Resistance to streptolysin O in mammalian cells treated with oxygenated derivatives of cholesterol. Cholesterol content of resistant cells and recovery of streptolysin O sensitivity

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Different sensitivity to streptolysin-O of cells in macrophage lineage

We have surveyed the sensitivity of cells in macrophage lineage to Streptolysin-O (SLO). SLO had cytotoxic activity on immature myeloid cell lines such as M1 and WEHI-3BD+. SLO was toxic to the cells after a 2-hr incubation. However, mature macrophage cell lines such as A640-BB-2, J774, and P388D1 were not sensitive to the same dose of SLO. After M1 cells were treated with leukemia inhibitory factor (LIF), a differentiation-inducer to macrophage, these cells became insensitive to SLO in one day. This cytotoxic action of SLO was inhibited by pretreatment with anti-Streptolysin-O antibody or cholesterol. These results indicate that SLO has different effects on macrophage lineage.

The thiol-activated toxin streptolysin O does not require a thiol group for cytolytic activity

Site-directed mutagenesis of the TGC codon in a cloned streptolysin O (SLO) gene exchanged the single Cys residue in SLO for either Ala or Ser. The parent wild-type SLO (SLO.Cys-530) and the SLO.Ala-530 and SLO.Ser-530 mutant toxins, expressed in Escherichia coli, were purified and analyzed. Wild-type SLO.Cys-530 and the SLO.Ala-530 mutant showed no significant differences in their specific hemolytic activities, while the SLO.Ser-530 mutant had a reduced (ca. 25%), but still considerable, specific hemolytic activity as compared with that of wild-type SLO. The parent and mutant toxins extracted from lysed erythrocyte membranes had similar sedimentation profiles on sucrose density gradients, suggesting that the mutations did not affect the ability of SLO to form oligomers in membranes. These results show that the widely held assumption that the in vitro cytolytic activity of SLO requires an essential Cys residue is not true.

Protease-nicked theta-toxin of Clostridium perfringens, a new membrane probe with no cytolytic effect, reveals two classes of cholesterol as toxin-binding sites on sheep erythrocytes

A nicked theta-toxin (C theta), obtained by limited proteolysis with subtilisin Carlsberg, causes almost no hemolysis while it retains a nearly intact cholesterol binding site below 20 degrees C. Neither electron microscopic evidence for the formation of arc- and ring-shaped structures on the membrane nor toxin-stimulated influx of extracellular Ca2+ are detected in C theta-treated cells below 20 degrees C. Thus, event(s) in the lytic process are responsible for the temperature dependency of hemolysis, which is also supported by the observation that C theta requires higher Arrhenius activation energy for hemolysis than the native toxin. Using C theta as a probe due to its high affinity for membrane cholesterol without causing any obvious membrane changes, we demonstrated the possible existence of high- and low-affinity sites for theta-toxin on sheep erythrocytes. Both binding sites disappear by simultaneous treatment of the cells with sublytic doses of digitonin. Furthermore, C theta binds only to cholesterol among the chloroform/methanol-extractable, lipid components of sheep and human erythrocytes but not to the protein components derived from them. These results strongly suggest that cholesterol is an essential component of the both high- and low-affinity sites, and also imply that the modes of existence of cholesterol in the red cell membrane are heterogeneous.

Effect of perfringolysin O on the lateral diffusion constant of membrane proteins of hepatocytes as revealed by fluorescence recovery after photobleaching

Perfringolysin O is a thiol-activated cytolytic exotoxin the primary receptor of which is the membrane cholesterol on the cell surface. The effect of perfringolysin O was tested in various hepatocyte preparations. (i) Smears of fresh liver exposed to a mild H2O2 (1.0 mM) injury for 10 min at 37 degrees C, develop a 'peroxide-induced autofluorescence' (PIAF) on the membrane proteins. PIAF is suitable for measuring the average lateral diffusion constant (D) of the membrane proteins by means of fluorescence recovery after photobleaching technique (FRAP). Incubation for 5 min with 600 or 2000 units/ml of the perfringolysin O resulted in a significant increase (32 and 46%, respectively) of D as compared to the controls of the same age group (13-14 months). Various tests like heat denaturation of cholesterol saturation of perfringolysin O before its application as well as thiol-activation of the smears with dithiothreitol revealed that the increase of D is a specific toxin effect due mot probably to the reaction of perfringolysin O with cholesterol. (ii) Isolated hepatocytes were exposed to perfringolysin O and their viability as well as the release of two cytosolic enzymes (lactate dehydrogenase and glutamic-pyruvic transaminase) were measured; 40-60 units/ml of perfringolysin O in 30 min reduced the viability of the hepatocytes to zero and caused a release of about 70% of both cytosolic enzymes. The significance of the results is discussed from the points of view of both the toxin-effect and the FRAP method.

Cold-labile hemolysin produced by limited proteolysis of theta-toxin from Clostridium perfringens

A nicked toxin whose hemolytic activity is temperature dependent was obtained by limited proteolysis of theta-toxin (Mr 54,000) with subtilisin. The nicked toxin (C theta) is a complex of two fragments: the N-terminal fragment (Mr 15,000) with basic isoelectric point and the C-terminal fragment (Mr 39,000) with the single cysteinyl residue of the toxin whose reduced form is essential for the hemolytic activity. C theta hemolyzes erythrocytes only at temperatures above 25 degrees C, whereas the native toxin hemolyzes them even at 10 degrees C. At temperatures below 25 degrees C, C theta does not hemolyze them although it does bind to membrane cholesterol and although no distinct difference was observed between the secondary structure of C theta and that of native toxin. It was found that C theta binds to the cells only in a reversible manner at low temperature, while the native one binds irreversibly to the cells within 10 min, which explains the cold lability of C theta on hemolysis. The structural basis of the cold lability was discussed through comparison of C theta with another nicked derivative of theta-toxin that was also obtained.

Approximate dimensions of membrane lesions produced by streptolysin S and streptolysin O

Membrane lesions produced by the streptococcal membranolysins streptolysin S and streptolysin O were investigated. Escape of labeled marker molecules of various sizes from resealed sheep erythrocyte ghosts treated with the toxins for 30 min allowed estimation of the sizes of the primary channels formed. Streptolysin S formed lesions ranging in size up to 45 A in diameter, and even high toxin concentrations did not result in larger channels. The lesions produced by streptolysin O exceeded 128 A in diameter. Kinetics experiments demonstrated that the primary streptolysin O lesions were formed rapidly (1-2 min), but release of marker molecules from streptolysin S-treated vesicles began only after a 5-15-min lag period. Label release from large unilamellar liposomes treated with streptolysin S suggested that membrane fluidity does not affect the size of the streptolysin S lesions.