Differentiate between intrinsic and extrinsic apoptotic pathways.

The intrinsic and extrinsic apoptotic pathways are the two main signaling pathways that lead to apoptosis (programmed cell death). Although both pathways ultimately lead to cell death, they are triggered by different stimuli and involve distinct molecular mechanisms before converging on common effector molecules. Here’s a detailed differentiation:

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1. Triggering Mechanism:

• Intrinsic Pathway (Mitochondrial Pathway):

  • Triggered by Internal Signals: The intrinsic pathway is activated in response to internal cellular stress. Common stressors include:
    • DNA damage (from radiation, chemicals, or replication errors)
    • Oxidative stress
    • Hypoxia (low oxygen levels)
    • Oncogene activation
    • Growth factor deprivation
    • Cytotoxic drugs
  • This pathway is regulated primarily by the mitochondria and proteins from the BCL-2 family.

• Extrinsic Pathway (Death Receptor Pathway):

  • Triggered by External Signals: The extrinsic pathway is activated by extracellular death ligands binding to death receptors on the cell surface. These ligands are typically part of the tumor necrosis factor (TNF) family or Fas ligand (FasL). Common ligands and receptors include:
    • Fas ligand (FasL) binding to the Fas receptor (CD95)
    • TNF-α binding to TNF receptors (TNFR1)
  • This pathway does not involve the mitochondria initially but relies on surface receptor-ligand interactions.

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2. Key Initiating Components:

• Intrinsic Pathway:

  • The key component is the mitochondria. Cellular stress triggers a change in the balance of BCL-2 family proteins:
    • Pro-apoptotic proteins (e.g., BAX, BAK, PUMA, NOXA) promote apoptosis.
    • Anti-apoptotic proteins (e.g., BCL-2, MCL-1) inhibit apoptosis by blocking pro-apoptotic proteins.
  • When pro-apoptotic signals dominate, mitochondrial outer membrane permeabilization (MOMP) occurs, leading to the release of cytochrome c and other factors from the mitochondria.

• Extrinsic Pathway:

  • The key components are death receptors located on the cell surface. The binding of a death ligand (like FasL or TNF-α) to its receptor (like Fas or TNFR) forms a death-inducing signaling complex (DISC).
  • DISC recruits and activates caspase-8, the initiator caspase for the extrinsic pathway.

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3. Initiator Caspases:

• Intrinsic Pathway:
  • The mitochondrial release of cytochrome c leads to the formation of the apoptosome, a multiprotein complex. The apoptosome activates caspase-9, the initiator caspase in the intrinsic pathway.
• Extrinsic Pathway:
  • Upon the formation of the DISC, caspase-8 is activated, which serves as the initiator caspase for this pathway.

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4. Role of Mitochondria:

• Intrinsic Pathway:

  • The mitochondria play a central role. When the balance between pro- and anti-apoptotic BCL-2 family members shifts in favor of apoptosis, mitochondrial outer membrane permeabilization (MOMP) occurs. This allows the release of apoptogenic factors like:
    • Cytochrome c: Activates the apoptosome, leading to caspase-9 activation.
    • Smac/DIABLO: Inhibits inhibitor of apoptosis proteins (IAPs), enhancing caspase activity.
    • Apoptosis-inducing factor (AIF): Causes DNA fragmentation and cell death.

• Extrinsic Pathway:

  • The mitochondria are not directly involved in the initial stages. However, in some cases, a crosstalk between the extrinsic and intrinsic pathways occurs. Caspase-8 can cleave the pro-apoptotic protein BID into its truncated form (tBID), which translocates to the mitochondria and promotes mitochondrial membrane permeabilization, linking the extrinsic pathway to the intrinsic pathway.

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5. Effector Caspases and Execution Phase:

• Intrinsic Pathway:

  • Caspase-9, activated by the apoptosome, goes on to activate effector caspases such as caspase-3 and caspase-7.
  • These effector caspases degrade cellular components, leading to the classic apoptotic morphological changes (e.g., DNA fragmentation, membrane blebbing).

• Extrinsic Pathway:

  • Caspase-8, activated by DISC, can directly activate effector caspases (e.g., caspase-3) or cleave BID (in some cases), indirectly engaging the intrinsic pathway for mitochondrial involvement.
  • This leads to the same execution phase, involving the breakdown of cellular components by caspases.

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6. Crosstalk Between Pathways:

• Intrinsic Pathway:
  • Functions independently but can be influenced by caspase-8 cleavage of BID in the extrinsic pathway.
• Extrinsic Pathway:
  • Can function independently but also activate the intrinsic pathway via BID cleavage and mitochondrial involvement, especially in certain cell types.

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7. Examples of Apoptosis Inducers:

• Intrinsic Pathway:
  • Internal stressors like DNA damage, hypoxia, oncogene activation, chemotherapeutic agents, and radiation can trigger the intrinsic pathway.
• Extrinsic Pathway:
  • External factors such as Fas ligand (FasL), TNF-α, and TRAIL (TNF-related apoptosis-inducing ligand) can activate the extrinsic pathway by binding to death receptors on the cell surface.

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Summary of Differences:

Feature	Intrinsic Pathway	Extrinsic Pathway
Trigger	Internal stress (DNA damage, hypoxia, etc.)	External signals (death ligands binding to receptors)
Key Components	Mitochondria, BCL-2 family proteins	Death receptors (Fas, TNFR) and death ligands
Initiator Caspase	Caspase-9	Caspase-8
Effector Caspases	Caspase-3, Caspase-7	Caspase-3, Caspase-7
Role of Mitochondria	Central to the pathway (cytochrome c release)	Not directly involved unless through BID cleavage
Cross-Talk	Can be influenced by extrinsic pathway through BID	Can activate intrinsic pathway via tBID and mitochondria

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Conclusion:

The intrinsic apoptotic pathway is primarily triggered by internal cellular stress and is regulated by the mitochondria and BCL-2 family proteins. The extrinsic apoptotic pathway, on the other hand, is initiated by external signals through death receptors on the cell surface. Both pathways lead to the activation of caspases, ultimately causing cell death. However, there can be crosstalk between the two, particularly when caspase-8 from the extrinsic pathway activates the intrinsic pathway via BID cleavage.