A study of the subcellular trafficking of tracer-tagged GLUT4 in rat adipose cells shows that 1) insulin does not alter GLUT4 endocytosis but instead increases the rate of exocytosis, 2) all of the cells' GLUT4 is involved in the recycling process, and 3) plasma membrane intermediate states may be present in the GLUT4 trafficking pathways. A mathematical analysis has been carried out which shows that this kinetic behavior is consistent with a 3-state model, with two intracellular compartments, and the presence of occluded states in the plasma membrane. Peptides from the alpha 1 domain of the major histocompatibility complex class I antigen (MHC class I) enhance cellular glucose uptake above that of maximal insulin stimulation, prolong the effect of insulin, and inhibit insulin receptor internalization in rat adipose cells, perhaps through regulation of the internalization process of cell surface integral membrane proteins. We also have observed that K+ depletion inhibits GLUT4 internalization strongly suggesting the involvement of a coated pit mechanism. Staurosporine, a widely used protein kinase C inhibitor, completely inhibits both PMA- and insulin-stimulate glucose transport activity in isolated rat adipocytes, possibly at a step that regulates the specific translocation of the glucose transporters to the plasma membrane. We have used a photoaffinity label to quantify cell surface GLUT4 glucose transporters in isolated rat soleus muscles; the insulin- stimulated increase in glucose transport activity appears to be accounted for by an increase in surface-accessible GLUT4 content. Autoradiography of photolabeled soleus muscles supports the concept that glucose transport may occur predominately across the T-tubule membrane. We have used the same photoaffinity label to quantitate the glucose transporters on the surface of BC3H-1 cells. This technique demonstrates that glucose prefeeding 1) decreases the total number of GLUT1 and 2) redistributes the majority of the remaining transporters to an intracellular site where they can now be translocated to the cell surface in response to insulin and PMA. We have observed that LDL receptors localized in intracellular compartments of isolated rat adipose cells seem to be functionally regulated since insulin acutely diminishes the number of receptors by accelerating their rate of degradation. We tested the hypothesis that sulfonylureas promote glucose uptake into 3T3-L1 cells or isolated rat adipocytes through specific high affinity receptors. We conclude that the anti-diabetogenic effects of sulfonylureas are not mediated by a direct action of sulfonylureas to increase glucose uptake into adipose tissue. We have observed that chronic consumption of a diet containing trans-fatty acids can influence lipid metabolism and result in decreased fat accumulation in murine adipose tissue.
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