Epithelial to mesenchymal transition in liver regeneration

Abstract

The process of liver regeneration is unique in regard to other organs. It presents the ability to recover up to 70% of the organ mass in a very short time frame of a few days, while also preserving quite efficiently the fine and unique architecture of the multicellular components (e.g. hepatocytes, sinusoiodal cells, etc) which is fundamental for its functions. Our investigations have shown, based on computer models generated using time series of confocal images from regenerating liver after CCl4 induced damage that a novel aspect which is hepatocytes movement in order to align within sinusoids is fundamental for efficient organ regeneration. One mechanism that would allow hepatocytes to move is epithelial to mesenchymal transition (EMT), a process which leads to downregulation of cell adhesion and increased motility. We have identified several EMT genes (e.g. Vimentin, Snail1, Integrin alpha2/beta6, MMP2, MMP9, MMP14) being transiently and strongly upregulated in mouse liver after a single dose of CCl4 (n=3 mice per time point). By immunofluorescence we confirmed that Snail1, but not Vimentin, is expressed in hepatocytes, indicating that not all EMT targets are expressed in hepatocytes. By stimulating primary hepatocytes in confluent culture with several cytokines and growth factors involved in regeneration we determined that TGFbeta, Insulin, IL-6 and Wnt3a can induce Snail1 expression (mRNA). In parallel, we have observed that hepatocytes proliferation in vitro in conventional monolayer or “collagen sandwich“ culture is always accompanied by features of EMT such as vimentin upregulation and acquisition of fibroblastoid morphology.Therefore, we propose that EMT is necessary for hepatocytes proliferation and in liver regeneration.