Response of cultured mammalian cells to diphtheria toxin. IV. Isolation of KB cells resistant to diphtheria toxin

Transforming growth factor beta and the cell surface in tumor progression

Type 1 transforming growth beta (TGF-beta 1) is a multifunctional regulator of cellular differentiation, motility and growth. It is capable of inhibiting or stimulating these processes depending on cell type, cell density, culture conditions and TGF-beta 1 concentration. TGF-beta 1 regulates growth, in part, by inducing the expression and secretion of various types of collagen, which participate in the control of cell adhesion and migration, as well as growth. TGF-beta 1 also regulates cell growth by controlling the response to epidermal growth factor (EGF) and other growth factors, in ways that can either decrease or increase their growth-promoting effects. Alterations in both negative and positive growth responses to TGF-beta 1 play important roles in tumor progression. Loss of sensitivity to growth inhibition by TGF-beta 1 can occur as a result of decreased expression of collagen. Acquisition of sensitivity to growth stimulation, and autocrine transformation by TGF-beta 1, are associated with aberrant EGF receptor regulation. Aberrant growth factor receptor regulation by TGF-beta 1 may be mediated by a protein kinase C (PKC)-dependent pathway which inhibits degradation of growth factor receptor/ligand complexes. The evidence reviewed is consistent with a minimal two-step mechanism for autocrine transformation, which involves production of growth factor and enhanced cellular response as a result of aberrant membrane traffic. Defects in membrane traffic regulation may provide an explanation for common alterations in tumor cell response to both multiple growth inhibitors and growth stimulators, and may also suggest novel approaches to cancer chemotherapy.

Observations on constitutional resistance to infection

The attention of most immunologists is held by the elaborate nonspecific and antigen-specific mechanisms of reactive immunity. There is, however, a more fundamental level of immunity to infection that is shared by all forms of life: constitutional resistance to infection. In this article, the nature and importance of constitutional immunity is illustrated using examples from population, cell and molecular biology.

Altered degradation of epidermal growth factor in a diphtheria toxin-resistant clone of KB cells

We have investigated the cellular fate of epidermal growth factor (EGF) in KB cells and a variant, KB-R2A, that was isolated based on its resistance to diphtheria toxin and subsequently was shown to be resistant to infection by RNA viruses (Moehring and Moehring, 1972, Infect. Immunity. 6:487-492). Both cell lines bind 125I-EGF and internalize the cell-bound hormone at the same rate. However, when the degradation of internalized 125I-EGF was measured by the release of low molecular weight (mw) hydrolysis products into the medium, the toxin-resistant KB-R2A cells degraded the hormone at a drastically reduced rate; 50% and 3% of the cell-bound 125I-EGF was degraded and released by 80 min in the KB and KB-R2A cells, respectively. To investigate the fate of cell-associated EGF prior to release into the medium, the radioactivity in extracts of cells labeled with 125I-EGF was fractionated by native gel electrophoresis. In KB cells three peaks of radioactivity other than native 125I-EGF were resolved. Time course and subcellular fractionation studies showed that the first processed product appeared while the hormone was located in the endocytic vesicles and the appearance of the other two peaks correlated with the arrival of the hormone in the lysosomal compartment. KB-R2A cells also produced the first intermediate but they produced only very low amounts of the other two peaks. These studies show that endocytic vesicles in both cell lines contain enzymes capable of processing EGF prior to the arrival of the hormone in the lysosomes and show that the KB-R2A cells have a lesion that prevents the complete degradation of the hormone. We propose that the KB-R2A cell line has a defective mechanism for the intracellular processing of a number of ligands that are internalized by the process of receptor-mediated endocytosis and that this defect is located beyond the initial endocytic step.

Autoradiographic assay of mutants resistant to diphtheria toxin in mammalian cells in vitro

Diphtheria toxin kills mammalian cells by ribosylating elongation factor 2, a protein factor necessary for protein synthesis. The frequency of cells able to form colonies in the presence of the toxin can be used as an assay for mutation to diphtheria toxin resistance. We report here that resistance to diphtheria toxin can also be detected autoradiographically in cells exposed to [3H]leucine after treatment with the toxin. In cultures of Chinese hamster ovary cells, the frequency of such resistant cells is increased by exposure of the cells to gamma-rays, ultraviolet light, ethylnitrosourea, mitomycin c, ethidium bromide, and 5-bromo-2'-deoxyuridine in a dose- and time-dependent manner. The resistant cells form discrete microcolonies if they are allowed to divide several times before intoxication, which indicates that they are genuine mutants. The assay is potentially adaptable to any cell population that can be intoxicated with diphtheria toxin and labeled with [3H]leucine, whether or not the cells can form colonies. It may be useful, therefore, for measuring mutation rates in slowly growing or nondividing cell populations such as breast, brain, and liver, as well as in cells that do divide but cannot be readily cloned, such as the colonic epithelium.

Binding and uptake of diphtheria toxin by toxin-resistant Chinese hamster ovary and mouse cells

We investigated two phenotypically distinct types of diphtheria toxin-resistant mutants of Chinese hamster cells and compared their resistance with that of naturally resistant mouse cells. All are resistant due to a defect in the process of internalization and delivery of toxin to its target in the cytosol, elongation factor 2. By cell hybridization studies, analysis of cross-resistance, and determination of specific binding sites for 125I-labeled diphtheria toxin, we showed that these cell strains fall into two distinct complementation groups. The Dipr group encompasses Chinese hamster strains that are resistant only to diphtheria toxin, as well as mouse LM cells. These strains possess a normal complement of high-affinity binding sites for diphtheria toxin, but these receptors are unable to deliver active toxin fragment A to the cytosol. Cells of the DPVr group have a broader spectrum of resistance, including Pseudomonas exotoxin A and several enveloped viruses as well as diphtheria toxin. In these studies, which investigate the resistance of these cells to diphtheria toxin, we demonstrate that they possess a reduced number of specific binding sites for this toxin and behave, phenotypically, like cells treated with the proton ionophore monensin. Their resistance is related to a defect in a mechanism required for release of active toxin from the endocytic vesicle.

Defective acidification of endosomes in Chinese hamster ovary cell mutants "cross-resistant" to toxins and viruses

Like many physiological ligands, several viruses and toxins enter mammalian cells through receptor-mediated endocytosis. Once internalized, the nucleic acids of several viruses and the toxic subunit of diphtheria toxin gain access to the cytosol of the host cell through an acidic intracellular compartment. In this report, we present evidence that one class of mutants of Chinese hamster ovary (CHO)-K1 cells, which is "cross-resistant" to Pseudomonas exotoxin A, diphtheria toxin, and several animal viruses, has a defect in acidification of the endosome. Cells were allowed to internalize fluorescein isothiocyanate-conjugated dextran before subcellular fractionation. Fluorescence measurements on subcellular fractions permitted measurement of the internal pH of the isolated endosomes and lysosomes. Our results show that (i) endosomes and lysosomes from CHO-K1 cells maintain an acidic pH, (ii) acidification of both endosomes and lysosomes is mediated by a Mg2+/ATP-dependent process, (iii) GTP can satisfy the ATP requirement for acidification of lysosomes but not of endosomes, and (iv) at least one class of mutants that is cross-resistant to toxins and animal viruses has a defect in the ATP-dependent acidification of their endosomes. These studies provide biochemical and genetic evidence that the mechanisms of acidification of endosomes and lysosomes are distinct and that a defect in acidification of endosomes is one biochemical basis for cross-resistance to toxins and viruses.

Strains of CHO-K1 cells resistant to Pseudomonas exotoxin A and cross-resistant to diphtheria toxin and viruses

We have investigated two phenotypically distinct types of mutants of CHO-K1 cells that are resistant to Pseudomonas exotoxin A due to a defect in the delivery of active toxin to the target site in the cell, elongation factor 2. Both types contain normal levels of toxin-sensitive elongation factor-2. Hybridization studies have shown that these cells fall into two distinct complementation groups. One group, designated DPVr, is resistant to Pseudomonas toxin, diphtheria toxin, and four enveloped RNA viruses. This group is also hypersensitive to ricin. The resistance of this group is apparently related to a defect in a mechanism for the acidification of endocytic vesicles. The other group, designated PVr, is resistant to Pseudomonas toxin and to three enveloped RNA viruses. The resistance of this group appears to be related to a defect in a cellular mechanism required for the maturation of Sindbis virus that is likewise required for the entry of active Pseudomonas toxin.

Binding and uptake of diphtheria toxin by toxin-resistant Chinese hamster ovary and mouse cells

We investigated two phenotypically distinct types of diphtheria toxin-resistant mutants of Chinese hamster cells and compared their resistance with that of naturally resistant mouse cells. All are resistant due to a defect in the process of internalization and delivery of toxin to its target in the cytosol, elongation factor 2. By cell hybridization studies, analysis of cross-resistance, and determination of specific binding sites for 125I-labeled diphtheria toxin, we showed that these cell strains fall into two distinct complementation groups. The Dipr group encompasses Chinese hamster strains that are resistant only to diphtheria toxin, as well as mouse LM cells. These strains possess a normal complement of high-affinity binding sites for diphtheria toxin, but these receptors are unable to deliver active toxin fragment A to the cytosol. Cells of the DPVr group have a broader spectrum of resistance, including Pseudomonas exotoxin A and several enveloped viruses as well as diphtheria toxin. In these studies, which investigate the resistance of these cells to diphtheria toxin, we demonstrate that they possess a reduced number of specific binding sites for this toxin and behave, phenotypically, like cells treated with the proton ionophore monensin. Their resistance is related to a defect in a mechanism required for release of active toxin from the endocytic vesicle.

Targeting potential of antibody conjugates

Although the use of conjugates of enzymes has been considered, their use has not been very actively pursued. Much more interest has focused on the possibilities offered by the use of toxins, their subunits or of ribosome inhibitors. Conjugates of holotoxin which were very active and specific in vitro have been prepared. High in vivo activity and some specificity together with reduced whole body toxicity has been described. When A chain subunits or ribosome inhibiting proteins are used, the results are more mixed. Some very active and specific conjugates are known but others have relatively low activity. The reasons for this may be associated with the particular antigen to which the antibody component is directed, the nature of the A chain or inhibitor and the type and physiological state of the target cell. Application to man seems likely in the first instance to involve the removal of undesirable cells from bone marrow transplants.

Ultrastructural localization of interferon receptors on the surfaces of cultured cells and erythrocytes

The bindings sites for interferon (IFN) on the limiting cell membranes of human and mouse fibroblasts and erythrocytes were revealed by an indirect immunoferritin technique. Mouse IFN-beta and human IFN-beta of high specific activity were used with the corresponding purified antibodies. Species-specific IFN binding was demonstrated by ferritin deposition on human erythrocytes and fibroblast membranes treated with human IFN and on mouse erythrocytes and fibroblast membranes treated with mouse IFN, but not on human erythrocytes or fibroblast membranes treated with mouse IFN. IFN binding sites on fibroblasts were located on regions of membranes between microvilli, whereas diphtheria toxin receptors were demonstrated mainly on microvilli. IFN binding altered the diphtheria toxin after IFN treatment. This reduced toxicity correlated with a decrease in the quantity of receptors for diphtheria toxin on the cell membrane. Thus, the species-specific binding of IFN appears to depend on membrane receptors in discrete regions of the limiting membrane which are present not only on functionally responsive fibroblasts but also on erythrocytes.

Increased toxicity of diphtheria toxin for human lymphoblastoid cells following covalent linkage to anti-(human lymphocyte) globulin or its F(ab')2 fragment

Anti-(human lymphocyte) globulin was reacted with a mixed anhydride derivative of chlorambucil to give a product which was in turn reacted with diphtheria toxin. The resulting conjugate was partially purified and was found to possess an ability similar to that of the native antibody to bind to the human lymphoblastoid cell lines, CLA4 and Daudi. Daudi cells, as had been observed previously with CLA4 cells, lacked the high sensitivity to diphtheria toxin normally characteristic of cells of human origin. Thus treatment with free toxin at a concentration of 1 microgram/ml was without effect upon their ability to incorporate [3H]leucine. By contrast, Daudi cells were highly sensitive to toxin conjugated to anti-(human lymphocyte) globulin or to its F(ab')2 fragment. Exposure for 24 h to a solution of conjugate containing toxin at a concentration of 0.5 ng/ml caused a reduction of 50% in the leucine uptake by Daudi cells. The toxicity of the conjugate could be blocked by diphtheria antitoxin or by pretreatment of the cells with non-conjugated antibody. Toxin linked to normal horse IgG or to its F(ab')2 fragment was without cytotoxic effect upon Daudi cells. Furthermore both the conjugate with anti-(human lymphocyte) globulin and that with normal IgG were approximately 100-fold less able than non-conjugated toxin to inhibit protein synthesis by a human fibroblast cell line to which the antibody showed no appreciable binding. Thus the conjugates are relatively ineffective against cells which lack an antigen to which the antibody moiety can bind. In contrast with the greatly increased toxicity of diphtheria toxin for human lymphoblastoid cells following its linkage to anti-(human lymphocyte) globulin, a conjugate of toxin linked to anti-(mouse lymphocyte) globulin was ineffective against murine spleen cells in vitro.

Biochemical and genetic characterization of three hamster cell mutants resistant to diphtheria toxin

We describe here three different hamster cell mutants which are resistant to diphtheria toxin and which provide models for investigating some of the functions required by the toxin inactivates elongation factor 2 (EF-2). Cell-free extracts from mutants Dtx(r)-3 was codominant. The evidence suggests that the codominant phenotype is the result of a mutation in a gene coding for EF-2. The recessive phenotype might arise by alteration of an enzyme which modifies the structure of EF-2 so that it becomes a substrate for reaction with the toxin. Another mutant, Dtx(r)-2, contained EF-2 that was sensitive to the toxin and this phenotype was recessive. Pseudomonas aeruginosa exotoxin is known to inactivate EF-2 as does diphtheria toxin and we tested the mutants for cross-resistance to pseudomonas exotoxin. Dtx(r)-1 and Dtx(r)-3 were cross-resistant while Dtx(r)-2 was not. It is known that diphtheria toxin does not penetrate to the cytoplasm of mouse cells and that these cell have a naturally occurring phenotype of diphtheria toxin resistance. We fused each of the mutants with mouse 3T3 cells and measured the resistance. We fused each of the mutants with mouse 3T3 cells and measured the resistance of the hybrid cells to diphtheria toxin. Intraspecies hybrids containing the genome of mutants Dtx(r)-1 and Dtx(r)-3 had some resistance while those formed with Dtx(r)-2 were as sensitive as hybrids derived from fusions between wild-type hamster cells and mouse 3T3 cells.

Isolation and characterization of mutants of human diploid fibroblasts resistant to diphtheria toxin

Stable mutants highly resistant to the protein synthesis inhibitor diphtheria toxin (dipr) have been selected in human diploid fibroblast cells at a frequency of 1-8 X 10(-6). Treatment of cells with mutagens, (e.g., ethylmethanesulfonate, nitrosoguanidine, and ICR-170), increased the frequencies of dipr mutants by 50- to 500-fold in different experiments, and the optimal expression time for dipr mutation was about 5 days. All mutants examined thus far have bred true, and no effects of cell density or cross feeding have been observed on the selection. Fluctuation analysis showed that the dipr mutation occurs in these fibroblasts at the rate of 5-6 X 10(-7) mutations per cell per generation. Protein synthesis in mutant extracts was resistant to diphtheria toxin, indicating that the dipr lesion in such mutants lies in the protein synthesis machinery. The characteristics of the dipr marker should make this system particularly useful for studies of quantitative mutagenesis in human diploid cells.

Studies of the diphtheria toxin receptor on Chinese hamster cells

Concanavalin A, wheat germ agglutinin and the ovalbumin glycopeptide are all inhibitors of the cytotoxic effect of diphtheria toxin on Chinese hamster cells. Ovalbumin glycopeptide loses its inhibitory property after treatment with beta-N-acetylglucosaminidase. This demonstrates the importance of the glycopeptide structure for the mechanism of inhibition. The glycopeptide may be a toxin cell-surface receptor analogue. Diphtheria toxin-resistant mutants were isolated in order to search for cells that might have an altered toxin receptor. One mutant was 10- to 15-fold more resistant to diphtheria toxin than wild-type cells when protein synthesis was measured as a function of toxin concentration. However, when protein synthesis was measured as a function of time at a high toxin concentration, the time before onset of inhibition was identical in the mutant and wild-type cells. We present evidence indicating that the resistance of this mutant can be accounted for by a decreased affinity of toxin for a cell-surface receptor.

Selection and characterization of cells resistant to diphtheria toxin and pseudomonas exotoxin A: presumptive translational mutants

Two classes of diphtheria toxin-resistant variants were selected from Chinese hamster ovary (CHO-K1) cells: permeability variants, in which uptake of toxin was impaired, and a new class of cytoplasmic variants, which were cross-resistant to Pseudomonas exotoxin. EF-2 prepared from the cytoplasmic variants was resistant to ADP-ribosylation by either toxin. The evidence presented suggests that these are translational variants possessing a mutationally altered EF-2 gene product. These studies also confirmed that Pseudomonas toxin ADP-ribosylates EF-2 in toxin-sensitive intact cells, as well as in cell-free systems.

Interaction of diphtheria toxin and its active subunit, fragment A, with toxin-sensitive and toxin-resistant cells

Diphtheria toxin and purified fragment A, the active subunit of the toxin, were tested on toxin-sensitive and permeability class toxin-resistant cultured mammalian cells. Protein synthesis was inhibited to the same degree in sensitive or resistant cells by active concentrations of purified fragment A. In contrast, resistant cells required a concentration of whole toxin 5 to 6 logs greater than that required by sensitive cells to achieve the same degree of inhibition. On a molar basis, the toxicity of fragment A was equivalent to that of whole toxin on resistant cells. These results are evidence for the existence of two independent mechanisms for the entry of toxin or its active moiety into cells. One mechanism is a highly efficient, toxin-specific entry mechanism, involving surface receptor and fragment B-mediated association, and is active in sensitive cells. The other is a less efficient, nonspecific mechanism, probably related to endocytosis, which is operative in sensitive and resistant cells but is inapparent in sensitive cells when they are exposed to whole toxin because of the action of the specific mechanism.

Comparison of diphtheria intoxication in human and nonhuman cell lines and their resistant variants

Differences in sensitivity to diphtheria toxin of several toxin-sensitive and toxin-resistant human and non-human cell lines were compared. A method is described whereby it is possible to compare the sensitivity of one cell line with another and obtain meaningful quantitative results. Based on the concentration of toxin required to produce 50% inhibition of protein synthesis after 24 h of exposure the ID50 (24) value toxin-resistant cells were found to be 105 to 106 times more resistant to toxin than toxin-sensitive cells. There was little variation in the ID50 (24) values for cells in each of the two groups. The toxin-resistant cells used in this study, naturally resistant as well as selected variants, possess elongation factor 2 which is susceptible to inactivation by toxin. It is suggested that they are capable of activation of toxin but either cannot bind toxin or are unable to transport toxin across the plasma membrane. Protein synthesis is inhibited when these resistant cells are exposed to high concentrations of toxin. Under these conditions it is likely that enough toxin is able to bypass the block in toxin-specific entry and reach the cytosol by a second, less efficient, nonspecific mechanism to catalyze the inactivation of elongation factor 2 and inhibit protein synthesis.

Chemical modulation of diphtheria toxin action on cultured mammalian cells

Ammonium chloride (4 times 10-3 M) rendered HEp-2 monolayers completely insensitive to the action of diphtheria toxin, as measured by de novo protein synthesis. Total protection was observed even with large amounts of toxin (400 minimum lethal doses/ml). Ammonium chloride did not reduce toxicity by direct action on the protein, nor did it prevent the adsorption of toxin to the cell membrane. Although the ammonium salt did not block the initial interaction between cell and toxin, it did maintain the toxin at a site amenable to neutralization with antitoxin. Surface-adsorbed toxin was inactivated by cellular enzymes or alternatively was desorbed from the membrane during a 12-h incubation in the presence of ammonium chloride. In addition, ammonium chloride provided protection to both toxin-sensitive guinea pig peritoneal macrophages and a partially toxin-resistant strain of HEp-2 cells. Sodium arsenite was effective in protecting cell monolayers from the action of diphtheria toxin; unlike ammonium chloride, its action was not dependent upon continued incubation with cells during exposure to toxin. Inhibitors of energy metabolism abolished toxin action either totally (sodium fluoride) or partially (dinitrophenol and sodium cyanide). Inhibitors of cellular proteases, on the other hand, did not modify toxin activity. The ability of several modifiers of membrane function to alter expression of toxicity for HEp-2 cells was also examined. One compound known to enhance endocytic activity, Tuftsin, had no effect, whereas poly-L-ornithine provided partial protection. Of the two compounds known to alter membrane fluidity, cytochalasin B provided partial protection for HEp-2 cell cultures, whereas colchicine had no effect. Agents that bind to sulfhydryl groups on the cell surface had no apparent effect on toxicity, suggesting that the initial toxin-cell interaction does not involve sulfhydryl groups. Those compounds that provide virtually full protection against the action of diphtheria toxic on cell monolayers (i.e., ammonium chloride, sodium fluoride, and sodium arsenite) had no inhibitory effect on the in vitro enzyme activity associated with fragment A of the toxin.

Diphtheria toxin: mode of action and structure

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Response of cultured mammalian cells to diphtheria toxin. V. Concurrent resistance to ribonucleic acid viruses in diphtheria toxin-resistant KB cell strains

It was determined that KB-R cell strains, isolated from the KB cell line and resistant to diphtheria toxin, also resist infection by poliovirus, Mengo virus, vesicular stomatitis virus, and Newcastle disease virus. This resistance manifests itself by reduction in yields of progeny virus (a reduction of more than 2 logs in some cases), reduced production of viral-specific ribonucleic acid (RNA), and delayed cytopathic effect. In three KB-R strains tested, resistance was related to a step which falls between adsorption of virus and uncoating or release of viral messenger RNA. In two of these three strains, a second resistance mechanism was also active, causing a reduced production of viral-specific RNA. A relationship between the resistance to diphtheria toxin and the resistance to viral infection of the KB-R strains is considered. It has been postulated that the native diphtheria toxin molecule must be "activated" at the surface of a susceptible cell by a proteolytic process before it can enter the cell and inhibit protein synthesis. It is also known that the eclipse of some viruses occurs at or near the cell membrane and involves proteolytic activity. Resistance to viruses and toxin in the KB-R strains may result from the loss or modification of related labilizing or activating principles associated with the surface receptors for these agents.