Lecture 6

What is the overall pathway of cell injury?

Cell injury → damage to DNA, lipids, or proteins → cell response (adaptation, reversible injury, or death)

How does DNA get damaged? (Physical, Chemical, Biological)

Physical: IR, UV
Chemical: Alkylations (eg Alflatoxin)

Biological: Nutrient Deficiency (Folic Acid (Vitamin B9) and Cobalamin (Vitamin

B12))

How does ionising radiation damage DNA?

Ionising Radiation damages DNA by reacting with water (hydrolysis) producing free radicals (OH-). These break the DNA strands

Toxicity: Acute damage to vascular endothelial cells → cells die = surrounding tissue shrinks (atrophy) → scarring, ulceration, and bleed

Misrepair: Mutations accumulate → chromosome translocations = blood cancers

How does UV radiation damage DNA?

UV radiation damages DNA by causing pyrimidine cross-linking and forming DNA adducts. The DNA cannot be read/copied.

Toxicity: Acute damage to keratinocytes on the skin → peeling sunburn

Misrepair: Chronic damage leads to accumulation of mutations → skin cancer

How does Alkylation damage DNA? (eg Aflaxtoxin)

Toxins like Aflatoxin B1, chemical group attaches to G on DNA strand forming DNA adducts which then leads to G to T transversions in DNA replication.

Toxicity: Leads to damage in the liver and acute aflatoxicosis (injury) at high doses

Misrepair: Chronic low doses can lead to hepatocellular carcinoma

How does dietary deficiency damage DNA? (eg Folic Acid)

Folic Acid (Vitamin B19) and Cobalamin (Vitamin B12) are needed for DNA synthesis (T and MET). When deficient (via diet or genetics), correct DNA cannot be synthesised or repaired.

Misrepair: faulty replication → abnormal cell division → megaloblastic anaemia

What are the 3 outcomes of DNA damage?

Repair → Survival

Misrepair → Mutation → Cancer

Severe Damage → Cell Death

How do Lipids get damaged? (Physical, Chemical, Biological)

Physical: Crystals

Chemical: Oxidants

Biological: Lipases

How do crystals cause lipid injury?

Macrophages will phagocytose the crystal (MSU, Silica, Asbestos etc.) and form a phagosome. When the phagosome fuses with the lysosome, it gets damaged, and its contents will leak, activating inflammasomes and causing inflammation.

What is the ROS formation sequence?

O2 → O2.- → H2O2 → .OH

Oxygen → Superoxide → Hydrogen Peroxide → Hydroxyl Radical

How do ROS damage lipids?

ROS steals an electron from the phospholipid membrane of a cell, causing lipid peroxidation. This causes a chain reaction across the membrane, resulting in membrane damage.

• Increased membrane rigidity

• Decreased activity of enzymes

• Altered activity of membrane receptors

• Increased membrane permeability

= cell dysfunction

What are key ROS enzymes and what do they convert?

• Superoxide Dismutase (SOD) converts Superoxide anion radical into O2 and Hydrogen Perioxide

• Catalase converts hydrogen peroxide into water and O2

• Hydroxyl radicals react with lipid hydrogens → lipid peroxidation

Where do ROS come from in the body?

Mitochondria during respiration

Neutrophils during inflammation

Enzyme reactions

Hypoxia/Hyperoxia

Signalling molecules

How do lipases/phospholipases cause cell injury?

Enzymes that breakdown fats get released in the wrong place. Eg acute haemorrhagic pancreatitis causes damage to exocrine, enzyme packed cells - these leak and damage the cells surrounding them.

→ Membrane Destruction and Cell Death

How do Proteins get damaged? (Physical, Chemical, Biological)

Physical: Heat

Chemical: Glycation

Biological: Proteases

How does heat damage proteins?

Heatstroke, fever and hyperthermia lead to protein denaturation (proteins lose 3D shape) so enzymes strop working.

Loss of structure → Loss of function → Cell Dysfunction

How do heat shock proteins (HSPs help)?

HSPs bind to denatured proteins, stop their aggregation (clumping), help refold them and destroy beyond repair proteins.

How does glycation damage proteins?

Sugar binds to proteins (non-enzymatically) and forms a schiff base which turns into a amadori product and then into an Advanced Glycation End Product (AGE) (Maillard Reaction). This is irreversible.

Why are AGEs harmful?

Inhibits protein function

Causes proteins to cross-link (stiff) and become insoluble (accumulate precipitate)

Generate ROS

Bind to cell surface receptors (RAGE) and signals to reduce blood flow and cause inflammation

Occurs in diabetes, aging, and chronic inflammation and results in cardiovascular disease (retinas/kidneys), neurodegeneration and cataracts.

How do proteases cause damage?

Proteases cleave (cut) structural proteins, so tissue breakdown = altered cell function

Targets:

collagen (joints) → arthritis

elastin (lungs) → emphysema

laminin (basement membrane) → cancer