From the Departments of Anatomy and Cell Biology and
¶ Medicine, and the College of Physicians and Surgeons, Columbia
University, New York, New York 10032, the ** Wallenberg Laboratory,
Göteborg Universitet, S-413 45, Göteborg, Sweden, the
Institut Pasteur de Lille, 59019 Lille, France, the
Department of Surgery, University of
California, San Francisco and the San Francisco Veterans Affairs
Medical Center, San Francisco, California 94121, and the
§§ Dorrance H. Hamilton Research Laboratories,
Division of Endocrinology, Diabetes, and Metabolic Diseases, Thomas
Jefferson University, Philadelphia, Pennsylvania 19107
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ABSTRACT |
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The subendothelial aggregation and retention of low density lipoprotein (LDL) are key events in atherogenesis, but the mechanisms in vivo are not known. Previous studies have shown that treatment of LDL with bacterial sphingomyelinase (SMase) in vitro leads to the formation of lesion-like LDL aggregates that become retained on extracellular matrix and stimulate macrophage foam cell formation. In addition, aggregated human lesional LDL, but not unaggregated lesional LDL or plasma LDL, shows evidence of hydrolysis by an arterial wall SMase in vivo, and several arterial wall cell types secrete a SMase (S-SMase). S-SMase, however, has a sharp acid pH optimum using a standard in vitro SM-micelle assay. Thus, a critical issue regarding the potential role of S-SMase in atherogenesis is whether the enzyme can hydrolyze lipoprotein-SM, particularly at neutral pH. We now show that S-SMase can hydrolyze and aggregate native plasma LDL at pH 5.5 but not at pH 7.4. Remarkably, LDL modified by oxidation, treatment with phospholipase A2, or enrichment with apolipoprotein CIII, which are modifications associated with increased atherogenesis, is hydrolyzed readily by S-SMase at pH 7.4. In addition, lipoproteins from the plasma of apolipoprotein E knock-out mice, which develop extensive atherosclerosis, are highly susceptible to hydrolysis and aggregation by S-SMase at pH 7.4; a high SM:PC ratio in these lipoproteins appears to be an important factor in their susceptibility to S-SMase. Most importantly, LDL extracted from human atherosclerotic lesions, which is enriched in sphingomyelin compared with plasma LDL, is hydrolyzed by S-SMase at pH 7.4 10-fold more than same donor plasma LDL, suggesting that LDL is modified in the arterial wall to increase its susceptibility to S-SMase. In summary, atherogenic lipoproteins are excellent substrates for S-SMase, even at neutral pH, making this enzyme a leading candidate for the arterial wall SMase that hydrolyzes LDL-SM and causes subendothelial LDL aggregation.
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INTRODUCTION |
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A critical event in early atherogenesis is the subendothelial retention of atherogenic lipoproteins, including LDL1 (1, 2), lipoprotein(a) (3), and triglyceride-rich lipoproteins (4). Retained lipoproteins likely trigger a series of biological responses, such as endothelial changes and recruitment of macrophages to the arterial wall, that are central to the initiation and progression of atherosclerosis (5).
Subendothelial lipoproteins are exposed to several modifying enzymes, including lipases (6-8), oxidizing enzymes (9), and proteases (10). The actions of these and other unknown factors lead to the several prominent lipoprotein modifications observed in vivo, including oxidation (11), enrichment with the phospholipid sphingomyelin (SM) (11-13), and self-aggregation (2, 14, 15). Lipoprotein aggregation is likely to be important in atherogenesis for at least two reasons. First, processes that promote lipoprotein aggregation before or during retention dramatically increase the amount of lipoprotein retained (16). Second, aggregated LDL, but not unaggregated LDL, is a potent inducer of macrophage foam cell formation (17-20).
Although the mechanism of lipoprotein aggregation in lesions is not known, several studies from our laboratories suggest a role for the enzyme sphingomyelinase (SMase). First, LDL treated with bacterial SMase forms lesion-like self-aggregates (20) due to enrichment in ceramide (7), the major product of SM hydrolysis; furthermore, these aggregates potently induce macrophage foam cell formation in vitro (16, 20). Second, aggregated LDL from human atherosclerotic lesions, but not unaggregated lesional LDL or plasma LDL, shows evidence of hydrolysis by an extracellular SMase, and LDL retained in rabbit aortic strips ex vivo is hydrolyzed by an extracellular, cation-dependent SMase (7). Third, and most important, we have found that several cell types present in atherosclerotic lesions, namely macrophages (21) and endothelial cells,2 secrete a Zn2+-activated SMase (S-SMase).
The cellular origins, secretion, and cation dependence make S-SMase a leading candidate for the arterial wall SMase that acts on retained lipoproteins. Nonetheless, two major issues regarding the relevance of S-SMase to atherogenesis needed to be addressed. First, mammalian SMases are much more selective than bacterial SMases in terms of the milieu in which the SM is presented to the enzyme (22).3 Second, studies on the molecular origin of S-SMase have revealed that it is a product of the same gene, the acid SMase (ASM) gene, that gives rise to lysosomal SMase (L-SMase) (21). Not surprisingly, therefore, S-SMase shares with L-SMase a sharp acid pH optimum when assayed under standard in vitro conditions using detergent-solubilized SM-micelles as a substrate (21, 23). Although it is possible that acidic enzymes are active in advanced atherosclerotic lesions, where local pockets of acidity may occur (24-28), a role for such enzymes in pre-lesional or early lesional events would require activity at neutral pH with physiologic substrates.
In this context, the goal of the current study was to test whether S-SMase can hydrolyze LDL-SM, particularly at neutral pH. Herein, we show that S-SMase can hydrolyze and aggregate native LDL at acid but not neutral pH. LDL modified by several means that have been shown to occur or might occur during atherogenesis, however, is an excellent substrate for S-SMase at pH 7.4. Most importantly, LDL extracted from human atherosclerotic lesions is efficiently hydrolyzed by S-SMase at neutral pH, suggesting that LDL is modified in the arterial wall to increase its susceptibility to S-SMase. Our results support a role for S-SMase in the subendothelial hydrolysis of LDL-SM, perhaps leading to lipoprotein aggregation and lesion initiation and progression.
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EXPERIMENTAL PROCEDURES |
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Materials--
sn-1,2-Diacylglycerol kinase (from
Escherichia coli) was purchased from Calbiochem. Cardiolipin
and 1,2-dioleoylglycerol were purchased from Avanti Polar Lipids
(Alabaster, AL). [9,10-3H]Palmitic acid and
[-32P]ATP were obtained from NEN Life Science
Products. Tissue culture media and reagents were purchased from Life
Technologies, Inc., and fetal bovine serum was from Gemini Bioproducts
(Calabasas, CA). Human native apoCIII (29), human recombinant
nonpancreatic soluble PLA2 (sPLA2) (30), and partially
purified phospholipid transfer protein (31) were prepared as described
previously. Soybean lipoxygenase and all other reagents were from
Sigma.
Mice-- LDL receptor-deficient (LDLr0) (32) and apolipoprotein E-deficient (E0) mice (33, 34) were purchased from Jackson Laboratories and crossed into the C57BL/6J background. LDL receptor-deficient mice expressing a human apoCIII transgene (LDLr0/CIII) were derived as described by Masucci-Magoulas et al. (35).
Lipoprotein Isolation and Modification-- Human and murine LDL (density, 1.020-1.063 g/ml) were isolated from fresh plasma by preparative ultracentrifugation as described previously (36). LDL (5 mg/ml) was oxidized by dialysis against 150 mM NaCl, 6 µM FeSO4, 0.04% azide for 36 h at room temperature followed by addition of EDTA (1 mM) and BHT (150 µM) and then dialysis against 150 mM NaCl, 0.3 mM EDTA (37). Alternatively, LDL (1 mg) was incubated with 275 units soybean lipoxygenase/ml and 60 µg of linoleic acid/ml in 50 mM Tris-HCl, pH 7.4, 0.04% azide for 24 h at 37 °C (38); LDL was re-isolated using a G-200 gel filtration column and then concentrated using a Centricon 30 (molecular weight cut-off = 30,000) ultrafiltration device. For modification by sPLA2, LDL (5 mg/ml) was incubated with 15 µg of pure human recombinant sPLA2/ml in 0.12 M Tris-HCl, pH 8.0, 12 mM CaCl2, 0.1 mM EDTA, 10 µM butylated hydroxytoluene for 14 h at 37 °C; control and modified LDL (50 µg protein) were then treated directly with S-SMase as described below. Acetyl-LDL was prepared by acetylation of LDL with acetic anhydride (39).
Isolation of LDL from Human Lesions-- LDL was extracted from abdominal aortic aneurysm plaque material as described previously (4). Briefly, aortic plaque was removed from individuals as part of the standard reconstructive surgery for abdominal aortic aneurysms at the San Francisco Veterans Affairs Medical Center. Plaque material, which ranged in weight from 2 to 12 g, was obtained in the operating room and immediately placed into ice-cold 7 mM citrate buffer, pH 7.4, containing 15 mM NaCl, 3 mM EDTA, 0.5 mM butylhydroxytoluene, 1 mM phenylmethylsulfonyl fluoride, 1.5 mg aprotinin/ml, 2 mM benzamidine, and 0.08 mg of gentamicin sulfate/ml. Blood and adherent thrombus were removed by blotting with absorbent gauze, scrubbing with a small brush, and sharp dissection as necessary. Loosely retained lipoproteins were extracted by mincing the plaque into 0.5-1.0-mm2 pieces and incubating them overnight on a Labquake shaker at 4 °C in a non-denaturing buffer (0.1 M citrate, pH 7.4, with 1 mg of EDTA/ml, 0.3 mg of benzamidine/ml, 0.08 mg of gentamicin sulfate/ml, 10 µg of aprotinin/ml, 10 µg of Trolox (an anti-oxidant)/ml, and 20 µg of phenylmethylsulfonyl fluoride/ml). The extracted material was cleared of particulate matter by centrifuging at 800 × g for 10 min, and 1.019 < d < 1.063-g/ml lipoproteins were isolated by sequential potassium bromide density ultracentrifugation (4, 36).
Synthesis of [3H]SM-- [N-palmitoyl-9,10-3H]SM was synthesized as follows (7, 40, 41): [9,10-3H]palmitic acid (25 mCi, 450 nmol) was stirred for 12 h at room temperature with an equimolar equivalent of N-hydroxysuccinimide and with 3 M eq of 1,3-dicyclohexylcarbodiimide in N,N-dimethylformamide. The reaction was run under dry argon in the dark. Sphingosylphosphorylcholine (300 nmol) and N,N-diisopropylethylamine (10 µl) were then added, and the reaction was stirred another 12 h at room temperature. The reaction was stopped by evaporating the N,N-dimethylformamide under a stream of N2. [N-palmitoyl-9,10-3H]SM was purified by preparative thin layer chromatography of the reaction products three consecutive times in chloroform:methanol (95:5) and then twice in chloroform:methanol:acetic acid:water (50:25:8:4). Greater than 95% of the [N-palmitoyl-9,10-3H]SM was converted to [N-palmitoyl-9,10-3H]ceramide after treatment with 10 milliunits of SMase/ml (Bacillus cereus) for 1 h at 37 °C, as assayed by TLC, indicating a pure, functional substrate.
[3H]SM Labeling of LDL-- Plasma LDL was labeled with [N-palmitoyl-9,10-3H]SM as follows (7). ~3.5 mCi (63 nmol) of [N-palmitoyl-9,10-3H]SM and 13 nmol of phosphatidylcholine (PC) were mixed in chloroform, and the solvent was removed first under a stream of nitrogen and then by lyophilization. The dried lipids were resuspended in 1 ml of 150 mM NaCl, 1 mM EDTA, 10 mM Tris-HCl, pH 7.5, and, to prepare [3H]SM/PC liposomes, sonicated for three 50-s pulses at 4 °C using a tapered microtip on a Branson 450 sonicator (setting 3). The liposomes were then incubated with 30 mg (by protein mass) of LDL, 50 µg of partially purified phospholipid transfer protein, 100 units of penicillin, and 100 µg of streptomycin for 18 h at 37 °C under argon. LDL was then separated from the liposomes after phospholipid transfer by centrifuging the mixture at density = 1.006 g/ml for 8 h at 35,000 rpm in a Beckman 50.3 rotor; the supernatant containing the liposomes was removed, and the LDL band at the bottom of the tube was harvested. The LDL solution was mixed with buffer containing 150 mM NaCl, 0.3 mM EDTA, pH 7.4, and centrifuged as before. This wash procedure was performed a total of four times, resulting in the removal of 95% of the unreacted [3H]SM/PC liposomes. All lipoproteins were stored under argon at 4 °C and were used within 2 weeks of preparation.
[3H]SM-Emulsions-- [3H]SM-emulsions with a lipid composition similar to human LDL were prepared as follows: 5.4 mg of cholesteryl oleate, 0.48 mg of triolein, 1.08 mg of free cholesterol, 2.04 mg of phosphatidylcholine, 0.96 mg of sphingomyelin, and 50 µCi of [N-palmitoyl-9,10-3H]sphingomyelin were all added in chloroform to a sonication vial, and the solvent was completely evaporated by exposure to a stream of nitrogen, followed by the high vacuum of a lyophilizer. The dried lipids were resuspended in 3 ml of buffer containing 150 mM NaCl, 0.3 mM EDTA, pH 7.4, and sonicated under a stream of argon at 40 °C until translucent (approximately 90 min). The sonicated material was then centrifuged twice at 15,000 × g to pellet any titanium shed from the sonication probe. SM-rich emulsions were prepared exactly as above except that 1.2 mg of phosphatidylcholine and 1.8 mg of SM were used, and sonication time was increased to 120 min. [3H]SM-emulsions were enriched with apoCIII based on the method of Ahmad et al. (42). Briefly, [3H]SM-emulsions (0.4 ml; 182 nmol of SM) were incubated with 200 µg of apoCIII (22.5 nmol) for 2 h at 40 °C. A portion of the emulsions was re-isolated from free apoCIII using ultrafiltration (29) as follows: the crude emulsion/apoCIII mixture was diluted to 2 ml with buffer containing 150 mM NaCl, 0.3 mM EDTA, pH 7.4, and ultrafiltered and concentrated to 0.2 ml using a Centricon 30 (molecular weight cut-off = 30,000); the concentrated emulsions were then diluted to 2 ml, and the process was repeated 5 times. [3H]SM-emulsions run through the same enrichment and re-isolation protocols in the absence of apoCIII served as the control for the experiments in Fig. 5A.
Ceramide Assay--
Ceramide was measured from LDL lipid
extracts using the method described by Schneider and Kennedy (43) and
adapted by Preiss et al. (44). In this method,
diacylglycerol (DAG) kinase phosphorylates ceramide and DAG using
[-32P]ATP. For ceramide measurement, the lipids were
first incubated with 0.1 N KOH in methanol for 1 h at
37 °C, which hydrolyzes DAG but not ceramide. The extracted lipids
were dried under nitrogen and then solubilized in 5 mM
cardiolipin, 7.5% octyl-
-glucopyranoside, and 1 mM
diethylenetriaminepentaacetic acid by bath sonication. This solution
was then added to reaction buffer (50 mM imidazole HCl, pH
6.6, 50 mM NaCl, 12.5 mM MgCl2, 1 mM EGTA) containing sn-1,2-DAG kinase (0.7 units/ml). The reaction was initiated by the addition of
[
-32P]ATP (final concentration = 10 mM). After incubation at room temperature for 60 min, the
reaction was stopped by lipid extraction with chloroform:methanol:HCl
(100:100:1, v/v/v) and 10 mM EDTA. Ceramide
1-[32P]phosphate in the organic phase was separated by
TLC using chloroform:methanol:acetic acid (65:15:5, v/v/v) and
visualized with autoradiography and identified by comparing with
standards. The spots corresponding to ceramide
1-[32P]phosphate were scraped and counted, and the mass
was calculated by comparison with a ceramide standard curve. To rule
out changes in the DAG kinase enzyme itself (cf. Ref. 45),
C2:0 ceramide was added to some of the lipid extracts and shown not to
undergo increased phosphorylation in samples in which LDL ceramide was found to be elevated.
Sphingomyelin (SM) and Phosphatidylcholine (PC) Assays-- Lipid extracts (46) of lipoproteins were chromatographed by TLC using chloroform:methanol:acetic acid:H2O (50:25:8:4, v/v/v/v). Individual phospholipid subclasses were visualized by iodine vapor staining, and the SM and PC spots were identified by comparison with standards. The spots were scraped, extracted twice with chloroform:methanol (2:1), and assayed for phosphate content by the method of Bartlett (47).
LDL Oxidation Assays-- LDL lipid peroxides were measured using the method of El-Saadani et al. (48). LDL (50-150 µg of protein), in a volume of no more than 100 µl, was added to 1 ml of color reagent (0.2 M KH2PO4, 0.12 M KI, 0.15 mM NaN3, 2 g of Triton X-100/liter, 0.1 g of benzalkonium chloride/liter, 10 µM ammonium molybdate, 20 µM BHT, 25 µM EDTA, pH 6.2) and incubated in the dark for 30 min at room temperature; light absorbance at 365 nm was then measured and lipid peroxides were quantified by comparison with a H2O2 standard curve. Thiobarbituric acid-reactive substances (TBARS) were measured using a standard method (49). Briefly, LDL (100 µl, 100-200 µg of protein) was mixed with 1 ml of 20% trichloroacetic acid and incubated on ice for 30 min. Following precipitation, 1 ml of 1% thiobarbituric acid was added, and the samples were heated at 95 °C for 45 min. After cooling, the samples were centrifuged at 1000 × g for 20 min, and the light absorbance at 532 nm was measured. TBARS were quantified by comparison with a malonaldehyde standard curve prepared using tetramethoxypropane. LDL electrophoretic mobility was assayed by loading 30 µg of LDL protein onto a polyacrylamide 0.75-27% gradient gel (Lipogel; Zaxis, Hudson, OH) and electrophoresing in 0.1 M Tris base, 0.1 M boric acid, 20 mM EDTA (upper chamber, pH 8.7; lower chamber, pH 8.3) for 12 h at 100 V. The gel was then stained with Sudan black and the bands visualized by counter-staining with methanol:acetic acid:water (10:7:83, v/v/v).
S-SMase-- The source of S-SMase was serum-free conditioned medium from DG44 Chinese hamster ovary cells stably transfected with the human acid SMase cDNA (21, 50). Our previous work demonstrated that S-SMase is the only detectable SMase secreted into the culture medium (21). Cells were plated in 100-mm dishes and cultured for 48 h in Dulbecco's modified Eagle's medium containing 10% fetal bovine serum, 292 µg of glutamine/ml, 100 units penicillin/ml, and 100 µg of streptomycin/ml. The cells were then changed to low protein serum-free media for 12 h, washed 3 times with phosphate-buffered saline, and finally incubated for 18 h in fresh serum-free media (6 ml per 100-mm dish). The "18-h conditioned medium" was then collected, centrifuged at 1000 × g to pellet any cells, and, except where indicated, ZnCl2 (final concentration = 100 µM) was added to fully activate and stabilize S-SMase; this S-SMase-containing conditioned medium was then used fresh to treat LDL and lipid emulsions.
S-SMase Treatment of LDL and Emulsions-- The standard incubation mixture consisted of up to 50 µl of sample (LDL or emulsions), 25 µl of S-SMase-containing conditioned media (see above), and a volume of assay buffer (0.1 M Tris-HCl, pH 7.4, 0.04% azide; or, where indicated, 0.1 M sodium acetate, pH 5.5, 0.04% azide) to bring the final volume to 200 µl. The reactions were incubated at 37 °C for no longer than 16 h and then extracted by the method of Bligh and Dyer (46). For the samples containing [N-palmitoyl-9-10-3H]SM-LDL or [N-palmitoyl-9-10-3H]SM-emulsions, the lower organic phase was harvested, evaporated under N2, and fractionated by TLC using chloroform:methanol (95:5). The ceramide spots were scraped and directly counted to quantify [3H]ceramide. In all other samples ceramide was determined as described above.
Statistics-- Unless otherwise indicated, results are given as means ± S.D. (n = 3); absent error bars in the figures signify S.D. values smaller than the graphic symbols.
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RESULTS |
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Hydrolysis and Aggregation of Native, Oxidized, and sPLA2-treated LDL by S-SMase-- We first sought to determine whether S-SMase could act on intact LDL and, if so, whether it would cause LDL aggregation. Although treatment of native LDL with S-SMase at pH 7.4 caused very little SM hydrolysis, S-SMase hydrolyzed significant amounts (nearly 90%) of LDL-SM at pH 5.5 (Fig. 1A), leading to LDL aggregation (Fig. 1B). Thus, S-SMase can hydrolyze the SM in intact LDL without the need for detergent, leading to LDL aggregation.
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Lipoproteins Enriched in ApoCIII Are Susceptible to S-SMase-induced Hydrolysis and Aggregation at Neutral pH-- Recent data suggest that apoCIII enrichment of lipoproteins promotes atherogenesis (35, 55, 56), although the mechanism is not known. Moreover, Ahmad et al. (42) have shown that apoCIII, but not other apolipoproteins, stimulates L-SMase hydrolysis of SM-liposomes. Given that L-SMase and S-SMase are such similar enzymes (21), we asked whether apoCIII could stimulate S-SMase hydrolysis of lipoprotein-SM at neutral pH. Because enriching native LDL with apoCIII in vitro proved extremely difficult, we first examined the effect of apoCIII on S-SMase hydrolysis of [3H]SM-labeled emulsions containing lipids in the same proportions as in LDL. For this experiment we treated three different types of emulsions with S-SMase at pH 7.4: control emulsions not enriched with apoCIII (control), emulsions mixed with apoCIII (+ CIII, emulsion-bound + free CIII), and emulsions mixed with apoCIII followed by re-isolation from free apoCIII (+ CIII, emulsion-bound only). Although S-SMase hydrolysis of the emulsions with bound plus free CIII was 2-fold greater than hydrolysis of the control emulsions, emulsions with only bound CIII were hydrolyzed approximately 10-fold more than control emulsions (Fig. 5A). Thus, apoCIII stimulates S-SMase hydrolysis of LDL-like emulsions, particularly in the absence of free apoCIII (see Ref. 42 and "Discussion").
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Plasma LDL from E0 Mice Is Efficiently Hydrolyzed by S-SMase at Neutral pH-- The E0 mouse develops widely distributed and complex atherosclerotic lesions like those seen in humans and thus has become a widely used model of atherosclerosis (33, 34). A likely factor contributing to the extensive atherosclerosis in these mice is the atherogenicity of E0 lipoproteins, but the mechanisms whereby apoE-deficient lipoproteins lead to foam cell formation and other lesional events is not known. In the context of our current study, we tested whether plasma LDL from these mice was susceptible to S-SMase at neutral pH; as a comparison, we used plasma LDL from LDLr0 mice, which also develop atherosclerosis, although to a lesser degree than E0 mice (32-34). Although the basal ceramide contents of native plasma LDL from these two strains of mice are equivalent (Fig. 6A, hatched bars), treatment with S-SMase at pH 7.4 resulted in generation of 4-5-fold more ceramide in LDL from E0 mice than in LDL from LDLr0 mice (Fig. 6A, solid bars), which resulted in aggregation of E0 LDL.4
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Human Atheroma-derived LDL Is Hydrolyzed by S-SMase at Neutral pH-- A critical issue regarding the physiologic relevance of the above findings is whether LDL modified in the arterial wall shows greater susceptibility to hydrolysis by S-SMase than native LDL at neutral pH. To address this issue, we compared two different samples of lesional LDL with same donor plasma LDL for their susceptibility to hydrolysis by S-SMase at neutral pH. First, the data in Fig. 7 confirm our previous finding that lesional LDL is enriched in ceramide compared with plasma LDL (compare first pair of bars with second pair of bars) (7). Most importantly, however, treatment of lesional LDL with S-SMase at pH 7.4 generates nearly 10-fold more ceramide than treatment of plasma LDL (compare third pair of bars with fourth pair of bars).5 LDL retained in the arterial wall, therefore, is an excellent substrate for S-SMase at neutral pH.
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DISCUSSION |
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Aggregated LDL retained in vivo shows evidence of hydrolysis by a SMase in the subendothelium (7). Although the mammalian SMase responsible for this extracellular process has yet to be identified, S-SMase is a leading candidate since it is the only SMase that we have been able to detect in the conditioned media of cultured arterial wall cells (7). Furthermore, preliminary immunohistochemistry studies indicate that S-SMase is present in both the pre-lesional artery and in atherosclerotic lesions.6 Macrophage-derived S-SMase (7) may be a source of the enzyme in established atherosclerotic lesions. Most recently, we have found that cultured human coronary artery endothelial cells secrete abundant amounts of S-SMase basolaterally,2 making the endothelium a potential source of the enzyme in the pre-lesional artery.
S-SMase, however, has an acid pH optimum using a standard in vitro, detergent-based SM-micelle assay (21, 23). Indeed, we found that S-SMase can hydrolyze and aggregate native plasma LDL but only at acid pH (Fig. 1). Although S-SMase-induced LDL aggregation under these conditions may be relevant in more advanced lesions, where an acidic environment may exist in the vicinity of lesion macrophages (27, 28) or as a result of hypoxia-induced metabolic acidosis (57), early lipoprotein aggregation likely occurs at a more neutral, physiologic pH. Thus, our finding that certain types of atherogenic lipoproteins are substrates for S-SMase at neutral pH may have relevance to early atherogenesis. Callahan et al. (51) demonstrated that for L-SMase, which is closely related to S-SMase (21), only the affinity for SM in detergent micelles (i.e., Km) is highly sensitive to changes in pH, whereas the maximal velocity (Vmax) for SM hydrolysis is pH-independent. Thus, a plausible explanation for our findings is that changes in the lipid surface of the various forms of modified lipoproteins increased the accessibility of their SM to S-SMase. The overall concept that "acid" SMase can function at neutral pH may also have relevance to ceramide-mediated cell-signaling processes, such as apoptosis, that are triggered by the hydrolysis of cellular SM by a product or products of the acid SMase gene (58-62).
What is the physiologic significance of subendothelial lipoprotein SM hydrolysis? One theoretical possibility is that SMase-treated lipoproteins may be able to deliver ceramide to cells and induce certain ceramide-mediated signal transduction events (63-66). In fact, oxidized LDL-induced stimulation of smooth muscle cell proliferation may involve the ceramide signaling pathway (67). The major physiological link that we have been studying, however, is SMase-induced lipoprotein self-aggregation (16, 20). In this regard, we have found three examples in which S-SMase hydrolysis led to lipoprotein aggregation as follows: native LDL at acid pH (Fig. 1B), LDL from LDLr0/CIII mice at neutral pH (see "Results"), and VLDL + LDL and LDL from E0 mice at neutral pH.4,5 In contrast, two of the lipoproteins that were hydrolyzed by S-SMase at neutral pH, namely oxidized LDL and sPLA2-treated LDL, did not show visible aggregation. In these two cases, the level of ceramide generated (~0.05 and 0.013 nmol of ceramide/µg of protein, respectively) was below the threshold level for lipoprotein aggregation in vitro (~0.08 nmol of ceramide/µg of protein) (7). Thus, more extensive exposure of these lipoproteins to S-SMase at neutral pH would likely be required for aggregation to occur. Furthermore, it is possible that other arterial wall factors, such as proteoglycans, collagen, and lipoprotein lipase (5, 68, 69), lower the threshold for ceramide-mediated lipoprotein aggregation in vivo. Interestingly, oxidized LDL treated with S-SMase at neutral pH contained similar levels of ceramide as aggregated lesional LDL (7) (also, compare Fig. 2A with Fig. 7), consistent with the idea that the threshold for ceramide-mediated aggregation may be lowered in vivo.
Our finding that S-SMase can hydrolyze and cause the aggregation of apoCIII-enriched lipoproteins at neutral pH is of particular interest since, through unknown mechanisms, apoCIII has been linked to certain forms of coronary heart disease (55). In addition, overexpression of apoCIII in mice significantly increases atherosclerosis (35). Although our samples of lesional LDL (1.019 < d < 1.063 g/ml) did not contain apoCIII, it is possible that less dense lesional lipoproteins or lipoproteins isolated from other individuals or from earlier lesions might have shown evidence of apoCIII. A possible mechanism for the apoCIII effect described herein may be gleaned from the finding that S-SMase-mediated hydrolysis of apoCIII-containing SM-emulsions was greater in the absence of free apoCIII (Fig. 5A). These data are consistent with the idea that apoCIII may act as a bridge between S-SMase and membrane SM, thereby stimulating SM hydrolysis.
A property of E0 LDL that likely plays a role in its susceptibility to S-SMase is the high SM:PC ratio in these particles (Fig. 6B). The mechanism of lipoprotein-SM enrichment appears to be a combination of increased SM synthesis and decreased plasma SM clearance in the E0 mice.4 There is evidence, however, that other factors may also be involved in the increased susceptibility of E0 lipoproteins. In particular, native human plasma LDL has an SM:PC ratio that is greater than that in E0 LDL and equal to that in the "control" emulsions in Fig. 6B and yet is substantially less susceptible to hydrolysis by S-SMase at neutral pH than these particles. Since E0 LDL contains mostly apoB48 (33, 34) and the emulsions contain no protein, it is possible that apoB100, the protein in human native plasma LDL, confers relative resistance to hydrolysis by S-SMase. If so, the degradation of apoB100 that is known to occur with LDL oxidation (52) may contribute to the enhanced susceptibility of oxidized LDL to hydrolysis by S-SMase. It is also possible that apoE itself may be an inhibitor of S-SMase hydrolysis, although we have recently found that apoE enrichment of the E0 lipoproteins does not block S-SMase-induced aggregation.4
A critically important finding in this report was that LDL extracted from human atherosclerotic lesions was hydrolyzed 10-fold more than same donor plasma LDL by S-SMase at neutral pH (Fig. 7). These data indicate that lesional LDL is modified in vivo, either before or after retention in lesions (see below), to a form that renders it more susceptible to hydrolysis by S-SMase. Although it is difficult to determine the specific characteristics of lesional LDL that increase its susceptibility to S-SMase, we have observed, as have others (11-13), that lesional LDL is enriched in SM (Table I), which we have shown is a potent stimulator of S-SMase hydrolysis of LDL (Fig. 6B). The mechanism of SM enrichment of lesional LDL is not known. The possibilities include SM enrichment of LDL retained in the subendothelium (e.g. by increased arterial wall synthesis of SM (70) followed by transfer of the SM to retained LDL) or selective uptake of SM-rich LDL from the plasma. Another property of lesional LDL that may increase its susceptibility to S-SMase is oxidation. Although we did not find evidence of TBARS or lipid peroxides in our samples of lesional LDL, more sensitive and/or specific techniques, such as immunostaining for oxidized epitopes (11) or evidence of myeloperoxidase-induced oxidation (9), may be required.
In summary, S-SMase, a product of the acid SMase gene (21), can efficiently hydrolyze and aggregate modified LDL at neutral pH. Most importantly, LDL extracted from atherosclerotic lesions, but not plasma LDL, is significantly hydrolyzed by S-SMase at neutral pH, indicating that modifications present in vivo can stimulate S-SMase hydrolysis of LDL. In previous reports, we have shown that SMase treatment of lipoproteins promotes lipoprotein aggregation and retention to matrix, which induces macrophage foam cell formation (16, 20), that lipoprotein-SM hydrolysis occurs in the subendothelium in vivo (7), and that S-SMase is secreted by both macrophages (21) and endothelial cells.2 Based on the sum of these data, we propose a model in which S-SMase secreted by arterial wall cells hydrolyzes lipoproteins retained in the subendothelium, which, in turn, promotes lesion initiation and progression.
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FOOTNOTES |
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* This study was supported in part by National Institutes of Health Grants HL57560 (to I. T.), HL 21006 (to I. T.), HL56984 (to I. T. and K. J. W.), and HL38956 (to K. J. W.); an Established Investigator award from the American Heart Association and Genentech (to K. J. W.); and the Research Service, Veterans Affairs (to J. H. R.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ Supported by National Institutes of Health Medical Scientist Training Grant 5T32GM07367.
¶¶ To whom correspondence and reprint requests should be addressed: Dept. of Medicine, Columbia University, 630 West 168th St., New York, NY 10032. Tel.: 212-305-9430; Fax: 212-305-5052; E-mail: iat1{at}columbia.edu.
1 The abbreviations used are: LDL, low-density lipoprotein; apo, apolipoprotein; L-SMase, lysosomal sphingomyelinase; PC, phosphatidylcholine; SM, sphingomyelin; SMase, sphingomyelinase; sPLA2, nonpancreatic secretory phospholipase A2; S-SMase; secretory sphingomyelinase; TBARS, thiobarbituric acid-reactive substances; DAG, diacylglycerol; LDLr0, LDL receptor-deficient; E0, apolipoprotein E-deficient.
2 Marathe, S., Schissel, S. L., Schuchman, E., Yellin, M., Beatini, N., Minzer, R., Williams, K. J., and Tabas, I. (1998) J. Biol. Chem. 273, in press.
3 S. L. Schissel and I. Tabas, unpublished data.
4 Jeong, T. S., Schissel, S. L., Tabas, I., Pownall, H. J., Tall, A. R., and Jiang, Y. C. (1998) J. Clin, Invest, in press.
5 Note that lesional LDL is already in an aggregated form when isolated (7), and so the effect of S-SMase treatment on aggregation could not be assessed.
6 S. Marathe and I. Tabas, unpublished data.
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