Infusion of exogenous platelet-activating factor produces intrauterine growth restriction in the rat

Toll-Like Receptor-4 Antagonist (+)-Naltrexone Protects Against Carbamyl-Platelet Activating Factor (cPAF)-Induced Preterm Labor in Mice

Spontaneous preterm labor is frequently caused by an inflammatory response in the gestational tissues elicited by either infectious or sterile agents. In sterile preterm labor, the key regulators of inflammation are not identified, but platelet-activating factor (PAF) is implicated as a potential rate-limiting effector agent. Since Toll-like receptor (TLR)-4 can amplify PAF signaling, we evaluated whether TLR4 contributes to inflammation and fetal loss in a mouse model of PAF-induced sterile preterm labor, and whether a small-molecule TLR4 inhibitor, (+)-naltrexone, can mitigate adverse PAF-induced effects. The administration of carbamyl (c)-PAF caused preterm labor and fetal loss in wild-type mice but not in TLR4-deficient mice. Treatment with (+)-naltrexone prevented preterm delivery and alleviated fetal demise in utero elicited after cPAF administered by i.p. or intrauterine routes. Pups born after cPAF and (+)-naltrexone treatment exhibited comparable rates of postnatal survival and growth to carrier-treated controls. (+)-Naltrexone suppressed the cPAF-induced expression of inflammatory cytokine genes Il1b, Il6, and Il10 in the decidua; Il6, Il12b, and Il10 in the myometrium; and Il1b and Il6 in the placenta. These data demonstrate that the TLR4 antagonist (+)-naltrexone inhibits the inflammatory cascade induced by cPAF, preventing preterm birth and perinatal death. The inhibition of TLR4 signaling warrants further investigation as a candidate strategy for fetal protection and delay of preterm birth elicited by sterile stimuli.

Platelet-activating factor: a role in preterm delivery and an essential interaction with Toll-like receptor signaling in mice

Platelet-activating factor (PAF), a potent phospholipid activator of inflammation that signals through its cognate receptor (platelet-activating factor receptor, PTAFR), has been shown to induce preterm delivery in mice. Toll-like receptors (TLRs) are transmembrane receptors that mediate innate immunity. We have shown previously that Escherichia coli-induced preterm delivery in mice requires TLR signaling via the adaptor protein myeloid differentiation primary response gene 88 (MyD88), but not an alternative adaptor, Toll/IL-1 receptor domain-containing adapter protein-inducing interferon-beta (TRIF). In the present work, we analyzed the role of endogenously produced PAF in labor using mice lacking (knockout [KO]) PAF acetylhydrolase (PAF-AH; the key degrading enzyme for PAF). PAF-AH KO mice are more susceptible to E. coli-induced preterm delivery and inflammation than controls. In peritoneal macrophages, the PTAFR agonist carbamyl PAF induces production of inflammatory markers previously demonstrated to be upregulated during bacterially induced labor, including: inducible nitric oxide synthase (Nos2), the chemokine Ccl5 (RANTES), tumor necrosis factor (Tnf), and level of their end-products (NO, CCL5, TNF) in a process dependent upon both IkappaB kinase and calcium/calmodulin-dependent protein kinase II. Interestingly, this induced expression was completely eliminated not only in macrophages deficient in PTAFR, but also in those lacking either TLR4, MyD88, or TRIF. The dependence of PAF effects upon TLR pathways appears to be related to production of PTAFR itself: PAF-induced expression of Ptafr mRNA was eliminated completely in TLR4 KO and partially in MyD88 and TRIF KO macrophages. We conclude that PAF signaling plays an important role in bacterially induced preterm delivery. Furthermore, in addition to its cognate receptor, PAF signaling in peritoneal macrophages requires TLR4, MyD88, and TRIF.

Unilateral uterine ischemia/reperfusion-induced bilateral fetal loss and fetal growth restriction in a murine model require intact complement component 5

The role of complement in ischemia/reperfusion-induced fetal growth restriction and fetal loss is unknown. C5-deficient or wild type timed-pregnant mice were subjected to unilateral uterine ischemia/reperfusion on gestation day 13, either by (1) partial flow restriction by right ovarian artery clamping for 30 min, or (2) total flow restriction by clamping both ovarian and uterine arteries for 5 min. Ischemia/reperfusion-challenged pregnancy outcomes were compared to sham-operated controls 5 days later. Ischemia/reperfusion-treated wild type mice exhibited significantly increased bilateral fetal loss, which was greater in total flow restriction than in partial flow restriction, and decreased fetal weights, which were the same in total flow restriction and partial flow restriction for the surviving fetuses. Placental weights were unchanged by treatments. Ischemia/reperfusion increased uterine, but not placental, myeloperoxidase activity, which correlated with fetal loss. In contrast, C5-deficient mice were protected from both fetal growth restriction and fetal loss, and exhibited no increase in myeloperoxidase activity. These results demonstrate that unilateral uterine ischemia/reperfusion results in bilateral fetal loss and fetal growth restriction, mediated by a systemic mechanism. In the current model, this pathological process is completely dependent on intact complement component 5.

The significance of endothelin in platelet-activating factor-induced fetal growth restriction

The significance of endothelin-1 (ET-1) in platelet-activating factor (PAF)-induced fetal growth restriction (FGR) was evaluated in timed-pregnant rats receiving intravenous carbamyl-PAF (c-PAF; 0.5, 1.0, or 2.5 µg/kg per h) or vehicle, with or without ET-1 receptor A (ET(A)) antagonist (10 or 20 mg/kg per d) for 7 days beginning on gestation day 14. Tissues were collected on day 21. Carbamyl-PAF reduced fetal weights dose dependently. Placental weights were significantly reduced but not dose dependently. ET(A) antagonism prevented FGR at the 0.5, but not the 1.0 and 2.5 µg/kg per h c-PAF doses. Correspondingly, placental, but not uterine, preproET-1 messenger RNA (mRNA) expression (determined by reverse transcription-polymerase chain reaction) was increased at 0.5 µg/kg per h but not at higher c-PAF doses. In summary, c-PAF infusion results in fetal and placental growth restriction in the rat. At low doses of c-PAF, ET-1 is central to the pathophysiology of PAF-induced FGR. At higher c-PAF doses, FGR is induced by mechanisms other than ET-1 action.

Endothelin and platelet-activating factor: significance in the pathophysiology of ischemia/reperfusion-induced fetal growth restriction in the rat

OBJECTIVE

The objective of the study was to evaluate the role of endothelin-1 and platelet-activating factor in ischemia/reperfusion-induced fetal growth restriction in the rat.

STUDY DESIGN

On day 17 of gestation, the right uterine and ovarian arteries were occluded for 30 minutes in experimental but not sham-operated rats. All rats received endothelin receptor A antagonist, A-127722 (10 mg/kg per day), platelet-activating factor antagonist, WEB-2086 (1 mg/kg), or vehicle. On gestational day 21, litter size, fetal viability, and fetal and placental weights were recorded. Reverse transcription-polymerase chain reaction for phospholipase A2-IIA and preproendothelin-1 messenger ribonucleic acid was performed on uterus and placentas from each uterine horn. Groups were compared statistically by analysis of variance.

RESULTS

Ischemia/reperfusion reduced fetal weights, in both the ischemic horn and the nonischemic horn (P < .001). Antagonism of either endothelin receptor A or platelet-activating factor normalized fetal growth in both horns. Neither placental weight nor the incidence of fetal demise was affected by ischemia/reperfusion. Phospholipase A2-IIA and preproendothelin-1 messenger ribonucleic acid expression did not differ between right and left uterine horns in any group. Uterine and placental tissues in the ischemia/reperfusion group exhibited increased phospholipase A2-IIA (P < .01) but not preproendothelin-1.

CONCLUSION

Endothelin-1 and platelet-activating factor are both important mediators in the pathophysiology of ischemia/reperfusion-induced fetal growth restriction in the rat, contributing to the fetal growth restriction observed in both the ischemic and nonischemic horns. Antagonism of either mediator produces normal fetal growth in this model of fetal growth restriction.