Glucocorticoids Sensitize Placental Inflammatory Responses Via Inhibiting Lipoxin A₄Biosynthesis

Objective:Inflammation dysregulation in placenta is implicated in the pathogenesis of numerous pregnancy complications. Glucocorticoids (GCs), universally considered anti-inflammatory, can also exert pro-inflammatory actions under some conditions. Lipoxin A4(LXA4),"stop signals" in inflammation, exerts dual anti-inflammatory and pro-resolving actions in vivo. Although the adverse effects of GCs on pregnancy have been extensively investigated, the effects of GCs on placental inflammation remain largely unclear. The purpose of the present study was to test the hypothesis that prior exposure to GC exacerbates, rather than inhibits, placental inflammatory responses to immunologic challenge. Furthermore, we examined whether GC-mediated sensitization of placental inflammatory responses is explained by the dysregulation of arachidonic acid (AA)-derived bioactive lipid mediators biosynthesis, particularly LXA4production. The modulation of LXA4on GC-mediated placental trophoblast5-lipoxygenase (5-LOX) activation was also investigated both in vivo and in vitro.Methods:1. Dexamethasone (Dex) and lipopolysaccharide (LPS) treatment:Pregnant rats were injected i.p. with50μg/kg LPS. Dex (5mg/kg) was administered s.c.1h post-LPS or24h pre-LPS to observe the modulation of Dex on LPS-stimulated placental inflammatory responses;2. Different doses of Dex treatment:Pregnant rats were injected s.c. with different doses of Dex (2.5,5,10and20mg/kg) to observe the effects of Dex on placental AA-derived bioactive lipid mediator metabolism, particularly LXA4biosynthesis;3. Dex, LPS and LXA4treatment:One hour after pregnant rats were injected s.c. with5mg/kg Dex, the rats were administered i.p with10μg/kg LXA4.23h later, the rats were then injected i.p. with50μg/kg LPS. This experiment was designed to observe the counter-regulation of LXA4on Dex-potentiated placental inflammation;4. Dex and LXA4treatment:One hour after pregnant rats were injected s.c. with5 mg/kg Dex, the rats were injected i.p. with10μg/kg LXA4to observe the counter-regulation of LXA4on Dex-mediated placental5-LOX activation;5. Cell treatment:HTR-8/SVneo cells were treated with Dex (10-6M,10-7M and10-8M), or Dex (10-6M) plus RU486(5μM), LXA4(200nM) or Boc2(10μM) for24h to observe the modulation of LXA4on Dex-mediated5-LOX activation in vitro.Results:In this study, we reported the opposing regulation of rat placental inflammation by Dex. When Dex was s.c. injected1h post LPS intraperitoneal challenge, neutrophil infiltration as well as pro-inflammatory cytokines interleukin1β (IL-1β), interleukin6(IL-6) and tumor necrosis factor a (TNF-a) expression in rat placenta were significantly attenuated. In contrast, Dex pretreatment for24h potentiated rat placental pro-inflammatory response to LPS and delayed inflammation resolution, which involved MAPKs and NF-κB activation. Mechanically, Dex pretreatment promoted5-LOX activation and increased pro-inflammatory leukotriene B4(LTB4) production whereas inhibited the anti-inflammatory and pro-resolving lipid mediator LXA4biosynthesis in rat placenta via downregulating15-LOX-1and15-LOX-2expression. Moreover, LXA4supplementation dampened Dex-potentiated placental inflammation, as well as suppressed Dex-mediated activation of inflammatory priming factor5-LOX both in vivo and in vitro.Conclusion:These findings suggest that prior exposure to GCs sensitizes and potentiates rat placental pro-inflammatory responses as well as delays resolution with the underlying mechanisms involving dysregulation of dual anti-inflammatory and pro-resolving LXA4biosynthesis as well as other AA-derived bioactive lipid mediator metabolism.