Unfractionated heparin displaces sFlt-1 from the placental extracellular matrix

Soluble vascular endothelial growth factor receptor-1 (sFlt-1) is an anti-angiogenic protein which is secreted by numerous cell types and acts as a decoy receptor for the angiogenic protein vascular endothelial growth factor (VEGF). Despite its physiologic importance in maintaining angiogenic balance, excess sFlt-1 levels are associated with the pathogenesis of many diseases, especially those with angiogenic imbalance, endothelial dysfunction, and hypertension. Although sFlt-1 is a soluble protein, it contains a binding site for the extracellular matrix component heparan sulfate. This allows cells to retain and localize sFlt-1 in order to prevent excessive VEGF signaling. During pregnancy, placental syncytiotrophoblasts develop a large extracellular matrix which contains significant amounts of heparan sulfate. Consequently, the placenta becomes a potential storage site for large amounts of sFlt-1 bound to extracellular heparan sulfate. Additionally, it should be noted that sFlt-1 can bind to the anticoagulant unfractionated heparin due to its molecular mimicry to heparan sulfate. However, it remains unknown whether unfractionated heparin can compete with heparan sulfate for binding of localized sFlt-1. In this study, we hypothesized that administration of unfractionated heparin would displace and solubilize placental extracellular matrix(ECM)-bound sFlt-1. If unfractionated heparin can displace this large reservoir of sFlt-1 in the placenta and mobilized it into the maternal circulation, we should be able to observe its effects on maternal angiogenic balance and blood pressure. To test this hypothesis, we utilized in vitro, ex vivo, and in vivo methods. Using the BeWo placental trophoblast cell line, we observed increased sFlt-1 in the media of cells treated with unfractionated heparin compared to controls. The increase in media sFlt-1 was found in conjunction with decreased localized cellular Flt (sFlt-1 and Flt-1) as measured by total cell fluorescence. Similar results were observed using ex vivo placental villous explants treated with unfractionated heparin. Real-time quantitative PCR of the explants showed no change in sFlt-1 or heparanase-1 mRNA expression, eliminating increased production and enzymatic cleavage of heparan sulfate as causes for sFlt-1 media increase. Timed-pregnant rats given a continuous infusion of unfractionated heparin exhibited an increased mean arterial pressure as well as decreased bioavailable VEGF compared to vehicle-treated animals. These data demonstrate that chronic unfractionated heparin treatment is able to displace matrix-bound sFlt-1 into the maternal circulation to such a degree that mean arterial pressure is significantly affected. Here we have shown that the placental ECM is a storage site for large quantities of sFlt-1, and that it should be carefully considered in future studies concerning angiogenic balance in pregnancy.