In vivo monitoring of liver oxidative stress defences by magnetic resonance measurements of glutathione turnover

Liver disease is common, causing over 10,000 deaths in the UK per year. Liver disease often occurs as a gradual decline in liver health, with fatty liver, fibrosis and cirrhosis leading towards liver failure. Part of this decline involves ?oxidative stress?. This stress occurs when the liver is overwhelmed by damaging chemical species caused by drugs, alcohol, viral infection and the effects of having a fatty liver.
We are developing new ways of measuring the degree of oxidative stress in the liver, and of measuring how well the liver copes with this stress. We use MRI scanners to measure the liver?s production of glutathione, which is a natural antioxidant that the liver uses for protection against the chemicals that cause oxidative stress. This allows us to gauge the liver?s defence mechanisms in a non-invasive way, using an MRI scanner instead of taking a liver biopsy sample with a needle.
Our studies will characterise how liver glutathione synthesis rate changes when liver oxidative stress occurs, and demonstrates that these measurements can be made in people in a robust and safe manner. Thus we are developing new methods to monitor the role that oxidative stress plays in progression of liver disease. If successful, these methods may play a role in future liver disease research projects, and we hope to use these techniques to assess the impact of therapeutic strategies such as antioxidant supplementation treatments or diet and lifestyle changes.

Oxidative stress is central to the development and progression of liver disease. The overproduction of reactive oxygen species, and the inability of cells to defend against such insults, plays a central role in the development of fibrosis, hepatitis and cirrhosis. The main causes of oxidative stress are excess alcohol and viral infection, with iron overload, fatty liver and drugs also playing key roles.

Tissue damage occurs if cellular oxidative stress defences are overwhelmed. Glutathione (a tripeptide of cysteine, glycine and glutamate) serves as an antioxidant synthesised by cells, forming a central component of oxidative stress defence. Glutathione turnover reflects the strength of these defences, and is highly responsive to oxidative stress insults. We have developed non-invasive methods to measure glutathione turnover with magnetic resonance techniques. We propose to characterise the changes in liver glutathione synthesis and concentration in response to chronic and acute oxidative stress insults in animal models of liver disease, demonstrating the utility of glutathione turnover rate measurements as a gauge of tissue oxidative stress. We will also demonstrate that these methods can be translated to studies of human liver.

Our methods measure glutathione turnover by introducing a non-radioactive isotope of carbon (13C) into the glutathione biosynthetic pathway. Studies on rats will compare treatment groups that represent normal, steatotic, acute injury (meditated by CCl4 insult), and combined steatotic/acute injury. Liver glutathione synthesis rate and concentration will be compared between treatment groups by MR spectroscopic imaging after infusion of [2-13C]-glycine. Analogous human studies will measure the rate of glutathione biosynthesis following ingestion of 13C-labelled glycine.

The proposed studies will provide a direct method of assessing liver oxidative stress without the need for invasive biospy sampling. We anticipate that development of these techniques and characterisation of response to insults will lead to studies of the real-time effects of antioxidant treatment, or stressors such as paracetamol and alcohol, on liver oxidative stress.