Serum amyloid A promotes cholesterol efflux mediated by scavenger receptor B-I

Serum amyloid A (SAA) is an acute phase protein whose expression is markedly up-regulated during inflammation and infection. The physiological function of SAA is unclear. In this study, we reported that SAA promotes cellular cholesterol efflux mediated by scavenger receptor B-I (SR-BI). In Chinese hamster ovary cells, SAA promoted cellular cholesterol efflux in an SR-BI-dependent manner, whereas apoA-I did not. Similarly, SAA, but not apoA-I, promoted cholesterol efflux from HepG2 cells in an SR-BI-dependent manner as shown by using the SR-BI inhibitor BLT-1. When SAA was overexpressed in HepG2 cells using adenovirus-mediated gene transfer, the endogenously expressed SAA promoted SR-BI-dependent efflux. To assess the effect of SAA on SR-BI-mediated efflux to high density lipoprotein (HDL), we compared normal HDL, acute phase HDL (AP-HDL, prepared from mice injected with lipopolysaccharide), and AdSAA-HDL (HDL prepared from mice overexpressing SAA). Both AP-HDL and AdSAA-HDL promoted 2-fold greater cholesterol efflux than normal HDL. Lipid-free SAA was shown to also stimulate ABCA1-dependent cholesterol efflux in fibroblasts, in line with an earlier report (Stonik, J. A., Remaley, A. T., Demosky, S. J., Neufeld, E. B., Bocharov, A., and Brewer, H. B. (2004) Biochem. Biophys. Res. Commun. 321, 936-941). When added to cells together, SAA and HDL exerted a synergistic effect in promoting ABCA1-dependent efflux, suggesting that SAA may remodel HDL in a manner that releases apoA-I or other efficient ABCA1 ligands from HDL. SAA also facilitated efflux by a process that was independent of SR-BI and ABCA1. We conclude that the acute phase protein SAA plays an important role in HDL cholesterol metabolism by promoting cellular cholesterol efflux through a number of different efflux pathways.     

Neutrophil association and degradation of normal and acute-phase high-density lipoprotein 3.

The interaction of normal and acute-phase high-density lipoproteins of the subclass 3 (N-HDL3 and AP-HDL3) with human neutrophils and the accompanying degradation of HDL3 apolipoproteins have been studied in vitro. The chemical composition of normal and acute-phase HDL3 was similar except that serum amyloid A protein (apo-SAA) was a major apolipoprotein in AP-HDL3 (approx. 30% of total apolipoproteins). 125I-labelled AP-HDL3 was degraded 5-10 times faster than 125I-labelled N-HDL3 during incubation with neutrophils or neutrophil-conditioned medium. Apo-SAA, like apolipoprotein A-II (apo-A-II), was more susceptible than apolipoprotein A-I (apo-A-I) to the action of proteases released from the cells. The amounts of cell-associated AP-HDL3 apolipoproteins at saturation were up to 2.8 times greater than N-HDL3 apolipoproteins; while apo-A-I was the major cell-associated apolipoprotein when N-HDL3 was bound, apo-SAA constituted 80% of the apolipoproteins bound in the case of AP-HDL3. The associated intact apo-SAA was mostly surface-bound as it was accessible to the action of exogenous trypsin. alpha 1-Antitrypsin-resistant (alpha 1-AT-resistant) cellular degradation of AP-HDL3 apolipoproteins also occurred; experiments in which pulse-chase labelling was performed or lysosomotropic agents were used indicated that insignificant intracellular degradation occurred which points to the involvement of cell-surface proteases in this degradation.     

Serum amyloid A (SAA)-induced remodeling of CSF-HDL

Inflammation is a risk factor for Alzheimer's disease. Serum amyloid A (SAA) is an acute phase protein that dissociates apolipoprotein AI (apoAI) from plasma HDL. In cerebrospinal fluid (CSF), the SAA concentration is much higher in subjects with Alzheimer's disease than in controls. CSF-HDL is rich in apoE, which plays an important role as a ligand for lipoprotein receptors in the central nervous system (CNS). To clarify whether SAA dissociates apoE from CSF-HDL, we added recombinant SAA to CSF and determined the apoE distribution in the CSF using native two-dimensional gel electrophoresis. We found that SAA dissociated apoE from CSF-HDL in a dose-dependent manner. This effect was more evident in apoE4 carriers than in apoE3 or apoE2 carriers. After a 24-h incubation at 37 degrees C, SAA continuously dissociated apoE from CSF-HDL. Amyloid beta (Abeta) fragments (1-42) were bound to large CSF-HDL but not to apoE dissociated by SAA. In conclusion, SAA dissociates apoE from CSF-HDL. We postulate that inflammation in the CNS may impair Abeta clearance due to the loss of apoE from CSF-HDL.     

LPS induces interleukin-6 and interleukin-8 but not tumor necrosis factor-alpha in human adipocytes

Adipose tissue-derived cytokines are presumably involved in obesity-associated pathologies including type 2 diabetes and atherosclerosis. Here we studied the lipopolysaccharide (LPS)-induced expression dynamics of tumor necrosis factor-alpha (TNFalpha), interleukin-6 (IL-6), IL-8 and IL-10 in human adipose tissue biopsies, in preadipocyte-derived adipocytes, and in mesenchymal stem cell (MSC)-derived adipocytes. TNFalpha, IL-6, IL-8 and IL-10 secretions by adipose tissue explants were increased 5.5-, 19.5-, 3.5- and 12.5-fold, respectively, by LPS (1 microg/mL) administration. Concordantly, IL-6 and IL-8 release was dose-dependently induced in MSC-derived adipocytes by LPS (>10 pg/mL). In contrast, TNFalpha and IL-10 remained undetectable even at the highest LPS dose (1 microg/mL) after 24h. In MSC- and preadipocyte-derived adipocytes, respectively, exposure to LPS evoked a weak and transient induction of TNFalpha mRNA whereas induction of IL-6 and IL-8 mRNA were pronounced and sustained for at least 24h. Basal glucose uptake, lipolysis and IL-6 mRNA were induced by exogenous TNFalpha (10 ng/mL) but not by IL-6 (10 ng/mL), IL-8 (100 ng/mL) and IL-10 (20 ng/mL). In this adipocyte model TNFalpha induces well known metabolic effects, but together with previous reports these data suggest that inflammation-induced TNFalpha may derive from non-adipocyte sources in adipose tissue, likely to be macrophages.     

Lipopolysaccharide binding protein and serum amyloid A secretion by human intestinal epithelial cells during the acute phase response.

The acute phase proteins LPS binding protein (LBP) and serum amyloid A (SAA) are produced by the liver and are present in the circulation. Both proteins have been shown to participate in the immune response to endotoxins. The intestinal mucosa forms a large surface that is continuously exposed to these microbial products. By secretion of antimicrobial and immunomodulating agents, the intestinal epithelium contributes to the defense against bacteria and their products. The aim of this study was to explore the influence of the inflammatory mediators TNF-alpha, IL-6, and IL-1beta on the release of LBP and SAA by intestinal epithelial cells (IEC). In addition, the induction of LBP and SAA release by cell lines of intestinal epithelial cells and hepatic cells was compared. The data obtained show that in addition to liver cells, IEC also expressed LBP mRNA and released bioactive LBP and SAA upon stimulation. Regulation of LBP and SAA release by IEC and hepatocytes was typical for class 1 acute phase proteins, although differences in regulation between the cell types were observed. Endotoxin did not induce LBP and SAA release. Glucocorticoids were demonstrated to strongly enhance the cytokine-induced release of LBP and SAA by IEC, corresponding to hepatocytes. The data from this study, which imply that human IEC can produce LBP and SAA, suggest a role for these proteins in the local defense mechanism of the gut to endotoxin. Furthermore, the results demonstrate that tissues other than the liver are involved in the acute phase response.     

Serum amyloid A generates high density lipoprotein with cellular lipid in an ABCA1- or ABCA7-dependent manner

Serum amyloid A (SAA) is an amphiphilic helical protein that is found associated with plasma HDL in various pathological conditions, such as acute or chronic inflammation. Cellular lipid release and generation of HDL by this protein were investigated, in comparison with the reactions by apolipoprotein A-I (apoA-I) and several types of cells that appear with various specific profiles of cholesterol and phospholipid release. SAA mediated cellular lipid release from these cells with the same profile as apoA-I. Upregulation of cellular ABCA1 protein by liver X receptor/retinoid X receptor agonists resulted in an increase of cellular lipid release by apoA-I and SAA. SAA reacted with the HEK293-derived clones that stably express human ABCA1 (293/2c) or ABCA7 (293/6c) to generate cholesterol-containing HDL in a similar manner to apoA-I. Dibutyryl cyclic AMP and phorbol 12-myristate 13-acetate, which differentiate apoA-I-mediated cellular lipid release between 293/2c and 293/6c, also exhibited the same differential effects on the SAA-mediated reactions. No evidence was found for the ABCA1/ABCA7-independent lipid release by SAA. Characterization of physicochemical properties of the HDL revealed that SAA-generated HDL particles had higher density, larger diameter, and slower electrophoretic mobility than those generated by apoA-I. These results demonstrate that SAA generates cholesterol-containing HDL directly with cellular lipid and that the reaction is mediated by ABCA1 and ABCA7.     

Expression of serum amyloid A protein in the absence of the acute phase response does not reduce HDL cholesterol or apoA-I levels in human apoA-I transgenic mice

Plasma concentrations of high density lipoprotein (HDL) cholesterol and its major apolipoprotein (apo)A-I are significantly decreased in inflammatory states. Plasma levels of the serum amyloid A (SAA) protein increase markedly during the acute phase response and are elevated in many chronic inflammatory states. Because SAA is associated with HDL and has been shown to be capable of displacing apoA-I from HDL in vitro, it is believed that expression of SAA is the primary cause of the reduced HDL cholesterol and apoA-I in inflammatory states. In order to directly test this hypothesis, we constructed recombinant adenoviruses expressing the murine SAA and human SAA1 genes (the major acute phase SAA proteins in both species). These recombinant adenoviruses were injected intravenously into wild-type and human apoA-I transgenic mice and the effects of SAA expression on HDL cholesterol and apoA-I were compared with mice injected with a control adenovirus. Plasma levels of SAA were comparable to those seen in the acute phase response in mice and humans. However, despite high plasma levels of murine or human SAA, no significant changes in HDL cholesterol or apoA-I levels were observed. SAA was found associated with HDL but did not specifically alter the cholesterol or human apoA-I distribution among lipoproteins. In summary, high plasma levels of SAA in the absence of a generalized acute phase response did not result in reduction of HDL cholesterol or apoA-I in mice, suggesting that there are components of the acute phase response other than SAA expression that may directly influence HDL metabolism.     

Impact of serum amyloid A on high density lipoprotein composition and levels

Serum amyloid A (SAA) is an acute-phase protein mainly associated with HDL. To study the role of SAA in mediating changes in HDL composition and metabolism during inflammation, we generated mice in which the two major acute-phase SAA isoforms, SAA1.1 and SAA2.1, were deleted [SAA knockout (SAAKO) mice], and induced an acute phase to compare lipid and apolipoprotein parameters between wild-type (WT) and SAAKO mice. Our data indicate that SAA does not affect apolipoprotein A-I (apoA-I) levels or clearance under steady-state conditions. HDL and plasma triglyceride levels following lipopolysaccharide administration, as well as the decline in liver expression of apoA-I and apoA-II, did not differ between both groups of mice. The expected size increase of WT acute-phase HDL was surprisingly also seen in SAAKO acute-phase HDL despite the absence of SAA. HDLs from both mice showed increased phospholipid and unesterified cholesterol content during the acute phase. We therefore conclude that in the mouse, SAA does not impact HDL levels, apoA-I clearance, or HDL size during the acute phase and that the increased size of acute-phase HDL in mice is associated with an increased content of surface lipids, particularly phospholipids, and not surface proteins. These data need to be transferred to humans with caution due to differences in apoA-I structure and remodeling functions.     

The adipokine SAA3 is induced by interleukin-1beta in mouse adipocytes

Serum amyloid A (SAA) 3 has been characterized as an inflammatory adipocyte-secreted acute-phase reactant. In the current study, regulation of SAA3 by the proinflammatory and insulin resistance-inducing cytokine interleukin (IL)-1beta was determined in 3T3-L1 and brown adipocytes. Interestingly, SAA3 mRNA and protein synthesis were dramatically increased by IL-1beta in a time-dependent fashion with maximal induction after 24 h. Furthermore, IL-1beta significantly induced SAA3 mRNA expression dose-dependently with maximal 36.4-fold upregulation seen at 2 ng/ml effector. Moreover, IL-1beta-induced SAA3 expression was mediated by nuclear factor-kappaB and janus kinase 2. Taken together, our data show a potent upregulation of SAA3 by IL-1beta.     

Effects of serum amyloid A protein (SAA) on composition, size, and density of high density lipoproteins in subjects with myocardial infarction

The acute phase reactant serum amyloid A protein (SAA) circulates in plasma as a constituent of high density lipoproteins (HDL). Advantage has been taken of the induction of SAA in human subjects with myocardial infarction to study the effect of SAA on the physical and chemical properties of HDL. HDL were isolated by sequential ultracentrifugation and assayed for chemical composition. Apolipoprotein composition was assessed by SDS polyacrylamide gel electrophoresis. Size distribution of HDL was determined by gradient gel electrophoresis and density distribution by density gradient ultracentrifugation. In studies of 18 subjects with myocardial infarction, SAA accounted for 8-87% (median 52%) of the HDL apolipoprotein. These SAA-enriched HDL had a density comparable to that of normal HDL subfraction-3 (HDL3). Their chemical composition differed from normal HDL3, however, with a reduced phospholipid (17% vs 24%) and an increased triglyceride (7.7% vs 1.6%) value. When separated by gradient gel electrophoresis, the SAA-enriched HDL were much larger than normal HDL3, having a radius of 4.5-5.3 nm that extended well into the size range of HDL2; particle size correlated with SAA content. This disassociation between particle density and particle size was also observed with the SAA-enriched HDL isolated from a subject with secondary amyloidosis and also with normal HDL that had been enriched with SAA during incubation in vitro. Thus, the presence of high levels of SAA has been found to be associated with phospholipid-depleted particles of a density comparable to HDL3 but a size larger than normal HDL3.     

Induction of inflammatory cytokine release from cultured human monocytes by C-reactive protein.

The human acute phase protein, C-reactive protein (CRP), is capable of specifically binding to and modulating the function of mononuclear phagocytes. To investigate whether CRP can also affect the capacity of these cells to produce inflammatory cytokines, enzyme immunoassays and Western blot techniques were used to quantitate interleukin 1 beta (IL-1 beta), interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-alpha) produced by freshly-isolated normal human monocytes. CRP induced the rapid release of each cytokine, with significantly elevated levels in culture supernatants at 4 hours and maximal levels of TNF-alpha at 8 hours, and of IL-1 beta and IL-6 at 16 hours of culture. The effects of CRP were dose-dependent; greater than 10-fold increases of each cytokine were observed following culture with greater than or equal to 50 micrograms/ml CRP, concentrations which are often found in the presence of moderate to severe inflammation or tissue injury. The induction of cytokine release by CRP was unaffected by inclusion of 25 micrograms/ml polymyxin-B in culture media, but was completely abrogated by prior boiling of the CRP, a procedure which had no effect on induction of monocyte cytokine release by lipopolysaccharide. The dose-dependent induction of inflammatory cytokines by CRP provides further support for the hypothesis that interaction with mononuclear phagocytes constitutes an important biological role for this acute phase protein.     

A cytokine-selective defect in interleukin-1 beta-mediated acute phase gene expression in a subclone of the human hepatoma cell line (HEPG2)

Several well-differentiated human hepatoma cell lines (HepG2, Hep3B) have been used to identify factors which regulate hepatic gene expression during the host response to inflammation/tissue injury (acute phase response). Studies in these cell lines, as well as in primary cultures of rat, rabbit, and mouse hepatocytes, have demonstrated that interleukin-1 beta (IL-1 beta), tumor necrosis factor (TNF-alpha), and interferon-beta 2 (IFN-beta 2) each mediate changes in expression of several hepatic acute phase genes. In this study we identify a subclone of the HepG2 cell line in which there is a selective defect in IL-1 beta-mediated acute phase gene expression. Recombinant human IL-1 beta mediates an increase in synthesis of the positive acute phase complement protein factor B and a decrease in synthesis of negative acute phase protein albumin in the parent uncloned HepG2 cell line (HG2Y), but not in the subclone HG2N. Recombinant human IFN-beta 2 and TNF-alpha, however, regulate acute phase protein synthesis in the subclone HG2N; i.e. IFN-beta 2 and TNF-alpha increase synthesis of factor B and decrease synthesis of albumin in both HG2Y and HG2N cells. Equilibrium binding analysis with 125I-rIL-1 beta at 4 degrees C showed that both HG2N and HG2Y cells bind IL-1 beta specifically and saturably. HG2N and HG2Y possess 3.8 and 4.0 x 10(3) plasma membrane receptors/cell with affinities of 0.96 and 1.07 x 10(-9) M, respectively. Thus, the defect in this subclone of the HepG2 cell line is likely to involve the signal transduction pathway for the biological activity of IL-1 beta and will be useful in elucidation of this signal transduction pathway.     

Purification and characterization of the immunostimulatory properties of recombinant human interleukin-1 beta

In this study we have used a new method for human recombinant IL-1 beta (rIL-1 beta) purification and investigated its immunostimulatory biological activity. The IL-1 beta gene was cloned using a novel mRNA preparation from activated human blood monocytes. The purification protocol consists of extraction and two chromatographic steps using the new Soloza cation exchange resin. The purified protein was characterized electrophoretically, by amino acid analysis and reverse phase chromatography. The protein migrated on SDS-PAGE with a molecular weight of 18.200 but demonstrated the minor presence of aggregates (dimers and trimers). Specific activity of purified rIL-1 beta in comitogenic assay on mouse thymocytes was 10(8) U/mg protein. rIL-1 beta increased in a dose dependent manner proliferation of Con A-stimulated murine thymocytes, splenocytes, PHA-stimulated human peripheral blood lymphocytes and transformed B-cell lines. Comitogenic activity depended on the degree of lymphocyte preactivation and was similar to that of natural human IL-1 beta. rIL-1 beta enhanced IL-2 production by murine spleen cells and EL-4 cell line and IL-2 receptor expression by human peripheral blood mononuclear cells. It induced PGE2 release from human blood monocytes but had no effect on human neutrophil chemotaxis, phagocytosis and respiratory burst.     

Molecular cloning and expression analysis of carp (Cyprinus carpio) interleukin-1 beta, high affinity immunoglobulin E Fc receptor gamma subunit and serum amyloid A

Suppression subtractive hybridisation (SSH) is a powerful means to identify genes of cytokines and other genes that express small amount of mRNA. In this study, cDNA of normal fish (carp) head kidney cells (HKC) was subtracted from pooled cDNA of HKC and peritoneal cell (PC) obtained from fish which had been injected with sodium alginate (SA) and scleroglucan (SG) 3-48 h earlier. This subtraction produced 248 clones of cDNA fragments. After sequencing some of the fragments of interest were used as probes, and yielded full-length cDNAs homologous to mammalian interleukin-1 beta (IL-1 beta), the gamma subunit of high affinity Fc receptor for IgE (Fc epsilon RI gamma) and serum amyloid A (SAA); these were cloned and sequenced. Carp IL-1 beta shows 21.8-24.7% amino acid identities to mammalian mature IL-1 beta, and lacks a signal sequence, which is consistent with mammalian IL-1 beta. Carp Fc epsilon RI gamma, which was the first cloned non-mammalian Fc receptor subunit, shows 39.3-40.4% amino acid identities to mammalian Fc epsilon RI gamma, and contains the immunoreceptor tyrosin-based activation motif characteristic of the signal transduction subunit of antigen- and Fc-receptors. Carp SAA is most similar to mammalian acute phase responsive type SAA with 53.0-55.3% amino acid identities. Both SA-elicited and SG-elicited PC expressed higher amounts of IL-1 beta and SAA mRNA compared to saline-injected fish HKC and PC, indicating that these proteins are associated with inflammatory responses, similar to mammalian homologues. Fc epsilon RI gamma was constitutively expressed in leucocytes and not immunopotentiator-responsive, but this indicates that Fc receptor including Fc epsilon RI gamma subunit is likely functional in the carp immune system.     

Testicular interleukin-6 response to systemic inflammation

Spermatogenesis is a highly controlled process of proliferation, meiosis, and differentiation. Systemic infection and chronic inflammation can impair testicular steroidogenesis and spermatogenesis. In this study, we examined the effect of systemic infection--intraperitoneal (i.p.) injection of lipopolysaccharide (LPS)--on the expression levels of IL-6 in the testis of sexually immature and adult mice. IL-6 levels in testicular homogenates of immature mice were significantly higher than in mature mice (both protein and RNA levels), before and after LPS injection. Injection of LPS (i.p.) into mature mice over 3 hours, significantly increased testicular IL-6 protein and mRNA levels (as demonstrated by ELISA and RT-PCR respectively) compared to the control group. Injection of LPS over 24 hours significantly increased IL-6 mRNA expression, but it did not significantly affect IL-6 protein levels in the homogenates. In contrast, stimulation of immature mice with LPS (2, 20 or 100 microg/mL) over 3 hours or LPS (2 or 20 microg/mL) over 24 hours, significantly increased testicular IL-6 (both protein and mRNA expression). The levels of testicular IL-6 (protein) in the homogenates were not significantly increased after stimulation with 100 microg/mL over 24 hours, but they were significantly increased at the mRNA level. Our results demonstrate, for the first time, the over-expression of IL-6 in testicular homogenates of mature and immature mice following systemic inflammation (i.p. injection of LPS). These results suggest the possibility of the involvement of systemic infection/inflammation, through the elevation of testicular IL-6, in testicular functions, which may affect male fertility. Also, high levels of IL-6 during pathological conditions, could play a role in protecting testicular tissue.     

Activation of AP-1 contributes to the beta-adrenoceptor-mediated myocardial induction of interleukin-6

The induction of proinflammatory cytokines in stressed myocardium is considered an innate immune response, but the role of beta-adrenergic signaling in this proinflammatory response and the mechanisms of cardioprotection by beta-blockers are not fully understood. In the present study, we analyzed interleukin-6 (IL-6) formation and promoter activation in beta-adrenoceptor-stimulated neonatal rat cardiomyocytes, in transgenic mice with cardiac overexpression of beta1-adrenoceptors, and in failing human myocardium. IL-6 formation and release in cultured cardiomyocytes under beta-adrenoceptor stimulation requires the activation of activating protein-1 (AP-1) binding sites and of cAMP response elements (CRE) in the IL-6 promoter, but this release (140 +/- 6 pg/mL medium under 10(-6) M isoproterenol vs. 81 +/- 3 pg/mL unstimulated, P < 0.05) is moderate compared with that under inflammatory stimulation (855 +/- 44 pg/mL, endotoxin 0.1microg/mL). Similarly, IL-6 is induced together with CRE- and AP-1 activation in the left ventricle (LV) of beta1-transgenic mice before the onset of failure. However, we observed IL-6 induction with activation of NF-kappaB in addition to CRE and AP-1 in beta1-transgenic mice at the age of 22 weeks and in explanted human LV after full development of failure. Treatment with beta-blockers lowered myocardial IL-6 as well as AP-1, NF-kappaB, and CRE activation. Therefore, the activation of AP-1 and CRE is part of beta-adrenergic signal transduction for IL-6 induction in nonfailing and failing cardiomyocytes, whereas NF-kappaB activation contributes only in overloaded failing myocardium.     

The Effect In vitro of High-Density Lipoprotein from Healthy and Infected Humans on the Oxidative Metabolism of Polymorphonuclear Leukocytes

We studied the effects in vitro of high-density lipoprotein from healthy (N-HDL) and from infected humans (AP-HDL) on the oxidative metabolism of human polymorphonuclear leukocytes (PMN). Products of the H₂O₂–MPO–halide system were monitored by luminol-enhanced chemiluminescence and superoxide anion formation was monitored by lucigenin-enhanced chemiluminescence during stimulation of human PMN with phorbol myristate acetate (PMA) or an opsonized stimulus (OS). The results showed that N-HDL and AP-HDL affect the oxidative metabolism of PMN in different ways. The posible role of this effect is discussed. © 1997 John Wiley & Sons, Ltd.