Serum microRNAs as novel biomarkers for primary sclerosing cholangitis and cholangiocarcinoma

The diagnosis of primary sclerosing cholangitis (PSC) is difficult due to the lack of sensitive and specific biomarkers, as is the early diagnosis of cholangiocarcinoma (CC), a complication of PSC. The aim of this study was to identify specific serum miRNAs as diagnostic biomarkers for PSC and CC. The levels of 667 miRNAs were evaluated in 90 human serum samples (30 PSC, 30 CC and 30 control subjects) to identify disease-associated candidate miRNAs (discovery phase). The deregulated miRNAs were validated in an independent cohort of 140 samples [40 PSC, 40 CC, 20 primary biliary cirrhosis (PBC) and 40 controls]. Receiver operating characteristic (ROC) curves were established and only miRNAs with an area under the curve (AUC) > 0·70 were considered useful as biomarkers. In the discovery phase we identified the following: 21 miRNAs expressed differentially in PSC, 33 in CC and 26 in both in comparison to control subjects as well as 24 miRNAs expressed differentially between PSC and CC. After the validation phase, miR-200c was found to be expressed differentially in PSC versus controls, whereas miR-483-5p and miR-194 showed deregulated expression in CC compared with controls. We also demonstrate a difference in the expression of miR-222 and miR-483-5p in CC versus PSC. Combination of these specific miRNAs further improved the specificity and accuracy of diagnosis. This study provides a basis for the use of miRNAs as biomarkers for the diagnosis of PSC and CC.

Differential transcriptional characteristics of small and large biliary epithelial cells derived from small and large bile ducts

Biliary epithelial cells (BEC) are morphologically and functionally heterogeneous. To investigate the molecular mechanism for their diversities, we test the hypothesis that large and small BEC have disparity in their target gene response to their transcriptional regulator, the biliary cell-enriched hepatocyte nuclear factor HNF6. The expression of the major HNF (HNF6, OC2, HNF1b, HNF1a, HNF4a, C/EBPb, and Foxa2) and representative biliary transport target genes that are HNF dependent were compared between SV40-transformed BEC derived from large (SV40LG) and small (SV40SM) ducts, before and after treatment with recombinant adenoviral vectors expressing HNF6 (AdHNF6) or control LacZ cDNA (AdLacZ). Large and small BEC were isolated from mouse liver treated with growth hormone, a known transcriptional activator of HNF6, and the effects on selected target genes were examined. Constitutive Foxa2, HNF1a, and HNF4a gene expression were 2.3-, 12.4-, and 2.6-fold, respectively, higher in SV40SM cells. This was associated with 2.7- and 4-fold higher baseline expression of HNF1a- and HNF4a-regulated ntcp and oatp1 genes, respectively. Following AdHNF6 infection, HNF6 gene expression was 1.4-fold higher (P = 0.02) in AdHNF6 SV40SM relative to AdHNF6 SV40LG cells, with a corresponding higher Foxa2 (4-fold), HNF1a (15-fold), and HNF4a (6-fold) gene expression in AdHNF6-SV40SM over AdHNF6-SV40LG. The net effects were upregulation of HNF6 target gene glucokinase and of Foxa2, HNF1a, and HNF4a target genes oatp1, ntcp, and mrp2 over AdLacZ control in both cells, but with higher levels in AdH6-SV40SM over AdH6-SV40LG of glucokinase, oatp1, ntcp, and mrp2 (by 1.8-, 3.4-, 2.4-, and 2.5-fold, respectively). In vivo, growth hormone-mediated increase in HNF6 expression was associated with similar higher upregulation of glucokinase and mrp2 in cholangiocytes from small vs. large BEC. Small and large BEC have a distinct profile of hepatocyte transcription factor and cognate target gene expression, as well as differential strength of response to transcriptional regulation, thus providing a potential molecular basis for their divergent function.

Polycystic liver diseases

Polycystic liver diseases (PCLDs) are genetic disorders with heterogeneous etiologies and a range of phenotypic presentations. PCLD exhibits both autosomal or recessive dominant pattern of inheritance and is characterized by the progressive development of multiple cysts, isolated or associated with polycystic kidney disease, that appear more extensive in women. Cholangiocytes have primary cilia, functionally important organelles (act as mechanosensors) that are involved in both normal developmental and pathological processes. The absence of polycystin-1, 2, and fibrocystin/polyductin, normally localized to primary cilia, represent a potential mechanism leading to cyst formation, associated with increased cell proliferation and apoptosis, enhanced fluid secretion, abnormal cell-matrix interactions, and alterations in cell polarity. Proliferative and secretive activities of cystic epithelium can be regulated by estrogens either directly or by synergizing growth factors including nerve growth factor, IGF1, FSH and VEGF. The abnormalities of primary cilia and the sensitivity to proliferative effects of estrogens and different growth factors in PCLD cystic epithelium provide the morpho-functional basis for future treatment targets, based on the possible modulation of the formation and progression of hepatic cysts.

Recent advances in the regulation of cholangiocyte proliferation and function during extrahepatic cholestasis

Bile duct epithelial cells (i.e., cholangiocytes), which line the intrahepatic biliary epithelium, are the target cells in a number of human cholestatic liver diseases (termed cholangiopathies). Cholangiocyte proliferation and death is present in virtually all human cholangiopathies. A number of recent studies have provided insights into the key mechanisms that regulate the proliferation and function of cholangiocytes during the pathogenesis of cholestatic liver diseases. In our review, we have summarised the most important of these recent studies over the past 3 years with a focus on those performed in the animal model of extrahepatic bile duct ligation. In the first part of the review, we provide relevant background on the biliary ductal system. We then proceed with a general discussion of the factors regulating biliary proliferation performed in the cholestatic animal model of bile duct ligation. Further characterisation of the factors that regulate cholangiocyte proliferation and function will help in elucidating the mechanisms regulating the pathogenesis of biliary tract diseases in humans and in devising new treatment approaches for these devastating diseases.

Exendin-4, a glucagon-like peptide 1 receptor agonist, protects cholangiocytes from apoptosis

BACKGROUND AND AIMS

Progression of chronic cholestatic disorders towards ductopenia results from the dysregulation of cholangiocyte survival, with cell death by apoptosis prevailing over compensatory proliferation. Currently, no therapy is available to sustain cholangiocyte survival in the course of those disorders. It was recently shown that cholangiocytes express the glucagon-like peptide-1 receptor (GLP-1R); its activation results in enhanced proliferative reaction to cholestasis. The GLP-1R selective agonist exendin-4 sustains pancreatic beta cell proliferation and prevents cell death by apoptosis. Exendin-4 is now employed in humans as a novel therapy for diabetes. The aim of the present study was to verify whether exendin-4 is effective in preventing cholangiocyte apoptosis.

METHODS

In vitro, tests were carried out to determine if exendin-4 is able to prevent apoptosis of cholangiocytes isolated from normal rats induced by glycochenodeoxycholic acid (GCDCA); in vivo, animals subjected to 1 week of bile duct ligation and to a single intraperitoneal injection of CCl(4) were treated with exendin-4 for 3 days.

RESULTS

Exendin-4 prevented GCDCA-induced Bax mitochondrial translocation, cytochrome c release and an increase in caspase 3 activity. Phosphatidylinositol 3-kinase, but not cAMP/protein kinase A or Ca(2+)/calmodulin-dependent protein kinase inhibitors, neutralised the effects of exendin-4. In vivo, exendin-4 administration prevented the increase in TUNEL (terminal deoxynucleotidyl transferase-mediated triphosphate end-labelling)-positive cholangiocytes and the loss of bile ducts observed in bile duct-ligated rats treated with CCl(4).

CONCLUSION

Exendin-4 prevents cholangiocyte apoptosis both in vitro and in vivo; such an effect is due to the ability of exendin-4 to counteract the activation of the mitochondrial pathway of apoptosis. These findings support the hypothesis that exendin-4 may be effective in slowing down the progression of cholangiopathies to ductopenia.

Human cholangiocarcinoma development is associated with dysregulation of opioidergic modulation of cholangiocyte growth

BACKGROUND/AIMS

Incidence of cholangiocarcinoma is increasing worldwide, yet remaining highly aggressive and with poor prognosis. The mechanisms that drive cholangiocyte transition towards malignant phenotype are obscure. Cholangiocyte benign proliferation is subjected to a self-limiting mechanism based on the autocrine release of endogenous opioid peptides. Despite the presence of both, ligands interact with delta opioid receptor (OR), but not with microOR, with the consequent inhibition of cell growth. We aimed to verify whether cholangiocarcinoma growth is associated with failure of opioidergic regulation of growth control.

METHODS

We evaluated the effects of OR selective agonists on cholangiocarcinoma cell proliferation, migration and apoptosis. Intracellular signals were also characterised.

RESULTS

Activation of microOR, but not deltaOR, increases cholangiocarcinoma cell growth. Such an effect is mediated by ERK1/2, PI3K and Ca(2+)-CamKIIalpha cascades, but not by cAMP/PKA and PKCalpha. microOR activation also enhances cholangiocarcinoma cell migration and reduces death by apoptosis. The anti-apoptotic effect of microOR was PI3K dependent.

CONCLUSIONS

Our data indicate that cholangiocarcinoma growth is associated with altered opioidergic regulation of cholangiocyte biology, thus opening new scenarios for future surveillance or early diagnostic strategies for cholangiocarcinoma.

Estrogens stimulate the proliferation of human cholangiocarcinoma by inducing the expression and secretion of vascular endothelial growth factor

BACKGROUND

Estrogens may induce the proliferation of neoplastic cells by activating neo-angiogenesis.

AIM

To evaluate the effect of estrogens on the expression of vascular endothelial growth factor (VEGF) and related receptors (VEGF-R) in human cholangiocarcinoma and the role played by VEGF in mediating the proliferative effects of estrogens.

METHODS

Seven biopsies of intra-hepatic cholangiocarcinoma and the HuH-28 cell lines were investigated. Cell proliferation was measured by both PCNA Western blot and MTS proliferation assay.

RESULTS

By immunohistochemistry, biopsies of human cholangiocarcinoma stained positively for VEGF-A and VEGF-C and related receptors. HuH-28 cells expressed VEGF-A, -C, and VEGFR-1, -2, -3 and, their protein level was enhanced by 17beta-estradiol in association with the stimulation of cell proliferation. 17beta-Estradiol-stimulated proliferation of HuH-28 cells was blocked by 70% by VEGF-TRAP, a receptor-based VEGF inhibitor. 17beta-Estradiol induced the secretion of VEGF in the supernatant of HuH-28 cells. The stimulatory effect of 17beta-estradiol on the protein expression of VEGF-A, VEGF-C and VEGFR-1, -2, -3 was blocked by antagonists of ER (Ici182,780) or insulin-like growth factor 1-receptor (alphaIR3).

CONCLUSIONS

With the limitations of experiments performed in a cell line, our study indicates that VEGF plays a major role in mediating the proliferative effects of estrogens on human cholangiocarcinoma.

Bile salts regulate proliferation and apoptosis of liver cells by modulating the IGF1 system

BACKGROUND

In different cell types, the insulin-like growth factor 1 and its receptor modulate growth, apoptosis and damage repair in cooperation with estrogen receptors.

AIM

To evaluate the involvement of the insulin-like growth factor 1 system and estrogen receptors in bile salts modulation of apoptosis/proliferation of hepatocytes and cholangiocytes. Primary cultures of rat hepatocytes and cholangiocytes were exposed to glycochenodeoxycholate or tauro-CDC in the presence or absence of insulin-like growth factor 1 receptor blocking antibody (alphaIR3), small interfering RNA for insulin-like growth factor 1, 17beta-estradiol or estrogen receptor antagonist (ICI 182,780). Proliferation was evaluated by proliferating cell nuclear antigen Western blot and apoptosis by measuring caspase-3 activity or annexin-V.

RESULTS

In hepatocytes, the insulin-like growth factor 1 receptor blocker enhanced glycochenodeoxycholate-induced apoptosis and caused tauro-CDC to promote apoptosis. 17Beta-estradiol or the estrogen receptor antagonist (ICI 182,780) did not influence the apoptotic effect of glycochenodeoxycholate. In cholangiocytes, both glycochenodeoxycholate and tauro-CDC induced proliferation at 100microM, while they induced apoptosis at 1mM with a more pronounced effect of glycochenodeoxycholate. Apoptosis induced by 1mM glycochenodeoxycholate or tauro-CDC in cholangiocytes was enhanced by blocking insulin-like growth factor 1 receptor or by silencing insulin-like growth factor 1. 17Beta-estradiol counteracts glycochenodeoxycholate-induced cholangiocyte apoptosis by enhancing insulin-like growth factor 1 secretion and activating the insulin-like growth factor 1 system.

CONCLUSIONS

Modulation of the IGF1 system could represent a potential strategy for the management of bile salts-induced liver injury.

Hepatic microcirculation and cholangiocyte physiopathology

BACKGROUND

The peribiliary plexus (PBP) plays a fundamental role in supporting the functions of the biliary epithelium. After common bile duct ligation (BDL) progressive PBP proliferation is demonstrated. We have, recently, demonstrated that the biliary epithelium express Vascular Endothelial Growth Factor (VEGF), both subtype -A and -B and VEGF receptors. Taking in consideration the wide extension of PBP during BDL, aim of our study is to investigate the role of VEGF in stimulating angiogenesis and also in the modulation of epithelial cells proliferation.

MATERIAL AND METHODS

Experimental studies were performed by evaluating the effects of: a) endogenous VEGF neutralization by chronic administration of anti VEGF-C antibody on cholangiocyte proliferation in BDL rats and; b) the hepatic artery ligation (HAL) immediately after BDL followed by treatment (7 days) with a recombinant of VEGF-A (administered through IP implanted minipumps) on cholangiocyte proliferative activities.

RESULTS

Both administration of antiVEGF-C antibody and HAL decreases cholangiocyte proliferation. The decrease of cholangiocyte proliferation was associated with depressed VEGF-A protein expression. 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.

CONCLUSION

These data suggest that VEGF-C modulates the proliferative activities of cholangiocytes in experimental cholestasis and that circulating factors (i.e., VEGF) in the blood supply of the intra-hepatic biliary epithelium, play an important role in the balance between cholangiocyte proliferation/loss.

Alfa and beta estrogen receptors and the biliary tree

This manuscript summarizes recent data showing that estrogens and their receptors play an important role in modulating cholangiocyte proliferation. We have recently demonstrated that rat cholangiocytes express both estrogen receptors (ER)-alpha and -beta subtypes, while hepatocytes only express ER-alpha. ER and especially the ER-beta subtype, are overexpressed in cholangiocytes proliferating after bile duct ligation (BDL) in the rat, in association with enlarged bile duct mass and with enhanced estradiol serum levels. Cholangiocyte proliferation, during BDL, is impaired by estrogen antagonists (tamoxifen, ICI 182,780) which furthermore, induce the overexpression of Fas antigen and activate apoptosis of proliferating cholangiocytes. 17beta-estradiol stimulates, in vitro cholangiocyte proliferation, and this effect is individually blocked by tamoxifen or ICI 182,780. Cholangiocyte proliferation during BDL was associated with an enhanced protein expression of phosphorylated extracellular regulated kinases (ERK)1/2 which is, in contrast, negatively modulated by tamoxifen in association with its antiproliferative effect. This indicates a major involvement of the ERK system in the estrogen modulation of cholangiocyte proliferation.

Bile acid feeding increased proliferative activity and apical bile acid transporter expression in both small and large rat cholangiocytes

Bile acids (BA) enter cholangiocytes by the Na(+)-dependent apical BA transporter (ABAT). By this mechanism, taurocholate (TC) and taurolithocholate (TLC) increase cholangiocyte proliferation. No in vivo studies exist regarding the anatomical sites involved in BA-regulation of cholangiocyte growth. Specific cholangiocyte subpopulations participate in BA-regulated proliferation. Proliferation was assessed in liver sections by determining the number of proliferating cellular nuclear antigen (PCNA)-positive cholangiocytes and cytokeratin-19 (CK-19)-positive ducts. We isolated small and large cholangiocytes from rats fed for 1 week TC, TLC, or BA control diet and determined PCNA and ABAT expression and BA transport activity. We evaluated if TC and TLC induction of ABAT expression was dependent on activation of PKC alpha. DNA replication was active only in large normal cholangiocytes. TC and TLC feeding increased proliferation of large cholangiocytes, induced the de novo activation of proliferation of small cholangiocytes, overexpression of ABAT and BA transport activity in large cholangiocytes, and de novo expression of ABAT and BA transport activity in small cholangiocytes. BA-stimulated ABAT expression was dependent on PKC activation in cholangiocytes. TC and TLC stimulate proliferation of small and large cholangiocytes associated with PKC-dependent up-regulation of ABAT.

Regulation of cholangiocyte bicarbonate secretion

The objective of this review article is to discuss the role of secretin and its receptor in the regulation of the secretory activity of intrahepatic bile duct epithelial cells (i.e., cholangiocytes). After a brief overview of cholangiocyte functions, we provide an historical background for the role of secretin and its receptor in the regulation of ductal secretion. We review the newly developed experimental in vivo and in vitro tools, which lead to understanding of the mechanisms of secretin regulation of cholangiocyte functions. After a description of the intracellular mechanisms by which secretin stimulates ductal secretion, we discuss the heterogeneous responses of different-sized intrahepatic bile ducts to gastrointestinal hormones. Furthermore, we outline the role of a number of cooperative factors (e.g., nerves, alkaline phosphatase, gastrointestinal hormones, neuropeptides, and bile acids) in the regulation of secretin-stimulated ductal secretion. Finally, we discuss other factors that may also play an important role in the regulation of secretin-stimulated ductal secretion.

Regression of cholangiocyte proliferation after cessation of ANIT feeding is coupled with increased apoptosis

Cholangiocyte proliferation and loss through apoptosis occur in cholestatic liver diseases. Our aim was to determine the mechanisms of apoptosis in an animal model of ductal hyperplasia. Rats were fed alpha-naphthylisothiocyanate (ANIT) for 2 wk and subsequently fed normal chow for 1, 2, and 4 wk. Proliferation was assessed in sections by morphometry and in small and large cholangiocytes by proliferating cellular nuclear antigen immunoblots and measurement of cAMP levels. Apoptosis and reactive oxygen species (ROS) levels were also assessed. ANIT feeding increased small and large cholangiocyte proliferation and apoptosis. Cessation of ANIT feeding was associated with decreased proliferation and a further increase in apoptosis in small and large cholangiocytes. Cholangiocytes from ANIT-fed rats or exposed to ANIT in vitro showed increased apoptosis and ROS generation. ANIT-induced duct injury results in enhanced proliferation and apoptosis in small and large cholangiocytes. The mechanism of ANIT-induced apoptosis may be due to ROS generation induced directly by ANIT. Our model has implications for understanding the pathophysiology of cholangiopathies (characterized by the coexistence of cholangiocyte apoptosis and proliferation).

Regulation of cholangiocyte proliferation

Intrahepatic bile duct epithelial cells (i.e., cholangiocytes) are the target cells of chronic cholestatic liver diseases (i.e., cholangiopathies), which makes these cells of great interest to clinical hepatologists. This review will focus on "typical" cholangiocyte proliferation, whereas "atypical" (extension of cholangiocyte proliferation into parenchyma), and premalignant "oval" cell proliferation are reviewed elsewhere. The bile duct ligated (BDL) rat model, where most of the known mechanisms of cholangiocyte proliferation have been illustrated, was the first and remains the prototype animal model for "typical" cholangiocyte proliferation. Following a short overview of cholangiocyte functions, we briefly discuss the: (i) in vivo models [i.e., BDL (Fig. 1 and 4), chronic alpha-naphthylisothiocyanate (ANIT) or bile acid feeding (Fig. 2), acute carbon tetrachloride (CCl4) feeding and partial hepatectomy; and (ii) in vitro experimental tools [e.g., purified cholangiocytes and isolated intrahepatic bile duct units (IBDU)] that are key to the understanding of the mechanisms of "typical" cholangiocyte growth. In the second part of the review, we discuss a number of potential factors or conditions [e.g., gastrointestinal hormones, nerves, estrogens, blood supply, and growth factors] as well as the intracellular mechanisms [e.g., adenosine 3',5'-monophosphate (cAMP), and protein kinase C (PKC)] that may regulate "typical" cholangiocyte hyperplasia.

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.

Gastrin inhibits cholangiocarcinoma growth through increased apoptosis by activation of Ca2+-dependent protein kinase C-alpha

BACKGROUND/AIMS

We determined the role of gastrin in the regulation of cholangiocarcinoma growth.

METHODS

We evaluated for the functional presence of cholecystokinin (CCK)-B/gastrin receptors in the cholangiocarcinoma cell lines, Mz-ChA-1, HuH-28 and TFK-1. We determined the effect of gastrin on the growth of Mz-ChA-1, HuH-28 and TFK-1 cells. We evaluated the effect of gastrin on growth and apoptosis of Mz-ChA-1 in the absence or presence of inhibitors for CCK-A (L-364, 718) and CCK-B/gastrin (L-365, 260) receptors, the intracellular Ca2+ chelator (BAPTA/AM), and the protein kinase C (PKC)-alpha inhibitor, H7. We evaluated if gastrin effects on Mz-ChA-1 growth and apoptosis are associated with membrane translocation of PKC-alpha.

RESULTS

Gastrin inhibited DNA synthesis of Mz-ChA-1, HuH-28 and TFK-1 cells in a dose- and time-dependent fashion. The antiproliferative effect of gastrin on Mz-ChA-1 cells was inhibited by L-365, 260, H7 and BAPTA/AM but not L-364, 718. Gastrin induced membrane translocation of PKC-alpha. The inhibition of growth of Mz-ChA-1 cells by gastrin was associated with increased apoptosis through a PKC-dependent mechanism.

CONCLUSIONS

Gastrin inhibits the growth of Mz-ChA-1, HuH-28 and TFK-1 cells. Gastrin inhibits growth and induces apoptosis in Mz-ChA-1 cells through the Ca2+-dependent PKC-alpha. The data suggest a therapeutic role for gastrin in the modulation of cholangiocarcinoma growth.

Estrogens stimulate proliferation of intrahepatic biliary epithelium in rats

BACKGROUND & AIMS

We investigated the expression of estrogen receptor (ER) alpha and beta subtypes in cholangiocytes of normal and bile duct-ligated (BDL) rats and evaluated the role and mechanisms of estrogens in the modulation of cholangiocyte proliferation.

METHODS

ER-alpha and ER-beta were analyzed by immunohistochemistry, reverse-transcription polymerase chain reaction, and Western blotting in normal and BDL rats. The effects of the ER antagonists tamoxifen and ICI 182,780 on cholangiocyte proliferation were evaluated.

RESULTS

Cholangiocytes expressed both ER-alpha and ER-beta subtypes, whereas hepatocytes expressed only ER-alpha. In association with a marked cholangiocyte proliferation and with enhanced estradiol serum levels, the immunoreactivity for ER-alpha involved a 3-fold higher percentage of cholangiocytes in 3-week BDL than in normal rats; immunoreactivity for ER-beta showed a 30-fold increase. Western blot analysis showed that during BDL, the total amount of ER-beta in cholangiocytes was markedly increased (5-fold), whereas that of ER-alpha decreased slightly (-25%). Treatment with tamoxifen or ICI 182,780 of 3-week BDL rats inhibited cholangiocyte proliferation and induced overexpression of Fas antigen and apoptosis in cholangiocytes. In vitro, 17 beta estradiol stimulated proliferation of cholangiocyte, an effect blocked to the same extent by tamoxifen or ICI 182,780.

CONCLUSIONS

This study suggests that estrogens and their receptors play a role in the modulation of cholangiocyte proliferation.

The function of alkaline phosphatase in the liver: regulation of intrahepatic biliary epithelium secretory activities in the rat

We studied the effects of alkaline phosphatase (AP) on the secretory processes of the rat intrahepatic biliary epithelium as well as the role of the intrahepatic biliary epithelium in the uptake and biliary secretion of exogenous AP. The effects of acute and chronic administration of AP on bile secretory parameters were investigated in vivo in normal and bile duct ligated (BDL) rats and in vitro in isolated rat bile duct units (IBDU). In vivo, acute AP administration decreased bile flow and biliary bicarbonate excretion and abolished secretin choleresis in BDL rats but not in normal rats. On the contrary, the AP inhibitor, levamisole, increased in BDL rat bile flow and biliary bicarbonate excretion. In vitro, basal and secretin-stimulated Cl(-)/HCO(3)(-) exchanger activity in IBDU was immediately inhibited by AP intraluminal microinjection (apical exposure) but only after a prolonged exposure to the basolateral pole. Levamisole increased the Cl(-)/HCO(3)(-) exchanger activity of IBDU. A significant basolateral uptake of AP occurs in IBDU with a progressive transport to the apical domain. AP chronic treatment increased AP and gamma-glutamyltranspeptidase (gamma-GT) activities in the intrahepatic bile ducts and hepatocyte canalicular pole, promoted enlargement of bile canaliculi, and decreased bile flow and biliary bicarbonate excretion. In conclusion, the intrahepatic biliary epithelium plays a role in the uptake and biliary secretion of serum AP. AP inhibits the secretory processes of the intrahepatic biliary epithelium and induces features of intrahepatic cholestasis after chronic administration. These findings indicate that AP plays an active role in down-regulating the secretory activities of the intrahepatic biliary epithelium.

Gastrin inhibits cholangiocyte growth in bile duct-ligated rats by interaction with cholecystokinin-B/Gastrin receptors via D-myo-inositol 1,4,5-triphosphate-, Ca(2+)-, and protein kinase C alpha-dependent mechanisms

We studied the role of gastrin in regulating cholangiocyte proliferation induced by bile duct ligation (BDL). In purified cholangiocytes, we evaluated (1) for the presence of cholecystokinin-B (CCK-B)/gastrin receptors, (2) the effect of gastrin on D-myo-Inositol 1,4,5-triphosphate (IP(3)) levels, and (3) the effect of gastrin on DNA synthesis and adenosine 3', 5'-monophosphate (cAMP) levels in the absence or presence of CCK-A (L-364,718) and CCK-B/gastrin (L-365,260) receptor inhibitors, 1, 2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetrakis(acetxymethyl ester) (BAPTA/AM; an intracellular Ca(2+) chelator), and 2 protein kinase C (PKC) inhibitors, 1-(5-Isoquinolinylsulfonyl)-2-methylpiperazine (H7) and staurosporin. To evaluate if gastrin effects on cholangiocyte proliferation are mediated by the isoform PKCalpha, we evaluated (1) for the presence of PKCalpha in cholangiocytes and (2) the effect of gastrin on the PKCalpha protein expression in a triton-soluble (containing cytoplasm + membrane) and a triton-insoluble (containing cytoskeleton) fraction. To evaluate the effects of gastrin in vivo, immediately following BDL, gastrin or bovine serum albumin (BSA) was infused by minipumps for 7 days to rats and we measured cholangiocyte growth and cAMP levels. We found CCK-B/gastrin receptors on cholangiocytes. Gastrin increased IP(3) levels. Gastrin inhibited DNA synthesis and cAMP synthesis in cholangiocytes. Gastrin effects on cholangiocyte functions were blocked by L-365,260, BAPTA/AM, H7, and staurosporin but not by L-364,718. Gastrin induced translocation of PKCalpha from cholangiocyte cytoskeleton to membrane. In vivo, gastrin decreased cholangiocyte growth and cAMP synthesis compared with controls. We concluded that gastrin inhibits cholangiocyte growth in BDL rats by interacting with CCK-B/gastrin receptors through a signal transduction pathway involving IP(3), Ca(2+), and PKCalpha.

Cholinergic system modulates growth, apoptosis, and secretion of cholangiocytes from bile duct-ligated rats

BACKGROUND & AIMS

To investigate the role of the cholinergic system in regulation of cholangiocyte functions, we evaluated the effects of vagotomy on cholangiocyte proliferation and secretion in rats that underwent bile duct ligation (BDL rats).

METHODS

After bile duct ligation (BDL), the vagus nerve was resected; 7 days later, expression of M3 acetylcholine receptor was evaluated. Cholangiocyte proliferation was assessed by morphometry and measurement of DNA synthesis. Apoptosis was evaluated by light microscopy and annexin-V staining. Ductal secretion was evaluated by measurement of secretin-induced choleresis, secretin receptor (SR) gene expression, and cyclic adenosine 3',5'-monophosphate (cAMP) levels.

RESULTS

Vagotomy decreased the expression of M3 acetylcholine receptors in cholangiocytes. DNA synthesis and ductal mass were markedly decreased, whereas cholangiocyte apoptosis was increased by vagotomy. Vagotomy decreased ductal secretion. Forskolin treatment prevented the decrease in cAMP levels induced by vagotomy, maintained cholangiocyte proliferation, and decreased cholangiocyte apoptosis caused by vagotomy in BDL rats. Cholangiocyte secretion was also maintained by forskolin.

CONCLUSIONS

Vagotomy impairs cholangiocyte proliferation and enhances apoptosis, leading to decreased ductal mass in response to BDL. Secretin-induced choleresis of BDL rats was virtually eliminated by vagotomy in association with decreased cholangiocyte cAMP levels. Maintenance of cAMP levels by forskolin administration prevents the effects of vagotomy on cholangiocyte proliferation, apoptosis, and secretion.

Expression of platelet-derived growth factor in newly formed cholangiocytes during experimental biliary fibrosis in rats

BACKGROUND/AIMS

Chronic cholestasis stimulates a fibroductular reaction which may progress to secondary biliary fibrosis and cirrhosis. Since platelet-derived growth factor has been indicated as a major fibrogenic factor in chronic liver disease, we analyzed its expression and that of its receptor beta subunit in a rat model of chronic cholestasis.

METHODS

Liver tissue samples collected at 7, 10, 21, and 28 days after induction of cholestasis obtained by bile duct ligation, were analyzed by immunohistochemistry, in situ hybridization and RNase protection assay for the expression of platelet-derived growth factor (PDGF)-B chain and receptor beta subunit. Furthermore, the expression of PDGF-B chain mRNA was analyzed in highly purified cholangiocytes from normal and cholestatic rat liver.

RESULTS

In cholestatic liver, platelet-derived growth factor-BB and B chain mRNA expression increased up to 4 weeks in epithelial cells of proliferating bile ducts, and periductular mesenchymal cells. The increased expression of PDGF-B chain mRNA was confirmed in highly purified cholangiocytes obtained from normal and cholestatic rat liver. The expression of the receptor beta subunit progressively increased after induction of cholestasis and was mainly localized to desmin-positive periductular hepatic stellate cells.

CONCLUSIONS

These data suggest that platelet-derived growth factor-B chain can be synthesized by cholangiocytes during chronic cholestasis. The presence of its receptor on periductular hepatic stellate cells raises the possibility that, in this experimental setting, this cytokine might contribute to fibrogenesis in vivo.

Acute carbon tetrachloride feeding induces damage of large but not small cholangiocytes from BDL rat liver

Bile duct damage and/or loss is limited to a range of duct sizes in cholangiopathies. We tested the hypothesis that CCl4 damages only large ducts. CCl4 or mineral oil was given to bile duct-ligated (BDL) rats, and 1, 2, and 7 days later small and large cholangiocytes were purified and evaluated for apoptosis, proliferation, and secretion. In situ, we measured apoptosis by morphometric and TUNEL analysis and the number of small and large ducts by morphometry. Two days after CCl4 administration, we found an increased number of small ducts and reduced number of large ducts. In vitro apoptosis was observed only in large cholangiocytes, and this was accompanied by loss of proliferation and secretion in large cholangiocytes and loss of choleretic effect of secretin. Small cholangiocytes de novo express the secretin receptor gene and secretin-induced cAMP response. Consistent with damage of large ducts, we detected cytochrome P-4502E1 (which CCl4 converts to its radicals) only in large cholangiocytes. CCl4 induces selective apoptosis of large ducts associated with loss of large cholangiocyte proliferation and secretion.

Acute carbon tetrachloride feeding selectively damages large, but not small, cholangiocytes from normal rat liver

The aim of this study was to develop a model of selective duct damage restricted to hormone-responsive segments corresponding to the ducts damaged in primary biliary cirrhosis (PBC). Carbon tetrachloride (CCl4) was fed by gavage to rats, and 2, 7, 14, and 28 days later, small and large cholangiocytes were isolated. Apoptosis was determined in situ by morphology and in purified cholangiocytes by assessment of nuclear fragmentation by 4, 6-diamidino-2-phenylindole (DAPI) staining. Cholangiocyte proliferation was evaluated in situ by morphometry of liver sections stained for cytokeratin-19 (CK-19) and by proliferating cellular nuclear antigen (PCNA) staining in liver sections and in purified cholangiocytes by PCNA gene expression. Ductal secretion was assessed by measurement of secretin receptor (SR) gene expression and secretin-induced cyclic adenosine 3',5'-monophosphate (cAMP) synthesis and secretin-induced choleresis. Two days after CCl4 administration, there was an increased number of small ducts, but a reduction of large ducts. Apoptosis, observed only in large ducts, was associated with decreased DNA synthesis and ductal secretion. Conversely, small cholangiocytes expressed de novo the SR gene and secretin-stimulated cAMP synthesis 2 days after CCl4 treatment. Proliferation of large cholangiocytes was delayed until 7 days, which was associated with a transient increase in ductal secretion in vivo. CCl4 effects on cholangiocytes were reversed by day 28. CCl4 treatment causes a decrease in large duct mass as a result of a higher rate of apoptosis and absence of initial proliferation in large cholangiocytes. These processes were concomitant with a decrease of ductal secretion in large cholangiocytes. Small cholangiocytes appear resistant to CCl4-induced apoptosis, and proliferate and transiently compensate for loss of proliferative and secretory activity of large cholangiocytes.

Differentiation-specific regulation of transgene expression in a diploid epithelial cell line derived from the normal F344 rat liver

To establish the differentiation potential of progenitor cells, non-parenchymal epithelial cells from the F344 rat liver (FNRL cells) were studied. These cells reacted with the OV-6 antibody marker of oval cells, but were negative for hepatocyte markers (albumin, transferrin, glycogen, glucose-6-phosphatase, H4 antigen), biliary markers (gamma glutamyl transpeptidase, cytokeratin-19), and alpha-fetoprotein, although exposure to sodium butyrate induced nascent albumin and alpha-fetoprotein mRNA transcription. When stably transduced, FNRL cells expressed a retroviral promotor-driven lacZ reporter in vitro, similar to transgene expression in hepatocyte-derived HepG2 cells. However, lacZ expression in FNRL cells was rapidly extinguished in intact animals, whereas the reporter remained active in HepG2 cells. Transplanted FNRL cells showed copious glucose-6-phosphatase expression; however, the cell differentiation programme remained incomplete, despite two-thirds partial hepatectomy, D-galactosamine treatment or bile duct ligation. Interestingly, lacZ expression resumed in cultures of FNRL cells explanted from recipients. Moreover, lacZ expression was down-regulated by gamma-interferon in FNRL cells, without affecting lacZ activity in HepG2 cells. The data indicate that although subpopulations of oval cells may not fully differentiate into mature hepatocytes, these cells might serve critical functions, such as glucose utilization, and help survival after liver injury. Also, introduced genes may be regulated in progenitor cells at multiple levels, including by interactions between regulatory sequences, differentiation-specific cellular factors, and extracellular signals; in vivo studies are thus especially important for analysing gene regulation in progenitor cells.

Bile acid feeding induces cholangiocyte proliferation and secretion: evidence for bile acid-regulated ductal secretion

BACKGROUND & AIMS

We have shown that taurocholate (TC) and taurolithocholate (TLC) interact in vitro with normal cholangiocytes, increasing DNA synthesis, secretin receptor (SR) gene expression, and adenosine 3',5'-cyclic monophosphate (cAMP) synthesis. To further extend these in vitro studies, we tested the hypothesis that bile acids (BAs) directly stimulate cholangiocyte proliferation and secretion in vivo.

METHODS

After feeding with TC or TLC (1% for 1-4 weeks), we assessed the following in vivo: (1) ductal proliferation by both morphometry and immunohistochemistry for proliferating cell nuclear antigen (PCNA) and measurement of [3H]thymidine incorporation; and (2) the effect of secretin on bile secretion and bicarbonate secretion in vivo. Genetic expression of H3-histone and SR and intracellular cAMP levels were measured in isolated cholangiocytes.

RESULTS

After BA feeding, there was an increased number of PCNA-positive cholangiocytes and an increased number of ducts compared with control rats. [3H]Thymidine incorporation, absent in control cholangiocytes, was increased in cholangiocytes from BA-fed rats. In BA-fed rats, there was increased SR gene expression (approximately 2.5-fold) and secretin-induced cAMP levels (approximately 3.0-fold) in cholangiocytes, which was associated with de novo secretin-stimulated bile flow and bicarbonate secretion.

CONCLUSIONS

These data indicate that elevated BA levels stimulate ductal secretion and cholangiocyte proliferation.

Endothelin-1 inhibits secretin-stimulated ductal secretion by interacting with ETA receptors on large cholangiocytes

We studied the expression of endothelin-1 (ET-1) receptors (ETA and ETB) and the effects of ET-1 on cholangiocyte secretion. The effects of ET-1 on cholangiocyte secretion were assessed in normal and bile duct-ligated (BDL) rats by measuring 1) basal and secretin-induced choleresis in vivo, 2) secretin receptor gene expression and cAMP levels in small and large cholangiocytes, and 3) luminal expansion in response to secretin in intrahepatic bile duct units (IBDU). ETA and ETB receptors were expressed by small and large cholangiocytes. ET-1 had no effect on basal bile flow or bicarbonate secretion in normal or BDL rats but decreased secretin-induced bicarbonate-rich choleresis in BDL rats. ET-1 decreased secretin receptor gene expression and secretin-stimulated cAMP synthesis in large cholangiocytes and secretin-induced luminal expansion in IBDU from normal or BDL rats. The inhibitory effects of ET-1 on secretin-induced cAMP synthesis and luminal duct expansion were blocked by specific inhibitors of the ETA (BQ-610) receptor. ET-1 inhibits secretin-induced ductal secretion by decreasing secretin receptor and cAMP synthesis, two important determinants of ductal secretion.

Heterogeneity of the proliferative capacity of rat cholangiocytes after bile duct ligation

We previously introduced the concept that intrahepatic bile duct epithelial cells, or cholangiocytes, are functionally heterogeneous. This concept is based on the observation that secretin receptor (SR) gene expression and secretin-induced cAMP synthesis are present in cholangiocytes derived from large (> 15 microns in diameter) but not small (< 15 microns in diameter) bile ducts. In work reported here, we tested the hypothesis that cholangiocytes are heterogeneous with regard to proliferative capacity. We assessed cholangiocyte proliferation in vivo by measurement of [3H]thymidine incorporation and in vitro by both [3H]thymidine incorporation and H3 histone gene expression in small (fraction 1) and large (fraction 2) cholangiocytes isolated from rats after bile duct ligation (BDL). In the two cholangiocyte subpopulations, we also studied basal somatostatin receptor (SSTR2) gene expression as well as the effects of somatostatin on 1) SR gene expression and secretin-induced cAMP synthesis and 2) [3H]thymidine incorporation and H3 histone gene expression. In normal rat liver, cholangiocytes, unlike hepatocytes, were mitotically dormant; after BDL, incorporation of [3H]thymidine markedly increased in cholangiocytes but not hepatocytes. When subpopulations of cholangiocytes were isolated after BDL, DNA synthesis assessed by both techniques was limited to large cholangiocytes, as was SSTR2 steady-state gene expression. In vitro, somatostatin inhibited SR gene expression and secretin-induced cAMP synthesis only in large cholangiocytes. Moreover, compared with no hormone, somatostatin inhibited DNA synthesis solely in large cholangiocytes. These results support the concept of the heterogeneity of cholangiocytes along the biliary tree, extend this concept to cholangiocyte proliferative activity, and imply that the proliferative compartment of cholangiocytes after BDL is located principally in the cholangiocytes lining large (> 15 microns) bile ducts.

Gastrin inhibits secretin-induced ductal secretion by interaction with specific receptors on rat cholangiocytes

We assessed the effect of gastrin on ductal secretion in normal and bile duct-ligated (BDL) rats. The effect of gastrin on ductal secretion was examined in the presence of proglumide, a specific antagonist for gastrin receptor (GR). We isolated pure cholangiocytes from normal and BDL rats and assessed gastrin effects on secretin receptor (SR) gene expression and intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels. We examined the presence of GR mRNA in cholangiocytes by reverse transcription polymerase chain reaction (RT-PCR). In normal or BDL rats, gastrin produced no changes in spontaneous bile secretion. Simultaneous infusion of gastrin inhibited secretin-induced choleresis and bicarbonate output in BDL rats. In the presence of proglumide gastrin did not inhibit secretin-induced choleresis in BDL rats. Gastrin decreased in cholangiocytes from BDL rats 1) SR gene expression and 2) secretin-induced cAMP levels. With the use of RT-PCR, GR mRNA was detected in cholangiocytes. Similar to what is shown for secretin and somatostatin, we propose that the opposing effects of secretin and gastrin on cholangiocyte secretory activity regulate ductal secretion in rats.

Functional expression of the apical Na+-dependent bile acid transporter in large but not small rat cholangiocytes

BACKGROUND & AIMS

Bile acids interact with cholangiocytes, resulting in cholangiocyte proliferation and increases in ductal bile secretion in large but not small cholangiocytes. It was proposed that for bile acids to exert these effects on cholangiocytes, a specific uptake mechanism must be present in cholangiocytes. The aim of this study was to show the expression of a bile acid transporter in cholangiocytes.

METHODS

Small and large cholangiocytes or intrahepatic bile duct units (IBDUs) were isolated from normal rats, and gene expression for the apical Na+-dependent bile acid transporter (ABAT) and the 14-kilodalton ileal cytosolic binding protein (IBABP) was assessed by ribonuclease-protection assays. Tissue and subcellular distribution of bile acid transporters was also studied. [14C]-Taurocholate uptake into cholangiocytes was determined.

RESULTS

Both ABAT and IBABP messenger RNAs were detected in large but not small cholangiocytes. By immunohistochemistry, ABAT was present in large but not small cholangiocytes. Immunofluorescence showed ABAT to be present in the apical membrane of large IBDUs. A Na+-dependent saturable uptake of taurocholate was present in large but not small cholangiocytes.

CONCLUSIONS

These proteins may mediate bile acid uptake from the duct lumen in large ducts, resulting in modification of canalicular bile secretion and modulation of ductal bile secretion and growth.

Role and mechanisms of action of acetylcholine in the regulation of rat cholangiocyte secretory functions

We investigated, in isolated bile duct units (IBDU) and cholangiocytes isolated from normal rat liver, the occurrence of acetylcholine (ACh) receptors, and the role and mechanisms of ACh in the regulation of the Cl-/HCO3- exchanger activity. The Cl-/HCO3- exchanger activity was evaluated measuring changes in intracellular pH induced by acute Cl- removal/readmission. M3 subtype ACh receptors were detected in IBDU and isolated cholangiocytes by immunofluorescence, immunoelectron microscopy, and reverse transcriptase PCR. M1 subtype ACh receptor mRNA was not detected by reverse transcriptase PCR and M2 subtype was negative by immunofluorescence. ACh (10 microM) showed no effect on the basal activity of the Cl-/HCO3- exchanger. When IBDU were exposed to ACh plus secretin, ACh significantly (P < 0.03) increased the maximal rate of alkalinization after Cl- removal and the maximal rate of recovery after Cl- readmission compared with secretin alone (50 nM), indicating that ACh potentiates the stimulatory effect of secretin on the Cl-/HCO3- exchanger activity. This effect of ACh was blocked by the M3 ACh receptor antagonist, 4-diphenyl-acetoxy-N-(2-chloroethyl)-piperidine (40 nM), by the intracellular Ca2+ chelator, 1,2-bis (2-Aminophenoxy)- ethane-N,N,N', N'-tetraacetic acid acetoxymethylester (50 microM), but not by the protein kinase C antagonist, staurosporine (0.1 microM). Intracellular cAMP levels, in isolated rat cholangiocytes, were unaffected by ACh alone, but were markedly higher after exposure to secretin plus ACh compared with secretin alone (P < 0.01). The ACh-induced potentiation of the secretin effect on both intracellular cAMP levels and the Cl-/HCO3- exchanger activity was individually abolished by two calcineurin inhibitors, FK-506 and cyclosporin A (100 nM).

CONCLUSIONS

M3 ACh receptors are markedly and diffusively represented in rat cholangiocytes. ACh did not influence the basal activity of the Cl-/HCO3- exchanger, but enhanced the stimulation by secretin of this anion exchanger by a Ca2+-dependent, protein kinase C-insensitive pathway that potentiates the secretin stimulation of adenylyl cyclase. Calcineurin most likely mediates the cross-talk between the calcium and adenylyl cyclase pathways. Since secretin targets cholangiocytes during parasympathetic predominance, coordinated regulation of Cl-/HCO3- exchanger by secretin (cAMP) and ACh (Ca2+) could play a major role in the regulation of ductal bicarbonate excretion in bile just when the bicarbonate requirement in the intestine is maximal.

gamma-Interferon inhibits secretin-induced choleresis and cholangiocyte proliferation in a murine model of cirrhosis

BACKGROUND/AIMS

Cholangiocyte proliferation is associated with increased secretin receptor gene expression and secretin-induced choleresis. Since gamma-interferon has antiproliferative effects, we tested the hypothesis that gamma-interferon inhibits ductal proliferation and secretin-stimulated choleresis associated with cirrhosis.

METHODS

Mice were treated with 0.1 ml of 25% carbon tetrachloride intraperitoneally twice weekly and 5% alcohol in drinking water for 12 weeks to induce cirrhosis and subsequently gamma-interferon 10(5) intramuscularly was administered daily for 10 weeks. We measured the effects of carbon tetrachloride and gamma-interferon on liver collagen content by morphometric analysis and hydroxyproline content. We measured the effects of gamma-interferon on ductal mass by morphometry and on ductal secretion by assessment of secretin receptor gene expression and secretin-induced choleresis.

RESULTS

Compared to controls, there was an increase in liver hydroxyproline content of carbon tetrachloride-treated mice with histologic evidence of cirrhosis. Gamma-interferon treatment significantly decreased collagen liver content with loss of histologic features of cirrhosis. Morphometry revealed an increased number of bile ducts in cirrhotic mice as compared to controls or cirrhotics who received gamma-interferon. Secretin receptor mRNA levels were higher in cirrhotic mice compared to controls but this increase was inhibited by gamma-interferon. Secretin stimulated ductal secretion in cirrhotic mice but not control or cirrhotic mice who received gamma-interferon.

CONCLUSIONS

We have established a murine model for cirrhosis and have shown, consistent with our hypothesis, that gamma-interferon decreases collagen content, ductal mass and secretin-induced choleresis incirrhotic mice.

Bile acids stimulate proliferative and secretory events in large but not small cholangiocytes

Accumulation of bile acids (BA) and cholangiocyte proliferation occur in cholestasis, but BA effects on the proliferative and secretory capacity of cholangiocytes are undefined. Cholangiocyte proliferation coupled with increased expression of H3 histone and secretin receptor (SR) genes and secretin-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) levels is limited to large cholangiocytes. We isolated pooled small and large cholangiocytes and studied the effect of taurocholic (TC) and taurolithocholic (TLC) acids on proliferation, by measurement of H3 histone gene expression, and secretion, by measurement of SR gene expression, cAMP levels, and Cl-/HCO3- exchanger activity. In pooled cholangiocytes, TC and TLC increased H3 histone (12-fold) and SR (3-fold) gene expression and both spontaneous (1.4-fold) and secretin-induced (4-fold) cAMP response. TC and TLC increased H3 histone (10-fold) and SR (2-fold) gene expression and secretin-induced cAMP response and Cl-/HCO3- exchanger activity (3-fold) only in large cholangiocytes. In large cholangiocytes, BA may have a signaling function in the modulation of ductal secretion.

Large but not small intrahepatic bile ducts are involved in secretin-regulated ductal bile secretion

We have shown that agonist-regulated ductal secretion is limited to large cholangiocytes. To directly study cholangiocyte heterogeneity along the length of the normal biliary tree, we defined the genetic and functional expression of agonist-induced ductal secretion in intrahepatic bile duct units (IBDU) of different sizes. Small IBDU (< 15-microns diam) were separated from large IBDU (> or = 15-microns diam), and then ducts of different sizes were characterized by morphometric analysis, gene expression, secretin-induced adenosine 3',5'-cyclic monophosphate (cAMP) synthesis, and secretion by change in luminal size in response to agonists. IBDU diameters ranged from 11 to 65 microns. Secretin increased ductal secretion solely in large IBDU. Forskolin induced a modest increase in ductal secretion in small IBDU but markedly increased ductal secretion in large IBDU. Secretion increased Cl-/HCO3- exchanger activity and cAMP levels in large but not small IBDU. Secretin receptor and Cl-/HCO3 exchanger mRNAs were detected only in large IBDU. We propose that agonist-induced ductal secretion occurs in large (> or = 15-microns diam) but not small (< 15-microns diam) intrahepatic ducts.

Molecular and functional heterogeneity of cholangiocytes from rat liver after bile duct ligation

Cholangiocytes, the epithelial cells that line intrahepatic bile ducts, participate in bile secretion via basal and agonist-stimulated transport of solutes and water. On the basis of subtle structural differences between cholangiocytes lining small vs. large bile ducts, as well as known phenotypic variations among transporting epithelia in other organs, we demonstrated that cholangiocytes are functionally heterogeneous along the intrahepatic biliary tree of normal rats. In studies reported here, we confirm and extend the concept of functional heterogeneity of cholangiocytes by employing the bile duct-ligated (BDL) rat model of cholestasis associated with selective cholangiocyte proliferation. Using novel isolation and separatory techniques, we prepared subpopulations of pure small, medium, and large cholangiocytes from BDL rats and compared them with regard to gene expression and basal or agonist-responsive transport activities. Although transcripts for gamma-glutamyl transpeptidase and cytokeratin 19, two cholangiocyte-specific proteins, and glyceraldehyde-3-phosphate dehydrogenase, a housekeeping gene, were in all three subpopulations, genes for several proteins involved in solute transport [Cl-/HCO3- exchanger, cystic fibrosis transmembrane conductance regulator (CFTR), and secretin receptor] were expressed only in medium and large cholangiocytes. Consistent with these findings, secretin increased intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP) and 36Cl- efflux rates in medium and large cholangiocytes but not in small cholangiocytes. Also, forskolin/8-(4-chlorophenylthio)-cAMP stimulated 36Cl- efflux rates only in medium and large cholangiocytes, consistent with selective functional expression of CFTR in these subpopulations. These results support the molecular and functional heterogeneity of cholangiocytes within the intrahepatic biliary ductal system and are consistent with the notion that hormone-regulated transport of solutes after BDL occurs principally in medium and large cholangiocytes in a fashion similar to that observed in normal rat liver.

Regrowth of the rat biliary tree after 70% partial hepatectomy is coupled to increased secretin-induced ductal secretion

BACKGROUND & AIMS

After partial hepatectomy, liver regeneration occurs with the return of hepatocyte mass to normal, Limited data exist regarding the renewal of the biliary tree after partial hepatectomy. This study tested the hypothesis that, after partial hepatectomy, the biliary tree regenerates by proliferation of the remaining cholangiocytes, leading to an increase in secretin-induced ductal bile secretion.

METHODS

After 70% partial hepatectomy, cholangiocyte proliferation was assessed in situ by morphometric analysis and In vitro by measurement of 3H-thymidine incorporation. Ductal secretion was estimated by measurement of secretin receptor gene expression and adenosine 3',5'-cyclic monophosphate (cAMP) levels in vitro and by the effect of secretin on ductal bile secretion in vivo.

RESULTS

DNA synthesis was undetectable in control cholangiocytes, increased and peaked at day 3 after partial hepatectomy, and returned to normal by day 28. Morphometric analysis showed regrowth of the biliary tree beginning at day 1 with restoration by day 10. The expression of secretin receptor gene and secretin-induced cAMP levels and secretin-induced bicarbonate-rich choleresis increased during the period of bile duct renewal.

CONCLUSIONS

After partial hepatectomy, the increase in secretin-induced ductal bile secretion observed during bile duct renewal results from proliferation of remaining cholangiocytes.

Morphological, molecular, and functional heterogeneity of cholangiocytes from normal rat liver

BACKGROUND & AIMS

While the lobular heterogeneity of hepatocytes is established, limited data suggest that bile duct epithelial cells, of cholangiocytes, are heterogeneous along the normal intrahepatic biliary tree. Thus, we tested the hypothesis that cholangiocytes are structurally and functionally heterogeneous in the biliary tract of normal rats.

METHODS

A series of in situ and in vitro experiments was performed in normal rats using complementary morphometric, molecular, and functional approaches.

RESULTS

In situ morphometry showed that (1) intrahepatic bile ducts are heterogeneous in external diameter (5-200 mum); (2) individual cholangiocytes lining bile ducts are heterogeneous in area (3-80 mum2); and (3) a significant relationship exists between bile duct diameter and cholangiocyte area. Using a novel approach developed by us, we isolated three subpopulations of small, medium, and large cholangiocytes and compared them at the molecular and functional level. The expression of two cholangiocyte-specific genes (gamma-glutamyl transpeptidase and cytokeratin 19) was similar among the three subpopulations. In contrast, secretion receptor, Cl-/HCO3- exchanger, and cystic fibrosis transmembrane conductance regulator messenger RNAs were differentially expressed, being present on medium and large but not small cholangiocytes. At the functional level, adenosine 3', 5'-cyclic monophosphate and intracellular pH responses and Cl-/HCO3- exchanger activity was enhanced by secretion in medium and large but not small cholangiocytes.

CONCLUSIONS

These data indicate that cholangiocytes are morphologically and functionally heterogeneous along the normal intrahepatic biliary tree and suggest that secretion-regulated transport of water and electrolytes occurs principally in medium and large ducts.

Somatostatin inhibits secretin-induced ductal hypercholeresis and exocytosis by cholangiocytes

Previous work from our laboratory has implicated hormone-induced plasma membrane movement (i.e., endo- and exocytosis) in water and electrolyte transport by the epithelial cells that line the ducts in the liver (i.e., cholangiocytes). To further explore the cellular mechanisms regulating ductal bile secretion, we infused somatostatin and/or secretin intravenously into rats 2 wk after either bile duct ligation (BDL), a procedure that induces selective proliferation of cholangiocytes, or sham surgery and measured bile flow and biliary constituents. We also determined the effect of somatostatin on basal and secretin-induced exocytosis by purified cholangiocytes isolated from rat liver after BDL. Finally, we studied the expression of the somatostatin receptor gene by both ribonuclease (RNase) protection and nuclear run-on assays using cDNA encoding for two subtypes of the somatostatin receptor gene (i.e., SSTR1 and SSTR2). In vivo, somatostatin infusion caused a dose-dependent bicarbonate-poor decrease (57% maximal decrease below baseline; P < 0.05) in bile flow in BDL but not in sham-operated rats; in contrast, secretin caused a dose-dependent bicarbonate-rich choleresis (228% maximal increase above baseline; P < 0.05) in BDL but not in sham-operated rats. Simultaneous or prior infusion of somatostatin inhibited the secretin-induced hypercholeresis in BDL rats. In vitro, somatostatin had no effect on basal exocytosis by cholangiocytes isolated from BDL rats; however, somatostatin inhitibed (88% maximal inhibition; P < 0.05) secretin-induced exocytosis by cholangiocytes in a dose-dependent fashion. In addition, somatostatin inhibited secretin-induced increases in levels of adenosine 3',5'-cyclic monophosphate (cAMP) in cholangiocytes isolated from BDL rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholangiocytes express the aquaporin CHIP and transport water via a channel-mediated mechanism

Cholangiocytes line the intrahepatic bile ducts and regulate salt and water secretion during bile formation, but the mechanism(s) regulating ductal water movement remains obscure. A water-selective channel, the aquaporin CHIP, was recently described in several epithelia, so we tested the hypothesis that osmotic water movement by cholangiocytes is mediated by CHIP. Isolated rodent cholangiocytes showed a rapid increase in volume in the presence of hypotonic extracellular buffers; the ratio of osmotic to diffusional permeability coefficients was > 10. The osmotically induced increase in cholangiocyte volume was inversely proportional to buffer osmolality, independent of temperature, and reversibly blocked by HgCl2. Also, the luminal area of isolated, enclosed bile duct units increased after exposure to hypotonic buffer and was reversibly inhibited by HgCl2. RNase protection assays, anti-CHIP immunoblots, and immunocytochemistry confirmed that CHIP transcript and protein were present in isolated cholangiocytes but not in hepatocytes. These results demonstrate that (i) isolated cholangiocytes and intact, polarized bile duct units manifest rapid, mercury-sensitive increases in cell size and luminal area, respectively, in response to osmotic gradients and (ii) isolated cholangiocytes express aquaporin CHIP at both the mRNA and the protein level. The data implicate aquaporin water channels in the transcellular movement of water across cholangiocytes lining intrahepatic bile ducts and provide a plausible molecular explanation for ductal water secretion.

Recent advances in the isolation of liver cells

The development of new and refined separation techniques--including FACS, FFE, CFE and isopyknic gradients--has had a profound impact on the ability of investigators to isolate specific cell types from the liver. Although some of these techniques, such as FFE, may be of limited preparative value, they are nonetheless important analytical tools that detect subtle differences among cell subpopulations. The isolation of highly purified preparations of liver cells in large yields requires the use of more conventional purification methods such as CFE and isopyknic centrifugation. Immunological approaches represent a key development for the isolation of specific liver cell types, especially when they are used in combination with other techniques. Excellent, reliable and relatively simple techniques now exist to isolate highly purified preparations of hepatocytes, cholangiocytes, KCs, SCs, FSC, myofibroblasts and pit cells. Additional work is necessary to refine techniques for the isolation of dendritic cells and lymphocytes.

Upregulation of secretin receptor gene expression in rat cholangiocytes after bile duct ligation

Secretion stimulates ductular bile secretion by binding to receptors on intrahepatic bile duct epithelial cells (i.e., cholangiocytes). In the rat, this choleretic effect increases after bile duct ligation (BDL). Although cholangiocyte proliferation induced by BDL contributes to secretin-induced hypercholeresis, the mechanisms modulating these alterations in secretin-induced ductular bile secretion are obscure. Thus we studied the expression of secretin receptor mRNA (SR-mRNA) in purified liver cells from normal and BDL rats. Northern blot analysis and RNase protection assays with mRNA from purified liver cells demonstrated SR-mRNA only in cholangiocytes; moreover, SR gene expression showed a seven- to ninefold increase in individual cholangiocytes from BDL rats compared with controls. This increase in SR-mRNA expression was related to a similar increase in the rate of transcription of SR-mRNA in cholangiocytes from BDL rats. Thus our studies indicate that 1) SR-mRNA is detected in liver only in cholangiocytes; 2) BDL causes an increase in SR-mRNA in individual cholangiocytes; and 3) the increase in SR-mRNA after BDL is partly related to an increase in the rate of transcription of SR-mRNA by cholangiocytes after BDL. Our data suggest that upregulation of the SR gene may contribute to secretin-induced hypercholeresis.

Butyrate synchronization of hepatocytes: modulation of cycling and cell cycle regulated gene expression

To develop a model for studies of liver growth control, we characterized cell cycle synchronization of liver-derived cells with sodium butyrate. Exposure of cultured HTC (rat hepatoma) cells to 5 mM butyrate arrested cell growth in a reversible manner. Flow cytometric analysis revealed that butyrate-treated HTC cells were restricted in G0/G1, as well as S/G2M phases. After release from butyrate arrest, HTC cells underwent synchronous cycles of DNA synthesis and transited through S phase. Inhibition of cell growth by butyrate was associated with a complex pattern of cell cycle regulated gene expression, including a decoupling of c-fos and c-jun gene expression. Transcription of c-fos, as well as c-jun increased with butyrate arrest, whereas steady rate mRNA levels of c-jun only were increased, suggesting additional regulation of c-fos. In addition, butyrate-arrested cells exhibited a transcriptionally determined accumulation of H3 histone, C-Ha-ras and ornithine decarboxylase mRNAs, suggesting that cell cycle-related check points following the onset of S phase were modulated. An increase in c-myc mRNA levels in butyrate-arrested cells was post-transcriptionally regulated. After release from butyrate-arrest, the abundance of immediate early, as well as S phase regulated, gene expression changed coordinately with S phase cell transitions. Thus, exposure of HTC cells to butyrate modulates cell cycle regulated gene expression, inhibits cycling, and results in accumulation of cells in specific compartments. Synchronization of liver cells with butyrate should, therefore, provide a useful model for defining cell cycle-related events in response to various mitogenic stimuli.

Models for hepatic progenitor cell activation

Activation of liver progenitor cells was studied in rat liver induced to regenerate after carbon tetrachloride (CCl4) or D-galactosamine (GalN) injury. A change in the concentration of histone-3 mRNA was used as a marker for cell proliferation and the fetal form of alpha-fetoprotein (AFP) mRNA as a marker for fetal hepatoblasts. gamma-Glutamyltranspeptidase (GGT) and glutathione-S-transferase P were used as markers for activation of putative liver progenitor cells. After CCl4 administration, the proliferative response was high but confined primarily to parenchymal cells. No changes in the relative expression of albumin, glutathione-S-transferase P or insulin-like growth factor-II were observed. On the other hand, the level of AFP mRNA was increased modestly and predominantly in the nonparenchymal cell (NPC) fraction. After GalN administration, proliferation of NPC began within 24 hr, primarily in the portal area around the bile ducts. Activated cells were bile "duct-like" in appearance, had scant cytoplasm, and a pale, oval-shaped nucleus. On Day 2, they formed rows and clusters, expanding from the portal zone and invading the parenchyma, as well as proliferating in regions of focal necrosis. On Days 3 and 5, NPC expressing histone-3 mRNA expanded further, forming pseudoducts and islet-like structures (NPC structures) throughout the hepatic lobule. Proliferating NPC were positive for GGT. Some GGT-positive cells on Days 3 and 5 were also positive for fetal AFP mRNA. Expression of fetal AFP mRNA lagged behind that of GGT by 24 hr, was highest on Day 5, and then declined. Expression of albumin mRNA and glucose 6-phosphatase decreased during the first 48 hr after GalN administration and then resumed. These findings indicate that after GalN injury, the liver responds with activation of putative progenitor cells that proliferate and then differentiate through the hepatocyte lineage, whereas the regenerative response after CCl4 administration is primarily through proliferation of preexisting hepatocytes.

Distribution of glucose-6-phosphatase activity in normal, hyperplastic, and preneoplastic rat liver

The significance of glucose-6-phosphatase (G6P) expression by bile duct-like cells proliferating during hepatocarcinogenesis in the histogenesis of hepatocellular carcinoma is not clear. To this end, we measured the histochemical and biochemical activity of G6P in normal rat liver, and in rat livers in which bile duct-like proliferation was induced by either hyperplastic (bile duct ligation for 14 days or feeding alpha-naphthylisothiocyanate for 28 days) or neoplastic (feeding a choline-devoid diet containing 0.1% ethionine for 60 days) regimens. In normal, hyperplastic, and preneoplastic livers, G6P histochemical activity was confined to the hepatocytes; proliferated bile duct-like cells, like normal bile ducts, did not display visible G6P staining. When the enzyme activity was determined biochemically, however, hydrolysis of glucose-6-phosphate was observed in both parenchymal and nonparenchymal liver cells isolated from all experimental animals. In elutriated nonparenchymal fractions, G6P activity was directly proportional to the number of cells positive for gamma-glutamyl transpeptidase and cytokeratin no. 19 (markers of bile duct cells) and inversely proportional to the number of cells positive for vimentin (marker of mesenchymal cells). These results indicate that, while by light microscopy hepatic G6P histochemical activity is detectable only in the hepatocytes, the biochemical activity is also expressed in proliferating bile duct-like cells. However, the nonparenchymal activity is observed during both neoplastic and hyperplastic liver growth, thus indicating that the presence of this enzyme in bile duct-like cells proliferating during hepatocarcinogenesis should not necessarily be construed as supporting their stem cell nature nor their neoplastic commitment.

Secretin stimulates bile ductular secretory activity through the cAMP system

Although convincing evidence has been obtained to support a ductular origin of secretin choleresis, the precise mechanism of the choleretic effect of the hormone is poorly understood. The present studies were carried out to 1) further clarify the anatomic site at which secretin stimulates bile flow and 2) establish the signal transduction system underlying this effect. To this end, parenchymal and nonparenchymal liver cells, the latter enriched in bile duct cells, were isolated from rats with ductular cell hyperplasia, and the effect of secretin on intracellular formation of both adenosine 3',5'-cyclic monophosphate (cAMP) and inositol phosphates (IPs) was compared with that observed with glucagon and [Tyr10,13,Phe22,Trp25]secretin (SG-secretin). In the pancreas, secretin stimulates both messenger systems, while SG-secretin activates only the cAMP cascade. In isolated hepatocytes, both secretin and SG-secretin failed to increase formation of cAMP and IPs, which were instead activated by glucagon. In isolated bile duct cells, secretin induced formation of both cAMP and IPs, while SG-secretin stimulated solely the cAMP system, as in the pancreas. Glucagon did not stimulate either messenger system in this cell preparation. In vivo, both secretin and SG-secretin stimulated a bicarbonate-rich fluid in rats with bile ductular cell hyperplasia and in normal guinea pigs, which was demonstrated to originate at the distal biliary epithelium. These findings support the existing view that glucagon stimulates canalicular bile flow, while secretin increases secretory activity at the bile ductules and/or ducts. More importantly, they indicate that stimulation of ductular secretory activity by secretin is mediated by the cAMP system and does not involve the IP signal transduction pathway.

Widespread differentiation stage-specific expression of the gene encoding phosphoprotein p19 (metablastin) in mammalian cells

p19 is a highly conserved 19 kD cytosolic protein that undergoes phosphorylation in response to diverse extracellular factors in mammalian cells. Its expression is abundant in brain and testis and is developmentally regulated. To gain insights regarding its function, we analyzed the expression of p19 mRNA in a variety of cell types during induction of differentiation. Murine erythroleukemia cells showed a moderate increase followed by a marked decrease in the abundance of p19 mRNA during induction of differentiation. In murine C2 myoblasts and primary fetal rat osteoblasts, p19 mRNA was abundant in replicating cells and decreased to undetectable levels during differentiation. In resting human peripheral blood lymphocytes, p19 mRNA was virtually undetectable but was strongly induced during blast transformation of both B and T cells. In rat liver, p19 mRNA was abundant on embryonic day 17 and decreased during early postnatal development. Upon fractionation of adult rat liver cells by centrifugal elutriation, p19 mRNA was not detected in hepatocytes while a low level was observed in a fraction enriched in non-parenchymal epithelial cells. CCl4-induced liver regeneration resulted in induction of p19 mRNA in hepatocytes. Primary cultures of embryonic and neonatal rat brain were analyzed by indirect immunofluorescence using co-staining with stage-specific markers. p19 expression was restricted to immature neurons and oligodendrocyte precursors. In contrast to the other cell types examined, the neuronal and glial precursors that express p19 were shown, using BrdU labeling, to be postmitotic both in primary culture and in vivo. The data demonstrate widespread, stage-specific expression of p19 and suggest that the protein exerts a general, lineage-independent function during induction of differentiation of mammalian cells. In view of the available evidence on the stimulation of serine phosphorylation of p19 by several growth factors, our working hypothesis is that phosphorylation of p19 may be involved in the mechanism by which growth factors control cell differentiation.

Distribution of albumin and alpha-fetoprotein mRNAs in normal, hyperplastic, and preneoplastic rat liver

The nature of bile duct-like (oval) cells proliferating during chemical hepatocarcinogenesis has been controversial. To investigate this issue further, the authors compared the hepatic distribution of albumin (ALB) and alpha-fetoprotein (AFP) mRNAs in rats in which oval cell proliferation was induced by feeding a choline-devoid diet containing 0.1% ethionine (CDE, a hepatocarcinogenic diet) with that in normal rats and in rats in which biliary epithelial cell hyperplasia was induced by either bile duct ligation or feeding alpha-naphthylisothiocyanate (ANIT). Northern blot analysis in parenchymal and nonparenchymal liver cells isolated from these animals demonstrated that ALB mRNA was present in the hepatocytes of both control and experimental animals, whereas this transcript was detected in nonparenchymal epithelial cells only in CDE-fed rats. Alpha-fetoprotein mRNA was not seen in either parenchymal or nonparenchymal cells isolated from normal or hyperplastic livers induced by bile duct ligation or ANIT feeding. In CDE-fed rats, however, both parenchymal and nonparenchymal cell populations displayed AFP message. In situ hybridization directly demonstrated nonparenchymal cell expression of both ALB and AFP transcripts in CDE-fed rats. Most surprisingly, ALB and AFP mRNAs were also detected by in situ hybridization in occasional nonparenchymal cells located in portal tracts near the limiting plate in normal liver, as well as under conditions associated with bile duct hyperplasia. Immunohistochemical studies of intermediate filament proteins, cytokeratin 19 (a marker of glandular epithelia), vimentin (a marker of mesenchymal lineage), and desmin (a marker of muscle cell differentiation) demonstrated that oval cells, as well as normal and hyperplastic bile duct cells, were positive for cytokeratin 19 and negative for both vimentin and desmin. Cytokeratin-positive oval cells formed duct profiles and were connected to preexisting ductules and ducts. These results are construed to suggest that oval cells proliferating during CDE hepatocarcinogenesis are derived from epithelial cells within the biliary tree. The presence of cells with similar morphologic appearance, periportal location, and AFP and ALB expression in normal liver suggests that these cells may be the progenitors of oval cells induced by some carcinogenic regimens.