Mitochondrial Catalase Overexpression Protects Insulin-Producing Cells Against Toxicity of Reactive Oxygen Species and Proinflammatory Cytokines

Abstract

Insulin-producing cells are known for their extremely low antioxidant equipment with hydrogen peroxide (H₂O₂)-inactivating enzymes. Therefore, catalase was stably overexpressed in mitochondria and for comparison in the cytoplasmic compartment of insulin-producing RINm5F cells and analyzed for its protective effect against toxicity of reactive oxygen species (ROS) and proinflammatory cytokines. Only mitochondrial overexpression of catalase provided protection against menadione toxicity, a chemical agent that preferentially generates superoxide radicals intramitochondrially. On the other hand, the cytoplasmic catalase overexpression provided better protection against H₂O₂ toxicity. Mitochondrial catalase overexpression also preferentially protected against the toxicity of interleukin-1β (IL-1β) and a proinflammatory cytokine mixture (IL-1β, tumor necrosis factor-α [TNF-α], and γ-interferon [IFN-γ]) that is more toxic than IL-1β alone. Thus, it can be concluded that targeted overexpression of catalase in the mitochondria provides particularly effective protection against cell death in all situations in which ROS are generated intramitochondrially. The observed higher rate of cell death after exposure to a cytokine mixture in comparison with the weaker effect of IL-1β alone may be due to an additive toxicity of TNF-α through ROS formation in mitochondria. The results emphasize the central role of mitochondrially generated ROS in the cytokine-mediated cell destruction of insulin-producing cells.