Cyclic AMP-specific phosphodiesterase 4 inhibitors promote ABCA1 expression and cholesterol efflux

Gypenoside XVII inhibits ox-LDL-induced macrophage inflammatory responses and promotes cholesterol efflux through activating the miR-182-5p/HDAC9 signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

The deposition of lipids in macrophages and the subsequent formation of foam cells significantly increase the risk of developing atherosclerosis (As). Targeting ATP-binding cassette transporter A1/G1 (ABCA1/ABCG1)-mediated reverse cholesterol transport is crucial for regulating foam cell formation. Therefore, the search for natural chemical components with the ability to regulate ABCA1/G1 is a potential drug target to combat the development of atherosclerosis. Gypenoside XVII (GP-17), a gypenoside monomer extracted from gynostemma pentaphyllum, presents an efficient anti-atherosclerosis function. However, the suppressed formation mechanism of foam cells by GP-17 remains elusive.

AIM OF STUDY

To explore the protective activities of GP-17 in ox-LDL-induced THP-1 macrophage-derived foam cells through modulating the promotion of cholesterol efflux and alleviation of inflammation.

MATERIALS AND METHODS

MTT was used to detect cell viability. Bodipy493/503 and oil red O staining were performed to measure cell lipid deposition. Enzymatic assay was used to measure intracellular cholesterol measurement. Cholesterol efflux/uptake were determined by cholesterol efflux assay and Dil-ox-LDL uptake assay. Inflammatory cytokines were measured by ELISA. Bioinformatics prediction and dual luciferase reporter assay were performed to validate miR-182-5p targeting HDAC9. Relative protein levels were evaluated by immunoblotting and relative gene levels were determined by quantitative real-time PCR.

RESULTS

Our results showed that GP-17 upregulated the expression of ABCA1, ABCG1 and miR-182-5p, but reduced HDAC9 expression levels in lipid-loaded macrophages, which promoted cholesterol efflux and inhibited lipid deposition. Additionally, GP-17 promoted the M2 phenotype of the macrophage and suppressed the inflammatory response in THP-1 macrophage-derived foam cells. Overexpression of HDAC9 or suppression of miR-182-5p eliminated the effects of ABCA1/G1 expression, lipid deposition and pro-inflammatory response.

CONCLUSION

These findings suggest that GP-17 exerts a beneficial effect on macrophage lipid deposition and inflammation responses through activating the miR-182-5p/HDAC9 signaling pathway.

Diaporisoindole B Reduces Lipid Accumulation in THP-1 Macrophage Cells via MAPKs and PPARγ-LXRα Pathways and Promotes the Reverse Cholesterol Transport by Upregulating SR-B1 and LDLR in HepG2 Cells

Diaporisoindole B (DPB), an isoprenylisoindole alkaloid isolated from the mangrove endophytic fungus Diaporthe sp. SYSU-HQ3, has been proved to have a good anti-inflammatory activity in macrophage cells. In this study, we found that DPB was able to reduce lipid accumulation in THP-1 macrophage-derived foam cells. DPB could inhibit the lipid influx-related gene CD36 and increase the expression of lipid efflux-related genes ATP binding cassette transporter A1 (ABCA1), ATP binding cassette transporter G1 (ABCG1), and scavenger receptor B1 (SR-B1). Moreover, DPB elevated low-density lipoprotein receptor (LDLR) protein expression in HepG2 cells, which can increase the transport of LDL. Meanwhile, DPB could downregulate the expression levels of proteins related to cholesterol and fatty acid synthesis. Further study showed that DPB could activate peroxisome proliferator-activated receptor gamma (PPARγ) and inhibit mitogen-activated protein kinase (MAPK) phosphorylation. Taken together, our findings demonstrated that DPB could reduce lipid accumulation in THP-1 macrophage cells by reducing the intake of lipids and promoting the efflux of lipids and also could promote the reverse cholesterol transport (RCT) mechanism by upregulating SR-B1 and LDLR in HepG2 cells.

Methotrexate monotherapy versus methotrexate and apremilast combination therapy in the treatment of palmoplantar psoriasis: A prospective, randomised, assessor-blinded, comparative study

Background

Palmoplantar psoriasis is a chronic debilitating condition which significantly impairs quality of life.

Objectives

To assess the efficacy and safety of the combination of apremilast and methotrexate compared with methotrexate monotherapy in the treatment of palmoplantar psoriasis. Also, to study the impact on treatment on the Dermatology Life Quality Index and Palmoplantar Quality of Life Index.

Methods

A total of 64 patients were randomised to two groups in a 1:1 ratio - Group A received both methotrexate and apremilast in combination, while Group B received only methotrexate, for 16 weeks. The primary endpoints were the mean score of Modified Palmoplantar Psoriasis Area and Severity Index at week 16, the proportion of patients achieving modified palmoplantar psoriasis area severity index-75 and/or Palmoplantar Psoriasis Physician Global Assessment score 0/1 at week 16.

Results

A significantly higher proportion of patients in Group A achieved Modified Palmoplantar Psoriasis Area and Severity Index-75 at week 16 (43% in Group A vs 30% in Group B). The Modified Palmoplantar Psoriasis Area and Severity Index score was significantly lower in the combination group at week 16 (4.03 ± 2.05 in Group A and 5.89 ± 2.31 in Group B, P-value = 0.002). About 80% of patients in the combination group with baseline Palmoplantar Psoriasis Physician Global Assessment ≥3 achieved Palmoplantar Psoriasis Physician Global Assessment 0/1 compared to 60% in Group B. The combination group showed a significantly higher reduction in Dermatology Life Quality Index and Palmoplantar Quality of Life Index scores compared to the methotrexate alone group (P-value = 0.025). No notable adverse events were observed.

Limitation

The limitations of the study were single blinding, small sample size and a lack of longer follow up to assess the rate of relapse. We did not account for attrition during sample size calculation. Also, due to the paucity of data regarding the use of apremilast in palmoplantar psoriasis, definitive comparisons could not be made with previous studies.

Conclusion

The combination of apremilast and methotrexate has superior efficacy and a similar safety profile as compared to methotrexate monotherapy for the treatment of moderate to severe palmoplantar psoriasis.

Geniposide Ameliorated Dexamethasone-Induced Cholesterol Accumulation in Osteoblasts by Mediating the GLP-1R/ABCA1 Axis

Background: Overexposure to glucocorticoid (GC) produces various clinical complications, including osteoporosis (OP), dyslipidemia, and hypercholesterolemia. Geniposide (GEN) is a natural iridoid compound isolated from Eucommia ulmoides. Our previous study found that GEN could alleviate dexamethasone (DEX)-induced differentiation inhibition of MC3T3-E1 cells. However, whether GEN protected against Dex-induced cholesterol accumulation in osteoblasts was still unclear. Methods: DEX was used to induce rat OP. Micro-CT data was obtained. The ALP activity and mineralization were determined by the staining assays, and the total intracellular cholesterol was determined by the ELISA kits. The protein expression was detected by western blot. Results: GEN ameliorated Dex-induced micro-structure damages and cell differentiation inhibition in the bone trabecula in rats. In MC3T3-E1 cells, Dex enhanced the total intracellular cholesterol, which reduced the activity of cell proliferation and differentiation. Effectively, GEN decreased DEX-induced cholesterol accumulation, enhanced cell differentiation, and upregulated the expression of the GLP-1R/ABCA1 axis. In addition, inhibition of ABAC1 expression reversed the actions of GEN. Treatment with Exendin9-39, a GLP-1R inhibitor, could abrogate the protective activity of GEN. Conclusions: GEN ameliorated Dex-induced accumulation of cholesterol and inhibition of cell differentiation by mediating the GLP-1R/ABCA1 axis in MC3T3-E1 cells.

Eicosapentaenoic acid membrane incorporation stimulates ABCA1-mediated cholesterol efflux from human THP-1 macrophages

A high intake in polyunsaturated fatty acids (PUFAs), especially eicosapentaenoic acid (EPA) (C20:5 n-3), is cardioprotective. Dietary PUFAs incorporate into membrane phospholipids, which may modify the function of membrane proteins. We investigated the consequences of the membrane incorporation of several PUFAs on the key antiatherogenic ABCA1-mediated cholesterol efflux pathway. Human THP-1 macrophages were incubated with EPA, arachidonic acid (AA) (C20:4 n-6) or docosahexaenoic acid (DHA) (C22:6 n-3) for a long time to mimic a chronic exposure. EPA 70 μM, but not AA 50 μM or DHA 15 μM, increased ABCA1-mediated cholesterol efflux to apolipoprotein (apo) AI by 28% without altering aqueous diffusion. No variation in ABCA1 expression or localization was observed after EPA treatment. EPA incorporation did not affect the phenotype of THP-1 macrophages. The membrane phospholipids composition of EPA cells displayed higher levels of both EPA and its elongation product docosapentaenoic acid, which was associated with drastic lower levels of AA. Treatment by EPA increased the ATPase activity of the transporter, likely through a PKA-dependent mechanism. Eicosanoids were not involved in the stimulated ABCA1-mediated cholesterol efflux from EPA-enriched macrophages. In addition, EPA supplementation increased the apo AI binding capacity from macrophages by 38%. Moreover, the increased apo AI binding in EPA-enriched macrophages can be competed. In conclusion, EPA membrane incorporation increased ABCA1 functionality in cholesterol-normal human THP-1 macrophages, likely through a combination of different mechanisms. This beneficial in vitro effect may partly contribute to the cardioprotective effect of a diet enriched with EPA highlighted by several recent clinical trials.

Apremilast and its role in psoriatic arthritis

Psoriatic arthritis (PsA) is a seronegative inflammatory arthritis often observed in patients with skin psoriasis. Treatment of PsA, especially peripheral PsA, has typically relied on disease-modifying anti-rheumatic agents (DMARDs); however, these agents have limited efficacy and considerable associated toxicity. More recently, monoclonal antibodies (biologic agents) have revolutionized management of immune-mediated diseases; however, these therapies carry a high cost and require parenteral administration. Apremilast, a novel oral DMARD, was approved by the European Union for psoriatic arthritis in 2015. Apremilast inhibits the function of phosphodiesterase-4, a regulator of cyclic adenosine monophosphate, leading to a broad inhibition of proinflammatory mediators and subsequent reduction in tumour necrosis factor-alpha (TNF-α) response. The PALACE and ACTIVE trials, phase III randomized controlled trials for apremilast, showed that apremilast is effective at improving various clinical and patient-reported outcome measures for psoriatic arthritis in both DMARD-naïve and DMARD-experienced PsA patients. Efficacy was limited in patients with previous biologic DMARD failure and the overall efficacy of apremilast appears to be less than biologics agents, though no head-to-head trials exist comparing apremilast to biologic DMARDs. Apremilast is generally well tolerated, with short-lived gastrointestinal side effects being the most commonly reported adverse events. Guidelines suggest a trial of apremilast in patients who have failed traditional oral DMARDs and for whom, biologics are contraindicated. More studies directly comparing apremilast to conventional DMARDs and biologic DMARDs are needed and will be crucial in informing clinical and economic decisions about apremilast role in management of PsA.

Brothers in Arms: ABCA1- and ABCG1-Mediated Cholesterol Efflux as Promising Targets in Cardiovascular Disease Treatment

Atherosclerosis is a leading cause of cardiovascular disease worldwide, and hypercholesterolemia is a major risk factor. Preventive treatments mainly focus on the effective reduction of low-density lipoprotein cholesterol, but their therapeutic value is limited by the inability to completely normalize atherosclerotic risk, probably due to the disease complexity and multifactorial pathogenesis. Consequently, high-density lipoprotein cholesterol gained much interest, as it appeared to be cardioprotective due to its major role in reverse cholesterol transport (RCT). RCT facilitates removal of cholesterol from peripheral tissues, including atherosclerotic plaques, and its subsequent hepatic clearance into bile. Therefore, RCT is expected to limit plaque formation and progression. Cellular cholesterol efflux is initiated and propagated by the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1. Their expression and function are expected to be rate-limiting for cholesterol efflux, which makes them interesting targets to stimulate RCT and lower atherosclerotic risk. This systematic review discusses the molecular mechanisms relevant for RCT and ABCA1 and ABCG1 function, followed by a critical overview of potential pharmacological strategies with small molecules to enhance cellular cholesterol efflux and RCT. These strategies include regulation of ABCA1 and ABCG1 expression, degradation, and mRNA stability. Various small molecules have been demonstrated to increase RCT, but the underlying mechanisms are often not completely understood and are rather unspecific, potentially causing adverse effects. Better understanding of these mechanisms could enable the development of safer drugs to increase RCT and provide more insight into its relation with atherosclerotic risk. SIGNIFICANCE STATEMENT: Hypercholesterolemia is an important risk factor of atherosclerosis, which is a leading pathological mechanism underlying cardiovascular disease. Cholesterol is removed from atherosclerotic plaques and subsequently cleared by the liver into bile. This transport is mediated by high-density lipoprotein particles, to which cholesterol is transferred via ATP-binding cassette transporters ABCA1 and ABCG1. Small-molecule pharmacological strategies stimulating these transporters may provide promising options for cardiovascular disease treatment.

Efficacy and Metabolic Effect on Serum Lipids of Apremilast in Psoriatic Arthritis: A Case Report

Psoriatic arthritis (PsA) is a chronic immune-mediated disease manifesting as joint inflammation with functional impairment associated with psoriasis. Recently, PsA has emerged as a systemic disease with several comorbidities, such as cardiovascular diseases and metabolic disorders. Apremilast is a targeted synthetic disease-modifying anti-rheumatic drug (tsDMARD) directed against phosphodiesterase 4 (PDE4) with demonstrated efficacy and safety in PsA and psoriasis. We report the case of a patient with PsA manifesting as arthritis, dactylitis, mild psoriasis and a significantly reduced health-related quality of life (HRQoL). Treatment with apremilast in association with methotrexate led to a quick improvement of joint and skin involvement with a stable amelioration of HRQoL. Furthermore, we observed a persistent favorable shift of serum lipid profile. Our observations suggest that apremilast is effective in controlling mild skin and joint involvement, including dactylitis, and suggest a potentially advantageous metabolic effect in patients with PsA.

Adenylyl cyclase 1 as a major isoform to generate cAMP signaling for apoA-1-mediated cholesterol efflux pathway

HDL apoA-1-mediated cholesterol efflux pathway requires multiple cellular proteins and signal transduction processes, including adenylyl cyclase (AC)/cAMP signaling. Due to the existence of multiple transmembrane AC isoforms, it was not known how many AC isoforms are expressed and which ones are essential for cholesterol efflux in macrophage foam cells. These questions were investigated in THP-1 macrophages in this study. Quantitative RT-PCR detected mRNAs for all nine transmembrane AC isoforms, but only the mRNA and protein of the AC1 isoform were consistently upregulated by cholesterol loading and apoA-1. AC1 shRNA interference decreased AC1 mRNA and protein levels, resulting in reduction of apoA-1-mediated cAMP production and cholesterol efflux, while the intracellular cholesterol levels remained high. Confocal microscopy showed that apoA-1 promoted translocation of cholesterol and formation of cholesterol-apoA-1 complexes (protrusions) on the cholesterol-loaded macrophage surface. AC1 shRNA-interfered macrophages showed no translocation of cholesterol to the cell surface. AC1 shRNA interference also disrupted cellular localization of the intracellular cholesterol indicator protein adipophillin, and the expression as well as surface translocation of ABCA1. Together, our results show that AC1 is a major isoform for apoA-1-activated cAMP signaling to promote cholesterol transport and exocytosis to the surface of THP-1 macrophage foam cells.

Intermedin Ameliorates Atherosclerosis by Increasing Cholesterol Efflux Through the cAMP-PKA Pathway in Macrophage RAW264.7 Cell Line

Background

The aim of this study was to explore the role of intermedin and its mechanism in cholesterol efflux of macrophage THP-1 and RAW264.7 cell lines in atherosclerosis (AS).

Material/Methods

ApoE−/− mice were fed with a high-fat diet, and the concentrations of total cholesterol (TC), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C) were measured. The lipidoses of the aortic sinus were analyzed by hematoxylin and eosin staining, and the cAMP level was detected by enzyme-linked immunosorbent assay (ELISA). The expressions of ATP-binding cassette transporter (ABCA1) were tested by real-time PCR and Western blot analysis.

Results

IMD decreased serum TC and LDL-C, and increased serum HDL-C level in apoE−/− mice and attenuated AS plaque areas. In vitro, IMD increased intracellular cAMP concentration in a dose-dependent manner in THP-1 and RAW264.7 cell lines, which enhanced the expression of ABCA1 and increased cholesterol efflux rate. However, this effect was inhibited by PKAI in the RAW 264.7 cell line but not in the THP-1 cell line.

Conclusions

IMD can ameliorate the development of AS in ApoE−/− mice and regulate cholesterol balance in the RAW264.7 cell line through the cAMP-PKA pathway.

Methyl protodioscin increases ABCA1 expression and cholesterol efflux while inhibiting gene expressions for synthesis of cholesterol and triglycerides by suppressing SREBP transcription and microRNA 33a/b levels

Sterol regulatory element-binding proteins (SREBPs) regulate homeostasis of LDL, HDL and triglycerides. This study was aimed to determine if inhibition of SREBPs by methyl protodioscin (MPD) regulates downstream gene and protein expressions of lipid metabolisms. In THP-1 macrophages, MPD increases levels of ABCA1 mRNA and protein in dose- and time-dependent manners, and apoA-1-mediated cholesterol efflux. The underlying mechanisms for the effects is that MPD inhibits the transcription of SREBP1c and SREBP2, and decreases levels of microRNA 33a/b hosted in the introns of SREBPs, which leads to reciprocally increase ABCA1 levels. In HepG2 cells, MPD shows the same effects as these observed in THP-1 macrophages. MPD also decreases the gene expressions of HMGCR, FAS and ACC for cholesterol and fatty acid synthesis. MPD further promotes LDL receptor through reducing the PCSK9 level. Collectively, the study demonstrates that MPD potentially increase HDL cholesterol while reducing LDL cholesterol and triglycerides.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

HDL regulation is exerted at multiple levels including regulation at the level of transcription initiation by transcription factors and signal transduction cascades; regulation at the posttranscriptional level by microRNAs and other noncoding RNAs which bind to the coding or noncoding regions of HDL genes regulating mRNA stability and translation; as well as regulation at the posttranslational level by protein modifications, intracellular trafficking, and degradation. The above mechanisms have drastic effects on several HDL-mediated processes including HDL biogenesis, remodeling, cholesterol efflux and uptake, as well as atheroprotective functions on the cells of the arterial wall. The emphasis is on mechanisms that operate in physiologically relevant tissues such as the liver (which accounts for 80% of the total HDL-C levels in the plasma), the macrophages, the adrenals, and the endothelium. Transcription factors that have a significant impact on HDL regulation such as hormone nuclear receptors and hepatocyte nuclear factors are extensively discussed both in terms of gene promoter recognition and regulation but also in terms of their impact on plasma HDL levels as was revealed by knockout studies. Understanding the different modes of regulation of this complex lipoprotein may provide useful insights for the development of novel HDL-raising therapies that could be used to fight against atherosclerosis which is the underlying cause of coronary heart disease.

Apremilast: A Phosphodiesterase 4 Inhibitor for the Treatment of Psoriatic Arthritis

Introduction

Psoriatic arthritis (PsA) is a spondyloarthritis that occurs in up to 30% of psoriasis patients. Patients with PsA are at risk for decreased quality of life due to both joint and skin symptoms, impaired physical function and disease progression. Treatments include non-steroidal anti-inflammatory drugs, conventional systemic disease-modifying anti-rheumatic drugs (DMARDs) such as methotrexate, and biologic agents, including tumor necrosis factor-α inhibitors. The most recently introduced treatment option is apremilast, an oral phosphodiesterase 4 inhibitor.

Methods

This review provides an in-depth discussion of apremilast’s mechanism of action, and evidence of its clinical efficacy and safety from the Psoriatic Arthritis Long-term Assessment of Clinical Efficacy (PALACE) phase III pivotal clinical trials (PALACE 1, 2, and 3).

Results

These trials demonstrate that apremilast is effective for the treatment of active PsA, despite prior conventional DMARDs or biologic treatment. The primary efficacy end point, a 20% improvement from baseline in modified American College of Rheumatology response criteria at Week 16, was achieved by significantly greater proportions of patients treated with apremilast 20 mg twice daily (BID) and apremilast 30 mg BID versus placebo in PALACE 1, 2, and 3. Improvements in this and other clinical and patient-reported end points, including swollen and tender joint counts, Psoriasis Area and Severity Index score, physical function, and quality of life, were maintained, extending over 52 weeks of treatment among patients initially randomized to apremilast. Apremilast’s safety profile has been acceptable, with diarrhea and nausea being the most common adverse events, with no evidence for an increased risk of infection or need for laboratory monitoring. The PALACE pivotal data indicate that apremilast presents a new option for the treatment of PsA that may be appropriate for use early in the treatment ladder. Ongoing PALACE open-label extension trials of up to 4 years will characterize the long-term clinical effects and safety of apremilast therapy.

Funding

Celgene Corporation, Summit, NJ, USA.

Electronic supplementary material

The online version of this article (doi:10.1007/s40744-014-0005-4) contains supplementary material, which is available to authorized users.

A lincRNA-DYNLRB2-2/GPR119/GLP-1R/ABCA1-dependent signal transduction pathway is essential for the regulation of cholesterol homeostasis

Accumulated evidence shows that G protein-coupled receptor 119 (GPR119) plays a key role in glucose and lipid metabolism. Here, we explored the effect of GPR119 on cholesterol metabolism and inflammation in THP-1 macrophages and atherosclerotic plaque progression in apoE(-/-) mice. We found that oxidized LDL (Ox-LDL) significantly induced long intervening noncoding RNA (lincRNA)-DYNLRB2-2 expression, resulting in the upregulation of GPR119 and ABCA1 expression through the glucagon-like peptide 1 receptor signaling pathway. GPR119 significantly decreased cellular cholesterol content and increased apoA-I-mediated cholesterol efflux in THP-1 macrophage-derived foam cells. In vivo, apoE(-/-) mice were randomly divided into two groups and infected with lentivirus (LV)-Mock or LV-GPR119 for 8 weeks. GPR119-treated mice showed decreased liver lipid content and plasma TG, interleukin (IL)-1β, IL-6, and TNF-α levels, whereas plasma levels of apoA-I were significantly increased. Consistent with this, atherosclerotic lesion development was significantly inhibited by infection of apoE(-/-) mice with LV-GPR119. Our findings clearly indicate that, Ox-LDL significantly induced lincRNA-DYNLRB2-2 expression, which promoted ABCA1-mediated cholesterol efflux and inhibited inflammation through GPR119 in THP-1 macrophage-derived foam cells. Moreover, GPR119 decreased lipid and serum inflammatory cytokine levels, decreasing atherosclerosis in apoE(-/-) mice. These suggest that GPR119 may be a promising candidate as a therapeutic agent.

Toll-like receptor 4 is up-regulated by mTOR activation during THP-1 macrophage foam cells formation

Macrophage foam cells formation is the most important process in atherosclerotic plaque formation and development. Toll-like receptor 4 (TLR4) is one of the important innate immune sensors of endogenous damage signals and crucial for regulating inflammation. Growing evidence indicates that TLR4 plays a very important role in macrophage foam cells formation. However, the underlying mechanisms regulating TLR4 expression in macrophage are not fully understood. In this study, we induced THP-1 macrophage foam cells formation with oxidative modified low-density lipoprotein (ox-LDL). We observed that TLR4 mRNA and protein expression were markedly up-regulated, and the phosphorylation of mammalian target of rapamycin (mTOR) and its downstream target p70S6K were promoted during foam cells formation. The mTOR inhibitor rapamycin blocked mTOR phosphorylation and inhibited TLR4 expression induced by ox-LDL. Silencing mTOR, rictor or raptor protein expression by small interfering RNA, also inhibited the up-regulation of TLR4 expression, respectively. Inhibition of mTOR with rapamycin reversed the down-regulation of cellular lipid efflux mediator ABCA1, which resulted from the activation of TLR4 by ligands. These data suggested that TRL4 expression was up-regulated by a mechanism dependent on mTOR signal pathway activation during THP-1 macrophage foam cells formation. Inhibition of ox-LDL induced mTOR activation reduced TLR4 expression, and improved the impaired lipid efflux.

Cilostazol enhances macrophage reverse cholesterol transport in vitro and in vivo

OBJECTIVE

Recent failure of an HDL-cholesterol raising strategy using a cholesteryl ester transfer protein inhibitor highlights the importance of the anti-atherogenic function rather than plasma concentration of HDL. Cilostazol, a selective inhibitor of phosphodiesterase 3, has been widely used in patients with atherosclerotic diseases and is known to increase HDL-cholesterol. However, it remains unclear whether cilostazol enhances anti-atherogenic properties by promoting reverse cholesterol transport (RCT), a major anti-atherogenic function of HDL.

METHODS AND RESULTS

We observed that treatment of THP-1 macrophages, human monocyte-derived macrophages, and RAW264.7 cells with cilostazol increased ABCA1 and ABCG1 expression in a concentration-dependent manner, translating into enhanced apoA-I- and HDL-mediated cholesterol efflux from the macrophages. However, other cyclic AMP (cAMP)-elevating agents did not increase ABCA1 gene expression in THP-1 macrophages. Cilostazol did not change intracellular cAMP levels in THP-1 macrophages and RAW264.7 cells, and a protein kinase A (PKA) inhibitor did not affect cilostazol-induced ABCA1 and ABCG1 expression. To further investigate RCT in vivo, (3)H-cholesterol-labeled and acetyl LDL-loaded RAW264.7 cells were intraperitoneally injected into mice and the appearance of the (3)H-tracer was monitored in plasma, liver, and feces. Supporting the in vitro data, cilostazol was found to significantly increase (3)H-tracer levels in both plasma and feces.

CONCLUSIONS

These findings indicate that cilostazol might provide anti-atherosclerotic effects by promoting RCT through increased ABCA1/G1 expression in macrophages.

Regulations of the key mediators in inflammation and atherosclerosis by aspirin in human macrophages

Although its role to prevent secondary cardiovascular complications has been well established, how acetyl salicylic acid (ASA, aspirin) regulates certain key molecules in the atherogenesis is still not known. Considering the role of matrix metalloproteinase-9 (MMP-9) to destabilize the atherosclerotic plaques, the roles of the scavenger receptor class BI (SR-BI) and ATP-binding cassette transporter A1 (ABCA1) to promote cholesterol efflux in the foam cells at the plaques, and the role of NF-κB in the overall inflammation related to the atherosclerosis, we addressed whether these molecules are all related to a common mechanism that may be regulated by acetyl salicylic acid. We investigated the effect of ASA to regulate the expressions and activities of these molecules in THP-1 macrophages. Our results showed that ASA inhibited MMP-9 mRNA expression, and caused the decrease in the MMP-9 activities from the cell culture supernatants. In addition, it inhibited the nuclear translocation of NF-κB p65 subunit, thus the activity of this inflammatory molecule. On the contrary, acetyl salicylic acid induced the expressions of ABCA1 and SR-BI, two molecules known to reduce the progression of atherosclerosis, at both mRNA and protein levels. It also stimulated the cholesterol efflux out of macrophages. These data suggest that acetyl salicylic acid may alleviate symptoms of atherosclerosis by two potential mechanisms: maintaining the plaque stability via inhibiting activities of inflammatory molecules MMP-9 and NF-κB, and increasing the cholesterol efflux through inducing expressions of ABCA1 and SR-BI.

Eicosapentaenoic acid reduces ABCA1 serine phosphorylation and impairs ABCA1-dependent cholesterol efflux through cyclic AMP/protein kinase A signaling pathway in THP-1 macrophage-derived foam cells

ABCA1 is a key mediator of cholesterol efflux to apoA-I in cholesterol loaded macrophages, a first step of RCT in vivo. Unsaturated fatty acids can inhibit cholesterol efflux from macrophages by increasing degradation of ABCA1. However, the detailed mechanisms of ABCA1 regulation by unsaturated fatty acids are not fully understood. In the present study, we investigated the effects of EPA on ABCA1 expression and ABCA1-dependent cholesterol efflux and examined the role of cAMP/PKA pathway on the regulation of ABCA1 by EPA in THP-1 macrophage-derived foam cells. Results showed that EPA significantly destabilized ABCA1 protein and reduced ABCA1-dependent cholesterol efflux but had no effect on ABCA1 mRNA expression. We also revealed that EPA markedly reduced cAMP level and PKA activity and ABCA1 serine phosphorylation. PKA-specific activation by PKA agonist markedly compensated the down-regulation of ABCA1 serine phosphorylation and ABCA1-mediated cholesterol efflux by EPA, while, siRNA of PKA leaded to reduce of ABCA1 serine phosphorylation and ABCA1-mediated cholesterol efflux more significantly than EPA. However, EPA-Induced enhancement of degradation rate of ABCA1 protein did not change by treatment with PKA agonist or PKA-siRNA. These results provide evidence that EPA may have dual negative effects on ABCA1 activity by decreasing ABCA1 protein level and by reducing PKA-mediated ABCA1 serine phosphorylation in THP-1 macrophage-derived foam cells.

cAMP-induced expression of ABCA1 is associated with MAP-kinase-pathway activation

Several lines of evidence suggest that the ATP binding cassette A1 (ABCA1) is also involved in other degenerative processes such as brain neurodegeneration. Cholesterol and cAMP activate ABCA1 in a cell-specific manner. We employed a cell culture model of murine monocytes (P388) and neuroblastoma cells (N2A) and studied the differential induction of the ABCA1-gene product by modifying the cholesterol acceptor and by inhibition of the MAP-kinase pathway. Our study reveals a rise of ABCA1-expression in both N2A and P388 by cAMP. This increase is accompanied by a higher activation of the MAP-kinase-pathway. The inhibition of the MAP-kinase activation disrupts the stimulating effect of cAMP but increases the base line expression of ABCA1. Our data suggest a negative feedback between the MAP-kinase-system and ABCA1. We conclude that the interaction of the MAP-kinase pathway and the ABCA1 system might affect the function of neuronal and microglial cells in the brain.

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.

Lipid efflux by the ATP-binding cassette transporters ABCA1 and ABCG1

Plasma levels of high-density lipoproteins (HDL) and apolipoprotein A-I (apoA-I) are inversely correlated with the risk of cardiovascular disease. One major atheroprotective mechanism of HDL and apoA-I is their role in reverse cholesterol transport, i.e., the transport of excess cholesterol from foam cells to the liver for secretion. The ATP-binding cassette transporters ABCA1 and ABCG1 play a pivotal role in this process by effluxing lipids from foam cells to apoA-I and HDL, respectively. In the liver, ABCA1 activity is one rate-limiting step in the formation of HDL. In macrophages, ABCA1 and ABCG1 prevent the excessive accumulation of lipids and thereby protect the arteries from developing atherosclerotic lesions. However, the mechanisms by which ABCA1 and ABCG1 mediate lipid removal are still unclear. Particularly, three questions remain controversial and are discussed in this review: (1) Do apoA-I and HDL directly interact with ABCA1 and ABCG1, respectively? (2) Does cholesterol efflux involve retroendocytosis of apoA-I or HDL? (3) Which lipids are directly transported by ABCA1 and ABCG1?

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.

Divergent action of calcium channel blockers on ATP-binding cassette protein expression

Calcium channel blockers (CCBs) are widely used in clinical practice, and have been reported to be effective in preventing the progression of atherosclerosis. We examined whether various types of calcium channel blockers affected the expression of ATP binding cassette transporter A1 (ABCA1), a factor contributing to anti-atherogenesis. Undifferentiated monocytic cell line, THP-1 cells were maintained in RPMI 1640 medium and treated with different kinds of calcium channel blockers. Among the calcium channel blockers tested, aranidipine and efonidipine increased ABCA1 protein expression without an increase in ABCA1 mRNA expression, whereas other calcium channel blockers (eg, nifedipine, amlodipine, and nicardipine) or T-type calcium channel blockers (eg, mibefradil and nickel chloride) failed to upregulate ABCA1 expression. H89, a protein kinase A inhibitor inhibited the aranidipine-induced ABCA1 protein expression, whereas genistein (a tyrosine kinase inhibitor), or AG490 (a JAK-2 inhibitor) had no effects. Neither of these inhibitors suppressed the efonidipine-induced ABCA1 protein expression. Intracellular cAMP levels were elevated only by aranidipine, but not by efonidipine. In conclusion, aranidipine and efonidipine have the ability to induce ABCA1 protein by distinct mechanisms; protein kinase A is involved in the aranidipine-induced ABCA1 upregulation. This non-class effect of calcium channel blockers may potentially offer beneficial action in the treatment of hypertensive subjects with atherosclerosis.

Identification of the cAMP-responsive enhancer of the murine ABCA1 gene: requirement for CREB1 and STAT3/4 elements

OBJECTIVE

To determine the mechanism by which expression of the murine ABCA1 gene is highly induced by cAMP analogues.

METHODS AND RESULTS

ABCA1 mRNA turnover cannot account for its induction by cAMP. Thus cAMP induction of ABCA1 mRNA occurs at a transcriptional level. Shotgun cloning DNA fragments from the murine ABCA1 locus identified a strong cAMP responsive enhancer located in the first intron, which led to 25- to 100-fold cAMP-mediated induction of reporter gene activity. Deletions and mutations of this enhancer led to the identification a cAMP-responsive element (CRE) that was essential for the cAMP induction. Furthermore, the capacity of this CRE site to mediate the cAMP induction required the presence of a STAT3/4 element located 81 bp away. A dominant-negative CREB expression vector inhibited the cAMP induction of ABCA1, demonstrating that CREB was required for cAMP induction of ABCA1 expression in RAW264.7 cells.

CONCLUSIONS

Phospho-CREB1 controls the cAMP-mediated induction of murine ABCA1 gene expression through a CRE site acting in cooperation with a nearby STAT element. This CRE site is not conserved in the human ABCA1 gene, explaining why human ABCA1 is not strongly stimulated by cAMP analogs.

ATP-binding cassette transporter A1: a cell cholesterol exporter that protects against cardiovascular disease

Blood high-density lipoprotein (HDL) levels are inversely related to risk for cardiovascular disease, implying that factors associated with HDL metabolism are atheroprotective. One of these factors is ATP-binding cassette transporter A1 (ABCA1), a cell membrane protein that mediates the transport of cholesterol, phospholipids, and other metabolites from cells to lipid-depleted HDL apolipoproteins. ABCA1 transcription is highly induced by sterols, a major substrate for cellular export, and its expression and activity are regulated posttranscriptionally by diverse processes. Liver ABCA1 initiates formation of HDL particles, and macrophage ABCA1 protects arteries from developing atherosclerotic lesions. ABCA1 mutations can cause a severe HDL deficiency syndrome characterized by cholesterol deposition in tissue macrophages and prevalent atherosclerosis. Genetic manipulations of ABCA1 expression in mice also affect plasma HDL levels and atherogenesis. Metabolites elevated in individuals with the metabolic syndrome and diabetes destabilize ABCA1 protein and decrease cholesterol export from macrophages. Moreover, oxidative modifications of HDL found in patients with cardiovascular disease reduce the ability of apolipoproteins to remove cellular cholesterol by the ABCA1 pathway. These observations raise the possibility that an impaired ABCA1 pathway contributes to the enhanced atherogenesis associated with common inflammatory and metabolic disorders. The ABCA1 pathway has therefore become an important new therapeutic target for treating cardiovascular disease.

Lipid efflux in human and mouse macrophagic cells: evidence for differential regulation of phospholipid and cholesterol efflux

ABCA1 is a critical regulator of lipid efflux from cells, which is highly regulated at the transcriptional and posttranslational levels. However, cells from different species and different tissues, and primary versus immortalized cells, show different modes of regulation. We have carried out a comparative analysis of basic signaling pathways of lipid efflux in mouse J774 cells, mouse peritoneal macrophages (MPMs), human THP-1 cells, and human monocyte-derived macrophages. Cyclic AMP (cAMP) was a potent stimulator of lipid efflux in mouse macrophages, but not in human macrophages. Moreover, this cAMP-inducible component of efflux from MPMs was inhibitable by H89 [a protein kinase A (PKA) inhibitor], but H89 did not affect basal efflux. On the other hand, cAMP failed to show any stimulatory effect in human macrophages, but basal efflux was inhibitable by H89. In MPMs and THP-1 cells, protein kinase C (PKC) inhibitors blocked cholesterol efflux but had no effect on phospholipid efflux, demonstrating the separation of the regulation of phospholipid efflux and cholesterol efflux in macrophages. We conclude that: 1) cAMP regulates lipid efflux predominantly in a PKA-dependent fashion; 2) cholesterol efflux is modulated by a PKC-dependent mechanism; and 3) mouse and human macrophages exhibit different modes of regulation of lipid efflux.

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.

Macrophage cholesterol efflux and the active domains of serum amyloid A 2.1

Serum amyloid A 2.1 (SAA2.1) suppresses ACAT and stimulates cholesteryl ester hydrolase (CEH) activities in cholesterol-laden macrophages, and in the presence of a cholesterol transporter and an extracellular acceptor, there is a marked increase in the rate of cholesterol export in culture and in vivo. The stimulation of CEH activity by SAA2.1 is not affected by chloroquine, suggesting that it operates on neutral CEH rather than the lysosomal form. With liposomes containing individual peptides of SAA2.1, residues 1-20 inhibit ACAT activity, residues 74-103 stimulate CEH activity, and each of residues 1-20 and 74-103 promotes macrophage cholesterol efflux to HDL in culture media. In combination, these peptides exhibit a profound effect, so that 55-70% of cholesterol is exported to media HDL in 24 h. The effect is also demonstrable in vivo. [3H]cholesterol-laden macrophages injected intravenously into mice were allowed to establish themselves for 24 h. Thereafter, the mice received a single intravenous injection of liposomes containing intact SAA1.1, SAA2.1, peptides composed of SAA2.1 residues 1-20, 21-50, 51-80, 74-103, or SAA1.1 residues 1-20. Only liposomes containing intact SAA2.1 or its residues 1-20 or 74-103 promoted the efflux of cholesterol in vivo. A single injection of each of the active peptides is effective in promoting cholesterol efflux in vivo for at least 4 days.

Structure-activity relationship: analyses of p-glycoprotein substrates and inhibitors

OBJECTIVE

A large number of structurally and functionally diverse compounds act as substrates or modulators of p-glycoprotein (p-gp). Some of them possess multiple drug resistance (MDR)-reversing activity, but only a small number of them have entered clinical study. In order to uncover the factors which exert a significant impact on the interaction between substrates/modulators and p-gp, we have performed structure-activity relationship (SAR) analyses, including molecular modelling, two-dimensional (2D) and three-dimensional (3D) parameter-frame-setting analysis, quantitative structure activity relationship (QSAR) analysis among substrates/modulators, as well as clinically promising MDR-reversing agents.

METHODS

The physicochemical parameters C log P, CMR and all regression equations were derived by using C log P version 4.0 and the latest CQSAR software, respectively. Molecular modelling and all other parameter calculations were performed by using HyperChem version 5.0 program, after geometry optimization and energy minimization using the AM1 semiempirical method.

RESULTS

SAR analyses indicate that MDR reversal activity is correlated with the lipophilicity (C log P), molecular weight (log Mw), longest chain (Nlc) of the molecule and the energy of the highest occupied orbital (Ehomo). In addition, the presence of a basic tertiary nitrogen atom in the structure is also an important contributor to p-gp inhibitory activity. Some separation in space is achieved for different subsets of p-gp substrates and inhibitors using Nlc, C log P and Ehomo as three independent parameters in the 3D-parameter-frame setting.

CONCLUSION

A highly effective p-gp modulator candidate should possess a log P value of 2.92 or higher, 18-atom-long or longer molecular axis, and a high Ehomo value, as well as at least one tertiary basic nitrogen atom. The results obtained may be useful in explaining drug-p-gp interactions for different compounds, including drug interactions and the development of new MDR chemosensitizers.

HDL apolipoproteins and ABCA1: partners in the removal of excess cellular cholesterol

It is widely believed that HDL protects against atherosclerosis by removing excess cholesterol from arterial cells. Lipid-poor HDL apolipoproteins promote efflux of cholesterol, phospholipids, and other lipophilic molecules from cells by an active process mediated by a cell-membrane transporter called the ATP binding cassette transporter A-1 (ABCA1). ABCA1 either directly or indirectly translocates phospholipids and cholesterol to the cell surface, where they appear to form lipid domains that interact with amphipathic alpha-helixes in apolipoproteins. This interaction solubilizes these lipids and generates nascent HDL particles that dissociate from the cell. Binding of apolipoproteins to ABCA1 may also enhance the activity of this lipid-transport pathway. Thus, the apolipoprotein/ABCA1 pathway efficiently clears cells of excess cholesterol that would otherwise accumulate as intracellular lipid droplets. ABCA1 expression is highly induced by cholesterol loading of cells and is also modulated by sterol-independent mechanisms at both the transcriptional and posttranslational level. Studies of human disease and animal models have shown that both an increased availability of apolipoproteins and an enhanced macrophage ABCA1 activity are atheroprotective. These findings implicate the apolipoprotein/ABCA1 pathway as an important therapeutic target for treating cardiovascular disease.

ATP-binding cassette transporter A1 and cholesterol trafficking

PURPOSE OF REVIEW

Two hallmarks of cardiovascular disease are the presence of sterol-laden macrophages in the artery wall and reduced plasma HDL levels. A cell membrane protein named ATP-binding cassette transporter A1 (ABCA1) mediates secretion of excess cholesterol from cells into the HDL metabolic pathway. The discovery of ABCA1 in 1999 triggered a deluge of studies conducted to characterize the properties of this important transporter. The present review summarizes the more recent of those studies and evaluates their implications for the role of ABCA1 in cholesterol transport, HDL metabolism, and atherogenesis.

RECENT FINDINGS

Cell culture experiments have shown that ABCA1 transports cholesterol, phospholipids, and other lipophilic molecules across the plasma membrane, where they are picked up by apolipoproteins containing little or no lipids, but the mechanisms involved are still unclear. It is now apparent that factors in addition to sterols modulate ABCA1 expression by diverse transcriptional and post-transcriptional processes. Studies in humans and mice with ABCA1 mutations revealed that the relative activity of ABCA1 determines plasma HDL levels and influences susceptibility to cardiovascular disease. Mouse models are beginning to provide insights into the function of ABCA1 in vivo but are also raising new questions regarding the contribution of ABCA1 to total cholesterol flux.

SUMMARY

Recent studies underscore the critical role of ABCA1 in clearing excess cholesterol from macrophages and generating HDL particles, implicating ABCA1 as an attractive new therapeutic target for treating cardiovascular disease.

Insights of high-density lipoprotein apolipoprotein-mediated lipid efflux from cells

High-density lipoprotein (HDL) protects against cardiovascular diseases by removal of excess lipids from cells. HDL apolipoprotein-mediated lipid efflux involves multiple cellular proteins to remove both cholesterol and phospholipids that are otherwise stored in the cells. This article reviews recent progress in the understanding of receptors, signal mediators, Golgi and vesicle transport related to the pathway and proposes a model of HDL apolipoprotein receptor-mediated exocytosis of cellular cholesterol. Such an exocytotic pathway could provide the most effective mechanism to remove excess cellular lipids and prevent atherogenesis.

ABCA1: regulation, trafficking and association with heteromeric proteins

HDL metabolism is crucial in maintaining cellular cholesterol and phospholipid homeostasis and prevention of atherosclerosis progression. Recent work identified the ATP-binding cassette transporter A1 (ABCA1) as the major regulator of plasma high density lipoprotein (HDL) cholesterol responsible for the removal of excess cholesterol from peripheral cells and tissues. Here we discuss some novel aspects of the ABCA1 network: 1) the cellular pathways involved in cholesterol and phospholipid efflux, 2) regulation of ABCA1, 3) sulfonylurea receptor 1 (SUR1)- or cystic fibrosis transmembrane conductance regulator (CFTR)-like function of ABCA1, 4) interaction of the ABCA1 C-terminus with beta2-syntrophin, 5) ABCA1 modulation of the Rho GTPase Cdc42, 6) localization of ABCA1 in plasma membrane microdomains and intracellular sites, 7) differential effects of prebeta-HDL precursors on ABCA1 mediated alpha-HDL particle formation and 8) ABCA1 in platelets and its relation to phosphatidylserine-flippase activity. A complex regulatory network and additional antiatherogenic features that may depend on the composition of prebeta-HDL precursor particles are believed to coordinate ABCA1 function in reverse cholesterol and phospholipid transport. Distinct prebeta-HDL ligand-specific receptor-clusters are involved that may modulate specific signaling pathways with varying outcomes related to prebeta-HDL particle composition, the cell-type and the cellular response status.