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Cell Death Mechanisms and Their Clinical Implications

Necrosis vs. Apoptosis

  • Definition of necrosis and apoptosis as two types of cell death.

  • Determinant: The deciding factor between which process occurs is the level of ATP (adenosine triphosphate).

    • Necrosis: Induced by sudden or severe loss of ATP production.

    • Apoptosis: Occurs during a gradual and less severe loss of ATP production.

  • Further discussion of pro-apoptotic proteins will be made in Lecture 6.

Role of Cytochrome c in Apoptosis

  • Cytochrome c: A vital protein that frequently triggers apoptosis in various cell types, though not universally applicable to all.

  • The Mitochondrial Permeability Transition Pore (MPTP):

    • A calcium-dependent pore that connects the mitochondrial matrix to the cytosol.

    • Composed of multiple molecules and is relatively non-selective (permeable to molecules up to 1.5 kDa).

Mitochondria and Cell Health

  • The MPTP is normally closed but opens during critical conditions to help mitigate oxidative stress and maintain calcium balance.

  • Long-term opening of the MPTP results in cell death, which is termed MPTP-mediated necrosis due to insufficient ATP levels necessary for apoptosis.

  • Key Takeaway: ATP is essential for apoptosis.

Flashback to the Electron Transport Chain (ETC)

  • The ETC comprises several multi-protein complexes facilitating redox reactions.

  • These reactions efficiently convert the potential energy from glucose oxidation into ATP.

  • Oxygen is the final electron acceptor in this process.

  • ATP Synthase:

    • Acts as a molecular motor, enabling protons to move through and couple ADP with inorganic phosphate, generating ATP.

Components of MPTP

  • The current best model suggests the following components are essential:

    • Outer Mitochondrial Membrane (OMM)

    • Inner Mitochondrial Membrane (IMM)

    • Intermembrane Space including ATP synthase

    • Adenosine Nucleotide Translocator (transports ADP across membranes)

    • Inorganic Phosphate Carrier

    • Voltage-dependent Anion Channel (VDAC)

    • Cyclophilin D (CYP D): Critical component that must always be present in the MPTP, playing a structural role.

  • Inducers of Pore Opening:

    • Reactive Oxygen Species (ROS)

    • Calcium levels (regulates MPTP opening)

    • Inorganic phosphate levels (indicates ATP imbalance)

    • Cyclosporine A (CSA): A drug that inhibits the pore's long-term opening, potentially having therapeutic applications.

Aging and MPTP Sensitivity

  • As organisms age, they exhibit increased sensitivity to prolonged MPTP opening, necessitating lower thresholds of ROS and calcium to trigger irreversible cell death.

  • Many diseases showcase heightened sensitivity to MPTP opening, including:

    • Cancer

    • Parkinson’s disease

    • Diabetes

Drugs and Therapeutics Related to MPTP

  • There is ongoing exploration of drugs that can stabilize the MPTP, allowing physiological openings while preventing tipping points that lead to cell death:

    • MyoQ: Reduces mitochondrial ROS

    • Edaravone: Used clinically for ischemic stroke patients

    • Cyclosporine A (CSA): Potential therapeutic applications but bears a narrow therapeutic index, requiring careful monitoring.

  • Therapeutic Index: The ratio between the dose of a drug that produces a therapeutic effect and the dose that results in toxicity. A ratio of two or less indicates a narrow therapeutic index; careful monitoring is required.

Calcium Homeostasis and Cell Injury

  • Role of Calcium: A crucial signaling molecule in multiple cell signaling pathways, typically maintained at low intracellular concentrations.

  • Implications of Elevated Calcium:

    • Causes activation of enzymes such as phospholipases and proteases, leading to substantial cellular damage and cell death via apoptosis or necrosis as they contribute to ATP depletion.

  • Calcium has an important role in activating caspases, which are essential for apoptosis.

Reactive Oxygen Species (ROS) and Oxidative Stress

  • Oxidative Stress: A condition arising from an imbalance between ROS production and antioxidant defenses, often leading to cellular damage.

    • Defined as free radicals with unpaired electrons, making them highly reactive.

    • ROS can result in further free radicals through chain reactions.

  • Primary ROS: Include superoxide, hydroxyl, and hydrogen peroxide.

  • Sources of ROS Generation:

    • Ionizing radiation

    • Drug metabolism in the liver

    • Catalysis involving iron and copper

      • Fenton Reaction: Specific to iron, leading to hydroxyl radicals when iron reacts with hydrogen peroxide.

      • Fenton-like Reaction: Similar reaction occurring with copper.

  • Certain ROS play a dual role in health, as innate immune cells use them to destroy pathogens.

  • Applications in Cancer Therapy: Radiation therapy effectiveness relies on ROS generation.

Mechanisms of ROS Removal

  • Body defenses include antioxidants, which can neutralize free radicals:

    • Enzymatic Systems:

      • Superoxide Dismutase (SOD): Converts superoxide radicals into hydrogen peroxide.

      • Catalase: Converts hydrogen peroxide into water and oxygen.

      • Glutathione Peroxidase: Reduces glutathione to scavenge free radicals.

  • Iron Sequestration: Through proteins like transferrin, which transport iron, and ferritin, which stores it, helps prevent oxidative damage.

Sources of Membrane Damage

  • ROS can cause extensive damage to membranes, leading to loss of selective permeability, indicating forms of cell injury except apoptosis.

  • Mechanisms of membrane damage:

    • Ischemia: Lack of blood flow leading to ATP depletion, causing membrane dysfunction and activation of damaging enzymes.

    • Phospholiapse leakage leading to further damage and loss of membrane integrity.

Transition to Irreversible Injury

  • Point of No Return:

    • Irreversible cell damage is indicated by mitochondrial dysfunction and loss of membrane integrity.

    • Cells undergoing necrosis release substances detectable in blood such as cardiac troponin, indicating myocardial infarction.

Clinical Applications of Cell Injury

  • Ischemia vs. Hypoxia: Ischemia is more damaging than simply hypoxic conditions due to absence of nutrients and oxygen supply.

    • Ischemic injuries evolve rapidly; within 60 seconds, cardiac cells cease contracting, although recovery is possible if oxygen is restored soon.

    • Time duration before irreversibility depends on the cell type.

  • HIF (Hypoxia Inducible Factor): Responds to low oxygen, promoting vascular endothelial growth factor (VEGF) for new blood vessel formation, critical for tissue viability under constrained oxygen supply conditions.

    • HIF also activates survival pathways, inhibiting apoptosis.

Hypothermia in Clinical Practice

  • Hypothermia can protect against ischemia-induced cell damage by slowing metabolism, improving survival rates in myocardial infarction cases

  • Experimental hypothermia utilization, while promising, faces practical challenges in real-time medical settings.

Ischemia-Reperfusion Injury

  • Restoration of blood flow after ischemic conditions can paradoxically worsen cell injury. This occurs because the rapid influx of oxygen leads to overwhelming ROS production without sufficient antioxidant defenses, impacting cell viability and adding damage via inflammation and immune response activation.