title: AMPK enhances insulin-stimulated GLUT4 regulation via lowering membrane cholesterol.
abstract: AMP-activated protein kinase (AMPK) enhances glucose transporter GLUT4 regulation. AMPK also suppresses energy-consuming pathways such as cholesterol synthesis. Interestingly, recent in vitro and in vivo data suggest that excess membrane cholesterol impairs GLUT4 regulation. Therefore, this study tested whether a beneficial, GLUT4-regulatory aspect of AMPK stimulation involved cholesterol lowering. Using L6 myotubes stably expressing an exofacial myc-epitope-tagged-GLUT4, AMPK stimulation by 5-aminoimidazole-4-carboxamide-1-β-d-ribonucleoside (AICAR; 45 min, 1 mm) or 2,4-dinitrophenol (DNP; 30 min, 200 μm) increased cell surface GLUT4myc labeling by approximately ≈ 25% (P < 0.05). Insulin (20 min, 100 nm) also increased GLUT4myc labeling by about 50% (P < 0.05), which was further enhanced (≈ 25%, P < 0.05) by AICAR or DNP. Consistent with AMPK-mediated suppression of cholesterol synthesis, AICAR and DNP decreased membrane cholesterol by 20-25% (P < 0.05). Whereas AMPK knockdown prevented the enhanced basal and insulin-stimulated GLUT4myc labeling by AICAR and DNP, cholesterol replenishment only blocked the AMPK-associated enhancement in insulin action. Cells cultured in a hyperinsulinemic milieu, resembling conditions in vivo that promote the progression/worsening of insulin resistance, displayed an increase in membrane cholesterol. This occurred concomitantly with a loss of cortical filamentous actin (F-actin) and defects in GLUT4 regulation by insulin. These derangements were prevented by AMPK stimulation. Examination of skeletal muscle from insulin-resistant Zucker rats revealed a similar elevation in membrane cholesterol and loss of F-actin. Lowering cholesterol to control levels restored F-actin structure and insulin sensitivity. In conclusion, these data suggest a novel aspect of GLUT4 regulation by AMPK involves membrane cholesterol lowering. Moreover, this AMPK-mediated process protected against hyperinsulinemia-induced insulin resistance.
authors.affiliation: Department of Cellular & Integrative Physiology, Indiana University School of Medicine, Indianapolis, Indiana 46202, USA.
authors.forename: Kirk M
authors.initials: KM
authors.lastname: Habegger
authors.forename: Nolan J
authors.initials: NJ
authors.lastname: Hoffman
authors.forename: Colin M
authors.initials: CM
authors.lastname: Ridenour
authors.forename: Joseph T
authors.initials: JT
authors.lastname: Brozinick
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Elmendorf
mesh_terms: D055372:AMP-Activated Protein Kinases / Q000378:metabolism*; D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism*; D002784:Cholesterol / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D051275:Glucose Transporter Type 4 / Q000378:metabolism*; D007004:Hypoglycemic Agents / Q000494:pharmacology*; D007328:Insulin / Q000494:pharmacology*; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011924:Rats, Zucker; D012265:Ribonucleotides / Q000494:pharmacology
chemical_list: D051275:Glucose Transporter Type 4; D007004:Hypoglycemic Agents; D007328:Insulin; D012265:Ribonucleotides; D000620:Aminoimidazole Carboxamide; D002784:Cholesterol; D055372:AMP-Activated Protein Kinases; C031143:AICA ribonucleotide; D005947:Glucose