Very low density lipoproteins promote triglyceride accumulation in macrophages

The vitamin E long-chain metabolite α-13'-COOH affects macrophage foam cell formation via modulation of the lipoprotein lipase system

The α-tocopherol-derived long-chain metabolite (α-LCM) α-13'-carboxychromanol (α-13'-COOH) is formed via enzymatic degradation of α-tocopherol (α-TOH) in the liver. In the last decade, α-13'-COOH has emerged as a new regulatory metabolite revealing more potent or even different effects compared with its vitamin precursor α-TOH. The detection of α-13'-COOH in human serum has further strengthened the concept of its physiological relevance as a potential regulatory molecule. Here, we present a new facet on the interaction of α-13'-COOH with macrophage foam cell formation. We found that α-13'-COOH (5 μM) increases angiopoietin-like 4 (ANGPTL4) mRNA expression in human THP-1 macrophages in a time- and dose-dependent manner, while α-TOH (100 μM) showed no effects. Interestingly, the mRNA level of lipoprotein lipase (LPL) was not influenced by α-13'-COOH, but α-TOH treatment led to a reduction of LPL mRNA expression. Both compounds also revealed different effects on protein level: while α-13'-COOH reduced the secreted amount of LPL protein via induction of ANGPTL4 cleavage, i.e. activation, the secreted amount of LPL in the α-TOH-treated samples was diminished due to the inhibition of mRNA expression. In line with this, both compounds reduced the catalytic activity of LPL. However, α-13'-COOH but not α-TOH attenuated VLDL-induced lipid accumulation by 35%. In conclusion, only α-13'-COOH revealed possible antiatherogenic effects due to the reduction of VLDL-induced foam cell formation in THP-1 macrophages. Our results provide further evidence for the role of α-13'-COOH as a functional metabolite of its vitamin E precursor.

Non-high density lipoprotein cholesterol versus low density lipoprotein cholesterol as a discriminating factor for myocardial infarction

Background

Serum total cholesterol (TC) and LDL cholesterol (LDL-C) have been used as major laboratory measures in clinical practice to assess cardiovascular risk in the general population and disease management as well as prognosis in patients. However, some studies have also reported the use of non-HDL cholesterol (non-HDL-C). As non-HDL-C can be calculated by subtracting HDL-C from TC, both of which do not require fasting blood sample in contrast to LDL-C which requires fasting blood sample, we aimed to compare non-HDL-C with LDL-C as a predictor of myocardial infarction (MI).

Methods

This hospital based cross sectional study was undertaken among 51 cases of MI and equal number of controls. MI was diagnosed based on the clinical history, ECG changes and biochemical parameters. 5 mL of fasting blood sample was collected from each research participant for the analysis of lipid profile. Non-HDL-C was calculated by using the equation; Non-HDL-C = TC – HDL-C. Statistical analysis was performed using SPSS 14.0.

Results

42 MI cases were dyslipidemic in contrast to 20 dyslipidemic subjects under control group. The differences in the median values of each lipid parameter were statistically significant between MI cases and controls. The lipid risk factors most strongly associated with MI were HDL-C (OR 5.85, 95% CI 2.41-14.23, P value = 0.000) followed by non-HDL-C (OR 3.77, 95% CI 1.64-8.66, P value = 0.002), LDL-C/HDL-C (OR 3.38, 95% CI 1.44-7.89, P value = 0.005), TC/HDL-C (OR 2.93, 95% CI 1.36-7.56, P value = 0.026), LDL-C (OR 2.70, 95% CI 1.20-6.10, P value = 0.017), TC (OR 2.68, 95% CI 1.04-6.97, P value = 0.042) and Tg (OR 2.54, 95% CI 1.01-6.39, P value = 0.047). Area under the receiver operating curve was greater for non-HDL-C than for LDL-C. Non-HDL-C was also found to be more sensitive and specific than LDL-C for MI.

Conclusions

HDL-C and non-HDL-C are better discriminating parameters than LDL-C for MI. Thus, we can simply perform test for HDL-C and non-HDL-C both of which do not require fasting blood sample rather than waiting for fasting blood sample to measure LDL-C.

Macrophage adipose triglyceride lipase deficiency attenuates atherosclerotic lesion development in low-density lipoprotein receptor knockout mice

OBJECTIVE

The consequences of macrophage triglyceride (TG) accumulation on atherosclerosis have not been studied in detail so far. Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme for the initial step in TG hydrolysis. Because ATGL knockout (KO) mice exhibit massive TG accumulation in macrophages, we used ATGL KO mice to study the effects of macrophage TG accumulation on atherogenesis.

METHODS AND RESULTS

Low-density lipoprotein receptor (LDLr) KO mice were transplanted with bone marrow from ATGL KO (ATGL KO→LDLr KO) or wild-type (WT→LDLr KO) mice and challenged with a Western-type diet for 9 weeks. Despite TG accumulation in ATGL KO macrophages, atherosclerosis in ATGL KO→LDLr KO mice was 43% reduced associated with decreased plasma monocyte chemoattractant protein-1 (MCP-1) and macrophage interleukin-6 concentrations. This coincided with a reduced amount of macrophages, possibly because of a 39% increase in intraplaque apoptosis and a decreased migratory capacity of ATGL KO macrophages. The reduced number of white blood cells might be due to a 36% decreased Lin(-)Sca-1(+)cKit(+) hematopoietic stem cell population.

CONCLUSIONS

We conclude that the attenuation of atherogenesis in ATGL KO→LDLr KO mice is due to decreased infiltration of less inflammatory macrophages into the arterial wall and increased macrophage apoptosis.

Efficient phagocytosis requires triacylglycerol hydrolysis by adipose triglyceride lipase

Macrophage phagocytosis is an essential biological process in host defense and requires large amounts of energy. To date, glucose is believed to represent the prime substrate for ATP production in macrophages. To investigate the relative contribution of free fatty acids (FFAs) in this process, we determined the phagocytosis rates in normal mouse macrophages and macrophages of adipose triglyceride lipase (ATGL)-deficient mice. ATGL was shown to be the rate-limiting enzyme for the hydrolysis of lipid droplet-associated triacylglycerol (TG) in many tissues. Here, we demonstrate that Atgl(-/-) macrophages fail to efficiently hydrolyze cellular TG stores leading to decreased cellular FFA concentrations and concomitant accumulation of lipid droplets, even in the absence of exogenous lipid loading. The reduced availability of FFAs results in decreased cellular ATP concentrations and impaired phagocytosis suggesting that fatty acids must first go through a cycle of esterification and re-hydrolysis before they are available as energy substrate. Exogenously added glucose cannot fully compensate for the phagocytotic defect in Atgl(-/-) macrophages. Hence, phagocytosis was also decreased in vivo when Atgl(-/-) mice were challenged with bacterial particles. These findings imply that phagocytosis in macrophages depends on the availability of FFAs and that ATGL is required for their hydrolytic release from cellular TG stores. This novel mechanism links ATGL-mediated lipolysis to macrophage function in host defense and opens the way to explore possible roles of ATGL in immune response, inflammation, and atherosclerosis.

Inhibition of long-chain acyl coenzyme A synthetases during fatty acid loading induces lipotoxicity in macrophages

OBJECTIVE

Obesity is often associated with hypertriglyceridemia and elevated free fatty acids (FFAs), which are independent risk factors for cardiovascular disease and diabetes. Although impairment of cholesterol homeostasis is known to induce toxicity in macrophages, the consequence of altered fatty acid homeostasis is not clear.

METHODS AND RESULTS

Long-chain acyl CoA synthetases (ACSLs) play a critical role in fatty acid homeostasis by channeling fatty acids to diverse metabolic pools. We treated mouse peritoneal macrophages (MPMs) with VLDL or FFAs in the presence of triacsin C, an inhibitor of the 3 ACSL isoforms present in macrophages. Treatment of macrophages with VLDL and triacsin C resulted in reduced TG accumulation but increased intracellular FFA levels, which induced lipotoxicity characterized by apoptosis. Treatment of MPMs with the saturated fatty acid stearic acid in the presence of triacsin C increased intracellular stearic acid and induced apoptosis. Stromal vascular cells collected from high-fat diet-fed mice displayed foam cell morphology and exhibited increased mRNA levels of macrophage markers and ACSL1. Importantly, all of these changes were associated with increased FFA level in AT.

CONCLUSIONS

Inhibition of ACSLs during fatty acid loading results in apoptosis via accumulation of FFAs. Our data have implications in understanding the consequences of dysregulated fatty acid metabolism in macrophages.

Triglyceride alters lysosomal cholesterol ester metabolism in cholesteryl ester-laden macrophage foam cells

In late-stage atherosclerosis, much of the cholesterol in macrophage foam cells resides within enlarged lysosomes. Similarly, human macrophages incubated in vitro with modified LDLs contain significant amounts of lysosomal free cholesterol and cholesteryl ester (CE), which disrupts lysosomal function similar to macrophages in atherosclerotic lesions. The lysosomal cholesterol cannot be removed, even in the presence of strong efflux promoters. Thus, efflux of sterol is prevented. In the artery wall, foam cells interact with triglyceride-rich particles (TRPs) in addition to modified LDLs. Little is known about how TRP metabolism affects macrophage cholesterol. Therefore, we explored the effect of TRP on intracellular CE metabolism. Triglyceride (TG), delivered to lysosomes in TRP, reduced CE accumulation by 50%. Increased TG levels within the cell, particularly within lysosomes, correlated with reductions in CE content. The volume of cholesterol-engorged lysosomes decreased after TRP treatment, indicating cholesterol was cleared. Lysosomal TG also reduced the cholesterol-induced inhibition of lysosomal acidification allowing lysosomes to remain active. Enhanced degradation and clearance of CE may be explained by movement of cholesterol out of the lysosome to sites where it is effluxed. Thus, our results show that introduction of TG into CE-laden foam cells influences CE metabolism and, potentially, atherogenesis.-Ullery-Ricewick, J. C., B. E. Cox, E. E. Griffin, and W. G. Jerome. Triglyceride alters lysosomal cholesterol ester metabolism in cholesteryl ester-laden macrophage foam cells.

The role of lipolysis in mediating the proinflammatory effects of very low density lipoproteins in mouse peritoneal macrophages

Hypertriglyceridemia is an important risk factor for atherosclerosis, especially in obesity. Macrophages are one of the primary cell types involved in atherogenesis and are thought to contribute to lesion formation through both lipid accumulation and proinflammatory gene expression. In this study, we sought to determine the direct impact of triglyceride (TG)-rich VLDL-induced lipid accumulation on macrophage proinflammatory processes. Incubation of mouse peritoneal macrophages with 100 microg/ml VLDL for 6 h led to 2.8- and 3.7-fold increases in intracellular TGs and FFAs, respectively (P < 0.05). The inflammatory proteins tumor necrosis factor-alpha, interleukin-1beta, monocyte chemoattractant protein-1, intercellular adhesion molecule-1, matrix metalloproteinase 3 (MMP3), and macrophage inflammatory protein-1alpha (MIP-1alpha) were all upregulated by at least 2-fold (P < 0.05) in a dose-dependent manner in VLDL-treated macrophages. The increase in inflammatory gene expression coincided with the phosphorylation of the mitogen-activated protein kinase (MAPK) pathway members extracellular signal-regulated kinase (ERK) 1/2, stress-activated protein kinase/c-Jun NH2-terminal kinase, and p38 MAPK and was ameliorated by U0126, an inhibitor of ERK1/2. Inhibition of extracellular TG hydrolysis with tetrahydrolipstatin (Orlistat) resulted in the absence of intracellular TG and FFA accumulation and was accompanied by the amelioration of ERK1/2 phosphorylation and MIP-1alpha gene expression. These data indicate that VLDL hydrolysis, and the subsequent accumulation of intracellular FFAs and TGs, plays a substantive role in mediating the proinflammatory effects of VLDL. These data have important implications for the direct proatherogenic effects of VLDL on macrophage-driven atherosclerosis.

Reduction of macrophage infiltration and chemoattractant gene expression changes in white adipose tissue of morbidly obese subjects after surgery-induced weight loss

In human obesity, the stroma vascular fraction (SVF) of white adipose tissue (WAT) is enriched in macrophages. These cells may contribute to low-grade inflammation and to its metabolic complications. Little is known about the effect of weight loss on macrophages and genes involved in macrophage attraction. We examined subcutaneous WAT (scWAT) of 7 lean and 17 morbidly obese subjects before and 3 months after bypass surgery. Immunomorphological changes of the number of scWAT-infiltrating macrophages were evaluated, along with concomitant changes in expression of SVF-overexpressed genes. The number of scWAT-infiltrating macrophages before surgery was higher in obese than in lean subjects (HAM56+/CD68+; 22.6 +/- 4.3 vs. 1.4 +/- 0.6%, P < 0.001). Typical "crowns" of macrophages were observed around adipocytes. Drastic weight loss resulted in a significant decrease in macrophage number (-11.63 +/- 2.3%, P < 0.001), and remaining macrophages stained positive for the anti-inflammatory protein interleukin 10. Genes involved in macrophage attraction (monocyte chemotactic protein [MCP]-1, plasminogen activator urokinase receptor [PLAUR], and colony-stimulating factor [CSF]-3) and hypoxia (hypoxia-inducible factor-1alpha [HIF-1alpha]), expression of which increases in obesity and decreases after surgery, were predominantly expressed in the SVF. We show that improvement of the inflammatory profile after weight loss is related to a reduced number of macrophages in scWAT. MCP-1, PLAUR, CSF-3, and HIF-1alpha may play roles in the attraction of macrophages in scWAT.

Non-high density lipoprotein cholesterol is the best discriminator of myocardial infarction in young individuals

BACKGROUND

Several studies have shown that non-high density lipoprotein (HDL) cholesterol is a strong and independent predictor of cardiovascular events. We investigated whether non-HDL cholesterol can discriminate young individuals with myocardial infarction (MI) from age- and sex-matched controls.

METHODS

We conducted a case-control study which included 100 consecutive patients who had survived their first MI before the age of 36 years and 100 age- and sex-matched healthy controls without a history of cardiovascular disease. Cardiovascular risk factors were reported and fasting lipids and apolipoproteins were measured.

RESULTS

Patients with premature MI had significantly higher levels of total cholesterol, low density lipoprotein cholesterol, triglycerides, apolipoprotein B, lipoprotein (a) and non-HDL cholesterol and significantly lower levels of HDL cholesterol and apolipoprotein A. Multivariate logistic regression analysis showed that for every 10mg/dl increase in non-HDL cholesterol levels, the odds of having a MI were increased by 34% after controlling for age, sex, body mass index, presence of hypertension, diabetes and smoking habits. Moreover, participants in the highest tertile of non-HDL cholesterol levels had 28-fold higher odds for having a MI (95% confidence interval, 7.5-104.1), compared to those in the lowest tertile. Finally, discriminant analysis showed that non-HDL cholesterol (lambda-Wilks=0.68) was the strongest discriminator for MI among all studied risk factors while smoking (lambda-Wilks=0.80) was the strongest discriminator for MI among the non-lipid risk factors.

CONCLUSIONS

Our study suggests that among conventional lipid and non-lipid risk factors non-HDL cholesterol is the best discriminator to predict the presence of MI in individuals under the age of 36 years.

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.

Chylomicron remnant induction of lipid accumulation in J774 macrophages is associated with up-regulation of triacylglycerol synthesis which is not dependent on oxidation of the particles

The influence of chylomicron remnants on lipid accumulation and synthesis and the activity and/or expression of mRNA for some of the key enzymes involved was investigated in the murine macrophage cell line J774. The effects of varying the polyunsaturated fatty acid (PUFA) composition and oxidation state of the remnants were also examined. Chylomicron remnants derived from corn oil (rich in n-6 PUFA) or fish oil (rich in n-3 PUFA) were prepared in vivo and oxidised by incubation with CuSO(4). The native and oxidised remnants caused a marked rise in intracellular triacylglycerol levels, but the rise induced by corn oil remnants (four- to sixfold) was greater than that observed with fish oil remnants (<2-fold). Triacylglycerol synthesis, as measured by the incorporation of [3H]oleate and [3H]glycerol into cellular triacylglycerol, was increased by all four remnant types tested, and corn oil remnants had a significantly greater effect than fish oil remnants. Oxidation of the remnants did not affect the results obtained. Although the incorporation of [3H]oleate into cholesteryl ester by the cells was not significantly changed by any of the four types of remnants tested, the activity and expression of mRNA for acyl Co-enzyme A: cholesterol acyltransferase (ACAT) was increased by corn oil, but not by fish or oxidised corn, remnants. Neutral cholesteryl ester hydrolase (nCEH) activity, however, was also raised by corn oil remnants. These studies indicate that chylomicron remnants induce the accumulation of triacylglycerol in J774 macrophages, and that increased synthesis of triacylglycerol plays a major role in this process. Furthermore, they demonstrate that these effects are enhanced when the remnants are enriched in n-6 PUFA as compared with n-3 PUFA, but not after oxidation of the particles, suggesting that the fatty acid composition of chylomicron remnants may be more important than their oxidation state in their ability to induce foam cell formation.

VLDL-induced triglyceride accumulation in human macrophages is mediated by modulation of LPL lipolytic activity in the absence of change in LPL mass

Mixed dyslipidemia of phenotype IIB is characterized by elevated levels of very low density lipoprotein (VLDL)-1 and VLDL-2 subfractions and of low density lipoprotein (LDL), which are associated with premature formation of atherosclerotic plaques, characterized by the presence of lipid-rich macrophage foam cells. Lipoprotein lipase (LPL) is a key factor in mediating macrophage lipid accumulation and foam-cell formation from native VLDL particles. The action of macrophage-derived LPL in the induction of intracellular lipid accumulation from triglyceride-rich lipoprotein (TRL) subfractions (VLDL-1, VLDL-2) is, however, indeterminate, as is the potential role of VLDL-1 and VLDL-2 in modulating macrophage LPL expression. We evaluated the role of LPL in the interaction of type IIB VLDL-1 and VLDL-2 with human macrophages. Both VLDL-1 and VLDL-2 subfractions induced significant accumulation of triglyceride (9.8-fold, P<0.0001, and 4.8-fold, P<0.0001, respectively) and of free cholesterol content (1.4-fold, P<0.001, and 1.2-fold, P=0.02, respectively). Specific inhibition (90%) of the lipolytic activity of endogenous LPL by tetrahydrolipstatin (THL) in the presence of VLDL-1 or VLDL-2 resulted in marked reduction in cellular loading of both triglycerides (-89%, P=0.008, and -89%, P=0.015, respectively) and free cholesterol (-76%, P=0.02, and -55%, P=0.06 respectively). Furthermore, VLDL-1 and VLDL-2 induced marked increase in macrophage-derived LPL enzyme activity (+81%, P=0.002, and +45%, P=0.02), but did not modulate macrophage-derived LPL mRNA and protein expression; consequently, LPL specific activity was significantly increased from 1.6 mU/microg at baseline to 4.1 mU/microg (P=0.01) and 3.1 mU/microg (P=0.05), in the presence of VLDL-1 and VLDL-2, respectively. We conclude that type IIB VLDL-1 and VLDL-2 induce triglyceride accumulation in human monocyte-macrophages primarily via the lipolytic action of LPL, which may involve stabilization and activation of the macrophage-secreted enzyme, rather than via modulation of enzyme production.

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

Elevated plasma levels of triacylglycerol-rich lipoproteins (TGRLP) are associated with increased risk of atherogenesis and abnormal reverse cholesterol transport, as illustrated in Type II diabetes. Here we examine the effect of plasma triacylglycerol-rich or cholesteryl ester-rich lipoproteins on the secretion of nascent apolipoprotein E (apoE)-containing lipoprotein E (LpE) particles by human (THP-1) macrophages. As expected, preincubation with low-density lipoprotein (LDL) yielded small but significant increases in total cellular cholesterol content and also the secretion of apoE by macrophages. By contrast, preincubation with TGRLP resulted in higher, dose-dependent, increases in apoE secretion that reflected, but were not dependent on, cellular triacylglycerol accumulation. Secreted apoE was incorporated into a pre-beta migrating LpE fraction that differed in lipid composition and flotation density depending on preincubation conditions. Specifically, the LpE-containing lipoprotein fraction produced by macrophages preincubated with TGRLP was cholesterol-poor, markedly heterogeneous and of higher peak flotation density (d 1.14-1.18) when compared with particles produced after preincubation with LDL. Both the conditioned medium and the isolated (d<1.21) LpE-containing fraction, yielded by macrophages preincubated with TGRLP, seemed poorer at inducing cholesterol efflux than the equivalent fractions from cells preincubated with LDL, as judged by [(3)H]cholesterol efflux from untreated 'naïve' macrophages. Thus, although the interaction of TGRLP with macrophages can enhance apoE output from these cells, the LpE particles produced seem to be relatively inefficient mediators of cholesterol efflux. These factors might contribute to the increased risk of atherosclerosis in individuals with Type II diabetes.

Regulation of serum-induced lipid accumulation in human monocyte-derived macrophages by interferon-gamma. Correlations with apolipoprotein E production, lipoprotein lipase activity and LDL receptor-related protein expression

The demonstration of lipid loaded macrophages in atherosclerotic tissue has led to the development of in vitro systems to elucidate the mechanisms involved in lipid accumulation. Here we have characterised the changes which occur in human monocyte-derived macrophage (MDM) lipids during culture in either human serum (HS) or foetal calf serum (FCS). MDM cultured in HS were rapidly converted to lipid filled foam cells, as assessed using HPLC analysis and oil red-O staining and compared with the same cells grown in FCS. However, the lipids which accumulated were predominantly triglycerides with smaller amounts of unesterified cholesterol (UC) and only traces of cholesteryl esters (CE). alpha-Tocopherol (alpha-TocH) was present at higher levels in MDM cultured in HS compared to the same cells grown in FCS. MDM lipid accumulation was dependent on the triglyceride-rich lipoprotein (TGRL) fraction of human serum; accordingly, supplementation of FCS with human TGRL also induced MDM lipid accumulation. The relationships between cellular lipid accumulation and secretion of apolipoprotein E (apo E) and lipoprotein lipase (LPL) as well as expression of the low density lipoprotein receptor-related protein (LRP) were also examined. MDM lipid accumulation was associated with increased apo E secretion but did not alter extracellular LPL activity. The lipid accumulation which was induced by HS was potently inhibited (but not reserved) by the inflammatory cytokine interferon-gamma (IFN gamma), and this was associated with decreased apo E production, LPL secretion and expression of LRP. These studies reveal striking differences in the lipid composition of MDM cultured in either HS or FCS, and indicate that oil red-O staining is not necessarily associated with cholesteryl ester accumulation in human macrophages. Furthermore, the effect that serum-induced lipid accumulation has on the specific MDM functions studied should be appreciated when developing in vitro macrophage models.

Regulation of lipoprotein lipase secretion in murine macrophages during foam cell formation in vitro. Effect of triglyceride-rich lipoproteins

Triglyceride rich-lipoproteins induce triglyceride accumulation in macrophages, leading to foam cell formation. The correlation between cell triglyceride accumulation and lipoprotein lipase (LPL) secretion in murine macrophages and the role that LPL plays in the accumulation process were examined. LPL secretion is defined as the extracellular LPL activity that accumulates during a 4-hour incubation of treated and untreated cells in a bovine serum albumin-containing RPMI-1640 medium. LPL secretion was suppressed (up to 70%) in a dose- and time-dependent manner when J774.1 cells were incubated with chylomicrons, very low density lipoproteins, and intermediate density lipoproteins but not with low or high density lipoproteins from normolipidemic and hypertriglyceridemic subjects. Oleic acid both suppressed LPL secretion and invoked triglyceride accumulation. Suppression of LPL secretion preceded gross triglyceride accumulation, was reversible, and was not the result of a reduction in LPL mRNA. P388D1 cells neither secreted LPL nor accumulated triglyceride. Inhibition of LPL secretion by tunicamycin in both peritoneal macrophages and J774.1 cells prevented a hypertriglyceridemic very low density lipoprotein-induced triglyceride accumulation, an effect that was counteracted by addition of exogenous LPL. The results suggest that 1) extracellular hydrolysis of lipoprotein triglyceride is a major factor in inducing foam cell formation and 2) LPL secretion may be regulated by cell energy needs, and when these needs are exceeded, LPL secretion is suppressed.

Lipoprotein receptor activity of peritoneal macrophages from insulin-deficient mice

Binding, uptake and degradation of 125I-labeled normal very low density lipoprotein (125I-n-VLDL) from normal swine plasma and 125I-labeled beta-migrating VLDL (125I-beta-VLDL) from hypercholesterolemic rabbit plasma by peritoneal macrophages of mice rendered insulin-deficient by streptozotocin (250 mg/kg) were studied. It was found that the amount of binding, uptake and degradation of 125I-n-VLDL by macrophages from the diabetic mice was 2-fold or 2.5-fold higher than by macrophages from normal mice, resulting from an increase in the binding capacity of VLDL receptors on the macrophages from the insulin-deficient rodents. In contrast, the binding, uptake and degradation of 125I-beta-VLDL by macrophages from diabetic mice were reduced to only about 45% of normal levels because of a decrease in the number and affinity of the receptors for beta-VLDL. These experimental results indicate that n-VLDL is more important than beta-VLDL in the pathogenesis of atherosclerosis in insulin-dependent diabetes.

Relationship of VLDL receptor and LPL in metabolism of VLDL by macrophage

Macrophages were incubated with 125I-VLDL for 5 h in presence or absence of lipoprotein lipase (LPL) inhibitor, benzene boronic acid (BBA). Both the uptake and degradation of 125I-VLDL by macrophages were saturable, and the uptake and degradation curves were virtually identical. When macrophages were incubated with 125I-VLDL for 10 h in presence of BBA, the uptake and degradation of 125I-VLDL were still saturable. However, in absence of BBA, the uptake and degradation were no longer saturable. The results suggest that with macrophages incubated with VLDL for a shorter period, VLDL was taken up predominantly via receptor pathway, with a longer period of incubation, LPL played a striking role in uptake of VLDL.

Effect of dietary fat saturation and cholesterol on LDL composition and metabolism. In vivo studies of receptor and nonreceptor-mediated catabolism of LDL in cebus monkeys

The mechanism(s) by which polyunsaturated fats reduce low density lipoprotein (LDL) cholesterol and apolipoprotein (apo) B were investigated in 20 cebus monkeys (Cebus albifrons) fed diets containing corn oil or coconut oil as fat (31% of calories) with or without dietary cholesterol (0.1% by weight) for 3 to 10 years. Coconut-oil feeding compared to corn-oil feeding resulted in significant increases in levels of plasma total cholesterol (176%), very low density lipoprotein (VLDL)-LDL cholesterol (236%), high density lipoprotein (HDL) cholesterol (148%), apo B (78%), and apo A-I (112%). The addition of dietary cholesterol to corn oil compared to corn oil alone resulted in smaller, but significant, increases in levels of total cholesterol (44%), HDL cholesterol (40%), and apo A-I (33%). Although the increases in VLDL-LDL cholesterol were of similar magnitude (52%), they barely failed to reach statistical significance (p less than 0.08), while the changes in apo B levels were negligible. The addition of dietary cholesterol to coconut oil, compared to coconut oil alone, resulted in no significant changes in lipoprotein cholesterol or apoproteins, although levels of VLDL-LDL cholesterol and apo B values increased 22% and 16%, respectively. Although hepatic free cholesterol content was not altered by diet, coconut-oil compared to corn-oil feeding resulted in significant increases in hepatic cholesteryl esters (236%) and triglycerides (325%), the latter increasing still further when dietary cholesterol was added to coconut oil (563%). To further assess the effects of these dietary changes on LDL metabolism, radioiodinated normal and glucosylated LDL kinetics were performed. The production rate of LDL apo B was not altered by diet. With corn-oil feeding, 63% of LDL catabolism was via the receptor-mediated pathway. Coconut-oil compared to corn-oil feeding resulted in a 50% decrease in receptor-mediated LDL apo B fractional catabolic rate (FCR) and a 27% reduction in nonreceptor-mediated LDL apo B FCR. The addition of dietary cholesterol to corn oil, compared to corn oil alone, resulted in no significant effect on LDL apo B catabolism. The addition of dietary cholesterol to coconut oil, compared to coconut oil alone, was associated with no significant change in nonreceptor catabolism of LDL apo B but with a 58% decrease in receptor-mediated catabolism of LDL (p less than 0.059). The diet-induced alterations of LDL catabolism were significantly correlated with hepatic lipids, which were enriched in saturated fatty acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of activation on lipid and lipoprotein metabolism in murine macrophages

The effects of activation on lipid and lipoprotein metabolism were examined in resident murine macrophages, inflammatory cells elicited by thioglycolate, primed cells elicited by pyran copolymer, and activated cells elicited by Corynebacterium parvum. Low density lipoprotein receptors were reduced by 70%, while scavenger receptors were reduced 60% in activated cells. Basal cholesteryl ester and triglyceride synthesis were increased fourfold in activated cells, whereas the activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase was high in resident cells and progressively declined by greater than 80% in activated cells. Activities of neutral cholesteryl esterase and neutral triglyceride lipase were increased two- to fourfold in inflammatory, primed, and activated macrophages. These results demonstrate the diverse changes in lipid and lipoprotein metabolism that occur with activation and emphasize how the behavior of macrophages in atherosclerotic lesions can be altered by activation.

Biogenesis of plasma lipoproteins in rat hepatoma McA-RH7777: importance of diffusion-mediated events during cell growth

Cultured McA-RH7777 rat hepatoma cells actively synthesize and secrete plasma lipoproteins. However, synthesis of [14C]triglyceride declines monotonically throughout the early growth period and remains low in postconfluent cultures; and net secretion of [14C]triglyceride is 10-fold more efficient in logarithmically growing cultures than in postconfluent cultures. Secretion of apolipoproteins associated with very low density and low density lipoproteins is selectively reduced in postconfluent cultures. The temporal reductions in [14C]triglyceride production are related more strongly to increasing cell concentration (cells/cm3 medium) than to increasing cell density (cells/cm2 growth surface). We have allowed cells to grow either retained within small circular corrals or unrestricted in culture dishes. When seeded at equal density (10(4) cells/cm2) but at one-fifth the cell concentration, corralled cells synthesize twice as much [14C]triglyceride per cell after 2 and 4 d, and are 10 times as efficient in [14C]triglyceride secretion by 6 d of growth, as noncorralled cells. When seeded at equal cell concentration (10(5) cells/dish) but at 5 times the cell density, corralled cells are only 20% less efficient at [14C]triglyceride synthesis and secretion than noncorralled cells. Conditioned medium depresses synthesis and secretion efficiency of [14C]triglyceride. Orotic acid exposure also inhibits synthesis of [14C]triglyceride and secretion of certain [35S]apolipoproteins in early cultures, but it has no significant effect on late cultures. We conclude that diffusion-mediated events are important regulators of triglyceride and apolipoprotein production in growing rat hepatoma cells, but that events associated with formation of cell-to-cell contacts play a minor role in regulation of plasma lipoprotein biogenesis.

Interaction of very-low-density lipoprotein isolated from type I (insulin-dependent) diabetic subjects with human monocyte-derived macrophages

Very-low-density lipoproteins (VLDL) (density less than 1.006 g/mL) were isolated from type I (insulin-dependent) diabetic patients in good to fair glycemic control and from age-, sex-, and race-matched, nondiabetic, control subjects. VLDL were incubated with human, monocyte-derived macrophages obtained from nondiabetic donors, and the rates of cellular cholesteryl ester synthesis and cholesterol accumulation were determined. VLDL isolated from diabetic patients stimulated significantly more cholesteryl ester synthesis than did VLDL isolated from control subjects (4.04 +/- 1.01 v 1.99 +/- 0.39 nmol 14C-cholesteryl oleate synthesized/mg cell protein/20 h; mean +/- SEM, P less than .05). The stimulation of cholesteryl ester synthesis in macrophages incubated with VLDL isolated from diabetic patients was paralleled by a significant increase in intracellular cholesteryl ester accumulation (P less than .05). The increase in cholesteryl ester synthesis and accumulation in macrophages were mediated by a significant increase in the receptor mediated, high affinity degradation (2.55 +/- 0.23 v 2.12 +/- 0.20 micrograms degraded/mg cell protein/20 h) and accumulation (283 +/- 35 v 242 +/- 33 ng/mg cell protein/20 h) of 125I-VLDL isolated from diabetic patients compared with VLDL from control subjects. To determine if changes in VLDL apoprotein composition were responsible for the observed changes in cellular rates of cholesteryl ester synthesis and accumulation, we also examined the apoprotein composition of the VLDL from both groups. There were no significant differences between the apoproteins B, E, and C content of VLDL from both groups. We also determined the chemical composition of VLDL isolated from both groups of subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced lipoprotein lipase secretion from human monocyte-derived macrophages caused by hypertriglyceridemic very low density lipoproteins

We studied the effects of very low density lipoprotein (VDL) obtained from hypertriglyceridemic subjects on the secretion of lipoprotein lipase and lipid accumulation in human monocyte-derived macrophages. The incubation of macrophages with VLDL obtained from different subjects caused different effects on the secretion of lipoprotein lipase (6.8 to 137.7 nM free fatty acid/min/mg cell protein) and triglyceride accumulation (184 to 507 micrograms/mg cell protein) in human monocyte-derived macrophages. VLDL from subjects with marked hypertriglyceridemia (approximately 1000 mg/dl) had a fourfold greater effect on lipoprotein lipase activity and a twofold greater effect on cellular triglyceride accumulation when compared with the effects of VLDL from normolipidemic subjects. Both lipoprotein lipase activity and triglyceride accumulation correlated positively with plasma VLDL triglyceride levels (r = 0.50 and 0.45, respectively, p less than 0.05). From these data, we suggest that the activity of lipoprotein lipase secreted from macrophages incubated with VLDL was dependent on triglyceride concentrations, and that the secretion of lipoprotein lipase enhanced by hypertriglyceridemic VLDL was closely related to the intracellular accumulation of triglyceride.

Hypertriglyceridaemia as a risk factor of coronary heart disease mortality in subjects with impaired glucose tolerance or diabetes. Results from the 11-year follow-up of the Paris Prospective Study

The Paris Prospective Study is a long-term investigation of the incidence of coronary heart disease in a large population of working men. The first follow-up examination involved 7,038 men, aged 43-54 years. Subjects with impaired glucose tolerance or diabetes (n = 943) were selected from the total population for a separate analysis of coronary heart disease mortality risk factors. During a mean follow-up of 11 years, 26 of these 943 subjects with abnormal glucose tolerance died from coronary heart disease. Univariate analysis showed that plasma triglyceride level (p less than 0.006), plasma cholesterol level (p less than 0.02), and plasma insulin level both fasting and 2-h post-glucose load (p less than 0.02), were significantly higher in subjects who died from coronary heart disease compared to those who did not. In multivariate regression analysis using the Cox model, plasma triglyceride level was the only factor positively and significantly associated with coronary death. The distribution of plasma triglyceride levels was clearly higher for the subjects who died from coronary heart disease compared to those who did not die from this cause or were alive at the end of the follow-up. This new epidemiological evidence that hypertriglyceridaemia is an important predictor of coronary heart disease mortality in subjects with impaired glucose tolerance or diabetes suggests a possible role of dyslipidaemia in the excessive occurrence of atherosclerotic vascular disease in this category of subjects.

Effects of feeding fish oil on the properties of lipoproteins isolated from rhesus monkeys consuming an atherogenic diet

This study examined plasma lipids and lipoproteins of rhesus monkeys fed fish oil incorporated into a highly atherogenic diet containing saturated fat and cholesterol. The animals were fed diets containing 2% cholesterol and either 25% coconut oil (group I), 25% fish oil/coconut oil (1:1; group II), or 25% fish oil/coconut oil (3:1; group III) for 12 months (n = 8/group). Adding menhaden fish oil to the diet increased plasma eicosapentaenoic acid and docosahexaenoic acid and decreased plasma linoleic acid in animals fed the fish oil containing diets. Plasma concentrations of all lipoprotein fractions were decreased in the fish oil groups. VLDL isolated from group I animals exhibited beta-mobility on agarose gels but the VLDL from groups II and III animals did not. The group I VLDL was more highly enriched in cholesteryl ester than was VLDL from groups II and III. Group I LDL had a small but significant increase in cholesteryl ester content compared to group III LDL. No differences in HDL composition were observed in the 3 groups. At least 6 times less apo E was recovered in VLDL, IDL, and LDL from group III animals than from group I animals. Assuming 1 molecule of apo B per lipoprotein particle, there were 50% fewer VLDL, IDL, and LDL particles in group III than in group I animals. Group III also had significantly lower molar ratios of apo E/apo B in VLDL, IDL, and LDL than did group I animals. When VLDL from all 3 groups were incubated with J774 macrophages at equal protein concentrations, only the VLDL from the group I animals stimulated cholesterol esterification. Thus, introducing fish oil into an atherogenic diet reduced the number of VLDL, IDL and LDL particles in plasma by as much as 50%, reduced the cholesteryl ester content of the circulating lipoprotein, and reduced the ability of the VLDL to stimulate cholesterol esterification in macrophages.

Effect of fatty acid supplementation on cholesterol and retinol esterification in J774 macrophages

J774 macrophages exposed to medium containing cholesterol-rich phospholipid dispersions accumulate cholesteryl ester. Supplementing this medium with 100 micrograms oleate/ml increased cellular cholesteryl ester contents 3-fold. Cell retinyl ester contents increased 8-fold when medium containing retinol dispersed in dimethyl sulfoxide was supplemented with oleate. These increases were not the result of increases in total lipid uptake by the cells but rather of redistribution of cholesterol and retinol into their respective ester pools. Effective oleate concentration of 15-30 micrograms/ml increased cellular retinyl and cholesteryl ester contents. The effective oleate concentration was reduced to 5 micrograms/ml when the fatty acid/albumin molar ratio was increased. The oleate-stimulated increase in cholesterol esterification was blocked by incubating cells with Sandoz 58-035, a specific inhibitor of acyl-CoA:cholesterol acyltransferase (ACAT), indicating that the effect of fatty acid exposure is mediated through changes in ACAT activity. When cholesterol or retinol was added to cells which had been exposed to oleate for 24 h to provide a triacylglycerol store, the cellular contents of cholesteryl or retinyl ester were also significantly increased compared to cells not previously exposed to oleate. The oleate-stimulated increase in the esterification of cholesterol and/or retinol was also observed in P388D1 macrophages, human (HepG2) and rat (Fu5AH) hepatomas, human fibroblasts, rabbit aortic smooth muscle cells and MCF-7 breast carcinoma cells. In addition to oleate, a number of other fatty acids increased retinol esterification in J774 macrophages; however, cellular cholesterol esterification in these cells was increased only by unsaturated fatty acids and was inhibited in the presence of saturated fatty acids. Although the cellular uptake of radiolabeled oleate and palmitate was similar, a significant difference in the distribution of these fatty acids among the lipid classes was observed. These data demonstrate that exogenous fatty acids are one factor that regulate cellular cholesteryl and retinyl ester contents in cultured cells.

Comparison of fatty acid and triacylglycerol metabolism of macrophages and smooth muscle cells

The response of macrophages and smooth muscle cells to culture in free fatty acid has been compared. Because oleate and linoleate promoted triacylglycerol enrichment of smooth muscle cells, whereas palmitate had little effect, oleate was used for these studies. The kinetics of the accumulation of triacylglycerol produced by oleate was comparable between smooth muscle cells and macrophages. When grown in increasing concentrations of oleic acid at various fatty acid to albumin molar ratios, the extent of triacylglycerol accumulation in both cell types was dependent on the concentration of oleate, the concentration of albumin, and the oleate to albumin molar ratio. However, macrophages contained 2.6-fold more triacylglycerol than smooth muscle cells in the presence of oleate at 0.36 mM or greater and at levels of albumin higher than 0.15 mM. The cellular triacylglycerol content of macrophages was linearly related to the oleate to albumin molar ratio at both a constant albumin concentration and a constant oleate concentration, whereas the accumulation of triacylglycerol in smooth muscle cells showed a curvilinear relationship. When cells were preloaded with triacylglycerol, smooth muscle cells showed a greater loss of lipid when exposed to albumin than macrophages did. Over a two-hr time period, macrophages incorporated twice as much labeled fatty acid as smooth muscle cells. Thus, while smooth muscle cells and macrophages showed similar responses to exogenous fatty acid and albumin, there were also significant quantitative distinctions.

[Lysosomal enzyme activity of monocytes/macrophages following incubation with postprandial hyperlipemic serum and its significance for the development of atherosclerosis]

Lipid accumulation in macrophages is a prominent feature of the atherosclerotic lesion. Decreased lysosomal function of these cells might play an important role in the pathogenesis of the atherosclerotic foam cell. In this investigation six normal volunteers were fed a meal with a high fat content (68.9% energy, P/S ratio 0.13). The hyperlipidemic postprandial serum was incubated with monocyte derived macrophages. The enzyme activity of cathepsin B, acid cholesterylester-hydrolase and N-acetyl-beta-hydrolase decreased significantly in these cells. Thus, inadequate response in enzyme activity of lysosomal enzymes in case of fat overload might contribute to the development of the atherosclerotic foam cell.

Modification of very low density lipoproteins leads to macrophage scavenger receptor uptake and cholesteryl ester deposition

Chemically modified low density lipoproteins (LDL) are recognized by the macrophage scavenger receptor and can lead to substantial cholesteryl ester accumulation in cultured macrophages. Uptake of modified lipoproteins in vivo could contribute to foam cell formation during generation of the atherosclerotic plaque lesion. In the present study, modification of human pre-beta migrating very low density lipoprotein (VLDL) by acetylation led to recognition by the macrophage scavenger receptor as demonstrated in cross-competition experiments with acetylated LDL (ALDL). Recognition by this alternative binding site was associated with increased cholesterol delivery to human macrophages as assessed by suppression of LDL receptor activity, stimulation of cholesterol esterification rates, and accumulation of intracellular cholesteryl ester. Subfractionation of acetylated very low density lipoprotein (AVLDL) by ultracentrifugation in a discontinuous NaCl gradient demonstrated that AVLDL subfractions were equally effective in competing for 125I-ALDL uptake by macrophages when compared on the basis of particle number. These results suggest that modification of VLDL with subsequent recognition by the macrophage scavenger receptor may be a mechanism by which VLDL particles participate in macrophage cholesteryl ester overload.

Regulation of lipoprotein lipase secretion by mouse peritoneal macrophages

Resident mouse peritoneal macrophages in culture spontaneously secrete lipoprotein lipase. Secretion of lipoprotein lipase was 10-fold greater in thioglycollate-elicited and 6-fold greater in mineral oil-elicited macrophages. The increase in enzyme secretion was dependent on protein synthesis and glycosylation of the newly synthesized enzyme. The addition of 1 to 500 ng per ml of lipopolysaccharide to the culture medium resulted in a dose-dependent inhibition of lipoprotein lipase secretion. Thus, secretion of macrophage lipoprotein lipase is a regulated process, and may be increased as part of the response to inflammation. This may have important implications in the atherogenic process in the arterial wall.

Regulation of the secretion of lipoprotein lipase by mouse macrophages

The regulation of the secretion of lipoprotein lipase was studied in primary cultures of mouse peritoneal macrophages and in the murine macrophage cell line J774. As previously reported, both cell types secrete a lipase with the characteristics of lipoprotein lipase. Incubation of macrophages with insulin, insulin-like growth factor, and L-thyroxine had no effect on lipoprotein lipase secretion. Incubation with dexamethasone and with several agents which increase intracellular cyclic AMP led to a decrease in lipoprotein lipase secretion by mouse peritoneal macrophages. These results suggest that the hormonal regulation of lipoprotein lipase in macrophages is different from that in adipose tissue and heart muscle. Incubation of the macrophages with heparin caused a marked increase in the secretion of lipoprotein lipase. Short incubations with heparin (5 min) caused a release of the enzyme into the media, while longer incubations caused a 2-8-fold increase in net lipoprotein lipase secretion which was maximal after 2-16 h depending on cell type, and persisted for 24 h. The effect of heparin was dose-dependent and specific (it was not duplicated by other glycosaminoglycans). The mechanism of heparin-induced increase in lipoprotein lipase secretion was explored. The increase was not caused by the release of a presynthesized intracellular pool of lipoprotein lipase or by the stabilization of lipoprotein lipase by heparin after secretion. The heparin-induced increase in lipoprotein lipase secretion was dependent on protein synthesis. The secretion of lipoprotein lipase by macrophages in response to low levels of heparin may be a significant factor in the formation of atherosclerotic lesions.

Metabolism of human plasma triacylglycerol-rich lipoproteins in rodent macrophages: capacity for interaction at beta-VLDL receptor

The capacity of human plasma triacylglycerol-rich lipoproteins to be metabolized by rat macrophages was studied with plasma triacylglycerol-rich lipoproteins obtained from subjects with fasting chylomicronemia or from normal subjects after a fat meal. Triacylglycerol-rich lipoproteins were separated by chromatography into two fractions designated TRL1 and TRL2; from their composition and changing concentration during alimentary lipemia, TRL1 contained a higher proportion of chylomicron remnants than TRL2. Degradation of 125I-labeled TRL1 was greater than that of 125I-labeled TRL2. In competition studies with 125I-labeled beta-VLDL from cholesterol-fed rabbits, unlabeled TRL1 displaced beta-VLDL as completely as did unlabeled beta-VLDL, being slightly more potent than TRL2, which contained less apolipoprotein E than TRL1. This reflected common interaction at receptors that probably included both beta-VLDL and B/E receptors, since: (1) in fresh macrophages, VLDL from hypertriglyceridemic subjects partially displaced beta-VLDL; (2) in B/E receptor-repressed macrophages, TRL1 maintained capacity to totally displace beta-VLDL. This was confirmed in experiments with J774 murine macrophages in which triacylglycerol-rich lipoproteins and beta-VLDL displaced each other equally, whereas LDL was ineffective in displacing beta-VLDL. Furthermore, monoclonal antibodies raised against apolipoprotein B48 and reacting strongly with LDL, failed to inhibit the binding of triacylglycerol-rich lipoprotein to the macrophages. This indicates an interaction through apolipoprotein E which is present in high concentration in triacylglycerol-rich lipoprotein as well as in beta-VLDL. It applies to triacylglycerol-rich particles derived from either the intestine (chylomicron remnants) or the liver (VLDL remnants from hypertriglyceridemic subjects).

Macrophage interaction with very-low-density lipoproteins results in triacylglycerol-enriched smooth muscle cells

Macrophage-conditioned medium containing very-low-density lipoproteins (VLDL) and its effects on smooth muscle cell triacylglycerol metabolism was investigated. Macrophages exposed to VLDL from normolipemic rats accumulated high levels of intracellular triacylglycerol, while similarly treated smooth muscle cells showed only slight changes. Medium, initially composed of VLDL and albumin, contained substantial levels of free fatty acids after exposure to macrophages. In the presence of albumin, the loss of VLDL triacylglycerol from the medium and the appearance of medium free fatty acids was consistent with a precursor-product relationship. The extent of medium fatty acid accumulation was dependent on the length of time of incubation with macrophages and was proportional to the concentration of VLDL and albumin added to the culture dish. This macrophage-conditioned medium, when given to smooth muscle cells, promoted a 6-12-fold increase in smooth muscle cell triacylglycerol levels over that produced by fresh VLDL and albumin. Similar increases in cell triacylglycerol levels could be produced using fresh medium approximating the oleate concentration and the fatty acid to albumin molar ratios found in macrophage-conditioned medium. In macrophage-conditioned medium with VLDL but without albumin, little free fatty acid was found. Other factors produced by macrophages did not seem to affect the metabolism of VLDL by smooth muscle cells since, in the absence of albumin, media with VLDL caused comparable responses in smooth muscle cell triacylglycerol accumulation whether or not the medium was previously exposed to macrophages. Thus, the minor changes in triacylglycerol content in smooth muscle cells promoted by medium containing VLDL and albumin were substantially enhanced by a prior exposure of the medium to macrophages, primarily due to the free fatty acids present in the macrophage-conditioned medium.

Metabolism of atherogenic lipoproteins by smooth muscle cells of different phenotype in culture

Smooth muscle cells of the rabbit aorta, when grown in vitro, express three distinguishable forms of phenotype (contractile, reversible synthetic, and irreversible synthetic). We compared the interactions of these three smooth muscle phenotypes with rabbit very low density lipoprotein (VLDL), low density lipoprotein (LDL), and very low density lipoprotein from cholesterol-fed rabbits (beta-VLDL). beta-VLDL showed saturable. high-affinity binding characteristics with each phenotype predominantly through the B/E receptor. The irreversible synthetic cells displayed the greatest binding capacity and the contractile cells, the least. Binding and degradation of normal VLDL was less than that of beta-VLDL and higher than that of LDL. Only the irreversible synthetic cells showed substantial (about threefold) cholesteryl ester formation and cholesterol accumulation, and then only when incubated with beta-VLDL. Substantial stainable lipid, shown chemically to include triglyceride, cholesterol and cholesteryl ester, was also observed only when irreversible synthetic cells were exposed to beta-VLDL. The high capacity of irreversible synthetic-state, smooth muscle cells to bind and accumulate beta-VLDL in contrast to the relative immunity of contractile cells may be relevant to the genesis of atherosclerosis in the rabbit and possibly also in humans.

Apo E-mediated uptake and degradation of normal very low density lipoproteins by human monocyte/macrophages: a saturable pathway distinct from the LDL receptor

Normal human fasting very low density lipoproteins (n-VLDL; d less than 1.006 g/ml) were demonstrated to be taken up and degraded by human monocyte-macrophages via a saturable process distinct from the previously described LDL and scavenger receptors. Through the use of apolipoprotein-phospholipid complexes, apolipoprotein E (apoE) was identified as the ligand mediating recognition of n-VLDL by this receptor.

Hypertriglyceridemia: lipoprotein receptors and atherosclerosis

We have shown first, that apoB mediates the binding of small VLDL Sf 20-60 and IDL, as well as LDL, to the LDL receptor. Second, apoE of an appropriate, accessible conformation is required for the binding of large VLDL to the LDL receptor; HTG-VLDL Sf greater than 60 but not normal VLDL Sf greater than 60 have this apoE population. Third, the same population of apoE that mediates binding of HTG-VLDL Sf greater than 60 to the LDL receptor modulates its binding to the beta-VLDL receptor, but it is not required for the latter interaction. Fourth, a domain of processed apoB or apoB-48 in association with a domain of the inaccessible apoE is required for binding to and uptake by the beta-VLDL receptor. Fifth, our observations suggest that the abnormal catabolism of VLDL that occurs in hypertriglyceridemia may be explained by the abnormal uptake of HTG-VLDL by either the LDL or the beta-VLDL receptor pathway. Finally, we suggest that plasma proteases may route apoB/E-containing lipoproteins to macrophages for disposal, and this results in foam cell formation.