The Notch signaling pathway plays an important role in the regulation of self-renewal and differentiation of hematopoietic progenitors. Tumor necrosis factor (TNF)-α induces apoptosis through activation of caspase pathway. A monoblastic leukemia cell line, U937, undergoes apoptosis following stimulation with TNF-α. We found that Notch activation induced by a recombinant Notch ligand, Delta-1, reduced the TNF-α-induced growth suppression and apoptosis in U937 cells. As the molecular mechanism involved, we showed Delta-1 stimulation partially suppressed the sequential activation of caspase-8, caspase-3, and, PARP induced by TNF-α. The TNF-α-induced activation of c-Jun N-terminal kinase (JNK), p38, and NF-κB was not affected by Delta-1 stimulation. The cells needed to be exposed to Delta-1 prior to TNF-α stimulation to reduce the suppressive effect of TNF-α. Therefore, we thought that Delta-1 stimulation might reduce the expression of TNF-receptor (R) 1 and proteins to modulate the activation of caspases such as FLIP and XIAP. However, Delta-1 stimulation did not affect their expression. The precise mechanism by which Notch signaling suppresses caspase activation has yet to be determined. This is the first report to show the relationship between Notch activation and TNF-R1 signaling. The findings suggest possible mechanisms by which Notch activation supports cell survival.

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