Ribosome-inactivating proteins up to date

Structural insights into the toxicity of type II ribosome inactivating proteins (RIPs): a molecular dynamics study

Ribosome Inactivating Proteins (RIPs) act by irreversibly depurinating the 28S rRNA ricin-sarcin loop (SRL) of the eukaryotic ribosome resulting in protein synthesis inhibition. In general, they consist of two variants: Type I which is single chained (∼30 kDa), and Type II, a more toxic variant which is a Type I N-glycosidase chain covalently linked to a lectin chain. These proteins are believed to play a pivotal role in defence mechanisms. Intriguingly, non-toxic variants of such toxic proteins do exist in nature. To explore their mode of action, in the present study we have selected three toxic (Ricin, Ebulin and HmRIP) as well as two non-toxic (BGSL and SGSL) RIPs and performed molecular docking and molecular dynamic simulations with the SRL loop. This study throws light on the structural stability and plasticity of the toxic and non-toxic RIP complexes. Furthermore, analysis of the active site cavity volume and binding free energy calculations reveal that the SRL, particularly the specific adenine (A4605), is relatively unstable in the case of non-toxic RIPs which is also supported by the free binding energy calculations, and the pocket size analysis indicates the abnormal increase in active site cavity volume of non-toxic RIPs with time. This first-of-its-kind comprehensive study of toxic and non-toxic RIPs gives insights about the mode of action and the dynamic nature of their interaction with the SRL loop. These observations will be helpful in the development of toxoids against RIPs and also in designing novel therapeutic approaches against human diseases.

Ribosome inactivating proteins - An unfathomed biomolecule for developing multi-stress tolerant transgenic plants

Transgenic crops would serve as a tool to overcome the forthcoming crisis in food security and environmental safety posed by degrading land and changing global climate. Commercial transgenic crops developed so far focus on single stress; however, sustaining crop yield to ensure food security requires transgenics tolerant to multiple environmental stresses. Here we argue and demonstrate the untapped potential of ribosome inactivating proteins (RIPs), translation inhibitors, as potential transgenes in developing transgenics to combat multiple stresses in the environment. Plant RIPs target the fundamental processes of the cell with very high specificity to the infecting pests. While controlling pathogens, RIPs also cause ectopic expression of pathogenesis-related proteins and trigger systemic acquired resistance. On the other hand, during abiotic stress, RIPs show antioxidant activity and trigger both enzyme-dependent and enzyme-independent metabolic pathways, alleviating abiotic stress such as drought, salinity, temperature, etc. RIPs express in response to specific environmental signals; therefore, their expression obviates additional physiological load on the transgenic plants instead of the constitutive expression. Based on evidence from its biological significance, ecological roles, laboratory- and controlled-environment success of its transgenics, and ethical merits, we unravel the potential of RIPs in developing transgenic plants showing co-tolerance to multiple environmental stresses.

Recombinant Ricin Toxin Binding Subunit B (RTB) Stimulates Production of TNF-α by Mouse Macrophages Through Activation of TLR4 Signaling Pathway

Ricin toxin binding subunit B (RTB) is a galactose-binding lectin protein derived from the beans of the castor oil plant (Ricinus communis). Our previous studies have reported a direct immunomodulatory effect of recombinant RTB, which stimulates RAW264.7 cells to produce cytokines including TNF-α. However, the role of RTB in innate immune response and its specific mechanism have not been reported in detail. In this work, the results showed that RTB treatment of macrophages significantly increased TLR4 protein levels. RTB also activated TLR4 downstream events, including MyD88, IRAK, and TRAF6, resulting in macrophage activation and TNF-α production. This process is reflected in the increase of IκB phosphorylation. TLR4 knockdown macrophages treated with RTB exhibited greatly reduced IκB phosphorylation and TNF-α secretion. Moreover, treatment with MyD88 inhibitor also suppressed TNF-α production. The docking of RT and TLR4 was simulated by computer, and the contact residues were concentrated on RTB. Our results suggest that recombinant RTB can activate mouse macrophages to secrete TNF-α through activation of NF-κB via the TLR4 signaling pathways.

How Ricin Damages the Ribosome

Ricin belongs to the group of ribosome-inactivating proteins (RIPs), i.e., toxins that have evolved to provide particular species with an advantage over other competitors in nature. Ricin possesses RNA N-glycosidase activity enabling the toxin to eliminate a single adenine base from the sarcin-ricin RNA loop (SRL), which is a highly conserved structure present on the large ribosomal subunit in all species from the three domains of life. The SRL belongs to the GTPase associated center (GAC), i.e., a ribosomal element involved in conferring unidirectional trajectory for the translational apparatus at the expense of GTP hydrolysis by translational GTPases (trGTPases). The SRL represents a critical element in the GAC, being the main triggering factor of GTP hydrolysis by trGTPases. Enzymatic removal of a single adenine base at the tip of SRL by ricin blocks GTP hydrolysis and, at the same time, impedes functioning of the translational machinery. Here, we discuss the consequences of SRL depurination by ricin for ribosomal performance, with emphasis on the mechanistic model overview of the SRL modus operandi.

Inhibition of ricin A-chain (RTA) catalytic activity by a viral genome-linked protein (VPg)

Ricin is a plant derived protein toxin produced by the castor bean plant (Ricinus communis). The Centers for Disease Control (CDC) classifies ricin as a Category B biological agent. Currently, there is neither an effective vaccine that can be used to protect against ricin exposure nor a therapeutic to reverse the effects once exposed. Here we quantitatively characterize interactions between catalytic ricin A-chain (RTA) and a viral genome-linked protein (VPg) from turnip mosaic virus (TuMV). VPg and its N-terminal truncated variant, VPg_1-110, bind to RTA and abolish ricin's catalytic depurination of 28S rRNA in vitro and in a cell-free rabbit reticulocyte translational system. RTA and VPg bind in a 1 to 1 stoichiometric ratio, and their binding affinity increases ten-fold as temperature elevates (5 °C to 37 °C). RTA-VPg binary complex formation is enthalpically driven and favored by entropy, resulting in an overall favorable energy, ΔG = -136.8 kJ/mol. Molecular modeling supports our experimental observations and predicts a major contribution of electrostatic interactions, suggesting an allosteric mechanism of downregulation of RTA activity through conformational changes in RTA structure, and/or disruption of binding with the ribosomal stalk. Fluorescence anisotropy studies show that heat affects the rate constant and the activation energy for the RTA-VPg complex, Ea = -62.1 kJ/mol. The thermodynamic and kinetic findings presented here are an initial lead study with promising results and provides a rational approach for synthesis of therapeutic peptides that successfully eliminate toxicity of ricin, and other cytotoxic RIPs.

Enzymatic Transition States and Drug Design

Transition state theory teaches that chemically stable mimics of enzymatic transition states will bind tightly to their cognate enzymes. Kinetic isotope effects combined with computational quantum chemistry provides enzymatic transition state information with sufficient fidelity to design transition state analogues. Examples are selected from various stages of drug development to demonstrate the application of transition state theory, inhibitor design, physicochemical characterization of transition state analogues, and their progress in drug development.

Fruticulosin: A novel type 2 ribosome-inactivating protein from Abrus fruticulosus seeds that exhibits toxic and antileishmanial activity

Plant ribosome-inactivating proteins (RIPs) are a family of toxins that inhibit protein synthesis. In this study, we have isolated a novel type 2 ribosome-inactivating protein (RIP) present in seeds of the Abrus fruticulosus, named of fruticulosin. Fruticulosin, shows characteristics common to other type 2 RIPs, as specificity by galactosides (d-galactose, N-acetyl-d-galactosamine, and d-lactose), mass of approximately 60 kDa and presence of the of disulfide bonds. The N-terminal amino acid sequence (26 residues) of A-chain fruticulosin, determined by Edman degradation, revealed high similarity of the A-chain with those of other type 2 RIPs. The secondary structure of fruticulosin was analysed by circular dichroism, which showed that fruticulosin contains α-helices (22.3%), β-sheets (43.5%), and random coils and corners (34.2%). Furthermore, fruticulosin showed high toxicity in Artemia sp. (3.12 μg/mL), inhibited in vitro protein synthesis by a cell-free system and showed RNA N-glycosidase activity. Fruticulosin presented biological activities such as agglutination and antileishmanial activity on promastigote forms of Leishmania major.

Purification, characterization and fine sugar specificity of a N-Acetylgalactosamine specific lectin from Adenia hondala

Plant lectins are gaining interest because of their interesting biological properties. Several Adenia species, that are being used in traditional medicine to treat many health ailments have shown presence of lectins or carbohydrate binding proteins. Here, we report the purification, characterization and biological significance of N-Acetyl galactosamine specific lectin from Adenia hondala (AHL) from Passifloraceae family. AHL was purified in a single step by affinity chromatography on asialofetuin Sepharose 4B column, characterized and its fine sugar specificity determined by glycan array analysis. AHL is human blood group non specific and also agglutinates rabbit erythrocytes. AHL is a glycoprotein with 12.5% of the carbohydrate, SDS-PAGE, MALDI-TOF-MS and ESI-MS analysis showed that AHL is a monomer of 31.6 kDa. AHL is devoid of DNase activity unlike other Ribosome inactivating proteins (RIPs). Glycan array analysis of AHL revealed its highest affinity for terminal lactosamine or polylactosamine of N- glycans, known to be over expressed in hepatocellular carcinoma and colon cancer. AHL showed strong binding to human hepatocellular carcinoma HepG2 cells with MFI of 59.1 expressing these glycans which was effectively blocked by 93.1% by asialofetuin. AHL showed dose and time dependent growth inhibitory effects on HepG2 cells with IC₅₀ of 4.8 μg/ml. AHL can be explored for its clinical potential.

Optimization of EnBase Fed-Batch Cultivation to Improve Soluble Fraction Ratio of α-Luffin Ribosome Inactivating Protein

Background

The increase of the protein expression via ribosomal manipulation is one of the suggested cellular mechanisms involved in EnBase fed-batch mode of cultivation. However, this system has not been implemented for cytotoxic proteins.

Objectives

Here, the expression pattern of α-Luffin, a ribosome inactivation protein (RIP) with an innate toxicity, was investigated in EnBase system and the effect of low temperature cultivation on the increase of α-Luffin solubility was determined.

Materials and Methods

The encoding cDNA for mature α-Luffin was synthesized and subcloned into pET28a plasmid under the control of T7 promoter. The E. coli expression yield in EnBase® Flo fed-batch system was compared with traditional batch mode at two temperatures: 25 °C and 30 °C. Sampling was performed at several time intervals and solubility of recombinant-protein was checked on SDS-PAGE in pellet and supernatant samples. The purification of recombinant protein was performed by Ni-NTA column.

Results

In fed-batch cultivation mode, the early incubation time was desirable at 30 °C whereas the maximum amount of soluble α-Luffin was achieved from the extended protein synthesis period (12 and 24h post induction) at 25 °C.

Conclusions

Our founding showed that EnBase had a greater efficacy in producing higher soluble protein ratios compared to batch cultivation growth rate, however for cytotoxic proteins, incubation temperature and time need to be optimized. Owing to the advantages of natural toxins from RIP family for producing anticancer immune-conjugates, well optimization of this protein expression is of importance regarding industrial aspects. The optimized condition proposed here is promising in terms of large scale soluble production of α-Luffin without the need for refolding.

Strategies to Improve the Clinical Utility of Saporin-Based Targeted Toxins

Plant Ribosome-inactivating proteins (RIPs) including the type I RIP Saporin have been used for the construction of Immunotoxins (ITxs) obtained via chemical conjugation of the toxic domain to whole antibodies or by generating genetic fusions to antibody fragments/targeting domains able to direct the chimeric toxin against a desired sub-population of cancer cells. The high enzymatic activity, stability and resistance to conjugation procedures and especially the possibility to express recombinant fusions in yeast, make Saporin a well-suited tool for anti-cancer therapy approaches. Previous clinical work on RIPs-based Immunotoxins (including Saporin) has shown that several critical issues must be taken into deeper consideration to fully exploit their therapeutic potential. This review focuses on possible combinatorial strategies (chemical and genetic) to augment Saporin-targeted toxin efficacy. Combinatorial approaches may facilitate RIP escape into the cytosolic compartment (where target ribosomes are), while genetic manipulations may minimize potential adverse effects such as vascular-leak syndrome or may identify T/B cell epitopes in order to decrease the immunogenicity following similar strategies as those used in the case of bacterial toxins such as Pseudomonas Exotoxin A or as for Type I RIP Bouganin. This review will further focus on strategies to improve recombinant production of Saporin-based chimeric toxins.

Ribosome Inactivating Proteins from an evolutionary perspective

Ribosome Inactivating Proteins (RIPs) are rRNA N-glycosidases that inhibit protein synthesis through the elimination of a single adenine residue from 28S rRNA. Many of these toxins have been characterized in depth from a biochemical and molecular point of view. In addition, their potential use in medicine as highly selective toxins is being explored. In contrast, the evolutionary history of RIP encoding genes has remained traditionally underexplored. In recent years, accumulation of large genomic data has fueled research on this issue and revealed unexpected information about the origin and evolution of RIP toxins. In this review we summarize the current evidence available on the occurrence of different evolutionary mechanisms (gene duplication and losses, horizontal gene transfer, synthesis de novo and domain combination) involved in the evolution of the RIP gene family. Finally, we propose a revised nomenclature for RIP genes based on their evolutionary history.

Neuroplasticity and Repair in Rodent Neurotoxic Models of Spinal Motoneuron Disease

Retrogradely transported toxins are widely used to set up protocols for selective lesioning of the nervous system. These methods could be collectively named "molecular neurosurgery" because they are able to destroy specific types of neurons by using targeted neurotoxins. Lectins such as ricin, volkensin, or modeccin and neuropeptide- or antibody-conjugated saporin represent the most effective toxins used for neuronal lesioning. Some of these specific neurotoxins could be used to induce selective depletion of spinal motoneurons. In this review, we extensively describe two rodent models of motoneuron degeneration induced by volkensin or cholera toxin-B saporin. In particular, we focus on the possible experimental use of these models to mimic neurodegenerative diseases, to dissect the molecular mechanisms of neuroplastic changes underlying the spontaneous functional recovery after motoneuron death, and finally to test different strategies of neural repair. The potential clinical applications of these approaches are also discussed.

Purification and partial characterization of a ribosome-inactivating protein from the latex of Euphorbia trigona Miller with cytotoxic activity toward human cancer cell lines

BACKGROUND

The objective of this study is to investigate the cytotoxic activity of three isolectins purified from the latex of Euphorbia trigona Miller.

HYPOTHESIS

Among lectins are the ribosome-inactivating proteins (RIPs), which are potent inhibitors of protein synthesis in cells and in cell-free systems.

RESULTS

Three isolectins, ETR1, ETR2 and ETR3, were purified by anion exchange chromatography. Both ETR1 and ETR3 yielded a single band on SDS-PAGE under reducing conditions, corresponding to a molecular weight of 32 g mol(-1), while ETR2 yielded two bands corresponding to 31 and 33 g mol(-1). When non-reducing conditions were used molecular weight decreased, indicating the presence of intrachain disulfide bonds. Size-exclusion chromatography revealed proteins of apparent molecular weight of 59-63 g mol(-1), suggesting a dimeric nature, with subunits not being held together by disulfide linkage. ETR1, ETR2 and ETR3 hemagglutinated human, sheep and rat erythrocytes and this hemagglutination was specifically inhibited by galactose and its derivatives. The lectins studied were thermostable up to 60 °C and their observed activity was maintained across pH range 5-12. These lectins, from the latex of Euphorbia trigona, are potent inhibitors of eukaryotic protein synthesis in a cell-free system. Flow cytometry analysis revealed the antiproliferative activity of them toward A549, HeLa, H116, HL-60 cell lines.

CONCLUSION

Euphorbia trigona isolectins are RIPs with cytotoxic activity toward human cancer cell lines.

Ribosome-inactivating and related proteins

Ribosome-inactivating proteins (RIPs) are toxins that act as N-glycosidases (EC 3.2.2.22). They are mainly produced by plants and classified as type 1 RIPs and type 2 RIPs. There are also RIPs and RIP related proteins that cannot be grouped into the classical type 1 and type 2 RIPs because of their different sizes, structures or functions. In addition, there is still not a uniform nomenclature or classification existing for RIPs. In this review, we give the current status of all known plant RIPs and we make a suggestion about how to unify those RIPs and RIP related proteins that cannot be classified as type 1 or type 2 RIPs.

Quantification of ricin, RCA and comparison of enzymatic activity in 18 Ricinus communis cultivars by isotope dilution mass spectrometry

The seeds of the Ricinus communis (Castor bean) plant are the source of the economically important commodity castor oil. Castor seeds also contain the proteins ricin and R. communis agglutinin (RCA), two toxic lectins that are hazardous to human health. Radial immunodiffusion (RID) and the enzyme linked immunosorbent assay (ELISA) are two antibody-based methods commonly used to quantify ricin and RCA; however, antibodies currently used in these methods cannot distinguish between ricin and RCA due to the high sequence homology of the respective proteins. In this study, a technique combining antibody-based affinity capture with liquid chromatography and multiple reaction monitoring (MRM) mass spectrometry (MS) was used to quantify the amounts of ricin and RCA independently in extracts prepared from the seeds of eighteen representative cultivars of R. communis which were propagated under identical conditions. Additionally, liquid chromatography and MRM-MS was used to determine rRNA N-glycosidase activity for each cultivar and the overall activity in these cultivars was compared to a purified ricin standard. Of the cultivars studied, the average ricin content was 9.3 mg/g seed, the average RCA content was 9.9 mg/g seed, and the enzymatic activity agreed with the activity of a purified ricin reference within 35% relative activity.

Pokeweed antiviral protein, a ribosome inactivating protein: activity, inhibition and prospects

Viruses employ an array of elaborate strategies to overcome plant defense mechanisms and must adapt to the requirements of the host translational systems. Pokeweed antiviral protein (PAP) from Phytolacca americana is a ribosome inactivating protein (RIP) and is an RNA N-glycosidase that removes specific purine residues from the sarcin/ricin (S/R) loop of large rRNA, arresting protein synthesis at the translocation step. PAP is thought to play an important role in the plant's defense mechanism against foreign pathogens. This review focuses on the structure, function, and the relationship of PAP to other RIPs, discusses molecular aspects of PAP antiviral activity, the novel inhibition of this plant toxin by a virus counteraction-a peptide linked to the viral genome (VPg), and possible applications of RIP-conjugated immunotoxins in cancer therapeutics.

Sequence comparison and phylogenetic analysis by the Maximum Likelihood method of ribosome-inactivating proteins from angiosperms

Ribosome-inactivating proteins (RIPs) from angiosperms are rRNA N-glycosidases that have been proposed as defence proteins against virus and fungi. They have been classified as type 1 RIPs, consisting of single-chain proteins, and type 2 RIPs, consisting of an A chain with RIP properties covalently linked to a B chain with lectin properties. In this work we have carried out a broad search of RIP sequence data banks from angiosperms in order to study their main structural characteristics and phylogenetic evolution. The comparison of the sequences revealed the presence, outside of the active site, of a novel structure that might be involved in the internal protein dynamics linked to enzyme catalysis. Also the B-chains presented another conserved structure that might function either supporting the beta-trefoil structure or in the communication between both sugar-binding sites. A systematic phylogenetic analysis of RIP sequences revealed that the most primitive type 1 RIPs were similar to that of the actual monocots (Poaceae and Asparagaceae). The primitive RIPs evolved to the dicot type 1 related RIPs (like those from Caryophyllales, Lamiales and Euphorbiales). The gene of a type 1 RIP related with the actual Euphorbiaceae type 1 RIPs fused with a double beta trefoil lectin gene similar to the actual Cucurbitaceae lectins to generate the type 2 RIPs and finally this gene underwent deletions rendering either type 1 RIPs (like those from Cucurbitaceae, Rosaceae and Iridaceae) or lectins without A chain (like those from Adoxaceae).

Cucurmosin induces the apoptosis of human pancreatic cancer CFPAC-1 cells by inactivating the PDGFR-β signalling pathway

BACKGROUND

Pancreatic cancer treatment is limited and effective drugs are needed. We investigated cucurmosin (CUS)-induced apoptosis in cystic fibrosis pancreatic adenocarcinoma cells (CFPAC-1) and a possible mechanism of action to evaluate the clinical application potential of this new Type I ribosome-inactivating protein.

METHODS

We analyzed the growth inhibition and apoptosis of CFPAC-1 cells via methylthiazol tetrazolium assay and fluorescence-activated cell sorting. Western blot was used to analyze the protein levels of caspase 3, bcl-2, caspase 9, platelet-derived growth factor receptor (PDGFR)-β, PI3K, Akt, p-Akt, the mammalian target of rapamycin (mTOR), p-mTOR, P70S6K-α, p-P70S6K-α, 4E-BP1, p-4E-BP1 and p-Bad after CUS intervention. The mRNA expression of PDGFR-β was analyzed using reverse transcription polymerase chain reaction.

RESULTS

CUS inhibited the proliferation of pancreatic cancer cells. The induction of apoptosis depended on the CUS dose and incubation time. The drug inhibited all of the examined proteins in the PI3K/Akt/mTOR signalling pathway and induced the active fragments of caspase 3 and caspase 9. CUS downregulated PDGFR-β expression but no significant change was observed at the mRNA level.

CONCLUSION

CUS strongly inhibits the growth of CFPAC-1 by inducing cell apoptosis. CUS downregulated the expression of PDGFR-β at the protein level and induced the apoptosis of CFPAC-1 through the PI3K/Akt/mTOR signalling pathway.

PANC-1 pancreatic cancer cell growth inhibited by cucurmosin alone and in combination with an epidermal growth factor receptor-targeted drug

OBJECTIVES

To investigate the inhibition of PANC-1 pancreatic cancer cell growth by cucurmosin (CUS) and its possible mechanism.

METHODS

We observed the inhibition of PANC-1 cell growth by sulforhodamine B and colony-forming experiments in vitro and established nonobese diabetic/severe combined immunodeficiency mouse subcutaneous tumor models in vivo. We used Western blot to analyze protein levels related to apoptosis and epidermal growth factor receptor (EGFR) signaling pathways after drug intervention, whereas the messenger RNA expression of EGFR was analyzed by quantitative real-time polymerase chain reaction.

RESULTS

Sulforhodamine B and colony-forming experiments indicated that CUS inhibited PANC-1 cell proliferation in a dose- and time-dependent manner. A stronger inhibitory effect was observed when CUS was combined with gefitinib. The subcutaneous tumor growth was also inhibited. Western blot showed that all the examined proteins decreased, except for 4E-BP1 and the active fragments of caspase 3 and caspase 9 increased. Epidermal growth factor receptor expression did not change significantly in quantitative real-time polymerase chain reaction.

CONCLUSIONS

Cucurmosin can strongly inhibit the growth of PANC-1 cells in vitro and in vivo. Cucurmosin can down-regulate EGFR protein expression, but not at the messenger RNA level. Cucurmosin can also inhibit the ras/raf and phosphatidylinositol 3-kinase/Akt downstream signaling pathways and enhance the sensitivity of the EGFR-targeted drug gefitinib.

Targeted cytolysins synergistically potentiate cytoplasmic delivery of gelonin immunotoxin

Targeted endocytic uptake is a first step toward tissue-specific cytoplasmic macromolecular delivery; however, inefficient escape from the endolysosomal compartment makes this generally impractical at present. We report here a targeted cytolysin approach that dramatically potentiates endosomal release of an independently targeted potent gelonin immunotoxin. Fibronectin domains engineered for affinity to EGF receptor or carcinoembryonic antigen were fused to the plant toxin gelonin or bacterial pore-forming cytolysins. These fusion proteins display synergistic activity in both antigen-specific cytotoxicity in vitro, enhancing potency by several orders of magnitude, and in tumor growth inhibition in vivo. In addition, the number of internalized gelonin molecules required to induce apoptosis is reduced from approximately 5 × 10(6) to less than 10(3). Targeted potentiation shows promise for enhancing cytoplasmic delivery of other macromolecular payloads such as DNA, siRNA, and miRNA.

Activation of RAW264.7 mouse macrophage cells in vitro through treatment with recombinant ricin toxin-binding subunit B: involvement of protein tyrosine, NF-κB and JAK-STAT kinase signaling pathways

Ricin toxin-binding subunit B (RTB) is a galactose-binding lectin protein. In the present study, we investigated the effects of RTB on inducible nitric oxide (NO) synthase (iNOS), interleukin (IL)-6 and tumor necrosis factor (TNF)-α, as well as the signal transduction mechanisms involved in recombinant RTB-induced macrophage activation. RAW264.7 macrophages were treated with RTB. The results revealed that the mRNA and protein expression of iNOS was increased in the recombinant RTB-treated macrophages. TNF-α production was observed to peak at 20 h, whereas the production of IL-6 peaked at 24 h. In another set of cultures, the cells were co-incubated with RTB and the tyrosine kinase inhibitor, genistein, the phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, the p42/44 inhibitor, PD98059, the p38 inhibitor, SB203580, the JNK inhibitor, SP600125, the protein kinase C (PKC) inhibitor, staurosporine, the JAK2 inhibitor, tyrphostin (AG490), or the NOS inhibitor, L-NMMA. The recombinant RTB-induced production of NO, TNF-α and IL-6 was inhibited in the macrophages treated with the pharmacological inhibitors genistein, LY294002, staurosporine, AG490, SB203580 and BAY 11-7082, indicating the possible involvement of protein tyrosine kinases, PI3K, PKC, JAK2, p38 mitogen-activated protein kinase (MAPK) and nuclear factor (NF)-κB in the above processes. A phosphoprotein analysis identified tyrosine phosphorylation targets that were uniquely induced by recombinant RTB and inhibited following treatment with genistein; some of these proteins are associated with the downstream cascades of activated JAK-STAT and NF-κB receptors. Our data may help to identify the most important target molecules for the development of novel drug therapies.

Immunomodulatory activity of recombinant Ricin toxin binding Subunit B (RTB)

Ricin toxin binding subunit B (RTB) is one of the subunits of the ricin protein. RTB has been used as adjuvant, but little is known about its mechanism. In this study, we found that RTB increased not only nitric oxide (NO) release, but also tumor necrosis factor (TNF)-α and interleukin (IL)-6 production in mouse macrophage cell line RAW264.7 cells. They subsequently exhibited enhanced ConA-induced T-cell and LPS-induced B-cell proliferative responses. We also examined the cytokines that were produced from splenocytes following in vitro RTB administration. Increased levels of IL-2, interferon (IFN)-γ and TNF-α were observed, while IL-4 and IL-5 were unaffected. These results demonstrate that recombinant RTB can act on the immune system and activate T-cells by introducing a Th1 immune response. Th1 cells might be the primary cellular target affected by RTB. Our results suggest that the recombinant RTB can promote the activation of macrophages and has a beneficial effect on immunomodulatory activity.

Anti-tumor action of trichosanthin, a type 1 ribosome-inactivating protein, employed in traditional Chinese medicine: a mini review

Trichosanthin (TCS) as a midterm abortifacient medicine has been used clinically in traditional Chinese medicine for centuries. Additionally, TCS manifests a host of pharmacological properties, for instance, anti-HIV and anti-tumor activities. TCS has been reported to inhibit cell growth of a diversity of cancers, including cervical cancer, choriocarcinoma, and leukemia/lymphoma, etc. This article purported to review the various anti-tumor activities of TCS and the mechanism of apoptosis it induced in these tumor cells. These research progresses provide an insight into cancer research and treatment as well as disclose new pharmacological properties of the ancient but popular Chinese medicine.

Distinct cellular responses induced by saporin and a transferrin-saporin conjugate in two different human glioblastoma cell lines

Glioblastoma multiforme (GBM) is the most common primary brain tumour in adults, with a median survival of ~12-18 months post-diagnosis. GBM usually recurs within 12 months post-resection, with poor prognosis. Thus, novel therapeutic strategies to target and kill GBM cells are urgently needed. The marked difference of tumour cells with respect to normal brain cells renders glioblastoma a good candidate for selective targeted therapies. Recent experimental strategies focus on over expressed cell surface receptors. Targeted toxins represent a new class of selective molecules composed by a potent protein toxin and a carrier ligand. Targeted toxins approaches against glioblastoma were under investigation in phase I and II clinical trials with several immunotoxins (IT)/ligand toxins such as IL4-Pseudomonas aeruginosa exotoxin A (IL4-PE, NBI-3001), tumour growth factor fused to PE38, a shorter PE variant, (TGF)alpha-TP-38, IL13-PE38, and a transferrin-C diphtheriae toxin mutant (Tf-CRM107). In this work, we studied the effects of the plant ribosome-inactivating saporin and of its chimera transferrin-saporin against two different GBM cell lines. The data obtained here indicate that cell proliferation is affected by the toxin treatments but that different mechanisms are used, directly linked to the presence of an active or inactive p53. A model is proposed for these alternative intracellular pathways.

Balsamin, a novel ribosome-inactivating protein from the seeds of Balsam apple Momordica balsamina

Plant seeds, a rich source of proteins, are considered important for their application as functional ingredients in a food system. A novel ribosome-inactivating protein (RIP), balsamin was purified from the seeds of Balsam apple, Momordica balsamina. Balsamin was purified by ion exchange chromatography on CM Sepharose and gel filtration on superdex-75. It has a molecular weight of 28 kDa as shown by SDS-PAGE analysis. Balsamin inhibits protein synthesis in a rabbit reticulocyte lysate-based cell free translation assay with an IC(50) of 90.6 ng ml(-1). It has RNA N-glycosidase activity and releases a 400-base long fragment termed the Endo fragment from 28S rRNA in the same manner as does saporin-6 from Saponaria officinalis. The N-terminal sequence analysis of the first 12 amino acids of balsamin revealed that it shares 83% similarity with type I RIP α-MMC from Momordica charantia and 50% similarity with β-MMC (from Momordica charantia), bryodin I (from Bryonia dioica) and luffin a (from Luffa cylindrica). Balsamin was further characterized by mass spectrometry. CD spectroscopic studies indicate that secondary structure of balsamin contains helix (23.5%), β-strand (24.6%), turn (20%) and random coil (31.9%). Thus RIPs activity expressed in vegetables like Momordica sp. advocates its usage in diet.

Structure-based design of ricin inhibitors

Ricin is a potent cytotoxin easily purified in large quantities. It presents a significant public health concern due to its potential use as a bioterrorism agent. For this reason, extensive efforts have been underway to develop antidotes against this deadly poison. The catalytic A subunit of the heterodimeric toxin has been biochemically and structurally well characterized, and is an attractive target for structure-based drug design. Aided by computer docking simulations, several ricin toxin A chain (RTA) inhibitors have been identified; the most promising leads belonging to the pterin family. Development of these lead compounds into potent drug candidates is a challenging prospect for numerous reasons, including poor solubility of pterins, the large and highly polar secondary binding pocket of RTA, as well as the enzyme’s near perfect catalytic efficiency and tight binding affinity for its natural substrate, the eukaryotic ribosome. To date, the most potent RTA inhibitors developed using this approach are only modest inhibitors with apparent IC₅₀ values in the 10⁻⁴ M range, leaving significant room for improvement. This review highlights the variety of techniques routinely employed in structure-based drug design projects, as well as the challenges faced in the design of RTA inhibitors.

Use of ribosome-inactivating proteins from Sambucus for the construction of immunotoxins and conjugates for cancer therapy

The type 2 ribosome-inactivating proteins (RIPs) isolated from some species belonging to the Sambucus genus, have the characteristic that although being even more active than ricin inhibiting protein synthesis in cell-free extracts, they lack the high toxicity of ricin and related type 2 RIPs to intact cells and animals. This is due to the fact that after internalization, they follow a different intracellular pathway that does not allow them to reach the cytosolic ribosomes. The lack of toxicity of type 2 RIPs from Sambucus make them good candidates as toxic moieties in the construction of immunotoxins and conjugates directed against specific targets. Up to now they have been conjugated with either transferrin or anti-CD105 to target either transferrin receptor- or endoglin-overexpressing cells, respectively.

Ribosome-inactivating proteins: from plant defense to tumor attack

Ribosome-inactivating proteins (RIPs) are EC3.2.32.22 N-glycosidases that recognize a universally conserved stem-loop structure in 23S/25S/28S rRNA, depurinating a single adenine (A4324 in rat) and irreversibly blocking protein translation, leading finally to cell death of intoxicated mammalian cells. Ricin, the plant RIP prototype that comprises a catalytic A subunit linked to a galactose-binding lectin B subunit to allow cell surface binding and toxin entry in most mammalian cells, shows a potency in the picomolar range. The most promising way to exploit plant RIPs as weapons against cancer cells is either by designing molecules in which the toxic domains are linked to selective tumor targeting domains or directly delivered as suicide genes for cancer gene therapy. Here, we will provide a comprehensive picture of plant RIPs and discuss successful designs and features of chimeric molecules having therapeutic potential.

Noradrenergic neurons of the area postrema mediate amylin's hypophagic action

Circulating amylin inhibits food intake via activation of the area postrema (AP). The aim of this study was to identify the neurochemical phenotype of the neurons mediating amylin's hypophagic action by immunohistochemical and feeding studies in rats. Expression of c-Fos protein was used as a marker for neuronal activation and dopamine-β-hydroxylase (DBH), the enzyme-catalyzing noradrenaline synthesis, as a marker for noradrenergic neurons. We found that ∼50% of amylin-activated AP neurons are noradrenergic. To clarify the functional role of these neurons in amylin's effect on eating, noradrenaline-containing neurons in the AP were lesioned using a saporin conjugated to an antibody against DBH. Amylin (5 or 20 μg/kg sc)-induced anorexia was observed in sham-lesioned rats with both amylin doses. Rats with a lesion of > 50% of the noradrenaline neurons were unresponsive to the low dose of amylin (5 μg/kg) and only displayed a reduction in food intake 60 min after injection of the high amylin dose (20 μg/kg). In a terminal experiment, the same rats received amylin (20 μg/kg) or saline. The AP and nucleus of the solitary tract (NTS) were stained for DBH to assess noradrenaline lesion success and for c-Fos expression to evaluate amylin-induced neuronal activation. In contrast to sham-lesioned animals, noradrenaline-lesioned rats did not show a significant increase in amylin-induced c-Fos expression in the AP and NTS. We conclude that the noradrenergic neurons in the AP mediate at least part of amylin's hypophagic effect.

Cloning and soluble expression of mature alpha-luffin from Luffa cylindrica and its antitumor activities in vitro

Luffin-a, a single-chain Type I ribosome-inactivating protein, which is known to be the most toxic of the luffin family and apparently possesses antitumor activity, was isolated from Luffa cylindrica seeds. In the present study, mature alpha-luffin was cloned from L. cylindrica and it was found that mature alpha-luffin shared 96% amino acid similarity with luffin-a. The recombinant mature alpha-luffin was successfully expressed in a partly soluble form in Escherichia coli after optimization of expression conditions. The effects of the recombinant protein on bacterial growth and its in vitro protein synthesis inhibition activity were tested. Then, its antitumor activities against different human cancer cell lines were evaluated by CCK-8 assay and flow cytometry. The results indicated that the recombinant alpha-luffin was slightly toxic to E. coli. It could inhibit protein synthesis in the rabbit reticulocyte lysate system. At the same time, it inhibited the growth of the tumor cell lines in a dose- and time-dependent manner. Additionally, recombinant alpha-luffin was able to induce cell death by apoptosis. The cytotoxicity of alpha-luffin towards tumor cells makes it a potential antitumor agent.

Differential abilities of the mushroom ribosome-inactivating proteins hypsin and velutin to perturb normal development of cultured mouse embryos

The teratogenicity of two fungal ribosome-inactivating proteins, hypsin from Hypsizigus mamoreus and velutin from Flammulina velutipes, was examined in this investigation using microinjection and postimplantation whole-embryo culture. The results demonstrated that hypsin induced abnormal embryonic development at 2.5 microM during the organogenesis period from E8.5 to E9.5. As its dosage increased, there was an increase in the total number of abnormal embryos, a drop in the final somite number, and a rise of abnormal structures. Structural abnormalities were detected: open cranial neural tube, abnormal branchial arches, absence of forelimb buds and twisted body axis. The otic and optic placodes were, however, less affected. Histological study of the abnormal embryos revealed a correlation of increased cell death with abnormal structures, suggesting that induction of cell death by hypsin may account for its teratogenicity. In contrast, velutin did not exert any adverse influence on mouse development.

Disinfection of waterborne coliform bacteria using Luffa cylindrica fruit and seed extracts

This study examined the potential of extracts of the plant Luffa cylindrica to act as disinfectants of drinking water. Aqueous extracts of the seeds and fruit of L. cylindrica were tested against total and faecal coliform bacteria in surface water by varying the extract doses and contact times. Inactivation of both faecal coliforms and total coliforms was highly variable and dose-dependent. The maximum coliform inactivation achieved in any trial was 86%. Fruit extracts were more successful at inactivating total coliforms than faecal coliforms. Seed extracts achieved higher coliform inactivation levels than fruit extracts generally. Overall, the antimicrobial potential of seeds and fruit from L. cylindrica was demonstrated; however the disinfection performance was less than would be required for these extracts to be considered reliable disinfectants for drinking water treatment.

Plant oxylipins: role of jasmonic acid during programmed cell death, defence and leaf senescence

Plants are continuously challenged by a variety of abiotic and biotic cues. To deter feeding insects, nematodes and fungal and bacterial pathogens, plants have evolved a plethora of defence strategies. A central player in many of these defence responses is jasmonic acid. It is the aim of this minireview to summarize recent findings that highlight the role of jasmonic acid during programmed cell death, plant defence and leaf senescence.

Autologous bone marrow transplantation with immunotoxin-purged marrow for advanced multiple myeloma

A system to purge the bone marrow of myeloma cells has been developed in our laboratories with the aim of treating with myeloablative radiochemotherapy patients suffering from advanced multiple myeloma. This system is based on the ex vivo incubation of the marrow with an immunotoxin composed of the 8A monoclonal antibody--that recognizes plasma cells and B-cell precursors--and the ribosome-inactivating protein momordin. 8 patients have so far been treated. 4 are surviving from 4 to 18 months after ABMT, whereas 4 died after 1 to 6 months, 2 from infections, 1 from relapsing disease and 1 from veno-occlusive disease. A marked tumour reduction was observed in all evaluable patients; however, none has achieved complete disappearance of the disease. The haemopoietic reconstitution was significantly delayed in 3 patients. These preliminary results show the feasibility of this approach in advanced MM patients with heavily infiltrated marrow. The place of ABMT in the treatment of MM remains to be determined; the selection of patients with still responding and less advanced disease would probably produce better results.

The X-ray structure of ricin A chain with a novel inhibitor

Ricin is a potent heterodimeric cytotoxin; the B chain binds eucaryotic cell surfaces aiding uptake and the A chain, RTA, reaches the cytoplasm where it enzymatically depurinates a key ribosomal adenine, inhibiting protein synthesis. Ricin is known to be an agent in bioterrorist repertoires and there is great interest in finding, or creating, efficacious inhibitors of the toxin as potential antidotes. We have previously identified two families of bicyclic RTA inhibitors, pterins and purines. Both classes have poor solubility which impairs inhibitor development. Here we report the use of 2-amino-4,6-dihydroxy-pyrimidines as RTA inhibitors. Unlike previously observed single ring inhibitor platforms, these displace Tyr 80 and bind deep in the RTA specificity pocket. These compounds are at least 10 times more soluble than pterin-based inhibitors and appear to be useful new class of ricin inhibitors.

Purification, characterization, and sequencing of antimicrobial peptides, Cy-AMP1, Cy-AMP2, and Cy-AMP3, from the Cycad (Cycas revoluta) seeds

Novel antimicrobial peptides (AMP), designated Cy-AMP1, Cy-AMP2, and Cy-AMP3, were purified from seeds of the cycad (Cycas revoluta) by a CM cellulofine column, ion-exchange HPLC on SP COSMOGEL, and reverse-phase HPLC. They had molecular masses of 4583.2 Da, 4568.9 Da and 9275.8 Da, respectively, by MALDI-TOF MS analysis. Half of the amino acid residues of Cy-AMP1 and Cy-AMP2 were cysteine, glycine and proline, and their sequences were similar. The sequence of Cy-AMP3 showed high homology to various lipid transfer proteins. For Cy-AMP1 and Cy-AMP2, the concentrations of peptides required for 50% inhibition (IC(50)) of the growth of plant pathogenic fungi, Gram-positive and Gram-negative bacteria were 7.0-8.9 microg/ml. The Cy-AMP3 had weak antimicrobial activity. The structural and antimicrobial characteristics of Cy-AMP1 and Cy-AMP2 indicated that they are a novel type of antimicrobial peptide belonging to a plant defensin family.

Protective effects of anti-ricin A-chain RNA aptamer against ricin toxicity

AIM: To investigate the therapeutic potential of an RNA ligand (aptamer) specific for the catalytic ricin A-chain (RTA), the protective effects of a 31-nucleotide RNA aptamer (31RA), which formed a high affinity complex with RTA, against ricin-induced toxicity in cell-based luciferase translation and cell cytotoxicity assays were evaluated.

METHODS: To test the therapeutic potential of anti-RTA aptamers in Chinese hamster ovary (CHO) AA8 cells stably transfected with a tetracycline regulatable promoter, ricin ribotoxicity was measured using luciferase and ricin-induced cytotoxicity was ascertained by MTS cell proliferation assay with tetrazolium compound [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium].

RESULTS: Inhibition of protein synthesis by ricin in CHO AA8 cells resulted in diminished luciferase activity and treatment with polyclonal antibody against deglycosylated RTA (dgA) neutralized the inhibitory effects of ricin on luciferase activity and protected against ricin-induced cytotoxicity as measured by MTS assay. The 31RA anti-RTA aptamer inhibited the translation of luciferase mRNA in cell-free reticulocyte translation assay. 31RA aptamer also partially neutralized the inhibitory effects of ricin on luciferase activity and partially protected against ricin-induced cytotoxicity in CHO AA8 cells.

CONCLUSION: We have shown that anti-RTA RNA aptamer can protect against ricin ribotoxicity in cell-based luciferase and cell cytotoxicity assays. Hence, RNA aptamer that inhibits RTA enzymatic activity represents a novel class of nucleic acid inhibitor that has the potential to be developed as a therapeutic agent for the treatment of ricin intoxication.

Inhalation Toxicology of Ricin Preparations: Animal Models, Prophylactic and Therapeutic Approaches to Protection

Ricin is a toxin and seed protein produced by the castor oil plant, Ricinus communis. The toxin is a dimeric protein consisting of an enzymic A chain and a B chain with lectin properties aiding the uptake of the whole molecule into cells. Ricin has been considered a possible military threat for several decades and is now also of some concern as a terrorist agent. The inhalation route is of primary concern in these situations, although previous attacks with ricin have used other approaches. Medical countermeasures against ricin are urgently required and the strategy adopted has been first to understand the nature of the problem, in this case the inhalation toxicology of ricin, followed by the preparation of vaccine antigens. Toxoided ricin and modified recombinant A chain components have been examined in terms of efficacy as potential vaccine candidates in protection of animal models against inhaled ricin, primarily in laboratories both in the United Kingdom and in the United States. One recombinant A chain vaccine has been taken through to clinical trials in the United States and should become commercially available in the next few years. Toxoided ricin has also been used as an antigen to prepare antitoxin antibodies for therapeutic treatment following poisoning. In this review, a synopsis of the inhalation toxicology of ricin and approaches to medical prophylaxis and therapy of poisoning is given, based on work conducted at our laboratory and at other research institutes.

Effect of food matrices on the biological activity of ricin

A cell-free translation assay was applied for the quick detection of ricin in food samples. Three economically important foods-ground beef, low-fat milk, and liquid chicken egg--were tested. The results indicated that ground beef had very little matrix effect on the assay, whereas low-fat milk and liquid chicken egg showed clear interference on the protein translation. A simple dilution in phosphate-buffered saline (PBS) effectively eliminated the translational inhibition from these foods. The concentrations inhibiting 50% of luciferase translation derived from the current study were 0.01 nM for the pure ricin A chain, 0.02 nM for pure ricin, and 0.087 nM for crude ricin in PBS. In most cases, the half inhibitory concentration values for ricin in food matrices were significantly lower than for those in PBS buffer, suggesting that some components in these food matrices might potentiate the activity of ricin. Thermal stability tests indicated that the ricin A chain was the least stable among the three forms of ricin in all matrices measured. The thermal stability of pure and crude ricins varied depending on the matrices. The specific activities of ricin in PBS buffer were confirmed by a neutralization test with ricin-specific and nonspecific antibodies. This study demonstrates that the cell-free translation assay is a rapid and sensitive method for detection of biologically active ricin toxin in ground beef, low-fat milk, and liquid chicken egg and that food matrices can greatly affect the thermal stability of ricin.