Molecular genetic mechanisms of regulation of cell proliferation. Introduction. Pathways of CDK regulation

. Chapter II
Cell reproduction. Problems of cell proliferation in medicine.
2.1. Life cycle of a cell.
The cellular theory states that cells arise from cells by dividing the original. This position excludes the formation of cells from non-cellular matter. Cell division is preceded by reduplication of their chromosomal apparatus, DNA synthesis in both eukaryotic and prokaryotic organisms.

The time a cell exists from division to division is called the cell or life cycle. Its magnitude varies significantly: for bacteria it is 20-30 minutes, for a shoe 1-2 times a day, for an amoeba about 1.5 days. Multicellular cells also have different abilities to divide. In early embryogenesis they divide frequently, and in the adult body they mostly lose this ability, as they become specialized. But even in an organism that has reached full development, many cells must divide to replace worn-out cells that are constantly sloughed off and, finally, new cells are needed to heal wounds.

Therefore, in some populations of cells, divisions must occur throughout life. Taking this into account, all cells can be divided into three categories:

1. By the time a child is born, nerve cells reach a highly specialized state, losing the ability to reproduce. During ontogenesis, their number continuously decreases. This circumstance also has one good side; if nerve cells divided, then higher nervous functions (memory, thinking) would be disrupted.

2. Another category of cells is also highly specialized, but due to their constant exfoliation, they are replaced by new ones and this function is performed by cells of the same line, but not yet specialized and have not lost the ability to divide. These cells are called renewing cells. An example is the constantly renewed cells of the intestinal epithelium, hematopoietic cells. Even bone tissue cells can be formed from unspecialized ones (this can be observed during the reparative regeneration of bone fractures). Populations of unspecialized cells that retain the ability to divide are usually called stem cells.

3. The third category of cells is an exception, when highly specialized cells under certain conditions can enter the mitotic cycle. We are talking about cells that have a long lifespan and where, after complete growth, cell division occurs rarely. An example is hepatocytes. But if 2/3 of the liver is removed from an experimental animal, then in less than two weeks it is restored to its previous size. The same are the cells of the glands that produce hormones: under normal conditions, only a few of them are able to reproduce, and under altered conditions, most of them can begin to divide.

The cell cycle means the repeated repetition of sequential events over a certain period of time. Typically, cyclic processes are graphically depicted as circles.

The cell cycle is divided into two parts: mitosis and the interval between the end of one mitosis and the beginning of the next - interphase. The autoradiography method made it possible to establish that in interphase the cell not only performs its specialized functions, but also synthesizes DNA. This period of interphase is called synthetic (S). It begins approximately 8 hours after mitosis and ends after 7-8 hours. The interval between the S-period and mitosis was called presynthetic (G1 - 4 hours) after the synthetic period, before mitosis itself - postsynthetic (G2). happening over the course of about an hour.

Thus, there are four stages in the steel cell cycle; mitosis, G1 period, S period, G2 period.

Establishing the fact of DNA duplication in interphase means that during interphase the cell cannot perform specialized functions; it is busy building cellular structures, synthesizing building materials that ensure the growth of daughter cells, accumulating energy expended during mitosis itself, and synthesizing specific enzymes for DNA replication . Therefore, interphase cells, in order to fulfill their functions prescribed by the genetic program (become highly specialized), must temporarily or permanently leave the cycle during the G0 period, or remain in an extended G1 (no significant differences in the state of cells of the G0 and G1 periods were noted, since it is possible to return from G0 cells in a cycle). It should be especially noted that in multicellular mature organisms, the majority of cells are in the G0 period.

As already mentioned, the increase in the number of cells occurs only due to the division of the original cell, which is preceded by a phase of accurate reproduction of genetic material, DNA molecules, chromosomes.

Mitotic division includes new cell states: interphase, decondensed and already reduplicated chromosomes pass into the compact form of mitotic chromosomes, an achromatic mitotic apparatus is formed, which is involved in chromosome transfer, chromosomes diverge to opposite poles and cytokinesis occurs. The process of indirect division is usually divided into the following main phases: prophase, metaphase, anaphase and telophase. The division is conditional, since mitosis is a continuous process and the change of phases occurs gradually. The only phase that has a real beginning is anaphase, in which

chromosomes begin to separate. The duration of individual phases is different (on average, prophase and telophase - 30-40", anaphase and metaphase - 7-15"). At the beginning of mitosis, a human cell contains 46 chromosomes, each of which consists of 2 identical halves - chromatids (a chromatid is also called the S-chromosome, and a chromosome consisting of 2 chromatids is called the d-chromosome).

One of the most remarkable phenomena observed in mitosis is the formation of the spindle. It ensures the alignment of d-chromosomes in one plane, in the middle of the cell, and the movement of S-chromosomes to the poles. The spindle is formed by the centrioles of the cell center. Microtubules are formed in the cytoplasm from the protein tubulin.

In the G1 period, each cell contains two centrioles; by the time of the transition to the G2 period, a daughter centriole is formed near each centriole and a total of two pairs are formed.

In prophase, one pair of centrioles begins to move to one pole, the other to the other.

Between pairs of centrioles, a set of interpolar and chromosomal microtubules begins to form towards each other.

At the end of prophase, the nuclear membrane disintegrates, the nucleolus ceases to exist, chromosomes (d) spiral, the spindle moves to the middle of the cell and d-chromosomes find themselves in the spaces between the microtubules of the spindle.

During prophase, D chromosomes undergo a path of condensation from thread-like structures to rod-shaped ones. The shortening and thickening of (d-chromosomes continues for some time in metaphase, as a result of which metaphase d-chromosomes have sufficient density. A centromere is clearly visible in the chromosomes, dividing them into equal or unequal arms, consisting of 2 adjacent S- chromosomes (chromatids). At the beginning of anaphase, the S-chromosomes (chromatids) begin to move from the equatorial plane to the poles. Anaphase begins with the splitting of the centromeric region of each chromosome, as a result of which the two S-chromosomes of each d-chromosome are completely separated from one another. Thanks therefore, each daughter cell receives an identical set of 46 S chromosomes.After centromere separation, one half of the 92 S chromosomes begins to move towards one pole, the other half towards the other.

To this day, it has not been established precisely under what forces the movement of chromosomes to the poles occurs. There are several versions:

1. The spindle contains actin-containing filaments (as well as other muscle proteins), it is possible that this force is generated in the same way as in muscle cells.

2. The movement of chromosomes is caused by the sliding of chromosomal microtubules along continuous (interpolar) microtubules with opposite polarity (McItosh, 1969, Margolis, 1978).

3. The speed of chromosome movement is regulated by kinetochore microtubules to ensure orderly segregation of chromatids. Most likely, all of the listed mechanisms for achieving a mathematically precise distribution of hereditary substance to daughter cells cooperate.

Towards the end of anaphase and the beginning of telophase, a constriction begins to form in the middle of the elongated cell; it forms the so-called cleavage furrow, which, going deeper, divides the cell into two daughter cells. Actin filaments take part in the formation of the furrow. But as the furrow deepens, the cells are connected to each other by a bundle of microtubules called the median body, the remainder of which is present for some time in interphase. Parallel to cytokinesis, chromosome decoiling occurs at each pole in the reverse order from the chromosomal to the nucleosomal level. Finally, the hereditary substance takes the form of clumps of chromatin, either tightly packed or decondensed. The nucleolus, nuclear envelope, surrounding chromatin and karyoplasm are formed again. Thus, as a result of mitotic cell division, the newly formed daughter cells are identical to each other and are a copy of the mother cell, which is important for the subsequent growth, development and differentiation of cells and tissues.
2.2. Mechanism of regulation of mitotic activity
Maintaining the number of cells at a certain, constant level ensures overall homeostasis. For example, the number of red and white blood cells in a healthy body is relatively stable, although these cells die, they are constantly replenished. Therefore, the rate at which new cells are formed must be regulated to match the rate at which they die.

To maintain homeostasis, it is necessary that the number of different specialized cells in the body and the functions they must perform be under the control of various regulatory mechanisms that maintain all this in a stable state.

In many cases, the cells are given a signal that they need to increase their functional activity, and this may require an increase in the number of cells. For example, if the Ca content in the blood drops, then the cells of the parathyroid gland increase the secretion of the hormone, and the calcium level reaches normal. But if the animal’s diet does not have enough calcium, then additional production of the hormone will not increase the content of this element in the blood. In this case, the cells of the thyroid gland begin to rapidly divide, so that an increase in their number leads to a further increase in the synthesis of the hormone. Thus, a decrease in a particular function can lead to an increase in the population of cells providing these functions.

In people who find themselves in high mountains, the number of red blood cells sharply increases (at an altitude of less than 02) in order to provide the body with the necessary amount of oxygen. Kidney cells react to a decrease in oxygen and increase the secretion of erythropoietin, which enhances hematopoiesis. After the formation of a sufficient number of additional red blood cells, hypoxia disappears and the cells producing this hormone reduce its secretion to normal levels.

Cells that are fully differentiated cannot divide, but their numbers can still be increased by the stem cells from which they originate. Nerve cells cannot divide under any circumstances, but they can increase their function by increasing their processes and multiplying the connections between them.

It should be noted that in adult individuals the ratio of the overall sizes of various organs remains more or less constant. When the existing ratio of organ sizes is artificially disrupted, it tends to normal (removal of one kidney leads to an increase in the other).

One of the concepts that explains this phenomenon is that cell proliferation is regulated by special substances called kelons. It is assumed that they have specificity for different types of cells and organ tissues. It is believed that a decrease in the number of kelons stimulates cell proliferation, for example, during regeneration. Currently, this problem is being carefully studied by various specialists. Evidence has been obtained that keylons are glycoproteins with a molecular weight of 30,000 – 50,000.

2.3. Irregular types of cell reproduction
Amitosis. Direct division, or amitosis, is described earlier than mitotic division, but is much less common. Amitosis is the division of a cell in which the nucleus is in an interphase state. In this case, chromosome condensation and spindle formation do not occur. Formally, amitosis should lead to the appearance of two cells, but most often it leads to the division of the nucleus and the appearance of bi- or multinucleated cells.

Amitotic division begins with fragmentation of the nucleoli, followed by division of the nucleus by constriction (or invagination). There may be multiple divisions of the nucleus, usually of unequal size (in pathological processes). Numerous observations have shown that amitosis almost always occurs in cells that are obsolete, degenerating and unable to produce full-fledged elements in the future. So, normally, amitotic division occurs in the embryonic membranes of animals, in the follicular cells of the ovary, and in giant trophoblast cells. Amitosis has a positive meaning in the process of tissue or organ regeneration (regenerative amitosis). Amitosis in aging cells is accompanied by disturbances in biosynthetic processes, including replication, DNA repair, as well as transcription and translation. The physicochemical properties of chromatin proteins in cell nuclei, the composition of the cytoplasm, the structure and functions of organelles change, which entails functional disorders at all subsequent levels - cellular, tissue, organ and organismal. As destruction increases and restoration fades, natural cell death occurs. Amitosis often occurs during inflammatory processes and malignant neoplasms (induced amitosis).

Endomitosis. When cells are exposed to substances that destroy spindle microtubules, division stops, and chromosomes will continue the cycle of their transformations: replicate, which will lead to the gradual formation of polyploid cells - 4 p. 8 p., etc. This transformation process is otherwise called endoreproduction. The ability of cells to undergo endomitosis is used in plant breeding to obtain cells with a multiple set of chromosomes. For this purpose, colchicine and vinblastine are used, which destroy the filaments of the achromatin spindle. Polyploid cells (and then adult plants) are large in size; the vegetative organs from such cells are large, with a large supply of nutrients. In humans, endoreproduction occurs in some hepatocytes and cardiomyocytes.

Another, rarer result of endomitosis is polytene cells. During polyteny in the S-period, as a result of replication and non-disjunction of chromosomal strands, a multi-stranded, polytene structure is formed. They differ from mitotic chromosomes in their larger size (200 times longer). Such cells are found in the salivary glands of dipteran insects and in the macronuclei of ciliates. On polytene chromosomes, swellings and puffs (transcription sites) are visible - an expression of gene activity. These chromosomes are the most important object of genetic research.
2.4. Problems of cell proliferation in medicine.
As is known, tissues with a high rate of cell turnover are more sensitive to the effects of various mutagens than tissues in which cells are renewed slowly. However, for example, radiation damage may not appear immediately and does not necessarily weaken with depth; sometimes it even damages deep-lying tissues much more than superficial ones. When cells are irradiated with X-rays or gamma rays, gross disturbances occur in the cell life cycle: mitotic chromosomes change shape, they break, followed by incorrect joining of fragments, and sometimes individual parts of chromosomes disappear altogether. Spindle anomalies may occur (not two poles in the cell, but three will form), which will lead to uneven divergence of chromatids. Sometimes cell damage (large doses of radiation) is so significant that all attempts by the cell to begin mitosis are unsuccessful and division stops.

This effect of radiation partly explains its use in tumor therapy. The goal of radiation is not to kill tumor cells in interphase, but to cause them to lose their ability to undergo mitosis, which will slow or stop tumor growth. Radiation in doses that are not lethal to the cell can cause mutations leading to increased proliferation of altered cells and give rise to malignant growth, as often happened to those who worked with X-rays, not knowing about their danger.

Cell proliferation is affected by many chemicals, including drugs. For example, the alkaloid colchicine (contained in colchicum corms) was the first drug that relieved joint pain due to gout. It turned out that it also has another effect - stopping division by binding to tubulin proteins from which microtubules are formed. Thus, colchicine, like many other drugs, blocks the formation of the spindle.

On this basis, alkaloids such as vinblastine and vincristine are used to treat certain types of malignant neoplasms, becoming part of the arsenal of modern chemotherapeutic anticancer drugs. It should be noted that the ability of substances such as colchicine to stop mitosis is used as a method for the subsequent identification of chromosomes in medical genetics.

Of great importance for medicine is the ability of differentiated (and germ) cells to maintain their potential for proliferation, which sometimes leads to the development of tumors in the ovaries, in the section of which cell layers, tissues, and organs are visible as a “mush”. Scraps of skin, hair follicles, hair, ugly teeth, pieces of bones, cartilage, nervous tissue, fragments of the eye, etc. are revealed, which requires urgent surgical intervention.

2.5. Pathology of cell reproduction
Mitotic cycle abnormalities.. The mitotic rhythm, usually adequate to the need for restoration of aging, dead cells, can be changed under pathological conditions. A slowdown of the rhythm is observed in aging or poorly vascularized tissues, an increase in the rhythm is observed in tissues under various types of inflammation, hormonal influences, in tumors, etc.

REGULATION OF THE CELL CYCLE

Introduction

Activation of proliferation

Cell cycle

Cell cycle regulation

Exogenous regulators of proliferation

Endogenous regulators of the cell cycle

Pathways of CDK regulation

G1 phase regulation

S phase regulation

G2 phase regulation

Regulation of mitosis

DNA damage

Ways to repair DNA double-strand breaks

Cellular response to DNA damage and its regulation

Tissue regeneration

Regulation of tissue regeneration

Conclusion

Bibliography

Introduction

The cell is the elementary unit of all living things. There is no life outside the cell. Cell reproduction occurs only through division of the original cell, which is preceded by the reproduction of its genetic material. Activation of cell division occurs due to the influence of external or internal factors on it. The process of cell division from the moment of its activation is called proliferation. In other words, proliferation is the multiplication of cells, i.e. an increase in the number of cells (in culture or tissue) that occurs through mitotic divisions. The duration of the cell's existence as such, from division to division, is usually called the cell cycle.

In the adult human body, cells of different tissues and organs have different abilities to divide. In addition, with aging, the intensity of cell proliferation decreases (i.e., the interval between mitoses increases). There are populations of cells that have completely lost the ability to divide. These are, as a rule, cells at the terminal stage of differentiation, for example, mature neurons, granular blood leukocytes, cardiomyocytes. In this regard, the exception is the immune B- and T-memory cells, which, being in the final stage of differentiation, are able to begin to proliferate when a certain stimulus appears in the body in the form of a previously encountered antigen. The body has constantly renewing tissues - various types of epithelium, hematopoietic tissues. In such tissues there are cells that constantly divide, replacing spent or dying cell types (for example, intestinal crypt cells, cells of the basal layer of the integumentary epithelium, hematopoietic cells of the bone marrow). There are also cells in the body that do not reproduce under normal conditions, but again acquire this property under certain conditions, in particular when it is necessary to regenerate tissues and organs. The process of cell proliferation is tightly regulated both by the cell itself (regulation of the cell cycle, cessation or slowdown of the synthesis of autocrine growth factors and their receptors) and its microenvironment (lack of stimulating contacts with neighboring cells and matrix, cessation of secretion and/or synthesis of paracrine growth factors). Dysregulation of proliferation leads to unlimited cell division, which in turn initiates the development of the oncological process in the body.

Activation of proliferation

The main function associated with the initiation of proliferation is assumed by the plasma membrane of the cell. It is on its surface that events occur that are associated with the transition of resting cells to an activated state that precedes division. The plasma membrane of cells, due to the receptor molecules located in it, perceives various extracellular mitogenic signals and ensures the transport into the cell of the necessary substances that take part in the initiation of the proliferative response. Mitogenic signals can be contacts between cells, between a cell and a matrix, as well as the interaction of cells with various compounds that stimulate their entry into the cell cycle, which are called growth factors. A cell that has received a mitogenic signal to proliferate starts the process of division.

CELL CYCLE

The entire cell cycle consists of 4 stages: presynthetic (G1), synthetic (S), postsynthetic (G2) and mitosis itself (M). In addition, there is a so-called G0 period, which characterizes the resting state of the cell. In the G1 period, cells have diploid DNA content per nucleus. During this period, cell growth begins, mainly due to the accumulation of cellular proteins, which is caused by an increase in the amount of RNA per cell. In addition, preparations for DNA synthesis begin. In the next S-period, the amount of DNA doubles and the number of chromosomes accordingly doubles. The post-synthetic G2 phase is also called premitotic. During this phase, active synthesis of mRNA (messenger RNA) occurs. This stage is followed by the cell division itself, or mitosis.

The division of all eukaryotic cells is associated with the condensation of duplicated (replicated) chromosomes. As a result of division, these chromosomes are transferred to daughter cells. This type of division of eukaryotic cells - mitosis (from the Greek mitos - threads) - is the only complete way to increase the number of cells. The process of mitotic division is divided into several stages: prophase, prometaphase, metaphase, anaphase, telophase.

REGULATION OF THE CELL CYCLE

The purpose of the regulatory mechanisms of the cell cycle is not to regulate the passage of the cell cycle as such, but to ensure, ultimately, the error-free distribution of hereditary material during the process of cell reproduction. The regulation of cell reproduction is based on the change in states of active proliferation and proliferative organ. Regulatory factors that control cell reproduction can be divided into two groups: extracellular (or exogenous) or intracellular (or endogenous). Exogenous factors are found in the cell microenvironment and interact with the cell surface. Factors that are synthesized by the cell itself and act inside it are classified as endogenous factors. This division is very arbitrary, since some factors, being endogenous in relation to the cell producing them, can leave it and act as exogenous regulators on other cells. If regulatory factors interact with the same cells that produce them, then this type of control is called autocrine. With paracrine control, the synthesis of regulators is carried out by other cells.

EXOGENOUS REGULATORS OF PROLIFERATION

In multicellular organisms, the regulation of the proliferation of various cell types occurs due to the action of not one growth factor, but a combination of them. In addition, some growth factors, being stimulators for some types of cells, behave as inhibitors in relation to others. Classic growth factors are polypeptides with a molecular weight of 7-70 kDa. To date, more than a hundred such growth factors are known. However, only a few of them will be discussed here.

Perhaps the largest body of literature is devoted to platelet-derived growth factor (PDGF). Released upon destruction of the vascular wall, PDGF is involved in the processes of thrombus formation and wound healing. PDGF is a potent growth factor for quiescent fibroblasts. Along with PDGF, epidermal growth factor (EGF), which is also capable of stimulating the proliferation of fibroblasts, has been studied no less thoroughly. But, besides this, it also has a stimulating effect on other types of cells, in particular on chondrocytes.

A large group of growth factors are cytokines (interleukins, tumor necrosis factors, colony-stimulating factors, etc.). All cytokines are multifunctional. They can either enhance or inhibit proliferative responses. For example, different subpopulations of CD4+ T lymphocytes, Th1 and Th2, producing a different spectrum of cytokines, are antagonists towards each other. That is, Th1 cytokines stimulate the proliferation of cells that produce them, but at the same time suppress the division of Th2 cells, and vice versa. Thus, normally the body maintains a constant balance of these two types of T-lymphocytes. The interaction of growth factors with their receptors on the cell surface leads to the launch of a whole cascade of events inside the cell. As a result, transcription factors are activated and proliferative response genes are expressed, which ultimately initiates DNA replication and the cell enters mitosis.

ENDOGENOUS REGULATORS OF THE CELL CYCLE

In normal eukaryotic cells, progression through the cell cycle is tightly regulated. The cause of cancer is cell transformation, usually associated with violations of the regulatory mechanisms of the cell cycle. One of the main results of cell cycle defects is genetic instability, since cells with defective cell cycle control lose the ability to correctly duplicate and distribute their genome between daughter cells. Genetic instability leads to the acquisition of new features that are responsible for tumor progression. Cyclin-dependent kinases (CDKs) and their regulatory subunits (cyclins) are major regulators of the cell cycle. Cell cycle progression is achieved through the sequential activation and deactivation of different cyclin-CDK complexes. The action of cyclin-CDK complexes is to phosphorylate a number of target proteins in accordance with the phase of the cell cycle in which a particular cyclin-CDK complex is active. For example, cyclin E-CDK2 is active in late G1 phase and phosphorylates proteins required for progression through late G1 phase and entry into S phase. Cyclin A-CDK2 is active in the S and G2 phases, it ensures the passage of the S phase and entry into mitosis. Cyclin A and cyclin E are central regulators of DNA replication. Therefore, misregulation of the expression of any of these cyclins leads to genetic instability. It has been shown that the accumulation of nuclear cyclin A occurs exclusively at the moment when the cell enters the S phase, i.e. at the moment of G1/S transition. On the other hand, it was shown that the level of cyclin E increased after passing the so-called restriction point (R-point) in late G1 phase, and then decreased significantly when the cell entered S phase.

WAYS OF REGULATION CDK

The activity of cyclin-dependent kinases (CDKs) is tightly regulated by at least four mechanisms:

1) The main way CDK is regulated is by binding to cyclin, i.e. In its free form, the kinase is not active, and only the complex with the corresponding cyclin has the necessary activities.

2) The activity of the cyclin-CDK complex is also regulated by reversible phosphorylation. In order to acquire activity, phosphorylation of CDK is necessary, which is carried out with the participation of the CDK activating complex (CAC), consisting of cyclin H, CDK7 and Mat1.

3) On the other hand, in the CDK molecule, in the region responsible for substrate binding, there are sites whose phosphorylation leads to inhibition of the activity of the cyclin-CDK complex. These sites are phosphorylated by a group of kinases, including Wee1 kinase, and dephosphorylated by Cdc25 phosphatases. The activity of these enzymes (Wee1 and Cdc25) varies significantly in response to various intracellular events, such as DNA damage.

4) Finally, some cyclin-CDK complexes may be inhibited due to binding to CDK inhibitors (CKIs). CDK inhibitors consist of two groups of proteins, INK4 and CIP/KIP. INK4 inhibitors (p15, p16, p18, p19) bind to and inactivate CDK4 and CDK6, preventing interaction with cyclin D. CIP/KIP inhibitors (p21, p27, p57) can bind to cyclin-CDK complexes containing CDK1, CDK2, CDK4 and CDK6. It is noteworthy that under certain conditions, CIP/KIP inhibitors can enhance the kinase activity of cyclin D-CDK4/6 complexes

REGULATION G 1 PHASE

In the G1 phase, at the so-called restriction point (restriction point, R-point), the cell decides whether to divide or not. The restriction point is the point in the cell cycle after which the cell becomes unresponsive to external signals until the completion of the entire cell cycle. The restriction point divides the G1 phase into two functionally distinct stages: G1pm (postmitotic stage) and G1ps (presynthetic stage). During G1pm, the cell evaluates the growth factors present in its environment. If the necessary growth factors are present in sufficient quantities, the cell enters G1ps. Cells that have entered the G1ps period continue to progress through the entire cell cycle normally, even in the absence of growth factors. If the necessary growth factors are absent in the G1pm period, then the cell enters a state of proliferative dormancy (G0 phase).

The main result of the cascade of signaling events that occurs due to the binding of growth factor to the receptor on the cell surface is the activation of the cyclin D-CDK4/6 complex. The activity of this complex increases significantly already in the early G1 period. This complex phosphorylates targets necessary for progression into S phase. The main substrate of the cyclin D-CDK4/6 complex is the retinoblastoma gene product (pRb). Unphosphorylated pRb binds and thereby inactivates transcription factors of the E2F group. Phosphorylation of pRb by cyclin D-CDK4/6 complexes leads to the release of E2F, which enters the nucleus and initiates the translation of protein genes necessary for DNA replication, in particular the cyclin E and cyclin A genes. At the end of the G1 phase, there is a short-term increase in the amount of cyclin E, which portends the accumulation of cyclin A and the transition to S phase.

The following factors can cause cell cycle arrest in the G1 phase: increased levels of CDK inhibitors, deprivation of growth factors, DNA damage, external influences, oncogenic activation

REGULATION S PHASES

S phase is the stage of the cell cycle when DNA synthesis occurs. Each of the two daughter cells that are formed at the end of the cell cycle must receive an exact copy of the DNA of the mother cell. Each base of the DNA molecules that make up the 46 chromosomes of a human cell must be copied only once. That is why DNA synthesis is extremely tightly regulated.

It has been shown that only DNA from cells in G1 or S phase can replicate. This suggests that DNA must be<лицензирована>for replication and that the piece of DNA that has been duplicated loses this<лицензию>. DNA replication begins at the binding site of proteins called ORC (Origin of replicating complex). Several components required for DNA synthesis bind to ORC in late M or early G1 phase, forming a prereplicative complex, which actually gives<лицензию>DNA for replication. At the G1/S transition stage, additional proteins necessary for DNA replication are added to the prereplicative complex, thus forming an initiation complex. When the replication process begins and a replication fork is formed, many components are separated from the initiation complex, and only the components of the post-replication complex remain at the replication initiation site.

Many studies have shown that the normal functioning of the initiation complex requires the activity of cyclin A-CDK2. In addition, for the successful completion of the S phase, the activity of the cyclin A-CDK2 complex is also required, which, in fact, is the main regulatory mechanism that ensures the successful completion of DNA synthesis. Arrest in S phase can be induced by DNA damage.

REGULATION G 2 PHASES

G2 phase is a stage of the cell cycle that begins after DNA synthesis is complete but before condensation begins. The main regulator of the G2 phase is the cyclin B-CDK2 complex. Cell cycle arrest in the G2 phase occurs due to inactivation of the cyclin B-CDK2 complex. The regulator of the G2/M transition is the cyclin B-CDK1 complex; its phosphorylation/dephosphorylation regulates entry into the M phase. DNA damage or the presence of unreplicated regions prevents the transition to M phase.

Proliferative processes in acute inflammation begin soon after the influence of the phlogogenic factor on the tissue and are more pronounced along the periphery of the inflammation zone. One of the conditions for the optimal course of proliferation is the attenuation of the processes of alteration and exudation.

Proliferation

Phagocytes also produce and release into the intercellular fluid a number of biologically active substances that regulate the development of either immunity, allergies, or tolerance states. Thus, inflammation is directly related to the formation of immunity or immunopathological reactions in the body.

Proliferation, a component of the inflammatory process and its final stage, is characterized by an increase in the number of stromal and, as a rule, parenchymal cells, as well as the formation of intercellular substance at the site of inflammation. These processes are aimed at the regeneration of altered and/or replacement of destroyed tissue elements. Various biologically active substances, especially those stimulating cell proliferation (mitogens), are essential at this stage of inflammation.

The forms and degree of proliferation of organ-specific cells are different and are determined by the nature of the cell populations (see the article “Cell Population” in the Appendix “Reference of Terms”).

In some organs and tissues (for example, liver, skin, gastrointestinal tract, respiratory tract), the cells have a high proliferative ability, sufficient to eliminate structural defects at the site of inflammation.

In other organs and tissues this ability is very limited (for example, in tissues of tendons, cartilage, ligaments, kidneys, etc.).

In a number of organs and tissues, parenchymal cells have virtually no proliferative activity (for example, cardiac muscle myocytes, neurons). In this regard, upon completion of the inflammatory process in the tissues of the myocardium and nervous system, stromal cells, mainly fibroblasts, which also form non-cellular structures, proliferate at the site of inflammation. As a result, a connective tissue scar is formed. At the same time, it is known that parenchymal cells of these tissues have a high ability for hypertrophy and hyperplasia of subcellular structures.

Activation of proliferative processes correlates with the formation of biologically active substances that have an anti-inflammatory effect (a kind of anti-inflammatory mediators). The most effective among them include:

Inhibitors of hydrolases, in particular proteases (for example, antitrypsin), microglobulin, plasmin or complement factors;

Antioxidants (eg, ceruloplasmin, haptoglobin, peroxidases, SOD);

Polyamines (eg putrescine, spermine, cadaverine);

Glucocorticoids;

Heparin (suppresses adhesion and aggregation of leukocytes, activity of kinins, biogenic amines, complement factors).

Replacement of dead and damaged tissue elements during inflammation is noted after their destruction and elimination (this process is called wound cleansing).

The proliferation reactions of both stromal and parenchymal cells are regulated by various factors. The most significant among them include:

Many inflammatory mediators (for example, TNF, which suppresses proliferation; leukotrienes, kinins, biogenic amines, which stimulate cell division).

Specific metabolic products of leukocytes (for example, monokines, lymphokines, ILs, growth factors), as well as platelets, that can activate cell proliferation.

Low molecular weight peptides released during tissue destruction, polyamines (putrescine, spermidine, spermine), as well as nucleic acid breakdown products that activate cell reproduction.

Hormones (GH, insulin, T4, corticoids, glucagon), many of them capable of both activating and suppressing proliferation depending on their concentration, activity, synergistic and antagonistic interactions; for example, glucocorticoids in low doses inhibit, and mineralocorticoids activate regeneration reactions.

Proliferation processes are also influenced by a number of other factors, for example, enzymes (collagenase, hyaluronidase), ions, neurotransmitters and others.

Proliferation is the final phase of the development of inflammation, ensuring reparative regeneration of tissue at the site of the alteration site.

Proliferation develops from the very beginning of inflammation along with the phenomena of alteration and exudation.

The proliferation of cellular elements begins along the periphery of the inflammation zone, while in the center of the lesion the phenomena of alteration and necrosis can still progress.

Proliferation of connective tissue and organ-specific cellular elements reaches full development after “cleaning” the damaged area of cellular detritus and infectious agents of inflammation by tissue macrophages and neutrophils. In this regard, it should be noted that the proliferation process is preceded by the formation of neutrophil and monocyte barriers, which form along the periphery of the alteration zone.

The restoration and replacement of damaged tissue begins with the release of fibrinogen molecules from the vessels and the formation of fibrin, which forms a kind of mesh, a framework for subsequent cell reproduction. Already along this framework, rapidly formed fibroblasts are distributed in the repair site.

The division, growth and movement of fibroblasts is possible only after they are bound to fibrin or collagen fibers. This connection is provided by a special protein - fibronectin.

The proliferation of fibroblasts begins along the periphery of the inflammation zone, ensuring the formation of a fibroblastic barrier. At first, fibroblasts are immature and do not have the ability to synthesize collagen. Maturation is preceded by internal structural and functional restructuring of fibroblasts: hypertrophy of the nucleus and nucleolus, hyperplasia of the ER, increased content of enzymes, especially alkaline phosphatase, nonspecific esterase, b-glucuronidase. Only after the restructuring begins collagenogenesis.

Intensively multiplying fibroblasts produce acidic mucopolysaccharides - the main component of the intercellular substance of connective tissue (hyaluronic acid, chondroitinsulfuric acid, glucosamine, galactosamine).

In this case, the inflammation zone is not only encapsulated, but also a gradual migration of cellular and acellular components of connective tissue occurs from the periphery to the center, the formation of a connective tissue skeleton at the site of primary and secondary alteration.

Along with fibroblasts, other tissue and hematogenous cells also multiply. Endothelial cells proliferate from tissue cells and form new capillaries. Around the newly formed capillaries, mast cells, macrophages, and neutrophils are concentrated, which release biologically active substances that promote the proliferation of capillaries.

Fibroblasts, together with newly formed vessels, form granulation tissue. It is essentially young connective tissue, rich in cells and thin-walled capillaries, the loops of which protrude above the surface of the tissue in the form of granules.

The main functions of granulation tissue are: protective - prevents the influence of environmental factors on the source of inflammation, and reparative - filling the defect and restoring the anatomical and functional usefulness of damaged tissues.

The formation of granulation tissue is not strictly necessary. This depends on the size and depth of the damage. Granulation tissue usually does not develop during the healing of bruised skin wounds or minor damage to the mucous membrane (Kuzin M.I., Kostyuchenok B.M. et al., 1990).

Granulation tissue gradually turns into fibrous tissue called scar.

In scar tissue, the number of vessels decreases, they become empty, the number of macrophages and mast cells decreases, and the activity of fibroblasts decreases.

A small part of the cellular elements, located among the collagen filaments, remains active. It is assumed that tissue macrophages that remain active take part in the resorption of scar tissue and ensure the formation of softer scars.

In parallel with the maturation of granulations, epithelization of the wound occurs. It begins in the first hours after damage, and within the first day 2-4 layers of basal epithelial cells are formed.

The rate of epithelialization is ensured by the following processes: migration, division and differentiation of cells. Epithelialization of small wounds is carried out mainly due to the migration of cells from the basal layer. Larger wounds are epithelialized due to migration and mitotic division of cells in the basal layer, as well as differentiation of the regenerating epidermis. The new epithelium forms the boundary between the damaged and underlying layers; it prevents dehydration of wound tissue, a decrease in electrolytes and proteins in it, and also prevents the invasion of microorganisms.

Organ-specific cellular elements of organs and tissues also participate in the process of proliferation. From the point of view of the possibilities of proliferation of organ-specific cellular elements, all organs and tissues can be classified into three groups:

The first group may include organs and tissues whose cellular elements have active or practically unlimited proliferation, sufficient to completely compensate for the structural defect in the area of inflammation (epithelium of the skin, mucous membranes of the respiratory tract, mucous membrane of the gastrointestinal tract, genitourinary system, hematopoietic tissue and etc.).

The second group includes tissues with limited regenerative abilities (tendons, cartilage, ligaments, bone tissue, peripheral nerve fibers).

The third group includes those organs and tissues where organ-specific cellular elements are not capable of proliferation (heart muscle, CNS cells).

Factors stimulating the development of proliferation processes are:

1. Procollagen and fibroblast collagenase interact according to the type of autoregulation and provide a dynamic balance between the processes of synthesis and destruction of connective tissue.

2. Fibronectin, produced by fibroblasts, determines the migration, proliferation and adhesion of connective tissue cells.

3. Fibroblast stimulating factor, secreted by tissue macrophages, ensures the proliferation of fibroblasts and their adhesive properties.

4. Cytokines of mononuclear cells stimulate proliferative processes in damaged tissue (IL-1, TNF, epidermal, platelet, fibroblastic growth factors, chemotactic factors). Some cytokines can inhibit fibroblast proliferation and collagen formation.

5. Calcitonin-related gene peptide stimulates the proliferation of endothelial cells, and substance P induces the production of TNF in macrophages, which leads to increased angiogenesis.

6. Group E prostaglandins potentiate regeneration by increasing blood supply.

7. Keylons and antikeylons produced by various cells, acting on the feedback principle, can activate and inhibit mitotic processes in the focus of inflammation (Bala Yu.M., Lifshits V.M., Sidelnikova V.I., 1988).

8. Polyamines (putrescine, spermidine, spermine), found in all mammalian cells, are vital for cell growth and division.

They provide stabilization of plasma membranes and the superhelical structure of DNA, protection of DNA from the action of nucleases, stimulation of transcription, methylation of RNA and its binding to ribosomes, activation of DNA ligases, endonucleases, protein kinases and many other cellular processes. Enhanced synthesis of polyamines, promoting proliferative processes, is noted in the focus of alteration (Berezov T.T., Fedoronchuk T.V., 1997).

9. Cyclic nucleotides: cAMP inhibits, and cGMP activates proliferation processes.

10. Moderate concentrations of biologically active substances and hydrogen ions are stimulants of regenerative processes.

Molecular genetic mechanisms of regulation of cell proliferation. Introduction. Pathways of CDK regulation

Introduction

Activation of proliferation

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