Selective inhibition of free apolipoprotein-mediated cellular lipid efflux by probucol

Inhibition of the cholesterol transporter ABCA1 by probucol decreases capacitation and tyrosine phosphorylation of dog spermatozoa, and is dose dependent

The ATP binding cassette (ABC) transporter molecule ABCA1 participates in the cholesterol transport within and through cell membranes. We recently demonstrated that in dog spermatozoa, capacitation could be decreased with probucol (PRO), an ABCA1 specific antagonist. In this study, a dose-effect relationship of PRO on dog sperm capacitation, tyrosine phosphorylation and cholesterol efflux from the sperm plasma membrane was investigated. A total of 16 ejaculates from dogs of different breeds, aged 2-4 years were used. Sperm motility and membrane integrity in the main fraction was determined by CASA. Samples were stained with a boron dipyrromethene difluoride (BODIPY) fluorophore (P9672, Sigma- Aldrich, A) diluted in DMSO at a final concentration of 0.4 μM. All samples were divided into 5 aliquots, with 0, 100, 250, 500 and 1000 μM of PRO. After incubation at 37 °C for 2 h, PI was added and flow cytometry performed. All aliquots were examined for capacitation and acrosome reaction by using the CTC assay and tyrosine phosphorylation (TP). Membrane integrity was measured in all aliquots to investigate the effect of PRO on cell membranes. Membrane integrity did not differ between controls (0 μM), and 100, 250 and 500 μM PRO, but decreased with 1000 μM PRO (p < 0.05). Increasing PRO concentration decreased the percentage alive cells with cholesterol efflux per PRO group (0 μM: 77.8 ± 10.6%, 100 μM: 63.7 ± 11.7%, 250 μM: 52.1 ± 12.9%, 500 μM: 37.7 ± 11.6%, 1000 μM: 33.1 ± 14.4%; p < 0.05), decreased head and entire tail phosphorylated cells (0 μM: 34.6%, 1000 μM: 5.1% p < 0.05); and decreased the percentage capacitated cells (maximum with PRO 500 μM: capacitated vs. control: 54.2 ± 17% vs 25 ± 7.7%, p < 0.05). Conclusion: PRO decreased the cholesterol efflux, and decreased tyrosine phosphorylation and capacitation in a dose-dependent manner. This suggests a strong involvement of the ABCA1 transporter in different functional aspects of sperm capacitation in dogs.

Probucol treatment is associated with an ABCA1-independent mechanism of cholesterol efflux to lipid poor apolipoproteins from foam cell macrophages

Objective

Probucol is a cholesterol-lowering agent whose ability to prevent atherosclerosis is currently under study. Herein, we investigate the putative mechanism of probucol by observation of changes in cellular cholesterol efflux and lipid droplet morphology in macrophages.

Results

The inhibitory activity of probucol was assessed in non-foam or foam cell macrophages expressing ABCA1 generated by treatment with fetal calf serum (FCS) alone or in combination with acetylated LDL, respectively. Probucol inhibited cholesterol efflux to apolipoprotein A-I (apoA-I) by 31.5±0.1% in THP-1 non-foam cells and by 18.5±0.2% in foam cells. In probucol-treated non-foam THP-1 cells, nascent high density lipoprotein (nHDL) particles with a diameter < 7 nm were generated, while in probucol-treated THP-1 foam cells nHDL particles of > 7 nm in diameter containing cholesterol were produced. Foam cells also displayed a significant accumulation of free cholesterol at the plasma membrane, as measured by percent cholestenone formed. Intracellularly, there was a significant decrease in lipid droplet number and an increase in size in probucol-treated THP-1 foam cells when compared to non-treated cells.

Conclusions

We report for the first time that probucol is unable to completely inhibit cholesterol efflux in foam cells to the same extent as in non-foam cells. Indeed, functional nHDL is released from foam cells in the presence of probucol. This difference in inhibitory effect could potentially be explained by changes in the plasma membrane pool as well as intracellular cholesterol storage independently of ABCA1.

Analysis of Low Molecular Weight Substances and Related Processes Influencing Cellular Cholesterol Efflux

Cholesterol efflux is the key process protecting the vascular system from the development of atherosclerotic lesions. Various extracellular and intracellular events affect the ability of the cell to efflux excess cholesterol. To explore the possible pathways and processes that promote or inhibit cholesterol efflux, we applied a combined cheminformatic and bioinformatic approach. We performed a comprehensive analysis of published data on the various substances influencing cholesterol efflux and found 153 low molecular weight substances that are included in the Chemical Entities of Biological Interest (ChEBI) database. Pathway enrichment was performed for substances identified within the Reactome database, and 45 substances were selected in 93 significant pathways. The most common pathways included the energy-dependent processes related to active cholesterol transport from the cell, lipoprotein metabolism and lipid transport, and signaling pathways. The activators and inhibitors of cholesterol efflux were non-uniformly distributed among the different pathways: the substances influencing ‘biological oxidations’ activate cholesterol efflux and the substances influencing ‘Signaling by GPCR and PTK6’ inhibit efflux. This analysis may be used in the search and design of efflux effectors for therapies targeting structural and functional high-density lipoprotein deficiency.

Zinc Increases ABCA1 by Attenuating Its Clearance Through the Modulation of Calmodulin Activity

Aim: We previously revealed that Ca⁺⁺-activated calmodulin binds to ABCA1 by the region near the PEST sequence and retards its calpain-mediated degradation to increase HDL biogenesis. Calmodulin activity is reportedly modulated also by other nutritional divalent cations; thus, we attempted to determine whether Zn⁺⁺ is involved in the regulation of ABCA1 stability through the modulation of calmodulin activity.

Methods: The effects of Zn⁺⁺ on ABCA1 expression was investigated in J774 mouse macrophage cell-line cells and HepG2 human hepatoma cell-line cells.

Results: Zn⁺⁺ increased ABCA1 expression, not by increasing the mRNA but by attenuating its decay rate, more prominently in the presence of cAMP. Accordingly, it enhanced cell cholesterol release with extracellular apolipo-protein A-I. Calmodulin binding to ABCA1 was increased by Zn⁺⁺ and Ca⁺⁺. Zn⁺⁺ suppressed calpain-mediated hydrolysis of the peptide of ABCA1 cytosolic loop, including the PEST sequence and the calmodulin-binding site, in a calmodulin-dependent fashion, in the presence of the minimum amount of Ca⁺⁺ to activate calpain, but not calmodulin. Calpain activity was not directly inhibited by Zn⁺⁺ at the concentration for enhancing calmodulin binding to ABCA1.

Conclusion: Nutritional divalent cation Zn⁺⁺ is involved in the regulation of ABCA1 activity and biogenesis of HDL through the modulation of calmodulin activity. The results were consistent with previous clinical findings that Zn⁺⁺ increased plasma HDL in the conditions of sympathetic activation, such as type 2 diabetes and chronic hemodialysis.

The ABCA1 blocking agent probucol decreases capacitation in ejaculated dog spermatozoa

BACKGROUND

The ATP binding cassette (ABC) transporters participate in the cholesterol and phospholipid transport within and through cell membranes of many cells including spermatozoa. Cholesterol efflux is important for capacitation of spermatozoa. ABCA1 expression has been assessed in canine spermatozoa previously but its role in capacitation still has to be determined. The aim of the study was to test whether inhibition of ABCA1 (1) decreases capacitation in ejaculated and epididymal canine sperm samples and (2) decreases cholesterol efflux in the same samples. Twenty-one ejaculates and sperm from 22 epididymal tails were collected from healthy dogs. Motility was measured by CASA and viability assessed after staining with SYBR-14/PI. Samples from ejaculated sperm and sperm from epididymal tails were aliquoted. One part was incubated with the ABCA1 inhibitor probucol, the other served as a negative control. In all samples, capacitation was evaluated by chlortetracyclin (CTC) assay and cholesterol was measured by cholesterol efflux assay and colorimetric enzymatic assay.

RESULTS

In ejaculated sperm, blockade of ABCA1 with 100 µM of probucol/mL of sample resulted in a significantly higher percentage of uncapacitated and acrosome reacted spermatozoa (P < 0.001 and P = 0.031), capacitation was significantly decreased (35% in probucol samples vs 54.2% in controls, P < 0.001). In probucol inhibited sperm samples from epididymal tails, the percentage of capacitated spermatozoa did not differ between groups but the percentage of acrosome reacted spermatozoa increased significantly (P = 0.014). The cholesterol measurement revealed significantly lower cholesterol concentration in the probucol group when compared to the controls (P = 0.035), however only in ejaculated sperm samples.

CONCLUSIONS

CTC assay and cholesterol measurement revealed significant differences between groups; we conclude that inhibition of ABCA1 significantly decreased capacitation and cholesterol efflux in ejaculated canine spermatozoa. The inhibition was not complete but ABCA1 is supposed to contribute to capacitation in canine ejaculated spermatozoa. ABCA1 is probably not important for capacitation of epididymal spermatozoa but might exert other functions during spermatozoa ripening.

Selective Correction of Genotype Yield by Probucol in HDL-Deficient Mice Propagation

Aim: Probucol is a controversial drug to inhibit ATP-binding cassette transporter A1 (ABCA1) and to exhibit some positive clinical effects such as regression of xanthomas. It reportedly rescues female infertility in scavenger receptor BI-deficient mice. Here, we investigated the effect of probucol on propagation in HDL-deficient mice as alternative models for impaired HDL-mediated cholesterol delivery.

Methods: Propagation of ABCA1-deficient (Abca1−/−) mice and lecithin: cholesterol acyltransferase (LCAT)-deficient (Lcat−/−) mice were quantitatively observed under the probucol treatment.

Results:Abca1−/− and Lcat−/− mice appear with negligible plasma HDL concentration. Upon backcrossing Abc1+/− with the Abc1−/− mice and cross-breeding between Abc1+/− mice, the numbers of Abc1−/− weaned pups were reduced to 54.7% and to 57.1% from those expected by Mendelian genetics, respectively. Similarly, Lcat-/-weaned pups decreased to 67.7% and to 35.9% but only in the male. Probucol severely reduced plasma HDL-cholesterol to 5% in the wild-type mice, but showed no effects on their propagation. Probucol corrected the deflections of the genotype distribution in the weaned pups recovery in the LCAT-deficient mice propagation but not in the ABCA1-deficient mice while plasma HDL was kept negligible. Probucol had no effect on cholesterol content in the steroidogenic organs of the HDL-deficient mice, while it somewhat increased plasma corticosterone and expression of adrenal cortex HMG-CoA reductase, StAR, cytochrome P450scc, and VKORC1 indicating increase in the synthesis of cholesterol and steroid hormones and in vitamin K turn-over. However, no evident mechanistic background was indicated.

Conclusions: Probucol corrected deflection of genotype distribution in propagation of the LCAT-deficient mice but not the ABCA1-deficient mice at the weaning stage, apparently not through normalization of hypoalphalipo-proteinemia.

Reconstituted Discoidal High-Density Lipoproteins: Bioinspired Nanodiscs with Many Unexpected Applications

PURPOSE OF REVIEW

Summarize the initial discovery of discoidal high-density lipoprotein (HDL) in human plasma and review more recent innovations that span the use of reconstituted nanodisc HDL for membrane protein characterization to its use as a drug carrier and a novel therapeutic agent for cardiovascular disease.

RECENT FINDINGS

Using a wide variety of biophysical techniques, the structure and composition of endogenous discoidal HDL have now largely been solved. This has led to the development of new methods for the in vitro reconstitution of nanodisc HDL, which have proven to have a wide variety of biomedical applications. Nanodisc HDL has been used as a platform for mimicking the plasma membrane for the reconstitution and investigation of the structures of several plasma membrane proteins, such as cytochrome P450s and ABC transporters. Nanodisc HDL has also been designed as drug carriers to transport amphipathic, as well as hydrophobic small molecules, and has potential therapeutic applications for several diseases. Finally, nanodisc HDL itself like native discoidal HDL can mediate cholesterol efflux from cells and are currently being tested in late-stage clinical trials for cardiovascular disease. The discovery of the characterization of native discoidal HDL has inspired a new field of synthetic nanodisc HDL, which has offered a growing number of unanticipated biomedical applications.

Charged Residues in the C-Terminal Domain of Apolipoprotein A-I Modulate Oligomerization

Charged residues of the C-terminal domain of human apolipoprotein A-I (apoA-I) were targeted by site-directed mutagenesis. A series of mutant proteins was engineered in which lysine residues (Lys 195, 206, 208, 226, 238, and 239) or glutamate residues (Glu 234 and 235) were replaced by glutamine. The amino acid substitutions did not result in changes in secondary structure content or protein stability. Cross-linking and size-exclusion chromatography showed that the mutations resulted in reduced self-association, generating a predominantly monomeric apoA-I when five or six lysine residues were substituted. The rate of phosphatidylcholine vesicle solubilization was enhanced for all variants, with approximately a threefold rate enhancement for apoA-I lacking Lys 206, 208, 238, and 239, or Glu 234 and 235. Single or double mutations did not change the ability to protect lipolyzed low density lipoprotein from aggregation, but variants lacking >4 lysine residues were less effective in preventing lipoprotein aggregation. ApoA-I mediated cellular lipid efflux from wild-type mice macrophage foam cells was decreased for the variant with five lysine mutations. However, this protein was more effective in releasing cellular phosphatidylcholine and sphingomyelin from Abca1-null mice macrophage foam cells. This suggests that the mutations caused changes in the interaction with ABCA1 transporters and that membrane microsolubilization was primarily responsible for lipid efflux in cells lacking ABCA1. Taken together, this study indicates that ionic interactions in the C-terminal domain of apoA-I favor self-association and that monomeric apoA-I is more active in solubilizing phospholipid bilayers.

Exposure to High Glucose Concentration Decreases Cell Surface ABCA1 and HDL Biogenesis in Hepatocytes

Aim: To study atherosclerosis risk in diabetes, we investigated ATP-binding cassette transporter A1 (ABCA1) expression and high-density lipoprotein (HDL) biogenesis in the liver and hepatocytes under hyperglycemic conditions.

Methods and Results: In streptozotocin-induced diabetic mice, plasma HDL decreased while ABCA1 protein increased without changing its mRNA in the liver, only in the animals that responded to the treatment to show hypoinsulinemia and fasting hyperglycemia but not in the poor responders not showing those. To study the mechanism for this finding, hepatocytes were isolated from the control and diabetic mice, and they showed no difference in expression of ABCA1 protein, its mRNA, and HDL biogenesis in 1 g/l d-glucose but showed decreased HDL biogenesis in 4.5 g/l d-glucose although ABCA1 protein increased without change in its mRNA. Similar findings were confirmed in HepG2 cells with d-glucose but not with l-glucose. Thus, these cell models reproduced the in vivo findings in hyperglycemia. Labeling of cell surface protein revealed that surface ABCA1 decreased in high concentration of d-glucose in HepG2 cells despite the increase of cellular ABCA1 while not with l-glucose. Immunostaining of ABCA1 in HepG2 cells demonstrated the decrease of surface ABCA1 but increase of intracellular ABCA1 with high d-glucose. Clearance of ABCA1 was retarded both in primary hepatocytes and HepG2 cells exposed to high d-glucose but not to l-glucose, being consistent with the decrease of surface ABCA1.

Conclusions: It is suggested that localization of ABCA1 to the cell surface is decreased in hepatocytes in hyperglycemic condition to cause decrease of HDL biogenesis.

Caveolin-1 facilitates internalization and degradation of ABCA1 and probucol oxidative products interfere with this reaction to increase HDL biogenesis

BACKGROUND AND AIMS

Expression of ATP binding cassette transporter (ABC) A1, a key membrane protein for biogenesis of high-density lipoprotein (HDL), is regulated not only by its gene transcription but also by its intracellular degradation to modulate plasma HDL concentration. We previously showed that inhibition of ABCA1 degradation by probucol oxidative products, spiroquinone (SQ) and diphenoquinone (DQ), increased HDL biogenesis and reverse cholesterol transport, and achieved reduction of atherosclerosis in animal models. The background mechanism has thus been investigated.

METHODS

Involvement of caveolin-1, a protein of multiple functions in cell biology, particularly in cholesterol trafficking, has been examined for its roles in ABCA1 degradation as well as the effects of SQ and DQ on the reaction.

RESULTS

ABCA1 protein was increased in caveolin-1-deficient mouse embryonic fibroblasts, not by increase of transcription but by decrease in its internalization and degradation. Transfection and expression of caveolin-1 normalized the protein level and the rate of degradation of ABCA1. Immunoprecipitation experiments demonstrated association between ABCA1 and caveolin-1 and SQ and DQ disrupted this interaction. The effects of SQ and DQ to increase ABCA1 and cell cholesterol release induced by apolipoprotein A-I were dependent on expression of caveolin-1. Fluorescence imaging of ABCA1 and caveolin-1 in cultured cells demonstrated their co-localization as well as its disruption by SQ and DQ, being consistent with the biochemical findings.

CONCLUSIONS

Caveolin-1 enhances internalization and degradation of ABCA1 by its association with ABCA1. Interference of this interaction by probucol oxidative products suppresses ABCA1 degradation and increase HDL biogenesis.

Probucol-Oxidized Products, Spiroquinone and Diphenoquinone, Promote Reverse Cholesterol Transport in Mice

OBJECTIVE

Oxidized products of probucol, spiroquinone and diphenoquinone, were shown to increase cell cholesterol release and plasma high-density lipoprotein (HDL) by inhibiting degradation of ATP-binding cassette transporter A1. We investigated whether these compounds enhance reverse cholesterol transport in mice.

APPROACH AND RESULTS

Spiroquinone and diphenoquinone increased ATP-binding cassette transporter A1 protein (2.8- and 2.6-fold, respectively, P<0.01) and apolipoprotein A-I-mediated cholesterol release (1.4- and 1.4-fold, P<0.01 and P<0.05, respectively) in RAW264.7 cells. However, diphenoquinone, but not spiroquinone, enhanced cholesterol efflux to HDL (+12%, P<0.05), whereas both increased ATP-binding cassette transporter G1 protein, by 1.8- and 1.6-fold, respectively. When given orally to mice, both compounds significantly increased plasma HDL-cholesterol, by 19% and 20%, respectively (P<0.05), accompanied by an increase in hepatic and macrophage ATP-binding cassette transporter A1 but not ATP-binding cassette transporter G1. We next evaluated in vivo reverse cholesterol transport by injecting RAW264.7 cells labeled with (3)H-cholesterol intraperitoneally into mice. Both spiroquinone and diphenoquinone increased fecal excretion of the macrophage-derived (3)H-tracer, by 25% and 28% (P<0.01 and P<0.05), respectively. spiroquinone/diphenoquinone did not affect fecal excretion of HDL-derived (3)H-cholesterol, implying that macrophage-to-plasma was the most important step in spiroquinone/diphenoquinone-mediated promotion of in vivo reverse cholesterol transport. Finally, spiroquinone significantly reduced aortic atherosclerosis in apolipoprotein E null mice when compared with the vehicle.

CONCLUSIONS

Spiroquinone and diphenoquinone increase functional ATP-binding cassette transporter A1 in both the macrophages and the liver, elevate plasma HDL-cholesterol, and promote overall reverse cholesterol transport in vivo. These compounds are promising as therapeutic reagents against atherosclerosis.

Association Between Cholesterol Efflux Capacity and Atherosclerotic Cardiovascular Disease in Patients With Familial Hypercholesterolemia

OBJECTIVE

Patients with familial hypercholesterolemia (FH) are at high risk for premature atherosclerotic cardiovascular disease (ASCVD), especially because of long-term exposure to high low-density lipoprotein cholesterol levels. It has been reported that low-density lipoprotein-lowering therapy delays the onset of ASCVD. However, it still remains difficult to prevent it. Therefore, novel biomarkers and therapeutic targets are necessary to evaluate and prevent atherosclerosis in FH. The aim of this study was to investigate associations of cholesterol efflux capacity with the presence of ASCVD and clinical features in patients with heterozygous FH.

APPROACH AND RESULTS

We measured cholesterol efflux capacity in 227 patients with heterozygous FH under pharmaceutical treatment. Seventy-six (33.5%) of them were known to have ASCVD. In a logistic-regression analysis adjusted for risk factors, increased efflux capacity was associated with decreased risk of ASCVD even after the addition of high-density lipoprotein cholesterol level as a covariate (odds ratio per 1-SD increase, 0.95; 95% confidence interval, 0.90-0.99; P<0.05). Decreased cholesterol efflux capacity was associated with the presence of corneal arcus after adjusting for age and sex. In addition, inverse relationships between cholesterol efflux capacity and Achilles tendon thickness, as well as carotid intima-media thickness, were observed after adjustment for age, sex, and traditional cardiovascular risk factors.

CONCLUSIONS

Cholesterol efflux capacity was independently and inversely associated with the presence of ASCVD in heterozygous FH. In view of residual risks after treatment with statins, cholesterol efflux capacity might be a novel biomarker and a therapeutic target for preventing atherosclerosis in patients with FH.

Spontaneous and diet-aggravated hemolysis and its correction by probucol in SR-BI knockout mice with LDL-R deficiency

BACKGROUND

High density lipoprotein receptor SR-BI plays a vital role in cholesterol homeostasis. Depletion of SR-BI causes plasma free cholesterol (FC) accumulation, which disrupts erythrocytes membrane and might induce hemolytic anemia. Here we explored the effects of hypercholesteremia, induced by depletion of low density lipoprotein receptor (LDL-R) and high fat diet (HFD) feeding, on plasma FC and possible hemolysis in SR-BI knockout (KO) mice, and the therapeutic effects of a lipid-lowering drug probucol.

METHODS AND RESULTS

To determine the effects of LDL-R depletion, SR-BI KO mice were cross-bred with LDL-R KO mice to generate SR-BI/LDL-R double KO (dKO) mice. Compared to control wild type (WT), SR-BI KO and LDL-R KO mice fed normal chow diet (NCD), dKO mice fed NCD had increased plasma FC and developed macrocytic anemia, splenomegaly, jaundice and renal tubular hemosiderin deposition, indicating spontaneous hemolysis. To determine the effects of HFD feeding and probucol therapy, dKO and LDL-R KO mice were fed HFD containing 0.5% cholesterol and 20% fat with or without 1% probucol. HFD further increased plasma FC and aggravated hemolysis while probucol almost normalized plasma FC and corrected hemolysis in dKO mice.

CONCLUSION

We demonstrated that in SR-BI KO mice, hypercholesteremia due to LDL-R deficiency significantly increased plasma FC and induced spontaneous hemolysis, which could be further exacerbated by HFD feeding. Probucol almost normalized plasma FC and corrected diet-aggravated hemolysis in SR-BI KO mice with LDL-R deficiency.

Preferential incorporation of shorter and less unsaturated acyl phospholipids into high density lipoprotein-like particles in the ABCA1- and ABCA7-mediated biogenesis with apoA-I

Molecular species of phosphatidylcholine (PC) and sphingomyelin (SPM) were globally analyzed for lipidomics in the nascent high-density lipoprotein (HDL)-like particles generated with human apolipoprotein A-I (apoA-I) form HEK293 cells where either human ATP binding cassette transporter (ABC) A1 or ABCA7 was transfected and overexpressed. SPM/PC ratio was higher in the ABCA1-mediated HDL than ABCA7-mediated HDL likely being related to their cholesterol content, while it was less than the ratio in the cell membrane in either case. Molecular species composition of hydrocarbon chain moiety in each phospholipid in the HDL largely reflected that in the cells the lipoprotein originated in, without remarkable difference between ABCA1 and ABCA7. Further analysis, however, revealed apparent preference for the molecules with shorter hydrocarbon chain length for both PC and SPM in their relative incorporation into HDL by ABCA1 and ABCA7. Likewise, it was in favor for less-unsaturated hydrocarbon chains of PC while this preference was not apparent for SPM. The results are consistent with the view that assembly of HDL particles with extracellular apoA-I is primarily with the cellular phospholipid molecules being regulated in part by their physicochemical nature.

Astaxanthin enhances ATP-binding cassette transporter A1/G1 expressions and cholesterol efflux from macrophages

ATP-binding cassette transporters (ABC) A1 and G1 are key molecules in cholesterol efflux from macrophages, which is an initial step of reverse cholesterol transport (RCT), a major anti-atherogenic property of high-density lipoprotein (HDL). Astaxanthin is one of the naturally occurring carotenoids responsible for the pink-red pigmentation in a variety of living organisms. Although astaxanthin is known to be a strong antioxidant, it remains unclear through what mechanism of action it affects cholesterol homeostasis in macrophages. We therefore investigated the effects of astaxanthin on cholesterol efflux and ABCA1/G1 expressions in macrophages. Astaxanthin enhanced both apolipoprotein (apo) A-I- and HDL-mediated cholesterol efflux from RAW264.7 cells. In supporting these enhanced cholesterol efflux mechanisms, astaxanthin promoted ABCA1/G1 expression in various macrophages. In contrast, peroxisome proliferator-activated receptor γ, liver X receptor (LXR) α and LXRβ levels remained unchanged by astaxanthin. An experiment using actinomycin D demonstrated that astaxanthin transcriptionally induced ABCA1/G1 expression, and oxysterol depletion caused by overexpression of cholesterol sulfotransferase further revealed that these inductions in ABCA1/G1 were independent of LXR-mediated pathways. Finally, we performed luciferase assays using human ABCA1/G1 promoter-reporter constructs to reveal that astaxanthin activated both promoters irrespective of the presence or absence of LXR-responsive elements, indicating LXR-independence of these activations. In conclusion, astaxanthin increased ABCA1/G1 expression, thereby enhancing apoA-I/HDL-mediated cholesterol efflux from the macrophages in an LXR-independent manner. In addition to the anti-oxidative properties, the potential cardioprotective properties of astaxanthin might therefore be associated with an enhanced anti-atherogenic function of HDL.

Calpain-mediated ABCA1 degradation: post-translational regulation of ABCA1 for HDL biogenesis

Helical apolipoproteins remove cellular phospholipid and cholesterol to generate nascent HDL and this reaction is the major source of plasma HDL. ABCA1 is mandatory and rate-limiting for this reaction. Besides regulation of the gene expression by transcriptional factors including LXR, AP2 and SREBP, the ABCA1 activity is regulated post-translationally by calpain-mediated proteolytic degradation of ABCA1 protein that occurs in the early endosome after its endocytosis. When the HDL biogenesis reaction is ongoing as helical apolipoproteins interact with ABCA1, ABCA1 becomes resistant to calpain and is recycled to cell surface after endocytosis. Biogenesis of HDL is most likely to take place on cell surface. Clearance rate of ABCA1 by this mechanism is also retarded by various factors that interact with ABCA1, such as α1-syntrophin, LXRβ and calmodulin. Physiological relevance of the retardation by these factors is not entirely clear. Pharmacological inhibition of the calpain-mediated ABCA1 degradation results in the increase of the ABCA1 activity and HDL biogenesis in vitro and in vivo, and potentially suppresses atherogenesis. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Coffee consumption enhances high-density lipoprotein-mediated cholesterol efflux in macrophages

RATIONALE

Association of habitual coffee consumption with coronary heart disease morbidity and mortality has not been established. We hypothesized that coffee may enhance reverse cholesterol transport (RCT) as the antiatherogenic properties of high-density lipoprotein (HDL).

OBJECTIVE

This study was to investigate whether the phenolic acids of coffee and coffee regulates RCT from macrophages in vitro, ex vivo and in vivo.

METHODS AND RESULTS

Caffeic acid and ferulic acid, the major phenolic acids of coffee, enhanced cholesterol efflux from THP-1 macrophages mediated by HDL, but not apoA-I. Furthermore, these phenolic acids increased both the mRNA and protein levels of ATP-binding cassette transporter (ABC)G1 and scavenger receptor class B type I (SR-BI), but not ABCA1. Eight healthy volunteers were recruited for the ex vivo study, and blood samples were taken before and 30 minutes after consumption of coffee or water in a crossover study. The mRNA as well as protein levels of ABCG1, SR-BI, and cholesterol efflux by HDL were increased in the macrophages differentiated under autologous sera obtained after coffee consumption compared to baseline sera. Finally, effects of coffee and phenolic acid on in vivo RCT were assessed by intraperitoneally injecting [(3)H]cholesterol-labeled acetyl low-density lipoprotein-loaded RAW264.7 cells into mice, then monitoring appearance of (3)H tracer in plasma, liver, and feces. Supporting in vitro and ex vivo data, ferulic acid was found to significantly increase the levels of (3)H tracer in feces.

CONCLUSIONS

Coffee intake might have an antiatherogenic property by increasing ABCG1 and SR-BI expression and enhancing HDL-mediated cholesterol efflux from the macrophages via its plasma phenolic acids.

Pharmacological inhibition of ABCA1 degradation increases HDL biogenesis and exhibits antiatherogenesis

Expression of ABCA1 is regulated by transcription of the gene and calpain-mediated proteolytic degradation, and inhibition ABCA1 degradation results in increased ABCA1 and HDL biogenesis in vitro. We examined whether this approach could be a potential antiatherogenic treatment. Although probucol inhibits both the activity and degradation of ABCA1, its oxidized products, spiroquinone and diphenoquinone, reduce degradation of ABCA1 without inhibiting its activity or altering transcription of the ABCA1 gene. Accordingly, both compounds enhanced apolipoprotein A-I/ABCA1-dependent generation of HDL in vitro, and increased hepatic ABCA1 and plasma HDL without increasing antioxidant activity in plasma when given to rabbits. Both compounds also decreased vascular lipid deposition in cholesterol-fed rabbits. We therefore conclude that stabilization of ABCA1 against calpain-mediated degradation is a novel and potentially important strategy to increase HDL formation and prevent atherosclerosis. Spiroquinone and diphenoquinone are potential seeds for development of such drugs.

Atorvastatin downregulates BMP-2 expression induced by oxidized low-density lipoprotein in human umbilical vein endothelial cells

BACKGROUND

Bone morphogenetic protein-2 (BMP-2) plays a key role both in vascular development and pathophysiological processes. However, the effects of oxidized low-density lipoprotein (ox-LDL) combined with atorvastatin on BMP-2 expression are entirely unknown in human umbilical vein endothelial cells (HUVECs). The present study investigates the effects of ox-LDL on BMP-2 expression. Furthermore, the influence of atorvastatin on ox-LDL-induced BMP-2 expression is also examined.

METHODS AND RESULTS

The HUVECs were treated by ox-LDL or combined with pyrrolidine dithiocarbamate (PDTC) or atorvastatin. The expression level of BMP-2 mRNA was examined by real-time PCR and RT-PCR analysis. The expression of BMP-2 protein was assayed by enzyme-linked immunosorbent assay. The malondialdehyde (MDA) and activities of total superoxide dismutase (SOD) were detected by routine methods. The activation of nuclear factor kappaB (NF-kappaB) in HUVECs was determined using an assay kit from active motif and western blot analysis. Ox-LDL treatment significantly increased BMP-2 expression, which is associated with NF-kappaB activation, but BMP-2 expression was suppressed by treatment with PDTC or atorvastatin. Furthermore, the increase in MDA levels and decrease in activities of total SOD caused by ox-LDL treatment were reversed by the treatment of PDTC or atorvastatin.

CONCLUSIONS

Ox-LDL-induced BMP-2 expression was suppressed by PDTC or atorvastatin treatment. The effects of atorvastatin might contribute to the mechanisms by inhibiting NF-kappaB activation.

Effect of probucol on HDL metabolism and class B type I scavenger receptor (SR-BI) expression in the liver of hypercholesterolemic rabbits

BACKGROUND

Scavenger receptor class B type I (SR-BI) is a major receptor for high-density lipoproteins (HDL) in the liver. Overexpression of SR-BI attenuated experimental atherosclerosis in murine models, concomitant with a reduction in plasma HDL-cholesterol levels. Probucol is known to be a potent hypolipidemic drug to regress xanthoma formation and carotid atherosclerosis in conjunction with a marked reduction in HDL-cholesterol levels. However, the mechanism by which probucol affects atherosclerosis is not completely understood, and the effect of probucol on the expression of SR-BI was controversial. The aim of this study was to know the effect of probucol on HDL metabolism and SR-BI expression in the liver.

METHODS

Sixteen rabbits fed with high cholesterol diet for 8 weeks were randomly divided into two groups: (1) high cholesterol group (n = 8): maintained high cholesterol diet for 6 weeks; (2) probucol group (n = 8): the same cholesterol diet plus 1% probucol for 6 weeks. Control group (n = 8) was fed with normal diet for 14 weeks. The classical in situ two steps perfusion of the liver with collagenase IV was used to isolate the parenchymal hepatocytes. The selective uptake of HDL by hepatocytes was performed using the double radiolabelled HDL. Immunohistochemistry and reverse transcription polymerase chain reaction (RT-PCR) were used to evaluate SR-BI expression in the liver.

RESULTS

Compared with control group, rabbits fed with high cholesterol diet showed higher levels of serum total cholesterol (TC), low-density lipoprotein (LDL) cholesterol and HDL-C, all of which were significantly reduced by probucol treatment. The selective uptake of HDL CEs in probucol group (249.68 +/- 60.13 ng/mg cell protein) was about two folds higher as compared with the control group (122.47 +/- 54.06 ng/mg cell protein, P < 0.01) and high cholesterol group (104.92 +/- 47.91 ng/mg cell protein, P < 0.01), but it could not be reproduced in vitro. The expression of SR-BI were significantly decreased in the high cholesterol group (0.48 +/- 0.06) as compared with control group (0.65 +/- 0.06, P < 0.01). Probucol increased SR-BI expression (0.68 +/- 0.06, P < 0.01) as compared with high cholesterol group. The expression of SR-BI was positively associated with the selective CEs uptake (r = 0.47, P = 0.032).

CONCLUSIONS

Probucol up-regulates SR-BI expression and enhance the uptake of HDL CEs by hepatocytes, which may help us to understand the anti-atherogenic properties and the HDL-C-lowering effect of probucol.

ABCA1 and biogenesis of HDL

Mammalian somatic cells do not catabolize cholesterol and therefore export it for sterol homeostasis at cell and whole body levels. This mechanism may reduce intracellularly accumulated excess cholesterol, and thereby would contribute to the prevention or cure of the initial stage of atherosclerotic vascular lesion. High-density lipoprotein (HDL) plays a central role in this reaction by removing cholesterol from cells and transporting it to the liver, the major cholesterol catabolic site. Two independent mechanisms have been identified for cellular cholesterol release. The first is non-specific diffusion-mediated cholesterol "efflux" from the cell surface, in which cholesterol is trapped by various extracellular acceptors including lipoproteins. Extracellular cholesterol esterification of HDL provides a driving force for the net removal of cell cholesterol by this pathway, and some cellular factors may enhance this reaction. The other mechanism is an apolipoprotein-mediated process to generate new HDL particles by removing cellular phospholipid and cholesterol. This reaction is mediated by a membrane protein ATP-binding cassette transporter A1 (ABCA1), and lipid-free or lipid-poor helical apolipoproteins recruit cellular phospholipid and cholesterol to assemble HDL particles. The reaction is composed of two elements: the assembly of HDL particles with phospholipid by apolipoprotein, and cholesterol enrichment in HDL. ABCA1 is essential for the former step and the latter requires further intracellular events. ABCA1 is a rate-limiting factor of HDL assembly and is regulated by transcriptional and post-transcriptional factors. Post-transcriptional regulation of ABCA1 involves modulation of its calpain-mediated degradation.

Probucol up-regulates paraoxonase 1 expression in hepatocytes of hypercholesterolemic rabbits

Paraoxonase 1 (PON1), an HDL-associated enzyme, has anti-oxidative property. Probucol, a hydrophobic antioxidant drug, inhibits progression of atherosclerosis and post-angioplasty restenosis. However, the mechanism by which probucol affects atherosclerosis is not completely understood. Sixteen rabbits fed with high cholesterol diet for 8 weeks were randomly divided into two groups: (1) starch group (n = 8): maintained high cholesterol diet plus starch (500 mg/kg/d) for 6 weeks; (2) probucol group (n = 8): the same cholesterol diet plus probucol (500 mg/kg/d) for 6 weeks. Control group (n = 8) was fed with normal diet for 14 weeks. The classic in-situ two-step perfusion of the liver with collagenase IV was used to isolate the parenchymal hepatocytes. The total activity of superoxide dismutase (SOD) and malondialdehyde (MDA) and PON1 concentrations in serum were measured after 0, 8, and 14 weeks of feeding. Reverse transcription polymerase chain reaction (RT-PCR) was used to detect the mRNA expression of PON1. Compared with control group, rabbits fed with high cholesterol diet showed higher levels of serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C), all of which were significantly reduced by probucol. Probucol significantly decreased the MDA concentration but was ineffective on SOD activity. High cholesterol diet decreased serum PON1 concentration and down-regulated PON1 mRNA expression in hepatocytes. Probucol significantly increased serum PON1 level and up-regulated the mRNA expression of PON1 as compared with starch group (0.65 +/- 0.06 versus 0.46 +/- 0.05, P = 0.001). The PON1 concentration is negatively associated with MDA concentration (r = -0.86, P = 0.003) but not with the level of HDL-C. In conclusion, probucol decreased MDA concentration, and increased PON1 serum level as well as mRNA expression in hepatocytes, which may help us to understand its antioxidant and anti-atherogenic effects.

Assembly of High-Density Lipoprotein

Mammalian somatic cells do not catabolize cholesterol and need to export it for its homeostasis at the levels of cells and whole bodies. This reaction may reduce intracellularly accumulated cholesterol in excess and would contribute to prevention or regression of the initial stage of atherosclerosis. High-density lipoprotein (HDL) is thought to play a main role in this reaction, and 2 independent mechanisms are proposed for this reaction. First, cholesterol is exchanged in a nonspecific physicochemical manner between cell surface and extracellular lipoproteins, and cholesterol esterification on HDL provides a driving force for net removal of cell cholesterol. Second, apolipoproteins directly interact with cells and generate HDL by removing cellular phospholipid and cholesterol. This reaction is a major source of plasma HDL and is mediated by a membrane protein, ABCA1. Lipid-free or lipid-poor helical apolipoproteins primarily recruit cellular phospholipid to assemble HDL particles, and cholesterol enrichment in these particles is regulated independently. ABCA1 is a rate-limiting factor of the HDL assembly and is regulated by transcriptional factors and posttranscriptional factors. Posttranscriptional regulation of ABCA1 includes modulation of its calpain-mediated degradation.

Probucol enhances the expression of human hepatic scavenger receptor class B type I, possibly through a species-specific mechanism

OBJECTIVE

Scavenger receptor class B type I (SR-BI) is a major receptor for high-density lipoproteins (HDL) in the liver, which is the terminus of reverse cholesterol transport. Overexpression of SR-BI attenuated experimental atherosclerosis in murine models, concomitant with a reduction in plasma HDL-cholesterol levels. Probucol is known to be a potent hypolipidemic drug to regress xanthoma formation and carotid atherosclerosis in conjunction with a marked reduction in HDL-cholesterol levels. The aim of the present study was to know the effect of probucol on the expression of SR-BI and the underlying mechanism.

METHODS AND RESULTS

We found that probucol increased the expression of SR-BI proteins in in vitro human liver cells and an in vivo rabbit model, but not in wild-type C57Bl6 mice. The decay curve of SR-BI protein was markedly retarded in probucol-treated HepG2 cells in the presence of cycloheximide, indicating that probucol may stabilize human SR-BI protein. To determine the underlying mechanism for the observed species-specific effect, we conducted the following host-swap experiments, in which SR-BI was transfected or expressed in heterologous cells or hosts. Probucol did not increase human SR-BI protein in the liver of transgenic mice carrying the entire human SR-BI genome. Although probucol could stabilize even murine SR-BI, when transfected into a human cell line, HepG2, human SR-BI was not stabilized in a mouse hepatoma cell line, Hepa 1-6, treated with probucol.

CONCLUSIONS

Probucol enhances hepatic SR-BI protein expression, possibly through species-specific stabilization of the protein.

Assembly of high density lipoprotein by the ABCA1/apolipoprotein pathway

PURPOSE OF REVIEW

Mammalian somatic cells do not catabolize cholesterol and therefore need to export it for sterol homeostasis at the levels of cells and whole bodies. This mechanism may reduce intracellularly accumulated cholesterol in excess, and thereby would contribute to the prevention or cure of the initial stage of atherosclerotic vascular lesions.

RECENT FINDINGS

HDL is thought to play a main role in this reaction on the basis of epidemiological evidence and in-vitro experimental data. Two independent mechanisms have been identified for this reaction. One is non-specific diffusion-mediated cholesterol 'efflux' from the cell surface, and cholesterol is trapped by various extracellular acceptors including lipoproteins. Extracellular cholesterol esterification on HDL provides a driving force for the net removal of cell cholesterol, and some cellular factors may enhance this reaction. The other mechanism is an apolipoprotein-mediated process to generate HDL by removing cellular phospholipid and cholesterol. This reaction is mediated by a membrane protein ABCA1, and lipid-free or lipid-poor helical apolipoproteins recruit cellular phospholipid and cholesterol to assemble HDL particles. The reaction is composed of two elements: the assembly of HDL particles with phospholipid by apolipoprotein, and cholesterol enrichment in HDL. ABCA1 is essential for the former step, and the latter step requires further intracellular events.

SUMMARY

ABCA1 is a rate-limiting factor of HDL assembly and is regulated by transcriptional factors and posttranscriptional factors. Posttranscriptional regulation of ABCA1 involves the modulation of its calpain-mediated degradation.

Modulation of macrophage function and metabolism

Several drugs or pharmacologically active molecules such as statins, calcium antagonists, and PPAR agonists have been shown to affect macrophage functions that contribute to atherosclerosis and modulate plaque stability. For example, the modulation of matrix metalloproteinase secretion and cholesterol metabolism in macrophages may help to prevent cardiovascular disease independently of the correction of risk factors.

On the hepatic mechanism of HDL assembly by the ABCA1/apoA-I pathway

The mechanism for the assembly of HDL with cellular lipid by ABCA1 and helical apolipoprotein was investigated in hepatocytes. Both HepG2 cells and mouse primary culture hepatocytes produced HDL with apolipoprotein A-I (apoA-I) whether endogenously synthesized or exogenously provided. Probucol, an ABCA1 inactivator, inhibited these reactions, as well as the reversible binding of apoA-I to HepG2. Primary cultured hepatocytes of ABCA1-deficient mice also lacked HDL production regardless of the presence of exogenous apoA-I. HepG2 cells secreted apoA-I into the medium even when ABCA1 was inactivated by probucol, but it was all in a free form as HDL production was inhibited. When a lipid-free apoA-I-specific monoclonal antibody, 725-1E2, was present in the culture medium, production of HDL was suppressed, whether with endogenous or exogenously added apoA-I, and the antibody did not influence HDL already produced by HepG2 cells. We conclude that the main mechanism for HDL assembly by endogenous apoA-I in HepG2 cells is an autocrine-like reaction in which apoA-I is secreted and then interacts with cellular ABCA1 to generate HDL.

Probucol inhibits ABCA1-mediated cellular lipid efflux

OBJECTIVE

ATP-binding cassette transporter A1 (ABCA1) mediates the efflux of lipids from cells to lipid-poor apolipoproteins. In this article, we characterize the effect of probucol on cellular ABCA1-mediated lipid efflux.

METHODS AND RESULTS

Probucol inhibited cholesterol efflux up to 80% in J774 macrophages expressing ABCA1. In Fu5AH hepatoma cells that contain scavenger receptor class B, type I, but not functional ABCA1, we observed no effect of probucol on cholesterol efflux. Probucol inhibited cholesterol efflux from normal human skin fibroblasts but not from fibroblasts from a Tangier patient. Fluorescent confocal microscopy and biotinylation assay demonstrated that in J774 cells probucol impaired the translocation of ABCA1 from intracellular compartments to the plasma membrane. Probucol also inhibited the formation of an ABCA1-linked cholesterol oxidase sensitive plasma membrane domain. Consistent with the inhibitory effect on ABCA1 translocation to the plasma membrane, probucol reduced cell surface-specific [125I]-labeled apolipoprotein-AI binding.

CONCLUSIONS

We conclude that probucol is an effective inhibitor of ABCA1-mediated cholesterol efflux without influencing scavenger receptor class B type I-mediated efflux. The inhibition of ABCA1 translocation to the plasma membrane may in part explain the reported in vivo high-density lipoprotein-lowering action of probucol.

Probucol inactivates ABCA1 in the plasma membrane with respect to its mediation of apolipoprotein binding and high density lipoprotein assembly and to its proteolytic degradation

Probucol has been shown to inhibit the release of cellular lipid by helical apolipoprotein and thereby to reduce plasma high density lipoprotein. We attempted to explore the underlying mechanism for this effect in human fibroblast WI-38. Probucol inhibited the apoA-I-mediated cellular lipid release and binding of apoA-I to the cells in a dose-dependent manner. It did not influence cellular uptake of low density lipoprotein, transport of cholesterol to the cell surface whether de novo synthesized or delivered as low density lipoprotein, and overall cellular content of cholesterol, although biosynthesis of lipids from acetate was somewhat increased. Probucol did not affect the mRNA level of ABCA1, and ABCA1 was recovered along with marker proteins for plasma membrane regardless of the presence of probucol. However, the protein level of ABCA1 increased, and the rate of its decay in the presence of cycloheximide was slower in the probucol-treated cells. ABCA1 in the probucol-treated cells was resistant to digestion by calpain but not by trypsin. We concluded that probucol inactivates ABCA1 in the plasma membrane with respect to its function in mediating binding of and lipid release by apolipoprotein and with respect to proteolytic degradation by calpain.

An inhibitor of acylCoA: cholesterol acyltransferase increases expression of ATP-binding cassette transporter A1 and thereby enhances the ApoA-I-mediated release of cholesterol from macrophages

The effect of inhibition of acylCoA: cholesterol acyltransferase (ACAT) was studied on high density lipoprotein (HDL) metabolism. An inhibitor of ACAT, MCC-147, was given mouse peritoneal macrophages and expression of ATP-binding cassette transporter A1 (ABCA1) was examined. ABCA1 was increased both at the mRNA and protein levels, only when the cells are cholesterol-loaded and thereby the inhibitor decreased esterified cholesterol and increased unesterified cholesterol. In this condition, the ACAT inhibitor increased reversible binding of apoA-I to the cells and enhanced apoA-I-mediated release of cellular cholesterol and phospholipid, but did not influence nonspecific cellular cholesterol efflux to lipid microemulsion. It was therefore concluded that the ACAT inhibitor increased the release of cholesterol from the cholesterol-loaded macrophages by increasing the expression of ABCA1, putatively through shifting cholesterol distribution from the esterified to the free compartments.

Verapamil increases the apolipoprotein-mediated release of cellular cholesterol by induction of ABCA1 expression via Liver X receptor-independent mechanism

OBJECTIVE

Release of cellular cholesterol and phospholipid mediated by helical apolipoprotein and ATP-binding cassette transporter (ABC) A1 is a major source of plasma HDL. We investigated the effect of calcium channel blockers on this reaction.

METHODS AND RESULTS

Expression of ABCA1, apoA-I-mediated cellular lipid release, and HDL production were enhanced in cAMP analogue-treated RAW264 cells by verapamil, and similar effects were also observed with other calcium channel blockers. The verapamil treatment resulted in rapid increase in ABCA1 protein and its mRNA, but not the ABCG1 mRNA, another target gene product of the nuclear receptor liver X receptor (LXR). By using the cells transfected with a mouse ABCA1 promoter-luciferase construct (-1238 to +57bp), verapamil was shown to enhance the transcriptional activity. However, it did not increase transcription of LXR response element-driven luciferase vector.

CONCLUSIONS

The data demonstrated that verapamil increases ABCA1 expression through LXR-independent mechanism and thereby increases apoA-I-mediated cellular lipid release and production of HDL.

Depletion of pre-beta-high density lipoprotein by human chymase impairs ATP-binding cassette transporter A1- but not scavenger receptor class B type I-mediated lipid efflux to high density lipoprotein

The ATP-binding cassette transporter A1 (ABCA1) mediates the efflux of cellular unesterified cholesterol and phospholipid to lipid-poor apolipoprotein A-I. Chymase, a protease secreted by mast cells, selectively cleaves pre-beta-migrating particles from high density lipoprotein (HDL)(3) and reduces the efflux of cholesterol from macrophages. To evaluate whether this effect is the result of reduction of ABCA1-dependent or -independent pathways of cholesterol efflux, in this study we examined the efflux of cholesterol to preparations of chymase-treated HDL(3) in two types of cell: 1) in J774 murine macrophages endogenously expressing low levels of scavenger receptor class B, type I (SR-BI), and high levels of ABCA1 upon treatment with cAMP; and 2) in Fu5AH rat hepatoma cells endogenously expressing high levels of the SR-BI and low levels of ABCA1. Treatment of HDL(3) with the human chymase resulted in rapid depletion of pre-beta-HDL and a concomitant decrease in the efflux of cholesterol and phospholipid (2-fold and 3-fold, respectively) from the ABCA1-expressing J774 cells. In contrast, efflux of free cholesterol from Fu5AH to chymase-treated and to untreated HDL(3) was similar. Incubation of HDL(3) with phospholipid transfer protein led to an increase in pre-beta-HDL contents as well as in ABCA1-mediated cholesterol efflux. A decreased cholesterol efflux to untreated HDL(3) but not to chymase-treated HDL(3) was observed in ABCA1-expressing J774 with probucol, an inhibitor of cholesterol efflux to lipid-poor apoA-I. Similar results were obtained using brefeldin and gliburide, two inhibitors of ABCA1-mediated efflux. These results indicate that chymase treatment of HDL(3) specifically impairs the ABCA1-dependent pathway without influencing either aqueous or SR-BI-facilitated diffusion and that this effect is caused by depletion of lipid-poor pre-beta-migrating particles in HDL(3). Our results are compatible with the view that HDL(3) promotes ABCA1-mediated lipid efflux entirely through its lipid-poor fraction with pre-beta mobility.

Apolipoprotein A-I activates protein kinase C alpha signaling to phosphorylate and stabilize ATP binding cassette transporter A1 for the high density lipoprotein assembly

ATP-binding cassette transporter A1 (ABCA1) plays an essential role in the helical apolipoprotein-mediated assembly of high density lipoprotein, and the apolipoporteins stabilize ABCA1 against calpain-mediated degradation during the reaction ((2002) J. Biol. Chem. 277, 22426-22429). Protein kinase C (PKC) inhibitors suppressed both ABCA1 stabilization and cellular lipid release mediated by apolipoprotein A-I (apoA-I) but not ABCA1 increase by calpain inhibitors. The increase of ABCA1 and the cellular lipid release by apoA-I were both suppressed by a phosphatidylcholine phospholipase C (PC-PLC) inhibitor but not by the inhibitors of phosphatidylinositol-PLC and phosphatidylinositol 3-kinase. A protein phosphatase inhibitor further enhanced the ABCA1 increase by apoA-I. Biochemical and microscopic evidence indicated that apoA-I activated PKC alpha, and phosphorylation of ABCA1 was directly demonstrated by apoA-I via PKC. Finally, digestion of sphingomyelin increased ABCA1, and a PC-PLC inhibitor suppressed it. We conclude that apoA-I activates PKC alpha by PC-PLC-mediated generation of diacylglycerol initiated by the removal of cellular sphingomyelin ((2002) J. Biol. Chem. 277, 44709-44714), and subsequently phosphorylates and stabilizes ABCA1.

Impaired ABCA1-dependent lipid efflux and hypoalphalipoproteinemia in human Niemann-Pick type C disease

The cholesterol trafficking defect in Niemann-Pick type C (NPC) disease leads to impaired regulation of cholesterol esterification, cholesterol synthesis, and low density lipoprotein receptor activity. The ATP-binding cassette transporter A1 (ABCA1), which mediates the rate-limiting step in high density lipoprotein (HDL) particle formation, is also regulated by cell cholesterol content. To determine whether the Niemann-Pick C1 protein alters the expression and activity of ABCA1, we determined the ability of apolipoprotein A-I (apoA-I) to deplete pools of cellular cholesterol and phospholipids in human fibroblasts derived from NPC1+/+, NPC1+/-, and NPC1-/- subjects. Efflux of low density lipoprotein-derived, non-lipoprotein, plasma membrane, and newly synthesized pools of cell cholesterol by apoA-I was diminished in NPC1-/- cells, as was efflux of phosphatidylcholine and sphingomyelin. NPC1+/- cells showed intermediate levels of lipid efflux compared with NPC1+/+ and NPC1-/- cells. Binding of apoA-I to cholesterol-loaded and non-cholesterol-loaded cells was highest for NPC1+/- cells, with NPC1+/+ and NPC1-/- cells showing similar levels of binding. ABCA1 mRNA and protein levels increased in response to cholesterol loading in NPC1+/+ and NPC1+/- cells but showed low levels at base line and in response to cholesterol loading in NPC1-/- cells. Consistent with impaired ABCA1-dependent lipid mobilization to apoA-I for HDL particle formation, we demonstrate for the first time decreased plasma HDL-cholesterol levels in 17 of 21 (81%) NPC1-/- subjects studied. These results indicate that the cholesterol trafficking defect in NPC disease results in reduced activity of ABCA1, which we suggest is responsible for the low HDL-cholesterol in the majority of NPC subjects and partially responsible for the overaccumulation of cellular lipids in this disorder.

Differential regulation of apolipoprotein A-I/ATP binding cassette transporter A1-mediated cholesterol and phospholipid release

We compared apolipoprotein A-I (apoA-I)-mediated release of cellular cholesterol and phospholipid among several fibroblast cell lines. ApoA-I induced phospholipid release from WI-38, MRC-5, BALB/3T3, L929 and CHO-K1, but not from COS-7, while cholesterol was released only from WI-38, MRC-5 and BALB/3T3 without correlation to the cellular cholesterol content. The reaction consequently generated cholesterol-rich high density lipoprotein (HDL) with WI-38, MRC-5 and BALB/3T3, cholesterol-poor HDL with L929 and CHO-K1, and no HDL from COS-7. In contrast, diffusion-mediated cholesterol efflux to cyclodextrin took place with all the cell lines tested in proportion to the cellular free cholesterol content. While caveolin-1 was expressed in all of these cell lines, ATP-binding cassette transporter (ABC) A1 was detected in all but COS-7. We concluded that (1) fibroblasts were categorized into three groups with respect to the interaction with apoA-I, (i) those to produce cholesterol-rich HDL, (ii) cholesterol-poor HDL and (iii) no HDL; (2) ABCAl was required for the assembly of HDL from phospholipid among the cell lines tested; (3) caveolin-1 alone did not induce cholesterol incorporation into the HDL generated.

Lipid accumulation in smooth muscle cells under LDL loading is independent of LDL receptor pathway and enhanced by hypoxic conditions

OBJECTIVE

The effect of a variety of hypoxic conditions on lipid accumulation in smooth muscle cells (SMCs) was studied in an arterial wall coculture and monocultivation model.

METHODS AND RESULTS

Low density lipoprotein (LDL) was loaded under various levels of oxygen tension. Oil red O staining of rabbit and human SMCs revealed that lipid accumulation was greater under lower oxygen tension. Cholesterol esters were shown to accumulate in an oxygen tension-dependent manner by high-performance liquid chromatographic analysis. Autoradiograms using radiolabeled LDL indicated that LDL uptake was more pronounced under hypoxia. This result holds in the case of LDL receptor-deficient rabbit SMCs. However, cholesterol biosynthesis and cellular cholesterol release were unaffected by oxygen tension.

CONCLUSIONS

Hypoxia significantly increases LDL uptake and enhances lipid accumulation in arterial SMCs, exclusive of LDL receptor activity. Although the molecular mechanism is not clear, the model is useful for studying lipid accumulation in arterial wall cells and the difficult-to-elucidate events in the initial stage of atherogenesis.

Helical apolipoproteins stabilize ATP-binding cassette transporter A1 by protecting it from thiol protease-mediated degradation

ATP-binding cassette transporter (ABC) A1 was increased by apolipoprotein A-I without an increase of its message in THP-1 cells. The pulse label study demonstrated that apoA-I retarded degradation of ABCA1. Similar changes were demonstrated by apoA-II, but the effect of high density lipoprotein was almost negligible on the basis of equivalent protein concentration. Thiol protease inhibitors (leupeptin and N-acetyl-Leu-Leu-norleucinal (ALLN)) increased ABCA1 and slowed its decay in the cells, whereas none of the proteosome-specific inhibitor lactacystin, other protease inhibitors, or the lysosomal inhibitor NH(4)Cl showed such effects. The effects of apoA-I and ALLN were additive for the increase of ABCA1, and the apoA-I-mediated cellular lipid release was enhanced by ALLN. The data suggest that ABCA1 is rapidly degraded by a thiol protease(s) in the cells unless helical apolipoproteins in their lipid-free form stabilize ABCA1 by protecting it from protease-mediated degradation.

In vivo metabolism of HDL, apo A-I, and lp A-I, and function of HDL--a clinical perspective

Serum levels of high density lipoprotein cholesterol (HDL-C) are inversely correlated with coronary heart disease (CHD). Kinetic studies indicate that the mechanism for the variation in HDL levels associated with various pathophysiologic states includes changes in the fractional catabolic rate (FCR) and/or the synthesis rate of HDL and its major proteins apolipoprotein (apo) A-I and apo A-II. The antiatherogenic effects of HDL are thought to be mainly due to its role in reverse cholesterol transport. HDL is an assembly of heterogeneous particles. HDL enlarges when it takes up cellular cholesterol, and shrinks when HDL cholesterol ester (CE) is transfered to low density lipoprotein (LDL) and very low density lipoprotein (VLDL) particles. The functional ability of HDL (to remove cellular cholesterol) has drawn considerable attention. The fractional esterification rate of cholesterol in HDL (FER(HDL)) has been established as a functional assay of HDL, and reflects the size of HDL particles. We investigated the clinical significance of FER(HDL) and its relationship to the quantity of HDL. FER(HDL) values were inversely correlated with levels of HDL-C and large lipoprotein containing apo A-I (LpA-I). The association between FER(HDL) and CHD changed with serum HDL-C levels: increased FER(HDL) values significantly increased the risk of CHD when serum HDL-C levels were low, while there was no such relationship when HDL-C levels were high. We concluded that the combination of HDL-C levels and FER(HDL) is a stronger indicator of CHD than either the HDL-C level (quantitative measure of HDL) or FER(HDL) (functional measure of HDL) alone.

Progesterone inhibits apolipoprotein-mediated cellular lipid release: a putative mechanism for the decrease of high-density lipoprotein

In order to investigate the mechanism for female gonadal hormones to regulate the plasma high-density lipoprotein (HDL) level, the effect of 17 beta-estradiol and progestogens was examined in vitro on the assembly of HDL by free apolipoprotein A-I (apoA-I) with cellular cholesterol and phospholipid. ApoA-I generated HDL particles by removing cholesterol and phospholipid from human fibroblasts, MRC-5. While 17 beta-estradiol did not influence this reaction, progesterone suppressed the removal by apoA-I of both cholesterol and phospholipid, with the extent of the inhibition more for cholesterol than phospholipid. Three other synthetic progestogens showed the similar inhibitory effect on the cellular cholesterol release. Cellular cholesterol de novo-synthesized from mevalonolactone entered more into the acyl-esterified cholesterol compartment and less to the unesterified compartment in the presence of progesterone. On the other hand, progesterone did not influence the overall mass ratio of free and esterified cholesterol in the cell. Cell-surface cholesterol was also uninfluenced by progesterone when probed by extracellular cholesterol oxidase reaction or by diffusion-mediated cellular cholesterol release to cyclodextrin. Neither caveolin-1 nor ABCA1 expression was influenced by progesterone. Progesterone thus seems primarily to alter the specific intracellular cholesterol compartment that is related to the apoA-I-mediated HDL assembly. This mechanism might contribute to the decrease of plasma HDL by administration of progestogen in women under hormone replacement therapy.

Effect of probucol in lecithin-cholesterol acyltransferase-deficient mice: inhibition of 2 independent cellular cholesterol-releasing pathways in vivo

Cellular cholesterol release takes place by at least 2 distinct mechanisms: the lecithin-cholesterol acyltransferase (LCAT)-driven net efflux by cholesterol diffusion and the generation of high density lipoprotein (HDL) with cellular cholesterol and phospholipid on the cell-apolipoprotein interaction. Therefore, LCAT deficiency impairs the former pathway, and the latter can be inhibited by probucol, which interferes with the apolipoprotein-cell interaction. Hence, probucol was given to the LCAT-deficient mice in the attempt to suppress both of these pathways. The mice were fed low (0.2%) and high (1.2%) cholesterol diets containing 0.5% probucol for 2 weeks. LCAT deficiency and probucol markedly decreased plasma HDL, and the effects were synergistic. Tissue cholesterol content was lower in the adrenal glands and ovaries in the LCAT-deficient mice and in the probucol-treated mice, suggesting that HDL is a main cholesterol provider for these organs. It was also moderately decreased in the spleen of the low cholesterol-fed female mice and in the thyroid gland of the low cholesterol-fed male mice. On the other hand, the esterified cholesterol content in the liver was substantially increased by the probucol treatment with a high cholesterol diet in the LCAT-deficient mice but not in the wild-type mice. Among the groups, there was no significant difference in the tissue cholesterol levels in other organs, such as the liver, spleen, thymus, brain, erythrocytes, thyroid gland, testis, and aorta, resulting from either LCAT deficiency or probucol. Thus, the apolipoprotein-mediated mechanism plays a significant role in the export of cellular cholesterol in the liver, indicating that the liver is a major site of the HDL assembly. Otherwise, tissue cholesterol homeostasis can largely be maintained in mice even when the assembly of new HDL is inhibited by probucol in the absence of LCAT. Nonspecific diffusion of cholesterol perhaps adequately maintains the homeostasis in the experimental condition.

Release of cellular cholesterol: molecular mechanism for cholesterol homeostasis in cells and in the body

Most mammalian somatic cells are unable to catabolize cholesterol and therefore need to export it in order to maintain sterol homeostasis. This mechanism may also function to reduce excessively accumulated cholesterol, which would thereby contribute to prevention or cure of the initial stage of atherosclerotic vascular lesion. High-density lipoprotein (HDL) has been believed to play a main role in this reaction based on epidemiological evidence and in vitro experimental data. At least two independent mechanisms are identified for this reaction. One is non-specific diffusion-mediated cholesterol 'efflux' from cell surface. Cholesterol molecules desorbed from cells can be trapped by various extracellular acceptors including various lipoproteins and albumin, and extracellular cholesterol esterification mainly on HDL may provide a driving force for the net removal of cell cholesterol by maintaining a cholesterol gradient between lipoprotein surface and cell membrane. The other is apolipoprotein-mediated process to generate new HDL by removing cellular phospholipid and cholesterol. The reaction is initiated by the interaction of lipid-free or lipid-poor helical apolipoproteins with cellular surface resulting in assembly of HDL particles with cellular phospholipid and incorporation of cellular cholesterol into the HDL being formed. Thus, HDL has dual functions as an active cholesterol acceptor in the diffusion-mediated pathway and as an apolipoprotein carrier for the HDL assembly reaction. The impairment of the apolipoprotein-mediated reaction was found in Tangier disease and other familial HDL deficiencies to strongly suggest that this is a main mechanism to produce plasma HDL. The causative mutations for this defect was identified in ATP binding cassette transporter protein A1, as a significant step for further understanding of the reaction and cholesterol homeostasis.

Enhancement of the cAMP-induced apolipoprotein-mediated cellular lipid release by calmodulin inhibitors W7 and W5 from RAW 264 mouse macrophage cell line cells

Apolipoprotein (apo) A-I generates high-density lipoprotein (HDL) by removing cellular cholesterol and phospholipid on the interaction with cells as a main source of plasma HDL. The reaction is induced by dibutylyl cyclic (dbc) adenosine monophosphate (AMP) in RAW 264, mouse macrophage cell line cells, and we investigated its pharmacologic modulation using this cell model. Release of cellular cholesterol and choline phospholipid by apoA-I was increased 9.9 and 4.2 times, respectively, by pretreatment of the cells with 300 microM dbcAMP for 24 h. Calmodulin inhibitors, W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide) and W5 (N(6-aminohexyl)-1-naphthalenesulfonamide), increased the apoA-I-mediated lipid release by 3 times from the dbcAMP-treated cells. The optimal drug concentrations (80 and 160 microM for W7 and W5, respectively) were not parallel with those reported for in vitro calmodulin inhibition (IC50, 28 and 240 microM for W7 and W5, respectively, toward phosphodiesterase activity), and in fact 40 microM W7 showed much stronger intracellular calmodulin inhibition than did 300 microM W5 using S7AAS2, a fluorescent peptide probe. Other calmodulin inhibitors such as amitriptyline, chlorpromazine, and trifluoperazine showed no effect on the apoA-I-mediated cholesterol release. In contrast to these results, neither dbcAMP nor W7 influenced the diffusion-mediated nonspecific cholesterol efflux to lipid microemulsion. We concluded that W7 and W5 increased the interaction of apoA-I with RAW 264 cells to generate more HDL. The effect did not seem directly correlated to their cal modulin inhibition or modulation of cAMP and protein kinase C.

Effects of probucol on cholesterol metabolism in mouse peritoneal macrophages: inhibition of HDL-mediated cholesterol efflux

Macrophage-derived foam cells are known to play an essential role in the development and progression of atherosclerotic lesions. Probucol prevents oxidative modification of low-density lipoprotein (LDL) and lowers plasma contents of LDL and high-density lipoprotein (HDL). A recent report using apoE -/- mice demonstrated that probucol treatment enhanced atherosclerosis in apoE -/- mice more rapidly than that in untreated apoE -/- mice, and a reduction in plasma cholesterol by probucol was not the cause of enhancement of atherosclerotic lesions in probucol-treated apoE -/- mice. Moreover, probucol was reported to inhibit apoA-I mediated cholesterol efflux from mouse macrophages. These reports suggested that probucol might directly affect cholesterol metabolism in mouse macrophages. Thus, we investigated the effects of probucol on cholesterol metabolism in mouse resident peritoneal macrophages. Probucol did not affect degradation of acetylated LDL (Ac-LDL), degradation of LDL and endogenous cholesterol synthesis in mouse macrophages. However, it significantly inhibited HDL-mediated cholesterol efflux. Moreover, probucol partially (30%) inhibited the binding of HDL to mouse macrophages, and significantly activated acyl-coenzyme A:cholesterol acyltransferase (ACAT). Our results suggested that probucol inhibited HDL-mediated cholesterol efflux by inhibiting the binding of HDL to mouse macrophages and reducing HDL-accessible free cholesterol content by ACAT activation, thereby worsening atherosclerotic lesions in apoE -/- mice. However, it remains unclear whether probucol inhibits HDL-mediated cholesterol efflux from human macrophages.