BACKGROUND: Angiocidin, first identified as a tumor-associated thrombospondin-1 (TSP-1) receptor, is a key mediator of tumor progression. TSP-1, an extracellular protein produced by stromal cells, up-regulates gelatinases and tumor cell invasion in epithelial malignancies. The authors recently developed 2 angiocidininhibitory peptides that block angiocidin-TSP-1 binding. They hypothesized that angiocidin mediates increased gelatinase expression and tumor cell invasion in sarcomas through its interaction with TSP-1. METHODS: Angiocidin, TSP-1, and gelatinase expression was evaluated in low-grade and high-grade sarcoma specimens. The authors established 3 distinct cell lines from a patient with an extraskeletal osteosarcoma: EXOS-N (normal mesenchymal), EXOS-P (primary osteosarcoma), and EXOS-M (lung metastasis). Each was evaluated for angiocidin, gelatinase, and gelatinase inhibitor (tissue inhibitors of metalloproteinase) expression and for invasive capacity. Their responsiveness to TSP-1 was determined. The role of angiocidin in up-regulating gelatinase expression and invasion was studied using the authors' angiocidin-inhibitory peptides. RESULTS: Expression of angiocidin, TSP-1, and gelatinases correlated with tumor grade. Angiocidin expression, gelatinase activity, and invasiveness in the EXOS cell lines correlated with phenotype; EXOS-N cells did not express angiocidin or gelatinases and were not invasive; EXOS-M cells were 5 times more invasive than EXOS-P cells and exhibited greater angiocidin and gelatinase expression. EXOS cell gelatinase activity and invasiveness increased 4- to 5-fold in response to TSP-1. Inhibition of angiocidin with the authors' inhibitory peptides blocked TSP-1-promoted increases in gelatinase activity and tumor cell invasion. CONCLUSIONS: Angiocidin promotes gelatinase up-regulation and tumor cell invasion in sarcomas. Angiocidin-inhibitory peptides are potent inhibitors of sarcoma cell invasion in vitro, suggesting a potential therapeutic role for these peptides in the treatment of sarcomas. © 2009 American Cancer Society.