The Notch signaling pathway has been recognized as a key factor for the 
pathogenesis of T-cell acute lymphoblastic leukemia (T-ALL), because of the high 
incidence of activating mutations of Notch1. Notch inhibition could serve as a 
new treatment strategy for T-ALL; however, the attempts to perturb Notch 
signaling pathways have been unsuccessful so far. In this study, we found that 
proteasome inhibitors exert cytotoxic effects on T-ALL cells with constitutive 
activation of Notch1 to a similar extent as myeloma cells. The proteasome 
inhibitor bortezomib repressed the transcription of Notch1 and downstream 
effectors including Hes1, GATA3, RUNX3 and nuclear factor-κB (NF-κB) (p65 and 
p50), coincided with downregulation of the major transactivator Sp1 and its 
dissociation from Notch1 promoter. Overexpression of the Notch1 intracellular 
domain (NICD) significantly ameliorated bortezomib-induced cytotoxicity against 
T-ALL cells. Drug combination studies revealed that bortezomib showed 
synergistic or additive effects with key drugs for the treatment of T-ALL such 
as dexamethasone (DEX), doxorubicin and cyclophosphamide, which were readily 
abolished by NICD overexpression. The synergy of bortezomib and DEX was 
confirmed in vivo using a murine xenograft model. Our findings provide a 
molecular basis and rationale for the inclusion of proteasome inhibitors in 
treatment strategies for T-ALL.