Melatonin modulates mir-340-5p expression in breast cancer molecular subtypes: Implications for subtype-specific regulation and therapeutic targeting

Background: Breast cancer (BC) comprises highly heterogeneous subtypes with distinct molecular and clinical features. Triple-negative breast cancer (TNBC) is characterized by poor prognosis and limited therapeutic options, whereas luminal A tumors exhibit hormone receptor positivity and more favorable outcomes. Emerging evidence suggests that melatonin (MEL) exerts oncostatic effects through the modulation of gene expression, including microRNAs (miRNAs), which regulate key oncogenic pathways. Among these, miR-340-5p has been implicated in tumor suppression and in the regulation of epithelial-mesenchymal transition (EMT). However, its modulation by MEL across BC molecular subtypes remains poorly understood. Therefore, we investigated the differential expression of miR-340-5p in BC cell lines representing distinct molecular profiles (luminal A and TN) following melatonin treatment. Methods: Human BC cell lines MCF-7 and T47D (luminal A) and MDA-MB-231 (TN) were cultured and either left untreated (control) or treated with 3000 μM MEL. Total RNA was extracted using the mirVana™ miRNA Isolation Kit. cDNA synthesis was performed with the High-Capacity cDNA Reverse Transcription Kit, and miR-340-5p expression was quantified by qRT-PCR using specific TaqMan® assays. Relative expression levels, endogenous control normalization, and data processing were performed using ExpressionSuite™ Software v1.3. Results: MEL treatment resulted in an upregulation of miR-340-5p in MDA-MB-231 (TN) cells compared with untreated controls. Conversely, luminal A cell lines (MCF-7 and T47D) exhibited a reduction in miR-340-5p expression following MEL treatment relative to control cells. Conclusion: Melatonin modulates miR-340-5p expression in a molecular subtype-dependent manner, increasing its expression in TN cells while reducing it in luminal A cells. This differential regulation highlights the potential of miR-340-5p as a biomarker for subtype-specific responses to melatonin and may contribute to distinguishing aggressive from hormone-responsive BC phenotypes. Additional studies are needed to identify downstream targets and elucidate the biological significance of this regulatory pathway.