The Role of Intercellular Contacts in EpithelialL-mesenchymal/-myofibroblast Transition

Epithelial mesenchymal/-myofibroblast transition (EMT/EMyT) has emerged as one of the central mechanisms in wound healing and tissue fibrosis. The main feature of EMyT is the activation of a myogenic program, leading to the induction of the α-smooth-muscle actin (SMA) gene in the transitioning epithelium. Recent research suggests that intercellular contacts are not merely passive targets, but are active contributors to EMT/EMyT. Indeed, our group showed previously that contact uncoupling or injury is necessary for TGFβ to induce EMyT (two-hit paradigm). Further, our previous work also revealed that Smad3, the main TGFβ-regulated transcription factor, binds to the Myocardin Related Transcription Factor (MRTF), the prime driver of SMA promoter, and inhibits MRTF’s transcriptional activity. During EMyT, Smad3 eventually degrades, which liberates the MRTF-driven myogenic program. However the mechanisms whereby cell contacts regulate the fate of Smad3 and MRTF during EMyT are poorly understood. Accordingly, the central aim of my studies was to explore the role of intercellular contacts, in particular that of Adherens Junction (AJs) in the induction of the myogenic reprogramming of the injured epithelium. This thesis describes two novel molecular mechanisms through which AJs impact EMyT. In the first part, we show β-catenin, an AJs component and transcriptional co-activator counteracts the inhibitory action of Smad3 on MRTF. Moreover we reveal that β-catenin is necessary to maintain MRTF stability via protecting MRTF from proteasomal degradation. Thus, β-catenin is an indispensable permissive factor for SMA expression. In the second part, we demonstrate that contact injury and TGFβ suppress the expression of the phosphatase PTEN. EMyT-related reduction or absence of PTEN potentiates Smad3 degradation. EMyT is associated with enhanced phosphorylation of the T179 residue in Smad3 linker region, and this event is necessary for Smad3 degradation. PTEN silencing increases the stimulatory effect of contact uncoupling and TGFβ on SMA promoter activity and SMA protein expression. Thus, the integrity of intercellular contacts regulates the level of PTEN, which in turn controls Smad3 stability through impacting on T179 phosphorylation. This new knowledge holds promises for targeted therapies and more effective prevention of the currently incurable fibroproliferative and fibrocontractile diseases.Ph