The loss of IAP expression during HL-60 cell differentiation is caspase-independent

Human promyelocytic leukaemia cells (HL-60) differentiate into neutrophil-like cells that die spontaneously by apoptosis when treated with retinoic acid (RA). Inhibitors of apoptosis proteins (IAP) bind to and inhibit caspases 3, 7, and 9 activity and the induction of apoptosis. In this study, we demonstrate that undifferentiated HL-60 cells express IAP. During their differentiation, IAP expression is decreased at the mRNA and protein levels. In addition, we show that there is a corresponding increase in the expression and functional activity of active caspases 3 and 9. This activity was associated with the cleavage of XIAP, NAIP, and cIAP-2. Most importantly, we demonstrate that blocking caspase activity does not alter the decrease in IAP protein expression during differentiation but prevents caspase activation, IAP cleavage, and the induction of apoptosis. This result shows that the loss of IAP expression is independent of the induction of apoptosis and is solely related to the differentiation process. However, IAP cleavage is caspase-dependent. Terminal differentiation results in an altered apoptotic phenotype that is associated with the induction of HL-60 cell apoptosis.