Programmed Cell Death (Apoptosis)

1. Programmed Cell Death (Apoptosis) Apoptosis: from a Greek word meaning "falling off," as leaves from a tree Molecular Biology of Cancer 1

2. What purposes does this massive cell death serve? In some cases, the answers are clear Mouse paws, for example, are sculpted by cell death during embryonic development 2 Molecular Biology of Cancer

3. 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. 3 Molecular Biology of Cancer

4. 4 Molecular Biology of Cancer • 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.

5. Cell death 5 • 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: 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 Cancer

6. Apoptosis is mediated by an intracellular proteolytic cascade Caspases: a family of proteases that have a cysteine at their active site and cleave their target proteins at specific aspartic acids. 6 Molecular Biology of Cancer Caspases are synthesized in the cell as inactive precursors, or procaspases, which are usually activated by cleavage at aspartic acids by other caspases

7. Once activated, caspases cleave, and thereby activate, other procaspases, resulting in an amplifying proteolytic cascade 7 Molecular Biology of Cancer • Some cleave the nuclear 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

8. How are procaspases activated to initiate the caspase cascade? 8 Molecular Biology of Cancer Adaptor protein Initiator procaspases aggregate Initiator procaspases • A general principle is that the activation is triggered by adaptor proteins • They bring multiple copies of specific procaspases, known as initiator procaspases, close together in a complex or aggregate. In other cases Conformational change In someSmall amount of protease activity they cleave each other Active caspases Active caspases procaspases

9. Apoptosis activation from outside the cell by the activation of cell surface death receptors 9 Molecular Biology of Cancer • For example: Fasligand on the surface of killer lymphocytes and the death receptor proteinFas on the surface of the target cell • Some stressed or damaged cells kill themselves by producing both the Fasligand and the Fas protein

10. Apoptosis activation from inside the damaged or stressed cells 10 • Mitochondria are induced to release the electron carrier protein cytochrome c into the cytosol • It binds and activates an adaptor protein called Apaf-1 Molecular Biology of Cancer

11. 11 Molecular Biology of Cancer • p53 usually can activate the transcription of genes (the Bcl-2 family) whose proteins promote the release of cytochromec from mitochondria.

12. The Bcl-2 family of intracellular proteins helps regulate the activation of procaspases. 12 Molecular Biology of Cancer Bcl-2 Bcl-XL Bad Bax Bid Bak