What goes on behind closed doors: physiological versus pharmacological steroid hormone actions

Abstract

Steroid-hormone-activated receptor proteins are among the best-understood class of factors for altering gene transcription in cells. Steroid receptors are of major importance in maintaining normal human physiology by responding to circulating concentrations of steroid in the nM range. Nonetheless, most studies of steroid receptor action have been conducted using the supra-physiological conditions of saturating concentrations (> or =100 nM) of potent synthetic steroid agonists. Here we summarize the recent developments arising from experiments using two clinically relevant conditions: subsaturating concentrations of agonist (to mimic the circulating concentrations in mammals) and saturating concentrations of antagonists (which are employed in endocrine therapies to block the actions of endogenous steroids). These studies have revealed new facets of steroid hormone action that could not be uncovered by conventional experiments with saturating concentrations of agonist steroids, such as a plethora of factors/conditions for the differential control of gene expression by physiological levels of steroid, a rational approach for examining the gene-specific variations in partial agonist activity of antisteroids, and a dissociation of steroid potency and efficacy that implies the existence of separate, and possibly novel, mechanistic steps and cofactors.

Fig. 1

Schematic of domains of rat GR with numbers above the drawing indicating the amino acids of the LBD with AF2 (cross hatched), AF1 (core is striped), and DBD (dashes) domains. These domains are almost identical in GRs of other species and are similarly organized in the other steroid receptors (115).

Fig. 2

Data plots for experiments on steroid hormone action. (A) Induction of gene expression of transiently transfected luciferase reporter genes A and B with a saturating concentration of agonist steroid. Raw luciferase data are plotted. Background levels with no added steroid are indicated by the dashed lines. (B) Dose-response curves for induction of transiently transfected luciferase reporter genes A and B by varying concentrations of agonist steroid, and bar graphs of residual agonist activity of an antisteroid with each gene. Raw luciferase data are plotted. (C) Normalized data for gene induction data of panel B. For each gene, the data are expressed as percent of maximal induction.

Fig. 3

Cyclical model of steroid hormone regulated gene transcription. Each circle of two reactions (single semi-circular arrow) represents a proposed, but usually unidentified, reaction in the currently uncharacterized series of steps from steroid (H) binding to receptor (R) to give the initial receptor-steroid complex (C) to the production of mRNA (beyond F). Other species correspond to both known (e.g., C′ = activated complex; * = biologically inert “activated complex”) and unknown (e.g., P′, E, etc.) factors and intermediates. This cyclical model predicts sigmoidal dose-response curves under a variety of conditions (from ref. 99).

Fig. 4

Proposed model by which changes in total activity (A_max) and in EC₅₀ and percent partial agonist activity can sometimes be expressed via different steps or pathways. The promoter-bound complex with GR is depicted as being a single multimeric species only for ease of drawing. The start of transcription is designated by the bent arrow. The mathematical model suggests that the various molecules, including GR, enter and leave the promoter regions sequentially so that only a few components are present at any one moment. The depicted arrows for pathways A and B may, depending upon the conditions, represent entirely different sequences in the reactions of Fig. 3 from receptor binding of steroid to production of protein product. Alternatively, A and B may represent other single cyclical steps in the overall pathway of Fig. 3, or anything in between. Similar different steps/processes may also dictate independent changes in percent partial agonist activity, although no mathematical model has yet been constructed for this.