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.