NOTCH1 Activation Depletes the Pool of Side Population Stem Cells in ATL

Background: HTLV-I infection is associated with the development of adult T-cell leukemia (ATL), a malignancy characterized by a highrate of disease relapse and poor survival. Previous studies reported the existence of side population (SP) cells in HTLV-I Tax transgenic mouse models. These studies showed that these ATL-like derived SP cells have both self-renewal and leukemia renewal capacity and represent Cancer Stem Cells (CSC)/ Leukemia-Initiating Cells (LIC). Since CSC/ LIC are resistant to conventional therapies, a better characterization is needed. Methods: We isolated, sorted and characterized SP cells from uncultured PBMCs from ATL patients and from ATL patient-derivedcell lines. We then identified several specific signaling pathways activated or suppressed in these cells. Expression of viral gene HBZ and Tax transcriptional activity was also investigated. Using gammasecretase inhibitor (GSI, Calbiochem) and stably transduced ATL celllines expressing TET-inducible NOTCH 1 intracellular domain (NICD), we characterized the role of activated NOTCH 1 in the maintenance of the SP cells in ATL. Results: Our studies confirm the existence of SP cells in ATL samples. These cells demonstrate lower activation of NOTCH1 and Tax, and reduced expression of STAT3, β-catenin/Wnt3 and viral HBZ. We further show that PI3K and the NOTCH1 signaling pathway have opposite functions, and constitutive activation of NOTCH1 signaling depletes the pool of SP cells in ATL-derived cell lines. Conclusions: Our results suggest that in ATL, a balance between activation of the NOTCH1 and PI3K signaling pathway is the key in the control of SP cells maintenance and may offer therapeutic opportunities.