IL-7 Activates a STAT5/PIM1 Axis to Promote T-Cell Acute Lymphoblastic Leukemia Proliferation and Viability in a Bcl-2-Independent Manner

Background: T-cell acute lymphoblastic leukemia (T-ALL) constitutes an aggressive subset of ALL, the most frequent childhood malignancy. Although risk-adjusted chemotherapeutic regimens are currently extremely effective, their efficacy is associated with significant long-term side effects. Moreover, a significant fraction of the patients still relapse despite intensive chemotherapy, prompting the need for a deeper understanding of T-ALL biology in order to develop novel therapies. Interleukin-7 is a cytokine essential for normal T-cell development, where it has a pivotal role in promoting thymocyte survival via Bcl-2 upregulation. In this normal setting, Bcl-2 is under the control of STAT5 mediated signaling. Previously, we have shown that IL-7 promotes T-ALL expansion in vivo and leukemia cell survival and proliferation in vitro by activating PI3K/Akt/mTOR signaling pathway, consequently leading to p27^kip1 downregulation and Bcl-2 upregulation. However, it is also known that T-cell lymphomas arising spontaneously in IL-7 transgenic mice depend on STAT5 activity and that leukemias displaying IL7R gain-of-function mutations are sensitive to JAK and STAT5 inhibitors. Thus, we investigated whether STAT5 could also be involved in the IL-7 pro-leukemia effects in human T-ALL cells.

Methods: We used an IL-7-dependent leukemia T-cell line (TAIL7), an IL-7-responsive T-ALL cell line (HPB-ALL, with or without shRNA-mediated STAT5 silencing), primary T-ALL samples collected at diagnosis and patient-derived xenografts (PDX) and treated them with inhibitors of STAT5 (N-((4-Oxo-4H-chromen-3-yl) methylene) nicotinohydrazide) and PIM (AZD1208). Analysis of viability, cell size, cell cycle, surface CD71 and Bcl-2 expression was performed by flow cytometry. Signaling pathway activation, STAT5, PIM1, Bcl-xL, Mcl-1 and cell cycle protein expression was performed by immunoblot analysis. Proliferation was assessed by 3H-Thymidine incorporation. STAT5 ChIP-seq and RNA-seq were performed on TAIL7 cells. ChIP-PCR of histone marks H3K4me3, H3K27me, H3K27ac was performed in TAIL7 and HPB-ALL.

Results: IL-7 induces JAK/STAT5 pathway activation in T-ALL cells and STAT5 genetic or pharmacological inactivation prevents IL-7-mediated T-ALL cell viability, growth and proliferation. At the molecular level, STAT5 is required for IL-7-induced downregulation of p27^kip1, and upregulation of Cyclin A and TfR/CD71. Surprisingly, STAT5 inhibition does not significantly affect IL-7-mediated Bcl-2 upregulation, suggesting that, contrary to normal T-cells, STAT5 promotes leukemia cell survival via a Bcl-2-independent mechanism. In addition, IL-7-mediated increase in transcription of BCL2, BCL2L1 (Bcl-xL) and MCL1 is not affected by STAT5 silencing. To understand how STAT5 mediates the survival effects of IL-7 in T-ALL cells without affecting BCL2 transcription, we performed STAT5 ChIP-seq together with RNA-seq. Data cross-analysis reveal a diverse IL-7-driven STAT5-dependent transcriptional program in T-ALL cells, which includes transcription of the serine/threonine kinase PIM1. PIM1 is involved in cell cycle regulation and apoptosis, thereby constituting a possible alternative to Bcl-2-dependent prevention of apoptosis. We show that STAT5 silencing prevents IL-7-mediated PIM1 expression and the upregulation of active chromatin marks, H3K4me3 and H3K27ac, at the STAT5 binding region in the PIM1 gene. Notably, pharmacological inhibition of PIM kinase abrogates IL-7-mediated T-ALL cell growth and proliferation, however, without affecting cell survival. In agreement, PIM inhibition does not affect expression of Bcl-2 or Bcl-2 family anti-apoptotic members Bcl-xL and MCL1.

Conclusion: Here we present evidence that T-ALL cells may have an alternative wiring of signaling networks downstream of IL-7 to that present in normal T-cells. In contrast to healthy lymphoid cells, IL-7-mediated control of survival of T-ALL cells via STAT5 does not rely on modulation of Bcl-2. Moreover, exploration of STAT5 downstream signaling reveals that PIM1 is required for IL-7-mediated proliferation of human T-ALL cells, indicating that strategies involving the use of PIM kinase small molecule inhibitors may have therapeutic potential against leukemias that rely on IL-7R and STAT5 signaling.