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Background Chemotherapy is a predominant strategy for breast cancer (BC) treatment and paclitaxel (PTX) has been known as a conventional chemotherapeutic drug. However, insensitivity of BC cells to PTX limits the anti-tumor effects of this agent. MicroRNAs are closely related to BC which are suggested as therapeutic factors in the combination therapy of BC. We examined the possible efficacy of miR-138-5p restoration in combination with PTX to impove BC treatment. Methods and Results The human breast cancer cell line MDA-MB-231 was transfected with miR-138-5p mimics and treated with PTX, in a combined or separate manner. The MTT assay was accomplished to determine inhibitory doses of PTX. Annexin V/PI assay and DAPI staining were applied to evaluate apoptosis. Flow cytometry was applied to determine cells arrested in different phases of the cell-cycle. Expression levels of molecular factors involved in cell migration, proliferation, apoptosis, and cell cycle were determined via western blotting and qRT-PCR. MiR-138-5p combined with PTX suppressed cell migration via modulating MMP2, E-cadherin, and vimentin and sustained colony formation and proliferation by downregulation of the PI3K/AKT pathway. qRT-PCR showed that miR-138-5p increases BC chemosensitivity to PTX by regulating the apoptosis factors, including Bcl-2, Bax, Caspase 3, and Caspase 9. Moreover, miR-138-5p restoration and paclitaxel therapy combined arrest the cells in the sub-G1 and G1 phases of cell cycle by regulating p21, CCND1, and CDK4. Conclusions Restored miR-138-5p intensified the chemosensitivity of MDA-MB-231 cell line to PTX, and the combination of miR-138-5p with PTX might represent a novel approach in BC treatment.

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