Heterogeneity of the intrahepatic biliary epithelium

Activation of interleukin-6/STAT3 in rat cholangiocyte proliferation induced by lipopolysaccharide

BACKGROUND

Cholangiocytes are exposed to endotoxins (lipopolysaccharide, LPS) in a variety of biliary inflammations. It is known that LPS enhances the release of interleukin (IL)-6, a potent cholangiocyte mitogen. However, the role of LPS in cholangiocyte proliferation in vivo is unknown. Aims To investigate whether LPS stimulates cholangiocyte proliferation in vivo via the IL-6/STAT3 pathway.

METHODS

Rats were randomized into four groups: the LPS group (injected intravenously with LPS 2.5 mg/kg), anti-IL-6 group (injected intravenously with anti-IL-6 0.5 mg/kg 1 h after LPS injection), RPM group (treated with RPM 0.4 mg/kg intraperitoneally 30 min before LPS injection), and control group. At 6, 12, 24, 48, and 72 h after LPS injection, LPS in plasma was detected by kinetic turbidimetric limulus test. IL-6 concentrations in liver homogenate and cholangiocyte proliferation were determined by ELISA or immunohistochemistry, respectively. Expression of IL-6 mRNA and phosphorylated-STAT3 (P-STAT3) protein in cholangiocytes was analyzed by real-time RT-PCR and western blotting.

RESULTS

Cholangiocytes responded to LPS by a marked increase in cell proliferation, IL-6 secretion, and P-STAT3 expression. Anti-IL-6 neutralizing antibody inhibited LPS-induced proliferation of cholangiocytes and decreased levels of IL-6 and STAT3. Furthermore, after being treated with RPM, STAT3 activation was also depressed, which resulted a decreased proliferation of cholangiocytes.

CONCLUSIONS

LPS promotes cholangiocyte proliferation through the IL-6/STAT3 pathway, while RPM shows a depressive effect in this pathway.

Cholangiocyte proliferation and liver fibrosis

Cholangiocyte proliferation is triggered during extrahepatic bile duct obstruction induced by bile duct ligation, which is a common in vivo model used for the study of cholangiocyte proliferation and liver fibrosis. The proliferative response of cholangiocytes during cholestasis is regulated by the complex interaction of several factors, including gastrointestinal hormones, neuroendocrine hormones and autocrine or paracrine signalling mechanisms. Activation of biliary proliferation (ductular reaction) is thought to have a key role in the initiation and progression of liver fibrosis. The first part of this review provides an overview of the primary functions of cholangiocytes in terms of secretin-stimulated bicarbonate secretion--a functional index of cholangiocyte growth. In the second section, we explore the important regulators, both inhibitory and stimulatory, that regulate the cholangiocyte proliferative response during cholestasis. We discuss the role of proliferating cholangiocytes in the induction of fibrosis either directly via epithelial mesenchymal transition or indirectly via the activation of other liver cell types. The possibility of targeting cholangiocyte proliferation as potential therapy for reducing and/or preventing liver fibrosis, and future avenues for research into how cholangiocytes participate in the process of liver fibrogenesis are described.

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.

Extracellular nucleotides stimulate Cl- currents in biliary epithelia through receptor-mediated IP3 and Ca2+ release

Extracellular ATP regulates bile formation by binding to P2 receptors on cholangiocytes and stimulating transepithelial Cl(-) secretion. However, the specific signaling pathways linking receptor binding to Cl(-) channel activation are not known. Consequently, the aim of these studies in human Mz-Cha-1 biliary cells and normal rat cholangiocyte monolayers was to assess the intracellular pathways responsible for ATP-stimulated increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) and membrane Cl(-) permeability. Exposure of cells to ATP resulted in a rapid increase in [Ca(2+)](i) and activation of membrane Cl(-) currents; both responses were abolished by prior depletion of intracellular Ca(2+). ATP-stimulated Cl(-) currents demonstrated mild outward rectification, reversal at E(Cl(-)), and a single-channel conductance of approximately 17 pS, where E is the equilibrium potential. The conductance response to ATP was inhibited by the Cl(-) channel inhibitors NPPB and DIDS but not the CFTR inhibitor CFTR(inh)-172. Both ATP-stimulated increases in [Ca(2+)](i) and Cl(-) channel activity were inhibited by the P2Y receptor antagonist suramin. The PLC inhibitor U73122 and the inositol 1,4,5-triphosphate (IP3) receptor inhibitor 2-APB both blocked the ATP-stimulated increase in [Ca(2+)](i) and membrane Cl(-) currents. Intracellular dialysis with purified IP3 activated Cl(-) currents with identical properties to those activated by ATP. Exposure of normal rat cholangiocyte monolayers to ATP increased short-circuit currents (I(sc)), reflecting transepithelial secretion. The I(sc) was unaffected by CFTR(inh)-172 but was significantly inhibited by U73122 or 2-APB. In summary, these findings indicate that the apical P2Y-IP3 receptor signaling complex is a dominant pathway mediating biliary epithelial Cl(-) transport and, therefore, may represent a potential target for increasing secretion in the treatment of cholestatic liver disease.

The stem cell niche of human livers: symmetry between development and regeneration

Human livers contain two pluripotent progenitors: hepatic stem cells and hepatoblasts. The hepatic stem cells uniquely express the combination of epithelial cell adhesion molecule (EpCAM), neural cell adhesion molecule (NCAM), cytokeratin (CK) 19, albumin +/-, and are negative for alpha-fetoprotein (AFP). They are precursors to hepatoblasts, which differ from hepatic stem cells in size, morphology, and in expressing the combination of EpCAM, intercellular cell adhesion molecule (ICAM-1), CK19, albumin++, and AFP++. The hepatic stem cells are located in vivo in stem cell niches: the ductal plates in fetal and neonatal livers and canals of Hering in pediatric and adult livers. The hepatoblasts are contiguous to the niches, decline in numbers with age, wax and wane in numbers with injury responses, and are proposed to be the liver's transit-amplifying cells. In adult livers, intermediates between hepatic stem cells and hepatoblasts and between hepatoblasts and adult parenchyma are observed. Amplification of one or both pluripotent cell subpopulations can occur in diseases; for example, hepatic stem cell amplification occurs in mild forms of liver failure, and hepatoblast amplification occurs in forms of cirrhosis. Liver is, therefore, similar to other tissues in that regenerative processes in postnatal tissues parallel those occurring in development and involve populations of stem cells and progenitor cells that can be identified by anatomic, antigenic, and biochemical profiles.

Small mouse cholangiocytes proliferate in response to H1 histamine receptor stimulation by activation of the IP3/CaMK I/CREB pathway

Cholangiopathies are characterized by the heterogeneous proliferation of different-sized cholangiocytes. Large cholangiocytes proliferate by a cAMP-dependent mechanism. The function of small cholangiocytes may depend on the activation of inositol trisphosphate (IP(3))/Ca(2+)-dependent signaling pathways; however, data supporting this speculation are lacking. Four histamine receptors exist (HRH1, HRH2, HRH3, and HRH4). In several cells: 1) activation of HRH1 increases intracellular Ca(2+) concentration levels; and 2) increased [Ca(2+)](i) levels are coupled with calmodulin-dependent stimulation of calmodulin-dependent protein kinase (CaMK) and activation of cAMP-response element binding protein (CREB). HRH1 agonists modulate small cholangiocyte proliferation by activation of IP(3)/Ca(2+)-dependent CaMK/CREB. We evaluated HRH1 expression in cholangiocytes. Small and large cholangiocytes were stimulated with histamine trifluoromethyl toluidide (HTMT dimaleate; HRH1 agonist) for 24-48 h with/without terfenadine, BAPTA/AM, or W7 before measuring proliferation. Expression of CaMK I, II, and IV was evaluated in small and large cholangiocytes. We measured IP(3), Ca(2+) and cAMP levels, phosphorylation of CaMK I, and activation of CREB (in the absence/presence of W7) in small cholangiocytes treated with HTMT dimaleate. CaMK I knockdown was performed in small cholangiocytes stimulated with HTMT dimaleate before measurement of proliferation and CREB activity. Small and large cholangiocytes express HRH1, CaMK I, and CaMK II. Small (but not large) cholangiocytes proliferate in response to HTMT dimaleate and are blocked by terfenadine (HRH1 antagonist), BAPTA/AM, and W7. In small cholangiocytes, HTMT dimaleate increased IP(3)/Ca(2+) levels, CaMK I phosphorylation, and CREB activity. Gene knockdown of CaMK I ablated the effects of HTMT dimaleate on small cholangiocyte proliferation and CREB activation. The IP(3)/Ca(2+)/CaMK I/CREB pathway is important in the regulation of small cholangiocyte function.

Molecular pathogenesis of intrahepatic cholestasis of pregnancy

Intrahepatic cholestasis of pregnancy (ICP) occurs mainly in the third trimester and is characterised by pruritus and elevated serum bile acid levels. ICP is associated with an increased perinatal risk and higher rates of foetal morbidity and mortality. Although the pathogenesis of this disease is unknown, a genetic hypersensitivity to female hormones (oestrogen and/or progesterone) or their metabolites is thought to impair bile secretory function. Recent data suggest that mutations or polymorphisms of genes expressing hepatobiliary transport proteins or their nuclear regulators may contribute to the development and/or severity of ICP. Unidentified environmental factors may also influence pathogenesis of the disease. This review summarises current knowledge on the potential mechanisms involved in ICP at the molecular level.

A CD133-expressing murine liver oval cell population with bilineage potential

Although oval cells are postulated to be adult liver stem cells, a well-defined phenotype of a bipotent liver stem cell remains elusive. The heterogeneity of cells within the oval cell fraction has hindered lineage potential studies. Our goal was to identify an enriched population of bipotent oval cells using a combination of flow cytometry and single cell gene expression in conjunction with lineage-specific liver injury models. Expression of cell surface markers on nonparenchymal, nonhematopoietic (CD45-) cells were characterized. Cell populations were isolated by flow cytometry for gene expression studies. 3,5-Diethoxycarbonyl-1,4-dihydrocollidine toxic injury induced cell cycling and expansion specifically in the subpopulation of oval cells in the periportal zone that express CD133. CD133+CD45- cells expressed hepatoblast and stem cell-associated genes, and single cells coexpressed both hepatocyte and cholangiocyte-associated genes, indicating bilineage potential. CD133+CD45- cells proliferated in response to liver injury. Following toxic hepatocyte damage, CD133+CD45- cells demonstrated upregulated expression of the hepatocyte gene Albumin. In contrast, toxic cholangiocyte injury resulted in upregulation of the cholangiocyte gene Ck19. After 21-28 days in culture, CD133+CD45- cells continued to generate cells of both hepatocyte and cholangiocyte lineages. Thus, CD133 expression identifies a population of oval cells in adult murine liver with the gene expression profile and function of primitive, bipotent liver stem cells. In response to lineage-specific injury, these cells demonstrate a lineage-appropriate genetic response. Disclosure of potential conflicts of interest is found at the end of this article.

Cholangiocyte biology

PURPOSE OF REVIEW

Cholangiocytes are increasingly recognized as biologically important epithelia because of the diverse array of cellular processes in which they participate. Collectively, these processes define normal function and, when disturbed, account for abnormalities that cause disease. In addition, the cholangiocyte is the target cell of diverse pathologic processes (i.e. the cholangiopathies). Advances in animal models, imaging and gene silencing have further defined the roles that cholangiocytes play in signaling, transport of water, ions and solutes, and alterations that result in cholestasis. The pace of advances in technology justifies a yearly summary to identify trends, and apprise the readership of the most significant developments in cholangiocyte biology.

RECENT FINDINGS

Recent progress includes insights into the molecular mechanisms of bile secretion, the development of new experimental models, technologies, hypotheses and therapies relevant to disease. Major advances have also included the identification of novel roles for receptors and a better understanding of mechanistic pathways and biologic processes.

SUMMARY

This compendium of current activities in cholangiocyte biology may promote collegial sharing and exchange of novel concepts, ideas, reagents and probes, thereby promoting positive advances in the field.