DIFFERENTIAL EXPRESSION OF UTERINE PROGESTERONE-RECEPTOR FORM-A AND FORM-B DURING THE MENSTRUAL-CYCLE

Recent studies suggest that the progesterone receptor isoforms (PR-A a nd PR-B) activate genes differentially and that PR-A may act as a repr essor of PR-B function. Hence, the absolute and relative expression of the two isoforms will determine the response to progesterone. We have measured their relative expression in the uterus of cycling women who underwent endometrial biopsy. PR isoforms were identified on blots of SDS-PAGE gels by reaction with the AB-52 antibody after immunoprecipi tation from endometrial extract. Both isoforms were highest in the per i-ovulatory phase, but levels of PR-A were always higher than those of PR-B. The ratio of PR-A to PR-B changed during the menstrual cycle. B etween days 2 and 8, PR-B is almost undetectable and the A:B ratio is >10:1. From days 9 to 13, the ratio is about 5:1, and it is about 2:1 between days 14 and 16. Thereafter, PR-B dwindles rapidly and is virtu ally undetectable at the end of the cycle. In various hypoestrogenic e nvironments, PR-B expression was reduced. However, exogenous estrogens in the follicular phase in the form of oral contraceptives, enhanced PR-B expression. These data support the possibility that progesterone acts through cycle-specific PR isoforms. (C) 1997 Published by Elsevie r Science Ltd. All rights reserved.