Cytotoxic Ribonuclease Research Articles

The cytotoxic ribonuclease α-sarcin: Three-dimensional structure, ribosomal rna identity elements for specific recognition, and an engineered change in the rna substrate that affects enzyme specificity

The cytotoxic ribonuclease α-sarcin: Three-dimensional structure, ribosomal rna identity elements for specific recognition, and an engineered change in the rna substrate that affects enzyme specificity

A Study of the Intracellular Routing of Cytotoxic Ribonucleases

Several ribonucleases serve as cytotoxic agents in host defense and in physiological cell death pathways. Although certain members of the pancreatic ribonuclease A superfamily can be toxic when applied to the outside of cells, they become thousands of times more toxic when artificially introduced into the cytosol, indicating that internalization is the rate-limiting step for cytotoxicity. We have used three agents that disrupt the Golgi apparatus by distinct mechanisms, retinoic acid, brefeldin A, and monensin, to probe the intracellular pathways ribonucleases take to reach the cytosol. Retinoic acid and monensin potentiate the cytotoxicity of bovine seminal RNase, Onconase, angiogenin, and human ribonuclease A 100 times or more. Retinoic acid-mediated potentiation of ribonucleases is completely blocked by brefeldin A. Ribonucleases appear to route more efficiently into the cytosol through the Golgi apparatus disrupted by monensin or retinoic acid. Intracellular RNA degradation by BS-RNase increased more than 100 times in the presence of retinoic acid confirming that the RNase reaches the cytosol and indicating that degradation of RNA is the intracellular lesion causing toxicity. As retinoic acid alone and Onconase are in clinical trials for cancer therapy, combinations of RNases and retinoic acid in vivo may offer new clinical utility.

A cytotoxic ribonuclease. Study of the mechanism of onconase cytotoxicity

Onconase, or P-30, is a protein initially purified from extracts of Rana pipiens oocytes and early embryos based upon its anticancer activity both in vitro and in vivo. It is a basic single-chain protein with an apparent molecular mass of 12,000 daltons and is homologous to RNase A. In cultured 9L glioma cells, onconase inhibits protein synthesis with an IC50 of about 10(-7) M. The inhibition of protein synthesis correlates with cell death determined by clonogenic assays. 125I-Labeled onconase binds to specific sites on cultured 9L glioma cells. Scatchard analysis of the binding data shows that onconase appears to bind to cells with two different affinities, one with a Kd of 6.2 x 10(-8) and another of 2.5 x 10(-7) M. Each cell could bind about 3 x 10(5) molecules of onconase at each of the two affinity sites. The low affinity Kd is similar to the IC50 for onconase toxicity. Onconase also demonstrates a saturability of cytotoxicity at a concentration that would saturate the low affinity binding site. Incubation at 4 degrees C increased the binding of onconase to cells relative to 37 degrees C binding and also increased the sensitivity of cells to onconase toxicity, indicating that receptor binding may be an initial step in cell toxicity. Onconase cytotoxicity can be blocked by metabolic inhibitors, NaN3 and 2-deoxyglucose, and cytotoxicity is potentiated 10-fold by monensin. Ribonuclease activity appears necessary for onconase toxicity because alkylated onconase, which only retains 2% of the ribonuclease activity, was at least 100-fold less potent in inhibiting protein synthesis in cells. Onconase inhibition of protein synthesis in 9L cells coincides with the degradation of cellular 28 S and 18 S rRNA. In contrast to RNase A, onconase is resistant to two RNase inhibitors, placental ribonuclease inhibitor and Inhibit-Ace. Northern hybridization with placental ribonuclease inhibitor cDNA probe indicates that 9L glioma cells contain endogenous placental ribonuclease inhibitor mRNA. Based on these results, we propose that onconase toxicity results from onconase binding to cell surface receptors, internalization to the cell cytosol where it degrades ribosomal RNA, inhibiting protein synthesis and causing cell death.

Cytotoxic ribonuclease chimeras. Targeted tumoricidal activity in vitro and in vivo.

Monoclonal antibodies to the transferrin receptor or to the T cell antigen, CD5, were chemically linked to mammalian RNase A and found to specifically inhibit protein synthesis in antigen-positive cells. Antibody-mediated specificity of these cytotoxic ribonuclease chimeras (CRCs) was demonstrated in three ways. 1) Toxicity was due to the chemical linkage of RNase to antibody, as the individual components added separately or in combination did not inhibit protein synthesis; 2) the anti-transferrin receptor CRCs inhibited protein synthesis in those cells expressing the human transferrin receptor (K562, U251, Jurkat cells) but had no detectable toxicity to cells lacking the human transferrin receptor (Vero or NIH 3T3 cells); 3) free antibody to either the human transferrin receptor (454A12 or 5E-9) or to the T cell antigen, CD5 (T101), blocked the cytotoxicity of the respective CRC. Two CRC species, designated P1 and P2, that differed in size and stoichiometry of RNase A to antibody, were purified by size-exclusion high performance liquid chromatography. The higher molecular weight P1 conjugate had an IC50 of 20-30 nM, whereas the P2 conjugate had a higher IC50 of 300-500 nM. Bioactivity could be reversibly increased more than 10-fold by freezing. The cytotoxicity of the CRCs was examined in vivo in a solid tumor animal model. Intratumoral injections of an anti-transferrin receptor CRC into established U251 human glioblastoma tumors grown in the flanks of nude mice prevented tumor growth, whereas RNase A mixed with antibody was ineffective. CRCs, therefore, express cytotoxicity in vitro and in vivo. Mammalian nucleases coupled to antibodies may be utilized as cell type-selective cytotoxins and have potential as pharmacologic reagents. The systemic toxicity and immunogenicity observed with mammalian derived cytotoxins may be significantly less than that of the currently employed plant- and bacterial-derived immunotoxins.

A chimaeric ribonuclease-inhibitor gene restores fertility to male sterile plants

Male fertility was restored to genetically engineered male sterile oilseed rape plants. Male sterile plants that express a chimaeric ribonuclease gene in the anther tapetal cell layer were crossed with male fertile plants that were transformed with a chimaeric tapetal-cell-specific ribonuclease-inhibitor gene. F1 progeny expressing both genes are restored to male fertility by the suppression of cytotoxic ribonuclease activity in the anther by the formation of cell-specific RNase/RNase inhibitor complexes. Genetically engineered male sterility and fertility restorer genes should facilitate hybrid seed production in crop plants.

31] Use of α-sarcin to analyze ribosomal RNA—protein interactions

Publisher Summary This chapter discusses the use of α-Sarcin to analyze ribosomal RNA–protein interaction. α-Sarcin is a cytotoxic ribonuclease that blocks protein synthesis by inactivating ribosomes. The cause of this inactivation is a single cleavage at a position within a highly conserved sequence of the large (23–28S) rRNA. The chapter establishes the suitability of α-sarcin by determining the association site for the three ribosomal proteins (L5, Ll8, and L25) that bind to Escherichia coli 5S rRNA. The chapter provides the definition of the L18 and L25 binding sites that is in good agreement with several earlier studies. The chapter goes on to locate the regions of association of Xenopus transcription factor IIIA and of rat ribosomal protein L5 on their cognate 5S rRNAs, thus providing an extensive analysis of protein–5S rRNA interactions. α-Sarcin has proved to be a valuable reagent for the identification of protein-binding sites on 5S rRNA. It is expect that the nuclease will be useful in work with complexes containing other ribonucleic acids.

Nuclease protection analysis of ribonucleoprotein complexes: use of the cytotoxic ribonuclease alpha-sarcin to determine the binding sites for Escherichia coli ribosomal proteins L5, L18, and L25 on 5S rRNA.

A rapid and convenient method has been devised to determine the binding sites for proteins on RNA. The procedure is an adaptation of one used to map DNA-protein complexes by protection against nuclease digestion. The method uses the cytotoxic ribonuclease alpha-sarcin, which hydrolyzes purines in both single- and double-stranded regions of RNA. It has been authenticated by confirming the binding sites for the Escherichia coli ribosomal proteins L18 and L25 on 5S rRNA and its value has been established by identifying the attachment site for protein L5. The procedure should be useful for the analysis of other ribonucleoprotein complexes.