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Acute myeloid leukemia-targeted toxins kill tumor cells by cell type-specific mechanisms and synergize with TRAIL to allow manipulation of the extent and mechanism of tumor cell death

Leukemia volume 22, pages 652–655 (2008)Cite this article

Targeted toxins are fusion proteins combining a targeting molecule that selectively recognizes and enters tumor cells through cell surface receptors with a bacterial toxin that kills the targeted cells. This strategy provides a way to selectively kill tumor cells based on their expression of particular growth factor or cytokine receptors. We have focused on the development of targeted diphtheria toxins (DT) to treat acute myeloid leukemia (AML) using two fusion toxins, DT—granulocyte–macrophage colony-stimulating factor (GMCSF) and DT-IL3, which selectively kill AML progenitor cells1, 2 through the GMCSF and interleukin-3 (IL-3) receptors respectively. In a clinical trial of DT-GMCSF in chemoresistant AML patients, several complete and partial remissions were obtained3 and clinical studies are currently being carried out using the DT-IL3 molecule (AE Frankel, unpublished).

Previous work from our lab and others4, 5 suggests that targeted toxins, like many other anticancer agents activate caspase-dependent apoptosis. In agreement with this, DT-GMCSF activated caspases in U937 cells as determined by enzymatic assays (Figure 1f) and western blotting to detect cleavage of caspase-8 and -3 and the caspase-8 substrate Bid (Figure 1g). However, when we examined the mechanism of DT-GMCSF-induced death of TF1HRas cells, we found no activation of caspases (Figure 1f) or cleavage of caspases and their substrates (Figure 1g). Additionally, the caspase inhibitor, zVAD-fmk, protects U937 cells against DT-GMCSF-induced cell death, but provides no protection to TF1HRas cells (Figure 1h) although in both U937 and TF1HRas cells the histone deacetylase (HDAC) inhibitor MS275 was inhibited by zVAD-fmk. As expected based on our previous work,4 DT-GMCSF activated the extrinsic apoptosis pathway in U937 cells leading to caspase-8 processing and cleavage of the caspase-8 substrate Bid (Figure 1g). To test if the TF1HRas cells were inherently unable to activate caspase-dependent apoptosis through the extrinsic pathway, we used TNF-related apoptosis inducing ligand (TRAIL). TRAIL treatment led to robust caspase activation in the TF1HRas cells (Figure 1e). These data show that DT-GMCSF induces caspase activation and apoptotic cell death in U937 cells, but that while it kills the TF1HRas cells with similar efficiency, and they can activate the extrinsic pathway and undergo apoptosis, the targeted toxin induces non-apoptotic cell death in these cells. Non-apoptotic cell death also took place when the TF1HRas cells were treated with DT-IL3 (data not shown).

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  1. Department of Pharmacology, University of Colorado at Denver and Health Sciences Center, Aurora, CO, USA

    H Horita & A Thorburn

  2. Department of Hematology/Oncology, Scott & White Cancer Research Institute, TX, USA

    A E Frankel

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  1. H Horita
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  2. A E Frankel
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  3. A Thorburn
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Correspondence to A Thorburn.

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Horita, H., Frankel, A. & Thorburn, A. Acute myeloid leukemia-targeted toxins kill tumor cells by cell type-specific mechanisms and synergize with TRAIL to allow manipulation of the extent and mechanism of tumor cell death. Leukemia 22, 652–655 (2008). https://doi.org/10.1038/sj.leu.2404956

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