Triacylglycerol-rich lipoproteins alter the secretion, and the cholesterol-effluxing function, of apolipoprotein E-containing lipoprotein particles from human (THP-1) macrophages

Interleukin-1beta and tumour necrosis factor-alpha impede neutral lipid turnover in macrophage-derived foam cells

Background

Pro-inflammatory cytokines can affect intracellular lipid metabolism. A variety of effects have been described for different cell types; hepatocyte lipid turnover pathways are inhibited during inflammation, whereas interleukin-1β (IL-1β) reduces intracellular cholesterol levels in fibroblasts. Levels of the pro-inflammatory cytokines IL-1β and tumour necrosis factor-α (TNF-α) are up-regulated at sites of formation of atherosclerotic plaques. Plaque formation is though to begin with infiltration of monocytes to the intimal layer of the vascular wall, followed by differentiation to macrophages and macrophage uptake of modified lipoproteins, resulting in accumulation of intracellular lipids. The lipid-filled cells are referred to as macrophage foam cells, a key feature of atherosclerotic plaques. We have investigated the effects of IL-1β and TNF-α on macrophage foam cells in order to assess whether presence of the pro-inflammatory cytokines improves or aggravates macrophage foam cell formation by affecting lipid accumulation and lipid turn-over in the cells.

Results

Differentiated primary human macrophages or THP-1 cells were lipid loaded by uptake of aggregated low density lipoproteins (AgLDL) or very low density lipoproteins (VLDL), and then incubated with IL-1β (0 – 5000 pg/ml) in lipoprotein-free media for 24 h. Cells incubated in absence of cytokine utilized accumulated neutral lipids, in particular triglycerides. Addition of exogenous IL-1β resulted in a dose-dependent retention of intracellular cholesterol and triglycerides. Exchanging IL-1β with TNF-α gave a similar response. Analysis of fatty acid efflux and intracellular fatty acid activation revealed a pattern of decreased lipid utilization in cytokine-stimulated cells.

Conclusion

IL-1β and TNF-α enhance macrophage foam cell formation, in part by inhibition of macrophage intracellular lipid catabolism. If present in vivo, these mechanisms will further augment the pro-atherogenic properties of the two cytokines.

Cellular functions of cholesterol probed with optical biosensors

Cholesterol is an essential constituent of cell membranes and the regulation of cholesterol concentration is critical for cell functions including signaling. In this paper, we applied resonant waveguide grating (RWG) biosensor to study the cellular functions of cholesterol through real time monitoring the dynamic mass redistribution (DMR) mediated by cholesterol depletion with methyl-beta-cyclodextrin (mbetaCD). In A431 cells, depletion of cholesterol by mbetaCD led to a DMR signature that was similar, but not identical to that induced by epidermal growth factor (EGF). To elucidate the cellular mechanisms of the DMR signal mediated by cholesterol depletion, a panel of modulators that specifically modulate the activities of various cellular targets were used to pretreat the cells. Results showed that the DMR signals triggered by cholesterol depletion are primarily linked to the transactivation of EGF receptor. Multiple signaling pathways including Ras/mitogenic activated protein (MAP) kinase, protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) acted synergically in the cell response, whereas the activation of protein kinase A (PKA) pathway was found to antagonize the cell response.

Triglyceride-rich lipoproteins inhibit cholesterol efflux to apolipoprotein (apo) A1 from human macrophage foam cells

High circulating levels of triglyceride-rich lipoproteins (TGRL) represent an independent risk factor for coronary artery disease. Here, we show that TGRL inhibit the efflux of cholesterol from 'foam cell' macrophages to lipid-poor apolipoprotein (apo) A1, and may thereby inhibit arterial reverse cholesterol transport and promote the formation of atherosclerotic lesions. Human (THP-1) monocyte-derived macrophages were pre-incubated (48 h) with acetylated low-density lipoprotein (AcLDL) to provide a foam cell model of cholesterol efflux to apoA1. Pre-incubation of macrophage 'foam cells' with TGRL (0-200 microg/ml, 0-24 h) inhibited the efflux of exogenously radiolabelled ([3H]), endogenously synthesised ([14C]) and cellular cholesterol mass to lipid-poor apoA1, but not control medium, during a (subsequent) efflux period. This inhibition is dependent upon the length of prior exposure to, and concentration of, TGRL employed, but is independent of changes in intracellular triglyceride accumulation or turnover of the cholesteryl ester pool. Despite the negative impact of TGRL on cholesterol efflux, major proteins involved in this process--namely apoE, ABCA1, SR-B1 and caveolin-1--were unaffected by TGRL pre-incubation, suggesting that exposure to these lipoproteins inhibits an alternate, and possibly novel, anti-atherogenic pathway.