Regulation of scavenger receptor class B type I in hamster liver and Hep3B cells by endotoxin and cytokines

The Composition of the HDL Particle and Its Capacity to Remove Cellular Cholesterol Are Associated with a Reduced Risk of Developing Active Inflammatory Rheumatoid Arthritis

In rheumatoid arthritis (RA), the risk of cardiovascular death is 50% higher compared to the general population. This increased risk is partly due to the systemic inflammation characteristic of RA and changes in the lipoprotein profiles. This study investigated plasma lipid levels, lipid ratios, and the composition and functionality of high-density lipoprotein (HDL) in control individuals and RA subjects based on the disease's inflammatory score (DAS28). This study included 50 control (CTR) individuals and 56 subjects with RA, divided into remission/low-activity disease (DAS28 < 3.2; n = 13) and active disease (DAS28 ≥ 3.2; n = 43). Plasma lipids (total cholesterol, TC; triglycerides, TG) and the HDL composition (TC; TG; phospholipids, PL) were determined using enzymatic methods; apolipoprotein B (apoB) and apoA-1 were measured by immunoturbidimetry. HDL-mediated cholesterol efflux and anti-inflammatory activity were assessed in bone marrow-derived macrophages. Comparisons were made using the Mann-Whitney test, and binary logistic regression was used to identify the predictors of active RA. A p-value < 0.05 was considered significant. TC, HDLc, and the TC/apoB ratio were higher in RA subjects compared to the CTR group. Subjects with active disease exhibited higher levels of TG and the TG/HDLc ratio and lower levels of HDLc, the TG/apoB ratio, TC, and apoA-1 in HDL particles compared to those with remission/low-activity RA. Increased levels of HDLc [odds ratio (OR) 0.931, 95% CI = 0.882-0.984], TC/apoB (OR 0.314, 95% CI = 0.126-0.78), HDL content in TC (OR 0.912, 95% CI = 0.853-0.976), PL (OR 0.973, 95% CI = 0.947-1.000), and apoA-1 (OR 0.932, 95% CI = 0.882-0.985) were associated with a decreased risk of active disease, but BMI (OR 1.169, 95% CI = 1.004-1.360) and TG (OR 1.031, 95% CI = 1.005-1.057) were positively associated with active disease. A reduction in HDL-mediated cholesterol efflux increased the OR for active RA by 26.2%. The plasma levels of HDLc, along with the composition and functionality of HDL, influence the inflammatory score in RA and may affect the development of cardiovascular disease.

Microglia in Alzheimer's Disease: A Target for Therapeutic Intervention

Alzheimer’s disease (AD) is one of the most common types of age-related dementia worldwide. In addition to extracellular amyloid plaques and intracellular neurofibrillary tangles, dysregulated microglia also play deleterious roles in the AD pathogenesis. Numerous studies have demonstrated that unbridled microglial activity induces a chronic neuroinflammatory environment, promotes β-amyloid accumulation and tau pathology, and impairs microglia-associated mitophagy. Thus, targeting microglia may pave the way for new therapeutic interventions. This review provides a thorough overview of the pathophysiological role of the microglia in AD and illustrates the potential avenues for microglia-targeted therapies, including microglial modification, immunoreceptors, and anti-inflammatory drugs.

The Role of Oxidative Stress and Inflammation in Cardiometabolic Health of Children During Cancer Treatment and Potential Impact of Key Nutrients

Significance: The 5-year survival rate of childhood cancers is now reaching 84%. However, treatments cause numerous acute and long-term side effects. These include cardiometabolic complications, namely hypertension, dyslipidemia, hyperglycemia, insulin resistance, and increased fat mass. Recent Advances: Many antineoplastic treatments can induce oxidative stress (OxS) and trigger an inflammatory response, which may cause acute and chronic side effects. Critical Issues: Clinical studies have reported a state of heightened OxS and inflammation during cancer treatment in children as the result of treatment cytotoxic action on both cancerous and noncancerous cells. Higher levels of OxS and inflammation are associated with treatment side effects and with the development of cardiometabolic complications. Key nutrients (omega-3 polyunsaturated fatty acids, dietary antioxidants, probiotics, and prebiotics) have the potential to modulate inflammatory and oxidative responses and, therefore, could be considered in the search for adverse complication prevention means as long as antineoplastic treatment efficiency is maintained. Future Directions: There is a need to better understand the relationship between cardiometabolic complications, OxS, inflammation and diet during pediatric cancer treatment, which represents the ultimate goal of this review. Antioxid. Redox Signal. 35, 293-318.

New insights into the emerging effects of inflammatory response on HDL particles structure and function

According to the increasing results, it has been well-demonstrated that the chronic inflammatory response, including systemic lupus erythematosus, rheumatoid arthritis, and inflammatory bowel disease are associated with an increased risk of atherosclerotic cardiovascular disease. The mechanism whereby inflammatory response up-regulates the risk of cardio-metabolic disorder disease is multifactorial; furthermore, the alterations in high density lipoprotein (HDL) structure and function which occur under the inflammatory response could play an important modulatory function. On the other hand, the serum concentrations of HDL cholesterol (HDL-C) have been shown to be reduced significantly under inflammatory status with remarked alterations in HDL particles. Nevertheless, the potential mechanism whereby the inflammatory response reduces serum HDL-C levels is not simply defined but reduces apolipoprotein A1 production. The alterations in HDL structure mediated by the inflammatory response has been also confirmed to decrease the ability of HDL particle to play an important role in reverse cholesterol transport and protect the LDL particles from oxidation. Recently, it has been shown that under the inflammatory condition, diverse alterations in HDL structure could be observed which lead to changes in HDL function. In the current review, the emerging effects of inflammatory response on HDL particles structure and function are well-summarized to elucidate the potential mechanism whereby different inflammatory status modulates the pathogenic development of dyslipidemia.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

TNFα-Induced LDL Cholesterol Accumulation Involve Elevated LDLR Cell Surface Levels and SR-B1 Downregulation in Human Arterial Endothelial Cells

Excess lipid droplets are frequently observed in arterial endothelial cells at sites of advanced atherosclerotic plaques. Here, the role of tumor necrosis factor alpha (TNFα) in modulating the low-density lipoprotein (LDL) content in confluent primary human aortic endothelial cells (pHAECs) was investigated. TNFα promoted an up to 2 folds increase in cellular cholesterol, which was resistant to ACAT inhibition. The cholesterol increase was associated with increased ¹²⁵I-LDL surface binding. Using the non-hydrolysable label, Dil, TNFα could induce a massive increase in Dil-LDL by over 200 folds. The elevated intracellular Dil-LDL was blocked with excess unlabeled LDL and PCSK9, but not oxidized LDL (oxLDL), or apolipoprotein (apoE) depletion. Moreover, the TNFα-induced increase of LDL-derived lipids was elevated through lysosome inhibition. Using specific LDLR antibody, the Dil-LDL accumulation was reduced by over 99%. The effects of TNFα included an LDLR cell surface increase of 138%, and very large increases in ICAM-1 total and surface proteins, respectively. In contrast, that of scavenger receptor B1 (SR-B1) was reduced. Additionally, LDLR antibody bound rapidly in TNFα-treated cells by about 30 folds, inducing a migrating shift in the LDLR protein. The effect of TNFα on Dil-LDL accumulation was inhibited by the antioxidant tetramethythiourea (TMTU) dose-dependently, but not by inhibitors against NF-κB, stress kinases, ASK1, JNK, p38, or apoptosis caspases. Grown on Transwell inserts, TNFα did not enhance apical to basolateral LDL cholesterol or Dil release. It is concluded that TNFα promotes LDLR functions through combined increase at the cell surface and SR-B1 downregulation.

High-density lipoprotein functionality in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a heterogeneous disease which is characterized with excessive inflammation and autoantibodies, macrophage and complement activation, and subsequently immunologically mediated tissue damage. In spite of improved treatments of SLE, these patients experience premature atherosclerosis and the rate of mortality among them remains high. Autoantibodies and circulating immune complexes might contribute to the pathogenesis of atherosclerosis by injuring the endothelium, as well as inducing pro-inflammatory and pro-adhesive endothelial cell phenotypes, as well as altering the metabolism of lipoproteins involved in atherogenesis. Hence, high levels of atherogenic lipoproteins (like low-density lipoprotein (LDL) and very low-density lipoprotein (VLDL)) and low levels of high-density lipoprotein (HDL-C) are important risk factors for atherosclerotic cardiovascular complications in SLE patients but these traditional risk factors fail to fully explain the increased risk of cardiovascular disease (CVD) in these patients. The exact mechanism by which inflammation decreases HDL levels is not defined, but decreases in apoA-I production and lecithin cholesterol acyltransferase (LCAT) activity, as well as increased serum amyloid A (SAA), endothelial lipase and secretory phospholipase A2 activity (PLA2) could all contribute. In addition, during inflammation multiple changes in HDL structure occur, leading to alterations in HDL function which may be implicated in the CVD complications of SLE. Therefore, this review will aim to identify the mechanisms implicated in HDL dysfunction which occurs in SLE patients.

Quercetin induces the selective uptake of HDL-cholesterol via promoting SR-BI expression and the activation of the PPARγ/LXRα pathway

Reverse cholesterol transport (RCT) is the process to deliver cholesterol to the liver for further excretion and involves scavenger receptor class B type I (SR-BI)-mediated selective lipid uptake (SLU) from high-density lipoprotein cholesterol (HDL-C). The up-regulation of hepatic SR-BI expression accelerates HDL-C clearance in circulation and impedes the development of atherosclerosis (AS). In the present study, we explored the modulation of hepatic SR-BI expression and SR-BI-mediated SLU by quercetin, a natural flavonoid compound in the diet with a favorable role in cardiovascular disorders. We found that quercetin significantly increased the expression level of SR-BI in HepG2 cells in a concentration- and time-dependent manner. Besides, quercetin had stimulatory effects on the binding of 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil)-labeled HDL to hepatocytes and ¹²⁵I/³H-CE-HDL association. Treatment with small interfering RNA (siRNA) or SR-BI specific inhibitor, BLT-1, inhibited quercetin-induced Dil-HDL binding and selective HDL-C uptake. Treatment with quercetin increased both proliferator-activated receptor γ (PPARγ) and liver X receptor α (LXRα) levels. Additionally, the quercetin-induced expression of SR-BI, Dil-HDL binding and the selective uptake of HDL-C were significantly attenuated by treatment with PPARγ siRNA, LXRα siRNA, and their antagonists, respectively. In C57BL/6 mice, quercetin administration potently increased SR-BI, PPARγ and LXRα levels and lipid accumulation in the liver. Altogether, our results suggest that quercetin-induced up-regulation of SR-BI and subsequent lipid uptake in hepatocytes might contribute to its beneficial effects on cholesterol homeostasis and atherogenesis.

Effect of inflammation on HDL structure and function

PURPOSE OF REVIEW

Studies have shown that chronic inflammatory disorders, such as rheumatoid arthritis, systemic lupus erythematosus, and psoriasis are associated with an increased risk of atherosclerotic cardiovascular disease. The mechanism by which inflammation increases cardiovascular disease is likely multifactorial but changes in HDL structure and function that occur during inflammation could play a role.

RECENT FINDINGS

HDL levels decrease with inflammation and there are marked changes in HDL-associated proteins. Serum amyloid A markedly increases whereas apolipoprotein A-I, lecithin:cholesterol acyltransferase, cholesterol ester transfer protein, paraoxonase 1, and apolipoprotein M decrease. The exact mechanism by which inflammation decreases HDL levels is not defined but decreases in apolipoprotein A-I production, increases in serum amyloid A, increases in endothelial lipase and secretory phospholipase A2 activity, and decreases in lecithin:cholesterol acyltransferase activity could all contribute. The changes in HDL induced by inflammation reduce the ability of HDL to participate in reverse cholesterol transport and protect LDL from oxidation.

SUMMARY

During inflammation multiple changes in HDL structure occur leading to alterations in HDL function. In the short term, these changes may be beneficial resulting in an increase in cholesterol in peripheral cells to improve host defense and repair but over the long term these changes may increase the risk of atherosclerosis.

Pro-atherogenic properties of lipopolysaccharide from the periodontal pathogen Actinobacillus actinomycetemcomitans

An association between cardiovascular and periodontal disease may be due to lipopolysaccharide (LPS)-promoted release of inflammatory mediators, adverse alterations of the lipoprotein profile, and an imbalance in cholesterol homeostasis. Since periodontopathogenic potential differs between serotypes of a major periodontal pathogen, Actinobacillus actinomycetemcomitans, we studied the pro-atherogenic properties of LPS preparations from serotypes b and d strains on macrophages (RAW 264.7). A. actinomycetemcomitans LPS preparations induced a time-dependent release of tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β). LPS induced foam cell formation and cholesteryl ester accumulation from native low density lipoprotein in the following order: A. actinomycetemcomitans strains JP2 (serotype b) > Y4 (serotype b) > IDH781 (serotype d). mRNA expression levels of scavenger receptor class B, type-I, and ATP-binding cassette transporter-1, receptors mediating cholesterol efflux from macrophages, were decreased by LPS preparations. The results suggest that the pro-atherogenic potential of A. actinomycetemcomitans LPS may depend on the infecting strain and correlate with the periodontopathogenic potential of the pathogen.

Apolipoproteins modulate the inflammatory response to lipopolysaccharide

An increasing body of evidence demonstrates a close interplay between lipoprotein metabolism and sepsis. Sepsis results in an increase of plasma triglycerides within VLDL as a consequence of an enhanced hepatic VLDL production and/or inhibited peripheral and hepatic VLDL clearance. In contrast, sepsis decreases plasma cholesterol within LDL and mainly HDL. The decrease in HDL is accompanied by a loss of mainly apoAI-containing particles, an almost total loss of apoCI, and an increase in apoE-containing HDL, as related to the effect of LPS on a wide range of apolipoproteins, plasma enzymes, lipid transfer factors, and receptors that are involved in HDL metabolism. Reciprocally, all lipoprotein classes have been shown to bind LPS and to attenuate the biological response to LPS in vitro and in rodents. Moreover, triglyceride-rich lipoproteins protect rodents against death from LPS and bacterial sepsis. Accumulating evidence indicates that apolipoproteins such as apoE and apoAI, and not the lipid moieties of the particles, may be responsible for these protective effects of lipoproteins. Therefore, to increase our understanding of the complex interaction between lipoprotein metabolism and sepsis, further studies that address the specific roles of apolipoproteins in sepsis are warranted.

Hepatic scavenger receptor BI is associated with type 2 diabetes but unrelated to human and murine non-alcoholic fatty liver disease

Scavenger receptor, class B type I (SR-BI) is a physiologically relevant regulator of high density lipoprotein (HDL) metabolism. Low HDL is a common feature of patients with non-alcoholic fatty liver disease (NAFLD). Here, hepatic SR-BI expression was analyzed in human and murine NAFLD. In primary human hepatocytes NAFLD relevant factors like inflammatory cytokines, lipopolysaccharide and TGF-β did not affect SR-BI protein. Similarly, oleate and palmitate had no effect. The adipokines chemerin, adiponectin, leptin and omentin did not regulate SR-BI expression. Accordingly, hepatic SR-BI was not changed in human and murine fatty liver and non-alcoholic steatohepatits. SR-BI was higher in type 2 diabetes patients but not in those with hypercholesterolemia. The current study indicates a minor if any role of SR-BI in human and murine NAFLD.

Hepatic scavenger receptor BI protects against polymicrobial-induced sepsis through promoting LPS clearance in mice

Recent studies revealed that scavenger receptor BI (SR-BI or Scarb1) plays a critical protective role in sepsis. However, the mechanisms underlying this protection remain largely unknown. In this study, using Scarb1(I179N) mice, a mouse model specifically deficient in hepatic SR-BI, we report that hepatic SR-BI protects against cecal ligation and puncture (CLP)-induced sepsis as shown by 75% fatality in Scarb1(I179N) mice, but only 21% fatality in C57BL/6J control mice. The increase in fatality in Scarb1(I179N) mice was associated with an exacerbated inflammatory cytokine production. Further study demonstrated that hepatic SR-BI exerts its protection against sepsis through its role in promoting LPS clearance without affecting the inflammatory response in macrophages, the glucocorticoid production in adrenal glands, the leukocyte recruitment to peritoneum or the bacterial clearance in liver. Our findings reveal hepatic SR-BI as a critical protective factor in sepsis and point out that promoting hepatic SR-BI-mediated LPS clearance may provide a therapeutic approach for sepsis.

Chronically active: activation of microglial proteolysis in ageing and neurodegeneration

One of the microglial cell functions is the removal of modified extracellular proteins in the brain. The connection between protein oxidation, proteolysis, and microglial activation is the topic of this review. The effect of various activation agents on microglial cells with regard to changes in substrate uptake, proteolytic capacity and degradation efficiency of different types of oxidized protein materials is reviewed. It is shown that different activation stimuli initiate substrate-specific modulation for uptake and proteolysis, influencing an array of factors including receptor expression, lysosomal pH, and proteasome subunit composition. Age-related alterations in activation and proteolytic capacity in microglial cells are also discussed. In ageing, proteolytic effectiveness is diminished, while microglial cells are chronically activated and lose the oxidative burst ability, possibly supporting a 'vicious circle' of macrophage-induced neurodegeneration.

MCP-1 impacts RCT by repressing ABCA1, ABCG1, and SR-BI through PI3K/Akt posttranslational regulation in HepG2 cells

Monocyte chemoattractant protein-1 (MCP-1) plays crucial roles at multiple stages of atherosclerosis. We hypothesized that MCP-1 might impair the reverse cholesterol transport (RCT) capacity of HepG2 cells by decreasing the cell-surface protein expression of ATP binding cassette A1 (ABCA1), ATP binding cassette G1 (ABCG1), and scavenger receptor class B type I (SR-BI). MCP-1 reduced the total protein and mRNA levels of ABCA1 and SR-BI, but not of ABCG1. MCP-1 decreased the cell-surface protein expression of ABCA1, ABCG1, and SR-BI in dose-dependent and time-dependent manners, as measured using cell-surface biotinylation. We further studied the phosphoinositide 3-kinase (PI3K)/serine/threonine protein kinase Akt pathway in regulating receptor trafficking. Both the translation and transcription of ABCA1, ABCG1, and SR-BI were not found to be regulated by the PI3K/Akt pathway. However, the cell-surface protein expression of ABCA1, ABCG1, and SR-BI could be regulated by PI3K activity, and PI3K activation corrected the MCP-1-induced decreases in the cell-surface protein expression of ABCA1, ABCG1, and SR-BI. Moreover, we found that MCP-1 decreased the lipid uptake by HepG2 cells and the ABCA1-mediated cholesterol efflux to apoA-I, which could be reversed by PI3K activation. Our data suggest that MCP-1 impairs RCT activity in HepG2 cells by a PI3K/Akt-mediated posttranslational regulation of ABCA1, ABCG1, and SR-BI cell-surface expression.

Inflammation modulates human HDL composition and function in vivo

OBJECTIVES

Inflammation may directly impair HDL functions, in particular reverse cholesterol transport (RCT), but limited data support this concept in humans.

METHODS AND RESULTS

We employed low-dose human endotoxemia to assess the effects of inflammation on HDL and RCT-related parameters in vivo. Endotoxemia induced remodelling of HDL with depletion of pre-β1a HDL particles determined by 2-D gel electrophoresis (-32.2±9.3% at 24 h, p<0.05) as well as small (-23.0±5.1%, p<0.01, at 24 h) and medium (-57.6±8.0% at 16 h, p<0.001) HDL estimated by nuclear magnetic resonance (NMR). This was associated with induction of class II secretory phospholipase A2 (~36 fold increase) and suppression of lecithin:cholesterol acyltransferase activity (-20.8±3.4% at 24 h, p<0.01) and cholesterol ester transfer protein mass (-22.2±6.8% at 24 h, p<0.001). The HDL fraction, isolated following endotoxemia, had reduced capacity to efflux cholesterol in vitro from SR-BI and ABCA1, but not ABCG1 transporter cell models.

CONCLUSIONS

These data support the concept that "atherogenic-HDL dysfunction" and impaired RCT occur in human inflammatory syndromes, largely independent of changes in plasma HDL-C and ApoA-I levels.

Autoimmune-mediated reduction of high-density lipoprotein-cholesterol and paraoxonase 1 activity in systemic lupus erythematosus-prone gld mice

OBJECTIVE

To characterize modifications of high-density lipoprotein (HDL) in autoimmune gld mice that may be relevant to premature atherosclerosis in systemic lupus erythematosus, and to assess their relationship to specific aspects of autoimmune disease.

METHODS

HDL cholesterol (HDL-C), apolipoprotein A-I (Apo A-I), paraoxonase 1 (PON1) activity, hepatic gene expression, and HDL biogenesis were measured in aging female gld and wild-type congenic mice. Autoantibodies, lymphoid organs, and cytokines were analyzed by enzyme-linked immunosorbent assay, flow cytometry, and multiplex assay, respectively.

RESULTS

Plasma HDL-C, HDL Apo A-I, and HDL-associated PON1 activity were reduced in aging gld mice in association with the development of autoimmunity, independent of changes in hepatic Apo A-I and PON1 expression or HDL biogenesis. Hepatic induction of the acute-phase reactant serum amyloid A1 resulted in its incorporation into HDL in gld mice. Deletion of the lipid-sensitive receptor G2A in gld mice (G2A-/- gld) attenuated reductions in HDL-C and PON1 activity without altering hepatic Apo A-I and PON1 expression, HDL biogenesis, or levels of acute-phase proinflammatory cytokines. Plasma anti-Apo A-I autoantibodies were elevated in aging gld mice commensurate with detectable increases in Apo A-I immune complexes. Autoantibody levels were lower in aging G2A-/- gld mice compared with gld mice, and anti-Apo A-I autoantibody levels were significantly related to HDL-C concentrations (r=-0.645, P<0.00004) and PON1 activity (r=-0.555, P<0.0007) among autoimmune gld and G2A-/- gld mice.

CONCLUSION

Autoantibodies against Apo A-I contribute to reducing HDL-C and PON1 activity in autoimmune gld mice independently of hepatic HDL biogenesis, suggesting that functional impairment and premature clearance of HDL immune complexes may be principal mechanisms involved.

Scavenger receptors and their potential as therapeutic targets in the treatment of cardiovascular disease

Scavenger receptors act as membrane-bound and soluble proteins that bind to macromolecular complexes and pathogens. This diverse supergroup of proteins mediates binding to modified lipoprotein particles which regulate the initiation and progression of atherosclerotic plaques. In vascular tissues, scavenger receptors are implicated in regulating intracellular signaling, lipid accumulation, foam cell development, and cellular apoptosis or necrosis linked to the pathophysiology of atherosclerosis. One approach is using gene therapy to modulate scavenger receptor function in atherosclerosis. Ectopic expression of membrane-bound scavenger receptors using viral vectors can modify lipid profiles and reduce the incidence of atherosclerosis. Alternatively, expression of soluble scavenger receptors can also block plaque initiation and progression. Inhibition of scavenger receptor expression using a combined gene therapy and RNA interference strategy also holds promise for long-term therapy. Here we review our current understanding of the gene delivery by viral vectors to cells and tissues in gene therapy strategies and its application to the modulation of scavenger receptor function in atherosclerosis.

ADRP/ADFP and Mal1 expression are increased in macrophages treated with TLR agonists

Activation of macrophages by TLR agonists enhances foam cell formation, but the underlying mechanisms are not understood. We examined the effects of TLR agonists on ADRP/ADFP, a protein associated with forming lipid droplets, and Mal1 a fatty acid-binding protein, in two mouse macrophage cell lines and human monocytes. Low doses of LPS, a TLR4 agonist increased both mRNA and protein levels of ADRP/ADFP and Mal1 in RAW 264.7 macrophages. Following pretreatment with Intralipid, fatty acids, or acetyl-LDL to increase triglyceride or cholesterol ester storage, LPS treatment still increased ADRP/ADFP and Mal1 mRNA levels. LPS also induced ADRP/ADFP and Mal1 in J774 macrophages and ADRP/ADFP in human monocytes. Zymosan, a fungal product that activates TLR2, poly-I:C, a viral mimetic that activates TLR3, and imiquimod, a TLR7 agonist, also increased ADRP/ADFP. Zymosan, but not poly-I:C or imiquimod, induced Mal1. In contrast, neither gene was induced by TNFalpha, IL-1beta, IL-6, or interferon-gamma. Thus TLR agonists induce ADRP/ADFP and Mal1, which likely contributes to macrophage triglyceride and cholesterol ester storage leading to foam cell formation.

Inflammation impairs reverse cholesterol transport in vivo

BACKGROUND

Inflammation is proposed to impair reverse cholesterol transport (RCT), a major atheroprotective function of high-density lipoprotein (HDL). The present study presents the first integrated functional evidence that inflammation retards numerous components of RCT.

METHODS AND RESULTS

We used subacute endotoxemia in the rodent macrophage-to-feces RCT model to assess the effects of inflammation on RCT in vivo and performed proof of concept experimental endotoxemia studies in humans. Endotoxemia (3 mg/kg SC) reduced (3)H-cholesterol movement from macrophage to plasma and (3)H-cholesterol associated with HDL fractions. At 48 hours, bile and fecal counts were markedly reduced consistent with downregulation of hepatic expression of ABCG5, ABCG8, and ABCB11 biliary transporters. Low-dose lipopolysaccharide (0.3 mg/kg SC) also reduced bile and fecal counts, as well as expression of biliary transporters, but in the absence of effects on plasma or liver counts. In vitro, lipopolysaccharide impaired (3)H-cholesterol efflux from human macrophages to apolipoprotein A-I and serum coincident with reduced expression of the cholesterol transporter ABCA1. During human (3 ng/kg; n=20) and murine endotoxemia (3 mg/kg SC), ex vivo macrophage cholesterol efflux to acute phase HDL was attenuated.

CONCLUSIONS

We provide the first in vivo evidence that inflammation impairs RCT at multiple steps in the RCT pathway, particularly cholesterol flux through liver to bile and feces. Attenuation of RCT and HDL efflux function, independent of HDL cholesterol levels, may contribute to atherosclerosis in chronic inflammatory states including obesity, metabolic syndrome, and type 2 diabetes.

SR-BI protects against endotoxemia in mice through its roles in glucocorticoid production and hepatic clearance

Septic shock results from an uncontrolled inflammatory response, mediated primarily by LPS. Cholesterol transport plays an important role in the host response to LPS, as LPS is neutralized by lipoproteins and adrenal cholesterol uptake is required for antiinflammatory glucocorticoid synthesis. In this study, we show that scavenger receptor B-I (SR-BI), an HDL receptor that mediates HDL cholesterol ester uptake into cells, is required for the normal antiinflammatory response to LPS-induced endotoxic shock. Despite elevated plasma HDL levels, SR-BI-null mice displayed an uncontrollable inflammatory cytokine response and a markedly higher lethality rate than control mice in response to LPS. In addition, SR-BI-null mice showed a lack of inducible glucocorticoid synthesis in response to LPS, bacterial infection, stress, or ACTH. Glucocorticoid insufficiency in SR-BI-null mice was due to primary adrenal malfunction resulting from deficient cholesterol delivery from HDL. Furthermore, corticosterone supplementation decreased the sensitivity of SR-BI-null mice to LPS. Plasma from control and SR-BI-null mice exhibited a similar ability to neutralize LPS, whereas SR-BI-null mice showed decreased plasma clearance of LPS into the liver and hepatocytes compared with normal mice. We conclude that SR-BI in mice is required for the antiinflammatory response to LPS-induced endotoxic shock, likely through its essential role in facilitating glucocorticoid production and LPS hepatic clearance.

Genetic polymorphisms of tumor necrosis factor-alpha modify the association between dietary polyunsaturated fatty acids and fasting HDL-cholesterol and apo A-I concentrations

BACKGROUND

Heterogeneity in circulating lipid concentrations in response to dietary polyunsaturated fatty acids (PUFAs) may be due, in part, to genetic variations. Tumor necrosis factor-alpha (TNF-alpha) is a proinflammatory cytokine that can induce hyperlipidemia and is known to be modulated by dietary PUFAs.

OBJECTIVE

The objective was to determine whether TNF-alpha genotypes modify the association between dietary PUFA intake and serum lipid concentrations.

DESIGN

The study involved 53 men and 56 women aged 42-75 y with type 2 diabetes. Dietary intakes were assessed with the use of a 3-d food record, and blood samples were collected to determine fasting serum lipids. DNA was isolated from blood for genotyping by polymerase chain reaction-restriction fragment length polymorphism for the TNF-alpha -238G-->A and -308G-->A polymorphisms.

RESULTS

PUFA intake was positively associated with serum HDL cholesterol in carriers of the -238A allele (beta = 0.06 +/- 0.03 mmol/L per 1% of energy from PUFAs; P = 0.03), but negatively associated in those with the -238GG genotype (beta = -0.03 +/- 0.01, P = 0.03) (P = 0.004 for interaction). PUFA intake was inversely associated with HDL cholesterol in carriers of the -308A allele (beta = -0.07 +/- 0.02, P = 0.002), but not in those with the -308GG genotype (beta = 0.02 +/- 0.02, P = 0.13) (P = 0.001 for interaction). A stronger gene x diet interaction was observed when the polymorphisms at the 2 positions (-238/-308) were combined (P = 0.0003). Similar effects were observed for apolipoprotein A-I, but not with other dietary fatty acids and serum lipids.

CONCLUSION

TNF-alpha genotypes modify the relation between dietary PUFA intake and HDL-cholesterol concentrations. These findings suggest that genetic variations affecting inflammation may explain some of the inconsistencies between previous studies relating PUFA intake and circulating HDL.

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

During the acute-phase reaction, SAA (serum amyloid A) replaces apoA-I (apolipoprotein A-I) as the major HDL (high-density lipoprotein)-associated apolipoprotein. A remarkable portion of SAA exists in a lipid-free/lipid-poor form and promotes ABCA1 (ATP-binding cassette transporter A1)-dependent cellular cholesterol efflux. In contrast with lipid-free apoA-I and apoE, lipid-free SAA was recently reported to mobilize SR-BI (scavenger receptor class B, type I)-dependent cellular cholesterol efflux [Van der Westhuyzen, Cai, de Beer and de Beer (2005) J. Biol. Chem. 280, 35890-35895]. This unique property could strongly affect cellular cholesterol mobilization during inflammation. However, in the present study, we show that overexpression of SR-BI in HEK-293 cells (human embryonic kidney cells) (devoid of ABCA1) failed to mobilize cholesterol to lipid-free or lipid-poor SAA. Only reconstituted vesicles containing phospholipids and SAA promoted SR-BI-mediated cholesterol efflux. Cholesterol efflux from HEK-293 and HEK-293[SR-BI] cells to lipid-free and lipid-poor SAA was minimal, while efficient efflux was observed from fibroblasts and CHO cells (Chinese-hamster ovary cells) both expressing functional ABCA1. Overexpression of SR-BI in CHO cells strongly attenuated cholesterol efflux to lipid-free SAA even in the presence of an SR-BI-blocking IgG. This implies that SR-BI attenuates ABCA1-mediated cholesterol efflux in a way that is not dependent on SR-BI-mediated re-uptake of cholesterol. The present in vitro experiments demonstrate that the lipidation status of SAA is a critical factor governing cholesterol acceptor properties of this amphipathic apolipoprotein. In addition, we demonstrate that SAA mediates cellular cholesterol efflux via the ABCA1 and/or SR-BI pathway in a similar way to apoA-I.

Impact of vitamin A on high-density lipoprotein-cholesterol and scavenger receptor class BI in the obese rat

OBJECTIVE

Scavenger receptor class BI (SR-BI), authentic high-density lipoprotein (HDL) receptors expressed in liver, are known to play an important role in HDL-cholesterol (C) metabolism and reverse cholesterol transport. Interestingly, obese rats of WNIN/Ob strain have abnormally elevated levels of serum HDL-C compared with their lean counterparts. Based on the well-established role of SR-B1 in HDL-C metabolism, it was hypothesized that these obese rats may have an underexpression of hepatic SR-B1 receptors. In view of the significant role of vitamin A in energy expenditure and obesity, we also tested whether vitamin A supplementation can correct abnormal HDL-C metabolism.

RESEARCH METHODS AND PROCEDURES

To test this hypothesis, 7-month-old male lean and obese rats of WNIN/Ob strain were divided into two groups; each group was subdivided into two subgroups consisting of six lean and six obese rats and received diets containing either 2.6 or 129 mg vitamin A/kg diet for 2 months.

RESULTS

At the end, obese rats receiving normal levels of vitamin A diet showed high serum HDL-C and lower hepatic SR-BI expression levels compared with lean counterparts. Furthermore, chronic dietary vitamin A supplementation resulted in overexpression of hepatic SR-BI receptors (protein and gene) with concomitant reduction in serum HDL-C levels in obese rats.

DISCUSSION

Thus, our observations highlight the role of vitamin A in reverse cholesterol transport through up-regulation of hepatic SR-BI receptors and, thereby, HDL-C homeostasis in obese rats of WNIN/Ob strain.

Asymmetrical regulation of scavenger receptor class B type I by apical and basolateral stimuli using Caco-2 cells

Cholesterol uptake and the mechanisms that regulate cholesterol translocation from the intestinal lumen into enterocytes remain for the most part unclear. Since scavenger receptor class B type I (SR-BI) has been suggested to play a role in cholesterol absorption, we investigated cellular SR-BI modulation by various potential effectors administered in both apical and basolateral sides of Caco-2 cells. With differentiation, Caco-2 cells increased SR-BI protein expression. Western blot analysis showed the ability of cholesterol and oxysterols in both cell compartments to reduce SR-BI protein expression. Among the n-3, n-6, and n-9 fatty acid families, only eicosapentaenoic acid was able to lower SR-BI protein expression on both sides, whereas apical alpha-linolenic acid decreased SR-BI abundance and basolateral arachidonic acid (AA) raised it. Epidermal growth factor and growth hormone, either in the apical or basolateral medium, diminished SR-BI cellular content, while insulin displayed the same effect only on the basolateral side. In the presence of proinflammatory agents (LPS, TNF-alpha, IFN-gamma), Caco-2 cells exhibited differential behavior. SR-BI was downregulated by lipopolysaccharide on both sides. Finally, WY-14643 fibrate diminished SR-BI protein expression when it was added to the apical medium. Biotinylation studies in response to selected stimuli revealed that regulatory modifications in SR-BI protein expression occurred for the most part at the apical cell surface irrespective of the effector location. Our data indicate that various effectors supplied to the apical and basolateral compartments may impact on SR-BI at the apical membrane, thus suggesting potential regulation of intestinal cholesterol absorption and distribution in various intracellular pools.

Biochemistry and cell biology of mammalian scavenger receptors

Scavenger receptors are integral membrane proteins that bind a wide variety of ligands including modified or oxidised low-density lipoproteins, apoptotic cells and pathogens. Modified low-density lipoprotein accumulation is thought to be an early event in vascular disease and thus scavenger receptor function is critical in this context. The scavenger receptor family has at least eight different subclasses (A-H) which bear little sequence homology to each other but recognize common ligands. Here we review our current understanding of the scavenger receptor subclasses with emphasis on their genetics, protein structure, biochemical properties, membrane trafficking, intracellular signalling and links to disease states. We also highlight emerging areas where scavenger receptors play roles in cell and animal physiology.

Expression of serum amyloid A transcripts in human trophoblast and fetal-derived trophoblast-like choriocarcinoma cells

The placenta comprises a highly specialized trophoblast layer, which arises from the embryo and differentiates during embryonic development to perform specialized functions, e.g., synthesis of pregnancy-associated hormones, growth factors and cytokines. As there is no evidence of maternal acute-phase protein transplacental transfer and trophoblast plays an important role in regulating immune responses at the feto-maternal interface, the expression of acute-phase serum amyloid A (A-SAA) was investigated in human first trimester trophoblast and trophoblast-like JAR and Jeg-3 choriocarcinoma cells. We here show expression of cytokine receptors and cytokine-dependent induction of A-SAA in JAR and Jeg-3 cells. While interleukin-1alpha/beta is a major agonist for A-SAA expression in JAR, tumor necrosis factor-alpha is the predominant agonist in Jeg-3. First trimester trophoblast and JAR/Jeg-3 cells further express the human homolog of SAA-activating factor-1, a transcription factor involved in cytokine-mediated induction of A-SAA genes. A-SAA1 and A-SAA2 transcripts were increased in first trimester trophoblast during pregnancy weeks 10 and 12 suggesting that A-SAA plays a role during early fetal development.

Parotid secretory protein is an HDL-associated protein with anticandidal activity

High-density lipoprotein (HDL) is part of innate immunity, protecting against infection and inflammation. Using a proteomic approach, we identified an amino acid sequence in a hamster HDL protein that showed homology to rat and mouse parotid secretory protein (PSP), a salivary protein secreted from the parotid glands. We cloned the cDNA encoding a putative hamster homolog of rat and mouse PSP. Searches for conserved domains of the protein showed that the COOH terminus of hamster PSP contains a region homologous to the NH2termini of a family of HDL-associated proteins, including LPS-binding protein, cholesteryl ester transfer protein, and phospholipid transfer protein. In mice, PSP was also associated with HDL but was not detected in very-low-density lipoprotein, low-density lipoprotein, or lipoprotein-deficient sera. In addition to salivary glands, we found that PSP mRNA was expressed in lung, testis, and ovary. The level of PSP in HDL was increased after endotoxin injection in hamsters, but not in mice. Recombinant PSP inhibits growth of Candida albicans in culture. In summary, our results showed that PSP is a novel anticandidal protein associated with HDL.

Apolipoproteins A-IV and A-V are acute-phase proteins in mouse HDL

BACKGROUND

Infection and inflammation are associated with atherosclerosis. During infection and inflammation, HDL decreases and there are changes in the levels of several HDL-associated proteins. To identify changes in the protein composition of HDL during infection and inflammation, a proteomic approach was utilized.

METHODS AND RESULTS

Using two-dimensional gel electrophoresis and mass spectrometry, we found the expected increases in apolipoprotein (apo) SAA and apo E, as well as a decrease in apo A-I on HDL isolated from mice injected with endotoxin. We identified apo A-IV and apo A-V as positive acute-phase proteins in mouse HDL. We also found an increase in hepatic mRNA levels of apo A-IV and apo A-V after injection of endotoxin. Interleukin-6 increased apo A-IV and apo A-V mRNA levels in Hep3B cells. Additionally, we demonstrated that the protein levels of apo A-II in acute-phase HDL and the hepatic mRNA levels of apo A-II were decreased.

CONCLUSIONS

Apo A-IV and A-V are positive acute-phase proteins that increase in the serum during inflammation while apo A-II is a negative acute-phase protein in mice. Similar to other positive and negative acute-phase proteins, changes in hepatic production account for the changes in serum levels. However, the changes in apo A-IV and apo A-V, two apolipoproteins whose activities are not fully understood, may serve functions other than regulating lipid metabolism during the acute-phase response (APR). Coupled with the other changes in HDL proteins that occur, these changes are likely to alter the functional properties of HDL perhaps increasing the risk of atherosclerosis.

The role of scavenger receptor class B type I (SR-BI) in lipid trafficking. defining the rules for lipid traders

The scavenger receptor class B type I (SR-BI) is a 509-amino acid, 82 kDa glycoprotein, with two cytoplasmic C- and N-terminal domains separated by a large extracellular domain. The aim of this review is to define the role of SR-BI as a lipoprotein receptor responsible for selective uptake of cholesteryl esters (CE) from high density lipoprotein (HDL) and low density lipoprotein (LDL) and free cholesterol (FC) efflux to lipoprotein acceptors. These activities depend on lipoprotein binding to its extracellular domain and subsequent lipid exchange at the plasma membrane. CE selective uptake supplies cholesterol to liver and steroidogenic tissues, for biliary cholesterol secretion and steroid hormone synthesis. Genetically modified mice have confirmed SR-BI's major role in tissue cholesterol uptake and in reverse cholesterol transport, i.e. cholesterol turnover. Accordingly, cellular cholesterol level, estrogens and trophic hormones regulate SR-BI expression by both transcriptional and post-transcriptional mechanisms. Importantly, mouse SR-BI overexpression has both corrective and preventive effects on atherosclerosis. Human SR-BI has very similar tissue distribution, binding properties and lipid transfer activities compared to rodent SR-BI. However, human plasma has most of its cholesterol in LDL. Thus, there is considerable interest to develop anti-atherogenic strategies involving human SR-BI-mediated increases in reverse cholesterol transport through HDL and/or LDL.

Scavenger receptor expressed by endothelial cells I (SREC-I) mediates the uptake of acetylated low density lipoproteins by macrophages stimulated with lipopolysaccharide

Scavenger receptor expressed by endothelial cells I (SREC-I) is a novel endocytic receptor for acetylated low density lipoprotein (LDL). Here we show that SREC-I is expressed in a wide variety of tissues, including macrophages and aortas. Lipopolysaccharide (LPS) robustly stimulated the expression of SREC-I in macrophages. In an initial attempt to clarify the role of SREC-I in the uptake of modified lipoproteins as well as in the development of atherosclerosis, we generated mice with a targeted disruption of the SREC-I gene by homologous recombination in embryonic stem cells. To exclude the overwhelming effect of the type A scavenger receptor (SR-A) on the uptake of Ac-LDL, we further generated mice lacking both SR-A and SREC-I (SR-A(-/-);SREC-I(-/-)) by cross-breeding and compared the uptake and degradation of Ac-LDL in the isolated macrophages. The contribution of SR-A and SREC-I to the overall degradation of Ac-LDL was 85 and 5%, respectively, in a non-stimulated condition. LPS increased the uptake and degradation of Ac-LDL by 1.8-fold. In this condition, the contribution of SR-A and SREC-I to the overall degradation of Ac-LDL was 90 and 6%, respectively. LPS increased the absolute contribution of SR-A and SREC-I by 1.9- and 2.3-fold, respectively. On the other hand, LPS decreased the absolute contribution of other pathways by 31%. Consistently, LPS did not increase the expression of other members of the scavenger receptor family such as CD36. In conclusion, SREC-I serves as a major endocytic receptor for Ac-LDL in LPS-stimulated macrophages lacking SR-A, suggesting that it has a key role in the development of atherosclerosis in concert with SR-A.

Effects of infection and inflammation on lipid and lipoprotein metabolism: mechanisms and consequences to the host

Infection and inflammation induce the acute-phase response (APR), leading to multiple alterations in lipid and lipoprotein metabolism. Plasma triglyceride levels increase from increased VLDL secretion as a result of adipose tissue lipolysis, increased de novo hepatic fatty acid synthesis, and suppression of fatty acid oxidation. With more severe infection, VLDL clearance decreases secondary to decreased lipoprotein lipase and apolipoprotein E in VLDL. In rodents, hypercholesterolemia occurs attributable to increased hepatic cholesterol synthesis and decreased LDL clearance, conversion of cholesterol to bile acids, and secretion of cholesterol into the bile. Marked alterations in proteins important in HDL metabolism lead to decreased reverse cholesterol transport and increased cholesterol delivery to immune cells. Oxidation of LDL and VLDL increases, whereas HDL becomes a proinflammatory molecule. Lipoproteins become enriched in ceramide, glucosylceramide, and sphingomyelin, enhancing uptake by macrophages. Thus, many of the changes in lipoproteins are proatherogenic. The molecular mechanisms underlying the decrease in many of the proteins during the APR involve coordinated decreases in several nuclear hormone receptors, including peroxisome proliferator-activated receptor, liver X receptor, farnesoid X receptor, and retinoid X receptor. APR-induced alterations initially protect the host from the harmful effects of bacteria, viruses, and parasites. However, if prolonged, these changes in the structure and function of lipoproteins will contribute to atherogenesis.

Endotoxin down-regulates ABCG5 and ABCG8 in mouse liver and ABCA1 and ABCG1 in J774 murine macrophages: differential role of LXR

Several of the ATP binding cassette (ABC) transporters have recently been shown to play important roles in reverse cholesterol transport (RCT) and prevention of atherosclerosis. In the liver, ABCG5 and ABCG8 have been proposed to efflux sterols into the bile for excretion. ABCG5 and ABCG8 also limit absorption of dietary cholesterol and plant sterols in the intestine. In macrophages, ABCA1 and ABCG1 mediate cholesterol removal from these cells to HDL. Many of these ABC transporters are regulated by the liver X receptor (LXR). We have previously shown that endotoxin (lipopolysaccharide) down-regulates LXR in rodent liver. In the present study, we examined the in vivo and in vitro regulation of these ABC transporters by endotoxin. We found that endotoxin significantly decreased mRNA levels of ABCG5 and ABCG8 in the liver, but not in the small intestine. When endotoxin or cytokines (tumor necrosis factor and interleukin-1) were incubated with J774 murine macrophages, the mRNA levels of ABCA1 were decreased. This effect was rapid and sustained, and was associated with a reduction in ABCA1 protein levels. Endotoxin and cytokines also decreased ABCG1 mRNA levels in J774 cells. Although LXR is a positive regulator of ABCA1 and ABCG1, we did not observe a reduction in protein levels of LXR or in binding of nuclear proteins to an LXR response element in J774 cells. The decrease in ABCG5 and ABCG8 levels in the liver as well as a reduction in ABCA1 and ABCG1 in macrophages during the host response to infection and inflammation coupled with other previously described changes in the RCT pathway may aggravate atherosclerosis.

Binding and internalization of lipopolysaccharide by Cla-1, a human orthologue of rodent scavenger receptor B1

Scavenger receptor, class B, type I (SR-BI) mediates selective uptake of high density lipoprotein (HDL) cholesteryl ester. SR-BI recognizes HDL, low density lipoprotein (LDL), exchangeable apolipoproteins, and protein-free lipid vesicles containing negatively charged phospholipids. Lipopolysaccharides (LPS) are highly glycosylated anionic phospholipids contributing to septic shock. Despite significant structural similarities between anionic phospholipids and LPS, the role of SR-BI in LPS uptake is unknown. Cla-1, the human SR-BI orthologue, was determined to be a LPS-binding protein and endocytic receptor mediating the binding and internalization of lipoprotein-free, monomerized LPS. LPS strongly competed with HDL, lipidfree apoA-I and apoA-II for HDL binding to the mouse RAW cells. Stably transfected HeLa cells expressing Cla-1-bound LPS with a Kd of about 16 microg/ml, and had a 3-4-fold increase in binding capacity and LPS uptake. Bodipy-labeled LPS uptake was found to initially accumulate in the plasma membrane and subsequently in a perinuclear region identified predominantly as the Golgi complex. Bodipy-LPS and Alexa-apoA-I had staining that colocalized on the cell surface and intracellularly indicating similar transport mechanisms. When associated with HDL, LPS uptake was increased in Cla-1 overexpressing HeLa cells by 5-10-fold. Cla-1-associated 3H-LPS uptake exceeded 125I-apolipoprotein uptake by 5-fold indicating a selective LPS uptake. Upon interacting with Cla-1 overexpressing HeLa cells, the complex (Bodipy-LPS/Alexa 488 apolipoprotein-labeled HDL) bound and was internalized as a holoparticle. Intracellularly, LPS and apolipoproteins were sorted to different intracellular compartments. With LPS-associated HDL, intracellular LPS co-localized predominantly with transferrin, indicating delivery to an endocytic recycling compartment. Our study reveals a close similarity between Cla-1-mediated selective LPS uptake and the recently described selective lipid sorting by rodent SR-BI. In summary, Cla-1 was found to bind and internalize monomerized and HDL-associated LPS, indicating that Cla-1 may play important role in septic shock by affecting LPS cellular uptake and clearance.

The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues

Because cholesterol is a precursor for the synthesis of steroid hormones, steroidogenic tissues have evolved multiple pathways to ensure adequate supplies of cholesterol. These include synthesis, storage as cholesteryl esters, and import from lipoproteins. In addition to endocytosis via members of the low-density lipoprotein receptor superfamily, steroidogenic cells acquire cholesterol from lipoproteins by selective lipid uptake. This pathway, which does not involve lysosomal degradation of the lipoprotein, is mediated by the scavenger receptor class B type I (SR-BI). SR-BI is highly expressed in steroidogenic cells, where its expression is regulated by various trophic hormones, as well as in the liver. Studies of genetically manipulated strains of mice have established that SR-BI plays a key role in regulating lipoprotein metabolism and cholesterol transport to steroidogenic tissues and to the liver for biliary secretion. In addition, analysis of SR-BI-deficient mice has shown that SR-BI expression is important for alpha-tocopherol and nitric oxide metabolism, as well as normal red blood cell maturation and female fertility. These mouse models have also revealed that SR-BI can protect against atherosclerosis. If SR-BI plays similar physiological and pathophysiological roles in humans, it may be an attractive target for therapeutic intervention in cardiovascular and reproductive diseases.

Scavenger receptor class B, type I on non-malignant and malignant human epithelial cells mediates cholesteryl ester-uptake from high density lipoproteins

Hepatoma cell lines serve as a suitable model to study hepatic clearance of lipoprotein-associated cholesteryl esters (CEs). The present study aimed at investigating holoparticle-association of and selective CE-uptake from human high density lipoprotein subclass 3 (HDL3) by non-malignant adult (Chang-liver) and non-malignant fetal (WRL-68) epithelial cell lines as well as a hepatocellular carcinoma (HUH-7) cell line. Binding properties of 125I-HDL3 at 4 and 37 degrees C were similar for all three cell lines while degradation rates were highest for Chang-liver cells. Calculating the selective uptake of HDL3-associated CEs as the difference between [3H]CE- and 125I-HDL3 cell-association revealed that the selective lipid uptake and holoparticle-association was similar in Chang-liver while in WRL-68 and HUH-7 cells pronounced capacity for lipid tracer uptake in excess of holoparticle uptake was measured. Using RT-PCR, Northern and Western blot analysis, as well as immunocytochemical technique pronounced expression of scavenger receptor class B, type I (SR-BI) but not SR-BII (a splice variant of SR-BI less efficient for selective CE-uptake than SR-BI) could be identified in HUH-7 and WRL-68 cells. A polyclonal antiserum raised against SR-BI significantly decreased cell-association of [3H]CE-HDL3 in HUH-7 and WRL-68. The present findings suggest that the capacity for selective cholesteryl ester-uptake from high density lipoprotein by malignant and normal epithelial cells from the liver depends on expression of the scavenger receptor class B, type I.

Plasma C-reactive protein in subjects with hypo/hyperalphalipoproteinemias

Hypoalphalipoproteinemia (Hypo-A), a lipid disorder characterized by low high-density lipoprotein (HDL)-cholesterol (HDL-C) levels, is frequently associated with an increased risk of suffering future coronary heart disease (CHD). Conversely, hyperalphalipoproteinemia (Hyper-A) is a characterized by high HDL-C concentrations and is possibly associated with longevity and protection against CHD. Whether plasma C-reactive protein (CRP) level, an emerging marker of CHD risk, may be influenced by either extremely low or high HDL-C concentrations is yet to be determined. Plasma levels of lipids and CRP have been measured in 52 middle-aged men and women, clinically free of CHD, including 20 subjects with Hypo-A, 12 with Hyper-A, and 20 healthy normolipemic age-matched controls. CRP levels were the highest in Hypo-A [0.22 mg/dL (interquartile range, 0.15 to 0.44)], the lowest in Hyper-A [0.03 mg/dL (0.02 to 0.07)], and intermediate in the control group [0.10 mg/dL (0.05 to 0.20)]. Differences in plasma CRP concentrations were significant between Hypo-A and the other 2 groups, as well as between Hyper-A and controls. Plasma CRP levels showed a particularly strong correlation with plasma HDL-C concentrations (r = -.66, P <.001). In multivariate models, HDL-C represented the only significant predictor of circulating levels of CRP. In conclusion, in subjects with Hypo-A or Hyper-A, HDL-C levels may account for plasma CRP variations independent of other potential cardiovascular risk factors.

Locus on chromosome 6p linked to elevated HDL cholesterol serum levels and to protection against premature atherosclerosis in a kindred with familial hypercholesterolemia

Heterozygous familial hypercholesterolemia (FH) is a highly atherogenic genetic disorder leading to premature coronary heart disease (CHD), usually before 60 years of age. We studied an extended multigenerational kindred with FH linked to chromosome 1p32 in which atherosclerotic complications were either delayed or prevented in individuals with elevated HDL cholesterol (HDL-C) levels or hyperalphalipoproteinemia (HA). Premature CHD was observed in FH individuals without HA. The study of this family established that the HA trait in the family also followed an autosomal dominant mode of inheritance with a pattern of segregation independent from FH. We identified a locus on chromosome 6 linked to elevated HDL-C levels (HA) in this family. Haplotype analysis refined the localization to a 7.32-cM interval (73 to 80 cM from pter) flanked by markers D6S1280 and D6S1275. Parametric 2-point and multipoint analyses yielded maximum LOD scores of 3.05 and 3.17, respectively. This finding was confirmed with a nonparametric multipoint score of 3.78 (P=0.0009). We propose that this locus, linked to elevated HDL-C levels, confers protection against premature CHD within an FH context.

Scavenger receptor class BI and selective cholesteryl ester uptake: partners in the regulation of steroidogenesis

The steroidogenic tissues have a special requirement for cholesterol, which is used as a substrate for steroid hormone biosynthesis. In many species this cholesterol is obtained from plasma lipoproteins by a unique pathway in which circulating lipoproteins bind to the surface of the steroidogenic cells and contribute their cholesteryl esters to the cells by a 'selective' process in which the whole lipoprotein particle does not enter the cell. This review describes the lipoprotein selective cholesteryl ester uptake process and its specific partnership with the HDL receptor, scavenger receptor class BI (SR-BI). It describes the characteristics of the selective pathway, and the molecular properties, localization, regulation, anchoring sites and potential mechanisms of action of SR-BI in facilitating cholesteryl ester uptake by steroidogenic cells.