Mechanisms of Cell Death: Apoptosis, Autophagy, and Necrosis Flashcards
Programmed cell death
-i.e. death by suicide
Apoptosis
Catabolic process involving the degradation of a cell’s own components through the lisosomal machinery
-i.e. Death by self-cannibalism
Autophagy
Defined as the premature death of cells by external factors
-i.e. Death by accident or murder
Necrosis
Most of what we know about apoptosis was discovered in
C. elegans
An integral part of both plan and animal tissue development
Apoptosis
In C. elegans, out of 1090 somatic cells, 131 of them underwent apoptosis. The different cells died at different times in
-remarkably accurate
Development
In normal development, apoptosis is required for
Tissue sculpting
Apoptosis is important in the immune system after infection for for eliminating used
T and B cells
A balance between proliferation and cell death is required to maintain
Tissue homeostasis
Crucial for eliminating cells that have been damaged by UV, radiation, chemical toxins, or viral infection
-i.e. it can prevent cancer
Apoptosis
An example of apoptosis function during tissue development is the development of the
Fingers and toes
Problems in apoptosis during finger and toe development results in
-2 or more fingers/toes are joined together
Syndactily
Lack of apoptosis during embryo development also disrupts
Normal brain development
Humans who are born with tails have deficiencies in
Apoptosis
What are five morphological markers of apoptosis?
- ) Electron dense nucleus
- ) Nuclear fragmentation
- ) Large, clear vacuoles
- ) Blebs at the surface
- ) Loss of cell adhesion
What is one very common biochemical marker of apoptosis?
Phosphatidylserine flipping from inner leaflet to outer leaflet
Phosphatidylserine flipping from inner leaflet to outer leaflet enables
Apoptotic cells to enter cell
Enables us to track nicks in DNA
Terminal Transferase dUTP Nick-End Labeling (TUNEL)
Used to add labeled dUTP to 3’ terminal end of the DNA fragments
Terminal Transferase
Highly sensitive and can detect fewer than 100 cells
-Fast: can be completed in 3 hours
TUNEL Assay
Has high reproducibility with good precision
TUNEL Assay
One of the disadvantages of TUNEL Assays are that we don’t know the minimum number of strand breaks necessary for detection. Thus we may miss the
Early stages of apoptosis
Designed to detect apoptotic cells that undergo extensive DNA degradation during the late stages of apoptosis
-Based on the ability of dUPT to label blunt ends of double-stranded DNA breaks independent of a template
Terminal Transferase dUTP Nick-End Labeling (TUNEL Assay)
Can generate false positives in a TUNEL assay
Necrotic cells
Another disadvantage of a TUNEL assay is that the detergent used to permeabilize cells can make them
Frgile
A distinctive feature of DNA degraded by caspase-activated DNAse (CAD)
DNA laddering
Cleaves genomic DNA at internucleosomal linker regions, resulting in DNA fragments that are multiples of 180-185 bps in length
Caspase-activated DNAse (CAD)
Must be cleaved to become active
Caspases
What are the two major apoptotic pathways?
- ) Cell-extrinsic pathway
2. ) Cell-intrinsic pathway
The key enzymes that give rise to all of the morphological and biochemical changes arising from apoptosis
Caspases
What are the two types of apoptotic caspases?
- ) Initiator (apical) caspases (#’s 2, 8, 9, and 10)
2. ) Effector (executioner) caspases (#’s 3, 6, and 7)
Regulated at a post-translational level, ensuring that they can be rapidly activated
Caspases
What are the steps of the caspase cascade?
Pro-apoptotic stimulus —> initiator caspases —> effector caspases —> apoptosis
Initiator caspase is activated by
2 proteolytic cleavages
Initiator caspase then cleaves and activates
Effector caspase
Cleaves nuclear lamins, inhibitors of DNAases, and Actin
Effector Caspases
The intrinsic apoptosis pathway is also known as the
Mitochondrial mediated pathway
The extrinsic apoptosis pathway is also known as the
Death receptor mediated pathway
Family of proteins that regulates the integrity of the mitochondrial membrane
BCL2
What are the two pro-apoptotic BCL2 proteins?
BAX and BAK
What are the three anti-apoptotic BCL2 proteins?
BCL2, BCL-XL, and MCL1
The regulators/sensors of the intrinsic pathway are “BH3-only proteins” and include
BAD, BID, and PUMA
Characterized by permeabilisation of the mitochondria and release of cytochrome c into the cytoplasm
Intrinsic apoptotic pathway
Forms a multi-protein complex known as the ‘apoptosome’ and initiates activation of the caspase cascade through caspase 9
Cytochrome C
Cytochrome C activates
-activates caspase 9
Apoptotic protease activating factor 1 (APAF1)
Antagonism of BCL2 causes the activation of the BAX/BAK channel which causes the mitochondria to release
Cytochrome C
Responsive to DNA damage by UV radiation
-one of the most studied genes of all time
p53
Can activate DNA repair proteins, arrest cell at G1/S checkpoint, and initiate apoptosis
p53
p53 is a transcription factor that regulates
Cell cycle and apoptosis
Acts as a ubiquitin ligase and covalently attaches ubiquitin to p53, thus marking it for degredation
Mdm2
Also transports p53 from nucleus to cytosol
Mdm2
The critical event leading to the activation of p53 is
Phorphorylation of its N-terminus
The protein kinases known to target this transcriptional activation domain of p53 can be broken down into what two groups?
- ) Members of MAPK family
2. ) Checkpoint kinases
Oncogenes also stimulate p53 activation, mediated by the protein
p14ARF
Contains many pro-apoptotic proteins such as cytochrome C
Mitochondria
Can be release from the mitochondria in response to pro-apoptotic signals
Cytochrome C
Translocate to the mitochondrial membrane and stimulate the formation of pores allowing Cytochrome c to leak out
-normally found in the cytosol
Pro-apoptotic molecules (i.e. Bad, Bax, Bid)
Interacts with APAF-1 to form the apoptosome
Cytochrome C
Interaction between cytochrome C and APAF-1 activate
-promotes caspase cascade
Caspase 9
Promote apoptosis by tying up Bcl2 and freeing Bax/Bak
Bid and Bad
Binds Bax/Bak and prevents pore formation
-anti-apoptotic
Bcl-2
Promote pore formation, release of cytochrome C and APAF1, and initiation of apoptosis
Bax and Bak
Results from a chromosomal translocation between the 14th and 18th chromosomes, which places the BCL-2 gene next to the IgG heavy chain locus
Follicular Lymphoma
The fusion gene of BCL-2 and IgG heavy chain leads to
High expression of BCL-2
High expression of BCL-2 then results in a decreased propensity of cells for undergoing
Apoptosis
Occurs in several cancers and is linked to poor disease outcome
Increased Bcl-2 protein expression
Triggered by the p53 tumor-suppressor in
response to DNA damage and other types of severe cell stress
Intrinsic pathway
Conventional anticancer therapies, such as chemotherapy, activate the intrinsic pathway via
p53
p53 activates the intrinsic pathway through transcriptional upregulation of pro-apoptotic members of the BCL2 family of proteins such as
PUMA and Bax
The extrinsic apoptotic pathway is apoptosis via the pro-apoptotic receptors
DR4 and DR5 (Fas receptors)
Activate the pro-apoptotic receptors DR4 and DR5
Endogenous Apo2L/TRAIL
The activated pro-apoptotic receptors D4 and D5 then recruit
Fas-associated death domain (FADD)
Recruits initiator caspase 8 and/or 10 to the death-inducing signaling complex (DISC)
FADD
The DISC activates caspases 8 and 10 and releases them into the cytoplasm, where they activate
Caspases 3, 6, an 7
Triggers apoptosis in response to the activation of pro-apoptotic receptors, such as DR4 and DR5, by specific pro-apoptotic ligands, such as Apo2L/TRAIL
Extrinsic apoptotic pathway
The extrinsic apoptotic pathway stimulates apoptosis independently of
p53
Ligand-induced activation of DR4 and DR5 leads to the rapid assembly of the death-inducing signaling complex (DISC) and the recruitment of initiator caspases 8 and 10 through the adaptor
Fas-associate death domain (FADD)
The extrinsic and intrinsic apoptosis pathways converge on
Caspases 3, 6, and 7
Also, caspases 8 and 10 (extrinsic) activate Bid,which activates
Bax and Bak (intrinsic)
Responsible for degrading cellular proteins and organelles and recycling them
-A survival pathway
Autophagy
Autophagy is actually a response to
Cellular starvation
Occurs in many disease states, including cancer, inflammatory bowel disease, and neurodegenerative disorders
Disregulation of Autophagy
The sequestration of cellular organelles into cytoplasmic autophagic vacuoles (autophagosomes) that fuse with lysosomes and digest the enclosed material
Autophagy
What are the four stages of autophagy?
- ) Induction
- ) Autophagosome formation
- ) Autophagosome-lysosome fusion
- ) Autophagosome breakdown
A key regulator of autophagic induction is
-has an inhibitory affect
mTOR (mammalian target of rapamycin)
A serine-theronine kinase
mTOR
Many of the genes that are upregulated in response to mTOR deactivation participate in
Autophagosome formation
Involves formation of a membrane around a targeted portion of the cell
Autophagosome formation
In autophagosome-lysosome fusion, the autophagosome fuses w/ the lysosome releasing its contents into the lysosome for
Degredation by proteases
Inevitably breaks down the autophagosome
Lysosome
Has contrasting roles during cancer development, progression, and treatment
Autophagy
Typically caused by factors external to the cell or tissue, such as infection, toxins, or trauma
Necrosis
Unlike apoptosis and autophagy, is almost always detrimental and can be fatal
Necrosis
What are the five types of necrosis?
- ) Coagulation necrosis
- ) Liquefactive necrosis
- ) Enzymatic fat necrosis
- ) Caseous necrosis
- ) Gangrenous necrosis
Cell destruction leading to escape of hydrolases
Liquefactive necrosis
Typically begins with cell swelling, chromatin digestion, and disruption of the plasma and organelle membranes
Necrosis
Charactrized by extensive DNA hydrolysis, vacuolation of the endoplasmic reticulum, organelle breakdown, and cell lysis
Late necrosis
What is the cause of inflammation in necrosis?
Release of intracellular contents after plasma membrane rupture