Disorders of apoptosis signaling in tumor cells

One of the functions of apoptosis is to prevent malignant tumor growth. Apoptosis is part of tissue homeostasis, maintaining the balance between cell formation and death. Excessive apoptosis leads to tissue hypotrophy (e.g., in ischemia). Decreased apoptosis (as well as increased cell replication) leads to tumor formation. All tumor cells had to suppress apoptosis during their transformation.

Apoptosis is triggered via two pathways: the extrinsic and the intrinsic pathways. In some cells, both pathways must be activated (especially the extrinsic pathway, which subsequently activates the intrinsic pathway). In other cells, the activation of the intrinsic pathway only is sufficient. Both pathways meet at the point of activation of the executioner (effector) caspases 3, 7, and 6, and then apoptosis ensues.



Extrinsic pathway
It begins with the binding of the DR (Death Receptor) ligand to the DR (Fas ligand to Fas receptor) and subsequent trimerization of these receptors (the ligand is either present on an apoptosis-triggering cell or is secreted in an apocrine manner). Other ligands for specific death receptors also exist. These ligands can be cytokines, growth factors, hormones, or toxins. The initiator procaspase 8 or 10 binds to these receptors via the FADD (Fas-associated protein with death domain) protein, which proteolytically activates the procaspases to their active form upon DR activation. This whole complex is called DISC (Death inducing signaling complex). The activated initiator caspases have 2 tasks: to activate executioner procaspases 3 and 7 and cleave the Bid protein to the t-Bid (its active form). Caspase 3 then activates procaspase 6.

Intrinsic pathway
It is also known as the mitochondrial pathway. Due to the change in the permeability of the inner mitochondrial membrane, cytochrome c is released into the cytosol, where it binds to the inactive APAF1 (apoptotic protease activating factor 1) protein and thus changes its conformation. After binding to ATP, the APAF1-cyt c-ATP complex associates in a pentamer capable of binding procaspase 9, which upon binding it, it is transformed to the active caspase 9. The formed complex is called the apoptosome and it transforms procaspases 3 and 7 to their active forms.

The change in mitochondrial membrane permeability is regulated by the Bcl family of proteins. Proapoptotic Bax and Bak form homodimers or heterodimers, which induce an increase in the permeability of the inner mitochondrial membrane. However, it should be impermeable in a healthy cell, and therefore there are anti-apoptotic proteins. These are Bcl-2 and Bcl-xl, which with Bak and Bax form Bax/Bcl-2 heterodimers. Elevated levels of Bcl-2 were found first in the B lymphoma (B-Cell Lymphoma).

The intrinsic pathway is induced by O2 and nutrient deficiencies, virus infections, glucocorticoids, heat, and radiation (DNA or mitochondrial damage).

Losing the activation signal
DR expression can either be reduced or normal, but the synthesized receptors are non-functional, such as in the case of decoy receptors. These receptors lack the cytosolic death domain (DD), which prevents FADD binding and DISC formation.

Signal shift
TRADD (Tumor necrosis factor Receptor Associated Death Domain) is a membrane protein that mediates DISC formation by binding FADD after binding the DR tumor necrosis factor receptor (TNFR). Instead, TRADD can bind TRAF (TNF Receptor Associated Factor), which leads to the expression of the transcription factor NFkB via a cascade of several kinases. The effect of NFkB is to active the anti-apoptotic factors XIAP, FLIP, and Bcl-2, inhibiting apoptosis.

Inactivation of DISC by the FLIP protein
The FLIP protein has a very similar structure to procaspase 8. It contains a death effector domain (DED) that binds to the FADD protein. It functions as an anti-apoptotic protein as it can displace one or two procaspases 8 in the DISC. Thus, homodimers cannot be formed or proteolytically activated. This inhibition directly correlates to the concentration of the FLIP protein. However, even with insufficient activation of caspase 8, the Bid protein can still be cleaved and the intrinsic pathway can be triggered.

Caspase inhibitors
IAP (Inhibitors of Apoptosis Proteins) act by directly binding to the procaspase active sites, preventing their activation and transformation to caspases. These include HIAP, XIAP, SURVIVIN, and LIVIN. Increased expression of SURVIVIN has been demonstrated in many types of tumors.

Disorders of mitochondrial signaling associated with the tumor suppressor p53
When DNA is damaged, there is increased expression of the TP53 gene, the product of which is the transcription factor p53. The p53 protein plays a key role in suppressing tumor processes. Firstly, it acts by increasing the expression of GADD45, which acts via p21 to arrest the cell cycle. Secondly, it increases the expression of Bax and PUMA and suppresses the expression of Bcl-2. This helps initiate apoptosis by the alteration of the Bax/Bcl-2 ratio, which leads to increased inner mitochondrial membrane permeability and the release of proapoptotic factors from the mitochondria. Another proapoptotic effect is the upregulation of FAS, DR5, or APAF1.

Other possibilities of influencing apoptosis
Point mutation of the Ras gene can lead to overly active Akt kinase (via MAP kinases). This is associated with prolonged cell survival. It weakens the internal activation of apoptosis by phosphorylating caspase 9 and Bad, which inactivates them. It also inactivates Fas L and increases the expression of NFkB via IKK kinase.

Apoptosis signaling is a very complicated process regulated by many proteins. This is advantageous as there are many ways to achieve apoptosis in the case of the failure of a particular protein. The process by which tumors arise is complex and requires multiple mutations. In other words, a single mutation is certainly not enough to suppress apoptosis. However, there are some exceptions such as mutations of p53 and Bcl family of proteins, whose defects can be fatal in terms of oncogenesis.

Related articles

 * Apoptosis
 * Apoptosis and clinical consequences of its regulation disorders
 * Caspases

Source

 * VERMACH, Petr. Vypracované otázky ke zkoušce z patobiochemie. 2010.