29th Annual Meeting of the American Society of Photobiology

Downtown Marriot

Chicago, Il.

Mitochondrial Reactive Oxygen Species in Cell Injury

Abstract-
Oxidative stress is a frequent feature in a wide variety of pathological conditions. Oxidative stress results from over production of reactive oxygen species (ROS) relative to the antioxidant defenses within a cell. Mitochondria are both source of ROS generation in pathophysiology and a target of ROS toxicity. We investigated the sequence of cellular events during oxidant chemical-induced cell death in hepatocytes using confocal microscopy. The oxidant chemical, tert-butylhydroperoxide, caused rapid oxidation of mitochondrial NAD(P)H. NAD(P)H oxidation disrupted the balance of mitochondrial Ca²⁺ uptake and release, leading to a net increase of mitochondrial Ca²⁺. Increased mitochondrial Ca²⁺ stimulated intramitochondrial ROS formation. The combination of increased Ca²⁺ and oxidative stress from ROS caused opening of the permeability transition pore of the mitochondrial inner membrane and onset of the mitochondrial permeability transition (MPT). As a consequence, cellular ATP became profoundly depleted and necrotic cell death ensued. Besides necrotic cell death, ROS play an important role in apoptotic cell death. An example is photodynamic therapy (PDT)-induced apoptosis of tumor cells. In human epidermoid carcinoma cells, PDT with the phthalocyanine photosensitizer Pc 4 induced azide-sensitive mitochondrial singlet oxygen production, which led further to lipid hydroperoxide formation and peroxidation chain reaction by an iron-dependent reaction. Mitochondrial ROS further induced mitochondrial inner membrane permeabilization resulting in mitochondrial depolarization, swelling, cytochrome c release, caspase-3 activation, and apoptotic death. These findings illustrate that mitochondrial ROS formation plays a critical role in necrosis after acute oxidative stress and apoptosis after PDT (supported by NIH grants RO1 NS39469, PO1 CA48735, P30 CA43703).

Keywords: mitochondria, calcium, membrane potential, reactive oxygen species