Dormancy exit correlates with CREB1 reactivation that is a potential target to overcome endocrine therapy resistance in ER+ breast cancer cells.

Estrogen receptor-positive (ER⁺) breast cancers frequently recur years after initial treatment with anti-estrogen therapies such as aromatase inhibitors (AIs), often due to emergence from a dormant state. The molecular mechanisms underlying dormancy and recurrence, especially following endocrine therapy, remain poorly understood. In this study, we identify the transcription factor CREB1 as a critical regulator of tumor dormancy and recurrence in ER⁺ breast cancer. By analyzing longitudinal tumor samples from patients treated with AIs, we identified more than 1,000 “dormancy-downregulated genes” (DDGs) that are downregulated and over 1,000 “dormancy-upregulated genes” (DUGs) that are upregulated during therapy-induced dormancy; notably, DDGs are re-expressed and DUGs are downregulated upon tumor recurrence. Transcriptional profiling and pathway enrichment analyses implicated CREB1, along with ER and E2F1, as central regulators of these genes. These findings were validated in estrogen-deprived ER⁺ cell lines and patient-derived xenograft (PDX) models, where dormancy correlated with reduced CREB1 activity and tumor recurrence coincided with CREB1 reactivation. CREB1 inhibition—via small molecule or siRNA knockdown—effectively reversed CREB1, ER, and E2F1 transcriptional programs by downregulating the DDGs while upregulating the DUGs, and reduced cell proliferation and survival in endocrine- and CDK4/6 inhibitor-resistant ER⁺ breast cancer cells. These findings establish CREB1 as a master regulator of transcriptional reprograming in relapsed ER⁺ breast cancers and highlight it as a promising therapeutic target for overcoming resistance to current endocrine and CDK4/6-based therapies.