Resistance of activated stellate cells to cell death in liver fibrosis: mechanisms and targets for intervention

In the normal liver, the hepatic stellate cell has a quiescent (i.e. non-proliferating) phenotype. It is the main storage site for vitamin A (retinoids) and it produces the appropriate quality and quantity of extracellular matrix. In chronic liver injury, a sustained wound healing response takes place in the liver. This sustained wound healing response is characterized by a phenotypic transformation of the stellate cell from a quiescent phenotype to an "activated" phenotype. This activated phenotype is characterized by loss of retinoids, increased proliferation, increased production of extracellular matrix (ECM), in particular fibrillar collagens, and increased responsiveness to growth factors, cytokines and chemokines. The replacement of normal low density ECM by high density fibrillar collagen-rich ECM contributes to the capillarization of the sinusoids and the replacement of functional liver cells (hepatocytes) by scar tissue. Together, these changes may lead to portal hypertension and loss of liver function. Cirrhosis, the advanced stage of liver fibrosis, is frequently accompanied by the development of liver tumors and hence the prognosis for patients with liver cirrhosis is poor (1-3). It has proven to be extremely difficult to effectively treat liver fibrosis. Other than removing the causative agent (alcohol, hepatitis B or C virus), there are no treatment options. For many patients, liver transplantation is the only effective treatment. However, this option is complicated and not without risk due to the shortage of donor livers, peri- and postoperative mortality and, in case of viral hepatitis, re-infection of the donor liver by the virus. Although the stellate cell has been identified as a key player in fibrogenesis, current treatments are not directed towards stellate cells. Some drugs that affect stellate cell proliferation or viability have been used in experimental models of liver fibrosis, e.g. pentoxifylline (4, 5) and gliotoxin (6), but human trials are (still) lacking. The activated stellate cell is an attractive target for intervention, since activation of stellate cells is a common feature of all chronic liver diseases regardless of their etiology. Therefore, any therapy aimed at the stellate cell could in principle be used for a wide variety of chronic liver diseases. These therapies should reduce the number of activated stellate cells, either by reversing the activated phenotype into the quiescent phenotype, or by inducing cell death of the activated stellate cell. A paradox emerges from the study of chronically injured livers: in these livers all cell types are present in the same "intrahepatic" environment and all cell types are exposed to the same cocktail of bile acids, toxic reactive oxygen species and apoptotic cytokines (e.g. tumor necrosis factor and Fas-ligand). Yet, whereas the hepatocytes, the functional parenchymal liver cells, perish in these conditions, the stellate cells become activated and proliferate. Also the Kupffer cells, the resident macrophages of the liver, become activated and synthesize increased amounts of cytokines. This is not only a paradox (one cell type perishes, the other thrives), it also points towards a strategy for the development of interventions that aim to reduce the number of activated stellate cells. The elucidation of the mechanisms that explain the unusual resistance to cell death of activated stellate cells and their survival in the chronically injured liver may yield strategies to interfere with these survival mechanisms. Therefore, the major goal of the research described in this thesis was to uncover the survival mechanisms of activated stellate cells