Metastasis involves critical interactions between cancer and stromal cells. Intratumoral hypoxia promotes metastasis through activation of hypoxia-inducible factors (HIFs). We demonstrate that
IntroductionAn important advance in cancer biology has been the appreciation that, in addition to somatic mutations in oncogenes and tumor suppressor genes within cancer cells, a major mechanism driving disease progression is the interaction of cancer cells with the tumor microenvironment. The tumor stroma consists of extracellular matrix and various mesenchymal cell types, including vascular ECs and pericytes, fibroblasts, myofibroblasts, and various cells of bone marrow origin, including tumor-associated macrophages, bone marrow-derived angiogenic cells, neutrophils, mast cells, myeloid-derived suppressor cells, and mesenchymal stem cells (MSCs), which are recruited to the tumor and enhance primary tumor growth and/or promote metastasis (1). The molecular mechanisms by which stromal cells are attracted to, and communicate with, cancer cells are only understood in a limited number of contexts. For example, breast cancer cell (BCC) production of colony-stimulating factor 1 (CSF1) induces homing of CSF1 receptor-expressing tumor-associated macrophages that secrete epidermal growth factor, which binds to its receptor on cancer cells and stimulates their invasive properties (2, 3).The combination of cancer cell proliferation and stromal cell recruitment results in an imbalance between O 2 consumption and delivery. Tumor vasculature is structurally and functionally abnormal, leading to spatial and temporal heterogeneity in perfusion,