Mechanism of Cell Injury Dr Shoaib Raza

Principals relevant to cell Injury Cellular response depends upon the nature, duration and severity of injurious stimulus Consequences depend upon the type, state and adaptability of the injured cell. Cell injury result from different biochemical mechanisms acting on several cellular components Any injurious stimulus may simultaneously trigger multiple interconnected mechanisms that damage the cell.

Biochemical Mechanisms Affected during Cell Injury Depletion of ATP (Adenosine triphosphate) Mitochondrial damage Influx of calcium and loss of calcium homeostasis Accumulation of oxygen derived free radicals (Oxidative stress)

Depletion of ATP Frequently associated with hypoxic and chemical injury Decreased oxygen supply, mitochondrial damage and certain toxin (e.g. cyanide) cause ATP depletion Tissues with greater glycolytic activity (liver) are able to survive better than are tissues with limited capacity (brain)

Depletion of ATP: consequences Reduced activity of Na+-K+ pump Cellular swelling & dilation of ER Increased anaerobic glycolysis Reduced pH, inactivation of enzymes, etc. Failure of the Ca++ pump Ca++ influx Detachment of ribosome from RER Decreased protein synthesis Irreversible damage to mitochondrial and lysosomal membranes Necrosis

Mitochondrial Damage Mitochondria may be damaged by: Increased cytosolic Ca++ Reactive oxygen species Oxygen deprivation Two major consequences of mitochondrial damage are:

Consequences of mitochondrial damage Formation of high conductance channel in the mitochondrial membrane (mitochondrial permeability transition pore) Failure of oxidative phosphorylation Progressive depletion of ATP Cytochrome-C and other pro-apoptotic proteins are released Activation of apoptosis

Loss of Calcium Homeostasis Cytosolic free Ca is usually maintained in very low concentrations (0.1 µmol) compared with extracellular levels of 1.3 mmol Most intracellular Ca is stored in mitochondria and ER Cytosolic Ca is increased in injury Release of Ca from intracellular stores Increased influx across plasma membrane

Cell Injury due to ↑Ca Opening of mitochondrial permeability transition Failure of ATP generation Enzyme activation Phospholipase, protease, endonuclease, etc. Induction of apoptosis Activation of caspases Increasing mitochondrial permeability

Free Radicals These are the chemical species that have a single unpaired electron in an outer orbit. Mechanism of cell damage in Chemical, radiation, ischemia-reperfusion injury, aging, microbial killing by phagocytes Free radicals initiate autocatalytic reactions Reactive oxygen species (ROS)

Generation of Free Radicals May be generated in several ways: Red-Ox reaction superoxide (O2-), hydrogen peroxide (H2O2), hydroxyl ion (O-H ) Absorption of radiant energy (UV, x-rays) E.g. water is converted into H+ and HO- free radical Rapid burst of ROS are produced in activated leukocytes during inflammation Enzymatic metabolism of exogenous chemicals or drugs (CCl3) Transitional metals: (Fenton reaction) H2O2 + Fe++ ------→ Fe +++ + OH. + OH-

Removal of Free Radicals Generally unstable and decay spontaneously In addition cell have: Antioxidants Vitamin A, E, Ascorbic Acid, etc. Binding and transport protein Ferritin, ceruloplasmin, remove transitional elements from their stores A series of enzymes

Enzymatic degradation of Free Radicals Catalase Present in peroxisomes, decomposes H2O2 Superoxide dismutase Converts superoxide into H2O2 Glutathione peroxidase H2O2 + 2SGH → GSSG + 2H2O 2OH + 2GSH → GSSG + 2H2O Free Radicals induced cell injury is due to imbalance between their production and decaying

Pathologic effects of Free Radicals Lipid peroxidation of membranes Oxidative modification of proteins Lesions in DNA They cause death either by necrosis or apoptosis

Defects in Membrane Permeability Mechanism of membrane damage: Reactive oxygen species Lipid peroxidation Decreased phospholipid synthesis Increased phospholipid breakdown Ca++ dependent enzyme activation Cytoskeletal abnormalities Activation of proteases

Consequences of Membrane Damage Most important sites of membrane damage are: Mitochondrial membrane: Mitochondrial permeability transition pore Plasma membrane: Loss of osmotic balance Influx of fluids, ions & cellular contents Lysosomal membrane: Leakage and activation of acid hydrolases, RNases, DNases, proteases, glucosidases, phosphatases and cathepsins.

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