Purpose: Follistatin-related gene 3 (FSTL3 ), an established oncogene, can modulate target gene expression via members of the transforming growth factor β (TGF-β) superfamily. The present study was conducted to evaluate the expression of FSTL3  in gastric cancer (GC) and to determine its prognostic significance. We also evaluated the possible mechanisms involved in the oncogenic role of FSTL3  in gastric carcinogenesis and development.
Methods: We obtained data from the Human Protein Atlas, MethSurv, cBioPortal, UALCAN, TIMER, GEPIA, STRING, GeneMANIA, ONCOMINE, and MEXPRESS databases and examined it using R software. RNAi was used to establish stable FSTL3 -knockdown (shFSTL3) and overexpression (OE) cell strains. Western blot; enzyme-linked immunosorbent (ELISA); and immunohistochemical (ICH), immunofluorescence, and phalloidin staining were used for examining protein expression. Cell invasion and migration were determined using transwell and scratch-wound assays. After tumor-associated macrophage (TAM) generation, co-culturing of cancer cells with TAMs was performed to confirm the relationship between FSTL3  and TAMs.
Results: In GC patients, FSTL3  mRNA and protein levels were upregulated. FSTL3  expression was significantly linked to cancer stage as well as to pathological tumor grade in GC. Moreover, a high expression of FSTL3  was associated with a dismal survival duration in patients with GC. Furthermore, functional enrichment analysis demonstrated that FSTL3  overexpression could activate epithelial–mesenchymal transition (EMT) by promoting F-actin expression and BMP/SMAD signaling. Finally, immunofluorescence staining confirmed that the overexpression of FSTL3 promoted the proliferation of M2 TAMs.
Conclusion: Taken together, our findings suggest that FSTL3  may be involved in GC progression via the promotion of BMP/SMAD signaling-mediated EMT and M2 macrophage activation.
Keywords: FSTL3, biomarker, gastric cancer, EMT, M2 macrophages