Morphological characterization of β-VLDL and acetylated-LDL binding and internalization by cultured pigeon monocytes

Abstract

The ultrastructure of binding, internalization, and translocation of β-migrating very low density lipoprotein (β-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 β-VLDL-gold and Ac-LDL-gold were documented. Cells exposed to either β-VLDL-gold or Ac-LDL-gold for 2 hr at 4°C had the label over noncoated regions of the plasma membrane. Upon warming the cells to 37°C for 2 min, 35% of the surface-bound β-VLDL-gold was within coated pits on the cell surface. Although coated pits occupied less than 2% of the surface, binding of β-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°C. Both the β-VLDL-gold and the Ac-LDL-gold were internalized when the cells were rewarmed at 37°C. Most of the internalized gold particles for both lipoproteins were located in electron-lucent vesicles; however, 9% of the intracellular β-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 β-VLDL-gold were within electron-dense and electron-lucent lysosomes. These results suggested that the receptor-mediated binding and internalization of β-VLDL and Ac-LDL by pigeon monocyte macrophages proceeded by separate, distinct routes: β-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.