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

Dynamic effects of Hg2+-induced changes in cell volume

Using a microfluidic volume sensor, we studied the dynamic effects of Hg2+ on hypotonic stress-induced volume changes in CHO cells. A hypotonic challenge to control cells caused them to swell but did not evoke a significant regulatory volume decrease (RVD). Treatment with 100 muM HgCl2 caused a substantial increase in the steady-state volume following osmotic stress. Continuous hypotonic challenge following a single 10-min exposure to HgCl2 produced a biphasic volume increase with a steady-state volume 100% larger than control cells. Repeated hypotonic challenges to cells exposed once to Hg2+ resulted in a sequential approach to the same steady-state volume. Stimulation after reaching steady state caused a reduction in peak cell volume. Repeated stimulation was different than continuous stimulation resulting in a more rapid approach to steady state. Substituting extracellular Na+ with impermeant NMDG+ in the hypotonic solution produced a rapid RVD-like volume decrease and eliminated the Hg2+-induced excess swelling. The volume decrease in the presence of Hg2+ was inhibited by tetraethylammonium and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium, blockers of K+ and Cl(-) channels, respectively, suggesting that part of the Hg2+ effect was increasing NaCl influx over KCl efflux. The presence of multiple phases of steady-state volume and their sensitivity to the stimulation history suggests that factors beyond solute fluxes, such as modification of mechanical stress within the cytoskeleton also plays a role in the response to hypotonic stress.

Effects of tributyltin on barrier functions in human intestinal Caco-2 cells

The effect of tributyltin (TBT) on human intestinal epithelial cell functions was investigated by using human intestinal Caco-2 cell monolayers. We paid particular attention to the effect of TBT on two barrier functions: the tight junction as a physical barrier and MDR1/P-glycoprotein as a biological barrier. A loss of monolayer integrity was apparent from the TBT treatment and the paracellular permeability was increased by TBT. On the other hand, the activity of P-glycoprotein, which was examined by measuring the accumulation of Rhodamine-123 and daunomycin, was increased by prolonged TBT treatment in a concentration-dependent manner (1-100 nM). Furthermore, it was clarified by Western and Northern blots that this increase was accompanied by the increased expression of MDR1 mRNA and protein. The activation of a multidrug resistance transporter P-glycoprotein by TBT would cause a disorder of the human intestines by changing the drug pharmacokinetics.

Organotin compounds alter the physical organization of phosphatidylcholine membranes

Organotin compounds have a broad range of biological activities and are ubiquitous contaminants in the environment. Their toxicity mainly lies in their action on the membrane. In this contribution we study the interaction of tributyltin and triphenyltin with model membranes composed of phosphatidylcholines of different acyl chain lengths using differential scanning calorimetry, (31)P-nuclear magnetic resonance, X-ray diffraction and infrared spectroscopy. Organotin compounds broaden the main gel to liquid-crystalline phase transition, shift the transition temperature to lower values and induce the appearance of a new peak below the main transition peak. These effects are more pronounced in the case of tributyltin and are quantitatively larger as the phosphatidylcholine acyl chain length decreases. Both tributyltin and triphenyltin increase the enthalpy change of the transition in all the phosphatidylcholine systems studied except in dilauroylphosphatidylcholine. Organotin compounds do not affect the macroscopic bilayer organization of the phospholipid but do affect the degree of hydration of its carbonyl moiety. The above evidence supports the idea that organotin compounds are located in the upper part of the phospholipid palisade near the lipid/water interface.

Analysis of enzyme kinetics in individual living cells utilizing fluorescence intensity and polarization measurements

BACKGROUND

The Cellscan mark-S (CS-S) scanning cytometer was used for tracing enzymatic reactions in the same individual cells under various physiological conditions over periods of minutes. On-line reagent addition and changes in the experimental conditions (buffers, ions, substrates and inhibitors) were performed.

METHODS

Kinetic events were monitored by fluorescence intensity (FI) and fluorescence polarization (FP) measurements of fluorescein diacetate (FDA) and chloromethyl fluorescein diacetate (CMFDA) intracellular hydrolysis. FP measurements have been used to assess the intracellular marker's mobility restrictions.

RESULTS

Kinetic measurement along 1000 s of FDA labeled individual Jurkat T cells, indicated variation of 65% for FI(t) and approximately 10% for FP(t). While FI increased linearly with time, FP(t) decreased nonlinearly and asymptotically, reaching a constant value. The FP(t) of CMFDA-labeled cells was different from that of FDA-labeled cells. Average cellular Km of 3.9 microM was calculated from individual cell FDA hydrolysis curves.

CONCLUSIONS

(1) Analysis of the reaction kinetics of intracellular enzymes can be refined by using FP measurements of the products of fluorogenic substrates in addition to the FI measurements. (2) Subpopulations or individual cells could be classified according to their reaction rates. (3) A specific dependence of FP(t) on type of enzyme substrate is suggested.

Nucleoside-mediated mitigation of 5-fluorouracil-induced toxicity in synchronized murine erythroleukemic cells

5-Fluorouracil (5-FU) is a chemotherapeutic agent known to retard embryonic growth and induce cleft palate and limb deformities. The predominant mechanism underlying its toxic action is thought to be inhibition of thymidylate synthetase (TS), and hence thymidine triphosphate (dTTP) synthesis, resulting in alteration of the balance of deoxynucleotide (dNTP) pools and disruption of DNA synthesis. Indeed, previously we demonstrated retarded cell-cycle progression concurrent with a 60% decrease in TS activity in rat whole embryos following maternal exposure to 40 mg/kg 5-FU on Gestational Day 14 and in the murine erythroleukemic cell (MELC) suspension culture following exposure to 5-25 microM 5-FU for 2 hr. In the study described herein, we used high-performance liquid chromatography (HPLC) to demonstrate in both of these model systems that 5-FU exposure results in similar patterns of dNTP perturbations: a prolonged decrease in dTTP and dGTP levels and an increase in dCTP and dATP. In addition, we used centrifugal elutriation to synchronize MELC in the phases of the cell cycle (G0/G1 and early S) most sensitive to 5-FU to investigate the ability of nucleoside supplementation to mitigate 5-FU-induced toxicity. Our data indicate that following a 2-hr exposure to 5-25 microM 5-FU, supplementation with 1-10 microM thymidine (TdR) for 24 hr partially reverses 5-FU-induced toxicity as evidenced by increased cellular proliferation and cell-cycle progression and amelioration of 5-FU-induced perturbations of protein synthesis and cellular membrane permeability compared to unsupplemented 5-FU-exposed cells. However, TdR concentrations >/=100 microM inhibited growth or were cytotoxic. In comparison, supplementation with 10 microM-10 mM of deoxycytidine (CdR) was not toxic, but effected a dose-dependent recovery from 5-FU-induced toxicity. At 1-100 microM, neither deoxyadenosine nor deoxyguanosine supplementation reduced 5-FU-induced toxicity; at higher concentrations, both purine nucleotides inhibited cell growth. Although these results support the hypothesis that 5-FU disrupts the MELC cell cycle by depleting dTTP (a perturbation that is reversible by TdR supplementation), they also indicate that CdR supplementation offers an additional recovery pathway.

Probing chromatin structure in the early phases of apoptosis

A typical flow cytometric marker of apoptosis is the appearance of a hypodiploid peak. This phenomenon is related to the chromatin fragmentation and loss that occurs during the late stages of the process. We describe herein the changes occurring at the chromatin level in purified nuclei preparations obtained from human peripheral blood mononuclear cells in a time-course study, including the simultaneous evaluation of nuclear proteins and DNA stainability, light-scattering properties, and spectrophotometric determination of the protein content. An augmentation of fluoroscein isothiocyanate (FITC) stainability was noticed as early as 1 h after irradiation. As this phenomenon is not correlated to changes in actual protein content, one can conclude that modifications of basic protein accessibility occur from the early phases of the apoptotic process. Also DNA stainability augmented with time, generating the transient appearance of a hyperdiploid peak that preceded the appearance of the hypodiploid peak typical of the late stages of the process, and that shared with the latter the same light-scattering properties. Chromatin status was further explored by staining apoptotic nuclei using DNA probes with peculiar molecular weight. Propidium iodide (PI) and ethidium bromide (EB), but not the much bulkier 7-aminoactinomycin D (7-AAD), identified the nuclei with a transient increase in DNA stainability confirming that an increased dye accessibility to binding sites was responsible for the phenomenon. Remarkably, all dyes identified the same proportion of hypodiploid nuclei when an apoptotic nucleus shed its fragmented chromatin. Control experiments included differential interference contrast and fluorescence microscopy that showed the purity of nuclei preparations and the typical morphological apoptotic features. Finally, the simultaneous evaluation of DNA by PI and nuclear proteins by FITC in a time course study allowed a thorough assessment of changes occurring at the chromatin level in the diverse stages of apoptosis. It is suggested that proteolysis precedes endonucleolysis and probably renders it easier the final endonucleolytic step leading to DNA fragmentation and loss.

The effect of tributyltin chloride on vascular responses to atrial natriuretic peptide

The effects of tributylin chloride (TBT) on vascular smooth muscle responses to norepinephrine, nitroprusside (SNP) and atrial natriuretic peptide (ANP) were studied in isolated aortic rings of rats. TBT did not interfere with norepinephrine-induced contraction or SNP-induced vasorelaxation. However, TBT produced a dose-dependent inhibition of ANP-induced vasorelaxation. Inhibition was not observed with inorganic tin chloride, SnCl2. The inhibition of vasorelaxation was accompanied by a parallel inhibition of ANP-induced cGMP generation. SNP-induced generation of cGMP was not affected by TBT. TBT did not interfere with binding of ANP to its receptor in bovine adrenal glands suggesting that the effects of TBT were mediated by direct interaction with membrane-bound guanylate cyclase.

The stimulation of arachidonic acid metabolism by organic lead and tin compounds in human HL-60 leukemia cells

The liberation of fatty acids, above all arachidonic acid, in human blood cells is involved in numerous health problems or physiological mechanisms. The activity of cellular phospholipases leads to lipid metabolites such as eicosanoids, platelet activating factor, diacylglycerol, and inositolphosphates that are capable of mediating such pathological symptoms. The results presented here demonstrate that organic heavy metal compounds induce arachidonic acid liberation or its rearrangement within the lipid classes of HL-60 cells before a loss in viability can be detected. Four of the compounds tested, triethyllead (Et3Pb+), diethyllead (Et2Pb2+), trimethyllead (Me3Pb+), and trimethyltin (Me3Sn+), show a threshold concentration at which the viability of the cells is drastically decreased after 60 to 180 min incubation, whereas dibutyltin (But2Sn2+) induces a constant increase of cell death during the whole incubation time. In the case of threshold concentrations, the compounds stimulate a loss of arachidonic acid within the phospholipids and an increase of free fatty acid and eicosanoids before cell death could be detected. An important fact is the rearrangement of arachidonic acid within the lipid classes of these cells induced by metal concentrations that were not able to kill the cells within the given time. Primarily affected is phosphatidylethanolamine which loses arachidonic acid and, to a minor extent, phosphatidylcholine. Portions of the liberated fatty acid were then metabolized and/or shifted into neutral lipids and other phospholipids. All compounds tested show comparable effects, although at different concentrations. The toxicities of the compounds can be ordered as follows: Et3Pb+ greater than or equal to Et2Pb2+ greater than But2Sn2+ greater than or equal to Me3Pb+ much greater than Me3Sn+ greater than or equal to Pb2+. The cellular shape change following incubation with metal compounds is a further strong indication of a change in the membrane lipids. The cells lose their characteristic microvilli and/or blebs and become round without a loss in viability.

An efficient multiple-exposure analysis of the toxicity of crisnatol, a DNA intercalator in phase II clinical trials

To investigate the toxicity and mechanism of action of crisnatol (CRS), a new DNA intercalator currently in phase II clinical trials, we analyzed cellular and nuclear flow cytometric (FCM) parameters of murine erythroleukemic cells (MELC) exposed to a range of CRS concentrations over three exposure conditions: short-term (4 h), long-term (24 h), and short-term with recovery (4 h+/19 h-). At 0.5-1.0 microM CRS, 4 h exposure results in a reversible G2-phase block, while 24 h exposure results in greater than G2 polyploidy. At 5-10 microM CRS concentrations, cells exhibit persistent retardation of S-phase progression or irreversible G2 and/or greater than G2 blocks, depending on duration of exposure. Cells terminally blocked in G2 exhibit increased nuclear/cellular volumes and increased nuclear fluorescein isothiocyanate (protein) staining, suggestive of unbalanced growth. At 25-50 microM CRS concentrations, MELC exhibit severe membrane perturbation (loss of viability) regardless of exposure. In contrast, following similar exposures to an inactive isomer of CRS, MELC exhibit minimal cell cycle effects, suggesting that cell cycle kinetics may be a useful criterion for assessing potential efficacy. Similar analyses with different classes of chemotherapeutic agents reveal that the range of induced cellular/nuclear perturbations varies with the class of compound used. Taken together, these results suggest that drug toxicity can vary with both concentration and duration of exposure and, as such, a selective multiple-exposure FCM analysis may better represent the spectrum of drug action for drug development and pharmacodynamic studies.

Tributyltin stimulates apoptosis in rat thymocytes

Treatment of rat thymocytes with micromolar concentrations of tributyltin caused a rapid increase in the cytosolic free Ca2+ concentration that was inhibited by Ni2+, which blocks Ca2+ influx through membrane channels. The elevation of cytosolic Ca2+ was associated with extensive DNA fragmentation, which was prevented by pretreatment of the cells with either of the intracellular Ca2+ chelators quin-2 or 1,2-bis(2-amino-phenoxy)ethane-N',N',N',N',-tetraacetic acid. Loss of thymocyte viability, which followed DNA fragmentation, was also prevented by the two Ca2+ chelators or by removing extracellular Ca2+ with ethylene glycol bis(beta-aminoethyl ether)N,N'-tetraacetic acid. The pattern of DNA fragmentation was characteristic of that produced by agents which activate a Ca2(+)- and Mg2(+)-dependent endogenous endonuclease during apoptosis or programmed cell death. Additional studies showed that other organotin compounds, including trimethyltin, triphenyltin, and dibutyltin had minimal effects on cytosolic Ca2+, DNA fragmentation, and cell viability. These results are consistent with a greater susceptibility of thymocytes to tributyltin and provide a basis for understanding its selective immunotoxicity in vivo.

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 effect of organotin compounds on chloride secretion by the in vitro perfused rectal gland of Squalus acanthias

The effects of various organotins on membrane function and electrolyte transport were studied in the marine elasmobranch, Squalus acanthias. The isolated perfused rectal gland was used as a model of electrolyte transport. This gland can be stimulated to secrete chloride by atrial natriuretic peptide, veratrine, and vasoactive intestinal polypeptide although the mechanism of action of each secretagogue is different. By analysis of the inhibitory effect of an organotin in the presence of each secretagogue, the mechanism of inhibition can be inferred. Tributyltin (TBT) produced a reversible inhibition of epithelial transport at 10(-8) to 10(-7) M which resulted from inhibition of stimulus-secretion coupling in VIP-containing neurons within the gland. The transporting epithelial cells were unaffected at these concentrations. Trimethytin (TMT) produced inhibition at 10(-7) M which was not reversible and which affected primarily the transporting epithelial cells. Triethyltin and triphenyltin were without effect. The inhibitory effect of TBT and TMT was not affected by simultaneous administration of dithiothreitol. TBT also produced inhibition of oxygen consumption, Na+,K-ATPase, and proton ATPase in dispersed rectal gland cells. These results indicate that organotins are toxic to cell membrane functions which are intimately involved in the movement of electrolytes. This is the first evidence of toxicity to membrane transport functions in a marine species which is at risk from environmental exposure.

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.

Cell cycle alterations and cell death in cyclophosphamide teratogenesis

Litters of pregnant mice treated with cyclophosphamide (CP) exhibit malformations of the limbs ranging from oligodactyly to amelia. Previous studies have indicated that cell death occurs in limb buds shortly after maternal exposure. We have investigated the relationship of cell death, cell cycle perturbation, and embryo/fetal toxicity in the mouse using vital staining and flow cytometry (FCM). CP (20, 30, and 40 mg/kg) was investigated via intraperitoneal administration to Swiss-Webster mice on day 10 of gestation. At 4, 8, or 28 hours later, embryos were removed. Cell death was identified with Nile blue sulphate (NBS). Two embryos per litter were stained with NBS, and the remaining embryos were frozen at -70 degrees C prior to FCM analysis. After thawing, the forelimb buds were removed for the isolation of nuclei. Tissues were dissociated through a wire mesh followed by cytolysis with 0.1% nonidet P-40 in PBS with 0.5 mg/ml RNase. Nuclei were stained with the fluorescent nucleic acid probe propidium iodide and analyzed (10,000 nuclei per sample) for propidium iodide fluorescence by FCM. NBS revealed a dose-related increase in cell death by 8 hours after dosing. CP-induced cell death was greatest in areas of rapid cell proliferation (DNA synthesis). FCM analysis revealed retardation of progression through the S-phase of the cell cycle by 4 hours post-exposure at all doses. This retardation occurred earlier in S-phase with increasing dose and persisted through 8 hours. At 28 hours, cell cycle histograms were normal in the low-dose embryos, but remained perturbed in the intermediate- and high-dose embryos. On day 17 of gestation, the last group of dams was killed. A high incidence of fetal malformations, including limb defects, occurred at the 20 mg/kg dose, and fetal mortality was observed at 30 and 40 mg/kg. The pattern and magnitude of cell death correlated with cell cycle perturbation and fetal toxicity at term, suggesting a relationship between cell cycle perturbation, cell death, and malformations produced by CP.

Metal-induced alteration of the cell membrane/cytoplasm complex studied by flow cytometry and detergent lysis

Flow cytometric analysis of the cell cycle is most effectively accomplished with membrane-/cytoplasm-free ("clean") nuclei. Non-ionic detergents (e.g. NP40 or Triton X-100) commonly are employed to solubilize cell membranes/cytoplasm to produce "clean" nuclei. Treatment of murine erythroleukemic cells (MELC) with tri-n-butyltin methoxide, cadmium acetate, zinc sulfate, or lead acetate alters the properties of the cell membrane/cytoplasm complex making it resistant to NP40 dissolution. On a molar basis, the organotin compound was more effective in inducing resistance to detergent-mediated dissolution than the inorganic metal compounds. Resistance to NP40-mediated dissolution was manifested as an increase in the flow cytometric parameters 90 degrees scatter and fluorescein isothiocyanate (FITC) fluorescence and was confirmed by light microscopy.

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.