Immune cell infiltration into the tumor microenvironment is associated with cancer prognosis. Tumor-associated macrophages play essential roles in tumor initiation, progression, and metastasis. Follistatin-like protein 1 (FSTL1), a widely expressed glycoprotein in human and mouse tissues, is a tumor suppressor in various cancers and a regulator of macrophage polarization. However, the mechanism by which FSTL1 affects crosstalk between breast cancer cells and macrophages remains unclear. By analyzing public data, we found that FSTL1 expression was significantly low in breast cancer tissues compared to normal breast tissues, and high expression of FSTL1 in patients indicated prolonged survival. Using flow cytometry, we found that total and M2-like macrophages dramatically increased in the metastatic lung tissues during breast cancer lung metastasis in Fstl1^+/− mice. Transwell assay in vitro and q-PCR experimental results showed that FSTL1 inhibited macrophage migration toward 4T1 cells by decreasing CSF1, VEGF-α, and TGF-β secretion in 4T1 cells. We demonstrated that FSTL1 inhibited M2-like tumor-associated macrophage recruitment toward the lungs by suppressing CSF1, VEGF-α, and TGF-β secretion in 4T1 cells. Therefore, we identified a potential therapeutic strategy for triple-negative breast cancer.

Immune cell infiltration in the tumor microenvironment is associated with cancer prognosis. Tumor-associated macrophages (TAMs) play an essential role in tumor initiation, progression, and metastasis. Follistatin-like protein 1 (Fstl1), a glycoprotein widely expressed in human and mouse tissues, is a tumor suppressor in various cancers and a regulator in macrophage polarization. However, how Fstl1 affects the crosstalk between breast cancer cells and macrophages remains unclear. Analyzing the public data, we found that Fstl1 expression significantly decreases in breast cancer tissues compared with normal breast tissues, and high expression of Fstl1 in patients means prolonged survival. Using flow cytometry, we found that total macrophages and M2-like macrophages increased in the metastatic lung tissues during breast cancer lung metastasis of Fstl1+/- mice. Cellular experiments in vitro showed that Fstl1 inhibited macrophage migration towards 4T1 cells via decreasing colony stimulating factor 1 (CSF-1), transforming growth factor-β1 (TGFB1) and vascular endothelial growth factor-α (VEGF-α) expression of 4T1 cells. We demonstrated that Fstl1 inhibited M2-like tumor-associated macrophages recruitment towards the lung and suppressed TGFB1/Smad2/3 pathway and relevant cytokines expression of 4T1 cells. Thereby we find a potential therapeutic strategy contributing to breast cancer.