Apoptosis is a biological mechanism which is
one type of programmed cell death.
Apoptosis is used by multicellular organisms
to remove cells that are not needed by the
Apoptosis has distinctive morphological
characteristics such as plasma membrane
blebbing, cell shrinkage, chromatin
condensation and DNA fragmentation, and
begins with the enzymes of the caspase
proteases and form a complex group of
Cysteine protease activation of multi sub-unit
Improve this chart Apoptosis Necrosis
Natural: Yes caused by factors external to the
cell or tissue, such as infection,
toxins, or trauma
Effects: Beneficial as well as detrimental Detrimental
Introduction: Apoptosis programmed cell death
(PCD) in humans & multicellular
organisms. PCD involves a series
of biochemical events leading
to a cell destruction and death.
Necrosis is the premature death
of cells and living tissue.
result: Can prevent tumor formation
(homeostasis between cell death
rate and mitosis rate)
Necrosis results in inflammation,
which could become chronic.
definition: programmed cell death the cell or tissue damage due to
process: membrane blebbing, shrinkage of
cell, nuclear collapse, apoptopic
body formation. Then, engulf by
white blood cells
membrane disruption, respiratory
poisons and hypoxia which cause
ATP depletion, metabolic
collapse, cell swelling and rupture
leading to inflammation.
Programmed cell destruction in
embryonic development for the
purpose of sculpting of tissue.
Mouse paws, for example, are sculpted
by cell death during embryonic
They start out as spadelike structures
and the individual digits separate only
as the cells between them die.
• In other cases, cells die when the
structure they form is no longer
• When a tadpole changes into a frog,
Sculpting the digits in the developing mouse
paw by apoptosis.
Apoptosis during the
metamorphosis of a
tadpole into a frog. As
a tadpole changes into
a frog, the cells in the
tadpole tail are induced
to undergo apoptosis;
as a consequence, the
tail is lost. All the
changes that occur
including the induction
of apoptosis in the tail,
are stimulated by an
increase in thyroid
hormone in the blood.
In adult tissues, cell death exactly balances cell
division. If this were not so, the tissue would
grow or shrink. If part of liver is removed in an
adult rat for ex liver cell proliferation increase to
make up the loss.
Conversely, if a rat is treated with the drug
phenobarbital which stimulates liver cell
division (and thereby liver enlargement) and
then the phenobarbital treatment is stopped.
Apoptosis in the liver greatly increases until the
liver has returned to its original size, usually
within a week or so.
Thus, the liver is kept at a constant size
through the regulation of both the cell death
rate and the cell birth rate.
Hormonal regulation of
physiological cell turnover and
Physiological cell turnover plays an important
role in maintaining normal tissue function and
architecture. This is achieved by the dynamic
balance of cellular regeneration and
elimination, occurring periodically in tissues
such as the uterus and mammary gland, or at
constant rates in tissues such as the
gastrointestinal tract and adipose tissue.
Apoptosis has been identified as the
prevalent mode of physiological cell loss in
Cell turnover is precisely regulated by the
interplay of various endocrine and
They modulate tissue and cell-specific
responses on proliferation and apoptosis,
either directly, or by altering expression
and function of key cell proliferative and/or
Several similarities exist among the
various tissues with regard to the
intermediates that regulate tissue
homeostasis, enabling a better
understanding of the general mechanisms
involved in the process.
Age‐related thymic involution
The thymus continues to grow between birth and
puberty and then begins to atrophy. This thymic
involution is directed by the high levels of
Proportional to thymic size, thymic activity (T-cell
output) is most active before puberty.
Upon atrophy, the size and activity are dramatically
reduced, and the organ is primarily replaced
with fat (a phenomenon known as "organ
involution“). The atrophy is due to the increased
circulating level of sex hormones, and chemical or
physical castration of an adult results in the thymus
increasing in size and activity.
Patients with the autoimmune disease Myasthenia
gravis commonly (70%) are found to have thymic
hyperplasia or malignancy.
Age related thymic
Birth about 15 grams;
puberty about 35 grams
twenty-five years 25 grams
sixty years less than 15 grams
seventy years as low as 5 grams
Cell death by apoptosis during
involution of the lactating
breast in mice and rats.
The role of cell death in involution of lactating
breast was investigated in mice and rats by light
and electron microscopy.
Apoptosis, recognized by sharply demarcated
compaction of chromatin against the nuclear
envelope and by shrinkage and budding of the
whole cell to form membrane-bounded apoptotic
bodies, was responsible for major loss of cells in
In the mouse, rapid involution during the first 2
days was associated with shedding of large
numbers of apoptotic bodies derived from
alveolar epithelial cells into alveolar lumens.
This was followed by more gradual regression,
during which the bodies were mostly
phagocytosed by macrophages within the
Apoptosis of myoepithelial cells was observed in
mice, the resulting apoptotic bodies being
phagocytosed by intraepithelial macrophages,
but was not detected in rats.
Apoptosis of capillary endothelial cells caused
rapid regression of the capillary beds in both
mice and rats.
Intraepithelial macrophages increased in
number during involution, developed
cytoplasmic lipofuscin pigment, and either
remained within the epithelium or migrated to
the interstitium and regional nodes.
Cell loss by apoptosis has been demonstrated
during involution and atrophy of a variety of
Apoptosis in human
endometrium and endometriosis
Apoptosis plays a critical role in maintaining
tissue homeostasis and represents a normal
function to eliminate excess or dysfunctional
Apoptosis helps to maintain cellular
homeostasis during the menstrual cycle by
eliminating senescent cells from the
functional layer of the uterine endometrium
during the late secretory and menstrual
phase of the cycle.
The BCL‐2 family and Fas/FasL system
have been extensively studied in human
endometrium and endometriotic tissues.
The machinery responsible for apoptosis
depends on the family of proteases that
have a cysteine at their active site and
cleave their target proteins at specific
The activation of a group of enzymes
belonging to the cysteine protease family
named caspases. The “c” of “caspase” refers to
a cysteine protease, while the “aspase” refers to
the enzyme’s unique property to cleave after
aspartic acid residues.
Caspases, or cysteine-aspartic
proteases or cysteine-dependent aspartate-directed
proteases are a family of cysteine proteases that
play essential roles in apoptosis (programmed cell
death), necrosis, and inflammation.
Caspases are essential in cells for apoptosis, or
programmed cell death, in development and most
other stages of adult life, and have been termed
"executioner" proteins for their roles in the cell.
Some caspases are also required in the immune
system for the maturation of lymphocytes. Failure of
apoptosis is one of the main contributions
to tumour development and autoimmune diseases;
this, coupled with the unwanted apoptosis that
occurs with ischemia or Alzheimer's disease, has
stimulated interest in caspases as potential
therapeutic targets since they were discovered in
TYPES OF CASPASES
As of November 2009, twelve caspases have been identified in
There are two types of apoptotic caspases: initiator (apical)
caspases and effector (executioner) caspases.
(1) Initiator caspases
CASP2, CASP8, CASP9, and CASP10
They cleave inactive pro-forms of effector caspases, thereby
CASP3, CASP6, CASP7
These in turn cleave other protein substrates within the cell, to trigger
the apoptotic process. The initiation of this cascade reaction is
regulated by caspase inhibitors.
CASP 1,CASP4 ,CASP 5, CASP 11, CASP 12 , which are
overexpressed in some cases of vitiligo and associated autoimmune
diseases caused by NALP1 variants, are not currently classified as
initiator or effector , because they are inflammatory enzymes that are
Location: short arm of chromosome 17
Subcellular location: Nucleus (kinetochore).
These are essential for cell growth regulation and
apoptosis induced by genotoxic and non-genotoxic
In normal unstressed cells, the level of p53 protein
is downregulated via the binding of proteins such
as MDM2, COP1, JNK that promote p53
degradation via the ubiquitin/proteasome pathway.
As most of these genes are up regulated by p53,
this lead to a regulation loop that will keep p53
level very low in a normal cells.
After genotoxic or non-genotoxic stresses,
Downstream signalling includes a large series
of genes that are activated by the
transactivating properties of p53. This occurs
via specific DNA binding of the p53 protein to a
p53 response element (p53 RE) that isfound
either in the promoter or in the intron of target
Regardless of the type of stress, the final
outcome of p53 activation is either cell cycle
arrest and DNA repair or apoptosis, but the
mechanism leading to the choice between
these fates has not yet been elucidated .
The p53 pathways has been conveniently
divided into five parts :
p21 is a potent cyclin dependent kinase inhibitor
(CKI). The p21 protein binds to and inhibits the
activity of cyclin- CDK2, -CDK1, and -
CDK4/6complexes, and thus functions as a
regulator of cell cycle progression at G1and S
Location: Chromsome no. 6(in humans).
Subcellular location: Cytoplasm , nucleus.
P21 is transcriptionally activated by p53.
Sometimes p21 is expressed without being
induced by p53.
This kind of induction plays a big role in p53
independent differentiation which is promoted by
. Expression of p21 is mainly
dependent on two factors:
1) stimulus provided
2) type of the cell.
Growth arrest by p21 can promote
p21 therefore prevents cell
Mouse double minute 2 homolog (MDM2)
also known as E3 ubiquitin-protein ligase.
Mdm2 is a protein that in humans is encoded
by the MDM2 gene.
Mdm2 is an important negative regulator of
the p53 tumor suppressor.
Mdm2 protein functions both as an E3
ubiquitin ligase that recognizes the N-terminal
trans-activation domain (TAD) of
the p53 tumor suppressor and
an inhibitor of p53 transcriptional activation.
The key target of Mdm2 is the p53 tumor
suppressor. Mdm2 has been identified as a
p53 interacting protein that represses p53
Mdm2 is a p53 responsive gene—that is, its
transcription can be activated by p53. Thus
when p53 is stabilized, the transcription of
Mdm2 is also induced, resulting in higher
Mdm2 protein levels.
Location: chrmosome no. 12
Subcellular location: Nucleus , Nucleoplasm
The serine-threonine protein kinase AKT1.
Location: chromsome no. 14.
Subcellular location: Cytoplasm, Cell
AKT1 and the related AKT2 are activated
by platelet-derived growth factor. The activation
is rapid and specific, and it is abrogated by
mutations in the pleckstrin homology domain of
It was shown that the activation occurs
through phosphatidylinositol 3-kinase.
In the developing nervous system AKT is a
critical mediator of growth factor-induced
Survival factors can suppress apoptosis in a
transcription-independent manner by
activating the serine/ threonine kinase AKT1,
which then phosphorylates (pAkt) and
inactivates components of the apoptotic
Mice lacking Akt1 display a 25% reduction in
body mass, indicating that Akt1 is critical for
transmitting growth promoting signals, most
likely via the igf1 receptor.
Mice lacking Akt1 are also resistant to cancer:
they experience considerable delay in tumor
growth initiated by the large T antigen or the
Neu oncogene . A single-nucleotide
polymorphism in this gene causes Proteus
Apoptosis Triggered via
2. Extrinsic pathway
bcl-2 (B-cell lymphoma 2), encoded by
the BCL2 gene, is the founding member of
the Bcl-2 family of regulator proteins that
regulate cell death .
Damage to the Bcl-2 gene has been identified as
a cause of a number of cancers including -
* breast and prostate cancer ,
* chronic lymphocytic leukaemia,
* lung cancer.
a possible cause of autoimmunity. It is also a
cause of resistance to cancer treatments.
Antibodies to Bcl-2 can be used with immuno
histochemistry to identify cells containing the
Caspases are regulated at a post-translational
level, ensuring that they can be rapidly activated
They are first synthesized as inactive pro-
caspases, that consist of a prodomain, a small
subunit and a large subunit.
initiator caspases possess a longer prodomain
than the effector caspases, whose prodomain is
The prodomain of the initiator caspases contain
domains such as a CARD domain (e.g.,
caspases-2 and -9) or a death effector
domain (DED) (caspases-8 and -10) that enables
the caspases to interact with other molecules
that regulate their activation.
These molecules respond to stimuli that cause the
clustering of the initiator caspases. Such clustering
allows them to activate automatically, so that they can
proceed to activate the effector caspases.
The caspase cascade can be activated by:
granzyme B (released by cytotoxic T
lymphocytes and NK cells), which is known to activate
caspase-3 and -7
death receptors (like Fas , TRAIL receptors and TNF
receptor ), which can activate caspase-8 and -10
the apoptosome (regulated by cytochrome c and
the Bcl-2 family), which activates caspase-9
Apoptosis involves the degradation of cellular components
by a group of cysteine proteases called caspases.
The first mechanism through which the caspases get
activated is –intrinsic pathway .
(which means mitochondrial mediated).
Pathway is characterized by permeabilization of the
then the release of cytochrome c into the cytoplasm
Cytochrome c forms a multi protein complex called
Initiates the activation of the caspases cascade through
Intrinsic pathway (damage):
Cytochrome c release
Pro-caspase 9 cleavage
Pro-execution caspase (3) cleavage
Caspase (3) cleavage of cellular proteins,
Tumors arises more from intrinsic pathway
Mitochondrial proteins are known as SMAC’s and
they are released into the cytosol following an
increase in permeability.
Cytochrome c is released from mitochondria due
to formation of a channel, ie MAC , in the outer
mitochondrial membrane .
Once the cytochrome is released , it binds to
APAF-1 and ATP , then it binds to pro caspase 9
to create a protein complex called as
the apoptosome cleaves the pro-caspase to its
active form of caspase9 , which in turn activates
the effector caspase3.
The intrinsic signalling pathway involves non-receptor–
mediated intracellular signals.
inducing activities in the mitochondria that initiate
Stimuli for the intrinsic pathway include viral infections or
damage to the cell by toxins, free radicals, or radiation.
Damage to the cellular DNA can also induce the
activation of the intrinsic pathway for programmed cell
These stimuli induce changes in the inner mitochondrial
membrane that result in the loss of transmembrane
potential, causing the release of pro-apoptotic proteins
into the cytosol
Pro-apoptotic proteins activate caspases that mediate
the destruction of the cell through many pathways.
These proteins also translocate into the cellular
nucleus, inducing DNA fragmentation, a hallmark
The regulation of pro-apoptotic events in the
mitochondria occurs through activity of members of
the Bcl-2 family of proteins and the tumor suppressor
Members of the Bcl-2 family of proteins may be pro-
The anti-apoptotic proteins are Bcl-2, Bcl-x, Bcl-xL,
Bcl-XS, Bcl-w, and BAG. Some of these proteins are
currently under investigation as potential targets for
Pro-apoptotic proteins include Bcl-10, Bax, Bak,
Bid, Bad, Bim, Bik, and Blk.
It has been suggested that up regulation of these
proteins or their increased activation may offer an
approach for cancer therapy.
Cellular pathways that modulate the activities of the
p53 protein are also currently being evaluated as
targets for potential anticancer therapies
The extrinsic pathway is activated by death receptors on
the plasma membrane such as TNF receptor1 and CD95.
As ligands bind to these receptors , the death inducing
signalling complex (DISC) is formed leading to initiation of
the caspase cascade through caspase 8.
Molecules that stimulate the activity of these pro-apoptotic
proteins or activate these receptors are currently under
evaluation for their therapeutic potential in the treatment of
The signal transduction of the extrinsic pathway involves
several caspases which are proteases with specific cellular
Once activated, the caspases affect several cellular
functions as part of a process that results in the death of