Distinguishing Androgen Receptor Agonists and Antagonists: Distinct Mechanisms of Activation by Medroxyprogesterone Acetate and Dihydrotestosterone

Abstract

Natural and pharmacological androgen receptor (AR) ligands were tested for their ability to induce the AR NH2-terminal and carboxyl-terminal (N/C) interaction in a two-hybrid protein assay to determine whether N/C complex formation distinguishes in vivo AR agonists from antagonists. High-affinity agonists such as dihydrotestosterone, mibolerone, testosterone, and methyltrienolone at concentrations between 0.1 and 1 nM induce the N/C interaction more than 40-fold. The lower affinity anabolic steroids, oxandrolone and fluoxymesterone, require concentrations of 10-100 nM for up to 23-fold induction of the N/C interaction. However no N/C interaction was detected in the presence of the antagonists, hydroxyflutamide, cyproterone acetate, or RU56187, at concentrations up to 1 microM, or with 1 microM estradiol, progesterone, or medroxyprogesterone acetate; each of these steroids at 1-500 nM inhibited the dihydrotestosterone-induced N/C interaction, with medroxyprogesterone acetate being the most effective. In transient and stable cotransfection assays using the mouse mammary tumor virus reporter vector, all ligands displayed concentration-dependent AR agonist activity that paralleled induction of the N/C interaction, with antagonists and weaker agonists failing to induce the N/C interaction. AR dimerization and DNA binding in mobility shift assays and AR stabilization reflected, but were not dependent on, the N/C interaction. The results indicate that the N/C interaction facilitates agonist potency at low physiological ligand concentrations as detected in transcription, dimerization/DNA binding, and stabilization assays. However the N/C interaction is not required for agonist activity at sufficiently high ligand concentrations, nor does its inhibition imply antagonist activity.

Modulation of Androgen Receptor Activation Function 2 by Testosterone and Dihydrotestosterone

“…Improved FXXLF motif binding contributes to the slower dissociation rate of T and nearly equivalent T-induced transcriptional response as DHT-bound WT AR. Optimal AR FXXLF motif binding and slow ligand dissociation are thus hallmarks of the fully stabilized AR consistent with properties of the AR N/C interaction (8,57) and AR stabilization (7,58). Greater AF2 activity of T-bound AR-H874Y is supported by the effects of MAGE-11, an AR coregulator that binds the AR FXXLF motif and targets AF2 for activation by SRC/p160 coactivators (23).…”

Discovery of an Orally-Active Nonsteroidal Androgen Receptor Pure Antagonist and the Structure-Activity Relationships of Its Derivatives

“…14) But this compound also showed agonistic activities in RGA. 15,16) Coupled with our previous results, we speculated that these agonistic activities are caused by the lack of a side chain to inhibit the folding of helix 12. The nitrogen atom on position 1 in thiohydantoin seemed appropriate for the introduction of a side chain and we therefore hypothesized that our strategy could be applied to thiohydantoin to discover an orally active nonsteroidal AR pure antagonist.…”

Repressors of Androgen and Progesterone Receptor Action

“…Our finding that SMRT inhibits N/C-terminal interactions implies that SMRT bound to the N terminus prevents the N/C-terminal interaction presumably through steric interference. That SMRT inhibits the activity of partial antagonistbound AR, which does not induce N/C-terminal interactions (44), and AR lacking its hormone binding domain suggests that binding of SMRT also interferes with the formation of productive AR coactivator complexes. Our observation that SMRT blocks the activity of PR lacking its hormone binding domain FIG.…”