Long-term insulin treatment of 3T3-L1 adipocytes results in mis-targeting of GLUT4: implications for insulin-stimulated glucose transport.

AIMS/HYPOTHESIS: Insulin stimulates glucose transport in adipose and muscle tissue by the translocation of a specialised pool of intracellular GLUT4-containing vesicles to the cell surface. It is well established that defective insulin-stimulated GLUT4 translocation is associated with insulin resistance. Long-term insulin treatment (500 nmol/l for 24 h) of 3T3-L1 adipocytes has previously been shown to decrease cellular GLUT4 content and reduce insulin-stimulated GLUT4 translocation. Here, we test the hypothesis that the insulin resistance observed after long-term insulin treatment arises by the selective loss of GLUT4 from a specific intracellular compartment.
METHODS: Using iodixanol gradient centrifugation we have separated intracellular GLUT4 containing membranes into two distinct populations corresponding to recycling endosomes and a distinct intracellular compartment which probably represents GLUT4 storage vesicles (GSVs).
RESULTS: A short-term insulin stimulation reduced the content of GLUT4 in the GSV fraction (51 +/- 3.5%) with only a modest decrease from the endosomal fraction (23 +/- 2.6%). Long-term insulin treatment decreased cellular GLUT4 content by about 40% and diminished the ability of a short-term insulin challenge to promote GLUT4 translocation. We further show that this depletion of cellular GLUT4 is selectively from the GSV fraction (68 +/- 7% decrease compared to untreated cells). CONCLUSIONS/
INTERPRETATION: Such data argue that long-term insulin treatment results in the mis-targeting of GLUT4 such that it no longer accesses the GSV compartment. These data imply that defective targeting of GLUT4 away from the GSV compartment plays an important role in the aetiology of insulin resistance.