MAINTENANCE OF CALCIUM HOMEOSTASIS IN THE ENDOPLASMIC-RETICULUM BY BCL-2

The oncogene bcl-2 encodes a 26-kD protein localized to intracellular membranes, including the ER, mitochondria, and perinuclear membrane, b ut its mechanism of action is unknown. We have been investigating the hypothesis that Bcl-2 regulates the movement of calcium ions (Ca2+) th rough the ER membrane. Earlier findings in this laboratory indicated t hat Bcl-2 reduces Ca2+ efflux from the ER lumen in WEHI7.2 lymphoma ce lls treated with the Ca2+-ATPase inhibitor thapsigargin (TG) but does not prevent capacitative entry of extracellular calcium. In this repor t, we show that sustained elevation of cytosolic Ca2+ due to capacitat ive entry is not required for induction of apoptosis by TG, suggesting that ER calcium pool depletion may trigger apoptosis. Bcl-2 overexpre ssion maintains Ca2+ uptake in the ER of TG-treated cells and prevents a TG-imposed delay in intralumenal processing of the endogenous glyco protein cathepsin D. Also, Bcl-2 overexpression preserves the ER Ca2pool in untreated cells when extracellular Ca2+ is low. However, low e xtracellular Ca2+ reduces the antiapoptotic action of Bcl-2, suggestin g that cytosolic Ca2+ elevation due to capacitative entry may be requi red for optimal ER pool filling and apoptosis inhibition by Bcl-2. In summary, the findings suggest that Bcl-2 maintains Ca2+ homeostasis wi thin the ER, thereby inhibiting apoptosis induction by TG.