LAUSR

Breast cancer has been considered as a less immune-sensitive disease. Patients with metastatic breast cancer have a particularly low objective response rate (ORR) to anti-PD-1/PD-L1 therapy (3%~24%). It will be valuable to improve the treatment efficacy of anti-PD-1 immunotherapy in breast cancer. Recent studies reported that exosomal PD-L1 derived from tumor cells mediated immune surveillance evasion by interacting with the PD-1 receptor on T cells and contributed to the low ORR in anti-PD-1/PD-L1 immunotherapy.The RNA-binding protein Hu antigen R (HuR) is a member of the embryonic lethal abnormal vision (ELAV) family that is overexpressed in a variety of cancers and promotes tumorigenesis by interacting with a subset of oncogenic mRNAs, including PD-L1 mRNA. HuR also plays an important role in the exosome secretion process by regulating RAB27B mRNA. In this study, we aim to identify HuR biological functions in the cancer-immune system and evaluate the combination strategy using a HuR inhibitor with an anti-PD-1 antibody in breast cancer. We show here the direct binding of HuR protein to PD-L1 mRNA in human breast cancer cell line MDA-MB-231 by Ribonucleoprotein immunoprecipitation (RNP-IP). We also show that PD-L1 protein expression level is decreased in HuR knockout clones and in exosomes derived from HuR knockout clones. HuR knockout downregulates exosome secretion with decreased size in MDA-MB-231 cells. KH-3 is a small molecule identified from several chemical libraries that disrupts the binding of HuR and its target mRNAs.To test the combination strategy using a HuR inhibitor and anti-PD-1 antibody in vivo, we established the EMT6 orthotopic mouse breast cancer model. A combination treatment with HuR inhibitor KH-3 and anti-PD-1 antibody inhibits tumor growth and prolongs survival in this breast cancer model. In conclusion, we preliminarily explored the role of HuR in the cancer-immune system, we showed that HuR may be a promising therapeutic target to overcome immune evasion and improve immunotherapy response in breast cancer. Citation Format: Qi Zhang, Xiaoqing Wu, Lan Lan, Lanjing Wei, Liang Xu. Targeting RNA-binding protein HuR to improve anti-PD-1 immunotherapy response in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1560.