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The influence of particle size and multiple apoprotein E-receptor interactions on the endocytic targeting of
I Tabas1, J N Myers, T L Innerarity
1Department of Medicine, Columbia University, New York 10032.
Abstract:
Low density lipoprotein (LDL) and beta-very low density lipoprotein (beta-VLDL) are internalized by the same receptor in mouse peritoneal macrophages and yet their endocytic patterns differ; beta-VLDL is targeted to both widely distributed and perinuclear vesicles, whereas LDL is targeted almost entirely to perinuclear lysosomes. This endocytic divergence may have important metabolic consequences since beta-VLDL is catabolized slower than LDL and is a more potent stimulator of acyl-CoA/cholesterol acyl transferase (ACAT) than LDL. The goal of this study was to explore the determinants of beta-VLDL responsible for its pattern of endocytic targeting. Fluorescence microscopy experiments revealed that large, intestinally derived, apoprotein (Apo) E-rich beta-VLDL was targeted mostly to widely distributed vesicles, whereas small, hepatically derived beta-VLDL was targeted more centrally (like LDL). Furthermore, the large beta-VLDL had a higher ACAT-stimulatory potential than the smaller beta-VLDL. The basis for these differences was not due to fundamental differences in the means of uptake; both large and small beta-VLDL were internalized by receptor-mediated endocytosis (i.e., not phagocytosis) involving the interaction of Apo E of the beta-VLDL with the macrophage LDL receptor. However, large beta-VLDL was much more resistant to acid-mediated release from LDL receptors than small beta-VLDL. Furthermore, partial neutralization of the multiple Apo Es on these particles by immunotitration resulted in a more perinuclear endocytic pattern, a lower ACAT-stimulatory potential, and an increased sensitivity to acid-mediated receptor release. These data are consistent with the hypothesis that the interaction of the multivalent Apo Es of large beta-VLDL with multiple macrophage LDL receptors leads to a diminished or retarded release of the beta-VLDL from its receptor in the acidic sorting endosome which, in turn, may lead to the widely distributed endocytic pattern of large beta-VLDL. These findings may represent a physiologically relevant example of a previously described laboratory phenomenon whereby receptor cross-linking by multivalent ligands leads to a change in receptor targeting.
Insights
Large beta-very low density lipoprotein (beta-VLDL) targets different cellular vesicles than low density lipoprotein (LDL) due to its apoprotein E structure, impacting cholesterol metabolism.
Area of Science:
- Lipid metabolism and lipoprotein trafficking
- Cellular biology and receptor-mediated endocytosis
- Atherosclerosis and cardiovascular disease research
Background:
- Low density lipoprotein (LDL) and beta-very low density lipoprotein (beta-VLDL) utilize the same LDL receptor in mouse macrophages.
- Despite shared receptors, LDL and beta-VLDL exhibit distinct endocytic patterns, influencing their metabolic fate and cellular effects.
- Beta-VLDL catabolism is slower and it stimulates acyl-CoA/cholesterol acyl transferase (ACAT) more potently than LDL, suggesting distinct cellular processing.
Purpose of the Study:
- To investigate the structural determinants of beta-VLDL responsible for its unique endocytic targeting.
- To understand how variations in beta-VLDL influence its interaction with macrophage LDL receptors and subsequent intracellular trafficking.
Main Methods:
- Utilized fluorescence microscopy to visualize and compare the endocytic patterns of different beta-VLDL subclasses.
- Employed receptor-mediated endocytosis assays to confirm uptake mechanisms and LDL receptor involvement.
- Performed acid-mediated release experiments and immunotitration to assess receptor-ligand interactions and apoprotein E (Apo E) functionality.
Main Results:
- Large, intestinally derived, apoprotein E-rich beta-VLDL predominantly targeted widely distributed vesicles, unlike small, hepatically derived beta-VLDL which localized centrally.
- Large beta-VLDL exhibited higher ACAT-stimulatory potential compared to smaller beta-VLDL particles.
- Particle size and Apo E multivalency influenced beta-VLDL's resistance to acid-mediated release from LDL receptors, correlating with endocytic pattern and ACAT stimulation.
Conclusions:
- The multivalent interaction of Apo E on large beta-VLDL with multiple LDL receptors retards release in acidic endosomes, leading to widespread vesicle targeting.
- This altered endocytic pathway for large beta-VLDL may explain its slower catabolism and greater ACAT stimulation compared to LDL.
- Findings suggest a physiological link between ligand multivalency, receptor dynamics, and endocytic targeting, extending previous laboratory observations.