FOXA2

Abstract: The numerous functions of the liver are controlled primarily at the transcriptional level by the concerted actions of a limited number of hepatocyte-enriched transcription factors (hepatocyte nuclear factor 1alpha [HNF1alpha], -1beta, -3alpha, -3beta, -3gamma, -4alpha, and -6 and members of the c/ebp family). Of these, only HNF4alpha (nuclear receptor 2A1) and HNF1alpha appear to be correlated with the differentiated phenotype of cultured hepatoma cells. HNF1alpha-null mice are viable, indicating that this factor is not an absolute requirement for the formation of an active hepatic parenchyma. In contrast, HNF4alpha-null mice die during embryogenesis. Moreover, recent in vitro experiments using tetraploid aggregation suggest that HNF4alpha is indispensable for hepatocyte differentiation. However, the function of HNF4alpha in the maintenance of hepatocyte differentiation and function is less well understood. To address the function of HNF4alpha in the mature hepatocyte, a conditional gene knockout was produced using the Cre-loxP system. Mice lacking hepatic HNF4alpha expression accumulated lipid in the liver and exhibited greatly reduced serum cholesterol and triglyceride levels and increased serum bile acid concentrations. The observed phenotypes may be explained by (i) a selective disruption of very-low-density lipoprotein secretion due to decreased expression of genes encoding apolipoprotein B and microsomal triglyceride transfer protein, (ii) an increase in hepatic cholesterol uptake due to increased expression of the major high-density lipoprotein receptor, scavenger receptor BI, and (iii) a decrease in bile acid uptake to the liver due to down-regulation of the major basolateral bile acid transporters sodium taurocholate cotransporter protein and organic anion transporter protein 1. These data indicate that HNF4alpha is central to the maintenance of hepatocyte differentiation and is a major in vivo regulator of genes involved in the control of lipid homeostasis.

Abstract: Genetic determinants of longevity include the forkhead-related transcription factor DAF-16 in the worm Caenorhabditis elegans and the p66shc locus in mice. We demonstrate that p66shc regulates intracellular oxidant levels in mammalian cells and that hydrogen peroxide can negatively regulate forkhead activity. In p66shc-/- cells, the activity of the mammalian forkhead homolog FKHRL1 is increased and redox-dependent forkhead inactivation is reduced. In addition, expression of FKHRL1 results in an increase in both hydrogen peroxide scavenging and oxidative stress resistance. These results demonstrate an important functional relation between three distinct elements linked to aging: forkhead proteins, p66shc, and intracellular oxidants.

Abstract: The location and timing of cellular differentiation must be stringently controlled for proper organ formation. Normally, hepatocytes differentiate from hepatic progenitor cells to form the liver during development. However, previous studies have shown that the hepatic program can also be activated in non-hepatic lineage cells after exposure to particular stimuli or fusion with hepatocytes. These unexpected findings suggest that factors critical to hepatocyte differentiation exist and become activated to induce hepatocyte-specific properties in different cell types. Here, by screening the effects of twelve candidate factors, we identify three specific combinations of two transcription factors, comprising Hnf4α plus Foxa1, Foxa2 or Foxa3, that can convert mouse embryonic and adult fibroblasts into cells that closely resemble hepatocytes in vitro. The induced hepatocyte-like (iHep) cells have multiple hepatocyte-specific features and reconstitute damaged hepatic tissues after transplantation. The generation of iHep cells may provide insights into the molecular nature of hepatocyte differentiation and potential therapies for liver diseases.