Bile acid depletion and repletion regulate cholangiocyte growth and secretion by a phosphatidylinositol 3-kinase-dependent pathway in rats

Ductular Reactions in Liver Injury, Regeneration, and Disease Progression-An Overview

Ductular reaction (DR) is a complex cellular response that occurs in the liver during chronic injuries. DR mainly consists of hyper-proliferative or reactive cholangiocytes and, to a lesser extent, de-differentiated hepatocytes and liver progenitors presenting a close spatial interaction with periportal mesenchyme and immune cells. The underlying pathology of DRs leads to extensive tissue remodeling in chronic liver diseases. DR initiates as a tissue-regeneration mechanism in the liver; however, its close association with progressive fibrosis and inflammation in many chronic liver diseases makes it a more complicated pathological response than a simple regenerative process. An in-depth understanding of the cellular physiology of DRs and their contribution to tissue repair, inflammation, and progressive fibrosis can help scientists develop cell-type specific targeted therapies to manage liver fibrosis and chronic liver diseases effectively.

Secondary bile acids and the biliary epithelia: The good and the bad

The biliary tract has been considered for several decades a passive system just leading the hepatic bile to the intestine. Nowadays several researches demonstrated an important role of biliary epithelia (i.e. cholangiocytes) in bile formation. The study of biliary processes therefore maintains a continuous interest since the possible important implications regarding chronic cholestatic human diseases, such as primary biliary cholangitis or primary sclerosing cholangitis. Bile acids (BAs), produced by the liver, are the most represented organic molecules in bile. The physiologic importance of BAs was initially attributed to their behavior as natural detergents but several studies now demonstrate they are also important signaling molecules. In this minireview the effect of BAs on the biliary epithelia are reported focusing in particular on secondary (deriving by bacterial manipulation of primary molecules) ones. This class of BAs is demonstrated to have relevant biological effects, ranging from toxic to therapeutic ones. In this family ursodeoxycholic and lithocholic acid present the most interesting features. The molecular mechanisms linking ursodeoxycholic acid to its beneficial effects on the biliary tract are discussed in details as well as data on the processes leading to lithocholic damage. These findings suggest that expansion of research in the field of BAs/cholangiocytes interaction may increase our understanding of cholestatic diseases and should be helpful in designing more effective therapies for biliary disorders.

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.

MiR-200c-3p targets SESN1 and represses the IL-6/AKT loop to prevent cholangiocyte activation and cholestatic liver fibrosis

Cholestasis causes ductular reaction in the liver where the reactive cholangiocytes not only proliferate but also gain a neuroendocrine-like phenotype, leading to inflammatory cell infiltration and extracellular matrix deposition and contributing to the development and progression of cholestatic liver fibrosis. This study aims to elucidate the role of miR-200c in cholestasis-induced biliary liver fibrosis and cholangiocyte activation. We found that miR-200c was extremely abundant in cholangiocytes but was reduced by cholestasis in a bile duct ligation (BDL) mouse model; miR-200c was also decreased by bile acids in vitro. Phenotypically, loss of miR-200c exacerbated cholestatic liver injury, including periductular fibrosis, intrahepatic inflammation, and biliary hyperplasia in both the BDL model and the 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) model. We identified sestrin 1 (SESN1) as a target of miR-200c. Sesn1^-/--BDL mice showed mitigation of cholestatic liver injury. On a molecular level, the pro-proliferative IL-6/AKT feedback loop was activated in Mir200c^-/- livers but was inhibited in Sesn1^-/- livers upon cholestasis in mice. Furthermore, rescuing expression of miR-200c by the adeno-associated virus serotype 8 ameliorated BDL-induced liver injury in Mir200c^-/- mice. Taken together, this study demonstrates that miR-200c restrains the proliferative and neuroendocrine-like activation of cholangiocytes by targeting SESN1 and inhibiting the IL-6/AKT feedback loop to protect against cholestatic liver fibrosis. Our findings provide mechanistic insights regarding biliary liver fibrosis, which may help to reveal novel therapeutic targets for the treatment of cholestatic liver injury and liver fibrosis.

Bile Acid Toxicity and Protein Kinases

If the bile acids reach to pathological concentrations due to cholestasis, accumulation of hydrophobic bile acids within the hepatocyte may result in cell death. Thus, hydrophobic bile acids induce apoptosis in hepatocytes, while hydrophilic bile acids increase intracellular adenosine 3',5'-monophosphate (cAMP) levels and activate mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways to protect hepatocytes from apoptosis.Two apoptotic pathways have been described in bile acids-induced death. Both are controlled by multiple protein kinase signaling pathways. In mitochondria-controlled pathway, caspase-8 is activated with death domain-independent manner, whereas, Fas-dependent classical pathway involves ligand-independent oligomerization of Fas.Hydrophobic bile acids dose-dependently upregulate the inflammatory response by further stimulating production of inflammatory cytokines. Death receptor-mediated apoptosis is regulated at the cell surface by the receptor expression, at the death-inducing signaling complex (DISC) by expression of procaspase-8, the death receptors Fas-associated death domain (FADD), and cellular FADD-like interleukin 1-beta (IL-1β)-converting enzyme (FLICE) inhibitory protein (cFLIP). Bile acids prevent cFLIP recruitment to the DISC and thereby enhance initiator caspase activation and lead to cholestatic apoptosis. At mitochondria, the expression of B-cell leukemia/lymphoma-2 (Bcl-2) family proteins contribute to apoptosis by regulating mitochondrial cytochrome c release via Bcl-2, Bcl-2 homology 3 (BH3) interacting domain death agonist (Bid), or Bcl-2 associated protein x (Bax). Fas receptor CD95 activation by hydrophobic bile acids is initiated by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species (ROS) signaling. However, activation of necroptosis by ligands of death receptors requires the kinase activity of receptor interacting protein1 (RIP1), which mediates the activation of RIP3 and mixed lineage kinase domain-like protein (MLKL). In this chapter, mainly the effect of protein kinases signal transduction on the mechanisms of hydrophobic bile acids-induced inflammation, apoptosis, necroptosis and necrosis are discussed.

Investigation of 23 Bile Acids in Liver Bile in Benign and Malignant Biliary Stenosis: A Pilot Study

Differential diagnosis between benign and malignant biliary stenosis can be difficult in clinical practice. Histology of biopsy specimens is often indeterminate. Laboratory markers (serum bilirubin > 75 μmol/L, carbohydrate antigen 19-9 > 400 U/mL) and the length of stenosis (>15 mm) can be helpful but are not specific enough. The aim of this study was to investigate bile acids in liver bile of patients with benign and malignant biliary stenosis and controls without stenosis. A total of 73 patients entered the study: 7 subjects with benign biliary stenosis (6 men, 1 woman; 68 ± 13 years old), 21 with malignant biliary stenosis (15 men, 6 women; 72 ± 14 years old), and 45 patients without biliary stenosis (22 men, 23 women; 70 ± 13 years old); out of those, 25 subjects have and 20 do not have choledocholithiasis. Twenty-three different bile acids were investigated by high-performance liquid chromatography/mass spectrometry. Serum total bilirubin was significantly higher in patients with malignant biliary stenosis compared with nonstenotic controls (p = 0.005). Significant relationship (r > 0.7) was found between several pairs of bile acids. Significantly lower bile acid concentrations in malignant biliary stenosis compared to controls without stenosis were found for GLCA (p = 0.032), GUDCA (p = 0.032), GCDCA (p = 0.006), GDCA (p = 0.031), GHCA (p = 0.005), TUDCA (p = 0.044), and TDCA (p = 0.036). Significant difference in cholic acid was found between benign and malignant stenosis (p = 0.022). Analysis of bile acids might be helpful in the differential diagnosis of malignant and benign biliary stenosis. More patients need to be enrolled in further studies so that the real diagnostic yield of bile acids can be determined.

Dual Role of Bile Acids on the Biliary Epithelium: Friend or Foe?

Bile acids are a family of amphipathic compounds predominantly known for their role in solubilizing and absorbing hydrophobic compounds (including liposoluble vitamins) in the intestine. Bile acids also are key signaling molecules and inflammatory agents that activate transcriptional factors and cell signaling pathways that regulate lipid, glucose, and energy metabolism in various human disorders, including chronic liver diseases. However, in the last decade increased awareness has been founded on the physiological and chemical heterogeneity of this category of compounds and their possible beneficial or injurious effects on the biliary tree. In this review, we provide an update on the current understanding of the molecular mechanism involving bile acid and biliary epithelium. The last achievements of the research in this field are summarized, focusing on the molecular aspects and the elements with relevance regarding human liver diseases.

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.

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.

The inhibition of Akt-Pdpk1 interaction efficiently suppresses the growth of murine primary liver tumor cells

The lack of primary liver tumor cells has hampered testing of potential chemotherapeutic agents in vitro. To overcome this issue we developed a primary mouse liver tumor cell line K07074. The K07074 cells were immortal, exhibited a biliary phenotype, formed colonies in soft agar and displayed an increase in Hedgehog, Notch and Akt signaling. To study the effect of single and combined inhibition of the liver tumor-related pathways on the growth of K07074 cells we treated these with small-molecule antitumor agents. While the inhibition of Akt and Notch pathways strongly inhibited the growth of K07074 cells the inhibition of Wnt and Hedgehog pathways was less efficient in cell growth suppression. Interestingly, the inhibition of Akt pathway at the level of Akt-Pdpk1 interaction was sufficient to suppress the growth of tumor cells and no significant additive effect could be detected when co-treated with the inhibitors of Wnt, Hedgehog or Notch pathways. Only when suboptimal doses of Akt-Pdpk1 interaction inhibitor NSC156529 were used an additive effect with Notch inhibition was seen. We conclude that the Akt pathway inhibitor NSC156529 is potentially useful as single treatment for liver tumors with hyperactivated Akt signaling.

Inhibition of mast cell-derived histamine secretion by cromolyn sodium treatment decreases biliary hyperplasia in cholestatic rodents

Cholangiopathies are characterized by dysregulation of the balance between biliary growth and loss. We have shown that histamine (HA) stimulates biliary growth via autocrine mechanisms. To evaluate the paracrine effects of mast cell (MC) stabilization on biliary proliferation, sham or BDL rats were treated by IP-implanted osmotic pumps filled with saline or cromolyn sodium (24 mg/kg BW/day (inhibits MC histamine release)) for 1 week. Serum, liver blocks and cholangiocytes were collected. Histidine decarboxylase (HDC) expression was measured using real-time PCR in cholangiocytes. Intrahepatic bile duct mass (IBDM) was evaluated by IHC for CK-19. MC number was determined using toluidine blue staining and correlated to IBDM. Proliferation was evaluated by PCNA expression in liver sections and purified cholangiocytes. We assessed apoptosis using real-time PCR and IHC for BAX. Expression of MC stem factor receptor, c-kit, and the proteases chymase and tryptase were measured by real-time PCR. HA levels were measured in serum by EIA. In vitro, MCs and cholangiocytes were treated with 0.1% BSA (basal) or cromolyn (25 μM) for up to 48 h prior to assessing HDC expression, HA levels and chymase and tryptase expression. Supernatants from MCs treated with or without cromolyn were added to cholangiocytes before measuring (i) proliferation by MTT assays, (ii) HDC gene expression by real-time PCR and (iii) HA release by EIA. In vivo, cromolyn treatment decreased BDL-induced: (i) IBDM, MC number, and biliary proliferation; (ii) HDC and MC marker expression; and (iii) HA levels. Cromolyn treatment increased cholangiocyte apoptosis. In vitro, cromolyn decreased HA release and chymase and tryptase expression in MCs but not in cholangiocytes. Cromolyn-treated MC supernatants decreased biliary proliferation and HA release. These studies provide evidence that MC histamine is key to biliary proliferation and may be a therapeutic target for the treatment of cholangiopathies.

Protective effects of rutin on liver injury induced by biliary obstruction in rats

Rutin has been shown to possess beneficial health effects, including hepatoprotection. However, to date, it has not been demonstrated to have a hepatoprotective effect against cholestatic liver injury. This is the first report to show a protective effect of rutin on cholestatic liver injury. Cholestasis was produced by bile duct ligation (BDL) in male Sprague-Dawley rats for 3 weeks. Daily oral administration of rutin was started 1 week before injury and was maintained for 4 weeks. In comparison with the control group, the BDL group showed liver injury as evidenced by histological changes and elevation in serum biochemicals, ductular reaction, fibrosis, inflammation, and oxidative stress. These pathophysiological changes were attenuated by rutin supplementation. Rutin alleviated BDL-induced transforming growth factor β1 (TGF-β1), interleukin-1β, connective tissue growth factor, and collagen expression. The antifibrotic effect of rutin was accompanied by reductions in α-smooth muscle actin-positive matrix-producing cells and Smad2/3 activity critical to the fibrogenic potential of TGF-β1. Rutin attenuated BDL-induced oxidative stress, leukocyte accumulation, NF-κB activation, and proinflammatory cytokine production. Further studies demonstrated an inhibitory effect of rutin on the redox-sensitive intracellular signaling molecule extracellular signal-regulated kinase (ERK). Rutin also attenuated BDL-induced reduction in NF-E2-related factor 2 (Nrf2), heme oxygenase-1 (HO-1), and AMP-activated protein kinase (AMPK). Taken together, the beneficial effects of rutin were shown to be associated with antioxidative and anti-inflammatory effects as well as the downregulation of NF-κB and TGF-β/Smad signaling, probably via interference of ERK activation and/or enhancement of Nrf2, HO-1, and AMPK activity.

Physiology of cholangiocytes

Cholangiocytes are epithelial cells that line the intra- and extrahepatic ducts of the biliary tree. The main physiologic function of cholangiocytes is modification of hepatocyte-derived bile, an intricate process regulated by hormones, peptides, nucleotides, neurotransmitters, and other molecules through intracellular signaling pathways and cascades. The mechanisms and regulation of bile modification are reviewed herein.

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.

Apoptosis of human cholangiocarcinoma cells induced by ESC-3 from Crocodylus siamensis bile

AIM: To investigate the effects of ESC-3 isolated from crocodile bile on the growth and apoptosis induction of human cholangiocarcinoma cells.

METHODS: ESC-3 was isolated from crocodile bile by Sephadex LH-20 and RP-18 reversed-phase column. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay was conducted to determine the effects of ESC-3 on the proliferation of human cholangiocarcinoma cell lines (QBC939, Sk-ChA-1 and MZ-ChA-1). Giemsa staining, Hoechst 33258 and acridine orange/ethidium bromide staining showed the morphological changes of Mz-ChA-1 cells exposed to ESC-3 at different concentrations. Flow cytometry with regular propidium iodide (PI) staining was performed to analyze the cell cycle distribution of Mz-ChA-1 cells and to assess apoptosis by annexin v-fluorescein isothiocyanate (V-FITC)/PI staining. Rh123 staining was used to detect the alteration of mitochondrial membrane potential (ΔΨm). The protein levels of Bax, Bcl-2, Cdk2, cytochrome c and caspase-3 were further confirmed by Western blotting.

RESULTS: ESC-3 significantly inhibited the growth of three human cholangiocarcinoma cell lines and arrested Mz-ChA-1 cell cycle at G0/G1 phase. Mz-ChA-1 cells showed typical apoptotic morphological changes after treated with ESC-3 (10 μg/mL) for 48 h. Cell death assay indicated that Mz-ChA-1 cells underwent apoptosis in a dose-dependent manner induced by ESC-3. In addition, ESC-3 treatment could downregulate the protein level of Bcl-2 and upregulate the Bax, leading to the increase in the ratio of Bax to Bcl-2 in Mz-ChA-1 cells. Meanwhile, cytochrome c was released from the mitochondria into the cytosol, which subsequently initiated the activation of caspase-3. All these events were associated with the collapse of the mitochondrial membrane potential.

CONCLUSION: ESC-3, the active ingredient of crocodile bile, induced apoptosis in Mz-ChA-1 cells through the mitochondria-dependent pathway and may be a potential chemotherapeutic drug for the treatment of cholangiocarcinoma.

Beneficial effect of docosahexaenoic acid on cholestatic liver injury in rats

Bile duct obstruction and subsequent cholestasis are associated with hepatocellular injury, cholangiocyte proliferation, stellate cell activation, Kupffer cell activation, oxidative stress, inflammation and fibrosis. Docosahexaenoic acid (DHA) is an essential polyunsaturated fatty acid that has been shown to possess health beneficial effects, including hepatoprotection. However, the molecular mechanism of DHA-mediated hepatoprotection is not fully understood. In the present study, we report the protective effect of DHA on cholestatic liver injury. Cholestasis was produced by bile duct ligation (BDL) in male Sprague-Dawley rats for 3 weeks. Daily administration of DHA was started 2 weeks before injury and lasted for 5 weeks. In comparison with the control group, the BDL group showed hepatic damage as evidenced by histological changes and elevation in serum biochemicals, ductular reaction, fibrosis, inflammation and oxidative stress. These pathophysiological changes were attenuated by chronic DHA supplementation. DHA alleviated BDL-induced transforming growth factor beta-1 (TGF-β1), intereukin-1beta, connective tissue growth factor and collagen expression. The anti-fibrotic effect of DHA was accompanied by reductions in α-smooth muscle actin-positive matrix-producing cells and Smad 2/3 activity critical to the fibrogenic potential of TGF-β1. DHA also attenuated BDL-induced leukocyte accumulation and nuclear factor-κB (NF-κB) activation. Further studies demonstrated an inhibitory effect of DHA on redox-sensitive intracellular signaling molecule extracellular signal-regulated kinase (ERK). Taken together, the hepatoprotective, anti-inflammatory and anti-fibrotic effects of DHA seem to be multifactorial. The beneficial effects of chronic DHA supplementation are associated with anti-oxidative and anti-inflammatory potential as well as down-regulation of NF-κB and transforming growth factor beta/Smad signaling probably via interference with ERK activation.

Mechanisms of oxysterol-induced carcinogenesis

Oxysterols are oxidation products of cholesterol that are generated by enzymatic reactions mediated by cytochrome P450 family enzymes or by non-enzymatic reactions involving reactive oxygen and nitrogen species. Oxysterols play various regulatory roles in normal cellular processes such as cholesterol homeostasis by acting as intermediates in cholesterol catabolism. Pathological effects of oxysterols have also been described, and various reports have implicated oxysterols in several disease states, including atherosclerosis, neurological disease, and cancer. Numerous studies show that oxysterols are associated with various types of cancer, including cancers of the colon, lung, skin, breast and bile ducts. The molecular mechanisms whereby oxysterols contribute to the initiation and progression of cancer are an area of active investigation. This review focuses on the current state of knowledge regarding the role of oxysterols in carcinogenesis. Mutagenicity of oxysterols has been described in both nuclear and mitochondrial DNA. Certain oxysterols such as cholesterol-epoxide and cholestanetriol have been shown to be mutagenic and genotoxic. Oxysterols possess pro-oxidative and pro-inflammatory properties that can contribute to carcinogenesis. Oxysterols can induce the production of inflammatory cytokines such as interleukin-8 and interleukin-1β. Certain oxysterols are also involved in the induction of cyclo-oxygenase-2 expression. Inflammatory effects can also be mediated through the activation of liver-X-receptor, a nuclear receptor for oxysterols. Thus, several distinct molecular mechanisms have been described showing that oxysterols contribute to the initiation and progression of cancers arising in various organ systems.

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.

Clinical implications of novel aspects of biliary pathophysiology

Cholangiocytes are the epithelial cells that line the biliary tree; they are the target of chronic diseases termed cholangiopathies, which represent a daily challenge for clinicians, since definitive medical treatments are not available yet. It is generally accepted that the progression of injury in the course of cholangiopathies, and promotion and progression of cholangiocarcinoma are at least in part due to the failure of the cholangiocytes' mechanisms of adaptation to injury. Recently, several studies on the pathophysiology of the biliary epithelium have shed some light on the mechanisms that govern cholangiocyte response to injury. These studies provide novel information to help interpret some of the clinical aspects of cholangiopathies and cholangiocarcinoma; the purpose of this review is thus to describe some of these novel findings, focusing on their significance from a clinical perspective.

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.

Bile-acid-induced cell injury and protection

Several studies have characterized the cellular and molecular mechanisms of hepatocyte injury caused by the retention of hydrophobic bile acids (BAs) in cholestatic diseases. BAs may disrupt cell membranes through their detergent action on lipid components and can promote the generation of reactive oxygen species that, in turn, oxidatively modify lipids, proteins, and nucleic acids, and eventually cause hepatocyte necrosis and apoptosis. Several pathways are involved in triggering hepatocyte apoptosis. Toxic BAs can activate hepatocyte death receptors directly and induce oxidative damage, thereby causing mitochondrial dysfunction, and induce endoplasmic reticulum stress. When these compounds are taken up and accumulate inside biliary cells, they can also cause apoptosis. Regarding extrahepatic tissues, the accumulation of BAs in the systemic circulation may contribute to endothelial injury in the kidney and lungs. In gastrointestinal cells, BAs may behave as cancer promoters through an indirect mechanism involving oxidative stress and DNA damage, as well as acting as selection agents for apoptosis-resistant cells. The accumulation of BAs may have also deleterious effects on placental and fetal cells. However, other BAs, such as ursodeoxycholic acid, have been shown to modulate BA-induced injury in hepatocytes. The major beneficial effects of treatment with ursodeoxycholic acid are protection against cytotoxicity due to more toxic BAs; the stimulation of hepatobiliary secretion; antioxidant activity, due in part to an enhancement in glutathione levels; and the inhibition of liver cell apoptosis. Other natural BAs or their derivatives, such as cholyl-N-methylglycine or cholylsarcosine, have also aroused pharmacological interest owing to their protective properties.

Control of cholangiocyte adaptive responses by visceral hormones and neuropeptides

Cholangiocytes, the epithelial cells lining the biliary tree, are the target cells in several liver diseases, termed cholangiopathies. Cholangiopathies are a challenge for clinicians and an enigma for scientists, as the pathogenetic mechanisms by which they develop, and the therapeutic tools for these diseases are still undefined. Several studies demonstrate that many visceral hormones, neuropeptides, and neurotransmitters modulate the adaptive changes of cholangiocytes to chronic cholestatic injury. The aim of this review is to present the recent findings that contributed to clarify the role of visceral hormones and neuropeptides in the regulation of the pathophysiology of cholestasis. These studies helped to shed light on some aspects of cholangiocyte pathophysiology, revealing novel perspectives for the clinical managements of cholangiopathies.

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

Cholangiocytes are the target cells in cholestatic models of ductal hyperplasia including bile duct ligation (BDL). We have shown that: (i) cholangiocytes express VEGFR-2 and VEGFR-3; (ii) VEGF-A and VEGF-C stimulate cholangiocyte proliferation via an autocrine mechanism; and (iii) chronic administration of VEGF-A prevents cholangiocyte damage induced by hepatic artery ligation. Caffeic acid phenethyl ester (CAPE) induces growth inhibition in different cells. Taurocholic acid (TC) protects cholangiocytes against injury induced by parasympathetic or sympathetic denervation. The aims of this study were to determine if: (i) CAPE induces bile duct damage; and (ii) TC prevents CAPE-induced bile duct damage by increasing cholangiocyte VEGF expression.

METHODS

Normal and BDL rats (immediately after surgery) were fed 1% TC or control diet in the absence/presence of daily IP injections of CAPE (10 mg/Kg BW). One week later, we evaluated: (i) cholangiocyte apoptosis, proliferation and ductal mass in liver sections; (ii) functional activity by measuring secretin-stimulated bile and bicarbonate secretion; and (iii) VEGF-A/C and VEGFR-2/R-3 expression in liver sections. In vitro, BDL cholangiocytes were exposed to CAPE (40 microM) in the absence/presence of TC (40 microM) with and without pretreatment with VEGF receptor inhibitors before evaluating cholangiocyte apoptosis and proliferation.

RESULTS

Chronic CAPE administration to BDL rats increased cholangiocyte apoptosis and decreased ductal mass. This effect was associated with reduced expression of VEGF-A, VEGF-C, VEGFR-2 and VEGFR-3. In vivo, TC feeding partly prevented CAPE-induced changes in cholangiocyte apoptosis and growth and loss of ductal secretion. The protective effect of TC was associated with enhanced VEGF-A, VEGF-C, VEGFR-2 and VEGFR-3. In vitro, TC partially prevented CAPE-induced increases in apoptosis and decreases in cholangiocyte proliferation. These changes were reversed by pretreatment with VEGF-receptor inhibitors.

CONCLUSION

Manipulation of cholangiocyte VEGF expression by bile acids may be important in preventing the impairment of cholangiocyte proliferation by exogenous agents.

Altered bile composition after liver transplantation is associated with the development of nonanastomotic biliary strictures

BACKGROUND/AIMS

Nonanastomotic biliary strictures are troublesome complications after liver transplantation. The pathogenesis of NAS is not completely clear, but experimental studies suggest that bile salt toxicity is involved.

METHODS

In one hundred and eleven adult liver transplants, bile samples were collected daily posttransplantation for determination of bile composition. Expression of bile transporters was studied perioperatively.

RESULTS

Nonanastomotic biliary strictures were detected in 14 patients (13%) within one year after transplantation. Patient and donor characteristics and postoperative serum liver enzymes were similar between patients who developed nonanastomotic biliary strictures and those who did not. Secretions of bile salts, phospholipids and cholesterol were significantly lower in patients who developed strictures. In parallel, biliary phospholipids/bile salt ratio was lower in patients developing strictures, suggestive for increased bile cytotoxicity. There were no differences in bile salt pool composition or in hepatobiliary transporter expression.

CONCLUSIONS

Although patients who develop nonanastomotic biliary strictures are initially clinically indiscernible from patients who do not develop nonanastomotic biliary strictures, the biliary bile salts and phospholipids secretion, as well as biliary phospholipids/bile salt ratio in the first week after transplantation, was significantly lower in the former group. This supports the concept that bile cytotoxicity is involved in the pathogenesis of nonanastomotic biliary strictures.

Endocrine and paracrine role of bile acids

Bile acids are not only important for the absorption of dietary lipids and fat soluble vitamins but are signalling molecules with diverse endocrine and paracrine functions. Bile acids regulate bile acid, lipid and glucose metabolism and modulate temperature and energy homeostasis. Furthermore, bile acids can not only promote cell proliferation and liver regeneration but can also induce programmed cell death. Bile acid functions are mediated through different pathways which comprise the activation of nuclear hormone receptors, of intracellular kinases and of the plasma membrane-bound, G-protein coupled bile acid receptor TGR5/Gpbar-1.

Non invasive in vivo investigation of hepatobiliary structure and function in STII medaka (Oryzias latipes): methodology and applications

Background

A novel transparent stock of medaka (Oryzias latipes; STII), recessive for all pigments found in chromatophores, permits transcutaneous imaging of internal organs and tissues in living individuals. Findings presented describe the development of methodologies for non invasive in vivo investigation in STII medaka, and the successful application of these methodologies to in vivo study of hepatobiliary structure, function, and xenobiotic response, in both 2 and 3 dimensions.

Results

Using brightfield, and widefield and confocal fluorescence microscopy, coupled with the in vivo application of fluorescent probes, structural and functional features of the hepatobiliary system, and xenobiotic induced toxicity, were imaged at the cellular level, with high resolution (< 1 μm), in living individuals. The findings presented demonstrate; (1) phenotypic response to xenobiotic exposure can be investigated/imaged in vivo with high resolution (< 1 μm), (2) hepatobiliary transport of solutes from blood to bile can be qualitatively and quantitatively studied/imaged in vivo, (3) hepatobiliary architecture in this lower vertebrate liver can be studied in 3 dimensions, and (4) non invasive in vivo imaging/description of hepatobiliary development in this model can be investigated.

Conclusion

The non-invasive in vivo methodologies described are a unique means by which to investigate biological structure, function and xenobiotic response with high resolution in STII medaka. In vivo methodologies also provide the future opportunity to integrate molecular mechanisms (e.g., genomic, proteomic) of disease and toxicity with phenotypic changes at the cellular and system levels of biological organization. While our focus has been the hepatobiliary system, other organ systems are equally amenable to in vivo study, and we consider the potential for discovery, within the context of in vivo investigation in STII medaka, as significant.

Non invasive high resolution in vivo imaging of alpha-naphthylisothiocyanate (ANIT) induced hepatobiliary toxicity in STII medaka

A novel transparent stock of medaka (Oryzias latipes; STII), homozygous recessive for all four pigments (iridophores, xanthophores, leucophores, melanophores), permits transcutaneous, high resolution (<1 microm) imaging of internal organs and tissues in living individuals. We applied this model to in vivo investigation of alpha -naphthylisothiocyanate (ANIT) induced hepatobiliary toxicity. Distinct phenotypic responses to ANIT involving all aspects of intrahepatic biliary passageways (IHBPs), particularly bile preductular epithelial cells (BPDECs), associated with transitional passageways between canaliculi and bile ductules, were observed. Alterations included: attenuation/dilation of bile canaliculi, bile preductular lesions, hydropic vacuolation of hepatocytes and BPDECs, mild BPDEC hypertrophy, and biliary epithelial cell (BEC) hyperplasia. Ex vivo histological, immunohistochemical, and ultrastructural studies were employed to aid in interpretation of, and verify, in vivo findings. 3D reconstructions from in vivo investigations provided quantitative morphometric and volumetric evaluation of ANIT exposed and untreated livers. The findings presented show for the first time in vivo evaluation of toxicity in the STII medaka hepatobiliary system, and, in conjunction with prior in vivo work characterizing normalcy, advance our comparative understanding of this lower vertebrate hepatobiliary system and its response to toxic insult.

Novel interaction of bile acid and neural signaling in the regulation of cholangiocyte function

Cholangiocytes, the epithelial cells that line the intrahepatic biliary tree, are the target of cholangiopathies, a wide array of chronic disorders that are characterized by the progressive vanishing of bile ducts, leading to ductopenia and liver failure. The loss of bile ducts is a consequence of cholangiocyte death by apoptosis and impaired proliferative response of these cells to injury. The factors that regulate cholangiocyte proliferation and survival are poorly understood. In this regard, a major role is played by the interaction between bile acids and the autonomic nervous system. It has been shown that adrenergic and cholinergic denervation of the liver results in the induction of cell death and impaired proliferative responses of the biliary epithelium to cholestasis. In addition,bile acids have been shown to enter cholangiocytes through the apical, Na(+)-dependent bile acid transporter, ASBT, which has a marked impact on cholangiocyte pathobiology. Recent evidence shows that bile acids and autonomic innervation interact in modulating cholangiocyte response to liver injury. In this review, we describe the recent advances in understanding the molecular mechanisms by which such events occur.

Glucagon-like peptide-1 and its receptor agonist exendin-4 modulate cholangiocyte adaptive response to cholestasis

BACKGROUND & AIMS

Cholangiopathies are characterized by progressive dysregulation of the balance between proliferation and death of cholangiocytes. In the course of cholestasis, cholangiocytes undergo a neuroendocrine transdifferentiation and their biology is regulated by neuroendocrine hormones. Glucagon-like peptide-1 (GLP-1), secreted by neuroendocrine cells, sustains beta-cell survival in experimental diabetes and induces the neuroendocrine transdifferentiation of pancreatic ductal cells. GLP-1 receptor (GLP-1R) selective agonist exendin-4 is used in humans as a novel therapeutic tool for diabetes. The aim of this study was to define if GLP-1 modulates cholangiocyte biologic response to cholestasis.

METHODS

Expression of GLP-1R in cholangiocytes was determined. Effects on cholangiocyte proliferation of the in vitro and in vivo exposure to GLP-1 or exendin-4, together with the intracellular signals, were then studied. Synthesis of GLP-1 by cholangiocytes and the effects of GLP-1R blockage on their growth were also determined.

RESULTS

Cholangiocytes express the GLP-1 receptor, which is up-regulated in the course of cholestasis. GLP-1 and exendin-4 increase cholangiocyte growth both in vitro and in vivo. The GLP-1R signal is mediated by the phosphatidyl-inositol-3-kinase, cAMP/Protein Kinase A, and Ca(2+)-CamKIIalpha but not by the ERK1/2 and PKCalpha pathways. Proliferating cholangiocytes synthesize GLP-1: neutralization of its action by GLP-1R antagonist blunts cholangiocyte response to cholestasis.

CONCLUSIONS

GLP-1 is required for the cholangiocyte adaptive response to cholestasis. Cholangiocytes are susceptible to the activation of GLP-1R and respond with increased proliferation and functional activity. Exendin-4 availability for employment in humans and these data may open novel perspectives for the medical treatment of cholangiopathies.

Bile salts regulate proliferation and apoptosis of liver cells by modulating the IGF1 system

BACKGROUND

In different cell types, the insulin-like growth factor 1 and its receptor modulate growth, apoptosis and damage repair in cooperation with estrogen receptors.

AIM

To evaluate the involvement of the insulin-like growth factor 1 system and estrogen receptors in bile salts modulation of apoptosis/proliferation of hepatocytes and cholangiocytes. Primary cultures of rat hepatocytes and cholangiocytes were exposed to glycochenodeoxycholate or tauro-CDC in the presence or absence of insulin-like growth factor 1 receptor blocking antibody (alphaIR3), small interfering RNA for insulin-like growth factor 1, 17beta-estradiol or estrogen receptor antagonist (ICI 182,780). Proliferation was evaluated by proliferating cell nuclear antigen Western blot and apoptosis by measuring caspase-3 activity or annexin-V.

RESULTS

In hepatocytes, the insulin-like growth factor 1 receptor blocker enhanced glycochenodeoxycholate-induced apoptosis and caused tauro-CDC to promote apoptosis. 17Beta-estradiol or the estrogen receptor antagonist (ICI 182,780) did not influence the apoptotic effect of glycochenodeoxycholate. In cholangiocytes, both glycochenodeoxycholate and tauro-CDC induced proliferation at 100microM, while they induced apoptosis at 1mM with a more pronounced effect of glycochenodeoxycholate. Apoptosis induced by 1mM glycochenodeoxycholate or tauro-CDC in cholangiocytes was enhanced by blocking insulin-like growth factor 1 receptor or by silencing insulin-like growth factor 1. 17Beta-estradiol counteracts glycochenodeoxycholate-induced cholangiocyte apoptosis by enhancing insulin-like growth factor 1 secretion and activating the insulin-like growth factor 1 system.

CONCLUSIONS

Modulation of the IGF1 system could represent a potential strategy for the management of bile salts-induced liver injury.

Cytoprotective effects of taurocholic acid feeding on the biliary tree after adrenergic denervation of the liver

BACKGROUND

Cholangiopathies impair the balance between proliferation and apoptosis of cholangiocytes leading to the disappearance of bile ducts and liver failure. Taurocholic acid (TC) is essential for cholangiocyte proliferative and functional response to cholestasis. Bile acids and neurotransmitters co-operatively regulate the biological response of the biliary epithelium to cholestasis. Adrenergic denervation of the liver during cholestasis results in the damage of bile ducts.

AIM

To verify whether TC feeding prevents the damage of the biliary tree induced by adrenergic denervation in the course of cholestasis.

METHODS

Rats subjected to bile duct ligation (BDL) and to adrenergic denervation were fed a TC-enriched diet, in the absence or presence of daily administration of the phosphatidyl-inositol-3-kinase (PI3K) inhibitor wortmannin for 1 week.

RESULTS

TC prevented the induction of cholangiocyte apoptosis induced by adrenergic denervation. TC also restored cholangiocyte proliferation and functional activity, reduced after adrenergic denervation. TC prevented AKT dephosphorylation induced by adrenergic denervation. The cytoprotective effects of TC were abolished by the simultaneous administration of wortmannin.

SUMMARY/CONCLUSIONS

TC administration prevents the damage of the biliary tree induced by the adrenergic denervation of the liver. These novel findings open novel perspectives in the understanding of the potential of bile acids especially in post-transplant liver disease.

Proliferating cholangiocytes: a neuroendocrine compartment in the diseased liver

In the last 15 years, the intrahepatic biliary tree has become the object of extensive studies, which highlighted the extraordinary biologic properties of cholangiocytes involved in bile formation, proliferation, injury repair, fibrosis, angiogenesis, and regulation of blood flow. Proliferation is a "typical" property of cholangiocytes and is key as a mechanism of repair responsible for maintaining the integrity of the biliary tree. Cholangiocyte proliferation occurs virtually in all pathologic conditions of liver injury where it is associated with inflammation, regeneration, and repair, thus conditioning the evolution of liver damage. Interestingly, proliferating cholangiocytes acquire the phenotype of neuroendocrine cells, and secrete different cytokines, growth factors, neuropeptides, and hormones, which represent potential mechanisms for cross talk with other liver cells. Many studies suggest the generation of a neuroendocrine compartment in the injured liver, mostly constituted by cells with cholangiocyte features, which functionally conditions the progression of liver disease. These insights on cholangiocyte pathophysiology will provide new potential strategies for the management of chronic liver diseases. The purpose of this review is to summarize the recent findings on the mechanisms regulating cholangiocyte proliferation and the significance of the neuroendocrine regulation of cholangiocyte biology.

Administration of r-VEGF-A prevents hepatic artery ligation-induced bile duct damage in bile duct ligated rats

The hepatic artery, through the peribiliary plexus, nourishes the intrahepatic biliary tree. During obstructive cholestasis, the nutritional demands of intrahepatic bile ducts are increased as a consequence of enhanced proliferation; in fact, the peribiliary plexus (PBP) displays adaptive expansion. The effects of hepatic artery ligation (HAL) on cholangiocyte functions during cholestasis are unknown, although ischemic lesions of the biliary tree complicate the course of transplanted livers and are encountered in cholangiopathies. We evaluated the effects of HAL on cholangiocyte functions in experimental cholestasis induced by bile duct ligation (BDL). By using BDL and BDL + HAL rats or BDL + HAL rats treated with recombinant-vascular endothelial growth factor-A (r-VEGF-A) for 1 wk, we evaluated liver morphology, the degree of portal inflammation and periductular fibrosis, microcirculation, cholangiocyte apoptosis, proliferation, and secretion. Microcirculation was evaluated using a scanning electron microscopy vascular corrosion cast technique. HAL induced in BDL rats 1) the disappearance of the PBP, 2) increased apoptosis and impaired cholangiocyte proliferation and secretin-stimulated ductal secretion, and 3) decreased cholangiocyte VEGF secretion. The effects of HAL on the PBP and cholangiocyte functions were prevented by r-VEGF-A, which, by maintaining the integrity of the PBP and cholangiocyte proliferation, prevents damage of bile ducts following ischemic injury.

Bile acid interactions with cholangiocytes

Cholangiocytes are exposed to high concentrations of bile acids at their apical membrane. A selective transporter for bile acids, the Apical Sodium Bile Acid Cotransporter (ASBT) (also referred to as Ibat; gene name Slc10a2) is localized on the cholangiocyte apical membrane. On the basolateral membrane, four transport systems have been identified (t-ASBT, multidrug resistance (MDR)3, an unidentified anion exchanger system and organic solute transporter (Ost) heteromeric transporter, Ostα-Ostβ. Together, these transporters unidirectionally move bile acids from ductal bile to the circulation. Bile acids absorbed by cholangiocytes recycle via the peribiliary plexus back to hepatocytes for re-secretion into bile. This recycling of bile acids between hepatocytes and cholangiocytes is referred to as the cholehepatic shunt pathway. Recent studies suggest that the cholehepatic shunt pathway may contribute in overall hepatobiliary transport of bile acids and to the adaptation to chronic cholestasis due to extrahepatic obstruction. ASBT is acutely regulated by an adenosine 3', 5’-monophosphate (cAMP)-dependent translocation to the apical membrane and by phosphorylation-dependent ubiquitination and proteasome degradation. ASBT is chronically regulated by changes in gene expression in response to biliary bile acid concentration and inflammatory cytokines. Another potential function of cholangiocyte ASBT is to allow cholangiocytes to sample biliary bile acids in order to activate intracellular signaling pathways. Bile acids trigger changes in intracellular calcium, protein kinase C (PKC), phosphoinositide 3-kinase (PI3K), mitogen-activated protein (MAP) kinase and extracellular signal-regulated protein kinase (ERK) intracellular signals. Bile acids significantly alter cholangiocyte secretion, proliferation and survival. Different bile acids have differential effects on cholangiocyte intracellular signals, and in some instances trigger opposing effects on cholangiocyte secretion, proliferation and survival. Based upon these concepts and observations, the cholangiocyte has been proposed to be the principle target cell for bile acids in the liver.

Nervous and Neuroendocrine regulation of the pathophysiology of cholestasis and of biliary carcinogenesis

Cholangiocytes, the epithelial cells lining the biliary ducts, are the target cells in several liver diseases. Cholangiopathies and cholangiocarcinoma generate interest in many scientists since the genesis. The developing mechanisms, and the therapeutic tools of these diseases are still undefined. Several studies demonstrate that many hormones, neuropeptides and neurotransmitters regulate malignant and non-malignant cholangiocyte pathophysiology in the course of chronic biliary diseases. The aim of this review is to present the findings of several studies published in the recent years that contributed to clarifying the role of nervous and neuroendocrine regulation of the pathophysiologic events associated with cholestasis and cholangiocarcinoma development. This manuscript is organized into two parts. The first part offers an overview of the innervation of the liver and the origin of neuroendocrine hormones, neurotransmitters and neuropeptides affecting cholangiocyte function and metabolism. The first section also reviews the effects played by several neuroendocrine hormones and nervous system on cholangiocyte growth, survival and functional activity in the course of cholestasis. In the second section, we summarize the results of some studies describing the role of nervous system and neuroendocrine hormones in the regulation of malignant cholangiocyte growth.

Heterogeneity of the intrahepatic biliary epithelium

The objectives of this review are to outline the recent findings related to the morphological heterogeneity of the biliary epithelium and the heterogeneous pathophysiological responses of different sized bile ducts to liver gastrointestinal hormones and peptides and liver injury/toxins with changes in apoptotic, proliferative and secretory activities. The knowledge of biliary function is rapidly increasing because of the recognition that biliary epithelial cells (cholangiocytes) are the targets of human cholangiopathies, which are characterized by proliferation/damage of bile ducts within a small range of sizes. The unique anatomy, morphology, innervation and vascularization of the biliary epithelium are consistent with function of cholangiocytes within different regions of the biliary tree. The in vivo models [e.g., bile duct ligation (BDL), partial hepatectomy, feeding of bile acids, carbon tetrachloride (CCl₄) or α-naphthylisothiocyanate (ANIT)] and the in vivo experimental tools [e.g., freshly isolated small and large cholangiocytes or intrahepatic bile duct units (IBDU) and primary cultures of small and large murine cholangiocytes] have allowed us to demonstrate the morphological and functional heterogeneity of the intrahepatic biliary epithelium. These models demonstrated the differential secretory activities and the heterogeneous apoptotic and proliferative responses of different sized ducts. Similar to animal models of cholangiocyte proliferation/injury restricted to specific sized ducts, in human liver diseases bile duct damage predominates specific sized bile ducts. Future studies related to the functional heterogeneity of the intrahepatic biliary epithelium may disclose new pathophysiological treatments for patients with cholangiopathies.

Cholangiocyte anion exchange and biliary bicarbonate excretion

Primary canalicular bile undergoes a process of fluidization and alkalinization along the biliary tract that is influenced by several factors including hormones, innervation/neuropeptides, and biliary constituents. The excretion of bicarbonate at both the canaliculi and the bile ducts is an important contributor to the generation of the so-called bile-salt independent flow. Bicarbonate is secreted from hepatocytes and cholangiocytes through parallel mechanisms which involve chloride efflux through activation of Cl^- channels, and further bicarbonate secretion via AE2/SLC4A2-mediated Cl^-/HCO₃^- exchange. Glucagon and secretin are two relevant hormones which seem to act very similarly in their target cells (hepatocytes for the former and cholangiocytes for the latter). These hormones interact with their specific G protein-coupled receptors, causing increases in intracellular levels of cAMP and activation of cAMP-dependent Cl^- and HCO₃^- secretory mechanisms. Both hepatocytes and cholangiocytes appear to have cAMP-responsive intracellular vesicles in which AE2/SLC4A2 colocalizes with cell specific Cl^- channels (CFTR in cholangiocytes and not yet determined in hepatocytes) and aquaporins (AQP8 in hepatocytes and AQP1 in cholangiocytes). cAMP-induced coordinated trafficking of these vesicles to either canalicular or cholangiocyte lumenal membranes and further exocytosis results in increased osmotic forces and passive movement of water with net bicarbonate-rich hydrocholeresis.

Bile salt exposure increases proliferation through p38 and ERK MAPK pathways in a non-neoplastic Barrett's cell line

Bile reflux has been implicated in the neoplastic progression of Barrett's esophagus (BE). Bile salts increase proliferation in a Barrett's-associated adenocarcinoma cell line (SEG-1 cells) by activating ERK and p38 MAPK pathways. However, it is not clear that these findings in cancer cells are applicable to non-neoplastic cells of benign BE. We examined the effect of bile salts on three human cell lines: normal esophageal squamous (NES) cells, non-neoplastic Barrett's cells (BAR cells), and SEG-1 cells. We hypothesized that bile salt exposure activates proproliferative and antiapoptotic pathways to promote increased growth in BE. NES, BAR, and SEG-1 cells were exposed to glycochenodeoxycholic acid (GCDA) at a neutral pH for 5 min. Proliferation was measured by Coulter counter cell counts and a 5-bromo-2'-deoxyuridine (BrdU) incorporation assay. GCDA-induced MAPK activation was examined by Western blot analysis for phosphorylated ERK and p38. Apoptosis was measured by TdT-mediated dUTP nick-end labeling and annexin V staining after GCDA and UV-B exposure. Statistical significance was determined by ANOVA. NES cells exposed to 5 min of GCDA did not increase cell number. In BAR cells, GCDA exposure increased cell number by 31%, increased phosphorylated p38 and ERK levels by two- to three-fold, increased BrdU incorporation by 30%, and decreased UV-induced apoptosis by 15-20%. In conclusion, in a non-neoplastic Barrett's cell line, GCDA exposure induces proliferation by activation of both ERK and p38 MAPK pathways. These findings suggest a potential mechanism whereby bile reflux may facilitate the neoplastic progression of BE.

Tannic acid inhibits cholangiocyte proliferation after bile duct ligation via a cyclic adenosine 5',3'-monophosphate-dependent pathway

Chronic cholestatic diseases are characterized by morphological changes involving cholangiocyte proliferation and functional alterations of secretory capacity. The plant polyphenol tannic acid inhibits the growth of malignant human cholangiocytes. However, the mechanisms by which tannic acid limits excessive cholangiocyte proliferation are unknown. In this study we assessed the effect of tannic acid on cholangiocyte proliferation after bile duct ligation in rats. Tannic acid feeding decreased cholangiocyte proliferation and ductal mass in vivo after bile duct ligation. These changes were associated with functional changes in bile secretion and with decreases of intracellular cyclic adenosine 5',3'-monophosphate. The anti-proliferative effect of tannic acid was associated with a reduction of ERK1,2 phosphorylation. Additionally, tannic acid feeding decreased protein kinase A phosphorylation and activity. Similar changes were observed in isolated cholangiocytes during in vitro incubation with tannic acid. Furthermore, forskolin abolished the anti-proliferative effect of tannic acid on cholangiocyte proliferation after bile duct ligation. In conclusion, the anti-proliferative effects of tannic acid in cholangiocytes involve modulation of ERK1,2 by a cyclic adenosine 5',3'-monophosphate-protein kinase A-dependent pathway. These data suggest that tannic acid may be useful in limiting excessive cholangiocyte proliferation and modulating secretion during cholestasis.

Bile duct proliferation associated with bile salt-induced hypercholeresis in Mdr2 P-glycoprotein-deficient mice

BACKGROUND/AIMS

Bile flow consists of bile salt-dependent bile flow (BSDF), generated by canalicular secretion of bile salts, and bile salt-independent flow (BSIF), probably of combined canalicular and ductular origin. Bile salt transport proteins have been identified in cholangiocytes, suggesting a role in control of BSDF and/or in control of bile salt synthesis through cholehepatic shunting.

METHODS

We studied effects of bile duct proliferation under non-cholestatic conditions in multidrug resistance-2 P-glycoprotein (Abcb4)-deficient multidrug resistance gene-2 (Mdr2(-/-)) mice. BSDF and BSIF were determined in wild-type and Mdr2(-/-) mice during infusion of step-wise increasing dosages of tauroursodeoxycholate (TUDC). Cholate synthesis rate was determined by 2H4-cholate dilution. Results were related to expression of transport proteins in liver and intestine.

RESULTS

During TUDC infusion, BSDF was increased by approximately 50% and BSIF by approximately 100% in Mdr2(-/-) mice compared with controls. Cholate synthesis rate was unaffected in Mdr2(-/-) mice. Hepatic expression of the apical sodium-dependent bile salt transporter (Asbt), its truncated form (tAsbt) and the multidrug resistance-related protein 3 were upregulated in Mdr2(-/-) mice.

CONCLUSIONS

Bile duct proliferation in Mdr2(-/-) mice enhances cholehepatic shunting of bile salts, which is associated with a disproportionally high bile flow but does not affect bile salt synthesis.

Secretin activation of the apical Na+-dependent bile acid transporter is associated with cholehepatic shunting in rats

The role of the cholangiocyte apical Na(+)-dependent bile acid transporter (ASBT) in bile formation is unknown. Bile acid absorption by bile ducts results in cholehepatic shunting, a pathway that amplifies the canalicular osmotic effects of bile acids. We tested in isolated cholangiocytes if secretin enhances ASBT translocation to the apical membrane from latent preexisting intracellular stores. In vivo, in bile duct-ligated rats, we tested if increased ASBT activity (induced by secretin pretreatment) results in cholehepatic shunting of bile acids. We determined the increment in taurocholate-dependent bile flow and biliary lipid secretion and taurocholate (TC) biliary transit time during high ASBT activity. Secretin stimulated colchicine-sensitive ASBT translocation to the cholangiocyte plasma membrane and (3)H-TC uptake in purified cholangiocytes. Consistent with increased ASBT promoting cholehepatic shunting, with secretin pretreatment, we found TC induced greater-than-expected biliary lipid secretion and bile flow and there was a prolongation of the TC biliary transit time. Colchicine ablated secretin pretreatment-dependent bile acid-induced choleresis, increased biliary lipid secretion, and the prolongation of the TC biliary transit. In conclusion, secretin stimulates cholehepatic shunting of conjugated bile acids and is associated with increased cholangiocyte apical membrane ASBT. Bile acid transport by cholangiocyte ASBT can contribute to hepatobiliary secretion in vivo.

Pathophysiology of cholangiopathies

The diseases of the intrahepatic biliary tree are a large group of potentially evolutive congenital and acquired liver disorders affecting both the adult and pediatric populations. They represent a relevant cause of liver-related morbidity and mortality and an important indication for liver transplantation, particularly in children. While the practical approach to patients affected by biliary tree diseases has not significantly changed yet, the conceptual approach to the pathophysiology of cholangiopathies has witnessed important advances that will be discussed. The primary cell target of the pathogenetic sequence of these disorders is the biliary epithelium. Cholangiocytes have multifaceted functions, not limited to bile production. Their capability to secrete a range of different pro-inflammatory mediators, cytokines, and chemokines indicates a major role of cholangiocytes in the inflammatory reaction. Furthermore, paracrine secretion of growth factors and peptides mediates an extensive cross-talk with other liver cell types, including hepatocytes, stellate, and endothelial and inflammatory cells. Cholangiopathies share a number of pathogenetic mechanisms, including inflammation, cholestasis, fibrosis, apoptosis, altered development, and neoplastic transformation. These basic disease mechanisms will be discussed in detail, along with the distinct features of a number of cholangiopathies. Furthermore, an increase in the biliary cell compartment is a common response to many forms of liver injury, from cholangiopathies to viral and fulminant hepatitis. Elucidation of these pathophysiologic mechanisms will likely provide clues for future therapeutic strategies. Furthermore, understanding the role of cholangiocytes in liver regeneration/repair and the mechanisms of cholangiocyte activation and their relationship with liver progenitor cell will be of further interest.

Autocrine/paracrine regulation of the growth of the biliary tree by the neuroendocrine hormone serotonin

BACKGROUND & AIMS

The biliary tree is the target of cholangiopathies that are chronic cholestatic liver diseases characterized by loss of proliferative response and enhanced apoptosis of cholangiocytes, the epithelial cells lining the biliary tree. The endogenous factors that regulate cholangiocyte proliferation are poorly understood. Therefore, we studied the role of the neuroendocrine hormone serotonin as a modulator of cholangiocyte proliferation.

METHODS

The presence of the serotonin 1A and 1B receptors on cholangiocytes was evaluated. We then tested whether the activation of such receptors by the administration of the selective agonists modifies cholangiocyte proliferation and functional activity both in vivo and in vitro. In addition, the intracellular signal mediating the serotonin receptor action in cholangiocytes was characterized. We studied the expression and secretion of serotonin by cholangiocytes and the effects of the neutralization of the secreted hormone on the growth of the biliary tree.

RESULTS

Cholangiocytes express the serotonin 1A and 1B receptors. Their activation markedly inhibits the growth and choleretic activity of the biliary tree in the bile duct-ligated rat, a model of chronic cholestasis. Such changes are mediated by enhanced d -myo-inositol 1,4,5-triphosphate/Ca 2+ /protein kinase C signaling and the consequent inhibition of the adenosine 3',5'-cyclic monophosphate/protein kinase A/Src/extracellular signal-regulated kinase 1/2 cascade. Cholangiocytes secrete serotonin, the blockage of which enhances cholangiocyte proliferation in the course of cholestasis.

CONCLUSIONS

We observed the existence of an autocrine loop based on serotonin that limits the growth of the biliary tree in the course of chronic cholestasis. Our novel findings might open new approaches for the management of cholangiopathies.

Bile salt exposure causes phosphatidyl-inositol-3-kinase-mediated proliferation in a Barrett's adenocarcinoma cell line

BACKGROUND

The mechanisms by which gastroesophageal reflux promotes malignant progression in Barrett's esophagus are poorly understood. The phosphatidylinositol-3-kinase (PI3 kinase)/Akt pathway regulates proliferation and apoptosis. We hypothesized that the PI3 kinase/Akt pathway mediates the pro-proliferative and antiapoptotic effects of bile.

METHODS

The Barrett's adenocarcinoma cell line, SEG-1, was exposed to the conjugated bile salt, glycochenodeoxycholic acid (GCDA). Cell number was measured by the MTT incorporation assay and by Coulter counter. PI3 kinase/Akt activity was inferred from Western blots of phosphorylated and total Akt. Proliferation and apoptosis were determined by BrdU incorporation and cell death ELISA.

RESULTS

A dose-dependent cell number increase was seen with a 20-minute exposure to GCDA. On Western blot, 200 micromol/L GCDA caused a 3-fold increase in Akt phosphorylation within 20 minutes, which was inhibited by 90% with the addition of PI3 kinase inhibitor, LY294002. LY294002 produced dose-dependent inhibition of GCDA-induced cell number increases. 200 micromol/L GCDA decreased apoptosis by 25%. Addition of LY294002 did not completely inhibit the antiapoptotic effect of bile.

CONCLUSIONS

Bile salts activate the PI3 kinase/Akt signaling pathway and stimulate cell growth in SEG-1. The majority of this PI3 kinase-mediated effect is secondary to increases in proliferation rather than to decreases in apoptosis.