There are conflicting reports regarding the effects of Notch activation on nuclear factor-κB (NF-κB) activity. The relationships are cell type-dependent and have not been fully elucidated. We examined the effects of Notch activation induced by a recombinant Notch ligand, Delta-like1 (Dll1), on the NF-κB activity in two acute myeloid leukemia (AML) cell lines. We found that Delta1-induced Notch activation activated the NF-κB pathway in THP-1 cells. Regarding the possible mechanisms, Dll1 stimulation increased the mRNA and protein expression levels of some components of the NF-κB pathway and induced phosphorylation of IKKα/β, IκB and RelA proteins after 24 or 48 h of stimulation. Since the phosphorylation required a long time, it did not appear to be caused by physical interactions between Notch and NF-κB proteins, but rather by indirect effects. One possible mechanism for the indirect effects was the observed induction of IL-1β expression by Dll1 stimulation. On the other hand, Notch activation did not affect NF-κB activity in TMD7 cells. RelA was phosphorylated without stimulation, indicating that NF-κB was constitutively activated in TMD7 cells. To the best of our knowledge, this is the first study to investigate AML cells and use a recombinant Notch ligand to activate Notch. The present findings lead to better understanding of Notch functions, which have not been fully elucidated.

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