Signal peptide-regulated toxicity of a plant ribosome-inactivating protein during cell stress.

The fate of the type I ribosome-inactivating protein (RIP) saporin when initially targeted to the endoplasmic reticulum (ER) in tobacco protoplasts has been examined. We find that saporin expression causes a marked decrease in protein synthesis, indicating that a fraction of the toxin reaches the cytosol and inactivates tobacco ribosomes. We determined that saporin is largely secreted but some is retained intracellularly, most likely in a vacuolar compartment, thus behaving very differently from the prototype RIP ricin A chain. We also find that the signal peptide can interfere with the catalytic activity of saporin when the protein fails to be targeted to the ER membrane, and that saporin toxicity undergoes signal sequence-specific regulation when the host cell is subjected to ER stress. Replacement of the saporin signal peptide with that of the ER chaperone BiP reduces saporin toxicity and makes it independent of cell stress. We propose that this stress-induced toxicity may have a role in pathogen defence.

Renal excretion of recombinant immunotoxins containing pseudomonas exotoxin.

Recombinant immunotoxins BL22 (CAT-3888) and LMB-2, composed of Fv fragments of anti-CD22 and CD25 MAbs, respectively, have produced major responses in patients with hematologic malignancies, and are also associated with renal toxicity, particularly with BL22. Characterization of the renal excretion of recombinant immunotoxins, which have 2-4 h half-lives in plasma, has not been reported in humans. To study the renal excretion of recombinant immunotoxins, urine from patients treated with BL22 was collected and the recombinant protein visualized after trichloroacetic acid (TCA) precipitation or anion exchange chromatography. BL22 viewed by immunoblot was found in the urine of patients within 8 h after dosing as an intact protein, and progressively degraded to fragments of <20 kDa within 1 day. We studied the stability of BL22 and LMB-2 added to urine at different time points and pH. When exposed to urine ex vivo, BL22 time-dependent proteolysis was similar to that observed in treated patients. By N-terminal sequencing, proteolysis was documented at positions 348-349 and 350-351 of BL22, and 339-340 and 341-342 of LMB-2, and other proteolytic sites were observed as well. Our data suggest that BL22 is excreted into the urine in a potentially cytotoxic form, even after its plasma level declines, and may remain intact long enough to cause renal toxicity.

Pichia pastoris as a host for secretion of toxic saporin chimeras.

Most of the targeting moieties, such as antibody fragments or growth factor domains, used to construct targeted toxins for anticancer therapy derive from secretory proteins. These normally fold in the oxidative environment of the endoplasmic reticulum, and hence their folding in bacterial cells can be quite inefficient. For instance, only low amounts of properly folded antimetastatic chimera constituted by the amino-terminal fragment of human urokinase (ATF) fused to the plant ribosome-inactivating protein saporin could be recovered. ATF-saporin was instead secreted efficiently when expressed in eukaryotic cells protected from autointoxication with neutralizing anti-saporin antibodies. Pichia pastoris is a microbial eukaryotic host where these domains can fold into a transport-competent conformation and reach the extracellular medium. We show here that despite some host toxicity codon-usage optimization greatly increased the expression levels of active saporin but not those of an active-site mutant SAP-KQ in GS115 (his4) strain. The lack of any toxicity associated with expression of the latter confirmed that toxicity is due to saporin catalytic activity. Nevertheless, GS115 (his4) cells in flask culture secreted 3.5 mg/L of a histidine-tagged ATF-saporin chimera showing an IC(50) of 6 x 10(-11) M against U937 cells, thus demonstrating the suitability of this expression platform for secretion of toxic saporin-based chimeras.

Initial characterization of an immunotoxin constructed from domains II and III of cholera exotoxin.

Immunotoxins are antibody-toxin fusion proteins under development as cancer therapeutics. In early clinical trials, immunotoxins constructed with domains II and III of Pseudomonas exotoxin (termed PE38), have produced a high rate of complete remissions in Hairy Cell Leukemia and objective responses in other malignancies. Cholera exotoxin (also known as cholix toxin) has a very similar three-dimensional structure to Pseudomonas exotoxin (PE) and when domains II and III of each are compared at the primary sequence level, they are 36% identical and 50% similar. Here we report on the construction and activity of an immunotoxin made with domains II and III of cholera exotoxin (here termed CET40). In cell viability assays, the CET40 immunotoxin was equipotent to tenfold less active compared to a PE-based immunotoxin made with the same single-chain Fv. A major limitation of toxin-based immunotoxins is the development of neutralizing antibodies to the toxin portion of the immunotoxin. Because of structure and sequence similarities, we evaluated a CET40 immunotoxin for the presence of PE-related epitopes. In western blots, three-of-three anti-PE antibody preparations failed to react with the CET40 immunotoxin. More importantly, in neutralization studies neither these antibodies nor those from patients with neutralizing titers to PE38, neutralized the CET40-immunotoxin. We propose that the use of modular components such as antibody Fvs and toxin domains will allow a greater flexibility in how these agents are designed and deployed including the sequential administration of a second immunotoxin after patients have developed neutralizing antibodies to the first.

Overexpression of parkin in the rat nigrostriatal dopamine system protects against methamphetamine neurotoxicity.

Methamphetamine (METH) is a central nervous system psychostimulant with a high potential for abuse. At high doses, METH causes a selective degeneration of dopaminergic terminals in the striatum, sparing other striatal terminals and cell bodies. We previously detected a deficit in parkin after binge METH in rat striatal synaptosomes. Parkin is an ubiquitin-protein E3 ligase capable of protecting dopamine neurons from diverse cellular insults. Whether the deficit in parkin mediates the toxicity of METH and whether parkin can protect from toxicity of the drug is unknown. The present study investigated whether overexpression of parkin attenuates degeneration of striatal dopaminergic terminals exposed to binge METH. Parkin overexpression in rat nigrostriatal dopamine system was achieved by microinjection of adeno-associated viral transfer vector 2/6 encoding rat parkin (AAV2/6-parkin) into the substantia nigra pars compacta. The microinjections of AAV2/6-parkin dose-dependently increased parkin levels in both the substantia nigra pars compacta and striatum. The levels of dopamine synthesizing enzyme, tyrosine hydroxylase, remained at the control levels; therefore, tyrosine hydroxylase immunoreactivity was used as an index of dopaminergic terminal integrity. In METH-exposed rats, the increase in parkin levels attenuated METH-induced decreases in striatal tyrosine hydroxylase immunoreactivity in a dose-dependent manner, indicating that parkin can protect striatal dopaminergic terminals against METH neurotoxicity.

Enhancing immunotoxin cell-killing activity via combination therapy with ABT-737.

Immunotoxins are antibody-toxin fusion proteins directed to kill cancer cells displaying specific target antigens on their surface. Remarkably, immunotoxins directed to CD22 on hairy cell leukemia have produced complete remissions in approximately 60% of patients enrolled in phase I/II trials. For reasons that are not yet clear, 40% of patients responded less well. In addition, patients with other CD22-positive malignancies have not yet achieved complete remissions. In trying to understand 'resistance' to immunotoxin therapy, a number of challenging issues have been raised. These include insufficient dosing, the production of neutralizing anti-immunotoxin antibodies, poor access to malignant cells, and resistance to toxin killing. In designing immunotoxins, we employ truncated Pseudomonas exotoxin, which enzymatically inactivates protein synthesis and produces cell death in sensitive cells. To begin to address toxin resistance we have explored combination therapy with the BH3-only mimetic, ABT-737. Our results indicate that immunotoxin-ABT combinations often exhibit greater killing activity than either compound alone and in some instances overcome resistance. Expression of high levels of prosurvival Bcl-2 proteins may contribute to toxin resistance.

ABT-737 overcomes resistance to immunotoxin-mediated apoptosis and enhances the delivery of pseudomonas exotoxin-based proteins to the cell cytosol.

Pseudomonas exotoxin (PE)-based immunotoxins (antibody-toxin fusion proteins) have achieved frequent complete remissions in patients with hairy cell leukemia but far fewer objective responses in other cancers. To address possible mechanisms of resistance, we investigated immunotoxin activity in a model system using the colon cancer cell line, DLD1. Despite causing complete inhibition of protein synthesis, there was no evidence that an immunotoxin targeted to the transferrin receptor caused apoptosis in these cells. To address a possible protective role of prosurvival Bcl-2 proteins, the BH3-only mimetic, ABT-737, was tested alone or in combination with immunotoxins. Neither the immunotoxin nor ABT-737 alone activated caspase 3, whereas the combination exhibited substantial activation. In other epithelial cell lines, ABT-737 enhanced the cytotoxicity of PE-related immunotoxins by as much as 20-fold, but did not enhance diphtheria toxin or cycloheximide. Because PE translocates to the cytosol via the endoplasmic reticulum (ER) and the other toxins do not, ABT-737-mediated effects on the ER were investigated. ABT-737 treatment stimulated increased levels of ER stress response factor, ATF4. Because of its activity in the ER, ABT-737 might be particularly well suited for enhancing the activity of immunotoxins that translocate from the ER to the cell cytosol.

N-terminal deletion affects catalytic activity of saporin toxin.

Single-chain ribosome inactivating proteins (RIPs) are cytotoxic components of macromolecular pharmaceutics for immunotherapy of cancer and other human diseases. Saporin belongs to a family of single-chain RIPs sharing sequence and structure homology. In a preliminary attempt to define an active saporin polypeptide of minimum size we have generated proteins with deletions at the N-terminus and at the C-terminus. An N-terminal (sapDelta1-20) deletion mutant of saporin displayed defective catalytic activity, drastically reduced cytotoxicity but increased ability to interact with liposomes inducing their permeabilization at low pH. A C-terminal (sapDelta239-253) deletion mutant showed instead a moderate reduction in cytotoxic activity. A substantial alteration of secondary structure was evidenced by Fourier transformed infrared spectroscopy (FTIR) in the sapDelta1-20 mutant. It can be hypothesized that the defective functions of sapDelta1-20 are due to alterations of its spatial configuration.