Proteolytic cleavage of PR-B: A novel mechanism for functional progesterone withdrawal in human parturition

PROJECT SUMMARY/ABSTRACT Spontaneous preterm (<37 weeks of gestation) labor (PTL) that leads to preterm birth (PTB) affects 10- 15% of pregnancies and causes 80-90% of neonatal mortality and morbidity. Currently, there are no effective therapies to prevent PTL/PTB, and the mechanism by which labor is initiated is unknown. Our research focuses on the steroid hormone progesterone and how its action in uterine cells is regulated to maintain pregnancy and induce labor and parturition. For most of pregnancy, P4, acting via the nuclear P4 receptors (PRs) affects expression of specific genes in myometrial cells, the net effect of which is to promote uterine relaxation and quiescence (i.e., pro-gestation). A common trait among viviparous species is that loss/disruption of P4/PR signaling induces labor, leading to parturition. The human PR exists as at least 2 isoforms: PR-A and PR-B, that share amino acid sequence except that PR-B has an extra 164 N-terminal (NT) amino acids. We found that in myometrial cells, pro-pregnancy actions of P4 are mediated by PR-B transcriptional activity, whereas PR-A inhibits the activity of PR-B and directly promotes expression of pro- labor genes. This led to the hypothesis that labor ensues when P4 signaling in myometrial transitions from PR-B to PR-A. The proposed studies explore a novel mechanism for this transition: proteolytic cleavage of the PR-B NT region leading to depletion of PR-B and accumulation of a PR-A-like cleavage product. The central hypothesis is that PR-B NT cleavage decreases PR-B transcriptional activity, due to loss of the NT region of PR-B, and production of a PR-A analogue cleavage product (PR-BΔNT) that promotes expression of pro-labor genes. This hypothesis will be tested by achieving 2 Specific Aims: 1) identify the amino acid locus for PR-B NT cleavage in myometrial cells; and 2) determine the effect of PR-B NT cleavage and PR- BΔNT on myometrial cell P4 responsiveness. PR-B NT cleavage is a hitherto unrecognized mechanism that may cause the loss of P4/PR-B signaling in myometrial cells that initiates labor. The research has groundbreaking potential for understanding the pathophysiology of PTL/PTB and may lead to novel therapeutic strategies (e.g., inhibition of the PR-B NT cleavage protease) to prevent PTL/PTB. PR-B NT cleavage may also have relevance for other P4/PR-related pathologies such as endometriosis, leiomyoma, and breast cancer.