Molecular mechanisms and therapeutic implications of the carbon monoxide/hmox1 and the hydrogen sulfide/CSE pathways in the prevention of pre-eclampsia and fetal growth restriction

Abstract

The exact aetiology of preeclampsia is unknown, but there is a good association with an imbalance in angiogenic growth factors and abnormal placentation (Ahmad and Ahmed, 2004). The incidence of preeclampsia is reduced by a third in smokers, but not in snuff users. Soluble Flt-1 (sFlt-1) and soluble endoglin (sEng) are increased prior to the clinical onset of preeclampsia. Animals exposed to high circulating levels of sFlt-1 and sEng elicit severe preeclampsia-like symptoms. Smokers have reduced circulating sFlt-1 and cigarette smoke extract decreases sFlt-1 release from placental villous explants. An anti-inflammatory enzyme, heme oxygenase-1 (HO-1) and its metabolite carbon monoxide (CO), inhibit sFlt-1 and sEng release. Women with preeclampsia exhale less CO than women with normal pregnancies and HO expression decreases as the severity of preeclampsia increases. In contrast, sFlt-1 levels increase with increasing severity. More importantly, chorionic villous sampling from women at eleven weeks gestation shows that HO-1 mRNA expression is decreased in women who go on to develop preeclampsia. Collectively, these facts provide compelling evidence to support the proposition that the pathogenesis of preeclampsia is largely due to loss of HO activity. This results in an increase in inflammation and excessive elevation of the two key anti-angiogenic factors responsible for the clinical signs of preeclampsia. The identification of a protective role for HO-1 in pregnancy, offers HO/CO pathway as a target for the treatment of preeclampsia. The cardiovascular drugs, statins, stimulate HO-1 expression and inhibit sFlt-1 release in vivo and in vitro, suggesting statins have the potential to ameliorate preeclampsia. The StAmP trial is underway to address this and if positive, its outcome will lead to the very first therapeutic intervention to prolong affected pregnancies. Hydrogen sulphide (H2S), a gaseous messenger produced mainly by cystathionineY-lyase (CSE), is pro-angiogenic vasodilator (Yang et al., 2008; Papapetropoulos et al., 2009). We hypothesized that a reduction in CSE activity may alter the angiogenic balance in pregnancy and induce abnormal placentation and maternal hypertension. Plasma levels of H2S were significantly decreased in preeclamptic women (p<-0.01), which was associated with reduced CSE message and protein expression in human placenta as determined by real - time PCR and immunohistochemistry. Inhibition of CSE activity by DL-propargylglycine (PAG) in first trimester (8-12weeks gestation) human placental explants had reduced placenta growth factor (PIGF) production as assessed by ELISA and inhibited trophoblast invasion in vitro. Endothelial CSE knockdown by siRNA transfection increased the endogenous release of soluble fms-Like tyrosine kinase-1 (sFlt-1) and soluble endoglin, (sEng) from human umbilical vein endothelial cells while adenoviral-mediated CSE overexpression inhibited their release. Administration of PAG to pregnant mice induced hypertension, liver damage, and promoted abnormal labyrinth vascularisation in the placenta and decreased fetal growth. Finally, a slow releasing, H2S-generating compound, GYY4137, inhibited circulating sFlt-1 and sEng levels and restored fetal growth that was compromised by PAG-treatment demonstrating that the effect of CSE inhibitor was due to inhibition of H2S production. These results imply that endogenous H2S is required for healthy placental vasculature and a decrease in of CSE/ H2S activity may contribute to the pathogenesis of preeclampsia. This work was supported by grants from the Medical Research Council (G0601295 and G0700288) and Aston University StrategicFunds.