Key Points• Bone marrow adipocytes support AML survival.• AML induces adipocyte lipolysis of triglyceride to free fatty acids and subsequent transport by FABP4. Abstract

SummaryThe transcription factor Meis1 drives myeloid leukemogenesis in the context of Hox gene overexpression but is currently considered undruggable. We therefore investigated whether myeloid progenitor cells transformed by Hoxa9 and Meis1 become addicted to targetable signaling pathways. A comprehensive (phospho)proteomic analysis revealed that Meis1 increased Syk protein expression and activity. Syk upregulation occurs through a Meis1-dependent feedback loop. By dissecting this loop, we show that Syk is a direct target of miR-146a, whose expression is indirectly regulated by Meis1 through the transcription factor PU.1. In the context of Hoxa9 overexpression, Syk signaling induces Meis1, recapitulating several leukemogenic features of Hoxa9/Meis1-driven leukemia. Finally, Syk inhibition disrupts the identified regulatory loop, prolonging survival of mice with Hoxa9/Meis1-driven leukemia.

Hypomethylating agents decitabine and azacytidine are regarded as interchangeable in the treatment of acute myeloid leukemia (AML). However, their mechanisms of action remain incompletely understood, and predictive biomarkers for HMA efficacy are lacking. Here, we show that the bioactive metabolite decitabine triphosphate, but not azacytidine triphosphate, functions as activator and substrate of the triphosphohydrolase SAMHD1 and is subject to SAMHD1-mediated inactivation. Retrospective immunohistochemical analysis of bone marrow specimens from AML patients at diagnosis revealed that SAMHD1 expression in leukemic cells inversely correlates with clinical response to decitabine, but not to azacytidine. SAMHD1 ablation increases the antileukemic activity of decitabine in AML cell lines, primary leukemic blasts, and xenograft models. AML cells acquire resistance to decitabine partly by SAMHD1 up-regulation. Together, our data suggest that SAMHD1 is a biomarker for the stratified use of hypomethylating agents in AML patients and a potential target for the treatment of decitabine-resistant leukemia.

Burkitt's lymphoma (BL) is a highly proliferative B-cell neoplasm and is treated with intensive chemotherapy that, because of its toxicity, is often not suitable for the elderly or for patients with endemic BL in developing countries. BL cell survival relies on signals transduced by B-cell antigen receptors (BCRs). However, tonic as well as activated BCR signaling networks and their relevance for targeted therapies in BL remain elusive. We have systematically characterized and compared tonic and activated BCR signaling in BL by quantitative phosphoproteomics to identify novel BCR effectors and potential drug targets. We identified and quantified ∼16,000 phospho-sites in BL cells. Among these sites, 909 were related to tonic BCR signaling, whereas 984 phospho-sites were regulated upon BCR engagement. The majority of the identified BCR signaling effectors have not been described in the context of B cells or lymphomas yet. Most of these newly identified BCR effectors are predicted to be involved in the regulation of kinases, transcription, and cytoskeleton dynamics. Although tonic and activated BCR signaling shared a considerable number of effector proteins, we identified distinct phosphorylation events in tonic BCR signaling. We investigated the functional relevance of some newly identified BCR effectors and show that ACTN4 and ARFGEF2, which have been described as regulators of membrane-trafficking and cytoskeletonrelated processes, respectively, are crucial for BL cell survival. Thus, this study provides a comprehensive dataset for tonic and activated BCR signaling and identifies effector proteins that may be relevant for BL cell survival and thus may help to develop new BL treatments.lymphoma | B-cell receptor | phosphoproteome | cancer biology

Key Points• Integrin signaling promotes proliferative signals in AML cells that are mediated by the kinase Syk and the transcription factors STAT3 and STAT5.Spleen tyrosine kinase (Syk) induces cell survival and proliferation in a high proportion of acute myeloid leukemia (AML) blasts, but the underlying molecular events of Syk signaling have not been investigated. Proteomic techniques have allowed us to identify the multiprotein complex that is nucleated by constitutively active Syk in AML cells. This complex differs from the B-lymphoid Syk interactome with respect to several proteins, especially the integrin receptor Mac-1, the Fc-g receptor I (FcgRI), and the transcription factors STAT3 and STAT5. We show in several AML cell line models that tonic signals derived from the Fc-g chain lead to Syk-dependent activation of STAT3 and STAT5, which in turn induces cell survival and proliferation. Moreover, stimulation of Mac-1 or FcgRI intensifies the constitutive Syk-mediated STAT3/5 activation in AML cells, a scenario likely to take place in the bone marrow niche. In accordance with these findings, we observed that b 2 integrins, including Mac-1, trigger proliferation of AML cells in an AML cell/stroma coculture model. Taken together, we identified an oncogenic integrin/Syk/STAT3/5 signaling axis that might serve as a therapeutic target of AML in the future. (Blood. 2013;121(19):3889-3899)