GF 6

GF 6

1. DNA

   • Physical: Radiation
     • X/¥ rays, has sufficient energy to break chemical bonds. The main target of ionising radiation in cells is water, which undergoes radiolysis to H and the hydroxyl radical OH. Ionising radiation and OH cause DNA strand breaks. The incorrect rejoining of broken ends (mis-repair) causes chromosome translocations.
     • Acute toxic effects include damage to vascular endothelial cells. Death of endothelial cells may lead to ulceration,scarring and atrophy shrinkage of surrounding cells.
     • Ultraviolet radiation damages DNA bases. Adjacent pyrimidines (C, T) become linked. Mis-repair generates C to T mutations contributing to cancer development. Excessive damage leads to death of keratinocytes (peeling sunburn).
   • Chemical: Alkylation
     • In undeveloped countries, the fungal product aflatoxin B1 accumulates in poorly stored food.
     • Metabolites alkylate (react covalently with)
       • Proteins to cause liver injury (aflatoxicosis), acutely, at high doses.
       • DNA, leading to G to T mutations and liver cancer, chronically, a low doses.
   • Biological: Dietary Deficiency
     • Folic acid (vitamin B9) and cyanocobalamin (vitamin B12) are needed to DNA synthesis and repair.
     • In autoimmune gastritis, lack of intrinsic factor prevents B12 absorption, leading to B12 deficiency and megaloblastic anaemia.

2. LIPIDS

   • Lipids are vital components of cells: they comprise membranes, that maintain compartments within cells, and provide the boundary between cells and the external environment.
   • Physical: Crystals
     • Cells take up crystals - silica, asbestos, monosodium urate, cholesterol and hydroxyapatite - into lysosomes.
     • Crystals puncture lysosomal membranes, releasing hydrolytic enzymes
     • These activate inflammasome (sensors of danger signals) to generate damaging inflammation.
   • Chemical: Oxidants
     • Free radicals (atoms or molecules with unpaired electrons) and reactive oxygen species (ROS) participate in oxidative reactions.
       • Superoxide is relatively stable, and is detoxified by superoxide dismutase to O2 and H2O2 ~1% of the oxygen consumed by mitochondria is converted to superoxide.
       • Hydrogen peroxide is detoxified by catalase to O2 and water.
       • Hydroxyl radical is the most destructive radical. It can damage all biological molecules and is the common effector of many types of injury.
     • ROS injure cells under conditions of...
       • Oxygen therapy: exposure of premature babies (who have inadequate antioxidant defences) to super-physiological oxygen concentrations may lead to lung damage.
       • Inflammation: mediated by neutrophils and macrophages
       • UV Radiation: excites biological molecules (photosensitisers) which transfer energy or electrons to O2 (producing singlet oxygen O2, and O2 respectively).
       • Damaged Mitochondria: which are not able to reduce O2 completely.
       • Radiotherapy
       • Membrane damage occurs in chain reactions involving carbon-centerer radicals, lipid pertly radicals, and lipid hydroperoxide. L-H represents any covalent bond involving H, in any lipid molecule.
   • Biological: Lipases
     • Acute Haemorrhagic Pancreatitis: may arise from damage to exocrine cells (that synthesise digestive enzymes) or from blockage to the ducts that deliver these enzymes to the duodenum. Activated enzymes (phospholipases, lipase) are released and digest cell membranes and triglycerides.

3. PROTEINS

   • Physical: Heat
     • Prolonged temperatures >42C occur in high fever, heatstroke arising from high ambient temperatures or exertion, or susceptible people treated with certain anaesthetics (malignant hyperthermia).
     • High temperatures denature proteins.
     • The presence of denatured proteins can be demonstrated by production of heat shock proteins.
   • Chemical: Glycation
     • In 1912 the chemist Maillard heated-reducing sugars and amino acids to produce a yellowish-brown product.
     • The non-enzymatic addition of sugars to proteins is called glycation or the Maillard reaction (cf the enzymatic addition of sugars to proteins, which is glycosylation).
     • The reaction between reducing sugars (such as glucose) + amino groups (at the N-terminus of proteins, or on lysine and arginine residues) generates
       • Reversible early-stage Schiff bases
       • Irreversible rearrangements to Amadori products
       • Further rearrangements to advanced glycation end products, AGE.
         • AGE accumulates during ageing, diabetes, and chronic inflammation. They promote CVD (inc. retinal and kidney damage, retinopathy and nephropathy), neurodegeneration and cataract formation.
     • AGE injure cells
       • Inhibiting protein function
       • Cross-linking and precipitating proteins (blocking axonal transport in neurons)
       • Generating ROS
       • Binding to receptors of AGE (RAGE) on vascular and inflammatory cells, reducing blood flow and causing inflammation.
   • Biological: Proteases
     • Proteolytic cleavage of ECM proteins occurs during inflammation
       • Collagen in Arthritis
       • Elastin in Emphysema
       • Laminin during Cancer Invasion

4. REVERSIBLE EFFECTS OF CELLS

   • Injury may be reversible in which case cells adapt and recover. Severe injury may lead to irreversible damage (cell death)
   • Acute Intracellular Oedema (Hydropic Change)
     • Cell injury may compromise the ability of cells to regulate ion concentrations in the cytoplasm.
       • Plasma membranes become more permeable to Na+
       • The Na+ pump is damaged
       • ATP synthesis is disrupted and the Na+/K+ ATPase inhibited
       • As a result, K+ leaks out of the cell and K+ leaks in. Cells and organelles (ER, mitochondria) swell osmotically. This is initially reversible.
   • Abnormal Storage
     • Abnormalities in the capacity of cells to recycle components result in accumulation of products such as fat and glycogen. During fatty change in liver cells, triglycerides accumulate.
       • Normal liver cells are unable to metabolise increased concentrations of fatty acids arriving from adipose tissue (diabetes)
       • Abnormal liver cells have a reduced ability to oxidise fatty acids (alcohol damage), or to export triglycerides complexed with lipid-acceptor proteins as VLDL (kwashiorkor, severe malnutrition.
   • Adaptive Responses
     • Cells may actively adapt to stress through a variety of mechanisms
       • The activation of transcription factors
       • Their binding to response elements in gene promoters.
       • The transcription of genes encoding protective proteins.
   • Examples
     • DNA Damage Response
       • The p53 transcription factor regulates repair, cell cycle arrest or (with severe damage) cell suicide.
     • Antioxidant Response
       • Oxidative stress activates Nrf family transcription factors, which induce genes encoding SOD1 and catalase (among others).
     • Heat Shock Response
       • Agents that damage proteins activate heat shock factors, which induce the expression of molecular chaperones (heat shock proteins). These bind to denatured proteins.
         • Prevent their aggregation (which is cytotoxic)
         • Aid in their renaturation
         • Promote proteolytic destruction of proteins that are too damaged.
     • Hypoxia Response
       • Hypoxia-inducible factors induce a genetic programme to help cells adapt to hypoxia.
       • Targets include glucose transporters, glycolytic enzymes and factors to increase production of erythrocytes
     • Unfolded Protein Response
       • Many stressors induce the production of unfolded proteins in the ER (ER stress, virus infection, hypoxia, ROS, metabolic abnormalities such as hyperlipidemia, inflammation)
       • The UPR induces synthesis of chaperones