Or evasion of programmed cell death of tumor cells is the most important property of the malignant phenotype.

Normally, the apoptotic program is present in a latent form in all cells of the body, since it is quite obvious that in the body, under the influence of various factors, DNA damage constantly occurs as a cell passes through the cell cycle, i.e. mutations occur.

It is known that during life in the human body 10 16 cell divisions occur. Spontaneous mutations occur with a frequency of 10 6 per gene per cell cycle.

Thus, during a person’s life, each gene may well undergo mutation about 10 billion times (10 16 x 10 6 = 10 10), and up to 1 million somatic mutations occur in the body every day.

And among them, undoubtedly, are possible, leading to cancer. From this perspective, the problem with cancer is not so much why it occurs, but why it occurs so rarely.

And cancer occurs, despite constant exposure to carcinogenic factors, relatively rarely because the body has defense mechanisms aimed at preserving the normal genotype of the cell. It should be noted that the fate of cells with certain genetic damage may be different.

Some of the mutated cells die due to vital damage to their genome, some are restored, some are destroyed by the body itself through apoptosis, and, finally, some of the mutated cells will survive and, in the process of reproduction, can become a source of accumulation of potentially oncogenic mutations and the development of cancer.

Normally, the genetic fund of a cell, despite its fragility, is protected by a powerful enzyme apparatus, which often ensures the recognition of mutated and altered DNA sections and their restoration.

DNA repair involves “cutting out” mutated nucleotides using endo- and exonucleases, synthesizing a normal section of DNA with the participation of DNA polymerase, and inserting the restored section into the DNA chain under the action of the enzyme ligase. Thus, the original genetically programmed nucleotide sequence of the damaged strand is recreated (Fig. 3.12).

Figure 3.12. DNA damage repair scheme and mutation formation [Novik A.L., 2004].

If the activity of repair and restoration systems is insufficient and DNA damage is preserved, then programmed cell death is induced in such cells, which leads to the destruction, including mutant cells capable of malignant transformation.

Apoptosis (from the Greek apoptosis - falling off) is programmed cell death or “cell death as a result of self-destruction” - an active, genetically controlled process. The term was proposed by Kerr J. et al. (1972) to designate changes occurring in a cell during its physiological death and leading to a decrease in the number of cells, as opposed to mitosis, which ensures an increase in their number.

Biological significance of apoptosis

The biological significance of apoptosis is that it is a key mechanism for maintaining genetic homeostasis, which the body uses to remove cells whose survival is undesirable: foreign, defective, with defects in the genome; mutant or infected with a virus; with inadequate receptor specificity to various life activity regulators, etc.

In the body, at every unit of time, millions of cells complete their cycle, working out “their time.” To prevent “clogging” of the body from “spent”, “worn out” cells that have managed to fulfill their function, a special mechanism for their elimination has been developed in the course of evolution - apoptosis.

The ability to initiate self-destruction (apoptosis) is an integral property of cells to maintain tissue homeostasis by maintaining a certain balance between proliferation (mitosis) and death.

Apoptosis plays an extremely important role in embryogenesis, in particular in regulating the amount of mesodermal tissue during the formation of organs and skeleton. The destruction of foreign cells by immune cells is also based on the apoptotic mechanism.

Cell death by type of apoptosis occurs during many physiological processes: age-related involution of organs (thymus), atrophy (prostate gland after castration), regression of hyperplasia in the normal functioning of the ovaries and testes, and, finally, in the destruction of mutant cells.

Mechanism of apoptosis activation

Mature differentiated cells are normally resistant to the induction of apoptosis, but become sensitive to it after their activation. This activation is caused by various external influences through specific receptors and intracellular signals caused by the expression of certain proto-oncogenes.

They can be physiological - activation of special killer cytokines, changes in hormonal status (cyclical changes in the endometrium, etc.), and non-physiological - intracellular damage or unfavorable conditions (lack of growth factors, DNA damage, hypoxia, etc.).

In the mechanisms of activation of apoptosis, there are two main stages: the induction phase (decision making) and the execution phase (execution of the sentence). In the first phase, the apoptosis sensor system monitors deviations from the norm in the intra- and extracellular environment and determines the further fate of the cell: to live or die.

A class of sensors are cell surface receptors that bind survival or death signals. Various cytokines act as such signals.

When anomalies are detected (for example, DNA damage, lack of growth factors, hypoxia, etc.), the second phase of apoptosis is triggered through sensory regulators - execution of the sentence. It begins with the activation of caspases + enzymes of the cysteine ​​proteinase family (the so-called execution caspases).

There are two fundamentally different pathways for caspase activation. One of them is triggered in response to an active death signal transmitted by specific killer cytokines of the TNF group (tumor necrosis factor) to the corresponding receptors (the most studied are Fas), called death receptors.

Apoptosis caused by activated death receptors is called instructive apoptosis. In the second pathway of caspase activation, mitochondria play a key role—mitochondrial apoptosis.

In this case, various damaging effects cause an increase in the permeability of the mitochondrial membrane and the release of mitochondrial proteins (mainly cytochrome C) into the cytoplasm, which activate caspases through a corresponding cascade of reactions.

A key role in the regulation of mitochondrial membrane permeability to cytochrome C is played by proteins of the bcl-2 family, which have either pro-apoptotic or anti-apoptotic activities.

Thus, in human cells, in response to damage, there are two mechanisms that trigger apoptosis: instructive, caused by death receptors, and mitochondrial, caused by increased membrane permeability. There is mutual regulation between them, which makes it possible to more reliably achieve the final effect.

As a result, caspases activated in one way or another proteolytically cleave the key structural components of the cell, which leads to DNA fragmentation and cell destruction. In this case, the cytoplasmic and nuclear skeletons are destroyed, chromosomes are degraded, the nucleus is fragmented, but without rupture of the cell membrane.

Therefore, such a cell can be utilized by phagocytes and neighboring cells, and even their massive death does not lead to any pathological processes. The process of proteolysis lasts 30-120 minutes, then the wrinkled cell is absorbed by macrophages and usually disappears within 24 hours (Fig. 3.13).

Rice. 3.13. Phagocytosis of an apoptotic cell by a macrophage [Filchenkov A.A., Stoika R.S., 1999]. 1 - fragmented core; 2 - fragments of cytoplasm (apoptotic bodies): 3 - fragments of an apoptotic cell are captured by a macrophage.

The task of apoptosis is the disposal of cell fragments before its contents enter the extracellular environment and cause an inflammatory process. External morphological manifestations of apoptotic cell death in the form of karyopyknosis (shrinkage of the nucleus), karyorrhexis (disintegration of the nucleus into parts), condensation (compression) of the cell, etc. have been known for a long time and only recently have it been shown that these are particular manifestations of apoptosis. There is no inflammatory process around cells that have undergone apoptosis.

Cell death by the type of apoptosis should be distinguished from necrosis, another form of cell death in the body. Necrosis is initiated by non-physiological agents, and apoptosis is initiated by both physiological and non-physiological agents. Unlike necrosis, apoptosis occurs not only in pathologically altered tissues, but also in normal tissues.

Necrosis occurs when cells are exposed to extreme factors and can therefore be called pathological death. In necrosis, morphological changes as a response to lethal cell damage almost always begin with damage to the plasma membrane, which does not happen with apoptosis.

Due to membrane rupture, water and ion molecules enter the cell from the extracellular space and cause swelling of the structures. At the same time, the entry of cytoplasmic contents (including lysosomal enzymes) into the extracellular space causes tissue damage and the development of a pronounced inflammatory process, which does not occur during apoptosis.

In addition, during apoptosis, single cells die, and during necrosis, groups of them die. The destruction of cells by apoptosis, compared with necrosis, ensures minimal tissue damage. There are other differences between these processes. Figure 3.14 provides a schematic representation of the two forms of cell death.

Rice. 3.14. Schematic representation of two forms of cell death [after Wyllle A. et al., 1998].

Like other physiological processes, apoptosis is regulated by a large number of genes. The key role in launching the apoptosis program belongs to the p53 suppressor gene. Due to its special significance, p53 was called the gene of the 20th century. p53 maintains the stability of the genetic apparatus and controls the cell cycle.

Normally, with damage to the DNA structure or other forms of genotoxic stress, rapid activation of p53 is observed. Its protein blocks the cell cycle in the G1 phase before DNA duplication and mitosis, initiates and participates in DNA repair processes. This allows the cell to repair the damaged section of DNA, which prevents the appearance of mutant cells.

In cases of severe irreversible damage, p53 initiates a program of apoptosis and thereby prevents pathological proliferation. It is important to emphasize that p53-dependent apoptosis eliminates from the body not only damaged cells, but also those cells in which unregulated stimulation of proliferation is observed.

If p53 mutates, it is inactivated and stops triggering the apoptotic cascade, which allows cells with damaged DNA to persist during mitosis, and this in turn leads to the survival of cells that have undergone tumor transformation (Figure 3.15).

Rice. 3.15. Regulatory influence of the p53 antioncogene. Damage to the gene creates conditions for pathological cell proliferation.

It is assumed that the increase in the frequency of neoplasia with age is associated not with the accumulation of mutations in the cell genome, but with age-related disorders of the DNA repair system.

Naturally, apoptosis is considered as a powerful antitumor defense. Inhibition of the process greatly facilitates the transformation of a normal cell into a cancerous one, since various mutations will easily accumulate in cells incapable of apoptosis.

Such mutant cells, despite DNA damage, will continue to actively reproduce. The accumulation of a critical number of mutations will inevitably lead to the appearance of a neoplastic cell and the formation of a malignant tumor (Fig. 3.16).

Rice. 3.16. Disruption of the processes of proliferation (P) and apoptosis (A) of cells during oncogenesis [Filchenkov A.A., Stoika R.S., 1999].

Acquired resistance to apoptosis is a feature of most, if not all, tumor clones. Avoiding apoptosis dramatically increases the viability of the neoplastic cell, making it less sensitive to antitumor immunity factors and therapeutic effects. Tumor cells acquire resistance to apoptosis in various ways.

Today it has been established that the loss of expression of the death receptor Fas on the cell surface can weaken the induction of apoptosis; disruption of the apoptogenic signal to mitochondria and inhibition of the permeability of the mitochondrial membrane to cytochrome C; blocking the activation and/or sharp decrease in the lifetime of execution caspases.

Obviously, along with proteins that enable apoptosis, there are proteins that prevent it, and there is a delicate balance between the two. Genes that promote apoptosis are classified as suppressor genes (except p53, BAX, PML, etc.). Genes that block the work of this protective mechanism are proto-oncogenes (BCL1, BCL2, etc.).

The latter, when activated, neutralize apoptotic activity and will sharply increase the appearance of constantly proliferating mutant cell clones, and, consequently, the likelihood of their subsequent development of malignant tumors.

It is believed that the ratio of the number of different forms of oncoproteins of the BCL group and p53 determines the rheostat of cell life and death. In this regard, it should be noted that due to the existence of the apoptosis mechanism, it is fundamentally impossible to achieve immortality of the organism.

Over time, atrophy of organ cells, regulators of the body’s vital functions occurs, and a number of diseases develop, which are united under the common name -

In the process of the emergence of multicellular living organisms, mechanisms were needed to regulate proper growth and development; one of such regulators is apoptosis.

Apoptosis is a form of programmed cell death, which manifests itself in a decrease in cell size, fragmentation and condensation of chromatin, compaction of membranes (external and cytoplasmic) without leakage of cell contents into its environment.

The process is two-phase:

1. The first phase is called latent and is based on the conduction of apoptosis signals. In other words, the “problem solving phase.” Depending on the nature of the action of the stimuli, it can be divided into 2 types:

a) DNA damage through exposure to toxins, radiation and other factors;

b) activation of “cell death region” receptors (CDR).

The “cell death region” is the receptors on the membranes of all cells that perceive stimuli to activate apoptosis. If the number of activated receptors increases, then the number of physiologically dying cells also increases. The most studied RKCs include CD95 (Fas, Apo1), TNFR1 (p55, CD120a), as well as CAR1, D3, DR4, DR5, etc. This process is not accompanied by DNA damage.

2. The second phase is called “effector” because in it the destruction of cellular ultrastructures occurs. The main performers of the effector phase are endonucleases, cysteine ​​proteases (caspases), lysosomal and serine proteases.

Farber E. (1994) proposed a classification of programmed cell death (PCD):

Programmed developmental cell death is death that occurs during normal cell development and/or metamorphosis.

Programmed physiological death of differentiated cells of mature organisms during the destruction of hyperplastic tissues as a result of exogenous and endogenous damage to organs and tissues. It manifests itself when restoration of cellular composition is necessary.

Programmed biochemical cell death after the action of pathogens of various origins. This type of death is not physiological, since it represents the body's response (active or passive) to a damaging agent.

All forms of PCH are based on a genetically determined program of cell death. This is supported by the involvement of many genes underlying this program at the cellular level and the presence of specific genes that control this process.

There are several regulators of apoptosis, one of which occurs with the participation of cytokines. Cytokines are proteins through which communication occurs with specific receptors on target cells and their differentiation and proliferation are regulated. The process of apoptosis starts when a specific receptor approaches its ligand - extracellular death protein (TNF-a, FasL, TRAIL, Apo-3L). The most studied ligand is FasL, which usually attaches to activated T lymphocytes and NK cells through interaction with specific APO1/CD95/Fas cell receptors. In the testicles and eye tissues, FasL provides protection against autoimmune damage to one's own cells. The principle of action is to activate a specific protease - caspase 8, which in turn triggers the process of HCG. An alternative pathway is the mitochondrial pathway of apoptosis activation with the participation of proteins of the Bcl-2 family. This pathway of apoptosis begins with DNA damage or exposure of the cell to toxic agents. The key event in this pathway is an increase in the permeability of the outer mitochondrial membrane, which is characterized by the release of apoptogenic proteins (cytochrome C, procaspase -2, -3, and -9, AIF (apoptosis-inducing factor) from the intermembrane space into the cell cytoplasm due to rupture of the mitochondrial membrane or opening highly permeable channels on the outer membrane of mitochondria.

The most important “receptor” for DNA damage is the so-called “genome guardian” protein p53. Typically, this protein is in an inactive state and is activated due to hypoxia, oncogene activation, DNA damage, or exposure to other cytotoxic agents. The role of the gene in the process of PCH is very important, since the development of 50% of tumors is caused by a mutation of the p53 gene. Regulation of apoptosis by the p53 protein occurs in several ways: activation of the Bax or Bid genes; activation of the formation of free forms of oxygen, which leads to peroxidation, which leads to the release of cytochrome C from mitochondria; induction of Fas mRNA, as well as release of Fas to the cell surface from the Golgi apparatus; stimulation of APAF-1 formation; stimulation of caspase 6 expression; the transition of part of the molecules of the p53 gene itself into mitochondria with the subsequent release of cytochrome C.

An essential mechanism of apoptosis is the synthesis and activation of proapoptotic compounds of the Bcl-2 family. For the first time, a protein of the Bcl-2 family was described as an oncogene in B-cell lymphoma, which led to the formation of a tumor clone by increasing the survival of tumor cells. . Currently, the Bcl-2 family includes a group of proteins with similar morphological compositions and is divided into two groups: apoptosis inducers and apoptosis inhibitors. The decision to kill a cell is made based on the relative dominance of active suppressors or promoters of apoptosis. The mechanism of action is based on the action of pro-apoptotic proteins of the Bcl-2 family, which form temporary megachannels in place of physiological ones (for Ca2+, O2, Na+/K+), through which cytochrome C and other apoptotic factors begin to flow. Cytochrome C is required for the formation of apoptosome, in which caspase 9 is activated.

There is another, stress-induced apoptosis pathway that activates caspase 9 through the Apaf-1 complex (apoptotic protease-activating factor). Conformational changes in Apaf-1 induced by cytochrome C from damaged mitochondria and ATP allow the recruitment of caspase 9 profactor through their common domain. Caspase 9 of the apoptosome, in turn, causes the activation of effector K(3,7), which initiates intense proteolysis and releases the associated DNase, which destroys chromatin. Particularly noteworthy is the role of the Bid protein, which is a link between two apoptotic pathways - mitochondrial and the “death receptor” pathway (effects of K8).

Currently, studying the process of apoptotic cell death is of great interest in medicine. Disruption of physiological death processes plays an important role in the development of pathological conditions, including cancer and autoimmune diseases.

Currently, many diseases are known that are associated with increased apoptosis: follicular lymphoma, cancer of the reproductive system in women and men (ovaries, prostate), glomerulonephritis, viral infections (adenovirus, herpes virus, poxvirus). As well as diseases associated with inhibition of apoptosis processes: AIDS, neurodegenerative diseases (Alzheimer's, Parkinson's), toxic liver diseases, cerebellar degenerations, etc.

The study of the mechanisms of apoptosis gives us ideas about the development of certain diseases and their course. Already now we can use this knowledge to prevent diseases at various stages of pathogenesis (correction and regulation).

Apoptosis is a very important process in the ontogenesis of every living organism. This process allows us to maintain internal homeostasis, control the proper growth and development of the body; without the mechanism of apoptosis, there would be chaos in our body, a large number of genetic changes, and random cell division.

Bibliography:

1. Vladimirskaya E.B. Mechanisms of apoptotic cell death / E.B. Vladimirskaya // Hematology and transfusiology. – 2002. – T.47, No. 2, - P. 35 - 40.

2. Robinson M.V. Apoptosis of cells of the immune system / M.V. Robinson, M.A. Trufakin // Advances in modern biology. – 1991. –Vol. 3 issue. 2. – pp. 246 – 259.

3. Adams J.M. Ways of dying: multiple pathways to apoptosis / J.M. Adams // Genes and Development/ - 2003. – N 17. – P. 2481 – 2495.

4. Itoh K. Central role of mitochondria and p53 in Fas-mediated apoptosis of rheumatoid synovial fibroblasts / K. Itoh, H. Hase, H. Kojima et al. // Reumatology. – 2004. – N 43. – P.277-285.

5. & Newton K. Caspases signal not only apoptosis but also antigen-induced activation in cells of the immune system / K. Newton, A. Strasser // Genes and Development. – 2003. – Vol.17, N7. – P.819 – 825.

The term “apoptosis” should be understood as the physiological process of cell death, which is triggered in response to physiological signals or is ensured by the inclusion of a special genetic program. Morphologically, this process is characterized by compaction of chromatin, division of DNA into fragments and changes in the structure of the cell membrane. As a result, the cell is destroyed and phagocytosed without signs of inflammation, which has virtually no effect on surrounding tissues.

Biological role

Programmed cell death is extremely important for the normal functioning of the body.

Programmed cell death plays an important role in the normal functioning of living organisms, it ensures:

• development during embryogenesis;
• regulation of the number of cells and their composition in a mature organism;
• cell differentiation;
• destruction of old cells that cease to perform their functions;
• hormonal changes;
• suppression of tumor growth;
• culling of cells with genetic defects;
• elimination of foreign agents (viruses, bacteria, fungi, etc.).

Dysregulation of cell death leads to the development of:

• viral infections;
• neurodegenerative diseases (,);
• blood pathologies (,).

It should be noted that in some of them the apoptosis function is reduced, while in others, on the contrary, it is increased.

• Inhibition of apoptosis is believed to be important for tumor progression. Cancer cells can become resistant to it due to increased expression of anti-apoptotic factors or as a result of mutations in genes.
• A decrease in apoptosis is observed in autoimmune processes, when autoaggressive T cells are not destroyed by the immune system. This leads to damage to the body's own tissues.
• Increased apoptosis also negatively affects human health. This may be associated with increased death of bone marrow precursor cells of the red and white hematopoietic lineage, which results in aplastic anemia.

Thus, apoptosis acts as a general mechanism of cell death, both during physiological and pathological processes.

Development mechanisms

Programmed cell death occurs in a succession of 3 stages:

1. Inductor.
2. Effective.
3. Degradation.

At the first stage, signal reception and the initial stages of its transmission occur. This is carried out using a receptor mechanism under the influence of external factors or through internal activation.

Receptors that trigger apoptosis are called death receptors. They have special domains within them, interaction with which induces special intracellular signals.

The internal pathway of activation of this process is associated with changes occurring in mitochondria. It is sensitive to deficiencies of growth factors, hormones or cytokines. It can also be affected by:

• hypoxia;
• hypothermia;
• virus invasion;
• irradiation;
• free radicals.

All these factors can cause restructuring of the inner mitochondrial membrane, as a result of which pores open and pro-apoptotic substances are released. By their structure, these are proteins that trigger the caspase-dependent apoptosis pathway and induce DNA division into fragments with condensation of peripheral chromatin regions.

During the effector stage, the main apoptotic enzymes, caspases, are activated. They have proteolytic activity and break down proteins at the aspartic residue. As a result of their activity, massive protein destruction occurs in the cell and irreversible changes develop.

At the last stage, the basic mechanisms of cell death are realized. This activates endonucleases, whose activity leads to DNA degradation. After this, the cytoskeleton is reorganized and the cell is transformed into apoptotic bodies, on the surface of which markers for phagocytosis appear. At the last stage, such cells are absorbed by macrophages.

Regulation of apoptosis

Impaired apoptosis is one of the factors that increases the risk of developing AIDS.

Each of the mechanisms of apoptosis has its own regulation:

• The mitochondrial pathway is regulated by proteins from the Bcl-2 family. They affect mitochondrial membrane permeability and can attenuate or stimulate apoptosis. This is done by controlling the release of cytochrome C.
• Regulation of the cell death receptor mechanism occurs by controlling the activity of caspases.

Apoptosis allows the body to maintain physiological balance and resist various external influences. Thus, every day in the human body, tens of billions of cells die as a result of programmed death, but these losses are quickly compensated for by cell proliferation. The total mass of cells that are destroyed annually by apoptosis is equal to the weight of the human body.

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What is apoptosis?

Apoptosis– physiological cell death, which is a kind of genetically programmed self-destruction.

The term "apoptosis" is translated from Greek as "falling off." The authors of the term gave this name to the process of programmed cell death because the autumn fall of wilted leaves is associated with it. In addition, the name itself characterizes the process as physiological, gradual and absolutely painless.

In animals, the most striking example of apoptosis is usually the disappearance of the frog's tail during metamorphosis from tadpole to adult.

As the frog grows up, the tail completely disappears, as its cells undergo gradual apoptosis - programmed death, and the absorption of destroyed elements by other cells.

The phenomenon of genetically programmed cell death occurs in all eukaryotes (organisms whose cells have a nucleus). Prokaryotes (bacteria) have a peculiar analogue of apoptosis. We can say that this phenomenon is characteristic of all living things, with the exception of such special precellular life forms as viruses.

Both individual cells (usually defective) and entire conglomerates can undergo apoptosis. The latter is especially characteristic of embryogenesis. For example, the experiments of researchers have proven that due to apoptosis during embryogenesis, the membranes between the toes of chickens disappear.

Scientists say that in humans, congenital anomalies such as fused fingers and toes also arise due to disruption of normal apoptosis in the early stages of embryogenesis.

History of the discovery of the theory of apoptosis

The study of the mechanisms and significance of genetically programmed cell death began in the sixties of the last century. Scientists were interested in the fact that the cellular composition of most organs throughout the life of the organism is almost the same, but the life cycle of different types of cells differs significantly. In this case, many cells are constantly replaced.

Thus, the relative constancy of the cellular composition of all organisms is maintained by the dynamic balance of two opposing processes - cell proliferation (division and growth) and the physiological death of obsolete cells.

The authorship of the term belongs to British scientists - J. Kerr, E. Wiley and A. Kerry, who first put forward and substantiated the concept of the fundamental difference between the physiological death of cells (apoptosis) and their pathological death (necrosis).

In 2002, scientists from the Cambridge laboratory, biologists S. Brenner, J. Sulston and R. Horwitz, received the Nobel Prize in Physiology or Medicine for discovering the basic mechanisms of genetic regulation of organ development and studying programmed cell death.

Today, tens of thousands of scientific works are devoted to the theory of apoptosis, revealing the basic mechanisms of its development at the physiological, genetic and biochemical levels. An active search for its regulators is underway.

Of particular interest are studies that make it possible to practically apply the regulation of apoptosis in the treatment of oncological, autoimmune and neurodystrophic diseases.

Mechanism

The mechanism of apoptosis development has not been fully studied to date. It has been proven that the process can be induced by low concentrations of most substances that cause necrosis.

However, in most cases, genetically programmed cell death occurs when signals are received from molecules - cellular regulators, such as:

• hormones;
• antigens;
• monoclonal antibodies, etc.

Signals for apoptosis are perceived by specialized cellular receptors, which trigger successive stages of intracellular complex biochemical processes.

It is typical that the signal for the development of apoptosis can be either the presence of activating substances or the absence of certain compounds that prevent the development of programmed cell death.

The cell's response to a signal depends not only on its strength, but also on the general initial state of the cell, the morphological features of its differentiation, and the stage of the life cycle.

One of the basic mechanisms of apoptosis at the stage of its implementation is DNA degradation, resulting in nuclear fragmentation. In response to DNA damage, protective reactions are launched aimed at its restoration.

Unsuccessful attempts to restore DNA lead to complete energy depletion of the cell, which becomes the direct cause of its death.

Mechanism of apoptosis - video

Phases and stages

There are three physiological phases of apoptosis:
1. Signaling (activation of specialized receptors).
2. Effector (formation of a single apoptosis pathway from heterogeneous effector signals, and the launch of a cascade of complex biochemical reactions).
3. Dehydration (literally dehydration - cell death).

In addition, two stages of the process are morphologically distinguished:
1. First stage – preapoptosis. At this stage, the size of the cell decreases due to its shrinkage, and reversible changes occur in the nucleus (chromatin compaction and its accumulation along the periphery of the nucleus). In the case of exposure to certain specific regulators, apoptosis can be stopped, and the cell will resume its normal functioning.

2. The second stage is apoptosis itself. Inside the cell, gross changes occur in all its organelles, but the most significant transformations develop in the nucleus and on the surface of its outer membrane. The cell membrane loses its villi and normal folding, bubbles form on its surface - the cell seems to be boiling, and as a result disintegrates into so-called apoptotic bodies, absorbed by tissue macrophages and/or neighboring cells.

The morphologically determined process of apoptosis usually takes from one to three hours.

Cell necrosis and apoptosis. Similarities and differences

The terms necrosis and apoptosis refer to the complete cessation of cell activity. However, apoptosis refers to physiological death, and necrosis refers to its pathological death.

Apoptosis is a genetically programmed cessation of existence, that is, by definition, it has an internal cause of development, while necrosis occurs as a result of the influence of extremely strong factors external to the cell:

• lack of nutrients;
• poisoning with toxins, etc.

Apoptosis is characterized by a gradual and staged process, while necrosis occurs more acutely, and it is almost impossible to clearly distinguish the stages.

In addition, cell death during the processes of necrosis and apoptosis differs morphologically - the first is characterized by its swelling, and during the second, the cell shrinks and its membranes thicken.

During apoptosis, the death of cellular organelles occurs, but the membrane remains intact, so that so-called apoptotic bodies are formed, which are subsequently absorbed by specialized cells - macrophages or neighboring cells.

In necrosis, the cell membrane ruptures and the contents of the cell come out. An inflammatory reaction begins.

If a sufficiently large number of cells have undergone necrosis, inflammation manifests itself in characteristic clinical symptoms known since ancient times, such as:

• pain;
• redness (dilation of blood vessels in the affected area);
• swelling (inflammatory edema);
• local and sometimes general increase in temperature;
• more or less pronounced dysfunction of the organ in which necrosis occurred.

Biological significance

The biological significance of apoptosis is as follows:
1. Implementation of normal development of the body during embryogenesis.
2. Preventing the proliferation of mutated cells.

3. Regulation of the immune system.
4. Preventing premature aging of the body.

This process plays a leading role in embryogenesis, since many organs and tissues undergo significant transformations during embryonic development. Many birth defects result from insufficient apoptotic activity.

As a programmed self-destruction of defective cells, this process is a powerful natural defense against cancer. For example, the human papillomavirus blocks cellular receptors responsible for apoptosis and, thus, leads to the development of cancer of the cervix and some other organs.

Thanks to this process, physiological regulation of T-lymphocyte clones responsible for the cellular immunity of the body occurs. Cells that are unable to recognize the proteins of their own body (and about 97% of them mature in total) undergo apoptosis.

Insufficiency of apoptosis leads to severe autoimmune diseases, while its enhancement is possible in immunodeficiency states. For example, the severity of AIDS correlates with the intensification of this process in T-lymphocytes.

In addition, this mechanism is of great importance for the functioning of the nervous system: it is responsible for the normal formation of neurons, and it can also cause the early destruction of nerve cells in Alzheimer's disease.

One of the theories of aging of the body is the theory of apoptosis. It has already been proven that it is the basis of premature aging of tissues, where cell death remains irreversible (nervous tissue, myocardial cells). On the other hand, insufficient apoptosis can contribute to the accumulation in the body of aging cells, which normally physiologically die off and are replaced by new ones (early aging of connective tissue).

The role of the theory of apoptosis in medicine

The role of the theory of apoptosis in medicine is the possibility of finding ways to regulate this process for the treatment and prevention of many pathological conditions caused by weakening or, conversely, strengthening of apoptosis.

Research is carried out simultaneously in many directions. First of all, it should be noted scientific research in such an important field of medicine as oncology. Since tumor growth is caused by a defect in the genetically programmed death of mutated cells, the possibility of specific regulation of apoptosis, with an increase in its activity in tumor cells, is being studied.

The action of some chemotherapeutic drugs widely used in oncology is based on enhancing the processes of apoptosis. Since tumor cells are more prone to this process, a dose of the substance is selected that is sufficient to kill pathological cells, but is relatively harmless to normal ones.

Also extremely important for medicine are studies studying the role of apoptosis in the degeneration of cardiac muscle tissue under the influence of circulatory failure. A group of Chinese scientists (Lv X, Wan J, Yang J, Cheng H, Li Y, Ao Y, Peng R) published new experimental data that prove the possibility of artificially reducing apoptosis in cardiomyocytes with the introduction of certain inhibitor substances.

If theoretical research on laboratory objects can be applied to clinical practice, this will be a big step forward in the fight against coronary heart disease. This pathology ranks first among the causes of death in all highly developed countries, so the transition from theory to practice would be difficult to overestimate.

Another very promising direction is the development of methods for regulating this process to slow down the aging of the body. Theoretical research is being conducted towards creating a program that combines increasing the activity of apoptosis of aging cells and simultaneously increasing the proliferation of young cellular elements. Some progress has been made here at the theoretical level, but the transition from theory to practical solutions is still far away.

In addition, large-scale scientific research is carried out in the following areas:

• allergology;
• immunology;
• therapy of infectious diseases;
• transplantology;

Thus, in the near future we will witness the introduction into practice of fundamentally new medical drugs that will overcome many diseases.

Apoptosis is programmed cell death (initiated under the influence of extra- or intracellular factors) in the development of which special and genetically programmed intracellular mechanisms take an active role. It, unlike necrosis, is an active process that requires certain energy consumption. Initially, they tried to distinguish between the concepts " programmed cell death" And " apoptosis": the first term included the elimination of cells in embryogenesis, and the second - the programmed death of only mature differentiated cells. It has now become clear that there is no practicality in this (the mechanisms of development of cell death are the same) and the two concepts have become synonymous, although this association is not indisputable.

Before we begin to present material on the role of apoptosis for the life of a cell (and organism) in normal and pathological conditions, we will consider the mechanism of apoptosis. Their implementation can be presented in the form of a gradual development of the following stages:

Stage 1 – initiation (induction) stage .

Depending on the origin of the signal stimulating apoptosis, there are:

intracellular stimuli of apoptosis. Among them, the most well-known include various types of radiation, excess H +, nitric oxide, free radicals of oxygen and lipids, hyperthermia, etc. All of them can cause various chromosome damage(DNA breaks, disturbances in its conformation, etc.) and intracellular membranes(especially mitochondria). That is, in this case, the reason for apoptosis is “the unsatisfactory state of the cell itself” (Mushkambirov N.P., Kuznetsov S.L., 2003). Moreover, the damage to cell structures should be quite strong, but not destructive. The cell must retain energy and material resources to activate apoptosis genes and its effector mechanisms. The intracellular pathway for stimulating programmed cell death can be designated as “ apoptosis from within»;

transmembrane stimuli of apoptosis, i.e., in this case it is activated by external “signaling”, which is transmitted through membrane or (less often) intracellular receptors. A cell may be quite viable, but from the standpoint of the whole organism or the “erroneous” stimulation of apoptosis, it must die. This type of apoptosis is called " apoptosis on command».

Transmembrane stimuli are divided into:

« negative» signals. For the normal functioning of a cell, the regulation of its division and reproduction, it is necessary to influence it through the receptors of various biologically active substances: growth factors, cytokines, hormones. Among other effects, they suppress cell death mechanisms. And naturally, the deficiency or absence of these biologically active substances activates the mechanisms of programmed cell death;

« positive» signals. Signaling molecules, such as TNFα, glucocorticoids, some antigens, adhesion proteins, etc., after interaction with cellular receptors, can trigger the apoptosis program.

On cell membranes there is a group of receptors whose task is to transmit a signal for the development of apoptosis is the main, perhaps even the only function. These are, for example, proteins of the DR group (death receptos - “ death receptors"): DR 3, DR 4, DR 5. The most well studied is the Fas receptor, which appears on the surface of cells (hepatocytes) spontaneously or under the influence of activation (mature lymphocytes). The Fas receptor, when interacting with the Fas receptor (ligand) of the killer T cell, launches the target cell death program. However, the interaction of the Fas receptor with the Fas ligand in areas isolated from the immune system ends in the death of the T-killer itself (see below).

It should be remembered that some apoptosis signaling molecules, depending on the situation, can, on the contrary, block the development of programmed cell death. Ambivalence(dual manifestation of opposite qualities) is characteristic of TNF, IL-2, interferon γ, etc.

On the membranes of erythrocytes, platelets, leukocytes, as well as lung and skin cells, special marker antigens. They synthesize physiological autoantibodies, and they, fulfilling the role opsonins, promote phagocytosis of these cells, i.e. cell death occurs by autophagocytosis. It turned out that marker antigens appear on the surface of “old” (which have gone through their ontogenetic development) and damaged cells, while young and undamaged cells do not have them. These antigens are called “marker antigens of aging and damaged cells” or “third band protein.” The appearance of the third band protein is controlled by the cell genome. Therefore, autophagocytosis can be considered as a variant of programmed cell death.

Mixed signals. This is the combined effect of signals of the first and second groups. For example, apoptosis occurs in lymphocytes activated by mitogone (positive signal) but not in contact with antigen (negative signal).

Stage 2 – programming stage (control and integration of apoptosis mechanisms).

This stage is characterized by two diametrically opposed processes observed after initiation. Either happens:

implementation of the trigger signal for apoptosis through activation of its program (effectors are caspases and endonucleases);

the effect of the apoptosis trigger is blocked.

There are two main, but not mutually exclusive, options for executing the programming stage (Fig. 14):

Rice. 14. Caspase cascade and its targets

R – membrane receptor; K – caspase; AIF – mitochondrial protease; Quote C – cytochrome c; Apaf-1 – cytoplasmic protein; IAPs – caspase inhibitors

1. Direct signal transmission (direct pathway of activation of effector mechanisms of apoptosis bypassing the cell genome) is realized through:

adapter proteins. For example, this is how apoptosis is triggered by killer T cells. It activates caspase-8 (adapter protein). TNF may act similarly;

cytochrome C and protease AIF (mitochondrial protease). They exit the damaged mitochondria and activate caspase-9;

granzymes. Killer T cells synthesize the protein perforin, which forms channels in the plasmalemma of the target cell. Proteolytic enzymes enter the cell through these channels. granzymes, secreted by the same T-killer and they trigger the caspase network cascade.

2. Indirect signal transmission. It is implemented using the cell genome by:

repression of genes that control the synthesis of proteins that inhibit apoptosis (genes Bcl-2, Bcl-XL, etc.). Bcl-2 proteins in normal cells are part of the mitochondrial membrane and close the channels through which cytochrome C and AIF protease exit these organelles;

expression, activation of genes that control the synthesis of apoptosis activator proteins (genes Bax, Bad, Bak, Rb, P 53, etc.). They, in turn, activate caspases (k-8, k-9).

In Fig. Figure 14 shows an approximate diagram of the caspase principle of caspase activation. It can be seen that no matter where the cascade starts, its key point is caspase 3. It is also activated by caspases 8 and 9. In total, there are more than 10 enzymes in the caspase family. Localized in the cytoplasm of the cell in an inactive state (procaspases). The position of all caspases in this cascade has not been fully elucidated, so a number of them are missing from the diagram. As soon as caspases 3,7,6 (possibly their other types) are activated, stage 3 of apoptosis occurs.

Stage 3 – program implementation stage (executive, effector).

The direct executors (“executioners” of the cell) are the above-mentioned caspases and endonucleases. The places of application of their action (proteolysis) are (Fig. 14):

cytoplasmic proteins – cytoskeletal proteins (fodrin and actin). The hydrolysis of fodrin explains the change in the cell surface - “corrugation” of the plasmalemma (the appearance of invaginations and protrusions on it);

proteins of some cytoplasmic regulatory enzymes: phospholipase A 2, protein kinase C, etc.;

nuclear proteins. Proteolysis of nuclear proteins plays a major role in the development of apoptosis. Structural proteins, proteins of replication and repair enzymes (DNA-protein kinases, etc.), regulatory proteins (pRb, etc.), and endonuclease inhibitor proteins are destroyed.

Inactivation of the last group – endonuclease inhibitor proteins leads to activation of endonucleases, the second "gun » apoptosis. Currently, endonucleases and, in particular, Ca 2+ , Mg 2+ -dependent endonuclease, is considered as the central enzyme of programmed cell death. It does not cleave DNA in random places, but only in linker regions (connecting regions between nucleosomes). Therefore, chromatin is not lysed, but only fragmented, which determines the distinctive, structural feature of apoptosis.

Due to the destruction of protein and chromatin in the cell, various fragments are formed and budded from it - apoptotic bodies. They contain remnants of cytoplasm, organelles, chromatin, etc.

Stage 4 – stage removal of apoptotic bodies (cell fragments).

Ligands are expressed on the surface of apoptotic bodies and are recognized by phagocyte receptors. The process of detection, absorption and metabolization of fragments of a dead cell occurs relatively quickly. This helps to avoid the contents of the dead cell from entering the environment and thus, as noted above, the inflammatory process does not develop. The cell passes away “calmly”, without disturbing its “neighbors” (“silent suicide”).

Programmed cell death is important for many physiological processes . Associated with apoptosis:

maintaining normal morphogenesis processes– programmed cell death during embryogenesis (implantation, organogenesis) and metamorphosis;

maintaining cellular homeostasis(including the elimination of cells with genetic disorders and infected with viruses). Apoptosis explains the physiological involution and balancing of mitoses in mature tissues and organs. For example, cell death in actively proliferating and self-renewing populations - intestinal epithelial cells, mature leukocytes, erythrocytes. Hormone-dependent involution - death of the endometrium at the end of the menstrual cycle;

selection of cell varieties within a population. For example, the formation of an antigen-specific component of the immune system and control of the implementation of its effector mechanisms. With the help of apoptosis, lymphocyte clones that are unnecessary and dangerous to the body (autoaggressive) are culled. Relatively recently (Griffith T.S., 1997) showed the importance of programmed cell death in the protection of “immunologically privileged” areas (internal environments of the eye and testes). When passing the histo-hematological barriers of these zones (which happens rarely), effector T-lymphocytes die (see above). The activation of the mechanisms of their death is ensured by the interaction of the Fas ligand of barrier cells with the Fas receptors of the T lymphocyte, thereby preventing the development of autoaggression.

Role of apoptosis in pathology and types of various diseases associated with impaired apoptosis are presented in the form of a diagram (Fig. 15) and Table 1.

Of course, the importance of apoptosis in pathology is less than that of necrosis (perhaps this is due to the lack of such knowledge). However, its problem in pathology also has a slightly different nature: it is assessed by the severity of apoptosis - intensification or weakening in certain diseases.