Preterm birth (PTB) affects 10-15% of pregnancies in the US and causes the majority of neonatal mortality and morbidity. To prevent PTB a clearer understanding is needed of the hormonal control of human parturition. In this regard, the steroid hormone progesterone (P4) acting via the nuclear P4 receptor (PR) isoforms, PR-A and PR-B, is a critical factor. For most of pregnancy P4/PR promotes uterine quiescence and blocks labor, and disruption of P4/PR signaling triggers parturition. The mechanism for these critical P4/PR actions are, however, unclear. Infection/inflammation in the uterine and gestational tissues are major drivers of term and preterm parturition, but the mechanism for these effects are also uncertain. The proposed research addresses these major knowledge gaps by exploring a mechanism linking inflammation and P4/PR signaling in myometrial cells via a phosphorylated form of PR-A that we hypothesize plays a central role in the causal pathway for inflammation-induced parturition. Our published and preliminary data suggest that P4/PR-B inhibits myometrial cell responsiveness to pro-inflammatory stimuli by interacting with and repressing the transcriptional activity of the activator protein 1 (AP-1) transcription factors at promotors of a subset of IL-1ß-responsive genes. Our data also suggest that P4/PR-A upon phosphorylation at the serine-344/345 locus (pSer344/345-PRA) interacts with AP-1 to disrupt P4/PR-B anti-inflammatory activity. We also found that generation of pSer344/345-PRA in myometrial cells is catalyzed by mitogen activated protein kinases (MAPKs) in response to pro-inflammatory stimuli. Based on those data we hypothesize that: 1) P4/PR-B exerts anti-inflammatory activity in myometrial cells by binding to AP-1 to inhibit transcription at a subset of inflammatory gene promoters; 2) pSer344/345-PRA inhibits P4/PR-B anti-inflammatory activity by disrupting the PR-B/AP-1 interaction, and 3) generation of pSer344/345-PRA in myometrial cells is catalyzed by specific MAPKs in response to pro-inflammation stimuli. This hypothesis will be tested in human myometrial cell lines and human myometrium obtained from c-section deliveries, and Rhesus macaque and mouse models of inflammation induced parturition. Two Specific Aims will be achieved: Specific Aim 1: Determine the mechanism by which P4/PR-B exerts anti-inflammatory activity in myometrial cells and how this is affected by pSer344/345-PRA; and Specific Aim 2: Determine how pSer344/345- PRA generation is controlled in myometrial cells. The proposed research is novel and groundbreaking and will advance understanding of the fundamental biology of human parturition and contribute to the development of effective P4/PR-based anti-inflammatory therapies to promote uterine quiescence and prevent preterm birth.