Taurocholate feeding to bile duct ligated rats prevents caffeic acid-induced bile duct damage by changes in cholangiocyte VEGF expression

Cholangiokines: undervalued modulators in the hepatic microenvironment

The biliary epithelial cells, also known as cholangiocytes, line the intra- and extrahepatic bile ducts, forming a barrier between intra- and extra-ductal environments. Cholangiocytes are mostly known to modulate bile composition and transportation. In hepatobiliary diseases, bile duct injury leads to drastic alterations in cholangiocyte phenotypes and their release of soluble mediators, which can vary depending on the original insult and cellular states (quiescence, senescence, or proliferation). The cholangiocyte-secreted cytokines (also termed cholangiokines) drive ductular cell proliferation, portal inflammation and fibrosis, and carcinogenesis. Hence, despite the previous consensus that cholangiocytes are bystanders in liver diseases, their diverse secretome plays critical roles in modulating the intrahepatic microenvironment. This review summarizes recent insights into the cholangiokines under both physiological and pathological conditions, especially as they occur during liver injury-regeneration, inflammation, fibrosis and malignant transformation processes.

The Effects of Taurocholic Acid on Biliary Damage and Liver Fibrosis Are Mediated by Calcitonin-Gene-Related Peptide Signaling

BACKGROUND & AIMS

Cholangiocytes are the target cells of liver diseases that are characterized by biliary senescence (evidenced by enhanced levels of senescence-associated secretory phenotype, SASP, e.g., TGF-β1), and liver inflammation and fibrosis accompanied by altered bile acid (BA) homeostasis. Taurocholic acid (TC) stimulates biliary hyperplasia by activation of 3',5'-cyclic cyclic adenosine monophosphate (cAMP) signaling, thereby preventing biliary damage (caused by cholinergic/adrenergic denervation) through enhanced liver angiogenesis. Also: (i) α-calcitonin gene-related peptide (α-CGRP, which activates the calcitonin receptor-like receptor, CRLR), stimulates biliary proliferation/senescence and liver fibrosis by enhanced biliary secretion of SASPs; and (ii) knock-out of α-CGRP reduces these phenotypes by decreased cAMP levels in cholestatic models. We aimed to demonstrate that TC effects on liver phenotypes are dependent on changes in the α-CGRP/CALCRL/cAMP/PKA/ERK1/2/TGF-β1/VEGF axis.

METHODS

Wild-type and α-CGRP-/- mice were fed with a control (BAC) or TC diet for 1 or 2 wk. We measured: (i) CGRP levels by both ELISA kits in serum and by qPCR in isolated cholangiocytes (CALCA gene for α-CGRP); (ii) CALCRL immunoreactivity by immunohistochemistry (IHC) in liver sections; (iii) liver histology, intrahepatic biliary mass, biliary senescence (by β-GAL staining and double immunofluorescence (IF) for p16/CK19), and liver fibrosis (by Red Sirius staining and double IF for collagen/CK19 in liver sections), as well as by qPCR for senescence markers in isolated cholangiocytes; and (iv) phosphorylation of PKA/ERK1/2, immunoreactivity of TGF-β1/TGF- βRI and angiogenic factors by IHC/immunofluorescence in liver sections and qPCR in isolated cholangiocytes. We measured changes in BA composition in total liver by liquid chromatography/mass spectrometry.

RESULTS

TC feeding increased CALCA expression, biliary damage, and liver inflammation and fibrosis, as well as phenotypes that were associated with enhanced immunoreactivity of the PKA/ERK1/2/TGF-β1/TGF-βRI/VEGF axis compared to BAC-fed mice and phenotypes that were reversed in α-CGRP-/- mice fed TC coupled with changes in hepatic BA composition.

CONCLUSION

Modulation of the TC/ α-CGRP/CALCRL/PKA/ERK1/2/TGF-β1/VEGF axis may be important in the management of cholangiopathies characterized by BA accumulation.

Mechanism of cholangiocellular damage and repair during cholestasis

The mechanism of damage of the biliary epithelium remains partially unexplored. However, recently many works have offered new evidence regarding the cholangiocytes' damage process, which is the main target in a broad spectrum of pathologies ranging from acute cholestasis, cholangiopathies to cholangiocarcinoma. This is encouraging since some works addressed this epithelium's relevance in health and disease until a few years ago. The biliary tree in the liver, comprised of cholangiocytes, is a pipeline for bile flow and regulates key hepatic processes such as proliferation, regeneration, immune response, and signaling. This review aimed to compile the most recent advances on the mechanisms of cholangiocellular damage during cholestasis, which, although it is present in many cholangiopathies, is not necessarily a common or conserved process in all of them, having a relevant role cAMP and PKA during obstructive cholestasis, as well as Ca²⁺-dependent PKC in functional cholestasis. Cholangiocellular damage could vary according to the type of cholestasis, the aggressor, or the bile ducts' location where it develops and what kind of damage can favor cholangiocellular carcinoma development.

New insights on the role of vascular endothelial growth factor in biliary pathophysiology

The family of vascular endothelial growth factors (VEGFs) includes 5 members (VEGF-A to -D, and placenta growth factor), which regulate several critical biological processes. VEGF-A exerts a variety of biological effects through high-affinity binding to tyrosine kinase receptors (VEGFR-1, -2 and -3), co-receptors and accessory proteins. In addition to its fundamental function in angiogenesis and endothelial cell biology, VEGF/VEGFR signalling also plays a role in other cell types including epithelial cells. This review provides an overview of VEGF signalling in biliary epithelial cell biology in both normal and pathologic conditions. VEGF/VEGFR-2 signalling stimulates bile duct proliferation in an autocrine and paracrine fashion. VEGF/VEGFR-1/VEGFR-2 and angiopoietins are involved at different stages of biliary development. In certain conditions, cholangiocytes maintain the ability to secrete VEGF-A, and to express a functional VEGFR-2 receptor. For example, in polycystic liver disease, VEGF secreted by cystic cells stimulates cyst growth and vascular remodelling through a PKA/RAS/ERK/HIF1α-dependent mechanism, unveiling a new level of complexity in VEFG/VEGFR-2 regulation in epithelial cells. VEGF/VEGFR-2 signalling is also reactivated during the liver repair process. In this context, pro-angiogenic factors mediate the interactions between epithelial, mesenchymal and inflammatory cells. This process takes place during the wound healing response, however, in chronic biliary diseases, it may lead to pathological neo-angiogenesis, a condition strictly linked with fibrosis progression, the development of cirrhosis and related complications, and cholangiocarcinoma. Novel observations indicate that in cholangiocarcinoma, VEGF is a determinant of lymphangiogenesis and of the immune response to the tumour. Better insights into the role of VEGF signalling in biliary pathophysiology might help in the search for effective therapeutic strategies.

Viral Hepatitis and Iron Dysregulation: Molecular Pathways and the Role of Lactoferrin

The liver is a frontline immune site specifically designed to check and detect potential pathogens from the bloodstream to maintain a general state of immune hyporesponsiveness. One of the main functions of the liver is the regulation of iron homeostasis. The liver detects changes in systemic iron requirements and can regulate its concentration. Pathological states lead to the dysregulation of iron homeostasis which, in turn, can promote infectious and inflammatory processes. In this context, hepatic viruses deviate hepatocytes' iron metabolism in order to better replicate. Indeed, some viruses are able to alter the expression of iron-related proteins or exploit host receptors to enter inside host cells. Lactoferrin (Lf), a multifunctional iron-binding glycoprotein belonging to the innate immunity, is endowed with potent antiviral activity, mainly related to its ability to block viral entry into host cells by interacting with viral and/or cell surface receptors. Moreover, Lf can act as an iron scavenger by both direct iron-chelation or the modulation of the main iron-related proteins. In this review, the complex interplay between viral hepatitis, iron homeostasis, and inflammation as well as the role of Lf are outlined.

The Contribution of the Adipose Tissue-Liver Axis in Pediatric Patients with Nonalcoholic Fatty Liver Disease after Laparoscopic Sleeve Gastrectomy

OBJECTIVE

To evaluate the histopathologic modifications in liver and visceral adipose tissue (VAT), and to correlate these changes with clinical measures, adipokine production, and proinflammatory cytokines in a population of adolescents with obesity with nonalcoholic fatty liver disease (NAFLD) who underwent laparoscopic sleeve gastrectomy (LSG).

STUDY DESIGN

Twenty adolescents with obesity who underwent LSG and with biopsy-proven NAFLD were included. Patients underwent clinical evaluation and blood tests at baseline and 1 year after the surgical procedure. Liver and VAT specimens were processed for routine histology, immunohistochemistry, and immunofluorescence.

RESULTS

In adolescents with obesity and NAFLD, hepatic histologic alterations were uncorrelated with VAT inflammation. LSG induced in both liver and VAT tissue histopathology amelioration and macrophage profile modification that were correlated with body mass index and improvement in insulin resistance. The adipokine profile in liver and VAT was associated with weight loss and histologic improvement after LSG. Serum proinflammatory cytokines were correlated with liver and VAT histopathology and IL-1β and IL-6 levels were independently predicted by liver necroinflammatory grade.

CONCLUSIONS

This study suggests a unique adipose tissue/fatty liver crosstalk in pediatric patients. LSG induces a similar pattern of histologic improvement in the liver and in VAT. Besides VAT, our results strengthen the role of the liver in adipocytokine production and its contribution to systemic inflammation in pediatric patients with NAFLD.

Laparoscopic Sleeve Gastrectomy Improves Nonalcoholic Fatty Liver Disease-Related Liver Damage in Adolescents by Reshaping Cellular Interactions and Hepatic Adipocytokine Production

OBJECTIVES

To investigate whether the modulation of local cellular cross-talks and the modification of hepatic adipocytokine expression could mechanistically explain the improvement of liver histopathology after laparoscopic sleeve gastrectomy (LSG) in adolescents with nonalcoholic fatty liver disease (NAFLD).

STUDY DESIGN

Twenty obese (body mass index of ≥35 kg/m²) adolescents who underwent LSG and with biopsy-proven NAFLD were included. At baseline (T0) and 1 year after treatment, patients underwent clinical evaluation, blood tests, and liver biopsy. Hepatic progenitor cells, hepatic stellate cells (HSCs), macrophages, and adipocytokines were evaluated by immunohistochemistry and immunofluorescence.

RESULTS

Liver biopsy samples after LSG demonstrated a significant improvement of NAFLD Activity Score and fibrosis. Immunohistochemistry indicated a significant reduction of hepatocyte cell cycle arrest, ductular reaction, activated HSC, and macrophage number after LSG compared with T0. The activation state of HSC was accompanied by modification in the expression of the autophagy marker LC3. Hepatocyte expression of adiponectin was significant higher after LSG than into T0. Moreover, LSG caused decreased resistin expression in Sox9⁺ hepatic progenitor cells compared with T0. The number of S100A9⁺ macrophages was also reduced by LSG correlating with resistin expression. Finally, serum levels of proinflammatory cytokines significantly correlated with macrophages and activated HSC numbers.

CONCLUSIONS

The histologic improvement induced by LSG is associated with the reduced activation of local cellular compartments (hepatic progenitor cells, HSCs, and macrophages), thus, strengthening the role of cellular interactions and hepatic adipocytokine production in the pathogenesis of NAFLD.

Serum from CCl4-induced acute rat injury model induces differentiation of ADSCs towards hepatic cells and reduces liver fibrosis

Cellular therapies hold promise to alleviate liver diseases. This study explored the potential of allogenic serum isolated from rat with acute CCl₄ injury to differentiate adipose derived stem cells (ADSCs) towards hepatic lineage. Acute liver injury was induced by CCl₄ which caused significant increase in serum levels of VEGF, SDF1α and EGF. ADSCs were preconditioned with 3% serum isolated from normal and acute liver injury models. ADSCs showed enhanced expression of hepatic markers (AFP, albumin, CK8 and CK19). These differentiated ADSCs were transplanted intra-hepatically in CCl₄-induced liver fibrosis model. After one month of transplantation, fibrosis and liver functions (alkaline phosphatase, ALAT and bilirubin) showed marked improvement in acute injury group. Elevated expression of hepatic (AFP, albumin, CK 18 and HNF4a) and pro survival markers (PCNA and VEGF) and improvement in liver architecture as deduced from results of alpha smooth muscle actin, Sirius red and Masson's trichome staining was observed.

Regulators of Cholangiocyte Proliferation

Cholangiocytes, a small population of cells within the normal liver, have been the focus of a significant amount of research over the past two decades because of their involvement in cholangiopathies such as primary sclerosing cholangitis and primary biliary cholangitis. This article summarizes landmark studies in the field of cholangiocyte physiology and aims to provide an updated review of biliary pathogenesis. The historical approach of rodent extrahepatic bile duct ligation and the relatively recent utilization of transgenic mice have led to significant discoveries in cholangiocyte pathophysiology. Cholangiocyte physiology is a complex system based on heterogeneity within the biliary tree and a number of signaling pathways that serve to regulate bile composition. Studies have expanded the list of neuropeptides, neurotransmitters, and hormones that have been shown to be key regulators of proliferation and biliary damage. The peptide histamine and hormones, such as melatonin and angiotensin, angiotensin, as well as numerous sex hormones, have been implicated in cholangiocyte proliferation during cholestasis. Numerous pathways promote cholangiocyte proliferation during cholestasis, and there is growing evidence to suggest that cholangiocyte proliferation may promote hepatic fibrosis. These pathways may represent significant therapeutic potential for a subset of cholestatic liver diseases that currently lack effective therapies.

Isolation and characterization of hepatic mast cells from cholestatic rats

Mast cells (MCs) are immune cells that release histamine and other mediators. MC number increases after bile duct ligation (BDL) and blocking mast cell-derived histamine decreases biliary proliferation. We aimed to isolate and characterize MCs from cholestatic livers. Rats were subjected to BDL starting at 6 h and up to 14 days. MC infiltration was evaluated by toluidine blue. BDL rats were perfused using standard collagenase perfusion. Following enzymatic digestion, tissue was passed through a fine gauge needle. Suspensions were incubated with MAb AA4, washed and incubated with goat anti-mouse-coated Dynal beads. MCs were stained with toluidine blue, and in isolated MCs the expression of FCɛRI and MC proteases was measured. The expression of histidine decarboxylase, histamine receptors, VEGF receptors, and TIE 1 and 2 was evaluated by qPCR. Histamine and VEGF-A secretion was measured in MC supernatants. MC purity was evaluated by CK-19, CK-8, albumin, VAP-1, and α-SMA expression. In vitro, cholangiocytes and HSCs were treated with isolated MC supernatants from BDL rats treated with either NaCl or cromolyn sodium (to block MC histamine release) and biliary proliferation and hepatic fibrosis were measured. MCs infiltrate the liver and surround bile ducts starting at day 2. We isolated a virtually pure preparation of mature, functional MCs. TEM images reveal distinct secretory granules and isolated MCs secrete histamine. MCs express FCɛRI, chymase, tryptase, RMCP-I, and RMCP-II, but were virtually void of other cell markers. Biliary proliferation and fibrosis increased following treatment with MC supernatants from BDL rats+NaCl and these parameters decreased in cells treated with MC supernatants from BDL+cromolyn sodium. In conclusion, we have isolated and characterized MCs from cholestatic livers. MCs regulate cholestatic liver injury and hepatic fibrosis. This tool provides a better understanding of the paracrine influence of mast cells on biliary/liver pathologies.

Pathogenesis of Kupffer Cells in Cholestatic Liver Injury

Kupffer cells are the resident macrophages in the liver. They are located in hepatic sinusoid, which allows them to remove foreign materials, pathogens, and apoptotic cells efficiently. Activated Kupffer cells secrete various mediators, including cytokines and chemokines, to initiate immune responses, inflammation, or recruitment of other liver cells. Bile duct ligation (BDL) surgery in rodents is often studied as an animal model of cholestatic liver disease, characterized by obstruction of bile flow. BDL mice show altered functional activities of Kupffer cells compared with sham-operated mice, including elevated cytokine secretion and impaired bacterial clearance. Various mediators produced by other liver cells can regulate Kupffer cell activation, which suggest that Kupffer cells orchestrate with other liver cells to relay inflammatory signals and to maintain liver homeostasis during BDL-induced liver injury. Blocking or depletion of Kupffer cells, an approach for the treatment of liver diseases, has shown controversial implications. Procedures in Kupffer cell research have limitations and may produce various results in Kupffer cell research. It is important, however, to reveal underlying mechanisms of activation and functions of Kupffer cells, followed by hepatic inflammation and fibrosis. This review summarizes present Kupffer cell studies in cholestatic liver injury.

Peribiliary Glands as a Niche of Extrapancreatic Precursors Yielding Insulin-Producing Cells in Experimental and Human Diabetes

Peribiliary glands (PBGs) are niches in the biliary tree and containing heterogeneous endodermal stem/progenitors cells that can differentiate, in vitro and in vivo, toward pancreatic islets. The aim of this study was to evaluate, in experimental and human diabetes, proliferation of cells in PBGs and differentiation of the biliary tree stem/progenitor cells (BTSCs) toward insulin-producing cells. Diabetes was generated in mice by intraperitoneal injection of a single dose of 200 mg/kg (N = 12) or 120 mg/kg (N = 12) of streptozotocin. Liver, pancreas, and extrahepatic biliary trees were en bloc dissected and examined. Cells in PBGs proliferated in experimental diabetes, and their proliferation was greatest in the PBGs of the hepatopancreatic ampulla, and inversely correlated with the pancreatic islet area. In rodents, the cell proliferation in PBGs was characterized by the expansion of Sox9-positive stem/progenitor cells that gave rise to insulin-producing cells. Insulin-producing cells were located mostly in PBGs in the portion of the biliary tree closest to the duodenum, and their appearance was associated with upregulation of MafA and Gli1 gene expression. In patients with type 2 diabetes, PBGs at the level of the hepatopancreatic ampulla contained cells showing signs of proliferation and pancreatic fate commitment. In vitro, high glucose concentrations induced the differentiation of human BTSCs cultures toward pancreatic beta cell fates. The cells in PBGs respond to diabetes with proliferation and differentiation towards insulin-producing cells indicating that PBG niches may rescue pancreatic islet impairment in diabetes. These findings offer important implications for the pathophysiology and complications of this disease. Stem Cells 2016;34:1332-1342.

Ischemia reperfusion of the hepatic artery induces the functional damage of large bile ducts by changes in the expression of angiogenic factors

Liver transplantation and cholangiocarcinoma induce biliary dysfunction following ischemia reperfusion (IR). The function of the intrahepatic biliary tree is regulated by both autocrine and paracrine factors. The aim of the study was to demonstrate that IR-induced damage of cholangiocytes is associated with altered expression of biliary angiogenic factors. Normal and bile duct ligation rats underwent 24-h sham or hepatic reperfusion after 30 min of transient occlusion of the hepatic artery (HAIR) or portal vein (PVIR) before collecting liver blocks and cholangiocyte RNA or protein. We evaluated liver histology, biliary apoptosis, proliferation and expression of VEGF-A/C, VEGFR-2/3, Ang-1/2, and Tie-1/2 in liver sections and isolated small and large cholangiocytes. Normal rat intrahepatic cholangiocyte cultures (NRICC) were maintained under standard conditions in normoxic or under a hypoxic atmosphere for 4 h and then transferred to normal conditions for selected times. Subsequently, we measured changes in biliary proliferation and apoptosis and the expression of VEGF-A/C and VEGFR-2/3. In vivo, HAIR (but not PVIR) induced damage of large bile ducts and decreased proliferation and secretin-stimulated cAMP levels. HAIR-induced damage of large bile ducts was associated with increased expression of VEGF-A/C, VEGFR-2/3, Ang-1/2, and Tie-1/2. In vitro, under hypoxic conditions, there was increased apoptosis and reduced proliferation of NRICC concomitant with enhanced expression of VEGF-A/C and VEGFR-2/3. The functional damage of large bile ducts by HAIR and hypoxia is associated with increased expression of angiogenic factors in small cholangiocytes, presumably due to a compensatory mechanism in response to biliary damage.

Functional and structural features of cholangiocytes in health and disease

Cholangiocytes are the epithelial cells that line the bile ducts. Along the biliary tree, two different kinds of cholangiocytes exist; small and large cholangiocytes. Each type has important differences in their biological role in physiological and pathological conditions. In response to injury, cholangiocytes become reactive and acquire a neuroendocrine-like phenotype with the secretion of a number of peptides. These molecules act in an autocrine/paracrine fashion to modulate cholangiocyte biology and determine the evolution of biliary damage. The failure of such mechanisms is believed to influence the progression of cholangiopathies, a group of diseases that selectively target biliary cells. Therefore, the understanding of mechanisms regulating cholangiocyte response to injury is expected to foster the development of new therapeutic options to treat biliary diseases. In the present review, we will discuss the most recent findings in the mechanisms driving cholangiocyte adaptation to damage, with particular emphasis on molecular pathways that are susceptible of therapeutic intervention. Morphogenic pathways (Hippo, Notch, Hedgehog), which have been recently shown to regulate biliary ontogenesis and response to injury, will also be reviewed. In addition, the results of ongoing clinical trials evaluating new drugs for the treatment of cholangiopathies will be discussed.

Bile acid signaling and biliary functions

This review focuses on various components of bile acid signaling in relation to cholangiocytes. Their roles as targets for potential therapies for cholangiopathies are also explored. While many factors are involved in these complex signaling pathways, this review emphasizes the roles of transmembrane G protein coupled receptor (TGR5), farnesoid X receptor (FXR), ursodeoxycholic acid (UDCA) and the bicarbonate umbrella. Following a general background on cholangiocytes and bile acids, we will expand the review and include sections that are most recently known (within 5-7 years) regarding the field of bile acid signaling and cholangiocyte function. These findings all demonstrate that bile acids influence biliary functions which can, in turn, regulate the cholangiocyte response during pathological events.

The physiological roles of secretin and its receptor

Secretin is secreted by S cells in the small intestine and affects the function of a number of organ systems. Secretin receptors (SR) are expressed in the basolateral domain of several cell types. In addition to regulating the secretion of a number of epithelia (e.g., in the pancreas and biliary epithelium in the liver), secretin exerts trophic effects in several cell types. In this article, we will provide a comprehensive review on the multiple roles of secretin and SR signaling in the regulation of epithelial functions in various organ systems with particular emphasis in the liver. We will discuss the role of secretin and its receptor in health and biliary disease pathogenesis. Finally, we propose future areas of research for the further evaluation of the secretin/secretin receptor axis in liver pathophysiology.

Knockout of histidine decarboxylase decreases bile duct ligation-induced biliary hyperplasia via downregulation of the histidine decarboxylase/VEGF axis through PKA-ERK1/2 signaling

Histidine is converted to histamine by histidine decarboxylase (HDC). We have shown that cholangiocytes 1) express HDC, 2) secrete histamine, and 3) proliferate after histamine treatment via ERK1/2 signaling. In bile duct-ligated (BDL) rodents, there is enhanced biliary hyperplasia, HDC expression, and histamine secretion. This studied aimed to demonstrate that knockdown of HDC inhibits biliary proliferation via downregulation of PKA/ERK1/2 signaling. HDC(-/-) mice and matching wild-type (WT) were subjected to sham or BDL. After 1 wk, serum, liver blocks, and cholangiocytes were collected. Immunohistochemistry was performed for 1) hematoxylin and eosin, 2) intrahepatic bile duct mass (IBDM) by cytokeratin-19, and 3) HDC biliary expression. We measured serum and cholangiocyte histamine levels by enzyme immunoassay. In total liver or cholangiocytes, we studied: 1) HDC and VEGF/HIF-1α expression and 2) PCNA and PKA/ERK1/2 protein expression. In vitro, cholangiocytes were stably transfected with shRNA-HDC plasmids (or control). After transfection we evaluated pPKA, pERK1/2, and cholangiocyte proliferation by immunoblots and MTT assay. In BDL HDC(-/-) mice, there was decreased IBDM, PCNA, VEGF, and HDC expression compared with BDL WT mice. Histamine levels were decreased in BDL HDC(-/-). BDL HDC(-/-) livers were void of necrosis and inflammation compared with BDL WT. PKA/ERK1/2 protein expression (increased in WT BDL) was lower in BDL HDC(-/-) cholangiocytes. In vitro, knockdown of HDC decreased proliferation and protein expression of PKA/ERK1/2 compared with control. In conclusion, loss of HDC decreases BDL-induced biliary mass and VEGF/HIF-1α expression via PKA/ERK1/2 signaling. Our data suggest that HDC is a key regulator of biliary proliferation.

Epithelial VEGF signaling is required in the mouse liver for proper sinusoid endothelial cell identity and hepatocyte zonation in vivo

Vascular endothelial growth factor (VEGF) is crucial for vascular development in several organs. However, the specific contribution of epithelial-VEGF signaling in the liver has not been tested. We used a mouse model to specifically delete Vegf from the liver epithelial lineages during midgestational development and assessed the cell identities and architectures of epithelial and endothelial tissues. We find that without epithelial-derived VEGF, the zonal endothelial and hepatocyte cell identities are altered. We also find decreased portal vein and hepatic artery branching coincident with an increase in hepatic hypoxia postnatally. Together, these data indicate that VEGF secreted from the hepatic epithelium is required for normal differentiation of cells and establishment of three-dimensional vascular branching and zonal architectures in both epithelial and endothelial hepatic tissues.

Recent advances in the morphological and functional heterogeneity of the biliary epithelium

This review focuses on the recent advances related to the heterogeneity of different-sized bile ducts with regard to the morphological and phenotypical characteristics, and the differential secretory, apoptotic and proliferative responses of small and large cholangiocytes to gastrointestinal hormones/peptides, neuropeptides and toxins. We describe several in vivo and in vitro models used for evaluating biliary heterogeneity. Subsequently, we discuss the heterogeneous proliferative and apoptotic responses of small and large cholangiocytes to liver injury and the mechanisms regulating the differentiation of small into large (more differentiated) cholangiocytes. Following a discussion on the heterogeneity of stem/progenitor cells in the biliary epithelium, we outline the heterogeneity of bile ducts in human cholangiopathies. After a summary section, we discuss the future perspectives that will further advance the field of the functional heterogeneity of the biliary epithelium.

Histamine and histamine receptor regulation of gastrointestinal cancers

Histamine is a neurotransmitter released throughout the body that regulates multiple physiological responses. Primarily histamine is acknowledged for its role in inflammatory reactions to foreign pathogens that enter the body. Aside from inflammatory responses, histamine expression and synthesis has been detected in various cancer cell lines and multiple malignancies. Through experimentation histamine has demonstrated its ability to manage proliferation and angiogenesis in these cancerous cells, in either a positive or inhibitory manner. Regulation of angiogenesis and proliferation have been proven to be carried out by the stimulation or inhibition of numerous pathways and secondary response elements, such as VEGFA/C, IP₃/Ca²⁺, G-proteins, cAMP, and many more. The activation of these different response pathways is linked to the binding of ligands to the histamine receptors H1-H4HR. These receptors exhibit various effects dependent on whether it binds an agonist, antagonist, or its specific ligand, histamine. In cancer cell lines and different tumor cells the binding of these different compounds has shown to be one of the main components in exerting proliferative or antiproliferative changes in the microenvironment. It is also known that the histamine receptors have varying degrees of expression in different forms of cancer, and this expression can impact the tumor in various ways. This clearly indicates the significance of histamine receptors in cancer formation, and one of the aims of this review is to cover this topic concisely and in depth. Histamine is produced from numerous cells such as basophils and mast cells and is synthesized from the enzyme histidine decarboxylase (HDC). In this review we will prominently discuss the function of mast cells and HDC in histamine expression in various gastrointestinal carcinomas. We also briefly discuss current studies to support these claims. In this review we hope to give the reader a clear and comprehensible overview of histamine in various gastrointestinal cancers, and how its regulation can affect the cancer cells in varying ways.

Perspective: TGR5 (Gpbar-1) in liver physiology and disease

Bile acids are signaling molecules with diverse endocrine functions. Bile acid effects are mediated through the nuclear receptor farnesoid X receptor (FXR), the G-protein coupled receptor TGR5 (Gpbar-1) and various other bile acid sensing molecules. TGR5 is almost ubiquitously expressed and has been detected in different non-parenchymal cells of human and rodent liver. Here, TGR5 has anti-inflammatory, anti-apoptotic and choleretic functions. Mice with targeted deletion of TGR5 are protected from the development of cholesterol gallstones. Administration of specific TGR5 agonists lowers serum and liver triglyceride levels thereby reducing liver steatosis. Furthermore, activation of TGR5 promotes intestinal glucagon-like peptide-1 (GLP-1) release, thereby modulating glucose homeostasis and energy expenditure in brown adipose tissue and skeletal muscle. Additionally, TGR5 exerts anti-inflammatory actions resulting in decreased liver injury in animal models of sepsis. These beneficial effects make TGR5 an attractive therapeutic target for metabolic diseases, such as diabetes, obesity, atherosclerosis and steatohepatitis.

Histamine stimulates the proliferation of small and large cholangiocytes by activation of both IP3/Ca2+ and cAMP-dependent signaling mechanisms

Although large cholangiocytes exert their functions by activation of cyclic adenosine 3',5'-monophosphate (cAMP), Ca(2+)-dependent signaling regulates the function of small cholangiocytes. Histamine interacts with four receptors, H1-H4HRs. H1HR acts by Gαq activating IP(3)/Ca(2+), whereas H2HR activates Gα(s) stimulating cAMP. We hypothesize that histamine increases biliary growth by activating H1HR on small and H2HR on large cholangiocytes. The expression of H1-H4HRs was evaluated in liver sections, isolated and cultured (normal rat intrahepatic cholangiocyte culture (NRIC)) cholangiocytes. In vivo, normal rats were treated with histamine or H1-H4HR agonists for 1 week. We evaluated: (1) intrahepatic bile duct mass (IBDM); (2) the effects of histamine, H1HR or H2HR agonists on NRIC proliferation, IP(3) and cAMP levels and PKCα and protein kinase A (PKA) phosphorylation; and (3) PKCα silencing on H1HR-stimulated NRIC proliferation. Small and large cholangiocytes express H1-H4HRs. Histamine and the H1HR agonist increased small IBDM, whereas histamine and the H2HR agonist increased large IBDM. H1HR agonists stimulated IP(3) levels, as well as PKCα phosphorylation and NRIC proliferation, whereas H2HR agonists increased cAMP levels, as well as PKA phosphorylation and NRIC proliferation. The H1HR agonist did not increase proliferation in PKCα siRNA-transfected NRICs. The activation of differential signaling mechanisms targeting small and large cholangiocytes is important for repopulation of the biliary epithelium during pathologies affecting different-sized bile ducts.

The efficiency of CAPE on retardation of hepatic fibrosis in biliary obstructed rats

The aim of this study was to evaluate the possible protective effects of caffeic acid phenethyl ester (CAPE) against cholestatic oxidative stress and liver damage in the common bile duct ligated rats. A total of 18 male Sprague-Dawley rats were divided into three groups: control, bile duct ligation (BDL) and BDL + received CAPE; each group contain 6 animals. The rats in CAPE treated groups were given CAPE (10 μmol/kg) once a day intraperitoneally (i.p) for 2 weeks starting just after BDL operation. The changes demonstrating the bile duct proliferation and fibrosis in expanded portal tracts include the extension of proliferated bile ducts into lobules, inflammatory cell infiltration into the widened portal areas were observed in BDL group. Treatment of BDL with CAPE attenuated alterations in liver histology. The proliferating cell nuclear antigen and the activity of TUNEL in the BDL were observed to be reduced with the QE treatment. The application of BDL clearly increased the tissue hydroxyproline (HP) content, malondialdehyde (MDA) levels and decreased the antioxidant enzyme (superoxide dismutase (SOD), glutathione peroxidase (GPx)) activities. CAPE treatment significantly decreased the elevated tissue HP content, and MDA levels and raised the reduced of SOD, and GPx enzymes in the tissues. The data indicate that CAPE attenuates BDL-induced cholestatic liver injury, bile duct proliferation, and fibrosis. The hepatoprotective effect of CAPE is associated with antioxidative potential.

Taurocholic acid prevents biliary damage induced by hepatic artery ligation in cholestatic rats

BACKGROUND

Ischemic injury by hepatic artery ligation (HAL) during obstructive cholestasis induced by bile duct ligation (BDL) results in bile duct damage, which can be prevented by administration of VEGF-A. The potential regulation of VEGF and VEGF receptor expression and secretion by bile acids in BDL with HAL is unknown.

AIMS

We evaluated whether taurocholic acid (TC) can prevent HAL-induced cholangiocyte damage via the alteration of VEGFR-2 and/or VEGF-A expression.

METHODS

Utilizing BDL, BDL+TC, BDL+HAL, BDL+HAL+TC, and BDL+HAL+wortmannin+TC treated rats, we evaluated cholangiocyte apoptosis, proliferation, and secretion as well VEGF-A and VEGFR-2 expression by immunohistochemistry. In vitro, we evaluated the effects of TC on cholangiocyte secretion of VEGF-A and the dependence of TC-induced proliferation on the activity of VEGFR-2.

RESULTS

In BDL rats with HAL, chronic feeding of TC prevented HAL-induced loss of bile ducts and HAL-induced decreased cholangiocyte secretion. TC also prevented HAL-inhibited VEGF-A and VEGFR-2 expression in liver sections and HAL-induced circulating VEGF-A levels, which were blocked by wortmannin administration. In vitro, TC stimulated increased VEGF-A secretion by cholangiocytes, which was blocked by wortmannin and stimulated cholangiocyte proliferation that was blocked by VEGFR-2 kinase inhibitor.

CONCLUSION

TC prevented HAL-induced biliary damage by upregulation of VEGF-A expression.

Histamine regulation of hyperplastic and neoplastic cell growth in cholangiocytes

Histamine has long been known to be involved in inflammatory events. The discovery of antihistamines dates back to the first half of the 20^th century when a Swiss-Italian pharmacologist, Daniel Bovet began his work. In 1957 he was awarded a Nobel Prize for his production of antihistamines for allergy relief. Since that time, histamine has been found to play a role in other events besides allergic reaction. Possibly unbelievable to Bovet and his peers, histamine has now been marked as playing a role in liver pathologies including hepatobiliary diseases.

Bile salts and cholestasis

Bile salts have a crucial role in hepatobiliary and intestinal homeostasis and digestion. Primary bile salts are synthesized by the liver from cholesterol, and may be modified by the intestinal flora to form secondary and tertiary bile salts. Bile salts are efficiently reabsorbed from the intestinal lumen to undergo enterohepatic circulation. In addition to their function as a surfactant involved in the absorption of dietary lipids and fat-soluble vitamins bile salts are potent signaling molecules in both the liver and intestine. Under physiological conditions the bile salt pool is tightly regulated, but the adaptive capacity may fall short under cholestatic conditions. Elevated serum and tissue levels of potentially toxic hydrophobic bile salts during cholestasis may cause mitochondrial damage, apoptosis or necrosis in susceptible cell types. Therapeutic nontoxic bile salts may restore impaired hepatobiliary secretion in cholestatic disorders. The hydrophilic bile salt ursodeoxycholate is today regarded as the effective standard treatment of primary biliary cirrhosis and intrahepatic cholestasis of pregnancy, and is implicated for use in various other cholestatic conditions. Novel therapeutic bile salts that are currently under evaluation may also prove valuable in the treatment of these diseases.

Vascular factors, angiogenesis and biliary tract disease

PURPOSE OF REVIEW

Recent studies have brought to light that angiogenesis and the expression of pro-angiogenic factors such as vascular endothelial growth factors (VEGFs) participate in the pathogenesis of biliary tract diseases. This review summarizes recent progress that has been accomplished in the field, which expands our understanding of the relationship between vascular growth and the biliary tract, particularly the molecular mechanisms that underlie the pathogenesis of biliary tract diseases.

RECENT FINDINGS

Angiogenesis and the expression of vascular factors play a key role in the pathogenesis of primary biliary cirrhosis, cholangiocarcinoma, liver cysts, and in the progression of biliary fibrosis in animal models. Inhibition of angiogenesis limits fibrosis in animal models, whereas the bile acid, taurocholate, has protective effects in animal models of bile duct and peribiliary vascular plexus damage.

SUMMARY

A widening body of information indicates that the expression of pro-angiogenic factors such as VEGFs and angiogenesis play an important role in a variety of biliary tract diseases. Further characterization of the link between angiogenesis and vascular growth factor expression will help in elucidating the mechanisms regulating the pathogenesis of biliary tract diseases and in devising new treatment approaches for these devastating diseases.

Polycystic liver diseases

Polycystic liver diseases (PCLDs) are genetic disorders with heterogeneous etiologies and a range of phenotypic presentations. PCLD exhibits both autosomal or recessive dominant pattern of inheritance and is characterized by the progressive development of multiple cysts, isolated or associated with polycystic kidney disease, that appear more extensive in women. Cholangiocytes have primary cilia, functionally important organelles (act as mechanosensors) that are involved in both normal developmental and pathological processes. The absence of polycystin-1, 2, and fibrocystin/polyductin, normally localized to primary cilia, represent a potential mechanism leading to cyst formation, associated with increased cell proliferation and apoptosis, enhanced fluid secretion, abnormal cell-matrix interactions, and alterations in cell polarity. Proliferative and secretive activities of cystic epithelium can be regulated by estrogens either directly or by synergizing growth factors including nerve growth factor, IGF1, FSH and VEGF. The abnormalities of primary cilia and the sensitivity to proliferative effects of estrogens and different growth factors in PCLD cystic epithelium provide the morpho-functional basis for future treatment targets, based on the possible modulation of the formation and progression of hepatic cysts.

Recent advances in the regulation of cholangiocyte proliferation and function during extrahepatic cholestasis

Bile duct epithelial cells (i.e., cholangiocytes), which line the intrahepatic biliary epithelium, are the target cells in a number of human cholestatic liver diseases (termed cholangiopathies). Cholangiocyte proliferation and death is present in virtually all human cholangiopathies. A number of recent studies have provided insights into the key mechanisms that regulate the proliferation and function of cholangiocytes during the pathogenesis of cholestatic liver diseases. In our review, we have summarised the most important of these recent studies over the past 3 years with a focus on those performed in the animal model of extrahepatic bile duct ligation. In the first part of the review, we provide relevant background on the biliary ductal system. We then proceed with a general discussion of the factors regulating biliary proliferation performed in the cholestatic animal model of bile duct ligation. Further characterisation of the factors that regulate cholangiocyte proliferation and function will help in elucidating the mechanisms regulating the pathogenesis of biliary tract diseases in humans and in devising new treatment approaches for these devastating diseases.