Purpose: Triple-negative breast cancer (TNBC), characterized by the absence of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2, is a highly heterogeneous disease. Recent studies suggest that there are links between TNBC and the epithelial-mesenchymal transition (EMT). To identify prognostic biomarkers and novel therapeutic targets, vimentin, one of the most major factors associated with EMT was investigated in TNBC. Materials and methods: Sporadic invasive ductal carcinoma specimens were obtained from 659 Japanese patients, and 90 (14 %) cases were diagnosed as TNBC. The vimentin mRNA and protein expression levels were evaluated by quantitative reverse transcriptase-polymerase chain reaction and immunohistochemistry. Results: The mRNA expression of vimentin was significantly upregulated in the basal-type breast cancer cell line. Immunohistochemically, the vimentin expression was significantly higher (p = 0.0042) in TNBC compared with the other subtypes. Vimentin expression was associated with a younger age (p = 0.016), high nuclear grade (p = 0.023) and high Ki67 expression (p < 0.0001), and a poorer prognosis in terms of both the recurrence-free survival (RFS) (p = 0.0058) and overall survival (OS) (p = 0.013) in TNBC patients. A multivariate analysis showed that vimentin expression was an independent prognostic factor for the RFS (p = 0.043). Vimentin expression was also associated with a significantly shorter RFS (p = 0.021) and OS (p = 0.017) in patients with basal-like breast cancer (BLBC). Conclusions: The elevated expression of vimentin contributes to the aggressive phenotype and poor prognosis in TNBC. Vimentin expression might be useful as a biomarker for the prognosis of TNBC.
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