Interplay between 7-ketocholesterol and tamoxifen shapes stress responses in breast cancer cells.

7-ketocholesterol (7-KC) is a highly abundant and biologically active lipid oxidation product that perturbs membrane integrity, sterol homeostasis, mitochondrial function, and redox balance. In parallel, tamoxifen, a cornerstone therapy for estrogen receptor-positive breast cancer, induces not only estrogen receptor antagonism but also pronounced metabolic and organelle-associated stress. Here, we investigated transcriptional responses of breast cancer cell line models to tamoxifen, 7-KC, and their combination. Tamoxifen elicited a shared antiproliferative response in MCF-7 and BT-20 cells, characterized by suppression of cell cycle progression, DNA replication, and mitosis. However, the downstream stress responses diverged markedly between the two models. MCF-7 cells activated adaptive programs, including unfolded protein response, autophagy, and metabolic reprogramming toward glycolysis, consistent with cytostatic survival. In contrast, BT-20 cells exhibited suppression of metabolic and redox pathways accompanied by inflammatory and apoptotic signaling, indicating impaired stress adaptation. Combined tamoxifen and 7-KC treatment further amplified these divergent stress-response phenotypes. Analysis of the correlation of 16 oppositely regulated genes with clinical data of breast cancer patients validated ST8SIA6 as the main candidate associated with adaptive stress tolerance. Overall, our findings indicate that the capacity to integrate metabolic and redox stress determines tumor cell type-specific responses to combined endocrine and oxysterol-induced stress in breast cancer.