Identification of megalin/gp330 as a receptor for lipoprotein(a) in vitro

Lipoprotein(a) [Lp(a)] is an atherogenic lipoprotein of unknown physiologic ai function. The mechanism of Lp(a) atherogenicity as well as its catabolic pathways are only incompletely understood at present. In this report, we s how that the low density lipoprotein receptor (LDLR) gene family member meg alin/glycoprotein (gp) 330 is capable of binding and mediating the cellular uptake and degradation of Lp(a) in vitro. A mouse embryonic yolk sac cell line with native expression of megalin/gp330 but genetically deficient in L DLR-related protein (LRP) and a control cell line carrying a double knockou t for both LRP and megalin/gp330 were compared with regard to their ability to bind, internalize, and degrade dioctadecyltetramethylindocarbocyanine p erchlorate (DiI)-fluorescence-labeled Lp(a) as well as equimolar amounts of I-125-Iabeled Lp(a) and LDL. Uptake and degradation of radiolabeled Lp(a) by the megalin/gp330-expressing cells were, on average, 2-fold higher than that of control cells. This difference could be completely abolished by add ition of the receptor-associated protein, an inhibitor of ligand binding to megalin/gp330. Mutual suppression of the uptake of I-125-Lp(a) and of I-12 5-LDL by both unlabeled Lp(a) and LDL suggested that Lp(a) uptake is mediat ed at least partially by apolipoprotein B100. Binding and uptake of Dil-Lp( a) resulted in strong signals on megalin/gp330-expressing cells versus back ground only on control cells, in addition, we show that purified megalin/gp 330, immobilized on a sensor chip, directly binds Lp(a) in a Ca2+-dependent manner with an affinity similar to that for LDL. We conclude that megalin/ gp330 binds Lp(a) in vitro and is capable of mediating its cellular uptake and degradation.