Abstract
Some driver gene mutations, including epidermal growth factor receptor (EGFR), have been reported to be involved in expression regulation of the immunosuppressive checkpoint protein programmed cell death ligand 1 (PD-L1), but the underlying mechanism remains obscure. We investigated the potential role and precise mechanism of EGFR mutants in PD-L1 expression regulation in non-small-cell lung cancer (NSCLC) cells. Examination of pivotal EGFR signaling effectors in 8 NSCLC cell lines indicated apparent associations between PD-L1 overexpression and phosphorylation of AKT and ERK, especially with increased protein levels of phospho-IκBα (p-IκBα) and hypoxia-inducible factor-1α (HIF-1α). Flow cytometry results showed stronger membrane co-expression of EGFR and PD-L1 in NSCLC cells with EGFR mutants compared with cells carrying WT EGFR. Additionally, ectopic expression or depletion of EGFR mutants and treatment with EGFR pathway inhibitors targeting MEK/ERK, PI3K/AKT, mTOR/S6, IκBα, and HIF-1α indicated strong accordance among protein levels of PD-L1, p-IκBα, and HIF-1α in NSCLC cells. Further treatment with pathway inhibitors significantly inhibited xenograft tumor growth and p-IκBα, HIF-1α, and PD-L1 expression of NSCLC cells carrying EGFR mutant in nude mice. Moreover, immunohistochemical analysis revealed obviously increased protein levels of p-IκBα, HIF-1α, and PD-L1 in NSCLC tissues with EGFR mutants compared with tissues carrying WT EGFR. Non-small-cell lung cancer tissues with either p-IκBα or HIF-1α positive staining were more likely to possess elevated PD-L1 expression compared with tissues scored negative for both p-IκBα and HIF-1α. Our findings showed important roles of phosphorylation activation of AKT and ERK and potential interplay and cooperation between NF-κB and HIF-1α in PD-L1 expression regulation by EGFR mutants in NSCLC.