Role of αvβ6 Integrin in Acute Biliary Fibrosis

The injury response in epithelia is highly focal or diffuse, depending on the nature of the injury factor and whether the process is acute or chronic. In the liver, the response may be localized to a portion of the lobule: the pericentral area (zone 3) in alcohol-induced injury, or the periportal area (zone 1) in chronic hepatitis C. Acute biliary injury, caused by mechanical obstruction of the common bile duct, autoimmune disease, and some toxins, is a particularly striking example of focality. The initial reaction is strictly periductal, consisting of ductular proliferation and the appearance of myofibroblasts and collagenous matrix that encircle the ducts.1 The pathogenesis of the acute periductal reaction is poorly understood.2 Studies of fibrosing liver injury have focused on the role of the stellate cell, and indeed activation of stellate cells does occur in biliary obstruction.3 However, this is a relatively late event and may be distinct and separate from the acute periductal reaction. Of the numerous cytokines that modulate the injury response, transforming growth factor beta (TGF-β) is particularly prominent.4 It is produced as a precursor, part of which is a “latency-associated peptide” (LAP). The latter is cleaved from the precursor but remains associated noncovalently with mature TGF, blocking its activity. Thus, a key event in the regulation of TGF is liberating the active cytokine from the LAP complex. This has been a subject of considerable investigation. Although proteases have been implicated,5 other mechanisms exist.6 LAP has an RGD (arg-glyasp) domain that has been shown to associate with integrins, specifically α5β1, αvβ6, and αvβ8.7 In binding assays, LAP has relatively high affinity for αv/β6, and binding causes the peptide to separate from TGFβ, presumably through an allosteric conformational change. This occurs at the cell surface, potentially serving to confine the action of TGF-β to the cell displaying αvβ6.7 Expression of αvβ6 is limited to epithelial cells,8 particularly duct epithelium. It is present during development, becoming minimal in resting normal adult epithelia. It increases in acute duct-associated injury in the adult, subsiding as the injury resolves.9 These observations suggest that αvβ6 could play a role in ductal injury in the liver, although the question has not been examined. An early survey of various tissues failed to detect αvβ6 mRNA in liver.8 However, expression of this integrin in the basal state is expected to be low. Moreover, if it is present in a small subset of liver cells and not in hepatocytes, it may escape detection by northern blot because of the dominance of hepatocyte RNA in whole-liver extracts. We report that after acute biliary obstruction, expression of αvβ6 is strikingly increased specifically on cholangiocytes in both murine and human liver. Moreover, studies of mice lacking this integrin show that its expression is directly involved in the fibrogenic response. Although inflammation is present in bile duct obstruction, it does not appear to have a primary role in regulating αvβ6. In αvβ6-null mice subjected to bile duct ligation, fibrosis was reduced whereas inflammation was unaffected. Also, in a transgenic mouse model of immunemediated biliary inflammation, there was minimal stimulation of αvβ6 expression and no periductal fibrosis. With regard to the origin of the periductal myofibroblasts, activated stellate cells have been implicated as the principal fibrogenic cell type in multiple forms of liver injury. However, our studies of biliary obstruction failed to show stellate cells migrating to the portal area during the acute phase of the injury. We did see histochemical change suggestive of epithelial-to-mesenchymal transition (EMT) of biliary epithelial cells, as a novel source of fibrogenic cells in this type of injury.