Caspase inhibition blocks cell death and results in cell cycle arrest in cytokine-deprived hematopoietic cells

Cytokine deprivation has been classically used to study molecular processes of apoptosis. Following interleukin (IL)-3 withdrawal in FL5.12 cells, Bax undergoes a conformational change that results in its mitochondria targeting, cytochrome c release, activation of caspase-9, and apoptosis. Cells overexpressing Casp9DN (dominant negative caspase-9) or treated with the caspase inhibitor Q-VD-OPh increased viability but failed to increase clonogenic survival. We find that caspase-inhibited cells had a significant fraction of viable cells (herein termed "rescued" cells) that failed to initiate cell division after IL-3 add back. The "rescued" cells had reduced mitochondrial potential, stained for active Bax, and had reduced staining with dihydroethidium, an agent sensitive to superoxide levels. Readdition of IL-3 after deprivation demonstrated that Bax activation was reversed, whereas altered 5,5', 6,6'-tetrachloro-1,1', 3,3'-tetraethylbenzimidazolylcarbocyanine iodide and dihydroethidium staining persisted for days. Furthermore, the "rescued" cells were resistant to rotenone, an inhibitor of mitochondrial respiration. The cells were highly sensitive to 2-deoxyglucose, an inhibitor of glycolysis and proposed anti-cancer agent. We conclude that the inhibition of caspase-9 allows cells to retain viability, but cells have prolonged mitochondrial dysfunction and enter a unique nondividing state that shares some properties with malignant cells.