Recombinant hybrid toxin with dual enzymatic activities. Potential use in preparing highly effective immunotoxins

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.

Modes of action of ADP-ribosylated elongation factor 2 in inhibiting the polypeptide elongation cycle: a modeling study

Despite the fact that ADP-ribosylation of eukaryotic elongation factor 2 (EF2) leads to inhibition of protein synthesis, the mechanism by which ADP-ribosylated EF2 (ADPR•EF2) causes this inhibition remains controversial. Here, we applied modeling approaches to investigate the consequences of various modes of ADPR•EF2 inhibitory actions on the two coupled processes, the polypeptide chain elongation and ADP-ribosylation of EF2. Modeling of experimental data indicates that ADPR•EF2 fully blocks the late-phase translocation of tRNAs; but the impairment in the translocation upstream process, mainly the GTP-dependent factor binding with the pretranslocation ribosome and/or the guanine nucleotide exchange in EF2, is responsible for the overall inhibition kinetics. The reduced ADPR•EF2-ribosome association spares the ribosome to bind and shield native EF2 against toxin attack, thereby deferring the inhibition of protein synthesis inhibition and inactivation of EF2. Minimum association with the ribosome also keeps ADPR•EF2 in an accessible state for toxins to catalyze the reverse reaction when nicotinamide becomes available. Our work underscores the importance of unveiling the interactions between ADPR•EF2 and the ribosome, and argues against that toxins inhibit protein synthesis through converting native EF2 to a competitive inhibitor to actively disable the ribosome.

Arg9-peptide facilitates the internalization of an anti-CEA immunotoxin and potentiates its specific cytotoxicity to target cells

Arginines-containing membrane translocational signals (MTS), such as Tat(47-57) and VP22(267-300), and synthetic arginine-rich peptides have been reported to be efficient carriers for transporting various types of biomolecules into cytoplasmic and nuclear compartments of living cells. Among those arginines-containing MTS, a 9-mer arginine peptide (Arg9) was proved the most economical and efficient. We fused Arg9-peptide to an anti-CEA (Carcinoembryonic antigen) immunotoxin, PE35/CEA(Fv)/KDEL, at the position between the toxin moiety and the binding moiety. Strong binding and internalization of this fusion protein was observed in all detected cell lines, but little cytotoxicity to the cells that lack the CEA molecules on the cell surface was detected. However, the cytotoxicity besides the binding activity of the fusion protein to specific tumor cells expressing large amount of CEA molecules on the cell surface was improved markedly, indicating that the Arg9-peptide is capable of facilitating the receptor-mediated endocytosis of this immunotoxin, which leads to the increase of the specific cytotoxicity of this immunotoxin.

Modulation of molecular mechanisms involved in protein synthesis machinery as a new tool for the control of cell proliferation

In the past years, the attention of scientists has focused mainly on the study of the genetic information and alterations that regulate eukaryotic cell proliferation and that lead to neoplastic transformation. All therapeutic strategies against cancer are, to date, directed at DNA either with cytotoxic drugs or gene therapy. Little or no interest has been aroused by protein synthesis mechanisms. However, an increasing body of data is emerging about the involvement of translational processes and factors in control of cell proliferation, indicating that protein synthesis can be an additional target for anticancer strategies. In this paper we review the novel insights on the biochemical and molecular events leading to protein biosynthesis and we describe their involvement in cell proliferation and tumorigenesis. A possible mechanistic explanation is given by the interactions that occur between protein synthesis machinery and the proliferative signal transduction pathways and that are therefore suitable targets for indirect modulation of protein synthesis. We briefly describe the molecular tools used to block protein synthesis and the attempts made at increasing their efficacy. Finally, we propose a new multimodal strategy against cancer based on the simultaneous intervention on protein synthesis and signal transduction.

A deletion within the translocation domain of Pseudomonas exotoxin A enhances translocation efficiency and cytotoxicity concomitantly

Pseudomonas exotoxin A (PE) is a cytotoxin composed of three structural domains. Domain I is responsible for cell binding, domain II for membrane translocation enabling access to the cytosol, and domain III for the catalytic inactivation of protein synthesis, which results in cell death. To investigate the role of the six alpha-helices (A-F) that form the translocation domain, we deleted them successively one at a time. All mutants showed native cell-binding and catalytic activities, indicating that deletions specifically affected translocation activity. This step of the intoxication procedure was examined directly using a cell-free translocation assay, and indirectly by monitoring cytotoxicity. Translocation activity and log(cytotoxicity) were highly correlated, directly indicating that translocation is rate limiting for PE intoxication. Deletion of B, C and D helices resulted in non-toxic and non-translocating molecules, whereas mutants lacking the A or E helix displayed significant cytotoxicity albeit 500-fold lower than native PE. We concluded that B, C and D helices, which make up the core of domain II, are essential, whereas the more peripheral A and E helices are comparatively dispensable. The last helix (F) is inhibitory for translocation because its deletion produced a mutant displaying a translocation activity 60% higher than PE, along with a three- to sixfold increase in cytotoxicity in all tested cell lines. This toxin is the most in vitro active PE mutant obtained until now. Finally, partial duplication of domain II did not give rise to a more actively translocated PE, but rather to a threefold less active molecule.

The emerging role of ricin A-chain immunotoxins in leukemia and lymphoma

Since MRD is the major cause for relapses of malignant diseases, strategies utilizing ITs to target tumor cells surviving conventional treatment have attracted scientific and clinical interest. Many different ITs against various blood-borne as well as solid malignancies have demonstrated specific potent anti-tumor effects in vitro and in animal models. Some of these have already undergone clinical phase I/II-trials. The dose-limiting toxicities of RTA ITs include manifestation of VLS presenting as decreased urinary sodium excretion, hypoalbuminemia, fatigue, hypotonia, myalgia, pulmonary edema, or rhabdomyolysis. Problems encountered clinically include the development of HAMA, HARA, and HACA and the selection of antigen-deficient malignant clones. Most clinical trials performed with ITs so far were conducted in heavily pretreated patients presenting with high tumor burdens. Thus, the responses observed with ITs in these trials are very encouraging and warrant further exploration.

Immunotoxins: an update

The use of immunotoxins (ITs) in the therapy of cancer, graft-vs-host disease (GvHD), autoimmune diseases, and AIDS has been ongoing for the past two decades. ITs contain a targeting moiety for delivery and a toxic moiety for cytotoxicity. Theoretically, one molecule of a toxin, routed to the appropriate cellular compartment, will be lethal to a cell. Newly developed MoAbs, toxins, and molecular biological technologies have enabled researchers to construct ITs that can effectively kill many different cell types. In fact, phase I/II clinical trials have given promising results. Although nonspecific toxicity and immunogenicity still limit the use of IT therapy, these agents hold enormous promise in an optimal setting to treat minimal disease.

Recent developments in immunotoxin therapy

Immunotoxin (IT) research has been ongoing for 15 years. During the past 2 years, work has focused on several areas: on improvements and developments in first- and second-generation ITs; the preparation of new immunotoxin constructs with anti-tumor activity; novel animal models for preclinical evaluation of immunotoxins; and clinical trials, which are now entering Phase II or III in humans.