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Inhibitory effects of HepG2 cell-derived apolipoprotein A-I-containing lipoproteins on cholesteryl ester accumulation
1Department of Biochemistry, Kumamoto University School of Medicine, Japan.
Abstract:
We investigated the mechanisms of inhibitory effects on foam cell formation of apolipoprotein A-I-containing lipoproteins secreted by HepG2 cells (HepG2-HDL) using mouse peritoneal macrophages. When macrophages were incubated with acetylated low-density lipoprotein (acetyl-LDL) in the presence of HepG2-HDL, cholesterol ester (CE) accumulation in cells was reduced by 63%. This inhibitory capacity was almost similar to that of plasma high-density lipoprotein (HDL). When macrophages were converted to foam cells with acetyl-LDL and then reacted with HepG2-HDL or plasma HDL, the HDL-induced CE reduction was 2.2-fold greater than HepG2-HDL. Similar results were obtained using apo E-free HepG2-HDL. Since the inhibitory effect of HDL on acetyl-LDL-induced CE accumulation in macrophages is due largely to its cholesterol efflux capacity, these results suggest the presence of an additional mechanism for the inhibition of CE accumulation by HepG2-HDL. To investigate the mechanism, acetyl-LDL was reisolated from HepG2-HDL by Sephacryl S-300 gel filtration after incubation in a cell-free system. Reisolated acetyl-LDL showed a significant reduction in electrophoretic mobility. The extent of CE accumulation by reisolated acetyl-LDL was reduced by 20% compared with control acetyl-LDL. Moreover, its endocytic degradation by macrophages was reduced by 28%. HepG2-HDL also inhibited macrophage degradation of acetyl-LDL as well as oxidized LDL, a likely atherogenic lipoprotein. This inhibitory effect was ascribed to the HepG2-HDL subfraction containing pre-beta HDL. Our results indicated that apo A-I-containing lipoproteins as a physiological model of nascent HDL may inhibit foam cell formation by reducing ligand activity of atherogenic lipoproteins. These data possibly suggest inhibitory function of nascent HDL for the formation of foam cells in vivo.
Insights
HepG2-HDL, a model for nascent high-density lipoprotein (HDL), inhibits foam cell formation by altering atherogenic lipoproteins, suggesting a novel mechanism beyond cholesterol efflux.
Area of Science:
- Biochemistry
- Cell Biology
- Cardiovascular Research
Background:
- Foam cell formation is a key process in atherosclerosis.
- High-density lipoprotein (HDL) plays a role in cholesterol homeostasis and possesses anti-atherogenic properties.
- HepG2 cell-secreted apolipoprotein A-I-containing lipoproteins (HepG2-HDL) serve as a model for nascent HDL.
Purpose of the Study:
- To investigate the inhibitory mechanisms of HepG2-HDL on foam cell formation.
- To elucidate how HepG2-HDL affects the processing of atherogenic lipoproteins by macrophages.
Main Methods:
- Incubation of mouse peritoneal macrophages with acetylated low-density lipoprotein (acetyl-LDL) and HepG2-HDL.
- Measurement of intracellular cholesterol ester (CE) accumulation.
- Analysis of acetyl-LDL reisolated after incubation with HepG2-HDL using gel filtration and macrophage assays.
Main Results:
- HepG2-HDL significantly reduced CE accumulation in macrophages treated with acetyl-LDL, comparable to plasma HDL.
- HepG2-HDL altered the properties of acetyl-LDL, reducing its CE accumulation and enhancing its degradation by macrophages.
- A subfraction of HepG2-HDL containing pre-beta HDL was identified as responsible for inhibiting the degradation of atherogenic lipoproteins.
Conclusions:
- Apo A-I-containing lipoproteins, like HepG2-HDL, inhibit foam cell formation through mechanisms beyond cholesterol efflux, including modulating atherogenic lipoprotein activity.
- Nascent HDL may possess an inhibitory function against foam cell formation in vivo by reducing the ligand activity of atherogenic lipoproteins.
- These findings suggest a novel role for nascent HDL in preventing atherosclerosis development.