Lipid composition and physical state effects on cellular cholesteryl ester clearance

Deciphering the Role of Lipid Droplets in Cardiovascular Disease: A Report From the 2017 National Heart, Lung, and Blood Institute Workshop

Lipid droplets (LDs) are distinct and dynamic organelles that affect the health of cells and organs. Much progress has been made in understanding how these structures are formed, how they interact with other cellular organelles, how they are used for storage of triacylglycerol in adipose tissue, and how they regulate lipolysis. Our understanding of the biology of LDs in the heart and vascular tissue is relatively primitive in comparison with LDs in adipose tissue and liver. The National Heart, Lung, and Blood Institute convened a working group to discuss how LDs affect cardiovascular diseases. The goal of the working group was to examine the current state of knowledge on the cell biology of LDs, including current methods to study them in cells and organs and reflect on how LDs influence the development and progression of cardiovascular diseases. This review summarizes the working group discussion and recommendations on research areas ripe for future investigation that will likely improve our understanding of atherosclerosis and heart function.

Eicosapentaenoic acid membrane incorporation impairs cholesterol efflux from cholesterol-loaded human macrophages by reducing the cholesteryl ester mobilization from lipid droplets

A diet containing a high n-3/n-6 polyunsaturated fatty acids (PUFA) ratio has cardioprotective properties. PUFAs incorporation into membranes influences the function of membrane proteins. We investigated the impact of the membrane incorporation of PUFAs, especially eicosapentaenoic acid (EPA) (C20:5 n-3), on the anti-atherogenic cholesterol efflux pathways. We used cholesteryl esters (CE)-loaded human monocyte-derived macrophages (HMDM) to mimic foam cells exposed to the FAs for a long period of time to ensure their incorporation into cellular membranes. Phospholipid fraction of EPA cells exhibited high levels of EPA and its elongation product docosapentaenoic acid (DPA) (C22:5 n-3), which was associated with a decreased level of arachidonic acid (AA) (C20:4 n-6). EPA 70μM reduced ABCA1-mediated cholesterol efflux to apolipoprotein (apo) AI by 30% without any alteration in ABCA1 expression. The other tested PUFAs, DPA, docosahexaenoic acid (DHA) (C22:6 n-3), and AA, were also able to reduce ABCA1 functionality while the monounsaturated oleic FA slightly decreased efflux and the saturated palmitic FA had no impact. Moreover, EPA also reduced cholesterol efflux to HDL mediated by the Cla-1 and ABCG1 pathways. EPA incorporation did not hinder efflux in free cholesterol-loaded HMDM and did not promote esterification of cholesterol. Conversely, EPA reduced the neutral hydrolysis of cytoplasmic CE by 24%. The reduced CE hydrolysis was likely attributed to the increase in cellular TG contents and/or the decrease in apo E secretion after EPA treatment. In conclusion, EPA membrane incorporation reduces cholesterol efflux in human foam cells by reducing the cholesteryl ester mobilization from lipid droplets.

Polarized THG microscopy identifies compositionally different lipid droplets in mammalian cells

Cells store excess lipids as two major compounds, triacylglycerols (TAGs) and cholesteryl esters (CEs), inside lipid droplets (LDs). The degree of lipid ordering is considered to play a major role in the mobility and enzymatic processing of lipids in LDs. Here, we provide evidence that polarized third-harmonic generation (THG) microscopy distinguishes between native TAG- and CE-enriched LDs in cells due to the different ordering of the two lipid species. We first demonstrate that the responses from synthetic TAG- and CE-enriched LDs using THG microscopy with linear and circular polarizations differ according to their different intrinsic ordering. We then employ simulations to dissect how polarization effects influence the THG from an isotropic LD. Finally, we induce TAG- and CE-enriched LDs in murine macrophages and demonstrate that polarized THG responses increase in a nonlinear fashion with increasing CE/TAG ratio. This suggests that with an increasing CE content, there is a rather sharp transition toward increased LD ordering. Our results demonstrate that polarized THG microscopy enables label-free quantitative analysis of LD ordering and discriminates between compositionally different LDs in intact mammalian cells.

Cytoskeleton disruption in J774 macrophages: consequences for lipid droplet formation and cholesterol flux

Macrophages store excess unesterified cholesterol (free, FC) in the form of cholesteryl ester (CE) in cytoplasmic lipid droplets. The hydrolysis of droplet-CE in peripheral foam cells is critical to HDL-promoted reverse cholesterol transport because it represents the first step in cellular cholesterol clearance, as only FC is effluxed from cells to HDL. Cytoplasmic lipid droplets move within the cell utilizing the cytoskeletal network, but, little is known about the influence of the cytoskeleton on lipid droplet formation. To understand this role we employed cytochalasin D (cyt.D) to promote actin depolymerization in J774 macrophages. Incubating J774 with acetylated LDL creates foam cells having a 4-fold increase in cellular cholesterol content (30-40% cholesterol present as cholesteryl ester (CE)) in cytoplasmic droplets. Lipid droplets formed in the presence of cyt.D are smaller in diameter. CE-deposition and -hydrolysis are decreased when cells are cholesterol-enriched in the presence of cyt.D or latrunculin A, another cytoskeleton disrupting agent. However, when lipid droplets formed in the presence of cyt.D are isolated and incubated with an exogenous CE hydrolase, the CE is more rapidly metabolized compared to droplets from control cells. This is apparently due to the smaller size and altered lipid composition of the droplets formed in the presence of cyt.D. Cytoskeletal proteins found on CE droplets influence droplet lipid composition and maturation in model foam cells. In J774 macrophages, cytoskeletal proteins are apparently involved in facilitating the interaction of lipid droplets and a cytosolic neutral CE hydrolase and may play a role in foam cell formation. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Lysosomes, cholesterol and atherosclerosis

Cholesterol-engorged macrophage foam cells are a critical component of the atherosclerotic lesion. Reducing the sterol deposits in lesions reduces clinical events. Sterol accumulations within lysosomes have proven to be particularly hard to mobilize out of foam cells. Moreover, excess sterol accumulation in lysosomes has untoward effects, including a complete disruption of lysosome function. Recently, we demonstrated that treatment of sterol-engorged macrophages in culture with triglyceride-containing particles can reverse many of the effects of cholesterol on lysosomes and dramatically reduce the sterol burden in these cells. This article describes what is known about lysosomal sterol engorgement, discusses the possible mechanisms by which triglyceride could produce its effects, and evaluates the possible positive and negative effects of reducing the lysosomal cholesterol levels in foam cells.

Lipogenesis in arterial wall and vascular smooth muscle cells of Psammomys obesus: its regulation and abnormalities in diabetes

AIM

Lipogenesis is expressed in vascular smooth muscle cells (VSMCs), and such in situ lipogenesis could be providing the fatty acids for triglyceride synthesis and cholesterol esterification, and contributing to lipid accumulation in the arterial wall. This study investigated both the expression and regulation of lipogenesis in VSMCs to determine if they are modified in Psammomys obesus gerbils fed a high-fat diet as a model of insulin resistance and diabetes.

METHODS

Aortas were collected from diabetic and non-diabetic P. obesus for histological examination, measurement of lipogenic gene expression and VSMC culture.

RESULTS

The aortas of diabetic animals exhibited lipid deposits and foam cells as well as disorganization of elastic fibres. However, lipogenic gene expression was not modified. VSMCs in vitro from the aortas of diabetic animals had, compared with cells from non-diabetic animals, lower mRNA levels of SREBP-1c and ChREBP. An adipogenic medium stimulated moderate FAS and ACC1 expression in cells from both diabetic and non-diabetic animals, but glucose and insulin on their own had no such stimulatory action. Also, triiodothyronine (T3) had a clear stimulatory action, while angiotensin II had a moderate effect, in cells from non-diabetic P. obesus, but not from diabetic animals, whereas LXR agonists stimulated lipogenesis in cells from both animal groups.

CONCLUSION

Lipogenesis is expressed in the arterial walls and VSMCs of P. obesus. However, its expression was not increased in diabetes, and did not respond to either T3 or angiotensin II. Therefore, lipogenesis in situ is unlikely to contribute to the accumulation of lipids in the arterial walls of diabetic P. obesus gerbils.

Lipogenesis in arterial wall and vascular smooth muscular cells: regulation and abnormalities in insulin-resistance

Background

Vascular smooth muscular cells (VSMC) express lipogenic genes. Therefore in situ lipogenesis could provide fatty acids for triglycerides synthesis and cholesterol esterification and contribute to lipid accumulation in arterial wall with aging and during atheroma.

Methods

We investigated expression of lipogenic genes in human and rat arterial walls, its regulation in cultured VSMC and determined if it is modified during insulin-resistance and diabetes, situations with increased risk for atheroma.

Results

Zucker obese (ZO) and diabetic (ZDF) rats accumulated more triglycerides in their aortas than their respective control rats, and this triglycerides content increased with age in ZDF and control rats. However the expression in aortas of lipogenic genes, or of genes involved in fatty acids uptake, was not higher in ZDF and ZO rats and did not increase with age. Expression of lipogenesis-related genes was not increased in human arterial wall (carotid endarterectomy) of diabetic compared to non-diabetic patients. In vitro, glucose and adipogenic medium (ADM) stimulated moderately the expression and activity of lipogenesis in VSMC from control rats. LXR agonists, but not PXR agonist, stimulated also lipogenesis in VSMC but not in arterial wall in vivo. Lipogenic genes expression was lower in VSMC from ZO rats and not stimulated by glucose or ADM.

Conclusion

Lipogenic genes are expressed in arterial wall and VSMC; this expression is stimulated (VSMC) by glucose, ADM and LXR agonists. During insulin-resistance and diabetes, this expression is not increased and resists to the actions of glucose and ADM. It is unlikely that this metabolic pathway contribute to lipid accumulation of arterial wall during insulin-resistance and diabetes and thus to the increased risk of atheroma observed in these situations.

15(S)-Lipoxygenase-1 associates with neutral lipid droplets in macrophage foam cells: evidence of lipid droplet metabolism

15(S)-lipoxygenase-1 (15-LO-1) was present in the whole-cell homogenate of an acute human monocytic leukemia cell line (THP-1). Additionally, 15-LO-1 was detected on neutral lipid droplets isolated from THP-1 foam cells. To investigate if 15-LO-1 is active on lipid droplets, we used the mouse leukemic monocytic macrophage cell line (RAW 264.7), which are stably transfected with human 15-LO-1. The RAW 15-LO-1 cells were incubated with acetylated low density lipoprotein to generate foam cells. 15(S)-hydroxyeicosatetraenoic acid [15(S)-HETE], the major 15-LO-1 metabolite of arachidonic acid, was produced in the 15-LO-1 RAW but not in the mock transfected cells when incubated with arachidonic acid. Lipid droplets were isolated from the cells and incubated with arachidonic acid, and production of 15(S)-HETE was measured over 2 h. 15(S)-HETE was produced in the incubations with the lipid droplets, and this production was attenuated when the lipid droplet fraction was subjected to enzyme inactivation through heating. Efflux of 15(S)-HETE from cholesteryl ester-enriched 15-LO RAW cells, when lipid droplets are present, was significantly reduced compared with that from cells enriched with free cholesterol (lipid droplets are absent). We propose that 15-LO-1 is present and functional on cytoplasmic neutral lipid droplets in macrophage foam cells, and these droplets may act to accumulate the anti-inflammatory lipid mediator 15(S)-HETE.

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.

Lipid droplet changes in proliferating and quiescent 3T3 fibroblasts

Lipid droplets (LDs) are fat-storing organelles present in virtually all eukaryotic cells and involved in many aspects of cell biology related to lipid metabolism and cholesterol homeostasis. In this study, we investigated the presence of LDs in proliferating and quiescent (contact-inhibited) 3T3 fibroblasts to verify a correlation with cell growth. LDs were characterized by Nile red staining, positivity to adipophilin and negativity to perilipin. LDs were numerous in proliferating cells, but very few in quiescent cells. However, the fraction of quiescent cells, which resumed proliferation after scratch-wound assay, also resumed the formation of LDs. In proliferating cells, the number of LDs correlated with the DNA content, suggesting a continuous accumulation of LDs during cell growth. These findings were supported by biochemical data showing much higher rates of cholesterol esterification and triglyceride synthesis in proliferating cells. Both filipin staining and the fluorescent cholesterol analog dehydroergosterol revealed the presence of an intense traffic of free cholesterol, mediated by acidic vesicles, in proliferating cells. Nile red ratiometric measurements revealed a different lipid composition of LDs in proliferating and quiescent cells. Changes in the number and composition of LDs were also found in growing cells treated with inhibitors of cholesterol esterification (Sandoz 58-035), endosomal cholesterol efflux (U18666A) and V-ATPase (bafilomycin-A1).

Wild-type ApoA-I and the Milano variant have similar abilities to stimulate cellular lipid mobilization and efflux

OBJECTIVE

The present study is a comparative investigation of cellular lipid mobilization and efflux to lipid-free human apoA-I and apoA-I(Milano), reconstituted high-density lipoprotein (rHDL) particles containing these proteins and serum isolated from mice expressing human apoA-I or apoA-I(Milano).

METHODS AND RESULTS

Cholesterol and phospholipid efflux to these acceptors was measured in cell systems designed to assess the contributions of ATP-binding cassette A1 (ABCA1), scavenger receptor type BI (SRBI), and cellular lipid content to cholesterol and phospholipid efflux. Acceptors containing the Milano variant of apoA-I showed no functional increase in lipid efflux in all assays when compared with wild-type apoA-I. In fact, in some systems, acceptors containing the Milano variant of apoA-I promoted significantly less efflux than the acceptors containing wild-type apoA-I (apoA-I(wt)). Additionally, intracellular cholesteryl ester hydrolysis in macrophage foam cells was not different in the presence of either apoA-I(Milano) or apoA-I(wt).

CONCLUSION

Collectively these studies suggest that if the Milano variant of apoA-I offers greater atheroprotection than wild-type apoA-I, it is not attributable to greater cellular lipid mobilization.

Dietary fat-induced alterations in atherosclerosis are abolished by ACAT2-deficiency in ApoB100 only, LDLr-/- mice

OBJECTIVES

The enzyme acyl-coenzymeA (CoA):cholesterol O-acyltransferase 2 (ACAT2) in the liver synthesizes cholesteryl esters (CE) from cholesterol and fatty acyl-CoA, which get incorporated into apoB-containing lipoproteins that are secreted into the bloodstream. Dietary fatty acid composition influences the amount and fatty acid composition of CE within apoB-containing lipoproteins. We hypothesized that when ACAT2 activity is removed by gene deletion, hepatic CE synthesis and secretion would be minimal and, as a result, dietary fat-related differences in atherosclerosis would be eliminated.

METHODS AND RESULTS

Groups of female apoB100 only, LDLr-/- mice with and without ACAT2 were fed diets enriched in either omega-3 or omega-6 polyunsaturated fat, saturated fat, and cis or trans monounsaturated fat. After 20 weeks on diet, mice fed diets enriched in monounsaturated or saturated fat exhibited significantly higher amounts of plasma cholesterol, larger LDL particles enriched in monounsaturated CE, and more atherosclerosis than mice fed polyunsaturated fat. The dietary fat-induced shifts in plasma cholesterol, LDL size, LDL CE composition, and atherosclerosis were not observed in ACAT2-/- mice. Regardless of the diet fed, the ACAT2-/- mice were protected from atherosclerosis.

CONCLUSIONS

The results indicate that in apoB100 only, LDLr-/- mice, ACAT2 plays an essential role in facilitating dietary fat type-specific atherosclerosis through its various effects on plasma lipoprotein concentration and composition.

Redistribution of macrophage cholesteryl ester hydrolase from cytoplasm to lipid droplets upon lipid loading

Hydrolysis of intracellular cholesteryl esters (CEs) represents the first step in the removal of cholesterol from lipid-laden foam cells associated with atherosclerotic lesions. Neutral cholesteryl ester hydrolase (CEH) catalyzes this reaction, and we recently cloned the cDNA for the human macrophage CEH and demonstrated increased mobilization of intracellular CE droplets by CEH overexpression. The present study was undertaken to test the hypothesis that for CE hydrolysis, CEH must become associated with the surface of the cytoplasmic lipid droplets. Our data show the redistribution of CEH from cytosol to lipid droplets upon lipid loading of human THP-1 macrophages. Depletion of triacylglycerol (TG) by incubation with the acyl-CoA synthetase inhibitor Triacsin D had no effect on CEH association with the lipid droplets, suggesting that CEH associates with mixed (CE + TG) as well as TG-depleted CE droplets. However, CEH had 2.5-fold higher activity when mixed droplets were used as substrate in an in vitro assay, consistent with the reported higher cholesterol efflux from cells containing mixed isotropic droplets. Perilipin as well as adipophilin, two lipid droplet-associated proteins, were also present on the lipid droplets in THP-1 macrophages. In conclusion, CEH associates with its intracellular substrate (lipid droplets) and hydrolyzes CE more efficiently from mixed droplets.

Effects of LDL enriched with different dietary fatty acids on cholesteryl ester accumulation and turnover in THP-1 macrophages

LDL enriched with either saturated, monounsaturated, n-6 polyunsaturated, or n-3 polyunsaturated fatty acids were used to study the effects of dietary fatty acids on macrophage cholesteryl ester (CE) accumulation, physical state, hydrolysis, and cholesterol efflux. Incubation of THP-1 macrophages with acetylated LDL (AcLDL) from each of the four diet groups resulted in both CE and triglyceride (TG) accumulation, in addition to alterations of cellular CE, TG, and phospholipid fatty acyl compositions reflective of the individual LDLs. Incubation with monounsaturated LDL resulted in significantly higher total and CE accumulation when compared with the other groups. After TG depletion, intracellular anisotropic lipid droplets were visible in all four groups, with 71% of the cells incubated with monounsaturated AcLDL containing anisotropic lipid droplets, compared with 30% of cells incubated with n-3 AcLDL. These physical state differences translated into higher rates of both CE hydrolysis and cholesterol efflux in the n-3 group. These data suggest that monounsaturated fatty acids may enhance atherosclerosis by increasing both cholesterol delivery to macrophage foam cells and the percentage of anisotropic lipid droplets, while n-3 PUFAs decrease atherosclerosis by creating more fluid cellular CE droplets that accelerate the rate of CE hydrolysis and the efflux of cholesterol from the cell.

Transgenic overexpression of human lecithin: cholesterol acyltransferase (LCAT) in mice does not increase aortic cholesterol deposition

Results from several atherosclerosis studies using morphometric procedures have proven controversial with regard to whether over-expression of human LCAT in transgenic (Tg) mice is atherogenic. The purpose of the present study was to determine the effect of 10-fold over-expression of human LCAT on aortic free and esterified cholesterol (EC) deposition as well as plasma lipoprotein cholesteryl ester (CE) fatty acid composition in mice fed an atherogenic diet containing cholic acid. C57Bl/6 (control) and human LCAT-Tg mice were fed chow or an atherogenic diet (15% of calories from palm oil, 1.0% cholesterol and 0.5% cholic acid) for 24 weeks before measurement of aortic cholesterol content. Compared with the chow diet, control and LCAT-Tg mice fed the atherogenic diet had a 2-fold increase in plasma total, free and EC, a 7-fold increase in plasma apoB lipoprotein cholesterol, and a 40-50-fold increase in hepatic cholesterol content. The aortic EC content was increased in control (0.7 vs. 1.2 mg/g protein) and LCAT-Tg (0.3 vs. 1.5 mg/g protein) mice fed the atherogenic diet compared with those consuming the chow diet; however, there was no difference in aortic free (14.4+/-6.8 vs. 18.5+/-7.7 mg/g protein) or esterified (1.2+/-1.0 vs. 1.5+/-1.2 mg/g protein) cholesterol content between atherogenic diet-fed control and LCAT-Tg mice, respectively. LCAT-Tg mice fed the atherogenic diet had a 2-fold increase in the ratio of saturated+monounsaturated to polyunsaturated CE species in plasma apoB lipoproteins compared with control mice (9.4+/-2.4 vs. 4.9+/-0.7). We conclude that over-expression of human LCAT in Tg mice fed an atherogenic diet containing cholic acid does not result in increased aortic cholesterol deposition compared with control mice, even though the CE fatty acid saturation index of plasma apoB lipoproteins was doubled.

Lecithin:cholesterol acyltransferase deficiency increases atherosclerosis in the low density lipoprotein receptor and apolipoprotein E knockout mice

The purpose of the present study was to test the hypothesis that lecithin:cholesterol acyltransferase (LCAT) deficiency would accelerate atherosclerosis development in low density lipoprotein (LDL) receptor (LDLr-/-) and apoE (apoE-/-) knockout mice. After 16 weeks of atherogenic diet (0.1% cholesterol, 10% calories from palm oil) consumption, LDLr-/- LCAT-/- double knockout mice, compared with LDLr-/- mice, had similar plasma concentrations of free (FC), esterified (EC), and apoB lipoprotein cholesterol, increased plasma concentrations of phospholipid and triglyceride, decreased HDL cholesterol, and 2-fold more aortic FC (142 +/- 28 versus 61 +/- 20 mg/g protein) and EC (102 +/- 27 versus 61+/- 27 mg/g). ApoE-/- LCAT-/- mice fed the atherogenic diet, compared with apoE-/- mice, had higher concentrations of plasma FC, EC, apoB lipoprotein cholesterol, and phospholipid, and significantly more aortic FC (149 +/- 62 versus 109 +/- 33 mg/g) and EC (101 +/- 23 versus 69 +/- 20 mg/g) than did the apoE-/- mice. LCAT deficiency resulted in a 12-fold increase in the ratio of saturated + monounsaturated to polyunsaturated cholesteryl esters in apoB lipoproteins in LDLr-/- mice and a 3-fold increase in the apoE-/- mice compared with their counterparts with active LCAT. We conclude that LCAT deficiency in LDLr-/- and apoE-/- mice fed an atherogenic diet resulted in increased aortic cholesterol deposition, likely due to a reduction in plasma HDL, an increased saturation of cholesteryl esters in apoB lipoproteins and, in the apoE-/- background, an increased plasma concentration of apoB lipoproteins.

Correlation of serum triglyceride and its reduction by omega-3 fatty acids with lipid transfer activity and the neutral lipid compositions of high-density and low-density lipoproteins

Serum triglyceride (TG) and high-density lipoprotein cholesterol (HDL-C) concentrations are inversely correlated and mechanistically linked by means of lipid transfer activities. Phospholipid transfer activity (PLTA) moves phospholipids among serum lipoproteins; cholesteryl ester transfer activity (CETA), which exchanges cholesteryl esters (CE) and TG among lipoproteins, is stimulated by nonesterified fatty acids (NEFA). The aims of this study were (a) to develop a quantitative model that correlates the neutral lipid (NL = CE + TG) compositions of HDL and LDL with serum TG concentration; (b) identify the serum lipid determinants of CETA and PLTA, and; (c) identify the effects of serum TG reductions on the neutral lipid compositions of HDL and LDL, serum NEFA concentrations, and on PLTA and CETA. These aims were addressed in 40 hypertriglyceridemic subjects before and after treatment with an 85% concentrate of omega-3 fatty acids (Omacor) and in 16 untreated normolipidemic subjects. In vivo, the NL compositions of LDL and HDL were described by a mathematical model having the form of adsorption isotherms: HDL - (TG/NL) = (0.90 +/- 0.07) serum TG/(7.0 +/- 1.2 mmol/l + serum TG) and LDL - (TG/NL) = (0.65 +/- 0.08) serum TG/(4.9 +/- 1.5 mmol/l + serum TG). Reduction of serum TG was associated with reductions in HDL - (TG/NL), serum NEFA concentration, and serum CETA but not PLTA. These data suggest that both hypertriglyceridemia and the attendant elevated serum CETA but not PLTA are determinants of HDL and LDL composition and structure and that serum TG concentrations are good predictors of the NL compositions of HDL and LDL.

Dietary monounsaturated fatty acids promote aortic atherosclerosis in LDL receptor-null, human ApoB100-overexpressing transgenic mice

In mice with genetically engineered high levels of plasma low density lipoprotein (LDL), we tested the hypothesis that an increase in the dietary content of monounsaturated fatty acids but not of polyunsaturated fatty acids would promote atherosclerosis. The mouse model used was an LDL receptor-null, human apoB100-overexpressing strain. Six experimental groups of 19 to 38 mice of both sexes were established when the animals had reached 8 weeks of age. For the next 16 weeks, individual groups were fed either a commercial diet or prepared diets including fat as 10% of energy, with 5 different fatty acid enrichment patterns including the following: saturated (sat), cis and trans monounsaturated (mono), and n-3 and n-6 polyunsaturated (poly). Highly significant differences (ANOVA, P<0. 0001) in LDL cholesterol (in mg/dL) were found, with the rank order at 16 weeks being trans mono (mean, 1390)>sat (922)=cis mono (869)=n-6 poly (868)>n-3 poly (652)>commercial diet (526). Significant elevations in very low density lipoprotein cholesterol were also found in the trans and cis mono and sat groups, and triacylglycerol concentrations were also elevated in all groups. High density lipoprotein cholesterol concentrations were consistently low (20 to 50 mg/dL) in all groups. Highly significant differences (ANOVA, P<0.0001) in atherosclerosis, quantified by measurement of aortic cholesteryl ester concentration (mg/g protein) among dietary fatty acid groups were found, with the order being trans mono (mean, 50.4)>sat (35.6)=cis mono (34.6)>n-6 poly (18. 3)=n-3 poly (9.7)=commercial diet (7.8). Therefore, in this mouse model of hypercholesterolemia, dietary cis or trans monounsaturated fat did not protect against atherosclerosis development, whereas aortic atherosclerosis in either of the polyunsaturated fat groups was significantly less than in the saturated fat group.

Degradation of low density lipoprotein cholesterol esters by lysosomal lipase in vitro. Effect of core physical state and basis of species selectivity

The effect of the physical state of low density lipoprotein (LDL) core and the selectivity of the degradation of LDL cholesterol esters (CEs) by the lysosomal acid lipase (LAL) in vitro were investigated. The physical state of LDL was modulated by varying temperature or the triglyceride content of the core. Normal LDL showed an abrupt increase of CE hydrolysis at 24 degrees C and another deviation occurred close to 36 degrees C. 1H-NMR measurements showed that these temperatures coincide with the onset and end temperatures of the LDL core lipid transition, respectively. Enrichment of LDL with triglycerides abolished the abrupt changes both in the CE hydrolysis and in the physical state of LDL lipids. These findings show that there is a correlation between the physical state of LDL lipids and the rate of LAL-mediated hydrolysis of the CEs in the particle. The relative rates of hydrolysis of different CE species were also compared. With native LDL, increasing the length of a saturated acyl chain from 14 to 20 carbons reduced the rate of degradation of CE modestly, while increasing acyl chain unsaturation increased the rate of degradation markedly. However, cholesterol oleate was hydrolyzed more slowly than cholesterol stearate. Essentially the same order of hydrolytic susceptibility was observed when the CE species were incorporated into triglyceride-enriched LDL, reconstituted high density lipoprotein particles or in detergent/phospholipid micelles. These results indicate that the selective hydrolysis of CE species in LDL is determined mainly by the ease with which the CE molecule can emerge from the surface layer reach the active site of LAL. Slower degradation of the more saturated CEs by LAL could lead, under certain conditions, to their accumulation in lysosomes and eventually, to cell death, lysis and deposition of crystalline, poorly mobilizable lipids to the arterial intima.

Increased cellular triglyceride levels in human monocytic and rat smooth muscle cells after lovastatin

Beta-hydroxy-beta-methyl-glutaryl-coenzyme A (HMG-CoA) reductase inhibitors reduce plasma LDL cholesterol by upregulating hepatic LDL receptors. However, their effects on lipid metabolism in extrahepatic cells may also contribute to their therapeutic benefit. We examined the effects of lovastatin (LOV) on cellular lipid levels in the human monocytic Mono Mac 6sr and cultured rat smooth muscle cells. In both cell types, LOV produced a dose-dependent increase in cellular triglycerides. This increase was observed in cells grown in the absence of exogenous lipids in the culture medium, but was more pronounced after additions of oleic acid (50 to 200 microM) and VLDL (50 to 200 microg ml-1). In Mono Mac 6sr cells grown in medium containing 10% delipidated FCS for the last 16 h, the LOV-induced rise in triglyceride levels was completely reversed by 2 mM mevalonic acid and was associated with a decrease in cellular cholesterol. However, when cells were maintained in lipoprotein-replete medium, the LOV-induced rise in triglycerides did not correlate with cellular cholesterol. LOV also reduced cellular cholesterol esterification and increased the synthesis of fatty acids and their incorporation into triglycerides and phospholipids. Increased triglyceride levels were also seen in Mono Mac 6sr cells treated with the lanosterol demethylase inhibitor RS-21607 and the acylcoenzyme A:cholesterol acyltransferase inhibitor SaH 58035. Our findings suggest that the LOV-induced triglyceride accumulation involves changes in intracellular cholesterol pools regulating cellular fatty acid concentrations. Although decreased cholesterol levels in cells participating in plaque formation are beneficial, the impact of the herein described shift in intracellular neutral lipid metabolism on other cellular functions warrants further investigation.

Lipoproteins and cellular cholesterol homeostasis

Cholesterol homeostasis in peripheral cells involves a balance between the influx and efflux processes. The acquisition of cholesterol by such cells is mediated by a variety of receptor and non-receptor processes involving both normal and modified lipoproteins. The offsetting efflux process is mediated by HDL and especially particles containing only apo A-I. An efficient reverse cholesterol transport by HDL of cholesterol from peripheral cells to the liver protects against the development of atherosclerosis. In cells that do not contain excess cholesterol, the cholesterol is distributed as unesterified cholesterol molecules between the plasma membrane and the membranes of the intracellular organelles. In cholesterol-loaded cells such as macrophage foam cells, the membranes became enriched in unesterified cholesterol and, in addition, cytoplasmic CE droplets and lysosomal cholesterol crystals can form. The ways in which cholesterol molecules move between intracellular sites and the plasma membrane to become available for efflux to extracellular acceptor particles are becoming known. Cholesterol molecules in the plasma membrane can desorb and diffuse through the aqueous phase and be sequestered by HDL particles. The cell cholesterol available for efflux can exist in different kinetic pools, and these pools, such as those in various domains in the plasma membrane, require further definition. The cholesterol molecules present in intracellular pools also efflux with different kinetics and by different pathways. Thus, newly synthesized cholesterol is actively transported by a vesicle system from the ER to the plasma membrane, whereas lysosomal cholesterol seems to be transported to the plasma membrane by a protein-mediated, diffusional process. Clearance of cytoplasmic CE is dependent upon the rate of turnover of the CE cycle and the magnitude of the cholesterol gradient between the plasma membrane and the extracellular acceptor particle. It can be expected that the interdependence of the pathways and the molecular mechanisms underlying the intracellular trafficking of cholesterol will be elucidated in the near future.

Cellular cholesterol efflux mediated by cyclodextrins. Demonstration Of kinetic pools and mechanism of efflux

The efflux of cholesterol from cells in culture to cyclodextrin acceptors has been reported to be substantially more rapid than efflux induced by other known acceptors of cholesterol (Kilsdonk, E. P. C., Yancey, P., Stoudt, G., Bangerter, F. W., Johnson, W. J., Phillips, M. C., and Rothblat, G. H. (1995) J. Biol. Chem. 270, 17250-17256). In this study, we compared the kinetics of cholesterol efflux from cells with 2-hydroxypropyl-beta-cyclodextrins and with discoidal high density lipoprotein (HDL) particles to probe the mechanisms governing the remarkably rapid rates of cyclodextrin-mediated efflux. The rate of cholesterol efflux was enhanced by shaking cells growing in a monolayer and further enhanced by placing cells in suspension to achieve maximal efflux rates. The extent of efflux was dependent on cyclodextrin concentration, and maximal efflux was observed at concentrations >50 mM. For several cell types, biexponential kinetics of cellular cholesterol efflux were observed, indicating the existence of two kinetic pools of cholesterol: a fast pool (half-time (t1/2) approximately 19-23 s) and a slow pool with t1/2 of 15-30 min. Two distinct kinetic pools of cholesterol were also observed with model membranes (large unilamellar cholesterol-containing vesicles), implying that the cellular pools are in the plasma membrane. Cellular cholesterol content was altered by incubating cells with solutions of cyclodextrins complexed with increasing levels of cholesterol. The number of kinetic pools was unaffected by raising the cellular cholesterol content, but the size of the fast pool increased. After depleting cells of the fast pool of cholesterol, this pool was completely restored after a 40-min recovery period. The temperature dependence of cyclodextrin-mediated cholesterol efflux from cells and model membranes was compared; the activation energies were 7 kcal/mol and 2 kcal/mol, respectively. The equivalent activation energy observed with apo-HDL-phospholipid acceptor particles was 20 kcal/mol. It seems that cyclodextrin molecules are substantially more efficient than phospholipid acceptors, because cholesterol molecules desorbing from a membrane surface can diffuse directly into the hydrophobic core of a cyclodextrin molecule without having to desorb completely into the aqueous phase before being sequestered by the acceptor.

Cell toxicity induced by inhibition of acyl coenzyme A:cholesterol acyltransferase and accumulation of unesterified cholesterol

Considerable evidence supports the involvement of acyl-CoA:cholesterol acyltransferase (ACAT) in the maintenance of intracellular cholesterol homeostasis. A number of recently developed ACAT inhibitors may have potential use as pharmacological agents to reduce the development of atherosclerosis. Recently, however, reports arose describing cytotoxic effects following administration of a specific ACAT inhibitor to experimental animals. In order to address the specific intracellular mechanisms involved with the cytotoxic effect, we examined the consequences of ACAT inhibition in cholesterol-enriched mouse peritoneal macrophages. Mouse peritoneal macrophages were cholesterol-enriched by incubation with acetylated low density lipoprotein and free cholesterol:phospholipid dispersions prior to the addition of an ACAT inhibitor, either Sandoz 58-035 or Pfizer CP-113,818. The adenine pool of the macrophages was radiolabeled prior to addition of the ACAT inhibitors, in order to monitor the release of radiolabeled adenine, a technique shown to be a sensitive method to monitor drug-induced toxicity. The ACAT inhibitors were added for up to 48 h and at concentrations up to 2 micrograms/ml. These conditions resulted in an approximately 2-fold increase in adenine release. The increase in cell toxicity paralleled an increase in the cellular free cholesterol content. Reducing the cellular free cholesterol content, by the addition of extracellular acceptors, decreased the cytotoxic effects of the ACAT inhibitors. Addition of an intracellular cholesterol transport inhibitor, either progesterone or U18666A, together with CP-113,818 blocked the toxic effect of CP-113,818. These results suggest that ACAT inhibition of cholesterol-enriched macrophages increases cell toxicity due to the buildup of cellular free cholesterol. Removal of free cholesterol by the addition of extracellular cholesterol acceptors or by blocking intracellular sterol transport relieves the ACAT inhibitor-induced toxicity.

Efflux of cholesterol from cholesterol loaded macrophages by incubation with synthetic HDL-particles

Cells from the mouse monocyte/macrophage cell line J774A.1 were incubated with acetylated human low density lipoprotein for 2 days, resulting in an intracellular accumulation of mainly cholesteryl esters. These in vitro foam cell models were used to study the capability of synthetic HDL-particles to promote efflux of cholesterol. The synthetic HDL-particles were prepared from recombinant human pro-apolipoprotein A-I or human apolipoprotein A-I and phosphatidylcholine. Both types of reconstituted complexes were found to have a discoidal structure. A 24 h incubation of lipid loaded J774A.1 cells with these two types of discoidal complexes resulted in an equivalent and marked egress of cholesterol. The effect was the same whether the origin of phosphatidylcholine was egg yolk or soybean.

Molecular motions and thermotropic phase behavior of triacylglycerols and cholesteryl esters in herpesvirus-infected arterial smooth muscle cells: a deuterium nuclear magnetic resonance study

The physical state of lipids in arterial smooth muscle cells (SMC) may contribute to lipid accumulation following injury. We have previously demonstrated that herpes simplex virus (HSV) infection alters the physical state of the neutral lipid accumulating in arterial SMC, as determined by differential scanning calorimetry (Biochem. J. 268 (1990) 693-697). To more precisely determine the molecular packing of neutral lipids in HSV-infected cells, the influence of HSV-infection on the thermotropic and phase-behavior of the lipids in intact arterial smooth muscle cells and in cell-free lipid extracts was evaluated using [2H]-NMR, employing U-[2H]-oleic acid incorporated into cells. Inspection of the [2H]-line-widths indicate that the lipid of HSV-infected cells exhibited more restricted motion or a greater chemical shift dispersity than lipids from uninfected cells, as evidenced by significant broadening of the -CD = CD- signals at 25 degrees C and 45 degrees C. Fatty acid compositional analysis of the neutral lipids of control and HSV-infected cells following C18:1 supplementation (an amount added similar to the NMR experiments) reveals that: (1) there is approximately 55-fold more triacylglycerols (TG) than cholesteryl esters (CE) in control cells and 40-fold more TG than CE in the HSV-infected cells; (2) HSV infection significantly increases the C18:1 content of CE, and C18:3 and C20:4 in TG; and (3) HSV-infection does not alter the ratio of TG to CE. These data support the hypothesis that the greater restriction of the neutral lipids in HSV-infected cells may be due to the rigidifying effects of C18:1 on lipid mobility. Thus, alterations in the physical state of neutral lipids in HSV-infected cells may lead to reduced CE hydrolysis which, in turn, may contribute to or exacerbate lipid accumulation.

Cholesterol transport between cells and high-density lipoproteins

Various types of studies in humans and animals suggest strongly that HDL is anti-atherogenic. The anti-atherogenic potential of HDL is thought to be due to its participation in reverse cholesterol transport, the process by which cholesterol is removed from non-hepatic cells and transported to the liver for elimination from the body. Extensive studies in cell culture systems have demonstrated that HDL is an important mediator of sterol transport between cells and the plasma compartment. The topic of this review is the mechanisms that account for sterol movement between HDL and cells. The most prominent and easily measured aspect of sterol movement between HDL and cells is the rapid bidirectional transfer of cholesterol between the lipoprotein and the plasma membrane. This movement occurs by unmediated diffusion, and in most situations its rate in each direction is limited by the rate of desorption of sterol molecules from the donor surface into the adjacent water phase. The net transfer of sterol mass out of cells occurs when there is either a relative enrichment of sterol within the plasma membrane or a depletion of sterol in HDL. Recent studies suggest that certain minor subfractions of HDL (with pre-beta mobility on agarose gel electrophoresis and containing apoprotein A-I but no apo A-II) are unusually efficient at promoting efflux of cell sterol. To what extent efflux to these HDL fractions is balanced by influx from the lipoprotein has not yet been established clearly. The prevention and reversal of atherosclerosis require the mobilization of cholesterol from internal (non-plasma membrane) cellular locations. To some extent, this may involve the retroendocytosis of HDL. However, most mobilization probably involves the transport of internal sterol to the plasma membrane, followed by desorption to extracellular HDL. Several laboratories are investigating the transport of sterol from intracellular locations to the plasma membrane. Studies on biosynthetic sterol (probably originating mostly in the smooth endoplasmic reticulum) suggest that there is rapid transport to the plasma membrane in lipid-rich vesicles. Important features of this transport are that it bypasses the Golgi apparatus and may be positively regulated by the specific binding of HDL to the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of cholesteryl ester lipid droplets in a J774 macrophage foam cell model

J774 macrophages rapidly incorporated [3H]cholesteryl oleate droplets by a non-saturable phagocytic process. In less than 2 h, foam cell morphology was acquired. The extent of loading obtained after 2 h was a linear function of the mass of cholesteryl oleate provided to the cells. The cholesteryl oleate incorporated was hydrolyzed in the cells at a linear rate over 24 h and the fractional hydrolysis was constant over a wide range of cellular esterified cholesterol contents. The rate of hydrolysis was influenced by the physical state of the cholesteryl ester; cholesteryl oleate in isotropic droplets was hydrolyzed 2-3-fold more rapidly than cholesteryl oleate in anisotropic droplets. The hydrolysis of both types of droplets was inhibited by lysosomotropic agents, indicating that hydrolysis occurred in the lysosomes. Only a small fraction (less than 10% after 24 h) of the free [3H]cholesterol generated in the lysosomes was esterified by ACAT resulting in a doubling of the cell free cholesterol content. Electron microscopy of cells treated with digitonin revealed the accumulation of free cholesterol in lipid-laden lysosomes. ACAT was active as endogenous free [14C]cholesterol was esterified in a linear manner over 24 h and was responsive to the presence of lysosomally-derived cholesterol, as the extent of esterification of the endogenous pool was directly proportional to the mass of [3H]cholesterol generated in the lysosomes.

Oxysterol mediated changes in fatty acid distribution and lipid synthesis in cultured bovine aortic smooth muscle cells

We studied the actions of oxysterols on fatty acid distribution and lipid synthesis in cultured bovine aortic smooth muscle cells. Cultures were labeled with [1-14C]arachidonate or [1-14C]oleate. During a 24-hr incubation, 25- or 22R-hydroxycholesterol enhanced the incorporation of label into triglycerides, concomitant with a reduction in the labeling of phospholipids. Cholestantriol or 20-hydroxycholesterol had the opposite effects. They caused a higher incorporation of radiolabel into phospholipids and a reduction of labeling of triacylglycerols. Similar changes were seen in cells labeled with [1-14C]acetate. Therefore, we conclude that oxysterols can promote changes in the distribution of fatty acids between neutral lipids and phospholipids through mechanisms that still need to be clarified.

Effect of substrate physical state on the activity of acid cholesteryl ester hydrolase

The effects of the physicochemical properties of the substrate vehicle on the activity of acid cholesteryl ester hydrolase (ACEH; EC 3.1.1.13) isolated from rat liver lysosomes have been studied. In particular, the influence of the physical state of the neutral lipid core of substrate emulsion particles on the enzymatic activity has been probed in the light of previous studies on the clearance of cholesteryl esters (CE) from lipid-loaded cells which indicated that inclusions that are in the isotropic (liquid) state can be hydrolyzed faster than those in the anisotropic (liquid-crystalline) state. In the present study, such lipid inclusions were isolated from cultured cells and used as substrates for the hydrolase. No appreciable difference between the hydrolysis rates of isotropic and anisotropic inclusions was observed; the Vmax values were 93.0 +/- 6.7 and 84.0 +/- 3.3 nmol CE/mg.h, respectively. To elucidate the factors which affect the activity of ACEH, model inclusions were prepared by sonication and used as substrates. The physical state of these models was varied in a systematic way by changes of droplet composition and incubation temperature. The rate of hydrolysis was found to be insensitive to the physical state of the core of the model inclusions in good agreement with the results obtained with cellular inclusions. However, the activity of ACEH is sensitive to such interfacial properties of the lipid droplets as surface area available to the enzyme, net surface charge and surface solubility of the substrate CE molecules. The enzymatic activity is also sensitive to the amount of free cholesterol present in the emulsion droplets. The interfacial concentration and molecular packing of substrate CE molecules in the droplet surface significantly affect the hydrolytic activity of ACEH.

Influence of high density lipoprotein on esterified cholesterol stores in macrophages and hepatoma cells

The ability of high density lipoproteins (HDL) to induce the clearance of cholesteryl esters from cultured cells has been explored. Studies using the J774 mouse macrophage cell line showed that these cells are not stimulated to clear esterified cholesterol upon exposure to HDL. This was observed over a wide range of HDL concentrations (10 to 1000 micrograms/ml HDL protein), and the lack of stimulation was not influenced by a number of factors relating to the preparation of the HDL, such as HDL subfraction, varying extents of lecithin:cholesterol acyltransferase modification, or heparin-Sepharose chromatography to remove particles containing apo E. Neither the method of loading the cells with esterified cholesterol nor the physical state of the lipid droplets affected the inability of HDL to elicit esterified cholesterol clearance. In the presence of the acyl CoA:cholesterol acyltransferase inhibitor, Sandoz 58-035, where a high level of intracellular free cholesterol was generated, efflux of only a small fraction of the excess free cholesterol to HDL was observed. J774 cells were able to clear esterified cholesterol efficiently in the presence of cholesterol-free apolipoprotein HDL/phospholipid particles, indicating that the cells have the capacity to clear esterified cholesterol. Fu5AH hepatoma cells and P388.D1 mouse macrophage cells also failed to clear esterified cholesterol in response to HDL. In contrast, mouse peritoneal macrophages cleared esterified cholesterol efficiently to HDL, indicating that there are fundamental differences between mouse peritoneal macrophages and the other cells types studied in regard to cholesterol metabolism as influenced by HDL.

Limitations of the lipid state hypothesis for atherosclerosis are revealed by X-ray diffraction measurements

The lipid state hypothesis proposes that liquid crystalline states of cholesteryl esters play a role in the development and persistence of the fatty streak lesions characteristic of atherosclerosis. We have tested several corollaries suggested by this hypothesis and find that the ensemble of droplets in atherosclerotic tissue are predominantly in the isotropic (fluid) state at 37.0 degrees C. Furthermore, the liquid-crystalline state transition behavior of these droplets is not influenced significantly by the distribution of component cholesteryl ester species. There are no significant correlations between the transition behavior of the droplets and the age, sex, or race of the subjects from which tissue samples were taken. These results show that the lipid state hypothesis is weak, and that the origin and persistence of fatty streak lesions in humans is probably dominated by other factors.