CK2 Basal Activity Regulates IL-7-Mediated JAK/STAT and PI3K/Akt Pathway Activation and Pro-Survival Effects in T-Cell Acute Lymphoblastic Leukemia Cells

Abstract 1242

Interleukin 7 (IL-7) and its receptor (IL-7R) are essential for normal T-cell development and homeostasis. However, IL-7/IL-7R-mediated signaling may also partake in leukemia development, as we recently demonstrated by the identification of IL-7Rα gain-of-function mutations in around 9% of T-cell acute lymphoblastic leukemia (T-ALL) patients (Zenatti et al, Nat Genet 2011). Moreover, we have shown that IL-7 produced in the leukemia milieu by stromal cells can accelerate leukemia progression in vivo (Silva et al, Cancer Res 2011) and have a relevant contribution to the viability and proliferation of T-ALL cells via activation of PI3K/Akt signaling pathway (Barata et al, J Exp Med 2004). On the other hand, the pleiotropic serine/threonine Casein Kinase 2 (CK2) is overexpressed and hyperactivated in leukemia, including in T-ALL. CK2 phosphorylates and thereby inactivates PTEN, contributing to constitutive activation of PI3K/Akt pathway in the vast majority of primary T-ALL cases (Silva et al, J Clin Invest 2008). Here, we show for the first time, that CK2 activity crosstalks with IL-7-mediated signaling in T-ALL. To determine the involvement of CK2 in IL-7 mediated signaling, we treated HPB-ALL (IL-7-responsive) and TAIL7 (IL-7-dependent) cells, with two CK2-specific pharmacological inhibitors: 4,5,6,7-tetrabromobenzotriazole (TBB) and CX-4945, the latter being in phase I clinical trials for several cancers. The efficacy of the inhibitors was first confirmed by assessing the phosphorylation of Akt at S129, a CK2-specific phosphorylation site, or by CK2 in vitro kinase activity assays, after lysis of TAIL7 cells treated with or without each drug. Stimulation of T-ALL cell lines with IL-7 had a minor positive effect on CK2 activity, which was not statistically significant. However, treatment with TBB or CX-4945 revealed that CK2 inhibition completely abrogated IL-7-mediated activation of PI3K/Akt pathway, as demonstrated by Akt S473 phosphorylation. Remarkably, CK2 inhibition also prevented STAT5 activation, as measured by STAT5 Y694/Y699 phosphorylation. These results suggest that although CK2 activity is not regulated by IL-7, it is fundamental for the activation of two major IL-7-dependent survival pathways. In agreement, we found that both TBB and CX-4945 completely prevented IL-7 viability effects not only in T-ALL cell lines but also in primary leukemia cells collected from patients at diagnosis, as determined by flow cytometry analysis of FSCxSSC and Annexin V-PE/7AAD staining. Accordingly, IL-7-induced Bcl-2 upregulation and maintenance of mitochondrial transmembrane potential were both reversed by treatment with the CK2 antagonists. Furthermore, CK2 inhibition completely abrogated T-ALL cell proliferation, as determined by evaluation of DNA synthesis upon ³H thymidine incorporation. In summary, our study contributes to the elucidation of the mechanisms involved in IL-7-induced viability and proliferation in T-ALL, identifying CK2 as a master regulator not only of cell-intrinsic but also of growth factor-dependent activation of pro-survival pathways in this malignancy.