Cell Injury and Adaptation 1

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Cell Injury and Adaptation 1

• Basic Cell Pathology
• Robbins (7th edition), Chapter 1

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Cell Injury and Adaptation

• Basic and clinical science
• Injurious agent
• Structural change.
• Functional change.
• Effects on cells, tissues, and
  organs

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Cell Injury, Adaptation, and Cell Death

• Normal cellular function
• Physiologic parameters.
• Maintained by homeostasis.
• Physiologic stress or pathologic stimuli.
• Adaptation.
• Hypertrophy. Atrophy.
• Hyperplasia. Metaplasia.
• Lack of adaptation.
• Further cell injury.
• Cell death.
• Necrosis.
• Apoptosis.

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Cell Injury, Adaptation, and Cell Death

• Severe or persistent cell stress
• Reversible injury can give way to
  irreversible injury and cellular death.
• Necrosis.
• Apoptosis.
• Necrosis
• Cellular swelling, protein denaturation,
  organellar breakdown.
• Cellular death, perhaps with tissue dysfunction.
• Apoptosis
• Cellular suicide may also be physiologic, as
  in embryo.
• Cellular shrinkage, with elimination by adjacent
  cells or macrophages (larger types of
  infiltrating phagocytes).
• Minimal disruption of surrounding tissue, yet
  there can be tissue dysfunction when cells are
  lost.

Not always clear which occurred.
Effect will differ if dead cells are
parenchymal, stromal (supportive), or vascular.
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Normal, Adapted, and Injured Cells

• Relationship between cell states
  Normal, adapted, reversible injury and
  irreversible injury, illustrated here
  in myocardium
• Injury, such as increased mechanical load
  with hyper- tension or stenotic heart
  valve.
• Cells adapt by increasing size
  (hypertrophy).
• Stress, such as malnutrition.
• Atrophy of individual cells.
• Stress, such as ischemia.
• Myocardial cells are at least transiently
  noncontractile.
• Potentially lethal.

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How Does Atrophy Occur?

• Autophagy (awtofagy)
• A means of decreasing cell parts
  during starvation and of recycling the parts
  (e.g., amino acids).
• Mechanism for atrophy of cell.
• Organelle Lysosome.
• Extralysosomal (cytosol) Ubiquitin
  conjugation of proteins, then
  chaperone protein-mediated path to
  proteolytic organelles (proteasomes).
• Proteasomes degrade damaged or excessive
  cytosolic proteins marked for destruction, or
  re-fold them properly in some cases, but if
  mutant proteins cannot be degraded, a signal for
  apoptosis may be sent ? Cellular death.

proteasome
Cuervo Dice (1998) J Mol Med 766-12.
Finkbeiner et al. (2006) J Neurosci
2610349-10357.
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Injured and Atrophic Cells

• Autophagy
• Autophagy can ? Atrophy of cell.
• Lysosomes.
• Microautophagy of small cytosolic areas.
• Macroautophagy of cytoplasmic regions.
• Chaperone-mediated autophagy.
• Nonlysosomal Proteasomes.
• Targeted protein degradation.
• Chaperone proteins guide specific proteins
  to lysosomes.
• Proteasomes degrade only ubiquitinated
  proteins.
• Result of autophagy Minor physiologic change,
  a smaller more efficient cell, or atrophic cell
  trying to survive the stress.
• Starvation Autophagy Atrophy.

Cuervo Dice (1998) J Mol Med 766-12.
Finkbeiner et al. (2006) J Neurosci
2610349-10357.
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Cellular Adaptation

• Atrophy and hypertrophy
• Example Neurogenic atrophy (after nerve
  injury).
• Denervated muscle cells become small and angular.
• Unaffected muscle cells hypertrophy
  (compensatory).
• Hyperplasia
• Graves disease in thyroid gland.
• Duct epithelial hyperplasia in breast.

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Cellular Adaptation

• Metaplasia (protective change)
• Example Vitamin A deficiency.
• Columnar to keratinized squamous epithelium.
• Possibility of being precancerous.
• Example Reflux (Barrett) esophagitis.
• Early Squamous immaturity.
• Later Intestinal metaplasia (precancerous).

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Cell Injury and Adaptation

• Injury with inadequate adaptation Cellular
  death
• Example Huntington disease.
• Genetic mutation with death of caudate nucleus
  neurons in young or older adults.
• Below, loss of neurons (apoptosis) results in
  significant atrophy of caudate nuclei (atrophy is
  at tissue/organ level).
• Huntington brain Normal brain

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Injured Cells Causes

• Examples
• Single nucleotide substitution in DNA ? Enzyme
  defect.
• Environmental toxin.
• Auto accident, etc.
• Basic causes fall into these groups
• Oxygen deprivation. Genetic defects.
• Chemical agents. Nutritional imbalances.
• Infectious agents. Physical agents.
• Immunologic Aging. reactions.

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Injured Cells Causes

• Oxygen deprivation
• Hypoxia, or decreased O2 (?pO2) in the blood,
  interrupts aerobic respiration in cells.
• Ischemia is a decrease or loss of blood supply, a
  consequence of which is hypoxia, and also loss of
  nutrients such as glucose.
• Causes range from choking, to pneumonia with ?
  pulmonary function, to carbon monoxide (CO)
  poisoning wherein CO binds so avidly to
  hemoglobin that O2 has no binding site.
• Chemical agents
• Everything in moderation ??? Blood glucose ?
  shift of H2O into blood ? hyperosmotic coma.
• Oxygen toxicity with very high partial pressures.
• Poisons Alter membrane permeability or enzyme
  function.
• Organic and inorganic environmental agents.
• Therapeutic drugs.

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Injured Cells Causes

• Infectious agents
• Viruses.
• Chlamydia.
• Rickettsia.
• Bacteria.
• Gram positive.
• Gram negative.
• Mycobacteria.
• Spirochetes.
• Fungi.
• Parasites.
• Protozoans.
• Worms.

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Injured Cells Causes

• Immunologic reactions
• Anaphylaxis.
• Exposure to foreign protein resulting in
  urticaria, pruritus and angioedema with
  life-threatening vascular collapse (shock).
• Loss of tolerance by T cells that should not
  attack the self.
• May lead to autoimmune disease if a foreign
  antigen (e.g., viral protein) looks like (mimics)
  a self-antigen to the immune system and the
  self-antigen is no longer tolerated but is
  attacked.
• Genetic defects
• Single amino acid substitution (hemoglobin S in
  sickle cells).
• Enzyme deficiencies.
• Anatomic malformations
• (but many are sporadic).
• Metachromatic Cyclops
• leukodystrophy
  (trisomy 13 in
• (loss of one enzyme) some cases)

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Injured Cells Causes

• Nutritional imbalances ? Aging
• Protein-calorie. Cellular
  senescence.
• Vitamins. Degeneration.
• Excess. Poor healing.
• Type 2 diabetes mellitus.
• Atherosclerosis.
• Increased vulnerability to cancer.
• Physical agents
• Trauma.
• Temperature.
• Radiation.
• Electric shock.
• Atmospheric pressure.

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Injured Cells Mechanisms

• Injury type, duration and severity dictate
  cellular response
• For example, a low-dose toxin or brief ischemia
  can cause reversible injury, but ? toxin or ?
  ischemia can ? cell death.
• Type, status, adaptability and genetic makeup
  influence consequences of cellular injury
• Muscle ischemia.
• Skeletal striated muscle (such as in the leg)
  suffers 2 3 hour of complete ischemia with
  reversible injury.
• Cardiac striated muscle dies after 20 30
  minutes of ischemia.
• Hepatocyte ischemia.
• Liver cells survive ischemia better when filled
  with glycogen.
• Genetic polymorphisms.
• Different forms (isoforms) of the same enzyme
  can metabolize a toxin, as an example, more
  efficiently than other forms.

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Injured Cells Mechanisms

• Vulnerable intracellular systems
• ATP generation.
• Mitochondrial aerobic respiration.
• Cell membrane integrity.
• Ionic homeostasis.
• Osmotic homeostasis.
• Protein synthesis.
• Cell structure and function.
• Integrity of the genetic apparatus.
• Access to DNA.
• Interactions with DNA.
• DNA replication.
• DNA repair mechanisms.

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Injured Cells Mechanisms

• Secondary effects and consequences (example)
• Cyanide (CN) ? Poisoning of cytochrome oxidase ?
  ? ATP ? ? Activity of Na/K ATPase (plasma
  membranes Na pump) ? Loss of ability to
  maintain intracellular osmotic balance ?
  Rapid cellular swelling ? Dysfunction or rupture
  and cell death.
• Normal Na pump activity is to
  rid a cell of excess Na in exchange for K.
• Cellular structure and biochemical
  components for function are so
  integrally connected that an
  injury at one cellular level will
  affect all levels,
  and usually Together very
  rapidly.
• Cell death

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Injured Cells Mechanisms

• If the cell dies at this point, the later
  changes are not found.
• Time from cellular injury to loss of a specific
  activity varies, and time to death is variable,
  but both are basically dependent on most or all
  systems being intact up to an observable end
  point.

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Injured Cells Mechanisms

• Myocardial cells are noncontractile 1 2
  min after ischemia die after
  20 30 minutes
• Electron microscopy has changes by 2
  3 hr.
• Light microscopy, no change for 6
  12 hr.
• Gross specimen is still normal-appearing by 12
  hr.
• Cerebral neurons (do not store glycogen)
  nonfunctional in seconds after ischemia, die in 5
  10 minutes
• EM, changes after minutes to an hour.
• LM, ischemic changes in 3 hours shows infarction
  in 6 8 hr.
• Gross brain changes by 12 hr.

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Injured CellsGeneral Biochemical Mechanisms

• Pathogenic mechanisms
• Cyanide specfically inactivates
  mitochondrial cytochrome c oxidase.
• Bacterial phospholipases digest cell
  membrane phospholipids.
• Many mechanisms are incompletely understood.
• Basic biochemical routes through which cellular
  injury occurs
• ATP depletion.
• Oxygen deprivation or generation of reactive
  oxygen species.
• Loss of calcium homeostatsis.
• Defects in plasma membrane permeability.
• Mitochondrial damage.

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Injured Cells Biochemical Mechanisms

• ATP depletion
• Virtually every cellular process
  requires high-energy phosphates.
• Decrease of aerobic or anaerobic
  glycolysis deprives cell of energy.
• Rate and amount of ATP loss influences rate and
  extent of cellular decline or death.
• O2 ?/? O2-, H2O2, OH (highly reactive), NO
• ? O2 ? ? Cellular function.
• ? Reactive oxygen species ? Lipid
  peroxidation ??? Cell death signal.
• Small amounts of reactive oxygen
  species are produced by cellular
  activity it is the amount that can
  overwhelm antioxidants.

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Injured Cells Biochemical Mechanisms

• Loss of Ca2 homeostasis
• ATP-dependent Ca2 transporters.
• Intracellular cytosolic Ca2extracellular Ca2
  110,000.
• Small Ca2 stores in mitochondria and endoplasmic
  reticulum.
• Cellular injury ? Net influx of
  extracellular Ca2 across plasma
  membrane.
• Ca2 also released from intracellular
  stores.
• ? Cytosolic Ca2 activates many
  enzymes.
• ATPases (less ATP).
• Phospholipases.
• Membrane damage.
• Proteases.
• Structural damage.
• Endonucleases (nuclear damage).

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Injured Cells Biochemical Mechanisms

• Defects in plasma membrane permeability
• Direct injury.
• Bacterial toxins.
• Viral proteins.
• Complement components.
• Cytolytic lymphocytes.
• Physical agents.
• Chemical agents.
• Secondary injury.
• Loss of ATP synthesis.
• Ca2-mediated phospholipase activation.
• Loss of ability to maintain concentration
  gradients of electrolytes and various
  metabolites.
• Variable effects on cellular metabolism.
• When irreversible, can be lethal to cell.

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Injured Cells Biochemical Mechanisms

• Mitochondrial damage
• Critical to cellular energy metabolism and
  function.
• Most cellular injuries include
  mitochondria, directly or indirectly.
• High-conductance channels of inner
  mitochondrial membrane (permeability
  transition).
• Form in response to
• ? Cytosolic Ca2.
• Oxidative stress.
• Lipid breakdown products.
• Nonselective pores dissipate proton gradient
  across membrane.
• Also, cytochrome c, an electron transport chain
  protein, leaks into cytosol and activates
  apoptotic pathways.

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Three Common Forms of Cell Injury

• Ischemic/hypoxic
• Free radical injury
• Toxic injury

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Ischemic/Hypoxic Injury

• Diminished blood flow is the most
  common type of cellular injury
  in clinical medicine
• Tissue injury exacerbated by lack of
  removal of metabolites that inhibit
  anaerobic glycolysis.
• Hypoxia alone, with continued glucose
  supply, allows a low generation
  of ATP by glycolysis
• Since oxidative phosphor- ylation
  supplies vastly more ATP than anaerobic
  glycolysis, hypoxia greatly diminishes all
  metabolic processes in the cell.

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Ischemic/Hypoxic Injury

• ? ATP
• ? Na/K ATPase activity
• The Na pump.
• ? Cytosolic Na
• Water follows Na into cell.
• ? Cytosolic K
• Result Cellular swelling
• Lactic acid (measured in acutely ill
  patients as lactic acidosis),
  inorganic phosphates (no longer on
  ATP) and purine nucleosides accumulate to
  further increase the osmotic draw of H2O into the
  cell

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Ischemia/Reperusion Injury

• Ischemia ? Paradoxical further injury after
  ischemia
• This type of injury results in loss of cells in
  addition to those lost at the end of the
  ischemic/hypoxic episode.
• Following myocardial infarct or cerebral infarct.
• Therapeutic intervention is possible.
• Mechanisms, so far as they are understood
• ? Extracellular Ca2 around injured cells after
  reperfusion.
• Loss of control over ionic environment ? ?
  Intracellular Ca2.
• Inflammatory cells, including PMN marginated
  within blood vessel lumina, release high levels
  of reactive oxygen species.
• Membrane damage, including mitochondrial
  transition (pores).
• Damaged mitochondria incompletely reduce O2,
  increasing free radicals.
• Compromised antioxidant defense (e.g., superoxide
  dismutase, glutathione system).

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Free Radical-Induced Cellular Injury
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Free Radical-Induced Cellular Injury

• What is a free radical?
• Has a single unpaired electron.
• Extremely unstable.
• Readily reacts with most other chemicals.
• Avidly degrades nucleic acids and membrane
  molecules.
• Initiates autolysis (by lysosomal enzymes and
  proteasomes).
• Molecules damaged by free radicals are converted
  into more free radicals
• Downhill spiral of injury.

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Free Radical-Induced Cellular Injury

• Damage induced by free radicals
• Induced in injured cells in general.
• Sources.
• Chemical injury.
• Radiation injury.
• Oxygen toxicity.
• Cellular aging.
• Inflammatory cell- induced damage.
• In phagocytic activity.
• Phagocytic killing of microbes.
• Phagocytic killing of tumor cells.
• Bystander damage of uninjured cells.

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Free Radical-Induced Cellular Injury

• Reduction-oxidation (redox) reactions
• Normal physiologic activity produces
  free radicals.
• At the end of the mitochondrial
  respiratory chain, O2 accepts electrons
  in forming water with protons
  (hydrogen).
• During this process, toxic intermediates
  form O2-, H2O2, OH.
• Cellular (P-450) oxidases also produce free
  radicals.
• Fenton reaction
• Ferrous iron catalyzes free radical formation, a
  major process in areas of hemorrhage.
• Fe2 H2O2 ? Fe3 OH OH-.
• Most cellular iron is ferric, but O2- donates an
  electron ? Fe2.

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Free Radical-Induced Cellular Injury

• NO is a widespread chemical mediator
• Can act as a free radical.
• Can be converted into peroxynitrite.
• O2- NO H ? ONOOH (peroxynitrite) ? OH
  NO2.
• Ionizing radiation (UV light, X-rays)
• H2O hydrolyzed into OH and H (hydrogen) free
  radicals.
• Breakdown of exogenous chemicals (CCl4).
• Cellular injury by free radicals particular-
  ly occurs through these reactions

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Free Radical-Induced Cellular Injury

• Lipid peroxidation of membranes
• Free radicals membrane lipid double bonds ?
  Peroxides, which are reactive and damage
  other molecules.
• DNA fragmentation
• Free radicals thymine (nuclear and
  mitochondrial DNA) ? Single strand
  breaks in DNA (an apoptotic type of DNA
  strand break, and may produce malignant
  transformation of cell).
• Cross-linking of proteins
• Enhanced degradation or loss of enzymatic
  activity.
• Free radicals may directly cause protein
  fragmentation.

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Free Radical-Induced Cellular Injury

• Getting rid of free radicals
• Spontaneous decay.
• Enzyme systems.
• Superoxide dismutases.
• Glutathione peroxidase.
• Catalase.
• Vitamins A, C and E, and b-carotene.
• Sequestered heavy metals
• Free ionized iron and copper are not readily
  available to catalyze the formation of reactive
  oxygen species because the cell partitions them
  in storage and as transport proteins.
• Ferritin, transferrin, ceruloplasmin.

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Chemical-Induced Cellular Injury

• Combining with a critical molecular component or
  cellular organelle
• Mercury (HgCl2) combines with cell membrane
  sulfydryl groups, inhibiting membrane transport
  and permeability control.
• Similar cytotoxicity from many anticancer agents
  and antibiotics.
• Most damage is to cells that use or handle the
  chemical.
• Conversion to a toxic metabolite
• Oxidases (e.g., liver P-450) form reactive
  intermediates.
• Damage mostly by free radical formation.
• CCl4 ? CCl3 ? lipid peroxidation ? ? lipid
  transport ? fatty liver, with Ca2 influx and
  liver cell death.
• Acetaminophen is similarly converted in liver to
  a liver toxin.
• Intermediates also covalently bind to proteins
  and lipids.

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Cellular Adaptation to Injury

• End of Part 1