Inhibition of STAT5 by IL-1β or the Inhibitor of Janus Kinases Stimulates Dormant Human Leukemia Stem Cells and Sensitizes Them to Anti-Leukemia Agents.

Abstract 1597

Poster Board I-623

Leukemia stem cells (LSCs) are in a quiescent state, which maybe contributes to their drug-resistant character. It is essential to eradicate LSCs to develop curative therapy for leukemia. To verify molecular mechanisms by which LSCs maintain a dormant state, we examined the activity of the major pro-survival signal pathways in LSCs (CD34⁺/CD38^- compartment) and non-LSCs (CD34⁺/CD38⁺ compartment) counterparts from patients with acute myelogenous leukemia (AML, n=3) and acute lymphoblastic leukemia (ALL, n=1) by FACS. Interestingly, LSCs compartment expressed a greater amount of the phosphorylated forms of STAT5 (p-STAT5) than non-LSCs counterparts in all patients. Further studies found that levels of interleukin-1b (IL-1b) were significantly down-regulated in LSCs compared with non-LSCs (p<0.05) and normal hematopoietic stem cells (p<0.01), as measured by real-time RT-PCR. Importantly, exposure of LSCs to IL-1b (50 ng/ml) dephosphorylated STAT5 and decreased the population of cells in a dormant state, as assessed by cell cycle analysis. In addition, blockade of JAK2/STAT5 signaling by the specific JAK2 inhibitor AZ960 stimulated cell-cycling in LSCs in conjunction with down-regulation of cyclin-dependent kinase inhibitor p21^waf1. As expected, IL-1b and AZ960 sensitized LSCs to cytarabine and the inhibitor of the FLT3 kinase, as assessed by clonogenic assay. Moreover, this study found that LSCs expressed a greater amount of anti-apoptotic protein Bcl-xL than non-LSCs counterparts. Exposure of LSCs to AZ960 potently induced apoptosis in parallel with down-regulation of Bcl-xL, as assessed by induction of the cleaved forms of PARP. Taken together, JAK2/STAT5 signaling may be a promising molecular target to eradicate LSCs in individuals with AML and ALL.