Showing papers by "Michael F. Hirshman published in 2013"

Brown adipose tissue regulates glucose homeostasis and insulin sensitivity

Abstract: Brown adipose tissue (BAT) is known to function in the dissipation of chemical energy in response to cold or excess feeding, and also has the capacity to modulate energy balance. To test the hypothesis that BAT is fundamental to the regulation of glucose homeostasis, we transplanted BAT from male donor mice into the visceral cavity of age- and sex-matched recipient mice. By 8–12 weeks following transplantation, recipient mice had improved glucose tolerance, increased insulin sensitivity, lower body weight, decreased fat mass, and a complete reversal of high-fat diet–induced insulin resistance. Increasing the quantity of BAT transplanted into recipient mice further improved the metabolic effects of transplantation. BAT transplantation increased insulin-stimulated glucose uptake in vivo into endogenous BAT, white adipose tissue (WAT), and heart muscle but, surprisingly, not skeletal muscle. The improved metabolic profile was lost when the BAT used for transplantation was obtained from Il6–knockout mice, demonstrating that BAT-derived IL-6 is required for the profound effects of BAT transplantation on glucose homeostasis and insulin sensitivity. These findings reveal a previously under-appreciated role for BAT in glucose metabolism.

Tribbles 3 mediates endoplasmic reticulum stress-induced insulin resistance in skeletal muscle

Abstract: Endoplasmic reticulum stress has been linked to insulin resistance in multiple tissues but the role of endoplasmic reticulum stress in skeletal muscle has not been explored. Endoplasmic reticulum stress has been reported to increase tribbles 3 expression in multiple cell lines. Here, we report that high-fat feeding in mice, and obesity and type 2 diabetes in humans significantly increases tribbles 3 and endoplasmic reticulum stress markers in skeletal muscle. Overexpression of tribbles 3 in C2C12 myotubes and mouse tibialis anterior muscles significantly impairs insulin signalling. Incubation of C2C12 cells and mouse skeletal muscle with endoplasmic reticulum stressors thapsigargin and tunicamycin increases tribbles 3 and impairs insulin signalling and glucose uptake, effects reversed in cells overexpressing RNAi for tribbles 3 and in muscles from tribbles 3 knockout mice. Furthermore, tribbles 3 knockout mice are protected from high-fat diet-induced insulin resistance in skeletal muscle. These data demonstrate that tribbles 3 mediates endoplasmic reticulum stress-induced insulin resistance in skeletal muscle.

Resistance to aerobic exercise training causes metabolic dysfunction and reveals novel exercise-regulated signaling networks

Abstract: Low aerobic exercise capacity is a risk factor for diabetes and a strong predictor of mortality, yet some individuals are “exercise-resistant” and unable to improve exercise capacity through exercise training. To test the hypothesis that resistance to aerobic exercise training underlies metabolic disease risk, we used selective breeding for 15 generations to develop rat models of low and high aerobic response to training. Before exercise training, rats selected as low and high responders had similar exercise capacities. However, after 8 weeks of treadmill training, low responders failed to improve their exercise capacity, whereas high responders improved by 54%. Remarkably, low responders to aerobic training exhibited pronounced metabolic dysfunction characterized by insulin resistance and increased adiposity, demonstrating that the exercise-resistant phenotype segregates with disease risk. Low responders had impaired exercise-induced angiogenesis in muscle; however, mitochondrial capacity was intact and increased normally with exercise training, demonstrating that mitochondria are not limiting for aerobic adaptation or responsible for metabolic dysfunction in low responders. Low responders had increased stress/inflammatory signaling and altered transforming growth factor-β signaling, characterized by hyperphosphorylation of a novel exercise-regulated phosphorylation site on SMAD2. Using this powerful biological model system, we have discovered key pathways for low exercise training response that may represent novel targets for the treatment of metabolic disease.

Contraction and AICAR Stimulate IL-6 Vesicle Depletion From Skeletal Muscle Fibers In Vivo

Abstract: Recent studies suggest that interleukin 6 (IL-6) is released from contracting skeletal muscles; however, the cellular origin, secretion kinetics, and signaling mechanisms regulating IL-6 secretion are unknown. To address these questions, we developed imaging methodology to study IL-6 in fixed mouse muscle fibers and in live animals in vivo. Using confocal imaging to visualize endogenous IL-6 protein in fixed muscle fibers, we found IL-6 in small vesicle structures distributed throughout the fibers under basal (resting) conditions. To determine the kinetics of IL-6 secretion, intact quadriceps muscles were transfected with enhanced green fluorescent protein (EGFP)-tagged IL-6 (IL-6-EGFP), and 5 days later anesthetized mice were imaged before and after muscle contractions in situ. Contractions decreased IL-6-EGFP-containing vesicles and protein by 62% (P < 0.05), occurring rapidly and progressively over 25 min of contraction. However, contraction-mediated IL-6-EGFP reduction was normal in muscle-specific AMP-activated protein kinase (AMPK) α2-inactive transgenic mice. In contrast, the AMPK activator AICAR decreased IL-6-EGFP vesicles, an effect that was inhibited in the transgenic mice. In conclusion, resting skeletal muscles contain IL-6-positive vesicles that are expressed throughout myofibers. Contractions stimulate the rapid reduction of IL-6 in myofibers, occurring through an AMPKα2-independent mechanism. This novel imaging methodology clearly establishes IL-6 as a contraction-stimulated myokine and can be used to characterize the secretion kinetics of other putative myokines.

Insulin stimulation regulates AS160 and TBC1D1 phosphorylation sites in human skeletal muscle

Abstract: Individuals with obesity and type 2 diabetes (T2D) are typically insulin resistant, exhibiting impaired skeletal muscle glucose uptake. Animal and cell culture experiments have shown that site-specific phosphorylation of the Rab-GTPase-activating proteins AS160 and TBC1D1 is critical for GLUT4 translocation facilitating glucose uptake, but their regulation in human skeletal muscle is not well understood. Here, lean, obese and T2D subjects underwent a euglycemic-hyperinsulinemic clamp, and vastus lateralis muscle biopsies were obtained before, and at 30 and 180 min post insulin infusion. Obese and T2D subjects had higher body mass indexes and fasting insulin concentrations, and T2D subjects showed insulin resistance. Consistent with the clamp findings, T2D subjects had impaired insulin-stimulated phosphorylation of AS160 Thr642, a site previously shown to be important in glucose uptake in rodents. Interestingly, insulin-stimulated phosphorylation of TBC1D1 Thr590, a site shown to be regulated by insulin in rodents, was only increased in T2D subjects, although the functional significance of this difference is unknown. These data show that insulin differentially regulates AS160 and TBC1D1 phosphorylation in human skeletal muscle. Impaired insulin-stimulated glucose uptake in T2D subjects is accompanied by dysregulation of AS160 and TBC1D1 phosphorylation in skeletal muscle, suggesting that these proteins may regulate glucose uptake in humans.

Exercise Training-Induced Adaptations Associated with Increases in Skeletal Muscle Glycogen Content

Abstract: Chronic exercise training results in numerous skeletal muscle adaptations, including increases in insulin sensitivity and glycogen content. To understand the mechanism leading to increased muscle glycogen, we studied the effects of exercise training on glycogen regulatory proteins in rat skeletal muscle. Female Sprague Dawley rats performed voluntary wheel running for 1, 4 or 7 weeks. After 7 weeks of training, insulin-stimulated glucose uptake was increased in epitrochlearis muscle. As compared with sedentary control rats, muscle glycogen did not change after 1 week of training, but increased significantly after 4 and 7 weeks. The increases in muscle glycogen were accompanied by elevated glycogen synthase activity and protein expression. To assess the regulation of glycogen synthase, we examined its major activator, protein phosphatase 1 (PP1), and its major deactivator, glycogen synthase kinase (GSK)-3. Consistent with glycogen synthase activity, PP1 activity was unchanged after 1 week of training but significantly increased after 4 and 7 weeks of training. Protein expression of R(GL)(G(M)), another regulatory PP1 subunit, significantly decreased after 4 and 7 weeks of training. Unlike PP1 activity, GSK-3 phosphorylation did not follow the pattern of glycogen synthase activity. The ~ 40% decrease in GSK-3α phosphorylation after 1 week of exercise training persisted until 7 weeks, and may function as a negative feedback mechanism in response to elevated glycogen. Our findings suggest that exercise training-induced increases in muscle glycogen content could be regulated by multiple mechanisms, including enhanced insulin sensitivity, glycogen synthase expression, allosteric activation of glycogen synthase, and PP1 activity.

Akt2 influences glycogen synthase activity in human skeletal muscle through regulation of NH₂-terminal (sites 2 + 2a) phosphorylation.

Abstract: Type 2 diabetes is characterized by reduced muscle glycogen synthesis. The key enzyme in this process, glycogen synthase (GS), is activated via proximal insulin signaling, but the exact molecular e...