GPER is inactivated by promoter methylation and potentially functions as a tumor suppressor in breast cancer

Purpose: More than 50% of breast cancer patients express high levels of membrane-bound G-protein coupled estrogen receptor (GPER) which is favorable for their survival. We investigated the role of GPER as potential tumor suppressor in breast cancer cells with special focus on triple-negative breast cancer (TNBC).

Methods: After stimulation of breast cancer cell lines MCF-7, SK-BR-3 and TNBC cell lines MDA-MB-231 and MDA-MB-468 with specific GPER ligand G-1 we analyzed cell cycle distribution and apoptosis. Methylation status of the GPER promoter was investigated by methylation-specific PCR and 5-Aza-2'-deoxicytidine (5-Aza) treatment. Stress-induced expression of GPER and p53 was analyzed after exposition to γ-radiation.

Results: Specific GPER stimulation inhibited the growth of all four breast cancer cell lines in a concentration dependent manner via cell cycle arrest in G₂/M-phase, increased histone H3 phosphorylation and caspase-3 induced cell apoptosis. Inactivation of DNA-methyltransferase by 5-Aza increased GPER expression in vitro. Methylation specific analysis of the GPER promoter in breast cancer cell lines and primary breast cancer tissue derived from patients showed that GPER is inactivated by the epigenetic mechanism of promoter methylation. Furthermore, expression of GPER after exposition to γ-radiation was induced in TNBC cell lines but reduced in MCF-7, which is inversely correlating with expression of the functional tumor suppressor p53.

Conclusion: Our data confirm the protective role of GPER as potential tumor suppressor. Because of its cell surface localization GPER is a very promising new therapeutic target for breast cancer especially for TNBC, where an optimal adjuvant therapy is still missing.