Synthesis and secretion of protein C inhibitor by the human hepatoma-derived cell line, Hep G2

The site of synthesis of protein C inhibitor, a recently identified human plasma inhibitor against activated protein C, is not known. We have studied the production and secretion of protein C inhibitor by an established human liver cell line derived from hepatocellular carcinoma (Hep G2). The concentration of protein C inhibitor, as measured by a specific radioimmunoassay, increased in the medium of Hep G2 cells with time. There was no evidence for a significant intracellular pool of this protein. Protein C inhibitor secreted from Hep G2 cells (G2 protein C inhibitor) inhibited the activity of purified activated protein C in a functional assay. De novo synthesis of protein C inhibitor was demonstrated by the presence of specific immunoprecipitable radioactivity in the medium after 5 h of labeling of the cells with [35S]methionine. Analysis of the immunoprecipitates by SDS-polyacrylamide gel electrophoresis showed a peak of radioactivity corresponding to Mr 57 000. These results indicate that the liver is a site of protein C inhibitor production.