Flashcards in Hepatotoxicity Deck (43):
What is the liver?
The largest organ in the body which receives dual blood supply – nutrient rich but poorly Oxygenated blood via the hepatic portal vein and well oxygenated blood through the hepatic arteriole
How is the liver divided functionally?
It is divided into functional lobules centred around a central vein and outlined by portal triads
How is the liver lobule divided up into zones?
There is zone 1 which is periportal, zone 3 which is centrilobular -around the central vein- as well as zone 2 which covers the region between zone 1 and 2
These zones reflect both an oxygen gradient (high in zone 1) and a metabolic gradient- formed by differential enzyme expression-
What are the functional cells of the liver?
The hepatocytes which lie in plates which are 1 cell thick separated by vascular spaces- sinusoids- which combined with the high permeability of the blood vessels- due to endothelial fenestrations- allows the hepatocytes to be bathed by nutrients and O2 in the space of disse (in between the hepatocytes and endothelial cells)
The hepatocytes form the bile duct through making tight junctions with one another
What physiological functions does the liver perform?
Carbohydrate metabolism (postprandial glucose consumption and glycogen storage), Lipid metabolism (triglyceride synthesis and uptake, LDL/HDL cholesterol synthesis), Protein metabolism (synthesis and deanimation of amino acids and conversion of ammonia to urea) Nutrient uptake and storage, synthesis and release of hormones, binding proteins
Protective clearance functions through detoxification of xenobiotics
What is unique about the regenerative functions of the liver?
The liver can fully restore itself after significant tissue loss, as even if 70% of the liver is re3moved it will be fully restored in 5-7 days and theoretically a single hepatocyte could produce 50 livers
How does the liver biotransform xenobiotics?
Non-polar xenobiotics are treated as toxins as they will be unable to be excreted in the urine and could accumulate to toxic levels
To account for this the liver converts them to polar products to increase their excretion in the bile and urine
This can unfortunately result in the production of toxic products like free radicals in the cell
What are examples of xenobiotics which are toxic to the liver?
Industrial chemicals such as halogenated aromatic compounds, nitroaromatic compounds, aliphatic hydrocarbons and inorganic compounds such as copper and iron
What results from damage to the bile duct epithelium?
This can impede bile excretion leading to choleostasis
What results from damage to the function of hepatocytes?
This can result in a lack of bile transport as well as altered function leading to accumulation of fat associated with steatosis
What pattern of enzyme changes can indicate damage to the bile duct epithelium/choleostasis?
High Alkaline Phosphatase as it is expressed at high levels in the cannicular membrane of the hepatocyte, and high levels of gamma glutamyl transferase ass this is constitutively expressed in the bilaiary epithelium but not in hepatocytes
Why are alanine aminotransferase and aspartate amino transferase used for routine monitoring of hepatocyte injury?
They are easy to measure
What are the sensitive enzyme markers for hepatocyte injury?
The mitochondrial enzyme ornithine carbamoyltransferase and lactate dehydrogenase
What results from damage to the endothelial cells in the liver?
What results from damage to the sinusoidal cells?
This can result in fibrosis and altered uptake of nutrients due to altered bloodflow
What is associated with chronic hepatocellular death?
Abberant tissue repair which combined with fibrosis can lead to cirrhosis
What can result in cholestasis?
Damage to the bile ductules or hepatocellular cannicular membrane
What are the symptoms of cholestasis?
Pruitis, Jaundice and malabsorption of lipids and vitameins
Why is cholesterol an important biological molecule?
It is used in the synthesis of steroid horomones and bile acids (via the CYP7A1 enzyme)
What is the link between loss of bile acids and malabsorption of vitamins?
Major bile acids are chenodeoxycholic acid and cholic acid which are conjugated to taurine or glycine forming taurocholic or glycocholic acid which are secreted into the bile duct and stored in the gall bladder where they will be released into the intestine to emulsify fats and fat soluble vitamins to allow them to be absorbed
What is steatorrhea?
If there is malabsorption of dietry fat then there is also excessive excretion of faecal fat which leads to floating, foul smelling faeces
What is bilirubin?
A water insoluble compound contained in bile which is formed from the breakdown of haemoglobin (found in erythrocytes) by kupfer cells
Hepatocytes can conjugate this with glucorinde to form water soluble bilirubin glucuronide for excretion
How can damaged biliary excretion cause cholestatic damage to the liver?
There is an accumulation of bile which can be due to extra or intra-hepatic obstruction
Intrahepatic accumulation can lead to hepato-cellular toxic damage
This is usually first observed in zone 1 -as this is where the bile is usually excreted- as yellow-greenish accumulations of bile salts
What can lead to intra-hepatic obstruction of the bile duct?
Selective damage of the canaliculi membrane transporters, direct damage to hepatocytes or latered uptake of bile salts across the basolateral membrane
What is bland cholestasis?
There is arrested bile flow but no obvious hepatocellular damage (potentially suggesting damage to cannicular transporters)
What is hepatocanalicular cholestasis?
This is when there is apparent hepatocellular damage along with a failure to appropriately excrete bile
What are some of the suggested mechanisms through which toxins may cause cholestasis?
Physicochemical changes to the bile at high drug concentrations such as an increased viscosity of bile or precipitation of drug solids
Some toxin may damage the bile efflux pumps
What are the clinical features of choleostasis?
The defective bilirubin excretion leads to its accumulation in the skin and sclera leading to jaundice
The defective biliary excretion of cholesterol bile salt leads to accumulation in the skin and pruritis (itching)
Defective production/delivery of bile leads to fat malabsorption, vitamin deficiency and steatorrhea
What causes venocclusive damage to the liver resulting in nutmeg liver?
Damage to endothelial cells especially the efferent hepatic vein which can lead to centrilobular necrosis casuing hepatic congestion, this can be caused by oncotherapeutic agnets such as thioguanine and X-Rays
What can cause steatosis?
There may be mitochondrial damage leading to an impairment of its function of fat oxidation, causing an accumulation of fat
This mitochondrial damage may also result in decreased energy production, open of permeability pores with Ca2+ influx, cell odema and lysis
What occurs when fatty acids undergoes beta-oxidation?
This transfomrs them into acetyl-coA subunits undergo further degradation via the krebs cycle to fomr CO2 and generate NADH and FADH2 which are reoxidised by the respiratory chain to generate ATP
If this process is impaired then the fatty acids will build up leading to steatosis
What is steatosis?
An imbalance between storage of fat and its removal where lipoprotein transport is disrupted an/or fatty acids accumulate
What is the result of steatosis on the liver?
It will be slightly enlarged and have a pale yellow appearance
What reasons, other than impaired beta-oxidation which can cause fat to accumulate in hepatocytes?
Increased rate of delivery (excessive dietary input) increased synthesis or decreased export from the liver
In children with Reyes syndrome it can be a toxic effect of aspirin
How can aspirin cause steatosis?
Reyes syndrome occurs when there is a combination of viral illness and aspirin intake which can cause encephalopathy with microvascular hepatic steatosis
Aspirin is hydrolysed into salicylic acid at the outer surface of the mitochondrion salicylic acid is converted to salicylic acid-coA which can sequester acetyl coA disrupting the TCA cycle and therefore beta oxidation and cellular energy levels
What can cause necrotic damage to hepatocytes?
Damage to the mitochondria/cell membranes leading to ion flux and then cell lysis
What does the area of necrosis potentially say about the cause of necrosis?
Where the toxin enters or forms in the liver often resulting in zonal necrosis, with zonal 1 showing periportal damage and zone 3 showing centrilobular damage
There may also be nonzonal necrosis which may be diffuse, focal or massive and may include apoptotic cell damage as a rare idiosyncratic response to a drug
How can iron Fe2+ cause damage?
This is a direct acting toxin as high levels of these irons react with membrane lipid hydroperoxides to produce free radicals as well as catalysing the formation of OH free radicals from hydrogen peroxide
What region of the liver does iron cause damage?
Zone 1 hepatocellular necrosis as this is the area with the highest level of O2 and where iron will enter the liver through the hepatic portal vein
How does CCL4 cause damage?
This is an indirect toxin requiring metabolic activation by CYP2E1 to generate the trichloromethyl free radical which can then cause peroxidation of membrane lipids
Where does CCL4 cause liver damage?
Zone 3 hepatocellular necrosis as this is where most of the CYP enzymes are located which can activate this indirect toxin
It is also dependent on oxygen gradient as high O2 will inactive the enzyme which leads to bioactivation of CCL4
How can paracetamol cause liver damage?
This is and indirect acting toxin which causes zone 3 necrosis due to the high levels of CYP enzymes