title: Improved insulin sensitivity by rapamycin is associated with reduction of mTOR and S6K1 activities in L6 myotubes.
abstract: This study was designed to evaluate the role of mammalian target of rapamycin (mTOR)/p70S61 kinase (S6K1) pathways in ER stress-induced insulin resistance in L6 myotubes. Pretreatment with 5μg/ml of tunicamycin or 600nM thapsigargin for 3h decreased insulin-mediated tyrosine phosphorylation of IRS-1 and glucose uptake, and increased the level of mTOR/S6K1 phosphorylation in L6 myotubes. However, the inhibition of mTOR activity by rapamycin (inhibitor of several intracellular pathways including S6K1 pathways) reversed the ER stress-reduced tyrosine phosphorylation of IRS-1 and glucose uptake. Furthermore, pretreatment of cells with rapamycin decreased ER stress-induced phosphorylation of mTOR and S6K1. Interestingly, inhibition of mTOR by rapamycin did not affect ER stress markers such as PERK and JNK activity under the ER stress condition. Similar results were obtained with or without pretreatment with tunicamycin in the absence or presence of S6K1 RNAi. Moreover, S6K1 RNAi-mediated knockdown preserved insulin-stimulated Akt phosphorylation and glucose uptake in ER-stressed L6 myotubes, which was blocked by the phosphatidylinositol 3-kinase inhibitor wortmannin. Taken together, these results suggest that rapamycin improved ER stress-induced insulin resistance via inhibition of mTOR/S6K1 hyperphosphorylation in L6 myotubes.
authors.affiliation: College of Pharmacy, Yeungnam University, Gyeongsan 712-749, Republic of Korea.
authors.forename: Seung-Lark
authors.initials: SL
authors.lastname: Hwang
authors.forename: Xian
authors.initials: X
authors.lastname: Li
authors.forename: Jai-Youl
authors.initials: JY
authors.lastname: Lee
authors.forename: Hyeun Wook
authors.initials: HW
authors.lastname: Chang
mesh_terms: D000818:Animals; D002460:Cell Line; D059865:Endoplasmic Reticulum Stress / Q000187:drug effects; D055785:Gene Knockdown Techniques; D005947:Glucose / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins / Q000378:metabolism; D007333:Insulin Resistance; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000201:enzymology; D010766:Phosphorylation / Q000187:drug effects; D051381:Rats; D019893:Ribosomal Protein S6 Kinases / Q000037:antagonists & inhibitors* / Q000235:genetics; D020123:Sirolimus / Q000494:pharmacology*; D058570:TOR Serine-Threonine Kinases / Q000037:antagonists & inhibitors*; D014443:Tyrosine / Q000378:metabolism
chemical_list: D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; C526296:Irs1 protein, rat; D014443:Tyrosine; D019893:Ribosomal Protein S6 Kinases; C526901:Rps6kb1 protein, rat; D058570:TOR Serine-Threonine Kinases; D005947:Glucose; D020123:Sirolimus