Cell death process initiated by a combination of aluminium and iron in suspension-cultured tobacco cells (Nicotiana tabacum): Apoptosis-like cell death mediated by calcium and proteinase

Aluminium (Al) enhances ferrous ion (Fe²⁺) -mediated lipid peroxidation which is the primary factor leading to cell death in nutrient medium in tobacco cells. Under these conditions, cell death processes were examined. Cells which had been treated with Al and Fe²⁺ together exhibited cell shrinkage and DNA ladders which were detected by agarose-gel electrophoresis in the form of bands corresponding to increasing multiples of ~150 bp fragments. In the calcium (Ca)-free medium, a combination of Al and Fe²⁺ enhanced only partly the peroxidation of lipids and hardly promoted cell death, indicating that Ca²⁺ further stimulates the peroxidation of lipids enhanced by a combination of Al and Fe²⁺ and is required for cell death. N-Tosyl-L-phenylalanylchloromethyl ketone, an inhibitor of some cysteine and serine proteinases, had no effect on the peroxidation of lipids but almost completely blocked cell death enhanced by a combination of Al and Fe²⁺, suggesting that proteinase acts downstream from the peroxidation of lipids. On the basis of these findings, we conclude that cell death initiated by a combination of Al and Fe²⁺ is involved in the apoptosis-like cell death program mediated by extracellular Ca²⁺ and endogenous proteinase.