Conformation of apolipoprotein AI in reconstituted lipoprotein particles and particle-membrane interaction: effect of cholesterol

Apoptotic cell death induced by dendritic derivatives of aminolevulinic acid in endothelial and foam cells co-cultures

Photodynamic therapy (PDT) is an effective procedure for the treatment of lesions diseases based on the selectivity of a photosensitising compound with the ability to accumulate in the target cell. Atherosclerotic plaque is a suitable target for PDT because of the preferential accumulation of photosensitisers in atherosclerotic plaques. Dendrimers are hyperbranched polymers conjugated to drugs. The dendrimers of ALA hold ester bonds that inside the cells are cleaved and release ALA, yielding PpIX production. The dendrimer 6m-ALA was chosen to perform this study since in previous studies it induced the highest porphyrin macrophage: endothelial cell ratio (Rodriguez et al. in Photochem Photobiol Sci 14:1617-1627, 2015). We transformed Raw 264.7 macrophages to foam cells by exposure to oxidised LDLs, and we employed a co-culture model of HMEC-1 endothelial cells and foam cells to study the affinity of ALA dendrimers for the foam cells. In this work it was proposed an in vitro model of atheromatous plaque, the aim was to study the selectivity of an ALA dendrimer for the foam cells as compared to the endothelial cells in a co-culture system and the type of cell death triggered by the photodynamic treatment. The ALA dendrimer 6m-ALA showed selectivity PDT response for foam cells against endothelial cells. A light dose of 1 J/cm² eliminate foam cells, whereas less than 50% of HMEC-1 is killed, and apoptosis cell death is involved in this process, and no necrosis is present. We propose the use of ALA dendrimers as pro-photosensitisers to be employed in photoangioplasty to aid in the treatment of obstructive cardiovascular diseases, and these molecules can also be employed as a theranostic agent.

Understanding the role of apolipoproteinA-I in atherosclerosis. Post-translational modifications synergize dysfunction?

BACKGROUND

The identification of dysfunctional human apolipoprotein A-I (apoA-I) in atherosclerotic plaques suggests that protein structure and function may be hampered under a chronic pro inflammatory scenario. Moreover, the fact that natural mutants of this protein elicit severe cardiovascular diseases (CVD) strongly indicates that the native folding could shift due to the mutation, yielding a structure more prone to misfold or misfunction. To understand the events that determine the failure of apoA-I structural flexibility to fulfill its protective role, we took advantage of the study of a natural variant with a deletion of the residue lysine 107 (K107del) associated with atherosclerosis.

METHODS

Biophysical approaches, such as electrophoresis, fluorescence and spectroscopy were used to characterize proteins structure and function, either in native conformation or under oxidation or intramolecular crosslinking.

RESULTS

K107del structure was more flexible than the protein with the native sequence (Wt) but interactions with artificial membranes were preserved. Instead, structural restrictions by intramolecular crosslinking impaired the Wt and K107del lipid solubilization function. In addition, controlled oxidation decreased the yield of the native dimer conformation for both variants.

CONCLUSIONS

We conclude that even though mutations may alter protein structure and spatial arrangement, the highly flexible conformation compensates the mild shift from the native folding. Instead, post translational apoA-I modifications (probably chronic and progressive) are required to raise a protein conformation with significant loss of function and increased aggregation tendency.

GENERAL SIGNIFICANCE

The results learnt from this variant strength a close association between amyloidosis and atherosclerosis.

Decreased OxLDL uptake and cholesterol efflux in THP1 cells elicited by cortisol and by cortisone through 11β-hydroxysteroid dehydrogenase type 1

BACKGROUND AND AIMS

Data about glucocorticoids role in the development of atherosclerosis are controversial showing different effects in human than in experimental animal models. Atherosclerosis is the result of a chronic inflammatory response to an injured endothelium where an uncontrolled uptake of OxLDL by macrophages triggers the development of foam cells, the main component of fatty streaks in atherosclerotic plaque. There are few data about the direct effect of glucocorticoids in macrophages of atherosclerotic plaque. The aim of the study was to elucidate the role of glucocorticoids in the development of foam cells in atherosclerosis initiation.

METHODS

For this purpose we used THP1 cells differentiated to macrophages with phorbol esters and incubated with OxLDL alone or with cortisol or cortisone. THP1 cells were also incubated with cortisone plus an inhibitor of 11β-hydroxysteroid dehydrogenase 1 (11βHSD1) activity to determine the role of this enzyme on glucocorticoid action in this process.

RESULTS

Ours results showed that cortisol and cortisone decreased significantly the inflammation promoted by OxLDL, and also diminished the expression of genes involved in influx and efflux of cholesterol resulting in a reduced lipid accumulation. Likewise cortisol and cortisone decreased 11βHSD1 expression in THP1 cells. The presence of the inhibitor of 11βHSD1 abolished all the effects elicited by cortisone.

CONCLUSION

Our results indicate a direct effect of glucocorticoids on macrophages braking atherosclerosis initiation, reducing pro-inflammatory markers and OxLDL uptake and cholesterol re-esterification, but also inhibiting cholesterol output. These effects appear to be mediated, at least in part, by 11βHSD1 activity.

HDL particle subpopulations: Focus on biological function

Low levels of high-density lipoprotein-cholesterol (HDL-C) constitute an independent biomarker of cardiovascular morbi-mortality. However, recent advances have drastically modified the classical and limited view of HDL as a carrier of 'good cholesterol', and have revealed unexpected levels of complexity in the circulating HDL particle pool. HDL particles are indeed highly heterogeneous in structure, intravascular metabolism and biological activity. This review describes recent progress in our understanding of HDL subpopulations and their biological activities, and focuses on relationships between the structural, compositional and functional heterogeneity of HDL particles.

Definition of human apolipoprotein A-I epitopes recognized by autoantibodies present in patients with cardiovascular diseases

Autoantibodies to apolipoprotein A-I (anti-apoA-I IgG) have been shown to be both markers and mediators of cardiovascular disease, promoting atherogenesis and unstable atherosclerotic plaque. Previous studies have shown that high levels of anti-apoA-I IgGs are independently associated with major adverse cardiovascular events in patients with myocardial infarction. Autoantibody responses to apoA-I can be polyclonal and it is likely that more than one epitope may exist. To identify the specific immunoreactive peptides in apoA-I, we have developed a set of methodologies and procedures to isolate, purify, and identify novel apoA-I endogenous epitopes. First, we generated high purity apoA-I from human plasma, using thiophilic interaction chromatography followed by enzymatic digestion specifically at lysine or arginine residues. Immunoreactivity to the different peptides generated was tested by ELISA using serum obtained from patients with acute myocardial infarction and high titers of autoantibodies to native apoA-I. The immunoreactive peptides were further sequenced by mass spectrometry. Our approach successfully identified two novel immunoreactive peptides, recognized by autoantibodies from patients suffering from myocardial infarction, who contain a high titer of anti-apoA-I IgG. The discovery of these epitopes may open innovative prognostic and therapeutic opportunities potentially suitable to improve current cardiovascular risk stratification.

Role of plasma membrane lipid composition on cellular homeostasis: learning from cell line models expressing fatty acid desaturases

Experimental evidence has suggested that plasma membrane (PM)-associated signaling and hence cell metabolism and viability depend on lipid composition and organization. The aim of the present work is to develop a cell model to study the endogenous polyunsaturated fatty acids (PUFAs) effect on PM properties and analyze its influence on cholesterol (Chol) homeostasis. We have previously shown that by using a cell line over-expressing stearoyl-CoA-desaturase, membrane composition and organization coordinate cellular pathways involved in Chol efflux and cell viability by different mechanisms. Now, we expanded our studies to a cell model over-expressing both Δ5 and Δ6 desaturases, which resulted in a permanently higher PUFA content in PM. Furthermore, this cell line showed increased PM fluidity, Chol storage, and mitochondrial activity. In addition, human apolipoprotein A-I-mediated Chol removal was less efficient in these cells than in the corresponding control. Taken together, our results suggested that the cell functionality is preserved by regulating PM organization and Chol exportation and homeostasis.

Effect of reconstituted discoidal high-density lipoproteins on lipid mobilization in RAW 264.7 and CHOK1 cells

Reconstituted discoidal high-density lipoproteins (rHDL) resemble nascent HDL, which are formed at the early reverse cholesterol transport steps, and constitute the initial cholesterol (Chol) acceptors from cell membranes. We have used different sized rHDL containing or not Chol, to test their abilities to promote cholesterol and phospholipid efflux from two different cell lines: Raw 264.7 macrophages and CHOK1 cells. All rHDL and lipid-free apolipoprotein A-I (apoA-I) were found to be bound to CHO and RAW cells. In RAW cells, a positive correlation between cellular binding and Chol removal was found for 78 and 96 Å rHDL. Chol-free rHDL were more effective than Chol-containing ones in binding to RAW cells and promoting Chol removal. These results were more evident in the 96 Å rHDL. On the other hand, rHDL binding to CHO cells was relatively independent of disc size and Chol content. In spite of the fact that apoA-I and rHDL promoted Chol efflux from both cellular lines, only in CHOK1 cells this result was also associated to decrease Chol esterification. Among choline-containing phospholipids, only phosphatidylcholine (PC) (but not sphingomyelin) was detected to be effuxed from both cellular lines. With the only exception of Chol-free 96 Å discs, the other rHDL as well as apoA-I promoted PC efflux from RAW cells. Chol-containing rHDL were more active than Chol-free ones of comparable size to promote PC efflux from RAW macrophages. Regarding CHO cells, only apoA-I and Chol-free 78 Å rHDL were active enough to remove PC.

Interaction of enterocyte FABPs with phospholipid membranes: clues for specific physiological roles

Intestinal and liver fatty acid binding proteins (IFABP and LFABP, respectively) are cytosolic soluble proteins with the capacity to bind and transport hydrophobic ligands between different sub-cellular compartments. Their functions are still not clear but they are supposed to be involved in lipid trafficking and metabolism, cell growth, and regulation of several other processes, like cell differentiation. Here we investigated the interaction of these proteins with different models of phospholipid membrane vesicles in order to achieve further insight into their specificity within the enterocyte. A combination of biophysical and biochemical techniques allowed us to determine affinities of these proteins to membranes, the way phospholipid composition and vesicle size and curvature modulate such interaction, as well as the effect of protein binding on the integrity of the membrane structure. We demonstrate here that, besides their apparently opposite ligand transfer mechanisms, both LFABP and IFABP are able to interact with phospholipid membranes, but the factors that modulate such interactions are different for each protein, further implying different roles for IFABP and LFABP in the intracellular context. These results contribute to the proposed central role of intestinal FABPs in the lipid traffic within enterocytes as well as in the regulation of more complex cellular processes.

Membrane insertion topology of the central apolipoprotein A-I region. Fluorescence studies using single tryptophan mutants

Apolipoprotein A-I (apoAI) contains several amphipathic α-helices. To carry out its function, it exchanges between lipid-free and different lipidated states as bound to membranes or to lipoprotein complexes of different morphology, size, and composition. When bound to membranes or to spherical lipoprotein surfaces, it is thought that most α-helices arrange with their long axis parallel to the membrane surface. However, we previously found that a central region spanning residues 87-112 is exclusively labeled by photoactivable reagents deeply located into the membrane (Córsico et al. (2001) J. Biol. Chem. 276, 16978-16985). A pair of amphipathic α-helical repeats with a particular charge distribution is predicted in this region. In order to study their insertion topology, three single tryptophan mutants, each one containing the tryptophan residue at a selected position in the hydrophobic face of the central Y-helices (W@93, W@104, and W@108), were used. From the accessibility to quenchers located at different membrane depths, distances from the bilayer center of 13.4, 10.5, and 15.7 Å were estimated for positions 93, 104, and 108, respectively. Reported data also indicate that distances between homologous positions (in particular for W@93 and W@104) are very short in dimers in aqueous solution, but they are larger in membrane-bound dimers. Data indicate that an intermolecular central Y-helix bundle would penetrate the membrane perpendicularly to the membrane surface. Intermolecular helix-helix interactions would occur through the hydrophilic helix faces in the membrane-bound bundle but through the hydrophobic faces in the case of dimers in solution.

Contribution of the C-terminal end of apolipoprotein AI to neutralization of lipopolysaccharide endotoxic effect

It is well known that high density lipoprotein (HDL) binds bacterial lipopolysaccharide (LPS) and neutralizes its toxicity. The aim of this work was to study changes in the apolipoprotein (apo) AI structure after its interaction with LPS as well as to determine the protein domain involved in that interaction. The presented data indicate that LPS does not lead to major changes in the structure of apoAI, judging from Trp fluorescence spectra. However, analysis of denaturation behavior and binding of ANS show that LPS induces a loosened protein conformation. Further evidence for an apoAI—LPS specific interaction was obtained by incubation of the protein with 125I-ASD-LPS. The results show that multiple regions of the protein were able to interact with LPS, according to its amphiphatic nature. Finally, the contribution of the purified C-terminal fragment of the protein in the endotoxin neutralization was evaluated in comparison with the effect of apoAI. In both cases, the same decrease in tumor necrosis factor-α released was observed. This result suggests that the C-terminal half of apoAI is the main domain responsible of the neutralization effect of this protein. Our data may provide innovative pharmacological tools in endotoxin neutralization therapies.

Membrane organization and regulation of cellular cholesterol homeostasis

An excess of intracellular free cholesterol (Chol) is cytotoxic, and its homeostasis is crucial for cell viability. Apolipoprotein A-I (apoA-I) is a highly efficient Chol acceptor because it activates complex cellular pathways that tend to mobilize and export Chol from cellular depots. We hypothesize that membrane composition and/or organization is strongly involved in Chol homeostasis. To test this hypothesis, we constructed a cell line overexpressing stearoyl coenzyme A (CoA) desaturase (SCD cells), which modifies plasma membrane (PM) composition by the enrichment of monounsaturated fatty acids, and determined this effect on membrane properties, cell viability, and Chol homeostasis. PM in SCD cells has a higher ratio of phospholipids to sphingomyelin and is slightly enriched in Chol. These cells showed an increase in the ratio of cholesteryl esters to free Chol; they were more resistant to Chol toxicity, and they exported more caveolin than control cells. The data suggest that cell functionality is preserved by regulating membrane fluidity and Chol exportation and storage.

High-density lipoprotein metabolism and the human embryo

BACKGROUND

High-density lipoprotein (HDL) appears to be the dominant lipoprotein particle in human follicular fluid (FF). The reported anti-atherogenic properties of HDL have been attributed in part to reverse cholesterol transport. The discoveries of the scavenger receptor class B type I (SR-BI) and the ATP-binding cassette A1 lipid (ABCA1) transporter have generated studies aimed at unraveling the pathways of HDL biogenesis, remodeling and catabolism. The production of SR-BI and ABCA1 knockout mice as well as other lipoprotein metabolism-associated mutants has resulted in reduced or absent fertility, leading us to postulate the existence of a human hepatic-ovarian HDL-associated axis of fertility. Here, we review an evolving literature on the role of HDL metabolism on mammalian fertility and oocyte development.

METHODS

An extensive online search was conducted of published articles relevant to the section topics discussed. All relevant English language articles contained in Pubmed/Medline, with no specific time frame for publication, were considered for this narrative review. Cardiovascular literature was highly cited due to the wealth of relevant knowledge on HDL metabolism, and the dearth thereof in the reproductive field.

RESULTS

Various vertebrate models demonstrate a role for HDL in embryo development and fertility. In our clinical studies, FF levels of HDL cholesterol and apolipoprotein AI levels were negatively associated with embryo fragmentation, but not with embryo cell cleavage rate. However, the HDL component, paraoxonase 1 arylesterase activity, was positively associated with embryo cell cleavage rate.

CONCLUSIONS

HDL contributes to intra-follicular cholesterol homeostasis which appears to be important for successful oocyte and embryo development.

Structure and stability of crustacean lipovitellin: influence of lipid content and composition

Lipovitellin (LV) is essential in crustacean eggs for embryo viability and development. Two LV were isolated from eggs of Macrobrachium borellii. corresponding to early (LVe ) and late (LVl) embryo developing stages. They differ in lipid composition but not in lipid/protein ratio or apoprotein composition. Structural information was obtained by fluorescence spectroscopy, far-UV circular dichroism, partial trypsinolysis and electron microscopy applied to LVe and LVl and two partially delipidated forms of LVe generated by phospholipase A2 (LVp) or Triton X-100 (LVt) treatment. All LV forms contained two apoprotein subunits of 94 and 112 kDa, being the 112k Da subunit more accessible to trypsinolysis in all. Only in LVp, different cleavage sites appeared. Secondary structure was similar in LVe and LVl, but LVp and LVt showed a small increase in beta-sheet at expense of alpha-helix. Electron microscopy revealed a spheroidal morphology in all LV and a decreased size in LVp. Delipidated LVs were more resistant to denaturation with guanidinium-HCl. Acrylamide quenching of tryptophan fluorescence was more efficient in delipidated LVs, probably due to apolipoprotein rearrangement, as reinforced by fluorescence anisotropy. It is concluded that LV stability, shape, and apoprotein conformation is not largely affected by the changes in lipid composition that take place during embryogenesis.

Interaction of high density lipoprotein particles with membranes containing cholesterol

In this study, free cholesterol (FC) efflux mediated by human HDL was investigated using fluorescence methodologies. The accessibility of FC to HDL may depend on whether it is located in regions rich in unsaturated phospholipids or in domains containing high levels of FC and sphingomyelin, known as "lipid rafts." Laurdan generalized polarization and two-photon microscopy were used to quantify FC removal from different pools in the bilayer of giant unilamellar vesicles (GUVs). GUVs made of POPC and FC were observed after incubation with reconstituted particles containing apolipoprotein A-I and POPC [78A diameter reconstituted high density lipoprotein (rHDL)]. Fluorescence correlation spectroscopy data show an increase in rHDL size during the incubation period. GUVs made of two "raft-like" mixtures [DOPC/DPPC/FC (1:1:1) and POPC/SPM/FC (6:1:1)] were used to model liquid-ordered/liquid-disordered phase coexistence. Through these experiments, we conclude that rHDL preferentially removes cholesterol from the more fluid phases. These data, and their extrapolation to in vivo systems, show the significant role that phase separation plays in the regulation of cholesterol homeostasis.

Functional independence of a peptide with the sequence of human apolipoprotein A-I central region

Previous results [J. Biol. Chem. 276 (2001) 16978] indicated that an apolipoprotein A-I (apoAI) central region swings away from lipid contact in discoidal high density lipoproteins (HDL), but it is able to penetrate into the bilayer of lipid vesicles. In this work, we have studied the interaction with lipid membranes of a synthetic peptide with the sequence of apoAI region between residues 77 and 120 (AI 77-120). Like apoAI, AI 77-120 binds to phospholipid vesicles and shows selectivity for cholesterol-containing membranes. Moreover, AI 77-120 promotes cholesterol desorption from membranes in a similar fashion as apoAI and can stimulate cholesterol efflux from Chinese hamster ovary cells. AI 77-120 has a considerable alpha-helical content in water solution, and its secondary structure is not largely modified after binding to membranes. Both apoA-I and AI 77-120 are oligomeric in the lipid-bound state, suggesting that dimerization of the central domain could be required for the membrane binding activity of apoA-I in HDL.

Oxidation of methionine residues affects the structure and stability of apolipoprotein A-I in reconstituted high density lipoprotein particles

To determine the effect of oxidative damage to lipid-bound apolipoprotein A-I (apo A-I) on its structure and stability that might be related to previously observed functional disorders of oxidized apo A-I in high density lipoproteins (HDL), we prepared homogeneous reconstituted HDL (rHDL) particles containing unoxidized apo A-I and its commonly occurring oxidized form (Met-112, 148 bis-sulfoxide). The size of the obtained discoidal rHDL particles ranged from 9.0 to 10.0 nm and did not depend upon the content of the oxidized protein. Using circular dichroism methods, no change in the secondary structure of lipid-bound oxidized apo A-I was found. Isothermal and thermal denaturation experiments showed a significant destabilization of the oxidized protein to denaturation by guanidine hydrochloride or heat. This effect was observed with and without co-reconstituted apolipoprotein A-II. Limited tryptic digestion indicated that the central region of oxidatively damaged apo A-I becomes exposed to proteolysis in the rHDL particles. Implications of these data for apolipoprotein function are discussed.

Evidence for a central apolipoprotein A-I domain loosely bound to lipids in discoidal lipoproteins that is capable of penetrating the bilayer of phospholipid vesicles

Previous evidence indicated that discoidal reconstituted high density lipoproteins (rHDL) of apolipoprotein A-I (apoA-I) can interact with lipid membranes (Tricerri, M. A., Córsico, B., Toledo, J. D., Garda, H. A., and Brenner, R. R. (1998) Biochim. Biophys. Acta 1391, 67-78). With the aim of studying this interaction, photoactivable reagents and protein cleavage with CNBr and hydroxylamine were used. The generic hydrophobic reagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine gave information on the apoA-I regions in contact with the lipid phase in the rHDL discs. Two protein regions loosely bound to lipids were detected: a C-terminal domain and a central one located between residues 87 and 112. They consist of class Y amphipathic alpha-helices that have a different distribution of the charged residues in their polar faces by comparison with class A helices, which predominate in the rest of the apoA-I molecule. The phospholipid analog 1-O-hexadecanoyl-2-O-[9-[[[2-[125I]iodo-4-(trifluoro-methyl-3-H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, which does not undergo significant exchange between membranes and lipoproteins, was used to identify the apoA-I domain directly involved in the interaction of rHDL discs with membranes. By incubating either rHDL or lipid-free apoA-I with lipid vesicles containing 125I-TID-PC, only the 87-112 apoA-I segment becomes labeled after photoactivation. These results indicate that the central domain formed by two type Y helices swings away from lipid contact in the discoidal lipoproteins and is able to insert into membrane bilayers, a process that may be of great importance for the mechanism of cholesterol exchange between high density lipoproteins and cell membranes.

Cholesterol flux between lipid vesicles and apolipoprotein AI discs of variable size and composition

Reconstituted discoidal high-density lipoproteins (rHDLs) of apolipoprotein AI are able to induce leakage of the internal aqueous space of lipid vesicles (A. Tricerri et al., 1998, Biochim. Biophys. Acta 1391, 67-78) and such interaction depends on the cholesterol content of vesicles and rHDL as well as the rHDL size. With the aim of knowing if this rHDL/vesicle interaction plays some role in the cholesterol exchange, the time course for bidirectional radiolabeled cholesterol transfer between 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) vesicles and different sized rHDLs was measured. The results show that size increase in the rHDL decreases the rate constant for cholesterol transfer from POPC/cholesterol vesicles and that the initial presence of cholesterol in the vesicles results in an increased rate constant for cholesterol transfer from the rHDLs. This cannot be explained by a simple aqueous diffusion mechanism. The existing correlation between rHDL/vesicle interaction and cholesterol transfer rate suggests that besides the aqueous diffusion, another mechanism involving the binding or interaction between donor and acceptor may occur. This fact may be of physiological relevance since the relative high affinity of small cholesterol-poor discs for cell membranes could facilitate the cholesterol efflux, while the decreased membrane affinity as a consequence of cholesterol enrichment and increase in size would decrease the rate of transfer in the opposite direction.