Expression of apolipoprotein A-1 mRNA in normal intrahepatic biliary tree

Our previous study demonstrated that apolipoprotein A-1 (apo A-1) immunoreactive peptides were located diffusely in the cytoplasm, not only of human normal hepatocytes, but also of intrahepatic bile ducts and peribiliary glands. It is important to determine whether the presence of these immunoreactive peptides in intrahepatic biliary tree is caused by pinocytosis from the bile, or by intracellular protein synthesis. Thus, we investigated whether apo A-1 is synthesized by cells that line the biliary tree. Normal human liver samples obtained at surgery were used; and the expression and distribution of apo A-1 mRNA in normal human liver tissues were examined, using in situ hybridization histochemistry with a 35S-labeled oligonucleotide probe specific for apo A-1. On the autoradiogram, many silver grains were found to be distributed uniformly in hepatocytes. In addition, an appreciable apo A-1 mRNA signal was also observed in both the surface epithelial lining of the bile ducts and the epithelial cells of the peribiliary glands. In conclusion, these findings suggest that the apo A-1 found in bile is secreted both by hepatocytes and by intrahepatic bile duct cells and peribiliary glands.