and in the regeneration of liver parenchyma. Small hepatocytes located in pericentral positions are also believed to act as progenitor cells on certain occasions. Distinct subpopulations of mature hepatocytes and stem/progenitor cell compartments are differentially activated depending on the nature and duration of the liver damage versus different human pathologies.
Cholangiocyte Reaction to Biliary Damage
BECs are usually quiescent, but following a liver insult they activate and/or proliferate. A typical element of the repair response to liver damage is the ductular reaction (DR), a stereotyped histopathologic lesion of the biliary epithelium that plays a fundamental role in the progression of hepatic fibrosis. The DR is characterized by a marked proliferation of cholangiocytes, leading to formation of reactive ductular cells (RDCs), with poor cytoplasm and arranged in cell cords without a lumen or in richly anastomosed small‐diameter ducts (<10 μm) with almost unrecognizable lumens. RDCs are activated epithelial cells that secrete a vast array of factors, including cytokines, chemokines, growth factors, and angiogenic factors. They may derive from hepatocytes undergoing a process of ductular metaplasia, or from activation of the hepatic progenitor cell compartment and/or from proliferation and dedifferentiation of preexisting cholangiocytes. The increase in RDCs is generally associated with a significant increase in inflammatory infiltrate and portal fibrosis.
RDCs are considered the major driver of portal fibrosis during parenchymal and/or biliary injury. The deposition of fibrosis follows paracrine cross‐talk mediated by the ability of RDCs to secrete profibrotic and proinflammatory growth factors, and cross‐talk with cells of mesenchymal origin, in particular Kupffer cells and portal fibroblasts, which are the main effectors of fibrosis, as stimulators of the deposition of extracellular matrix by activated myofibroblasts. In addition, RDCs also establish paracrine communications with endothelial cells that provide the vascular support necessary for the growth and arborization of the ductal structures themselves [8].
Protective Role of Biliary HCO3− Secretion
The cholangiocyte is exposed to millimolar concentrations of hydrophobic bile salts, which are toxic to other cells such as hepatocytes at micromolar levels. Resistance against these noxious compounds and their cytolytic potential is therefore essential. One of the strategies that cholangiocytes have developed to survive is the biliary HCO3− umbrella.
Biliary HCO3− secretion sustains bile flow and confers its appropriate viscosity, generates part of the alkaline tide necessary for optimal digestion of various nutrients within the intestine, and protects the apical surface of cholangiocytes against protonated apolar hydrophobic BA monomers by maintaining an alkaline pH above the apical membrane. Isoforms of the Cl−/HCO3− exchanger, AE2, are responsible for the vast majority of biliary HCO3− secretion. Dysfunction of any of the elements involved in HCO3− formation might weaken the biliary HCO3− umbrella and contribute to the development of chronic cholestatic liver disease such as sclerosing cholangitis.
Cholangiocytes and Immunity
BECs are a first line of defense in liver innate immunity: they can present antigen to immune cells, be a target of immune‐mediated aggression, or be the initiators of an inflammatory reaction that then progresses to adaptive immune activation [9]. The contribution of BECs to liver immune responses was initially believed to be limited to the secretion of immunoglobulin A (IgA) into the bile, but it is now clear that their role in the immune response is far more complex. The biliary epithelium stands as a first line of defense against bacteria, fungi and other pathogens by secreting antimicrobial peptides such as defensin and cathelicidin. A major role in epithelial innate immunity in BECs is played by Toll‐like receptors (TLRs) and by nuclear receptors. TLRs can recognize pathogen‐associated molecular patterns (PAMPs), i.e. bacterial elements such as lipopolysaccharide (LPS), DNA and RNA fragments, but also respond to endogenous components or damage‐associated molecular patterns (DAMPs), such as hyaluronan and high mobility group box 1 (HMGB1) that are released from damaged cells. TLR4‐mediated signaling is the better studied in cholangiocytes. Once activated by LPS or other ligands, TLR4 activates two different pathways, one mediated by NF‐κB, which stimulate the expression of a number of proinflammatory cytokines and chemokines, and one mediated by mitogen‐activated protein kinase (MAPK)/activator protein 1 (AP‐1), which requires the nuclearization of the AP‐1 complex. In normal cholangiocytes, TLR4 signaling is repressed by protective mechanisms aimed at maintaining LPS tolerance. Since the biliary epithelium is continuously in contact with bacterial products of intestinal origin, changes in one or more regulatory checkpoints may trigger an exaggerated inflammatory response in the liver. For example in cystic fibrosis‐related liver disease, a decrease in LPS tolerance plays a major role in the development of the disease. A continuous stress in the absence of a correct modulation of the TLR‐mediated responses could be the trigger for a chronic inflammatory or autoimmune response.
Nuclear receptors, particularly the retinoid X receptor (RXR), have recently been involved in the immune response of BECs. This is a receptor superfamily that includes the glucocorticoid receptor, the retinoic acid receptor, the VDR, the liver X receptors, and the peroxisome proliferator‐activated receptors (PPARs). Nuclear receptors control several cell functions including cell proliferation and apoptosis, cell metabolism, cell–cell interaction, detoxification from BAs, and bile secretion.
In addition, continuous exposure to DAMPs and PAMPs could promote cellular senescence. Cell senescence is a mechanism of irreversible cell arrest in G1 stage induced by different stimuli. The main causes responsible for the onset of senescence are DNA damage (particularly but not exclusively) to the telomeres, the activation of mitogenic signals induced by oncogene activation, epigenetic modifications, and expression of tumor suppressor genes. All these signals lead to different physiologic responses generally leading to tumor suppression; however, in some cases it could promote cancer development or induce a fibrosing response and mediate age‐related degenerative diseases. Once senescent, cells not only cease proliferation but assume a senescence‐associated secretory phenotype (SASP) characterized by the secretion of a plethora of peptides with profibrogenic, proinflammatory, and tumorigenic properties. This indicates that senescence could not only act as a barrier to tumor growth, but also paracrinally stimulate the activation of aberrant reparative/regenerative responses. In chronic biliary diseases, cholangiocyte senescence is likely the result of ongoing inflammation, a sort of “exhaustion” of the activated cholangiocytes. This is particularly important in PSC, given the association with cholangiocarcinoma.
Biochemical Markers and Patterns of Hepatic Injury
Contrary to the kidney, no single test can be used to assess liver function. The liver biochemical tests, or liver function tests (LFTs), provide indirect evidence of hepatobiliary disease. LFTs that more accurately reflect liver synthesis include serum albumin, serum bilirubin, and prothrombin time, which is standardized to the international normalized ratio (INR). The enzymes aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are markers of hepatocellular disease, whereas alkaline phosphatase (ALP) and gamma‐glutamyltranspeptidase (GGT) are markers of cholestasis. The clinical evaluation of patients with abnormal LFTs involves an accurate interpretation of the pattern of liver damage in the context of an accurately collected medical history and physical examination. The severity and pattern of LFT abnormality, assessed by serial measurements, can be distinctive or aspecific. Different patterns of damage can be observed: hepatocellular damage, with predominant elevations in AST and ALT; cholestatic damage, with predominant increases in ALP, GGT, and bilirubin; and infiltrative damage with ALP and GGT increased disproportionately to bilirubin. These patterns are valuable in directing specific serologic tests, imaging, and liver biopsy. However, they are not diagnostic for a specific cause, nor are they able to distinguish whether cholestasis is intrahepatic or extrahepatic.
