Abstract
The acetyl low density lipoprotein (LDL), or scavenger, receptor, which binds modified forms of LDL, was thought to be expressed only on macrophages and endothelial cells. We demonstrate that rabbit fibroblasts and smooth muscle cells bind, internalize, and degrade acetoacetylated LDL, a ligand for the acetyl LDL receptor. Degradation is specific in that unlabeled acetoacetylated LDL and fucoidin, a known competitor for binding to the acetyl LDL receptor, are effective competitors, while native LDL is not. The acetyl LDL receptor on these cells is readily regulated. Higher levels of degradation are observed in cells preincubated with serum than in cells preincubated with plasma. This up-regulation of the acetyl LDL receptor is most likely due to the presence of platelet secretory products in serum since secretion products derived from thrombin-stimulated platelets also cause an increase in degradation. In addition, preincubation of rabbit fibroblasts with phorbol esters results in a 16-20-fold increase in specific degradation. These results indicate that rabbit fibroblasts and smooth muscle cells express the acetyl LDL receptor and that increased receptor expression appears to be mediated through activation of the protein kinase C pathway.
Highlights
The acetyl low density lipoprotein (LDL), or scavenger, receptor, which binds modified forms of LDL, was thought to be expressed only on macrophages and endothelial cells
Rabbit fibroblasts and smooth muscle cells were incubated for 5 h at 37 “C with DiI-labeled AcAc LDL (5 pg/ml) and viewed under the fluorescence microscope
From fluorescence-activated cell sorter (FACS) analysis it was apparent that the rabbit fibroblasts preincubated with fetal bovine serum internalized more DiI-labeled AcAc LDL than those preincubated with human lipoprotein-deficient serum
Summary
Foundation Laboratories for of California, San Francisco, Cardiovascular California. The acetyl low density lipoprotein (LDL), or scavenger, receptor, which binds modified forms of LDL, was thought to be expressed only on macrophages and endothelial cells. These results indicate that rabbit fibroblasts and smooth muscle cells express the acetyl LDL receptor and that increased receptor expression appears to be mediated through activation of the protein kinase C pathway. LDL [6] and oxidized LDL [7], two other ligands for the acetyl LDL receptor, may be generated in uiuo [8, 9] These modified forms of LDL have been postulated to contribute to the accumulation of cholesteryl esters in foam cells of atherosclerotic lesions [10, 11]. In this study we have investigated the expression of the acetyl LDL receptor by rabbit fibroblasts and smooth muscle cells We found that these cells degrade AcAc LDL and that degradation is increased by preincubating the cells with serum-containing medium, platelet secretory products, or phorbol esters. Increased degradation appears to be mediated through activation of protein kinase C
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