Context: Increased myometrial sensitivity to oxytocin at term is mediated through increased oxytocin receptor (OTR) expression. OTR promoter contains putative transcription factor-binding sites for activating protein-1 (AP-1), CCAAT/enhancer-binding protein (C/EBP), and nuclear factor-kappaB (NF-kappaB), which may be activated by IL-1beta, whose concentrations increase with labor.
Objective: The objective of this study was to examine the effect of IL-1beta on OTR expression and the roles of AP-1, C/EBP, and NF-kappaB in OTR promoter function.
Results: IL-1beta induces an increase in OTR mRNA concentrations and OTR ligand binding in myometrial cells, which is maximal at 4 h and decreased after 20 h. IL-1beta activates the transcription factors AP-1 C/EBPbeta, and NF-kappaB. Using computer-based analysis and EMSA studies, we have identified three AP-1, nine C/EBP, and three NF-kappaB DNA-binding sites in the OTR promoter. In transient transfection studies, OTR promoter activity was increased by C/EBPbeta and NF-kappaB, but not by AP-1. C/EBPbeta and NF-kappaB together had a synergistic action in the induction of OTR promoter activity. Site-directed mutagenesis of each individual C/EBP and NF-kappaB site had no effect on the ability of C/EBPbeta, NF-kappaB, or their combination to activate OTR promoter. However, mutation of both NF-kappaB sites inhibited promoter activation by NF-kappaB alone, but not that by the combination of C/EBPbeta and NF-kappaB. Deletion studies showed that a region between -851 and -656 of the OTR confers responsiveness to the combination of C/EBPbeta and NF-kappaB.
Conclusion: IL-1beta has a biphasic effect on OTR expression in myometrial cells, and C/EBP and NF-kappaB play synergistic roles in OTR promoter activation.