A possible role of oxidative stress in the switch mechanism of the cell death mode from apoptosis to necrosis – studies on ρ0 cells

Apoptosis is induced not only during morphogenesis and embryogenesis but also under various pathological conditions, especially related to oxidative stress. Apoptotic cells are phagocytized by neighboring cells while necrotic cells cause local and general reactions sometimes lethal to our bodies. Data have been accumulated to demonstrate that the switch of the cell death mode from apoptosis to necrosis does occur. However, detailed mechanisms involved in the switch mechanism remain unsolved although decreases in the intracellular level of ATP and a burst in the cellular level of reactive oxygen species (ROS) have been proposed. Recently, we have shown that the population of apoptotic cells reaches maximum in human osteosarcoma 143B cells treated for 6 h with menadione (MEN) while necrotic cells become predominant at 9 h of the treatment. In the present study we have attempted to clarify the role of cellular ATP in the switch mechanism using ρ0 cells derived from human osteosarcoma ρ+ cells. Results are summarized as follows: (1) Apoptotic and necrotic changes in ρ0 cells are much faster than ρ+ cells after the treatment with MEN. (2) Cellular level of ATP in ρ0 cells remains essentially in the same level before and after the MEN-treatment while intracellular levels of superoxide continuously increase after the MEN-treatment. (3) ρ+ cells treated with MEN in the presence of antimycin A plus oligomycin show similar changes to those of MEN-treated ρ0 cells. (4) MEN-induced increases in the cellular level of superoxide are distinctly suppressed by inhibitors of NADPH oxidase. These results suggest that the intracellular level of superoxide may be a key factor directly related to the switch mechanism from apoptosis to necrosis, and that decreases in cellular level of ATP accelerate both apoptotic and necrotic changes of the cells.