NOW, ONLY ON THIS WEBSITE:
THE FIRST BOOK WRITTEN BY:
MOHAMMED HAMMDI ALI
CANCER
THE SCARING MONSTER
الأحد، 26 أكتوبر 2008
السبت، 5 يوليو 2008
CANCER: Dance of the death
Cell cycle
The cell cycle is the process by which a cell grows, duplicates its DNA, and divides into identical daughter cells. -Also known as cell division cycle. In a typical cell cycle, the parent cell doubles its volume, mass, and complement of chromosomes, then sorts it's doubled contents to opposite sides of the cell, and finally divides in half to yield two genetically identical offspring.
- This idea fits well with the behavior of many unicellular organisms, but for multicellular organisms the daughter cells may differ from their parent cell and from each other in terms of size, shape, and differentiation state.
- Cell cycle duration varies according to cell type and organism. Embryonic cells that do not need to grow between divisions can complete a cell cycle in as little as 8 min, whereas it usually takes 10–24 hours of the most rapidly dividing somatic cells. Many somatic cells divide much less frequently; liver cells divide about once a year, postmitotic cells are incapable of cell division even after maximal stimulation, and include mature neurons, striated muscle cells, and heart muscle cells.
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Eukaryotic phases
-The cell cycle is divided into two main parts: interphase and mitosis
During interphase, the cell grows and replicates its chromosomes.
- Interphase subdivided into three phases: gap phase 1 (G1), synthesis (S), and gap phase 2 (G2). Interphase is followed by mitosis (nuclear division"karyokinesis", and cell division "cytokinesis"). In plants and algae, cytokinesis is accompanied by the formation of a new cell wall.
Gap 1 phase "G1": (G indicating gap or growth)
Gap phase 1 begins at the completion of mitosis and cytokinesis and lasts until the beginning of S phase. This phase is generally the longest of the four cell cycle phases and is quite variable in length. During this phase, the cell chooses either to replicate its DNA or to exit the cell cycle and enter a quiescent state (the G0 phase). Some conditions cause cells to enter the G0 phase.G0 phase: The term "post-mitotic" is sometimes used to refer to both quiescent and senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). This is very common for cells that are fully differentiated. Cellular senescence is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable; it is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis. Some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G0 phase semi-permanently and can only be induced to begin dividing again under very specific circumstances; other types, such as epithelial cells, continue to divide throughout an organism's life. p53 prevents the cell from replicating by stopping the cell cycle at G1, or interphase, to give the cell time to repair.
The Synthetic phase "S":
Replication of the chromosomes occurs (DNA synthesis phase), it takes 6 hours. During S phase, each chromosome replicates exactly once to form a pair of physically linked sister chromatids, in animal cells, a pair of centrioles is also duplicated during S phase.
Gaps 2 phase "G2":
Follows S phase is called gap phase 2. Some cells can exit the cell cycle from G2 phase, just as they can from G1 phase. Considered as preparation phase for mitosis and usually takes 4 hours. Significant protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis. Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis
The mitotic phase "M": mitosis.
Cell divisions phase, takes from 1 to 2 hours. It includes: nuclear division "karyokinesis" followed by cytoplasmic division "cytokinesis". During the mitosis (M) phase, the duplicated chromosomes are segregated, migrating to opposite poles of the cell. The cell then divides into two daughter cells, each having the same genetic components as the parental cell. Mitosis is divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis usually begins during anaphase and ends at a point after the completion of mitosis.
Note: Mitosis is prevented if DNA damage has occurred or if genomic replication is not complete.
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Regulation of cell cycle:
including detecting and repairing genetic damage, and provision of various checks to prevent uncontrolled cell division.The mammalian cell cycle is regulated by a group of protein kinases called cyclin-dependent kinases (CDKs). These proteins catalyze the attachment of phosphate groups to specific serine or threonine amino acids in a target protein. The phosphate groups alter the target protein's properties, such as its interaction with other proteins. (The alteration of protein activity by the attachment of phosphate groups occurs frequently in cells.).
Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin. When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle.
The number of CDKs in a cell remains constant during the cell cycle, the level of cyclins oscillate, CDKs are constitutively expressed in cells whereas cyclins are synthesized at specific stages of the cell cycle, in response to various molecular signals.
There are G1 cyclins, S-phase cyclins, and G2/M cyclins, each of which interact differently with CDK subunits to regulate the various phases of the cell cycle.
Cyclin D is the first cyclin produced in the cell cycle, in response to extracellular signals (eg. growth factors). Cyclin D binds to existing CDK4, forming the active cyclin D-CDK4 complex. Cyclin D-CDK4 complex in turn phosphorylates the retinoblastoma susceptibility protein (RBp).
Cell cycle inhibitors
CDKs can also associate with inhibitory subunits called CDK inhibitors (CKIs). CKIs cause the cell cycle to halt " stop". Two families of CKIs have been identified, based on their amino acid sequence similarity and the specificity of their interactions with CDKs. One of the families of CKIs, the INK family, includes four proteins (p15, p16 p18 and p20). These CKIs exclusively bind complexes of cyclin D and CDK4, as well as complexes of cyclin D and CDK6, to block cells that are in the G1 phase of the cell cycle. The other family of CKIs, the Cip/Kip family, consists of three proteins (p21, p27, and p57). These inhibitors bind to all complexes of cyclins and CDKs that function during the G1 phase and during the transition from the G1 to the S phase. They act preferentially, however, to block the activity of complexes containing CDK2.
Active complexes of cyclins and CDKs exert their biological effects by phosphorylating proteins. During the G1 phase, a major target of cyclin/CDK complexes is the retinoblastoma protein (pRb). PRb is a growth-suppressing protein whose activity is controlled by whether or not it is phosphorylated.
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Deregulation and Cancer:
Deregulation of cell cycle control proteins plays a key role in the development of cancer. Overactivation of proteins that favor cell cycle progression, namely cyclins and CDKs, and the inactivation of proteins that impede cell cycle progression, such as CKIs, can result in uncontrolled cell proliferation.
In human tumors, it is genes encoding the proteins that control the transition from the G1 to the S phase that are most commonly altered. These genes include those for cyclins, CKIs, and pRb. Such mutations overcome the inhibitory effects of pRb on the cell cycle, causing cells to have a growth advantage. In some cancers, this occurs after the direct mutation of the pRb gene, resulting in the protein's loss of function. In a larger set of cancers, pRb is indirectly inactivated by the hyper-activation of CDKs. This may result from over expression of cyclins, from an activating mutation in CDK4, or from inactivation of CKIs.
There is much evidence to suggest that cyclins can act as oncogenes to induce cells to become cancerous.
In particular the G1 cyclins, cyclin D1, and cyclin E have been implicated in the development of cancer.
Cyclin D1 protein is frequently detected in human breast cancer, and increasing evidence suggests that cyclin E plays an important role in the pathogenesis of breast cancer.
CKIs antagonize the function of cyclins, and considerable evidence suggests that these proteins function as tumor suppressors. CKI function is often altered in cancer cells. The gene encoding p16, a protein that belongs to the INK family of CKIs, is mutated, deleted, or inactivated in a large number of human malignancies and tumors. Such alterations prevent the inhibition of cyclin D/CDK4 and cyclin D/CDK6 complexes during G1.
Decreased expression of p21 and p27, proteins that belong to the Cip/Kip family of CKIs, also has been demonstrated in numerous human tumors. In contrast to the genetic mutations observed with p16, the decrease in p27 levels in tumors is due to enhanced degradation of the p27 protein. One of the proteins required for the degradation of p27, Skp2, has oncogenic properties. Skp2 over expression is observed in several human cancers and likely contributes to the uncontrolled progression of the cell cycle by increasing the degradation of p27. Understanding of the fine details of cell cycle regulation is likely to lead to specific cancer therapies targeting one or more of these important proteins.
Role of cell cycle in tumor formation
A disregulation of the cell cycle components may lead to tumor formation. As mentioned above, some genes like the cell cycle inhibitors, RB, p53 etc., when they mutate, may cause the cell to multiply uncontrollably, forming a tumor. Although the duration of cell cycle in tumor cells is equal to or longer than that of normal cell cycle, the proportion of cells that are in active cell division (versus quiescent cells in G0 phase) in tumor cells are much more compared to that in normal cells. Thus there is a net increase in cell number as the number of cells that die by apoptosis or senescence remains the same.
The cells which are actively undergoing cell cycle are targeted in cancer therapy as the DNA is relatively exposed during cell division and hence susceptible to damage by drugs or radiation. This fact is made use of in cancer treatment; by a process known as debulking, a significant mass of the tumor is removed which pushes a significant number of the remaining tumor cells from G0 to G1 phase (due to increased availability of nutrients, oxygen, growth factors etc.). Radiation or chemotherapy following the debulking procedure kills these cells which have newly entered the cell cycle
Synchronization of cell cultures:
Several methods can be used to synchronise cell cultures by halting the cell cycle at a particular phase. For example, Serum starvation and treatment with Thymidine or Aphidicolin halt the cell in the G1 phase, Mitotic shake-off, treatment with colchicine and treatment with Nocodazole halt the cell in M phase and treatment with 5-fluorodeoxyuridine halts the cell in S phase.
Observation:
There are numerous ways to observe the cell cycle occurring. Onion bulbs or garlic root tips are often used.
A sample of root tip is fixed in a mixture of 99% of 70% aqueous industrial methylated spirit and 1% glacial ethanoic acid for two hours. Treat the root tips in 1 molar hydrochloric acid at 60C for 6 -7 minutes. Rinse thoroughly with water. Add Schiff's reagent and leave for one hour. Rinse again in distilled water. Observe under a microscope.
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Death is an inevitable fact of life for organisms
Apoptosis "programmed cell death" " PCD" "Dance of Death"
Apoptosis is a process by which cells in a multicellular organism commit suicide.
Apoptosis is a form of death that the cell itself initiates, regulates, and executes. using an elaborate arsenal of cellular and molecular machinery.
Apoptotic cell
Why Cells Commit Suicide:
Why do cells commit apoptosis? There are 2 major reasons:
First, apoptosis is one means by which a developing organism shapes its tissues and organs. For instance, a human fetus has webbed hands and feet early on its development. Later, apoptosis removes skin cells, revealing individual fingers and toes. A fetus's eyelids form an opening by the process of apoptosis. During metamorphosis, tadpoles lose their tails through apoptosis. In young children, apoptosis is involved in the processes that literally shape the connections between brain cells, and in mature females, apoptosis of cells in the uterus causes the uterine lining to slough off at each menstrual cycle. Between 50 billion and 70 billion cells die each day due to apoptosis in the average human adult. For an average child between the ages of 8 to 14, approximately 20 billion to 30 billion cells die a day.
Second, Cells may also commit suicide in times of distress,_ damaged beyond repair, infected with a virus_ for the good of the organism as a whole. For example, in the case of a viral infection, certain cells of the immune system, called cytotoxic T lymphocytes, bind to infected cells and trigger them to undergo apoptosis. Also, cells that have suffered damage to their DNA, which can make them prone to becoming cancerous, are induced to commit apoptosis.
More strikingly, they found that many of these genes were mutated in tumors from cancer patients. Other genes often found to be mutated in cancers are those which regulate the cell cycle, which is the complex set of processes controlling how and when cells divide. These two findings led cancer researchers to recognize that cancer, a disease of uncontrolled cell proliferation, can result either from too much cell division or not enough apoptosis.
A cell can be triggered to undergo apoptosis either by external signaling molecules, such as so-called "death activator" proteins, or through molecules that reside within the cell and monitor events that might commit the cell to suicide, such as damage to DNA. There are several biochemical pathways that lead to apoptosis. One of the major pathways involves inducing mitochondria to leak one of their proteins, cytochrome c, into the cystosol. This in turn activates a set of related proteases (enzymes that degrade proteins) called caspases. Ultimately, the caspases degrade proteins in the cell and activate enzymes that degrade other cell constituents, such as the DNA. Cells undergoing apoptosis exhibit characteristic morphological and biochemical traits, which can be recognized by microscopic examination or biochemical assays. Apoptosis can occur in as little as twenty minutes, after which the cell "corpse" typically becomes engulfed and completely degraded by neighboring phagocytic cells that are present in the tissue and attracted to the apoptotic cell.
The aberrant inhibition or initiation of apoptosis contributes to many disease processes, including cancer
apoptotic neutrophil undergoes a series of changes including violent membrane blebbing, called zeiosis, (animation to the right), chromatin condensation and fragmentation of DNA creating a vacuolar nucleus. Apoptotic cells shrink in size, break into smaller pieces called apoptotic bodies that other body cells recognize and eat it.
Apoptosis also plays a role in preventing cancer; if a cell is unable to undergo apoptosis, due to mutation or biochemical inhibition, it can continue dividing and develop into a tumour. For example, infection by papillomaviruses causes a viral gene to interfere with the cell's p53 protein, an important member of the apoptotic pathway. This interference in the apoptotic capability of the cell plays a critical role in the development of cervical cancer.
A) Section of mouse liver stained to B) section of mouse liver showing an
show cells undergoing apoptosis (orange) apoptotic cell indicated by an arrow
Homeostasis:
The number of cells is kept relatively constant through cell death and division.
Homeostasis is achieved when the rate of mitosis (cell division) in the tissue is balanced by cell death. If this equilibrium is disturbed, one of two potentially fatal disorders occurs:
* The cells are dividing faster than they die, effectively developing a tumor.
The cells are dividing slower than they die, which results in a disorder of cell loss.*
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Mechanism of apoptosis:
The process of apoptosis is controlled by a diverse range of cell signals, which may originate either extracellularly (extrinsic inducers) or intracellularly (intrinsic inducers).
Extracellular signals may include hormones, growth factors, nitric oxideHYPERLINK \l "wp-_note-NO" or cytokines, and therefore must either cross the plasma membrane or transduce to effect a response. These signals may positively or negatively induce apoptosis; in this context the binding and subsequent initiation of apoptosis by a molecule is termed positive, whereas the active repression of apoptosis by a molecule is termed negative.
Intracellular signals: a response initiated by a cell in response to stress, and may ultimately result in cell suicide. The binding of nuclear receptors by glucocorticoids, heat, radiation, nutrient deprivation, viral infection and hypoxia are all factors which can lead to the release of intracellular apoptotic signals by a damaged cell. A number of cellular components, such as poly ADP ribose polymerase, may also help regulate apoptosis.
Mitochondrial regulation:
1) Apoptotic proteins which target mitochondria affect them in different ways; they may cause mitochondrial swelling through the formation of membrane pores, or they may increase the permeability of the mitochondrial membrane and cause apoptotic effectors to leak out.
2) Nitric oxide (NO) is able to induce apoptosis by helping to dissipate the membrane potential of mitochondria and therefore make it more permeable
Roles of mitochondria in apoptosis:
Mitochondrial proteins known as SMACs (second mitochondria-derived activator of caspases) are released into the cytosol following an increase in permeability.
A)SMAC binds to inhibitor of apoptosis proteins (IAPs) and deactivates them, preventing the IAPs from arresting the apoptotic process and therefore allowing apoptosis to proceed.
B)IAP also normally suppresses the activity of a group of cysteine proteases called caspases, which carry out the degradation of the cell, therefore the actual degradation enzymes can be seen to be indirectly regulated by mitochondrial permeability.
C)Cytochrome c is also released from mitochondria due to increased permeability of the outer mitochondrial membrane, and serves a regulatory function as it precedes morphological change associated with apoptosis.
D) Once cytochrome c is released it binds with Apaf-1 and ATP, which then bind to pro-caspase-9 to create a protein complex known as an apoptosome.
E) The apoptosome cleaves the pro-caspase to its active form of caspase-9, which in turn activates the effector caspase-3.
The mitochondrial permeability is itself subject to regulation by various proteins, as Bcl-2 proteins, Bcl-2 proteins are able to promote or inhibit apoptosis by either direct action on mitochondrial permeability, or indirectly through other proteins. Importantly, the actions of some Bcl-2 proteins are able to halt apoptosis even if cytochrome c has been released by the mitochondria.
WHAT'S OCCUR DURING APOPTOSIS:
1-binding of specific signals proteins (death signals) (Extracellular signals) to specific receptors (death receptors) on the cell membrane: this binding initiate apoptosis,
Examples of these proteins (death signals) a)tumour necrosis factor (TNF)
b)Fas-ligand (Fas-L)
a) tumour necrosis factor (TNF):
- TNF is a cytokine produced mainly by activated macrophages (the major extrinsic mediator of apoptosis)
Most cells in the human body have two receptors for TNF: TNF-R1 and TNF-R2. -
-The binding of TNF to TNF-R1 has been shown to initiate the pathway that leads to caspase activation
- Binding of this receptor can also indirectly lead to the activation of transcription factors involved in cell survival and inflammatory responses
b) Fas-ligand (Fas-L):
-transmembrane protein
-bind to Fas receptor, this binding results in the formation of the death-inducing signaling complex (DISC),which contains caspase-8 and caspase-10.
- In some types of cells(type I) processed caspase-8 directly activates other members of the caspase family, and triggers the execution of apoptosis In other types of cells (type II), the Fas-DISC starts a feedback loop that spirals into increasing release of pro-apoptotic factors from mitochondria and the amplified activation of caspase-8.
- Following TNF-R1 and Fas activation in mammalian cells a balance between pro-apoptotic (BAX,[19] BID, BAK, or BAD) and anti-apoptotic (Bcl-Xl and Bcl-2) members of the Bcl-2 family is established.
- This balance is the proportion of pro-apoptotic homodimers that form in the outer- membrane of the mitochondrion.
- The pro-apoptotic homodimers are required to make the mitochondrial membrane permeable for the release of caspase activators such as cytochrome c and SMAC.
Mechanical change during apoptosis:
Cell shrinkage and rounding due to the breakdown of the proteinaceous cytoskeleton by caspases.
The cytoplasm appears dense, and the organelles appear tightly packed.
Chromatin undergoes condensation into compact patches against the nuclear envelope in a process known as pyknosis, a hallmark of apoptosis.
The nuclear envelope becomes discontinuous and the DNA inside it is fragmented in a process referred to as karyorrhexis. The nucleus breaks into several discrete chromatin bodies or nucleosomal units due to the degradation of DNA.
The cell membrane shows irregular buds known as blebs.
The cell breaks apart into several vesicles called apoptotic bodies, which are then phagocytosed.
Apoptosis progresses quickly and its products are quickly removed, making it difficult to detect or visualize. During karyorrhexis, endonuclease activation leaves short DNA fragments, regularly spaced in size. These give a characteristic "laddered" appearance on agar gel after electrophoresis. Tests for DNA laddering differentiate apoptosis from ischemic or toxic cell death.
Removal of dead cells:
-Dying cells that undergo the final stages of apoptosis display phagocytotic molecules, such as phosphatidylserine on their cell surface.
-Phosphatidylserine is normally found on the cytosolic surface of the plasma membrane, but is redistributed during apoptosis to the extracellular surface by a hypothetical protein known as scramblase.
- These molecules mark the cell for phagocytosis by cells possessing the appropriate receptors, such as macrophages.
- The phagocyte reorganizes its cytoskeleton (of dying cell) for engulfment of the cell. -The removal of dying cells by phagocytes occurs in an orderly manner without eliciting an inflammatory response
p53 dysregulation:
p53 prevents the cell from replicating by stopping the cell cycle at G1, or interphase, to give the cell time to repair, however it will induce apoptosis if damage is extensive and repair efforts fail. Logically, any disruption to the regulation of the p53 or interferon genes will result in impaired apoptosis and the possible formation of tumors.
Viral infection:
Viruses have developed strategies to evade apoptosis to permit their survival within the host cell. Most viruses encode proteins that can inhibit apoptosis.
Several viruses encode viral homologs of Bcl-2. These homologs can inhibit pro-apoptotic proteins such as BAX and BAK, which are essential for the activation of apoptosis. Examples of viral Bcl-2 proteins include the Epstein-Barr virus BHRF1 protein and the adenovirus E1B 19K protein.
Some viruses express caspase inhibitors that inhibit caspase activity and an example is the CrmA protein of cowpox viruses. Whilst a number of viruses can block the effects of TNF and Fas. For example the M-T2 protein of myxoma viruses can bind TNF preventing it from binding the TNF receptor and inducing a response.
Furthermore, many viruses express p53 inhibitors that can bind p53 and inhibit its transcriptional transactivation activity. Consequently p53 cannot induce apoptosis since it cannot induce the expression of pro-apoptotic proteins. The adenovirus E1B-55K protein and the hepatitis B virus HBx protein are examples of viral proteins that can perform such a function.
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Necrosis
"Traumatic cell death"
Death or disintegration of a cell or tissue due to disease, physical injury, or chemical injury.
Death of cells resulting from, for example, loss of blood supply, bacterial toxins, or physical or chemical agents.
Necrosis is less orderly than apoptosis "programmed cell death".
In contrast apoptosis, cleanup of cell debris by phagocytes of the immune system is generally more difficult, as the disorderly death generally does not send cell signals which tell nearby phagocytes to engulf the dying cell. This lack of signaling, makes it harder for the immune system to locate and recycle dead cells which have died through necrosis than if the cell had undergone apoptosis. The release of intracellular content after cellular membrane damage is the cause of inflammation in necrosis.
There are many causes of necrosis including exposure to injury, infection, cancer, infarction, poisons, bites from some spiders such as brown recluses and inflammation. Severe damage to one essential system in the cell leads to secondary damage to other systems, a so-called "cascade of effects". Necrosis is accompanied by the release of special enzymes, that are stored by lysosomes, which are capable of digesting cell components or the entire cell itself. The injuries received by the cell may compromise the lysosome membrane, or may initiate an unorganized chain reaction which causes the release in enzymes. Unlike apoptosis, cells that die by necrosis may release harmful chemicals that damage other cells. In biopsy, necrosis is halted by fixation or freezing.
Morphologic patterns:
There are seven distinctive morphologic patterns of necrosis:
· Coagulative necrosis is typically seen in hypoxic environments (e.g. myocardial infarction, infarct of the spleen). Cell outlines remain after cell death and can be observed by light microscopy.
· Liquefactive necrosis is usually associated with cellular destruction and pus formation (e.g. pneumonia). This is typical of bacterial or, sometimes, fungal infections because of their ability to stimulate an inflammatory reaction. Curiously, ischemia (restriction of blood supply) in the brain produces liquefactive rather than coagulative necrosis.
· Gummatous necrosis is restricted to necrosis involving spirochaetal infections (e.g. syphilis).
· Haemorrhagic necrosis is due to blockage of the venous drainage of an organ or tissue (e.g. in testicular torsion).
· Caseous necrosis is a specific form of coagulation necrosis typically caused by mycobacteria (e.g. tuberculosis).
· Fatty necrosis results from the action of lipases on fatty tissues (e.g. acute pancreatitis, breast tissue necrosis).
· Fibrinoid necrosis is caused by immune-mediated vascular damage. It is marked by deposition of fibrin-like proteinaceous material in arterial walls, which appears smudgy and eosinophilic on light microscopy
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Oncogene
Oncogenesis: conversion of well regulated cell into a cancerous cell.
Oncogene: Gene that can cause cancer, it is a sequence of DNA that has been altered or mutated from its original form, "the proto-oncogene".
Proto-oncogenes promote the specialization and division of normal cells. A change in their genetic sequence can result in uncontrolled cell growth, ultimately causing the formation of a cancerous tumour.
Oncogenes were first discovered in certain a chicken retrovirus and were later identified as cancer-causing agents in many animals.
Proto-oncogene
A proto-oncogene is a normal gene that can become an oncogene due to mutations. Proto-oncogenes code for proteins that help to regulate cell growth and differentiation. Proto-oncogenes are often involved in signal transduction and execution of mitogenic signals, usually through their protein products. Upon activation, a proto-oncogene (or its product) becomes a tumor inducing agent, "an oncogene". Examples of proto-oncogenes: RAS, WNT, MYC, and ERK.
Oncogenes forming by major 4 ways (3 due to change of proto-oncogenes to oncogenes,and 1 from viral infection)
In humans, proto-oncogenes can be transformed into oncogenes in three ways:
1)- single mutations (called point mutations) (alteration of a single nucleotide base pair), mutation within a proto-oncogene can cause a change in the protein structure.
2)-translocation (in which a segment of the chromosome breaks off and attaches to another chromosome). a proto-oncogene on one chromosome might be moved to another chromosome, For example, in the translocation between chromosomes 9 and 22 that is found in CML, a protooncogene on chromosome 9, called c-Abl, is moved to chromosome 22, where it is fused to another gene called Bcr. c-Abl is a nuclear enzyme called "tyrosine kinase," which adds a phosphate molecule to proteins at an amino acid called tyrosine. The fusion of Bcr and c-Abl genes creates an oncogene, called Bcr/Abl, which makes a highly overactive tyrosine kinase variant that is found in the cytoplasm instead of the nucleus. These changes in the activity and cellular location of the c-Abl proto-oncogene lead to chronic myelogenous leukemia.
3)-amplification (increase in the number of copies of the proto-oncogene). Oncogenes can be created by the localized amplification of small chromosomal domains, DNA amplification increases by several-fold a specific region of a chromosome. This can produce many copies of any genes that lie in the amplified region.
4)-oncogenes may transported by viruses that carry oncogenes in their genome and transferred into the cell by infection. .(viruses inserted into the chromosome near a specific gene.)
Although most human cancer is not associated with infectious agents such as viruses, some cancers have been shown to be caused at least in part by viruses.
The places that become altered in the DNA of cancer cells are called oncogenes and tumor suppressor genes. Oncogenes and tumor suppressor genes are particular locations on DNA that control a cell's ability to perform its biological functions and to control its growth.
ONCOGENIC VIRUSES: viruses cause cancers.
1)DNA viruses
Epstein-barr virus (EBV): may cause –Burkett's lymphoma.
_nasopharyngeal carcinoma.
Human papilloma virus (HPV): may cause cervical carcinoma.
Hepatitis B virus (HBV): may cause hepatocellular carcinoma.
Human herpes virus (HHV): may cause Kaposi sarcoma.
2)RNA viruses "retroviruses"
Hepatitis C virus (HCV): may cause hepatocellular carcinoma.
Human T cell lymphotropic virus: may cause T-cell leukemia.
retroviruses carry an RNA genome that, once inside a cell, is copied into DNA, which then is inserted randomly into the genome of a host cell. Some retroviruses are slow to cause tumors. After infection and spread to a large number of cells, a DNA copy of the viral genome, by chance, integrates into a host cell's DNA next to a normal gene that plays an important role in cell growth. If this viral integration disrupts the expression or structure of the normal cellular gene, it induces abnormal growth signals that can lead to cancer. Other retroviruses cause tumors to appear very quickly. In the process of copying viral RNA into DNA, RNA that is expressed from cellular genes can be mistakenly copied into the viral genome. The progeny of the virus transfer the cellular gene to many other cells. If this "captured" cellular RNA is from a gene that stimulates cell growth, it then causes abnormal growth stimulation, leading to cancer. This process is termed "gene capture."
Short-Circuiting Normal Cell Growth Mechanisms
Normal cell growth is controlled by the availability of growth factors, which are hormone-like molecules that bind to specific receptors embedded in the surface membrane of cells. When this happens, the receptor stimulates a signaling cascade(growth signals)to tell cells to divide. Many gene products in this signaling pathway are proto-oncogenes that can become oncogenes when activated by the different mechanisms described above. When a signaling proto-oncogene is activated, the signaling cascade becomes "short-circuited" and cells behave as if they are continually stimulated by their growth factor.
For example, the v-sis oncogene from a monkey cancer virus known as simian sarcoma retrovirus (SSV) comes from a gene that stimulates growth of different cell types. Cells infected with SSV are, therefore, constantly bathed in the v-sis growth factor and stimulated to proliferate. Other oncogenes are mutated growth factor receptors where mutation leaves the receptor in the "on" status even in the absence of the growth factor. Two examples of mutated receptor onco-genes include v-erbB, found in a bird retrovirus that causes various cancers, and v-fms, which is carried by a mouse retrovirus that causes leukemia.
Inside the cell, connect cell surface growth receptors to the nucleus also can cause cancer when their activity is altered by mutation or overexpression. The Ras proto-oncogene is an example of a signal-transmitting molecule inside cells that can mutate into an oncogene.
In the nucleus, these normal growth signals trigger other proteins, called transcription factors, that regulate gene expression needed for cell growth. Many transcription factors are proto-oncogenes. Two examples of proto-oncogene transcription factors are c-Fos and c-Jun, both of which were first identified as retroviral oncogenes.
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Carcinogen
A carcinogen is an agent that can cause cancer. Carcinogens can be chemicals, viruses, hormone, ionizing radiation, or solid materials.
Carcinogens produce cancer by changing the information that cells receive from their DNA, causing immature cells to accumulate in the body rather than differentiate into normal functional cells.
Carcinogens may be genotoxic: meaning that they interact physically with DNA to damage or change its structure,Ultraviolet light and ionizing radiation are genotoxic carcinogens.
Nongenotoxic carcinogens, or promoters Other carcinogens change how DNA expresses its information without changing its structure directly, or may create a situation in a cell or tissue that makes it more susceptible to DNA damage from other sources, Arsenic and estrogen are nongenotoxic carcinogens.
Still other carcinogens, such as nickel, may interfere with cell division, changing the number or structure of chromosomes in new cells after a cell divides.
The places that become altered in the DNA of cancer cells are called oncogenes and tumor suppressor genes. Oncogenes and tumor suppressor genes are particular locations on DNA that control a cell's ability to perform its biological functions and to control its growth.
Some carcinogens have been identified from studies of people exposed to various substances over time. These include cancer in cigarette smokers and leukemia in people breathing benzene in the workplace. Carcinogens have also been identified using laboratory animals exposed over time, usually to high doses. Saccharin was found to be a carcinogen through experiments to produce bladder cancer in rats, and aflatoxin was found to produce liver cancer in rats. Some substances that are carcinogens in laboratory animals, like saccharin, are not carcinogens in people because of differences in how they are metabolized or differences in how they produce cancer.
soot was acknowledged as one of the first carcinogenic agents. Soot is a complex mixture of chemicals that arises from the combustion of organic material. As scientists and physicians separated soot's individual components, it became clear that chemicals called polycyclic aromatic hydrocarbons (PAHs) were among its principal carcinogenic compounds. The story became even more intriguing when it was shown that many PAHs behave as procarcinogens. Procarcinogens do not cause cancer per se, but they can be converted to active carcinogens by enzymes located in organs like the liver and lung. The implications of this discovery are noteworthy. For example, cigarette smoke contains a wide variety of procarcinogenic PAHs that are turned into active carcinogens in lung cells. Since smokers draw these PAHs deep into their lungs with each inhale on a cigarette, one reason that cigarette smoking correlates so highly with the induction of lung cancer becomes very clear.
How do carcinogens cause cancer? many carcinogens, particularly chemicals and radiation, is that they act as mutagens. Mutagens are agents that generate changes in DNA, sometimes by reacting with the DNA building blocks, guanine, adenine, thymine, and cytosine, which results in damaged DNA. When such damage remains in chromosomes, genes are often mutated in a way that impairs their normal function and enhances cancer induction. Cells try to prevent such mutations by repairing DNA damage, but they are not always successful. In fact, some individuals are susceptible to hereditary skin and colon cancers because they lack the ability to remove damaged DNA from chromosomes.
A mutagen is not the same as a carcinogen. Carcinogens are agents that cause cancer. While many mutagens are carcinogens as well, many others are not.
There are two general classes of genes that contribute to malignant tumor formation when they are mutated by carcinogens: oncogenes and tumor suppressor genes.
1) Protooncogenes encode proteins that are often involved in regulating normal cell growth and division. When a proto-oncogene is mutated by exposure to a carcinogen, the protein it encodes may lose its ability to govern cell growth and division, often giving rise to the rapid, unrestrained cell proliferation that is characteristic of cancer. In such a case, the mutations in the protooncogene convert it into an actual oncogene.
Ex: proto-oncogene, called ras, which is an abbreviation for "rat sarcoma." "Ras" is written as Ras when biologists refer to the protein, and as ras when they refer to the gene that encodes the protein. The ras gene encodes Ras protein, which acts to regulate cell growth. Normally, Ras protein cycles between an "off" and "on" form. Many carcinogens induce mutations in the ras proto-oncogene, converting it to a ras oncogene, which encodes a form of the Ras protein that is locked in the "on" state. By abolishing Ras protein's regulatory off/on cycle, the accumulated mutations in the ras gene contribute to the formation of malignancies.
Not all oncogenes arise from mutations in normal cellular protooncogenes. In the early twentieth century, Peyton Rous discovered a carcinogenic virus that now bears his name, the Rous sarcoma virus. This virus harbors a gene called v-src (viral-sarcoma) that is a mutant form of a normal cellular proto-oncogene called c-src (cell-sarcoma). Like Ras protein, c-Src protein helps to regulate cell growth. When cells are infected by Rous sarcoma virus, the v-src gene, which is classified as an oncogene, High amounts of mutant v-Src protein encoded by the v-src oncogene are made in the cell, and they dominate the normal cellular c-Src protein, an event that contributes to abnormal cell growth and proliferation, eventually leading to cancer.
2) Tumor suppressor genes encode proteins that tend to repress cancer formation. When tumor suppressor genes are mutated by carcinogens, they often lose their ability to stem tumor formation, resulting in cancer. Some hereditary forms of breast cancer are linked to mutations in a tumor suppressor gene called BRCA-1. BRCA is derived from BReast CAncer. The BRCA-1 gene encodes BRCA-1 protein, which participates in controlling cell division, preventing cells from growing out of control, thus contributing to the suppression of tumor formation. Mutations in the BRCA-1 gene result in altered BRCA-1 protein that no longer functions correctly in cell-growth regulation, contributing to the formation of tumors, particularly in breast tissue.
Generally: oncogenes or tumor suppressor genes encode proteins (involved in cell growth). And finally mutated by (exposure to) carcinogen.
Reducing Exposure to carcinogen:
1)variety of toxicological assessments, including the Ames test, are used to identify potential mutagens and carcinogens. When possible, established carcinogens, such as asbestos, are removed from the environment, home, and workplace.
2) advising the use of sunblock to shield skin from the cancer-causing effects of ultraviolet radiation in sunlight.
TYPES OF CARCINOGENS:
1-radiation (alpha, beta, or gamma, and the energy), Carcinogenity of radiation depends of the type of radiation, type of exposure and penetration. For example, alpha radiation has low penetration and is not a hazard outside the body, but are carcinogenic when inhaled or ingested.
Not all types of electromagnetic radiation are carcinogenic. Low-energy waves on the electromagnetic spectrum are generally not, including radio waves, microwave radiation, infrared radiation, and visible light. Higher-energy radiation, including ultraviolet radiation (present in sunlight), x-rays, and gamma radiation, generally is carcinogenic, if received in sufficient doses.
2-Cooking food at high temperatures, for example broiling or barbecuing meats, can lead to the formation of minute quantities of many potent carcinogens that are comparable to those found in cigarette smoke (i.e., benzopyrene).[1] Charring of food resembles coking and tobacco pyrolysis and produces similar carcinogens. animal carcinogen Acrylamide. is generated in fried or overheated carbohydrate foods (such as french fries and potato chips).
The hazard symbol for carcinogenic chemicals in the Globally Harmonized System
Prepared by: Mohammed Hammdi Ali
The cell cycle is the process by which a cell grows, duplicates its DNA, and divides into identical daughter cells. -Also known as cell division cycle. In a typical cell cycle, the parent cell doubles its volume, mass, and complement of chromosomes, then sorts it's doubled contents to opposite sides of the cell, and finally divides in half to yield two genetically identical offspring.
- This idea fits well with the behavior of many unicellular organisms, but for multicellular organisms the daughter cells may differ from their parent cell and from each other in terms of size, shape, and differentiation state.
- Cell cycle duration varies according to cell type and organism. Embryonic cells that do not need to grow between divisions can complete a cell cycle in as little as 8 min, whereas it usually takes 10–24 hours of the most rapidly dividing somatic cells. Many somatic cells divide much less frequently; liver cells divide about once a year, postmitotic cells are incapable of cell division even after maximal stimulation, and include mature neurons, striated muscle cells, and heart muscle cells.
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Eukaryotic phases
-The cell cycle is divided into two main parts: interphase and mitosis
During interphase, the cell grows and replicates its chromosomes.
- Interphase subdivided into three phases: gap phase 1 (G1), synthesis (S), and gap phase 2 (G2). Interphase is followed by mitosis (nuclear division"karyokinesis", and cell division "cytokinesis"). In plants and algae, cytokinesis is accompanied by the formation of a new cell wall.
Gap 1 phase "G1": (G indicating gap or growth)
Gap phase 1 begins at the completion of mitosis and cytokinesis and lasts until the beginning of S phase. This phase is generally the longest of the four cell cycle phases and is quite variable in length. During this phase, the cell chooses either to replicate its DNA or to exit the cell cycle and enter a quiescent state (the G0 phase). Some conditions cause cells to enter the G0 phase.G0 phase: The term "post-mitotic" is sometimes used to refer to both quiescent and senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain quiescent for long periods of time, possibly indefinitely (as is often the case for neurons). This is very common for cells that are fully differentiated. Cellular senescence is a state that occurs in response to DNA damage or degradation that would make a cell's progeny nonviable; it is often a biochemical alternative to the self-destruction of such a damaged cell by apoptosis. Some cell types in mature organisms, such as parenchymal cells of the liver and kidney, enter the G0 phase semi-permanently and can only be induced to begin dividing again under very specific circumstances; other types, such as epithelial cells, continue to divide throughout an organism's life. p53 prevents the cell from replicating by stopping the cell cycle at G1, or interphase, to give the cell time to repair.
The Synthetic phase "S":
Replication of the chromosomes occurs (DNA synthesis phase), it takes 6 hours. During S phase, each chromosome replicates exactly once to form a pair of physically linked sister chromatids, in animal cells, a pair of centrioles is also duplicated during S phase.
Gaps 2 phase "G2":
Follows S phase is called gap phase 2. Some cells can exit the cell cycle from G2 phase, just as they can from G1 phase. Considered as preparation phase for mitosis and usually takes 4 hours. Significant protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis. Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis
The mitotic phase "M": mitosis.
Cell divisions phase, takes from 1 to 2 hours. It includes: nuclear division "karyokinesis" followed by cytoplasmic division "cytokinesis". During the mitosis (M) phase, the duplicated chromosomes are segregated, migrating to opposite poles of the cell. The cell then divides into two daughter cells, each having the same genetic components as the parental cell. Mitosis is divided into five stages: prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis usually begins during anaphase and ends at a point after the completion of mitosis.
Note: Mitosis is prevented if DNA damage has occurred or if genomic replication is not complete.
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Regulation of cell cycle:
including detecting and repairing genetic damage, and provision of various checks to prevent uncontrolled cell division.The mammalian cell cycle is regulated by a group of protein kinases called cyclin-dependent kinases (CDKs). These proteins catalyze the attachment of phosphate groups to specific serine or threonine amino acids in a target protein. The phosphate groups alter the target protein's properties, such as its interaction with other proteins. (The alteration of protein activity by the attachment of phosphate groups occurs frequently in cells.).
Cyclins form the regulatory subunits and CDKs the catalytic subunits of an activated heterodimer; cyclins have no catalytic activity and CDKs are inactive in the absence of a partner cyclin. When activated by a bound cyclin, CDKs perform a common biochemical reaction called phosphorylation that activates or inactivates target proteins to orchestrate coordinated entry into the next phase of the cell cycle.
The number of CDKs in a cell remains constant during the cell cycle, the level of cyclins oscillate, CDKs are constitutively expressed in cells whereas cyclins are synthesized at specific stages of the cell cycle, in response to various molecular signals.
There are G1 cyclins, S-phase cyclins, and G2/M cyclins, each of which interact differently with CDK subunits to regulate the various phases of the cell cycle.
Cyclin D is the first cyclin produced in the cell cycle, in response to extracellular signals (eg. growth factors). Cyclin D binds to existing CDK4, forming the active cyclin D-CDK4 complex. Cyclin D-CDK4 complex in turn phosphorylates the retinoblastoma susceptibility protein (RBp).
Cell cycle inhibitors
CDKs can also associate with inhibitory subunits called CDK inhibitors (CKIs). CKIs cause the cell cycle to halt " stop". Two families of CKIs have been identified, based on their amino acid sequence similarity and the specificity of their interactions with CDKs. One of the families of CKIs, the INK family, includes four proteins (p15, p16 p18 and p20). These CKIs exclusively bind complexes of cyclin D and CDK4, as well as complexes of cyclin D and CDK6, to block cells that are in the G1 phase of the cell cycle. The other family of CKIs, the Cip/Kip family, consists of three proteins (p21, p27, and p57). These inhibitors bind to all complexes of cyclins and CDKs that function during the G1 phase and during the transition from the G1 to the S phase. They act preferentially, however, to block the activity of complexes containing CDK2.
Active complexes of cyclins and CDKs exert their biological effects by phosphorylating proteins. During the G1 phase, a major target of cyclin/CDK complexes is the retinoblastoma protein (pRb). PRb is a growth-suppressing protein whose activity is controlled by whether or not it is phosphorylated.
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Deregulation and Cancer:
Deregulation of cell cycle control proteins plays a key role in the development of cancer. Overactivation of proteins that favor cell cycle progression, namely cyclins and CDKs, and the inactivation of proteins that impede cell cycle progression, such as CKIs, can result in uncontrolled cell proliferation.
In human tumors, it is genes encoding the proteins that control the transition from the G1 to the S phase that are most commonly altered. These genes include those for cyclins, CKIs, and pRb. Such mutations overcome the inhibitory effects of pRb on the cell cycle, causing cells to have a growth advantage. In some cancers, this occurs after the direct mutation of the pRb gene, resulting in the protein's loss of function. In a larger set of cancers, pRb is indirectly inactivated by the hyper-activation of CDKs. This may result from over expression of cyclins, from an activating mutation in CDK4, or from inactivation of CKIs.
There is much evidence to suggest that cyclins can act as oncogenes to induce cells to become cancerous.
In particular the G1 cyclins, cyclin D1, and cyclin E have been implicated in the development of cancer.
Cyclin D1 protein is frequently detected in human breast cancer, and increasing evidence suggests that cyclin E plays an important role in the pathogenesis of breast cancer.
CKIs antagonize the function of cyclins, and considerable evidence suggests that these proteins function as tumor suppressors. CKI function is often altered in cancer cells. The gene encoding p16, a protein that belongs to the INK family of CKIs, is mutated, deleted, or inactivated in a large number of human malignancies and tumors. Such alterations prevent the inhibition of cyclin D/CDK4 and cyclin D/CDK6 complexes during G1.
Decreased expression of p21 and p27, proteins that belong to the Cip/Kip family of CKIs, also has been demonstrated in numerous human tumors. In contrast to the genetic mutations observed with p16, the decrease in p27 levels in tumors is due to enhanced degradation of the p27 protein. One of the proteins required for the degradation of p27, Skp2, has oncogenic properties. Skp2 over expression is observed in several human cancers and likely contributes to the uncontrolled progression of the cell cycle by increasing the degradation of p27. Understanding of the fine details of cell cycle regulation is likely to lead to specific cancer therapies targeting one or more of these important proteins.
Role of cell cycle in tumor formation
A disregulation of the cell cycle components may lead to tumor formation. As mentioned above, some genes like the cell cycle inhibitors, RB, p53 etc., when they mutate, may cause the cell to multiply uncontrollably, forming a tumor. Although the duration of cell cycle in tumor cells is equal to or longer than that of normal cell cycle, the proportion of cells that are in active cell division (versus quiescent cells in G0 phase) in tumor cells are much more compared to that in normal cells. Thus there is a net increase in cell number as the number of cells that die by apoptosis or senescence remains the same.
The cells which are actively undergoing cell cycle are targeted in cancer therapy as the DNA is relatively exposed during cell division and hence susceptible to damage by drugs or radiation. This fact is made use of in cancer treatment; by a process known as debulking, a significant mass of the tumor is removed which pushes a significant number of the remaining tumor cells from G0 to G1 phase (due to increased availability of nutrients, oxygen, growth factors etc.). Radiation or chemotherapy following the debulking procedure kills these cells which have newly entered the cell cycle
Synchronization of cell cultures:
Several methods can be used to synchronise cell cultures by halting the cell cycle at a particular phase. For example, Serum starvation and treatment with Thymidine or Aphidicolin halt the cell in the G1 phase, Mitotic shake-off, treatment with colchicine and treatment with Nocodazole halt the cell in M phase and treatment with 5-fluorodeoxyuridine halts the cell in S phase.
Observation:
There are numerous ways to observe the cell cycle occurring. Onion bulbs or garlic root tips are often used.
A sample of root tip is fixed in a mixture of 99% of 70% aqueous industrial methylated spirit and 1% glacial ethanoic acid for two hours. Treat the root tips in 1 molar hydrochloric acid at 60C for 6 -7 minutes. Rinse thoroughly with water. Add Schiff's reagent and leave for one hour. Rinse again in distilled water. Observe under a microscope.
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Death is an inevitable fact of life for organisms
Apoptosis "programmed cell death" " PCD" "Dance of Death"
Apoptosis is a process by which cells in a multicellular organism commit suicide.
Apoptosis is a form of death that the cell itself initiates, regulates, and executes. using an elaborate arsenal of cellular and molecular machinery.
Apoptotic cell
Why Cells Commit Suicide:
Why do cells commit apoptosis? There are 2 major reasons:
First, apoptosis is one means by which a developing organism shapes its tissues and organs. For instance, a human fetus has webbed hands and feet early on its development. Later, apoptosis removes skin cells, revealing individual fingers and toes. A fetus's eyelids form an opening by the process of apoptosis. During metamorphosis, tadpoles lose their tails through apoptosis. In young children, apoptosis is involved in the processes that literally shape the connections between brain cells, and in mature females, apoptosis of cells in the uterus causes the uterine lining to slough off at each menstrual cycle. Between 50 billion and 70 billion cells die each day due to apoptosis in the average human adult. For an average child between the ages of 8 to 14, approximately 20 billion to 30 billion cells die a day.
Second, Cells may also commit suicide in times of distress,_ damaged beyond repair, infected with a virus_ for the good of the organism as a whole. For example, in the case of a viral infection, certain cells of the immune system, called cytotoxic T lymphocytes, bind to infected cells and trigger them to undergo apoptosis. Also, cells that have suffered damage to their DNA, which can make them prone to becoming cancerous, are induced to commit apoptosis.
More strikingly, they found that many of these genes were mutated in tumors from cancer patients. Other genes often found to be mutated in cancers are those which regulate the cell cycle, which is the complex set of processes controlling how and when cells divide. These two findings led cancer researchers to recognize that cancer, a disease of uncontrolled cell proliferation, can result either from too much cell division or not enough apoptosis.
A cell can be triggered to undergo apoptosis either by external signaling molecules, such as so-called "death activator" proteins, or through molecules that reside within the cell and monitor events that might commit the cell to suicide, such as damage to DNA. There are several biochemical pathways that lead to apoptosis. One of the major pathways involves inducing mitochondria to leak one of their proteins, cytochrome c, into the cystosol. This in turn activates a set of related proteases (enzymes that degrade proteins) called caspases. Ultimately, the caspases degrade proteins in the cell and activate enzymes that degrade other cell constituents, such as the DNA. Cells undergoing apoptosis exhibit characteristic morphological and biochemical traits, which can be recognized by microscopic examination or biochemical assays. Apoptosis can occur in as little as twenty minutes, after which the cell "corpse" typically becomes engulfed and completely degraded by neighboring phagocytic cells that are present in the tissue and attracted to the apoptotic cell.
The aberrant inhibition or initiation of apoptosis contributes to many disease processes, including cancer
apoptotic neutrophil undergoes a series of changes including violent membrane blebbing, called zeiosis, (animation to the right), chromatin condensation and fragmentation of DNA creating a vacuolar nucleus. Apoptotic cells shrink in size, break into smaller pieces called apoptotic bodies that other body cells recognize and eat it.
Apoptosis also plays a role in preventing cancer; if a cell is unable to undergo apoptosis, due to mutation or biochemical inhibition, it can continue dividing and develop into a tumour. For example, infection by papillomaviruses causes a viral gene to interfere with the cell's p53 protein, an important member of the apoptotic pathway. This interference in the apoptotic capability of the cell plays a critical role in the development of cervical cancer.
A) Section of mouse liver stained to B) section of mouse liver showing an
show cells undergoing apoptosis (orange) apoptotic cell indicated by an arrow
Homeostasis:
The number of cells is kept relatively constant through cell death and division.
Homeostasis is achieved when the rate of mitosis (cell division) in the tissue is balanced by cell death. If this equilibrium is disturbed, one of two potentially fatal disorders occurs:
* The cells are dividing faster than they die, effectively developing a tumor.
The cells are dividing slower than they die, which results in a disorder of cell loss.*
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Mechanism of apoptosis:
The process of apoptosis is controlled by a diverse range of cell signals, which may originate either extracellularly (extrinsic inducers) or intracellularly (intrinsic inducers).
Extracellular signals may include hormones, growth factors, nitric oxideHYPERLINK \l "wp-_note-NO" or cytokines, and therefore must either cross the plasma membrane or transduce to effect a response. These signals may positively or negatively induce apoptosis; in this context the binding and subsequent initiation of apoptosis by a molecule is termed positive, whereas the active repression of apoptosis by a molecule is termed negative.
Intracellular signals: a response initiated by a cell in response to stress, and may ultimately result in cell suicide. The binding of nuclear receptors by glucocorticoids, heat, radiation, nutrient deprivation, viral infection and hypoxia are all factors which can lead to the release of intracellular apoptotic signals by a damaged cell. A number of cellular components, such as poly ADP ribose polymerase, may also help regulate apoptosis.
Mitochondrial regulation:
1) Apoptotic proteins which target mitochondria affect them in different ways; they may cause mitochondrial swelling through the formation of membrane pores, or they may increase the permeability of the mitochondrial membrane and cause apoptotic effectors to leak out.
2) Nitric oxide (NO) is able to induce apoptosis by helping to dissipate the membrane potential of mitochondria and therefore make it more permeable
Roles of mitochondria in apoptosis:
Mitochondrial proteins known as SMACs (second mitochondria-derived activator of caspases) are released into the cytosol following an increase in permeability.
A)SMAC binds to inhibitor of apoptosis proteins (IAPs) and deactivates them, preventing the IAPs from arresting the apoptotic process and therefore allowing apoptosis to proceed.
B)IAP also normally suppresses the activity of a group of cysteine proteases called caspases, which carry out the degradation of the cell, therefore the actual degradation enzymes can be seen to be indirectly regulated by mitochondrial permeability.
C)Cytochrome c is also released from mitochondria due to increased permeability of the outer mitochondrial membrane, and serves a regulatory function as it precedes morphological change associated with apoptosis.
D) Once cytochrome c is released it binds with Apaf-1 and ATP, which then bind to pro-caspase-9 to create a protein complex known as an apoptosome.
E) The apoptosome cleaves the pro-caspase to its active form of caspase-9, which in turn activates the effector caspase-3.
The mitochondrial permeability is itself subject to regulation by various proteins, as Bcl-2 proteins, Bcl-2 proteins are able to promote or inhibit apoptosis by either direct action on mitochondrial permeability, or indirectly through other proteins. Importantly, the actions of some Bcl-2 proteins are able to halt apoptosis even if cytochrome c has been released by the mitochondria.
WHAT'S OCCUR DURING APOPTOSIS:
1-binding of specific signals proteins (death signals) (Extracellular signals) to specific receptors (death receptors) on the cell membrane: this binding initiate apoptosis,
Examples of these proteins (death signals) a)tumour necrosis factor (TNF)
b)Fas-ligand (Fas-L)
a) tumour necrosis factor (TNF):
- TNF is a cytokine produced mainly by activated macrophages (the major extrinsic mediator of apoptosis)
Most cells in the human body have two receptors for TNF: TNF-R1 and TNF-R2. -
-The binding of TNF to TNF-R1 has been shown to initiate the pathway that leads to caspase activation
- Binding of this receptor can also indirectly lead to the activation of transcription factors involved in cell survival and inflammatory responses
b) Fas-ligand (Fas-L):
-transmembrane protein
-bind to Fas receptor, this binding results in the formation of the death-inducing signaling complex (DISC),which contains caspase-8 and caspase-10.
- In some types of cells(type I) processed caspase-8 directly activates other members of the caspase family, and triggers the execution of apoptosis In other types of cells (type II), the Fas-DISC starts a feedback loop that spirals into increasing release of pro-apoptotic factors from mitochondria and the amplified activation of caspase-8.
- Following TNF-R1 and Fas activation in mammalian cells a balance between pro-apoptotic (BAX,[19] BID, BAK, or BAD) and anti-apoptotic (Bcl-Xl and Bcl-2) members of the Bcl-2 family is established.
- This balance is the proportion of pro-apoptotic homodimers that form in the outer- membrane of the mitochondrion.
- The pro-apoptotic homodimers are required to make the mitochondrial membrane permeable for the release of caspase activators such as cytochrome c and SMAC.
Mechanical change during apoptosis:
Cell shrinkage and rounding due to the breakdown of the proteinaceous cytoskeleton by caspases.
The cytoplasm appears dense, and the organelles appear tightly packed.
Chromatin undergoes condensation into compact patches against the nuclear envelope in a process known as pyknosis, a hallmark of apoptosis.
The nuclear envelope becomes discontinuous and the DNA inside it is fragmented in a process referred to as karyorrhexis. The nucleus breaks into several discrete chromatin bodies or nucleosomal units due to the degradation of DNA.
The cell membrane shows irregular buds known as blebs.
The cell breaks apart into several vesicles called apoptotic bodies, which are then phagocytosed.
Apoptosis progresses quickly and its products are quickly removed, making it difficult to detect or visualize. During karyorrhexis, endonuclease activation leaves short DNA fragments, regularly spaced in size. These give a characteristic "laddered" appearance on agar gel after electrophoresis. Tests for DNA laddering differentiate apoptosis from ischemic or toxic cell death.
Removal of dead cells:
-Dying cells that undergo the final stages of apoptosis display phagocytotic molecules, such as phosphatidylserine on their cell surface.
-Phosphatidylserine is normally found on the cytosolic surface of the plasma membrane, but is redistributed during apoptosis to the extracellular surface by a hypothetical protein known as scramblase.
- These molecules mark the cell for phagocytosis by cells possessing the appropriate receptors, such as macrophages.
- The phagocyte reorganizes its cytoskeleton (of dying cell) for engulfment of the cell. -The removal of dying cells by phagocytes occurs in an orderly manner without eliciting an inflammatory response
p53 dysregulation:
p53 prevents the cell from replicating by stopping the cell cycle at G1, or interphase, to give the cell time to repair, however it will induce apoptosis if damage is extensive and repair efforts fail. Logically, any disruption to the regulation of the p53 or interferon genes will result in impaired apoptosis and the possible formation of tumors.
Viral infection:
Viruses have developed strategies to evade apoptosis to permit their survival within the host cell. Most viruses encode proteins that can inhibit apoptosis.
Several viruses encode viral homologs of Bcl-2. These homologs can inhibit pro-apoptotic proteins such as BAX and BAK, which are essential for the activation of apoptosis. Examples of viral Bcl-2 proteins include the Epstein-Barr virus BHRF1 protein and the adenovirus E1B 19K protein.
Some viruses express caspase inhibitors that inhibit caspase activity and an example is the CrmA protein of cowpox viruses. Whilst a number of viruses can block the effects of TNF and Fas. For example the M-T2 protein of myxoma viruses can bind TNF preventing it from binding the TNF receptor and inducing a response.
Furthermore, many viruses express p53 inhibitors that can bind p53 and inhibit its transcriptional transactivation activity. Consequently p53 cannot induce apoptosis since it cannot induce the expression of pro-apoptotic proteins. The adenovirus E1B-55K protein and the hepatitis B virus HBx protein are examples of viral proteins that can perform such a function.
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Necrosis
"Traumatic cell death"
Death or disintegration of a cell or tissue due to disease, physical injury, or chemical injury.
Death of cells resulting from, for example, loss of blood supply, bacterial toxins, or physical or chemical agents.
Necrosis is less orderly than apoptosis "programmed cell death".
In contrast apoptosis, cleanup of cell debris by phagocytes of the immune system is generally more difficult, as the disorderly death generally does not send cell signals which tell nearby phagocytes to engulf the dying cell. This lack of signaling, makes it harder for the immune system to locate and recycle dead cells which have died through necrosis than if the cell had undergone apoptosis. The release of intracellular content after cellular membrane damage is the cause of inflammation in necrosis.
There are many causes of necrosis including exposure to injury, infection, cancer, infarction, poisons, bites from some spiders such as brown recluses and inflammation. Severe damage to one essential system in the cell leads to secondary damage to other systems, a so-called "cascade of effects". Necrosis is accompanied by the release of special enzymes, that are stored by lysosomes, which are capable of digesting cell components or the entire cell itself. The injuries received by the cell may compromise the lysosome membrane, or may initiate an unorganized chain reaction which causes the release in enzymes. Unlike apoptosis, cells that die by necrosis may release harmful chemicals that damage other cells. In biopsy, necrosis is halted by fixation or freezing.
Morphologic patterns:
There are seven distinctive morphologic patterns of necrosis:
· Coagulative necrosis is typically seen in hypoxic environments (e.g. myocardial infarction, infarct of the spleen). Cell outlines remain after cell death and can be observed by light microscopy.
· Liquefactive necrosis is usually associated with cellular destruction and pus formation (e.g. pneumonia). This is typical of bacterial or, sometimes, fungal infections because of their ability to stimulate an inflammatory reaction. Curiously, ischemia (restriction of blood supply) in the brain produces liquefactive rather than coagulative necrosis.
· Gummatous necrosis is restricted to necrosis involving spirochaetal infections (e.g. syphilis).
· Haemorrhagic necrosis is due to blockage of the venous drainage of an organ or tissue (e.g. in testicular torsion).
· Caseous necrosis is a specific form of coagulation necrosis typically caused by mycobacteria (e.g. tuberculosis).
· Fatty necrosis results from the action of lipases on fatty tissues (e.g. acute pancreatitis, breast tissue necrosis).
· Fibrinoid necrosis is caused by immune-mediated vascular damage. It is marked by deposition of fibrin-like proteinaceous material in arterial walls, which appears smudgy and eosinophilic on light microscopy
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Oncogene
Oncogenesis: conversion of well regulated cell into a cancerous cell.
Oncogene: Gene that can cause cancer, it is a sequence of DNA that has been altered or mutated from its original form, "the proto-oncogene".
Proto-oncogenes promote the specialization and division of normal cells. A change in their genetic sequence can result in uncontrolled cell growth, ultimately causing the formation of a cancerous tumour.
Oncogenes were first discovered in certain a chicken retrovirus and were later identified as cancer-causing agents in many animals.
Proto-oncogene
A proto-oncogene is a normal gene that can become an oncogene due to mutations. Proto-oncogenes code for proteins that help to regulate cell growth and differentiation. Proto-oncogenes are often involved in signal transduction and execution of mitogenic signals, usually through their protein products. Upon activation, a proto-oncogene (or its product) becomes a tumor inducing agent, "an oncogene". Examples of proto-oncogenes: RAS, WNT, MYC, and ERK.
Oncogenes forming by major 4 ways (3 due to change of proto-oncogenes to oncogenes,and 1 from viral infection)
In humans, proto-oncogenes can be transformed into oncogenes in three ways:
1)- single mutations (called point mutations) (alteration of a single nucleotide base pair), mutation within a proto-oncogene can cause a change in the protein structure.
2)-translocation (in which a segment of the chromosome breaks off and attaches to another chromosome). a proto-oncogene on one chromosome might be moved to another chromosome, For example, in the translocation between chromosomes 9 and 22 that is found in CML, a protooncogene on chromosome 9, called c-Abl, is moved to chromosome 22, where it is fused to another gene called Bcr. c-Abl is a nuclear enzyme called "tyrosine kinase," which adds a phosphate molecule to proteins at an amino acid called tyrosine. The fusion of Bcr and c-Abl genes creates an oncogene, called Bcr/Abl, which makes a highly overactive tyrosine kinase variant that is found in the cytoplasm instead of the nucleus. These changes in the activity and cellular location of the c-Abl proto-oncogene lead to chronic myelogenous leukemia.
3)-amplification (increase in the number of copies of the proto-oncogene). Oncogenes can be created by the localized amplification of small chromosomal domains, DNA amplification increases by several-fold a specific region of a chromosome. This can produce many copies of any genes that lie in the amplified region.
4)-oncogenes may transported by viruses that carry oncogenes in their genome and transferred into the cell by infection. .(viruses inserted into the chromosome near a specific gene.)
Although most human cancer is not associated with infectious agents such as viruses, some cancers have been shown to be caused at least in part by viruses.
The places that become altered in the DNA of cancer cells are called oncogenes and tumor suppressor genes. Oncogenes and tumor suppressor genes are particular locations on DNA that control a cell's ability to perform its biological functions and to control its growth.
ONCOGENIC VIRUSES: viruses cause cancers.
1)DNA viruses
Epstein-barr virus (EBV): may cause –Burkett's lymphoma.
_nasopharyngeal carcinoma.
Human papilloma virus (HPV): may cause cervical carcinoma.
Hepatitis B virus (HBV): may cause hepatocellular carcinoma.
Human herpes virus (HHV): may cause Kaposi sarcoma.
2)RNA viruses "retroviruses"
Hepatitis C virus (HCV): may cause hepatocellular carcinoma.
Human T cell lymphotropic virus: may cause T-cell leukemia.
retroviruses carry an RNA genome that, once inside a cell, is copied into DNA, which then is inserted randomly into the genome of a host cell. Some retroviruses are slow to cause tumors. After infection and spread to a large number of cells, a DNA copy of the viral genome, by chance, integrates into a host cell's DNA next to a normal gene that plays an important role in cell growth. If this viral integration disrupts the expression or structure of the normal cellular gene, it induces abnormal growth signals that can lead to cancer. Other retroviruses cause tumors to appear very quickly. In the process of copying viral RNA into DNA, RNA that is expressed from cellular genes can be mistakenly copied into the viral genome. The progeny of the virus transfer the cellular gene to many other cells. If this "captured" cellular RNA is from a gene that stimulates cell growth, it then causes abnormal growth stimulation, leading to cancer. This process is termed "gene capture."
Short-Circuiting Normal Cell Growth Mechanisms
Normal cell growth is controlled by the availability of growth factors, which are hormone-like molecules that bind to specific receptors embedded in the surface membrane of cells. When this happens, the receptor stimulates a signaling cascade(growth signals)to tell cells to divide. Many gene products in this signaling pathway are proto-oncogenes that can become oncogenes when activated by the different mechanisms described above. When a signaling proto-oncogene is activated, the signaling cascade becomes "short-circuited" and cells behave as if they are continually stimulated by their growth factor.
For example, the v-sis oncogene from a monkey cancer virus known as simian sarcoma retrovirus (SSV) comes from a gene that stimulates growth of different cell types. Cells infected with SSV are, therefore, constantly bathed in the v-sis growth factor and stimulated to proliferate. Other oncogenes are mutated growth factor receptors where mutation leaves the receptor in the "on" status even in the absence of the growth factor. Two examples of mutated receptor onco-genes include v-erbB, found in a bird retrovirus that causes various cancers, and v-fms, which is carried by a mouse retrovirus that causes leukemia.
Inside the cell, connect cell surface growth receptors to the nucleus also can cause cancer when their activity is altered by mutation or overexpression. The Ras proto-oncogene is an example of a signal-transmitting molecule inside cells that can mutate into an oncogene.
In the nucleus, these normal growth signals trigger other proteins, called transcription factors, that regulate gene expression needed for cell growth. Many transcription factors are proto-oncogenes. Two examples of proto-oncogene transcription factors are c-Fos and c-Jun, both of which were first identified as retroviral oncogenes.
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Carcinogen
A carcinogen is an agent that can cause cancer. Carcinogens can be chemicals, viruses, hormone, ionizing radiation, or solid materials.
Carcinogens produce cancer by changing the information that cells receive from their DNA, causing immature cells to accumulate in the body rather than differentiate into normal functional cells.
Carcinogens may be genotoxic: meaning that they interact physically with DNA to damage or change its structure,Ultraviolet light and ionizing radiation are genotoxic carcinogens.
Nongenotoxic carcinogens, or promoters Other carcinogens change how DNA expresses its information without changing its structure directly, or may create a situation in a cell or tissue that makes it more susceptible to DNA damage from other sources, Arsenic and estrogen are nongenotoxic carcinogens.
Still other carcinogens, such as nickel, may interfere with cell division, changing the number or structure of chromosomes in new cells after a cell divides.
The places that become altered in the DNA of cancer cells are called oncogenes and tumor suppressor genes. Oncogenes and tumor suppressor genes are particular locations on DNA that control a cell's ability to perform its biological functions and to control its growth.
Some carcinogens have been identified from studies of people exposed to various substances over time. These include cancer in cigarette smokers and leukemia in people breathing benzene in the workplace. Carcinogens have also been identified using laboratory animals exposed over time, usually to high doses. Saccharin was found to be a carcinogen through experiments to produce bladder cancer in rats, and aflatoxin was found to produce liver cancer in rats. Some substances that are carcinogens in laboratory animals, like saccharin, are not carcinogens in people because of differences in how they are metabolized or differences in how they produce cancer.
soot was acknowledged as one of the first carcinogenic agents. Soot is a complex mixture of chemicals that arises from the combustion of organic material. As scientists and physicians separated soot's individual components, it became clear that chemicals called polycyclic aromatic hydrocarbons (PAHs) were among its principal carcinogenic compounds. The story became even more intriguing when it was shown that many PAHs behave as procarcinogens. Procarcinogens do not cause cancer per se, but they can be converted to active carcinogens by enzymes located in organs like the liver and lung. The implications of this discovery are noteworthy. For example, cigarette smoke contains a wide variety of procarcinogenic PAHs that are turned into active carcinogens in lung cells. Since smokers draw these PAHs deep into their lungs with each inhale on a cigarette, one reason that cigarette smoking correlates so highly with the induction of lung cancer becomes very clear.
How do carcinogens cause cancer? many carcinogens, particularly chemicals and radiation, is that they act as mutagens. Mutagens are agents that generate changes in DNA, sometimes by reacting with the DNA building blocks, guanine, adenine, thymine, and cytosine, which results in damaged DNA. When such damage remains in chromosomes, genes are often mutated in a way that impairs their normal function and enhances cancer induction. Cells try to prevent such mutations by repairing DNA damage, but they are not always successful. In fact, some individuals are susceptible to hereditary skin and colon cancers because they lack the ability to remove damaged DNA from chromosomes.
A mutagen is not the same as a carcinogen. Carcinogens are agents that cause cancer. While many mutagens are carcinogens as well, many others are not.
There are two general classes of genes that contribute to malignant tumor formation when they are mutated by carcinogens: oncogenes and tumor suppressor genes.
1) Protooncogenes encode proteins that are often involved in regulating normal cell growth and division. When a proto-oncogene is mutated by exposure to a carcinogen, the protein it encodes may lose its ability to govern cell growth and division, often giving rise to the rapid, unrestrained cell proliferation that is characteristic of cancer. In such a case, the mutations in the protooncogene convert it into an actual oncogene.
Ex: proto-oncogene, called ras, which is an abbreviation for "rat sarcoma." "Ras" is written as Ras when biologists refer to the protein, and as ras when they refer to the gene that encodes the protein. The ras gene encodes Ras protein, which acts to regulate cell growth. Normally, Ras protein cycles between an "off" and "on" form. Many carcinogens induce mutations in the ras proto-oncogene, converting it to a ras oncogene, which encodes a form of the Ras protein that is locked in the "on" state. By abolishing Ras protein's regulatory off/on cycle, the accumulated mutations in the ras gene contribute to the formation of malignancies.
Not all oncogenes arise from mutations in normal cellular protooncogenes. In the early twentieth century, Peyton Rous discovered a carcinogenic virus that now bears his name, the Rous sarcoma virus. This virus harbors a gene called v-src (viral-sarcoma) that is a mutant form of a normal cellular proto-oncogene called c-src (cell-sarcoma). Like Ras protein, c-Src protein helps to regulate cell growth. When cells are infected by Rous sarcoma virus, the v-src gene, which is classified as an oncogene, High amounts of mutant v-Src protein encoded by the v-src oncogene are made in the cell, and they dominate the normal cellular c-Src protein, an event that contributes to abnormal cell growth and proliferation, eventually leading to cancer.
2) Tumor suppressor genes encode proteins that tend to repress cancer formation. When tumor suppressor genes are mutated by carcinogens, they often lose their ability to stem tumor formation, resulting in cancer. Some hereditary forms of breast cancer are linked to mutations in a tumor suppressor gene called BRCA-1. BRCA is derived from BReast CAncer. The BRCA-1 gene encodes BRCA-1 protein, which participates in controlling cell division, preventing cells from growing out of control, thus contributing to the suppression of tumor formation. Mutations in the BRCA-1 gene result in altered BRCA-1 protein that no longer functions correctly in cell-growth regulation, contributing to the formation of tumors, particularly in breast tissue.
Generally: oncogenes or tumor suppressor genes encode proteins (involved in cell growth). And finally mutated by (exposure to) carcinogen.
Reducing Exposure to carcinogen:
1)variety of toxicological assessments, including the Ames test, are used to identify potential mutagens and carcinogens. When possible, established carcinogens, such as asbestos, are removed from the environment, home, and workplace.
2) advising the use of sunblock to shield skin from the cancer-causing effects of ultraviolet radiation in sunlight.
TYPES OF CARCINOGENS:
1-radiation (alpha, beta, or gamma, and the energy), Carcinogenity of radiation depends of the type of radiation, type of exposure and penetration. For example, alpha radiation has low penetration and is not a hazard outside the body, but are carcinogenic when inhaled or ingested.
Not all types of electromagnetic radiation are carcinogenic. Low-energy waves on the electromagnetic spectrum are generally not, including radio waves, microwave radiation, infrared radiation, and visible light. Higher-energy radiation, including ultraviolet radiation (present in sunlight), x-rays, and gamma radiation, generally is carcinogenic, if received in sufficient doses.
2-Cooking food at high temperatures, for example broiling or barbecuing meats, can lead to the formation of minute quantities of many potent carcinogens that are comparable to those found in cigarette smoke (i.e., benzopyrene).[1] Charring of food resembles coking and tobacco pyrolysis and produces similar carcinogens. animal carcinogen Acrylamide. is generated in fried or overheated carbohydrate foods (such as french fries and potato chips).
The hazard symbol for carcinogenic chemicals in the Globally Harmonized System
Prepared by: Mohammed Hammdi Ali
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