WNT/BETA-CATENIN SIGNALING IN LIVER HOMEOSTASIS AND REGENERATION

Liver-specific β-catenin knockout (β-catenin-LKO) mice have revealed an essential role of β-catenin in metabolic zonation where it regulates pericentral gene expression and in initiating liver regeneration (LR) after partial hepatectomy (PH), by regulating expression of Cyclin-D1. However what regulates β-catenin activity in these events remains an enigma. Here, we investigate to what extent β-catenin activation is Wnt-signaling dependent and the potential cell source of Wnt ligands. We studied liver-specific Lrp5/6 KO (Lrp-LKO) mice where Wnt-signaling was abolished in hepatocytes while the β-catenin gene remained intact. Intriguingly, like β-catenin-LKO mice, Lrp-LKO exhibited a defect in metabolic zonation observed as lack of glutamine synthetase (GS), Cyp1a2 and Cyp2e1. Lrp-LKO also displayed a significant delay in initiation of LR due to absence of β-catenin-TCF4 association and lack of Cyclin-D1. Given the important role of Wnt signaling, we further aimed to address the cellular source of Wnt ligands in liver. Q-PCR analysis showed distinct Wnt expression pattern in individual hepatic cell types before and after PH. To elucidate the role of different liver cell populations in Wnt secretion, we investigated conditional Wntless (Wls) KO mice, which lacked ability to secrete Wnts from hepatic cell populations, including liver epithelial cells (Wls-LKO), hepatic stellate cells (Wls-SKO), macrophages including Kupffer cells (Wls-MKO), and endothelial cells (Wls-EKO). Wls-LKO, Wls-SKO and Wls-MKO did not show any defect in hepatic zonation. Wls-EKO had decreased mRNA expression of zonal genes, which suggests that endothelial cells derived Wnts may be important for maintaining zonation. After PH, Wls-LKO and Wls-SKO showed normal initiation of LR; however, Wls-MKO and Wls-EKO showed a significant deficit in LR. At later time points after PH, Wls-LKO had temporarily enhanced LR. Our experiments show that hepatocytes are capable of secreting Wnt5a during late stage of LR, which has an antagonistic role on Wnt/β-catenin pathway, while the hepatocytes from Wls-LKO cannot. Conclusion: Wnt-signaling is the major upstream effector of β-catenin activity in pericentral hepatocytes and during LR. Hepatocytes, cholangiocytes, hepatic stellate cells or macrophages are not the source of Wnts in regulating hepatic zonation, while endothelial cells may be involved in this process. On the other hand, Kupffer cells and endothelial cells are major contributing sources of Wnt secretion necessary for β-catenin activation during LR. Hepatocytes might secrete inhibitory Wnts involved in the termination of LR