Liver repair mechanisms in non alcoholic steatohepatitis (NASH): defining the role of hepatic progenitor cells, ductular reaction and Notch signaling

BACKGROUND: In case of chronic or severe liver injuries, functional hepatic repair relies upon the contribution of hepatic progenitor cells (HPC). HPC expansion leads to ductular reaction (DR), a fundamental phenomenon in liver repair that coordinates the cross-talk among epithelial cells with inflammatory/mesenchymal cells. Chronic insults turn this process into pathologic DR-driven repair, eventually responsible for the fibrotic evolution of the original injury. HPC activation during liver injury is mediated by morphogens, including Notch signaling. Notch activation needs to be finely regulated to avoid “pathologic” liver repair or abnormal proliferation/differentiation that may promote hepatic cancer, depending on the type of hepatic damage. We previously demonstrated that after cholestatic challenges Notch is required for biliary specification of HPC and the remodeling of ductular reactive structures into branching biliary tubules. We have thus hypothesized that activation of Notch signaling may also be involved in liver repair mechanisms activated in case of hepatocellular damage, as in non-alcoholic steatohepatitis (NASH). Brisk HPC/DR activation occurs in NASH, which correlates with the extent of liver fibrosis and the risk of progression to cirrhosis. HPC localize in intimate contact with fat laden hepatocytes, suggestive of cell-cell interactions mediated by the Notch pathway. We aimed to characterize the HPC response in this clinical setting focusing on the involvement of Notch in HPC activation. RESULTS: NASH was reproduced in mice with methionine-choline deficient (MCD) diet for 4 up to 8 weeks. HPC/DR were identified by cytokeratin19 (CK19) expression. The number of CK19+ve cells increased progressively from the 4th to the 8th week of treatment, when HPC also appeared in closer contact with fat laden hepatocytes. We studied the contribution of the different hepatic cell types to Notch expression on laser capture microdissected CK19+ve (mainly HPC and cholangiocytes) and CK19-ve (hepatocytes) cells. Notch was not modulated in CK19+ve cells, while in the CK19-ve population, a decrease in the expression of Numb (an endogenous inhibitor of Notch) occurred after MCD feeding, consistent with a possible dysregulation of Notch activation. Accordingly, in MCD diet treated mice we reported increasing expression of Sox9 (a Notch-dependent biliary marker) in cells with morphologic features of hepatocytes, consistent with aberrant Notch activation in a subpopulation of hepatocytes. Similarly, in vitro stimulation with the Jag1 ligand promoted the differentiation of a hepatocytic cell line toward a more progenitor-like phenotype. We further demonstrated hepatocytes dedifferentiation by performing hepatocyte lineage tracing experiments: R26R-YFP reporter mice were infected with AAV8-TBG-Cre to obtain efficient and specific Cre expression only in hepatocytes. After 8 weeks of MCD diet, we identified a subpopulation of YFP+ve hepatocytes that was also Sox9+ve and CK19+ve. Interestingly, in the MCD model, Jag1 expression correlated with the expression of the profibrotic mediator procollagen. Therefore, we studied Notch signaling during hepatic stellate cells (HSC) activation, and we reported that TGF-β1 treatment strongly upregulated Jag1 expression in HSC during HSC transactivation process. Moreover, αSMA positive activated-HSC localized around Sox9+ve cells in MCD-diet fed mice. CONCLUSIONS: Our results suggest that downregulation of Notch occurs in hepatocytes as a physiological response to hepatic steatosis. However, aberrant Notch activation in a subset of hepatocytes is responsible of their dedifferentiation into HPC-like cells. Notch-dependent hepatocytes reprogramming is promoted by TGF-β1-induced Jag1 on HSC. These data suggest that Jag1 could play a crucial role in NASH-related liver repair mechanisms, by driving cirrhotic evolution and potentially leading to malignant progression.