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）, mitogenactivated protein （MAP） kinase and extracellular signalregulated 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.

Cholangiocytes and blood supply

The microvascular supply of the biliary tree, the peribiliary plexus （PBP）, stems from the hepatic artery branches and flows into the hepatic sinusoids. A detailed three-dimensional study of the PBP has been performed by using the Scanning Electron Microscopy vascular corrosion casts （SEMvcc） technique. Considering that the PBP plays a fundamental role in supporting the secretory and absorptive functions of the biliary epithelium, their organization in either normalcy and pathology is explored. The normal liver shows the PBP arranged around extra-and intrahepatic biliary tree. In the small portal tract PBP was characterized by a single layer of capillaries which progressively continued with the extrahepatic PBP where it showed a morecomplex vascular network. After common duct ligation （BDL）, progressive modifications of bile duct and PBP proliferation are observed. The PBP presents a three-dimensional network arranged around many bile ducts and appears as bundles of vessels, composed by capillaries of homogeneous diameter with a typical round mesh structure. The PBP network is easily distinguishable from the sinusoidal network which appears normal. Considering the enormous extension of the PBP during BDL, the possible role played by the Vascular Endothelial Growth Factor （VEGF） is evaluated. VEGF-A,VEGF-C and their related receptors appeared highly immunopositive in proliferating cholangiocytes of BDL rats. The administration of anti-VEGF-A or anti-VEGF-C antibodies to BDL rats as well as hepatic artery ligation induced a reduced bile duct mass. The administration of rVEGF-A to BDL hepatic artery ligated rats prevented the decrease of cholangiocyte proliferation and VEGF-A expression as compared to BDL control rats. These data suggest the role of arterial blood supply of the biliary tree in conditions of cholangiocyte proliferation, such as it occurs during chronic cholestasis. On the other hand,the role played by VEGF as a tool of cross-talk between cholangiocytes and PBP endothelial cells suggests that manipulation of VEGF release and function could represent a therapeutic strategy for human pathological conditions characterized by damage of hepatic artery or the biliary tree.

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 （CCI4） 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.

Role of sex hormones in the modulation of cholangiocyte function

Over the last years,cholangiocytes,the cells that line the biliary tree,have been considered an important object of study for their biological properties which involves bile formation,proliferation,injury repair,fibrosis and angiogenesis.Cholangiocyte proliferation occurs in all pathologic conditions of liver injury where it is associated with inflammation and regeneration.During these processes,biliary cells start to secrete different cytokines,growth factors,neuropeptides and hormones which represent potential mechanisms for cross talk with other liver cells.Several studies suggest that hormones,and in particular,sex hormones,play a fundamental role in the modulation of the growth of this compartment in the injured liver which functionally conditions the progression of liver disease.Understanding the mechanisms of action and the intracellular pathways of these compounds 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 role of sex hormones in cholangiocyte proliferation and biology.

Role of the melatonin/menin signaling in the regulation of cholangiocarcinoma growth

Objective:There is decreased expression of arylalkylamine N-acetyltransferase(the rate-limiting enzyme regulating melatonin synthesis)and the clock gene,Per1 and increased expression of the MT1 melatonin receptor subtype in cholangiocarcinoma(CCA).This study tested the hypothesis that the inhibitory effects of the melatonin/MT1/Per1 axis on CCA growth are mediated by inhibition of miR-24.

Emerging Therapeutic Strategies in The Fight Against Primary Biliary Cholangitis

The liver has a vital role in many metabolic and regulatory processes in the body.Primary biliary cholangitis(PBC),previously known as primary biliary cirrhosis,is a chronic cholestatic autoimmune disease of the intrahepatic bile ducts associated with loss of tolerance to mitochondrial antigens.At this time there is no definitive cure for PBC;however,ursodeoxycholic acid(UDCA)has been shown to reduce injury when administered as the first line of treatment.Additional therapeutics can be given concurrently or as an alternative to UDCA to manage the symptoms and further curb disease progression.Currently,a liver transplant is the only potentially curative option when the patient has developed end-stage liver disease or intractable pruritus.This review aims to delineate the pathogenesis of primary biliary cholangitis and shed light on current therapeutic strategies in the treatment of PBC.

Functional roles of gut bacteria imbalance in cholangiopathies

Cholangiopathies are caused by bile duct damage or inflammation followed by cholestasis leading to liver fibrosis.Bile duct epithelial cells,cholangiocytes,are a primary target for cholangiopathies.Ductular reaction is often observed in cholangiopathies and the proliferation of cholangiocytes is associated with ductular reaction and liver fibrogenesis.Accumulating evidence suggests that patients with chol-angiopathies have different gut bacterial profiles from healthy individuals,indicating the association between gut microbiota and cholangiopathies.Bile acids are produced by hepatocytes and modified by gut bacteria.Bile acids regulate cholangiocyte proliferation but effects vary depending on the type of bile acids.Recent studies suggest that therapies targeting gut bacteria,such as antibiotics administration and gut bacteria depletion or therapies using gut bacteria-associated bile acids,such as ursodeoxycholic acid(UDCA)administration,may be useful for treatments of cholangiopathies,although data are contro-versial depending on animal models or cohorts.This review summarizes current understandings of functional roles of gut bacterial imbalance and strategies for treatments of cholangiopathies targeting gut bacteria.

Adoptive transfer of Pfkfb3-disrupted hematopoietic cells to wild-type mice exacerbates diet-induced hepatic steatosis and inflammation

Background and objectives:Hepatic steatosis and inflammation are key characteristics of non-alcoholic fatty liver disease(NAFLD).However,whether and how hepatic steatosis and liver inflammation are differentially regulated remains to be elucidated.Considering that disruption of 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3(Pfkfb3/iPfk2)dissociates fat deposition and inflammation,the present study examined a role for Pfkfb3/iPfk2 in hematopoietic cells in regulating hepatic steatosis and inflammation in mice.Methods:Pfkfb3-disrupted(Pfkfb3^(+/-))mice and wild-type(WT)littermates were fed a high-fat diet(HFD)and examined for NAFLD phenotype.Also,bone marrow cells isolated from Pfkfb3^(+/-)mice andWT mice were differentiated into macrophages for analysis of macrophage activation status and for bone marrow transplantation(BMT)to generate chimeric(WT/BMT-Pfkfb3^(+/-))mice in which Pfkfb3 was disrupted only in hematopoietic cells and control chimeric(WT/BMT-WT)mice.The latter were also fed an HFD and examined for NAFLD phenotype.In vitro,hepatocytes were co-cultured with bone marrowderived macrophages and examined for hepatocyte fat deposition and proinflammatory responses.Results:After the feeding period,HFD-fed Pfkfb3^(+/-)mice displayed increased severity of liver inflammation in the absence of hepatic steatosis compared with HFD-fed WT mice.When inflammatory activation was analyzed,Pfkfb3^(+/-)macrophages revealed increased proinflammatory activation and decreased anti-proinflammatory activation.When NAFLD phenotype was analyzed in the chimeric mice,WT/BMT-Pfkfb3^(+/-) mice displayed increases in the severity of HFD-induced hepatic steatosis and inflammation compared with WT/BMT-WT mice.At the cellular level,hepatocytes co-cultured with Pfkfb3^(+/-) macrophages revealed increased fat deposition and proinflammatory responses compared with hepatocytes co-cultured with WT macrophages.Conclusions:Pfkfb3 disruption only in hematopoietic cells exacerbates HFD-induced hepatic steatosis and inflammation whereas the Pfkfb3/iPfk2 in nonhematopoietic cells appeared to be needed for HFD feeding to induce hepatic steatosis.As such,the Pfkfb3/iPfk2 plays a unique role in regulating NAFLD pathophysiology.

Diagnostic and therapeutic potentials of microRNAs in cholangiopathies

Cholangiopathies are a group of rare,devastating diseases that arise from damaged cholangiocytes,the cells that line the intra-and extra-hepatic bile ducts of the biliary epithelium.Cholangiopathies result in significant morbidity and mortality and are a major cause of liver transplantation.A better understanding of the underlying pathogenesis that influences cholangiocyte dysregulation and cholangiopathy progression is necessary,considering the dismal prognosis associated with these diseases.MicroRNAs are a class of small,non-coding RNAs that regulate post-transcriptional mRNA expression of specific genes.The role of microRNAs has expanded to include the initiation and development of many diseases,including cholangiopathies.Understanding microRNA regulation of cholangiopathies may provide diagnostic and therapeutic benefit for these diseases.In this review,the authors primarily focus on studies published within the last five years that help determine the diagnostic and therapeutic potential of microRNAs in cholangiopathies.