Demonstration of insulin-responsive trafficking of GLUT4 and vpTR in fibroblasts.
Insulin-responsive trafficking of the GLUT4 glucose transporter and the insulin-regulated aminopeptidase (IRAP) in adipose and muscle cells is well established. Insulin regulation of GLUT4 trafficking in these cells underlies the role that adipose tissue and muscle play in the maintenance of whole body glucose homeostasis. GLUT4 is expressed in a very limited number of tissues, most highly in adipose and muscle, while IRAP is expressed in many tissues. IRAP's physiological role in any of the tissues in which it is expressed, however, is unknown. The fact that IRAP, which traffics by the same insulin-regulated pathway as GLUT4, is expressed in 'non-insulin responsive' tissues raises the question of whether these other cell types also have a specialized insulin-regulated trafficking pathway. The existence of an insulin-responsive pathway in other cell types would allow regulation of IRAP activity at the plasma membrane as a potentially important physiological function of insulin. To address this question we use reporter molecules for both GLUT4 and IRAP trafficking to measure insulin-stimulated translocation in undifferentiated cells by quantitative fluorescence microscopy. One reporter (vpTR), a chimera between the intracellular domain of IRAP and the extracellular and transmembrane domains of the transferrin receptor, has been previously characterized. The other is a GLUT4 construct with an exofacial HA epitope and a C-terminal GFP. By comparing these reporters to the transferrin receptor, a marker for general endocytic trafficking, we demonstrate the existence of a specialized, insulin-regulated trafficking pathway in two undifferentiated cell types, neither of which normally express GLUT4. The magnitude of translocation in these undifferentiated cells (approximately threefold) is similar to that reported for the translocation of GLUT4 in muscle cells. Thus, undifferentiated cells have the necessary retention and translocation machinery for an insulin response that is large enough to be physiologically important.
Lampson
M A
MA
Program in Physiology, Biophysics and Molecular Medicine and Department of Biochemistry, Weill Graduate School of Medical Sciences Cornell University, New York, NY 10021, USA.
Racz
A
A
Cushman
S W
SW
McGraw
T E
TE
D016475:3T3 Cells; D000818:Animals; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D006224:Cricetinae; D051275:Glucose Transporter Type 4; D049452:Green Fluorescent Proteins; D006801:Humans; D053590:Interleukin 1 Receptor Antagonist Protein; D008164:Luminescent Proteins / Q000032:analysis / Q000737:chemistry / Q000235:genetics; D051379:Mice; D009004:Monosaccharide Transport Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D009124:Muscle Proteins; D017433:Protein Structure, Secondary; D011990:Receptors, Transferrin / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000378:metabolism; D012795:Sialoglycoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D014168:Transferrin / Q000378:metabolism
D051275:Glucose Transporter Type 4; C507763:IL1RN protein, human; C507764:Il1rn protein, mouse; D053590:Interleukin 1 Receptor Antagonist Protein; D008164:Luminescent Proteins; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D011990:Receptors, Transferrin; D011993:Recombinant Fusion Proteins; C495692:SLC2A4 protein, human; D012795:Sialoglycoproteins; C495693:Slc2a4 protein, mouse; D014168:Transferrin; D049452:Green Fluorescent Proteins