Distinguishing androgen receptor agonists and antagonists: Distinct mechanisms of activation by medroxyprogesterone acetate and dihydrotestosterone

Natural and pharmacological androgen receptor (AR) ligands were tested for their ability to induce the AR NH2-terminal and carboxyl-terminal (N/C) int eraction in a two-hybrid protein assay to determine whether N/C complex for mation distinguishes in vivo AR agonists from antagonists. High-affinity ag onists such as dihydrotestosterone, mibolerone, testosterone, and methyltri enolone at concentrations between 0.1 and 1 nM induce the N/C interaction m ote than 40-fold. The lower affinity anabolic steroids, oxandrolone and flu oxymesterone, require concentrations of 10-100 nM for up to 23-fold inducti on of the N/C interaction. However no N/C interaction was detected in the p resence of the antagonists, hydroxyflutamide, cyproterone acetate, or RU561 87, at concentrations up to 1 mu M, or with 1 mu M estradiol, progesterone, or medroxyprogesterone acetate; each of these steroids at 1-500 nM inhibit ed the dihydrotestosterone-induced N/C interaction, with medroxyprogesteron e acetate being the most effective, in transient and stable cotransfection assays using the mouse mammary tumor virus reporter vector, all ligands dis played concentration-dependent AR agonist activity that paralleled inductio n of the N/C interaction, with antagonists and weaker agonists failing to i nduce the N/C interaction. AR dimerization and DNA binding in mobility shif t assays and AR stabilization reflected, but were not dependent on, the N/C interaction. The results indicate that the N/C interaction facilitates ago nist potency at low physiological ligand concentrations as detected in tran scription, dimerization/DNA binding, and stabilization assays. However the N/C interaction is not required for agonist activity at sufficiently high l igand concentrations, nor does its inhibition imply antagonist activity.