MST1 inhibits the progression of breast cancer by regulating the Hippo signaling pathway and may serve as a prognostic biomarker.

Breast cancer (BCa) is the most common malignancy threatening the health of women worldwide, and the incidence rate has significantly increased in the last 10 years. Mammalian STE20‑like protein kinase 1 (MST1) is involved in the development of various types of malignant tumor. The present study aimed to investigate the role of MST1 in BCa and its potential involvement in the poor prognosis of patients with BCa. Reverse transcription‑quantitative PCR and immunohistochemistry were used to analyze the expression levels of MST1 in BCa, and the clinicopathological characteristics and prognosis of patients with BCa were further analyzed by statistical analysis. MST1 was overexpressed in BCa cell lines (MCF‑7, MDA‑MB‑231 and SKBR3). Cell Counting Kit‑8, 5‑ethynyl‑2'‑deoxyuridine and flow cytometry assays were used to analyze cell proliferation and apoptosis, respectively, and a wound healing assay was used to analyze cell migration. The results of the present study revealed that the downregulated expression levels of MST1 in BCa were closely associated with the poor prognosis of patients, and MST1 may be an independent risk factor for BCa. The overexpression of MST1 significantly inhibited the proliferation and migration, and promoted the apoptosis of BCa cells. In addition, the overexpression of MST1 significantly activated the Hippo signaling pathway. Treatment with XMU‑MP‑1 downregulated the expression levels of MST1 and partially reversed the inhibitory effects of MST1 on proliferation, migration and apoptosis‑related proteins, and inhibited the Hippo signaling pathway. In conclusion, the results of the present study suggested that MST1 expression levels may be downregulated in BCa and closely associated with tumor size and clinical stage, as well as the poor prognosis of affected patients. Furthermore, MST1 may inhibit the progression of BCa by targeting the Hippo signaling pathway.