Structures of distant diphtheria toxin homologs reveal functional determinants of an evolutionarily conserved toxin scaffold

Neuropilin-1 Binding Peptide as Fusion to Diphtheria Toxin Induces Apoptosis in Non-small Cell Lung Cancer Cell Line

BACKGROUND

Targeted cancer therapy can be considered as a new strategy to overcome the side effects of current cancer treatments. Neuropilin-1 (NRP-1) is a transmembrane glycoprotein that is expressed in endothelial cells and tumor vessels to stimulate angiogenesis progression. Targeted diphtheria toxin (DT)- based therapeutics are promising tools for cancer treatment. This study aimed to construct a novel NRP-1 binding peptide (as three repeats) (CRGDK) as a fusion to truncated DT (DTA) (DTA-triCRGDK) for targeted delivery of DT into NRP-1 expressing cells.

METHODS

The concept of DTA-triCRGDK was designed, synthesized and cloned into the bacterial host. Expression of DTA-triCRGDK was induced by Isopropyl ß-D-1-thiogalactopyranoside (IPTG) and purification was performed using Ni-NTA chromatography. Biological activity of DTA-triCRGDK was evaluated using MTT, apoptosis, and wound healing assays. In addition, expression levels of apoptotic Bax, Bcl2, and Casp3 genes were determined by Real-time PCR.

RESULTS

Cytotoxicity analysis showed the IC50 values of DTA-triCRGDK for A549 and MRC5 were 0.43 nM and 4.12 nM after 24 h, respectively. Bcl2 expression levels decreased 0.4 and 0.72 fold in A549 and MRC5, respectively. However, Bax and Casp3 expression level increased by 6.75 and 8.19 in A549 and 2.51 and 3.6 in MRC5 cells.

CONCLUSION

Taken together, DTA-triCRGDK is a promising tool for targeted therapy of NRP-1 overexpressing cancer cells.

Unconventional structure and mechanisms for membrane interaction and translocation of the NF-κB-targeting toxin AIP56

Bacterial AB toxins are secreted key virulence factors that are internalized by target cells through receptor-mediated endocytosis, translocating their enzymatic domain to the cytosol from endosomes (short-trip) or the endoplasmic reticulum (long-trip). To accomplish this, bacterial AB toxins evolved a multidomain structure organized into either a single polypeptide chain or non-covalently associated polypeptide chains. The prototypical short-trip single-chain toxin is characterized by a receptor-binding domain that confers cellular specificity and a translocation domain responsible for pore formation whereby the catalytic domain translocates to the cytosol in an endosomal acidification-dependent way. In this work, the determination of the three-dimensional structure of AIP56 shows that, instead of a two-domain organization suggested by previous studies, AIP56 has three-domains: a non-LEE encoded effector C (NleC)-like catalytic domain associated with a small middle domain that contains the linker-peptide, followed by the receptor-binding domain. In contrast to prototypical single-chain AB toxins, AIP56 does not comprise a typical structurally complex translocation domain; instead, the elements involved in translocation are scattered across its domains. Thus, the catalytic domain contains a helical hairpin that serves as a molecular switch for triggering the conformational changes necessary for membrane insertion only upon endosomal acidification, whereas the middle and receptor-binding domains are required for pore formation.

Cancer Drug Delivery Systems Using Bacterial Toxin Translocation Mechanisms

Recent advances in targeted cancer therapy hold great promise for both research and clinical applications and push the boundaries in finding new treatments for various currently incurable cancers. However, these therapies require specific cell-targeting mechanisms for the efficient delivery of drug cargo across the cell membrane to reach intracellular targets and avoid diffusion to unwanted tissues. Traditional drug delivery systems suffer from a limited ability to travel across the barriers posed by cell membranes and, therefore, there is a need for high doses, which are associated with adverse reactions and safety concerns. Bacterial toxins have evolved naturally to specifically target cell subtypes via their receptor binding module, penetrating the cell membrane efficiently through the membrane translocation process and then successfully delivering the toxic cargo into the host cytosol. They have, thus, been harnessed for the delivery of various drugs. In this review, we focus on bacterial toxin translocation mechanisms and recent progress in the targeted delivery systems of cancer therapy drugs that have been inspired by the receptor binding and membrane translocation processes of the anthrax toxin protective antigen, diphtheria toxin, and Pseudomonas exotoxin A. We also discuss the challenges and limitations of these studies that should be addressed before bacterial toxin-based drug delivery systems can become a viable new generation of drug delivery approaches in clinical translation.

Strategies to mitigate the on- and off-target toxicities of recombinant immunotoxins: an antibody engineering perspective

Targeted cancer therapies using immunotoxins has achieved remarkable efficacies in hematological malignancies. However, the clinical development of immunotoxins is also faced with many challenges like anti-drug antibodies and dose-limiting toxicity issues. Such a poor efficacy/safety ratio is also the major hurdle in the research and development of antibody-drug conjugates. From an antibody engineering perspective, various strategies were summarized/proposed to tackle the notorious on target off tumor toxicity issues, including passive strategy (XTENylation of immunotoxins) and active strategies (modulating the affinity and valency of the targeting moiety of immunotoxins, conditionally activating immunotoxins in the tumor microenvironments and reconstituting split toxin to reduce systemic toxicity etc.). By modulating the functional characteristics of the targeting moiety and the toxic moiety of immunotoxins, selective tumor targeting can be augmented while sparing the healthy cells in normal tissues expressing the same target of interest. If successful, the improved therapeutic index will likely help to address the dose-limiting toxicities commonly observed in the clinical trials of various immunotoxins.