Biosynthesis of lipopolysaccharide-binding protein in rabbit hepatocytes

Studies reported here show that the recently discovered acute-phase protein, lipopolysaccharide-binding protein (LBP), is synthesized by hepatocytes. For these studies, explanted rabbit hepatocytes were grown in the presence of 35S-methionine. Biosynthetically labelled LBP in the cells and supernatant was identified using immunoprecipitation with rat anti-rabbit LBP antibody. This antibody immunoprecipitates both the LBP polypeptide and the glycosylated protein. With a cell-free translation system a comparison of RNA from normal rabbit liver with that isolated from acute-phase rabbit liver indicated that a translatable LBP message is only found in the RNA from acute-phase liver. Studies with explanted rabbit hepatocytes showed that several forms of LBP distinguishable by migration in SDS-PAGE are secreted into the extracellular medium. This heterogeneity probably is a result of differences in glycosylation of LBP since pretreatment of the hepatocytes with tunicamycin results in accumulation of a single polypeptide with an apparent mass of 50 kD in SDS-PAGE. Explanted rabbit hepatocytes spontaneously synthesize and secrete LBP and express LBP mRNA as detected by cell-free translation; thus, it was not possible to upregulate the expression of LBP. Nevertheless, these studies form the basis for future investigations on the regulation of LBP biosynthesis.