Journal of Hepatology
Volume 33, Issue 2 , Pages 333-340, August 2000

Regulation and deregulation of cholangiocyte proliferation

Division of Gastroenterology, Department of Clinical Medicine, University of Rome “La Sapienza”, Rome, Italy

Article Outline

 

Cholangiocytes are the epithelial cells which line the intrahepatic biliary tree, a network of interconnecting ducts of increasing diameter from the duct of Hering to the extrahepatic bile ducts 1., 2., 3., 4., 5., 6.. Cholangiocytes determine the final bile composition through a series of secretory and absorptive processes regulated by a number of hormones and neuropeptides 2., 3., 4., 5.. The intrahepatic bile ducts are the target of damage in a group of chronic cholestatic liver diseases recently classified as Vanishing Bile Duct Syndromes 4., 7., 8., 9.. These diseases are characterized by the progressive disappearance of intrahepatic bile ducts, which leads to a severe ductopenic condition in the terminal stages 7., 8., 9.. The residual bile ducts tend to proliferate as a compensatory mechanism (8). Thus, the course of these diseases is characterized by a balance between damage (loss) of bile ducts and compensatory proliferation of the residual ducts. The terminal decompensated stages are characterized by the inefficacy of proliferation to balance for the loss of intrahepatic bile ducts. Therefore, a therapeutic strategy designed to support efficacious cholangiocyte proliferation could delay progression to ductopenia. This represents a challenge for the future.

The aim of this review is to focus on current knowledge of the regulation and dysregulation of cholangiocyte proliferation.

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Cholangiocyte Proliferation 

A peculiar property of cholangiocytes is the capacity to proliferate, as evidenced in specific experimental conditions as well as in different human pathological conditions 1., 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12.. Neoplastic proliferation leads to cholangiocarcinoma, which is one of the worst malignancies (13).

Cholangiocyte proliferation is usually classified as type I (“typical”), type II(“atypical”) and type III (oval cell) proliferation. Type I or “typical” cholangiocyte proliferation is a hyperplastic reaction resulting in an increased number of intrahepatic bile ducts, which, however, remain confined to portal spaces 7., 8., 9.. The proliferating cholangiocytes form a well-differentiated three-dimensional network of tubular structures with a well-defined lumen 7., 8., 9.. In the rat, “typical” cholangiocyte proliferation is observed after bile duct ligation (BDL) (6), 70% hepatectomy (14), chronic α-naphthylisothiocyanate feeding (15), chronic L-proline treatment (16), prolonged oral administration of lithocholate (LCA), chenodeoxycholate and taurocholate (TCA) 17., 18.. In humans, a “typical” proliferation may be observed in acute severe obstructive cholestasis (19) as well as in the early phases of chronic cholestatic liver diseases 7., 8., 9.. It is quite accepted that the “typical” ductal proliferation results from the elongation of pre-existing bile ducts located within portal areas. This conclusion arises from a number of observations: i) in the BDL rat proliferating ducts retain immunohistochemical, ultrastructural and functional characteristics of their normal counterparts and appear, morphologically, as elongation of pre-existing ducts 6., 8.; ii) thymidine labeling studies were consistent with a proliferation of pre-existing ducts 20., 21.; iii) in humans with extrahepatic biliary obstruction of recent onset, proliferating parenchymal and bile duct cells express their respective normal cytokine profile (19). However, in young rats submitted to BDL, proliferating ductular cells show characteristics of progenitor cells given the expression of full length α-fetoprotein and stem cell factor, c-kit, thus indicating that, in “typical” proliferation as well, the participation of a stem cell compartment cannot be completely ruled out (22).

Type II or “atypical” ductular proliferation occurs in rodents in association with oval cell proliferation after chronic administration of chemicals such as D-galactosamine (23) and carbon tetrachloride (CCl4) (24). In humans, it occurs after massive hepatic necrosis, in alcoholic liver diseases, long-standing extrahepatic biliary obstruction, focal nodular hyperplasia and chronic cholestatic liver diseases [(i.e. primary biliary cirrhosis (PBC), primary sclerosing cholangitis (PSC)] 7., 8., 9., 25., 26., 27.. The process is characterized by an irregular proliferation of intrahepatic bile ducts which are not confined to portal areas but spread into periportal and parenchymal regions as wedge-shaped extensions creating an irregular portal-parenchymal interface(“biliary piecemeal necrosis”). These proliferating ductular structures are arranged in a three-dimensional network of tortuous and irregular conduits that do not possess a well-defined lumen and are associated with edema and neutrophil infiltration 7., 8., 9., 25., 26., 27.. This implies that the neoformed bile ducts are often functionally inefficient, andthe lack of anatomic contact with the canalicular spaces and peribiliary plexus around them further supports this concept. At variance with “typical” proliferation, in “atypical” proliferation, transitional or intermediate cells with phenotypical characteristics of both hepatocytes and cholangiocytes have been documented (28). This favors the view that this type II hyperplasia originates from ductular metaplasia or transformation of hepatic liver cell cords (i.e., retrodifferentiation of hepatocytes into cholangiocytes) and not from replication of pre-existing cholangiocytes 9., 25., 26., 27.. Three-dimensional reconstruction studies support this concept (29). As demonstrated by Hillan et al. (30), when hepatocytes are transplanted into the spleen of BDL rats, but not normal rats, they can give rise to bile duct structures, suggesting that a pattern of hormones/growth factors during BDL may trigger the ductal metaplasia of hepatocytes. The alternative hypothesis that the “atypical” proliferation may arise from a stem cell compartment has also been considered (31). Independently of their origin, proliferating cholangiocytes, especially in the “atypical” proliferation, acquire phenotypical features of a neuroendocrine epithelium: 1) expression of neuroendocrine markers (chromogranin A, glycolipid A2-B4, S-100 protein, neural cell adhesion molecule) and acquisition of neuroendocrine granules 8., 32.; 2) expression of parathyroid hormonerelated peptide (PTHrP), which is encoded by a growth factor regulated “early response” gene and which is involved in the growth and differentiation of the cell (33); 3) increased expression of and response to endothelin (34); 4) enhanced response to hormones/neuropeptides such as secretin, somatostatin and acetylcholine 1., 2., 3., 4., 5., 6..

Type III ductular hyperplasia, also called “oval cell” proliferation, occurs in the early stages of carcinogenesis in rat liver 35., 36., 37., 38. and is caused by a number of chemicals including ethionine and 2-acetyl-aminofluorene. This type of proliferation induces the formation of disorganized tubular structures with a poorly defined duct lumen which randomly spread into hepatic lobules, creating a distorted hepatic architecture 7., 8., 9.. Oval cells represent a subpopulation of nonparenchymal cells which are heterogeneous with regard to proteins and cell surface markers and which are unable to form recognizable ductular structures. Oval cells express phenotypes of parenchymal or neoplastic cells (i.e., α-fetoprotein, albumin, and G-6-PO4) 7., 8., 9., 35., 36., 37., 38. and may differentiate into hepatocytes, bile duct epithelial cells, enterocytes and exocrine pancreatic cells 35., 36., 37., 38..

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Regulation of Cholangiocyte Proliferation 

Several growth factors, hormones and neuropetides as well as bile salts (BS) are involved in the regulation of cholangiocyte proliferation (Table 1). In the BDL rat, early studies suggested that the increased pressure in the biliary tree triggers duct proliferation (6). However, by using the model of selective ligation of lobar ducts, Polimeno et al. (39) showed that proliferation also occurs in the non-ligated lobes, suggesting that humoral factors play a more important role.

TABLE 1. Regulation of cholangiocyte proliferation
StimulationInhibition
Cytokines/growth factorsIL1α, IL6, TGF-α TNF-α, EGF, HGF, IGF-1TGF-β1, TGF-β2

Hormones/neuropeptidesEstrogens, ACh, PTHrPSomatostatin, gastrin

Bile saltsLCA, TLCA, TCAUDCA, TUDCA

IL=interleukin; TGF=transforming growth factor; TNF=tumor necrosis factor; EGF=epidermal growth factor; HGF=hepatocyte growth factor; IGF=insulin-like growth factor; ACh=acetylcholine; PTHrP=parathyroid hormone-related peptide; LCA=lithocholate; TLCA=taurolithocholate; TCA=taurocholate; UDCA=ursodeoxycholate; TUDCA=tauroursodeoxycholate.

Growth factors and cytokines 

Studies on the role and effect of growth factors in modulating cholangiocyte proliferation have been performed in vitro by using cell cultures, or in vivo in the BDL rat, in the model of partial hepatectomy or by studying cholangiocyte proliferation after injury.

Epidermal growth factor (EGF), hepatocyte growth factor (HGF), insulin-like growth factor 1 (IGF1), the interleukin-6 (IL-6)/gp-80 ligand/receptor system, IL 1α, and tumor necrosis factor α (TNFα) stimulate in vitro proliferation of cholangiocyte or cholangiocyte cell lines 40., 41..

In the BDL model, selective and “typical” cholangiocyte proliferation is associated with the increased intrahepatic expression of EGF, IL-6, basic fibroblast growth factor, and transforming growth factor β (TGFβ) 42., 43.. In the same model, Polimeno et al. (39) showed an increased expression of the oncogene cerb-B2, while TGF-α receptor expression did not change.

In the model of partial hepatectomy, where cholangiocytes proliferate in conjunction with the other parenchymal and mesenchymal cell types, a sharp increase in HGF serum levels occurs as a primary phenomenon capable of triggering a cascade of events including increased expression of TGFα and acidic fibroblast growth factor (44). The parathyroid hormone-related peptide (PTHrP) produced by proliferating ductal cells and released into the portal spaces (33), together with other growth factors, may generate complex loops of growth factors and hormones (see above) which will lead to proliferation.

Ductal regeneration after injury has mainly been investigated by using necrogenic agents like D-galactosamine or CCl4, with regeneration occurring in damaged tissue 23., 24.. After acute CCl4 administration, only the large cholangiocytes are damaged with loss of proliferative and secretory activities (24). Small cholangiocytes in bile ductules and small bile ducts, resistant to CCl4 damage, transiently proliferate and secrete de novo, thus compensating for the loss of large cholangiocytes (24). Small cholangiocytes could in this model represent a “reserve” compartment which drives regeneration of bile ducts after injury. They can differentiate to hepatocytes, suggesting that this compartment could be of relevance in the regeneration of hepatocytes as well (45).

In understanding the type and entity of the ductal regeneration occurring in human pathological conditions, we must consider the complex interactions between cholangiocytes and the inflammatory and stromal compartment. Endotoxins, which are always released into the systemic circulation and portal spaces of jaundiced patients, might activate Kupffer cells and macrophages with release of mediators like TGF-α, TNF-α, IL-1-α, IL-6, TGF-β (8). Cholangiocytes express receptors for most of these mediators which may influence their proliferation but, in addition, cholangiocytes may release Il-6, thus generating an autocrine loop able to amplify the proliferative response 7., 8., 9., 41., 42., 46.. Furthermore, under stimulation, cholangiocytes produce chemotactic factors (IL-8, monocyte chemotactic protein-1) with the recruitment of inflammatory cells influencing the entire reparative process (47). On the other hand, cholangiocytes express TGF-β1, TGF-β2 and TIMP, which can counteregulate the proliferative response and at the same time stimulate stellate cells to secrete biomatrix components 7., 8., 9., 46., 47., 48., 49.. The complexity of the process is enhanced by the fact that bile duct tight junction permeability is increased by TNF-α (50), with exudation into portal spaces of bile constituents like BS able to influence cholangiocyte proliferation. Thus, the type and entity of ductal proliferation in human pathology will be the result of these complex cell interactions.

Neuropeptides/hormones 

Cholangiocyte proliferation is regulated by the parasympathetic and sympathetic nervous systems in a coordinated fashion. Cholangiocytes express M3 acetylcholine (Ach) receptors which, through Ca++ and calcineurin positively modulate adenyl cyclase (51) and, by these pathways, the secretory processes. The parasympathetic nervous system also plays an important role in modulating cholangiocyte proliferation (51). In fact, in the BDL model, vagotomy induces the disappearance of M3 Ach receptor, a marked impairment of cholangiocyte proliferation and activation of apoptotic cell death (51). Vagotomy was associated with decreased cAMP intracellular levels (51). When BDL rats were treated with forskolin, an activator of adenyl cyclase, cAMP intracellular levels were maintained and vagotomy failed to impair cholangiocyte proliferation and did not induce apoptosis. We have also shown (51) that vagotomy lacked effect in normal rats, which indicates that the role of parasympathetic nervous system is of specific relevance in proliferating cholangiocytes where the cAMP levels are critical in sustaining proliferative machinery and preventing apoptosis (52) (Fig. 1). Thus, as demonstrated in other cell types and specifically in neural tissues (53), Ach acts in proliferating cholangiocytes like a trophic/growth factor which modulates cAMP levels. The role of the parasympathetic nervous system could be crucial in the transplanted liver, which virtually lacks parasympathetic innervation. The proliferative and secretory responses of the biliary epithelium to ischemic, toxic or immunological injury, which frequently occur in the early post-transplant period, when reinnervation is in progress, should be markedly affected by the lack of a parasympathetic tone, thus favoring evolution to chronic damage rather than repair.

  • View full-size image.
  • Fig. 1. 

    Regulation of cholangiocyte proliferation through the cAMP system. The cAMP intracellular levels are crucial for sustaining cholangiocyte proliferation, as demonstrated by studies using the hyperplastic model of bile duct ligated rats 6., 51. and studies in vitro (52). Acetylcholine, by acting on M3 receptor subtype, via Ca++, calmodulin (CM) and calcineurin (CN), induces a sensitization of specific isoforms of adenyl cyclase (AC), thus amplifying the cAMP system and sustaining proliferation (51). Gastrin, by acting on CCK-B receptors, inhibits cholangiocyte proliferation by downregulating the cAMP system via IP3, Ca++ and membrane translocation of PKCα (60). Somatostatin inhibits cholangiocyte proliferation by depressing AC activity via SSTR2 receptors 56., 57..

An intact sympathetic innervation is required for hepatocyte and cholangiocyte proliferation following partial hepatectomy (54). Rat cholangiocytes express α-1a and α-1b adrenergic receptors as well as D2 dopaminergic receptors (55). Altogether, these observations suggest that the sympathetic innervation of the biliary tree may play a role, together with the parasympathetic nervous system, in the modulation of cholangiocyte functions including proliferation.

SSTR2 somatostatin receptors have been identified in rat and human cholangiocytes as well as cholangiocarcinoma cell lines. Somatostatin inhibits cholangiocyte hyperplastic reaction in BDL rats (56) by selectively interacting with SSTR2 receptors and decreasing intracellular cAMP levels (Fig. 1). Somatostatin and its analogues also inhibit the growth of human cholangiocarcinoma cell lines expressing SSTR2 receptor subtypes (57), thus encouraging clinical studies to assess their impact in the diagnosis and treatment of this malignancy.

Gastrin exerts trophic effects on different epithelia and induces the growth of different normal and neoplastic tissues expressing gastrin receptors. Rat cholangiocytes express CCK-B/gastrin receptors (58) but, at variance with many different tissues and similarly to pancreatic cell lines (59), gastrin inhibits cholangiocyte proliferation in BDL rats. Gastrin counteregulates cAMP synthesis by acting through CCK-B receptors, IP3-, Ca++ and PKC-dependent pathways (60) (Fig. 1). The antiproliferative effect of gastrin is also associated with a membrane translocation of the Ca++-dependent PKC isoform, PKC-α (60). A similar antiproliferative capacity of gastrin was also shown in a cholangiocarcinoma cell line (61).

Insulin receptors are expressed in rat cholangiocytes which respond to insulin with changes in secretory activities (62). Given the general effect of insulin on cell growth and proliferation, a role in the modulation of cholangiocyte proliferation is likely, although yet not explored.

Estrogens are important inducers of cell growth and differentiation. Cholangiocytes express estrogen receptors (ER) of α and β subtypes, while hepatocytes express only ER-α (63). In vitro, 17β estradiol stimulates cholangiocyte proliferation (63). In the BDL model, where estradiol serum levels are increased, selective cholangiocyte proliferation is associated with a marked increase in the expression of ER, particularly ERβ, while hepatocytes, which do not proliferate after BDL, showed a marked decrease of ER-α expression (63). Administration of estrogen antagonists in BDL rats blocks cholangiocyte proliferation and induces Fasmediated apoptosis of proliferating cholangiocytes (63). We suggested that the increased serum levels of estrogens during BDL play a key role in determining the selective cholangiocyte proliferation, and that this effect is mediated by the upregulation of ER-β, exclusively expressed in cholangiocytes (63). When the mitogenic effect of endogenous estrogens was blocked, cholangiocytes stopped proliferating and underwent apoptosis, resulting in a marked decrease of bile duct mass (ductopenia). Estrogens probably act by potentiating the effects of growth factors. The role of sex hormones in modulating cholangiocyte functions is of relevance in human cholangiopathies, since estrogens have been considered to play a pathogenic role in these diseases which preferentially affect the female sex 64., 65.. Furthermore, estrogens may play a role in modulating the growth of cholangiocarcinoma. In fact, Sampson et al. (66) showed that ER are expressed in cholangiocarcinoma cell lines where tamoxifen block cell growth and proliferation and triggers apoptosis of Fas-positive cells.

Bile salts 

A pioneering study by Palmer et al. (17) showed that feeding LCA to rodents induces a hyperplastic reaction of intrahepatic bile ducts. BS transporters have recently been identified in cholangiocytes which are constantly subjected to an intracellular traffic of BS (3). These, by affecting intracellular signaling molecules, may influence a number of cell functions including proliferation. In vitro, TCA and tauro-LCA (TLCA) stimulate the proliferative capacity of cholangiocytes (67). In the BDL model, cholangiocyte proliferation is extremely sensitive to the experimental perturbation of BS pool composition 18., 68., 69.. In fact, external bile drainage decreases cholangiocyte proliferation while bile acid feeding enhances proliferation 18., 69.. The hyperplastic ductal reaction induced by feeding of TCA or TLCA to normal rats (18) is confined to portal areas and resembles the BDL model. Ursodeoxycholate (UDCA) and TUDCA, on the contrary, have been shown to inhibit cholangiocyte proliferative capacity (68). Due to the fact that profound changes in the qualitative and quantitative BS pool composition occur during chronic cholestatic liver diseases, and since these diseases are frequently treated with UDCA (70), the role of BS in modulating proliferative cholangiocyte response is clinically relevant.

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Deregulation of Cholangiocyte Proliferation: Apoptosis 

Apoptosis represents a key mechanism in the maintenance of tissue homeostasis. After removal of obstruction in BDL rats, enhanced ductal proliferation is reversed by apoptosis which restores normal bile duct mass (71). Interestingly, changes in the balance between proliferation and apoptosis in BDL and post-BDL phases were associated with parallel changes in expression of B-cell-lymphoma/leukemia 2 (bcl-2) (antiapoptotic) and bax (pro-apoptotic) proteins, the former being overexpressed during proliferation and the latter during apoptosis 72., 73..

Apoptosis may be activated by a variety of stimuli, including withdrawal of growth factors, immunologically-mediated processes, nitric oxide, xenobiotics and infectious agents. The complex regulation of apoptotic machinery in cholangiocytes has been the object of recent investigations using different approaches.

In rodent cholangiocytes, a model of apoptosis has been developed by Que et al. (74), who demonstrated that beauvericin, a K+ ionophore, activates the apoptotic cascade through a caspase 3-sensitive pathway. In cholangiocyte cell lines, glyco-UDCA prevents beauvericin-induced apoptosis by inhibiting caspase 3, through the blockage of cytochrome C mitochondrial release (75).

In the BDL model of bile duct hyperplasia, vagotomy induces activation of cholangiocyte apoptotic cell death (51). This resembles the apoptotic effect observed in neural tissue by withdrawal of nerve growth factor which, like acetylcholine, modulates the cAMP pathways 51., 53.. The same mechanism of apoptosis, i.e. withdrawal of growth factors, could be implied in the Fas-mediated apoptosis induced in BDL rats by estrogen antagonists which, by blocking the estrogen mitogenic effects, are responsible for Fas-mediated activation of the apoptotic cascade (63).

An experimental model simulating apoptosis in the course of oxidative stress has been proposed by Celli et al. (76), who showed that depletion of glutathione in a human cholangiocyte cell line induced an increased degradation of the anti-apoptotic protein bcl-2, thus providing a link between oxidative stress occurring in inflammatory cholangiopathies and apoptosis.

Apoptosis induced by toxic injury has been investigated by Le Sage et al. (24), who showed that a single gavage dose of the toxin CCl4 to normal or BDL rats induced apoptosis of large bile ducts, while small ducts were unaffected. The resistance of small bile ducts to CCl4-induced apoptosis could be due to overexpression of bcl-2, but also to the lack of cytochrome P-4502E1, which converts CCl4 to toxic radicals and which is expressed only by large bile ducts (24).

From experimental studies to the human pathology, the occurrence of apoptosis associated with an increased expression of Fas antigen has been demonstrated in cholangiocytes of PBC and PSC patients (77), but its relevance remains a matter of debate. Recent preliminary studies based on a rigorous quantitative morphologic approach underscored the role of apoptosis as a mechanism of damage in the biliary epithelium of PBC or PSC patients, showing that apoptotic cholangiocytes are a very rare feature (78). Rather, these studies indicate that necrosis is the main mechanism of cholangiocyte cell death in PBC and PSC. It is, however, important to point out that apoptotic cholangiocytes could be rapidly shed into bile, so that their identification is very difficult. More direct evidence exists that cholangiocyte apoptosis could be of relevance in AIDS-related opportunistic cholangitis. Chen et al. (79) showed that cryptosporidum has a direct cytopathic effect on human cholangiocytes and, in an in vitro model of cryptosporidiosis in cultured human cholangiocytes, apoptosis was activated by a Fas/Fas-ligand dependent mechanism.

In contrast to ductopenic conditions, where apoptosis could be pathologically activated, defective apoptotic mechanisms certainly play a major role in the development and growth of cholangiocarcinoma. The expression of the anti-apoptotic protein, Bcl-2, was estimated to be 15-fold higher in malignant than non-malignant human cholangiocyte cell lines (80). In human cholangiocarcinoma bcl2 protein was detected by some studies but not all. A recent study using in situ hybridization showed high levels of bcl2-mRNA in 90% of biopsies of human cholangiocarcinoma, while less than 10% stained positively for bcl2 protein, suggesting a post-transcriptional defect (81). Potential mechanisms for the abnormal regulation of bcl2 are mutations in p53 or Ras tumor suppressor genes, which have an incidence in cholangiocarcinoma of approx. 30% of cases (82).

Escape from immune surveillance, a major mechanism for tumor growth and progression, has also been proposed in cholangiocarcinoma by Que et al. (83) who, in experiments performed on a human cholangiocarcinoma cell line, demonstrated an increased expression of Fas ligand, which may induce apoptosis of invading T-cells as well as abnormal Fas-receptor signaling, induced by expression of FLICE inhibitor. Certainly, aberrant Fas antigen expression could be a key factor in the suppression of apoptosis. Fas antigen is heterogeneously expressed in cholangiocarcionoma cells which only partially respond to anti-Fas agonistic antibodies undergoing apoptotic cell death (84). Only Fas-negative cells survived and were able to produce tumors when injected into nude mice (84). These studies suggest a critical role of Fas as candidate oncogene in the growth of cholangiocarcinoma, stimulating possible future therapeutic strategies. IL6 has recently been shown to play a role in the growth of cholangiocarcinoma by acting through signaling pathways involving both p38 and p44/p42 MAP kinases (85). Tamoxifen (66) induces apoptosis of Fas-positive cells in cholangiocarcinoma, and somatostatin (57) inhibits cell growth by decreasing intracellular cAMP, thus suggesting possible therapeutic strategies for a neoplasm which may currently be treated only by surgical resection.

Cholangiocarcinoma complicates the course of PSC with a very high frequency. Chronic inflammation and/or infections may be risk factors. Some cytokines and nitric oxide should be good candidates as mediators of the neoplastic transformation and growth of cholangiocytes. Jaiswal et al. (86) showed that an excessive production of nitric oxide (NO) by cholangiocytes, and activation of iNOS in response to inflammatory cytokines, might induce DNA damage and impair DNA repair mechanisms. IL6 sustains the proliferation of cholangiocarcinoma (85) cell lines, and this may contribute to the autocrine/paracrine stimulation of neoplastic cell growth.

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Conclusions and Future Perspectives 

Current evidence shows that a number of cytokines, growth factors, hormones, neuropeptides as well as BS are involved in the regulation or dysregulation of cholangiocyte proliferation. An obvious supposition is that the pattern of these humoral factors induced by injury, inflammation and cholestasis in the portal spaces and surrounding parenchyma will affect the type and entity of the proliferative ductal response. A challenge for the future is to clarify and modulate the complex interrelationships between humoral factors, neoformed bile ducts, surrounding biomatrix and neovascularization, and to reach a restitutio ad integrum rather than evolution toward ductopenia or neoplastic transformation.

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Acknowledgements 

Supported by the grant MURST (40% funds) #9806210866.

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PII: S0168-8278(00)80377-3

Journal of Hepatology
Volume 33, Issue 2 , Pages 333-340, August 2000

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