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.

From teratogens to potential therapeutics: natural inhibitors of the Hedgehog signaling network come of age.

Steroidal alkaloids from Veratrum californicum (Durand) are known to exert teratogenic effects (e.g., cyclopia, holoprosencephaly) by blocking the Hedgehog (Hh) signaling pathway, which plays a considerable role in embryonic development and organogenesis. Most surprisingly, recent studies demonstrate that this complex signaling network is active even in the healthy adult organism, where it seems to control important aspects of basic metabolism and interorgan homeostasis. Abnormal activation of Hh signaling, however, can lead to the development of different tumors, psoriasis, and other diseases. This review provides an overview of how the principle teratogenic and hazardous constituent of Veratrum californicum, cyclopamine, interferes with Hh signaling and can potentially serve as a beneficial therapeutic for different tumors and psoriasis.

Hedgehog signaling is a potent regulator of liver lipid metabolism and reveals a GLI-code associated with steatosis.

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease in industrialized countries and is increasing in prevalence. The pathomechanisms, however, are poorly understood. This study assessed the unexpected role of the Hedgehog pathway in adult liver lipid metabolism. Using transgenic mice with conditional hepatocyte-specific deletion of Smoothened in adult mice, we showed that hepatocellular inhibition of Hedgehog signaling leads to steatosis by altering the abundance of the transcription factors GLI1 and GLI3. This steatotic 'Gli-code' caused the modulation of a complex network of lipogenic transcription factors and enzymes, including SREBP1 and PNPLA3, as demonstrated by microarray analysis and siRNA experiments and could be confirmed in other steatotic mouse models as well as in steatotic human livers. Conversely, activation of the Hedgehog pathway reversed the "Gli-code" and mitigated hepatic steatosis. Collectively, our results reveal that dysfunctions in the Hedgehog pathway play an important role in hepatic steatosis and beyond.

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/ß-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/ß-catenin signalling, it seems to act on a very low level forming a porto-central gradient in the lobules opposite to that of Wnt/ß-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.

Hedgehog signaling in glioblastoma multiforme.

Glioblastoma multiforme (GBM) is the most malignant brain tumor in adults with a median survival of 14.6 mo under the best available treatment. New treatment strategies are therefore urgently required, for which a profound understanding of tumor biology is necessary. Much effort has been devoted to tumor-specific aberrant signaling processes. Recently it was discovered that the transcription factor Gli1, which is activated by hedgehog signaling, is a highly predictive marker in GBM, as determined by immunohistochemistry. To determine whether GBM cells have transcriptionally active Gli1, we performed experiments with reporter genes with cells isolated from surgically removed human tumors and cell lines. We also determined whether the hedgehog signaling inhibitor cyclopamine influences reporter gene expression and cell viability, and we determined the expression of Gli1, SHH and Patched1 by quantitative real-time RT-PCR. Reporter gene analysis of nine cultures and four cell lines demonstrated a significantly enhanced transcriptional activity in six tumor cell cultures and all cell lines. Analysis of cell viability in the presence of cyclopamine revealed a response of all cell cultures with the exception of one primary culture and one cell line, but only one cell line responded to cyclopamine with reduced hedgehog signaling activity. This indicates that the toxicity of cyclopamine toward GBM cells is independent from hedgehog signaling. Since no correlation between hedgehog activity and SHH, Gli1 and Patched1 mRNA levels was observed we conclude that other mechanisms aside from transcriptional regulation of these factors are responsible for hedgehog activity in tumor cells derived from GBM.