While the factors that cause preeclampsia are unclear, placental ischemia, which can be initiated as a result of insufficient trophoblastic invasion of uterine spiral

arteries, as well as impaired placental blood flow, is central to the disorder [83, 89, 97, 156]. As a result of underperfusion in the latter half of gestation, the placenta releases many factors which contribute to the multifaceted maternal syndrome, including endothelial dysfunction (reviewed in [50]). Angiogenic growth factors play a central role in normal fetal and placental vascular development. VEGF is an important endothelial-cell-specific growth factor expressed in numerous tissues including the placenta [12, 24]. It promotes angiogenesis by binding to two receptor tyrosine kinases, VEGF receptor 1 and VEGF receptor 2 (reviewed in [44]). It is also an important permeability factor due to its ability to induce vascular leakage [26, 27]. VEGF expression is induced Nutlin 3a by various growth factors [39, 106, 109], inflammatory cytokines [25, 61, 112], and hypoxia [128]. In early pregnancy, vascular development and permeability in the endometrium, placenta, and embryo are modulated by VEGF [19, 36, 137]. Furthermore, VEGF has been found in the serum of pregnant women throughout gestation and is believed to play a role in modification of the maternal systemic vasculature by inducing the production of the vasodilators

NO and prostacyclin (PGI2) HAS1 by endothelial cells [43, Apoptosis inhibitor 58, 152, 151]. PIGF is an angiogenic factor within the VEGF family which interacts with VEGF receptor 1 and Nrp-1 (reviewed in [140]). It functions independently or as a heterodimer with VEGF and is strongly expressed in the placenta, where it is an important facilitator of angiogenesis [18, 24]. Like VEGF, PlGF is a powerful vasodilator and may be involved in the reduction of peripheral vascular resistance during pregnancy [99]. The concentration of circulating PlGF is significantly

lower in women with preeclamptic pregnancies compared to those with normal pregnancies [87, 119]. In preeclampsia, antiangiogenic factors including sFlt-1 and sEng impede the activity of proangiogenic factors and promote vascular dysfunction. sFlt-1 is a splice variant of VEGF receptor 1, produced by the placenta, which binds VEGF and PlGF, thereby inhibiting interaction with their receptors (reviewed in [94]). While serum sFlt-1 levels increase during the last two months of normal pregnancy, this increase occurs earlier and is significantly greater in women with preeclampsia [66, 73, 88]. The increase in circulating sFlt-1 is associated with a decrease in free VEGF and PlGF, resulting in inhibition of vasodilator activity and endothelial dysfunction [84]. In rats, sFlt-1 is capable of blocking VEGF and PlGF-mediated relaxation of renal vessels in vitro, and administration in vivo contributes to hypertension, proteinuria, and glomerular endotheliosis [84].