ApoptosisApoptosis(Programmed Cell Death)(Programmed Cell Death)

Apoptosis vs Necrosis

• Level of stress, change in environment

stress

apoptosis necrosis

Modes of Cell Death

• A pathological response to injury

• Chromatin clumping• Mitochondria swelling and

rupture• Plasma membrane lyses• Cell contents spill out

• A normal physiological response to specific suicide signals

• Chromatin condenses• Internucleosomal

cleavage leads to laddering of DNA

• Cytoplasma shrinks without membrane rupture

• Blebbing of plasma and nuclear membranes

• No spillage

Necrosis Apoptosis

Tumor Suppressor Genes

• Tumor-suppressor genes, function like brakes, keep cell numbers down, either by:

1. Inhibiting progress through the cell cycle and thereby preventing cell birth, or

2. Promoting apoptosis

• When cellular tumor suppressor genes are rendered non-functional through mutation, the cell becomes malignant. Example: pRb, p53, and p16INK4a.

Oncogenes

• Oncogenes stimulate cells to grow (or refuse to die).

• Mutated oncogenes can stimulate cells even when they are not receiving growth signals. Example: Ras, Akt, Survivin.

• Amplification of oncogenes is also found in cancer. Example: MDM2.

Apoptosis Signaling Pathway

Cell, 2002;108:153-64

Tumor-derived mutations affecting apoptosis

Protein Role in apoptosisReferences

p53 Mutated or altered expression in many cancers. Initiates the intrinsic apoptotic pathway.  

Vogelstein et al., 2000

p19ARF Mutated or altered expression in many cancers. Blocks MDM2 inhibition of p53. Enhances drug-induced apoptosis by p53.

Sherr and Weber, 2000

pRb Mutated in some cancers, and functionally disrupted in many cancers. Inhibits E2F-mediated transcription.

Harbour and Dean, 2000

FLIP Overexpressed in some cancers. Prevents activation of caspase-8 and apoptosis induced by some chemotherapeutic drugs.    

Tepper and Seldin, 1999

Tumor-derived mutations affecting apoptosis

Protein Role in apoptosisReferences

Apaf-1 Mutated and transcriptionally silenced in melanoma and leukemia cell lines. Necessary for activation of caspase-9 following cytochrome c release. Apaf-1-/- cells are chemoresistant.

Soengas et al., 2001

CD-95/Fas

Mutated and down-regulated in lymphoid and solid tumors. Initiates the extrinsic apoptotic pathway. Loss of function is associated with resistance to drug-induced cell death.   

Muschen et al., 2000

TRAIL Mutated in metastatic breast cancers. Initiate the extrinsic apoptotic pathway. Mutations lead to suppression of death receptor-mediated apoptosis.

Shin et al., 2001

Caspase-8

Gene silenced in neuroblastomas. Activates both extrinsic and intrinsic apoptotic pathways. Silencing results in resistance to drug-induced apoptosis.

Teitz et al., 2000

Tumor-derived mutations affecting apoptosis

Protein Role in apoptosis References

Bcl2 Frequently overexpressed in many tumors. Antagonises Bax and/or Bak and inhibits mitochondrial membrane disruption. Inhibits drug-induced apoptosis.

Reed, 1999

MDM2 Overexpressed in some tumors. Negative regulator of p53. Inhibits drug-induced p53 activation. 

Sherr and Weber, 2000

IAPs Frequently overexpressed in cancer. Down regulation of XIAP induces apoptosis in chemoresistant tumors. 

Deveraux and Reed, 1999

NFB Transcriptionally activates expression of anti-apoptotic members of the Bcl-2 and IAP families. Can inhibit both the extrinsic and intrinsic death pathways and induce drug-resistance.  

Baldwin, 2001

Tumor-derived mutations affecting apoptosis

Protein Role in apoptosis Ref

Myc Induces proliferation in the presence of survival factors, such as Bcl-2, and apoptosis in the absence of survival factors.

Evan and Vousden, 2001

Akt Frequently amplified in cancers. Phosphorylates Bad. Hyperactivation induces resistance to a range of apoptotic stimuli including drugs.   

Datta et al., 1999

PI3K Overexpressed in some cancers. Responsible for activation of Akt and downstream phosphorylation of Bad. Inhibition of PI3K enhances chemotherapeutic drug-induced apoptosis.    

Roymans and Slegers, 2001

Ras Mutated or deregulated in many cancers. Activates PI3K and downstream pathways. Induces proliferation and inhibits c-myc and drug-induced apoptosis.   

el-Deiry, 1997

Tumor-derived mutations affecting apoptosis

Protein Role in apoptosisReferences

Bax Mutated or decreased expression in some tumors. Mediates mitochondrial membrane damage. Sufficient but not necessary for drug-induced apoptosis.    

Rampino et al., 1997

Bak Mutated or decreased expression in some tumors. Mediates mitochondrial membrane damage. Sufficient but not necessary for drug-induced apoptosis. 

Kondo et al., 2000

PTEN Mutated or altered expression in cancers. Regulates Akt activation and subsequent phosphorylation of Bad. Loss of PTEN results in resistance to many apoptotic stimuli.

Di Cristofano and Pandolfi, 2000

Role of p53 in Apoptosis

1. p53 induces apoptosis through transcriptional activation of proapoptotic genes, such as Puma, Noxa, p53AIP, Bax, Apaf-1, etc.

2. It can also directly induce apoptosis by localizing to mitochondria via interaction with Bcl-2 family protein.

Nat Rev Cancer, 2002;2:594-604

p53 in DNA Repair and Apoptosis

Tumor Suppressor p53

• Mutated/inactivated in a majority of human cancers

• p53 -/- knockout mice embryos develop normally to term but after a few weeks the young mice develop numerous malignant tumors

• adenovirus introduced wild-type p53 markedly enhanced the anti-tumor effect of a common chemotherapeutic agent, cisplatin, in human non-small cell lung cancer cells, and in human colon cancer cells.

• Effectiveness of cancer therapy correlates with the ability to induce p53-dependent apoptotic response

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