Loss of the Bcl-2 phosphorylation loop domain increases resistance of human leukemia cells (U937) to paclitaxel-mediated mitochondrial dysfunction and apoptosis

The impact of ectopic expression of an N-terminal phosphorylation loop deletant Bcl-2 protein (Bcl-2 Delta(32-80)) on the response of U937 monoblastic leukemia cells to paclitaxel was examined. In contrast to recent findings in HL-60 cells (Fang et al, Cancer Res. 58, 3202, 1998), U937 cells overexpressing Bcl-2 Delta(32-80) were significantly more resistant than those overexpressing full-length protein to caspase-3 and -9 activation, PARP degradation, and apoptosis induced by paclitaxel (500 nM; 18 h), Bcl-2 Delta(32-80) was also more effective than its full-length counterpart in opposing paclitaxel-mediated mitochondrial dysfunction, e.g., loss of mitochondrial membrane potential (Delta psi(m)) and cytochrome c release into the cytoplasm, Enhanced resistance of U937/Bcl-2 Delta(32-80) cells to paclitaxel was observed primarily in the G(2)M population. Together, these findings demonstrate that deletion of the Bcl-2 phosphorylation loop domain increases resistance of U937 leukemia cells to paclitaxel-mediated mitochondrial damage and apoptosis and suggest that factors other than, or in addition to, phosphorylation contribute to Bcl-2-related cytoprotectivity against paclitaxel in this model system. (C) 1999 Academic Press.