Mechanisms of cholangiocyte responses to injury

Cholangiocytes, epithelial cells that line the biliary epithelium, are the primary target cells for cholangiopathies including primary sclerosing cholangitis and primary biliary cholangitis. Quiescent cholangiocytes respond to biliary damage and acquire an activated neuroendocrine phenotype to maintain the homeostasis of the liver. The typical response of cholangiocytes is proliferation leading to bile duct hyperplasia, which is a characteristic of cholestatic liver diseases. Current studies have identified various signaling pathways that are associated with cholangiocyte proliferation/loss and liver fibrosis in cholangiopathies using human samples and rodent models. Although recent studies have demonstrated that extracellular vesicles and microRNAs could be mediators that regulate these messenger/receptor axes, further studies are required to confirm their roles. This review summarizes current studies of biliary response and cholangiocyte proliferation during cholestatic liver injury with particular emphasis on the secretin/secretin receptor axis. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.     

Progesterone stimulates the proliferation of female and male cholangiocytes via autocrine/paracrine mechanisms

During cholestatic liver diseases, cholangiocytes express neuroendocrine phenotypes and respond to a number of hormones and neuropeptides by paracrine and autocrine mechanisms. We examined whether the neuroendocrine hormone progesterone is produced by and targeted to cholangiocytes, thereby regulating biliary proliferation during cholestasis. Nuclear (PR-A and PR-B) and membrane (PRGMC1, PRGMC2, and mPRalpha) progesterone receptor expression was evaluated in liver sections and cholangiocytes from normal and bile duct ligation (BDL) rats, and NRC cells (normal rat cholangiocyte line). In vivo, normal rats were chronically treated with progesterone for 1 wk, or immediately after BDL, rats were treated with a neutralizing progesterone antibody for 1 wk. Cholangiocyte growth was measured by evaluating the number of bile ducts in liver sections. The expression of the progesterone synthesis pathway was evaluated in liver sections, cholangiocytes and NRC. Progesterone secretion was evaluated in supernatants from normal and BDL cholangiocytes and NRC. In vitro, NRC were stimulated with progesterone and cholangiocyte supernatants in the presence or absence of antiprogesterone antibody. Aminoglutethimide was used to block progesterone synthesis. Cholangiocytes and NRC express the PR-B nuclear receptor and PRGMC1, PRGMC2, and mPRalpha. In vivo, progesterone increased the number of bile ducts of normal rats, whereas antiprogesterone antibody inhibited cholangiocyte growth stimulated by BDL. Normal and BDL cholangiocytes expressed the biosynthetic pathway for and secrete progesterone. In vitro, 1) progesterone increased NRC proliferation; 2) cholangiocyte supernatants increased NRC proliferation, which was partially inhibited by preincubation with antiprogesterone; and 3) inhibition of progesterone steroidogenesis prevented NRC proliferation. In conclusion, progesterone may be an important autocrine/paracrine regulator of cholangiocyte proliferation.     

Paracrine modulation of cholangiocyte serotonin synthesis orchestrates biliary remodeling in adults

Paracrine signaling between cholangiocytes and stromal cells regulates biliary remodeling. Cholangiocytes have neuroepithelial characteristics and serotonin receptor agonists inhibit their growth, but whether they are capable of serotonin biosynthesis is unknown. We hypothesized that cholangiocytes synthesize serotonin and that cross talk between liver myofibroblasts (MF) and cholangiocytes regulates this process to influence biliary remodeling. Transwell cultures of cholangiocytes ± MF, and tryptophan hydroxylase-2 knockin (TPH2KI) mice with an inactivating mutation of the neuronal tryptophan hydroxylase (TPH) isoform, TPH2, were evaluated. Results in the cell culture models confirm that cholangiocytes have serotonin receptors and demonstrate for the first time that these cells express TPH2 and produce serotonin, which autoinhibits their growth but stimulates MF production of TGF-β(1). Increased TGF-β(1), in turn, counteracts autocrine inhibition of cholangiocyte growth by repressing cholangiocyte TPH2 expression. Studies of TPH2KI mice confirm that TPH2-mediated production of serotonin plays an important role in remodeling damaged bile ducts because mice with decreased TPH2 function have reduced biliary serotonin levels and exhibit excessive cholangiocyte proliferation, accumulation of aberrant ductules and liver progenitors, and increased liver fibrosis after bile duct ligation. This new evidence that cholangiocytes express the so-called neuronal isoform of TPH, synthesize serotonin de novo, and deploy serotonin as an autocrine/paracrine signal to regulate regeneration of the biliary tree complements earlier work that revealed that passive release of serotonin from platelets stimulates hepatocyte proliferation. Given the prevalent use of serotonin-modulating drugs, these findings have potentially important implications for recovery from various types of liver damage.     

Role of inflammation and proinflammatory cytokines in cholangiocyte pathophysiology

Cholangiocytes, the epithelial cells lining the bile ducts, are an important subset of liver cells. They are involved in the modification of bile volume and composition, and respond to endogenous and exogenous stimuli. Along the biliary tree, two different kinds of cholangiocytes exist: small and large cholangiocytes. Each type has different features and biological role in physiologic and pathologic conditions, and their immunobiology is important for understanding biliary diseases. Cholangiocytes provide the first line of defence against luminal microbes in the hepatobiliary system. Indeed, they express a variety of pattern recognition receptors and may start an antimicrobial defence activating a set of intracellular signalling cascades. In response to injury, cholangiocytes that are normally quiescent become reactive and acquire a neuroendocrine-like phenotype with the release of proinflammatory mediators and antimicrobial peptides, which support biliary epithelial integrity. These molecules act in an autocrine/paracrine manner to modulate cholangiocyte biology and determine the evolution of biliary damage. Failure or dysregulation of such mechanisms may influence the progression of cholangiopathies, a group of diseases that selectively target biliary cells. In this review, we focus on the response of cholangiocytes in inflammatory conditions, with a particular focus on the mechanism driving cholangiocytes adaptation to damage. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.     

Wnt signaling in biliary development,proliferation, and fibrosis

Biliary fibrosis is an important pathological indicator of hepatobiliary damage. Cholangiocyte is the key cell type involved in this process. To reveal the pathogenesis of biliary fibrosis, it is essential to understand the normal development as well as the aberrant generation and proliferation of cholangiocytes. Numerous reports suggest that the Wnt signaling pathway is implicated in the physiological and pathological processes of cholangiocyte development and ductular reaction. In this review, we summarize the effects of Wnt pathway in cholangiocyte development from embryonic stem cells, as well as the underlying mechanisms of cholangiocyte responses to adult ductal damage. Wnt signaling pathway is regulated in a step-wise manner during each of the liver differentiation stages from embryonic stem cells to functional mature cholangiocytes. With the modulation of Wnt pathway, cholangiocytes can also be generated from adult liver progenitor cells and mature hepatocytes to repair liver damage. Non-canonical Wnt signaling is triggered in the active ductal cells during biliary fibrosis. Targeted control of the Wnt signaling may hold the great potential to reduce and/or reverse the biliary fibrogenic process.     

New insights on the molecular and cell biology of human cholangiopathies

Cholangiopathies are diseases of high social impact representing the main indication for liver transplantation in the infanthood and the third in adulthood. Despite the heterogeneous etiology and pathogenesis, cholangiopathies share many different common morphological features and, chronically progress toward a ductupenic condition clinically evidenced by the classical features of a cholestatic syndrome. The primary target of damage in the course of cholangiopathies are cholangiocytes, the epithelia cells lining the biliary tree. A bulk of researches performed in the last decade, highlighted the extraordinary biological properties of cholangiocytes involved in a number of important processes such as bile formation, proliferation, injury repair, fibrosis, angiogenesis and regulation of blood flow. Recent advances on the molecular and cell biology of human cholangiopathies are opening new potential therapeutic perspectives for these diseases.     

Calcium signaling and the secretory activity of bile duct epithelia

Cytosolic calcium (Ca_i²⁺) is a second messenger that is important for the regulation of secretion in many types of tissues. Bile duct epithelial cells, or cholangiocytes, are polarized epithelia that line the biliary tree in liver and are responsible for secretion of bicarbonate and other solutes into bile. Ca_i²⁺ signaling plays an important role in the regulation of secretion by cholangiocytes, and this review discusses the machinery involved in the formation of Ca²⁺ signals in cholangiocytes, along with the evidence that these signals regulate ductular secretion. Finally, this review discusses the evidence that impairments in cholangiocyte Ca²⁺ signaling play a primary role in the pathogenesis of cholestatic disorders, in which hepatic bile secretion is impaired.     

Protective role of melatonin in early‐stage and end‐stage liver cirrhosis

The liver is composed of hepatocytes, cholangiocytes, Kupffer cells, sinusoidal endothelial cells, hepatic stellate cells (HSCs) and dendritic cells; all these functional and interstitial cells contribute to the synthesis and secretion functions of liver tissue. However, various hepatotoxic factors including infection, chemicals, high‐fat diet consumption, surgical procedures and genetic mutations, as well as biliary tract diseases such as sclerosing cholangitis and bile duct ligation, ultimately progress into liver cirrhosis after activation of fibrogenesis. Melatonin (MT), a special hormone isolated from the pineal gland, participates in regulating multiple physiological functions including sleep promotion, circadian rhythms and neuroendocrine processes. Current evidence shows that MT protects against liver injury by inhibiting oxidation, inflammation, HSC proliferation and hepatocyte apoptosis, thereby inhibiting the progression of liver cirrhosis. In this review, we summarize the circadian rhythm of liver cirrhosis and its potential mechanisms as well as the therapeutic effects of MT on liver cirrhosis and earlier‐stage liver diseases including liver steatosis, nonalcoholic fatty liver disease and liver fibrosis. Given that MT is an antioxidative and anti‐inflammatory agent that is effective in eliminating liver injury, it is a potential agent with which to reverse liver cirrhosis in its early stage.     

Castration inhibits biliary proliferation induced by bile duct obstruction: novel role for the autocrine trophic effect of testosterone

Increased cholangiocyte growth is critical for the maintenance of biliary mass during liver injury by bile duct ligation (BDL). Circulating levels of testosterone decline following castration and during cholestasis. Cholangiocytes secrete sex hormones sustaining cholangiocyte growth by autocrine mechanisms. We tested the hypothesis that testosterone is an autocrine trophic factor stimulating biliary growth. The expression of androgen receptor (AR) was determined in liver sections, male cholangiocytes, and cholangiocyte cultures [normal rat intrahepatic cholangiocyte cultures (NRICC)]. Normal or BDL (immediately after surgery) rats were treated with testosterone or antitestosterone antibody or underwent surgical castration (followed by administration of testosterone) for 1 wk. We evaluated testosterone serum levels; intrahepatic bile duct mass (IBDM) in liver sections of female and male rats following the administration of testosterone; and secretin-stimulated cAMP levels and bile secretion. We evaluated the expression of 17β-hydroxysteroid dehydrogenase 3 (17β-HSD3, the enzyme regulating testosterone synthesis) in cholangiocytes. We evaluated the effect of testosterone on the proliferation of NRICC in the absence/presence of flutamide (AR antagonist) and antitestosterone antibody and the expression of 17β-HSD3. Proliferation of NRICC was evaluated following stable knock down of 17β-HSD3. We found that cholangiocytes and NRICC expressed AR. Testosterone serum levels decreased in castrated rats (prevented by the administration of testosterone) and rats receiving antitestosterone antibody. Castration decreased IBDM and secretin-stimulated cAMP levels and ductal secretion of BDL rats. Testosterone increased 17β-HSD3 expression and proliferation in NRICC that was blocked by flutamide and antitestosterone antibody. Knock down of 17β-HSD3 blocks the proliferation of NRICC. Drug targeting of 17β-HSD3 may be important for managing cholangiopathies.     

Interaction of CD44 and hyaluronic acid enhances biliary epithelial proliferation in cholestatic livers

Biliary epithelia express high levels of CD44 in hepatobiliary diseases. The role of CD44-hyaluronic acid interaction in biliary pathology, however, is unclear. A rat model of hepatic cholestasis induced by bile duct ligation was employed for characterization of hepatic CD44 expression and extracellular hyaluronan distribution. Cell culture experiments were employed to determine whether hyaluronan can regulate cholangiocyte growth through interacting with adhesion molecule CD44. Biliary epithelial cells were found to express the highest level of CD44 mRNA among four major types of nonparenchymal liver cells, including Kupffer, hepatic stellate, and liver sinusoidal endothelial cells isolated from cholestatic livers. CD44-positive biliary epithelia lining the intrahepatic bile ducts were geographically associated with extracellular hyaluronan accumulated in the portal tracts of the livers, suggesting a role for CD44 and hyaluronan in the development of biliary proliferation. Cellular proliferation assays demonstrated that cholangiocyte propagation was accelerated by hyaluronan treatment and antagonized by small interfering RNA CD44 or anti-CD44 antibody. The study provides compelling evidence to suggest that proliferative biliary epithelia lining the intrahepatic bile ducts are a prime source of hepatic CD44. CD44-hyaluronan interaction, by enhancing biliary proliferation, may play a pathogenic role in the development of cholestatic liver diseases.     

Bombesin and neurotensin exert antiproliferative effects on oval cells and augment the regenerative response of the cholestatic rat liver

The regenerative capacity of the cholestatic liver is significantly attenuated. Oval cells are hepatic stem cells involved in liver's regeneration following diverse types of injury. The present study investigated the effect of the neuropeptides bombesin (BBS) and neurotensin (NT) on oval cell proliferation as well as on hepatocyte and cholangiocyte proliferation and apoptosis in the cholestatic rat liver. Seventy male Wistar rats were randomly divided into five groups: controls, sham operated, bile duct ligated (BDL), BDL + BBS (30 μg/kg/d), BDL + NT (300 μg/kg/d). Ten days later, alpha-fetoprotein (AFP) mRNA (in situ hybridization), cytokeratin-19 and Ki67 antigen expression (immunohistochemistry) and apoptosis (TUNEL) were evaluated on liver tissue samples. Cells with morphologic features of oval cells that were cytokeratin-19(+) and AFP mRNA(+) were scored in morphometric analysis and their proliferation was recorded. In addition, the proliferation and apoptotic rates of hepatocytes and cholangiocytes were determined. Alanine aminotransferase (ALT) levels and hepatic oxidative stress (lipid peroxidation and glutathione redox state) were also estimated. The neuropeptides BBS and NT significantly reduced ALT levels and hepatic oxidative stress. Both agents exerted similar and cell type-specific effects on oval cells, hepatocytes and cholangiocytes: (a) oval cell proliferation and accumulation in the cholestatic liver was attenuated, (b) hepatocyte proliferation was increased along with a decreased rate of their apoptosis and (c) cholangiocyte proliferation was attenuated and their apoptosis was increased. These observations might be of potential value in patients with extrahepatic cholestasis.     

Efficient generation of biliary epithelial cells from rabbit intrahepatic bile duct by Y-27632 and Matrigel

Efficient culture of primary biliary epithelial cells (BECs) from adult liver is useful for both experimental studies and clinical applications of tissue engineering. However, an effective culture system for long-term proliferation of adult BECs is still unachieved. Laboratory rabbit has been used in a large number of studies; however, there are no reports of BECs from normal adult rabbit. As little as 5 g of normal rabbit liver tissue were minced, digested, and then clonally cultured in medium containing FBS and ITS. Cells were characterized by cell morphology, immunoassaying, and growth rate assay. Different combination of growth factors and substrates, including Y-27632 and Matrigel, were employed to assess their effect on cell proliferation. In the primary culture, the BECs cellular sheets consisting of cuboidal cells, as well as fibroblast-like cells and other hepatic cells, emerged with time of culture. The BECs cellular sheets were then manually split into cells clumps for further characterization. The subcultured cells had typical cell morphology of cholangiocytes, expressed the specific markers of BECs, including GGT, cytokeratin (CK18), and CK19, and possessed the capacity to form duct-like structure in three-dimensional Matrigel. Y-27632 and Matrigel-treated BECs had a steady growth rate as well as colony-formation capacity. The BECs were maintained in Y-27632 and Matrigel culture system for more than 3 mo. This is the first example, to our knowledge, of the successful culture of BECs from normal adult rabbit liver. Furthermore, our results indicate that treatment of BECs with Y-27632 and Matrigel is a simple method for efficient output of BECs.     

Interleukin-5 inhibition of biliary cell chloride currents and bile flow

Recent studies have detected significant elevations of interleukin (IL)-5 mRNA in the liver parenchyma of patients with both primary biliary cirrhosis and acute rejection after liver transplantation. In both of these disorders, intrahepatic biliary epithelial cells (BECs) are the targets of injury. We hypothesized that BECs may themselves express IL-5 receptors that may modulate key biliary functions. RNAs coding for IL-5alpha and -beta receptors were amplified by RT/PCR from a biliary cell line derived from a human cholangiocarcinoma (Mz-ChA-1) and verified by DNA sequencing. IL-5 receptor distribution was detected immunocytochemically on Mz-ChA-1 cells, immortalized murine BEC, bile duct-ligated rat liver, and isolated cholangiocytes. Patch-clamp studies on Mz-ChA-1 cells showed that IL-5 inhibits 5'-N-ethylcarboxamidoadenosine-stimulated chloride currents. Additional functional studies showed that IL-5 inhibits secretin-induced bile flow. We conclude that BECs express IL-5 receptors and that IL-5 modulates BEC chloride currents and fluid secretion. Since IL-5 has previously been associated with cholestatic liver disease, we speculate that IL-5 may contribute to liver injury through its effects on biliary secretion.     

Pathobiology of biliary epithelia

Cholangiocytes are epithelial cells that line the intra- and extrahepatic biliary tree. They serve predominantly to mediate the content of luminal biliary fluid, which is controlled via numerous signaling pathways influenced by endogenous (e.g., bile acids, nucleotides, hormones, neurotransmitters) and exogenous (e.g., microbes/microbial products, drugs etc.) molecules. When injured, cholangiocytes undergo apoptosis/lysis, repair and proliferation. They also become senescent, a form of cell cycle arrest, which may prevent propagation of injury and/or malignant transformation. Senescent cholangiocytes can undergo further transformation to a senescence-associated secretory phenotype (SASP), where they begin secreting pro-inflammatory and pro-fibrotic signals that may contribute to disease initiation and progression. These and other concepts related to cholangiocyte pathobiology will be reviewed herein. This article is part of a Special Issue entitled: Cholangiocytes in Health and Disease edited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.     

Prolactin stimulates the proliferation of normal female cholangiocytes by differential regulation of Ca2+-dependent PKC isoforms

Background

Prolactin promotes proliferation of several cells. Prolactin receptor exists as two isoforms: long and short, which activate different transduction pathways including the Ca²⁺-dependent PKC-signaling. No information exists on the role of prolactin in the regulation of the growth of female cholangiocytes. The rationale for using cholangiocytes from female rats is based on the fact that women are preferentially affected by specific cholangiopathies including primary biliary cirrhosis. We propose to evaluate the role and mechanisms of action by which prolactin regulates the growth of female cholangiocytes.

Results

Normal cholangiocytes express both isoforms (long and short) of prolactin receptors, whose expression increased following BDL. The administration of prolactin to normal female rats increased cholangiocyte proliferation. In purified normal female cholangiocytes, prolactin stimulated cholangiocyte proliferation, which was associated with increased [Ca²⁺]_i levels and PKCβ-I phosphorylation but decreased PKCα phosphorylation. Administration of an anti-prolactin antibody to BDL female rats decreased cholangiocyte proliferation. Normal female cholangiocytes express and secrete prolactin, which was increased in BDL rats. The data show that prolactin stimulates normal cholangiocyte growth by an autocrine mechanism involving phosphorylation of PKCβ-I and dephosphorylation of PKCα.

Conclusion

We suggest that in female rats: (i) prolactin has a trophic effect on the growth of normal cholangiocytes by phosphorylation of PKCβ-I and dephosphorylation of PKCα; and (iii) cholangiocytes express and secrete prolactin, which by an autocrine mechanism participate in regulation of cholangiocyte proliferation. Prolactin may be an important therapeutic approach for the management of cholangiopathies affecting female patients.     

Knockout of the neurokinin-1 receptor reduces cholangiocyte proliferation in bile duct-ligated mice

In bile duct-ligated (BDL) rats, cholangiocyte proliferation is regulated by neuroendocrine factors such as α-calcitonin gene-related peptide (α-CGRP). There is no evidence that the sensory neuropeptide substance P (SP) regulates cholangiocyte hyperplasia. Wild-type (WT, (+/+)) and NK-1 receptor (NK-1R) knockout (NK-1R(-/-)) mice underwent sham or BDL for 1 wk. Then we evaluated 1) NK-1R expression, transaminases, and bilirubin serum levels; 2) necrosis, hepatocyte apoptosis and steatosis, and the number of cholangiocytes positive by CK-19 and terminal deoxynucleotidyl transferase biotin-dUTP nick-end labeling in liver sections; 3) mRNA expression for collagen 1α and α-smooth muscle (α-SMA) actin in total liver samples; and 4) PCNA expression and PKA phosphorylation in cholangiocytes. In cholangiocyte lines, we determined the effects of SP on cAMP and D-myo-inositol 1,4,5-trisphosphate levels, proliferation, and PKA phosphorylation. Cholangiocytes express NK-1R with expression being upregulated following BDL. In normal NK-1R(-/-) mice, there was higher hepatocyte apoptosis and scattered hepatocyte steatosis compared with controls. In NK-1R (-)/(-) BDL mice, there was a decrease in serum transaminases and bilirubin levels and the number of CK-19-positive cholangiocytes and enhanced biliary apoptosis compared with controls. In total liver samples, the expression of collagen 1α and α-SMA increased in BDL compared with normal mice and decreased in BDL NK-1R(-/-) compared with BDL mice. In cholangiocytes from BDL NK-1R (-)/(-) mice there was decreased PCNA expression and PKA phosphorylation. In vitro, SP increased cAMP levels, proliferation, and PKA phosphorylation of cholangiocytes. Targeting of NK-1R may be important in the inhibition of biliary hyperplasia in cholangiopathies.     

Pinealectomy or light exposure exacerbates biliary damage and liver fibrosis in cholestatic rats through decreased melatonin synthesis

Melatonin, a neuroendocrine hormone synthesized by the pineal gland and cholangiocytes, decreases biliary hyperplasia and liver fibrosis during cholestasis-induced biliary injury via melatonin-dependent autocrine signaling through increased biliary arylalkylamine N-acetyltransferase (AANAT) expression and melatonin secretion, downregulation of miR-200b and specific circadian clock genes. Melatonin synthesis is decreased by pinealectomy (PINX) or chronic exposure to light. We evaluated the effect of PINX or prolonged light exposure on melatonin-dependent modulation of biliary damage/ductular reaction/liver fibrosis. Studies were performed in male rats with/without BDL for 1 week with 12:12 h dark/light cycles, continuous light or after 1 week of PINX. The expression of AANAT and melatonin levels in serum and cholangiocyte supernatant were increased in BDL rats, while decreased in BDL rats following PINX or continuous light exposure. BDL-induced increase in serum chemistry, ductular reaction, liver fibrosis, inflammation, angiogenesis and ROS generation were significantly enhanced by PINX or light exposure. Concomitant with enhanced liver fibrosis, we observed increased biliary senescence and enhanced clock genes and miR-200b expression in total liver and cholangiocytes. In vitro, the expression of AANAT, clock genes and miR-200b was increased in PSC human cholangiocyte cell lines (hPSCL). The proliferation and activation of HHStecs (human hepatic stellate cell lines) were increased after stimulating with BDL cholangiocyte supernatant and further enhanced when stimulated with BDL rats following PINX or continuous light exposure cholangiocyte supernatant via intracellular ROS generation. Conclusion: Melatonin plays an important role in the protection of liver against cholestasis-induced damage and ductular reaction.     

Pathophysiologic implications of innate immunity and autoinflammation in the biliary epithelium

The most studied physiological function of biliary epithelial cells (cholangiocytes) is to regulate bile flow and composition, in particular the hydration and alkalinity of the primary bile secreted by hepatocytes. After almost three decades of studies it is now become clear that cholangiocytes are also involved in epithelial innate immunity, in inflammation, and in the reparative processes in response to liver damage. An increasing number of evidence highlights the ability of cholangiocyte to undergo changes in phenotype and function in response to liver damage. By participating actively to the immune and inflammatory responses, cholangiocytes represent a first defense line against liver injury from different causes. Indeed, cholangiocytes express a number of receptors able to recognize pathogen- or damage-associated molecular patterns (PAMPs/DAMPs), such as Toll-like receptors (TLR), which modulate their pro-inflammatory behavior. Cholangiocytes can be both the targets and the initiators of the inflammatory process. Derangements of the signals controlling these mechanisms are at the basis of the pathogenesis of different cholangiopathies, both hereditary and acquired, such as cystic fibrosis-related liver disease and sclerosing cholangitis. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.