Hepatic cell differentiation requires a HNF6 - miR-122 feedback loop

(eng) Hepatocyte differentiation and maturation during embryogenesis is controlled by a network of liver-enriched transcription factors (LETFs). Whether LETFs control microRNA (miRNA) expression during development, and whether such control is required for hepatocyte differentiation has not yet been investigated. Here we show that two LETFs, namely Hepatocyte Nuclear Factor (HNF) 6 and Onecut2 (OC2), redundantly stimulate the expression of several miRNAs during development, and that the hepatocyte-specific miR-122 is the most critically dependent on HNF6 and OC2. Using in vivo and in vitro gain- and loss-of-function experiments, we further demonstrate that accurate levels of miR-122 are required to ensure proper progression of hepatocyte differentiation and maturation from hepatoblasts: miR-122 stimulates the expression of hepatocyte-specific genes and of most LETFs, including HNF6. This reveals the existence of a HNF6 – miR-122 positive feedback loop. Moreover, stimulation of hepatocyte maturation by miR-122 is abolished in an HNF6-null background, revealing that a transcription factor can mediate microRNA function. Confirming direct regulation we provide evidence that all hepatocyte-specific genes whose expression is stimulated by miR-122 are occupied in vivo by HNF6. MiR-122 also promotes hepatocyte maturation by directly repressing specific mRNAs. Mir-122 participates to liver-specific disallowance of the enzyme 2-oxoacid CoA transferase (OXCT1) during hepatocyte maturation. OXCT1 is involved in ketone body catabolism and needs to be repressed in liver, where ketone bodies are produced, in order to avoid hepatic catabolism of these metabolites. We show that OXCT1 mRNA is a direct target of miR-122 and that the rise in miR-122 level during hepatocyte maturation induces repression of OXCT1 protein production during liver development and in adulthood. Together, our findings indicate that the expression level of the liver specific miR-122 is controlled by a HNF6 - miR-122 positive feedback loop and that miR-122 controls hepatocyte differentiation and maturation by indirect stimulation of hepatocyte-specific genes and by direct repression of a ubiquitous gene.