Modulation of the intracellular stability and toxicity of diphtheria toxin through degradation by the N-end rule pathway

Cellular recovery from exposure to sub-optimal concentrations of AB toxins that inhibit protein synthesis

Ricin, Shiga toxin, exotoxin A, and diphtheria toxin are AB-type protein toxins that act within the host cytosol and kill the host cell through pathways involving the inhibition of protein synthesis. It is thought that a single molecule of cytosolic toxin is sufficient to kill the host cell. Intoxication is therefore viewed as an irreversible process. Using flow cytometry and a fluorescent reporter system to monitor protein synthesis, we show a single molecule of cytosolic toxin is not sufficient for complete inhibition of protein synthesis or cell death. Furthermore, cells can recover from intoxication: cells with a partial loss of protein synthesis will, upon removal of the toxin, increase the level of protein production and survive the toxin challenge. Thus, in contrast to the prevailing model, ongoing toxin delivery to the cytosol appears to be required for the death of cells exposed to sub-optimal toxin concentrations.

Towards Engineering Novel PE-Based Immunotoxins by Targeting Them to the Nucleus

Exotoxin A (PE) from Pseudomonas aeruginosa is a bacterial ADP-ribosyltransferase, which can permanently inhibit translation in the attacked cells. Consequently, this toxin is frequently used in immunotoxins for targeted cancer therapies. In this study, we propose a novel modification to PE by incorporating the NLS sequence at its C-terminus, to make it a selective agent against fast-proliferating cancer cells, as a nucleus-accumulated toxin should be separated from its natural substrate (eEF2) in slowly dividing cells. Here, we report the cytotoxic activity and selected biochemical properties of newly designed PE mutein using two cellular models: A549 and HepG2. We also present a newly developed protocol for efficient purification of recombinant PE and its muteins with very high purity and activity. We found that furin cleavage is not critical for the activity of PE in the analyzed cell lines. Surprisingly, we observed increased toxicity of the toxin accumulated in the nucleus. This might be explained by unexpected nuclease activity of PE and its potential ability to cleave chromosomal DNA, which seems to be a putative alternative intoxication mechanism. Further experimental investigations should address this newly detected activity to identify catalytic residues and elucidate the molecular mechanism responsible for this action.

A d-Amino Acid at the N-Terminus of a Protein Abrogates Its Degradation by the N-End Rule Pathway

Eukaryotes have evolved the ubiquitin (Ub)/proteasome system to degrade polypeptides. The Ub/proteasome system is one way that cells regulate cytosolic protein and amino acids levels through the recognition and ubiquitination of a protein's N-terminus via E1, E2, and E3 enzymes. The process by which the N-terminus stimulates intracellular protein degradation is referred to as the N-end rule. Characterization of the N-end rule has been limited to only the natural l-amino acids. Using a cytosolic delivery platform derived from anthrax lethal toxin, we probed the stability of mixed chirality proteins, containing one d-amino acid on the N-terminus of otherwise all l-proteins. In all cases, we observed that one N-terminal d-amino acid stabilized the cargo protein to proteasomal degradation with respect to the N-end rule. We found that since the mixed chirality proteins were not polyubiquitinated, they evaded N-end-mediated proteasomal degradation. Evidently, a subtle change on the N-terminus of a natural protein can enhance its intracellular lifetime.

Channel-forming bacterial toxins in biosensing and macromolecule delivery

To intoxicate cells, pore-forming bacterial toxins are evolved to allow for the transmembrane traffic of different substrates, ranging from small inorganic ions to cell-specific polypeptides. Recent developments in single-channel electrical recordings, X-ray crystallography, protein engineering, and computational methods have generated a large body of knowledge about the basic principles of channel-mediated molecular transport. These discoveries provide a robust framework for expansion of the described principles and methods toward use of biological nanopores in the growing field of nanobiotechnology. This article, written for a special volume on "Intracellular Traffic and Transport of Bacterial Protein Toxins", reviews the current state of applications of pore-forming bacterial toxins in small- and macromolecule-sensing, targeted cancer therapy, and drug delivery. We discuss the electrophysiological studies that explore molecular details of channel-facilitated protein and polymer transport across cellular membranes using both natural and foreign substrates. The review focuses on the structurally and functionally different bacterial toxins: gramicidin A of Bacillus brevis, α-hemolysin of Staphylococcus aureus, and binary toxin of Bacillus anthracis, which have found their "second life" in a variety of developing medical and technological applications.

Anthrax toxin-mediated delivery of the Pseudomonas exotoxin A enzymatic domain to the cytosol of tumor cells via cleavable ubiquitin fusions

Anthrax toxin proteins from Bacillus anthracis constitute a highly efficient system for delivering cytotoxic enzymes to the cytosol of tumor cells. However, exogenous proteins delivered to the cytosol of cells are subject to ubiquitination on lysines and proteasomal degradation, which limit their potency. We created fusion proteins containing modified ubiquitins with their C-terminal regions fused to the Pseudomonas exotoxin A catalytic domain (PEIII) in order to achieve delivery and release of PEIII to the cytosol. Fusion proteins in which all seven lysines of wild-type ubiquitin were retained while the site cleaved by cytosolic deubiquitinating enzymes (DUBs) was removed were nontoxic, apparently due to rapid ubiquitination and proteasomal degradation. Fusion proteins in which all lysines of wild-type ubiquitin were substituted by arginine had high potency, exceeding that of a simple fusion lacking ubiquitin. This variant was less toxic to nontumor tissues in mice than the fusion protein lacking ubiquitin and was very efficient for tumor treatment in mice. The potency of these proteins was highly dependent on the number of lysines retained in the ubiquitin domain and on retention of the C-terminal ubiquitin sequence cleaved by DUBs. It appears that rapid cytosolic release of a cytotoxic enzyme (e.g., PEIII) that is itself resistant to ubiquitination is an effective strategy for enhancing the potency of tumor-targeting toxins.

IMPORTANCE

Bacterial toxins typically have highly efficient mechanisms for cellular delivery of their enzymatic components. Cytosolic delivery of therapeutic enzymes and drugs is an important topic in molecular medicine. We describe anthrax toxin fusion proteins containing ubiquitin as a cytosolic cleavable linker that improves the delivery of an enzyme to mammalian cells. The ubiquitin linker allowed modulation of potency in cells and in mice. This effective strategy for enhancing the intracellular potency of an enzyme may be useful for the cytosolic delivery and release of internalized drugs.

Increasing stability and toxicity of Pseudomonas exotoxin by attaching an antiproteasic Peptide

Trypsin-like activities are present within the endocytic pathway and allow cells to inactivate a fraction of incoming toxins, such as Pseudomonas exotoxin (PE), that require endocytic uptake before reaching the cytosol to inactivate protein synthesis. PE is a favorite toxin for building immunotoxins. The latter are promising molecules to fight cancer or transplant rejection, and producing more active toxins is a key challenge. More broadly, increasing protein stability is a potentially useful approach to improve the efficiency of therapeutic proteins. We report here that fusing an antiproteasic peptide (bovine pancreatic trypsin inhibitor, BPTI) to PE increases its toxicity to human cancer cell lines by 20-40-fold. Confocal microscopic examination of toxin endocytosis, digestion, and immunoprecipitation experiments showed that the fused antiproteasic peptide specifically protects PE from trypsin-like activities. Hence, the attached BPTI acts as a bodyguard for the toxin within the endocytic pathway. Moreover, it increased the PE elimination half-time in mice by 70%, indicating that the fused BPTI stabilizes the toxin in vivo. This BPTI-fusion approach may be useful for protecting other circulating or internalized proteins of therapeutic interest from premature degradation.

The N-end rule pathway and regulation by proteolysis

The N-end rule relates the regulation of the in vivo half-life of a protein to the identity of its N-terminal residue. Degradation signals (degrons) that are targeted by the N-end rule pathway include a set called N-degrons. The main determinant of an N-degron is a destabilizing N-terminal residue of a protein. In eukaryotes, the N-end rule pathway is a part of the ubiquitin system and consists of two branches, the Ac/N-end rule and the Arg/N-end rule pathways. The Ac/N-end rule pathway targets proteins containing N(α) -terminally acetylated (Nt-acetylated) residues. The Arg/N-end rule pathway recognizes unacetylated N-terminal residues and involves N-terminal arginylation. Together, these branches target for degradation a majority of cellular proteins. For example, more than 80% of human proteins are cotranslationally Nt-acetylated. Thus most proteins harbor a specific degradation signal, termed (Ac)N-degron, from the moment of their birth. Specific N-end rule pathways are also present in prokaryotes and in mitochondria. Enzymes that produce N-degrons include methionine-aminopeptidases, caspases, calpains, Nt-acetylases, Nt-amidases, arginyl-transferases and leucyl-transferases. Regulated degradation of specific proteins by the N-end rule pathway mediates a legion of physiological functions, including the sensing of heme, oxygen, and nitric oxide; selective elimination of misfolded proteins; the regulation of DNA repair, segregation and condensation; the signaling by G proteins; the regulation of peptide import, fat metabolism, viral and bacterial infections, apoptosis, meiosis, spermatogenesis, neurogenesis, and cardiovascular development; and the functioning of adult organs, including the pancreas and the brain. Discovered 25 years ago, this pathway continues to be a fount of biological insights.

Toxin-based therapeutic approaches

Protein toxins confer a defense against predation/grazing or a superior pathogenic competence upon the producing organism. Such toxins have been perfected through evolution in poisonous animals/plants and pathogenic bacteria. Over the past five decades, a lot of effort has been invested in studying their mechanism of action, the way they contribute to pathogenicity and in the development of antidotes that neutralize their action. In parallel, many research groups turned to explore the pharmaceutical potential of such toxins when they are used to efficiently impair essential cellular processes and/or damage the integrity of their target cells. The following review summarizes major advances in the field of toxin based therapeutics and offers a comprehensive description of the mode of action of each applied toxin.

Sensitivity of cancer cells to truncated diphtheria toxin

BACKGROUND

Diphtheria toxin (DT) has been utilized as a prospective anti-cancer agent for the targeted delivery of cytotoxic therapy to otherwise untreatable neoplasia. DT is an extremely potent toxin for which the entry of a single molecule into a cell can be lethal. DT has been targeted to cancer cells by deleting the cell receptor-binding domain and combining the remaining catalytic portion with targeting proteins that selectively bind to the surface of cancer cells. It has been assumed that "receptorless" DT cannot bind to and kill cells. In the present study, we report that "receptorless" recombinant DT385 is in fact cytotoxic to a variety of cancer cell lines.

METHODS

In vitro cytotoxicity of DT385 was measured by cell proliferation, cell staining and apoptosis assays. For in vivo studies, the chick chorioallantoic membrane (CAM) system was used to evaluate the effect of DT385 on angiogenesis. The CAM and mouse model system was used to evaluate the effect of DT385 on HEp3 and Lewis lung carcinoma (LLC) tumor growth, respectively.

RESULTS

Of 18 human cancer cell lines tested, 15 were affected by DT385 with IC(50) ranging from 0.12-2.8 microM. Furthermore, high concentrations of DT385 failed to affect growth arrested cells. The cellular toxicity of DT385 was due to the inhibition of protein synthesis and induction of apoptosis. In vivo, DT385 diminished angiogenesis and decreased tumor growth in the CAM system, and inhibited the subcutaneous growth of LLC tumors in mice.

CONCLUSION

DT385 possesses anti-angiogenic and anti-tumor activity and may have potential as a therapeutic agent.

Processing of Pseudomonas aeruginosa exotoxin A is dispensable for cell intoxication

Exotoxin A is a major virulence factor of Pseudomonas aeruginosa. This toxin binds to a specific receptor on animal cells, allowing endocytosis of the toxin. Once in endosomes, the exotoxin can be processed by furin to generate a C-terminal toxin fragment that lacks the receptor binding domain and is retrogradely transported to the endoplasmic reticulum for retrotranslocation to the cytosol through the Sec61 channel. The toxin then blocks protein synthesis by ADP ribosylation of elongation factor 2, thereby triggering cell death. A shorter intracellular route has also been described for this toxin. It involves direct translocation of the entire toxin from endosomes to the cytosol and therefore does not rely on furin-mediated cleavage. To examine the implications of endosomal translocation in the intoxication process, we investigated whether the toxin required furin-mediated processing in order to kill cells. We used three different approaches. We first fused to the N terminus of the toxin proteins with different unfolding abilities so that they inhibited or did not inhibit endosomal translocation of the chimera. We then assayed the amount of toxin fragments delivered to the cytosol during cell intoxication. Finally we used furin inhibitors and examined the fate and intracellular localization of the toxin and its receptor. The results showed that exotoxin cytotoxicity results largely from endosomal translocation of the entire toxin. We found that the C-terminal fragment was unstable in the cytosol.

Role of N-terminal amino acids in the potency of anthrax lethal factor

Anthrax lethal factor (LF) is a Zn(+2)-dependent metalloprotease that cleaves several MAPK kinases and is responsible for the lethality of anthrax lethal toxin (LT). We observed that a recombinant LF (LF-HMA) which differs from wild type LF (LF-A) by the addition of two residues (His-Met) to the native Ala (A) terminus as a result of cloning manipulations has 3-fold lower potency toward cultured cells and experimental animals. We hypothesized that the "N-end rule", which relates the half-life of proteins in cells to the identity of their N-terminal residue, might be operative in the case of LF, so that the N-terminal residue of LF would determine the cytosolic stability and thereby the potency of LF. Mutational studies that replaced the native N-terminal residue of LF with known N-end rule stabilizing or destabilizing residues confirmed that the N-terminal residue plays a significant role in determining the potency of LT for cultured cells and experimental animals. The fact that a commercially-available LF preparation (LF-HMA) that is widely used in basic research studies and for evaluation of vaccines and therapeutics is 3-fold less potent than native LF (LF-A) should be considered when comparing published studies and in the design of future experiments.

An improved mouse line for Cre-induced cell ablation due to diphtheria toxin A, expressed from the Rosa26 locus

The means to specifically ablate cells inside of a living organism have recently been improved and facilitated by stable mouse lines, carrying conditional expression constructs for diphtheria toxin (DT) or diphtheria toxin receptor, that could be activated upon Cre-mediated recombination or the application of diphtheria toxin, respectively. We have lately described the R26:lacZ/DT-A line (Brockschnieder et al., 2004, Mol Cell Biol 24:7636-7642) in which a loxP-conditional DTA allele was introduced into the ubiquitously expressed Rosa26 locus. This strain allowed the ablation of a wide spectrum of cell types by crossing it to tissue specific Cre lines. Unexpectedly, homozygous (but not heterozygous) animals of the R26:lacZ/DT-A line developed some degenerative abnormalities in a variety of tissues. The defects were most probably caused by leaky expression of small amounts of toxin from the unrecombined lacZ(flox)DT-A cassette. Here we show that insertion of an additional transcriptional regulatory sequence (bovine growth hormone polyadenylation signal, bpA) following the lacZ open reading frame prevented the formation of any defects in homozygous mice. The modification did not affect the functionality of the lacZ(flox)DTA allele, as exemplified by the complete ablation of oligodendrocytes upon Cre-mediated recombination. The novel R26:lacZbpA(flox)DTA line is expected to greatly facilitate the reliable generation of cell type ablated mice.

Construction of tumor-specific toxins using ubiquitin fusion technique

The use of cytotoxic agents to eliminate cancer cells is limited because of their nonselective toxicity and unwanted side effects. One of the strategies to overcome these limitations is to use latent prodrugs that become toxic in situ after being enzymatically activated in target cells. In this work we describe a method for producing tumor-specific toxins by using a ubiquitin fusion technique. The method is illustrated by the production of recombinant toxins by in-frame fusion of ubiquitin to saporin, a toxin from the plant Saponaria officinalis. Ubiquitin-fused toxins were rapidly degraded via the ubiquitin-proteasome system, significantly reducing their nonspecific toxicity. The insertion of the protease-cleavage sequence between ubiquitin and saporin led to the removal of ubiquitin by the protease and resulted in protease-dependent stabilization of the toxin. We engineered toxins that can be stabilized by specific proteases such as deubiquitinating enzymes and prostate-specific antigen (PSA). Both constructs were activated in vitro and in cultured cells by the appropriate enzyme. Processing by the protease resulted in a greater than 10-fold increase in the toxicity of these constructs. Importantly, the PSA-cleavable toxin was able to kill specifically the PSA-producing prostate cancer cells. The ubiquitin fusion technique is thus a versatile and reliable method for obtaining selective cytotoxic agents and can easily be adapted for different kinds of toxins and activating proteases.

Ubiquitin fusion technique and related methods

The ubiquitin fusion technique, developed in 1986, is still the method of choice for producing a desired N-terminal residue in a protein of interest in vivo. This technique is also used as a tool for protein expression. Over the past two decades, several otherwise unrelated methods were invented that have in common the use of ubiquitin fusions as a component of design. I describe the original ubiquitin fusion technique, its current applications, and other methods that use the properties of ubiquitin fusions.

Glycosylation of onconase increases its conformational stability and toxicity for cancer cells

Onconase (ONC) is currently in Phase III clinical trials as a cancer chemotherapeutic agent. Despite the finding that ONC contains an N-linked glycosylation site (-N69-V70-T71-), only the non-glycosylated form of the protein has been identified to date. We employed the Pichia pastoris expression system to produce recombinant glycosylated ONC (gONC) protein. Approximately 10 mg of ONC protein was secreted per liter of culture media, of which about 80% was glycosylated at N69. CD spectroscopic analyses revealed that the secondary structure of gONC is identical to that of ONC. We found that gONC contains a high-mannose core structure. Importantly, glycosylation of ONC at N69 greatly increased its toxicity for K-562 cancer cells. Specifically, the IC50 value of gONC was 50-fold lower than that of ONC. Glycosylation increased both the Tm of ONC and its resistance to proteinase K, suggesting that the elevated cytotoxicity of gONC is related to higher conformational stability.

A class of mutant CHO cells resistant to cholera toxin rapidly degrades the catalytic polypeptide of cholera toxin and exhibits increased endoplasmic reticulum-associated degradation

After binding to the eukaryotic cell surface, cholera toxin undergoes retrograde transport to the endoplasmic reticulum. The catalytic A1 polypeptide of cholera toxin (CTA1) then crosses the endoplasmic reticulum membrane and enters the cytosol in a process that may involve the quality control mechanism known as endoplasmic reticulum-associated degradation. Other toxins such as Pseudomonas exotoxin A and ricin are also thought to exploit endoplasmic reticulum-associated degradation for entry into the cytosol. To test this model, we mutagenized Chinese hamster ovary cells and selected clones that survived a prolonged coincubation with Pseudomonas exotoxin A and ricin. These lethal endoplasmic reticulum-translocating toxins bind different surface receptors and target different cytosolic substrates, so resistance to both would likely result from disruption of a shared trafficking or translocation event. Here we characterize two Pseudomonas exotoxin A/ricin-resistant clones that exhibited increased endoplasmic reticulum-associated degradation. Both clones acquired the following unselected traits: (i) resistance to cholera toxin; (ii) increased degradation of an endoplasmic reticulum-localized CTA1 construct; (iii) increased degradation of an established endoplasmic reticulum-associated degradation substrate, the Z variant of alpha1-antitrypsin (alpha1AT-Z); and (iv) reduced secretion of both alpha1AT-Z and the transport-competent protein alpha1AT-M. Proteosome inhibition partially rescued the alpha1AT-M secretion deficiencies. However, the mutant clones did not exhibit increased proteosomal activity against cytosolic proteins, including a second CTA1 construct that was expressed in the cytosol rather than in the endoplasmic reticulum. These results suggested that accelerated endoplasmic reticulum-associated degradation in the mutant clones produced a cholera toxin/Pseudomonas exotoxin A/ricin-resistant phenotype by increasing the coupling efficiency between toxin translocation and toxin degradation.

Designing immunotoxins for cancer therapy

Immunotoxins are therapeutic agents with a high degree of specificity and unique mechanism of action. An immunotoxin is a chimeric protein consisting of a targeting moiety linked to a toxin. The targeting moiety selectively binds to a tumor cell and targets it for death via the attached toxin. Generally, immunotoxins are specifically potent against cancer cells in vitro and in animal models of human malignancies. However, immunotoxins can be limited clinically by immunogenicity, toxicity, and instability. In this review, we offer ways to overcome these limitations to create "ideal immunotoxins" for cancer therapy. These include producing single chain targeting/toxin fusion proteins of fully human origin that are extracellularly stable but once internalized, can be cleaved by intracellular proteases to free the toxin and facilitate its translocation to the cytosol.

Design of toxins that can be activated by cell-specific proteases and their potential use in targeted cell killing

Protein toxins designed to eliminate specific cell types, e.g. disease-associated cells, have mainly made by linking the active domain of the toxin to a protein that only binds to certain cells. A different approach for the construction of toxins capable of killing disease-associated cells is suggested here, based on the knowledge that many of these cells express specific proteases that are not expressed in normal tissue. The construction of toxins that become activated through cleavage by the protease (HIV-1 PR) expressed by the HIV-1 virus is described. These toxins contain a signal for degradation by the N-end rule pathway, which is cleaved off by HIV-1 PR, resulting in increased toxicity. Alternative strategies for the construction of toxins that can be activated by proteases are discussed.

Penetration of protein toxins into cells

AB toxins deliver their enzymatically active A domain to the cytosol. Some AB-toxins are able to penetrate cellular membranes from endosomes where the low pH triggers their translocation. One such toxin is diphtheria toxin and important features of its translocation mechanism have been unraveled during the last year. Other toxins depend on retrograde transport through the secretory pathway to the ER before translocation, and recent findings suggest that these toxins take advantage of the ER translocation machinery normally used for transport of cellular proteins. In addition, the intracellular targets of many of these toxins have been identified recently.

Conformational stability is a determinant of ribonuclease A cytotoxicity

Onconasetrade mark, a homolog of bovine pancreatic ribonuclease A (RNase A) with high conformational stability, is cytotoxic and has efficacy as a cancer chemotherapeutic agent. Unlike wild-type RNase A, the G88R variant is toxic to cancer cells. Here, variants in which disulfide bonds were removed from or added to G88R RNase A were used to probe the relationship between conformational stability and cytotoxicity in a methodical manner. The conformational stability of the C40A/G88R/C95A and C65A/C72A/G88R variants is less than that of G88R RNase A. In contrast, a new disulfide bond that links the N and C termini (residues 4 and 118) increases the conformational stability of G88R RNase A and C65A/C72A/G88R RNase A. These changes have little effect on the ribonucleolytic activity of the enzyme or on its ability to evade the cytosolic ribonuclease inhibitor protein. The changes do, however, have a substantial effect on toxicity toward human erythroleukemia cells. Specifically, conformational stability correlates directly with cytotoxicity as well as with resistance to proteolysis. These data indicate that conformational stability is a key determinant of RNase A cytotoxicity and suggest that cytotoxicity relies on avoiding proteolysis. This finding suggests a means to produce new cancer chemotherapeutic agents based on mammalian ribonucleases.

Short-lived green fluorescent proteins for quantifying ubiquitin/proteasome-dependent proteolysis in living cells

The ubiquitin/proteasome-dependent proteolytic pathway is an attractive target for therapeutics because of its critical involvement in cell cycle progression and antigen presentation. However, dissection of the pathway and development of modulators are hampered by the complexity of the system and the lack of easily detectable authentic substrates. We have developed a convenient reporter system by producing N-end rule and ubiquitin fusion degradation (UFD)-targeted green fluorescent proteins that allow quantification of ubiquitin/proteasome-dependent proteolysis in living cells. Accumulation of these reporters serves as an early predictor of G2/M arrest and apoptosis in cells treated with proteasome inhibitors. Comparison of reporter accumulation and cleavage of fluorogenic substrates demonstrates that the rate-limiting chymotrypsin-like activity of the proteasome can be substantially curtailed without significant effect on ubiquitin-dependent proteolysis. These reporters provide a new powerful tool for elucidation of the ubiquitin/proteasome pathway and for high throughput screening of compounds that selectively modify proteolysis in vivo.

Diphtheria toxin fused to human interleukin-3 is toxic to blasts from patients with myeloid leukemias

Leukemic blasts from patients with acute phase chronic myeloid leukemic and refractory acute myeloid leukemia are highly resistant to a number of cytotoxic drugs. To overcome multi-drug resistance, we engineered a diphtheria fusion protein by fusing human interleukin-3 (IL3) to a truncated form of diphtheria toxin (DT) with a (G4S)2 linker (L), expressed and purified the recombinant protein, and tested the cytotoxicity of the DTLIL3 molecule on human leukemias and normal progenitors. The DTLIL3 construct was more cytotoxic to interleukin-3 receptor (IL3R) bearing human myeloid leukemia cell lines than receptor-negative cell lines based on assays of cytotoxicity using thymidine incorporation, growth in semi-solid medium and induction of apoptosis. Exposure of mononuclear cells to 680 pM DTLIL3 for 48 h in culture reduced the number of cells capable of forming colonies in semi-solid medium (colony-forming units leukemia) > or =10-fold in 4/11 (36%) patients with myeloid acute phase chronic myeloid leukemia (CML) and 3/9 (33%) patients with acute myeloid leukemia (AML). Normal myeloid progenitors (colony-forming unit granulocyte-macrophage) from five different donors treated and assayed under identical conditions showed intermediate sensitivity with three- to five-fold reductions in colonies. The sensitivity to DTLIL3 of leukemic progenitors from a number of acute phase CML patients suggests that this agent could have therapeutic potential for some patients with this disease.

Requirement for prolonged action in the cytosol for optimal protein synthesis inhibition by diphtheria toxin

Diphtheria toxin A-fragment enters the cytosol of target cells, where it inhibits protein synthesis by catalyzing ADP-ribosylation of elongation factor 2 (EF-2). We have here analyzed toxin-induced protein synthesis inhibition in single cells by autoradiography and compared it with inhibition of protein synthesis in the whole cell culture. The data show that half-maximal protein synthesis inhibition in the whole cell population after a short incubation time is achieved by partially inhibiting protein synthesis in basically all the cells, while half-maximal protein synthesis inhibition after a long incubation time is due to a complete protein synthesis block in about half the cells in the population. We have also compared stable and unstable A-fragment mutants with respect to the kinetics of cell intoxication. While the toxicity of the stable mutants increased with time, the unstable mutants showed a similar toxicity at early and late time points. When studying the kinetics of cell intoxication by toxins with short cytosolic half-life, we could not detect any recovery of protein synthesis at late time points when all the mutant A-fragments should be degraded. This indicates that the ADP-ribosylation of EF-2 cannot be reversed by an endogenous activity in the cells. The data indicate that entry of toxin into a cell is not associated with an immediate block in protein synthesis, and that prolonged action of single A-fragment molecules in the cytosol is sufficient to obtain complete protein synthesis inhibition at low toxin concentrations.

Stability of the Listeria monocytogenes ActA protein in mammalian cells is regulated by the N-end rule pathway

Upon infection of mammalian cells, Listeria monocytogenes lyses the phagosome and enters the cytosol, where it secretes proteins necessary for its intracellular growth cycle. Consequently, bacterial proteins exposed to the cytosol are potential targets for degradation by host cytosolic proteases. One pathway for degradation of host cytosolic proteins, the N-end rule pathway, involves recognition of the N-terminal amino acid and is mediated by the proteasome. However, very few natural N-end rule substrates have been identified. We have examined the L. monocytogenes ActA protein as a potential target for this pathway. ActA is an essential determinant of L. monocytogenes pathogenesis that is required to induce actin-based motility and cell-to-cell spread. We show that the half-life of a secreted form of ActA can be altered in the mammalian cytosol by changing the N-terminal amino acid. Moreover, the introduction of a destabilizing N-terminus into the functional, surface-bound form of ActA results in a small-plaque phenotype in L2 cells, which is partially reversible by an inhibitor of the proteasome. These results indicate that the L. monocytogenes ActA protein is a natural N-end rule substrate, and that optimal function of ActA in mediating cell-to-cell spread is dependent upon its intracellular turnover rate.

Toxins that are activated by HIV type-1 protease through removal of a signal for degradation by the N-end-rule pathway

Diphtheria toxin enters the cytosol of mammalian cells where it inhibits cellular protein synthesis, leading to cell death. Recently we found that the addition of a signal for N-end-rule-mediated protein degradation to diphtheria toxin substantially reduced its intracellular stability and toxicity. These results prompted us to construct a toxin containing a degradation signal that is removable through the action of a viral protease. In principle, such a toxin would be preferentially stabilized, and thus activated, in cells expressing the viral protease in the cytosol, i.e. virus-infected cells, thereby providing a specific eradication of these cells. In the present work we describe the construction of toxins that contain a signal for N-end-rule-mediated degradation just upstream of a cleavage site for the protease from HIV type 1 (HIV-1 PR). We show that the toxins are cleaved by HIV-1 PR exclusively at the introduced sites, and thereby are converted from unstable to stable proteins. Furthermore, this cleavage substantially increased the ability of the toxins to inhibit cellular protein synthesis. However, the toxins were unable to selectively eradicate HIV-1-infected cells, apparently due to low cytosolic HIV-1 PR activity, since we could not detect cleavage of the toxins by HIV-1 PR in infected cells. Alternative strategies for the construction of toxins that can specifically be activated by viral proteases are discussed.

Characterization of membrane translocation by anthrax protective antigen

Solving the crystallographic structure of the ring-shaped heptamer formed by protective antigen (PA), the B moiety of anthrax toxin, has focused attention on understanding how this oligomer mediates membrane translocation of the toxin's A moieties. We have developed an assay for translocation in which radiolabeled ligands are bound to proteolytically activated PA (PA63) at the surface of CHO or L6 cells, and translocation across the plasma membrane is induced by lowering the pH. The cells are then treated with Pronase E to degrade residual surface-bound material, and protected ligands are quantified after fractionation by SDS-PAGE. Translocation was most efficient (35%-50%) with LFN, the N-terminal PA binding domain of the anthrax lethal factor (LF). Intact LF, edema factor (EF), or fusion proteins containing LFN fused to certain heterologous proteins [the diphtheria toxin A chain (DTA) or dihydrofolate reductase (DHFR)] were less efficiently translocated (15%-20%); and LFN fusions to several other proteins were not translocated at all. LFN with different N-terminal residues was found to be degraded according to the N-end rule by the proteasome, and translocation of LFN fused to a mutant form of DHFR with a low affinity for methotrexate (MTX) protected cells from the effects of MTX. Both results are consistent with a cytosolic location of protected proteins. Evidence that a protein must unfold to be translocated was obtained in experiments showing that (i) translocation of LFNDTA was blocked by introduction of an artificial disulfide into the DTA moiety, and (ii) translocation of LFNDHFR and LFNDTA was blocked by their ligands (MTX and adenine, respectively). These results demonstrate that the acid-induced translocation by anthrax toxin closely resembles that of diphtheria toxin, despite the fact that these two toxins are unrelated and form pores by different mechanisms.