Lec 6. Cell injury- II (Ischemia and Hypoxic Injury).pdf

Cell Injury – II Ischemia and Hypoxic Injury Autophagy Pathology Unit of BMS (UCM) Learning Objectives By the end of this lecture, the students should be able to:  Cell Path Inj.IV.1. Define ischemia.  Cell Path Inj.IV.2. Explain why ischemia results in faster tissue injury than isolated hypoxia....

Cell Injury – II Ischemia and Hypoxic Injury Autophagy Pathology Unit of BMS (UCM) Learning Objectives By the end of this lecture, the students should be able to:  Cell Path Inj.IV.1. Define ischemia.  Cell Path Inj.IV.2. Explain why ischemia results in faster tissue injury than isolated hypoxia.  Cell Path Inj.IV.3. Discuss the mechanisms contributing to ischemia- reperfusion injury.  Cell Path Inj.IV.4. Describe the effects on injured cells with reperfusion after ischemia.  Cell Path Inj.IV.5. Explain the major mechanisms of chemical injury.  Cell Path Inj.IV.6. Define autophagy and describe its purpose.  Cell Path Inj.IV.7. Outline the functional phases of autophagy Ischemic cell Injury  Ischemia: reduction in blood flow to a tissue or organ, usually due to occlusion of an artery, but can also be caused by severe blood loss, sepsis, or any cause of hypotension (abnormally decreased blood pressure)  Ischemia causes tissue injury at faster rate than hypoxia alone:  deprives tissues of oxygen (hypoxia)  deprives tissues of glucose (substrate for anaerobic glycolysis)  diminishes removal of injurious substances  Cardiovascular disease (#1 cause of death in U.S.) and cerebrovascular disease (#3 cause of death) have an ischemic pathogenesis in most cases. Outcomes of Ischemic Injury  Reversible: Injury may be avoided or reduced if adequate blood flow restored. Effort-related chest pain due to myocardial ischemia (“angina”) may be relieved by rest, which decreases the myocytes’ need for oxygen and glucose  Irreversible: ischemia produces cell damage that cannot be repaired. In cardiac myocytes, total deprivation of blood flow causes irreversible injury in about 30-40 minutes (based on electron microscopy studies in animal models).  Clinical interventions to limit irreversible ischemic injury 1. Restoration of blood flow soon after ischemia occurs 2. Induction of hypothermia (core body temperature 92° F.) to reduce metabolic needs of injured cells, suppress formation of ROS, and inhibit inflammatory response Ischemia-Reperfusion Injury  Reperfusion: restoration of blood flow after a period of ischemia (animal & human studies)  Effects on injured cells with reperfusion after ischemia: 1. reperfusion promotes survival of some (not all) reversibly injured cells 2. reperfusion accelerates cell injury and death in a subpopulation of reversibly injured cells Ischemia-Reperfusion Injury: Several mechanisms A. Oxidative stress  New damage may be initiated during reoxygenation by increased generation of reactive oxygen and nitrogen species.  These free radicals may be produced in reperfused tissue as a result of incomplete reduction of oxygen by damaged mitochondria, or because of the action of oxidases in leukocytes, endothelial cells, or parenchymal cells.  Cellular antioxidant defense mechanisms may be compromised by ischemia, favoring the accumulation of free radicals Ischemia-Reperfusion Injury: Several mechanisms B. Intracellular calcium overload  As mentioned earlier, intracellular and mitochondrial calcium overload begins during acute ischemia; it is exacerbated during reperfusion due to influx of calcium resulting from cell membrane damage and ROS mediated injury to sarcoplasmic reticulum.  Calcium overload favors opening of the mitochondrial permeability transition pore with resultant depletion of ATP.  This in turn causes further cell injury Ischemia-Reperfusion Injury: Several mechanisms C. Inflammation  Ischemic injury is associated with inflammation as a result of “dangers signals” released from dead cells, cytokines secreted by resident immune cells such as macrophages, and increased expression of adhesion molecules by hypoxic parenchymal and endothelial cells, all of which act to recruit circulating neutrophils to reperfused tissue.  The inflammation causes additional tissue injury.  The importance of neutrophil influx in reperfusion injury has been demonstrated experimentally by the salutary effects of treatment with antibodies that block cytokines or adhesion molecules and thereby reduce neutrophil extravasation Ischemia-Reperfusion Injury: Several mechanisms D. Activation of the complement system  May contribute to ischemia-reperfusion injury.  Some IgM antibodies have a propensity to deposit in ischemic tissues, for unknown reasons, and when blood flow is resumed, complement proteins bind to the deposited antibodies, are activated, and cause more cell injury and inflammation Summary of mechanism for I-R injury  Reperfusion brings oxygen, generating additional reactive oxygen and nitrogen species through oxidases in the injured parenchymal cells, endothelial cells, and leukocytes  Injured mitochondriae produce more ROS, because of inability to completely reduce the oxygen supplied by reperfusion  Inflammation is amplified by restoration of blood flow.  hypoxic parenchymal cells produce cytokines that activate endothelial cells  damaged endothelial cells increase the expression of adhesion molecules to recruit neutrophils into injured tissue  more cellular injury and necrosis Chemical-Induced Cellular Injury  Direct toxicity:  Some chemicals can injure cells directly by combining with critical molecular components.  For example:  Mercuric chloride poisoning  mercury binds to the sulfhydryl groups of cell membrane proteins, causing increased membrane permeability and inhibition of ion transport.  the greatest damage is to the cells that use, absorb, excrete, or concentrate the chemicals—in the case of mercuric chloride, the cells of the gastrointestinal tract and kidney.  Cyanide poisons mitochondrial cytochrome oxidase and thus inhibits oxidative phosphorylation.  Many antineoplastic chemotherapeutic agents and antibiotics also induce cell damage by direct cytotoxic effects  In-Direct toxicity:  Conversion to toxic metabolites.  Most toxic chemicals are not biologically active in their native form but must be converted to reactive toxic metabolites, which then act on target molecules.  This modification is usually accomplished by the cytochrome P-450 mixed- function oxidases in the smooth ER of the liver and other organs.  The toxic metabolites cause membrane damage and cell injury mainly by:  formation of free radicals and subsequent lipid peroxidation;  direct covalent binding to membrane proteins and lipids may also contribute.  For instance, Carbon tetrachloride (CCl4), used in the dry cleaning industry, is converted by cytochrome P-450 to the highly reactive free radical ˙CCl3, which causes lipid peroxidation and damages many cellular structures.  Acetaminophen, an analgesic drug, is also converted to a toxic product during detoxification in the liver, leading to cell injury. Chemical-Induced Cellular Injury  Combining with a critical molecular component or cellular organelle  –Mercury combines with cell membrane, inhibiting membrane transport and permeability control  –Similar cytotoxicity from many anticancer agents and antibiotics  Conversion to a toxic metabolite eg Ccl4  Damage mostly by free radical formation  CCl4 → CCl3 → autocatalytic lipid peroxidation and rapid breakdown of endoplasmic reticulum → ↓ lipid transport → “fatty liver,” with Ca2+ influx and liver cell death Chemical Injury: Acetominophen Overdose Therapeutic dose: 250-500 milligrams Hepatocytes produce small amounts of toxic metabolite that is neutralized by glutathione; no cellular injury observed Toxic dose: 15-25 grams Hepatocyte glutathione depleted by toxic metabolite (N-acetyl-p-benzoquinoneimnine). NAPQI binds to proteins, damaging cell membranes and mitochondriae  liver cell necrosis occurs 2-4 days post-overdose Treatment of overdose with antioxidant N-acetylcysteine (a metabolic precursor of glutathione) replenishes GSH stores and reduces oxidative damage to hepatocytes by toxic metabolites. Drug therapy is based on plasma acetominophen levels correlated with time after ingestion. Autophagy Definition autophagy: digestion of cell’s endogenous cytoplasmic components & recycling of digested materials for intracellular use. Autophagy enhances cell survival in times of nutrient deprivation. Heterophagy: lysosomal digestion exogenous materials Residual bodies: persistent autophagic vacuoles with remains of organelles that can’t be digested or excreted Lipofuscin: lipid-derived granules resistant to autophagy Below: Fig. 2-28, PBD 9th, 2015 Functional phases of autophagy  Cellular stresses, such as nutrient deprivation, activate an autophagy pathway that proceeds through several phases  (initiation, nucleation, and elongation of isolation membrane)  Eventually creates double-membrane-bound vacuoles (autophagosome) in which cytoplasmic materials including organelles are sequestered and then degraded following fusion of the vesicles with lysosomes.  In the final stage, the digested materials are released for recycling of metabolites References (textbooks only)  Kumar, Abbas, Aster: Pathologic Basis of Disease, 9th edition, Elsevier, 2015.  Rubin, Strayer: Rubin’s Pathology: Clinicopathologic Foundations of Medicine, 6th ed., Lippincott, Williams, and Wilkins, 2011.