Immunosuppressive activity of bovine seminal RNase on T-cell proliferation

Strengths and Challenges of Secretory Ribonucleases as AntiTumor Agents

Approaches to develop effective drugs to kill cancer cells are mainly focused either on the improvement of the currently used chemotherapeutics or on the development of targeted therapies aimed at the selective destruction of cancer cells by steering specific molecules and/or enhancing the immune response. The former strategy is limited by its genotoxicity and severe side effects, while the second one is not always effective due to tumor cell heterogeneity and variability of targets in cancer cells. Between these two strategies, several approaches target different types of RNA in tumor cells. RNA degradation alters gene expression at different levels inducing cell death. However, unlike DNA targeting, it is a pleotropic but a non-genotoxic process. Among the ways to destroy RNA, we find the use of ribonucleases with antitumor properties. In the last few years, there has been a significant progress in the understanding of the mechanism by which these enzymes kill cancer cells and in the development of more effective variants. All the approaches seek to maintain the requirements of the ribonucleases to be specifically cytotoxic for tumor cells. These requirements start with the competence of the enzymes to interact with the cell membrane, a process that is critical for their internalization and selectivity for tumor cells and continue with the downstream effects mainly relying on changes in the RNA molecular profile, which are not only due to the ribonucleolytic activity of these enzymes. Although the great improvements achieved in the antitumor activity by designing new ribonuclease variants, some drawbacks still need to be addressed. In the present review, we will focus on the known mechanisms used by ribonucleases to kill cancer cells and on recent strategies to solve the shortcomings that they show as antitumor agents, mainly their pharmacokinetics.

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.

Species-specific and collection method-dependent differences in endometrial susceptibility to seminal plasma-induced RNA degradation

This study aimed to determine the effect of bull seminal plasma (SP) and sperm on endometrial function. Bovine endometrial explants were incubated with: ejaculated sperm with or without SP, epididymal sperm, or SP alone. Neither ejaculated nor epididymal sperm induced differential expression of IL1A, IL1B, IL6, IL8, PTGES2, TNFA, and LIF. Interestingly, SP had a detrimental effect on endometrial RNA integrity. Addition of an RNase inactivation reagent to SP blocked this effect, evidencing a role for a SP-RNase. Because bulls deposit the ejaculate in the vagina, we hypothesized that the bovine endometrium is more sensitive to SP-RNase than vaginal and cervical tissues (which come into contact with SP during mating), or to endometrium from intrauterine ejaculators (such as the horse). In addition, due to differences in SP-RNase abundance depending on SP collection method (i.e., with an artificial vagina, AV, or by electroejaculation, EE), this effect was also tested. Bull SP, collected by AV, degrades RNA of mare endometrium, and bovine vagina, cervix and endometrium. However, stallion SP or bull SP collected by EE did not elicit this effect. Thus, results do not support a role for SP in modulating endometrial function to establish pregnancy in cattle.

Immune Modulation by Human Secreted RNases at the Extracellular Space

The ribonuclease A superfamily is a vertebrate-specific family of proteins that encompasses eight functional members in humans. The proteins are secreted by diverse innate immune cells, from blood cells to epithelial cells and their levels in our body fluids correlate with infection and inflammation processes. Recent studies ascribe a prominent role to secretory RNases in the extracellular space. Extracellular RNases endowed with immuno-modulatory and antimicrobial properties can participate in a wide variety of host defense tasks, from performing cellular housekeeping to maintaining body fluid sterility. Their expression and secretion are induced in response to a variety of injury stimuli. The secreted proteins can target damaged cells and facilitate their removal from the focus of infection or inflammation. Following tissue damage, RNases can participate in clearing RNA from cellular debris or work as signaling molecules to regulate the host response and contribute to tissue remodeling and repair. We provide here an overall perspective on the current knowledge of human RNases’ biological properties and their role in health and disease. The review also includes a brief description of other vertebrate family members and unrelated extracellular RNases that share common mechanisms of action. A better knowledge of RNase mechanism of actions and an understanding of their physiological roles should facilitate the development of novel therapeutics.

A study of ribonuclease activity in venom of vietnam cobra

BACKGROUND

Ribonuclease (RNase) is one of the few toxic proteins that are present constantly in snake venoms of all types. However, to date this RNase is still poorly studied in comparison not only with other toxic proteins of snake venom, but also with the enzymes of RNase group. The objective of this paper was to investigate some properties of RNase from venom of Vietnam cobra Naja atra.

METHODS

Kinetic methods and gel filtration chromatography were used to investigate RNase from venom of Vietnam cobra.

RESULTS

RNase from venom of Vietnam cobra Naja atra has some characteristic properties. This RNase is a thermostable enzyme and has high conformational stability. This is the only acidic enzyme of the RNase A superfamily exhibiting a high catalytic activity in the pH range of 1-4, with pHopt = 2.58 ± 0.35. Its activity is considerably reduced with increasing ionic strength of reaction mixture. Venom proteins are separated by gel filtration into four peaks with ribonucleolytic activity, which is abnormally distributed among the isoforms: only a small part of the RNase activity is present in fractions of proteins with molecular weights of 12-15 kDa and more than 30 kDa, but most of the enzyme activity is detected in fractions of polypeptides, having molecular weights of less than 9 kDa, that is unexpected.

CONCLUSIONS

RNase from the venom of Vietnam cobra is a unique member of RNase A superfamily according to its acidic optimum pH (pHopt = 2.58 ± 0.35) and extremely low molecular weights of its major isoforms (approximately 8.95 kDa for RNase III and 5.93 kDa for RNase IV).

The mammalian secreted RNases: Mechanisms of action in host defence

The mammalian ribonucleaseA family comprises a large group of structurally similar proteins which are secreted by a range of tissues and immune cells. Their physiological role is unclear. It has been suggested that some of these RNases contribute to host defence, notably eosinophil-derived neurotoxin, eosinophil cationic protein, eosinophil-associated RNases, RNase4, angiogenin (RNase5), RNase7, RNase8 and bovine seminal RNase. This review summarises data supporting the involvement of these proteins in host defence, focusing on their antimicrobial, cytotoxic and immunomodulatory activities. The extent to which the data support possible mechanisms of action for these proteins is discussed. This compilation of findings and current hypotheses on the physiological role of these RNases will provide a stimulus for further research and development of ideas on the contribution of the RNases to host defence.

Characterization of the angiogenic activity of zebrafish ribonucleases

Ribonucleases identified from zebrafish possess angiogenic and bactericidal activities. Zebrafish RNases have three intramolecular disulfide bonds, a characteristic structural feature of angiogenin, different from the typical four disulfide bonds of the other members of the RNase A superfamily. They also have a higher degree of sequence homology to angiogenin than to RNase A. It has been proposed that all RNases evolved from these angiogenin-like progenitors. In the present study, we characterize, in detail, the function of zebrafish RNases in various steps in the process of angiogenesis. We report that zebrafish RNase-1, -2 and -3 bind to the cell surface specifically and are able to compete with human angiogenin. Similar to human angiogenin, all three zebrafish RNases are able to induce phosphorylation of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase. They also undergo nuclear translocation, accumulate in the nucleolus and stimulate rRNA transcription. However, zebrafish RNase-3 is defective in cleaving rRNA precursor, even though it has been reported to have an open active site and has higher enzymatic activity toward more classic RNase substrates such as yeast tRNA and synthetic oligonucleotides. Taken together with the findings that zebrafish RNase-3 is less angiogenic than zebrafish RNase-1 and -2 as well as human angiogenin, these results suggest that zebrafish RNase-1 is the ortholog of human angiogenin and that the ribonucleolytic activity of zebrafish RNases toward the rRNA precursor substrate is functionally important for their angiogenic activity.

Natural and chemically induced oligomeric ribonucleases: structural study by immobilized metal ion affinity electrophoresis and their functional relationship

Oligomerization can endow proteins with novel structural and catalytic properties. The native dimer of bovine seminal ribonucleases (BS-RNase) binds, melts and catalyses the hydrolysis of double-stranded ribonucleic acids 30-fold better than its pancreatic homologue, the monomeric RNase A. Chemically induced oligomers of pancreatic RNase A are also found to show an increase in enzyme activity on double-stranded poly(A).poly(U) (Libonati, M. Bertoldi, M. and Sorrentino, S. (1996) Biochem. J. 318, 287-290) and, therefore, can be considered as potential immunosuppressive and cytotoxic agents. We report here a study on the relationship between surface histidine topography in oligomeric forms of these ribonucleases and their catalytic properties. Subtle changes in structure conformation of both BS-RNase and oligomeric RNase A are shown to result in a modification of the affinity of these proteins toward the immobilized transition-metal chelate, IDA-Cu(II). Because, such conformational change has been shown to correlate with an improvement of the newly acquired biological activities upon oligomerization, we can conclude that surface histidines topography constitutes an exquisite probe for the study of protein structure/function relationship.

Bovine seminal RNase induces apoptosis in normal proliferating lymphocytes

Bovine seminal ribonuclease is a member of the RISBAses (ribonucleases with special biological actions) family. It exerts specific anti-tumor, embryotoxic, aspermatogenic and immunosuppressive activities. The cytotoxic effect of bovine seminal ribonuclease on tumor cells is accompanied by the induction of apoptosis. We provide ultrastructural and flow cytometry evidence of apoptotic death following bovine seminal ribonuclease treatment, in normal cells and phytohemagglutinin-stimulated lymphocytes. Transmission and scanning electron microscopy, which were fully supported by flow cytometry data, showed typical features of apoptosis, such as decreased cell size, chromatin condensation, fragmentation in micronuclei, and the presence of apoptotic bodies.

Ribonucleases and their antitumor activity

The antitumor effect of ribonucleases was studied with animal ribonucleolytic enzymes, bovine pancreatic RNase A, bovine seminal RNase (BS-RNase), onconase and angiogenin. While bovine pancreatic RNase A exerts a minor antitumor effect, BS-RNase and onconase exert significant effects. Angiogenin, as RNase, works in an opposite way, it initiates vascularization of tumors and subsequent tumor growth. Ribonunclease inhibitors are not able to inhibit the antitumor effectiveness of BS-RNase or onconase. However, they do so in the case of pancreatic RNases. Conjugation of BS-RNase with antibodies against tumor antigens (preparation of immunotoxins) like the conjugation of the enzyme with polymers enhances the antitumor activity of the ribonuclease. After conjugation with polymers, the half-life of BS-RNase in blood is extended and its immunogenicity reduced. Recombinant RNases have the same functional activity as the native enzymes. The synthetic genes have also been modified, some of them with gene sequences typical for the BS-RNase parts. Recent experimental efforts are directed to the preparation of 'humanized antitumor ribonuclease' that would be structurally similar to human enzyme with minimal immunogenicity and side effects. The angiogenesis of tumors is attempted to be minimized by specific antibodies or anti-angiogenic substances.

Thermodynamic stability of the two isoforms of bovine seminal ribonuclease

Bovine seminal ribonuclease (BS-RNase) is a dimeric protein with two identical subunits linked by two disulfide bridges, each subunit showing 80% of sequence identity with pancreatic RNase A. BS-RNase exists in two different quaternary conformations in solution: the MxM form, in which each subunit exchanges its alpha-helical N-terminal segment with its partner, and the M=M form with no exchange. By differential scanning microcalorimetry (DSC), the denaturation of the two dimeric forms of BS-RNase was found to be more complex than a simple two-state process. Monomeric derivatives of the dimeric protein follow instead a simple two-state mechanism, but are distinctly less stable than RNase A. The three-state N if I if D denaturation process of the two quaternary isoforms was interpreted by identifying in the dimers a central highly structured core, enclosing the covalently bonded subunit interface, which unfolds only after the periphery (mainly the N-terminal peptide) unfolds. Circular dichroism spectra of the two forms in the far-ultraviolet region show large differences between the secondary structure of the isoforms and that of the native BS-RNase mixture at equilibrium. This has been attributed to the presence in the equilibrium mixture of intermediate forms with displaced and disordered N-terminal alpha-helical segments.

Bovine seminal ribonuclease attached to nanoparticles made of polylactic acid kills leukemia and lymphoma cell lines in vitro

Bovine seminal ribonuclease (BS-RNase) is a protein with a number of biological effects. It shows antitumoral, aspermatogenic, antiembryonic, immunosuppressive and antiviral properties. The cytotoxic effects appear to be specific for tumor cells as non-malignant cells seem to be unaffected in vitro. Unfortunately, the in vivo application of BS-RNase so far was successful only when it was administered intratumorally. Therefore, the objective of the present investigation was to improve the properties of BS-RNase by attachment to nanoparticles made of polylactic acid (PLA-NP) using an adsorption method. This preparation was tested in vitro against leukemia (MOLT-4) and lymphoma (H9) cell lines sensitive and resistant to cytarabine. No difference between the nanoparticle preparation and pure BS-RNase was found in these tests. To examine the in vivo effects, the preparations were tested for their aspermatogenic and antiembryonal efficacy compared to the pure BS-RNase as a rapid test for antitumoral activity. The aspermatogenic and antiembryonal effects were enhanced by the nanoparticle preparation. Consequently, BS-RNase loaded adsorptively to PLA-NP holds promise for the in vivo use as an antitumoral agent. Further research will investigate the efficacy of this preparations in an in vivo tumor model.

Poly[N-(2-Hydroypropyl)Methacrylamide] Conjugates of Bovine Seminal Ribonuclease. Synthesis, Physicochemical, and Preliminary Biological Evaluation

The synthesis of three conjugates of poly(HPMA) with bovine seminal ribonuclease (BS-RNase) differing in their structure is described. Two conjugates contained BS-RNase conjugated with the polymer via functional group situated at the end of the polymer chain (star-shaped conjugate I) or attached to the poly(HPMA) carrier via biodegradable oligopeptide spacers randomly distributed along the polymer chain ("classic" conjugate II). These two conjugates differ in structure, molecular weight, and molecular weight distribution. In addition, a conjugate combining the activity of two compounds, BS-RNase and doxorubicin, both attached to the same polymer chain via biodegradable spacers was synthesized ("classic" conjugate III). Biological activity of all BS-RNase conjugates was compared with that of free BS-RNase and to the polymer-bound anticancer drug doxorubicin (conjugate IV). Unlike the bovine pancreatic ribonuclease (RNase A), BS-RNase displays a potent antitumor activity when tested in vitro and, if administered intratumorally, also in vivo. BS-RNase in its polymer-conjugated forms (conjugates I, II and III) tested on various human tumor cell lines has lost at least part of its antitumor activity. In in vivo experiments (nude mice bearing human melanoma), intratumoral (i.t.) therapy with BS-RNase or with its conjugates II and III showed a significant antitumor effect. Intravenous (i.v.) application of free BS-RNase was totally in-effective, while both BS-RNase conjugates II and III caused significant inhibition of tumor growth. BS-RNase bound to a star-shaped polymer (conjugate I) and administered i.t. or i.v. at the same concentration showed very high toxicity. Our results demonstrate that modification of BS-RNase with poly(HPMA) can prevent it from degrading or inactivating events occurring in blood vessels after intravenous application, significantly enhancing its potential for therapeutical application.

Evolution of oligomeric proteins. The unusual case of a dimeric ribonuclease

The model system made up of a monomeric and a dimeric ribonuclease of the pancreatic-type superfamily has recently attracted the attention of investigators interested in the evolution of oligomeric proteins. In this system, bovine pancreatic ribonuclease (RNase A) is the monomeric prototype, and bovine seminal ribonuclease (BS-RNase) is the dimeric counterpart. However, this evolutionary case is unusual, as BS-RNase is the only dimeric member of the whole large superfamily comprising more than 100 identified members from amphibia, aves, reptilia and mammalia. Furthermore, although the seminal-type RNase gene can be traced back to the divergence of the ruminants, it is expressed only in a single species (Bos taurus). These unusual findings are discussed, as well as previous hypotheses on the evolution of seminal RNase. Furthermore, a new 'minimalist' hypothesis is proposed, in line with basic principles of structural biology and molecular evolution.

Structural changes to ribonuclease A and their effects on biological activity

Bovine seminal ribonuclease (BS RNase) displays immunosuppressive and antitumor activities on mammalian cells, whereas bovine pancreatic ribonuclease (RNase A) is not cytotoxic. To learn more about the mechanism of BS RNase cytotoxicity, various mutants and hybrid proteins were prepared. A series of RNase A variants substituted with amino acid residues from BS RNase were prepared. Concerning quaternary structure, a significant impact was achieved in the variant TM (Q28L K31C S32C), which forms a dimer joined covalently by two intersubunit disulfide bonds. This variant is more efficient than RNase A but less active than BS RNase. Introduction of cationic residues at positions 55, 62, and 64 or substitution at positions 111 and 113 enhanced the immunosuppressive activity of RNase A but did not confer its antitumor activity. The substitution at positions 28, 31, 32, 55, 62, 64, 111, and 113 in variant T13 exerted the best immunosuppressive and antitumor effect observed among the round of the RNase A variants. Replacement of the active-site histidine residues H12 and H119 with asparagine led to the loss of both catalytic and biological activities. Five previously prepared hybrid enzymes (SRA 1-5), synthesized by introducing 16 amino acid residues from RNase A into BS RNase, exerted the same immunosuppressive activities as did the wild-type BS RNase. However, the substitution at positions 111, 113, and 115 in variant SRA 5 caused a marked decrease in its antitumor effect, indicating that these residues play an important role in antitumor efficiency. A different mechanism of action of RNases on tumor cells and/or on blastogenic transformed lymphocytes has been assumed.

Origin of the catalytic activity of bovine seminal ribonuclease against double-stranded RNA

Bovine seminal ribonuclease (RNase) binds, melts, and (in the case of RNA) catalyzes the hydrolysis of double-stranded nucleic acid 30-fold better under physiological conditions than its pancreatic homologue, the well-known RNase A. Reported here are site-directed mutagenesis experiments that identify the sequence determinants of this enhanced catalytic activity. These experiments have been guided in part by experimental reconstructions of ancestral RNases from extinct organisms that were intermediates in the evolution of the RNase superfamily. It is shown that the enhanced interactions between bovine seminal RNase and double-stranded nucleic acid do not arise from the increased number of basic residues carried by the seminal enzyme. Rather, a combination of a dimeric structure and the introduction of two glycine residues at positions 38 and 111 on the periphery of the active site confers the full catalytic activity of bovine seminal RNase against duplex RNA. A structural model is presented to explain these data, the use of evolutionary reconstructions to guide protein engineering experiments is discussed, and a new variant of RNase A, A(Q28L K31C S32C D38G E111G), which contains all of the elements identified in these experiments as being important for duplex activity, is prepared. This is the most powerful catalyst within this subfamily yet observed, some 46-fold more active against duplex RNA than RNase A.

Glycoprotein Erns of pestiviruses induces apoptosis in lymphocytes of several species

Classical swine fever virus and bovine virus diarrhea virus are members of the genus pestivirus, which belongs to the family of the Flaviviridae. Recently, envelope glycoprotein Erns was identified as an RNase. RNases can express different biological actions. They have been shown to be neurotoxic, antihelminthic, and immunosuppressive. We studied the immunosuppressive properties of Erns in vitro. The glycoprotein totally inhibited concanavalin A-induced proliferation of porcine, bovine, ovine, and human lymphocytes. We then studied the direct cytotoxic effects of Erns on lymphocytes and epithelial cells in protein synthesis assays. Erns strongly inhibited the protein synthesis of lymphocytes of different species, without cell membrane damage. This suggested an apoptotic process, and indeed apoptosis of lymphocytes was detected after incubation with Erns. Pestivirus infections are characterized by leukopenia and immunosuppression. Our results suggest that Erns plays an important role in the pathogenesis of pestiviruses.

An angiogenic protein from bovine serum and milk--purification and primary structure of angiogenin-2

Bovine serum and milk contain a basic angiogenic protein that binds tightly to placental ribonuclease inhibitor. It was purified from both sources by ion-exchange and reversed-phase chromatographies. Its amino acid sequence revealed that it is a member of the ribonuclease superfamily. It contains 123 amino acids in a single polypeptide chain, is cross-linked by three disulfide bonds, is glycosylated at Asn33, and is 57% identical to bovine angiogenin. The amino-terminal and carboxyl-terminal residues are pyroglutamic acid and proline, respectively. The protein has ribonucleolytic activity that is similar to, but somewhat lower than, that of bovine angiogenin, i.e. very low relative to RNase. It is angiogenically potent on chicken chorioallantoic membrane, but less so than angiogenin. The sequence and activities demonstrate that this protein is a second, distinct, member of the angiogenin sub-family of pancreatic ribonucleases, and is referred to as angiogenin-2.

Immunosuppressive effect of bovine seminal ribonuclease on a model of corneal transplantation in rabbit

BACKGROUND

Bovine seminal ribonuclease (BS RNase) was determined to have a specific suppressive effect on the proliferation of T lymphocytes in vitro. Its immunosuppressive effect was proven in skin grafting in mice as well.

METHODS

The immunosuppressive effect of BS RNase was evaluated in tissue cultures and on a model of corneal transplantation in rabbits. The penetration of BS RNase into the anterior chamber was detected by immunoblotting of anterior chamber fluid obtained from animals treated either topically or subconjunctivally.

RESULTS

In vitro blastic transformation of mouse T lymphocytes was significantly inhibited by BS RNase (concentrations 15-250 micrograms/ml). No such effect was observed on B lymphocytes. In the rabbit model of corneal graft rejection, BS RNase injected subconjunctivally prolonged mean graft survival time significantly (33.4 days) compared with placebo (salt solution; MST 17.7 days). No BS RNase was detected by immunoblotting in anterior chamber fluid after either topical or subconjunctival application.

CONCLUSION

BS RNase showed significant immunosuppressive effect both in the blastic transformation test and in the rabbit high-risk model of corneal transplantation. Negative results of anterior chamber fluid immunoblotting indicate poor absorption of the drug.

Immunosuppressive activity of bovine seminal ribonuclease and its mode of action

Two preparations of dimeric BS RNase-native and recombinant proteins caused identical immunosuppressive effects on MLC-stimulated human lymphocytes. The monomers of RNase A and BS RNase were ten times less active. The inhibitory effect on MLC-stimmulation was followed by 90% inhibition of cell-mediated lympholysis (CML) caused by BS RNase (10 micrograms/ml). This effect indicated that BS RNase suppressed the recognition phase of the cytotoxic reaction, resulting in inhibition of generation of cytotoxic effector cells. BS RNase exerted a similar effect on generation of cytotoxic LAK cells. Cytotoxic activity of LAK cells or CTLs against K562 target cells was abrogated only when BS RNase was added at the beginning of the sensitizing phase, but the cytotoxicity of effector cells in the destruction phase was not influenced. The effect of RNase A on the generation of cytotoxic cells was much less pronounced. To get more information about the site of action, the effect of BS RNase on early lymphocyte stimulation by PHA was investigated by using fluorescein cell probes. BS RNase (100 micrograms/ml) prevented a shift in fluorescein emission occurring within one hour of activation using fluorescein diacetate as a marker for changes in the cytoplasmic matrix. On the contrary, it did not block the shift in fluorescence emission when tested with diphenylhexatrien as a marker for changes in membrane fluidity. Furthermore the effect of BS RNase on expression of membrane antigens expressed on activated human lymphocytes was estimated. BS RNase significantly inhibited the expression of CD25, CD38 and CD71 antigens on PHA-, Con A- and MLC-stimulated human T and B lymphocytes. No substantial change in expression of these antigens was observed on IL-2-stimulated cells, but DNA synthesis was totally abrogated. These results indicate that the mode of action of BS RNase on activated T and B lymphocytes is based mainly on the suppressed expression of receptors for interleukin-2-alpha-chain and transferrin.

Mechanism of ribonuclease cytotoxicity

Bovine seminal ribonuclease (BS-RNase), a dimeric homolog of bovine pancreatic ribonuclease A (RNase A), is toxic to mammalian cells. In contrast to dimeric BS-RNase, a monomeric BS-RNase and RNase A are not cytotoxic and are bound tightly by cytosolic ribonuclease inhibitor. To elucidate the mechanism of ribonuclease cytotoxicity, we constructed a series of hybrid and semisynthetic enzymes and examined their properties. In five hybrid enzymes, divergent residues in BS-RNase were replaced with the analogous residues of RNase A so as to diminish an interaction with a putative cellular receptor. In a semisynthetic enzyme, the disulfide bonds that cross-link the monomeric subunits of dimeric BS-RNase were replaced with thioether bonds, which can withstand the reducing environment of the cytosol. Each hybrid and semisynthetic enzyme had ribonucleolytic and cytotoxic activities comparable with those of wild-type BS-RNase. These results suggest that dimeric BS-RNase (pI = 10.3) enters cells by adsorptive rather than receptor-mediated endocytosis and then evades cytosolic ribonuclease inhibitor so as to degrade cellular RNA. This mechanism accounts for the need for a cytosolic ribonuclease inhibitor and for the cytotoxicity of other homologs of RNase A.

Immunosuppressive activity of angiogenin in comparison with bovine seminal ribonuclease and pancreatic ribonuclease

Angiogenin, a member of the pancreatic-like ribonuclease family with a special biological action (RISBAses), is a basic protein that induces blood vessel formation. Another member of these special ribonucleases, bovine seminal ribonuclease (BS RNase), displays biological properties, including aspermatogenic, embryotoxic, antitumor and immunosuppressive activities. The effects of two angiogenin preparations tested on the biological activities mentioned above are reported and compared with those of BS RNase and RNase A. In contrast to RNase A, which was ineffective in all biological activities tested, angiogenin suppressed significantly the proliferation of human lymphocytes stimulated by phytohemagglutinin or concanavalin A or by allogenic human lymphocytes (mixed lymphocyte culture). However, angiogenin did not affect the growth of human tumor cell lines, development of cow and mouse embryos and spermatogenicity in mice. On the basis of these results, angiogenin is the first monomeric ribonuclease described so far that displays immunosuppressive activity similar to that of the dimeric BS RNase. The immunosuppressive activity of angiogenin might synergize with the effect on neovascularization of tumor tissues and thus contribute to the development of tumor.

Catalytic activity of bovine seminal ribonuclease is essential for its immunosuppressive and other biological activities

Bovine seminal ribonuclease (BS-RNase) is a homologue of RNase A with special biological properties, including potent immunosuppressive activity. A mutant BS-RNase was created in which His-119, the active-site residue that acts as a general acid during catalysis, was changed to an aspartic acid. H119D BS-RNase formed a dimer with quaternary structure similar to that of the wild-type enzyme but with values of kcat. and kcat./Km for the cleavage of UpA [uridylyl(3'-->5')adenosine] that were 4 x 10(3)-fold lower. The mutant protein also demonstrated dramatically decreased immunosuppressive, anti-tumour, aspermatogenic, and embryotoxic activities. The catalytic activity of BS-RNase is therefore necessary for its special biological properties.

Structural basis for the biological activities of bovine seminal ribonuclease

Bovine seminal ribonuclease (BS-RNase) is a homolog of RNase A with special biological properties that include specific antitumor, aspermatogenic, and immuno-suppressive activities. Unlike RNase A, BS-RNase is a dimer cross-linked by disulfide bonds between Cys31 of one subunit and Cys32 of the other. At equilibrium, this dimer is a mixture of two distinct quaternary forms, M = M and M x M. The conversion of M = M to M x M entails the exchange of NH2-terminal alpha-helices between subunits. Here, the cytotoxic activities of purified M x M were shown to be greater than those of purified M = M, despite extensive equilibration of M = M and M x M during the time course of the assays. Replacing Cys31 or Cys32 with a serine residue did not compromise the enzymatic activity of dimeric BS-RNase, but reduced both the fraction of M x M at equilibrium and the cytotoxicity. We conclude that the M x M form is responsible for the special biological properties of BS-RNase. Since cytosolic ribonuclease inhibitor binds tightly to monomeric but not dimeric BS-RNase and only the M x M form can remain dimeric in the reducing environment of the cytosol, we propose that BS-RNase has evolved its M x M form to retain its lethal enzymatic activity in vivo.

A misfolded but active dimer of bovine seminal ribonuclease

Bovine seminal ribonuclease (BS-RNase) is an unusual homolog of RNase A. Isolated from bulls as a dimer, BS-RNase has special biological properties including antispermatogenic, antitumor and immunosuppressive activities. The structural bases for these properties are unknown. Four forms of BS-RNase were isolated after folding and air oxidation of the denatured and reduced protein produced in Escherichia coli: two dimers (M = M and M x I, where x signifies an active site composed of residues from both subunits) and two monomers (M and I). Considerable ribonuclease activity was generated by air oxidation of an equimolar mixture of two inactive mutant proteins ([H12D]BS-RNase and [H119D]BS-RNase) prepared by site-directed mutagenesis. This activity came from a dimer (M x I) with a composite active site. 1H-NMR spectroscopy revealed that this dimer contained one correctly folded subunit (M), and one incorrectly folded subunit (I). Form I, which is a poor catalyst, was activated by ribonuclease S-protein, suggesting that the C-terminal portion of I was not folded properly. Electrospray-ionization mass spectrometry and sulfhydryl group titration indicated that I contains a single oxidized sulfhydryl group, which cannot participate in a disulfide bond. These results show that quaternary structure in BS-RNase is attained by the initial formation of two monomers, M and I, which then combine with another M to form M = M and M x I, respectively. Adventitious oxidation can thus lead to the formation of a misfolded but active enzyme (M x I).

Aspermatogenic effect of the bull seminal ribonuclease (BS RNase) in the presence of anti-BS RNase antibodies in mice

Injection of mouse scrotum with the bull seminal ribonuclease (BS RNase) isolated from bull seminal vesicle fluid inhibited spermatogenesis and caused a decrease in the weight of the testes. Long-term injection of BS RNase evoked the production of antibodies which reached the titre 524,448. These antibodies did not prevent the aspermatogenic action of BS RNase in vivo when a twofold higher amount of this enzyme was injected into mouse scrotum. Aspermatogenesis was reversible in both the first and second part of the experiment. During the period of aspermatogenesis the males were sterile. Increasing the amount of BS RNase injections in the second part of experiments caused aspermatogenesis around 3 months. No malformations were observed among offspring of males recovered from the first stage of aspermatogenesis. The antigen-antibody complex prepared in vitro and injected into testes of mice evoked the same degree of aspermatogenesis as the enzyme itself.

Identification of a structural glycoprotein of an RNA virus as a ribonuclease

One of the three structural glycoproteins of classical swine fever virus (CSFV) is E0, a disulfide-bonded homodimer that induces virus-neutralizing antibodies and occurs in a virion-bound as well as a secreted form. E0 was shown to be similar to a family of fungal and plant ribonucleases. Purified E0 from CSFV-infected cells was a potent ribonuclease specific for uridine and inhibitable by zinc ions.

Expression of native dimers of bovine seminal ribonuclease in a eukaryotic cell system

Bovine seminal ribonuclease, a uniquely dimeric pancreatic-like RNase, with its dimeric structure stabilized by two intersubunit disulfides, and endowed with special, i.e. non-catalytic, actions (antitumor, immunosuppressive, antispermatogenic), was stably expressed in Chinese hamster ovary cells. The recombinant protein, secreted in the culture medium as a correctly folded dimeric enzyme, was purified to homogeneity and found to be fully active both in its catalytic and antitumor activities.

Seminal RNase: a unique member of the ribonuclease superfamily

The RNase found in bull semen, although a member of the mammalian superfamily of ribonucleases, possesses some unusual properties. Besides its unique structure and enzymic properties, it displays antispermatogenic, antitumor and immunosuppressive activities. Seminal RNase belongs to an interesting group of RNases, the RISBASES (RIbonucleases with Special, i.e. non catalytic, Biological Actions) other members of which include angiogenin, selectively neurotoxic RNases, a lectin and the self-incompatibility factors from a flowering plant.