Apoptosis suppression by bcl-2 is correlated with the regulation of nuclear and cytosolic Ca²⁺

Bcl-2 expression is associated with the progression of prostate cancer from androgen-dependence to androgen-independence. Bcl-2 is an integral membrane protein which localizes to mitochondria, endoplasmic reticulum, and the nuclear envelope. Using spectrofluorometry and laser confocal microscopy, the ability of bcl-2 to intracellular Ca²⁺ was examined in the G prostate carcinoma cell line following apoptosis induction by adriamycin. Adriamycin and thapsigargin, an endoplasmic reticulum Ca²⁺-pump inhibitor, were effective inducers of apoptosis in control, but not bcl-2 transfected, cells. Treatment with adriamycin was accompanied by a sustained rise in cytoplasmic Ca²⁺ in control and bcl-2 transfected cells. An increase in intranuclear Ca²⁺ was observed in control cells only. Apoptosis induction by thapsigargin was associated with an increase in cytoplasmic Ca²⁺ in control cells that was not detected in the resistant bcl-2 transfectants. Ca²⁺ was excluded from nuclei isolated from bcl-2 expressing cells, but was sequestered in control nuclei, following the addition of ATP. These findings suggest that bcl-2 may regulate levels of intranuclear Ca²⁺ independently of cytosolic Ca²⁺ levels. The ability of bcl-2 to modulate, directly or indirectly, sustained increases in both cytosolic and intranuclear Ca²⁺ may provide a common basis for bcl-2 function in different subcellular compartments.