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Mice with an impaired Type I interferon (IFN) signaling (IFNAR1- and IFNβ-deficient mice) display an increased susceptibility toward v-ABL-induced B-cell leukemia/lymphoma. The enhanced leukemogenesis in the absence of an intact Type I IFN signaling is caused by alterations within the tumor environment. Deletion of Ifnar1 in tumor cells (as obtained in Ifnar1(f/f) CD19-Cre mice) failed to impact on disease latency or type. In line with this observation, the initial transformation and proliferative capacity of tumor cells were unaltered irrespective of whether the cells expressed IFNAR1 or not. v-ABL-induced leukemogenesis is mainly subjected to natural killer (NK) cell-mediated tumor surveillance. Thus, we concentrated on NK cell functions in IFNAR1 deficient animals. Ifnar1(-/-) NK cells displayed maturation defects as well as an impaired cytolytic activity. When we deleted Ifnar1 selectively in mature NK cells (by crossing Ncr1-iCre mice to Ifnar1(f/f) animals), maturation was not altered. However, NK cells derived from Ifnar1(f/f) Ncr1-iCre mice showed a significant cytolytic defect in vitro against the hematopoietic cell lines YAC-1 and RMA-S, but not against the melanoma cell line B16F10. Interestingly, this defect was not related to an in vivo phenotype as v-ABL-induced leukemogenesis was unaltered in Ifnar1(f/f )Ncr1-iCre compared with Ifnar1(f/f) control mice. Moreover, the ability of Ifnar1(f/f) Ncr1-iCre NK cells to kill B16F10 melanoma cells was unaltered, both in vitro and in vivo. Our data reveal that despite the necessity for Type I IFN in NK cell maturation the expression of IFNAR1 on mature murine NK cells is not required for efficient tumor surveillance.

Constitutive activation of STAT5 is critical for the maintenance of chronic myeloid leukemia (CML) characterized by the BCR-ABL oncoprotein. Tyrosine kinase inhibitors (TKIs) for the STAT5-activating kinase JAK2 have been discussed as a treatment option for CML patients. Using murine leukemia models combined with inducible ablation of JAK2, we show JAK2 dependence for initial lymphoid transformation, which is lost once leukemia is established. In contrast, initial myeloid transformation and leukemia maintenance were independent of JAK2. Nevertheless, several JAK2 TKIs induced apoptosis in BCR-ABL(+) cells irrespective of the presence of JAK2. This is caused by the previously unknown direct 'off-target' inhibition of BCR-ABL. Cellular and enzymatic analyses suggest that BCR-ABL phosphorylates STAT5 directly. Our findings suggest uncoupling of the canonical JAK2-STAT5 module upon BCR-ABL expression, thereby making JAK2 targeting dispensable. Thus, attempts to pharmacologically target STAT5 in BCR-ABL(+) diseases need to focus on STAT5 itself.

The transcription factor c-JUN and its upstream kinase JNK1 have been implicated in BCR-ABL–induced leukemogenesis. JNK1 has been shown to regulate BCL2 expression, thereby altering leukemogenesis, but the impact of c-JUN remained unclear. In this study, we show that JNK1 and c-JUN promote leukemogenesis via separate pathways, because lack of c-JUN impairs proliferation of p185BCR-ABL–transformed cells without affecting their viability. The decreased proliferation of c-JunΔ/Δ cells is associated with the loss of cyclin-dependent kinase 6 (CDK6) expression. In c-JunΔ/Δ cells, CDK6 expression becomes down-regulated upon BCR-ABL–induced transformation, which correlates with CpG island methylation within the 5′ region of Cdk6. We verified the impact of Cdk6 deficiency using Cdk6−/− mice that developed BCR-ABL–induced B-lymphoid leukemia with significantly increased latency and an attenuated disease phenotype. In addition, we show that reexpression of CDK6 in BCR-ABL–transformed c-JunΔ/Δ cells reconstitutes proliferation and tumor formation in Nu/Nu mice. In summary, our study reveals a novel function for the activating protein 1 (AP-1) transcription factor c-JUN in leukemogenesis by antagonizing promoter methylation. Moreover, we identify CDK6 as relevant and critical target of AP-1–regulated DNA methylation on BCR-ABL–induced transformation, thereby accelerating leukemogenesis.

In BCR-ABL1+ leukemia, drug resistance is often associated with up-regulation of BCR-ABL1 or multidrug transporters as well as BCR-ABL1 mutations. Here we show that the expression level of the transcription factor STAT5 is another parameter that determines the sensitivity of BCR-ABL1+ cells against tyrosine kinase inhibitors (TKIs), such as imatinib, nilotinib, or dasatinib. Abelson-transformed cells, expressing high levels of STAT5, were found to be significantly less sensitive to TKI-induced apoptosis in vitro and in vivo but not to other cytotoxic drugs, such as hydroxyurea, interferon-β, or Aca-dC. The STAT5-mediated protection requires tyrosine phosphorylation of STAT5 independent of JAK2 and transcriptional activity. In support of this concept, under imatinib treatment and with disease progression, STAT5 mRNA and protein levels increased in patients with Ph+ chronic myeloid leukemia. Based on our data, we propose a model in which disease progression in BCR-ABL1+ leukemia leads to up-regulated STAT5 expression. This may be in part the result of clonal selection of cells with high STAT5 levels. STAT5 then accounts for the resistance against TKIs, thereby explaining the dose escalation frequently required in patients reaching accelerated phase. It also suggests that STAT5 may serve as an attractive target to overcome imatinib resistance in BCR-ABL1+ leukemia.

The tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) induces apoptosis specifically in cancer cells with little effect on normal cells. We have previously shown that TRAIL signaling is altered in most ovarian cancer patients and that resistance to TRAIL contributes to ovarian cancer progression. In this study, we investigated whether resistance to TRAIL may be overcome by a monoclonal TRAILR2 (DR5) agonistic antibody (AD5-10). We found that the joint presence of AD5-10 with TRAIL and natural killer (NK) cells expressing TRAIL resensitizes ovarian cancer cells to apoptosis in vitro and in vivo, respectively. The combination of AD5-10 with carboplatin exerts a more than additive effect in vitro, which may at least partially be explained by the fact that carboplatin triggers DR5 expression on ovarian cancer cells. Moreover, AD5-10 restores the sensitivity of platin-resistant ovarian cancer to carboplatin in vivo. In addition, we found that TRAIL expression and NK cells are abundant in the tumor microenvironment and that depletion of NK cells abolishes the antitumor activity of AD5-10. This indicates that NK-mediated immunosurveillance against ovarian cancer might be mediated by TRAIL and that apoptosis induced by AD5-10 requires the presence of NK cells. In conclusion, this study indicates a key role and strong antitumorigenic effect of DR5 and highlights a novel link between NK-mediated immunosurveillance and activation of DR5-mediated apoptosis in ovarian cancer. Mol Cancer Ther; 9(4); 1007–18. ©2010 AACR.

The PI3Kδ isoform is a candidate drug target in leukemia. Here, we explored its role in Abelson-induced leukemia. Frank leukemia emerges if the tumor cells have managed to outwit the immune system. The absence of PI3Kδ affected both the tumor cells and the NK cells. Abelsontransformed PI3Kδ-/cells induced leukemia in RAG2-/animals with a significantly increased latency, implicating PI3Kδ in tumor progression. NK cell function, however, was also contingent on PI3Kδ. PI3Kδ-/NK cells failed to lyse target cells. Capacitance measurements revealed the underlying defect: in PI3Kδ-/NK cells lytic granules did not fuse with the cell membrane. Accordingly, transplanted leukemic cells killed PI3Kδ-/animals more rapidly, both in syngeneic (PI3Kδ-/-) or immunocompromised (RAG2-/PI3Kδ-/-) animals. Our observations define a dual function of PI3Kδ in leukemia and document that the action of PI3Kδ in the NK compartment is as relevant to the survival of the mice as the delayed tumor progression. from 13th Scientific Symposium of the Austrian Pharmacological Society (APHAR). Joint Meeting with the Austrian Society of Toxicology (ASTOX) and the Hungarian Society for Experimental and Clinical Pharmacology (MFT) Vienna, Austria. 22–24 November 2007

Cancers adopt diverse strategies to safeguard their survival by evading tumor surveillance. Natural killer (NK) cells and CD8 T-lymphocytes are capable of destroying tumor cells thus playing a critical role in tumor immunity. In previous studies, we showed that NK cell derived TRAIL is essential for the cytotoxic activity of the DR5 agonistic antibody AD5-10, since depletion of NK cells abolished the antitumor activity of AD5-10 in a xenograft ovarian cancer (OC) mouse model. In the present study, we investigated whether NK cells are associated with clinical outcome as well as with the immune infiltrate showing anti-tumor immunity in OC. We analyzed NK cell infiltration in human primary ovarian tumor tissues using immunohistochemical analysis of a representative population of OC patients on a tissue microarray. NK tumor-infiltrating cells were detected within tumor tissues, whereas CD3 tumor-infiltrating T-lymphocytes were almost undetectable. NK cells in addition inversely correlated with FIGO stage (r=−0.51, p When we analyzed the cytotoxic capacity of human NK cells against OC cells in vitro, we found that human NK cells were capable to induce apoptosis in a concentration-and time-dependent manner in several OC cell lines. Interestingly, the addition of the DR5 agonistic antibody AD5-10 sensitized TRAIL resistant OC cell lines to NK cells mediated cytolysis. To study tumor surveillance of OC in vivo, we are currently setting up mouse models. Transformed murine ovarian surface epithelial (MOSE) cell lines derived from C57BL/6 mice have been implanted intraperitoneally into syngenic wild type, RAG2 -/-, RAG2 -/- γ -/-, NK depleted, CD8 depleted C57BL/6 recipient mice. The outcome of these studies will be presented. In summary, our findings point at a critical role for NK cell mediated tumor surveillance for OC development. Importantly, NK cell infiltration correlates with prognosis in human ovarian carcinoma. Thus, our study may open novel therapeutic opportunities by interfering with host immunity in the future. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3661. doi:10.1158/1538-7445.AM2011-3661

Abstract 511 Persistent activation of the transcription factor Stat5 is a signaling hallmark of Chronic Myelogenous Leukemia (CML). In mouse models, Stat5 was required for initial myeloid and lymphoid transformation (by Bcr-Abl p210 or p185 and v-Abl). Most importantly, we and others recently showed that Stat5 was also required for maintenance of Bcr-Abl-dependent leukemia in vivo and for engraftment and reconstitution of Bcr-Abl p210-positive leukemia in secondary recipients. Therefore, Stat5 is of central functional importance in the Bcr-Abl signaling network and represents a possible critical vulnerable node in CML. In contrast to the well-studied functional role of Stat5 in disease models, the molecular mechanism of Bcr-Abl dependent Stat5 activation, including the tyrosine kinase(s) that phosphorylate and activate Stat5, remain only partially understood. In particular, conflicting views on the involvement of the Jak2 kinase and its potential role as a drug target in CML exist. We used conditional Jak2 knock-out mice to study the contribution of Jak2 in Bcr-Abl-dependent transformation and leukemogenicity. Jak2 ablation did not compromise the Bcr-Abl p210-mediated transforming capability in primary murine bone marrow- or fetal liver-derived hematopoietic cells in vitro. In contrast, initial lymphoid transformation by v-abl and Bcr-Abl p185 was abolished in Jak2 knock-out mice. Jak2 deletion did not have an effect on maintenance of lymphoid leukemia cells in vitro, whereas deletion of Stat5 induced a G1 arrest and subsequent apoptosis. In line with this, ablation of Jak2 expression after leukemia induction did not alter disease latency or disease phenotype. Consistently, we did not observe a decrease in Stat5 activation upon siRNA-mediated knock-down of Jak2 alone or all four Jak kinases (Jak1, Jak2, Jak3 and Tyk2) in CML cell lines. Using a panel of pharmacological inhibitors, we found that neither Jak2-selective, nor pan-Jak kinase inhibitors or Src family kinase-selective inhibitors led to a decrease in Stat5 phosphorylation, while the highly selective Bcr-Abl inhibitor nilotinib completely abrogated Stat5 phosphorylation. To study possible contributions of other tyrosine kinases in the Bcr-Abl dependent activation of Stat5, we used Ba/F3 cells expressing the TKI-resistant Bcr-Abl mutant T315I in combination with different broad-specificity tyrosine kinase inhibitors, like dasatinib. At dasatinib concentrations that inhibited several dozens of tyrosine kinases, Stat5 phosphorylation in Ba/F3 Bcr-Abl T315I cells was unaffected, excluding a role for most tyrosine kinases other than Bcr-Abl in Stat5 activation and pointing towards a direct phosphorylation of Stat5 by Bcr-Abl. Together, this data excludes a role of Jak and Src kinases in the activation of Stat5 in Bcr-Abl positive cell lines. Finally, in comprehensive enzyme kinetic analysis experiments using recombinant kinase, Stat5 had a similar KM value for Bcr-Abl as the canonical direct Bcr-Abl substrate CrkL and displayed only mildly lower kinase substrate parameters (vmax, kcat) than CrkL, fully compatible with direct phosphorylation of Stat5 by Bcr-Abl. Together with our earlier data on the pivotal role of Stat5 in the transcriptional and signaling network of Bcr-Abl, we propose a hypersensitive switch-like behavior of the Bcr-Abl-Stat5 kinase substrate pair that mechanistically rationalizes the central functional role of Stat5 in the signaling of CML cells. In summary, we provide compelling evidence that activation of Stat5 by Bcr-Abl is likely to be direct and that targeting of Jak2 in CML may not be of therapeutic benefit, as Jak2 is not required for CML initiation or maintenance. Disclosures: Hantschel: Novartis: Honoraria; Bristol-Myers Squibb: Honoraria.