CCL2 and cancer: Activation by growth factors, such as transforming growth factor-β (TGF-β), PDGFs and fibroblast growth factor-2, results in the production of cytokines and chemokines important for cancer progression [14], leading to the hypothesis that fibroblasts activated by a product(s) of 4T1 cells significantly contribute to the overall MCP-1 production in 4T1 tumors.