Abstract
Insulin causes the exocytic translocation of GLUT4 glucose transporters to stimulate glucose uptake in fat and muscle. Previous results support a model in which TUG traps GLUT4 in intracellular, insulin-responsive vesicles termed GLUT4 storage vesicles (GSVs). Insulin triggers TUG cleavage to release the GSVs; GLUT4 then recycles through endosomes during ongoing insulin exposure. The TUG C terminus binds a GSV anchoring site comprising Golgin-160 and possibly other proteins. Here, we report that the TUG C terminus is acetylated. The TUG C-terminal peptide bound the Golgin-160-associated protein, ACBD3 (acyl-CoA-binding domain-containing 3), and acetylation reduced binding of TUG to ACBD3 but not to Golgin-160. Mutation of the acetylated residues impaired insulin-responsive GLUT4 trafficking in 3T3-L1 adipocytes. ACBD3 overexpression enhanced the translocation of GSV cargos, GLUT4 and insulin-regulated aminopeptidase (IRAP), and ACBD3 was required for intracellular retention of these cargos in unstimulated cells. Sirtuin 2 (SIRT2), a NAD(+)-dependent deacetylase, bound TUG and deacetylated the TUG peptide. SIRT2 overexpression reduced TUG acetylation and redistributed GLUT4 and IRAP to the plasma membrane in 3T3-L1 adipocytes. Mutation of the acetylated residues in TUG abrogated these effects. In mice, SIRT2 deletion increased TUG acetylation and proteolytic processing. During glucose tolerance tests, glucose disposal was enhanced in SIRT2 knock-out mice, compared with wild type controls, without any effect on insulin concentrations. Together, these data support a model in which TUG acetylation modulates its interaction with Golgi matrix proteins and is regulated by SIRT2. Moreover, acetylation of TUG enhances its function to trap GSVs within unstimulated cells and enhances insulin-stimulated glucose uptake.
Highlights
Insulin stimulates glucose uptake by triggering TUG proteolysis, which liberates intracellular storage vesicles containing GLUT4
Results reported here support a model in which the retention of GLUT4 in insulin-responsive GLUT4 storage vesicles (GSVs) is mediated by acetylated TUG
Binding of insulin-regulated aminopeptidase (IRAP) to p115, another Golgi matrix protein that colocalizes with Golgin-160, may contribute to GSV intracellular retention in unstimulated cells [27, 42]
Summary
Insulin stimulates glucose uptake by triggering TUG proteolysis, which liberates intracellular storage vesicles containing GLUT4. Glucose disposal was enhanced in SIRT2 knock-out mice, compared with wild type controls, without any effect on insulin concentrations Together, these data support a model in which TUG acetylation modulates its interaction with Golgi matrix proteins and is regulated by SIRT2. Insulin liberates GSVs by triggering site-specific endoproteolytic cleavage of TUG, which separates regions that bind GLUT4 from regions that bind Golgi proteins [17, 18] Data imply that this action accounts for a substantial portion of the acute effect of insulin in fat and muscle [14]. The data support the notion that GSVs are controlled by acetylated TUG and that SIRT2 modulates insulin sensitivity by regulating this pool of GSVs
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