The J774 Macrophage Model of Native LDL-Induced Foam Cell Formation
The mechanism of cholesteryl ester (CE) accumulation in macrophage (Mp)-derived atherosclerotic foam cells is unknown. We have shown that mouse J774 Mp, unlike mouse peritoneal Mp (MPMp). accumulates large amounts of CE from unmodified low-density lipoprotein (LDL), a property related to sluggish down-regulation of the J774 LDL receptor and HMG-CoA reductase. We have further shown that down-regulation can be enhanced and CE accumulation diminished by ACAT inhibition of J774 Mp. We hypothesized that foam cell formation in J774 Mp might be related to diversion, perhaps by overactive ACAT, of intracellular cholesterol away from a regulatory pool. We now report a direct comparison of cholesterol metabolism and whole-cell ACAT activity in foam-cell-forming J774 Mp with that in non-foam-cell-forming MPMp. Our initial observation, necessary in allowing us to make this comparison, was that MPMp preincubated in LPDS-containing medium display seven-fold more chloroquine-inhibitable [125I]LDL degradation than freshly isolated MPMp and the same amount of [125I]LDL degradation as similarly treated J774 Mp. Despite this similar degree of LDL receptor activity, LPDS-preincubated MPMp accumulate only very small amounts of LDL-derived CE (15.6 μg/mg cell protein at 1 mg LDL/24 hr) compared to J774 Mp (130.5 μg/mg). When the two murine Mp were preincubated with LDL (100 μg/ml) and then assayed for [125I]LDL degradation. MPMp receptor activity was down-regulated by 72% in comparison to only 44% down-regulation of J774 Mp receptor activity. Furthermore, whereas preincubation of J774 Mp with LDL plus the ACAT inhibitor 58-035 led to a 1.8-fold enhancement of LDL-mediated receptor down-regulation (to a level of receptor activity similar to that in MPMp preincubated with LDL alone). ACAT-inhibited MPMp showed no enhancement of receptor down-regulation. Finally, we sought to determine if both the relatively diminished LDL-mediated receptor down-regulation and greater responsiveness to 58-035 in J774 Mp versus MPMp could be related to greater ACAT activity in the J774 Mp. The two murine Mp were preincubated in LPDS to induce equivalent LDL receptor activity and then tested for whole-cell ACAT activity in the absence or presence of LDL by monitoring the incorporation of [14C]oleate into cholesteryl [l4C]oleate. At all LDL concentrations tested (0-200 μg/ml), J774 ACAT activity was 20 to 30-fold greater than MPMp activity.
In summary: (1) LPDS-preincubated MPMp degrade similar amounts of [125I]LDL as J774 Mp; (2) J774 Mp accumulate 20-fold more CE than MPMp; (3) LDL-mediated receptor down-regulation in MPMp is almost two-fold greater than that in J774 Mp and, unlike regulation in J774 Mp, is unaffected by ACAT inhibition; and (4) J774 Mp display 20-to 30-fold greater whole-cell ACAT activity than MPMp.
Thus, despite similar amounts of LDL-cholesterol reaching the lysosomal compartments of the two murine MP, the fates of the intracellular cholesterol are markedly different, especially in regard to its esterification by ACAT. We suggest that the elevated esterification of cholesterol by ACAT in J774 Mp, whether by the ACAT enzyme itself or by increased substrate delivery, is a key characteristic responsible for LDL-induced foam cell formation in these cells.
KeywordsCholesteryl Ester Foam Cell Foam Cell Formation Mouse Peritoneal Macrophage J774 Cell
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- Hammond, M. E., Koo, C., Mahley, R. W., and Innerarity, T. L., 1985, Characterization of mouse peritoneal macrophage receptors for ß-VLDL, Arteriosclerosis 5:506a.Google Scholar
- Hashimoto, S., Drevon, C. A., Weinstein, D. B., Bernett, J. S., Dayton, S., and Steinberg, D., 1983, Activity of acyl-CoA : cholesterol acyltransferase and 3-hydroxy-3-methylglutaryl reductase in subfractions of hepatic microsomes enriched with cholesterol, Biochim. Biophys. Acta 754:126– 133.PubMedGoogle Scholar
- Tabas, I., Weiland, D. A., and Tall, A. R., 1986, Inhibitions of acyl coenzyme A : cholesterol ester transferase in J774 macrophages enhances down-regulation of the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl coenzyme A reductase and prevents low density lipoprotein-induced cholesterol accumulation, J. Biol. Chem. 261:3147–3155.PubMedGoogle Scholar
- Tabas, I., Boykow, G. C., Tall, A. R., 1986, Foam cell-forming J774 macrophages have markedly elevated LDL-induced acyl coenzyme A: cholesterol acyl transferase activity compared to mouse peritoneal macrophages despite similar LDL receptor activity, J. Clin. Invest. 79:418–426.CrossRefGoogle Scholar