Progestin-mediated changes in progesterone receptor forms in the normal human endometrium

Monoclonal antibodies to the human progesterone receptor (PR) were used to detect changes in PR form during the menstrual cycle. In proliferative phase samples, two PR proteins with mol wt (Mr) of 116,000 and 81,000 were detected on protein blots probed with anti-PR monoclonal antibodies. The 116,000 Mr protein was comprised of triplet isoforms, while the 81,000 Mr protein was a singlet. By contrast, protein blot analysis of secretory phase samples revealed a doublet isoform at 116,000 Mr and an 81,000 Mr singlet. Organ culture of human endometrium was used to mimic these changes in PR form in vitro. Although the triplet to doublet conversion was not realized in organ culture, time- and dose-related changes in PR form were achieved which were similar to the in vivo state. Furthermore, these changes preceded the progestin-mediated induction of the enzyme estradiol dehydrogenase in parallel cultures, demonstrating that this is a useful system in which to study the relationship between receptor modulation and initiation and maintenance of biological response.