Subtype-dependent modulation of mir-655-3p by melatonin in breast cancer cell lines

Background: Breast cancer (BC) is a highly heterogeneous disease composed of distinct molecular subtypes that influence tumor behavior and therapeutic response. Melatonin (MEL) has been widely investigated for its oncostatic properties, partly mediated through the modulation of microRNA (miRNA) expression-key post-transcriptional regulators of genes involved in proliferation, invasion, and metastasis. Among these, miR-655-3p has been described as a potential tumor-suppressive miRNA associated with reduced aggressiveness in several cancer types. However, the effects of MEL on miR-655-3p expression across different BC subtypes remain poorly defined. Therefore, we aimed to evaluate the impact of MEL treatment on miR-655-3p expression in BC cell lines with distinct molecular profiles, including luminal A and triple-negative (TN) subtypes. Methods: Human BC cell lines MCF-7 and T47D (luminal A; ER+/PR+) and MDA-MB-231 (TN) were cultured under standard conditions and treated with 3000 μM MEL or left untreated (control group - CC). Total RNA was extracted using the mirVana™ miRNA Isolation Kit. cDNA synthesis was performed with the High-Capacity cDNA Reverse Transcription Kit, and miR-655-3p expression was quantified by qRT-PCR using specific TaqMan® assays. Relative expression levels and normalization to endogenous controls were obtained using ExpressionSuite™ Software v1.3. Results: MEL treatment resulted in a significant upregulation of miR-655-3p in MDA-MB-231 (TN) cells compared with untreated controls. In contrast, luminal A cell lines (MCF-7 and T47D) exhibited a significant decrease in miR-655-3p expression under the same MEL exposure conditions. Conclusion: Melatonin differentially modulates miR-655-3p expression across BC molecular subtypes, increasing its expression in TN cells while reducing it in luminal A cells. These opposite expression patterns suggest that MEL's regulatory effects on miR-655-3p are subtype-dependent, potentially reflecting differences in signaling pathways associated with hormone receptor status. Together, these findings indicate that miR-655-3p may serve as a biomarker of subtype-specific responses to melatonin and a potential therapeutic target involved in the modulation of tumor aggressiveness. Further studies are warranted to identify its direct gene targets and elucidate the biological relevance of this regulation.