Estrogens and the pathophysiology of the biliary tree

The scientific framework concerning estrogen effects on different tissues has expanded enormously during the last decades, when estrogen receptor （ER） subtypes were identified. Estrogens are not only essential for the female reproductive system, but they also control fundamental functions in other tissues including the cardiovascular system, bone, brain and liver. Recently, estrogens have been shown to target the biliary tree, where they modulate the proliferative and secretory activities of cholangiocytes, the epithelial cells lining bile ducts. By acting on both estrogen receptors （ER-α） and （ER-β） subtypes, and by activating either genomic or non-genomic pathways, estrogens play a key role in the complex loop of growth factors and cytokines, which modulates the proliferative response of cholangiocytes to damage. Specifically, estrogens activate intracellular signalling cascades JERK1/2 （extracellular regulated kinases 1/2, PI3-kinase/AKT （phosphatidylinositol-3＇ kinase/AKT）] typical of growth factors such as insulin like growth factor （IGF1）, nerve growth factor （NGF） and vascular endothelial growth factor （VEGF）, thus potentiating their action. In addition, estrogens stimulate the secretion of different growth factors in proliferating cholangiocytes. This review specifically deals with the recent advances related to the role and mechanisms by which estrogens modulate cholangiocyte functions in normal and pathological conditions.

Cholangiocytes and blood supply

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

Role of sex hormones in the modulation of cholangiocyte function

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

Histamine regulation of hyperplastic and neoplastic cell growth in cholangiocytes

Histamine has long been known to be involved in inflammatory events.The discovery of antihistamines dates back to the first half of the 20th century when a Swiss-Italian pharmacologist,Daniel Bovet began his work.In 1957 he was awarded a Nobel Prize for his production of antihistamines for allergy relief.Since that time,histamine has been found to play a role in other events besides allergic reaction.Possiblyun believable to Bovet and his peers,histamine has now been marked as playing a role in liver pathologies including hepatobiliary diseases.