Local Retention Versus Systemic Release of Soluble VEGF Receptor-1 Are Mediated by Heparin-Binding and Regulated by Heparanase

Abstract

Rationale:The vascular endothelial growth factor (VEGF) decoy receptor soluble VEGF-R1 (sVEGF-R1) is thought to protect the cells that produce it from adverse VEGF signaling. To accomplish this role, a mechanism for pericellular retention of sVEGF-R1 is required. Local retention may also prevent the accumulation of high circulating levels of sVEGF-R1 and resulting interference with homeostatic VEGF functions in remote organs.Objective: To reveal natural storage depots of sVEGF-R1 and determine mechanisms underlying its pericellular retention. To uncover natural mechanisms regulating its systemic release. Methods and Results:We show that both the canonical and human-specific isoforms of sVEGF-R1 are strongly bound to heparin. sVEGF-R1 produced by vascular smooth muscle cells is stored in the vessel wall and can be displaced from isolated mouse aorta by heparin. Another major reservoir of sVEGF-R1 is the placenta. Heparin increases the level of sVEGF-R1 released by cultured human placental villi, and pregnant women treated with low molecular weight heparin showed markedly elevated levels of sVEGF-R1 in the circulation. Heparanase is expressed in human placenta at the same locales as sVEGF-R1, and its transgenic overexpression in mice resulted in a marked increase in the levels of circulating sVEGF-R1. Conversely, heparanase inhibition, by either a neutralizing antibody or by inhibition of its maturation, reduced the amounts of sVEGF-R1 released from human placental villi, indicating a natural role of heparanase in sVEGF-R1 release.Conclusions: Together, the findings uncover a new level of regulation governing sVEGF-R1 retention versus release and suggest that manipulations of the heparin/heparanase system could be harnessed for reducing unwarranted release of sVEGF-R1 in pathologies such as preeclampsia. (Circ Res.

Current Theories on the Prevention of Severe Preeclampsia With Low-Molecular Weight Heparin

“…76 Because sFlt-1 binds to the cell surface or extracellular matrix via heparan sulfate proteoglycans, LMWH may displace sFlt-1 from these heparin-binding sites via competitive binding, leading to higher levels of sFlt-1 while yielding an improvement in endothelial function. 74 This proposed mechanism would be consistent with previous reports that heparin increases the bioavailability of NO in patients with coronary artery disease, whereas concurrently increasing levels of circulating MPO. 58 Levels of circulating MPO are also elevated in PE, with both sFlt-1 and MPO bound to the endothelium via heparan sulfate proteoglycans and displaced by LMWH.…”

“…Clinical and animal studies have determined that the LMWH administration significantly increases serum levels of sFlt-1. 52,73,74 Although large, these heparin-induced elevations in circulating sFlt-1 are likely not pathogenic because women who require heparin anticoagulation throughout pregnancy do not seem to have an increased risk of developing PE. 75 In functional terms, endothelium-bound sFlt-1 becomes available for enhanced renal excretion because of mobilization by circulating heparin.…”

“…59 Women with PE have demonstrated elevated levels of circulating sFlt-1, however, these circulating levels may only represent a portion of the total, endothelium-bound sFlt-1 levels as suggested in previous molecular work. 74 Consequently, there is potential benefit in the release of this protein into the circulation where it can be cleared. Heparins, therefore, seem to exert beneficial effects on the endothelium in clinical states of vascular dysfunction, however, it is currently unknown if these agents have a similar effect on women with PE.…”

Is heparin a placental anticoagulant in high-risk pregnancies?

“…71 sFlt1 is locally retained on the surface of placental villi, where it can be released via the heparanase action of LMWH. 70,72 Interestingly, LMWH induces transcription and translation of sFlt1 in floating first-trimester villous explants, further contributing to the LMWH-induced secretion in a manner that antagonizes VEGF receptor phosphorylation. 73 This disparity in clinical and in vitro observations suggests more complex interactions between the placenta and LMWH to mediate a preventive role of heparin in women at high risk of severe PE.…”

“…We demonstrated that both unfractionated heparin (UFH) and LMWH, in comparable doses used clinically in pregnancy, are capable of reversing the natural antiangiogenic tendency of first-trimester placental villi. 97 These heparins exert this proangiogenic effect despite increasing secretion of sFLT1 70,72 that at least in vitro impairs VEGF receptor signaling. 75 This paradox between in vitro and in vivo data in the context of sFLT1 may be because of the restoration additional functions of the outer syncytiotrophoblast that override the endothelial-specific effects of excess sFLT1.…”