Lecture 10 - Hepatotoxicity Flashcards
(27 cards)
Liver physiology
Right lobe - bigger than the left lobe, contains three zones
Left lobe - smaller lobe, contains three zones
Hepatic vein - taking blood back to the heart
Hepatic artery - brings fresh blood from the heart
Portal vein - brings blood from the intestines
Common bile duct - connected to the gall bladder, sends bile to the intestines
Gall bladder - located within the right lobe
Diaphragm - located above the left lobe
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Liver zones: what are they and what are their key features?
Zone 1 - periportal:
* Most proximal to the portal triad
* Highly perfused hepatocytes
* Rich in mitochondria
* Perform beta oxidation, bile & cholesterol formation, gluconeogenesis and amino acid catabolism
Zone 2 - midzonal:
* Mixture of functions of zone 1 and zone 3
Zone 3 - centrolobular:
* Adjacent to central vein
* “Innermost zone”
* Lowest perfusion
* Rich in cytochrome p450 enzymes
* Involved in detoxification, ketogenesis, lipogenesis, glycolysis and formation of glutamine
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Liver cells: what are the types and what are their key features?
Hepatocytes:
* 80% of the total liver volume
* Synthesis and Metabolism
Stellate (Ito) Cells:
* Stores fat and vitamin A
* Control turnover of the extracellular matrix
* Regulate contractility of the sinusoids
Sinusoidal Endothelial Cells:
* Endocytic
* Antigen-presenting cells
Kupffer cells:
* Macrophage
Functions of the liver
- Detoxification - removing body waste, hormones, drugs, chemicals, foreign substances
- Biosynthesis - plasma protein synthesis: Clotting factors, albumin, transporter proteins, immune factors
- Nutrient Metabolism and storage - storing vitamins, minerals, sugars and fats
Metabolism of xenobiotics: what types of biotransformations occur, and are they always good?
- Phase I = reduction; oxidation; hydroxylation; hydrolysis
- Phase II = conjugation (amino acid and glutathione); sulfonation; methylation; acetylation; glucuronidation
Sometimes biotransformation backfires and makes a substance (more) toxic
Phase 1 metabolism: what is it, what is the most common type of reaction, the most common type of enzymes, and where are they located?
Convert a parent drug to more polar (water soluble) active metabolites by unmasking or inserting a polar functional group (-OH, -SH, -NH2)
Oxidation
The cytochrome P450 family of monooxygenases is the most prominent family of enzymes involved
They are located on the smooth endoplasmic reticulum and mitochondria
Metabolism of xenobiotics: what does oxidation result in, how does this cause a xenobiotic to be metabolised, and why may it result in hepatotoxicity?
Activates the molecule to become electrophilic or release a free radical intermediate
The reactive metabolite can be scavenged by glutathione or immediately undergo a phase II conjugation reaction
If there is insufficient antioxidants or glutathione is depleted, then the reactive intermediate can cause hepatotoxicity
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CYP450 enzymes: how many are there, which are the main ones responsible for drug metabolism, and are they variable among people?
- > 50 CYP450 enzymes have been identified
- 18 gene families identified
The three most important for metabolism - Cyp1, 2, 3
6 main isoforms are responsible for more than 90% of drug metabolism - CYP1A2; CYP2C9; CYP2C19; CYP2D6; CYP2E1; CYP3A4
Genetic polymorphisms exist - poor, rapid and ultrarapid metabolisers
Phase II metabolism: what is it, what are the most common types of reaction, and what type of metabolites can be formed?
Convert a parent drug to more polar (water soluble) inactive metabolites by conjugation of subgroups to -OH, -SH, -NH2 functional groups on drug
- Glucuronidation is the most common (important) type of Phase II metabolism
- Glutathione conjugation is the next most important
Acylglucuronides - hepatotoxic metabolites
Hepatotoxicity: what is it, and what are the main hepatotoxins?
The capacity of a substance to have damaging effects on the liver
Hepatotoxins/toxicants:
* Metals + Metalloids - arsenic; cadmium; copper; iron; lead; manganese; gold; thallium
* Plant toxins - pyrrolizidines
* Mycotoxins - aflotoxins; ochratoxins
* Bacterial Toxins - mycrocrystins (cyanobacteria); E. coli; Clostridium botulinus
* Toxicants - medicines; haloalkanes; nitroaromatic compounds
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Symptoms of liver damage: less serious to most serious consequences
- Nausea, vomiting, fatigue
- Disrupted metabolism of nutrients; impaired metabolism and elimination
- Heptomegaly; Jaundice
- Hepatic encephalopathy; portal hypertension; oedema, ascites, hypoproteinemia, clotting problems
Detection of Hepatotoxcity
- Blood Tests
- Liver function tests (LFTs)
Types of Liver Damage: what are the specific histopathologies and what do they mean?
- Steatosis - lipid droplets in hepatocytes
- Cell death - apoptosis and necrosis
- Cholestasis - reduced bile acid secretion
- Bile duct damage - damage to epithelium
- Fibrosis and cirrhosis - inflammation
- Tumours
Risk factors: what are the main risk factors and how can they increase the likelihood of hepatotoxicity?
- Gender - females, higher risk of hepatotoxicity
- Age - 60+ typically has a higher risk of hepatotoxicity (mostly regarding cholestasis)
- Nutrition - poor nutrition has a higher risk of hepatotoxicity
- Disease - higher risk of hepatotoxicity
- Infections - higher risk of hepatotoxicity
- Medications - some medications may result in higher risk of hepatotoxicity
- Genetic - Some polymorphisms in drug metabolism enzymes may reduce the risk of hepatotoxicity
DILI: what is it, what are the types, and what are their key features?
Drug-induced liver injury reactions
Intrinsic DILI:
* Affects all individuals at the same dose
* Reproducible
* Occurs dose-dependently
* Predictable
Idiosyncratic DILI:
* Happens in only a small number of people
* Not reproducible
* Variable time of onset
* No relationship to dose
* Not predictable
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DILI: what are mechanism of intrinsic and idiosyncratic DILI?
- Drug administered (lipophilic drugs tend to have the highest risk of hepatoxicity)
- Reactive metabolite formed - covalent binding to phase 2 detoxification enzymes, stress kinase activation, mitochondria stress (ROS release), and ER stress
Intrinsic DILI - drug is present at a lethal dose to hepatocytes:
* Cells start dying through apoptosis/necrosis
Idiosyncratic DILI - genetic susceptibility (occurs regardless of dose):
* Adaptive immune response
DILI: why does it result in apoptosis?
Generation of ROS/damage to mitochondria/ER causes cells to swiftly undergo apoptosis
Drugs or produced metabolites may result in inhibited bile secretion - bile build-up within cells may promote apoptosis
DILI: what if the drug is present at sub-lethal amounts?
Sub-lethal amounts - adaptive responses to defend against the drugs
Paracetamol: what is it, how frequent are hepatotoxic injuries with it, what DILI is it, and why?
Routinely used as an analgesic since 1950s
Paracetamol overdose :
* Is the most common drug overdose
* Is the greatest cause of acute liver failure in the UK
* Can occur by dose or length of continuous use
Intrinsic drug-induced liver injury (DILI) as it is dose-dependent
Paracetamol: what enzymes are involved in its normal metabolism, how much of paracetamol metabolism are they responsible for, and what is the molecular mechanism behind them?
Phenosulfotransferase (40%)
Paracetamol is converted to paracetamol sulphate (safely removed from the body)
UDP-glucuronosyl-transferase (50%):
Paracetamol is converted to paracetamol glucuronide (safely removed from the body)
CYP450 (5-10%):
Paracetamol undergoes an oxidation reaction to form NAPQI (a potentially toxic metabolite) which can then undergo a phase II reaction by GSH to form paracetamol glutathione conjugate
Paracetamol metabolism: do enzymes metabolise all of it before it is removed from the body?
No, <5% of paracetamol is excreted unchanged in the urine
Paracetamol-induced hepatotoxicity: what is the mechanism behind it?
If the phase II enzymes (phenosulfotransferase and UDP-glucuronosyl-transferase) are saturated (can even happen at low drug concentrations if the enzymes have been saturated by other causes) and unable to metabolise paracetamol, CYP450 will metabolise it and form higher levels of NAPI, which can start binding to other cellular proteins and other macromolecules and cause hepatotoxicity
NAPQI: what is the mechanism behind its damage to hepatocytes?
- NAPQI can react with proteins’ sulphhydryl groups in the mitochondria (ie glutathione peroxidase - the alpha subunit of ATP synthase, HMG CoA synthase, glycine amidinotransferase, etc.)
- NAPQI binding to proteins results in increased superoxide (O₂⁻) production, which reacts with nitric oxide (NO) and produces peroxynitrite radical (ONOO⁻), which reacts with protein tyrosine residues
- The production of these products induces mitochondrial permeability transition and may result in MOMP, which will result in apoptosis
CCl₄: what is it, where is it found, and how may it be toxic?
Carbon tetrachloride - classic hepatotoxicant that is a colourless, volatile, and sweet-smelling liquid – industrial/environmental origin
Toxicity can still occur near industrial sites or sites of previous use - toxic by inhalation, ingestion and lesser degree absorption through skin
