G-protein-coupled estrogen receptor GPER-1 expression in hormone receptor-positive breast cancer is associated with poor benefit of tamoxifen

The role of G-protein-coupled estrogen receptor 1 (GPER-1) in the development of tamoxifen resistance in breast cancer is a highly controversial issue. The aim of this study was to determine the expression of GPER-1 in the clinical routine under conditions of endocrine treatment.

GPER-1 expression was analyzed in 442 patients with primary invasive breast cancer. GPER-1 score of > 3 was determined as positive. Expression data were correlated with clinical and pathological characteristics and patient survival.

GPER-1 expression was observed in 352 (80.9%) cases, and positively correlated with estrogen and progesterone receptor status (p = 0.0001). GPER-1 positivity was associated with an increased grade of differentiation (p = 0.0001) and with a low level of Ki-67 expression (p = 0.0001). High GPER-1 expression was associated with a decreased level upon systemic treatment (p = 0.011). In the whole cohort, GPER-1 expression was associated with prolonged disease-free survival (DFS). DFS between tamoxifen- and aromatase inhibitor-treated GPER-1-positive patients was similar (p = 0.090). Notably, after matching the analysis for the most important prognostic factors, DFS for tamoxifen-treated GPER-1-positive patients was 69.1%, which is a percentage that is significantly lower compared to DFS for GPER-1-positive patients treated with aromatase inhibitors (92.7%) (p = 0.005).

GPER-1 expression is a favorable prognostic factor in breast cancer patients. Its predictive role for poor benefit form tamoxifen treatment should be investigated in further studies.