ß-catenin signaling in murine liver zonation and regeneration: a Wnt-Wnt situation!

UNLABELLED: Liver-specific ß-catenin knockout (ß-Catenin-LKO) mice have revealed an essential role of ß-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 ß-catenin activity in these events remains an enigma. Here we investigate to what extent ß-catenin activation is Wnt-signaling-dependent and the potential cell source of Wnts. We studied liver-specific Lrp5/6 KO (Lrp-LKO) mice where Wnt-signaling was abolished in hepatocytes while the ß-catenin gene remained intact. Intriguingly, like ß-catenin-LKO mice, Lrp-LKO exhibited a defect in metabolic zonation observed as a lack of glutamine synthetase (GS), Cyp1a2, and Cyp2e1. Lrp-LKO also displayed a significant delay in initiation of LR due to the absence of ß-catenin-TCF4 association and lack of Cyclin-D1. To address the source of Wnt proteins in liver, we investigated conditional Wntless (Wls) KO mice, which lacked the ability to secrete Wnts from either liver epithelial cells (Wls-LKO), or macrophages including Kupffer cells (Wls-MKO), or endothelial cells (Wls-EKO). While Wls-EKO was embryonic lethal precluding further analysis in adult hepatic homeostasis and growth, Wls-LKO and Wls-MKO were viable but did not show any defect in hepatic zonation. Wls-LKO showed normal initiation of LR; however, Wls-MKO showed a significant but temporal deficit in LR that was associated with decreased ß-catenin-TCF4 association and diminished Cyclin-D1 expression. CONCLUSION: Wnt-signaling is the major upstream effector of ß-catenin activity in pericentral hepatocytes and during LR. Hepatocytes, cholangiocytes, or macrophages are not the source of Wnts in regulating hepatic zonation. However, Kupffer cells are a major contributing source of Wnt secretion necessary for ß-catenin activation during LR.

Modulation of ß-catenin signaling by glucagon receptor activation.

The glucagon receptor (GCGR) is a member of the class B G protein-coupled receptor family. Activation of GCGR by glucagon leads to increased glucose production by the liver. Thus, glucagon is a key component of glucose homeostasis by counteracting the effect of insulin. In this report, we found that in addition to activation of the classic cAMP/protein kinase A (PKA) pathway, activation of GCGR also induced ß-catenin stabilization and activated ß-catenin-mediated transcription. Activation of ß-catenin signaling was PKA-dependent, consistent with previous reports on the parathyroid hormone receptor type 1 (PTH1R) and glucagon-like peptide 1 (GLP-1R) receptors. Since low-density-lipoprotein receptor-related protein 5 (Lrp5) is an essential co-receptor required for Wnt protein mediated ß-catenin signaling, we examined the role of Lrp5 in glucagon-induced ß-catenin signaling. Cotransfection with Lrp5 enhanced the glucagon-induced ß-catenin stabilization and TCF promoter-mediated transcription. Inhibiting Lrp5/6 function using Dickkopf-1(DKK1) or by expression of the Lrp5 extracellular domain blocked glucagon-induced ß-catenin signaling. Furthermore, we showed that Lrp5 physically interacted with GCGR by immunoprecipitation and bioluminescence resonance energy transfer assays. Together, these results reveal an unexpected crosstalk between glucagon and ß-catenin signaling, and may help to explain the metabolic phenotypes of Lrp5/6 mutations.