Chemically and biologically synthesized CPP-modified gelonin for enhanced anti-tumor activity

The ineffectiveness of small molecule drugs against cancer has generated significant interest in more potent macromolecular agents. Gelonin, a plant-derived toxin that inhibits protein translation, has attracted much attention in this regard. Due to its inability to internalize into cells, however, gelonin exerts only limited tumoricidal effect. To overcome this cell membrane barrier, we modified gelonin, via both chemical conjugation and genetic recombination methods, with low molecular weight protamine (LMWP), a cell-penetrating peptide (CPP) which was shown to efficiently ferry various cargoes into cells. Results confirmed that gelonin-LMWP chemical conjugate (cG-L) and recombinant gelonin-LMWP chimera (rG-L) possessed N-glycosidase activity equivalent to that of unmodified recombinant gelonin (rGel); however, unlike rGel, both gelonin-LMWPs were able to internalize into cells. Cytotoxicity studies further demonstrated that cG-L and rG-L exhibited significantly improved tumoricidal effects, with IC50 values being 120-fold lower than that of rGel. Moreover, when tested against a CT26 s.c. xenograft tumor mouse model, significant inhibition of tumor growth was observed with rG-L doses as low as 2 μg/tumor, while no detectable therapeutic effects were seen with rGel at 10-fold higher doses. Overall, this study demonstrated the potential of utilizing CPP-modified gelonin as a highly potent anticancer drug to overcome limitations of current chemotherapeutic agents.

Curcin C, a novel type I ribosome-inactivating protein from the post-germinating cotyledons of Jatropha curcas

A novel type I ribosome-inactivating protein (RIP), designated as curcin C, was purified from Jatropha curcas, an important feedback source of bio-fuel. Molecular mass and isoelectric point of curcin C were 31.398 kDa and 7.12 as detected by MALTI-TOF assay and capillary electrophoresis assay, respectively. N-terminal sequence and LC-MS/MS analyses confirmed that curcin C is a type I RIP having high homology, but not the exactly the same with curcin, another type 1 RIP isolated from the endosperm of J. curcas. It exhibited N-glycosidase activity and in vitro translation inhibition activity. Moreover, curcin C displayed a strong selectively anti-tumor activity on human cancer cells. Its cytotoxicity against osteosarcoma cell line U20S is even higher than that of Paclitaxel with IC₅₀ of 0.019 μM. Purification and identification of curcin C not only suggested its potential in natural anticancer drug development, but also provide chance to understanding different cytotoxic action among different RIPs.

A comparison between constitutive and inducible transgenic expression of the PhRIP I gene for broad-spectrum resistance against phytopathogens in potato

OBJECTIVES

To engineer broad spectrum resistance in potato using different expression strategies.

RESULTS

The previously identified Ribosome-Inactivating Protein from Phytolacca heterotepala was expressed in potato under a constitutive or a wound-inducible promoter. Leaves and tubers of the plants constitutively expressing the transgene were resistant to Botrytis cinerea and Rhizoctonia solani, respectively. The wound-inducible promoter was useful in driving the expression upon wounding and fungal damage, and conferred resistance to B. cinerea. The observed differences between the expression strategies are discussed considering the benefits and features offered by the two systems.

CONCLUSIONS

Evidence is provided of the possible impact of promoter sequences to engineer BSR in plants, highlighting that the selection of a suitable expression strategy has to balance specific needs and target species.

A novel adenine-releasing assay for ribosome-inactivating proteins

Ribosome-inactivating proteins (RIPs) are toxic enzymes that are mostly biosynthesized by plants. RIPs are N-glycosidases that cleave an essential adenine molecule from the 28S rRNA. This is followed by the irreversible inhibition of protein synthesis leading to cell death. By fusing RIPs to cancer cell specific targeting ligands RIPs have been utilized for targeted anti-tumor therapy. The anti-tumoral efficiency of such conjugates depends significantly on the N-glycosidase activity of the RIP domain. Different methods have been developed in order to determine the N-glycosidase activity of RIPs and RIP domain containing anti-tumor toxins. However the existing methods are elaborate and include radioassays, HPLC and enzymatic conversion assays. Here, a simple and cost effective N-glycosidase assay is presented, which is based on the direct determination of the released adenine by thin-layer chromatography (TLC) and TLC-densitometry. An adenine based single stranded oligonucleotide is used as substrate. Following TLC development the released adenine is quantified on silica glass plates by UV absorbance at 260nm.

Treatment of unresectable stage IV metastatic melanoma with aviscumine after anti-neoplastic treatment failure: a phase II, multi-centre study

Background

Aviscumine, a recombinant plant protein, is an immune modulator that induces ribotoxic stress at the 28S ribosomal RNA subunit. In this way cytokine release and T-cell responses are enhanced. This phase II trial was conducted to test the efficacy and safety of aviscumine in patients with systemically pre-treated metastatic melanoma stage IV.

Methods

A total of 32 patients with progressive stage IV melanoma after failure of standard therapy were enrolled onto a single-arm, multi-centre, open-label, phase II trial. All patients had an ECOG performance status of 0 or 1. Patients received 350 ng aviscumine twice weekly by subcutaneous injection until progression. The primary end points were progression-free survival (PFS) and overall survival (OS). Safety was assessed as adverse events (AEs). Tumor response was assessed every eight weeks and survival of patients was followed up to one year after the end of therapy. Thirty one patients (intent-to-treat population (ITT)) were assessed for efficacy; safety was assessed in the whole population.

Results

One patient achieved a partial response (PR) and 10 patients showed stable disease/no change (SD). The median progression-free survival (mPFS) was 63 days (95% CI 57–85) and median overall survival (mOS) was 335 days (95% CI 210–604). In total 210 treatment-emergent adverse events were recorded. Grade 1 or 2 AEs occurred in 72% of patients and were mostly application-site effects such as pruritus Grade 3–4 treatment-emergent drug-related adverse events occurred in 9% of patients.

Conclusion

These results suggest that aviscumine may have a clinical impact in patients with previously treated metastatic melanoma and provide rationale for further clinical evaluation of this agent. In the light of effective new immune checkpoint blockers it might be a candidate for combinations with these agents.

Trial registration

ClinicalTrials.gov: NCT00658437

Genome-wide screening of Oryza sativa ssp. japonica and indica reveals a complex family of proteins with ribosome-inactivating protein domains

Ribosome-inactivating proteins (RIPs) are cytotoxic enzymes capable of halting protein synthesis by irreversible modification of ribosomes. Although RIPs are widespread they are not ubiquitous in the plant kingdom. The physiological importance of RIPs is not fully elucidated, but evidence suggests a role in the protection of the plant against biotic and abiotic stresses. Searches in the rice genome revealed a large and highly complex family of proteins with a RIP domain. A comparative analysis retrieved 38 RIP sequences from the genome sequence of Oryza sativa subspecies japonica and 34 sequences from the subspecies indica. The RIP sequences are scattered over different chromosomes but are mostly found on the third chromosome. The phylogenetic tree revealed the pairwise clustering of RIPs from japonica and indica. Molecular modeling and sequence analysis yielded information on the catalytic site of the enzyme, and suggested that a large part of RIP domains probably possess N-glycosidase activity. Several RIPs are differentially expressed in plant tissues and in response to specific abiotic stresses. This study provides an overview of RIP motifs in rice and will help to understand their biological role(s) and evolutionary relationships.

Recombinant pebulin protein, a type 2 ribosome-inactivating protein isolated from dwarf elder (Sambucus ebulus L.) shows anticancer and antifungal activities in vitro

In this study, encoding sequence of a new type 2 RIP (pebulin) was isolated and cloned from dwarf elder (Sambucus ebulus L.) native to the northern regions of Iran. The nucleotide sequence of pebulin was ligated to the pET-28a(+) expression plasmid and cloned into the E. coli strain BL21 (DE3) in order to express heterologously of recombinant protein. The recombinant pebulin protein was mainly produced in the form of insoluble inclusion bodies probably because to absence of N-glycosylation process in E. coli. Therefore, in order to increase the expression of recombinant protein in soluble form, co-expression of the target protein with the pG-Tf2 chaperone plasmid and incubation of bacterial culture under low temperature were used to enhance solubility and accumulation of recombinant protein. After purification of the recombinant protein using affinity chromatography method, the bioactivity of pebulin was analyzed by hemagglutination, anticancer, and antifungal assays. The results of the hemagglutination assay showed that purified pebulin agglutinated erythrocytes in all human blood groups. In addition, pebulin considerably inhibited the proliferation of cancer cell lines MCF-7 and HT-29 in a time- and dose-dependent manner and indicated remarkably growth-inhibiting effect against the plant pathogenic fungi such as Alternaria solani and Fusarium oxysporum.

Involvement of OsRIP1, a ribosome-inactivating protein from rice, in plant defense against Nilaparvata lugens

Rice is the most important staple food in the world, but rice production is challenged by several biotic stress factors like viruses, bacteria, fungi and pest insects. One of the most notorious pest insects is Nilaparvata lugens, commonly known as the brown planthopper, which feeds on rice phloem sap and can cause serious damage to rice fields. In order to protect themselves, plants express a wide array of defense proteins such as ribosome-inactivating proteins (RIPs). This study shows that the expression of 'OsRIP1' is highly induced in rice plants infested with N. lugens, with transcript levels more than 100-fold upregulated in infested plants compared to non-infested plants. Furthermore, recombinant OsRIP1 was toxic for brown planthoppers when administered through liquid artificial diet. OsRIP1 inactivated insect ribosomes in vitro, suggesting that its toxicity relates to the enzymatic activity of OsRIP1. Over-expression of OsRIP1 in transgenic rice plants did not affect the performance of insects reared on these plants, most likely due to insufficient concentrations of OsRIP1 in the phloem. The data obtained in this research indicate that OsRIP1 can play a role in plant defense against herbivorous insects.

Pokeweed antiviral protein restores levels of cellular APOBEC3G during HIV-1 infection by depurinating Vif mRNA

Pokeweed antiviral protein (PAP) is an RNA glycosidase that inhibits production of human immunodeficiency virus type 1 (HIV-1) when expressed in human culture cells. Previously, we showed that the expression of PAP reduced the levels of several viral proteins, including virion infectivity factor (Vif). However, the mechanism causing Vif reduction and the consequences of the inhibition were not determined. Here we show that the Vif mRNA is directly depurinated by PAP. Because of depurination at two specific sites within the Vif ORF, Vif levels decrease during infections and the progeny viruses that are generated are ∼ 10-fold less infectious and compromised for proviral integration. These results are consistent with PAP activity inhibiting translation of Vif, which in turn reduces the effect of Vif to inactivate the host restriction factor APOBEC3G (apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like editing complex 3G). Our findings identify Vif mRNA as a new substrate for PAP and demonstrate that derepression of innate immunity against HIV-1 contributes to its antiviral activity.

A Spectroscopic Study on Secondary Structure and Thermal Unfolding of the Plant Toxin Gelonin Confirms Some Typical Structural Characteristics and Unravels the Sequence of Thermal Unfolding Events

Gelonin from the Indian plant Gelonium multiflorum belongs to the type I ribosome-inactivating proteins (RIPs). Like other members of RIPs, this toxin glycoprotein inhibits protein synthesis of eukaryotic cells; hence, it is largely used in the construction of immunotoxins composed of cell-targeted antibodies. Lysosomal degradation is one of the main issues in targeted tumor therapies, especially for type I RIP-based toxins, as they lack the translocation domains. The result is an attenuated cytosolic delivery and a decrease of the antitumor efficacy of these plant-derived toxins; therefore, strategies to permit their release from endosomal vesicles or modifications of the toxins to make them resistant to degradation are necessary to improve their efficacy. Using infrared spectroscopy, we thoroughly analyzed both the secondary structure and the thermal unfolding of gelonin. Moreover, by the combination of two-dimensional correlation spectroscopy and phase diagram method, it was possible to deduce the sequence of events during the unfolding, confirming the typical characteristic of the RIP members to denature in two steps, as a sequential loss of tertiary and secondary structure was detected at 58 °C and at 65 °C, respectively. Additionally, some discrepancies in the unfolding process between gelonin and saporin-S6, another type I RIP protein, were detected.

The type-1 ribosome-inactivating protein OsRIP1 triggers caspase-independent apoptotic-like death in HeLa cells

Ribosome-inactivating proteins (RIPs) are capable of removing a specific adenine from 28S ribosomal RNA, thus inhibiting protein biosynthesis in an irreversible manner. In this study, recombinant OsRIP1, a type 1 RIP from rice (Oryza sativa L.), was investigated for its anti-proliferative properties. Human cervical cancer HeLa cells were incubated in the presence of OsRIP1 for 24-72 h. OsRIP1 treatment yielded an anti-proliferation response of the HeLa cells and resulted in apoptotic-like blebbing of the plasma membrane without causing DNA fragmentation. OsRIP1 labeled with FITC accumulated at the cell surface. Pull-down assays identified ASPP1 (Apoptosis-Stimulating Protein of p53 1) and IFITM3 (interferon-induced transmembrane protein 3) as potential interaction partners for OsRIP1. Transcript levels for several critical genes related to different signaling pathways were quantified by RT-qPCR. OsRIP1 provoked HeLa cells to undergo caspase-independent cell death, associated with a significant transcriptional upregulation of the apoptotic gene PUMA, interferon regulatory factor 1 (IRF1) and the autophagy-related marker LC3. No changes in caspase activities were observed. Together, these data suggest that apoptotic-like events were involved in OsRIP1-driven caspase-independent cell death that might trigger the IRF1 signaling pathway and LC3-mediated autophagy.

Atomic-resolution structures of type I ribosome inactivating protein alpha-momorcharin with different substrate analogs

Alpha-momorcharin (Alpha-MMC) from the seed of bitter melon is a type I ribosome inactivating protein (RIP) that removes a specific adenine from 28S rRNA and inhibits protein biosynthesis. Here, we report seven crystal complex structures of alpha-MMC with different substrate analogs (adenine, AMP, cAMP, dAMP, ADP, GMP, and xanthosine) at 1.08 Å to 1.52 Å resolution. These structures reveal that not only adenine, but also guanine and their analogs can effectively bind to alpha-MMC. The side chain of Tyr93 adopts two conformations, serving as a switch to open and close the substrate binding pocket of alpha-MMC. Although adenine, AMP, GMP, and guanine are located in a similar active site in different RIPs, residues involved in the interaction between RIPs and substrate analogs are slightly different. Complex structures of alpha-MMC with different substrate analogs solved in this study provide useful information on its enzymatic mechanisms and may enable the development of new inhibitors to treat the poisoning of alpha-MMC.

Construction of Minicircle Suicide Genes Coding for Ribosome-Inactivating Proteins

Due to the lower risks of adverse effects, nonviral gene therapy is a suitable alternative to transfect cancer cells with a suicide gene to let them kill themselves by expressing toxic ribosome-inactivating proteins. Plasmids are stable and easy-to-produce vectors, but they have some disadvantages due to the bacterial backbone. Applying the minicircle technology, this problem can be solved with manageable effort in a well-equipped laboratory. With the described methodology, minicircle-DNA can be produced at low costs. The cell killing properties are monitored following transfection using the CytoSMART^® Omni system-a camera based live cell imaging device.

Comparative antiviral resistance induction by CIP-29, a ribosome-inactivating protein from Clerodendrum inerme, and salicylic acid, a chemical

CIP-29, a basic glycoprotein purified from the leaves of Clerodendrum inerme, induces resistance against viruses in plants. It is a type-I ribosome-inactivating protein (RIP) with polynucleotide:adenosine glycosidase activity. Present study was made to compare the resistance induced against TMV by CIP-29 and salicylic acid (SA), a chemical elicitor of systemic acquired resistance (SAR). Foliar treatments with CIP-29 (20 µg mL-1) and SA (2.5 mM) on Nicotiana tabacum cv. Xanthi-nc resulted in a percent reduction of TMV lesion number by 99 and 64 on remote leaves and decrease in lesion diameter by 2.67- and 1.97-fold, respectively. CIP-29-treated (resistant) tobacco leaf extract inhibited TMV by 93%, as against 25% by SA, suggesting induction of a virus inhibitory agent (VIA) in the former. In semi-quantitative gene expression analysis of treated and inoculated (+V) sets of tobacco Xanthi-nc, PR1a (PR-1) and Osmotin (PR-5) gene transcripts were significantly enhanced only in SA-treated set, and showed pronounced accumulation in both CIP-29+V and SA+V. Acidic chitinase (PR-3) transcripts were detectable only in SA and SA+V. Expression of allene oxide cyclase (AOC) gene was prominent in CIP-29+V and DW+V, while 1-aminocyclopropane-1-carboxylate oxidase (ACO) and ethylene responsive factor 1 (ERF1) genes were expressed in greater intensity in CIP-29+V. In qPCR analysis of ACO and ERF1 genes, a respective fold increase of 377.4 and 25.1 was observed in the post-inoculated CIP-29-treated sets compared to an increase by 22.5- and 10.2-fold in SA-treated sets. Between 14 and 28 days post-inoculation with TMV, symptomatic plants of Solanum lycopersicum increased from 0 to 20% in the CIP-29 treated set, compared to an increase from 30 to 90% in SA-treated set. Greater delay in symptom progression and accumulation of TMV was noted in CIP-29 compared to SA-treated sets, while growth parameters were significantly higher following SA treatment. This study clearly points to different mechanisms governing phytoprotein and chemical induced resistance.

Apoptosis and necroptosis induced by stenodactylin in neuroblastoma cells can be completely prevented through caspase inhibition plus catalase or necrostatin-1

BACKGROUND

Stenodactylin is a highly toxic plant lectin purified from the caudex of Adenia stenodactyla, with molecular structure, intracellular routing and enzyme activity similar to those of ricin, a well-known type 2 ribosome-inactivating protein. However, in contrast with ricin, stenodactylin is retrogradely transported not only in peripheral nerves but also in the central nervous system.

PURPOSE

Stenodactylin properties make it a potential candidate for application in neurobiology and in experimental therapies against cancer. Thus, it is necessary to better clarify the toxic activity of this compound.

STUDY DESIGN

We investigated the mechanism of stenodactylin-induced cell death in the neuroblastoma-derived cell line, NB100, evaluating the implications of different death pathways and the involvement of oxidative stress.

METHODS

Stenodactylin cytotoxicity was determined by evaluating protein synthesis and other viability parameters. Cell death pathways and oxidative stress were analysed through flow cytometry and microscopy. Inhibitors of apoptosis, oxidative stress and necroptosis were tested to evaluate their protective effect against stenodactylin cytotoxicity.

RESULTS

Stenodactylin efficiently blocked protein synthesis and reduced the viability of neuroblastoma cells at an extremely low concentration and over a short time (1 pM, 24 h). Stenodactylin induced the strong and rapid activation of apoptosis and the production of free radicals. Here, for the first time, a complete and long lasting protection from the lethal effect induced by a toxic type 2 ribosome-inactivating protein has been obtained by combining the caspase inhibitor Z-VAD-fmk, to either the hydrogen peroxide scavenger catalase or the necroptotic inhibitor necrostatin-1.

CONCLUSION

In respect to stenodactylin cytotoxicity, our results: (i) confirm the high toxicity to nervous cells, (ii) indicate that multiple cell death pathways can be induced, (iii) show that apoptosis is the main death pathway, (iv) demonstrate the involvement of necroptosis and (v) oxidative stress.

In addition to its endosomal escape effect, platycodin D also synergizes with ribosomal inactivation protein to induce apoptosis in hepatoma cells through AKT and MAPK signaling pathways

Efficient endosomal escape after cellular uptake is a major challenge for the clinical application of therapeutic proteins. To overcome this obstacle, several strategies have been used to help protein drugs escape from endosomes without affecting the integrity of the cell membrane. Among them, some triterpenoid saponins with special structures were used to greatly enhance the anti-tumor therapeutic effect of protein toxins. Herein, we demonstrated that platycodin D (PD), polygalacin D (PGD) and platycodin D2 (PD2) from Platycodonis Radix significantly enhanced the ability of MHBP (a type I ribosome-inactivating protein toxin MAP30 fused with a cell-penetrating peptide HBP) to induce apoptosis in hepatoma cells. Based on the results of co-localization of endocytosed EGFP-HBP with a lysosomal probe and Galectin-9 vesicle membrane damage sensor, we demonstrated that PD, PGD and PD2 have the ability to promote endosomal escape of endocytic proteins without affecting the integrity of the plasma membrane. Meanwhile, we observed that cholesterol metabolism plays an important role in the activity of PD by RNA-seq analysis and KEGG pathway enrichment analysis, and confirm that PD, PGD and PD2 enhance the anti-tumor activity of MHBP by inducing the redistribution of free cholesterol and inhibiting the activity of cathepsin B and cathepsin D. Finally, we found that PD synergized with MHBP to induce caspase-dependent apoptosis through inhibiting Akt and ERK1/2 signaling pathways and activating JNK and p38 MAPK signaling pathways. This study provides new insights into the application of PD in cancer therapy and provides efficient and promising strategies for the cytosolic delivery of therapeutic proteins.

Opportunities and challenges for ribosome-inactivating proteins in traditional Chinese medicine plants

Ribosome-inactivating proteins (RIPs) are a class of cytotoxic rRNA N-glycosylase, which widely exist in higher plants in different taxonomy, including many traditional Chinese medicinal materials and vegetables and fruits. In this paper, the traditional Chinese medicinal plants containing RIPs protein were sorted out, and their pharmacological effects and clinical applications were analyzed. Since many RIPs in traditional Chinese medicine plants exhibit antiviral and antitumor activities and show great clinical application potential, people's interest in these proteins is on the rise. This paper summarizes the possible mechanism of RIPs's anti-virus and anti-tumor effects, and discusses its potential problems and risks, laying a foundation for subsequent research on how to exert its anti-virus and anti-tumor effects.

Targeting hexokinase 2 to enhance anticancer efficacy of trichosanthin in HeLa and SCC25 cell models

Background and purpose

Trichosanthin (TCS) is a plant-based ribosome-inactivating protein exhibiting a range of pharmacological properties, including abortifacient and anticancer. However, the routine clinical use in cancer treatment was hampered by its antigenicity. Hexokinase 2 (HK2) is a pivotal regulator of glycolysis, where aberrant expression is observed in many cancers. This study investigates the anticancer effects and mechanisms of TCS in combination with benserazide (Benz), a HK2 inhibitor, in Hela and SCC25 cancer models.

Experimental approach

MTT, colony-formation and cell cycle assays were performed to assess the cytotoxic effects of TCS and Benz in HeLa and SCC25 cells. Seahorse assay, western blotting, flow cytometry analysis and RNA sequencing were employed to investigate the pharmacological effects of the combo treatment. SCC25 cell xenograft mouse model was established for in vivo efficacy study.

Key results

Combined use of TCS and Benz exhibited synergistic anticancer effects in both Hela and SCC25 cell models. The observed synergistic effects were attributed to the modulation of glycolysis by targeting HK2, leading to reduced lactate production and increased ROS accumulation which further inhibited colony formation and cell cycle progression, as well as triggered apoptosis. Moreover, this combination effectively inhibited NFκB/ERK signalling pathways, which were found to be significantly activated upon single use of TCS. It was found that the combination significantly suppressed the tumour growth in SCC25 cell xenograft mouse model.

Conclusion

Our findings suggested that targeting HK2 and modulating glycolysis may offer a promising avenue for improving the therapeutic outcomes of TCS-based anticancer treatments.