Exercise training increases components of the c-Cbl-associated protein/c-Cbl signaling cascade in muscle of obese Zucker rats

Insulin-stimulated endoproteolytic TUG cleavage links energy expenditure with glucose uptake

TUG tethering proteins bind and sequester GLUT4 glucose transporters intracellularly, and insulin stimulates TUG cleavage to translocate GLUT4 to the cell surface and increase glucose uptake. This effect of insulin is independent of phosphatidylinositol 3-kinase, and its physiological relevance remains uncertain. Here we show that this TUG cleavage pathway regulates both insulin-stimulated glucose uptake in muscle and organism-level energy expenditure. Using mice with muscle-specific Tug (Aspscr1)-knockout and muscle-specific constitutive TUG cleavage, we show that, after GLUT4 release, the TUG C-terminal cleavage product enters the nucleus, binds peroxisome proliferator-activated receptor (PPAR)γ and its coactivator PGC-1α and regulates gene expression to promote lipid oxidation and thermogenesis. This pathway acts in muscle and adipose cells to upregulate sarcolipin and uncoupling protein 1 (UCP1), respectively. The PPARγ2 Pro12Ala polymorphism, which reduces diabetes risk, enhances TUG binding. The ATE1 arginyltransferase, which mediates a specific protein degradation pathway and controls thermogenesis, regulates the stability of the TUG product. We conclude that insulin-stimulated TUG cleavage coordinates whole-body energy expenditure with glucose uptake, that this mechanism might contribute to the thermic effect of food and that its attenuation could promote obesity.

Skeletal Muscle CAP Expression Increases after Dietary Restriction and Aerobic Training in Women with a History of Gestational Diabetes

The purpose is to determine the effects of 6 months caloric restriction and aerobic training (3x/wk) (CR+AEX) on c-CBL associated protein (CAP) gene expression in women with a history of GDM. CAP is involved in cell signaling and protein ubiquitination, and is linked to the development of insulin resistance. Obese (BMI=32 ± 1 kg/m², % fat=46 ± 2, X ± SEM), sedentary (VO2 max=21.2 ± 1.2 ml/kg/min), women aged 52 ± 2 years participated in 6 months D+WL (n=10) with body composition, fitness (VO2 max), and glucose tolerance testing. Insulin sensitivity was assessed during the last 30 min of 2-hour hyperinsulinemic-euglycemic clamps (40 mU.m^-2.min^-1) pre and post interventions. Vastus lateralis skeletal muscle biopsies (n=7) were conducted and CAP, GLUT4 and glycogen synthase (GS) gene expression measured by RT-PCR. No change in FFM by DXA was observed, but body weight decreased 8% with losses of total body fat mass (P<0.05) and a 10% increase in VO2 max (P<0.01). Glucose and insulin areas under the curve by OGTT decreased (P<0.05). Glucose utilization during the clamp increased 27% (23.1 ± 3.8 vs. 29.4 ± 3.6 umol.kg.min^-1, P<0.05). Vastus lateralis skeletal muscle CAP expression increased 21% (P<0.05) but GLUT4 did not. Results suggest that changes in CAP could be involved in the improvement in glucose metabolism with caloric restriction and aerobic training in women with a history of gestational diabetes.

Regulation of glucose transporter translocation in health and diabetes

To enhance glucose uptake into muscle and fat cells, insulin stimulates the translocation of GLUT4 glucose transporters from intracellular membranes to the cell surface. This response requires the intersection of insulin signaling and vesicle trafficking pathways, and it is compromised in the setting of overnutrition to cause insulin resistance. Insulin signals through AS160/Tbc1D4 and Tbc1D1 to modulate Rab GTPases and through the Rho GTPase TC10α to act on other targets. In unstimulated cells, GLUT4 is incorporated into specialized storage vesicles containing IRAP, LRP1, sortilin, and VAMP2, which are sequestered by TUG, Ubc9, and other proteins. Insulin mobilizes these vesicles directly to the plasma membrane, and it modulates the trafficking itinerary so that cargo recycles from endosomes during ongoing insulin exposure. Knowledge of how signaling and trafficking pathways are coordinated will be essential to understanding the pathogenesis of diabetes and the metabolic syndrome and may also inform a wide range of other physiologies.

Ponsin interacts with Nck adapter proteins: implications for a role in cytoskeletal remodelling during differentiation of skeletal muscle cells

Skeletal muscle differentiation is a complex process: It is characterised by changes in gene expression and protein composition. Simultaneously, a dramatic remodelling of the cytoskeleton and associated cell-matrix contacts, the costameres, occurs. The expression and localisation of the protein ponsin at cell-matrix contacts marks the establishment of costameres. In this report we show that skeletal muscle cells are characterised by a novel ponsin isoform, which contains a large insertion in its carboxy-terminus. This skeletal muscle-specific module binds the adapter proteins Nck1 and Nck2, and increased co-localisation of ponsin with Nck2 is observed at remodelling cell-matrix contacts of differentiating skeletal muscle cells. Since this ponsin insertion can be phosphorylated, it may adjust the interaction affinity with Nck adapter proteins. The novel ponsin isoform and its interaction with Nck1/2 provide exciting insight into the convergence of signalling pathways at the costameres, and its crucial role for skeletal muscle differentiation and re-generation.

Activation of atypical protein kinase Czeta toward TC10 is regulated by high-fat diet and aerobic exercise in skeletal muscle

We determined whether sustained aerobic exercise reverses high-fat diet-induced impairments in the c-Cbl associated protein (CAP)/Casitas b-lineage lymphoma (c-Cbl) signaling cascade in rodent skeletal muscle. Sprague-Dawley rats were placed into either control (n = 16) or high-fat-fed (n = 32) diet groups for 4 weeks. During a subsequent 4-week experimental period, 16 high-fat-fed rats remained sedentary, 16 high-fat-fed rats completed 4 weeks of exercise training, and control animals were sedentary and remained on the control diet. After the intervention period, animals were subjected to hind limb perfusions in the presence (n = 8 per group) or absence (n = 8 per group) of insulin. In the plasma membrane fractions, neither high-fat feeding nor exercise training altered adaptor protein with PH and SH2 domains, (APS), c-Cbl, or TC10 protein concentrations. In contrast, CAP protein concentration and insulin-stimulated plasma membrane c-Cbl tyrosine phosphorylation were reduced by high-fat feeding; but exercise training reversed these impairments. Of note was that insulin-stimulated atypical protein kinase Czeta kinase activity toward TC10 was reduced by high-fat feeding but normalized by exercise training. We conclude that sustained (4 weeks) exercise training can reverse high-fat diet-induced impairments on the CAP/c-Cbl pathway in high-fat-fed rodent skeletal muscle. We also provide the first evidence that the CAP/c-Cbl insulin signaling cascade in skeletal muscle may directly interact with components of the classic (phosphoinositide 3-kinase dependent) insulin signaling cascade.