Endoxifen Resistance in ER+ Breast Cancer Involves Translational Adaptation and Potential Contribution of ABCC Transporters

Background: Endocrine therapy with tamoxifen remains a cornerstone in the treatment of estrogen receptor-positive (ER+) breast cancer; however, the emergence of resistance to its active metabolites, 4-hydroxytamoxifen (4-OHTAM) and Endoxifen, represents a major clinical limitation. Increasing evidence suggests that drug efflux transporters, redox-adaptive signaling, and translational control mechanisms may converge to promote chemoresistance. This study aimed to investigate the coordinated expression patterns of ABCC transporters, the eukaryotic initiation factor 4F (eIF4F) complex, and NRF2 signaling in tamoxifen-metabolite-resistant MCF-7 breast cancer cells. Methods: MCF-7 cell variants resistant to 4-OHTAM (Variant B) or Endoxifen (Variant C) were established through prolonged drug exposure. Cytotoxicity assays assessed cellular viability and chemoresistance. Protein expression and molecular interactions were analyzed using Western blotting and co-immunoprecipitation. Flow cytometry was employed to evaluate transporter-associated fluorescence intensity. In silico molecular docking was performed to estimate the binding affinity of tamoxifen metabolites to ABCC transporters. Results: Endoxifen-resistant cells exhibited the most pronounced chemoresistant phenotype. Analysis of ABCC transporters revealed modest but consistent increases in fluorescence intensity across resistant variants; however, these differences did not reach statistical significance. Dysregulation of the eIF4F complex was observed, with increased eIF4E and reduced eIF4A levels, suggesting altered translational control associated with resistant phenotypes. Increased NRF2 protein expression was detected in resistant variants, consistent with enhanced redox-adaptive capacity. Analysis of ABCC transporters revealed modest but consistent increases in fluorescence intensity across resistant variants; however, these differences did not reach statistical significance. Molecular docking demonstrated strong binding affinity between Endoxifen and ABCC2, supporting a potential role for transporter-mediated efflux. Conclusions: Tamoxifen-metabolite resistance in ER+ breast cancer is associated with coordinated trends in ABCC transporter-associated signals, altered eIF4F complex expression, and sustained NRF2 signaling. These findings suggest the presence of a multifactorial adaptive network that may contribute to endocrine resistance. Targeting components of this network warrants further mechanistic investigation.