Receptor translocation inhibitor and apparent saturability of the nuclear acceptor

Abstract

Steroid hormone-receptor complexes bind to nuclear acceptor(s). In various cell-free systems, this interaction has been considered as saturable, and this has led many research groups to postulate the existence of a relatively small population of acceptor sites with a high affinity for the complexes. Using rat liver nuclei, chromatin or DNA as acceptor, we have shown that at physiological concentrations of glucocorticoid receptor there is no actual saturation of the acceptor. A pseudo-saturation was observed, which was due to the inhibitory effect of non-receptor macromolecules present in the cytosol. Rather than a competition for the acceptor, the mechanism of this inhibition seems to consist of a direct interaction between the receptor and these unknown inhibitors. By repeated incubations with cytosol, it is possible to bind up to 21 pmol of 3H-dexamethasone-receptor complexes per mg DNA to the nuclei whereas the tissue concentration of receptor is 5.7 pmol/mg DNA. When rat liver slices are incubated at 37°C with 3H-dexamethasone, the equilibrium distribution of steroidreceptor complexes between nuclei and cytosol at all hormone concentrations is 9 1 . It is therefore proposed that the interaction of steroid-receptor complexes with the nuclear acceptor is not a limiting factor in steroid hormone action since at all hormone concentrations a constant proportion of the complexes are bound to the nuclei. The relevance of these findings to dose-response curves for steroid hormones is discussed. If it is accepted that the biological response is proportional to the amount of the ternary complexes (acceptor-receptor-steroid), then it may be calculated that the dose-response curve and the curve showing the degree of saturation of receptor at various hormone concentrations can be superimposed if one requirement is met. It is necessary that the cellular concentration of unbound steroid-receptor complexes never approaches the equilibrium dissociation constant of the interaction of these complexes with the acceptor sites implicated in the biological response. Two cases in which such a situation could possibly be present are described.