The Role of Endocytic Pathways on Estrogen Receptor α Intracellular Trafficking and 17β-estradiol Signaling

Research performed during the last decades clearly demonstrated that the endocytic pathways act as a fine spatio-temporal regulator of the cell signaling. It is now accepted that the principal sorting signal for most of the endocytic cargos is the post-translational modification with ubiquitin (Ub). Indeed, nonproteolytic ubiquitination of the cargo protein (e.g., monoubiquitination) triggers its internalization and its incorporation into specific endocytic vesicles through which cargo is destined either to lysosomes for degradation, to plasma membrane for recycling or to other intracellular fates. Thus, cargo ubiquitination and intracellular trafficking are intrinsically connected and contribute to the regulation of many cellular functions. The nuclear receptor estrogen receptor α (ERα) regulates the physiological effects triggered by 17β-estradiol (E2). It is now clear that the ERα drives the nuclear (gene transcription) and extra-nuclear (activation of signaling kinase cascades) E2-dependent effects because it has a wide intracellular localization both in the nucleus, in the cytoplasm and at the plasma membrane of the E2 target cells. Remarkably, many laboratories, including our own, have reported that ERα is monoubiquitinated and receptor monoubiquitination is required for the receptor- dependent regulation of E2 cellular effects and that an endocytic intracellular trafficking of the ERα can occur. Thus, this evidence suggests the possibility of a Ub-based endo-membrane shuttling of the activated ERα for the regulation of the E2-triggered effects. In light of these novel discoveries, this review will focus on the role of the endocytic trafficking pathways in E2:ERα signaling by particularly highlighting how endocytic proteins and non-degradative ubiquitination could impact on the modulation of E2-dependent physiological processes.