Progesterone receptor distribution in the human endometrium. Analysis using monoclonal antibodies to the human...

Two monoclonal antibodies to the human progesterone receptor (PR), JZB39 and KD68, were used in determining the immunohistochemical distribution of PR in the human endometrium throughout the menstrual cycle and after menopause. These antibodies recognized PR, as demonstrated by a downfield shift in the radiolabeled progestin binding peak when KD68 or JZB39 was added to high salt sucrose density gradients. The specificity of both antibodies for PR was confirmed with Western immunoblots and competition studies performed with purified receptor. Progesterone receptor was identified with these antibodies and the peroxidase-antiperoxidase technique in the nuclei of epithelial cells, stromal cells, and myometrial smooth muscle cells. The receptor content of endometrial epithelium and stroma varied with the menstrual cycle. The variation was most marked in the epithelium, which demonstrated very strong PR immunostaining during the proliferative phase and postovulation Days 1-3 of the early secretory phase, but PR immunostaining decreased sharply at postovulation Day 4 and remained relatively weak or absent during the mid and late secretory phase. In contrast, stromal cell nuclei were moderately to strongly immunostained even during the secretory phase. Progesterone receptor was not localized in vascular smooth muscle cells or endothelial cells. Specific cytoplasmic staining for PR was not identified in any of these cases, even prior to ovulation, when circulating levels of progesterone are low, indicating that both the steroid-occupied and -unoccupied forms of human progesterone receptor, like rabbit and guinea pig PR, and estrogen receptor, is a nuclear protein.