The role of elastin degradation in the pathogenesis of liver fibrosis

Long term or chronic liver disease, regardless of cause, results in scarring of the liver (termed fibrosis and cirrhosis). This scarring causes the liver to fail and is associated with significant illness and death. In the UK, the burden of cirrhosis has been identified by the Chief Medical Officer as a major challenge of the next decade. Currently there are no effective treatments for liver scarring. We have a programme of research investigating the factors which regulate recovery from scarring to inform and develop anti-scarring (so called anti-fibrotic) therapies.

We have recently demonstrated that if it is allowed to, the liver has a significant ability to break the scar tissue with a return towards more normal structure and function. In advanced liver disease though, the scar tissue persists and appears to be resistant to break-down. The presence of a protein called elastin seems to be critical to preventing effective breakdown of the scar tissue. In addition, we have preliminary evidence to suggest that a specialised form of inflammatory cell, the macrophage, produces a specific protein (enzyme) which breaks down elastin. Taken together these results suggest that elastin characterises irreversible scarring (cirrhosis) and that it accumulates as a result of a failure of the normal processes of breakdown.

Work we are undertaking will determine the cell source of elastin and the elastin degrading proteins and their relative levels at different stages of liver scarring (reversible and irreversible).
In addition, we will use experimental models to determine the mechanisms that make scarring resistant to breakdown and use infusions of purified inflammatory cells and specific population of stem cells to enhance scar breakdown.

Ultimately, by understanding the mechanisms regulating the turnover of elastin in liver scarring we will be better able to design effective anti-scarring therapies applicable in the clinic.

Currently there are no effective treatments for liver fibrosis and cirrhosis which represent major health challenges worldwide. Therefore, there is a pressing need to develop antifibrotic therapies. We have recently demonstrated that experimental liver fibrosis is reversible and that reversal is characterised by matrix degradation and apoptosis of activated (myofibroblast-like) hepatic stellate cells (the major cell mediator of fibrosis). However, following prolonged experimental injury, fibrosis develops which is not degraded even after one year of recovery. Characterisation of the septa which fail to undergo degradation indicates that they contain elastin, in contrast to those which rapidly become remodelled, which do not. These septa are also crosslinked by tissue transglutaminase (tTG), for which elastin is a substrate.
In pilot studies, we have shown elastase (MMP-12) is expressed in the liver by tissue macrophages and after conditional depletion of macrophages, hepatic elastin degradation is impaired; indicating that the macrophages are critical to spontaneous recovery from fibrosis and are an important source of MMP-12.
We propose to investigate the hypotheses that: A failure of elastin degradation characterises mature liver fibrosis. The presence of elastin facilitates matrix cross-linking which becomes resistant to MMP mediated degradation and persists even during spontaneous resolution. Initially, we propose to quantitate the relative expression, distribution and cellular source of elastin and MMP-12 in rodent models of reversible and irreversible fibrosis. These will be completed by studies of cirrhotic human liver tissue. MMP-12 knockout mice and wild type mice will be examined after induction of liver fibrosis by CCl4 injection and during spontaneous recovery to examine whether elastin persistence results from a failure of elastin degradation. We will go on to ?rescue? the phenotype of MMP-12 knockout mice by restoring a wild type macrophage genotype via either haemopoetic stem cell infusion or direct macrophage injection from wild type animals. We will use DTR mice to examine in detail the effect of selective macrophage depletion on the degradation of elastin and fibrotic tissue in livers undergoing spontaneous recovery from experimentally induced liver fibrosis. Finally we will use a novel model to determine the relative role of stellate cell/myofibroblast derived MMP-12 vs macrophage derived MMP-12 in matrix turnover by seeding the hepatic scar with MMP-12 knockout stem cell derived stellate cells or macrophages. By completion of these studies we will have defined the role of elastin and regulation of its degradation by MMP-12 in the spontaneous resolution of liver fibrosis.