Inhibition of platelet aggregation by OxHDL depends on platelet SR-BI. (A) Confluent monolayers of SR-BI–overexpressing or control, vector-transfected cells were incubated with 5 μg/mL [¹²⁵I]-HDL, [¹²⁵I]-NO₂HDL, or [¹²⁵I]-Cu²⁺-HDL for 3 hours at 37°C, and bound radioactivity was quantified. Data shown are means plus or minus SEM from 3 independent experiments. (B) Immunoblot analysis of SR-BI protein in platelets isolated from WT and SR-BI^−/− mice. β-actin was used as a loading control, and the molecular weight markers are shown on the left side of the panel. (C) Murine platelets isolated by gel filtration were preincubated with or without anti–SR-BI blocking antibody (20 μg/mL) or nonimmune isotype control antibody for 5 minutes, followed by incubation with 200 μg/mL murine OxHDL or native HDL for 5 minutes at 37°C and stimulation with 0.1 U/mL thrombin. (D) Platelets prepared from CD36^−/− mice were preincubated with or without anti–SR-BI blocking antibody for 5 minutes, followed by incubation with 200 μg/mL murine OxHDL or native HDL for 5 minutes at 37°C and stimulation with 0.1 U/mL thrombin. (E) Platelets isolated from SR-BI^−/− mice were incubated with or without 200 μg/mL murine OxHDL for 5 minutes at 37°C, followed by stimulation with 0.1 U/mL thrombin; platelet aggregation was optically monitored. (F) Data from panels C-E are presented as means plus or minus SEM of 3 to 5 independent experiments. NA indicates no addition; α-SR-BI, anti–SR-BI blocking antibody; and NI, nonimmune antibody. *P < .001.