Incorporation of lycopene into chylomicron remnant-like particles enhances their induction of lipid accumulation in macrophages

The oxidative state of chylomicron remnants influences their modulation of human monocyte activation

Chylomicron remnants (CMRs) contribute directly to human monocyte activation in vitro, by increasing reactive oxygen species (ROS) production and cell migration. In this study, the effects of the oxidative state of CMR on the degree of monocyte activation was investigated. CMR-like particles (CRLPs) were prepared in three different oxidative states, normal (CRLPs), protected from oxidation by incorporation of the antioxidant, probucol (pCRLPs), or oxidised with CuSO(4) (oxCRLPs). Lipid accumulation and ROS production were significantly increased in primary human monocytes incubated with CRLPs, whilst secretion on monocyte chemoattractant protein-1 was reduced, but oxCRLPs had no additional effect. In contrast, pCRLPs were taken up by monocytes to a lesser extent and had no significant effect on ROS or MCP-1 secretion. These studies suggest that the oxidative state of CMRs modulates their stimulation of the activation of peripheral blood human monocytes and that dietary antioxidants may provide some protection against these atherogenic effects.

Paraoxonase 2 (PON2) decreases high glucose-induced macrophage triglycerides (TG) accumulation, via inhibition of NADPH-oxidase and DGAT1 activity: studies in PON2-deficient mice

OBJECTIVE

The present study investigates the role of paraoxonase 2 (PON2) in the attenuation of macrophage triglycerides (TG) biosynthesis, and oxidative stress, under diabetic conditions.

METHODS

Peritoneal macrophages (MPM) from PON2-deficient and from C57BL/6 control mice were harvested and cultured under normal (5mM) or high glucose concentration (30mM), and evaluated for cellular TG metabolism as well as for their oxidative stress.

RESULTS

In PON2-deficient MPM vs. control MPM, under diabetic conditions (high glucose concentration), we observed substantial increment in TG accumulation (3 fold), TG biosynthesis (2.6 fold) and microsomal diacylglycerol acyltransferase1 (DGAT1) activity (+60%). Furthermore, in these cells we have demonstrated increased oxidative stress, as expressed by significant increment in cellular oxidative stress (+25%), macrophage-mediated LDL oxidation (+41%) and expression of the receptor for advanced glycation end products - RAGE (+18%). Apocynin, an NADPH-oxidase inhibitor, abolished the increment in MPM TG accumulation, MPM TG biosynthesis, and microsomal DGAT1 activity, as a result of PON2-deficiency, under diabetic conditions.

CONCLUSION

We conclude that PON2 has a significant protective role against macrophage triglyceride accumulation, macrophage TG biosynthesis, microsomal DGAT1 activity and macrophage oxidative stress, under high glucose concentrations. We suggest that this protective effect may be mediated by PON2 through the attenuation of NADPH-oxidase activity. The use of appropriate means to increase macrophage PON2 expression can lead to attenuation in macrophage TG accumulation and in cellular oxidative stress, under diabetic conditions, and thus may contribute to the decrement in macrophage atherogenicity and foam cell formation, attenuating the development of vascular complications in diabetes mellitus.

Paraoxonase 2 attenuates macrophage triglyceride accumulation via inhibition of diacylglycerol acyltransferase 1

This study questioned the role of paraoxonase 2 (PON2) in attenuation of macrophage lipids accumulation. Mouse peritoneal macrophages (MPMs) harvested from PON2-deficient mice versus control C57BL/6 mice, look like foam cells and were larger in size and filled with lipid droplets. Macrophage triglyceride (but not cholesterol) content, biosynthesis rate, and microsomal acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1) activity (not mRNA and protein) in PON2-deficient versus control MPM were all significantly increased by 4.6-, 3.6-, and 4.4-fold, respectively. Similarly, microsomal DGAT1 activity and cellular triglyceride content were significantly decreased in human PON2-transfected cells as well as upon incubation of PON2-deficient MPM with recombinant PON2. In all the above experimental systems, PON2 also decreased macrophage oxidative state. Incubation of PON2-deficient MPM with the free radicals generator 2,2'-amidinopropane hydrochloride increased cellular oxidative stress and DGAT1 activity by 2.2- and 3.4-fold, respectively, whereas incubation of microsomes from PON2-deficient MPM with superoxide dismutase decreased DGAT1 activity by 40%. We thus conclude that PON2 attenuates macrophage triglyceride accumulation and foam cell formation via inhibition of microsomal DGAT1 activity, which appears to be sensitive to oxidative state.

Effects of lycopene on the induction of foam cell formation by modified LDL

The effect of lycopene on macrophage foam cell formation induced by modified low-density lipoprotein (LDL) was studied. Human monocyte-derived macrophages (HMDM) were incubated with lycopene in the presence or absence of native LDL (nLDL) or LDL modified by oxidation (oxLDL), aggregation (aggLDL), or acetylation (acLDL). The cholesterol content, lipid synthesis, scavenger receptor activity, and the secretion of inflammatory [interleukin (IL)-1beta and tumor necrosis factor (TNF)-alpha] and anti-inflammatory (IL-10) cytokines was determined. Lycopene was found to decrease the synthesis of cholesterol ester in incubations without LDL or with oxLDL while triacylglycerol synthesis was reduced in the presence of oxLDL and aggLDL. Scavenger receptor activity as assessed by the uptake of acLDL was decreased by approximately 30% by lycopene. In addition, lycopene inhibited IL-10 secretion by up to 74% regardless of the presence of nLDL or aggLDL but did not affect IL-1beta or TNF-alpha release. Lycopene also reduced the relative abundance of mRNA transcripts for scavenger receptor A (SR-A) in THP-1 macrophages treated with aggLDL. These findings suggest that lycopene may reduce macrophage foam cell formation induced by modified LDL by decreasing lipid synthesis and downregulating the activity and expression of SR-A. However, these effects are accompanied by impaired secretion of the anti-inflammatory cytokine IL-10, suggesting that lycopene may also exert a concomitant proinflammatory effect.

Mechanisms involved in chylomicron remnant lipid uptake by macrophages

Although it is clear that chylomicron remnants are atherogenic, events leading to their internalization by macrophages are still debated. The lack of apoE (apolipoprotein E) in CRLPs (chylomicron remnant-like particles) reduces macrophage TAG (triacylglycerol) content by approx. 50%, suggesting that, as well as apoE-mediated endocytic uptake, apoE receptor-independent mechanisms are involved in the induction of foam cells by chylomicron remnants. Evaluation of the radioactivity associated with macrophages after incubation with CRLPs containing radiolabelled lipids suggests that the TAG and cholesterol carried by the particles have different kinetics of internalization. In addition, inhibition-based experiments indicate that cholesteryl ester-selective uptake and the extracellular lipoprotein lipase hydrolysis of TAG contribute to cholesterol and TAG accumulation respectively. Thus plasma TAG and cholesterol carried by remnant particles have to be considered two independent and non-interchangeable risk factors for athero-related diseases. In addition, the interaction between CRLPs and macrophages is modulated by dietary oxidized lipids and other lipophilic components. The presence of oxidized lipids, such as 7β-hydroxycholesterol and 7-oxocholesterol, the major cholesterol oxidation products found in atherosclerotic lesions, in CRLPs interferes with the mechanisms of their internalization, but does not cause quantitative changes of accumulated lipids, while the presence of the plant carotenoid, lycopene, or the antioxidant drug, probucol, enhances lipid accumulation in macrophages by increasing the rate of uptake of the particles and raising the intracellular synthesis of TAG. In conclusion, several mechanisms contribute to the macrophage uptake of postprandial lipoproteins, however, little is known of the balance and modulation between the different pathways.

The induction of macrophage foam cell formation by chylomicron remnants

The accumulation of foam cells in the artery wall causes fatty streaks, the first lesions in atherosclerosis. LDL (low-density lipoprotein) plays a major role in foam cell formation, although prior oxidation of the particles is required. Recent studies, however, have provided considerable evidence to indicate that CMRs (chylomicron remnants), which carry dietary lipids in the blood, induce foam cell formation without oxidation. We have shown that CMRs are taken up by macrophages and induce accumulation of both triacylglycerol and cholesterol, and that the rate of uptake and amount of lipid accumulated is influenced by the type of dietary fat in the particles. Furthermore, oxidation of CMRs, in striking contrast with LDL, inhibits, rather than enhances, their uptake and induction of lipid accumulation. In addition, the lipid accumulated after exposure of macrophages to CMRs is resistant to efflux, and this may be due to its sequestration in lysosomes. These findings demonstrate that CMRs induce pro-atherogenic changes in macrophages, and that their effects may be modulated by dietary factors including oxidized fats, lipophilic antioxidants and the type of fat present.

Oxidation of chylomicron remnant-like particles inhibits their uptake by THP-1 macrophages by apolipoprotein E-dependent processes

The influence of the oxidative state of chylomicron remnants (CMR) on the mechanisms of their uptake and induction of lipid accumulation by macrophages derived from the human monocyte cell line, THP-1, during foam cell formation was investigated using chylomicron-remnant-like particles (CRLPs) at 3 different levels of oxidation. The oxidative state of CRLPs was varied by exposure to CuSO₄ (oxCRLPs) or incorporation of the antioxidant, probucol (pCRLPs) into the particles. oxCRLPs caused significantly less accumulation of triacylglycerol in the macrophages than CRLPs, and their rate of uptake was lower, while pCRLPs caused more lipid accumulation and were taken up faster. Uptake of all 3 types of particles was inhibited to a similar extent when entry via the low density lipoprotein (LDL) receptor related protein (80–90%), LDL receptor (− 30–40%), CD36 (− 40%) and phagocytosis (− 35–40%) was blocked using lactoferrin, excess LDL, anti-CD36 and cytochalasin D, respectively, but blocking scavenger receptors-A or -B1 using poly inosinic acid or excess HDL had no effect. These findings show that oxidation of CRLPs lowers their rate of uptake and induction of lipid accumulation in macrophages. However, oxidation does not change the main pathways of internalisation of CRLPs into THP-1 macrophages, which occur mainly via the LRP with some contribution from the LDLr, while CD36 and phagocytosis have only a minor role, regardless of the oxidative state of the particles. Thus, the effects of CMR oxidation on foam cell formation contrast sharply with those of LDL oxidation and this may be important in the role of dietary oxidized lipids and antioxidants in modulating atherosclerosis.

Incorporation of lycopene into chylomicron remnant-like particles inhibits their uptake by HepG2 cells

The influence of the incorporation of the antioxidant tomato pigment, lycopene, into chylomicron remnant-like particles (CRLPs) on their uptake by the liver cells was investigated. CRLPs or CRLPs containing lycopene (lycCRLPs) radiolabelled with [(3)H]triacylglycerol were incubated with cells of the human liver hepatoma cell line, HepG2, and the radioactivity taken up by the cells was determined. LycCRLPs were taken up significantly more slowly than CRLPs over a concentration range of 5-60 microg cholesterol/ml and a time course of 2-6 h. Pre-incubation of the hepatocytes with an excess of low density lipoprotein (LDL) inhibited the uptake of CRLPs by about 50%, but had no effect on the uptake of lycCRLPs, and under these conditions the CRLPs and lycCRLPs were taken up at similar rates. In HepG2 cells pre-treated with suramin, which inhibits uptake via the LDL receptor-related protein (LRP), the uptake of CRLPs was also inhibited (-37%) to a greater extent than that of lycCRLPs (-24%), so that the values for the two types of particle were no longer significantly different. Heparinase increased the uptake of lycCRLPs (about 2 fold), but not CRLPs, bringing it to a level equivalent to that seen with the control particles. These findings demonstrate that the incorporation of lycopene into CRLPs decreases their uptake by HepG2 cells and suggest that this effect is due to differential interaction with the LDL receptor and the LRP-receptor-mediated pathways, and may also involve binding of the particles to HSPG.

Fatty acid composition of chylomicron remnant-like particles influences their uptake and induction of lipid accumulation in macrophages

The influence of the fatty acid composition of chylomicron remnant-like particles (CRLPs) on their uptake and induction of lipid accumulation in macrophages was studied. CRLPs containing triacylglycerol enriched in saturated, monounsaturated, n-6 or n-3 polyunsaturated fatty acids derived from palm, olive, corn or fish oil, respectively, and macrophages derived from the human monocyte cell line THP-1 were used. Lipid accumulation (triacylglycerol and cholesterol) in the cells was measured after incubation with CRLPs for 5, 24 and 48 h, and uptake over 24 h was determined using CRLPs radiolabelled with [3H]triolein. Total lipid accumulation in the macrophages was significantly greater with palm CRLPs than with the other three types of particle. This was mainly due to increased triacylglycerol concentrations, whereas changes in cholesterol concentrations did not reach significance. There were no significant differences in lipid accumulation after incubation with olive, corn or fish CRLPs. Palm and olive CRLPs were taken up by the cells at a similar rate, which was considerably faster than that observed with corn and fish CRLPs. These findings demonstrate that CRLPs enriched in saturated or monounsaturated fatty acids are taken up more rapidly by macrophages than those enriched in n-6 or n-3 polyunsaturated fatty acids, and that the faster uptake rate results in greater lipid accumulation in the case of saturated fatty acid-rich particles, but not monounsaturated fatty acid-rich particles. Thus, dietary saturated fatty acids carried in chylomicron remnants may enhance their propensity to induce macrophage foam cell formation.

Dietary lutein and fat interact to modify macrophage properties in chicks hatched from carotenoid deplete or replete eggs

Two experiments were conducted to study the interaction between dietary lutein and fat levels in broiler chicks hatched from lutein depleted (Experiment I) and repleted (Experiment II) eggs. In both experiments, a 2 x 3 factorial arrangement of treatments resulted in six dietary treatments (fat at 3% and 6% and lutein at 0, 25 and 50 mg/kg feed) that were fed for 23 days to 18 birds per treatment (in three replications). In Experiment I, the anti-dinitrophenyl-keyhole-lympet-hemocyanin (anti-DNP-KLM) serum antibody response at day 22 and macrophage phagocytotic index at day 17 did not differ among treatment groups (p > 0.05). The concavalin A and phytohaemagglutinin-P lymphocyte proliferation index at day 19 was greater in birds fed 50 mg of lutein and 3% fat than in birds fed all other diets (p < 0.05). Independent of the level of dietary fat, dietary lutein increased macrophage (day 23) nitrite production measured 46 h after in vitro stimulation with LPS (p < 0.05). Among the birds fed lutein at 25 or 50 mg/kg feed, birds fed 3% fat had higher LPS-induced nitrite production compared to the birds fed 6% fat after 46 (p = 0.014) or 70 h (p < 0.001). In Experiment II, macrophage nitrite production was measured at 54 h after LPS stimulation on days 11, 15, 19 and 23. An interaction between dietary lutein and fat levels on nitrite production was observed on day 19 (p = 0.012), where macrophages from birds fed 0 mg lutein and 3% fat had the highest nitrite production (p = 0.012). Macrophages from birds fed lutein at 25 and 50 mg/kg diet and 3% fat had higher (p = 0.012) nitrite production than birds fed 6% fat. Thus, in birds hatched from lutein deplete and replete eggs, modulation of macrophage nitrite production by lutein is dependent on the level of dietary fat.

Protection of chylomicron remnants from oxidation by incorporation of probucol into the particles enhances their uptake by human macrophages and increases lipid accumulation in the cells

The effects of protection of chylomicron remnants from oxidation on their uptake and induction of lipid accumulation in macrophages were investigated using chylomicron remnant-like particles (CRLPs) containing the lipophilic antioxidant drug, probucol, and macrophages derived from the human monocyte cell line, THP-1. The total lipid content of THP-1 macrophages was markedly higher (x2.2) after 48 h of incubation of THP-1 macrophages with CRLPs containing probucol (pCRLPs) when compared to CRLPs without probucol, and this was because of increases in triacylglycerol (x2.3) and cholesterol (x1.8) levels, while cholesteryl ester concentrations were not significantly changed. Determination of the uptake of CRLPs and pCRLPs by the cells using particles labelled with the fluorescent probe 1,1'-dioctadecyl-3,3,3'3'-tetramethylindo-carbocyanine perchlorate showed that pCRLPs are taken up at a faster rate than CRLPs. The synthesis of triacylglycerol, as measured by the incorporation of [(3)H]oleate and [(3)H]glycerol, was also increased in macrophages incubated with pCRLPs as compared to CRLPs without probucol, but phospholipid and cholesteryl ester formation from [(3)H]oleate was unaffected. In addition, no differences between the effects of CRLPs and pCRLPs on the expression of mRNA for a range of genes believed to be involved in lipoprotein uptake, intracellular lipid metabolism and the efflux of cholesterol from macrophages was detected. These results suggest that antioxidants carried in chylomicron remnants enhance lipid accumulation in macrophages by increasing the rate of uptake of the particles and raising the intracellular synthesis of triacylglycerol, but not cholesteryl ester, and that these effects are brought about by changes at the post-transcriptional level. Antioxidants carried in chylomicron remnants therefore may promote the development of atherosclerosis, and this is likely to be particularly important in conditions where clearance of remnants from the circulation is delayed.