Burek et al: Calcium induced cell deathantiapoptotic Bcl-2 family members including Bcl-2 itselfand Bcl-X L , protect cells from calcium by lowering theCa 2+ -storage capacity of ER. Thus, the death stimuli thatcause the release of calcium from ER will be less efficientin elevating the cytoplasmic calcium concentration andtherefore, will less effectively activate the calciumdependentsignaling pathways.The death inducted by the calcium ionophore A23187was a mixture of necrosis and apoptosis. A critical factorthat influences the form of cell death (apoptotic ornecrotic) is the cellular ATP content. Stimuli that undernormal condition induce apoptosis will cause classicalnecrotic cell death if the cellular concentration of ATPdrops below 10-15 % of the normal level (Nieminen et al,1994; Los et al, 2002). One of the mechanisms that causesevere ATP depletion is the uncoupling of phosphorylativeoxidation and ATP production caused by mitochondrialpermeability transition (MPT). MPT may be triggered by arising Ca 2+ level and the subsequent activation of thehypothetical permeability transition pore componentcyclophilin D. Once the pH and electrical gradient acrossthe inner mitochondrial membrane collapses the finalenzyme of the mitochondrial respiratory chain, the F 1 F 0 -ATPase, that normally converts ADP to ATP, reverses andconsumes ATP while trying to restore the gradient. Thismechanism is among the strongest depletors of cellularATP, since it also consumes ATP produced by thecompensatory, glycolytic pathway (reviewed in Lemasterset al, 2002; Hajnoczky et al, 2003). The above mechanismpermits both necrotic- and apoptotic death. A strongincrease of Ca 2+ concentration would cause a significantportion of mitochondria to collapse, massive ATPdepletion would follow, thus, cells would die by necrosis.A less pronounced rise of calcium concentration wouldresult in a slow and asynchronous MPT occurrence.Affected mitochondria would release proapoptoticmolecules like cytochrome c, AIF and endonuclease G.While the depletion of ATP would not be significant, thecell would have enough energy to die in an orderly,apoptotic fashion. This is exactly what we observed in ourexperimental system. While low concentrations of thecalcium ionophore A23187 induce apoptosis, intermediateand higher concentrations of it cause substantial necrosis.In summary, we are presenting here evidence for anew caspase-8-dependent calcium-induced death pathway.Since it is FADD-independent, we hypothesize that theBap31 ER-localized adaptor molecule is involved in thepathway. 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