The effect of normo (NTG)- and hypertriglyceridemic (HTG)-VLDL on cultured human umbilical vein endothelial cell (HUVEC) surface-localized fibrinolysis was examined following preincubation with NTG-, HTG-VLDL, LDL (1-20 μg/mL) or buffer (control). Ligand binding assays, using 125I-labeled tcu-PA, t- PA, or Glu-plasminogen (Glu-Pmg) were carried out in the absence/presence of lipoproteins. Scatchard analyses showed that HTG-VLDL decreased the B(max) for 125I-labeled Glu-Pmg ligand binding ~35% [(2.11 ± 0.39)-(1.40 ± 0.32) x 106 sites/cell, p < 0.05] and increased the K(d,app) ~5-fold (0.32 ± 0.03 to 1.74 ± 0.08 μM, p < 0.01), while NTG-VLDL, LDL, and buffer had no effect. 125I-labeled PA ligand binding was unaffected by these lipoproteins. Receptor-bound PA activation of cell-bound 125I-labeled Glu- Pmg was measured by quantitation of either the M(r) 20 kDa light- or M(r) 60 kDa heavy-chain of 125I-labeled plasmin, following SDS-PAGE. Kinetic analysis of these data (HTG-VLDL vs controls) indicated that HTG-VLDL decreased the V(max) of tcu-PA- and t-PA-mediated activation of plasminogen ~2.7-fold (0.317 ± 0.023 vs 0.869 ± 0.068 nM s-1, p < 0.01) and ~2.9- fold (0.391 ± 0.098 vs 1.152 ± 0.265 nM s-1, p < 0.01), respectively. Increasing concentrations of the HTG-VLDL increased I/V(max), yielding a series of parallel plots, typical for uncompetitive inhibition with a K(i) for inhibition of ~10 μg/mL. The combined ligand binding and kinetic data best fit an uncompetitive inhibition model in which the binding of the large HTG-VLDL particle to the EC surface may directly affect Glu-Pmg binding and activation, thus contributing to early fibrin deposition and the increased thrombotic risk associated with HTG.
