The deubiquitinating enzyme USP25 binds tankyrase and regulates trafficking of the facilitative glucose transporter GLUT4 in adipocytes

Jessica B A Sadler,Gwyn W Gould,Cassie R Welburn,Nai-Wen Chi,Christopher A Lamb,Nia J Bryant,Iain S Adamson,Dimitrios Kioumourtzoglou

doi:10.1038/s41598-019-40596-5

Jessica B A Sadler, Gwyn W Gould + Show 6 more

Open Access

https://doi.org/10.1038/s41598-019-40596-5

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Abstract

Key to whole body glucose homeostasis is the ability of fat and muscle cells to sequester the facilitative glucose transporter GLUT4 in an intracellular compartment from where it can be mobilized in response to insulin. We have previously demonstrated that this process requires ubiquitination of GLUT4 while numerous other studies have identified several molecules that are also required, including the insulin-responsive aminopeptidase IRAP and its binding partner, the scaffolding protein tankyrase. In addition to binding IRAP, Tankyrase has also been shown to bind the deubiquinating enzyme USP25. Here we demonstrate that USP25 and Tankyrase interact, and colocalise with GLUT4 in insulin-sensitive cells. Furthermore depletion of USP25 from adipocytes reduces cellular levels of GLUT4 and concomitantly blunts the ability of insulin to stimulate glucose transport. Collectively, these data support our model that sorting of GLUT4 into its insulin-sensitive store involves a cycle of ubiquitination and subsequent deubiquitination.

Highlights

The facilitative glucose transporter GLUT4 expressed in fat and muscle is responsible for the increased rate of glucose transport into these cells in response to insulin[1]
Our finding that only ~0.1% of cellular GLUT4 in adipocytes is ubiquitinated at steady state led us to suggest that GLUT4 ubiquitination is a transient modification that is reversed once its role of directing the transporter into GLUT4 storage vesicles (GSVs) has been fulfilled[5]
We identified USP25 as a candidate DUB to regulate this process through its interaction with IRAP, via TNKS13,14

Summary

Introduction

The facilitative glucose transporter GLUT4 expressed in fat and muscle is responsible for the increased rate of glucose transport into these cells in response to insulin[1]. We have previously reported that ubiquitination of GLUT4 is required for its sorting into GSVs; with a ubiquitin-resistant version of GLUT4 failing to enter GSVs and not exhibiting insulin-stimulated translocation to the cell surface[5]. IRAP binds to the scaffolding protein tankyrase (TNKS)[13] whose knockdown abrogates insulin-stimulated GLUT4 delivery to the cell surface of adipocytes[11]. Binding partners of TNKS, including IRAP, contain a conserved TNKS binding motif (RXX(P/A)DG)[14] This motif resides in the C-terminus of USP2514, a deubiquitinase that has been identified as a TNKS binding protein in HEK293 cells[15]. USP25, positioned, could serve to deubiquitinate GLUT4, rescuing the transporter from lysosomal degradation

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