Seminal RNase: a unique member of the ribonuclease superfamily

Carboxymethyl tethered poly(disubstituted)triazoles built on nucleoside skeletons: A unique class of ribonuclease A inhibitors designed using chemical logic

Molecular docking of N-1,4-disubstituted-1,2,3-triazole tethered carboxymethylated thymidine and uridine with ribonuclease A, indicated their possible binding with the P₁, B₁ and P₂ subsites with varied efficiencies. This theoretical study in combination of our earlier experimental observations was used as the guiding principles for designing a range of 1,4-disubstituted 1, 2, 3- triazole tethered carboxymethylated pyrimidine nucleosides. Triazoles are biologically important molecules and at the same time easily accessible through less complicated synthetic routes as reported about two decades back in the context of "click" reactions. Regioselective propargylation of the nucleosides under controlled conditions followed by the use of CuAAC strategy afforded mono-, bis-, tris- and tetratriazolyl pyrimidine nucleosides. Although the characteristics of nucleosides were lost in these densely functionalized polyheterocycles, the catalytic efficiency of ribonuclease A was significantly reduced by these molecules which were investigated experimentally and by docking studies. Triazoles as linkers helped one or more acidic groups to reach the P₁ subsite of ribonuclease A. Enzyme kinetics showed that the efficiency of inhibition reached the highest point with an optimum number of functional groups and were not linearly dependent on the number of triazole tethered carboxymethyl groups. The location of the triazole ring in the molecule affected the efficiency and nature of inhibition which were the result of the overall structure of the modified nucleosides. Thus, the tris-triazolylated thymidine derivative (T-3', 5', N-tris-CH₂TzCH₂COOH) as opposed to tetra-triazolylated uridine (U-2', 3', 5', N-tetrakis-CH₂TzCH₂COOH) emerged as the best inhibitor with an inhibition constant value of 2.3 ± 0.05 µM.

Ribonuclease 4 is associated with aggressiveness and progression of prostate cancer

Prostate specific antigen screening has resulted in a decrease in prostate cancer-related deaths. However, it also has led to over-treatment affecting the quality of life of many patients. New biomarkers are needed to distinguish prostate cancer from benign prostate hyperplasia (BPH) and to predict aggressiveness of the disease. Here, we report that ribonuclease 4 (RNASE4) serves as such a biomarker as well as a therapeutic target. RNASE4 protein level in the plasma is elevated in prostate cancer patients and is positively correlated with disease stage, grade, and Gleason score. Plasma RNASE4 level can be used to predict biopsy outcome and to enhance diagnosis accuracy. RNASE4 protein in prostate cancer tissues is enhanced and can differentiate prostate cancer and BPH. RNASE4 stimulates prostate cancer cell proliferation, induces tumor angiogenesis, and activates receptor tyrosine kinase AXL as well as AKT and S6K. An RNASE4-specific monoclonal antibody inhibits the growth of xenograft human prostate cancer cell tumors in athymic mice.

Emerging biological functions of ribonuclease 1 and angiogenin

Pancreatic-type ribonucleases (ptRNases) are a large family of vertebrate-specific secretory endoribonucleases. These enzymes catalyze the degradation of many RNA substrates and thereby mediate a variety of biological functions. Though the homology of ptRNases has informed biochemical characterization and evolutionary analyses, the understanding of their biological roles is incomplete. Here, we review the functions of two ptRNases: RNase 1 and angiogenin. RNase 1, which is an abundant ptRNase with high catalytic activity, has newly discovered roles in inflammation and blood coagulation. Angiogenin, which promotes neovascularization, is now known to play roles in the progression of cancer and amyotrophic lateral sclerosis, as well as in the cellular stress response. Ongoing work is illuminating the biology of these and other ptRNases.

Enzymes as a Reservoir of Host Defence Peptides

Host defence peptides (HDPs) are powerful modulators of cellular responses to various types of insults caused by pathogen agents. To date, a wide range of HDPs, from species of different kingdoms including bacteria, plant and animal with extreme diversity in structure and biological activity, have been described. Apart from a limited number of peptides ribosomally synthesized, a large number of promising and multifunctional HDPs have been identified within protein precursors, with properties not necessarily related to innate immunity, consolidating the fascinating hypothesis that proteins have a second or even multiple biological mission in the form of one or more bio-active peptides. Among these precursors, enzymes constitute certainly an interesting group, because most of them are mainly globular and characterized by a fine specific internal structure closely related to their catalytic properties and also because they are yet little considered as potential HDP releasing proteins. In this regard, the main aim of the present review is to describe a panel of HDPs, identified in all canonical classes of enzymes, and to provide a detailed description on hydrolases and their corresponding HDPs, as there seems to exist a striking link between these structurally sophisticated catalysts and their high content in cationic and amphipathic cryptic peptides.

Design of configuration-restricted triazolylated β-d-ribofuranosides: a unique family of crescent-shaped RNase A inhibitors

Seven new carbohydrate-bistriazole hybrid molecules were designed taking into consideration the crescent shaped active site of ribonuclease A (RNase A). In this case, the β-d-ribofuranose structure was used as the basic building unit; both the C1 and C4 arms protruding out towards the β-face of the tetrahydrofuran moiety of the ribose sugar provided an overall "U" shape to the basic building block. Several combinations of bistriazole moieties were constructed on the two arms of this basic building block. These mono- and/or bis-substituted 1,2,3-triazole units were linked to acidic functional groups because of the overall basic nature of the hydrolytic site of RNase A. All these compounds were efficient competitive inhibitors of RNase A with inhibition constants (Ki) in the micromolar range. In contrast to the carboxylic acid-modified hybrid molecules, molecules carrying sulfonic acids were found to be more potent because of the stronger interactions with the positively charged active site. The most efficient inhibitor of the series was the disulfonic acid-functionalized carbohydrate-bis-triazole hybrid molecule. Docking studies disclosed that the molecule, because of its well defined "U" shape with flexible arms, fits effectively in the active site; moreover, in all cases, besides the acid groups, the triazole and sugar rings also actively participated in creating the hydrogen bonding network in the cavity of the enzyme active site.

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.

Surveillance of Tumour Development: The Relationship Between Tumour-Associated RNAs and Ribonucleases

Tumour progression is accompanied by rapid cell proliferation, loss of differentiation, the reprogramming of energy metabolism, loss of adhesion, escape of immune surveillance, induction of angiogenesis, and metastasis. Both coding and regulatory RNAs expressed by tumour cells and circulating in the blood are involved in all stages of tumour progression. Among the important tumour-associated RNAs are intracellular coding RNAs that determine the routes of metabolic pathways, cell cycle control, angiogenesis, adhesion, apoptosis and pathways responsible for transformation, and intracellular and extracellular non-coding RNAs involved in regulation of the expression of their proto-oncogenic and oncosuppressing mRNAs. Considering the diversity/variability of biological functions of RNAs, it becomes evident that extracellular RNAs represent important regulators of cell-to-cell communication and intracellular cascades that maintain cell proliferation and differentiation. In connection with the elucidation of such an important role for RNA, a surge in interest in RNA-degrading enzymes has increased. Natural ribonucleases (RNases) participate in various cellular processes including miRNA biogenesis, RNA decay and degradation that has determined their principal role in the sustention of RNA homeostasis in cells. Findings were obtained on the contribution of some endogenous ribonucleases in the maintenance of normal cell RNA homeostasis, which thus prevents cell transformation. These findings directed attention to exogenous ribonucleases as tools to compensate for the malfunction of endogenous ones. Recently a number of proteins with ribonuclease activity were discovered whose intracellular function remains unknown. Thus, the comprehensive investigation of physiological roles of RNases is still required. In this review we focused on the control mechanisms of cell transformation by endogenous ribonucleases, and the possibility of replacing malfunctioning enzymes with exogenous ones.

RNA binding proteins: Diversity from microsurgeons to cowboys

The conformation and mechanism of proteins that degrade and bind RNA, which has provided key insights into post-transcriptional gene regulation, is explored here. During the twentieth century's last decades, the characterization of ribonucleases and RNA binding domains revealed the diversity of their reaction mechanisms and modes of RNA recognition, and the bases of protein folding, substrate specificity and binding affinity. More recent research showed how these domains combine through oligomerization or genetic recombination to create larger proteins with highly specific and readily programmable ribonucleolytic activity. In the last 15 years, the study of the capacity of proteins, usually disordered, to pool RNAs into discrete, non-aqueous microdroplets to facilitate their transport, modification and degradation - analogous to cowboys herding cattle - has advanced our comprehension of gene expression. Finally, the current uses of RNA binding proteins and the future applications of protein/RNA microdroplets are highlighted.

Functional insights into the role of seminal plasma proteins on sperm motility of buffalo

The objective of the present study was to describe the proteins from the seminal plasma of buffalo and correlate these proteins with sperm motility. Ejaculates from sixteen Murrah buffalo were used. Semen collection was performed by electroejaculation, and the ejaculate was evaluated by macroscopic (volume) and microscopic analysis (subjective motility and vigor, as well as sperm concentration). After the analysis, the samples were centrifuged (800g for 10 min and 10,000 for 30 min at 4 °C), and the supernatant (seminal plasma) was used to determine total protein concentration by the Bradford method. Based on total protein concentration, an aliquot (50 μg) was taken to conduct protein in-solution digestion for nano-LC-ESI-Q-TOF mass spectrometry analysis. Samples were divided into two groups, minimal (little sperm motility) and greater (typical sperm motility), based on non-hierarchical clustering considering motility and emPAI protein value. The data were analyzed by multivariate statistical analysis using principal component analysis (PCA) and partial analysis of minimum squares discrimination (PLS-DA). Forty-eight proteins were detected in the seminal plasma, and fifteen were common to two groups. There were six proteins that were significantly different between the groups. The main functions of proteins in seminal plasma were catalytic and binding activity. Spermadhesin protein, ribonuclease, 14-3-3 protein zeta/delta and acrosin inhibitor were in greater amounts in seminal plasma from the group with greater sperm motility; prosaposin and peptide YY were in greater amounts in the group with little sperm motility. The proteins detected in the greater motility group were correlated with sperm protection, including protection against oxidative stress, lipid peroxidation, protease inhibition and prevention of premature capacitation and acrosome reaction. In the group with little sperm motility, one of the identified proteins is considered to be an antifertility factor, whereas the function of other identified protein is not definitive. Results from the present study add to the knowledge base about the molecular processes related with sperm motility, and these findings can be used for determining potential markers of semen quality.

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 ribonuclease inhibitor resistant dimer of human pancreatic ribonuclease displays specific antitumor activity

Human pancreatic ribonuclease (HPR) and bovine seminal ribonuclease (BS-RNase) are members of the RNase A superfamily. HPR is monomeric, whereas BS-RNase is dimeric. BS-RNase has strong antitumor and cytotoxic activities. However, HPR lacks cytotoxic activity as it is inactivated by intracellular cytosolic ribonuclease inhibitor (RI). Earlier, an RI-resistant cytotoxic variant of HPR, termed HPR-KNE was generated which contained three residues Lys7, Asn71 and Glu111 of HPR, known to interact with RI, mutated to alanine. In this study, we have engineered HPR to develop two dimeric RI-resistant molecules having anti-tumor activity. By incorporating two cysteines in HPR and HPR-KNE, we generated disulfide linked dimeric HPR, and a dimer of HPR-KNE, termed as HPR-D and HPR-KNE-D respectively. HPR-KNE-D was resistant towards inhibition by RI, and was found to be highly toxic to a variety of cells. On J774A.1 cells HPR-KNE-D was >375-fold more cytotoxic than HPR, and 15-fold more toxic than HPR-D. Further, on U373 cells HPR-KNE-D was >65-fold more cytotoxic than HPR, and 9-fold more toxic than HPR-D. The study demonstrates that combining dimerization and RI-resistance results in providing potent anti-tumor activity to HPR. The cytotoxic variants of HPR will be useful in designing protein therapeutics with low immunogenicity.

Abundance of RNase4 and RNase5 mRNA and protein in host defence related tissues and secretions in cattle

Members of the RNaseA family are present in various tissues and secretions but their function is not well understood. Some of the RNases are proposed to participate in host defence. RNase4 and RNase5 are present in cows' milk and have antimicrobial activity. However, their presence in many tissues and secretions has not been characterised. We hypothesised that these two RNases are present in a range of tissues and secretions where they could contribute to host defence. We therefore, determined the relative abundance of RNase4 and RNase5 mRNA as well as protein levels in a range of host defence related and other tissues as well as a range of secretions in cattle, using real time PCR and western blotting. The two RNases were found to be expressed in liver, lung, pancreas, mammary gland, placenta, endometrium, small intestine, seminal vesicle, salivary gland, kidney, spleen, lymph node, skin as well as testes. Corresponding proteins were also detected in many of the above tissues, as well as in seminal fluid, mammary secretions and saliva. This study provides evidence for the presence of RNase4 and RNase5 in a range of tissues and secretions, as well as some major organs in cattle. The data are consistent with the idea that these proteins could contribute to host defence in these locations. This work contributes to growing body of data suggesting that these proteins contribute to the physiology of the organism in a more complex way than acting merely as digestive enzymes.

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RNase4 and RNase5 are present in several tissues and secretions in cattle.

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mRNA and protein levels of the RNases correlate in various tissues analysed.

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The RNases could contribute to host defence in these tissues and secretions.

Exploring the mechanisms of action of human secretory RNase 3 and RNase 7 against Candida albicans

Human antimicrobial RNases, which belong to the vertebrate RNase A superfamily and are secreted upon infection, display a wide spectrum of antipathogen activities. In this work, we examined the antifungal activity of the eosinophil RNase 3 and the skin-derived RNase 7, two proteins expressed by innate cell types that are directly involved in the host defense against fungal infection. Candida albicans has been selected as a suitable working model for testing RNase activities toward a eukaryotic pathogen. We explored the distinct levels of action of both RNases on yeast by combining cell viability and membrane model assays together with protein labeling and confocal microscopy. Site-directed mutagenesis was applied to ablate either the protein active site or the key anchoring region for cell binding. This is the first integrated study that highlights the RNases' dual mechanism of action. Along with an overall membrane-destabilization process, the RNases could internalize and target cellular RNA. The data support the contribution of the enzymatic activity for the antipathogen action of both antimicrobial proteins, which can be envisaged as suitable templates for the development of novel antifungal drugs. We suggest that both human RNases work as multitasking antimicrobial proteins that provide a first line immune barrier.

Changes in brain ribonuclease (BRB) messenger RNA in granulosa cells (GCs) of dominant vs subordinate ovarian follicles of cattle and the regulation of BRB gene expression in bovine GCs

Brain ribonuclease (BRB) is a member of the ribonuclease A superfamily that is constitutively expressed in a range of tissues and is the functional homolog of human ribonuclease 1. This study was designed to characterize BRB gene expression in granulosa cells (GCs) during development of bovine dominant ovarian follicles and to determine the hormonal regulation of BRB in GCs. Estrous cycles of Holstein cows (n = 18) were synchronized, and cows were ovariectomized on either day 3 to 4 or day 5 to 6 after ovulation during dominant follicle growth and selection. Ovaries were collected, follicular fluid (FFL) was aspirated, and GCs were collected for RNA isolation and quantitative polymerase chain reaction. Follicles were categorized as small (1-5 mm; pooled per ovary), medium (5-8 mm; individually collected), or large (8.1-17 mm; individually collected) based on surface diameter. Estradiol (E2) and progesterone (P4) levels were measured by radioimmunoassay (RIA) in FFL. Abundance of BRB messenger RNA (mRNA) in GCs was 8.6- to 11.8-fold greater (P < 0.05) in small (n = 31), medium (n = 66), and large (n = 33) subordinate E2-inactive (FFL E2 < P4) follicles than in large (n = 16) dominant E2-active (FFL E2 > P4) follicles. In the largest 4 follicles, GCs BRB mRNA abundance was negatively correlated (P < 0.01) with FFL E2 (r = -0.65) and E2:P4 ratio (r = -0.46). In experiment 2, GCs from large (8-22 mm diameter) and small (1-5 mm diameter) follicles were treated with insulin-like growth factor 1 (IGF1; 0 or 30 ng/mL) and/or tumor necrosis factor alpha (0 or 30 ng/mL); IGF1 increased (P < 0.05) BRB mRNA abundance, and tumor necrosis factor alpha decreased (P < 0.001) the IGF1-induced BRB mRNA abundance in large-follicle GCs. In experiment 3 to 6, E2, follicle-stimulating hormone, fibroblast growth factor 9, cortisol, wingless 3A, or sonic hedgehog did not affect (P > 0.10) abundance of BRB mRNA in GCs; thyroxine and luteinizing hormone increased (P < 0.05), whereas prostaglandin E2 (PGE2) decreased (P < 0.05) BRB mRNA abundance in small-follicle GCs. Treatment of small-follicle GCs with recombinant human RNase1 increased (P < 0.05) GCs numbers and E2 production. In conclusion, BRB is a hormonally and developmentally regulated gene in bovine GCs and may regulate E2 production during follicular growth in cattle.

Ribonuclease A inhibition by carboxymethylsulfonyl-modified xylo- and arabinopyrimidines

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

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.

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.

Mapping, phylogenetic and expression analysis of the RNase (RNase A) locus in cattle

The mammalian secreted ribonucleases (RNases) comprise a large family of structurally related proteins displaying considerable sequence variation, and have been used in evolutionary studies. RNase 1 (RNase A) has been assumed to play a role in digestion, while other members have been suggested to contribute to host defence. Using the recently assembled bovine genome sequence, we characterised the complete repertoire of genes present in the RNaseA family locus in cattle, and compared this with the equivalent locus in the human and mouse genomes. Several additions and corrections to the earlier analysis of the RNase locus in the mouse genome are presented. The bovine locus encodes 19 RNases, of which only six have unambiguous equivalent genes in the other two species. Chromosomal mapping and phylogenetic analysis indicate that a number of distinct gene duplication events have occurred in the cattle lineage since divergence from the human and mouse lineages. Substitution analysis suggests that some of these duplicated genes are under evolutionary pressure for purifying selection and may therefore be important to the physiology of cattle. Expression analysis revealed that individual RNases have a wide pattern of expression, including diverse mucosal epithelia and immune-related cells and tissues. These data clarify the full repertoire of bovine RNases and their relationships to those in humans and mice. They also suggest that RNase gene duplication within the bovine lineage accompanied by altered tissue-specific expression has contributed a survival advantage.

Genetic diversity of human RNase 8

BACKGROUND

Ribonuclease 8 is a member of the RNase A family of secretory ribonucleases; orthologs of this gene have been found only in primate genomes. RNase 8 is a divergent paralog of RNase 7, which is lysine-enriched, highly conserved, has prominent antimicrobial activity, and is expressed in both normal and diseased skin; in contrast, the physiologic function of RNase 8 remains uncertain. Here, we examine the genetic diversity of human RNase 8, a subject of significant interest given the existence of functional pseudogenes (coding sequences that are otherwise intact but with mutations in elements crucial for ribonucleolytic activity) in non-human primate genomes.

RESULTS

RNase 8 expression was detected in adult human lung, spleen and testis tissue by quantitative reverse-transcription PCR. Only two single-nucleotide polymorphisms and four unique alleles were identified within the RNase 8 coding sequence; nucleotide sequence diversity (π = 0.00122 ± 0.00009 per site) was unremarkable for a human nuclear gene. We isolated transcripts encoding RNase 8 via rapid amplification of cDNA ends (RACE) and RT-PCR which included a distal potential translational start site followed by sequence encoding an additional 30 amino acids that are conserved in the genomes of several higher primates. The distal translational start site is functional and promotes RNase 8 synthesis in transfected COS-7 cells.

CONCLUSIONS

These results suggest that RNase 8 may diverge considerably from typical RNase A family ribonucleases and may likewise exhibit unique function. This finding prompts a reconsideration of what we have previously termed functional pseudogenes, as RNase 8 may be responding to constraints that promote significant functional divergence from the canonical structure and enzymatic activity characteristic of the RNase A family.

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.

RNase 1 genes from the family Sciuridae define a novel rodent ribonuclease cluster

The RNase A ribonucleases are a complex group of functionally diverse secretory proteins with conserved enzymatic activity. We have identified novel RNase 1 genes from four species of squirrel (order Rodentia, family Sciuridae). Squirrel RNase 1 genes encode typical RNase A ribonucleases, each with eight cysteines, a conserved CKXXNTF signature motif, and a canonical His(12)-Lys(41)-His(119) catalytic triad. Two alleles encode Callosciurus prevostii RNase 1, which include a Ser(18)<-->Pro, analogous to the sequence polymorphisms found among the RNase 1 duplications in the genome of Rattus exulans. Interestingly, although the squirrel RNase 1 genes are closely related to one another (77-95% amino acid sequence identity), the cluster as a whole is distinct and divergent from the clusters including RNase 1 genes from other rodent species. We examined the specific sites at which Sciuridae RNase 1s diverge from Muridae/Cricetidae RNase 1s and determined that the divergent sites are located on the external surface, with complete sparing of the catalytic crevice. The full significance of these findings awaits a more complete understanding of biological role of mammalian RNase 1s.

The solution structure and dynamics of human pancreatic ribonuclease determined by NMR spectroscopy provide insight into its remarkable biological activities and inhibition

Human pancreatic ribonuclease (RNase 1) is expressed in many tissues; has several important enzymatic and biological activities, including efficient cleavage of single-stranded RNA, double-stranded RNA and double-stranded RNA-DNA hybrids, digestion of dietary RNA, regulation of vascular homeostasis, inactivation of the HIV, activation of immature dendritic cells and induction of cytokine production; and furthermore shows potential as an anti-tumor agent. The solution structure and dynamics of uncomplexed, wild-type RNase 1 have been determined by NMR spectroscopy methods to better understand these activities. The family of 20 structures determined on the basis of 6115 unambiguous nuclear Overhauser enhancements is well resolved (pairwise backbone RMSD=1.07 A) and has the classic RNase A type of tertiary structure. Important structural differences compared with previously determined crystal structures of RNase 1 variants or inhibitor-bound complexes are observed in the conformation of loop regions and side chains implicated in the enzymatic as well as biological activities and binding to the cytoplasmic RNase inhibitor. Multiple side chain conformations observed for key surface residues are proposed to be crucial for membrane binding as well as translocation and efficient RNA hydrolysis. (15)N-(1)H relaxation measurements interpreted with the standard and our extended Lipari-Szabo formalism reveal rigid regions and identify more dynamic loop regions. Some of the most dynamic areas are key for binding to the cytoplasmic RNase inhibitor. This finding and the important differences observed between the structure in solution and that bound to the inhibitor are indications that RNase 1 to inhibitor binding can be better described by the "induced fit" model rather than the rigid "lock-into-key" mechanism. Translational diffusion measurements reveal that RNase 1 is predominantly dimeric above 1 mM concentration; the possible implications of this dimeric state for the remarkable biological properties of RNase 1 are discussed.

Molecular cloning and characterization of the human RNase kappa, an ortholog of Cc RNase

A novel protein family, designated hereafter as RNase κ (kappa) family, has been recently introduced with the characterization of the specific Cc RNase, isolated from the insect Ceratitis capitata. The human ortholog of this family consists of 98 amino acids and shares > 98% identity with its mammalian counterparts. This RNase is encoded by a single-copy gene found to be expressed in a wide spectrum of normal and cancer tissues. The cDNA of the human ribonuclease has been isolated and subcloned into a variety of prokaryotic expression vectors, but most efforts to express it caused a severe toxic effect. On the other hand, the expression of the human RNase by the use of the methylotrophic yeast Pichia pastoris system resulted in the production of a highly active recombinant enzyme. Using a 30-mer 5′-end-labeled RNA probe as substrate, the purified enzyme seems to preferentially cleave ApU and ApG phosphodiester bonds, while it hydrolyzes UpU bonds at a lower rate. Based on amino acid sequence alignment and substrate specificity data, as well as the complete resistance of the recombinant protein to the placental ribonuclease inhibitor, we concluded that the human RNase κ is a novel endoribonuclease distinct from other known ribonucleases.

Molecular paleoscience: systems biology from the past

Experimental paleomolecular biology, paleobiochemistry, and paleogenetics are closely related emerging fields that infer the sequences of ancient genes and proteins from now-extinct organisms, and then resurrect them for study in the laboratory. The goal of paleogenetics is to use information from natural history to solve the conundrum of modern genomics: How can we understand deeply the function of biomolecular structures uncovered and described by modern chemical biology? Reviewed here are the first 20 cases where biomolecular resurrections have been achieved. These show how paleogenetics can lead to an understanding of the function of biomolecules, analyze changing function, and put meaning to genomic sequences, all in ways that are not possible with traditional molecular biological studies.

Effect of wheat leaf ribonuclease on tumor cells and tissues

The antiproliferative and antitumor effect of wheat leaf ribonuclease was tested in vitro on the human ML-2 cell line and in vivo on athymic nude mice bearing human melanoma tumors. The antiproliferative activity of this plant ribonuclease was negligible in comparison with bovine seminal ribonuclease. In the experiments in vivo, a significant decrease of the tumor size, however, was observed in the mice treated with wheat leaf ribonuclease (27 kDa) compared with the control RNase A and polyethylene glycol. In nude mice injected intratumoraly with wheat leaf ribonuclease, the tumor size decreased from 100% in the control mice to 39% in treated mice. In the mice treated with polyethylene glycol-conjugated wheat leaf ribonuclease, the tumor reduction was observed from 100 to 28%, whereas in counterparts treated with polyethylene glycol-conjugated bovine seminal ribonuclease the tumor inhibition was reduced from 100 to 33%. Certain aspermatogenic and embryotoxic activity of wheat leaf ribonuclease and bovine seminal ribonuclease also appeared, but was lower in comparison with the effect of onconase. Mutual immunological cross-reactivity between wheat leaf ribonuclease antigens on one side and animal RNases (bovine seminal ribonuclease, RNase A, human HP-RNase and onconase) on the other side proved a certain structural similarity between animal and plant ribonucleases. Immunogenicity of wheat leaf ribonuclease was weaker in comparison with bovine seminal ribonuclease (titer of antibodies 160-320 against 1280-2560 in bovine seminal ribonuclease). Interestingly, immunosuppressive effect of wheat leaf ribonuclease tested on mixed lymphocyte culture-stimulated human lymphocytes reached the same level as that of bovine seminal RNase. The antibodies against wheat leaf ribonuclease produced in the injected mice did not inactivate the biological effect of this plant RNase in vivo. This is probably the first paper in which plant ribonuclease was used as antiproliferative and antitumor drug against animal and human normal and tumor cells and tissues in comparison with animal ribonucleases.

Copper complexes of (−)-epicatechin gallate and (−)-epigallocatechin gallate act as inhibitors of Ribonuclease A

Green tea polyphenols, which have the ability to inhibit angiogenesis, form complexes with Cu(II), a known potent stimulator of blood vessel proliferation. Copper complexes of (-)-epicatechin gallate and (-)-epigallocatechin gallate were found to inhibit the enzymatic activity of Ribonuclease A (RNase A) as revealed by an agarose gel based assay and urea denatured gel electrophoresis. The copper complexes were found to be non-competitive inhibitors of RNase A with inhibition constants in the micromolar range. Changes in the secondary structure of the protein are found to occur due to the interaction as revealed from Fourier transform infrared and circular dichroism studies.

Duplication and divergence of 2 distinct pancreatic ribonuclease genes in leaf-eating African and Asian colobine monkeys

Unique among primates, the colobine monkeys have adapted to a predominantly leaf-eating diet by evolving a foregut that utilizes bacterial fermentation to breakdown and absorb nutrients from such a food source. It has been hypothesized that pancreatic ribonuclease (pRNase) has been recruited to perform a role as a digestive enzyme in foregut fermenters, such as artiodactyl ruminants and the colobines. We present molecular analyses of 23 pRNase gene sequences generated from 8 primate taxa, including 2 African and 2 Asian colobine species. The pRNase gene is single copy in all noncolobine primate species assayed but has duplicated more than once in both the African and Asian colobine monkeys. Phylogenetic reconstructions show that the pRNase-coding and noncoding regions are under different evolutionary constraints, with high levels of concerted evolution among gene duplicates occurring predominantly in the noncoding regions. Our data suggest that 2 functionally distinct pRNases have been selected for in the colobine monkeys, with one group adapting to the role of a digestive enzyme by evolving at an increased rate with loss of positive charge, namely arginine residues. Conclusions relating our data to general hypotheses of evolution following gene duplication are discussed.

Cytotoxicity of human RNase-based immunotoxins requires cytosolic access and resistance to ribonuclease inhibition

Immunotoxins are targeted therapeutics designed to kill cancer cells. The targeting moiety of an immunotoxin selectively binds to a tumor cell and targets it for death via an attached toxin. Because the toxins are typically of plant or bacterial origin, their clinical use is limited by immunogenicity and nonspecific toxicity. To circumvent these problems, we have begun to engineer immunotoxins containing human pancreatic ribonuclease. Here we describe the generation of ribonuclease mutants designed to evade a ubiquitous cytosolic inhibitor that would otherwise block cytotoxicity. Two mutants retained catalytic activity and were relatively resistant to the inhibitor. To deliver them to human T leukemic cells, these ribonuclease variants were fused to a single chain Fv fragment specific for CD 7. The ribonuclease-sFv fusion proteins bound CD 7(+) T cells and were internalized yet were not cytotoxic. Transfection of the proteins directly into the cytosol reduced cell viability, suggesting that the failure of the immunotoxins to kill cells when added externally resulted from the inability of the ribonuclease moiety to access the cytosol efficiently. Our results indicate appropriate intracellular routing, as well as resistance to inhibition, is critical to the cytotoxicity of human ribonuclease-based immunotoxins.

Increased plasma levels of angiogenin and the risk of bladder carcinoma: from initiation ot recurrence

BACKGROUND

Angiogenesis is a well known prerequisite for tumor growth and metastasis. It is believed that angiogenin initiates cell migration and aids cell proliferation. Based on this, the authors hypothesized that individuals who had increased plasma levels of angiogenin were at an elevated risk for carcinoma of the urinary bladder.

METHODS

In this ongoing case-control study, the authors used an enzyme-linked immunosorbent assay to compare plasma levels of angiogenin in 209 patients with bladder carcinoma and in 208 healthy control participants who were matched according to age (+/- 5 years), gender, and ethnicity.

RESULTS

The mean plasma angiogenin concentration was significantly higher in patients compared with controls (343.2 ng/mL vs. 308.0 ng/mL, respectively; P < 0.01). High plasma angiogenin levels were associated with a two-fold increased risk for bladder carcinoma. Moreover, in patients who had superficial bladder carcinoma, plasma angiogenin levels were significantly higher among those who had recurrent disease than in those who were without recurrence (P < 0.01). Similarly, patients who had superficial bladder carcinoma with higher angiogenin levels had a shorter recurrence-free survival than patients who had lower angiogenin levels (P < 0.01). Finally, elevated angiogenin levels were associated with an increased recurrence risk, with hazard ratio of 2.85.

CONCLUSIONS

The results of this study demonstrated that the plasma levels of angiogenin were significantly higher in patients who had bladder carcinoma compared with healthy control participants and in patients with superficial bladder carcinoma who had recurrent disease compared with patients who were without recurrence. Therefore, an elevated plasma level of angiogenin may serve as a novel predictor for the risk of bladder carcinoma.

A Dimeric Angiogenin Immunofusion Protein Mediates Selective Toxicity Toward CD22+ Tumor Cells

To improve selective cytotoxicity and pharmacokinetics of an anti-CD22 antibody single chain Fv (scFv)-ribonuclease fusion protein, a dimeric derivative was generated. Human angiogenin was fused via a (G4S)3 spacer peptide to the carboxy-terminal end of the stable dimeric anti-CD22 VL-VH zero-linker scFv MLT-7. The dimeric fusion protein and a monovalent counterpart were produced as soluble proteins in the periplasm of Escherichia coli. Comparative studies with homogeneously purified fusion proteins revealed that both constructs specifically bound to the target antigen and retained ribonucleolytic activity. However, they exhibited a markedly different capability for killing CD22+ tumor cells. The monomeric construct inhibited protein synthesis of target cells in a dose-dependent manner, but 50% inhibition (IC50) could be achieved only at the highest tested concentration (>350 nM). In contrast, the dimeric fusion protein efficiently killed CD22+ Raji and Daudi tumor cell lines with IC50 values of 74 nM and 118 nM, respectively. These results show that the therapeutic potential of scFv-ANG fusion proteins can be markedly enhanced by engineering dimeric derivatives.

Green tea polyphenols as inhibitors of ribonuclease A

Ribonucleases (RNases), which are essential for cleavage of RNA, may be cytotoxic due to undesired cleavage of RNA in the cell. The quest for small molecule inhibitors of members of the ribonuclease superfamily has become indispensable with a growing number exhibiting unusual biological properties. Thus, inhibitors of RNases may serve as potential drug candidates. Green tea catechins (GTC), particularly its major constituent (-)-epigallocatechin-3-gallate (EGCG), have reported potential against cell proliferation and angiogenesis induced by several growth factors including angiogenin, a member of the RNase superfamily. This study reports the inhibition of bovine pancreatic ribonuclease A (RNase A) by EGCG and GTC. This has been checked qualitatively by an agarose gel based assay. Enzyme kinetic studies with cytidine 2',3' cyclic monophosphate as the substrate have also been conducted. Results indicate substantial inhibitory activity of a noncompetitive nature with an inhibition constant of approximately 80 microM for EGCG and approximately 100 microM for GTC measured in gallic acid equivalents.

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.

Role of the hinge peptide and the intersubunit interface in the swapping of N-termini in dimeric bovine seminal RNase

Bovine seminal ribonuclease (BS-RNase) is the only known dimeric enzyme characterized by an equilibrium between two different 3D structures: MxM, with exchange (or swapping) of the N-terminal 1-20 residues, and M=M, without exchange. As a consequence, the hinge region 16-22 has a different tertiary structure in the two forms. In the native protein, the equilibrium ratio between MxM and M=M is about 7 : 3. Kinetic analysis of the swapping process for a recombinant sample shows that it folds mainly in the M=M form, then undergoes interconversion into the MxM form, reaching the same 7 : 3 equilibrium ratio. To investigate the role of the regions that are most affected structurally by the swapping, we expressed variant proteins by replacing two crucial residues with the corresponding ones from RNase A: Pro19, within the hinge peptide, and Leu28, located at the interface between subunits. We compared the structural properties of the monomeric forms of P19A-BS-RNase, L28Q-BS-RNase and P19A/L28Q-BS-RNase variants with those of the parent protein, and investigated the exchange kinetics of the corresponding dimers. The P19A mutation slightly increases the thermal stability of the monomer, but it does not alter the swapping tendency of the dimer. In contrast, the L28Q mutation significantly affects both the dimerization and swapping processes but not the thermal stability of the monomer. Overall, these results suggest that the structural determinants that control the exchange of N-terminal arms in BS-RNase may not be located within the hinge peptide, and point to a crucial role of the interface residues.

Discovery in silico and characterization in vitro of novel genes exclusively expressed in the mouse epididymis

Epididymal proteins interact with sperm during their passage through the epididymis and thus contribute to the maturation and fertilizing capacity of the spermatozoa. In the present study we have discovered five novel epididymis-specific genes through in silico analysis of expressed sequence tags (ESTs) at the UniGene library collection. The strategy used is a powerful way to discover novel epididymis-specific genes. The full-length cDNA sequences were determined, and computational tools were used to characterize the genomic structures and to predict putative functions for the encoded proteins. In vitro analyses revealed that all five genes characterized were highly expressed in the defined areas of the epididymis, and they were not expressed at significant levels in any other tissue. Three of the genes were named on the basis of their putative functions: Spint4 (serine protease inhibitor, Kunitz type 4), and Rnase9 and Rnase10 (ribonuclease, Rnase A family 9 and 10), while for the ESTs AV381130 and AV381126 no putative functions could be predicted. The expression of Spint4, Rnase9, and AV381130 was found to be under a direct or indirect regulation by androgens, while the expression of Rnase10 is regulated by a testicular factor(s) other than androgen. None of the genes were expressed in the immature epididymis, while mRNAs were detected from d 17 onward, at the time of maturation of epididymal epithelium. However, the expression of AV381130 was not detected until d 30 after birth, indicating a close connection between gene expression and puberty.

Antitumor activity and other biological actions of oligomers of ribonuclease A

Dimers, trimers, and tetramers of bovine ribonuclease A, obtained by lyophilization of the enzyme from 40% acetic acid solutions, were purified and isolated by cation exchange chromatography. The two conformers constituting each aggregated species were assayed for their antitumor, aspermatogenic, or embryotoxic activities in comparison with monomeric RNase A and bovine seminal RNase, which is dimeric in nature. The antitumor action was tested in vitro on ML-2 (human myeloid leukemia) and HL-60 (human myeloid cell line) cells and in vivo on the growth of human non-pigmented melanoma (line UB900518) transplanted subcutaneously in nude mice. RNase A oligomers display a definite antitumor activity that increases as a function of the size of the oligomers. On ML-2 and HL-60 cells, dimers and trimers generally show a lower activity than bovine seminal RNase; the activity of tetramers, instead, is similar to or higher than that of the seminal enzyme. The growth of human melanoma in nude mice is inhibited by RNase A oligomers in the order dimers < trimers < tetramers. The action of the two tetramers is very strong, blocking almost completely the growth of melanoma. RNase A dimers, trimers, and tetramers display aspermatogenic effects similar to those of bovine seminal RNase, but, contrarily, they do not show any embryotoxic activity.

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.

Human endothelial cells selectively express large amounts of pancreatic-type ribonuclease (RNase 1)

Pyrimidine-specific ribonucleases are a superfamily of structurally related enzymes with distinct catalytic and biological properties. We used a combination of enzymatic and non-enzymatic assays to investigate the release of such enzymes by isolated cells in serum-free and serum-containing media. We found that human endothelial cells typically expressed large amounts of a pancreatic-type RNase that is related to, if not identical to, human pancreatic RNase. This enzyme exhibits pyrimidine-specific catalytic activity, with a marked preference for poly(C) substrate over poly(U) substrate. It was potently inhibited by placental RNase inhibitor, the selective pancreatic-type RNase inhibitor Inhibit-Ace, and a polyclonal antibody against human pancreatic RNase. The enzyme isolated from medium conditioned by immortalized umbilical vein endothelial cells (EA.hy926) possesses an amino-terminal sequence identical to that of pancreatic RNase, and shows molecular heterogeneity (molecular weights 18,000-26,000) due to different degrees of N-glycosylation. Endothelial cells from arteries, veins, and capillaries secreted up to 100 ng of this RNase daily per million cells, whereas levels were low or undetectable in media conditioned by other cell types examined. The corresponding messenger RNA was detected by RT-PCR in most cell types tested so far, and level of its expression was in keeping with the amounts of protein. The selective strong release of pancreatic-type RNase by endothelial cells suggests that it is endowed with non-digestive functions and involved in vascular homeostasis.

Residues involved in the catalysis, base specificity, and cytotoxicity of ribonuclease from Rana catesbeiana based upon mutagenesis and X-ray crystallography

The Rana catesbeiana (bullfrog) ribonucleases, which belong to the RNase A superfamily, exert cytotoxicity toward tumor cells. RC-RNase, the most active among frog ribonucleases, has a unique base preference for pyrimidine-guanine rather than pyrimidine-adenine in RNase A. Residues of RC-RNase involved in base specificity and catalytic activity were determined by site-directed mutagenesis, k(cat)/K(m) analysis toward dinucleotides, and cleavage site analysis of RNA substrate. The results show that Pyr-1 (N-terminal pyroglutamate), Lys-9, and Asn-38 along with His-10, Lys-35, and His-103 are involved in catalytic activity, whereas Pyr-1, Thr-39, Thr-70, Lys-95, and Glu-97 are involved in base specificity. The cytotoxicity of RC-RNase is correlated, but not proportional to, its catalytic activity. The crystal structure of the RC-RNase.d(ACGA) complex was determined at 1.80 A resolution. Residues Lys-9, His-10, Lys-35, and His-103 interacted directly with catalytic phosphate at the P(1) site, and Lys-9 was stabilized by hydrogen bonds contributed by Pyr-1, Tyr-28, and Asn-38. Thr-70 acts as a hydrogen bond donor for cytosine through Thr-39 and determines B(1) base specificity. Interestingly, Pyr-1 along with Lys-95 and Glu-97 form four hydrogen bonds with guanine at B(2) site and determine B(2) base specificity.

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.

Isolation of a novel thermolabile heterodimeric ribonuclease with antifungal and antiproliferative activities from roots of the sanchi ginseng Panax notoginseng

An isolation procedure, consisting of ion exchange chromatography on CM-Sepharose, affinity chromatography on Affi-gel blue gel, and fast protein liquid chromatography on Mono S, was utilized to purify a base-nonspecific, heterodimeric ribonuclease (RNase) with diverse activities from roots of the sanchi ginseng Panax notoginseng. The RNase is unique in that it consists of two different nonglycoprotein subunits with a molecular weight of 27 and 29 kDa, respectively. The latter subunit is characterized by an N-terminal sequence showing remarkable similarity to that of the bitter gourd RNase. The Panax notoginseng RNase demonstrates potent RNase and translation-inhibitory activities. In addition, it exhibits antiproliferative activity toward leukemia L1210 cells and antifungal activity against Physalospora piricola and Coprinus comatus. Its RNase activity is not heat-resistant, unlike most RNases which are thermostable.

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.

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.