Breast tumors are characterized by an extensive desmoplastic stroma, abundantly populated by fibroblasts. It is known that fibroblasts, among other stromal cell types populating the neoplastic microenvironment are important for facilitating tumorigenesis. Cancer Associated Fibroblasts (CAFs) are an activated sub-population of stromal fibroblasts, which can have different characteristics in different tumor types. CAFs have been shown to promote tumor growth by directly stimulating tumor cell proliferation and by enhancing angiogenesis and inflammation. Osteopontin (OPN) is a phosphorylated glycoprotein known in its bi-functional form as a secreted and non-secreted protein. Several physiologic roles have been attributed to OPN in inflammation, tumor progression and metastasis. Binding of secreted OPN to its cell surface receptors stimulates cell adhesion, migration, and specific signaling functions.
We show that breast tumor cell-derived OPN can functionally activate mammary fibroblasts. Using a proteomic analysis we identified OPN as a factor secreted by a mammary carcinoma-derived cell line (MET-1). Tumor cell-derived OPN in conditioned medium activated a pro-inflammatory gene expression signature and enhanced motility in normal mammary fibroblasts (NMFs), characteristic of CAFs. Recombinant OPN was sufficient to induce this “reprogramming” of NMFs and neutralizing antibodies to OPN could block this NMF activation. Furthermore, we show that this activation of mammary fibroblasts by secreted OPN depends on both its known promoters: CD44 and αVb3. Thus, we demonstrate a functional role for paracrine signaling by tumor-derived OPN in educating NMFs to become pro-inflammatory CAFs.
