BackgroundHuman uterus undergoes distinct molecular and functional changes during pregnancy and parturition. Hydrogen sulfide (H2S) has recently been shown to play a key role in the control of smooth muscle tension. The role of endogenous H2S produced locally in the control of uterine contractility during labour is unknown.Methodology/Principal FindingsHuman myometrium biopsies were obtained from pregnant women undergoing cesarean section at term. Immunohistochemistry analysis showed that cystathionine-γ-lyase (CSE) and cystathionine-β-synthetase (CBS), the principle enzymes responsible for H2S generation, were mainly localized to smooth muscle cells of human pregnant myometrium. The mRNA and protein expression of CBS as well as H2S production rate were down-regulated in labouring tissues compared to nonlabouring tissues. Cumulative administration of L-cysteine (10−7–10−2 mol/L), a precursor of H2S, caused a dose-dependent decrease in the amplitude of spontaneous contractions in nonlabouring and labouring myometrium strips. L-cysteine at high concentration (10−3 mol/L) increased the frequency of spontaneous contractions and induced tonic contraction. These effects of L-cysteine were blocked by the inhibitors of CBS and CSE. Pre-treatment of myometrium strips with glibenclamide, an inhibitor of ATP-sensitive potassium (KATP) channels, abolished the inhibitory effect of L-cysteine on spontaneous contraction amplitude. The effects of L-cysteine on the amplitude of spontaneous contractions and baseline muscle tone were less potent in labouring tissues than that in nonlabouring strips.Conclusion/SignificanceH2S generated by CSE and CBS locally exerts dual effects on the contractility of pregnant myometrium. Expression of H2S synthetic enzymes is down-regulated during labour, suggesting that H2S is one of the factors involved in the transition of pregnant uterus from quiescence to contractile state after onset of parturition.

CRH induces the production of chemokines and cytokines in myometrium at term and subsequently results in the cascade of inflammation in uterus. The inflammation induced by CRH can lead to activation of uterus.

Prostaglandin F2α (PGF2α) plays a critical role in the initiation and process of parturition. Since human labor has been described as an inflammatory event, we investigated the role of PGF2α in the inflammatory process using cultured human uterine smooth muscle cells (HUSMCs) isolated from term pregnant women as a model. Using a multiplex assay, HUSMCs treated with PGF2α changed their output of a number of cytokines and chemokines, with a distinct response pattern that differed between HUSMCs isolated from the upper and lower segment region of the uterus. Confirmatory enzyme-linked immunosorbent assays (ELISAs) showed that PGF2α stimulated increased output of interleukin (IL) 1β, IL6, IL8 (CXCL8) and monocyte chemotactic protein-1 (MCP1, also known as chemokine (c-c motif) ligand 2, CCL2) by HUSMCs isolated from both upper and lower uterine segments. In contrast, PGF2α inhibited tumor necrosis factor α (TNFα) release by HUMSCs from the lower uterine segment while the output of TNFα was undetectable in the upper segment. Small interfering (si) RNA mediated knockdown of the PGF2α receptor prevented the changes in cytokine and chemokine output by the HUSMCs. Since the PGF2α receptor (PTGFR) couples via the Gq protein and subsequently activates the phospholipase C (PLC) and protein kinase C (PKC) signaling pathways, we examined the role of these pathways in PGF2α modulation of the cytokines. Inhibition of PLC and PKC reversed the effects of PGF2α. PGF2α activated multiple signaling pathways including extracellular signal-regulated kinases (ERK) 1/2, phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K), P38, calcineurin/nuclear factor of activated T-cells (NFAT) and NF-κB signaling. Inhibition of ERK reversed PGF2α-induced IL1β, IL6 and CCL2 output, while inhibition of PI3K blocked the effect of PGF2α on IL6, CXCL8 and CCL2 output and inhibition of NF-κB reversed PGF2α-induced IL1β and CCL2 output. NFAT was involved in PGF2α modulation of CCL2 and TNFα output. In conclusion, our results support a role of PGF2α in creating an inflammatory environment during the late stage of human pregnancy.

A major cause of preterm labor in pregnant women is intra-amniotic infection, which is mediated by an inflammatory process. Hydrogen sulfide (H2S), a gaseous transmitter, has been implicated to be involved in inflammatory responses. We sought to investigate whether H2S affects infectious preterm birth using the mouse model of lipopolysaccharides (LPS)-induced preterm birth. Administration of LPS at 0.4 mg/kg with two injections intraperitoneally (i.p.) on gestational day 14.5 induced preterm labor. LPS significantly increased leukocyte infiltration in uterus, stimulated the expression of pro-inflammatory cytokines interleukin 1β (IL-1β), IL-6, tumor necrosis factor α (TNF-α), CCL2 and CXCL15 in myometrium. Administration of NaHS (i.p.) delayed the onset of labor induced by LPS in a dose-dependent manner. NaHS prevented leukocyte infiltration into intrauterine tissues and inhibited the production of pro-inflammatory cytokines in myometrium and decreased the levels of these cytokines in maternal circulation. H2S also decreased LPS-activated extracellular signal-regulated kinase (ERK) 1/2/ nuclear factor (NF)-κB signaling pathways in myometrium. This study provides new in vivo evidence for the roles of H2S in attenuating inflammation, and a potential novel therapeutic strategy for infection-related preterm labor.

Estrogens and corticotrophin-releasing hormone (CRH) produced by the placenta play pivotal roles in the control of parturition in human and other primates. There is a strong correlation between maternal CRH and estrogen concentrations throughout gestation. To investigate whether CRH produced locally in the placenta could modulate estrogen production, we obtained human placental trophoblasts from uncomplicated term pregnancies and cultured them for 72 h. Cells were then treated with CRH and with a CRH receptor antagonist, alpha-helical CRH9-41. The results showed that CRH stimulated, but alpha-helical CRH9-41 inhibited, the production of estradiol in a time- and dose-dependent manner. Consistent with this thesis, CRH increased whereas alpha-helical CRH decreased the mRNA levels of STS, CYP19A1, and HSD17B1, the key enzymes for estrogen synthesis. These results suggest that, in the placenta, endogenously produced CRH exhibits a tonic stimulatory effect on estrogen production.