PCBP1-mediated regulation of WNT signaling is critical for breast tumorigenesis.

Loss of expression or protein kinase B (Akt1)-mediated post-translational modification of the RNA binding protein Poly r(C) binding protein 1 (PCBP1) is closely related to metastatic advancement of breast cancer. However, the role of PCBP1 in tumorigenesis is not completely defined. Using a xenograft orthotopic model of breast tumorigenesis (4T1-Pcbp1-/-), we show here that PCBP1 knockdown-induced tumorigenesis is inhibited by activation of the WNT signaling via treating with the glycogen synthase kinase 3 beta inhibitor TWS119, but not the Akt2/Akt3 inhibitor GSK690693. Mass cytometry-based evaluation of the tumor microenvironment (TME) revealed significantly more regulatory T cells (Tregs) and significantly less cytotoxic T cells in 4T1-Pcbp1-/-mice treated with saline control in comparison to mice treated with TWS119. Infiltrating cytotoxic T cells were phenotypically and functionally exhausted. Treatment with TWS119 resulted in rescue of cytotoxic T cell function and inhibition of suppressor activity of Tregs. Using cytotoxic T cells isolated from healthy donors, we show that TWS119-induced WNT signaling-mediated inhibition of cytotoxic T cell expansion is reliant on expression of PCBP1. In conclusion, decreased PCBP1 expression favors breast tumorigenesis by potentiating skewing of tumor infiltrating T cells towards Tregs, thereby effectively suppressing anti-tumor immunity.