Elevated concentration of a dexamethasone-receptor translocation inhibitor in Novikoff hepatoma cells

Despite the apparent similarity of the dexamethasone-receptor concentration in the rat liver and Novikoff hepatoma cytoplasms, the in vitro translocation of the dexamethasone-receptor complex into liver or hepatoma nuclei was three times greater from liver as compared to hepatoma cytosol; both cytosols showed an apparent saturation of nuclear receptor sites. Although the competitive interaction of the receptor complex with nuclei suggested that they were qualitatively similar, the translocation of additional dexamethasone-receptor from liver cytosol into nuclei presaturated with the receptor complex from hepatoma cytosol suggested that they were different. However, the latter observation, as well as the initial differential nuclear uptake of the dexamethasone-receptor complex from liver and hepatoma cytosols, can be accounted for by a higher concentration of a translocation inhibitor in the hepatoma cytosol. Thus hepatoma cytosol, at a protein concentration of 6 mg/ml and free of activated receptor complex, was four times more inhibitory to nuclear uptake of the activated complex than was a comparable preparation of liver cytosol. A study of the kinetics of nuclear uptake of the partially purified dexamethasone-receptor complex from hepatoma confirmed that, in the absence of cytoplasmic translocation inhibitor, nuclear acceptor sites were not limiting in vitro. Evidence is presented which suggests, but does not prove, that the cytoplasmic translocation inhibitor functions as such in the intact cell.