Contents lists available at ScienceDirect Seminars in Cell & Developmental Biology journal homepage: www.elsevier.com/locate/semcdb Review When ubiquitin meets E-cadherin: Plasticity of the epithelial cellular barrier Carlos A. Niño a, ⁎ , Simona Sala a , Simona Polo a,b, ⁎ a IFOM, Fondazione Istituto FIRC di Oncologia Molecolare, Via Adamello 16, Milan 20139, Italy b Dipartimento di oncologia ed emato-oncologia, Universita’ degli Studi di Milano, Via Santa Sofia 9/1, Milan 20122, Italy ARTICLE INFO Keywords: E-cadherin Adherens junctions Cellular plasticity Endocytosis Trafficking Ubiquitination ABSTRACT Cellular plasticity is, by definition, the ability of cells to adapt to a dynamic micro-environment by changing their phenotype. E-cadherin is the key organizer of the epithelial cell barrier, and it is required at the cell surface to preserve epithelial tissue integrity and homeostasis, since it not only organizes the adherens junctions, but also transfers intracellular signals that provide cues to regulate cell survival, morphology and polarity. As such, de-regulation of E-cadherin has deleterious effects on cells and whole tissues. The availability of cadherin at the cellular junctions is determined by the rates of new protein synthesis and degradation, as well as of internalization and recycling. Indeed, E-cadherin is subjected to a constant and a signal-mediated turnover due to trafficking and recycling between the cell surface and the cytoplasm. Importantly, the turnover of E-cadherin is required for both cell adhesion and cell plasticity within a tissue. Understanding the pathways and molecular mechanisms that E-cadherin undertakes to move in and out of adherens junctions, through which epithelial cells communicate with each other, has, thus, been a major re- search focus over the past decade, but several issues remain unresolved. Here, we review major advances and remaining open questions in the understanding of E-cadherin trafficking, with a particular focus on its ubi- quitination. 1. Introduction E-cadherin is the main cell-cell adhesion molecule in epithelial tis- sues and it is considered to be the master regulator of cellular junctions. From the very early stages of development, cells retain E-cadherin and adherens junctions (AJs) as both are crucial for maintaining tissue homeostasis, as well as regulating permeability and barrier function of the epithelia. Indeed, de-regulation of E-cadherin-mediated cell-cell adhesion is a key step during the epithelial-to-mesenchymal transition (EMT), a process where loss of contact inhibition is associated with increased cell motility and advanced stages of cancer [1–4]. However, recent data demonstrated that a partial EMT, resulting in a hybrid epithelial/mesenchymal phenotype with retention of E-cadherin, is essential for cancer cell dissemination [5–8]. Moreover, E-cadherin and E-cadherin-based AJs are required for collective cancer cell invasion and migration, survival in circulation and metastatic outgrowth (for reviews, see [9–13]). In textbooks, AJs and cell contacts in general are depicted as static structures. This picture is misleading, however, because AJs are sub- jected to various mechanical forces and are very dynamic structures, which are continuously formed, broken and rearranged. The assembly and disassembly of AJs are regulated by diverse intracellular signaling pathways, which affect the transcriptional expression of E-cadherin, as well as its endocytosis, recycling and degradation. Understanding the molecular mechanisms at the basis of this plastic behavior is of great interest and is extensively studied by several researchers. In this review, we will focus on the trafficking aspect of E-cadherin that is critically modulated by the post-translational modifier, ubiquitin. The highly conserved ubiquitin (Ub) protein consists of 76 amino acids and is covalently attached to other proteins, typically via an isopeptide bond that links the C-terminal glycine residue at the ε-amino radical of lysine residues on substrates. Modifications are carried out by the sequential actions of three classes of enzymes. First, a Ub-activating enzyme (E1) catalyzes the formation of an energy-rich thioester bond between its active-site cysteine and the C-terminal glycine residue of Ub. The activated Ub is then delivered to a similar cysteine residue at the active site of a Ub-conjugating enzyme (E2). Finally, a Ub ligase (E3) catalyzes the linkage of Ub to the target protein. The over 600 E3s estimated in the human proteome have pivotal roles in conferring specificity in ubiquitination, since they select substrates and modifica- tion sites, and govern the types of modifications that are generated. In addition to monoubiquitination, the presence of seven internal lysine residues (K6, K11, K27, K29, K33, K48 and K63) and the N-terminal methionine residue on Ub allows the assembly of eight homotypic and https://doi.org/10.1016/j.semcdb.2018.12.005 Received 5 September 2018; Received in revised form 28 November 2018; Accepted 13 December 2018 ⁎ Corresponding authors at: IFOM, Fondazione Istituto FIRC di Oncologia Molecolare, Via Adamello 16, Milan 20139, Italy. E-mail addresses: carlos.nino@ifom.eu (C.A. Niño), simona.polo@ifom.eu (S. Polo). Seminars in Cell and Developmental Biology xxx (xxxx) xxx–xxx 1084-9521/ © 2018 Elsevier Ltd. All rights reserved. Please cite this article as: Niño, C.A., Seminars in Cell and Developmental Biology, https://doi.org/10.1016/j.semcdb.2018.12.005