ized-LDL receptor activity in macrophages

Platelet secretory products were shown to modulate the interaction between lipoproteins and their receptors on macrophages. Preincubation of macrophages for 2 h at 37°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 37OC demonstrated enhanced uptake of Ox-LDL, preincubation of macrophages with PCM at 4OC 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 heatand 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.-Fuhrman, E., G. J. Brook, and M. Aviram. Activated platelets secrete a protein-like factor that stimulates oxidized-LDL receptor activity in macrophages. J Lipid Res. 1991. 32: 1113-1123 Supplementary key words cholesterol scavenger receptor Lipid-laden macrophages are characteristic constituents of the atherosclerotic plaque (1, 2). Macrophages accumulate cholesterol after incubation with chemically (3, 4) or biologically (5) modified LDL, but not with native LDL (6). Modification of LDL by oxidative processes results in its enhanced uptake by macrophages (7). LDL oxidation in vitro can be achieved by incubation in cell-free medium under oxidative conditions (8) and by incubation with endothelial cells (8, 9), smooth muscle cells (10, ll), or monocyte derived macrophages (12). Thus, LDL oxidation may occur within the arterial wall, and indeed, evidence for the in vivo existence of Ox-LDL has been presented (13, 14). Recently platelets were shown to enhance in vitro oxidation of LDL (15). Blood platelets play an important role in the development of atherosclerosis (16, 17) and they are found in close association with foam cells in the developing atherosclerotic lesion (18). Platelet aggregation and release may be implicated early in atherogenesis as well as in the more advanced stages. In early lesions platelets adhere to regions of the arterial wall where blood flow is disturbed (19). The latter also permits the accumulation of agents that can injure the endothelium, which may in turn cause further platelet activation and degranulation (20-22). Platelet involvement in the more advanced atherosclerotic lesion may occur subsequent to oxidation of LDL in the subendothelial space where the concentration of antioxidants is low. Ox-LDL can be highly cytotoxic, causing loss of endothelial cells overlying the fatty streak (ZO), thus leading to platelet adherence and activation. The secretory products then released may react with macrophages with subsequent effect on cellular uptake of Ox-LDL. Activated platelets release a variety of substances that have been shown to enhance cholesterol esAbbreviations: LDL, low density lipoprotein; Ox-LDL, oxidized LDL; PCM, platelet conditioned medium; MPM, mouse peritoneal macrophages; HMDM, human monocyte-derived macrophages; PBS, phosphate-buffered saline; PRP, platelet-rich plasma; MDA, malondialdehyde. 'Address for correspondence and reprints: Dr. M. Aviram, Lipid Research Laboratory, Rambam Medical Center, Haifa, Israel. Journal of Lipid Research Volume 32, 1991 1113 by gest, on O cber 9, 2017 w w w .j.org D ow nladed fom terification in murine macrophages (23) and in human monocyte-derived macrophages (24), to modify LDL to a form taken up rapidly by macrophages (25), and also to affect the uptake of lipoproteins via the scavenger receptor (26, 27). Recent study in our laboratory revealed that preincubation of macrophages with PCM resulted in up to 40% inhibition in cellular uptake of native LDL (Fuhrman, B., J. G. Brook, and M. Aviram, unpublished data). Since Ox-LDL may exist in vivo and can lead to cholesterol accumulation in macrophages, the present study was undertaken to elucidate the effect of platelet secretory products on Ox-LDL uptake by macrophages.