Plasmin-mediated macrophage reversal of low density lipoprotein aggregation

Evidence suggests that aggregated low density lipoprotein (AgLDL) accumulat es in atherosclerotic lesions. Previously, we showed that AgLDL induces and enters surface-connected compartments (SCC) in human monocyte-derived macr ophages by a process we have named patocytosis, Most AgLDL taken up by thes e macrophages in the absence of serum is stored in SCC and remains undegrad ed, We now show that macrophages released AgLDL (prepared by vortexing or t reatment with phospholipase C or sphingomyelinase) from their SCC when expo sed to 10% human lipoprotein-deficient serum (LPDS), Macrophages also took up AgLDL in the presence of LPDS, but subsequently released it. In both cas es, the released AgLDL was disaggregated, Although the AgLDL that macrophag es took up could not pass through a 0.45-mum filter, >60% of AgLDL could pa ss this filter after release from the macrophages. Disaggregation of AgLDL was verified by gel-filtration chromatography and electron microscopy that also showed particles larger than LDL, reflecting fusion of LDL that aggreg ates. The factor in serum that mediated AgLDL release and disaggregation wa s plasmin generated from plasminogen by macrophage urokinase plasminogen ac tivator. AgLDL release was decreased >90% by inhibitors of plasmin (E-amino caproic acid and antiplasminogen mAb), and also by inhibitors of urokinase plasminogen activator (plasminogen activator inhibitor-1 and anti-urokinas e plasminogen activator mAb), Moreover, plasminogen could substitute for LP DS and produce similar macrophage release and disaggregation of AgLDL. Beca use only plasmin bound to the macrophage surface is protected from serum pl asmin inhibitors, interaction of AgLDL with macrophages was necessary for r eversal of its aggregation by LPDS. The released disaggregated LDL particle s were competent to stimulate LDL receptor-mediated endocytosis in cultured fibroblasts, Macrophage-mediated disaggregation of aggregated and fused LD L is a mechanism for transforming LDL into lipoprotein structures size-cons istent with lipid particles found in atherosclerotic lesions.