Threshold Levels of Hepatocyte Nuclear Factor 6 (HNF-6) Acting in Synergy with HNF-4 and PGC-1α Are Required for Time-Specific Gene Expression during Liver Development

Abstract

During liver development, hepatocytes undergo a maturation process that leads to the fully differentiated state. This relies at least in part on the coordinated action of liver-enriched transcription factors (LETFs), but little is known about the dynamics of this coordination. In this context we investigate here the role of the LETF hepatocyte nuclear factor 6 (HNF-6; also called Onecut-1) during hepatocyte differentiation. We show that HNF-6 knockout mouse fetuses have delayed expression of glucose-6-phosphatase (g6pc), which catalyzes the final step of gluconeogenesis and is a late marker of hepatocyte maturation. Using a combination of in vivo and in vitro gain- and loss-of-function approaches, we demonstrate that HNF-6 stimulates endogenous g6pc gene expression directly via a synergistic and interdependent action with HNF-4 and that it involves coordinate recruitment of the coactivator PGC-1α. The expression of HNF-6, HNF-4, and PGC-1α rises steadily during liver development and precedes that of g6pc. We provide evidence that threshold levels of HNF-6 are required to allow synergism between HNF-6, HNF-4, and PGC-1α to induce time-specific expression of g6pc. Our observations on the regulation of g6pc by HNF-6 provide a model whereby synergism, interdependency, and threshold concentrations of LETFs and coactivators determine time-specific expression of genes during liver development.

We thank B. M. Spiegelman and S. A. Duncan for plasmids, F. Clotman for critical reading of the manuscript, I. Talianidis for advice on chromatin immunoprecipitation, and members of the Lemaigre and Weiss laboratories for discussions.

This work was supported by grants from the Belgian State Program on Interuniversity Poles of Attraction, the Actions de Recherche Concertées of the French Community of Belgium, and the Belgian Fund for Scientific Medical Research and by a Marie Curie Fellowship of the European Community program “Improving Human Research Potential and the Socio-Economic Knowledge Base” (contract HPMF-CT-2001-01405). J.-B.B. holds a fellowship from the Fonds pour la Formation à la Recherche dans l'Industrie et l'Agriculture, and N.P.-R. held a fellowship from Télévie.