As a VEGF receptor, Flt-1 is highly expressed in the invading extravillous trophoblasts in the first trimester, which implies that VEGF-Flt-1 interactions lead to early trophoblast invasion
[60][43]. As gestational age develops, VEGF-Flt-1 interaction also guides trophoblast differentiation and migration
[61][44]. Soluble FMS-like tyrosine kinase I (sFlt-1) is a truncated protein resulting from splicing of Flt-1 which lacks the cytoplasmic and transmembrane domain but keeps the ligand-binding domain
[62][45]. Therefore, sFlt-1 antagonizes and inhibits VEGF and PlGF by binding to them and blocking their interaction with Flt-1 for proangiogenic function. In preeclampsia, placental ischemia resulting from RUPP may stimulate upregulation of sFlt-1 by binding of hypoxia inducible factor (HIF) to the promotor of Flt-1 gene
[55][38]. The elevated maternal serum level of sFlt-1 in preeclampsia has been found to be associated with severe endothelial dysfunction and inhibition of VEGF and PlGF by sFlt-1 serves a major pathogenic role in hypertension and proteinuria
[5][1]. VEGF is responsible for decreasing vascular tone and blood pressure by inducing nitric oxide and prostacyclins that have a vasodilatory effect in endothelial cells, which is blocked by sFlt-1. In addition, several molecular mechanisms of sFlt-1 found to be responsible for renal dysfunction are related to glomerular capillary endotheliosis, dysregulation of the glomerular filtration apparatus, and podocyte loss
[63][46]. Therefore, excess of sFlt-1 results in the characteristic antiangiogenic state of preeclampsia which manifests as the clinical syndrome of endothelial dysfunction. In fact, maternal serum level of sFlt-1 to PlGF ratio (sFlt-1/PlGF ratio) can be used as a reliable biomarker for predicting development and severity of preeclampsia
[64][47]. Moreover, a recent systematic review and meta-analysis on the performance of the sFlt-1/PlGF ratio in predicting adverse outcomes in women diagnosed or suspected of preeclampsia showed that the sFlt-1/PlGF ratio performs better in predicting women with early onset preeclampsia in comparison to those with late onset
[65][48]; this relates to our previous topic in chapter 4 which described that defective uteroplacental vascular remodeling is mostly seen in the early onset type of preeclampsia.
3.4.4. Soluble Endoglin (sEng)
Soluble endoglin (sEng), a coreceptor for transforming growth factor-β1 (TGF-β1), is another antiangiogenic factor released by the placenta that acts in synergy with sFlt-1. Endoglin (Eng) is an angiogenic receptor expressed mainly on the surface of placental syncytiotrophoblast and endothelial cells which serves as a co-receptor of angiogenic TGF-β signaling
[66][49]. TGF-β is known to contribute to angiogenesis and appropriate vascular relaxation by increasing VEGF
[67,68][50][51]. However, in preeclampsia sEng is released in excessive quantity and binds to free TGF-β1 which inhibits the pro-angiogenic TGF-β1 signaling in the vasculature. The circulating level of sEng is elevated in patients with preeclampsia two-to-three months prior to the onset of clinical symptoms and its serum levels seem to be correlated with the severity of the disease
[69][52].
3.5. Activin A
Activin A is a dimeric glycoprotein belonging to the TGF-β family produced by the placenta and fetal membranes
[70][53]. In preeclampsia, the serum level of activin A is elevated (up to 10-fold) compared to normal pregnancy and it is found to be resulting from increased placental production triggered by oxidative stress
[71,72][54][55]. In fact, circulating levels of activin A have shown to rise months prior to the onset of the clinical manifestation of preeclampsia, which is earlier than the elevation of sFlt-1 or sEng
[73][56]. Recent studies have shown that elevated activin A in preeclampsia may be responsible for the endothelial dysfunction, which was shown as hypertension, proteiunuria, fetal growth restriction, and preterm littering in activin administered mice
[74][57]. An in vitro study using human umbilical vein endothelial cells (HUVECs) has suggested that activin A up-regulates transcription of endothelial vasoconstrictors such as ET-1
[75][58]. Moreover, an elevated activin A level had been reported to be strongly correlated with myocardial dysfunction at 1 year after preeclamptic pregnancy, and a recent follow up study confirmed that the activin A level still remained elevated with impaired cardiac function 10 years after preeclamptic pregnancy, implying its potential use as a tool for monitoring women at risk for postpartum CVD
[76,77][59][60].
3.6. Hypoxia Inducible Factor
Hypoxia inducible factor (HIF) is a heterodimer consisting of HIF1-α and HIF2-α subunits, which are regulated by oxygen, and a constitutively expressed HIF1-β subunit. In a hypoxic environment, HIF-1 regulates transcription of various genes, including VEGF, TGF-β3, and NOS, by binding at their promotor and enhancer regions
[36][19]. HIF expression is shown to be higher in normal pregnancy, probably due to high estrogen and progesterone levels; however, HIF-1α and HIF-2α is overexpressed further in preeclampsia in response to RUPP
[78,79][61][62]. Moreover, HIF-1α upregulates anti-angiogenic factors such as sFlt-1, sEng, and ET-1 expressions and AngII and AngII-converting enzyme (ACE) expressions in the lungs and kidney which add on to the abnormal placentation and development of preeclampsia
[80][63]. An animal study with RUPP rats showed that inhibition of HIF-1α using siRNA reversed the high blood pressure, renal damage, proteinuria, and elevated serum sFlt-1 level
[81][64]. Therefore, the efficacy of using maternal serum level of HIF-1α as a predictive marker for preeclampsia has been questioned. A recent prospective study showed that high serum HIF-1α level (above 1.45 MoM) in the first trimester of pregnancy (11–13
+6 weeks of gestation) was related to development of preeclampsia, which requires further confirmation with large-scaled studies
[82][65].
3.7. MicroRNAs
MicroRNAs(miRNAs) are small (<25 nucleotides), single-stranded, non-coding RNAs that regulate gene expression by inhibiting translation. These molecules bind to the untranslated lesion of a target gene and silence their expression
[83][66]. During pregnancy, miRNAs are profusely expressed in the placenta, mainly from villous trophoblasts, and play pivotal role in several processes including trophoblast proliferation, immune tolerance, and angiogenesis
[84][67].
Specifically, miR-210 has been reported to be overexpressed in placentas of preeclampsia
[85][68]. Studies have shown that miR-210 is strongly linked with hypoxia related to RUPP which leads to inadequate trophoblast invasion and failure of spiral artery remodeling in preeclampsia
[86][69]. miR-210 is upregulated by HIF which overexpresses it in response uteroplacental hypoxia in order to regulate genes involved in various pathways including angiogenesis, inflammation, and cell proliferation
[87][70]. Another miRNA involved in preeclampsia is miR-155, which has been shown to inhibit cysteine-rich protein 61 (CYR61), an essential angiogenic factor in pregnancy
[88,89][71][72]. A crucial function of CYR61 is related to inducing the expression of VEGF, which is a major pro-angiogenic factor as previously mentioned
[87][70]. Previous studies have shown that CYR61 gene expression is downregulated in preeclamptic placentas compared to those of normal pregnancy, and suggested that increased miR-155 causes inhibition of the CYR61-VEGF pathways, which leads to reduced placental angiogenesis
[90][73].
Additionally, miR-125b is known to be an anti-angiogenic factor which decreases VEGF expression when it is overexpressed
[91][74]. A recent case-control study showed that the maternal plasma level of miR-125b at 12 weeks of gestation is significantly elevated compared to those in normal pregnancy. Moreover, the same study revealed that miR-125b targets trophoblast cell surface antigen-2 (Trop-2) protein in placental tissue, suggesting miR-125b might be involved in development of preeclampsia via modulating Trop-2 expression in the syncitiotrophoblast
[92][75].
The role of miR-21 in preeclampsia has been also newly studied, since it regulates the forkhead box M1 protein (FOXM1), which is expressed in cytotrophoblasts for proliferation and differentiation, responsible for the early placental development
[93][76]. In fact, a study showed that miR-21 is elevated with reduced FOXM1 expression in patients with preeclampsia compared to those in normotensive pregnant women, implying that miR-21 may impede the early placental invasion leading to preeclampsia
[94][77]. These results demonstrate that various miRNAs are involved in the pathway of preeclampsia which implies their potential to become possible future therapeutic targets for treatment of preeclampsia.