Endocytosed beta-VLDL and LDL are delivered to different intracellular vesicles in mouse peritoneal macrophages

Cholesterol transfer at the plasma membrane

Cholesterol homeostasis is of central importance for life. Therefore, cells have developed a divergent set of pathways to meet their cholesterol needs. In this review, we focus on the direct transfer of cholesterol from lipoprotein particles to the cell membrane. More molecular details on the transfer of lipoprotein-derived lipids were gained by recent studies using phospholipid bilayers. While amphiphilic lipids are transferred right after contact of the lipoprotein particle with the membrane, the transfer of core lipids is restricted. Amphiphilic lipid transfer gains special importance in genetic diseases impairing lipoprotein metabolism like familial hypercholesterolemia. Taken together, these data indicate that there is a constant exchange of amphiphilic lipids between lipoprotein particles and the cell membrane.

Synthesis and Live-Cell Imaging of Fluorescent Sterols for Analysis of Intracellular Cholesterol Transport

Cellular cholesterol homeostasis relies on precise control of the sterol content of organelle membranes. Obtaining insight into cholesterol trafficking pathways and kinetics by live-cell imaging relies on two conditions. First, one needs to develop suitable analogs that resemble cholesterol as closely as possible with respect to their biophysical and biochemical properties. Second, the cholesterol analogs should have good fluorescence properties. This interferes, however, often with the first requirement, such that the imaging instrumentation must be optimized to collect photons from suboptimal fluorophores, but good cholesterol mimics, such as the intrinsically fluorescent sterols, cholestatrienol (CTL) or dehydroergosterol (DHE). CTL differs from cholesterol only in having two additional double bonds in the ring system, which is why it is slightly fluorescent in the ultraviolet (UV). In the first part of this protocol, we describe how to synthesize and image CTL in living cells relative to caveolin, a structural component of caveolae. In the second part, we explain in detail how to perform time-lapse experiments of commercially available BODIPY-tagged cholesterol (TopFluor-cholesterol^®; TF-Chol) in comparison to DHE. Finally, using two-photon time-lapse imaging data of TF-Chol, we demonstrate how to use our imaging toolbox SpatTrack for tracking sterol rich vesicles in living cells over time.

Very-low and low-density lipoproteins induce neutral lipid accumulation and impair migration in monocyte subsets

Blood monocytes are heterogeneous effector cells of the innate immune system. In circulation these cells are constantly in contact with lipid-rich lipoproteins, yet this interaction is poorly characterised. Our aim was to examine the functional effect of hyperlipidaemia on blood monocytes. In the Ldlr^−/− mouse monocytes rapidly accumulate cytoplasmic neutral lipid vesicles during hyperlipidaemia. Functional analysis in vivo revealed impaired monocyte chemotaxis towards peritonitis following high fat diet due to retention of monocytes in the greater omentum. In vitro assays using human monocytes confirmed neutral lipid vesicle accumulation after exposure to LDL or VLDL. Neutral lipid accumulation did not inhibit phagocytosis, endothelial adhesion, intravascular crawling and transmigration. However, lipid loading led to a migratory defect towards C5a and disruption of cytoskeletal rearrangement, including an inhibition of RHOA signaling. These data demonstrate distinct effects of hyperlipidaemia on the chemotaxis and cytoskeletal regulation of monocyte subpopulations. These data emphasise the functional consequences of blood monocyte lipid accumulation and reveal important implications for treating inflammation, infection and atherosclerosis in the context of dyslipidaemia.

Specific cellular incorporation of a pyrene-labelled cholesterol: lipoprotein-mediated delivery toward ordered intracellular membranes

In the aim of testing tools for tracing cell trafficking of exogenous cholesterol, two fluorescent derivatives of cholesterol, 22-nitrobenzoxadiazole-cholesterol (NBD-Chol) and 21-methylpyrenyl-cholesterol (Pyr-met-Chol), with distinctive chemico-physical characteristics, have been compared for their cell incorporation properties, using two cell models differently handling cholesterol, with two incorporation routes. In the Caco-2 cell model, the cholesterol probes were delivered in bile salt micelles, as a model of intestinal absorption. The two probes displayed contrasting behaviors for cell uptake characteristics, cell staining, and efflux kinetics. In particular, Pyr-met-Chol cell incorporation involved SR-BI, while that of NBD-Chol appeared purely passive. In the PC-3 cell model, which overexpresses lipoprotein receptors, the cholesterol probes were delivered via the serum components, as a model of systemic delivery. We showed that Pyr-met-Chol-labelled purified LDL or HDL were able to specifically deliver Pyr-met-Chol to the PC-3 cells, while NBD-Chol incorporation was independent of lipoproteins. Observations by fluorescence microscopy evidenced that, while NBD-Chol readily stained the cytosolic lipid droplets, Pyr-met-Chol labelling led to the intense staining of intracellular structures of membranous nature, in agreement with the absence of detectable esterification of Pyr-met-Chol. A 48 h incubation of PC-3 cells with either Pyr-met-Chol-labelled LDL or HDL gave same staining patterns, mainly colocalizing with Lamp1, caveolin-1 and CD63. These data indicated convergent trafficking downwards their respective receptors, LDL-R and SR-BI, toward the cholesterol-rich internal membrane compartments, late endosomes and multivesicular bodies. Interestingly, Pyr-met-Chol staining of these structures exhibited a high excimer fluorescence emission, revealing their ordered membrane environment, and indicating that Pyr-met-Chol behaves as a fair cholesterol tracer regarding its preferential incorporation into cholesterol-rich domains. We conclude that, while NBD-Chol is a valuable marker of cholesterol esterification, Pyr-met-Chol is a reliable new lipoprotein fluorescent marker which allows to probe specific intracellular trafficking of cholesterol-rich membranes.

Group A Streptococcal vaccine candidate: contribution of epitope to size, antigen presenting cell interaction and immunogenicity

Aim: Utilize lipopeptide vaccine delivery system to develop a vaccine candidate against Group A Streptococcus. Materials & methods: Lipopeptides synthesized by solid-phase peptide synthesis-bearing carboxyl (C)-terminal and amino (N)-terminal Group A Streptococcus peptide epitopes. Nanoparticles formed were evaluated in vivo. Results: Immune responses were induced in mice without additional adjuvant. We demonstrated for the first time that incorporation of the C-terminal epitope significantly enhanced the N-terminal epitope-specific antibody response and correlated with forming smaller nanoparticles. Antigen-presenting cells had increased uptake and maturation by smaller, more immunogenic nanoparticles. Antibodies raised by vaccination recognized isolates. Conclusion: Demonstrated the lipopeptidic nanoparticles to induce an immune response which can be influenced by the combined effect of epitope choice and size.

Homocysteine-mediated cholesterol efflux via ABCA1 and ACAT1 DNA methylation in THP-1 monocyte-derived foam cells

Homocysteine (Hcy) has been recognized as a prevalent risk factor for cardiovascular events. Cholesterol-loaded foam cells are a central component of atherosclerotic lesions. ATP-binding cassette transporter A1 (ABCA1), which mediates the efflux of cellular cholesterol and phospholipids, is the rate-limiting step in lipid metabolism. Acyl-coenzyme A:cholesterol acyltransferase-1 (ACAT1) promotes accumulation of cholesterol ester in macrophages, thereby resulting in the foam cell formation, a hallmark of early stage in atherosclerosis. In this study, cultured monocyte-derived foam cells were incubated with clinical relevant concentrations of Hcy for 24 h. Both increased number of foam cells and accumulation of cholesterol were found, and the mRNA and protein expression levels of ABCA1 were decreased, while ACAT1 expression was increased in the presence of Hcy. Furthermore, the DNA methylation level of ABCA1 gene was increased whereas ACAT1 DNA methylation was decreased by using different concentrations of Hcy. Moreover, our results showed that DNA methyltransferase (DNMT) activity and DNA methyltransferase 1 (DNMT1) mRNA expression were increased by Hcy. It is indicated that DNA methylation has the function to regulate the expression of ABCA1 and ACAT1 via DNMT. In conclusion, these results suggest that ABCA1 and ACAT1 DNA methylation induced by Hcy may play a potential role in ABCA1 and ACAT1 expression and the accumulation of cholesterol in monocyte-derived foam cells.

Combined light and electron microscopy using diaminobenzidine photooxidation to monitor trafficking of lipids derived from lipoprotein particles

Diaminobenzidine (DAB) photooxidation is a method for conversion of fluorescent signals into electron-dense precipitates that are visible in the electron microscope. Recently, we have applied this method to analyze organelles involved in holo-high density lipoprotein (HDL) particle uptake at the ultrastructural level. In the present work we extended the spectrum of molecules visualized via photooxidation to monitor the uptake of HDL-derived lipids in HepG2 cells. By the combined light-electron microscopic method and with the aid of the DAB photooxidation technique, it became possible for the first time to visualize different intracellular pathways of lipoprotein particle-derived lipids and analyze the compartments involved at the ultrastructural level. HDL-Alexa 568 was used to visualize holo-HDL particle uptake. Reconstituted HDL particles containing the fluorescent cholesterol analogues Bodipy-cholesterol, Bodipy-cholesteryl oleate, or cholesteryl Bodipy-ester were used to visualize uptake of the HDL-associated sterol. In Bodipy-cholesteryl oleate and cholesteryl Bodipy-ester, the cholesterol moiety or the fatty acid moiety is fluorescently labeled, respectively; in contrast, Bodipy-cholesterol is an analogue of free cholesterol. The cellular compartments involved in their intracellular routes after uptake were analyzed in the fluorescence and electron microscope after DAB photooxidation. Bodipy-cholesterol was found to be localized in tubular endosomes and multivesicular bodies (MVBs), in the trans-Golgi network, and in stacked Golgi cisternae. In contrast, HepG2 cells incubated with HDL containing Bodipy-cholesteryl oleate or cholesteryl Bodipyester gave an uptake pattern comparable to that of holo-HDL particles, with MVBs being involved. Bodipy-cholesteryl oleate was also found in lysosomes. These results indicate that HDL-derived cholesterol and cholesteryl ester are transported by different intracellular pathways in HepG2 cells. Thus, the DAB photooxidation method enables the analysis of intracellular transport of lipoprotein particle-derived lipids at the light and at the ultrastructural level.

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.

Characterization of endocytic compartments after holo-high density lipoprotein particle uptake in HepG2 cells

Holo-high density lipoprotein (HDL) particle uptake, besides selective lipid uptake, constitutes an alternative pathway to regulate cellular cholesterol homeostasis. In the current study, the cellular path of holo-HDL particles was investigated in human liver carcinoma cells (HepG2) using combined light and electron microscopical methods. The apolipoprotein moiety of HDL was visualized with different markers: horseradish peroxidase, colloidal gold and the fluorochrome Alexa(568), used in fluorescence microscopy and after photooxidation correlatively at the ultrastructural level. Time course experiments showed a rapid uptake of holo-HDL particles, an accumulation in endosomal compartments, with a plateau after 1-2 h of continuous uptake, and a clearance 1-2 h upon replacement by unlabeled HDL. Correlative microscopy, using HDL-Alexa(568)-driven diaminobenzidine (DAB) photooxidation, identified the fluorescent organelles as DAB-positive multivesicular bodies (MVBs) in the electron microscope; their luminal contents but not the internal vesicles were stained. Labeled MVBs increased in numbers and changed shapes along with the duration of uptake, from polymorphic organelles with multiple surface domains and differently shaped protrusions dominating at early times of uptake to compact bodies with mainly tubular appendices and densely packed vesicles after later times. Differently shaped and labeled surface domains and appendices, as revealed by three dimensional reconstructions, as well as images of homotypic fusions indicate the dynamics of the HDL-positive MVBs. Double staining visualized by confocal microscopy, along with the electron microscopic data, shows that holo-HDL particles after temporal storage in MVBs are only to a minor degree transported to lysosomes, which suggests that different mechanisms are involved in cellular HDL clearance, including resecretion.

Scavenger receptor, Class B, Type I provides an alternative means for beta-VLDL uptake independent of the LDL receptor in tissue culture

Recent evidence suggests that scavenger receptor, class B, type I (SR-BI) plays a physiological role in VLDL metabolism. SR-BI was reported to mediate beta-VLDL uptake; however, cellular details of this process are not well characterized. In the present study we show that SR-BI delivers cholesterol derived from beta-VLDL to LDL receptor negative SR-BI over-expressing Chinese Hamster Ovarian cells (ldlA7-SRBI). Cell association of beta-VLDL was approximately 3 times higher after SR-BI over-expression, which was competed by beta-VLDL, but only to a lesser extent by HDL and LDL. Almost all of the associated beta-VLDL was located intracellularly, and therefore could not be released by a 50-fold excess of unlabeled beta-VLDL. beta-VLDL was degraded at a rate of 6 ng beta-VLDL/mg cell protein and hour. In contrast to ldlA7 cells, beta-VLDL association was competed by LDL in cells with a functional LDL receptor like CHO and HepG2 cells, indicating a strong impact of the LDL receptor in beta-VLDL uptake. beta-VLDL degradation was similar to ldlA7-SRBI cells. When beta-VLDL uptake was followed using fluorescence microscopy, beta-VLDL showed a different uptake pattern in SR-BI over-expressing cells, ldlA7-SRBI, compared to LDL receptor containing cells, CHO and HepG2.

Photooxidation technology for correlated light and electron microscopy

The combination of the capabilities of light microscopical techniques with the power of resolution of electron microscopy along with technical advances has led to a gradual decline of the gap between classical light and electron microscopy. Among the correlative techniques using the synergistic opportunities, photooxidation methods have been established as valuable tools for visualizing cell structures at both light and electron microscopic level. Fluorescent dyes are used to oxidize the substrate diaminobenzidine, which in its oxidized state forms fine granular precipitates. Stained with osmium, the diaminobenzidine precipitates are well discernible in the electron microscope, thus labelling and defining the cellular structures, which at light microscopy level are recorded by fluorescent probes. The underlying photooxidation reaction is based on the excitation of free oxygen radicals that form upon illumination of fluorochromes; this is a central step in the procedure, which mainly influences the success of the method. This article summarizes basic steps of the technology and progresses, shows efforts and elaborated pathways, and focuses on methodical solutions as to the applicability of different fluorochromes, as well as conditions for fine structural localizations of the reaction products.

The role of calcium in lipoprotein release by the low-density lipoprotein receptor

The LDL receptor (LDLR) mediates efficient endocytosis of VLDL, VLDL remnants, and LDL. As part of the uptake process, the LDLR releases lipoproteins in endosomes. Released lipoproteins are subsequently trafficked to lysosomes for degradation, while the LDLR recycles back to the cell surface for further rounds of uptake. Endosomes have at least two features that can promote lipoprotein release: an acidic pH and low concentrations of free calcium. The relative contributions of acidic pH and low free calcium to lipoprotein release are not known. Here, we generated fibroblasts that express either normal LDLR or an LDLR variant that is unable to employ the acid-dependent release mechanism to determine the relative contributions of acidic pH and low free calcium on lipoprotein release. We show that endosomal concentrations of free calcium can drive lipoprotein release at rates that are similar to those of acid-dependent release and that the calcium-dependent and acid-dependent mechanisms can cooperate during lipoprotein release. Assessment of lipoprotein uptake by these two cell lines showed that LDL uptake requires the acid-dependent mechanism, while uptake of the VLDL remnant, beta-VLDL, does not. We propose that endosomes use both the acid-dependent and calcium-dependent release mechanisms to drive lipoprotein release and that the acid-dependent process is only required for LDL release.

Remnant lipoproteins as strong key particles to atherogenesis

Recent epidemiologic studies have revealed that hypertriglyceridemia is associated with atherosclerosis independent of other coronary risk factors. However, it is difficult to select patients at high risk for coronary artery disease using only serum triglyceride levels compared with low-density lipoprotein cholesterol levels since multiple factors are associated with elevating triglycerides. Atherosclerotic diseases with high triglyceride levels can be found in patients with familial combined hyperlipidemia, diabetes mellitus, and metabolic syndrome, in which remnant lipoproteins accumulate in the circulating blood. Recent researches have paid attention to remnant lipoproteins as atherogenic particles with the development of methods for measuring remnant cholesterol levels and apolipoprotein B-48 levels directly from human serum. Measurement of these parameters in addition to serum triglycerides may help to distinguish high-risk patients and enable us to prevent or suppress the progression of atherosclerotic diseases in those patients. However, questions remain to be answered to evaluate the significance of remnant lipoproteins. Here, we focus on three issues: the underlying problems in measuring remnant lipoprotein cholesterol, the assessment of postprandial hyperlipidemia as an atherogenic condition, and finally a review of our experimental and clinical findings about the mechanisms by which remnant lipoproteins induce atherosclerosis.

Aggregated LDL in contact with macrophages induces local increases in free cholesterol levels that regulate local actin polymerization

OBJECTIVE

Interaction of macrophages with aggregated matrix-anchored lipoprotein deposits is an important initial step in atherogenesis. Aggregated lipoproteins require different cellular uptake processes than those used for endocytosis of monomeric lipoproteins. In this study, we tested the hypothesis that engagement of aggregated LDL (agLDL) by macrophages could lead to local increases in free cholesterol levels and that these increases in free cholesterol regulate signals that control cellular actin.

METHODS AND RESULTS

AgLDL resides for prolonged periods in surface-connected compartments. Although agLDL is still extracellular, we demonstrate that an increase in free cholesterol occurs at sites of contact between agLDL and cells because of hydrolysis of agLDL-derived cholesteryl ester. This increase in free cholesterol causes enhanced actin polymerization around the agLDL. Inhibition of cholesteryl ester hydrolysis results in decreased actin polymerization.

CONCLUSIONS

We describe a novel process that occurs during agLDL-macrophage interactions in which local release of free cholesterol causes local actin polymerization, promoting a pathological positive feedback loop for increased catabolism of agLDL and eventual foam cell formation.

The epidermal growth factor homology domain of the LDL receptor drives lipoprotein release through an allosteric mechanism involving H190, H562, and H586

The low density lipoprotein (LDL) receptor (LDLR) mediates efficient endocytosis of VLDL, VLDL remnants, and LDL. As part of the endocytic process, the LDLR releases lipoproteins in endosomes. The release process correlates with an acid-dependent conformational change in the receptor from an extended, "open" state to a compact, "closed" state. The closed state has an intramolecular contact involving H190, H562, and H586. The current model for lipoprotein release holds that protonation of these histidines drives the conformational change that is associated with release. We tested the roles of H190, H562, and H586 on LDLR conformation and on lipoprotein binding, uptake, and release using variants in which the three histidines were replaced with alanine (AAA variant) or in which the histidines were replaced with charged residues that can form ionic contacts at neutral pH (DRK variant). Contrary to expectation, both the AAA and the DRK variants exhibited normal acid-dependent transitions from open to closed conformations. Despite this similarity, both the AAA and DRK mutations modulated lipoprotein release, indicating that H190, H562, and H586 act subsequent to the conformational transition. These observations also suggest that the intramolecular contact does not drive release through a competitive mechanism. In support of this possibility, mutagenesis experiments showed that beta-VLDL binding was inhibited by mutations at D203 and E208, which are exposed in the closed conformation of the LDLR. We propose that H190, H562, and H586 are part of an allosteric mechanism that drives lipoprotein release.

Lipolysis stimulated lipoprotein receptor: a novel molecular link between hyperlipidemia, weight gain, and atherosclerosis in mice

The lipolysis-stimulated lipoprotein receptor, LSR, is a multimeric protein complex in the liver that undergoes conformational changes upon binding of free fatty acids, thereby revealing a binding site (s) that recognizes both apoB and apoE. Complete inactivation of the LSR gene is embryonic lethal in mice. Here we show that removal of a single LSR allele (LSR(-/+)) caused statistically significant increases in both plasma triglyceride and cholesterol levels, a 2-fold increase in plasma triglyceride changes during the post-prandial phase, and delayed clearance of lipid emulsions or a high fat meal. The longer postprandial lipoprotein clearance time observed in LSR(-/+) mice was further increased in LSR(-/+) mice lacking functional low density lipoprotein (LDL) receptors. LSR(-/+) mice placed on a Western-type diet displayed higher plasma triglycerides and cholesterol levels, increased triglyceride-rich lipoproteins and LDL, and increased aorta lipid content, as compared with control mice on the same diet. Furthermore, a direct correlation was observed between the hyperlipidemia and weight gain but only in the LSR(-/+) mice. Knockdown of LSR expression by small interfering RNA in mouse Hepa1-6 cells led to decreased internalization of both DiI-labeled cyclohexanedione-LDL and very low density lipoprotein in the presence of oleate. These data led us to conclude that LSR contributes to the physiological clearance of atherogenic triglyceride-rich lipoproteins and LDL. We propose that LSR cooperates with the LDL receptor in the final hepatic processing of apoB-containing lipoproteins and represents a novel therapeutic target for the treatment of hyperlipidemia associated with obesity and atherosclerosis.

Folate and ApoE DNA Methylation Induced by Homocysteine in Human Monocytes

Homocysteine (Hcy) is an important and independent risk factor for arteriosclerosis, and apolipoprotein E (ApoE) is an important gene of anti atherosclerosis, but the characteristics and their key links that are involved in their pathogenic mechanisms are still poorly understood. The objective of the present study was to investigate the effects of Hcy and folate on ApoE as well as the underlying mechanism of ApoE expression induced by Hcy in monocytes. When clinically relevant concentrations of Hcy and folate were added to the cultured monocytes for 4 days, we found that clinically relevant Hcy (100 microM) may increase the levels of total cholesterol (TC), free cholesterol (FC), and cholesteryl ester (CE), and also decrease ApoE mRNA, protein expressions, leading to 34.28%, 45.00% in cultured primary human monocytes in comparison to the positive group. The effects of Hcy were primarily mediated by C-5 MTase, because Hcy could upregulate the activity of C-5 MTase and then accelerate DNA methylation of ApoE. However, folate decreased the levels of TC, FC, and CE (p < 0.001) and increased the ApoE expression; as to say, folate primarily repressed the effects of DNA methylation induced by Hcy and reduced anti atherosclerosis. In conclusion, these results suggested that ApoE DNA methylation that is induced by Hcy may play a potential role for ApoE expression in atherosclerosis. Folate has beneficial effects for anti atherosclerosis, and it may become a therapeutic target for preventing Hcy-induced atherosclerosis.

Identification of a VLDL-induced, FDNPVY-independent internalization mechanism for the LDLR

The low-density lipoprotein (LDL) receptor (LDLR) binds to and internalizes lipoproteins that contain apolipoproteinB100 (apoB100) or apolipoproteinE (apoE). Internalization of the apoB100 lipoprotein ligand, LDL, requires the FDNPVY(807) sequence on the LDLR cytoplasmic domain, which binds to the endocytic machinery of coated pits. We show here that inactivation of the FDNPVY(807) sequence by mutation of Y807 to cysteine prevented the uptake of LDL; however, this mutation did not prevent LDLR-dependent uptake of the apoE lipoprotein ligand, beta-VLDL. Comparison of the surface localization of the LDLR-Y807C using LDLR-immunogold, LDL-gold and beta-VLDL-gold probes revealed enrichment of LDLR-Y807C-bound beta-VLDL in coated pits, suggesting that beta-VLDL binding promoted the internalization of the LDLR-Y807C. Consistent with this possibility, treatment with monensin, which traps internalized LDLR in endosomes, resulted in the loss of surface LDLR-Y807C only when beta-VLDL was present. Reconstitution experiments in which LDLR variants were introduced into LDLR-deficient cells showed that the HIC(818) sequence is involved in beta-VLDL uptake by the LDLR-Y807C. Together, these experiments demonstrate that the LDLR has a very low-density lipoprotein (VLDL)-induced, FDNPVY-independent internalization mechanism.

Disruption of LDL but not VLDL clearance in autosomal recessive hypercholesterolemia

Genetic defects in LDL clearance result in severe hypercholesterolemia and premature atherosclerosis. Mutations in the LDL receptor (LDLR) cause familial hypercholesterolemia (FH), the most severe form of genetic hypercholesterolemia. A phenocopy of FH, autosomal recessive hypercholesterolemia (ARH), is due to mutations in an adaptor protein involved in LDLR internalization. Despite comparable reductions in LDL clearance rates, plasma LDL levels are substantially lower in ARH than in FH. To determine the metabolic basis for this difference, we examined the synthesis and catabolism of VLDL in murine models of FH (Ldlr(-/-)) and ARH (Arh(-/-)). The hyperlipidemic response to a high-sucrose diet was greatly attenuated in Arh(-/-) mice compared with Ldlr(-/-) mice despite similar rates of VLDL secretion. The rate of VLDL clearance was significantly higher in Arh(-/-) mice than in Ldlr(-/-) mice, suggesting that LDLR-dependent uptake of VLDL is maintained in the absence of ARH. Consistent with these findings, hepatocytes from Arh(-/-) mice (but not Ldlr(-/-) mice) internalized beta-migrating VLDL (beta-VLDL). These results demonstrate that ARH is not required for LDLR-dependent uptake of VLDL by the liver. The preservation of VLDL remnant clearance attenuates the phenotype of ARH and likely contributes to greater responsiveness to statins in ARH compared with FH.

Intracellular trafficking of recycling apolipoprotein E in Chinese hamster ovary cells

We have investigated apolipoprotein E (apoE) recycling in Chinese hamster ovary (CHO) cells, a peripheral cell that does not produce lipoproteins or express apoE. Using a pulse-chase protocol in which cells were pulsed with 125I-apoE-VLDL and chased for different periods, approximately 30% of the apoE internalized during the pulse was resecreted within a 4 h chase in a relatively lipid-free state. The addition of lysosomotropic agents or brefeldin A had no effect on apoE recycling. Unlike previous results with hepatocytes and macrophages, neither apoA-I nor upregulation of ABCA1 stimulated apoE recycling. However, cyclodextrin, which extracts cholesterol from plasma membrane lipid rafts, increased recycling. Confocal studies revealed that apoE, internalized during a 1 h pulse, colocalizes with early endosomal antigen-1, Rab5, Rab11a, and lysobisphosphatidic acid but not with lysosomal-associated membrane protein-1. Colocalization of apoE and Rab11a persisted even after cells had been chased for 1 h, suggesting a pool of apoE within the endosomal recycling compartment (ERC). Our data suggest that apoE recycling in CHO cells is linked to cellular cholesterol removal via the ERC and phospholipid-containing acceptors in a pathway alternative to the ABCA1-apoA-I axis.

Apolipoprotein E recycling: implications for dyslipidemia and atherosclerosis

After receptor-mediated endocytosis, the intracellular fate of triglyceride-rich lipoproteins (TRLs) is far more complex than the classical degradation pathway of low-density lipoproteins. Once internalized, TRLs disintegrate in peripheral endosomes, followed by a differential sorting of TRL components. Although core lipids and apolipoprotein B are targeted to lysosomes, the majority of TRL-derived apolipoprotein E (apoE) remains in peripheral recycling endosomes. This pool of TRL-derived apoE is then mobilized by high-density lipoproteins (HDLs) or HDL-derived apoA-I to be recycled back to the plasma membrane, followed by apoE resecretion and the subsequent formation of apoE-containing HDL. The HDL-induced recycling of apoE is accompanied by cholesterol efflux and involves the internalization and targeting of HDL-derived apoA-I to endosomes containing both apoE and cholesterol. These findings point to a yet unknown intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism. Recent studies provide first evidence that impaired recycling of TRL-derived apoE4, but not apoE3, is associated with intracellular cholesterol accumulation, which might explain some well-documented effects of apoE4 on HDL metabolism. This review summarizes the current understanding of apoE recycling and its potential role in the regulation of plasma apoE levels in the postprandial state.

An investigation by electron microscopy of chylomicron remnant uptake by human monocyte-derived macrophages

Human monocyte-derived macrophages (HMM) internalise proatherogenic chylomicron remnants via several high affinity receptor pathways. However, the endocytic ultrastructures responsible for the uptake of chylomicron remnants by macrophages have not previously been described. In this study, we have utilised transmission electron microscopy together with colloidal gold-labelling of chylomicron remnants to investigate the pathways involved in macrophage uptake of chylomicron remnants. We found that macrophages internalise chylomicron remnants via surface-connected compartments of up to 600 nm as well as non-clathrin coated pits. Chylomicron remnants were found to be distributed internally in a number of endocytic vesicles including early cysternal endosomes, spherical late endosomes and tubular vesicular compartments. Uptake of chylomicron remnants by HMM via phagocytosis or macropinocytosis was excluded based on the observations that lipoproteins were not found in phagolysosomes nor modified by inhibitors of these two processes, respectively. The latter observation contrasts with previous reports of chylomicron remnant internalisation by macrophages of other species.

Elevated plasma membrane cholesterol content alters macrophage signaling and function

OBJECTIVE

During atherogenesis, macrophages migrate into the subendothelial space where they ingest deposited lipoproteins, accumulate lipids, and transform into foam cells. It is unclear why these macrophages do not remove their lipid loads from the region. This study was aimed at testing the hypothesis that macrophage behavior is altered when membrane cholesterol levels are elevated, as might be the case for cells in contact with lipoproteins within atherosclerotic lesions.

METHODS AND RESULTS

We examined the effects of elevating membrane cholesterol on macrophage behavior. J774 macrophages were treated with either acetylated low-density lipoprotein (ac-LDL) and ACAT inhibitor or cholesterol-chelated methyl-beta-cyclodextrin (chol-MbetaCD) to increase membrane cholesterol levels. Our results show that elevating the membrane cholesterol of J774 macrophages induced dramatic ruffling, stimulated cell spreading, and affected F-actin organization. Cellular adhesion was required for these effects, and Rac-mediated signaling pathways were involved. Additionally, 3-dimensional transwell chemotaxis assays showed that migration of J774 macrophages was significantly inhibited when membrane cholesterol levels were raised.

CONCLUSIONS

These findings indicate that increased membrane cholesterol causes dramatic effects on macrophage cellular functions related to the actin cytoskeleton. They should provide new insights into the early steps of atherogenesis.

Cholesterol-induced macrophage apoptosis requires ER stress pathways and engagement of the type A scavenger receptor

Macrophage death in advanced atherosclerosis promotes necrosis and plaque destabilization. A likely cause of macrophage death is accumulation of free cholesterol (FC) in the ER, leading to activation of the unfolded protein response (UPR) and C/EBP homologous protein (CHOP)-induced apoptosis. Here we show that p38 MAPK signaling is necessary for CHOP induction and apoptosis. Additionally, two other signaling pathways must cooperate with p38-CHOP to effect apoptosis. One involves the type A scavenger receptor (SRA). As evidence, FC loading by non-SRA mechanisms activates p38 and CHOP, but not apoptosis unless the SRA is engaged. The other pathway involves c-Jun NH2-terminal kinase (JNK)2, which is activated by cholesterol trafficking to the ER, but is independent of CHOP. Thus, FC-induced apoptosis requires cholesterol trafficking to the ER, which triggers p38-CHOP and JNK2, and engagement of the SRA. These findings have important implications for understanding how the UPR, MAPKs, and the SRA might conspire to cause macrophage death, lesional necrosis, and plaque destabilization in advanced atherosclerotic lesions.

Aggregated LDL and lipid dispersions induce lysosomal cholesteryl ester accumulation in macrophage foam cells

Macrophage foam cells in atherosclerotic lesions accumulate substantial cholesterol stores within large, swollen lysosomes. Previous studies with mildly oxidized low density lipoprotein (OxLDL)-treated THP-1 macrophages suggest an initial buildup of free cholesterol (FC), followed by an inhibition of lysosomal cholesteryl ester (CE) hydrolysis and a subsequent lysosomal accumulation of unhydrolyzed lipoprotein CE. We examined whether other potential sources of cholesterol found within atherosclerotic lesions could also induce similar lysosomal accumulation. Biochemical analysis combined with microscopic analysis showed that treatment of THP-1 macrophages with aggregated low density lipoprotein (AggLDL) or CE-rich lipid dispersions (DISP) produced a similar lysosomal accumulation of both FC and CE. Co-treatment with an ACAT inhibitor, CP113,818, confirmed that the CE accumulation was primarily the result of the inhibition of lysosomal CE hydrolysis. The rate of unhydrolyzed CE buildup was more rapid with DISP than with AggLDL. However, with both treatments, FC appeared to accumulate in lysosomes before the inhibition in hydrolysis and CE accumulation, a sequence shared with mildly OxLDL. Thus, lysosomal accumulation of FC and CE can be attributable to more general mechanisms than just the inhibition of hydrolysis by oxidized lipids.

Physiological relevance of apolipoprotein E recycling: studies in primary mouse hepatocytes

Studies in our laboratory have shown that a fraction of apolipoprotein (apo) E internalized by hepatocytes escapes degradation and is resecreted. Although the intracellular routing is not fully understood, our studies suggest that a portion of apoE recycles through the Golgi apparatus. Given the role of the Golgi apparatus in lipoprotein secretion and the fact that apoE modulates the hepatic secretion of very low-density lipoprotein, we hypothesized that recycling apoE has an effect on hepatic very low-density lipoprotein assembly and/or secretion. To test this hypothesis, apoE-/- mice were transplanted with bone marrow from wild-type mice. In this model, extrahepatic (macrophage-derived) apoE is internalized by the hepatocytes in vivo and is resecreted when the hepatocytes are placed in culture. Unexpectedly, our studies demonstrate that recycling apoE has little effect on hepatic lipid content or hepatocyte triglyceride secretion. In addition, recycling apoE has little effect on the expression of enzymes and proteins involved in lipid synthesis as well as plasma lipoprotein apoproteins. We conclude that the physiological relevance of apoE recycling may not be related to cell-specific functions, such as lipoprotein assembly in the liver. Rather, recycling may provide a mechanism for modulating general cellular effects such as intracellular cholesterol transport or cholesterol efflux.

Comparison of monomeric and oligomeric transferrin as potential carrier in oral delivery of protein drugs

Oligomeric transferrin (Tf) was used to investigate the effect that cross-linking of transferrin receptors (TfR) has on intracellular trafficking of the Tf-TfR complex and to determine whether or not the Tf-oligomer would be a better carrier than monomeric Tf for the oral delivery of protein drugs. The intracellular retention and transcytosis of the Tf-oligomer was determined by performing pulse chase studies on enterocyte-like Caco-2 cells. The intracellular retention of the Tf-oligomer was 2-fold higher than that of monomeric Tf while there was no significant difference in transcytosis. However, in vivo studies in CF-1 mice showed that the plasma concentrations of Tf from the orally administered Tf-oligomer were approximately 2-, 3- and 60-fold higher than that of orally administered monomeric Tf at 24, 48 and 72 h post-administration, respectively. In addition, the retention of the Tf-oligomer in the intestine was higher than that of monomeric Tf, which was consistent with in vitro studies. Insulin (In), when conjugated to the Tf-oligomer (Agg-Tf-S-S-In), was more effective than monomeric Tf-In conjugate (Mono-Tf-S-S-In) in reducing blood glucose levels when orally administered to streptozotocin (STZ)-induced diabetic rats. Post-oral administration of Agg-Tf-In, a delayed onset and prolonged hypoglycemic effect was observed. These results demonstrate that the cross-linking of TfR induced by the binding of the oligomeric Tf alters the intracellular trafficking and increases the intracellular retention of Tf-TfR complexes in polarized Caco-2 cells. The alteration of TfR trafficking could conceivably have caused the increase of insulin transport across the intestinal barrier when Agg-Tf-S-S-In was administered orally to STZ-induced diabetic rats. The delayed onset and prolonged effect of Agg-Tf-S-S-In in hypoglycemia strongly suggests that the Tf-oligomer can act as a sustained release carrier in the oral delivery of protein and peptide drugs.

The recycling of apolipoprotein E in macrophages

The ability of apolipoprotein E (apoE) to be spared degradation in lysosomes and to recycle to the cell surface has been demonstrated by our group and others, but its physiologic relevance is unknown. In this study, we characterized apoE recycling in primary murine macrophages and probed the effects of HDL and apoA-I on this process. In cells pulsed with (125)I.apoE bound to VLDL, intact apoE was found in the chase medium for up to 24 h after the pulse. Approximately 27 +/- 5% of the apoE internalized during the pulse was recycled after 4 h of chase. Addition of apoA-I and HDL increased apoE recycling to 45 +/- 3% and 46 +/- 3%, respectively, similar to the amount of apoE recycled after pulsing the cells with (125)I.apoE.HDL. In addition, apoA-I-producing macrophages from transgenic mice showed increased apoE recycling at 4 h (38 +/- 3%). Increased ABCA1 expression potentiated apoE recycling, suggesting that recycling occurs via ABCA1. Finally, in the presence of apoA-I, recycled apoE exited the cells on HDL-like particles. These results suggest that apoE recycling in macrophages may be part of a larger signaling loop activated by HDL and directed at maximizing cholesterol losses from the cell.

Transferrin-oligomers as potential carriers in anticancer drug delivery

PURPOSE

To investigate if the cross-linking of transferrin receptor (TfR) induced by Tf-oligomers alters the endocytosis of receptor-ligand complexes in cultured tumor cells and hence increases intracellular drug release.

METHODS

An average of 3.5 Tf molecules per aggregate were cross-linked either by using homobifunctional linker (1, 11-bis-maleimidotetraethyleneglycol) [Tf(3.5-BM(PEO)4)] or heterobifunction linker [succinimidyl 4-(-p-maleimidophenyl)-butyrate] (Tf(3.5-SMPB)). Cell surface binding and competition experiments with 125I-Tf for TfR binding were studied to demonstrate that Tf-oligomers maintain specificity of the TfR-binding. To determine the degradation of Tf-oligomers in TfR-mediated endocytosis, cultured tumor cells were pulsed for 15 min with 125I-Tf-oligomers and chased for 2 h at 37 degrees C in the presence of excess unlabeled Tf. The chase medium was subjected to TCA precipitation to separate the intact and degraded Tf. To investigate if the alteration of TfR-trafficking facilitates the intracellular release of the drug from the Tf-conjugated form, methotrexate (MTX) was conjugated to Tf-oligomer (Agg-Tf-MTX) and its antiproliferative activity was compared with monomeric-Tf-MTX (Mono-Tf-MTX) in human colon carcinoma (Caco-2) cells, human breast adenocarcinoma (MCF-7) cells, wild-type Chinese hamster ovary (CHO) cells, and MTX-resistant CHO (CHO-MTX-RII) cells.

RESULTS

TfR-mediated degradation of Tf-oligomers was higher than that of monomeric Tf in both Caco-2 and MCF-7 cells. The IC50 of Agg-Tf-MTX was lower than that of Mono-Tf-MTX in both tumor cell lines. The IC50 of MTX and Mono-Tf-MTX in CHO-MTX-RII cells was higher than that in wild-type CHO cells, whereas the Agg-Tf-MTX was almost identical in both the resistant and wild-type cells.

CONCLUSIONS

Cross-linking of TfR induced by oligomeric Tf binding alters the intracellular trafficking of Tf-TfR complexes, redirects them out of the recycling pathway, and targets them to intracellular degradation in cultured tumor cells. The alteration of TfR-trafficking facilitates the intracellular release of the drug from the Tf-conjugated form. Consequently, Agg-Tf-MTX is more effective than Mono-Tf-MTX as a TfR-mediated antiproliferative agent in tumor cells, as well as in MTX-resistant transport deficient cells. Therefore, Tf-oligomers are potentially effective TfR-targeting carriers for intracellular delivery of anticancer drugs.

Impaired Recycling of Apolipoprotein E4 Is Associated with Intracellular Cholesterol Accumulation

After internalization of triglyceride-rich lipoproteins (TRL) in hepatoma cells, TRL particles are immediately disintegrated in the early endosomal compartment. This involves the targeting of lipids and apoprotein B along the degradative pathway and the recycling of TRL-derived apoE through recycling endosomes. Re-secretion of apoE is accompanied by the concomitant association of apoE and cellular cholesterol with high-density lipoproteins (HDL). Since epidemiological data showed that apoE3 and apoE4 have differential effects on HDL metabolism, we investigated whether the intracellular processing of TRL-derived apoE4 differs from apoE3-TRL. In this study, we demonstrated by radioactive and immunofluorescence uptake experiments that cell-surface binding and internalization of TRL-derived apoE4 are increased compared with apoE3 in hepatoma cells. Pulse-chase experiments revealed that HDL-induced recycling, but not disintegration and degradation, of apoE4-enriched TRL is strongly reduced in these cells. Furthermore, impaired HDL-induced apoE4 recycling is associated with reduced cholesterol efflux. Studies performed in Tangier fibroblasts showed that apoE recycling does not depend on ATP-binding cassette transporter A1 activity. These studies provide initial evidence that impaired recycling of apoE4 could interfere with intracellular cholesterol transport and contribute to the pathophysiological lipoprotein profile observed in apoE4 homozygotes.

The recycling of apolipoprotein E and its amino-terminal 22 kDa fragment: evidence for multiple redundant pathways

A portion of apolipoprotein E (apoE) internalized by hepatocytes is spared degradation and is recycled. To investigate the intracellular routing of recycling apoE, primary hepatocyte cultures from LDL receptor-deficient mice and mice deficient in receptor-associated protein [a model of depressed expression of LDL receptor-related protein (LRP)] were incubated with human VLDL containing 125I-labeled human recombinant apoE3. Approximately 30% of the internalized intact apoE was recycled after 4 h. The N-terminal 22 kDa fragment of apoE was also resecreted, demonstrating that this apoE domain contains sufficient sequence to recycle. The 22 kDa fragment has reduced affinity for lipoproteins, suggesting that apoE recycling is linked to the ability of apoE to bind directly to a recycling receptor. Finally, apoE was found to recycle equally well in the presence of brefeldin A, a drug that blocks transport from the endoplasmic reticulum and leads to collapse of the Golgi stacks. Our studies demonstrate that apoE recycling occurs 1) in the absence of the LDL receptor or under conditions of markedly reduced LRP expression; 2) when apoE lacks the carboxyl-terminal domain, which allows binding to the lipoprotein; and 3) in the absence of an intact Golgi apparatus. We conclude that apoE recycling occurs through multiple redundant pathways.

Decreased Macrophage Paraoxonase 2 Expression in Patients With Hypercholesterolemia Is the Result of Their Increased Cellular Cholesterol Content: Effect of Atorvastatin Therapy

OBJECTIVE

To analyze paraoxonase2 (PON2) expression in human monocyte-derived macrophages (HMDM) from patients with hypercholesterolemia in relation to cellular cholesterol and oxidative stress.

METHODS AND RESULTS

Ten healthy subjects (controls) and 10 patients with hypercholesterolema who received 20-mg/d atorvastatin participated in the study. The patients' versus controls' HMDM demonstrated increased cholesterol content (270%) and oxidative stress (30% to 45%). Atorvastatin therapy reduced these parameters (59% and 25%, respectively). The patients' versus controls' macrophage-PON2 mRNA expression and PON2 activity were lower (100% and 40%, respectively), and atorvastatin therapy increased these parameters (76% and 200%, respectively). Untreated patient HMDM incubation with atorvastatin (0 to 10 micromol/L) resulted in a dose-dependent reduction in cellular cholesterol content and in cell-mediated low-density lipoprotein (LDL) oxidation up to 79% and 66%, respectively. In parallel, PON2 mRNA expression and PON2 activity increased dose-dependently up to 3.6- and 2.1-fold, respectively. On incubation of control HMDM with acetylated-LDL or aggregated-LDL, cellular cholesterol content increased (77% and 100%), and macrophage-PON2 activity decreased (49% and 22%), respectively. In contrast, oxidized LDL increased both cellular oxidative stress and PON2 expression.

CONCLUSIONS

HMDM-PON2 expression is reduced in patients with hypercholesterolemia as a result of their increased cellular cholesterol content. Atorvastatin therapy reduced both macrophage oxidative stress and cholesterol content, and upregulated PON2 expression, thus contributing to attenuation of foam cells formation.

Recycling of apoprotein E is associated with cholesterol efflux and high density lipoprotein internalization

After receptor-mediated endocytosis of triglyceride-rich lipoproteins (TRL) into the liver, TRL particles are immediately disintegrated in peripheral endosomal compartments. Whereas core lipids and apoprotein B are delivered for degradation into lysosomes, TRL-derived apoE is efficiently recycled back to the plasma membrane. This is followed by apoE re-secretion and association of apoE with high density lipoproteins (HDL). Because HDL and apoE can independently promote cholesterol efflux, we investigated whether recycling of TRL-derived apoE in human hepatoma cells and fibroblasts could be linked to intracellular cholesterol transport. In this study we demonstrate that HDL(3) does not only act as an extracellular acceptor for recycled apoE but also stimulates the recycling of internalized TRL-derived apoE. Furthermore, radioactive pulse-chase experiments indicate that apoE recycling is accompanied by cholesterol efflux. Confocal imaging reveals co-localization of apoE and cholesterol in early endosome antigen 1-positive endosomes. During apoE re-secretion, HDL(3)-derived apoA-I is found in these early endosome antigen 1, cholesterol-containing endosomes. As shown by time-lapse fluorescence microscopy, apoE recycling involves the intracellular trafficking of apoA-I to pre-existing and TRL-derived apoE/cholesterol-containing endosomes in the periphery. Thus, these studies provide evidence for a new intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism.

The recycling of apolipoprotein E in primary cultures of mouse hepatocytes. Evidence for a physiologic connection to high density lipoprotein metabolism

Internalization of apoE-containing very low density protein (VLDL) by hepatocytes in vivo and in vitro leads to apoE recycling and resecretion. Because of the role of apoE in VLDL metabolism, apoE recycling may influence lipoprotein assembly or remnant uptake. However, apoE is also a HDL protein, and apoE recycling may be related to reverse cholesterol transport. To investigate apoE recycling, apoE(-/-) mouse hepatocytes were incubated (pulsed) with wild-type mouse lipoproteins, and cells and media were collected at chase periods up to 24 h. When cells were pulsed with VLDL, apoE was resecreted within 30 min. Although the mass of apoE in the media decreased with time, it could be detected up to 24 h after the pulse. Intact intracellular apoE was also detectable 24 h after the pulse. ApoE was also resecreted when cells were pulsed with HDL. When apoA-I was included in the chase media after a pulse with VLDL, apoE resecretion increased 4-fold. Furthermore, human apoE was resecreted from wild-type mouse hepatocytes after a pulse with human VLDL. Finally, apoE was resecreted from mouse peritoneal macrophages after pulsing with VLDL. We conclude that 1) HDL apoE recycles in a quantitatively comparable fashion to VLDL apoE; 2) apoE recycling can be modulated by extracellular apoA-I but is not affected by endogenous apoE; and 3) recycling occurs in macrophages as well as in hepatocytes, suggesting that the process is not cell-specific.

Assay methods of modified lipoproteins in plasma

Modified lipoproteins, especially oxidatively modified low-density lipoprotein (Ox-LDL), are present in the plasma of patients with atherosclerosis and related diseases. The modification of LDL is believed to play an important role in the development of atherosclerosis. Thus, measurement of plasma Ox-LDL is essential not only for investigating its relevance to atherosclerotic diseases, but also for diagnosis. Chromatographic methods are effective for indirectly measuring the oxidatively modified state of LDL or directly measuring the modified LDL. Indirect determination can be done by estimating the LDL subfraction, LDL particle size, oxidized amino acids in apolipoprotein B, lipid hydroperoxide or F(2)-isoprostane in LDL. Direct determination of the modified LDL in plasma can be done with chromatographic methods such as anion-exchange chromatography and size-exclusion chromatography. Other methods for estimating the modified state of LDL include electromigration methods such as agarose gel, polyacrylamide gradient gel and capillary electrophoresis. Recently, enzyme-linked immunosorbent assay methods of malondialdehyde (MDA)-LDL and autoantibodies against Ox-LDL have been developed to assess Ox-LDL in plasma. This review article summarizes the detection and assay methods of modified lipoproteins in plasma.

Sequestration of aggregated low-density lipoproteins by macrophages

PURPOSE OF REVIEW

Evidence suggests that much of the LDL in atherosclerotic plaques is aggregated. Aggregation of LDL could be an important factor that determines how this lipoprotein is metabolized by plaque macrophages and the fate of aggregated LDL cholesterol within plaques. This review discusses a novel endocytic pathway by which macrophages process aggregated LDL.

RECENT FINDINGS

Recently, it has been shown that aggregated LDL can be sequestered in macrophage surface-connected compartments and plasma membrane invaginations by a process termed patocytosis. In contrast to rapid degradation of LDL and aggregated LDL taken up by macrophages through pinocytosis and phagocytosis, respectively, aggregated LDL sequestered in macrophages undergoes only limited degradation. Macrophages can disaggregate and release sequestered aggregated LDL by activating plasminogen to plasmin. Plasmin degrades LDL apolipoprotein B sufficiently to disaggregate the aggregated LDL, releasing it from the macrophage surface-connected compartments. In contrast, activating macrophages with phorbol-myristate-acetate stimulates degradation of aggregated LDL and inhibits plasminogen-mediated release of the aggregated lipoprotein from macrophage surface-connected compartments.

SUMMARY

Macrophage sequestration of aggregated LDL is a unique endocytic pathway relevant not only to the processing of aggregated LDL in atherosclerotic plaques but also for the processing of other materials, such as hydrophobic particles that trigger this endocytic pathway. Macrophage sequestration of aggregated LDL can result in different fates for the aggregated LDL, depending on the state of macrophage activation and the functioning of the plasminogen-based fibrinolytic system. Patocytosis of aggregated LDL should be considered in addition to phagocytosis as a possible uptake pathway in studies of macrophage processing of aggregated LDL.

Acid sphingomyelinase-deficient macrophages have defective cholesterol trafficking and efflux

Cholesterol efflux from macrophage foam cells, a key step in reverse cholesterol transport, requires trafficking of cholesterol from intracellular sites to the plasma membrane. Sphingomyelin is a cholesterol-binding molecule that transiently exists with cholesterol in endosomes and lysosomes but is rapidly hydrolyzed by lysosomal sphingomyelinase (L-SMase), a product of the acid sphingomyelinase (ASM) gene. We therefore hypothesized that sphingomyelin hydrolysis by L-SMase enables cholesterol efflux by preventing cholesterol sequestration by sphingomyelin. Macrophages from wild-type and ASM knockout mice were incubated with [(3)H]cholesteryl ester-labeled acetyl-LDL and then exposed to apolipoprotein A-I or high density lipoprotein. In both cases, [(3)H]cholesterol efflux was decreased substantially in the ASM knockout macrophages. Similar results were shown for ASM knockout macrophages labeled long-term with [(3)H]cholesterol added directly to medium, but not for those labeled for a short period, suggesting defective efflux from intracellular stores but not from the plasma membrane. Cholesterol trafficking to acyl-coenzyme A:cholesterol acyltransferase (ACAT) was also defective in ASM knockout macrophages. Using filipin to probe cholesterol in macrophages incubated with acetyl-LDL, we found there was modest staining in the plasma membrane of wild-type macrophages but bright, perinuclear fluorescence in ASM knockout macrophages. Last, when wild-type macrophages were incubated with excess sphingomyelin to "saturate" L-SMase, [(3)H]cholesterol efflux was decreased. Thus, sphingomyelin accumulation due to L-SMase deficiency leads to defective cholesterol trafficking and efflux, which we propose is due to sequestration of cholesterol by sphingomyelin and possibly other mechanisms. This model may explain the low plasma high density lipoprotein found in ASM-deficient humans and may implicate L-SMase deficiency and/or sphingomyelin enrichment of lipoproteins as novel atherosclerosis risk factors.

Recycling of apolipoprotein E and lipoprotein lipase through endosomal compartments in vivo

We have recently described a novel recycling pathway of triglyceride-rich lipoprotein (TRL)-associated apolipoprotein (apo) E in human hepatoma cells. We now demonstrate that not only TRL-derived apoE but also lipoprotein lipase (LPL) is efficiently recycled in vitro and in vivo. Similar recycling kinetics of apoE and LPL in normal and low density lipoprotein receptor-negative human fibroblasts also indicate that the low density lipoprotein receptor-related protein seems to be involved. Intracellular sorting mechanisms are responsible for reduced lysosomal degradation of both ligands after receptor-mediated internalization. Immediately after internalization in rat liver, TRLs are disintegrated, and apoE and LPL are found in endosomal compartments, whereas TRL-derived phospholipids accumulate in the perinuclear region of hepatocytes. Subsequently, substantial amounts of both proteins can be found in purified recycling endosomes, indicating a potential resecretion of these TRL components. Pulse-chase experiments of perfused rat livers with radiolabeled TRLs demonstrated a serum-induced release of internalized apoE and LPL into the perfusate. Analysis of the secreted proteins identified approximately 80% of the recycled TRL-derived proteins in the high density lipoprotein fractions. These results provide the first evidence that recycling of TRL-derived apoE and LPL could play an important role in the modulation of lipoproteins in vivo.

The uptake and degradation of matrix-bound lipoproteins by macrophages require an intact actin Cytoskeleton, Rho family GTPases, and myosin ATPase activity

A key cellular event in atherogenesis is the interaction of macrophages with lipoproteins in the subendothelium. In vivo, these lipoproteins are bound to matrix and often aggregated, yet most cell-culture experiments explore these events using soluble monomeric lipoproteins. We hypothesized that the internalization and degradation of matrix-retained and aggregated low density lipoprotein (LDL) by macrophages may involve the actin-myosin cytoskeleton in a manner that distinguishes this process from the endocytosis of soluble LDL. To explore these ideas, we plated macrophages on sphingomyelinase-aggregated LDL bound to smooth muscle cell-derived matrix in the presence of lipoprotein lipase. The macrophages internalized and degraded the LDL, which was mediated partially by the LDL receptor-related protein. Cytochalasin D and latrunculin A, which block actin polymerization, markedly inhibited the uptake and degradation of matrix-retained LDL but not soluble LDL. Inhibition of Rho family GTPases by Clostridium difficile toxin B blocked the degradation of matrix-retained and aggregated LDL by >90% without any inhibition of soluble LDL degradation. However, specific inhibition of Rho had no effect, suggesting the importance of Rac1 and Cdc42. Degradation of matrix-retained, but not soluble, LDL was also blocked by inhibitors of tyrosine kinase, phosphatidylinositol 3-kinase, and myosin ATPase. These findings define fundamental cytoskeletal pathways that may be involved in macrophage foam cell formation in vivo but have been missed by the use of previous cell culture models.

Intracellular cholesterol and phospholipid trafficking: comparable mechanisms in macrophages and neuronal cells

During the past ten years considerable evidences have accumulated that in addition to monocytes/macrophages, that are implicated in innate immunity and atherogenesis, neuronal cells also exhibit an extensive cellular metabolism. The present study focuses on the major protein players that establish cellular distribution of cholesterol and phospholipids. Evidences are provided that neuronal cells and monocytes/macrophages are equipped with comparable intracellular lipid trafficking mechanisms. Selected examples are presented that trafficking dysfunctions lead to disease development, such as Tangier disease and Niemann-Pick disease type C, or contribute to the pathogenesis of diseases such as Alzheimer disease and atherosclerosis.

A recycling pathway for resecretion of internalized apolipoprotein E in liver cells

We have investigated the recycling of apoE in livers of apoE(-)/- mice transplanted with wild type bone marrow (apoE(+/+) --> apoE(-)/-), a model in which circulating apoE is derived exclusively from macrophages. Nascent Golgi lipoproteins were recovered from livers of apoE(+/+) --> apoE(-)/- mice 8 weeks after transplantation. ApoE was identified with nascent d < 1.006 and with d 1.006-1.210 g/ml lipoproteins at a level approximately 6% that of nascent lipoproteins from C57BL/6 mice. Hepatocytes from apoE(+/+) --> apoE(-)/- mice were isolated and cultured in media free of exogenous apoE. ApoE was found in the media primarily on the d < 1.006 g/ml fraction, indicating a resecretion of internalized apoprotein. Secretion of apoE from C57BL/6 hepatocytes was consistent with constitutive production, whereas the majority of apoE secreted from apoE(+/+) --> apoE(-)/- hepatocytes was recovered in the last 24 h of culture. This suggests that release may be triggered by accumulation of an acceptor, such as very low density lipoproteins, in the media. In agreement with the in vivo data, total recovery of apoE from apoE(+/+) --> apoE(-)/- hepatocytes was approximately 6% that of the apoE recovered from C57BL/6 hepatocytes. Since plasma apoE levels in the transplanted mice are approximately 10% of control levels, the findings indicate that up to 60% of the internalized apoE may be reutilized under physiologic conditions. These studies provide definitive evidence for the sparing of apoE and its routing through the secretory pathway and demonstrate that internalized apoE can be resecreted in a quantitatively significant fashion.

Intracellular metabolism of triglyceride-rich lipoproteins

Over the past 10 years, many advances have been made in our understanding of the intravascular metabolism of triglyceride-rich lipoproteins. It is now known that the complex extracellular interactions of triglyceride-rich lipoprotein-associated apolipoprotein E, lipoprotein lipase and hepatic lipase with heparan sulfate proteoglycans and lipoprotein receptors facilitate the hepatocellular uptake of triglyceride-rich lipoproteins. Recent studies have also revealed that the intracellular fate of internalized triglyceride-rich lipoproteins is highly complex. The dissociation of triglyceride-rich lipoprotein components within intracellular endosomal compartments involves the recycling of apolipoprotein E, whereas the remaining lipid core associated with apolipoprotein B is susceptible to lysosomal degradation. Apolipoprotein E recycling is an important newly discovered feature of lipoprotein metabolism, and will be discussed in the context of its intracellular transport mechanisms and cholesterol efflux. Current concepts concerning its potential relevance with regard to lipoprotein metabolism and atherosclerosis will also be discussed.

Apolipoprotein E regulates dietary cholesterol absorption and biliary cholesterol excretion: studies in C57BL/6 apolipoprotein E knockout mice

The present study examined the role of apolipoprotein E (apoE) in the regulation of dietary cholesterol absorption and biliary cholesterol excretion. Increasing dietary cholesterol from 0.02% to 0.5% in C57BL/6 wild-type mice decreased the percentage of dietary cholesterol that is absorbed by 25%, and this decrease was associated with a 2-fold increase in gallbladder biliary cholesterol concentration. In contrast, increasing dietary cholesterol from 0. 02% to 0.5% in C57BL/6 apoE knockout mice produced no significant suppression of the percentage dietary cholesterol absorption and increased gallbladder biliary cholesterol concentration only 16%. Whereas in wild-type mice, the increase in dietary cholesterol increased the hepatic excretion of biliary cholesterol 4-fold, there was only a 2-fold increase in apoE knockout mice. On both the low- and the high-cholesterol diets, whole liver and isolated hepatocyte cholesterol content was higher in the apoE knockout mice. These results suggest that, in response to dietary cholesterol, apoE may play a critical role in decreasing the percentage absorption of dietary cholesterol and increasing biliary cholesterol excretion. These observations suggest a mechanism whereby the absence of apoE contributes to the propensity for tissue cholesterol deposition and accelerated atherogenesis in apoE knockout mice.

Apolipoprotein E regulates dietary cholesterol absorption and biliary cholesterol excretion: Studies in C57BL/6 apolipoprotein E knockout mice

The present study examined the role of apolipoprotein E (apoE) in the regulation of dietary cholesterol absorption and biliary cholesterol excretion. Increasing dietary cholesterol from 0.02% to 0.5% in C57BL/6 wild-type mice decreased the percentage of dietary cholesterol that is absorbed by 25%, and this decrease was associated with a 2-fold increase in gallbladder biliary cholesterol concentration. In contrast, increasing dietary cholesterol from 0. 02% to 0.5% in C57BL/6 apoE knockout mice produced no significant suppression of the percentage dietary cholesterol absorption and increased gallbladder biliary cholesterol concentration only 16%. Whereas in wild-type mice, the increase in dietary cholesterol increased the hepatic excretion of biliary cholesterol 4-fold, there was only a 2-fold increase in apoE knockout mice. On both the low- and the high-cholesterol diets, whole liver and isolated hepatocyte cholesterol content was higher in the apoE knockout mice. These results suggest that, in response to dietary cholesterol, apoE may play a critical role in decreasing the percentage absorption of dietary cholesterol and increasing biliary cholesterol excretion. These observations suggest a mechanism whereby the absence of apoE contributes to the propensity for tissue cholesterol deposition and accelerated atherogenesis in apoE knockout mice.

Apolipoprotein E is resistant to intracellular degradation in vitro and in vivo. Evidence for retroendocytosis

Apolipoprotein E (apoE) is an important determinant for the uptake of triglyceride-rich lipoproteins and emulsions by the liver, but the intracellular pathway of apoE following particle internalization is poorly defined. In the present study, we investigated whether retroendocytosis is a unique feature of apoE as compared with apoB by studying the intracellular fate of very low density lipoprotein-sized apoE-containing triglyceride-rich emulsion particles and LDL after LDLr-mediated uptake. Incubation of HepG2 cells with [(3)H]cholesteryl oleate-labeled particles at 37 degrees C led to a rapid release of [(3)H]cholesterol within 30 min for both LDL and emulsion particles. In contrast, emulsion-derived (125)I-apoE was more resistant to degradation (>/=120 min) than LDL-derived (125)I-apoB (30 min). Incubation at 18 degrees C, which allows endosomal uptake but prevents lysosomal degradation, with subsequent incubation at 37 degrees C resulted in a time-dependent release of intact apoE from the cells (up to 14% of the endocytosed apoE at 4 h). The release of apoE was accelerated by the presence of protein-free emulsion (20%) or high density lipoprotein (26%). Retroendocytosis of intact particles could be excluded since little intact [(3)H]cholesteryl oleate was released (<3%). In contrast, the degradation of LDL was complete with virtually no secretion of intact apoB into the medium. The intracellular stability of apoE was also demonstrated after hepatic uptake in C57Bl/6 mice. Intravenous injection of (125)I-apoE and [(3)H]cholesteryl oleate-labeled emulsions resulted in efficient LDLr-mediated uptake of both components by the liver (45-50% of the injected dose after 20 min). At 1 h after injection, only 15-20% of the hepatic (125)I-apoE was degraded, whereas 75% of the [(3)H]cholesteryl oleate was hydrolyzed. From these data we conclude that following LDLr-mediated internalization by liver cells, apoE can escape degradation and can be resecreted. This sequence of events may allow apoE to participate in its hypothesized intracellular functions such as mediator of the post-lysosomal trafficking of lipids and very low density lipoprotein assembly.

The pathogenesis of foam cell formation: modified LDL stimulates uptake of co-incubated LDL via macropinocytosis

Previously, modified LDLs were shown to stimulate macropinocytosis in pigeon macrophages. Simultaneous intracellular trafficking of LDL and AcLDL, differentially labeled with colloidal gold, was done to determine whether uptake of LDL, which does not cause foam cell formation, was internalized via a separate route from AcLDL, which stimulates foam cell formation. AcLDL and LDL were followed at either low (12 microg/mL) concentrations near the saturation of high affinity binding sites or high (50 to 150 microg/mL) lipoprotein concentrations used to induce foam cell formation. The colloidal gold distribution and percentage of co-labeling as observed by transmission electron microscopy were determined for organelles involved with coated-pit endocytosis or macropinocytosis. LDL simultaneously incubated with AcLDL on macrophages at the low concentration was predominately internalized via coated-pit endocytosis. AcLDL was internalized via both coated-pit endocytosis and macropinocytosis at low concentration. At higher lipoprotein concentrations (50 to 150 microg/mL), AcLDL continued to be internalized via macropinocytosis. Interestingly, a significant portion of the co-incubated LDL, at high concentrations, also trafficked via macropinocytosis. LDL internalized by macropinosomes at high lipoprotein concentrations suggests that AcLDL-stimulated macropinocytosis might increase uptake of co-incubated lipoproteins. When (125)I-LDL was incubated with cold AcLDL, LDL degradation at 37 degrees C doubled, without a corresponding increase in cell association or total binding of LDL at 4 degrees C. These studies suggest that modified LDL-stimulated macropinocytosis is a mechanism for increased degradation of co-incubated LDL potentially leading to foam cell formation.