Characterization of LDL and VLDL Binding Sites on Human Basophils and Mast Cells

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Abstract
Abstract Recent data suggest that basophils and mast cells play a potential role in the processing and accumulation of plasma lipoproteins. This study investigated the interactions of 111 In-low-density lipoprotein (LDL), 111 In-acetyl-LDL, and 111 In-very-low-density lipoprotein (VLDL) with purified primary human blood basophils, immortalized human basophils (KU812 cell line), and a human mast cell line, HMC-1. Binding sites for 111 In-LDL resolved into curvilinear Scatchard plots indicating two classes of specific binding sites on primary basophils (B max1 , 7404 sites/cell; K d1 , 1.9 nmol/L; B max2 , 39 611 sites/cell; K d2 , 29 nmol/L), on KU812 cells (B max1 , 8290±2690 sites/cell; K d1 , 2.4±0.6 nmol/L; B max2 , 46 470 sites/cell; K d2 , 33.4±7.8 nmol/L), and on HMC-1 cells (B max1 , 7840±360 sites/cell; K d1 , 1.8±0.8 nmol/L; B max2 , 61 450±9900 sites/cell; K d2 , 28.4±9.4 nmol/L). On KU812 cells, binding of 111 In-LDL was displaced by apolipoprotein (apo)-E–rich high-density lipoprotein (HDL) (IC 50 , 14±6 nmol/L), LDL (IC 50 , 29±11 nmol/L), VLDL (IC 50 , 55±21 nmol/L), HDL 2 (IC 50 , 420±140 nmol/L), and heparin (IC 50 , 67±28 nmol/L), whereas no competition was produced by HDL, HDL 3 , or acetyl-LDL (IC 50 , >1 μmol/L). Western blot analysis using the monoclonal antibody C7 confirmed the presence of the LDL receptor on human basophils and HMC-1 cells. 111 In-acetyl-LDL binding sites (scavenger receptor) could be detected neither on human basophils nor on HMC-1 cells. 111 In-VLDL bound to a single class of high-affinity binding sites on primary basophils (B max , 4320 sites/cell; K d , 10 nmol/L), KU812 cells (B max , 4020±840 sites/cell; K d , 8±3 nmol/L), and HMC-1 cells (B max , 6143±1866 sites/cell; K d , 4±2 nmol/L). 111 In-VLDL binding was displaced by VLDL>LDL>apoE-rich HDL but not by heparin (IC 50 >1 mmol/L). In the presence of prostaglandin E 1 , the number of 111 In-LDL receptors increased by 150% ( P <.05) in the high-affinity range and by 170% ( P <.01) in the low-affinity range, whereas the number of 111 In-VLDL binding sites remained unchanged. VLDL, LDL, HDL, and the subclasses HDL 2 and HDL 3 inhibited immunological histamine release by primary normal basophils (n=3) and mast cells (n=3). Our results provide evidence for the existence of LDL and VLDL binding sites on human basophils and HMC-1 mast cells. The exact biological and pathophysiological roles of these sites remain to be elucidated.