Chapter 2- Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death Flashcards
(118 cards)
1
Q
_______ is devoted to the study of structural, biochemical, and functional changes in cells, tissues, and organs that underlie disease
A
Pathology
2
Q
Four aspects of a disease process that form the core of pathology
A
- Cause (etiology)
- Biochemical and molecular mechanisms of its development (pathogenesis)
- Structural alterations induced in the cells and organs of the body (morphologic changes)
- Functional consequences of these changes (clinical manifestations)
3
Q
All disease etiologies can be classified into two classes
A
Genetic (inherited mutations, disease-associated gene variants, or polymorphisms)
Acquired (infectious, nutritional, chemical, physical)
Most common afflictions (i.e. cancer, atherosclerosis) are multifactorial, involving external triggers and a genetically susceptible individual
4
Q
Pathogenesis definition
A
Sequence of cellular, biochemical, molecular events that follow the exposure of cells or tissues to an injurious agent
5
Q
Morphologic change definition
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Structural alterations in cells or tissues that are either characteristic of a disease or diagnostic of an etiologic process
6
Q
Definition of cellular adaptation
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Reversible functional and structural responses to changes in physiologic states and some nonlethal pathologic stimuli, allowing the cell to survive and continue functioning (e.g. hypertrophy, hyperplasia, atrophy, metaplasia)
7
Q
When does cell injury occur?
A
When the limits of adaptive responses are exceeded, or if cells are exposed to injurious agents or stress, deprived of essential nutrients, or become compromised by mutations that affect essential cellular constituents
8
Q
Cell injury is reversible up to a certain point, but if the stimulus persists or is severe enough from the beginning, the cell suffers irreversible injury and ultimately undergoes cell death.
A
Adaptation, reversible injury, and cell death may be stages of progressive impairment following different types of insults. (i.e. Increased hemodynamic loads –> cardiac hypertrophy –> reversible injury (fat accumulation, cellular swelling) –> irreversible injury / cell death)
9
Q
A
Stages of the cellular response to stress and injurious stimuli.
10
Q
Two principal pathways of cell death
A
Necrosis
Apoptosis
11
Q
TABLE: Cellular responses to injury
A
12
Q
Nutrient depletion triggers an adaptive cellular response which can lead to cell death called:
A
Autophagy
13
Q
Another definition of adaptation
A
Reversible changes in size, number, phenotype, metabolic activity, or functions of cells in response to changes in their environment
14
Q
Hypertrophy definition
A
Increase in size of cells resulting in an increase in organ size Examples: Physiologic uterine hypertrophy during pregnancy Pathologic hypertrophy of cardiac muscle
15
Q
What is the most common stimulus for hypertrophy of muscle?
A
Increased work load (i.e. “pumping iron”)
In the heart, stimulus is usually chronic hemodynamic overload (e.g. hypertension, faulty valves)
16
Q
Hypertrophy is the result of increased production of cellular ______ (very broad)
A
Proteins
17
Q
Three basic steps in molecular pathogenesis of cardiac hypertrophy:
A
- Integrated actions of mechanical sensors, growth factors (TGF-b, IGF1, FGF), and vasoactive agents (a-adrenergic agonists, endothelin-1, angiotensin II)
- These signals result in complex transduction. Two important pathways involved are: - Phosphoinositide 3-kinase (PI3k)/AKT pathway (important in physiologic hypertrophy) - Downstream signaling of G-protein-coupled receptors (induced by GFs and vasoactive agents) - important in pathologic hypertrophy
- Activation of transcription factors such as GATA4, nuclear factor of activated T cells (NFAT), and myocyte enhancer factor 2 (MEF2). These work to increase synthesis of muscle proteins
18
Q
A
Biochemical mechanisms of myocardial hypertrophy. The major known signaling pathways and their functional effects are shown. Mechanical sensors appear to be the major triggers for physiologic hypertrophy, and agonists and growth factors may be more important in pathologic states. ANF, Atrial natriuretic factor; GATA4, transcription factor that binds to DNA sequence GATA; IGF1, insulin-like growth factor; NFAT, nuclear factor activated T cells; MEF2, myocardial enhancing factor 2.
19
Q
Hyperplasia definition
A
Increase in number of cells in an organ or tissue in response to a stimulus. This can only take place in tissue with cells capable of dividing, and frequently occurs together with hypertrophy as stimuli are similar
20
Q
Physiologic hyperplasia due to the action of hormones or growth factors occurs in several circumstances: when there is a need to increase functional capacity of hormone sensitive organs; when there is need for compensatory increase after damage or resection. Give some examples
A
- Hormonal hyperplasia of the glandular epithelium in the female breast at puberty and during pregnancy (usually accompanied by hypertrophy)
- epatocyte hyperplasia secondary to partial hepatectomy -
- Marrow undergoes remarkable hyperplasia in response to peripheral cytopenias
21
Q
Most forms of pathologic hyperplasia are caused by excessive or inappropriate actions of hormones or growth factors acting on target cells. Give examples
A
- Endometrial hyperplasia post-menstration, due to imbalances in estrogen, progesterone - common cause of abnormal menstrual bleeding
- Benign prostatic hyperplasia occurs in response to hormonal stimulation by androgens
In these instances, the process remains controlled and it regresses if hormonal stimulation is eliminated. However, pathologic hyperplasia provides a fertile soil in which cancer can arise.
22
Q
Hyperplasia is a characteristic response to certain _____ infections
A
Viral
Papillomaviruses and others can cause warts and mucosal lesions of hyperplastic epithelium. Some of these are precursors to cancer
23
Q
Hyperplasia is the result of growth factor-driven proliferation of mature cells, and in some cases, by increased output of new cells from tissue stem cells.
A
Got that, you little bitch?
24
Q
Atrophy definition
A
Reduction in the size of an organ or tissue due to a decrease in cell size and number
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Physiologic vs. pathologic atrophy
Physiologic atrophy is common during normal development. - Embryonic structures (notochord, thyroglossal duct) undergo atrophy during fetal development. - Decrease in uterine size post-parturition Pathologic atrophy has several causes and can be localized or generalized.
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Common causes of pathologic atrophy
1. Decreased workload (atrophy of disuse): With prolonged disuse, cells can decrease in size and eventually in number (due to apoptosis)
2. Loss of innervation (denervation atrophy): normal metabolism and function of skeletal muscle are dependent on its nerve supply.
3. Diminished blood supply: Ischemia results in atrophy. This is what happens with senile atrophy (brain atrophy due to atherosclerosis)
4. Inadequate nutrition: Profound protein-calorie malnutrition (marasmus) --\> skeletal muscle protein utilizaiton for energy --\> cachexia
5. Loss of endocrine stimulation: (i.e. loss of estrogen after menopause --\> physiologic atrophy of endometrium vaginal epithelium breast)
6. Pressure: Tissue compression (enlarging benign tumor, others) can cause atrophy
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Fundamental cellular changes of atrophy
Initial decrease in cell and organelle size (may reduce needs enough to permit survival)
Early on, there is diminished function but minimal cell death.
It may progress to point where cells are dying by apoptosis
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Atrophy results from decreased protein synthesis and increased protein degradation in cells due to reduced metabolic activity. What pathway is mainly responsible for degradation of cellular proteins?
Ubiquitin-proteasome pathway Nutrient deficiency / disuse --\> activate ubiquitin ligases --\> target proteins for proteasomal degradation
This is thought to be responsible for cancer cachexia.
Often accompanied by autophagy, marked by appearance of increased numbers of autophagic vacuoles. Starved cells eat their own components to decrease nutrient demand. Lipofuscin granules in hepatocytes are examples of residual bodies (autophagic debris that resisted digestion)
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Metaplasia definition and examples
Reversible change in which one differentiated cell type (epithelial or mesenchymal) is replaced by another cell type.
Most common epithelial metaplasia is columnar to squamous (respiratory tract secondary to chronic irritation)
Squamous metaplasia of salivary, pancreatic, biliary ducts with stones
Squamous metaplasia of respiratory epithelium with Vitamin A deficiency
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If influences that predispose to metaplasia are persistent, it can initiate malignant transformation in metaplastic epithelium
TRUE
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Connective tissue metaplasia
Formation of cartilage, bone, adipose in tissues that normally do not contain these elements (e.g. myositis ossificans after intramuscular hemorrhage)
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Metaplasia does not result from a change in the phenotype of an already differentiated cell type; instead it is the result of a reprogramming of stem cells that are known to exist in normal tissues, or of undifferentiated mesenchymal cells present in connective tissue
This was bolded
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KEY CONCEPTS: Cellular adaptations to stress
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What are the hallmarks of reversible cellular injury?
1. Reduced oxidative phosphorylation --\> depletion of energy (ATP) stores
2. Cellular swelling due to [ion] changes and water influx - Various intracellular organelles (mitochondria, cytoskeleton) may show alterations
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When continuing cellular damage becomes irreversible, the cell cannot recover and it dies. Give brief descriptions of the two principle types of cell death
1. Necrosis - "accidental", unregulated cell death due to cell membrane damage and loss of ion homeostasis. When membrane damage is severe, lysosomal enzymes enter cytoplasm and digest the cell. This causes cellular content leakage and inflammation. Seen with many common injuries (e.g. ischemia, toxins, infections, trauma)
2. Apoptosis: When the cell's DNA or proteins are damaged beyond repair, the cell kills itself. This is characterized by nuclear dissolution, fragmentation of cell without loss of membrane integrity, and rapid removal of cellular debris. No inflammatory response is expected
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Necrosis is always a pathologic process. Is this true for apoptosis?
NO!
Apoptosis serves many normal functions and is not necessarily associated with cell injury.
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Causes of cell injury
1. Oxygen deprivation: Hypoxia --\> decreased aerobic oxidative respiration. This can result from ischemia, cardiorespiratory failure, decreased O2-carrying capacity of the blood (anemia, metHgbemia). This can lead to adaptation, injury, or death, depending on severity of hypoxia.
2. Physical agents: mechanical trauma, extreme temepratures, sudden atmospheric pressure changes, radiation, electric shock
3. Chemical agents and drugs: Too many to list. This can be due to hypertonic concentrations of glucose or salt. Even very high [O2] is toxic. Trace amounts of poisons (arsenic, cyanide, etc.) can cause massive cell death. Environmental and air polluants, CO, asbestos, alcohol, and many therapeutic drugs all can cause cell injury/death
4. Infectious agents: Viruses, rickettsiae, bacteria, fungi, parasites can cause cell injury and death in many ways
5. Immunologic reactions: Injurious reactions to endogenous self antigens during autoimmune disease. May be due to external agents (viruses, environmental substances)
6. Genetic derangements: This can be due to a variety of mechanisms (protein deficiencies, damaged DNA or misfolded protein accumulations. Common polymorphisms in DNA sequences can also influence susceptibility of cells to injury.
7. Nutritional imbalances: Protein-calorie deficiencies cause high numbers of human deaths. Specific vitamin deficiencies and excesses are important causes of cell injury (e.g. high cholesterol -\> atherosclerosis, obesity -\> diabetes, cancer). Apparently obesity is rampant in the United States... I haven't noticed that though, I think our plane seats and doorways are just too small.
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Sequential development of biochemical and morphologic changes in cell injury. Cells may become rapidly nonfunctional after the onset of injury, although they may still be viable, with potentially reversible damage; a longer duration of injury may lead to irreversible injury and cell death. Note that irreversible biochemical alterations may cause cell death, and typically this precedes ultrastructural, light microscopic, and grossly visible morphologic changes.
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TABLE: Features of apoptosis and necrosis
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Schematic illustration of the morphologic changes in cell injury culminating in necrosis or apoptosis
41
What are a few morphologic changes that characterize reversible injury?
1. Cellular and organellar swelling
2. Blebbing of plasma membrane
3. Detachment of ribosomes from the ER
4. Clumping of nuclear chromatin
These changes are associated with decreased ATP generation, loss of membrane integrity, defects in protein synthesis, cytoskeletal damage, and DNA damage
42
What are two characteristic features typically associated with necrosis?
1. Severe mitochondrial damage with ATP depletion
2. Rupture of lysosomal and plasma membranes
43
Two features of reversible cell injury can be recognized under the light microscope:
1. Cellular swelling (a.k.a. hydropic change, vacuolar degeneration): cells incapable of maintaining ionic and fluid homeostasis due to failure of energy-dependent ion pumps in plasma membrane
2. Fatty change: hypoxic injury, various forms of toxic or metabolic injury cause this. Lipid vacuoles seen in cytoplasm, seen mostly in hepatocytes and myocardial cells (these are dependent on fat metabolism)
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The morphologic appearance of necrosis as well as necroptosis is the result of \_\_\_\_\_\_\_\_\_\_\_\_\_
Denaturation of intracellular proteins and enzymatic digestion of the lethally injured cell by the cell's own lysosomal enzymes
45
Morphologic changes associated with necrotic tissue on H&E slides
1. Increased eosinophilia (loss of cytoplasmicc RNA and increased denatured proteins)
2. Glassy, homogeneous appearance (loss of glycogen particles)
3. Vacuolated, moth-eaten cytoplasm (enzymes digesting organelles)
4. Myelin figures - whorled phospholipid masses derived from damaged membranes
5. Nuclear changes:
- Karyolysis (faded chromatin color): enzymatic degradation of DNA
- Pyknosis (nuclear shrinkage): Condensed chromatin into a solid mass
- Karyorrhexis (nuclear fragmentation)
46
What are six patterns of tissue necrosis?
1. Coagulative necrosis
2. Liquefactive necrosis
3. Gangrenous necrosis
4. Caseous necrosis
5. Fat necrosis
6. Fibrinoid necrosis
47
Coagulative necrosis
Archesticture of dead tissues is preserved for at least some days.
Affected tissues exhibit a firm texture
Eosinophilic, anucleate cells persist for days or weeks due to degradation of enzymes, blocking proteolysis
Leukocytes come in and digest the dead cells
Often due to ischemia from an obstructed vessel.
Localized area of coagulative necrosis: **Infarct**
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Liquefactive necrosis
Characterized by digestion of dead cells into a liquid, viscous mass
Seen in focal bacterial or fungal infections (microbes stimulate leukocyte accumulation and enzyme liberation by the leuks)
Necrotic material is creamy yellow (**pus**)
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Gangrenous necrosis
Usually applied to a limb that has lost its blood supply and undergone (typically coagulative) necrosis
**Wet gangrene**: when bacterial infection is superimposed, there is more liquefactive necrosis due to enzymes from leukocytes
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Caseous necrosis
Most often seen in foci of tuberculous infection
Caseous (cheese-like) is used due to the friable appearance
**Granuloma**: necrotic area (structureless collection of lysed cells and amorphous debris) enclosed with a distinctive inflammatory border (macs and lymphs)
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Fat necrosis
Pretty non-specific
Local areas of fat destruction (typically from release of activated pancreatic lipases - acute pancreatitis)
Fatty acids from lipase activity bind calcium to produce a chalky white area (saponification)
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Fibrinoid necrosis
Usually seen in immune reactions involving blood vessels
Antigen-antibody complex deposition in artery walls combine with fibrin and result in a bright pink and amorphous appearance called fibrinoid
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KEY CONCEPTS: Morphologic alterations in injured cells and tissues
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The molecular pathways that lead to cell injury are complex.
Several principles that are relevant to most forms of cell injury include:
1. The cellular response to injurious stimuli depends on the nature o fthe injury, its duration, and its severity.
2. The consequences of cell injury depend on the cell type, state of the cell, and adaptability of the injured cell. (e.g. striated skeletal muscle in the leg can be at rest and be preserved with ischemia... not so much with cardiomyocytes)
3. Cell injury results from different biochemical mechanisms acting on several essential cellular components. Cellular components that are most frequently damaged are mitochondria, cell membranes, ER/golgi, and DNA
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The principal biochemical mechanisms and sites of damage in cell injury. ATP, Adenosine triphosphate; ROS, reactive oxygen species.
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Mechanisms of cell injury: Depletion of ATP
- Reduction of ATP levels is a fundamental cause of necrotic cell death
- Two major routes of ATP production:
1. Oxidative phosphorylation of ADP in mitochondria
2. Glycolytic pathway (can be anaerobic)
*Major causes of depletion of ATP are reduced supply of oxygen and nutrients, mitochondrial damage, and actions of some toxins (e.g. cyanide)*
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Depletion of cellular ATP to 5-10% of normal has widespread effects on critical cellular systems, including:
1. Reduced Na-K ATPase activity: sodium accumulates in cell --\> net gain of solute and water --\> cell swelling and ER dilation
2. Cellular energy metabolism is altered: Limited oxygen supply to cells (e.g. ischemia) --\> decrease in ATP, increase in AMP --\> PFK and phosphorylase stimulation --\> increased anaerobic glycolysis --\> glycogen store depletion, lactic acid and Pi accumulation --\> Decreased cellular pH --\> decreased activity of many enzymes
3. Failure of Ca2+ pump --\> influx of Ca2+ --\> damage to numerous cellular components
4. Prolonged or worsening ATP depletion --\> Detachment of ribosomes from rough ER and dissociation of polysomes --\> reduction in protein synthesis
5. O2 or glucose-deprived cells --\> Misfolded protein accumulation in ER --\> unfolded protein response (ER stress response) -- \>cell injury or cell death
6. Irreversible damage to mitochondrial and lysosomal membranes --\> necrosis
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Mechanisms of cell injury: Mitochondrial damage
Mitochondria are critical players in cell injury and cell death by all pathways.
Mitochondria can be damaged by increased intracellular [Ca2+] and hypoxia, so are sensitive to all types of injurious stimuli (hypoxia, toxins)
Mutations of mitochondrial genes are causes of some inherited disorders.
Three major consequences of mitochondria damage:
1. Formation of a high-conductance channel in mitochondrial membrane (*mitochondrial permeability transition pore*), --\> loss of mitochondrial membrane potential --\> failure of oxidative phosphorylation and progressive ATP depletion --\> necrosis
* *Cyclophilin D** (targeted by cyclosporine) is a structural component of the transition pore
2. Abnormal oxidative phosphorylation --\> reactive oxygen species formation --\> many deleterious effects
3. Protein sequestration between inner and outer membranes of mitochondria that can activate apoptosis pathways (e.g. cytochrome c, caspases. Increased permeability may result in leakage of these into cytosol and apoptosis
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Mechanisms of cell injury: Influx of Calcium and loss of Calcium homeostasis
Calcium ions are important mediators of cell injury; depleting intracellular [Ca2+] protects agents harmful stimuli
Intracytoplasmic [Ca2+] is very low compared to extracellular levels; most is sequestered in mitochondria and ER
Ischemia, toxins can cause increased cytosolic [Ca2+] first due to intracellular Ca2+ release, and then due to influx across plasma membrane
Increased cytoplasmic [Ca2+] causes cell injury by several mechanisms:
1. Ca2+ accumulation in mitochondria --\> opening of mitochondrial permeability transition pore --\> failure of ATP generation
2. Increased cytosolic [Ca2+] activates phospholipases (membrane damage), proteases (cytoskeletal/membrane protein break down), endonucleases (DNA, chromatin fragmentation), and ATPases (hastening ATP depletion)
3. Induction of apoptosis by direct activation of caspases and increasing mitochondrial permeability
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Mechanisms of cell injury: Accumulation of oxygen-derived free radicals (oxidative stress)
Cell injury induced by free radicals is an imporant mechanism of cell damage in many pathologic conditions (chemical / radiation injury, ischemia-reperfusion injury, cellular aging, microbial killing)
Free radicals: chemical species that have an unpaired, highly reactive electron which 'attacks' lipids, proteins, carbs, nucleic acids
Reactive oxygen species (ROS) are oxygen-derived free radicals produced normally during mitochondrial respiration and energy generation, but are degraded and removed by cellular defense systems, achieving a steady state of ROS with no cellular damage
Increased production and excess of ROS is **oxidative stress** (implicatetd in cell injury, cancer, aging, degenerative diseases)
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Several ways free radicals can be generated:
1. Reduction-oxidation reaction during normal metabolic processes: Enzymes in the ER, cytosol, mitochondria, peroxisomes, lysosomes catalyze the reduction of molecular O2 (4 e- transfer to H2 to generate 2 water molecules. Small amounts of partially-reduced intermediates (Superoxide anion: (O2*- with 1 e-), hydrogen peroxide (H2O2, with 2 e-), and hydroxyl ions (\*OH, with 3 e-) are produced by this process.
2. Absorption of radiant energy (e.g. UV light, x-rays): Ionizing radiation can hydrolyze water into \*OH and H free radicals
3. Inflammation causes rapid bursts of ROS in leukocytes: Precisely controlled reaction by NADPH oxidase (some intracellular oxidases e.g. xanthine oxidase) can generate O2*-
4. Enzymatic metabolism of exogenous chemicals or drugs: Generate free radicals that are not ROS but have similar effects (e.g. CCl4 --\> \*CCl3)
5. Transition metals (iron, copper): donate free electrons during intracellular reactions and catylze free radical formation (e.g. Fenton reaction (H2O2 + Fe2+ --\> Fe3+ + \*OH + OH-))
6. Nitric oxide (NO): important chemical mediator generated by endothelial cells, macrophages, neurons, other cell types, can act as a free radical
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Removal of free radicals
Free radicals are unstable and decay spontaneously. O2*- (superoxide) decays to O2 and H2O2 in the presence of water.
Cells have also developed enzymatic and nonenzymatic ways to remove free radicals:
1. Antioxidants block free radical formation or inactivate (scavenge) free radicals. Vitamains E and A and ascorbic acid and glutathione in cytosol are examples
2. Free iron and copper can catalyze formation of ROS; normal binding of these molecules to transport or storage proteins (e.g. transferrin, ferritin, lactoferrin, ceruloplasmin) prevents this process
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Series of enzymes that act as free radical scavengers and how they work
1. Catalse (in peroxisomes) decopmoses H2O2 (2H2O2 --\> O2 + 2 H2O)
2. Superoxide dismutases (SODs) are in many cell types, and convert O2*- to H2O2 (2O2*- + 2H --\> H2O2 + O2). This group includes manganese-SOD in mitochondria and copper-zinc-SOD in cytosol
3. Glutathione peroxidase catalyzes free radical breakdown
(H2O2 + 2GSH --\> GSSG [glutathione homodimer] + 2H2O)
OR
(2*OH + 2GSH --\> GSSG + 2H2O)
intracellular ratio of oxidized glutathione (GSSG) to reduces glutathione (GSH) reflects oxidative state of the cell and it's ability to detoxify ROS
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Pathologic effects of free radicals
1. Lipid peroxidation in membranes: In aerobic conditions, free radicals can cause peroxidation of lipids within plasma and organellar membranes. Double bonds in unsaturated fatty acids attacked by ROS (particularly *OH) releasing unstable peroxides and initiating propagation, an autocatalytic chain reaction causing membrane damage
2. Oxidative modification of proteins: Free radicals promote oxidation of amino acid side chains, formation of covalent pr-pr cross links (e.g. disulfide bonds), and oxidation of protein backbone. This can disrupt conformation of structural proteins and enhance proteasomal degradation of misfolded proteins --\> havoc inside the cell
3. Lesions in DNA: free radicals single- and double- stranded breaks in DNA, cross-linking of DNA strands, and adduct formation. Oxidative DNA damage has been implicated in cell aging and malignant transformation of cells.
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TABLE: Properties of the principal free radicals involved in cell injury
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Mechansims of cell injury: Defects in membrane permeability
Early loss of selective membrane permeability, leading ultimately to overt membrane damage, is a consistent feature of most forms of cell injury (except apoptosis).
Mechanisms of membrane damage:
1. ROS: lipid peroxidation of membrane lipids
2. Decreased phospholipid synthesis: Defective mitochondrial function and hypoxia both decrease ATP production and energy-dependent biosynthetic pathways
3. Increased phospholipid breakdown: Presumptively due to activation of Ca2+-dependent phospholipases secondary to increased cytosolic and mitochondrial [Ca2+]. This leads to accumulation of lipid breakdown products (e.g. unesterified free fatty acids, acyl carnitine, lysophospholipids) which hurt membranes.
4. Cytoskeletal abnormalities: Activation of proteases by increased cytosolic [Ca2+] --\> cytoskeletal damage (particularly in myocardial cells)
Consequences of membrane damage
1. Mitochondrial membrane damage: opening of mitochondrial permeability transition pore --\> decreased ATP generation, release of pro-apoptotic proteins
2. Plasma membrane damage: Loss of osmotic balance and influx of fluids and ions, loss of cellular contents (including metabolites vital for ATP reconstitution)
3. Injury to lysosomal membranes: leakage of enzymes into cytoplasm and activation of acid hydrolases in acidic intracellular pH of injured cell. RNases, DNases, proteases, phosphatases, glucosidasese get activated and digest cellular proteins, RNA, DNA, glycogen, causing necrosis
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Mechanisms of membrane damage in cell injury. Decreased O2 and increased cytosolic Ca2+ are typically seen in ischemia but may accompany other forms of cell injury. Reactive oxygen species, which are often produced on reperfusion of ischemic tissues, also cause membrane damage (not shown).
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Mechanisms of cell injury: Damage to DNA and proteins
Cells have mechanisms that repair damage to DNA, but if DNA damage is too severe to be corrected (e.g. DNA damaging drugs, radiation, oxidative stress), cell initiates apoptosis
A similar reaction is triggered by misfolded protein accumulation (e.g. inherited mutations ofr acquired triggers like free radicals)
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Reversible vs. Irreversible cell injury
While the 'point of no return' at which damage becomes irreversible is still alrgely undefined, there are two phenomena that consistently characterize irreversibility; what are they?
1. Inability to reverse mitochondrial dysfunction (lack of oxidative phosphorylation and ATP generation) even after resolution of the original injury
2. Profound disturbances in membrane function
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KEY CONCEPTS: Mechanisms of cell injury
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Ischemia and hypoxic injury
Ischemia is the most common type of cell injury in clinical medicine and it results from hypoxia induced by reduced blood flow, most commonly due to a mechanical arterial obstruction.
In addition to hypoxia, ischemic tissues also have decreased delivery of substrates for glycolysis, limiting anaerobic as well as aerobic metabolism; *ischemia leads to more rapid and severe cell injury than hypoxia in the absence of ischemia*.
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Mechanisms of ischemic cell injury
Decreased O2 tension in cells --\> decreased oxidative phosphorylation and ATP generation --\> failure of Na-K pump, efflux of K, influx of Na and water, cell swelling
Also an influx of Ca2+ --\> many deleterious effects
Progressive loss of glycogen, decreased protein synthesis
Continued hypoxia --\> worsening ATP depletion --\> cytoskeleton dispersion --\> loss of microvilli, cell surface 'blebbing', myelin figure formation in cytoplasm (from degenerating membranes)
Mitochondria are usually swollen and ER remains dilated due to too much water, Na, Cl in the cella nd decreased K
***If oxygen is restored, all of the above disturbances are reversible***
Persistent ischemia --\> irreversible injury and necrosis ensue
Irreversible injury associated with severe mitochondrial swelling, extensive damage to plasma membranes, and lysosomal swleling
Massive influx of Ca2+ occurs, especially when ischemic area is re-perfused.
Death mainly is by necrosis, but apoptosis can contribute due to leaky mitochondria
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Very important transcription factor that has developed in mammalian cells as a protective response to deal with hypoxic stress
**Hypoxia-inducible factor-1 (HIF-1)**
Promotes new blood vessel formation
Stimulates cell survival pathways
Enhances anaerobic glycolysis
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Ischemia-Reperfusion injury
**Restoration of blood flow to ischemic tissues can promote recovery of cells if they are reversibly injured, but can also paradoxically exacerbate the injury and cause cell death.**
Several mechanisms have been proposed:
1. Oxidative stress: new damage during reoxygenation due to ROS and nitrogen species generation (may be produced in reperfused tissue as a result of incomplete reduction of oxygen by damaged mitochonrdia). Cellular antioxidant defenses may be compromised in ischemic tissue
2. Intracellular calcium overload: Begins during acute ischemia, and is exacerbated by reperfusion due to Ca influx from cell membrane damage and ROS mediated injury to sarcoplasmic reticulum. Leads to opening of mitochondrial permeability transition pore and ATP depletion
3. Inflammation: "Danger signals" from dead cells, cytokines from resident immune cells, and increased expression of adhesion molecules by hypoxic epithelial/endothelial cells all result in inflammation and additional tissue injury
4. Activation of the complement system may contribute
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Chemical (Toxic) injury
**Chemical injury remains a frequent problem in clinical medicine and is a major limitation to drug therapy.**
The liver is a frequent target of drug toxicity due to its significant metabolic capacity.
Chemicals induce cell injury by one of two general mechanisms:
1. Direct toxicity: Some chemicals injure cell directly by combining with critical molecular components (e.g. mercury binds sulfhydryl groups --\> membrane damage of GI and kidney cells, which absorb, excrete, or concentrate mercury). Cyanide poisons mitochondrial cytochrome oxidase --\> inhibits oxidative phosphorylation
2. Conversion to toxic metabolites: Most toxic chemicals must be converted to reactive toxic metabolites, which then act on target molecules. Usually done by cytochrome P-450 mixed-function oxidases in smooth ER of hepatocytes and other cells. Toxic metabolites cause membrane damage and cell injury primarily by free radical formation and subsequent lipid peroxiation. (examples: acetaminophen --\> metabolized to toxic product in liver; CCl4 (bleach) converted by cytochrome P-450 to highly reactive *CCl3 --\> lipid peroxidation
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KEY CONCEPTS: ischemic and toxic injury
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Apoptosis - definition
**Apoptosis is a pathway of cell death that is induced by a tightly regulated suicide program in which cells destined to die activate intrinsic enzymes that degrade the cells' own nuclear DNA and nuclear and cytoplasmic proteins.**
Apoptotic cells break up into fragments (a.k.a. apoptotic bodies), which contain nuclear and cytoplasmic portions.
\*Plasma membrane remains intact but the structure is altered in a way that becomes 'tasty' for phagocytes --\> phosphatidylserine flips to outer leaflet of plasma membrane
Dead cell and contents are devoured rapidly before contents leak out
**THIS DOES NOT LEAD TO INFLAMMATION, CONTRARY TO NECROSIS**
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Physiologic apoptosis
**Death by apoptosis is a normal phenomenon that serves to eliminate cells that are no longer needed, and to maintain a steady number of various cell populations in tissues.**
Here are some physiologic situations in which apoptosis is important:
1. Destruction of cells during embryogenesis (implantation, organogenesis, developmental involution, metamorphosis)
2. Involution of hormone-dependent tissues upon hormone withdrawal (e.g. endometrial cell breakdown during menstrual cycle, ovarian follicular atresia in menopause, regression of lactating breast after weaning, prostatic atrophy after castration)
3. Cell loss in proliferating cell populations (e.g. immature lymphocytes in bone marrow and thymus and B lymphocytes in germinal centers that fail to express useful antigen receptors, epithelial cells in intestinal crypts to maintain a constant number)
4. Elimination of potentially harmful self-reactive lymphocytes before or after they have completed maturation
5. Death of host cells that have served their useful purpose (e.g. neutrophils in APR, lymphocytes at the end of an immune response)
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Apoptosis in pathologic conditions
Apoptosis eliminates cells that are injured beyond repair without eliciting a host reaction, thus limiting collateral damage.
Examples:
1. DNA damage: Radiation, cytotoxic chemo drugs, hypoxia can damage DNA, directly or via free radical production. If repair mechanisms cannot cope, cell triggers intrinsic mechanisms inducing apoptosis.
2. Accumulation of misfolded proteins: Improperly folded proteins may arise due to gene mutations encoding them or damage caused by free radicals. Excess accumulation of these in ER leads to ER stress (a.k.a. unfolded protein response) --\> apoptosis
3. Cell death in certain infections (particularly viral) is largely due to apoptosis: May be induced by virus (e.g. adenovirus, HIV) or by host immune response (e.g. viral hepatitis). Cytotoxic T lymphocytes are important to induce apoptosis of viral-infected cells
4. Pathologic atrophy in parenchymal organs after duct obstruction: occurs in pancreas, parotid gland, kidney
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Morphologic features of apoptosis
Morphologic features of apoptosis.
A, Apoptosis of an epidermal cell in an immune reaction. The cell is reduced in size and contains brightly eosinophilic cytoplasm and a condensed nucleus.
B, This electron micrograph of cultured cells undergoing apoptosis shows some nuclei with peripheral crescents of compacted chromatin, and others that are uniformly dense or fragmented.
C, These images of cultured cells undergoing apoptosis show blebbing and formation of apoptotic bodies (left panel, phase contrast micrograph), a stain for DNA showing nuclear fragmentation (middle panel), and activation of caspase-3 (right panel, immunofluorescence stain with an antibody specific for the active form of caspase-3, revealed as red color).
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Mechanisms of apoptosis - broad
**Apoptosis results from the activation of enzymes called caspases (named because they are cysteine proteases that cleave proteins after aspartic resides)**
These caspases exist as inactive proenzymes (zymogens) and must undergo cleavage to become active.
Apoptosis is divided into the initiation phase (some caspases become active) and execution phase (other caspases triggere cellular degradation)
Activation of caspases depends on a finely tuned balance of pro- and anti- apoptotic proteins
Tow pathways converge on caspase activation:
1. Mitochondrial (intrinsic) pathway
2. Death receptor (extrinsic) pathway
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Intrinsic (mitochondrial) pathway of apoptosis
**This is the major mechanism of apoptosis in all mammalian cells.**
Permeability of mitochondrial outer membrane --\> release of pro-apoptotic molecules from intermembrane space
Mitochondrial cytochrome c is essential for cell life, but when it is released into the cytoplasm, it initiates apoptosis.
Release of pro-apoptotic proteins is tightly controlled by BCL2 family of proteins, which have three subgroups (pro-apoptotic, anti-apoptotic, sensors)
Growth factors and other survival signals stimulate production of anti-apoptotic proteins (e.g. BCL2), preventing leakage of death-inducing proteins.
Deprivation of survival signals, DNA damage, or ER stress are sensed by BH3-only proteins (sensors) which activate BAX and BAK, causing leakage of death-inducing proteins. BH-3 only proteins also bind and block anti-apoptotic proteins.
Once in the cytosol, cytochrome c binds APAF-1 (apoptosis-activating factor-1) to form the apoptosome, which binds caspase-9, the critical initiator of the intrinsic pathway, which cleaves other caspase-9s (autoamplification). Caspase-9 also cleaves other caspases, mediating the second (execution phase) of apoptosis.
Other mitochondrial proteins (Smac/Diablo) enter cytoplasm, neutralize IAPs (inhibitors of apoptosis)
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Intrinsic pathway of apoptosis: BCL family of mitochondrial proteins
1. Anti-apoptotic: BCL2, BCL-XL, MCL1 are the most important. These possess four BH domains and exist in outer mitochondrial membranes, ER membranes, and cytosol. These keep the outer membrane of the mitochondria impermeable and prevent leakage of cytochrome c and other death-inducing proteins into the cytosol
2. Pro-apoptotic: BAX and BAK are the two main members. These also have 4 BH domains. When activated, these oligomerize in outer mitochondrial membrane and promote permeability. They may form a channel through which cytochrome c leaks from intermembranous space
3. Sensors: BAD, BIM, BID, Puma, Noxa are the main ones. These have one BH domain (the third of the four in the other groups... sometimes called BH3-only proteins). Act as sensors of cellular stress and damage and regulate balance between the other two groups.
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The intrinsic (mitochondrial) pathway of apoptosis.
A, Cell viability is maintained by the induction of anti-apoptotic proteins such as BCL2 by survival signals. These proteins maintain the integrity of mitochondrial membranes and prevent leakage of mitochondrial proteins.
B, Loss of survival signals, DNA damage, and other insults activate sensors that antagonize the anti-apoptotic proteins and activate the pro-apoptotic proteins BAX and BAK, which form channels in the mitochondrial membrane. The subsequent leakage of cytochrome c (and other proteins, not shown) leads to caspase activation and apoptosis.
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The extrinsic (death-receptor-initiated) pathway of apoptosis
**This pathway is initiated by engagement of plasma membrane death receptors on a variety of cells.**
Death receptors of the TNF family have a cytoplasmic domain (death domain) that's involved in pr-pr interactions. Best known death receptors are type 1 TNF (TNFR1) and a related protein called Fas (CD95)
Ligand for Fas (FasL) is expressed on T cells that recognize self-antigens and on some cytotoxic T lymphocytes which kill virus-infected and tumor cells.
FasL binds Fas --\> 3+ Fas molecules combine, and their death domains form a binding site for an adaptor protein with a death domain called FADD (Fas-associated death domain) --\> FADD binds an inactive caspase-8 via a death domain -- \> multiple pro-caspase-8 molecules come and cleave one another to generate active caspase-8 --\> activation of execution phase
This pathway of apoptosis can be inhibited by FLIP which binds pro-caspase-8, but does not cleave it
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The extrinsic (death receptor initiated) pathway of apoptosis, illustrated by the events following Fas engagement. FADD, Fas-associated death domain; FasL, Fas ligand.
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Mechanisms of apoptosis. The two pathways of apoptosis differ in their induction and regulation, and both culminate in the activation of caspases. In the mitochondrial pathway, proteins of the BCL2 family, which regulate mitochondrial permeability, become imbalanced and leakage of various substances from mitochondria leads to caspase activation. In death receptor pathway, signals from plasma membrane receptors lead to the assembly of adaptor proteins into a “death-including signaling complex,” which activates caspases, and the end result is the same.
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Execution phase of apoptosis
**The tow initiating pathways converge to a cascade of caspase activation, which mediates the final phase of apoptosis.**
Mitochondrial pathway --\> initiator caspase-9 activation
Death-receptor pathway --\> initiator caspases-8 (and -10 in humans) activation
After initiator caspase activation, cell death set in motion by sequential activation of executioner caspases (caspase-3, 6, others). These act on many cell components (e.g. cleave inhibitors of cytoplasmic DNase --\> cleavage of DNA; degradation of structural components of nuclear matrix)
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Removal of dead cells after apoptosis
Formation of apoptotic bodies breaks cells into 'bite-sized' fragments for phagocytes
Phosphatidylserine is flipped to outer leaflet of membrane, where it's recognized by several macrophage receptors as an 'eat me' signal
Cells dying by apoptosis release soluble factors that recruit phagocytes.
Some apoptotic bodies are coated by **thrombospondin**, an adhesive glycoprotein recognized by phagocytes
Apoptotic bodies can also be coated with antibodies and complement (C1q) which are recognized by phagocytes)
Very efficient process; dead cells disappear, often within minutes, without leaving a trace and NO INFLAMMATION
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Examples of apoptosis
1. Growth factor deprivation: Hormone-sensitive cells deprived of their hormone, lymphocytes that are not stimulated by antigens/cytokines, neurons deprived of nerve growth factor all die by apoptosis, triggered by the intrinsic pathway
2. DNA damage: Cell exposure to radiation or chemotherapeutic agents --\> DNA damage (genotoxic stress) --\> apoptosis. This involves tumor-suppressor gene (TP53) which encodes p53 protein, which normally arrests the cell cycle at G1 to allow time for repair. If damage is too great to repair, p53 triggers apoptosis. Mutations in TP53 can lead to inappropriate cell survival and neoplastic transofrmation ('life or death switch' following genotoxic stress.
3. Protein misfolding: Unfolded/misfolded proteins may accumulate, despite chaperone activity, due to inherited mutations or stresses --\> unfolded protein response --\> increase chaperone production, enhance proteasomal degradation of abnormal proteins, slows protein translation --\> IF THERE ARE TOO MANY MISFOLDED PROTEINS TO COMPENSATE FOR --\> activation of caspases and apoptosis (ER stress)
4. Apoptosis induced by TNF receptor family: FasL on T cells binds Fas on the same or neighboring lymphocytes (helps prevent autoimmune disease)
5. Cytotoxic T Lymphocyte-Mediated apoptosis: CTLs secrete perforin (transmembrane, pore-forming molecule) which promotes entry of CTL-derived serine proteases (granzymes) which cleave proteins at aspartate residues, activating cellular caspases
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The unfolded protein response and endoplasmic reticulum (ER) stress.
A, In healthy cells, newly synthesized proteins are folded with the help of chaperones and are then incorporated into the cell or secreted.
B, Various external stresses or mutations induce a state called ER stress, in which the cell is unable to cope with the load of misfolded proteins. Accumulation of these proteins in the ER triggers the unfolded protein response, which tries to restore protein homeostasis; if this response is inadequate, the cell dies by apoptosis.
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Disorders associated with dysregulated apoptosis
1. *Disorders associated with defective apoptosis and increased cell survival:* Mutations in TP53 gene increase susceptibility to mutations due to defective DNA repair, which can lead to cancer. TP53 mutations are the most common genetic abnormality found in human cancers. Defective apoptosis can also lead to failure of elimination of lymphocytes that recognize self-antigen --\> autoimmune disorders
2. *Disorders associated with increased apoptosis and excessive cell death:*
(1) neurodegenerative diseases - apoptosis of specific sets of neurons due to mutations and misfolded proteins
(2) Ischemic injury - myocardial infarction, stroke
(3) Death of virus-infected cells
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KEY CONCEPTS: apoptosis
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Necroptosis (a.k.a. programmed necrosis, caspase-independent programmed cell death)
Hybrid form of cell death that shares aspects of necrosis and apoptosis
Morphologically, and to an extent, biochemically, it resembles necrosis
- Morphologic: swelling of cells and organelles, ultimately plasma membrane rupture
Biochemical: Loss of ATP, ROS generation, lysosomal enzyme release
Mechanistically, it is similar to apoptosis
- triggered by genetically programmed signal transudction events leading in cell death
This DOES NOT result in caspase activation
Process:
Ligation of a recptor (usually TNFR1) --\> recruitment of RIP1, RIP3 (receptor-interacting proteins), caspase-8 into a multiprotein complex (a.k.a. necrosome)**, CASPASE-8 IS NOT ACTIVATED** --\> unclear terminal events of signal transduction --\> permeabilization of lysosomal membranes, generation of ROS, mitochondrial damage, decreased ATP levels
This process is involved in both physiologic and pathologic cell death
- Physiologic: formation of boen growth plate
- Pathologic: steatohepatitis, acute pancreatitis, reperfucsion injury, neurodegenerative disease (e.g. Parkinson)
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Molecular mechanism of TNF-mediated necroptosis. Cross-linking of TNFR1 by TNF causes recruitment of RIP1 and RIP3 along with caspase 8. Activation of the caspase leads to apoptosis as described in the text. Inhibition of caspase 8, as may occur in some viral infections, allows RIP1 and RIP3 to initiate signals that affect mitochondrial generation of ATP and ROS. This is followed by events typical of necrosis.
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Pyroptosis
Another form of programmed cell death
Accompanied by release of fever-inducing cytokine, IL-1
Microbial products taht enter infected cells' cytoplasm activate a multiprotein complex called the *inflammasome*, which functions to activate caspase-1 (a.k.a. interleukin-1b converting enzyme) which cleaves and activates IL-1, a mediator of inflammation
Caspase-1 and caspase-11 induce death of cells
Unlike apoptosis, this is characterized by cell swelling, loss of plasma membrane integrity, and release of inflammatory mediators
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KEY CONCEPTS: necroptosis and pyroptosis
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Autophagy
**A process in which a cell eats its own contents**
Three types are based on how material is delivered to lysosome for degradation:
1. Chaperone-mediated autophagy: direct translocation across lysosomal membrane by chaperone proteins)
2. Microautophagy: inward invagination of lysosomal membrane for delivery
3. Macroautophagy (a.k.a. autophagy): sequestration and transportation of portions of cytosol in a double membrane-bound autophagic vacuole (autophagosome)
This is an evolutionarily conserved survival mechanism; in states of nutrient deprivation, starved cell lives by cannibalizing itself and recycling digested contents.
Autophagy is implicated in many physiologic states (e.g. aging, exercise) and pathologic processes.
Several steps:
1. Formation of an isolation membrane (phagophore - derived from ER) and its nucleation
2. Elongation of the vesicle
3. Maturation of autophagosome, its fusion with lysosomes, eventual degradation of contents
Numerous genes related to autophagy (Atgs) have required products for autophagosome creation.
Starvation, depletion of growth factors --\> activation of initiation complex (4 proteins) --\> assembly of nucleation complex --\> nucleation of autophagosomal membrane --\> further elongation of membrane and enclosure of 'cargo' (requires several ubiquitin-like conjugation systems, e.g. microtubule-associated protein light chain 3 (LC3) - marker used to identify cells in autophagy) --\> Autophagosome fuses with endosomes and lysosomes to form autophagolysosome --\> inner membrane and enclosed cytosolic gargoes are degraded by lysosomal enzymes
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Autophagy. Cellular stresses, such as nutrient deprivation, activate an autophagy pathway that proceeds through several phases (initiation, nucleation, and elongation of isolation membrane) and 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. See text for details.
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KEY CONCEPTS: Autophagy
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Four main pathways of abnormal intracellular accumulations
1. Inadequate removal of a normal substance, secondary to packaging and transport mechanistic defects (e.g. fatty change in liver)
2. Accumulation of an abnormal endogenous substance as a result of genetic/acquired defects in its folding, packaging, transport, or secretion (e.g. mutated forms of a1-antitrypsin)
3. Failure to degrade a metabolite due to inherited enzyme deficiencies (e.g. storage disease)
4. Deposition and accumulation of an abnormal exogenous substance when the cell has neither the enzymatic machinery to degrade it nor the ability to transport it (e.g. carbon, silica particles)
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Mechanisms of intracellular accumulations discussed in the text
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Intracellular lipid accumulations
1. Steatosis (fatty change): **These terms describe abnormal accumulations of triglycerides within parenchymal cells**; it is often seen in the liver due to its involvement in fat metabolism, but also occurs in heart, muscle, and kidney. Causes: toxins, protein malnutrition, diabetes mellitus, obesity, anorexia
2. Cholesterol and cholesterol ester accumulations:
(1) Atherosclerosis:**In atherosclerotic plaques, smooth muscle cells and macrophages in intimal layer of aorta and large arteries are filled with lipid vacuoles made up of cholesterol and its esters**.
(2) Xanthomas: **Intracellular accumulation of cholesterol within macrophages is also characteristic of acquired and hereditary hyperlipidemic states.** Clusters of foamy cells seen in skin and tendons
(3) Cholesterolosis: focal accumulations of cholesterol-laden macrophages in lamina propria of gallbladder.
(4) Niemann-Pick disease, type C: lysosomal storage disease caused by mutations in cholesterol-trafficking enzyme --\> cholesterol accumulation in multiple organs
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Intracellular accumulations of proteins
**These usually appear as rounded, eosinophilic droplets, vacuoles, or aggregates in cytoplasm**.
Different causes:
1. Reabsorption droplets in proximal renal tubules: associated with significant proteinuria; increased PCT cell pinocytosis of proteins--\> reversible protein accumulations in cells
2. Normally secreted proteins may accumulate if produced in excessive amounts (e.g. plasma eclls in active Ig syntehsis (a.k.a. mott cells, russell bodies)
3. Defective intracellular transport and secretion of critical proteins: in a1-antitrypsin deficiency, mutations in the gene --\> slow protein folding --\> buildup of partially folded intermediates in ER of liver
4. Accumulation of cytoskeletal proteins: several microtubules, thin actin filaments, thick myosin filaments, and intermediate filaments. Accumulations of types of intermediate filaments (keratin filaments in epithelial cells, and neurofilaments in neurons) are asscoaited with cell injury.
5. Aggregation of abnormal proteins (a.k.a. proteinopathies, protein-aggregation diseases): Abnormal/misfolded proteins may deposit in tissues and interfere with function. These can be intra- or extra-cellular or both (i.e. amyloidosis)
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Intracellular hyaline accumulation
**The term hyaline usually refers to an alteration within cells or in the extracellular space that gives a homogeneous, glassy, pink appearance in routine histologic sections stained with H&E**.
Produced by a variety of alterations
Examples:
Certain protein accumulations (Russell bodies, reabsorption droplets, alcoholic hyaline)
*Extracellular hyaline* may be collagenous fibrous tissue in old scars or hyalinized arteriolar walls due to hypertension and Diabetes melltius.
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Intracellular glycogen accumulation
**Glycogen is a readily available energy source stored in the cytoplasm of healthy cells. Excessive intracellular deposits of glycogen are seen in patients with a glucose/glycogen metabolism abnormality (e.g. hepatocytes, renal tubular cells, islets of langerhans, cardiac muscle cells in patients with diabetes mellitus)**
Glycogen masses appear as clear cytoplasmic vacuoles.
It will stian rose-violet with PAS or Best Carmine stain.
Glycogen may accumulate in cells due to glycogen storage diseases
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Intracellular pigment accumulations
Exogenous pigments
**The most common exogenous pigment is carbon, a ubiquitous air pollutant in urban areas**.
Macrophages in the alveoli pick it up and transport it to lymph nodes. Accumulation in the lungs - anthracosis
Endogenous pigments
1. **Lipofuscin is an insoluble yellow-brown pigment, also known as lipochrome of wear-and-tear pigment.**
It is not harmful to the cell or its functions. It is derived from lipid peroxidation.
2. Melanin - black/brown pigment formed by tyrosinase catalyzing tyrosine in melanocytes.
3. **Hemosiderin - Hgb-derived, golden-yellow-to-brown, granular or crystalline pigment, one of the major storage forms of iron**
Local excesses are due to local hemorrhages, and cause accumulation in cells
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Pathologic calcification - definition
**Pathologic calcification is the abnormal tissue deposition of calcium salts, together with smaller amounts of iron, magnesium, and other mineral salts.**
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Dystrophic calcification vs. metastatic calcification
1. **Dystrophic calcification is encountered in areas of necrosis (coagulative, liquefactive, caseous, or fat necrosis)**
Always present in atheromas of atherosclerosis.
Commonly develops in aging heart valves
Look like fine white granules
2. **Metastatic calcification may occur in normal tissues whenever there is hypercalcemia.**
Four principal causes of hypercalcemia:
(1) increased PTH secretion with subsequent bone resorption (i.e. hyperparathyroidism or PTH-rp from other tumors)
(2) Resorption of bone tissue secondary to primary bone marrow tumors or diffuse skeletal metastasis
(3) vitamin-D related disorders
(4) Renal failure --\> phosphate retention --\> 2ndary hyperPTH
Most common sites: interstitial tissues of gastric mucosa, kidneys, lungs, systemic arteries, pulmonary veins (all these excrete acid, giving them an internal alkaline component, predisposing to calcification)
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KEY CONCEPTS: Abnormal intracellular depositions and calcifications
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What is cellular aging the result of?
**Cellular aging is the result of a progresive decline in cellular function and viability caused by genetic abnormalities and the accumulation of cellular and molecular damage due to the effects of exposure to exogenous influences.**
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What are four mechanisms involved in cell aging?
1. DNA damage
2. Cellular senescence
3. Defective protein homeostasis
4. Deregulated nutrient sensing
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Cellular aging: DNA damage
DNA damage is caused by many exogenous (physical, chemical biologic) and endogenous factors (ROS)
Most DNA damage is reparied by DNA repair enzymes, but some persists and accumulates as cells age. This leads to mutations
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Cellular aging: Cellular senescence
**All normal cells have a limited capacity for replication, and after a fixed number of divisions, cells become arrested in a terminally nondividing state (senescence)**.
Two mechanisms underlie cellular senescence:
1. Telomere attrition: **Progressively shortening telomeres ultimately results in cell cycle arrest.**
With each replication, a small part of the telomere is not duplicated. Telomerase is an RNA-protein complex which adds nucleotides to chromosomes. It's activity is strong in germ cells, very strong in cancer cells, and low in stem cells, but is absent in most somatic tissues.
2. Activation of tumor suppressor genes: Activation of tumor suppressor genes (i.e. CDKN2A locus) is involved in cellular senescence. This locus encodes p16, which protects the cell from uncontrolled mitogenic signals
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Cellular aging: Defective protein homeostasis
Two mechanisms:
1. Maintain proteins in their correctly folded conformations, mediated by chaperone molecules
2. Degrade misfolded proteins by autophagy-lysosome system and ubiquitin-proteasome system
Both of these are impaired with cell aging
Administration of rapamycin (inhibits mTOR pathway) increases life span of middle-aged mice.
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Cellular aging: Deregulated nutrient sensing
Eating less increases longevity - stop eating a whole bag of popcorn and/or a thousand goldfish every night you fat idiot.
Two major neurohormonal circuits regulate metabolism:
1. Insuin and insulin-like growth factor 1(IGF-1) signaling: IGF-1 is produced by many cells in response to growth hormone secretion by the pituitary. It promotes an anabolic state, cell growth, replication. It targets AKT and its downstream target, mTOR
2. Sirtuins - family of NAD-dependent protein deacetylases. Promote expression of several genes whose products increase longevity (inhibit metabolic activity, reduce apoptosis, stimulate protein folding, inhibit harmful effects of ROS)
Caloric restriction --\> reduces signaling intensity of IGF-1, and increases sirtuins --\> increases longevity
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Mechanisms that cause and counteract cellular aging. DNA damage, replicative senescence, and decreased and misfolded proteins are among the best described mechanisms of cellular aging. Nutrient sensing exemplified by calorie restriction, counteracts aging by activating various signaling pathways and transcription factors. IG, Insulin-like growth factor; TOR, target of rapamycin
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KEY CONCEPTS: Cellular aging
