Stat1ioning lrf1 in mammary tumorigenesis and therapy response

Stat1 has been defined as a suppressor of mammary carcinogenesis, as loss of Stat1 in mouse models promotes mammary tumor development.1-4 Consistently, Stat1 expression is frequently lost in human breast cancer. We previously reported that the tumor-suppressing activity of Stat1 in mammary cancer is attributed to both cell-intrinsic and CTL-mediated immune-mechanisms.1 Some aspects of the cell intrinsic tumor-suppressing effect of Stat1 are mediated via the regulation of the downstream transcription factor Irf1. Accordingly, mouse models deficient for Irf1 spontaneously develop mammary tumors. However, it remains unclear, how Stat1 and Irf1 relate to each other in mammary tumorigenesis. Despite being consecutive members of one signaling cascade Stat1 and Irf1 exert distinct functions in mammary tumor development, progression and response to therapy as evident from our preliminary data. We observe different tumor phenotypes upon loss of Stat1 and Irf1; the loss of Irf1 leads to highly malignant, estrogen-receptor (ER) negative tumors that show a mosaic expression of Stat1. Contrary, the loss of Stat1 itself inflicts a milder, ER-positive phenotype and leads to a partial down-regulation of Irf1. We thus propose that Stat1 and Irf1 represent distinct signaling nodes in the network driving mammary carcinogenesis and tumor progression and that loss of either factor shapes tumor formation in its own way with precise consequences for mammary tumor formation, progression and treatment response of patients. We furthermore propose that the loss of Stat1 and/or Irf1 set the course at an early stage of tumor development and that the importance of Stat1 shifts at later stages towards regulating chemotherapeutic responses. This will be tested by re- introducing Stat1 and/or Irf1 at consecutive tumor stages and the investigation of the impact of Stat1 re-expression on Irf1 expression, mammary tumor progression and chemosensitivity. Our aim is to define the time frame in tumor development at which effects are still reversible and where re- activation of Stat1 might have therapeutic relevance. It will also allow us to gain first insights on a potential involvement of Irf1, which we propose to be largely independent players. We will employ mouse models as well as study human patient samples before and after neoadjuvant chemotherapy. A novel mouse model that allows for the re-introduction of Stat1 expression will enable us to test reversibility at consecutive tumor stages. These in vivo studies will be completed by the analysis of human breast cancer samples from a phase II neoadjuvant clinical study for Stat1 and Irf1 expression. The staining results will be correlated to tumor response to preoperative therapy and overall clinical outcome. In summary, this study will enable the establishment of Stat1 and Irf1 as predictive biomarkers of breast cancer that allow a more precise prediction of treatment response and represent a further step in personalized medicine. Moreover, the identification of clear-cut predictive factors, either host- or tumor-related, will define directions for robust targets for novel therapeutic strategies.

Multiple forms of solid as well as hematological cancer harbor alterations in the JAK-STAT signaling pathway. In particular, breast cancer and leukemia patients display a constitutive activation of this pathway. Before we embarked on the project, we had identified STAT1 as a key factor for the initiation of breast cancer. We aimed at identifying those factors within the STAT1-dependent network whose depletion halt the disease. We identified two critical players: STAT1 and CDK8, a downstream partner of STAT1. Loss of CDK8 sufficed to lower significantly the invasiveness of the tumors. In this project, we went a step further and asked how the loss of STAT1 or CDK8 influences the development of hematological cancer. Both factors turned out to play crucial parts in leukemia - however, in an opposing manner. The sole loss of STAT1 sufficed to elicit a fatal myeloproliferative disease. Eradication of the malignant myeloid cell pool evoked a different, likewise fatal disease: a leukemic B-cell lymphoma. We joined efforts with colleagues from the General Hospital in Vienna and discovered a comparable picture observed in human patients suffering from a myeloproliferative disease. The inhibition of the myeloid cells via inhibition of the JAK2/STAT1 pathway inflicted a fatal leukemic B-cell lymphoma. These observations indicate a tumor-suppressing role of JAK2/STAT1. Targeting STAT1 would result in unwanted effects and would negatively influence leukemia development. We conclude that STAT1 is rather not suited as a therapeutic target to combat leukemia. On the contrary, CDK8 appears to be a valuable drug target, especially when combined with mTOR inhibition. Depletion of CDK8 at the start or during the maintenance of leukemia significantly prolonged the life span of diseased mice. To drive leukemia, CDK8 seems to rely on a partner: the mTOR pathway. In a close collaboration with scientists from the Harvard Medical School, we applied a new generation inhibitor, a so-called "dual inhibitor". This dual inhibitor blocked CDK8 as well as mTOR activity and initiated leukemic cell death. Hence, combined blocking of CDK8 and mTOR might represent a constructive way to target leukemia.