Cell Injury, Cell Death, and Adaptations

Etiology and Pathogenesis

Etiology: The causes of disease.
Pathogenesis: The mechanisms of disease.

Causes of Cell Injury

Hypoxia and ischemia
Toxins
Infectious agents
Immunologic reactions
Genetic abnormalities
Nutritional imbalances
Physical agents
Aging

Outcomes of Cell Injury

Reversible Cell Injury:
- Autophagy
- Intracellular accumulations
Cellular Adaptation:
- Pathological calcification
- ER stress
- DNA damage
- Inflammation
- Hypertrophy
- Hyperplasia
- Atrophy
- Metaplasia
Key Factors in Cell Injury:
- ATP
- ROS (Reactive Oxygen Species)
- Mitochondrial dysfunction
- Membrane permeability
- Physiological vs. pathological necrosis
Cell Death:
- Apoptosis
  - Causes
  - Patterns
  - Types
  - Mechanisms
    - Mitochondrial pathway
    - Death receptor pathway
Irreversible

Hypoxia and Ischemia

Hypoxia: Not enough oxygen due to low $O_2$ concentrations.
- Oxygen deprivation
- CO poisoning
- Anemia
- Respiratory problems
- Altitude sickness
Ischemia: Not enough oxygen due to reduced blood supply.
- Oxygen deprivation + deficiency of essential nutrients + buildup of toxic metabolites
- Occlusion of artery (blood clot, embolism, atherosclerotic plaque) or vein (stuwing)
Anoxia: No oxygen at all.
- Example: Intestinal ischemia (necrosis)

Aging

Results from a combination of multiple, progressive cellular alterations.
'Inflammaging'
Progressive decline in lifespan and functional activity of cells.
Factors influencing aging:
- Physical activity
- Calorie restriction
- Stress
- Chronic diseases (inflammation)
Progressive shortening of telomeres leads to cell cycle arrest.
Diminished ability of cells to respond to stress = cellular senescence.

Limited Cellular Responses

Cell adaptation
Cell death
Inflammation
Fibrosis
Neoplastic changes

Cellular Adaptation

Adaptations are generally reversible processes.
Changes in number, size, phenotype, metabolic activity, or function of cells.
Physiological adaptations: normal stimulation by hormones or endogenous mediators.
Pathological adaptations: responses to stress, loss of normal function.
Various Forms:
1. Hypertrophy
2. Hyperplasia
3. Atrophy
4. Metaplasia

1. Hypertrophy

Increase in cell size (no new cells!) -> increased organ size
- Associated with increased amounts of structural proteins and organelles
- Increased functional demand or due to growth factor or hormone stimulation
Occurs in cells with a limited dividing capacity, e.g., heart and muscle cells
Reversible process, but if stress is too much or not relieved and organ can’t enlarge further -> progression towards more degenerative organ changes can occur
Examples:
- Physiologic hypertrophy: increased workload striated muscle cells (heart and skeletal)
- Pathological hypertrophy: cardiac enlargement resulting from hypertension or aortic valve disease

2. Hyperplasia

Increase in cell number (proliferation)
Stimulated by growth factors or hormones
Examples:
- Physiological hyperplasia:
  - Proliferation of glandular epithelium of the female breast at puberty and pregnancy (hormonal hyperplasia)
  - Residual tissue growth after loss of part of the organ, e.g., liver regeneration (compensatory hyperplasia)
- Pathological hyperplasia:
  - Benign prostatic hyperplasia
  - Papilloma virus-induced wart formation or mucosal lesions
Reversible but… if it persists: a fertile soil for cancer
Patients with hyperplasia of the endometrium: increased risk of developing endometrial cancer
Hyperplasia and hypertrophy can occur together resulting in an enlarged organ, e.g., uterus enlargement during pregnancy (smooth muscle hypertrophy and hyperplasia)

3. Atrophy

Decrease in cell size resulting in a decrease in organ size
- Diminished cell function but not death!
- Associated with decreased protein synthesis and increased protein degradation (ubiquitin-proteasome pathway)
- Atrophy often associated with autophagy (=cell/body eats itself)
Common causes:
- Decreased workload
- Loss of innervation
- Diminished blood supply
- Inadequate nutrition
- Loss of endocrine stimulation
- Aging (senile atrophy)
Examples:
- Physiologic atrophy: decreased workload (immobilization to permit fracture healing)
- Pathological atrophy: aging and reduced blood supply -> brain atrophy
Atrophy: Decrease in cell size associated with decreased protein synthesis + increased degradation of cell organelles
Degradation can occur in 2 cellular compartments:
- Lysosomes
- Proteasomes

Autophagy

Adaptation to nutrient deprivation
Survival mechanism
‘Self-eating’: lysosomal digestion of the cell’s own organelles and recycling in order to save energy and nutrients
Autophagy seen in starvation, in ischemic injury, and in some types of myopathies, cancer.
If stress is too severe -> apoptosis

Proteasome Degradation: The Ubiquitin Pathway

Proteasome: degrades all proteins that are ubiquitinated
Proteasome = enzyme complex to degrade intracellular proteins
Ubiquitin (Ub) is coupled to it = signal to send it to proteasome

4. Metaplasia

Reversible!
Change in phenotype of differentiated cells
Often in response to chronic irritation
Better able to withstand the adverse environment
Happens in Epithelial or mesenchymal cells:
Example:
- Change in lung epithelium due to smoking cigarettes
  - -> loss of function
  - -> increased risk for cancer
Normal ciliated columnar epithelium is replaced by stratified squamous epithelium.
Increased risk of cancer: Increased growth, altered shape & function, immature cells, growth of new cell types (=Cancer) (will be a topic in lectures on cancer)
The order of cells: hyperplasia > metaplasia > dysplasia >>> neoplasia

Cellular Adaptations - Summary

Hypertrophy: increased cell and organ size, often in response to increased workload; induced by growth factors produced in response to mechanical stress or other stimuli; occurs in tissues incapable of cell division
Hyperplasia: increased cell numbers in response to hormones and other growth factors; occurs in tissues whose cells are able to divide or contain abundant tissue stem cells
Atrophy: decreased cell and organ size, as a result of decreased nutrient supply or disuse; associated with decreased synthesis of cellular building blocks and increased breakdown of cellular organelles
Metaplasia: change in phenotype of differentiated cells, often in response to chronic irritation, that makes cells better able to withstand the stress; usually induced by an altered differentiation pathway of tissue stem cells; may result in reduced functions or increased propensity for malignant transformation

Cell Injury: Reversible vs. Irreversible

Reversible Cell Injury:
- Cellular swelling due to uptake of water:
  - Failure of energy-dependent ion pumps in plasma membrane
- Fatty change:
  - Principally in organs involved in lipid metabolism (liver)
    *Morphological characteristics of Reversible injury:
    *Accumulation of water (Failure of $Na^+/K^+/Ca^{2+}$ pumps)
    *Accumulation of fat
Irreversible Cell Injury:
- Inability to restore mitochondrial function (-> no oxidative phosphorylation and ATP generation)
- Loss of structure and functions of intracellular and plasma membranes
- Loss of DNA and chromatin
  Irreversible injury leads to cell death -> necrosis or apoptosis

Necrosis and Apoptosis

Necrosis: Major path of cell death.
- Induced by many factors: ischemia, exposure to toxins, infections, trauma
Apoptosis: When the injury is less severe or cells need to be eliminated during normal processes.
- Activation of a regulated set of molecular pathways leading to death (programmed cell death)

Necrosis

Cellular membranes fall apart and cellular enzymes leak out of cells, digestion of cells.
- Inflammatory reaction (chapter 3)
- Leakage of cellular enzymes (diagnostics)
Fate of the necrotic cells:
- Phagocytosis by other cells
- Further degradation into fatty acids, and these fatty acids can bind calcium salts -> calcification
Characteristics:
- Cytoplasmic changes:
  - Eosinophilia, vacuolated
- Nuclear changes:
  - Pyknosis, karyorrhexis, and karyolysis
Cytoplasmic Changes:
*Eosinophilia (denatured proteins + loss of lighter stained glycogen particles)
*Cytoplasm: vacuoles (digestion of organelles) “moth-eaten”
*Nuclear Changes:
*Pyknosis: condensation (reduced DNA transcription)
*Karyorrhexis-Nucleus: degradation (Dnases)
*Karyolysis-Nucleus: dissolution

Apoptosis

Programmed cell death: activation of enzymes that degrade nuclear DNA and cytoplasmic proteins
Point-of-no-return: mitochondrial membrane permeability
Plasma membrane remains intact!
Apoptotic bodies -> phagocytosis

Initiation: 2 Pathways:

Mitochondrial (intrinsic) pathway
Death receptor (extrinsic) pathway

Apoptosis - Mechanism

Induction phase: proteases disrupt cell connections + activation of pro- + anti-apoptotic proteins
Effector phase: mitochondrial wall gets permeable release cytochrome C point of no return
Degradation phase: activation caspases -Degradation of proteins and DNA -Formation apoptotic body
Phagocytic phase: clearance of cell fragments

Necrosis vs. Apoptosis

Feature	Necrosis	Apoptosis
Cell size	Enlarged (swelling)	Reduced (shrinkage)
Nucleus	Pyknosis → karyorrhexis → karyolysis	Fragmentation into nucleosome-sized fragments
Plasma membrane	Disrupted	Intact; altered structure, especially orientation of lipids
Cellular contents	Enzymatic digestion; may leak out of cell	Intact; may be released in apoptotic bodies
Adjacent inflammation	Frequent	No

Biochemical Mechanisms of Cell Injury

ATP depletion
Oxidative stress
Misfolded proteins
DNA damage
Membrane damage

1. ATP Depletion

ATP necessary for membrane transport, protein synthesis, lipogenesis, and de/re-acylation reactions for phospholipid turnover
Per day: 50-75 kg ATP is used (human)
Loss of ATP leads to:
1. Reduced activity of $Na^+/K^+/Ca^{2+}$ membrane pumps
  - Cell and internal cell organelles will swell
  - Vacuolization
2. Increase in anaerobic glycolysis
3. Structural disruption of the protein synthetic apparatus
4. Decrease in enzyme activity
5. Failure of $Ca^{2+}$ membrane pumps
  *Increased intracellular $Ca^{2+}$ levels
- $Ca^{2+}$ is a second messenger if it’s too much intracellular $Ca^{2+}$ it is destructive! $Ca^{2+}$ over-activation of several enzymes that need calcium:
  - Phospholipases -> membrane blebbing
  - Proteases -> cytoskeletal destruction
  - Endonucleases -> nuclear damage
  - ATPases -> reduced ATP

2. Oxidative Stress

Damage induced by ROS (reactive oxygen species): free radicals
Extremely unstable and very reactive with various cellular molecules (nucleic acids, cellular proteins, and lipids)
Causes: Radiation therapy in cancer
ROS production in physiological situations:
1. Mitochondrial respiration:
  - All cells
  - Small amounts
2. In phagocytic leukocytes:
  - In a phagosome or phagolysosome
  - Large amounts
  - Weapon for destruction of microbes
- MPO = myeloperoxidase formation of hypochlorite (HOCL-)
Removal of free radicals:
- Enzymatic systems
 - Superoxide dismutase (SOD): $2O2^- + 2H^+ \rightarrow H2O2 + O2$
 - Glutathione (GSH) peroxidase: $2GSH + H2O2 \rightarrow GS-SG + 2 H_2O$
 - Catalase: $2H2O2 \rightarrow 2H2O + O2$
- Non-enzymatic systems
 - Vitamin E, A, and C
 - Beta-carotene

3. Misfolded Proteins

Neurodegenerative diseases: Alzheimer's disease, Huntington's disease, Parkinson's disease
Accumulation of misfolded proteins in cells: ER stress
Unfolded Protein Response (aim: restore homeostasis)
Unfolded protein response ---> apoptosis
- Severe cellular stress: apoptosis

Intracellular Accumulations

Inadequate removal
Degradation failure
- Accumulation in lysosomes
- Inherited enzyme deficiencies
- E.g., Glycogen storage disease
  What is commonly deposited?
  *Lipids (Fatty livers) – Triglycerides & cholesterol
  *Proteins – Hyaline (large protein complexes) – Immunoglobulin (Autoimmune diseases) – Neurofibrillary tangles (Alzheimer)
  *Glycogen (in liver)
  *Non-degradable compounds – Exogenous: carbon, silica, asbestos – Endogenous: iron (hemochromatosis), etc
PAS staining (stains polysaccharides like glycogen)

Extracellular Accumulations: Pathological Calcification

Abnormal deposition of calcium salts
Dystrophic calcification:
- Deposits in injured or dead tissue
- Calcium metabolism is normal: calcium is released from death cells
- In general irreversible
- Example: arterial lesions in advanced atherosclerosis and damaged heart valves
Metastatic calcification:
- In normal tissues (no death cells); affects all tissues
- Abnormal calcium metabolism, reversible after correction of disorder
- Hypercalcemia as a result of:
  - Increased secretion of parathyroid hormone
  - Destruction of bone
  - Vitamin D related disorders
  - Renal failure