Suppression of interleukin-1 beta and LDL scavenger receptor expression in macrophages by a selective protein kinase C inhibitor

Smooth Muscle Cell—Macrophage Interactions Leading to Foam Cell Formation in Atherosclerosis: Location, Location, Location

Cholesterol-overloaded cells or “foam cells” in the artery wall are the biochemical hallmark of atherosclerosis, and are responsible for much of the growth, inflammation and susceptibility to rupture of atherosclerotic lesions. While it has previously been thought that macrophages are the main contributor to the foam cell population, recent evidence indicates arterial smooth muscle cells (SMCs) are the source of the majority of foam cells in both human and murine atherosclerosis. This review outlines the timeline, site of appearance and proximity of SMCs and macrophages with lipids in human and mouse atherosclerosis, and likely interactions between SMCs and macrophages that promote foam cell formation and removal by both cell types. An understanding of these SMC-macrophage interactions in foam cell formation and regression is expected to provide new therapeutic targets to reduce the burden of atherosclerosis for the prevention of coronary heart disease, stroke and peripheral vascular disease.

Induction of tryptophan 2,3-dioxygenase expression in human monocytic leukemia/lymphoma cell lines THP-1 and U937

Tumor-associated macrophages are immune cells with diverse functions in tumor development. Among other functions, they downregulate immune-mediated tumor rejection by depriving lymphocytes of nutrients. The essential amino acid tryptophan is metabolized by the enzymes indoleamine 2,3-dioxygenase 1 and tryptophan 2,3-dioxygenase (TDO). Indoleamine 2,3-dioxygenase 1 is expressed in a large number of human tumors, and inhibitors are in development to improve immunotherapy. Tryptophan 2,3-dioxygenase was also found in human tumors and preclinical working models confirmed its immunosuppressive power. We explored a potential expression of TDO by macrophages. This enzyme could be induced in two human cell lines, THP-1 and U937, by incubation with phorbol myristate acetate, lipopolysaccharide, and interferon gamma. Phorbol-myristate-acetate-mediated induction was inhibited by rottlerin, a protein kinase C inhibitor. In contrast to these monocytic cell lines, other cell lines or fresh human monocytes isolated from peripheral blood mononuclear cells and differentiated into proinflammatory or anti-inflammatory macrophages could not be induced to express TDO. Our results suggest that TDO might play an immunosuppressive role in human monocytic leukemias but not in untransformed macrophages.

Macrophage scavenger receptors and atherosclerosis

The fatty streak is the earliest visible atherosclerotic lesion. Microscopically, this lesion consists of a subendothelial collection of macrophage foam cells that are characterized by their content of numerous lipid droplets. Investigations into the mechanisms responsible for foam cell formation have led to the identification of macrophage scavenger receptors that are capable of recognizing and internalizing modified forms of low-density lipoprotein (LDL). The recent cloning of a macrophage scavenger receptor that is capable of recognizing acetylated and oxidized LDL has permitted the application of molecular approaches to the study of scavenger receptor expression and the determinants of ligand recognition. These studies suggest the possibility that treatment strategies that inhibit LDL modification or influence macrophage gene expression might be effective in the prevention or treatment of atherosclerosis.

Anti-atherosclerotic effects of vitamin E--myth or reality?

Atherosclerosis and its complications such as coronary heart disease, myocardial infarction and stroke are the leading causes of death in the developed world. High blood pressure, diabetes, smoking and a diet high in cholesterol and lipids clearly increase the likelihood of premature atherosclerosis, albeit other factors, such as the individual genetic makeup, may play an additional role. Several epidemiological studies and intervention trials have been performed with vitamin E, and some of them showed that it prevents atherosclerosis. For a long time, vitamin E was assumed to act by decreasing the oxidation of LDL, a key step in atherosclerosis initiation. However, at the cellular level, vitamin E acts by inhibition of smooth muscle cell proliferation, platelet aggregation, monocyte adhesion, oxLDL uptake and cytokine production, all reactions implied in the progression of atherosclerosis. Recent research revealed that these effects are not the result of the antioxidant activity of vitamin E, but rather of precise molecular actions of this compound. It is assumed that specific interactions of vitamin E with enzymes and proteins are at the basis of its non-antioxidant effects. Vitamin E influences the activity of several enzymes (e.g. PKC, PP2A, COX-2, 5-lipooxygenase, nitric oxide synthase, NADPH-oxidase, superoxide dismutase, phopholipase A2) and modulates the expression of genes that are involved in atherosclerosis (e.g. scavenger receptors, integrins, selectins, cytokines, cyclins). These interactions promise to reveal the biological properties of vitamin E and allow designing better strategies for the protection against atherosclerosis progression.

Effect of glucose concentration on foam cell formation in THP-1 cells

We investigated whether a high glucose condition could affect cholesterol ester (CE) synthesis and accumulation of cholesterol in arterial wall cells by using the human monocytic cell line THP-1. After 24-hour PMA treatment, cells were grown in control (200 mg/dl of glucose) or high glucose concentration (400, 600, 800, or 1,600 mg/dl) medium for 6 days. CE synthesis was then investigated in cells incubated with 50 microg/ml of native, glycated, acetylated, or oxidized LDL. Cells grown in 400 mg/dl of glucose showed a significant increase of CE synthesis regardless of whether they were incubated with native, glycated or oxidized LDL, compared with cells grown in 200 mg/dl of glucose. In parallel with the studies of CE synthesis, the intracellular accumulation of CE also increased in cells grown in 400 mg/dl of glucose when incubated with oxidized LDL (50 microg/ml), compared with that in cells grown in 200 mg/dl of glucose. The amount of oxidized LDL associated with cells grown in 400 mg/dl of glucose was markedly higher than that in cells grown in 200 mg/dl of glucose. This suggests that there is an optimal glucose concentration (400 mg/dl) which increases the number of some scavenger receptors (receptors for oxidized LDL) expressed on cells, and might increase and stimulate CE synthesis, resulting in intracellular accumulation of CE in macrophage. A high blood glucose concentration could change the metabolism of arterial wall cells and play an important role in the pathogenesis of vascular complications of diabetes mellitus.

C-type natriuretic peptide is synthesized and secreted from leukemia cell lines, peripheral blood cells, and peritoneal macrophages

OBJECTIVE

C-type natriuretic peptide (CNP) is the third member of the natriuretic peptide family. Cultured endothelial cells secrete CNP, and its secretion rate from the endothelial cells is augmented by lipopolysaccharide, interleukin-1beta, and tumor necrosis factor-alpha, which participate in the pathophysiology of inflammation. In this study, we investigated the regulation of CNP secretion from monocytes and macrophages to estimate its contribution to the progression of inflammation.

MATERIALS AND METHODS

CNP secretion rates from two human leukemia cell lines (THP-1 and HL-60), human peripheral blood lymphocytes, granulocytes, monocytes, monocyte-derived macrophages, and mouse peritoneal macrophages were measured under conditions with or without stimulation. Immunoreactive CNP levels in the culture media of these cells were measured by a specific radioimmunoassay.

RESULTS

The secretion rates of CNP from THP-1 and HL-60 cells were augmented according to the degree of their differentiation into macrophage-like cells under the stimulation with phorbol ester. Peripheral blood monocytes also increased the CNP secretion rate after their differentiation into macrophages. Retinoic acid elicited synergistic effects on the CNP secretion rate from HL-60 cells when administered with lipopolysaccharide, interferon-gamma, interleukin-1beta, tumor necrosis factor-alpha, or phorbol ester. In contrast, the phorbol ester-stimulated CNP secretion rate from THP-1 cells was suppressed with dexamethasone, which inhibits monocyte differentiation into macrophage.

CONCLUSIONS

The secretion rate of CNP from monocytes was shown to be regulated based on the degree of their differentiation. This study provides evidence that the monocyte/macrophage system is one of the sources of CNP, especially under inflammatory conditions.

Vitamin E reduces cholesterol esterification and uptake of acetylated low density lipoprotein in macrophages

The effects of vitamin E on cholesteryl ester (CE) metabolism in J774 cells were examined. Pretreatment of J774 cells with vitamin E at concentrations above 50 microM significantly decreased acetylated low density lipoprotein (LDL)-induced incorporation of [14C]oleate into CE in cells in a dose-dependent manner. This was partly due to vitamin E also significantly inhibiting the uptake of [3H]CE-labeled acetylated LDL by J774 cells. A trend existed toward suppression of acyl-CoA:cholesterol acyltransferase (ACAT) activity in the cell lysate at high vitamin E concentration, but there was no effect on hydrolysis of CE. These data indicate that vitamin E reduces the uptake of modified LDL and suppresses ACAT activity, resulting in less cholesterol esterification in macrophages: a novel mechanism underlying the antiatherogenic properties of vitamin E.

Cholesterol metabolism and efflux in human THP-1 macrophages

This study has investigated in detail factors regulating accumulation, esterification, and mobilization of cholesterol in human THP-1 macrophages. Human THP-1 monocytes were differentiated into macrophages and then cholesterol enriched by exposure to acetylated LDL (AcLDL), together with [3H]free cholesterol (FC). Although THP-1 macrophages accumulated FC and esterified cholesterol (EC), assessed by both mass and radioactivity, cellular EC always demonstrated a much lower specific activity (cpm/ microg) than did cellular FC, and several potential causes of this finding were investigated. Inhibition of acyl-CoA:cholesterol acyltransferase (ACAT) during loading decreased cell [3H]EC by 95+/-1.4% but decreased cell EC mass by only 66.0+/-4.0%, indicating that some intracellular undegraded AcLDL-derived EC was present in these cells. Esterification of [3H]oleate to EC in THP-1 cells loaded with AcLDL was 2.0 nmol x mg-1 x h-1, consistent with previous literature. However, EC, triglyceride, and phospholipid fractions respectively contained 1.0+/-0.07%, 80.0+/-0.5%, and 18.9+/-0.3% of cell [3H]oleate, indicating triglycerides were much more metabolically active than EC. In addition, the mass of triglyceride in THP-1 macrophages exceeded that of EC both before and after exposure to AcLDL. Esterification of nonlipoprotein-derived cholesterol was compared in THP-1 cells and nonhuman Fu5AH, CHO, and RAW macrophage cells. Whereas the nonhuman cell lines all esterified over 30% of 2-hydroxypropyl-beta-cyclodextrin (hp-ss-CD)-delivered cholesterol within 6 hours, THP-1 cells esterified <8.0% of incorporated cholesterol. Kinetics of cholesterol efflux from AcLDL-loaded THP-1 cells were first investigated after loading with only FC, and interactions between efflux and EC hydrolysis were further assessed after loading cells with both EC and FC. Over 24 hours, human apolipoprotein (apo) A-I, apoHDL reconstituted with phosphatidylcholine, and HDL3 respectively removed 46.6+/-3.7%, 61. 3+/-3.4%, and 76.4+/-10.1% of [3H]FC from FC-enriched THP-1 cells. Cholesterol efflux to apoA-I was saturated by 24 hours and was enhanced by using apoA-I-phospholipid instead of pure apoA-I. Kinetic modeling identified that 97% of effluxed FC derived from a slow pool, with a T1/2 ranging from 27.7 hours for HDL to 69.3 hours for apoA-I. Although efflux enhanced net clearance of EC, hydrolysis of EC during concurrent inhibition of ACAT was unaffected by cholesterol efflux. Supplementation of THP-1 cultures with cAMP to stimulate hormone-sensitive lipase did not significantly enhance net hydrolysis of EC or cholesterol efflux. In conclusion, human THP-1 macrophages contain a large and metabolically active pool of triglyceride and a relatively inactive pool of EC. The low specific activity of EC relative to FC is contributed to by reduced esterification of FC, slow hydrolysis of EC, and accumulated lipoprotein EC. The relative inactivity of the EC pool may further contribute to already impaired cholesterol efflux from these cells. Net cholesterol efflux from human macrophages is achieved by pure apoA-I and is substantially further enhanced by the presence of phospholipid in acceptor particles.

Production of adrenomedullin in macrophage cell line and peritoneal macrophage

We demonstrate that adrenomedullin (AM) is produced and secreted from cultured murine monocyte/macrophage cell line (RAW 264.7) as well as mouse peritoneal macrophage. Immunoreactive (IR) AM secreted from RAW 264.7 cells was chromatographically identified to be native AM. To elucidate the regulation mechanism of AM production in macrophage, we examined the effects of various substances inducing differentiation or activation of monocyte/macrophage. Phorbol ester (TPA), retinoic acid (RA), lipopolysaccharide (LPS), and interferon-gamma (IFN-gamma) increased AM production 1.5-7-fold in RAW 264.7 cells in a dose- as well as time-dependent manner. By LPS stimulation, the AM mRNA level in RAW 264.7 cells was augmented up to 7-fold after 14 h incubation. RA exerted a synergistic effect when administered with TPA, LPS, or IFN-gamma, whereas IFN-gamma completely suppressed AM production in RAW 264.7 cells stimulated with LPS. Dexamethasone, hydrocortisone, estradiol, and transforming growth factor-beta dose-dependently suppressed AM production in RAW 264.7 cells. AM production was also investigated in mouse peritoneal macrophage. Primary mouse macrophage secreted IR-AM at a rate similar to that of RAW 264.7 cells, and its production was enhanced 9-fold by LPS stimulation. AM was found to increase basal secretion of tumor necrosis factor alpha (TNF-alpha) from RAW 264.7 cells, whereas AM suppressed the secretion of TNF-alpha and interleukin-6 from that stimulated with LPS. Thus, macrophage should be recognized as one of the major sources of AM circulating in the blood. Especially in cases of sepsis and inflammation, AM production in macrophage is augmented, and the secreted AM is deduced to function as a modulator of cytokine production.

Adrenomedullin production is correlated with differentiation in human leukemia cell lines and peripheral blood monocytes

We demonstrated that adrenomedullin (AM) is produced and secreted from human leukemia cell lines (THP-1 and HL-60) as well as peripheral blood granulocytes, lymphocytes, monocytes and monocyte-derived macrophages. Immunoreactive AM accumulated in the culture media of THP-1 and HL-60 cells increased according to their differentiation into macrophage-like cells. Retinoic acid exerted synergistic effects on AM secretion from THP-1 and HL-60 cells when administered with tumor necrosis factor-alpha, lipopolysaccharide or 12-O-tetradecanoyl phorbol-13-acetate. AM was shown to increase the scavenger receptor activity on THP-1 cells. Thus, monocytes/macrophages should be recognized as sources of AM, and the secreted AM may modulate the function of macrophages.

The macrosialin promoter directs high levels of transcriptional activity in macrophages dependent on combinatorial interactions between PU.1 and c-Jun

Macrosialin is a transmembrane glycoprotein that is highly expressed in macrophages. In the present studies, macrosialin mRNA levels are shown to be markedly up-regulated during macrophage differentiation of bone marrow progenitor cells in response to macrophage colony-stimulating factor and granulocyte-macrophage colony-stimulating factor. To investigate the mechanisms responsible for regulation of macrosialin expression, we have isolated the macrosialin gene and performed an initial analysis of its transcriptional regulatory elements. The macrosialin promoter and 7.0 kilobase pairs of 5'-flanking information direct high levels of reporter gene activity in monocyte/macrophage-like cells, but little or no expression in nonmyeloid cells. This pattern of expression is dependent on regulatory elements located between -7.0 and -2.5 kilobase pairs from the transcriptional start site that exhibit strong enhancer activity in macrophages and repressor activity in nonmyeloid cells. Analysis of the proximal macrosialin promoter indicates that combinatorial interactions between at least four classes of transcriptional activators, including PU.1/Spi-1 and members of the AP-1 family are required for basal promoter function. PU.1/Spi-1 and c-Jun act synergistically to activate the macrosialin promoter in a nonmyeloid cell line, suggesting that combinatorial interactions between these proteins are involved in regulating macrosialin expression during macrophage differentiation.

Receptor associated phosphorylation following monoclonal antibody or synthetic peptide binding to nonspecific cytotoxic cells

We have previously shown that crosslinkage of a receptor protein on catfish nonspecific cytotoxic cells (NCC) with anti-receptor monoclonal antibody or with a synthetic peptide activates cytotoxicity and initiates signalling responses. Receptor linked signalling was associated with the production of increased levels of expression of 50-60 and 20-30 kDa phosphoproteins determined by immunoprecipitation with anti-phosphoserine and anti-phosphotyrosine mabs. These proteins are components of a macromolecular protein complex (>200 kDa) determined by reducing and nonreducing SDS-PAGE. The calcium ionophore A23187 treatment produced the same pattern of phosphoprotein expression as peptide or mab. Maximum phosphoserine expression occurred at 15'-30' post-mab binding. We now show that synthetic peptide or mab treatment initiated the same serine and tyrosine phosphorylation profiles. The PKC specific inhibitor MDL 29,152 produced 50% inhibition of NCC lysis of IM-9 target cells, and completely inhibited serine phosphorylation of peptide activated cells but had no effect on tyrosine phosphorylation of the phosphointermediates. Genistein pretreatment of NCC inhibited cytotoxicity and tyrosine phosphorylation. Sequential immunprecipitation of the phosphointermediate demonstrated that the phosphorylated serine and tyrosine residues were on the same 50-60 kDa protein. These data indicate that both proximal and distal signalling events required for NCC activation may be associated with ATPase phosphorylation.

Regulation of scavenger receptor expression in smooth muscle cells by protein kinase C: a role for oxidative stress

Phorbol esters increase scavenger-receptor mRNA expression and receptor activity in smooth muscle cells (SMCs). Our present results demonstrate that activation of protein kinase C (PKC) mediates this increase in receptor expression. This conclusion is based on the findings that (1) phorbol esters induced translocation of PKC-alpha from the cytosol to the membrane fraction; (2) PKC inhibitors blocked the effect of phorbol esters on receptor expression; (3) diacylglycerol, a physiological PKC agonist, enhanced scavenger-receptor activity; and (4) in cotransfected human SMCs, constitutively active PKC-alpha stimulated the expression of a reporter gene under control of the scavenger-receptor promoter. Phorbol ester treatment of SMCs increased intracellular reactive oxygen, and the increase in receptor activity was reduced 30% by the antioxidant N-acetyl cysteine (NAC), suggesting a role for reactive oxygen in phorbol ester-mediated receptor regulation. Furthermore, direct treatment of SMCs with reactive oxygen species increased scavenger-receptor activity. In rabbit SMCs, 100 micromol/L H2O2 alone slightly increased scavenger-receptor mRNA and protein expression. In combination, 100 micromol/L H2O2 and 10 micromol/L vanadate, which promotes formation of OH and enhances the inhibition of protein tyrosine phosphatase by H2O2, increased scavenger-receptor mRNA expression 25-fold in rabbit SMCs and 8-fold in human SMCs. NAC reduced the effect of H2O2 and vanadate by 93%. The increase in SMC scavenger-receptor expression occurs at the level of gene transcription. Receptor mRNA half-life was unchanged after treatment with either phorbol esters or reactive oxygen (approximately 14.5 hours), and induction by phorbol esters increased SMC scavenger-receptor mRNA transcription, as determined by nuclear run-on assay. Multiple cytokines and growth factors that contribute to the generation of reactive oxygen species are present in atherosclerotic lesions. These factors may all contribute to the upregulation of SMC scavenger-receptor activity and therefore to the formation of smooth muscle foam cells.

Uremic serum enhances scavenger receptor expression and activity in the human monocytic cell line U937

The macrophage scavenger receptor (SR) plays a leading role in atherogenesis, but little is known about the relevance of SR to atherosclerosis in uremia. In this study, the impact of uremic serum on SR expression and activity was examined in the human monocytic cell line U937. The cells were cultured with serum from ten healthy subjects, ten hemodialysis (HD) and ten continuous ambulatory peritoneal dialysis (CAPD) patients. SR mRNA expression was examined using reverse transcriptase-polymerase chain reaction followed by Southern blot. SR protein amount was evaluated by ligand blot. SR activity was analyzed by cellular uptake of fluorescently labeled acetylated low-density lipoprotein using flow cytometry. Uremic serum dose-dependently enhanced SR activity primarily by increasing the amount of receptor protein. Heat-inactivated uremic serum had a stimulatory effect, but ultrafiltrate of uremic serum, which included molecules with a molecular weight less than ten kDa, had no effect. The serum levels of macrophage-colony stimulating factor (M-CSF), an activator of SR, were fourfold higher in uremia and significantly correlated with SR activity in cells treated with uremic serum. Pre-treatment of uremic serum with a neutralizing antibody to M-CSF abolished the effect of uremic serum on SR activity. In conclusion, uremic serum contains a factor(s) that enhances SR expression and activity in U937 cells. Elevated M-CSF in uremic serum could be responsible for this enhancement.

Lovastatin inhibits gene expression of type-I scavenger receptor in THP-1 human macrophages

Lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, inhibits the synthesis of mevalonic acid and is widely used as an anti-atherosclerotic drug. The macrophage scavenger receptor (SCR), a trimeric membrane glycoprotein, is postulated to play a key role in atheroma macrophage foam cell formation. HMG-CoA reductase is involved in the control of the synthesis of glycoproteins and farnesylated proteins, including ras proteins, which are involved in the transcriptional regulation of SCR gene expression. Accordingly, we examined whether lovastatin alters the gene expression of SCRs in THP-1 cell derived human macrophages. Lovastatin (5-15 microM) caused a significant dose-related reduction in steady state levels of type-I SCR mRNA in phorbol 12-myristate 13-acetate (PMA)-treated THP-1 cells. The addition of exogenous mevalonate (1 mM) completely restored the lovastatin-induced decrease of type-I SCR mRNA levels. While the addition of the isoprenoid end-product, isopentenyl adenine (50 microM), had little effect on the type-I SCR mRNA levels in lovastatin treated cells, the addition of isoprenoid farnesol (5 microM) largely restored the lovastatin-induced decrease of type-I SCR mRNA levels. Actinomycin D treatment showed that degradation rates of type-I SCR mRNA did not differ between the THP-1 derived cells with and without lovastatin treatment. Nuclear run-on assays showed that lovastatin markedly decreased the transcription of SCR gene in the cells. These results suggest that lovastatin inhibits the transcription of type-I SCR gene by affecting mevalonate metabolism, possibly through the farnesyl-pyrophosphate related end-product(s) in the THP-1-derived macrophages.

Inhibition of macrophage scavenger receptor activity by tumor necrosis factor-alpha is transcriptionally and post-transcriptionally regulated

Regulation of expression of the scavenger receptor is thought to play a critical role in the accumulation of lipid by macrophages in atherosclerosis. Tumor necrosis factor-alpha (TNF-alpha) has been shown to suppress macrophage scavenger receptor function (van Lenten, B.J., and Fogelman, A.M. (1992) J. Immunol. 148, 112-6). However, the mechanism by which it does so is unknown. We evaluated the mechanism by which TNF-alpha inhibited macrophage scavenger receptor surface expression and binding of acetylated low density lipoprotein (aLDL). Binding of aLDL to phorbol 12-myristate 13-acetate (PMA)-differentiated THP-1 macrophages was suppressed by TNF-alpha in a dose-dependent manner. Inhibition of aLDL binding was paralleled by a reduction of macrophage scavenger receptor protein as detected by the Western blot. TNF-alpha partially decreased macrophage scavenger receptor mRNA steady state levels in PMA-differentiated THP-1 macrophages, a result that was confirmed by reverse transcription-polymerase chain reaction. PMA increased the luciferase activity driven by the macrophage scavenger receptor promoter in the transfected cells, whereas TNF-alpha partially reduced luciferase activity. However, macrophage scavenger receptor mRNA half-life was dramatically reduced in cells treated with TNF-alpha relative to untreated cells. Reduction in macrophage scavenger receptor message in response to TNF-alpha was dependent on new protein synthesis because it was blocked by cycloheximide. These results indicate that TNF-alpha regulates macrophage scavenger receptor expression in PMA-differentiated THP-1 macrophages by transcriptional and post-transcriptional mechanisms but principally by destabilization of macrophage scavenger receptor mRNA.

Enhanced scavenger receptor expression in monocyte-macrophages in dialysis patients

The macrophage scavenger receptor (SR), which has two isoforms named type I and II, plays a leading role in the atherosclerotic process. To elucidate the mechanism of atherosclerosis in dialysis patients, SR expression was studied in monocyte-macrophages from thirteen dialysis patients and eight healthy controls. SR mRNA expression was examined in four hemodialysis patients and four controls matched for age and sex. Monocytes were allowed to differentiate into macrophages in in vitro cultures for seven days and SR type I and II mRNA expression were analyzed with the reverse transcription-polymerase chain reaction and quantitated using radiation densities of Southern blots. Only SR type I expression increased during differentiation and was accelerated by one or two days and enhanced after five days in patients, as compared to controls. To detect SR protein expression, uptake of fluorescently labeled acetylated low-density lipoprotein (Ac-LDL) was evaluated by flow cytometry. The mean fluorescence intensity of labeled cells was significantly higher in hemodialysis and continuous ambulatory peritoneal dialysis patients than in controls, although the number of SR-positive cells remained constant. In conclusion, SR expression is enhanced in macrophages from dialysis patients, probably by up-regulation of type I, which may contribute to the development of atherosclerosis in these patients.

Dietary marine lipids suppress continuous expression of interleukin-1 beta gene transcription

n-3 Polyunsaturated fatty acids abundant in marine lipids suppress certain inflammatory and immune reactions, and dietary marine lipid supplements have antiinflammatory effects in experimental and human autoimmune disease. Previous work by other investigators demonstrated that dietary marine lipid supplements suppressed production of cytokines from stimulated human peripheral blood mononuclear cells ex vivo. The present study further documents the ability of n-3 fatty acids to inhibit cytokine formation, and in part defines the mechanism of the inhibition of production of interleukin-1 beta (IL-1 beta) by dietary n-3 fatty acid. Female BALB/c mice were each fed a fat-free balanced diet to which was added either a refined fish oil (FO) preparation as a source of n-3 fatty acid, or beef tallow (BT), which consisted primarily of saturated and monoenoic fatty acids. After ingesting the experimental diets for periods ranging from 3 to 12 wk. spleen cell preparations were stimulated ex vivo with either lipopolysaccharide (LPS) or phorbol 12-myristate 13-acetate (PMA), and proIL-1 beta mRNA (IL-1 beta mRNA) was measured by northern analysis. Levels of IL-1 beta mRNA in both LPS- and PMA-stimulated cells from BT-fed mice were elevated to a greater extent than in cells from FO-fed mice, at most concentrations of LPS and PMA. Stability of LPS-stimulated mRNA levels after actinomycin D was similar for BT and FO groups, indicating that lower levels of IL-1 mRNA with FO groups was related to suppressed IL-1 gene transcription and not due to accelerated transcript degradation. Nuclear run-on transcription assays revealed a more transient expression of the IL-1 beta gene in LPS-stimulated spleen cells from FO-fed mice compared to cells from BT-fed mice. We conclude that dietary marine lipids reduce transient expression of the IL-1 beta gene in stimulated splenic monocytic cells. Preliminary results from nuclear run-on transcription assays indicate that n-3 fatty acids may not change the initial rate of gene transcription but may promote more rapid shutting down of transcription of this gene after induction than do alternative lipids.

Synergistic effects of growth factors on the regulation of smooth muscle cell scavenger receptor activity

Rabbit smooth muscle cells (SMC) express types I and II scavenger receptors (ScR) that are up-regulated by platelet secretion products. In the current studies we investigated the effect of growth factors secreted by platelets on ScR activity in rabbit and human SMC. Platelet-derived growth factor (PDGF BB) and transforming growth factor beta 1 (TGF-beta 1) at 10 ng/ml increased ScR activity in rabbit SMC (by approximately 4- and 2-fold, respectively) but not in human SMC. Epidermal growth factor (EGF) or insulin-like growth factor I (IGF-I) alone had little effect on SMC ScR activity. The growth factors had synergistic effects on ScR activity and on types I and II ScR mRNA expression. In rabbit SMC, PDGF BB, EGF, and TGF-beta 1 together stimulated ScR activity 12-fold. In human SMC, EGF and TGF-beta 1, together with either IGF-I or PDGF BB, stimulated receptor activity approximately 7-fold. Growth factor-mediated induction of ScR activity in rabbit and human SMC was blocked by the tyrosine kinase inhibitor tyrphostin 47, whereas the induction of ScR activity in rabbit but not human SMC was blocked by the protein kinase C inhibitor MDL.29,152. Studies using neutralizing antibodies demonstrated that TGF-beta 1 is the predominant factor in in vitro preparations of platelet secretory products which regulates ScR activity. The growth factors that act synergistically in regulating ScR activity in vitro are all present in atherosclerotic lesions, where they are produced by macrophages, endothelial cells, SMC, and platelets. The data suggest that these growth factors may regulate ScR activity in SMC in vivo and contribute to foam cell formation.

Immune complexes of LDL induce atherogenic responses in human monocytic cells

The ability of immune complexes of LDL or acetylated LDL (acLDL), together with antibodies to LDL, to induce a proatherogenic phenotype in human monocytic cells has been explored. Treatment of THP-1 monocytic cells or peripheral human monocytes with LDL immune complexes containing intact anti-LDL markedly enhanced the ability of these cells to subsequently bind and take up LDL, whereas aggregated LDL or LDL immune complexes prepared with F(ab')2 fragments of anti-LDL had no significant effect. Activation of THP-1 cells with intact LDL immune complexes also stimulated mRNA expression for the scavenger receptor. Additionally, activation of THP-1 cells with insoluble immune complexes of LDL or LDL stimulated generation of reactive oxygen intermediates that, in turn, could oxidize exogenous LDL. These results indicate that the binding of lipoprotein immune complexes to Fc receptors on monocytic cells activates a series of responses that could accelerate the initiation or progression of atherosclerosis.

Estrogen modulates the inducible expression of platelet-derived growth factor mRNA by monocyte/macrophages

We examined the effects of estrogen, 12-O-tetradecanoylphorbol 13 acetate (TPA), and lipopolysaccharide (LPS) on the gene expression of platelet-derived growth factor (PDGF) by the monocyte/macrophage cell line, THP-1. THP-1 cells were exposed to TPA for 48 or 96 hours to induce differentiation. Some were treated with LPS in the last 3 hours and/or ethinyl estradiol (estrogen) (10(-9) M) in the last 20 hours. Total cellular RNA was isolated and cDNA was synthesized and then coamplified (with an internal control, beta-actin, product size 1126 bp) using polymerase chain reaction (PCR) and a set of primers for PDGF-A (product size 225 bp), PDGF-B (217 bp), or PDGF beta-receptor (PDGF-R) (228 bp). The products were separated on an agarose gel and the ratios of radioactivity incorporated into PDGF PCR products to beta-actin products were used to assess the relative changes in the levels of PDGF mRNA abundance in response to various inducers. TPA induced the expression of PDGF-A mRNA, whereas LPS had no effect. Treatment of TPA-stimulated cells with estrogen caused a 61% and 190% increase in PDGF-A mRNA (p < 0.05) at 48 and 96 hours, respectively. Addition of estrogen to cells treated with both TPA and LPS did not cause any significant change in the amounts of the transcripts. In contrast to PDGF-A mRNA, attempts to visualize and estimate PDGF-B and PDGF-R mRNA were unsuccessful. This was probably due to low levels of these transcripts in THP-1 cells. The results indicate that estrogen modulates PDGF-A gene expression by monocyte/macrophages and suggest that estrogen may influence atherogenesis at the vascular level.

Cytokine and growth factor regulation of macrophage scavenger receptor expression and function

Regulation of macrophage scavenger receptor (MSR) activity may be an important determinant of the extent of atherogenesis and the efficacy of host defense. The effect of M-CSF on this pathway was studied using a recently developed monoclonal antibody to murine MSR. M-CSF markedly and selectively increased MSR synthesis in murine macrophages (M phi); post-translationally the receptor appeared more stable and shifted to a predominantly surface distribution. Functionally M-CSF enhanced modified lipoprotein uptake and increased divalent cation-independent adhesion in vitro. These results suggest a plausible mechanism whereby M-CSF production in the atheromatous plaque microenvironment could promote the recruitment and retention of mononuclear phagocytes and subsequent foam cell formation. In addition, the Th1 cytokine (gamma-interferon) and Th2 cytokine (interleukin-4) had differential effects on MSR glycosylation in vitro suggesting a further possible regulatory role by these lymphokines on macrophage MSR function.