Transforming Growth Factor β/NR4A1-Inducible Breast Cancer Cell Migration and Epithelial-to-Mesenchymal Transition Is p38α (Mitogen-Activated Protein Kinase 14) Dependent

Transforming growth factor beta (TGF-beta)-induced migration of triplenegative breast cancer (TNBC) cells is dependent on nuclear export of the orphan receptor NR4A1, which plays a role in proteasome-dependent degradation of SMAD7. In this study, we show that TGF-beta induces p38 alpha (mitogen-activated protein kinase 14 [MAPK14]), which in turn phosphorylates NR4A1, resulting in nuclear export of the receptor. TGF-beta/p38 alpha and NR4A1 also play essential roles in the induction of epithelial-to-mesenchymal transition (EMT) and induction of beta-catenin in TNBC cells, and these TGF-beta-induced responses and nuclear export of NR4A1 are blocked by NR4A1 antagonists, the p38 inhibitor SB202190, and kinase-dead [p38(KD)] and dominant-negative [p38(DN)] forms of p38 alpha. Inhibition of NR4A1 nuclear export results in nuclear export of TGF-beta-induced beta-catenin, which then undergoes proteasome-dependent degradation. TGF-beta-induced beta-catenin also regulates NR4A1 expression through formation of the beta-catenin-TCF-3/TCF-4/LEF-1 complex on the NR4A1 promoter. Thus, TGF-beta-induced nuclear export of NR4A1 in TNBC cells plays an essential role in cell migration, SMAD7 degradation, EMT, and induction of beta-catenin, and all of these pathways are inhibited by bis-indole-derived NR4A1 antagonists that inhibit nuclear export of the receptor and thereby block TGF-beta-induced migration and EMT.