In vitro-differentiated embryonic stem cell macrophages: a model system for studying atherosclerosis-associated macrophage functions

Efficient Method to Differentiate Mouse Embryonic Stem Cells into Macrophages in vitro

Macrophages are key cells in the innate immune system and play a role in a variety of diseases. However, macrophages are terminally differentiated and difficult to manipulate genetically via transfection or through CRISPR-Cas9 gene editing. To overcome this limitation, we provide a simplified protocol for the generation of mouse embryonic stem cells-derived macrophages (ESDM). Thus, genetic manipulation can be performed using embryonic stem cells, selecting for the desired changes, and finally producing macrophages to study the effects of the previous genetic manipulation. These studies can contribute to many areas of research, including atherosclerosis and inflammation. Production of ESDM has been previously achieved using embryoid body (EB) intermediates. Here, we optimized the EB method using a simplified medium, reducing the number of recombinant proteins and medium recipes required. Our EB-based differentiation protocol consists of three stages: 1) floating EB formation; 2) adherence of EBs and release of floating macrophage progenitors; and, 3) terminal differentiation of harvested macrophage progenitors. The advantages of this protocol include achieving independent floating EBs in stage 1 by using a rocker within the tissue culture incubator, as well as the exclusion of small EBs and cell clusters when harvesting macrophage progenitors via cell filtration.

Designer macrophages: Pitfalls and opportunities for modelling macrophage phenotypes from pluripotent stem cells

Macrophages are phagocytic immune cells resident in every tissue that are not only important for host defence, but are also involved in tissue homeostasis, injury, and disease. Despite increasingly sophisticated methods for in vitro macrophage isolation, expansion and activation over the past three decades, these have largely been restricted to modelling bone-marrow or blood-derived cells. The in vitro derivation of macrophages from human pluripotent stem cells provides new opportunities to study macrophage biology, including the factors that impact human myeloid development and those that induce macrophage activation. While sharing many of the functional characteristics of monocyte-derived macrophages, stem cell-derived macrophages may offer new opportunities to understand the role of development or tissue context in innate immune cell function. Immune responsiveness to pathogenic challenge is known to be impacted by a macrophage's history of prior exposure, as well as ontogeny and tissue context. Therefore, we explore the factors of in vitro derivation likely to influence macrophage phenotype and function.

Molecular cloning and characterization of mouse LITAF cDNA: role in the regulation of tumor necrosis factor-α (TNF-α) gene expression

The inflammatory response to bacteria and bacterial products, such as lipopolysaccharides (LPSs), is mediated by a variety of secreted factors, but cytotoxic effects of LPS have been ascribed to the tumor necrosis factor alpha (TNF-α) activity. TNF-α is probably the most pleiotropic cytokine and, given the deleterious effects to the host of this factor, it has been postulated that its expression must be tightly regulated. Our laboratory has recently isolated, cloned and characterized a novel human transcription factor named LITAF or LPS-induced TNF-alpha factor. The present study reports the isolation, cloning and characterization of the mouse LITAF cDNA. Chromosomal localization revealed that mouse LITAF mapped to mouse chromosome 16, in a region highly homologous with the area on which human LITAF was previously located. Northern blot analysis shows that mouse LITAF is already expressed at embryonic day 7 of development, and is highly expressed in adult liver, heart and kidney. Moreover, upon LPS stimulation, we show that: (i) LITAF expression is increased in a mouse monocyte/macrophage cell line; and (ii) TNF-α expression is reduced in ES cell-derived macrophages lacking one copy of LITAF gene. Taken together, these results highlight the important role of LITAF in the regulation of TNF-α gene expression and suggest a potential role of LITAF in mouse organogenesis.

Attenuated Innate Immunity in Embryonic Stem Cells and Its Implications in Developmental Biology and Regenerative Medicine

Embryonic stem cells (ESCs) represent a promising cell source for regenerative medicine. Intensive research over the past 2 decades has led to the feasibility of using ESC-differentiated cells (ESC-DCs) in regenerative medicine. However, increasing evidence indicates that ESC-DCs generated by current differentiation methods may not have equivalent cellular functions to their in vivo counterparts. Recent studies have revealed that both human and mouse ESCs as well as some types of ESC-DCs lack or have attenuated innate immune responses to a wide range of infectious agents. These findings raise important concerns for their therapeutic applications since ESC-DCs, when implanted to a wound site of a patient, where they would likely be exposed to pathogens and inflammatory cytokines. Understanding whether an attenuated immune response is beneficial or harmful to the interaction between host and grafted cells becomes an important issue for ESC-based therapy. A substantial amount of recent evidence has demonstrated that the lack of innate antiviral responses is a common feature to ESCs and other types of pluripotent cells. This has led to the hypothesis that mammals may have adapted different antiviral mechanisms at different stages of organismal development. The underdeveloped innate immunity represents a unique and uncharacterized property of ESCs that may have important implications in developmental biology, immunology, and in regenerative medicine.

Advances in cellular technology in the hematology field: What have we learned so far?

Despite the advances in the hematology field, blood transfusion-related iatrogenesis is still a major issue to be considered during such procedures due to blood antigenic incompatibility. This places pluripotent stem cells as a possible ally in the production of more suitable blood products. The present review article aims to provide a comprehensive summary of the state-of-the-art concerning the differentiation of both embryonic stem cells and induced pluripotent stem cells to hematopoietic cell lines. Here, we review the most recently published protocols to achieve the production of blood cells for future application in hemotherapy, cancer therapy and basic research.

Directed differentiation of embryonic stem cells using a bead-based combinatorial screening method

We have developed a rapid, bead-based combinatorial screening method to determine optimal combinations of variables that direct stem cell differentiation to produce known or novel cell types having pre-determined characteristics. Here we describe three experiments comprising stepwise exposure of mouse or human embryonic cells to 10,000 combinations of serum-free differentiation media, through which we discovered multiple novel, efficient and robust protocols to generate a number of specific hematopoietic and neural lineages. We further demonstrate that the technology can be used to optimize existing protocols in order to substitute costly growth factors with bioactive small molecules and/or increase cell yield, and to identify in vitro conditions for the production of rare developmental intermediates such as an embryonic lymphoid progenitor cell that has not previously been reported.

Mesodermal and hematopoietic differentiation from ES and iPS cells

Embryonic stem (ES) and induced pluripotent stem (iPS) cells can differentiate into any type of tissue when grown in a suitable culture environment and are considered valuable tools for regenerative medicine. In the field of hematology, generation of hematopoietic stem cells (HSCs) and mature hematopoietic cells (HCs) from ES and iPS cells through mesodermal cells, the ancestors of HCs, can facilitate transplantation and transfusion therapy. Several studies report generation of functional HCs from both mouse and human ES and iPS cells. This approach will likely be applied to individual patient-derived iPS cells for regenerative medicine approaches and drug screening in the future. Here, we summarize current studies of HC-generation from ES and iPS cells.

Therapeutic effect of human iPS-cell-derived myeloid cells expressing IFN-β against peritoneally disseminated cancer in xenograft models

We recently developed a method to generate myeloid cells with proliferation capacity from human iPS cells. iPS-ML (iPS-cell-derived myeloid/macrophage line), generated by introducing proliferation and anti-senescence factors into iPS-cell-derived myeloid cells, grew continuously in an M-CSF-dependent manner. A large number of cells exhibiting macrophage-like properties can be readily obtained by using this technology. In the current study, we evaluated the possible application of iPS-ML in anti-cancer therapy. We established a model of peritoneally disseminated gastric cancer by intraperitoneally injecting NUGC-4 human gastric cancer cells into SCID mice. When iPS-ML were injected intraperitoneally into the mice with pre-established peritoneal NUGC-4 tumors, iPS-ML massively accumulated and infiltrated into the tumor tissues. iPS-ML expressing IFN-β (iPS-ML/IFN-β) significantly inhibited the intra-peritoneal growth of NUGC-4 cancer. Furthermore, iPS-ML/IFN-β also inhibited the growth of human pancreatic cancer MIAPaCa-2 in a similar model. iPS-ML are therefore a promising treatment agent for peritoneally disseminated cancers, for which no standard treatment is currently available.

Antibacterial capacity of differentiated murine embryonic stem cells during defined in vitro inflammatory conditions

Embryonic stem (ES) cells are a powerful model for the development of cells responsible for the cellular immune response. Therefore, we analyzed the defense and phagocytic capacity of embryoid bodies (EBs) derived from ES cells using in the vitro inflammatory conditions caused by Escherichia coli. Further, we used this phagocytic activity to purify activated immune cells. Our data show that spontaneously differentiated 18-day-old EBs of the cell line CGR8 contained immune cells, which were positive for CD45, CD68, CD11b, F4/80, and CD19. Exposure of these EBs to E. coli with defined infection doses of bacterial colony-forming units (CFUs) led to a significant time-dependent reduction of CFUs, indicating the immune responses exerted by EBs. This was paralleled by an upregulation of inflammatory cytokines, that is, IL-1β and TNF-α. Western blot analysis of infected EBs indicated an upregulation of CD14 and cytochrome b-245 heavy chain (NOX2). Silencing of NOX2 significantly reduced the antibacterial capacity of EBs, which was partially explained by reduction of F4/80-positive cells. To identify, isolate, and further cultivate phagocytic active cells from differentiated EBs, a cocultivation assay of differentiated ES cells with green fluorescent protein (GFP)-labeled E. coli was established. Colocalization of GFP-labeled E. coli with cells positive for CD45, CD68, and F4/80 revealed time-dependent phagocytotic uptake, which was underlined by colocalization with the LysoTracker-Red(®) dye as well as preincubation with cytochalasin D. In conclusion, a primitive immune response with efficient phagocytosis was responsible for the antibacterial capacity of differentiated EBs.

Generation of a novel mouse model for the inducible depletion of macrophages in vivo

Macrophages play an essential role in tissue homeostasis, innate immunity, inflammation, and wound repair. Macrophages are also essential during development, severely limiting the use of mouse models in which these cells have been constitutively deleted. Consequently, we have developed a transgenic model of inducible macrophage depletion in which macrophage-specific induction of the cytotoxic diphtheria toxin A chain (DTA) is achieved by administration of doxycycline. Induction of the DTA protein in transgenic animals resulted in a significant 50% reduction in CD68+ macrophages of the liver, spleen, and bone over a period of 6 weeks. Pertinently, the macrophages remaining after doxycycline treatment were substantially smaller and are functionally impaired as shown by reduced inflammatory cytokine production in response to lipopolysaccharide. This inducible model of macrophage depletion can now be utilized to determine the role of macrophages in both development and animal models of chronic inflammatory diseases.

HIV-1 Resistant CDK2-Knockdown Macrophage-Like Cells Generated from 293T Cell-Derived Human Induced Pluripotent Stem Cells

A major challenge in studies of human diseases involving macrophages is low yield and heterogeneity of the primary cells and limited ability of these cells for transfections and genetic manipulations. To address this issue, we developed a simple and efficient three steps method for somatic 293T cells reprogramming into monocytes and macrophage-like cells. First, 293T cells were reprogrammed into induced pluripotent stem cells (iPSCs) through a transfection-mediated expression of two factors, Oct-4 and Sox2, resulting in a high yield of iPSC. Second, the obtained iPSC were differentiated into monocytes using IL-3 and M-CSF treatment. And third, monocytes were differentiated into macrophage-like cells in the presence of M-CSF. As an example, we developed HIV-1-resistant macrophage-like cells from 293T cells with knockdown of CDK2, a factor critical for HIV-1 transcription. Our study provides a proof-of-principle approach that can be used to study the role of host cell factors in HIV-1 infection of human macrophages.

Control of leucocyte differentiation from embryonic stem cells upon vasculogenesis and confrontation with tumour tissue

Embryonic stem (ES) cells spontaneously differentiate capillary-like structures as well as leucocytes such as monocytes/macrophages, neutrophils, natural killer (NK) cells and cytototoxic T lymphocytes. The interplay between vasculogenesis and leucocyte differentiation as well as the population of tumour tissues with ES cell-derived leucocytes and endothelial cells is, however, not sufficiently specified. In the present study, gene expression of the cell surface markers CD68 and CD14 (expressed on monocytes and macrophages), Mac-1 (CD11b) (expressed on granulocytes, monocytes and NK cells) and CD16 (expressed on neutrophils) was investigated in murine CGR8 ES cells in relation to the endothelial cell markers CD31 and vascular endothelial (VE)-cadherin. Expression of leucocyte markers increased from day 7–8 of cell culture on. Furthermore, addition of macrophage colony-stimulating factor to the cell culture medium resulted in a threefold increase in the number of CD68⁺ monocytes/macrophages. Treatment of embryoid bodies with lipopolysaccharide (LPS) up-regulated CD14 thus suggesting functionality of the CD14 LPS receptor. Differentiation of vascular structures positive for CD31 and VE-cadherin preceded leucocyte differentiation by 2 days (i.e. from day 5–6 on) suggesting that vasculogenesis may be a determinant of leucocyte differentiation. Consequently the Flk-1 antagonist SU5416 which inhibits vasculogenesis of ES cells significantly blunted leucocyte differentiation. Confrontation culture of embryoid bodies with multicellular breast tumour spheroids initiated significant increase of leucocyte cell numbers and invasion of leucocytes into the tumour tissue. In summary our data demonstrate that during ES cell differentiation vasculogenesis precedes leucocyte differentiation, and point towards the direction that leucocyte cell invasion into tumour tissue may initiate the pro-inflammatory microenvironment necessary for tumour vascularization.

Generation of dendritic cells and macrophages from human induced pluripotent stem cells aiming at cell therapy

This report describes generation of dendritic cells (DCs) and macrophages from human induced pluripotent stem (iPS) cells. iPS cell-derived DC (iPS-DC) exhibited the morphology of typical DC and function of T-cell stimulation and antigen presentation. iPS-DC loaded with cytomegalovirus (CMV) peptide induced vigorous expansion of CMV-specific autologous CD8+ T cells. Macrophages (iPS-MP) with activity of zymosan phagocytosis and C5a-induced chemotaxis were also generated from iPS cells. Genetically modified iPS-MPs were generated by the introduction of expression vectors into undifferentiated iPS cells, isolation of transfectant iPS cell clone and subsequent differentiation. By this procedure, we generated iPS-MP expressing a membrane-bound form of single chain antibody (scFv) specific to amyloid β (Aβ), the causal protein of Alzheimer's disease. The scFv-transfectant iPS-MP exhibited efficient Aβ-specific phagocytosis activity. iPS-MP expressing CD20-specific scFv engulfed and killed BALL-1 B-cell leukemia cells. Anti-BALL-1 effect of iPS-MP in vivo was demonstrated in a xeno-transplantation model using severe combined immunodeficient mice. In addition, we established a xeno-free culture protocol to generate iPS-DC and iPS-MP. Collectively, we demonstrated the possibility of application of iPS-DC and macrophages to cell therapy.

Autologous blood cell therapies from pluripotent stem cells

The discovery of human embryonic stem cells (hESCs) raised promises for a universal resource for cell based therapies in regenerative medicine. Recently, fast-paced progress has been made towards the generation of pluripotent stem cells (PSCs) amenable for clinical applications, culminating in reprogramming of adult somatic cells to autologous PSCs that can be indefinitely expanded in vitro. However, besides the efficient generation of bona fide, clinically safe PSCs (e.g., without the use of oncoproteins and gene transfer based on viruses inserting randomly into the genome), a major challenge in the field remains how to efficiently differentiate PSCs to specific lineages and how to select cells that will function normally upon transplantation in adults. In this review, we analyse the in vitro differentiation potential of PSCs to the hematopoietic lineage by discussing blood cell types that can be currently obtained, limitations in derivation of adult-type HSCs and prospects for clinical application of PSCs-derived blood cells.

Characterization of dendritic cells and macrophages generated by directed differentiation from mouse induced pluripotent stem cells

Methods have been established to generate dendritic cells (DCs) from mouse and human embryonic stem (ES) cells. We designated them as ES-DCs and mouse models have demonstrated the induction of anti-cancer immunity and prevention of autoimmune disease by in vivo administration of genetically engineered ES-DCs. For the future clinical application of ES-DCs, the histoincompatibility between patients to be treated and available human ES cells and the ethical concerns associated with human ES cells may be serious obstacles. However, recently developed induced pluripotent stem (iPS) cell technology is expected to resolve these issues. This report describes the generation and characterization of DCs derived from mouse iPS cells. The iPS cell-derived DCs (iPS-DCs) possessed the characteristics of DCs including the capacity of T-cell-stimulation, antigen-processing and presentation and cytokine production. DNA microarray analyses revealed the upregulation of genes related to antigen-presenting functions during differentiation into iPS-DCs and similarity in gene expression profile in iPS-DCs and bone marrow cell-derived DCs. Genetically modified iPS-DCs expressing antigenic protein primed T-cells specific to the antigen in vivo and elicited efficient antigen-specific anti-tumor immunity. In addition, macrophages were generated from iPS cells (iPS-MP). iPS-MP were comparable with bone marrow cell-derived macrophages in the cell surface phenotype, functions, and gene expression profiles.

LPS-induced suppression of macrophage cholesterol efflux is mediated by adipocyte enhancer-binding protein 1

Macrophages facilitate clearance of cholesterol from the body via reverse cholesterol transport (RCT). The first event in RCT is internalization of modified low density lipoprotein by macrophages, upon which PPARgamma1 and LXRalpha signaling pathways are turned on, leading to the transactivation of a cascade of genes (e.g. ABCA1 and ABCG1), whose products promote macrophage cholesterol efflux. Down-regulation of macrophage cholesterol efflux mediators leads to an imbalance in cholesterol homeostasis, promoting foam cell formation. Lipopolysaccharide (LPS) has been shown to suppress PPARgamma1 and its downstream target genes in macrophages, inducing foam cell formation; a key mechanism proposed to underlie bacterial infection-induced atherosclerosis. Herein, we show that adipocyte enhancer-binding protein 1 (AEBP1) is up-regulated during monocyte differentiation. Moreover, we provide experimental evidence suggesting that AEBP1 expression is induced by LPS, and that LPS-induced down-regulation of pivotal macrophage cholesterol efflux mediators, leading to foam cell formation, is largely mediated by AEBP1. Although AEBP1-independent pathways seem to contribute to these LPS effects, such pathways can only mediate lesser and delayed effects of LPS on macrophage cholesterol efflux and development of foam cells. We speculate that AEBP1 may serve as a potential therapeutic target for the prevention/treatment of bacterial infection-induced atherosclerosis.

Homogeneous monocytes and macrophages from human embryonic stem cells following coculture-free differentiation in M-CSF and IL-3

Objective

To develop a simple and efficient method for producing homogeneous populations of monocytes and macrophages from human embryonic stem cells (hES).

Materials and Methods

Human embryonic stem cell lines KCL001, KCL002, and HUES-2 were differentiated into monocytes by coculture-free differentiation with two growth factors using a three-step method. The method involved embryoid body (EB) formation in hES media, directed differentiation with macrophage colony-stimulating factor and interleukin (IL)-3, and harvest of nonadherent monocytes from the culture supernatants. hES monocytes (esMCs) were analyzed by microscopy, flow cytometry, transcriptome analysis, and tested for the ability to differentiate into macrophages. hES monocyte–derived macrophages (esMDM) were analyzed for phagocytosis and endocytosis by microscopy and flow cytometry, cytokine secretion by multiplex cytokine assay, and for interferon (IFN)-γ and IL-4 activation by flow cytometry.

Results

Homogeneous esMCs (>90% CD14-positive) that did not require any additional purification steps were produced after 18.7 ± 7.7 days (mean ± SD, n = 19). Production continued for several months when growth factors were replaced, with a total yield of 3.4 × 10⁵ ± 2.0 esMCs (mean ± SD, n = 9) per EB. Transcriptome analysis of the esMC and the esMDM revealed a distinct myeloid signature that correlated with primary adult blood–derived monocytes and spleen tissue samples but not with other tissue samples tested. We found that esMCs and esMDMs expressed well-defined markers of the mononuclear phagocyte system including PU-1, C/EBPα, EMR1, and EMR2, MPEG1, CD1c, CD4, CD18, CD32, CD33, CD68, cathepsins and serine carboxypeptidase. Finally, esMCs differentiated into functional macrophages that could endocytose acetylated low-density lipoprotein, phagocytose opsonized yeast particles, secrete specific cytokines in response to lipopolysaccharide, and be activated differentially with IFN-γ and IL-4.

Conclusions

We have developed a simple and efficient method for producing homogeneous populations of monocytes and macrophages from hES cells. esMCs have a myeloid signature and can differentiate into functional macrophages. The method should prove useful in answering experimental questions regarding monocyte and macrophage development and biology.

Quantitative expansion of ES cell-derived myeloid progenitors capable of differentiating into macrophages

Macrophages participate in physiologic and pathologic processes through elaboration of distinct activation programs. Studies with macrophage cell systems have revealed much concerning the importance of this pleiotropic cell; however, these studies are inherently limited by three factors: heterogeneity of the target cell population, poor capacity to elaborate various activation programs, and lack of a genetically tractable model system for loss- and gain-of-function studies. Although definitive, hematopoietic lineages can be isolated from embryonic stem (ES) cells, these isolation procedures are inefficient and time-consuming and require elaborate cell-sorting protocols. We therefore examined whether myeloid precursors, capable of differentiating into macrophages, could be conditionally expanded in vitro. Here, we report methods for selective isolation and immortalization of ES cell-derived myeloid precursors by estrogen-regulated HoxA9 protein. Using this new macrophage differentiation system, an unlimited number of custom-designed macrophages with defined functional characteristics can be generated from any targeted ES cell. In combination with knockout or small interfering RNA knockdown technologies, this macrophage differentiation system provides a powerful tool for high throughput analysis of regulatory mechanisms controlling macrophage activation in health and disease.

Adipocyte enhancer-binding protein 1 is a potential novel atherogenic factor involved in macrophage cholesterol homeostasis and inflammation

Peroxisome proliferator-activated receptor gamma1 (PPARgamma1) and liver X receptor alpha (LXRalpha) play pivotal roles in macrophage cholesterol homeostasis and inflammation, key biological processes in atherogenesis. Herein we identify adipocyte enhancer-binding protein 1 (AEBP1) as a transcriptional repressor that impedes macrophage cholesterol efflux, promoting foam cell formation, via PPARgamma1 and LXRalpha down-regulation. Contrary to AEBP1 deficiency, AEBP1 overexpression in macrophages is accompanied by decreased expression of PPARgamma1, LXRalpha, and their target genes ATP-binding cassette A1, ATP-binding cassette G1, apolipoprotein E, and CD36, with concomitant elevation in IL-6, TNF-alpha, monocyte chemoattractant protein 1, and inducible NO synthase levels. AEBP1, but not the C-terminally truncated DNA-binding domain mutant (AEBP1DeltaSty), represses PPARgamma1 and LXRalpha in vitro. Expectedly, AEBP1-overexpressing transgenic (AEBP1TG) macrophages accumulate considerable amounts of lipids compared with AEBP1 nontransgenic macrophages, making them precursors for foam cells. Indeed, AEBP1-overexpressing transgenic macrophages exhibit diminished cholesterol efflux compared with AEBP1 nontransgenic macrophages, whereas AEBP1-knockout (AEBP1-/-) macrophages exhibit enhanced cholesterol efflux compared with wild-type (AEBP1+/+) macrophages. Our in vitro and ex vivo experimental data strongly suggest that AEBP1 plays critical regulatory roles in macrophage cholesterol homeostasis, foam cell formation, and proinflammation. Thereby, we speculate that AEBP1 may be critically implicated in the development of atherosclerosis, and it may serve as a molecular target toward developing antiinflammatory, antiatherogenic therapeutic approaches.

Designer blood: creating hematopoietic lineages from embryonic stem cells

Embryonic stem (ES) cells exhibit the remarkable capacity to become virtually any differentiated tissue upon appropriate manipulation in culture, a property that has been beneficial for studies of hematopoiesis. Until recently, the majority of this work used murine ES cells for basic research to elucidate fundamental properties of blood-cell development and establish methods to derive specific mature lineages. Now, the advent of human ES cells sets the stage for more applied pursuits to generate transplantable cells for treating blood disorders. Current efforts are directed toward adapting in vitro hematopoietic differentiation methods developed for murine ES cells to human lines, identifying the key interspecies differences in biologic properties of ES cells, and generating ES cell-derived hematopoietic stem cells that are competent to repopulate adult hosts. The ultimate medical goal is to create patient-specific and generic ES cell lines that can be expanded in vitro, genetically altered, and differentiated into cell types that can be used to treat hematopoietic diseases.

Gene expression profiling shows that macrophages derived from mouse embryonic stem cells is an improved in vitro model for studies of vascular disease

Macrophages (Mphi) play an important role in the initiation and progress of the atherogenic process. They contribute to the growth of atherosclerotic plaque by affecting lipoprotein metabolism, matrix homeostasis, lipoprotein modification and cholesterol accumulation. Access to in vitro Mphi models is therefore important for understanding the mechanisms involved in the transition of the relatively simple fatty streak to more complex type of lesions. The aim of the present work was to compare the expression profile of macrophages differentiated from the hematopoietic lineage to peritoneal mouse macrophages and two commonly used mouse macrophage cell lines (J774.A1 and RAW264.7). Our results showed that the embryonic stem cell-derived macrophages (ES Mphi) had a more similar expression profile to peritoneal macrophages than the two mouse macrophage cell lines. The ES Mphi had unchanged expression of the majority of cholesterol efflux mediators when compared to mouse peritoneal macrophages, whereas the cell lines showed altered expression of several of these genes. A key gene in this process is apolipoprotein E, which is expressed in ES Mphi but not in macrophage cell lines. In conclusion, ES Mphi are likely to provide a better in vitro model than mouse Mphi cell lines to study macrophage involvement in atherosclerosis.

Activation of signaling pathways by putative scavenger receptor class A (SR-A) ligands requires CD14 but not SR-A

Macrophage scavenger class A type I and type II receptors (SR-A) are trimeric, integral membrane glycoproteins that bind an unusually broad array of macromolecular ligands. These ligands include modified proteins and lipoproteins, nucleic acids, and a variety of plant and microbial cell wall constituents, such as fucoidan and lipoteichoic acid. Early studies of SR-A functions indicated that the receptors bound, internalized, and degraded their ligands without provoking any macrophage activating signaling events. More recent studies have provided evidence that several SR-A ligands can activate macrophage gene expression via utilization of a receptor-linked, PI3-kinase pathway. To investigate the role of SR-A in engaging signal transduction events, we employed macrophages taken from mice lacking these receptors. Using either fucoidan or lipoteichoic acid, we confirm that both ligands stimulate tyrosine phosphorylation of PI3-kinase and production of modest levels of the cytokine, TNFalpha. However, macrophages taken from SR-A null mice did not differ from wild type macrophages in these responses, indicating that these signaling events arise independently of SR-A activity. Employing mice lacking CD14, a GPI anchored receptor that binds bacterial lipopolysaccharide and signals via activation of Toll-like receptors, we show that the fucoidan and lipoteichoic acid responses are largely abrogated when CD14 is absent. These data do not provide support for direct SR-A involvement in signal transduction events and suggest that the early characterization of these receptors as initiators of a non-phlogistic, pathogen clearance pathway was correct.

Scavenger receptors class A-I/II and CD36 are the principal receptors responsible for the uptake of modified low density lipoprotein leading to lipid loading in macrophages

Modification of low density lipoprotein (LDL) can result in the avid uptake of these lipoproteins via a family of macrophage transmembrane proteins referred to as scavenger receptors (SRs). The genetic inactivation of either of two SR family members, SR-A or CD36, has been shown previously to reduce oxidized LDL uptake in vitro and atherosclerotic lesions in mice. Several other SRs are reported to bind modified LDL, but their contribution to macrophage lipid accumulation is uncertain. We generated mice lacking both SR-A and CD36 to determine their combined impact on macrophage lipid uptake and to assess the contribution of other SRs to this process. We show that SR-A and CD36 account for 75-90% of degradation of LDL modified by acetylation or oxidation. Cholesteryl ester derived from modified lipoproteins fails to accumulate in macrophages taken from the double null mice, as assessed by histochemistry and gas chromatography-mass spectrometry. These results demonstrate that SR-A and CD36 are responsible for the preponderance of modified LDL uptake in macrophages and that other scavenger receptors do not compensate for their absence.

PPARgamma is not a critical mediator of primary monocyte differentiation or foam cell formation

In the present report we clarify the role of PPARgamma in differentiation and function of human-derived monocyte/macrophages in vitro. Rosiglitazone, a selective PPARgamma activator, had no effect on the kinetics of appearance of monocyte/macrophage differentiation markers or on cell size or granularity. Depletion of PPARgamma by more than 90% using antisense oligonucleotides did not influence accumulation of oxidized LDL or prevent the upregulation of CD36 that normally accompanies oxLDL treatment. In contrast, PPARgamma depletion reduced the expression of ABCA1 and LXRalpha mRNAs. Metalloproteinase-9 expression, a marker of atherosclerotic plaque vulnerability, was suppressed by rosiglitazone. We conclude that activation of PPARgamma does not affect monocyte/macrophage differentiation. In addition, PPARgamma is not absolutely required for oxLDL-driven lipid accumulation, but is required for full expression of ABCA1 and LXRalpha. Our data support a role for rosiglitazone as a potential directly acting antiatherosclerotic agent.

The role of PPAR-gamma in macrophage differentiation and cholesterol uptake

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma), the transcription factor target of the anti-diabetic thiazolidinedione (TZD) drugs, is reported to mediate macrophage differentiation and inflammatory responses. Using PPAR-gamma-deficient stem cells, we demonstrate that PPAR-gamma is neither essential for myeloid development, nor for such mature macrophage functions as phagocytosis and inflammatory cytokine production. PPAR-gamma is required for basal expression of CD36, but not for expression of the other major scavenger receptor responsible for uptake of modified lipoproteins, SR-A. In wild-type macrophages, TZD treatment divergently regulated CD36 and class A macrophage-scavenger receptor expression and failed to induce significant cellular cholesterol accumulation, indicating that TZDs may not exacerbate macrophage foam-cell formation.

Sites of action of protein kinase C and phosphatidylinositol 3-kinase are distinct in oxidized low density lipoprotein-induced macrophage proliferation

Oxidized low density lipoprotein (Ox-LDL) can induce macrophage proliferation in vitro. To explore the mechanisms involved in this process, we reported that activation of protein kinase C (PKC) is involved in its signaling pathway (Matsumura, T., Sakai, M., Kobori, S., Biwa, T., Takemura, T., Matsuda, H., Hakamata, H., Horiuchi, S., and Shichiri, M. (1997) Arterioscler. Thromb. Vasc. Biol. 17, 3013-3020) and that expression of granulocyte/macrophage colony-stimulating factor (GM-CSF) and its subsequent release in the culture medium are important (Biwa, T., Hakamata, H., Sakai, M., Miyazaki, A., Suzuki, H., Kodama, T., Shichiri, M., and Horiuchi, S. (1998) J. Biol. Chem. 273, 28305-28313). However, a recent study also demonstrated the involvement of phosphatidylinositol 3-kinase (PI3K) in this process. In the present study, we investigated the role of PKC and PI3K in Ox-LDL-induced macrophage proliferation. Ox-LDL-induced macrophage proliferation was inhibited by 90% by a PKC inhibitor, calphostin C, and 50% by a PI3K inhibitor, wortmannin. Ox-LDL-induced expression of GM-CSF and its subsequent release were inhibited by calphostin C but not by wortmannin, whereas recombinant GM-CSF-induced macrophage proliferation was inhibited by wortmannin by 50% but not by calphostin C. Ox-LDL activated PI3K at two time points (10 min and 4 h), and the activation at the second but not first point was significantly inhibited by calphostin C and anti-GM-CSF antibody. Our results suggest that PKC plays a role upstream in the signaling pathway to GM-CSF induction, whereas PI3K is involved, at least in part, downstream in the signaling pathway after GM-CSF induction.

Shortened microsatellite d(CA)21 sequence down-regulates promoter activity of matrix metalloproteinase 9 gene

One characteristic elements in the promoter of the matrix metalloproteinase 9 (MMP-9) gene is the d(CA) repeat. To investigate whether this element regulates the transcription of the MMP-9 gene and its enzymatic activities, we sequenced the promoter region isolated from esophageal carcinoma cell lines. TE9 cells with low MMP-9 enzymatic activity had the number of d(CA) repeats shortened from 21 to 14 or 18. TE8, TE10 and TE11 cells with high MMP-9 activities had 21 or 23 d(CA) repeats. Luciferase assays using MMP-9 promoter containing 18, 14 or 0 d(CA) repeats showed transcriptional activities which were 50, 50 or 5%, respectively, of the level achieved with promoter containing 21 d(CA) repeats. Sequence analysis of the promoter of 223 Japanese subjects revealed that most had two alleles with 20, 21 or 22 d(CA) repeats, whereas six had one or two alleles with 14, 18 or 19 d(CA) repeats. We postulate that length alteration of the d(CA) repeat causes phenotypic differences among carcinoma cells and that microsatellite instability may contribute to the polymorphism of d(CA) repeat length.