Ubiquitin-like modification is a conserved biochemical mechanism that is used by cells for various purposes. These include targeted degradation of proteins, endocytosis of receptors, regulation of protein localization, and signal transduction. Apart from a role of ubiquitin itself to modulate insulin receptors and IRS proteins, ubiquitin-like modifications have not been described to play a prominent role in insulin signaling. TUG was identified as a target of insulin signaling, and is implicated regulating GLUT4 glucose transporter trafficking and glucose uptake in adipocytes. According to the proposed model, TUG acts by """"""""tethering"""""""" GLUT4 transporters intracellularly in the absence of insulin, excluding them from the plasma membrane and limiting glucose uptake. Insulin """"""""untethers"""""""" the transporters to redistribute GLUT4 and to enhance glucose transport into cells. The mechanism by which insulin acts on TUG and GLUT4 is not well understood. The present proposal will test the hypothesis that insulin stimulates rapid and site-specific cleavage of TUG to liberate an amino terminal fragment, TUGUL, that is a new ubiquitin-like modifier. Using cultured 3T3-L1 adipocytes, several biochemical and cell biological approaches will be used in Aim 1 to test whether insulin stimulates TUG cleavage, and whether this is required for insulin to mobilize GLUT4.
Aim 2 will test the hypothesis that TUGUL functions as a ubiquitin-like modifier, and will study the role of the putative TUG C terminal cleavage product.
Aim 3 will study how the insulin signal may act on TUG to cause its processing. It is anticipated that, together, accomplishment of these aims will begin to define a novel pathway for insulin regulated ubiquitin-like modification. Furthermore, it is anticipated that the results will have importance for understanding how insulin stimulates glucose uptake. Type 2 diabetes is a major public health problem that results in part from a defect in the ability of insulin to stimulate glucose uptake. It is anticipated that the proposed studies of insulin action and glucose uptake will lead to a greater understanding of mechanisms that may contribute to the development of type 2 diabetes.
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