GLUT4 Is Not Necessary for Overload-Induced Glucose Uptake or Hypertrophic Growth in Mouse Skeletal Muscle

Abstract

GLUT4 is necessary for acute insulin- and contraction-induced skeletal muscle glucose uptake, but its role in chronic muscle loading (overload)-induced glucose uptake is unknown. Our goal was to determine whether GLUT4 is required for overload-induced glucose uptake. Overload was induced in mouse plantaris muscle by unilateral synergist ablation. After 5 days, muscle weights and ex vivo [3H]-2-deoxy-d-glucose uptake were assessed. Overload-induced muscle glucose uptake and hypertrophic growth were not impaired in muscle-specific GLUT4 knockout mice, demonstrating that GLUT4 is not necessary for these processes. To assess which transporters mediate overload-induced glucose uptake, chemical inhibitors were used. The facilitative GLUT inhibitor cytochalasin B, but not the sodium-dependent glucose cotransport inhibitor phloridzin, prevented overload-induced uptake demonstrating that GLUTs mediate this effect. To assess which GLUT, hexose competition experiments were performed. Overload-induced [3H]-2-deoxy-d-glucose uptake was not inhibited by d-fructose, demonstrating that the fructose-transporting GLUT2, GLUT5, GLUT8, and GLUT12 do not mediate this effect. To assess additional GLUTs, immunoblots were performed. Overload increased GLUT1, GLUT3, GLUT6, and GLUT10 protein levels twofold to fivefold. Collectively, these results demonstrate that GLUT4 is not necessary for overload-induced muscle glucose uptake or hypertrophic growth and suggest that GLUT1, GLUT3, GLUT6, and/or GLUT10 mediate overload-induced glucose uptake.

Glucose transporters in adipose tissue, liver, and skeletal muscle in metabolic health and disease

“…[10] GLUT8 Mediates fructose-induced de novo lipogenesis [44]; overexpression linked to decreased PPARγ expression levels [43]; expression correlates with circulating insulin in diabetic mice [77]; involved in trehalose-induced autophagy [150] GLUT9 High-capacity uric acid (UA) transporter; hepatic inactivation of the gene in adult mice leads to severe hyperuricemia and hyperuricosuria [177] Muscle GLUT1 Contributes to basal glucose transport and fiber type-specific expression [106,146]; increased surface expression in metabolic stress [195,216]; increased overload-induced muscle glucose uptake or hypertrophic growth [153] GLUT4 Most abundant GLUT isoform, responsible for bulk of insulin-and contraction-stimulated glucose uptake [50,131,148];…”

“…insulin/contraction-regulated subcellular distribution between intracellular compartments and cell surface [38,58,67,229]; knockout mice display systemic insulin resistance and a mild diabetic phenotype [115]; overexpression improves insulin sensitivity [19,237]; upregulated in response to exercise [185]; abundance in diabetic skeletal muscle is mostly unchanged [174] GLUT10 Localized in mitochondria, involved in mitochondrial dehydroascorbic acid (DHA) transport, may protect from oxidative stress [126]; increased in overload-induced muscle glucose uptake or hypertrophic growth [153] GLUT12 May act as insulin-responsive glucose transporter similar to GLUT4 [225]; upregulated in humans after intensive exercise training [224] Adipose GLUT1 Contributes to basal glucose transport, undergoes recycling through internal membrane compartments [94]; abundance unaffected in type 2 diabetes [105] GLUT8 Expression increases markedly during fat cell differentiation [206]; recycles between endosomal compartments and cell surface, mostly intracellular, in mature adipocytes unresponsive to insulin [9,128] GLUT4 Most abundant GLUT isoform, responsible for bulk of insulin stimulated glucose uptake [104]; activity associated with activation of nuclear transcription factor carbohydrate-response element-binding protein (ChREBP), enhanced lipogenesis and production of branched fatty acid esters of hydroxy fatty acids (FAHFAs) and secretion of retinol binding protein 4 (RBP4) [91,160,261]; reduced abundance in type 2 diabetes [69,219] GLUT10 Mitochondrial DHA transport, may protect from oxidative stress [126] 2 diabetes accelerates NAFLD progression towards even more fatal liver disorders such as cirrhosis, hepatocellular carcinoma, and non-alcoholic steatohepatitis (NASH) [248]. Importantly, NAFLD can be considered as a reliable predictor for the development of type 2 diabetes [12].…”

“…In mice, chronic muscle loading resulted in an approx. 2-fold increase in protein [153]. However, a possible role of GLUT10 in metabolism remains to be investigated.…”

Structure, function and regulation of mammalian glucose transporters of the SLC2 family

“…The insulin-stimulated increase in the V max for transport in insulin-target tissues occurs rapidly and is considered to be almost entirely due to the rapid translocation of GLUT4 to the cell surface from an intracellular reservoir compartment, but often it is also associated with a smaller fold translocation of GLUT1 [92], and possibly other GLUTs [145].…”

Mechanical stretch activates glycometabolism‐related enzymes via estrogen in C2C12 myoblasts

“…However, would disparate modes of exercise have the same effects or not? It has been shown that acute exercise induces the rise in GLUT4 protein expression, whereas long‐term exercise loading has no significant effects on GLUT4 protein (Mcmillin, Schmidt, Kahn, & Witczak, 2017; Wu, Jiang, Wei, & Jiang, 2018). Our results also showed that mechanical stretch under our experimental conditions (15% magnitude, 1 Hz frequency, and 6‐hr duration) increased the expression of PI3K and p‐AKT proteins in the myoblasts and the amount of GLUT4 in the cell membrane was also elevated after the stretching.…”