title: Silica-induced caspase activation in mouse alveolar macrophages is dependent upon mitochondrial integrity and aspartic proteolysis.
abstract: Although silica has been documented to cause apoptotic cell death, the cellular pathways leading to caspase activation have not been extensively investigated. Here we demonstrate in a mouse macrophage cell line (MH-S cells) that alpha-quartz silica exposure (12.5 mug/cm2 to 50 mug/cm2) elicited activation of both caspase 3 and caspase 9, whereas anatase titanium dioxide (TiO2), a non-fibrogenic particle, did not. Silica exposure in vitro also induced apoptosis after 6 h, as measured by the appearance of subdiploid cell fragments in a flow cytometric analysis. Exposure to TiO 2 did not elicit significant apoptosis. Silica-induced apoptosis and caspase 3 activation were, in part, caspase 9 dependent, as determined by their sensitivity to either a general caspase inhibitor (Z-VAD-FMK) or a specific caspase 9 inhibitor (Z-LEHD-FMK). Silica exposure in vitro also elicited significant mitochondrial depolarization after 2 and 6 h of exposure. Cyclosporin A, an inhibitor of the mitochondrial permeability pore, partially decreased mitochondrial depolarization, caspase 3 activation, and caspase 9 activation, suggesting a role for mitochondrial dysfunction in these events. Pepstatin A, an inhibitor of cathepsin D, also decreased mitochondrial depolarization, caspase 3 activation, and caspase 9 activation, whereas leupeptin, an inhibitor of cathepsin B, had no effect. These data suggest that short-term silica exposure in vitro induces both caspase 3 and caspase 9 activity, which appears to participate in apoptosis. Activation of these caspases seems to be dependent, in part, on aspartic proteolysis and loss of mitochondrial integrity.
authors.affiliation: Department of Pharmaceutical Sciences, University of Connecticut, Storrs 06269, USA.
authors.forename: M
authors.initials: M
authors.lastname: Thibodeau
authors.forename: C
authors.initials: C
authors.lastname: Giardina
authors.forename: A K
authors.initials: AK
authors.lastname: Hubbard
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D053148:Caspase 3; D053453:Caspase 9; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism*; D002402:Cathepsin D / Q000037:antagonists & inhibitors*; D002460:Cell Line; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005434:Flow Cytometry; D007473:Ion Channels / Q000378:metabolism*; D016676:Macrophages, Alveolar / Q000187:drug effects* / Q000201:enzymology; D051379:Mice; D033681:Mitochondrial Membrane Transport Proteins; D000083162:Mitochondrial Permeability Transition Pore; D012822:Silicon Dioxide / Q000633:toxicity*
chemical_list: D061945:Caspase Inhibitors; D004791:Enzyme Inhibitors; D007473:Ion Channels; D033681:Mitochondrial Membrane Transport Proteins; D000083162:Mitochondrial Permeability Transition Pore; D012822:Silicon Dioxide; C505409:Casp3 protein, mouse; C506434:Casp9 protein, mouse; D053148:Caspase 3; D053453:Caspase 9; D020169:Caspases; D002402:Cathepsin D