Processes that promote cancer progression such as angiogenesis require a functional interplay between malignant and nonmalignant cells in the tumor microenvironment. The metalloprotease aminopeptidase N (APN; CD13) is often overexpressed in tumor cells and has been implicated in angiogenesis and cancer progression. Our previous studies of APN-null mice revealed impaired neoangiogenesis in model systems without cancer cells and suggested the hypothesis that APN expressed by nonmalignant cells might promote tumor growth. We tested this hypothesis by comparing the effects of APN deficiency in allografted malignant (tumor) and nonmalignant (host) cells on tumor growth and metastasis in APNnull mice. In two independent tumor graft models, APN activity in both the tumors and the host cells cooperate to promote tumor vascularization and growth. Loss of APN expression by the host and/ or the malignant cells also impaired lung metastasis in experimental mouse models. Thus, cooperation in APN expression by both cancer cells and nonmalignant stromal cells within the tumor microenvironment promotes angiogenesis, tumor growth, and metastasis.lung cancer | melanoma | proteolytic activity | shRNA | tumorigenesis A minopeptidase N (APN, CD13; EC 3.4.11.2) is a widely expressed type II membrane-bound metalloprotease (1, 2). It functions in the enzymatic cleavage of peptides, in endocytosis, and as a signaling molecule and has been implicated in the regulation of complex and diverse processes, including cell migration, cell survival, viral uptake, and angiogenesis (3). APN has also been linked specifically to cancer, having been identified as a cellsurface marker for malignant myeloid cells (4-7) and reaching high levels of expression in association with the progression of tumors, including breast, ovarian, and prostate cancer (8-14). Indeed, vascular endothelial growth factor (VEGF), a key angiogenesis regulator, induces the expression of APN at an early stage of tumor growth (15), again highlighting the role of this enzyme in angiogenesis, a process crucial for sustained growth of most solid tumors (16). Studies of bestatin, a CD13 inhibitor and antiangiogenic agent, also suggest that APN enzymatic activity is relevant for tumorigenesis (17,18). Nevertheless, the substrates of APN in the context of angiogenesis are still unknown. The only well-defined substrate is angiotensin III in the renin-angiotensin pathway, in which APN cleaves the NH 2 -terminal arginine residue of angiotensin III to form angiotensin IV. Consistent with several lines of evidence, we have previously identified APN as a target for inhibition of tumor vascularization and growth (18)(19)(20).Tumor growth relies on a complex microenvironment in which malignant cells cooperate with various other cell types: endothelial cells of the blood and lymphatic circulation, mesenchymal stromal cells/cancer-associated fibroblasts, and a variety of bone marrow-derived cells such as myeloid-derived suppressor cells and lymphocytes (21, 22). Some of these cell populations c...