1. Mechanism of Cell Injury
Dr Shoaib Raza

2. 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.

3. 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)

4. 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)

5. 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

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

9. 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

10. 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

11. 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

13. 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)

14. 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-

15. 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

16. 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

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

18. 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

19. 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.

20. Any question?