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Qi-Xing Tian,1,* Zhi-Hui Zhang,1,* Qing-Lin Ye,1 Shen Xu,1 Qian Hong,1 Wei-Yang Xing,1 Lei Chen,1 De-Xin Yu,1 De-Xiang Xu,2,3 Dong-Dong Xie1 1Department of Urology, Second Affiliated Hospital, Anhui Medical University, Hefei, 230601, People’s Republic of China; 2Department of Toxicology, Anhui Medical University, Hefei, 230032, People’s Republic of China; 3Laboratory of Environmental Toxicology, Anhui Medical University, Hefei, 230032, People’s Republic of China*These authors contributed equally to this workCorrespondence: De-Xiang XuDepartment of Toxicology, Anhui Medical University, Hefei, 230032, People’s Republic of ChinaTel +86 138 6674 1832Email xudex@126.comDong-Dong XieDepartment of Urology, Second Affiliated Hospital, Anhui Medical University, Hefei, 230601, People’s Republic of ChinaTel +86 138 6670 0010Email xiedd_urology@163.comPurpose: Gram-negative bacteria are usually found in prostate cancer (PCa) tissues. This study aims to investigate the role of lipopolysaccharide (LPS), a glycolipid compound found in the outer membrane of gram-negative bacteria, on the migration and invasion of PCa cells, and to evaluate the protective effect of melatonin.Materials and Methods: DU145, PC-3 and LNCaP cells were incubated with LPS in the presence or absence of melatonin. Wound healing and Transwell assays were used to analyze migration and invasion of PCa cells. RT-PCR and Western blotting were used to assess the mRNA and protein levels, respectively. Co-IP was used to analyze β-catenin ubiquitination.Results: Our results showed that LPS promoted migration and invasion of PCa cells. In addition, LPS stimulated inflammatory reaction and induced epithelial–mesenchymal transition (EMT) in PCa cells by activating several TLR4 downstream pathways. Specifically, LPS promoted NF-κB/IL-6/STAT3 signal transduction. In addition, LPS upregulated phosphorylation levels of cytoplasmic AKTSer473 and GSK-3βSer9. Moreover, LPS induced phosphorylation of GSK-3βSer9 in the “disruption complex”, and then inhibited phosphorylation and ubiquitination of cytoplasmic β-catenin, leading to β-catenin nuclear translocation. Interestingly, melatonin inhibited invasion and migration not only in LPS-stimulated but also in LPS-unstimulated PCa cells. Melatonin suppressed PCa cells migration and invasion by blocking EMT mediated by IL-6/STAT3, AKT/GSK-3β and β-catenin pathways.Conclusion: This study provides evidence that melatonin inhibits migration and invasion through blocking multiple TLR4 downstream EMT-associated pathways both in LPS-stimulated and -unstimulated PCa cells. Our results provide new insights into the role of bacterial infection in PCa metastasis and a potential therapeutic agent.Keywords: β-catenin, EMT, lipopolysaccharide, melatonin, prostate cancer