title: Fat-induced membrane cholesterol accrual provokes cortical filamentous actin destabilisation and glucose transport dysfunction in skeletal muscle.
abstract: OBJECTIVE
  Diminished cortical filamentous actin (F-actin) has been implicated in skeletal muscle insulin resistance, yet the mechanism(s) is unknown. Here we tested the hypothesis that changes in membrane cholesterol could be a causative factor, as organised F-actin structure emanates from cholesterol-enriched raft microdomains at the plasma membrane.
  
  
  METHODS
  Skeletal muscle samples from high-fat-fed animals and insulin-sensitive and insulin-resistant human participants were evaluated. The study also used L6 myotubes to directly determine the impact of fatty acids (FAs) on membrane/cytoskeletal variables and insulin action.
  
  
  RESULTS
  High-fat-fed insulin-resistant animals displayed elevated levels of membrane cholesterol and reduced F-actin structure compared with normal chow-fed animals. Moreover, human muscle biopsies revealed an inverse correlation between membrane cholesterol and whole-body glucose disposal. Palmitate-induced insulin-resistant myotubes displayed membrane cholesterol accrual and F-actin loss. Cholesterol lowering protected against the palmitate-induced defects, whereas characteristically measured defects in insulin signalling were not corrected. Conversely, cholesterol loading of L6 myotube membranes provoked a palmitate-like cytoskeletal/GLUT4 derangement. Mechanistically, we observed a palmitate-induced increase in O-linked glycosylation, an end-product of the hexosamine biosynthesis pathway (HBP). Consistent with HBP activity affecting the transcription of various genes, we observed an increase in Hmgcr, a gene that encodes 3-hydroxy-3-methyl-glutaryl coenzyme A reductase, the rate-limiting enzyme in cholesterol synthesis. In line with increased HBP activity transcriptionally provoking a membrane cholesterol-based insulin-resistant state, HBP inhibition attenuated Hmgcr expression and prevented membrane cholesterol accrual, F-actin loss and GLUT4/glucose transport dysfunction.
  
  
  CONCLUSIONS
  Our results suggest a novel cholesterolgenic-based mechanism of FA-induced membrane/cytoskeletal disorder and insulin resistance.
authors: Department of Cellular and Integrative Physiology, Indiana University School of Medicine, VanNuys Medical Science Building Rm 308A, 635 Barnhill Drive, Indianapolis, IN 46202-5120, USA.
authors: K M
authors: KM
authors: Habegger
authors: B A
authors: BA
authors: Penque
authors: W
authors: W
authors: Sealls
authors: L
authors: L
authors: Tackett
authors: L N
authors: LN
authors: Bell
authors: E K
authors: EK
authors: Blue
authors: P J
authors: PJ
authors: Gallagher
authors: M
authors: M
authors: Sturek
authors: M A
authors: MA
authors: Alloosh
authors: H O
authors: HO
authors: Steinberg
authors: R V
authors: RV
authors: Considine
authors: J S
authors: JS
authors: Elmendorf
mesh_terms: D000199:Actins / Q000378:metabolism*; D000328:Adult; D000818:Animals; D001692:Biological Transport; D001707:Biopsy, Needle / Q000379:methods; D002462:Cell Membrane / Q000378:metabolism; D002784:Cholesterol / Q000378:metabolism*; D003599:Cytoskeleton / Q000378:metabolism; D005227:Fatty Acids / Q000378:metabolism; D005260:Female; D005947:Glucose / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000378:metabolism; D008297:Male; D021962:Membrane Microdomains / Q000378:metabolism; D051379:Mice; D008810:Mice, Inbred C57BL; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000378:metabolism; D019308:Palmitic Acid / Q000378:metabolism; D051381:Rats
chemical_list: D000199:Actins; D005227:Fatty Acids; D007328:Insulin; D019308:Palmitic Acid; D002784:Cholesterol; D005947:Glucose