Introduction: Disease recurrence is an important obstacle in estrogen receptor positive (ER⁺) tamoxifen treated breast carcinoma patients. Tamoxifen resistance-related molecular mechanisms are not fully understood. Alteration in DNA methylation which contributes to transcriptional regulation of cancer-related genes plays a crucial role in tamoxifen response. In the present study, the contribution of promoter methylation and mRNA expression of PAX2 and AIB1 in the development of breast carcinoma and tamoxifen refractory was assessed. Methods: Methylation specific-high resolution melting (MS-HRM) analysis and Real-time quantitative PCR (RT-qPCR) experiment were performed to analyze the promoter methylation and mRNA expression levels of PAX2 and AIB1 genes in 102 breast tumors and adjacent normal breast specimens. Results: We indicated that PAX2 expression is decreased in breast tissues due to hypermethylation in its promoter region. Compared to the adjacent normal tissues, the tumors exhibited significantly lower relative mRNA levels of PAX2 and increased expression of AIB1. Aberrant promoter methylation of PAX2 and overexpression of AIB1 was observed in tamoxifen resistance patients compared to the sensitive ones. Cox regression analysis exhibited that the increased promoter methylation status of PAX2 and overexpression of AIB1 remained as unfavorable identifiers which influence patients' survival independently. Conclusions: Our results revealed that the aberration in PAX2 promoter methylation and AIB1 overexpression are associated with the tamoxifen response in breast carcinoma patients. Further research is needed to demonstrate the potential of using PAX2 and AIB1 expression and their methylation-mediated regulation as predictive or prognostic biomarkers or as a new target therapy for better disease management.

Patients with estrogen receptor-positive (ER⁺) breast carcinoma often receive tamoxifen as an anti-estrogen therapy; however, a significant number still experience disease recurrence during treatment. The molecular mechanisms behind tamoxifen resistance are not yet fully understood. TAM mainly works by inhibiting estrogen receptor signaling pathways, inducing apoptosis, and causing growth arrest. Factors that influence TAM response include impaired bioactivation of tamoxifen and changes in the expression of tumor suppressor genes and proto-oncogenes, which are essential for regulating the cell cycle. Additionally, host factors, such as the processes involved in TAM bioactivation and drug transporters, play a crucial role in the therapeutic response. Understanding the interplay between tumor characteristics and host factors could provide valuable insights into TAM efficacy. Variations in gene expression associated with tamoxifen resistance may result from a combination of genetic mutations and epigenetic modifications. Clarifying the precise mechanisms of TAM response is crucial for improving treatment efficacy. Several therapeutic strategies have been explored to enhance TAM response, including drug replacement, epigenetic therapy, natural compounds, and innovative approaches like Antisense Oligonucleotides (ASOs). Combining these treatment modalities may improve patient survival more effectively than using tamoxifen alone. Graphical Abstract

Multidrug resistance (MDR) is a significant barrier to cancer treatment. Various mechanisms contribute to MDR development, including failure of apoptosis, angiogenesis, progression of cancer stem cells and drug efflux. Resistant cells often exhibit multiple genetic and epigenetic alterations. Identifying suitable chemical compounds to modulate or inhibit MDR is a slow and complex process. Natural products possess unique characteristics including, no toxicity, selective effects and good tolerance in the human body that make them more effective MDR modulators than synthetic drugs. Biologically active components from plants, bacteria, marine organisms, insects and honey can enhance cancer treatment outcomes by inducing apoptosis, inhibiting ATP-binding cassette transporters, reducing angiogenesis, eliminating cancer stem cells and restoring epigenetic patterns. Natural compounds may also serve as effective adjuvants to traditional chemotherapy, helping to manage drug resistance in patients.