Fibrinogen-like protein 1 induces the formation of an immunosuppressive microenvironment by upregulating fibronectin 1 to promote immune evasion in bladder cancer

Objectives: The role of fibrinogen-like protein 1 (FGL1) in the immune microenvironment in bladder cancer has been extensively studied, but the specific regulatory pathways require further investigation. Materials and Methods: The overexpression vector of FGL1 or small interfering RNA was transfected into RT4 cells, and cell growth was observed. Tumor cells with altered FGL1 expression were co-incubated with peripheral blood mononuclear cells to simulate the immune microenvironment of bladder cancer in vitro , and the number and function of immune cells were monitored. The potential interacting proteins of FGL1 were predicted using an online bioinformatics database. Fibronectin 1 (FN1) was selected as the follow-up research object and was overexpressed or knocked down to explore its effect on tumor cell growth and immune microenvironment. A mouse model of transplanted tumor in vitro was established to explore FGL1 knockdown on tumor growth and immune microenvironment in vivo . Results: FGL1 was abnormally overexpressed in bladder cancer and enhanced RT4 cells’ migration, invasion, and proliferation. Its knockdown inhibited tumor cell growth. FGL1 decreased CD4 and CD8+T cell proportion, increased the number of Treg cells, promoted CD8+T cell apoptosis, inhibited interferon-gamma, and upregulated the secretion level of interleukin-10. Its knockdown had the opposite effect. FN1 was identified as an interacting protein of FGL1 and was positively regulated by FGL1 to promote the growth of RT4 cells. In the FGL1-induced tumor immunosuppressive microenvironment, blocking FN1 reversed the situation. In vivo studies showed that FGL1 silencing inhibited the growth of subcutaneously transplanted tumors in mice, which was related to the inhibition of FGL1 and FN1 expression. Conclusion: FGL1 promotes bladder cancer immune evasion by upregulating FN1. Our results provide a new reference for the immunotherapy of bladder cancer.