TOPK has been suggested to contribute to invasion of lung, prostate, gastric, pancreatic or breast cancer cells. However, how TOPK mediates TGE-beta 1/Smad signaling leading to epithelial-mesenchymal transition (EMT) and invasion of breast cancer cells remains unknown. Here we report that TOPK upregulates T-box transcription factor TBX3 to enhance TGE-beta 1-induced EMT and invasion of MDA-MB-231 breast cancer cells. Expression of endogenous TOPK was promoted by TGE-beta 1 treatment of MDA-MB-231 cells timedependently. In addition, knockdown of TOPK attenuated TOE-Pt-induced phosphorylation or transcriptional activity of Smad3. Meanwhile, levels of both mRNA and protein of TBX3 induced by TGE-beta 1 were abolished by TOPK depletion. Also, knockdown of TBX3 inhibited TGE-beta 1 induction of EMT-related genes Snail, Slug or Fibronectin. Furthermore, ablation of TOPK or TBX3 suppressed TGE-beta 1-induced MDA-MB-231 cell invasion. Collectively, we conclude that TOPK positively regulates TBX3 in TGE-beta 1/Smad signaling pathway, thereby enhancing EMT and invasion of breast cancer cells, implying a mechanistic role of TOPK in TGE-beta 1/Smad signaling. (C) 2019 Elsevier Inc. All rights reserved.