Hedgehog signalling pathway in adult liver: a major new player in hepatocyte metabolism and zonation? Med Hypotheses. 2013;80:589-594. [PMID: 23433827 DOI: 10.1016/j.mehy.2013.01.032]

Spatial metabolomics and its application in the liver

Hepatocytes work in highly structured, repetitive hepatic lobules. Blood flow across the radial axis of the lobule generates oxygen, nutrient, and hormone gradients, which result in zoned spatial variability and functional diversity. This large heterogeneity suggests that hepatocytes in different lobule zones may have distinct gene expression profiles, metabolic features, regenerative capacity, and susceptibility to damage. Here, we describe the principles of liver zonation, introduce metabolomic approaches to study the spatial heterogeneity of the liver, and highlight the possibility of exploring the spatial metabolic profile, leading to a deeper understanding of the tissue metabolic organization. Spatial metabolomics can also reveal intercellular heterogeneity and its contribution to liver disease. These approaches facilitate the global characterization of liver metabolic function with high spatial resolution along physiological and pathological time scales. This review summarizes the state of the art for spatially resolved metabolomic analysis and the challenges that hinder the achievement of metabolome coverage at the single-cell level. We also discuss several major contributions to the understanding of liver spatial metabolism and conclude with our opinion on the future developments and applications of these exciting new technologies.

Spatial hepatocyte plasticity of gluconeogenesis during the metabolic transitions between fed, fasted and starvation states

The liver acts as a master regulator of metabolic homeostasis in part by performing gluconeogenesis. This process is dysregulated in type 2 diabetes, leading to elevated hepatic glucose output. The parenchymal cells of the liver (hepatocytes) are heterogeneous, existing on an axis between the portal triad and the central vein, and perform distinct functions depending on location in the lobule. Here, using single cell analysis of hepatocytes across the liver lobule, we demonstrate that gluconeogenic gene expression ( Pck1 and G6pc ) is relatively low in the fed state and gradually increases first in the periportal hepatocytes during the initial fasting period. As the time of fasting progresses, pericentral hepatocyte gluconeogenic gene expression increases, and following entry into the starvation state, the pericentral hepatocytes show similar gluconeogenic gene expression to the periportal hepatocytes. Similarly, pyruvate-dependent gluconeogenic activity is approximately 10-fold higher in the periportal hepatocytes during the initial fasting state but only 1.5-fold higher in the starvation state. In parallel, starvation suppresses canonical beta-catenin signaling and modulates expression of pericentral and periportal glutamine synthetase and glutaminase, resulting in an enhanced pericentral glutamine-dependent gluconeogenesis. These findings demonstrate that hepatocyte gluconeogenic gene expression and gluconeogenic activity are highly spatially and temporally plastic across the liver lobule, underscoring the critical importance of using well-defined feeding and fasting conditions to define the basis of hepatic insulin resistance and glucose production.

Intracellular spatiotemporal metabolism in connection to target engagement

The metabolism in eukaryotic cells is a highly ordered system involving various cellular compartments, which fluctuates based on physiological rhythms. Organelles, as the smallest independent sub-cell unit, are important contributors to cell metabolism and drug metabolism, collectively designated intracellular metabolism. However, disruption of intracellular spatiotemporal metabolism can lead to disease development and progression, as well as drug treatment interference. In this review, we systematically discuss spatiotemporal metabolism in cells and cell subpopulations. In particular, we focused on metabolism compartmentalization and physiological rhythms, including the variation and regulation of metabolic enzymes, metabolic pathways, and metabolites. Additionally, the intricate relationship among intracellular spatiotemporal metabolism, metabolism-related diseases, and drug therapy/toxicity has been discussed. Finally, approaches and strategies for intracellular spatiotemporal metabolism analysis and potential target identification are introduced, along with examples of potential new drug design based on this.

Opportunities and considerations for studying liver disease with microphysiological systems on a chip

Advances in microsystem engineering have enabled the development of highly controlled models of the liver that better recapitulate the unique in vivo biological conditions. In just a few short years, substantial progress has been made in creating complex mono- and multi-cellular models that mimic key metabolic, structural, and oxygen gradients crucial for liver function. Here we review: 1) the state-of-the-art in liver-centric microphysiological systems and 2) the array of liver diseases and pressing biological and therapeutic challenges which could be investigated with these systems. The engineering community has unique opportunities to innovate with new liver-on-a-chip devices and partner with biomedical researchers to usher in a new era of understanding of the molecular and cellular contributors to liver diseases and identify and test rational therapeutic modalities.

HepaRG cells undergo increased levels of post-differentiation patterning in physiologic conditions when maintained as 3D cultures in paper-based scaffolds

Monolayer cultures of hepatocytes lack many aspects of the liver sinusoid, including a tissue-level organization that results from extracellular matrix interactions and gradients of soluble molecules that span from the portal triad to the central vein. We measured the activity and transcript levels of drug-metabolizing enzymes in HepaRG cells maintained in three different culture configurations: as monolayers, seeded onto paper scaffolds that were pre-loaded with a collagen matrix, and when seeded directly into the paper scaffolds as a cell-laden gel. Drug metabolism was significantly decreased in the presence of the paper scaffolds compared to monolayer configurations when cells were exposed to standard culture conditions. Despite this decreased function, transcript levels suggest the cells undergo increased polarization and adopt a biliary-like character in the paper scaffolds, including the increased expression of transporter proteins (e.g., ABCB11 and SLOC1B1) and the KRT19 cholangiocyte marker. When exposed to representative periportal or perivenous culture conditions, we observed in vivo zonal-like patterns, including increased cytochrome P450 (CYP) activity and transcript levels in the perivenous condition. This increased CYP activity is more pronounced in the laden configuration, supporting the need to include multiple aspects of the liver microenvironment to observe the post-differentiation processing of hepatocytes.

HepaRG cells undergo increased levels of post-differentiation patterning in physiologic conditions when maintained as 3D cultures in paper-based scaffolds

Monolayer cultures of hepatocytes lack many aspects of the liver sinusoid, including a tissue-level organization that results from extracellular matrix interactions and gradients of soluble molecules that span from the portal triad to the central vein. We measured the activity and transcript levels of drug-metabolizing enzymes in HepaRG cells maintained in three different culture configurations: as monolayers, seeded onto paper scaffolds that were pre-loaded with a collagen matrix, and when seeded directly into the paper scaffolds as a cell-laden gel. Drug metabolism was significantly decreased in the presence of the paper scaffolds compared to monolayer configurations when cells were exposed to standard culture conditions. Despite this decreased function, transcript levels suggest the cells undergo increased polarization and adopt a biliary-like character in the paper scaffolds, including the increased expression of transporter proteins (e.g., ABCB11 and SLOC1B1) and the KRT19 cholangiocyte marker. When exposed to representative periportal or perivenous culture conditions, we observed in vivo zonal-like patterns, including increased cytochrome P450 (CYP) activity and transcript levels in the perivenous condition. This increased CYP activity is more pronounced in the laden configuration, supporting the need to include multiple aspects of the liver microenvironment to observe the post-differentiation processing of hepatocytes.

Repurposed itraconazole for use in the treatment of malignancies as a promising therapeutic strategy

Understanding cancer biology and the development of novel agents for cancer treatment has always been the goal of cancer researchers. However, the research and development of new drugs is hindered by its long development time, exorbitant cost, high regulatory hurdles, and staggering failure rates. Given the challenges involved drug development for cancer therapies, alternative strategies, in particular the repurposing of 'old' drugs that have been approved for other indications, are attractive. Itraconazole is an FDA-approved anti-fungal drug of the triazole class, and has been used clinically for more than 30 years. Recent drug repurposing screens revealed itraconazole exerts anti-cancer activity via inhibiting angiogenesis and multiple oncogenic signaling pathways. To explore the potential utilization of itraconazole in different types of malignancies, we retrieved the published literature relating to itraconazole in cancer and reviewed the mechanisms of itraconazole in preclinical and clinical cancer studies. Current research predicts the hedgehog signaling pathway as the main target by which itraconazole inhibits a variety of solid and hematological cancers. As clinical trial results become available, itraconazole could emerge as a new antitumor drug that can be used in combination with first-line antitumor drugs.

Effects of Ion-Transporting Proteins on the Digestive System Under Hypoxia

Hypoxia refers to a state of oxygen limitation, which mainly mediates pathological processes in the human body and participates in the regulation of normal physiological processes. In the hypoxic environment, the main regulator of human body homeostasis is the hypoxia-inducible factor family (HIF). HIF can regulate the expression of many hypoxia-induced genes and then participate in various physiological and pathological processes of the human body. Ion-transporting proteins are extremely important types of proteins. Ion-transporting proteins are distributed on cell membranes or organelles and strictly control the inflow or outflow of ions in cells or organelles. Changes in ions in cells are often closely related to extensive physiological and pathological processes in the human body. Numerous studies have confirmed that hypoxia and its regulatory factors can regulate the transcription and expression of ion-transporting protein-related genes. Under hypoxic stress, the regulation and interaction of ion-transporting proteins by hypoxia often leads to diseases of various human systems and even tumors. Using ion-transporting proteins and hypoxia as targets to explore the mechanism of digestive system diseases and targeted therapy is expected to become a new breakthrough point.

The Role of Liver Zonation in Physiology, Regeneration, and Disease

As blood flows from the portal triad to the central vein, cell-mediated depletion establishes gradients of soluble factors such as oxygen, nutrients, and hormones, which act through molecular pathways (e.g., Wnt/β-catenin, hedgehog) to spatially regulate hepatocyte functions along the sinusoid. Such "zonation" can lead to the compartmentalized initiation of several liver diseases, including alcoholic/non-alcoholic fatty liver diseases, chemical/drug-induced toxicity, and hepatocellular carcinoma, and can also modulate liver regeneration. Transgenic rodent models provide valuable information on the key molecular regulators of zonation, while in vitro models allow for subjecting cells to precisely controlled factor gradients and elucidating species-specific differences in zonation. Here, we discuss the latest advances in both in vivo and in vitro models of liver zonation and pending questions to be addressed moving forward. Ultimately, obtaining a deeper understanding of zonation can lead to the development of more effective therapeutics for liver diseases, microphysiological systems, and scalable cell-based therapies.

High-fat diet intake ameliorates the expression of hedgehog signaling pathway in adult rat liver

BACKGROUND

Disproportionate fatty diet intake provokes hepatic lipid accumulation that causes non-alcoholic fatty liver disease, triggering the embryonically conserved Hedgehog (Hh) pathway in the adult liver. The present study incorporates exploring the impact of chronically administered unsaturated (D-1) and saturated (D-2) fat-enriched diets on hematological parameters, liver functioning, and lipid profile in the rat model. Besides, hepatohistology and real time gene expression analysis of Hh signaling pathway genes i.e., Shh, Ihh, Hhip, Ptch1, Smo, Gli1, Gli2, and Gli3 were carried out.

METHODS AND RESULTS

Fifteen Rattus norvegicus (♂) of 200 ± 25 g weight were grouped into control, D-1, and D-2. Animals were fed on their respective diets for 16 weeks. Fatty diet intake resulted in neutropenia, lymphocytosis, monocytosis, polycythemia, and macrocytosis in both experimental groups. Altered liver injury biomarkers, hypertriglyceridemia, and significantly increased very-low-density lipoprotein VLDL were also noted in both high-fat diet (HFD) groups as compared to control. Hepatohistological examination showed disrupted microarchitecture, infiltration of inflammatory cells, cellular necrosis, widened sinusoidal spaces, and microvesicular steatotic hepatocytes in D-1 and D-2. Collagen deposition in both HFD groups marks the extent of fibrosis. Significant upregulation of hedgehog pathway genes was found in fatty diet groups. In comparison with the control group, Shh Ihh, Hhip, Ptch1, Smo, Gli1, Gli2, and Gli3 were upregulated in D-1. In D-2 Shh, Hhip, and Smo expressions were upregulated, Ihh exhibited downregulation as compared to control.

CONCLUSION

Excess fat deposits in liver due to chronic consumption of high-fat diet results in anomalous architecture and functioning. High-fat diet induced significant variations in Hh pathway genes expression; especially Shh, Ihh, Hhip, Ptch1, Smo, Gli1, Gli2, and Gli3 were upregulated. Infiltration of inflammatory cells ( ), widened sinusoidal spaces (▲), cellular necrosis, and micro vesicular steatotic hepatocytes (*) were shown in the liver. Significant collagen deposition in both HFD groups i.e. D-1 and D-2 confirmed liver fibrosis. Excessive intake of dietary fats impaired normal liver functioning and liver inflammation triggered Hh signaling in adult rats.

Hepatocyte activity of the cholesterol sensor smoothened regulates cholesterol and bile acid homeostasis in mice

Cellular cholesterol is regulated by at least two transcriptional mechanisms involving sterol-regulatory-element-binding proteins (SREBPs) and liver X receptors (LXRs). Although SREBP and LXR pathways are the predominant mechanisms that sense cholesterol in the endoplasmic reticulum and nucleus to alter sterol-regulated gene expression, evidence suggests cholesterol in plasma membrane can be sensed by proteins in the Hedgehog (Hh) pathway which regulate organ self-renewal and are a morphogenic driver during embryonic development. Cholesterol interacts with the G-protein-coupled receptor Smoothened (Smo), which impacts downstream Hh signaling. Although evidence suggests cholesterol influences Hh signaling, it is not known whether Smo-dependent sterol sensing impacts cholesterol homeostasis in vivo. We examined dietary-cholesterol-induced reorganization of whole-body sterol and bile acid (BA) homeostasis in adult mice with inducible hepatocyte-specific Smo deletion. These studies demonstrate Smo in hepatocytes plays a regulatory role in sensing and feedback regulation of cholesterol balance driven by excess dietary cholesterol.

TMEM9-v-ATPase Activates Wnt/β-Catenin Signaling Via APC Lysosomal Degradation for Liver Regeneration and Tumorigenesis

BACKGROUND AND AIMS

How Wnt signaling is orchestrated in liver regeneration and tumorigenesis remains elusive. Recently, we identified transmembrane protein 9 (TMEM9) as a Wnt signaling amplifier.

APPROACH AND RESULTS

TMEM9 facilitates v-ATPase assembly for vesicular acidification and lysosomal protein degradation. TMEM9 is highly expressed in regenerating liver and hepatocellular carcinoma (HCC) cells. TMEM9 expression is enriched in the hepatocytes around the central vein and acutely induced by injury. In mice, Tmem9 knockout impairs hepatic regeneration with aberrantly increased adenomatosis polyposis coli (Apc) and reduced Wnt signaling. Mechanistically, TMEM9 down-regulates APC through lysosomal protein degradation through v-ATPase. In HCC, TMEM9 is overexpressed and necessary to maintain β-catenin hyperactivation. TMEM9-up-regulated APC binds to and inhibits nuclear translocation of β-catenin, independent of HCC-associated β-catenin mutations. Pharmacological blockade of TMEM9-v-ATPase or lysosomal degradation suppresses Wnt/β-catenin through APC stabilization and β-catenin cytosolic retention.

CONCLUSIONS

Our results reveal that TMEM9 hyperactivates Wnt signaling for liver regeneration and tumorigenesis through lysosomal degradation of APC.

Adult Hepatocytes Are Hedgehog-Responsive Cells in the Setting of Liver Injury: Evidence for Smoothened-Mediated Activation of NF-κB/Epidermal Growth Factor Receptor/Akt in Hepatocytes that Counteract Fas-Induced Apoptosis

Although hedgehog (Hh) signaling pathway is inactive in adult healthy liver, it becomes activated during acute and chronic liver injury and, thus, modulates the reparative process and disease progression. We developed a novel mouse model with liver-specific knockout of Smoothened (Smo LKO), and animals were subjected to Fas-induced liver injury in vivo. Results showed that Smo deletion in hepatocytes enhances Fas-induced liver injury. Activation of Hh signaling in hepatocytes in the setting of Fas-induced injury was indicated by the fact that Jo2 treatment enhanced hepatic expression of Ptch1, Smo, and its downstream target Gli1 in control but not Smo LKO mice. Primary hepatocytes from control mice showed increased Hh signaling activation in response to Jo2 treatment in vitro. On the other hand, the Smo KO hepatocytes were devoid of Hh activation and were more susceptible to Jo2-induced apoptosis. The levels of NF-κB and related signaling molecules, including epidermal growth factor receptor and Akt, were lower in Smo KO livers/hepatocytes than in control livers/hepatocytes. Accordingly, hydrodynamic gene delivery of active NK-κB prevented Jo2-induced liver injury in the Smo LKO mice. Our findings provide important evidence that adult hepatocytes become responsive to Hh signaling through up-regulation of Smo in the setting of Fas-induced liver injury and that such alteration leads to activation of NF-κB/epidermal growth factor receptor/Akt, which counteracts Fas-induced hepatocyte apoptosis.

Tick-tock hedgehog-mutual crosstalk with liver circadian clock promotes liver steatosis

BACKGROUND & AIMS

The mammalian circadian clock controls various aspects of liver metabolism and integrates nutritional signals. Recently, we described Hedgehog (Hh) signaling as a novel regulator of liver lipid metabolism. Herein, we investigated crosstalk between hepatic Hh signaling and circadian rhythm.

METHODS

Diurnal rhythms of Hh signaling were investigated in liver and hepatocytes from mice with ablation of Smoothened (SAC-KO) and crossbreeds with PER2::LUC reporter mice. By using genome-wide screening, qPCR, immunostaining, ELISA and RNAi experiments in vitro we identified relevant transcriptional regulatory steps. Shotgun lipidomics and metabolic cages were used for analysis of metabolic alterations and behavior.

RESULTS

Hh signaling showed diurnal oscillations in liver and hepatocytes in vitro. Correspondingly, the level of Indian Hh, oscillated in serum. Depletion of the clock gene Bmal1 in hepatocytes resulted in significant alterations in the expression of Hh genes. Conversely, SAC-KO mice showed altered expression of clock genes, confirmed by RNAi against Gli1 and Gli3. Genome-wide screening revealed that SAC-KO hepatocytes showed time-dependent alterations in various genes, particularly those associated with lipid metabolism. The clock/hedgehog module further plays a role in rhythmicity of steatosis, and in the response of the liver to a high-fat diet or to differently timed starvation.

CONCLUSIONS

For the first time, Hh signaling in hepatocytes was found to be time-of-day dependent and to feed back on the circadian clock. Our findings suggest an integrative role of Hh signaling, mediated mainly by GLI factors, in maintaining homeostasis of hepatic lipid metabolism by balancing the circadian clock.

LAY SUMMARY

The results of our investigation show for the first time that the Hh signaling in hepatocytes is time-of-day dependent, leading to differences not only in transcript levels but also in the amount of Hh ligands in peripheral blood. Conversely, Hh signaling is able to feed back to the circadian clock.

Liver Zonation in Health and Disease: Hypoxia and Hypoxia-Inducible Transcription Factors as Concert Masters

The liver and its zonation contribute to whole body homeostasis. Acute and chronic, not always liver, diseases impair proper metabolic zonation. Various underlying pathways, such as β-catenin, hedgehog signaling, and the Hippo pathway, along with the physiologically occurring oxygen gradient, appear to be contributors. Interestingly, hypoxia and hypoxia-inducible transcription factors can orchestrate those pathways. In the current review, we connect novel findings of liver zonation in health and disease and provide a view about the dynamic interplay between these different pathways and cell-types to drive liver zonation and systemic homeostasis.

Argininosuccinate synthase 1 and periportal gene expression in sonic hedgehog hepatocellular adenomas

Genetic alterations define different molecular subclasses of hepatocellular adenoma (HCA) linked with risk factors, histology and clinical behavior. Recently, Argininosuccinate Synthase 1 (ASS1), a major periportal protein, was proposed as a marker of HCA with a high risk of hemorrhage. We aimed to assess the significance of ASS1 expression through the scope of the HCA molecular classification. ASS1 expression was evaluated using RNAseq, quantitative reverse transcriptase polymerase chain reaction (RT-PCR) and Immunohistochemistry. ASS1 and glioma-associated oncogene 1 (GLI1) expression were analyzed in vitro after modulation of GLI1 expression. Using RNAseq in 27 HCA and five nontumor liver samples, ASS1 expression was highly correlated with GLI1 expression (P<0.0001, R=0.75). In the overall series of 408 HCA, ASS1 overexpression was significantly associated with sonic hedgehog HCA (shHCA) compared to other molecular subgroups (P<0.0001), suggesting that sonic hedgehog signaling controls ASS1 expression. GLI1 expression silencing by siRNA induced a downregulation of ASS1 in PLC/PFR5 and SNU878 cell lines. In 390 HCA, we showed that ASS1 expression belonged to the periportal expression program that was maintained in shHCA but down-regulated in all the other HCA subtypes. In contrast, HCA with β-catenin activation showed an activation of a perivenous program. Despite the significant association between GLI1 and ASS1 expression, ASS1 mRNA expression was not associated with specific clinical features. At the protein level using immunohistochemistry, prostaglandin D synthase (PTGDS) was strongly and specifically overexpressed in shHCA.

CONCLUSION

ASS1 is associated with sonic hedgehog activation as part of a periportal program expressed in shHCA, a molecular subgroup defined by INHBE-GLI1 gene fusion. (Hepatology 2018).

Conditional loss of hepatocellular Hedgehog signaling in female mice leads to the persistence of hepatic steroidogenesis, androgenization and infertility

The Hedgehog signaling pathway is known to be involved in embryogenesis, tissue remodeling, and carcinogenesis. Because of its involvement in carcinogenesis, it seems an interesting target for cancer therapy. Indeed, Sonidegib, an approved inhibitor of the Hedgehog receptor Smoothened (Smo), is highly active against diverse carcinomas, but its use is also reported to be associated with several systemic side effects. Our former work in adult mice demonstrated hepatic Hedgehog signaling to play a key role in the insulin-like growth factor axis and lipid metabolism. The current work using mice with an embryonic and hepatocyte-specific Smo deletion describes an adverse impact of the hepatic Hedgehog pathway on female fertility. In female SAC-KO mice, we detected androgenization characterized by a 3.3-fold increase in testosterone at 12 weeks of age based on an impressive induction of steroidogenic gene expression in hepatocytes, but not in the classic steroidogenic organs (ovary and adrenal gland). Along with the elevated level of testosterone, the female SAC-KO mice showed infertility characterized by juvenile reproductive organs and acyclicity. The endocrine and reproductive alterations resembled polycystic ovarian syndrome and could be confirmed in a second mouse model with conditional deletion of Smo at 8 weeks of age after an extended period of 8 months. We conclude that the down-regulation of hepatic Hedgehog signaling leads to an impaired hormonal balance by the induction of steroidogenesis in the liver. These effects of Hedgehog signaling inhibition should be considered when using Hedgehog inhibitors as anti-cancer drugs.

Metabolic zonation of the liver: The oxygen gradient revisited

The liver has a multitude of functions which are necessary to maintain whole body homeostasis. This requires that various metabolic pathways can run in parallel in the most efficient manner and that futile cycles are kept to a minimum. To a large extent this is achieved due to a functional specialization of the liver parenchyma known as metabolic zonation which is often lost in liver diseases. Although this phenomenon is known for about 40 years, the underlying regulatory pathways are not yet fully elucidated. The physiologically occurring oxygen gradient was considered to be crucial for the appearance of zonation; however, a number of reports during the last decade indicating that β-catenin signaling, and the hedgehog (Hh) pathway contribute to metabolic zonation may have shifted this view. In the current review we connect these new observations with the concept that the oxygen gradient within the liver acinus is a regulator of zonation. This is underlined by a number of facts showing that the β-catenin and the Hh pathway can be modulated by the hypoxia signaling system and the hypoxia-inducible transcription factors (HIFs). Altogether, we provide a view by which the dynamic interplay between all these pathways can drive liver zonation and thus contribute to its physiological function.

Zonation of hepatic fatty acid metabolism - The diversity of its regulation and the benefit of modeling

A pronounced heterogeneity between hepatocytes in subcellular structure and enzyme activities was discovered more than 50years ago and initiated the idea of metabolic zonation. In the last decades zonation patterns of liver metabolism were extensively investigated for carbohydrate, nitrogen and lipid metabolism. The present review focuses on zonation patterns of the latter. We review recent findings regarding the zonation of fatty acid uptake and oxidation, ketogenesis, triglyceride synthesis and secretion, de novo lipogenesis, as well as bile acid and cholesterol metabolism. In doing so, we expose knowledge gaps and discuss contradictory experimental results, for example on the zonation pattern of fatty acid oxidation and de novo lipogenesis. Thus, possible rewarding directions of further research are identified. Furthermore, recent findings about the regulation of metabolic zonation are summarized, especially regarding the role of hormones, nerve innervation, morphogens, gender differences and the influence of the circadian clock. In the last part of the review, a short collection of models considering hepatic lipid metabolism is provided. We conclude that modeling, despite its proven benefit for understanding of hepatic carbohydrate and ammonia metabolisms, has so far been largely disregarded in the study of lipid metabolism; therefore some possible fields of modeling interest are presented.

Fuzzy modeling reveals a dynamic self-sustaining network of the GLI transcription factors controlling important metabolic regulators in adult mouse hepatocytes

The Hedgehog signalling-driven Gli transcription factors in hepatocytes form a regulatory network identified by a fuzzy-logic modelling approach. The network explains dynamic features important for hepatocyte function and fate.

Liver zonation: Novel aspects of its regulation and its impact on homeostasis

Liver zonation, the spatial separation of the immense spectrum of different metabolic pathways along the liver sinusoids, is fundamental for proper functioning of this organ. Recent progress in elucidating localization and interactions of different metabolic pathways by using “omics” techniques and novel approaches to couple them with refined spatial resolution and in characterizing novel master regulators of zonation by using transgenic mice has created the basis for a deeper understanding of core mechanisms of zonation and their impact on liver physiology, pathology and metabolic diseases. This review summarizes the fascinating technical achievements for investigating liver zonation and the elucidation of an emerging network of master regulators of zonation that keep the plethora of interrelated and sometimes opposing functions of the liver in balance with nutritional supply and specific requirements of the entire body. In addition, a brief overview is given on newly described zonated functions and novel details on how diverse the segmentation of metabolic zonation may be. From these facts and developments a few fundamental principles are inferred which seem to rule zonation of liver parenchyma. In addition, we identify important questions that still need to be answered as well as interesting fields of research such as the connection of zonation with circadian rhythm and gender dimorphism which need to be pushed further, in order to improve our understanding of metabolic zonation. Finally, an outlook is given on how disturbance of liver zonation and its regulation may impact on liver pathology and the development of metabolic diseases.

Canonical and non-canonical Hedgehog signalling and the control of metabolism

Obesity and diabetes represent key healthcare challenges of our day, affecting upwards of one billion people worldwide. These individuals are at higher risk for cancer, stroke, blindness, heart and cardiovascular disease, and to date, have no effective long-term treatment options available. Recent and accumulating evidence has implicated the developmental morphogen Hedgehog and its downstream signalling in metabolic control. Generally thought to be quiescent in adults, Hedgehog is associated with several human cancers, and as such, has already emerged as a therapeutic target in oncology. Here, we attempt to give a comprehensive overview of the key signalling events associated with both canonical and non-canonical Hedgehog signalling, and highlight the increasingly complex regulatory modalities that appear to link Hedgehog and control metabolism. We highlight these key findings and discuss their impact for therapeutic development, cancer and metabolic disease.

Hepatic Hedgehog signaling contributes to the regulation of IGF1 and IGFBP1 serum levels

Background

Hedgehog signaling plays an important role in embryonic development, organogenesis and cancer. In the adult liver, Hedgehog signaling in non-parenchymal cells has been found to play a role in certain disease states such as fibrosis and cirrhosis. However, whether the Hedgehog pathway is active in mature healthy hepatocytes and is of significance to liver function are controversial.

Findings

Two types of mice with distinct conditional hepatic deletion of the Smoothened gene, an essential co-receptor protein of the Hedgehog pathway, were generated for investigating the role of Hedgehog signaling in mature hepatocytes. The knockout animals (KO) were inconspicuous and healthy with no changes in serum transaminases, but showed a slower weight gain. The liver was smaller, but presented a normal architecture and cellular composition. By quantitative RT-PCR the downregulation of the expression of Indian hedgehog (Ihh) and the Gli3 transcription factor could be demonstrated in healthy mature hepatocytes from these mice, whereas Patched1 was upregulated. Strong alterations in gene expression were also observed for the IGF axis. While expression of Igf1 was downregulated, that of Igfbp1 was upregulated in the livers of both genders. Corresponding changes in the serum levels of both proteins could be detected by ELISA. By activating and inhibiting the transcriptional output of Hedgehog signaling in cultured hepatocytes through siRNAs against Ptch1 and Gli3, respectively, in combination with a ChIP assay evidence was collected indicating that Igf1 expression is directly dependent on the activator function of Gli3. In contrast, the mRNA level of Igfbp1 appears to be controlled through the repressor function of Gli3, while that of Igfbp2 and Igfbp3 did not change. Interestingly, body weight of the transgenic mice correlated well with IGF-I levels in both genders and also with IGFBP-1 levels in females, whereas it did not correlate with serum growth hormone levels.

Conclusions

Our results demonstrate for the first time that Hedgehog signaling is active in healthy mature mouse hepatocytes and that it has considerable importance for IGF-I homeostasis in the circulation. These findings may have various implications for mouse physiology including the regulation of body weight and size, glucose homeostasis and reproductive capacity.