Heme oxygenase-1 in placental development and pathology.

Pregnancy is accompanied by several adaptations in the mother, such as increased blood volume, higher cardiac output and reduced peripheral vascular resistance. Inability to accomplish these changes places both her and her pregnancy at risk of major placental complications such severe pre-eclampsia (sPE) or severe intra-uterine growth restriction (sIUGR). sPE is characterized by wide-spread maternal vascular dysfunction expressed as increased systemic vascular resistance; this state is accompanied by elevated levels of anti-angiogenic factors and lower production of vasodilatory gases. One of the key molecules implicated in sPE pathogenesis is heme oxygenase-1 (HO-1), a rate-limiting enzyme that breaks down heme into carbon monoxide (CO), biliverdin and free iron. CO and bilirubin (a downstream product of biliverdin processing) account for the angiogenic, vasodilatory and anti-oxidant properties of HO-1. These collective actions of the heme breakdown metabolites generated by HO-1 offer protection against cytotoxicity, inflammation, hypoxia and other forms of cellular stress that are central to the pathogenesis of sPE. Placental HO-1 expression and exhaled CO levels are both lower in women with sPE, consistent with a pathogenic role of HO-1. In vitro experiments demonstrate that induction of HO-1 downregulates secretion of the anti-angiogenic factor soluble fms-like tyrosine kinase-1 (sFLT-1) and increases CO production. Advancing our understanding of regulatory pathways promoting placental HO-1 expression may offer new pharmacological tools to reduce maternal and perinatal morbidity in severe placental insufficiency syndromes, especially in women at greatest risk of developing sPE.

Revisiting the housekeeping genes of human placental development and insufficiency syndromes.

Gene expression analysis using semi-quantitative RT-PCR is a common tool in placental research. However the comparison of steady-state gene transcription between different clinical groups is dependent upon comparison of target mRNA data with mRNA obtained from so-called housekeeping (HK) genes whose steady-state transcription does not differ significantly between the groups. In this communication, we evaluated the performance of candidate HK genes across nine clinical groups commonly used in placental research. We used the GeNorm method to evaluate qRT-PCR data to determine the performance of candidate HKs.

Heparin promotes soluble VEGF receptor expression in human placental villi to impair endothelial VEGF signaling.

BACKGROUND: Severe preeclampsia is characterized by hypertension, renal injury and placental dysfunction. Prothrombotic disorders are discovered in 10-20% of women with preeclampsia, providing the rationale for prescribing low-molecular-weight heparin (LMWH) in future pregnancies. Heparin has diverse molecular actions and appears to reduce the recurrence risk of preeclampsia in women without prothrombotic disorders. The placenta-derived anti-angiogenic splice-variant protein soluble vascular endothelial growth factor (VEGF) receptor-1 (sFLT1) is strongly implicated in the pathogenesis of the underlying endothelial dysfunction. As the placental syncytiotrophoblast is the principal source of sFLT1, we tested the hypothesis that heparin suppresses placental sFLT1 secretion. METHODS AND RESULTS: First trimester placental villi exposed to LMWH (0.25-25 IU mL(-1)) in an in vitro explant model significantly increased the expression and release of sFLT1 by the syncytiotrophoblast into culture media, reducing phosphorylation of FLT1 and KDR receptors in cultured human umbilical vein endothelial cells. This response was significantly diminished in placental villi from healthy term pregnancies. Placental villi from severely preeclamptic pregnancies had a higher baseline sFLT1 release, compared with first trimester placental villi and did not respond to LMWH treatment. LMWH promoted villous cytotrophoblast proliferation (BrdU incorporation) and impaired syncytial fusion-differentiation, causing syncytiotrophoblast apoptosis (by caspase 3&7 activity and TUNEL staining) and necrosis (ADP/ATP ratio). CONCLUSION: LMWH promotes sFLT1 synthesis and release from first trimester placental villi in a manner similar to that of severely preeclamptic placental villi, which antagonizes VEGF signaling in endothelial cells. These effects in part are mediated by an interaction between heparin and the cytotrophoblasts that regenerates the overlying syncytiotrophoblast responsible for sFLT1 secretion into the maternal blood.

Villous trophoblast abnormalities in extremely preterm deliveries with elevated second trimester maternal serum hCG or inhibin-A.

Elevated levels of the maternal prenatal screening markers hCG and inhibin-A, measured at 15-20 weeks gestation, increase the subsequent risk of severe pre-eclampsia and intra-uterine growth restriction (IUGR). Since both markers are produced by syncytiotrophoblast, we tested the hypothesis that these elevations were due to accelerated differentiation of the villous trophoblast compartment. We performed a retrospective study of 12 cases from our Placenta Clinic with total hCG and/or inhibin-A levels of ≥3.0 multiples of the median that subsequently delivered by 28 weeks gestation and compared their placental pathology findings with 24 gestational age-matched controls. Morphometric analysis demonstrated a 41% reduction in the volume ratio of Ki67 positive cytotrophoblast nuclei to total trophoblast in cases vs controls (Student's T-test; p = 0.028). Distal villous hypoplasia (DVH) was significantly more common in cases (10/12) than controls (4/24); Fisher's exact test, p = 0.002. Wave-like syncytial knot (WLSK) formation was significantly more common in cases (9/12) than controls (1/24); Fisher's exact test, p < 0.0001. WLSK formation was associated with DVH and resulted from accumulation of senescent/apoptotic syncytiotrophoblast nuclei along inherent lines of syncytial nuclear organization. Our data support the hypothesis that elevated second trimester maternal serum levels of total hCG and/or inhibin-A may result from premature accelerated differentiation of the villous cytotrophoblasts. The subsequent pathologic findings in the syncytiotrophoblast could render the pregnancy at risk of severe pre-eclampsia and IUGR.