The role of tumour necrosis factor superfamily receptors and the innate immune system in liver inflammation and epithelial to mesenchymal transition

Lead Research Organisation: University of Birmingham
Department Name: Clinical and Experimental Medicine

Abstract

As a consequence of its physiological functions the liver is exposed to a wide variety of pathogens and toxic insults. Consequently it is capable of protecting itself against infections entering from the blood by mounting a vigorous immune response. The earliest response triggered is called the 'innate immune response' and this is very old in evolutionary terms. We believe that this response will be critical in determining the outcome of liver damage. Such responses may be effective at removing the infectious organism but in the process may cause 'collateral' damage to the liver and to counteract this the liver can regenerate and thereby restore normal tissue architecture and function. If the balance between the immune response and the repair response is not carefully controlled the result is persistent, irreversible tissue damage, scarring and eventually cancerous change. The liver cell types most commonly targetted in these process, hepatocytes and cholangiocytes contribute to persistent damage by secreting factors that promote continuing activation of the immune system which leads to more damage and eventually to the development of a cancerous transformation of the liver cells. We propose that a family of proteins called tumour necrosis factor receptors found on the surface of liver cells are critical in determining whether injury results in removal of the injurious factor, regeneration, repair and full recovery or continuing damage, scarring and cancerous change. These receptors are activated by specific factors (ligands) in the local environment and the outcone of such activation is important for a wide range of cell functions including determining whether a cell dies or proliferates. If a cell continues in the proliferative state control is lost and it can become a cancerous cell. Thus how, when and where these receptors are activated will determine the balance between resolution/regeneration on one hand and persistent damage and cancerous change on the other. We have developed techniques to grow human cells out of liver tissue that is removed during liver transplantation allowing us to set up systems to study these processes using human liver cells rather than relying on animal models which do not always accurately represent what takes place in humans. We shall now use the human cells in tissue culture to investigate how the tumour necrosis factor receptors (TNFr) determine the balance between liver damage and repair and their involvement in cancerous change in the liver. We plan to carry out specific experiments to determine a) the factors that control whether TNF receptors are present in the liver b) how these receptors control the fate of liver cells c) how the 'innate' immune system can activate these processes. d) whether this response can lead to cancerous change in liver cells These studies will tell us a great deal about how the liver responds to injury and the role of the innate immune system in this process. The results will not only provide new information on how the normal liver functions and responds to damage but may also suggest new approaches to developing therapies in which the liver response to injury can be manipulated in favour of regeneration and repair without risking cancerous change.

Technical Summary

As a consequence of its physiological functions the liver is constantly exposed to pathogens and toxins. It is capable of mounting a vigorous inflammatory response followed by a regenerative response that promotes repair and restoration of normal tissue architecture and function. Failure to regulate these responses results in persistent inflammation, irreversible tissue damage, scarring and malignant change. Both hepatocytes and cholangiocytes are targets for toxic and immune-mediated damage but also contribute to persistent inflammation by expressing adhesion molecules and cytokines that promote continuing inflammation, epithelial to mesenchymal transition and malignant transformation. We propose that the epithelial response to damage is critical to the development and outcome of inflammation and that this response is dependent on activation of TNF receptor-mediated pathways that determine the balance between apoptosis and proliferation. We have developed coculture systems using primary human liver cells in which we can study the role of specific receptors and the selective activation of intracellular signalling pathways which modulate transcription factor activation giving rise to cell-specific functional outputs. We have identified the receptors CD95 and CD40 as central to this process and NFkB, AP-1, STAT3 and PI3kinase (Akt/wnt) as four signalling pathways that determine the outcome of TNFr activation on hepatocytes and cholangiocytes. We shall now use human cell-culture models to determine: 1. The factors that regulate expression of TNFr in the liver 2. How downstream intracellular signalling from TNFr modulates epithelial cell survival, function and epithelial to mesenchymal transition 3. The role of the innate immune components complement and Toll-like receptors in regulating these responses. These studies will elucidate the role of TNF receptors and the innate immune system in determining cell fate and the outcome of inflammation in the human liver.

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