Toxic, DNA-Damaging and Mutagenic Activity of Epichlorohydrin on Human Cells Cultured in Vitro

Epichlorohydrin (ECHH) highly inhibited the tritiated thymidine uptake by human lymphocytes cultured in vitro, although the corresponding cell viability was unaffected. Furthermore, it elicited unscheduled DNA synthesis, acting as a DNA-damaging agent after its metabolic activation. ECHH also showed a clear toxic and mutagenic activity toward a human epithelial-like cell line, causing a decrease in cell viability and an increase in mutants resistant to 0.05 Lf/ml of diphtheria toxin.

Inhibition of Cholera Toxin and Other AB Toxins by Polyphenolic Compounds

Cholera toxin (CT) is an AB-type protein toxin that contains a catalytic A1 subunit, an A2 linker, and a cell-binding B homopentamer. The CT holotoxin is released into the extracellular environment, but CTA1 attacks a target within the cytosol of a host cell. We recently reported that grape extract confers substantial resistance to CT. Here, we used a cell culture system to identify twelve individual phenolic compounds from grape extract that inhibit CT. Additional studies determined the mechanism of inhibition for a subset of the compounds: two inhibited CT binding to the cell surface and even stripped CT from the plasma membrane of a target cell; two inhibited the enzymatic activity of CTA1; and four blocked cytosolic toxin activity without directly affecting the enzymatic function of CTA1. Individual polyphenolic compounds from grape extract could also generate cellular resistance to diphtheria toxin, exotoxin A, and ricin. We have thus identified individual toxin inhibitors from grape extract and some of their mechanisms of inhibition against CT.

[Not Available]

The mechanisms of diabetes-associated impairment of cellular immune defense and its regulation by vitamin D3 are not fully elucidated. The study was devoted to investigating the functional state of T-cell immunity as well as humoral immune activity in response to artificial immunization in experimental diabetes and after prolonged administration of vitamin D3. It was established that diabetes is characterized by a 2.3 times decrease in blood serum 25OHD3 content. Vitamin D3 deficiency was accompanied by the failures in proliferative activity of T-lymphocytes and alterations of the regulatory (CD4+-postive lymphocytes) and cytotoxic (CD8+-positive lymphocytes) cell subpopulations. It was found an increase in the content of phosphorylated p65 subunit of nuclear factor κB in total lysates of spleen T lymphocytes and its enhanced translocation to the nucleus. In addition, it was shown intensification of humoral IgG response to administration of recombinant diphtheria toxin subunit B. Revealed impairments in the cellular link of the immune system were associated with an increase in splenocytes apoptosis, which was detected by Annexin V-GFP ability to bind phosphatidyl serine that is specifically located on the outer surface of plasmalemma in apoptosis. Prolonged vitamin D3 treatment (within 2 months) in a dose of 20 IU/animal leads to normalization of the proliferative activity and the ratio of T-cell subpopulations, reduces the formation of phosphorylated subunit of NF-κB - p65 and contributes to a balanced secretion of IgG against artificial antigen. These changes were accompanied by a decrease in apoptotic events in the total population of splenocytes. Our findings suggest an important role of vitamin D3 in the regulation of the immune system abnormalities related to type 1 diabetes.

A possible clinical adaptation of CRM197 in combination with conventional chemotherapeutic agents for ovarian cancer

AIM

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is a promising target for cancer therapy. We have already started a phase I study of CRM197, a specific HB-EGF inhibitor, for advanced ovarian cancer. In this study, we evaluated possible clinical adaptations of CRM197 in combination with conventional chemotherapeutic agents.

MATERIALS AND METHODS

CRM197, bevacizumab, and paclitaxel were intraperitoneally administered either alone or in combination with mice xenografted with ES2 human ovarian cancer cells. The tumor volumes and microvessel densities (MVD) were determined.

RESULTS

Enhanced antitumor effects were observed when paclitaxel was used in combination with bevacizumab or CRM197. The antitumor effect of paclitaxel/CRM197 was significantly higher than that of paclitaxel/bevacizumab. The tumor MVD of mice treated with paclitaxel/CRM197 was significantly lower than that of mice treated with paclitaxel/bevacizumab.

CONCLUSION

CRM197 in combination with paclitaxel significantly blocked tumor formation and angiogenesis. These results suggest that paclitaxel is a suitable candidate for CRM197 combination therapy.

Molecular characterization of diphtheria toxin repressor (dtxR) genes present in nontoxigenic Corynebacterium diphtheriae strains isolated in the United Kingdom

Nontoxigenic strains of Corynebacterium diphtheriae represent a potential reservoir for the emergence of toxigenic C. diphtheriae strains if they possessed functional diphtheria toxin repressor (dtxR) genes. We studied the predominant strain of nontoxigenic C. diphtheriae circulating in the United Kingdom to see if they possessed dtxR genes and ascertain whether they were functional. A total of 26 nontoxigenic C. diphtheriae strains isolated in the United Kingdom during 1995 and 4 nontoxigenic strains isolated in other countries were analyzed by PCR and direct sequencing to determine the presence and intactness of the dtxR genes. The functionality of the DtxR proteins was assayed by testing for the production of siderophore in medium containing high and low concentrations of iron. PCR amplification and sequence analysis of the dtxR genes revealed four variants of the predicted DtxR protein among the nontoxigenic strains isolated in the United Kingdom. Production of siderophore in medium containing a low concentration of iron and repression of siderophore production in medium containing a high concentration of iron demonstrated that in all the strains the dtxR genes were functional. These findings demonstrate that, if lysogenised by a bacteriophage, nontoxigenic strains circulating in the United Kingdom could produce toxin and therefore represent a potential reservoir for toxigenic C. diphtheriae.

Behavior of Diphtheria Toxin T Domain Containing Substitutions That Block Normal Membrane Insertion at Pro345 and Leu307: Control of Deep Membrane Insertion and Coupling between Deep Insertion of Hydrophobic Subdomains

Diphtheria toxin T domain aids the translocation of toxin A chain across membranes. T domain has two hydrophobic layers/subdomains that can insert deeply into membranes: helices TH8 and 9, which form a transmembrane hairpin, and helices TH5-7, which form a nonclassical, nontransmembrane structure. Substitutions were made at Pro345, a residue located near the turn between TH8 and 9. P345 is critical for toxicity and pore formation by the T domain. Fluorescence methods showed that hairpin-disrupting Gly or Glu substitutions at 345 did not insert into lipid bilayers as deeply as the wild-type protein, and consistent with previous studies, these mutations reduced pore formation activity as assayed by a novel biotin-streptavidin-based influx assay. Introducing Pro at positions 347 or 353 not only failed to compensate for substitutions at P345, but also they further disrupted deep insertion and/or pore formation. Substitution of P345 with Asn, a residue that promotes helical hairpin formation almost as well as Pro, resulted in somewhat more normal insertion and pore formation than other substitutions. Importantly, a P345E substitution disrupted deep insertion of TH5-7. This suggests that TH8 and 9 and TH5-7 undergo some sort of coordinated insertion into the lipid bilayer and/or that the membrane-inserted T domain has a distinct tertiary structure in which TH5-7 interact with TH8 and 9 instead of consisting of noninteracting hydrophobic segments. Intriguingly, a L307R substitution in TH6, which disrupted deep insertion of TH7, had only a weak effect on pore formation and deep insertion of TH8 and 9. This suggests that the TH8 and 9 region can insert independently of TH5-7 to some degree and that TH8 and 9 insertion may occur early in T-domain insertion.

Mechanistic aspects of the deoxyribonuclease activity of diphtheria toxin

Here we examined the intrinsic nuclease activity of diphtheria toxin (DTx) to determine the mechanism by which it catalyzes DNA degradation. Results show that DTx degrades double-stranded DNA (dsDNA) by non-processive, endonucleolytic attack, without apparent specificity for nucleotide sequence. Moreover, divalent cation composition determines whether supercoiled dsDNA is cleaved by the introduction of single-strand nicks or double-strand breaks. Circular single-stranded DNA (ssDNA) is also a substrate for endonucleolytic attack. Pre-incubation of DTx with a 2000-fold excess of NAD, the natural substrate for the toxin's ADP-ribosyltransferase (ADPrT) activity, inhibited the transfer of radiolabeled ADP-ribose to elongation factor 2 but had no effect on the degradation of radiolabeled DNA. Based on this result and the fact that compounds known to inhibit the ADPrT activity of DTx had no effect on its nuclease activity and pre-incubation of DTx with DNA had no effect on ADPrT activity, we conclude that the ADPrT and nuclease active sites of DTx are functionally and spatially distinct. Moreover, studies with an ADPrT-inactivated form of DTx indicate that nuclease activity alone can lead to target cell lysis.

Actin--an inhibitor of eukaryotic elongation factor activities

An inhibitor of diphtheria toxin- and endogenous transferase-dependent ADP-ribosylation of eukaryotic elongation factor 2 (eEF2) has been found in the cytoplasmic fraction from rat liver. We provide evidence that this cytoplasmic inhibitor corresponds to actin, which gives rise also to inhibition of polyphenylalanine (polyPhe) synthesis. Both globular monomeric (G-actin) and filamentous (F-actin) forms of actin appear to be inhibitory on the action of elongation factors 1 and 2 (eEF1 and eEF2) in polyPhe synthesis with the inhibitory effect of G-actin proving to be stronger. Some component(s) in the postribosomal supernatant (S-130) fraction and also DNase I prevent actin-promoted inhibition of polyPhe synthesis.

Adjuvant activity of Mycobacterium bovis BCG expressing CRM197 on the immune response induced by BCG expressing tetanus toxin fragment C

In order to develop a combined recombinant Mycobacterium bovis BCG (rBCG) vaccine against diphtheria, pertussis and tetanus (DPT), we have constructed different strains of rBCG expressing tetanus toxin fragment C (FC), driven by the up-regulated M. fortuitum beta-lactamase promoter, pBlaF*. Tetanus toxin FC was expressed in comparable levels in native form or in fusion with the beta-lactamase exportation signal sequence; however, in both constructs it was localized to the cytosol. Immunization of mice with rBCG-FC or its combination with rBCG expressing CRM197, induced anti-tetanus toxin antibodies with a Th2 immunoglobulin profile. Administration of a subimmunizing dose of the diphtheria-tetanus toxoid vaccine showed that rBCG-FC primed mice for production of an intense humoral response. Interestingly, the combination of rBCG-FC and rBCG-CRM197 reduced the time required for maturation of the immune response and increased anti-tetanus toxin antibody levels, suggesting adjuvant properties for rBCG-CRM197; this combination induced 75% protection in mice challenged with 100 minimum lethal doses (MLD) of tetanus toxin. Antisera from guinea pigs immunized with this combination were shown to neutralize tetanus toxin and diphtheria toxin. Our results suggest reciprocal adjuvant effects of rBCG-FC and rBCG-CRM197, which may contribute to induction of a more effective immune response against both diseases.

Diphtheria toxin catalyzed hydrolysis of NAD(+): molecular dynamics study of enzyme-bound substrate, transition state, and inhibitor

The mechanism of the diphtheria toxin-catalyzed hydrolysis of NAD(+) was investigated by quantum chemical calculations and molecular dynamics simulations. Several effects that could explain the 6000-fold rate acceleration (Delta Delta G(++) approximately 5 kcal/mol) by the enzyme were considered. First, the carboxamide arm of the enzyme-bound NAD(+) adopts a trans conformation while the most stable conformation is cis. The most stable conformation for the nicotinamide product has the amide carbonyl trans. The activation energy for the cleavage of the ribosidic bond is reduced by 2 kcal/mol due to the relaxation of this ground state conformational stress in the transition state. Second, molecular dynamics simulations to the nanosecond time range revealed that the carboxylate of Glu148 forms a hydrogen bond to the substrate's 2' hydroxyl group in E.S (approximately 17% of the time) and E.TS (approximately 57% of the time) complexes. This interaction is not seen in crystal structures. The ApUp inhibitor is held more tightly by the enzyme than the transition state and the substrate. Analysis of correlated motions reveals differences in the pattern of anticorrelated motions for protein backbone atoms when the transition state occupies the active site as compared to the E.NAD(+) complex.

Photoaffinity labeling of diphtheria toxin fragment A with 8-azidoadenosyl nicotinamide adenine dinucleotide

Diphtheria toxin fragment A (DT-A) is an important enzyme in the class of mono(ADP-ribosyl)transferases. To identify peptides and amino acid residues which form the NAD(+) binding site of DT-A using a photoaffinity approach, the photoprobes nicotinamide 8-azidoadenine dinucleotide (8-N(3)-NAD) and nicotinamide 2-azidoadenine dinucleotide (2-N(3)-NAD) were synthesized. Binding studies gave an IC(50) of 2.5 microM for 8-N(3)-NAD and 5.0 microM for 2-N(3)-NAD. Irradiation of DT-A and low concentrations of [alpha-(32)P]-8-N(3)-NAD with short-wavelength UV light resulted in rapid covalent incorporation of the photoprobe into the protein. The photoincorporation was shown to be specific for the active site with a stoichiometry of photoincorporation of 75-80%. After proteolytic digestion of photolabeled DT-A, derivatized peptides were isolated using immobilized boronate affinity chromatography followed by reversed phase HPLC. Radiolabeled peptides originating from two regions of the protein were identified. Chymotryptic digestion produced labeled peptides corresponding to His(21)-Gln(32) and Lys(33)-Phe(53). Lys-C digestion gave overlapping peptides Ser(11)-Lys(33) and Ser(40)-Lys(59). Tyr(27) was identified as the site of photoinsertion within the peptide His(21)-Gln(32) on the basis of the absence of PTH-Tyr at the predicted cycle during sequence analysis and by the lack of predicted chymotryptic cleavage at Tyr(27). Within the second modified peptide Ser(40)-Lys(59), Trp(50) is the most probable site of modification. Identification of Tyr(27) as a site of photoinsertion is in agreement with its placement in the NAD binding site of the X-ray structure of the proenzyme DT-NAD complex [Bell, C. E., and Eisenberg, D. (1996) Biochemistry 35, 1137]. Trp(50) is far from the adenine ring in the crystallographic model; however, site-directed mutagenesis studies suggest that Trp(50) is a major determinant of NAD binding affinity [Wilson, B. A., Blanke, S. R., Reich, K. A., and Collier, R. J. (1994) J. Biol. Chem. 269, 23296-23301].

Inhibition of the cytotoxicity of protein toxins by a novel plant metabolite, mansonone-D

We have studied the effect of several structurally related mansonones on the cytotoxicity of plant and bacterial toxins in Vero and BER-40, a brefeldin A-resistant mutant of Vero cells. Mansonone-D (MD), a sesquiterpenoid ortho-naphthoquinone, inhibited the cytotoxicity of ricin, modeccin, Pseudomonas toxin, and diphtheria toxin in Vero cells to different extents. The inhibition of ricin cytotoxicity was dose dependent and reversed upon removal of the drug. Protection of ricin cytotoxicity was also observed in the presence of cycloheximide, indicating that de novo protein synthesis is not required for the protective effect. Although MD inhibited the degradation and excretion of ricin, the binding and internalization of ricin was not affected. In contrast, MD strongly reduced the specific binding of diphtheria toxin in Vero cells. Fluorescence microscopic studies show that MD treatment dramatically alters the morphology of the Golgi apparatus in Vero cells. The kinetic studies reveal that the protection of ricin cytotoxicity is the consequence of decreased toxin translocation to the cytosol in MD-treated cells. The reactive ortho-quinone moiety of MD is important for the protective effect as thespesone, a para-naphthoquinone with a heterocyclic ring structure identical to that of MD, did not inhibit the cytotoxicity of toxins. Thespone, a dehydromansonone-D, lacking two hydrogens from the heterocyclic dihydrofuran ring of MD, inhibited the cytotoxicity of ricin, but was albeit less potent than MD. Neither mansonone-E nor mansonone-H with reactive ortho-quinone moiety, but with a different heterocyclic structure, had any effect on the cytotoxicity of ricin indicating that the protective effect of MD is specifically related to the overall structure of the metabolite.

Potency assay of diphtheria antitoxin in Vero cell microcultures

Laboratory conditions were established for the titration of diphtheria toxin and antitoxin in Vero Cell Cultures (CCT) and a comparison was made with the intradermal test (IDT) as currently used throughout the world. Working dilutions and cut off values were established by reading both the change in color of phenol red used as pH indicator and changes in cell viability in the microscope. CCT shows high reproducibility and higher detectability than IDT in the titration of both WHO Reference Standard and high titer horse antisera. In sera of guinea pigs immunized for potency testing of diphtheria toxoid the titre was approximately 10 times lower than in the IDT. The explanation is a subject of speculation. The Vero cell titration might be adopted as such for titration of diphtheria antitoxin. In the case of the toxoid potency test it could be used if the limit for the titration is adjusted to 0.2 IU considering the equivalence obtained between the two tests, by taking into account a ratio of 1:10 between CCT and IDT.

Diphtheria toxin endocytosis and membrane translocation are dependent on the intact membrane-anchored receptor (HB-EGF precursor): studies on the cell-associated receptor cleaved by a metalloprotease in phorbol-ester-treated cells

Preincubation of Vero cells with 1 microM phorbol 12-myristate 13-acetate (PMA) decreased the specific binding of diphtheria toxin by about 50%, whereas the toxic effect, endocytic uptake and membrane translocation were completely blocked. Toxin bound to PMA-treated cells was released upon incubation with heparinase. The effect of PMA was abrogated in the presence of EDTA or N-(DL-[2-(hydroxyaminocarbonyl)methyl]-4-methyl-pentanoyl)-L-3-(2' - naphthyl)-alanyl-L-alanine 2-aminoethyl-amide (TAPI), a specific inhibitor of matrix metalloproteases. The results indicate that PMA induces proteolytic cleavage of the diphtheria-toxin receptor [heparin-binding EGF-like growth factor (HB-EGF)-precursor] outside the membrane anchor, and that about 50% of the growth-factor ecto-domain remains associated with the cells, due to binding to surface proteoglycans containing heparan sulphates. Although the cleaved cell-associated HB-EGF binds diphtheria toxin, it does not serve as a functional receptor, since neither toxin internalization nor translocation occurs. Thus the intact HB-EGF precursor is of crucial importance for its function as the diphtheria-toxin receptor.

Ilimaquinone inhibits the cytotoxicities of ricin, diphtheria toxin, and other protein toxins in Vero cells

Ilimaquinone (IQ), a metabolite from sea sponges, has been shown to cause the breakdown of Golgi membranes into small vesicular structure and to inhibit protein transport without eliciting the retrograde transport of the Golgi enzymes to the endoplasmic reticulum [P. A. Takizawa, J. K. Yucel, B. Viet, D. J. Faulkner, T. Deerinck, G. Soto, M. Ellismann, and V. Malhotra, Cell (1993) 73, 1079-1090]. We have found that incubation of Vero cells with IQ inhibited the cytotoxicity of ricin in a dose-dependent manner. The inhibition was reversed upon the removal of IQ. Neither binding and internalization of 125I-ricin nor the translocation of ricin to the cytosol was affected by IQ. However, IQ significantly inhibited the recycling and degradation of internalized 125I-ricin. Preincubation with IQ also prevented the enhancement of ricin cytotoxicity by NH4Cl or nigericin. The inhibition of ricin cytotoxicity by IQ was observed in the presence of cycloheximide, indicating that de novo protein synthesis is not required for IQ-mediated protection of Vero cells from ricin cytotoxicity. In contrast to perinuclear distribution of TRITC-labeled ricin in Vero cells, TRITC-ricin appeared in numerous small vesicles dispersed throughout the cytoplasm in IQ-treated Vero cells. Double labeling with C6-NBD-ceramide and TRITC-labeled ricin showed that these ricin-containing vesicles were distinct from the IQ-induced breakdown product of the Golgi membranes. Like brefeldin A (BFA), IQ inhibited the cytotoxicities of abrin, modeccin, Pseudomonas toxin, and Shiga-like toxin in Vero cells. Unlike BFA, IQ also inhibited the cytotoxicity of diphtheria toxin (DT). Inhibition of DT cytotoxicity was the consequence of a decreased specific binding of the toxin in the IQ-treated cells.

Cerulenin inhibits the cytotoxicity of ricin, modeccin, Pseudomonas toxin, and diphtheria toxin in brefeldin A-resistant cell lines

We have found that cerulenin, an antibiotic that inhibits de novo fatty acid and cholesterol biosynthesis and fatty acylation of proteins, strongly inhibited the cytotoxicity of ricin, modeccin, Pseudomonas toxin, and diphtheria toxin in a brefeldin A (BFA)-resistant mutant of Vero cells (BER-40). The protective effect of cerulenin against ricin was also observed in two other BFA-resistant cell lines, Madin-Darby canine kidney, and PtK1 cells. In contrast to BER-40 cells, no significant effect of cerulenin was observed in Vero cells. Cerulenin did not affect the binding of ricin to the cell-surface receptors, but reduced significantly the internalization of ricin in BER-40 cells; no effect of cerulenin on the binding or internalization of ricin was observed in Vero, PtK1, and Madin-Darby canine kidney cells. Endocytic uptake of fluid-phase markers such as horseradish peroxidase and lucifer yellow was inhibited by cerulenin in BER-40 cells, but the endocytosis of transferrin via the coated pit/coated vesicle pathway was slightly increased. Cerulenin inhibited the degradation and excretion of ricin in BER-40 cells, and this effect of cerulenin was not observed in Vero cells. Furthermore, cerulenin inhibited the bulk protein secretion in a dose-dependent manner, with BER-40 cells being more susceptible than Vero cells. These results suggest that in addition to its effect on endocytosis, cerulenin interferes with the intracellular trafficking or processing of toxin molecules, and the vesicle transport system in BER-40 cells appears to be cerulenin-sensitive. Since addition of fatty acids and cholesterol did not reverse the effects of cerulenin, the protective effect of cerulenin against protein toxins is not due to an inhibition of de novo fatty acids and cholesterol biosynthesis.

An antibody that inhibits the binding of diphtheria toxin to cells revealed the association of a 27-kDa membrane protein with the diphtheria toxin receptor

A monoclonal antibody that blocks the binding of diphtheria toxin to Vero cells was isolated by immunizing mice with Vero cell membrane. The antibody inhibits the binding of diphtheria toxin and also CRM197, a mutant form of diphtheria toxin, to Vero cells, and consequently inhibits the cytotoxicity of diphtheria toxin. This antibody does not directly react with the receptor molecule of diphtheria toxin (DTR14.5). Immunoprecipitation and immunoblotting studies revealed that this antibody binds to a novel membrane protein of 27 kDa (DRAP27). When diphtheria toxin receptor was passed through an affinity column made with this antibody, the receptor was trapped only in the presence of DRAP27. These results indicate that DRAP27 and DTR14.5 closely associate in Vero cell membrane and that the inhibition of the binding of diphtheria toxin to the receptor is due to the binding of the antibody to the DRAP27 molecule. Binding studies using 125I-labeled antibody showed that there are many more molecules of DRAP27 on the cell surface than diphtheria toxin-binding sites. However, there is a correlation between the sensitivity of a cell line to diphtheria toxin and the number of DRAP27 molecules on the cell surface, suggesting that DRAP27 is involved in the entry of diphtheria toxin into the target cell.

Cell surface sulfhydryls are required for the cytotoxicity of diphtheria toxin but not of ricin in Chinese hamster ovary cells

A previous study on cleavage of disulfide bonds in endocytosed model compounds had shown that an initial phase of cleavage was totally inhibited by membrane-impermeant sulfhydryl inhibitors and thus was mediated by cell surface sulfhydryls (Feener, E. P., Shen, W.-C., and Ryser, H. J.-P. (1990) J. Biol. Chem. 265, 18780-18785). This paper uses the same inhibitors (5,5'-dithiobis(2-nitrobenzoic acid) and p-chloromercuriphenylsulfonic acid) to examine the role of surface sulfhydryls in the cytotoxicity of diphtheria toxin (DT). Since the interchain disulfide of endocytosed DT must be cleaved prior to translocation of chain A from endosomes to cytoplasm, it was postulated that surface sulfhydryls might mediate the cleavage of that disulfide bond as well. Both sulfhydryl blockers did indeed markedly inhibit DT cytotoxicity. This effect was not due to inactivation of unbound DT, inhibition of receptor-mediated endocytosis, or impairment of acidification of endosomes. We conclude that cell surface sulfhydryls susceptible to blockage by 5,5'-dithiobis(2-nitro-benzoic acid) and p-chloromercuriphenylsulfonic acid are required for the cytotoxicity of DT and, most likely, for the reductive cleavage of DT's interchain disulfides. Ricin cytotoxicity was not decreased; this is consistent with the view that ricin reaches the cytoplasm from a late endocytic structure and with the finding that endocytosed disulfides are also cleaved in a cell fraction containing elements of the Golgi apparatus (Feener, E. P., Shen, W.-C., and Ryser, H. J.-P. (1990) J. Biol. Chem. 265, 18780-18785).

The cytotoxic action of diphtheria toxin and its degradation in intact Vero cells are inhibited by bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase

The role of vacuolar-type H(+)-ATPase (V-ATPase) in the cytotoxic action of diphtheria toxin (DT) was studied by using bafilomycin A1, a specific inhibitor of V-ATPase. Studies with acridine orange showed that the acidification of intracellular acidic compartments was inhibited strongly when Vero cells were treated with 500 nM bafilomycin A1, indicating that bafilomycin effectively inhibits V-ATPase when it is added to the culture medium. The toxicity of DT to Vero cells, which was determined by the inhibition of protein synthesis by DT, was inhibited partially by bafilomycin at 10 nM and inhibited completely at 500 nM. Therefore, V-ATPase is involved in the expression of the toxicity of DT. Studies using 125I-labeled DT showed that bafilomycin inhibited the degradation of internalized DT, indicating that V-ATPase is also involved in this step. Subcellular fractionation revealed that 125I-DT accumulated mainly in the endosome fraction, and not in the lysosome fraction, when the cells were incubated with 125I-DT in the presence of bafilomycin. Under the cell fractionation conditions similar to those used for the DT-treated cells, we determined the location of 125I-labeled epidermal growth factor in the degradation pathway. The result suggests that bafilomycin A1 does not inhibit the transport of epidermal growth factor to lysosome.

Comparison of the intoxication pathways of tumor necrosis factor and diphtheria toxin

The mechanism by which tumor necrosis factor alpha (TNF) initiates tumor cell destruction is unknown. Having established that a brief drop in extracellular pH enhances the killing activity of TNF, our next objective was to explore whether TNF-induced cell death is dependent on endosomal acidification. Diphtheria toxin (DTx), a well-characterized acid-dependent cytotoxin, served as an indicator of the effectiveness of each treatment condition. Studies with lysosomotropic agents demonstrated that the cytotoxic pathway of TNF can operate independently of low pH exposure in contrast to the lethal pathway of DTx. When NH4Cl-treated cells were exposed to TNF at low pH, the level of killing increased two- to threefold over that attained with cells maintained at neutral pH (either with or without NH4Cl). Furthermore, inhibition of metabolic processes by sodium azide in combination with 2-deoxyglucose severely reduced DTx killing but stimulated TNF killing. Despite these differences, TNF and DTx provoked extensive internucleosomal DNA cleavage in prelytic target cells. Inhibitor of nuclear poly(ADP-ribose) transferase also evoked similar levels of cellular resistance to both cytotoxins. Models for DTx- and TNF-induced cytolysis are discussed in view of these new discoveries.

Inhibition of coated pit formation in Hep2 cells blocks the cytotoxicity of diphtheria toxin but not that of ricin toxin

It has been recently shown (Larkin, J. M., M. S. Brown, J. L. Goldstein, and R. G. W. Anderson, 1983, Cell, 33:273-285) that after a hypotonic shock followed by incubation in a K+-free medium, human fibroblasts arrest their coated pit formation and therefore arrest receptor-mediated endocytosis of low density lipoprotein. We have used this technique to study the endocytosis of transferrin, diphtheria toxin, and ricin toxin by three cell lines (Vero, Wi38/SV40, and Hep2 cells). Only Hep2 cells totally arrested internalization of [125I]transferrin, a ligand transported by coated pits and coated vesicles, after intracellular K+ depletion. Immunofluorescence studies using anti-clathrin antibodies showed that clathrin associated with the plasma membrane disappeared in Hep2 cells when the level of intracellular K+ was low. In the absence of functional coated pits, diphtheria toxin was unable to intoxicate Hep2 cells but the activity of ricin toxin was unaffected by this treatment. By measuring the rate of internalization of [125I]ricin toxin by Hep2 cells, with and without functional coated pits, we have shown that this labeled ligand was transported in both cases inside the cells. Hep2 cells with active coated pits internalized twice as much [125I]ricin toxin as Hep2 cells without coated pits. Entry of ricin toxin inside the cells was a slow process (8% of the bound toxin per 10 min at 37 degrees C) when compared to transferrin internalization (50% of the bound transferrin per 10 min at 37 degrees C). Using the indirect immunofluorescence technique on permeabilized cells, we have shown that Hep2 cells depleted in intracellular K+ accumulated ricin toxin in compartments that were predominantly localized around the cell nucleus. Our study indicates that in addition to the pathway of coated pits and coated vesicles used by diphtheria toxin and transferrin, another system of endocytosis for receptor-bound molecules takes place at the level of the cell membrane and is used by ricin toxin to enter the cytosol.

[Antitoxic properties of monoclonal antibodies against diphtheria toxin]

Antitoxin properties of monoclonal antibodies to diphtheria toxin were studied in chick fibroblast culture and during the dermonecrotic tests in guinea pigs. Antitoxic properties were shown for antibodies specific to B subunit of the toxin but not for the ones specific to a subunit. The mixture of monoclonal B subunit specific antibodies reveals a higher neutralizing activity as compared with the activity produced by any single monoclonal antibody tested. The protective activity demonstrated by antibodies is connected with the preventing of toxin molecules fixation on the cellular receptors.

Monoclonal antibodies against diphtheria toxin fragment A. Characterization and introduction into living cells

Monoclonal antibodies against fragment A of diphtheria toxin were isolated and characterized. Three antibodies with similar affinities for fragment A had different effects on the NAD: EF2-ADP ribose transferase activity of fragment A; i.e., antibody DA1 almost completely inhibited the enzymic activity at a molar ratio of one, whereas DA2 inhibited only partially and DA3 had no effect. However, when fragment A176 from the mutant toxin CRM176 (about 1/10 as active as wild type) was used, DA2 proved a more effective inhibitor than DA1. The affinities of these antibodies for the enzymically inactive mutant fragments, A197 and A228, were significantly less manifest than for wild-type fragment A. Binding of the antibodies to whole toxin and the chain termination mutant CRM45 was weak. When DA2 was introduced into Vero cells growing in monolayers, by using the red cell ghost fusion method, the cells became resistant to CRM176. The anti-fragment A antibodies may serve as the basis of a simple method for selection of cells into which other molecules have been co-introduced.

Regulation of toxinogenesis in Corynebacterium diphtheriae: mutations in the bacterial genome that alter the effects of iron on toxin production

Mutants of Corynebacterium diphtheriae C7(beta) that are resistant to the inhibitory effects of iron on toxinogenesis were identified by their ability to form colonies surrounded by toxin-antitoxin halos on agar medium containing both antitoxin and a high concentration of iron. Chromosomal mutations were essential for the altered phenotypes of four independently isolated mutant strains. During growth in deferrated liquid medium containing various amounts of added iron, these mutants differed from wild-type C. diphtheriae C7(beta) in several ways. Their growth rates were slower under low-iron conditions and were stimulated to various degrees under high-iron conditions. The concentrations of iron at which optimal toxin production occurred were higher for the mutants than for wild-type C. diphtheriae C7(beta). Toxin production by the mutants during growth in low-iron medium occurred throughout the period of exponential growth at nearly constant rates that were proportional to the bacterial growth rates. In contrast, toxin production by wild-type C. diphtheriae C7(beta) in similar low-iron cultures occurred predominantly during the late exponential phase, when iron was a growth-limiting nutrient. Additional studies demonstrated that these mutants had severe defects in their transport systems for ferric iron. We propose that the altered regulation of toxinogenesis by iron in our mutants was caused by the severe defects in their iron transport systems. As a consequence, the mutants exhibited a low-iron phenotype during growth under conditions that permitted wild-type C. diphtheriae C7(beta) to exhibit a high-iron phenotype.

Reconstitution of diphtheria toxin from two nontoxic cross-reacting mutant proteins

The isolation of a new type of mutant Corynephage beta, which carries a missense mutation in the structural gene for diphtheria toxin synthesis is described. The lysogenic C7(8)(beta(197))(tox-crm+) strain of Corynebacterium diphtheriae produces a nontoxic, extracellular protein of molecular weight 62,000. This protein is immunologically indistinguishable from toxin itself but inhibits the action of toxin on HeLa cells, probably by competing for attachment sites on the cell membrane. In contrast to fragment A derived from diphtheria toxin, fragment A(197) is unable to catalyze the inactivation of eucaryotic polypeptidyl-transfer RNA-transferase II. When mixtures of the two nontoxic mutant proteins, enzymically active crm(45) protein and enzymically inactive crm(197) protein, are subjected to mild treatment with trypsin in the presence of a thiol and then allowed to reoxidize after dialysis to remove excess thiol, "diphtheria toxin" is reconstituted in high yield.

Response of interferon-treated cells to diphtheria toxin

It has been reported that treatment with homologous interferon of cells in culture interferes with the process of diphtheria intoxication in these cells. This report both confirms and extends this observation. The most sensitive indicator of diphtheria toxin action in cells, i.e., the inhibition of incorporation of labeled amino acids into protein, was employed, and human, mouse, and chicken interferon-cell systems were studied. Interferon treatment afforded a distinct, reproducible protection from the action of toxin, but in no case could total protection be shown. The "antitoxic" component is stable at pH 2, is species-specific, is not sedimented at 100,000 x g, and is trypsin-sensitive. Human cell preparations are heat-labile. Experiments with cell-free amino acid incorporation systems suggest that the "anti-toxic" effect of interferon is concentrated at the level of the cell membrane and subcellular components are not directly involved.

Diphtheria toxin subunit active in vitro

Exposure of diphtheria toxin to dithiothreitol (and similar thiols) resulted in a subunit which was active in catalyzing the adenosine diphosphateribosylation of mammalian aminoacyl-transferase II in the presence of nicotinamide adenine dinucleotide. At the same time there was a marked increase in total ADP-ribosylation activity. A molecule which was apparently identical to the derived subunit in size and activity was detected in partially purified preparations of toxin.