Molecular characterization of hepatocystin, the protein that is defective in autosomal dominant polycystic liver disease

Background & Aims. Autosomal dominant polycystic liver disease is characterized by the presence of numerous cysts spread throughout the liver parenchyma. Recently, we discovered that polycystic liver disease is caused by mutations in the protein kinase C substrate 80K-H gene, which encodes a protein named hepatocystin. Previous studies have identified hepatocystin as a protein kinase C substrate, a component of a cytosolic signal transduction complex, a receptor for advanced glycation end products, a vacuolar protein, and the β subunit of endoplasmic reticulum glucosidase II. Thus, the exact localization and cellular function of hepatocystin remain unclear. Methods: The localization and biochemical properties of normal and polycystic liver disease mutant forms of hepatocystin were examined by using a combination of immunofluorescence microscopy, immunoblotting, metabolic labeling, immunoprecipitation, and carbohydrate analyses. Results: Normal hepatocystin localizes to the endoplasmic reticulum, where it assembles with the glucosidase II α subunit. The 1338-2A→G truncating mutation in hepatocystin observed in some polycystic liver disease patients produces a protein that is not retained in the endoplasmic reticulum but is secreted into the medium. This mutant protein fails to assemble with the glucosidase II α subunit. As a consequence, mutant hepatocystin is undetectable in liver cysts. In addition, levels of normal hepatocystin and of the glucosidase II α subunit are substantially reduced in liver and Epstein-Barr virus-immortalized B lymphoblasts from patients with polycystic liver disease. Conclusions: These findings are consistent with a role of hepatocystin in carbohydrate processing and quality control of newly synthesized glycoproteins in the endoplasmic reticulum. Therefore, altered endoplasmic reticulum processing of some key regulator of cell proliferation may underlie polycystic liver disease.