Estrogen Effects Differ Between Medium Maintenance and Replacement from Transcriptional and Clinical Perspectives in T47D Breast Cancer Cells

Background/Aim: Our most recent study revealed that the responsiveness of hormone receptor-positive breast cancer (HR+ BC) cells to estrogen or endocrine therapy can be altered by certain cell culture or ambient environmental conditions. Nevertheless, we were unable to investigate the relevant molecular mechanism and clinical relevance. Therefore, this study was planned as a follow-up. Materials and Methods: RNA sequencing was mainly used with T47D cells treated with or without 17-estradiol (E2) under medium maintenance (MTN; conventional culture method) and medium exchange (EXC; daily replacing the existing medium with fresh medium). Results: The role of E2 in transcription differed between MTN and EXC, and E2 played more important roles in transcription in terms of cancer development under EXC than under MTN, consistent with the previous functional effects of EXC. The novel concept of the "estrogen-responsive and proliferation-related gene (ERPG)" was introduced. The expression of ERPGs, which were distinguished from typical estrogen-responsive genes, was correlated with that of prognostic and predictive factors for HR+ BC. The transcriptional induction of ERPGs and typical estrogen-responsive genes regardless of E2 treatment under MTN was reminiscent of constitutive estrogen receptor (ER) activation. Additionally, phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) inhibitors were more effective under EXC than under MTN. Conclusion: This study, demonstrating the more important roles of estrogen in terms of cancer development under EXC than under MTN, supports the use of our research model in future studies to overcome endocrine resistance in HR+ BC.