Liver-specific loss of beta-catenin results in delayed hepatocyte proliferation after partial hepatectomy.

UNLABELLED: Recent studies have suggested that beta-catenin is involved in the regulation of hepatocyte proliferation in multiple contexts, including organ development and tumorigenesis. We explored the role of beta-catenin during liver regeneration using T cell factor/lymphoid enhancer factor (TCF/LEF)-reporter mice (TOPGal mice) and liver-specific beta-catenin knockout mice. Liver-specific beta-catenin knockout mice showed a delayed onset of DNA synthesis after hepatectomy, whereas recovery of liver mass was not affected. Among putative beta-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 beta-catenin/TCF signaling activity and hepatocyte proliferation. CONCLUSION: Our results indicate that beta-catenin is critical for the proper regulation of hepatocyte proliferation during liver regeneration; however, the activity of beta-catenin/TCF signaling does not correlate with hepatocyte proliferation, suggesting that this regulation might be indirect/secondary.

Liver-specific loss of beta-catenin blocks glutamine synthesis pathway activity and cytochrome p450 expression in mice.

There is accumulating evidence that Wnt/beta-catenin signaling is involved in the regulation of liver development and physiology. The presence of genetic alterations resulting in constitutive beta-catenin stabilization in human and murine liver tumors also implicates this pathway in hepatocyte proliferation. In the present study, we generated hepatocyte-specific beta-catenin knockout mice to explore the role of beta-catenin in liver function. Conditional knockout mice were born at the expected Mendelian ratio and developed normally to adulthood, indicating beta-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 beta-catenin knockout mice. Consequently, these mice were resistant to acetaminophen challenge, confirming the requirement of these cytochrome P450 enzymes for metabolism of xenobiotic substances. In conclusion, in addition to regulating hepatocyte proliferation, beta-catenin may also control multiple aspects of normal liver function.