Cholesterol metabolism in monocyte-derived macrophages from macrophage colony-stimulating factor administered rabbits

Acrolein-conjugated low-density lipoprotein induces macrophage foam cell formation

OBJECTIVE

Acrolein-conjugated lysine residues in proteins are present in human atherosclerotic lesions, and are detected in human low-density lipoprotein (LDL). These findings suggest that acrolein may contribute to macrophage foam cell formation and atherogenesis through modification of LDL. The purpose of this study is to determine whether acrolein-conjugated LDL (Acro-LDL) induces macrophage conversion to form foam cells.

METHODS

Acro-LDL was prepared by incubation of LDL with acrolein. Characteristics of Acro-LDL were examined by agarose gel electrophoresis and western blotting. Cholesterol contents of THP-1 macrophages incubated with Acro-LDL were determined by enzymatic method. Pathway of Acro-LDL uptake by THP-1 macrophages was determined using neutralizing antibody against scavenger receptors. Delivery of Acro-LDL into lysosome and formation of lipid droplet by incubation with Acro-LDL were demonstrated by confocal microscopy.

RESULTS

The mobility of Acro-LDL determined by agarose gel electrophoresis was increased by modification with acrolein, and the shift of mobility was dependent on the concentration of acrolein. Acrolein interacted with apolipoprotein B in LDL and Acro-LDL uptake by THP-1 macrophage was a more effective inducer of cholesterol accumulation than oxidized LDL uptake. Acro-LDL uptake was mediated by scavenger receptor class A type 1 (SR-A1), but not by CD36. As a result of Acro-LDL uptake, cholesterol ester accumulated in lipid droplets of macrophages, converting them to foam cells.

CONCLUSIONS

The results show that Acro-LDL uptake via SR-A1 receptors can mediate macrophage foam cell formation.

Marked induction of sterol 27-hydroxylase activity and mRNA levels during differentiation of human cultured monocytes into macrophages

Sterol 27-hydroxylase has been suggested to be involved in an alternative pathway for the elimination of cholesterol from macrophages and early atherosclerotic lesions. We have previously shown that human lung macrophages as well as monocyte-derived macrophages have a relatively high activity of sterol 27-hydroxylase (CYP27). This enzyme converts intracellular cholesterol into 27-hydroxycholesterol and cholestenoic acid that flux from cultured cells into the medium. It is shown here that human monocytes have very low CYP27 activity and CYP27 mRNA levels. During differentiation into macrophages, both CYP27 activity and CYP27 mRNA levels increase markedly after 4 days of culture in serum-free medium. Addition of macrophage-colony stimulating factor had no significant effect on the induction and addition of fetal calf serum had an inhibitory effect. Cholesterol synthesis was found to be a critical factor for the production of 27-oxygenated products by the macrophages cultured in serum-free medium. The increased capacity of the differentiated cells to eliminate intracellular cholesterol is of interest and supports the contention that CYP27 is an antiatherogenic factor.

Atherosclerotic plaques composed of a large core of foam cells covered with thin fibrous caps in twice-injured carotid arterial specimens obtained from high cholesterol diet-fed rabbits

We attempted to find atherosclerotic plaques including a large lipid core and thin fibrous cap in twice-injured arterial specimens obtained from high cholesterol diet (HCD) fed rabbits. Rabbits fed a HCD were subjected to carotid artery injury using a balloon catheter. After 2 or 4 weeks of cholesterol feeding, a second mild injury was induced in the same region as the first injury. The rabbits were given a standard diet for 2 weeks after the second injury. Typical atherosclerotic plaques with a fibrous cap formed by smooth muscle cells and extracellular matrix overlying a core formed by macrophage foam cells were observed in the lesion. Gelatin proteolytic activities were found in homogenates containing either media or intima from the injured artery, and activated matrix metalloproteinase-2 (MMP2) was detected. With prolongation of the HCD feeding period (interval between injuries) from 2 weeks to 4 weeks, typical plaque was observed more frequently. Furthermore, the neointimal area and the macrophage foam cells area increased, as did gelatin-proteolytic activity. Since the typical atherosclerotic plaques observed in the present study have some histopathological and pathogenic characteristics in common with unstable atherosclerotic plaque, we expect that the typical atherosclerotic plaque found in the present study will be useful for basic studies of plaque stabilization and prevention of acute coronary syndromes.

Ectopic expression of alpha1,6 fucosyltransferase in mice causes steatosis in the liver and kidney accompanied by a modification of lysosomal acid lipase

The alpha1,6 fucosyltransferase (alpha1,6 FucT) catalyzes the transfer of a fucose from GDP-fucose to the innermost GlcNAc residue of N-linked glycans via an alpha1,6 linkage. alpha1,6 FucT was overexpressed in transgenic mice under the control of a combined cytomegalovirus and chicken beta-actin promoter. Histologically numerous small vacuoles, in which lipid droplets had accumulated, were observed in hepatocytes and proximal renal tubular cells. Electron microscopic studies showed that the lipid droplets were membrane-bound and apparently localized within the lysosomes. Cholesterol esters and triglycerides were significantly increased in liver and kidney of the transgenic mice. Liver lysosomal acid lipase (LAL) activity was significantly lower in the transgenic mice compared to the wild mice, whereas LAL protein level, which was detected immunochemically, was increased, indicating that the specific activity of LAL was much lower in the transgenic mice. In all of the transgenic and nontransgenic mice examined, the activity of liver LAL was negatively correlated with the level of alpha1,6 FucT activity. As evidenced by lectin and immunoblot analysis, LAL was found to be more fucosylated in the transgenic mice, suggesting that the aberrant fucosylation of LAL causes an accumulation of inactive LAL in the lysosomes. Such an accumulation of inactive LAL could be a likely cause for a steatosis in the lysosomes of the liver and kidney in the case of the alpha1,6 FucT transgenic mice.

Effects of CSF-1 on cholesterol accumulation and efflux by macrophages

To assess whether human monocyte-specific colony-stimulating factor (CSF-1) might influence atherogenesis, CSF-1-induced macrophage responses that might contribute to enhanced clearance of low-density lipoprotein (LDL) or modified LDL were investigated. Careful account was made of cell preservation and increases in cell volume and protein (representing increased cell surface area, and thus endocytically active membrane) during culture with CSF-1. This permitted distinction between selective and nonspecific effects of CSF-1, the latter paralleling increases in cellular mass and volume. CSF-1 enhanced mouse peritoneal macrophage survival in vitro during exposure to lipoprotein-deficient serum with or without native LDL or acetylated LDL (Ac-LDL), as judged by maintenance of cellular DNA and cell numbers. In the presence of copper-oxidized LDL (Ox-LDL), such effects were very slight. In all conditions, CSF-1 increased cellular protein content. CSF-1 increased the uptake of both Ac-LDL and Ox-LDL calculated per culture, but this was entirely explicable by the increased cell protein, indicating that there was no selective enhancement of scavenger receptor or other routes for uptake of the modified LDLs. Similarly, CSF-1 also increased the accumulation of cholesterol and its esters nonspecifically. CSF-1 did have a marked and specific effect on the composition of cholesterol esters, decreasing the proportion of polyunsaturated esters relative to monounsaturated and saturated esters. Finally, cholesterol efflux induced by apolipoprotein A1 from Ac-LDL-loaded macrophages was not influenced by CSF-1. Thus, the enhanced macrophage catabolism of modified LDLs by CSF-1 is part of a nonspecific action on the cells but could contribute to a reduction in circulating cholesterol, observed in some situations of CSF-1 presentation in humans.

Genetic differences of lipid metabolism in macrophages from C57BL/6J and C3H/HeN mice

Cholesterol metabolism in macrophages from atherosclerosis-prone C57BL/6J mice was compared with that in macrophages from atherosclerosis-resistant C3H/HeN mice. Plasma total cholesterol levels of both types of mice were significantly increased, but HDL cholesterol level was increased only in C3H/HeN mice when a high-cholesterol diet (1% cholesterol) was fed for 5 weeks. After incubation of macrophages from male and female mice on the high-cholesterol diet with beta-VLDL for 24 hours, cholesterol content in macrophages from C57BL/6J was approximately 1.5- to 2.0-fold higher than in those from C3H/HeN mice. [3H]Cholesterol oleate-beta-VLDL incorporation into macrophages from C57BL/6J mice on the high-cholesterol diet was greater than incorporation into those from C3H/HeN mice. The release of [3H]cholesterol from macrophages from C57BL/6J mice on the high-cholesterol diet was one seventh that from macrophages from C57BL/6J mice on the basal diet or that from macrophages from C3H/HeN mice on the basal or high-cholesterol diet. Acid cholesterol esterase activity was almost the same in macrophages from any group. Acyl CoA:cholesterol acyltransferase activity in macrophages from C57BL/6J mice on the high-cholesterol diet increased compared with that from macrophages from C57BL/6J mice on the normal diet. Neutral cholesterol esterase activity in macrophages from C57BL/6J mice was about half of that in macrophages from C3H/HeN mice independent of the type of diet. There were no sex differences in these metabolisms. Considered with our previous data, these results suggested that a high-cholesterol diet may cause metabolic changes to accumulate cholesterol ester in macrophages from C57BL/6J mice in accordance with genetic abnormalities.