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Low density lipoprotein receptor activity in human monocyte-derived macrophages and its relation to atheromatous
Abstract:
Human peripheral monocytes, isolated from a previously unused source (a leukocyte concentrate byproduct of the plateletphoresis procedure for platelet transfusion), transformed into macrophages while cultured with 5% human serum or isolated lipoprotein fractions. Used for the study of their cholesterol (Chol) metabolism, these human monocyte-derived macrophages had a high-affinity receptor that is saturable, specific for low density lipoprotein (LDL) and maximally induced by incubation for as little as 24 hr in medium devoid of lipoproteins or Chol. Macrophages were shown to have separate receptor activity for 125I-labeled LDL and 125I-labeled acetylated LDL; macrophages that had been incubated with lipoprotein-depleted serum degraded native and acetylated LDL at similar rates. The receptor for LDL was functional in maintaining Chol homeostasis, as demonstrated by feedback inhibition of Chol synthesis after culture with LDL or very low density lipoprotein (VLDL). The Chol content of macrophages was doubled by incubation with medium containing VLDL compared to incubation with LDL (60 microgram of Chol per ml of medium). Incubation with native lipoproteins did not lead to accumulation of esterified Chol by macrophages. Changes in the Chol metabolism of macrophages, rather than modifications in circulating LDL, may cause these cells to store cholesteryl ester and take on the characteristics of cholesteryl ester-laden macrophages of atheromatous lesions.
Insights
Human macrophages efficiently process cholesterol from lipoproteins like LDL. Their metabolism, not circulating LDL, may drive cholesteryl ester accumulation, contributing to atheromatous lesions.
Area of Science:
- Biochemistry
- Cell Biology
- Immunology
Background:
- Human peripheral monocytes can be differentiated into macrophages.
- Macrophages play a role in cholesterol metabolism.
- Atheromatous lesions are characterized by cholesteryl ester-laden macrophages.
Purpose of the Study:
- To investigate cholesterol metabolism in human monocyte-derived macrophages.
- To characterize the low-density lipoprotein (LDL) receptor activity on macrophages.
- To explore the factors contributing to cholesteryl ester accumulation in macrophages.
Main Methods:
- Isolation of human peripheral monocytes from plateletpheresis byproduct.
- Culture of monocytes into macrophages with human serum or lipoprotein fractions.
- Assessment of cholesterol metabolism and lipoprotein uptake using radiolabeled LDL and acetylated LDL.
- Analysis of cholesterol synthesis feedback inhibition by LDL and very low-density lipoprotein (VLDL).
Main Results:
- Human monocyte-derived macrophages possess a saturable, high-affinity LDL receptor.
- LDL receptor activity is induced by lipoprotein-depleted medium.
- Macrophages exhibit distinct receptor activities for native LDL and acetylated LDL.
- Macrophage cholesterol synthesis is regulated by LDL and VLDL via feedback inhibition.
- VLDL significantly increases macrophage cholesterol content compared to LDL.
- Native lipoproteins do not cause cholesteryl ester accumulation in macrophages.
Conclusions:
- Macrophage cholesterol metabolism, specifically changes within the cell, is a key factor in cholesteryl ester accumulation.
- These cellular changes may lead to the characteristic cholesteryl ester-laden macrophages found in atheromatous lesions.
- The findings suggest a cellular mechanism for atheroma development distinct from circulating LDL modifications.