Diphtheria Receptor

Protein toxins acting intracellularly are often composed of two subunits (A/B model). One subunit is catalytic (A-subunit) and the other is responsible for binding and cell entry (B-subunit). Following binding to an extracellular membrane receptor, the toxins are endocytosed. From the endosomes, the A-subunit is directly (pH dqDendent) transferred into the cytosol (e.g., diphtheria toxin and anthrax toxin) or the toxin is transported in a retrograde manner via the golgi to the ER (e.g., cholera toxin), where translocation into the cytosol occurs [1]. 

Denileukin diftitox is a combination of the active sections of interleukin 2 and diphtheria toxin. It binds to high-affinity interleukin 2 receptors on the cancer cell (and other cells), and the toxin portion of the molecule inhibits protein synthesis to result in cell death. The pharmacokinetics of denileukin diftitox are best described by a two-compartment model, with an a half-life of 2 to 5 minutes and a terminal half-life of 70 to 80 minutes. Denileukin diftitox is used for the treatment of persistent or recurrent cutaneous T-cell lymphoma whose cells express the CD25 receptor. Side effects include vascular leak syndrome, fevers/chills, hypersensitivity reactions, hypotension, anorexia, diarrhea, and nausea and vomiting. 

Keen, J.H., Maxfield, F.R., Hardegree, M.C., and Habig, W.H. (1982) Receptor-mediated endocytosis of diphtheria toxin by cell in culture. Proc. Natl Acad. Sci. USA 79, 2912. 

Miskimins, W.K., and Shimizu, N. (1979) Synthesis of cytotoxic insulin cross-linked to diphtheria toxin fragment a capable of recognizing insulin receptors. Biochem. Biophys. Res. Comm. 91, 143. 

The IL-2 portion of the fusion protein facilitates product interaction with cells displaying cell surface IL-2 receptors, found in high levels on some leukaemia and lymphoma cells, including CTCL cells. Binding appears to trigger internalization of the receptor-fusion protein complex (Figure 9.B1). Sufficient quantities of the latter escape immediate cellular destruction to allow diphtheria toxin-mediated inhibition of cellular protein synthesis. Cell death usually results within hours. 

Proleukin is a recombinant form of IL-2. It is approved for the treatment of malignant melanoma and renal cell cancer. Ontak (denileukin diftitox) is a fusion protein for the treatment of persistent or recurrent T-cell lymphoma. Activated T cells express lL-2 receptors. Ontak has a fragment that binds to the IL-2 receptor while the other part presents a diphtheria toxin to kill the activated T cell. 

Classical bacterial exotoxins, such as diphtheria toxin, cholera toxin, clostridial neurotoxins, and the anthrax toxins are enzymes that modify their substrates within the cytosol of mammalian cells. To reach the cytosol, these toxins must first bind to different cell-surface receptors and become subsequently internalized by the cells. To this end, many bacterial exotoxins contain two functionally different domains. The binding (B-) domain binds to a cellular receptor and mediates uptake of the enzymatically active (A-) domain into the cytosol, where the A-domain modifies its specific substrate (see Figure 1). Thus, three important properties characterize the mode of action for any AB-type toxin selectivity, specificity, and potency. Because of their selectivity toward certain cell types and their specificity for cellular substrate molecules, most of the individual exotoxins are associated with a distinct disease. Because of their enzymatic nature, placement of very few A-domain molecules in the cytosol will normally cause a cytopathic effect. Therefore, bacterial AB-type exotoxins which include the potent neurotoxins from Clostridium tetani and C. botulinum are the most toxic substances known today. However, the individual AB-type toxins can greatly vary in terms of subunit composition and enzyme activity (see Table 2). 

C. botulinum toxins belong to the AB group of toxins, which also includes diphtheria toxin, pseudomonas exotoxin A, anthrax toxin, Shiga(like) toxin, cholera toxin, pertussis toxin, and plant toxins, e.g., ricin. Moiety A has an enzymatic activity and usually modified cellular-target entering cytosol. Moiety B consists of one or more components and binds the toxin to surface receptors, and is responsible for translocation of the A component into cells. AB toxins are produced in a non-active form and are activated by a split between two cysteine residues within a region (Falnes and Sandvig, 2000). 

A. General description Denileukin dif-titox is a recombinant, DNA-derived, interleukin-2 receptor specific ligand, cytotoxic fusion protein consisting of diphtheria toxin fragments A and B fused to interleukin-2. It is produced by expression of a recombinant fusion protein in Escherichia coli that contains nucleotide sequences for human interleukin-2, and sequences for the enzymatically active fragment A of diphtheria toxin and the membrane-translocating portion of diph-... 

Clinical pharmacology Denileukin diftitox is a fusion protein designed to direct the cytocidal action of diphtheria toxin to cells that express the IL-2 receptor. It binds avidly to the high-affinity interleukin-2 receptor on target-cell surfaces, where it is... 

IL-2 DAB389 (Ontak) Interleukin-2 (IL-2) Binds to IL-2 receptors expressed on cancer cells and T-cell lymphoma DAB389 is a binding-deficient variant of diphtheria toxin Cytotoxicity Approved for treatment of T-cell lymphoma... 

Receptor-mediated endocytosis is exploited by some toxins and viruses to gain entry to cells. Influenza virus (see Fig. 11-24), diphtheria toxin, and cholera toxin all enter cells in this way. 

Denileukin Diftitox. Denileukin Diftitox (Ontak) is formulated by combining interleukin-2 with diphtheria toxin.11 Certain leukemia and lymphoma cells have a surface receptor that has a high affinity for interleukin-2, thus attracting this drug directly to these cells. Upon binding with tbe receptor, the diphtheria toxin component of the drug inhibits cellular protein synthesis, which ultimately results in cell death. This... 

As will become clear, this chapter is focused on toxicants for which the receptor is a high-affinity recognition site of the type discussed in the previous paragraphs. It should be noted explicitly that other toxicants have receptors, but fall into more complex situations not appropriate for this chapter. For example, some toxicants inhibit enzymes, or are themselves enzymes. Such interesting compounds include the organophosphate and carbamate insecticides (acetylcholine esterase inhibitors) and diphtheria toxin (an enzyme). 

Archaea are systematically susceptible to ADP-ribosylation from NAD by the diphtheria toxin [118,166-168] although the rate of the ADP-ribosylation reaction is three orders of magnitude slower than that typically observed with eucaryal EF-2 [164,168]. Furthermore, in accordance with the sensitivity data, diphthamide occurs in archaea (//. halobium) but not in bacteria ( . coli) [164]. The archaeal EF-G-equivalent factor, therefore, resembles the eucaryal factor in having both the diphthamide that acts as the receptor for the ADP-ribose moiety of NAD, and the enzyme system which assists the post-translational conversion of histidine to diphthamide. 

DenUeukin diftitox is a fusion protein formed by binding human aldesleukin to the cytotoxic A chain of diphtheria toxin. This product binds to the aldesleukin receptor and inhibits protein synthesis, resulting in cell death. It has been approved for treatment of persistent or recurrent cutaneous T cell lymphoma and is being evaluated in patients with severe psoriasis. 

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