Modulation of mitochondrial Ca2+ homeostasis by Bcl-2

We have investigated the role of mitochondrial Ca2+ (Ca-m) homeostasis in c ell survival. Disruption of Ca-m homeostasis via depletion of the mitochond rial Ca2+ store was the earliest event that occurred during staurosporine-i nduced apoptosis in neuroblastoma cells (SH-SY5Y), The decrease of Ca-m pre ceded activation of the caspase cascade and DNA fragmentation. Overexpressi on of the anti-apoptosis protein Bcl-2 led to increased Ca-m load, increase d mitochondrial membrane potential (Delta Psi(m)), and inhibition of stauro sporine-induced apoptosis, On the other hand, ectopic expression of the pro -apoptotic protein Bik led to decreased Ca-m load and decreased Delta Psi(m ). Inhibition of calcium uptake into mitochondria by ruthenium red induced a dose-dependent apoptosis as determined by nuclear staining and DNA. ladde r assay. Similarly, reducing the Ca-m load by lowering the extracellular ca lcium concentration also led to apoptosis, We suggest that the anti-apoptot ic effect of Bcl-2 is related to its ability to maintain a threshold level of Ca-m and Delta Psi(m) while the pro-apoptotic protein Bik has the opposi te effect, Furthermore, both ER and mitochondrial Ca2+ stores are important , and the depletion of either one will result in apoptosis, Thus, our resul ts, for the first time, provide evidence that the maintenance of Ca-m homeo stasis is essential for cell survival.