BCL-2 ACTS SUBSEQUENT TO AND INDEPENDENT OF CA2-TREATED AND GLUCOCORTICOID-TREATED MOUSE LYMPHOMA-CELLS( FLUXES TO INHIBIT APOPTOSIS IN THAPSIGARGIN)

The mechanism by which Bcl-2 inhibits apoptosis is unknown. One propos al is that Bcl-2 regulates intracellular Ca2+ fluxes thought to mediat e apoptosis. In the present study, we investigated Bcl-2's mechanism o f action by determining the effect of Bcl-2 on intracellular Ca2+ flux es in the WEHI7.2 mouse lymphoma cell line, which does not express Bcl -2, and its stable transfectant, W.Hb12, which expresses a high level of Bcl-2. Treatment with the endoplasmic reticulum Ca2+-ATPase inhibit or thapsigargin produced marked alterations in intracellular Ca2+ home ostasis in both WEHI7.2 and W.Hb12 cells, including elevation of free cytosolic Ca2+, endoplasmic reticulum Ca2+ pool depletion, capacitativ e entry of extracellular Ca2+, and increased loading of Ca2+ into mito chondria. Similar changes in intracellular Ca2+ occurred spontaneously in both cell lines following exponential growth. In both situations, W.Hb12 cells maintained optimal viability despite marked alterations i n intracellular Ca2+, whereas WEHI7.2 cells underwent apoptosis. Treat ment with the glucocorticoid hormone, dexamethasone, induced apoptosis in WEHI7.2 cells, but not in W.HB12 cells, even though dexamethasone treatment did not alter intracellular Ca2+ homeostasis in either cell line. These findings indicate that Bcl-2 acts downstream from intracel lular Ca2+ fluxes in a pathway where Ca2+-dependent and Ca2+-independe nt death signals converge.