Metastasis-related miRNAs, active players in breast cancer invasion, and metastasis.

Breast cancer is the most common malignancy with the highest incidence among women in the world. Metastasis is the major reason for breast cancer-related deaths. The precise molecular circuitry that governs the metastasis process has not been completely understood. Discoveries of microRNAs (miRNAs) open a new avenue for cancer metastasis research. It has become clear that alterations of miRNA expression contribute to cancer pathogenesis. miRNAs control a wide array of physiological and pathological processes, including development, differentiation, cellular proliferation, programmed cell death, oncogenesis, and metastasis by modulating the expression of their cognate target genes through cleaving mRNA molecules or inhibiting their translation. Some miRNAs are associated with the invasive and metastatic phenotype of breast cancer cell lines or identified in metastatic tumor tissues and lymph nodes. Some miRNAs serve as metastasis suppressors and their expression is frequently downregulated or lost in both breast cancer cell lines and metastatic foci. Some miRNAs are considered to play key roles in the phenotype formation of breast cancer stem cells. This review will focus on recent discoveries related to the miRNAs involved in the metastasis of breast cancer and discuss the implications for the diagnosis, prognosis, and therapeutic strategies of breast cancer.