The cholangiopathies: Disorders of biliary epithelia Konstantinos N. Lazaridis, Mario Strazzabosco, Nicholas F. LaRusso  Gastroenterology  Volume 127, Issue 5, Pages 1565-1577 (November 2004) DOI: 10.1053/j.gastro.2004.08.006 Copyright © 2004 American Gastroenterological Association Terms and Conditions

Figure 1 The biology of biliary epithelia involves a number of interrelated activities in both health and disease. Transport capacity and, thus, contribution of bile formation is perhaps one of the pivotal functions of cholangiocytes. Disruption of ductal bile secretion leads to cholestasis. Being positioned between the hepatocytes and enterocytes, cholangiocytes are exposed to a number of endogenous and exogenous compounds, thus, interacting with the immune system, microbes, xenobiotics. Furthermore, biliary epithelia communicate with other liver cells (i.e., hepatocytes, mesenchymal, and endothelial cells) and participate in processes such as innate immunity and liver tissue damage/repair. Under certain conditions, the latter mechanisms may lead to hepatic inflammation and fibrosis. As dynamic cells, biliary epithelia undergo cell-cycle phenomena under the effect of internal and/or external stimuli aimed at maintaining tissue homeostasis. Dysregulation of cell-cycle mechanisms (i.e., proliferation, apoptosis, necrosis) in bile ducts may result in biliary hyperplasia followed by ductopenia and, in some cases, by malignant transformation (i.e., cholangiocarcinoma). Of note, biliary obstruction in animals causes cholangiocyte proliferation, which, in the early stage, leads to choleresis (i.e., increased bile flow). Nevertheless, in humans, the chronic interplay of cholestasis, biliary inflammation/fibrosis, and ductopenia lead to development of cholangiopathies (shaded box). Gastroenterology 2004 127, 1565-1577DOI: (10.1053/j.gastro.2004.08.006) Copyright © 2004 American Gastroenterological Association Terms and Conditions

Figure 2 Bile is originated by hepatocytes (ie, primary or hepatic bile) and is subsequently delivered into bile ducts. The canals of Hering provide the continuum between the hepatocyte canaliculus and the ductules or cholangioles (<15 μm), the small size bile ducts (15 μm to 300 μm), and the large bile ducts (300 μm to 800 μm) in which hepatic bile is modified by large size cholangiocytes (ie, ductal bile). Active biliary epithelial transport of electrolytes and solutes occurs mainly in large bile ducts and determines the vectorial water movement (ie, absorption or secretion) across cholangiocytes, thus altering ductal bile composition and flow. To achieve this notable task, biliary epithelia express an array of flux molecules located on their apical (ie, luminal) and/or basolateral plasma membrane domains. The periductular capillary plexus (not shown) located under the basolateral cholangiocyte domain facilitates the communication of bile ducts with the systemic circulation (ie, the cholehepatic shunt). NHE1 or SLC9A, Na+/H+ exchanger; NCB1 or SLC4A4, Na+/HCO3− cotransporter; KC, K+ channel; NCHE, Na+-dependent, Cl−/HCO3− exchanger; NHE2 or SLC9A2, Na+/H+ exchanger-2; AE2 or SLC4A2, Cl−/HCO3− exchanger; P2U, purinergic receptors; CCC, Ca2+-activated Cl− channel; MDR1a or ABCB1, multidrug resistance protein 1a; AQP, aquaporins (ie, water channels); GT, glutamate transporter; CFTR or ABCC7, cystic fibrosis transmembrane conductance regulator; SGLT1 or SLC5A1, Na+-dependent glucose cotransporter-1; ASBT or SLC10A2, apical Na+-dependent bile acid cotransporter; NKCC1 or SLC12A2, Na+/K+/2Cl− cotransporter; MRP3 or cMOAT2, or ABCC3, multidrug resistance protein 3; GLUT1 or SLC2A1, facilitated glucose transporter-1; t-ASBT, truncated ASBT; GLT, glutathiome; Glu: glucose; TCA, taurocholate. Gastroenterology 2004 127, 1565-1577DOI: (10.1053/j.gastro.2004.08.006) Copyright © 2004 American Gastroenterological Association Terms and Conditions

Figure 3 In pathologic conditions, cholangiocytes become “reactive.” Indeed, biliary epithelia abandon their differentiated epithelial phenotype, acquire neuroendocrine features, express adhesion molecules, and have the ability to produce and secrete proinflammatory and chemotactic mediators. “Reactive” cholangiocytes also produce growth factors able to activate mesenchymal cells and matrix production. Thus, “reactive” cholangiocytes likely play an important role not only in hepatic reparative processes but also in the progression of chronic liver damage and act as “the pace-maker of portal fibrosis.” (A) Transmission electron micrograph (TEM) showing the ultrastructural appearance of septal normal cholangiocytes immunoisolated using anti-egp35 antibodies (courtesy of Fabris et al.34). (B) TEM showing the ultrastructural appearance of “reactive” cholangiocytes immunoisolated from cholestatic human livers using anti-NCAM antibodies (courtesy of Fabris et al.34). M3 Ach-R, M3 acetylcholine receptors; NGF, nerve growth factor; NCAM, neutral cell adhesion molecule; ICAM-1, intracellular adhesion molecule 1; MHC II, major histocompatibility complex-class II molecules; TNF-α, tumor necrosis factor-α; IL-6 and IL-8, interleukin 6 and 8; MCP-1, monocyte chemotactic protein-1; CINC, cytokine-induced neutrophilic chemoattractant; HGF, human growth factor; PDGF-BB, platelet-derived growth factor-BB; CTGF, connective tissue growth factor; ET-1, endothelin-1; TGF-β2, transforming growth factor-β2; NO, nitric oxide. Gastroenterology 2004 127, 1565-1577DOI: (10.1053/j.gastro.2004.08.006) Copyright © 2004 American Gastroenterological Association Terms and Conditions

Figure 4 A cellular cross talk among cholangiocytes and other liver cells exists in normal conditions that are extended during the development of cholangiopathies. For example, “reactive” cholangiocytes produce and release bioactive molecules and mediators to communicate in a paracrine fashion with epithelial, endothelial, and mesechymal liver cells and circulating white cells. In cholangiopathies, biliary epithelial cells produce interleukins 1, 6, and 8 (IL-1, IL-6, IL-8) and interferon-γ (INF-γ) that have an effect on the function of polymorphonuclear cells (PMN), T cells, and Kupffer cells. Also, “reactive” cholangiocytes produce and secrete molecules such as endothelin-1 (ET-1), platelet-derived growth factor-BB (PDGF-BB), transforming growth factor-β2 (TGF-β2), connective tissue growth factor (CTGF), and nitric oxide (NO) that have an effect on “activated” stellate cells or myofibroblasts leading to liver reparative processes and/or fibrosis. Nevertheless, the mechanism(s) that contribute to the transformation of “quiescent” stellate cells and portal fibroblasts to “activated” stellate cells or myofibroblasts are still unknown. Of interest, in many cholangiopathies, the production and release of bioactive mediators from hepatocytes, endothelial and mesechymal cells of the liver and circulating white cells have an effect on cholangiocyte structure and function. These changes in biliary epithelia include the up-regulation of the major histocompatibility complex (MHC) class II molecules, proliferation of cholangiocytes, and alteration of their transport capacities (modified from Schuppan et al. Fibrogenesis in PBC. In: Lindor KD, Heathcote J, Poupon R, eds. Primary Biliary Cirrhosis: From Pathogenesis to Clinical Treatment. Kluwer Academic Press, Dordrecht, the Netherlands, 1998:64–75. Gastroenterology 2004 127, 1565-1577DOI: (10.1053/j.gastro.2004.08.006) Copyright © 2004 American Gastroenterological Association Terms and Conditions

Figure 5 A conceptual platform of the proposed pathogenetic model(s) of cholangiopathies. In this working hypothesis, the putative initial insult of biliary epithelia cells may be an interaction with an endogenous or exogenous substance and/or microorganism. The host’s initial response is perhaps an inflammatory reaction. It is anticipated that, in most cases, the inflammatory response is resolved, leading to resolution of the insult/damage to the biliary tree. Derangement of the host’s response is, however, likely dependent on putative genetic susceptibility, and other yet unknown factors may result in perpetuating this initial inflammatory response, leading to chronic inflammation of bile ducts and ultimately to the development of cholestasis, bile duct proliferation/ductopenia, and biliary/hepatic fibrosis, including potential malignant transformation of bile ducts. An interplay of these abnormal phenomena characterizes the clinical presentation and natural history of most cholangiopathies. OLT, orthotopic liver transplantation. Gastroenterology 2004 127, 1565-1577DOI: (10.1053/j.gastro.2004.08.006) Copyright © 2004 American Gastroenterological Association Terms and Conditions