Objectives: Preeclampsia (PE), a serious disorder of human pregnancy is characterized by systemic endothelial dysfunction. PE can be categorized into subtypes of early-onset (EPE, ≤34 weeks of gestation) and late-onset (LPE, >34 weeks). EPE is often severe and poses a greater risk, but the only effective treatment is premature delivery. We have reported that high temperature requirement factor A4 (HtrA4) is a placenta-specific protease that is significantly up-regulated in EPE [1]. We have also shown that HtrA4 is secreted into the maternal circulation and serum levels of HtrA4 are significantly elevated in EPE [1]. We have further demonstrated that high levels of HtrA4 induce endothelial dysfunction [1, 2]. However, the mechanisms of HtrA4 action on endothelial cells are unknown. In this study, we investigated whether HtrA4 can cleave the main cell surface receptor of VEGFA, the kinase insert domain receptor (KDR), thereby inhibiting VEGFA action.
Methods: We first determined whether HtrA4 can directly cleave recombinant KDR in vitro. We then determined whether HtrA4 can cleave KDR from the surface of human umbilical vein endothelial cells (HUVECs) using western blot analysis and immunocytochemistry. Finally, we examined whether HtrA4 can inhibit VEGFA action in HUVECs and in mouse aorta rings.
Results: HtrA4 directly cleaved recombinant KDR in vitro. HtrA4 also reduced the amount of intact KDR in HUVECs. Treatment of HUVECs with high levels of HtrA4 inhibited the VEGFA-induced phosphorylation of Akt kinase, which is essential for VEGFA signalling. Functionally, HtrA4 prevented VEGFA-induced tube formation in HUVECs and inhibited VEGFA-induced angiogenesis in mouse aorta rings.
Conclusion: High levels of HtrA4 in the maternal circulation may cleave the main receptor of VEGFA on endothelial cells to induce wide-spread impairment of angiogenesis. Our study thus suggests that HtrA4 may be an important causal factor of EPE development and a potential target for treatment.