STING mediates nuclear PD-L1 targeting-induced senescence in cancer cells.

Immune checkpoint molecule programmed death-ligand 1 (PD-L1) is overexpressed in cancer cells and imparts resistance to cancer therapy. Although membrane PD-L1 has been targeted for cancer immune therapy, nuclear PD-L1 was reported to confer cancer resistance. Therefore, it is important to regulate the nuclear PD-L1. The mechanisms underlying the therapeutic efficacy of PD-L1 targeting have not been well-established. Cellular senescence has been considered a pivotal mechanism to prevent cancer progression, and recently, PD-L1 inhibition was shown to be involved in cancer cell senescence. However, the relevance of PD-L1 targeting-induced senescence and the role of stimulator of interferon genes (STING) has not been reported. Therefore, we aimed to identify the role of PD-L1 in cancer progression and how it regulates cancer prevention. In this study, we found that PD-L1 depletion-induced senescence via strong induction of STING expression in mouse melanoma B16-F10 and colon cancer CT26 cells, and in human melanoma A375 and lung cancer A549 cells. Interestingly, nuclear PD-L1 silencing increased STING promoter activity, implying that PD-L1 negatively regulates STING expression via transcriptional modulation. Furthermore, we showed that PD-L1 binds to the STING promoter region, indicating that PD-L1 directly controls STING expression to promote cancer growth. In addition, when we combined PD-L1 silencing with the senescence-inducing chemotherapeutic agent doxorubicin, the effect of PD-L1-targeting was even more powerful. Overall, our findings can contribute to the understanding of the role of PD-L1 in cancer therapy by elucidating a novel mechanism for PD-L1 targeting in cancer cells.