ERβ localization influenced outcomes of EGFR-TKI treatment in NSCLC patients with EGFR mutations

Effects of estrogen receptor beta (ER beta) localization on epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) in advanced non-small cell lung cancer (NSCLC) are unknown. First, we analyzed the relationship between ER beta localization determined by immunohistochemistry and EGFR-TKI outcomes in 184 patients with advanced NSCLC and found that ER beta expression localized in the cytoplasm and/or nucleus. The frequency of cytoplasmic ER beta (c-ER beta) and nuclear ER beta (nER beta) co-expression was 12% (22/184). C-ER beta and n-ER beta co-expression was correlated with poor median progression-free survival compared to patients without co-expression. In subsequent in vitro experiments, PC9 cells transfected with ER beta isoform1 (ER beta 1, strong expression of both c-ER beta and n-ER beta) were more resistant to gefitinib than PC9 cells transfected with ER beta isoform2 or 5 (ER beta 2 or ER beta 5, strong expression of ER beta in cytoplasm but not nucleus). Resistance was identified due to interactions between ER beta 1 and other isoforms, and mediated by activation of non-genomic pathways. Moreover, gefitinib resistance was reversed by a combination treatment with gefitinib and fulvestrant, both in cell lines and in one NSCLC patient. These results suggested that c-ER beta and n-ER beta co-expression was a potential molecular indicator of EGFR-TKI resistance, which might be overcome by combining EGFR-TKI and ER antagonist.