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Trastuzumab has led to a marked improvement in the outcomes of patients with human epidermal growth factor receptor 2 (HER‑2)‑positive breast cancer. However, the effects of trastuzumab on HER‑2‑positive breast cancer are limited by the emergence of its cardiotoxicside effects. MicroRNA (miR)‑135b‑5p has been shown to inhibit tumor metastasis in breast cancer. The present study aimed to explore the effects of miR‑135b‑5p overexpression on the efficacy of trastuzumab in HER‑2‑positive breast cancer. Reverse transcription‑quantitative PCR was performed to detect the levels of miR‑135b‑5p. Cell viability was evaluated with a Cell Counting Kit‑8 assay. Annexin V/propidium iodide staining was employed to detect the number of apoptotic cells. Flow cytometry assay was performed to investigate the cell cycle. Western blotting was used to detect the expression levels of Bax, cleaved caspase‑3, Bcl‑2, cyclin D2, p27Kip1 and cyclin E1. Cell migration and invasion were detected by Transwell assay. Luciferase assays were conducted to identify the target gene of miR‑135b‑5p. In addition, an in vivo tumor xenograft model was established. miR‑135b‑5p agomir significantly enhanced the anti‑proliferative effect of trastuzumab on HER‑2‑positive breast cancer cells via the induction of apoptosis, whereas the anti‑metastatic effect of trastuzumab was enhanced by miR‑135b‑5p agomir treatment. Subsequently, luciferase assays indicated that cyclin D2 was the direct target of miR‑135b‑5p, whereas overexpression of the latter arrested cell cycleduring the G0/G1 phase. Moreover, miR‑135b‑5p agomir notably increased the antitumor effect of trastuzumab in vivo. The data demonstrated that miR‑135b‑5p sensitized HER‑2‑positive breast cancer cells to trastuzumab in vitro and in vivo by directly binding to cyclin D2. These results suggested that the combination of miR‑135b‑5p with trastuzumab may be a therapeutic strategy for patients with HER‑2‑positive breast cancer.

miR‑135b‑5p enhances the sensitivity of HER‑2 positive breast cancer to trastuzumab via binding to cyclin�D2