Leukotrienes in human umbilical plasma at birth

Prostaglandins and leukotriene B4 are potent inhibitors of 11beta-hydroxysteroid dehydrogenase type 2 activity in human choriocarcinoma JEG-3 cells

The 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) is responsible for the inactivation of glucocorticoids. This is the predominant isozyme in the human placenta, where it is proposed to protect the fetus from high levels of maternal cortisol. In the present study, we examined the effects of eicosanoids on the activity of 11beta-HSD2 in human choriocarcinoma JEG-3 cells, a well-established model for placental trophoblasts. Treatment of JEG-3 cells for 24 h with either prostaglandin (PG) E2 or F2alpha attenuated 11beta-HSD2 activity ( approximately 40%). Paradoxically, indomethacin, an inhibitor of cyclooxygenases, inhibited (approximately 40%) rather than stimulated the activity of this enzyme. This indicated that the arachidonic acid metabolism may be diverted to other pathway(s), the products of which may inhibit 11beta-HSD2 activity. To determine whether the lipoxygenase pathways were involved, the cells were treated with nordihydroguaretic acid (NDGA), a blocker of all three (5-, 12-, and 15-) lipoxygenases. NDGA caused a 3-fold increase in 11beta-HSD2 activity. To further delineate which specific lipoxygenase pathway was involved, the cells were incubated with zileuton, a selective inhibitor of 5-lipoxygenase. This resulted in a similar increase in 11beta-HSD2 activity, suggesting that the products of this pathway (e.g., leukotrienes) may be involved. Given that leukotriene B4 (LTB4) is the most biologically active product of the 5-lipoxygenase pathway, we treated the cells with LTB4, which inhibited 11beta-HSD2 activity in a time- and dose-dependent manner with a maximal effect (60% reduction) at 10 nM for 9 h. Semiquantitative reverse transcription-polymerase chain reaction analysis revealed that 11beta-HSD2 mRNA levels were not altered by the addition of LTB4, PGE2, or PGF2alpha, indicating an effect at the posttranscriptional level. In conclusion, these results demonstrate that prostaglandins and LTB4 are potent inhibitors of 11beta-HSD2 activity in JEG-3 cells, suggesting that placental 11beta-HSD2 activity is modulated by these locally produced eicosanoids. This is the first time that the products of arachidonic acid metabolism have been found to regulate the activity of 11beta-HSD2.

Interrelationships between oxytocin and eicosanoids in human fetal membranes at term gestation: which role for leukotriene B4?

The existence of a functional paracrine loop between oxytocin and prostaglandin F2-alpha in human placental cells has been demonstrated. The present study was undertaken to investigate further the possible interrelationships between oxytocin and eicosanoids in human intrauterine tissues at term gestation. Therefore, we evaluated the effect of leukotriene B4 (LTB4) on oxytocin (OT) production by explants of fetal membranes and amnion and the effect of oxytocin on the production of LTB4 and prostaglandin E2 (PGE2) by both fetal membranes and amnion. In all cases studied (n = 25), short-term cultures of tissue explants (fetal membranes or amnion) have been carried out. The production of eicosanoids and oxytocin in culture medium was evaluated. Oxytocin measurement was carried out by radioimmunoassay following extraction of the substance with Sep Pak C18 cartridges, PGE2 and LTB4 were measured by radioimmunoassay directly in culture medium. Results show that LTB4 has no significant stimulatory effect on oxytocin production by fetal membranes or amnion tissue. On the other hand, oxytocin stimulates PGE2 release by both fetal membranes and isolated amnion, but has no effect on LTB4 production by these tissues. Taken together, these findings suggest the following conclusions: (1) a paracrine loop between LTB4 and oxytocin is lacking in fetal membranes and amnion at term pregnancy; (2) oxytocin exerts a stimulatory effect on PGE2 release by both fetal membranes and amnion; (3) the interrelationships between oxytocin and the different eicosanoids in the above tissues seem to be highly selective.

Release of arachidonic acid metabolites by human fetal membranes: interrelationship between leukotriene B4 and prostaglandin E2

The objective of this study was to ascertain whether human fetal membranes metabolize arachidonic acid preferentially through the lipoxygenase rather than the cyclooxygenase pathway before labor and whether an interaction between lipoxygenase and cyclooxygenase products is present in these tissues. Reflected fetal membranes were obtained from 8 healthy women at term gestation who were delivered by elective repeat cesarean section before the onset of labor. Tissues were cultured either in the presence or in the absence of the calcium ionophore A23187 for 60 minutes. Leukotriene B4 (LTB4) and prostaglandin E2 (PGE2) were measured in culture medium by radioimmunoassays. Moreover, the effect of different concentrations of exogenously added LTB4 on PGE2 release was evaluated. The basal and stimulated output of LTB4 by tissues was significantly higher than that of PGE2. Addition of LTB4 significantly decreased PGE2 release by tissues. These findings suggest that in the above tissues: 1) the arachidonate lipoxygenase pathway is highly active before labor; 2) LTB4 might play a role in the regulation of PGE2 production.

The role of prostaglandins in obstetrical disorders

All pregnancy-associated tissues are capable of producing prostaglandins including PGI2 and TXA2. In normal pregnancy there is a dominance of PGI2 over TXA2 which may contribute to the maternal circulatory adaptation to pregnancy. Furthermore, both fetoplacental PGI2 and TXA2 production are important regulators of the fetal blood supply. It has been clearly established that in pre-eclampsia PGI2 production decreases in the fetoplacental tissues and quite probably also in the maternal tissues. The effect of this change may be further exaggerated by the simultaneous stimulation in pre-eclampsia of TXA2 production. The reason for PGI2 deficiency is not known. Other vasoactive agents, such as endothelin, may act in concert with prostaglandins. Relative PGI2 deficiency is likely to exist also in IUGR and lupus anticoagulant syndrome of pregnancy. In the latter, lupus anticoagulant may directly inhibit the synthesis of PGI2. One study suggests PGI2 deficiency also in early pregnancies of women with a history of repeated abortions. Prostaglandin production increases during full-term labour, and similar but smaller changes also occur in preterm labour. A silent bacterial infection may trigger the onset of preterm labour through cytokine-stimulated increase of prostaglandin production. No data were found on prostaglandin production in post-term pregnancies. That oligo-polyhydramnios is possibly prostaglandin mediated is suggested by the control of polyhydramnios by indomethacin treatment. Smoking decreases the production of PGI2 and possibly increases that of TXA2, which may lead to decreased blood flow and IUGR. Which constituent of cigarette smoke exerts this effect is not known. Ethanol consumption causes aberrations in prostaglandin metabolism which cannot be directly connected with fetal alcohol effects.

Influence of labor and oxytocin on in vitro leukotriene release by human fetal membranes and uterine decidua at term gestation

OBJECTIVE

The purpose of this study was to determine whether labor and oxytocin influence in vitro leukotriene release by human fetal membranes and uterine decidua at term gestation.

STUDY DESIGN

All women (N = 21) were studied in an academic clinical research environment. They were delivered either vaginally or before labor by cesarean section. Tissues were incubated with calcium ionophore A23187 alone or with calcium ionophore A23187 after preincubation with either oxytocin, BW755c, or cycloheximide. Leukotrienes in culture medium were determined by high-performance liquid chromatography. Statistical analysis was performed with analysis of variance.

RESULTS

Tissues obtained after vaginal delivery released significantly higher (p less than 0.05) quantities of leukotrienes LTB4, LTD4, and delta-6-trans-LTB4 than did tissues obtained during cesarean section. Leukotriene release was stimulated by oxytocin and was inhibited by both BW755c and cycloheximide.

CONCLUSION

These results suggest an activation of the arachidonate lipoxygenase pathway in these tissues during labor. Oxytocin could play a regulatory role in this process.

Evidence for 5-hydroxyeicosatetraenoic acid (5-HETE) and leukotriene C4(LTC4) in the onset of labor

Arachidonic acid is metabolized by cyclooxygenase leading to prostaglandins (PG), and by lipoxygenases leading to hydroxyeicosatetraenoic acids (HETE) and leukotrienes (LT). PGs are potent uterine constrictors. 5-HETE and LTC4 also stimulate uterine contractions, but their role in labor is not known. To estimate the activities of these pathways before parturition and their relationship to uterine contractility, amniotic fluid (AF) concentrations of 5-HETE, LTC4 and PGF2 alpha were determined in 5 chronically catheterized rhesus monkeys. Uterine contractility was continually assessed by changes in AF pressure.

RESULTS

AF concentrations at the time of intrauterine surgery were 4.4 +/- 0.8 ng/ml for 5-HETE and 2.5 +/- 0.7 ng/ml for LTC4. PGF2 alpha levels were nondetectable. Indomethacin infusion into the mother, although effective in suppressing PGF2 alpha levels, did not prevent the onset of premature labor and vaginal delivery in 2 animals. PGF2 alpha did not increase in 1 monkey with term labor. Approximately 1 week before delivery, 5-HETE and LTC4 concentrations increased sharply in all animals in association with a nocturnal pattern of labor contractions. Peak AF concentrations during labor were 5-HETE: 40 +/- 8 ng/ml, LTC4: 20 +/- 4 ng/ml, and PGF2 alpha: 5.4 +/- 2.1 ng/ml.

COMMENT

LTC4 and 5-HETE stimulate uterine contractility in vitro, and probably act in concert with PGF and PGE to stimulate uterine contractions during parturition. HETEs may also act as a signal to recruit white blood cells to the uterus and to activate them, there to augment PG and LT production and act as a first line of defense against any infection that might enter the uterus from the vagina during or after delivery.

CONCLUSIONS

1) 5-HETE and LTC4, but not PGF2 alpha, are associated with uterine contractility after preterm intrauterine surgery. Surprisingly, AF PGF2 alpha levels were nondetectable for 1 to 2 weeks after surgery. 2) 5-HETE and LTC4 are present in higher concentrations than PGF2 alpha in AF. 3) 5-HETE, LTC4 and PGF2 alpha all increase with the onset of labor. AF concentrations of 5-HETE and LTC4 are significantly higher than PGF2 alpha before and during term and preterm labor. 4) Labor can occur with suppressed PGF2 alpha levels, but with increasing 5-HETE and LTC4 levels. 5) These data suggest that 5-HETE and LTC4 are important components of the parturitional process, and they challenge the dogma that PGs are the universal mediators of labor.

Placental leukotriene B4 release in early pregnancy and in term and preterm labour

The production of leukotriene B4 (LTB4) by the human placenta in vitro was measured by radioimmunoassay. In early pregnancy (7-12 weeks gestation) LTB4 production rate (pmol/mg dry weight/2 h) was 3.7 (2.6-4.7) (median and range; n = 9) and at term it was 0.7 (0.4-2.3; n = 10) in placental tissue obtained at elective caesarean section, and 2.7 (1.3-3.6; n = 10) in samples following labour of spontaneous onset and vaginal delivery. In spontaneous preterm labour (26-36 weeks gestation) with normal placental histology LTB4 production was 0.7 (0.3-1.7; n = 14), but it was significantly higher in preterm placentas with inflammatory infiltration: 3.1 (0.8-4.8; n = 6). These data show that the production of LTB4 by human placenta is high in early pregnancy, but remains low during the third trimester, with a significant increase in spontaneous labour at term. LTB4 output is low in uncomplicated preterm labour but markedly increased in chorioamnionitis-associated preterm labour.