosis.

Lower levels of ROS production can lead to chronic remodeling of the heart, whereas high levels of ROS can directly lead to apoptosis in the cardiomyocytes [48]. It is therefore interesting that catalase overexpression inhibits cardiomyocyte apoptosis by protecting the cells from ROS [49]. Peroxisomal antioxidant enzymes and plasmalogens protect cardiomyocytes via the degradation and trapping of ROS and the maintenance of ROS homeostasis. Apoptosis of cardiac cells has been demonstrated in several cardiovascular diseases, including myocardial ischemia–reperfusion injury (I/R) and atherosclerosis [50, 51, 52]. Atherosclerosis, a major cause of heart failure and myocardial infarction, can likewise predispose to acute coronary heart disease. There is evidence that thrombosis and plaque rupture may be due to apoptosis of a large number of smooth muscle cells and macrophages in unstable atherosclerotic plaques [53, 54]. Rupture of atherosclerotic plaques with concomitant thrombus formation may lead to coronary artery occlusion, which affects the blood supply to the myocardium, resulting in myocardial infarction and leading to patient death. Reperfusion is an effective treatment for acute myocardial infarction, but it may cause reperfusion injury while restoring blood flow [55]. Studies in the last decade or so have shown that cardiac cell death occurring during reperfusion after myocardial infarction is mainly apoptosis, not cell necrosis, which breaks the long-held misconception [56, 57, 58]. Usually, what occurs during I/R is mostly cell apoptosis, whereas necrosis occurs more often after prolonged ischemia. In addition, apoptosis also plays an important role in myocardial remodeling after infarction. There is evidence that a large number of apoptotic cells can be detected in myocardium at the marginal zone of myocardial infarction [56]. Since the regenerative capacity of myocardium is limited, people show great interest in preventing apoptosis of myocardial cells during I/R.

There is also a connection between chronic heart failure and apoptosis [59]. It has been reported that patients with advanced heart failure have higher rates of cardiac myocyte apoptosis than normal subjects. Using transgenic mice with cardiac tissue-specific expression of caspase-8, it was found that apoptosis of cardiomyocytes, even at very low levels, can lead to fatal dilated cardiomyopathy as long as it occurs chronically [60]. In addition, the use of caspase inhibitors prevented left ventricular dilatation and improved ventricular function, suggesting that long-term apoptosis can lead to a significant reduction in cardiomyocyte numbers, which in turn gradually decreases cardiac contractile function. As a result, the remaining cardiomyocytes become overcompensated and contribute to cardiac hypertrophy, leading to the development of heart failure [61].

Regarding the major pathways involved in apoptotic signaling in the heart, the death receptor pathway, the mitochondrial, and ER-stress death pathways are all involved [62]. The cross-talk between death receptors and mitochondrial cell death pathways has been demonstrated in cardiomyocytes and the heart [63, 64]. For example, Date et al. found that overexpression of FasL of the death receptor pathway activated both caspase-8 and -9 in cardiac myocytes [65]. Cardiac-restricted overexpression of TNF-α promoted apoptosis, but when these mice overexpressing TNF-α were crossed with mice overexpressing Bcl-2 in the heart, both left ventricular remodeling and cardiac apoptosis in the progeny mice were be alleviated [66].

In recent years, ER stress pathway has been reported to be in cross-talk with both the death receptor pathway and the mitochondrial pathway [13, 67, 68]. One study found that application of TNF-α induced HL-1 myoblast cell lines that activated both caspase-3 and -12 [69]. Bcl-2, which targets ER, inhibited mitochondrial membrane depolarization in apoptotic cells and also inhibited cytochrome c release [70]. Caspase-8 cleaves BAP31, an ER-associated protein, and the cleaved fragment induces Ca²⁺ release from ER, into the mitochondria, and initiates apoptosis [71]. It has also been reported that Bik proteins can activate Bax/Bak in the ER membrane after localization to the mitochondria, initiating Ca²⁺ release [72].

Regardless of the causative factor, and regardless of which signal transduction pathway or pathways are involved, oxidative stress due to the interaction of peroxisomes and mitochondria plays a pivotal role in triggering apoptosis and thus contributing to the development of cardiovascular disease.

2.4.3 Neurological disorders

Apoptosis plays a key role in the normal development of the central nervous system and is involved in the pathogenesis of adult brain-related diseases, such as stroke [73] and neurodegenerative diseases [74].

There is growing evidence that the decline in peroxisome function with age may be associated with age-related neurodegenerative diseases such as Alzheimer’s disease (AD) and Parkinson’s disease (PD) [75]. In the brains of patients with Alzheimer’s disease and Parkinson’s disease, plasmin levels are significantly reduced [76, 77], which suggests peroxisome dysfunction in neurodegenerative diseases. The lack of peroxisome activity in aged cells accumulates cellular ROS, which can compromise the integrity of organelles including mitochondria and the peroxisome itself. Subsequent defects in energy production mediated by peroxisomal fatty acid metabolism and mitochondrial oxidative phosphorylation may lead to metabolic failure in aged postmitotic cells, thereby inducing apoptosis associated with neurodegeneration.

Huntington’s disease (HD), a prototypical neurodegenerative disorder, is caused by a mutation in the Huntingtin protein due to a repeat amplification of the CAG in the Huntington gene. Patients with this disease suffer from neuronal dysfunction due to massive apoptosis of nerve cells, which in turn manifests as mental cognitive and motor impairment, and even disability [74].

ROS can easily poison neurons due to their series of characteristics, such as rich in fatty acids, easy intracellular production of large amounts of hydroxyl radicals, weak antioxidant capacity, and low regenerative capacity. In addition, because of the high metabolic rates, neurons require a high energy supply from mitochondria, which are both the most important intracellular organelle for ROS production and also vulnerable to ROS attack. It has been shown that treatment of isolated cultured cerebellar granule neurons with hydrogen peroxide induces mitochondrial fission within 1 hour [78]. Furthermore, treatment of mice with nitric oxide in stroke leads to massive fission of neuronal mitochondria before the onset of neuronal loss [79]. In the presence of calcium, acute exposure to high levels of ROS can induce massive opening of mitochondrial membrane transition pores and increased permeability, which in turn causes cell Apoptosis or necrosis occurs. ROS production in mitochondria forms a vicious cycle with oxidative stress and is toxic to cells. There is some evidence in transgenic mouse models of HD that showed that Tauroursodeoxycholic acid (TUDCA), a hydrophilic bile acid with antioxidant properties, prevents the production of reactive oxygen species, mitigates mitochondrial insufficiency and apoptosis, in part, by inhibiting Bax translocation from cytosol to the mitochondria [80]. TUDCA prevented striatal degeneration and ameliorated locomotor and cognitive deficits in a 3-NP (3-nitropropionic acid) rat model of HD. Keene et al. [81] showed that systemically administered TUDCA significantly reduced striatal neuropathology, decreased striatal apoptosis, reduced the size of ubiquitinated neuronal intranuclear htt inclusions, and improved locomotor and sensorimotor deficits in the R6/2 transgenic HD mouse.

3. Conclusions

Apoptosis is a highly regulated cell death program that can be induced by a variety of physiological and pathological factors and has specific morphological and biochemical characteristics. The mechanism of its onset has not been completely elucidated to date, and it is now accepted that it is mediated by a number of pathways including the death receptor signaling pathway, the mitochondrial signaling pathway, and the endoplasmic reticulum signaling pathway. As an important way for the organism to maintain the numerical homeostasis of the cell population, apoptosis plays a key role in the pathogenesis of various human diseases. Peroxisomes and mitochondria are membrane-bound organelles in the cytoplasm of eukaryotic cells and are closely related to each other in their organelle synthesis and function. One of their important roles in cooperating with each other is to regulate the level and extent of apoptosis by maintaining the homeostasis of reactive oxygen species in the cell. Peroxisome dysfunction severely affects mitochondrial metabolism, cellular morphological stability, and biosynthesis, and therefore contributes directly or indirectly to a number of apoptosis-related diseases. Based on the available relevant findings, this chapter presents and summarizes the important potential role of peroxisomes in apoptosis-related diseases such as tumors, cardiovascular diseases, and neuropsychiatric disorders.

Acknowledgments

This work was supported in part by grants from National Natural Science Foundation of China (22176002), Anhui Provincial Natural Science Foundation (2008085 MB49), Natural Science Foundation of Anhui Provincial Department of Education (KJ2021A0215), Anhui Medical University Research Enhancement Program (2021xkjT004), and Open Project Fund of the Key Laboratory of the Ministry of Education for the Birth Population (JKZD20202).

Conflict of interest

The authors report no conflicts of interest.