Honokiol (HNK),an active compound isolated from traditional Chinese medicine Magnolia of Ocinalis,has shown potent anticancer activities.In the present study,we investigated the effects of HNK on breast cancer metastasis in vitro and in vivo,as well as the underlying molecular mechanisms.We showed that HNK (10-70 μmol/L) dose-dependently inhibited the viability of human mammary epithelial tumor cell lines MCF7,MDA-MB-231,and mouse mammary tumor cell line 4T1.In the transwell and scratch migration assays,HNK (10,20,30 μmol/L) dose-dependently suppressed the invasion and migration of the breast cancer cells.We demonstrated that HNK (10-50 μmol/L) dose-dependently upregulated the epithelial marker E-cadherin and downregulated the mesenchymal markers such as Snail,Slug,and vimentin at the protein level in breast cancer cells.Using a puromycin incorporation assay,we showed that HNK decreased the Snail translation efficiency in the breast cancer cells.In a mouse model of tumor metastasis,administration of HNK (50 mg/kg every day,intraperitoneal (i.p.),6 times per week for 30 days) significantly decreased the number of metastatic 4T1 cell-derived nodules and ameliorated the histological alterations in the lungs.In addition,HNK-treated mice showed decreased Snail expression and increased E-cadherin expression in metastatic nodules.In conclusion,HNK inhibits EMT in the breast cancer cells by downregulating Snail and Slug protein expression at the mRNA translation level.HNK has potential as an integrative medicine for combating breast cancer by targeting EMT.