Functional studies of Akt isoform specificity in skeletal muscle in vivo; maintained insulin sensitivity despite reduced insulin receptor substrate-1 expression.
The phosphoinositide 3-kinase/Akt pathway is thought to be essential for normal insulin action and glucose metabolism in skeletal muscle and has been shown to be dysregulated in insulin resistance. However, the specific roles of and signaling pathways triggered by Akt isoforms have not been fully assessed in muscle in vivo. We overexpressed constitutively active (ca-) Akt-1 or Akt-2 constructs in muscle using in vivo electrotransfer and, after 1 wk, assessed the roles of each isoform on glucose metabolism and fiber growth. We achieved greater than 2.5-fold increases in total Ser473 phosphorylation in muscles expressing ca-Akt-1 and ca-Akt-2, respectively. Both isoforms caused hypertrophy of muscle fibers, consistent with increases in p70S6kinase phosphorylation, and a 60% increase in glycogen accumulation, although only Akt-1 increased glycogen synthase kinase-3beta phosphorylation. Akt-2, but not Akt-1, increased basal glucose uptake (by 33%, P = 0.004) and incorporation into glycogen and lipids, suggesting a specific effect on glucose transport. Consistent with this, short hairpin RNA-mediated silencing of Akt-2 caused reductions in glycogen storage and glucose uptake. Consistent with Akt-mediated insulin receptor substrate 1 (IRS-1) degradation, we observed approximately 30% reductions in IRS-1 protein in muscle overexpressing ca-Akt-1 or ca-Akt-2. Despite this, we observed no decrease in insulin-stimulated glucose uptake. Furthermore, a 68% reduction in IRS-1 levels induced using short hairpin RNAs targeting IRS-1 also did not affect glucose disposal after a glucose load. These data indicate distinct roles for Akt-1 and Akt-2 in muscle glucose metabolism and that moderate reductions in IRS-1 expression do not result in the development of insulin resistance in skeletal muscle in vivo.
Cleasby
Mark E
ME
Diabetes and Obesity Program, Garvan Institute of Medical Research, Darlinghurst, New South Wales 2010, Australia. M.Cleasby@garvan.org.au
Reinten
Tracie A
TA
Cooney
Gregory J
GJ
James
David E
DE
Kraegen
Edward W
EW
D000818:Animals; D003847:Deoxyglucose / Q000737:chemistry; D015971:Gene Expression Regulation, Enzymologic; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D007328:Insulin / Q000378:metabolism*; D055504:Insulin Receptor Substrate Proteins; D051379:Mice; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000378:metabolism*; D010750:Phosphoproteins / Q000096:biosynthesis*; D010766:Phosphorylation; D020033:Protein Isoforms; D051057:Proto-Oncogene Proteins c-akt / Q000737:chemistry / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D038762:Ribosomal Protein S6 Kinases, 70-kDa / Q000378:metabolism
D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; C526295:Irs1 protein, mouse; C526296:Irs1 protein, rat; D010750:Phosphoproteins; D020033:Protein Isoforms; D003847:Deoxyglucose; D000071679:Glycogen Synthase Kinase 3 beta; C000605507:Gsk3b protein, mouse; C000605508:Gsk3b protein, rat; D051057:Proto-Oncogene Proteins c-akt; D038762:Ribosomal Protein S6 Kinases, 70-kDa; D038362:Glycogen Synthase Kinase 3