title: Regulation of skeletal muscle insulin-stimulated signaling through the MEK-REDD1-mTOR axis.
abstract: Recent findings in adipocytes suggest that mitogen-activated protein kinase (MAPK)/extracellular-regulated signaling kinase (ERK) kinase 1/2 (MEK1/2) signaling regulates regulated in development and DNA damage 1 (REDD1) protein expression. Similarly, our previous work show that a lack of REDD1 protein expression, and associated hyperactive basal mechanistic target of rapamycin (mTOR) signaling, limits skeletal muscle's response to insulin. Therefore, we sought to determine: 1) if MEK1/2 inhibition is sufficient to reduce REDD1 protein expression and subsequently insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation via negative feedback of hyperactive mTOR in REDD1 wild-type (WT) mice and 2) if rapamycin-mediated mTOR inhibition is sufficient to improve IRS-1 tyrosine phosphorylation in REDD1 knockout (KO) mice. REDD1 WT mice were injected with 10 mg/kg BW of the MEK1/2 non-competitive inhibitor, PD184352, 3 h prior to acute insulin treatment. In separate studies, REDD1 KO mice were injected with 5 mg/kg BW of the mTOR inhibitor, rapamycin, 3 h prior to acute insulin treatment. Following the inhibitor treatment period, markers of insulin signaling activation (IRS-1 Y1222, MEK1/2 S217/221, ERK1/2 T202/Y204), REDD1, and mTOR signaling activation (S6K1 T389, rpS6 S240/244) were examined in skeletal muscle collected before and after a 10 min insulin treatment. PD184352 treatment reduced MEK/ERK phosphorylation and REDD1 protein expression, independent of insulin. This reduction in REDD1 protein expression was associated with elevated basal S6K1 and rpS6 phosphorylation and reduced insulin stimulated IRS-1 phosphorylation. Conversely, rapamycin inhibited S6K1 and rpS6 activation, and significantly improved insulin -stimulated activation of IRS-1 and MEK1/2 in KO mice. These data support that REDD1 is required for normal insulin-stimulated signaling, and that a subtle balance exists between MEK1/2, REDD1, and mTOR for the proper regulation of insulin signaling.
authors.affiliation: Kinesiology Program, School of Behavioral Sciences and Education, Penn State Harrisburg, Middletown, PA, United States.
authors.forename: Cory M
authors.initials: CM
authors.lastname: Dungan
authors.affiliation: Kinesiology Program, School of Behavioral Sciences and Education, Penn State Harrisburg, Middletown, PA, United States. Electronic address: dlw125@psu.edu.
authors.forename: David L
authors.initials: DL
authors.lastname: Williamson
mesh_terms: D000818:Animals; D001549:Benzamides / Q000737:chemistry; D007328:Insulin / Q000378:metabolism*; D048369:MAP Kinase Kinase 1 / Q000378:metabolism*; D048370:MAP Kinase Kinase 2 / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D018482:Muscle, Skeletal / Q000378:metabolism*; D010766:Phosphorylation; D015398:Signal Transduction; D020123:Sirolimus / Q000737:chemistry; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*
chemical_list: C120227:2-(2-chloro-4-iodophenylamino)-N-cyclopropylmethoxy-3,4-difluorobenzamide; D001549:Benzamides; C469992:Ddit4 protein, mouse; D007328:Insulin; D014157:Transcription Factors; C546843:mTOR protein, mouse; D058570:TOR Serine-Threonine Kinases; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2; C482127:Map2k1 protein, mouse; C482129:Map2k2 protein, mouse; D020123:Sirolimus
keywords: Insulin signaling; PD184352; REDD1; Rapamycin; Skeletal muscle