De Novo Synthesis Of SAA1 In The Placenta Participates In Parturition

AimsPremature birth is the main cause of neonatal death,and prevention and treatment of premature birth remains challenges for obstetrician.High-risk factors of preterm birth include systemic or local infections,pregnancy complications and co-morbidities,twin pregnancy,previous preterm birth,and history of cervical and uterine surgery etc.However,the exact causes of preterm birth are complicated,and etiology of about 30%of preterm birth remains elusive.This is largely due to the incomprehensive elucidation of the mechanism underlying human parturition.Clarifying the specific mechanism of human labor initiation may provide new insights for prediction and treatment of preterm birth.Studies have found that sterile inflammation of intrauterine tissues exists not only in preterm births without obvious incentives or evidence of infection,but also is present innormal labor process at term.Serum amyloid A1(SAA1),an acute-phase response protein,is mainly synthesized by the liver in response to both sterile and non-sterile noxious stimuli and is involved in the regulation of the inflammatory process.Recently extra-hepatic tissues were found to be capable of synthesizing and secreting SAA1.Our previous studies have found that SAA1 can participate in labor by eliciting inflammation in the amnion.It is unclear whether the placenta can also be a source of SAA1 participating in parturition.Therefore,the main purpose of this study is to determine whether SAA1 synthesized in the placenta plays any roles in initiating preterm and term labor.MethodsThe expression and regulation of placental SAA1 and its role in induction of proinflammatory cytokines and cyclooxygenase-2(COX-2),the prostaglandin synthesis key enzyme,were explored in human placental tissue and cultured human primary placental trophoblasts by using RNA sequencing,in situ hybridization,immunohistochemistry,immunofluorescence,real-time quantitative PCR,Western blotting,ELISA,inhibitors for SAA1 receptor and signaling pathway.The relationship of SAA1 with normal parturition and preterm birth with chorioamnionitis was also investigated.In addition,the expression of SAA1 in mouse placenta and whether SAA1 can cause premature birth was studied in the mouse.Results1.There are four types of SAA genes in human,which are SAA1,SAA2,SAA3,and SAA4.SAA1 and SAA2 belongs to acute-phase response proteins.SAA3 is a pseudogene and SAA4 is constitutively expressed.Term placental trophoblast transcriptome sequencing showed that SAA1,SAA2 and SAA4 were all expressed in the placenta trophoblasts with SAA1 being the highest one.In situ hybridization,immunohistochemical and immunofluorescence staining of human placental tissue showed that SAA1 m RNA and protein were mainly distributed in the syncytiotrophoblast layer of the placental villi.Real-time quantitative PCR and ELISA assays found that the expression of SAA1 m RNA and its secretion were significantly increased during trophoblast syncytialization..2.SAA1 induced the expression of proinflammatory factors IL-1?,IL-8,TNF-?and COX-2 with increased PGF2? secretion in human placental trophoblast.All these factors are known to be crucial in parturition.However,SAA1 did not affect the expression of corticotropin-releasing hormone(CRH)and aromatase,the other two factors involved in human parturition.3.TLR4 antagonist(CLI095),NF?B inhibitor(JSH-23),p38 inhibitor(SB203580),ERK1/2inhibitor(PD98059),and JNK inhibitor(SP600125)could attenuate the induction of IL-8,TNF-? and COX-2 by SAA1.4.Lipopolysaccharides(LPS),TNF-? treatment and cortisol,the known factors involved in parturition,all could increase SAA1 m RNA and protein abundance in human primary placental trophoblasts.5.SAA1 m RNA and protein levels in the placenta tissue and SAA1 level in the maternal blood were significantly increased in labor as compared with those in the placenta tissue without labor.SAA1 level in the maternal blood was further increased in preterm labor with histologic chorioamnionitis.6.SAA1 was present in the placenta and yolk sac membrane of pregnant mice at E18.5,which was significantly increased following intraperitoneal administration of LPS.Intraperitoneal injection of SAA1 shortened the gestational age and increased the expression of IL-1?,TNF-? and COX-2 in the placental tissues.ConclusionsThis study has demonstrated that human placental tissue is able to express and secrete SAA1 which is increased during trophoblast syncytialization.Labor promoting factors such as LPS,cortisol and TNF-? all can induced SAA1 expression in human placental trophoblasts.SAA1 expression in the placental tissue and maternal blood is significantly increased in labor.SAA1 can induce IL-8,TNF-? and COX-2 expression via TLR4 and NF?B/MAPK signaling pathways.Intraperitoneal injection of SAA1 in mice can promote the expression of IL-1?,TNF-? and COX-2 protein abundance in the mouse placenta and induce preterm birth.This study provides evidence for the involvement of placental SAA1 in parturition and excessive SAA1 may result in preterm birth.