Ectopic programmed cell death ligand 1 (PD-L1) expression in non-small cell lung cancers (NSCLCs) is related to immune evasion by cancer, and it is a molecular target of immune checkpoint therapies. Since the precise mechanisms responsible for ectopic PD-L1 expression remains obscure, we analyzed the molecular mechanisms of ectopic PD-L1 expression in human lung cancers, focusing on the MAPK signal. Because we found a higher frequency of EGFR/KRAS mutations in NSCLC cell lines with high PD-L1 expression (p < 0.001), we evaluated the relationships between downstream signals and PD-L1 expression, particularly in three KRAS-mutant adenocarcinoma cell lines. The MEK inhibitor U0126 (20 μM) significantly decreased the surface PD-L1 levels by 50-60% compared with dimethyl sulfoxide (p < 0.0001). Phorbol 12-myristate 13-acetate stimulation (100 nM, 15 min) increased (p < 0.05) and two ERK2 siRNAs as well as KRAS siRNAs decreased (p < 0.05) PD-L1 expression. Post-transcriptional mechanism by miR-200s appears not to be in general under the downstream of MAPK in the PD-L1 expression. The transcriptional activity of the potential AP-1 site (+4785 to +5056 from the transcription start site) in the PD-L1 gene was demonstrated by luciferase assays, which was inhibited by U0126. The chromatin immunoprecipitation assay demonstrated the binding of cJUN to the AP-1 site. Two STAT3 siRNAs decreased PD-L1 expression by 10-32% in two of the three KRAS-mutant lung adenocarcinoma cell lines (p < 0.05), while the PI3K inhibitor LY294002 (40 μM) did not change the expression level. Supervised cluster analysis and gene set enrichment analysis between the PD-L1-high and -low NSCLCs revealed a correlation between PD-L1 expression and genes/pathways related to cell motility/adhesion. These results indicate that MAPK signaling is the dominant downstream signal responsible for ectopic PD-L1 expression at a transcriptional level, in which STAT3 is also involved to some extent. Furthermore, MAPK signaling may control the expression of PD-L1 and several genes related to enhanced cell motility. Our findings suggest that MAPK is important for determining PD-L1 expression, which could be useful for targeted therapies against lung cancers.

Citation Format: Hidetoshi Sumimoto, Atsushi Takano, Koji Teramoto, Yataro Daigo. RAS-MAPK signal is required for enhanced PD-L1 expression in human lung cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3708. doi:10.1158/1538-7445.AM2017-3708