title: Akt substrate TBC1D1 regulates GLUT1 expression through the mTOR pathway in 3T3-L1 adipocytes.
abstract: Multiple studies have suggested that the protein kinase Akt/PKB (protein kinase B) is required for insulin-stimulated glucose transport in skeletal muscle and adipose cells. In an attempt to understand links between Akt activation and glucose transport regulation, we applied mass spectrometry-based proteomics and bioinformatics approaches to identify potential Akt substrates containing the phospho-Akt substrate motif RXRXXpS/T. The present study describes the identification of the Rab GAP (GTPase-activating protein)-domain containing protein TBC1D1 [TBC (Tre-2/Bub2/Cdc16) domain family, member 1], which is closely related to TBC1D4 [TBC domain family, member 4, also denoted AS160 (Akt substrate of 160 kDa)], as an Akt substrate that is phosphorylated at Thr(590). RNAi (RNA interference)-mediated silencing of TBC1D1 elevated basal deoxyglucose uptake by approx. 61% in 3T3-L1 mouse embryo adipocytes, while the suppression of TBC1D4 and RapGAP220 under the same conditions had little effect on basal and insulin-stimulated deoxyglucose uptake. Silencing of TBC1D1 strongly increased expression of the GLUT1 glucose transporter but not GLUT4 in cultured adipocytes, whereas the decrease in TBC1D4 had no effect. Remarkably, loss of TBC1D1 in 3T3-L1 adipocytes activated the mTOR (mammalian target of rapamycin)-p70 S6 protein kinase pathway, and the increase in GLUT1 expression in the cells treated with TBC1D1 siRNA (small interfering RNA) was blocked by the mTOR inhibitor rapamycin. Furthermore, overexpression of the mutant TBC1D1-T590A, lacking the putative Akt/PKB phosphorylation site, inhibited insulin stimulation of p70 S6 kinase phosphorylation at Thr(389), a phosphorylation induced by mTOR. Taken together, our data suggest that TBC1D1 may be involved in controlling GLUT1 glucose transporter expression through the mTOR-p70 S6 kinase pathway.
authors.affiliation: Programme in Molecular Medicine, University of Massachusetts Medical School, 373 Plantation Street, Worcester, MA 01605, USA.
authors.forename: Qiong L
authors.initials: QL
authors.lastname: Zhou
authors.forename: Zhen Y
authors.initials: ZY
authors.lastname: Jiang
authors.forename: John
authors.initials: J
authors.lastname: Holik
authors.forename: Anil
authors.initials: A
authors.lastname: Chawla
authors.forename: G Nana
authors.initials: GN
authors.lastname: Hagan
authors.forename: John
authors.initials: J
authors.lastname: Leszyk
authors.forename: Michael P
authors.initials: MP
authors.lastname: Czech
mesh_terms: D041721:3T3-L1 Cells; D017667:Adipocytes; D000818:Animals; D000906:Antibodies / Q000276:immunology; D006224:Cricetinae; D020690:GTPase-Activating Proteins / Q000378:metabolism; D005786:Gene Expression Regulation; D051272:Glucose Transporter Type 1 / Q000235:genetics / Q000378:metabolism*; D007328:Insulin / Q000494:pharmacology; D051379:Mice; D009154:Mutation / Q000235:genetics; D009687:Nuclear Proteins / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D010766:Phosphorylation / Q000187:drug effects; D011494:Protein Kinases / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D013379:Substrate Specificity; D058570:TOR Serine-Threonine Kinases
chemical_list: D000906:Antibodies; D020690:GTPase-Activating Proteins; D051272:Glucose Transporter Type 1; D007328:Insulin; D009687:Nuclear Proteins; C095906:Tbc1d1 protein, mouse; D011494:Protein Kinases; C546843:mTOR protein, mouse; D051057:Proto-Oncogene Proteins c-akt; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D058570:TOR Serine-Threonine Kinases