Lecture 27

Question 1

principle intracellular targets of injurious stimuli

Answer 1

• mitochondria
• cell membranes
• machinery of protein synthesis and secretion
• DNA

pg 669

Question 2

cellular mechanisms of injury

Answer 2

• hypoxia/ischemia, radiation -> mitochondria -> decreased ATP, decreased energy-dependent functions OR increased ROS, damage to lipids, proteins, NAs -> cell injury -> NECROSIS
• ROS -> cellular membranes -> damage to lysosomal membranes, leakage of enzymes OR damage to plasma membrane, impaired transport functions, leakage of cellular contents -> NECROSIS
• radiation, mutations -> nucleus -> DNA damage, cell cycle arrest or activation of caspases -> APOPTOSIS

pg 670

Question 3

mitochondria as intracellular targets of injurious stimuli

Answer 3

• increased cytosolic calcium ions
• generation of reactive oxygen species (ROS)
• oxygen deprivation

pg 671

Question 4

three major consequences of mitochondrial damage

Answer 4

• ATP depletion (associated with both hypoxic and chemical (toxic) injury)
• reduction in activity of plasma membrane energy-dependent sodium pumps
• alteration of cellular energy metabolism

pg 671

Question 5

role of mitchondria in cell injury and death

Answer 5

• ATP required for all synthetic and degradative processes within the cell -> membrane transport, protein synthesis, lipogenesis, and the deacylation-reacylation reactions necessary for phospholipid turnover
• depletion of ATP to 5%-10% of normal levels has widespread effects on critical cellular systems
• ATP generated through: oxidative phosphorylation (MOST) and glycolysis

pg 672

Question 6

reversible cell damage early responses

Answer 6

includes injury to one of more vital cell systems

• mitochondria -> leading to inability to produce energy in the form of ATP
• cell membranes -> causing loss of fluid and ion homeostasis and accumulation of free radicals
• first ultrastructural evidence of sublethal cell damage is swelling of membrane-bound organelles (ER and mitochondria)

pg 674

Question 7

reduced activity of plasma membrane energy-dependent sodium pumps

Answer 7

• causes sodium ions to enter and accumulate inside cells
• intracellular potassium ion concentration falls
• cell swelling and ER dilation occurs due to osmotically-driven water accumulation

pg 675

Question 8

cellular energy metabolism is altered by mitochondrial injury

Answer 8

• supply of oxygen to cells is decreased as ischemia develops and oxidative phosphorylation stops
• adenosine monophosphate levels increase (AMP)
• conditions stimulate the use of glycogen to generate ATP and rapidly deplete glycogen levels
• anaerobic conditions increase as glycogen metabolism increases lactic acid levels and inorganic phosphates
• intracellular pH decreases and causes decreased activity of many cytosolic enzymes

pg 675

Question 9

machinery of protein synthesis and secretion is disrupted

Answer 9

• prolonged or worsening depletion of ATP leads to structural disruption of the protein synthetic apparatus occurs
• results in detachment of ribosomes from the rough ER and dissocation of polysomes
• consequent reduction in protein synthesis
• injurious effects caused by increased protein misfolding

pg 676

Question 10

irreversible damage to mitochondrial and lysosomal membranes -> cell undergoes necrosis

Answer 10

• incomplete oxidative phosphorylation also leads to formation of ROS, which have many deleterious effects
• leakage of mitochondrial proteins due to channel formation by pro-apoptotic BAX and BAK is the initital step in apoptosis by the intrinisic pathway
• this action of BAX and BAK is specific to mitochondrial membranes only and leads to damage of other organelles indirectly

pg 676

Question 11

mechanisms of membrane damage in cell injury

Answer 11

• decreased oxygen
• increased cytosolic calcium ions
• reactive oxygen species, often produced on reperfusion of ischemic tissues

pg 677

Question 12

cytoskeletal abnormalities

Answer 12

• cytoskeletal filaments anchor the plasma membrane to the cell interior
• proteases activated by cytosolic calcium may damage these tethers
• cell swelling, particularly in myocardial cells, may lead to detachment of the cell membrane from the cytoskeleton causing the cell to be susceptible to stretching and rupture

pg 677

Question 13

membrane damage

Answer 13

• early loss of selective membrane permeability, leading ultimately to overt membrane damage is a consistent feature of most forms of cell injury (except apoptosis)
• membrane damage may affect the integrity and functions of all cellular membranes
• in ischemic cells, membrane defects may be the result of ATP depletion and calcium-mediated activation of phospholipases
• plasma membrane can also be damaged directly by bacterial toxins, viral proteins, lytic complement components, and a variety of physical and chemical agents
• mitochondrial membranes are damaged by the opening of a mitochondrial permeability transition pore that leads to decreased ATP generation and release of proteins that trigger apoptotic death

pg 678

Question 14

plasma membrane damage

Answer 14

• results in loss of osmotic balance, influx of fluids and ions, and loss of cellular contents
• leakage of metabolites, such as glycolytic intermediates, that are vital for replacing lost ATP

pg 679

Question 15

injury to lysosomal membranes

Answer 15

• results in leakage of lysosomal enzymes into the cytoplasm
• activation of acid hydrolases, which degrade RNA, DNA, proteins, phosphoproteins, and glycogen (work together to cause membrane damage)
• drives the cell into necrosis

pg 679

Question 16

damage to DNA

Answer 16

• damage to nuclear DNA activates sensors that trigger p53-dependent pathways (p53 important for tumor suppression)
• DNA damage may be caused by: exposure to radiation, chemotherapeutic drugs, ROS, or may occur spontaneously as a part of aging

pg 680

Question 17

DNA damage activates p53

Answer 17

arrests cells in the G1 phase of the cell cycle and activates DNA repair mechanisms

• if these mechanisms fail to correct the DNA damage, p53 triggers apoptosis by the mitochondrial pathway
• the cell dies rather than survive with abnormal DNA that has the potential to induce malignant transformation
• mutations in p53 that interfere with its ability to arrest cell cycling or to induce apoptosis are associated with numerous cancers

pg 680

Question 18

pathologic effects of free radicals

Answer 18

affects of ROS and other free radicals are wide-ranging

• lipid peroxidation in membranes
• oxidative modifications of proteins
• lesions in DNA

pg 681

Question 19

lipid peroxidation in membranes

Answer 19

LOW HANGING FRUIT: in the presence of molecular oxygen (O2), free radicals may cause peroxidation of lipids within plasma and organellar membranes

• oxidative damage is initiated when the double bonds in UFA of membrane lipids are attacked by O2 derived free radicals
• lipid-free radical interactions yield peroxides, which are unstable and reactive
• an autocatalytic chain reaction follows (called propagation) that can result in extensive membrane damage

pg 682

Question 20

oxidative modification of proteins

Answer 20

• free radicals promote: oxidation of amino acid side chains, formation of covalent protein-protein crosslinks (disulfide bonds), and oxidation of the protein backbone
• oxidative modifications may also damage the active sites of enzymes, disrupt the conformation of structural proteins, and enhance proteasomal degradation of unfolded or misfolded proteins -> leads to severe disorder in the cell

pg 683

Question 21

lesions in DNA

Answer 21

• free radicals are capable of causing single and double strand breaks in DNA, crosslinking DNA strands, and forming adducts
• oxidative DNA damage has been implicated in cell aging and in malignant transformation of cells

pg 683

Question 22

pathologic effects of calcium homeostasis disturbance

Answer 22

ischemia and certain toxins cause an excessive increase in cytosolic Ca²⁺ (initially bc of release from intracellular stores and later due to increased influx across the plasma membrane)

• accumulation of Ca²⁺ in mitchondria results in opening of the mitochondrial permeability transition pore and failure of ATP generation
• increased cytosolic Ca²⁺ abnormally activates: phospholipases, proteases, endonucleases, and ATPases
• phospholipases: cause membrane damage
• proteases: break down both membrane and cytoskeletal proteins
• endonucleases are responsible for DNA and chromatin fragmentation
• ATPases hasten ATP depletion

pg 684

Question 23

ER stress

Answer 23

The accumulation of misfolded proteins in the ER can stress adaptive mechanisms and trigger apoptosis. If unfolded or misfolded proteins accumulate in the ER, they trigger a number of alterations that are collectively called the unfolded protein response.

pg 685-686

Question 24

Unfolded Protein Response

Answer 24

• activates signaling pathways that increase the production of chaperones
• enhances proteasomal degradation of abnormal proteins
• slows protein translation, thus reducing the load of misfolded proteins in the cell
• if this cytoprotective response is unable to cope with the accumulation of misfolded proteins, the cell activates caspases and induces apoptosis
• protein misfolding is thought to be the causative cellular abnormality in several neurodegenerative diseases

pg 685-686

Question 25

diseases caused by misfolding of proteins

Answer 25

• cystic fibrosis: loss of CFTR leads to defects in chloride transport
• familial hypercholesterolemia: loss of LDL receptor leads to hypercholesterolemia
• Tay-Sachs disease: lack of the lysosomal enzyme leads to storage of GM2 gangliosides in neurons
• α1-antitrypsin deficiency: storage of nonfunctional protein in hepatocytes causes apoptosis, absence of enzymatic activity in lungs causes destruction of elastic tissue giving rise to emphysema
• Creutzfeldt-Jacob disease: abnormal folding of prions (PrPsc) causes neuronal cell death
• Alzheimer disease: abnormal folding of αβ peptides causes aggregation within neurons and apoptosis

pg 687

Question 26

hypoxia and ischemia

Answer 26

ULTRA LOW HANGING FRUIT: ischemia is the most common cause of cell injury in clinical medicine and results from hypoxia induced by reduced blood flow (often due to a mechanical arterial obstruction)

• ischemia can also occur due to reduced venous drainage
• ischemia compromises the delivery of substrates for glycolysis (blood flow has stopped)
• ischemia causes more rapid and severe cell injury and tissue injury than hypoxia
• hypoxia maintains blood flow and energy production by anaerobic glycolysis can continue
• in ischemic tissues, aerobic metabolism ceases and anaerobic energy generation also fails after glycolytic substrates are exhausted or glycolysis is inhibited by the accumulation of metabolites which otherwise would be washed out by flowing blood

pg 688

Question 27

cellular adaptation

Answer 27

• cellular injury is reversible up to a certain point
• if severe enough, cell injury can lead to cell death

pg 689

Question 28

mechanisms of ischemic cell injury

Answer 28

• the sequence of events following hypoxia or ischemia reflects many of the biochemical alterations in cell injury
• as intracellular oxygen tension falls, oxidative phosphorylation fails and ATP generation decreases
• loss of ATP results initially in reversible cell injury and later in cell death by necrosis
• decrease in oxidative phosphorylation -> decreased ATP -> eventually: ER swelling, cellular swelling, loss of microvilli, blebs, clumping of nuclear chromatin, decreased protein synthesis

pg 690

Question 29

ischemia-reperfusion injury

Answer 29

• restoration of blood flow to ischemic tissues can promote recovery of cells if they are reversibly injured but can also paradoxically exacerbate (increase) cell injury and cause cell death
• as a consequence, reperfused tissues may sustain loss of viable cells in addition to those that are irreversibly damaged by the ischemia
• this process of ischemia-reperfusion injury is clinically important because it contributes to tissue damage during myocardial and cerebral infarction following therapies that restore blood flow

pg 691

Question 30

chemical (toxic) injury

Answer 30

• chemical injury remains a frequent problem in clinical medicine and is a major limitation to drug therapy
• many drugs are metabolized in the liver, making the liver a major target of drug toxicity
• toxic liver injury is often the reason for terminating the therapeutic use or development of a drug

pg 693

Question 31

direct toxicity

Answer 31

Some chemicals injure cells directly by combining with critical molecular components
Examples:

• in mercuric chloride poisioning, mercury binds to the sulfhydryl groups of cell membrane proteins, causing increased membrane permeability and inhibition of ion transport
• cyanide poisions mitochondrial cytochrome oxidase and thus inhibits oxidative phosphorylation
• many antineoplastic chemotherapeutic agents and antibiotics also induce cell damage by direct cytotoxic effects

pg 693

Question 32

conversion to toxic metabolites

Answer 32

• most toxic chemicals are not biologically active in their native form but must be converted to reactive toxic metabolites which then act on target molecules
• modification is usually accomplished by the cytochrome P-450 mixed function oxidases in the smooth ER of the liver and other organs
• membrane damage and cell injury occur mainly by formation of free radicals and subsequent lipid peroxidation

examples:

• carbon tetra-chloride (CCl4) is converted by cytochrome p-450 into a highly reactive free radical (carbon tri-chloride radical) which causes lipid peroxidation and extensive cellular structural damage
• acetaminophen is convered to a toxic product in the liver, leading to cell injury

pg 694

Question 33

hypertrophy

Answer 33

increase in size of cells resulting in an increase in size of the organ

pg 695

Question 34

pathologic hypertrophy

Answer 34

• most common stimulus for hypertrophy of skeletal and cardiac muscle is increased workload
• muscle cells respond by synthesizing MORE PROTEIN and increasing the number of microfilaments per cell; this increases the amount of force each cell and generate
• classic example: pathologic hypertrophy of the heart in response to pressure overload usually results from either hypertension or valvular disease

pg 696

Question 35

physiologic hypertrophy

Answer 35

increase in amount of protein in cells
Classic Example 1:

• massive physiologic growth of the uterus during pregnancy due to estrogenic hormonal stimulation, leading to hypertrophy of smooth muscle fibers

Classic Example 2:

• bulging muscles of bodybuilders engaged in “pumping iron” results from enlargment of individual skeletal muscle fibers in response to increased demand

pg 696

Question 36

hyperplasia

Answer 36

An increase in the number of cells within an organ or tissue in response to a stimulus; can happen at the same time as hypertrophy

• only takes place in tissues that contain cells capable of dividing (increase in cell number)
• physiologic hyperplasia: due to action of hormones or growth factors, when there is need for… increased functional capacity (female breast development at puberty and pregnancy) OR compensatory increase after damage or resection (liver regeneration after a liver lobe transplant)
• most forms of pathologic hyperplasia are caused by excessive or inappropriate actions of hormones or growth factors

pg 698-700

Question 37

forms of pathologic hyperplasia

Answer 37

• endometrial hyperplasia: due to relative or absolute increases in the amount of estrogen
• benign prostatic hyperplasia: induced by abnormal response to androgens
• certain viral infections: papillomavirus can cause skin warts and mucosal lesions composed of masses of hyperplastic epithelium
• hyperplasia can regress if the hormonal stimulation is eliminated

pg 700

Question 38

pathologic hyperplasia

Answer 38

if growth control mechanisms become deregulated (genetic abberations), hyperplasia can progress to cancerous growth:

• endometrial carcinoma
• prostatic carcinoma
• squamous cell carcinoma (papilloma virus)

pg 701

Question 39

metaplasia

Answer 39

• reversible change in which one differentiated cell type is replaced by another cell type
• most commonly columnar to squamous epithelium (in response to habitual smoking)

pg 702

Question 40

atrophy

Answer 40

reduction in size of an organ or tissue due to a decrease in cell size and number; two types of atrophy

• physiologic: common during normal embryonic development or decrease in size of uterus after giving birth
• pathologic: local or generalized
• causes: decreased workload (atrophy of disuse), loss of innervation, diminished blood supply, inadequate nutrition, loss of endocrine stimulation, pressure

pg 703

Question 41

mechanisms of atrophy

Answer 41

• decreased protein synthesis -> decreased metabolic activity
• increased protein degradation (by ubiquitin-proteasome pathway)
• nutrient deficiency
• ubiquitin ligases attach ubiquitin to cellular proteins, which targets the proteins for degradation in proteasomes
• often accompanied by increased autophagy

pg 704

Question 42

causes of cell injury

Answer 42

• oxygen deprivaiton (hypoxia)
• physical agents
• chemical agents & drugs
• infectious agents
• immunological reactions
• genetic derangements
• nutritional imbalances

pg 705

Question 43

reversible features of cell injury

Answer 43

• cellular swelling: cells become unable to maintain ionic and fluid homeostasis due to failure of energy-dependent ion pumps in the plasma membrane
• fatty change: due to hypoxic injury, various forms of toxic or metabolic injury; lipid vacuoles appear in the cytoplasm
• plasma membrane alterations: blebbing, blunting, loss of microvilli
• mitochondrial changes: swelling, appearance of small amorphous densities
• dilation of the ER: detachment of polysomes, intracytoplasmic myelin
• nuclear alterations: disaggregation of granular and fibrillar elements

pg 706