Regulated expression of the human acetylated low density lipoprotein receptor gene and isolation of promoter sequences

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.

Identification of a novel monocytic phenotype in Classic Hodgkin Lymphoma tumor microenvironment

Classic Hodgkin lymphoma (CHL) characteristically shows few malignant cells in a microenvironment comprised of mixed inflammatory cells. Although CHL is associated with a high cure rate, recent studies have associated poor prognosis with absolute monocyte count in peripheral blood and increased monocyte/macrophages in involved lymph nodes. Thus, the role of monocytic infiltration and macrophage differentiation in the tumor microenvironment of CHL may be more relevant than absolute macrophage numbers to defining prognosis in CHL patients and potentially have therapeutic implications. Most studies identify tumor-associated macrophages (TAMs) using markers (e.g., CD68) expressed by macrophages and other mononuclear phagocytes, such as monocytes. In contrast, Class A Scavenger Receptor (SR-A/CD204) is expressed by tissue macrophages but not monocytic precursors. In this study, we examined SR-A expression in CHL (n = 43), and compared its expression with that of other macrophage markers. We confirmed a high prevalence of mononuclear cells that stained with CD68, CD163, and CD14 in CHL lymph nodes. However, SR-A protein expression determined by immunohistochemistry was limited to macrophages localized in sclerotic bands characteristic of nodular sclerosis CHL. In contrast, SR-A protein was readily detectable in lymph nodes with metastatic tumor, extra-nodal CHL, T cell/histiocyte-rich large B cell lymphoma, and resident macrophages in non-malignant tissues, including spleen, lymph node, liver and lung. The results of SR-A protein expression paralleled the expression of SR-A mRNA determined by quantitative RT-PCR. These data provide evidence that tumor-infiltrating monocyte/macrophages in CHL have a unique phenotype that likely depends on the microenvironment of nodal CHL.

Zerumbone Suppresses Phorbol Ester-Induced Expression of Multiple Scavenger Receptor Genes in THP-1 Human Monocytic Cells

Unregulated uptake of oxidized low-density lipoproteins (ox-LDL) via macrophage scavenger receptors (SRs), such as lectin-like ox-LDL receptor-1 (LOX-1), is a key event in atherosclerosis. In the present study, we used differentiated Caco-2 cells as a model of the human small intestine to evaluate the suppressive effects of 16 traditional food items selected from Okinawa on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced LOX-1 mRNA expression in THP-1 human monocyte-like cells. Three Zingiberaceae plants, Curcuma aromatica Salisbury, Curcuma longa L., and Zingiber zerumbet Smith, markedly suppressed that expression. When added to the apical sides of Caco-2 monolayers, zerumbone, a sesquiterpene from Z. zerumbet Smith, was found to permeate into the basolateral medium as an intact structure in a time-dependent manner. alpha-Humulene, a structural analog of zerumbone lacking the alpha,beta-unsaturated carbonyl group, did not suppress LOX-1 mRNA expression, indicating that its electrophilic moiety might play pivotal roles in its activities. Further, zerumbone attenuated the expression of SR-A, SR-PSOX, and CD36, but not that of CD68 or CLA-1, leading to a blockade of DiI-acLDL uptake, while it also inhibited the transcriptional activities of activator protein-1 and nuclear factor-kappaB. Together, our results indicate that zerumbone is a potential phytochemical for regulating atherosclerosis with reasonable action mechanisms.

SR-A ligand and M-CSF dynamically regulate SR-A expression and function in primary macrophages via p38 MAPK activation

BACKGROUND

Inflammation is characterized by dynamic changes in the expression of cytokines, such as M-CSF, and modifications of lipids and proteins that result in the formation of ligands for Class A Scavenger Receptors (SR-A). These changes are associated with altered SR-A expression in macrophages; however, the intracellular signal pathways involved and the extent to which SR-A ligands regulate SR-A expression are not well defined. To address these questions, SR-A expression and function were examined in resident mouse peritoneal macrophages incubated with M-CSF or the selective SR-A ligand acetylated-LDL (AcLDL).

RESULTS

M-CSF increased SR-A expression and function, and required the specific activation of p38 MAPK, but not ERK1/2 or JNK. Increased SR-A expression and function returned to basal levels 72 hours after removing M-CSF. We next determined whether prolonged incubation of macrophages with SR-A ligand alters SR-A expression. In contrast to most receptors, which are down-regulated by chronic exposure to ligand, SR-A expression was reversibly increased by incubating macrophages with AcLDL. AcLDL activated p38 in wild-type macrophages but not in SR-A-/- macrophages, and p38 activation was specifically required for AcLDL-induced SR-A expression.

CONCLUSIONS

These results demonstrate that in resident macrophages SR-A expression and function can be dynamically regulated by changes in the macrophage microenvironment that are typical of inflammatory processes. In particular, our results indicate a previously unrecognized role for ligand binding to SR-A in up-regulating SR-A expression and activating p38 MAPK. In this way, SR-A may modulate inflammatory responses by enhancing macrophage uptake of modified protein/lipid, bacteria, and cell debris; and by regulating the production of inflammatory cytokines, growth factors, and proteolytic enzymes.

Human protein S inhibits the uptake of AcLDL and expression of SR-A through Mer receptor tyrosine kinase in human macrophages

Human protein S is an anticoagulation protein. However, it is unknown whether protein S could regulate the expression and function of macrophage scavenger receptor A (SR-A) in macrophages. Human THP-1 monocytes and peripheral blood monocytes were differentiated into macrophages and then treated with physiological concentrations of human protein S. We found that protein S significantly reduced acetylated low-density lipoprotein (AcLDL) uptake and binding by macrophages and decreased the intracellular cholesteryl ester content. Protein S suppressed the expression of the SR-A at both mRNA and protein levels. Protein S reduced the SR-A promoter activity primarily through inhibition in the binding of transcription factors to the AP-1 promoter element in macrophages. Furthermore, human protein S could bind and induce phosphorylation of Mer receptor tyrosine kinase (Mer RTK). Soluble Mer protein or tyrosine kinase inhibitor herbimycin A effectively blocked the effects of protein S on AcLDL uptake. Immunohistochemical analysis revealed that the level of protein S was substantially increased in human atherosclerotic arteries. Thus, human protein S can inhibit the expression and activity of SR-A through Mer RTK in macrophages, suggesting that human protein S is a modulator for macrophage functions in uptaking of modified lipoproteins.

Nobiletin, a citrus flavonoid, suppresses phorbol ester-induced expression of multiple scavenger receptor genes in THP-1 human monocytic cells

Unregulated uptake of oxidized low-density lipoproteins (ox-LDL) via macrophage scavenger receptors (SRs) such as lectin-like ox-LDL receptor-1 (LOX-1) is a key event in atherosclerosis. In this study, we examined the effects of five selected food phytochemicals on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced LOX-1 mRNA expression in THP-1 human monocyte-like cells. Nobiletin, a citrus polymethoxylated flavone, markedly reduced it in dose- and time-dependent manners. It also suppressed the phosphorylation of extracellular signal-regulated protein kinase (ERK) 1/2, c-Jun NH2-terminal kinase (JNK) 1/2, and c-Jun (Ser-63), thereby inhibiting the transcriptional activity of activator protein-1. Further nobiletin attenuated expression of SR-A, SR-PSOX, CD36, and CD68, but not CLA-1, mRNA, leading to the blockade of DiI-acLDL uptake. Together, our results suggest that nobiletin is a promising phytochemical for regulating atherosclerosis with reasonable action mechanisms.

Immunology and Endometriosis

PROBLEM

Accumulating data suggests that aberrant immune responses during retrograde menstruation may be involved in the development of endometriosis.

METHOD OF STUDY

The role of immunology in the etiology of endometriosis is reviewed and summarized from the available literature.

RESULTS

Immunologic factors may affect a woman's susceptibility to implantation of exfoliated endometrial cells. Immune alterations include increased number and activation of peritoneal macrophages, decreased T cell reactivity and natural killer cell cytotoxicity, increased circulating antibodies, and changes in the cytokine network.

CONCLUSION

There is substantial evidence that immunologic factors play a role in the pathogenesis of endometriosis and endometriosis-associated infertility. Decreased natural killer cell cytotoxicity leads to an increased likelihood of implantation of endometriotic tissue. In addition, macrophages and a complex network of locally produced cytokines modulate the growth and inflammatory behavior of ectopic endometrial implants.

A role for class A scavenger receptor in dendritic cell nibbling from live cells

Monocyte-derived dendritic cells (DC) possess the unique capacity to capture Ag from live cells through intimate cell contact, a process referred to as nibbling. We sought to define the receptor(s) mediating DC nibbling. Uptake of fluorescently labeled plasma membrane from live cells by DC was inhibited by protease treatment and by a panel of polyanionic ligands, implicating scavenger receptors (SR) in this process. Differential expression of SR on DC and macrophages correlated with the capacity to acquire membrane from live cells. Internalized membrane colocalized with SR ligand and entered the endosomal pathway. DC very efficiently acquired and internalized gp100 tumor Ag expressed at the surface of viable adenocarcinoma cells via recombinant adenoviral infection. Cross-presentation of gp100 by DC to MHC class I-restricted T cells was inhibited by polyanionic SR ligand and an Ab to type A SR (SR-A), whereas Ab to the class B SR CD36, which mediates uptake of apoptotic cells, induced no inhibition. DC capture of fluorescently labeled membrane from live cells was partially blocked by SR-A-specific Ab, suggesting that other SR may also be contributing to nibbling. DC maturation resulted in a switch in expression from type II SR-A (SR-AII) to the SR-AI splice variant. Finally, SR-A was identified on interdigitating DC isolated from monkey lymph nodes. These findings define a novel role for SR-A, and suggest that Ag uptake from live cells by DC may be important in the generation of immunity and in the maintenance of peripheral tolerance in vivo.

The many roles of the class A macrophage scavenger receptor

The class A macrophage scavenger receptor (SR-A) is the prototypic example of a group of plasma membrane receptors collectively known as scavenger receptors. SR-A displays the ability to bind and endocytose large quantities of modified lipoprotein. Hence, it is thought to be one of the main receptors involved in mediating lipid influx into macrophages (Mphi), which promotes their conversion into foam cells that are abundant in the atherosclerotic lesion. However, as a result of increased interest and research effort and through the development of specific reagents and animal models, it is now appreciated to be multifunctional. These roles include Mphi growth and maintenance, adhesion to the substratum, cell-cell interactions, phagocytosis, and host defense. In this review, we attempt to summarize the evidence and argue that these kinds of activities underlie the biological versatility of Mphi.

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.

Dual promoter structure of mouse and human fatty acid translocase/CD36 genes and unique transcriptional activation by peroxisome proliferator-activated receptor alpha and gamma ligands

Fatty acid translocase (FAT)/CD36 is a glycoprotein involved in multiple membrane functions including uptake of long-chain fatty acids and oxidized low density lipoprotein. In mice, expression of the gene is regulated by peroxisome proliferator-activated receptor (PPAR) alpha in the liver and by PPAR gamma in the adipose tissues (Motojima, K., Passilly, P. P., Peters, J. M., Gonzalez, F. J., and Latruffe, N. (1998) J. Biol. Chem. 273, 16710-16714). However, the time course of PPAR alpha ligand-induced expression of FAT/CD36 in the liver, and also in the cultured hepatoma cells, is significantly slower than those of other PPAR alpha target genes. To study the molecular mechanism of the slow transcriptional activation of the gene by a PPAR ligand, we first cloned the 5' ends of the mRNA and then the mouse gene promoter region from a genomic bacterial artificial chromosome library. Sequencing analyses showed that transcription of the gene starts at two initiation sites 16 kb apart and splicing occurs alternatively, producing at least three mRNA species with different 5'-noncoding regions. The PPAR alpha ligand-responsive promoter in the liver was identified as the new upstream promoter where we found several possible binding sites for lipid metabolism-related transcriptional factors but not for PPAR. Neither promoter responded to a PPAR alpha ligand in the in vitro or in vivo reporter assays using cultured hepatoma cells and the liver of living mice. We also have cloned the human FAT/CD36 gene from a bacterial artificial chromosome library and identified a new independent promoter that is located 13 kb upstream of the previously reported promoter. Only the upstream promoter responded to PPAR alpha and PPAR gamma ligands in a cell type-specific manner. The absence of PPRE in the responding upstream promoter region, the delayed activation by the ligand, and the results of the reporter assays all suggested that transcriptional activation of the FAT/CD36 gene by PPAR ligands is indirectly dependent on PPAR.

Macrophages, oxidation, and endometriosis

Retrograde menstruation has been suggested to be the cause for the presence of endometrial cells in the peritoneal cavity. However, little is known about the events that lead to the adhesion and growth of these cells that ultimately result in endometriosis, considering the fact that the disease occurs only in certain women despite the common occurrence of retrograde menstruation in most women. We postulate that, in normal women, the endometrial cells and tissue that arrive in the peritoneal cavity during menstruation are effectively removed by macrophages that are chemoattracted and become resident tissue macrophages in the peritoneal cavity. In contrast, the peritoneal macrophages in women with endometriosis are nonadherent and ineffectively scavenged, resulting in the sustained presence and growth of the endometrial cells. We also postulate that the peritoneal fluid is not a passive reservoir of the factors secreted by cells of the peritoneum, but actively promotes endometriosis. The peritoneal fluid is rich in lipoproteins, particularly low-density lipoprotein, which generates oxidized lipid components in a macrophage-rich inflammatory milieu. The oxidants exacerbate the growth of endometriosis by inducing chemoattractants such as MCP-1 and endometrial cell growth-promoting activity. We provide evidence for the presence of oxidative milieu in the peritoneal cavity of women with endometriosis, the nonscavenging properties of macrophages that are nonadherent, and the synergistic interaction between macrophages, oxidative stress, and the endometrial cells. For example, the peritoneal fluid lipoproteins of subjects with endometriosis have increased the propensity to undergo oxidation as compared with plasma lipoproteins, and the subjects also have increased titer of autoantibodies to oxidatively modified proteins. If the oxidative proinflammatory nature of the peritoneal fluid is an important mediator of endometriosis growth, anti-inflammatory agents and antioxidants might afford protection against endometriosis.

Regulation and modulation of abnormal immune responses in endometriosis

There is ample evidence demonstrating that endometriosis is accompanied by inflammatory reactions in the peritoneum, resulting in abnormal levels of a variety of cytokines and chemokines in the peritoneal fluid. Among the immunological parameters that have been shown to be altered in the peritoneal cavity of women with endometriosis, an increase in the number of activated nonadherent macrophages that show reduced surface expression of scavenger receptors has been observed. The cause-and-effect relationship between aberrant peritoneal macrophage activity and endometriosis is still unknown. We have demonstrated that steroid hormone receptor agonists and antagonists [e.g., retinoids, antiglucocorticoids, ligands to peroxisome proliferator activated receptors (PPARs)] can regulate macrophage functions in ways that could either suppress or stimulate the growth of ectopic endometrial lesions. Our studies include a number of relevant findings: (1) RU486, acting as an antioxidant, can suppress activation of NFkappaB, a nuclear transcription factor that affects the expression of several inflammatory genes such as those for MCP-1, GM-CSF, CSF-1, and various adhesion molecules; (2) IL-6 secretion from a variety of cell types including endometrial cells is inhibited by retinoic acid; and (3) retinoids and PPARgamma ligands can upregulate the expression of scavenger receptors in cells of the monocyte/macrophage lineage. These observations, combined with the possibility that macrophage activity may play a fundamental role in endometriosis, suggest that pharmacologic manipulation of macrophage function may provide a novel mechanism for treating this disease.

Ligands of macrophage scavenger receptor induce cytokine expression via differential modulation of protein kinase signaling pathways

Our previous works demonstrated that ligands of macrophage scavenger receptor (MSR) induce protein kinases (PKs) including protein-tyrosine kinase (PTK) and up-regulate urokinase-type plasminogen activator expression (Hsu, H. Y., Hajjar, D. P., Khan, K. M., and Falcone, D. J. (1998) J. Biol. Chem. 273, 1240--1246). To continue to investigate MSR ligand-mediated signal transductions, we focus on ligands, oxidized low density lipoprotein (OxLDL), and fucoidan induction of the cytokines tumor necrosis factor-alpha (TNF) and interleukin 1 beta (IL-1). In brief, in murine macrophages J774A.1, OxLDL and fucoidan up-regulate TNF production; additionally, fucoidan but not OxLDL induces IL-1 secretion, prointerleukin 1 (proIL-1, precursor of IL-1) protein, and proIL-1 message. Simultaneously, fucoidan stimulates activity of interleukin 1-converting enzyme. We further investigate the molecular mechanism by which ligand binding-induced PK-mediated mitogen-activated protein kinase (MAPK) in regulation of expression of proIL-1 and IL-1. Specifically, fucoidan stimulates activity of p21-activated kinase (PAK) and of the MAPKs extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38. Combined with PK inhibitors and genetic mutants of Rac1 and JNK in PK activity assays, Western blotting analyses, and IL-1 enzyme-linked immunosorbent assay, the role of individual PKs in the regulation of proIL-1/IL-1 was extensively dissected. Moreover, tyrosine phosphorylation of pp60Src as well as association between pp60Src and Hsp90 play important roles in fucoidan-induced proIL-1 expression. We are the first to establish two fucoidan-mediated signaling pathways: PTK(Src)/Rac1/PAK/JNK and PTK(Src)/Rac1/PAK/p38, but not PTK/phospholipase C-gamma 1/PKC/MEK1/ERK, playing critical roles in proIL-1/IL-1 regulation. Our current results indicate and suggest a model for MSR ligands differentially modulating specific PK signal transduction pathways, which regulate atherogenesis-related inflammatory cytokines TNF and IL-1.

Tumor necrosis factor-alpha -mediated protein kinases in regulation of scavenger receptor and foam cell formation on macrophage

We previously reported tumor necrosis factor-alpha (TNF) modulates transcriptional and post-transcriptional down-regulation of macrophage scavenger receptor (MSR) (Hsu, H. Y., Nicholson, A. C., and Hajjar, D. P. (1996) J. Biol. Chem. 271, 7767-7773); however, TNF-mediated signaling mechanisms are unknown. Here, we demonstrate that ligation of TNF receptor stimulates activity of p21-activated protein kinase (PAK) and mitogen-activated protein kinases (MAPK) as follows: ERK, JNK, and p38 in murine macrophage J774A.1 cells. Upon activation of protein kinases (PK), TNF rapidly increases MSR message and protein; later it markedly reduces MSR expression. Studies using PK inhibitors and dominant negative constructs demonstrate phosphatidylinositol 3-kinase/Rac1/PAK/JNK and phosphatidylinositol 3-kinase/Rac1/PAK/p38 pathways contribute to important roles in the late stage of TNF down-regulation of MSR expression and taking up of OxLDL. Alternatively, the PKC/MEK1/ERK pathway in the early stage plays a significant role in up-regulation of the MSR gene. By using anti-TNF-R1 agonist antibody, we further confirm TNF-R1-mediated MAPK in regulation of MSR. Furthermore, in MSR gene promoter-driven luciferase reporter assays with TNF, PKC activator increases, but antioxidant N-acetylcysteine, PK inhibitors, and dominant negative constructs decrease luciferase activity in MSR gene promoter-transfected cells. Our current results show the first evidence of crucial roles for TNF-mediated MAPK pathways in the transcriptional regulation of MSR gene and increase MSR expression; in contrast, with TNF longer treatment the pathways down-regulate MSR and foam cell formation probably via post-transcriptional process.

Lipopolysaccharide induces scavenger receptor A expression in mouse macrophages: a divergent response relative to human THP-1 monocyte/macrophages

Gene deletion studies indicate that the macrophage scavenger receptor A (SR-A) protects mice from LPS-induced endotoxemia. Paradoxically, cultured human monocyte-derived macrophages down-regulate SR-A expression when exposed to LPS. We found that human THP-1 monocyte/macrophages decrease SR-A expression in response to LPS independent of their differentiation status. In contrast, primary and elicited mouse peritoneal macrophages as well as the J774A.1 and RAW264.7 mouse macrophage lines increase SR-A expression in response to LPS. Exposure to LPS caused J774A.1 and RAW264.7 cells to increase SR-A transcripts by 3- and 5-fold, respectively. LPS caused a concomitant 3-fold increase in SR-A protein levels and increased cell membrane expression of the receptor. RAW264.7 cells increased SR-A transcript levels in response to LPS at concentrations as low as 1 ng/ml, and the response was saturated at 10 ng/ml. The LPS induction of SR-A transcripts required continual protein synthesis and began at 8 h, peaked by 16 h, and persisted for at least 48 h. LPS induction did not increase SR-A gene transcription or affect alternative transcript splicing, but mildly increased mature transcript stability and proceeded in the presence of actinomycin D. Finally, treatment of RAW264.7 cells with TNF-alpha did not induce SR-A transcript levels, indicating that a TNF-alpha autocrine/paracrine signaling mechanism alone is not sufficient to recapitulate the LPS induction of SR-A transcripts. The induction of SR-A expression by LPS-stimulated mouse macrophages is the opposite of the down-regulation of SR-A reported in human monocyte-derived macrophages and may have implications for the observed resistance mice show toward endotoxemia.

Scavenger receptor regulation and atherosclerosis

Atherosclerosis and its complications, such as coronary heart disease, heart 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. During atherosclerosis, uncontrolled cholesterol and lipid accumulation in macrophages and smooth muscle cells leads to foam cell formation and to the progression of the atherosclerotic plaque. This review will focus on foam cell formation within the atherosclerotic lesion, the involvement of the scavenger receptor genes in this process, and the possibility to interfere with scavenger receptor function to reduce the progression of atherosclerosis. To date, the regulatory mechanisms for the expression of scavenger receptor genes and their role in atherosclerosis are not well characterized. Knowledge on this subject could lead to a better understanding of the process, prevention and therapy of this disease.

Structure and function of type I and II macrophage scavenger receptors

Type I and II macrophage scavenger receptors are implicated in the pathologic deposition of cholesterol during the atherogenesis. There is a charged collagen structure of type I and II receptors identified as a ligand binding domain, which can recognize a wide range of negatively charged macromolecules including oxidized LDL as well as damaged or apoptotic cells and pathogenic micro-organisms. After binding these ligands can be either internalized by endocytosis, phagocytosis, or remain at cell surface and mediate the adhesion. Under physiological condition, scavenger receptors serve to scavenge or clean up cellular debris and other related materials, as well as playing a role in the hosts defence. In pathological condition, they mediate the recruitment, activation and transformation of macrophages and other cells, which may be related to the development of atherosclerosis and to disorders caused by the accumulation of denatured materials, such as Alzheimer's disease.

In Vivo Priming by DNA Injection Occurs Predominantly by Antigen Transfer

DNA vaccines can stimulate both humoral and cytolytic immune responses. Although bone marrow-derived elements present the expressed Ag, the mechanisms for acquiring immunogenic peptides have yet to be fully elucidated. APCs may become directly transfected by plasmid DNA or process extracellular proteins produced by other transfected cells. Using a transactivating plasmid system and bone marrow chimeras, we show that both mechanisms appear to be involved; however, the bulk of the immune response is dependent on expression of Ag by nonlymphoid tissues and transfer to APCs. These in vivo studies are the first to define the role of transfected nonlymphoid cells in generating Ag for presentation by bone marrow-derived APCs after needle injection with plasmid DNA.

Human mesangial cells express inducible macrophage scavenger receptor

BACKGROUND

Type A scavenger receptors (Scr) mediate the uptake of modified low-density lipoproteins by macrophages. The accumulation of lipids via this process is thought to lead to foam cell formation in atherosclerotic plaques. Human mesangial cells (HMCs) have not been previously shown to express Scr in normal culture. We therefore investigated whether there is an inducible form of Scr in a human mesangial cell line (HMCL).

METHODS

Scr activity was analyzed by cellular uptake of fluorescently labeled acetylated low-density lipoprotein using a flow cytometer. Scr mRNA expression was examined using reverse transcription-polymerase chain reaction, followed by Southern blotting. To investigate the molecular mechanism of Scr expression, several reporter gene constructs were designed. The first contained a full Scr promoter, the second a part of the Scr promoter that has both AP-1 and ets transcription factor binding sites. Other constructs were identical to the second, except that they contained either AP-1 or ets motif mutations.

RESULTS

Phorbol 12-Myristate 13-acetate (PMA) and angiotensin II (Ang II) increased both the percentage of Scr-positive cells and the Scr mean fluorescence intensity. PMA and Ang II also increased Scr mRNA and promoter activity in a time- and dose-responsive manner. Protein kinase C and calmodulin transduction pathways were involved in Scr up-regulation induced by PMA and Ang II. Additionally, a serine/threonine kinase was involved in PMA stimulation. Functional analysis showed that both AP-1 and ets motifs were specific response elements to PMA stimulation in HMCLs.

CONCLUSIONS

This study suggests that HMCs may express an inducible Scr, by which cells can acquire lipids and convert to foam cells in developing glomerulosclerosis.

Glucocorticoid inhibits oxidized LDL-induced macrophage growth by suppressing the expression of granulocyte/macrophage colony-stimulating factor

Glucocorticoid, an anti-inflammatory agent, inhibits the development of atherosclerosis in various experimental animal models. This is partially explained by its ability to inhibit smooth muscle cell migration and proliferation in the intima and to reduce chemotaxis of circulating monocytes and leukocytes into the subendothelial spaces. We have recently demonstrated that oxidized LDL (Ox-LDL) has a mitogenic activity for macrophages in vitro in which Ox-LDL-induced granulocyte/macrophage colony-stimulating factor (GM-CSF) production plays an important role. Proliferation of cellular components is one of the characteristic events in the development and progression of atherosclerotic lesions. In the present study, we investigated the effects of glucocorticoids on Ox-LDL-induced macrophage growth. Dexamethasone, prednisolone, and cortisol inhibited Ox-LDL-induced thymidine incorporation into macrophages by 85%, 70%, and 50%, respectively. Ox-LDL induced a significant production of GM-CSF by macrophages, which was effectively inhibited by dexamethasone, prednisolone, and cortisol by 80%, 65%, and 50%, respectively. Dexamethasone-mediated inhibition of Ox-LDL-induced GM-CSF mRNA expression and macrophage growth was significantly abrogated by RU-486, a glucocorticoid receptor antagonist. Our results suggest that the inhibitory effects of glucocorticoids on macrophage growth may be due to the inhibition of Ox-LDL-induced GM-CSF production through transactivation of the glucocorticoid receptor.

Activin/follistatin and atherosclerosis--a review

Activin-A, a member of the TGF-beta superfamily, has a variety of important biological functions. Concerning Møs, we demonstrated that MSR which has a key role in disposing of modified LDL is downregulated by activin-A. This leads to a decrease in binding, cell association, and degradation of Ac-LDL, resulting in the inhibition of foam cell formation. Follistatin, presumably by blocking the effect of intrinsic activin-A, upregulates MSR expression, thereby promoting Ac-LDL disposal and foam cell formation. Because both activin-A and MSR are induced during Mø differentiation, these results suggest that MSR expression is suppressed by simultaneous production of activin-A in an autocrine manner. In addition to Møs, activin-A and follistatin exert influences on SMCs and ECs. Examination of in vivo expression of activin-A and follistatin revealed that they are present in various atherosclerotic lesions, including human coronary arteries, suggesting that they are locally produced. Activin-A and follistatin are produced by Møs, SMCs, and ECs in vitro. Thus, the activin-A/follistatin system plays an important role in the development of atherosclerosis.

Expression pattern and cellular distribution of the murine homologue of AF10

We have cloned Af10, the murine homologue of the MLL partner gene AF10. The predicted open reading frame of Af10 contains 1069 aa which are 90% identical to those of AF10. Af10 contains an N-terminal cysteine-rich region with a LAP/PHD finger, a leucine zipper domain and a glutamine-rich region at the C-terminus, features also found in the human proteins AF10 and AF17. A single 5. 5-kb transcript was detected in murine tissues with the highest level of expression in the testes. A polyclonal antibody raised to the cysteine-rich region of AF10 was able to identify a double band of 140 kDa on Western analysis in mouse testicular extracts. After subcellular separation Af10 was identified in both the nuclear and cytoplasmic extracts, again as a double band of 140 kDa in size. In situ hybridisation studies were performed with sense and antisense digoxigenin-labelled oligonucleotides. High levels of expression were noted in postmeiotic germ cells, especially in spermatids from around stage VI to stage VIII. High levels of expression were also seen in the white matter of the cerebellum, extending into the granular layer. The expression in differentiated rather than in proliferating cells suggests that the role of Af10 may lie in the suppression of proliferation rather than in differentiation. Since the LAP/PHD finger domains are lost in the MLL-AF10 fusion, arguably such a function could be carried out by this domain.

Differential utilization of Ras signaling pathways by macrophage colony-stimulating factor (CSF) and granulocyte-macrophage CSF receptors during macrophage differentiation

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) independently stimulate the proliferation and differentiation of macrophages from bone marrow progenitor cells. Although the GM-CSF and M-CSF receptors are unrelated, both couple to Ras-dependent signal transduction pathways, suggesting that these pathways might account for common actions of GM-CSF and M-CSF on the expression of macrophage-specific genes. To test this hypothesis, we have investigated the mechanisms by which GM-CSF and M-CSF regulate the expression of the macrophage scavenger receptor A (SR-A) gene. We demonstrate that induction of the SR-A gene by M-CSF is dependent on AP-1 and cooperating Ets domain transcription factors that bind to sites in an M-CSF-dependent enhancer located 4.1 to 4.5 kb upstream of the transcriptional start site. In contrast, regulation by GM-CSF requires a separate enhancer located 4.5 to 4.8 kb upstream of the transcriptional start site that confers both immediate-early and sustained transcriptional responses. Results of a combination of DNA binding experiments and functional assays suggest that immediate transcriptional responses are mediated by DNA binding proteins that are constitutively bound to the GM-CSF enhancer and are activated by Ras. At 12 to 24 h after GM-CSF treatment, the GM-CSF enhancer becomes further occupied by additional DNA binding proteins that may contribute to sustained transcriptional responses. In concert, these studies indicate that GM-CSF and M-CSF differentially utilize Ras-dependent signal transduction pathways to regulate scavenger receptor gene expression, consistent with the distinct functional properties of M-CSF- and GM-CSF-derived macrophages.

Hormone-sensitive lipase overexpression increases cholesteryl ester hydrolysis in macrophage foam cells

Atherosclerosis is a complex physiopathologic process initiated by the formation of cholesterol-rich lesions in the arterial wall. Macrophages play a crucial role in this process because they accumulate large amounts of cholesterol esters (CEs) to form the foam cells that initiate the formation of the lesion and participate actively in the development of the lesion. Therefore, prevention or reversal of CE accumulation in macrophage foam cells could result in protection from multiple pathological effects. In this report, we show that the CE hydrolysis catalyzed by neutral cholesterol ester hydrolase (nCEH) can be modulated by overexpression of hormone-sensitive lipase (HSL) in macrophage foam cells. For these studies, RAW 264.7 cells, a murine macrophage cell line, were found to be a suitable model of foam cell formation. HSL expression and nCEH activity in these cells and in peritoneal macrophages were comparable. In addition, antibody titration showed that essentially all nCEH activity in murine macrophages was accounted for by HSL. To examine the effect of HSL overexpression on foam cell formation, RAW 264.7 cells were stably transfected with a rat HSL cDNA. The resulting HSL overexpression increased hydrolysis of cellular CEs 2- to 3-fold in lipid-laden cells in the presence of an acyl coenzyme A:cholesterol acyltransferase (ACAT) inhibitor. Furthermore, addition of cAMP produced a 5-fold higher rate of CE hydrolysis in cholesterol-laden, HSL-overexpressing cells than in control cells and resulted in nearly complete hydrolysis of cellular CEs in only 9 hours, compared with <50% hydrolysis in control cells. Thus, HSL overexpression stimulated the net hydrolysis of CEs, leading to faster hydrolysis of lipid deposits in model foam cells. These data suggest that HSL overexpression in macrophages, alone or in combination with ACAT inhibitors, may constitute a useful therapeutic approach for impeding CE accumulation in macrophages in vivo.

Macrophage scavenger receptor(s) and oxidatively modified proteins in endometriosis

OBJECTIVE

To determine whether cultured human peritoneal macrophages have functional scavenger receptor(s) and whether activation of macrophages in endometriosis may involve an increase in scavenger receptor activity.

DESIGN

A controlled clinical study comparing peritoneal fluid (PF) macrophages of women with endometriosis and controls without endometriosis.

SETTING

Women undergoing laparoscopic evaluation and treatment in a tertiary medical center.

PATIENT(S)

Twenty-one women undergoing evaluation for pelvic pain or infertility and 10 women undergoing elective laparoscopic tubal ligation.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Evidence for functional macrophage scavenger receptor and evidence of ligands for the scavenger receptor in PF.

RESULT(S)

Peritoneal macrophages of women with endometriosis degrade significantly more endothelial cell-low density lipoprotein (EC-LDL) and copper-oxidized LDL (Cu-LDL) than native LDL. Macrophages of women with endometriosis also incorporate more labeled oleic acid into cholesteryl ester in the presence of oxidized LDL (Ox-LDL) than in the presence of native LDL. Western blot analysis demonstrates the presence of adducts between lipid peroxidation products and proteins in PF of patients with and without endometriosis. The PF of women with endometriosis competes with labeled Ox-LDL for uptake by mouse peritoneal macrophages in a dose-dependent manner.

CONCLUSION(S)

We demonstrate for the first time that human macrophages have functional scavenger receptor(s) and that activation of macrophages in endometriosis involves an increase in scavenger receptor activity. Two lines of evidence indicate the presence of ligands for the scavenger receptor in PF.

Structure, organization, and chromosomal mapping of the gene encoding macrosialin, a macrophage-restricted protein

Murine macrosialin and its human homologue CD68 are heavily glycosylated transmembrane proteins expressed specifically in macrophages and macrophage-related cells. Macrosialin is predominantly a late endosomal protein but is also found on the cell surface where it binds oxidized LDL, an important factor in atherogenesis. We have cloned and sequenced the murine macrosialin gene (Cd68) and localized it by linkage analysis to chromosome 11. The gene is 1908 nucleotides long from the start site of transcription to the end of the 3'UTR. It has six exons, which range in size from 79 to 434 nucleotides. The promoter lacks a classical TATA box but contains other protein binding sites consistent with preferential monocyte/macrophage gene expression. Although the function of macrosialin is unknown, it might play a role in lipoprotein regulation given its binding of oxidized LDL in vitro and its colocalization to a region on chromosome 11 involved in the control of HDL levels.

Lipoproteins modulate expression of the macrophage scavenger receptor

Macrophage scavenger receptors (MSR) bind and internalize oxidized low density lipoprotein (OxLDL), a modified lipoprotein that is thought to be the proximal source of lipids that accumulate within cells of atherosclerotic lesions. The role of lipoproteins in modulating MSR expression are undetermined. We studied the effect of lipoproteins, native and modified LDL (acetylated LDL (AcLDL) and OxLDL) on the expression of the MSR in RAW cells, a murine macrophage cell line. Exposure to lipoproteins resulted in a marked induction of MSR mRNA expression (12- to 17-fold) with OxLDL and AcLDL having the greatest effects. Maximum induction occurred 1 hour after treatment with OxLDL and LDL. AcLDL induced a fourfold increase at 1 hour followed by a return to baseline and peak expression (sixfold) at 14 hours. Scavenger receptor function, as measured by 125I-AcLDL binding, was only modestly increased in response to lipoproteins. Incubation of macrophages with a cholesterol acceptor particle resulted in a dose-dependent decrease in MSR mRNA expression, which paralleled cholesterol loss from the cells. OxLDL did not affect MSR mRNA stability, implying that MSR mRNA was transcriptionally regulated by lipoproteins. Finally, peritoneal macrophages were isolated from mice following intraperitoneal injection of lipoproteins. Macrophage expression of MSR mRNA was significantly (16-fold) increased by LDL, AcLDL, or OxLDL relative to mice infused with phosphate-buffered saline. This demonstration that exposure to lipoproteins increases expression of the macrophage scavenger receptor implies that lipoproteins can further contribute to foam cell development in atherosclerosis.

PPARgamma promotes monocyte/macrophage differentiation and uptake of oxidized LDL

The formation of foam cells from macrophages in the arterial wall is characterized by dramatic changes in lipid metabolism, including increased expression of scavenger receptors and the uptake of oxidized low-density lipoprotein (oxLDL). We demonstrate here that the nuclear receptor PPARgamma is induced in human monocytes following exposure to oxLDL and is expressed at high levels in the foam cells of atherosclerotic lesions. Ligand activation of the PPARgamma:RXRalpha heterodimer in myelomonocytic cell lines induces changes characteristic of monocytic differentiation and promotes uptake of oxLDL through transcriptional induction of the scavenger receptor CD36. These results reveal a novel signaling pathway controlling differentiation and lipid metabolism in monocytic cells, and suggest that endogenous PPARgamma ligands may be important regulators of gene expression during atherogenesis.

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.

Role of activin-A and follistatin in foam cell formation of THP-1 macrophages

Macrophage (M phi) foam cell formation is a characteristic event that occurs in the early stage of atherosclerosis. To examine the roles of activin-A, a member of the transforming growth factor-beta superfamily, and follistatin, the binding protein for activin-A, in M phi function, we investigated their effects on foam cell formation of THP-1 M phi s. When THP-1 M phi s were treated with activin-A (5 nmol/L), foam cell formation and cellular cholesteryl ester accumulation were decreased. This downregulation was paralleled by a reduction in cell association and degradation of acetylated LDL. The inhibitory effect of activin-A on cell association and degradation was dose dependent, and the effect was blocked by concomitant addition of follistatin. Activin-A (5 nmol/L) also decreased the Bmax for acetylated LDL and scavenger receptor mRNA expression. Follistatin showed an effect opposite to that of activin-A and promoted M phi foam cell formation and cellular cholesteryl ester accumulation. It increased binding, cell association, and degradation of acetylated LDL and upregulated scavenger receptor mRNA expression. Because follistatin is the binding protein for activin-A, follistatin's effect is considered to be mediated by blocking the inhibitory effect of intrinsic activin-A. These results indicate that activin-A inhibits and follistatin promotes M phi foam cell formation by regulating scavenger receptor mRNA expression. We conclude that activin-A and follistatin play important roles in the process of atherosclerosis by regulating M phi foam cell formation.

Lipopolysaccharide decreases scavenger receptor mRNA in vivo

Lipopolysaccharide (LPS) downregulates scavenger receptor (ScR) activity in cultured macrophages through release of tumor necrosis factor-alpha (TNF-alpha). However, LPS administration in vivo stimulates cytokine release from both macrophages and lymphocytes, the combined effects of which could alter ScR expression differently from TNF-alpha in isolation. To investigate whether LPS regulates ScR in vivo, 10 microg/g was injected i.p. into Swiss Webster mice. Administration of LPS produced a profound decrease in hepatic ScR mRNA, with reductions of 74% +/- 8% at 2 h that returned to baseline levels by 6 h. Changes in ScR mRNA abundance coincided with changes in serum concentrations of TNF-alpha, which peaked at 2 h (1320 +/- 309 pg/ml) and returned to preinjection concentrations at 4 h. Serum concentrations of interferon-gamma (IFN-gamma) did not increase until 4 h after injection of LPS. There was no effect on ScR mRNA abundance following LPS administration to LPS-resistant strains of mice, C3H/HeJ and IFN-gamma receptor-/-. The LPS-induced reduction in ScR mRNA in Swiss Webster mice was not sufficiently sustained to affect receptor function, as determined by the kinetics of [(125)I]-acetylated LDL clearance from plasma. Therefore, as observed in cultured cells, LPS administration to mice decreases ScR mRNA despite the release of several cytokines in vivo.

Acetylated LDL endocytosis by the human monocytic Mono Mac 6sr cells is not mediated by the macrophage type I and II scavenger receptors

We recently reported that the human monocytic Mono Mac 6sr cell line constitutively takes up and degrades acetylated (acLDL) and oxidized LDL through receptor-specific pathways. The present studies were undertaken to further characterize the acLDL binding site on a functional and molecular basis. The degradation of acLDL increased during differentiation of Mono Mac 6sr cells with lipopolysaccharide (10 ng/mL, 72 hours) and low concentrations of phorbol 12-myristate 13-acetate (PMA; 0.1 to 1.0 ng/mL, 72 hours). Higher doses of PMA (5 or 10 ng/mL), however, decreased acLDL degradation. Scatchard plots of acLDL binding in untreated and LPS-differentiated Mono Mac 6sr cells were nonlinear and suggested the presence of more than one binding site. Although the ligand specificity of the acLDL receptor in Mono Mac 6sr cells resembles that of the macrophage type I and type II scavenger receptors, we did not detect mRNA of either receptor type in untreated or differentiated Mono Mac 6sr cells by means of Northern blotting and reverse transcription polymerase chain reaction. Furthermore, ligand blotting with 125I-acLDL failed to detect the 220-kD types I and II scavenger receptor protein. Thus, Mono Mac 6sr cells express an acLDL receptor that is distinct from the type I and type II scavenger receptor found in human monocyte-derived macrophages but that, like the latter, is induced during monocytic differentiation.

Nuclear integration of JAK/STAT and Ras/AP-1 signaling by CBP and p300

We report that interferon gamma (IFN-gamma) inhibits transcription of the macrophage scavenger receptor gene by antagonizing the Ras-dependent activities of AP-1 and cooperating ets domain transcription factors, apparently as a result of competition between AP-1/ets factors and activated STAT1 for limiting amounts of CBP and p300. Consistent with this model, STAT1 alpha interacts directly with CBP in cells, and microinjection of anti-CBP and anti-p300 antibodies blocks transcriptional responses to IFN-gamma. Cells lacking STAT1 fail to inhibit AP-1/ets activity, and overexpression of CBP both potentiates IFN-gamma-dependent transcription and relieves AP-1/ets repression. Thus, CBP and p300 integrate both positive and negative effects of IFN-gamma on gene expression by serving as essential coactivators of STAT1 alpha, modulating gene-specific responses to simultaneous activation of two or more signal transduction pathways.

The effect of RU 486 and related compounds on cultured macrophage differentiation and function

OBJECTIVE

Our purpose was to examine RU 486 and related compounds on macrophage scavenger receptors and cellular adhesion.

STUDY DESIGN

THP-1 cells were activated with phorbol myristate acetate and treated with dexamethasone, levonorgestrel, and RU 486 alone or in combination. Scavenger receptor activity was determined by counting adhered cells. In addition, fluorescently labeled acetyl low density lipoprotein uptake was determined.

RESULTS

Both dexamethasone and RU 486 significantly decreased activated macrophages (81% and 26% of control). Levonorgestrel stimulated adherent cells in activated monocytes (130% of control). RU 486 and dexamethasone were antagonistic when combined (p < 0.001). In contrast, dexamethasone could not overcome the stimulatory effect of levonorgestrel (p < 0.001). Fluorescent studies yielded similar results.

CONCLUSIONS

RU 486 is a known antiglucocorticoid with novel antioxidant properties. Levonorgestrel has antiglucocorticoid but no antioxidant activity. Glucocorticoids decrease scavenger receptors and antioxidants regulate inflammatory cytokines. RU 486 antagonized the inhibitory effect of dexamethasone on scavenger receptors, whereas levonorgestrel was stimulatory. It is therapeutically important to up-regulate scavenger receptor activity by antiglucocorticoids in the peritoneal cavity of women with endometriosis. However, because these mechanisms also induce inflammatory cytokines, a balance of antioxidants and antiglucocorticoids may prove beneficial.

The activity of the highly inducible mouse phenylalanine hydroxylase gene promoter is dependent upon a tissue-specific, hormone-inducible enhancer

Expression of the phenylalanine hydroxylase gene in livers and kidneys of rodents is activated at birth and is induced by glucocorticoids and cyclic AMP in the liver. Regulatory elements in a 10-kb fragment upstream of the mouse gene have been characterized. The promoter lacks TAATA and CCAAT consensus sequences and shows only extremely weak activity in transitory expression assays with phenylalanine hydroxylase-producing hepatoma cells. No key elements for regulation of promoter activity are localized within 2 kb of upstream sequences. However, a liver-specific DNase I-hypersensitive site at kb -3.5 comprises a tissue-specific and hormone-inducible enhancer. This enhancer contains multiple protein binding sites, including sites for ubiquitous factors (NF1 and AP1), the glucocorticoid receptor, and the hepatocyte-enriched transcription factors hepatocyte nuclear factor 1 (HNF1) and C/EBP. Mutation revealed that the last two sites are critical not only for basal activity but also for obtaining a maximal hormone response. Efficient transcription from the highly inducible promoter shows absolute dependence upon the enhancer at kb - 3.5, which in turn requires HNF1 and C/EBP as well as hormones. The regulatory region of the mouse phenylalanine hydroxylase gene differs totally from that of humans, even though the genes of both species are expressed essentially in the liver. Furthermore, the phenylalanine hydroxylase gene of mice shows an expression pattern very similar to those of the rodent tyrosine aminotransferase and phosphoenolpyruvate carboxykinase genes, yet each shows a different organization of its regulatory region.

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.

Effect of RU-486 and related compounds on the proliferation of cultured macrophages

PROBLEM

Macrophages are implicated in the pathophysiology of endometriosis and are influenced by anti-inflammatory steroids as well as anti-oxidants.

METHODS

We tested the effect of RU-486, an antiprogesterone, antiglucocorticoid and an antioxidant, on the proliferation of RAW macrophages.

RESULTS

The incorporation of 3H-thymidine was significantly inhibited by both progesterone and RU-486. Progesterone and RU-486, in combination, synergistically inhibited macrophage growth. In contrast, dexamethasone-stimulated growth was antagonized by RU-486 in a dose dependent manner. ZK 112,993 which is structurally related to RU-486 but lacks antioxidant properties, also inhibited thymidine incorporation. The synergistic effect of RU-486 and ZK 112,993 with progesterone implicate a mechanism of action separate from receptor bound antagonists. A cell permeable antioxidant, pyrrolidine dithiocarbamate was very effective in inhibiting the incorporation of 3H-thymidine into cells.

CONCLUSIONS

These results suggest novel therapeutic modalities in the management of endometriosis via antiglucocorticoid as well as antioxidant mechanisms.

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.

Scavenger receptor A gene regulatory elements target gene expression to macrophages and to foam cells of atherosclerotic lesions

Transcription of the macrophage scavenger receptor A gene is markedly upregulated during monocyte to macrophage differentiation. In these studies, we demonstrate that 291 bp of the proximal scavenger receptor promoter, in concert with a 400-bp upstream enhancer element, is sufficient to direct macrophage-specific expression of a human growth hormone reporter in transgenic mice. These regulatory elements, which contain binding sites for PU.1, AP-1, and cooperating ets-domain transcription factors, are also sufficient to mediate regulation of transgene expression during the in vitro differentiation of bone marrow progenitor cells in response to macrophage colony-stimulating factor. Mutation of the PU.1 binding site within the scavenger receptor promoter severely impairs transgene expression, consistent with a crucial role of PU.1 in regulating the expression of the scavenger receptor gene. The ability of the scavenger receptor promoter and enhancer to target gene expression to macrophages in vivo, including foam cells of atherosclerotic lesions, suggests that these regulatory elements will be of general utility in the study of macrophage differentiation and function by permitting specific modifications of macrophage gene expression.

Identification of a cell-type-specific and E2F-independent mechanism for repression of cdc2 transcription

Human myeloid leukemia cells, such as HL60, U937, and THP1 cells, undergo macrophage differentiation and growth arrest following treatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Surprisingly, we find that growth of a significant percentage of THP1 cells is arrested in the G2 phase of the cell cycle. G2 arrest correlates with cell-specific repression of the gene encoding p34cdc2, a crucial regulator of G2/M progression. Intriguingly, TPA-mediated repression of the cdc2 promoter was independent of the transcription factor E2F, distinguishing this pathway from mechanisms responsible for repression of cdc2 transcription in response to serum starvation. The region of the cdc2 promoter required for repression was located from bp -22 to -2 from the major transcriptional start site. This sequence, which we term the R box, directs the uncoupling of the basal promoter from upstream activators following TPA treatment. Analysis of THP1 nuclear proteins revealed a 55-kDa protein that was induced by TPA and interacted with the cdc2 promoter in an R-box-dependent manner. These observations provide evidence for the existence of cell-type- and promoter-specific pathways for the assembly of stable transcriptional initiation complexes that function to differentially regulate the expression of cell cycle control genes in mammalian cells.

Repression of the macrophage scavenger receptor in macrophage-smooth muscle cell heterokaryons

Macrophage scavenger receptors mediate the uptake of chemically modified LDL in an unregulated manner, leading to massive intracellular accumulation of lipid and thus a foamy cellular morphology. In atherosclerotic lesions, foam cells originate not only from macrophages but also from smooth muscle cells, yet smooth muscle cells do not normally express scavenger receptors, and when exposed to chemically modified LDL in vitro, lipid accumulation does not occur. The mechanism of conversion of smooth muscle cells into foam cells in the arterial wall is thus still under discussion. To investigate whether direct interaction between macrophages and smooth muscle cells may be involved and to explore the effects of components of the two cell types on the expression of scavenger receptors, we report here experiments using somatic cell hybrids formed by fusion of the two cell types. Immunofluorescent labeling and confocal microscopic techniques were applied to investigate and measure (1) lipid accumulation (using Nile Red staining), (2) the binding and uptake of acetylated LDL (using 1,1'-dioctadecyl-1-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate-labeled acetylated LDL), and (3) receptor expression (assessed using a specific anti-receptor antibody) in smooth muscle cell-macrophage heterokaryons, macrophage-macrophage homokaryons, smooth muscle cell-smooth muscle cell homokaryons, and unfused macrophages and smooth muscle cells. The results demonstrate that scavenger receptor expression becomes repressed in macrophage-smooth muscle cell heterokaryons but not in macrophage-macrophage homokaryons. One possible explanation for the observed repression would be the existence of a negative regulatory cytoplasmic factor produced by smooth muscle cells.