Enzyme kinetic studies in cell populations using fluorogenic substrates and flow cytometric techniques

Spectral flow cytometry: Fundamentals and future impact

Spectral flow cytometry has emerged as a significant player in the cytometry marketplace, with the potential for rapid growth. Despite a slow start, the technology has made significant strides in advancing various areas of single-cell analysis utilized by the scientific community. The integration of spectral cytometry into clinical laboratories and diagnostic processes is currently underway and is expected to garner a significant level of widespread acceptance in the near future. However, incorporating a new methodological approach into existing research programs can lead to misunderstandings or even misuse. This chapter offers an introductory yet comprehensive explanation of the scientific principles that form the foundation of spectral cytometry. Specifically, it delves into the unmixing processes that are utilized for data analysis. This overview is designed for those who are new to the field and seeking an informative guide to this exciting emerging technology.

Measuring enzyme activity in single cells

Seemingly identical cells can differ in their biochemical state, function and fate, and this variability plays an increasingly recognized role in organism-level outcomes. Cellular heterogeneity arises in part from variation in enzyme activity, which results from interplay between biological noise and multiple cellular processes. As a result, single-cell assays of enzyme activity, particularly those that measure product formation directly, are crucial. Recent innovations have yielded a range of techniques to obtain these data, including image-, flow- and separation-based assays. Research to date has focused on easy-to-measure glycosylases and clinically-relevant kinases. Expansion of these techniques to a wider range and larger number of enzymes will answer contemporary questions in proteomics and glycomics, specifically with respect to biological noise and cellular heterogeneity.

Mathematical modeling of a single-cell enzyme assay

A quantitative assay of beta-galactosidase activity in single cells of Saccharomyces cerevisiae has been developed using a fluorogenic substrate and flow cytometry [reported in Wittrup & Bailey, Cytometry, 9,394 (1988)]. The beta-galactosidase activity is expressed in yeast from the Escherichia coli lacZ gene under the control of the yeast GAL10 promoter, and is used as a marker for multicopy plasmid content. A nonfluorescent fluorogenic substrate is enzymatically cleaved by intracellular beta-galactosidase to form a fluorescent product. The accumulation of fluorescent product in single cells was found to depend on bulk substrate concentration and single-cell enzyme activity in a fashion that could not be described by a Michaelis-Menten kinetic rate form. It has been demonstrated that diffusion limitation rather than enzyme activity can determine the level of single-cell fluorescence under certain assay conditions, and a mathematical model has; been formulated which accounts for substrate and product diffusion. Guided by the mathematical model, the assay conditions were modified to allow measurement of single-cell enzyme activity rather than diffusion rates.

Development of a robust flow cytometric assay for determining numbers of viable bacteria

Several fluorescent probes were evaluated as indicators of bacterial viability by flow cytometry. The probes monitor a number of biological factors that are altered during loss of viability. The factors include alterations in membrane permeability, monitored by using fluorogenic substrates and fluorescent intercalating dyes such as propidium iodide, and changes in membrane potential, monitored by using fluorescent cationic and anionic potential-sensitive probes. Of the fluorescent reagents examined, the fluorescent anionic membrane potential probe bis-(1,3-dibutylbarbituric acid)trimethine oxonol [DiBAC(inf4)(3)] proved the best candidate for use as a general robust viability marker and is a promising choice for use in high-throughput assays. With this probe, live and dead cells within a population can be identified and counted 10 min after sampling. There was a close correlation between viable counts determined by flow cytometry and by standard CFU assays for samples of untreated cells. The results indicate that flow cytometry is a sensitive analytical technique that can rapidly monitor physiological changes of individual microorganisms as a result of external perturbations. The membrane potential probe DiBAC(inf4)(3) provided a robust flow cytometric indicator for bacterial cell viability.

Enzyme cytochemical techniques for metabolic mapping in living cells, with special reference to proteolysis

Specific enzymes play key roles in many pathophysiological processes and therefore are targets for therapeutic strategies. The activity of most enzymes is largely determined by many factors at the post-translational level. Therefore, it is essential to study the activity of target enzymes in living cells and tissues in a quantitative manner in relation to pathophysiological processes to understand its relevance and the potential impact of its targeting by drugs. Proteases, in particular, are crucial in every aspect of life and death of an organism and are therefore important targets. Enzyme activity in living cells can be studied with various tools. These can be endogenous fluorescent metabolites or synthetic chromogenic or fluorogenic substrates. The use of endogenous metabolites is rather limited and nonspecific because they are involved in many biological processes, but novel chromogenic and fluorogenic substrates have been developed to monitor activity of enzymes, and particularly proteases, in living cells and tissues. This review discusses these substrates and the methods in which they are applied, as well as their advantages and disadvantages for metabolic mapping in living cells.

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.

Proof without prejudice revisited: immunofluorescence histogram analysis using cumulative frequency subtraction plus ratio analysis of means

BACKGROUND

Apart from the work of Lampariello and colleagues (Cytometry 15:294-301, 1994; Cytometry 32:241-254, 1998), very little analytical work has been carried out for analysis of immunofluorescence distributions containing an overlapping mixture of labeled and unlabeled cells. The methods developed tend to rely on fitting theoretical distributions to the relevant populations. However, the method described here attempts to produce an analytical solution.

METHODS

A new method for immunofluorescence histogram analysis is presented. It uses cumulative frequency distribution subtraction of the test sample from the control to predict the mean of a labeled cell component embedded within a histogram containing unlabeled cells. Ratio analysis of means (RAM) was then carried out to calculate the labeled fraction. The results were submitted to Kolmogorov-Smirnov analysis and Student's t-test for validation at a given level of probability.

RESULTS

The method was developed with a data set exhibiting a small "positive" shoulder, which was predicted to contain a labeled fraction comprising 8.0% of the total at the 99% confidence limit. It was then tested with data analyzed and published previously where the Johnson Su family of distributions was used in curve fitting.

CONCLUSIONS

There was good agreement between the known and predicted proportions of labeled cells. However, the method is dependent on the symmetry of the distributions. Some minor systematic errors were encountered due, in part, to skewed experimental distributions.

Increased collagenase and dipeptidyl peptidase I activity in leucocytes from healthy elderly people

The incidence of infectious diseases increases with ageing. The enzymatic activity of leucocytes may have a relevant role in the morbidity and mortality due to infections in the elderly. In this study we have compared the activity of enzymes involved in the inflammatory response in leucocytes from young and elderly women. A total of 35 healthy females was studied, 20 volunteers aged 78-98 years (mean 89.1 years) and 15 young controls aged 19-34 years (mean 26 years). All of them were in good clinical condition, without any acute or chronic disease. Intracellular enzyme activity was analysed by flow cytometry in leucocytes from young and elderly women. The enzyme substrates employed were for oxidative burst, L-aminopeptidase, collagenase, cathepsin B, C, D and, G and dipeptidyl peptidase I. The intracellular enzyme activity assessed by flow cytometry in leucocytes from young and elderly women was similar, as far as oxidative burst, L-aminopeptidase, cathepsin B, C, D and G are concerned. An increased collagenase activity was detected in granulocytes from elders. The mean fluorescence channels for this enzyme corresponded to 86 +/- 23 and 60 +/- 15 in cells from elders and controls, respectively (P = 0.01224). An increased dipeptidyl peptidase I activity was detected in lymphocytes from elderly women. The corresponding values for this enzyme in elders and the young were 65.9 +/- 43.3 and 17.3 +/- 5, respectively (P = 0. 0036). The proper functional activity of intracellular enzymes involved in inflammatory responses is likely to be determinant for successful ageing.

Chemical synthesis and biological properties of novel fluorescent antifolates in Pgp- and MRP-overexpressing tumour cell lines

We have synthesised a series of fluorescent analogues of methylbenzoprim, a diaminopyrimidine antifolate which we have previously shown to exhibit in vivo antitumour activity in a methotrexate (MTX) "transport-resistant" tumour cell line. The analogues bear the dansyl, nitrobenzoxodiazole or methoxycoumarin fluorophores. The cytotoxicity of the compounds was evaluated using the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay against two human lung cancer cell lines, together with their multidrug resistant (MDR) sublines. H69/P is a small cell line and its multidrug resistant subline H69/LX4 overexpresses P-glycoprotein (Pgp). COR-L23/P is a large cell line and its multidrug resistant subline COR-L23/R overexpresses the multidrug resistance associated protein (MRP). IC50 values for the compounds (i.e. concentration to reduce cell growth by 50%) in the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay ranged from 0.20 to 0.81 microM in the H69 parental line and from 0.83 to 5.10 microM in the COR-L23 parent line. The MDR sublines both showed clear cross-resistance to each of the compounds, with resistance factors (ratio of IC50 value in resistant vs parental cell line) ranging from 16 to 137 in H69/LX4 and from 5 to 16 in COR-L23/R. For compounds (10) and (11) where drug accumulation was studied using flow cytometry, resistance was associated with an approximately 10-fold reduction in cellular drug accumulation over a period of 30 min. The drug resistance modifiers verapamil (used at 6.6 microM) and cyclosporin A (used at 4.2 microM) were tested for their ability to sensitise the resistant lines. Whereas verapamil showed little activity, cyclosporin A partially restored the activity of compound (10), and fully restored the activity of compound (11) in H69/LX4 cells. This sensitisation of H69/LX4 by cyclosporin A was associated with a partial restoration of the drug accumulation deficit in this line. Hence, these novel lipophilic antifolates appear to be substrates for both the P-glycoprotein and MRP resistance mechanisms. Therefore, although they have been designed to overcome one mechanism of methotrexate resistance, namely impaired drug transport, this has been achieved only at the cost of rendering them susceptible to alternative mechanisms.

Multivariate analyses of DNA index, p62c-myc, and clinicopathological status of patients with ovarian cancer

AIM

To determine if either DNA index or p62c-myc is an independent prognostic variable in ovarian cancer.

METHODS

Multivariate and univariate analyses of the relation between DNA index, p62c-myc, FIGO stage, histological type, tumour grade, completeness of surgery, and patient survival in ovarian cancer were examined.

RESULTS

Multivariate analysis showed significant association of survival only with stage and grade. There was no relation between survival and DNA index.

CONCLUSIONS

DNA index is not an independent prognostic variable in ovarian cancer.

Enzyme-assisted cell photosensitization: a proposal for an efficient approach to tumor therapy and diagnosis. The rose bengal fluorogenic substrate

Rose bengal, a xanthene derivative among the most efficient producer of singlet oxygen, was submitted to a chemical modification consisting in the introduction of an acetate group into the aromatic ring fluorophore structure. The acetate group acts as a quencher, thus inactivating both fluorescence and photosensitization properties of the molecule. In the modified structure, rose bengal acts as a fluorogenic substrate giving rise to the cellular reaction termed fluorochromasia. The acetate group is recognized by a carboxylic esterase activity that splits it. Removal of the quencher group results in restoring the native structure of photosensitizer inside the cells. The intracellular turnover of rose bengal acetate was studied in rat glioma-derived cultures cells, in terms of the balance of the processes of influx and enzyme hydrolysis of the fluorogenic substrate, and of the efflux of the fluorescent product. A large intracellular accumulation of photosensitizer is obtained when treatments are performed with the fluorogenic substrate, even at the drug concentration at which rose bengal does not enter the cells. The intracellular localization allows rose bengal to exert a more effective photosensitization effect. Provided that the quencher group is selected according to the metabolic properties of the tumor cells, the use of fluorogenic substrates as photosensitizer precursors could improve fluorescence diagnosis and the photodynamic therapy of tumors, exploiting the biological properties that distinguish pathological from normal conditions.

Flow cytometric method to measure urea-resistant alkaline phosphatase from blood neutrophils for use in the prenatal screening of Down's syndrome

The histochemical measurement of urea-resistant alkaline phosphatase from maternal blood neutrophils is known to have a high detection rate for the prenatal detection of Down's syndrome pregnancies. However, because the histochemical method is laborious and subjective to use, it has not gained widespread acceptance in prenatal screening programmes. We present a simple and objective method for the measurement of urea-resistant alkaline phosphatase by flow cytometry. The method should allow the design of larger studies aimed at evaluating the role of neutrophil urea-resistant alkaline phosphatase in the prenatal screening for Down's syndrome.

Recent advances in the isolation of liver cells

The development of new and refined separation techniques--including FACS, FFE, CFE and isopyknic gradients--has had a profound impact on the ability of investigators to isolate specific cell types from the liver. Although some of these techniques, such as FFE, may be of limited preparative value, they are nonetheless important analytical tools that detect subtle differences among cell subpopulations. The isolation of highly purified preparations of liver cells in large yields requires the use of more conventional purification methods such as CFE and isopyknic centrifugation. Immunological approaches represent a key development for the isolation of specific liver cell types, especially when they are used in combination with other techniques. Excellent, reliable and relatively simple techniques now exist to isolate highly purified preparations of hepatocytes, cholangiocytes, KCs, SCs, FSC, myofibroblasts and pit cells. Additional work is necessary to refine techniques for the isolation of dendritic cells and lymphocytes.

A comparison of rhodamine 123 accumulation and efflux in cells with P-glycoprotein-mediated and MRP-associated multidrug resistance phenotypes

Rhodamine 123 (Rh123) is a fluorescent dye which locates in the mitochondria of cells. It is a substrate for P-glycoprotein (Pgp) and can, therefore, be used as a molecular probe in studies of the multidrug resistance (MDR) phenotype. However, not all MDR cells overexpress Pgp. In some, the MDR phenotype is associated with expression of an alternative transporter molecule, the multidrug resistance-associated protein (MRP). We have studied the accumulation and efflux of Rh123 in MDR cells having both Pgp-mediated and MRP-associated phenotypes. In the mouse tumour parental cell line, EMT6/P, Rh123 accumulates rapidly to reach plateau levels by 90 min. Confocal microscopy confirms a localisation to the mitochondria. In the MDR subline, EMT6/AR1.0, which overexpresses Pgp and which is 10-fold resistant to Rh123 cytotoxicity, accumulation is dramatically reduced. Efflux of Rh123 from both resistant and parental lines is rapid but can be inhibited by reduced temperature or by the presence of cyclosporin A (5 micrograms/ml). Efflux from the parental line is probably due to the presence of very low, but detectable, levels of Pgp but the existence of other mechanisms cannot be ruled out. In contrast, the human lung cancer parental cell line COR-L23/P, and its MRP-associated (but Pgp-negative) MDR subline, COR-L23/R (which is 23-fold resistant to Rh123 cytotoxicity), accumulate Rh123 at similar rates for the first 30 min. The curves then diverge so that, at 180 min, the resistant cells contain only 70% of the Rh123 of parental cells. Confocal microscopy demonstrates a similar distribution of fluorescence in resistant and parental cells. Essentially no efflux of Rh123 occurs from parental cells, whereas 70% of the content is lost from resistant cells over a period of 150 min. Such efflux may again be inhibited by reduced temperature but cyclosporin A (5 micrograms/ml) has little effect. These observations should be borne in mind when interpreting Rh123 efflux data in terms of MDR mechanisms.

Multiparameter flow cytometric analysis of a novel cytotoxin (factor 2) induced tumor cell membrane permeability

An improved twin-probe multiparameter flow cytometric technique was applied to examine a novel cytotoxin, Factor (F2), induced tumor cell permeability. Ability to retain preloaded intracellular bis-carboxyethyl carboxyfluorescein (BCECF, green fluorescence) and to exclude extracellular propidium (red fluorescence) was measured simultaneously with forward and right-angle scatter. In addition to the two expected cell populations which were stained green negative, red positive ("membrane-damaged" and "non-viable", Region 2), and green positive, red negative ("membrane intact" and "viable", Region 3), a third population was seen which fluoresced neither green nor red and displayed intermediate light scatter characteristics (Region 1). K562 cells progressed from Region 3 to Region 1, and then from Region 1 to Region 2 after treatment with F2. These results suggest that sequential changes in membrane structure lead to increased permeability, first with respect to intracellular BCECF and then in turn to extracellular propidium. Flow cytometric changes caused by F2 were detectable 10 min after treatment with 2.5 U/ml of F2, and 5 min after 10 or 40 U/ml of F2. Flow cytometric analysis showed that F2-induced tumor cell lysis and growth inhibition were accompanied by rapid alternations in tumor cell membrane permeability. Flow cytometric analysis also distinguished F2 cytotoxicity from phorbol myristate acetate (PMA) associated cytotoxicity to K562 cells and determined that F2 produced spontaneously or induced by PMA and/or ciprofloxacin had a similar ability to induce tumor cell membrane permeability change.

Derivation and characterisation of a mouse tumour cell line with acquired resistance to cyclosporin A

Cyclosporin A (CsA) is an effective modifier of multidrug resistance. We have studied (a) the possibility that cells grown in increasing concentrations of CsA acquire cellular resistance to the agent and, (b) whether such cells have a multidrug resistant phenotype. Sublines of the EMT6 mouse tumour cell line were developed which were able to grow in 75 and 200 micrograms/ml of CsA, respectively. The resistant sublines grew slowly in the presence of CsA but reverted to control growth rates, whilst maintaining resistance, when the drug was removed. P-glycoprotein (Pgp) was not detectable in the resistant sublines by immunocytochemistry. The CsA-resistant cells were not cross-resistant to doxorubicin or vincristine but showed a clear degree of cross-resistance to the calcium transport blocker, verapamil. Cellular accumulation of both [3H]CsA and [3H]daunorubicin was significantly increased in the EMT6/CsA200R subline compared with the parent line. In the EMT6 parent line, which expresses very low levels of Pgp, 10-30-fold sensitisation to doxorubicin may be achieved using 0.1-5 microgram/ml of CsA. Similar sensitisation by CsA was also seen in the CsA-resistant sublines.

Fluid pinocytosis and esterase-oxidase in chronic myeloid leukemic granulocytes are differentially stimulated by chemotactic peptide

Binding of a chemotactic peptide, n-formyl-methionyl-leucyl-phenylalanine (FMLP) to polymorphonuclear leukocytes (PMNL) leads to a series of biochemical and functional events. We have studied stimulation of fluid phase pinocytosis (FP), esterase and oxidase by FMLP in CML PMNL, by flow cytometry. Stimulation of FP in CML PMNL was lower than that in normal PMNL but, stimulation of esterase and oxidase was comparable to that in normal PMNL. Thus, early response to FMLP which is dependent on the integrity of actin network seems to be defective in CML PMNL.

Time interval gating for analysis of cell function using flow cytometry

We propose a method which significantly shortens the time required for both the collection and analysis of data derived from multiple sample, flow cytometric kinetic assays. We have defined the term Time Interval Gating (TIG) to describe this method. TIG effectively allows one flow cytometer to concurrently monitor several samples over the course of a kinetic assay. Data for all samples are stored in a single FCS 2.0 compatible listmode data file which we refer to as the TIG data file. TIG is adaptable to most commerical flow cytometers. Standard listmode analysis software can be used to analyze the TIG data files and correlate any combination of tubes and/or time intervals from the assay. Results for the entire assay can be displayed on a single two parameter plot. This paper describes how TIG is applied to neutrophil oxidative burst measurement using a standard EPICS Elite flow cytometer. In this assay, 11 samples were each monitored for 30 min to identify the extent to which volatile organic chemicals (VOCs) inhibited the oxidation of DCFH in stimulated neutrophils. TIG makes the oxidative burst assay practical for high volume screening by reducing the overall flow cytometer and analysis time required by a factor of ten. In addition, TIG provides an organized approach to managing data acquisition on instruments equipped with automated sampling systems.

Characterisation of a mouse tumour cell line with in vitro derived resistance to verapamil

We have established a subline (EMT6/VRP) of the mouse tumour cell line EMT6/P with acquired resistance to the calcium transport blocker verapamil (VRP). The subline was 4-fold resistant to the cytoxicity of VRP alone compared with the parent line but of similar sensitivity to adriamycin, vincristine or colchicine. EMT6/VRP cells growing in 75 micrograms ml-1 VRP were morphologically different from and larger in diameter than EMT6/P cells, but these two parameters reverted almost to normal within 3 days of VRP removal, although resistance was retained. Expression of an mRNA coding for P-glycoprotein was similar in EMT6/VRP and the parent cell line, although considerable hyperexpression was seen in a multidrug resistant subline, EMT6/AR1.0. Cellular accumulation of both 3H-daunorubicin and 3H-VRP were greater in EMT6/VRP than in the parent line. Sensitisation to adriamycin by 3.3 micrograms ml-1 VRP was, however, somewhat reduced in EMT6/VRP (i.e. to 6.1-fold) compared with the 11-fold sensitisation seen in the parent line. It is clear that resistance to VRP seen in this cell line occurs via a different mechanism from the resistance to drugs such as adriamycin, vincristine and colchicine seen in multidrug resistant cell lines.

An improved method for the detection of cell surface antigens in samples of low viability using flow cytometry

A high non-specific background fluorescence signal was observed when cell surface antigen analysis was carried out using flow cytometry on a cell sample which contained a high proportion of dead and dying cells. To overcome this problem it was necessary to analyse the cells in three stages. First the intact cells were identified by their forward (FWD) and 90 degree scatter profile. These cells were gated-on, then analysed on the basis of their FWD scatter and propidium iodide (PI) signal, allowing the dead PI positive cells to be gated out. The PI negative cells were then displayed using their 90 degree scatter and fluorescence signals following staining with the irrelevant antibody control. This revealed a population of dead cells, which despite being PI negative, were non-specifically binding antibody molecules. Such multiparameter analysis permitted the successful analysis of cell surface antigens in preparations of low viability by gating out the high background fluorescence associated with dead PI positive and negative cells.

Multiparametric analysis of cell membrane permeability by two colour flow cytometry with complementary fluorescent probes

We describe an improved twin-probe multiparameter flow cytometric technique to examine cell membrane permeability. Ability to retain preloaded intracellular bis-carboxyethyl carboxy fluorescein (BCECF, green fluorescence) and to exclude extracellular propidium (red fluorescence) is measured, simultaneously with forward and right-angle scatter. This has significant advantages over an earlier method using fluorescein together with ethidium. In addition to the two expected cell populations which were stained green positive, red negative (by convention membrane "intact" and "viable," Region 1) and green negative, red positive ("membrane-damaged" and "non-viable," Region 3), a third population was seen which fluoresced neither green nor red and displayed intermediate light scatter characteristics (Region 2). This was true for each of 9 cell types in vitro. For EMT6 mouse mammary tumour cells held under sub-optimal conditions or treated with membrane-active drugs, progression from Region 1 to Region 2 was observed, followed by further progression from Region 2 to Region 3. Cells eventually accumulated in Region 3. These results suggest that sequential changes in membrane structure lead to increased permeability, first with respect to intracellular BCECF and in turn to extracellular propidium.

Flow cytometry chamber with 4 pi light collection suitable for epifluorescence microscopes

A compact, solid, spherico-ellipsoidal chamber (SEC), which has approaching 4 pi ("all around") light collection, has been developed for flow cytometry. This was mounted onto the stage of a standard fluorescence photomicroscope, and the camera was replaced by a photomultiplier. Both components can be added or removed in minutes. The increased light collection efficiency of the SEC (about 85%) compared with about 4% from standard chambers enabled a fluorescence microscope with a 50 W mercury vapour lamp to "double" as a flow cytometer. The system was tested with microbeads and cells stained for DNA with ethidium bromide, and results were comparable to those obtained with our laser-based instrument.

Flow cytometric comparison of the effects of trialkyltins on the murine erythroleukemic cell

Cellular effects of exposure to tributyltin (TBT), triethyltin (TET), or trimethyltin (TMT) were investigated by flow cytometry employing the murine erythroleukemic cell (MELC) as a model cellular system. Cell viability was investigated by the carboxyfluorescein diacetate (CFDA) uptake/propidium iodide (PI) exclusion method: above a critical concentration (exposure for 4 h), which was specific for each of the trialkyltin compounds, the cell becomes permeable to PI, indicating loss of viability. Cellular CF fluorescence (derived from intracellular hydrolysis of CFDA) increased as a function of alkyltin concentration below the critical concentration and decreased as viability decreased above the critical concentration. Relative membrane potential, monitored with a cyanine dye (DiOC6), correlated with viability (PI exclusion), remaining essentially unaltered below the critical concentration and decreasing above it. At/above 1 microM TBT, 5 microM TET, or 100 microM TMT, the cell cycle was blocked in the G2/M phase. The 90 degrees light scatter (a measure of refractive index), axial light loss (a measure of volume), and fluorescein isothiocyanate (FITC) fluorescence (a measure of protein content) of nuclei isolated from trialkyltin-treated MELC by detergent treatment, increased as a function of organotin dose. Fluorescence and interference microscopy revealed increased quantities of residual cytoplasmic tags adherent to the nuclei as a function of organotin dose, apparently resulting from increased cytoplasmic resistance to detergent-mediated solubilization. The effects of the trialkyltins correlated with their lipophilicity (octanol/water coefficient). These data support the hypothesis that fixation (protein denaturation, cross-linking, etc.) is an important mode of organotin cytotoxicity.

The MTT assay underestimates the growth inhibitory effects of interferons

The growth inhibitory effects of interferons, IFN-alpha and IFN-gamma on human lung cancer cell lines were studied using both a tetrazolium (MTT) colorimetric assay and direct cell counting. Significant discrepancies between the two assays were observed, the MTT assay consistently underestimating the growth inhibitory effects of the IFNs. There was no direct chemical effect of the IFNs on the tetrazolium reduction process. IFN treated cells showed increased cell size compared with control cells, although there was little or no change in cell cycle distribution. Mitochondrial activity was 30-50% greater in IFN-gamma treated cells (COR-L23) than the controls. Reduced formazan production per cell was observed in medium which had supported cell growth for several days. Differential 'medium conditioning' led to a difference in formazan production per cell between IFN and control cells and this was the major basis of the observed discrepancy. This discrepancy was not due to the differences in the glucose concentrations between these media. However, differences in pH between the media proved to be the major contributory factor of the discrepancy.

Fixation of the plasma membrane/cytoplasm complex: a mechanism of toxic interaction of tributyltin with the cell

Flow cytometric and light/fluorescence microscopic analysis of murine erythroleukemic cells (MELC) and electron microscopic investigation of porcine microsomal membrane preparations suggest that tributyltin (TBT) toxicity is mediated through fixation processes (protein denaturation, crosslinking, and so on) within the plasma membrane/cytoplasm complex. This hypothesis was derived from the following observations: 1. Exposure of the MELC to micromolar concentrations of TBT results in increased resistance to detergent-mediated cytolysis; 2. Exposure of porcine renal microsomal membrane preparations to similar concentrations results in inhibition of vanadate-mediated crystallization of Na+,K(+)-ATPase, a process requiring protein mobility within the membrane; 3. Flow cytometric and fluorescence microscopic analyses indicate that MELC exposed to submicromolar concentrations of TBT exhibit increased cellular carboxyfluorescein retention; and 4. Nuclei prepared from TBT-treated cells by detergent-mediated cytolysis exhibit increased axial light loss, 90 degrees light scatter, fluorescein isothiocyanate fluorescence, and the presence of adherent proteinaceous tags. The DNA distribution histogram of such nuclei also is perturbed.

Inhibition of cellular esterases by the antitumour imidazotetrazines mitozolomide and temozolomide: demonstration by flow cytometry and conventional spectrofluorimetry

Using flow cytometry and conventional spectrofluorimetry we have previously shown that chloroethylnitrosoureas (CNUs) can exhibit marked inhibition of cellular enzymes catalysing hydrolysis of fluorescein diacetate (FDA). More potent inhibition was seen for the carbamoylating CNUs, whereas alkylating agents were largely inactive. We now report results obtained with the developmental imidazotetrazines mitozolomide and temozolomide in comparison with BCNU, the novel alkylating agents clomesome and cyclodisone, and the active mitozolomide metabonate MCTIC. Inhibition of EMT6 mouse mammary-tumour esterases was seen for mitozolomide and temozolomide, and activity against purified porcine carboxylesterase was demonstrated. Flow cytometric analysis showed that inhibition occurred across the entire EMT6 cell population, with no evidence of a subpopulation resistant to enzyme inhibition. Inhibitory potency for the imidazotetrazines was much weaker than for BCNU. With EMT6 cells, I50 values from flow cytometry were 9.7 x 10(-3) M and 1.5 x 10(-3) M for mitozolomide and temozolomide compared with 3.7 x 10(-4) M for BCNU. These were higher than the ID50 values for in vitro antitumour activity (MTT assay), 8.5 x 10(-6) M in the case of mitozolomide and 1.2 x 10(-5) M for BCNU, but similar to that of 5.6 x 10(-4) M for the less toxic temozolomide. MCTIC and cyclodisone showed very low activity, but significant inhibition was seen for clomesome. The results are consistent with the view that the imidazotetrazines do not exhibit major carbamoylating ability, although significant effects are seen at cytotoxic concentrations of temozolomide. In addition, the potential for the generation of carbamoylating species at the enzyme active site cannot be ruled out.

Effects of tributyltin on biomembranes: alteration of flow cytometric parameters and inhibition of Na+, K+-ATPase two-dimensional crystallization

Carboxyfluorescein diacetate (CFDA) is a lipophilic nonfluorescent molecule that readily crosses the cell membrane. In the cytoplasm, it is hydrolyzed by nonspecific esterases to carboxyfluorescein (CF), a negatively charged fluorescent molecule, which is retained incompletely by cells with an intact plasma membrane. Exposure (4 hr) of the murine erythroleukemic cell (MELC) to micromolar quantities (0.1 to 5.0 microM) of tributyltin (TBT) results in increased cellular CF fluorescence. The increase occurs within a range below a critical value of the product (CPV) of the concentration (C) of TBT X duration (T) of exposure to TBT. Fluorescence increase is a sensitive indicator of the interaction of TBT with the cell: it is observed following exposure to 0.1 microM TBT for 4 hr at 37 degrees C. In the range above the CPV, cellular CF fluorescence is reduced apparently resulting from perturbation of membrane structure. For example, exposure of MELC to 2.5 microM TBT for 4 hr at 37 degrees C produces resistance to detergent-mediated cytolysis and inhibition of vanadate-mediated two-dimensional crystallization of Na+, K+-ATPase molecules in porcine renal microsomal membrane preparations, a process requiring molecular mobility within the membrane. Taken together, the increased cellular CF fluorescence and resistance of the MELC to cytolysis along with the inhibition of Na+, K+-ATPase crystallization in the microsomal membrane preparations suggest fixation (protein denaturation, cross-linking, etc.) at the level of the plasma membrane as a mode of toxic action of TBT.

Data compression: 8-dimensional flow cytometric data processing with 28K addressable computer memory

A method of data analysis for flow cytometry is presented which enables up to eight-dimensional data to be handled by a microcomputer with 28K addressable plus a further 32K non-addressable memory. The multi-parameter coordinates are coded into single numbers using a minimal modification of the array vector mapping equation. These code numbers, each of which corresponds to a given set of coordinates, and then ranked in ascending order according to magnitude and the frequency of each code number is found. Following this step the code is then decoded by integer arithmetic into its original coordinates which are then packed, together with the frequency, into two, three or four 16-bit words depending on the dimensionality of the data set. A five-dimensional data set is used as the illustration. Three regions were set on one two-dimensional data space and the five mono-dimensional histograms, plus a different bivariate distribution, were extracted in a single pass through the processed data file. In addition to considerable space saving the technique has two further attributes, namely, increased speed with which the user can appreciate multiparameter data and the ability to analyse such data sets with a microcomputer.

Inhibition of intracellular esterases by antitumour chloroethylnitrosoureas. Measurement by flow cytometry and correlation with molecular carbamoylation activity

Antitumour chloroethylnitrosoureas (Cnus) decompose in physiological conditions yielding alkylating species and organic isocyanates. While antitumour activity is mainly attributed to the alkylation of DNA, carbamoylation of intracellular proteins by isocyanates may also have pharmacological and toxicological relevance. We previously reported a novel dynamic flow cytoenzymological assay for esterase inhibition in intact murine cells by BCNU and related isocyanates, and proposed that this might form the basis of an assay for intracellular carbamoylation. We have now examined a wide range of Cnus, isocyanates, and alkylating agents for their ability to inhibit cellular esterases. BCNU, CCNU, their derived isocyanates, and the 4-OH metabolites of CCNU exhibited potent inhibitory activity (I50 values 5.5 x 10(-5)-7.3 x 10(-4) M). Chlorozotocin and GANU were relatively inactive (I50 much greater than 10(-2) M). ACNU, TCNU and the 2-OH metabolites of CCNU exhibited intermediate activity (I50 values 1.1 x 10(-3)-2.3 x 10(-2) M). Compounds not able to form isocyanates were essentially inactive. Poor membrane permeability was also implicated in the weak activity of chlorozotocin and GANU. There was overall a good correlation between esterase inhibition and chemical carbamoylating activity, but some particular differences were identified. We concluded that flow cytoenzymological assay appears to have the potential to provide useful measurement of intracellular protein carbamoylation by existing Cnus and novel derivatives, and also offers the advantage of cell subpopulation identification for in vivo evaluation of these agents.

Flow cytometric analysis of the cellular toxicity of tributyltin

Flow cytometric and light/fluorescence microscopic analyses indicate that tributyltin (TBT) alters the plasma membrane/cytoplasm complex of the murine erythroleukemic cell (MELC) in a dose-dependent and time-dependent manner. The flow cytometric parameter axial light loss, a measure of cell volume, decreases in cells exposed to 5 microM TBT relative to control cells or cells exposed to 50 microM TBT. The flow cytometric parameter 90 degrees light scatter, a function of refractive index and a measure of protein content, increases as a function of TBT concentration above 0.5 microM. Following exposure to TBT concentrations greater than 0.5 microM, but less than 50 microM, DNA distribution across the cell cycle cannot be resolved adequately by flow cytometry. Also, the cells become resistant to solubilization of the cell membrane/cytoplasm complex by nonionic detergents. Relative to logarithmically growing cells, MELC in the stationary phase of the growth cycle and butyric acid-differentiated cells exhibit decreased plasma membrane permeability resulting in increased carboxyfluorescein (CF) retention derived from the intracellular hydrolysis of carboxyfluorescein diacetate (CFDA). Similarly, cells exposed to TBT concentrations below 50 microM exhibit increased cellular CF retention. Viability in terms of CFDA hydrolysis/CF retention and propidium iodide (PI) exclusion is not decreased by exposure to TBT concentrations below 1 microM. At doses between 5 and 50 microM, however, cells exhibit both CF and PI fluorescence simultaneously and are programmed for death. At TBT concentrations greater than 1.0 microM, MELC plasma membrane potential, measured with the cyanine dye, 3,3'-dihexyloxacarbocyanine iodide (DiOC6) decreases at the same time that the uptake of PI is observed. In conjunction with other data, the concentration-dependent increase in CF fluorescence, resistance to detergent-mediated solubilization of the plasma membrane/cytoplasm complex, and increase in 90 degrees light scatter suggest fixation (protein denaturation, cross-linking, etc.) as a mechanism of the toxic action of TBT.

Intracellular pH measurements using flow cytometry with 1,4-diacetoxy-2,3-dicyanobenzene

1,4-Diacetoxy-2,3-dicyanobenzene (ADB) has been increasingly used for measurement of intracellular pH by flow cytometry. ADB rapidly enters cells and is cleaved to the fluorescent pH indicator 2,3-dicyano-hydroquinone (DCH). We have analyzed several potential problems that can affect its usefulness as a pH indicator. Hydrolysis of ADB in aqueous solutions reveals the temporary presence of a fluorescent species blue-shifted from DCH at the same pH. The presence of this species with DCH can lead to erroneous pH measurements. Stable pH measurements with ADB depend on the incubation conditions and esterase activity. Heated cells required 20 min for stable measurements, whereas control cells required 5 to 10 min. The reproducibility of pH measurements was excellent, with a resolution of less than or equal to 0.05 pH units in the range of 6.4 to 8.0. Absolute calibration curves of intracellular pH using the ionophore nigericin depended on matching the intracellular K+ concentration with the buffer, but relative measurements of intracellular pH were insensitive to K+. ADB was nontoxic to Chinese hamster ovary cells at up to 20 micrograms/ml. However, when cells loaded with dye were passed through a UV laser beam, concentrations of dye greater than 5 micrograms/ml were highly toxic. Viable cells could be sorted on the basis of intracellular pH if ADB were used at low concentrations.

A single-cell assay of beta-galactosidase activity in Saccharomyces cerevisiae

A novel assay of single-cell exogenous beta-galactosidase activity in Saccharomyces cerevisiae has been developed. Intracellular fluorescence due to the hydrolysis of resorufin-beta-D-galactopyranoside attains a steady state between production of resorufin and its subsequent leakage from the cell. The cells are permeabilized with Triton X-100, and the assay is performed at 0 degrees C. These conditions were chosen to minimize intercellular fluorescence communication. Free resorufin in the extracellular space is bound by bovine serum albumin to prevent its uptake by cells. Two regimes of fluorescence accumulation are observed, one limited by the rate of diffusion of substrate into the cell, and one limited by the rate of enzymatic cleavage of the substrate. A quantitative correlation between fluorescence and beta-galactosidase activity is obtained under optimized assay conditions.

Polar fluorescein derivatives as improved substrate probes for flow cytoenzymological assay of cellular esterases

Fluorescein esters are employed in assays of cell viability, membrane permeability and esterase activity. The ester most widely used, fluorescein diacetate (FDA), has the disadvantage of rapid cellular efflux of its hydrolysis product fluorescein. This is particularly problematic for flow cytoenzymology (FCE), where fluorescence is measured in individual cells allowing identification of subpopulations differing in esterase activity and/or membrane characteristics. We present a comparison of FDA with two potentially improved substrate probes for FCE, carboxyfluorescein diacetate (CFDA) and bis(carboxyethyl)-carboxyfluorescein-tetra acetoxy methyl ester (BCECF-AM). Substrates were characterized in terms of reaction and product efflux kinetics in EMT6 mouse mammary tumour cells, together with inhibition kinetics for the carbamoylating agent BCNU. Intact viable cells were analysed by FCE and spectrofluorimetry, and the latter was also used for cell sonicates and purified esterase. CFDA and BCECF-AM enter cells and are hydrolysed more slowly than FDA. CFDA and FDA hydrolyses obey Michaelis-Menten kinetics with Km values of around 19 and 2 microM, respectively, whereas BCECF-AM hydrolysis deviates from this classical behaviour. BCNU (5 X 10(-4) M) inhibits FDA and BCECF-AM hydrolyses by approximately 50%, compared to 30% for CFDA. CFDA may be partly hydrolysed by membrane-bound esterases. Efflux half-lives were 16 min, 94 min and greater than 2 h for products of FDA, CFDA and BCECF-AM, respectively. We conclude that BCECF-AM is the optimal substrate probe for FCE. This study emphasizes the need to optimize various parameters when selecting a substrate for flow cytoenzymological assay or when loading other reporter fluorochromes into cells via lipophilic esters.

Novel dynamic flow cytoenzymological determination of intracellular esterase inhibition by BCNU and related isocyanates

We present a novel dynamic flow cytoenzymological demonstration of the potent inhibition by the antitumour chloroethylnitrosourea BCNU of the intracellular hydrolysis of fluorescein diacetate by esterases of viable, intact murine and human tumour cells in vitro. The BCNU metabonate chloroethyl isocyanate and the related compound n-butyl isocyanate were also potent inhibitors. I50 values were in the range 4.2 X 10(-5)-2.0 X 10(-4) M. Generally similar quantitative results were obtained for intact cells and sonicates by conventional spectrofluorimetry, and inhibition of purified porcine liver carboxyl esterase (EC 3.1.1.1) was demonstrated. Little or no inhibitory activity was seen with the alkylating agents methyl methane-sulphonate, melphalan and nitrogen mustard. The results are consistent with carbamoylation of the esterase molecules by isocyanates, and this may provide a basis for the flow cytometric determination of intracellular carbamoylation in discrete subpopulations of heterogeneous samples.

Improved methodology for intracellular enzyme reaction and inhibition kinetics by flow cytometry

Flow cytoenzymology is the determination of enzyme activities or concentrations in single intact cells. Using the flow cytometer built and designed in our laboratory and recent modifications to hardware and software, we have developed an improved dynamic flow cytoenzymological procedure for the assay of cellular enzyme kinetics. The reaction mixture is sampled continuously, and the computer clock incorporates time as a parameter for kinetic determinations. Conditions for cellular esterase analysis were optimized and the rates of hydrolysis of two fluorogenic substrates, fluorescein diacetate (FDA) and 4-methylumbelliferone acetate (MUA), by esterases in EMT6 mouse mammary tumor cells were studied. Reaction kinetics were characterized, and Km values of 19 and 72 microM were obtained for the hydrolysis of FDA and MUA respectively. The kinetics of the cellular efflux of fluorescein were investigated, and a half-life of 7.5 min obtained. Enzyme inhibition kinetics were investigated using the competitive substrates p-nitrophenyl acetate and phenyl acetate, and the carbamoylating agents physostigmine and n-butyl isocyanate. The latter was particularly potent with an I50 of 4.8 X 10(-5) M for FDA hydrolysis compared with 6.5 X 10(-3) M for physostigmine. The I50 of 8.8 X 10(-5) M for n-butyl isocyanate inhibition of MUA hydrolysis was similar to that obtained with FDA as substrate. By monitoring FDA and MUA reactions separately and simultaneously, we showed them to act as competitive substrates. A comparison of flow cytoenzymological and conventional spectrofluorimetric analysis was also made, and differences identified in some cases.

Time, a quality-control parameter in flow cytometry

Time has been used as a quality-control parameter in our flow cytometer. This parameter was automatically incorporated into the list-mode data base by hooking the computer clock directly into the acquisition logic. Quality control can then be checked by "replaying" fluorescence or light scatter data vs. time and any instrumental drift can be observed and degraded data can be excised. The technique is illustrated with a number of DNA data sets from a tissue culture cell line in which artificial perturbations were introduced during data acquisition to simulate potential causes of instrumental drift.

Oncogene expression in ovarian cancer: a pilot study of c-myc oncoprotein in serous papillary ovarian cancer

The nuclear-associated protein product of the c-myc gene, p62c-myc, was assayed simultaneously with total DNA using flow cytometry in nuclei extracted from archival biopsies of serous papillary carcinoma of the ovary. The oncoprotein was probed with a synthetic peptide-induced mouse monoclonal antibody which was subsequently labeled with a fluorescent rabbit anti-mouse immunoglobulin and DNA was assayed using the nucleic acid fluorochrome propidium iodide. Serous papillary ovarian carcinoma expressed significantly higher p62c-myc levels compared with normal ovary (P less than 0.00003 Mann-Whitney U test). Biopsies classified as "borderline" low-potential malignancy exhibited levels between normal ovary and carcinoma. The difference between normal and "borderline" was significant at P less than 0.003, but no difference between "borderline" and frankly invasive biopsies was observed, P = 0.149. There was no difference among the histological grades of carcinomas. All normal ovaries had diploid DNA content as did 5/6 cases of "borderline" malignancy. The majority of cases of carcinoma, 28/36, were aneuploid. There was a statistically significant difference in the distribution of aneuploidy, P less than 0.005, between invasive carcinomas and those classified as "borderline" low-potential malignancy.

Flow cytometric quantitation of the c-myc oncoprotein in archival neoplastic biopsies of the colon

The c-myc oncogene encoded protein product, p62c-myc, was assayed simultaneously with DNA in populations of individual nuclei extracted from archival biopsies of colonic neoplasia. Both the protein and DNA were assayed fluorimetrically using flow cytometry with a synthetic peptide induced monoclonal antibody (MYC 1-6E10) for the protein and propidium iodide for DNA. The nuclear p62c-myc levels increased progressively from normal mucosa through polyps to carcinomas. However, there was a trend for the more poorly differentiated carcinomas to exhibit lower levels than moderately and well-differentiated tumours, p = 0.085. These results agree with those published previously with the same antibody using Western blotting for protein extracted from fresh frozen tissue and immunocytochemical assessment. Furthermore, flow cytometry is able to effect discriminations between subsets in heterogeneous populations using DNA as a second parameter which Western blot bulk studies cannot.

Flow cytometric quantitation of DNA and c-myc oncoprotein in archival biopsies of uterine cervix neoplasia

The c-myc nuclear associated oncoprotein has been quantitated simultaneously with DNA in nuclei extracted from archival biopsies of uterine cervix neoplasia. The oncoprotein and DNA were measured fluorimetrically in a flow cytometer using a mouse monoclonal antibody (MYC 1-6E10) and propidium iodide. Normal biopsies exhibited higher oncoprotein levels than carcinomas (P less than 0.00001). Furthermore, the maximum fluorescence signal in the normal tissue occurred at a lower antibody concentration compared with tumour tissue. There was no correlation between oncoprotein levels and histological grade, stage of disease, age of the patients or prognosis in the carcinomas. Aneuploidy, defined as a distinct second peak separate from the diploid distribution, was not a significant feature. The c-myc oncoprotein nuclear content does not appear to be a prognostic indicator in carcinoma of the cervix from the results of these studies but there is clearly diagnostic potential, particularly for automated analysis of cervical screening.

A pragmatic approach to the analysis of DNA histograms with a definable G1 peak

A method for DNA histogram analysis is described that depends only on the simple assumption that the data are normally distributed and a requirement that a G1 peak is present. A probability density function was derived from the assumption that extracted the S-phase component from the whole histogram. The model was tested with simulated data, and good agreement between predicted and known proportions in G1, S, and G1 + M was found. Good agreement was also found between duplicates of experimentally derived data. Some systematic errors are present in the analysis of certain types of histograms. However, these result in small errors when compared with biological and experimental variation and are less than the average of variation and are less than the average of algorithms in current use. The program required only two queued requests, those of the start and the end channels over which the analysis is to be performed. The algorithms perform rapidly on a microcomputer with only 28K addressable memory. Only two failures occurred in over 350 analyses and the method can be used for drug- and radiation-perturbed populations as well as with unperturbed.

Flow cytometric determination of cell cycle phase-specific changes in cellular phosphatase and glycosidase activities

The activities of two phosphatases (E.C. 3.1.3.1 and 3.1.4.1) and four glycosidases (E.C. 3.2.1.21, 3.2.1.30, 3.2.1.31 and 3.2.1.51) were measured by fluorescence spectrophotometry, and flow cytometry, in mitogen-stimulated lymphocytes, and in cultures of Molt-4-F and F-89 cell lines, synchronized by hydroxyurea or thymidine. All enzymes were active throughout the cycle but the activities of three enzymes were elevated at specific points in the cycle, alkaline phosphatase activity increased at G2 + M/G1 boundary and in early S-phase, the activity of beta-L fucosidase was elevated in G1 and late S-phase. Orthophosphate diesterase activity was elevated at the G1/S boundary, and during G2 + M. The increase in beta-L fucosidase activity was due to an increased number of cells showing activity, whilst the increase in orthophosphate diesterase activity was attributable to an increase in cellular enzyme activity. Only the activities of orthophosphate diesterase and beta-L fucosidase were measurable by flow cytometry, alkaline phosphatase activity was mainly extracellular, and therefore not detectable by flow cytometric methods employed.

Molecular calibration in flow cytometry with sub-attogram detection limit

A monoclonal antibody to complement receptor type 1 (CR1) has been double labelled with 125iodine and fluorescein isothiocyanate (FITC) and used for parallel radioimmune assay (RIA) and flow cytometric studies. Serial dilutions of the reagent with unlabelled antibody were used to obtain a linear correlation between radioactivity counts and fluorescence over the whole range of labelled antibody molecules bound per cell, namely 700-25,000. This has enabled us to calibrate our flow cytometer in terms of numbers of molecules per channel as opposed to the customary arbitrary fluorescence units. Normal human granulocytes from a single donor were found to have a mean of 25,000 CR1 molecules per cell with a normal distribution and standard deviation of 4775 molecules. The detection limit of the flow cytometer was 730 CR1 molecules per cell which is equivalent to between 125 and 150 molecules of free fluorescein. This represents less than 0.1 attogram (ag) of fluorescein per cell, where 1 ag = 10(-18) g. The resolution was 44 CR1 molecules per digitization step of the analogue-to-digital converters (ADC) which corresponds to the fluorescence from less than ten molecules of free fluorescein.