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The human placenta is a transient life sustaining organ which is responsible for mediating all the physiological exchanges between the mother and the fetus. The steroid hormone progesterone, often referred to as the hormone of pregnancy, is critical for the establishment and for maintaining the pregnancy. During the gestation period the human placenta produces progesterone which via interacting with the progesterone receptors exerts its many effects. Specific intracellular progesterone receptors (PRs) have been reported to mediate the genomic signalling of progesterone whereas recently two novel receptor families which are phylogenetically distinct to the nuclear receptor superfamily have been characterised, and shown to mediate progesterone’s non genomic actions. These are the multiple membrane progestin receptors (mPRα, mPRβ, and mPRγ) and progesterone membrane receptor component 1 (PGMRC-1). The rapid progesterone actions mediated via these non-classical progesterone receptors have received attention with main focus on their reproductive functions. Our aim is to elucidate the expression of the receptors in the human placenta, further understand the signalling pathways via which progesterone mediates its effects and lastly examine the functional relevance of these receptors in this organ. Choriocarcinoma cell lines are used frequently as placental models for investigations of steroid hormone actions, but until now little is known about the expression of progesterone receptors (PRs) in these cell lines. Quantitative RT-PCR revealed that in fully syncytialized BeWo cells (treated with 50 µM forskolin for 72 h) there was a significant down-regulation of mPRα and up-regulation of mPRβ and of the PGRMC1 when compared with non-syncytialized BeWo cells. Expression of all the mPR and PGRMC1 mRNAs was significantly lower in JEG-3 cells compared to non-syncytialized BeWo cells. Expression of PR-B was unaltered between the two BeWo states but was significantly higher in JEG-3 cells. Immunofluorescence analysis revealed that mPR proteins are differentially expressed in these choriocarcinoma cell lines as well as in the human placenta. The functionality of mPRs was investigated in vitro, using BeWo and JEG-3 cells that were treated with Org OD-02 (a specific mPR agonist), progesterone (P4) and R5020 (a specific nuclear PR agonist) in the presence or absence of the pro-inflammatory cytokine inteleukin-1β (IL-1β) at a concentration of 10ng/μl. The effect was more exacerbated in JEG-3 cells, where IL-1β induced 40% cell death when compared to BeWo cells that reached a modest but significant 15% cell death. When JEG-3 cells were treated with IL-1β and progesterone, there was a significant decrease in cell death at concentrations of 100nM and 1000nM. When cells were treated with 1000nM progesterone, IL-1β’s effect was completely abolished. Progesterone was also able to induce phosphorylation of ERK1/2 in these cells. Pretreatment of JEG-3 cells with a specific MAPK inhibitor (UO126) inhibited substantially the progesterone’s proliferative effect. Moreover, using the specific mPR agonist Org OD 02-0, we have shown that that the progestin antagonism of apoptotic effects of IL-1β on BeWo cells is mediated through mPRs. Quantitative PCR in clinical samples revealed a 2.8 fold decrease of mPRβ in labouring comparing to non-labouring tissues and 4.6 fold higher levels of mPRγ in preterm mPRγ compared to term placentas. The ratio of mPRα to PR-B was increased in term compared to preterm samples, whereas it was decreased in labour compared to non-labour placentas. There was also a high correlation between mPRα and PGRMC1 expression irrespective of pathologies. This study addressed many fundamental questions regarding how progestins exert their effect at placental level. It is evident that there is a higher order of complexity and changes in the ratios of placental progesterone receptors rather than individual fluctuations might affect subsequent signalling events.