IL-3 WITHDRAWAL ACTIVATES A CRMA-INSENSITIVE POLY(ADP-RIBOSE) POLYMERASE CLEAVAGE ENZYME IN FACTOR-DEPENDENT MYELOID PROGENITOR CELLS

Murine myeloid progenitor cells that are dependent on interleukin-3 (I L-3) undergo apoptosis when this essential cytokine is withdrawn. To d etermine whether IL-3 withdrawal leads to the activation of caspase pr oteases, known mediators of apoptosis, we studied proteolytic cleavage of the caspase substrate protein poly(ADP-ribose) polymerase (PARP) i n two IL-3-dependent myeloid progenitor cell lines, 32D and FDCP-1. We observed that IL-3 withdrawal leads to PARP cleavage in both cell lin es, with complete cleavage occurring by 24 h after cytokine removal. T he induced PARP cleavage activities were blocked by the caspase inhibi tors z-DEVD-fluoromethyl ketone (z-DEVD-FMK) and z-VAD-fluoromethyl ke tone (z-VAD-FMK), or by overexpression in 32D cells of Bcl-2 or BCR/AB L. By contrast, overexpression in 32D cells of cowpox virus CrmA prote in, an inhibitor of Fas-mediated PARP cleavage, failed to inhibit PARP cleavage following IL-3 withdrawal. CrmA also failed to block DNA fra gmentation and loss of cell viability. We propose that a CrmA-insensit ive caspase protease is activated in the IL-3-deprived myeloid precurs ors, and that activation of this protease may direct the cells on a pa th towards commitment to death.