A Saporin-6 cDNA containing a precursor sequence coding for a carboxyl-terminal extension

Hosts and Heterologous Expression Strategies of Recombinant Toxins for Therapeutic Purposes

The production of therapeutic recombinant toxins requires careful host cell selection. Bacteria, yeast, and mammalian cells are common choices, but no universal solution exists. Achieving the delicate balance in toxin production is crucial due to potential self-intoxication. Recombinant toxins from various sources find applications in antimicrobials, biotechnology, cancer drugs, and vaccines. "Toxin-based therapy" targets diseased cells using three strategies. Targeted cancer therapy, like antibody-toxin conjugates, fusion toxins, or "suicide gene therapy", can selectively eliminate cancer cells, leaving healthy cells unharmed. Notable toxins from various biological sources may be used as full-length toxins, as plant (saporin) or animal (melittin) toxins, or as isolated domains that are typical of bacterial toxins, including Pseudomonas Exotoxin A (PE) and diphtheria toxin (DT). This paper outlines toxin expression methods and system advantages and disadvantages, emphasizing host cell selection's critical role.

Ribosome-inactivating and related proteins

Ribosome-inactivating proteins (RIPs) are toxins that act as N-glycosidases (EC 3.2.2.22). They are mainly produced by plants and classified as type 1 RIPs and type 2 RIPs. There are also RIPs and RIP related proteins that cannot be grouped into the classical type 1 and type 2 RIPs because of their different sizes, structures or functions. In addition, there is still not a uniform nomenclature or classification existing for RIPs. In this review, we give the current status of all known plant RIPs and we make a suggestion about how to unify those RIPs and RIP related proteins that cannot be classified as type 1 or type 2 RIPs.

Shiga toxin 1 and ricin A chain bind to human polymorphonuclear leucocytes through a common receptor

The main cause of acute renal failure in children is HUS (haemolytic uraemic syndrome), a consequence of intestinal infections with Escherichia coli strains producing Stx (Shiga toxins). Stx released in the gut by the non-invasive bacteria reach the bloodstream and are targeted to cerebral and renal endothelium triggering HUS. PMN (polymorphonuclear leucocytes) seem to be involved in Stx delivery through an unidentified membrane receptor (Kd=10⁻⁸ M; 2×10⁵ binding sites) which does not allow internalization. Some experts in the field have defined the Stx-PMN interaction as non-specific and of little biological significance. In the present study, we show that the A chain of ricin, the well-known plant RIP (ribosome-inactivating protein), interacts with PMN (Kd=10⁻⁹ M; 2×10⁵ binding sites) competing for the same receptor that recognizes Stx, whereas diphtheria toxin and several agonists of TLRs (Toll-like receptors) or the mannose receptor were ineffective. No toxic effects of ricin A chain on PMN were observed, as assessed by measuring protein synthesis and the rate of spontaneous apoptosis of leucocytes. Moreover, two single-chain RIPs (gelonin and saporin S6) had the same competing effect. Thus RIPs and Stx1 share structural similarities, the same enzymatic activity and a common receptor on PMN. These observations reveal that the Stx-PMN interaction is specific, confirming that PMN recognize molecular patterns common to different foreign molecules.

Expression and purification of cysteine introduced recombinant saporin

Saporin, a ribosome inactivating protein is widely used for immunotoxin construction. Here we describe a mutation of saporin (sap)-3 DNA by introducing a cysteine residue, followed by protein expression and purification by ion exchange chromatography. The purified Cys255sap-3, sap-3 isomer and commercially purchased saporin, were tested for toxicity using assays measuring inhibition for protein synthesis. The IC(50) values showed that the toxicity of the Cys255sap-3 is equivalent to the sap-3 isomer and commercial saporin. Reactivity of Cys255sap-3 was confirmed by labeling with a thio-specific fluorescent probe as well as conjugation with a nonspecific mouse IgG. We have found that a single cysteine within saporin provides a method for antibody conjugation that ensures a uniform and reproducible modification of a saporin variant retaining high activity.

RIBOSOME-INACTIVATING PROTEINS: A Plant Perspective

Ribosome-inactivating proteins (RIPs) are toxic N-glycosidases that depurinate the universally conserved alpha-sarcin loop of large rRNAs. This depurination inactivates the ribosome, thereby blocking its further participation in protein synthesis. RIPs are widely distributed among different plant genera and within a variety of different tissues. Recent work has shown that enzymatic activity of at least some RIPs is not limited to site-specific action on the large rRNAs of ribosomes but extends to depurination and even nucleic acid scission of other targets. Characterization of the physiological effects of RIPs on mammalian cells has implicated apoptotic pathways. For plants, RIPs have been linked to defense by antiviral, antifungal, and insecticidal properties demonstrated in vitro and in transgenic plants. How these effects are brought about, however, remains unresolved. At the least, these results, together with others summarized here, point to a complex biological role. With genetic, genomic, molecular, and structural tools now available for integrating different experimental approaches, we should further our understanding of these multifunctional proteins and their physiological functions in plants.

Characterization of a saporin isoform with lower ribosome-inhibiting activity

We have expressed in Escherichia coli five isoforms of saporin, a single-chain ribosome-inactivating protein (RIP). Translation inhibition activities of the purified recombinant polypeptides in vitro were compared with those of recombinant dianthin 30, a less potent and closely related RIP, and of ricin A chain. Dianthin 30, and a saporin isoform encoded by a cDNA from leaf tissue (SAP-C), both had about one order of magnitude lower activity in translation inhibition assays than all other isoforms of saporin tested. We recently demonstrated that saporin extracted from seeds of Saponaria officinalis binds to alpha2-macroglobulin receptor (alpha2MR; also termed low density lipoprotein-receptor-related-protein), indicating a general mechanism of interaction of plant RIPs with the alpha2MR system [Cavallaro, Nykjaer, Nielsen and Soria (1995) Eur. J. Biochem. 232, 165-171]. Here we report that SAP-C bound to alpha2MR equally well as native saporin. However, the same isoform had about ten times lower cytotoxicity than the other saporin isoforms towards different cell lines. This indicates that the lower cell-killing ability of the SAP-C isoform is presumably due to its altered interaction with the protein synthesis machinery of target cells. Since saporin binding to the alpha2MR is competed by heparin, we also tested in cell-killing experiments Chinese hamster ovary cell lines defective for expression of either heparan sulphates or proteoglycans. No differences were observed in cytotoxicity using native saporin or the recombinant isoforms. Therefore saporin binding to the cell surface should not be mediated by interaction with proteoglycans, as is the case for other alpha2MR ligands.

Polynucleotide:adenosine glycosidase activity of ribosome-inactivating proteins: effect on DNA, RNA and poly(A)

Ribosome-inactivating proteins (RIP) are a family of plant enzymes for which a unique activity was determined: rRNAN-glycosidase at a specific universally conserved position, A4324in the case of rat ribosomes. Recently we have shown that the RIP from Saponaria officinalis have a much wider substrate specificity: they are actually polynucleotide:adenosine glycosidases. Here we extend studies on substrate specificity to most known RIP: 52 purified proteins, both type 1 (single-chain) and type 2 (two chain, an enzymatic chain and a lectin chain) were examined for adenine release on various substrates including RNAs from different sources, DNA, and poly(A). All RIP depurinated extensively DNA and some released adenine from all adenine-containing polynucleotides tested. From experimental evidence the entire class of plant proteins, up to now called ribosome-inactivating proteins, may be classified as polynucleotide:adenosine glycosidases. The newly identified substrates may be implicated in the biological role(s) of RIP.

Anti-CD30 immunotoxins with native and recombinant dianthin 30

Immunotoxins were prepared with a Ber-H2 (anti-CD30) monoclonal antibody and native or recombinant dianthin 30, a ribosome-inactivating protein from Dianthus caryophyllus (carnation). Both immunotoxins selectively inhibited protein synthesis by CD30+ cell lines D430B (lymphoblastoid, infected with Epstein-Barr virus), L428 and L540 (both from Hodgkin's lymphoma). IC50 values (concentrations, as dianthin, causing 50% inhibition) ranged from 324 pM to 479 pM (immunotoxin with native dianthin 30) or from 45 pM to 182 pM (immunotoxin with recombinant dianthin 30). The effect of either immunotoxin on protein synthesis by the CD30+ cell line K562 (from a chronic myeloid leukaemia) was not different from that of free dianthin (IC50 higher than nM).

Distribution and properties of major ribosome-inactivating proteins (28 S rRNA N-glycosidases) of the plant Saponaria officinalis L. (Caryophyllaceae)

We have studied the distribution of the protein synthesis inhibitory activity in the tissues of Saponaria officinalis L. (Caryophyllaceae). Seven major saporins, ribosome-inactivating proteins, were purified to apparent homogeneity from leaves, roots and seeds using a new procedure of RIPs isolation including ion-exchange and hydrophobic chromatography. They all catalysed the depurination of rat liver ribosomes, which generate the Endo's diagnostic rRNA fragment upon treatment with acid aniline, thus indicating that A4324 from the 28S rRNA has been released (Endo et al. (1987) J. Biol. Chem. 262, 5908-5912). The molecular mass of saporins by SDS-PAGE ranged between 30.2 and 31.6 kDa and by gel-filtration between 27.5 and 30.1 kDa. Amino acid composition and amino-terminal amino acid sequence indicate that all saporins may be considered isoforms. Only two saporins present in roots were glycosylated (SO-R1 and SO-R3). All saporins are very active on cell-free translation systems derived from rabbit reticulocyte lysates, rat liver, Triticum aestivum L., Cucumis sativus L. and Vicia sativa L. However, they are poor inhibitors of an Escherichia coli translation system. They inhibit protein synthesis in HeLa, BeWo and NB 100 cells, HeLa cells being the most resistant. The enzymatic activity of at least one saporin isoform was dependent on magnesium concentration in the standard rat liver cell-free system.

Ribosome–Inactivating Proteins from Plants: Present Status and Future Prospects

Plant ribosome-inactivating proteins (RIPs) are N-glycosidases which cleave the N-glycosidic bond of adenine in a specific ribosomal RNA sequence. Most commonly RIPs are single-chain proteins (type 1 RIPs), but some (type 2 RIPs) possess a galactose-specific lectin domain that binds to cell surfaces. The latter RIPs are potent toxins, the best known of which is ricin. RIPs have antiviral and abortifacient activities, and, in a widespread application, can also be linked to antibodies or ligands to form immunotoxins or conjugates specifically toxic to a given type of cell.

Sorting of proteins to the vacuoles of plant cells

The secretory system of plant cells sorts a large number of soluble proteins that either are secreted or accumulate in vacuoles. Secretion is a bulk-flow process that requires no information beyond the presence of a signal peptide necessary to enter the endoplasmic reticulum. Many vacuolar proteins are glycoproteins and the glycans are often modified as the proteins pass through the Golgi complex. Vacuolar targeting information is not contained in glycans as it is in animal cells; rather, targeting information is in polypeptide domains as it is in yeast cells. Several such domains have now been identified, but these show little or no amino acid sequence homology. We discuss the possibilities that targeting of protein to plant vacuoles may involve receptors as well as aggregation of protein at low pH.

Pokeweed antiviral protein: ribosome inactivation and therapeutic applications

Pokeweed antiviral protein (PAP) is a ribosome-inactivating protein (RIP) that inactivates ribosomes by the removal of a single adenine from ribosomal RNA. The studies summarized in our review concern the nature and application of this novel therapeutic agent. We describe how researchers continue to elucidate the structure and biologic activity of RIPs. Pokeweed antiviral protein is among the RIPs that have been conjugated to selective monoclonal antibodies for the treatment of several human cancers and viral diseases. Clinical trials using PAP immunotoxins for the treatment of leukemia have been particularly encouraging.