title: Insulin acutely regulates Munc18-c subcellular trafficking: altered response in insulin-resistant 3T3-L1 adipocytes.
abstract: Preincubation of 3T3-L1 adipocytes in high glucose or glucosamine decreases acute insulin (100 nm)-stimulated glucose transport provided that insulin (0.6 nm) is included during preincubation. GLUT4 expression is unchanged (Nelson, B. A., Robinson, K. A., and Buse, M. G. (2000) Diabetes 49, 981-991). Munc18-c, a Syntaxin 4-binding protein, is a proposed regulator of the docking/fusion of GLUT4-containing vesicles with the plasma membrane. We examined the subcellular distribution of Munc18-c in response to acute (15-min) insulin (100 nm) stimulation after preincubation in 5 or 25 mm glucose +/- 0.6 nm insulin. Immunoblotting detected Munc18-c mainly in the Triton X-100-soluble plasma membrane (TS-PM) and the Triton X-100-insoluble low density microsomal (TI-LDM) fraction. Under each condition except high glucose + insulin preincubation, acute insulin increased Munc18-c (50-200%) in TS-PM and decreased Munc18-c (60%) in TI-LDM. Munc18-c traffic was time-dependent with a lag time of 3 min compared with GLUT4. Preincubation with high glucose + 0.6 nm insulin significantly impaired acute insulin-stimulated Munc18-c trafficking and decreased basal Munc18-c in the TI-LDM. Preincubation with glucosamine + insulin had similar effects. Total cellular Munc18-c remained unchanged. In conclusion, acute insulin stimulation promotes the translocation of Munc18-c, apparently from a TI-LDM-associated compartment to the TS-PM. Chronically increased glucose flux or exposure to glucosamine disrupts this process, which may negatively impact the fusion of GLUT4-containing vesicles with the plasma membrane.
authors.affiliation: Department of Medicine, Division of Endocrinology, Diabetes and Medical Genetics, Medical University of South Carolina, Charleston, South Carolina 29425, USA.
authors.forename: Bryce A
authors.initials: BA
authors.lastname: Nelson
authors.forename: Katherine A
authors.initials: KA
authors.lastname: Robinson
authors.forename: Maria G
authors.initials: MG
authors.lastname: Buse
mesh_terms: D016475:3T3 Cells; D017667:Adipocytes / Q000378:metabolism* / Q000648:ultrastructure; D000818:Animals; D007328:Insulin / Q000502:physiology*; D007333:Insulin Resistance; D051379:Mice; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D021381:Protein Transport / Q000502:physiology*; D011506:Proteins / Q000378:metabolism*; D013347:Subcellular Fractions / Q000378:metabolism*; D033921:Vesicular Transport Proteins
chemical_list: D007328:Insulin; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; D011506:Proteins; D033921:Vesicular Transport Proteins