Disorders of apoptosis signaling in tumor cells

One of the function of apoptosis is to prevent malignant tissue growth. Apoptosis is part of the tissue homoestasis - balanced formation and death. Excessive apoptosis leads to tissue hypotrophy (eg in ischemia). Decrased death (as well as increased cell replication) leads to tumor tranformation. All transformed cells had to supress apoptosis during their transformation.

Apoptosis is triggered in two ways - internal and external. In some cells both must be activated (especially exrternal, which subsequently activates the internal), in ohters only internal is sufficient. Both pathways meet by activating executive caspases 3, 7 and 6, and then apoptosis occurs.



Extrinsic pathway
It begins with the binding of DR L (Death Receptor ligand) to Death Receptor, hense Fas L to Fas receptor and subsequent trimerization of these receptors (ligand is taken there either by apoptosis triggering cell or apocrine secretion, apoptosis triggering factors also include cytokines, growth factors, hormones, toxins). The initiator procaspase 8 or 10 binds to these receptors via the FADD protein, which is proteolytically activates to the active caspase homodimer upon DR activation. This whole complex is called DISC (Death inducing signaling complex). The caspase homodimer has 2 tasks: to activate executive propasses 3,7 and split to Bidprotein to t-Bid protein. Caspase 3 then cleaves procaspase 6. Caspase 3, 6, 7 are executors' protease of their own apoptosis.

Intrinsic pathway
Also known as mitochondrial. Due to the change in the permiability of the inner mitochondrial membrane, cytohrome c is released into the cytosol, where it binds to the inactive Apaf-1 protein and thus changes its conformation. After binding to ATP, the Apaf-1*cyt c*ATP complex associates in a pentamer capable of binding procaspase 9, which upon binding changes to active caspase 9. The entire complex is called the apoptosome. This apoptosome is able to cenvert procaspases 3, 7, to caspase 3, 7.

The change in mitochondrial membrane permiability is regulated by the Bcl family of proteins. Proapoptotic Bax, Bak they they form homodimers of heterodimers, which mean the permiability of the inner mitochondrial membrane. However, it should be impermeable in a healthy cell, and thereforethere are anti-apoptic proteins Bcl-2, Bcl-xl, which with Bak, Bax form Bax/Bcl-2 heterodimers and these are impermeable. Eleated levels of Bcl-2 were found first in the B lymphoma (B-Cell Lymphoma).

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

Losing activation signal
DR expression can either be reduced or non-functional, such as decoy receptors, are synthesized. These lack the cytosolic domain of DD. so that even FADD cannot bind and DISC activation is thus prevented.

Signal shift
TRADD (Tumor necrosis factor Receptor Associated Death Domain) does not create the desired FADD (Fas Associated Death Domain), which is able to bind procaspase 8 instead of TRADD bound to the TRAF (TNF Receptor Associated Factor), which over expresses the cascade of kinases transcription factor NFkB. The effect of NFkB is to active the anti-apoptotic factors XIAP, FLIP and Bcl-2. It is thought to negatively affect the course of apoptosis itself.

Inactivation of proteins FLIP DISC
The FLIP protein has a very similar structure to procaspase 8, contains a DED domain that binds to the FADD protein, and takes the place of one or two procaspases 8 in DISC. Thus, homodimers cannot be formed or proteolytically activated. This inhibition is directly to the concentraion of FLIP protein. It is not appropiate to immediately despair, even with insufficient activation of caspase 8, it can at least cleave proteins around the DISC, such as the Bid protein. This, like tBid, eleminates the Bcl-2 protein and thus causes the escape of cytochrome c from mitochondria.

Caspase inhibitors
IAP (Inhibitors of Apoptosis Proteins) by direct binding to procaspase active sites prevent their activation to caspases. There are, for example, HIAP, XIAP, SURVIVIN, LIVIN. Increased expression of SURVIVIN has been shown in many types of tumors.

Disorders of mitochondrial signaling associated with tumor suppressor p53
When DNA is damaged, the TP53 gene, the product of which is the transcription factor p53. The p53 protein plays a key role in suppresing tumor processes. Firstly, by expressing p21, GADD45 builds the cell cycle, secondly, by increasing the expression of Bax, PUMA and supressing Bcl-2, it significantly helps to release cytochrome c and other proapoptotic factors from mitochondria. (Alteration of Bax/Bcl-2 ratio). Another proapoptotic effect is upregulation of FAS, DR5 or Apaf 1.

Other possibilities of influencing apoptosis
Point mutation of the Ras gene can overactivate Akt kinase (via MAP kinases). This is generally associated with cell survival. It weakens the internal activation of apoptosis by phosphoralyting caspase 9, by phosphorylating Bad (phosphorylation of these proteins means inactivation). Also inactivates Fas L. It also positively affects NFkB via IKK kinase.

Apoptosis signaling is very complicated process regulated by many proteins. Fortunately, there are plenty of "options" that can surprise you in the event of a failure, whether it's internal and external activation and their interconnection, or caspase-independent apoptosis. The tumor process is multiple and a single mutation is certainly not enough to suppress apoptosis. Nevertheless, it is necessary to keep in mind the 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.