Verotoxin induces rapid elimination of human renal tumor xenografts in SCID mice

AB Toxins as High-Affinity Ligands for Cell Targeting in Cancer Therapy

Conventional targeted therapies for the treatment of cancer have limitations, including the development of acquired resistance. However, novel alternatives have emerged in the form of targeted therapies based on AB toxins. These biotoxins are a diverse group of highly poisonous molecules that show a nanomolar affinity for their target cell receptors, making them an invaluable source of ligands for biomedical applications. Bacterial AB toxins, in particular, are modular proteins that can be genetically engineered to develop high-affinity therapeutic compounds. These toxins consist of two distinct domains: a catalytically active domain and an innocuous domain that acts as a ligand, directing the catalytic domain to the target cells. Interestingly, many tumor cells show receptors on the surface that are recognized by AB toxins, making these high-affinity proteins promising tools for developing new methods for targeting anticancer therapies. Here we describe the structure and mechanisms of action of Diphtheria (Dtx), Anthrax (Atx), Shiga (Stx), and Cholera (Ctx) toxins, and review the potential uses of AB toxins in cancer therapy. We also discuss the main advances in this field, some successful results, and, finally, the possible development of innovative and precise applications in oncology based on engineered recombinant AB toxins.

The choanoflagellate pore-forming lectin SaroL-1 punches holes in cancer cells by targeting the tumor-related glycosphingolipid Gb3

Choanoflagellates are primitive protozoa used as models for animal evolution. They express a large variety of multi-domain proteins contributing to adhesion and cell communication, thereby providing a rich repertoire of molecules for biotechnology. Adhesion often involves proteins adopting a β-trefoil fold with carbohydrate-binding properties therefore classified as lectins. Sequence database screening with a dedicated method resulted in TrefLec, a database of 44714 β-trefoil candidate lectins across 4497 species. TrefLec was searched for original domain combinations, which led to single out SaroL-1 in the choanoflagellate Salpingoeca rosetta, that contains both β-trefoil and aerolysin-like pore-forming domains. Recombinant SaroL-1 is shown to bind galactose and derivatives, with a stronger affinity for cancer-related α-galactosylated epitopes such as the glycosphingolipid Gb3, when embedded in giant unilamellar vesicles or cell membranes. Crystal structures of complexes with Gb3 trisaccharide and GalNAc provided the basis for building a model of the oligomeric pore. Finally, recognition of the αGal epitope on glycolipids required for hemolysis of rabbit erythrocytes suggests that toxicity on cancer cells is achieved through carbohydrate-dependent pore-formation.

A curated lectin database, structural characterization, and in vitro assays show that choanoflagellate lectin SaroL-1 binds to cancer-related α-galactosylated epitopes and can be toxic to cancer cells through a carbohydrate-dependent pore-formation mechanism.

Novel lectin-based chimeric antigen receptors target Gb3-positive tumour cells

The link between cancer and aberrant glycosylation has recently become evident. Glycans and their altered forms, known as tumour-associated carbohydrate antigens (TACAs), are diverse, complex and difficult to target therapeutically. Lectins are naturally occurring glycan-binding proteins  that offer a unique opportunity to recognise TACAs. T cells expressing chimeric antigen receptors (CARs) have proven to be a successful immunotherapy against leukaemias, but so far have shown limited success in solid tumours. We developed a panel of lectin-CARs that recognise the glycosphingolipid globotriaosylceramide (Gb3), which is overexpressed in various cancers, such as Burkitt's lymphoma, colorectal, breast and pancreatic. We have selected the following lectins: Shiga toxin's B-subunit from Shigella dysenteriae, LecA from Pseudomonas aeruginosa, and the engineered lectin Mitsuba from Mytilus galloprovincialis as antigen-binding domains and fused them to a well-known second-generation CAR. The Gb3-binding lectin-CARs have demonstrated target-specific cytotoxicity against Burkitt's lymphoma-derived cell lines as well as solid tumour cells from colorectal and triple-negative breast cancer. Our findings reveal the big potential of lectin-based CARs as therapeutical applications to target Gb3 and other TACAs expressed in haematological malignancies and solid tumours.

Role of Globotriaosylceramide in Physiology and Pathology

At first glance, the biological function of globoside (Gb) clusters appears to be that of glycosphingolipid (GSL) receptors for bacterial toxins that mediate host-pathogen interaction. Indeed, certain bacterial toxin families have been evolutionarily arranged so that they can enter eukaryotic cells through GSL receptors. A closer look reveals this molecular arrangement allocated on a variety of eukaryotic cell membranes, with its role revolving around physiological regulation and pathological processes. What makes Gb such a ubiquitous functional arrangement? Perhaps its peculiarity is underpinned by the molecular structure itself, the nature of Gb-bound ligands, or the intracellular trafficking unleashed by those ligands. Moreover, Gb biological conspicuousness may not lie on intrinsic properties or on its enzymatic synthesis/degradation pathways. The present review traverses these biological aspects, focusing mainly on globotriaosylceramide (Gb3), a GSL molecule present in cell membranes of distinct cell types, and proposes a wrap-up discussion with a phylogenetic view and the physiological and pathological functional alternatives.

Shiga Toxins as Antitumor Tools

Shiga toxins (Stxs), also known as Shiga-like toxins (SLT) or verotoxins (VT), constitute a family of structurally and functionally related cytotoxic proteins produced by the enteric pathogens Shigella dysenteriae type 1 and Stx-producing Escherichia coli (STEC). Infection with these bacteria causes bloody diarrhea and other pathological manifestations that can lead to HUS (hemolytic and uremic syndrome). At the cellular level, Stxs bind to the cellular receptor Gb3 and inhibit protein synthesis by removing an adenine from the 28S rRNA. This triggers multiple cellular signaling pathways, including the ribotoxic stress response (RSR), unfolded protein response (UPR), autophagy and apoptosis. Stxs cause several pathologies of major public health concern, but their specific targeting of host cells and efficient delivery to the cytosol could potentially be exploited for biomedical purposes. Moreover, high levels of expression have been reported for the Stxs receptor, Gb3/CD77, in Burkitt's lymphoma (BL) cells and on various types of solid tumors. These properties have led to many attempts to develop Stxs as tools for biomedical applications, such as cancer treatment or imaging, and several engineered Stxs are currently being tested. We provide here an overview of these studies.

Primary Human Renal Proximal Tubular Epithelial Cells (pHRPTEpiCs): Shiga Toxin (Stx) Glycosphingolipid Receptors, Stx Susceptibility, and Interaction with Membrane Microdomains

Tubular epithelial cells of the human kidney are considered as targets of Shiga toxins (Stxs) in the Stx-mediated pathogenesis of hemolytic–uremic syndrome (HUS) caused by Stx-releasing enterohemorrhagic Escherichia coli (EHEC). Analysis of Stx-binding glycosphingolipids (GSLs) of primary human renal proximal tubular epithelial cells (pHRPTEpiCs) yielded globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer) with Cer (d18:1, C16:0), Cer (d18:1, C22:0), and Cer (d18:1, C24:1/C24:0) as the dominant lipoforms. Investigation of detergent-resistant membranes (DRMs) and nonDRMs, serving as equivalents for the liquid-ordered and liquid-disordered membrane phase, respectively, revealed the prevalence of Gb3Cer and Gb4Cer together with cholesterol and sphingomyelin in DRMs, suggesting lipid raft association. Stx1a and Stx2a exerted strong cellular damage with half-maximal cytotoxic doses (CD₅₀) of 1.31 × 10² pg/mL and 1.66 × 10³ pg/mL, respectively, indicating one order of magnitude higher cellular cytotoxicity of Stx1a. Surface acoustic wave (SAW) real-time interaction analysis using biosensor surfaces coated with DRM or nonDRM fractions gave stronger binding capability of Stx1a versus Stx2a that correlated with the lower cytotoxicity of Stx2a. Our study underlines the substantial role of proximal tubular epithelial cells of the human kidney being associated with the development of Stx-mediated HUS at least for Stx1a, while the impact of Stx2a remains somewhat ambiguous.

Therapeutic Uses of Bacterial Subunit Toxins

The B subunit pentamer verotoxin (VT aka Shiga toxin-Stx) binding to its cellular glycosphingolipid (GSL) receptor, globotriaosyl ceramide (Gb₃) mediates internalization and the subsequent receptor mediated retrograde intracellular traffic of the AB5 subunit holotoxin to the endoplasmic reticulum. Subunit separation and cytosolic A subunit transit via the ER retrotranslocon as a misfolded protein mimic, then inhibits protein synthesis to kill cells, which can cause hemolytic uremic syndrome clinically. This represents one of the most studied systems of prokaryotic hijacking of eukaryotic biology. Similarly, the interaction of cholera AB5 toxin with its GSL receptor, GM1 ganglioside, is the key component of the gastrointestinal pathogenesis of cholera and follows the same retrograde transport pathway for A subunit cytosol access. Although both VT and CT are the cause of major pathology worldwide, the toxin–receptor interaction is itself being manipulated to generate new approaches to control, rather than cause, disease. This arena comprises two areas: anti neoplasia, and protein misfolding diseases. CT/CTB subunit immunomodulatory function and anti-cancer toxin immunoconjugates will not be considered here. In the verotoxin case, it is clear that Gb₃ (and VT targeting) is upregulated in many human cancers and that there is a relationship between GSL expression and cancer drug resistance. While both verotoxin and cholera toxin similarly hijack the intracellular ERAD quality control system of nascent protein folding, the more widespread cell expression of GM1 makes cholera the toxin of choice as the means to more widely utilise ERAD targeting to ameliorate genetic diseases of protein misfolding. Gb₃ is primarily expressed in human renal tissue. Glomerular endothelial cells are the primary VT target but Gb₃ is expressed in other endothelial beds, notably brain endothelial cells which can mediate the encephalopathy primarily associated with VT2-producing E. coli infection. The Gb₃ levels can be regulated by cytokines released during EHEC infection, which complicate pathogenesis. Significantly Gb₃ is upregulated in the neovasculature of many tumours, irrespective of tumour Gb₃ status. Gb₃ is markedly increased in pancreatic, ovarian, breast, testicular, renal, astrocytic, gastric, colorectal, cervical, sarcoma and meningeal cancer relative to the normal tissue. VT has been shown to be effective in mouse xenograft models of renal, astrocytoma, ovarian, colorectal, meningioma, and breast cancer. These studies are herein reviewed. Both CT and VT (and several other bacterial toxins) access the cell cytosol via cell surface ->ER transport. Once in the ER they interface with the protein folding homeostatic quality control pathway of the cell -ERAD, (ER associated degradation), which ensures that only correctly folded nascent proteins are allowed to progress to their cellular destinations. Misfolded proteins are translocated through the ER membrane and degraded by cytosolic proteosome. VT and CT A subunits have a C terminal misfolded protein mimic sequence to hijack this transporter to enter the cytosol. This interface between exogenous toxin and genetically encoded endogenous mutant misfolded proteins, provides a new therapeutic basis for the treatment of such genetic diseases, e.g., Cystic fibrosis, Gaucher disease, Krabbe disease, Fabry disease, Tay-Sachs disease and many more. Studies showing the efficacy of this approach in animal models of such diseases are presented.

Potent in vitro antitumor activity of B-subunit of Shiga toxin conjugated to the diphtheria toxin against breast cancer

Immunotoxins are protein-based drugs consist of a target-specific binding domain and a cytotoxic domain to eliminate target cells. Such compounds are potentially therapeutic to combat diseases such as cancer. Generally, the B-subunit of Shiga toxin (STXB) receptor, globotriaosylceramide (Gb3), is expressed in high amounts on a number of human tumors cancer cells. In this study, we evaluated a new antitumor candidate called DT389-STXB chimeric protein, which genetically fused the DT to B-subunit of Shiga-like toxin (STXB). First a chimeric protein, encoding DT389-STXB was synthesized. The optimized chimeric protein expressed in E.coli BL21 (DE3) and confirmed by anti-His Western blot analysis. T47D, SKBR3, 4T1 and MCF7 cell lines were treated separately with purified DT389-STXB recombinant protein and functional activity of DT389-STXB was analyzed by the cell enzyme-linked immunosorbentassay (ELISA), MTT, ICC, Western blot and apoptosis tests. The results indicated that the recombinant DT389-STXB fusion protein with a molecular weight of 53 kDa was successfully expressed in E.coli BL21 (DE3) and the anti-His western-blot was used to confirm the presence of the protein. The DT389-STXB fusion protein attached to T47D, SKBR3 and 4T1 cell lines with the proper affinity and induced dose-dependent cytotoxicity against GB3-expressing cancer cells in vitro. Our results showed that DT389-STXB fusion protein may be a promising candidate for antitumor therapy agent against breast cancer; however, further studies are required to explore its efficacy in vivo for therapeutic applications.

Shiga Toxin (Stx)-Binding Glycosphingolipids of Primary Human Renal Cortical Epithelial Cells (pHRCEpiCs) and Stx-Mediated Cytotoxicity

Human kidney epithelial cells are supposed to be directly involved in the pathogenesis of the hemolytic–uremic syndrome (HUS) caused by Shiga toxin (Stx)-producing enterohemorrhagic Escherichia coli (EHEC). The characterization of the major and minor Stx-binding glycosphingolipids (GSLs) globotriaosylceramide (Gb3Cer) and globotetraosylceramide (Gb4Cer), respectively, of primary human renal cortical epithelial cells (pHRCEpiCs) revealed GSLs with Cer (d18:1, C16:0), Cer (d18:1, C22:0), and Cer (d18:1, C24:1/C24:0) as the dominant lipoforms. Using detergent-resistant membranes (DRMs) and non-DRMs, Gb3Cer and Gb4Cer prevailed in the DRM fractions, suggesting their association with microdomains in the liquid-ordered membrane phase. A preference of Gb3Cer and Gb4Cer endowed with C24:0 fatty acid accompanied by minor monounsaturated C24:1-harboring counterparts was observed in DRMs, whereas the C24:1 fatty acid increased in relation to the saturated equivalents in non-DRMs. A shift of the dominant phospholipid phosphatidylcholine with saturated fatty acids in the DRM to unsaturated species in the non-DRM fractions correlated with the GSL distribution. Cytotoxicity assays gave a moderate susceptibility of pHRCEpiCs to the Stx1a and Stx2a subtypes when compared to highly sensitive Vero-B4 cells. The results indicate that presence of Stx-binding GSLs per se and preferred occurrence in microdomains do not necessarily lead to a high cellular susceptibility towards Stx.

New opportunities and challenges of venom-based and bacteria-derived molecules for anticancer targeted therapy

Due to advances in detection and treatment of cancer, especially the rise in the targeted therapy, the five-year relative survival rate of all cancers has increased significantly. However, according to the analysis of the survival rate of cancer patients in 2019, the survival rate of most cancers is still less than five years. Therefore, to combat complex cancer and further improve the 5-year survival rate of cancer patients, it is necessary to develop some new anticancer drugs. Because of the adaptive evolution of toxic species for millions of years, the venom sac is a "treasure bank", which has millions of biomolecules with high affinity and stability awaiting further development. Complete utilization of venom-based and bacteria-derived drugs in the market is still staggering because of incomplete understanding regarding their mode of action. In this review, we focused on the currently identified targets for anticancer effects based on venomous and bacterial biomolecules, such as ion channels, membrane non-receptor molecules, integrins, and other related target molecules. This review will serve as the key for exploring the molecular mechanisms behind the anticancer potential of venom-based and bacteria-derived drugs and will also lay the path for the development of anticancer targeted therapy.

RAB5A and TRAPPC6B are novel targets for Shiga toxin 2a inactivation in kidney epithelial cells

The cardinal virulence factor of human-pathogenic enterohaemorrhagic Escherichia coli (EHEC) is Shiga toxin (Stx), which causes severe extraintestinal complications including kidney failure by damaging renal endothelial cells. In EHEC pathogenesis, the disturbance of the kidney epithelium by Stx becomes increasingly recognised, but how this exactly occurs is unknown. To explore this molecularly, we investigated the Stx receptor content and transcriptomic profile of two human renal epithelial cell lines: highly Stx-sensitive ACHN cells and largely Stx-insensitive Caki-2 cells. Though both lines exhibited the Stx receptor globotriaosylceramide, RNAseq revealed strikingly different transcriptomic responses to an Stx challenge. Using RNAi to silence factors involved in ACHN cells’ Stx response, the greatest protection occurred when silencing RAB5A and TRAPPC6B, two host factors that we newly link to Stx trafficking. Silencing these factors alongside YKT6 fully prevented the cytotoxic Stx effect. Overall, our approach reveals novel subcellular targets for potential therapies against Stx-mediated kidney failure.

Designing and Analyzing the Structure of DT-STXB Fusion Protein as an Anti-tumor Agent: An in Silico Approach

BACKGROUND & OBJECTIVE

A main contest in chemotherapy is to obtain regulator above the biodistribution of cytotoxic drugs. The utmost promising strategy comprises of drugs coupled with a tumor-targeting bearer that results in wide cytotoxic activity and particular delivery. The B-subunit of Shiga toxin (STxB) is nontoxic and possesses low immunogenicity that exactly binds to the globotriaosylceramide (Gb3/CD77). Gb3/CD77 extremely expresses on a number of human tumors such as pancreatic, colon, and breast cancer and acts as a functional receptor for Shiga toxin (STx). Then, this toxin can be applied to target Gb3-positive human tumors. In this study, we evaluated DT390-STXB chimeric protein as a new anti-tumor candidate via genetically fusing the DT390 fragment of DT538 (Native diphtheria toxin) to STxB.

METHODS

This study intended to investigate the DT390- STxB fusion protein structure in silico. Considering the Escherichia coli codon usage, the genomic construct was designed. The properties and the structure of the protein were determined by an in silico technique. The mRNA structure and the physicochemical characteristics, construction, and the stability of the designed chimeric protein were analyzed using computational and bioinformatics tools and servers. Hence, the GOR4 and I-TASSER online web servers were used to predict the secondary and tertiary structures of the designed protein.

RESULTS

The results demonstrated that codon adaptation index (CAI) of dt390-stxB chimeric gene raised from 0.6 in the wild type to 0.9 in the chimeric optimized gene. The mfold data revealed that the dt390-stxB mRNA was completely stable to be translated effectively in the novel host. The normal activity of the fusion protein determined by considering the secondary and tertiary structure of each construct. Energy calculation data indicated that the thermodynamic ensemble for mRNA structure was -427.40 kJ/mol. The stability index (SI) of DT390-STxB was 36.95, which is quite appropriate to preserve the stability of the construct. Ultimately, the DT390-STxB was classified as a steady fusion protein according to the Ramachandran plot.

CONCLUSION

Our results showed that DT390-STXB was a stable chimeric protein and it can be recruited as a candidate of novel anti-tumor agents for the development of breast cancer treatment.

Shiga-like toxin I exerts specific and potent anti-tumour efficacy against gastric cancer cell proliferation when driven by tumour-preferential Frizzled-7 promoter

OBJECTIVES

Tumour-targeted gene therapy is a promising approach for effective control of gastric cancer cell proliferation. Our study aims to develop a cancer therapy which combines tumour-targeting promoters with cytotoxins.

METHODS

The expression of globotriaosylceramide (Gb3), which is a Shiga-like toxin I (Stx1) receptor, was verified in gastric cancer compared with normal stomach tissues as assessed by flow cytometry and immunohistochemical analysis. We therefore constructed the recombinant pFZD7-Stx1 plasmid vectors with tumour-preferential Frizzled-7 promoter and Stx1. pFZD7-Stx1 was used to treat gastric cancer in vitro and in vivo. The gastric cancer cell proliferation and tumour growth were identified after the transfection with the pFZD7-Stx1.

RESULTS

Globotriaosylceramide was obviously increased in gastric cancer compared with normal stomach. The gastric cancer cell proliferation and tumour growth decreased significantly after the transfection with the pFZD7-Stx1.

CONCLUSION

Frizzled-7 promoter is preferentially active, and Gb3 is abundant in gastric cancer cells. Frizzled-7 promoter and Stx1 may be used to determine a novel and relatively specific and potent gastric cancer therapeutic strategy.

Shiga toxin-glycosphingolipid interaction: Status quo of research with focus on primary human brain and kidney endothelial cells

Shiga toxin (Stx)-mediated injury of the kidneys and the brain represent the major extraintestinal complications in humans upon infection by enterohemorrhagic Escherichia coli (EHEC). Damage of renal and cerebral endothelial cells is the key event in the pathogenesis of the life-threatening hemolytic uremic syndrome (HUS). Stxs are AB₅ toxins and the B-pentamers of the two clinically important Stx subtypes Stx1a and Stx2a preferentially bind to the glycosphingolipid globotriaosylceramide (Gb3Cer, Galα4Galβ4Glcβ1Cer) and to less extent to globotetraosylceramide (Gb4Cer, GalNAcβ3Galα4Galβ4Glcβ1), which are expected to reside in lipid rafts in the plasma membrane of the human endothelium. This review summarizes the current knowledge on the Stx glycosphingolipid receptors and their lipid membrane ensemble in primary human brain microvascular endothelial cells (pHBMECs) and primary human renal glomerular endothelial cells (pHRGECs). Increasing knowledge on the precise initial molecular mechanisms by which Stxs interact with cellular targets will help to develop specific therapeutics and/or preventive measures to combat EHEC-caused diseases.

Salmonella-Mediated Cytolethal Distending Toxin Transfer Inhibits Tumor Growth

The Salmonella enterica serovar Choleraesuis (S.C.) has potential as an antitumor agent because of its tumor-targeting characteristics. S.C. can also be used for specific delivery of therapeutic agents and continuous release during replication. Previously, we successfully used S.C. as a vector to transfer a therapeutic gene and oncolytic virus, which suggested that modified S.C. is suitable for incorporating other antitumor agents into a single system. Cytolethal distending toxin B (CdtB) produced by Campylobacter jejuni can induce tumor cell apoptosis. Here we coated CdtB with poly(allylamine hydrochloride) (PAH) to yield PAH-CdtB. Treatment of cells with PAH-coated CdtB induced apoptosis, demonstrating that the compound retained antitumor activity. Furthermore, S.C. coated with PAH-CdtB (CdtB-S.C.) maintained tumor-targeting activity and had an enhanced antitumor effect. Measurement of the cytotoxic effect of CdtB-S.C. in vitro in a tumor cell line showed increased apoptosis whereas treatment of tumor-bearing mice with CdtB-S.C. reduced tumor growth and prolonged survival. Taken together, our results provide evidence that Salmonella carrying CdtB could have application for the treatment of tumors.

Intracellular drug delivery: Potential usefulness of engineered Shiga toxin subunit B for targeted cancer therapy

A treasure trove of intracellular cancer drug targets remains hidden behind cell membranes. However, engineered pathogen-derived toxins such as Shiga toxins can deliver small or macromolecular drugs to specific intracellular organelles. After binding to ganglioglobotriaosylceramide (Gb3, CD77), the non-toxic subunit B (StxB) of the Shiga-holotoxin is endocytosed and delivers its payload by a unique retrograde trafficking pathway via the endoplasmic reticulum to the cytosol. This review provides an overview of biomedical applications of StxB-based drug delivery systems in targeted cancer diagnosis and therapy. Biotechnological production of the Stx-material is discussed from the perspective of developing efficacious and safe therapeutics.

Cytolethal Distending Toxin Enhances Radiosensitivity in Prostate Cancer Cells by Regulating Autophagy

Cytolethal distending toxin (CDT) produced by Campylobacter jejuni contains three subunits: CdtA, CdtB, and CdtC. Among these three toxin subunits, CdtB is the toxic moiety of CDT with DNase I activity, resulting in DNA double-strand breaks (DSB) and, consequently, cell cycle arrest at the G2/M stage and apoptosis. Radiation therapy is an effective modality for the treatment of localized prostate cancer (PCa). However, patients often develop radioresistance. Owing to its particular biochemical properties, we previously employed CdtB as a therapeutic agent for sensitizing radioresistant PCa cells to ionizing radiation (IR). In this study, we further demonstrated that CDT suppresses the IR-induced autophagy pathway in PCa cells by attenuating c-Myc expression and therefore sensitizes PCa cells to radiation. We further showed that CDT prevents the formation of autophagosomes via decreased high-mobility group box 1 (HMGB1) expression and the inhibition of acidic vesicular organelle (AVO) formation, which are associated with enhanced radiosensitivity in PCa cells. The results of this study reveal the detailed mechanism of CDT for the treatment of radioresistant PCa.

Endoplasmic Reticulum-Targeted Subunit Toxins Provide a New Approach to Rescue Misfolded Mutant Proteins and Revert Cell Models of Genetic Diseases

Many germ line diseases stem from a relatively minor disturbance in mutant protein endoplasmic reticulum (ER) 3D assembly. Chaperones are recruited which, on failure to correct folding, sort the mutant for retrotranslocation and cytosolic proteasomal degradation (ER-associated degradation-ERAD), to initiate/exacerbate deficiency-disease symptoms. Several bacterial (and plant) subunit toxins, retrograde transport to the ER after initial cell surface receptor binding/internalization. The A subunit has evolved to mimic a misfolded protein and hijack the ERAD membrane translocon (dislocon), to effect cytosolic access and cytopathology. We show such toxins compete for ERAD to rescue endogenous misfolded proteins. Cholera toxin or verotoxin (Shiga toxin) containing genetically inactivated (± an N-terminal polyleucine tail) A subunit can, within 2–4 hrs, temporarily increase F508delCFTR protein, the major cystic fibrosis (CF) mutant (5-10x), F508delCFTR Golgi maturation (<10x), cell surface expression (20x) and chloride transport (2x) in F508del CFTR transfected cells and patient-derived F508delCFTR bronchiolar epithelia, without apparent cytopathology. These toxoids also increase glucocerobrosidase (GCC) in N370SGCC Gaucher Disease fibroblasts (3x), another ERAD–exacerbated misfiling disease. We identify a new, potentially benign approach to the treatment of certain genetic protein misfolding diseases.

Shiga Toxins as Multi-Functional Proteins: Induction of Host Cellular Stress Responses, Role in Pathogenesis and Therapeutic Applications

Shiga toxins (Stxs) produced by Shiga toxin-producing bacteria Shigella dysenteriae serotype 1 and select serotypes of Escherichia coli are primary virulence factors in the pathogenesis of hemorrhagic colitis progressing to potentially fatal systemic complications, such as hemolytic uremic syndrome and central nervous system abnormalities. Current therapeutic options to treat patients infected with toxin-producing bacteria are limited. The structures of Stxs, toxin-receptor binding, intracellular transport and the mode of action of the toxins have been well defined. However, in the last decade, numerous studies have demonstrated that in addition to being potent protein synthesis inhibitors, Stxs are also multifunctional proteins capable of activating multiple cell stress signaling pathways, which may result in apoptosis, autophagy or activation of the innate immune response. Here, we briefly present the current understanding of Stx-activated signaling pathways and provide a concise review of therapeutic applications to target tumors by engineering the toxins.

Molecular Mechanisms and Potential Clinical Applications of Campylobacter jejuni Cytolethal Distending Toxin

Cytolethal distending toxin (CDT), a genotoxin produced by Campylobacter jejuni, is composed of three subunits: CdtA, CdtB, and CdtC. CdtB is a DNase that causes DNA double-strand breaks (DSB) in the nucleus resulting in cell cycle arrest at the G2/M stage and apoptosis. CdtA and CdtC bind to cholesterol-rich microdomains on the cytoplasmic membrane, a process required for the delivery of CdtB to cells. Although a unique motif associated with cholesterol-binding activity has been identified in other pathogens, the mechanism underlying the interaction between the CdtA and CdtC subunits and membrane cholesterol remains unclear. Also, the processes of cell uptake and delivery of CdtB in host cells and the translocation of CdtB into the nucleus are only partially understood. In this review, we focus on the underlying relationship among CDT, membrane cholesterol, and the intracellular trafficking pathway as a unique mechanism for C. jejuni-induced pathogenesis. Moreover, we discuss the clinical aspects of a possible therapeutic application of CDT in cancer therapy. Understanding the molecular mechanism of CDT-host interactions may provide insights into novel strategies to control C. jejuni infection and the development of potential clinical applications of CDT.

Shiga toxins: from structure and mechanism to applications

Shiga toxins are a group of type 2 ribosome-inactivating proteins (RIPs) produced in several types of bacteria. The toxins possess an AB5 structure, which comprises a catalytic A chain with N-glycosidase activity, and five identical B chains and recognize and bind to the target cells with specific carbohydrate moieties. In humans, the major molecular target which recognizes the Shiga toxins is the Gb3 receptor, which is mainly expressed on the cell surface of endothelial cells of the intestine, kidney, and the brain. This causes these organs to be susceptible to the toxicity of Shiga toxins. When a person is infected by Shiga toxin-producing bacteria, the toxin is produced in the gut, translocated to the circulatory system, and carried to the target cells. Toxicity of the toxin causes inflammatory responses and severe cell damages in the intestine, kidneys, and brain, bringing about the hemolytic uremic syndrome (HUS), which can be fatal. The Shiga toxin requires a couple of steps to exert its toxicity to the target cells. After binding with the target cell surface receptor, the toxin requires a complicated process to be transported into the cytosol of the cell before it can approach the ribosomes. The mechanisms for the interactions of the toxin with the cells are described in this review. The consequences of the toxin on the cells are also discussed. It gives an overview of the steps for the toxin to be produced and transported, expression of catalytic activity, and the effects of the toxin on the target cells, as well as effects on the human body.

Frizzled-7 promoter is highly active in tumors and promoter-driven Shiga-like toxin I inhibits hepatocellular carcinoma growth

Frizzled-7 protein plays a significant role in the formation of several malignant tumors. Up regulation of the Frizzled-7 in cancer cell lines is associated with nuclear accumulation of wild-type β-catenin from the Wnt/β-catenin pathway which is frequently activated in tumors. To analyze activity of the Frizzled-7 promoter in tumor cells, we constructed two recombinant plasmid vectors in which the Frizzled-7 promoter was used to drive the expression of green fluorescent protein (GFP) and Shiga-like toxin I (Stx1) (pFZD7-GFP/Stx1) genes. The Frizzled-7 protein was found to be expressed in the cancer cell lines but not in the normal cell lines. The GFP expression was restricted to the cancer cell lines and xenografts in the BALB/C mice but not to normal cell lines. Moreover, cell proliferation and tumor growth decreased significantly after transfection with the pFZD7-Stx1. Results from this study will help determine a highly effective strategy for gene therapy of tumors.

Shiga Toxins: Potent Poisons, Pathogenicity Determinants, and Pharmacological Agents

The Shiga toxins (Stxs), also known as Vero toxins and previously called Shiga-like toxins, are a family of potent protein synthesis inhibitors made by Shigella dysenteriae type 1 and some serogroups of Escherichia coli that cause bloody diarrhea in humans. Stxs act as virulence factors for both S. dysenteriae and E. coli and contribute to the disease process initiated by those organisms both directly and indirectly. A handful of methods exist for toxin purification, and the toxins can now even be purchased commercially. However, the Stxs are now classified as select agents, and specific rules govern the distribution of both the toxin and clones of the toxin. Toxin delivery into the host in S. dysenteriae type 1 is most likely aided by the invasiveness of that organism. Although the Stxs are made and produced by bacteria, they do not appear to act against either their host organism or other bacteria under normal circumstances, most likely because the A subunit is secreted from the cytoplasm as soon as it is synthesized and because the holotoxin cannot enter intact bacterial cells. The effectiveness of antibiotic therapy in patients infected with Stx-producing E. coli (STEC) such as O157:H7 as well as the potential risks of such treatment are areas of controversy. Several studies indicate that the course of the diarrhea stage of the disease is unaltered by antibiotic treatment. Several groups anticipate that a therapy that targets the Stxs is an important component of trying to alleviate disease caused by Stx-producing bacteria.

Sensitization of radio-resistant prostate cancer cells with a unique cytolethal distending toxin

Cytolethal distending toxin (CDT) produced by Campylobacter jejuni is a genotoxin that induces cell-cycle arrest and apoptosis in mammalian cells. Recent studies have demonstrated that prostate cancer (PCa) cells can acquire radio-resistance when DOC-2/DAB2 interactive protein (DAB2IP) is downregulated. In this study, we showed that CDT could induce cell death in DAB2IP-deficient PCa cells. A combination of CDT and radiotherapy significantly elicited cell death in DAB2IP-deficient PCa cells by inhibiting the repair of ionizing radiation (IR)-induced DNA double-strand break (DSB) during G2/M arrest, which is triggered by ataxia telangiectasia mutated (ATM)-dependent DNA damage checkpoint responses. We also found that CDT administration significantly increased the efficacy of radiotherapy in a xenograft mouse model. These results indicate that CDT can be a potent therapeutic agent for radio-resistant PCa.

RLIP76 Targeted Therapy for Kidney Cancer

Despite recent improvements in chemotherapeutic approaches to treating kidney cancer, this malignancy remains deadly if not found and removed at an early stage of the disease. Kidney cancer is highly drug-resistant, which may at least partially result from high expression of transporter proteins in the cell membranes of kidney cells. Although these transporter proteins can contribute to drug-resistance, targeting proteins from the ATP-binding cassette transporter family has not been effective in reversing drug-resistance in kidney cancer. Recent studies have identified RLIP76 as a key stress-defense protein that protects normal cells from damage caused by stress conditions, including heat, ultra-violet light, X-irradiation, and oxidant/electrophilic toxic chemicals, and is crucial for protecting cancer cells from apoptosis. RLIP76 is the predominant glutathione-electrophile-conjugate (GS-E) transporter in cells, and inhibiting it with antibodies or through siRNA or antisense causes apoptosis in many cancer cell types. To date, blocking of RLIP76, either alone or in combination with chemotherapeutic drugs, as a therapeutic strategy for kidney cancer has not yet been evaluated in human clinical trials, although there is considerable potential for RLIP76 to be developed as a therapeutic agent for kidney cancer. In the present review, we discuss the mechanisms underlying apoptosis caused by RLIP76 depletion, the role of RLIP76 in clathrin-dependent endocytosis deficiency, and the feasibility of RLIP76-targeted therapy for kidney cancer.

Human breast cancer and lymph node metastases express Gb3 and can be targeted by STxB-vectorized chemotherapeutic compounds

BACKGROUND

The B-subunit of Shiga toxin (STxB) specifically binds to the glycosphingolipid Gb3 that is highly expressed on a number of human tumors and has been shown to target tumor cells in mouse models and ex vivo on primary colon carcinoma specimen.

METHODS

Using a novel ex vivo STxB labeling (ESL) method we studied Gb3 expression in cytological specimens of primary human breast tumors from 107 patients, and in synchronous lymph node metastases from 20 patients. Fluorescent STxB was incubated with fine-needle aspiration (FNA) specimens, and Gb3 expression was evaluated by fluorescence microscopy. Furthermore, 11 patient-derived human breast cancer xenografts (HBCx) were evaluated for expression of Gb3 by ESL and FACS. In addition, the biodistribution of fluorescent STxB conjugate was studied after intravenous injection in a Gb3 positive HBCx model.

RESULTS

Gb3 expression was detected in 62 of 107 patients (57.9%), mainly in epithelial tumor cells. Gb3 positivity correlated with estrogen receptor expression (p≤0.01), whereas absence of Gb3 expression in primary tumors was correlated with the presence of lymph node metastases (p≤0.03). 65% of lymph node metastases were Gb3 positive and in 40% of tested patients, we observed a statistically significant increase of metastatic Gb3 expression (p≤0.04). Using concordant ESL and flow cytometry analysis, 6 out of 11 HBCx samples were scored positive. Intravenous injections of fluorescent STxB into HBC xenografted mice showed preferential STxB accumulation in epithelial cells and cells with endothelial morphology of the tumor.

CONCLUSION

The enhanced expression of Gb3 in primary breast carcinomas and its lymph node metastases indicate that the development of STxB-based therapeutic strategies is of interest in this pathology. Gb3 expressing HBCx can be used as a model for preclinical studies with STxB conjugates. Finally, the ESL technique on FNA represents a rapid and cost effective method for the stratification of patients in future clinical trials.

Anti-Gb3 monoclonal antibody inhibits angiogenesis and tumor development

Inhibiting the growth of tumor vasculature represents one of the relevant strategies against tumor progression. Between all the different pro-angiogenic molecular targets, plasma membrane glycosphingolipids have been under-investigated. In this present study, we explore the anti-angiogenic therapeutic advantage of a tumor immunotherapy targeting the globotriaosylceramide Gb3. In this purpose, a monoclonal antibody against Gb3, named 3E2 was developed and characterized. We first demonstrate that Gb3 is over-expressed in proliferative endothelial cells relative to quiescent cells. Then, we demonstrate that 3E2 inhibits endothelial cell proliferation in vitro by slowing endothelial cell proliferation and by increasing mitosis duration. Antibody 3E2 is further effective in inhibiting ex vivo angiogenesis in aorta ring assays. Moreover, 3E2 treatment inhibits NXS2 neuroblastoma development and liver metastases spreading in A/J mice. Immunohistology examination of the NXS2 metastases shows that only endothelial cells, but not cancer cells express Gb3. Finally, 3E2 treatment diminishes tumor vessels density, proving a specific therapeutic action of our monoclonal antibody to tumor vasculature. Our study demonstrates that Gb3 is a viable alternative target for immunotherapy and angiogenesis inhibition.

Ribosome-inactivating proteins with an emphasis on bacterial RIPs and their potential medical applications

Ribosome-inactivating proteins (RIPs) are toxic due to their N-glycosidase activity catalyzing depurination at the universally conserved α-sarcin loop of the 60S ribosomal subunit. In addition, RIPs have been shown to also have other enzymatic activities, including polynucleotide:adenosine glycosidase activity. RIPs are mainly produced by different plant species, but are additionally found in a number of bacteria, fungi, algae and some mammalian tissues. This review describes the occurrence of RIPs, with special emphasis on bacterial RIPs, including the Shiga toxin and RIP in Streptomyces coelicolor recently identified in S. coelicolor. The properties of RIPs, such as enzymatic activity and targeting specificity, and how their unique biological activity could be potentially turned into medical or agricultural tools to combat tumors, viruses and fungi, are highlighted.

Shiga toxins

Shiga toxins are virulence factors produced by the bacteria Shigella dysenteriae and certain strains of Escherichia coli. There is currently no available treatment for disease caused by these toxin-producing bacteria, and understanding the biology of the Shiga toxins might be instrumental in addressing this issue. In target cells, the toxins efficiently inhibit protein synthesis by inactivating ribosomes, and they may induce signaling leading to apoptosis. To reach their cytoplasmic target, Shiga toxins are endocytosed and transported by a retrograde pathway to the endoplasmic reticulum, before the enzymatically active moiety is translocated to the cytosol. The toxins thereby serve as powerful tools to investigate mechanisms of intracellular transport. Although Shiga toxins are a serious threat to human health, the toxins may be exploited for medical purposes such as cancer therapy or imaging.

Shiga toxin and its use in targeted cancer therapy and imaging

Shiga and the Shiga‐like toxins are related protein toxins produced by Shigella dysenteriae and certain strains of Escherichia coli. These toxins are composed of two non‐covalently attached, modular parts: the A moiety (StxA) containing the enzymatically active A₁ fragment, and the non‐toxic, pentameric binding moiety (StxB). Stx binds specifically to the glycosphingolipid globotriaosylceramide (Gb3) at the surface of target cells and is then internalized by endocytosis. Subsequently, in toxin‐sensitive cells, the Stx/Gb3 complex is transported in a retrograde manner via the Golgi apparatus to the endoplasmic reticulum, where the enzymatically active part of Stx is translocated to the cytosol, enabling it to irreversibly inhibit protein synthesis via modification of ribosomal 28S RNA. Whereas Gb3 shows a relatively restricted expression in normal human tissues, it has been reported to be highly expressed in many types of cancers. This review gives a brief introduction to Stx and its intracellular transport. Furthermore, after a description of Gb3 and the methods that are currently used to detect its cellular expression, we provide an updated overview of the published reports on Gb3 overexpression in human cancers. Finally, we discuss the possibility of utilizing Stx or StxB coupled to therapeutic compounds or contrast agents in targeted cancer therapy and imaging.

Biochemical, pathological and oncological relevance of Gb3Cer receptor

Glycosphingolipids are amphipathic molecules composed of hydrophilic oligosaccharide chain and a hydrophobic ceramide part, located primarily in the membrane microdomains of animal cells. Their oligosaccharide chains make them excellent candidates for the cell surface recognition molecules. Natural glycosphingolipid, globotriaosylceramide (Gal α1-4, Gal β1-4, Glc β1-1, ceramide), is also called CD77 and its expression was previously associated with proliferating centroblasts undergoing somatic hypermutation, but it has been demonstrate that globotriaosylceramide is not a reliable marker to discriminate human centroblasts from centrocytes. Globotriaosylceramide constitutes rare P k blood group antigen on erythrocytes, and it is also known as Burkitt's lymphoma antigen. On endothelial cells, globotriaosylceramide plays as the receptor for bacterial toxins of the Shiga family, also called verotoxins. Precise biological function and significance of globotriaosylceramide expression on endothelial cells remains to be the subject of many studies and it is believed globotriaosylceramide represents an example of a glycolipid antigen able to transduce a signal leading to apoptosis. In past decade, cancer researches put a great afford in determining new therapeutic agents such as bacterial toxins against tumor malignancies. Reports have demonstrated that verotoxin-1 induces apoptosis in solid tumor cell lines expressing globotriaosylceramide such as astrocytoma, renal cell carcinoma, colon cancer and breast cancer due to verotoxin-1 high specificity and apoptosis-inducing properties, and therefore, it is suggested to be an anticancer agent. Verotoxins have been investigated weather they could reduce treatment side-effects and toxicity to normal tissues and become a new oncological tool in cancer labeling.

Verotoxin-1 treatment or manipulation of its receptor globotriaosylceramide (gb3) for reversal of multidrug resistance to cancer chemotherapy

A major problem with anti-cancer drug treatment is the development of acquired multidrug resistance (MDR) of the tumor cells. Verotoxin-1 (VT-1) exerts its cytotoxicity by targeting the globotriaosylceramide membrane receptor (Gb3), a glycolipid associated with multidrug resistance. Gb3 is overexpressed in many human tumors and tumor cell lines with inherent or acquired MDR. Gb3 is co-expressed and interplays with the membrane efflux transporter P-gp encoded by the MDR1 gene. P-gp could act as a lipid flippase and stimulate Gb3 induction when tumor cells are exposed to cancer chemotherapy. Recent work has shown that apoptosis and inherent or acquired multidrug resistance in Gb3-expressing tumors could be affected by VT-1 holotoxin, a sub-toxic concentration of the holotoxin concomitant with chemotherapy or its Gb3-binding B-subunit coupled to cytotoxic or immunomodulatory drug, as well as chemical manipulation of Gb3 expression. The interplay between Gb3 and P-gp thus gives a possible physiological approach to augment the chemotherapeutic effect in multidrug resistant tumors.

Structure, biological functions and applications of the AB5 toxins

AB(5) toxins are important virulence factors for several major bacterial pathogens, including Bordetella pertussis, Vibrio cholerae, Shigella dysenteriae and at least two distinct pathotypes of Escherichia coli. The AB(5) toxins are so named because they comprise a catalytic A-subunit, which is responsible for disruption of essential host functions, and a pentameric B-subunit that binds to specific glycan receptors on the target cell surface. The molecular mechanisms by which the AB(5) toxins cause disease have been largely unravelled, including recent insights into a novel AB(5) toxin family, subtilase cytotoxin (SubAB). Furthermore, AB(5) toxins have become a valuable tool for studying fundamental cellular functions, and are now being investigated for potential applications in the clinical treatment of human diseases.

Shiga toxin A subunit mutant of Escherichia coli O157:H7 releases outer membrane vesicles containing the B-pentameric complex

Shiga toxins (STx) are secreted extracellularly through the outer membrane vesicles (OMVs) of Escherichia coli O157:H7. In an attempt to produce STxA-deficient OMVs from E. coli O157:H7, site-specific deletions of the stx1A and stx2A subunit genes were carried out. The STxA-deficient phenotype of the stx1A/stx2A mutant was confirmed by Vero cell cytotoxicity and VTEC-RPLA assay. Western blot analyses showed that the B (STxB) subunits were present without coupling to STxA in the OMVs of the STxA-deficient mutant. Furthermore, STxB was located in its homo-pentameric complexes, as revealed by immunoprecipitation and immunoblotting with anti-STxB antibodies. These results suggest that STxB alone can be oligomerized into the B pentamer in the periplasm, and subsequently entrapped into the OMVs. Determination of the median lethal dose concentration for the OMV preparations suggests that the STxA-deficient OMVs containing STxB complex could be safely used as vaccine delivery vehicles.

Shiga toxins--from cell biology to biomedical applications

Shiga toxin-producing Escherichia coli is an emergent pathogen that can induce haemolytic uraemic syndrome. The toxin has received considerable attention not only from microbiologists but also in the field of cell biology, where it has become a powerful tool to study intracellular trafficking. In this Review, we summarize the Shiga toxin family members and their structures, receptors, trafficking pathways and cellular targets. We discuss how Shiga toxin affects cells not only by inhibiting protein biosynthesis but also through the induction of signalling cascades that lead to apoptosis. Finally, we discuss how Shiga toxins might be exploited in cancer therapy and immunotherapy.

Cisplatin-induced expression of Gb3 enables verotoxin-1 treatment of cisplatin resistance in malignant pleural mesothelioma cells

Background:

A major problem with cisplatin treatment is the development of acquired-drug resistance of the tumour cells. Verotoxin-1 (VT-1) exerts its cytotoxicity by targeting the membrane glycolipid globotriasosylceramide (Gb3), a molecule associated with drug resistance. Cisplatin- and VT-1-induced apoptosis involves mitogen-activated protein kinase (MAPK) activation, and deactivation of MAPKs is associated with cisplatin resistance. This study aimed to investigate whether a sub-toxic concentration of VT-1 could enhance cisplatin-induced apoptosis and overcome acquired-cisplatin resistance in cultured cancer cell lines.

Method:

P31 and H1299 cells with corresponding cisplatin-resistant sub-lines (P31res/H1299res) were incubated with VT-1 and/or cisplatin followed by determination of Gb3 expression, cell viability, apoptosis, and signalling pathways.

Results:

Cells from the resistant sub-lines had elevated Gb3 expression compared with the parental cell lines, and cisplatin further increased Gb3 expression, whereas VT-1 reduced the percentage of Gb3-expressing cells. Combination of cisplatin and sub-toxic concentrations of VT-1 led to a super-additive increase of cytotoxicity and TUNEL staining, especially in the cisplatin-resistant sub-lines. Blockade of Gb3 synthesis by a Gb3 synthesis inhibitor not only led to eradicated TUNEL staining of P31 cells, but also sensitised P31res cells to the induction of apoptosis by cisplatin alone. Cisplatin- and VT-1-induced apoptosis involved the MAPK pathways with increased C-Jun N-terminal kinase and MAPK kinase-3 and -6 phosphorylation.

Conclusions:

We show the presence of Gb3 in acquired-cisplatin resistance in P31res and H1299res cells. Cisplatin up-regulated Gb3 expression in all cells and thus sensitised the cells to VT-1-induced cytotoxicity. A strong super-additive effect of combined cisplatin and a sub-toxic concentration of VT-1 in cisplatin-resistant malignant pleural mesothelioma cells were observed, indicating a new potential clinical-treatment approach.

RLIP76: a target for kidney cancer therapy

RLIP76 is a multifunctional transporter protein that serves as an energy-dependent efflux mechanism for endogenously generated toxic metabolites as well as exogenous toxins, including chemotherapy drugs. Our recent studies in cultured cells, syngeneic animal tumor model, and in xenograft model have shown that RLIP76 serves a major cancer-specific antiapoptotic role in a wide variety of histologic types of cancer, including leukemia, melanoma, colon, lung, prostate, and ovarian cancer. Results of present studies in cell culture and xenograft model of Caki-2 cells show that RLIP76 is an important anticancer for kidney cancer because inhibition of RLIP76 function by antibody or its depletion by small interfering RNA or antisense DNA caused marked and sustained regression of established human kidney xenografts of Caki-2 cells in nude mouse.

Expression of verotoxin-1 receptor Gb3 in breast cancer tissue and verotoxin-1 signal transduction to apoptosis

Background

The prerequisite for the potential use of the bacterial toxin verotoxin-1 in the treatment of breast cancer was investigated by first determining the expression of its receptor Gb3 (CD77) in clinical breast cancer tissue specimens. We then examined the cytotoxicity and mechanism of apoptosis induction of Escherichia coli verotoxin-1 (VT-1) in two human breast cancer cell lines.

Methods

Immunohistochemistry for Gb3 expression was performed on cryostat section from 25 breast cancer specimens. The human breast cancer cell lines T47D and MCF-7 were screened for Gb3 expression by flow cytometry. Fluorescein diacetate and LDH release was used to determine cell viability after VT-1 exposure. Apoptosis was studied by measuring caspase activity and DNA-fragmentation. Signal transduction studies were performed on T47D cells with immunoblotting.

Results

Gb3 expression was detected in the vascular endothelial cells of all tumours specimens, and in tumour cells in 17 of the specimens. We found no associations between tumour cell Gb3-expression and age, tumour size, TNM-classification, histological type, hormone receptor expression, or survival time. T47D cells strongly expressed Gb3 and were sensitive to the cytotoxicity, caspase activation and DNA fragmentation by VT-1, whereas MCF-7 cells with faint Gb3-expression were insensitive to VT-1. VT-1 (0.01 – 5 μg/L) exposure for 72 h resulted in a small percentage of viable T47D cells whereas the cytotoxicity of cells pre-treated with 2 μmol/L D, L-treo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP, an inhibitor of glucosylceramide synthesis) was eliminated (≤ 0.1 μg/L VT-1) or reduced (0.5 – 5 μg/L VT-1). VT-1 did not cause cellular LDH-release or cell cycle arrest. VT-1 induction of caspase-3 (0.1, 1, and 5 μg/L VT-1), -8, and -9 (1 and 5 μg/L VT-1) activity and DNA fragmentation of T47D cells was blocked by PPMP. Key components of MAP kinase signalling pathways that control mitochondrial function were investigated. VT-1 0.1 – 5 μg/L induced phosphorylation of JNK as well as MKK3/6 suggesting that survival signal pathways were overruled by VT-1-induced JNK activation leading to mitochondrial depolarization, caspase-9 activation and apoptosis.

Conclusion

The high specificity and apoptosis-inducing properties of verotoxin-1 indicates that the toxin potentially may be used for treatment of Gb3-expressing breast cancer.

Tracing the retrograde route in protein trafficking

Retrograde transport, in which proteins and lipids are shuttled between endosomes and biosynthetic/secretory compartments such as the Golgi apparatus, is crucial for a diverse range of cellular functions. Mechanistic studies that explore the molecular machinery involved in this retrograde trafficking route are shedding light on the functions of transport proteins and are providing fresh insights into possible new therapeutic directions.

Retrograde delivery of photosensitizer (TPPp-O-beta-GluOH)3 selectively potentiates its photodynamic activity

Photodynamic therapy involves administration of a photosensitizing drug and its subsequent activation by visible light of the appropriate wavelength. Several approaches to increasing the specificity of photosensitizers for cancerous tissues and, in particular, through their conjugation to ligands that are directed against tumor-associated antigens have been investigated. Here, we have studied the delivery of the photocytotoxic porphyrin compound TPP(p-O-beta-D-GluOH)3 into tumor cells that overexpress the glycosphingolipid Gb3, using the Gb3-binding nontoxic B-subunit of Shiga toxin (STxB) as a vector. To allow for site-directed chemical coupling, an STxB variant carrying a free sulfhydryl moiety at its C-terminal end has been used. Binding affinity, cellular uptake, singlet oxygen quantum yield, and phototoxicity of the conjugate have been examined. Despite some effect of coupling on both the photophysical properties of TPP(p-O-beta-D-GluOH)3 and the affinity of STxB for its receptor, the conjugate exhibited a higher photocytotoxic activity than the photosensitizer alone and was exquisitely selective for Gb3-expressing tumor cells. Furthermore, our data strongly suggest that STxB-mediated retrograde delivery of the photosensitizer to the biosynthetic/secretory pathway is critical for optimal cytotoxic activity. In conclusion, a strong rationale for using retrograde delivery tools such as STxB in combination with photosensitizing agents for the photodynamic therapy of tumors is presented.

Efficient antiproliferative and antiangiogenic effects on human ovarian cancer growth by gene transfer of attenuated mutants of Shiga-like toxin I

To evaluate the potential effect of anticancer and antiangiogenesis of Stx1(W203F) and Stx1(R170H), two attenuated mutants of Shiga-like toxin I (Stx1), in cancer gene therapy. Antiproliferative effects of these Stx1 mutants were tested in human ovarian carcinoma cell line SKOV3 and human umbilical vein endothelial cells (HUVECs) in vitro. Effect of these Stx1 mutants on inducing cell death and cell cycle arrest was analyzed in SKOV3 cells. Influence of these Stx1 mutants on endothelial cell function was analyzed in HUVECs. In vivo therapeutic effect of these Stx1 mutants on SKOV3 was explored using xenograft models in nude mice. These Stx1 mutants can inhibit the growth of SKOV3 or HUVECs and this effect can be abrogated by antibody specific for Stx1. They caused considerable cell death of SKOV3 cells in 24 h; neither caspase activity nor DNA fragmentation was observed, and necrosis is the major mode of cell death. These Stx1 mutants can induce cell cycle arrest of SKOV3 cells in G(2)-M or S phase depending on the dosage of gene transfer. Furthermore, they significantly decreased migration and capillary tube formation of HUVECs at low dose. In vivo study showed that Stx1(W203F) but not Stx1(R170H) significantly suppressed transplanted SKOV3 tumor growth in nude mice model. Interestingly, the microvessel densities of tumor treated with Stx1(W203F) and Stx1(R170H) were significantly reduced. This study suggests that genes encoding attenuated Stx1 can be selected as good candidates for the gene therapy of ovarian carcinoma because of their antiproliferative and antiangiogenic effects.

In vivo tumor targeting using a novel intestinal pathogen-based delivery approach

Efficient methods for tumor targeting are eagerly awaited and must satisfy several challenges: molecular specificity, transport through physiologic barriers, and capacity to withstand extracellular or intracellular degradation and inactivation by the immune system. Through interaction with its hosts, the intestinal pathogen-produced Shiga toxin has evolved molecular properties that are of interest in this context. Its nontoxic B-subunit binds to the cellular toxin receptor, glycosphingolipid Gb3, which is highly expressed on human cancers and has recently been reported to be involved in the formation of metastasis in colorectal cancers. Its function as a target for cancer therapy has already been addressed in xenograft experiments. We here show that after oral or i.v. injections in mice, the B-subunit targets spontaneous digestive Gb3-expressing adenocarcinomas. The nontumoral mucosa is devoid of labeling, with the exception of rare enteroendocrine and CD11b-positive cells. As opposed to other delivery tools that are often degraded or recycled on cancer cells, the B-subunit stably associates with these cells due to its trafficking via the retrograde transport route. This can be exploited for the in vivo delivery of contrast agents to tumors, as exemplified using fibered confocal fluorescence endoscopy and positron emission tomography (PET) imaging. In conclusion, the data presented in this manuscript lay the groundwork for a novel delivery technology that, in addition to its use for molecular imaging applications such as noninvasive PET, could also be exploited for targeted tumor therapies.

Synthesis of globo- and isoglobotriosides bearing a cinnamoylphenyl tag as novel electrophilic thiol-specific carbohydrate reagents

The galactosyl donor, 4,6-di-O-acetyl-2,3-di-O-benzyl-D-galactopyranosyl trichloroacetimidate, was efficiently coupled with regioselectively benzylated lactoside acceptors under standard conditions to stereoselectively afford the corresponding globotrioside and isoglobotrioside derivatives in very good yields. These glycosides were smoothly functionalized with a 6-(p-cinnamoylphenoxy)-hexyl tether tag as novel electrophilic thiol-specific carbohydrate reagents. Immobilization of the globotrioside conjugate to Thiopropyl Sepharose 6B for purification of B-subunit of Shiga toxin (StxB) and coupling of a model cysteine-containing protein (StxB-Z(n)-Cys) to the isoglobotrioside conjugate were both performed with high efficiency.

Protein toxins: intracellular trafficking for targeted therapy

The immunotoxin approach is based on the use of tumor-targeting ligands or antibodies that are linked to the catalytic (toxic) moieties of bacterial or plant protein toxins. In this review, we first discuss the current state of clinical development of immunotoxin approaches describing the results obtained with the two toxins most frequently used: diphtheria and Pseudomonas toxin-derived proteins. In the second part of the review, a novel concept will be presented in which the roles are inverted: nontoxic receptor-binding toxin moieties are used for the targeting of therapeutic and diagnostic compounds to cancer or immune cells. The cell biological basis of these novel types of toxin-based therapeutics will be discussed, and we will summarize ongoing preclinical and clinical testing.

Escherichia coliverotoxin 1 mediates apoptosis in human HCT116 colon cancer cells by inducing overexpression of the GADD family of genes and S phase arrest

The Escherichia coli verotoxin 1 (VT1) inhibits protein synthesis, cell proliferation, and damages endothelial cell in the hemolytic uremic syndrome. VT1 can specifically bind and act on endothelial cells as well as on many tumor cells because these cells express its high affinity receptor, globotriaosylceramide. This indicates that VT1 may have both antiangiogenic and antineoplastic activities. We investigated this potential of VT1 by incubating several colon cancer cell lines with VT1 for different time periods and found that HCT116 cells were especially sensitive to VT1. A combination of morphological studies, flow cytometry, DNA laddering and annexin V staining confirmed that VT1 irreversibly arrests these cells in S phase within 24 h and prolonged incubation triggers DNA fragmentation. Concomitant to the activation of the S phase checkpoint, increased levels of mRNA and proteins of growth arrest and DNA damage-inducible gene family that include GADD34, GADD45alpha, and GADD45beta was observed. Interestingly, no significant changes in expression of key cell cycle related proteins such as cdk2, cdk4, p21, p27, and p53 was found during the S phase arrest and apoptosis. We therefore suggest that GADD proteins might play an important role in VT1 induced S phase arrest and programmed cell death in HCT116 cells.