LDL receptor-related protein mediates uptake of aggregated LDL in human vascular smooth muscle cells.

Foam cell formation is a key event in the onset and progression of atherosclerotic lesions. We have previously reported that internalization of aggregated low density lipoproteins (agLDLs) by vascular smooth muscle cells (VSMCs) produces cholesteryl ester (CE) accumulation in these cells. The aim of this study was to analyze whether the low density lipoprotein receptor-related protein (LRP) mediates the uptake of agLDL by VSMCs. First, immunocytochemistry and fluorescence microscopic analysis with the use of anti-LRP antibodies indicated that there was a high expression of LRP in VSMCs. Confocal microscopic analysis with the use of agLDLs labeled with fluorochrome 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine and anti-LRP antibodies showed the colocalization of agLDL and LRP. The second approach was to analyze the effect of LRP ligands on agLDL internalization; lactoferrin strongly inhibited CE accumulation from agLDLs (85.0+/-5.7% at 25 microg/mL) by impairing agLDL binding. Coincubation of agLDL with anti-LRP antibodies decreased in a dose-dependent manner agLDL-derived CE accumulation (from 20% at 12.5 microg/mL to 80% at 50 microg/mL). The third approach was to evaluate whether antisense LRP oligodeoxynucleotides were able to block agLDL internalization. Treatment of VSMCs with 5 micromol/L antisense LRP oligodeoxynucleotides reduced agLDL-derived CE accumulation by 84+/-2%. In conclusion, these results from immunologic, biochemical, and molecular interventions demonstrate that LRP mediates the binding and internalization of agLDL in human VSMCs. Because LRP is highly expressed in VSMCs and the uptake of 1 LDL aggregate amounts to the deposition of several hundreds of LDL particles, the uptake of agLDL through LRP could have a crucial role for lipid deposition in VSMCs.