Three probe flow cytometry of a human foam-cell forming macrophage

Quantification of sterol-specific response in human macrophages using automated imaged-based analysis

BACKGROUND

The transformation of normal macrophage cells into lipid-laden foam cells is an important step in the progression of atherosclerosis. One major contributor to foam cell formation in vivo is the intracellular accumulation of cholesterol.

METHODS

Here, we report the effects of various combinations of low-density lipoprotein, sterols, lipids and other factors on human macrophages, using an automated image analysis program to quantitatively compare single cell properties, such as cell size and lipid content, in different conditions.

RESULTS

We observed that the addition of cholesterol caused an increase in average cell lipid content across a range of conditions. All of the sterol-lipid mixtures examined were capable of inducing increases in average cell lipid content, with variations in the distribution of the response, in cytotoxicity and in how the sterol-lipid combination interacted with other activating factors. For example, cholesterol and lipopolysaccharide acted synergistically to increase cell lipid content while also increasing cell survival compared with the addition of lipopolysaccharide alone. Additionally, ergosterol and cholesteryl hemisuccinate caused similar increases in lipid content but also exhibited considerably greater cytotoxicity than cholesterol.

CONCLUSIONS

The use of automated image analysis enables us to assess not only changes in average cell size and content, but also to rapidly and automatically compare population distributions based on simple fluorescence images. Our observations add to increasing understanding of the complex and multifactorial nature of foam-cell formation and provide a novel approach to assessing the heterogeneity of macrophage response to a variety of factors.

Cholesterol Efflux Capacity of Apolipoprotein A-I Varies with the Extent of Differentiation and Foam Cell Formation of THP-1 Cells

Apolipoprotein A-I (apoA-I), the main protein component of high-density lipoprotein (HDL), has many protective functions against atherosclerosis, one of them being cholesterol efflux capacity. Although cholesterol efflux capacity measurement is suggested to be a key biomarker for evaluating the risk of development of atherosclerosis, the assay has not been optimized till date. This study aims at investigating the effect of different states of cells on the cholesterol efflux capacity. We also studied the effect of apoA-I modification by homocysteine, a risk factor for atherosclerosis, on cholesterol efflux capacity in different states of cells. The cholesterol efflux capacity of apoA-I was greatly influenced by the extent of differentiation of THP-1 cells and attenuated by excessive foam cell formation. N-Homocysteinylated apoA-I indicated a lower cholesterol efflux capacity than normal apoA-I in the optimized condition, whereas no significant difference was observed in the cholesterol efflux capacity between apoA-I in the excessive cell differentiation or foam cell formation states. These results suggest that cholesterol efflux capacity of apoA-I varies depending on the state of cells. Therefore, the cholesterol efflux assay should be performed using protocols optimized according to the objective of the experiment.

Cholesterol selectively regulates IL-5 induced mitogen activated protein kinase signaling in human eosinophils

Eosinophils function contributes to human allergic and autoimmune diseases, many of which currently lack curative treatment. Development of more effective treatments for eosinophil-related diseases requires expanded understanding of eosinophil signaling and biology. Cell signaling requires integration of extracellular signals with intracellular responses, and is organized in part by cholesterol rich membrane microdomains (CRMMs), commonly referred to as lipid rafts. Formation of these organizational membrane domains is in turn dependent upon the amount of available cholesterol, which can fluctuate widely with a variety of disease states. We tested the hypothesis that manipulating membrane cholesterol content in primary human peripheral blood eosinophils (PBEos) would selectively alter signaling pathways that depend upon membrane-anchored signaling proteins localized within CRMMs (e.g., mitogen activated protein kinase [MAPK] pathway), while not affecting pathways that signal through soluble proteins, like the Janus Kinase/Signal Transducer and Activator of Transcription [JAK/STAT] pathway. Cholesterol levels were increased or decreased utilizing cholesterol-chelating methyl-β-cyclodextrin (MβCD), which can either extract membrane cholesterol or add exogenous membrane cholesterol depending on whether MβCD is preloaded with cholesterol. Human PBEos were pretreated with MβCD (cholesterol removal) or MβCD+Cholesterol (MβCD+Chol; cholesterol delivery); subsequent IL-5-stimulated signaling and physiological endpoints were assessed. MβCD reduced membrane cholesterol in PBEos, and attenuated an IL-5-stimulated p38 and extracellular-regulated kinase 1/2 phosphorylation (p-p38, p-ERK1/2), and an IL-5-dependent increase in interleukin-1β (IL-1β) mRNA levels. In contrast, MβCD+Chol treatment elevated PBEos membrane cholesterol levels and basal p-p38, but did not alter IL-5-stimulated phosphorylation of ERK1/2, STAT5, or STAT3. Furthermore, MβCD+Chol pretreatment attenuated an IL-5-induced increase in cell survival at 48 hours, measured as total cellular metabolism. The reduction in cell survival following cholesterol addition despite unaltered STAT phosphorylation contradicts the current dogma in which JAK/STAT activation is sufficient to promote eosinophil survival, and suggests an additional, unidentified mechanism critically regulates IL-5-mediated human PBEos survival.

Effect of culture conditions on the phenotype of THP-1 monocyte cell line

PROBLEM

Macrophage function has many implications in a variety of diseases. Understanding their biology becomes imperative when trying to elucidate immune cell interactions with their environment, and in vitro cell lines allow researchers to manipulate these interactions. A common cell line used is THP-1, a promyeloid cell line suggestive to outside factors, and therefore sensitive to culture conditions. In this study, we describe how culture conditions can alter THP-1 morphology and in turn affect their response to differentiation stimuli.

METHOD OF STUDY

THP-1 cells were cultured in two conditions and treated with phorbol 12-myristate 13-acetate (PMA) or MCSF. CD14 surface expression was determined by flow cytometry and cytokine/chemokine production determined by multiplex analysis.

RESULTS

Culture conditions of THP-1 affect their response to PMA. Highly confluent THP-1 cells differentiate into a heterogeneous population responsive to PMA as seen by an increase in CD14 expression. However, these cells, cultured in low confluence, remain as a homogenous population and do not gain CD14. Additionally, there are major differences in the constitutive cytokine profile.

CONCLUSION

We demonstrate that the culture conditions of THP-1 cells can alter their response PMA. This suggests that culture techniques may account for the discrepancy in the literature of both basal THP-1 phenotype and their response to PMA.

Characterization of native and oxidized human low-density lipoproteins by the Z-scan technique

The nonlinear optical response of human normal and oxidized by Cu2+ low-density lipoproteins particles (LDL), were investigated by the Z-scan technique as a function of temperature and concentration of LDL particles. The Z-scan signals increase linearly with concentration of normal LDL particles, following the usual Beer-Lambert law in a broad range of concentrations. The oxidized LDL particles do not show nonlinear optical response. On the other hand, normal LDL increases its nonlinear optical response as a function of temperature. These behaviors can be attributed to an absorbing element that is modified by the oxidative process. Contrarily, changes in the physical state of the cores and conformation of the ApoB100 protein due to an increase in temperature seems to enhance their nonlinear optical properties. This tendency is not due to aggregation of particles. The main contribution to the nonlinear optical response of normal LDL particles comes from the phospholipid fraction of the particles.

7-Ketocholesterol favors lipid accumulation and colocalizes with Nile Red positive cytoplasmic structures formed during 7-ketocholesterol-induced apoptosis: Analysis by flow cytometry, FRET biphoton spectral imaging microscopy, and subcellular fractionati

BACKGROUND

Oxidized low-density lipoproteins play key roles in atherosclerosis. Their toxicity is at least in part due to 7-ketocholesterol (7KC), which is a potent inducer of apoptosis. In this study on human promonocytic U937 cells, we determined the effects and the interactions of 7KC with cellular lipids during 7KC-induced apoptosis.

METHODS

Morphologic and functional changes were investigated by microscopic and flow cytometric methods after staining with propidium iodide, 3,3'-dihexyloxacarbocyanine iodide, and Hoechst 33342. Cellular lipid content was identified by using filipin to quantify free cholesterol and Nile Red (NR), which emit a yellow or orange-red fluorescence in the presence of neutral and polar lipids, respectively. After staining with NR, interactions of 7KC with cellular lipids were identified by fluorescence resonance energy transfer biphoton spectral imaging confocal microscopy and by subcellular fractionation, gas chromatography, and mass spectrometry.

RESULTS

During 7KC-induced apoptosis the fluorescence from filipin and the ratio of measured (orange-red vs. yellow) fluorescence of NR were enhanced. Spectral analysis of images obtained in biphoton mode and resulting factor images demonstrated the occurrence of fluorescence resonance energy transfer between 7KC and NR and the subsequent colocalization of 7KC and NR. These data were in agreement with biochemical characterization and demonstrated that 7KC and neutral and polar lipids accumulate in NR-stained cytoplasmic structures.

CONCLUSIONS

During 7KC-induced apoptosis, 7KC modifies the cellular content of neutral and polar lipids, favors free cholesterol accumulation, and colocalizes with neutral and polar lipids that are inside NR-stained cytoplasmic structures.

Microfluidics for flow cytometric analysis of cells and particles

This review describes recent developments in microfabricated flow cytometers and related microfluidic devices that can detect, analyze, and sort cells or particles. The high-speed analytical capabilities of flow cytometry depend on the cooperative use of microfluidics, optics and electronics. Along with the improvement of other components, replacement of conventional glass capillary-based fluidics with microfluidic sample handling systems operating in microfabricated structures enables volume- and power-efficient, inexpensive and flexible analysis of particulate samples. In this review, we present various efforts that take advantage of novel microscale flow phenomena and microfabrication techniques to build microfluidic cell analysis systems.

Glycoxidized low-density lipoprotein regulates the expression of scavenger receptors in THP-1 macrophages

Low-density lipoprotein (LDL) in patients with diabetes is subject to modification by both oxidation and glycation. In contrast to oxidized LDL, the biological effects of glycoxidized LDL have not been well characterised. In this study, the effects of oxidized, glycated, glycoxidized and oxidized LDL on scavenger receptor gene expressions, and the induction of oxidized LDL uptake and cholesteryl ester accumulation in THP-1 macrophages were compared. Modified LDL was incubated with THP-1 macrophages. Gene expression of scavenger receptor class A (SR-A), CD36 and scavenger receptor class B type I (SR-BI) was determined by quantitative reverse transcriptase PCR (RT-PCR). Glycoxidized LDL was able to significantly induce SR-A and CD36 expression by 3- and 4.5-fold, respectively, in macrophages whereas SR-BI expression was suppressed by glycoxidized LDL, glycated LDL and oxidized LDL. Incubation with glycoxidized LDL enhanced the uptake of DiI-labeled oxidized LDL by macrophages to a greater extent than that of glycated LDL or oxidized LDL. Glycoxidized LDL also induced a significant degree of intracellular cholesteryl ester accumulation. Taken together, our results would suggest that glycoxidized LDL might be an important candidate in the initiation of foam cell formation and might play a significant role in the pathogenesis of atherosclerosis in diabetes mellitus.

Membrane cholesterol regulates LFA-1 function and lipid raft heterogeneity

Many surface receptors and signaling molecules are thought to associate with unique membrane microdomains termed lipid rafts. We examined the involvement of lipid rafts in the activation of leukocyte function-associated antigen-1 (LFA-1). Depletion or sequestration of cholesterol with methyl-beta-cyclodextrin (MCD) or filipin, respectively, strongly inhibited LFA-1-mediated adhesion of T-cell lines and primary T cells. This inhibition was reversed by cholesterol reconstitution. LFA-1 on T-cell lines was detected in cold Triton X-100-insoluble lipid rafts, which were disrupted by MCD or filipin treatment. However, no LFA-1 on primary T cells was detected in lipid rafts isolated by the same procedures, and these rafts were resistant to cholesterol depletion or sequestration. Association of LFA-1 with lipid rafts of primary T cells could be detected only when they were isolated with another nonionic detergent, Brij 35. Upon treatment with MCD, LFA-1 in Brij 35-insoluble lipid rafts partially shifted to nonraft fractions. T-cell lines were found to have a high level of cholesterol and a low level of ganglioside GM1, a common marker for lipid rafts, whereas primary T cells have a much lower level of cholesterol and a very high amount of GM1. Cross-linking of LFA-1 on primary T cells induced cocapping of cholesterol but not GM1. These results suggest that lipid rafts of T cells are heterogenous, and LFA-1 associates with a subset of lipid rafts containing a high level of cholesterol. This association seems to regulate LFA-1 functions, possibly by facilitating LFA-1 clustering.

Nile Red binding to HepG2 cells: an improved assay for in vitro studies of hepatosteatosis

Nile Red is a fluorescent dye used extensively to study fat accumulation in many types of cells; unfortunately protocols that work well for most cells are not effective for studying drug-induced lipid accumulation in cultured liver cells and hepatocyte-derived cell lines. Using human hepatoma (HepG2) cells, we have developed a simple Nile Red binding assay as a screen for steatosis-inducing compounds. Increases in Nile Red binding in response to known hepatotoxic compounds were observed after incubating treated cells with 1 microM Nile Red for several hours, washing away free Nile Red, and then allowing redistribution, and/or clearance of the lipid-indicator dye. Several compounds known to cause hepatic fat accumulation in vivo were examined and most robustly increased Nile Red binding in HepG2 cells. These include estrogen and other steroids, ethionine, cyclosporin A, and valproic acid. Required concentrations for increased Nile Red binding were generally three-fold or more lower than the cytotoxic concentration determined by a resazurin reduction assay in the same cells. Qualitatively similar Nile Red binding results were obtained when primary canine or rat hepatocytes were used. Morphological differences in Nile Red staining were observed by confocal fluorescence microscopy in HepG2 cells after treatment with different compounds and likely reflect distinct toxicological mechanisms.

Current status of flow cytometry in cell and molecular biology

This review summarizes recent developments in flow cytometry (FC). It gives an overview of techniques currently available, in terms of apparatus and sample handling, a guide to evaluating applications, an overview of dyes and staining methods, an introduction to internet resources, and a broad listing of classic references and reviews in various fields of interest, as well as some recent interesting articles.

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

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

Lipoprotein(a) isoforms display differences in affinity for plasminogen-like binding to human mononuclear cells

Binding of lipoprotein(a) (Lp(a)) to membrane proteins of the monocyte-macrophage cell lineage may be an important event in atheroma formation. Since Lp(a) with distinct apolipoprotein(a) (apo(a)) isoforms may show differences in their affinity with regard to fibrin binding, the existence of such a functional behavior in the interaction of apo(a) in Lp(a) with these cells was explored using the monocytic cell line THP-1. Lp(a) preparations containing small size apo(a) isoforms (M(r) = 450,000 to 550,000) and high molecular mass isoforms (M(r) > or = 700,000) were purified from plasmas containing > 0.35 g/L of Lp(a) obtained from subjects (n = 14) with cardiovascular atherosclerotic disease. Binding of plasminogen to THP-1 cells was performed using the method of radioisotopic dilution. For binding of Lp(a) to cells, the THP-1 monocytic cells were incubated with varying concentrations of the different Lp(a) preparations; cells were then washed and the amount of Lp(a) bound was detected with a radiolabeled polyclonal antibody directed against apo(a). Binding due to kringle interactions with lysine residues was calculated by subtracting from the total bound the amount of Lp(a) bound (approximately 10%) in the presence of 6-aminohexanoic acid. Analysis of data with the Langmuir equation indicated identical and independent (non-interacting) sites and allowed evaluation of the Kd. Binding isotherms of small size isoforms showed saturation and a high affinity (Kd = 25.8 +/- 19 nmol/L) relative to that of plasminogen (Kd = 1750 +/- 760 nmol/L). A similar difference (Kd = 17.5 +/- 7.9 nmol/L versus Kd = 600 +/- 220 nmol/L) was found when binding experiments were performed with a fibrin surface. In contrast, binding isotherms of the high molecular mass isoforms did not show saturation at the highest Lp(a) concentrations used, thus indicating a lower affinity. In conclusion, these results show that apo(a) isoforms may display polymorphism-linked functional heterogeneity with regard to cell binding, which may explain the higher association with cardiovascular risk of small size isoforms. These qualitative differences in the binding of apo(a) isoforms to fibrin or cells may modulate the cardiovascular risk associated with high levels of Lp(a).

Human (THP-1) macrophages oxidize LDL by a thiol-dependent mechanism

The oxidative modification of low-density lipoprotein by macrophages may be an important mechanism in the pathogenesis of atherosclerosis. The human monocytic leukaemic cell line THP-1, when stimulated with phorbol ester, shares many properties with human monocyte-derived macrophages. Oxidation of LDL by these cells was characterised by depletion of alpha-tocopherol, increases in thiobarbituric acid reactive substances and increases in electrophoretic mobility. The LDL particles were also converted to a form which increased accumulation of cholesteryl esters within macrophages. The oxidative mechanism appeared to be dependent upon the presence of thiols in the cellular medium. Oxidation of LDL by THP-1 macrophages, and production of thiols by these cells, were dependent upon the presence of L-cystine in the medium. Furthermore, cellular oxidation of LDL could be partially mimicked by the addition of cysteine to Hams F10 medium. Macrophage-independent oxidation of LDL, mediated by the addition of copper ions, was inhibited by cystine and cysteine in phosphate buffered saline, but not in Hams F10 medium. The glutathione content of THP-1 macrophages was also dependent upon the presence of cysteine or cystine in the medium, but inhibition of glutathione synthesis by buthionine sulfoximine did not prevent the production of thiols or the oxidation of LDL by THP-1 macrophages.

Impaired mobilisation of cholesterol from stored cholesteryl esters in human (THP-1) macrophages

The formation of macrophage-derived foam cells is central to the development of fatty streaks within the arterial wall, and to the progression of atherosclerosis. The unregulated deposition of cholesteryl esters, as lipid droplets within the cytoplasm of these cells, is responsible for the formation of foam cells; this process is thought to be regulated by the balance between cholesterol esterification, by acyl CoA:cholesterol acyltransferase (ACAT), and hydrolysis, by neutral cholesteryl ester hydrolase (nCEH). This study examines the importance of the balance between these two enzymes in determining the efflux of cholesterol from human (THP-1) macrophages. The presence of modified lipoprotein, or of 25-hydroxycholesterol, markedly increased cholesterol esterification in these cells and these effects were potently inhibited by the presence of the ACAT inhibitor, 447C88. In the absence of HDL, an acceptor particle, there was little or no hydrolysis of the cholesteryl ester pool and no efflux of cholesterol to the extracellular milieu; addition of HDL led to a partial (36%) reduction in cholesteryl esters, an effect which was not enhanced by the inhibition of ACAT. This suggested that the stored cholesteryl esters in human (THP-1) macrophages, unlike those in mouse peritoneal macrophages, were relatively resistant to removal by efflux to HDL. Efflux of newly synthesised free cholesterol from these macrophages was increased by HDL in a saturable manner, suggesting that the lack of reduction of stored cholesteryl esters was due to impaired mobilisation of cholesteryl esters to free cholesterol via nCEH. Indeed, nCEH activity in these macrophages was much lower than in mouse peritoneal macrophages, and appeared to be down-regulated in the presence of 25-hydroxycholesterol or modified lipoproteins; this loss of nCEH activity was prevented by the ACAT inhibitor 447C88. The efflux of stored cholesteryl esters from THP-1 macrophages therefore appears to be limited by the activity of nCEH.

Changes in free cholesterol content, measured by filipin fluorescence and flow cytometry, correlate with changes in cholesterol biosynthesis in THP-1 macrophages

The free cholesterol content of cells can be monitored by the intensity of fluorescence emissions from the polyene antibiotic filipin. In a previous study (Hassall: Cytometry 13:381-388, 1992) using THP-1 macrophages, a decrease in filipin fluorescence in response to increasing concentrations of modified lipoprotein was observed, suggesting a reduction in the free cholesterol content of the cells. In this study, THP-1 macrophages were treated with a number of agents known to modulate cholesterol biosynthesis and cholesterol esterification. Changes in filipin fluorescence emissions were measured by flow cytometry, and correlated with changes in cholesterol biosynthesis measured by incorporation of [14C]acetate into cholesterol. A correlation between decreases in filipin fluorescence and reductions in cholesterol biosynthesis was apparent, even when cholesterol esterification was inhibited. These results suggest that the decreases in filipin fluorescence observed may be due, at least in part, to reduction in cholesterol biosynthesis.

Human (THP-1) macrophages oxidize LDL by a thiol-dependent mechanism

The oxidative modification of low-density lipoprotein by macrophages may be an important mechanism in the pathogenesis of atherosclerosis. The human monocytic leukaemia cell line THP-1, when stimulated with phorbol ester, shares many properties with human monocyte-derived macrophages. Oxidation of LDL by these cells was characterised by depletion of alpha-tocopherol, increases in thiobarbituric acid reactive substances and increases in electrophoretic mobility. The LDL particles were also converted to a form which increased accumulation of cholesteryl esters within macrophages. The oxidative mechanism appeared to be dependent upon the presence of thiols in the cellular medium. Oxidation of LDL by THP-1 macrophages, and production of thiols by these cells, were dependent upon the presence of L-cystine in the medium. Furthermore, cellular oxidation of LDL could be partially mimicked by the addition of cysteine to Hams F10 medium. Macrophage-independent oxidation of LDL, mediated by the addition of copper ions, was inhibited by cystine and cysteine in phosphate buffered saline, but not in Hams F10 medium. The glutathione content of THP-1 macrophages was also dependent upon the presence of cysteine or cystine in the medium, but inhibition of glutathione synthesis by buthionine sulfoximine did not prevent the production of thiols or the oxidation of LDL by THP-1 macrophages.

Spectra of fluorescent dyes used in flow cytometry

Flow cytometry uses a relatively small set of dyes for immunochemistry and nucleic acid detection, most of which have been known and used reliably in flow cytometry for several years. These can usually be combined to make simultaneous two-color measurements of multiple cell-surface antigens and nucleic acid content. Because of the overlap of dye spectra and difficulties in finding dyes with substantial Stokes shifts that can be excited by the argon laser, simultaneous three-color or more detection can be more difficult. A basic knowledge of the factors that go into producing the fluorescent signal, including the spectra of dyes and their overlap, is necessary in planning multicolor experiments. By contrast, there have been a number of new fluorescent probes developed for detecting ions, membrane potential, metabolism, organelles, and other properties of living cells as well as for determining cell viability, proliferation, and cell tracking. So far most of these physiological probes are being used only for fundamental research rather than for cell classification. However, as research activity expands in this area, its diagnostic potential is likely to be increasingly appreciated.

Peroxynitrite modification of low-density lipoprotein leads to recognition by the macrophage scavenger receptor

Peroxynitrite is an oxidant which could be formed in the vasculature by the reaction of superoxide with nitric oxide. It is capable of modifying amino acid residues and of initiating lipid peroxidation. In the present study we have shown that peroxynitrite converts low density lipoprotein to a form recognized by the macrophage scavenger receptor and that this process is associated with modification of the protein and lipid, and with the oxidation of alpha-tocopherol to alpha-tocopherol quinone.