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Activated platelets secrete a protein-like factor that stimulates oxidized-LDL receptor activity in macrophages
Abstract:
Platelet secretory products were shown to modulate the interaction between lipoproteins and their receptors on macrophages. Preincubation of macrophages for 2 h at 37 degrees C with platelet conditioned medium (PCM), followed by its removal and a further 5-h incubation in the presence of oxidized-LDL (Ox-LDL), resulted in increased cellular degradation of Ox-LDL (34%), stimulation of cellular cholesterol esterification (31%), and mass accumulation of esterified and nonesterified cholesterol (25% and 41%, respectively). These effects were found to be the result of a PCM-mediated increase in the number of Ox-LDL receptors on macrophages. PCM was shown to interact with the macrophage scavenger receptor. Enhanced Ox-LDL uptake by macrophages preincubated with PCM could not be reproduced when PCM remained in the incubation medium. Maintenance of PCM in the incubation medium reduced Ox-LDL uptake by macrophages (40%) and was shown to be PCM dose-dependent. Whereas incubation at 37 degrees C demonstrated enhanced uptake of Ox-LDL, preincubation of macrophages with PCM at 4 degrees C exhibited a 64% reduction in Ox-LDL-mediated cellular cholesterol esterification. Thus, PCM internalization by macrophages after its binding to the scavenger receptor is required to promote the enhancing effect of PCM on Ox-LDL uptake by macrophages. PCM activity was associated with platelet degranulation, and was recovered in the protein fraction of PCM. It was found to be heat- and trypsin-labile with a molecular weight greater than 25,000. PCM obtained from platelets derived from a patient with alpha granules deficiency failed to enhance the uptake of Ox-LDL by macrophages, suggesting that the active protein-like factor in PCM originated from platelet alpha granules. These results indicate that a platelet-secreted protein-like factor can modulate macrophage uptake of Ox-LDL with subsequent effect on foam cell formation.
Insights
Platelet-secreted factors enhance macrophage uptake of oxidized-low-density lipoprotein (Ox-LDL), promoting cholesterol accumulation and foam cell formation. This process involves platelet alpha granules and scavenger receptor interaction.
Area of Science:
- Biochemistry
- Cell Biology
- Immunology
Background:
- Platelet secretory products can influence macrophage behavior.
- Macrophage uptake of oxidized low-density lipoprotein (Ox-LDL) is critical in atherogenesis.
- Scavenger receptors on macrophages play a key role in Ox-LDL metabolism.
Purpose of the Study:
- To investigate the effect of platelet conditioned medium (PCM) on macrophage interaction with Ox-LDL.
- To elucidate the mechanism by which PCM modulates Ox-LDL uptake and metabolism.
- To identify the source and nature of the active factor in PCM.
Main Methods:
- Macrophage preincubation with PCM, followed by incubation with Ox-LDL.
- Measurement of Ox-LDL degradation, cholesterol esterification, and cholesterol mass.
- Analysis of PCM interaction with macrophage scavenger receptors.
- Investigation of PCM internalization and temperature dependence.
- Biochemical characterization of the active factor in PCM.
Main Results:
- PCM preincubation significantly increased Ox-LDL degradation, cholesterol esterification, and cholesterol accumulation in macrophages.
- PCM enhanced Ox-LDL uptake by increasing the number of Ox-LDL receptors on macrophages, involving scavenger receptor interaction.
- PCM internalization was essential for enhancing Ox-LDL uptake; its presence in the medium reduced uptake.
- The active factor is protein-like, heat- and trypsin-labile, originates from platelet alpha granules, and is >25,000 MW.
Conclusions:
- Platelet alpha-granule-derived factors modulate macrophage Ox-LDL uptake and metabolism.
- This modulation involves scavenger receptor interaction and internalization, impacting foam cell formation.
- Platelet secretory products represent a potential target for modulating atherogenic processes.