Modulation of GLUT4 and GLUT1 Recycling by Insulin in Rat Adipocytes:  Kinetic Analysis Based on the Involvement of Multiple Intracellular Compartments

Abstract

The trafficking kinetics of GLUT4 and GLUT1 in rat epididymal adipocytes were analyzed by a four-compartment model based upon steady-state pool sizes of three intracellular fractions and one plasma membrane fraction separated and assessed under both basal and insulin-stimulated states. The steady-state compartment sizes provided relative values of the kinetic coefficients characterizing the rate of each process in the loop. Absolute values of these coefficients were obtained by matching the simulated half-times to those observed experimentally and reported in the literature for both basal and insulin-stimulated states. Our analysis revealed that insulin modulates the GLUT4 trafficking at multiple steps in the rat adipocyte, not only reducing the endocytotic rate constant 3-4-fold and increasing the exocytotic rate 8-24-fold but also increasing the two rate coefficients coupling the three intracellular compartments 2-6-fold each. Furthermore, GLUT1 was completely segregated from GLUT4 in two of the three intracellular compartments, and its steady-state distribution is consistent with a four-compartment model of GLUT1 recycling involving an insulin sensitive endocytosis step in common with the GLUT4 system, but with all other processes being insensitive to insulin.

Cellular spelunking: exploring adipocyte caveolae

“…Whereas a very small percentage of insulin receptors are needed for maximal signaling, all Glut4 molecules are involved in trafficking and Glut4 attains a steady-state distribution, cell surface versus internal, ?10 min after insulin exposure, when exocytic and endocytic rates are equal (69)(70)(71). In this time frame, we see no evidence that any insulin-dependent changes occur in cell surface caveolae composition (34), consistent with the finding that bulk caveolae are not very dynamic structures (72,73).…”

Separation of Insulin Signaling into Distinct GLUT4 Translocation and Activation Steps

“…8). Despite the fact that insulin treatment triggers both GLUT4 and GLUT1 (to a lesser extent) translocation in 3T3-L1 adipocytes, it has been reported that these two glucose transporters are stored in separate intracellular compartments, utilizing different machinery for translocation (31,37,41). In fact, overexpressing the constitutively active form of Akt induces GLUT4, but not GLUT1, translocation (16).…”

“…It has been reported that the intracellular storage sites of these glucose transporters are different. Insulin-induced GLUT1 translocation is much weaker than that of GLUT4 and involves a different signal transduction pathway (16,31,37,41). In order to investigate whether PBP10 treatment primarily affects signals specific for GLUT4 translocation, we looked at the effect of PBP10 treatment on GLUT1 translocation.…”

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

“…From extensive research on GLUT4 translocation in adipocytes, it would seem that a net transporter redistribution to the sarcolemma upon exposure of cardiac tissue or cells to a given stimulus is caused by an increase in the rate of exocytosis without any change in the rate of endocytosis (which is regarded as noninducible in this translocation model). This is currently the most favored view of the regulation of transporter translocation (207,365,428), although a decreased endocytosis may also contribute to net GLUT4 translocation (263). Nevertheless, in general, endocytosis is regarded as an unregulatable (housekeeping) trafficking event, while in contrast exocytosis is highly inducible.…”