PF4 promotes the clustering of LDL and LDL-R detected by immuno-electron microscopy.

Electron micrographs of fibroblasts incubated with 5 nM LDL (A), 5 nM LDL plus 20 μg/mL PF4 (B), buffer (C), or 20 μg/mL PF4 alone (D) for 120 minutes at 37°C. After fixation, LDL was labeled with a polyclonal rabbit anti-apoB100 IgG and 18 nm Au-antirabbit IgG (A,B) from opposite sides of the grid as described in “Materials and methods.” LDL-Rs were labeled with a monoclonal antireceptor immunoglobulin and either 5 nm Au (A,B) or 10 nm (C,D) Au-labeled antimouse immunoglobulin. Clusters of LDL and/or LDL-R (arrowheads) are seen at the cell surface (B,D) in the presence of PF4. LDL-R is seen throughout the cell in the absence of both LDL and PF4. Micron bars correspond to 100 nm. Shown are representative images from 2 or more independent experiments.