Zonal gene expression in murine liver: Are tumors helping us to solve the mystery?

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Hedgehog signalling pathway in adult liver: a major new player in hepatocyte metabolism and zonation?

Metabolic Zonation, i.e. the heterogeneous distribution of different metabolic pathways in different zones of the lobules, forms the basis of proper function of the liver in metabolic homeostasis and its regulation. According to recent results, Metabolic Zonation is controlled by the Wnt/β-catenin signalling pathway. Here, we hypothesize that hedgehog signalling via Indian hedgehog ligands plays an equal share in this control although, up to now, hedgehog signalling is considered not to be active in healthy adult hepatocytes. We provide broad evidence taken mainly by analogy from other mature organs that hedgehog signalling in adult hepatocytes may particularly control liver lipid and cholesterol metabolism as well as certain aspects of hormone biosynthesis. Like Wnt/β-catenin signalling, it seems to act on a very low level forming a porto-central gradient in the lobules opposite to that of Wnt/β-catenin signalling with which it is interacting by mutual inhibition. Consequently, modulation of hedgehog signalling by endogenous and exogenous agents may considerably impact on liver lipid metabolism and beyond. If functioning improperly, it may possibly contribute to diseases like non-alcoholic fatty liver disease (NAFLD) and other diseases such as lipodystrophy.

Organ patterning in the adult stage: the role of Wnt/beta-catenin signaling in liver zonation and beyond

Wnt/beta-catenin signaling has been found to play key roles in metabolic zonation of adult liver, regeneration, and hepatocellular carcinogenesis. In this review, recent progress in this field is summarized, in particular the rapidly growing knowledge about the various interactions of beta-catenin with many transcription factors involved in controlling metabolism. These interactions may provide the basis for understanding how the wide range of activities of Wnt/beta-catenin signaling is differentially interpreted. Based on these results, a three-level mode for the molecular interpretation of beta-catenin activity gradients in liver is proposed favoring cell differentiation, metabolic zonation, and proliferation. While derangement of the combinatorial interplay of the various transcription factors with beta-catenin at the intermediary activity level may contribute to the development of metabolic diseases, extremely high activation of beta-catenin may eventually lead to initiation and progression of hepatocellular tumors.

Hepatocellular expression of glutamine synthetase: an indicator of morphogen actions as master regulators of zonation in adult liver

Glutamine synthetase (GS) has long been known to be expressed exclusively in pericentral hepatocytes most proximal to the central veins of liver lobuli. This enzyme as well as its peculiar distribution complementary to the periportal compartment for ureogenesis plays an important role in nitrogen metabolism, particularly in homeostasis of blood levels of ammonium ions and glutamine. Despite this fact and intensive studies in vivo and in vitro, many aspects of the regulation of its activity on the protein and on the genetic level remained enigmatic. Recent experimental advances using transgenic mice and new analytic tools have revealed the fundamental role of morphogens such as wingless-type MMTV integration site family member signals (Wnt), beta-catenin, and adenomatous polyposis coli in the regulation of this particular enzyme. In addition, novel information concerning the structure of transcription factor binding sites within regulatory regions of the GS gene and their interactions with signalling pathways could be collected. In this review we focus on all aspects of the regulation of GS in the liver and demonstrate how the new findings have changed our view of the determinants of liver zonation. What appeared as a simple response of hepatocytes to blood-derived factors and local cellular interactions must now be perceived as a fundamental mechanism of adult tissue patterning by morphogens that were considered mainly as regulators of developmental processes. Though GS may be the most obvious indicator of morphogen action among many other targets, elucidation of the complex regulation of the expression of the GS gene could pave the road for a better understanding of the mechanisms involved in patterning of liver parenchyma. Based on current knowledge we propose a new concept of how morphogens, hormones and other factors may act in concert, in order to restrict gene expression to small subpopulations of one differentiated cell type, the hepatocyte, in different anatomical locations. Although many details of this regulatory network are still missing, and an era of exciting new discoveries is still about to come, it can already be envisioned that similar mechanisms may well be active in other organs contributing to the fine-tuning of organ-specific functions.