GLUT4 facilitates insulin stimulation and cAMP-mediated inhibition of glucose transport.

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Abstract
The glucose transporter isoform GLUT4 is found only in cells that exhibit insulin-sensitive glucose transport. To investigate the function of this transporter, L6 myoblasts were stably transfected with GLUT4 cDNA. GLUT4 underwent insulin-dependent movement to the cell surface in myoblasts overexpressing the transporter. One cell line (243-6) expressed sufficient levels of the GLUT4 protein to study insulin-dependent glucose transport. Unlike wild-type L6 cells, 243-6 myoblasts exhibited two features that are characteristic of differentiated muscle fibers and adipocytes in vivo: a large insulin-stimulated component of glucose transport and inhibition of this stimulated component by cAMP. Relative to normal L6 cells, 243-6 cells responded to insulin or insulin-like growth factor 1 with a 5-fold larger increase in 2-deoxy[3H]glucose uptake. N6,O2'-Dibutyryladenosine 3',5'-cyclic monophosphate (Bt2cAMP) did not inhibit transport in normal L6 myoblasts, which express only GLUT1, but inhibited IGF-1/insulin-stimulated transport by 50% in 243-6 cells. The effect of cAMP was investigated further by using Chinese hamster ovary cells transiently expressing GLUT1 and GLUT4. Bt2cAMP inhibited glucose transport only in Chinese hamster ovary cells expressing GLUT4. These results indicate that cAMP-mediated inhibition of glucose transport is dependent on expression of the GLUT4 isozyme.