Distribution and subcellular localization of a hepatic proliferation inhibitor in rat liver

The subcellular and intralobular distributions of a protein which specifically inhibits the proliferation of normal liver cells were determined in rat liver, using a combination of immunological and biochemical techniques. The IgG fraction from an antiserum raised against the hepatic proliferation inhibitor was isolated by protein A-Sepharose CL-4B chromatography and shown to be highly specific for the antigen using electroimmunodiffusion and affinity chromatography. To determine the intracellular location of the inhibitor, subcellular fractions were prepared from adult rat livers by differential centrifugation. The cytoplasmic fraction contained the biologically active cytostatic inhibitor, whereas the nuclear and mitochondrial fractions were inactive. Cytoplasmic localization of the hepatic proliferation inhibitor was further confirmed by anion exchange high performance liquid chromatography and by double immunodiffusion with the anti-hepatic proliferation inhibitor IgG. When liver sections were subjected to histochemical staining mediated through the immune IgG and an avidin-biotinylated horseradish peroxidase complex, the parenchymal liver cells were stained, but endothelial and connective tissue cells were not. Although some staining was evident throughout the liver parenchyma, the most intensely stained cells were located in the centrilobular region. Moreover, an age-dependent increase in the staining intensity and/or in the number of cells containing the proliferation inhibitor was observed. Preliminary experiments showed that little, if any, staining occurred in hepatocellular carcinoma cells. This highly specific IgG can be used to monitor alterations in the content and location of hepatic proliferation inhibitor in proliferative disorders of the liver.