Regulation of the Polarity Protein Par6 by TGFß Receptors Controls Epithelial Cell Plasticity

TLDR

Epithelial‑to‑mesenchymal transition, driven by TGFβ, is central to development and cancer metastasis, yet the mechanisms that trigger early tight‑junction dissolution remain unclear. These studies define how an extracellular cue signals to the polarity machinery to control epithelial cell morphology. Par6 is phosphorylated by TβRII, and this modification is required for TGFβ‑induced EMT; phosphorylated Par6 recruits Smurf1, which ubiquitinates RhoA, leading to its degradation and loss of tight junctions.

Abstract

The transition of cells from an epithelial to a mesenchymal phenotype is a critical event during morphogenesis in multicellular organisms and underlies the pathology of many diseases, including the invasive phenotype associated with metastatic carcinomas. Transforming growth factor beta (TGFbeta) is a key regulator of epithelial-to-mesenchymal transition (EMT). However, the molecular mechanisms that control the dissolution of tight junctions, an early event in EMT, remain elusive. We demonstrate that Par6, a regulator of epithelial cell polarity and tight-junction assembly, interacts with TGFbeta receptors and is a substrate of the type II receptor, TbetaRII. Phosphorylation of Par6 is required for TGFbeta-dependent EMT in mammary gland epithelial cells and controls the interaction of Par6 with the E3 ubiquitin ligase Smurf1. Smurf1, in turn, targets the guanosine triphosphatase RhoA for degradation, thereby leading to a loss of tight junctions. These studies define how an extracellular cue signals to the polarity machinery to control epithelial cell morphology.