O‐glycan structures in apo(a) subunit of human lipoprotein(a) suppresses the pro‐angiogenic activity of galectin‐1 on human umbilical vein endothelial cells

Abstract

AbstractApolipoprotein(a) [apo(a)] is a highly polymorphic O‐glycoprotein circulating in human plasma as lipoprotein(a) [Lp(a)]. The O‐glycan structures of apo(a) subunit of Lp(a) serve as strong ligands of galectin‐1, an O‐glycan binding pro‐angiogenic lectin abundantly expressed in placental vascular tissues. But the pathophysiological significance of apo(a)‐galectin‐1 binding is not yet been revealed. Carbohydrate‐dependent binding of galectin‐1 to another O‐glycoprotein, neuropilin‐1 (NRP‐1) on endothelial cells activates vascular endothelial growth factor receptor 2 (VEGFR2) and mitogen‐activated protein kinase (MAPK) signaling. Using apo(a), isolated from human plasma, we demonstrated the potential of the O‐glycan structures of apo(a) in Lp(a) to inhibit angiogenic properties such as proliferation, migration, and tube‐formation in human umbilical vein endothelial cells (HUVECs) as well as neovascularization in chick chorioallantoic membrane. Further, in vitro protein‐protein interaction studies have confirmed apo(a) as a superior ligand to NRP‐1 for galectin‐1 binding. We also demonstrated that the protein levels of galectin‐1, NRP‐1, VEGFR2, and downstream proteins in MAPK signaling were reduced in HUVECs in the presence of apo(a) with intact O‐glycan structures compared to that of de‐O‐glycosylated apo(a). In conclusion, our study shows that apo(a)‐linked O‐glycans prevent the binding of galectin‐1 to NRP‐1 leading to the inhibition of galectin‐1/neuropilin‐1/VEGFR2/MAPK‐mediated angiogenic signaling pathway in endothelial cells. As higher plasma Lp(a) level in women is an independent risk factor for pre‐eclamsia, a pregnancy‐associated vascular complication, we propose that apo(a) O‐glycans‐mediated inhibition of the pro‐angiogenic activity of galectin‐1 may be one of the underlying molecular mechanism of pathogenesis of Lp(a) in pre‐eclampsia.