Cholangiocyte biology as relevant to cystic liver diseases

Polycystic liver diseases are hereditary disorders that affect the biliary epithelium, often in conjunction with the renal tubule epithelium. Characterized by the progressive formation of cysts throughout the liver and kidney, they can often lead to severe life-threatening complications. Polycystins and fibrocystin, the defective proteins in the dominant and in the recessive form of the disease, respectively, are mainly expressed in the primary (nonmotile) cilia of cholangiocytes, the epithelial cells that line the intrahepatic biliary tree. Important clues for understanding the pathogenesis of cystic diseases come from understanding the biology and pathobiology of cholangiocytes. In this chapter, cholangiocyte function and morphology is first briefly described, with particular emphasis on the regulation of their secretory properties and the complex intercellular signaling. Then, we discuss a number of possible mechanisms leading to cyst formation and progressive growth of the cysts. In both autosomal dominant and recessive forms, liver cysts arise from an aberrant development of intrahepatic bile duct epithelium. During cyst expansion, different factors, including excessive fluid secretion, extracellular matrix remodeling, increased proliferation of the epithelial cells lining the cyst, and aberrant hypervascularization around the cyst wall, variably take part in promoting progressive cyst growth. Many of these factors act via autocrine mechanisms. Each of them represents a possible target for therapies aimed at reducing the growth of liver cysts.