Modulation of mitochondrial Ca2+ homeostasis by Bcl-2

We have investigated the role of mitochondrial Ca2+ (Ca-m) homeostasis in cell survival. Disruption of Ca-m homeostasis via depletion of the mitochondrial Ca2+ store was the earliest event that occurred during staurosporine-induced apoptosis in neuroblastoma cells (SH-SY5Y), The decrease of Ca-m preceded activation of the caspase cascade and DNA fragmentation. Overexpression of the anti-apoptosis protein Bcl-2 led to increased Ca-m load, increased mitochondrial membrane potential (Delta Psi(m)), and inhibition of staurosporine-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. ladder assay. Similarly, reducing the Ca-m load by lowering the extracellular calcium concentration also led to apoptosis, We suggest that the anti-apoptotic 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 opposite effect, Furthermore, both ER and mitochondrial Ca2+ stores are important, and the depletion of either one will result in apoptosis, Thus, our results, for the first time, provide evidence that the maintenance of Ca-m homeostasis is essential for cell survival.