Sterol domains in phospholipid membranes: dehydroergosterol polarization measures molecular sterol transfer

Fluorescence techniques using dehydroergosterol to study cholesterol trafficking

Cholesterol itself has very few structural/chemical features suitable for real-time imaging in living cells. Thus, the advent of dehydroergosterol [ergosta-5,7,9(11),22-tetraen-3beta-ol, DHE] the fluorescent sterol most structurally and functionally similar to cholesterol to date, has proven to be a major asset for real-time probing/elucidating the sterol environment and intracellular sterol trafficking in living organisms. DHE is a naturally occurring, fluorescent sterol analog that faithfully mimics many of the properties of cholesterol. Because these properties are very sensitive to sterol structure and degradation, such studies require the use of extremely pure (>98%) quantities of fluorescent sterol. DHE is readily bound by cholesterol-binding proteins, is incorporated into lipoproteins (from the diet of animals or by exchange in vitro), and for real-time imaging studies is easily incorporated into cultured cells where it co-distributes with endogenous sterol. Incorporation from an ethanolic stock solution to cell culture media is effective, but this process forms an aqueous dispersion of DHE crystals which can result in endocytic cellular uptake and distribution into lysosomes which is problematic in imaging DHE at the plasma membrane of living cells. In contrast, monomeric DHE can be incorporated from unilamellar vesicles by exchange/fusion with the plasma membrane or from DHE-methyl-beta-cyclodextrin (DHE-MbetaCD) complexes by exchange with the plasma membrane. Both of the latter techniques can deliver large quantities of monomeric DHE with significant distribution into the plasma membrane. The properties and behavior of DHE in protein-binding, lipoproteins, model membranes, biological membranes, lipid rafts/caveolae, and real-time imaging in living cells indicate that this naturally occurring fluorescent sterol is a useful mimic for probing the properties of cholesterol in these systems.

Structure of dehydroergosterol monohydrate and interaction with sterol carrier protein-2

Dehydroergosterol [ergosta-5,7,9(11),22-tetraen-3beta-ol] is a naturally-occurring, fluorescent sterol utilized extensively to probe membrane cholesterol distribution, cholesterol-protein interactions, and intracellular cholesterol transport both in vitro and in vivo. In aqueous solutions, the low solubility of dehydroergosterol results in the formation of monohydrate crystals similar to cholesterol. Low temperature X-ray diffraction analysis reveals that dehydroergosterol monohydrate crystallizes in the space group P2(1) with four molecules in the unit cell and monoclinic crystal parameters a = 9.975(1) A, b = 7.4731(9) A, c = 34.054(4) A, and beta = 92.970(2) degrees somewhat similar to ergosterol monohydrate. The molecular arrangement is in a slightly closer packed bilayer structure resembling cholesterol monohydrate. Since dehydroergosterol fluorescence emission undergoes a quantum yield enhancement and red-shift of its maximum wavelength when crystallized, formation or disruption of microcrystals was monitored with high sensitivity using cuvette-based spectroscopy and multi-photon laser scanning imaging microscopy. This manuscript reports on the dynamical effect of sterol carrier protein-2 (SCP-2) interacting between aqueous dispersions of dehydroergosterol monohydrate microcrystal donors and acceptors consisting not only of model membranes but also vesicles derived from plasma membranes isolated by biochemical fractionation and affinity purification from Madin-Darby canine kidney cells. Furthermore, this study provides real-time measurements of the effect of increased SCP-2 levels on the rate of disappearance of dehydroergosterol microcrystals in living cells.

The potential of fluorescent and spin-labeled steroid analogs to mimic natural cholesterol

Cholesterol analogs are often used to investigate lipid trafficking and membrane organization of native cholesterol. Here, the potential of various spin (doxyl moiety) and fluorescent (7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) group) labeled cholesterol analogs as well as of fluorescent cholestatrienol and the naturally occurring dehydroergosterol to mimic the unique properties of native cholesterol in lipid membranes was studied in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes by electron paramagnetic resonance, nuclear magnetic resonance, and fluorescence spectroscopy. As cholesterol, all analogs undergo fluctuating motions of large amplitude parallel to the bilayer normal. Native cholesterol keeps a strict orientation in the membrane with the long axis parallel to the bilayer normal. Depending on the chemical modification or the position of the label, cholesterol analogs may adopt an "up-side-down" orientation in the membrane or may even fluctuate between "upright" and up-side-down orientation by rotational motions about the short axis not typical for native cholesterol. Those analogs are not able to induce a comparable condensation of phospholipid membranes as known for native cholesterol revealed by 2H nuclear magnetic resonance. However, cholesterol-induced lipid condensation is one of the key properties of native cholesterol, and, therefore, a well suited parameter to assess the potential of steroid analogs to mimic cholesterol. The study points to extreme caution when studying cholesterol behavior by the respective analogs. Among seven analogs investigated, only a spin-labeled cholesterol with the doxyl group at the end of the acyl chain and the fluorophore cholestatrienol mimic cholesterol satisfactorily. Dehydroergosterol has a similar upright orientation as cholesterol and could be used at low concentration (about 1 mol %), at which its lower potential to enhance lipid packing density does not perturb membrane organization.

ACBP and cholesterol differentially alter fatty acyl CoA utilization by microsomal ACAT

Microsomal acyl CoA:cholesterol acyltransferase (ACAT) is stimulated in vitro and/or in intact cells by proteins that bind and transfer both substrates, cholesterol, and fatty acyl CoA. To resolve the role of fatty acyl CoA binding independent of cholesterol binding/transfer, a protein that exclusively binds fatty acyl CoA (acyl CoA binding protein, ACBP) was compared. ACBP contains an endoplasmic reticulum retention motif and significantly colocalized with acyl-CoA cholesteryl acyltransferase 2 (ACAT2) and endoplasmic reticulum markers in L-cell fibroblasts and hepatoma cells, respectively. In the presence of exogenous cholesterol, ACAT was stimulated in the order: ACBP > sterol carrier protein-2 (SCP-2) > liver fatty acid binding protein (L-FABP). Stimulation was in the same order as the relative affinities of the proteins for fatty acyl CoA. In contrast, in the absence of exogenous cholesterol, these proteins inhibited microsomal ACAT, but in the same order: ACBP > SCP-2 > L-FABP. The extracellular protein BSA stimulated microsomal ACAT regardless of the presence or absence of exogenous cholesterol. Thus, ACBP was the most potent intracellular fatty acyl CoA binding protein in differentially modulating the activity of microsomal ACAT to form cholesteryl esters independent of cholesterol binding/transfer ability.

Role of the sterol carrier protein-2 N-terminal membrane binding domain in sterol transfer

Previous studies showed that the N-terminal 32 amino acids of sterol carrier protein-2 ((1-32)SCP(2)) comprise an amphipathic alpha-helix essential for SCP(2) binding to membranes [Huang et al. (1999) Biochemistry 38, 13231]. However, it is unclear whether membrane interaction of the (1-32)SCP(2) portion of SCP(2) is in itself sufficient to mediate intermembrane sterol transfer, possibly by altering membrane structure. In this study a fluorescent sterol exchange assay was used to resolve these issues and demonstrated that the SCP(2) N-terminal peptide (1-32)SCP(2) did not by itself enhance intermembrane sterol transfer but potentiated the ability of the SCP(2) protein to stimulate sterol transfer. Compared with SCP(2) acting alone, (1-32)SCP(2) potentiated the sterol transfer activity of SCP(2) by increasing the initial rate of sterol transfer by 2.9-fold and by decreasing the half-time of sterol transfer by 10-fold (from 11.6 to 1.2 min) without altering the size of the transferable fractions. The ability of a series of SCP(2) mutant N-terminal peptides to potentiate SCP(2)-mediated sterol transfer was directly correlated with membrane affinity of the respective peptide. N-Terminal peptide (1-32)SCP(2) did not potentiate intermembrane sterol transfer by binding sterol (dehydroergosterol), altering membrane fluidity (diphenylhexatriene) or membrane permeability (leakage assay). Instead, fluorescence lifetime measurements suggested that SCP(2) and (1-32)SCP(2) bound to membranes and thereby elicited a shift in membrane sterol microenvironment to become more polar. In summary, these data for the first time showed that while the N-terminal membrane binding domain of SCP(2) was itself inactive in mediating intermembrane sterol transfer, it nevertheless potentiated the ability of SCP(2) to enhance sterol transfer.

Molecular and fluorescent sterol approaches to probing lysosomal membrane lipid dynamics

Although the most exogenous lipids enter the cell via the LDL-receptor pathway, the mechanism(s) whereby lipids leave the lysosome for transport to intracellular sites are not clearly resolved. As shown herein, expression of sterol carrier protein-2 (SCP-2) in transfected L-cells altered lysosomal membrane lipid distribution, dynamics, and response to lipid transfer proteins. SCP-2 expression decreased the mass of cholesterol and lyso-bis-phosphatidic acid [LBPA], as well as the ratios of cholesterol/phospholipid and polyunsaturated/monounsaturated fatty acids esterified to lysosomal membrane phospholipids. Concomitantly, a fluorescent sterol transfer assay showed that SCP-2 expression decreased the initial rates of spontaneous and SCP-2-mediated sterol transfer 5.5- and 3.8-fold, respectively, from lysosomal membranes isolated from SCP-2 expressing cells as compared to controls. SCP-2, sphingomyelinase, low density lipoprotein, and high density lipoprotein directly enhanced the initial rates of sterol transfer from isolated lysosomal membranes by 50-, 12-, 4-, and 5-fold, respectively. In contrast, albumin and cholesterol esterase had no effect on lysosomal sterol transfer. Spontaneous sterol was very slow, t(1/2)>4 days, regardless of the source of the lysosomal membrane, while SCP-2 added in vitro induced formation of rapid and slowly transferable sterol pools in lysosomal membranes of control cells. In contrast, SCP-2 did not induce formation of a rapidly transferable sterol domain in lysosomal membranes isolated from SCP-2 expressing cells. These data suggest that SCP-2 expression selectively shifted the distribution of lipids (cholesterol, LBPA, esterified polyunsaturated fatty acids) away from lysosomal membranes. Furthermore, the cholesterol depleted lysosomal membrane isolated from SCP-2 expressing cells was resistant to additional direct action of SCP-2 to further enhance sterol transfer and induce rapidly transferable sterol pools in the lysosomal membrane.

Fluorescence studies of lipid regular distribution in membranes

This article reviews the use of fluorescent lipids and free probes in the studies of lipid regular distribution in model membranes. The first part of this article summarizes the evidence and physical properties for lipid regular distribution in pyrene-labeled phosphatidylcholine (PC)/unlabeled PC binary mixtures as revealed by the fluorescence of pyrene-labeled PC. The original and the extended hexagonal superlattice model are discussed. The second part focuses on the fluorescence studies of sterol regular distributions in membranes. The experimental evidence for sterol superlattice formation obtained from the fluorescent sterol (i.e. dehydroergosterol) and non-sterol fluorescent probes (e.g. DPH and Laurdan) are evaluated. Prospects and concerns are given with regard to the sterol regular distribution. The third part deals briefly with the evidence for polar headgroup superlattices. The emphasis of this article is placed on the new concept that membrane properties and activities, including the activities of surface acting enzymes, drug partitioning, and membrane free volume, are fine-tuned by minute changes in the concentration of bulky lipids (e.g. sterols and pyrene-containing acyl chains) in the vicinities of the critical mole fractions for superlattice formation.

Lysosomal membrane cholesterol dynamics

Although the majority of exogenous cholesterol and cholesterol ester enters the cell by LDL-receptor-mediated endocytosis and the lysosomal pathway, the assumption that cholesterol transfers out of the lysosome by rapid (minutes), spontaneous diffusion has heretofore not been tested. As shown herein, lysosomal membranes were unique among known organellar membranes in terms of cholesterol content, cholesterol dynamics, and response to cholesterol-mobilizing proteins. First, the lysosomal membrane cholesterol:phospholipid molar ratio, 0.38, was intermediate between those of the plasma membrane and other organellar membranes. Second, a fluorescence sterol exchange assay showed that the initial rate of spontaneous sterol transfer out of lysosomes and purified lysosomal membranes was extremely slow, t(1/2) >4 days. This was >100-fold longer than that reported in intact cells (2 min) and 40-60-fold longer than from any other known intracellular membrane. Third, when probed with several cholesterol-binding proteins, the initial rate of sterol transfer was maximally increased nearly 80-fold and the organization of cholesterol in the lysosomal membrane was rapidly altered. Nearly half of the essentially nonexchangeable sterol in the lysosomal membrane was converted to rapidly (t(1/2) = 6 min; fraction = 0.06) and slowly (t(1/2) = 154 min; fraction = 0.36) exchangeable sterol domains/pools. In summary, the data revealed that spontaneous cholesterol transfer out of the lysosome and lysosomal membrane was extremely slow, inconsistent with rapid spontaneous diffusion across the lysosomal membrane. In contrast, the very slow spontaneous transfer of sterol out of the lysosome and lysosomal membrane was consistent with cholesterol leaving the lysosome earlier in the endocytic process and/or with cholesterol transfer out of the lysosome being mediated by additional process(es) extrinsic to the lysosome and lysosomal membrane.

A potential role for sterol carrier protein-2 in cholesterol transfer to mitochondria

Mitochondrial cholesterol oxidation rapidly depletes cholesterol from the relatively cholesterol-poor mitochondrial membranes. However, almost nothing is known regarding potential mechanism(s) whereby the mitochondrial cholesterol pool is restored. Since most exogenous cholesterol enters the cell via the lysosomal pathway, this could be a source of mitochondrial cholesterol. In the present study, an in vitro fluorescent sterol transfer assay was used to examine whether the lysosomal membrane could be a putative cholesterol donor to mitochondria. First, it was shown that spontaneous sterol transfer from lysosomal to mitochondrial membranes was very slow (initial rate, 0.316 +/- 0.032 pmol/min). This was due, in part, to the fact that 90% of the lysosomal membrane sterol was not exchangeable, while the remaining 10% also had a relatively long half-time of exchange t(1/2) = 202 +/- 19 min. Second, the intracellular sterol carrier protein-2 (SCP-2) and its precursor (pro-SCP-2) increased the initial rate of sterol transfer from the lysosomal to mitochondrial membrane by 5.2- and 2.0-fold, respectively, but not in the reverse direction. The enhanced sterol transfer was due to a 3.5-fold increase in exchangeable sterol pool size and to induction of a very rapidly (t(1/2) = 4.1 +/- 0.6 min) exchangeable sterol pool. Confocal fluorescence imaging and indirect immunocytochemistry colocalized significant amounts of SCP-2 with the mitochondrial marker enzyme cytochrome oxidase in transfected L-cells overexpressing SCP-2. In summary, SCP-2 and pro-SCP-2 both stimulated molecular sterol transfer from lysosomal to mitochondrial membranes, suggesting a potential mechanism for replenishing mitochondrial cholesterol pools depleted by cholesterol oxidation.

The sterol carrier protein-2 amino terminus: a membrane interaction domain

Sterol carrier protein-2 (SCP2) is a small, 123 amino acid, protein postulated to play a role in intracellular transport and metabolism of lipids such as cholesterol, phospholipids, and branched chain fatty acids. While it is thought that interaction of SCP2 with membranes is necessary for lipid transfer, evidence for this possibility and identification of a membrane interaction domain within SCP2 has remained elusive. As shown herein with circular dichroism and a direct binding assay, SCP2 bound to small unilamellar vesicle (SUV) membranes to undergo significant alteration in secondary structure. The SCP2 amphipathic N-terminal 32 amino acids, comprised of two alpha-helical segments, were postulated to represent a putative phospholipid interaction site. This hypothesis was tested with a series of SCP2 N-terminal peptides, circular dichroism, and direct binding studies. The SCP2 N-terminal peptide (1-32)SCP2, primarily random coil in aqueous buffer, adopted alpha-helical structure upon interaction with membranes. The induction of alpha-helical structure in the peptide was maximal when the membranes contained a high mole percent of negatively charged phospholipid and of cholesterol. While deletion of the second alpha-helical segment within this peptide had no effect on formation of the first alpha-helix, it significantly weakened the peptide interaction with membranes. Substitution of Leu(20) with Glu(20) in the N-terminal peptide disrupted the alpha-helix structure and greatly weakened the peptide interaction with membranes. Finally, deletion of the first nine nonhelical amino acids had no effect either on formation of alpha-helix or on peptide binding to membranes. N-Terminal peptide (1-32)SCP2 competed with SCP2 for binding to SUV. These data were consistent with the N-terminus of SCP2 providing a membrane interaction domain that preferentially bound to membranes rich in anionic phospholipid and cholesterol.

Lipid specificity and location of the sterol carrier protein-2 fatty acid-binding site: a fluorescence displacement and energy transfer study

Although it was recently recognized that sterol carrier protein-2 (SCP-2) interacts with fatty acids, little is known regarding the specificity of SCP-2 for long-chain fatty acids or branched-chain fatty-acid-like molecules. Likewise the location of the fatty-acid binding site within SCP-2 is unresolved. A fluorescent cis-parinaric acid displacement assay was used to show that SCP-2 optimally interacted with 14-22 carbon chain lipidic molecules: polyunsaturated fatty acids > monounsaturated, saturated > branched-chain isoprenoids > branched-chain phytol-derived fatty acids. In contrast, the other major fatty-acid binding protein in liver, fatty-acid binding protein (L-FABP), displayed a much narrower carbon chain preference in general: polyunsaturated fatty acids > branched-chain phytol-derived fatty acids > 14- and 16-carbon saturated > branched-chain isoprenoids. However, both SCP-2 and L-FABP displayed a very similar unsaturated fatty-acid specificity profile. The presence and location of the SCP-2 lipid binding site were investigated by fluorescence energy transfer. The distance between the SCP-2 Trp50 and bound cis-parinaric acid was determined to be 40 A. Thus, the SCP-2 fatty-acid binding site appeared to be located on the opposite side of the SCP-2 Trp50. These findings not only contribute to our understanding of the SCP-2 ligand binding site but also provide evidence suggesting a potential role for SCP-2 and/or L-FABP in metabolism of branched-chain fatty acids and isoprenoids.

Photochemical reactions and phototoxicity of sterols: novel self-perpetuating mechanisms for lipid photooxidation

Sterols are important lipid components that may contribute to phototoxicity. We have found that phototoxic response in earthworms is related to sterols extractable with lipophilic solvents. The photochemically active compounds in worm lipids are 5,7,9(11),22-ergostatetraen-3 beta-ol (9-DHE) and 5,7,9(11)-cholestartien-3 beta-ol (9-DDHC), respectively. Human skin lipids are known to contain 9-DHE. We have also found 9-DDHC in human skin, which is reported here for the first time. In the presence of an excess of the corresponding 5,7-dienes (ergosterol of 7-dehydrocholesterol), these photoactive sterols constitute a self-regenerating source of singlet molecular oxygen (1O2) during irradiation in vivo or in vitro with UVA (315-400 nm). The quantum yield for photosensitization of 1O2 by 9-DHE was estimated to be 0.09. The 1O2 is scavenged by the dienes and the rate constant for 1O2 quenching by ergosterol was found to be 1.2 x 10(7) M-1 s-1 in methyl t-butyl ether (MTBE). This scavenging ultimately leads to the production of 5,8-endoperoxide and hydrogen peroxide. Photochemically induced superoxide radical was also produced on irradiation of sterol 5,7,9-trienes and trapped with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The production of singlet oxygen, peroxides and radicals by the sterols may be significant in the cell damaging and tumor promoting action of UVA light on skin.

Dehydroergosterol structural organization in aqueous medium and in a model system of membranes

The aggregation of delta 5,7,9(11),22-ergostatetraen-3 beta-ol (dehydroergosterol or DHE), a fluorescent analog of cholesterol, was studied by photophysical techniques. It was concluded that the aqueous dispersions of DHE consist of strongly fluorescent microcrystals, and no evidence for self-quenching in micellar-type aggregates was found. The organization of DHE in model systems of membranes (phospholipid vesicles) is strongly dependent on the vesicle type. In small unilamellar vesicles, no evidence for aggregation is obtained, and the fluorescence anisotropy is rationalized on the basis of a random distribution of fluorophores. On the contrary, in large unilamellar vesicles (LUVs), a steeper concentration depolarization was observed. To explain this, a model that takes into account transbilayer dimer formation was derived. This was further confirmed from observation of excitonic absorption bands of 22-(N-7-nitrobenz-2-oxa-1,3-diazol-4-yl-amino)-23,24-bisnor- 5-cholen-3 beta-ol (NBD-cholesterol) in LUV, which disappear upon sonication. It is concluded that, in agreement with recent works, sterol aggregation is a very efficient process in large vesicles (and probably in natural membranes), even at very low concentrations (approximately 5 mol%).

Spontaneous and protein-mediated sterol transfer between intracellular membranes

Relatively little is known regarding intracellular cholesterol trafficking pathways. To resolve some of these potential pathways, spontaneous and protein-mediated sterol transfer was examined between different donor-acceptor membrane pairs in vitro using L-cell fibroblast plasma membrane (PM) and microsomal (MICRO) and mitochondrial (MITO) membranes. Several new exciting insights were provided. First, the initial rate of spontaneous molecular sterol transfer was more dependent on the type of acceptor than donor membrane, i.e. spontaneous intracellular sterol trafficking was vectorial. Therefore, the rate of sterol desorption from the donor membrane was not necessarily the rate-limiting step in molecular sterol transfer. Second, the rate of molecular sterol transfer was not obligatorily correlated with the direction of the cholesterol gradient. For example, although PM had a 3.2-fold higher cholesterol/phospholipid ratio than MITO, spontaneous sterol transfer was 4-5-fold faster up (MITO to PM) rather than down (PM to MITO) the concentration gradient. Third, sterol carrier protein-2 differentially stimulated the initial rate of sterol transfer for all donor-acceptor combinations, being most effective with PM donors: PM-MICRO, 27-fold; and PM-MITO, 12-fold. Sterol carrier protein-2 was less effective in enhancing sterol transfer in the reverse direction, i.e. MICRO-PM and MITO-PM (5- and 4-fold, respectively). Fourth, liver fatty acid-binding protein was limited in stimulating the initial rate of sterol transfer from PM to PM (1.5-fold), from PM to MITO (3-fold), and from MICRO to MITO (3-fold). In summary, these observations present important insights into potential sterol trafficking pathways between the major membrane components of the cell.

Liver fatty acid binding protein enhances sterol transfer by membrane interaction

Among the large family of fatty acid binding proteins, the liver L-FABP is unique in that it not only binds fatty acids but also interacts with sterols to enhance sterol transfer between membranes. Nevertheless, the mechanism whereby L-FABP potentiates intermembrane sterol transfer is unknown. Both fluorescence and dialysis data indicate L-FABP mediated sterol transfer between L-cell fibroblast plasma membranes occurs by a direct membrane effect: First, dansylated-L-FABP (DNS-L-FABP) is bound to L-cell fibroblast plasma membranes as indicated by increased DNS-L-FABP steady state polarization and phase resolved limiting anisotropy. Second, coumarin-L-FABP (CPM-L-FABP) fluorescence lifetimes were significantly increased upon interaction with plasma membranes. Third, dialysis studies with 3H-cholesterol loaded plasma membranes showed that L-FABP added to the donor compartment of the dialysis cell stimulated 3H-cholesterol transfer whether or not the dialysis membrane was permeable to L-FABP. However, L-FABP mediated intermembrane sterol transfer did require a sterol binding site on L-FABP. Chemically blocking the ligand binding site also inhibited L-FABP activity in intermembrane sterol transfer. Finally, L-FABP did not act either as an aqueous carrier or in membrane fusion. The fact that L-FABP interacted with plasma membrane vesicles and required a sterol binding site was consistent with a mode of action whereby L-FABP binds to the membrane prior to releasing sterol from the bilayer.

Cholesterol interaction with recombinant human sterol carrier protein-2

The interaction of human recombinant sterol carrier protein-2 (SCP-2) with sterols was examined. Two independent ligand binding methods, Lipidex 1000 binding of [3H]cholesterol and a fluorescent dehydroergosterol binding assay, were used to determine the affinity of SCP-2 for sterols. Binding analysis indicated SCP-2 bound [3H]cholesterol and dehydroergosterol with a Kd of 0.3 and 1.7 microM, respectively, and suggested the presence of a single binding site. Phase fluorometry and circular dichroism were used to characterize the SCP-2 sterol binding site. Alterations in dehydroergosterol lifetime, SCP-2 tryptophan lifetime, and SCP-2 tryptophan quenching by acrylamide upon cholesterol binding demonstrated a shielding of the SCP-2 tryptophan from the aqueous solvent by bound sterol. Differential polarized phase fluorometry revealed decreased SCP-2 tryptophan rotational correlation time upon cholesterol binding. Circular dichroism of SCP-2 indicated that cholesterol elicited a small decrease in SCP-2 alpha helical content. The data suggest that SCP-2 binds sterols with affinity consistent with a lipid transfer protein that may act either as an aqueous carrier or at a membrane surface to enhance sterol desorption.

Sterol carrier protein-2 stimulates intermembrane sterol transfer by direct membrane interaction

It is unclear how the cytosolic sterol carrier protein-2 (SCP-2) binds sterols and enhances sterol transfer between membranes. Therefore, human recombinant SCP-2 was used in conjunction with phase fluorometry, dialysis, and chemical labeling techniques to show if a direct membrane effect accounted for this activity. SCP-2 directly interacted with L-cell fibroblast plasma membrane vesicles as determined by increased fluorescence anisotropy of coumarin-labeled protein (CPM-SCP-2). Furthermore, a new fluorescence lifetime component due to plasma membrane-bound CPM-SCP-2 was observed. Dialysis studies with 3H- cholesterol loaded plasma membranes indicated that SCP-2, added to the donor compartment, stimulated sterol transfer whether or not the dialysis membrane was permeable to SCP-2. Nevertheless, ligand-binding experiments indicated that chemically blocking the SCP-2 sterol binding site inhibited the ability of SCP-2 to enhance sterol transfer between plasma membrane vesicles. SCP-2 did not stimulate plasma membrane fusion. Addition of SCP-2 to plasma membranes increased the anisotropy plasma membrane proteins covalently reacted with CPM, but not that of lipids labeled with the fatty acid analogue octadecyl rhodamine B. In conclusion, the data are consistent with SCP-2 stimulating intermembrane sterol transfer by direct interaction with sterol in the membrane and enhancing its desorption from the membrane.

Cholesterol esterase: a cholesterol transfer protein

Rat pancreatic cholesterol esterase was examined for its ability to effect sterol transfer between small unilamellar vesicle (SUV) preparations. Sterol exchange was determined using SUV composed of palmitoyloleoylphosphatidylcholine/sterol (65:35) with or without 10 mol % phosphatidylserine or phosphatidic acid. This recently developed assay does not require separation of donor and acceptor vesicles (Butko et al., 1992). Cholesterol esterase stimulated cholesterol exchange when SUV contained phosphatidylserine and even more so in the presence of phosphatidic acid. Cholesterol esterase increased the initial rate of sterol transfer between phosphatidic acid-containing SUV by approximately 80%. The enzyme increased sterol exchange by significantly decreasing the half-times of sterol transfer and by significantly increasing the initial rates of sterol exchange. In the absence of negatively charged phospholipids, cholesterol esterase was ineffective at increasing sterol transfer. Monolayer studies showed that negatively charged phospholipids seem to play a key role in cholesterol esterase adsorption to lipid interfaces. Finally, a mutant cholesterol esterase lacking a histidine (435) residue essential for esterasic catalysis was found to be equally capable of increasing sterol transfer and binding to charged monolayers. In summary, cholesterol esterase enhances sterol transfer in SUV containing negatively charged phospholipids, independent of esterasic activity.

Evidence for regular distribution of sterols in liquid crystalline phosphatidylcholine bilayers

To investigate the lateral organization of sterols in membranes, the fluorescence intensity of dehydroergosterol at different mole fractions in liquid crystalline dimyristoyl phosphatidylcholine bilayers was examined. A number of intensity drops were observed at specific mole fractions, as predicted from a hexagonal super-lattice model. The fluorescence dips provide compelling evidence that a naturally occurring sterol is regularly distributed at fixed compositional fractions, consistent with the presence of hexagonal super-lattices in the fluid membranes. Regularly distributed regions, however, coexist with irregularly distributed regions. The extent of regular distribution varies periodically with sterol mole fraction and, consequently, similar variations take place in the membrane volume and lipid packing. This level of modulation in local membrane structure by minute changes in sterol concentration should have profound implications for the functional role of cholesterol content in cell membranes.

Regulation of membrane cholesterol domains by sterol carrier protein-2

Sterols are not randomly distributed in membranes but appear to be localized in multiple kinetic domains. Factors that regulate these sterol domains are not well-understood. A recently developed fluorescence polarization assay that measures molecular sterol transfer [Butko, P., Hapala, I., Nemecz, G., of Schroeder, F. (1992) J. Biochem. Biophys. Methods 24, 15-37] was used to examine the mechanism whereby anionic phospholipids and liver sterol carrier protein-2 (SCP2) enhance sterol transfer. Two exchangeable and one very slowly or nonexchangeable sterol domain were resolved in phosphatidylcholine (POPC)/sterol small unilamellar vesicles (SUV). Inclusion of 10 mol % anionic phospholipids enhanced sterol exchange primarily by redistribution of sterol domain sizes rather than by alteration of half-times of exchange. This effect was dependent primarily on the percent content rather than the net charge per anionic phospholipid. In contrast, SCP2 simultaneously altered both the distribution of sterol molecules between kinetic domains and the exchange half-times of exchangeable sterol domains. The effects of SCP2 were much more pronounced when 10% acidic phospholipid was incorporated in the SUV. Compared to spontaneous sterol exchange, in the presence of 1.5 microM SCP2, the rapidly exchanging pool was increased by 36 to 330%, depending on the SUV phospholipid composition. Concomitantly, exchange half-times for rapidly and slowly exchangeable sterol were reduced by 60 to 98% for 1t1/2 and 14 to 85% for 2t1/2, respectively. The stimulatory effect of SCP2 was saturable and dependent both on protein concentration and on content of acidic phospholipids in membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanistic studies of sterol carrier protein-2 effects on L-cell fibroblast plasma membrane sterol domains

The factors which regulate intermembrane sterol domains and exchange in biomembranes are not well understood. A new fluorescent sterol exchange assay allowed correlation of changes in polarization to sterol transfer. Analysis of spontaneous sterol exchange between L-cell plasma membranes indicated two exchangeable and one very slowly or nonexchangeable sterol domain. The exchangeable domains exhibited half-times of 23 and 140 min with fractional contributions of 5 and 30%, respectively. Sterol carrier protein-2 (SCP-2) enhanced sterol exchange between L-cell plasma membranes and altered sterol domain size in a concentration dependent manner. Previous model membrane studies indicate that SCP-2 alters sterol domains and exchange through interaction with anionic phospholipids. In contrast to these observations, the ionic shielding agents KCl, low pH, or neomycin were either totally or partially ineffective inhibitors of SCP-2 action in L-cell plasma membrane exchanges. Thus the mechanism of SCP-2 in sterol transfer appears to be less charge dependent in L-cell plasma membranes than in model membranes. The cholesterol lowering drug probucol was also capable of altering the sterol exchange kinetics.

Expression of rat L-FABP in mouse fibroblasts: role in fat absorption

Fatty acid-binding proteins (FABP) are abundant cytosolic proteins whose levels is responsive to nutritional, endocrine, and a variety of pathological states. Although FABPs have been investigated in vitro for several decades, little is known of their physiological function. Liver L-FABP binds both fatty acids and cholesterol. Competitive binding analysis and molecular modeling studies of L-FABP indicate the presence of two ligand binding pockets that accommodate one fatty acid each. One fatty acid binding site is identical to the cholesterol binding site. To test whether these observations obtained in vitro were physiologically relevant, the cDNA encoding L-FABP was transfected into L-cells, a cell line with very low endogenous FABP and sterol carrier proteins. Uptake of both ligands did not differ between control cells and low expression clones. In contrast, both fatty acid uptake and cholesterol uptake were stimulated in the high expression cells. In high expression cells, uptake of fluorescent cis-parinaric acid was enhanced more than that of trans-parinaric acid. This is consistent with the preferential binding of cis-fatty acids to L-FABP but in contrast to the preferential binding of trans-parinaric acid to the L-cell plasma membrane fatty acid transporter (PMFABP). These data show that the level of cytosolic fatty acids in intact cells can regulate both the extent and specificity of fatty acid uptake. Last, sphingomyelinase treatment of L-cells released cholesterol from the plasma membrane to the cytoplasm and stimulated microsomal acyl-CoA: cholesteryl acyl transferase (ACAT). This process was accelerated in high expression cells. These observations show for the first time in intact cells that L-FABP, a protein most prevalent in liver and intestine where much fat absorption takes place, may have a role in fatty acid and cholesterol absorption.

Expression of liver fatty acid binding protein alters plasma membrane lipid composition and structure in transfected L-cell fibroblasts

Liver fatty acid binding protein, L-FABP, is an abundant protein that binds fatty acids in vitro. The effects of L-FABP on plasma membrane lipid composition, distribution, and physical structure were determined in intact L-cell fibroblasts transfected with cDNA encoding L-FABP. L-FABP expression altered plasma membrane phospholipids by decreasing both phosphatidylethanolamine and esterified oleic acid content, and increasing sphingomyelin. L-FABP also binds sterols and stimulates sterol uptake and esterification. The fluorescent sterol dehydroergosterol was used to examine sterol distribution in the transfected cell plasma membrane. Dehydroergosterol codistributed equally with the cholesterol in both the bulk membrane and the individual bilayer leaflets. The sterol/phospholipid ratio was decreased in the inner leaflet due to sterol depletion. Concomitantly, intermembrane sterol transfer from the rapidly exchangeable lateral sterol domains as measured by exchange of dehydroergosterol, was reduced. The fluidity of the plasma membrane was measured with the fluorescent molecule diphenylhexatriene by multifrequency (1-250 MHz) phase and modulation fluorometry. Both the bulk plasma membrane and the plasma membrane outer leaflet lipids were fluidized in transfected cells. These alterations of plasma membrane structure and composition are consistent with a role for L-FABP in regulating intracellular sterol and fatty acid distribution and thereby membrane lipid domain structure.