Blood Cells With Reduced Mitochondrial Membrane Potential and Cytosolic Cytochrome C Can Survive and Maintain Clonogenicity Given Appropriate Signals to Suppress Apoptosis

Reduction of mitochondrial membrane potential (Ψ m ) and release of cytochrome c from mitochondria appear to be key events during apoptosis. Apoptosis was induced in IC.DP premast cells by the withdrawal of interleukin-3 (IL-3). Ψ m decreased by 12 hours and cytochrome c was detected in the cytosol at 18 hours. Despite these changes in the mitochondria after 18 hours of IL-3 deprivation, clonogenicity was unaffected when IL-3 was replenished at 18 hours. Activation of v-Abl tyrosine kinase (v-Abl TK) in IC.DP cells before IL-3 depletion led to increased levels of Bcl-X L , prevented reduction of Ψ m and the release of mitochondrial cytochrome c, and suppressed apoptosis. Activation of v-Abl TK 18 hours after withdrawal of IL-3 when ≤10% of the cells had died restored Ψ m in the remaining cells. More than 40% of cells thus rescued by v-Abl TK between 18 and 42 hours could subsequently form colonies in the presence of IL-3. These data suggest that reduction in Ψ m precedes loss of mitochondrial cytochrome c in IC.DP cells; that v-Abl TK activation, probably via upregulation of Bcl-X L, prevents loss of Ψ m and blocks the release of cytochrome c from mitochondria; and that neither of these mitochondrial events is sufficient for commitment to apoptosis.