بسم الله الرحمن الرحيم Apoptosis

1. بسم الله الرحمن الرحيمApoptosis Added by Engi Seif El-Eslam Shaker

2. Cell deathIs irreversible loss of cell function (the point of no return) We have different forms of cell death • Programmed cell death Cell death that occurs at a genetically scheduled time that typically mediates architectural changes during organogenesis and morphogenesis • OncosisPassive cell death caused by accidental factors such as ischemia and toxins & accompanied by cytoplasmic& nuclear swelling, blebbing ,loss of plasma membrane integrity & release of cellular contents into extracellular medium & often ppt significant immune response • NecrosisChanges that occur to a cell after death is reached, by any mechanism either oncosis or apoptosis

3. Apoptosis It is derived from the Greek Word (Apo=from, Ptosis= fall) 1)Def: • A genetically encoded program that, when activated, leads to cell death ccc by morphological & chemical changes & can be considered as a counter part of mitosis. • Apoptosis usually affect scattered individual cell (while necrosis affects cell group or whole tissues or organ compartments ) • When apoptosis fails to occur ,the resultcan be dysregulation of tissue homeostasis. • Signals of apoptosis are typically non-toxicin contrast to oncosis which is passive cell death caused by accidental factors such as ischemia and toxins.

4. 2)Changes that occur in the cell during Apoptosis: • Loss of cell adhesion • Cell shrinkage • Chromatin condensation & aggregation in the periphery in homogenous dense mass against the nuclear membrane • Karyorrhexis of the nucleus • Cells frequently develop cytoplasmic process that contain condensed nuclear fragments (Buds) • Separation of buds l.t formation of apoptotic bodies which are engulfed by nearby monocytes ,macropages, epithelial cells or vascular endothelium • Phagocytosis of apoptotic bodies does not induce the release of proteolytic enzymes or generation of reactive O2 metabolites .So removal of apoptotic bodies does not incur any inflammatory response. • Cytoplasmic changes occur late in apoptosis. Cells undergo apoptosis within 4 Hours but morphological features of apoptosis may be visible only during a fraction of this time

5. Differences between Apoptotic& Oncotic Necrosis

6. 3)Phases of Apoptosis : I. Induction phase: During theses phase ,the cell receive combination of signals that can elicit different responses( either induction or prevention of apoptosis) II. Effector phase: At this phase a point of no return is passed & cells become irreversibly programmed to death The decision process between death & life is influenced by a series of different protooncogenes & onco suppressor gene “bcl-2 gene family is the most important ones” Interleukin -1B converting enzyme like protease &protease antagonists may act during this phase III. Degradation phase: The process that give rise to manifestations of apoptosis

7. 4) Mechanism by which apoptosis occur: There are 3 different pathways by which a cell commits suicide A)Apoptosis triggered by internal signals In a healthy cell ,the outer membrane of mitocondria , the ER & the nuclear envelope express the protein Bcl-2 on their surface. Bcl-2 binds a moleculeof the protein Apaf-1, which is it self bound to a molecule of caspase 9 Internal damage to a cell causes (1)Bcl-2 to release Apaf-1 (2)A related protein ,Bax, to penetrate mitocondrial membrane causing cytochrome c to leak out Then the released cytochrome c & Apaf-1bind to molecule of caspase 9 . The resulting complex cytochrome c /Apaf-1 /caspase 9 (& ATP ) is called Apoptosome. then caspase 9 cleaves & activate other caspase .The sequential activation of one cascade by another creates an expanding cascade of proteolytic activity which leads to: (1)Digestion of structural protein in cytoplasm (2)Degradation of chromosomal DNA & death of cell

8. B) Apoptosis triggered by external factors: Occurs through binding to the complementary cell activator (Fasl &TNF respectively )transmits a signal to the cytoplasm that leads to activation of caspase 8 which intiates a cascade of caspase activation leading to phagocytosis of cell . C) Apoptosis inducing factors: Neuron have another way of self destruct unlike 2 methods described before in that it does not use caspase instead using (AIF) w is protein located normally in mitochondria & when the cell receive a signal telling it is time to die . AIF is released from the mitochondria & migrate into nucleus &binds to DNA w triggers the destruction of DNA & cell death

9. Rate of Cell Proliferation v v Rate of Cell death Disorder of cell accumulation Homeostasis Effects of different rates of cell death Disorder of cell loss

10. 5)Regulation of Apoptosis : A-Triggers of Apoptosis : I-Physical Inducers: • Removal of essential trophic factors e.growth factor deprivation • TNF family e.g Fas / Apo-1 are important inducer of apoptosis in the immune system • TGF-B • IL-4,IL-5,IL-10 • Topical steroid or elevated endogenous glucocorticoids Apoptosis

11. II-Damage related inducers • Free radicals & ROS • Bacterial Toxins • Viral infection • Oncogenes ,there are two categories of oncogenes: • 1st: Some oncogenes, when expressed, function as inducers of cell proliferation (e.g. myc) and some, when lost, cause depression of cell proliferation (e.g. p53, Rb). • 2nd: The other category of oncogenes prevents apoptosis when either overexpressed (e.g. bcL2) or when deleted (e.g. p53). • Therapy associated inducers • Ionizing & UVradiation • Chemotherapeutic drugs e.g apoptosis induced by DNA reactive drugs e.g Cisplatin & nitrogen mustard • cAMP elevators e.g Erythromycin at 10mg/ml &above • Tumor suppressor gene P53

12. Tumor suppressor gene P53 • functions as a tumor suppressor and as aregulator of apoptosis. • It has a role in cell response to DNA damage (e.g. from radiation exposure), causing an otherwise proliferating cell to enter into G1 arrest. It is at this pause that a cell decides whether to repair the DNA damage (allowing it to reenter the cell cycle), or commit suicide (if the damage is too great).

13. B-Inhibitors of Apoptosis : • Some of Bcl-2 Family especially bcl-2 &bcl-xl E.g basal cells, typically express bcL-2, which protects as them against apoptotic signals. Terminally differentiated cells (suprabasilar keratinocytes) loose bcL-2 expression. Melanocytes are protected against UV-induced apoptosis by expression of high-level bcL-2. • Hormones e.g Estrogen & androgen • Pharmacological Agents e.g Cystine protease inhibitor • Tumor promoters e.g α- hexachlorocyclohexane

14. 6)Physiological Role of Apoptosis : • Maintenance of Homeostasis • Elimination of unwanted cell population • Removal of damaged cells w is essential in maintenance of functional integrity of many organs • Dynamic balance among the highly changing & interactive cell population of immune system

15. 7)Overview of apoptosis in skin biology andskin diseases: 1)Sunburn cells: After UVR exposure,keratinocytes undergo apoptosis and forms large, rounded intraepidermal bodies which are phagocytosed efficiently and rapidly. Numerous sunburn cells are visible at 24 hs, but absent at 72 hrs of sun exposure. However ,melanocytes are protected against UV-induced apoptosis by expressionofhigh levels of bcL-2.

16. 2)Lichenoid tissue reactions: e.g. LP, LE,erythema multiforme, pityriasis lichenoid, FDE and GVHD. The Civatte bodies, a characteristic finding in lichenoid disorders, represents a large apoptotic body fragment which is extruded into the underlying upper dermis and phagocytosed.

17. 3)Skin tumors: A neoplasm represents unregulated tissue growth caused by a breakdown of normal homeostasis between cell proliferation and cell death. bcL-2 (a marker of inhibition of apoptosis) is not observed in SCC or its precursors, but is present in high proportion of BCC,melanomas and melanocytic naevi. P53 functions as a tumor suppressor and as a regulator of apoptosis. Cells lacking functional p53, duplicate their DNA before repair is complete and increasing the probability that they will become neoplastic.

18. 4)Immune system dysregulation Inappropriate persistence of inflammatory cells because of defective apoptosis could lead to chronic inflammatory disorders, e.g. elevated levels of granulocyte-macrophage colony-stimulating factor in atopic persons inhibit IL-4-mediated monocyte apoptosis, potentially fostering the chronicity of AD. Autoimmune disease occurs when the body fails to eliminate self-reactive immune cells. Fas antigen, a member of the TNF receptor superfamily, is expressed in many cell types, including keratinocytes and mediates apoptosis. Lacking Fas l.t loosing the ability to induce apoptosis of autoreactive T cells SLE.

19. 5)Hair disorders: Regression of hair follicle occurswhen a large majority of the cells synchro­nouslyundergo apoptosis. Apoptosis is important in most types of alopecias, e.g. AA, AGA. 6)Viral infectens: several viruses may prevent apoptosis of the infected cells survival of the virus, e.g. HPV expresses factor E6 p53 degration hindering apoptosis in the basal cell. 7)Wound healing: Aberrant apoptosis may play a role in pathologic wound healing (e.g. keloids, hypertrophic scars and pyogenic granuloma).

20. 8)Psoriasis: Decreased Apoptosis may be related to increase cell proliferation in psoriasis 9)Apoptosis & AIDS What cause disappearance of CD4+T cell in AIDS?? HIV invades CD4+ & one might assume that that it is the infection by HIV that cause great dying off of CD4+.However this does not appear to be the main culprit because fewer than 1 in 100,000 CD4+ T cell in Bl. of AIDS pt are actually infected with the virus . So ,What kill so many uninfected CD4+cells?? The answer is the Apoptosis but the exact mech. is not clear but mostly d.t expression of HIV gene in HIV infected cell cause cell to express high level of (Fasl) at its surface while preventing an interaction with its own Fas from causing it to self destruct. However, when the infected T cell encounters an uninfected one , the interaction of Fasl with Fas on the uninfected cell kills it by apoptosis

21. 8)Therapeutic and clinical applications ofPapoptosis: • Chemotherapeutics make neoplastic cellsmore susceptible to apoptosis. E.g: Paclitaxel (Taxol) induces apoptosis in malignant melanoma by interfering with mitosis. • PUVA selectively induces apoptosis in lymphocytes at doses that do not harm keratinocytes that is why it is effective in CTCL and psoriasis.

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