Breast cancer (BC) is the most frequently diagnosed cancer in women, affecting one in eight women. Despite improved treatment strategies, BC recurrence after initial therapy is still responsible for nearly 500,000 deaths worldwide each year. This is related to the fact that BC cells metastasize very early in the disease course, and often remain unnoticed at the time of diagnosis. Novel therapeutic approaches that block metastasis offer thus great promise. Hypoxia is a key characteristic of solid tumors including breast cancer. Hypoxia inducible factor (HIF) is the main transcription factor involved in the cellular hypoxic response. The oxygen sensor enzymes, HIF-prolyl hydroxylases (PHD1-3) prevent HIF activity under normoxic conditions. Initial results indicate that haplodeficiency of the oxygen sensor HIF-prolyl hydroxylase 2 (PHD2) in tumor and stromal cells reduced metastasis by 80% in a clinically relevant spontaneously arising PyMT BC mouse model. We propose to dissect the molecular mechanisms of how PHD2-blockade inhibits tumor cell dissemination and breast cancer metastasis. Using a multi-disciplinary approach of state-of-the-art conditional and cell type-specific mouse genetics, transplantation assays, in combination with cell biological and molecular analysis in vitro we will determine the effect of PHD2 haplodeficiency on tumor cells as well as on cancer-associated fibroblasts (CAFs). Preliminary data indicate that these stromal cells have reduced invasive properties in the background of PHD2 halpodeficiency compared to wildtype CAFs. Besides promising to yield novel fundamental insights in the role of PHD2 in metastasis, this proposal will also test the clinically relevant question if a induction of global PHD2-silencing at various stages after onset of tumor growth can halt metastatic progression. Such “genetic treatment” experiments will yield valuable insights for the design of future pharmacological PHD2-blockade treatment of BC patients.