TGF-B downstream of Smad3 and MAPK signaling antagonistically regulate the viability and partial epithelial-mesenchymal transition of liver progenitor cells

Background: Liver progenitor cells (LPCs) are a subpopulation of cells that contribute to liver regeneration, fibrosis and liver cancer initiation under different circumstances. Results: By performing adenoviral-mediated transfection, CCK-8 analyses, F -actin staining, transwell analyses, luciferase reporter analyses and Western blotting, we observed that TGF-B promoted cytostasis and partial epithelial-mesenchymal transition (EMT) in LPCs. In addition, we confirmed that TGF-B activated the Smad and MAPK pathways, including the Erk, JNK and p38 MAPK signaling pathways, and revealed that TGFBSmad signaling induced growth inhibition and partial EMT, whereas TGFB-MAPK signaling had the opposite effects on LPCs. We further found that the activity of Smad and MAPK signaling downstream of TGF-B was mutually restricted in LPCs. Mechanistically, we found that TGF-B activated Smad signaling through serine phosphorylation of both the C -terminal and linker regions of Smad2 and 3 in LPCs. Additionally, TGFB-MAPK signaling inhibited the phosphorylation of Smad3 but not Smad2 at the C -terminus, and it reinforced the linker phosphorylation of Smad3 at T179 and S213. We then found that overexpression of mutated Smad3 at linker phosphorylation sites intensifies TGF-B-induced cytostasis and EMT, mimicking the effects of MAPK inhibition in LPCs, whereas mutation of Smad3 at the C -terminus caused LPCs to blunt TGF-B-induced cytostasis and partial EMT. Conclusion: These results suggested that TGF-B downstream of Smad3 and MAPK signaling were mutually antagonistic in regulating the viability and partial EMT of LPCs. This antagonism may help LPCs overcome the cytostatic effect of TGF-B under fibrotic conditions and maintain partial EMT and progenitor phenotypes.