History
Described first by Kerr et al., (1972).
Apoptosis derived from Greek "falling off" (as
for autumn leaves).
Importance
Essential for physiological and developmental process
 Embryonic development.
 Endocrine-dependent tissue atrophy,
 Normal tissue turnover,
 Sexual differentiation and clonal
selection of immune system.
Cells death types
There are two distinct modes of cell death:
1. Necrosis
Results from toxins and ischemia injury by agents
Affects cells in groups rather than singly
Evokes exudative inflammation when it develops.
2. Apoptosis
The cells undergo cytoplasmic shrinkage,
nuclear condensation and/or fragmentation.
membrane blebbing. Moreover,
inflammation does not occur while it is evident in necrosis.
the second p is pronounced as in sym-ptom.
Apoptosis and Cancer
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Apoptosis of DNA-damaged cells may
constitute a physiological antineoplastic
mechanism to protect the organism from
cancer development by eliminating cells
that might otherwise replicate the
damaged DNA and lead to mutations and
to cancer
What we will gain?
The understanding of discrete apoptotic
pathways will obviously contribute not only
to the explanations of the development of
cancer, but also the development of new
strategies toward the prevention and therapy
of the disease.
Genetic Alterations
Keeping apoptosis under control.
Activation mechanism
autoactivated protease cascade
Negative regulator
ced-9 and bcl-2 family proteins
Apoptosis switch
Cell Death
Apoptosis switch
Cell Death
the cell swells
components of the cell (e.g. the DNA in the nucleus) stay pretty much the same the
basic i
the cell disintergrates, leaving a mess behind nternal
tends to result from some some external force (e.g. an injury or infection
the cell swells
the basic internal components of the cell (e.g. the DNA in the nucleus)
stay pretty much the same
the cell disintergrates, leaving a mess behind
tends to result from some some external force (e.g. an injury or
infection
Lonising radiation effect DNA
damage and induces
P53 expression
I-V Evolutionary conserved region
transactivation
Oligomerisation
Binding sites on transcribed protein o
Mutation hotspot zones
5-terminus
Gene X
3-terminus
P53 Gene
Normal cells arrest in G1 or G2 phase
to effect DNA repair mechanisms
Cell cycles normally
if repear of gene X
is successful
Cancer cells do not arrest in G1 or G2
phase and replication of damaged DNA
results in an accumulation of mutations
Apoptosis occurs if repair of gene X is unsuccessful
prevent continuation of genetic defect and possible
development of tumour cell population
Death Receptor-ligand interactions
Apaf-1
Cytochrome C
Regulated by
cytokines and
hormones
Mitochondria
Cell Injury
Initiator Caspases
Execution Caspases
DNA
Fragmentation
Nucleus
• Cell death programmes, far from being sinister,
are essential to proper growth and development.
In order to make the spaces between your
fingers, for example, certain cells had to die and
be removed during your development.
• Apoptosis is particularly interesting to humans
because problems with cell death not going on
as planned are implicated in diseases like
Alzheimer's, and because the ability to target
and kill cells selectively would be of great benefit
in the treatement of diseases like cancers.
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But before we get too excited about this, does apoptosis occur in
single celled organisms?
it has been documented in bacteria (Yarmolinsky )1995
it has been documented in protozoans( Welburn et al. 1997)
I have published some evidence for it in phytoplankton (Berges and
Falkowski )1998
But why would a unicellular organism kill itself?
in a time of starvation, it would liberate nutrients for neighbouring
(genetically identical) cells (altruism; Yarmolinsky )1995
it is a response to disease, i.e. a way to prevent a viral infection from
spreading to other cells (sacrifice in the face of foes; Mittler and Lam 1996)
it reflect a degree of 'social organisation' among unicellular populations
(Raff 1992)
there is no reason: it is just an obscure pathway that developed in
unicellular organisms and simply persisted until multicellular organisms
arose, when it became useful
APOPTOSIS: Overall scheme
General Causes
External
Exogenous oxidative stress: Ischemia (Mild)
Excitotoxins: Glutamate; Ion fluxes
Trophic factors: Reduced receptor occupancy
Stimuli: TNF-a; Corticosteroids (Lymphoid system)
Toxins: b-Amyloid
Internal
Toxic substances: Mutated SOD1
Byproducts of cell metabolism
Cell signals
Cell stress
Mitochondrial permeability increased
Release of cytochrome c
Extracellular signal: Ligand-Receptor
Caspase activation
Expression of "Death" genes: Reversible
Terminal apoptotic changes: Irreversible
Cell Death
External link: R&D
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APOPTOSIS: Disease related proteins
Autoimmune lymphoproliferative syndrome
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Tumor necrosis factor ligand superfamily, Member 6 (TNFSF6; CD95L; CD178) : Also Systemic lupus
erythematosus, susceptibility
Tumor necrosis factor receptor superfamily, Member 6 (TNFRSF6; FAS antigen; CD95)
Caspase 8
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Caspase 10 (Apoptosis-related cysteine protease)
Neoplasms
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Tumor protein p53
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Lymphadenopathy ,splenomegaly & defective CD95-induced apoptosis of peripheral blood lymphocytes
Recurrent sinopulmonary & herpes simplex virus infections
Poor responses to immunization
Defects in activation of T lymphocytes, B lymphocytes, & natural killer cells leading to immunodeficiency
Li-Fraumeni syndrome
Other neoplasms: Pancreatic; Hepatocellular ;Histiocytoma ;Osteosarcoma; Breast; Nasopharyngeal
Tumor necrosis factor receptor superfamily, Member 6 (TNFRSF6; FAS antigen; CD95) : Squamous cell
Tumor necrosis factor receptor superfamily, Member 10B (TNFRSF10B) : Squamous cell
BCL2-associated X protein (BAX) : T-Cell acute lymphoblastic leukemia; Colorectal
Mouse disorders
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Caspase 9 : Knockout
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Enlarged & malformed cerebrum: Caused by reduced apoptosis during brain development
For every cell, there is a time to live and a time to die.
• There are two ways in which cells die:
• they are killed by injurious agents
• they are induced to commit suicide
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Death by injury
Cells that are damaged by injury, such as by
mechanical damage
exposure to toxic chemicals
undergo a characteristic series of changes:
they (and their organelles like mitochondria)
swell (because the ability of the plasma
membrane to control the passage of ions and
water is disrupted)
• the cell contents leak out, leading to
• inflammation of surrounding tissues
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Death by suicide
Cells that are induced to commit suicide:
shrink
have their mitochondria break down with the release of cytochrome c
develop bubble-like blebs on their surface
have the chromatin (DNA and protein) in their nucleus degraded
break into small, membrane-wrapped, fragments
The phospholipid phosphatidylserine, which is normally hidden within the plasma
membrane is exposed on the surface.
This is bound by receptors on phagocytic cells like macrophages and dendritic cells
which then engulf the cell fragments.
The phagocytic cells secrete cytokines that inhibit inflammation.
The pattern of events in death by suicide is so orderly that the process is often called
programmed cell death or PCD. The cellular machinery of programmed cell death
turns out to be as intrinsic to the cell as, say, mitosis. Programmed cell death is also
called apoptosis. (There is no consensus yet on how to pronounce it; some
say APE oh TOE sis; some say uh POP tuh sis.)
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Why should a cell commit suicide?
There are two different reasons. 1. Programmed cell death is as needed for proper development as mitosis
is.
Examples:
The resorption of the tadpole tail at the time of its metamorphosis into a frog occurs by apoptosis.
The formation of the fingers and toes of the fetus requires the removal, by apoptosis, of the tissue between them.
The sloughing off of the inner lining of the uterus (the endometrium) at the start of menstruation occurs by
apoptosis.
The formation of the proper connections (synapses) between neurons in the brain requires that surplus cells be
eliminated by apoptosis
2. Programmed cell death is needed to destroy cells that represent a threat to the integrity of the
organism.
Examples:
Cells infected with viruses
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Cells of the immune system
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One of the methods by which cytotoxic T lymphocytes (CTLs) kill virus-infected cells is by inducing apoptosis [diagram of the
mechanism]. (And some viruses mount countermeasures to thwart it.)
As cell-mediated immune responses wane, the effector cells must be removed to prevent them from attacking body constituents.
CTLs induce apoptosis in each other and even in themselves. Defects in the apoptotic machinery is associated with autoimmune
diseases such as lupus erythematosus and rheumatoid arthritis.
Cells with DNA damage
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Damage to its genome can cause a cell
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to disrupt proper embryonic development leading to birth defects
to become cancerous.
Cells respond to DNA damage by increasing their production of p53. p53 is a potent inducer of apoptosis. Is it any wonder that
mutations in the p53 gene, producing a defective protein, are so often found in cancer cells (that represent a lethal threat to the
organism if permitted to live)?
Cancer cells
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Radiation and chemicals used in cancer therapy induce apoptosis in some types of cancer cells
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The Mechanisms of Apoptosis
There are 3 different mechanisms by which a cell commits suicide by apoptosis.
one generated by signals arising within the cell
another triggered by death activators binding to receptors at the cell surface.
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a third that may be triggered by dangerous reactive oxygen species.
1. Apoptosis triggered by internal signals: the intrinsic or mitochondrial pathway
In a healthy cell, the outer membranes of its mitochondria express the protein Bcl-2 on their surface.
Bcl-2 is bound to a molecule of the protein Apaf-1.
Internal damage to the cell (e.g., from reactive oxygen species) causes
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cytochrome c
Apaf-1
caspase 9
(and ATP)
is called the apoptosome.
These aggregate in the cytosol.
Caspase 9 is one of a family of over a dozen caspases. They are all proteases. They get their name because they
cleave proteins - mostly each other - at aspartic acid (Asp) residues).
Caspase 9 cleaves and, in so doing, activates other caspases.
The sequential activation of one caspase by another creates an expanding cascade of proteolytic activity (rather
like that in blood clotting and complement activation) which leads to
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Bcl-2 to release Apaf-1;
a related protein, Bax, to penetrate mitochondrial membranes, causing cytochrome c to leak out.
The released cytochrome c and Apaf-1 bind to molecules of caspase 9.
The resulting complex of
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TNF-a
Lymphotoxin
Fas ligand (FasL)
digestion of structural proteins in the cytoplasm
degradation of chromosomal DNA and
phagocytosis of the cell
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Apoptosis and Cancer
Some cancer-causing viruses use tricks to prevent apoptosis of the cells they have transformed.
Several human papilloma viruses (HPV) have been implicated in causing cervical cancer. One of them
produces a protein (E6) that binds and inactivates the apoptosis promoter p53.
Epstein-Barr Virus (EBV), the cause of mononucleosis and a cause of Burkitt's lymphoma
– produces a protein similar to Bcl-2
– produces another protein that causes the cell to increase its own production of Bcl-2. Both these
actions make the cell more resistant to apoptosis (thus enabling the cancer cell to continue to
proliferate).
Even cancer cells produced without the participation of viruses may have tricks to avoid apoptosis.
Some B-cell leukemias and lymphomas express high levels of Bcl-2, thus blocking apoptotic signals they
may receive. The high levels result from a translocation of the BCL-2 gene into an enhancer region for
antibody production. [Discussion].
Melanoma (the most dangerous type of skin cancer) cells avoid apoptosis by inhibiting the expression of
the gene encoding Apaf-1.
Some cancer cells, especially lung and colon cancer cells, secrete elevated levels of a soluble "decoy"
molecule that binds to FasL, plugging it up so it cannot bind Fas. Thus, cytotoxic T cells (CTL) cannot kill
the cancer cells by the mechanism shown above.
Other cancer cells express high levels of FasL, and can kill any cytotoxic T cells (CTL) that try to kill them
because CTL also express Fas (but are protected from their own FasL).
The modes of cell death
Necrosis
A pathological response to cellular injury.
Chromatin clumps
Mitochondria swell and rupture
Plasma membrane lyses
Cell contents spill out
General inflammatory response is triggered.
Apoptosis
Apoptosis: a normal physiological response to specific suicide
signals, or lack of survival signals.
When?
In embryonic and fetal development:
• Tissue developmental programs which control sculpting of embryonic
form.
• Developmental organization of the nervous system;
• Elimination of self-reactive components of the immune system.
In the adult:
• On stimulation by T-lymphocytes.
• In response to DNA damage or abnormality, e.g. by radiation or viral
infection or transformation.
• In certain organs and tissues, on withdrawal of supporting hormones
In addition, there are often apoptotic centres in tumours, accounting for
the paradox of slow gross enlargement in the face of rapid cell
proliferation, and the rare spontaneous remission.