Notch1 is required for hypoxia-induced proliferation, invasion and chemoresistance of T-cell acute lymphoblastic leukemia cells

Background: Notch1 is a potent regulator known to play an oncogenic role in many malignancies including T-cell acute lymphoblastic leukemia (T-ALL). Tumor hypoxia and increased hypoxia-inducible factor-1 alpha (HIF-1 alpha) activity can act as major stimuli for tumor aggressiveness and progression. Although hypoxia-mediated activation of the Notch1 pathway plays an important role in tumor cell survival and invasiveness, the interaction between HIF-1 alpha and Notch1 has not yet been identified in T-ALL. This study was designed to investigate whether hypoxia activates Notch1 signalling through HIF-1 alpha stabilization and to determine the contribution of hypoxia and HIF-1 alpha to proliferation, invasion and chemoresistance in T-ALL. Methods: T-ALL cell lines (Jurkat, Sup-T1) transfected with HIF-1 alpha or Notch1 small interference RNA (siRNA) were incubated in normoxic or hypoxic conditions. Their potential for proliferation and invasion was measured by WST-8 and transwell assays. Flow cytometry was used to detect apoptosis and assess cell cycle regulation. Expression and regulation of components of the HIF-1 alpha and Notch1 pathways and of genes related to proliferation, invasion and apoptosis were assessed by quantitative real-time PCR or Western blot. Results: Hypoxia potentiated Notch1 signalling via stabilization and activation of the transcription factor HIF-1 alpha. Hypoxia/HIF-1 alpha-activated Notch1 signalling altered expression of cell cycle regulatory proteins and accelerated cell proliferation. Hypoxia-induced Notch1 activation increased the expression of matrix metalloproteinase-2 (MMP2) and MMP9, which increased invasiveness. Of greater clinical significance, knockdown of Notch1 prevented the protective effect of hypoxia/HIF-1 alpha against dexamethasone-induced apoptosis. This sensitization correlated with losing the effect of hypoxia/HIF-1 alpha on Bcl-2 and Bcl-xL expression. Conclusions: Notch1 signalling is required for hypoxia/HIF-1 alpha-induced proliferation, invasion and chemoresistance in T-ALL. Pharmacological inhibitors of HIF-1 alpha or Notch1 signalling may be attractive interventions for T-ALL treatment.