Ribonucleases of different origins with a wide spectrum of medicinal applications

Edodin: A New Type of Toxin from Shiitake Mushroom (Lentinula edodes) That Inactivates Mammalian Ribosomes

Ribosome-inactivating proteins (RIPs) are a group of proteins with rRNA N-glycosylase activity that irreversibly inhibit protein synthesis and consequently cause cell death. Recently, an RIP called ledodin has been found in shiitake; it is cytotoxic, strongly inhibits protein synthesis, and shows rRNA N-glycosylase activity. In this work, we isolated and characterized a 50 kDa cytotoxic protein from shiitake that we named edodin. Edodin inhibits protein synthesis in a mammalian cell-free system, but not in insect-, yeast-, and bacteria-derived systems. It exhibits rRNA N-glycosylase and DNA-nicking activities, which relate it to plant RIPs. It was also shown to be toxic to HeLa and COLO 320 cells. Its structure is not related to other RIPs found in plants, bacteria, or fungi, but, instead, it presents the characteristic structure of the fold type I of pyridoxal phosphate-dependent enzymes. Homologous sequences have been found in other fungi of the class Agaricomycetes; thus, edodin could be a new type of toxin present in many fungi, some of them edible, which makes them of great interest in health, both for their involvement in food safety and for their potential biomedical and biotechnological applications.

Essential Role of Enzymatic Activity in the Leishmanicidal Mechanism of the Eosinophil Cationic Protein (RNase 3)

The recruitment of eosinophils into Leishmania lesions is frequently associated with a favorable evolution. A feasible effector for this process is eosinophil cationic protein (ECP, RNase 3), one of the main human eosinophil granule proteins, endowed with a broad spectrum of antimicrobial activity, including parasites. ECP was active on Leishmania promastigotes and axenic amastigotes (LC₅₀’s = 3 and 16 μM, respectively) but, in contrast to the irreversible membrane damage caused on bacteria and reproduced by its N-terminal peptides, it only induced a mild and transient plasma membrane destabilization on Leishmania donovani promastigotes. To assess the contribution of RNase activity to the overall leishmanicidal activity of ECP, parasites were challenged in parallel with a single-mutant version, ECP-H15A, devoid of RNase activity, that fully preserves the conformation and liposome permeabilization ability. ECP-H15A showed a similar uptake to ECP on promastigotes, but with higher LC₅₀’s (>25 μM) for both parasite stages. ECP-treated promastigotes showed a degraded RNA pattern, absent in ECP-H15A-treated samples. Moreover ECP, but not ECP-H15A, reduced more than 2-fold the parasite burden of infected macrophages. Altogether, our results suggest that ECP enters the Leishmania cytoplasm by an endocytic pathway, ultimately leading to RNA degradation as a key contribution to the leishmanicidal mechanism. Thus, ECP combines both membrane destabilization and enzymatic activities to effect parasite killing. Taken together, our data highlight the microbicidal versatility of ECP as an innate immunity component and support the development of cell-penetrating RNases as putative leishmanicidal agents.

Alginate-Based Hydrogel as Delivery System for Therapeutic Bacterial RNase

To deliver therapeutic proteins into a living body, it is important to maintain their target activity in the gastrointestinal tract after oral administration. Secreted ribonuclease from Bacillus pumilus (binase) has antitumor and antiviral activity, which makes it a promising therapeutic agent. This globular protein of small molecular weight (12.2 kDa) is considered as a potential agent that induces apoptosis of tumor cells expressing certain oncogenes, including colorectal and duodenum cancer. The most important problem of its usage is the preservation of its structure and target activity, which could be lost during oral administration. Here, we developed alginate microspheres reinforced with divalent cations and analyzed the enzyme release from them. Using methods of scanning electron microscopy, measurements of fluorescence, enzyme catalytic activity, and determination of viability of the duodenum adenocarcinoma tumor cell line, we characterized obtained microspheres and chose calcium as a biogenic ion-strengthening microsphere structure. Among such modified additivities as beta-casein, gelatin, and carbon nanotubes introduced into microspheres, only gelatin showed a pronounced increase in their stability and provided data on the prolonged action of enzyme release from microspheres into tumor cell culture medium during 48 h in an amount of about 70% of the loaded quantity.

Role of the Ribonuclease ONCONASE in miRNA Biogenesis and tRNA Processing: Focus on Cancer and Viral Infections

The majority of transcribed RNAs do not codify for proteins, nevertheless they display crucial regulatory functions by affecting the cellular protein expression profile. MicroRNAs (miRNAs) and transfer RNA-derived small RNAs (tsRNAs) are effectors of interfering mechanisms, so that their biogenesis is a tightly regulated process. Onconase (ONC) is an amphibian ribonuclease known for cytotoxicity against tumors and antiviral activity. Additionally, ONC administration in patients resulted in clinical effectiveness and in a well-tolerated feature, at least for lung carcinoma and malignant mesothelioma. Moreover, the ONC therapeutic effects are actually potentiated by cotreatment with many conventional antitumor drugs. This review not only aims to describe the ONC activity occurring either in different tumors or in viral infections but also to analyze the molecular mechanisms underlying ONC pleiotropic and cellular-specific effects. In cancer, data suggest that ONC affects malignant phenotypes by generating tRNA fragments and miRNAs able to downregulate oncogenes expression and upregulate tumor-suppressor proteins. In cells infected by viruses, ONC hampers viral spread by digesting the primer tRNAs necessary for viral DNA replication. In this scenario, new therapeutic tools might be developed by exploiting the action of ONC-elicited RNA derivatives.

Host Defence RNases as Antiviral Agents against Enveloped Single Stranded RNA Viruses

Owing to the recent outbreak of Coronavirus Disease of 2019 (COVID-19), it is urgent to develop effective and safe drugs to treat the present pandemic and prevent other viral infections that might come in the future. Proteins from our own innate immune system can serve as ideal sources of novel drug candidates thanks to their safety and immune regulation versatility. Some host defense RNases equipped with antiviral activity have been reported over time. Here, we try to summarize the currently available information on human RNases that can target viral pathogens, with special focus on enveloped single-stranded RNA (ssRNA) viruses. Overall, host RNases can fight viruses by a combined multifaceted strategy, including the enzymatic target of the viral genome, recognition of virus unique patterns, immune modulation, control of stress granule formation, and induction of autophagy/apoptosis pathways. The review also includes a detailed description of representative enveloped ssRNA viruses and their strategies to interact with the host and evade immune recognition. For comparative purposes, we also provide an exhaustive revision of the currently approved or experimental antiviral drugs. Finally, we sum up the current perspectives of drug development to achieve successful eradication of viral infections.

Barnase-Barstar Pair: Contemporary Application in Cancer Research and Nanotechnology

Barnase is an extracellular ribonuclease secreted by Bacillus amyloliquefaciens that was originally studied as a small stable enzyme with robust folding. The identification of barnase intracellular inhibitor barstar led to the discovery of an incredibly strong protein-protein interaction. Together, barnase and barstar provide a fully genetically encoded toxin-antitoxin pair having an extremely low dissociation constant. Moreover, compared to other dimerization systems, the barnase-barstar module provides the exact one-to-one ratio of the complex components and possesses high stability of each component in a complex and high solubility in aqueous solutions without self-aggregation. The unique properties of barnase and barstar allow the application of this pair for the engineering of different variants of targeted anticancer compounds and cytotoxic supramolecular complexes. Using barnase in suicide gene therapy has also found its niche in anticancer therapy. The application of barnase and barstar in contemporary experimental cancer therapy is reflected in the review.

Water-soluble pillar[5]arene sulfo-derivatives self-assemble into biocompatible nanosystems to stabilize therapeutic proteins

Pillar[5]arenes containing sulfonate fragments have been shown to form supramolecular complexes with therapeutic proteins to facilitate targeted transport with an increased duration of action and enhanced bioavailability. Regioselective synthesis was used to obtain a water-soluble pillar[5]arene containing the fluorescent label FITC and nine sulfoethoxy fragments. The pillar[5]arene formed complexes with the therapeutic proteins binase, bleomycin, and lysozyme in a 1:2 ratio as demonstrated by UV-vis and fluorescence spectroscopy. The formation of stable spherical nanosized macrocycle/binase complexes with an average particle size of 200 nm was established by dynamic light scattering and transmission electron microscopy. Flow cytometry demonstrated the ability of macrocycle/binase complexes to penetrate into tumor cells where they exhibited significant cytotoxicity towards A549 cells at 10^-5-10^-6 M while maintaining the enzymatic activity of binase.

Transcriptomic alterations in malignant pleural mesothelioma cells in response to long‑term treatment with bullfrog sialic acid‑binding lectin

Malignant pleural mesothelioma (MPM) is a universally lethal type of cancer that is increasing in incidence worldwide; therefore, the development of new drugs for MPM is an urgent task. Bullfrog sialic acid-binding lectin (cSBL) is a multifunctional protein that has carbohydrate-binding and ribonuclease activities. cSBL exerts marked antitumor activity against numerous types of cancer cells, with low toxicity to normal cells. Although in vitro and in vivo studies revealed that cSBL was effective against MPM, the mechanism by which cSBL exerts antitumor effects is not fully understood. To further understand the mechanism of action of cSBL, the present study aimed to identify the key molecules whose expression was affected by cSBL. The present study established cSBL-resistant MPM cells. Microarray analyses revealed that there were significant pleiotropic changes in the expression profiles of several genes, including multiple genes involved in metabolic pathways in cSBL-resistant cells. Furthermore, the expression of some members of the aldo-keto reductase family was revealed to be markedly downregulated in these cells. Among these, it was particularly interesting that cSBL action reduced the level of AKR1B10, which has been reported as a biomarker candidate for MPM prognosis. These findings revealed novel aspects of the effect of cSBL, which may contribute to the development of new therapeutic strategies for MPM.

Mushrooms and immunity

In the wide field of nutraceuticals, the effects of mushrooms on immunity, cancer and including autoimmunity have been proposed for centuries but in recent years a growing interest has led scientists to elucidate which specific compounds have bioactive properties and through which mechanisms. Glucans and specific proteins are responsible for most of the biological effects of mushrooms, particularly in terms of immunomodulatory and anti-tumor results. Proteins with bioactive effects include lectins, fungal immunomodulatory proteins (FIPs), ribosome inactivating proteins (RIPs), ribonucleases, laccases, among others. At the present status of knowledge, numerous studies have been performed on cell lines and murine models while only a few clinical trials have been conducted. As in most cases of dietary components, the multitude of variables implicated in the final effect and an inadequate standardization are expected to affect the observed differences, thus making the available evidence insufficient to justify the treatment of human diseases with mushrooms extracts. We will herein provide a comprehensive review and critically discussion the biochemical changes induced by different mushroom compounds as observed in in vitro studies, particularly on macrophages, dendritic cells, T cells, and NK cells, compared to in vivo and human studies. Additional effects are represented by lipids which constitute a minor part of mushrooms but may have a role in reducing serum cholesterol levels or phenols acting as antioxidant and reducing agents. Human studies provide a minority of available data, as well illustrated by a placebo-controlled study of athletes treated with β-glucan from Pleurotus ostreatus. Variables influencing study outcomes include different mushrooms strains, growing conditions, developmental stage, part of mushroom used, extraction method, and storage conditions. We foresee that future rigorous research will be needed to determine the potential of mushroom compounds for human health to reproduce the effects of some compounds such as lentinan which a metaanalysis demonstrated to increase the efficacy of chemotherapy in the treatment of lung cancer and in the improvement of the patients quality of life.

Antitumour Activity of the Ribonuclease Binase from Bacillus pumilus in the RLS40 Tumour Model Is Associated with the Reorganisation of the miRNA Network and Reversion of Cancer-Related Cascades to Normal Functioning

The important role of miRNA in cell proliferation and differentiation has raised interest in exogenous ribonucleases (RNases) as tools to control tumour-associated intracellular and extracellular miRNAs. In this work, we evaluated the effects of the RNase binase from Bacillus pumilus on small non-coding regulatory RNAs in the context of mouse RLS₄₀ lymphosarcoma inhibition. In vitro binase exhibited cytotoxicity towards RLS₄₀ cells via apoptosis induction through caspase-3/caspase-7 activation and decreased the levels of miR-21a, let-7g, miR-31 and miR-155. Intraperitoneal injections of binase in RLS₄₀-bearing mice resulted in the retardation of primary tumour growth by up to 60% and inhibition of metastasis in the liver by up to 86%, with a decrease in reactive inflammatory infiltration and mitosis in tumour tissue. In the blood serum of binase-treated mice, decreases in the levels of most studied miRNAs were observed, excluding let-7g, while in tumour tissue, the levels of oncomirs miR-21, miR-10b, miR-31 and miR-155, and the oncosuppressor let-7g, were upregulated. Analysis of binase-susceptible miRNAs and their regulatory networks showed that the main modulated events were transcription and translation control, the cell cycle, cell proliferation, adhesion and invasion, apoptosis and autophagy, as well as some other tumour-related cascades, with an impact on the observed antitumour effects.

RNases Disrupt the Adaptive Potential of Malignant Cells: Perspectives for Therapy

Exogenous RNases are selectively toxic to tumor cells. The reasons for this selectivity are not quite clear and should be searched for in the properties that distinguish malignant from normal cells. During onco-transformation, cells acquire properties allowing them to adapt to the altered microenvironment, such as resistance to hypoxia, changes in intracellular pH, disruption of ion transport, reduced adhesion and increased mobility, and production of specific exosomes. These adaptation mechanisms distinguish malignant cells from normal ones and give them a competitive advantage, ensuring survival and spread in the organism. Here, we analyze if the directed cytotoxic effect of exogenous RNases is linked to the disruption of the adaptive potential of tumor cells and how it can be used in anticancer therapy.

Emerging Antitumor Activities of the Bitter Melon (Momordica charantia)

Bitter melon or bitter gourd (Momordica charantia) is a common vegetable in Asia and it is distinctive for its bitter taste. As an ingredient in folk medicine, research from different laboratories in recent years supports its potential medicinal applications with anti-tumor, anti-diabetic, anti-HIV activities in both in vitro and animal studies. In this short review, we summarize herein the recent progress in the antitumor aspect of bitter melon with a focus on the underlying molecular mechanisms. Further mechanistic studies as well as clinical trials are necessary to further verify its medicinal applications.

A Human Ribonuclease Variant and ERK-Pathway Inhibitors Exhibit Highly Synergistic Toxicity for Cancer Cells

Pancreatic-type ribonucleases (ptRNases) are prevalent secretory enzymes that catalyze the cleavage of RNA. Ribonuclease inhibitor (RI) is a cytosolic protein that has femtomolar affinity for ptRNases, affording protection from the toxic catalytic activity of ptRNases, which can invade human cells. A human ptRNase variant that is resistant to inhibition by RI is a cytotoxin that is undergoing a clinical trial as a cancer chemotherapeutic agent. We find that the ptRNase and protein kinases in the ERK pathway exhibit strongly synergistic toxicity toward lung cancer cells (including a KRAS^G12C variant) and melanoma cells (including BRAF^V600E variants). The synergism arises from inhibiting the phosphorylation of RI and thereby diminishing its affinity for the ptRNase. These findings link seemingly unrelated cellular processes, and suggest that the use of a kinase inhibitor to unleash a cytotoxic enzyme could lead to beneficial manifestations in the clinic.

Improving the specific antitumor efficacy of ONC by fusion with N-terminal domain of transferrin

Onconase (ONC) as a novel anti-tumor drug has a significant killing effect on a variety of tumor cells. Drug delivery system mediated by transferrin (TF) and TF receptor (TfR), which can significantly increase the amount of drug uptake in the tumor cells, enhance the initiative target efficiency of drugs and reduce its toxic side effects. It has been widely used in drug delivery and clinical trials. In this study, the rONC-TFn was expressed in Escherichia coli by linking ONC with the N-terminal domain of TF (TFn). ELISA and competitive binding analysis demonstrated that rONC-TFn can bind to TfR. The rONC-TFn protein showed much higher cytotoxicity to the cultured HepG2 and Hela cells than rONC. These results suggested that the N-terminal domain protein of TF promoted the tumor targeting of ONC, and thus the rONC-TFn fusion protein may be further developed as a potential targeted anti-tumor drug.

A Sensitive In Vitro Approach to Assess the Hybridization-Dependent Toxic Potential of High Affinity Gapmer Oligonucleotides

The successful development of high-affinity gapmer antisense oligonucleotide (ASO) therapeutics containing locked nucleic acid (LNA) or constrained ethyl (cEt) substitutions has been hampered by the risk of hepatotoxicity. Here, we present an in vitro approach using transfected mouse fibroblasts to predict the potential hepatic liabilities of LNA-modified ASOs (LNA-ASOs), validated by assessing 236 different LNA-ASOs with known hepatotoxic potential. This in vitro assay accurately reflects in vivo findings and relates hepatotoxicity to RNase H1 activity, off-target RNA downregulation, and LNA-ASO-binding affinity. We further demonstrate that the hybridization-dependent toxic potential of LNA-ASOs is also evident in different cell types from different species, which indicates probable translatability of the in vitro results to humans. Additionally, we show that the melting temperature (T_m) of LNA-ASOs maintained below a threshold level of about 55°C greatly diminished the hepatotoxic potential. In summary, we have established a sensitive in vitro screening approach for assessing the hybridization-dependent toxic potential of LNA-ASOs, enabling prioritization of candidate molecules in drug discovery and early development.

Binase treatment increases interferon sensitivity and apoptosis in SiHa cervical carcinoma cells by downregulating E6 and E7 human papilloma virus oncoproteins

In this study, we determined whether binase, a ribonuclease from Bacillus pumilus, increases interferon sensitivity and apoptosis in SiHa cervical cancer cells infected with high-risk human papilloma virus (HPV) strain 16. Binase treatment increased SiHa cell apoptosis in a time- and concentration-dependent manner, as determined by flow cytometry, WST tests and real time xCelligence cell index analysis. Binase-treated SiHa cells showed reduced expression of E6 and E7 viral oncoproteins and increased expression of their intracellular targets, p53 and pRb. Combined treatment with binase and IFNα2b enhanced the interferon sensitivity of HPV-positive SiHa cells. By contrast, combined treatment with binase and IFNα2b in HPV-negative C33A cervical cancer cells, which do no expess E6 and E7, elicited no changes in interferon sensitivity or p53 and pRb expression. These findings suggest binase enhances interferon sensitivity and apoptosis in HPV-positive SiHa cervical cancer cells by suppressing E6 and E7 viral protein expression.

A nuclear-directed human pancreatic ribonuclease (PE5) targets the metabolic phenotype of cancer cells

Ribonucleases represent a new class of antitumor RNA-damaging drugs. However, many wild-type members of the vertebrate secreted ribonuclease family are not cytotoxic because they are not able to evade the cytosolic ribonuclease inhibitor. We previously engineered the human pancreatic ribonuclease to direct it to the cell nucleus where the inhibitor is not present. The best characterized variant is PE5 that kills cancer cells through apoptosis mediated by the p21^WAF1/CIP1 induction and the inactivation of JNK. Here, we have used microarray-derived transcriptional profiling to identify PE5 regulated genes on the NCI/ADR-RES ovarian cancer cell line. RT-qPCR analyses have confirmed the expression microarray findings. The results show that PE5 cause pleiotropic effects. Among them, it is remarkable the down-regulation of multiple genes that code for enzymes involved in deregulated metabolic pathways in cancer cells.

A Ribonuclease Isolated from Wild Ganoderma Lucidum Suppressed Autophagy and Triggered Apoptosis in Colorectal Cancer Cells

The mushroom Ganoderma lucidum (G. lucidum) has been consumed in China as a medicine for promoting health and longevity for thousands of years. Due to its paramount and multiple pharmaceutical effects, G. lucidum has received considerable attention from researchers and its chemical constituents as well as their respective functions were gradually unveiled by using modern research methods. Herein, we reported the isolation of a protein (Ganoderma lucidum ribonuclease, GLR) with anti-colorectal cancer activities from G. lucidum. This protein is a 17.4-kDa RNA degrading enzyme (ribonuclease) and was purified by using liquid chromatography procedures. GLR manifested potent anti-proliferative and anti-colony formation activities on HT29 and HCT116 colorectal cancer cells by inducing cell cycle arrest in G1 phase through the regulation of cyclin D1 and P53 expression. GLR was demonstrated to induce cell apoptosis in HCT116 cells by activating unfolded protein response and caspase-9 regulated pathways. Besides, the ability to undergo autophagy which is a stress adaption mechanism to cope with metabolic crisis was significantly suppressed by GLR treatment in HCT116 cells. The activation of apoptosis in GLR-treated HT29 cells was, however, independent of caspase-9 and the suppression of autophagy was also relatively minor. Thus the apoptosis of HT29 cells triggered by GLR was much milder than that in HCT116 cells. Our findings show that the RNase from G. lucidum may be one of the bioactive components that contribute to the anti-colorectal cancer activity of G. lucidum.

Carboxymethylsulfonylated Ribopyrimidines: Rational Design of Ribonuclease A Inhibitors Employing Chemical Logic

Hydrolysis of RNA by ribonuclease A crucially depends on the participation of the 2'-OH group as well as the positioning of the internucleotide bond at two different sites of the enzyme. Therefore, ribopyrimidines were modified with -SO2CH2CO2H, an acidic functional group, which was expected to interact with the phosphate binding site. These ribonucleosides were designed to understand the influence of the 2'-OH group of these inhibitors on ribonuclease A inhibition along with the effect of the -SO2CH2CO2H group. The "down" configuration of the 2'-OH group enhanced the inhibitory properties (Ki =1.75 μm) and also imparted important Val43 H-bonding with the furanose oxygen atom of the inhibitors. One of the most important aspects of this work is that there was no serendipitous discovery of the inhibitors. The inhibitors reported in this manuscript were obtained by design by employing chemical logic.

Isolation of Rhp-PSP, a member of YER057c/YjgF/UK114 protein family with antiviral properties, from the photosynthetic bacterium Rhodopseudomonas palustris strain JSC-3b

Rhodopseudomonas palustris strain JSC-3b isolated from a water canal adjacent to a vegetable field produces a protein that was purified by bioactivity-guided fractionation based on ammonium sulfate precipitation, ion-exchange absorption and size exclusion. The protein was further identified as an endoribonuclease L-PSP (Liver-Perchloric acid-soluble protein) by shotgun mass spectrometry analysis and gene identification, and it is member of YER057c/YjgF/UK114 protein family. Herein, this protein is designated Rhp-PSP. Rhp-PSP exhibited significant inhibitory activities against tobacco mosaic virus (TMV) in vivo and in vitro. To our knowledge, this represents the first report on the antiviral activity of a protein of the YER057c/YjgF/UK114 family and also the first antiviral protein isolated from R. palustris. Our research provides insight into the potential of photosynthetic bacterial resources in biological control of plant virus diseases and sustainable agriculture.

Microbial ribonucleases (RNases): production and application potential

Ribonuclease (RNase) is hydrolytic enzyme that catalyzes the cleavage of phosphodiester bonds in RNA. RNases play an important role in the metabolism of cellular RNAs, such as mRNA and rRNA or tRNA maturation. Besides their cellular roles, RNases possess biological activity, cell stimulating properties, cytotoxicity and genotoxicity. Cytotoxic effect of particular microbial RNases was comparable to that of animal derived counterparts. In this respect, microbial RNases have a therapeutic potential as anti-tumor drugs. The significant development of DNA vaccines and the progress of gene therapy trials increased the need for RNases in downstream processes. In addition, RNases are used in different fields, such as food industry for single cell protein preparations, and in some molecular biological studies for the synthesis of specific nucleotides, identifying RNA metabolism and the relationship between protein structure and function. In some cases, the use of bovine or other animal-derived RNases have increased the difficulties due to the safety and regulatory issues. Microbial RNases have promising potential mainly for pharmaceutical purposes as well as downstream processing. Therefore, an effort has been given to determination of optimum fermentation conditions to maximize RNase production from different bacterial and fungal producers. Also immobilization or strain development experiments have been carried out.

Human recombinant truncated RNASET2, devoid of RNase activity; A potential cancer therapeutic agent

Human RNASET2 has been implicated in antitumorigenic and antiangiogenic activities, independent of its ribonuclease capacities. We constructed a truncated version of human RNASET2, starting at E50 (trT2-50) and devoid of ribonuclease activity. trT2-50 maintained its ability to bind actin and to inhibit angiogenesis and tumorigenesis. trT2-50 binds to cell surface actin and formed a complex with actin in vitro. The antiangiogenic effect of this protein was demonstrated in human umbilical vein endothelial cells (HUVECs) by its ability to arrest tube formation on Matrigel, induced by angiogenic factors. Immunofluorescence staining of HUVECs showed nuclear and cytosolic RNASET2 protein that was no longer detectable inside the cell following trT2-50 treatment. This effect was associated with disruption of the intracellular actin network. trT2-50 co-localized with angiogenin, suggesting that both molecules bind (or compete) for similar cellular epitopes. Moreover, trT2-50 led to a significant inhibition of tumor development. Histological analysis demonstrated abundant necrotic tissue and a substantial loss of endothelial structure in trT2-50-treated tumors. Collectively, the present results indicate that trT2-50, a molecule engineered to be deficient of its catalytic activity, still maintained its actin binding and anticancer-related biological activities. We therefore suggest that trT2-50 may serve as a potential cancer therapeutic agent.

Killing of cancer cells through the use of eukaryotic expression vectors harbouring genes encoding nucleases and ribonuclease inhibitor

Cancer gene therapy vectors are promising tools for killing cancer cells with the purpose of eradicating malignant tumours entirely. Different delivery methods of vectors into the cancer cells, including both non-viral and viral, as well as promoters for the targeted expression of genes encoding anticancer proteins were developed for effective and selective killing of cancer cells without harming healthy cells. Many vectors have been created to kill cancer cells, and some vectors suppress malignant tumours with high efficiency. This review is focused on vectors bearing genes for nucleases such as deoxyribonucleases (caspase-activated DNase, deoxyribonuclease I-like 3, endonuclease G) and ribonucleases (human polynucleotide phosphorylase, ribonuclease L, α-sarcin, barnase), as well as vectors harbouring gene encoding ribonuclease inhibitor. The data concerning the functionality and the efficacy of such vectors are presented.

Rana catesbeiana ribonuclease induces cell apoptosis via the caspase-9/-3 signaling pathway in human glioblastoma DBTRG, GBM8901 and GBM8401 cell lines

Human glioblastoma multiforme is one of the most aggressive malignant brain tumor types, and the mean survival time of patients with a brain tumor is <2 years when traditional therapies are administered. Thus, numerous studies have focused on the development of novel treatments for brain tumors. Frog ribonucleases, such as Onconase and Rana catesbeiana ribonuclease (RC-RNase), exert antitumor effects on various tumor cells, including cervical cancer, breast cancer, hepatoma, leukemia, pancreatic cancer and prostate cancer cells. In addition, frog Onconase has been applied as a treatment in clinical trials. However, the antitumor effects of frog ribonucleases on brain tumors are unclear. Previous studies have indicated that RC-RNase demonstrates a decreased cytotoxic effect in normal cells compared with Onconase. Therefore, the present study investigated the ability of RC-RNase to exert antitumor activities on human glioblastoma. It was found that RC-RNase inhibits the growth of the human glioblastoma DBTRG, GBM8901 and GBM8401 cells. In addition, the present study revealed that RC-RNase induces caspase-9/-3 activity and triggers the apoptotic cell death pathway in human glioblastoma cells. Notably, it was also demonstrated that RC-RNase effectively inhibits the growth of human glioblastoma tumors in a nude mouse model. Overall, the present study indicates that RC-RNase may be a potential agent for the treatment of human glioblastoma.

Carboxylated acyclonucleosides: synthesis and RNase A inhibition

Strategically designed carboxylated acyclonucleosides have been probed as a new class of RNase A inhibitors. Several experimental and theoretical studies have been performed to compile relevant qualitative and quantitative information regarding the nature and extent of inhibition. The inhibition constant (Ki) values were determined using a UV-based kinetics experiment. The changes in the secondary structure of the enzyme upon binding with the inhibitors were obtained from circular dichroism studies. The binding constants for enzyme-inhibitor interactions were determined with the help of fluorescence spectroscopy. Docking studies were performed to reveal the possible binding sites of the inhibitors within the enzyme. The cytosine analogues were found to possess better inhibitory properties in comparison to the corresponding uracil derivatives. An increment in the number of carboxylic acid groups (-COOH) in the inhibitor backbone was found to result in better inhibition.

Cytotoxicity of RNase Sa to the acute myeloid leukemia Kasumi-1 cells depends on the net charge

The majority of known cytotoxic RNases are basic proteins which destroy intracellular RNA. Cationization of RNases is considered to be an effective strategy for strengthening their antitumor properties. We constructed a set of RNase Sa variants consisting of charge reversal mutants, charge neutralization mutants, and variants with positively charged cluster at the N-terminus. All constructs retain a high level of catalytic activity and differ in net charge. Using acute myeloid leukemia cells Kasumi-1 we have shown that (i) cytotoxicity of RNase Sa mutants is linearly enhanced by cationization, (ii) the ability of cytotoxic mutants to induce cell death is caused by induction of apoptosis and (iii) localization of positive charge on N-terminus does not contribute to RNase Sa cytotoxicity. Capacity to induce apoptosis in malignant cells and the absence of necrotic effects make the RNase Sa mutants with high positive charge a suitable anti-cancer agent.

Ribonuclease A inhibition by carboxymethylsulfonyl-modified xylo- and arabinopyrimidines

A group of acidic nucleosides were synthesized to develop a new class of ribonuclease A (RNase A) inhibitors. Our recent study on carboxymethylsulfonyl-modified nucleosides revealed some interesting results in RNase A inhibition. This positive outcome triggered an investigation of the role played by secondary sugar hydroxy groups in inhibiting RNase A activity. Uridines and cytidines modified with SO2 CH2 COOH groups at the 2'- and 3'-positions show good inhibitory properties with low inhibition constant (Ki ) values in the range of 109-17 μM. The present work resulted in a set of inhibitors that undergo more effective interactions with the RNase A active site, as visualized by docking studies.

Leczyme: a new candidate drug for cancer therapy

Sialic acid-binding lectin (SBL), isolated from oocytes of Rana catesbeiana, is leczyme and has both lectin and ribonuclease (RNase) activities. A remarkable antitumor effect of SBL has also been reported. SBL agglutinates various kinds of tumor cells but not normal cells. SBL agglutination activity is not affected by mono- or oligosaccharides. However, SBL-induced agglutination and antitumor effects are inhibited by sialomucin but not asialomucin. In addition, SBL has very little effect on sialidase-treated cells. SBL causes cancer-selective induction of apoptosis by multiple signaling pathways, which target RNA. Synergistic antitumor effects with other molecules, such as tumor necrosis factor-related apoptosis ligand (TRAIL) and interferon-γ (IFN-γ), have been reported. Thus, SBL may be a novel candidate molecule for anticancer drug development. Sialoglycoconjugates on the tumor cell surface may be associated with lectin activity and antitumor effects of SBL. We review the properties of SBL, particularly its lectin, RNase, and antitumor activities, and comprehensively examine the potential application of SBL for clinical purposes.

Purification and characterization of iso-ribonucleases from a novel thermophilic fungus

A thermophilic fungus previously isolated from composted horse manure was found to produce extracellular iso-RNases that were purified 127.6-fold using a combination of size exclusion chromatography and a novel affinity membrane purification system. The extent of purification was determined electrophoretically using 4%–15% gradient polyacrylamide gels. RNase activity was dependent on the presence of a metal co-factor with significantly more activity with Zn²⁺ or Mn²⁺ than Mg²⁺. The RNases exhibited maximum activity at both pH 3.0 and pH 7.0 with no activity at pH 2.0 or 10.0. The optimal temperature for the iso-RNase was 70 °C. The molecular weight of the iso-RNase was determined to be 69 kDa using a Sephadex G-75 column.

Fluorogenic probe for constitutive cellular endocytosis

Endocytosis is a fundamental process of eukaryotic cells that is critical for nutrient uptake, signal transduction, and growth. We have developed a molecular probe to quantify endocytosis. The probe is a lipid conjugated to a fluorophore that is masked with an enzyme-activatable moiety known as the trimethyl lock. The probe is not fluorescent when incorporated into the plasma membrane of human cells but becomes fluorescent upon internalization into endosomes, where cellular esterases activate the trimethyl lock. Using this probe, we found that human breast cancer cells undergo constitutive endocytosis more rapidly than do matched noncancerous cells. These data reveal a possible phenotypic distinction of cancer cells that could be the basis for chemotherapeutic intervention.

Sialic acid-binding lectin (leczyme) induces caspase-dependent apoptosis-mediated mitochondrial perturbation in Jurkat cells

Sialic acid binding lectin (SBL) isolated from Rana catesbeiana oocytes is a multifunctional protein which has lectin activity, ribonuclease activity and antitumor activity. However, the mechanism of antitumor effects of SBL is unclear to date and the validity for human leukemia cells has not been fully studied. We report here that SBL shows cytotoxicity for some human leukemia cell lines including multidrug-resistant (MDR) cells. The precise mechanisms of SBL-induced apoptotic signals were analyzed by combinational usage of specific caspase inhibitors and the mitochondrial membrane depolarization detector JC-1. It was demonstrated that SBL causes mitochondrial perturbation and the apoptotic signal is amplified by caspases and cell death is executed in a caspase-dependent manner. The efficacy of this combinational usage was shown for the first time, to distinguish the apoptotic pathway in detail. SBL selectively kills tumor cells, is able to exhibit cytotoxicity regardless of P-glycoprotein expression and has potential as an alternative to conventional DNA-damaging anticancer drugs.

Ribonuclease binase inhibits primary tumor growth and metastases via apoptosis induction in tumor cells

Exogenous ribonucleases are known to inhibit tumor growth via apoptosis induction in tumor cells, allowing to consider them as promising anticancer drugs for clinical application. In this work the antitumor potential of binase was evaluated in vivo and the mechanism of cytotoxic effect of binase on tumor cells was comprehensively studied in vitro. We investigated tumoricidal activity of binase using three murine tumor models of Lewis lung carcinoma (LLC), lymphosarcoma RLS 40 and melanoma B-16. We show for the first time that intraperitoneal injection of binase at a dose range 0.1-5 mg/kg results in retardation of primary tumor growth up to 45% in LLC and RLS 40 and inhibits metastasis up to 50% in LLC and RLS 40 and up to 70% in B-16 melanoma. Binase does not exhibit overall toxic effect and displays a general systemic and immunomodulatory effects. Treatment of RLS 40-bearing animals with binase together with polychemotherapy revealed that binase decreases the hepatotoxicity of polychemotherapy while maintaining its antitumor effect. It was demonstrated that the cytotoxic effect of binase is realized via the induction of the intrinsic and extrinsic apoptotic pathways. Activation of intrinsic apoptotic pathway is manifested by a drop of mitochondrial potential, increase in calcium concentration and inhibition of respiratory activity. Subsequent synthesis of TNF-α in the cells under the action of binase triggers extrinsic apoptotic pathway through the binding of TNF with cell-death receptors and activation of caspase 8. Thus binase is a potential anticancer therapeutics inducing apoptosis in cancer cells.

Increased alkalotolerant and thermostable ribonuclease (RNase) production from alkaliphilic Streptomyces sp. M49-1 by optimizing the growth conditions using response surface methodology

Total of 171 alkaliphilic actinomycetes were evaluated for extracellular RNase production and Streptomyces sp. M49-1 was selected for further experiments. Fermentation optimization for RNase production was implemented in two steps using response surface methodology with central composite design. In the first step, the effect of independent fermentation variables including temperature, initial pH and process time were investigated. After identification of carbon and nitrogen sources affecting the production by one variable at a time method, concentrations of glucose and yeast extract and also inoculum size were chosen for the second central composite design. A maximum RNase activity was obtained under optimal conditions of 4.14 % glucose concentration, 4.63 % yeast extract concentration, 6.7 × 10⁶ spores as inoculum size for 50 ml medium, 42.9 °C, 91.2 h process time and medium initial pH 9.0. Optimum activity of the enzyme is achieved at pH 11 and temperature 60 °C. The enzyme is highly stable at pH range 9.0-12.0 and at 90 °C after 2 h. Statistical optimization experiments provide 2.25 fold increases in the activity of alkalotolerant and thermostable RNase and shortened the fermentation time compared to that of unoptimized condition. The members of Streptomyces can be promising qualified RNase producer for pharmaceutical industries.

Increased ribonuclease expression reduces inflammation and prolongs survival in TLR7 transgenic mice

TLR7 activation is implicated in the pathogenesis of systemic lupus erythematosus. Mice that overexpress TLR7 develop a lupus-like disease with autoantibodies and glomerulonephritis and early death. To determine whether degradation of the TLR7 ligand RNA would alter the course of disease, we created RNase A transgenic (Tg) mice. We then crossed the RNase Tg to TLR7 Tg mice to create TLR7 × RNase double Tg (DTg) mice. DTg mice had a significantly increased survival associated with reduced activation of T and B lymphocytes and reduced kidney deposition of IgG and C3. We observed massive hepatic inflammation and cell death in TLR7 Tg mice. In contrast, hepatic inflammation and necrosis were strikingly reduced in DTg mice. These findings indicate that high concentrations of serum RNase protect against immune activation and inflammation associated with TLR7 stimulation and that RNase may be a useful therapeutic strategy in the prevention or treatment of inflammation in systemic lupus erythematosus and, possibly, liver diseases.

Nucleotide binding architecture for secreted cytotoxic endoribonucleases

Vertebrate secreted RNases are small cationic protein endowed with an endoribonuclease activity that belong to the RNase A superfamily and display diverse cytotoxic activities. In an effort to unravel their mechanism of action, we have analysed their nucleotide binding recognition patterns. General shared features with other nucleotide binding proteins were deduced from overall statistics on the available structure complexes at the Protein Data Bank and compared with the particularities of selected representative endoribonuclease families. Results were compared with other endoribonuclease representative families and with the overall protein-nucleotide interaction features. Preferred amino acids and atom types involved in pair bonding interactions were identified, defining the spatial motives for phosphate, base and ribose building blocks. Together with the conserved catalytic triad at the active site, variability was observed for secondary binding subsites that may contribute to the proper substrate alignment and could explain the distinct substrate preference patterns. Highly conserved binding patterns were identified for the pyrimidine and purine subsites at the main and secondary base subsites. Particular substitution could be ascribed to specific adenine or guanine specificities. Distribution of evolutionary conserved residues were compared to search for the structure determinants that underlie their diverse catalytic efficiency and those that may account for putative physiological substrate targets or other non-catalytic biological activities that contribute to the antipathogen role of the RNases involved in the host defence system. A side by side comparison with another endoribonuclease superfamily of secreted cytotoxic proteins, the microbial RNases, was carried on to analyse the common features and peculiarities that rule their substrate recognition. The data provides the structural basis for the development of applied therapies targeting cellular nucleotide polymers.

Trichosanthin inhibits breast cancer cell proliferation in both cell lines and nude mice by promotion of apoptosis

Breast cancer ranks as a common and severe neoplasia in women with increasing incidence as well as high risk of metastasis and relapse. Translational and laboratory-based clinical investigations of new/novel drugs are in progress. Medicinal plants are rich sources of biologically active natural products for drug development. The 27-kDa trichosanthin (TCS) is a ribosome inactivating protein purified from tubers of the Chinese herbal plant Trichosanthes kirilowii Maximowicz (common name Tian Hua Fen). In this study, we extended the potential medicinal applications of TCS from HIV, ferticide, hydatidiform moles, invasive moles, to breast cancer. We found that TCS manifested anti-proliferative and apoptosis-inducing activities in both estrogen-dependent human MCF-7 cells and estrogen-independent MDA-MB-231 cells. Flow cytometric analysis disclosed that TCS induced cell cycle arrest. Further studies revealed that TCS-induced tumor cell apoptosis was attributed to activation of both caspase-8 and caspase-9 regulated pathways. The subsequent events including caspase-3 activation, and increased PARP cleavage. With regard to cell morphology, stereotypical apoptotic features were observed. Moreover, in comparison with control, TCS- treated nude mice bearing MDA-MB-231 xenograft tumors exhibited significantly reduced tumor volume and tumor weight, due to the potent effect of TCS on tumor cell apoptosis as determined by the increase of caspase-3 activation, PARP cleavage, and DNA fragmentation using immunohistochemistry. Considering the clinical efficacy and relative safety of TCS on other human diseases, this work opens up new therapeutic avenues for patients with estrogen-dependent and/or estrogen-independent breast cancers.

A novel aspartic protease with HIV-1 reverse transcriptase inhibitory activity from fresh fruiting bodies of the wild mushroom Xylaria hypoxylon

A novel aspartic protease with HIV-1 RT inhibitory activity was isolated and characterized from fruiting bodies of the wild mushroom Xylaria hypoxylon. The purification protocol comprised distilled water homogenization and extraction step, three ion exchange chromatographic steps (on DEAE-cellulose, Q-Sepharose, and CM-cellulose in succession), and final purification was by FPLC on Superdex 75. The protease was adsorbed on all the three ion exchangers. It was a monomeric protein with a molecular mass of 43 kDa as estimated by SDS-PAGE and FPLC. Its N-terminal amino acid sequence was HYTELLSQVV, which exhibited no sequence homology to other proteases reported. The activity of the protease was adversely affected by Pepstatin A, indicating that it is an aspartic protease. The protease activity was maximal or nearly so in the pH range 6–8 and in the temperature range 35–60°C. The purified enzyme exhibited HIV-1 RT inhibitory activity with an IC₅₀ value of 8.3 μM, but was devoid of antifungal, ribonuclease, and hemagglutinating activities.

Apoptosis of epithelial cells and macrophages due to nonpigmented Serratia marcescens strains

Serratia marcescens strains are opportunistic pathogens that are increasingly recognized as a cause of severe nosocomial infections. In this study we observed interactions between nonpigmented strains with human epithelial and macrophage-like cells. The strains revealed hemolytic activity only after the contact of the cells with erythrocytes. The contact of the bacteria with the host cells was also essential to their cytotoxicity. Moreover, all strains revealed adherence ability and were invasive to epithelial cells. Analyses of cellular morphology and DNA fragmentation of the HEp-2 and J774 cells exhibited typical features of cells undergoing apoptosis. We observed morphological changes, including condensation of nuclear chromatin and formation of membrane-bound apoptotic bodies. The lowest apoptotic index in HEp-2 cells did not exceed 25%, whereas the highest reached 59% at 24 h and 72% at 48 h after infection. Most of the strains (60%) induced fragmentation of nuclear DNA. The process depended on the activation of caspases, and was completely blocked by the pan-caspase inhibitor z-VAD-fmk. This study provided new insights into the mechanisms of nonpigmented S. marcescens pathogenesis. The results revealed that the strains produce cell-contact toxins that facilitate bacterial invasion, induce hemolysis, cytotoxicity, and apoptosis of host cells.

Rational design and evaluation of mammalian ribonuclease cytotoxins

Mammalian pancreatic-type ribonucleases (ptRNases) comprise an enzyme family that is remarkably well suited for therapeutic exploitation. ptRNases are robust and prodigious catalysts of RNA cleavage that can naturally access the cytosol. Instilling cytotoxic activity requires endowing them with the ability to evade a cytosolic inhibitor protein while retaining other key attributes. These efforts have informed our understanding of ptRNase-based cytotoxins, as well as the action of protein-based drugs with cytosolic targets. Here, we address the most pressing problems encountered in the design of cytotoxic ptRNases, along with potential solutions. In addition, we describe assays that can be used to evaluate a successful design in vitro, in cellulo, and in vivo. The emerging information validates the continuing development of ptRNases as chemotherapeutic agents.

A novel ribonuclease with potent HIV-1 reverse transcriptase inhibitory activity from cultured mushroom Schizophyllum commune

A 20-kDa ribonuclease (RNase) was purified from fresh fruiting bodies of cultured Schizophyllum commune mushrooms. The RNase was not adsorbed on Affi-gel blue gel but adsorbed on DEAE-cellulose and CM-cellulose. It exhibited maximal RNase activity at pH 6.0 and 70°C. It demonstrated the highest ribonucleolytic activity toward poly (U) (379.5 μ/mg), the second highest activity toward poly (C) (244.7 μ/mg), less activity toward poly (A) (167.4 μ/mg), and much weaker activity toward poly (G) (114.5 μ/mg). The RNase inhibited HIV-1 reverse transcriptase with an IC(50) of 65 μM. No effect on [(3)H-methyl]-thymidine uptake by lymphoma MBL2 cells and leukemia L1210 cells was observed at 100 μM concentration of the RNase. A comparison of RNases from S. commune and Volvariella volvacea revealed that they demonstrated some similarities in N-terminal amino acid sequence, optimum pH and polyhomoribonucleotide specificity. However, some differences in chromatographic behavior and molecular mass were observed.

Arginine residues are more effective than lysine residues in eliciting the cellular uptake of onconase

Onconase is an amphibian member of the pancreatic ribonuclease family of enzymes that is in clinical trials for the treatment of cancer. Onconase, which has an abundance of lysine residues, is internalized by cancer cells through endocytosis in a mechanism similar to that of cell-penetrating peptides. Here, we compare the effect of lysine versus arginine residues on the biochemical attributes necessary for Onconase to elicit its cytotoxic activity. In the variant R-Onconase, 10 of the 12 lysine residues in Onconase are replaced with arginine, leaving only the two active-site lysines intact. Cytometric assays quantifying internalization showed a 3-fold increase in the internalization of R-Onconase compared with Onconase. R-Onconase also showed greater affinity for heparin and a 2-fold increase in ribonucleolytic activity. Nonetheless, arginine substitution endowed only a slight increase in toxicity toward human cancer cells. Analysis of denaturation induced with guanidine-HCl showed that R-Onconase has less conformational stability than does the wild-type enzyme; moreover, R-Onconase is more susceptible to proteolytic degradation. These data indicate that arginine residues are more effective than lysine in eliciting cellular internalization but can compromise other aspects of protein structure and function.

Momordica Charantia lectin, a type II ribosome inactivating protein, exhibits antitumor activity toward human nasopharyngeal carcinoma cells in vitro and in vivo

The incidence of nasopharyngeal carcinoma (NPC) remains high in endemic regions, including southern China, northern Africa, and North America. One of the promising therapeutic approaches on NPC is drug screening from natural products, such as components from traditional Chinese medicine. In this study, the antitumor activity of Momordica charantia lectin (MCL), a type II ribosome inactivating protein from bitter gourd, on NPC was investigated. MCL evinced potent cytotoxicity toward NPC CNE-1 (IC(50) = 6.9) and CNE-2 (IC(50) = 7.4) cells but minimally affected normal NP 69 cells. Further investigation disclosed that MCL induced apoptosis, DNA fragmentation, G(1)-phase arrest, and mitochondrial injury in both types of NPC cells. The reduction of cyclin D1 and phosphoretinoblastoma (Rb) protein expression contributed to arrest at G(1)-phase of the cell cycle. These events were associated with regulation of mitogen-activated protein kinases (MAPK; including p38 MAPK, JNK, and ERK) phosphorylation and promoted downstream nitric oxide (NO) production. Concurrent administration of the p38 MAPK inhibitor SB-203580 significantly diminished NO production and lethality of MCL toward NPC cells. Further studies revealed that MCL increased cytochrome c release into the cytosol, activated caspases-8, -9, and -3, and enhanced production of cleaved PARP, subsequently leading to DNA fragmentation and apoptosis. Finally, an intraperitoneal injection of MCL (1.0 mg/kg/d) led to an average of 45% remission of NPC xenograft tumors subcutaneously inoculated in nude mice. This is the first article that unveils the potential of a type II RIP, MCL, for prevention and therapy of NPC.