There is accumulating evidence that Wnt/␤-catenin signaling is involved in the regulation of liver development and physiology. The presence of genetic alterations resulting in constitutive ␤-catenin stabilization in human and murine liver tumors also implicates this pathway in hepatocyte proliferation. In the present study, we generated hepatocyte-specific ␤-catenin knockout mice to explore the role of ␤-catenin in liver function. Conditional knockout mice were born at the expected Mendelian ratio and developed normally to adulthood, indicating ␤-catenin is dispensable for essential liver function under normal breeding conditions. However, the liver mass of knockout mice was 20% less than those of mice in the control groups. Expression analysis revealed loss of genes required for glutamine synthesis in knockout mice. Loss of the liver glutamine synthesis pathway did not affect the blood ammonia level in mice fed a standard diet, yet, knockout mice showed significantly elevated blood ammonia levels with high-protein dietary feeding. Furthermore, the expression of two cytochrome P450 enzymes, CYP1A2 and CYP2E1, was almost completely abolished in livers from hepatocyte-specific ␤-catenin knockout mice. Consequently, these mice were resistant to acetaminophen challenge, confirming the requirement of these cytochrome P450 enzymes for metabolism of xenobiotic substances. ␤ -Catenin is an adapter protein that fulfills two distinct roles in cells. As a part of an adherens junction complex, it interacts with transmembrane proteins of the cadherin family to promote cell-cell adhesion. In addition, ␤-catenin is an integral part of the canonical Wnt signaling pathway known to regulate cell proliferation, differentiation, and stem cell maintenance in a wide variety of tissues. 1-3 Wnt ligands are secreted proteins that bind to cognate frizzled receptors expressed in the cell membrane of Wnt-responsive cells. In the absence of Wnt signals, cytoplasmic ␤-catenin is phosphorylated by glycogen synthase kinase 3␤, a modification that triggers rapid proteosomal degradation of ␤-catenin. Upon stimulation with Wnt ligands, cytoplasmic ␤-catenin is stabilized and translocates to the nucleus, where it forms active transcription complexes with members of the TCF/LEF1 transcription factor family.There is accumulating evidence that indicates a role for Wnt/␤-catenin signaling in hepatocyte proliferation. Micsenyi et al. reported nuclear/cytoplasmic localization of ␤-catenin in hepatocytes during early developmental stages, which gradually decrease with progression of organ development. During liver development, the level of nuclear/cytoplasmic ␤-catenin expression correlates well with the proliferative activity of hepatocytes. 4 Suppression of ␤-catenin by antisense oligonucleotides in ex vivo liver cultures results in decreased cell proliferation and increased apoptosis of hepatocytes. 5 Conversely, transgenic expression of N-terminally truncated ␤-catenin results in increased hepatocyte proliferation and hepatomegaly, 6 a finding that ...

Recent studies have suggested that ␤-catenin is involved in the regulation of hepatocyte proliferation in multiple contexts, including organ development and tumorigenesis. We explored the role of ␤-catenin during liver regeneration using T cell factor/lymphoid enhancer factor (TCF/LEF)-reporter mice (TOPGal mice) and liver-specific ␤-catenin knockout mice. Liver-specific ␤-catenin knockout mice showed a delayed onset of DNA synthesis after hepatectomy, whereas recovery of liver mass was not affected. Among putative ␤-catenin target genes examined, the induction of Ccnd1 expression was reduced, whereas the expression of Myc and Egfr was unaffected. Furthermore, cyclin D1 protein levels were not induced, and the expression of cyclins A, E, and proliferating cell nuclear antigen was delayed. Intriguingly, the analysis of TOPGal mice showed that hepatocytes with active TCF/LEF transcription are confined to the pericentral zone and are not increased in number during regeneration, indicating an uncoupling between ␤-catenin/TCF signaling activity and hepatocyte proliferation. Conclusion: Our results indicate that ␤-catenin is critical for the proper regulation of hepatocyte proliferation during liver regeneration; however, the activity of ␤-catenin/TCF signaling does not correlate with hepatocyte proliferation, suggesting that this regulation might be indirect/secondary. (HEPATOLOGY 2007;45:361-368.)