3124 – TARGETING THE NOTCH/IL-7/SKP2 CIRCUITRY IN T-ALL

Acute Lymphoblastic Leukemia (ALL) is the most common pediatric cancer. Despite marked clinical successes in the treatment of childhood T-ALL, leukemia relapse, refractory disease and induction failure (≈ 30% of patients) remain significant clinical problems, often life-threatening. Thus, new therapeutic strategies are needed. Activating Notch mutations have been identified in over 50% of the cases, placing Notch signaling as a central player in T-ALL. Notch regulates the differentiation, proliferation and survival of both, normal and malignant T-cells, however its therapeutic targeting has failed. Moreover, little is known about its downstream mediators and the crosstalk with other microenvironmental cues, like the IL-7 signaling pathway. IL-7 is an essential cytokine for normal lymphoid development, and its continuous signal can lead to T-cell neoplasms. Our laboratory has focused on study the crosstalk between these two signaling pathways in T-ALL. Here we show that primary T-ALL cells carrying Notch activating mutations require IL-7 for a robust proliferation and are still responsive to ligand-dependent Notch stimulation. We found that Notch activation hypersensitize T-ALL cells to IL-7 by mediating direct transcription of IL-7Rα and thus, up-regulating its surface expression. Notch blockade significantly inhibited IL-7-mediated proliferation of T-ALL whereas IL-7 stimulation greatly potentiated Notch signaling. Investigation the downstream mechanisms common to both pathways lead us to the identification of SKP2. We discovered that SKP2, a key cell cycle regulator for cell cycle entry associated with poor prognosis in cancers, is a downstream target of Notch playing an important role in T-ALL. We demonstrated that genetic ablation or pharmacological blockade of Skp2, significantly delayed T-ALL progression and increased survival in in vivo T-ALL models. SKP2 mediates mitogenic responses to various cytokines, and we showed that is upregulated following IL-7 stimulation in human T-ALL cells, suggesting that Notch and IL-7 signaling cooperate and converge in SKP2. In conclusion, our studies indicated a cooperative effect between an oncogenic event (Notch signaling) and microenvironmental cues (IL-7 signaling) for leukemia maintenance and progression, and identify the Notch/IL-7/SKP2 circuitry as a critical network and potential therapeutic target in T-ALL.