YB-1 promotes cell proliferation and metastasis by targeting cell-intrinsic PD-1/PD-L1 pathway in breast cancer

Programmed cell death 1 (PD-1) suppresses T effector functions by inhibiting signaling downstream of the T cell receptor and helping tumor cells escape the immune response. However, the effect and mechanism of cell-intrinsic PD-1 in cancer cells are still unknown. Here, we found that PD-1 is aberrantly upregulated in TNBC patients and cell lines. Cell-intrinsic PD-1 in TNBC cells significantly facilitated tumor growth and metastasis in vitro and in vivo. Further studies indicated that PD-1 effect on TNBC cell growth depends on the cell-intrinsic-PD-1/PD-L1 pathway independent of adaptive immunity. In addition, we further found that the activation of cell-intrinsic PD-1/PD-L1 pathway in TNBC cells is regulated by the gene expression regulator YB-1. Mechanistically, the results of protein degradation analysis, mRNA translationally active analysis, CLICK chemistry and L-azidohomoalanine (AHA) incorporation assays, immunoprecipitation assay and Dual-Luciferase reporter assay showed that YB-1 promotes PD-1 expression through the translational activation pathway. We provide in vitro and in vivo evidence that silencing YB-1 expression in TNBC cells inhibits cell proliferation, tumorigenesis, and metastasis. However, this inhibition can be rescued by simultaneous exogenous expression of PD-1 and PD-L1 proteins. In conclusion, our results identify TNBC cell-intrinsic functions of the PD-1/PD-L1 axis in tumor growth and metastasis; and revealed PD-1/PD-L1 is a critical effector of YB-1-mediated TNBC proliferation and metastasis in vitro and in vivo.