Giving the liver a new way to deal with paracetamol overdose
A study published today in Stem Cells Translational Medicine shows that microRNAs could be used to treat paracetamol overdose. Lead researcher Dr David Hay from the MRC Centre for Regenerative Medicine at the University of Edinburgh spoke to Sylvie Kruiniger about his findings, made possible by growing and testing their own stem cell-derived liver cells.
Why is it important to study paracetamol overdose?
Taken at recommended levels, paracetamol is usually safe, effective and is used widely in adults and children, either alone or in combination with other drugs.
However, it can damage the liver and the risk of liver damage increases with doses over the recommended levels. Each year we see around 200 deaths involving paracetamol (National Office for Statistics).
What happens in your liver when you take paracetamol?
When the liver processes a recommended dose of paracetamol, most of the drug is broken down by acid into water-soluble forms that can be passed in the urine or exported to the bile: this is called the sulfation pathway.
Around five per cent is turned into a toxin called N Acetyl-p-Benzo Quinone Imine (NAPQI). At this low level, the liver can clear the toxin with an antioxidant that reacts with NAPQI so it can be excreted in urine and bile.
So what happens if you take too much?
If we take too much paracetamol, the liver cannot process enough of the paracetamol using the sulfation pathway and so more of the NAPQI toxin is produced.
When the liver cells try to cope with the toxin by deploying the antioxidants, its store is quickly depleted. As a result, liver cells die and the liver becomes inflamed which, untreated, can lead to organ failure and death.
How might your findings change how we treat overdose in the future?
Our work focused on the major cell-type found in the liver, the hepatocyte. This cell makes up 70-80 per cent of the liver and is the main site of damage in paracetamol poisoning.
Current clinical practice for treating paracetamol overdose is to substitute the antioxidant by giving a compound which is highly efficient at neutralising the NAPQI toxin. However, this compound can cause allergic reactions and nausea in patients.
Instead of replacing or boosting the antioxidant levels, we wanted to find a way for the liver to process more of the paracetamol safely. We aimed to do this by increasing the levels of proteins involved in the sulfation pathway.
How does it work?
We found that by introducing a microRNA inhibitor, antagomir 324, we could increase the number of proteins used in the sulfation pathway and this meant that the number of cells dying decreased.
MicroRNAs are small non-coding RNAs – cellular components that fine tune ‘gene expression’ which determines how many proteins are produced. Non-coding RNAs have already been used safely in pre-clinical studies to treat atherosclerosis and virus replication in mice and primates; phase I clinical trials using them are underway to treat cancer.
We also exposed the liver cells to plasma from patients with liver failure caused by an overdose of paracetamol and found that antagomir 324 supported the cells.
Where did you get your liver cells from?
The liver ‘model’ used in our studies was generated from stem cells. First, we established that our stem cell-derived liver cells made the correct genes to process paracetamol, then we performed large-scale characterisation of the stem cell-based liver model.
The benefit of our system is that we can produce a limitless number of liver cells that can be compared between different experiments. This is something that cannot be done with cells derived from donor organs, as the banks of those cells eventually run out.
What implications might this have for animal research?
Generating liver cells from stem cells allows us to recreate human physiology ‘in a dish’. These liver models have been validated in collaboration with the pharmaceutical industry and will ultimately lead to a reduction in the use of animals in academic research and industrial drug development, though animal models are likely to remain important for pre-clinical research.
What are the next steps for your research?
One key question for developing any targeted therapy is how to deliver the microRNA to a specific cell, in our case to liver cells. As we move to the pre-clinical stage, we will explore the use of liposomes and nanomaterials which are available at clinical grade deliver antagomir 324 in a rodent model.
Our future work will also study antagomir 324’s protective nature and any side effects that may arise. MicroRNAs can target multiple parts of the cell and this might mean that it damages cells or reduces its effectiveness as a therapy. If these preclinical assessments are successful, we will be in a position to take these studies toward clinical trials.
Is there potential for developing other treatments using your liver model?
I think our stem cell-derived system has an important role to play in modern medicine. We have successfully used stem cell-derived liver cell models to study how humans process drugs, liver injury caused by drugs and virus infection and replication.
The ability to model human health and disease ‘in a dish’, from a renewable resource, has allowed us to move away from surrogate liver models, for example liver cancer cell lines, which can have chromosome abnormalities.
We believe that this is an important shift that will lead to the identification of new medicines and the repositioning of existing drugs to treat human liver disease.