Glut4-mediated glucose uptake represents the rate-limiting step of insulin-stimulated glucose disposal, and type 2 diabetes is associated with impaired translocation of Glut4 to the cell surface. The insulin-responsive pool of Glut4 is localized in intracellular membrane vesicles, or IRVs, that deliver Glut4 to the plasma membrane. Proteomics analyses performed in our and other laboratories have shown that, in addition to Glut4, these vesicles contain three major proteins, sortilin, IRAP, and LRP1. Our hypothesis is that sortilin represents the proactive component of the IRVs which is responsible for their formation and insulin responsiveness, whereas other IRV proteins including Glut4 may just be passive cargo in this compartment. However, sortilin cannot perform its biological functions without regulatory proteins that interact with its cytoplasmic tail.
In Specific Aim 1, w propose to isolate and to identify these proteins by mass-spectrometry. Another important biological role of sortilin is to maintain normal levels of the Glut4 protein in adipocytes by retrieving it from endosomes. The specific mechanism of Glut4 retrieval is unknown and will be explored in Specific Aim 2. In addition to its role in the Glut4 pathway, sortilin plays an important role in protein targeting from Golgi to lysosomes. In the previous funding period, we have found that sortilin interacts with adiponectin and facilitates its lysosomal degradation thus regulating adiponectin secretion at a previously unknown post-translational level.
In Specific Aim 3, we will test the hypothesis that sortilin targets adiponectin for degradation in lysosomes in an acyl CoA-dependent fashion. In parallel, we will analyze expression levels of sortilin, adiponectin, Glut4, and several other proteins in fat samples obtained from insulin-sensitive and insulin-resistant patients undergoing bariatric surgery.
Impaired translocation of Glut4 to the cell surface represents the key factor for the development of type 2 diabetes mellitus. Recent studies point out to the cell biology of Glut4 recycling as a potential site of primary diabetes-related abnormalities. We propose to explore several unknown aspects of Glut4 traffic which will shed light on the molecular nature of insulin resistance and diabetes.
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