Plant ribonucleases and nucleases as antiproliferative agens targeting human tumors growing in mice.

The antiproliferative and antitumor effect of leaf ribonuclease was tested in vitro on the human ML-2 tumor cell line and in vivo on athymic nude mice bearing human melanoma tumors. The antiproliferative activity of this plant ribonuclease in vitro studies was negligible. In the experiments in vivo a significant decrease of the tumor size, however was observed. From nucleases the mung bean nuclease (PhA) was studied first from nucleases. The antitumor effect of this enzyme on ML2 human tumor cell line was almost non-effective. However, significant antitumor activity was detected on human melanoma tumors in vivo. The antitumor effect of black pine pollen nuclease (PN) tested in vitro was also negligible. On the other side, in the experiments in vivo a significant decrease of the human melanoma tumor size was observed too. Recombinant plant nucleases of tomato (TBN1) and hop (HBN1) (submitted to patenting under no. PV 2008-384;Z7585) were isolated to homogeneity and examined for their antitumor effects and cytotoxicity. Although antiproliferative effects of both recombinant nucleases were not significant on the ML-2 cell culture in vitro, the nucleases were strongly cytostatic in vivo after their administration intravenously as stabilized conjugates with polyethylene glycol (PEG). Recombinant both nucleases were as effective against human melanoma tumors as previously studied pine pollen (PN) and mung bean nucleases and their effects were reached at about ten times lower concentrations compared to the use of bovine seminal RNase (BS-RNase).

Mapping the ribonucleolytic active site of bovine seminal ribonuclease. The binding of pyrimidinyl phosphonucleotide inhibitors.

Bovine seminal ribonuclease (BS-RNase) is a 27kDa homodimeric enzyme and a member of the pancreatic RNase A superfamily. It is the only RNase with a quaternary structure and it is a mixture of two dimeric forms. In the most abundant form the active site is formed by the swapping of the N-terminal segments. BS-RNase is a potent antitumor agent with severe side effects such as aspermatogenicity, and immunosuppression. As a first step towards the design of potent inhibitors of this enzyme we mapped its active site through the study of the binding of uridine 2'-phosphate (U2'p), uridine 3'-phosphate (U3'p), uridine 5'-diphosphate (UDP), cytidine 3'-phosphate (C3'p), and cytidine 5-phosphate (C5'p), by kinetics, and X-ray crystallography. These phosphonucleotides are potent inhibitors with C3'p being the most potent with a K(i) value of 22 microM. Absorption, distribution, metabolism, and excretion pharmacokinetic property predictions reveal U2'p, U3'p, and C5'p as the most promising with respect to oral bioavailability. In vivo studies on the aspermatogenic effect have shown that C3'p and C5'p inhibit significantly this biological action of BS-RNase.

Antitumor effects and cytotoxicity of recombinant plant nucleases.

Recombinant plant nucleases R-TBN1 and R-HBN1 were isolated to homogeneity and examined for their antitumor effects and cytotoxicity. Although antiproliferative effects of both recombinant nucleases were not significant on the ML-2 cell culture in vitro, the nucleases were strongly cytostatic in vivo after their administration intravenously as stabilized conjugates with polyethylene glycol (PEG). Recombinant nucleases were as effective against melanoma tumors as previously studied pine pollen (PN) and mung bean nucleases and their effects were reached at about 10 times lower concentrations compared to the use of bovine seminal RNase (BS-RNase). Because the recombinant nucleases R-HBN1 and R-TBN1 share only 67.4% amino acid identity and showed only partial immunochemical cross-reactivity, their similar anticancerogenic effects can be mainly explained by their catalytical similarity. Both recombinant nucleases showed lower degree of aspermatogenesis compared to BS-RNAse and PN nuclease. Unlike BS-RNase, aspermatogenesis induced by both recombinant nucleases could not be prevented by the homologous antibody complexes. Owing to relatively low cytotoxicity on the one hand, and high efficiency at low protein levels on the other, recombinant plant nucleases R-HBN1 and R-TBN1 appear to be stable biochemical agents that can be targeted as potential antitumor cytostatics.

Cytotoxicity of polyspermine-ribonuclease A and polyspermine-dimeric ribonuclease A.

Polyspermine-ribonuclease A (PS-RNase A) and polyspermine-dimeric ribonuclease A (PS-dimeric RNase A) were prepared by cross-linking ribonuclease A or its covalently linked dimer to polyspermine (PS) using dimethyl suberimidate. The two RNase A derivatives were tested for a possible antitumor action. The in vitro and in vivo cytotoxic activity of PS-RNase A, although strong, is not higher than that known for free polyspermine. PS-dimeric RNase A, which was characterized by mass spectroscopy, titration of free amine groups, and enzymatic assays, proved instead to be a definitely more efficient antitumor agent, both in vitro and in vivo. This result could tentatively be explained in view of the importance of positive charges for ribonuclease activity, considering the higher basicity of PS-dimeric RNase A compared to that of PS-(monomeric)RNase A. It must be also taken into account that the dimeric RNase A moiety of PS-dimeric RNase A could evade the cytoplasmic ribonuclease inhibitor, which instead could trap the monomeric RNase A moiety of the other derivative. The two RNase A derivatives degrade poly(A).poly(U) under conditions where native RNase A is inactive. The results of this work demonstrate once again the importance of positive charges for the functions of mammalian pancreatic type ribonucleases in general, in particular for RNase A derivatives, and the potential therapeutic use of the ribonuclease A derivatives.

The cytosolic ribonuclease inhibitor contributes to intracellular redox homeostasis.

The hypothesis that the cytosolic RNase inhibitor (cRI) has a role in the protection of the cellular redox homeostasis was investigated testing the effects of oxidants and anti-oxidants on normal, primary endothelial HUVE cells, and malignant HeLa cells, before and after their engineering into cRI-deprived cells. We found that cRI plays an important, possibly a key, physiological role in the protection of cells from redox stress, as demonstrated by decreased GSH levels as well as increased oxidant-induced DNA damage in cRI deprived cells.

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.

Effect of hyaluronidase and PEG chain conjugation on the biologic and antitumor activity of RNase A.

Subcutaneous application of bovine RNase A conjugated to HYase (bovine hyaluronidase), polyethylene glycol (PEG) and HYase+PEG resulted in a marked reduction of the width of the spermatogenic layers of the mouse testes. The number of sperms in caput epididymidis was significantly decreased in mice injected with conjugated RNase A. There was not any significant embryotoxic effect of free RNase A even conjugated with HYse, PEG and HYse+PEG. The immunogenicity, expressed in production of antibodies against free RNase A or conjugates with PEG, was very low. However, the immunogenic action of this enzyme conjugated only to HYase was much higher and produced the same immunogenicity as HYase itself. The immunogenic effect of RNase A+HYase conjugate decreased when PEG was joined to this conjugate. The inhibitory effect of RNase A conjugated to HYase, PEG and HYase+PEG on human ML-2 cells studied in vitro, was practically ineffective. On the other side, when RNase A conjugated to HYase or PEG was administered intraperitoneally into the mice bearing human melanoma, the antitumor effect was pronounced.

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.

Comprehensive comparison of the cytotoxic activities of onconase and bovine seminal ribonuclease.

Onconase (ONC) and bovine seminal ribonuclease (BS-RNase) are homologs of bovine pancreatic ribonuclease (RNase A). Unlike RNase A, ONC and BS-RNase can evade the cytosolic ribonuclease inhibitor protein and are potent cytotoxins. Here, the endogenous cytotoxic activities of ONC and BS-RNase are compared in a wide variety of assays. Injections of ONC into one or both testes of mice and rats evokes a stronger aspermatogenic activity than does the injection of BS-RNase. Epididymides exposed to ONC lose mass and all sperm. Testicular tissue is gradually colonized by immunite and fibrocytic cells. Yet, Leydig cells are always present and functional in the ligamented parts of testicles injected with ONC or BS-RNase. ONC is likewise more toxic to mouse embryos than is BS-RNase, both in vitro and in vivo. The antiproliferative effect of ONC on human tumor cell line ML-2 and lymphocytes in a mixed lymphocyte culture is also more pronounced than is that of BS-RNase. The number of granulocyte-macrophage colony-forming units is repressed almost completely by ONC, whereas a five-fold higher dose of BS-RNase does not cause substantial inhibition. In mice, ONC is less immunogenic than BS-RNase but more immunogenic than RNase A. Together, these data indicate that ONC is a pluripotent cytotoxin, and serves as the benchmark with which to gauge the cytotoxicity of other ribonucleases.

PEG chains increase aspermatogenic and antitumor activity of RNase A and BS-RNase enzymes.

RNase A (bovine pancreatic ribonuclease) and BS-RNase (bovine seminal ribonuclease) are monomeric and dimeric enzymes, respectively, with aspermatogenic and antitumor activities. While the aspermatogenic and, in some experimental situations, the antitumor effects of the RNase A are only minor, the activity of BS-RNase in these phenomena is very significant. These differences can be annulled by means of conjugation of the enzymes with PEG (polyethylene glycol) chains. Aspermatogenic activity was studied histologically following subcutaneous injections of RNase A and BS-RNase conjugates in ICR mice, and the antitumor activity in athymic nude mice with growing human melanoma with i.p. injection of these conjugated ribonucleases. The experiments proved that RNase A, when conjugated to PEG, produced identical aspermatogenic and antitumour effects as BS-RNase conjugated to this polymer. Immunogenicity of RNase A and BS-RNase did not change substantially after the conjugation with PEG polymers. Binding of produced antibodies to both ribonucleases attached to PEG, however, was substantially reduced.

Coupling of the antitumoral enzyme bovine seminal ribonuclease to polyethylene glycol chains increases its systemic efficacy in mice.

Bovine seminal ribonuclease (BS-RNase) is an antitumoral active enzyme exhibiting specific antitumoral action against a number of different cancer cell lines. However, its systemic use is limited by its pharmacokinetic properties and antigenicity. Therefore, it was conjugated to polyethylene glycol (PEG) chains to overcome these problems. Measurement of aspermatogenic effects of the preparation after s.c. injection and injection into the scrotum was chosen as a model for the distribution of the enzyme in the body mediated by the linkage to PEG chains. Additionally, the antigenicity of BS-RNase coupled to PEG chains (BS-RNase-PEG) was compared to that of free BS-RNase, as antigenicity is known to be one of the main obstacles in the use of protein-based drugs. BS-RNase-PEG caused aspermatogenic effects after systemic administration to mice in very low concentrations at which free BS-RNase is not effective. Moreover, BS-RNase possessed a very low antigenicity as long as it was coupled to the PEG chains. In order to investigate the antitumoral efficacy of BS-RNase-PEG in vivo, preliminary experiments on the effect of the conjugate on neuroblastoma growth in mice were performed in a UKF-NB-3 xeno-transplantate model, demonstrating a drastically increased anti-tumoral activity of the conjugate compared to the free enzyme.

Ribonucleases endowed with specific toxicity for spermatogenic layers.

Bovine seminal ribonuclease (BS-RNase) is a dimer in which the subunits are cross-linked by disulfide bonds between Cys31 of one subunit and Cys32 of the other. Dimeric BS-RNase is resistant to ribonuclease inhibitor (RI), a protein endogenous to mammalian cells, and is toxic to a variety of cell types. Monomeric BS-RNase (like its homolog, RNase A) is bound tightly by RI and is not cytotoxic. The three-dimensional structure of the RI·RNase A complex suggests that carboxymethylation of C32S BS-RNase (to give MCM31) or C31S BS-RNase (MCM32) could diminish affinity for RI. We find that MCM31 and MCM32 are not only resistant to RI, but are also aspermatogenic to mice. In contrast to the aspermatogenic activity of dimeric BS-RNase, that of MCM31 and MCM32 is directed only at spermatogenic layers. Intratesticular injection of MCM31 or MCM32 affects neither the diameter of seminiferous tubules nor the weight of testes. Also in contrast to wild-type BS-RNase, MCM31 and MCM32 are not toxic to other cell types. Direct immunofluorescence reveals that MCM31 and MCM32 bind only to spermatogonia and primary spermatocytes. This cell specificity makes MCM31 and MCM32 of potential use in seminoma therapy and contraception.