Key Points CDK6 acts as a transcriptional regulator to suppress Egr1 in HSCs and LSCs, allowing their activation. Cdk6 −/− HSCs fail to contribute to repopulation in competitive transplants, and BCR-ABLp210+Cdk6−/− LSCs fail to inflict disease.

Natural killer (NK) cells are tightly regulated by the JAK-STAT signaling pathway and cannot survive in the absence of STAT5. We now report that STAT5-defi cient NK cells can be rescued by overexpression of BCL2. Our experiments defi ne STAT5 as a master regulator of NK-cell proliferation and lytic functions. Although NK cells are generally responsible for killing tumor cells, the rescued STAT5-defi cient NK cells promote tumor formation by producing enhanced levels of the angiogenic factor VEGFA. The importance of VEGFA produced by NK cells was verifi ed by experiments with a conditional knockout of VEGFA in NK cells. We show that STAT5 normally represses the transcription of VEGFA in NK cells, in both mice and humans. These fi ndings reveal that STAT5-directed therapies may have negative effects: In addition to impairing NK-cell-mediated tumor surveillance, they may even promote tumor growth by enhancing angiogenesis. SIGNIFICANCE:The importance of the immune system in effective cancer treatment is widely recognized. We show that the new signal interceptors targeting the JAK-STAT5 pathway may have dangerous side effects that must be taken into account in clinical trials: inhibiting JAK-STAT5 has the potential to promote tumor growth by enhancing NK-cell-mediated angiogenesis. Cancer Discov; 6(4); ©2016 AACR . Ni and Cerwenka, p. 347. See related commentary by INTRODUCTIONNatural killer (NK) cells are innate lymphocytes that develop from a common lymphoid progenitor in the bone marrow ( 1 ). They represent the fi rst line of defense against infected, stressed, and malignant cells. Recent evidence has assigned distinct features and functions to tissue-specifi c NK cells ( 2 ). NK cells have organ-specifi c properties, such as distinct profi les of receptor expression or cytokine production ( 3 ). Uterine NK cells secrete high levels of VEGFA and are involved in placental vascularization. The physiologic functions of other organ-specifi c NK-cell subsets are less well understood ( 4 ). STAT5 Is a Key Regulator in NK Cells and Acts as a Molecular Switch from Tumor Surveillance to Tumor PromotionDagmarAll aspects of NK-cell development are regulated by cytokines, their downstream signaling pathways, and transcriptional regulators. These include key cytokines such as IL2, IL12, IL15, IL18, and IL21 ( 5 ), most of which signal via the common γ chain ( 5 ) and activate the JAK-STAT pathway ( 6 ). JAK kinases (JAK1-3 and TYK2) bind to cytokine receptors and are activated by ligand/receptor binding. The activated kinase phosphorylates STAT transcription factors refs. 6,7 ).Consistent with its function as the major STAT protein downstream of IL7, IL2, and IL15, STAT5 is absolutely essential for conventional NK-cell development and survival; Stat5 Δ / Δ Ncr1-iCre Tg mice lack NK cells ( 8 ). It is also important for lymphoid cell development ( 9 ): STAT5 is constitutively active in a plethora of lymphoid malignancies ( 10 ). Recent studies have described somatic Stat5b mutations as active drivers of lymphoid mali...

Inhibition of Janus-kinase 1/2 (JAK1/2) is a mainstay to treat myeloproliferative neoplasms (MPN). Sporadic observations reported the co-incidence of B-cell non-Hodgkin lymphomas during treatment of MPN with JAK1/2 inhibitors. We assessed 626 patients with MPN, including 69 with myelofibrosis receiving JAK1/2 inhibitors for lymphoma development. B-cell lymphomas evolved in 4 (5.8%) of 69 patients receiving JAK1/2 inhibition compared with 2 (0.36%) of 557 with conventional treatment (16-fold increased risk). A similar 15-fold increase was observed in an independent cohort of 929 patients with MPN. Considering primary myelofibrosis only (N = 216), 3 lymphomas were observed in 31 inhibitor-treated patients (9.7%) vs 1 (0.54%) of 185 control patients. Lymphomas were of aggressive B-cell type, extranodal, or leukemic with high MYC expression in the absence of V617F or other MPN-associated mutations. Median time from initiation of inhibitor therapy to lymphoma diagnosis was 25 months. Clonal immunoglobulin gene rearrangements were already detected in the bone marrow during myelofibrosis in 16.3% of patients. Lymphomas occurring during JAK1/2 inhibitor treatment were preceded by a preexisting B-cell clone in all 3 patients tested. Sequencing verified clonal identity in 2 patients. The effects of JAK1/2 inhibition were mirrored in mice: 16 of 24 mice developed a spontaneous myeloid hyperplasia with the concomitant presence of aberrant B cells. Transplantations of bone marrow from diseased mice unmasked the outgrowth of a malignant B-cell clone evolving into aggressive B-cell leukemia-lymphoma. We conclude that JAK/STAT1 pathway inhibition in myelofibrosis is associated with an elevated frequency of aggressive B-cell lymphomas. Detection of a preexisting B-cell clone may identify individuals at risk.

STAT5B is often mutated in hematopoietic malignancies. The most frequent STAT5B mutation, Asp642His (N642H), has been found in over 90 leukemia and lymphoma patients. Here, we used the Vav1 promoter to generate transgenic mouse models that expressed either human STAT5B or STAT5BN642H in the hematopoietic compartment. While STAT5B-expressing mice lacked a hematopoietic phenotype, the STAT5BN642H-expressing mice rapidly developed T cell neoplasms. Neoplasia manifested as transplantable CD8+ lymphoma or leukemia, indicating that the STAT5BN642H mutation drives cancer development. Persistent and enhanced levels of STAT5BN642H tyrosine phosphorylation in transformed CD8+ T cells led to profound changes in gene expression that were accompanied by alterations in DNA methylation at potential histone methyltransferase EZH2-binding sites. Aurora kinase genes were enriched in STAT5BN642H-expressing CD8+ T cells, which were exquisitely sensitive to JAK and Aurora kinase inhibitors. Together, our data suggest that JAK and Aurora kinase inhibitors should be further explored as potential therapeutics for lymphoma and leukemia patients with the STAT5BN642H mutation who respond poorly to conventional chemotherapy.

Key Points• CDK6 directly regulates transcription of FLT3 and PIM1 in a kinase-dependent manner.• CDK6 kinase inhibition impairs not only FLT3-dependent cell growth in vitro but also FLT3-driven leukemogenesis in vivo.Up to 30% of patients with acute myeloid leukemia have constitutively activating internal tandem duplications (ITDs) of the FLT3 receptor tyrosine kinase. Such mutations are associated with a poor prognosis and a high propensity to relapse after remission. FLT3 inhibitors are being developed as targeted therapy for FLT3-ITD 1 acute myeloid leukemia;however, their use is complicated by rapid development of resistance, which illustrates the need for additional therapeutic targets. We show that the US Food and Drug Administration-approved CDK4/6 kinase inhibitor palbociclib induces apoptosis of FLT3-ITD leukemic cells. The effect is specific for FLT3-mutant cells and is ascribed to the transcriptional activity of CDK6: CDK6 but not its functional homolog CDK4 is found at the promoters of the FLT3 and PIM1 genes, another important leukemogenic driver. There CDK6 regulates transcription in a kinase-dependent manner. Of potential clinical relevance, combined treatment with palbociclib and FLT3 inhibitors results in synergistic cytotoxicity. Simultaneously targeting two critical signaling nodes in leukemogenesis could represent a therapeutic breakthrough, leading to complete remission and overcoming resistance to FLT3 inhibitors. (Blood. 2016; 127(23):2890-2902