Transition metal-induced apoptosis in lymphocytes via hydroxyl radical generation, mitochondria dysfunction, and caspase-3 activation: an in vitro model for neurodegeneration

Abstract

Redox transition metals have been implicated as crucial players in pathogenesis of neurodegenerative diseases. Intracellular signaling mechanism(s) responsible for oxidative stress and death in single-cell model exposed to metals has not yet been fully elucidated. The objective of the study was to determine the mechanism by which metals induced apoptosis in human peripheral blood lymphocytes (PBL). PBL were exposed to 50, 100, 250, 500, and 1,000 μM (Fe 2+ ), (Mn 2+ ), (Cu 2+ ), and (Zn 2+ )-(SO 4 ). Apoptotic/necrotic morphology was assessed with acridine orange/ethidium bromide staining. Further evaluations comprised production of H 2 O 2 , generation of hydroxyl radical (·OH), disruption of mitochondrial transmembrane potential (ΔΨ m ), caspase-3 activation, and activation of NF-κB and p53 transcriptional factors. Morphologic analysis showed that 500 μM provoked maximal percentage of apoptosis (22–30% AO/EB) and minimal necrosis (3–7%), whereas low concentrations were innocuous but 1,000 μM induced mainly necrosis (>40% AO/EB). Metals generated both H 2 O 2 and (·OH) by Fenton reaction. Hydroxyl scavengers protected PBL from metal-induced apoptosis. All metals induced mitochondrial depolarization (17–62% nonfluorescent cells) and activated caspase-3 concomitantly with apoptotic morphology (25–32% AO/EB) at 24 h, and neither NF-κB nor p53 transcription factor showed activation. This study provides evidence that redox-active (Fe 2+ ), (Mn 2+ ), (Cu 2+ ), and (Zn 2+ ) ion-induced apoptosis in PBL by (H 2 O 2 )/(·OH) generation, resulting in mitochondria depolarization, caspase-3 activation, and nuclear fragmentation independent of NF-κB and p53 transcription factors activation. Our data highlight the potential use of lymphocytes as a model to screen antioxidant strategies designed to remove H 2 O 2 /·OH associated with metal-catalyzed reactions in neurodegenerative disorders.