Cell Injury Lec1 - DNA Repair Mechanisms PDF

DNA Repair Mechanisms:  Repair Activation: During the cell cycle arrest, DNA repair mechanisms are activated to correct the damage. These mechanisms include nucleotide excision repair, base excision repair, and double-strand break repair, depending on the type of damage. ...

DNA Repair Mechanisms:  Repair Activation: During the cell cycle arrest, DNA repair mechanisms are activated to correct the damage. These mechanisms include nucleotide excision repair, base excision repair, and double-strand break repair, depending on the type of damage.  Successful Repair: If the DNA damage is repaired successfully, the cell can safely proceed through the cell cycle and continue normal function. Outcome of Failed DNA Repair:  Induction of Apoptosis: If the DNA damage is too extensive or cannot be repaired, p53 initiates apoptosis through the mitochondrial pathway. This programmed cell death is a protective mechanism to prevent the proliferation of cells with potentially oncogenic mutations.  Role in Cancer Prevention: By triggering apoptosis, p53 ensures that cells with significant DNA damage do not survive, which is crucial for preventing malignant transformation. C ANCER AND P 53 M UTATIONS :  Impact of p53 Mutations: Mutations in the p53 gene can impair its ability to arrest the cell cycle or induce apoptosis. These mutations are commonly associated with various types of cancer because they allow cells with damaged DNA to evade apoptosis and continue dividing, potentially leading to tumor formation. OXIDATIVE STRESS AND ACCUMULATION OF OXYGEN-DERIVED FREE RADICALS Overview:  Free Radicals and ROS: Free radicals, especially reactive oxygen species (ROS), play a significant role in cell injury across various pathological conditions, including chemical and radiation injury, ischemia-reperfusion injury, cellular aging, and microbial killing by phagocytes. Free radicals have an unpaired electron, making them highly reactive,  capable of attacking and modifying essential molecules like proteins, lipids, carbohydrates, and nucleic acids. Generation of Free Radicals: 1. Normal Metabolic Processes: During cellular respiration, molecular oxygen (O₂) is reduced through a series of redox reactions, producing small amounts of partially reduced intermediates like superoxide anion (O₂ −), hydrogen peroxide (H₂O₂), and hydroxyl radicals ( OH). 2. Radiant Energy: Ultraviolet light and X-rays can cause the formation of free radicals by hydrolyzing water into OH and hydrogen free radicals. 3. Inflammation: Activated leukocytes, such as neutrophils and macrophages, produce ROS in controlled bursts during inflammation, primarily through the activity of NADPH oxidase. 4. Enzymatic Metabolism: Certain exogenous chemicals or drugs can be metabolized into free radicals. For example, carbon tetrachloride (CCl₄) can be converted into the free radical CCl₃. 5. Transition Metals: Metals like iron and copper can catalyze free radical formation through reactions like the Fenton reaction, where H₂O₂ is converted into OH in the presence of Fe²⁺. 6. Nitric Oxide (NO): NO, produced by various cell types, can act as a free radical and be converted into reactive species like peroxynitrite anion (ONOO⁻). Removal of Free Radicals: 1. Spontaneous Decay: Free radicals are inherently unstable and often decay spontaneously. For example, O₂ − can spontaneously dismutate into O₂ and H₂O₂. 2. Antioxidants: Non-enzymatic antioxidants, such as vitamins E and A, ascorbic acid, and glutathione, can neutralize free radicals. These molecules either prevent the formation of free radicals or directly scavenge them. 3. Metal Binding Proteins: Iron and copper are sequestered by proteins like transferrin, ferritin, lactoferrin, and ceruloplasmin, which prevent them from catalyzing harmful free radical reactions. 4. Enzymatic Defense Mechanisms: o Catalase: Found in peroxisomes, catalase decomposes H₂O₂ into water and oxygen. o Superoxide Dismutases (SODs): These enzymes convert O₂ − into H₂O₂ and O₂. They are found in various cellular locations, including mitochondria and the cytoplasm. o Glutathione Peroxidase: This enzyme reduces H₂O₂ and other peroxides, protecting cells from oxidative damage. Pathological Effects of Free Radicals: 1. Lipid Peroxidation: ROS, particularly OH, can attack unsaturated fatty acids in cell membranes, initiating a chain reaction that leads to the formation of lipid peroxides. This process compromises membrane integrity, leading to cellular dysfunction. 2. Oxidative Modification of Proteins: Free radicals can oxidize amino acid side chains, form disulfide bonds, and damage the protein backbone, leading to enzyme inactivation, structural protein disruption, and increased degradation of misfolded proteins. 3. DNA Damage: ROS can cause single and double-strand breaks in DNA, cross-linking of DNA strands, and the formation of DNA adducts. This damage is associated with cellular aging and the development of cancers. Physiological and Pathological Roles:  While traditionally associated with necrosis and cell injury, ROS can also induce apoptosis under certain conditions. Moreover, when produced in controlled amounts, ROS can serve important physiological functions in cellular signaling pathways. DISTURBANCE IN CALCIUM HOMEOSTASIS AND ITS ROLE IN CELL INJURY CALCIUM AS A SIGNALING M OLECULE :  Calcium ions (Ca²⁺) normally act as essential second messengers in various cellular signaling pathways, regulating processes like muscle contraction, neurotransmitter release, and cell proliferation. However, when cytosolic Ca²⁺ levels become excessively elevated, they contribute to cell injury. Normal Calcium Homeostasis:  Cytosolic Ca²⁺ Concentration: Under normal conditions, the concentration of free Ca²⁺ in the cytosol is kept very low (~0.1 μmol) compared to extracellular levels (~1.3 mmol). Intracellular Ca²⁺ is primarily stored in the endoplasmic reticulum (ER) and mitochondria.  Maintenance of Low Cytosolic Ca²⁺: Cells maintain low cytosolic Ca²⁺ through active transport mechanisms that pump Ca²⁺ into the ER, mitochondria, and out of the cell. Causes of Calcium Disturbance:  Ischemia and Toxins: Ischemia (lack of blood supply) and exposure to certain toxins can lead to an abnormal increase in cytosolic Ca²⁺. This occurs initially through the release of Ca²⁺ from intracellular stores (ER and mitochondria) and later through increased influx across the plasma membrane due to damaged membrane channels and transporters. Mechanisms of Calcium-Induced Cell Injury: 1. Mitochondrial Damage: o Mitochondrial Permeability Transition Pore: Excessive Ca²⁺ accumulation in the mitochondria leads to the failure of ATP generation, which is critical for cell survival. 2. Enzyme Activation: o Phospholipases: These enzymes are activated by elevated Ca²⁺ levels and cause the breakdown of membrane phospholipids, leading to membrane damage and increased permeability. o Proteases: Ca²⁺-activated proteases degrade cytoskeletal and membrane proteins, compromising the structural integrity of the cell and its ability to maintain shape and function. o Endonucleases: These enzymes fragment DNA and chromatin, potentially leading to DNA damage and disrupting normal cellular function. o ATPases: Increased Ca²⁺ activates ATPases, which consume ATP at a faster rate, exacerbating the depletion of cellular energy reserves and hastening cell death. CLINICOPATHOLOGIC CORRELATIONS : HYPOXIA, ISCHEMIA, AND REPERFUSION INJURY HYPOXIA AND ISCHEMIA:  Ischemia is a common cause of cell injury in clinical settings, typically due to reduced blood flow caused by arterial obstruction or reduced venous drainage. Unlike hypoxia, where blood flow is maintained but oxygen is limited, ischemia cuts off blood supply entirely, leading to more severe tissue injury.  Key Differences: o Hypoxia: Maintained blood flow allows some energy production through anaerobic glycolysis, despite the reduced oxygen supply. o Ischemia: The absence of blood flow prevents both aerobic and anaerobic metabolism, leading to rapid and severe cell and tissue injury. Mechanisms of Ischemic Cell Injury: 1. Sequence of Events: o Oxygen Deprivation: As oxygen levels decrease, oxidative phosphorylation in mitochondria ceases, resulting in decreased ATP production. o ATP Depletion: The loss of ATP initially causes reversible cell injury, such as cell and organelle swelling. Prolonged ischemia leads to irreversible injury and cell death by necrosis. 2. Cellular Response to Hypoxia: o Hypoxia-Inducible Factor-1 (HIF-1): Cells respond to hypoxia by activating HIF-1, which promotes angiogenesis, enhances glycolysis, and stimulates survival pathways. Although investigational drugs are being developed to enhance HIF-1 signaling, effective therapies for ischemic injury are still lacking. o Therapeutic Hypothermia: Lowering the body’s core temperature to about 92°F is used in cases of ischemic brain and spinal cord injury. Hypothermia reduces metabolic demands, decreases swelling, suppresses free radical formation, and inhibits inflammatory responses, helping to minimize cell and tissue injury. Reperfusion Injury:  Reperfusion Injury occurs when blood flow is restored to ischemic tissue. While reperfusion is necessary to save tissue, it can paradoxically cause further damage through several mechanisms: 1. Oxidative Stress: o Free Radical Generation: Reoxygenation leads to the production of reactive oxygen and nitrogen species (ROS/RNS) in reperfused tissues, particularly from leukocytes, damaged endothelial cells, and parenchymal cells. The compromised antioxidant defenses during ischemia make cells more vulnerable to this oxidative stress. 2. Intracellular Calcium Overload: o Calcium Influx: During reperfusion, there is an influx of calcium into cells, exacerbating the calcium overload that began during ischemia. This leads to the opening of the mitochondrial permeability transition pore, further depleting ATP and causing additional cell injury. 3. Inflammation: o Inflammatory Response: Ischemia triggers an inflammatory response characterized by the release of "danger signals" from dead cells, cytokine secretion by immune cells, and increased expression of adhesion molecules by hypoxic cells. This attracts neutrophils to the reperfused tissue, causing further damage. Blocking cytokines or adhesion molecules has been shown experimentally to reduce this inflammation and injury. 4. Complement System Activation: o Complement Activation: Some IgM antibodies tend to deposit in ischemic tissues. Upon reperfusion, complement proteins bind to these antibodies, triggering their activation and exacerbating cell injury and inflammation. Chemical (Toxic) Injury Overview: Chemical injury is a significant issue in clinical medicine and often limits drug therapy, particularly affecting the liver due to its role in drug metabolism. Toxic liver injury is a common reason for discontinuing or halting the development of certain drugs. There are two primary mechanisms by which chemicals cause cell injury: 1. D IRECT T OXICITY :  Mechanism: Some chemicals directly injure cells by interacting with and damaging critical molecular components within the cell.  Examples: o Mercuric Chloride Poisoning: leading to increased membrane permeability and disrupted ion transport. This primarily affects cells that are involved in the absorption, excretion, or concentration of these chemicals, such as those in the gastrointestinal tract and kidneys. o Cyanide Poisoning: Cyanide inhibits mitochondrial cytochrome oxidase, blocking oxidative phosphorylation and leading to cell death. o Chemotherapeutic Agents and Antibiotics: Many of these drugs exert their effects through direct cytotoxic mechanisms, damaging cells they target. 2. C ONVERSION TO T OXIC M ETABOLITES :  Mechanism: Many chemicals are not toxic in their original form but become harmful after being metabolized into reactive toxic metabolites. This conversion often occurs via the cytochrome P-450 enzyme system in the liver.  Examples: o Carbon Tetrachloride (CCl₄): Used historically in the dry cleaning industry, CCl₄ is converted by cytochrome P-450 enzymes into the reactive free radical CCl₃. This radical causes lipid peroxidation, leading to widespread cellular damage. o Acetaminophen: Commonly used as an analgesic, acetaminophen is metabolized in the liver to a toxic product that can cause significant liver injury, particularly in cases of overdose. REVERSIBLE CELL INJURY:  Definition: Reversible cell injury refers to functional and structural changes in cells during early stages or mild forms of injury that can be corrected if the harmful stimulus is removed. Key Features of Reversible Cell Injury: 1. Cellular Swelling: o Description: Cellular swelling is the earliest sign of almost all types of cell injury. It occurs when cells lose the ability to maintain ion and fluid balance, leading to the accumulation of water within the cell. o Effects: When many cells in an organ are affected, it can cause the organ to become pale, swollen (increased turgor), and heavier. o Microscopic Appearance: Under the microscope, small clear vacuoles may be visible in the cytoplasm. This type of injury is sometimes referred to as hydropic change or vacuolar degeneration. 2. Fatty Change: o Description: Fatty change occurs in cells of organs that are heavily involved in lipid metabolism, such as the liver. It happens when toxic injury disrupts metabolic pathways, leading to the accumulation of triglyceride-filled lipid vacuoles within the cell. o Affected Organs: The liver is the most common organ affected, but fatty change can also occur in the heart, kidneys, and muscles. Conclusion: Reversible cell injury represents an early stage of cellular response to damage, characterized by changes such as cellular swelling and fatty change. These alterations can be reversed if the damaging stimulus is removed before the injury progresses to an irreversible state.