Abstract P6-05-09: G protein-coupled estrogen receptor expressed in the plasma membrane is associated with worse breast cancer outcome, but does not contribute to tamoxifen resistance
Authors: Tutzauer Julia , Sjöström Martin , Bendahl Pär-Ola , Rydén Lisa , Fernö Mårten , Leeb-Lundberg Fredrik , Alkner Sara 
Published: March 2020

Introduction - G protein-coupled estrogen receptor (GPER), or GPR30, is a membrane receptor reported to mediate rapid, non-genomic estrogen responses. Multiple studies suggest that GPR30 expression associates with breast cancer outcome, although the clinical implications appear to depend on receptor subcellular localization and level of expression. Specifically, high plasma membrane localization of GPR30 has been reported to associate with a worse prognosis. Furthermore, in vitro studies show that tamoxifen up-regulates GPR30 in breast cancer cells, suggesting that GPR30 acts as a mediator of tamoxifen resistance. To further clarify the prognostic and treatment resistance role of GPR30 in vivo, we studied the receptor in patients developing contralateral breast cancer with or without tamoxifen treatment for the first primary tumor, serving as an in vivo model of tamoxifen resistance.

Patients and methods - In a cohort of 688 patients with metachronous contralateral breast cancer, total and plasma membrane specific GPR30 expression were evaluated by immunohistochemistry. Total GPR30 was evaluated in five levels (0-4) and plasma membrane staining as positive or negative. Evaluation was successful for 559 first primary tumors and 595 contralateral tumors. In addition, matched lymph node and distant metastases were evaluated (lymph node matched to first primary tumor, n=213, matched to contralateral tumor, n=196, and distant metastasis, n=197). The relationship between GPR30 and breast cancer mortality was assessed by the Cox proportional hazards model, and illustrated by curves of cumulative incidence. The association between GPR30 and breast cancer outcome in relation to tamoxifen was assessed by comparing groups either untreated or treated with tamoxifen after the first primary tumor.

Results - GPR30 expression in the plasma membrane associated with increased risk of breast cancer mortality both when expressed in the contralateral tumor (HR=1.7, p=0.03) and matched lymph node (HR=2.0; p=0.02). Additionally, GPR30 plasma membrane expression associated with high Ki67 staining both in the first primary tumor (p&lt;0.0001) and contralateral tumor (p&lt;0.0001). In both the first primary tumor and contralateral tumor, GPR30 plasma membrane expression associated with estrogen receptor α (ER)-negativity (p&lt;0.0001 and p&lt;0.0001, respectively) and progesterone receptor (PR)-negativity (p=0.0007 and p&lt;0.0001, respectively). Furthermore, ER and PR expression associated with total GPR30 expression in a biphasic manner in both the first primary tumor and contralateral tumor, as previously observed in three cohorts of primary breast cancer. The highest total and plasma membrane GPR30 expression was observed in triple-negative breast cancer. Total GPR30 expression of the first primary tumor and contralateral tumor did not correlate, but it was significantly lower in the matched lymph node (first primary tumor p&lt;0.0001, contralateral tumor p&lt;0.0001). No clear evidence was found that tamoxifen treatment during contralateral tumor development correlated with GPR30 expression, or that GPR30 expression correlated with response to tamoxifen treatment.

Conclusion - GPR30 expression in the plasma membrane associates with increased risk of breast cancer death when expressed in the contralateral tumor and match lymph node, and correlates strongly with multiple clinicopathological markers of poor outcome. On the other hand, prior tamoxifen treatment does not appear to affect GPR30 expression, suggesting that GPR30 does not mediate tamoxifen resistance. Future studies should aim to characterize the pathophysiological mechanisms and function of GPR30 when located in the plasma membrane. Here, GPR30 could be an novel target for breast cancer treatments.

Citation Format: Julia Tutzauer, Martin Sjöström, Pär-Ola Bendahl, Lisa Rydén, Mårten Fernö, Fredrik Leeb-Lundberg, Sara Alkner. G protein-coupled estrogen receptor expressed in the plasma membrane is associated with worse breast cancer outcome, but does not contribute to tamoxifen resistance [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-05-09.