apoptosis as a regulated process of programmed cell death, including the role of caspases, and the consequences of disruption to the regulation of the cell cycle and apoptosis with reference to the development of cancer

What this dot point is asking

VCAA wants apoptosis as a regulated, controlled cell-death programme using caspases, and the consequences when the regulation of the cell cycle and apoptosis breaks down, leading to cancer.

The answer

Apoptosis

Apoptosis is programmed cell death: a regulated, energy-requiring process by which the cell dismantles itself in a controlled way. It is essential for development, tissue homeostasis and elimination of damaged or infected cells.

Examples in healthy biology:

• Removal of webbing between fingers during embryonic development.
• Daily turnover of intestinal lining cells.
• Removal of self-reactive T and B lymphocytes during immune development.
• Elimination of cells with irreparable DNA damage.

Apoptosis is the opposite of necrosis, the chaotic death of a cell after injury (which spills contents and causes inflammation).

Mechanism: caspases

The central executioners of apoptosis are caspases: cysteine proteases that cleave target proteins. They exist normally as inactive pro-caspases and must be activated by cleavage.

There are two classes:

• Initiator caspases (caspase-2, -8, -9, -10): activated first by upstream signals; their job is to activate effector caspases.
• Effector (executioner) caspases (caspase-3, -6, -7): activated by initiator caspases; their job is to cleave hundreds of structural and regulatory proteins, dismantling the cell.

The caspase cascade is amplifying: one activated initiator caspase activates many effector caspases, ensuring the cell commits irreversibly to death.

Two pathways

Extrinsic (death receptor) pathway. Triggered by external signals binding to death receptors on the plasma membrane (such as Fas binding FasL on a cytotoxic T cell). The death-receptor complex activates caspase-8 (initiator), which activates caspase-3 (effector).

Intrinsic (mitochondrial) pathway. Triggered by internal damage signals (such as severe DNA damage detected by p53, or oxidative stress). The mitochondrial outer membrane becomes permeable, releasing cytochrome c into the cytoplasm. Cytochrome c binds Apaf-1 to form the apoptosome, which activates caspase-9 (initiator), which activates caspase-3 (effector).

Both pathways converge on the same set of effector caspases.

Visible morphology

A cell undergoing apoptosis shows distinctive changes:

1. Cell shrinkage and detachment from neighbours.
2. Chromatin condensation and nuclear fragmentation.
3. Membrane blebbing: bubble-like protrusions.
4. Fragmentation into apoptotic bodies: small membrane-bound packets of cell contents.
5. Phagocytosis of apoptotic bodies by neighbouring cells or macrophages, without inflammation.

Disruption to the cell cycle and apoptosis

Cancer is the disease of unregulated cell division and resistance to apoptosis. It arises when two classes of genes mutate.

Proto-oncogenes. Normal genes whose products promote cell division (growth factors, growth-factor receptors, signal-transduction proteins, cyclins). A gain-of-function mutation turns a proto-oncogene into an oncogene, producing a hyperactive or constantly-on version that pushes the cell to divide even without a normal signal. Examples: Ras (a signalling switch stuck "on"); HER2 (a growth-factor receptor over-expressed in some breast cancers).

Tumour suppressor genes. Normal genes whose products inhibit cell division or trigger apoptosis when damage is detected. They act as brakes. A loss-of-function mutation removes the brake. Examples:

• p53 ("the guardian of the genome"). Detects DNA damage; pauses the cycle at the G1 checkpoint to allow repair; if repair fails, triggers apoptosis via the intrinsic pathway. Over half of human cancers have p53 mutations.
• RB (retinoblastoma). Restrains entry into S phase. Loss of RB allows cells to enter S phase inappropriately.

How cancer develops

Cancer typically requires multiple mutations over time (the multi-hit hypothesis):

1. A proto-oncogene mutates to an oncogene: the cell divides faster than normal.
2. A tumour suppressor (such as p53) is lost: damaged DNA is no longer repaired or eliminated.
3. The cell ignores checkpoints (G1, G2, M).
4. Apoptosis is resisted (loss of p53 prevents the intrinsic pathway from triggering).
5. The cell accumulates more mutations, and the colony grows into a benign tumour.
6. Further mutations (in genes controlling adhesion, angiogenesis and migration) can produce a malignant tumour that invades surrounding tissue and metastasises through blood or lymph.

Carcinogens are agents that increase the rate of these mutations: UV radiation (skin cancer), tobacco smoke (lung cancer), some viruses (HPV in cervical cancer, EBV in some lymphomas), and chemicals (asbestos, benzene).

Worked example

A skin cell sustains UV damage that creates thymine dimers in its DNA. Normally, the G1 checkpoint detects the damage; p53 pauses the cycle and recruits repair enzymes. If the damage is unrepairable, p53 triggers apoptosis via the intrinsic pathway: cytochrome c leaves the mitochondria, caspase-9 activates caspase-3, the cell dismantles itself and is engulfed by phagocytes. If the cell already has a p53 mutation, no pause and no apoptosis occur; the damaged cell divides, passing the damage on. Over many years, additional mutations accumulate and a melanoma develops.

Common traps

Confusing apoptosis with necrosis. Apoptosis is regulated, energy-using, controlled and non-inflammatory; necrosis is uncontrolled, passive, and inflammatory.

Saying caspases "kill the cell". Caspases are proteases; they cleave specific target proteins. The cleavage cascade dismantles the cell from inside.

Forgetting that cancer is multi-step. A single mutation rarely causes cancer; multiple mutations across both proto-oncogenes and tumour suppressors over time are required.

Confusing proto-oncogenes and oncogenes. Proto-oncogenes are the normal versions (good). Oncogenes are the mutated, over-active versions (bad).

Treating apoptosis as a bad thing. Apoptosis is usually a healthy housekeeping process. The problem in cancer is the loss of apoptosis, not its presence.

In one sentence

Apoptosis is a regulated programme of cell death driven by initiator caspases (activated by the extrinsic death-receptor pathway or the intrinsic mitochondrial pathway via cytochrome c) that activate effector caspases to dismantle the cell into apoptotic bodies; when cell-cycle regulation breaks down through gain-of-function mutations in proto-oncogenes (becoming oncogenes) and loss-of-function mutations in tumour suppressors (especially p53), cells ignore checkpoints, resist apoptosis, and grow into a tumour.