Association of carboxyl esterase with facilitative glucose transporter isoform 4 (GLUT4) intracellular compartments in rat adipocytes and its possible role in insulin-induced GLUT4 recruitment

Facilitative glucose transporter isoform 4 (GLUT4) in rat adipocytes is largely sequestered in intracellular sites, and insulin recruits GLUT4 from these sites to the cell surface. The process is known to involve multiple intracellular compartments and associated proteins, many of which are yet to be identified. Recently, we purified three distinct insulin-sensitive intracellular GLUT4 compartments (G4T(L), G4H, and G4L) in rat adipocytes and unraveled several new resident proteins in these compartments. Here, we describe one of them, a 62-kDa protein, purified and identified as rat adipose tissue carboxyl esterase (p62/CE) by matrix-assisted laser desorption/ionization time of flight mass spectroscopy, reverse transcription-polymerase chain reaction, gene cloning, and immunological and enzymatic activity measurements. p62/CE in rat adipocytes was 80% cytosolic and 20% microsome-associated. It was found in all of the three insulin-sensitive intracellular GLUT4 compartments, and particularly enriched in G4T(L), a compartment thought to represent GLUT4 endocytic vesicles. Significantly, an antibody against p62/CE introduced into rat adipocytes completely abolished the insulin-induced GLUT4 recruitment to the plasma membrane in host cells without affecting the basal GLUT4 distribution. Together, these findings suggest that p62/CE plays a key role in insulin-induced GLUT4 recruitment in rat adipocytes, probably by hydrolyzing acylglycerols or acyl-CoA esters to the respective free acids that are required for GLUT4 transport vesicle budding and/or fusion.