Endothelial regulation of liver fibrosis

Liver fibrosis is a scarring process of the liver in response to sustained liver injury and can progress into cirrhosis, hepatocellular carcinoma (HCC) and liver failure as end stage disease. Moreover, liver fibrosis dictates the long-term prognosis and mortality in non-alcoholic steatohepatitis (NASH) patients. Parenchymal liver cells such as hepatocytes and non-parenchymal liver cells such as hepatic stellate cells (HSC) and endothelial cells (EC) are involved in liver fibrogenesis. Liver sinusoidal endothelial cells (LSEC) are a discontinuous endothelium exhibiting fenestrations and lacking a basement membrane and form the capillary bed of the liver. Sinusoidal capillarization leads to the activation of HSC, which in turn produce extracellular matrix (ECM) proteins resulting in liver fibrosis. Therefore, the physiological differentiation of LSEC is crucial for liver homeostasis and identification of molecules crucial for the homeostasis of LSEC is indispensable. Recently, we identified transcription factor GATA4 as a master regulator of LSEC differentiation. LSEC-specific loss of GATA4 leads to sinusoidal capillarization and perisinusoidal liver fibrosis in Gata4LSEC-KO mouse. We wondered, if endothelial GATA4 plays a regulatory role in the development of liver fibrosis. Therefore, the aim of this study was to establish two different liver fibrosis models and to analyze the role of GATA4 in the development of liver fibrosis. Chronic toxic carbon tetrachloride (CCl4) model was compared with diet-induced liver fibrosis NASH models. Gata4 was rather increased in endothelial cells of mice with bridging peri-central liver fibrosis in the CCl4 model, whereas downregulation of endothelial Gata4 was seen in the choline-deficient l-amino acid-defined (CDAA) diet-induced NASH model with perisinusoidal liver fibrosis. Downregulation of Gata4 in LSEC of CDAA fed mice was accompanied with sinusoidal transdifferentiation of mLSEC and downregulation of GATA4 dependent genes. This data indicates that GATA4 in LSEC seems to play an important role in preventing development of diet-induced perisinusoidal liver fibrosis in NASH but not in toxic-induced bridging fibrosis. For maintenance of Gata4 expression during perisinusoidal liver fibrosis models on the one hand a LSEC-specific Gata4 knockin mouse (Gata4LSEC-KI) was generated and used in CDAA model. On the other hand, LSEC-specific lentivirus expressing Gata4 was generated for application of Gata4-therapy. However, the Gata4LSEC-KI mouse was not able to reduce perisinusoidal liver fibrosis in CDAA model as no functional GATA4 overexpression in LSEC was induced. The lentivirus was able to induce functional GATA4 expression in mouse brain cell line bEnd.3, but not in primary LSEC in vitro, indicating that the virus generated is not sufficient to overactivate Gata4 in primary LSEC. Taken together, these data indicate that downregulation of GATA4 is an important factor during CDAA-mediated perisinusoidal liver fibrosis