EFFECTS OF NORADRENALINE ON THE CELL-SURFACE GLUCOSE TRANSPORTERS IN CULTURED BROWN ADIPOCYTES - NOVEL MECHANISM FOR SELECTIVE ACTIVATION OF GLUT1 GLUCOSE TRANSPORTERS
Y. Shimizu et al., EFFECTS OF NORADRENALINE ON THE CELL-SURFACE GLUCOSE TRANSPORTERS IN CULTURED BROWN ADIPOCYTES - NOVEL MECHANISM FOR SELECTIVE ACTIVATION OF GLUT1 GLUCOSE TRANSPORTERS, Biochemical journal, 330, 1998, pp. 397-403
Glucose transport into rat brown adipocytes has been shown to be stimu
lated directly by the sympathetic neurotransmitter, noradrenaline, wit
hout a significant increase in the protein content of either GLUT1 or
GLUT4 glucose transporter in the plasma membrane [Shimizu, Kielar, Min
okoshi and Shimazu (1996) Biochem. J. 314, 485-490]. In the present st
udy, we labelled the exofacial glucose-binding sites of GLUT1 and GLUT
4 with a membrane-impermeant photoaffinity reagent, 2-N-[4-(1 l]-[2-H-
3]1,3-bis-(D-mannos-4-yloxy)-2-propylamine (ATB-[H-3]BMPA), to determi
ne which isoform is responsible for the noradrenaline-induced increase
in glucose transport into intact brown adipocytes in culture. Insulin
stimulated the rate of hexose transport by increasing ATB-[H-3]BMPA-l
abelled cell-surface GLUT4. In contrast, the noradrenaline-induced inc
rease in glucose transport was not accompanied by an increased ATB-[H-
3]BMPA labelling of GLUT4, nor with an increased amount of GLUT4 in th
e plasma membrane fraction as assessed by Western blotting, indicating
that noradrenaline does not promote the translocation of GLUT4. Howev
er, noradrenaline induced an increase in photoaffinity labelling of ce
ll-surface GLUT1 without an apparent increase in the immunoreactive GL
UT1 protein in the plasma membrane. This is suggestive of an increased
affinity of GLUT1 for the ligand. In fact, the K-i value of non-radio
active ATB-BMPA for 2-deoxy-D-glucose uptake was significantly decreas
ed after treatment of the cells with noradrenaline. The increased phot
oaffinity labelling of GLUT1 and increased glucose transport caused by
noradrenaline were inhibited by a cAMP antagonist, cAMP-S Rp-isomer.
These results demonstrate that noradrenaline stimulates glucose transp
ort in brown adipocytes by enhancing the functional activity of GLUT1
through a cAMP-dependent mechanism.