Tamoxifen resistance induction results in the upregulation of ABCG2 expression and mitoxantrone resistance in MCF-7 breast cancer cells.

Cancer endocrine therapy can promote evolutionary dynamics and lead to changes in the gene expression profile of tumor cells. We aimed to assess the effect of tamoxifen (TAM)-resistance induction on ABCG2 pump mRNA, protein, and activity in ER + MCF-7 breast cancer cells. We also evaluated if the resistance to TAM leads to the cross-resistance toward mitoxantrone (MX), a well-known substrate of the ABCG2 pump. The ABCG2 mRNA and protein expression were compared in MCF-7 and its TAM-resistant derivative MCF-7/TAMR cells using RT-qPCR and western blot methods, respectively. Cross-resistance of MCF-7/TAMR cells toward MX was evaluated by the MTT method. Flow cytometry was applied to compare ABCG2 function between cell lines using MX accumulation assay. ABCG2 mRNA expression was also analyzed in tamoxifen-sensitive (TAM-S) and tamoxifen-resistant (TAM-R) breast tumor tissues. The levels of ABCG2 mRNA, protein, and activity were significantly higher in MCF-7/TAMR cells compared to TAM-sensitive MCF-7 cells. MX was also less toxic in MCF-7/TAMR compared to MCF-7 cells. ABCG2 was also upregulated in tissue samples obtained from TAM-R cancer patients compared to TAM-S patients. Prolonged exposure of ER + breast cancer cells to the active form of TAM and clonal evolution imposed by the selective pressure of the drug can lead to higher expression of the ABCG2 pump in the emerged TAM-resistant cells. Therefore, in choosing a sequential therapy for a patient who develops resistance to TAM, the possibility of the cross-resistance of the evolved tumor to chemotherapy drugs that are ABCG2 substrates should be considered. Prolonged exposure of MCF-7 breast cancer cells to tamoxifen can cause resistance to it and an increase in the expression of the ABCG2 mRNA and protein levels in the cells. Tamoxifen resistance can lead to cross-resistance to mitoxantrone.

Expression analysis elucidates the roles of Nicastrin, Notch4, and Hes1 in prognosis and endocrine-therapy resistance in ER-positive breast cancer patients.

Background and purpose: Although some proposed mechanisms responsible for tamoxifen resistance have already been present, further study is needed to determine the mechanisms underlying tamoxifen resistance more clearly. The critical role of Notch signaling has been described in promoting resistance in therapeutics, but there is little information about its role in tamoxifen resistance progression. Experimental approach: In the present study, the expression of Notch pathway genes, including Notch4, nicastrin and the Notch downstream target Hes1 was evaluated using quantitative RT-PCR in 36 tamoxifen-resistant (TAM-R) and 36 tamoxifen-sensitive (TAM-S) patients. Expression data were correlated with the clinical outcome and survival of patients. Findings/Results: mRNA levels of Notch4 (fold change = 2.7), nicastrin (fold change = 6.71), and Hes1 (fold change= 7.07) were significantly higher in TAM-R breast carcinoma patients compared to sensitive cases. We confirmed all these genes were co-expressed. Hence, it seems that Notch signaling is involved in tamoxifen resistance in our TAM-R patients. Obtained results showed that Hes1, nicastrin, and Notch4 mRNA upregulation was correlated with the N stage. The extracapsular nodal extension was associated with nicastrin and Notch4 overexpression. Moreover, nicastrin overexpression was correlated with perineural invasion. Hes1 upregulation was also associated with nipple involvement. Finally, the Cox regression proportional hazard test revealed that overexpression of nicastrin was an independent worse survival factor. Conclusion and implications: Presumably, upregulation of the Notch pathway may be involved in tamoxifen resistance in breast cancer patients.