Background: Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer. Metformin has been shown to have the potential to inhibit the proliferation of malignant cells. This study aimed to investigate the regulatory effect of metformin on the expression of programmed death protein ligand 1(PD-L1) and mechanisms in TNBC.
Methods: Mouse breast cancer cell line 4T1 was co-cultured with metformin, and the effect of metformin on cell proliferation was detected by MTT assay. The effect of metformin on the expression of JNK, RSK2 and CREB was detected by MAPK pathway protein chip. BALB/c mice were inoculated with 4T1 cells with knockdown/overexpression of C-Jun N-terminal kinase (JNK), and administered with metformin. The weight of tumor tissue was observed at the end of the experiment. The expression of PD-L1 in tumor cells was observed by immunofluorescence staining and the level of INF-γwas quantitatively determined by ELISA.
Results: Metformin inhibited the viability of 4T1 cells and increased the phosphorylation of JNK to reduce the phosphorylation of RSK2 and CREB. Metformin and JNK knockdown reduced the expression of PD-L1 in tumor cells, but there was no significant difference in the weight of tumor tissue. Metformin can reduce the level of INF-γ in tumor tissues, but JNK has no effect.
Conclusion: Metformin can inhibit the expression of PD-L1 in triple-negative breast cancer mice and improve the tumor microenvironment, but does not reduce the size of the tumor.

Keywords: triple negative breast cancer, metformin, cell programmed death ligand 1, C-Jun amino-terminal kinase