Breast cancer is the most common cancer among women worldwide. About 10-20% of breast cancers are triple-negative breast cancers (TNBC). Dihydroartemisinin (DA) is a semi-synthetic derivative of artemisinin and is widely used as a clinical antimalarial drug with great safety and economic. In this study, we found an effective anticancer effect of DA in TNBC in vitro and in vivo. DA significantly induced cell death and oxidative stress in TNBC cells. DA also inhibited the cell migration and invasion ability of TNBC cells. In the established MDA-MB-231 xenograft nude mice model, DA administration significantly retarded the tumor growth. Moreover, we found that treating MDA-MB-231 cells with DA significantly reduced the cellular PD-L1 mRNA and protein level. The PD-L1 level of MDA-MB-231 cells decreased by DA reduced the binding of PD-1 protein and sensitized cells to activated T cells. In mechanism, DA reduced FoxO3a levels in MDA-MB-231 cells, in line with the increase of FoxO3a ser644 phosphorylation and the phosphorylation activation level of IKK protein. FoxO3a overexpression or knockdown increased or decreased PD-L1 protein levels. Dual luciferase reporter gene test and ChIP test results show that FoxO3a, as a transcription factor, bound the promoter region of PD-L1 and improved its transcriptional activity. DA treatment also reduced the PD-L1 gene promoter activity.

In conclusion, these findings suggest that DA exert an antitumor effect in TNBC cells and sensitize tumor cells to activated T cells via IKKβ/FoxO3a/PD-L1 pathway. DA is a potentially safe and economical tumor PD- L1 inhibitory drug.