Apoptosis -Cell aging and death

Question 1

What is the theory of aging?

Answer 1

• All humans die
• The safest age is around puberty [10-15 yrs old have lowest risk of dying]
• Death rate increase with age

Question 2

Accumulated mutation hypothesis.

Answer 2

• Cells accumulate mutations as they age and that leads to lethal damage eventually.
• Somatic mutations tend to accumulate during aging
• No direct evidence that these mutations cause aging.
• Despite enormous mutation load there is no acceleration in aging, hiroshima and nagasaki

Question 3

Telomere depletion hypothesis.

Answer 3

fibroblast cells
growing in tissue culture divided a limited number of
times
• After 50 population doubling, cell division stops with
cell cycle blocked before DNA replication
• If cell sample is taken after 20 divisions and frozen it
will resume growth for 30 more doubling and then
stops.

• Hayflick limit explained by Cooke = At the ends of chromosomes, there is an extra length of DNA called telomeres.
• Shorter TTAGGG in old somatic cells

Question 4

After 50 cell cycles, what happens to the telomeres?

Answer 4

Eventually, after 50 cycles, the telomere cap used up
and cell line would enter senescence.

-Cooke’s postulate in 1998 showed that telomerase builds the TTAGGG caps and causes Telomeres to not deplete. Caused cells not to senesce at the Hayflick limit

Question 5

What do cancer cells avoid?

Answer 5

They avoid Telomeric shortening.

Question 6

Wear and tear hypothesis.

Answer 6

-Cells wear out over time, accumulating damage until they are
no longer able to function.
- There is evidence that cells accumulate damage
-e.g Free radicals. They are very reactive and cause damage to the cell.
-Most damaging free radical reactions is the glycation
of glucose to collagen and elastin which is not replaced
-accum = reduced flexibility esp in joints.

Question 7

Gene clock hypothesis.

Answer 7

• Some aspects of aging are genetically controlled.
• Genes regulate body development and also the rate of aging.
• Mutations in genes can cause premature ageing in the young. e.g Progeria [retarded growth]
• Atherosclerosis and strokes lead to death by 12.

Question 8

Mechanisms of cell death.

Answer 8

Cells die by either: 1. Necrosis or 2. Apoptosis.

Question 9

Necrosis

Answer 9

Cells damaged by acute injury. e.g. No oxygen supply or poisoned by toxins. They then die by necrosis or ‘accidental’ cell death.

• The cells burst and trigger the inflammatory response.
• This response is natural and desired but in this case it can be life threatening.

Question 10

Apoptosis

Answer 10

-programmed cell death .
- Is a type of cell death that occurs during the normal
turnover of cells in the adult and also under a number
of other circumstances in healthy cells
-examples are:
1. Cell death in bone marrow and intestines every hour
2. Excess neurons during development
3. Loss of webbing between fingers in embryo
4. Epi lining of the GIT [death after fixed time]
5. Shedding of endometrium during menstrual cycle
6. Death of active mature lymphocytes.

Question 11

The Apoptotic Process

Answer 11

1. Cells withdraws from neighboring cells and shrink
2. Nuclear chromatin condenses against nuclear
   membrane
3. Cell breaks up into several membrane bound fragments
   called apoptotic bodies
4. Apoptotic bodies contain tightly packed cellular
   organelles and some genetic material
5. Apoptotic bodies phagocytosed by local macrophages
   # No inflammatory response

Question 12

Apoptosis can also occur after chronic?

Answer 12

Apoptosis also occurs after chronic cell injury through
viruses, toxins, and genetic mutation
– But no potentially harmful inflammation

Question 13

What are some of the triggers for apoptosis?

Answer 13

1. heat
2. radiation
3. nutrient deprivation
4. viral infection
5. hypoxia
6. increased intracellular calcium concentration

Question 14

Intracellular Proteolytic Cascade

Answer 14

Depends on proteases that have cysteine at their
active site and cleave target proteins at aspartic
acids
• Called Caspases
• Synthesized as inactive precursors – procaspases
• Activated by cleavage of aspartic acids by other
caspases
• Once activated they cleave and activate other
procaspases –amplification of proteolytic cascade

Question 15

Caspase cascade

Answer 15

Activated caspases then cleave other key proteins in
the cell
• Cleaves nuclear lamins – causes irreversible
breakdown of the nuclear lamina
• Cleaves an inactive DNA degrading enzyme/protein
which releases DNase to lyse DNA in the nucleus
• The cell thereby dismantles itself safely and neatly
• Cell remnants engulfed and digested by other cells
• Cascade once started is irreversible.

Question 16

Activation of Caspase cascade

Answer 16

Adaptor Proteins
• All nucleated cells have the ability to undergo
apoptosis
• The process has to be tightly controlled to ensure
apoptosis only occurs when needed
• Adaptor proteins bring procaspases together –the
procaspases cleave themselves in this conformation
and activate themselves and further procaspases
• This amplifies the downstream death signal.

Question 17

The extrinsic pathway of the Proteolytic cascade

Answer 17

Activation of death receptors on cell surface can also
cause procaspase activation
• Killer lymphocytes produce a protein called Fas ligand
• Binds to death receptor Fas on surface of target cell
which recruits intracellular adaptor proteins which
accumulate the procaspase-8 molecules
• Tumour necrosis factor produced by macrophages
induces a TNF-receptor associated death domain
(TRADD) that initiates direct and indirect apoptosis
• Procaspase 8 cleaves and activates each other and
activate downstream procaspases to induce apoptosis
• Some stressed or damaged cells produce their own Fas
and Fas ligand to trigger an intracellular cascade.

Question 18

the intrinsic pathway of the Proteolytic cascade

Answer 18

Stressed or damaged cells can also release proteins
that cause procaspase activation from within
• Best known is when mitochondria release
cytochrome c into the cytosol
• Cytochrome c binds to and activates Apoptotic
protease activating factor (Apaf-1) which triggers
caspase cascade via procaspase 9
• Used in most forms of apoptosis to initiate or
accelerate apoptosis
• DNA damage initiates p53 to promote expression of
proteins that cause release of cytochrome c
• Bcl-2 family of proteins

Question 19

What are the regulators of Apoptosis?

Answer 19

Bcl-2 Family of intracellular proteins regulate
procaspases
• Some inhibit apoptosis eg Bcl-2 and Bcl-XL
inhibit
release of Cytochrome c
• Some are promoters eg. Bad bind to and inactivate
the death inhibitors
• Bax and Bak stimulate release of cytochrome c
• If genes for Bax and Bak are inactivated cells
become resistant to most apoptotic signals
• Inhibitors of Apoptosis (IAPs) – bind to procaspases
to inhibit their activation and bind to caspases to
inhibit activity

Question 20

Apoptotic triggers

Answer 20

• FAS pathway
– Cell surface receptors called death receptors
– Bind to FAS ligands which are autocrine or paracrine signals
– Induces activation of caspase 8 which activates cascades of
other caspases which eventually breaks down chromatin
• The BAX pathway
• Caspase-8 activates the channel protein Bax which is
inserted into the mitochondrial membrane
• Releases cytochrome c
• Cytochrome c activates cytoplasmic proteases
• P53- When sufficient DNA damage occurs to a cell, it
triggers its own demise, p53 pathway activates pro-apoptotic pathways to trigger apoptosis

Question 21

Failure of Apoptosis

Answer 21

Apoptosis is closely regulated and balanced by
cellular growth preventing both shrinkage and
excessive growth of the body
• Abnormalities of apoptosis is directly responsible
for some cancers
• Inhibitors of apoptosis can cause inflammatory
diseases, autoimmune diseases, viral infections
• Neurodegenerative disorders such as Alzheimer’s
disease and autoimmune diseases also related to
defective apoptotic processes
• Drugs in cancer therapy target apoptotic pathways

Question 22

HIV and apoptosis

Answer 22

• HIV enzymes deactivate Bcl-2 and activate
procaspase-8
• Increase proteins that mediate Fas apoptosis
• Decrease the amount of CD4 glycoprotein on the cell
membrane
• Released viral particles induce apoptosis in nearby
cells
• HIV decreases the production of molecules that
mark a cell for apoptosis, giving the infected cell time to
to replicate virus
• Infected CD4+ cell receives death signal from
cytotoxic T cell