PD-L1 Inhibitor Regulates the miR-33a-5p/PTEN Signaling Pathway and Can Be Targeted to Sensitize Glioblastomas to Radiation

Background: Glioblastoma (GBM) is the most common and lethal brain tumor in adults. Ionizing radiation (IR) is a standard-of-care treatment for GBM patients and results in DNA damage. However, the clinical efficacy of IR is limited due to therapeutic resistance. The programmed death ligand 1 (PD-L1) blockade has a shown the potential to enhance the efficacy of radiotherapy by inhibiting DNA damage and repair responses. The miR-33a-5p is an essential microRNA that promotes GBM growth and self-renewal. In this study, we investigated whether a PD-L1 inhibitor exerted radio-sensitive effects to impart an anti-tumor function in GBM cells by modulating miR-33a-5p. Methods: U87 GBM cells were pretreated with PD-L1 inhibitor. The PD-L1 inhibitor-induced radio-sensitivity in GBM cells was assessed by assaying cellular apoptosis, proliferation, and migration. Luciferase reporter assays were also used to confirm the target of miR-33a-5p. The expression level of phosphatase and tensin homolog (PTEN) was detected by western blotting, and was also silenced using small interfering RNAs. The levels of DNA damage following radiation was measured by the presence of γ-H2AX foci and the mRNA of the DNA damage-related genes, BRCA1, NBS1, RAD50, and MRE11. Finding: Our results demonstrated that the PD-L1 inhibitor significantly decreased the expression of the target gene, miR-33a-5p. In addition, pretreatment of U87 cells with the PD-L1 inhibitor increased radio-sensitivity, as indicated by increased apoptosis, and decreased proliferation and migration of GBM cells. Additionally, PD-L1 inhibitor treatment suppressed the expression of the DNA damage response-related genes, BRCA1, NBS1, RAD50, and MRE11. However, radiosensitivity was reverted by the forced downregulation of PTEN, which is the target of miR-33a-5p. Interpretation: Our results demonstrated a novel role for the PD-L1 inhibitor in inducing radio- sensitivity in GBM cells, where modulating miR-33a-5p and inducing DNA damage was crucial for antitumor immunotherapies to treat GBM. Funding Statement: This study was supported by the national natural science foundation of China (grant no. 81671205 to STL; grant no. 81600278 to WZX), the Program from Shanghai Committee of Science and technology (grant no. 18XD1402700 to STL) and the Foundation for Interdisciplinary Research of Shanghai JiaoTong University (grant no. YG2017MS68 to JZ). Declaration of Interests: The authors declare that they have no competing interests. Ethics Approval Statement: The authors stated: "Not applicable."