Endoglin

Abstract: Preeclampsia is a pregnancy-specific hypertensive syndrome that causes substantial maternal and fetal morbidity and mortality. Maternal endothelial dysfunction mediated by excess placenta-derived soluble VEGF receptor 1 (sVEGFR1 or sFlt1) is emerging as a prominent component in disease pathogenesis. We report a novel placenta-derived soluble TGF-beta coreceptor, endoglin (sEng), which is elevated in the sera of preeclamptic individuals, correlates with disease severity and falls after delivery. sEng inhibits formation of capillary tubes in vitro and induces vascular permeability and hypertension in vivo. Its effects in pregnant rats are amplified by coadministration of sFlt1, leading to severe preeclampsia including the HELLP (hemolysis, elevated liver enzymes, low platelets) syndrome and restriction of fetal growth. sEng impairs binding of TGF-beta1 to its receptors and downstream signaling including effects on activation of eNOS and vasodilation, suggesting that sEng leads to dysregulated TGF-beta signaling in the vasculature. Our results suggest that sEng may act in concert with sFlt1 to induce severe preeclampsia.

Abstract: Smad proteins play a key role in the intracellular signalling of transforming growth factor beta (TGF beta), which elicits a large variety of cellular responses. Upon TGF beta receptor activation, Smad2 and Smad3 become phosphorylated and form heteromeric complexes with Smad4. These complexes translocate to the nucleus where they control expression of target genes. However, the mechanism by which Smads mediate transcriptional regulation is largely unknown. Human plasminogen activator inhibitor-1 (PAI-1) is a gene that is potently induced by TGF beta. Here we report the identification of Smad3/Smad4 binding sequences, termed CAGA boxes, within the promoter of the human PAI-1 gene. The CAGA boxes confer TGF beta and activin, but not bone morphogenetic protein (BMP) stimulation to a heterologous promoter reporter construct. Importantly, mutation of the three CAGA boxes present in the PAI-1 promoter was found to abolish TGF beta responsiveness. Thus, CAGA elements are essential and sufficient for the induction by TGF beta. In addition, TGFbeta induces the binding of a Smad3/Smad4-containing nuclear complex to CAGA boxes. Furthermore, bacterially expressed Smad3 and Smad4 proteins, but not Smad1 nor Smad2 protein, bind directly to this sequence in vitro. The presence of this box in TGF beta-responsive regions of several other genes suggests that this may be a widely used motif in TGF beta-regulated transcription.

Abstract: TGF-β signals from the membrane to the nucleus through serine/threonine kinase receptors and their downstream effectors, termed SMAD proteins1. The activated TGF-β receptor induces phosphorylation of two such proteins, Smad2 and Smad3 (refs 2, 3, 5, 6), which form hetero-oligomeric complex(es) with Smad4/DPC4 (refs 5, 6, 7, 8, 9, 10) that translocate to the nucleus2,4,5,7, where they then regulate transcriptional responses11,12. However, the mechanisms by which the intracellular signals of TGF-β are switched off are unclear. Here we report the identification of Smad7, which is related to Smad6 (ref. 13). Transfection of Smad7 blocks responses mediated by TGF-β in mammalian cells, and injection of Smad7 RNA into Xenopus embryos blocks activin/TGF-β signalling. Smad7 associates stably with the TGF-β receptor complex, but is not phosphorylated upon TGF-β stimulation. TGFβ-mediated phosphorylation of Smad2 and Smad3 is inhibited by Smad7, indicating that the antagonistic effect of Smad7 is exerted at this important regulatory step. TGF-β rapidly induces expression of Smad7 mRNA, suggesting that Smad7 may participate in a negative feedback loop to control TGF-β responses.

Abstract: Background Alterations in circulating soluble fms-like tyrosine kinase 1 (sFlt1), an antiangiogenic protein, and placental growth factor (PlGF), a proangiogenic protein, appear to be involved in th...