Metformin suppresses cancer cell growth in endometrial carcinoma by inhibiting PD-L1

Endometrial cancer is the most common cancer of the female reproductive system in the developed countries. Metformin is a widely used medication that has been prescribed to treat type 2 diabetes. In recent years, metformin has been found to improve the survival prognosis of cancer patients clinically. We aimed to investigate inhibition of metformin on the proliferation of endometrial carcinoma. Metformin was used to treat endometrial cancer cell lines Ishikawa and RL95-2. The expression of programmed death-ligand 1 (PD-L1) in the treated cells was assessed by western blot. The tumor cell proliferation was evaluated by colony formation assay. The binding between PD-L1 and AMP-activated protein kinase (AMPK) was identified by co-immunoprecipitation. Ishikawa and RL95-2 cells were co-cultured with activated T cells to detect the survival of Ishikawa and RL95-2 cells in the presence or absence of metformin. Our results showed that metformin treatment on endometrial cancer cells Ishikawa and RL95-2 decreased the expression level of PD-L1 protein. Metformin treatment significantly activated T cells against Ishikawa and RL95-2 cells. We demonstrated that the inhibition of PD-L1 by metformin is dependent on the AMPK signaling protein, and that metformin promotes direct binding of the AMPK protein to the PD-L1 protein. We confirmed that metformin, a conventional medication used in diabetes therapy, holds anti-tumor activity in endometrial cancer. The suppression of metformin is relevant to the inhibition of PD-L1 expression and the activation of AMPK signaling protein, providing a novel mechanism in the anti-tumor property of metformin.