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

Hormone and Metabolic Research Unit, Institute of Cellular Pathology and Université Catholique de Louvain, 75 Avenue Hippocrate, B-1200 Brussels, Belgium,¹ Département de Biologie du Développement, Institut Pasteur, Rue du Dr. Roux 25, 75724 Paris Cedex 15, France²

Received 22 December 2005/ Returned for modification 7 February 2006/ Accepted 25 May 2006

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-1. The expression of HNF-6, HNF-4, and PGC-1 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-1 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.

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