Adipocytes and myocytes contain intracellular GLUT4-containing vesicles which fuse to the plasma membrane upon insulin stimulation. The resulting increase in plasma membrane GLUT4 is responsible for the 10 to 20-fold increase in glucose transport in the insulin-stimulated state. GLUT4-containing vesicle movement from the cytoplasm to fusion with the plasma membrane, a process termed translocation, is believed to be abnormal in both insulin resistance and non-insulin dependent diabetes mellitus (NIDDM). See B. Kahn, J. Clin. Invest., 89, 1367-1374, 1992.
The normal molecular mechanisms of insulin-stimulated translocation of GLUT4-containing vesicles to the plasma membrane and the concomitant increase in glucose uptake by the affected cells are still largely unknown. Proposed mechanisms include both GLUT4-containing vesicle docking and fusion with the plasma membrane.
The GLUT4-containing vesicles are only a minor fraction of the total intracellular vesicle population within adipocytes [James et al, J. Biol. Chem., 262, 11817-11824, (1987)]. These GLUT4 vesicles can be isolated from the low density microsomal fraction using immunoaffinity methods with antibodies to the cytoplasmic-oriented C-terminus of GLUT4 [James, et al, J. Biol. Chem., 262, 11817-11824, (1987); Thoidis, et al, J. Biol. Chem., 268, 11691-11696, (1993)]. GLUT4-enriched vesicles have been shown to have several synaptic vesicle-like proteins including vesicle associated membrane protein (VAMP) [Cain et al, J. Biol. Chem., 267, 11681-11684, (1992)] and secretory carrier associated membrane protein (SCAMP) [Laurie et al, J. Biol. Chem., 268, 19110-19117, (1993)], and certain ras analog proteins originally identified in brain (Rab proteins) [Cormont, et al, J. Biol. Chem., 268, 19491-19497, (1993)].
Presently the pharmacologic therapy for NIDDM may not target the insulin resistance which is the key pathophysiological abnormality of the disease. The existing therapies are 1) diet and exercise; 2) sulfonylureas, which stimulate insulin secretion; 3) .alpha.-glucosidase inhibitors, which inhibit the enzymatic digestion of complex carbohydrates and thereby slow the postprandial absorption of glucose; 4) metformin, whose mechanism likely includes improvement of hepatic insulin sensitivity, and 5) insulin injections.
A therapeutic agent which targets insulin resistance and improves insulin sensitivity would have significant advantages over the therapies listed above. Such a therapeutic agent is the subject of the present invention. It is an objective of this invention to increase insulin sensitivity by modulating the aminopeptidase associated with GLUT4 vesicles.
USPTO Backgrounds
USPTO Backgrounds