TGF-β/SMAD4/14-3-3σ/TFEB axis promotes mesenchymal-epithelial transition and inhibits autophagy in colorectal cancer.

14-3-3σ is a p53-inducible gene with tumor suppressive properties and SMAD4 is a transcription factor encoded by a tumor suppressor gene, which is commonly inactivated in colorectal cancer (CRC). Here, 14-3-3σ was characterized as direct transcriptional target of SMAD4. TGF-β treatment of tumoroids derived from CRC patients and mouse models resulted in a SMAD4-dependent induction of 14-3-3σ. In murine, intestinal epithelia, the apical expression of 14-3-3σ was dependent on Smad4. Ectopic SMAD4 or 14-3-3σ promoted mesenchymal-to-epithelial transition (MET) and suppressed invasion, migration, and autophagy of CRC cells. As experimental inactivation of 14-3-3σ abolished these tumor-suppressive functions of SMAD4, 14-3-3σ mediates these effects of SMAD4. Inhibition of autophagy and promotion of MET by SMAD4 was mediated by inhibition of TFEB via binding and sequestration of TFEB by 14-3-3σ. The association of 14-3-3σ and TFEB was dependent on phosphorylation of the TFEB serine 211 residue, which is a target of mTORC1. Taken together, the TGF-β/SMAD4/14-3-3σ/TFEB axes characterized here antagonizes epithelial plasticity and autophagy. Thereby, it may ultimately suppress the progression of CRC and other types of cancer.