# Hsa-miR-181a-5p Enhances the Chemosensitivity of Breast Cancer Cells to Tamoxifen by Regulation of the OSBPL3/RAS Signaling Pathway

## TL;DR

This study shows that miR-181a-5p improves breast cancer cells' response to tamoxifen by reducing OSBPL3 levels and affecting RAS signaling.

## Contribution

The study identifies miR-181a-5p as a novel regulator of OSBPL3, linking it to RAS signaling and tamoxifen chemosensitivity in breast cancer.

## Key findings

- miR-181a-5p suppresses OSBPL3 by targeting its 3’-UTR, reducing cancer cell invasion and proliferation.
- OSBPL3 knockdown enhances tamoxifen-induced apoptosis and reduces RAS signaling regulators.
- miR-181a-5p upregulation increases chemosensitivity to tamoxifen in breast cancer cells.

## Abstract

Background: Despite tamoxifen being a broad-spectrum therapeutic agent for breast cancer (BC) management, the emergence of chemoresistance significantly compromises its clinical effectiveness. OSBPL3 has been identified as a metastasis-promoting protein implicated in BC progression, yet the upstream regulatory mechanisms controlling its expression remain poorly understood, particularly regarding microRNA-mediated regulation.

Methods: MCF-7 cells underwent lentiviral infection for stable gene modification, followed by transient transfection with hsa-miR-181a-5p mimics (or corresponding negative controls), antisense miR-181a-5p inhibitors (anti-miR-181a-5p), and OSBPL3-specific small interfering RNAs (siRNAs). The direct binding interaction between miR-181a-5p and OSBPL3 was validated using a dual-luciferase reporter system containing wild-type or mutant 3'UTR sequences. Tamoxifen sensitivity was evaluated through functional assessments, including CCK-8 assay, Annexin V-FITC/PI apoptosis assay, and Transwell migration/invasion assays. Gene expression was assessed by Western blot analysis.

Results: Our findings revealed that miR-181a-5p inhibits OSBPL3 expression by specifically targeting the 3’-UTR of OSBPL3. At the pharmacologically active concentration of 5 µM tamoxifen, overexpression of miR-181a-5p led to the suppression of OSBPL3 expression, which in turn inhibited the invasion, migration, and proliferation of BC cells, while increasing their apoptosis. These effects were further enhanced when OSBPL3 was concurrently interfered with. Additionally, knockdown of OSBPL3 in MCF-7 cells resulted in the coordinated downregulation of five positive regulators of RAS signaling, leading to the activation of the RAS signaling pathway.

Conclusions: These results suggested that the upregulation of miR-181a-5p enhanced chemosensitivity to tamoxifen by negatively regulating OSBPL3 through the RAS signaling pathway in BC cells. Therefore, a treatment strategy based on the miR-181a-5p/OSBPL3 axis may represent a potential approach to overcoming tamoxifen resistance in BC.