Morphological characterization of beta-VLDL and acetylated-LDL binding and internalization by cultured pigeon monocytes.
The ultrastructure of binding, internalization, and translocation of beta-migrating very low density lipoprotein (beta-VLDL) and acetylated low density lipoprotein (Ac-LDL) by cultured pigeon monocytes was examined using lipoprotein-gold conjugates. Through morphometry, differences in the binding and uptake of beta-VLDL-gold and Ac-LDL-gold were documented. Cells exposed to either beta-VLDL-gold or Ac-LDL-gold for 2 hr at 4 degrees C had the label over noncoated regions of the plasma membrane. Upon warming the cells to 37 degrees C for 2 min, 35% of the surface-bound beta-VLDL-gold was within coated pits on the cell surface. Although coated pits occupied less than 2% of the surface, binding of beta-VLDL-gold was 53 times more concentrated in coated pits as compared to noncoated membrane regions. In contrast, Ac-LDL-gold neither bound to coated pits nor relocated into coated regions of the membrane upon warming to 37 degrees C. Both the beta-VLDL-gold and the Ac-LDL-gold were internalized when the cells were rewarmed at 37 degrees C. Most of the internalized gold particles for both lipoproteins were located in electron-lucent vesicles; however, 9% of the intracellular beta-VLDL-gold was observed within coated vesicles at early times. Upon prolonged rewarming (30-90 min), both lipoprotein-gold conjugates were within acid phosphatase-positive lysosomes. Ultimately 83% of the Ac-LDL-gold and 90% of the beta-VLDL-gold were within electron-dense and electron-lucent lysosomes. These results suggested that the receptor-mediated binding and internalization of beta-VLDL and Ac-LDL by pigeon monocyte macrophages proceeded by separate, distinct routes; beta-VLDL by both coated and noncoated pathways while Ac-LDL was internalized exclusively by noncoated mechanisms. Regardless of these internalization differences, both lipoproteins were delivered to lysosomes for degradation.
Henson
D A
DA
Department of Pathology, Wake Forest University, Bowman Gray School of Medicine, Winston-Salem, North Carolina 27103.
St Clair
R W
RW
Lewis
J C
JC
D000135:Acid Phosphatase / Q000378:metabolism; D000818:Animals; D015815:Cell Adhesion Molecules; D002451:Cell Compartmentation; D002478:Cells, Cultured; D003034:Coated Pits, Cell-Membrane / Q000378:metabolism; D010856:Columbidae; D004577:Electron Probe Microanalysis; D004705:Endocytosis; D005487:Foam Cells / Q000378:metabolism; D007150:Immunohistochemistry; D066298:In Vitro Techniques; D008077:Lipoproteins, LDL / Q000378:metabolism*; D008079:Lipoproteins, VLDL / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism; D008264:Macrophages / Q000378:metabolism; D009000:Monocytes / Q000378:metabolism*; D015388:Organelles / Q000378:metabolism; D011973:Receptors, LDL / Q000378:metabolism*; D051116:Receptors, Scavenger
D015815:Cell Adhesion Molecules; D008077:Lipoproteins, LDL; D008079:Lipoproteins, VLDL; D011973:Receptors, LDL; D051116:Receptors, Scavenger; C036053:VLDL receptor; C032629:acetyl-LDL; D000135:Acid Phosphatase