Objective: To investigate the regulatory effect of the activation of G protein-coupled estrogen receptor (GPER) on expression of fibroblast growth factor 2 (FGF2) in cancer-associated fibroblasts (CAFs) in breast cancer and the effect of its induced paracrine on growth of cancer cells.  
Methods: The expression localizations of GPER in CAFs and CAFs with GPER mutation cultured alone or co-cultured with ER+ breast cancer MCF-7 cells or ER- breast cancer MDA-MB-468 cells were determined by immunofluorescence assays. The mRNA expression and secretion levels of FGF2 in CAFs cultured alone or co-cultured with breast cancer cells and CAFs with GPER mutation co-cultured with breast cancer cells after treatment with 17 β-estradiolum (E2) or GPER specific agonist G1 were detected by real-time qPCR and ELISA, respectively; the changes in proliferative abilities in the two types of breast cancer cells co-cultured with CAFs after treatment with E2 or G1 were examined by flow cytometry and CCK-8 assays, and the interventional effects of FGF2 neutralizing antibody were also observed.
Results: In alone culture condition, GPER was located in the nucleus of the CAFs or CAFs with GPER mutation, while evident cytoplasmic translocation of GPER was seen in CAFs but not in CAFs with GPER mutation after co-culture with the both types of breast cancer cells. After treatment with E2 or G1, the expression levels of FGF2 mRNA in CAFs and FGF2 contents in the culture supernatants were significantly increased in co-culture conditions (all P<0.05), but above changes were not observed in CAFs cultured alone and CAFs with GPER mutation under co-culture conditions. The proliferative abilities were significantly enhanced in both types of breast cancer cells in co-culture with CAFs after treatment with E2 or G1 (all P<0.05), but these effects were abolished by adding FGF2 neutralizing antibody.
Conclusion: In breast cancer microenvironment, there is cytoplasmic translocation of GPER, which may contribute to the activation of estrogen/GPER/FGF2 pathway, and thereby promote the progression of breast cancer.