Programmed Cell Death Programmed Cell Death (Apoptosis)(Apoptosis)

Apoptosis: from a Greek word meaning "falling offfalling off," as leaves from a tree

Molecular Biology of Molecular Biology of CancerCancer 11

What purposes does this massive What purposes does this massive cell death serve?cell death serve?

In some cases, the answers are clear Mouse paws, for example, are sculpted by cell death

during embryonic development

Molecular Biology of CancerMolecular Biology of Cancer22

In other cases, cells die when the structure they form is no longer needed When a tadpole changes into a frog, the cells in the tail die, and the tail, which is not

needed in the frog, disappears

In many other cases, cell death helps regulate cell numbers. In the developing nervous system, for example, cell death adjusts the number of nerve

cells to match the number of target cells that require innervation.

Molecular Biology of CancerMolecular Biology of Cancer33

In adult tissues, cell death exactly balances cell division If part of the liver is removed in an adult rat, liver cell

proliferation increases to make up the loss. Conversely, if the rat liver cell division is stimulated by

phenobarbital treatmnt and then the phenobarbital treatment is stopped liver enlargement occur first apoptosis in the liver greatly increases until the liver has returned

to its original size Thus, the liver is kept at a constant size through the

regulation of both the cell death rate and the cell birth rate.

Molecular Biology of CancerMolecular Biology of Cancer44

Cell deathCell death

• Cell necrosis: Cell necrosis: acute injury typically causes the cells to swell and burst. – They spill their contents all over their neighbors causing a potentially

damaging inflammatory response.• Apoptosis: Apoptosis: causes cells to die neatly, without damaging its

neighbors.– The cell shrinks and condenses– The cytoskeleton collapses– the nuclear envelope disassembles– the nuclear DNA breaks up into fragments– Most importantly, the cell surface is altered, displaying

properties that cause the dying cell to be rapidly phagocytosed, either by a neighboring cell or by a macrophage before any leakage of its contents occurs• This not only avoids the damaging consequences of cell necrosis

but also allows the organic components of the dead cell to be recycled by the cell that ingests it.

Molecular Biology of CancerMolecular Biology of Cancer55

Apoptosis is mediated by an intracellular Apoptosis is mediated by an intracellular proteolytic cascadeproteolytic cascade

Molecular Biology of CancerMolecular Biology of Cancer66

• CCaspaaspasesses: a family of proteaproteasesses that have a ccysteineysteine at their active site and cleave their target proteins at specific aspaaspartic acidsrtic acids.

A.A. CaspasesCaspases are synthesized in the cell as inactive precursors, or procaspasesprocaspases, which are usually activated by cleavage at aspartic acidsaspartic acids by other caspasescaspases

Molecular Biology of CancerMolecular Biology of Cancer77

B. Once activated, caspases cleave, and thereby activate, other procaspases, resulting in an amplifying amplifying proteolytic proteolytic cascadecascade

Some cleave the nuclear laminsnuclear lamins, causing the irreversible breakdown of the nuclear lamina

another cleaves a protein that normally holds a DNAse inactive freeing the DNAse to cut up the DNA in the cell nucleus

How are procaspases activated to How are procaspases activated to initiate the caspase cascade?initiate the caspase cascade?

Molecular Biology of CancerMolecular Biology of Cancer88

Initiator procaspasesInitiator procaspasesInitiator procaspases Initiator procaspases

aggregateaggregate

Active caspasesActive caspases

procaspasesprocaspases Active Active caspasescaspases

In some Small amount of protease activity they cleave each other

In other cases Conformational

change

Adaptor proteinAdaptor protein

A general principle is that the activation is triggered by adaptor adaptor proteinsproteins They bring multiple copies

of specific procaspasesprocaspases, known as initiator initiator procaspasesprocaspases, close together in a complex or aggregate.

Apoptosis activation from outside the cell by the Apoptosis activation from outside the cell by the activation of cell surface death receptorsactivation of cell surface death receptors

Molecular Biology of CancerMolecular Biology of Cancer99

• For example: Fas ligandFas ligand on the surface of killer lymphocytes and the death receptor proteinreceptor protein FasFas on the surface of the target cell

• Some stressed or damaged cells kill themselves by producing both the Fas Fas ligandligand and the Fas proteinFas protein

Apoptosis activation from inside the damaged or Apoptosis activation from inside the damaged or stressed cellsstressed cells

• Mitochondria are induced to release the electron carrier protein cytochrome ccytochrome c into the cytosol

• It binds and activates an adaptor protein called Apaf-1Apaf-1

Molecular Biology of CancerMolecular Biology of Cancer1010

Molecular Biology of CancerMolecular Biology of Cancer1111

p53p53 usually can activate the transcription of genes (the Bcl-Bcl-22 family) whose proteins promote the release of cytochrome cytochrome cc from mitochondria.

The Bcl-2 family of intracellular proteins The Bcl-2 family of intracellular proteins helps regulate the activation of procaspases.helps regulate the activation of procaspases.

Molecular Biology of CancerMolecular Biology of Cancer1212

Bcl-2Bcl-2 Bcl-XLBcl-XL

BadBadBaxBax

BakBak

BidBid