Ribonucleases and their antitumor activity

Assessing Partial Inhibition of Ribonuclease A Activity by Curcumin through Fluorescence Spectroscopy and Theoretical Studies

Molecular interactions and controlled expression of enzymatic activities are fundamental to all cellular functions in an organism. The active polyphenol in turmeric known as curcumin (CCM) is known to exhibit diverse pharmacological activities. Ribonucleases (RNases) are the hydrolytic enzymes that plays important role in ribonucleic acid (RNA) metabolism. Uncontrolled and unwanted cleavage of RNA by RNases may be the cause of cell death leading to disease states. The protein ribonuclease A (RNase A) in the superfamily of RNases cleaves the RNA besides its role in different diseases like autoimmune diseases, and pancreatic disorders. Interaction of CCM with RNase A have been reported along with the possible role of CCM to inhibit the RNase A enzymatic activity. The interaction strength was found to be 104 M-1 order from spectroscopic results. Quenching of RNase A fluorescence by CCM was 104 M-1 order. Non-radiative energy transfer from RNase A (donor) to CCM (acceptor) suggested a distance of 2.42 nm between the donor-acceptor pair. Circular dichroism studies revealed no structural changes in RNase A after binding. Binding-induced conformational variation in protein was observed from synchronous fluorescence studies. Agarose gel electrophoresis revealed a partial inhibition of the RNase A activity by CCM though not significant. Molecular docking and molecular dynamics studies suggested the residues of RNase A involved in the interaction with supporting the experimental finding for the partial inhibition of the enzyme activity. This study may help in designing new CCM analogues or related structures to understand their differential inhibition of the RNase A activity.

Thermodynamics of the Association of Aminoglycoside Antibiotics with Human Angiogenin

BACKGROUND

The body needs to maintain a firm balance between the inducers and inhibitors of angiogenesis, the process of proliferation of blood vessels from pre-existing ones. Human angiogenin (hAng), being a potent inducer of angiogenesis, is a cause of tumor cell proliferation, therefore its inhibition becomes a vital area of research. Aminoglycosides are linked ring systems consisting of amino sugars and an aminocyclitol ring and are in use in clinical practices for a long time. These compounds have found clinical uses as antibacterial agents that inhibit bacterial protein synthesis.

OBJECTIVE

Gentamycin C1, Kanamycin A, Neomycin B, Paromomycin I, and Streptomycin A are commonly used aminoglycoside antibiotics that have been used for the present study. Among these, Neomycin has reported inhibitory activity against angiogenin-induced angiogenesis on the chicken chorioallantoic membrane. This study focuses on the thermodynamic parameters involved in the interactions of these antibiotics with hAng.

METHODS

Agarose gel-based assay, Fluorescence quenching studies and Docking studies.

RESULTS

Anti-ribonucleolytic effect of the antibiotics was observed qualitatively using an agarose gelbased assay, which shows that Neomycin exhibits the most efficient inhibition of hAng. Fluorescence quenching studies at different temperatures, using Stern-Volmer and van't Hoff equations provide information about the thermodynamics of binding, which furthermore highlights the higher binding constant of Neomycin. Docking studies showed that the antibiotics preferably interact with the nuclear translocation site, except Streptomycin, which shows affinity towards the ribonucleolytic site of the protein with very less affinity value.

CONCLUSION

The study has shown the highly spontaneous formation of Neomycin-hAng complex, giving an exothermic reaction with increase in the degree of freedom of the protein-ligand complex.

Anticancer Activity of Reconstituted Ribonuclease S-Decorated Artificial Viral Capsid

Ribonuclease S (RNase S) is an enzyme that exhibits anticancer activity by degrading RNAs within cancer cells; however, the cellular uptake efficiency is low due to its small molecular size. Here we generated RNase S-decorated artificial viral capsids with a size of 70-170 nm by self-assembly of the β-annulus-S-peptide followed by reconstitution with S-protein at neutral pH. The RNase S-decorated artificial viral capsids are efficiently taken up by HepG2 cells and exhibit higher RNA degradation activity in cells compared with RNase S alone. Cell viability assays revealed that RNase S-decorated capsids have high anticancer activity comparable to that of standard anticancer drugs.

A Nuclear-Directed Ribonuclease Variant Targets Cancer Stem Cells and Inhibits Migration and Invasion of Breast Cancer Cells

Simple Summary

During the past decades the achievements made in treating cancers have significantly improved the survival of patients. However, cancer is still one of the leading causes of mortality. It is suggested that treatment failure is mediated by a subpopulation of tumor cells named cancer stem cells that can survive after treatment and promote cancer relapse. Targeting these cells is important to improve cancer therapy. The aim of our study is to determine the effect of a human ribonuclease variant on breast cancer cells grown in 3D and on cancer stem cells. Moreover, we study its effect on the ability of breast cancer cells to migrate and produce metastasis, responsible for about 90% of cancer deaths. We show that this ribonuclease arrests tumor cells grown in 3D without affecting normal breast cells, and this significantly inhibits cancer stem cell development. Additionally, it reduces the migratory and invasive capacities of tumor cells.

Abstract

Despite the significant advances in cancer research made in recent years, this disease remains one of the leading causes of death worldwide. In part, this is due to the fact that after therapy, a subpopulation of self-renewing tumor cells can survive and promote cancer relapse, resistance to therapies and metastasis. Targeting these cancer stem cells (CSCs) is therefore essential to improve the clinical outcome of cancer patients. In this sense, multi-targeted drugs may be promising agents targeting CSC-associated multifocal effects. We have previously constructed different human pancreatic ribonuclease (RNase) variants that are cytotoxic for tumor cells due to a non-classical nuclear localization signal introduced in their sequence. These cytotoxic RNases affect the expression of multiple genes involved in deregulated metabolic and signaling pathways in cancer cells and are highly cytotoxic for multidrug-resistant tumor cell lines. Here, we show that these cytotoxic nuclear-directed RNases are highly selective for tumor cell lines grown in 3D, inhibit CSCs’ development and diminish the self-renewal capacity of the CSCs population. Moreover, these human RNase variants reduce the migration and invasiveness of highly invasive breast cancer cells and downregulate N-cadherin expression.

The Selectivity for Tumor Cells of Nuclear-Directed Cytotoxic RNases Is Mediated by the Nuclear/Cytoplasmic Distribution of p27KIP1

Although single targeted anti-cancer drugs are envisaged as safer treatments because they do not affect normal cells, cancer is a very complex disease to be eradicated with a single targeted drug. Alternatively, multi-targeted drugs may be more effective and the tumor cells may be less prone to develop drug resistance although these drugs may be less specific for cancer cells. We have previously developed a new strategy to endow human pancreatic ribonuclease with antitumor action by introducing in its sequence a non-classical nuclear localization signal. These engineered proteins cleave multiple species of nuclear RNA promoting apoptosis of tumor cells. Interestingly, these enzymes, on ovarian cancer cells, affect the expression of multiple genes implicated in metabolic and signaling pathways that are critic for the development of cancer. Since most of these targeted pathways are not highly relevant for non-proliferating cells, we envisioned the possibility that nuclear directed-ribonucleases were specific for tumor cells. Here, we show that these enzymes are much more cytotoxic for tumor cells in vitro. Although the mechanism of selectivity of NLSPE5 is not fully understood, herein we show that p27^KIP1 displays an important role on the higher resistance of non-tumor cells to these ribonucleases.

Biological Activities of Secretory RNases: Focus on Their Oligomerization to Design Antitumor Drugs

Ribonucleases (RNases) are a large number of enzymes gathered into different bacterial or eukaryotic superfamilies. Bovine pancreatic RNase A, bovine seminal BS-RNase, human pancreatic RNase 1, angiogenin (RNase 5), and amphibian onconase belong to the pancreatic type superfamily, while binase and barnase are in the bacterial RNase N1/T1 family. In physiological conditions, most RNases secreted in the extracellular space counteract the undesired effects of extracellular RNAs and become protective against infections. Instead, if they enter the cell, RNases can digest intracellular RNAs, becoming cytotoxic and having advantageous effects against malignant cells. Their biological activities have been investigated either in vitro, toward a number of different cancer cell lines, or in some cases in vivo to test their potential therapeutic use. However, immunogenicity or other undesired effects have sometimes been associated with their action. Nevertheless, the use of RNases in therapy remains an appealing strategy against some still incurable tumors, such as mesothelioma, melanoma, or pancreatic cancer. The RNase inhibitor (RI) present inside almost all cells is the most efficacious sentry to counteract the ribonucleolytic action against intracellular RNAs because it forms a tight, irreversible and enzymatically inactive complex with many monomeric RNases. Therefore, dimerization or multimerization could represent a useful strategy for RNases to exert a remarkable cytotoxic activity by evading the interaction with RI by steric hindrance. Indeed, the majority of the mentioned RNases can hetero-dimerize with antibody derivatives, or even homo-dimerize or multimerize, spontaneously or artificially. This can occur through weak interactions or upon introducing covalent bonds. Immuno-RNases, in particular, are fusion proteins representing promising drugs by combining high target specificity with easy delivery in tumors. The results concerning the biological features of many RNases reported in the literature are described and discussed in this review. Furthermore, the activities displayed by some RNases forming oligomeric complexes, the mechanisms driving toward these supramolecular structures, and the biological rebounds connected are analyzed. These aspects are offered with the perspective to suggest possible efficacious therapeutic applications for RNases oligomeric derivatives that could contemporarily lack, or strongly reduce, immunogenicity and other undesired side-effects.

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.

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.

Bovine brain ribonuclease is the functional homolog of human ribonuclease 1

Mounting evidence suggests that human pancreatic ribonuclease (RNase 1) plays important roles in vivo, ranging from regulating blood clotting and inflammation to directly counteracting tumorigenic cells. Understanding these putative roles has been pursued with continual comparisons of human RNase 1 to bovine RNase A, an enzyme that appears to function primarily in the ruminant gut. Our results imply a different physiology for human RNase 1. We demonstrate distinct functional differences between human RNase 1 and bovine RNase A. Moreover, we characterize another RNase 1 homolog, bovine brain ribonuclease, and find pronounced similarities between that enzyme and human RNase 1. We report that human RNase 1 and bovine brain ribonuclease share high catalytic activity against double-stranded RNA substrates, a rare quality among ribonucleases. Both human RNase 1 and bovine brain RNase are readily endocytosed by mammalian cells, aided by tight interactions with cell surface glycans. Finally, we show that both human RNase 1 and bovine brain RNase are secreted from endothelial cells in a regulated manner, implying a potential role in vascular homeostasis. Our results suggest that brain ribonuclease, not RNase A, is the true bovine homolog of human RNase 1, and provide fundamental insight into the ancestral roles and functional adaptations of RNase 1 in mammals.

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.

Internalization of Bacillus intermedius ribonuclease (BINASE) induces human alveolar adenocarcinoma cell death

Ribonuclease (RNase) treatment represents a novel mechanism based approach to anticancer therapy as an alternative to the DNA damaging drugs commonly used in clinical practice. Apart from their ribonucleolytic activity, cytotoxic effects have attracted a considerable attention to RNases because of their potential as selective agents for treatment of certain malignancies. Among these enzymes, Binase, an RNase from Bacillus intermedius, has shown promising results. Here, we have found that binase selectively attacked human A549 alveolar adenocarcinoma cells to trigger an apoptotic response, whereas normal lung epithelial cells LEK were not affected by the ribonuclease. The tumor transformation led to the modification of certain cellular characteristics causing cell sensitivity to binase. Although a general mode for RNases cytotoxicity includes their penetration into the cell, translocation to the cytosol and degradation of ribonucleic acid, many aspects of this process have not been fully elucidated. Our data revealed the following time-dependent changes induced by binase in A549 cells: (a) fast permanent internalization of the enzyme during the first hours of treatment; (b) temporary increase in cellular permeability for macromolecules during the 4-6 h of treatment; (c) apoptotic alterations in population after 24 h and (d) DNA fragmentation and cell death after 72 h of treatment with binase. Elucidation of these molecular strategies used by this promising toxin provides us essential information for the development of new anticancer drugs.

Plant multifunctional nuclease TBN1 with unexpected phospholipase activity: structural study and reaction-mechanism analysis

Type I plant nucleases play an important role in apoptotic processes and cell senescence. Recently, they have also been indicated to be potent anticancer agents in in vivo studies. The first structure of tomato nuclease I (TBN1) has been determined, its oligomerization and activity profiles have been analyzed and its unexpected activity towards phospholipids has been discovered, and conclusions are drawn regarding its catalytic mechanism. The structure-solution process required X-ray diffraction data from two crystal forms. The first form was used for phase determination; the second form was used for model building and refinement. TBN1 is mainly α-helical and is stabilized by four disulfide bridges. Three observed oligosaccharides are crucial for its stability and solubility. The active site is localized at the bottom of the positively charged groove and contains a zinc cluster that is essential for enzymatic activity. An equilibrium between monomers, dimers and higher oligomers of TBN1 was observed in solution. Principles of the reaction mechanism of the phosphodiesterase activity are suggested, with central roles for the zinc cluster, the nucleobase-binding pocket (Phe-site) and Asp70, Arg73 and Asn167. Based on the distribution of surface residues, possible binding sites for dsDNA and other nucleic acids with secondary structure were identified. The phospholipase activity of TBN1, which is reported for the first time for a nuclease, significantly broadens the substrate promiscuity of the enzyme, and the resulting release of diacylglycerol, which is an important second messenger, can be related to the role of TBN1 in apoptosis.

Biochemical properties of three plant nucleases with anticancer potential

Biochemical and structural properties of three recombinant (R), highly homologous, plant bifunctional nucleases from tomato (R-TBN1), hop (R-HBN1) and Arabis brassica (R-ABN1) were determined. These nucleases cleave single- and double-stranded substrates, as well as both RNA and DNA with nearly the same efficiency. In addition, they are able to cleave several artificial substrates and highly stable viroid RNA. They also possess 3'-nucleotidase activity; therefore, they can be classified as nuclease I family members. Interestingly, poly(G) is resistant to cleavage and moreover it inhibits dsDNase, ssDNase and RNase activity of the studied nucleases. All three nucleases exhibit zinc-dependence and a strong stimulatory effect of Zn²+ for dsDNA cleavage. 3-D models, predicted on the basis of experimental structure of P1 nuclease, show nine amino acid residues responsible for interactions with zinc atoms, located in the same positions as in P1 nuclease. It was also shown that R-TBN1, R-HBN1, and R-ABN1 are all N-glycosylated. Oligosaccharidic chains constitute about 16% of their MW. In addition, an anticancer potential of the R-ABN1 is compared in this work with previously tested R-TBN1, and R-HBN1. R-ABN1 injected intravenously showed 70% inhibitory effect on growth of human prostate carcinoma in athymic mice.

Inhibition of metastasis development by daily administration of ultralow doses of RNase A and DNase I

Recent data on the involvement of miRNA and circulating tumor-derived DNA in regulation of tumorigenesis showed a great prospect for these molecules as a novel class of therapeutic targets and gave a new start for the study of enzymes cleaving nucleic acids as potential antitumor and antimetastatic agents. In the present paper using two murine tumor models with pulmonary or liver metastases we studied the antimetastatic potential of RNase A and DNase I and performed a search for possible molecular targets of the enzymes. Herein, we show for the first time that daily administration of ultralow doses of RNase A (0.5-50 μg/kg) and DNase I (0.02-2.3 mg/kg) inhibits the development of metastasis to 60-90% and RNase A exerts 30% retardation of tumor growth. Remarkably, the increase in RNase A dose from 50 μg/kg to 10mg/kg leads to a disappearance of antitumor and antimetastatic effects. Simultaneous treatment of tumor-bearing animals with RNase A and DNase I leads to an additive effect and results in almost total absence of metastases. The use of RNase A as an adjuvant in conjunction with conventional cytostatic cyclophosphamide results in a reliable enhancement of antitumor and antimetastatic effect of the therapy compared with the use of these agents individually. The search for possible molecular mechanism of antimetastatic effect of nucleases showed that daily administration of the enzymes reduced the pathologically increased level of extracellular nucleic acids and increased nuclease activity of the blood plasma of tumor-bearing mice back to the level of healthy animals. Thus, we unequivocally show that the proposed protocol of treatment of tumor-bearing animals with RNase A and DNase I has a general systemic and immunomodulatory effect, leads to a drastic suppression of metastasis development, and in perspective may become an effective component of intensive complex therapy of cancer.

Potentiation of ribonuclease cytotoxicity by a poly(amidoamine) dendrimer

Variants of bovine pancreatic ribonuclease (RNase A) engineered to evade the endogenous ribonuclease inhibitor protein (RI) are toxic to human cancer cells. Increasing the basicity of these variants facilitates their entry into the cytosol and thus increases their cytotoxicity. The installation of additional positive charge also has the deleterious consequence of decreasing ribonucleolytic activity or conformational stability. Here, we report that the same benefit can be availed by co-treating cells with a cationic dendrimer. We find that adding the generation 2 poly(amidoamine) dendrimer in trans increases the cytotoxicity of RI-evasive RNase A variants without decreasing their activity or stability. The increased cytotoxicity is not due to increased RI-evasion or cellular internalization, but likely results from improved translocation into the cytosol after endocytosis. These data indicate that co-treatment with highly cationic molecules could enhance the efficacy of ribonucleases as chemotherapeutic agents.

Toward an antitumor form of bovine pancreatic ribonuclease: the crystal structure of three noncovalent dimeric mutants

The cytotoxic action of bovine seminal ribonuclease (BS-RNase) depends on its noncovalent swapped dimeric form (NCD-BS), which presents a compact structure that allows the molecule to escape ribonuclease inhibitor (RI). A key role in the acquisition of this structure has been attributed to the concomitant presence of a proline in position 19 and a leucine in position 28. The introduction of Leu28, Cys31, and Cys32 and, in addition, of Pro19 in the sequence of bovine pancreatic ribonuclease (RNase A) has produced two dimeric variants LCC and PLCC, which do exhibit a cytotoxic activity, though at a much lower level than BS-RNase. The crystal structure analysis of the noncovalent swapped form (NCD) of LCC and PLCC, complexed with the substrate analogue 2 '-deoxycytidylyl(3 ',5 ')-2 '-deoxyguanosine, has revealed that, differently from NCD-BS, the dimers adopt an opened quaternary structure, with the two Leu residues fully exposed to the solvent, that does not hinder the binding of RI. Similar results have been obtained for a third mutant of the pancreatic enzyme, engineered with the hinge peptide sequence of the seminal enzyme (residues 16-22) and the two cysteines in position 31 and 32, but lacking the hydrophobic Leu residue in position 28. The comparison of these three structures with those previously reported for other ribonuclease swapped dimers strongly suggests that, in addition to Pro19 and Leu28, the presence of a glycine at the N-terminal end of the hinge peptide is also important to push the swapped form of RNase A dimer into the compact quaternary organization observed for NCD-BS.

Ribonucleases as potential modalities in anticancer therapy

Antitumor ribonucleases are small (10-28 kDa) basic proteins. They were found among members of both, ribonuclease A and T1 superfamilies. Their cytotoxic properties are conferred by enzymatic activity, i.e., the ability to catalyze cleavages of phosphodiester bonds in RNA. They bind to negatively charged cell membrane, enter cells by endocytosis and translocate to cytosol where they evade mammalian protein ribonuclease inhibitor and degrade RNA. Here, we discuss structures, functions and mechanisms of antitumor activity of several cytotoxic ribonucleases with particular emphasis to the amphibian Onconase, the only enzyme of this class that reached clinical trials. Onconase is the smallest, very stable, less catalytically efficient and more cytotoxic than most RNase A homologues. Its cytostatic, cytotoxic and anticancer effects were extensively studied. It targets tRNA, rRNA, mRNA as well as the non-coding RNA (microRNAs). Numerous cancer lines are sensitive to Onconase; their treatment with 10-100 nM enzyme leads to suppression of cell cycle progression, predominantly through G(1), followed by apoptosis or cell senescence. Onconase also has anticancer properties in animal models. Many effects of this enzyme are consistent with the microRNAs, one of its critical targets. Onconase sensitizes cells to a variety of anticancer modalities and this property is of particular interest, suggesting its application as an adjunct to chemotherapy or radiotherapy in treatment of different tumors. Cytotoxic RNases as exemplified by Onconase represent a new class of antitumor agents, with an entirely different mechanism of action than the drugs currently used in the clinic. Further studies on animal models including human tumors grafted on severe combined immunodefficient (SCID) mice and clinical trials are needed to explore clinical potential of cytotoxic RNases.

Onconase cytotoxicity relies on the distribution of its positive charge

Onconase (ONC) is a member of the ribonuclease A superfamily that is toxic to cancer cells in vitro and in vivo. ONC is now in Phase IIIb clinical trials for the treatment of malignant mesothelioma. Internalization of ONC to the cytosol of cancer cells is essential for its cytotoxic activity, despite the apparent absence of a cell-surface receptor protein. Endocytosis and cytotoxicity do, however, appear to correlate with the net positive charge of ribonucleases. To dissect the contribution made by the endogenous arginine and lysine residues of ONC to its cytotoxicity, 22 variants were created in which cationic residues were replaced with alanine. Variants with the same net charge (+2 to +5) as well as equivalent catalytic activity and conformational stability were found to exhibit large (> 10-fold) differences in toxicity for the cells of a human leukemia line. In addition, a more cationic ONC variant could be either much more or much less cytotoxic than a less cationic variant, again depending on the distribution of its cationic residues. The endocytosis of variants with widely divergent cytotoxic activity was quantified by flow cytometry using a small-molecule fluorogenic label, and was found to vary by twofold or less. This small difference in endocytosis did not account for the large difference in cytotoxicity, implicating the distribution of cationic residues as being critical for lipid-bilayer translocation subsequent to endocytosis. This finding has fundamental implications for understanding the interaction of ribonucleases and other proteins with mammalian cells.

New anti-CD30 human pancreatic ribonuclease-based immunotoxin reveals strong and specific cytotoxicityin vivo

Although the therapy of Hodgkin lymphoma and anaplastic large cell lymphoma has been considerably improved during the last decades, high therapeutic toxicity, relapses, secondary tumors, and primary treatment failure(s) occur. Both malignancies are well suited for CD30-targeted immunotherapy because of their strong CD30 expression. We constructed an immunotoxin composed of a single chain variable fragment of a CD30 antibody fused to the human pancreatic ribonuclease, showing CD30-specific binding and ribonucleolytic activity resistant to the inhibitor RNasin. This immunotoxin revealed CD30-specific anti-tumor activity in BALB/c mice that were challenged with CD30-positive or CD30-negative syngeneic tumor cells.

Design and characterization of an HIV-specific ribonuclease zymogen

Ribonucleases are evoking medical interest because of their intrinsic cytotoxic activity. Most notably, ranpirnase, which is an amphibian ribonuclease, is in advanced clinical trials as a chemotherapeutic agent for the treatment of cancer. Here, we describe a strategy to create a novel antiviral agent based on bovine pancreatic ribonuclease (RNase A), a mammalian homologue of ranpirnase. Specifically, we have linked the N- and C-termini of RNase A with an amino acid sequence that is recognized and cleaved by human immunodeficiency virus (HIV) protease. This linkage obstructs the active site, forming an HIV-specific RNase A zymogen. Cleavage by HIV-1 protease increases ribonucleolytic activity by 50-fold. By relying on the proper function of HIV-1 protease, rather than its inhibition, our approach will not engender known mechanisms of resistance. Thus, we report an initial step toward a new class of agents for the treatment of HIV/AIDS.

Human antimicrobial proteins effectors of innate immunity

We live in a world populated by an enormous number of micro-organisms. This necessitates the existence of highly effective mechanisms to control microbial growth. Through many research efforts, a chemical defense system based on the production of antimicrobial proteins (AMPs) has been identified. AMPs are endogenous, small proteins exhibiting antimicrobial activity against a wide variety of micro-organisms. The wide distribution of these molecules in the plant and animal kingdom reflects their biological significance. Various human AMPs show a potent effect on pathogenic micro-organisms including antibiotic-resistant bacteria. Thus, there is great interest in understanding the role of AMPs within innate immunity and evaluating their use and/or specific induction to fend off infections. In this review, we provide an overview of the characteristics of human AMPs and discuss examples where AMPs may be involved in the pathogenesis of infectious and inflammatory diseases.

Ribonucleases as novel chemotherapeutics : the ranpirnase example

Ranpirnase, a cytotoxic ribonuclease from the frog Rana pipiens, is the archetype of a novel class of cancer chemotherapeutic agents based on homologs and variants of bovine pancreatic ribonuclease (RNase A). Ranpirnase in combination with doxorubicin is in clinical trials for the treatment of unresectable malignant mesothelioma and other cancers. The putative mechanism for ranpirnase-mediated cytotoxicity involves binding to anionic components of the extracellular membrane, cytosolic internalization, and degradation of transfer RNA leading to apoptosis. The maintenance of ribonucleolytic activity in the presence of the cytosolic ribonuclease inhibitor protein is a key aspect of the cytotoxic activity of ranpirnase. The basis for its specific toxicity for cancer cells is not known. This review describes the development of ranpirnase as a cancer chemotherapeutic agent.

Cytostatic and cytotoxic properties of Amphinase: a novel cytotoxic ribonuclease from Rana pipiens oocytes

Onconase (Onc), is a novel amphibian cytotoxic ribonuclease with antitumor activity, and is currently in a confirmatory phase III clinical trial for the treatment of malignant mesothelioma. It was recently reported that Rana pipiens oocytes contain still another ribonuclease, named Amphinase (Amph). Amph shows 38-40% amino acid sequence identity with onconase, presents as four variants varying between themselves from 87-99% in amino acid sequence identity and has a molecular mass approximately 13,000. In the present study we describe the effects of Amph on growth of several tumor cell lines. All four variants demonstrated cytostatic and cytotoxic activity against human promyelocytic HL-60-, Jurkat T-cell- and U-937 monocytic leukemia cells. The pattern of Amph activity to certain extent resembled that of Onc. Thus, cell proliferation was suppressed at 0.5-10.0 mug/ml (40-80 nM) Amph concentration with distinct accumulation of cells in G(1) phase of the cell cycle. In addition, the cells were undergoing apoptosis, which manifested by DNA fragmentation (presence of "sub-G1" cells, TUNEL-positivity), caspases and serine proteases activation as well as activation of transglutaminase. The cytostatic and cytotoxic effects of Amph required its ribonuclease activity: the enzymatically inactive Amph-2 having histidine at the active site alkylated was ineffective. The effectiveness and cell cycle specificity was generally similar for all four Amph variants and at the equimolar concentrations was somewhat more pronounced than that of Onc. The observed cytostatic and cytotoxic activity of Amph against tumor cell lines suggests that similar to Onc this cytotoxic ribonuclease may have antitumor activity and find an application in clinical oncology.

Cytotoxic ribonucleases: the dichotomy of Coulombic forces

Cells tightly regulate their contents. Still, nonspecific Coulombic interactions between cationic molecules and anionic membrane components can lead to adventitious endocytosis. Here, we characterize this process in a natural system. To do so, we create variants of human pancreatic ribonuclease (RNase 1) that differ in net molecular charge. By conjugating a small-molecule latent fluorophore to these variants and using flow cytometry, we are able to determine the kinetic mechanism for RNase 1 internalization into live human cells. We find that internalization increases with solution concentration and is not saturable. Internalization also increases with time to a steady-state level, which varies linearly with molecular charge. In contrast, the rate constant for internalization (t1/2 = 2 h) is independent of charge. We conclude that internalization involves an extracellular equilibrium complex between the cationic proteins and abundant anionic cell-surface molecules, followed by rate-limiting internalization. The enhanced internalization of more cationic variants of RNase 1 is, however, countered by their increased affinity for the cytosolic ribonuclease inhibitor protein, which is anionic. Thus, Coulombic forces mediate extracellular and intracellular equilibria in a dichotomous manner that both endangers cells and defends them from the potentially lethal enzymatic activity of ribonucleases.

Binding assay and preliminary X-ray crystallographic analysis of ACTIBIND, a protein with anticarcinogenic and antiangiogenic activities

ACTIBIND is a T2 RNase extracellular glycoprotein produced by the mould Aspergillus niger B1 (CMI CC 324626) that possesses anticarcinogenic and antiangiogenic activities. ACTIBIND was found to be an actin-binding protein that interacts with rabbit muscle actin in a 1:2 molar ratio (ACTIBIND:actin) with a binding constant of 16.17 x 10(4) M(-1). Autoclave-treated ACTIBIND (EI-ACTIBIND) lost its RNase activity, but its actin-binding ability was conserved. ACTIBIND crystals were grown using 20% PEG 3350, 0.2 M ammonium dihydrogen phosphate solution at room temperature (293 K). One to four single crystals appeared in each droplet within a few days and grew to approximate dimensions of 0.5 x 0.5 x 0.5 mm after about two weeks. Diffraction studies of these crystals at low temperature (100 K) indicated that they belong to the P3(1)21 space group, with unit-cell parameters a = 78, b = 78, c = 104 A.

A novel cross-linked RNase A dimer with enhanced enzymatic properties

A new cross-linked ribonuclease A (RNase A) dimer composed of monomeric units covalently linked by a single amide bond between the side-chains of Lys(66) and Glu(9) is described. The dimer was prepared in the absence of water by incubating a lyophilized preparation of RNase, sealed under vacuum, in an oven at 85 degrees C. It was determined that the in vacuo procedure does not induce any significant conformational changes to the overall structure of RNase A, yet the amide cross-link has an increased acid lability, indicating that it is exposed and conformationally strained. Examination of X-ray crystallographic structures indicates that Lys(66) and Glu(9) are not close enough for the in vacuo dimer to adopt any of the known domain-swapped conformations. Therefore, the in vacuo RNase A dimer appears to be a novel dimeric structure. The in vacuo RNase A dimer also exhibits a twofold increase in activity over monomeric RNase A on a per monomer basis. This doubling of enzymatic activity was shown using dsRNA and ssRNA as substrates. In addition to this enhanced ability to degrade RNA, the dimer is not inhibited by the cellular ribonuclease inhibitor protein (cRI).

The RNase a superfamily: Generation of diversity and innate host defense

The Ribonuclease A superfamily includes an extensive network of distinct and divergent gene lineages. Although all ribonucleases of this superfamily share invariant structural and catalytic elements and some degree of enzymatic activity, the primary sequences have diverged significantly, ostensibly to promote novel function. We will review the literature on the evolution and biology of the RNase A ribonuclease lineages that have been characterized specifically as involved in host defense including: (1) RNases 2 and RNases 3, also known as the eosinophil ribonucleases, which are rapidly-evolving cationic proteins released from eosinophilic leukocytes, (2) RNase 7, an anti-pathogen ribonuclease identified in human skin, and (3) RNase 5, also known as angiogenin, another rapidly-evolving ribonuclease known to promote blood vessel growth with recently-discovered antibacterial activity. Interestingly, some of the characterized anti-pathogen activities do not depend on ribonuclease activity per se. We discuss the ways in which the anti-pathogen activities characterized in vitro might translate into experimental confirmation in vivo. We will also consider the possibility that other ribonucleases, such as the dimeric bovine seminal ribonuclease and the frog oocyte ribonucleases, may have host defense functions and therapeutic value that remain to be explored. (190 words).

Evolution and Function of Leukocyte RNase A Ribonucleases of the Avian Species, Gallus gallus

In this study, we explore the evolution and function of two closely related RNase A ribonucleases from the chicken, Gallus gallus. Separated by approximately 10 kb on chromosome 6, the coding sequences of RNases A-1 and A-2 are diverging under positive selection pressure (dN > dS) but remain similar to one another (81% amino acid identity) and to the mammalian angiogenins. Immunoreactive RNases A-1 and A-2 (both approximately 16 kDa) were detected in peripheral blood granulocytes and bone marrow. Recombinant proteins are ribonucleolytically active (kcat = 2.6 and 0.056 s(-1), respectively), and surprisingly, both interact with human placental ribonuclease inhibitor. RNase A-2, the more cationic (pI 11.0), is both angiogenic and bactericidal; RNase A-1 (pI 10.2) has neither activity. We demonstrated via point mutation of the catalytic His110 that ablation of ribonuclease activity has no impact on the bactericidal activity of RNase A-2. We determined that the divergent domains II (amino acids 71-76) and III (amino acids 89-104) of RNase A-2 are both important for bactericidal activity. Furthermore, we demonstrated that these cationic domains can function as independent bactericidal peptides without the tertiary structure imposed by the RNase A backbone. These results suggest that ribonucleolytic activity may not be a crucial constraint limiting the ongoing evolution of this gene family and that the ribonuclease backbone may be merely serving as a scaffold to support the evolution of novel, nonribonucleolytic proteins.

ACTIBIND, an actin-binding fungal T2-RNase with antiangiogenic and anticarcinogenic characteristics

BACKGROUND

ACTIBIND is an Aspergillus niger extracellular ribonuclease (T2-ribonuclease [RNase]) that possesses actin-binding activity. In plants, ACTIBIND inhibits the elongation and alters the orientation of pollen tubes by interfering with the intracellular actin network. The question rose whether ACTIBIND can also affect mammalian cancer development.

METHODS

Cell colony formation was performed in human colon (HT-29, Caco-2, RSB), breast (ZR-75-1), and ovarian (2780) cancer cells in the presence or absence of 1 muM ACTIBIND. In HT-29 and ZR-75-1 cells, the effect of ACTIBIND on cell migration was studied by microscopic observations and by invasion assay through Matrigel. Tube formation was assessed in human umbilical vein endothelial cells (HUVEC) in the presence of angiogenin or basic fibroblast growth factor (bFGF) (1 microg/mL each) following overnight incubation with 1 or 10 microM ACTIBIND. In an athymic mouse xenograft model, HT-29 cells were injected subcutaneously, followed by subcutaneous (0.4-8 mg/mouse/injection) or intraperitoneal (0.001-1 mg/mouse/injection) injections of ACTIBIND. In a rat dimethylhydrazine (DMH)-colorectal carcinogenesis model, ACTIBIND was released directly into the colon via osmotic micropumps (250 microg/rat/day) or given orally via microcapsules (1.6 mg/rat/day). Aberrant crypt foci, tumors in the distal colon, and tumor blood vessels were examined.

RESULTS

ACTIBIND had an anticlonogenic effect unrelated to its ribonuclease activity. It also inhibited angiogenin-induced HUVEC tube formation in a dose-responsive manner. ACTIBIND was found to bind actin in vitro. It also bound to cancer cell surfaces, leading to disruption of the internal actin network and inhibiting cell motility and invasiveness through Matrigel-coated filters. In mice, ACTIBIND inhibited HT-29 xenograft tumor development, given either as a subcutaneous or intraperitoneal treatment. In rats, ACTIBIND exerted preventive and therapeutic effects on developing colonic tumors induced by DMH. It also reduced the degree of tumor observation.

CONCLUSIONS

This study indicated that ACTIBIND is an effective antiangiogenic and anticarcinogenic factor.

A recombinant human RNASET2 glycoprotein with antitumorigenic and antiangiogenic characteristics

BACKGROUND

Human RNASET2 is a T2-RNase glycoprotein encoded by the RNASET2 gene, which is located on chromosome 6 (6q27). Deletion in 6q27 is associated with several human malignancies.

METHODS

A synthetic RNASET2 gene that was optimized for expression in the yeast Pichia pastoris was designed according to the cDNA sequence and was cloned under the control of the methanol-induced promoter fused to the alpha-mating secretion peptide. The recombinant protein was purified from the culture supernatant of transformed P. pastoris through an affinity Sepharose-concanavalin A column. Actin-binding activity was examined by membrane blotting using monoclonal mouse antiactin immunoglobulin M and by cross-linking in solution to G-actin using 1-[3-(dimethylamino)propyl]-3-ethyl-carboimide methiodide. The antiangiogenic activity of RNASET2 (from 0.5 microM to 10 microM) was assessed by a human umbilical vein endothelial (HUVE) cell assay in the presence of 1 microg/mL angiogenin, basic fibroblast growth factor (bFGF), or recombinant human vascular endothelial growth factor (VEGF). Cell colony formation was examined in human colon HT29 cancer cells to assess the antitumorigenic activity of RNASET2 or the enzymatic-inactivated RNASET2 (EI-RNASET2) (1 microM each). In an athymic mouse xenograft model, LS174T human cancer cells were injected subcutaneously. When tumors were palpable, the mice were treated for 3 weeks with RNASET2 (1 mg/kg), paclitaxel (10 mg/kg or 15 mg/kg), or a combination of the 2 drugs.

RESULTS

The recombinant RNASET2 was identified as a 27-kilodalton glycoprotein that possessed the ability to bind actin in vitro. RNASET2 significantly inhibited clonogenicity in HT29 cells. EI-RNASET2 produced a similar effect, suggesting that its antitumorigenic activity is unrelated to its RNase activity. In HUVE cells, RNASET2 inhibited angiogenin-, bFGF-, and VEGF-induced tube formation in a dose-dependent manner. In athymic mice, RNASET2 inhibited the development of an LS174T-derived xenograft by 40%. A synergistic effect was obtained with combined RNASET2 and paclitaxel treatments.

CONCLUSIONS

The current results suggested that RNASET2 represents a new class of antitumorigenic and antiangiogenic drugs, and the findings of this study emphasize the advantage of using agents like RNASET2 in combined therapy.

Disruption of shape-complementarity markers to create cytotoxic variants of ribonuclease A

Onconase (ONC), an amphibian member of the bovine pancreatic ribonuclease A (RNase A) superfamily, is in phase III clinical trials as a treatment for malignant mesothelioma. RNase A is a far more efficient catalyst of RNA cleavage than ONC but is not cytotoxic. The innate ability of ONC to evade the cytosolic ribonuclease inhibitor protein (RI) is likely to be a primary reason for its cytotoxicity. In contrast, the non-covalent interaction between RNase A and RI is one of the strongest known, with the RI.RNase A complex having a K(d) value in the femtomolar range. Here, we report on the use of the fast atomic density evaluation (FADE) algorithm to identify regions in the molecular interface of the RI.RNase A complex that exhibit a high degree of geometric complementarity. Guided by these "knobs" and "holes", we designed variants of RNase A that evade RI. The D38R/R39D/N67R/G88R substitution increased the K(d) value of the pRI.RNase A complex by 20 x 10(6)-fold (to 1.4 microM) with little change to catalytic activity or conformational stability. This and two related variants of RNase A were more toxic to human cancer cells than was ONC. Notably, these cytotoxic variants exerted their toxic activity on cancer cells selectively, and more selectively than did ONC. Substitutions that further diminish affinity for RI (which has a cytosolic concentration of 4 microM) are unlikely to produce a substantial increase in cytotoxic activity. These results demonstrate the utility of the FADE algorithm in the examination of protein-protein interfaces and represent a landmark towards the goal of developing chemotherapeutics based on mammalian ribonucleases.

The Importance of Dynamic Effects on the Enzyme Activity

Onconase (ONC), a member of the RNase A superfamily extracted from oocytes of Rana pipiens, is an effective cancer killer. It is currently used in treatment of various forms of cancer. ONC antitumor properties depend on its ribonucleolytic activity that is low in comparison with other members of the superfamily. The most damaging side effect from Onconase treatment is renal toxicity, which seems to be caused by the unusual stability of the enzyme. Therefore, mutants with reduced thermal stability and/or increased catalytic activity may have significant implications for human cancer chemotherapy. In this context, we have determined the crystal structures of two Onconase mutants (M23L-ONC and C87S,des103-104-ONC) and performed molecular dynamic simulations of ONC and C87S,des103-104-ONC with the aim of explaining on structural grounds the modifications of the activity and thermal stability of the mutants. The results also provide the molecular bases to explain the lower catalytic activity of Onconase compared with RNase A and the unusually high thermal stability of the amphibian enzyme.

Train A, an RNase A-Like Protein Without RNase Activity, Is Secreted and Reabsorbed by the Same Epididymal Cells under Testicular Control1

Most of the proteins secreted in the epididymis are produced by the proximal region, and several of them are secreted in abundance. Many of these major proteins have now been identified, including a new epididymis-specific RNase A-like Train A protein, which has been recently described in several mammals. This protein is expressed and secreted exclusively in the initial part of the epididymis. RNase A activity was analyzed in the fluids from the testis and from different epididymal regions, but in no case was the Train A protein found to have RNase A activity. The protein was present only in the luminal fluid of the epididymal region that secreted it. Using an in vitro/in vivo microperfusion technique and immunogold electron microscopy labeling, we demonstrated that the epithelium that secreted it specifically reabsorbed the protein that was present in the lumen of the tubule. Thus, the presence of Train A protein in epididymal fluid was the result of a steady state between secretion and absorption. The transcription and translation of Train A mRNA were simultaneous and actively regulated by testicular factors. The function of this protein is unknown, but it does not seem to interact directly with sperm. As for other members of the RNase family (e.g., angiogenin), its biological activity might be expressed after its cellular reabsorption. This new compound might therefore participate in an unknown function in the epithelial cells of this first part of the epididymis by an autocrine pathway.

Oligomerization of bovine ribonuclease A: structural and functional features of its multimers

Bovine pancreatic RNase A (ribonuclease A) aggregates to form various types of catalytically active oligomers during lyophilization from aqueous acetic acid solutions. Each oligomeric species is present in at least two conformational isomers. The structures of two dimers and one of the two trimers have been solved, while plausible models have been proposed for the structures of a second trimer and two tetrameric conformers. In this review, these structures, as well as the general conditions for RNase A oligomerization, based on the well known 3D (three-dimensional) domain-swapping mechanism, are described and discussed. Attention is also focused on some functional properties of the RNase A oligomers. Their enzymic activities, particularly their ability to degrade double-stranded RNAs and polyadenylate, are summarized and discussed. The same is true for the remarkable antitumour activity of the oligomers, displayed in vitro and in vivo, in contrast with monomeric RNase A, which lacks these activities. The RNase A multimers also show an aspermatogenic action, but lack any detectable embryotoxicity. The fact that both activity against double-stranded RNA and the antitumour action increase with the size of the oligomer suggests that these activities may share a common structural requirement, such as a high number or density of positive charges present on the RNase A oligomers.

Subtle functional collective motions in pancreatic-like ribonucleases: from ribonuclease A to angiogenin

The analysis of the dynamic behavior of enzymes is fundamental to structural biology. A direct relationship between protein flexibility and biological function has been shown for bovine pancreatic ribonuclease (RNase A) (Rasmussen et al., Nature 1992;357:423-424). More recently, crystallographic studies have shown that functional motions in RNase A involve the enzyme beta-sheet regions that move concertedly on substrate binding and release (Vitagliano et al., Proteins 2002;46:97-104). These motions have been shown to correspond to intrinsic dynamic properties of the native enzyme by molecular dynamics (MD) simulations. To unveil the occurrence of these collective motions in other members of pancreatic-like superfamily, we carried out MD simulations on human angiogenin (Ang). Essential dynamics (ED) analyses performed on the trajectories reveal that Ang exhibits collective motions similar to RNase A, despite the limited sequence identity (33%) of the two proteins. Furthermore, we show that these collective motions are also present in ensembles of experimentally determined structures of both Ang and RNase A. Finally, these subtle concerted beta-sheet motions were also observed for other two members of the pancreatic-like superfamily by comparing the ligand-bound and ligand-free structures of these enzymes. Taken together, these findings suggest that pancreatic-like ribonucleases share an evolutionary conserved dynamic behavior consisting of subtle beta-sheet motions, which are essential for substrate binding and release.

Cc RNase: the Ceratitis capitata ortholog of a novel highly conserved protein family in metazoans

Complementary DNA encoding a protein, designated Cc RNase, was isolated from the insect Ceratitis capitata. Deduced amino acid sequence analysis demonstrates that the Cc RNase has strong sequence homology with other uncharacterized proteins predicted from EST sequences belonging to different animal species, therefore defining a new protein family, which is conserved from Caenorhabditis elegans to humans. Phylogenetic analysis data in addition to extensive homolog searches in all available complete genomes suggested that all family members are true orthologs. Proteins belonging to this family are composed of 95-101 amino acids. The C.capitata orthologous protein was expressed in Escherichia coli. Despite the fact that the amino acid sequence of Cc RNase does not share any significant similarities with other known ribonucleases, our data give strong evidence in support of the assignment of enzymatic activity to the recombinant protein. The expressed molecule exhibits ribonucleolytic activity against poly(C) and poly(U) synthetic substrates, as well as rRNA. It is also demonstrated that expression of Cc RNase in E.coli inhibits growth of the host cells.

Cytotoxic ribonucleases: molecular weapons and their targets

Many ribonucleases (RNases) are highly cytotoxic. In some cases, they attack selectively malignant cells, triggering apoptotic response, and therefore are considered as alternative chemotherapeutic drugs. Factors that determine the cytotoxicity of RNases, primarily of those of microbial origin, are reviewed here. These factors include catalytic activity, ability to escape natural inhibitors, stability, and efficiency of internalization. The latter is, in turn, determined by positive charge on the molecule and interaction with cell membrane. Cellular targets and molecular determinants of RNases decisive for their cytotoxic action are characterized.

Ribonuclease inhibitor as an intracellular sentry

Onconase (ONC) is a homolog of RNase A that is in clinical trials as a cancer chemotherapeutic agent. The toxicity of ONC and RNase A variants relies on their ability to evade the cytosolic ribonuclease inhibitor protein (RI) and degrade cellular RNA. We find that these ribonucleases are more toxic for more rapidly growing cells. The enhanced cytotoxicity does not arise from variation in the endogenous level of RI, which is virtually constant. Overproduction of RI diminishes the potency of toxic RNase A variants, but has no effect on the cytotoxicity of ONC. Thus, RI constrains the cytotoxicity of RNase A. These data provide new insights for the development of an optimal ribonuclease-based cancer chemotherapy.

Secretory ribonucleases are internalized by a dynamin-independent endocytic pathway

Cytosolic internalization is a requirement for the toxicity of secretory ribonucleases. Here, we investigate the mechanism of internalization of Onconase (ONC), a toxic protein, and ribonuclease A (RNase A), a nontoxic homolog. Microscopy studies indicate that both ribonucleases readily bind to the cell surface and are internalized via acidic vesicles. Blocking dynamin-dependent endocytosis prevents transferrin internalization but does not hinder RNase A internalization. ONC and G88R RNase A, which is a toxic variant, demonstrate enhanced cytotoxicity in the absence of clathrin- and dynamin-mediated endocytosis. The cytosolic entry of ribonucleases does not require an acidic environment or transport to the ER and probably occurs from endosomes. Thus, common proteins - secretory ribonucleases - enter the cytosol by a pathway that is distinct from that of other known toxins.

X-ray structure of two crystalline forms of a streptomycete ribonuclease with cytotoxic activity

Ribonuclease (RNase) Sa3 is secreted by the Gram-positive bacterium Streptomyces aureofaciens. The enzyme catalyzes the cleavage of RNA on the 3' side of guanosine residues. Here, x-ray diffraction analysis was used to determine the three-dimensional structure of two distinct crystalline forms of RNase Sa3 to a resolution of 2.0 and 1.7 A. These two structures are similar to each other as well as to that of a homolog, RNase Sa. All of the key active-site residues of RNase Sa (Asn(42), Glu(44), Glu(57), Arg(72), and His(88)) are located in the putative active site of RNase Sa3. Also herein, RNase Sa3 is shown to be toxic to human erythroleukemia cells in culture. Like onconase, which is an amphibian ribonuclease in Phase III clinical trials as a cancer chemotherapeutic, RNase Sa3 is not inhibited by the cytosolic ribonuclease inhibitor protein. Thus, a prokaryotic ribonuclease can be toxic to mammalian cells.

Induction of differentiation rescues HL-60 cells from Rana catesbeiana ribonuclease-induced cell death

Rana catesbeiana ribonuclease (RC-RNase) exerted strong anti-tumor activity and its cytotoxicity was shown to correlate with differentiation stages of three different hepatoma cell lines. In this study, we demonstrate different RC-RNase cytotoxicity in undifferentiated HL-60 cells and in those that had been induced to differentiate by retinoic acid or dimethylsulfoxide. RC-RNase showed cytotoxicity in undifferentiated HL-60 cells, but not in HL-60 cells undergoing terminal differentiation. Furthermore, the caspase-9/caspase-3 pathway was activated when RC-RNase induced death in undifferentiated HL-60 cells and induction of differentiation led to a reversal of the caspase activation pathway.