Lecture 4 - Mechanisms of Toxicity 2

what are chemicals that specifically interact with protein targets?

a. chemicals that inhibit cellular respirations

b. chemicals that inhibit production of cellular building blocks

what are chemicals that inhibit cellular respiration?

inhibitors of proteins or enzymes involved in oxygen consumption, fuel utilization and ATP production will cause energy depletion and cell death

what compound inhibits mitochondrial cytochrome c oxidase to prevent cellular respiration?

cyanide

where is cyanide found?

in plants, cherries and apple seeds

what does carbon monoxide do?

displaces oxygen from hemoglobin causing hypoxia

what can be used to treat cyanide poisoning?

Vitamin B-12

what are sources of CO?

anything you burn (petroleum fuel)

what is an example of a chemical that inhibits the production of cellular building blocks (nucleotides, lipids, amino acids)?

amanitin from Deathcap mushrooms

what organs are particularly susceptible to toxin damage?

liver and kidney

what is a common cause of hepatic necrosis?

acetaminophen poisoning

what is a common cause of hepatic inflammation (hepatitis)?

halothane can covalently bind to liver proteins and trigger an autoimmune reaction

what is a common cause of chronic liver damage (cirrhosis)?

long-term sub-clinical toxicant causes cellular toxicity and inflammation

what can cause nephrotoxicity?

a. changes in glomerular filtration rate (GFR)

b. allergic nephritis

c. chronic nephritis

what causes changes in glomerular filtration rates?

• largely due to drugs that alter blood flow:

  • NSAIDS (aspirin) reduce prostaglandins which in turn reduces blood flow/GFR ACE inhibitors (ramipril) increase blood flow/GFR

what causes allergic nephritis?

allergic reaction to NSAIDs (fenoprofen) and antibiotics (metacillin)

what causes chronic nephritis?

long-term NSAID and acetaminophen use

what are examples of renal tox in dogs?

raisins/grapes

what are examples of renal tox in cats?

lily flower

what are molecular mechanisms of cellular dysfunction?

a. chemicals that cause DNA adducts
b. chemicals that cause protein adducts

c. chemicals that cause oxidative stress

d. all of the above can also cause inflammation which can lead to cellular dysfunction

what happens with chemicals that cause DNA adducts?

a. can lead to DNA mutations which can activate cell death pathways

b. if mutations activate oncogenes or inactivate tumor suppressors, it can lead to uncontrolled cell proliferation and cancer (benzopyrene)

what happens with chemicals that cause protein adducts?

a. can lead to protein dysfunction which can activate cell death pathways

b. can also lead to autoimmunity

c. if protein adducts activate oncogenes or inactivate tumor suppressors, it can lead to uncontrolled cell proliferation and cancer (diclofenac glucuronidation metabolite)

what happens with chemicals that cause oxidative stress?

can oxidize the DNA or proteins leading to DNA mutations or protein dysfunction and all of the above (benzene, CCl4)

apoptosis?

one of the main forms of programmed cell death (not as dangerous to organism as necrosis)

characteristics of apoptosis?

• active form of cell death enabling individual cells to commit suicide

• caspase-dependent

what happens to dying cells during apoptosis?

dying cells shrink and condense and then fragment, releasing small membrane-bound apoptotic bodies, which are phagocytosed by immune cells (macrophages)

during apoptosis where are intracellular constituents not released?

where they might have deleterious effects on neighboring cells

what is necrosis?

unprogrammed cell death (dangerous)

Necrosis passive or active? why?

passive form of cell death induced by accidental damage of tissue and does not involve activation of any specific cellular program

what happens to plasma membrane during necrosis?

early loss of plasma membrane integrity and swelling of the cell body followed by bursting of cell

when do cells necrotize?

in response to tissue damage (injury by chemicals/viruses, infection, cancer, inflammation, ischemia(death due to blockage of blood to tissue))

mechanism of necrosis?

agents that disrupt the normal functions of the cell (esp. energy producing function of the mitochondria and protein synthesize)

what are the three primary metabolic disorders jeopardizing cell survival?

1. ATP depletion

2. Sustained rise in intracellular Ca2+

3. Overproduction of ROS, RNS

why is ATP important in a cell?

central role in cellular maintenance both as a chemical for biosynthesis and as the major source of energy

what are all the ways ATP is used?

1. ATP (energy) drives ion transporters such as Na+/K+-ATPase (plasma membrane), Ca2+-ATPase (ER and plasma membrane) to maintain cellular ion gradients (most important for necrosis!)

2. used in biosynthetic reactions (phosphorylation and adenylation)

3. used for signal transduction regulation (phosphorylation of receptor tyrosine kinase and kinase pathways

4. incorporated into DNA

5. Muscle contraction (actin/myosin interaction) and neurotransmission

6. polymerization of cytoskeleton (actin and tubule polymerization)

7. cell division

8. maintenance of cell morphology

If ATP depleted what is the immediate next step?

compromised ion pumps (Na/K (3/2) ATPase and Ca2+-ATPase)

when the ion pumps are compromised due to ATP depletion what happens next?

1. loss of ionic and volume regulatory controls → cell swelling (water influx) (rise in intracellular Na+) → cell lysis

2. Ca2+/Na levels rise intracellularly, opening of voltage-gated channels, depolarize membranes, Ca2+ and Na+ influx into the cell

what is the end result of ATP depletion?

Necrosis

what are the agents that impair ATP synthesis?

1. Inhibitors of electron transport

2. Inhibitors of oxygen delivery

3. inhibitors of ADP phosphorylation

4. chemicals causing mitochondrial DNA damage

what are inhibitors of electron transport that impair ATP synthesis?

1. Cyanide

2. Rotenone

3. Paraquat

what does cyanide inhibit? Action?

cytochrome C oxidase → blocks mitochondria in cell membrane

what does Rotenone inhibit? Importance?

inhibits complex I → insecticide that may be an environmental cause of parkinson’s disease

what does Paraquat inhibit? Importance?

inhibits complex I → herbicide, but also causes lung hemorrhaging in humans

what are the inhibitors of oxygen delivery?

a. ischemic agents → ergot alkaloids, cocaine

b. Carbon monoxide → displaces oxygen from hemoglobin

what are examples of inhibitors of ADP phosphorylation?

DDT, DIM, phytochemicals

what are examples of chemicals causing mitochondrial DNA damage?

antivirals

what is Ca2+ involved in?

a. signal transduction regulation and exocytosis

b. muscle contraction

c. cytoskeletal polymerization

d. neurotransmission and synaptic plasticity

e. enzyme induction

f. transporters

how regulated is Ca2+?

• highly regulated

• the 10K-fold difference between extracellular and cytosolic Ca2+

• Ca2+ sources are from outside cell or Ca2+ stores in ER or mitochondria

what is the 10K-fold difference between extracellular and cytosolic Ca2+ maintained by?

• impermeability of the plasma membrane to Ca2+

• transport mechanisms that remove Ca2+ from the cytoplasm

what are the 4 mechanisms to Ca2+ elimination?

1. Extracellular Ca2+ATPase

2. ER Ca2+ATPase

3. Extracellular Na+/Ca2+ exchanger

4. Mitochondrial Ca2+ uniporter

what is the excitotoxicity consequence of increased intracellular Ca2+

1. depletion of energy reserves

2. dysfunction of microfilaments

3. activation of hydrolytic enzymes

4. generation of ROS/RNS

what does depletion of energy reserves mean?

decreased mitochondrial ATP production and increased loss of ATP by activation of Ca2+ATPase

what happens with dysfunction of microfilaments?

impaired cell motility, disruption in cell morphology, cellular functions

what happens with activation of hydrolytic enzymes and generation of ROS/RNS?

disintegration of membranes, proteins, DNA, etc…

what are the three points of the triangle when the three things jeopardize the cell survival?

a) Decrease ATP → Cell osmolarity disruption (transporter disruption) cell swelling

b) Increased Ca → microfilamental dissociation, membrane blebbing, cell destruction

c) increased ROS/RNS → lipid peroxidation, membrane destruction/cell swelling, cell lysis