Tumourigenesis caused by the Bcr/Abl oncoprotein is a multi-step process proceeding from initial to tumour-maintaining events and finally results in a complex tumour-supporting network. A key to successful cancer therapy is the identification of critical functional nodes in an oncogenic network required for disease maintenance. So far, the transcription factors Stat3 and Stat5a/b have been implicated in bcr/abl-induced initial transformation. However, to qualify as a potential drug target, a signalling pathway must be required for the maintenance of the leukaemic state. Data on the roles of Stat3 or Stat5a/b in leukaemia maintenance are elusive. Here, we show that both, Stat3 and Stat5 are necessary for initial transformation. However, Stat5-but not Stat3-deletion induces G0/G1 cell cycle arrest and apoptosis of imatinib-sensitive and imatinib-resistant stable leukaemic cells in vitro. Accordingly, Stat5-abrogation led to effective elimination of myeloid and lymphoid leukaemia maintenance in vivo. Hence, we identified Stat5 as a vulnerable point in the oncogenic network downstream of Bcr/Abl representing a case of non-oncogene addiction (NOA).

In E-myc transgenic animals lymphoma formation requires additional genetic alterations, which frequently comprise loss of p53 or overexpression of BCL-2. We describe that the nature of the "second hit" affects the ability of the immune system to contain lymphoma development. Tumors with disrupted p53 signaling killed the host more rapidly than BCL-2 overexpressing ones. Relaxing immunologic control, using Tyk2 ؊/؊ mice or by Ab-mediated depletion of CD8 ؉ T or natural killer (

The transcription factors and proto-oncogenes STAT3 and STAT5 are highly activated in hematological malignancies and represent promising therapeutic targets. Whereas the importance of STAT5 as tumor promoter is beyond doubt, the role of STAT3 in hematological cancers is less well understood. Both, enforced as well as attenuated expression of STAT3 were reported in hematopoietic malignancies. Recent evidence implicates STAT3 as key player for tumor immune surveillance as it both mediates the production of and response to inflammatory cytokines. Here we investigated the effects of STAT3 deletion in a BCR/ABL-induced lymphoma model, which is tightly controlled by natural killer (NK) cells in vivo. Upon STAT3 deletion tumor growth is significantly enhanced when compared to STAT3-expressing controls. The increased tumor size upon loss of STAT3 was accompanied by reduced NK cell infiltration and decreased levels of the cytokine IFN-γ and the chemokine RANTES. Upon transplantation into NK cell-deficient mice differences in lymphoma size were abolished indicating that STAT3 expression in the tumor cells controls NK cell-dependent tumor surveillance. Our findings indicate that STAT3 inhibition in lymphoma patients will impair NK cell-mediated tumor surveillance, which needs to be taken into account when testing STAT3 inhibitors in preclinical or clinical trials.

The hedgehog (Hh) signaling pathway plays fundamental roles during embryonic development and tumorigenesis. Previously, we have shown that ablation of the tumor suppressor and negative regulator, Suppressor of fused (Sufu), within this pathway causes embryonic lethality around E9.5 in the mouse. In this study, we examine how lack of Sufu influences early cell fate determination processes. We established embryonic stem cell (ESC) lines from preimplantation Sufu(-/-) and wild-type mouse embryos and show that these ESCs express the typical pluripotency markers, alkaline phosphatase, SSEA-1, Oct4, Sox2, and Nanog. We demonstrate that these ESCs express all core Hh pathway components and that glioma-associated protein (Gli)1 mRNA levels are increased in Sufu(-/-) ESCs. Upon spontaneous differentiation of Sufu(-/-) ESCs into embryoid bodies (EBs) in vitro, the Hh pathway is strongly upregulated as indicated by an increase in both Gli1 and patched1 (Ptch1) gene expression. Interestingly, developing Sufu(-/-) EBs were smaller than their wild-type counterparts and showed decreased expression of the ectodermal markers, Fgf5 and Sox1. In vivo teratoma formation revealed that Sufu(-/-) ESCs have a limited capacity for differentiation as the resulting tumors lacked the mesodermal derivatives, cartilage and bone. However, Sufu(-/-) ESCs were able to develop into chondrocytes and osteocytes in vitro, which suggests a differential response of ESCs compared with in vivo conditions. Our findings suggest a regulatory function of the Hh signaling pathway in early mesodermal cell fate determination and emphasize the role of Sufu as a key molecule in this process.

LGR5 is a known marker of embryonic and adult stem cells in several tissues. In a mouse model, Lgr5+ cells have shown tumour-initiating properties, while in human cancers, such as basal cell carcinoma and colon cancer, LGR5 expression levels are increased: however, the effect of increased LGR5 expression is not fully understood. To study the effects of elevated LGR5 expression levels we generated a novel tetracycline-responsive, conditional transgenic mouse line expressing human LGR5, designated TRELGR5. In this transgenic line, LGR5 expression can be induced in any tissue depending on the expression pattern of the chosen transcriptional regulator. For the current study, we used transgenic mice with a tetracycline-regulated transcriptional transactivator linked to the bovine keratin 5 promoter (K5tTA) to drive expression of LGR5 in the epidermis. As expected, expression of human LGR5 was induced in the skin of double transgenic mice (K5tTA;TRELGR5). Inducing LGR5 expression during embryogenesis and early development resulted in macroscopically and microscopically detectable phenotypic changes, including kink tail, sparse fur coat and enlarged sebaceous glands. The fur and sebaceous gland phenotypes were reversible upon discontinued expression of transgenic LGR5, but this was not observed for the kink tail phenotype. There were no apparent phenotypic changes if LGR5 expression was induced at three weeks of age. The results demonstrate that increased expression of LGR5 during embryogenesis and the neonatal period alter skin development and homeostasis.