M. Lachaal et al., BREFELDIN-A INHIBITS INSULIN-INDUCED GLUCOSE-TRANSPORT STIMULATION AND GLUT4 RECRUITMENT IN RAT ADIPOCYTES, The Journal of biological chemistry, 269(38), 1994, pp. 23689-23693
GLUT4, the major insulin-responsive glucose transporter isoform in rat
adipocytes, rapidly recycles between an intracellular pool and the pl
asma membrane in the basal and insulin-stimulated states. To gain insi
ght into the route of this GLUT4 recycling, we studied the effects of
brefeldin A (BFA) on glucose transport and glucose transporter subcell
ular distribution in rat adipocytes in the absence and in the presence
of insulin. 3-O-Methyl-D-glucose equilibrium exchange measurements re
vealed that BFA inhibits insulin-stimulated glucose transport by as mu
ch as 80%, whereas the inactive BFA analog, B36, was without effect. T
he inhibition was reversible and was a saturable function of BFA conce
ntration with an apparent K-i of less than 1 mu M. In the absence of i
nsulin, on the other hand, BFA caused a slight (up to 2-fold) increase
in glucose transport. Subcellular fractionation and semiquantitative
immunoblotting analysis revealed that BFA inhibits insulin-induced red
istribution of GLUT4 from microsomes to the plasma membranes, with a d
ose dependence similar to that for glucose transport inhibition. BFA a
lso caused a slight increase in the plasma membrane GLUT4 level in the
absence of insulin. BFA did not affect the subcellular distribution o
f GLUT1 in these experiments. These findings strongly suggest that GLU
T4 recycling in rat adipocytes involves a BFA-sensitive, coat protein-
mediated, membrane budding step, which is distinct between the constit
utive and the insulin induced pathways.