Nuclear Binding of Hydrocortisone-Receptors in the Embryonic Chick Retina and its Relationship to Glutamine Synthetase Induction

Abstract

Binding of hydrocortisone (HC) to cytoplasmic receptors and of the resulting receptor-hydrocortisone complex (R-HC) to nuclear acceptor sites has been studied in neural retina cells of the chick embryo, in which this hormone induces glutamine synthetase (GS). These studies were done in a cell-free system, as well as in intact retina tissue in culture. Optimal conditions, specificity, and the quantitative aspects of R-HC binding to nuclei in the cell-free system were determined. Isolated nuclei retained their binding specificity for R-HC prepared from retina cytosol; at saturation, the total number of nuclear acceptor sites for R-HC was estimated to be in the range of 1500 per nucleus. These sites were resistant to RNAse but sensitive to DNAse. When retina tissue was cultured in the presence of progressively higher doses (0–90 nM) of HC, increasing amounts of receptors were translocated from the cytoplasm to the nuclei. Following incubation of retina with 9–90 nM HC, the cytoplasm was almost completely depleted of receptors and the nuclei became saturated with R-HC. From the amount of R-HC bound to nuclei at saturation, the estimated total number of R-HC acceptor sites per nucleus was comparable to that derived from the cell-free system. Increases in the level of GS induction in the retina correlated well with amounts of R-HC complex bound by nuclei.