TPEN Induces Apoptosis Independently of Zinc Chelator Activity in a Model of Acute Lymphoblastic Leukemia and Ex Vivo Acute Leukemia Cells through Oxidative Stress and Mitochondria Caspase-3- and AIF-Dependent Pathways
Figure 8
Proposed scheme of TPEN-induced apoptosis in Jurkat cell line and ALL cells. TPEN generates (1, by a yet unknown mechanism), which in turn dismutates into H2O2 (2). This last compound serves as signal molecule. Indeed, it indirectly activates NF-κB (4) and c-Jun (6) transcription factors through activation of kinases (v.gr. ASK1, MEKK1, SHIP-1 (3) and JNK (5)). Once NF-κB (4) through phosphorylation of its inhibitor IκBα, it translocates into the nucleus and transcribes several antiapoptotic genes (7) and proapoptotic genes, amongst them the p53 (8). This transcription factors balance the cell decision to death by several actions: p53 either directly impact mitochondria or transcribes proapoptotic genes such as Bax contributing to the permeabilization of the outer mitochondrial membrane by antagonizing antiapoptotic proteins (9). Simultaneously, p53 induces expression of prooxidant genes (e.g., p53-induced gene-3 (PIG3), proline oxidase (PO), (10), which generate more H2O2 (2), and represses the transcription of antioxidant genes. Thus, H2O2 over-production and further activation of NF-κB induce upregulation of proapoptotic genes (e.g., p53), which in turn amplify the initial H2O2-induced cell death signal. Mitochondrial damage allows the release of apoptogenic factors (11) such as AIF, and cytochrome C, which together with dATP, pro-caspase-9, and apoptotic protease activating factor-1; that is, the apoptosome complex triggers activation of caspase-3 protease (12). This protease in turn activates the endonuclease DFF40/CAD, by cutting the nuclease’s inhibitor DFF45/ICAD. Finally, DFF40/CAD causes nuclear chromatin fragmentation (stage II nuclei morphology, (13), typical of apoptosis. Alternatively, AIF translocates into the nucleus to generate chromatin fragmentation (stage I nuclei morphology (14)). Noticeably, zinc chelator TPEN further warrants killing of the cell by either directly activating caspase-3 (15) or by releasing caspase-3 from X-linked inhibitor of apoptosis (XIAP, 15). Interestingly, TPEN toxicity can be blocked by the antioxidant N-acetyl-cysteine (NAC) (16) and zinc ions (17), probably by changing its tridimensional structure.
