Prospero-related homeobox 1 (Prox1) is a stable hepatocyte marker during liver development, injury and regeneration, and is absent from "oval cells"

Pannexin1 deletion in lymphatic endothelium affects lymphatic function in a sex-dependent manner

The lymphatic network of capillaries and collecting vessels ensures tissue fluid homeostasis, absorption of dietary fats and trafficking of immune cells. Pannexin1 (Panx1) channels allow for the passage of ions and small metabolites between the cytosol and extracellular environment. Panx1 channels regulate the pathophysiological function of several tissues in a sex-dependent manner. Here, we studied the role of Panx1 in lymphatic function, and potential sex-dependent differences therein, in Prox1-CreERT2Panx1fl/fl and Panx1fl/fl control mice. Panx1 expression was higher in lymphatic endothelial cells (LECs) of male mice. Lymphatic vessel morphology was not affected in Prox1-CreERT2Panx1fl/fl male and female mice. Lymphatic drainage was decreased by 25% in male Prox1-CreERT2Panx1fl/fl mice, but was similar in females of both genotypes. Accordingly, only male Prox1-CreERT2Panx1fl/fl mice exhibited tail swelling, pointing to interstitial fluid accumulation in males upon Panx1 deletion in LECs. Moreover, serum triglyceride and free fatty acid levels raised less in Prox1-CreERT2Panx1fl/fl mice of both sexes in an oral lipid tolerance test. Finally, the percentage of migratory dendritic cells arriving in draining lymph nodes was increased in Prox1-CreERT2Panx1fl/fl female mice, but was comparable between male mice of both genotypes. Our results point to a LEC-specific role for Panx1 in the functions of the lymphatic system.

Triptolide inhibits the progression of Glioblastoma U251 cells via targeting PROX1

Background

Glioblastoma multiforme (GBM) is the most lethal brain cancer in adults, characterized by rapid growth, extensive invasiveness, and poor prognosis, and there is still a lack of effective treatments. Here, we aimed to explore the role of triptolide (TPL), purified from Tripterygium wilfordii Hook F, on glioblastoma cell growth, apoptosis, proliferation, migration and invasion, as well as potential underlying mechanisms.

Methods

The publicly available clinical data of Brain Lower Grade Glioma (LGG) from The Cancer Genome Atlas (TCGA) had been screened to observe PROX1 expression. The Kaplan-Meier analysis was used to analyze the relationship between PROX1 expression and GBM prognosis. CCK8, cell cycle, EDU, apoptosis, wound healing, and transwell assays were performed to detect the effects of TPL on glioblastoma U251 cell viability, cell cycle, proliferation, apoptosis, migration and invasion, respectively. Further, a soft agar colony assay was used to calculate the growth of glioblastoma cells. The qRT-PCR and western blot were conducted to quantify PROX1 mRNA and protein levels. The transcriptional regulation of TPL was detected by Dual luciferase reporter assay.

Results

We found that TPL inhibited glioblastoma cell viability, proliferation, cell cycle, migration and invasion, but enhanced apoptosis in a dose-dependent manner. The expression of cell cycle inhibitor, P21, and pro-apoptosis factor, Bax was increased, while invasion-related factors MMP2 and MMP9 were silenced after TPL treatments. Mechanistically, TPL showed transcriptional inhibition of PROX1 appearance. Moreover, ectopic expression of PROX1 partially rescued the effects of TPL on glioblastoma cell viability, proliferation, apoptosis, migration and invasion, and on the expression of cell function-related genes.

Conclusion

This study verified that TPL inhibited the progression of glioblastoma cells by transcriptionally depressing the expression of PROX1.

Mechanism of paracrine communications between hepatic progenitor cells and endothelial cells

Hepatic progenitor cells (HPCs) are facultative tissue-specific stem cells lining reactive ductules, which are ubiquitously observed in chronic liver diseases and cancer. Although previous research mainly focused on their contribution to liver regeneration, it turned out that in vivo differentiation of HPCs into hepatocytes only occurs after extreme injury. While recent correlative evidence implies the association of HPCs with disease progression, their exact role in pathogenesis remains largely unknown. Our previous research demonstrated that HPCs expressing angiogenic paracrine factors accumulate in the peritumoral area and are positively correlated with the extent of intratumoral cell proliferation and angiogenesis in the livers of patients with liver cancer. Given the crucial roles of angiogenesis in liver disease progression and carcinogenesis, we aimed to test the hypothesis that HPCs secrete paracrine factors to communicate with endothelial cells, to determine molecular mechanisms mediating HPCs-endothelial interactions, and to understand how the paracrine function of HPCs is regulated. HPCs promoted viability and tubulogenesis of human umbilical vein endothelial cells (HUVECs) and upregulated genes known to be involved in angiogenesis, endothelial cell function, and disease progression in a paracrine manner. The paracrine function of HPCs as well as expression of colony stimulating factor 1 (CSF1) were inhibited upon differentiation of HPCs toward hepatocytes. Inhibition of CSF1 receptor partly suppressed the paracrine effects of HPCs on HUVECs. Taken together, our study indicates that inhibition of the paracrine function of HPCs through modulation of their differentiation status and inhibition of CSF1 signaling is a promising strategy for inhibition of angiogenesis during pathological progression.

Emerging Roles for Lymphatics in Chronic Liver Disease

Chronic liver disease (CLD) is a global health epidemic causing ∼2 million deaths annually worldwide. As the incidence of CLD is expected to rise over the next decade, understanding the cellular and molecular mediators of CLD is critical for developing novel therapeutics. Common characteristics of CLD include steatosis, inflammation, and cholesterol accumulation in the liver. While the lymphatic system in the liver has largely been overlooked, the liver lymphatics, as in other organs, are thought to play a critical role in maintaining normal hepatic function by assisting in the removal of protein, cholesterol, and immune infiltrate. Lymphatic growth, permeability, and/or hyperplasia in non-liver organs has been demonstrated to be caused by obesity or hypercholesterolemia in humans and animal models. While it is still unclear if changes in permeability occur in liver lymphatics, the lymphatics do expand in number and size in all disease etiologies tested. This is consistent with the lymphatic endothelial cells (LEC) upregulating proliferation specific genes, however, other transcriptional changes occur in liver LECs that are dependent on the inflammatory mediators that are specific to the disease etiology. Whether these changes induce lymphatic dysfunction or if they impact liver function has yet to be directly addressed. Here, we will review what is known about liver lymphatics in health and disease, what can be learned from recent work on the influence of obesity and hypercholesterolemia on the lymphatics in other organs, changes that occur in LECs in the liver during disease and outstanding questions in the field.

Calcitonin-Receptor-Like Receptor Signaling Governs Intestinal Lymphatic Innervation and Lipid Uptake

The absorption of dietary fat requires complex neuroendocrine-mediated regulation of chylomicron trafficking through enterocytes and intestinal lymphatic vessels. Calcitonin-receptor-like receptor (Calcrl) is a G protein-coupled receptor that can bind either a lymphangiogenic ligand adrenomedullin, with coreceptor RAMP2, or the neuropeptide CGRP, with coreceptor RAMP1. The extent to which this common GPCR controls lipid absorption via lymphatics or enteric innervation remains unclear. We used conditional and inducible genetic deletion of Calcrl in lymphatics to elucidate the pathophysiological consequences of this receptor pathway under conditions of high-fat diet. Inefficient absorption of dietary fat coupled with altered lymphatic endothelial junctions in Calcrl ^fl/fl /Prox1-CreER ^T2 mice results in excessive, transcellular lipid accumulation and abnormal enterocyte chylomicron processing and failure to gain weight. Interestingly, Calcrl ^fl/fl /Prox1-CreER ^T2 animals show reduced and disorganized mucosal and submucosal innervation. Consistently, mice with genetic loss of the CGRP coreceptor RAMP1 also displayed mucosal and submucosal innervation deficits, substantiating the CGRP-biased function of Calcrl in the neurolymphocrine axis. Thus, the common Calcrl receptor is a critical regulator of lipid absorption through its cell-specific functions in neurolymphocrine crosstalk.

Digestion of the Murine Liver for a Flow Cytometric Analysis of Lymphatic Endothelial Cells

Within the liver, lymphatic vessels are found within the portal triad, and their described function is to remove interstitial fluid from the liver to the lymph nodes where cellular debris and antigens can be surveyed. We are very interested in understanding how the lymphatic vasculature might be involved in inflammation and immune cell function within the liver. However, very little has been published establishing digestion protocols for the isolation of lymphatic endothelial cells (LECs) from the liver or specific markers that can be used to evaluate liver LECs on a per cell basis. Therefore, we optimized a method for the digestion and staining of the liver in order to evaluate the LEC population in the liver. We are confident that the method outlined here will be useful for the identification and isolation of LECs from the liver and will strengthen our understanding of how LECs respond to the liver microenvironment.

Utilizing lymphatic cell markers to visualize human lymphatic abnormalities

In vivo visualization of the human lymphatic system is limited by the mode of delivery of tracing agents, depth of field and size of the area examined, and specificity of the cell markers used to distinguish lymphatic endothelium from the blood vessels and the surrounding tissues. These limitations are particularly problematic when imaging human lymphatic abnormalities. First, limited understanding of the lymphatic disease aetiology exists with respect to genetic causes and phenotypic presentations. Second, the ability of a tracer to reach the entire lymphatic network within the diseased tissue is suboptimal. Third, what is known about the expression of lymphatic endothelial cell (LEC) markers, such as podoplanin, lymphatic vessel endothelial hyaluronan receptor, Drosophila melanogaster homeobox gene prospero-1 and vascular endothelial growth factor receptor-3 in rodent lymphatic vessels and healthy human LECs may not necessarily apply in human lymphatic disease settings. The aim of this review is to highlight challenges in visualizing lymphatic vessels in human lymphatic abnormalities with respect to distribution patterns of the cellular markers currently employed to visualize abnormal human lymphatic vessels in experimental settings. Allowing for these limitations within new diagnostic visualization technologies is likely to improve our ability to image human lymphatic diseases.

The Hepatic Lymphatic Vascular System: Structure, Function, Markers, and Lymphangiogenesis

The lymphatic vascular system has been minimally explored in the liver despite its essential functions including maintenance of tissue fluid homeostasis. The discovery of specific markers for lymphatic endothelial cells has advanced the study of lymphatics by methods including imaging, cell isolation, and transgenic animal models and has resulted in rapid progress in lymphatic vascular research during the last decade. These studies have yielded concrete evidence that lymphatic vessel dysfunction plays an important role in the pathogenesis of many diseases. This article reviews the current knowledge of the structure, function, and markers of the hepatic lymphatic vascular system as well as factors associated with hepatic lymphangiogenesis and compares liver lymphatics with those in other tissues.

Rapamycin up-regulates triglycerides in hepatocytes by down-regulating Prox1

Background

Although the prolonged use of rapamycin may cause unwanted side effects such as hyperlipidemia, the underlying mechanism remains unknown. Prox1 is a transcription factor responsible for the development of several tissues including lymphatics and liver. There is growing evidences that Prox1 participates in metabolism in addition to embryogenesis. However, whether Prox1 is directly related to lipid metabolism is currently unknown.

Methods

HepG2 human hepatoma cells were treated with rapamycin and total lipids were analyzed by thin layer chromatography. The effect of rapamycin on the expression of Prox1 was determined by western blotting. To investigate the role of Prox1 in triglycerides regulation, siRNA and overexpression system were employed. Rapamycin was injected into mice for 2 weeks and total lipids and proteins in liver were measured by thin layer chromatography and western blot analysis, respectively.

Results

Rapamycin up-regulated the amount of triglyceride and down-regulated the expression of Prox1 in HepG2 cells by reducing protein half-life but did not affect its transcript. The loss-of-function of Prox1 was coincident with the increase of triglycerides in HepG2 cells treated with rapamycin. The up-regulation of triglycerides by rapamycin in HepG2 cells reverted to normal levels by the compensation of Prox1 using the overexpression system. Rapamycin also down-regulated Prox1 expression but increased triglycerides in mouse liver.

Conclusion

This study suggests that rapamycin can increase the amount of triglycerides by down-regulating Prox1 expression in hepatocytes, which means that the mammalian target of rapamycin (mTOR) signaling is important for the regulation of triglycerides by maintaining Prox1 expression.

Lipid Abundance in Zebrafish Embryos Is Regulated by Complementary Actions of the Endocannabinoid System and Retinoic Acid Pathway

The endocannabinoid system (ECS) and retinoic acid (RA) signaling have been associated with influencing lipid metabolism. We hypothesized that modulation of these pathways could modify lipid abundance in developing vertebrates and that these pathways could have a combinatorial effect on lipid levels. Zebrafish embryos were exposed to chemical treatments altering the activity of the ECS and RA pathway. Embryos were stained with the neutral lipid dye Oil-Red-O (ORO) and underwent whole-mount in situ hybridization (WISH). Mouse 3T3-L1 fibroblasts were differentiated under exposure to RA-modulating chemicals and subsequently stained with ORO and analyzed for gene expression by qRT-PCR. ECS activation and RA exposure increased lipid abundance and the expression of lipoprotein lipase. In addition, RA treatment increased expression of CCAAT/enhancer-binding protein alpha. Both ECS receptors and RA receptor subtypes were separately involved in modulating lipid abundance. Finally, increased ECS or RA activity ameliorated the reduced lipid abundance caused by peroxisome proliferator-activated receptor gamma (PPARγ) inhibition. Therefore, the ECS and RA pathway influence lipid abundance in zebrafish embryos and have an additive effect when treated simultaneously. Furthermore, we demonstrated that these pathways act downstream or independently of PPARγ to influence lipid levels. Our study shows for the first time that the RA and ECS pathways have additive function in lipid abundance during vertebrate development.

The new era of the lymphatic system: no longer secondary to the blood vascular system

The blood and lymphatic systems are the two major circulatory systems in our body. Although the blood system has been studied extensively, the lymphatic system has received much less scientific and medical attention because of its elusive morphology and mysterious pathophysiology. However, a series of landmark discoveries made in the past decade has begun to change the previous misconception of the lymphatic system to be secondary to the more essential blood vascular system. In this article, we review the current understanding of the development and pathology of the lymphatic system. We hope to convince readers that the lymphatic system is no less essential than the blood circulatory system for human health and well-being.

Transcriptional activation of the Prox1 gene by HIF-1α and HIF-2α in response to hypoxia

Prox1 encodes a homeobox transcription factor critical to organ development, but its regulation is poorly understood. Here, we show that Prox1 expression is induced by hypoxia, and controlled by a hypoxia-response element (HRE) at the Prox1 promoter/regulatory region and HIF-1α/HIF-2α. EMSA and ChIP assays demonstrated the direct interaction of the HRE with HIF-1α or HIF-2α. Overexpression of HIF-1α or HIF-2α increased activation of the Prox1 promoter, whereas knockdown of HIF-1α or HIF-2α inhibited the activation. These data reveal a novel molecular mechanism for regulation of Prox1 expression in response to hypoxia and provide new insights into Prox1-controlled processes such as lymphangiogenesis.

Immunohistochemical endothelial markers: a review

A relatively large number of new endothelial markers that can assist in the diagnosis and classification of endothelial and vascular neoplasms have become available over the past few years. The expression of these markers, however, differs considerably among the various tumors. A selection of markers that have potential diagnostic utility or are of current interest among pathologists are reviewed and compared with some of the more traditional markers that have been employed in diagnostic pathology.

Prox1 transcription factor as a marker for vascular tumors-evaluation of 314 vascular endothelial and 1086 nonvascular tumors

Prox1, a transcription factor important in the regulation and maintenance of the lymphatic endothelial phenotype, is consistently expressed in lymphangiomas and Kaposi sarcoma and has also been reported in Kaposiform hemangioendothelioma. However, information on its distribution in vascular tumors, such as angiosarcoma, is limited. In this study, we examined selected normal tissues and 314 vascular endothelial and 1086 nonvascular tumors to get an insight into the biology of these tumors and on potential diagnostic use of Prox1 as an immunohistochemical marker. In adult tissues, Prox1 was essentially restricted to lymphatic endothelia, with expression in subsets of pancreatic and gastrointestinal epithelia. However, it was also detected in embryonic liver and heart. Prox1 was consistently expressed in lymphangiomas, venous hemangiomas, Kaposi sarcoma, in endothelia of spindle cell hemangioma, Kaposiform hemangioendothelioma, and retiform hemangioendothelioma, and in half of epithelioid hemangioendotheliomas. It was present in most cutaneous angiosarcomas from different sites but was less commonly expressed in deep soft tissue and visceral angiosarcomas. In contrast, Prox1 was generally absent in capillary and cavernous hemangiomas. In positive hemangiomas and angiosarcomas it was coexpressed with podoplanin, another marker of the lymphatic endothelial phenotype. There was an inverse correlation with CD34 expression. The expression in mesenchymal nonendothelial neoplasm was limited. Prox1 was detected in 5 of 27 synovial sarcomas, specifically in the epithelia of biphasic tumors. Four of 16 Ewing sarcomas and 5 of 15 paragangliomas were also positive. All melanomas and undifferentiated sarcomas were negative. Among epithelial neoplasms, Prox1 was detected in 18 of 38 colonic carcinomas and 10 of 15 cholangiocarcinomas and in a minority of pulmonary, prostatic, and endometrial adenocarcinomas. The common Prox1 expression in angiosarcoma and its rare presence in nonvascular mesenchymal tumors make this marker suitable for the diagnosis of angiosarcoma and Kaposi sarcoma. However, the presence of Prox1 in some malignant epithelial tumors necessitates caution in applying Prox1 as a marker for vascular tumors. Common Prox1 expression in angiosarcoma may reflect the lymphatic endothelial phenotype in these tumors. Its patterns of expression in hemangiomas and angiosarcoma may be diagnostically useful and offer a new parameter in the biological classification of vascular tumors.

Foxl1-Cre-marked adult hepatic progenitors have clonogenic and bilineage differentiation potential

Isolation of hepatic progenitor cells is a promising approach for cell replacement therapy of chronic liver disease. The winged helix transcription factor Foxl1 is a marker for progenitor cells and their descendants in the mouse liver in vivo. Here, we purify progenitor cells from Foxl1-Cre; RosaYFP mice and evaluate their proliferative and differentiation potential in vitro. Treatment of Foxl1-Cre; RosaYFP mice with a 3,5-diethoxycarbonyl-1,4-dihydrocollidine diet led to an increase of the percentage of YFP-labeled Foxl1(+) cells. Clonogenic assays demonstrated that up to 3.6% of Foxl1(+) cells had proliferative potential. Foxl1(+) cells differentiated into cholangiocytes and hepatocytes in vitro, depending on the culture condition employed. Microarray analyses indicated that Foxl1(+) cells express stem cell markers such as Prom1 as well as differentiation markers such as Ck19 and Hnf4a. Thus, the Foxl1-Cre; RosaYFP model allows for easy isolation of adult hepatic progenitor cells that can be expanded and differentiated in culture.

Directed hepatic differentiation from embryonic stem cells

The liver is the largest internal organ in mammals, and is important for the maintenance of normal physiological functions of other tissues and organs. Hepatitis, cirrhosis, liver cancer and other chronic liver diseases are serious threats to human health, and these problems are compounded by a scarcity of liver donors for transplantation therapies. Directed differentiation of embryonic stem cells to liver cells is a promising strategy for obtaining hepatocytes that can be used for cell transplantation. In vitro hepatocyte differentiation of embryonic stem cells requires a profound understanding of normal development during embryonic hepatogenesis. Here we provide a simple description of hepatogenesis in vivo and discuss directed differentiation of embryonic stem cells into hepatocytes in vitro.

A role for all-trans-retinoic acid in the early steps of lymphatic vasculature development

The molecular mechanisms that regulate the earliest steps of lymphatic vascular system development are unknown. To identify regulators of lymphatic competence and commitment, we used an in vitro vascular assay with mouse embryonic stem cell-derived embryoid bodies (EBs). We found that incubation with retinoic acid (RA) and, more potently, with RA in combination with cAMP, induced the expression of the lymphatic competence marker LYVE-1 in the vascular structures of the EBs. This effect was dependent on RA receptor (RAR)-α and protein kinase A signaling. RA-cAMP incubation also promoted the development of CD31+/LYVE-1+/Prox1+ cell clusters. In situ studies revealed that RAR-α is expressed by endothelial cells of the cardinal vein in ED 9.5-11.5 mouse embryos. Timed exposure of mouse and Xenopus embryos to excess of RA upregulated LYVE-1 and VEGFR-3 on embryonic veins and increased formation of Prox1-positive lymphatic progenitors. These findings indicate that RA signaling mediates the earliest steps of lymphatic vasculature development.

Transcriptional profiling of human embryonic stem cells differentiating to definitive and primitive endoderm and further toward the hepatic lineage

Human embryonic stem cells (hESC) can differentiate into a variety of specialized cell types, and they constitute a useful model system to study embryonic development in vitro. In order to fully utilize the potential of these cells, the mechanisms that regulate the developmental processes of specific lineage differentiation need to be better defined. The aim of this study was to explore the molecular program involved in the differentiation of hESC toward definitive endoderm (DE) and further into the hepatic lineage, and to compare that with primitive endoderm (PrE) differentiation. To that end, we applied two protocols: a specific DE differentiation protocol and an intrinsic differentiation protocol that mainly mediates PrE formation. We collected hESC, hESC-derived DE, DE-derived hepatocyte-progenitors (DE-Prog), DE-derived hepatocyte-like cells (DE-Hep), and the corresponding PrE derivatives. The samples were analyzed using microarrays, and we identified sets of genes that were exclusively up-regulated in DE derivatives (compared to PrE derivatives) at discrete developmental stages. We also investigated known protein interactions among the set of up-regulated genes in DE-Hep. The results demonstrate important differences between DE and PrE differentiation on the transcriptional level. In particular, our results identify a unique molecular program, exclusively activated during development of DE and the subsequent differentiation of DE toward the hepatic lineage. We identified key genes and pathways of potential importance for future efforts to improve hepatic differentiation from hESC. These results reveal new opportunities for rational design of specific interventions with the purpose of generating enriched populations of DE derivatives, including functional hepatocytes.

The homeobox protein Prox1 is a negative modulator of ERR{alpha}/PGC-1{alpha} bioenergetic functions

Estrogen-related receptor alpha (ERRalpha) and proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) play central roles in the transcriptional control of energy homeostasis, but little is known about factors regulating their activity. Here we identified the homeobox protein prospero-related homeobox 1 (Prox1) as one such factor. Prox1 interacts with ERRalpha and PGC-1alpha, occupies promoters of metabolic genes on a genome-wide scale, and inhibits the activity of the ERRalpha/PGC-1alpha complex. DNA motif analysis suggests that Prox1 interacts with the genome through tethering to ERRalpha and other factors. Importantly, ablation of Prox1 and ERRalpha have opposite effects on the respiratory capacity of liver cells, revealing an unexpected role for Prox1 in the control of energy homeostasis.

Hepatic stem cells and liver development

The liver consists of many cell types with specialized functions. Hepatocytes are one of the main players in the organ and therefore are the most vulnerable cells to damage. Since they are not everlasting cells, they need to be replenished throughout life. Although the capacity of hepatocytes to contribute to their own maintenance has long been recognized, recent studies have indicated the presence of both intrahepatic and extrahepatic stem/progenitor cell populations that serve to maintain the normal organ and to regenerate damaged parenchyma in response to a variety of insults.The intrahepatic compartment most likely derives primarily from the biliary tree, particularly the most proximal branches, i.e. the canals of Hering and smallest ductules. The extrahepatic compartment is at least in part derived from diverse populations of cells from the bone marrow. Embryonic stem cells (ES's) are considered as a part of the extrahepatic compartment. Due to their pluripotent capabilities, ES cell-derived cells form a potential future source of hepatocytes, to replace or restore hepatic tissues that have been damaged by disease or injury. Progressing knowledge about stem cells in the liver would allow a better understanding of the mechanisms of hepatic homeostasis and regeneration. Although a human stem cell-derived cell type equivalent to primary hepatocytes does not yet exist, the promising results obtained with extrahepatic stem cells would open the way to cell-based therapy for liver diseases.

Hepatoblast and mesenchymal cell-specific gene-expression in fetal rat liver and in cultured fetal rat liver cells

The aim of this study was to determine whether passaged rat fetal liver cells are functional hepatoblasts. Hepatocyte/hepatoblast- and liver myofibroblast-gene-expressions were studied in adult and fetal rat liver tissues as well as in primary and passaged cultures of isolated rat fetal liver cells at both the mRNA and protein level. Desmin- and Alpha-Smooth Muscle Actin (SMA)-positive cells were located in the walls of liver vessels, whereas Desmin-positive/SMA-negative cells were distributed within the liver parenchyma. Primary cultures contained Prox1-positive hepatoblasts, Desmin/SMA-positive myofibroblasts and only a few Desmin-positive/SMA-negative cells. Albumin and alpha-fetoprotein (AFP) could be detected in the primary cultures and to a lesser extent after the first passage. The number of Desmin-positive/SMA-negative cells decreased with successive passage, such that after the second passage, only Desmin/SMA-positive cells could be detected. SMA-gene-expression increased during the passages, suggesting that myofibroblasts become the major cell population of fetal liver cell cultures over time. This observation needs to be taken into account, should passaged fetal liver cells be used for liver cell transplantation. Moreover it contradicts the concept of epithelial-mesenchymal transformation and suggests rather that selective overgrowth of mesenchymal cells occurs in culture.

Thy-1 is expressed in myofibroblasts but not found in hepatic stellate cells following liver injury

Thy-1 (CD90) is an adhesion molecule induced in fibroblast populations associated with wound healing and fibrosis. In this study the question whether Thy-1-gene-expression can be induced in hepatic stellate cells (HSC) in vivo, under conditions of liver injury or liver regeneration was addressed. Acute and chronic rat liver injury was induced by the administration of CCl4. For comparison, cirrhotic human liver, and rat 67% partial hepatectomy (PH) was studied as well. Thy-1-gene-expression was examined also in isolated human liver myofibroblasts. Thy-1-mRNA expression was significantly upregulated in chronic liver injury. Thy-1+ cells were detected in the periportal area of rat liver specimens in normal-, injured- and regenerative-conditions. In chronic human and rat liver injury, Thy-1+ cells were located predominantly in scar tissue. In the pericentral necrotic zone after CCl4-treatment, no induction of Thy-1 was found. Gremlin and Thy-1 showed comparable localization in the periportal areas. Thy-1 was not detected in either normal or capillarized sinusoids, in isolated rat HSC, and was neither inducible by inflammatory cytokines in isolated HSC, nor upregulated in treated myofibroblasts. Based upon these data Thy-1 is not a marker of "activated" sinusoidal HSC, but it is a marker of "activated" (myo)fibroblasts found in portal areas and in scar tissue.

Triiodothyronine accelerates differentiation of rat liver progenitor cells into hepatocytes

The 2-acetaminofluorene/partial hepatectomy (AAF/Phx) model is widely used to induce oval/progenitor cell proliferation in the rat liver. We have used this model to study the impact of a primary hepatocyte mitogen, triiodothyronine (T3) on the liver regenerating by the recruitment of oval/progenitor cells. Administration of T3 transiently accelerates the proliferation of the oval cells, which is followed by rapid differentiation into small hepatocytes. The oval cell origin of the small hepatocytes has been proven by tracing retrovirally transduced and BrdU marked oval cells. The differentiating oval cells become positive for hepatocyte nuclear factor-4 and start to express hepatocyte specific connexin 32, alpha1 integrin, Prox1, cytochrom P450s, and form CD 26 positive bile canaliculi. At the same time oval cell specific OV-6 and alpha-fetoprotein expression is lost. The upregulation of hepatocyte specific mRNAs: albumin, tyrosine aminotransferase and tryptophan 2,3-dioxygenase detected by real-time PCR also proves hepatocytic maturation. The hepatocytic conversion of oval cells occurs on the seventh day after the Phx in this model while the first small hepatocytes appear 5 days later without T3 treatment. The administration of the primary hepatocyte mitogen T3 accelerates the differentiation of hepatic progenitor cells into hepatocytes in vivo, and that may have therapeutic potential.

Prospero-related homeobox 1 and liver receptor homolog 1 coordinately regulate long-term proliferation of murine fetal hepatoblasts

During early to late-fetal liver development, bipotential hepatoblasts proliferate and differentiate into hepatocytes and cholangiocytes. The prospero-related homeobox 1 gene (Prox1) is expressed in hepatoblasts, and the inactivation of Prox1 causes defective early liver development, in particular, faulty migration of fetal hepatoblasts. Prox1 binds to another hepatocyte-enriched transcription factor, liver receptor homolog 1 (Lrh1), and suppresses its transcriptional activity. However, the molecular mechanism by which Prox1 and Lrh1 regulate the characteristics of fetal hepatic cells remains unknown. We investigated the contribution of Prox1 and Lrh1 in early liver development. Embryonic day 13 liver-derived CD45-Ter119-Dlk+ cells were purified as fetal hepatic stem/progenitor cells, and formation of colonies derived from single cells was detected under low-density culture conditions. We found that overexpression of Prox1 using retrovirus infection induced migration and proliferation of fetal hepatic stem/progenitor cells. In contrast, overexpression of Lrh1 suppressed colony formation. Prox1 induced the long-term proliferation of fetal hepatic stem/progenitor cells, which exhibited both high proliferative activity and bipotency for differentiation. Prox1 up-regulated expression of cyclins D2, E1, and E2, whereas it suppressed expression of p16(ink4a), the cdk inhibitor. In addition, overexpression of Prox1 significantly inhibited the proximal promoter activity of p16(ink4a).

CONCLUSION

These results suggested that Prox1 and Lrh1 coordinately regulate development of hepatic stem/progenitor cells and that Prox1 induces fetal hepatocytic proliferation through the suppression of the promoter activity of p16(ink4a).

Altered regulation of Prox1-gene-expression in liver tumors

Background

Prospero-related homeobox 1 (Prox1) transcription factor was described as a tumor-suppressor gene in liver tumors. In contrast, Prox1 knock out in murine embryos drastically reduces proliferation of hepatoblasts.

Methods

We have studied the expression of Prox1 in normal liver, liver cirrhosis and peritumoral liver samples in comparison to hepatocellular (HCC) and cholangiocellular carcinoma (CCC) at mRNA, protein and functional levels.

Results

Prox1 was found in hepatocytes of normal liver, while normal bile duct epithelial cells were negative. However, Prox1⁺ cells, which co-expressed biliary epithelial makers and showed ductular morphology, could be detected within fibrotic septa of cirrhotic livers, and in both HCC and CCC. Two Prox1 mRNA isoforms (2.9 kb and 7.9 kb) were identified with a prevalence of the longer isoform in several HCC samples and the shorter in most CCC samples. Evidence was provided that Myc-associated zinc finger protein (MAZ) might significantly contribute to the gene expression of Prox1 in HCC, while neo-expression of Prox1 in CCC remains to be resolved. A point mutation in the prospero domain of Prox1 was found in one HCC sample.

Conclusion

Our study shows dysregulation of Prox1 in liver cirrhosis, HCC and CCC, such as neo-expression in cells with biliary epithelial phenotype in liver cirrhosis, and in CCC. Altered Prox1 mRNA expression is partly regulated by MAZ, and mutation of the prospero domain in HCC indicates an involvement for Prox1 during tumor progression.

Activation of stem cells in hepatic diseases

The liver has enormous regenerative capacity. Following acute liver injury, hepatocyte division regenerates the parenchyma but, if this capacity is overwhelmed during massive or chronic liver injury, the intrinsic hepatic progenitor cells (HPCs) termed oval cells are activated. These HPCs are bipotential and can regenerate both biliary epithelia and hepatocytes. Multiple signalling pathways contribute to the complex mechanism controlling the behaviour of the HPCs. These signals are delivered primarily by the surrounding microenvironment. During liver disease, stem cells extrinsic to the liver are activated and bone-marrow-derived cells play a role in the generation of fibrosis during liver injury and its resolution. Here, we review our current understanding of the role of stem cells during liver disease and their mechanisms of activation.

Recent progress in histochemistry

The progress in discerning the structure and function of cells and tissues in health and disease has been achieved to a large extent by the continued development of new reagents for histochemistry, the improvement of existing techniques and new imaging techniques. This review will highlight some advancements made in these fields.

Thy-1 is an in vivo and in vitro marker of liver myofibroblasts

Thy-1, a glycophosphatidylinositol-linked glycoprotein of the outer membrane leaflet, has been described in myofibroblasts of several organs. Previous studies have shown that, in fetal liver, Thy-1 is expressed in a subpopulation of ductular/progenitor cells. The aim of this study has been to investigate whether the liver myofibroblasts belong to the Thy-1-positive subpopulation of the adult liver. The expression of Thy-1 has been studied in normal rat liver, in the rat liver regeneration model following 2-acetylaminofluorene treatment and partial hepatectomy (AAF/PH), and in isolated rat liver cells, at the mRNA and protein levels. In normal rat liver, Thy-1 is detected in sparse cells of the periportal area, whereas 7 days after PH in the AAF/PH model, a marked increase of the number of Thy-1-positive cells is detectable by immunohistochemistry. Comparative immunohistochemical analysis has revealed the co-localization of Thy-1 and smooth muscle actin, but not of Thy-1 and cytokeratin-19, both in normal rat liver and in the AAF/PH model. Investigation of isolated rat liver cell populations has confirmed that liver myofibroblasts are Thy-1-positive cells, whereas hepatocytes, hepatic stellate cells, and liver macrophages are not. Thy-1 is the first cell surface marker for identifying liver myofibroblasts in vivo and in vitro.

The histochemistry and cell biology vade mecum: a review of 2005–2006

The procurement of new knowledge and understanding in the ever expanding discipline of cell biology continues to advance at a breakneck pace. The progress in discerning the physiology of cells and tissues in health and disease has been driven to a large extent by the continued development of new probes and imaging techniques. The recent introduction of semi-conductor quantum dots as stable, specific markers for both fluorescence light microscopy and electron microscopy, as well as a virtual treasure-trove of new fluorescent proteins, has in conjunction with newly introduced spectral imaging systems, opened vistas into the seemingly unlimited possibilities for experimental design. Although it oftentimes proves difficult to predict what the future will hold with respect to advances in disciplines such as cell biology and histochemistry, it is facile to look back on what has already occurred. In this spirit, this review will highlight some advancements made in these areas in the past 2 years.