Apoptotic Pathways: Caspases, Bax, and Beyond

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Introduction

Cell death: Apoptosis is a normal and tightly controlled process by which the body rids itself of cells it no longer needs or those that are damaged and therefore dysfunctional. This mechanism is essential in the regulation of tissue balance and anti-cancer activity and as a means to regulate the immune response. Apoptosis is initiated by stimulation pathways, and the most important of them are caspases and proteins of the Bcl-2cl-2 family, particularly Bax. These molecules facilitate the process of lytic cell necrosis without causing inflammation as it would occur in necrosis. Knowing apoptotic mechanisms is critical for maintaining therapeutic progress for such conditions as cancer, cardiovascular diseases, neurodegenerative disorders, etc. In this article, the crucial protein caspases and Bax involved in apoptosis will be discussed along with some other proteins that are involved in the regulation and the execution of programmed cell death.

Caspases: The Executioners of Apoptosis

Caspases, also known as cysteine-aspartic proteases, are considered molecular switches of apoptosis. These proteases are present in cells in an inactive form and are cleaved when apoptotic signals are received. Caspases can be categorized into two broad groups based on their role in the apoptotic process: Initiator caspases include caspase-8 and caspase-9 and the apoptosome, and executioner caspases include caspase-3, caspase-6, and caspase-7.

Initiator Caspases

The initiator caspases, caspase 8 and caspase 9, are considered executioners of apoptosis signal receptors. These caspases are recruited to form part of bigger signalosomes; for instance, the death-inducing signaling complex in the case of caspase 8 and the apoptosome for caspase 9. On the activation of these caspases, they become self-cleaved and convert into their active forms. They then proceed to cleave and activate other effector executioner caspases hereinafter mentioned.

Caspase 8 is part of the extrinsic or death receptor-mediated form. It is recruited to the DISC when receptors, for example, Fas or TNF receptors, are bound to their respective ligands. This recruitment results in the dimerization and activation of caspase 8, and subsequently, the death signal is scaffolded to the activation of executioner caspases.

However, Caspase-9 is an important activator of the intrinsic or mitochondrial apoptosis pathway. In humans, it is activated when cytochrome c, a protein that was released from damaged mitochondria, binds to an apparatus known as the apoptosome. Staining revealed that the apoptosome promotes the activation of caspase-9, which in turn activates the executioner caspases, causing the dismantling of the cell.

Executioner Caspases

The agents who carry on to destroy the apoptotic phase are executioner caspases, of which caspases 3, 6, and 7 are the most common. When initiator caspases are activated, they target enzymes that bind to and control functions within the cell, breaking them down and bringing about the ordered destruction of the cell. Caspace-3, for example, is one example of a caspase that cleaves proteins like PARP (poly ADP-ribose polymerase), which plays a role in DNA repair, so that, once it has been triggered, the cell won’t be able to repair itself.

They degrade cytoskeletal proteins, nuclear proteins, and other molecules essential to maintaining cellular integrity through these executioner caspases. Consequently, the cell shrinks, has DNA fragmentation, babbles its membrane, and finally develops apoptotic bodies engulfed by neighboring cells or macrophages.

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Bax: The Key Regulator of the Intrinsic Pathway

Although caspases are the predominant executioner of apoptosis, members of the Bcl-2 protein family, and especially Bax, are essential regulators of the intrinsic apoptotic pathway. Bax is a pro-apoptotic member of the Bcl2 family of both pro- and anti-apoptotic proteins.

The study shows how Bax is involved in controlling the mitochondrial membrane permeability critical to the release of cytochrome c, a key event of the intrinsic pathway. Bax behaves normally in the cytosol in an inactive form under normal conditions. Yet while Bax can receive apoptotic stimuli, upon receiving them, Bax itself changes into a form that allows it to translocate to the mitochondrial membrane, turn in, and go between the outer mitochondrial membrane. This action allows the release of cytochrome c into the cytoplasm, followed by its binding to Apaf-1 (apoptotic protease activating factor 1), forming the apoptosome and induction of caspase-9 activation.

The Interplay of Bax and Bcl-2

Bax is inactivated by anti-apoptotic proteins of the Bcl-2 family, for example, Bcl-2 and Bcl-; this consequently cancels apoptosis. These proteins interact with Bax and inhibit its ability to form, possess pro-apoptotic activity, and insert into the mitochondrial membrane, preventing cytochrome c release. This model shows that depending on the ratio of proteins that promote apoptosis, such as Bax, and proteins that protect a cell from apoptosis, such as Bcl-2, a cell will either die or live.

Notably, the silencing of death-promoting proteins and overexpression of anti-apoptotic proteins such as Bcl-2 contributes to a shift in balance in favor of survival in many forms of cancer. This has applied particular importance to Bcl-2 and Bax proteins, making it possible to use drugs such as BH3 mimetics to block the action of anti-apoptotic Bcl-2 proteins in cancer therapy.

Beyond Caspases and Bax: Additional Regulators of Apoptosis

However, caspases and Bax are considered to be core components of the apoptotic machinery; however, many other molecules participate and modulate the process of apoptosis. Some of these include:

IAPs (Inhibitors of Apoptosis Proteins)

Members of the inhibitors of the apoptosis proteins family are important in the regulation of apoptosis, especially by directly inhibiting caspases. For instance, the X-linked inhibitor of apoptosis protein (XIAP) directly binds with and suppresses caspases 3. 7, and 9, which suggests that caspase would not be activated. These proteins include IAPs, and since tumor cells use them to keep off apoptosis, they are also candidates for cancer treatment.

Smac/DIABLO

Smac/DIABLO is an intermembrane space protein of the mitochondria that translocates to the cytosol during apoptosis. This is due to the ability of Smac/DAB2 to interact with IAPs and counter their action to enable caspases to function optimally. This molecule provides an example of the tightly regulated balance between apoptotic promoters and inhibitors in the determination of cell survival.

Apoptosis-Inducing Factor (AIF)

However, apoptosis also follows caspase-independent systems or pathways to its execution. AIF is one of the factors being released from mitochondria in response to apoptotic signals. AIF translocates to the nucleus and causes chromatin condensation and large-scale DNA fragmentation. Apoptotic cell death is achieved without the help of caspases.

Fas and the Death Receptor Pathway

The extrinsic apoptotic pathway starts with the interaction of death ligands, FasL and TNF-alpha, with their corresponding receptors on the cell membrane. This interaction leads to the formation of the DISC and the activation of caspase-8 as an executioner caspase, which were discussed earlier in this chapter. Apoptosis regulated by Fas ligand, a cell surface receptor, is critical in regulating the immune system’s stability, also called homeostasis; alterations in the pathway have been associated with autoimmunity and cancer.

Mitochondrial Outer Membrane Permeabilization (MOMP)

Therefore, it is concluded that MOMP is an important process in intrinsic apoptosis signals. Bax and Bak control mitochondrially located MOMP, and their homo-oligomerization leads to the formation of channels in the mitochondrial outer membrane and the liberation of cytochrome c and other apoptogenic factors. The opening and closing of the MOMP channel is therefore dependent on a delicate balance in the cell composition of proteins that either promote or inhibit apoptosis.

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Therapeutic Implications and Future Directions

Due to the enhancement of knowledge regarding the molecular signals of apoptosis, efforts have been made toward identifying treatments for diseases associated with excessive/deficient apoptosis. In many malignancies, cells refuse to die through apoptosis; therefore, Bax, caspases, and Bcl-2 family members hold various therapeutic options for cancer therapy. Chinese drugs like the BH3 mimetics that block the anti-apoptotic proteins are in different phases of clinical trials testing for their efficiency in inducing apoptosis in cancer cells.

Whereas in neurodegenerative diseases and ischemic injuries, apoptosis plays a crucial role in cell death, the targets are to block apoptosis and, therefore, save the healthy cells. Apopoptics such as caspase inhibitors and IAP antagonists are somewhat under investigation for having the capacity to mitigate undesirable cell death in such circumstances.

Most of the apoptotic research is to explore the molecular mechanisms governing apoptosis and to discover new targets of action. This knowledge gives us the capacity to fine-tune apoptosis for therapeutic advantage as we become enlightened more on the pathways involved here.

References

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