Determination of toxin-induced leakage of different-size nucleotides through the plasma membrane of human diploid fibroblasts

Staphylococcus aureus α-toxin: nearly a century of intrigue

Staphylococcus aureus secretes a number of host-injurious toxins, among the most prominent of which is the small β-barrel pore-forming toxin α-hemolysin. Initially named based on its properties as a red blood cell lytic toxin, early studies suggested a far greater complexity of α-hemolysin action as nucleated cells also exhibited distinct responses to intoxication. The hemolysin, most aptly referred to as α-toxin based on its broad range of cellular specificity, has long been recognized as an important cause of injury in the context of both skin necrosis and lethal infection. The recent identification of ADAM10 as a cellular receptor for α-toxin has provided keen insight on the biology of toxin action during disease pathogenesis, demonstrating the molecular mechanisms by which the toxin causes tissue barrier disruption at host interfaces lined by epithelial or endothelial cells. This review highlights both the historical studies that laid the groundwork for nearly a century of research on α-toxin and key findings on the structural and functional biology of the toxin, in addition to discussing emerging observations that have significantly expanded our understanding of this toxin in S. aureus disease. The identification of ADAM10 as a proteinaceous receptor for the toxin not only provides a greater appreciation of truths uncovered by many historic studies, but now affords the opportunity to more extensively probe and understand the role of α-toxin in modulation of the complex interaction of S. aureus with its human host.

Cellular effects of an anionic surfactant detected in V79 fibroblasts by different cytotoxicity tests

Several cytotoxicity tests were employed to detect the cellular effects of low concentrations of the anionic surfactant linear alkylbenzene sulphonate (LAS). When added to growth medium containing 5% foetal calf serum, LAS did not affect V79 cell growth, nor did it alter the permeability of cell membranes. The inactivity depended on the serum component of the medium. When treatments were carried out in serum-free saline, LAS inhibited cell proliferation, made the plasma membrane permeable to otherwise-undiffusible compounds, and reduced the uptake of tritiated thymidine. The alterations in membrane permeability were evaluated from the release of cytoplasmic molecules of different size (lactate dehydrogenase, adenine nucleotides, RNA) into the medium. The sensitivity of the spectrophotometric lactate dehydrogenase assay was inadequate for the conditions of treatment required to detect the cytotoxicity of LAS. In cultures pre-incubated with tritiated adenine instead, the release of labelled ATP pool components was time and dose dependent and allowed discrimination between levels of membrane damage causing the same degree of trypan blue staining. Also, macromolecular nucleic acids were detected outside the treated cells at doses of 4-6 mg LAS/litre, which indicated severe membrane damage.

Acrylonitrile-induced reversible inhibition of uridine uptake by isolated rat intestinal epithelial cells

Isolated rat intestinal epithelial cells (IEC) were used to investigate the mechanism(s) of acrylonitrile (VCN)-induced gastro-intestinal damage. The isolated cells were 93% structurally intact for 60 min, as indicated by trypan blue exclusion. Uridine uptake by isolated IEC was linear from 6-20 min, after which a steady state was reached for up to 40 min. Exposure of isolated IEC to various concentrations of VCN reduced the ability of the cells to take up uridine in a concentration-dependent manner. A concentration of 82 mum VCN inhibited the [(3)H]uridine uptake of the cells by 50% (IU(50)). A time-course study indicated that the maximal inhibition of uridine uptake occurred at 15 min after exposure to VCN. The VCN-induced inhibition of uridine uptake was found to be reversible. IEC exposed to two sublethal doses of VCN (41 and 82 mum) for 15 min regained normal uridine uptake activity within 50 min after removal of VCN. The present study provides a sensitive approach for the detection and evaluation of cytotoxic risk of sublethal doses of the gastro-intestinal toxin VCN using IEC as target cells. The observed in vitro cytotoxicity of sublethal doses of VCN will be used to investigate further the mechanism of VCN-induced gastro-intestinal damage.

delta-hemolysin, an update on a membrane-interacting peptide

delta-hemolysin is a hemolytic peptide produced by Staphylococcus, and it has been studied for nearly 50 years. Therefore, it has become a model in the study of peptides interacting with membranes. In this review, we report some recent findings and compare them with previous works. delta-hemolysin is a 26 amino acid peptide, somewhat hydrophobic and presenting a zero net charge. Study of its structure has shown that delta-hemolysin is alpha-helical and amphipathic, such as many antimicrobial peptides (e.g. magainin and melittin). However, delta-hemolysin had not displayed any reported antimicrobial activity until a recent publication showed its high potency against Legionella. Its mode of action is based on direct interaction with target membranes. In accordance with its concentration, delta-hemolysin may slightly perturb a membrane or lead to cell lysis. Peptide charge plays an important role in its interaction with membranes, as is shown in the study of peptide variants. Some positively charged variants become highly hemolytic and even active against Escherichia coli and Staphylococcus aureus. Finally, it has recently been demonstrated that peptide preferentially binds to lipid-disordered domains. It has been postulated that as a result, enrichment in lipid-ordered domains might increase peptide concentration in lipid-disordered domains and thereby improve its activity.

Exotoxins of Staphylococcus aureus

This article reviews the literature regarding the structure and function of two types of exotoxins expressed by Staphylococcus aureus, pyrogenic toxin superantigens (PTSAgs) and hemolysins. The molecular basis of PTSAg toxicity is presented in the context of two diseases known to be caused by these exotoxins: toxic shock syndrome and staphylococcal food poisoning. The family of staphylococcal PTSAgs presently includes toxic shock syndrome toxin-1 (TSST-1) and most of the staphylococcal enterotoxins (SEs) (SEA, SEB, SEC, SED, SEE, SEG, and SEH). As the name implies, the PTSAgs are multifunctional proteins that invariably exhibit lethal activity, pyrogenicity, superantigenicity, and the capacity to induce lethal hypersensitivity to endotoxin. Other properties exhibited by one or more staphylococcal PTSAgs include emetic activity (SEs) and penetration across mucosal barriers (TSST-1). A detailed review of the molecular mechanisms underlying the toxicity of the staphylococcal hemolysins is also presented.

Exotoxins of Staphylococcus aureus

This article reviews the literature regarding the structure and function of two types of exotoxins expressed by Staphylococcus aureus, pyrogenic toxin superantigens (PTSAgs) and hemolysins. The molecular basis of PTSAg toxicity is presented in the context of two diseases known to be caused by these exotoxins: toxic shock syndrome and staphylococcal food poisoning. The family of staphylococcal PTSAgs presently includes toxic shock syndrome toxin-1 (TSST-1) and most of the staphylococcal enterotoxins (SEs) (SEA, SEB, SEC, SED, SEE, SEG, and SEH). As the name implies, the PTSAgs are multifunctional proteins that invariably exhibit lethal activity, pyrogenicity, superantigenicity, and the capacity to induce lethal hypersensitivity to endotoxin. Other properties exhibited by one or more staphylococcal PTSAgs include emetic activity (SEs) and penetration across mucosal barriers (TSST-1). A detailed review of the molecular mechanisms underlying the toxicity of the staphylococcal hemolysins is also presented.

Staphylococcal alpha-toxin kills human keratinocytes by permeabilizing the plasma membrane for monovalent ions

Incubation of human keratinocytes with nanomolar concentrations of Staphylococcus aureus alpha-toxin leads to irreversible depletion of cellular ATP. The toxin forms hexamers in the target cell membranes, and rapid transmembrane flux of K+, Na+, and 86Rb+ is observed. Unexpectedly, pores formed in keratinocytes through application of low but lethal doses of alpha-toxin appeared to be considerably smaller than those formed in erythrocyte membranes. They permitted neither rapid influx of Ca2+ or propidium iodide, nor efflux of carboxyfluorescein. Larger pores allowing flux of all three markers did form when the toxin was applied at high concentrations. Flux of monovalent ions and reduction in cellular ATP levels evoked by low toxin doses correlated temporally with a fall in oxygen consumption, which was interpreted to reflect breakdown of mitochondrial respiration. The lethal event could not be thwarted by manipulating the extracellular K+ or Ca2+ concentrations. Realization that alpha-toxin may form very small pores in nucleated cells is important for future research on cellular toxin effects and membrane repair processes.

Alpha-toxin of Staphylococcus aureus

Alpha-toxin, the major cytotoxic agent elaborated by Staphylococcus aureus, was the first bacterial exotoxin to be identified as a pore former. The protein is secreted as a single-chain, water-soluble molecule of Mr 33,000. At low concentrations (less than 100 nM), the toxin binds to as yet unidentified, high-affinity acceptor sites that have been detected on a variety of cells including rabbit erythrocytes, human platelets, monocytes and endothelial cells. At high concentrations, the toxin additionally binds via nonspecific absorption to lipid bilayers; it can thus damage both cells lacking significant numbers of the acceptor and protein-free artificial lipid bilayers. Membrane damage occurs in both cases after membrane-bound toxin molecules collide via lateral diffusion to form ring-structured hexamers. The latter insert spontaneously into the lipid bilayer to form discrete transmembrane pores of effective diameter 1 to 2 nm. A hypothetical model is advanced in which the pore is lined by amphiphilic beta-sheets, one surface of which interacts with lipids whereas the other repels apolar membrane constitutents to force open an aqueous passage. The detrimental effects of alpha-toxin are due not only to the death of susceptible targets, but also to the presence of secondary cellular reactions that can be triggered via Ca2+ influx through the pores. Well-studied phenomena include the stimulation of arachidonic acid metabolism, triggering of granule exocytosis, and contractile dysfunction. Such processes cause profound long-range disturbances such as development of pulmonary edema and promotion of blood coagulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cryptic alpha-toxin gene in toxic shock syndrome and septicaemia strains of Staphylococcus aureus

The majority of clinical isolates of Staphylococcus aureus that produce toxic shock syndrome toxin-1 (TSST-1) fail to express alpha-toxin, despite having a copy of the hla gene in the chromosome. The hla gene was cloned from an Hla- TSST-1+ strain, Todd 555, which had been isolated from a case of toxic shock syndrome in the USA. Of the 630 bases of the Todd 555 gene sequenced, 46 differed from the hla gene sequence of strain Wood 46. The defect in alpha-toxin expression was shown to be due to a nonsense mutation which converted a CAG glutamine codon in the equivalent position in the functional Wood 46 sequence to a TAG stop codon. The same mutation was present in the hla gene cloned from a human septicaemia strain (V37) isolated in Dublin. The nonsense mutation of Todd 555 was suppressed by the supE44 mutation in Escherichia coli resulting in haemolytic activity in cell lysates. Hybrid hla genes were formed by splicing fragments of hla from Todd 555 and Wood 46. Expression of one such chimaeric hla gene in S. aureus demonstrated that the Todd 555 hla gene has a functional agr-regulated promoter. The silent hla gene may be a cryptic gene in S. aureus.

Release of interleukin-1 beta associated with potent cytocidal action of staphylococcal alpha-toxin on human monocytes

The pathogenetic relevance of Staphylococcus aureus alpha-toxin in humans has been debated because human cells have been thought to display a natural resistance toward the cytotoxic action of this cytolysin. Following our previous demonstration that human platelets represent sensitive targets for toxin attack, we have now identified monocytes as a second, highly vulnerable human cell species that succumb to attack by low doses (20 ng/ml) of alpha-toxin. The cytotoxic action of alpha-toxin is reflected in a rapid depletion of cellular ATP that is essentially complete within 30 min. The presence of human plasma proteins affords some protection of monocytes against the action of the toxin. In 10% autologous serum, ATP depletion commences at 80 to 300 ng of toxin per ml. Subcytolytic doses stimulate the release of tumor necrosis factor alpha, a process that is slightly accentuated in the presence of 50% serum. Cytocidal toxin doses unfailingly cause the release of large amounts of interleukin-1 beta from cultured cells, with levels of this monokine generally exceeding 10 ng/ml in the cell supernatants 60 min after application of toxin. Initial evidence suggests that this is due to processing of intracellular interleukin-1 rather than to de novo synthesis of the cytokine. All noted effects are abrogated in the presence of a neutralizing monoclonal antibody against alpha-toxin. Through its capacity to provoke cytokine release from monocytes and its attack on platelets, alpha-toxin may initiate cellular events that are relevant to the pathogenesis of staphylococcal infection.

Staphylococcal alpha toxin promotes blood coagulation via attack on human platelets

Staphylococcus aureus plays a major role as a bacterial pathogen in human medicine, causing diseases that range from superficial skin and wound to systemic nosocomial infections . The majority of S. aureus strains produces a toxin, a proteinaceous exotoxin whose hemolytic, dermonecrotic, and lethal properties have long been known (1-6). The toxin is secreted as a single- chained, nonglycosylated polypeptide with a M(r) of 3.4 x 10(4) (7, 8). The protein spontaneously binds to lipid monolayers and bilayers (9-14), producing functional transmembrane pores that have been sized to 1.5-2.0-nm diameters (15-18). The majority of pores formed at high toxin concentrations (20 mug/ml) is visible in the electron microscope as circularized rings with central pores of approximately 2 nm in diameter. The rings have been isolated, and molecular weight determinations indicate that they represent hexamers of the native toxin (7). We have proposed that transmembrane leakiness is due to embedment of these ring structures in the bilayer, with molecular flux occurring through the central channels (15, 19). Pore formation is dissectable into two steps (20, 21). Toxin monomers first bind to the bilayer without invoking bilayer leakiness . Membrane-bound monomers then laterally diffuse and associate to form non-covalently bonded oligomers that generate the pores. When toxin pores form in membranes of nucleated cells, they may elicit detrimental secondary effects by serving as nonphysiologic calcium channels, influx of this cation triggering diverse reactions, including release of potent lipid mediators originating from the arachidonate cascade (22-24). That alpha toxin represents an important factor of staphylococcal pathogenicity has been clearly established in several models of animal infections through the use of genetically engineered bacterial strains deleted of an active alpha toxin gene (25-27). Whether the toxin is pathogenetically relevant in human disease, however, is a matter of continuing debate. Doubts surrounding this issue originate from two main findings. First, whereas 60 percent hemolysis of washed rabbit erythrocytes is effected by approximately 75 ng/ml alpha toxin, approximately 100-fold concentrations are required to effect similar lysis of human cells (4-6, 13). The general consensus is that human cells display a natural resistance towards toxin attack. The reason for the wide inter-species variations in susceptibility towards alpha toxin is unknown but does not seem to be due to the presence or absence of high-affinity binding sites on the respective target cells (20, 21). Second, low-density lipoprotein (28) and neutralizing antibodies present in plasma of all healthy human individuals inactivate a substantial fraction of alpha toxin in vitro. These inactivating mechanisms presumably further raise the concentration threshold required for effective toxin attack, and it is most unlikely that such high toxin levels will ever be encountered during infections in the human organism. The aforegoing arguments rest on the validity of two general assumptions. First, the noted natural resistance of human erythrocytes to alpha toxin must be exhibited by other human cells. Second, toxin neutralization by plasma components, usually tested and quantified after their preincubation with toxin in vitro, must be similarly effective under natural conditions, and protection afforded by these components must not be restricted to specific cell species.

Oligomerization of 3H-labelled staphylococcal alpha-toxin and fragments on adrenocortical Y1 tumour cells

Staphylococcus aureus alpha-toxin has previously been shown to bind to erythrocyte membranes and the isolated membranes contain the toxin in both monomeric and hexameric form. The hexamers are believed to form the ring-shaped structures observed by electron microscopy on toxin-treated erythrocytes. It has not previously been shown that hexamers are formed also on nucleated mammalian cells although it has been assumed that hexamers in both systems create transmembrane channels, responsible for the toxin-induced membrane damage. Here we demonstrate by autoradiography that 3H-alpha-toxin bound to and formed high molecular weight complexes-presumably hexamers-on cultured adrenocortical Y1 tumour cells. The binding kinetics suggested a non-specific association of alpha-toxin with the membrane, rather than specific receptor-binding. The pH during toxin binding did not influence the subsequently induced membrane damage. Non-membrane damaging alpha-toxin fragment preparations also bound firmly to the cell membranes. Upon contact with Y1 cells the fragments formed complexes of the same apparent molecular size as those generated from intact alpha-toxin. Two interpretations are possible: either the fragment oligomers are somehow defective i.e. not able to form transmembrane structures or the functional relevance of toxin oligomerization for alpha-toxin-induced membrane damage must be questioned.

Production and characterization of monoclonal antibodies against Staphylococcus aureus alpha-toxin

Murine monoclonal antibodies against staphylococcal alpha-toxin were produced using a well-characterized alpha-toxin fragment preparation as immunizing agent. Three monoclonal antibodies were selected for anti-alpha-toxin activity in an ELISA using alpha-toxin as antigen. The monoclonal antibodies (MAbs) belonged to different immunoglobulin classes/subclasses and showed different abilities to neutralize the hemolytic, cell-membrane-damaging, dermonecrotizing and lethal action of alpha-toxin. One MAb was superior to mouse polyclonal antiserum in all test systems except for hemolysis, whereas another MAb neutralized essentially as the polyclonal serum. The third MAb did not neutralize the hemolytic or dermonecrotic effect but still inhibited the lethal and membrane-damaging effect of alpha-toxin. These results indicate that the three MAbs recognize different epitopes on the toxin molecule and that different biological activities might correspond to these epitopes.

Cell membrane interaction of Bacillus thuringiensis subsp. israelensis cytolytic toxins

Two toxic polypeptides of 24 and 25 kilodaltons (kDa) were purified from parasporal proteinaceous crystals of Bacillus thuringiensis subsp. israelensis. Both of these polypeptides, which are antigenically similar and have identical N terminals, lysed human erythrocytes and cultured mosquito cells. Although the 24-kDa peptide was more toxic than the 25-kDa peptide, both were less toxic than the crude alkali-solubilized crystal toxin. However, a 1:1 mixture of these 24- and 25-kDa proteins was more toxic than either of these polypeptides individually, indicating a possible interaction between these proteins at the cell membrane. Both the 24- and the 25-kDa proteins were inactivated by aqueous suspensions of dioleolylphosphatidylcholine, indicating the involvement of phospholipids in the cytotoxic action of these toxins. Thus the role of cell membrane phospholipids in mediating the toxin action was studied by using phospholipases as probes. Treatment of erythrocytes with high levels of phospholipase D increased their susceptibility to the toxin; however, phospholipase A2-treated erythrocytes were less susceptible to the toxin. These erythrocytes also bound less 125I-labeled 25-kDa toxin. These results support the role of fatty acyl residues at the syn-2 position of membrane phospholipids in toxin action. The cytolytic toxin of B. thuringiensis subsp. israelensis is thought to damage cell membranes in a detergentlike manner. However, there was a difference between the cytolytic action of this toxin and that of a nonionic detergent such as Triton X-100 because phospholipase A2-treated erythrocytes were more susceptible to Triton X-100, whereas such erythrocytes were less sensitive to the toxin. Thus, the cytolytic toxin apparently did not act as a nonspecific detergent, but rather interacted with phospholipid receptors on the cell membrane. Such an interaction of the toxin with phospholipid receptors probably results in the increased cell permeability, thereby causing cell lysis.

The comparative study of cardiovascular toxins utilizing a sensitive indicator of sublethal toxic injury

This study measures the inhibition of [3H]uridine uptake by smooth muscle cells to determine sublethal toxic injury by the cardiovascular toxins, allylamine, isoproterenol, and beta-aminopropionitrile. The exposure period or the concentration of toxin which inhibited 30% of [3H]uridine uptake by smooth muscle cells could be utilized as an endpoint for ranking toxicity. Cytotoxicity of the three toxins to smooth muscle cells were ranked as: allylamine greater than isoproterenol greater than beta-aminopropionitrile. Recovery of cells utilizing [3H]uridine uptake inhibition as a method for assessing comparative cytotoxicity and for screening of agents potentially injurious to vascular cells.

Nuclear magnetic resonance investigation of the conformation of delta-haemolysin bound to dodecylphosphocholine micelles

delta-Haemolysin in mixed micelles with perdeuterated dodecylphosphocholine was investigated with two-dimensional proton nuclear magnetic resonance experiments at 500 MHz. A single set of resonance lines was observed for the micelle-bound polypeptide, indicating that delta-haemolysin adopts a single conformation in this environment. Nearly complete, sequence-specific assignments were obtained for the segment 5-23 of this 26-residue polypeptide chain. From the sequential connectivities and numerous medium-range nuclear Overhauser effects this central portion of the molecule was found to form an extended helix with pronounced amphipathic distribution of polar and nonpolar amino acid side-chains.

Crystallization of the delta toxin of Staphylococcus aureus

Delta toxin is a small cytolytic polypeptide produced and secreted by the organism Staphylococcus aureus and belongs to a family of surface-active toxins that exhibit pronounced effects on a wide variety of cellular membranes. Although this class of proteins has been much studied by a wide variety of physical techniques, no consensus has been reached on their mode of action. Therefore, in order to investigate their role in causing membrane damage, a structural analysis of the delta toxin has been initiated. Crystals of this protein have been grown by dialysis against mixtures of 2-methylpentan-2,4-diol and water. These crystals are relatively insensitive to radiation damage and diffract to high resolution. The results of this study should provide a valuable insight into the cytolytic properties of these molecules.

Early events in the action of staphylococcal alpha-toxin on the plasma membrane of adrenocortical Y1 tumor cells

The early events in staphylococcal alpha-toxin action on mouse adrenocortical (Y1) tumor cells were studied. Cell-bound toxin could be partially neutralized by anti-alpha-toxin and inactivated by trypsin added within 10 min at 37 degrees C after the end of the binding step. Likewise, cell-bound toxin was capable of lysing rabbit erythrocytes (RRBC) added to the cells within 10 min after binding at 37 degrees C. After this time, the Y1 cells could not be rescued from intoxication by antibodies or trypsin, and the toxin was not accessible for lysis of RRBC. However, at 0 to 4 degrees C, the cell-bound toxin remained accessible to antibodies for at least 4 h. CaCl2 (30 mM) did not affect binding of the toxin to Y1 cells but completely prevented the intoxication if added within 10 min at 37 degrees C after the end of the binding step. The intoxication was independent of metabolic energy, active receptor clustering on the cell surface, and endocytosis of the toxin. Therefore, alpha-toxin interacted with the Y1 cell membrane in at least three separable steps: binding, a conformational change at the cell surface, and membrane damage. These early events appear to be similar to those occurring on RRBC treated with alpha-toxin.

Effect of thiol-activated toxins (streptolysin O, alveolysin, and theta toxin) on the generation of leukotrienes and leukotriene-inducing and -metabolizing enzymes from human polymorphonuclear granulocytes

The generation of leukotrienes (LTC4, LTD4, LTE4, and LTB4; 12-epi-LTB4 isomer) from human granulocytes by thiol-activated toxins (streptolysin O, alveolysin from Bacillus alvei, and theta toxin from Clostridium perfringens) is described. The release occurs under noncytolytic conditions. Although LTB4 is the major component after calcium ionophore stimulation, more LTC4 as compared with LTB4 is released with the toxins. The 5-lipoxygenase pathway of toxin-mediated activation can effectively be inhibited by caffeic acid, a lipoxygenase inhibitor. The toxins also induce the release of leukotriene-metabolizing enzymes such as gamma-glutamyltranspeptidase, which transfers LTC4 into LTD4, and dipeptidase, which metabolizes LTD4, into LTE4. Dipeptidase activity is more pronounced than the gamma-glutamyltranspeptidase activity but still does not reach the levels obtained when cells were triggered with opsonized zymosan.

Action of bacterial cytotoxins on normal mammalian cells and cells with altered membrane lipid composition

Cytotoxic proteins produced by a number of bacteria, as well as one from a marine invertebrate, were tested for their ability to disrupt the permeability barrier of mammalian cells. Agents were tested individually and in combination shown to have synergistic disruptive actions on erythrocytes. Toxins included the lipid-hydrolyzing enzymes sphingomyelinases C and D and cholesterol oxidase, as well as the non-enzymatic agents, helianthus toxin, streptolysin O and saponin. Cells treated included cultured human skin fibroblasts, normal human erythrocytes and erythrocytes enhanced and depleted in membrane cholesterol. Fibroblasts were disrupted by helianthus toxin and by the combination of sphingomyelinase C and cholesterol oxidase. Thin layer chromatographic analysis of the treated cells confirmed the enzymatic alteration of membrane lipids by the lipid hydroxylases. Human erythrocytes having an increased content of membrane cholesterol were more sensitive than normal cells to agents which interact with membrane sterol. Conversely, cholesterol-depleted cells were more resistant to these as well as other agents. Results are discussed in relation to biochemical mechanisms of action of the agents tested, and to their possible significance in bacterial pathogenesis.

Evaluation of the biological activity of cigarette-smoke condensate fractions using six in vitro short-term tests

The biological activity of the volatile part of the particulate phase of cigarette-smoke condensate, the semivolatile fraction, has been examined, since the constituents of this material are accessible to selective filtration. Such a process offers a possibility to reduce the biological activity of total cigarette smoke without appreciably affecting the taste. Cigarette-smoke condensate, obtained from domestic American blend type cigarettes, was therefore separated into a nonvolatile and a semivolatile fraction, and the latter was fractionated by liquid-liquid extractions into four subfractions; acids, phenols, bases, and neutrals. The biological activity of these fractions was investigated using six in vitro short-term tests, of which two, the Ames test and the induction of sister chromatid exchanges, provided information on their genotoxicity, and the other four provided information on their cytotoxicity by measuring inhibition of cell growth, inhibition of oxidative metabolism, membrane damage, and ciliotoxicity. Sister chromatid exchanges were found to be induced by the total condensate, the nonvolatile and the semivolatile fractions, and the subfractions derived from the semivolatile fraction, except the bases. The Ames test showed the total condensate and the nonvolatile fraction to contain direct-acting base-pair mutagens as well as indirect-acting frameshift mutagens. While the semivolatile fraction was found nonmutagenic, two of its subfractions, acids and phenols, were shown to contain base-pair mutagens, which did not require metabolic activation. The total condensate and the nonvolatile and semivolatile fractions showed similar activity in the four cytotoxicity tests. Of the semivolatile subfractions, the acids and the phenols exhibited the highest activity and the bases the lowest; the toxicity observed for the neutrals varied with the test system used.

Chromatofocusing: a new method for purification of staphylococcal enterotoxins B and C1

A new chromatographic procedure was developed which obtained highly purified preparations of staphylococcal enterotoxins B and C1 in yields of 60% from cultures of Staphylococcus aureus and which is faster than any of the separation methods used previously. The procedure involves chromatography on carboxymethylcellulose, removal of alpha-toxin by adsorption to rabbit erythrocyte membranes, and finally, chromatofocusing as the fundamental new step. Enterotoxins were obtained in highly purified form and behaved in a homogeneous manner as determined by ultracentrifugation and electrophoresis on polyacrylamide gel in the presence of sodium dodecyl sulfate, with molecular weights of 34,000 for staphylococcal enterotoxin B and 30,000 for staphylococcal enterotoxin C1. Using chromatofocusing as the final purification step, we isolated three B and six C1 distinct but immunologically identical enterotoxin fractions, which were found to be devoid of any impurities and to possess a marked degree of toxicity in monkeys.

Membrane damage by staphylococcal alpha-toxin to different types of cultured mammalian cell

Staphylococcal alpha-toxin was shown to be more membrane-damaging to epithelial-like cells than to neuroblasts or normal fibroblasts. Mouse adrenal cortex tumor (Y1Ac) epithelioid cells and human embryonal lung (MRC-5) fibroblasts were used for further comparison. Alpha-toxin was considerably more cytotoxic to adrenal cells than to fibroblasts. This difference did not depend on the presence fibronectin on the fibroblast surface, or on a general difference in the response to other membrane-damaging hemolytic toxins and detergents. Incubation of adrenal cells at 0 degree C with alpha-toxin induced some irreversible change, and membrane damage and a cytotoxic effect developed upon further incubation in toxin-free growth medium. In fibroblasts the membrane damage progressed slowly and only in the continued presence of the toxin. Toxin-induced damage to transport and synthetic functions in fibroblasts was reversible upon removal of the toxin after prolonged exposure. It is proposed that adrenal cells may carry a cell-surface receptor to which alpha-toxin binds specifically, thereby allowing the toxin to exert its cell damaging effect.

The influence of bacterial exotoxins and endotoxins on the phagocytic activity of human macrophages in culture

The effect of bacterial exotoxins and endotoxins on phagocytosis was tested on human macrophages in monolayer cultures by determining the rate of zymosan particle ingestion at different toxin concentrations and incubation times. The exotoxins tested were staphylococcal alpha-toxin and diphtheria-toxin. The endotoxins used were lipopolysaccharides from Salmonella typhi, Salmonella typhimurium, Shigella flexneri and Serratia marcescens. Phagocytosis was significantly impaired after prolonged incubation with diphtheria toxin whereas alpha-toxin was ineffective. Endotoxin-treated macrophages showed a wide range of phagocytic activity. Enhancement of phagocytosis was observed with a low concentration of endotoxin (1 microgram/ml) from S. typhi, S. typhimurium and S. flexneri. Higher concentrations (2.5 and 5 micrograms/ml) depressed phagocytosis to varying extents, except for S. typhi lipopolysaccharide, which did not induce a significant decrease in phagocytosis in comparison to the controls.

Survival of cultured cells after functional and structural disorganization of plasma membrane by bacterial haemolysins and phospholipases

Lesions were induced in the plasma membranes of cultured human fibroblasts by membrane damaging toxins of bacterial origin (haemolysins). Structural disorganization of the membrane was measured as leakage of a radiolabelled small cytoplasmic marker and functional membrane damage was measured as decreased uptake of aminoisobutyrate. Cell survival was scored 24 and 48 hr later by measuring uptake of Trypan Blue and by light microscopical evaluation of cell morphology and proliferation. The membrane damage induced by most bacterial toxins was reversible upon removal of the toxin, since toxin-treated cells recovered and excluded Trypan Blue although they had been permeable to the dye immediately after the toxin treatment. Among ten bacterial toxins tested, the only exception of this general behavior was the Aeromonas hydrophila beta-haemolysin, which irreversibly damaged human fibroblasts. Thus, the action of bacterial haemolysins on cultured cells generally seems restricted to a plasma membrane permeabilization, which is reversible regardless of the mechanism of membrane damaging action of the toxin or of the relative size of the structural lesions induced. Furthermore, the use of Trypan Blue uptake as a measure of cell death caused by membrane damaging agents appears to be of limited value.

Role of serum in inhibition of cultured lymphocytes by lysophosphatidylcholine

Serum was heated at various temperatures to inactivate components which might be involved in the regulation of lysophosphatidylcholine (lysoPC) levels in rabbit lymph-node cell cultures. Cells cultured in medium containing serum preheated for 20 min at 66 degrees C ("66 degrees C-serum") were inhibited much more by exogenous lysoPC (5 microM) than were cells cultured in medium containing control serum ("38 degrees C-serum"). This was observed over a 20 h culture period as a slow increase in inhibition of cell labelling with [3H] uridine, which reflected cytotoxic cell damage. Heating serum at 66 degrees C caused (i) conversion of monomeric albumin to highly polymeric forms which were deficient in lysoPC-binding activity, (ii) transfer of lysoPC from albumin to lipoproteins, predominantly high density lipoproteins, and (iii) inhibition of two lysoPC metabolizing activities (which were detected only at low levels in control serum). Addition of albumin to cultures containing 66 degrees C-serum decreased the toxicity of lysoPC to the same extent as did the addition of control serum with an equivalent albumin content. Thus, albumin was the major heat-labile factor protecting cells against lysoPC. However, cell inhibition by lysoPC was dependent on the sequence of heating serum and lysoPC addition. Inhibition was small when lysoPC was added before heating the serum. This could not be explained by a detectable difference in the binding of lysoPC to serum components. Furthermore, although radioactive labelling of cells with [14C] lysoPC was increased in 66 degrees C-serum, this did not correlate with cell inhibition. Increased labelling with [14C] lysoPC occurred several hours before significant cell inhibition was evident and was not affected by the sequence of heating and lysoPC addition. Since preincubation of lysoPC with 66 degrees C-serum increased the inhibition, it is suggested that the heated serum lysoPC generates another factor which is responsible for the cytotoxic effects observed.

Membrane lesions in cultured mouse neuroblastoma cells exposed to metal compounds

Tritiated 2-deoxy-D-glucose (dGlc) was rapidly taken up into cultured mouse neuroblastoma C1300 cells (clone 41A3). Upon perfusion the preloaded cultures slowly released radioactivity as [3H] 2-deoxy-D-glucose-6-phosphate ([3H]dGlc-6-P) (rate const. = 0.017 min-1) from a pool corresponding to 74% (t1/2 = 41 min) of the total radioactivity incorporated. Destruction of the plasma membrane of the cells by means of Triton X-100 (1.0%) resulted in a rapid and total release of the radioactivity. CH3HgCl, HgCl2, (C2H5)3SnCl and K2Cr2O7 all caused an increase in the passive cell membrane permeability to [3H]dGlc-6-P. A membrane toxic concentration (MTC) was defined as the concentration of the tested metal compound giving rise to an increase in the relative efflux from 1.0 to 1.2 during 60 min perfusion. Using this MTC-value, the membrane toxicity of the compounds could be ranked in the following order: CH3HgCl (MTC = 9 x 10(-7) M) greater than HgCl2 (MTC = 6 x 10(-6) M) greater than (C2H5)3SnCl (MTC = 3 x 10(-4) M) greater than K2Cr2O7 (MTC = 7 x 10(-4) M). Since this differential toxicity is in accordance with other reports it is concluded that 2-deoxy-D-glucose (dGlc) may be used together with 41A3 cells to screen metal compounds for their membrane toxicity.

Membrane-damaging and cytotoxic effects on human fibroblasts of alpha- and beta-hemolysins from Aeromonas hydrophila

The effects of two hemolysins (alpha and beta) from Aeromonas hydrophila on human lung fibroblasts were investigated. The toxins differed distinctly in regard to the morphological changes they produced. The alpha-hemolysin caused rounding of the cells. The beta-hemolysin caused a striking vacuolization of the cytoplasm in cells which remained spread out on the growth surface. The toxins also differed as to relative size of the initial lesions they induced in the fibroblast membrane, scored by leakage of different-sized cytoplasmic markers. The alpha-hemolysin induced larger lesions than did the beta-hemolysin. It was indirectly demonstrated that the alpha-hemolysin did not bind, or bound only transiently, to the fibroblasts. By contrast, the beta-hemolysin bound rapidly and firmly. The cytopathogenic response to the alpha-hemolysin was reversible, whereas cells treated with small amounts of the beta-hemolysin for only 1 min invariably died within a few hours. Thus, the two hemolysins from A. hydrophila, despite many biochemical similarities, show essential dissimilarities in their interactions with cultured cells.

Membrane-damaging action of alveolysin from Bacillus alvei

We investigated membrane damage to human diploid, embryonic lung fibroblasts caused by highly purified alveolysin by measuring leakage of cytoplasmic markers and uptake of various metabolites, and we observed a leakage pattern typical of SH-activated cytolysins. However, the mode of membrane interaction resembled the mode of membrane interaction of theta-toxin from Clostridium perfringens rather than that of streptolysin O in the following respects: the activity on fibroblast membranes was high compared with the activity on sheep erythrocytes; the toxin did not bind irreversibly to fibroblast cytoplasmic membranes; considerable membrane damage was caused at 0 degrees C; and inhibition of amino acid uptake occurred in the absence of significant structural membrane damage. These findings imply that data on membrane effects caused by streptolysin O cannot be related indiscriminately to other SH-activated cytolysins. With regard to the mode of membrane interaction, two apparently different groups of SH-activated cytolysins exist.

Staphylococcal enterotoxins fail to disrupt membrane integrity or synthetic functions of Henle 407 intestinal cells

The potential cytotoxic activity of purified staphylococcal enterotoxins for mammalian cells was evaluated. The effects of staphylococcal enterotoxins A (SEA) and B (SEB) on cell membrane integrity as measured by leakage of labeled cytoplasmic constituents ([3H]uridine), amino acid transport (lysine and aminoisobutyric acid), and macromolecular synthesis (protein, ribonucleic acid, and deoxyribonucleic acid) was evaluated for a human intestinal epithelial cell (Henle 407). No evidence of cytotoxicity by any of these criteria could be detected for cell monolayers incubated with SEA for periods of between 30 min and 24 h. Purified staphylococcal hemolysins (alpha- and delta-toxins) were shown to exert cytotoxicity by the leakage and amino acid uptake assays. In efforts to detect synergistic effects between enterotoxin and the staphylococcal cytotoxins, membrane functions were evaluated after sequential or combined treatment with enterotoxin and alpha-toxin or with enterotoxin and delta-toxin. In no instance could a contribution to cytotoxicity by the staphylococcal enterotoxin be detected. That the assays were sufficiently sensitive to detect synergistic effects was shown by the greater than additive effects achieved with a combination of alpha- and delta-toxins. The data, contrary to previous reports, showed that staphylococcal enterotoxins did not behave as bacterial cytotoxins.

Staphylococcal delta toxin stimulates endogenous phospholipase A2 activity and prostaglandin synthesis in fibroblasts

Delta toxin, one of at least four toxins produced by pathogenic strains of the skin bacterium Staphylococcus aureus, is an amphipathic polypeptide possessing hemolytic and cytolytic activity. Delta toxin stimulates high levels of phospholipase A2 activity in 3T3 mouse fibroblasts with concomitant synthesis and release of prostaglandins. Alpha toxin, another hemolytic toxin produced by strains of S. aureus, did not stimulate phospholipase A2 or prostaglandin release in these cells. Analysis of the release of lactate dehydrogenase and beta-galactosidase (cytoplasmic and lysosomal marker enzymes, respectively) from delta-toxin-treated cells indicated that cytolytic concentrations of the toxin damage the cell-surface membrane more extensively than lysosomal membranes. During a 30 min exposure, delta toxin stimulated 3T3 cells to hydrolyze up to 32% of the lipids biosynthetically labeled by incorporation of [3H]arachidonic acid. A relatively high percentage of the free arachidonic acid formed in delta-toxin-treated 3T3 cells was converted to prostaglandins (up to 41.3% and 8.3% converted to chromatographically identifiable prostaglandins E2 and F2 alpha, respectively, in 30 min), with optimal conversion occurring at sublytic toxin concentrations. The degree of activation of phospholipase A2 in 3T3 cells by a range of concentrations of delta toxin correlates with cytotoxicity assessed by failure to exclude trypan blue dye. Analysis of the calcium dependency of the toxin-activated phospholipase A2 was consistent with a cell-surface, Ca2+-dependent enzyme. The phospholipase A2 exhibits a degree of specificity for substrate lipids containing polyunsaturated fatty acid residues which can serve as precursors for prostaglandin formation. Enzymatic activity was not inhibited by diisopropylfluorophosphate (5 mM), N-ethylmaleimide (5 mM) or p-bromophenacylbromide (0.1 mM). Delta toxin did not activate detectable phospholipase A2 in subcellular preparations containing plasma membrane.

Interaction of streptolysin O from Streptococcus pyogenes and theta-toxin from Clostridium perfringens with human fibroblasts

The membrane-damaging properties on human diploid embryonic lung fibroblasts of streptolysin O (from Streptococcus pyogenes) and theta-toxin (from Clostridium perfringens) were compared. The results are consistent with the suggested mechanism for hemolysis by streptolysin O involving one fixation site and one lytic site of this cytolysin. However, the membrane-damaging activity of the two toxins differed with respect to (i) relative cytolytic activity on human diploid lung fibroblasts compared with that on sheep erythrocytes, (ii) binding to the fibroblast membrane, (iii) activity at 0 degrees C, (iv) membrane repair after more than 30 min, and (v) effect on influx of amino acids. It is concluded that the mechanism of membrane damage caused by theta-toxin differs from that of cytoplasmic membrane. These results question the current concept that all thiol-activated, cholesterol-inactivated bacterial toxins are similar both structurally and functionally.

Characterization of membrane permeability alterations induced in Vero cells by Clostridium perfringens enterotoxin

Alterations in plasma membrane permeability induced by Clostridium perfringens enterotoxin were studied using Vero (African green monkey kidney) cells which were radioactively labeled with four markers of different molecular size. The markers were alpha-amino[14C]isobutyric acid (Mr 103), 3H-labeled nucleotide (Mr approx. 300), 51Cr label (Mr approx. 3000) and [3H]RNA (Mr>25000). Over a 2h period, enterotoxin caused significant release of aminoisobutyric acid, nucleotides and 51Cr label but not RNA. The effects of enterotoxin on label release were dose- and time-dependent. The rate of release of markers was dependent upon their size. Permeability alterations could be detected within 15 min with a high dose of enterotoxin. Gel chromatography of released material was used to determine that markers of Mr 3000 but not 25000 leaked from permeabilized cells. It was concluded that enterotoxin is producing functional 'holes' of limited size in the membrane. Permeability changes due to enterotoxin treatment differed between confluent and nonconfluent (growing) cells. We propose that the primary action of the enterotoxin is to interact with the plasma membrane and produce functional 'holes' of defined size. The resultant alterations in membrane permeability cause the loss of essential cellular substances which inhibits processes such as macromolecular synthesis and eventually leads to cell deterioration and death.

Cultured human fibroblasts as a model for evaluation of potential in vivo toxicity of membrane damaging antibiotics

The toxic side effects of certain antimicrobial agents are probably related to their membrane damaging properties. Thus it should be possible to use measurement of membrane damage in vitro for evaluation of the potential toxicity in vivo of such antibiotics. We estimated the membrane damage induced in cultured human fibroblasts by anti-microbial agents, such as polyene antibiotics, sodium fusidate and polymyxin B as well as derivatives of some of these. Degree and character of membrane damage was determined on basis of leakage of three defined cytoplasmic markers from prelabelled cells after treatment with test substance. By comparing the minimal inhibitory concentrations against the target microbial cells (MIC) with the amounts needed to cause membrane damage of human cells (ED50) a 'therapeutic dose range' was obtained (ED50/MIC). The therapeutic dose range and the character of induced membrane damage were compared with the relative toxicities in vivo of each test substance. Highly toxic agents caused large functional 'holes' and/or showed a narrow therapeutic dose range, whereas less toxic substances induced smaller functional holes and/or had a larger therapeutic dose range. These parameters, evaluated in the presented model system, should be useful for an indication of potential toxicity in vivo.

Two species of full-length cDNA are synthesized in high yield by melittin-treated avian retrovirus particles

A method of activating endogenous cDNA synthesis in avian retroviruses that results in the formation of two species of full-length cDNA in high yield is described. Tests of biological activity show infectivity of at least the same order of magnitude as for full-length cDNA made by other procedures. Melittin, the major component of bee venom, is used as an alternative to nonionic detergents to make the viral envelope permeable and thus activate the endogenous RNA-dependent DNA polymerase. This compound is a toxic peptide known to interact with phospholipid membranes. It appears to be less disruptive to the viral structure than detergents, resulting in a more efficient transcription of the viral genome. Preliminary tests indicate that this method will also prove useful for studying enzymatic activities associated with other enveloped viruses.

Screening and characterization of membrane damaging effects in tissue culture

A leakage test system for specific detection, measurement and characterization of membrane damage in human lung fibroblasts is discussed. Examples of applications with various types of substances are presented, i.e. tobacco smoke compounds; animal, plant and microbial cytolysins; membrane damaging antibiotics.

Effect of tobacco smoke compounds on the plasma membrane of cultured human lung fibroblasts

The ability of compounds derived from tobacco and tobacco smoke to increase the permeability of the membranes of human lung fibroblasts has been studied by measuring the release of an intracellular marker after short term exposure. Of the 464 compounds tested, about 25% gave rise to severe membrane damage. The most active compounds, when divided according to functionality, were found within the groups of amines, strong acids and alkylated phenols, whereas nitriles and polycyclic aromatic hydrocarbons were found completely inactive. A pronounced effect of the chain length on the activity was observed for the aliphatic alcohols, aldehydes and acids, and all monocyclic aromatic compounds but benzonitriles and benzoic acids showed an increase in activity with increasing alkylsubstitution. It is concluded that tobacco smoke contains a number of membrane damaging substances. These membrane active compounds could not only cause direct toxic reactions but also potentiate the toxic effect by promoting the cell membrane penetration of other toxic substances in tobacco smoke.

Classification of microbial, plant and animal cytolysins based on their membrane-damaging effects of human fibroblasts

38 cytolytic agents of mainly microbial origin were investigated with respect to membrane-damaging activity on human diploid fibroblasts. Increased plasma membrane permeability was measured as leakage of three defined cytoplasmic markers of various sizes: alpha-aminoisobutyric acid, uridine nucleotides and ribosomal RNA. The relative leakages of these markers, caused by different concentrations of the various cytolysins, yielded a leakage pattern for each substance. Five distinct types of leakage patterns were obtained. These were transformed into numerical expressions by calculating the ratios between the amounts of cytolysin needed to release 50% of the nucleotide and ribosomal RNA markers and the amounts required to release 50% of the alpha-aminoisobutyric acid marker (ED50 ratios). A classification of the cytolysins into five groups was arrived at on the basis of the different types of leakage patterns with the aid of reference cytolysins with well-known mechanisms of membrane interaction. These groups comprised: (1) detergent-like agents, (2) agents interacting with only certain constituents of the cell membrane, (3) agents interacting with specific receptor molecules in the membrane, (4) agents inducing small functional holes of a definable size, and (5) agents inducing only a very limited increase in plasma membrane permeability. The system may be useful for characterization and differentiation of new cytolytic agents of various sources as it divides membrane-damaging agents into separate groups on the basis of their principal function on intact human cells.

Nonenteric toxins of Staphylococcus aureus

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