PF4 promotes aggregation of LDL on the cell surface, outside coated pits.

Electron micrographs of fibroblasts incubated with 5 nM Au-LDL alone (A,C) or 5 nM Au-LDL in the presence of 20 μg/mL PF4 (B,D) for 30 (A,B) or 120 (C,D) minutes at 37°C. The size of the Au particles used was 18 nm. At 30 minutes (A,B) most Au-LDL (arrow) is on the cell surface. In the absence of PF4, LDL is seen in coated pits, whereas, when PF4 is present, LDL was exclusively located outside coated pits. At 120 minutes LDL is primarily located intracellularly in the absence of PF4 (C). LDL is present in clusters on the cell surface in the presence of PF4 (D, arrowhead). The micron bar corresponds to 500 nm. Shown are representative images from 2 or more independent experiments.