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Macrophage-derived factors increase low density lipoprotein uptake and receptor number in cultured human liver cells
R I Grove1, C Mazzucco, N Allegretto
1Oncogen, Bristol-Myers Squibb PRI, Seattle, WA 98121.
Abstract:
Recent evidence suggests the possibility that macrophages can influence lipoprotein metabolism. Therefore we investigated the ability of cultured macrophages to alter low density lipoprotein (LDL) uptake in a human liver cell line (HepG2). Conditioned media from phlogogenic-induced mouse peritoneal macrophages or from a human macrophage cell line stimulated with endotoxin increased HepG2 LDL uptake by as much as 60-70%. The increase was due, in part, to a significant macrophage-induced 40% increase in the number of LDL receptors per cell. Although macrophage conditioned media inhibited HepG2 cholesterol synthesis, the LDL receptor up-regulation did not appear to be due to the effects on cholesterol synthesis. The LDL receptor stimulatory activity was sensitive to proteolysis and heat. Its molecular mass was approximately 20 kDa based on gel filtration. Several macrophage secretory proteins were tested in HepG2 cultures for LDL uptake stimulation. Of these, oncostatin M (approximately 18 kDa by gel filtration) gave the strongest response. The rank order for LDL uptake stimulation was oncostatin M much greater than interleukin 6 = interleukin 1 = transforming growth factor-beta 1. A neutralizing antibody directed against oncostatin M inhibited the ability of conditioned media to up-regulate LDL receptors by 85%. Thus, our results indicate that macrophages can secrete several proteins that up-regulate LDL receptors in HepG2 cells and that most of the up-regulatory activity in macrophage conditioned media appears to be due to oncostatin M.
Insights
Macrophages secrete proteins, notably oncostatin M, that significantly increase low-density lipoprotein (LDL) uptake in liver cells by up-regulating LDL receptors. This finding impacts understanding of lipoprotein metabolism and macrophage function.
Area of Science:
- Cell Biology
- Immunology
- Metabolic Research
Background:
- Macrophages play a role in lipoprotein metabolism.
- Investigating macrophage influence on low-density lipoprotein (LDL) uptake is crucial for understanding lipid regulation.
Purpose of the Study:
- To determine if cultured macrophages can alter LDL uptake in a human liver cell line (HepG2).
- To identify the specific macrophage-secreted factors responsible for modulating LDL receptor expression and uptake.
Main Methods:
- Utilized conditioned media from activated mouse and human macrophages applied to HepG2 cells.
- Quantified changes in HepG2 LDL uptake and LDL receptor numbers.
- Characterized the molecular properties of the stimulatory activity and tested specific cytokines (oncostatin M, IL-6, IL-1, TGF-β1).
- Employed neutralizing antibodies against oncostatin M to assess its role.
Main Results:
- Macrophage conditioned media increased HepG2 LDL uptake by 60-70%.
- This increase was partly due to a 40% rise in LDL receptors per HepG2 cell.
- Oncostatin M demonstrated the strongest LDL uptake stimulation among tested cytokines.
- An anti-oncostatin M antibody blocked 85% of the conditioned media's LDL receptor up-regulatory activity.
Conclusions:
- Macrophages secrete proteins that up-regulate LDL receptors in liver cells.
- Oncostatin M is a primary mediator of this LDL receptor up-regulation by macrophages.
- Findings suggest a novel mechanism by which macrophages influence lipoprotein metabolism.