Notch-driven expression of IGF1R promotes the growth, viability, and transplantability of T-ALL cells.

NOTCH1 is activated by mutation in more than 50% of human T-cell acute lymphoblastic leukemias (T-ALLs) and inhibition of Notch signaling causes cell-cycle/ growth arrest, providing rationale for NOTCH1 as a therapeutic target. The tumor suppressor phosphatase and tensin homolog (PTEN) is also mutated or lost in up to 20% of cases. It was recently observed among human T-ALL cell lines that PTEN loss correlated with resistance to Notch inhibition, raising concern that patients with PTEN-negative disease may fail Notch inhibitor therapy. As these studies were limited to established cell lines, we addressed this issue using a genetically defined mouse retroviral transduction/bone marrow transplantation model and observed primary murine leukemias to remain dependent on NOTCH1 signaling despite Pten loss, with or without additional deletion of p16 Ink4a /p19 Arf IntroductionThe 4 mammalian Notch genes (NOTCH1-4) encode a family of highly conserved type I transmembrane receptors that are normally activated by ligands of the Delta/Serrate/Lag-2 family expressed on the surface of neighboring cells. Once activated by ligand, the Notch receptors undergo proteolytic cleavage first by a disintegrin and metalloprotease, then by a ␥-secretase, which releases the intracellular domain (ICN) from the plasma membrane to translocate to the nucleus to stimulate transcription of downstream target genes in complex with the DNA-binding factor CBF1(RBPJ)/ suppressor of hairless/Lag-1 and coactivators of the Mastermind family. Although regulated NOTCH1 signaling is important for normal T-cell development, 1 it is frequently activated by mutation in the human cancer T-cell acute lymphoblastic leukemia (T-ALL). 2 The potent oncogenicity of activated NOTCH1 has been demonstrated in murine bone marrow transduction/transplantation models and several transgenic mouse lines. 3 Activating NOTCH1 mutations occur in more than 50% of primary human T-ALLs and cluster in the heterodimerization (HD) and C-terminal proline-, glutamic acid-, serine-, and threonine-rich (PEST) domains. 4 HD mutations result in weakened association or complete dissociation of the receptor subunits, and thus lead to heightened/constitutive activation of the receptor. 5 PEST domain mutations often generate premature stop codons that delete the PEST degron, and thus enhance signaling by reducing turnover/ prolonging half-life of activated ICN. 6 When present together, the HD and PEST mutations occur in cis, and stimulate signaling in a synergistic fashion. 4 Interestingly, a similar overall frequency of Notch1 mutations (mostly PEST, but some HD) has been observed in various mouse models of T-ALL, underscoring the importance of Notch1 signaling in T-cell leukemogenesis. In addition, both human and murine T-ALL cells bearing NOTCH1 mutations are frequently sensitive to treatment with inhibitors of Notch signaling including ␥-secretase inhibitors (GSIs) that induce G1 cell-cycle/ growth arrest and in some cases apoptosis. 4,7-10 Based on these findings, GSIs and other inhi...

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Insulin-like growth factor 1 receptor (IGF1R) is a prevalent signaling pathway in human cancer that supports cell growth/survival and thus contributes to aggressive biological behavior. Much work has gone into development of IGF1R inhibitors; however, candidate agents including small molecule tyrosine kinase inhibitors and blocking antibodies have yet to fulfill their promise clinically. Understanding cellular features that define sensitivity versus resistance are important for effective patient selection and anticipation of outgrowth of a resistant clone. We previously identified an important role for IGF signaling in T-cell acute lymphoblastic leukemia (T-ALL) relying primarily upon genetically defined mouse models. We present here an assessment of IGF1R dependence in human T-ALL using a broad panel of 27 established cell lines that capture a spectrum of the genetic variation that might be encountered in clinical practice. We observed that a subset of cell lines are sensitive to IGF1R inhibition and are characterized by high levels of surface IGF1R expression and PTEN positivity. Interestingly, lentiviral expression or knock-down of PTEN in PTEN-negative/positive cell lines, respectively, had limited effects on their response to IGF1R inhibition, suggesting that PTEN contributes to, but does not define IGF dependence. Additionally, we characterize downstream PI3K/AKT signaling as dominant over RAS/RAF/MEK/ERK in mediating growth and/or survival in this context. Finally, we demonstrate that IGF and interleukin-7 (IL-7) fulfill non-overlapping roles in supporting T-ALL growth. These findings are significant in that they reveal cellular features and downstream mechanisms that may determine the response of an individual patient’s tumor to IGF1R inhibitor therapy.