Separation of insulin signaling into distinct GLUT4 translocation and activation steps.
GLUT4 (glucose transporter 4) plays a pivotal role in insulin-induced glucose uptake to maintain normal blood glucose levels. Here, we report that a cell-permeable phosphoinositide-binding peptide induced GLUT4 translocation to the plasma membrane without inhibiting IRAP (insulin-responsive aminopeptidase) endocytosis. However, unlike insulin treatment, the peptide treatment did not increase glucose uptake in 3T3-L1 adipocytes, indicating that GLUT4 translocation and activation are separate events. GLUT4 activation can occur at the plasma membrane, since insulin was able to increase glucose uptake with a shorter time lag when inactive GLUT4 was first translocated to the plasma membrane by pretreating the cells with this peptide. Inhibition of phosphatidylinositol (PI) 3-kinase activity failed to inhibit GLUT4 translocation by the peptide but did inhibit glucose uptake when insulin was added following peptide treatment. Insulin, but not the peptide, stimulated GLUT1 translocation. Surprisingly, the peptide pretreatment inhibited insulin-induced GLUT1 translocation, suggesting that the peptide treatment has both a stimulatory effect on GLUT4 translocation and an inhibitory effect on insulin-induced GLUT1 translocation. These results suggest that GLUT4 requires translocation to the plasma membrane, as well as activation at the plasma membrane, to initiate glucose uptake, and both of these steps normally require PI 3-kinase activation.
Department of Physiology, Institute for Medicine and Engineering, University of Pennsylvania, 1080 Vagelos Research Laboratories, 3340 Smith Walk, Philadelphia, PA 19104, USA. funaki@mail.med.upenn.edu
Makoto
M
Funaki
Paramjeet
P
Randhawa
Paul A
PA
Janmey
D016475:3T3 Cells; D017667:Adipocytes / Q000166:cytology / Q000378:metabolism; D000626:Aminopeptidases / Q000378:metabolism; D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D010122:Cystinyl Aminopeptidase; D004705:Endocytosis / Q000502:physiology; D004789:Enzyme Activation; D005456:Fluorescent Dyes / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D051275:Glucose Transporter Type 4; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D051379:Mice; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D009124:Muscle Proteins; D010455:Peptides / Q000378:metabolism*; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D000081082:Phosphoinositide-3 Kinase Inhibitors; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D021381:Protein Transport / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt; D015398:Signal Transduction / Q000502:physiology*
D000626:Aminopeptidases; D010122:Cystinyl Aminopeptidase; D005456:Fluorescent Dyes; D005947:Glucose; D051275:Glucose Transporter Type 4; D007328:Insulin; D009004:Monosaccharide Transport Proteins; D009124:Muscle Proteins; D010455:Peptides; D019869:Phosphatidylinositol 3-Kinases; D000081082:Phosphoinositide-3 Kinase Inhibitors; D017346:Protein Serine-Threonine Kinases; D011518:Proto-Oncogene Proteins; D051057:Proto-Oncogene Proteins c-akt; C495692:SLC2A4 protein, human; C495693:Slc2a4 protein, mouse; C105964:leucyl-cystinyl aminopeptidase