Decreases in insulin-responsive glucose transport and associated levels of
cell surface GLUT4 occur in rat adipocytes maintained in culture for 20 h u
nder hyperinsulinaemic and hyperglycaemic conditions. We have investigated
whether this defect is due to reduced signalling from the insulin receptor,
GLUT4 expression or impaired GLUT4 trafficking. The effects of chronic ins
ulin treatment on glucose transport and GLUT4 trafficking were ameliorated
by inclusion of metformin in the culture medium. In comparison with the acu
te effects of insulin, chronic insulin treatment attenuated changes in sign
alling processes leading to glucose transport. These included insulin recep
tor tyrosine phosphorylation, phosphoinositide 3-kinase activity and Akt ac
tivity, which were all reduced by 60-70%. Inclusion of metformin in the cul
ture medium prevented the effects of the chronic insulin treatment on these
signalling processes. In comparison with cells maintained in culture witho
ut insulin, the total expression of GLUT4 protein was not significantly alt
ered by chronic insulin treatment, although the level of GLUT1 expression w
as increased Trafficking rate constants for wortmannin-induced cell-surface
loss of GLUT4 and GLUT1 were assessed by 2-N-4-(1-azi-2,2,2-trifluoroethyl
)benzoyl-1,3-bis(D-mannose-4-yloxy)-2-propylamine (ATB-BMPA) photolabelling
. In comparison with cells acutely treated with insulin, chronic insulin tr
eatment resulted in a doubling of the rate constants for GLUT4 endocytosis.
These results suggest that the GLUT4 endocytosis process is very sensitive
to the perturbations in signalling that occur under hyperinsulinaemic and
hyperglycaemic conditions, and that the resulting elevation of endocytosis
accounts for the reduced levels of net GLUT4 translocation observed.