Stimulation of glucose transport by thyroid hormone in 3T3-L1 adipocytes: increased abundance of GLUT1 and GLUT4 glucose transporter proteins

In 3T3-L1 adipocytes we have examined the effect of tri-iodothyronine (T-3) on glucose transport, total protein content and subcellular distribution o f GLUT1 and GLUT4 glucose transporters. Cells incubated in T-3-depleted ser um were used as controls. Cells treated with T-3 (50 nM) for three days had a 3.6-fold increase in glucose uptake (P<0.05), and also presented a highe r insulin sensitivity, without changes in insulin binding. The two glucose carriers, GLUT1 and GLUT4, increased by 87% (P<0.05) and 90% (P<0.05), resp ectively, in cells treated with T-3. Under non-insulin-stimulated condition s, plasma membrane fractions obtained from cells exposed to T-3, were enric hed with both GLUT1 (3.29 +/- 0.69 vs 1.20 +/- 0.29 arbitrary units (A.U.)/ 5 mu g protein, P<0.05) and GLUT4 (3.50 +/- 1.16 vs 0.82 +/- 0.28 A.U./5 mu g protein, P<0.03). The incubation of cells with insulin produced the tran slocation of both glucose transporters to plasma membranes, and again cells treated with T-3 presented a higher amount of GLUT1 and GLUT4 in the plasm a membrane fractions (P<0.05 and P<0.03 respectively). These data indicate that T-3 has a direct stimulatory effect on glucose transport in 3T3-L1 adi pocytes due to an increase in GLUT1 and GLUT4, and by favouring their parti tioning to plasma membranes. The effect of T-3 on glucose uptake induced by insulin can also be explained by the high expression of both glucose trans porters.