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1 Programme in Cell Biology,
Insulin stimulates glucose uptake into muscle
and fat cells via recruitment of the glucose transporter 4 (GLUT-4)
from intracellular store(s) to the cell surface. Robust stimulation of
glucose uptake by insulin coincides with the expression of GLUT-4
during differentiation of muscle and fat cells, but it is not known if
GLUT-4 expression suffices to confer insulin sensitivity to glucose
uptake. We have therefore examined the effect of expression of a myc
epitope-tagged GLUT-4 (GLUT-4myc) into L6 myoblasts, which do not
express endogenous GLUT-4 until differentiated into myotubes. Ectopic
expression of GLUT-4myc markedly improved insulin sensitivity of
glucose uptake in L6 myoblasts. The GLUT-4myc protein distributed
equally to the cell surface and intracellular compartments in
myoblasts, and the intracellular fraction of GLUT-4myc further
increased in myotubes. In myoblasts, the intracellular GLUT-4myc
compartment contained the majority of the insulin-regulatable amino
peptidase (IRAP) but less than half of the GLUT-1, suggesting
segregation of GLUT-4myc and IRAP to a specific cellular locus. Insulin
stimulation of phosphatidylinositol 3-kinase and protein kinase B-
activities was similar for L6-GLUT-4myc myoblasts and myotubes. At both
stages, GLUT-4myc responded to insulin by translocating to the cell
surface. These results suggest that GLUT-4myc segregates into a
specific compartment in L6 myoblasts and confers insulin sensitivity to these cells. L6-GLUT-4myc myoblasts, which are easily transfectable with various constructs, are a useful resource to study insulin action.
GLUT-4 translocation; GLUT-4 vesicles; insulin action; muscle cells
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