Fig 2: Recombinant mouse FSTL1 protein inhibited several cytokines produced by 4T1 TNBC cells. (A) FSTL1 (1 μg/mL)-treated 4T1 cells with or without IL4 (20 ng/mL) in vitro. Cytokine secretions: (B) CCL2, (C) CCL5, (D) CSF1, (E) VEGF-α, and (F) TGF-β. (G) Schematic representation of the effect of FSTL1 on breast cancer metastatic tumor growth. The model depicts the suppression of CSF1, VEGF-α, and TGF-β secretion in 4T1 breast cancer cells by FSTL1, which decreases macrophage recruitment toward TNBC cells. * p < 0.05, ** p < 0.01, *** p < 0.001.

Fig 3: Lower FSTL1 expression in breast cancer tissues compared to normal breast tissues, and high expression of FSTL1 meant prolonged survival. (A–D) FSTL1 mRNA expression decreased in primary breast cancer. (E,F) FSTL1 had no significant effect on the survival rate of patients with breast invasive carcinoma (BRCA) and TNBC. (G,H) High FSTL1 expression could increase the survival rate of patients with breast cancer and those with positive nodal metastasis. *** p < 0.001.

Fig 4: Total and M2 macrophage ratios increased in breast cancer lung metastasis in Fstl1+/− mice. (A,B) Total (F4/80+) and M2 (F4/80+CD11c-CD206+) macrophage ratios in WT and Fstl1+/− mice on the 0th and 28th day. (C–E) Expression of several M2-related macrophage markers (IL4, IL13, Arg-1, and IL10) in WT and Fstl1+/− mice on the 0th and 28th day. (F,G) Western blot of metastasis-related markers (TGFβ and MMP9) of WT and Fstl1+/− mice on the 28th day. * p < 0.05, ** p < 0.01, *** p < 0.001.

Fig 5: FSTL1 had no effect on the proliferation and EMT markers of 4T1 cells. (A) The proliferation ability of 4T1 TNBC cells showed no change after 600 ng/mL FSTL1 treatment for 1–4 d. (B) Under different FSTL1 concentration treatments, expression of 4T1 EMT markers, including E-cadherin and N-cadherin, showed no change after 12 h treatment.