Cellular Response to Stress & Injury

Cellular Adaptations to Stress:

• Cells respond to stress by adapting, undergoing reversible injury, or succumbing to irreversible damage and death.

Overview of Cell Injury:

• Cells maintain homeostasis but face constant potential harm.
• Cellular responses to stress fall into three categories: adaptations, reversible injury, and cell death.

Adaptations:

• Adaptations are alterations enabling cells to manage stress without damage, enhancing survival in altered environments.
• Examples include muscle mass increase with workload.

Reversible Injury:

• Reversible injury involves structural and functional abnormalities correctable upon removal of the injurious agent.
• If the stress continues, injury could become irreversible, leading to cell death.

Cell Death:

• Cell death results from injury, occurring through necrosis and apoptosis pathways upon exposure to damaging agents.

Causes of Cell Injury:

• Diverse insults cause cell injury/death, leading to disease.
• Insults include:
  • Infectious pathogens: Injure cells through toxins or immune responses.
  • Hypoxia and ischemia: Reduce oxygen/blood supply, causing deprivation, nutrient loss, and metabolite buildup.
  • Toxins: Environmental and therapeutic drugs.
  • Environmental insults: Physical trauma, radiation exposure, nutritional imbalances.
  • Genetic abnormalities: Impair protein function or cause damaged DNA/misfolded proteins.
  • Immunologic reactions against self or environmental antigens trigger inflammation.
  • Aging: Slow, progressive cell injury.

Reversible Cell Injury:

• Reversible injury shows functional and structural changes not permanent.
• Early changes affect cytoplasmic structures, not nuclei.
  • Cell swelling: Water influx due to ATP-dependent pump failure from decreased ATP or membrane damage.
    • Loss of intracellular K+ and Na+ influx causes plasma membrane alterations, like blebbing and swelling of organelles.
  • Fatty change: Toxic injury disrupts liver/heart metabolism, causing triglyceride accumulation.
  • Eosinophilia: Injured cell cytoplasm appears redder in H&E stains due to RNA loss.
  • Myelin figures: Phospholipids from damaged membranes appear in cytosol (collections of phospholipids in concentric layers).
  • Mitochondria may swell.
  • ER may dilate, detaching ribosomes and halting protein synthesis.
  • Nuclear chromatin may clump.

Irreversible Cell Injury and Cell Death:

• Persistent noxious exposures result in cell death beyond a "point of no return”.
• Irreversibility indicators:
  • Inability to restore mitochondrial function.
  • Altered plasma and intracellular membrane structure/function.
  • DNA damage and loss of chromatin integrity.

Necrosis:

• Necrosis is "accidental" cell death, damaging many cellular components, causing cells to "fall apart" and causing local inflammation.
• Involves severe injury leading to cells spilling contents into the extracellular space, causing local inflammation.

Hallmarks of Necrosis:

• Dissolution of membranes due to lipid damage and phospholipase activity.
• Leakage of lysosomal enzymes digests the cell.
• Local inflammation results from released cell contents and triggers phagocytosis and cytokine production by macrophages.

DAMPs (Damage-Associated Molecular Patterns):

-Released factors include ATP and uric acid, recognized by receptors, trigger inflammation by triggering phagocytosis.

Causes of Necrosis:

• Causes include ischemia, microbial toxins, burns, and enzyme leakage (e.g., pancreatitis).
• Initiating triggers irreparably damage cellular components, culminating in membrane damage.

Morphology of Necrosis:

• Cells show increased cytoplasmic eosinophilia.
• Nuclei undergo chromatin condensation (pyknosis), fragmentation (karyorrhexis), and dissolution (karyolysis).

Patterns of Necrosis:

• Coagulative Necrosis: Tissue architecture preserved; characteristic of hypoxia-induced cell death (infarction) in solid organs like the heart and kidneys.
• Liquefactive Necrosis: Dead cells digested by released enzymes; seen in bacterial/fungal infections and brain infarcts.
• Gangrenous Necrosis: Clinical term for soft tissue death, often in limbs with lost blood supply, resulting from ischemia.
  • Dry gangrene: Dead tissue intact; wet gangrene: Tissue liquefies from bacterial infection.
• Caseous Necrosis: Occurs due to tuberculosis and some fungal infections; dead tissue creates a cheesy consistency.
• Fat Necrosis: Focal fat destruction from pancreatic lipase release, seen in acute pancreatitis, resulting in chalky white areas (fat saponification).
• Fibrinoid Necrosis: Microscopic finding in immune reactions where complexes deposit in blood vessel walls, appearing bright pink.

Laboratory Diagnosis of Necrosis:

• Detecting elevated serum levels of intracellular proteins (enzymes) leaked from necrotic cells (e.g., troponin for myocardial infarction).

Apoptosis:

• Apoptosis is "regulated" cell death via defined molecular pathways, eliminating cells precisely without inflammation.
• A cellular suicide to eliminate unneeded/damaged cells without harmful inflammation.
• Enzymes dismantle nucleus and cytoplasm, generating fragments cleared by phagocytes.

Causes of Apoptosis:

• Physiologic apoptosis:
  • Cell death during organism development.
  • Leukocyte death after immune responses.
  • Dysfunctional lymphocyte elimination and hormone-responsive tissue loss matched by cell proliferation.
  • Lymphocyte recognition of self-antigens.
• Pathologic apoptosis:
  • Severe DNA damage by radiation or cytotoxic drugs.
  • Misfolded protein accumulation causing ER stress.
  • Certain infectious agents, trigger immune responses that destroy infected cells.

Mechanisms of Apoptosis:

• Two pathways:
  • Mitochondrial (intrinsic) pathway.
  • Death receptor (extrinsic) pathway.
• Both result in activation of caspases (cysteine proteases cleaving proteins after aspartic acid) and clearance by phagocytes.

Mitochondrial (Intrinsic) Pathway:

• Most common, initiated by molecular sensors detecting lack of survival signals or damage.
• Sensors induce BAX/BAK protein dimerization, increasing mitochondrial membrane permeability.
• This permits pro-apoptotic factor leakage (cytochrome c) into the cytosol, activating caspase-9 and a cascade of caspases, leading to enzymatic breakdown and phagocytosis. No inflammation occurs.
• BCL-2 family molecules normally prevent BAX/BAK dimerization and are activated by growth factors, promoting cell survival/proliferation.

Death Receptor (Extrinsic) Pathway:

• Death receptors are plasma membrane receptors of the tumor necrosis factor (TNF) receptor family.
• Receptors such as TNF receptor and FAS (CD95) have cytoplasmic death domains interacting with proteins.
• FAS ligand (FASL) on activated T lymphocytes cross-links FAS and binds adaptor proteins, recruiting and activating caspase-8, which activates downstream caspases for cell death.
• The death receptor pathway eliminates self-reactive lymphocytes and kills target cells via cytotoxic T lymphocytes.

Clearance of Apoptotic Fragments:

• Apoptotic cells express molecules recognized by phagocyte receptors.
• Phagocytes ingest/destroy apoptotic cell fragments rapidly, before membrane damage and contents release.
• This is efficient, leaving virtually no inflammation.

Morphology of Apoptotic Cells:

• In H&E-stained sections, nuclei appear pyknotic, and cells are shrunken, lying in vacuoles.
• Cells are removed so efficiently they are often not identified in histologic specimens.

Other Pathways of Cell Death:

• Necroptosis: Induced by kinases responding to TNF, leading to membrane injury, like necrosis, although it is regulated (like apoptosis).
• Pyroptosis: Induced by bacterial toxins; dying cell releases cytokines inducing inflammation and fever.
• Autophagy: "Self-eating," where nutrient-starved cells digest own organelles, and if deficiency continues, it can trigger apoptosis.

Mechanisms of Cell Injury and Death:

• Injury degree depends on agent type/severity/duration, plus target cell adaptability/genetic makeup.
  • Small toxin doses or brief ischemia leads to reversible injury; larger doses/prolonged ischemia leads to necrosis.
  • Individuals' genetic makeup impacts reaction to injurious agents. One goal of precision medicine is to use genetics to predict reactions to stimuli.
• Cell injury results from abnormalities in essential cellular components, including mitochondria, membranes, and DNA.

Mitochondrial Injuries:

• Sites of oxidative phosphorylation/ATP production.
• Injury (hypoxia, ischemia, radiation) impairs oxidative phosphorylation, increasing ROS and decreasing ATP.
• Mitochondria sequester cytochrome c, triggering apoptosis when released.

Cellular Membrane Injuries:

• Maintain cell/organelle structure, enable transport.
• Damage, by ROS, to membrane that lysosome is contained in releases digestive enzymes, leading to necrosis.
• Damage to plasma membrane to release cellular constituents, furthering necrosis.

Nucleus Injuries:

• Store genetic material.
• Nuclear damage interferes with transcription or proliferation.
• Irreparable DNA damage triggers apoptosis.

ER (Endoplasmic Reticulum) and Cytoskeleton:

• Other components that suffer damage upon exposure to various injurious agents include the ER (one site of protein synthesis and post-translation processing) and the cytoskeleton (the structural scaffold and “motor” of cells).

Extrinsic Factors in Cell Injury:

• In addition to intrinsic damage, cells may be damaged extrinsically, such as by the products of leukocytes during inflammation.

Oxidative Stress:

• Cellular abnormalities induced by ROS (reactive oxygen) and free radicals.

Free Radicals:

• Molecules with unpaired electron(s) that react with organic/inorganic molecules, converting them into free radicals that can cause damage.
• Include ROS and nitric oxide.

Reactive Oxygen Species (ROS):

• Normally produced in small amounts during mitochondrial respiration and energy generation (redox).
• Generated when oxygen is partially reduced, including superoxide O2⨪ (converted to hydrogen peroxide H2O2) and hydroxyl radical produced via Fenton reaction.
• UV light/radiation, toxins, aging, and deprivation increase ROS production.

Leukocyte Production of ROS:

• ROS are produced in phagocytic leukocytes to destroy microbes and other substances during inflammation and in the "respiratory" burst.