Sarayu Malireddy

Background: ERα+ breast cancer accounts for roughly 70% to 80% of all breast cancer cases and is characterized by estrogen-driven proliferation.^1,2 Tamoxifen, a selective estrogen receptor modulator, is considered a first-line endocrine therapy for ERα+ breast cancer, acting as a competitive ligand that blocks estrogen-ERα binding, thereby inhibiting growth-stimulating gene transcription.^1,2,3 Despite clinical efficacy, around 50% of patients develop acquired resistance to tamoxifen, presenting a clinical challenge.⁴ Research shows that microRNAs (miRNAs) play a central role in modulating tamoxifen resistance.^4,5 A few specific microRNAs have been found to mediate the expression and signaling status of ERα, the drug target.^6,7 Further investigations are likely to identify additional miRNA-based therapeutic approaches that restore sensitivity to anti-hormones in ERα+ breast cancer.

Objective: The objective of this investigation was to determine the mechanism(s) by which the miR-221/222 cluster and miR-27a regulate ERα expression levels and thus mediate tamoxifen resistance in ERα+ breast cancer.

Search Methods: An online search of the PubMed database for years 2018-2026, using the following MeSH search terms: “breast cancer”, “ERα+ breast cancer,” “tamoxifen resistance,” and “miRNA,” was conducted.

Results: Studies show how miRNA can mediate tamoxifen resistance by depriving tamoxifen of its drug target, ERα, through different routes, direct suppression of expression and indirect homeostatic disruption, the latter consisting of the breakdown of a self-regulating feedback loop that normally maintains stable receptor levels. ^6,7 The miR-221/222 cluster was upregulated in MCF-7TamR cells, an acquired tamoxifen-resistant breast cancer model. Synthetic miRNA sponges—oligonucleotide expression cassettes containing multi-antisense binding sites—demonstrated that the cluster inhibits ERα expression by physically binding to the 3’ untranslated regions (3’UTR) of ESR1 (ERα) and PTEN mRNA.⁶ Transfection of miR-221/222-suppressing sponges into resistant cells restored the physical expression of ERα mRNA levels approximately 23-fold and resensitized cells to 15 μM tamoxifen treatment, a dose at which resistant cells maintained viability.⁶ In contrast, miR-27a enables sufficient ERα re-expression in an indirect manner; miR-27a inhibits ZBTB10 (a protein repressor), which suppresses members of the Sp-protein family.⁷ Because Sp-proteins are required for ESR1 gene transcription, miR-27a enables adequate receptor expression. Resistant TAM6 cells, tamoxifen-resistant clones, exhibited a proportional decrease (p<0.05) of approximately 50%, in both miR-27a and ERα expression compared to parental MCF7 wildtype cells.⁷  Restoration of miR-27a expression through use of mimics restored the feedback loop, reactivating ERα signaling with increased ERE-luciferase reporter activity (p < 0.05) and resensitizing cells to 20 μM tamoxifen.⁷

Conclusions: Overcoming tamoxifen resistance requires strategies that ensure sufficient expression of the drug target, ERα. Tamoxifen resensitization was demonstrated with miRNAs (miR-221/222 and miR-27a) that leverage relationships between microRNAs and ERα, resulting in reexpression of ERα. Downregulation of oncomiRs like the miR-221/222 cluster via sponge technology or upregulation of sensitizing miRNAs like miR-27a maintain ERα homeostasis through direct and indirect mechanisms, respectively. Early identification of miRNAs as biomarkers and combined administration of miRNA-targeting agents with tamoxifen offer a promising approach to resensitize resistant ERα+ breast cancers.

Works Cited:

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