The lipidation status of acute-phase protein serum amyloid A determines cholesterol mobilization via scavenger receptor class B, type I

Discovering the deep evolutionary roots of serum amyloid A protein family

Deep evolutionary origin of the conserved animal serum amyloid A (SAA) apolipoprotein family leading to yet unknown highly similar SAA-like sequences occurring in certain bacterial genomes is demonstrated in this contribution. Horizontal gene transfer event of corresponding genes between gut bacteria and non-vertebrate animals was discovered in the reconstructed phylogenetic tree obtained with maximum likelihood and neighbor-joining methods, respectively. This detailed phylogeny based on totally 128 complete sequences comprised diverse serum amyloid A isoforms from various animal vertebrate and non-vertebrate phyla and also corresponding genes coding for highly similar proteins from animal gut bacteria. Typical largely conserved sequence motifs and a peculiar structural fold consisting mainly of four α-helices in a bundle within all reconstructed clades of the SAA protein family are discussed with respect to their supposed biological functions in various organisms that contain corresponding genes.

Alterations of HDL's to piHDL's Proteome in Patients with Chronic Inflammatory Diseases, and HDL-Targeted Therapies

Chronic inflammatory diseases, such as rheumatoid arthritis, steatohepatitis, periodontitis, chronic kidney disease, and others are associated with an increased risk of atherosclerotic cardiovascular disease, which persists even after accounting for traditional cardiac risk factors. The common factor linking these diseases to accelerated atherosclerosis is chronic systemic low-grade inflammation triggering changes in lipoprotein structure and metabolism. HDL, an independent marker of cardiovascular risk, is a lipoprotein particle with numerous important anti-atherogenic properties. Besides the essential role in reverse cholesterol transport, HDL possesses antioxidative, anti-inflammatory, antiapoptotic, and antithrombotic properties. Inflammation and inflammation-associated pathologies can cause modifications in HDL's proteome and lipidome, transforming HDL from atheroprotective into a pro-atherosclerotic lipoprotein. Therefore, a simple increase in HDL concentration in patients with inflammatory diseases has not led to the desired anti-atherogenic outcome. In this review, the functions of individual protein components of HDL, rendering them either anti-inflammatory or pro-inflammatory are described in detail. Alterations of HDL proteome (such as replacing atheroprotective proteins by pro-inflammatory proteins, or posttranslational modifications) in patients with chronic inflammatory diseases and their impact on cardiovascular health are discussed. Finally, molecular, and clinical aspects of HDL-targeted therapies, including those used in therapeutical practice, drugs in clinical trials, and experimental drugs are comprehensively summarised.

Lycopene-rich diets modulate HDL functionality and associated inflammatory markers without affecting lipoprotein size and distribution in moderately overweight, disease-free, middle-aged adults: A randomized controlled trial

Background

The consumption of lycopene-rich foods may lower cardiovascular disease (CVD) risk. Lycopene circulates in the blood bound to lipoproteins, including high-density lipoproteins (HDLs). Preliminary data from our group showed that increased consumption of tomato-based food or lycopene supplement in middle-aged subjects led to functional changes to HDL's sub-fractions, HDL₂ and HDL₃. These changes were also associated with a decrease in serum amyloid A (SAA), potentially enhancing their anti-atherogenic properties.

Objective

We carried out a comprehensive randomized controlled intervention trial with healthy middle-aged volunteers to assess whether the consumption of tomato-based foods or lycopene supplements affects HDL functionality and associated inflammatory markers, and lipoprotein subfractions size and distribution.

Design

Volunteers (225, aged 40–65 years) were randomly assigned to one of three dietary intervention groups and asked to consume a control diet (low in tomato-based foods, <10 mg lycopene/week), a lycopene-rich diet (224–350 mg lycopene/week), or the control diet with a lycopene supplement (70 mg lycopene/week). HDL₂ and HDL₃ were isolated by ultracentrifugation. Compliance was monitored by assessing lycopene concentration in serum. Systemic and HDL-associated inflammation was assessed by measuring SAA concentrations. HDL functionality was determined by monitoring paraoxonase-1 (PON-1), cholesteryl ester transfer protein (CETP), and lecithin cholesterol acyltransferase (LCAT) activities. The lipoprotein subfractions profile was assessed by NMR.

Results

Lycopene in serum and HDL significantly increased following consumption of both the high tomato diet and lycopene supplement (p ≤ 0.001 for both). Lycopene, either as a tomato-rich food or a supplement, enhanced both serum- and HDL₃-PON-1 activities (p ≤ 0.001 and p = 0.036, respectively), while significantly reducing HDL₃-SAA-related inflammation (p = 0.001). Lycopene supplement also significantly increased HDL₃-LCAT activity (p = 0.05), and reduced the activity of both HDL₂- and HDL₃-CETP (p = 0.005 and p = 0.002, respectively). These changes were not associated with changes in the subclasses distribution for all lipoprotein fractions or the size of lipoprotein subclasses.

Conclusion

Our results showed that dietary lycopene can significantly enhance HDL functionality, without associated changes in particle size and distribution, by modulating the activity of HDL-associated enzymes. Concomitantly, dietary lycopene significantly decreased serum- and HDL₃-associated SAA, confirming that SAA may represent a sensitive inflammatory biomarker to dietary change.

Clinical Trial Register

(https://www.isrctn.com), ISRCTN34203810.

Utilizing the LoxP-Stop-LoxP System to Control Transgenic ABC-Transporter Expression In Vitro

ABCA1 and ABCG1 are two ABC-transporters well-recognized to promote the efflux of cholesterol to apoAI and HDL, respectively. As these two ABC-transporters are critical to cholesterol metabolism, several studies have assessed the impact of ABCA1 and ABCG1 expression on cellular cholesterol homeostasis through ABC-transporter ablation or overexpressing ABCA1/ABCG1. However, for the latter, there are currently no well-established in vitro models to effectively induce long-term ABC-transporter expression in a variety of cultured cells. Therefore, we performed proof-of-principle in vitro studies to determine whether a LoxP-Stop-LoxP (LSL) system would provide Cre-inducible ABC-transporter expression. In our studies, we transfected HEK293 cells and the HEK293-derived cell line 293-Cre cells with ABCA1-LSL and ABCG1-LSL-based plasmids. Our results showed that while the ABCA1/ABCG1 protein expression was absent in the transfected HEK293 cells, the ABCA1 and ABCG1 protein expression was detected in the 293-Cre cells transfected with ABCA1-LSL and ABCG1-LSL, respectively. When we measured cholesterol efflux in transfected 293-Cre cells, we observed an enhanced apoAI-mediated cholesterol efflux in 293-Cre cells overexpressing ABCA1, and an HDL2-mediated cholesterol efflux in 293-Cre cells constitutively expressing ABCG1. We also observed an appreciable increase in HDL3-mediated cholesterol efflux in ABCA1-overexpressing 293-Cre cells, which suggests that ABCA1 is capable of effluxing cholesterol to small HDL particles. Our proof-of-concept experiments demonstrate that the LSL-system can be used to effectively regulate ABC-transporter expression in vitro, which, in turn, allows ABCA1/ABCG1-overexpression to be extensively studied at the cellular level.

Combined LXR and RXR Agonist Therapy Increases ABCA1 Protein Expression and Enhances ApoAI-Mediated Cholesterol Efflux in Cultured Endothelial Cells

Endothelial ABCA1 expression protects against atherosclerosis and this atheroprotective effect is partially attributed to enhancing apoAI-mediated cholesterol efflux. ABCA1 is a target gene for LXR and RXR; therefore, treating endothelial cells with LXR and/or RXR agonists may increase ABCA1 expression. We tested whether treating cultured immortalized mouse aortic endothelial cells (iMAEC) with the endogenous LXR agonist 22(R)-hydroxycholesterol, synthetic LXR agonist GW3965, endogenous RXR agonist 9-cis-retinoic acid, or synthetic RXR agonist SR11237 increases ABCA1 protein expression. We observed a significant increase in ABCA1 protein expression in iMAEC treated with either GW3965 or SR11237 alone, but no significant increase in ABCA1 protein was observed in iMAEC treated with either 22(R)-hydroxycholesterol or 9-cis-retionic acid alone. However, we observed significant increases in both ABCA1 protein expression and apoAI-mediated cholesterol efflux when iMAEC were treated with a combination of either 22(R)-hydroxycholesterol and 9-cis-retinoic acid or GW3965 and SR11237. Furthermore, treating iMAEC with either 22(R)-hydroxycholesterol and 9-cis-retinoic acid or GW3965 and SR11237 did not trigger an inflammatory response, based on VCAM-1, ICAM-1, CCL2, and IL-6 mRNA expression. Based on our findings, delivering LXR and RXR agonists precisely to endothelial cells may be a promising atheroprotective approach.

High-Density Lipoproteins and Serum Amyloid A (SAA)

PURPOSE OF REVIEW

Serum amyloid A (SAA) is a highly sensitive acute phase reactant that has been linked to a number of chronic inflammatory diseases. During a systemic inflammatory response, liver-derived SAA is primarily found on high-density lipoprotein (HDL). The purpose of this review is to discuss recent literature addressing the pathophysiological functions of SAA and the significance of its association with HDL.

RECENT FINDINGS

Studies in gene-targeted mice establish that SAA contributes to atherosclerosis and some metastatic cancers. Accumulating evidence indicates that the lipidation state of SAA profoundly affects its bioactivities, with lipid-poor, but not HDL-associated, SAA capable of inducing inflammatory responses in vitro and in vivo. Factors that modulate the equilibrium between lipid-free and HDL-associated SAA have been identified. HDL may serve to limit SAA's bioactivities in vivo. Understanding the factors leading to the release of systemic SAA from HDL may provide insights into chronic disease mechanisms.

Serum amyloid A is not incorporated into HDL during HDL biogenesis

Liver-derived serum amyloid A (SAA) is present in plasma where it is mainly associated with HDL and from which it is cleared more rapidly than are the other major HDL-associated apolipoproteins. Although evidence suggests that lipid-free and HDL-associated forms of SAA have different activities, the pathways by which SAA associates and disassociates with HDL are poorly understood. In this study, we investigated SAA lipidation by hepatocytes and how this lipidation relates to the formation of nascent HDL particles. We also examined hepatocyte-mediated clearance of lipid-free and HDL-associated SAA. We prepared hepatocytes from mice injected with lipopolysaccharide or an SAA-expressing adenoviral vector. Alternatively, we incubated primary hepatocytes from SAA-deficient mice with purified SAA. We analyzed conditioned media to determine the lipidation status of endogenously produced and exogenously added SAA. Examining the migration of lipidated species, we found that SAA is lipidated and forms nascent particles that are distinct from apoA-I-containing particles and that apoA-I lipidation is unaltered when SAA is overexpressed or added to the cells, indicating that SAA is not incorporated into apoA-I-containing HDL during HDL biogenesis. Like apoA-I formation, generation of SAA-containing particles was dependent on ABCA1, but not on scavenger receptor class B type I. Hepatocytes degraded significantly more SAA than apoA-I. Taken together, our results indicate that SAA's lipidation and metabolism by the liver is independent of apoA-I and that SAA is not incorporated into HDL during HDL biogenesis.

Serum amyloid A - a review

Serum amyloid A (SAA) proteins were isolated and named over 50 years ago. They are small (104 amino acids) and have a striking relationship to the acute phase response with serum levels rising as much as 1000-fold in 24 hours. SAA proteins are encoded in a family of closely-related genes and have been remarkably conserved throughout vertebrate evolution. Amino-terminal fragments of SAA can form highly organized, insoluble fibrils that accumulate in “secondary” amyloid disease. Despite their evolutionary preservation and dynamic synthesis pattern SAA proteins have lacked well-defined physiologic roles. However, considering an array of many, often unrelated, reports now permits a more coordinated perspective. Protein studies have elucidated basic SAA structure and fibril formation. Appreciating SAA’s lipophilicity helps relate it to lipid transport and metabolism as well as atherosclerosis. SAA’s function as a cytokine-like protein has become recognized in cell-cell communication as well as feedback in inflammatory, immunologic, neoplastic and protective pathways. SAA likely has a critical role in control and possibly propagation of the primordial acute phase response. Appreciating the many cellular and molecular interactions for SAA suggests possibilities for improved understanding of pathophysiology as well as treatment and disease prevention.

Development of quantitative mass spectrometric immunoassay for serum amyloid A

OBJECTIVE

Proteins can exist as multiple proteoforms in vivo that can have important roles in physiological and pathological states.

METHODS

We present the development and characterization of mass spectrometric immunoassay (MSIA) for quantitative determination of serum amyloid A (SAA) proteoforms.

RESULTS

Intra- and inter-day precision revealed CVs <10%. Against existing SAA ELISA, the developed MSIA showed good correlation according to the Altman-Bland plot. Individual concentrations of the SAA proteoforms across a cohort of 170 samples revealed 7 diverse SAA polymorphic types and 12 different proteoforms.

CONCLUSION

The new SAA MSIA enables parallel analysis of SAA polymorphisms and quantification of all expressed SAA proteoforms, in a high-throughput and time-efficient manner.

Serum amyloid A1: Structure, function and gene polymorphism

Inducible expression of serum amyloid A (SAA) is a hallmark of the acute-phase response, which is a conserved reaction of vertebrates to environmental challenges such as tissue injury, infection and surgery. Human SAA1 is encoded by one of the four SAA genes and is the best-characterized SAA protein. Initially known as a major precursor of amyloid A (AA), SAA1 has been found to play an important role in lipid metabolism and contributes to bacterial clearance, the regulation of inflammation and tumor pathogenesis. SAA1 has five polymorphic coding alleles (SAA1.1-SAA1.5) that encode distinct proteins with minor amino acid substitutions. Single nucleotide polymorphism (SNP) has been identified in both the coding and non-coding regions of human SAA1. Despite high levels of sequence homology among these variants, SAA1 polymorphisms have been reported as risk factors of cardiovascular diseases and several types of cancer. A recently solved crystal structure of SAA1.1 reveals a hexameric bundle with each of the SAA1 subunits assuming a 4-helix structure stabilized by the C-terminal tail. Analysis of the native SAA1.1 structure has led to the identification of a competing site for high-density lipoprotein (HDL) and heparin, thus providing the structural basis for a role of heparin and heparan sulfate in the conversion of SAA1 to AA. In this brief review, we compares human SAA1 with other forms of human and mouse SAAs, and discuss how structural and genetic studies of SAA1 have advanced our understanding of the physiological functions of the SAA proteins.

Residual Cardiovascular Risk in Chronic Kidney Disease: Role of High-density Lipoprotein

Although reducing low-density lipoprotein-cholesterol (LDL-C) levels with lipid-lowering agents (statins) decreases cardiovascular disease (CVD) risk, a substantial residual risk (up to 70% of baseline) remains after treatment in most patient populations. High-density lipoprotein (HDL) is a potential contributor to residual risk, and low HDL-cholesterol (HDL-C) is an established risk factor for CVD. However, in contrast to conventional lipid-lowering therapies, recent studies show that pharmacologic increases in HDL-C levels do not bring about clinical benefits. These observations have given rise to the concept of dysfunctional HDL where increases in serum HDL-C may not be beneficial because HDL loss of function is not corrected by or even intensified by the therapy. Chronic kidney disease (CKD) increases CVD risk, and patients whose CKD progresses to end-stage renal disease (ESRD) requiring dialysis are at the highest CVD risk of any patient type studied. The ESRD population is also unique in its lack of significant benefit from standard lipid-lowering interventions. Recent studies indicate that HDL-C levels do not predict CVD in the CKD population. Moreover, CKD profoundly alters metabolism and composition of HDL particles and impairs their protective effects on functions such as cellular cholesterol efflux, endothelial protection, and control of inflammation and oxidation. Thus, CKD-induced perturbations in HDL may contribute to the excess CVD in CKD patients. Understanding the mechanisms of vascular protection in renal disease can present new therapeutic targets for intervention in this population.

High-density lipoprotein: structural and functional changes under uremic conditions and the therapeutic consequences

High-density lipoprotein (HDL) has attracted interest as a therapeutic target in cardiovascular diseases in recent years. Although many functional mechanisms of the vascular protective effects of HDL have been identified, increasing the HDL plasma level has not been successful in all patient cohorts with increased cardiovascular risk. The composition of the HDL particle is very complex and includes diverse lipids and proteins that can be modified in disease conditions. In patients with chronic kidney disease (CKD), the accumulation of uremic toxins, high oxidative stress, and chronic micro-inflammatory conditions contribute to changes in the HDL composition and may also account for protein/lipid modifications. These conditions are associated with a decreased protective function of HDL. Therefore, the HDL quantity and the functional quality of the particle must be considered. This review summarizes the current knowledge of dyslipidemia in CKD patients, the effects of lipid-modulating therapy, and the structural modifications of HDL that are associated with dysfunction.

Impact of serum amyloid A on cellular cholesterol efflux to serum in type 2 diabetes mellitus

OBJECTIVE

Serum amyloid A (SAA) is an acute phase response protein and has apolipoprotein properties. Since type 2 diabetes is associated with chronic subclinical inflammation, the objective of this study is to investigate the changes in SAA level in type 2 diabetic patients and to evaluate the relationship between SAA and the capacity of serum to induce cellular cholesterol efflux via the two known cholesterol transporters, scavenger receptor class B type I (SR-BI) and ATP-binding cassette transporter G1 (ABCG1).

METHODS

264 patients with type 2 diabetes mellitus (42% with normoalbuminuria, 30% microalbuminuria, and 28% proteinuria) and 275 non-diabetic controls were recruited. SAA was measured by ELISA. SR-BI and ABCG1-mediated cholesterol efflux to serum were determined by measuring the transfer of [(3)H]cholesterol from Fu5AH rat hepatoma cells expressing SR-BI and from human ABCG1-transfected CHO-K1 cells to the medium containing the tested serum respectively.

RESULTS

SAA was significantly increased in diabetic patients with incipient or overt nephropathy. Both SR-BI and ABCG1-mediated cholesterol efflux to serum were significantly impaired in all three groups of diabetic patients (p < 0.01). SAA inversely correlated with SR-BI-mediated cholesterol efflux (r = -0.36, p < 0.01) but did not correlate with ABCG1-mediated cholesterol efflux. Stepwise linear regression analysis showed that HDL, the presence or absence of diabetes, and log(SAA) were significant independent determinants of SR-BI-mediated cholesterol efflux to serum.

CONCLUSION

SAA was increased in type 2 diabetic patients with incipient or overt nephropathy, and SAA was associated with impairment of SR-BI-mediated cholesterol efflux to serum.

The Impairment of Macrophage-to-Feces Reverse Cholesterol Transport during Inflammation Does Not Depend on Serum Amyloid A

Studies suggest that inflammation impairs reverse cholesterol transport (RCT). We investigated whether serum amyloid A (SAA) contributes to this impairment using an established macrophage-to-feces RCT model. Wild-type (WT) mice and mice deficient in SAA1.1 and SAA2.1 (SAAKO) were injected intraperitoneally with ³H-cholesterol-labeled J774 macrophages 4 hr after administration of LPS or buffered saline. ³H-cholesterol in plasma 4 hr after macrophage injection was significantly reduced in both WT and SAAKO mice injected with LPS, but this was not associated with a reduced capacity of serum from LPS-injected mice to promote macrophage cholesterol efflux in vitro. Hepatic accumulation of ³H-cholesterol was unaltered in either WT or SAAKO mice by LPS treatment. Radioactivity present in bile and feces of LPS-injected WT mice 24 hr after macrophage injection was reduced by 36% (P < 0.05) and 80% (P < 0.001), respectively. In contrast, in SAAKO mice, LPS did not significantly reduce macrophage-derived ³H-cholesterol in bile, and fecal excretion was reduced by only 45% (P < 0.05). Injection of cholesterol-loaded allogeneic J774 cells, but not syngeneic bone-marrow-derived macrophages, transiently induced SAA in C57BL/6 mice. Our study confirms reports that acute inflammation impairs steps in the RCT pathway and establishes that SAA plays only a minor role in this impairment.

Serum amyloid A opposes lipoxin A₄ to mediate glucocorticoid refractory lung inflammation in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) will soon be the third most common cause of death globally. Despite smoking cessation, neutrophilic mucosal inflammation persistently damages the airways and fails to protect from recurrent infections. This maladaptive and excess inflammation is also refractory to glucocorticosteroids (GC). Here, we identify serum amyloid A (SAA) as a candidate mediator of GC refractory inflammation in COPD. Extrahepatic SAA was detected locally in COPD bronchoalveolar lavage fluid, which correlated with IL-8 and neutrophil elastase, consistent with neutrophil recruitment and activation. Immunohistochemistry detected SAA was in close proximity to airway epithelium, and in vitro SAA triggered release of IL-8 and other proinflammatory mediators by airway epithelial cells in an ALX/FPR2 (formyl peptide receptor 2) receptor-dependent manner. Lipoxin A(4) (LXA(4)) can also interact with ALX/FPR2 receptors and lead to allosteric inhibition of SAA-initiated epithelial responses (pA(2) 13 nM). During acute exacerbation, peripheral blood SAA levels increased dramatically and were disproportionately increased relative to LXA(4). Human lung macrophages (CD68(+)) colocalized with SAA and GCs markedly increased SAA in vitro (THP-1, pEC(50) 43 nM). To determine its direct actions, SAA was administered into murine lung, leading to induction of CXC chemokine ligand 1/2 and a neutrophilic response that was inhibited by 15-epi-LXA(4) but not dexamethasone. Taken together, these findings identify SAA as a therapeutic target for inhibition and implicate SAA as a mediator of GC-resistant lung inflammation that can overwhelm organ protective signaling by lipoxins at ALX/FPR2 receptors.

The role of humoral innate immunity in hepatitis C virus infection

Infection with Hepatitis C Virus (HCV) causes chronic disease in approximately 80% of cases, resulting in chronic inflammation and cirrhosis. Current treatments are not completely effective, and a vaccine has yet to be developed. Spontaneous resolution of infection is associated with effective host adaptive immunity to HCV, including production of both HCV-specific T cells and neutralizing antibodies. However, the supporting role of soluble innate factors in protection against HCV is less well understood. The innate immune system provides an immediate line of defense against infections, triggering inflammation and playing a critical role in activating adaptive immunity. Innate immunity comprises both cellular and humoral components, the humoral arm consisting of pattern recognition molecules such as complement C1q, collectins and ficolins. These molecules activate the complement cascade, neutralize pathogens, and recruit antigen presenting cells. Here we review the current understanding of anti-viral components of the humoral innate immune system that play a similar role to antibodies, describing their role in immunity to HCV and their potential contribution to HCV pathogenesis.

Carrier-free cellular uptake and the gene-silencing activity of the lipophilic siRNAs is strongly affected by the length of the linker between siRNA and lipophilic group

The conjugation of siRNA to molecules, which can be internalized into the cell via natural transport mechanisms, can result in the enhancement of siRNA cellular uptake. Herein, the carrier-free cellular uptake of nuclease-resistant anti-MDR1 siRNA equipped with lipophilic residues (cholesterol, lithocholic acid, oleyl alcohol and litocholic acid oleylamide) attached to the 5′-end of the sense strand via oligomethylene linker of various length was investigated. A convenient combination of H-phosphonate and phosphoramidite methods was developed for the synthesis of 5′-lipophilic conjugates of siRNAs. It was found that lipophilic siRNA are able to effectively penetrate into HEK293, HepG2 and KB-8-5 cancer cells when used in a micromolar concentration range. The efficiency of the uptake is dependent upon the type of lipophilic moiety, the length of the linker between the moiety and the siRNA and cell type. Among all the conjugates tested, the cholesterol-conjugated siRNAs with linkers containing from 6 to 10 carbon atoms demonstrate the optimal uptake and gene silencing properties: the shortening of the linker reduces the efficiency of the cellular uptake of siRNA conjugates, whereas the lengthening of the linker facilitates the uptake but retards the gene silencing effect and decreases the efficiency of the silencing.

Regulation of ABCA1 functions by signaling pathways

ATP-binding cassette transporter A1 (ABCA1) is an integral cell membrane protein that protects cardiovascular disease by at least two mechanisms: by export of excess cholesterol from cells and by suppression of inflammation. ABCA1 exports cholesterol and phospholipids from cells by multiple steps that involve forming cell surface lipid domains, binding of apolipoproteins to ABCA1, activating signaling pathways, and solubilizing these lipids by apolipoproteins. ABCA1 executes its anti-inflammatory effect by modifying cell membrane lipid rafts and directly activating signaling pathways. The interaction of apolipoproteins with ABCA1 activates multiple signaling pathways, including Janus kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3), protein kinase A, Rho family G protein CDC42 and protein kinase C. Activating protein kinase A and Rho family G protein CDC42 regulates ABCA1-mediated lipid efflux, activating PKC stabilizes ABCA1 protein, and activating JAK2/STAT3 regulates both ABCA1-mediated lipid efflux and anti-inflammation. Thus, ABCA1 behaves both as a lipid exporter and a signaling receptor. Targeting ABCA1 receptor-like property using agonists for ABCA1 protein could become a promising new therapeutic target for increasing ABCA1 function and treating cardiovascular disease. 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).

Uremia alters HDL composition and function

Functional impairment of HDL may contribute to the excess cardiovascular mortality experienced by patients with renal disease, but the effect of advanced renal disease on the composition and function of HDL is not well understood. Here, we used mass spectrometry and biochemical analyses to study alterations in the proteome and lipid composition of HDL isolated from patients on maintenance hemodialysis. We identified a significant increase in the amount of acute phase protein serum amyloid A1, albumin, lipoprotein-associated phospholipase A2, and apoC-III composing uremic HDL. Furthermore, uremic HDL contained reduced phospholipid and increased triglyceride and lysophospholipid. With regard to function, these changes impaired the ability of uremic HDL to promote cholesterol efflux from macrophages. In summary, the altered composition of HDL in renal disease seems to inhibit its cardioprotective properties. Assessing HDL composition and function in renal disease may help identify patients at increased risk for cardiovascular disease.

Protein carbamylation renders high-density lipoprotein dysfunctional

Carbamylation of proteins through reactive cyanate has been demonstrated to predict an increased cardiovascular risk. Cyanate is formed in vivo by breakdown of urea and at sites of inflammation by the phagocyte protein myeloperoxidase. Because myeloperoxidase (MPO) associates with high-density lipoprotein (HDL) in human atherosclerotic intima, we examined in the present study whether cyanate specifically targets HDL. Mass spectrometry analysis revealed that protein carbamylation is a major posttranslational modification of HDL. The carbamyllysine content of lesion-derived HDL was more than 20-fold higher in comparison with 3-chlorotyrosine levels, a specific oxidation product of MPO. Notably, the carbamyllysine content of lesion-derived HDL was five- to eightfold higher when compared with lesion-derived low-density lipoprotein (LDL) or total lesion protein and increased with lesion severity. The carbamyllysine content of HDL, but not of LDL, correlated with levels of 3-chlorotyrosine, suggesting that MPO mediated carbamylation in the vessel wall. Remarkably, one carbamyllysine residue per HDL-associated apolipoprotein A-I was sufficient to induce cholesterol accumulation and lipid-droplet formation in macrophages through a pathway requiring the HDL-receptor scavenger receptor class B, type I. The present results raise the possibility that HDL carbamylation contributes to foam cell formation in atherosclerotic lesions.

Deactivation of the lipopolysaccharide antagonist eritoran (E5564) by high-density lipoprotein-associated apolipoproteins

Lipid A, the active moiety of LPS, exerts its effects through interaction with TLR4, triggering a signalling cascade that results in the release of pro-inflammatory cytokines. Eritoran is a lipid A analogue that competes with LPS for binding to TLR4; however, after intravenous administration, it undergoes a time-dependent deactivation as a consequence of binding to high-density lipoproteins (HDLs). The site of eritoran association with HDL remains unknown. Therefore the aim of this study was to determine if HDL-associated apolipoproteins A1, A2, serum amyloid A (SAA) and C1, inhibit the ability of eritoran to block LPS-induced TNF-α release from whole blood. Eritoran activity after LPS stimulation in human whole blood was assessed in the presence of reconstituted HDL (rHDL) containing different apos. In rHDL, the major apolipoproteins in both the healthy and septic state, A1 and SAA, caused a significant reduction in eritoran antagonistic activity and had a greater effect than minor apolipoproteins A2 and C1. Apolipoproteins associated with HDL are likely to facilitate eritoran deactivation. Apolipoproteins A1 and SAA should be of particular focus as they are the major apos found on HDL in both the healthy and septic state. Further evaluation of the physical association between apolipoproteins and eritoran should be explored.

Vitamin D intestinal absorption is not a simple passive diffusion: evidences for involvement of cholesterol transporters

SCOPE

It is assumed that vitamin D is absorbed by passive diffusion. However, since cholecalciferol (vitamin D(3) ) and cholesterol display similar structures, we hypothesized that common absorption pathways may exist.

METHODS AND RESULTS

Cholecalciferol apical transport was first examined in human Caco-2 and transfected Human embryonic kidney (HEK) cells. Cholecalciferol uptake was then valuated ex vivo and in vivo, using either wild-type mice, mice overexpressing Scavenger Receptor class B type I (SR-BI) at the intestinal level or mice treated or not with ezetimibe. Cholecalciferol uptake was concentration-, temperature- and direction-dependent, and was significantly impaired by a co-incubation with cholesterol or tocopherol in Caco-2 cells. Moreover Block Lipid Transport-1 (SR-BI inhibitor) and ezetimibe glucuronide (Niemann-Pick C1 Like 1 inhibitor) significantly decreased cholecalciferol transport. Transfection of HEK cells with SR-BI, Cluster Determinant 36 and Niemann-Pick C1 Like 1 significantly enhanced vitamin D uptake, which was significantly decreased by the addition of Block Lipid Transport-1, sulfo-N-succinimidyl oleate (Cluster Determinant 36 inhibitor) or ezetimibe glucuronide, respectively. Similar results were obtained in mouse intestinal explants. In vivo, cholecalciferol uptake in proximal intestinal fragments was 60% higher in mice overexpressing SR-BI than in wild-type mice (p<0.05), while ezetimibe effect remained non-significant.

CONCLUSION

These data show for the first time that vitamin D intestinal absorption is not passive only but involves, at least partly, some cholesterol transporters.

Influence of long-term leflunomide treatment on serum amyloid concentration in rheumatoid arthritis patients

Rheumatoid arthritis (RA) is a chronic, inflammatory disease that requires intervention with disease-modifying antirheumatic drugs (DMARDs) to stop disease progression. Leflunomide (LEF) is a DMARD with anti-inflammatory and immunomodulatory properties. As its primary mode of action, LEF reversibly inhibits dihydroorotate dehydrogenase, a key enzyme in de novo biosynthesis of pyrimidine in cells. Serum amyloid A protein (SAA) is elevated in inflammatory states and high SAA levels indicate a risk of developing secondary amyloidosis. The aim of this study was to investigate the effects of long-term LEF treatment on SAA levels and disease activity in a group of RA patients. The study group consisted of 50 consecutive RA patients (43 F, 7 M) treated with leflunomide. All patients had a clinical evaluation and SAA measurements taken at two consecutive visits during months 0, 1, 3, 6 and 12. Mean SAA concentrations decreased significantly in the first months of LEF therapy (up to the 6th month) with a more pronounced effect in patients with higher SAA levels. However, by the 12(th) month of treatment, the mean SAA level did not differ significantly from the SAA level at the start of treatment. At the same time though, other clinical and laboratory parameters of RA activity indicated that the disease activity decreased. Results demonstrated that in patients with active RA LEF therapy provided a significant, long-term reduction of inflammatory activity, as measured by the classic parameters of disease activity. During the treatment, SAA concentrations decreased significantly, followed by a slight increase, in spite of a reduction in other classical indicators of inflammatory response.

Anti-obesity effect of phosphatidylinositol on diet-induced obesity in mice

The aim of this study is to investigate the biodistribution of phosphatidylinositol (PI) after oral administration and its anti-obesity effect. When a suspension of radiolabeled PI was orally administered to mice and the biodistribution was examined, PI radioactivity accumulated in the liver compared to myo-inositol radioactivity at 48 h or later after administration. Then, a PI suspension was orally administered to diet-induced obesity (DIO) mice every 4 days, and the anti-obesity effect of PI was examined. As a result, PI suppressed the body weight increase of DIO mice and significantly reduced the plasma levels of aspartate aminotransferase (AST) and cholesterol. Furthermore, PI regulated the expression of some genes in the liver involved in lipid synthesis and metabolism. The present study demonstrated that PI accumulated in the liver after oral administration and exerted its anti-obesity effect on DIO by regulating the expression of certain genes involved in lipid metabolism in the liver.

ATP-binding membrane cassette transporter A1 (ABCA1): a possible link between inflammation and reverse cholesterol transport

Atherosclerosis is characterized by a chronic inflammatory condition that involves numerous cellular and molecular inflammatory components. A wide array of inflammatory mediators, such as cytokines and proteins produced by macrophages and other cells, play a critical role in the development and progression of the disease. ATP-binding membrane cassette transporter A1 (ABCA1) is crucial for cellular cholesterol efflux and reverse cholesterol transport (RCT) and is also identified as an important target in antiatherosclerosis treatment. Evidence from several recent studies indicates that inflammation, along with other atherogenic-related mediators, plays distinct regulating roles in ABCA1 expression. Proatherogenic cytokines such as interferon (IFN)-γ and interleukin (IL)-1β have been shown to inhibit the expression of ABCA1, while antiatherogenic cytokines, including IL-10 and transforming growth factor (TGF)-β1, have been shown to promote the expression of ABCA1. Moreover, some cytokines such as tumor necrosis factor (TNF)-α seem to regulate ABCA1 expression in species-specific and dose-dependent manners. Inflammatory proteins such as C-reactive protein (CRP) and cyclooxygenase (COX)-2 are likely to inhibit ABCA1 expression during inflammation, and inflammation induced by lipopolysaccharide (LPS) was also found to block the expression of ABCA1. Interestingly, recent experiments revealed ABCA1 can function as an antiinflammatory receptor to suppress the expression of inflammatory factors, suggesting that ABCA1 may be the molecular basis for the interaction between inflammation and RCT. This review aims to summarize recent findings on the role of inflammatory cytokines, inflammatory proteins, inflammatory lipids, and the endotoxin-mediated inflammatory process in expression of ABCA1. Also covered is the current understanding of the function of ABCA1 in modulating the immune response and inflammation through its direct and indirect antiinflammatory mechanisms including lipid transport, high-density lipoprotein (HDL) formation and apoptosis.

High density lipoprotein structure-function and role in reverse cholesterol transport

High density lipoprotein (HDL) possesses important anti-atherogenic properties and this review addresses the molecular mechanisms underlying these functions. The structures and cholesterol transport abilities of HDL particles are determined by the properties of their exchangeable apolipoprotein (apo) components. ApoA-I and apoE, which are the best characterized in structural terms, contain a series of amphipathic alpha-helical repeats. The helices located in the amino-terminal two-thirds of the molecule adopt a helix bundle structure while the carboxy-terminal segment forms a separately folded, relatively disorganized, domain. The latter domain initiates lipid binding and this interaction induces changes in conformation; the alpha-helix content increases and the amino-terminal helix bundle can open subsequently. These conformational changes alter the abilities of apoA-I and apoE to function as ligands for their receptors. The apoA-I and apoE molecules possess detergent-like properties and they can solubilize vesicular phospholipid to create discoidal HDL particles with hydrodynamic diameters of ~10 nm. In the case of apoA-I, such a particle is stabilized by two protein molecules arranged in an anti-parallel, double-belt, conformation around the edge of the disc. The abilities of apoA-I and apoE to solubilize phospholipid and stabilize HDL particles enable these proteins to be partners with ABCA1 in mediating efflux of cellular phospholipid and cholesterol, and the biogenesis of HDL particles. ApoA-I-containing nascent HDL particles play a critical role in cholesterol transport in the circulation whereas apoE-containing HDL particles mediate cholesterol transport in the brain. The mechanisms by which HDL particles are remodeled by lipases and lipid transfer proteins, and interact with SR-BI to deliver cholesterol to cells, are reviewed.

Detection of up-regulated serum amyloid A transcript and of amyloid AA aggregates in skeletal muscle lesions of rainbow trout infected with Flavobacterium psychrophilum

Serum amyloid A (SAA) is a family of acute-phase proteins, recognized as important effectors of innate immunity in higher vertebrates. Under pro-inflammatory conditions, up-regulation of saa transcripts occurs not only in the liver, but also in several extrahepatic tissues of a wide variety of vertebrates. SAA is also known as the precursor to amyloid A (AA), a major component of amyloid fibrils deposited in liver, kidney and spleen of humans suffering chronic inflammatory diseases. Here we show the up-regulation of saa transcription in lesions affecting skin, adipose tissue and skeletal muscle of rainbow trout naturally and experimentally infected with Flavobacterium psychrophilum, the causative agent of cold water disease (CWD). Using an antiserum against a trout acute SAA peptide that was previously shown to specifically recognize intact recombinant trout SAA and peptides derived from it, we showed by confocal microscopy analysis extensive colocalization of SAA and thioflavin T (ThT) staining in the skeletal muscle fibers of infected fish, suggesting for the first time the presence of AA-derived aggregates in the skeletal muscle of a lower vertebrate. These findings support the idea that SAA and/or its derivatives could constitute relevant markers for fish health and also for fish meat quality control.

Plasma-advanced oxidation protein products are potent high-density lipoprotein receptor antagonists in vivo

Advanced oxidation protein products (AOPPs) are carried by oxidized plasma proteins, especially albumin and accumulate in subjects with renal disease and coronary artery disease. AOPPs represent an excellent novel marker of oxidative stress and their roles in the development of cardiovascular disease might be of great importance. Here, we show that in vitro-generated AOPP-albumin binds with high affinity to the high-density lipoprotein (HDL) receptor scavenger receptor class B type I (SR-BI). Already an equimolar concentration of AOPP-albumin to HDL blocked HDL association to SR-BI and effectively inhibited SR-BI-mediated cholesterol ester (CE) uptake. Interestingly, albumin extensively modified by advanced glycation end products (AGE-albumin), which is an established SR-BI ligand known to accumulate in renal disease, only weakly interfered with HDL binding to SR-BI. Furthermore, AOPP-albumin administration increased the plasma half-life of [3H]CE-HDL in control mice 1.6-fold (P=0.01) and 8-fold (P=0.0003) in mice infected with adenoviral vectors encoding human SR-BI. Moreover, albumin isolated from hemodialysis patients, but not albumin isolated from healthy controls, markedly inhibited SR-BI-mediated HDL-CE transfer in vitro dependent on the AOPP content of albumin. These results indicate that AOPP-albumin effectively blocks SR-BI in vitro and in vivo. Thus, depressed plasma clearance of HDL-cholesterol may contribute to the abnormal composition of HDL and the high cardiovascular risk observed in patients with chronic renal failure.

Serum amyloid A: an acute-phase protein involved in tumour pathogenesis

The synthesis of acute-phase protein serum amyloid A (SAA) is largely regulated by inflammation- associated cytokines and a high concentration of circulating SAA may represent an ideal marker for acute and chronic inflammatory diseases. However, SAA is also synthesized in extrahepatic tissues, e.g. human carcinoma metastases and cancer cell lines. An increasing body of in vitro data supports the concept of involvement of SAA in carcinogenesis and neoplastic diseases. Accumulating evidence suggests that SAA might be included in a group of biomarkers to detect a pattern of physiological events that reflect the growth of malignancy and host response. This review is meant to provide a broad overview of the many ways that SAA could contribute to tumour development, and accelerate tumour progression and metastasis, and to gain a better understanding of this acute-phase reactant as a possible link between chronic inflammation and neoplasia.

Expression of serum amyloid A transcripts in human bone tissues, differentiated osteoblast-like stem cells and human osteosarcoma cell lines

Although the liver is the primary site of cytokine-mediated expression of acute-phase serum amyloid A (SAA) protein, extrahepatic production has also been reported. Besides its role in amyloidosis and lipid homeostasis during the acute-phase, SAA has recently been assumed to contribute to bone and cartilage destruction. However, expression of SAA in human osteogenic tissue has not been studied. Therefore, we first show that SAA1 (coding for the major SAA isoform) but not SAA2 transcripts are expressed in human trabecular and cortical bone fractions and bone marrow. Next, we show expression of (i) IL-1, IL-6, and TNF receptor transcripts; (ii) the human homolog of SAA-activating factor-1 (SAF-1, a transcription factor involved in cytokine-mediated induction of SAA genes); and (iii) SAA1/2 transcripts in non-differentiated and, to a higher extent, in osteoblast-like differentiated human mesenchymal stem cells. Third, we provide evidence that human osteoblast-like cells of tumor origin (MG-63 and SAOS-2) express SAF-1 under basal conditions. SAA1/2 transcripts are expressed under basal conditions (SAOS-2) and cytokine-mediated conditions (MG-63 and SAOS-2). RT-PCR, Western blot analysis, and immunofluorescence technique confirmed cytokine-mediated expression of SAA on RNA and protein level in osteosarcoma cell lines while SAA4, a protein of unknown function, is constitutively expressed in all osteogenic tissues investigated.

Macrophage scavenger receptors and host-derived ligands

The scavenger receptors are a large family of molecules that are structurally diverse and have been implicated in a range of functions. They are expressed by myeloid cells, selected endothelial cells and some epithelial cells and recognise many different ligands, including microbial pathogens as well as endogenous and modified host-derived molecules. This review will focus on the eight classes of scavenger receptors (class A-H) in terms of their structure, expression and recognition of host-derived ligands. Scavenger receptors have been implicated in a range of physiological and pathological processes, such as atherosclerosis and Alzheimer's disease, and function in adhesion and tissue maintenance. More recently, some of the scavenger receptors have been shown to mediate binding and endocytosis of chaperone proteins, such as the heat shock proteins, thereby playing an important role in antigen cross-presentation.

Serum amyloid A: a typical acute-phase reactant in rainbow trout?

Acute serum amyloid A (A-SAA) has been considered a major acute-phase reactant and an effector of innate immunity in all vertebrates. The work presented here shows that the expression of A-SAA is strongly induced in a wide variety of immune-relevant tissues in rainbow trout, either naturally infected with Flavobacterium psychrophilum or challenged with lipopolysaccharide (LPS) or CpG oligonucleotides (CpG ODN). Nevertheless, A-SAA was undetectable by Western blot either in the plasma or in high-density lipoprotein (HDL) of infected or challenged fish, using either an anti-mouse SAA1 IgG or an anti-trout A-SAA peptide serum, which recognise both the intact recombinant trout A-SAA and fragments derived from it. However, the anti-peptide serum was the immunoreactive in all primary defence barriers and in mononuclear cells of head kidney, spleen and liver. These findings reveal that, unlike mammalian SAA, trout A-SAA does not increase significantly in the plasma of diseased fish, suggesting it is more likely to be involved in local defence.

Identification of the molecular target of small molecule inhibitors of HDL receptor SR-BI activity

Scavenger receptor, class B, type I (SR-BI), controls high-density lipoprotein (HDL) metabolism by mediating cellular selective uptake of lipids from HDL without the concomitant degradation of the lipoprotein particle. We previously identified in a high-throughput chemical screen of intact cells five compounds (BLT-1-5) that inhibit SR-BI-dependent lipid transport from HDL, but do not block HDL binding to SR-BI on the cell surface. Although these BLTs are widely used to examine the diverse functions of SR-BI, their direct target(s), SR-BI itself or some other component of the SR-BI pathway, has not been identified. Here we show that SR-BI in the context of a membrane lipid environment is the target of BLT-1, -3, -4, and -5. The analysis using intact cells and an in vitro system of purified SR-BI reconstituted into liposomes was aided by information derived from structure-activity relationship (SAR) analysis of the most potent of these BLTs, the thiosemicarbazone BLT-1. We found that the sulfur atom of BLT-1 was crucially important for its inhibitory activity, because changing it to an oxygen atom resulted in the isostructural, but essentially inactive, semicarbazone derivative BLT-1sc. SAR analysis also established the importance of BLT-1's hydrophobic tail. BLTs and their corresponding inactive compounds can be used to explore the mechanism and function of SR-BI-mediated selective lipid uptake in diverse mammalian experimental models. Consequently, BLTs may help determine the therapeutic potential of SR-BI-targeted pharmaceutical drugs.

Biogenesis of HDL by SAA is dependent on ABCA1 in the liver in vivo

Serum amyloid A (SAA) was markedly increased in the plasma and in the liver upon acute inflammation induced by intraperitoneal injection of lipopolysaccharide (LPS) in mice, and SAA in the plasma was exclusively associated with HDL. In contrast, no HDL was present in the plasma and only a small amount of SAA was found in the VLDL/LDL fraction (d < 1.063 g/ml) after the induction of inflammation in ABCA1-knockout (KO) mice, although SAA increased in the liver. Primary hepatocytes isolated from LPS-treated wild-type (WT) and ABCA1-KO mice both secreted SAA into the medium. SAA secreted from WT hepatocytes was associated with HDL, whereas SAA from ABCA1-KO hepatocytes was recovered in the fraction that was >1.21 g/ml. The behavior of apolipoprotein A-I (apoA-I) was the same as that of SAA in HDL biogenesis by WT and ABCA1-KO mouse hepatocytes. Lipid-free SAA and apoA-I both stabilized ABCA1 and caused cellular lipid release in WT mouse-derived fibroblasts, but not in ABCA1-KO mouse-derived fibroblasts, in vitro when added exogenously. We conclude that both SAA and apoA-I generate HDL largely in hepatocytes only in the presence of ABCA1, likely being secreted in a lipid-free form to interact with cellular ABCA1. In the absence of ABCA1, nonlipidated SAA is seemingly removed rapidly from the extracellular space.

Influence of HDL-cholesterol-elevating drugs on the in vitro activity of the HDL receptor SR-BI

Treatment of atherosclerotic disease often focuses on reducing plasma LDL-cholesterol or increasing plasma HDL-cholesterol. We examined in vitro the effects on HDL receptor [scavenger receptor class B type I (SR-BI)] activity of three classes of clinical and experimental plasma HDL-cholesterol-elevating compounds: niacin, fibrates, and HDL376. Fenofibrate (FF) and HDL376 were potent (IC(50) approximately 1 microM), direct inhibitors of SR-BI-mediated lipid transport in cells and in liposomes reconstituted with purified SR-BI. FF, a prodrug, was a more potent inhibitor of SR-BI than an activator of peroxisome proliferator-activated receptor alpha, a target of its active fenofibric acid (FFA) derivative. Nevertheless, FFA, four other fibrates (clofibrate, gemfibrozil, ciprofibrate, and bezafibrate), and niacin had little, if any, effect on SR-BI, suggesting that they do not directly target SR-BI in vivo. However, similarities of HDL376 treatment and SR-BI gene knockout on HDL metabolism in vivo (increased HDL-cholesterol and HDL particle sizes) and structure-activity relationship analysis suggest that SR-BI may be a target of HDL376 in vivo. HDL376 and other inhibitors may help elucidate SR-BI function in diverse mammalian models and determine the therapeutic potential of SR-BI-directed pharmaceuticals.