Mechanisms for increased insulin-stimulated Akt phosphorylation and glucose uptake in fast- and slow-twitch skeletal muscles of calorie-restricted rats

Abstract

Sajan MP, Farese RV, Cartee GD. Mechanisms for increased insulin-stimulated Akt phosphorylation and glucose uptake in fastand slow-twitch skeletal muscles of calorie-restricted rats. Am J Physiol Endocrinol Metab 300: E966 -E978, 2011. First published March 8, 2011 doi:10.1152/ajpendo.00659.2010.-Calorie restriction [CR; ϳ65% of ad libitum (AL) intake] improves insulin-stimulated glucose uptake (GU) and Akt phosphorylation in skeletal muscle. We aimed to elucidate the effects of CR on 1) processes that regulate Akt phosphorylation [insulin receptor (IR) tyrosine phosphorylation, IR substrate 1-phosphatidylinositol 3-kinase (IRS-PI3K) activity, and Akt binding to regulatory proteins (heat shock protein 90, Appl1, protein phosphatase 2A)]; 2) Akt substrate of 160-kDa (AS160) phosphorylation on key phosphorylation sites; and 3) atypical PKC (aPKC) activity. Isolated epitrochlearis (fast-twitch) and soleus (slow-twitch) muscles from AL or CR (6 mo duration) 9-moold male F344BN rats were incubated with 0, 1.2, or 30 nM insulin and 2-deoxy-[ 3 H]glucose. Some CR effects were independent of insulin dose or muscle type: CR caused activation of Akt (Thr 308 and Ser 473 ) and GU in both muscles at both insulin doses without CR effects on IRS1-PI3K, Akt-PP2A, or Akt-Appl1. Several muscle-and insulin dose-specific CR effects were revealed. Akt-HSP90 binding was increased in the epitrochlearis; AS160 phosphorylation (Ser 588 and Thr 642 ) was greater for CR epitrochlearis at 1.2 nM insulin; and IR phosphorylation and aPKC activity were greater for CR in both muscles with 30 nM insulin. On the basis of these data, our working hypothesis for improved insulin-stimulated GU with CR is as follows:

Sirt1 enhances skeletal muscle insulin sensitivity in mice during caloric restriction

“…Taken together, these results identify Sirt1 as a key molecular convergence point that links reduced nutrient intake during CR to improvements in skeletal muscle insulin sensitivity. Enhanced insulin-stimulated glucose uptake after brief CR is due to greater Glut4 recruitment to the cell surface (7) and is paralleled by changes in insulin signaling, particularly at the level of PI3K-Akt (3-5, 8, 9), and As160, a key downstream target of Akt (9). Indeed, studies using the PI3K inhibitor, wortmannin (6), and mice with KO of Akt2 (3), reveal that PI3K and Akt2 are essential for the ability of brief CR to enhance skeletal muscle insulin sensitivity.…”

“…Indeed, studies using the PI3K inhibitor, wortmannin (6), and mice with KO of Akt2 (3), reveal that PI3K and Akt2 are essential for the ability of brief CR to enhance skeletal muscle insulin sensitivity. In contrast, CR does not appear to augment insulin action upstream of PI3K at the level of the IR or Irs1 in response to submaximal insulin (9) but has been shown to enhance IR-Irs1 activation in response to supraphysiological insulin stimulation (8,9,11,23), although this is not a universal finding (24,25). In addition, mice with KO of Irs1 exhibit the expected increase in skeletal muscle insulin sensitivity after CR (26).…”

“…Sirt1 may also mediate PI3K function during CR through binding to p85α (37) or insulin signaling via regulation of protein tyrosine phosphatase 1B expression (38). In addition, CR increases the association of heat shock protein 90 with Akt in skeletal muscle (9), which may facilitate Akt phosphorylation by reducing dephosphorylation of Akt by protein phosphatase 2A (39). Interestingly, lysine acetylation of the PH domain of Akt was recently shown to regulate Akt activity, and Sirt1 was shown to deacetylate Akt and enhance Akt activation (40).…”

“…Indeed, Akt2 is essential for the full effect of CR on skeletal muscle insulin sensitivity (3), and the phosphoinositide 3-kinase (PI3K) inhibitor, wortmannin, inhibits the ability of CR to increase skeletal muscle insulin sensitivity (6), which suggests that PI3K signaling contributes to improved insulin signaling after CR. However, most studies suggest that insulin-stimulated PI3K activity is not enhanced after CR (6,(9)(10)(11). Regardless, the molecular convergence point linking CR to enhanced insulin-stimulated PI3K-Akt signaling in skeletal muscle remains to be determined.…”

Improved insulin sensitivity with calorie restriction does not require reduced JNK1/2, p38, or ERK1/2 phosphorylation in skeletal muscle of 9-month-old rats

“…Although CR profoundly enhances the ability of insulin to activate Akt, CR does not uniformly enhance insulin-mediated activation of every protein that is regulated by insulin. For example, most (3,4,10,54), but not all (59) studies have reported little or no CR-related effect on various markers of insulin receptor activation by physiologic insulin concentrations. In this context, our fourth aim was to determine the influence of CR on insulin-mediated ERK phosphorylation in skeletal muscle.…”