Convergence of Wnt Signaling on the HNF4α-Driven Transcription in Controlling Liver Zonation

BACKGROUND & AIMS: In each heparocyte, the specific repertoire of gene expression is influenced by its exact location along the portocentrovenular axis of the hepatic lobule and provides a reason for the liver functions compartmentalization defined metabolic zonation. So far, few molecular players controlling genetic programs of periportal (PP) and perivenular (PV) hepatocytes have been identified; the elucidation of zonation mechanisms remains a challenge for experimental hepatology. Recently, a key role in induction and maintenance of the hepatocyte heterogeneity has been ascribed to Wnt/beta-catenin pathway. We sought to clarify how this wide-ranging stimulus integrates with hepatocyte specificity. METHODS: Reverse transcriptase polymerase chain reaction (RT-PCR) allowed the transcriptional profiling of hepatocytes derived from in vitro differentiation of liver stem cells. The GSK3 beta inhibitor 6-bromoindirubin-3'-oxime (BIO) was used for beta-catenin stabilization. Co-immunoprecipitations were used to study biochemical protein interactions while ChIP assays allowed the in vivo inspection of PV and PP genes regulatory regions. RESULTS: We found that spontaneous differentiation of liver stem cells gives rise to PP hepatocytes that, after Writ pathway activation, switch into PV hepatocytes. Next, we showed that the Writ downstream player LEF1 interacts with the liver-enriched transcriptional factor HNF4 alpha. Finally, we unveiled that the BIO induced activation of PV genes correlates with LEF1 binding to both its own and HNF4 alpha consensus, and the repression of PP genes correlates with HNF4 alpha displacement from its own consensus. CONCLUSION: Our data show a direct and hitherto unknown convergence of the canonical Wnt signaling on the HNF4 alpha-driven transcription providing evidences of a mechanism controlling liver zonated gene expression.