Characterization of a Specific Progesterone Receptor in Decidualized Hamster Uterus*

Abstract

Decidualization in the hamster depends on progesterone action. The purpose of this study was to characterize progesterone-binding components in the cytosol fraction of hamster deciduomal tissue on day 7 of pseudopregnancy. Density gradient centrifugation studies performed with deciduomal and myometrial cytosol revealed two progesterone-binding components. Component 1 had a 6–7 S sedimentation coefficient, was heat labile and was insensitive to cortisol competition. Component 2 had a 4–5 S sedimentation coefficient, was heat stable, and sensitive to cortisol competition. Serum contained a progesterone binder with properties similar to those of component 2. Competitive binding studies revealed that component 1 possessed hormonal binding specificity (progesterone > 5α-pregnanedione > deoxycorticosterone > testosterone > estradiol > cortisol), and the binding specificity of component 2 was similar to that of corticosteroid-binding globulin (CBG). Neither of these components bound [3H]dexamethasone, but a [3H]dexamethasone binder with a 7–8 S sedimentation coefficient was identified in cytosol as a presumptive glucocorticoid receptor. Thus, the physicochemical properties of component 1 were consistent with those expected of a progesterone receptor, and those of component 2 and serum CBG were similar. An assay was developed for the measurement of components 1 and 2. Component 1 was assayed using cytosol preincubated with an excess of unlabeled cortisol, and component 2 was determined after preincubation of cytosol at 37 C for 90 min. Specific [3H]progesterone binding data were subjected to Scatchard plot analysis to provide an estimate of the binding affinity (KA) and quantity of binding sites. Component 1 had a high binding affinity for progesterone (KA = 1.04 ± 0.15 (SE) × 109 M-1) with a concentration of 38.6 ± 8.4 pmol/g deciduomal tissue and 29.1 ± 6.5 pmol/g myometrial tissue. Component 2 exhibited a lower binding affinity for progesterone (KA = 3.32 ± 0.89 × 107 M-1). Treatment of the pseudopregnant hamster with progesterone for 2 h before death caused a significant depletion of component 1 from the cytosol fraction in both uterine compartments, whereas similar treatment with cortisol for 2 h had no effect. Estrogen treatment initiated 24 h before autopsy increased component 1 levels. These results demonstrate the existence of a specific progesterone receptor (component 1) in the cytosol fraction of deciduomal tissue in the pseudopregnant hamster. The source and functional significance of component 2 in cytosol remains to be determined. (Endocrinology102: 443, 1978)