Download LESSON 2.5 WORKBOOK

LESSON 2.5 WORKBOOK
How do cells die?
DEFINITIONS OF TERMS
Anti-oxidants – a molecule that
inhibits the activity of reactive
oxygen species preventing them
damaging DNA.
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.5
Cells can have three fates: They can divide, they can differentiate or they can die.
But while cell death following trauma is a passive process cells also have a built-in
program to actively destroy themselves, called apoptosis. In a normal cell, prosurvival signals prevent cells destroying themselves, but when a cell ages or when
its DNA becomes too damaged to repair, the cell can no longer respond to prosurvival signals are and the apoptosis program is switched on. This lesson investigates what happens during apoptosis and how cancer cells can cheat death.
Cell responses to damage: apoptosis
With the exception of stem cells that can replicate indefinitely, cells, like people have finite lifespans. For
most cells lifespan is dictated by the amount of DNA damage they accumulate. Each time a cell divides
its DNA, errors in replication cause the DNA to mutate randomly. External events can also damage
DNA in addition to the normal errors in replication it accumulates at each cell division. Normally the cell
will stop cell cycle progression temporarily so that DNA repair proteins can fix any DNA damage. DNA
repair enzymes are not 100% successful however, and some mutations will escape, eventually leading to
functional defects that are the equivalent of cellular aging. As cells age they will acquire so many mutations
that they can no longer function normally at all. Then the cell will die.
What kind of external events damage DNA? Cellular DNA is particularly vulnerable to the byproducts of the
normal respiration processes cells need to survive: When oxygen is broken down in the cell it produces
reactive oxygen species (ROS), compounds that react with DNA causing mutations or breaks in DNA.
Any compound that behaves like ROS in this way are called mutagens. Some, but not all carcinogens are
mutagens. Another significant mutagen that originates outside the cell is the UV radiation in sunlight. UV
radiation is obviously particularly dangerous for skin cells. The DNA damage that mutagens like ROS and
UV radiation cause is one of the major causes of cellular aging. As a result many so-called anti-aging products feature anti-oxidants, compounds that eliminate the ROS produced by oxidation reactions, thereby
hopefully delaying the aging process. Similarly certain foods (like blueberries) that are high in anti-oxidants
are recommended because they can scavenge the ROS produced as a result of cellular metabolism,
thereby staving off cellular aging.
Notes:
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LESSON READINGS
DEFINITIONS OF TERMS
Apoptosis – the process of programmed cell death in multicellular organisms characterized by
cell shrinkage, nuclear fragmentation, degradation of proteins,
and release of cellular “blebs”.
This is an organized and planned
death of a cell.
Necrosis – the premature death
of a normal cell in which it bursts,
randomly releasing fragments of
proteins and DNA into surrounding tissue. This unplanned death
is disorganized.
Phagocytic cells – cells that are
responsible for consuming foreign
particles, bacteria, and cellular
blebs. These cells break down
contents of what they consume
and recycle them for other cells
to use.
Wo r k b o o k
Lesson 2.5
The more mutations cellular DNA
accumulates, whether due to errors
of replication or damage by ROS
or other mutagens the more likely
they are to prevent the cell from
functioning normally, and the more
unlikely DNA repair mechanisms
will be able to fix them during the
cell cycle. At that point the cell will
activate a program to kill itself called
apoptosis.
Figure 1: An apoptotic cell (a) and a necrotic cell
(b) show exhibit two types of cell death. Apoptosis is
a regulated dismantling of the cell, while necrosis is an
unregulated explosion of cellular contents.
During apoptosis, the cell shrinks
itself down in size, and produces enzymes that break down all the contents of the cell, including cellular
proteins and DNA. The cell debris this breakdown causes is then released from the dying cell in little
packets known as cellular ‘blebs’. The cellular blebs end up outside the cells where cells of the immune
system called phagocytes (from the Greek meaning ‘cell eater’) consume them by phagocytosis.
Macrophages are an example of such immune system phagocytes. Once macrohpage has phagocytosed a bleb containing protein and DNA debris from an apoptosing cell it recycles the contents of the
bleb for its own use. Apoptosis is therefore like cellular suicide: a cell takes its own life because it is no
longer functional. As it dies it is cannibalized by its neighbors to the ultimate benefit of the community as
a whole. While we are young the gap left by the cell that has apoptosed is rapidly filled by a new cell that
has just been produced, but as we get older we come less efficient at replacing damaged cells and tissue.
In this way cellular aging translates into bodily aging.
The scenario in which ROS damage DNA and activate apoptosis is known as intrinsic apoptosis. This
means that the signal to start producing the enzymes that digest the damaged cell’s DNA and proteins
comes from inside the cell itself – in this case the intrinsic signal is provided by DNA damage.
A second type of apoptosis is known as extrinsic apoptosis. In this case the signal to start producing
the apoptosis enzymes comes from outside the cell. Examples of signals like this are lack of oxygen and
tissue damage. A good example of an extrinsic signal occurs when a cell is infected with a virus and
activates the immune pathway that ends up with killer T cells. The killer T cells then send a signal to the
infected cell telling it to initiate apoptosis.
MC Questions:
1. When is apoptosis activated? (Circle
all correct.)
aa. A cell has accumulated too
much DNA damage.
bb. A cell cannot enter the cell cycle.
cc. A cell has grown too large.
dd. A cell has received signals to
die.
2. Which of the following is a difference
between apoptosis and necrosis?
(Circle all correct.)
aa. Apoptosis is only activated from
external signals necrosis is not.
bb. Necrosis activates immune
response, apoptosis does not.
cc. Apoptosis causes the cell to
break down, necrosis does not.
dd. Apoptosis is organized, necrosis
is not.
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LESSON READINGS
Not all cell death occurs through apoptosis. When a normal cell is physically damaged by trauma it can
suddenly burst and release its contents explosively to the environment. This is known as necrosis. In this
case also release of the cell contents recruits immune phagocytes to the area and they consume the cell
debris.
The best way to describe the difference between apoptosis and necrosis is that apoptosis dismantles the
cell in an organized fashion whereas necrosis is an unorganized cellular explosion.
Apoptosis: a struggle between life and death
DEFINITIONS OF TERMS
The severe DNA damage that triggers apoptosis is detected by a specific protein called p53. The protein
p53 plays a critical role in controlling DNA damage within normal cells:
■■ When DNA damage has occurred p53 is the protein that stops the cell cycle progressing.
■■ When DNA damage is fixable p53 is the protein that activates transcription of the DNA repair
enzymes that can repair the DNA.
Caspases – a family of proteins
involved in apoptosis that are
responsible for breaking down
proteins and DNA in the cell.
Wo r k b o o k
Lesson 2.5
MC Questions:
3. Which of the following is a direct
activity of p53? (Circle all correct.)
aa. Control entrance of cell into cell
cycle.
bb. Repair DNA damage.
cc. Activate apoptosis.
dd. Break down proteins for
apoptosis.
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However if DNA damage is irreparable, p53 is the protein that activates apoptosis. How?
Cellular proteins and DNA are broken down during apoptosis because the dying cells start to express a
series of enzymes: proteases called caspases that break down proteins) and DNAases (that break down
DNA) that normal cells don’t produce.
Signaling the cell to produce these
enzymes is a critical first step in switching
on the apoptosis program and p53 can
control expression of these enzymes both
directly and indirectly:
■■ p53 controls expression of apoptosis enzymes directly by activating
expression of transcription factors
that bind to the protease and DNAase
genes. When the transcription factor
proteins are expressed and bind to
the genes this permits the enzyme
genes to be transcribed (we learned
how transcription factors work in the
last lesson.
Figure 2: The process by which apoptosis is
triggered by p53 activation which leads to degradation of proteins and DNA by caspases, forming
'blebs' that are consumed by other cells.
4. Which proteins are directly
responsible for degrading the
contents of the cell for apoptosis?
(Circle all correct.)
aa. p53
bb. Rb
cc. DNA repair proteins
dd. Caspases
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LESSON READINGS
■■ p53 can activate apoptosis indirectly by damaging mitochondria. Mitochondria are the organelles chiefly responsible for generating energy in cells. To generate energy they need an intact
membrane. P53 can send signals to the mitochondria that cause their membranes to become leaky.
Apoptosis occurs in cells with leaky mitochondria.
p53 is called a pro-apoptosis factor, but it is not the only one. Both the intrinsic and extrinsic apoptosis
pathways also activate other pro-apoptotic factors that can poke holes in mitochondria and make them
leak (these include the proteins Bax and Bak, as seen in Figure 3).
DEFINITIONS OF TERMS
MC Questions:
5. During which step of apoptosis is
there a competition between proapoptotic factors and pro-survival
factors? (Circle all correct.)
aa. Extrinsic signaling pathway.
bb. DNA damage.
cc. Making holes in mitochondria.
dd. Degrading cellular proteins.
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Pro-apoptotis factors – proteins
that promote apoptosis by making mitochondrial membranes
leaky.
Pro-survival factors – proteins
that inhibit apoptosis by preventing mitochondria leaking, thereby
promoting cell survival.
Figure 3: Pro-apoptotic factors like Bax or Bak make holes
in the mitochondria to release contents. Release of contents
activates caspases which leads to apoptosis. Alternatively, prosurvival factors, like Bcl-2, inhibit formation of mitochondrial
pores and prevent apoptosis.
Apoptosis happens when a cell’s DNA is too damaged to repair, but the mechanisms that lead to
apoptosis are very sensitive and are already primed and ready to go in cells with normal amounts of DNA
damage. They are prevented in these normal cells because surrounding tissues secrete survival signals
to keep apoptosis under control so that it is only activated when necessary. These signals are called
pro-survival factors (an example is Bcl-2 in Figure 3). Bcl-2 can promote cell survival by activating
mechanisms that repair the holes in mitochondria to prevent them leaking.
Wo r k b o o k
Lesson 2.5
Bcl-2 is a pro-survival signals because it opposes the indirect effects of p53. Other pro-survival signals
can also turn down the volume on p53 so it behaves appropriately. Because p53 controls DNA damage
in both normal and dysfunctional cells it is important that it doesn’t overreact to normal damage by starting
apoptosis in normal cells.
6. Which is the most common way that
cancer cells subvert the activation of
apoptosis? (Circle all correct.)
aa. Inactivating pro-survival
proteins.
bb. Inactivating pro-apoptosis
proteins.
cc. Hyperactivating pro-survival
proteins.
dd. Hyperactivating pro-apoptosis
proteins.
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LESSON READINGS
DEFINITIONS OF TERMS
NF-кB – a transcription factor
that promotes expression of prosurvival proteins. This protein is
often overactive in cancer cells.
Tumor suppressor protein – a
protein that controls cell proliferation so that cells don’t proliferate abnormally. This includes
proteins that prevent growth and
survival.
How cancer cells cheat death
MC Questions:
Because apoptosis destroys cells with serious mutations that affect their functions it is a critical mechanism for preventing tumor formation. As we will see in later units, both the intrinsic apoptosis pathway (via
p53) and the extrinsic apoptosis pathways (via immune cells) are pretty efficient at killing off cells that have
accumulated tumorigenic DNA damage. However, some of these cells can go on to accumulate further
mutations that either inhibit pro-apoptotic pathways, or hyperactivate pro-survival pathways. In either case
a cell that is resistant to apoptosis control will result.
7. What is the direct activity of NF-кB?
(Circle all correct.)
aa. Bind pro-survival receptors.
bb. Activate expression of prosurvival genes.
cc. Mutate pro-apoptotic proteins.
dd. Prevent leakage of mitochondrial
contents.
Tumor cells most commonly resist apoptosis is by inhibiting pro-apoptotic proteins like p53. As a result
upwards of 60-80% of cancers have mutations that inactivate p53. This shows just how important p53 is
for controlling DNA damage in cells.
When tumor cells are investigated to see what mutations they have, mutations that disrupt the apoptosis
pathways often coincide with mutations that activate survival pathways. The transcription factor, NF-кB
(pronounced NF-kappa B) activates one of the most powerful pro-survival pathways. NF-кB binds to
genes that prevent apoptosis being initiated. So when the NF-кB pathway is overactive, as it is in many
tumors, apoptosis can’t be switched on. So there are two ways that tumor cells interfere with apoptosis:
1. They can switch off pathways that switch apoptosis on.
2. They can switch on pathways that switch apoptosis off.
In both cases the cancer cell cheats death.
Apoptosis is only one of many mechanisms that cells used to control their growth. Proteins that control cell
growth are called tumor suppressors and we have learned about several in this unit:
■■ The retinoblastoma protein (Rb) makes sure the cell passes through the cell cycle ‘gate’ with intact
DNA
■■ The cell cycle checkpoint proteins (INK proteins) make sure the hand-off between the different
stages of the cell cycle doesn’t occur until the cell is ready.
■■ The pro-apoptosis proteins (p53, Bax/Bak) make sure that cells that are too damaged cells die.
Wo r k b o o k
Lesson 2.5
These regulators of the community of cells promote the ‘laws’ that keep the tissue community intact. Now
that we know what the normal laws are, the next unit will help us understand how cancer cells violate
them.
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STUDENT RESPONSES
Describe 2-3 ways that cancer cells can disrupt apoptosis. Hint: Think of the steps necessary for apoptosis to occur from the
initial apoptosis stimuli (either activation of apoptotic signaling or DNA damage) to cell blebbing.
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Remember to identify your
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Wo r k b o o k
Lesson 2.5
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TERMS
TERM
For a complete list of defined
terms, see the Glossary.
Wo r k b o o k
Lesson 2.5
DEFINITION
Anti-oxidants
A molecule that inhibits the activity of reactive oxygen species preventing them damaging DNA.
Apoptosis
The process of programmed cell death in multicellular organisms characterized by cell shrinkage, nuclear
fragmentation, degradation of proteins, and release of cellular 'blebs'. This is an organized and planned
death of a cell.
Caspases
A family of proteins involved in apoptosis that are responsible for breaking down proteins and DNA in the
cell.
Necrosis
The premature death of a normal cell in which it bursts, randomly releasing fragments of proteins and DNA
into surrounding tissue. This unplanned death is disorganized.
NF-кB
A transcription factor that promotes expression of pro-survival proteins. This protein is often overactive in
cancer cells.
Phagocytic cells
Cells that are responsible for consuming foreign particles, bacteria, and cellular blebs. These cells break
down contents of what they consume and recycle them for other cells to use.
Reactive oxygen species
(ROS)
Chemically reactive molecules of oxygen that can mutate or break DNA to cause damage.
Pro-apoptotic factors
Proteins that promote apoptosis by making mitochondrial membranes leaky.
Pro-survival factors
Proteins that inhibit apoptosis by preventing mitochondria leaking, thereby promoting cell survival.
Tumor suppressor
protein
A protein that controls cell proliferation so that cells don’t proliferate abnormally. This includes proteins that
prevent growth and survival.
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