Two glucose transporter isoforms are sorted differentially and are expressed in distinct cellular compartments.
Rat GLUT4 (adipocyte/muscle-type glucose transporter) was expressed in two fibroblastic cell lines, Chinese hamster ovary (CHO) cells and 3T3-L1 fibroblasts, under the control of the methallothionein I promoter. Although immunoblotting with a GLUT4-specific anti-peptide antibody demonstrated that the amount of GLUT4 expressed was comparable with that in 3T3-L1 adipocytes and rat adipose tissues, no increase in 2-deoxy-D-glucose uptake was observed in the basal state in fibroblasts. Immunocytochemical studies showed that the expressed GLUT4 appeared to be localized in a specific region in the cytoplasm. These results were in marked contrast to those obtained in CHO cells expressing GLUT1 (HepG2/erythrocyte-type glucose transporter) using the same expression vector. In this case the expressed GLUT1 protein appeared to reside mainly on the plasma membranes, and a significant increase in glucose uptake was observed. Although insulin increased glucose uptake in CHO cells and 3T3-L1 fibroblasts as well as in the cells expressing rat GLUT4, an increment due to insulin above basal values was small, at most 2-fold, and no significant differences were observed in insulin-stimulated glucose uptake between transfected and parental cells. In addition, no apparent differences in the subcellular distribution of expressed GLUT4 were observed between the insulin-stimulated and the basal state. These results indicate that in fibroblastic cell lines GLUT1 and GLUT4 proteins are sorted in a different fashion, and the expression of GLUT4 protein per se is not enough to produce a large insulin-induced increase in glucose transport activity such as that observed in rat adipocytes and 3T3-L1 adipocytes. Thus unidentified aspects of the cellular environment which are present in the adipocytes but not in fibroblastic cell lines may be required for a large insulin-induced increase in glucose transport activity to be observed.
Shibasaki
Y
Y
Third Department of Internal Medicine, Faculty of Medicine, University of Tokyo, Japan.
Asano
T
T
Lin
J L
JL
Tsukuda
K
K
Katagiri
H
H
Ishihara
H
H
Yazaki
Y
Y
Oka
Y
Y
D000273:Adipose Tissue / Q000378:metabolism; D000345:Affinity Labels; D000818:Animals; D001386:Azides; D015153:Blotting, Western; D016466:CHO Cells; D002460:Cell Line; D003001:Cloning, Molecular; D006224:Cricetinae; D004247:DNA / Q000235:genetics; D003847:Deoxyglucose / Q000378:metabolism; D004187:Disaccharides; D005347:Fibroblasts / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D015870:Gene Expression; D006027:Glycosides; D008668:Metallothionein / Q000235:genetics; D009004:Monosaccharide Transport Proteins / Q000235:genetics*; D011401:Promoter Regions, Genetic; D011437:Propylamines; D051381:Rats; D014162:Transfection
D000345:Affinity Labels; D001386:Azides; D004187:Disaccharides; D006027:Glycosides; D009004:Monosaccharide Transport Proteins; D011437:Propylamines; C065412:2-N-(4-(1-azitrifluoroethyl)benzoyl)-1,3-bis-(mannos-4-yloxy)-2-propylamine; D004247:DNA; D008668:Metallothionein; D003847:Deoxyglucose