INDUCTION PROPERTIES OF A TRANSIENTLY TRANSFECTED GLUCOCORTICOID-RESPONSIVE GENE VARY WITH GLUCOCORTICOID RECEPTOR CONCENTRATION

Transient transfections of steroid receptors have yielded much of the data used to construct the current models of steroid hormone action, T hese experiments invariably examine the ability of receptors to regula te transcription when occupied by saturating concentrations of steroid , We now report that other induction properties of a transiently trans fected gene are not constant but vary with the concentration of transi ently transfected glucocorticoid receptors, Thus, the percentage of ma ximal induction seen with subsaturating concentrations of glucocortico id could be dramatically increased, and an antiglucocorticoid could be converted into a partial glucocorticoid, simply by increasing the con centration of glucocorticoid receptors. This behavior was observed in HeLa cells, containing endogenous receptors, or in CV-1 cells, contain ing almost no endogenous receptor, with either homologous or heterolog ous receptors, These increases were relatively insensitive to the conc entration of reporter gene, suggesting the titration of some transcrip tion factor(s) involved in regulating the position of the glucocortico id dose-response curve and the agonist activity of an antiglucocortico id. This property of transfected glucocorticoid receptors required a f ull-length, functionally active receptor but was retained, albeit redu ced in magnitude, in the absence of binding to a glucocorticoid respon se element, Furthermore, this phenomenon was specific in that the A fo rm of the human progesterone receptor had no effect under the same con ditions, These variations in induction properties of antiglucocorticoi ds and of subsaturating concentrations of glucocorticoid, in a manner that was proportional to the amount of transfected receptor, reveal pr ocesses that are not operative with saturating concentrations of gluco corticoid. These variations also demonstrate that caution should be ex ercised in making mechanistic conclusions based solely on experiments conducted with saturating concentrations of glucocorticoid.