Insulin-induced Effects on the Subcellular Localization of AKT1, AKT2 and AS160 in Rat Skeletal Muscle

Ginger nanoparticles mediated induction of Foxa2 prevents high-fat diet-induced insulin resistance

Rationale: The obesity epidemic has expanded globally, due in large part to the increased consumption of high-fat diets (HFD), and has increased the risk of major chronic diseases, including type 2 diabetes. Diet manipulation is the foundation of prevention and treatment of obesity and diabetes. The molecular mechanisms that mediate the diet-based prevention of insulin resistance, however, remain to be identified. Here, we report that treatment with orally administered ginger-derived nanoparticles (GDNP) prevents insulin resistance by restoring homeostasis in gut epithelial Foxa2 mediated signaling in mice fed a high-fat diet (HFD). Methods: Ginger-derived nanoparticles (GDNP) were added into drinking water to treat high-fat diet fed mice for at least one year or throughout their life span. A micro array profile of intestinal, liver and fat tissue of GDNP treated mice was used to analyze their gene expression profile. Genes associated with metabolism or insulin signaling were further quantified using the real time polymerase chain reaction (RT-PCR). Surface plasmon resonance (SPR) was used for determining the interaction between Foxa2 protein and phosphatic acid lipid nanoparticles. Results: HFD-feeding inhibited the expression of Foxa2; the GDNPs increased the expression of Foxa2 and protected Foxa2 against Akt-1 mediated phosphorylation and subsequent inactivation of Foxa2. Increasing expression of Foxa2 leads to altering the composition of intestinal epithelial cell (IEC) exosomes of mice fed a HFD and prevents IEC exosome mediated insulin resistance. Collectively, oral administration of GDNP prevents insulin resistance in HFD mice. Interestingly, oral administration of GDNP also extended the life span of the mice and inhibited skin inflammation. Conclusion: Our findings showed that GDNP treatment can prevent HFD-induced obesity and insulin resistance via protecting the Foxa2 from Akt-1 mediated phosphorylation. GDNP treatment provides an alternative approach based on diet manipulation for the development of therapeutic interventions for obesity.

Role of Akt isoforms in neuronal insulin signaling and resistance

The aim of the present study was to determine the role of Akt isoforms in insulin signaling and resistance in neuronal cells. By silencing Akt isoforms individually and in pairs, in Neuro-2a and HT22 cells we observed that, in insulin-sensitive condition, Akt isoforms differentially reduced activation of AS160 and glucose uptake with Akt2 playing the major role. Under insulin-resistant condition, phosphorylation of all isoforms and glucose uptake were severely affected. Over-expression of individual isoforms in insulin-sensitive and resistant cells differentially reversed AS160 phosphorylation with concomitant reversal in glucose uptake indicating a compensatory role of Akt isoforms in controlling neuronal insulin signaling. Post-insulin stimulation Akt2 translocated to the membrane the most followed by Akt3 and Akt1, decreasing glucose uptake in the similar order in insulin-sensitive cells. None of the Akt isoforms translocated in insulin-resistant cells or high-fat-diet mediated diabetic mice brain cells. Based on our data, insulin-dependent differential translocation of Akt isoforms to the plasma membrane turns out to be the key factor in determining Akt isoform specificity. Thus, isoforms play parallel with predominant role by Akt2, and compensatory yet novel role by Akt1 and Akt3 to regulate neuronal insulin signaling, glucose uptake, and insulin-resistance.

AKT ISOFORMS-AS160-GLUT4: The defining axis of insulin resistance

The Akt isoforms-AS160-GLUT4 axis is the primary axis that governs glucose homeostasis in the body. The first step on the path to insulin resistance is deregulated Akt isoforms. This could be Akt isoform expression, its phosphorylation, or improper isoform-specific redistribution to the plasma membrane in a specific tissue system. The second step is deregulated AS160 expression, its phosphorylation, improper dissociation from glucose transporter storage vesicles (GSVs), or its inability to bind to 14-3-3 proteins, thus not allowing it to execute its function. The final step is improper GLUT4 translocation and aberrant glucose uptake. These processes lead to insulin resistance in a tissue-specific way affecting the whole-body glucose homeostasis, eventually progressing to an overt diabetic phenotype. Thus, the relationship between these three key proteins and their proper regulation comes out as the defining axis of insulin signaling and -resistance. This review summarizes the role of this central axis in insulin resistance and disease in a new light.

Redox regulation of the insulin signalling pathway

The peptide hormone insulin is a key regulator of energy metabolism, proliferation and survival. Binding of insulin to its receptor activates the PI3K/AKT signalling pathway, which mediates fundamental cellular responses. Oxidants, in particular H₂O₂, have been recognised as insulin-mimetics. Treatment of cells with insulin leads to increased intracellular H₂O₂ levels affecting the activity of downstream signalling components, thereby amplifying insulin-mediated signal transduction. Specific molecular targets of insulin-stimulated H₂O₂ include phosphatases and kinases, whose activity can be altered via redox modifications of critical cysteine residues. Over the past decades, several of these redox-sensitive cysteines have been identified and their impact on insulin signalling evaluated. The aim of this review is to summarise the current knowledge on the redox regulation of the insulin signalling pathway.

High-intensity muscle contraction-mediated increases in Akt1 and Akt2 phosphorylation do not contribute to mTORC1 activation and muscle protein synthesis

High-intensity muscle contraction (HiMC) is known to induce muscle protein synthesis, a process in which mechanistic target of rapamycin (mTOR) is reported to play a critical role. However, the mechanistic details have not been completely elucidated. Here, we investigated whether Akt plays a role in regulating HiMC-induced mTORC1 activation and muscle protein synthesis using a rodent model of resistance exercise and MK2206 (an Akt kinase inhibitor). The right gastrocnemius muscle of male C57BL/6J mice aged 10 wk was isometrically contracted via percutaneous electrical stimulation (100 Hz, 5 sets of 10 3-s contractions, 7-s rest between contractions, and 3-min rest between sets), while the left gastrocnemius muscle served as a control. Vehicle or MK2206 was injected intraperitoneally 6 h before contraction. MK2206 inhibited both resting and HiMC-induced phosphorylation of Akt1 Ser-473 and Akt2 Ser-474. MK2206 also inhibited the resting phosphorylation of p70S6K and 4E-BP1, which are downstream targets of mTORC1; however, it did not inhibit the HiMC-induced increase in phosphorylation of these targets. Similarly, MK2206 inhibited the resting muscle protein synthesis, but not the resistance exercise-induced muscle protein synthesis. On the basis of these observations, we conclude that although Akt2 regulates resting mTORC1 activity and muscle protein synthesis, HiMC-induced increases in mTORC1 activity and muscle protein synthesis are Akt-independent processes.

NEW & NOTEWORTHY Akt is well known to be an upstream regulator of mechanistic target of rapamycin (mTOR) and has three isoforms in mammals, namely, Akt1, Akt2, and Akt3. We found that high-intensity muscle contraction (HiMC) increases Akt1 and Akt2 phosphorylation; however, HiMC-induced increases in mTORC1 activity and muscle protein synthesis are Akt-independent processes.

TBC1D4 Is Necessary for Enhancing Muscle Insulin Sensitivity in Response to AICAR and Contraction

Muscle insulin sensitivity for stimulating glucose uptake is enhanced in the period after a single bout of exercise. We recently demonstrated that AMPK is necessary for AICAR, contraction, and exercise to enhance muscle and whole-body insulin sensitivity in mice. Correlative observations from both human and rodent skeletal muscle suggest that regulation of the phosphorylation status of TBC1D4 may relay this insulin sensitization. However, the necessity of TBC1D4 for this phenomenon has not been proven. Thus, the purpose of this study was to determine whether TBC1D4 is necessary for enhancing muscle insulin sensitivity in response to AICAR and contraction. We found that immediately after contraction and AICAR stimulation, phosphorylation of AMPKα-Thr172 and downstream targets were increased similarly in glycolytic skeletal muscle from wild-type and TBC1D4-deficient mice. In contrast, 3 h after contraction or 6 h after AICAR stimulation, enhanced insulin-stimulated glucose uptake was evident in muscle from wild-type mice only. The enhanced insulin sensitivity in muscle from wild-type mice was associated with improved insulin-stimulated phosphorylation of TBC1D4 (Thr649 and Ser711) but not of TBC1D1. These results provide genetic evidence linking signaling through TBC1D4 to enhanced muscle insulin sensitivity after activation of the cellular energy sensor AMPK.

Whole body glucoregulation and tissue-specific glucose uptake in a novel Akt substrate of 160 kDa knockout rat model

Akt substrate of 160 kDa (also called AS160 or TBC1D4) is a Rab GTPase activating protein and key regulator of insulin-stimulated glucose uptake which is expressed by multiple tissues, including skeletal muscle, white adipose tissue (WAT) and the heart. This study introduces a novel rat AS160-knockout (AS160-KO) model that was created using CRISPR/Cas9 technology. We compared male AS160-KO versus wildtype (WT) rats for numerous metabolism-related endpoints. Body mass, body composition, energy expenditure and physical activity did not differ between genotypes. Oral glucose intolerance was detected in AS160-KO versus WT rats (P<0.005). A hyperinsulinemic-euglycemic clamp (HEC) revealed insulin resistance for glucose infusion rate (P<0.05) with unaltered hepatic glucose production in AS160-KO versus WT rats. Genotype-effects on glucose uptake during the HEC: 1) was significantly lower in epitrochlearis (P<0.01) and extensor digitorum longus (P<0.05) of AS160-KO versus WT rats, and tended to be lower for AS160-KO versus WT rats in the soleus (P<0.06) and gastrocnemius (P<0.08); 2) tended to be greater for AS160-KO versus WT rats in white adipose tissue (P = 0.09); and 3) was significantly greater in the heart (P<0.005) of AS160-KO versus WT rats. GLUT4 protein abundance was significantly lower for AS160-KO versus WT rats in each tissue analyzed (P<0.01–0.001) except the gastrocnemius. Ex vivo insulin-stimulated glucose uptake was significantly lower (P<0.001) for AS160-KO versus WT rats in isolated epitrochlearis or soleus. Insulin-stimulated Akt phosphorylation (in vivo or ex vivo) did not differ between genotypes for any tissue tested. Ex vivo AICAR-stimulated glucose uptake by isolated epitrochlearis was significantly lower for AS160-KO versus WT rats (P<0.01) without genotype-induced alteration in AMP-activated protein phosphorylation. This unique AS160-KO rat model, which elucidated striking genotype-related modifications in glucoregulation, will enable future research aimed at understanding AS160’s roles in numerous physiological processes in response to various interventions (e.g., diet and/or exercise).

Exercise training reduces the insulin-sensitizing effect of a single bout of exercise in human skeletal muscle

KEY POINTS

A single bout of exercise is capable of increasing insulin sensitivity in human skeletal muscle. Whether this ability is affected by training status is not clear. Studies in mice suggest that the AMPK-TBC1D4 signalling axis is important for the increased insulin-stimulated glucose uptake after a single bout of exercise. The present study is the first longitudinal intervention study to show that, although exercise training increases insulin-stimulated glucose uptake in skeletal muscle at rest, it diminishes the ability of a single bout of exercise to enhance muscle insulin-stimulated glucose uptake. The present study provides novel data indicating that AMPK in human skeletal muscle is important for the insulin-sensitizing effect of a single bout of exercise.

ABSTRACT

Not only chronic exercise training, but also a single bout of exercise, increases insulin-stimulated glucose uptake in skeletal muscle. However, it is not well described how adaptations to exercise training affect the ability of a single bout of exercise to increase insulin sensitivity. Rodent studies suggest that the insulin-sensitizing effect of a single bout of exercise is AMPK-dependent (presumably via the α₂ β₂ γ₃ AMPK complex). Whether this is also the case in humans is unknown. Previous studies have shown that exercise training decreases the expression of the α₂ β₂ γ₃ AMPK complex and diminishes the activation of this complex during exercise. Thus, we hypothesized that exercise training diminishes the ability of a single bout of exercise to enhance muscle insulin sensitivity. We investigated nine healthy male subjects who performed one-legged knee-extensor exercise at the same relative intensity before and after 12 weeks of exercise training. Training increased V ̇ O 2 peak and expression of mitochondrial proteins in muscle, whereas the expression of AMPKγ3 was decreased. Training also increased whole body and muscle insulin sensitivity. Interestingly, insulin-stimulated glucose uptake in the acutely exercised leg was not enhanced further by training. Thus, the increase in insulin-stimulated glucose uptake following a single bout of one-legged exercise was lower in the trained vs. untrained state. This was associated with reduced signalling via confirmed α₂ β₂ γ₃ AMPK downstream targets (ACC and TBC1D4). These results suggest that the insulin-sensitizing effect of a single bout of exercise is also AMPK-dependent in human skeletal muscle.

Postexercise improvement in glucose uptake occurs concomitant with greater γ3-AMPK activation and AS160 phosphorylation in rat skeletal muscle

A single exercise session can increase insulin-stimulated glucose uptake (GU) by skeletal muscle, concomitant with greater Akt substrate of 160 kDa (AS160) phosphorylation on Akt-phosphosites (Thr⁶⁴² and Ser⁵⁸⁸) that regulate insulin-stimulated GU. Recent research using mouse skeletal muscle suggested that ex vivo 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) or electrically stimulated contractile activity-inducing increased γ3-AMPK activity and AS160 phosphorylation on a consensus AMPK-motif (Ser⁷⁰⁴) resulted in greater AS160 Thr⁶⁴² phosphorylation and GU by insulin-stimulated muscle. Our primary goal was to determine whether in vivo exercise that increases insulin-stimulated GU in rat skeletal muscle would also increase γ3-AMPK activity and AS160 site-selective phosphorylation (Ser⁵⁸⁸, Thr⁶⁴², and Ser⁷⁰⁴) immediately postexercise (IPEX) and/or 3 h postexercise (3hPEX). Epitrochlearis muscles isolated from sedentary and exercised (2-h swim exercise; studied IPEX and 3hPEX) rats were incubated with 2-deoxyglucose to determine GU (without insulin at IPEX; without or with insulin at 3hPEX). Muscles were also assessed for γ1-AMPK activity, γ3-AMPK activity, phosphorylated AMPK (pAMPK), and phosphorylated AS160 (pAS160). IPEX versus sedentary had greater γ3-AMPK activity, pAS160 (Ser⁵⁸⁸, Thr⁶⁴², Ser⁷⁰⁴), and GU with unaltered γ1-AMPK activity. 3hPEX versus sedentary had greater γ3-AMPK activity, pAS160 Ser⁷⁰⁴, and GU with or without insulin; greater pAS160 Thr⁶⁴² only with insulin; and unaltered γ1-AMPK activity. These results using an in vivo exercise protocol that increased insulin-stimulated GU in rat skeletal muscle are consistent with the hypothesis that in vivo exercise-induced enhancement of γ3-AMPK activation and AS160 Ser⁷⁰⁴ IPEX and 3hPEX are important for greater pAS160 Thr⁶⁴² and enhanced insulin-stimulated GU by skeletal muscle.

The cell biology of systemic insulin function

Insulin is the paramount anabolic hormone, promoting carbon energy deposition in the body. Its synthesis, quality control, delivery, and action are exquisitely regulated by highly orchestrated intracellular mechanisms in different organs or "stations" of its bodily journey. In this Beyond the Cell review, we focus on these five stages of the journey of insulin through the body and the captivating cell biology that underlies the interaction of insulin with each organ. We first analyze insulin's biosynthesis in and export from the β-cells of the pancreas. Next, we focus on its first pass and partial clearance in the liver with its temporality and periodicity linked to secretion. Continuing the journey, we briefly describe insulin's action on the blood vasculature and its still-debated mechanisms of exit from the capillary beds. Once in the parenchymal interstitium of muscle and adipose tissue, insulin promotes glucose uptake into myofibers and adipocytes, and we elaborate on the intricate signaling and vesicle traffic mechanisms that underlie this fundamental function. Finally, we touch upon the renal degradation of insulin to end its action. Cellular discernment of insulin's availability and action should prove critical to understanding its pivotal physiological functions and how their failure leads to diabetes.

The effects of heat stress on morphological properties and intracellular signaling of denervated and intact soleus muscles in rats

The effects of heat stress on the morphological properties and intracellular signaling of innervated and denervated soleus muscles were investigated. Heat stress was applied to rats by immersing their hindlimbs in a warm water bath (42°C, 30 min/day, every other day following unilateral denervation) under anesthesia. During 14 days of experimental period, heat stress for a total of seven times promoted growth‐related hypertrophy in sham‐operated muscles and attenuated atrophy in denervated muscles. In denervated muscles, the transcription of ubiquitin ligase, atrogin‐1/muscle atrophy F‐box (Atrogin‐1), and muscle RING‐finger protein‐1 (MuRF‐1), genes was upregulated and ubiquitination of proteins was also increased. Intermittent heat stress inhibited the upregulation of Atrogin‐1, but not MuRF‐1 transcription. And the denervation‐caused reduction in phosphorylated protein kinase B (Akt), 70‐kDa heat‐shock protein (HSP70), and peroxisome proliferator‐activated receptor γ coactivator‐1α (PGC‐1α), which are negative regulators of Atrogin‐1 and MuRF‐1 transcription, was mitigated. In sham‐operated muscles, repeated application of heat stress did not affect Atrogin‐1 and MuRF‐1 transcription, but increased the level of phosphorylated Akt and HSP70, but not PGC‐1α. Furthermore, the phosphorylation of Akt and ribosomal protein S6, which is known to stimulate protein synthesis, was increased immediately after a single heat stress particularly in the sham‐operated muscles. The effect of a heat stress was suppressed in denervated muscles. These results indicated that the beneficial effects of heat stress on the morphological properties of muscles were brought regardless of innervation. However, the responses of intracellular signaling to heat stress were distinct between the innervated and denervated muscles.

Thick Filament Protein Network, Functions, and Disease Association

Sarcomeres consist of highly ordered arrays of thick myosin and thin actin filaments along with accessory proteins. Thick filaments occupy the center of sarcomeres where they partially overlap with thin filaments. The sliding of thick filaments past thin filaments is a highly regulated process that occurs in an ATP-dependent manner driving muscle contraction. In addition to myosin that makes up the backbone of the thick filament, four other proteins which are intimately bound to the thick filament, myosin binding protein-C, titin, myomesin, and obscurin play important structural and regulatory roles. Consistent with this, mutations in the respective genes have been associated with idiopathic and congenital forms of skeletal and cardiac myopathies. In this review, we aim to summarize our current knowledge on the molecular structure, subcellular localization, interacting partners, function, modulation via posttranslational modifications, and disease involvement of these five major proteins that comprise the thick filament of striated muscle cells. © 2018 American Physiological Society. Compr Physiol 8:631-709, 2018.

Biochemical and cellular properties of insulin receptor signalling

The mechanism of insulin action is a central theme in biology and medicine. In addition to the rather rare condition of insulin deficiency caused by autoimmune destruction of pancreatic β-cells, genetic and acquired abnormalities of insulin action underlie the far more common conditions of type 2 diabetes, obesity and insulin resistance. The latter predisposes to diseases ranging from hypertension to Alzheimer disease and cancer. Hence, understanding the biochemical and cellular properties of insulin receptor signalling is arguably a priority in biomedical research. In the past decade, major progress has led to the delineation of mechanisms of glucose transport, lipid synthesis, storage and mobilization. In addition to direct effects of insulin on signalling kinases and metabolic enzymes, the discovery of mechanisms of insulin-regulated gene transcription has led to a reassessment of the general principles of insulin action. These advances will accelerate the discovery of new treatment modalities for diabetes.

Exosomes derived from pancreatic cancer cells induce insulin resistance in C2C12 myotube cells through the PI3K/Akt/FoxO1 pathway

Prospective epidemiological studies have consistently suggested that pancreatic cancer-associated new-onset diabetes mellitus (PC-DM) represents a potential platform for early diagnose of pancreatic cancer (PC). Despite the studies performed, the mechanism behind this phenomenon remains ambiguous. In this study, we explored the effects of two types of exosomes released by murine pancreatic cancer and ductal epithelial cells on murine skeletal muscle cells. The results show that PC-derived exosomes can readily enter C2C12 myotubes, triggering lipidosis and glucose intake inhibition. We also demonstrate that PC-derived exosomes can inhibit insulin and PI3K/Akt signalling, in which insulin-induced FoxO1 nuclear exclusion is preserved and Glut4 trafficking is impaired. Microarray and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses show that exosomal microRNAs (miRNAs) probably play key roles in this process, an assumption that is corroborated by in vitro studies. These results confirm that the insulin resistance (IR) of skeletal muscle cells is governed by PC-derived exosomes through the insulin and PI3K/Akt/FoxO1 signalling pathways, where exosomal miRNAs potentially contribute to this phenomenon. These novel findings pave the way towards a comprehensive understanding of the cancer theories: "metabolic reprogramming" and "metabolic crosstalk".

Heterotypic endosomal fusion as an initial trigger for insulin-induced glucose transporter 4 (GLUT4) translocation in skeletal muscle

KEY POINTS

Comprehensive imaging analyses of glucose transporter 4 (GLUT4) behaviour in mouse skeletal muscle was conducted. Quantum dot-based single molecule nanometry revealed that GLUT4 molecules in skeletal myofibres are governed by regulatory systems involving 'static retention' and 'stimulus-dependent liberation'. Vital imaging analyses and super-resolution microscopy-based morphometry demonstrated that insulin liberates the GLUT4 molecule from its static state by triggering acute heterotypic endomembrane fusion arising from the very small GLUT4-containing vesicles in skeletal myofibres. Prior exposure to exercise-mimetic stimuli potentiated this insulin-responsive endomembrane fusion event involving GLUT4-containing vesicles, suggesting that this endomembranous regulation process is a potential site related to the effects of exercise.

ABSTRACT

Skeletal muscle is the major systemic glucose disposal site. Both insulin and exercise facilitate translocation of the glucose transporter glucose transporter 4 (GLUT4) via distinct signalling pathways and exercise also enhances insulin sensitivity. However, the trafficking mechanisms controlling GLUT4 mobilization in skeletal muscle remain poorly understood as a resuly of technical limitations. In the present study, which employs various imaging techniques on isolated skeletal myofibres, we show that one of the initial triggers of insulin-induced GLUT4 translocation is heterotypic endomembrane fusion arising from very small static GLUT4-containing vesicles with a subset of transferrin receptor-containing endosomes. Importantly, pretreatment with exercise-mimetic stimuli potentiated the susceptibility to insulin responsiveness, as indicated by these acute endomembranous activities. We also found that AS160 exhibited stripe-like localization close to sarcomeric α-actinin and that insulin induced a reduction of the stripe-like localization accompanying changes in its detergent solubility. The results of the present study thus provide a conceptual framework indicating that GLUT4 protein trafficking via heterotypic fusion is a critical feature of GLUT4 translocation in skeletal muscles and also suggest that the efficacy of the endomembranous fusion process in response to insulin is involved in the benefits of exercise.