Cell injury and Death

Question 1

Under what conditions can atrophy occur? [7] Give an example for each.

Answer 1

1. Reduced functional demand e.g. Muscles
2. Inadequate supply of oxygen (interference of blood supply to tissues is ischaemia) e.g. Ischaemic necrosis of the heart, brain, kidney
3. Insufficient nutrients e.g. In muscles and adipose tissue
4. Interruption of trophic signals. Many cells depend on chemical mediators e.g. Endocrine and nervous system (endometrium atrophies in absence of oestrogen)
5. Persistent cell injury. Most commonly caused by chronic infalmmation (prolonged viral/bacterial infections or autoimmune disease) e.g. Gastric mucosa during chronic gastritis.
6. Increased pressure (physical Injury) e.g. Bed sores
7. Chronic disease e.g. Cancer, Congestive heart failure, AIDS (tissue loss from alterations in cytokines and other mediators)

Question 2

Define hypertrophy. Give a example.

Answer 2

An adaptive change that results in an increase in cellular size to satisfy increased functional demand or trophic signals. In some cases hyperplasia may occur.

e.g. increased muscle size from increased endurance

Question 3

What is the difference between physiologic hypertrophy and pathologic hypertrophy of the heart?

Answer 3

In physiologic hypertrophy the cell number and/or size increases due to increased excersize, and in pathologic hypertrophy the increase is due to vascular resistance.

Question 4

Historically, neurons, cardiac mycotes and skeletal muscle cells were considered incapable of mitosis. This is only partially true. Explain why.

Answer 4

Although cardiac mycotes and neurons can’t undergo mitosis, studies show they have progenitor cells that proliferate in response to cell injury or death, or increased functional demand.

Question 5

The most understood mechanism for cells to destroy specific proteins is the ubiquitin (Ub)-proteasomal apparatus. How does this work? (Basic)

Answer 5

1. Proteins to be degraded are marked by attaching small chains of Ub molecules to them (ubiquitination)

Question 6

What are the three types of proteasome and what do they do?

Answer 6

There are two main types of proteasome and one variant:

1. 20S proteasomes degrade oxidised proteins
2. 26S proteasomes degrade proteins bound to Ub
3. The Immunoproteasome (variant) is formed in response to interferon-γ (INF-γ). They are important in processing proteins to 8/9-amino acid peptides to be attached to major histocompatability complex I for tpresentation to the immune system as antigens.

Question 7

What are the three types of autophagy and what do they do?

Answer 7

1. Macroautophagy: autophagosome surrounds bulk cytosol and organelles
2. Microautophagy: selected organelles/foreign material is engulfed by lysosomes
3. Chaperone-mediated autophagy: selected cellular macromolecules are conducted to lysosomes by chaperones

Question 8

Define hyperplasia. Give a example.

Answer 8

The increase in the number of cells in an organ or tissue.

e.g. increased number of endometrial cells during puberty, increased number of erythrocytes from blood loss/menstruation, chronic injury such as prolonged pressure; calluses.

Question 9

What factors promote hyperplasia?

Answer 9

1. Hormonal stimulation
2. Increassed functional demand
3. Chronic injury

Question 10

Define metaplasia. Give a example.

Answer 10

The conversion of one differentiated cell type to another.

e.g. bronchial epithelium changing to squamous epithelium from prolonged exposure to tobacco smoke, squamous epithelium of the oesophagus replaced by glandular epithelium of the stomach (Barrett epithelium)

Question 11

Define dysplasia

Answer 11

The disordered growth and maturation of the cellular components of a tissue

Question 12

When does cell injury occur?

Answer 12

When environmental changes exceed the cell’s capacity to maintain normal homeostasis.

Question 13

Define hydropic swelling

Answer 13

A acute, reversible increase in cell volume due to increased water content.

Question 14

What changes in organelles are seen in hydropic swelling?

Answer 14

1. the cisternae of the endoplasmic reticulum become distended
2. the mitochondria swell
3. the plasma membrane blebs
4. nucleolar change

Question 15

List the types of reactive oxygen species (ROS). [6]

Answer 15

1. Hydrogen peroxide (H₂O₂)
2. Superoxide anion (O₂^-)
3. Hydroxyl radical (OH•)
4. Peroxynitrite (ONOO^-•)
5. Lipid peroxide radicals (RCOO•)
6. Hypochlorus acid (HOCl)

Question 16

How is superoxide (O₂^-) produced and what does it do?

Answer 16

• O₂^- is produced primarily by leaks in the mitochondrial electron transport chain or as part of the inflammatory response.
• used in cellular oxidative defences against pathogens, signalling intermediates that elicit the release of degradative enzymes.

Question 17

How is peroxynitrite (ONOO^-) formed and what does it do?

Answer 17

• Formed from the interaction between superoxide (O₂^-) and nitric oxide (NO).
• NO + O₂^- → ONOO^-
• Peroxynitrite attacks a wide range of important molecules; lipids, proteins, DNA.

Question 18

How is hydrogen peroxide (H₂O₂) formed and what does it do?

Answer 18

• Superoxide (O₂^-) anions are catabolised by superoxide dismutase (SOD) to produce H₂O₂.
• Also produced directly from serveral oxidases in the peroxisome.
• H₂O₂ is largely metabolised to H₂O by catalase or glutathione peroxidase.
• When produced in excess it is converted into OH•
• In neutrophils myeloperoxidase converts it into the potent radical hypochlorite (OCl^-).

Question 19

How is the hydroxyl radical (OH•) formed and what does it do?

Answer 19

• OH• is formed by either (1) radiolysis of water, (2) the Fenton reaction (H₂O₂ + Fe²⁺), (3) the Haber-Weiss reaction (O₂^- + H₂O₂)
• It is the most damaging ROS; lipid peroxidation destroys membrane integrity, protein interations causing fragmentation, cross-linking, aggregation, eventually degredation, DNA damage causing strand breaks, modified bases, and cross-linking.

Question 20

List some molecules that can be abnormally retained in the cell.

Answer 20

1. Degraded phospholipids
2. Substances that cannot be metabolised: endogenous substrates that cannot be metabolised due to having a key enzyme missing, insoluble endogenous insoluble pigments, normal protein aggregates, exogenous particulates e.g.carbon.
3. Normal body constituents: iron, copper, cholesterol.
4. Abnormal proteins e.g. Lewy bodies in Parkinson’s disease.

Question 21

How is ionising radiation damaging?

Answer 21

It causes the radiolysis of water which in turn forms OH•. This is then free to interact with DNA and inhibit DNA replication.

Question 22

List the ways in which a virus can cause cell injury [3].

Answer 22

1. Direct toxicity: undermine cellular enzymes, deplete cellular nutrients, thereby disrupting normal homeostatic mechanisms.
2. Manipulation of apoptosis: many viruses have evolved mechanisms to upregulate antiapoptotic proteins and inhibit proapoptotic ones.
3. Imunologically mediated cytotoxicity: the immune system eliminates virus-infected cells by inucing apoptosis or by lysing the cell with complement.

Question 23

Acetaminophen, an important constituent of many analgesics, is a well-studied _________, which is metabolized by the mixed-function _______system of the hepatocyte and causes liver cell ________. The drug is innocuous in recommended doses, but when consumed to excess, it is highly toxic to the _____. Most acetaminophen is enzymatically converted in the liver to nontoxic __________or ________metabolites. Less than 5% of acetaminophen is ordinarily metabolized by isoforms of cytochrome ____to _-______-_-_____________ _____ (NAPQI), a highly reactive quinone (Fig. 1-15). However, when large doses of acetaminophen overwhelm the ____________pathway, toxic amounts of NAPQI are formed. The conjugation of NAPQI with sulfhydryl groups on liver proteins causes extensive cellular ___________and subsequent injury. At the same time, NAPQI depletes the antioxidant _________(GSH), rendering the cell more susceptible to ____-_______–induced injury.

Answer 23

Acetaminophen, an important constituent of many analgesics, is a well-studied hepatotoxin, which is metabolized by the mixed-function oxidase system of the hepatocyte and causes liver cell necrosis. The drug is innocuous in recommended doses, but when consumed to excess, it is highly toxic to the liver. Most acetaminophen is enzymatically converted in the liver to nontoxic glucuronide or sulfate metabolites. Less than 5% of acetaminophen is ordinarily metabolized by isoforms of cytochrome P450 to N -acetyl-p -benzoquinone imine (NAPQI), a highly reactive quinone (Fig. 1-15). However, when large doses of acetaminophen overwhelm the glucuronidation pathway, toxic amounts of NAPQI are formed. The conjugation of NAPQI with sulfhydryl groups on liver proteins causes extensive cellular dysfunction and subsequent injury. At the same time, NAPQI depletes the antioxidant glutathione (GSH), rendering the cell more susceptible to free-radical–induced injury.

Question 24

What’s the difference between necrosis, apoptosis, and autophagy?

Answer 24

• Necrosis is often defined as accidental and passive cell death caused by a hostile environment to which the cells could not adapt.
• Apoptosis is progammed cell suicide where the cell participates in its own demise.
• Autophagic cell death is elicited when the cell is in a stressful environment and requires digestion of the portion of the cell’s macromelecular constituents.

Question 25

Outline the different types of **necrosis**. [4]

Answer 25

1. **Coagulative necrosis**: light microscopic alterations made in a dead cell; pyknosis, karyorrhexis, and karyolysis. 2. **Liquefactive necrosis**: where the rate of dissolution is much faster than the rate of repair. The result is ofen known as an abscess. 3. **Fat necrosis**: specificallt affects adipose tissue and most commonly arises from pancreatitis or trauma. Digestive enzymes are released from the damaged acinar cells and digest the surrounding adipose cells. 4. **Caseous necrosis**: characteristic of tuberculosis. Granulomas are formed containing aggregates of macrophages and other inflammatory cells.

Question 26

Outline the steps of necrosis. [8]

Answer 26

1. **The blood supply is interrupted**: decreased O₂ and glucose. 2. **Anaerobic glycolysis**: overproduction of lactate, decreased pH and ATP. 3. Lack of ATP **distorts membrane pumps**, decreased pH skews ionic balance inside cell. 4. **Ca²⁺ accumulates** inside the cell. 5. **Phospholipase A₂ (PLA₂) and proteases are activated** due to Ca²⁺, which disrupts the plasma membrane, cytoskeleton, and causes swelling. 6. Lack of O₂ **impairs mitochondrial electron-transport**: decreased ATP, facilitates ROS production. 7. **Mitochondrial damage releases cytochrome c (Cyt c)** into the cytosol: initiates apoptotic cascade. 8. **Cell dies**.

Question 27

What are the different **signalling pathways** for apoptosis [4]? Briefly decribe each.

Answer 27

1. **Extrinsic pathway apoptosis**: activation of certain membrane receptors by their ligands. 2. **Intrinsic pathway apoptosis**: triggered by diverse intracellular stresses, central role for mitochondria. 3. **Inflammatory and infectious apoptosis**: initiated by infectious agents. 4. **Perforin/granzyme pathway apoptosis**: caused by interaction of cytotoxic T cells with their target, activated by transfer of granzyme B from killer to victim.

Question 28

Give an example of **extrinsic** apoptosis. What are the main steps?

Answer 28

TNF-α binding to TNFR. FasL binding to Fas. 1. TNFR and Fas, death receptors, become activated upon binding their ligands. 2. The activated death receptor Fas, FADD (a docking protein), and procaspase-8 associate to form a death-inducing signalling complex (DISC). 3. DISC activates downstream signalling elements: procaspase-8 → caspase-8 ⇒ caspase-3,-6,-7. 4. Caspase-3 destabilises the cytoskeleton ad activates CAD, which causes nuclear fragmentation. 5.

Question 29

What are the steps in **immunologic** apoptosis?

Answer 29

1. Granzyme and perforin are released from cytotoxic lymphocytes. 2. Perforin makes a hole in the target cell's membrane through which granzyme B enters.

Question 30

What are the main steps in **intrinsic** apoptosis?

Answer 30

1. **p53** is activaded by stimuli such as ultraviolet radiation (UV): increases production of **proapoptotic** proteins. 2. These **outcompete** Bax and Bak by binding to **antiapoptotic** components, Bcl-2 and Bcl-X_L. 3. Free Bax and Bak homo-oligomerise and open the **mitochondrial permeability transition pore (MPTP).** 4. **Cyt c** leaves the mitochondria through the MPTP into the cytosol. 5. Cyt c, dATP, and Apaf-1 homopolymerise into a complex with procaspase-9 → **caspase-9**, also known as the '**apoptosome**'. 6. Caspase-9 activates procaspase-3 → **caspase-3,** which the activates enzymes that degrade DNA.

Question 31

After it binds to areas of DNA damage, **\_\_\_**activates proteins, particularly **\_\_\_**^WAF/CIP1 , that arrest the cell in stage **\_\_**of the cell cycle, allowing time for DNA repair to proceed. It also directs DNA repair enzymes to the site of injury. If DNA damage cannot be repaired, **\_\_\_**activates mechanisms that lead to apoptosis, principally by altering the balance of **\_\_\_-\_** family members at the mitochondria.

Answer 31

After it binds to areas of DNA damage, **p53** activates proteins, particularly **p21** ^WAF/CIP1 , that arrest the cell in stage **G₁** of the cell cycle, allowing time for DNA repair to proceed. It also directs DNA repair enzymes to the site of injury. If DNA damage cannot be repaired, **p53** activates mechanisms that lead to apoptosis, principally by altering the balance of **Bcl-2** family members at the mitochondria.