The Mitochondria as an Essential Regulator of Apoptosis

Abstract

The process of apoptosis refers to programmed cell death, an essential mechanism for eliminating damaged cells. It is initiated through the intrinsic mitochondrial or the extrinsic death receptor pathways. This paper aims to unveil the pivotal role of mitochondria in regulating the apoptotic process. This paper is a systematic review based on thoroughly examining nineteen articles that align with my search criteria and cross-examining them to find principal concepts to clarify the answer to the research question. Through rigorous analysis, it became apparent that the mitochondria house the intrinsic apoptotic pathway and facilitate communication with the extrinsic apoptotic pathway through crosstalk, which cements the mitochondria’s integral role in controlled cell death.

Keywords: Apoptosis, Programmed cell death, Intrinsic apoptosis, Mitochondrial apoptosis

Introduction

Apoptosis refers to the fundamental and highly regulated process of controlled cell death. This process can occur through the two main apoptotic pathways: the intrinsic and extrinsic, each with a unique apoptotic process. The intrinsic pathway occurs at the outer mitochondrial membrane and is activated by signals inside the mitochondria. Those signals amplify the intrinsic pathway through cytochrome c¹. Factors outside the cell activate the extrinsic pathway¹. Generally, these main apoptotic pathways are initiated by cellular damaging factors such as excessive reactive oxygen species (ROS), Deoxyribonucleic Acid (DNA) damage, and mitochondrial dysfunction¹. The primary enzymes involved in both apoptotic processes are caspases. Caspases are proteases critical in orchestrating cell death by degrading cellular components.

Numerous key enzymes regulate the intrinsic mitochondrial pathway including cytochrome c, caspase 9, caspase 3, OMA1 zinc metalloendopeptidase (OMA1), and B-cell lymphoma 2 (Bcl-2) family proteins². In contrast, many death receptors in the Tumor necrosis factor receptor (TNFR) family, which are pro-apoptotic cell membrane proteins, regulate the extrinsic apoptotic pathway³. These receptors include: Fas, TNFR1, TNFR2, death receptor 4 (DR4), and DR5 ect³. BH3 Interacting Domain Death Agonist (BID) is the pro-apoptotic protein that connects the intrinsic and the extrinsic pathways⁴. BID binds to and activates the proteins Bcl-2-associated X (BAX) and Bcl-2 antagonist killer 1 (BAK) in the intrinsic mitochondrial pathway⁴. These Bcl-2 family proteins oligomerize and create pores in the mitochondrial membrane, forming mitochondrial outer membrane permeability (MOMP)⁴. MOMP triggers the release of cytochrome c, which facilitates the activation of caspases and ultimately initiates apoptosis⁴.

Moreover, in the extrinsic pathway, death receptor TNFR proteins bind to their corresponding death ligands and trigger the formation of Fas-associated death domain protein (FADD) and caspase 8, creating the death-inducing signaling complex (DISC)⁴. Activation of caspase 8 or caspase 10 at the signaling complex leads to the cleavage of BID and presents communication with the mitochondria⁵. The mutual crosstalk between apoptosis and autophagy regulates cell death or survival⁵. This interaction is also imperative for regulating mitochondrial DNA (mtDNA) release, which is released when there is an increase in cell death stressors⁵.

The intrinsic mitochondrial pathway is characterized by pro-apoptotic and anti-apoptotic Bcl-2 family members, which modulate the release of cytochrome c from the mitochondria². Anti-apoptotic Bcl-2 members halt the release of apoptotic factors into the cytosol, unlike pro-apoptotic members, which initiate the release and activity of apoptotic factors². Interactions between Bcl-2 family members control the beginning steps of the intrinsic apoptotic process². For instance, pro-apoptotic members such as BAX and BAK will bind to anti-apoptotic members, which negatively regulates their activity, allowing for the progression of apoptosis¹. During the mitochondrial outer membrane permeabilization process, Bcl-2 family members BAX and BAK form pores in the outer mitochondrial membrane⁴. This permeabilization activates OMA1, which initiates cytochrome c release from the intermembrane space⁴. Cytochrome c binds to Apoptotic protease activating factor-1 (APAF1) to form the apoptosome (a complex of proteins that moderate the initiation of caspases)⁴,⁶. Procaspase 9, an active enzyme that initiates cell death, binds to this apoptotic complex⁴. Caspase 9 then gets activated by dimerization and activates caspase 3, which executes programmed cell death^4,5.

Conversely, the extrinsic pathway consists of death receptors crucial in initiating apoptosis³. When bound to ligands such as Fas Cell Surface Death Receptor Ligand (FASL), Tumor necrosis factor (TNF), or TNF-related apoptosis-inducing ligand (TRAIL), the formation of a death-inducing signaling complex DISC occurs³. This complex activates caspase 8, which triggers the apoptotic signal, killing the cell⁴.

In addition to housing the intrinsic apoptotic pathway, the mitochondria play additional roles in the overall maintenance of cellular health. For example, the generation of Adenosine Triphosphate (ATP) through the electron transport chain (ETC) occurs in the inner mitochondrial membrane⁷. The ETC contains five protein complexes that work together to facilitate oxidative phosphorylation⁷. Oxidative phosphorylation is a metabolic process that generates energy for a cell⁷. The process is characterized by the transfer of electrons among the first four protein complexes⁷. This electron transfer powers the simultaneous pumping of protons to generate a potential difference, which is then used to power the synthesis of ATP by the fifth and final protein complex⁷. Electrons that pass through this chain ultimately convert oxygen to water⁷. During this conversion process, an increased output of ROS occurs⁷. ROS produced without sufficient antioxidant counteraction may lead to oxidative stress and mitochondrial dysfunction⁷. ROS reacts with proteins and nucleic acids, causing damage to oxidative phosphorylation proteins and mitochondrial DNA⁷. This disturbance to mitochondrial homeostasis results in dysfunctional mitochondria, which can trigger apoptosis through the intrinsic pathway⁷. Generally, dysfunctional mitochondria will overproduce ROS and release pro-apoptotic cytochrome c, which initiates intrinsic apoptosis¹. Usually, cells will attempt to remove this dysfunctional mitochondria through mitophagy, a process imperative in mitochondrial quality control⁸. A lack of mitophagy due to the deletion of the minimal essential region (MER) domain can result in damaged mitochondria accumulating, creating cellular stress and disease in the worst cases⁸. Over time, this increase in cellular stress results in mitochondrial disease or other types of disease like certain cancers in the host⁸. This paper will primarily focus on the crucial role of mitochondria in regulating apoptosis and the implications that dysfunctional mitochondria can have on this vital process. Hypothetically, mitochondrial dysfunction may trigger the initiation of the intrinsic pathway, and this review seeks to elucidate this possibility.

Purpose of Apoptosis and Consequences of Dysregulation

As mentioned previously, apoptosis occurs via the intrinsic mitochondrial pathway and the extrinsic death receptor pathway. Each pathway contains several enzymes that jointly regulate and activate the apoptotic process⁴. The intrinsic mitochondrial pathway includes cytochrome c, caspase 9, caspase 3, OMA1, and Bcl-2 family proteins⁴. Contrastingly, the extrinsic death receptor pathway is regulated by the receptors Fas, TNFR1, TNFR2, DR4, and DR5, etc⁴. These receptors are considered pro-apoptotic death receptors belonging to the TNFR family⁴. Though apoptosis can be highly beneficial to the functionality of an organism when left unchecked, apoptosis can have crippling repercussions for the health of the organism. Dysregulated apoptosis can result in the damage of tissues due to the uncontrolled loss of healthy cells⁹.

Apoptotic Pathways

Apoptosis can occur via either the intrinsic mitochondrial pathway or extrinsic death receptor pathway¹. Both pathways are activated by damaging cellular factors such as the excessive production of ROS, mutations in the mtDNA, and overall cellular stress¹. The intrinsic pathway is triggered by signals within the cell and is located in the mitochondria¹. The activation of pro-apoptotic proteins and caspases takes place in the cytosol¹. Stress signals are produced when there are inappropriate amounts of ROS and mtDNA mutations¹¹. This signal, which indicates dysfunction, initiates the intrinsic mitochondrial pathway¹¹.

The apoptotic process begins on the surface of the mitochondria, where the BAX and BAK proteins are located¹¹. These pro apoptotic proteins are activated by an apoptotic signal from oxidative stress or mtDNA damage which marks the start of the intrinsic cell death process⁵. BAX and BAK can activate OMA1, which cleaves OPA1¹¹. This cleaved OPA1 then prompts mitochondrial cristae remodeling, which are the inner folds of the mitochondria¹¹. During this process, the cristae junctions narrow to prevent cytochrome c release¹. In the outer mitochondrial membrane, BAX and BAK, triggered by mitochondrion permeabilizing peptide truncated BID (tBID), oligomerize and form pores, creating macropores⁹. These pores lead to MOMP⁹. The formation of MOMP initiates the release of intermembrane space (IMS) proteins, including cytochrome c, into the cytosol, which forms the apoptosome⁹. The apoptosome is a complex of proteins that mediate the initiation of caspases⁹. Caspases belong to a family of cysteine proteases that cleave many other proteins during the apoptotic process⁹. The apoptosome is formed when cytochrome c binds to APAF1¹². The release of cytochrome c is positively regulated by pro-apoptotic Bcl-2 family members BAX, BAK, BID, and a p53 modulator of apoptosis (PUMA)⁴. Conversely, cytochrome c release is negatively regulated by anti-apoptotic Bcl-2 family members BCL-XL, Bcl-w, A1, and MCL 1⁴. The apoptosome initiates the activation of caspases and is bound to the death-inducing enzyme Procaspase 9¹². The apoptosome activates caspase 9, which activates caspase 3, leading to the execution of cell death¹².

Moreover, the extrinsic death receptor pathway is also regulated by various ligands, proteins, and receptors³. External stimuli such as DNA damage signals activate the following receptors⁶. The process begins with the initiation of death receptors (also known as cellular membrane proteins) such as TNFR1, TNFR2, DR4, and DR5³. DR4 and DR5 directly bind to FADD, whereas TNFR1 has to bind to the adapter protein TRADD, which then recruits FADD³. The binding of a death receptor such as FAS-R, TNF-R1, TNF-R2, TRAIL-R1, and TRAIL-R2 to FADD and caspase 8 results in a trimerization and the creation of the DISC³. The activation of caspase 8 or caspase 10 by the DISC results in the cleavage of BID⁴. tBID eventually relocates to the mitochondria³. It prompts cytochrome c release and caspase 9 activation, which triggers caspase 3⁴. The activation of caspase 3 ultimately induces the apoptotic signal, which incites cell death⁴.

Mitochondrial Apoptosis

Because the intrinsic mitochondrial apoptotic pathway is activated by signals from the mitochondria and is located in the outer mitochondrial membrane, the mitochondria arguably play a critical role in supporting apoptosis⁴. Mitochondria are the source of the pro-apoptotic factor cytochrome c, which primarily activates caspase 9 and caspase 3 during the apoptotic process⁴. The mitochondria release cytochrome c under damaging conditions, such as excessive ROS and mtDNA damage/mutations, which negatively interfere with mitochondrial function⁸. These stressors may be alleviated by mitochondrial fusion, which is a process that entails the generation of a singular mitochondrion by combining dysfunctional mitochondria, which is necessary because mitochondria are imperative to the apoptotic process⁹. In the presence of stressors such as excessive amounts of ROS, mitochondrial fusion is upregulated¹³.

Decreased or increased apoptosis occurring in both apoptotic pathways can cause numerous types of cancers and neurodegenerative diseases⁸. Apoptosis occurs in an adult human body in large amounts, and around ten billion cells are made daily to make up for the amount lost due to cell death¹⁴. This increase in apoptosis in adults could be due to age-related cellular decline, which elicits the need for increases in apoptosis¹⁴. There is a baseline level of activity in both apoptotic pathways to maintain cellular homeostasis and to ensure that the amount of dysfunctional cells does not increase through cellular proliferation⁴. The amount of apoptosis also depends on the activation of caspases, whose activity positively upregulates mitochondrial apoptosis¹. Cell-death-inducing signals and stimuli generally activate caspases¹⁵.

The level of apoptotic activity of the intrinsic mitochondrial pathway depends on signals from the Bcl-2 family members in the mitochondria, which are activated due to death stimuli such as DNA damage¹. In healthy mitochondria, the apoptotic process is not necessarily activated since cellularly stressful and damaging factors trigger apoptosis^1,5. However, excessive ROS significantly impacts cell division and proliferation, which can trigger apoptosis⁸. Dysfunctional mitochondria can also induce cell death because they increase the production of ROS, which creates an excess amount of oxidative stress, damaging the cell and causing mutations to mtDNA⁸. This impairment can activate the apoptotic signal, which kills the cell⁸.

Relationship Between Mitochondrial Apoptosis and Other Apoptotic Pathways

Although the intrinsic mitochondrial pathway is triggered mainly by signals inside the mitochondria and the extrinsic death receptor pathway is activated by external stimuli, both pathways may be activated due to damaging factors⁹. The intrinsic pathway is triggered by ROS and mtDNA mutations, which are stressors that negatively interfere with the function of mitochondria⁹. Conversely, extracellular ligands such as TNF and Fas-L can trigger the extrinsic pathway⁴. The intrinsic and extrinsic pathways ultimately activate the same caspase 3, which, when cleaved by an initiator caspase, triggers the execution pathway of apoptosis¹⁴. This pathway is where the formation of apoptotic bodies and the irreversible death of the cell occur¹⁴.

The intrinsic mitochondrial pathway is triggered by stress signals inside the cell, such as oxidative stress and mtDNA mutations, while external factors trigger the extrinsic death receptor pathway⁶. The apoptosome is the protein complex formed in the intrinsic pathway, while the DISC is the protein complex formed in the extrinsic pathway⁶. Both the apoptosome and DISC result in the initiation or activation of specific caspases that are the gateway to the final executioner pathway of apoptosis, ultimately resulting in cell death⁶. The apoptosome activates caspase 9, which triggers caspase 3, causing cell death via the executioner pathway⁶. On the contrary, the DISC is where caspase 8 is activated and separates from the signaling complex to initiate the execution of the apoptotic pathway⁶.

While the extrinsic pathway is regulated by death ligands such as TNF, FAS-L, and TRAIL, the intrinsic pathway activates apoptotic proteins from the Bcl-2 family to function⁶. The intrinsic and extrinsic pathways of apoptosis each consist of numerous different caspases, which assist in the function of each pathway respectively⁴. In the intrinsic pathway, the cleavage of caspase 9 at the apoptosome activates caspase 3, which initiates the execution of cell death⁴. Similarly, in the extrinsic pathway, caspases 8 and 10 are activated at DISC,  which triggers the cleavage of BID and initiates the apoptotic signal, killing the cell⁴.

The intrinsic mitochondrial and the extrinsic death receptor pathways merge and induce the final apoptotic execution pathway⁶. This phase of the apoptotic process occurs due to executioner caspases, such as caspase 3 and caspase 7⁶. When activated, these caspases carry out apoptosis to kill the cell⁶. The pro-apoptotic protein BID and the final executioner apoptotic pathway link the intrinsic and extrinsic apoptotic pathways^4,6. In the intrinsic pathway, pro-apoptotic factors BID, BAX, and BAK positively regulate the release of cytochrome c, which binds to APAF1 and forms the apoptosome¹². Procaspase 9, found only in the intrinsic pathway, binds to this complex, cleaves, and activates caspase 3, which is activated in both the intrinsic and extrinsic pathway⁴. Caspase 3 then activates the executioner pathway, which completes cell death⁴. In the extrinsic pathway, death receptors bind to FAS-L, TNF, and TRAlL ligands, forming the DISC complex and causing the death receptors to trimerize, activating caspases 8 and 10 and cleavage of BID⁷. The extrinsic death receptor pathway communicates with the intrinsic mitochondrial pathway through the cleavage of BID by caspase 8, resulting in tBID⁴. tBID translocates to the mitochondria, which stimulates the release of cytochrome c^1,4. This escalates the cytochrome c death signal, leading to apoptosis⁴. Furthermore, extrinsic apoptosis can occur without the mitochondria’s involvement. This is the case when caspase 3 is activated by caspase 8, leading to the executioner pathway⁴. This process still leads to apoptotic cell death without involving the mitochondria⁴.

How is Apoptosis Handled When Mitochondria Are Dysfunctional?

Dysfunctional mitochondria, due to mitochondrial ROS, mtDNA damage, and calcium signaling can indirectly lead to cell death⁹. An additional mechanism for counteracting mitochondrial dysfunction is mitochondrial fission. This process will occur in a cell to separate the damaged mitochondrial parts of a mitochondrion, which are then removed through mitophagy⁹. The process of mitochondrial fission is characterized by the detachment of damaged parts of the mitochondria by DRP1⁹. These damaged parts are then removed through mitophagy to assure mitochondrial quality⁹. Ensuring mitochondrial function and quality is necessary for apoptosis since the mitochondria are responsible for housing the intrinsic apoptotic pathway⁹.

Following fission, the process of mitophagy occurs to remove unwanted dysfunctional mitochondria to maintain mitochondrial homeostasis and function, as well as the cell’s overall health⁸. A cell will undergo mitophagy to remove only the defective mitochondria in a cell, whereas apoptosis is the death of the entire cell⁸.Mitophagy can be categorized as either ubiquitin-mediated mitophagy or receptor-mediated mitophagy⁸. The PTEN-induced kinase 1 (PINK)/Parkin pathway is considered ubiquitin-mediated mitophagy, while Bcl-2 and adenovirus E1B19 kDa-interacting protein 3 (BNIP3) mitophagy and FUN14 domain-containing protein 1 (FUNDC-1) mitophagy are considered receptor-mediated mitophagy⁸. PINK1/Parkin mitophagy begins when PINK1 is collected at the outer mitochondrial membrane in response to mitochondrial damage and dysfunction⁸. This collection prompts the recruitment of Parkin, which modifies elements of the outer membrane⁸. Then, phosphorylated Poly-ubiquitination chains located on mitochondrial proteins modify other proteins such as TBK1, which phosphorylates optineurin (OPTN) and then signals the process of autophagy⁸. Additionally, receptor-mediated mitophagy includes BNIP3 and FUNDC1, which are mitophagy receptors⁸. BNIP3 and FUNDC-1 interact with OPA1 and dynamin-related protein 1 (DRP1) in order to trigger mitophagy⁸. Moreover, the apoptotic protein BNIP3 has the ability to attach itself to proteins called Ras homolog enriched in brain (RHEB) to prevent the activation of mTOR to initiate mitophagy⁸. Mitophagy with FUNDC-1 depends on hypoxia-induced dephosphorylation⁸. Excessive accumulation of unhealthy cells can lead to disease. When a cell holds an excessive amount of mitochondrial dysfunction, processes such as mitophagy cannot fix the damage⁹. This leads to an accumulation of dysfunctional mitochondria, increasing oxidative stress and triggering apoptosis⁹.

In mitochondrial dysfunction, the intrinsic pathway is initiated to eliminate damaging factors such as mtDNA damage and dysregulated ROS production through cell death⁴. It is unlikely that the extrinsic pathway compensates for the loss of apoptosis directed by the intrinsic pathway during mitochondrial dysfunction since the intrinsic pathway is triggered by mitochondrial dysfunction. In contrast, factors outside the mitochondria trigger the extrinsic pathway.

Discussion

The strictly regulated process of programmed cell death is known as apoptosis. This paper strived to clarify the critical role of the mitochondria in the apoptotic process and the potential implications that mitochondrial dysfunction can have on this fundamental procedure. The two primary pathways in which the apoptotic process occurs are the intrinsic mitochondrial and the extrinsic death receptor pathways. Both pathways are activated in response to cellular stressors and are connected by the pro-apoptotic protein BID.  BID connects the extrinsic and intrinsic apoptotic pathways when it is cleaved by caspase-8. In the extrinsic pathway, the DISC complex, which functions as the site where cell death is initiated, triggers the cleavage of BID by caspase 8 to create tBID, which translocates to the mitochondria, prompting the release of cytochrome c. tBID binds to APAF-1 to form the apoptosome, which activates Procaspase-9. As a result, caspase 3 and the apoptotic death signal are activated, resulting in cell death.

The extrinsic apoptotic pathway begins with the initiation of cellular membrane death receptors, which bind to ligands to form a DISC where caspase 8 or caspase 10 are activated. Caspase 8 can trigger the apoptotic death signal in mitochondria-independent apoptosis. Additionally, activated caspase 8 has the ability to cleave BID, which causes the mitochondria to release cytochrome c. This cytochrome c attaches to APAF-1 to create the apoptosome, leading to the activation of Procaspase-9. Consequently, the activation of caspase 3 occurs, and the apoptotic death signal is initiated, resulting in cell death. The articles reviewed don’t explicitly state precisely where the extrinsic apoptotic pathway is located. I hypothesize that the extrinsic death receptor pathway may be located near the mitochondria’s outer membrane, where the intrinsic pathway is located. This may be due to BID’s communication between the two apoptotic pathways. tBID in the extrinsic pathway can translocate to the mitochondrial intrinsic pathway to release cytochrome c. For tBID to move to the intrinsic pathway (facilitating crosstalk between the two apoptotic pathways), the extrinsic pathway may have to be located close to the mitochondria.

Both apoptotic pathways are necessary for maintaining cellular health since different factors trigger them. The intrinsic mitochondrial pathway is triggered by stressors inside the mitochondria, such as mtDNA mutations, excessive ROS levels, and mitochondrial dysfunction. On the other hand, the extrinsic pathway is triggered by signals outside of the mitochondria, such as DNA damage. Cells can be characterized as “damaged” due to the accumulation of dysfunctional mitochondria or overall cellular damage. This is why both apoptotic pathways are necessary so that each pathway can account for signals coming from inside the mitochondria or from anywhere else in the cell to maintain the host’s health.

Suppose apoptosis is halted due to either of the two pathways being impaired. In that case, it can contribute to the development of various types of cancers and neurodegenerative diseases due to the overaccumulation of damaged cells. These can include Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease¹⁶. The functionality of the intrinsic pathway can be compromised due to deformities in BAX and BAK, such as the eradication of BAX¹⁶. Since Bcl-2 proteins are essential to the apoptotic process, the deletion of BAX may hinder the completion of the cell death process. The functionality of apoptosis may also be hindered in the apoptosome with the deactivation of APAF1¹⁶. Cancer cells evade cell death because they upregulate anti-apoptotic proteins, which prevent apoptosis. Suppose we could manipulate the expression of pro-apoptotic Bcl-2 family proteins, which would elicit the release of cytochrome c. In that case, this may allow the apoptotic process to commence by creating the apoptosome, eradicating cancerous cells.

Additionally, understanding the mechanisms that cause upregulation of pro-apoptotic factors in the intrinsic pathway can be used to comprehend why cancerous cells prevent apoptosis through the upregulation of anti-apoptotic Bcl-2 proteins. The levels at which Bcl-2 family members are expressed depend on their transcriptional regulation¹⁷. For instance, the pro-apoptotic protein BAX is made from mRNA, which p53 regulates to induce apoptosis. DNA damage activates p53, which causes it to facilitate transcription of the genes of BAX and trigger apoptosis¹⁷. P53, activated by stress signals, initiates the expression of PUMA. This creates the PUMA-BclXL complex, which releases p53 to activate BAX¹⁸. If cancer cells can be stopped downregulating pro-apoptotic proteins by providing them with a pro-apoptotic protein such as p53, it may eventually allow for MOMP to occur, making apoptosis inevitable. This idea could help cancer treatment by stopping cancerous cells from overriding cell death.

The mitochondria house the intrinsic apoptotic pathway, making it unquestionably essential to intrinsic apoptosis. The mitochondria also can send signals through ROS to indicate a disturbance. This signal triggers the apoptotic process. Since mitochondria are critical in maintaining overall cellular health, damaged mitochondria are removed to prevent disturbances in maintaining homeostasis. The presence of mitochondrial dysfunction due to mtDNA damage and excessive levels of ROS can ultimately lead to cell death to stop dysfunctional mitochondria from multiplying. Various cancers may result from uncontrolled cell proliferation of unhealthy cells, and unhealthy cells might directly result from mitochondrial dysfunction. Cells utilize the process of mitophagy to remove dysfunctional mitochondria and maintain cellular homeostasis. Mitophagy is imperative in mitochondrial quality control. This process eliminates mitochondrial damage by removing all of the impaired mitochondria. Similarly, mitochondrial fusion and fission are integral to ensuring the health of mitochondria so they can facilitate the apoptotic process. Mitochondrial fusion allows damaged mitochondria to combine to create a singular mitochondrion⁹. Combining may improve mitochondrial function by allowing the mitochondria to compensate for each other’s damage. Alternatively, fission allows the eradication of dysfunctional parts of mitochondria by separating the exact damaged part and then removing it through mitophagy⁹. This process indicates that mitochondria are on the verge of elimination due to their damage and are activated by oxidative stress¹⁹.

Conclusion

The term “apoptosis” refers to the closely monitored process of programmed cell death. The two pathways that enable the occurrence of the apoptotic process are the intrinsic mitochondrial and the extrinsic death receptor pathways. These pathways are activated by cellular damage and malfunction, such as mitochondrial dysfunction and other external stressors. This paper aimed to explore the critical role of mitochondria in regulating apoptosis. The mitochondria house the intrinsic apoptotic pathway and utilize Bcl-2 family proteins and numerous caspases to execute cell death. In the presence of mitochondrial dysfunction, the mitochondria signal to the intrinsic apoptotic pathway that there is a disturbance within the organelle, initiating the process. Additional investigation and research may be necessary to determine the precise location of the cell’s extrinsic death receptor apoptotic pathway.

Methodology

To determine the role of the mitochondria in regulating apoptosis, a systematic review was conducted. The following search engines were utilized to gather over twenty relevant research articles: Google Scholar and PubMed. A rigorous quality assessment of all data presented was conducted. Papers referenced were found only through the reputable databases previously mentioned to ensure the merit of the research and to prevent the use of illegitimate evidence. Papers used also had to include the following keywords and follow the search criteria. Keywords used include apoptosis, mitochondrial dysfunction, intrinsic pathway, and extrinsic pathway. Search criteria were predicated on the following items: articles must have been published within the last decade, have been cited at least three times, must include at least two of the keywords described above, and should be either a primary research article or a literature review. A rigorous literature review of the chosen articles was completed by thoroughly examining the papers aligned with the search criteria and creating concise summaries containing the overall findings of each report. Summaries were then analyzed to find key recurring themes and differences across the studied papers. These themes or differences were used to generate answers to the research question presented in this paper. Discrepancies identified were considered potential knowledge gaps and were further investigated and resolved through educated inferences. Ideas for future investigations and questions that require further exploration were proposed at the end of the analysis.

Acknowledgments

Thank you to my mentor, Kaydine Edwards, for her invaluable guidance and support while forming this research paper.

Abbreviation	Full Name
ROS	Reactive oxygen species
DNA	Deoxyribonucleic Acid
OMA-1	OMA1 zinc metalloendopeptidase
Bcl-2	B-cell lymphoma 2
TNFR	Tumor necrosis factor receptor
DR4	Death receptor 4
BID	BH3 Interacting Domain Death Agonist
BAX	Bcl-2-associated X
BAK	Bcl-2 antagonist killer 1
MOMP	Mitochondrial outer membrane permeability
FADD	Fas-associated death domain protein
DISC	Death-inducing signaling complex
mtDNA	Mitochondrial DNA
APAF-1	Apoptotic protease activating factor-1
FAS-L	Fas Cell Surface Death Receptor Ligand
TNF	Tumor necrosis factor
TRAIL	TNF-related apoptosis-inducing ligand
ATP	Adenosine Triphosphate
ETC	Electron transport chain
MER	Minimal essential region
OPA1	Optic atrophy 1
tBID	truncated BID
IMS	Intermembrane space
PUMA	p53 modulator of apoptosis
PINK	PTEN-induced kinase 1
BNIP3	Bcl-2 and adenovirus E1B19 kDa-interacting protein 3
FUNDC-1	FUN14 domain-containing protein 1
OPTN	Optineurin
DRP1	Dynamin-related protein 1
RHEB	Ras homolog enriched in brain

References

1. C. Wang, R. J. Youle. The role of mitochondria in apoptosis. Annual Review of Genetics. 43, 95–118 (2009). [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩]
2. J. Estaquier, F. Vallette, J.-L. Vayssiere, B. Mignotte. The mitochondrial pathways of apoptosis. Advances in Experimental Medicine and Biology. 942, 157–183 (2011). [↩] [↩] [↩] [↩]
3. S.-M. Park, R. Schickel, M. E. Peter. Nonapoptotic functions of fadd-binding death receptors and their signaling molecules. Current Opinion in Cell Biology. 17, 610–616 (2005). [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩]
4. B.A. Carneiro, W.S. El-Deiry. Targeting apoptosis in cancer therapy. Nature Reviews Clinical Oncology. 17, 395–417 (2020). [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩]
5. A. Picca, R. Calvani, H. J.Coelho-Junior, E. Marzetti. Cell death and inflammation: the role of mitochondria in health and disease. Cells. 10, 537 (2021). [↩] [↩] [↩] [↩] [↩] [↩]
6. R. Jan, G.-S. Chaudhry. Understanding apoptosis and apoptotic pathways targeted cancer therapeutics. Advanced Pharmaceutical Bulletin. 9, 205–218 (2019). [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩]
7. J. M. Winter, T. Yadav, J. Rutter. Stressed to death: mitochondrial stress responses connect respiration and apoptosis in cancer. Molecular Cell. 82, 3321–3332 (2022). [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩]
8. A. Li, M. Gao, B. Liu, Y. Qin, L. Chen, H. Liu, H. Wu, G. Gong. Mitochondrial autophagy: molecular mechanisms and implications for cardiovascular disease. Cell Death & Disease. 13, 444 (2022). [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩]
9. Q. Chu, X. Gu, Q. Zheng, J. Wang, H. Zhu. Mitochondrial mechanisms of apoptosis and necroptosis in liver diseases. Analytical Cellular Pathology. 2021, 8900122 (2021). [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩] [↩]
10. L. Galluzzi, C. Brenner, E. Morselli, T. Zahia, G. Kroemer. Viral control of mitochondrial apoptosis. PLoS pathogens. 4, e1000018 (2008). [↩]
11. X. Jiang, H. Jiang, Z. Shen, X. Wang. Activation of mitochondrial protease oma1 by bax and bak promotes cytochrome c release during apoptosis. Proceedings of the National Academy of Sciences. 111, 14782–14787 (2014). [↩] [↩] [↩] [↩] [↩]
12. Q. Hu, D. Wu, W. Chen, Z. Yan, C. Yan, T. He, Q. Liang, Y. Shi. Molecular determinants of caspase-9 activation by the apaf-1 apoptosome. Proceedings of the National Academy of Sciences of the United States of America. 111, 16254–16261 (2014). [↩] [↩] [↩] [↩]
13. R. J. Youle, A.M. van der Bliek. Mitochondrial fission, fusion, and stress. Science. 337, 1062–1065 (2012). [↩]
14. S. Elmore. Apoptosis: a review of programmed cell death. Toxicologic Pathology. 35, 495–516 (2007). [↩] [↩] [↩] [↩]
15. Y. Shi. Caspase activation, inhibition, and reactivation: a mechanistic view. Protein Science. 13, 1979–1987 (2004). [↩]
16. B. Favaloro, N. Allocati, V. Graziano, C. Di Ilio, V. De Laurenzi. Role of apoptosis in disease. Aging. 4, 330–349 (2012). [↩] [↩] [↩]
17. R. Singh, A. Letai, K. Sarosiek. Regulation of apoptosis in health and disease: the balancing act of bcl-2 family proteins. Nature Reviews Molecular Cell Biology. 20, 175–193 (2019). [↩] [↩]
18. J. Chen. The cell-cycle arrest and apoptotic functions of p53 in tumor initiation and progression. Cold Spring Harbor Perspectives in Medicine. 6, a026104 (2016). [↩]
19. M. Frank, S. Duvezin-Caubet, S. Koob, A. Occhipinti, R. Jagasia, A. Petcherski, M.O. Ruonala, M. Priault, B. Salin, A.S. Reichert. Mitophagy is triggered by mild oxidative stress in a mitochondrial fission dependent manner. Biochimica et Biophysica Acta (BBA) – Molecular Cell Research. 1823, 2297–2310 (2012). [↩]